Development of novel renal cell carcinoma therapy targeting signal transduction pathways

众所周知，VHL基因失活是人透明细胞肾癌的易感基因。但关于VHL失活在肿瘤发生过程中的关键作用的细节仍不清楚。MET蛋白是一种肝细胞生长因子/分散因子酪氨酸激酶受体(HGF/SF)，调节细胞生长、细胞形态和细胞运动。我们证明了在CCRC细胞中过度表达的MET蛋白在没有HGF/SF的情况下被磷酸化。外源野生型VHL基因的拯救抑制了CCRC细胞MET蛋白的这种结构性磷酸化，但不降低MET蛋白的表达水平。有趣的是，野生型VHL基因仅在高细胞密度条件下抑制MET蛋白的磷酸化。另外，VHL基因失活激活的MET蛋白还可以改变细胞的粘附性，包括N-钙粘附素和--连接素。当MET抑制剂K252a抑制CCRC细胞MET蛋白的激活时，可抑制CCRC细胞的体外生长和CCRC细胞在裸鼠体内的成瘤作用。根据这些结果，我们得出结论：VHL基因失活导致MET蛋白的结构性磷酸化，并改变细胞间黏附结构，触发细胞从接触抑制中释放出来，最终导致肿瘤的发生。这是CCRC肿瘤发生的机制之一，MET蛋白有可能成为CCRC治疗的分子靶点。

It is well known that inactivation of VHL gene predisposes for human clear cell renal carcinoma (CCRC). But details about critical roles of VHL inactivation during tumorigenesis are still unknown. MET protein is a tyrosine kinase receptor for hepatocyte growth factor/scatter factor (HGF/SF) to regulate cell growth, cell morphology, and cell motility. We demonstrated that MET protein overexpressed in CCRC cells was phosphorylated without HGF/SF. This constitutive phosphorylation of MET protein in CCRC cells was inhibited by the rescue of exogenous wild-type VHL gene without decrease in expression level of MET protein. Interestingly wild-type VHL gene suppressed the phosphorylation of MET protein only under high cell density conditions. Additionally MET protein activated by the inactivation of VHL gene modified the cell adherence including N-cadherin and □-catenin. When activation of MET protein in CCRC cells was inhibited by the MET inhibitor K252a, the growth of CCRC cells in vitro and the tumorigenesis induced by CCRC cells in nude mice were suppressed. From these results we concluded that inactivation of VHL gene induced constitutive phosphorylation of MET protein and modified intercellular adherence structure to trigger the cell growth released from contact inhibition, finally resulting in tumorigenesis. This is one of the mechanisms of CCRC oncogenesis and MET protein has a potential as a molecular target for novel CCRC therapies.