Retinoic acid inhibition of cell cycle progression in MCF-7 human breast cancer cells

Cell cycle analysis indicates that retinoic acid (RA) inhibition of MCF- 7 cell growth occurs through induction of G1 arrest with a concomitant 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 after 24 h, RA also inhibited cyclin B1 and CDC2 expression, possibly responsible for the reduction of the proportion of cells in G2 + M and S phases. RA did not induce p16 and p27 expression, but obviously reduced p21 level in MCF-7 cells. The retinoid markedly reduced pRB protein level and abrogated pRB phosphorylation after 48 h; it also reduced transcription factor E2F1 expression at both the mRNA and protein levels. E2F1 promoter activity was reduced by 60%, which is probably responsible, at least in part, for the reduction of E2F1 expression in RAtreared MCF-7 cells. These observations demonstrate a marked effect of RA on some of the key cell cycle regulatory proteins in MCF-7 cells. Cyclin D3 and CDK4 are likely the early targets of RA, followed by reduced 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 cell growth, which begins at Day 3 of RA treatment.