Overexpression of the Bcl-2 Protein Increases the Half-life of p21

Bcl-2 and Bax are homologous proteins which can heterodimerize with each other. These proteins have opposing effects on cell survival when overexpressed in cells, with Bcl-2 blocking and Bax promoting apoptosis. Here we demonstrate that gene transfer-mediated elevations in Bcl-2 protein levels result in a marked increase in the steady-state levels of endogenous p21 protein as determined by immunoblotting in the Jurkat T-cell and 697 pre-B-cell leukemia cell lines, but not in several other cell lines including CEM T-cell leukemia, 32D.3 myeloid progenitor, PC12 pheochromocytoma, and NIH-3T3 fibroblasts. Steady-state levels of p21 protein were also elevated in the lymph nodes of Bcl-2 transgenic mice in which a BCL-2 transgene is expressed at high levels in B-cells. Northern blot analysis of BCL-2 -transfected and control-transfected Jurkat and 697 leukemia cells revealed no Bcl-2-induced alterations in the steady-state levels of BAX mRNAs. In contrast, L-[ S]methionine pulse-chase analysis indicated a marked increase in the half-life ( t ) of the p21 protein in BCL-2 -transfected 697 cells compared to control-transfected cells ( t >24 h versus 4 h), whereas the rate of Bax degradation was unaltered in Bcl-2-transfected CEM cells. The results demonstrate that levels of the proapoptotic p21 protein can be post-translationally regulated by Bcl-2, probably in a tissue-specific fashion, and suggest the existence of a feedback mechanism that may help to maintain the ratio of Bcl-2 to Bax protein in physiologically appropriate ranges.