Regeneration Of Sinusoidal Endothelial Cells: A Possible Communication System Through Vascular Endothelial Growth Factor Among Liver Cells

Vascular endothelial growth factor (VEGF) has been shown to induce proliferation of sinusoidal endothelial cells in primary culture. To elucidate the mechanisms of sinusoidal endothelial cell regeneration in vivo, expressions of VEGF and its receptors, flt-1 and KDR/flk-1, were studied in rat livers. Northern blot analysis revealed that VEGF mRNA expression diminished in hepatocytes in primary culture Following a transient increase of its expression. In these cells, the expression increased again following a peak of DNA synthesis when cultured with EGF. In 70% resected liver, mitosis was maximal at 36 hours after the operation in hepatocytes and at 96 hours in sinusoidal endothelial cells. In such liver, VEGF mRNA expression increased in hepatocytes at the G1 phase of the cell cycle and became prominent in the cells following the: M phase. Similar increase of VEGF expression was also seen immunohistochemically. On the other hand, mRNA expression of VEGF receptors was upregulated in the liver between 72 and 168 hours following 70% resection. When rats received carbon tetrachloride (CCl4), liver necrosis developed between 1 and 3 days following intoxication and the liver became normal on histology at 7 days. Immunohistochemical examination using SE-1, a monoclonal antibody against rat sinusoidal endothelial cells, revealed that vascular endothelial cells proliferated in the necrotic areas and the proliferation of sinusoidal endothelial cells followed. In these rats, mRNA expressions of VEGF receptors were increased in the liver later than 1 day of intoxication with a peak between 1 and 3 days. VEGF mRNA expression was minimal in Kupffer cells from normal rats, but marked in activated Kupffer cells and hepatic macrophages isolated from rats between 1 and 3 days after intoxication. VEGF mRNA expression was increased in activated stellate cells from CCl4-intoxicated rats and in stellate cells from normal rats activated by culture. Also, regenerating hepatoeytes isolated from rats at 7 days after intoxication showed marked increase of VEGF mRNA expression compared to hepatocytes from normal rats. Increased VEGF expressions in activated sinusoidal cells and regenerating hepatocytes were also detected immunohistochemically. These data suggest that VEGF expressed in regenerating hepatocytes may contribute to proliferation of sinusoidal endothelial cells in partially resected liver, probably through VEGF receptors upregulated on the cells. In injured liver, increased expression of VEGF may derive from activated Kupffer cells, hepatic macrophages and stellate cells as well as regenerating hepatocytes. These activated sinusoidal cells might induce proliferation of vascular endothelial cells by producing PDGF and basic FGF as well as VEGF It seems that VEGF derived from regenerating hepatocytes is essential for sinusoidal endothelial cell proliferation in reconstruction of the hepatic sinusoids.