Physiological role of protein-tyrosine phosphatase which regulates the function of hepatocyte growth factor (HGF) receptor

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Hepatocyte growth factor (HGF) is now known to be a multi-functional growth factor acting as a mitogen, motogen, and morphogen for various types of epithelial cells. The receptor for HGF is a transmenbrane tyrosine kinase and the receptor phosphorylation on tyrosine is critical for both kinase stimulation and association with intracellular signal transducers. However, it is not clear for the negative factor regulating the function of the HGF receptor. The purpose of this project is to identify the protein-tyrosine phosphatase (PTPase) which regulates negatively the function of HGF receptor. In this project, we obtained following results :1. The cell lysates from primary cultured rat hepatocytes had no ability hydrolyse p-nitrophenol phosphate, a chromogenic molecule that is structurally related to phosphotyrosine, but rapidly hydrolysed phosphotyrosine of C-terminal fragment (53-65) of hirudin. Thus, we used this fragment as a substrate to identify the PTPase (s) as a functional negative regulator for HGF receptor in primary cultured hepatocytes.2. HGF (20ng/ml) stimulated the PTPase activity of primary cultured hepatocytes withine 10 min. The effect of HGF on the PTPase activity was time-and dose-dependent.The stimulation of the PTPase activity by HGF was completely inhibited by vanadate, a PTPase inhibitor, but not by okadaic acid, a specific inhibitor for Ser/Thr phosphatase.3. The PTPase activity elevated in density-dependent growth-arrested hepatocytes, suggesting that density-dependent inhibition of hepatocyte growth involves the regulated elevation of the PTPase activity in primary cultured hepatocytes.