Insights into the Molecular Mechanism of p53 Inhibition by HTLV Type 1 Tax

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Abstract
The p53 protein plays a pivotal role in transmitting signals from many forms of genotoxic stress to genes and factors that control aspects of the cell cycle and death. Although mutated in approximately 60% of all human cancers, only a minority of human T-lymphotropic virus type 1 (HTLV-1)-transformed cells carry p53 mutations. Nevertheless, the p53 protein in HTLV-1-transformed cells is functionally inactive. We have previously demonstrated that the HTLV-1 Tax protein can inhibit p53 trans-activation function. Tax does not accomplish this by directly binding to p53, but rather by a unique mechanism that includes constitutive phosphorylation of p53 at Ser-15 and Ser-392. Analysis of Tax mutants in lymphocytes demonstrates that Tax-induced p53 inhibition correlates with the ability of Tax to activate NF-κB, but not p300 binding or CREB trans-activation. Consistent with these results, expression of the I-κBα(S32,36A) mutant that blocks NF-κB activation blocks Tax-mediated p53 inhibition. We further demonstrate the importance of Tax activation of NF-κB in p53 inhibition, using p65 knockout (KO) mouse embryo fibroblasts (MEFs). In the absence of p65 Tax could not inhibit p53. Tax does activate IKKβ in the p65 KO MEFs, indicating that prenuclear events of NF-κB activation are not sufficient for Tax-mediated p53 inhibition, but rather NF-κB transcriptional activation is critical. Importantly, using phosphospecific antibodies, we demonstrate that phosphorylation of p53 at Ser-15 and Ser-392 correlates with Tax-mediated inhibition. In addition, mutation of p53 at Ser-15 and Ser-392 to alanines renders p53 resistant to Tax inhibition. This report reviews p53 inhibition by Tax and presents our current model.