Kinetic analysis of human T-cell leukemia virus type I Tax-mediated activation of NF-kappa B.

The human T-cell leukemia virus type I (HTLV-I) Tax protein induces the expression of cellular genes, at least in part, by activating the endogenous NF-kappa B transcription factors. Induced expression of cellular genes is thought to be important for transformation of T cells to continued growth, a prelude to the establishment of adult T-cell leukemia. However, neither underlying mechanisms nor kinetics of the Tax-mediated activation of NF-kappa B are understood. We have analyzed a permanently transfected Jurkat T-cell line in which the expression of Tax is entirely dependent on addition of heavy metals. The initial NF-kappa B binding activity seen after induction of Tax is due almost exclusively to p50/p65 heterodimers. At later times, NF-kappa B complexes containing c-Rel and/or p52 accumulate. The early activation of p50/p65 complexes is a posttranslational event, since neither mRNA nor protein levels of NF-kappa B subunits had increased at that time. We demonstrate for the first time a Tax-induced proteolytic degradation of the NF-kappa B inhibitor, I kappa B-alpha, which may trigger the initial nuclear translocation of NF-kappa B. As nuclear NF-kappa B rapidly and potently stimulates resynthesis of I kappa B-alpha, the steady-state level of I kappa B-alpha does not significantly change. Thus, the dramatic Tax-induced increase in the I kappa B-alpha turnover may continually weaken inhibition and activate NF-kappa B. Additional, distinct actions of Tax may contribute further to the high levels of NF-kappa B activity seen.