Synergistic effects of 8-Cl-cAMP and retinoic acids in the inhibition of growth and induction of apoptosis in ovarian cancer cells: induction of retinoic acid receptor beta.

Both cAMP and retinoids play a role in cell differentiation and the control of cell growth. A site-selective cAMP analog, 8-Cl-cAMP and retinoic acid synergistically inhibit growth and induce apoptosis in certain cancer cells. In advanced or recurrent malignant diseases, retinoic acid (RA) is not effective even at doses that are toxic to the host. The objective of our present study was to examine the mechanism(s) of synergistic effects of retinoic acid (9-cis, 13-cis or all-trans RA) and 8-Cl-cAMP on apoptosis in human ovarian cancer NIH: OVCAR-3 and OVCAR-8 cells. RA induced growth inhibition and apoptosis in OVCAR-3 and OVCAR-8 cells. 8-Cl-cAMP acted synergistically with RA in inducing and activating retinoic acid receptor beta (RARbeta) which correlates with growth inhibition and apoptosis in both cell types. In addition, induction of apoptosis by RA plus 8-Cl-cAMP requires caspase-3 activation followed by cleavage of anti-poly(ADP-ribose) polymerase. Furthermore, mutations in CRE-related motif within the RARbeta promoter resulted in loss of both transcriptional activation of RARbeta and synergy between RA and 8-Cl-cAMP. RARbeta expression appears to be associated with induction of apoptosis. Introduction of the RARbeta gene into OVCAR-3 cells resulted in gain of RA sensitivity. Loss of RARbeta expression, therefore, may contribute to the tumorigenicity of human ovarian cancer cells. Thus, combined treatment with RA and 8-Cl-cAMP may provide an effective means for inducing RARbeta expression leading to apoptosis in ovarian cancer cells.