The Past and Future of Inflammation as a Target to Cancer Prevention

Abstract

Inflammation is an essential defense mechanism in which innate immune cells are coordinately activated on encounter of harmful stimuli, including pathogens, tissue injury, and toxic compounds and metabolites to neutralize and eliminate the instigator and initiate healing and regeneration. Properly terminated inflammation is vital to health, but uncontrolled runaway inflammation that becomes chronic begets a variety of inflammatory and metabolic diseases and increases cancer risk. Making damaged tissues behave as “wounds that do not heal” and sustaining the production of growth factors whose physiological function is tissue healing, chronic inflammation accelerates cancer emergence from premalignant lesions. In 1863, Rudolf Virchow, a leading German pathologist, suggested a possible association between inflammation and tumor formation, but it took another 140 years to fully elucidate and appreciate the tumorigenic role of inflammation. Key findings outlined molecular events in the inflammatory cascade that promote cancer onset and progression and enabled a better appreciation of when and where inflammation should be inhibited. These efforts triggered ongoing research work to discover and develop inflammation-reducing chemopreventive strategies for decreasing cancer risk and incidence.

A brief history of a cancer hallmark: inflammation

Inflammation, from the Latin word flamma for fire, is an evolutionarily important process that entails the recruitment of leukocytes and plasma proteins to sites of injury (sterile inflammation) or infection to eliminate the instigator and mitigate the injury by clearing damaged cells, initiating tissue repair, and restoring homeostasis(1,2). If properly terminated, such inflammation is beneficial. However, persistent injury or infection trigger delayed-resolving or non-resolving chronic inflammation. Although the signs of chronic inflammation may not be as alarming as those of acute inflammation, chronic inflammation often causes further damage (fibrosis, necrosis, organ dysfunction, and genetic alterations) and begets degenerative diseases and cancer(3).

Although inflammation precedes and often facilitates the activation of adaptive immunity, chronic inflammation is often immunosuppressive(4). Further confusing the distinction between inflammation that should be inhibited and anti-tumor immunity that should be boosted is the popular reference to cancers with a high T cell content as “inflamed tumors”. However, T cells that reside within actively growing tumors are exhausted or dysfunctional due to cytokines associated with chronic inflammation that promote T cell dysfunctionality and sustain a tumor-permissive and immunosuppressive tumor microenvironment (TME)(5–7). The adaptive immune system itself, also has a dichotomous role in cancer onset and progression(5). Through anti-tumorigenic immunosurveillance (elimination phase), the immune system detects and eliminates microbial pathogens as well as malignant cells that express neoantigens(8). This process, however, can enhance tumor heterogeneity by selecting for aggressive clones that are less immunogenic (escape phase)(6).

The inflammatory process was documented in ancient Egyptian and Greek scriptures, and the Roman doctor, Aulus Celsus (30 BC-45 AD) described its four cardinal signs: redness, warmth, swelling, and pain (Fig.1). Galen, the physician and surgeon of the Roman emperor Marcus Aurelius, introduced the fifth sign, loss of tissue function. While these early concepts were largely intuitive they provided the framework for critical experimentation later on(9). In the 19^th century, upon his observation of tumor infiltrating leukocytes, the German pathologist Rudolf Virchow formulated the ‘chronic irritation theory’, linking for the first time the origin of cancer with severe tissue inflammation (Fig.1)(10). However, only in the past 30 years the role of inflammation in tumor development and its mechanisms of action have become sufficiently clear and robust(11), allowing us to propose that existing and newly developed anti-inflammatory drugs can be added to the cancer prevention armamentarium(12). Prior to any mechanistic insight, epidemiological studies noted that 15–20% of cancer deaths occur in the context of infection and inflammation(13). In 1989 Brenner and Karin showed that the prototypical inflammatory cytokine TNF activated the oncogenic transcription factor AP-1 (JUN:FOS) through the stimulation of JNK activity(14) and 10 years later, inflammatory cytokines were found to interfere with TP53 tumor suppressive activity(15) (Fig.1). But it wasn’t until the early 2000s, following their earlier watershed discoveries of the kinase (IKK) and the ubiquitin ligase (bTrCP), that execute the key steps in NF-kB signaling(16), that Karin at UCSD and Ben-Neriah at the Hebrew University decided to test the hypothesis that NF-kB activation in cancer progenitors by tumor microenvironmental (TME) signals protects the emergent tumor from death to enable its expansion(17). Ablating IKKβ in intestinal epithelial cells (IEC) and lamina propria macrophages Karin’s lab showed that NF-κB activation in IEC by macrophage-produced factors, e.g., IL-6, endowed colorectal cancer (CRC) progenitors with a survival advantage and stimulated their proliferation(18). Using a different but complimentary approach, Ben-Neriah’s group expressed a non-degradable IkB in hepatocytes along with TNF neutralizing antibodies to show that NF-κB in hepatocellular carcinoma (HCC) progenitors converted TNF generated death signals to HCC promoting proliferative instructions(19). While similar findings were obtained in other malignancies, Karin’s lab established the concept of tumor elicited inflammation, showing that gut barrier disruption in early adenomas led to entry of microbial signals from the gut lumen that triggered an IL-23-IL-17 cytokine cascade stimulating the proliferation of CRC progenitors(20) (Fig.1). These results were consistent with the epidemiological studies of Galon and Wolf showing that IL-17 and IL-23 in the tumor bed increase the aggressivity of human CRC(21). Adding to this framework, Colotta et al., (Fig.1), showed that cancer-related inflammation increased the accumulation of random genetic alterations in cancer cells(22), although whether this is due to enhanced mutagenesis or increased survival and proliferation of mutated cancer progenitors remains unknown. Despite encountering some resistance from the cancer genetics camp, the overwhelming data supporting the oncogenic role of localized indolent inflammation, led to its acceptance into the cancer hallmarks pantheon(23) (Fig.1).

Figure 1.

A brief history of the 7th hallmark of cancer. This timeline illustrates the important events that have taken place in the field of research linking Inflammation and Cancer.

Inflammatory mechanisms in tumor initiation and promotion

Persistent perturbation of tissue structure and homeostasis caused by inflammatory agents and oncogenic events leads to a dysfunctional interaction between innate and adaptive immunity that dismantles cancer immunosurveillance(5). Following an initial increase in the number of local tissue macrophages and dendritic cells (DCs), immune cells from bone marrow (monocytes, neutrophils, and monocyte-derived cells) and secondary lymphoid tissues (lymphoid cells) arrive to the perturbed tissue. Recruited or locally amplified inflammatory cells further undergo local activation, differentiation, polarization, or dysfunction (as observed for T lymphocyte), instructed by chemokines and growth factors reciprocally produced and received by the infiltrating cells(5). The complex crosstalk between innate or adaptive immune cells, and the interactions between cancer and immune cells in the TME is dynamic and affected by the tissue context(24,25).

Inflammation may affect tumor initiation; oncogenic mutations in cancer progenitors were linked to the ability of activated neutrophils or macrophages to produce reactive oxygen species (ROS) and nitrogen intermediates (RNI)(11), which can cause DNA damage, genomic instability and loss of function mutations in TP53 and other cancer-related genes in intestinal epithelial cells(26) (Fig.2). Inflammatory cytokines, such as TNF, were also proposed to stimulate ROS accumulation in neighboring epithelial cells(11). However, given the high reactivity of ROS and RNI, it is unlikely that they can travel between cells and not react with extracellular molecules. A more plausible inflammation-dependent mutagenic mechanism is the induction of AID (Activation induced cytidine deaminase), a member of APOBEC family, which is expressed in mature B cells on CD154:CD40 engagement and activation of NF-κB(27). AID promotes immunoglobulin gene class switching by catalyzing cytosine deamination, and is a critical immunoregulatory molecule; however, AID off-target activity increases mutation burden, translocations, and oncogenesis(27). Nonetheless, inflammation mostly affects tumor promotion through the production of growth factors and cytokines that activate downstream transcription factors, NF-κB, STAT3, and AP-1, which stimulate stem cell renewal, expansion of transit amplifying cells and boost cell survival(11). Inflammation-driven cell survival may also be important in the context of cancer immunosurveillance and the tumor elimination phase(5). AP-1, NF-κB, and STAT3 are activated in most tumors both within cancer cells and inflammatory cells and induce the expression of other inflammatory mediators, including TNF, IL-1, IL-6, cyclooxygenase-2 (COX-2), and 5-lipo-oxygenase(28). Circulating IL-6 and TNF correlate with clinicopathological features and clinical outcome in prostate cancer(29).

Figure 2. Inflammation-induced reprogramming is needed for the expansion of cancer progenitors.

Cytokines, chemokines, and reactive oxygen species (ROS) promote the accumulation of mutations and/or epigenetic alterations of genes and signaling pathways involved in tumor suppressors and oncogenes.

Reactive oxygen species (ROS), Mutant (Mut), tumor protein (p53), Snail Family Transcriptional Repressor 2 (SLUG, product of SNAI2), DNA methyltransferases (DNMTs).

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COX-1 and −2 play important roles in tumorigenesis, mainly by catalyzing the production of Prostaglandin E2 (PGE₂), a major mediator of inflammation and angiogenesis(30). In this context, COX-2 is generally upregulated only under pathological conditions, and was shown to promote aggressive tumor formation from stem/progenitor cells in murine skin and enhance the formation of esophageal squamous cell carcinoma (SCC) near the squamocolumnar junction(30). COX-2 also leads to induction of vascular endothelial growth factor (VEGF), different protooncogenes, the antiapoptotic protein BCL-2, epidermal growth factor receptor (EGFR) and matrix metalloproteases (MMP-2 and MMP-9), and suppresses IL-12 production, leading to immunosuppression(31).

Inflammation Reprograms Cancer Cell Progenitors

Inflammation-induced reprogramming leads to expansion of cancer progenitors. Inflammatory mediators, cytokines, chemokines, ROS and RNI induce tumor promoting epigenetic alterations(5,32) (Fig.2). The protective cytokine IL-22 induces DNA damage response (DDR) genes to counteract possible inflammation-elicited genotoxic insults(5). Chronic inflammation contributes to myeloproliferative neoplasia, promoting the survival and genetic evolution of TP53-mutated hematopoietic stem/progenitor cells (HSPCs)(33). Proinflammatory cytokines activate the epigenetic machinery in epithelial cells, including DNMT1, DNMT3, and disruptor of telomeric silencing 1 (DOTL1)(5), and in a gerbil gastric cancer model the expression of several inflammation-related genes correlates with their methylation levels (Fig.2). Accordingly, suppression of inflammation with cyclosporin A in H. pylori-infected gastric mucosa cells showed blocks of altered DNA methylation(34). In a nicotine-addicted mouse model, inflammation-driven changes in cytosine methylation and hydroxymethylation led to imbalanced DNA methylation–demethylation dynamics, which in turn gave rise to a shift in lung cancer promoting histone acetylation(35). In NSCLC, IL-1β-induced epithelial–mesenchymal transition (EMT) promoted SLUG-dependent epigenetic modifications of the E-cadherin and CDH1 promoters and SLUG accumulation further enhanced H3K27Me3 and H3K9Me2/3 (Fig.2), contributing to suppression of EMT memory(36). In the pancreas, an acute inflammatory event primed sustained transcriptional and epigenetic reprogramming that activated multiple transcriptional programs, facilitating acinar-to-ductal metaplasia upon subsequent inflammatory challenges(37). Moreover, tissue damage collaborates with oncogenic mutations (e.g., mKras) to unleash an epigenetic remodeling program that involves an ‘acinar-to-neoplasia’ chromatin switch that contributes to early transcriptional dysregulation in human PDAC(38). Injury and inflammation also cause epigenetic silencing of genomic loci linked to cell type specification. In contrast, neoplastic progression couples dedifferentiation to a distinctive chromatin remodeling program that diverts DNA accessibility from normal lineage-specifying to cancer-defining loci(38) (Fig.2). Injury-induced transcriptomic perturbation in the colonic epithelium induces interferon stimulated genes, cell division of a small but highly proliferative set of epithelial founder progenitor cells and progression from IBD to CRC(39). IL-6/STAT3 signaling stimulates the generation of HCC progenitor cells (HcPC)(40) and promotes cell identity conversion in liver injury by re-activating reprogramming/progenitor-related genes in mature hepatocytes(41).

Inflammatory events also influence cellular metabolism with TNF and IL-17 stimulating glycolysis by inducing SLC2A1, a glucose transporter, and hexokinase 2 (HK2), as well as HIF-1α and MYC in CRC(42). IL-6 and stroma-derived factor-1α secreted by cancer associated fibroblasts (CAF) induce purine nucleotide synthesis and ROS scavenging through the NRF2 pathway in PDAC, while TNF and TGF-β increase glucose uptake and lactate secretion(32). Inflammation-induced miRNA alterations were also described to affect tumorigenesis(36). IL-1β/NF-κB induce expression of miR-181a and miR-425, which negatively regulate PTEN, downregulate miR-101, and upregulate Lin28B thus affecting cell proliferation and migration(36). Congruently, autocrine IL-6 production by HcPC in premalignant lesions depends on elevated LIN28 expression(40).

Anti-inflammatory agents with anti-cancer activity

Epidemiological studies linking Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) to reduced cancer risk further support the link between inflammation and cancer(43,44). Although NSAIDs and their precursors have been used for millennia for the amelioration of inflammatory symptoms, their mechanism of action remained unknown until John Vane and Bengt Samuelson(45) discovered their ability to inhibit COX-catalyzed prostaglandin synthesis(46,47). Later on, Philip Needleman(48) and Harvey Herschman discovered and cloned COX-2(49), which is upregulated in CRC(50) and other cancers along with its products(51). Some of the first reports documenting NSAIDs anticancer activity in rodent models came from Pollard et al., and Narisawa et al., who used indomethacin to inhibit the growth of carcinogen-induced intestinal tumors(52,53). Shortly thereafter, Waddell and Loughry reported that sulindac reduced the formation of colonic adenomas in patients with familial adenomatous polyposis (FAP)(54). In 1999, the COX-2 selective inhibitor celecoxib (Celebrex) was approved by FDA for the treatment of FAP(55), followed in 2015, by aspirin, as an adjuvant for reversal of chemoradiotherapy resistance(56,57) (Table 1).

Table 1.

Summary of the most important class of anti-inflammatory drugs, properties and anti-cancer activity.

Drugs	Targets	Clinical Use	Chemoprevention Type of Cancer	Study, Year (Reference), Clinical trial
Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), Celecoxib (Celebrex), Aspirin	Ability to block certain prostaglandins (PGs) synthesis through the cyclooxygenase enzymes (COX-1 and COX-2) inhibition.	Most commonly prescribed pain medications. It is a highly effective drug class for pain and inflammation.	Intestinal tumors, familial adenomatous polyposis (FAP), Adjuvant for reversal of chemoradiotherapy resistance, human prostate cancer.	Narisawa, T., et al., (1981). Cancer Res 41, 1954-1957(52). Pollard, M., and Luckert, P.H. (1980). Cancer Treat Rep 64, 1323-1327(53). Waddell, W.R., and Loughry, R.W. (1983). J Surg Oncol 24, 83-87(54). Restivo, A., et al., (2015). Br J Cancer 113, 1133-1139(56). Clinical trial, NCT02840162, NCT05411718, ASPIRED-NCT02394769.
CORTICOSTEROIDS Compound E, VAMP (vincristine, amethopterin, 6-mercaptopurine, and prednisone) Budesonide	Through the glucocorticoid receptor, leading to most anti-inflammatory and immunosuppressive effects.	Infectious and inflammatory disorders, allergic and autoimmune diseases, shock, lowering of hypercalcemia, promotion of water excretion, treatment of pathologic hypoglycemia, suppression of excess adrenocortical secretion, prevention of graft rejection, neurological disorders, hematologic disorders, skin disorders, and corticosteroid replacement therapy.	Lymphosarcoma, leukemia, lung cancer, breast cancer.	Heilman FR, Kendall EC. (1944). Endocrinology 34(6):416–20(61). Freireich, Emil J., Myron Karon, and Emil Frei III. (1964), Proc. Am. Assoc. Cancer Res. Vol. 5(164). Veronesi G, et al., (2015) Ann Oncol. 26(5):1025-1030(165). Clinical trial: NCT01540552.
STATINS, ML-236B (mevastatin) Simvastatin, lovastatin, and pravastatin fluvastatin, pitavastatin, cerivastatin, atorvastatin and rosuvastatin	Inhibition of HMG-CoA reductase (HMGR), the rate-limiting enzyme in cholesterol biosynthesis, statins exhibit many pleiotropic effects downstream of the mevalonate pathway.	Anti-inflammatory properties unrelated to their cholesterol lowering action. Treatment of hypercholesterolemia-induced cardiovascular-associated diseases.	Breast, colorectal, lung, and prostate cancers, hepatocellular carcinoma.	Endo, A., et al., (1976). J Antibiot (Tokyo) 29, 1346-1348(64). Jain, M.K., and Ridker, P.M. (2005). Nat Rev Drug Discov 4, 977-987(68). Zielinski, S.L. (2005). J Natl Cancer Inst 97, 1172-1173(71). Wang, Y., et al., (2022). Can J Gastroenterol Hepatol(72). Clinical trial: NCT00914017, NCT05028829.
METFORMIN	Mild and transient inhibition of the mitochondrial respiratory-chain complex 1. In addition, the resulting decrease in hepatic energy status activates the AMP-activated protein kinase (AMPK), a cellular metabolic sensor, providing a generally accepted mechanism for metformin action on hepatic gluconeogenic program.	First-line oral glucose-lowering agent in the management of type 2 diabetes (T2D).	Colorectal cancer, lung cancer in overweight or obese individuals at high risk for lung cancer, colorectal cancer risk reduction among patients with a history of colorectal adenomas and elevated body mass Index, prevention hepatocellular carcinoma.	Higurashi T, et al., Lancet Oncol. (2016);17(4):475-483(166). Bowker SL et al. (2006), Diabetes Care. 29:254-8(167). Soranna D, et al., (2012) 17(6):813-22(168). Clinical trial, NCT04931017, NCT02319200, NCT01312467, NCT05023967.

In 2010, a retrospective study with 20-year follow-up of five randomized trials concluded that aspirin taken reduced long-term incidence and mortality due to colorectal cancer, and mostly cancers of the proximal colon(58). However, conflicting findings were demonstrated in the recently published Aspirin in Reducing Events in the Elderly (ASPREE) Trial(59). In contrast with previous study conducted on young- to middle-aged or high-risk populations, the ASPREE trial showed association with higher all-cause mortality among healthy older adults who received daily low-dose aspirin as primary prevention, compared to those who received a placebo, in particular higher CRC mortality rate, though not CRC incidence at a median follow-up of 4.7 years(59). Furthermore, the increased of lung cancer risk associated with high frequency of aspirin use, and prostate cancer risk associated with high-dose aspirin use have cast further doubt on the overall cancer preventive value of low-dose Aspirin(60). In regards of non-aspirin NSAIDs (NA-NSAIDs) epidemiological studies explored their ability to reduce adenoma formation in high-risk patient populations, and their effect on CRC-related mortality. There is also emerging evidence from observational studies that genetic and molecular variations may influence the chemopreventive effects of NA-NSAIDs in CRC(59). Ongoing trials are further evaluating the use of NSAIDs as chemopreventives. A phase IIa trial at the MD Anderson Cancer Center (NCT05411718) is testing if immune interception with naproxen or aspirin in patients with Lynch Syndrome can prevent the development of cancerous cells in the colon (Table 1). The ASPirin Intervention for the REDuction of Colorectal Cancer Risk (ASPIRED, NCT02394769) at Massachusetts General Hospital, is a double-blind, placebo-controlled, randomized clinical trial aiming to measure the effects of daily low-dose aspirin on various biomarkers associated with CRC, and whether age influences these effects (Table 1).

Corticosteroids are produced by the adrenal gland and regulate metabolism, electrolyte balance, inflammation, and stress responses. Corticosteroids are prescribed in advanced cancer for pain relief, the resetting of metabolism, appetite enhancement and improved wellbeing(61). After their recognition as potent anti-inflammatory agents by Philip S Hench, who treated rheumatoid arthritis with compound E, Heilman and Kendal used compound E to shrink lymphosarcoma in mice(61). This paved the way for the VAMP (vincristine, amethopterin, 6-mercaptopurine, and prednisone) combo which increased 5 year survival rates for children with leukemia(62) (Table 1). Since then, natural and synthetic glucocorticoids have become the most prescribed immunosuppressive medications(63), but in addition to blocking inflammation such drugs also interfere with cancer immunosurveillance and should therefore be used sparingly. Moreover, the metabolic and psychological side effects of glucocorticoids make them unsuitable for chemoprevention.

Statins were first developed as cholesterol-lowering drugs in the 1970s, following Endo and Kuroda identification of ML-236B (mevastatin) produced by Penicillium citrinium(64). Simvastatin, lovastatin, and pravastatin are also fungal derived, whereas fluvastatin, pitavastatin, cerivastatin, atorvastatin and rosuvastatin are synthetic compounds containing a fluoride side group(65). By inhibiting HMG-CoA reductase and decreasing the production of mevalonate, statins decrease the serum level of low-density lipoprotein (LDL) cholesterol, and as secondary effect, serum triglycerides(66). Considering the essential contribution of cholesterol to cellular homeostasis, proper membrane fluidity and formation of lipid rafts, as well as in vesicular trafficking and signal transduction(67), this group of drugs was investigated for potential beneficial effects on inflammation and inflammation-related cancers(68). Several pleiotropic cholesterol-independent functions of statins were identified: inhibition of IL-6 mediated inflammation by interfering with the function of RAS superfamily proteins(66); induction of Kruppel-like factor 2 (KLF2) expression, which negatively regulates the pro-inflammatory monocyte activation by suppressing NF-κB activation(69); induction of pro-apoptotic BAX; cell cycle regulation; up-regulation of the cell cycle inhibitors p21 and p27; and AMPK activation. Statins also decrease Skp2 expression and STAT3 phosphorylation; repress CD44 expression; downregulate MMP2 and MMP9; and inhibit androgen receptor overexpression(70). Furthermore, high cholesterol levels are associated with different types of cancer, resulting in enhanced synthesis of protumorigenic sex hormones, cell proliferation and migration(70). In 2005, statins were found to reduce the risks of breast, colorectal, lung, and prostate cancers(71). Pitavastatin and fluvastatin have been tested on glioblastoma in a xenograft mouse model, and daily atorvastatin administration suppressed tumor growth in mice carrying TKI-resistant lung cancer xenografts(70). Use of statins in prostate cancer therapy has increased patients’ survival rate and decreased tumor progression and recurrence, and a 72% risk reduction has been registered in the onset of estrogen-negative breast cancer cases between statins users(70). Currently, lipophilic statins are being evaluated for their cancer suppressive activity in HCC(72,73) (Table 1). A phase II trial is testing whether atorvastatin reduces the risk of developing liver carcinoma in patients with liver fibrosis that have spread to other places in the body (fibrosis) (NCT05028829, Table 1). Studies analyzing the effects of statins on the prevention or prognosis of colorectal neoplasia have shown controversial results, which were proposed to be due to heterogeneity amongst drugs, or to effects restricted to a particular subgroup of patients(74). By reducing the risk of proximal colon cancer in men, and rectal cancer in both sexes, it appears that statin use blunts CRC progression and aggressiveness(70). Although these studies do not support an overall preventive effect of statins in CRC, they highlight a protective association with rectal and other types of cancer that merit further research. Of note, statins are relatively safe, well-tolerated and widely used, and are orally administered, properties which indicate suitability for chemoprevention.

Metformin, a natural product, is the preferred first-line oral glucose-lowering agent for the management of type 2 diabetes (T2D). In 2007, metformin was described to have anti-inflammatory properties(75), and the erroneous assumption that it directly activates AMPK (AMP activated protein kinase) which is naturally activated by LKB1 (liver kinase B1), a well-recognized tumor suppressive protein kinase, opened the way to observational studies investigating metformin for cancer prevention and treatment in patients with or without T2D(76,77). Nevertheless, outcomes from late-phase efficacy studies of metformin as a cancer therapeutic have been disappointing. Yet, new studies assessing metformin in select populations including its combination with immunotherapy, and as potential chemopreventive agent are still ongoing(78). Currently, a phase IIb trial is studying the combined effect of prolonged nightly fasting and metformin hydrochloride extended release in decreasing cell proliferation and breast cancer biomarkers (NCT05023967, Table 1).

Mechanism-based development of preventive agents

The explosive growth in mechanistic investigations into the inflammation and cancer link led to the identification of multiple signaling pathways, cytokines, chemokines, and cell types that serve as critical lynchpins (Fig.3). In addition to the JNK-AP-1 and IKK-NF-κB pathways, the mitogen-activated protein kinases (MAPK), Janus kinase (JAK)-STAT3 and cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) pathways offer fertile grounds for novel interceptions and development of novel chemopreventives. Inflammatory cytokines, including TNF, IL-1, IL-6, IL-17 and IL-23, for which clinically approved neutralizing antibodies and decoy receptors exist(79) (Fig.3), should also be considered. A case in point is the CANTOS (Canakinumab Anti-inflammatory Thrombosis Outcomes Study) trial in which an IL-1β blocking antibody (Fig.3) was tested for lowering the risk of secondary infarcts in a large cohort enriched in smokers. The trial revealed a modest cardiovascular benefit but a strong lung cancer preventive effect by canakinumab(80). Based on these findings, Novartis launched the CANOPY study (NCT03447769, NCT03968419, NCT03631199, NCT03626545) with three large-scale, randomized, Phase III clinical trials and a Phase II clinical trial to investigate canakinumab as a potential treatment option in non-small cell lung cancer (NSCLC), but the trial did not meet its primary endpoint, suggesting that IL-1β blockade is more suitable for cancer prevention rather than therapy(81).

Figure 3. Targeting multiple signaling pathways, cytokines, chemokines, downstream inflammation: possible strategies for chemoprevention.

Schematic representation of the major targetable events during the inflammatory process leading to cancer, and the corresponding compounds with cancer-preventive properties.

Tumor necrosis factor α (TNF-α), Tumor necrosis factor receptor 1 (TNFR1), Interleukin-1 beta (IL-1β), Interleukin-1 receptor (IL-1R), Interleukin-17 (IL-17, also known as IL-17A), Interleukin-17 receptor (IL-17R), mitogen-activated protein kinase (MAPK), c-Jun NH2-terminal kinase (JNK), FBJ osteosarcoma oncogene (FOS), Jun Proto-Oncogene (C-JUN), transcription factor activator protein 1 (AP-1), IκB kinase complex (IKK), Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), cyclic GMP-AMP synthase (cGAS), Stimulator of interferon genes (STING), Cyclic guanosine monophosphate–adenosine monophosphate (cyclic GMP-AMP, cGAMP), phosphate group (P).

Created with BioRender.com (Agreement # ZW266D22X4).

Recently, blockers of both C-C and C-X-C motif chemokines, such as MSX-122, AMD3100, BPRCX807, WZ811, motixafortide, BPRCX714, were shown to inhibit recruitment of cancer promoting immune cells(82,83) (Fig.3). Inflammatory leukocytes are important sources of growth factors, including EGF and PDGF family members, VEGF and TGF-β isotypes, for which neutralizing antibodies and receptor inhibitors already exist or are being developed. The NLRP3 and AIM2 inflammasomes, which produce tumor and metastasis promoting IL-1β are other worthwhile targets(84,85). Numerous bioactive lipids, including prostaglandins, leukotrienes, thromboxanes, and specialized resolvins (SPM)(12,86), should also be considered.

The interest in developing promising chemopreventive strategies, in which the above agents could find a useful application, has been growing due to positive results in the chemoprevention of breast, prostate and colon cancers, and the availability of FDA-approved agents for the treatment of precancerous lesions or cancer risk reduction(87). Neutralizing inflammatory cytokines or antagonizing their receptor function is a relatively safe therapeutic strategy for the treatment of autoimmune diseases(88), and if the cytokine being blocked does not play a key role in in immunosurveillance, it should be considered for cancer prevention. For instance, the IL-18-neutralizing humanized monoclonal antibody GSK1070806, has been tested in cohorts of healthy and obese subjects for its safety, tolerability, pharmacokinetics and pharmacodynamic profile(89).

Ruxolitinib is a selective oral inhibitor of JAK1 and 2 which activate STAT3 with potent anti-inflammatory and immunosuppressive effects (Fig.3). At the cellular level ruxolitinib targets components of both the innate and adaptive immune system, such as natural killer cells, dendritic cells, T helper, and regulatory T cells. As for macrophages, ruxolitinib was shown to prevent the up-regulation of various proinflammatory cytokines by inhibiting LPS/TLR4/IFN type I signaling pathway(90). By targeting IL-6/JAK/STAT3 signaling cascade in inflammatory human liver cells, ruxolitinib inhibits production of C-reactive protein (CRP), serving as a promising candidate for modulation of inflammatory responses in liver cells and associated conditions(91). However, earlier data suggested that JAK inhibition may increase the inflammatory potential of macrophages exposed to TLR4 agonists, thereby enhancing inflammatory cytokine production(90). An active trial is testing effect of ruxolitinib before surgery on prevention of breast cancer recurrence in patients with high risk and precancerous breast conditions (NCT02928978). However, long term use of JAK inhibitors may have unfavorable metabolic side effects(92) and can even lead to liver damage(93) making such drugs unsuitable for general chemoprevention.

In addition to the above considerations, cancer chemopreventive agents need to be safe for general use, and if not they need to be tested in subpopulations with high cancer risk through proper, long-term phase I, II, and III studies(87). An important case-in-point is the COX-2 inhibitor Rofecoxib (Vioxx), which was approved for the short-term treatment of signs and symptoms of rheumatoid arthritis, osteoarthritis, management of acute pain, and treatment of primary dysmenorrhea, but when tested for its long-term ability to prevent colon polyps was found to double heart attack and stroke risk, and was pulled off the market(94).

Inflammatory markers and cancer risk

Markers of inflammation, such as leukocytes (monocytes, macrophages, neutrophils, lymphocytes), serum proteins (e.g., C-reactive protein), and cytokines (e.g., IL-6) can serves as signs of an ongoing cancer-related inflammatory response. Being detectable in blood samples, inflammatory markers represent an easy strategy to monitor cancer progression and response to anti-inflammatory intervention(91). For instance, elevated CRP was found to be associated with several cancers, including breast, lung, gastric, and colorectal cancers, HCC, and renal carcinoma(91). CRP is an acute response protein synthesized in the liver in response to various inflammatory stimuli, whose function remains unknown(91). A prospective cohort study of UK primary care patients using data from electronic health records in the Clinical Practice Research Datalink, found a 3.53% overall one-year cancer incidence in patients with elevated CRP, that was twice as high as the risk-incidence rate in individuals with normal CRP(95). However, with a sensitivity of 46.1% for CRP(95), those markers cannot be considered a reliable indicator for initial cancer diagnosis(96). Other studies have also explored the potential utility of CRP level as a diagnostic tool in assessing disease status and progression, including cancer(96), revealing CRP as potential biomarker to assess overall cancer risk and 12 site-specific cancers, but no association was observed for genetically-predicted CRP and cancer risk(97). Other markers including neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), lymphocyte-to-monocyte ratio (LMR) and the systemic immune-inflammation index (SII) based on peripheral lymphocyte, neutrophil, monocyte and platelet counts, have been investigated as prognostic markers in newly diagnosed cancer patients but have yielded inconsistent results. Nonetheless, two recent studies have considered those as pre-diagnostic biomarkers of cancer incidence risk with potential for early disease identification in the last year prior to clinical diagnosis(98). Furthermore, increased risk of pancreatic cancer was associated with pre-diagnostic serum levels of haptoglobin, CRP and leukocytes(99), and higher levels of fibrinogen, CRP, and blood inflammation z-score were associated also with increased CRC risk(100). Additionally, a recent study has assessed the role of circulating cytokines on the risk of cancer using a two-sample Mendelian randomization (MR) approach, and obtained evidence for 48 cytokine-cancer associations including tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) and cutaneous T-cell attracting chemokine (CTACK) with the risk of several types of cancer(101).

Risk mitigation and avoidance

Cancer risk is influenced by genetic and environmental factors, that are either non-modifiable (sex, age, genetic polymorphisms, ethnicity) or modifiable (lifestyle factors, risky behaviors, bad habits)(12). Here we discuss lifestyle and behavioral factors that increase cancer risk through inflammation-related effects(102). A careful assessment of the mechanisms by which such behaviors increase cancer risk is needed for the eventual implementation of risk reducing behaviors, that can be as effective as smoking cessation has been.

Obesity and Cancer

Obesity, defined by a body mass index (BMI) >30 kg/m², significantly increases the risk of 13 different cancers, most notably breast, ovarian, liver and colorectal(103). Diet-induced obesity promotes HCC development in mice by inducing low-grade inflammation and increasing IL-6 production by steatotic hepatocytes(104). Obesity also stimulates production of tumor promoting adipokines, increases lipotoxic tissue damage and triggers insulin resistance, accompanied by elevated IGF-1 production(105). High fat (HFD) and energy-dense diets alter IEC homeostasis, increase the number of intestinal stem cells, cause intestinal dysbiosis with diminished barrier function, thereby causing tumor promoting endotoxemia and inflammation(106). Excess lipids alter macrophage polarization(107), and inhibit autophagy to trigger oncogenic NRF2 and mTORC1 activation(108). HFD also stimulates generation of the complement fragment C5a to promote obesity-related inflammation and insulin resistance(109). C5a has a pro-tumorigenic role in many cancer types, such as HNSCC(110), squamous cell carcinomas (SCCs)(111), and colorectal tumorigenesis(112). Despite this wealth of information, more precise, behavior-modifying, and easily digested facts that can be conveyed to lay audiences and show unequivocally how obesity increases cancer risk are needed. In this regard, HFD and other energy-dense diets were recently shown to cause hepatocyte DNA damage in rodent models(113,114), important results that if backed by human studies can lead to better dietary habits.

Alcohol and Cancer

According to WHO estimates nearly 4% of cancers diagnosed worldwide in 2020 were linked to alcohol consumption. In the US, about 75,000 cancer cases and 19,000 cancer deaths are estimated to be alcohol linked, each year. Alcohol drinking increases the risk of cancers of the upper aerodigestive tract, liver, colorectum, and breast, and has been linked to inflammation, oxidative stress, and DNA damage(115). Chronic alcohol consumption increases pro-inflammatory monocytes and macrophages in the TME(116). Alcohol-induced NF-κB activation augments production of inflammatory mediators as well as ROS(115). Alcohol abuse leads to HCC-promoting liver inflammation, alcoholic hepatitis(117), and negatively impacts the host immune system by blocking NK cells (Natural killer cells) egress from the bone marrow, suppressing T cell anti-tumor responses and activating hepatocyte-killing NKT cells(115). Ethanol consumption also induces microbial dysbiosis and intestinal bacterial overgrowth(118), barrier dysfunction, and consequent hepatic translocation of inflammation provoking microbial components(119). Bacterial components, such as endotoxin (LPS) and metabolites such as trimethylamine N-oxide (TMAO), as well as modified bile acids (BA) also cause intestinal and systemic inflammation(120).

Smoking and Cancer

Tobacco smoking increases cancer risk at multiple organ sites, especially head and neck, and lung, accounting for about 1/3^rd of all cancer-related deaths in the US(121). There is also a strong link between smoking and bladder, pancreatic, renal, colon, liver, and stomach cancers. Free radicals from tobacco smoke induce chronic inflammation(121), and smoke-derived carcinogens, such as polycyclic aromatic hydrocarbons (PAHs) and N-nitrosamines(122), form mutation-inducing DNA adducts. Tobacco smoking also results in abnormal methylation of genomic loci that increase cancer risk, such as DUSP4 (Dual Specificity Protein Phosphatase 4) and AKT3(123). Smoking-induced lipid peroxidation products also damage DNA and proteins, thereby leading to aberrant cell behavior(124). Using a mouse model of smoke distillate-induced lung cancer we demonstrated that IKK-NF-κB activation is an important part of the tumorigenic process(125). Cigarette smoke also promotes CRC progression via gut metabolites(126), and increases HIF-1α and METTL3, whose downstream targets stimulate cell cycle progression and suppresses cell death, to promote the growth of non-small cell lung carcinomas (NSCLC)(127).

Environmental Risks, Air Pollution, and Cancer

The WHO attributes ~20% of all cancers to environmental risk factors, predominantly air pollution, and chemical and radiation exposure(128). Many environmental carcinogens also induce tumor-promoting inflammation and disrupt its resolution(129). Traditionally carcinogens were classified as genotoxic, causing cancer via somatic mutations or DNA rearrangements, and non-genotoxic mechanism(129). Curiously, some studies suggest that DNA adducts or mutations alone do not trigger cancer onset(130), highlighting the essential contribution of non-cell-autonomous mechanisms, including inflammation(129). The International Agency for Research on Cancer (IARC) identified air pollution as a group 1 carcinogen for lung cancer. Genotoxicity and molecular biomarker data suggest that air pollution also contributes to cancer at other sites(131). Air pollution consists of particulate matter [PM] < than 2.5 μm in diameter, as well as carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide, while PM can contain carcinogenic chemicals such as benzene, formaldehyde, and polycyclic aromatic hydrocarbons. A recent study, conducted in the UK, has shown that 3-years of high PM_2.5 exposure increase EGFR-driven lung cancer in nonsmokers via the macrophage-IL-1β axis(132). These results provide additional evidence that a major trigger of cancer development is not only the acquisition of driver mutations but also intrinsic and extrinsic mechanisms that promote inflammatory changes and allow pre-existing mutated clones to expand(132).

Infections and Cancer

Bacterial infections account for 15% of all cancers, whereas viruses account for an additional 10%, and 11 pathogens were classified as human carcinogens(133). After Helicobacter pylori, the four most prominent infection-related carcinogens are viral: HPV, HBV, HCV, and EBV(134). H. pylori is a gram-negative bacterium that exclusively colonizes the human gastric mucosa and causes chronic gastritis, followed by an epithelial apoptosis, atrophy, compensatory hyperproliferation and metaplasia(135). Metaplastic lesions can evolve into dysplasia, in situ carcinoma and invasive adenocarcinoma. Treatment of H. pylori infections reduces gastric cancer risk but can also lead to acid reflux that increases the risk of esophageal cancer(136). H. pylori also increases CRC, cholangiocarcinoma (CCA), gall bladder carcinoma, HCC, and pancreatic cancer (PDAC) risk(137). The rate of progression from superficial gastritis to atrophic metaplasia, and ultimately to cancer relates both to the virulence of the infecting H. pylori strain and environmental and dietary factors. In a landmark mouse study by Enzler et al., antibiotics treatment was found to prevent lymphomagenesis and solid tumor development, underlying the interplay between infectious agents and cytokine-mediated regulation of immune homeostasis as a critical determinant of cancer susceptibility(138). Whereas some human oncoviruses, including HPV, HBV, and EBV, integrate into the host genome and induce genetic alterations(139), most, if not all, cancer causing viruses also trigger chronic inflammation by inducing cell death, immune activation or inflammatory signaling by viral proteins(140).

Chronic Inflammatory Diseases and Cancer

Like persistent viral infections, chronic inflammatory diseases increase cancer risk. Chronic obstructive pulmonary disease (COPD) is a chronic bronchitis and/or emphysema that can further progress to pulmonary heart disease and respiratory failure. Numerous studies identify COPD as an important lung cancer risk factor. COPD-induced lung hypoxia can cause cell death, activate HIF transcription factors which stimulate angiogenesis and reprogram metabolism(141). Non-Alcoholic Steatohepatitis (NASH) is the aggressive form of Non-Alcoholic Fatty Liver Disease (NAFLD), the main cause of chronic liver complications and increased HCC risk. NASH is the consequence of fat accumulation and hepatocyte death, liver damage and activation of hepatic immune, parenchymal, and endothelial cells. NAFLD affects ~1/3^rd of adults in affluent societies and about 20% of these individuals suffer from NASH, a cause of cirrhosis and HCC(142). Two relapsing inflammatory bowel diseases (IBD), Crohn’s disease (CD) and ulcerative colitis (UC), are associated with a 3-fold increase in CRC risk. Although the exact etiology of IBD is unknown, it may result from an inappropriate immune response to intestinal microbes that is augmented by environmental factors(143). Like NASH, IBD is associated with increased production of tumor promoting and NF-kB and STAT3 activating cytokines(18,144). Chronic pancreatitis (CP) increases PDAC risk by 7.6–68.1-fold(145). Often, PDAC is diagnosed as early as 1–2 years after CP diagnosis. Risk factors for CP progression to PDAC include newly developed diabetes, obesity, pancreatic duct dilatation, old age, and smoking(145). Chronic inflammation is also present in breast cancer and may be associated with multiple cellular changes that accelerate tumor progression(146).

Additional Preventive Strategies Targeting Tumor-Promoting Inflammation

Although facing multiple challenges in its implementation(147), cancer prevention is an effective way to reduce the societal and economic burden of this lethal disease. Given that 30-50% of cancers may be preventable(148–150), it is critical to invest more efforts in a comprehensive, coordinated cancer prevention and control program. As demonstrated by the success of smoking cessation campaigns, education and behavioral modification are good starting points. Here we discuss preventive strategies that may lower the impact of inflammation-dependent tumor promotion, some of which do not involve pharmacological interception.

Physical activity

Physical activity as a means for lowering cancer risk was first explored in the early 20th century, with the finding that cancer mortality rates among men decreased with increased physical activity(151). One quarter of all cancer cases worldwide are linked to obesity and sedentary lifestyle(151), with the largest enhancement observed for ovarian, endometrial, liver, colon, breast, rectal and prostate cancers(152). The ability of physical activity to lower cancer risk could be due to the anti-inflammatory activity of skeletal muscle(151). Several ‘myokines’ were proposed to mediate exercise-induced protection against chronic diseases, including cancer(151). Curiously, IL-6 produced by monocytes or macrophages is proinflammatory and protumorigenic, whereas skeletal muscle-derived IL-6 has an anti-inflammatory activity(151). Exactly what is different between the two IL-6 sources is unknown and some of the effects could be due to differential engagement of the soluble IL-6 receptor. Exercise also reduces the burden of obesity and induces changes in microbiota composition and diversity, further enhancing weight loss and a reducing obesity-associated pathologies(151).

Vaccines

Vaccines can prevent virus induced cancers with minimal adverse effects, as demonstrated by the success of HBV and HPV vaccines(153). Vaccination of adolescent girls against HPV before the onset of sexual activity is the most effective long-term strategy for reducing cervical cancer risk(154). A recent cohort study of 1.6 million girls and women found that HPV vaccination was associated with an 88% lower risk of invasive cervical cancer, results that were confirmed in several other studies(155). Even as HPV vaccine coverage in the US has steadily increased, it is still below the Healthy People 2030 target goal of 80% for adolescents aged 13–15 years, with a drop in the number of adolescent HPV vaccinations during the COVID-19 pandemic(155). HBV vaccination remains the cornerstone of public health policy of HCC prevention, with the WHO setting a 90% vaccination target to achieve HBV elimination by 2030(156). Despite the ongoing decline in HBV, HCV has infected more than 150 million individuals, becoming a major cause of liver cirrhosis and cancer(157). Unfortunately, no effective HCV vaccines were generated. Nonetheless, direct-acting antivirals (DAAs), targeting viral protease, polymerase, or non-structural proteins, have proven highly effective, but their high costs have limited their use to wealthy countries(157).

Although most cancers are not caused by viruses, their elimination can be improved by development of prophylactic vaccines against oncoproteins and conserved neoantigens. This approach led to the Nous-209 (NCT05078866) and Tri-Ad5 (NCT05419011) vaccine trials to prevent or delay the onset of Lynch-syndrome-associated cancers(158). Both approaches use proteins unique to or overexpressed in cancer cells but rarely expressed in healthy cells, to generate a strong immune response(158). Cancer vaccines composed of commonly recurring frameshift peptide (FSP) neoantigens selected through prediction algorithms have been clinically evaluated and proven safe and immunogenic(159). Other strategies include DNA-based therapeutic vaccines targeting antigens common to many cancers: hTERT, a subunit of the telomerase complex(160), and mutated KRAS peptides (NCT05013216)(161).

Epilogue-the Future of Inflammation Reducing Cancer Prevention

Several clinical trials targeting inflammatory mediators have been designed. COMBAT, a phase IIa trial, was completed using the combination of a CXCR4 antagonist with pembrolizumab and chemotherapy for the treatment of metastatic PDAC(162). Blocking CXCR2 signaling inhibited metastasis and improved T cell invasion in the KPC mouse PDAC model(163). However, it is unlikely that such treatments can be used for cancer prevention, even in high-risk sub-populations. When developing preventive strategies that curtail the impact of tumor-promoting inflammation, it should be considered that in addition to low cost, cancer preventives need to be assessed for their safety and tolerability as they are being used on healthy individuals. Moreover, successful prevention trials require large cohorts and a lot of time, resulting in high initial expenses. Whether Pharma or governmental agencies should shoulder this cost, remains a matter of debate, but it is our hope that the pace of cancer prevention will pick up.