Requirement of CRTC1 coactivator for hepatitis B virus transcription

Tang, Hei-Man Vincent; Gao, Wei-Wei; Chan, Chi-Ping; Cheng, Yun; Chaudhary, Vidyanath; Deng, Jian-Jun; Yuen, Kit-San; Wong, Chun-Ming; Ng, Irene Oi-Lin; Kok, Kin-Hang; Zhou, Jie; Jin, Dong-Yan

Although CRTC2 level was elevated in HepG2 cells expressing HBx (Figure 7) , it was unchanged in HepG2.2.15 or pHBV1.3D-transfected HepG2 cells (Figure 1 ). Further investigations are required to resolve this discrepancy and to clarify whether and how other viral proteins might affect CRTC2 protein expression. Second, the slow-migrating form of HBx detected in the presence of CRTC1/2/3 (Figure 7) should be further characterized. Particularly, whether it represented mono-or multi-ubiquitinated HBx should be clarified. Finally, exactly how stability of HBx and CRTCs in HBV-infected cells would be regulated through ubiquitination and proteolysis should be clarified. HBx has previously been shown to stabilize AIB1 oncoprotein by preventing its ubiquitination and proteosomal degradation (42) . In view of the minimal to mild effect of MG132 on the mutual stabilization of HBx and CRTC1 (Figure 7) , a role for polyubiquitination and proteosome was suggested. However, the abrogation of the mutual stabilization effect by ubiquitin-K0 pointed to another direction. This discrepancy should be resolved in future work. The possibility that HBx and CRTC1 might be mono-or multi-ubiquitinated and then degraded by the lysosome should be experimentally validated. Alternatively, K63-linked or other types of polyubiquitination of CRTC1 suppressed by ubiquitin-K0 might be required for its stabilization. Our data cannot exclude that multiple mechanisms might operate in the control of HBx and CRTC stability. Further investigations should be performed to determine the involvement of proteosome, lysosome and different types of ubiquitination in the destruction of HBx and CRTCs in HBV-infected cells.