Mechanism of hepatitis B virus X mediating SAMHD1 degradation to promote proliferation of hepatoma cells through AKT/p27 pathway

Objective To investigate the molecular mechanism of hepatitis B virus X (HBx) regulating the degradation of SAMHD1 and promoting the proliferation of hepatocellular carcinoma (HCC) cells. Methods The SAMHD1 expression in hepatoma and para-cancerous tissues collected from 15 HCC patients was detected by Western blotting. A stable hepatocellular carcinoma cell line of HepAD38 with SAMHD1 knockout was constructed using CRISP9/Cas9 technology, and the effects of SAMHD1 knockout on the change of proliferation and cell cycle were measured by MTT assay and flow cytometry, respectively, and its effects on related cyclin expression and Akt/p27 pathway were also detected by Western blotting. The impacts of HBx protein overexpression on the mRNA and protein expression of SAMHD1 were subsequently examined. In addition, the molecular mechanism of HBx regulating the degradation of SAMHD1 was further investigated through ubiquitin-proteasome pathway using cycloheximide (CHX) and MG132 treatment along with immunoprecipitation (IP) analysis. Results ① The expression of SAMHD1 was significantly lower in liver cancer tissues than the para-cancerous tissues (P < 0.01).②A stable SAMHD1 knockout cell line HepAD38 (KO-SAMHD1) was successfully constructed, and MTT assay showed that the KO-SAMHD1 group had obviously faster cell proliferation (P < 0.01), and flow cytometry indicated the group had larger amounts of cells arrested in G2/M phase when compared with the control group. ③Western blot analysis indicated that the expression of related cyclins was changed in the KO-SAMHD1 group when compared with the control group, including the expression of p27 protein decreased and p-Akt protein increased. ④In vitro cell experiments showed that HBx overexpression induced decreased expression of SAMHD1 in Huh7.0 cells. ⑤The results of immunoprecipitation suggested that HBx were combined and interacted with SAMHD1. CHX and ubiquitination IP assay indicated an earlier decay time of SAMHD1 protein and enhanced ubiquitination of SAMHD1 induced by HBx. Conclusion HBx interacts with SAMHD1 and degrades it by regulating its ubiquitination, and thereby affects the Akt/p27 pathway to modulate the cell cycle and promote the proliferation of HCC cells.