G1 phase-dependent bcl-2 expression correlates with chemoresistance of human cancer cells1

Abstract

Recent experiments suggest an interconnection between cell proliferation and programmed cell death (apoptosis) although the detailed molecular mechanisms remain unclear. We hypothesized that (i) some apoptosis regulators are periodically expressed during the cell cycle progression, and (ii) if so, cellular response to chemotherapeutic agents should be cell cycle-dependent. To test the first hypothesis, we synchronized multiple human tumor and transformed cell lines and measured protein levels of the cell death inhibitor Bcl-2 and inducer Bax in different phases of the cell cycle. In all the five tested cell lines, levels of Bcl-2 protein were very high in the G1 phase, dramatically decreased in S, G2 and M phase, and increased again in the G1 phase of the next cycle. In contrast, levels of Bax protein remained relatively unchanged in most of the cell lines although S phase-associated Bax expression was detected in human breast carcinoma MCF-7 cells. Similar to its protein levels, Bcl-2 mRNA expression was also G1 phase-specific, whereas the level of the Bcl-2 cleavage activity remained constitutive, indicating that cell cycle-dependent regulation of Bcl-2 is on its mRNA level. To test the second hypothesis, aliquots of the synchronized human tumor or transformed cells were treated with the anticancer drug etoposide or cisplatin, followed by measurement of apoptotic cell death. We demonstrated that the cells containing a high G1 population and a high Bcl-2 protein level were much more resistant to chemotherapy-induced apoptosis than the cells containing a high S phase population and a low Bcl-2 protein level. Furthermore, constitutive overexpression of Bcl-2 protein in human Jurkat T cells partially inhibited the G1 to S transition and completely blocked the S phase-associated chemosensitivity. Therefore, chemosensitivity and apoptotic commitment of human tumor cells are regulated by their cell cycle-dependent Bcl-p2 expression.