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To seek an endothelium-specific growth factor by examining the mitogenic effects of hepatocyte growth factor (HGF) on endothelial cells and on vascular smooth muscle cells (VSMC).Rat and human endothelial cells and VSMC were employed. DNA, RNA and protein synthesis were measured by using [3H]-thymidine, uridine and leucine. Coculture of endothelial cells with VSMC was also performed to study the role of endothelial cells.Coculture of endothelial cells with VSMC resulted in a significant decrease in DNA synthesis of VSMC. HGF, as well as basic fibroblast growth factor (bFGF), stimulated DNA, RNA and protein synthesis by endothelial cells in a dose-dependent manner. Interestingly, co-incubation of endothelial cells with HGF and bFGF resulted in an additive stimulation of DNA synthesis. Similarly, HGF and interleukin-1 α and -6 stimulated DNA synthesis by coronary endothelial cells, whereas interleukin-1 β and transforming growth factor-β (TGF-β) did not. However, HGF showed markedly different actions from bFGF on VSMC growth. bFGF, TGF-β, interleukin-1α, -1β and -6 stimulated DNA synthesis in VSMC significantly, whereas HGF did not. Finally, we examined the mitogenic effect of HGF on human aortic endothelial cells and VSMC. Incubation with HGF increased DNA synthesis and growth by endothelial cells in a dosedependent manner, whose degree was significantly greater than those with bFGF, vascular endothelial growth factor (VEGF) and interleukin-6. Addition of HGF and VEGF showed no additive effect on DNA synthesis in endothelial cells, in contrast to those of bFGF and HGF. On the other hand, bFGF, but not HGF and VEGF, stimulated DNA synthesis in VSMC.These results demonstrate that HGF can exert stimulating effects on endothelial cell growth, but not on VSMC growth, in an additive manner with bFGF but not with VEGF. These characteristics of HGF as an endotheliumspecific growth factor may provide the opportunity for a new therapeutic strategy for vascular diseases in which the abnormalities are vasoconstriction and pathological growth.