9-CIS RETINOIC ACID INHIBITS GROWTH OF BREAST-CANCER CELLS AND DOWN-REGULATES ESTROGEN-RECEPTOR RNA AND PROTEIN

All-trans retinoic acid (tRA) inhibits growth of estrogen receptor-pos itive (ER+) breast cancer cells in vitro, and a variety of retinoids i nhibit development of breast cancer in animal models, 9-cis retinoic a cid (9-cis RA) is a naturally occurring high affinity ligand for the r etinoid X receptors, as well as the retinoic acid receptors (RARs). Wh ether 9-cis RA has a different spectrum of biological activity from tR A, which only binds RARs with high affinity, is largely unknown, We st udied the effects of 9-cis RA on growth and gene expression in ER+ and ER- human breast cancer cells. 9-cis RA inhibited the growth in monol ayer culture of several ER+, but not ER-, cell lines in a dose-depende nt manner. Growth inhibition and morphological changes by 9-cis RA wer e similar to those of tRA, suggesting that the ability to bind both RA R and retinoid X receptors did not significantly augment growth inhibi tion or confer sensitivity to tRA-resistant lines, MCF-7 cells exposed to 9-cis RA showed a dose-dependent accumulation in G(1). Northern an alyses showed that RAR-alpha and RAR-beta were not significantly regul ated, while RAR-gamma was up-regulated and retinoid X receptor alpha w as down-regulated by 9-cis RA, Since interactions between tRA and ER-d ependent transcription have recently been reported, we investigated wh ether these retinoids regulate expression of ER itself or estrogen-res ponsive genes. Both 9-cis RA and tRA induce down-regulation of ER mRNA and protein in MCF-7 cells. 9-cis RA down-regulates expression of the estrogen-responsive genes PR and pS2 in MCF-7 cells as reported previ ously for tRA, In several ER-positive subclones, we found that the deg ree of ER expression and regulation, but not always estrogen-sensitivi ty, correlates with the growth inhibitory effects of 9-cis RA, Further , in an ER-, retinoid-unresponsive breast cancer cell line, induced ER expression confers responsiveness to retinoid growth inhibition. Thes e data, combined with reports of additive growth inhibition of tRA and tamoxifen in vitro, suggest that 9-cis RA might augment the ability o f tamoxifen to inhibit growth of ER+ breast cancer cells in vivo.