Human T-cell lymphotropic virus type 1 Tax₁ activation of NF-κB: Involvement of the protein kinase C pathway

Human T-cell lymphotropic virus type 1 Tax₁ induces the activation and nuclear localization of the cellular transcription factor, NF-κB. Treatment of cells with calphostin C, a protein kinase C (PKC) inhibitor, blocked induction of NF-κB DNA binding activity in human T-cell lymphotropic virus type 1-transformed C81 cells and Tax₁-stimulated murine pre-B cells, suggesting that PKC was an important intermediate in the NF-κB induction pathway. We further demonstrate that Tax₁ associates with, and activates, PKC. PKC was coimmunoprecipitated with anti-Tax₁ sera from Tax₁-expressing MT4 extracts and Jurkat extracts in the presence of exogenous Tax₁ protein. In addition, the glutathione-S-transferase-Tax₁ protein bound specifically to the alpha, delta, and eta PKC isoenzymes synthesized in rabbit reticulocyte lysates. The addition of Tax₁ to in vitro kinase reaction mixtures leads to the phosphorylation of Tax₁ and an 18-fold increase in the autophosphorylation of PKC. Transfection of Jurkat cells with wild-type Tax₁ stimulated membrane translocation of PKC. In contrast, Tax₁ mutant M22, which fails to stimulate NF-κB-dependent transcription, failed to stimulate membrane translocation of PKC. Tax₁ did not directly increase PKC phosphorylation of IκBα. Our results are consistent with a model in which Tax₁ interacts with PKC and stimulates membrane translocation and triggering of the PKC pathway. Subsequent steps in the PKC cascade likely stimulate phosphorylation of IκBα.