PPARβ/δ (peroxisome-proliferator-activated receptor β/δ) is one of three PPARs in the nuclear hormone receptor superfamily that are collectively involved in the control of lipid homoeostasis among other functions. PPARβ/δ not only acts as a ligand-activated transcription factor, but also affects signal transduction by interacting with other transcription factors such as NF-κB (nuclear factor κB). Constitutive expression of PPARβ/δ in the gastrointestinal tract is very high compared with other tissues and its potential physiological roles in this tissue include homoeostatic regulation of intestinal cell proliferation/differentiation and modulation of inflammation associated with inflammatory bowel disease and colon cancer. Analysis of mouse epithelial cells in the intestine and colon has clearly demonstrated that ligand activation of PPARβ/δ induces terminal differentiation. The PPARβ/δ target genes mediating this effect are currently unknown. Emerging evidence suggests that PPARβ/δ can suppress inflammatory bowel disease through PPARβ/δ-dependent and ligand-independent down-regulation of inflammatory signalling. However, the role of PPARβ/δ in colon carcinogenesis remains controversial, as conflicting evidence suggests that ligand activation of PPARβ/δ can either potentiate or attenuate this disease. In the present review, we summarize the role of PPARβ/δ in gastrointestinal physiology and disease with an emphasis on findings in experimental models using both high-affinity ligands and null-mouse models.

The expression patterns of PPARbeta/delta have been described, but the majority of these data are based on mRNA data. To date, there are no reports that have quantitatively examined the expression of PPARbeta/delta protein in mouse tissues. In the present study, a highly specific PPARbeta/delta antibody was developed, characterized, and used to examine tissue expression patterns of PPARbeta/delta. As compared to commercially available anti-PPARbeta/delta antibodies, one of six polyclonal anti-PPARbeta/delta antibodies developed was significantly more effective for immunoprecipitation of in vitro-translated PPARbeta/delta. This antibody was used for quantitative Western blot analysis using radioactive detection methods. Expression of PPARbeta/delta was highest in colon, small intestine, liver, and keratinocytes as compared to other tissues including heart, spleen, skeletal muscle, lung, brain, and thymus. Interestingly, PPARbeta/delta expression was localized in the nucleus and RXRalpha can be co-immunoprecipitated with nuclear PPARbeta/delta. Results from these studies demonstrate that PPARbeta/delta expression is highest in intestinal epithelium, liver, and keratinocytes, consistent with significant biological roles in these tissues.

Ligands for peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) increase skeletal muscle fatty acid catabolism, improve insulin sensitivity, increase serum high-density lipoprotein cholesterol, elicit anti-inflammatory activity and induce terminal differentiation. Contradictory findings are also reported suggesting that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation, by inhibiting apoptosis through phosphorylation of Akt and by increasing cyclooxygenase-2 (COX2) and vascular endothelial growth factor (VEGF) expression. The contradictory findings could be due to differences in the model system (cancer cell line versus in vivo), differences in cell culture conditions (with and without serum) or differences in ligands. The present study examined the effect of two different PPARbeta/delta ligands (GW0742 and GW501516) in human cancer cell lines (HT29, HCT116, LS-174T, HepG2 and HuH7) cultured in the presence or absence of serum and compared in vitro analysis with in vivo analysis. Neither PPARbeta/delta ligand increased cell growth or phosphorylation of Akt and no increase in the expression of VEGF or COX2 were detected in any cancer cell line in the presence or absence of serum. Similarly, liver, colon and colon polyps from mice administered these PPARbeta/delta ligands in vivo did not exhibit changes in these markers. Results from these studies demonstrate that serum withdrawal and/or differences in ligands do not underlie the disparity in responses reported in the literature. The quantitative nature of the present findings are inconsistent with the hypothesis that cancer cell lines respond differentially as compared with normal cells, and provide further evidence that PPARbeta/delta ligands do not potentiate tumorigenesis.

Cyclooxygenase (COX) 2-derived prostaglandin E(2) (PGE(2)) promotes colorectal carcinoma growth and invasion, and inhibition of COX2 by non-steroidal anti-inflammatory drugs is known to inhibit these processes. There is controversy regarding the effect of ligand activation of peroxisome proliferator-activated receptor (PPAR)-beta/delta on colon carcinogenesis, although collective evidence from independent laboratories suggest that ligand activation of PPARbeta/delta leads to the induction of terminal differentiation coupled with inhibition of cell growth in a variety of models. The present study examined the hypothesis that ligand activation of PPARbeta/delta and inhibition of COX2 attenuate colon cancer through independent mechanisms and that combining these two mechanisms will enhance this inhibition. Colon cancer was induced by administering azoxymethane to wild-type and PPARbeta/delta-null mice. Cohorts of mice were treated with GW0742 (a PPARbeta/delta ligand), nimesulide (a COX2 inhibitor) or a combination of GW0742 and nimesulide. Inhibition of COX2 by nimesulide attenuated colon cancer and ligand activation of PPARbeta/delta by GW0742 had inhibitory effects. However, the combined treatment of GW0742 and nimesulide did not cause an enhancement in the attenuation of colon cancer. Mechanistically, the effects of these compounds occurred through independent mechanisms as increased levels of differentiation markers as a result of ligand activation of PPARbeta/delta were not found with COX2 inhibition, and a reduction in PGE(2) levels resulting from COX2 inhibition was not observed in response to ligand activation of PPARbeta/delta. Results from these studies effectively dissociate COX2 inhibition and PPARbeta/delta activity during colon carcinogenesis.

The development of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) ligands for the treatment of diseases including metabolic syndrome, diabetes and obesity has been hampered due to contradictory findings on their potential safety. For example, while some reports show that ligand activation of PPARβ/δ promotes the induction of terminal differentiation and inhibition of cell growth, other reports suggest that PPARβ/δ ligands potentiate tumorigenesis by increasing cell proliferation. Some of the contradictory findings could be due in part to differences in the ligand examined, the presence or absence of serum in cell cultures, differences in cell lines, or differences in the method used to quantify cell growth. For these reasons, this study examined the effect of ligand activation of PPARβ/δ on cell growth of two human cancer cell lines, MCF7 (breast cancer) and UACC903 (melanoma) in the presence or absence of serum using two highly specific PPARβ/δ ligands, GW0742 or GW501516. Culturing cells in the presence of either GW0742 or GW501516 caused upregulation of the known PPARβ/δ target gene angiopoetin-like protein 4 (ANGPTL4). Inhibition of cell growth was observed in both cell lines cultured in the presence of either GW0742 or GW501516, and the presence or absence of serum had little influence on this inhibition. Results from the present studies demonstrate that ligand activation of PPARβ/δ inhibits the growth of both MCF7 and UACC903 cell lines and provide further evidence that PPARβ/δ ligands are not mitogenic in human cancer cell lines.