Estrogen receptor-negative breast cancer cells transfected with the estrogen receptor exhibit increased RARα 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 RARα 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 RARα and RARE‐mediated RA‐induced CAT gene expression, but their growth was now inhibited by RA. Estrogen enhanced RARα 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 RARα mRNA and cycloheximide failed to block estrogen‐mediated enhancement of RARα gene expression. Our data strongly suggest that ER‐mediated enhancement of RARα 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 RARα which was detected only when the full‐length RARα cDNA was used as a probe; the RARα1 and RARα2 specific probes failed to hybridize with the HBC specific RARα message.