Apoptosis induced by atRA in MEPM cells is mediated through activation of caspase and RAR

We have previously demonstrated that all-trans retinoic (atRA) induced growth inhibition and Apoptosis in mouse embryonic palate mesenchymal cells (MEPM). In the present study, we investigated the molecular mechanisms of atRA-induced Apoptosis and its putative action pathway. atRA-induced Apoptosis is associated with activation of the initiator caspase-9 and the effector Caspase-3, but not of the effector Caspase-8. A broad Caspase Inhibitor (z-VAD-fmk), caspase-9 inhibitor z-LEHD-fmk and Caspase-3 inhibitor (z-DEVD-fmk) blocked atRA-induced DNA fragmentation and sub-G1 fraction, but not Caspase-8 inhibitor z-IETD-fmk. We further showed that atRA dose-dependently promoted mRNA expression of retinoic acid receptor beta (RAR-beta) and gamma. A weaker increase in RAR-alpha mRNA was seen only at the highest concentration of atRA (5 muM). The pan RAR antagonist, BMS493, completely abrogated atRA-induced DNA fragmentation, Sub-G1 fraction, and Caspase-3 activation. Taken together, these findings show that caspase-mediated induction of Apoptosis by atRA is an RAR-dependent signaling pathway.