Reduction of both RAR and RXR levels is required to maximally alter sensitivity of CA-OV3 ovarian tumor cells to growth suppression by all-trans-retinoic acid.

We wished to determine the effect of altering the levels or functional activity of retinoid receptors, in particular retinoic acid receptor-alpha (RAR-alpha) and retinoid X receptor-alpha (RXR-alpha) on the growth sensitivity of ovarian tumor cells to all-trans-retinoic acid (all-trans-RA). We found that CA-OV3 cells could be made resistant to all-trans-RA growth inhibition by overexpressing RAR-beta(R269Q), an efficient dominant negative mutant which inhibits the function of all RAR subtypes. Antisense technology was then used to prepare stable transfectants of the retinoid-sensitive ovarian carcinoma cell line CA-OV3 in which expression of RAR-alpha, RXR-alpha, or both RAR-alpha and RXR-alpha was reduced. The effect of all-trans-RA on ovarian tumor cell growth was determined by MTT assay, autoradiographic analysis of DNA synthesis, and anchorage-independent colony formation in soft agar. Our results show that cell lines expressing reduced levels of either RAR-alpha alone or RXR-alpha alone exhibited a small decrease in sensitivity to growth inhibition by all-trans-RA. However, maximum RA resistance was obtained in cell lines in which the levels of both RAR-alpha and RXR-alpha were reduced. These results demonstrate the importance of both retinoid nuclear receptors and retinoid-X receptors in general, and RAR-alpha and RXR-alpha in particular, as mediators of ovarian carcinoma cell growth inhibition by retinoids.