Combined systemic elimination of MET and epidermal growth factor receptor signaling completely abolishes liver regeneration and leads to liver decompensation.

Receptor tyrosine kinases MET and epidermal growth factor receptor (EGFR) are critically involved in initiation of liver regeneration. Other cytokines and signaling molecules also participate in the early part of the process. Regeneration employs effective redundancy schemes to compensate for the missing signals. Elimination of any single extracellular signaling pathway only delays but does not abolish the process. Our present study, however, shows that combined systemic elimination of MET and EGFR signaling (MET knockout + EGFR-inhibited mice) abolishes liver regeneration, prevents restoration of liver mass, and leads to liver decompensation. MET knockout or simply EGFR-inhibited mice had distinct and signaling-specific alterations in Ser/Thr phosphorylation of mammalian target of rapamycin, AKT, extracellular signal-regulated kinases 1/2, phosphatase and tensin homolog, adenosine monophosphate-activated protein kinase α, etc. In the combined MET and EGFR signaling elimination of MET knockout + EGFR-inhibited mice, however, alterations dependent on either MET or EGFR combined to create shutdown of many programs vital to hepatocytes. These included decrease in expression of enzymes related to fatty acid metabolism, urea cycle, cell replication, and mitochondrial functions and increase in expression of glycolysis enzymes. There was, however, increased expression of genes of plasma proteins. Hepatocyte average volume decreased to 35% of control, with a proportional decrease in the dimensions of the hepatic lobules. Mice died at 15-18 days after hepatectomy with ascites, increased plasma ammonia, and very small livers. CONCLUSION: MET and EGFR separately control many nonoverlapping signaling endpoints, allowing for compensation when only one of the signals is blocked, though the combined elimination of the signals is not tolerated; the results provide critical new information on interactive MET and EGFR signaling and the contribution of their combined absence to regeneration arrest and liver decompensation. (Hepatology 2016;64:1711-1724).

Novel Death Defying Domain in Met entraps the active site of caspase-3 and blocks apoptosis in hepatocytes.

UNLABELLED: Met, the transmembrane tyrosine kinase receptor for hepatocyte growth factor (HGF), is known to function as a potent antiapoptotic mediator in normal and neoplastic cells. Herein we report that the intracellular cytoplasmic tail of Met has evolved to harbor a tandem pair of caspase-3 cleavage sites, which bait, trap, and disable the active site of caspase-3, thereby blocking the execution of apoptosis. We call this caspase-3 cleavage motif the Death Defying Domain (DDD). This site consists of the following sequence: DNAD-DEVD-T (where the hyphens denote caspase cleavage sites). Through functional and mechanistic studies, we show that upon DDD cleavage by caspase-3 the resulting DEVD-T peptide acts as a competitive inhibitor and entraps the active site of caspase-3 akin to DEVD-CHO, which is a potent, synthetic inhibitor of caspase-3 activity. By gain- and loss-of-function studies using restoration of DDD expression in DDD-deficient hepatocytic cells, we found that both caspase-3 sites in DDD are necessary for inhibition of caspase-3 and promotion of cell survival. Employing mutagenesis studies, we show that DDD could operate independently of Met's enzymatic activity as determined by using kinase-dead human Met mutant constructs. Studies of both human liver cancer tissues and cell lines uncovered that DDD cleavage and entrapment of caspase-3 by DDD occur in vivo, further proving that this site has physiological and pathophysiological relevance. CONCLUSION: Met can directly inhibit caspase-3 by way of a novel mechanism and promote hepatocyte survival. The results presented here will further our understanding of the mechanisms that control not only normal tissue homeostasis but also abnormal tissue growth such as cancer and degenerative diseases in which apoptotic caspases are at play.