15-Deoxy-Δ^12,14-prostaglandin J₂-induced apoptosis does not require PPARγ in breast cancer cells

Naturally occurring derivatives of arachidonic acid are potent agonists for the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARγ) and block cancer cell proliferation through the induction of apoptosis. We have previously reported that induction of apoptosis using cyclopentenone prostaglandins of the J series, including 15deoxyΔ^12,14PGJ₂ (15dPGJ₂), is associated with a high degree of PPAR-response element (PPRE) activity and requires early de novo gene expression in breast cancer cells. In the current study, we used pharmacologic and genetic approaches to test the hypothesis that PPARγ is required for 15dPGJ₂-induced apoptosis. The PPARγ agonists 15dPGJ₂, trogliltazone (TGZ), and GW7845, a synthetic and highly selective tyrosine-based PPARγ agonist, all increased transcriptional activity of PPARγ, and expression of CD36, a PPARγ-dependent gene. Transcriptional activity and CD36 expression was reduced by GW9662, a selective and irreversible PPARγ antagonist, but GW9662 did not block apoptosis induced by 15dPGJ₂. Moreover, dominant negative expression of PPARγ blocked PPRE transcriptional activity, but did not block 15dPGJ₂-induced apoptosis. These studies show that while 15dPGJ₂ activates PPRE-mediated transcription, PPARγ is not required for 15dPGJ₂-induced apoptosis in breast cancer cells. Other likely mechanisms through which cyclopentenone prostaglandins induce apoptosis of cancer cells are discussed.