[Analysis of the hepatocyte growth factor receptor in regeneration and oncogenesis of hepatocytes].

The Met proto-oncogene product is a tyrosine kinase receptor whose ligand is hepatocyte growth factor (HGF). The Met protein is first synthesized in the hepatocytes as a single chain precursor, or p170MET proreceptor, and is then processed to a mature heterodimer receptor consisting of an extracellular alpha subunit (p50 alpha MET) and a transmembrane beta subunit (p 145 beta MET). The beta subunit has a protein kinase domain which is activated through phosphorylation on tyrosine residue by the binding of HGF to the receptor. In order to elucidate the function of the Met gene product in hepatic disorders, we analyzed the expression and tyrosine phosphorylation of the Met protein on regeneration and carcinogenesis of the liver. For studies on carcinogenesis we used human hepatoma tissues, and for studies on regeneration we used rat hepatectomy. Two antibodies were used for western blotting; a mouse monoclonal anti-phosphotyrosine antibody, which recognizes phosphorylated tyrosine residue in proteins, and a polyclonal rabbit anti-Met antibody, which recognizes the C-terminus of both the Met beta chains and proreceptor. To compare the amount of protein in each experiment, the results of western blotting were evaluated using an image analyzing system. In experiments involving rats with partial hepatectomy, a decreased expression of the proreceptor with a decreased amount of tyrosine phosphorylation was observed within 12 hours of hepatectomy. However, there were no significant changes of the Met beta subunit during the experiment. These data suggest that the Met proreceptor is decreased in the early stages of liver regeneration. In experiments on human samples surgically removed from 18 patients with hepatocellular carcinoma, the met proteins, p 145 beta MET and p 160 MET proreceptor, were expressed both in cancer tissues (12/18, and 10/18, respectively) and in non-cancer tissues (8/18, and 15/18, respectively). From the comparative analyses of the intensity of the signals in cancerous region against those of non-cancerous region in the 18 individual cases, it was demonstrated that expression of p 160 MET proreceptor was increased in non-cancerous region more significantly than in cancerous region (p < 0.05). On the contrary, expression of p145 beta MET was increased in cancerous region more significantly than in non-cancerous region (p < 0.05), except for a few cases of poorly differentiated carcinomas in which p 145 beta MET signal was not detected. These findings suggested that a processing pathway from the proreceptor to the mature Met receptor is amplified in carcinogenesis of the liver.(ABSTRACT TRUNCATED AT 400 WORDS)