Hepatitis B surface antigen induces nuclear accumulation of YAP, thereby driving BMI1-associated hepatocarcinogenesis

Background and aim Chronic hepatitis B virus (HBV) infection is one of the foremost sources of hepatocellular carcinoma (HCC). Efficient restraint of HBV viremia and necroinflammation via nucleos(t)ide analogue treatment could reduce the HCC incidence. However, hepatocarcinogenesis still occurs in the absence of active hepatitis, correlating with high hepatitis B surface antigen (HBsAg) serum levels. Nevertheless, the molecular mechanisms leading to neoplastic transformation remain elusive.

Methods Hemizygous HBsAg-transgenic mice (tg(Alb1HBV)44Bri) were investigated as a model of hepatocarcinogenesis driven by accumulation of HBsAg.

Results Gene set enrichment analysis suggested that signatures in HBsAg-transgenic mice correlated with YAP-downstream, cell cycle, DNA damage and spindle events (GSE84429). Flow cytometry revealed that polyploidy and aneuploidy frequently occurred in hepatocytes of these mice. Quantitative PCR, western blot and immunohistochemical staining revealed the downregulation of MST1/2, loss of YAP phosphorylation and the induction of BMI1 expression. Bioinformatics analysis of the Bmi1 promoter indicated the presence of several TEAD4 bind sites. Chromatin immunoprecipitation and the analysis of mutated binding site in DLR assays confirmed that the YAP/TEAD4 transcription factor complex was able to bind and activate the Bmi1 promoter. Loss and gain of function of Yap or Bmi1 in vitro confirmed the Yap-regulated Bmi1 expression, which led to suppression of p16INK4a, p19ARF, p53 and induction of Cyclin D1, here a CCK-8 assay indicated the direct proliferative effect.

Conclusion Our findings reveal an HBsAg-mediated inactivation of Hippo pathway, resulting in increased BMI1 expression, thereby promoting proliferative hepatocarcinogenesis through alterations in the cell cycle and chromosomal stability.

Publication History

Article published online:
26 January 2022