HIV-1 Tat induces microvascular endothelial apoptosis through caspase activation

HIV-1 Tat, in addition to its critical role in viral transcription, is secr eted from infected cells and can act as a proto-cytokine. We studied the ef fects of HIV-1 Tat in primary human microvascular endothelial cells of lung origin and found that it caused apoptosis. This apoptosis occurred without induction of either Fas or TNF, known mediators of programmed cell death. Tat, like Fas ligand, induced cleavage of chromatin structure, as evidenced by changes in DNA laddering, incorporation of fluorescein into the nicked chromosomal DNA (TUNEL assay), and mono- or oligonucleosomes. Furthermore, Tat treatment caused cleavage of poly(A/DP)-ribose polymerase, a substrate of caspases. Caspase-3, but not caspase-9, was activated following treatmen t of primary human microvascular endothelial cells of lung origin with eith er Tat or anti-Fas agonist Ab (anti-Fas). Inhibition of caspase-3 activity markedly reduced apoptosis. Although Fas-mediated apoptosis involved change s in Bcl-2, Bax, and Bad regulatory proteins, such alterations were not obs erved with Tat. Taken together, these data demonstrate that HIV-1 Tat is ab le to activate apoptosis in microvascular endothelium by a mechanism distin ct from TNF secretion or the Fas pathway.