HIV-1 Tat induces tyrosine phosphorylation of p125(FAK) and its association with phosphoinositide 3-kinase in PC12 cells

Objective: To evaluate the signal transduction potential of HIV-1 Tat in a neuronal cell model. Methods: The tyrosine phosphorylation levels of the focal adhesion kinase p125(FAK) and its association with phosphoinositide 3-kinase (PI 3-K) were evaluated in serum-starved rat pheochromocytoma PC12 cells, either treated with low concentrations (0.1-1 nM) of extracellular HIV-1 Tat protein or stably transfected with tat cDNA. Results: Extracellular Tat induced a rapid increase of p125(FAK) tyrosine phosphorylation and p125(FAK)-associated PI 3-K activity. By using recombinant mutated Tat proteins, it was found that deletion of amino acids 73-86 encoded by the second exon of the tat gene resulted in a significant decrease of the ability of Tat to induce p125(FAK) tyrosine phosphorylation. Paradoxically, mutations in the basic region encoded by the first exon of tat, which is essential for nuclear localization and HIV-1 LTR transactivation, increased the ability of Tat to stimulate p125(FAK) tyrosine phosphorylation. Moreover, in comparison with cells transfected with a control vector, PC12 cells stably transfected with tat cDNA showed greater amounts of p125(FAK) protein, an increase in p125(FAK) tyrosine phosphorylation and higher levels of p125(FAK)-associated PI 3-K activity. The addition of anti-Tat neutralizing antibody to rat-transfected PC12 cells in culture blocked both the p125(FAK) tyrosine phosphorylation and its association with PI 3-K but did not affect the total amount of p125(FAK). Conclusion: HIV-1 Tat protein enhanced both the expression and the functionality of p125(FAK) in PC12 neuronal cells. Whereas the first event required intracellular Tat, the increased p125(FAK) phosphorylation was strictly dependent upon extracellular Tat.