Bcl-2 inhibition of T-cell proliferation is related to prolonged T-cell survival.

Bcl-2 promotes oncogenesis by inhibiting cell death. Bcl-2 also inhibits proliferation and suppresses tumorigenesis in some settings. To clarify the role of the antiproliferative function of Bcl-2, mice expressing a mutant form of Bcl-2 reported to lack antiproliferative activity were generated (tyrosine 28 to alanine, Bcl-2-Y28A). As expected, both wild type (WT) and Bcl-2-Y28A inhibited apoptosis similarly. In contrast to previous results in cell lines, Bcl-2-Y28A inhibited T-cell proliferation identical to WT-Bcl-2. Significantly, both Bcl-2-Y28A and WT-Bcl-2 inhibited proliferation of T cells isolated from older animals, but not proliferation of T cells from immature mice. Instead, inhibition of cell activation correlated with T-cell size, p27 levels, and RNA content, all indicators of quiescent G0 arrest. Consistent with this model, Bcl-2 inhibition of T-cell proliferation was reversed by expression of Bax, again correlating cell proliferation with cell size. These experiments do not support genetically separate effects of Bcl-2 on apoptosis and proliferation. Instead, the data support a model in which Bcl-2 and Bax regulate T-cell proliferation by changes in T-cell size and by increasing the markers of quiescent G0 arrest. These changes likely result from prolonged T-cell survival.

Bcl-xL/Bcl-2 coordinately regulates apoptosis, cell cycle arrest and cell cycle entry.

Bcl-x(L) and Bcl-2 inhibit both apoptosis and proliferation. In investigating the relationship between these two functions of Bcl-x(L) and Bcl-2, an analysis of 24 Bcl-x(L) and Bcl-2 mutant alleles, including substitutions at residue Y28 previously reported to selectively abolish the cell cycle activity, showed that cell cycle delay and anti-apoptosis co-segregated in all cases. In determining whether Bcl-2 and Bcl-x(L) act in G(0) or G(1), forward scatter and pyronin Y fluorescence measurements indicated that Bcl-2 and Bcl-x(L) cells arrested more effectively in G(0) than controls, and were delayed in G(0)-G(1) transition. The cell cycle effects of Bcl-2 and Bcl-x(L) were reversed by Bad, a molecule that counters the survival function of Bcl-2 and Bcl-x(L). When control and Bcl-x(L) cells of equivalent size and pyronin Y fluorescence were compared, the kinetics of cell cycle entry were similar, demonstrating that the ability of Bcl-x(L) and Bcl-2 cells to enhance G(0) arrest contributes significantly to cell cycle delay. Our data suggest that cell cycle effects and increased survival both result from intrinsic functions of Bcl-2 and Bcl-x(L).