Hepatocyte-specific Bid depletion reduces tumor development by suppressing inflammation-related compensatory proliferation

Liver cancer is a major health-care concern and its oncogenic mechanisms are still largely unclear. Persistent hepatocyte cell death is a common feature among various chronic liver diseases, the blocking of which presents as logical treatment. Therefore, we aimed at investigating tumor development in mice with hepatocyte-specific Bid depletion - a BH3-only Bcl-2 family member that amplifies apoptotic death signals. Hepatocyte-specific conditional Bid-knockout mice (Bid Δhep) were injected with 25 mg/kg diethylnitrosamine (DEN) at 14 days of age, and liver tumorigenesis was investigated 9 months later. Additionally, different models of acute liver injury were used including: acute high-dose DEN challenge, 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet and carbon tetrachloride (CCL4) injection. Bid Δhep mice developed significantly fewer tumors, showed smaller maximal and average tumor size and reduced tumor incidence. In the acute DEN model, 48 h post injection we observed a significant reduction in liver injury in Bid Δhep animals, assessed via serum transaminases and liver histopathology. Furthermore, TNF-α, IL-1ß, cJUN and IL-6 mRNA expression was reduced. These findings were accompanied by reduced compensatory hepatocyte proliferation in Bid Δhep mice when compared with controls by immunohistochemistry for Ki67 and proliferating cell nuclear antigen 48 h after DEN injection. In the acute CCL4 model, Bid Δhep mice displayed reductions in liver injury and inflammation when compared with controls. No differences in liver injury and serum bilirubin levels were detected in Bid Δhep and Bid flo/flo mice fed with DDC, which induces bile duct injury and a ductular reaction. Our study demonstrates that in DEN-induced hepatocellular carcinoma, the inhibition of hepatocyte death pathways through Bid deletion protects animals from tumorigenesis. These results suggest that reducing hepatocyte cell death, liver inflammation and compensatory proliferation has a stronger beneficial effect than the potential side effect of enhancing tumor cell survival.