ESTROGEN RECEPTOR-NEGATIVE BREAST-CANCER CELLS TRANSFECTED WITH THE ESTROGEN-RECEPTOR EXHIBIT INCREASED RAR-ALPHA GENE-EXPRESSION AND SENSITIVITY TO GROWTH-INHIBITION BY RETINOIC ACID

We and others have shown previously that retinoic acid (RA) selectivel y inhibits the growth of estrogen receptor (ER)-positive human breast carcinoma (HBC) cells and ER-negative cells are refractory to RA inhib ition of growth. The ER-negative cells inherently express lower levels of RARalpha and retinoic acid response element (RARE)-mediated RA-ind uced CAT activity. In this study we report that when ER-negative MDA-M B-231 cells were transfected with the ER gene they not only expressed higher levels of RARalpha and RARE-mediated RA-induced CAT gene expres sion, but their growth was now inhibited by RA. Estrogen enhanced RARa lpha gene expression not only in established ER-positive cell lines bu t also in ER-transfected MDA-MB-231 cells. The estrogen effect appears to be direct and at the gene transcription level since it did not alt er the stability of RARalpha mRNA and cycloheximide failed to block es trogen-mediated enhancement of RARalpha gene expression. Our data stro ngly suggest that ER-mediated enhancement of RARalpha levels plays an important role in RA inhibition of HBC growth. In addition, we also re port here that HBC cells appear to express a unique isoform(s) of RARa lpha which was detected only when the full-length RARalpha cDNA was us ed as a probe; the RARalpha1 and RARalpha2 specific probes failed to h ybridize with the HBC specific RARalpha message.