p38 MAPK mediates TNF-induced apoptosis in endothelial cells via phosphorylation and downregulation of Bcl-xL

The role of p38 mitogen-activated protein kinase (MAPK) in apoptosis is a matter of debate. Here, we investigated the involvement of p38 MAPK in endothelial apoptosis induced by tumor necrosis factor a (TNF). We found that activation of p38 MAPK preceded activation of caspase-3, and the early phase of p38 MAPK stimulation did not depend on caspase activity, as shown by pretreatment with the caspase inhibitors z-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD-fmk) and Boc-Asp(OMe)-fluoromethylketone (BAF). The p38 MAPK inhibitor SB203580 significantly attenuated TNF-induced apoptosis in endothelial cells, suggesting that p38 MAPK is essential for apoptotic signaling. Furthermore, we observed a time-dependent increase in active p38 MAPK in the mitochondrial subfraction of cells exposed to TNF. Notably, the level of Bcl-xL protein was reduced in cells undergoing TNF-induced apoptosis, and this reduction was prevented by treatment with SB203580. Immunoprecipitation experiments revealed p38 MAPK-dependent serine - threonine phosphorylation of Bcl-xL in TNF-treated cells. Exposure to lactacystin prevented both the downregulation of Bcl-xL and activation of caspase-3. Taken together, our results suggest that TNF-induced p38 MAPK-mediated phosphorylation of Bcl-xL in endothelial cells leads to degradation of Bcl-xL in proteasomes and subsequent induction of apoptosis.