The peroxisome proliferator-activated receptor γ (PPARγ) ligands 15-deoxy-Δ12,14-prostaglandin J2 and ciglitazone induce human B lymphocyte and B cell lymphoma apoptosis by PPARγ-independent mechanisms

Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a transcription factor important for adipogenesis and more recently has been shown to be an anticancer target. PPAR gamma ligands, including the endogenous ligand 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)) and synthetic ligands like ciglitazone and troglitazone, all induce apoptosis in normal and malignant human B lymphocytes, but the dependency of PPARy for apoptosis induction is unknown. In this study, we used a PPARy dominant-negative approach and a small molecule irreversible PPARy antagonist and found that these inhibitors prevented PPARy activation but did not prevent B cell apoptosis induced by 15d-PGJ(2) or ciglitazone. In addition, a PPARy agonist that is a structural analog of 15d-PGJ(2), and lacks the electrophilic carbon of the 15d-PGJ(2) cyclopentenone ring, activated PPARy but did not kill B lymphocytes, further supporting a non-PPAR gamma-mediated mechanism. To further investigate the apoptotic mechanism, the effects of 15d-PGJ(2) and ciglitazone on reactive oxygen species were investigated. 15d-PGJ(2), but not ciglitazone, potently induced reactive oxygen species in B lymphocytes, implicating the reactive nature of the 15d-PGJ(2) structure in the apoptosis mechanism. In addition, 15d-PGJ(2) caused an almost complete depletion of intracellular glutathione. Moreover, incubation with glutathione reduced ethyl ester, an antioxidant, prevented apoptosis induced by 15d-PGJ(2), but not by ciglitazone. These findings indicate that the expression of PPARy may not be predictive of whether a normal or malignant B lineage cell is sensitive to PPARy agonists. Furthermore, these new findings support continued investigation into the use of PPARy agonists as agents to attenuate normal B cell responses and as anti-B cell lymphoma agents.