Takahashi T, Shibuya M
Department of Genetics, Institute of Medical Science, University of Tokyo, Minato-ku, Japan.
Oncogene. 1997 May 1;14(17):2079-89. doi: 10.1038/sj.onc.1201047.
KDR/Flk-1 tyrosine kinase, one of the two receptors for Vascular Endothelial Growth Factor (VEGF) has been shown to generate the major part of mitotic signals in endothelial cells, although the mechanisms are poorly understood. Here we examined the processing and signal transduction of KDR/Flk-1. Both in endothelial cells and in NIH3T3 cells expressing KDR/Flk-1, an immature form of KDR/Flk-1 with a molecular mass of about 150 kDa was glycosylated to create a 200 kDa intermediate, and after further glycosylation a mature 230 kDa was expressed on the cell surface. Only this 230 kDa form was rapidly and transiently phosphorylated on tyrosine residues in the presence of VEGF. As a major substrate of KDR/Flk-1, PLC-gamma was found to be rapidly tyrosine-phosphorylated and associated with KDR/Flk-1 both in endothelial cells and NIH3T3 cells. Interestingly, however, a prompt activation of MAP kinase and subsequent strong mitotic signaling were generated only in the endothelial cell background. Activation of MAP kinase in NIH3T3 cells overexpressing KDR/Flk-1 showed a slower response as maximum levels were only attained after 20 min compared to 5 min in sinusoidal endothelial cells. These results suggest that the KDR/Flk-1 utilizes cell type-specific signal transduction pathway(s) for MAP kinase activation and the mitotic response in endothelial cells.
KDR/Flk-1酪氨酸激酶是血管内皮生长因子（VEGF）的两种受体之一，已被证明可在内皮细胞中产生大部分有丝分裂信号，尽管其机制尚不清楚。在此，我们研究了KDR/Flk-1的加工和信号转导。在内皮细胞和表达KDR/Flk-1的NIH3T3细胞中，分子量约为150 kDa的未成熟形式的KDR/Flk-1被糖基化形成200 kDa的中间体，进一步糖基化后，成熟的230 kDa形式在细胞表面表达。只有这种230 kDa的形式在VEGF存在下能迅速且短暂地在酪氨酸残基上磷酸化。作为KDR/Flk-1的主要底物，PLC-γ在内皮细胞和NIH3T3细胞中均被发现能迅速酪氨酸磷酸化并与KDR/Flk-1结合。然而，有趣的是，只有在内皮细胞背景下才会迅速激活MAP激酶并随后产生强烈的有丝分裂信号。过表达KDR/Flk-1的NIH3T3细胞中MAP激酶的激活显示出较慢的反应，因为与窦状内皮细胞中的5分钟相比，最大水平仅在20分钟后才能达到。这些结果表明，KDR/Flk-1利用细胞类型特异性信号转导途径来激活内皮细胞中的MAP激酶并引发有丝分裂反应。
