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Neurotrophins prevent HIV Tat-induced neuronal apoptosis via a nuclear factor-kappaB (NF-kappaB)-dependent mechanism.

HIV-1 associated dementia is thought to be caused by neuronal damage and death in response to the production of soluble neurotoxic factors by virally infected mononuclear phagocytes. These neurotoxins include HIV-1 Tat. The ability of neurotrophins to promote cell survival prompted us to examine whether neurotrophins might also be capable of opposing the pro-apoptotic effects of Tat. Here, we show that Tat-induced neuronal apoptosis in primary cultures of rat cerebellar granule cells and in neuronally differentiated human SK-N-MC cells is profoundly inhibited by brain-derived neurotrophic factor, nerve growth factor and activity-dependent neurotrophic factor nonamer peptide. These neurotrophins activated the transcription factor NF-kappaB, and inhibition of NF-kappaB activation using a super-repressor IkappaB-alpha mutant was found to block the survival-promoting activity of the neurotrophins. Reporter gene assays and immunoblot experiments revealed that the neurotrophins also up-regulated the expression of Bcl-2, at both the transcriptional and protein levels. Overexpression of the super-repressor IkappaB-alpha mutant prevented this induction of Bcl-2 expression. Moreover, overexpression of either Bcl-2, alone, or the RelA subunit of NF-kappaB, alone, protected neurons from Tat-induced apoptosis. These findings suggest that the activation of NF-kappaB by neurotrophic factors may promote survival of neurons exposed to Tat, via regulation of anti-apoptotic genes including Bcl-2.