G(1) phase-dependent expression of Bcl-2 mRNA and protein correlates with chemoresistance of human cancer cells

Recent experiments suggest an interconnection between cell proliferation an d programmed cell death (apoptosis), although the detailed molecular mechan isms remain unclear. We have hypothesized that expression of some apoptosis regulators is cell cycle-dependent, which in turn influences tumor cell ch emosensitivity in a cell cycle-dependent fashion. To test these hypotheses, we synchronized human leukemia Jurkat T, Neo (using aphidicolin), breast c ancer MCF-7, normal fibroblast, and simian virus 40-transformed cells (by a phidicolin or serum starvation), and measured levels of several Bcl-2 famil y proteins. The highest expression of Bcl-2 protein was found in the G(1) p hase of all the five cell lines tested. In contrast, levels of Bax protein remained relatively unchanged in four of the cell lines, and levels of Bcl- X-L, Bcl-X-S, and Bak proteins showed little or no cell cycle-dependent cha nges in Jurkat T cells. Similar to the changes in Bcl-2 protein levels, its mRNA expression was also G(1) phase-specific, whereas the level of a Bcl-2 cleavage activity remained constitutive. When treated with an anticancer d rug (etoposide or cisplatin) or the kinase inhibitor staurosporin, the cell s containing a high G(1) population and a high Bcl-2 protein level were muc h more resistant to the induced apoptosis than the cells containing a high S phase population and a low Bcl-2 protein level. Constitutive overexpressi on of Bcl-2 protein in Jurkat T cells completely blocked the S phase-associ ated sensitivity to these apoptosis stimuli. The cell cycle-dependent Bcl-2 protein expression seems to contribute to the regulation of chemosensitivi ty and apoptotic commitment of human tumor cells.