Regulation of peroxisome proliferator-activated receptor-β/δ by the APC/β-CATENIN pathway and nonsteroidal antiinflammatory drugs

Studies indicate that peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) can either attenuate or potentiate colon cancer. One hypothesis suggests that PPARβ/δ is upregulated by the adenomatous polyposis coli (APC)/β-CATENIN pathway and a related hypothesis suggests that PPARβ/δ is downregulated by nonsteroidal antiinflammatory drugs (NSAIDs). The present study examined these possibilities using in vivo and in vitro models. While APC/β-CATENIN-dependent expression of CYCLIN D1 was observed in vivo and in vitro, expression of PPARβ/δ was not different in colon or intestinal polyps from wild-type or Apc^min heterozygous mice or in human colon cancer cell lines with mutations in APC and/or β-CATENIN. No difference in the level of PPARβ/δ was found in colon from wild-type or Apc^min heterozygous mice following treatment with NO-donating aspirin (NO-ASA). NSAIDs inhibited cell growth in RKO (wild-type APC) and DLD1 (mutant APC) human colon cancer cell lines but expression of PPARβ/δ was not downregulated in these cell lines in response to a broad concentration range of celecoxib, indomethacin, NS-398, or nimesulide. However, indomethacin caused an increase in PPARβ/δ mRNA and protein that was accompanied with increased expression of a known PPARβ/δ target gene. Interestingly, expression of PPARα was also increased in the human colon cancer cell lines by several NSAIDs at the highest concentration examined. Results from these studies provide additional evidence indicating that PPARβ/δ is not upregulated by the APC/β-CATENIN pathway. Further, these studies suggest that increased PPARβ/δ and/or PPARα by NSAIDs in human colon cancer cell lines could contribute to the mechanisms underlying the chemopreventive effects of NSAIDs.