Growth of human breast adenocarcinoma MCF-7 cells as a tumor on nude mice is dependent on estrogen. It has been shown that estrogen withdrawal (EW) induces a partial regression of the tumor via an inhibition of cell proliferation and an induction of apoptosis. We investigated in this in vivo model the underlying molecular mechanisms of the hormone-dependent regulation of cell cycle machinery and apoptosis. We found that, 2 days after EW, the tumor protein levels of p21 rose, whereas those of Rb proteins decreased in parallel with the decrease in the proportion of tumor cells in S phase and the increase of the tumor apoptotic index. Between 3 and 7 days after EW, apoptosis was inhibited and tumor proliferation returned to the control value. There was a concomitant decline in p21 and an elevation of Rb tumor protein content. Slight variations of cyclin D protein level were observed in MCF-7 tumors over the time course following EW treatment. Bcl-2 overexpression not only inhibited apoptosis induced by EW but also modulated hormone-dependent cell cycle regulation. First, the analysis of phosphorylation status of Rb protein and the measurement of the proportion of tumor cells in S phase indicated that Bcl-2 overexpression results in a decrease of DNA synthesis induced by estradiol. Furthermore, after EW, Bcl-2-induced inhibition of hormone-dependent apoptosis was associated with an inhibition of Rb protein downregulation, a sustained level of p21 protein, and a prolonged inhibition of cell cycle progression. These results suggest that, in human hormone-dependent breast cancers, cross-talk exists between the signaling pathways which lead to regulation of cell cycle progression and apoptosis.
Copyright 2000 Academic Press.