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

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.