Regulation of tissue regeneration and homeostasis through ON-OFFcontrol of the Met/HGF receptor

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HGF (hepatocyte growth factor) and Met/HGF receptor tyrosine kinase play important roles in tissue regeneration. In non-injured normal tissues, Met activation/tyrosine phosphorylation is suppressed even following binding with HGF. Thus, HGF-dependent Met receptor activation may be regulated by not only HGF-binding but also cellular context such as cell-cell contact and cellular injury. In this research, suppressive mechanisms of Met activation through cell-cell contact and Ser985 phosphorylation in the juxtamembrane domain of the Met were investigated.1. In hepatocytes cultured at a sparse cell density, HGF induced prolonged Met activation and a marked mitogenic response. In contrast, HGF induced transient Met activation but failed to induce mitogenic response in confluent hepatocytes with tight cell-cell contact. Expression of the protein tyrosine phosphatase, LAR increased following HGF-stimulation specifically in confluent hepatocytes. LAR associated with Met and dephosphorylated ty … More rosine-phosphorylated Met. Thus functional association of LAR and Met underlies the inhibitory mechanism by which HGF-dependent Met activation is suppressed by cell-cell contact.2. In hepatocytes in culture, HGF-dependent Met tyrosine phosphorylation/activation was inhibited when Ser985 was phosphorylated. In the normal liver, Met Ser985 was phosphorylated, however, Ser985 was dephosphorylated after hepatic injury such as partial hepatectomy. In reciprocal manner to Ser985 phosphorylation, Met was activated after hepatic injury. The results suggest that Ser985 in the juxtamembrane domain of Met is regulated by tissue injury and that Ser985 phosphorylation in the juxtamembrane domain of Met may be a mechanism by which HGF-dependent Met activation is suppressed in non-injured organs. Because Met receptor deleted with the juxtamembrane is naturally expressed as a splicing variant, the juxtamembrane of the Met may play an important role in regulation of HGF-dependent Met activation in response to tissue injury.3. Physiological significance of the juxtamembrane domain of Met in the development and tissue regeneration is investigated using the transgenic mice which express only variant Met deleted with the juxtamembrane domain. Less