HTLV-1 HBZ suppresses AP-1 activity by impairing both the DNA-binding ability and the stability of c-Jun protein.

Disruption of transcriptional control of cellular genes by human T-cell leukemia virus type-1 (HTLV-1) is thought to be associated, at least in part, with the development of adult T-cell leukemia. It has been reported that activating protein-1 (AP-1) is dysregulated by HTLV-1 infection. HTLV-1-encoded Tax elevates AP-1 activity through the induction of AP-1 family member gene expression, including c-Jun, JunD, c-Fos, and Fra-1. However, the precise mechanism by which HTLV-1 regulates AP-1 activity remains to be addressed. Recently, a novel viral protein named HTLV-1 basic leucine-zipper factor, HBZ, has been shown to interact with c-Jun and repress c-Jun-mediated transcription by abrogating its DNA-binding activity. In the course of investigating HBZ function, we found that HBZ reduced the steady-state levels of c-Jun, and the levels were restored by treatment with a proteasome inhibitor. Together, this indicates that HBZ promotes c-Jun degradation through a proteasome-dependent pathway. Furthermore, HBZ deletion mutants revealed that both the N-terminal and leucine-zipper region of HBZ were required for the elimination of c-Jun. These results suggest dual effects of HBZ on the suppression of AP-1 activity by inhibiting c-Jun function, which may contribute to the dysregulation of cell proliferation.