Overexpression of the Rho-guanine nucleotide exchange factor ECT2 inhibits nuclear translocation of nuclear receptor CAR in the mouse liver

Effects of dexamethasone to reverse decreased hepatic midazolam metabolism in rats with acute renal failure

The inductive effects of dexamethasone on hepatic midazolam metabolism were examined in Wistar rats with acute renal failure (ARF) to clarify whether the ARF-related decrease in the hepatic expression of drug-metabolizing enzymes is caused by an impairment in the translation/polypeptide formation process.ARF was induced with intramuscular glycerol injection. Dexamethasone was orally administered. Pooled liver microsomes from five rats were prepared with ultracentrifugation for each of four groups, namely, control and ARF rats, control rats with dexamethasone treatment and ARF rats with dexamethasone treatment.Hepatic drug-metabolizing activity was examined in an incubation study with the microsomes, where midazolam was employed as a substrate of cytochrome P450 (CYP) 3A enzymes. The hepatic protein and mRNA expressions of CYP3A23/3A1 and 3A2 enzymes were also evaluated.With dexamethasone treatment, the hepatic metabolic rate of midazolam increased 1.4 times in control rats, while it increased 19.6 times in ARF rats, reflecting the greater induction of hepatic protein expressions of CYP3A enzymes in ARF rats than in control rats.The hepatic protein expression process for CYP3A23/3A1 and 3A2 responds well to dexamethasone treatment in ARF rats, indicating that the translation/polypeptide formation process is not impaired in the presence of ARF.

Coordinated regulation of nuclear receptor CAR by CCRP/DNAJC7, HSP70 and the ubiquitin-proteasome system

The constitutive active/androstane receptor (CAR) plays an important role as a coordinate transcription factor in the regulation of various hepatic metabolic pathways for chemicals such as drugs, glucose, fatty acids, bilirubin, and bile acids. Currently, it is known that in its inactive state, CAR is retained in the cytoplasm in a protein complex with HSP90 and the tetratricopeptide repeat protein cytosoplasmic CAR retention protein (CCRP). Upon activation by phenobarbital (PB) or the PB-like inducer 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP), CAR translocates into the nucleus. We have identified two new components to the cytoplasmic regulation of CAR: ubiquitin-dependent degradation of CCRP and protein-protein interaction with HSP70. Treatment with the proteasome inhibitor MG132 (5 µM) causes CAR to accumulate in the cytoplasm of transfected HepG2 cells. In the presence of MG132, TCPOBOP increases CCRP ubiquitination in HepG2 cells co-expressing CAR, while CAR ubiquitination was not detected. MG132 treatment of HepG2 also attenuated of TCPOBOP-induced CAR transcriptional activation on reporter constructs which contain CAR-binding DNA elements derived from the human CYP2B6 gene. The elevation of cytoplasmic CAR protein with MG132 correlated with an increase of HSP70, and to a lesser extent HSP60. Both CCRP and CAR were found to interact with endogenous HSP70 in HepG2 cells by immunoprecipitation analysis. Induction of HSP70 levels by heat shock also increased cytoplasmic CAR levels, similar to the effect of MG132. Lastly, heat shock attenuated TCPOBOP-induced CAR transcriptional activation, also similar to the effect of MG132. Collectively, these data suggest that ubiquitin-proteasomal regulation of CCRP and HSP70 are important contributors to the regulation of cytoplasmic CAR levels, and hence the ability of CAR to respond to PB or PB-like inducers.

Molecular mechanisms of drug transporter regulation

Interindividual differences in drug transporter expression can result in variability in drug response. This variation in gene expression is determined, in part, by the actions of nuclear hormone receptors that act as xenobiotic- and endobiotic-sensing transcription factors. Among the ligand-activated nuclear receptors, signaling through the pregnane X receptor (PXR), constitutive androstane receptor (CAR), farnesoid X receptor (FXR), and vitamin D receptor (VDR) constitute major pathways regulating drug transporter expression in tissues. Hence, these endobiotic- and xenobiotic-sensing nuclear receptors are intrinsically involved in environmental influences of drug response. Moreover, because nuclear receptor genes are polymorphic, these transcription factors are also thought to contribute to heritability of variable drug action. In this chapter, the molecular aspects of drug transporter gene regulation by ligand-activated nuclear receptors will be reviewed including their clinical relevance.

Hepatic OATP1B transporters and nuclear receptors PXR and CAR: interplay, regulation of drug disposition genes, and single nucleotide polymorphisms

Drug uptake transporters are now increasingly recognized as clinically relevant determinants of variable drug responsiveness and unexpected drug-drug interactions. Emerging evidence strongly suggests members of the organic anion transporting polypeptide (OATP) family appear to be particularly important to the disposition of many drugs in clinical use today. Specifically, the liver-enriched OATP1B subfamily members OATP1B1 and OATP1B3 exhibit broad substrate specificity and the ability to transport drugs which are ligands for xenobiotic sensing nuclear receptors such as the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Accordingly, OATP1B transporters may indirectly regulate expression of drug metabolism genes via modulation of the intracellular concentration of PXR and CAR ligands. Moreover, a number of functionally important single nucleotide polymorphisms (SNPs) in OATP1B transporters have been described. In this review, a brief summary of known SNPs in PXR and CAR will be followed by an in-depth outline of OATP1B1 and OATP1B3 transporters particularly in relation to the known SNPs in these OATPs and the interplay between OATP1B transporters with PXR and CAR, both in vitro and in vivo.

Discovery of substituted sulfonamides and thiazolidin-4-one derivatives as agonists of human constitutive androstane receptor

The constitutive androstane receptor (CAR; NR1I3) is a nuclear receptor responsible for the recognition of potentially toxic endo- and exogenous compounds whose elimination from the body is accelerated by the CAR-mediated inducible expression of metabolizing enzymes and transporters. Despite the importance of CAR, few human agonists are known so far. Following a sequential virtual screening procedure using a 3D pharmacophore and molecular docking approach, we identified 17 novel agonists that could activate human CAR in vitro and enhance its association with the nuclear receptor co-activator SRC1. Selected agonists also increased the expression of the human CAR target CYP2B6 mRNA in primary hepatocytes. Composed of substituted sulfonamides and thiazolidin-4-one derivatives, these agonists represent two novel chemotypes capable of human CAR activation, thus broadening the agonist spectrum of CAR.

Nuclear receptor CAR requires early growth response 1 to activate the human cytochrome P450 2B6 gene

The nuclear receptor CAR (constitutive active/androstane receptor) is a drug-sensing transcription factor, regulating the hepatic genes that encode various drug-metabolizing enzymes. We have now characterized the novel regulatory mechanism by which the signal molecule EGR1 (early growth response 1) determines CAR-mediated activation of the human CYP2B6 (cytochrome P450 2B6) gene. The CYP2B6 enzyme metabolizes commonly used therapeutics and also activates pro-drugs. The CAR directly binds to the distal enhancer element of the CYP2B6 promoter, which is essential in converging to its drug-sensing function onto promoter activity. However, this binding alone is not sufficient to activate the CYP2B6 promoter; the promoter requires EGR1 to enable CAR to activate the CYP2B6 promoter. Upon stimulation by protein kinase C, EGR1 directly binds to the proximal promoter and coordinates the nearby HNF4alpha (hepatocyte-enriched nuclear factor 4alpha) with CAR at the distal enhancer element to activate the promoter. Thus, synergy of drug activation and the stimulation of cellular signal are necessary for CAR to activate the CYP2B6 gene.