This study demonstrates that human immunodeficiency virus type 1 (HIV‐1) Tat protein amplifies the activity of tumor necrosis factor (TNF), a cytokine that stimulates HIV‐1 replication through activation of NF‐kappa B. In HeLa cells stably transfected with the HIV‐1 tat gene (HeLa‐tat cells), expression of the Tat protein enhanced both TNF‐induced activation of NF‐kappa B and TNF‐mediated cytotoxicity. A similar potentiation of TNF effects was observed in Jurkat T cells and HeLa cells treated with soluble Tat protein. TNF‐mediated activation of NF‐kappa B and cytotoxicity involves the intracellular formation of reactive oxygen intermediates. Therefore, Tat‐mediated effects on the cellular redox state were analyzed. In both T cells and HeLa cells HIV‐1 Tat suppressed the expression of Mn‐dependent superoxide dismutase (Mn‐SOD), a mitochondrial enzyme that is part of the cellular defense system against oxidative stress. Thus, Mn‐SOD RNA protein levels and activity were markedly reduced in the presence of Tat. Decreased Mn‐SOD expression was associated with decreased levels of glutathione and a lower ratio of reduced:oxidized glutathione. A truncated Tat protein (Tat1‐72), known to transactivate the HIV‐1 long terminal repeat (LTR), no longer affected Mn‐SOD expression, the cellular redox state or TNF‐mediated cytotoxicity. Thus, our experiments demonstrate that the C‐terminal region of HIV‐1 Tat is required to suppress Mn‐SOD expression and to induce pro‐oxidative conditions reflected by a drop in reduced glutathione (GSH) and the GSH:oxidized GSH (GSSG) ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

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