T cell activation-induced and HIV tat-enhanced CD95(APO-1/Fas) ligand transcription involves NF-kappaB.

CD95(APO-1 / Fas) ligand (CD95L) gene expression is critically involved in activation-induced T cell apoptosis. We and other have previously shown that HIV-1 Tat which is essential for efficient HIV gene expression sensitizes CD95-mediated apoptosis and up-regulates CD95L expression in T cells. In the present study we have investigated the regulatory mechanism for CD95L expression. Two NF-kappaB binding sites are localized at - 537 to - 521 and - 57 to - 47 (relative to the transcription start site) of the human CD95L promoter. We show that both elements bind to NF-kappaB and SP-1 transcription factors and NF-kappaB is involved in transactivation of the CD95L promoter upon T cell activation. Mutations at each NF-kappaB site by two base pair substitutions resulted in 30 - 70 % reduction of the promoter activity. The effect of Tat on the human CD95L promoter activity was mapped to the same sites. Mutation of each NF-kappaB site also impaired the effect of Tat on CD95L promotor activity. We also show that ectopic expression of Tat protein in Jurkat T cells greatly increases NF-kappaB binding to its target DNA. Our studies provide evidence that Tat-enhanced CD95L expression is regulated at least in part by the NF-kappaB sites of the promoter.