Loss of hepatic mTORC1 signaling affects liver homeostasis which is partially compensated by Wnt/β‐catenin pathway activation
Authors: Hu Shikai , Cao Catherine , Delgado Evan , Singh Sucha , Poddar Minakshi , Stolz Donna B. , Bell Aaron , Monga Satdarshan 
Published: May 2022

Backgrounds

Liver has an innate capacity to regenerate following partial hepatectomy (PH). Pathways including the mTORC1 and the Wnt/β‐catenin pathways have been shown to become activated after PH to enable liver regeneration (LR). 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 mTOR and Wnt signaling that enables LR following PH when mTORC1 pathway is interrupted genetically.

Methods

We generated hepatocyte‐specific Raptor knockout (Raptor∆HC) mice by delivering AAV8‐TBG‐Cre to 6‐week‐old Raptorflox/flox mice. Two weeks after AAV8 injection, animals were either harvested at baseline, or subjected to partial hepatectomy (PH) for analysis to address changes and compensations during the process of LR.

Results

Acute deletion of hepatocyte Raptor abolished mTORC1 activity and led to compensatory activation of mTORC2 and its downstream target p‐AKT (S473). Raptor∆HC mice had baseline liver injury as shown by elevated serum levels of ALT, AST, and ALP. Raptor∆HC mice exhibited decreased levels of pericentral enzyme (Glul, Cyp2e1), periportal enzyme (G6pc), and main secretory proteins (Alb, Trf, Ttr), suggesting global impairment of metabolic, synthetic, and secretory functions. These mice were also under metabolic stress as seen by ER whorl formation, mitochondrial swelling, and increased autophagy, by transmission electron microscopy (TEM). There was an increase in cell death seen as enhanced apoptotic body formation by TUNEL and cleaved caspase‐3 staining and an associated increase in macrophage infiltration in the immediate proximity of dying hepatocytes. An attempt at compensatory liver regeneration was concurrently evident through activation of the Wnt/β‐catenin pathway leading to enhanced Cyclin D1, Ki67, Axin2 and c‐Myc. β‐Catenin protein increased 5‐fold in Raptor∆HC mice and appears to be due to both increased gene transcription and decreased phosphorylation‐mediated degradation. Further, stabilization of β‐catenin protein appears to be due to both cell‐extrinsic mechanism seen as increased p‐LPR6 (S1490) suggesting Wnt stimulation, and cell‐intrinsic mechanism seen as decreased GSK3β activity which was phosphorylated and inhibited by compensatory elevation of AKT activity in the Raptor∆HC mice. Despite activation of the Wnt pathways, hepatocytes failed to finish cell cycle seen as an overall decrease in BrdU incorporation after a 14‐day continuous pulse. Indeed, when Raptor∆HC mice were subjected to PH, they all died within 48h.

Conclusions

mTORC1 activity is essential for homeostatic liver functions and for LR after PH. Stabilization of β‐catenin due to multiple mechanisms including Wnt secretion likely from macrophages, secondary to AKT mediated GSK3β inhibition and via de novo β‐catenin transcription, attempts to compensate to maintain homeostasis in the Raptor∆HC mice.