A novel xenobiotic responsive element regulated by aryl hydrocarbon receptor is involved in the induction of BCRP/ABCG2 in LS174T cells.

Induction of the breast cancer resistance protein (BCRP/ABCG2) expression has been found in various tissues and cell-types after exposure to chemicals including 17ß-estradiol, rosiglitazone, imatinib, as well as aryl hydrocarbon receptor (AhR) activators such as 2,3,7,8-tetrachlorodibenzodioxin, 3-methylcholanthrene (3MC), and omeprazole. However, the mechanism(s) underlying AhR-related induction of ABCG2 is largely unknown. Here, we demonstrate the AhR-dependent induction of ABCG2 expression in human colon adenocarcinoma LS174T cells. Importantly, a novel distal AhR-responsive element (AhRE5) located -2357/-2333bp upstream of the ABCG2 transcriptional start site has been identified and characterized as a functional unit pivotal to 3MC-mediated induction of ABCG2. Cell-based reporter assays revealed that deletion of AhRE5 and 4 dramatically attenuated 3MC-induced activation of ABCG2 reporter activity, while further deletion of the proximal AhRE3 and 2 only moderately changed the luciferase activities. Notably, site-directed mutation of the AhRE5 in the BCRP-3.8kb reporter construct alone resulted in approximately 80% decrease in 3MC activation of the ABCG2 promoter; additional mutation of the AhRE4 site had negligible effect on the ABCG2 promoter activity. Moreover, chromatin immunoprecipitation assays demonstrated that treatment with 3MC significantly enhanced the recruitment of AhR to the AhRE5 occupied region, and mutation of the AhRE5 site clearly dissociated AhR protein from this promoter region. Together, these data show that the novel distal AhRE5 is critical for AhR-mediated transcriptional activation of ABCG2 gene expression in LS174T cells, and it may offer new strategies for early identification of ABCG2 inducers, which would be of benefit for preventing transporter-associated drug-drug interactions.

A single amino acid controls the functional switch of human constitutive androstane receptor (CAR) 1 to the xenobiotic-sensitive splicing variant CAR3.

The constitutive androstane receptor (CAR) is constitutively activated in immortalized cell lines independent of xenobiotic stimuli. This feature of CAR has limited its use as a sensor for xenobiotic-induced expression of drug-metabolizing enzymes. Recent reports, however, reveal that a splicing variant of human CAR (hCAR3), which contains an insertion of five amino acids (APYLT), exhibits low basal but xenobiotic-inducible activities in cell-based reporter assays. Nonetheless, the underlying mechanisms of this functional shift are not well understood. We have now generated chimeric constructs containing various residues of the five amino acids of hCAR3 and examined their response to typical hCAR activators. Our results showed that the retention of alanine (hCAR1+A) alone is sufficient to confer the constitutively activated hCAR1 to the xenobiotic-sensitive hCAR3. It is noteworthy that hCAR1+A was significantly activated by a series of known hCAR activators, and displayed activation superior to that of hCAR3. Moreover, intracellular localization assays revealed that hCAR1+A exhibits nuclear accumulation upon 6-(4-chlorophenyl) imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl) oxime (CITCO) treatment in COS1 cells, which differs from the spontaneous nuclear distribution of hCAR1 and the nontranslocatable hCAR3. Mammalian two-hybrid and glutathione S-transferase pull-down assays further demonstrated that hCAR1+A interacts with the coactivator SRC-1 and GRIP-1 at low level before activation, while at significantly enhanced level in the presence of CITCO. Thus, the alanine residue in the insertion of hCAR3 seems in charge of the xenobiotic response of hCAR3 through direct and indirect mechanisms. Activation of hCAR1+A may represent a sensitive avenue for the identification of hCAR activators.

CYP2B6: new insights into a historically overlooked cytochrome P450 isozyme.

Human CYP2B6 has been thought to account for a minor portion (<1%) of total hepatic cytochrome P450 (CYP) content and to have a minor function in human drug metabolism. Recent studies, however, indicate that the average relative contribution of CYP2B6 to total hepatic CYP content ranges from 2% to 10%. An increased interest in CYP2B6 research has been stimulated by the identification of an ever-increasing substrate list for this enzyme, polymorphic and ethnic variations in expression levels, and evidence for cross-regulation with CYP3A4, UGT1A1 and several hepatic drug transporters by the nuclear receptors pregnane X receptor and constitutive androstane receptor. Moreover, 20- to 250-fold interindividual variation in CYP2B6 expression has been demonstrated, presumably due to transcriptional regulation and polymorphisms. These individual differences may result in variable systemic exposure to drugs metabolized by CYP2B6, including the antineoplastics cyclophosphamide and ifosfamide, the antiretrovirals nevirapine and efavirenz, the anesthetics propofol and ketamine, the synthetic opioid methadone, and the anti-Parkinsonian selegiline. The potential clinical significance of CYP2B6 further enforces the need for a comprehensive review of this xenobiotic metabolizing enzyme. This communication summarizes recent advances in our understanding of this traditionally neglected enzyme and provides an overall picture of CYP2B6 with respect to expression, localization, substrate-specificity, inhibition, regulation, polymorphisms and clinical significance. Emphasis is given to nuclear receptor mediated transcriptional regulation, genetic polymorphisms, and their clinical significance.

Unique transcription start sites and distinct promoter regions differentiate the pregnane X receptor (PXR) isoforms PXR 1 and PXR 2.

The pregnane X receptor (PXR) is known as the xenosensing receptor responsible for coordinated regulation of metabolic genes in response to diverse xenobiotic challenges. In particular, the ability of the PXR to regulate CYP3A4, the enzyme capable of metabolizing more than 60% of all pharmaceuticals, defines its metabolic importance. Currently the list of PXR ligands and target genes is extensive, yet investigations into the regulation and expression of PXRs are few. After an initial review of available sequence data, we discovered discrepancies in the 5' untranslated region (UTR) and transcriptional start site (TSS) characterizations of the human PXR gene and subsequently endeavored to define TSSs and proximal promoters for isoforms PXR 1 and PXR 2. Reverse transcriptase-polymerase chain reaction and primer extension experiments performed on RNA from human liver identified two TSSs for each receptor isoform. These results extended the 5'UTR sequence of each isoform and defined new proximal promoters for both. Candidate response elements for liver-enriched transcription factors and other receptors were found in both proximal promoters. Quantitative PCR from human liver illustrated a highly variable expression profile for total PXRs; yet PXR 2 expression represented a consistent 2 to 5% of total PXR expression, despite the observed variability. Transfection experiments demonstrated that PXR 1 and PXR 2 had comparable abilities to transcriptionally activate the CYP3A4 promoter. Collectively, comparable function, consistent expression, and independent regulation suggest that PXR 2 is capable of contributing to the cumulative function of PXRs and should be included in the larger investigations of PXR expression and regulation.

Mechanisms of cytochrome P450 induction.

Cytochrome P450s (CYPs) are important heme-containing proteins that play important roles in the metabolism of xenobiotics and endogenous compounds. The oxidative metabolisms of drugs, environmental chemicals, hormones, and fatty acids by CYP enzymes are critical pathways aiding in their excretion from the body, but in some cases metabolism may lead to bioactivation and enhanced toxicity. The expression and activity levels of CYPs can be elevated by a process of induction involving the activation of key transcription factors. The mechanisms by which CYP3A4, 2B6, and 1A1 are induced involving the activation of the transcription factors pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AhR) will be discussed.