Nitric oxide inhibits HIV Tat-induced NF-κB activation

To evaluate the roles of nitric oxide (NO) on human immunodeficiency virus (HIV) Tat-induced transactivation of HIV long terminal repeat (HIV- LTR), we examined the effect of NO in the regulation of nuclear factor (NF)- κB, a key transcription factor involved in HIV gene expression and viral replication. In the present study, we demonstrate that HIV Tat activates NF- κB and that this activation can be attenuated by endogenous or exogenous NO. Inhibition of endogenous NO production with the NO synthase (NOS) inhibitor L-NMMA causes a significant increase in Tat-induced NF-κB activity. In addition, NO attenuates signal-initiated degradation of IκBα, an intracellular inhibitor of NF-κB, and blocks the DNA binding activity of the NF-κB p50/p50 homodimer and p50/p65 heterodimer. To determine how NO is induced by HIV Tat, reverse transcription polymerase chain reaction was used to demonstrate the induction of NOS-2 and NOS-3 mRNA by Tat. Although a putative NF-κB binding site was identified in the -74 GGAGAGCCCCC -64 region of the NOS-3 gene promoter, gel mobility shift assays and site-directed mutation analyses suggest that the putative NF-κB site is not of primary importance. Rather, several Sp-1 sites adjoining the putative NF-κB binding site in the promoter region of NOS-3 gene are required for the induction of NOS-3 gene expression by Tat.