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) selectively inhibits the growth of estrogen receptor (ER)-positive human breast carcinoma (HBC) cells and ER-negative cells are refractory to RA inhibition of growth. The ER-negative cells inherently express lower levels of RARalpha and retinoic acid response element (RARE)-mediated RA-induced CAT activity. In this study we report that when ER-negative MDA-MB-231 cells were transfected with the ER gene they not only expressed higher levels of RARalpha and RARE-mediated RA-induced CAT gene expression, but their growth was now inhibited by RA. Estrogen enhanced RARalpha gene expression not only in established ER-positive cell lines but 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 alter the stability of RARalpha mRNA and cycloheximide failed to block estrogen-mediated enhancement of RARalpha gene expression. Our data strongly suggest that ER-mediated enhancement of RARalpha levels plays an important role in RA inhibition of HBC growth. In addition, we also report here that HBC cells appear to express a unique isoform(s) of RARalpha which was detected only when the full-length RARalpha cDNA was used as a probe; the RARalpha1 and RARalpha2 specific probes failed to hybridize with the HBC specific RARalpha message.