Cytoplasmic forms of human T-Cell leukemia virus type 1 tax induce NF-κB activation

Human T-cell leukemia virus type 1 (HTLV-1) Tax targets I-kappa B alpha and I-kappa B beta for phosphorylation, ubiquitination, and proteasome-mediated degradation, causing the nuclear translocation of NF-kappa B/Rel proteins and transcription induction of many cellular genes. The mechanism by which a nuclear protein such as Tax stimulates I-kappa B phosphorylation and degradation remains unclear. Here, we describe two cytoplasmic mutants of Tax, designated Tax Delta N81 and Tax Delta N109, from which the domains important for cyclic AMP response element binding factor (CREB) and serum response factor (SRF) binding and nuclear transport have been removed. These mutants were unable to trans activate from the HTLV-1 21-bp repeats or the serum response element in the c-fos promoter. In contrast, they activated NF-kappa B reporters, suggesting that activation of NF-kappa B by Tax occurs in the cytoplasm. Incorporation of the nuclear localization signal (NLS) of the simian virus 40 large T antigen into Tax Delta N81 and Tax Delta N109 redirected both proteins predominantly to the nucleus yet did not restore trans activation via CREB or SRF. The NLS fusion had little effect on Tax Delta N81 but reduced NF-kappa B trans activation by Tax Delta N109, possibly because of its proximity to the NF-kappa B-activating domain of Tax. In contrast to wild-type Tax, the cytoplasmic Tax Delta N mutants are not cytotoxic. Stable expression of Tax Delta N109 in HeLa cells resulted in a significant reduction in the intracellular level of I-kappa B alpha, with the constitutive presence of NF-kappa B in the nucleus and concomitant activation of the NF-kappa B enhancer. These results are suggestive of a potential application of the Tax Delta N109-like mutants in targeting I-kappa B degradation and NF-kappa B activation. Interestingly, a Tax species with a molecular mass similar to that of Tax Delta N109 was identified in many HTLV-1-transformed T cells, suggesting that Tax Delta N109-like species might play a role in HTLV-1-induced leukemogenesis.