A1 functions at the mitochondria to delay endothelial apoptosis in response to tumor necrosis factor

Tumor necrosis factor (TNF) does not cause endothelial apoptosis unless the expression of cytoprotective genes is blocked. We have previously demonstr ated that one of the TNF-inducible cytoprotective genes is the Bcl-2 family member, Al. Al is induced by the action of the transcription factor, NF ka ppa B, in response to inflammatory mediators. In this report we demonstrate that, as with other cell types, inhibition of NF kappa B initiates microva scular endothelial apoptosis in response to TNF. Al is able to inhibit this apoptosis over 24 h, We demonstrate that Al is localized to and functions at the mitochondria. Whereas Al is able to inhibit mitochondrial depolariza tion, loss of cytochrome c, cleavage of caspase 9, BID, and poly(ADP-ribose ) polymerase, it does not block caspase 8 or caspase 3 cleavage. In contras t, Al is not able to prevent endothelial apoptosis by TNF over 72 h, when N F kappa B signaling is blocked. On the other hand, the caspase inhibitor, b enzyloxycarbonyl-VAD-formylmethyl ketone, completely blocks TNF-induced end othelial apoptosis over 72 h. Our findings indicate that Al is able to main tain temporary survival of endothelial cells in response to TNF by maintain ing mitochondrial viability and function. However, a mitochondria-independe nt caspase pathway eventually results in endothelial death despite mitochon drial protection by A1.