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 (RAR al pha) expression in the estrogen receptor (ER)-positive human breast ca rcinoma (HBC) cells results in their sensitivity to RA-mediated growth inhibition (A. K. Rishi et al., Cancer Res., 55: 4999-5006, 1995). Mo st ER-negative HBCs are known to express lower levels of RAR alpha and are resistant to RA-mediated inhibition of growth. We show that ER-ne gative SKBR-3 and MDA-MB-435 HBCs express approximately 2-fold higher levels of RAR alpha isoform 1 mRNA when compared to the ER-negative MD A-MB-231 and MDA-MB-468 HBCs. SKBR-3 cells are sensitive to growth inh ibition by RA, and by using RAR alpha-selective synthetic retinoids, w e demonstrate that the antiproliferative effects of RA in the SKBR-3 c ell line are accomplished, in part, via activation of RAR alpha. Both MDA-MB-231 and MDA-MB-468 HBCs are not growth inhibited by RA or any o f the retinoids tested. Transient transfection experiments using a 5.0 -kb RAR alpha 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 fra gment of RAR alpha promoter that contains unique cis elements responsi ble for mediating an estradiol-independent 2.5-fold enhancement of RAR alpha gene expression in SKBR-3 and MDA-MB-435 cells.