The retinoic acid receptor antagonist, BMS453, inhibits normal breast cellgrowth by inducing active TGF beta and causing cell cycle arrest

We have previously shown that a retinoic acid receptor (RAR) antagonist BMS 453, which does not activate RAR-dependent gene transcription in breast cel ls, inhibits normal breast cell growth. In this study we have investigated the mechanisms by which this retinoid receptor antagonist inhibits cell gro wth. Both all traits retinoic acid (atRA) and BMS453 inhibited the prolifer ation of normal breast cell growth without significantly inducing apoptosis . Both retinoids caused a GI block in the cell cycle with an increase in th e proportion of cells in G0/G1 and a decrease in the proportion of cells in S phase. We then investigated the effects of the retinoids on molecules th at regulate the G1 to S transition. These studies demonstrated that both at RA and BMS453 induce Rb hypophosphorylation and decrease CDK2 kinase activi ty. We then studied the effect of the retinoids on the expression of CDK in hibitors. atRA and BMS453 increased total p21 protein levels and CDK2-bound p21 protein, but did not change CDK4-bound p21. These results suggest that atRA and BMS453 increase p21, decrease CDK2 kinase activity, which in turn leads to hypophosphorylation of Rb and GI arrest. Because transforming gro wth factor beta (TGF beta) has been proposed as a mediator of retinoid-indu ced growth inhibition, we next investigated whether TGF beta mediates the a nti-proliferative effect of atRA and BMS453 in normal breast cells. These s tudies showed that atRA and BMS453 increased total TGF beta activity by 3-5 -fold. However, BMS453 increased active TGF beta activity by 33-fold while atRA increased active TGF beta activity by only threefold. These results su ggest that BMS453 treatment induces conversion of latent TGF beta to active TGF beta. To investigate whether this increase in active TGF beta mediates the anti-proliferative effects of these retinolds, a TGF beta -blocking an tibody was used in an attempt to prevent retinoid-induced growth inhibition . Results from these experiments showed that the anti-TGF beta antibody pre vented the inhibition of cell proliferation induced by BMS453, but did not prevent the inhibition of cell proliferation induced by atRA. 2001 These re sults demonstrate that BMS453 inhibits breast cell growth predominantly thr ough the induction of active TGF beta, while atRA inhibits growth through o ther mechanisms. These results suggest that retinoid analogs that increase active TGF beta may be promising agents for the prevention of breast cancer .