Impact of higher-order heme degradation products on hepatic function and hemodynamics

- Plasma levels of bilirubin oxidation products rise in cholestatic liver disease.
- The regio-isomers Z-BOX A and B exhibit distinct pharmacokinetics and -dynamics.
- Z-BOX A and B differentially affect the hepatocellular glutathione redox state.
- Z-BOX A and B differentially modulate the activity of Rev-erbα and Rev-erbβ.

Background & AimsBiliverdin and bilirubin were previously considered end products of heme catabolism; now, however, there is evidence for further degradation to diverse bioactive products. Z-BOX A and Z-BOX B arise upon oxidation with unknown implications for hepatocellular function and integrity. We studied the impact of Z-BOX A and B on hepatic functions and explored their alterations in health and cholestatic conditions.MethodsFunctional implications and mechanisms were investigated in rats, hepatocytic HepG2 and HepaRG cells, human immortalized hepatocytes, and isolated perfused livers. Z-BOX A and B were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in acute and acute-on-chronic liver failure and hereditary unconjugated hyperbilirubinemia.ResultsZ-BOX A and B are found in similar amounts in humans and rodents under physiological conditions. Serum concentrations increased ∼20-fold during cholestatic liver failure in humans (p < 0.001) and in hereditary deficiency of bilirubin glucuronidation in rats (p < 0.001). Pharmacokinetic studies revealed shorter serum half-life of Z-BOX A compared to its regio-isomer Z-BOX B (p = 0.035). While both compounds were taken up by hepatocytes, Z-BOX A was enriched ∼100-fold and excreted in bile. Despite their reported vasoconstrictive properties in the brain vasculature, BOXes did not affect portal hemodynamics. Both Z-BOX A and B showed dose-dependent cytotoxicity, affected the glutathione redox state, and differentially modulated activity of Rev-erbα and Rev-erbβ. Moreover, BOXes-triggered remodeling of the hepatocellular cytoskeleton.ConclusionsOur data provide evidence that higher-order heme degradation products, namely Z-BOX A and B, impair hepatocellular integrity and might mediate intra- and extrahepatic cytotoxic effects previously attributed to hyperbilirubinemia.Lay summaryDegradation of the blood pigment heme yields the bile pigment bilirubin and the oxidation products Z-BOX A and Z-BOX B. Serum concentrations of these bioactive molecules increase in jaundice and can impair liver function and integrity. Amounts of Z-BOX A and Z-BOX B that are observed during liver failure in humans have profound effects on hepatic function when added to cultured liver cells or infused into healthy rats.

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