ESTRADIOL INDUCTION OF RETINOIC ACID RECEPTORS IN HUMAN BREAST-CANCERCELLS

Retinoic acid inhibits proliferation and steroid receptor gene express ion in human breast cancer cell lines. Retinoic acid receptors (RAR)al pha, -beta, and -gamma are expressed in these cells and the expression of RARalpha is significantly greater in estrogen receptor (ER)-positi ve cells. This study was undertaken to determine whether the same rela tionship between RARalpha and ER gene expression was present in human breast cancers and to explore the possibility that the higher level of RARalpha in ER-positive cells was due to estrogen regulation of RARal pha gene expression. RARalpha and ER mRNA expression were determined b y Northern blot analysis in 116 primary breast tumors; 94 (81%) tumors were ER-positive and of these 87 (93%) were also RARa-positive. The c oexpression of ER and RARalpha was statistically significant (P = 0.00 52 by chi2 contingency analysis). There was also a positive correlatio n (by linear regression analysis) between the levels of expression of ER and RARalpha mRNA (r2 = 0.251, P = 0.0001), which confirmed the rel ationship previously documented in breast cancer cell lines and sugges ted that RARalpha expression may be modulated in breast cancer in vivo by estrogens acting via the ER. The ability of estradiol to regulate RARalpha gene expression was examined in vitro using T-47D cells which had been rendered sensitive to estrogen by repeated passage in steroi d-depleted medium. Estradiol increased RARalpha gene expression, but n ot that of RARbeta or RARgamma, in a concentration-dependent manner, w ith the effect being maximal at 10(-10) M and less marked at higher co ncentrations. The effect was rapid, being detectable 1 h after and max imal 6 h after treatment with 10(-10) M estradiol. Co-treatment of cel ls with estradiol and antiestrogens (tamoxifen or ICI 164384, 4 x 10(- 7) M for 6 h) inhibited the estradiol induction of RARalpha gene expre ssion, demonstrating that the effect was ER mediated. The estradiol se nsitivity of the effect was underscored by the demonstration that addi tion of untreated serum to cells growing under steroid-depleted condit ions was sufficient to induce maximal RARalpha gene expression. This e ffect was totally abolished by addition of ICI 164384. In summary, the demonstration that estradiol increased RARalpha mRNA levels in breast cancer cells supports the hypothesis that the correlation between RAR alpha and ER gene expression in breast tumors and breast cancer cell l ines is due to estradiol augmentation of RARalpha gene expression.