MECHANISMS OF LIVER REGENERATION AFTER ACETAMINOPHEN-INDUCED HEPATOTOXICITY IN MICE

Abstract

Overdose of acetaminophen (APAP) is the major cause of acute liver failure (ALF) in the western world with very limited treatment options. Recent studies suggest that liver regeneration is a critical determinant of final recovery and overall survival following APAP overdose. Stimulating liver regeneration in patients of APAP-induced ALF holds a great therapeutic potential. However, development of novel regenerative therapies for ALF is hampered because the mechanisms of liver regeneration after APAP-induced ALF are not completely known. The focus of this research was to investigate the mechanisms of liver regeneration after APAP-induced hepatotoxicity in mice. Firstly, we identified potential regulators of liver regeneration following APAP-induced hepatotoxicity using a novel incremental dose model. Liver injury and regeneration were studied in C57BL/6 mice treated with either 300 mg/kg (APAP300) or 600 mg/kg (APAP600) APAP. Mice treated with APAP300 developed extensive liver injury initially, followed by robust liver regeneration, resulting in resolution of the injury. In contrast, mice treated with APAP600 exhibited significant liver injury, but substantially delayed and attenuated liver regeneration resulting in sustained injury and decreased survival. The inhibition of liver regeneration in APAP600 group was associated with cell cycle arrest and decreased Cyclin D1 expression. Further analysis revealed that several known regenerative pathways including IL-6/STAT-3, growth factor signaling via EGFR/c-Met/MAPK pathways were activated even in APAP600 group where regeneration was inhibited. However, canonical Wnt/β-catenin and NF-κB pathways were activated only in APAP300 treated mice, where liver regeneration was stimulated. Next, we investigated role of Wnt/β-catenin in further detail. ChIP analysis revealed decreased binding of β-catenin to Cyclin D1 promoter in APAP600 group correlating with decreased cyclin D1 induction and impaired liver regeneration. Overexpression of a stable form of β-catenin (S45D) in mice resulted in improved liver regeneration following APAP overdose. Inactivation of GSK3, an upstream negative regulator β-catenin, was found to be positively associated with β-catenin activation and liver regeneration. GSK3 inactivation was remarkably reduced in APAP600 group, where liver regeneration was attenuated and delayed. Treatment with a selective GSK3 inhibitor (L803-mts), as late as 4 hr after APAP600, resulted in early initiation of liver regeneration and improved survival in mice. Early hepatocyte proliferation after GSK3 inhibition was due to rapid induction of cyclin D1 secondary to increased activation of β-catenin signaling. Finally, we investigated the role of EGFR signaling in liver regeneration, which was dose-dependently activated after APAP overdose. Surprisingly, early EGFR inhibition (1 hr post-APAP) by treatment with an irreversible EGFR inhibitor, canertinib, strikingly attenuated APAP hepatotoxicity suggesting role of EGFR in development of APAP hepatotoxicity. Delayed EGFR inhibition, 12 hr post-APAP, did not alter initial injury but caused cell cycle arrest, remarkable impairment of liver regeneration and decreased survival after APAP overdose (APAP300) in mice. Overall, these studies comprehensively investigated the mechanisms of liver regeneration after APAP-induced ALF and will pave the road for future therapeutic and mechanistic research on liver regeneration after APAP-induced ALF.