Posttranslational Modification of Bcl-2 Facilitates Its Proteasome-Dependent Degradation: Molecular Characterization of the Involved Signaling Pathway

The ratio of proapoptotic versus antiapoptotic Bcl-2 members is a critical determinant that plays a significant role in altering susceptibility to apoptosis. Therefore, a reduction of antiapoptotic protein levels in response to proximal signal transduction events may switch on the apoptotic pathway. In endothelial cells, tumor necrosis factor alpha (TNF-α) induces dephosphorylation and subsequent ubiquitin-dependent degradation of the antiapoptotic protein Bcl-2. Here, we investigate the role of different putative phosphorylation sites to facilitate Bcl-2 degradation. Mutation of the consensus protein kinase B/Akt site or of potential protein kinase C or cyclic AMP-dependent protein kinase sites does not affect Bcl-2 stability. In contrast, inactivation of the three consensus mitogen-activated protein (MAP) kinase sites leads to a Bcl-2 protein that is ubiquitinated and subsequently degraded by the 26S proteasome. Inactivation of these sites within Bcl-2 revealed that dephosphorylation of Ser87 appears to play a major role. A Ser-to-Ala substitution at this position results in 50% degradation, whereas replacement of Thr74 with Ala leads to 25% degradation, as assessed by pulse-chase studies. We further demonstrated that incubation with TNF-α induces dephosphorylation of Ser87 of Bcl-2 in intact cells. Furthermore, MAP kinase triggers phosphorylation of Bcl-2, whereas a reduction in Bcl-2 phosphorylation was observed in the presence of MAP kinase-specific phosphatases or the MAP kinase-specific inhibitor PD98059. Moreover, we show that oxidative stress mediates TNF-α-stimulated proteolytic degradation of Bcl-2 by reducing MAP kinase activity. Taken together, these results demonstrate a direct protective role for Bcl-2 phosphorylation by MAP kinase against apoptotic challenges to endothelial cells and other cells.