RETINOIC ACID INHIBITION OF CELL-CYCLE PROGRESSION IN MCF-7 HUMAN BREAST-CANCER CELLS

Cell cycle analysis indicates that retinoic acid (RA) inhibition of MC F-7 cell growth occurs through induction of G1 arrest with a concomita nt reduction in the proportion of cells in S and G2 + M phases. RA did not affect cyclins D1, A, and E and cyclin-dependent kinase 2 (CDK2) expression, but significantly reduced cyclin D3 and CDK4 expression af ter 24 h. RA also inhibited cyclin B1 and CDC2 expression, possibly re sponsible for the reduction of the proportion of cells in G2 + M and S phases, RA did not induce p16 and p27 expression, but obviously reduc ed p21 level in MCF-7 cells, The retinoid markedly reduced pRB protein level and abrogated pRB phosphorylation after 48 h; it also reduced t ranscription factor E2F1 expression at both the mRNA and protein level s. E2F1 promoter activity was reduced by 60%, which is probably respon sible, at least in part, for the reduction of E2F1 expression in RA-tr eated MCF-7 cells. These observations demonstrate a marked effect of R A on some of the key cell cycle regulatory proteins in MCF-7 cells. Cy clin D3 and CDK4 are likely the early targets of RA, followed by reduc ed pRB expression and phosphorylation, as well as by the inhibition of the E2F1 transcription factor which controls progression from G1 to S phase. Most of these events precede the observed reduction in MCF-7 c ell growth, which begins at Day 3 of RA treatment. (C) 1997 Academic P ress.