15-Deoxy-Δ^12,14-prostaglandin J₂ induces death receptor 5 expression through mRNA stabilization independently of PPARγ and potentiates TRAIL-induced apoptosis

15-Deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂), the terminal derivative of the PGJ series, is emerging as a potent antineoplastic agent among cyclopentenone prostaglandins derivatives and also known as the endogenous ligand of peroxisome proliferator-activated receptor γ (PPARγ). On the other hand, death receptor 5 (DR5) is a specific receptor for tumor necrosis factor - related apoptosis-inducing ligand (TRAIL), which is one of the most promising candidates for new cancer therapeutics. Here, we report that 15d-PGJ₂ induces DR5 expression at both mRNA and protein levels, resulting in the synergistic sensitization of TRAIL-induced apoptosis in human neo-plastic cells, such as Jurkat human leukemia cells or PC3 human prostate cancer cells. 15d-PGJ₂ significantly increased DR5 mRNA stability, whereas it did not activate DR5 promoter activity. Synthetic PPARγ agonists, such as pioglitazone or rosiglitazone, did not mimic the DR5-inducing effects of 15d-PGJ₂, and a potent PPARγ inhibitor GW9662 failed to block DR5 induction by 15d-PGJ₂, suggesting PPARγ-independent mechanisms. Cotreatment with 15d-PGJ₂ and TRAIL enhanced the sequential activation of caspase-8, caspase-10, caspase-9, caspase-3, and Bid. DR5/Fc chimera protein, zVAD-fmk pancaspase inhibitor, and caspase-8 inhibitor efficiently blocked the activation of these apoptotic signal mediators and the induction of apoptotic cell death enhanced by cotreatment with 15d-PGJ₂ and TRAIL. Moreover, a double-stranded small interfering RNA targeting DR5 gene, which suppressed DR5 up-regulation by 15d-PGJ₂, significantly attenuated apoptosis induced by cotreatment with 15d-PGJ₂ and TRAIL. These results suggest that 15d-PGJ₂ is a potent sensitizer of TRAIL-mediated cancer therapeutics through DR5 up-regulation.