Involvement of hepatic stellate cell cytoglobin in acute hepatocyte damage through the regulation of CYP2E1-mediated xenobiotic metabolism

Abstract

Oxygen (O 2 ) is required for cytochrome P450 (CYP)-dependent drug metabolism. Cytoglobin (CYGB) is a unique globin expressed exclusively in hepatic stellate cells (HSCs). However, its role in O 2 -dependent metabolism in neighboring hepatocytes remains unknown. This study provides evidence that CYGB in HSCs is involved in acetaminophen (N-acetyl-p-aminophenol; APAP)-induced hepatotoxicity. Serum alanine aminotransferase levels were higher in wild-type mice than in Cygb-null mice. Wild-type mice exhibited more severe hepatocyte necrosis around the central vein area compared with Cygb-null mice, thus indicating that CYGB deficiency protects against APAP-induced liver damage. Although no difference in the hepatic expression of CYP2E1, a key enzyme involved in APAP toxicity, was observed between wild-type and Cygb-null mice, the serum levels of the APAP metabolites cysteinyl-APAP and N-acetyl-cysteinyl-APAP were decreased in Cygb-null mice, suggesting reduced APAP metabolism in the livers of Cygb-null mice. In primary cultures, APAP-induced hepatocyte damage was increased by co-culturing with wild-type HSCs but not with Cygb-null HSCs. In addition, cell damage was markedly alleviated under low O 2 condition (5% O 2 ), suggesting the requirement of O 2 for APAP toxicity. Carbon tetrachloride-induced liver injury (CYP2E1-dependent), but not lipopolysaccharide/ D-galactosamine-induced injury (CYP2E1-independent), was similarly alleviated in Cygb-null mice. Considering the function of CYGB as O 2 carrier, these results strongly support the hypothesis that HSCs are involved in the CYP2E1-mediated xenobiotic activation by augmenting O 2 supply to hepatocytes. In conclusion, CYGB in HSCs contributes to the CYP-mediated metabolism of xenobiotics in hepatocytes by supplying O 2 for enzymatic oxidation. The metabolism of xenobiotics, including clinical drugs, occurs primarily in the liver. Most xenobiotic detoxification metabolism involves cytochrome P450 (CYP) enzymes, which are predominantly expressed in the liver, 1 and these processes occasionally result in acute hepatocyte damage. For instance, acetaminophen (N-acetyl-p-aminophenol; APAP) and carbon tetrachloride (CCl 4 ) are metabolized by CYP to generate toxic intermediates.APAP is commonly used as an antipyretic and analgesic drug. CYP-mediated APAP metabolism is known to generate N-acetyl-p-benzoquinone imine (NAPQI), which binds to cellular macromolecules and initiates hepatocyte damage if present at high concentrations under glutathione (GSH) depletion. 2 CYP1A2, CYP2A6, CYP2E1, and CYP3A have been identified as enzymes that generate NAPQI from APAP. 3 Among these CYPs, CYP2E1 has the lowest K m for APAP, and the CYP2E1-dependent metabolism of APAP can result in hepatotoxicity. 4 CYP2E1 is a monooxygenase that requires molecular oxygen (O 2 ) for its enzymatic activity. A recent study reported that the APAP-induced damage to primary mouse hepatocytes was reduced under a low O 2 concentration (5% O 2 ) compared with normoxic conditions (20% O 2 ). 5 Thus, APAP-indu...

The Hemoglobin Homolog Cytoglobin in Smooth Muscle Inhibits Apoptosis and Regulates Vascular Remodeling

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