Comparison of the mechanism of induction of apoptosis in ovarian carcinoma cells by the conformationally restricted synthetic retinoids CD437 and 4-HPR

All-trans-retinoic acid (ATRA) has been shown to inhibit the growth of a number of ovarian tumor cell lines while others have been found to be resistant to retinoid suppression of growth. Interestingly, two synthetic retinoids, CD437 and 4-HPR, inhibit the growth of both ATRA-sensitive (CA-OV-3) and ATRA-resistant (SK-OV-3) ovarian tumor cells. However, in contrast to ATRA, both induce apoptosis. Our goal was to elucidate the mechanism by which these two synthetic retinoids induce apoptosis in ovarian tumor cells. Since it has been documented that apoptosis induction is often mediated by the activation of a cascade of proteases known as caspases, we initially studied the role of caspases in induction of apoptosis by CD437 and 4-HPR. We found that both retinoids induced caspase-3 and caspase-9 enzyme activity. Furthermore, using caspase specific inhibitors we determined that caspase-3 and caspase-9 activity was essential for the induction of apoptosis by these synthetic retinoids since these inhibitors completely blocked CD437 and 4-HPR induced apoptosis. Interestingly, we found that treatment with bongkriekic acid (BA), a mitochondrial membrane depolarization inhibitor, blocked apoptosis, caspase-9 activation and caspase-3 activation induced by both retinoids. Finally, we were able to determine that CD437 treatment induced the translocation of TR3, a nuclear orphan receptor, whereas, 4-HPR did not. Our results suggest that CD437 and 4-HPR initially activate separate pathways to induce mitochondrial depolarization but both utilize mitochondrial depolarization, caspase-9 activation, and caspase-3 activation in the later stages of apoptosis induction.