Induction of nuclear NF-κB DNA binding activity after exposure of lymphoid cells to soluble Tax₁ protein

We demonstrate that purified HTLV-I Tax₁ protein can be taken up by 70Z/3 lymphoid cells and localized in both the nuclear and cytoplasmic compartments. Introduction of the Tax₁ protein into the growth medium of 70Z/3 cells resulted in the rapid and transient induction of NF-κB binding activity in the nuclear fraction. Tax₁ activation of NF-κB was not sensitive to either staurosporin or prolonged stimulation with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, suggesting that Tax₁-dependent NF-κB activation did not require the protein kinase C pathway. Purified Tax₁ did not directly increase NF-κB binding activity in 70Z/3 cytoplasmic extracts, suggesting that NF-κB induction may require cellular factors. Western blot and competitive radioimmunoassays demonstrated that Tax₁ protein was present in the tissue culture media of HTLV-I-transformed cell lines. These results show that extracellular Tax₁ may regulate cellular gene expression in noninfected cells.