Synergistic effects of retinoic acid and 8-Cl-cAMP on apoptosis require caspase-3 activation in human ovarian cancer cells

We investigated the intracellular mechanisms of retinoic acid (9-cis-RA, 13 -cis-RA or all-trans-RA) and a cyclic AMP analog 8-Cl-cAMP on growth-inhibi tion and apoptosis in human ovarian cancer NIH: OVCAR-3 and OVCGR-8 cells, The cyclic AMP analog, 8-Cl-cAMP, acted synergistically with RA in inducing and activating retinoic acid receptor beta (RAR beta) which correlated wit h the growth inhibition, cell cycle arrest, and apoptosis in both cell type s. In addition, combined treatment of cells with RA plus 8-Cl-cAMP resulted in the release of cytochrome c, loss in mitochondrial membrane potential a nd activation of caspase-3 followed by cleavage of anti-poly(ADP-ribose)pol ymerase and DNA-dependent protein kinase (catalytic subunit), Interestingly , inhibition of caspase-3 activation blocked RA plus 8-Cl-cAMP induced apop tosis, Furthermore, mutations in a CRE-related motif within the RAR beta pr omoter resulted in loss of both transcriptional activation of RAR beta and synergy between RA and 8-Cl-cAMP, Thus, RAR beta can mediate RA and/or cycl ic AMP action in ovarian cancer cells by promoting apoptosis, Loss of RAR b eta expression, therefore, may contribute to the tumorigenicity of human ov arian cancer cells, These findings suggest that RA and 8-Cl-cAMP act in a s ynergistic fashion in inducing apoptosis via caspase-3 activation, and may have potential for combination biotherapy for the treatment of malignant di sease such as ovarian cancer.