Cross-talk between Peroxisome Proliferator-Activated Receptor δ and Cytosolic Phospholipase A2α/Cyclooxygenase-2/Prostaglandin E2 Signaling Pathways in Human Hepatocellular Carcinoma Cells

Abstract

Peroxisome proliferator-activated receptor delta (PPARdelta) is a nuclear transcription factor that is recently implicated in tumorigenesis besides lipid metabolism. This study describes the cross-talk between the PPARdelta and prostaglandin (PG) signaling pathways that coordinately regulate human hepatocellular carcinoma (HCC) cell growth. Activation of PPARdelta by its pharmacologic ligand, GW501516, enhanced the growth of three human HCC cell lines (HuH7, HepG2, and Hep3B), whereas inhibition of PPARdelta by small interfering RNA prevented growth. PPARdelta activation up-regulates the expression of cyclooxygenase (COX)-2, a rate-limiting enzyme for PG synthesis, and tumor growth. PPARdelta activation or PGE(2) treatment also induced the phosphorylation of cytosolic phospholipase A(2)alpha (cPLA(2)alpha), a key enzyme that releases arachidonic acid substrate for PG production via COX. Activation of cPLA(2)alpha by the calcium ionophore A23187 enhanced PPARdelta binding to PPARdelta response element (DRE) and increased PPARdelta reporter activity, which was blocked by the selective cPLA(2)alpha inhibitors. Consistent with this, addition of arachidonic acid to isolated nuclear extracts enhanced the binding of PPARdelta to DRE in vitro, suggesting a direct role of arachidonic acid for PPARdelta activation in the nucleus. Thus, PPARdelta induces COX-2 expression and the COX-2-derived PGE(2) further activates PPARdelta via cPLA(2)alpha. Such an interaction forms a novel feed-forward growth-promoting signaling that may play a role in hepatocarcinogenesis.