The human immunodeficiency virus type 1 (HIV-1) Tat is a virally encoded protein that dramatically up-regulates viral replication through interactions with the HIV-1 5' long terminal repeat (LTR) and cellular transcription factors. The HIV-1 LTR is divided into three major regions: modulatory, core and TAR. The modulatory region contains numerous cis-acting sequences for the binding of transcription factors including NF-kappaB, NF-AT, and AP-1. In several reports, Tat has been found to induce NF-kappaB activation of the HIV-1 LTR, while in other studies Tat has been reported to have no effect on activation of NF-kappaB. These discrepancies may arise from differences in experimental conditions such as the source of Tat (exogenous versus endogenous), the detection methods for NF-kappaB activation (DNA binding capability versus IkappaB degradation), and the types of reporters used (HIV-1 versus non-HIV-1 derived). To reconcile these differences we examined the effect of endogenous Tat on NF-kappaB activation, on IkappaB degradation and its interaction with upstream MAP3Ks. We demonstrate that although an 80% reduction in Tat-induced HIV-1 LTR activity can be detected if the kappaB binding sites are mutated, surprisingly endogenous Tat (expressed intracellularly by transfection) lacks direct effect on IkappaB degradation. Further analysis demonstrates that although Tat alone lacks direct effect on IkappaBalpha degradation or dissociation from NF-kappaB, Tat can substantially enhance the capacity of NF-kappaB-inducing kinase (NIK), but not MEKK1, to accelerate degradation of IkappaB. We propose a model to explain these collective experimental findings.
Copyright 2001 Academic Press.