IN-VIVO AND IN-VITRO MODULATORY EFFECT OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 (HIV-1) TAT PROTEIN ON PROTEIN-KINASE-C ACTIVITY

Extracellular HIV-1 Tat protein shows a pleiotropic activity on the su rvival/proliferation of different cell types, which may be relevant to the pathogenesis of the immune suppression as well as of the frequent neoplastic disorders observed during the course of HIV-1 disease. The refore, we investigated the effect of recombinant Tat on the protein k inase C (PKC) activity in Jurkat CD4(+) T lymphoma cells by using a se rine substituted specific PKC peptide substrate, which allowed the eva luation of the whole catalytic activity of both Ca++-dependent and Ca+-independent PKC isoforms. High concentrations of recombinant Tat (1 mu g/ml) induced an early (5 min) stimulation followed by a secondary (30-60 min) inhibition of PKC in whole Jurkat cell homogenates. Immuno -localization experiments showed that recombinant Tat protein was rapi dly taken up by Jurkat cells within the first 5 min from the addition in culture, thus suggesting the possibility that the secondary inhibit ory phase of Tat on PKC activity in Jurkat cells could be due to a dir ect interaction between the two proteins. Consistently, PKC immunoprec ipitated from Jurkat cells or purified from rat brain was significantl y inhibited by the addition of high (0.1-1 mu g) but not low (1-10 ng) doses of Tat in a cell-free in vitro assay. The inhibition of PKC cat alytic activity mediated by 1 mu g of Tat was at least partially due t o competition among substrates. The present data may help in understan ding the opposite effects on the survival/proliferation of different c ell types observed in the presence of picomolar (stimulation) vs nanom olar (inhibition) concentrations of recombinant Tat.