Expression of the Bcl-2 Homologue Mcl-1 Correlates with Survival of Peripheral Blood B Lymphocytes¹

Normal peripheral blood B lymphocytes undergo spontaneous apoptosis in vitro, and this process is regulated positively and negatively by several immunomodulatory stimuli. We have shown previously that Bcl-2 protein levels are unaltered by these factors, suggesting a Bcl-2-independent regulation of apoptosis in this system. Here, we have investigated the possibility that the three recently identified Bcl-2 homologues, Bax, Bcl-x, and Mcl-1, could be involved instead. Freshly isolated cells expressed both Bax and Mcl-1 protein, but only low levels of Bcl-x_L and no detectable Bcl-x_S, as determined by Western blot analysis. Upon culture of cells with apoptotic or survival stimuli, Bax and Bcl-x_L protein levels remained relatively unchanged. By contrast, Mcl-1 levels decreased markedly in cells undergoing apoptosis in medium and, even more dramatically, after treatment with the apoptotic stimuli transforming growth factor β1 and forskolin. This decrease was rapid and preceded cell death. Furthermore, all the survival stimuli tested (interleukin 4, anti-IgM antibodies, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate) prevented the decline in Mcl-1 levels. This striking correlation between cell survival and Mcl-1 expression in peripheral blood B cells suggests the possible involvement of Mcl-1, instead of Bcl-2, in the regulation of apoptosis in these cells. The present study is the first one linking this novel Bcl-2 homologue to the control of cell death in normal cells.