HIV-1 Tat protein down-regulates CREB transcription factor expression in PC12 neuronal cells through a phosphatidylinositol 3-kinase/AKT/cyclic nucleoside phosphodiesterase pathway

The addition of low concentrations (0.1-1 nM) of extracellular HIV-1 Tat pr otein to PC12 neuronal cells stimulated a rapid (peak at 5 min) elevation o f the cAMP intracellular levels, which in turn induced the phosphorylation of CREB transcription factor (peak at 15 min) on serine-133 (Ser-133). On t he contrary, at later time points (60-120 min) Tat induced a significant de cline of intracellular cAMP with respect to the basal levels observed in co ntrol cells treated with bovine serum albumin. In blocking experiments perf ormed with pharmacological inhibitors, Tat decreased the intracellular leve ls of cAMP and CREB Ser-133 phosphorylation through a signal transduction p athway involving the sequential activation of phosphatidylinositol 3-kinase , AKT, and cyclic nucleoside phosphodiesterases. Moreover, in transient tra nsfection experiments, Tat inhibited transcription of CREB promoter in a ma nner strictly dependent on the presence of the cAMP-responsive elements (CR E) in the CREB promoter. Consistently, the expression of endogenous CREB pr otein was significantly reduced in PC12 cells by prolonged (24-48 h) treatm ent with Tat. This decline in the expression of CREB, which plays an essent ial role in the survival and function of neuronal cells, anticipated a prog ressive increase of apoptosis in Tat-treated cells. Although obtained in a neuronal cell Line, our findings might help to explain some aspects of the pathogenesis of HIV-l-associated dementia.