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

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Schaffner CA, Mwinyi J, Gai Z, Thasler WE, Eloranta JJ, Kullak-Ublick GA

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

Liver Int. 2015 Apr;35(4):1152-61. doi: 10.1111/liv.12558. Epub 2014 Apr 29.

PubMed ID
24703425 [ View in PubMed
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Abstract

BACKGROUND & AIMS: The organic solute transporters alpha and beta (OSTalpha-OSTbeta) 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 OSTalpha-OSTbeta. METHODS: OSTalpha-OSTbeta expression was measured in Huh7 cells and primary human hepatocytes (PHH) exposed to chenodeoxycholic acid (CDCA), hypoxia or both. OSTalpha-OSTbeta promoter activity was analysed in luciferase reporter gene assays. Binding of hypoxia-inducible factor-1 alpha (HIF-1alpha) to the OSTalpha-OSTbeta gene promoters was studied in electrophoretic mobility shift assays (EMSA). RESULTS: Expression of OSTalpha and OSTbeta increased in PHH under conditions of hypoxia. Exposure of Huh7 cells or PHH to CDCA (50 muM) enhanced the effect of hypoxia on OSTalpha mRNA levels. In luciferase assays and EMSA, the inducing effect of low oxygen could be assigned to HIF-1alpha, which binds to hypoxia responsive elements (HRE) in the OSTalpha and OSTbeta gene promoters. Site-directed mutagenesis of either the predicted HRE or the bile acid responsive FXR binding site abolished inducibility of the OSTalpha 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 OSTalpha expression was associated with an increase in HIF-1alpha protein levels. CONCLUSION: OSTalpha-OSTbeta expression is induced by hypoxia. FXR and HIF-1alpha bind in close proximity to the OSTalpha gene promoter and produce synergistic effects on OSTalpha expression.