EXOGENOUS HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 TAT PROTEIN SELECTIVELYSTIMULATES A PHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHOLIPASE-C NUCLEAR PATHWAY IN THE JURKAT T-CELL LINE

We investigated the effect of extracellular Tat protein of human immun odeficiency virus-type 1 (HIV-1) on the phosphatidylinositol (PI) cycl e, which represents a major signal transduction pathway in lymphoid ce lls. Recombinant Tat, recombinant HIV-1 p24 and cross-linked anti-CD3 monoclonal antibody (mAb) were added in culture for 1-60 min to Jurkat lymphoblastoid CD4(+) T cells. The stimulation of T cell receptor by cross-linked anti-CD3 mAb resulted in a rapid increase of the phosphat idylinositol-specific phospholipase C (PI-PLC) activity in whole cell lysates. On the other hand, Tat protein, either alone or in combinatio n with anti-CD3 mAb, showed little effect on the PI turnover of whole cell extracts. Tat, however, selectively stimulated a nuclear-specific PI-PLC with a peak of activity after 30 min from the addition in cult ure to Jurkat cells. Interestingly, this time corresponded to that req uired for the uptake and nuclear localization of recombinant Tat prote in, as demonstrated by electron microscope immunocytochemistry experim ents with anti-Tat mAb. Moreover, exogenous Tat reached the nucleus of Jurkat cells in a bioactive form, as shown in a HIV-1. long terminal repeat-chloramphenicol acetyl transferase transactivation assay. The s pecific increase of a nuclear PI-PLC activity was further demonstrated by the ability of Tat to stimulate PI turnover also when added direct ly to isolated nuclei. As a whole, these data demonstrate that Tat sel ectively stimulates a nuclear polyphosphoinositide hydrolysis, which a ppears to be independent of the cellular PI turnover. The relevance of these findings for a better understanding of the biological functions of extracellular Tat is discussed.