Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands as bifunctional regulators of cell proliferation

Peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the ligand-activated nuclear receptor superfamily, plays a key role in mediating differentiation of adipocytes and regulating fat metabolism. PPARgamma has been implicated in the pathophysiology of atherosclerosis, inflammation, obesity, diabetes, immune response, and ageing. Recently, it has been shown that activation of PPARgamma by J(2) series cyclopentenone prostaglandins (cyPGs), especially 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) or synthetic agents, such as antidiabetic thiazolidinediones, causes anti-proliferation, apoptosis, differentiation, and anti-inflammation of certain types of cancer cells. The anti-proliferative effects of PPARgamma activators are associated with de novo synthesis of proteins involved in regulating the cell cycle and cell survival/death. Anti-inflammatory effects of 15d-PGJ(2) are associated with interruption of nuclear factor-kappaB and subsequent blockade of inflammatory gene expression. Furthermore, 15d-PGJ(2) at nontoxic doses induce expression of phase II detoxification or stress-responding enzymes, which may confer cellular resistance or adaptation to oxidative stress. The presence of a reactive alpha,beta-unsaturated carbonyl moiety in the cyclopentenone ring of 15d-PGJ(2) is important for part of biological functions this cyPG has. Recently, attention has been focused on the anti-proliferative activity of nonsteroidal anti-inflammatory drugs (NSAIDs) in cancerous or transformed cells, which is mediated through interaction with PPARgamma irrespective of their ability to inhibit COX-2. Despite the fact that abnormally elevated COX-2 is associated with resistance to cell death, induction of apoptosis by certain NSAIDs is accompanied by up-regulation of COX-2 expression. This commentary focuses on dual effects of the typical PPARgamma agonist 15d-PGJ(2) on cell proliferation and growth, and its possible involvement in the NSAID-induced COX-2 expression and apoptosis.