Assessment of Human Pregnane X Receptor Involvement in Pesticide-Mediated Activation of CYP3A4 Gene

Differential modulation of xenobiotic-metabolizing enzymes in rats following single and concurrent exposure to chlorpyrifos, arsenic, and ascorbic acid

The present study was undertaken to evaluate the subacute toxicity of arsenic (As) and chlorpyrifos (CPF) alone or in combination. In addition, the ameliorative effect of ascorbic acid on As and/or CPF-induced hepatic microsomal xenobiotic metabolizing enzymes in rats was examined. Rats were divided into 9 groups of 6 animals each: control (deionized water), vehicle control (groundnut oil), ascorbic acid (100 mg/kg body weight), As (40 ppm in water), CPF (5 mg/kg body weight), As (40 ppm) + CPF (5 mg/kg body weight), As + ascorbic acid, CPF + ascorbic acid, and As + CPF + ascorbic acid. After 28 d of exposure, rats were sacrificed and liver was extracted for isolation of hepatic microsomes. Exposure to As or CPF alone as well as both of these in combination significantly altered microsomal proteins and activity of phase I and phase II xenobiotic-metabolizing enzymes. Cytochrome P-450 and cytochrome b 5 levels and activities of aniline p-hydroxylase (APH) and uridine diphosphate glucuronosyltransferase (UGT) were significantly decreased in groups treated with As, CPF, and As plus CPF, while glutathione S-transferase (GST) was not markedly altered. Enzymatic activity of aminopyrine N-demethylase (ANDM) was also significantly reduced in As- and CPF-only groups. Co-administration of ascorbic acid effectively countered the As- and CPF-induced alterations in xenobiotic-metabolizing enzymes.

Mechanisms of CYP3A induction by glucocorticoids in human fetal liver cells

Human fetal liver (HFL) cells express major drug metabolic enzymes CYP3A4, CYP3A5 and CYP3A7. In the fetal hepatocytes, betamethasone and dexamethasone (DEX) markedly enhanced the expression levels of CYP3A4 and CYP3A7 mRNAs and slightly increased the expression level of CYP3A5 mRNA. Interestingly, a high correlation between the CYP3A induction ability and the intensity of anti-inflammatory effect was observed. Human glucocorticoid receptor (GR)-small interfering RNA clearly attenuated the expression level of GR mRNA, and diminished the DEX-stimulated CYP3A4, CYP3A5 and CYP3A7 expression in HFL cells. These findings indicate that GR mediates the induction of CYP3A4 and CYP3A7 expression in human fetal hepatocytes as well as the CYP3A5.

Polycyclic aromatic hydrocarbons activate CYP3A4 gene transcription through human pregnane X receptor

Aryl hydrocarbon receptor (AhR) activators have been shown to induce members of the cytochrome P450 (P450) 1 family. Here we demonstrate that the AhR activators induce CYP3A4 through human pregnane X receptor (PXR). AhR activators, polycyclic aromatic hydrocarbons (PAHs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increased CYP3A4 reporter activity and CYP3A4 mRNA expression in HepG2 cells. The CYP3A4 reporter activity was also increased by treatment with cigarette tar. The increased CYP3A4 reporter activity was clearly knocked down by the introduction of human PXR-small interfering RNA, but not by that of human AhR-small interfering RNA. The CYP3A4 reporter activity enhanced by overexpression of human PXR was further increased by treatment with PAHs and TCDD as well as by treatment with rifampicin. These results suggest that PAHs contained in cigarette smoke induce CYP3A4 in human liver.

Unveiling a new essential cis element for the transactivation of the CYP3A4 gene by xenobiotics

Pregnane X receptor (PXR) has been shown to form a heterodimer with retinoid X receptor alpha (RXRalpha) and to bind to the distal nuclear receptor-binding element 1 and an everted repeat separated by six nucleotides in the proximal promoter of the CYP3A4 gene. In the present study, a new rifampicin-responsive region, located at -7.6 kilobases upstream from the transcription initiation site, has been identified using reporter assays in HepG2 cells. This region contains a cluster of possible nuclear receptor-binding half-sites, AG(G/T)TCA-like sequence. Of these putative half-sites, we focused six half-sites and termed them alpha-eta half-sites. Introduction of a mutation into either an alpha or beta half-site of CYP3A4 reporter genes almost completely diminished the rifampicin-induced transcription. In electrophoretic mobility shift assays, PXR/RXRalpha heterodimer bound to the direct repeat separated by four nucleotides (DR4) formed with alpha and beta half-sites. HepG2-based transactivation assays with the reporter gene constructs with or without mutations in the PXR binding element(s) demonstrated that this DR4 motif is essential for the transcriptional activation not only by rifampicin but also by various human PXR activators. In addition, reporter assays performed in human hepatocytes and mice with adenoviruses expressing luciferase derived from various CYP3A4 reporter genes and that expressing human PXR supported the results of experiments in HepG2 cells. These results suggest the obligatory role of the newly identified direct repeat separated by four nucleotides-type PXR binding element of the CYP3A4 gene for xenobiotic induction of CYP3A4.

PXR-mediated transcriptional activation of CYP3A4 by cryptotanshinone and tanshinone IIA

Danshen (Radix Salvia miltiorrhiza) is a famous Traditional Chinese Medicine used widely for the treatment of coronary heart disease and cerebrovascular disease. Diterpenoid tanshinones including tanshinone I, tanshinone IIA and cryptotanshinone are the major bioactive components from Danshen herb. Previous reports have demonstrated that Danshen extracts could induce the expression of CYP3A in rodents, however, the constituents responsible for Danshen-mediated CYP3A induction and the underlying molecular mechanisms remain unknown. The discovery of a family of nuclear receptors such as pregnane X receptor (PXR), constitutive androstane receptor (CAR) and glucocorticoid receptor (GR) gives insight into the molecular explanation of CYP3A induction by xenobiotics. In the present study, interactions between Danshen constituents and human PXR were evaluated using a reporter gene assay. Our observations showed that Danshen ethanol extract could activate human PXR and induce the CYP3A4 reporter construct in HepG2 cells. Tanshinone IIA and cryptotanshinone were identified as efficacious PXR agonists, and cryptotanshinone activated the CYP3A4 promoter more strongly than tanshinone IIA. Furthermore, CAR and GR were also involved in the induction of CYP3A4 expression by tanshinones, though their roles seemed not as important as PXR. Treatment of LS174T cells with cryptotanshinone or tanshinone IIA resulted in a significant increase of CYP3A4 mRNA, which was consistent with the results from the reporter gene assay. Collectively, activation of PXR and the resultant CYP3A4 induction mediated by cryptotanshinone and tanshinone IIA provide a molecular mechanism for previously observed CYP3A induction by Danshen extracts, and our findings also suggest that caution should be taken when Danshen products are used in combination with therapeutic drugs metabolized by CYP3A4.

Role of human hepatocyte nuclear factor 4alpha in the expression of drug-metabolizing enzymes and transporters in human hepatocytes assessed by use of small interfering RNA

Hepatocyte nuclear factor 4alpha (HNF4alpha) is an important transcription factor in hepatic gene expression. Here, we have investigated the role of HNF4alpha in the expression of drug-metabolizing enzymes and transporters in human hepatocytes using an adenovirus expressing human HNF4alpha-small interfering RNA (hHNF4alpha-siRNA). The hHNF4alpha-siRNA effectively reduced the mRNA and nuclear protein levels of hHNF4alpha in a concentration-dependent manner. The hHNF4alpha-siRNA also decreased the mRNA levels of CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, UGT1A1, UGT1A9, SULT2A1, ABCB1, ABCB11, ABCC2, OATP1B1 and OCT1, as well as those of PXR and CAR. To discern the role of these nuclear receptors, we co-infected hepatocytes with hHNF4alpha-siRNA and PXR- or CAR-expressing adenovirus. The hHNF4alpha-siRNA-induced reductions of the enzyme and transporter mRNA levels were not restored except CYP2B6 mRNA levels, which were returned to the control level by overexpressing CAR. Furthermore, although hHNF4alpha-siRNA did not significantly affect the fold-induction of CYP2B6, CYP2C8, CYP2C9, or CYP3A4 mRNA levels following treatment with CYP inducers, the levels in hHNF4alpha-suppressed cells fell significantly compared to the control. These results suggest that HNF4alpha plays a dominant role in the expression of drug-metabolizing enzymes and transporters in human hepatocytes, and that HNF4alpha expression levels is a possible determinant for inter-individual variations in the expression of these enzymes and transporters.