Characterization and development of a peptide substrate-based phosphate transfer assay for the human vascular endothelial growth factor receptor-2 tyrosine kinase

Vascular endothelial growth factor (VEGF), a critical regulator in angiogenesis, exerts its angiogenic effect via binding to its receptor, VEGF receptor-2 tyrosine kinase (VEGFR2) or kinase insert domain-containing receptor (Kdr), on the surface of endothelial cells. Kdr-mediated signaling plays an important role in the proliferation, migration, differentiation, and survival of endothelial cells. Therefore, the inhibition of this signaling pathway represents a promising therapeutic approach for the discovery of novel anticancer agents by destabilizing the progression of solid tumors via abrogating tumor-induced angiogenesis. To explore Kdr as an anticancer target and further characterize the enzyme, we purified a cytoplasmic domain of human Kdr (Kdr–CD) and characterized its autophosphorylation activity. We also designed and synthesized peptides containing amino acid sequences corresponding to the autophosphorylation sites of Kdr and developed a simple, robust, high-throughput assay for measuring the phosphate transfer activity of the enzyme. This assay was validated by the experiments showing that the phosphate transfer activity of the purified Kdr–CD required Mg 2+ or Mn 2+ and preactivation by adenosine 5′-triphosphate (ATP) and was inhibited by known Kdr inhibitors. Using this assay, we examined effects of Mg 2+ and Mn 2+ on the enzyme activity; optimized the concentrations of Kdr–CD, peptide and ATP substrates, and metal ions in the assay; and determined the kinetic properties of the enzyme for the peptide and ATP as well as IC 50 values of two known Kdr inhibitors. Thus, the results of these studies have validated the utilities of this assay for biochemical characterizations of the enzyme and its inhibitors. This approach of designing peptides corresponding to the autophosphorylation sites of Kdr as substrates for the enzyme has general practical implications to other kinases.