The organic solute transporters alpha and beta are induced by hypoxia in human hepatocytes

Abstract

Background & Aims: The organic solute transporters alpha and beta (OSTa- OSTb) form a heterodimeric transporter located at the basolateral membrane of intestinal epithelial cells and hepatocytes. Liver injury caused by ischaemia- reperfusion, cancer, inflammation or cholestasis can induce a state of hypoxia in hepatocytes. Here, we studied the effect of hypoxia on the expression of OSTa-OSTb. Methods: OSTa-OSTb expression was measured in Huh7 cells and primary human hepatocytes (PHH) exposed to chenodeoxycholic acid (CDCA), hypoxia or both. OSTa-OSTb promoter activity was analysed in luciferase reporter gene assays. Binding of hypoxia-inducible factor- 1 alpha (HIF-1a) to the OSTa-OSTb gene promoters was studied in electrophoretic mobility shift assays (EMSA). Results: Expression of OSTa and OSTb increased in PHH under conditions of hypoxia. Exposure of Huh7 cells or PHH to CDCA (50 lM) enhanced the effect of hypoxia on OSTa mRNA levels. In luciferase assays and EMSA, the inducing effect of low oxygen could be assigned to HIF-1a, which binds to hypoxia responsive elements (HRE) in the OSTa and OSTb gene promoters. Site-directed mutagenesis of either the predicted HRE or the bile acid responsive FXR binding site abolished inducibility of the OSTa promoter, indicating that both elements need to be intact for induction by hypoxia and CDCA. In a rat model of chronic renal failure, the known increase in hepatic OSTa expression was associated with an increase in HIF-1a protein levels. Conclusion: OSTa-OSTb expression is induced by hypoxia. FXR and HIF-1a bind in close proximity to the OSTa gene promoter and produce synergistic effects on OSTa expression.