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 prolifer ation 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 h epatocytes in primary culture Following a transient increase of its express ion. In these cells, the expression increased again following a peak of DNA synthesis when cultured with EGF. In 70% resected liver, mitosis was maxim al at 36 hours after the operation in hepatocytes and at 96 hours in sinuso idal endothelial cells. In such liver, VEGF mRNA expression increased in he patocytes at the G1 phase of the cell cycle and became prominent in the cel ls following the: M phase. Similar increase of VEGF expression was also see n immunohistochemically. On the other hand, mRNA expression of VEGF recepto rs was upregulated in the liver between 72 and 168 hours following 70% rese ction. When rats received carbon tetrachloride (CCl4), liver necrosis devel oped between 1 and 3 days following intoxication and the liver became norma l on histology at 7 days. Immunohistochemical examination using SE-1, a mon oclonal antibody against rat sinusoidal endothelial cells, revealed that va scular endothelial cells proliferated in the necrotic areas and the prolife ration of sinusoidal endothelial cells followed. In these rats, mRNA expres sions of VEGF receptors were increased in the liver later than 1 day of int oxication with a peak between 1 and 3 days. VEGF mRNA expression was minima l in Kupffer cells from normal rats, but marked in activated Kupffer cells and hepatic macrophages isolated from rats between 1 and 3 days after intox ication. VEGF mRNA expression was increased in activated stellate cells fro m 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 he patocytes from normal rats. Increased VEGF expressions in activated sinusoi dal cells and regenerating hepatocytes were also detected immunohistochemic ally. These data suggest that VEGF expressed in regenerating hepatocytes ma y contribute to proliferation of sinusoidal endothelial cells in partially resected liver, probably through VEGF receptors upregulated on the cells. I n injured liver, increased expression of VEGF may derive from activated Kup ffer 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.