Mechanisms of cell cycle arrest in response to TGF-beta in progestin-dependent and -independent growth of mammary tumors

TGF-beta1 modulation of cell cycle components was assessed in an experiment al model in which the synthetic progestin medroxyprogesterone acetate (MPA) induced mammary tumors in Balb/c mice. TGF-beta1 inhibited both MPA-induce d proliferation of progestin-dependent C4HD epithelial cells and proliferat ion of the progestin-independent variant cell type C4HI, arresting cells in G, phase of the cell cycle. Progestin-independent 60 epithelial cells evid enced reduced response to TGF-beta1 antiproliferative effects. TGF-beta1 in hibition of cyclins D1 and A expression and up-regulation of p21(CIP1) leve ls were the common findings in all three cell types. In addition, a signifi cant content reduction of cyclin D1/cdk4 and cyclin A/cdk2 complexes was fo und after TGF-beta1 inhibition of MPA-dependent and -independent proliferat ion, TGF-beta1 inhibited cyclin D2 expression and up-regulated p27(KIP1) le vels only when acting as inhibitor of MPA-induced proliferation of C4HD cel ls. Regulation of these two cell cycle components resulted in decreased cyc lin D2/cdk2 complex and in increased p27(KIP1) association with cdk2 in C4H D cells treated with TGF-beta1. These two molecular mechanisms, unobserved in progestin-independent growth of C4HI or 60 cells, were associated with a significantly higher degree of inhibition of cdk2 kinase activity in C4HD cells compared to that found in TGF-P-treated C4HI or 60 cells. Reduced sen sitivity of 60 cells to the growth-inhibitory effects of TGF-beta1 correlat ed with significantly lower levels of p15(INK4B) p21(CIP1), and p27(KIP1) e xpressed in these cells, compared to the levels present in CIHD or C4HI cel ls, and correlated as well with lack of expression of p16(INK4). Thus, comm on targets were found to exist in TGF-beta1 inhibitory action on breast can cer cells, but regulation of specific targets was found when TGF-beta1-inhi bited proliferation driven by the progesterone receptor. (C) 2001 Academic Press.