Regulation of the human retinoic acid receptor alpha gene in the estrogen receptor negative human breast carcinoma cell lines SKBR-3 and MDA-MB-435.

Estradiol-mediated enhancement of retinoic acid receptor alpha (RARalpha) expression in the estrogen receptor (ER)-positive human breast carcinoma (HBC) cells results in their sensitivity to RA-mediated growth inhibition (A. K. Rishi et al., Cancer Res., 55: 4999-5006, 1995). Most ER-negative HBCs are known to express lower levels of RARalpha and are resistant to RA-mediated inhibition of growth. We show that ER-negative SKBR-3 and MDA-MB-435 HBCs express approximately 2-fold higher levels of RARalpha isoform 1 mRNA when compared to the ER-negative MDA-MB-231 and MDA-MB-468 HBCs. SKBR-3 cells are sensitive to growth inhibition by RA, and by using RARalpha-selective synthetic retinoids, we demonstrate that the antiproliferative effects of RA in the SKBR-3 cell line are accomplished, in part, via activation of RARalpha. Both MDA-MB-231 and MDA-MB-468 HBCs are not growth inhibited by RA or any of the retinoids tested. Transient transfection experiments using a 5.0-kb RARalpha promoter fragment fused to the luciferase reporter gene showed 2-3-fold higher transcriptional activation in SKBR-3 cells when compared to MDA-MB-468 cells. We report identification of a 72-bp fragment of RARalpha promoter that contains unique cis elements responsible for mediating an estradiol-independent 2.5-fold enhancement of RARalpha gene expression in SKBR-3 and MDA-MB-435 cells.