Hepatocyte growth factor (HGF) ameliorates liver injuries in hepatectomized cholestatic rats. On the other hand, the protein level of organic anion-transporting polypeptide (Oatp1), which is responsible for the uptake of bile salts into hepatocytes, decreases in cholestatic humans and rats. However, the relationship between the ameliorative effects of HGF and the decrease in Oatp1 levels in cholestasis remains to be understood. Therefore, in order to investigate this relationship, we evaluated the effects of HGF on the function and protein level of Oatp1. HGF treatment significantly increased the uptake of radiolabeled estradiol 17β-d-glucuronide ([3H]E217βG), a predominant Oatp1 substrate, in primary cultured rat hepatocytes. Additionally, there was an increase in the Oatp1 protein levels. The increased [3H]E217βG uptake was significantly inhibited by simultaneous incubation with the HGF receptor antibody and treatment with non-radiolabeled E217βG. However, inhibition by taurocholic acid, a Na+-taurocholate co-transporting polypeptide (Ntcp) substrate, was weaker than that caused by non-radiolabeled E217βG. Further, the increase was not altered by replacing Na+ in the medium with Li+. In the inhibition study, the increased [3H]E217βG uptake was inhibited by Oatp1 substrates, including bromosulfophthalein, ochratoxin A, and ouabain, but not by digoxin, which is an Oatp2-specific substrate. Furthermore, HGF did not alter the Oatp1 mRNA expression. In contrast, HGF treatment suppressed the ubiquitination of Oatp1 protein. In conclusion, this is the first report suggesting that HGF regulates Oatp1 protein level and that the ameliorative effects of HGF in cholestasis was induced, at least in part, by correcting the down-regulation of the Oatp1 protein level.
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