Inflammation, microbiota, and pancreatic cancer

Abstract

Pancreatic cancer (PC) is a malignancy of gastrointestinal tract threatening the life of people around the world. In spite of the advances in the treatment of PC, the overall survival of this disease in advanced stage is less than 12%. Moreover, PC cells have aggressive behaviour in proliferation and metastasis as well as capable of developing therapy resistance. Therefore, highlighting the underlying molecular mechanisms in PC pathogenesis can provide new insights for its treatment. In the present review, inflammation and related pathways as well as role of gut microbiome in the regulation of PC pathogenesis are highlighted. The various kinds of interleukins and chemokines are able to regulate angiogenesis, metastasis, proliferation, inflammation and therapy resistance in PC cells. Furthermore, a number of molecular pathways including NF-κB, TLRs and TGF-β demonstrate dysregulation in PC aggravating inflammation and tumorigenesis. Therapeutic regulation of these pathways can reverse inflammation and progression of PC. Both chronic and acute pancreatitis have been shown to be risk factors in the development of PC, further highlighting the role of inflammation. Finally, the composition of gut microbiota can be a risk factor for PC development through affecting pathways such as NF-κB to mediate inflammation.

Graphical Abstract

Introduction

In spite of significant attempts for the introduction of novel therapeutics, there are still major issues and obstacles in the treatment of pancreatic cancer (PC). Although PC is a rare cancer, it is responsible for only 3% of all cancer cases in the USA and it accounts for 8% of estimated deaths [1]. Based on the report of American Cancer Society, PC will be the second leading cause of death by 2030, requiring effective therapeutics. Originally, PC results from pancreas tissue responsible for digestion and endocrine activities. Up to 85% of PC cases result from exocrine cells known as exocrine tumors. The diagnosis of these tumor types at the early stages is challenging and the diagnosis of most cancer patients occurs during the organ metastasis including liver metastasis. In this level, surgical resection is not preferred and the patients undergo chemotherapy-based regimens showing low efficacy in this stage. Regarding the late diagnosis of cancer patients and lack of potential therapeutic strategies, the 5-year survival rate of the PC patients is less than 12% [2]. Folfirinox and gemcitabine are commonly applied chemotherapy drugs for PC [3, 4]. However, drug resistance and adverse impacts compromise the efficacy of PC chemotherapy. Moreover, PC patients demonstrate a poor prognosis. In most of the cases, the reason of PC development is the mutations in the oncogenes in exocrine cells. These tumors demonstrate mutation in KRAS as oncogenic factor, resulting in alterations in the downstream targets to promote survival [5]. Moreover, the mutation of p53 as onco-suppressor factor in observed in PC [6]. The pancreatic ductal adenocarcinoma (PDAC) is the most prevalent type of PC that 5-year survival rate is less than 10% and the median survival time is suggested to be 5–6 months [7]. Clinically, the poor efficacy of the current therapeutics for PC elimination is due to the deep location of the pancreas tissue lacking screening and diagnostic tools along with issues in tissue biopsy, aggressive nature and poor outcomes in chemotherapy and radiotherapy. Along with the heterogeneous nature of tumor microenvironment (TME) in PC and presence of immunosuppression, PC stem cells (PCSCs) have been also mentioned as accelerators of tumorigenesis [8]. Regarding the challenges faced in PC, especially for its treatment, it is suggested to develop novel therapeutics based on targeting specific and vital mechanisms in this malignant disease.

In addition to the genetic mutations, molecular factors in PC progression have been emphasized. Notably, induction of RAF/MEK/ERK pathway, caused by the KRAS mutation can enhance the proliferation and survival of cancer [9]. Another essential pathway is suggested to be PI3K/Akt axis in which promotes the growth, metabolism and apoptosis evasion [10]. An example of this case is the upregulation of mTORC1 by fructose in enhancing the PC malignancy via the autophagy suppression [11]. The metastasis of PC can be elevated by TGF-β/SMAD axis that is associated with the induction of EMT mechanism [12]. There is also frequent induction of Wnt in PC, increasing drug resistance [13, 14]. The cytokines including IL-6 have been shown to stimulate JAK/STAT axis for promoting the survival and proliferation [15]. The metastasis and carcinogenic interactions in the TME of PC can be increased by Hedgehog pathway [16]. The PC progression can be also regulated by the RBFOX2 that controls the alternative splicing of cytoskeletal remodelling-associated transcripts such as ABI1 [17].

Since 17th, the function of inflammation and its association with tumorigenesis have been unveiled. Since then, significant attempts have been followed to highlight the function of inflammation in tumor and the experiments have demonstrated the function of cytokines, physiological process and cells in controlling inflammation and cancer. A number of certain tumors and solid cancers can be mediated by inflammation, initiating local inflammation in enhancing cancer proliferation and dissemination. As a result, the inflammatory pathways have been of importance in understanding tumorigenesis and controlling cancer progression [18]. FBXO38 has been shown to increase FGL1 degradation through ubiquitination to increase anti-cancer immunity and disrupt inflammation. However, inflammation acts as double-edged sword in cancer. Moreover, presence of inflammation accounts for the progression of a number of certain tumors including colorectal cancer, making inflammation as as therapeutic target and therefore, understanding the mechanisms regulating inflammation is of importance [19, 20]. Regarding this, emphasizing on inflammation in PC is essential in its suppression and the direction of future therapeutics. This paper will comprehensively discuss the inflammation and related cytokines in the PC malignancy along with chemokines and versatile function of gut microbiota in tumorigenesis to highlight a new emerging therapeutic target for PC therapy.

NF-kB and pancreatic cancer

NF-κB was identified in 1986 by David Baltimore’s group that has been comprised of five DNA-binding proteins having different heterodimers and homodimers [21]. NF-κB axis is able to regulate innate and adaptive immune system and it can control growth, suppress apoptosis and increase metastasis and angiogenesis [22]. The viral and bacterial infections, necrosis, DNA damage and pro-inflammatory factors can mediate NF-κB or be regulated by this pathway [22]. NF-κB upregulation can occur in the malignant cells and TME [22] and NF-κB stimulation demonstrates association with genetic alterations [23, 24]. The increasing evidences have displayed the role of NF-κB in controlling PC progression. CRIP1 has been shown to bind to NF-κB/p65 for increasing its nuclear transfer to upregulate CXCL1/2. The CXC1/2 then increases myeloid-derived suppressor cell (MDSC) migration to create an immunosuppressive TME [25]. As a result, targeting MDSCs is of importance in PC therapy. CXCR4 elevates CAR-T cell infiltration that disrupts STAT3 axis for decreasing levels of inflammation factors such as TNF-α, IL-6 and IL-17. Such reduction in the levels of inflammatory factors can impair SDF-1α secretion in cancer-associated fibroblasts through NF-κB axis to diminish MDSC migration into TME [26]. Notably, NF-κB can also promote levels of CXCL1 to provide an immunosuppressive TME in PC [27]. In addition, the ILs can control the levels of NF-κB in tumorigenesis. IL-1β induces NF-κB axis to elevate the levels of ESE3. At the next step, ESE3 induces pancreatic stellate cells (PSCs) to trigger chemoresistance [28]. In spite of these discussions, the function of NF-κB is beyond controlling immune system and it can also increase the proliferation and metastasis of PC. Notably, the phosphorylation of NF-κB/JNK/ERK axis can be performed by PRKCI-RIPK2 to enhance the proliferation and invasion of PC, suppress apoptosis and promote autophagosome formation [29].

Alkaliptosis is a new kind of cell death depending on the pH and it was first recognized for the elimination of PDAC [30]. JTC801 is a commonly used drug for the induction of alkaliptosis that its function is based on the stimulation of NF-κB [30]. According to this concept, it can be perceived that NF-κB can be modulated for increasing cell death in PC. Based on the results, ACSS2 mediates the generation of acetyl-coenzyme A to induce histone acetylation for providing CA9 downregulation by NF-κB in enhancing alkaliptosis [31]. This can highlight the dual function of NF-κB and even in some certain conditions, the induction of NF-κB can mediate cell death in PC. NF-κB has shown potential in generating feedback loops with ILs. The upregulation of Pin1 and IL-18 is observed and they mediate poor prognosis. Notably, Pin1 increases NF-κB expression to promote IL-18 levels. Notably, both IL-18 and Pin1 have potential to upregulate NF-κB, showing the presence of positive feedback loop [32]. The inhibitors of NF-κB are suppressed for its activation. According to this, TNF-α attachment to TNFR receptor on the surface of PC cells stimulates the degradation of IκB to trigger nuclear translocation of p65/p50 [33]. Table 1 and Fig. 1 summarize the NF-κB function in PC.

Table 1.

PC malignancy regulation by NF-κB

Pathway	Remark	Refs.
HOXA10/NF-κB	HOXA10 stimulates NF-κB to promote tumorigenesis	[34]
miR-365a-3p/NF-κB	miR-365a-3p downregulates c-Rel-related NF-κB axis to impair carcinogenesis	[35]
NF-κB/NUAK2	Silencing NUAK2 impairs tumorigenesis and increases apoptosis NF-κB binds to NUAK2 to increase its levels NUAK2 silencing decreases the levels of p-SMAD2/3 and SMAD2/3	[36]
NF-κB	Metallothionein-1G inhibits NF-κB to diminish secretion of activin A for decreasing stemness	[37]
miR-146a-5p/TRAF6/NF-κB p65	miR-146a-5p downregulates TRAF6 to suppress NF-κB axis for reducing levels of P-gp in reversing chemoresistance	[38]
TLR4/MAPKs/NF-κB	Polysaccharide increases cytotoxicity in PC therapy through TLR4/MAPKs/NF-κB control	[39]
NF-κB/STAT3 and NF-κB/CSN5	Shikonin suppresses NF-κB/STAT3 and NF-κB/CSN5 axis to increase PD-L1 degradation in impairing immune evasion	[40]
NF-κB	The recombinant defensin/HSA fusion protein can suppress NF-κB	[41]
FAK NF-κB	Downregulation of NF-κB and FAK can reduce metastasis of PC	[42]
TLR4/NF-κB	Triptolide suppresses TLR4/NF-κB axis to increase gemcitabine sensitivity	[43]
NF-κB	Silicene-mesoporous silica nanoparticles increase NF-κB expression to impair growth	[44]
YEATS2/TAK1/NF-κB	Cinobufacini disrupts YEATS2/TAK1/NF-κB axis to impair tumorigenesis	[45]
MyD88-dependent NF-κB	CircCUL2 stimulates MyD88-dependent NF-κB axis to mediate an inflammatory CAF phenotype	[46]
TNF/NF-κB	SIK3 stimulates TNF/NF-κB axis to support cancer cells against T cells	[47]

Fig. 1.

The function of NF-κB in the PC malignancy control. The downregulation of NF-κB by miR-365a-3p suppresses cancer progression, while HOXA10 upregulates NF-κB in tumorigenesis. Moreover, proliferation, metastasis and autophagosome formation can be increased by NF-κB, while it suppresses apoptosis. NUAK2 upregulates NF-κB in apoptosis inhibition. Furthermore, NF-κB upregulates CXCL1 in promoting immunosuppression in tumor microenvironment. NF-κB increases activin A expression in accelerating stemness. SIK3 upregulates TNF/NF-κB in protecting the tumor cells against the function of T cells. Both genomic and epigenetic factors participate in the regulation of NF-κB. The circCUL2 stimulates MyD88/NF-κB axis to mediate an inflammatory phenotype in cancer-associated fibroblasts. (Created by Bioreder.com)

Interleukins

IL-1 and pancreatic cancer

IL-1/IL-1 receptor axis is able to affect PC malignancy. The PC cells and macrophages are able to release L-1α and IL-1β and they are essential for the TSLP secretion by cancer-associated fibroblasts. The PC cells release alarmins to stimulate secretion of IL-1β in macrophages [48]. The therapeutic compounds are able to affect IL-1 levels. The (-)-epigallocatechin-3-gallate (EGCG) suppresses NF-κB to reduce IL-1RI and impair the function of IL-1 in the suppression of tumorigenesis in PC [49]. The obesity and the inflammation caused by macrophages can enhance risk of PC development. Notably, the expression of FABP4 in macrophages can upregulate NLRP3 inflammassome to increase IL-1β levels in the process of tumorigenesis [50]. Another aspect that is affected by IL-1 is the metastasis of PC cells. TNFSF9 facilitates metastasis and invasion of PC. TNFSF9 influences Src/FAK/p-Akt/IL-1β axis to enhance the M2 macrophages, accelerating metastasis [51].

IL-6 and pancreatic cancer

According to the studies, IL-6 exerts an oncogenic function in PDAC. This has been confirmed by that fact that suppression of IL-6 and PD-L1 can impair the tumorigenesis in PC and increases the T cells in the TME [52]. IL-6 and related molecular factors can create positive feedback loop in the progression of PC. Notably, the levels of IL-6 enhance in PC. PC cells increase generation of IL-6 in fibroblasts through Jagged/Notch and in turn, IL-6 is able to stimulate Jagged-1/2. The stimulation of Jagges-1/2 by IL-6 relies on the stimulation of STAT3 and in turn, jagged-1/2 enhances the levels of IL-6 through NF-κB [53]. The IL-1β derived from neutrophils is able to mediate stromal inflammation and creates an inflammatory CAF phenotype and CAF-tumor cell IL-6/STAT3 axis [54]. In addition, IL-6 participates in the immunosuppression in human cancers. The expansion of myeloid-derived suppressor cells (MDSCs) is enhanced by IL-6 and this creates an immunosuppressive TME in PDAC [55]. One of the most common downstream targets of STAT3 in PC is STAT3. According to the studies, STAT3 is able to significantly increase progression of PC. In line with this, Baker and colleagues demonstrated that IL-6 can stimulate the phosphorylation of STAT3 and Pim-1 kinase. On the other hand, the upregulation of Pim-1 cannot significantly affect the growth of PC cells [56].

Cancer-associated fibroblasts (CAFs) are the main immunosuppressive factors causing tumorigenesis and cancer metastasis. CAFs participate in the immune evasion through the secretion of cytokines, chemokines and growth factors [57]. The infiltration of immune cells in the TME can be directly or indirectly affected by CAFs. As heterogenous stromal population, CAFs are divided into three uniqute subtypes regulating carcinogenesis and cancer immunity. CAF-driven IL-6 elevates PC malignancy. The application of nintedanib can impair the activation of CAFs through reducing levels of PDGFRβ. This disrupts the capcaity of CAFs in the IL-6 release and the co-application of nintedanib along with CAF inhibitor can increase the activity of natural killer (NK) cell [58]. The major hallmarks of PC including proliferation and metastasis can be regulated by IL-6. This can originate from the IL-6 action in controlling epigenetic factors such as microRNAs (miRNAs). The miR-455-5p level is shown to be reduced in PC cells. IL-6 causes methylation of miR-455-5p to downregulate it to increase levels of IGF-1R in facilitating growth, invasion and EMT [59].

IL-8 and pancreatic cancer

Smoking can enhance risk of PC development. Nicotine is one of the additive components of tobacco and it can enhance the secretion of IL-8 from PC stroma and it enhances cancer-mediated cachexia [60]. Moreover, IL-8 increases PC metastasis. The miR-623 is able to downregulate MMP1 and impair the IL-8-mediated invasion of PC [61]. Furthermore, IL-8 participates in controlling PC metabolism. The cells prefer to use glucose and they stimulate glycolysis. Tumor-associated macrophages are able to mediate glycolysis through IL-8 release. Then, induction of STAT3 occurs to enhance GLUT3 levels for stimulation of glycolysis-mediated PC progression [62]. The regulation of IL-8 has been beneficial in controlling immune system. Anti-IL-8 antibody can stimulate myeloid cells and this elevates the potential of anti-PD-1 antibody in the exertion of anti-cancer immunity [63]. The final impact would be the impact of IL-8 on angiogenesis and the PC cells increase IL-8 secretion to increase gemcitabine resistance through angiogenesis [64].

IL-10 and pancreatic cancer

PC malignancy is also regulated by IL-10. The CAR-T cells expressing IL-10 can resist against dysfunction and they enhance suppression of solid tumors including PC and metastasis [65]. The M2 polarized macrophages are responsible for the progression of PDAC. The M2 polarized macrophages can increase TLR4 levels to upregulate IL-10 for EMT induction and increasing invasion [66]. However, a few researches have investigated the role of IL-8 in PC and more studies regarding the interaction of IL-8 with other cells in TME including MDSCs, T cells and its impact on angiogenesis and therapy resistance along with inflammation should be highlighted.

IL-11 and pancreatic cancer

Until now, only one experiment has evaluated the role of IL-11 in controlling PC progression. According to this study, the PC patients upregulation of IL-11 compared to healthy individuals. The IL-11 is a reliable diagnostic and prognostic factor for PC with high accuracy with AUC of 0.901, sensitivity of 97.7% and specificity of 70%. The patients with distant metastasis demonstrated low median levels of IL-11p. The prognosis of patients with high levels of IL-11 is favourable and it can improve overall survival [67].

IL-17 and pancreatic cancer

In the TME of PC, IL-17 displays interaction with the cells and cytokines adjacent to the tumor and they can regulate a number of biological events including chronic pancreatitis, acinar-ductal metaplasia, PanIN and advanced carcinogenesis [68]. There is a remarkable enhancement in the levels of IL-17 and Th17 cells in the peripheral blood of PC patients and they have positive relationship with the stage of cancer [69, 70]. Regarding the plasticity of T cells, there is possibility for Th17 and Treg cells to transform into each other. In spite of decrease in number of Treg cells, there is an enhancement in IL-17A in peripheral blood of PC patients in stable and remission stages [71]. However, PC patients at advanced and unresectable stage demonstrate increase in Treg cells and reduction in IL-17A levels. More importantly, Th17 cells and IL-17A demonstrate upregulation in the TME of PC [69]. It has been shown that the number of TH17 and IL-17 producing-gamma-delta T cells elevates in the dense fibro-inflammatory stroma around the Kras-mediated murine PanINs. Moreover, the number of cells generating IL-17 enhances along with presence of chronic pancreatitis [72]. Therefore, IL-17 can be considered as a target in PC therapy [73]. The overall idea is that IL-17 contributes to the progression of PanIN. Moreover, antibodies for therapeutic targeting of IL-17 or IL-17 receptor have been utilized in clinical trials for the treatment of autoimmune diseases including psoriasis and rheumatoid arthritis and they can be used in the future to avoid PanIN and treat human cancers, especially PC [74]. Table 2 summarizes the role of ILs in the regulation of PC progression along with Fig. 2.

Table 2.

The versatile activity of ILs in PCprogression

Interleukin	Remark	References
IL-6	MUC16 C terminal-accelerated secretion of tumor-derived IL-6 enhances the levels of Treg cells	[75]
IL-6	Mesothelin is able to increase autocrine IL-6/sIL-6R trans-signaling to enhance proliferation	[76]
IL-6	Quercetin downregulates STAT3 to impair IL-6-mediaed EMT	[77]
IL-6	Silencing IL-6 prevents proliferation and recurrence	[78]
IL-6	ZIP4 affects CREB to induce IL-6/STAT3	[79]
IL-6	IL-6 upregulates VEGF through paracrine and autocrine manners	[80]
IL-10	IL-10 shows an increase in the levels, while IFN-γ expression decreases	[81]
IL-10	IL-10-expressing CAR T cells resist against dysfunction and promote durable clearance	[65]
IL-10	M2 polarized macrophages stimulate TLR4/IL-10 axis to enhance EMT	[66]
IL-6 and IL-1β	High levels of IL-6 and IL-1β mediate short survival rate of patients	[82]
IL-1β	Upregulation of IL-1β and NO can induce drug resistance through reducing caspase levels	[83]
IL-8	CXCL8/IL-8 and CXCL12/SDF-1alpha coordinate to induce angiogenesis and metastasis	[84]
IL-8	Mutant KRAS-induced PAI-1 overexpression activates pancreatic stellate cells to increase tumorigenesis via IL-8	[85]
IL-8	Laminin-1 activated by integrin can increase levels of IL-8 and CXCR4	[86]
IL-8	BAG3 downregulation increases levels of HuR and AGO2 to increase mRNA degradation of IL-8 in impairing metastasis	[87]
IL-8	TRAF2 and Bcl-xL upregulation increases potential of TRAIL in upregulating uPA and IL-8	[88]
IL-8	IL-1α increases secretion of IL-8 through MAPK and ROS	[89]
IL-6	P2X7 receptor induces IL-6 expression Toculizumab prevents the STAT3 upregulation	[90]
IL-6	Bone marrow mesenchymal stromal cell-released IL-6 increases proliferation	[91]
IL-6	Suppressing IL-6/GP130 axis impairs proliferation, glucose metabolism and colony formation	[92]
IL-6	IL-6 enhances the liver metastasis through niche formation	[93]
IL-6	SPON1 upregulation increases IL-6 trans-signaling in tumorigenesis	[94]
IL-6	HIC1 disrupts IL-6/STAT3 to diminish invasion	[95]
IL-6	Curcumin suppresses EMT and metastsasi through inhibiting IL-6/ERK/NF‑κB	[96]
IL-6	Tyrphostin B42 impairs trichostatin A-mediated resistance through disrupting IL-6/JAK2/STAT3	[97]

Fig. 2.

The versatile function of interleukins in the pancreatic cancer. MUC16 is able to increase tumor-derived IL-6 in promoting number of Treg cells. Furthermore, mesothelin can mediate IL-6/sIL-6R to facilitate growth of tumor. The upregulation of IL-1β and NO prevents caspase cascade to mediate drug resistance. The M2 polarized macrophages upregulate TLR4 to increase IL-10-induced EMT. Furthermore, IL-6 increases VEGF expression and promotes STAT3 expression to induce EMT. The angiogenesis and metastasis can be accelerated by the function of CXCL8/IL-8 and CXCL12/SDF-1α. The function of IL-6 in niche formation can promote liver metastasis of pancreatic cancer cells. Integrin increases laminin-1 levels to promote IL-8 and CXCR4 levels. Notably, the ILs can affect the levels of each other in pancreatic cancer such as IL-1α that affects ROS and MAPK for increasing secretion of IL-8. (Created by Biorender.com)

Chemokines and pancreatic cancer

Chemokine can modulate the carcinogenesis process in PC. There are different kinds of families of chemokines that can affect cancer progression [98]. CCL2 dysregulation has been shown in PC. Upregulation of CCL2 mediates poor prognosis of PC patients [99]. CCL2 accelerates the immune cell infiltration in TME. The tumor-derived CCL2 can increase recruitment of CCR2⁺ macrophages in enhancing proliferation of cancer cells. Moreover, CCL2 is able to recruit the MDSCs in providing an immunosuppressive TME in PC [100]. The CCL2-accelerated recruitment of immune cells can induce MAPK axis to provide an immunosuppressive TME. The MDSCs have ability of impairing the CD⁴⁺ and CD⁸⁺ T cell proliferation. The reduction of CCL2 levels impairs immunosuppression and it stimulates CD⁸⁺ T cells to mediate TME remodelling [101]. Tumor secretome containing chemokines participate in the development of therapy resistance in PDAC [102–106]. The dysregulation of chemokines and their abundancy can result in the cell–cell communications in TME to mediate drug resistance. The CXCR4-CXCL12 accelerates drug resistance in PDAC [107].

GABRP has been shown to be upregulated in PDAC and it can elevate growth and metastasis. Notably, GABRP increases macrophage infiltration in TME and it interacts with KCNN4 to increase entry of Ca²⁺ into the cells for stimulation of NF-κB axis and enhancing macrophage infiltration through upregulation of CXCL5 and CCL20 [108]. The function of all chemokines is not oncogenic and a number of them are able to suppress PC progression. CXCL12 chemokine has been shown to impair proliferation and invasion of PC [109]. CXCR7 increases CXCL12 levels to induce phosphorylation of ERK1/2 for increase in the proliferation of PC [110]. In addition to CXCR7, CXCR4 has been also shown to enhance growth of PC through phosphorylation of AKT and ERK [111]. The upregulation of CCR9 can occur in a paracrine way to finally enhance the invasion [112]. The neovascularization participates in the emergence of new functional blood vessels for increasing progression of PC. Accordingly, triptonide has been shown to downregulate VE-cadherin and chemokine ligand 2 genes in impairing PC progression through suppressing vasculogenic mimicry [113]. Another chemokine that can stimulate angiogenesis and increase the progression of PC is CXCR2 [114]. CCL25 can stimulate CCR9 in PC that subsequent promotes proliferation of PC [115]. Regarding to these observations, the suppression of CXCR4 and NCOA3 has been shown to suppress metastasis and carcinogenesis in PC [116]. Some specific conditions in the TME of PC such as hypoxia can increase CXCR4 expression in tumorigenesis. However, miR-150 is able to downregulate CXCR4 for impairing proliferation and metastasis of PC [117]. Therefore, these studies highlight the fact that chemokines are potent regulators of tumorigenesis in PC and they can affect proliferation, metastasis and neovascularization. However, the studies have simply evaluated the expression and final impact on tumorigenesis, while underlying molecular pathways have not been largely understood.

Toll-like receptors and pancreatic cancer

The immune interactions can be regulated by toll-like receptors (TLRs) and they participate in the regulation of tumorigenesis [118]. TLR4 is the most common type with dysregulation in PC and influences the tumor site, lymph node metastasis and pathological stage [119]. Exposure to palmitic acid can increase the expression of TLR4 to promote the generation of ROS in the stimulation of NF-κB. Then, expression levels of MMP-9 enhance to promote metastasis of PC [120]. One of the mechanisms significantly increasing the metastasis of PC is EMT. The TLR4 participate in the increase in PC metastasis through EMT induction. In this case, M2 polarized macrophages are able to stimulate TLR4/IL-10 axis to mediate EMT for increasing metastasis and invasion of PC [66].

There are different factors regulating TLR4 in PC to affect the progression of this malignant disease. TRIM15 has been recognized as an oncogenic factor in PC. As an ubiquitin ligase, TRIM15 has been shown to upregulate IGF2BP for the induction of TLR4 axis in increasing progression of PC [121]. One of the mechanisms employed by the TLRs is inflammation. Notably, the application of intrabodies for the suppression of TLR2 and TLR9 can impair inflammation-induced progression and proliferation of PC [122]. In this case, if TLRs demonstrate upregulation, it can cause the immunosuppression of TME. The intratumoral administration of TLR9 agonist can provide an immunopermissive TME in PC [123]. This can be followed for the synergistic cancer immunotherapy. Notably, the application of TLR9 agonist and STING upregulation can cause anti-cancer immunity and it promotes levels of CD4 and CD8 T along with stimulation of IL-12 and type I IFN to exert anti-cancer function in PC [124]. Robinin can downregulate TLR2 to disrupt PI3K/Akt for reducing tumorigenesis, EMT and inflammation in PC therapy [125]. Hence, TLRs are critical regulators of PC progression and promising candidates for cancer therapy [126–129].

TGF-β and pancreatic cancer

Transforming growth factor-beta (TGF-β) is one of the 12 essential pathways in PC and can regulate response to radiation therapy. Somatostatin has been emerged as an agent in the PC therapy. Somatostatin stimulates apoptosis, while it suppresses proliferation and invasion of PC. Notably, somatostatin impairs Bcl-2 expression, while it enhances caspase-3 and Bax expression. Moreover, somatostatin impaired the TGF-β-driven EMT [130]. Therefore, TGF-β participates in the EMT induction. Notably, PC cells demonstrate liver metastasis, while metformin administration can impair TGF-β1-mediated EMT in impairing metastasis [131]. This idea has been also conceived in another experiment showing that application of riboflavin can suppresses TβR1 in EMT suppression and reducing metastasis [132]. TGF-β participates in two main characteristics of PC cells including EMT and stemness. In addition to EMT, TGF-β is able to increase elevels of ABCG2, CD24 and CD44 to enhance stemness and this is also coordinated through interaction with FZD7 [133]. In low-grade PC, ESE1 disrupts TGF-β-mediated EMT in impairing invasion of PC [134]. These studies provide a fact that invasion and stemness of PC cells depend on the overexpression of TGF-β. Therefore, therapeutic targeting of TGF-β can significantly impair PC progression (Table 3).

Table 3.

The role of TGF-β in PC

Molecular pathway	Remark	References
LASP2/TGF-β	The overexpression of LASP2 contributes to the suppression of TGF-β-driven EMT to impair metastasis	[135]
TGF-β/Smad	Paeonol is able to suppress TGF-β1/Smad axis and EMT in impairing PC metastasis	[136]
TGF-β/Smad SHH	Quercetin is able to downregulate SHH and TGF-β/Smad in impairing metastasis of pancreatic cancer	[137]
PHLPP2/Akt/TGF-β	Circ ANAPC7 has been shown to increase PHLPP2 levels in impairing Akt phosphorylation to downregulate TGF-β in disrupting tumor growth and muscle wasting	[138]
TGF-β	The suppression of cholesterol pathway stimulates TGF-β to enhance basal differentiation in PC	[139]
CircEIF3I	CircEIF3I promotes SMAD3 recruitment to endosomes in induction of TGF-β for upregulation of MMPs in PC metastasis	[140]
Numb-PRRL	Numb-PRRL enhances TGF-β1- and EGF-mediated EMT in increasing metastasis of PC	[141]
TGF-β1	TGF-β1 increases the lysosomal degradation of Tap63α in enhancing metastasis and invasion of PC	[142]
TGF-β/SMAD2/3	The inhibition of TGF-β/SMAD2/3 axis by itraconazole participates in impairing metastasis of cancer cells	[143]
TGF-β/Smad	TGF-β/Smad suppression by magnolol impairs the development of PC	[144]

TNF-α and pancreatic cancer

The tumor necrosis factor-α (TNF-α) is a signalling protein that mediates metabolic derangements [145]. The activated macrophages are able to release TNF-α in addition to other cells including CD⁴⁺ cells, neutrophils, mast cells, eosinophils and neurons. TNF-α can regulate biological mechanisms including induction of apoptosis, cachectic condition and inflammatory response. Cxcl1 facilitates tolerogenic pathways and exacerbates stromal inflammation via neutrophil-derived TNF, hence promoting PC development. TNF can trigger the Cxcl1 release from cancer cells and CAFs to cause T cell dysfunction and mediate inflammatory CAF polarization through interaction with transmembrane TNF-TNFR2 [146]. Therefore, TNF-α can exert as oncogenic factor in PC. Notably, Ccr2 downregulation can decrease the recruitment of monocytes to reduce TNF-α levels, whereas it increases IL-33 expression in impairing metastasis and enhancing survival rate of mice [147]. The expression of TNF-α on some of the cells in the TME including macrophages can significantly increase the aggressive behaviour of PC cells. BRD4-induced c-JUN/AP1 expression is involved in sustaining the basal-like neoplastic state in PC that promotes CCL2 levels in the recruitment of TNF-α-secreting macrophages. These cells are able to change classical neoplastic cells into aggressive phenotypes through lineage reprogramming to significantly increase tumorigenesis [148]. However, such macrophages may demonstrate dual function in PC. Regarding to this, an experiment has shown that TNF-α can trigger cell death in M0 macrophages and this can impair PC progression [149]. To further validate the anti-cancer efficacy of TNF-α, salicylates were utilized to enhance TNF-α-induced apoptosis, while inhibiting NF-κB. [150].

Pancreatitis and pancreatic cancer

There are lobules in the adult pancreas tissue that have been separated by the thin layers of connective tissue. The pancreas tissue shows two exocrine and endorine functions. The exocrine role of pancreas tissue is related to the release of digestive enzymes including amylase, lipase and trypsin by acinar cells that comprise up to 85% of the pancreatic mass and they are the major components of the lobules. Around the intercalated ducts, there are acinar cells in the clusters that can secrete enzymes into the ducts and the function of ducts is to integrate them into the larger intralobules. Then, they are transferred into interlobular ducts that finally reach to duodenum. The cells present in the small pancreatic islets are responsible for the endocrine function of pancreas that have been comprised of endocrine cells and they directly drain into the arterioles. Other cells in the pancreas tissue are comprised of centroacinar cells at the tip of ducts possessing characteristics of both acinar and embryonic stem cells. The extracellular matrix is preserved by the PSCs and fibroblast-like cells [151]. The presence of inflammation in pancreas tissue results in the development of pancreatitis that is categorized into acute and chronic pancreatitis [152]. The rapid disease of pancreas is acute pancreatitis that has a number of clinical and morphological presentations. The onset of acute pancreatitis is severe epigastric pain that suddenly occurs that can reach to back and cause abdominal pain with severe issues upon eating, abdominal tenderness, nausea, vomiting, fever and rapid pulse [153]. The diagnosis of acute pancreatitis is based on three criteria including abdominal pain, serum lipase or amylase levels three times higher than the normal level and radiographic evidence of acute pancreatitis [153]. Clinically, acute pancreatitis can be categorized into three types including mild, moderate and severe [153]. On the other hand, the fibroinflammatory condition related to the repetitive episodes of pancreatic inflammation with different intensities and durations is known as chronic pancreatitis resulting in the damage in pancreas and loss of function (may occur permananetly) [154]. The presence of inflammation in repetitive episodes can lead to the excessive fibrotic tissue accumulation, insufficiencies of exocrine and endocrine activity, chronic pain and decreased life and mental health quality [154]. Imaging and presence of some clinical features including pain, nausea, vomiting and steatorrhea can result in the diagnosis of chronic pancreatitis [154].

Historically, chronic pancreatitis and PDAC were supposed to be unrelated, since they are resulted from the different cells including acinar and ductal cells [155]. However, subsequent experiments using mouse models demonstrated a similar origin in acinar cells. According to the model, PDAC has been suggested to be developed from the duct cells through PanINs lesions [156]. Increasing evidences demonstrate that PC can be resulted from acinar cells by acinar metaplasia resulting in ductal metaplasia. The different are involved in this process including oxidative damage and inflammation, among others [157–159]. Based on the animal models, adult mice with expression of KRAS oncogene in the acinar cells show PanIN and PDAC lesions with higher penetrance compared to acute and chronic inflammation [160]. The presence of sporadic episodes of pancreatitis can result in the PDAC development in these mice. The long inflammatory periods (three months) or presence of chronic pancreatitis can enhance risk of tumor development and decrease the onset latency. More information on PC and pancreatitis can be found in these studies [158, 159, 161, 162].

The clinical studies have also highlighted the association between pancreatitis and PC [163]. According to the Danish cohort study [164], the age more than 50 years is a predictor of PC in the patients having acute pancreatitis and the highest risk was observed in patients aged 56–70. Another cohort study in Sweden demonstrated that [165] acute pancreatitis increases risk of PC in the first few years and this risk reduces gradually overtime. In addition, clinical studies have highlighted the association between chronic pancreatitis and PC development. Currently, multiple studies have highlighted the presence of chronic pancreatitis can increase risk of PC development [166–170]. A clinical study has shown that with aging, risk of PC development from chronic pancreatitis increases [167]. In addition to age, the increasing evidences have shown that elevation in the chronic pancreatitis duration enhances risk of PC development [171–173].

Microbiome and pancreatic cancer

By definition, a commensal micobiome is responsible for preserving the symbiotic relationship of healthy individuals and can support against the diseases through nutritive, inflammatory-modulating activity, hormonal homeostasis, detoxification and metabolic impacts of bacterial metabolites [174–177]. An imabalanced micrbiome is known as dysbiosis that is involved in the pathogenesis of different diseases [176]. The increasing evidences have shown that changes in the micrbiome can result in the development of human cancers [175, 176, 178, 179]. Helicobacter pylori (H. pylori) is a classical pathogen that has shown inconsistent and paradoxical associations [180, 181]. H. pylori has been mentioned as a risk factor for PC and its function is under debate [182–185]. Since the clinical prognosis of PC is poor because of late diagnosis and therapeutic resistance, there are still studies for understanding the risk factor, screening biomarkers and TME elements that affect prognosis [186, 187]. In the current section, the role of microbiota in the regulation of PC progression is evaluated.

One of the important functions of microbiota is related to the regulation of chemotherapy response in PC [188]. The efficacy of chemotherapy agents such as gemcitabine in the treatment of PC can be affected by gut microbiota such as Escherichia coli. This bacterium reduces the potential of gemcitabine in cancer chemotherapy and a combination of Escherichia coli and gemcitabine in mice can enhance the PC volume [189]. Therefore, the presence of specific bacteria in the gut can affect the efficacy of chemotherapy regimen and gemcitabine is among them. Notably, the intratumoral bacteria also play a function in chemoresistance in which bacterial DNA has been recognized in 76% of PC samples. Most of these bacteria were suggested to be Gammaproteobacteria [190]. Therefore, therapeutic targeting of gut microbiota can provide new insights for improving the response of PC patients to chemotherapy [188]. However, it is worth addressing that a number of intra-tumoral bacteria have been shown to be beneficial for improving the clinical results [191].

In order to identify the function of microbiome in the development of carcinogenesis, an animal model of mice was utilized [192, 193]. These animals were shown to spontaneously develop PC. Notably, in the animals lacking microbiome, there was a reduction in the pancreatic dysplasia. The increase in tumorigenesis was observed upon the ablation of bacterial gut of Kras mice with antibiotics and then, repopulation with feces derived from mice expressing mutant intrapancreatic Kras and Trp53 genes [192]. In another model of mice with Kras mutation and partial downregulation of PTEN as tumor suppressor, the ablation of bacteria with antibodies can lead to reduced rate of PDAC carcinogenesis [194]. In the next level, for understanding the fact that presence of pancreatic or intestinal microbiota can affect the PDAC progression, an animal model of PC was utilized. In WT mice, the application of oral antibiotics led to a significant reduction (50%) in the size of pancreatic tumor [192]. Therefore, the intestinal microbiota is able to increase development of PC.

The disturbances in the gut microbiota can cause inflammation [195]. The PC micrbiome can cause innate and adaptive immune system reactions leading to the immune inhibition and PC evasion [179]. As an instance of such microbes, Helicobacter pylori can be mentioned [196]. H. pylori has been considered as a bacterial carcinogen that is able to changes gastric cancer progression and its function in PC patients is under debate exerting an indirect function through affecting inflammatory mechanisms [196]. The secretion of bacterial proteins including lipopolysaccharide chains by bacterial microbes can stimulate the immune responses through NF-kB overexpression [197]. LPS has been shown to stimulate severe pancreatitis and can affect the CD⁴⁺ T cells for the secretion of TNF-1, IL-1β and IL-8 [198]. In addition to bacteria, viruses can also be present in the gut microbiota and an experiment has recognized up to 100,000 viruses [199]. The similar blood vessels and ducts can provide the transfer of HBV among these two organs. The HBV infection marker that is Hepatitis B surface antigen can be found in the pancreatic juice [200].

The pancreas is known as a sterile organ and there are bacteria and fungi inside the pancreatic tumor tissue and cystic fluid [201]. Geller and colleagues [190] have shown that there are bacteria inside the healthy pancreatic tissue from the organ donors and PDAC specimens at the ribonucleic acid (RNA) and desoxyribonucleic acid (DNA) level based on the bacterial S16 rRNA sequence. The detection of bacteria occurred in 76% of PDAC samples, while it was 15% in non-tumoral samples. Moreover, Gammaproteobacteria was suggested to be most abundant species (52%) with the members of Enterobacteriaceae and Pseudomonadaceae families. These bacteria can be also found in the duodenum of healthy individuals, highlighting that they have ability of being transferred from this organ [202]. Other studies also highlighted that the phyla Proteobacteria and Firmicutes can be found in PDAC samples [192, 194]. An interesting study by Riquelme and colleagues [203] showed that the high presence of intratumor bacterial diversity mediates the long survival rate. An intratumor microbiome signature was developed from Pseudoxanthomonas-Streptomyces-Saccharopolyspora-Bacillus clausii that is a precise predictor of long-term survival in two independent cohorts.

There have been multiple kinds of studies providing the difference between the microbiome of PDAC patients and healthy individuals using oral and fecal samples [204]. An experiment performed by Farrell and colleagues [185] demonstrated that based on the analysis of oral microbiota, there are decreased levels of Neisseria elongata and Streptococcus mitis in PDAC patients compared to the normal individuals. Another experiment performed by Fan and colleagues [205] highlighted that risk of PDAC development enhances by Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans. Michaud and colleagues focused on the levels of antibodies and they showed that increased levels of antibodies against oral bacteria Porphyromonas gingivalis can elevate risk of PDAC development [206]. According to this study, the plasma levels of antibody of Porphyromonas gingivalis were evaluated ten years before the diagnosis of PDAC and this shows that gut microbiota can be used as a diagnostic factor for PDAC (Fig. 3).

Fig. 3.

Gut microbiome in the PC [212]. There are a number of risk factors for PC including obesity, chronic pancreatitis, aging, smoking and diabetes mellitus that can affect TME. Moreover, these risk factors can affect inflammation and immune responses in the development of PC. On the other hand, microbial metabolites including lipopolysaccharide can upregulate TLRs and NLRs to induce NF-κB and MAPK for accelerating inflammation. Therefore, dysregulation of gut microbiota, known as dysbiosis participates in the progression of PC. (Created by Biorender.com)

Notably, gut microbiota has been demonstrated to affect the progression of PC through various kinds of mechanisms such as immune response modulation, metabolic pathways and inflammation. Accordingly, dysbiosis or an imbalance in the gut microbiota can elevate the chronic inflammation that is considered as a risk factor for PC. The metabolites including short-chain fatty acids (SCFAs) and lipopolysaccharides (LPS) can be generated by gut microbiota exerting anti-inflammatory activity or enhancing inflammation, respectively. LPS is considered as the outer membrane of Gram-negative bacteria and it is able to induce TLR4 axis in promoting the NF-κB level and enhancing the generation of pro-inflammatory cytokines including TNF-α and IL-6 that are implicated in PC progression. In addition, the efficacy of chemotherapy and immunotherapy can be affected by gut microbiota. A number of bacterial species have been shown to exert anti-cancer immunity through increasing T cell infiltration in TME, whereas other kinds of bacteria have been shown to cause therapy resistance through the induction of immunesuppressive pathway. In addition to the gut microbiota, the intratumoral microbiota contribute to the PC progression and there are specific bacteria within PC causing poor prognosis. There is abundance of gammaproteobacteria in PC compared to normal tissues that is able to in enhancing carcinogenesis through immuno-suppression and enhancing drug resistance. The metabolism of chemotherapy agents including gemcitabine can be performed by intratumoral bacteria, further reducing the efficacy and causing chemoresistance. The activity of immune cells in the TME can be affected by intratumoral microbiota including macrophages and MDSCs to further cause tumorigenesis and immune evasion. More information about the function of microbiota in the progression of PC can be found in these studies [207–212]. Table 4 highlights the role of microbiota in the progression of PC.

Table 4.

The function of microbiota in the pancreatic cancer malignancy

Microbiota	Highlights	References
Gut microbiota	There is enrichment of 3-IAA in the PDAC patients responding to chemotherapy that can promote efficacy of this therapy As a microbiota-derived metabolite, 3-IAA has been shown to reduce ROS-degrading enzymes including GPX3 and GPX7, enhancing ROS generation and inhibiting autophagy in tumor cells	[213]
Fecal microbiota	Fecal microbiota has demonstrated better performance for the classification and prognosis of PDAC patients with AUROC of 0.84 The combination with microbiome-based predictions with serum levels of carbohydrate antigen (CA) 19–9 improved the AUROC to 0.94	[214]
Gut microbiota	TMAO is a microbiota-derived metabolite that enhances anti-cancer immunity and diminishes tumor growth in mouse models Induction of immunostimulatory TAM phenotype Stimulation of T cell responses Enhancing the IFN pathway causing anti-cancer impacts in a type I IFN-dependent manner	[215]
Tumor microbiome (enriched in anaerobic Bacteroidales and microaerophilic Campylobacterales)	The production of Col1 homotrimer by PC cells to induce oncogenesis and promote proliferation and growth via α3β1 integrin Col1 homotrimer is associated with an anaerobic Bacteroidales-enriched tumor microbiome in hypoxic and immunosuppressive tumors Deletion of Col1 homotrimers reprograms the tumor microbiome, increasing microaerophilic Campylobacterales	[216]
Tepidimonas Leuconostoc Sutterella Comamonas Turicibacter Streptococcus Akkermansia	There is higher abundance of Tepidimonas in tumor tissues compared to the normal tissues Increased in size of tumors has shown decreased levels of Leuconostoc and Sutterella Enhanced lymph node metastasis demonstrated high levels of Comamonas and Turicibacter Reduction in the levels of Streptococcus and Akkermansia is related to the tumor recurrence	[217]
Acinetobacter Pseudomonas Sphingopyxis	The microbial communities increase the aggressive basal-like PDAC subtype causing poor prognosis and increasing inflammatory responses High abundance of Acinetobacter, Pseudomonas, and Sphingopyxis is enhances carcinogenesis The microbial genes stimulate pathogen-associated inflammation in tumorigenesis Host genetics shape the tumor microbiome, affecting tumorigenesis	[218]

Clinical translation

The pre-clinical studies have highlighted the role of inflammation and related pathways along with gut microbiota in the regulation of PC progression. Notably, clinical translation of these findings can significantly improve the purpose for the treatment of patients. The dysregulation of NF-κB has been implicated in PC as a factor in the inflammation and progression of regulation. Therefore, the application of NF-κB inhibitors in the clinic can resolve the inflammation and malignancy of PC. Moreover, these inhibitors can be beneficial in improving response of PC patients to chemotherapy. The TLR agonists including TLR9 can be combined with immune checkpoint inhibitors in enhancing anti-cancer immunity. TGF-β inhibitors, such as galunisertib can be also considered for the clinic in the treatment of PC patients. One of the strategies utilized for the transfer of healthy gut microbiota is fecal microbiota transplantation that can increase the response of PC cells to chemotherapy. The application of probiotics and prebiotics can be considered as a promising clinical strategy for PC management. In addition, the microbiota-derived metabolites including SCFAs can be considered as regulators of immune response and inflammation in PC patients. Considering such strategies can be beneficial in addressing the current challenges for the treatment of PC patients such as chemoresistanc and immune evasion.

There are multiple clinical studies highlighting the role of microbiota in the progression of PC. A clinical trial has focused on understanding the features of oral flora and serum metabolome in PC patients compared to chronic pancreatitis patients and healthy individuals by collecting throat swabs, saliva, feces, and serum samples with the aim of designing a predictive model for PC according to the metabolic and microbiota differences (NCT06411470). Another clinical study has been developed for understanding the microbial and metabolic differences between pancreatic head cancer (PHC) and pancreatic body/tail cancer (PBTC) using 16S rRNA sequencing and untargeted metabolomics to highlight the differences involved in the eriology, pathogenesis and clinical outcomes of PDAC at the different anatomical sites. Such results are beneficial for providing an explanation of poor prognosis in PBTC compared to PHC and highlighting the new biomarkers (NCT06147154). Another clinical trial investigated the impacts of MS-20 as a fermented fermented soy milk derivative on the gut microbiota and the risk/severity of cachexia in Pc patients during chemotherapy. This is a double-blind, placebo-controlled trial involving 40 participants and its aim is to understand the function of MS-20 in changing gut microbiota profiles and mitigating cachexia for improving survival of patients. Moreover, this study has been dedicated to understand the alterations in cachexia staging, muscle mass and progression-free survival over a 12-week treatment period and an 8-week follow-up (NCT04600154).

Conclusion and perspectives

The present review highlighted the function of inflammation-related pathways and gut microbiota in the progression of PC. Although various kinds of biological mechanisms have been understood to regulate tumorigenesis, the function of inflammation and gut microbiota in PC progression is of importance. It was revealed that presence of chronic and acute pancreatitis can increase risk of PC development highlighting the importance of inflammation. Therefore, the molecular pathways related to inflammation including NF-κB, TGF-β, TNF-α and others were evaluated showing their dysregulation in PC progression. Notably, the TME components including macrophages can be M2 polarized during inflammation to enhance tumorigenesis. Moreover, M2 polarized macrophages can mediate inflammation. On the other hand, the inflammatory phenotype in CAFs can enhance tumorigenesis. The chemokines and ILs are critical regulators of inflammation and their levels change during PC progression. At final, the gut microbiota composition changes during PC progression and the metabolic produced by these microbes including LPS can stimulate NF-κB and MAPK to increase inflammation for PC progression.

Author contributions

XiaoLiang Chen, Writing-original draft, Feixia Sun, Writing-original draft; Xuqin Wang, Writing-original draft; Xiaoqiang Feng, Writing-original draft; Amir Reza Aref, English Editing, Yu Tian, Conceptualization, Writing-review editing, Milad Ashrafizadeh, Conceptualization, Writing-review editing; Dengfeng Wu, Conceptualization, Writing-review editing.

Funding

None.

Availability of data and materials

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.