Human T-cell lymphotropic virus type 1 tax inhibits transforming growth factor-beta signaling by blocking the association of Smad proteins with Smad-binding element.

The human T-cell lymphotropic virus type 1 (HTLV-1) oncoprotein Tax is implicated in various clinical manifestations associated with infection by HTLV-1, including an aggressive and fatal T-cell malignancy. Because many human HTLV-1-infected T-cell lines are resistant to the growth inhibitory activity of transforming growth factor beta (TGF-beta), we examined the possibility that the HTVL-1-Tax oncoprotein regulates TGF-beta signaling. We show that Tax significantly decreases transcriptional activity and growth inhibition in response to TGF-beta. Tax inhibits TGF-beta-induced plasminogen activator inhibitor-1 expression and Smad2 phosphorylation. Competitive interaction studies show that Tax inhibits TGF-beta signaling, in part, by disrupting the interaction of the Smads with the transcriptional co-activator p300. Tax directly interacts with Smad2, Smad3, and Smad4; the Smad MH2 domain binds to Tax. Furthermore, Tax inhibits Smad3.Smad4 complex formation and its DNA binding. These results suggest that suppression of Smad-mediated signaling by Tax may contribute to HTLV-1-associated leukemogenesis.