INHIBITION OF RETINOIC ACID RECEPTOR FUNCTION IN NORMAL HUMAN MAMMARYEPITHELIAL-CELLS RESULTS IN INCREASED CELLULAR PROLIFERATION AND INHIBITS THE FORMATION OF A POLARIZED EPITHELIUM IN-VITRO

The expression of retinoic acid receptor-beta (RAR beta) mRNA is absen t or down-regulated in a majority of breast cancers, suggesting that l oss of retinoic acid receptor function may be a critical event in brea st cancer carcinogenesis. We developed an in vitro system to investiga te whether the loss of retinoic acid receptor (RAR) function might aff ect the proliferation and structural differentiation of normal culture d human mammary epithelial cells (HMECs). Utilizing a truncated retino ic acid receptor (RAR)-alpha construct exhibiting dominant-negative ac tivity against retinoic acid receptor isoforms alpha, beta, and gamma (DNRAR), we inhibited normal retinoic acid receptor function in HMECs. Suppression of RAR function in HMECs resulted in reduced growth inhib ition mediated by all-trans-retinoic acid (ATRA). Moreover, the doubli ng time of HMECs expressing the DNRAR was significantly shortened, ass ociated with a decrease in the percentage of cells in G(1) and an incr ease in the percentage of cells in S-phase relative to controls. In ad dition, HMECs expressing the DNRAR cultured in prepared extracellular matrix exhibited a loss of extracellular matrix-induced growth arrest and formation of a polarized ductal epthelium. Our results suggest tha t ATRA and RARs may play an important role in regulating the prolifera tion of HMECs and in promoting differentiation. (C) 1997 Academic Pres s.