In an effort to understand the role of key eicosanoid-forming enzymes in the activation of peroxisome proliferator-activated receptor (PPAR), this study was designed to evaluate the possible contributions of cytosolic phospholipase A2 (cPLA2) and group IIA secretory phospholipase A2 (sPLA2) in the regulation of PPAR-mediated gene transcription in a human hepatoma cell line (HepG2). The HepG2 cells express both PPAR-α and -γ but not PPAR-β. Overexpression of cPLA2, but not group IIA sPLA2 in the HepG2 cells, caused a significantly increased PPAR-α/γ-mediated reporter activity. Antisense inhibition of cPLA2 resulted in a significantly decreased PPAR-α/γ activity. The PPAR-α/γ-induced gene transcription in the HepG2 cells was inhibited by the cPLA2 inhibitors methyl arachidonyl fluorophosphonate and arachidonyltrifluoromethyl ketone, but not by the sPLA2 inhibitor LY311727. The expression of PPAR-α-mediated endogenous gene apolipoprotein A-II was increased in cells with overexpression of cPLA2, decreased in cells with antisense inhibition of cPLA2, but unaltered in cells with overexpression of group IIA sPLA2. The above results demonstrated an important role of cPLA2, but not group IIA sPLA2 in the control of PPAR activation. The cPLA2-mediated PPAR activation was likely mediated by arachidonic acid and prostaglandin E2. This study reveals a novel intracellular function of cPLA2 in PPAR activation in HepG2 cells. The cPLA2 thus may represent a potential therapeutic target for the control of PPAR-related liver and metabolic disorders such as obesity, lipid metabolic disorders, diabetes mellitus, and atherosclerosis.
Downregulation of c-fos gene transcription in cells transformed by E1A and cHa-ras oncogenes: a role of sustained activation of MAP/ERK kinase cascade and of inactive chromatin structure at c-fos promoter
