PPARα

PPARα is highly expressed in mouse colonic epithelial cells facing the intestinal lumen, and its expression is induced by glucocorticoids (GCs). Subsequent studies have further demonstrated that PPARα mediates the antiinflammatory effects of GC in a mouse model of chemically induced colitis. In this study, treatment with dexamethasone, a potent synthetic member of the GC class of steroid drugs, suppressed the formation of dinitrobenzene sulfuric acid (DNBS)–induced colitis in wild-type mice but not in PPARα knockout mice. Consistent with these results, the deletion of PPARα promoted the severity of colitis in DNBS-treated mice, whereas activation of PPARα by its agonist significantly reduced colonic inflammation in this mouse model. However, there is no report thus far on the precise role of PPARα in genetic models of IBD (transgenic and knockout models).

PPARγ

Although PPARγ is predominantly present in the liver and adipose tissue, it is also expressed in the intestinal epithelium, immune cells, and adipocytes. However, patients with UC, but not those with CD, show decreased PPARγ levels in colonic epithelial cells compared with normal controls. This observation raises the hypothesis that microbe-host interactions, chronic inflammation, and/or genetic predisposition may lead to low PPARγ levels in colonic epithelial cells, which in turn may result in unrestrained inflammation. Several lines of evidence support the notion that PPARγ may serve as a new therapeutic target in IBD. In mouse models of chemically induced colitis, 5-ASA treatment had a beneficial effect on colitis only in wild-type mice and not in heterozygous PPARγ ^+/− mice, demonstrating that PPARγ mediates the antiinflammatory effect of 5-ASA. Furthermore, treatment with a PPARγ ligand, thiazolidinedione, markedly reduced colonic inflammation in mouse models of chemically induced colitis and interleukin (IL)-10 deficient mice (a genetic model of colitis), suggesting that activation of PPARγ suppresses inflammation in IBD.

Because PPARγ is expressed in intestinal epithelial cells, macrophages, and T and B lymphocytes, it is critical to understand the contribution of PPARγ present in each of these cell types to this protection. The results from 2 studies showed that the disruption of PPARγ in colonic epithelial cells worsened colonic inflammatory lesions in dextran sulfate sodium (DSS)–treated mice, indicating that PPARγ expression in epithelial cells is required for the prevention of experimental IBD. Similarly, mice with deficiency of PPARγ in CD4 T lymphocytes are more sensitive to trinitrobenzene sulfonic acid–induced colitis because the deficiency of PPARγ in regulatory T cells impaired their ability to prevent effector CD4 T lymphocyte–induced colitis. Moreover, mice with a targeted disruption of PPARγ in macrophages displayed an increased susceptibility to DSS-induced colitis compared with wild-type littermates, demonstrating that PPARγ is required for macrophage-mediated protection against colitis. Consistent with these results, an increase in PPARγ expression by adenovirus-mediated gene transfer attenuated colonic inflammation induced by DSS in mice. In addition, a recent study showed that the antiinflammatory effects of PPARγ on IBD are via maintenance of innate antimicrobial immunity in the mouse colon. The results from 1 randomized placebo-controlled trial and 1 open-label trial showed that a PPARγ agonist, rosiglitazone, has therapeutic efficacy in humans with UC. Collectively, all of these studies support a rationale to develop PPARγ agonists as potential therapeutic and prophylactic agents against IBD.

PPARδ

Little is known about the role of PPARδ in IBD, and the results from 2 mouse models of IBD are controversial. Deletion of PPARδ significantly exacerbated colitis, whereas treatment with a PPARδ agonist did not affect the clinical symptoms in the DSS-treated mouse model. This study implies that PPARδ, like PPARγ, exerts antiinflammatory effects in IBD via a ligand-independent mechanism. In contrast with this observation, administration of a PPARδ agonist caused enhanced colitis in IL-10–deficient mice (a genetic model of colitis), suggesting that PPARδ has a proinflammatory effect. Therefore, further studies are necessary to clarify the biologic functions of PPARδ in the modulation of IBD.

PPARα

Less is known about the role of PPARα in human cancer, although long-term administration of a PPARα agonist induces the development of hepatocarcinomas in mice but not in PPARα-null animals, conclusively demonstrating that PPARα mediates these effects in promoting liver cancer. In spite of the fact that activation of PPARα by exogenous agonists generally causes inhibition of tumor cell growth in cell lines derived from CRC, melanoma, and glial brain tumors, the physiologic significance of PPARα in the regulation of CRC progression is also less well characterized than that of PPARγ and PPARδ.

PPARγ

Because of the elevated expression of PPARγ in CRC and its involvement in regulating cellular differentiation, PPARγ has become a point of interest in CRC studies. However, studies of PPARγ mutation in human colon tumor samples and CRC cell lines have produced controversial results. One study has shown that 8% of the patients with primary human CRCs had a loss-of-function mutation in 1 allele of the PPARγ gene. Recent data revealed that a Pro12Ala polymorphism in the PPARγ gene is associated with an increased risk of CRC. These results suggest a putative role for this receptor as a tumor suppressor. In contrast, another study showed that the mutant PPARγ gene was not detected in human colon carcinoma samples or CRC cell lines, suggesting that PPARγ mutations in human CRC may be a rare event.

It is well established in numerous in vitro studies that activation of PPARγ results in growth arrest of colon carcinoma cells through induction of cell cycle arrest or/and apoptosis. However, the effect of PPARγ on CRC progression in vivo is controversial because of conflicting results from different mouse models of colon cancer. Although PPARγ agonists inhibit colorectal carcinogenesis in xenograft models and azoxymethane (AOM)-induced colon cancer model, these drugs are reported to have either tumor-promoting or tumor-inhibiting effects in a mouse model of FAP, the Apc ^Min/+ mouse. Multiple studies showed that administration of PPARγ agonists significantly increases the number of colon adenomas in the Apc ^Min/+ and even in wild-type C57BL/6 mice. However, other studies showed that treatment of 2 different Apc mutant models ( Apc ^Min/+ and Apc ^Δ1309 ) with a PPARγ agonist pioglitazone reduced the polyp number in small and large intestines in a dose-dependent manner. These divergent effects of PPARγ might be related to drug doses and bioavailability and/or the animal models that are used. These paradoxic observations seem to have been resolved by genetic studies, showing that the heterozygous disruption of PPARγ is sufficient to increase tumor numbers in AOM-treated mice and that intestinal-specific PPARγ knockout promotes tumor growth in Apc ^Min/+ mice. Thus, genetic evidence supports the hypothesis that PPARγ serves as a tumor suppressor in CRC. In addition, a combined treatment of mice with a selective COX-2 inhibitor and a PPARγ agonist significantly inhibited the incidence and multiplicity of inflammation-associated colonic adenocarcinoma induced by AOM or DSS. A retrospective cohort study revealed that treatment of diabetic patients with a PPARγ agonist (thiazolidinedione) exhibited a mild trend toward risk reduction of CRC, although this difference did not reach statistical significance. Collectively, these findings further support the rationale of developing PPARγ agonists as antitumor agents.

PPARδ

The role of PPARδ in colorectal carcinogenesis is more controversial than that of PPARγ. The first evidence linking PPARδ to carcinogenesis actually came from studies on gastrointestinal cancer. PPARδ was identified as a direct transcriptional target of APC/β-catenin/Tcf pathway and as a repression target of NSAIDs. A large case-control study showed that the protective effect of NSAIDs against colorectal adenomas was modulated by a polymorphism in the PPAR gene. Moreover, COX-2-derived PGI ₂ directly transactivates PPARδ, and COX-2-derived PGE ₂ indirectly induces PPARδ activation in CRC, hepatocellular carcinoma, and cholangiocarcinoma cells. In addition, PPARδ expression and activity are also induced by oncogenic K-Ras. These studies indicate that PPARδ is a focal point of crosstalk between oncogenic signaling pathways.

Similar to PPARγ, investigation of PPARδ expression in human and mouse colonic tumor samples and CRC cell lines generated controversial results. Some reports showed that PPARδ is elevated in most human CRCs and in tumors arising in the Apc ^Min/+ mice and AOM-treated rats, in agreement with the observations that activation of the β-catenin/Tcf pathway by APC mutation or K-Ras upregulates PPARδ expression. PPARδ proteins are accumulated only in human CRC cells with highly malignant morphology. Downregulation of PPARδ is correlated with antitumor effects of dietary fish oil and pectin in rats treated with radiation and AOM. However, other reports showed that PPARδ expression is lower in human cancer tissues and adenomas from the Apc ^Min/+ mice than in normal control tissues.

In a murine xenograft cancer model, the disruption of both PPARδ alleles by deletion of its exons 4 and 6 in human HCT116 colon carcinoma cells decreased tumorigenicity, suggesting that activation of PPARδ promotes tumor growth. To further determine whether PPARδ attenuates or promotes intestinal tumor growth, 3 mouse models of CRC were used, including AOM-treated, Apc ^Min/+ , and Mlh -null mice. Mlh is a DNA mismatch repair gene that is involved in hereditary nonpolyposis CRC. Conflicting data were obtained from studies in AOM-treated and Apc ^Min/+ mice. For example, activation of PPARδ by a selective synthetic PPARδ agonist (GW501516) or a PPARδ endogenous activator (PGE ₂ ) accelerated intestinal adenoma growth in Apc ^Min/+ mice by promoting tumor cell survival. In contrast, another PPARδ ligand (GW0742) inhibited colon carcinogenesis in AOM-treated mice but promoted small intestinal polyp growth in Apc ^Min/+ mice. It is not clear whether PPARδ mediates the effects of GW0742 in Apc ^Min/+ mice in this study. A genetic study showed that loss of PPARδ by deletion of its exons 4 and 5 attenuated small and large intestinal adenoma growth and demonstrated that PPARδ mediated the tumor-promoting effects of the PPARδ ligand (GW501516) and PGE ₂ in Apc ^Min/+ mice. In a recent study with a tissue-specific deletion of PPARδ exon 4 in the colon, loss of PPARd inhibited colonic carcinogenesis in AOM-treated mice, further confirming the notion that PPARδ serves as a tumor accelerator. On the other hand, several other studies have shown different results when using PPARδ mutant mice generated by germline deletion of PPARδ exon 8. Deletion of PPARδ exon 8 enhances polyp growth in Apc ^Min/+ and AOM-treated mice in the absence of exogenous PPARδ stimulation. In Mlh -null mice, no significant differences are evident in the number and size of intestinal adenomas between wild-type and PPARδ mutant mice (deletion of PPARδ exon 8). The conflicting results regarding the effect of PPARδ on intestinal tumorigenesis in Apc ^Min/+ and AOM-treated mice could be attributed to differences in the specific targeting strategy used to delete PPARδ. Deletion of PPARδ exon 4 and/or 5, which encode an essential portion of the DNA binding domain, is thought to disrupt PPARδ’s function as a nuclear transcription factor and inhibit tumorigenesis. The deletion of exon 8, the last PPARδ exon, is postulated to generate a hypomorphic allele, which retains some aporeceptor function. Indeed, the observation that the high rates of embryonic mortality and abnormal placental development occurred in mice in which PPARδ exons 4 and 5 were deleted but not in mice in which PPARδ exon 8 was deleted supports this hypothesis. Taken together, not enough evidence is available to establish whether PPARδ has pro- or antitumorigenic effect on CRC progression, and the role of PPARδ in cancer biology remains unclear.

PPARα

Less is known about the role of PPARα in human cancer, although long-term administration of a PPARα agonist induces the development of hepatocarcinomas in mice but not in PPARα-null animals, conclusively demonstrating that PPARα mediates these effects in promoting liver cancer. In spite of the fact that activation of PPARα by exogenous agonists generally causes inhibition of tumor cell growth in cell lines derived from CRC, melanoma, and glial brain tumors, the physiologic significance of PPARα in the regulation of CRC progression is also less well characterized than that of PPARγ and PPARδ.

PPARγ

Because of the elevated expression of PPARγ in CRC and its involvement in regulating cellular differentiation, PPARγ has become a point of interest in CRC studies. However, studies of PPARγ mutation in human colon tumor samples and CRC cell lines have produced controversial results. One study has shown that 8% of the patients with primary human CRCs had a loss-of-function mutation in 1 allele of the PPARγ gene. Recent data revealed that a Pro12Ala polymorphism in the PPARγ gene is associated with an increased risk of CRC. These results suggest a putative role for this receptor as a tumor suppressor. In contrast, another study showed that the mutant PPARγ gene was not detected in human colon carcinoma samples or CRC cell lines, suggesting that PPARγ mutations in human CRC may be a rare event.

It is well established in numerous in vitro studies that activation of PPARγ results in growth arrest of colon carcinoma cells through induction of cell cycle arrest or/and apoptosis. However, the effect of PPARγ on CRC progression in vivo is controversial because of conflicting results from different mouse models of colon cancer. Although PPARγ agonists inhibit colorectal carcinogenesis in xenograft models and azoxymethane (AOM)-induced colon cancer model, these drugs are reported to have either tumor-promoting or tumor-inhibiting effects in a mouse model of FAP, the Apc ^Min/+ mouse. Multiple studies showed that administration of PPARγ agonists significantly increases the number of colon adenomas in the Apc ^Min/+ and even in wild-type C57BL/6 mice. However, other studies showed that treatment of 2 different Apc mutant models ( Apc ^Min/+ and Apc ^Δ1309 ) with a PPARγ agonist pioglitazone reduced the polyp number in small and large intestines in a dose-dependent manner. These divergent effects of PPARγ might be related to drug doses and bioavailability and/or the animal models that are used. These paradoxic observations seem to have been resolved by genetic studies, showing that the heterozygous disruption of PPARγ is sufficient to increase tumor numbers in AOM-treated mice and that intestinal-specific PPARγ knockout promotes tumor growth in Apc ^Min/+ mice. Thus, genetic evidence supports the hypothesis that PPARγ serves as a tumor suppressor in CRC. In addition, a combined treatment of mice with a selective COX-2 inhibitor and a PPARγ agonist significantly inhibited the incidence and multiplicity of inflammation-associated colonic adenocarcinoma induced by AOM or DSS. A retrospective cohort study revealed that treatment of diabetic patients with a PPARγ agonist (thiazolidinedione) exhibited a mild trend toward risk reduction of CRC, although this difference did not reach statistical significance. Collectively, these findings further support the rationale of developing PPARγ agonists as antitumor agents.

PPARδ

The role of PPARδ in colorectal carcinogenesis is more controversial than that of PPARγ. The first evidence linking PPARδ to carcinogenesis actually came from studies on gastrointestinal cancer. PPARδ was identified as a direct transcriptional target of APC/β-catenin/Tcf pathway and as a repression target of NSAIDs. A large case-control study showed that the protective effect of NSAIDs against colorectal adenomas was modulated by a polymorphism in the PPAR gene. Moreover, COX-2-derived PGI ₂ directly transactivates PPARδ, and COX-2-derived PGE ₂ indirectly induces PPARδ activation in CRC, hepatocellular carcinoma, and cholangiocarcinoma cells. In addition, PPARδ expression and activity are also induced by oncogenic K-Ras. These studies indicate that PPARδ is a focal point of crosstalk between oncogenic signaling pathways.

Similar to PPARγ, investigation of PPARδ expression in human and mouse colonic tumor samples and CRC cell lines generated controversial results. Some reports showed that PPARδ is elevated in most human CRCs and in tumors arising in the Apc ^Min/+ mice and AOM-treated rats, in agreement with the observations that activation of the β-catenin/Tcf pathway by APC mutation or K-Ras upregulates PPARδ expression. PPARδ proteins are accumulated only in human CRC cells with highly malignant morphology. Downregulation of PPARδ is correlated with antitumor effects of dietary fish oil and pectin in rats treated with radiation and AOM. However, other reports showed that PPARδ expression is lower in human cancer tissues and adenomas from the Apc ^Min/+ mice than in normal control tissues.

In a murine xenograft cancer model, the disruption of both PPARδ alleles by deletion of its exons 4 and 6 in human HCT116 colon carcinoma cells decreased tumorigenicity, suggesting that activation of PPARδ promotes tumor growth. To further determine whether PPARδ attenuates or promotes intestinal tumor growth, 3 mouse models of CRC were used, including AOM-treated, Apc ^Min/+ , and Mlh -null mice. Mlh is a DNA mismatch repair gene that is involved in hereditary nonpolyposis CRC. Conflicting data were obtained from studies in AOM-treated and Apc ^Min/+ mice. For example, activation of PPARδ by a selective synthetic PPARδ agonist (GW501516) or a PPARδ endogenous activator (PGE ₂ ) accelerated intestinal adenoma growth in Apc ^Min/+ mice by promoting tumor cell survival. In contrast, another PPARδ ligand (GW0742) inhibited colon carcinogenesis in AOM-treated mice but promoted small intestinal polyp growth in Apc ^Min/+ mice. It is not clear whether PPARδ mediates the effects of GW0742 in Apc ^Min/+ mice in this study. A genetic study showed that loss of PPARδ by deletion of its exons 4 and 5 attenuated small and large intestinal adenoma growth and demonstrated that PPARδ mediated the tumor-promoting effects of the PPARδ ligand (GW501516) and PGE ₂ in Apc ^Min/+ mice. In a recent study with a tissue-specific deletion of PPARδ exon 4 in the colon, loss of PPARd inhibited colonic carcinogenesis in AOM-treated mice, further confirming the notion that PPARδ serves as a tumor accelerator. On the other hand, several other studies have shown different results when using PPARδ mutant mice generated by germline deletion of PPARδ exon 8. Deletion of PPARδ exon 8 enhances polyp growth in Apc ^Min/+ and AOM-treated mice in the absence of exogenous PPARδ stimulation. In Mlh -null mice, no significant differences are evident in the number and size of intestinal adenomas between wild-type and PPARδ mutant mice (deletion of PPARδ exon 8). The conflicting results regarding the effect of PPARδ on intestinal tumorigenesis in Apc ^Min/+ and AOM-treated mice could be attributed to differences in the specific targeting strategy used to delete PPARδ. Deletion of PPARδ exon 4 and/or 5, which encode an essential portion of the DNA binding domain, is thought to disrupt PPARδ’s function as a nuclear transcription factor and inhibit tumorigenesis. The deletion of exon 8, the last PPARδ exon, is postulated to generate a hypomorphic allele, which retains some aporeceptor function. Indeed, the observation that the high rates of embryonic mortality and abnormal placental development occurred in mice in which PPARδ exons 4 and 5 were deleted but not in mice in which PPARδ exon 8 was deleted supports this hypothesis. Taken together, not enough evidence is available to establish whether PPARδ has pro- or antitumorigenic effect on CRC progression, and the role of PPARδ in cancer biology remains unclear.