Cancer Therapy Vol 3, page 5 have been observed to be constitutively overexpressed (Koochekpouret al, 1997; Fukuda et al, 1998; Hendrix et al, 1998; Birchmeier et al, 2003). These observations are further strengthened by the evidence of c-MET and/or HGF expression in carcinomas, and other types of human solid tumors and their metastasis (Birchmeieret al, 2003). Furthermore, mouse and human cells that ectopically overexpress HGF or c-MET become tumorigenic and metastatic in athymic nude mice (Rong et al, 1994). A large number of sporadic and germline mutations of cMET have been identified in human renal papillary carcinomas (Danilkovitch-Miagkova et al, 2002) and homologous c-MET mutations produce distinct tumor profiles in mice (Graveel et al, 2004). These mutations occur within the c-MET kinase domain, often making it capable of constitutive signaling. Mutations in the c-MET juxtamembrane domain, a region important for c-Cbl binding have been observed in gastric and lung cancers (Lee et al, 2000; Ma et al, 2003). Recently, mutations in extracellular semaphorin domain that is important for HGF binding, were identified in lung cancers (Ma et al, 2003; Ma et al, 2005). The role of c-MET in physiological processes such as proliferation, survival, invasion and angiogenesis could point to its involvement in corresponding stages during tumor progression. c-MET signaling has also repeatedly emerged as a pathway that is exploited by several pathogens including Listeria monocytogenes, Plasmodium spp. and Helicobacter pylori (Figure 2) (Shen et al, 2000; Carrolo et al, 2003; Churin et al, 2003). InlB, a listerial protein was identified as a bacterial agonist for c-MET and shown to mimic HGF-induced c-MET activation, endocytosis (Ireton et al, 1999; Li et al, 2005) and signaling (Shen et al, 2000). H. pylori CagA protein also activated c-MET, although by a distinct mechanism. The CagA- induced cMET signaling could be important for H. pylori-induced cancer onset and tumor progression (Churinet al, 2003). Contrary to Listeria and H pylori, Plasmodium, the
causative agent for malaria, did not directly interact with c-MET, but exploited HGF-c-MET signaling to make the host cell more susceptible to infection (Carrollo et al, 2003).
IV. Regulating activity
c-MET
enzymatic
HGF-mediated dimerization facilitates c-MET autophosphorylation. The kinetics of c-MET autophosphorylation has not been extensively studied, although the phosphotyrosine sites and their role in c-MET signaling is well-characterized (as reviewed above). Phosphorylation of Y1231, Y1234 and Y1235 in the kinase domain AL has been reported to modulate c-MET catalytic activity (Rodrigues et al, 1994). The correlation between AL phosphorylation and increased kinase catalytic activity has been extensively documented in a number of RTKs (Cobbet et al, 1989; Parast et al, 1998; Murray et al, 2001). In IR, where insulin binding induces receptor activation of a constitutive dimeric receptor, autophosphorylation kinetics were observed to follow a two phase model where the ligand activated receptor had a prolonged fast phase compared to the non-ligand stimulated receptor (Kohanski, 1993). Murray et al, determined the kinetic parameters for phosphorylated and unphosphorylated Tie2 cytoplasmic kinase domain and showed that phosphorylation resulted in a 2-5-fold decrease in substrate KM relative to the unphosphorylated kinase (Murray et al, 2001). Parast et al also showed an order of magnitude increase in the catalytic activity of the phosphorylated VEGFR2 tyrosine kinase domain versus unphosphorylated receptor (Parast et al, 1998). The crystal structure of IR in its phosphorylated and unphosphorylated forms provided a structural interpretation for how AL phosphorylation might modulate kinase activity (Hubbard et al, 1994; Hubbard, 1997). In the phosphorylated state, the AL adopted a
Figure 2. c-MET signaling and function. HGF-mediated c-MET signaling is important for several physiological processes including cell proliferation, differentiation and survival. Aberrant regulation of c-MET signaling by HGF/c-MET overexpression or c-MET mutations is associated with tumorigenesis and metastasis. Furthermore, c-MET signaling has been shown to be exploited by several pathogens, including Listeria monocytogenes, Helicobacter pylori and malarianparasite Plasmodium spp for tissue invasion and pathogen dissemination.
5