A novel positive feedback loop between peroxisome proliferator-activated receptor-delta and prostaglandin E2 signaling pathways for human cholangiocarcinoma cell growth.

Peroxisome proliferator-activated receptor-delta (PPARdelta) is a nuclear receptor implicated in lipid oxidation and the pathogenesis of obesity and diabetes. This study was designed to examine the potential effect of PPARdelta on human cholangiocarcinoma cell growth and its mechanism of actions. Overexpression of PPARdelta or activation of PPARdelta by its pharmacological ligand, GW501516, at low doses (0.5-50 nM) promoted the growth of three human cholangiocarcinoma cell lines (CCLP1, HuCCT1, and SG231). This effect was mediated by induction of cyclooxygenase-2 (COX-2) gene expression and production of prostaglandin E2 (PGE2) that in turn transactivated epidermal growth factor receptor (EGFR) and Akt. In support of this, inhibition of COX-2, EGFR, and Akt prevented the PPARdelta-induced cell growth. Furthermore, PPARdelta activation or PGE2 treatment induced the phosphorylation of cytosolic phospholipase A2alpha (cPLA2alpha), a key enzyme that releases arachidonic acid (AA) substrate for PG production via COX. Overexpression or activation of cPLA2alpha enhanced PPARdelta binding to PPARdelta response element (DRE) and increased PPARdelta reporter activity, indicating a novel role of cPLA2alpha for PPARdelta activation. Consistent with this, AA enhanced the binding of PPARdelta to DRE, in vitro, suggesting a direct role of AA for PPARdelta activation. In contrast, although PGE2 treatment increased the DRE reporter activity in intact cells, it failed to induce PPARdelta binding to DRE in cell-free system, suggesting that cPLA2alpha-mediated AA release is required for PGE2-induced PPARdelta activation. Taken together, these observations reveal that PPARdelta induces COX-2 expression in human cholangiocarcinoma cells and that the COX-2-derived PGE2 further activates PPARdelta through phosphorylation of cPLA2alpha. This positive feedback loop plays an important role for cholangiocarcinoma cell growth and may be targeted for chemoprevention and treatment.