Tumor necrosis factor alpha (TNF-␣) is a candidate human immunodeficiency virus type 1-induced neurotoxin that contributes to the pathogenesis of AIDS dementia complex. We report here on the effects of exogenous TNF-␣ on SK-N-MC human neuroblastoma cells differentiated to a neuronal phenotype with retinoic acid. TNF-␣ caused a dose-dependent loss of viability and a corresponding increase in apoptosis in differentiated SK-N-MC cells but not in undifferentiated cultures. Importantly, intracellular signalling via TNF receptors, as measured by activation of the transcription factor NF-B, was unaltered by retinoic acid treatment. Finally, overexpression of bcl-2 or crmA conferred resistance to apoptosis mediated by TNF-␣, as did the addition of the antioxidant N-acetylcysteine. These results suggest that TNF-␣ induces apoptosis in neuronal cells by a pathway that involves formation of reactive oxygen intermediates and which can be blocked by specific genetic interventions.

Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system may result in neuronal apoptosis in vulnerable brain regions, including cerebral cortex and basal ganglia. The mechanisms for neuronal loss are likely to be multifactorial and indirect, since HIV-1 productively infects brain-resident macrophages and microglia but does not cause cytolytic infection of neurons in the central nervous system. HIV-1 infection of macrophages and microglia leads to production and release of diffusible factors that result in neuronal cell death, including the HIV-1 regulatory protein Tat. We demonstrate in this report that recombinant Tat 1-86 and Tat peptides containing the basic region induce neuronal apoptosis in approximately 50% of vulnerable neurons in both rat and human neuronal cultures, and this apoptotic cell death is mediated by release of the pro-inflammatory cytokine tumor necrosis factor ␣, and by activation of glutamate receptors of the non-N-methyl-D-aspartate subtype. Finally, we show that Tat-induced apoptosis of human neuronal cell cultures occurs in the absence of activation of the transcription factor NFB. These findings further define cellular pathways activated by Tat, that dysregulate production of tumor necrosis factor ␣, and lead to activation of glutamate receptors and neuronal death during HIV-1 infection of the central nervous system. It has been proposed that the mechanisms by which human immunodeficiency virus type 1 (HIV-1) 1 induces widespread neuronal dysfunction and death in the developing and adult central nervous system may involve activation of glutamate receptor subtypes (1, 2). However, the sequence of events that lead to glutamate receptor activation in the setting of HIV-1 infection and low-level chronic inflammation in the central nervous system are complex and poorly understood. Many previous studies on the neuropathogenesis of HIV-1 infection have focused on the neurotoxic effects of the HIV-1 envelope protein, gp120 (3, 4). These studies have revealed that gp120 causes neuronal toxicity in large part via indirect mechanism(s), possibly mediated by intermediary glial cells that in turn produce cellular metabolites that ultimately lead to excitotoxic activation of glutamate receptors. In contrast, the HIV-1 regulatory protein Tat is soluble, secreted, and efficiently taken up by many cell types, including astrocytes and neurons (5, 6). Several studies have implicated both full-length Tat and basic Tat peptides (i.e. Tat amino acid residues 31 to 61 (7)) as mediators of neuronal death. In vitro studies have demonstrated that Tat-induced neuronal cell death occurs via apoptosis (8), and involves activation of a non-NMDA subtype of glutamate receptors (9). In addition, in vivo experiments have revealed that Tat-mediated neurotoxicity can be prevented by pentoxifylline, an agent that blocks the transcription of the pro-inflammatory cytokine tumor necrosis factor ␣ (TNF␣) (10).Previous studies from this laboratory have shown that TNF␣-induced neurotoxicity can be pr...

Neuronal loss in HIV encephalopathy remains a mystery since HIV-1 productively infects macrophage and microglia and only rarely infects neurons in the central nervous system. Apoptosis is a mechanism which may account for the loss of neurons in HIV-1 infected brain. Putative toxic factors that result in neuronal cell death in HIV-1 infection include the regulatory protein Tat, since this protein is known to be released from HIV-1 infected cells. Here we show that Tat induces cell death by apoptosis in cultured human fetal neuronsproducing characteristic morphological and biochemical features associated with apoptosis. These findings suggest that Tat may play an important role as a secreted, soluble neurotoxin in HIV-1 associated dementia.

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