Pharmacological activation of the constitutive androstane receptor (CAR) protects the liver during cholestasis. The current study evaluates how activation of CAR influences genes involved in bile acid biosynthesis as a mechanism of hepatoprotection during bile acid-induced liver injury. CAR activators phenobarbital (PB) and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) or corn oil (CO) were administered to C57BL/6 wild-type (WT) and CAR knockout (CAR-null) mice before and during induction of intrahepatic cholestasis using the secondary bile acid, lithocholic acid (LCA). In LCA-treated WT and all the CAR-null groups (excluding controls), histology revealed severe multifocal necrosis. This pathology was absent in WT mice pretreated with PB and TCPOBOP, indicating CAR-dependent hepatoprotection. Decreases in total hepatic bile acids and hepatic monohydroxy, dihydroxy, and trihydroxy bile acids in PB- and TCPOBOP-pretreated WT mice correlated with hepatoprotection. In comparison, concentrations of monohydroxylated and dihydroxylated bile acids were increased in all the treated CAR-null mice compared with CO controls. Along with several other enzymes (Cyp7b1, Cyp27a1, Cyp39a1), Cyp8b1 expression was increased in hepatoprotected mice, which could be suggestive of a shift in the bile acid biosynthesis pathway toward the formation of less toxic bile acids. In CAR-null mice, these changes in gene expression were not different among treatment groups. These results suggest CAR mediates a shift in bile acid biosynthesis toward the formation of less toxic bile acids, as well as a decrease in hepatic bile acid concentrations. We propose that these combined CAR-mediated effects may contribute to the hepatoprotection observed during LCA-induced liver injury.
Non-alcoholic fatty liver disease (NAFLD) encompasses a spectrum of diagnoses ranging from simple fatty liver (SFL), to non-alcoholic steatohepatitis (NASH). This study aimed to determine the effect of moderate and severe NAFLD on hepatic transporter expression and function in vivo. Rats were fed a high-fat diet (SFL model) or a methionine-choline-deficient diet (NASH model) for eight weeks. Hepatic uptake transporter function was determined by bromosulfophthalein (BSP) disposition. Transporter expression was determined by branched DNA signal amplification assay and western blotting; inflammation was identified by immunostaining of liver slices for interleukin 1 beta (IL-1beta). MC- rats showed significant retention of BSP in the plasma when compared to control rats. Hepatic NTCP, OATP1a1, 1a4, 1b2 and 2b1; and OAT 2 and 3 mRNA levels were significantly decreased in high-fat and MC- diet rats when compared to control. Protein expression of OATP1a1 was significantly decreased in high-fat animals, while OATP1a1 and OATP1b2 expressions were significantly lower in MC- rats when compared to control. Liver tissue from high-fat and MC- rats stained positive for IL-1beta, a pro-inflammatory cytokine known to decrease expression of NTCP, OATP and OAT transporters, suggesting a plausible mechanism for the observed transporter alterations. These data suggest that different stages of NAFLD result in altered hepatic uptake transporter expression that can lead to a functional impairment of xenobiotic uptake from the blood. Furthermore, NAFLD may alter the plasma retention time of clinically relevant drugs that are reliant on these transporters and may increase the potential drug toxicity.
Efflux transporters are responsible for the excretion of numerous xenobiotics and endobiotics and thus play an essential role in proper liver and kidney function. Nonalcoholic fatty liver diseases (NAFLDs) comprise a spectrum of disorders that range from simple fatty liver (SFL) to nonalcoholic steatohepatitis (NASH). Although the precise events leading to NAFLD are unclear, even less is known about the effects on efflux transporter expression and drug disposition. The purpose of this study was to determine the effect of NAFLD on efflux transporter expression in rat liver as well as on acetaminophen (APAP) metabolite excretion. To simulate SFL and NASH, rats were fed either a high-fat (HF) or a methionine- and choline-deficient (MCD) diet for 8 weeks. In the livers of MCD rats, there were striking increases in both mRNA and protein levels of multidrug resistance-associated protein (Mrp) 3, Mrp4, and breast cancer resistance protein, as well as increased Mrp2 protein. After administration of a nontoxic dose of APAP, biliary concentrations of APAP-sulfate, APAP-glucuronide (APAP-GLUC), and APAP-glutathione were reduced in MCD rats. The effects of the HF diet on both transporter expression and APAP disposition were by comparison far less dramatic than the MCD diet-induced alterations. Whereas APAP-sulfate levels were also decreased in MCD rat plasma, the levels of the Mrp3 substrate APAP-GLUC were elevated. Urinary elimination of APAP metabolites was identical between groups, except for APAP-GLUC, the concentration of which was 80% higher in MCD rats. These studies correlate increased hepatic Mrp3 protein in the MCD model of NASH with increased urinary elimination of APAP-GLUC. Furthermore, the proportional shift in elimination of APAP metabolites from bile to urine indicates that MCD-induced alterations in efflux transporter expression can affect the route of drug elimination.
Myeloid cell leukemia-1 (Mcl-1) is an anti-apoptotic protein that is regulated by the constitutive androstane receptor (CAR). Activation of CAR can protect the liver against bile acid-induced toxicity and it may have a role in cell death via apoptosis by altering expression of Bcl-2 family proteins such as myeloid cell leukemia-1 (Mcl-1). Our aim was to determine if activation of CAR reduces hepatocellular apoptosis during cholestasis as a mechanism of hepatoprotection. CAR(+/+) (WT) and CAR(-/-) (CAR-null) mice were pre-treated with compounds known to activate CAR prior to induction of intrahepatic cholestasis using the secondary bile acid lithocholic acid (LCA). Pre-treatment with the CAR activators phenobarbital (PB) and TCPOBOP (TC), as well as the non-CAR activator pregnenolone 16alpha-carbontrile (PCN), protected against LCA-induced liver injury in WT mice, whereas liver injury was more extensive without CAR (CAR-null). Unexpectedly, expression of anti-apoptotic Mcl-1 and Bcl-x(L) was not increased in hepatoprotected mice. Compared to unprotected groups, apoptosis was decreased in hepatoprotected mice as evidenced by the absence of cleaved caspase 3 (cCasp3). In contrast to the cytoplasmic localization in the injured livers (LCA and oltipraz), Mcl-1 protein was localized in the nucleus of hepatoprotected livers to potentially promote cell survival. This study demonstrates that although apoptosis is reduced in hepatoprotected mice pre-treated with CAR and non-CAR activators; hepatoprotection is not directly a result of CAR-induced Mcl-1 expression.
Characteristic pathological changes define the progression of steatosis to nonalcoholic steatohepatitis (NASH) and are correlated to metabolic pathways. A common rodent model of NASH is the methionine and choline deficient (MCD) diet. The objective of this study was to perform full metabolomic analyses on liver samples to determine which pathways are altered most pronouncedly in this condition in humans, and to compare these changes to rodent models of nonalcoholic fatty liver disease (NAFLD).