The biological basis for the pleiotropic activity of extracellular human immunodeficiency virus (HIV)-1 Tat protein on lymphoid T cell survival is not well understood. We have here demonstrated that the addition in culture of 0.1-10 nM Tat protein to 36-h serum-starved lymphoblastoid Jurkat T cells rapidly stimulates the catalytic activity of phosphatidylinositol 3-kinase (PI 3-K). The peak of activation was observed 30 min after Tat addition. Extracellular Tat also stimulated the catalytic activity of the Akt/PKB kinase, a major target of PI 3-K lipid products. Pretreatment of serum-starved Jurkat cells with 100 nM wortmannin (WT) or 10 microM LY294002, two unrelated pharmacological inhibitors of PI 3-K, markedly suppressed the catalytic activity of both PI 3-K and Akt/PKB in Jurkat cells. Moreover, at low concentrations (0.1-1 nM), extracellular Tat showed a small but reproducible protection of Jurkat cells from apoptosis induced by serum deprivation (p < 0.05), while the combination of Tat plus 100 nM WT significantly (p < 0.05) increased the percentage of apoptosis with respect to cells left untreated or treated with Tat alone. Taken together, these data suggest that the anti-apoptotic activity of low concentrations of Tat protein on Jurkat cells is mediated by a PI 3-kinase/Akt pathway.