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

Abstract

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

Disease‐Associated Changes in Drug Transporters May Impact the Pharmacokinetics and/or Toxicity of Drugs: A White Paper From the International Transporter Consortium

“…The similar changes in nonsepsis patients may be explained by ischemia/reperfusion‐related inflammation and cytokine release, because a large proportion of nonsepsis patients were admitted to the ICU postcardiac surgery. These findings also are consistent with hypoxia‐associated increases in hepatic basolateral organic anion transporter α and β (OSTα/β) expression, which may contribute to increased efflux of bile acids from hepatocytes into the systemic circulation …”

Organic solute transporter OSTα/β is overexpressed in nonalcoholic steatohepatitis and modulated by drugs associated with liver injury

“…The recent discovery that bile acid concentrations are increased in the serum and liver of patients with NASH (22,37) supports the finding that OST␣/␤ is induced in patients with NASH. In vitro data also support this hypothesis, because both hypoxia and elevated bile acid concentrations, conditions similar to patients with NASH (37,46), induced OST␣ and OST␤ expression (52). OST␣/␤ induction is mediated through the bile acid-activated farnesoid X receptor (FXR, NR1H4) pathway (7,65).…”

Changes in Biliary Levels of Arginine and its Methylated Derivatives after Hepatic Ischaemia/Reperfusion

“…have also demonstrated that no difference in mRNA of CAT‐2 occurs after kidney I/R; on the contrary, a decrease, although not significant, in mRNA CAT‐2A was found . The role of I/R injury in the regulation of hepatic transporters is not new in the literature, though not sufficiently explored: a previous study has demonstrated that hypoxia regulates the expression of hepatobiliary transporter genes ; further data in human hepatocytes have demonstrated that the organic solute transporters alpha and beta are induced by hypoxia .…”