Expression and Regulation of Retinoic Acid Receptors in Human Breast Cancer Cells1

Retinoic acid is known to inhibit mammary carcinogenesis in rodents and to inhibit proliferation and steroid hormone receptor gene expression in human breast cancer cells. Since these effects are likely to be mediated by nuclear retinoic acid receptors (RARs) the present study was initiated to determine the expression and regulation of RARs in human breast cancer cell lines. Differential cellular gene expression of the RARs was determined by Northern blot analysis of total RNA prepared from S IK* and 6 ER" cell lines. RARa was detected as mRNA species of 2.7 and 3.4 kilobases in all cell lines and the level of gene expression was greater in ER+ cell lines (P < 0.001). RAR0 mRNA (3.7 kilobases) was detected in seven of the eleven lines tested and was expressed most commonly in IK cell lines. RAR-ymRNA was expressed in all cell lines as a transcript of 3.4 kilobases at levels that were similar in both IK* and IK cell lines. Retinoic acid failed to regulate the expression of the RARa and RAR7 genes. The effect of steroid hormones on RARa and RAR-ymRNA levels was also examined. In four PR* cell lines (T-47D, BT 474, MCF7M, and MDA-MB-361), progestins markedly decreased RARa mRNA levels. The progestin effect on RARa levels in T-47D cells was detectable at concentrations of 0.05 UMand was maximal at 1 MM16a-ethyl-21hydroxy-19-nor-4-pregnene-3,20-dione ORG 2058, whereas dihydrotestosterone and dexamethasonc were without effect. RARa and RAR> mRNA levels were rapidly decreased by progestin, and the effect was maximal 3-6 h after ORG 2058 treatment. However, the mRNA loss was transient, and recovery of RARa and K \ K> mRNA levels was noted 12-24 h after retinoic acid treatment. Although R\Ry mRNA returned to control levels by 24 h, the decrease in RARa mRNA was maintained at around 50% control until at least 48 h. In summary, RARa and RAR-y were expressed in all human breast cancer cell lines and were regulated by progestins in the PR+ T-47D cell line. The previously reported ability of retinoic acid to down-regulate PR mRNA and the present demonstra tion that progestins down-regulate RARa and RAR-ymRNA suggest that mutual regulation may be a mechanism through which PR and the RARs interact in human breast cancer cells.