Objective: To investigate the intracellular signals elicited by extracellular HIV-1 Tat protein in lymphoid CD4 T cells.
Methods: CD4 Jurkat T cells were treated with a series of glutathione S-transferase (GST)-Tat fusion proteins: full-length two-exon GST-Tat (GST-Tat2E); one-exon Tat, in which the second exon of Tat was deleted (GST-Tat1E); two-exon Tat, in which the seven arginine residues have been changed to alanine residues (GST-TatArgmut), GST-TatΔN, which shows a deletion of the N-terminal 21 amino acids. The cells were either treated with soluble GST-Tat proteins or seeded on plates coated with GST-Tat proteins immobilized on plastic. At various time points, Jurkat cells were lysed and examined for c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) activity.
Results: Soluble and immobilized GST-Tat2E, but not GST-Tat1E, GST-TatArgmut and GST-TatΔN, activated JNK in a dose-dependent manner, induced a rapid phosphorylation of c-Jun on Ser63 and promoted the de novo synthesis of c-Jun protein. Moreover, both GST-Tat2E and GST-Tat1E also stimulated ERK/MAPK. However, the activation of JNK was maximal at concentrations of 100nM of GST-Tat2E and was blocked by the S6-kinase inhibitor rapamycin, whereas the activation of ERK/MAPK was already maximal at 1nM of GST-Tat2E and was enhanced by rapamycin.
Conclusions: Tat-mediated activation of JNK requires the second exon of Tat, which is dispensable for the activation of ERK/MAPK. The ability to stimulate JNK and ERK/MAPK does not require Tat internalization.