51.9 - Enabling regeneration in a β-catenin-deficient liver: Unraveling a novel compensatory role of mTOR

Shikai Hu (Tsinghua University, University of Pittsburgh), Catherine Cao (University of Pittsburgh), Sucha Singh (University of Pittsburgh), Minakshi Poddar (University of Pittsburgh), Satdarshan Monga (University of Pittsburgh, University of Pittsburgh, University of Pittsburgh)

Backgrounds: Liver can regenerate to maintain its essential functions upon injury. Several signaling pathways including the mTORC1 and Wnt/β-catenin pathways have been shown to independently stimulate liver regeneration (LR) to promote repair. However, how one pathway could compensate during deficiency of another is less well understood. Here, we report discovery and characterization of a compensatory interplay between the mTORC1 and the Wnt/β-catenin pathway that enables LR following liver injury when Wnt/β-catenin pathway is interrupted genetically. 

Methods:  We generated hepatocyte-specific β-catenin knockout (β-catenin∆HC) mice by delivering AAV8-TBG-Cre to 6-week-old Ctnnb1flox/flox mice. Two weeks after AAV8 injection, animals were either harvested at baseline, or subjected to partial hepatectomy (PH) for analysis at various time points to address changes and compensations during the process of LR. LR was also compared between β-catenin∆HC and Albumin-Cre-driven β-catenin knockouts from both hepatocytes and cholangiocytes (β-catenin∆Liver). 

Results:  At baseline, a small subset of β-catenin∆HC mice had smaller liver size and mild fibrosis, but overall, there was no impact on body weight, liver size, and liver weight to body weight ratio (LW/BW). After PH, LR was more severely delayed in β-catenin∆HC mice compared to β-catenin∆Liver, likely due to chronic adaptive changes that only occur in the latter. After PH, 14-day continuous BrdU labeling through drinking water showed only 31% positively labeled hepatocytes in β-catenin∆HC mice versus 92% positively labeled hepatocytes in controls. The scant LR in β-catenin∆HC mice started around the periportal region at 40h and increased at 72h and peaked at 96h seen by positive staining for Cyclin D1, Ki67, BrdU (5h labeling). Western blots using whole cell lysates showed a clear increase in p-IGF1R β-Y1135/1136 and p-Insulin receptor (IR) β-Y1150/1151 at 72h, and activation of mTORC1 activity seen as increased p-mTORC1 (S2448), p-p70S6K (T389), p-S6 (S235/236) and p-S6 (S240/244) at 72h and 96h. More importantly, careful quantification showed 97% of mitotic hepatocytes to be positive for p-S6 (S235/236) in β-catenin∆HC. Pharmacological inhibition of IGF1R and IR by linsitinib, or inhibition of mTORC1 by rapamycin completely blocked LR in β-catenin∆HC, further confirming that IGFR/IR-mTORC1 pathway drove LR to ensure liver repair in β-catenin∆HC mice. 

Conclusions:  Our data reaffirms an important role of β-catenin in normal LR after PH. In the absence of β-catenin in hepatocytes, there is spontaneous activation of IGFR/IR driven mTORC1 pathway, which in turn drives hepatocyte proliferation in β-catenin∆HC mice. Our studies have discovered a novel axis of compensation, once again supporting existence of redundant mechanisms that enable LR when one pathway is inhibited.