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

肝细胞生长因子(HGF)和Met/HGF受体酪氨酸激酶在组织再生中起重要作用。在未损伤的正常组织中，Met激活/酪氨酸磷酸化即使在与HGF结合后也受到抑制。因此，HGF依赖的Met受体的激活可能不仅受HGF结合的调节，而且还受细胞环境的调节，如细胞与细胞的接触和细胞损伤。在本研究中，我们研究了细胞间接触抑制Met激活和Met膜旁区域Ser985磷酸化的机制。在稀疏细胞密度培养的肝细胞中，HGF诱导延长Met的激活和显著的有丝分裂反应。相反，HGF可在细胞间紧密接触的融合肝细胞中诱导一过性Met激活，但不能诱导有丝分裂反应。肝细胞生长因子刺激后蛋白酪氨酸磷酸酶LAR的表达增加，尤其是融合肝细胞。与蛋氨酸和去磷酸化Ty…相关的LAR更多松香磷酸化的甲硫氨酸。因此，LAR和Met的功能结合是细胞间接触抑制HGF依赖的Met激活的抑制机制的基础。在培养的肝细胞中，当Ser985被磷酸化时，HGF依赖的Met酪氨酸的磷酸化/激活被抑制。在正常肝脏中，Met Ser985被磷酸化，而在肝损伤后，如肝部分切除，Ser985被去磷酸化。与Ser985磷酸化相反，Met在肝损伤后被激活。结果提示，Met膜旁区域的Ser985受组织损伤的调节，Met膜旁区域Ser985的磷酸化可能是非损伤器官抑制HGF依赖的Met激活的机制之一。由于被膜旁缺失的Met受体是以剪接变异体的形式自然表达，Met膜旁可能在调节HGF依赖的Met在组织损伤反应中的激活中发挥重要作用。利用只表达突变Met的转基因小鼠研究了Met膜旁结构域在发育和组织再生中的生理意义。较少

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