Terpenes: Modulating anti-inflammatory signaling in inflammatory bowel disease

Abstract

Inflammatory Bowel Disease (IBD) are autoimmune diseases characterized by chronic intestinal inflammation. Considered a western disease, IBD incidence in newly developed countries is skyrocketing. Accordingly, global prevalence is steadily increasing. There are two major IBD phenotypes, ulcerative colitis (UC) and Crohn’s disease (CD). UC manifests as uninterrupted inflammation localized in the colon and rectum. Meanwhile, CD presents as interrupted inflammation that can occur throughout the digestive tract. As a result, therapeutics have focused on anti-inflammatory approaches for its treatment. Unfortunately, only 50% of patients benefit from current Food and Drug Administration approved treatments, and all are associated with serious adverse effects. Thus, there is a need for safer and novel therapeutics to increase the efficacy in this population. One aspect that is critical in understanding IBD is how food and phytochemicals therein may be associated with modifying the pathogenesis of IBD. A variety of retrospective and prospective studies, and clinical trials have shown benefits of plant-rich diets on the prevention and symptomatic improvement of IBD. The Mediterranean diet is rich in vegetables, fruits, legumes, and herbs; and characterized by the abundance of anti-inflammatory phytochemicals. An understudied phytochemical class enriched in this diet is terpenes; isoprene-based molecules are widely available in Mediterranean herbs and citrus fruits. Various terpenes have been evaluated in different IBD models. However, some present contradictory or inconclusive results. Therefore, in this review we evaluated preclinical studies of terpenes modulating basic inflammatory signaling related to IBD.

1. Introduction

Inflammatory Bowel Disease (IBD) are polygenic autoimmune disorders characterized by intestinal chronic inflammation. In 2017, an estimated 6.8 million people in 192 countries and territories were diagnosed with IBD; the United States and Europe accounted for 3 million of these newly diagnosed IBD patients.[1] Until the mid-20^th century, high income, and highly industrialized countries, including the United States, United Kingdom, and Australia, had a significant increase in IBD incidence.[1,2] However, by the end of the 20th century, incidence started to plateau, and prevalence took the lead; this was in part due to better treatments and patients’ increased life expectancy. Traditionally, IBD has been considered a disease of the West and most often associated with a low nutrition diet and exposure to environmental toxins. In the beginning of the 21^st century, the global IBD prevalence and incidence scenario progressed. Countries including China, South Korea, Japan, and India developed further economically and as a result, there was a steep increase in IBD incidence. Interestingly, as countries develop their industrial market, their populations begin to experience increased incidence much like their counterparts in the West. Regarding India, there is a sharp increase in incidence and as of now, a plateau of incidence has not been reached. Based on these populations studies, the evidence suggests that IBD is no longer described as a Western disease significant efforts will be needed to reduce the global impact of IBD on the healthcare industry.

IBD is a highly heterogenous disease classified into four types: Crohn’s disease (CD), ulcerative colitis (UC), microscopic colitis, and intermediate colitis. Within the major forms, CD and UC, multiple subtypes can be observed and are indicated by their location in the digestive tract and/or mutated molecular pathways.[3] For consistency, in this review we will be referring to IBD as Crohn’s disease and ulcerative colitis. IBD is a polygenic disease which becomes active when susceptible individuals are triggered by environmental factors. Researchers have identified more than 240 genetic risk loci for IBD with around 40 unique to CD and around 30 to UC. These loci include genes involved in epithelial barrier function, microbial defense, immune system, and cellular homeostasis. The first CD mutation found was a polymorphism in the nucleotide binding oligomerization domain containing 2 (NOD2) gene which encodes for an intracellular peptidoglycan sensor.[3,4] This mutation is prevalent in CD patients of European descent (30%), however, it is not as common in other ethnic groups. Another well-known CD mutation is polymorphisms in the autophagy related 16 like 1 gene that leads to abnormal Paneth cell formation and like NOD2, knockout studies show no causation. These alterations account for 8–13% of known CD and 4–7% of known UC disease variance, and 70% of them are present in other immune-modulated disease; therefore suggesting the importance of the environment in IBD progression.[3,5]

Terpenes are the largest class of plant-derived secondary metabolites and are classified based on the number of isoprene units in their backbone.[6,7] Isoprene is the most abundant hydrocarbon in the planet, thus the high number of terpene species is estimated to exceed 300,000 terpene molecules. From a phytochemistry perspective, this explains the high amount of encoded terpene synthases in the plant genome and the role of plants as major producers of terpenes. Terpenes are often differentiated from terpenoids -oxidized, hydrated, or dehydrated terpenes- however, in this review, we will use the broader term terpenes.[7]

The most basic terpenes, hemiterpenes, have one isoprene unit (5C); meanwhile, monoterpenes have 2 units (10 C) and sesquiterpenes have 3 units (15 C).[6] These three subclasses have low molecular weight conferring them volatility and aromaticity. Bigger species grouped as diterpenes (20 C), sesterterpenes (25 C), triterpenes (30 C), tetraterpenes (40 C), and polyterpenes (40 C to 5 × 10⁴ C) can be aromatic but not volatile. Nonetheless, as shown in Figure 1, their structural diversity is richer due to the vast number of possible combinations. In addition to their abundance, terpenes typically display superior pharmacokinetics and are metabolized by phase two enzymes.[8–11] For example, carnosic acid and carnosol from rosemary had a T_max of 0.25 h and a C_max of 54.016 and 5.008 μM, respectively when administered through a rosemary oil extract at 100 mg/kg.[8]

Figure 1.

Chemical structure of exemplary terpenes. a) geraniol, b) oleuropein, c) asperuloside, d) carnosic acid, e) neorogioldiol, f) CX-10, g) nimbolide, h) CDDO-Im, i) ganoderic acid C1, j) alantolactone, k) tussilagone, l) nerolidol, m) glycyrrhizin, n) compound K, and o) kalopanaxsaponin A.

Due to their abundance and above-mentioned characteristics, terpenes have been studied in multiple disease states.[6] As a class, they have the following medicinal properties: antioxidant, anti-inflammatory, anti-tumor, anti-coagulant, anti-bacterial, anti-fungal, anti-viral, anti-parasitic, anti-aggregatory, analgesic, and sedative. The most popular terpene-derived drugs include the anti-cancer diterpene paclitaxel, and the anti-malarial sesquiterpene artemisinin. In this review we describe terpenes and the targeting of selected cell signaling pathways for the treatment of IBD as shown in Table 1 and Figure 2. In 2020, Araruna et al., published a review focusing on terpene modulation of IBD, however, they only included 12 compounds.[12] By focusing on basic inflammation pathways we present a comprehensive article that reviewed 80 phytochemicals as of May 1, 2023.

Table 1.

Summary of the terpenes that modulate inflammation in IBD pre-clinical models.

Compound	Category	Source	Targets	References
Acanthoic acid	Diterpene	Acanthropanax koreanum	↓ NF-ƙB, MAPKs, TNF-α, IL-8, chymase	(Kang et al., 2010); (Kim et al., 2004)
Alantolactone	Sesquiterpene	Inula helenium L. and Inula japonica	↑ PXR; ↓ NF-ƙB, IL-1β	(Ren et al., 2019)
AL-1	Diterpene	Synthetic conjugate	↑ IL-10, PPARγ; ↓ NF-ƙB, IƘB, IL-1β, IL-6, TNF-α, IFN-γ, JNK, ERK, CXCL10, PGE2, COX-2	(Guo et al., 2019); (Jiang et al., 2020); (Jing, et al., 2019); (Yang, et al., 2016)
Amyrin	Triterpene	Protium kleinii	↑ IL-10; ↓ NF-ƘB, CREB, IL-8 TNF-α, IL-1β, VEGF, CXCL1, ICAM-1, VCAM-1, PCAM-1, β2-integrin, and P-selectin	(Matos et al., 2013); (Vitor et al., 2009)
Andrographolide	Diterpene	Andrographis paniculata	↑ ACC, GSK3β; ↓ NF-ƘB, MAPKs, IL-1β, IL-6, TNF-α, IL-17A, IL-23, RORɣt, STAT3, Th17	(Kim et al., 2019); (Zhu et al., 2018)
Asperuloside	Monoterpene	Hedyotis diffusa	↑ Nrf2, IL-10; ↓ NF-ƘB, IL-6, TNF-α	(Chen et al., 2021)
Astragaloside IV	Triterpene	Radix astragalus	↓ NF-ƘB, IƘB, IL-6, TNF-α, IL-1β, NO	(Wu and Chen, 2019)
Betulin	Triterpene	Plants from the Betula L. genus	↓ NF-ƘB, IL-1β, IL-6, TNF-α, TLR4, caspase 3, caspase 8	(El-Sherbiny et al., 2021)
Borneol	Monoterpene	Salvia officinalis, Valeriana officinalis, Chamaemelum nobile, Salvia rosmarinus, and Lavandula angustifolia	↓ IL-1β, IL-6	(Juhás et al., 2008)
Brusatol	Triterpene	Brucea javanica	↑ IL-10, IL-4, SOD, GPX; ↓ NF-ƘB, IFN-γ, IL-1β, IL-6, TNF-α, TLR4	(Zhou et al., 2017)
Caesaldekarine	Diterpene	Caesalpinia bonduc	↓ TNF-α, IL-1β, IL-6	(Liu et al., 2023)
Carnosic acid	Diterpene	Salvia rosmarinus	↑ Nrf2, SOD2, GPX2, HO-1, GSH; ↓ NF-ƘB, MAPKs, IL-6, TNF-α, IL-17A, IFN-γ, IL-1β, IL-18, caspase 1	(Yang et al., 2017)
Catalposide	Monoterpene	Catalpa ovata	↓ NF-ƘB, MAPKs, TNF-α, IL-1β, IL-8, ICAM-1	(Kim et al., 2004)
Celastrol	Triterpene	Tripterygium wilfordii Hook F	↑ IL-10, SOD, GST, GSH; ↓ IFN-β, IL-17A, IL-1β, IL-6, IL-8, TNF-α, IFN-γ, IL-23, IL-17A, CXCL1, CXCL2, mTOR, CD98	(Pinna et al., 2004); (Shaker et al., 2014); (Zhao et al., 2015);
Compound K	Saponin	Bacterial metabolism of ginsenosides	↑ IL-10, PXR, CYP3A4; ↓ NF-ƘB, TNF-α, IL-1β, IL-6, IL-17, IL-23, COX-2, iNOS, IRAK-1	(Joh et al., 2011); (Lee, et al., 2015); (Li et al., 2014); (Zhang et al., 2015)
Deacetylnomilin	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Dehydrocostus lactone	Sesquiterpene	Aucklandia lappa Decne	↑ SOD; ↓ TNF-α, IL-6, IL-17, IL-1β, IL-23, IL-6Rβ, STAT3, MCP-1, COX-2, iNOS	(Zhou et al., 2020)
Diammonium glycyrrhizinate	Saponin	Glycyrrhizin salt	↓ NF-ƘB, TNF-α	(Yuan et al., 2006)
Euphol	Triterpene	Euphorbia tirucalli	↓ NF-ƘB, IL-6, TNF-α, IL-1β, IL-18, CXCL1, MIP-2, iNOS, VEGF, ICAM-1, P-selectin, E-selectin	(Dutra et al., 2011)
(E)-β-caryophyllene	Sesquiterpene	Cannabis genus	↓ IL-6	(Cho et al., 2007)
Ganoderic acid C1	Triterpene	Ganoderma lucidum	↓ NF-ƙB, TNF-α, IFN-γ, IL-17A	(Liu et al., 2015)
Genipin	Monoterpene	Gardenia jasminoides	↑ Nrf2; ↓ NF-ƘB, TNF-α, IL-1β	(Li et al., 2020)
Geniposide	Monoterpene	Gardenia jasminoides	↑ PPARγ; ↓ NF-ƘB, IƘB, TNF-α, IL-1β, IL-6	(Zhang, et al., 2017)
Gentiopicroside	Monoterpene	Gentiana lutea	↓ COX-2, iNOS, TNF-α, IL-1β	(Niu et al., 2016)
Geraniol	Monoterpene	Dietary plants	↑ SOD, GSH, PPARγ; ↓ NF-ƘB, TNF-α, IL-1β, IL-6, Nitrites, TBARS, NO, ICAM-1, p38, PGE2, caspase 3	(Medicherla et al., 2015); (Soubh et al., 2015)
Ginsenoside Rb1	Saponin	Panax ginseng Meyer	↑ PXR, IL-10; ↓ NF-ƘB, IL-1β, IFN-γ, TNF-α, IL-6, COX-2, iNOS, IRAK-1	(Joh et al., 2011); Zhang et al., 2015)
Ginsenoside Re	Saponin	Panax ginseng Meyer	↑ IL-10; ↓ NF-ƘB, IL-1β, TNF-α, IL-6, iNOS, COX-2	(Lee et al., 2012)
Ginsenoside Rd	Saponin	Panax ginseng Meyer	↑ SOD, GPX; ↓ MAPKs, IL-1β, TNF-α, IL-6, NO, iNOS	(Yang et al., 2012); (Yang et al., 2012)
Ginsenoside Rf	Saponin	Panax ginseng Meyer	↓ NF-ƘB, MAPKs, IL-1β, IL-6	(Ahn et al., 2016)
Ginsenoside Rg1	Saponin	Panax ginseng Meyer	↑ IL-10; ↓ NF-ƘB, IL-1β, TNF-α, IL-17, IL-23, COX-2, iNOS	(Lee, et al., 2015)
Ginsenoside Rh1	Saponin	Panax ginseng Meyer	↑ IL-10; ↓ NF-ƘB, IL-1β, TNF-α, IL-17, IL-23, COX-2, iNOS	(Lee, et al., 2015)
Ginsenoside Rh2	Saponin	Panax ginseng Meyer	↑ PTEN; ↓ NF-ƘB, MAPKs, IL-1β, IL-6, TNF-α, STAT3, miRNA-214	(Chen et al., 2021)
Glycyrrhetic acid	Saponin	Glycyrrhiza uralensis	↓ NF-ƘB, TNF-α, IL-1β, IL-6	(Jeon et al., 2016)
Glycyrrhizin	Saponin	Glycyrrhiza galabra L.	↑ IL-10, SOD, PPARγ; ↓ NF-ƘB, TNF-α, IL-1β, IL-6, IL-17, IL-12, IFN-γ	(Liu et al., 2011); (Sethurman et al., 2015); (Sun et al., 2009); (Wu et al., 2022)
Ichangensin	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Ichangin	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Kalopanaxsaponin A	Saponin	Kalopanax pictus	↑ IL-10, IRAK-1; ↓ NF-ƘB, IL-6, TNF-α, IL-1β	(Joh and Kim, 2011)
Libertellenone M	Diterpene	Stibella fimetaris	↓ IL-1β, IL-18, caspase 1	(Fan et al., 2020)
Limonene	Monoterpene	Citrus fruits	↑ SOD, GSH, p-ERK 1/2; ↓ NF-ƘB, TNF-α, IL-1β, TGF-β	(d’Alessio et al., 2013); (Yu et al., 2017)
Limonin	Triterpene	Rutaceace and Meliaceae family	↑ IL-10, PTEN, PDLIM2; ↓ NF-ƘB, IL-6, TNF-α, IL-17, STAT3, miRNA-214	(Liu et al., 2019)
Loganin	Monoterpene	Flos lonicerae and Cornus fruit	↓ NF-ƘB, JAK, IL-1β, IL-6, TNF-α, STAT3, TLR4	(Wang et al., 2019); (Yuan, et al, 2020)
Lupeol	Triterpene	Dietary plants	↓ NF-ƘB, IL-8	(Lee et al., 2016)
Magnesium isoglycyrrhizinate	Saponin	Glycyrrhizin salt	↓ NF-ƘB, IL-1β, IL-17, IFN-γ, TNF-α, ROS	(Cui et al., 2021)
Menthol	Monoterpene	Mentha genus	↑ GSH; ↓ IL-1β, IL-6, IL-1, IL-23, TNF-α	(Bastaki et al., 2018); (Ghasemi-Pirbaluti et al., 2017)
Methylnomilinate	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Methyldeacetylnomilinate	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Monotropein	Monoterpene	Morinda officinali	↓ NF-ƙB	(Shin et al., 2013)
Morroniside	Monoterpene	Flos lonicerae and Cornus fruit	↓ NF-ƘB, IL-1β, IL-6, TNF-α, STAT3	(Yuan, et al, 2020)
Nagilactone B	Diterpene	Podicarpus macrophyllus	↓ IL-8	(Kim et al, 2021)
Neorogioldiol	Diterpene	Laurencia glandulifera	↓ IL-1β, IL-6, TNF-α	(Daskalaki et al., 2019)
0, 11,15-cyclo-14-bromo-14,15-dihydrorogiol-3,11-diol	Diterpene	Laurencia glandulifera	↓ IL-1β, IL-6, TNF-α	(Daskalaki et al., 2019)
Nerol	Monoterpene	Agastache Mexicana and citral	↓ IL-13, TNF-α	(González-Ramirez et al., 2016)
Nerolidol	Sesquiterpene	Ornamental and edible plants	↑ SOD, CAT, GSH, IL-6; ↓ IL-1β, IL-23	(Bastaki et al., 2021)
Nimbolide	Triterpene	Azadirachta indica	↓ NF-ƘB, IL-8	(Seo et al., 2016)
Nomilin	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Nomilinate A ring lactone	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Notoginsenoside R1	Saponin	Panax notoginseng	↑ PXR; ↓ NF-ƙB, IL-1β, IFN-γ, TNF-α, IL-6, IL-2, IL-1α	(Zhang et al., 2015)
Oleuropein	Monoterpene	Olea europaea	↓ NF-ƘB, TNF-α, IL-1β, IL-6, ROS	(Giner et al., 2011); (Huguet-Casquero et al., 2020), (Larussa et al., 2017)
Oleanolic acid	Triterpene	Dietary plants	↓ IL-8, IL-23, CXCL1, CXCL10, CXCL11, sICAM,	(Mueller et al., 2013)
Paeoniflorin	Monoterpene	Paeonia lactiflora	↑ Nrf2; ↓ NF-ƘB, MAPKs, MMP-9, IL-6, IL-17, IFN-γ, TNF-α, TLR4	(Wu et al., 2019); (Zhang et al., 2014)
Parthenolide	Sesquiterpene	Tanacetum parthenium	↓ NF-ƘB, TNF-α, IL-1β	(Zhao et al., 2012)
Perillaldehyde	Monoterpene	Perilla frutescens	↑ IL-1β; ↓ MMP-9, IL-6, TNF-α	(Uemura et al., 2018)
Phenylpropyl triterpenes	Triterpenes	Osamanthus fragrans var. aurantiacus	↓ IL-8	(Jeong et al., 2020)
Plumericin	Monoterpene	Himatantus scuba	↑ Nrf2; ↓ NF-ƘB, IL-1β, TNF-α, nitrotyrosine, ROS, caspase 1	(Rapa et al., 2020)
Pristimerin	Triterpene	Tripterygium wilfordii Hook F	↑ Nrf2; ↓ NF-ƘB, IL-6, TNF-α, IL-17, IL-1β, miRNA-155	(Tian et al., 2021)
Saikosaponin A	Saponin	Bupleurum falcatum L.	↓ NF-ƘB, IƘB, TNF-α, IL-1β	(Zhou, et al., 2019)
Saikosaponin D	Saponin	Bupleurum falcatum L.	↑ IL-10; ↓ IƘB, IL-6, TNF-α, IL-1β	(Li et al., 2020)
Sudachinoid	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
Tanshinone IIA	Diterpene	Salvia miltiorrhiza Bunge	↑ GSH, PXR; ↓ NF-ƙB, TNF-α, IL-1β, IL-6	(Bai et al., 2008); (Zhang et al., 2015)
Thymol	Monoterpene	Lamiaceae family	↓ NF-ƘB, TNF-α, IL-6, IL-1, COX-2, NO	(Chamanara et al., 2019); (Tahmasebi et al., 2019)
Triptolide	Diterpene	Tripterygium wilfordii Hook F	↓ NF-ƘB, TNF-α, IFN-γ, IL-4, IL-12, IL-6, IL-17, IL-23, IL-6Rβ, STAT3, RORɣt, MyD88, TLR2, TLR4	(Li et al., 2020); (Wei et al., 2008); (Yu et al., 2011)
Tussilagone	Sesquiterpene	Tussilago farfara	↑ Nrf2; ↓ NF-ƘB, IL-6, TNF-α, cPLA2α	(Cheon et al., 2018)
Unnamed	Triterpenes	Alnus japonica	↓ IL-8	(Lee et al., 2017)
Unnamed	Triterpenes	Lycopodium clavatum	↓ IL-8	(Jo et al., 2020)
Ursolic acid	Triterpene	Dietary plants	↑ SOD, IL-10, IRAK-1, IRAK-4, IL-1Ra; ↓ NF-ƘB, IL-8, IL-23, TNF-α, IL-1β, IL-6, TLR4, CXCL1, CXCL10, sICAM	(Chun et al., 2014); (Jang et al., 2014); (Liu et al., 2016); (Mueller et al., 2013)
1-O-methylichangensin	Triterpene	Citrus junos	↓ IL-8	(Shin et al., 2020)
1-[2-cyano-3, 12-dioxo-oleana-1, 9 (11)-dien-28-oyll-dien-28-oyl] imidazole	Triterpene	Synthetic	↓ IL-16, IL-17, IL-17A, IL-17F, STAT3	(Fitzpatrick et al., 2014)
1, 8-cineole	Monoterpene	Myrtaceae, Lamiaceae, and Zingiberaceae families	↑ Nrf2, SOD, CAT, NQO1, HO-1, PPARγ; ↓ NF-ƘB, MAPKs, IL-6, TNF-α, IL-17A, IFN-γ, IL-1β, IL-8, Nitrites, iNOS, COX-2, CXCL1	(Venkataraman et al., 2023)
3, 14, 19-triacetylandrographolide	Diterpene	Andrographolide synthetic derivate	↓ NF-ƙB, MAPKs, TNF-α, IL-6	(Gao et al., 2018)
14-O-acetylneoline	Diterpene	Acontium laciniatum	↓ IFN-γ	(Wangchuck et al., 2015)

Figure 2.

Cell signaling pathways modulated by terpenes in IBD models. Transcriptionally targeted genes are examples of transcribed proteins and not an exhaustive record.

2. Major pathological characteristics of IBD and therapeutic strategies

IBD patients present with a variety of clinical symptoms with the most common being rectal bleeding, anemia, abdominal pain, incontinence, diarrhea, and weight loss.[13] Additionally, those with IBD are at a higher risk of other autoimmune diseases including psoriasis, sclerosing cholangitis, and ankylosing spondylitis. Both Crohn’s disease and ulcerative colitis have specific risk loci translating into pathological and symptomatic differences. CD is characterized by non-continuous progressive intramural inflammation that can occur at any part of the digestive tract. The most common complications are fistulas, strictures, dysplasia, neoplasia, and intestinal granulomas. Conversely, UC is characterized by superficial (mucosa-bound) continuous inflammation localized to the colon and rectum. The most common observed complications in UC patients are ulcers, mucosal erosion, fibrosis, and polyps.[13] These pathophysiological traits are explained by molecular damage to the intestinal tissue. The hallmark description of IBD is that microorganisms, including commensal bacteria, infiltrate into the lamina propria, the intestinal tissue where immune cells reside, activating resident immune cells, thus commencing an inflammatory response that further breaks down the mucosal barrier and recruits exogenic innate immune cells which causes more inflammation. While IBD was discovered a century ago the full etiology remains elusive. There are likely several variables that contribute to the etiology including changes in the microbiome, mucosal barrier proteins or intestinal immune cells that originate the cycle further emphasizing that IBD is a complex and multifactorial disease.

The intestinal mucosal barrier is the gateway to a diversely populated space, the lamina propria.[14] In IBD, mutations in mucous bound proteins, goblet cell secreted proteins, along with tight and adherent junction proteins are common, contributing to structural changes of the gastrointestinal tract.[14] The small intestine has one discontinuous mucosal layer, while the colon is organized in three layers (glycocalyx, inner layer and outer layer). The intestinal mucosa is the first line of defense, it seizes luminal pathogens for screening by resident dendritic cells and holds anti-microbial peptides and immunoglobulins. In UC the most abundant mucin protein (MUC2) is downregulated, and goblet cell (epithelial cells that produce mucous) population decreases, thus eroding the first shield.[14] On the contrary, in CD, mucous production increases yet, the proteins are defective and cannot seize pathogens. These events allow a higher number of pathogens and commensal bacteria to interact with the epithelium. The epithelium is primarily composed of a single layer of ciliated epithelial cells held together by tight and adherent junction proteins. These proteins create a space between cells called the paracellular space which allows passage of small molecules to the lamina propria. In IBD the formation of tight and adherent junction proteins critical as a barrier are downregulated, while pore promoting tight and adherent junctions are upregulated.[14] Consequently, as the paracellular space becomes wider commensal bacteria and pathogens can infiltrate into the lamina propria. After crossing the epithelium, commensal bacteria display a proinflammatory phenotype causing resident immune cells to counter them. Subsequently, this hyperactivation of the intestinal immune system exacerbates epithelial cell and mucous dwindling and perpetuates the inflammatory cycle.

There are five classes of Food and Drug Administration (FDA) approved drugs for the treatment of IBD with the primary goal of addressing inflammation.[15] The first drug used to treat IBD patients was the aminosalicylate sulfasalazine, a prodrug to 5-aminosalycilic acid (5-ASA) approved in the 1940s for both UC and CD treatment.[15] In 2007, a colon-targeted aminosalicylate, mesalamine, was approved by the FDA presenting less adverse effects.[16] The second oldest drug are corticosteroids, steroidal hormones that bind the glucocorticoid receptor which plays a central role in inflammation.[15] Immunomodulatory drugs have been used since the 1960s for the treatment of both UC and CD and are divided in three subgroups, thiopurines, calcineurin inhibitors, and the molecule methotrexate.[17] The most commonly prescribed thiopurines are azathioprine, 6-marcaptopurine, and 6-thioguanine.[15,17] Newer drugs target unique cell signaling pathways that modulate the innate and adaptive immune response to inflammation. They have less off target effects, thus decreasing the number of adverse effects. This new generation consists of small molecules and monoclonal antibodies; all small molecules are used to treat UC, and the antibodies are mixed. Details of all agents approved in the United States are found in table 2.[15,18–20]

Table 2.

Summary of the FDA approved drugs used for the treatment of IBD. Listed adverse effects do not constitute an exhaustive list.

Name	Drug Class	Mechanism of Action	Adverse Effects	Notes
Sulfasalazine	Aminosalicylate	Disrupts arachidonic acid conversion into proinflammatory prostaglandins and leukotrienes. It can also scavenge ROS and decrease T-cell pro-inflammatory cytokine production(Cai et al., 2021)	Flatulence, nausea, abdominal pain, diarrhea, headaches, infertility, hemolytic anemia, photosensitization, granulocytosis(Cai et al., 2021)	Most prescribed but benefit in CD patients is limited
Mesalamine	Aminosalicylate	Induces Treg proliferation in the colon through TGFβ activation by AhR(Oh-oka et al., 2017)	Flatulence, nausea, abdominal pain, diarrhea, headaches, aplastic anemia, bone marrow suppression, NMSC, renal failure(Nakashima and Preuss, 2023)
Prednisone, Prednisolone	Corticosteroid 1st generation	Inhibit the activation of pro-inflammatory transcription factors and promotes transcription of anti-inflammatory proteins(Cai et al., 2021)	Increased risk of opportunistic infections, diabetes mellitus, hypertension, ocular damage, venous thrombosis, osteoporosis(Cai et al., 2021)	Not used for maintenance because their inefficacy to do so and long-term serious adverse effects
Budesonide	Corticosteroid 2nd generation	Inhibits the activation of pro-inflammatory transcription factors and promotes transcription of anti-inflammatory proteins(Cai et al., 2021)	Increased risk of opportunistic infections, diabetes mellitus, hypertension, ocular damage, venous thrombosis, osteoporosis(Cai et al., 2021)	Decreased adverse effects compared to 1st generation because of targeted delivery to the colon
Methotrexate	Immunomodulator	Inhibits enzymes involved in T-cell DNA synthesis(Cai et al., 2021)	Increased risk of serious and opportunistic infections, fatigue, peritoneal abscess, hypoalbuminemia, atypical pneumonia(Cai et al., 2021; Magro et al., 2020)	Clinical efficacy only observed in CD patients
Azathioprine	Immunomodulator Thiopurine	Inhibits T-cell proliferation and activation(Cai et al., 2021)	Increased risk of serious and opportunistic infections, bone marrow suppression, liver injury, lymphomas, gastrointestinal intolerance(Cai et al., 2021; Magro et al., 2020)	Most prescribed thiopurine
6-marcaptopurine	Immunomodulator Thiopurine	Inhibits T-cell proliferation and activation(Cai et al., 2021)	Increased risk of serious and opportunistic infections, bone marrow suppression, liver injury, lymphomas, gastrointestinal intolerance(Cai et al., 2021; Magro et al., 2020)
6-thioguanine	Immunomodulator Thiopurine	Inhibits T-cell proliferation and activation(Cai et al., 2021)	Increased risk of serious and opportunistic infections, bone marrow suppression, liver injury, lymphomas, gastrointestinal intolerance(Cai et al., 2021; Magro et al., 2020)
Cyclosporin A	Immunomodulator Calcineurin inhibitor	Inhibit activation of NFAT(Cai et al., 2021)	Tremors, renal function damage, hot flashes, increased susceptibility to infections, and hyperkalemia(Cai et al., 2021)	Clinical efficacy only observed in UC patients
Ozanimod	Small molecule	Selective inhibitor of S1RP1 and S1RP5(Cai et al., 2021)	Increased risk of opportunistic and serious infections, liver injury, headache, PML, cardiovascular events, fetal harm, pulmonary function decline, macular edema, increased blood pressure, PRES(Sandborn et al., 2021)	Approved in 2022. Contraindicated for patients with heart conditions
Tofacitinib	Small molecule	Nonselective inhibitor of all JAKs (1, 2, and 3)(Cai et al., 2021)	Rash, diarrhea, headache, elevated cholesterol, increased risk of opportunistic and serious infections, lymphomas, lung cancer, cardiovascular events, thrombosis(Sandborn et al., 2022a)	Fast therapeutic onset
Upadacitinib	Small molecule	Inhibits JAK-1(Cai et al., 2021)	Acne, rash, folliculitis, increased risk of opportunistic and serious infections, cardiovascular events, NMSC, thrombosis(Irani et al., 2023; Sandborn et al., 2020)	Approved in 2022
Infliximab	Monoclonal antibody	Binds secreted TNF-α(Cai et al., 2021)	Headache, abdominal pain, increased risk of opportunistic and serious infections, cervical cancer, leukemia, lymphoma, hepatotoxicity, hepatitis B reactivation, higher risk of CNS diseases(Hanauer et al., 2002)	Chimeric antibody
Adalimumab	Monoclonal antibody	Binds secreted TNF-α(Cai et al., 2021)	Headache, rash, increased risk of opportunistic and serious infections, tuberculosis, lymphoma, hepatitis B reactivation, higher risk of CNS diseases, cytopenia, CHF(Burmester et al., 2020)	Fully humanized antibody and most prescribed to CD patients
Vedolizumab	Monoclonal antibody	Binds to the α4β7 integrin(Cai et al., 2021)	Increased risk of opportunistic and serious infections, liver injury, headache, arthralgia, anemia PML, abdominal pain(Colombel et al., 2017; Loftus et al., 2020)	Recombinant human antibody used for anti-TNF-α non-responsive patients because it does not modulate mucosal inflammation
Ustekinumab	Monoclonal antibody	Binds to the p40 subunit of IL-12 and IL-23(Cai et al., 2021)	Increased risk of opportunistic and serious infections, vomiting, diarrhea, pruritus, NMSC(Abreu et al., 2022; Sandborn et al., 2022b)	Fully humanized antibody
Risankizumab	Monoclonal antibody	Binds to the p19 subunit of IL-23(Cai et al., 2021)	Headaches, arthralgia, anemia, arthropathy, increased risk of serious and opportunistic infections, hepatotoxicity, tuberculosis(D’Haens et al., 2022; Ferrante et al., 2022)	Fully humanized antibody

3. Terpenes modulating inflammatory cytokines and chemokines

3.1. Terpenes modulating cytokines

Levels of interleukin 8 (IL-8) pro-inflammatory cytokine were measured from lipopolysaccharide (LPS) insulted HT-29 human adenocarcinoma cells treated with different terpenes. Pre-treatment with 100 μM nagilactone B, a norditerpenoid dilactone from Podicarpus macrophyllus; 100 μM of unidentified triterpenes from the Lycopodium clavatum moss, 10 and 25 μM of unidentified Alnus japonica triterpenes, and 5 and 10 μM phenylpropyl triterpenes isolated from Osamanthus fragrans var. aurantiacus by Jeong et al., dose-dependently diminished IL-8 levels.[21–24] Meanwhile, sudachinoid, ichangensin, 1-O-methylichangensin, nomilin, deacetylnomilin, methylnomilinate, methyldeacetylnomilinate, nomilinate A ring lactone, ichangin limonoids isolated from Citrus junos -commonly known as yuzu- dose-dependently reduced IL-8 levels at 1 and 10 μM.[25] Contrasting the above-mentioned terpenes, celastrol, a Tripterygium wilfordii Hook F triterpene, was tested in colon tissue from CD patients.[26] 0.1, 1, and 10 μM celastrol downregulated IL-8 as well as pro-inflammatory cytokines tumor necrosis factor α (TNF-α), interleukin 6 (IL-6), and interleukin 1β (IL-1β). Interestingly, while there was a dose dependent reduction of IL-6, IL-8, and IL-1β; 0.1 μM was as effective as 1 and 10 μM at reducing TNF-α levels, suggesting a more potent effect towards this cytokine which is central in IBD pathogenesis and the target of some biologics.

Borneol, a camphor derivative isolated from Salvia officinalis (sage), Valeriana officinalis (valerian), Chamaemelum nobile (chamomile), Salvia rosmarinus (rosemary), and Lavandula angustifolia (lavender) was orally administered to mice with the oxidizing agent trinitrobenzene sulfonic acid (TNBS) which induces acute colitis through colonic ulceration and consequent immune response.[27] 135 and 270 mg/kg of borneol was preventively and diagnostically administered through the feed.[28] Unexpectedly, borneol had no effect on colon weight, mortality, histological score, and macroscopic damage, yet at both concentrations, it similarly downregulated IL-1β and IL-6 in tissue. The addition of borneol to the feed might have dampened the effects of borneol. The used formulation represents the consumption of borneol through the diet, so to imitate a drug, oral gavage would have been a better option which would have delivered a higher quantity of borneol to the tissue. Like borneol, O, 11,15-cyclo-14-bromo-14,15-dihydrorogiol-3,11-diol and neorogioldiol, metabolites from the brominated algal compound neorogioltriol found in the Laurencia genus, did not alter colon length in acutely colitic mice co-treated with the epithelial damaging detergent dextran sulfate sodium (DSS).[27,29] Still, the intraperitoneal 0.25 mg dose decreased histological damage, as well as TNF-α, IL-1β, and IL-6 expression. In this case, the lack of macroscopical change might have been due to intermittent administration of the test compounds; the mice were dosed every two days. Oleuropein, a monoterpene from Olea europaea -commonly known as olive was another terpene that showed molecular modulation without clinical observations. A dose of 500 mg/kg of the monoterpene were loaded into nanostructured lipid carriers and administered orally to mice with DSS-induced acute colitis.[30] Intriguingly, neither oleuropein suspension (500 mg/kg) nor encapsulation improved colitis clinical score (stool consistency, blood in stool, and body weight), yet it reduced pathohistological score, neutrophil tissue infiltration marker myeloperoxidase (MPO), and reactive oxygen species (ROS) levels in tissue. Oleuropein also downregulated TNF-α, and IL-6, still the free suspension was better than its encapsulated counterpart. Nanoparticles can deliver a compound in higher quantities, however, if the compound does not have the efficacy to treat a disease, then its concentration will not influence the results completely. Therefore, oleuropein might not be effective enough in the DSS model to exert changes that transcend into symptomatic amelioration.

Oxazolone is another molecule used to induce colitis by producing ulcerations in the distal colon.[27] 10, 30, 100, 177.8, and 300 mg/kg of nerol, the cis-isomer of geraniol found in Agastache Mexicana and citral, was orally and diagnostically administered to mice with oxazolone-induced acute colitis.[31] Although nerol reduced the clinical score from 30 to 300 mg/kg, the authors only presented histologic and molecular data for the 177.8 mg/kg dose. At the chosen dose, nerol improved histological damage and lesion severity, while downregulating TNF-α and interleukin13 expression. Acetic acid can also be used to mimic colitis in vivo; it produces superficial tissue damage, ulceration and inflammation.[32] In rats with acetic acid-induced acute colitis, the alcohol monoterpene from the Mentha genus, menthol, was administered intraperitoneally.[33] In this model, menthol at 50 and 80 mg/kg ameliorated weight loss, macroscopic damage, ulcer area, colon shortening and weight, histological score and MPO. In addition, menthol treatment downregulated IL-6, IL-1β, and TNF-α. In another study with acetic acid injured rats, one group was administered menthol diagnostically and the other with both preventively and diagnostically.[34] In the two groups, 50 mg/kg menthol reduced weight loss, ulcer score, levels of lipid oxidation marker malondialdehyde (MDA), MPO, lymphocytic infiltration, fecal colon inflammatory biomarker calprotectin, as well as expression of interleukin 1 (IL-1), interleukin 23 (IL-23), and TNF-α. Additionally, menthol increased antioxidant molecule glutathione (GSH) levels in both groups. In this study, there was no significant difference between menthol preventive or concomitant treatment, suggesting that menthol could be used as a preventive tool.

Caesaldekarine is a cassane diterpene abundant in the seed kernels of Caesalpinia bonduc.[35] In mice with DSS-induced acute colitis, 20 mg/kg intraperitoneal or oral caesaldekarine diagnostic treatment decreased clinical score, colon shortening, MPO, histological damage, as well as tissue and serum IL-6, TNF-α, and IL-1β. Overall, the intraperitoneal administration was more potent than the oral route, suggesting that hepatic metabolites drive the anti-inflammatory effects. An example of this occurrence is carnosic acid which after oral administration rapidly becomes glucuronidated.[10] The diterpene alkaloid 14-O-acetylneoline which is isolated from Acontium laciniatum was preventively administered to mice with TNBS-induced acute colitis.[36] In this model, 20 μg intraperitoneal 14-O-acetylneoline consistently reduced clinical score, macroscopic score, colon shortening, histological damage, and interferon γ (IFN-γ). From the 3 doses, 10 and 50 μg were inconsistent throughout the tested metrics, suggesting that at those concentrations, 14-O-acetylneoline is not a good agent to target both, molecular and physiological symptoms. This demonstrates the importance of a correct dose, one that can successfully alter multiple disease parameters. (E)-β-caryophyllene (BCP) is a bicyclic sesquiterpene found in many essential oils like clove and species from the Cannabis genus.[37] 12.5, 25, and 50 mg/kg BCP was orally co-administered to mice with DSS-induced acute colitis. However, only 50 mg/kg showed improved colon elongation, histological score, and clinical score while diminishing MPO and IL-6 levels.

Ginsenosides are triterpenoid steroidal compounds and the main bioactive components in the root of Panax ginseng C.A. Meyer, commonly known as ginseng.[38] There is over 100 known ginsenosides which are classified based on their backbone: 20(S)-protopanaxatriol (PPT) and 20(S)-protopanaxadiol (PPD). Mice with relapsing TNBS-induced acute colitis were treated orally with 10, 20, and 40 mg/kg of ginsenoside Rd after relapse.[39] All Rd doses similarly reduced weight loss, macroscopic damage, histological damage, colon weight and length, MPO, TNF-α, IL-1β, and IL-6. Kinases need to be phosphorylated before their activation, so a more accurate measurement of kinases is that of their phosphorylated self. In this model, all Rd doses equivalently reduced activated mitogen-activated protein kinases (MAPKs) p38 and c-Jun N-terminal kinase (JNK) by decreasing their phosphorylated/dephosphorylated ratio.

Perillaldehyde is a volatile monoterpene and the major component of the Perilla frutescens leaf essential oil. [40] Preventive administration of 50, 100, and 200 mg/kg oral perillaldehyde to mice followed by acute DSS co-treatment reduced clinical score, histological score, colon shortening, as well as expression of TNF-α, IL-6, and metalloproteinase 9 (MMP-9) protein involved in tissue inflammation and apoptosis.[40] From the three studied doses, 100 mg/kg was the only one with consistent action thought clinical and molecular metrics, therefore, perillaldehyde has a narrow dosing efficiency which limits its potential clinical use. Strangely, IL-1β levels increased after perillaldehyde treatment, however, the authors did not present an explanation for this event. There is strong consensus on the pro-inflammatory and pro-apoptotic behavior of IL-1β. However, in IBD, although present, this cytokine does not play a central role in driving inflammation.[41] Therefore, perillaldehyde might modulate a pathway involved in IL-1β expression or cleavage from pro-IL-1β. Nevertheless, the possibility of inflammation or apoptosis promotion by perillaldehyde should be addressed.

Mice with DSS-induced acute colitis were diagnostically treated with 100 or 200 mg/kg of oral gentiopicroside, a secoiridoid found in Gentiana lutea, reducing colon shortening, clinical score, histological score, MPO, TNF-α, IL-1β, IL-6, cyclooxygenase 2 (COX-2), and the nitric oxide (NO) synthesizer inducible nitric oxide synthetase (iNOS).[42] Only the 200 mg/kg dose modulated molecular markers. Furthermore, compared to the positive control, sulfasalazine, gentiopicroside at 200 mg/kg, was more effective. Another compound that was more effective than sulfasalazine was ginsenoside Rd, which was orally administered to rats with relapsing TNBS-induced acute colitis after relapse.[43] In this model, 10, 20, and 40 mg/kg Rd equivalently reduced macroscopic damage, colon shortening and weight, weight loss, MPO, MDA, NO levels, and iNOS expression. Meanwhile, it upregulated antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPX).

Glycyrrhizin (GCZ) is a saponin responsible for the sweet taste of Glycyrrhiza glabra L. root, one of three Glycyrrhiza plants referred to as licorice.[44] Its two glucuronic acid moieties confer GCZ colon specificity, thus it was studied in mice with TNBS-induced acute colitis.[44,45] 90 mg/kg diagnostic oral GCZ treatment decreased weight loss, mortality, colon shortening, histological score, MPO, and expression of IL-6, interleukin 12 (IL-12), interleukin 17 (IL-17), TNF-α, and IFN-γ.[45] Additionally, GCZ upregulated interleukin 10 (IL-10) secretion. Most in vivo GCZ studies use either oral or intraperitoneal administration, however, when the targeted tissue is the colon, intrarectal administration can be a better alternative because of its directness and wide safety profile.[46] Therefore, rats with TNBS-induced acute colitis were diagnostically treated with intrarectal (2, 10, and 50 mg/kg) or oral (10 mg/kg) GCZ.[47] Both regimens lowered colon shortening, colonic lesions, histological damage, MPO, MDA, and lymphocyte proliferation in the spleen. Furthermore, it downregulated TNF-α and IL-1β, while upregulating SOD. Interestingly, both routes were equivalent at 10 mg/kg, suggesting that 1) the Bacteroidetes and Firmicutes needed for GCZ metabolism are found in the colon and 2) it is a viable therapeutic route for IBD localized in the colon.[48,49] GCZ has low water solubility resulting in low oral bioavailability, thus folic acid-zein-pectin nanoparticles were used for colon-targeted diagnostical delivery of GCZ in mice with DSS-induced acute colitis.[50] As expected, the orally delivered nanoparticles were superior to the GCZ free suspension in reducing clinical score, colon shortening, mortality, histological damage, MPO, macrophage tissue infiltration, and expression of IL-6 and TNF-α. The success of the GCZ nanoparticles over unencapsulated GCZ was due to the accumulation of nanoparticles in the colon of diseased mice over healthy, which allowed for a higher concentration of GCZ to reach the targeted organ. Of note, both treatment groups were administered the same dose of GCZ, 40 mg/kg.

IL-17 is the primary cytokine secreted by T helper 17 (Th17) and is involved in both UC and CD pathogenesis.[51] Like many other pro-inflammatory cytokines, IL-17 expression is principally mediated by the transcription factor signal transducer and activator of transcription 3 (STAT3).[52] In mice with DSS-induced or TNBS-induced acute colitis, the synthetic triterpene 1-[2-cyano-3, 12-dioxo-oleana-1, 9 (11)-dien-28-oyll-dien-28-oyl] imidazole (CDDO-Im) was orally co-administered in the DSS model and diagnostically administered in the TNBS model.[53] In both models, 20 mg/kg CDDO-Im improved clinical score, colon shortening, histological score, colon weight, reduced IL-17 and IL-6 expression, and STAT3 binding to IL-17 promoter region. Additionally, CDDO-Im diminished secretion of interleukin 17A (IL-17A) and interleukin 17F (IL-17F) in IL-1β and IL-23 splenocytes and colonic strips from TNBS and DSS mice. IL-17F is a known pro-inflammatory cytokine in the DSS model, however, the role of IL-17A is contended since it has been found to be both protective and pathogenic in said model.[51] Andrographolide, a diterpene isolated from Andrographis paniniculata, was also tested for its IL-17 modulating properties. Mice with TNBS-induced acute colitis were diagnostically treated with 100 mg/kg oral andrographolide.[54] Besides improving histological damage, andrographolide downregulated TNF-α, IL-1β, IL-6, IL-17A, and IL-23. Furthermore, it reduced the ratio of p-STAT3/STAT3, Th17 cell population, and expression of the main Th17 differentiation transcription factor retinoic acid receptor-related orphan receptor Ɣ (RORɣt). Thus, the data suggests that andrographolide is immunoregulatory lowering inflammation by decreasing T effector cell differentiation into Th17 cells.

IL-6 is another cytokine involved in IBD, whereas IL-17 is secreted by Th17 cells, IL-6 polarizes naïve T-cells into Th17 cells.[51] In addition, IL-6 initiates the signaling pathway that activates STAT3 creating an inflammatory loop.[52] Nerolidol, an aliphatic sesquiterpene alcohol common in ornamental plants was orally administered to rat with acetic acid colitis preventively or both preventively and diagnostically.[55] 50 mg/kg nerolidol decreased macroscopic damage, histological damage, weight loss, MPO, MDA, calprotectin levels, and expression of IL-1β and IL-23 (another Th17 polarizing cytokine). It also increased GSH levels and upregulated catalase (CAT) and SOD. Surprisingly, nerolidol did not affect TNF-α levels but upregulated IL-6. IL-6 is a pleiotropic cytokine with both pro- and anti-inflammatory capabilities. For example, IL-10 and IL-6 deficient (IL-10^−/−/IL-6^−/−) mice presented with significantly more colitis symptoms and inflammation than IL-10^−/−.[56] Hence, nerolidol might promote activation of IL-6 mediated anti-inflammatory pathways. In the macroscopic analysis, the group treated preventively and diagnostically revealed superior results than the group that was only treated preventively. However, for the molecular analysis, except GSH, the preventive-only group was more effective in a few analyses. This may suggest that the diagnostic treatment has a superficial effect or chemical effect, for example as an ROS scavenger.

3.2. Terpenes modulating chemokines and cell adhesion molecules

Chemokines are another family of small proteins secreted by innate immune cells that mediate multiple cell processes, the principal one being immune cell movement and recruitment better known as chemotaxis.[57] Chemokine receptors (CKRs) are found in the cell surface of many cell types, but the ones that mediate chemotaxis connect resident innate immune cells (chemokine) to circulating immune cells (CKRs). In IBD, hyperactivation of resident innate immune cells drives up chemotaxis and population of recruited circulating pro-inflammatory immune cells perpetuating the immunoinflammatory cycle.[58] Hence, chemokine expression is a reliable indicator of inflammation. After chemokine secretion and consequent arrival of immune cells, cell adhesion molecules (CAMs) are responsible for their arrest at the endothelial cells of the designated tissue.[59,60] CAMs are transmembrane proteins that mediate physical connections between cells or with the extracellular matrix (ECM). There are four families of CAMs: immunoglobulin-like, integrins, cadherins, and selectins. Cadherins are the only ones that are not directly involved with immune cells, they bind to the ECM, tight junctions, and other adherent junctions. The other subfamilies are expressed in endothelial, immune cells, and platelets; selectins being the only ones involved in leukocyte rolling to their destined tissue. Contrasting chemokines, CAMs indicate the population of circulating immune cells that made it to the tissue contributing to local inflammation. Therefore, CAM expression in intestinal tissue is also utilized to measure inflammation, although they are not adequate for mechanism of action deciphering because there is only a handful of them.

Catalposide, an iridoid glycoside from Catalpa ovata, was administered preventively and for a second time before relapse to mice with TNBS-induced relapsing acute colitis.[61] 100 μg/mL intrarectal catalposide improved body weight, macroscopic damage, colon length, histological damage, and downregulated NF-ƙB, TNF-α, IL-1β, and intercellular adhesion molecule 1 (ICAM-1), an immunoglobulin-like CAM. In stimulated T84 human colon epithelial cells, 5–100 μM ursolic acid and oleanolic acid, triterpene carboxylic acids available in many dietary and medicinal plants, downregulated IL-8, IL-23, chemokines C-X-C motif chemokine ligand 10 (CXCL10) and C-X-C motif chemokine ligand 1 (CXCL1), and soluble intercellular adhesion molecule (ICAM) -proteolytically cleaved and circulating ICAM.[62] In addition, ursolic acid upregulated anti-inflammatory cytokine interleukin 1 receptor antagonist (IL-1Ra), while oleanolic acid downregulated chemokine C-X-C motif chemokine ligand 11. AL-1 is a synthetic water soluble andrographolide-lipoic acid conjugate that confers andrographolide water solubility as well as colon specificity.[63] In mice with DSS-induced acute colitis, 45 mg/kg diagnostic AL-1 oral treatment diminished histological score, colon shortening, as well as TNF-α, IL-6, IFN-γ, and CXCL10 expression.[64] Another study that investigated AL-1 diagnostically treated mice with DSS-induced acute colitis with oral 5, 15, and 45 mg/kg.[65] In this model, AL-1 decreased clinical score, colon shortening, macroscopic damage, and expression of TNF-α, IL-6, and IL-1β. It also upregulated IL-10.

Euphol a triterpene from Euphorbia tirucalli was investigated for its immunosuppressive activity in mice with DSS-induced acute colitis.[66] 30 mg/kg oral euphol was preventively administered showing improved clinical score, colon length, macroscopic damage, reduced proliferative cells, histological damage, MPO, as well as expression of phosphorylated p65 (p-p65), TNF-α, IL-6, and IL-1β. Euphol downregulated chemokines CXCL1, macrophage inflammatory molecule 2, and monocyte chemoattractant protein 1 (MCP-1), and CAMs; ICAM-1, vascular cell adhesion protein 1 (VCAM-1), lymphocyte function-associated antigen 1, P-selectin, and E-selectin, as well as angiogenesis related proteins iNOS and vascular endothelial growth factor (VEGF). For circulating immune cells to enter their targeted tissues, they need to cross the capillaries, so chemokines also regulate vascularity.[57] Together, the data suggests that euphol suppresses inflammation by decreasing the number of proliferative immune cells and consequently the amount of secreted pro-inflammatory mediators and recruited cells. Another triterpene studied for its immunosuppressive abilities was amyrin, a racemic mixture of α-amyrin and β-amyrin found in Protium kleinii and other plants.[67] 10 mg/kg oral co-administration of amyrin to mice with DSS-induced acute colitis reduced clinical score, macroscopic damage, colon shortening, MPO, histological damage, pro-inflammatory macrophage population and tissue infiltration, proliferative cells, and expression of TNF-α, IL-1β, CXCL1, ICAM-1, VCAM-1, platelet endothelial cell adhesion molecule 1, β2-integrin, and P-selectin.[67] Like euphol, amyrin reduced numbers of immune cells and their secreted compounds, however, for amyrin the pro-inflammatory macrophage population was measured giving more insight into its immunosuppressive effect.

CD98 is a transmembranal glycoprotein expressed by intestinal epithelial cells under inflammatory stress.[68] Activation of CD98 prevents epithelial barrier reconstruction as well as tissue homeostasis, therefore contributing to barrier erosion and the inflammatory cycle. CD98 expression is regulated by IFN-γ, and its activation also promotes overexpression of pro-inflammatory cytokines and chemokines. Celastrol (2 mg/kg) was orally and diagnostically administered to mice with DSS-induced acute colitis decreasing clinical score, MPO, histological score, MDA, as well as the expression of IL-1β, IFN-γ, IL-23, IL-17A, and CD98.[69] Meanwhile, it upregulated IL-10, SOD, glutathione s transferase, and GSH levels. To further assess the inhibitory mechanism of celastrol on CD98, the authors compared macrophage population between celastrol treated and DSS-only treated mice observing no significant difference. Thus, celastrol decreased CD98 by downregulating IFN-γ expression in the already present macrophage population. Interestingly, celastrol significantly upregulated TNF-α as well as nitric oxide in both the cecum and colon of mice compared to DSS-only treatment. Recent work has uncovered a more complicated role for TNF-α in UC where it also has non-pathogenic functions.[70] For example, it triggers steroidogenic enzymes in intestinal epithelial cells to produce glucocorticoids.[71] Moreover, TNF-α-deficient mice subjected to DSS treatment presented worsen colitis symptoms.[72] Although nitric oxide is a type of RNS, basal concentrations are beneficial promoting blood flow and consequent ulceration healing.[73] Therefore, celastrol might be stimulating the anti-inflammatory properties of said molecules overshadowing their harmful effects.

4. Terpenes targeting selected cell signaling pathways in IBD

4.1. NF-ƙB Pathway

NF-ƙB is the quintessential pro-inflammatory transcription factor and is activated and upregulated in inflammatory diseases including IBD.[74] The canonical NF-ƙB signaling pathway starts with activation of pattern recognition receptors (PRRs), cytokine receptors, tumor necrosis factor receptors (TNFRs), T-cell receptors (TCRs), or B cell receptors.[74] Their activation prompts phosphorylation of NF-ƙB repressor protein nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor α (IκBα) by IκB kinase (IKK) and its consequent ubiquitination by the proteasome. After separation from IκBα, NF-ƙB translocates into the nucleus where it binds promoter regions of target genes. Target genes of NF-ƙB can be classified under pro-inflammatory cytokines, chemokines, adhesion molecules, and enzymes, as well as anti-apoptotic proteins and cell cycle regulators.[74] There are five members in the NF-ƙB family: p50, p52, p65, RelB and c-Rel which form heterodimers with p65 in the cytoplasm.[74] Because p65 is the subunit that is always activated, researchers use it as a marker for NF-ƙB pathway activation.

The importance of NF-ƙB in inflammation is well known being expressed in almost every cell, thus it is a commonly evaluated target in IBD research. For example, in LPS insulted colon tissue from UC patients 3 mM oleuropein downregulated COX-2 as well as IL-17, both NF-ƙB targets.[75] The iridoid glycoside monotropein is extracted from the roots of Morinda officinali.[76] Diagnostic administration of 100 and 200 mg/kg monotropein to mice with DSS-induced acute colitis dose-dependently ameliorated clinical score while decreasing MPO, expression of iNOS, COX-2, and phosphorylated nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor α (p-IƙBα), and nuclear translocation of both NF-ƙB subunits. Meanwhile, it upregulated IƙBα. Oral 10 and 20 mg/kg tanshinone IIA (Tan IIA) diagnostic treatment of mice with TNBS-induced acute colitis similarly reduced clinical score, MPO, nuclear p65, and expression of TNF-α and IL-1β.[77] Additionally, the diterpene from Salvia miltiorrhiza Bunge increased GSH levels. The data suggests that Tan IIA ameliorates colitis inflammation by downregulating NF-ƙB and its promoted cytokines, TNF-α and IL-1β. 20-O-β-D-glucopyranosyl-20(S)-protopanaxadiol better known as compound K is metabolized by microbiota from ginsenosides Rb1, Rc, and Rb2.[78] In mice with DSS-induced acute colitis, oral co-treatment with 5, 10 and 20 mg/kg intraperitoneal compound K dose-dependently decreased weight loss, colon shortening, histological damage, and expression of TNF-α, IL-6, IL-1β, nuclear p65, and p-IƙBα.[78] Compound K also upregulated IL-10 and IƙBα.

4 and 8 mg/kg of oral saikosaponin D (SSD) -the major bioactive compound in Bupleurum falcatum L.- were preventively and diagnostically administered to mice with DSS-induced acute colitis.[79] SSD lessened clinical score, colon shortening, histological score, and expression of TNF-α, IL-6, IL-1β, and p-IƙB. Additionally, SSD upregulated IL-10. In another study, the epimer of SSD, saikosaponin A (SSA) also showed NF-ƙB pathway modulation.[80] In mice with DSS-induced acute colitis, preventive and diagnostic administration of 5, 10, and 20 mg/kg oral SKA dose-dependently ameliorated clinical score, colon shortening, histological score, MPO, and expression of TNF-α, IL-1β, p-p65, and p-IƙB. Therefore, both SSD and SSA inhibited NF-ƙB-dependent pro-inflammatory cytokine production. Another saponin studied for its anti-inflammatory effects was ginsenoside Re which was diagnostically administered to mice with TNBS-induced acute colitis at 10 and 20 mg/kg.[81] Oral Re treatment dose-dependently reduced macroscopic damage, colon shortening, MPO, histological damage, and expression of TNF-α, IL-6, IL-1β, iNOS, COX-2, and p-p65, while upregulating IL-10.

Mice with DSS-induced acute colitis were orally co-administered 80 and 60 mg/kg loganin, iridoid glycoside from Cornus offcinalis, dose-dependently diminished clinical score, histological score, colon shortening, NF-κB activity, as well as p-p65, p-STAT3, IL-1β, IL-6, and TNF-α.[82] Morroniside is another iridoid glycoside from C. officinalis. At 90 and 180 mg/kg, it showed the same modulation as loganin, but at a lesser degree.[82] The results from the in vivo study were confirmed in HCT116 cells, were both compounds modulated pro-inflammatory cytokines and proteins in the same trend. Magnesium isoglycyrrhizinate (MIG) is a commercially available GCZ salt.[83] MIG was intraperitoneally administered to mice with DSS-induced acute colitis at 1.25, 2.5, and 5 mg/kg. Oral MIG treatment diminished clinical score, colon shortening, colon tissue permeability, histological score, ROS levels, and expression of IL-17, TNF-α, IFN-γ, IL-1β, p-p65. For the clinical metrics, only 2.5 and 5 mg/kg showed significant improvement, however, for the molecular metrics, only the 5 mg/kg dose consistently showed beneficial results. To validate the results, the authors also employed a chronic colitis DSS model using 5 mg/kg MIG observing the same trend as in the acute model.

Thymol, a monoterpene phenol, is abundantly found in plants from the Lamiaceae family. In rats with acetic acid-induced acute colitis, diagnostic administration of 30 and 100 mg/kg by oral gavage ameliorated mucosal injury, ulcer index, histological damage.[84] It also decreased MPO levels, p-p65, and TNF-α tissue expression. Although clinical symptoms were improved by the 30 mg/kg dose, molecular markers were only improved at 100 mg/kg. These results were validated by another group where diagnostic 100 mg/kg thymol orally administered to the same model decreased clinical score, mortality, MPO, MDA, and, NO levels, as well as expression of IL-6, IL-1, TNF-α, COX-2, and p65.[85] However, thymol did not modify IƙB, suggesting that it might modulate NF-ƙB through another pathway. For most markers, thymol was better than the positive control prednisolone. Parthenolide is a sesquiterpene lactone isolated from Tanacetum parthenium, commonly known as feverfew. In mice with DSS-induced acute colitis, diagnostic rectal administration of 5, 10, and 15 mg/kg parthenolide dose-dependently reduced clinical score, MPO, histological score, and expression of TNF-α, IL-1β, p-p65, and p-IƙBα, while upregulating IƙBα.[86] Hence, parthenolide reduced cytokine expression through NF-ƙB. Nimbolide is one if the main compounds in the Azadirachta indica (neem) extract. In TNF-α insulted COLO 205 cells, 20 and 200 nM nimbolide dose-dependently reduced IL-8 levels and p-IκBα expression.[87] Moreover, in both a DSS-induced acute colitis mouse model and a chronic CD model (IL-10^−/− mice), 0.2 and 1 mg/kg oral preventive (acute) and diagnostic (chronic) treatments similarly reduced clinical score, histological damage, and p-IκBα expression.[87] IL-10^−/− mice spontaneously develop CD-like enterocolitis, therefore, nimbolide’s effects could be upon pathways shared by UC and CD, thus showing promise in the treatment of both diseases. Lupeol, a triterpene found in various dietary plants at 50 μM and100 μM (in vitro), and at 30 mg/kg (in vivo); as well as ursolic acid at 10 μM and50 μM (in vitro), and at 10 and 20 mg/kg (in vivo) were tested in the same way as nimbolide presenting similar results.[88,89] The only difference was that lupeol (50 μM) and ursolic acid (10 and 50 μM) were tested for their NF-κB DNA binding inhibition in vitro, and both diminished it. IL-8 is an NF-ƙB target gene; therefore, its downregulation validates the three compound’s NF-ƙB inhibition.

Mice with DSS-induced acute colitis were orally co-treated with 100 mg/kg of GCZ’s aglycone metabolite glycyrrhetic acid (GA). GA reduced clinical score, colon shortening, histological damage, and expression of TNF-α, IL-6, IL-1β, COX-2, and prostaglandin E2 (PGE₂).[90] Additionally, it decreased phosphorylated/dephosphorylated ratio of p65 and IƙBα. Thus, GA regulates key proteins at different stages of the inflammatory cycle. GCZ’s salt, diammonium glycyrrhizinate (DG) at 40 mg/kg was intraabdominally administered in rats with acetic acid-induced acute colitis.[91] DG ameliorated clinical score, histological score, MPO, and downregulated TNF-α, p65, and NF-ƙB target gene ICAM-1. Together, the data indicates that even in its stable and metabolized form, GCZ has potential as an anti-inflammatory therapy in IBD. AL-1 was diagnostically tested in mice with DSS-induced acute colitis. 5, 15, and 45 mg/kg oral AL-1 recued clinical score, histological score, colon shortening, MPO, and expression of TNF-α, IL-6, IL-1β, IFN-γ, and PGE₂.[92] Additionally, AL-1 lowered the phosphorylation ratio of p65, JNK, and extracellular signal regulated kinase while upregulating IL-10. COX-2, iNOS, as well as p38 and IƙBα phosphorylation were also evaluated. The authors claimed that AL-1 reduced their expression, however there was no statistical test present, thus we concluded that neither AL-1 nor positive controls modulated said proteins in the used model. Although for most biological markers all doses were beneficial, 5 and 45 mg/kg had equivalent responses, therefore AL-1 demonstrated a wide therapeutic dose range.

In rats with TNBS-induced acute colitis, 10 and 100 mg/kg orally administered limonene, a monoterpene mostly found in citrus fruits, dose-dependently reduced weight loss, colon shortening, macroscopic damage, histological score, expression of serum TNF-α, and NF-ƙB nuclear translocation, while increasing epithelial integrity.[93] Additionally, TNF-α and IL-6 serum levels diminished in healthy elderly (65–85 years) individuals taking encapsulated orange peel extract with 95% limonene.[93] TNF-α is one of the main NF-ƙB activating cytokines, thus limonene might decrease NF-ƙB by decreasing its upstream activator or vice versa, since TNF-α is a NF-ƙB target gene.[74] Using a different administration technique, feed, diagnostic 1% oleuropein decreased clinical score, colon shortening, NO levels, MPO, histological score, and expression of nuclear p65, TNF-α, IL-1β, IL-6, iNOS, MMP-9, and COX-2 in mice with DSS-induced acute colitis.[94] All of the downregulated proteins are transcribed by NF-ƙB and involved in inflammatory processes, attesting to oleuropein’s NF-ƙB inhibition.[74] Additionally, iNOS downregulation was validated by NO concentration drop.

Tripterygium wilfordii Hook F is commonly used in traditional Chinese medicine to treat Crohn’s disease symptoms, thus triptolide, a diterpene from said plant was tested in IL-10^−/− mice.[95] Oral 0.07 mg/kg triptolide was diagnostically administered to IL-10^−/− mice reducing colon shortening, weight loss, histological score, serum amyloid A levels, NF-ƙB DNA binding activity, and expression of IFN-γ, the pleiotropic cytokine interleukin 4 (IL-4), TNF-α, IL-12, and IL-23.[96] The IL-12/23 axis is essential in IBD progression, especially CD; IL-12 polarizes Th17 cells and IL-23 T helper 1 (Th1) cells.[97] Meanwhile, serum amyloid A is a mucosal inflammation biomarker that shows better accuracy than C-reactive protein.[98] IL-12 and IL-23 are NF-ƙB target genes, hence the presented results, propose that triptolide improves CD inflammation by downregulating the NF-ƙB pathway and its downstream and upstream cytokines.[74] Food allergy herbal formula 2 is a regimen of nine Chinese herbal medicines based on the herbal formula Wu Mei Wan and it is used to treat allergies.[99] However, in colon tissue from CD patients it downregulated pro-inflammatory cytokines.[99] Its main ingredient is the fungus Ganoderma lucidum from which ganoderic acid C1 (GAC1) is the most abundant compound. Colon tissue from pediatric CD patients was treated with 20 μg/mL GAC1 where it downregulated mucosal IFN-γ, TNF-α, IL-17A, as well as p-IκBα from the mucosa and lamina propria.[100] Like triptolide, GAC1 shows promise in the NF-ƙB pathway modulation of CD inflammation.

Mice with DSS-induced acute colitis were preventive and diagnostically administered 50 and 100 mg/kg geraniol, a monoterpene found in many dietary plants.[101] Both dosages of oral geraniol similarly decreased clinical score, histological damage, MPO, p65 DNA binding, and expression of TNF-α, IL-6, IL-1β, iNOS, COX-2, p65, and p-IƙBα. Additionally, geraniol diminished nitrites and thiobarbituric acid reactive substances (TBARS) levels, while increasing SOD and GSH levels. TBARS are byproducts of lipid peroxidation much like MDA, so they are utilized as a marker for lipid peroxidation.[102] Thus, geraniol lessens inflammation by promoting antioxidant pathways and suppressing pro-inflammatory ones. In mice with DSS-induced acute colitis, 20 mg/kg of diagnostic ursolic acid treatment ameliorated clinical score, histological score, MDA, MPO, and expression of TNF-α, IL-1β, and nuclear p65.[103] Additionally, ursolic acid upregulated SOD suggesting that uroslic acid lowers inflammation by modulating antioxidant pathways and blocking NF-ƙB.

In TNF-α insulted HT29 cells, 50 μM ginsenoside Rf downregulated IL-6, IL-1β, p65, IKK, iNOS, and phosphorylated MAPKs; extracellular signal regulated kinase (p-ERK), p38 (p-p38), and phosphorylated c-Jun N-terminal kinase (p-JNK).[104] This suggests that Rf prevents p65 phosphorylation consequently decreasing its target genes and their pathways. Catalposide was also investigated in TNF-α insulted HT-29 cells. In this cell line, 200 ng/mL catalposide downregulated IL-8, and phosphorylated extracellular signal regulated kinase 1 and 2 (p-ERK 1/2) and p-p38.[61] It also decreased NF-ƙB activation and IκBα degradation. Meanwhile, 10 and 100 μM acanthoic acid, a diterpene isolated from Acanthropanax koreanum, dose-dependently decreased nuclear NF-ƙB, downregulated p-ERK 1/2, p-p38, and phosphorylated c-Jun N-terminal kinase 1 and 2, while upregulating cytosolic IκBα.[105] Besides IL-8 being an NF-ƙB target gene, its messenger RNA is stabilized by MAPKs, thus their decrease might have contributed to IL-8 depletion.[61] Acanthoic acid was also investigated in mice with DSS-induced acute colitis. 100 and 300 mg/kg of diagnostic intraperitoneal acanthonic acid treatments equivalently reduced clinical score, colon shortening, MPO, histological damage, and expression of TNF-α, COX-2, NF-κB, and the proteinase chymase.[106] Hence, acanthonic acid’s NF-ƙB inhibition was translated to an in vivo model, strengthening its role in said pathway. Furthermore, the survival and positive outcome of the higher dosage proved its safety at a wide dose range.

3, 14, 19-triacetylandrographolide (CX-10) is an andrographolide synthetic derivate and in mice with DSS-induced acute colitis, orally co-administered 200 mg/kg CX-10 consistently diminished clinical score, colon shortening and weight, MPO, histological score, and expression of p65, p-IκBα, TNF-α, and IL-6.[107] In addition, CX-10 increased IκBα and reduced phosphorylation/dephosphorylation ratio of p38, p-ERK, and p-JNK. Together, the data implies that CX-10 modulates pro-inflammatory cytokine expression by inhibiting IκBα dissociation from NF-κB, and inhibition of the MAPK pathway. In rats with DSS-induced acute colitis, 50 and 100 mg/kg diagnostic oral limonene treatment equivalently reduced clinical score, TNF-α, IL-6, IL-1β, iNOS, COX-2, PGE₂, metalloproteinase 2, MMP-9, NF-κB, p-ERK 1/2, and transforming growth factor β (TGFβ), while raising SOD and GSH levels.[108] Although TGFβ is commonly regarded as an anti-inflammatory mediator, under the right conditions it can promote generation of pro-inflammatory actors, hence, limonene might decrease inflammation by inhibiting TGFβ mediated pro-inflammatory processes.

Intestinal fibroblasts are key components of tissue healing promoting ECM maintenance, epithelial differentiation, and intestinal stem cell maintenance.[109] Therefore, they are a good target for IBD therapy. In LPS insulted CCD-18C0 human colon fibroblast cells, 10 and 50 μM astragaloside IV (ASIV), major bioactive compound in Radix astragalus, dose-dependently downregulated TNFα, IL-1β, IL-6, p-p65, and p-IκB.[110] The obtained results were achieved in vivo using 200 mg/kg oral ASIV. Preventive and diagnostic administration of ASIV to mice with DSS-induced acute colitis reduced clinical score, colon shortening, histological score, MPO, NO levels, and expression of TNFα, IL-1β, and IL-6. Together, the in vitro and in vivo data suggest that ASIV lowers inflammation by inhibiting NF-ƙB transcription activity.

4.2. CREB Pathway

The cyclic adenosine monophosphate response element binding (CREB) protein is a transcription factor in a variety of cellular processes.[111,112] CREB activation depends on phosphorylation by adenosine monophosphate (AMP)-dependent protein kinase A, protein kinase C, pp90 ribosomal S6 kinase, or calmodulin kinases followed by nuclear migration and binding to cofactor CREB-binding protein. Like NF-ƙB, CREB is a pro-inflammatory protein that promotes transcription of inflammatory proteins like COX-2 and cytokines. Nonetheless, it can also transcribe anti-inflammatory proteins like IL-10, superoxide dismutase 2 (SOD2), and forkhead box P3; as with most transcription factors, the end products are dictated by the most upstream step in signaling cascades, the stimulating ligand. In mice with TNBS-induced acute colitis, 3 mg/kg intraperitoneal amyrin reduced macroscopic damage, histological damage, IL-1β, VEGF, COX-2, p-65, and phosphorylated cyclic adenosine monophosphate response element binding, while increasing IL-10.[113] Based on the results, diagnostic amyrin might downregulate COX-2 though both NF-ƙB and CREB. All tested proteins are targets of both transcription factors, so no further differentiations could not be made.[74,111,112]

4.3. Nrf2 Pathway

The nuclear factor erythroid 2–related factor 2 (Nrf2) is a transcription factor that promotes transcription of antioxidant proteins like SOD, CAT, GPX, and heme oxygenase 1 (HO-1).[114] In the cytoplasm, it is bound to its repressor Kelch-like ECH-associated protein 1 (Keap1), who after ligand binding disassociates allowing Nrf2 translocation to the nucleus.[114] Its activity is negatively correlated to that of NF-κB, disassociated Keap1 inhibits IƙBα ubiquitination, therefore suppressing NF-κB activation.[115]

LPS insulted Caco-2 cells, treatment with 100 μM paeoniflorin, a monoterpene isolated from Paeonia lactiflora (peony), decreased permeability, NF-κB nuclear translocation, and downregulated TNF-α, IL-6, iNOS, COX-2, MMP-9, nuclear p65, and cytoplasmic IƙBα; while increasing transepithelial electrical resistance, and expression of seal promoting tight junction proteins zonula occludens (ZO-1), occludin, and claudin 5.[116] Furthermore, paeoniflorin upregulated nuclear Nrf2 and cytoplasmic HO-1. To understand the role of Nrf2 in the observed anti-inflammatory trend, small interfering RNA for Nrf2 (siNrf2) was co-treated with paeoniflorin. siNrf2 effectively abolished paeoniflorin’s observed effects, indicant of paeoniflorin’s dependency on Nrf2 for its antioxidant and ani-inflammatory regulation. The anti-colitic effects of preventive or diagnostic treatment of genipin, an iridoid glycoside mostly found in Gardenia jasminoids Ellis, were investigated in mice with DSS-induced acute colitis.[117] Oral genipin at 2.5, 5, and 10 mg/kg dose-dependently reduced clinical score, colon shortening MPO, histological damage, MDA, and expression of TNF-α, IL-1β, p-p65, and p-IƙBα; while upregulating Nrf2 and HO-1.

Preventive administration of 125 and 500 mg/kg asperuloside, a glycosylated iridoid from Hedyotis diffusa, to mice with DSS-induced chronic colitis revealed reduction in clinical score, colon shortening, MPO, histological damage, MDA, p-p65/p65 ratio, and expression of TNF-α, IL-6, p65, and p-p65.[118] Asperuloside also upregulated Nrf2, SOD, GPX, HO-1, and another Nrf2 target gene, NAD(P)H quinone dehydrogenase 1 (NQO1). Of note, for all parameters except Nrf2 and NF-ƙB, where the highest dose was more effective, both dosages were equivalent. 1,8-cineole is a monoterpene oxide found in the essential oils of the plant families: Myrtaceae, Lamiaceae, and Zingiberaceae.[119] 100 and 200 mg/kg oral 1,8-cineole was diagnostically administered to mice with DSS-induced acute colitis observing reduced clinical score, colon shortening, MPO, histological score, tissue nitrite, IL-6, TNF-α, IL-1β, IL-17A, iNOS, and COX-2.[119] Furthermore, 1,8-cineole upregulated cytosolic and nuclear Nrf2 as well as its transcriptionally targeted proteins, NQO1 and HO-1. It also increased SOD and CAT enzymatic activity. Although in most parameters both doses had a beneficial effect, in all of them, 200 mg/kg was superior to 100 mg/kg, thus 1,8-cineole needs higher concentrations to exert good pharmacological modulation.

Plumericin, a spirolactone iridoid isolated from Himatantus scuba, showed anti-inflammatory and antioxidant effects in LPS and IFN-γ insulted IEC-6 cells at 1 and 2 μM.[120] Plumericin treatment reduced ROS levels and nitrotyrosine -a nitrosative stress marker- levels, as well as expression of TNF-α, IL-1β, iNOS, COX-2, and caspase 1. Meanwhile, it downregulated Nrf2, HO-1, NQO1, and SOD. To confirm these results, mice with dinitrobenzene sulfonic acid-induced acute colitis were intraperitoneally administered 3 mg/kg plumericin.[120] Diagnostic plumericin treatment improved macroscopic damage, body weight, colon length, histological score, MDA, MPO, nitrotyrosine levels, and expression of TNF-α, IL-1β, and nuclear p65; while increasing Nrf2, SOD, and IƙBα. Caspase 1 is an inflammatory protein involved in toll-like receptor (TLR) and nod-like receptor (NLR) pathways, and TLR is upstream of NF-ƙΒ, so plumericin might be modulating NF-ƙΒ through caspase 1 inhibition.[121] Cytosolic phospholipase A2α (cPLA2α) initiates the NF-ƙΒ pathway, so its inhibition is a viable route for IBD treatment. Tussilagone a sesquiterpene found in Tussilago farfara was diagnostically tested at 0.5 and 2.5 mg/kg in mice with DSS-indued acute colitis where it dose-dependently decreased clinical score, colon shortening, histological damage, MPO, and expression of TNF-α, IL-6, COX-2, iNOS, nuclear p65, and cPLA2α.[122] In addition, tussilagone upregulated nuclear Nrf2, and HO-1. Together, the data suggests that tussilagone decreased activity of the NF-ƙB by inhibiting its upstream mediator cPLA2α and suppression through Nrf2 activation.

Rosemary possess multiple known and unknown diterpenes; the most abundant being carnosic acid. In mice with DSS-induced acute colitis, 50 and 100 mg/kg diagnostic carnosic acid treatment dose-dependently diminished clinical score, colon shortening, histological score, MPO, macrophage infiltration, and expression of TNF-α, IFN-γ, IL-1β, IL-6, IL-17, interleukin 18 (IL-18), iNOS, COX-2, p-IƙBα, p65, p-JNK, and phosphorylated c-Jun.[123] Caspase 1 is found in the cytoplasm as its inactive form pro-caspase-1 and after nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome assembly, it gets cleaved to its active form cleaved caspase 1.[121] Carnosic acid was able to diminish caspase 1 and cleaved caspase 1 expression, yet it did not modulate the NLRP3 inflammasome nor its components, apoptosis-associated speck-like protein containing a CARD and phosphorylated signal transducer and activator of transcription 3 (p-STA3). On the other hand, carnosic acid upregulated Nrf2, SOD2, HO-1, and other Nrf2 transcriptionally targeted proteins, glutamate-cysteine ligase modifier subunit and glutathione peroxidase 2 (GPX2), while increasing GSH levels and binding of activating methylation, H3K4Me3, to HO-1 and GPX2 promoter regions. Meanwhile, it downregulated iNOS and binding of repressing methylation, H3K27Me3, to HO-1 and GPX2 promoter region. Overall, the 100 mg/kg dose modulated all the tested parameters, while 50 mg/kg did not. Since Nrf2 is a cytoplasm-based transcription factor, positive modulators can either bind to Nrf2 or disturb its interaction with Keap1 or Nrf2 degrader, Cullin3.[124] In pull down assays, carnosic acid interrupted binding between Cullin3 and Nrf2, while slightly but significantly promoting Nrf2 binding to Keap1. Additionally, carnosic acid decreased Nrf2 ubiquitination. Together the data indicates that carnosic acid lessens inflammation by Nrf2 upregulation due to its stabilization.

Pristimerin, the methyl ester derivate of celastrol, was tested in mice with DSS-induced acute colitis.[125] 0.4 and 0.8 mg/kg diagnostic oral administration of pristimerin lessened clinical score, colon shortening, histological score, and expression of TNF-α, IL-1β, IL-6, IL-17, p65, and micro RNA 155 (miRNA-155). Conversely, pristimerin augmented GSH levels, as well as Nrf2, SOD, and GPX expression. In HT-29 cells, 0.5 μM pristimerin downregulated p65 and miRNA-155, and upregulated Nrf2. 0.25 μM pristimerin also modulated miRNA-155 and Nrf2, but not NF-ƙB, suggesting that the latest has more safeguards to ensure its effective activation. Additionally, a miRNA-155 inhibitor showed similar effects to pristimerin. Therefore, pristimerin regulates Nrf2 and NF-ƙΒ pathways through miRNA-155 inhibition.

4.4. PPARγ Pathway

Peroxisome proliferator-activated receptors (PPARs) are four transcription factors that mediate multiple cellular processes maintaining homeostasis.[126] One of the regulated processes is NF-ƙB mediated inflammation, where PPARs impede its activity by inhibiting IƙBα phosphorylation. Peroxisome proliferator-activated receptor γ (PPARγ) is the most abundant PPAR in the intestines -especially the colon- and can be activated by a variety of molecules such as unsaturated fatty acids, amino salicylic acids, and prostaglandin derivates. Within the intestinal tissue, PPARγ is expressed by intestinal epithelial and intestinal immune cells, therefore it is the target of choice in an intestinal autoimmune disease like IBD.[126]

Geniposide at 50, 100, and 200 mg/kg was administered orally to mice with DSS-induced acute colitis in a preventive and diagnostic manner.[127] Besides improving clinical score, colon shortening, MPO, and histological score; geniposide diminished p65 and IƙB phosphorylation and expression of TNF-α, IL-6, and IL-1β. On the other hand, geniposide upregulated PPARɣ. From the administered doses, only 100 and 200 mg/kg had consistent effects across markers, with 200 mg/kg being the best. In addition, the in vivo results were recreated in LPS insulted Caco-2 cells. Therefore, the data suggests that geniposide diminishes inflammation through PPARɣ’s NF-ƙB inhibition. 5, 15, and 45 mg/kg preventive AL-1 treatment to mice with TNBS-induced acute colitis, reduced clinical score, histological score, colon shortening, MPO, macroscopic damage, and expression of TNF-α, IL-6, IL-1β, COX-2, p-p65, and p-IƙB and also upregulated PPARɣ.[128] Although all doses showed improvement of biological targets, the 45 mg/kg dose was consistently better than the others. The results suggest that AL-1 suppresses the NF-ƙB pathway by repressing PPARɣ pathway.

Orally administered GCZ was tested preventively and diagnostically in rats with acetic acid-induced acute colitis for its anti-colitic and antioxidant potential.[129] Besides ameliorating macroscopic and histological damage, 100 mg/kg GCZ downregulated TNF-α, and upregulated CAT, SOD, GPX, and PPARγ. Hence, the data suggests that GCZ PPARγ activation decreased NF-ƙB activity thus downregulating its target gene TNF-α and upregulating Nrf2 target genes. Rats with TNBS-induced acute colitis were preventively treated with 250 mg/kg geraniol by oral gavage.[130] Geraniol treatment decreased clinical score, MPO, MDA, NO levels, and the expression of ICAM-1, p38, p65, IL-1β, PGE₂, and caspase 3. It also increased total antioxidant capacity and PPARγ expression. Caspase 3 is not only involved in apoptosis, it also cleaves PPARγ inactivating it.[131] Thus, geraniol might be indirectly promoting PPARγ accumulation by downregulating caspase 3. Interestingly, geraniol treatment was more effective than the positive control 500 mg/kg sulfasalazine, yet their combination was superior to both individually. This suggests the potential of terpenes in combination with FDA approved drugs.

1,8-cineole was previously described as an Nrf2 pathway modulator, yet, in the same study the authors also evaluated its effects on the PPARγ pathway.[119] Through molecular docking simulations they observed good affinity to the receptor’s ligand binding domain. Additionally, it downregulated p-p65 while only upregulating the expression of the γ subtype. As with clinical and antioxidant markers, 200 mg/kg was superior to 100 mg/kg. To further study the PPARγ modulation, TNF-α insulted HT-29 cells were treated with 40 μM 1,8-cineole.[119] In vitro, 1, 8-cineole downregulated inflammatory markers IL-8 and CXCL1, while upregulating PPARγ and its promoter region. In cells with silenced PPARγ, 1, 8-cineole had a dampened effect on IL-8 levels but did differentially impact CXCL1 expression. Moreover, the effects of 1, 8-cineole were equivalent to that of the synthetic PPARγ agonist GW1929 highlighting its promise as IBD therapy.

4.5. IL-6/STAT3 Pathway

The dose commonly used to test triptolide, 0.07 mg/kg, was diagnostically administered to IL-10^−/− mice observing reduction in histological score, MPO, and expression of p-STAT3, IL-6, IL-17, interleukin 6 receptor β (IL-6Rβ), component of the IL-6 signaling pathway, as well as RORɣt.[132] Additionally, in tissue from CD patients, 20 ng/mL triptolide downregulated IL-6 and IL-17. IL-6 is involved in Th17 differentiation and mutations in STAT3 impair said process. Thus, triptolide might be compromising Th17 differentiation by blocking the IL-6/STAT3 transcription pathway. The sesquiterpene isolated from Aucklandia lappa Decne dehydrocostus lactone (DL) was diagnostically administered to mice with DSS-induced chronic inflammation at 10, 15, and 20 mg/kg.[133] In this model, DL decreased clinical score, histological score, MPO, as well as expression of IL-6, IL-23, IL-17, TNF-α, IL-1β, IL6R-β, COX-2, iNOS, MCP-1 and STAT3. Additionally, it upregulated SOD. Thus, DL blocks the IL-6/STAT3 pathway leading to a reduction in pro-inflammatory cytokines and chemokines.

Contradicting the in vitro experiment by Shin et al., mice with DSS-induced acute colitis treated with 40, 80, and 160 mg/kg limonin, a triterpene in the Rutaceace and Meliaceae families, not only ameliorated clinical score, colon shortening and histological damage, it also modulated the STAT3/(micro RNA 214) miRNA-214 pro-inflammatory pathway.[134] Limonin reduced expression of IL-6, TNF-α, and miRNA-214, and reduced p-STAT3/STAT3 ratio. Additionally, it upregulated IL-10 and miRNA-214 target genes, phosphatase and tensin homolog (PTEN) and PDZ And LIM Domain 2 (PDLIM2). Interestingly, limonin only reduced miRNA-214 levels in diseased mice. The same modulation trend was observed in IL-6 insulted NCM460 cells treated with 20 μg/mL limonin. It is important to mention that the 80 mg/kg dose was the most potent in most tests, indicating that limonin might be modulating some targets indirectly reflecting the inconsistencies. PTEN and PDLIM2 inhibit STAT3 and facilitate NF-ƙB ubiquitination, respectively. Moreover, they are not only activated by miRNA-214.[135] Hence limonin modulated miRNA-214 levels but not its transcription activity, still promoting inflammation reduction. So, miRNA-214 might be directly regulating a key protein in the STAT3 pathway. Diagnostic administration of 50 mg/kg ginsenoside Rh2 to mice with DSS-induced acute colitis decreased clinical score, colon shortening, and expression of TNF-α, IL-1β, IL-6, p-STAT3, and miRNA-214, while upregulating PTEN.[136] The observed results were confirmed in NCM460 cells were 10 μM Rh2 downregulated p-STAT3 and miRNA-214, while upregulating PTEN. miRNA-214 is known to activate the IL-6/STAT3 pathway and inhibit PTEN, who does not only inhibit STAT3, but also serves as a positive feedback activator of PPARγ, since it is both a PPARγ upstream protein (through the PI3K/Akt pathway) and target gene.[135] Thus, Rh2 lessens inflammation by inhibiting miRNA-214 activation of IL-6 and STAT3.

4.6. Toll-like Receptor Pathway

TLRs are a family of proteins under the PRRs. There are 10 TLRs encoded in the human genome, half are transmembrane and the other half intracellular.[137] They recognize pathogen associated molecular patterns (PAMPs) such as liposaccharides, and lipoproteins and damage associated molecular patterns (DAMPs) such as glycoproteins, mitochondrial products, DNA, and RNA.[137] Due to their pathogen sensing abilities, TLRs are highly expressed in barrier tissues and antigen presenting cells, especially dendritic cells. TLRs are essential in host protection against exogenous pathogens, however, because of the dysbiosis and mucosal barrier deterioration, TLRs are hyperactivated initiating inflammatory pathways like NF-ƙB, IL-18, and MAPKs. Therefore, TLR regulation is an encouraging IBD therapy target. Myeloid differentiation primary response 88 (MyD88) is a Toll/IL-1 receptor (TIR) that resides in the cytoplasm and binds to toll-like receptor 4 (TLR4) upon LPS binding.[137] As a TIR, it initiates TLR signaling cascade by recruiting other kinases.

0.07 mg/kg of oral triptolide was diagnostically administered to IL-10^−/− mice.[138] In this model, triptolide reduced histological score as well as TLR4, toll-like receptor 2 (TLR2), MyD88, and p65 expression. Additionally, in TNF-α insulted CD patient colon tissue, triptolide downregulated TLR4 and TLR2.[138] Thus, triptolide modulates the TLR signaling pathway possibly by inhibiting TLRs, although more experiments are needed to define its molecular mechanism. Libertellenone M, a pimarane diterpene isolated from the marine fungus Stibella fimetaris, was diagnostically administered to mice with DSS-induced acute colitis.[139] Intraperitoneal 10 and 20 mg/kg libertellenone M treatment dose-dependently reduced clinical score, colon shortening, histological score, and expression of IL-18, IL-1β, and cleaved caspase 1. IL-18 is transcribed as the inactive pro-IL-18 which is cleaved by caspase 1.[140] Thus, the data suggests that libertellenone M downregulates IL-18 transcription through inhibition of pro-caspase 1 cleavage by TLR4. Additionally, libertellenone M was more efficient than the positive control 5-ASA. 8 mg/kg intraperitoneal betulin, a triterpene from the Betula L. genus, preventive and diagnostic administration in rats with acetic acid-induced acute colitis lessened macroscopic damage, histological damage, serum lactate dehydrogenase and C-reactive protein levels, macrophage population, and expression of IL-6, TNF-α, IL-1β, NF-ƙB, TLR4, caspase 3, and caspase 8.[141] Caspase 1 is the caspase commonly associated with TLR4 because of its role in NLRP3 inflammasome activation.[142] Meanwhile, caspases 3, 8 and 9 are linked to apoptosis and cell death. TLR4 can promote apoptosis through two main pathways, NF-ƙB and its transcription of pro-apoptotic proteins and cytokines or direct recruitment and activation of caspase 8.[143] Therefore, the data suggests that betulin is not only inhibiting NF-ƙB-facilitated apoptosis but might also be regulating caspase 8.

Oral ginsenoside Rb1 or compound K were preventively and diagnostically administered to mice with TNBS-induced acute colitis.[144] Both compounds at 10 and 20 mg/kg ameliorated macroscopic damage, weight loss, colon shortening, histological score, MPO levels, and expression of TNF-α, IL-6, IL-1β, iNOS, COX-2, and p-p65, while upregulating IL-10. There was no difference between the two doses of neither compound, thus 20 mg/kg constitutes the pharmacokinetic plateau. Rb1 is the most abundant ginsenoside in P. ginseng C.A. Meyer, therefore most of the compound k produced in vivo comes from Rb1. As expected, there was no significant difference between Rb1 and compound K, validating the metabolic faith of Rb1. Interleukin 1 receptor associated kinases (IRAK) are a family of kinases that become phosphorylated by TLRs initiating the signaling cascades that culminate in activation of transcription factors like NF-ƙB.[137] In a cell-free assay, both Rb1 and compound k inhibited interleukin 1 receptor associated kinase 1 (IRAK-1). Nevertheless, compound k at the highest concentration, 10 μM, was approximately 15% more efficient than Rb1, once more validating that compound k is the compound that drives the bioactivity of P. ginseng C.A. Meyer.[144] Ginsenosides Rg1 and Rh1,as well as compound k were diagnostically administered to mice with TNBS-induced acute colitis.[145] 20 mg/kg oral Rg1, Rh1, and compound k macroscopic damage, colon shortening, histological damage, MPO, and expression of TNF-α, IL-1β, IL-17, IL-23, COX-2, iNOS, p-p65. Additionally, they upregulated IL-10. Unlike in the study that tested ginsenoside Rb1 and compound k, Rh1 and Rg1 were less effective than compound k. These two ginsenosides are less abundant than Rb1 and must be converted to other ginsenosides before becoming compound k. Therefore, the yield of conversion should be lower, resulting in less available compound k and bioactivity.

Kalopanaxsaponin A (KA) is the best known bioactive saponin in Kalopanax pictus.[146] In mice with TNBS-induced acute colitis, 10 and 20 mg/kg KA was administered in two different regimes, preventively or diagnostically.[146] Both treatment courses dose-dependently reduced macroscopic damage, colon shortening, MPO, histological damage, and expression of TNF-α, IL-1β, IL-6, p-p65, iNOS, and COX-2. Meanwhile, it upregulated IL-10 and IRAK-1. Therefore, KA might be inhibiting NF-ƙB via inactivation of TLR signaling cascade. Oral ursolic acid treatment was diagnostically administered to mice with TNBS-induced acute colitis.[147] In this model, 10 and 20 mg/kg ursolic acid treatment equivalently ameliorated macroscopic damage, weight loss, colon shortening, MPO, histological damage, and expression of TNF-α, IL-1β, IL-6, COX-2, iNOS, p-p65, p-IƙBα, phosphorylated IκB kinase β, and TLR4. On the contrary, ursolic acid upregulated IL-10, and the inactive IRAK1 and interleukin 1 receptor associated kinase 4. Therefore, ursolic acid inhibits the TLR4 signaling pathway from start all the way to transcription.

TLR4 is sensitive to LPS, mediating LPS-induced NF-ƙB activation.[137] In LPS insulted Caco-2 cells, 10, 20, and 40 μM loganin downregulated TNF-α, IL-1β, IL-6, p-p65, p-STAT3, phosphorylated janus kinase, and TLR4.[148] The janus kinase (JAK)/STAT3 kinases are downstream TLR4, furthermore, co-treatment with a JAK/STAT3 activator mitigated loganin’s effects. So loganin’s TLR4 inhibition was validated by JAK and STAT3 downregulation as wells as that of NF-ƙB. In mice with DSS-induced acute colitis, 50 mg/kg preventive and diagnostic paeoniflorin administration decreased weight loss, bloody diarrhea, colon shortening, histological damage, MPO, and expression of TNF-α, IFN-γ, IL-17, IL-6, p-p65, COX-2, p-p65, p-IƙBα, and MCP-1.[149] Paeoniflorin also downregulated TLR4, without affecting TLR2 nor TLR5 levels. Thus, it is inhibiting NF-ƙB stimulation at one of its most upstream proteins which would also inhibit the other TLR-facilitated pathways.

The triterpene found in Brucea javanica, brusatol, has low water solubility and rapid first pass clearance, thus brusatol in a self-micro emulsifying drug delivery system (SMEDDS) was diagnostically administered to mice with DSS-induced acute colitis.[150] Both the free brusatol suspension (0.25, 0.5, and 1 mg/kg) and loaded SMEDDS (1 mg/kg) dose-dependently reduced clinical score, colon shortening, histological score, MPO, MDA, PGE₂ levels, and expression of TNF-α, IFN-γ, IL-1β, IL-6, p65, TLR4, and MyD88. Treatments also upregulated IL-10, IL-4, GPX, and SOD. As expected, brusatol-SMEDDS was more effective than the brusatol suspension. Furthermore, the lowest dose of brusatol-SMEDDS had similar potency to the highest concentration of the brusatol suspension. Thus, brusatol lowered inflammation through inhibition of TLR4 signaling.

4.7. mTOR Pathway

The mechanistic target of rapamycin (mTOR) is an intracellular kinase that can be found in two different complexes, mechanistic target of rapamycin complex 1 (mTORC1) and mechanistic target of rapamycin complex 2.[135] mTOR complexes are activated by a variety of receptors that respond to metabolites, growth factors, and immune cell components. Expectedly, mTOR facilitates the transcription of metabolism, immune cell inductors, as well as proliferation and migration modulating proteins.

Andrographolide was diagnostically administered to mice with DSS-induced acute colitis. 100 mg/kg oral andrographolide treatment diminished clinical score, colon shortening, histological score, macrophage population, TNF-α, IL-1β, IL-6, p-IƙBα, p-ERK, p-JNK, p-p38, and phosphorylated ribosomal protein S6 kinase beta 1.[151] Meanwhile, andrographolide upregulated phosphorylated acetyl-CoA carboxylase and phosphorylated glycogen synthase kinase 3 β. Unexpectedly, andrographolide did not modulate mTOR nor one of its deactivating kinases, adenosine monophosphate activated protein kinase (AMPK). Still, it decreased dephosphorylation (activation) of AMPK target protein, acetyl-CoA carboxylase (ACC). Hence, andrographolide might be regulating ACC through another protein. Ribosomal protein S6 kinase β 1 (p70S6K) is an mTORC1 target protein, and glycogen synthase kinase 3 β(GSK3β)a more downstream member of the pathway. This confirms that andrographolide did modulate the mTOR pathway. In conclusion, andrographolide decreased colitis associated inflammation by decreasing T-cell effector induction (GSK3β), immune cell proliferation (p70S6K), and fatty acid synthesis/immune cell energy source ACC. 2 mg/kg celastrol was orally administered to IL-10^−/− mice displaying amelioration of histological score, MPO, and expression of TNF-α, IFN-β, IL-1β, IL-17A, CXCL1, C-X-C motif chemokine ligand 2, mTOR, phosphoinositide 3-kinase (PI3K), Akt, and p70S6K.[152] Additionally, celastrol increased autophagy in the colonic tissue. As mentioned previously, IBD results from a decline in immunological tolerance, yet autophagy boosts immunological tolerance in CD. mTOR inhibits autophagy, thus celastrol boosts autophagy by inhibiting mTOR, its downstream protein Akt, and Akt inductor PI3K. mTOR also promotes pro-inflammatory immune cell proliferation and migration, yet celastrol also reduced that signaling cascade as proven by the diminished cytokine and chemokines, as well as downregulation of p70S6K.

4.8. PXR Pathway

Pregnane X receptor (PXR) is a transcription factor activated by pathogenic stimuli such as microbial metabolites.[153] Its main job is to eliminate the stimulating molecule by promoting transcription of metabolizing enzymes like cytochrome P450 3A4 (CYP3A4). Additionally, PXR suppresses NF-ƙB activation, and promotes tight junctions. Moreover, PXR mutation confers susceptibility to IBD, showcasing its importance in the anti-inflammatory response.[154]

Mice with DSS-induced acute colitis were diagnostically administered 10 and 20 mg/kg Tan IIA reducing weight loss, histological score, and expression of TNF-α, IL-6, iNOS, and MCP-1.[155] Meanwhile, it increased GSH levels, and expression of PXR, CYP3A4, glutathione s transferase α 1, and PXR target gene macrophage inflammatory molecule 1 α (MIP-1α). All tested parameters were positively modulated by the highest dose. Additionally, Tan IIA treatment of PXR-silenced mice showed none of the above-mentioned improvements. Therefore, Tan IIA suppressed inflammation through PXR activation. Alantolactone is a sesquiterpene lactone isolated from Inula helenium L. and Inula japonica. In mice with DSS-induced acute colitis, 50 mg/kg alantolactone diminished weight loss, colon shortening, histological damage, MPO, NO levels, and expression of TNF-α, IL-6, IFN-γ, PGE₂, COX-2, iNOS, p-p65, p-IƙBα, and ICAM-1.[156] In HT-29 cells, 10 and 25 μM alantolactone dose-dependently increased PXR activity and in cells with silenced PXR, it could not activate NF-ƙB. Thus, alantolactone’s interaction with PXR was measured observing that alantolactone binds PXR in the ligand binding domain (LBD) and that it promotes CYP3A4 transcription.

TNF-α insulted LS174T human colon cancer cells treated with 10 μM ginsenoside Rb1 and 10 μM compound K individually showed decreased p65 nuclear translocation, p-p65/p65 ratio, and expression of IL-1β and iNOS.[157] On the other hand, both compounds upregulated PXR and CYP3A4. Ginsenosides are not PXR agonists, they stimulate p65 interaction with PXR, and as anticipated, Rb1 and compound K increased PXR-p65 interaction. Notoginsenoside R1 is a saponin structurally like ginsenosides but isolated from Panax notoginseng. Diagnostic administration of 25 mg/kg R1 to mice with DSS- or TNBS-induced acute colitis reduced colon shortening, weight loss, MPO, histological score, and expression of IL-6 and TNF-α.[158] Only in the DSS model more extensive analysis were performed. R1 also downregulated p-p65, pIKKα/β, iNOS, COX-2, ICAM-1, IL-1β, interleukin 2, interleukin 1 α, and IFN-γ. In an HT-29 based luciferase assay, 25 μM R1 dose-dependently increased PXR-mediated activation of its target gene CYP3A4. In another luciferase assay where PXR was silenced, R1 did not modulate NF-ƙB activity. To further elucidate R1’s role in PXR modulation, PXR target gene expression was measured in HT-29 cells as well as in samples from the DSS mouse experiment and found that R1 upregulated CYP3A4, MIP1α, and UDP-glucuronosyltransferase 1–1. To understand if R1 interacted with PXR, PXR was mutated on the LBD. Unlike ginsenosides, R1 binds PXR in the LBD. This result was confirmed with a competitive proximity assay.

5. Future Directions

Botanical natural products, including terpenes, are reported to exert their therapeutic effects through several mechanisms like modulation of inflammatory and other cell signaling pathways. There are more than 300,000 terpene compounds indicating to the importance of carefully select promising terpenes taking into consideration their unique chemical and physical properties that will influence their pharmacokinetic and pharmacodynamic properties in the clinic.

No terpene has been clinically studied in IBD patients, but from the above-mentioned terpenes, food-derived carnosic acid, GCZ, compound K, and oleuropein are the most promising candidates as IBD treatment based on their safety, pharmacokinetics, and modulated inflammatory pathways. Yet, more work is needed to evaluate these agents in clinical trials as well as continue to evaluate agents in preclinical models.[159] Carnosic acid has superior pharmacokinetics, but has never been studied in clinical trials, thus its safety and dosing need to be assessed before conducting more specialized trials. However, it is important to point out that rosemary extracts standardized to carnosic acid have undergone extensive safety studies by the European Food Safety Authority.[160] On the other hand, GCZ has been clinically tested in autoimmune and inflammatory diseases.[161–163] Therefore, study conditions can be extrapolated to IBD. Compound K is attractive for IBD therapy because it is already metabolized and since IBD patients have an altered microbiota, it should be more effective than the other ginsenosides. Additionally, like GCZ, it could function as an anti-inflammatory agent as well as a prebiotic. There are multiple clinical trials assessing compound K’s pharmacokinetic and metabolic profiles, therefore, specialized trials could be performed extrapolating from the available dosing strategies.[164–166] Finally, oleuropein has been studied in UC patient samples, furthermore, its clinical safety and dosing profile has been established.[167] Accordingly, advanced clinical trials will be the next step. For the time being first line therapy continues to be the 5 classes of drugs used for IBD including 5-ASA, corticosteroids, immune modulators, among others. However, it is important to point out that there appears to be a wide variety of terpenes that may show promise for the treatment and or prevention of IBD.

Abbreviations:

ACC

acetyl-CoA carboxylase

AMP

adenosine monophosphate

AMPK

adenosine monophosphate activated protein kinase

AhR

aryl hydrocarbon receptor

AZA

azathioprine

BCP

(E)-β-caryophyllene

C

Carbon

CAMs

cell adhesion molecules

CAT

catalase

CD

Crohn’s disease

CDDO-Im

1-[2-cyano-3, 12-dioxo-oleana-1, 9 (11)-dien-28-oyll-dien-28-oyl] imidazole

CKRs

chemokine receptors

COX-2

cyclooxygenase 2

cPLA2α

cytosolic phospholipase A2α

CREB

cyclic adenosine monophosphate response element binding

CXCL1

C-X-C motif chemokine ligand 1

CXCL2

C-X-C motif chemokine ligand 2

CXCL10

C-X-C motif chemokine ligand 10

CXCL11

C-X-C motif chemokine ligand 11

CX-10

3, 14, 19-triacetyl andrographolide

CYP3A4

cytochrome P450 3A4

DAMPs

damage associated molecular patterns

DL

dehydrocostus lactone

DG

diammonium glycyrrhizinate

DSS

dextran sulfate sodium

ECM

extracellular matrix

ERK

extracellular signal-regulated kinase

FDA

Food and Drug Administration

GA

glycyrrhetic acid

GAC1

ganoderic acid C1

GCZ

Glycyrrhizin

GPX

glutathione peroxidase

GPX2

glutathione peroxidase 2

GSH

glutathione

GSK3β

glycogen synthase kinase 3 β

HO-1

heme oxygenase 1

IBD

Inflammatory Bowel Disease

ICAM

intercellular adhesion molecule

ICAM-1

intercellular adhesion molecule 1

IFN-γ

interferon γ

IKK

IκB kinase

IL-1

interleukin 1

IL-1Ra

interleukin 1 receptor antagonist

IL-1α

interleukin 1 α

IL-1β

interleukin 1β

IL-2

interleukin 2

IL-4

interleukin 4

IL-6

interleukin 6

IL-6^−/−

interleukin 6 deficient

IL-8

interleukin 8

IL-10

interleukin 10

IL-10^−/−

interleukin 10 deficient

IL-12

interleukin 12

IL-17

interleukin 17

IL-17A

interleukin 17A

IL-17F

interleukin 17F

IL-18

interleukin 18

IL-23

interleukin 23

iNOS

inducible nitric oxide synthase

IRAK-1

interleukin 1 receptor associated kinase 1

IRAK-4

interleukin 1 receptor associated kinase 4

IκBα

nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor alpha

JAK

janus kinase

JAK-1

janus kinase 1

JNK

c-Jun N-terminal kinase

KA

kalopanaxsaponin A

Keap1

Kelch-like ECH-associated protein 1

LBD

ligand binding domain

MAPK

mitogen-activated protein kinase

MCP-1

monocyte chemoattractant protein 1

miRNA-155

micro RNA 155

miRNA-214

micro RNA 214

MMP-9

metalloproteinase 9

MPO

myeloperoxidase

mTOR

mechanistic target of rapamycin

mTORC1

mechanistic target of rapamycin complex 1

MUC2

mucin 2

MyD88

myeloid differentiation primary response 88

NFAT

nuclear factor of activated T lymphocytes

NF-ƙB

nuclear factor kappa-light-chain-enhancer of activated B-cells

NLR

nod-like receptor

NLRP3

nod-like receptor family pyrin domain containing 3

NO

nitric oxide

NOD2

nucleotide binding oligomerization domain containing 2

Nrf2

nuclear factor erythroid 2–related factor 2

NQO1

NAD(P)H quinone dehydrogenase 1

PAMPs

pathogen associated molecular patterns

PCAM-1

platelet endothelial cell adhesion molecule 1

p-CREB

phosphorylated cyclic adenosine monophosphate response element binding

PDLIM2

PDZ And LIM Domain 2

p-ERK

phosphorylated extracellular signal regulated kinase

p-ERK 1/2

phosphorylated extracellular signal regulated kinase 1 and 2

PGE₂

prostaglandin E2

p-IκBα

phosphorylated nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor α

PI3K

phosphoinositide 3-kinase

p-JNK

phosphorylated c-Jun N-terminal kinase

PPARs

peroxisome proliferator activated receptors

PPARγ

peroxisome proliferator-activated receptor γ

PPD

20(S)-protopanaxadiol

PPT

20(S)-protopanaxatriol

p-p38

phosphorylated p38

p-p65

phosphorylated p65

PRRs

pattern recognition receptors

p-STAT3

phosphorylated signal transducer and activator of transcription 3

PTEN

phosphatase and tensin homolog

PXR

pregnane X receptor

p70S6K

ribosomal protein S6 kinase β 1

RORɣt

retinoic acid receptor-related orphan receptor Ɣ

ROS

reactive oxygen species

SSA

saikosaponin A

SSD

saikosaponin D

sICAM

soluble intercellular adhesion molecule

siNrf2

small interfering RNA for Nrf2

SMEDDS

self-micro emulsifying drug delivery system

SOD

superoxide dismutase

SOD2

superoxide dismutase 2

STAT3

signal transducer and activator of transcription 3

S1PR1

sphingosine-1-phosphate receptor 1

S1PR5

sphingosine-1-phosphate receptor 5

Tan IIA

tanshinone IIA

TBARS

thiobarbituric acid reactive substances

TCRs

T-cell receptors

TGFβ

transformation growth factor β

Th1

T helper 1

Th17

T helper 17

TIR

toll/IL-1 receptor

TNBS

trinitrobenzenesulfonic acid

TNF-α

tumor necrosis factor α

TNFRs

tumor necrosis factor receptors

TLR

toll-like receptor

TLR4

toll-like receptor 4

Treg

regulatory T cells

UC

ulcerative colitis

VCAM-1

vascular cell adhesion protein 1

VEGF

vascular endothelial growth factor

ZO-1

zonula occludens

5-ASA

5-aminosalycilic acid

Data Availability

No data was used for the research described in the article.