Pro-inflammatory prostaglandins and progression of colorectal cancer

Abstract

Chronic inflammation is a risk factor for several gastrointestinal malignancies, including esophageal, gastric, hepatic, pancreatic and colorectal cancer. It has long been known that long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs) reduces the relative risk of developing colorectal cancer. NSAIDs exert their anti-inflammatory and anti-tumor effects primarily by inhibiting activity of cyclooxygenase (COX) enzymes. Cyclooxygenase enzymes catalyze the conversion of arachidonic acid into prostanoids, including prostaglandins (PGs) and thromboxanes (TXs). Emerging evidence demonstrates that prostaglandins play an important role in inflammation and cancer. In this review, we highlight recent breakthroughs in our understanding of the roles of the different prostaglandins in colorectal cancer (CRC) and inflammatory bowel disease (IBD). These findings may provide a rationale for the development of new anti-inflammatory therapeutic approaches to cancer prevention and/or treatment.

Introduction

Clinical and epidemiologic studies suggest that chronic inflammation caused by infectious or autoimmune diseases is clearly associated with increased risk of cancer, including colorectal cancer (CRC) [1] and chronic inflammation contributes to the development of approximately 15–20% of malignancies worldwide [2]. Experimental studies indicate that inflammation promotes tumor growth, in part, through stimulation of proliferation and angiogenesis as well as inhibition of apoptosis and immune surveillance. The best evidence for the link between inflammation and tumor progression come from recent epidemiologic studies and clinical trials showing that long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs) reduced the relative risk of developing colorectal cancer by 40–50% [3]. NSAIDs are thought to exert their anti-inflammatory, analgesic and anti-pyretic effects mainly by targeting cyclooxygenase (COX) enzymes [4]. The COX enzyme exists in three isoforms commonly referred to as COX-1, COX-2, and COX-3. COX-1 is constitutively expressed in a broad range of cells and tissues and its expression remains constant under most physiological or pathological conditions. COX-1 contributes to maintenance of the gastric mucosa, regulation of renal blood flow in the afferent vessels of the kidney, and regulation of platelet aggregation. By contrast, COX-2 is an immediate-early response gene normally absent from most cells but is induced mainly at sites of inflammation in response to inflammatory stimuli including pro-inflammatory cytokines such as IL-1α/β, IFN-γ, and TNF-α produced by inflammatory cells [5]. COX-2 is involved in inflammation and tumorigenesis. COX-3 is an acetaminophen-sensitive splice variant of COX-1 and its function remains unknown. Therefore, COX-2 is a pro-inflammatory mediator and NSAIDs and specific COX-2 inhibitors are currently used to treat acute pain as well as the signs and symptoms of osteoarthritis and rheumatoid arthritis [6].

The gastrointestinal mucosa forms a complex, semi-permeable barrier between the host and the largest source of foreign antigens. The mucosal immune system has the ability to mount an immune response to pathogens while maintaining tolerance to the vast array of benign luminal antigens from food and commensal bacteria. Many inflammatory processes are self-limiting, supporting the existence of endogenous anti-inflammatory mechanisms. An abnormal mucosal immune response is thought to result in chronic inflammation such as arthritis and inflammatory bowel disease (IBD). IBD in humans is a complex class of disorders that has been grouped into two major forms, ulcerative colitis (UC) and Crohn’s disease (CD). These chronic conditions of the gut ordinarily affect individuals between the age of 16 and 40 and can cause significant morbidity. Chronic IBD (especially pan-colitis) significantly increases the risk of developing colorectal cancer [7]. IBD is thought to result from inappropriate activation of immune responses and/or a deficient feedback system that normally down-regulates the mucosal response to luminal factors.

The first evidence linking COX-2 to carcinogenesis emerged from studies on CRC [8]. Several subsequent reports confirmed that elevated COX-2 expression was found in approximately 50% of adenomas and 85% of adenocarcinomas [9, 10]. Similarly, COX-2 is induced in large intestinal epithelium in active human IBD and in inflamed tissues of IL-10 deficient mice (a mouse model of IBD) [11, 12]. An increasingly large body of evidence from population-based studies and clinical trials have shown that regular use of NSAIDs, including aspirin and related drugs, over a 10–15 year period reduces the relative risk of developing colorectal cancer and adenomas by 40–50% [3]. In particularly, aspirin specifically prevents the subgroup of colon cancers in which COX-2 is most highly induced [13]. Furthermore, NSAID use leads to the regression of preexisting adenomas in patients with the hereditary colon cancer syndrome, familial adenomatous polyposis coli (FAP) [14, 15]. Direct molecular evidence that COX-2 plays a key role in colorectal carcinogenesis was obtained from studies in animal models. Genetic studies demonstrate that deletion of COX-2 gene results in decreased tumor formation in both the small intestine and colon of Apc^Min mice (a mouse model of CRC) [16] as well as in Apc^Δ716 mice, another Apc mutant model [17]. Although the effects of COX-2 overexpression upon colorectal carcinogenesis in transgenic mouse models have not been reported, overexpression of COX-2 in transgenic mice using a murine mammary tumor virus (MMTV) promoter induced breast carcinomas formation [18]. Moreover, transgenic mice with COX-2 expression driven by the keratin-5 promoter did not develop skin cancer spontaneously, but were much more sensitive to carcinogen-induced tumor formation [19].

The COX enzymes convert free arachidonic acid into prostanoids, including prostaglandins (PGs) and thromboxanes (TXs). The key regulatory step in this process is the enzymatic conversion of arachidonate to PGG₂, which is then reduced to an unstable endoperoxide intermediate, PGH₂. Specific PG synthases in turn metabolize PGH₂ to at least five structurally related bioactive lipid molecules, including PGE₂, PGD₂, PGF_2α, PGI₂, and thromboxane A₂ (TxA₂), in a cell type-specific manner. PGs are unstable compounds that are rapidly metabolized in vivo [20]. PGE₂ and PGF_2〈 are rapidly metabolized to a stable 13,14-dihydro-15-keto-PGA₂ (PGEM) and 13,14-dihydro-15-keto-PGF₂〈 in vivo by the enzyme 15-hydroxy prostaglandin dehydrogenase (15-PGDH), respectively. PGI₂ is non-enzymatically hydrated to 6-keto PGF_1〈, and then quickly converted to the major urinary metabolite, 2,3-dinor-6-keto PGF_1〈 and PGD₂ is also rapidly metabolized to 15-deoxy-^Δ12, ^Δ14PGJ₂ (15dPGJ₂) and 11β-PGF_2〈 in vivo. Moreover, TXA₂ is rapidly hydrolyzed to TXB₂ in a non-enzymatic manner. Prostaglandins exert their cellular functions by binding cell surface receptors that belong to the family of seven transmembrane G protein-coupled rhodopsin-type receptors. These cell surface receptors are designated DP for the PGD₂ receptor, EP (EP1, EP2, EP3, and EP4) for the PGE₂ receptors, FP for the PGF_2α receptor, IP for the PGI₂ receptor, and TP for the TxA₂ receptor. In some cases, however, certain prostaglandins and their metabolites bind nuclear receptors such as peroxisome proliferator-activated receptors (PPARs). It has been shown that PGI₂ can transactivate PPARδ [21], while 15dPGJ₂ is a natural ligand for PPARγ [22]. Moreover, recent studies show that PGE₂ indirectly induces PPARδ activation in certain contexts [23].

Prostaglandins and colorectal cancer

The prostanoids are involved in a variety of pathophysiologic processes, including modulation of the inflammatory reaction, gastrointestinal cytoprotection and ulceration, angiogenesis, cancer, hemostasis and thrombosis, renal hemodynamics, and progression of kidney disease. Multiple lines of evidence demonstrate that overexpression of COX-2 in epithelial, mesenchymal, and inflammatory cells leads to the production of multiple PGs, which in turn promote angiogenesis, protect against apoptosis, stimulate proliferation, induce metastasis, and suppress cell-mediated immune responses. A great effort has been made to identify which of the individual PGs are directly involved in CRC. A large body of evidence has demonstrated that PGE₂ mediates the tumor-promoting effects of COX-2 in colorectal cancer. Less evidence supports a role for other prostaglandins in CRC.

PGE₂

Recent research has indicated that PGE₂ is a key mediator of acute inflammatory responses [24], stem cell differentiation [25], arthritis [26], and inflammatory bowel disease (IBD) [27]. Direct evidence came from observations that blocking either PGE₂ synthases or PGE₂ receptors impaired both acute and chronic inflammatory responses. Mice deficient in PGE₂ synthase displayed a marked reduction in acute pain during the inflammatory response and collagen antibody-induced arthritis (an animal model of rheumatoid arthritis) [28]. EP1 receptor-deficient mice exhibited a reduction of their pain-sensitivity responses in two acute prostaglandin-dependent models [29]. Moreover, EP4 receptor-deficient mice showed decreased incidence and severity of collagen antibody-induced arthritis [30] with a possible contribution of EP₂ [31]. These studies support the notion that PGE₂ mediates the effects of COX-2 during acute pain and arthritis. In an experimental model for IBD, PGE₂ appears to have a dual effect. High levels of PGE₂ exacerbate the inflammatory process [27]. On the other hand, PGE₂ signaling suppresses colitis symptoms and mucosal damage by protecting the integrity of the epithelial intestinal wall, presumably through the enhancement of epithelial survival and regeneration [32]. Moreover, a genetic study reveals that only EP4-deficient mice and not mice deficient in either EP1, EP2, EP3, DP, FP, IP, or TP are more sensitive to DSS treatment and developed severe colitis [33]. Further studies are necessary to define the role of PGE₂ in IBD.

Pro-inflammatory PGE₂ plays a predominant role in promoting colorectal tumor growth. PGE₂ is the most abundant PG found in human colorectal cancer tissues [34]. The steady-state cellular levels of PGE₂ depend on the relative rates of COX-2/PGE synthase-dependent biosynthesis and 15-hydroxyprostaglandin dehydrogenase (15-PGDH)-dependent degradation. 15-PGDH is highly expressed in normal colon mucosa but is ubiquitously lost in most human colorectal cancers [35]. A recent study showed that PGE₂ treatment dramatically increased both small and large intestinal adenoma burden in Apc^Min/+ mice and significantly enhanced colon carcinogen azoxymethane (AOM)-induced colon tumor incidence and multiplicity [23, 36]. Furthermore, elevated endogenous PGE₂ via loss of 15-PGDH promoted colon tumor growth in Apc^Min/+ and AOM mouse models [37]. Similarly, increased endogenous PGE₂ by overexpressing both COX-2 and microsomal prostaglandin E synthase-1 in the stomach is sufficient to induce gastric tumor formation [38]. PGE₂ also protects small intestinal adenomas from NSAID-induced regression in Apc^Min/+ mice [39]. The central role of PGE₂ in colorectal tumorigenesis has been further confirmed by evaluating mice with homozygous deletion of PGE₂ receptors [40–42].

To understand mechanism(s) underlying effects of PGE₂ on cancer progression, researchers have been investigating precisely how PGE₂ promotes tumor growth and its signaling pathways. Several reports have shown that PGE₂ promotes colorectal tumor growth by stimulating angiogenesis, cell invasion, cell growth and inhibiting apoptosis [43]. These essential cellular processes are regulated by PGE₂ activated signaling pathways, including EGFR-PI3K-Akt, Ras-MAPK, PPARδ, VEGF, Bcl-2, chemokines and their receptors [43, 44]. PGE₂ also activates canonical Wnt signaling by activating Tcf-4 transcription factors via stabilizing β-catenin in CRC cells [45]. Conversely, many of the downstream pathways of PGE₂ also upregulates COX-2 expression. Such feedback loops may amplify the activity of the COX-2 pathway and may magnify the potency of COX-2 inhibitors.

TxA₂

Although the role of TxA₂ in atherogenesis has been established [46], few studies have delineated the biological function of TxA₂ in CRC and IBD. TxA₂ is the only other COX-2 derived prostaglandins implicated in oncogenesis. TxA₂ has been shown to promote tumor growth and tumor-associated angiogenesis [47]. Moreover, it has been reported that TxA₂ synthase inhibitor blocks colorectal carcinoma liver metastasis in an in vivo study [48]. However, disruption of TP receptor doesn’t affect colon tumor formation in AOM-treated mice [41]. Additional research is needed to determine the role of TxA₂ signaling in CRC. In an animal model of IBD, conflicting results have emerged regarding the role of TxA₂-TP system in IBD. Treatment of a TP antagonist significantly reduced the colonic damage [49], while deletion of TP didn’t affect colitis formation caused by DSS [33]. These conflicting data dictate the need of additional research to address the role of TxA₂ in CRC and IBD.

PGD₂

Although PGD₂ have been implicated as an anti-inflammatory regulator of the IBD [50], the role of PGD₂ in colon cancer is not defined. PGD₂ and/or its metabolites may have tumor inhibitory effects. Recent study shows that disruption of the gene for hematopoietic PGD synthase in Apc^Min/+ mice accelerates intestinal tumor growth, while Apc^Min/+ mice with transgenic human hematopoietic PGD synthase exhibit fewer intestinal adenomas than controls [51]. These results suggest that PGD₂ serves as tumor suppressor in colorectal cancer. However, the evidence from DP receptor studies in CRC mouse models don’t support the hypothesis that PGD₂ has tumor inhibitory effects. For example, disruption of the DP receptor didn’t affect colon tumor formation in AOM-treated mice [41]. Treatment with a selective DP antagonist significantly reduced susceptibility of post-colitis rats to the development of aberrant crypt foci [52]. One possible explanation for the differences in phenotype caused by PGD synthase versus DP in mouse models may be due to a fact that PGD₂ exerts its biological functions through either DP or/and PPARγ receptor. It is possible that the anti-tumor effect of PGD₂ is mediated by PPARγ since PGD₂ can be metabolized to 15dPGJ₂, which is a endogenous ligand for the PPARγ. Activation of PPARγ results in growth arrest of colon carcinoma cells through induction of cell differentiation and inhibition of cell proliferation or induction of apoptosis. Alternatively, overexpression of PGD synthase may shift the conversion of PGH₂ away from PGE₂, which in turn suppresses tumor growth.

PGF_2α

Although the emerging data support the role of PGF_2α in acute and chronic inflammation [53], However, one study showed that FP is not involved in the progression of chronic inflammation in IBD [33]. Furthermore, there are no clear studies showing that PGF_2α participates in IBD. Similarly, the observation that PGF_2α failed to induce cell proliferation in CRC cell lines and deletion of FP receptor didn’t affect colon tumor formation in AOM-treated mice suggests that PGF_2α is not involved in colorectal cancer progression [41, 54].

PGI₂

In normal physiological processes, COX-1-derived PGI₂ is a major prostaglandin product in the gastrointestinal tract and plays a role in the cytoprotection of gastric mucosal surfaces and in maintaining normal vasculature [55]. Under pathological conditions, PGI₂ serves as a mediator of acute and chronic inflammation. Loss or inhibition of IP (the prostacyclin receptor) reduces pain and inflammation in a chronic model of inflammatory arthritis [56]. In an animal model of IBD, another study showed that IP is not involved in chronic inflammation [33]. There is also no other report showing that PGI₂ is implicated in IBD. Although one study showed that the IP receptor is not involved in colon tumor formation in AOM-treated mice [41], little is known about the role of PGI₂ and IP in CRC. Since PGI₂ can activate PPARδ in CRC cells [21] and activation of PPARδ accelerates intestinal tumor growth in Apc^Min/+ mice [57], it is conceivable that PGI₂ may participate in the colon tumor progression through PPARδ. Further work is necessary to explore the role of PGI₂ and IP in colon carcinogenesis.

Conclusions

Prolonged use of high doses of NSAIDs (except for aspirin) is associated with unacceptable cardiovascular side effects [58–60]. The mechanism of the side effects of NSAIDs is unclear. A potential explanation for the cardiovascular side effects of NSAIDs is proposed based on the following observations. The synthesis of PGI₂ depends principally on COX-2 activity in the vascular wall, whereas the production of TxA₂ in platelets is dependent on COX-1. Since PGI₂ antagonizes the biological functions of TxA₂ produced by platelets, inhibition of PGI₂ may shift the homeostatic balance toward more the pro-thrombotic TxA₂ effects. The potential inhibition of endothelial cell–derived COX-2 activity and subsequent PGI₂ production by COX-2 selective inhibitors may promote platelet aggregation and lead to increased the risk of coronary thrombosis and stroke [61]. In addition, PGI₂ appears to serve an important role in protecting cardiomyocytes from oxidant stress, which is a key risk factor for atherogenesis [62]. Therefore, it will be important to develop chemopreventive agents that do not inhibit production of other prostanoids, such as the anti-thrombotic PGI₂. Given that PGE₂ appears to be the main pro-carcinogenic eicosanoid, more selective pharmacological inhibition of PGE₂ production downstream of COX-2 may be superior and result in fewer side effects. Thus, it is now crucial to evaluate whether PGE₂ receptor antagonists and inhibitors of PGE₂ synthases have better specificity for the prevention of colon cancer and result in minimal adverse effects. Of course, another option may be to modulate the expression levels of 15-PGDH in order to decrease the levels of PGE₂ in the tumor microenvironment.