Human Ah receptor (AHR) gene: localization to 7p15 and suggestive correlation of polymorphism with CYP1A1 inducibility

Investigation of the effects of oxidative stress, inflammation on the pathway of tryptophan/kynurenine in OCD

OBJECTIVES

Recent studies have shown that the distribution of the tryptophan/kynurenine pathway (KP) plays a role in the development of obsessive-compulsive disorder (OCD). We aimed to reveal the relationship between CYP1A1 rs464903 and aryl hydrocarbon receptor (AhR) rs10249788 associated with the KP and interferon gamma (IFN γ) and oxidative stress in OCD.

METHODS

In our study, the serum and DNAs of 150 samples, including 100 OCD patients and 50 controls, were used. The activity of glutathione peroxidase (GSH-Px), and the levels of IFN γ, thiobarbituric acid reactive substances (TBARS), tryptophan, and kynurenine were determined by biochemical methods. AhR rs10249788 and cytochrome P450 family CYP1A1 rs4646903, which interact directly with the KP, were analysed by polymerase chain reaction followed by restriction fragment length polymorphism. P < 0.05 was considered statistically significant.

RESULT

There were no significant differences between groups in CYP1A1 rs4646903 and AhR rs10249788 while tryptophan and IFN γ were found to be higher in controls (p < 0.001, for both), and TBARS and indolamine-2,3-dioxygenase were found to be higher in OCD (p < 0.001, for both). There were significant correlations between IFN γ and TBARS and GSH-Px (p = 0.028, p = 0.020, respectively) in the OCD group.

CONCLUSIONS

For the first time studied in OCD, it has been shown that IFN γ, tryptophan, oxidative stress parameters, and gene variants of CYP1A1 rs4646903 anAhR rs10249788 are shown effective on the KP.

The Aryl Hydrocarbon Receptor (AHR): A Novel Therapeutic Target for Pulmonary Diseases?

The aryl hydrocarbon receptor (AHR) is a cytoplasmic transcription factor that is well-known for regulating xenobiotic metabolism. Studies in knockout and transgenic mice indicate that the AHR plays a vital role in the development of liver and regulation of reproductive, cardiovascular, hematopoietic, and immune homeostasis. In this focused review on lung diseases associated with acute injury and alveolar development, we reviewed and summarized the current literature on the mechanistic role(s) and therapeutic potential of the AHR in acute lung injury, chronic obstructive pulmonary disease, and bronchopulmonary dysplasia (BPD). Pre-clinical studies indicate that endogenous AHR activation is necessary to protect neonatal and adult lungs against hyperoxia- and cigarette smoke-induced injury. Our goal is to provide insight into the high translational potential of the AHR in the meaningful management of infants and adults with these lung disorders that lack curative therapies.

Nuclear Receptors in Myocardial and Cerebral Ischemia-Mechanisms of Action and Therapeutic Strategies

Nearly 18 million people died from cardiovascular diseases in 2019, of these 85% were due to heart attack and stroke. The available therapies although efficacious, have narrow therapeutic window and long list of contraindications. Therefore, there is still an urgent need to find novel molecular targets that could protect the brain and heart against ischemia without evoking major side effects. Nuclear receptors are one of the promising targets for anti-ischemic drugs. Modulation of estrogen receptors (ERs) and peroxisome proliferator-activated receptors (PPARs) by their ligands is known to exert neuro-, and cardioprotective effects through anti-apoptotic, anti-inflammatory or anti-oxidant action. Recently, it has been shown that the expression of aryl hydrocarbon receptor (AhR) is strongly increased after brain or heart ischemia and evokes an activation of apoptosis or inflammation in injury site. We hypothesize that activation of ERs and PPARs and inhibition of AhR signaling pathways could be a promising strategy to protect the heart and the brain against ischemia. In this Review, we will discuss currently available knowledge on the mechanisms of action of ERs, PPARs and AhR in experimental models of stroke and myocardial infarction and future perspectives to use them as novel targets in cardiovascular diseases.

Aromatic hydrocarbon receptors in mitochondrial biogenesis and function

Mitochondria are ubiquitous membrane-bound organelles that not only play a key role in maintaining cellular energy homeostasis and metabolism but also in signaling and apoptosis. Aryl hydrocarbons receptors (AhRs) are ligand-activated transcription factors that recognize a wide variety of xenobiotics, including polyaromatic hydrocarbons and dioxins, and activate diverse detoxification pathways. These receptors are also activated by natural dietary compounds and endogenous metabolites. In addition, AhRs can modulate the expression of a diverse array of genes related to mitochondrial biogenesis and function. The aim of the present review is to analyze scientific data available on the AhR signaling pathway and its interaction with the intracellular signaling pathways involved in mitochondrial functions, especially those related to cell cycle progression and apoptosis. Various evidence have reported the crosstalk between the AhR signaling pathway and the nuclear factor κB (NF-κB), tyrosine kinase receptor signaling and mitogen-activated protein kinases (MAPKs). The AhR signaling pathway seems to promote cell cycle progression in the absence of exogenous ligands, whereas the presence of exogenous ligands induces cell cycle arrest. However, its effects on apoptosis are controversial since activation or overexpression of AhR has been observed to induce or inhibit apoptosis depending on the cell type. Regarding the mitochondria, although activation by endogenous ligands is related to mitochondrial dysfunction, the effects of endogenous ligands are not well understood but point towards antiapoptotic effects and inducers of mitochondrial biogenesis.

Microengineered systems with iPSC-derived cardiac and hepatic cells to evaluate drug adverse effects

Hepatic and cardiac drug adverse effects are among the leading causes of attrition in drug development programs, in part due to predictive failures of current animal or in vitro models. Hepatocytes and cardiomyocytes differentiated from human induced pluripotent stem cells (iPSCs) hold promise for predicting clinical drug effects, given their human-specific properties and their ability to harbor genetically determined characteristics that underlie inter-individual variations in drug response. Currently, the fetal-like properties and heterogeneity of hepatocytes and cardiomyocytes differentiated from iPSCs make them physiologically different from their counterparts isolated from primary tissues and limit their use for predicting clinical drug effects. To address this hurdle, there have been ongoing advances in differentiation and maturation protocols to improve the quality and use of iPSC-differentiated lineages. Among these are in vitro hepatic and cardiac cellular microsystems that can further enhance the physiology of cultured cells, can be used to better predict drug adverse effects, and investigate drug metabolism, pharmacokinetics, and pharmacodynamics to facilitate successful drug development. In this article, we discuss how cellular microsystems can establish microenvironments for these applications and propose how they could be used for potentially controlling the differentiation of hepatocytes or cardiomyocytes. The physiological relevance of cells is enhanced in cellular microsystems by simulating properties of tissue microenvironments, such as structural dimensionality, media flow, microfluidic control of media composition, and co-cultures with interacting cell types. Recent studies demonstrated that these properties also affect iPSC differentiations and we further elaborate on how they could control differentiation efficiency in microengineered devices. In summary, we describe recent advances in the field of cellular microsystems that can control the differentiation and maturation of hepatocytes and cardiomyocytes for drug evaluation. We also propose how future research with iPSCs within engineered microenvironments could enable their differentiation for scalable evaluations of drug effects.

Enhanced nuclear translocation and activation of aryl hydrocarbon receptor (AhR) in THP-1 monocytic cell line by a novel niosomal formulation of indole-3-carbinol

Although niosomes structurally resemble liposomes, they are composed of nonionic surfactants which result in less toxicity and more stability. Here, we developed a novel niosomal formulation of I3C and investigated the nuclear translocation and activation of AhR among human acute myeloid leukaemia (AML) monocytic THP-1 cell line. Niosomal vesicles comprised of nonionic surfactants, cholesterol and I3C were prepared using thin film hydration (TFH) method and characterized according to the entrapment efficiency (EE %), size and zeta potential, by Dynamic light scattering method (DLS), and the surface morphology visualized by Transmission electron microscopy (TEM). In vitro release of I3C was evaluated and MTS assay was used to evaluate the viability of THP-1 cells. The nuclear translocation of AhR was assessed by immunocytochemistry (ICC) and Real-time RT-PCR was conducted using AhR target genes. The ratio of Cholesterol:Span 60 (1:1) niosomal formulations with the highest significant EE% were selected. I3C exerted cytotoxic effects on THP-1 cells in a dose- and time-dependent manner, while administration of niosomal I3C reduced these effects. Both niosomal and free I3C formulations facilitated the nuclear translocation of AhR. CYP1A1 was overexpressed in response to both free and niosomal I3C treatments, while IL1β was overexpressed merely in niosomal I3C-treated THP-1 cells. Niosomal formulation of I3C resulted in reduced cytotoxicity effects by enhancing the functional effects of I3C on AhR in THP-1 cells, including its nuclear translocation and overexpression of the target genes.

Role of aryl hydrocarbon receptor polymorphisms on TCDD-mediated CYP1B1 induction and IgM suppression by human B cells

Previous studies have demonstrated that most of the intraspecies variation in sensitivity to the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), including suppression of antibody responses, in murine models is due to single nucleotide polymorphisms (SNPs) within the aryl hydrocarbon receptor (AhR) gene. The underlying reason for variation in sensitivity to TCDD-induced suppression of IgM responses among humans is not well understood, but is thought, in part, to be a result of different polymorphic forms of the AhR expressed by different individuals. In this study, the functional properties of six (P517S, R554K, V570I, V570I+P517S, R554K+V570I and P517S+R554K+V570I) human AhR variants were examined in the human B cell line, SKW 6.4. TCDD-induced Cyp1B1 and Cyp1A2 mRNA expression levels and Cyp1B1-regulated reporter gene activity, used for comparative purposes, were markedly lower in SKW cells containing the R554K SNP than in SKW-AHR(+) (control AhR) cells. Furthermore, all AhR variants were able to mediate TCDD-induced suppression of the IgM response; however, a combined P517S+R554K+V570I variant partially reduced sensitivity to TCDD-mediated suppression of IgM secretion. Collectively, our findings show that the R554K human AhR SNP alone altered sensitivity of human B cells to TCDD-mediated induction of Cyp1B1 and Cyp1A2. By contrast, attenuation of TCDD-induced IgM suppression required a combination of all three SNPs P517S, R554K, and V570I.

Indoor air pollution and neural tube defects: effect modification by maternal genes

BACKGROUND

Gene-environment interactions have been implicated in the development of neural tube defects (NTDs).

METHODS

We conducted a case-control study to investigate (1) the association of aryl hydrocarbon receptor (AHR) genetic variants and phase I metabolic enzymes with the risk of NTDs and (2) the interaction of these variants with maternal exposure to indoor air pollution from smoking and coal combustion or with placental polycyclic aromatic hydrocarbons (PAHs). Blood samples were collected from 534 mothers of fetuses or newborns with NTDs and 534 control mothers who had healthy term newborns and were assayed for 12 polymorphisms in the AHR and cytochrome P450 (CYP) genes. Information on maternal exposure was collected, and placental levels of PAHs were analyzed.

RESULTS

Maternal exposure to indoor air pollution was associated with an increased NTD risk. However, no increased NTD risk was observed for individual genetic variants. For mothers with the CYP1B1 rs2855658 GG variant, exposure to indoor air pollution led to a dose-response relationship for NTD risk, with odds ratios (ORs) of 3.0 (95% confidence interval = 1.6-5.7) and 8.1 (3.8-17) for medium and high levels of exposure, respectively. For mothers with GA or AA genotypes, this trend was less apparent. Placental PAHs were associated with an increased risk of NTDs, with an OR of 16 (3.3-75) for high levels compared with low levels of exposure among mothers with the GG genotype; there was no association for mothers with GA or AA genotypes.

CONCLUSIONS

The CYP1B1 variant modifies the effect of indoor air pollution on NTD risk.

Single nucleotide polymorphisms in the human aryl hydrocarbon receptor nuclear translocator (ARNT) gene

Species' variation(s) in gene homologues can result in differences among species in their quantitative and qualitative susceptibility and responsiveness to environmental contaminants. In the case of dioxin-like compounds (DLCs), it has been hypothesized that single nucleotide polymorphisms (SNPs) in genes associated with aryl hydrocarbon receptor (AHR)-regulated pathways may result in greater susceptibility to DLC toxicity. A key step in the activation of AHR involves heterodimerization with the AHR nuclear translocator (ARNT) protein before binding to its DNA response element. The objective of this study was to identify SNPs in the human ARNT gene that could potentially affect the sensitivity of AHR-dependent gene transcription. Results from DNA sequencing of 101 human samples demonstrated the presence of five unique SNPs at the ARNT locus, including three non-synonymous SNPs, of which two were novel: V304M and T462A. The genetic frequencies of the non-synonymous SNPs were very low (≤0.02), and the novel SNPs occurred in the Per-ARNT-Sim (PAS) functional domain. In silico analysis indicated that V304M was the only SNP identified in the current population with the potential to significantly alter ARNT protein function. Our findings indicated a very limited occurrence of SNPs with predicted functional consequence in key domains of human ARNT.

Genetic architecture of susceptibility to PCB126-induced developmental cardiotoxicity in zebrafish

Variability in risk of developmental defects caused by dioxin-like compounds (DLCs) has been demonstrated within and among several vertebrate species. Beyond our knowledge of the aryl hydrocarbon receptor (AHR) and its role in mediating toxicity for this class of compounds, little else is known concerning precise downstream targets influencing this vulnerability. In the present study, zebrafish with divergent genetic backgrounds were screened for susceptibility to developmental cardiotoxicity caused by the prototypical DLC, 3,3',4,4',5-pentachlorobiphenyl (PCB126); a range up to ∼40-fold differences was observed. Differentially sensitive zebrafish were chosen for a genetic cross, and the recombinant generation was used for genome-wide quantitative trait loci (QTL) mapping. Multiple QTLs were identified--several acting alone, one additively, and two others via epistatic interaction. Together, these QTLs account for 24% of the phenotypic variance observed in cardioteratogenicity resulting from PCB126 exposure (logarithm of the odds = 13.55, p = 1.89 × 10⁻¹⁰). Candidate genes in these QTL regions include the following: ahr2, bcor, and capn1 (Chr 22); e2f1 and pdyn (Chr 23); ctnnt2, plcg1, eno3, tgm1, and tgm2 (interacting on Chr 23); and vezf1 (Chr 15). These data demonstrate that DLC-induced cardiac teratogenicity is a multifactorial complex trait influenced by gene × gene and gene × environment interactions. The identified QTLs harbor many DLC-responsive genes critical to cardiovascular development and provide insight into the genetic basis of susceptibility to AHR-mediated developmental toxicity.

The human AHR: identification of single nucleotide polymorphisms from six ethnic populations

BACKGROUND

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related dioxin-like chemicals are mediated through binding-dependent activation of the cytosolic aryl hydrocarbon receptor (AHR). The human AHR is a low-affinity receptor relative to most rodents, but some reports suggest that there may be individuals with polymorphic high-affinity receptors, thereby possibly increasing the sensitivity to dioxins in such people.

METHODS

Although no polymorphisms have been reported in the ligand binding region of the AHR in the over 100 reported sequences, we sequenced 108 additional human AHR genes in an effort to further identify single single nucleotide polymorphisms (SNPs) within the open reading frames of the AHR locus. The DNA was sequenced from six ethnic populations that included Japanese, Chinese, European/Caucasian, African-American, South East Asian, and Hispanic.

RESULTS

Six exonic SNPs were identified; four had been described as previously reported and two seem to be novel. Four of the SNPs identified lead to amino acid changes in the AHR protein and two of the SNPs lead to synonymous substitutions. An additional four SNPs have been reported elsewhere that were not identified in the current analysis. With these new sequences, more than 200 human AHR gene sequences have been analyzed for SNPs.

CONCLUSION

The results indicate a very limited presence of polymorphisms in the core ligand binding region of the human AHR. Other regions, such as the transactivation domain, seem to be slightly more polymorphic in the human population and the impact on functionality should be further examined.

An Interethnic Comparison of Polymorphisms of the Genes Encoding Drug-Metabolizing Enzymes and Drug Transporters: Experience in Singapore

Much of the interindividual variability in drug response is attributable to the presence of single nucleotide polymorphisms (SNPs) in genes encoding drug-metabolizing enzymes and drug transporters. In recent years, we have investigated the polymorphisms in a number of genes encoding phase I and II drug-metabolizing enzymes including CYPIA1, CYP3A4, CYP3A5, GSTM1, NAT2, UGT1A1, and TPMT and drug transporter (MDR1) in three distinct Asian populations in Singapore, namely the Chinese, Malays, and Indians. Significant differences in the frequencies of common alleles encoding these proteins have been observed among these three ethnic groups. For example, the frequency of the variant A2455G polymorphism of CYP1A1 was 28% in Chinese and 31% in Malays, but only 18% in Indians. CYP3A4*4 was detected in two of 110 Chinese subjects, but absent in Indians and Malays. Many Chinese and Malays (61-63%) were homozygous for the GSTM1*0 null genotype compared with 33% of Indians. The frequency of the UGTIA1*28 allele was highest in the Indian population (35%) compared to similar frequencies that were found in the Chinese (16%) and Malay (19%) populations. More importantly, our experience over the years has shown that the pharmacogenetics of these drug-metabolizing enzymes and MDR1 in the Asian populations are different from these in the Caucasian and African populations. For example, the CYP3A4*1B allele, which contains an A-290G substitution in the promoter region of CYP3A4, is absent in all three Asian populations of Singapore studied, but occurs in more than 54% of Africans and 5% of Caucasians. There were no difference in genotype and allelic variant frequencies in exon 12 of MDR1 between the Chinese, Malay, and Indian populations. When compared with other ethnic groups, the distribution of the wild-type C allele in exon 12 in the Malays (34.2%) and Indians (32.8%) was relatively high and similar to the Japanese (38.55%) and Caucasians (41%) but different from African-Americans (15%). The frequency of wild-type TT genotype in Asians (43.5% to 52.1%) and Japanese (61.5%) was much higher than those found in Caucasians (13.3%). All the proteins we studied represent the primary hepatic or extrahepatic enzymes, and their polymorphic expression may be implicated in disease risk and the disposition of drugs or endogenous substances. As such, dose requirements of certain drugs may not be optimal for Asian populations, and a second look at the factors responsible for this difference is necessary.

For dioxin-induced birth defects, mouse or human CYP1A2 in maternal liver protects whereas mouse CYP1A1 and CYP1B1 are inconsequential

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) induces cleft palate and hydronephrosis in mice, when exposed in utero; these effects are mediated by the aryl hydrocarbon receptor. The Cyp1a1, Cyp1a2, and Cyp1b1 genes are up-regulated by the aryl hydrocarbon receptor. To elucidate their roles in dioxin-induced teratogenesis, we compared Cyp1a1(-/-), Cyp1a2(-/-), and Cyp1b1(-/-) knock-out mice with Cyp1(+/+) wild-type mice. Dioxin was administered (25 microg/kg, gavage) on gestational day 10, and embryos were examined on gestational day 18. The incidence of cleft palate and hydronephrosis was not significantly different in fetuses from Cyp1a1(-/-), Cyp1b1(-/-), and Cyp1(+/+) wild-type mice. To fetuses carried by Cyp1a2(-/-) dams, however, this dose of dioxin was lethal; this effect was absolutely dependent on the maternal Cyp1a2 genotype and independent of the embryonic Cyp1a2 genotype. Dioxin levels were highest in adipose tissue, mammary gland, and circulating blood of Cyp1a2(-/-) mothers, compared with that in the Cyp1(+/+) mothers, who showed highest dioxin levels in liver. More dioxin reached the embryos from Cyp1a2(-/-) dams, compared with that from Cyp1(+/+) dams. Fetuses from Cyp1a2(-/-) dams exhibited a approximately 6-fold increased sensitivity to cleft palate, hydronephrosis, and lethality. Using the humanized hCYP1A1_1A2 transgenic mouse (expressing the human CYP1A1 and CYP1A2 genes in the absence of mouse Cyp1a2 gene), the teratogenic effects of dioxin reverted to the wild-type phenotype. These data indicate that maternal mouse hepatic CYP1A2, by sequestering dioxin and thus altering the pharmacokinetics, protects the embryos from toxicity and birth defects; substitution of the human CYP1A2 trans-gene provides the same protection. In contrast, neither CYP1A1 nor CYP1B1 appears to play a role in dioxin-mediated teratogenesis.

Impact of Cyp1a2 or Ahr gene knockout in mice: Implications for biomonitoring studies

Studies of the impact of phase 1 enzyme polymorphisms on genetic damage have yielded mixed results. We studied how genetic damage would be altered when specific genes were ablated under low dose conditions.

METHODS

Knockouts (KO) were generated from c57bl6/J mice with mutations in Cyp1a2 or Ahr receptor that eliminated gene product function. Animals were treated topically with either 4-aminobiphenyl (4ABP) 10mg/kg, benzo(a)pyrene (BaP) 33.3mg/kg or dibenzo(c,g)carbazole (DBC) 8 mg/kg, and sacrificed after 24h. DNA from livers, skin and/or urinary bladders were isolated and (32)P-post labelled.

RESULTS

Cyp1a2-/- mice did not differ in 4ABP DNA adduct levels in either urinary bladder or liver compared to wildtype. There was a sex difference in the organ affected. Cyp1a2 knockout reduced skin BAP adduct levels 50% and AHR knockout reduced skin BAP adduct levels by 90%. There was no impact of either knockout on the levels of DBC-DNA adducts in any tissue.

CONCLUSIONS

Ablation of specific metabolizing enzymes had compound- and tissue-specific effects in mice. Phenotypic variability in single CYP enzymes may have minor impact in humans at low doses, but variation in the ability to induce the family of CYPs may have a greater impact.

Human response to dioxin: aryl hydrocarbon receptor (AhR) molecular structure, function, and dose-response data for enzyme induction indicate an impaired human AhR

The aryl hydrocarbon receptor (AhR) mediates nearly all studied adverse effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and many related compounds. Binding of TCDD or related ligands to AhR is the key initiating event in downstream biochemical responses. The binding affinity of AhR for TCDD is specific to species and strain, and studies of human AhR demonstrate binding affinities approximately an order of magnitude or more lower than those observed in the most sensitive laboratory strains and species. Molecular genetic studies confirmed that human AhR shares key mutations with the DBA mouse strain that result in an "impaired" AhR (with respect to TCDD binding and responsiveness). Despite a number of polymorphisms in human AhR, the key "DBA-type" mutations appear to be a constant feature of the human AhR, and no polymorphisms have been identified that compensate for the impaired binding function conferred by these mutations. Consistent with the impaired binding status of the human AhR, human cells have consistently required approximately 10-fold higher concentrations of TCDD in vitro than rodent cells to respond with enzyme induction. Recent studies of in vivo enzyme induction-related endpoints in human populations with moderately and highly increased TCDD body burdens detected no relationship between these endpoints and TCDD body burdens at body-burden levels up to 250 ng TEQ/kg body weight, or approximately 25 times above the upper range of current general population background body burdens, while marked elevations in enzyme activity were observed in persons with body burdens above 750 ng TEQ/kg. In contrast, the more sensitive laboratory rodent strains and species exposed to TCDD exhibit significant enzyme induction at body burdens below 50 ng/kg. These interspecies data on the most sensitive and best understood response to binding of TCDD and related compounds to the AhR are consistent with the binding affinity and molecular structure data and support the hypothesis that the human AhR is less functional than the AhR of the more sensitive laboratory animals at a molecular level. Quantitative risk assessments involving interspecies extrapolation from sensitive laboratory species and strains should take these fundamental differences into account when margins of exposure and safety factors are considered.

Comparison of TCDD and PCB CYP1A induction sensitivities in fresh hepatocytes from human donors, sprague-dawley rats, and rhesus monkeys and HepG2 cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals induce cytochrome P450 1A (CYP1A) gene expression and, at sufficient exposures, cause toxicity. Human health risks from such exposures are typically estimated from animal studies. We tested whether animal models predict human sensitivity by characterizing CYP1A gene expression in cultures of fresh hepatocytes from human donors, rats, and rhesus monkeys and HepG2 human hepatoma cells. We exposed the cells to three aryl hydrocarbon receptor (AhR) ligands of current environmental interest and measured 7-ethoxyresorufin-O-deethylase (EROD) activity and concentrations of CYP1A1 and CYP1A2 mRNA. We found that human cells are about 10-1000 times less sensitive to TCDD, 3,3',4,4',5-pentachlorobiphenyl (PCB 126), and Aroclor 1254 than rat and monkey cells, that relative potencies among these chemicals are different across species, and that gene expression thresholds exist for these chemicals. Newly calculated rat-human interspecies relative potency factors for PCB 126 were more than 100 times lower than the current rodent-derived value. We propose that human-derived values be used to improve the accuracy of estimates of human health risks.

Functional analysis of six human aryl hydrocarbon receptor variants in a Japanese population

Aryl hydrocarbon receptor (AhR) is an important transcriptional regulator involved in the induction of CYP1A1, CYP1A2, CYP1B1, UGT1A1, and UGT1A6. In this study, functional properties of four novel naturally occurring human AhR variants (K401R, N487D, I514T, and K17T/R554K) were examined along with the single variants K17T and R554K. The luciferase reporter assay using the CYP1A1 promoter reporter in HeLa cells treated with beta-naphthoflavone or 3-methylcholanthrene, which are known as typical agonists for AhR, showed that reporter activities of the K401R and N487D variants were reduced to 40 to 58% of those of wild-type (WT) but not of the other variants. Similarly, the K401R and N487D variants also reduced the omeprazole-induced reporter activities to approximately 56 and 74% of those of the WT, respectively. The reduced activities of the two variants were probably caused by the reduced protein expression levels, since the protein levels of the K401R and N487D variants were approximately 52 and 47% of the WT, respectively, without any changes in their mRNA levels. The reduced protein levels were recovered by treatment with a proteasome inhibitor MG132 [N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal], suggesting that the reduced protein levels were caused by the accelerated proteasomal degradation by a proteasome. Together, the current data demonstrate that the K401R and N487D variants reduce their apparent transcriptional activities, both ligand-induced and omeprazole-induced activation, probably through reduced protein expression. Thus, these two variants may influence drug metabolism through reduced induction of CYP1A1 and other target enzymes.

Metabolism-based drug-drug interactions: what determines individual variability in cytochrome P450 induction?

Individual variability in cytochrome P450 (P450) induction comprises an important component contributing to the difficulties in assessing and predicting metabolism-based drug-drug interactions in humans. In this article, we outline the major factors responsible for the individual variability in P450 induction, including variable transporter activity and metabolism of inducers in vivo, genetic variations of P450 genes and their regulatory regions, genetic variations of receptors and regulatory proteins required for induction, and different physiological and environmental elements. With a better understanding of the major determinants in P450 induction and a profile of the phenotypes of these determinants in each individual, it is believed that the individual variability in induction-mediated drug-drug interactions can be adequately evaluated.

Aryl hydrocarbon receptor polymorphisms and dioxin resistance in Atlantic killifish (Fundulus heteroclitus)

The aryl hydrocarbon receptor (AHR) gene encodes a ligand-activated transcription factor through which planar halogenated aromatic hydrocarbons (HAHs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as well as polynuclear aromatic hydrocarbons (PAHs) cause altered gene expression and toxicity. To understand the role of AHR genetic variability in differential sensitivity to HAHs and PAHs, we are currently studying a population of the teleost Fundulus heteroclitus (Atlantic killifish) that has evolved genetic resistance to the toxic and biochemical effects of these compounds. Here, we report that the killifish AHR1 locus is highly polymorphic and that the frequencies of the major allele types differ between dioxin-sensitive and dioxin-resistant populations. Twenty-five single nucleotide polymorphisms (SNPs), nine of which are non-synonymous, were identified in the AHR1 coding sequence. Seven identified alleles were assigned to three groups, designated AHR1*1, AHR1*2 and AHR1*3. AHR1*1 alleles were under-represented in a population of dioxin- and polychlorinated biphenyl (PCB)-resistant fish from a PCB-contaminated Superfund site (New Bedford Harbor, Massachusetts, USA) compared to dioxin-sensitive fish from a less contaminated reference site (Scorton Creek, Massachusetts, USA). To determine the possible role of these AHR1 variants in differential HAH sensitivity, we expressed representative variant proteins from the two most divergent allelic groups (AHR1*1 and AHR1*3) by in-vitro transcription and translation and assessed their functional properties. AHR1*1A and AHR1*3A proteins displayed similar binding capacities and affinities for [H]TCDD. In transient transfection assays using mammalian cells, AHR1*1A and AHR1*3A exhibited similar abilities to support TCDD-dependent transactivation of a luciferase reporter gene under control of AHR-responsive enhancer elements. We discuss the possibility of other functional differences in AHR1 variants or their interaction with other killifish loci (AHR2, AHRR) that may contribute to differences in dioxin sensitivity.

Cyp1a2 protects against reactive oxygen production in mouse liver microsomes

H(2)O(2) production was evaluated in liver microsomes prepared from Cyp1a1/1a2(+/+) wild-type and Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice pretreated with 5 microg dioxin (TCDD)/kg body wt or vehicle alone. NADPH-dependent H(2)O(2) production in TCDD-induced microsomes from wild-type mice was about one-third of that in noninduced microsomes. In Cyp1a2(-/-) mice, H(2)O(2) production was the same for induced and noninduced microsomes, with levels significantly higher than those in wild-type mice. Cyp1a1(-/-) microsomes displayed markedly lower levels of H(2)O(2) production in both induced and noninduced microsomes, compared with those in wild-type and Cyp1a2(-/-) microsomes. The CYP1A2 inhibitor furafylline in vitro exacerbated microsomal H(2)O(2) production proportional to the degree of CYP1A2 inhibition, and the CYP2E1 inhibitor diethyldithiocarbamate decreased H(2)O(2) production proportional to the degree of CYP2E1 inhibition. Microsomal H(2)O(2) production was strongly correlated to NADPH-stimulated production of thiobarbituric acid-reactive substances, as well as to decreases in microsomal membrane polarization anisotropy, indicative of peroxidation of unsaturated membrane lipids. Our results suggest that possibly acting as an "electron sink," CYP1A2 might decrease CYP2E1-and CYP1A1-mediated H(2)O(2) production and oxidative stress. In this regard, CYP1A2 may be considered an antioxidant enzyme.

Genetic Variations of the AHR Gene Encoding Aryl Hydrocarbon Receptor in a Japanese Population

Aryl hydrocarbon receptor (AhR), encoded by the AHR gene, is a transcriptional factor that induces various drug metabolizing enzymes in response to diverse endogenous and exogenous ligands. In order to identify genetic variations of the AHR gene, genomic DNA from 242 Japanese individuals was sequenced. We identified 32 single nucleotide variations, including 25 novel ones [7 were in the coding exons, 7 in the introns, 1 in the 5'-untranslated region (UTR), 5 in the 3'-UTR, 2 in the 5'-flanking region, and 3 in the 3'-flanking region] and a GGGGC repeat polymorphism (a novel microsatellite marker) in the promoter region. The novel nonsynonymous variations were 50A>C (Lys17Thr), 1202A>G (Lys401Arg), 1459A>G (Asn487Asp), and 1541T>C (Ile514Thr). The allele frequencies were 0.010 for 1459A>G (Asn487Asp) and 0.002 for the other 3 variations. Also detected in this analysis was the known nonsynonymous single nucleotide polymorphism 1661G>A (Arg554Lys) at a 0.444 frequency.

Distinct response to dioxin in an arylhydrocarbon receptor (AHR)-humanized mouse

There are large inter- and intraspecies differences in susceptibility to dioxin-induced toxicities. A critical question in risk assessment of dioxin and related compounds is whether humans are sensitive or resistant to their toxicities. The diverse responses of mammals to dioxin are strongly influenced by functional polymorphisms of the arylhydrocarbon receptor (AHR). To characterize responses mediated by the human AHR (hAHR), we generated a mouse possessing hAHR instead of mouse AHR. Responses of these mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene were compared with the responses of naturally sensitive (C57BL6J) and resistant (DBA2) mice. Mice homozygous for hAHR exhibited weaker induction of AHR target genes such as cyp1a1 and cyp1a2 than did C57BL6J (Ahr(b-1/b-1)) mice. DBA2 (Ahr(d/d)) mice were less responsive to induction of cyp genes than C57BL6J mice. hAHR and DBA2 AHR exhibit similar ligand-binding affinities and homozygous hAHR and Ahr(d/d) mice displayed comparable induction of AHR target genes by 3-methylcholanthrene. However, when TCDD was administered, a greatly diminished response was observed in homozygous hAHR mice compared with Ahr(d/d) mice, indicating that hAHR expressed in mice is functionally less responsive to TCDD than DBA2 AHR. After maternal exposure to TCDD, homozygous hAHR fetuses developed embryonic hydronephrosis, but not cleft palate, whereas fetuses possessing Ahr(b-1) or Ahr(d) developed both anomalies. These results suggest that hAHR may define the specificity of the responses to various AHR ligands. Thus, the hAHR knock-in mouse is a humanized model mouse that may better predict the biological effects of bioaccumulative environmental toxicants like TCDD in humans.

Genetics of endometriosis

Endometriosis long has been recognized as showing heritable tendencies, with recurrence risks of 5% to 7% for first-degree relatives. The risk indicates that polygenic and multifactorial etiology is far more likely to be the cause than mendelian inheritance. The current task is to determine the number and location of genes responsible for endometriosis. Molecular advances of the past decade make identification and elucidation of these genes a reality. The authors review the basis for concluding that endometriosis is a genetic disorder of polygenic/multifactorial inheritance. Genome-wide strategies for identifying causative genes are considered and available data on association or linkage to putative candidate genes systematically reviewed.

Aryl hydrocarbon receptors: diversity and evolution

Animals have evolved inducible enzymatic defenses to facilitate the biotransformation and elimination of toxic compounds encountered in the environment. The sensory component of this system consists of soluble receptors that regulate the expression of certain isoforms of cytochrome P450, other enzymes, and transporters in response to environmental chemicals. These receptors include several members of the steroid/nuclear receptor superfamily as well as the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS gene superfamily. In addition to its adaptive functions, the AHR serves poorly understood physiological roles; interference with those roles by dioxins and related chemicals causes toxicity. One approach to understanding the physiological significance of the AHR is to characterize its structure, function, and regulation in diverse species, including mammals, birds, fish, and invertebrates. These animal groups include model species with unique features that can be exploited to broaden our understanding of AHR function. Studies carried out in diverse species also provide phylogenetic information that allows inferences about the evolutionary history of the AHR. This review summarizes the current understanding of AHR diversity among animal species and the evolution of the AHR signaling pathway, as inferred from molecular studies in vertebrate and invertebrate animals. The AHR gene has undergone duplication and diversification in vertebrate animals, resulting in at least three members of an AHR gene family: AHR1, AHR2, and AHR repressor. The inability of invertebrate AHR homologs to bind dioxins and related chemicals, along with other evidence, suggests that the adaptive role of the AHR as a regulator of xenobiotic metabolizing enzymes may have been a vertebrate innovation. The physiological functions of the AHR during development appear to be ancestral to the adaptive functions. Sensitivity to the developmental toxicity of dioxins and related chemicals may have had its origin in the evolution of dioxin-binding capacity of the AHR in the vertebrate lineage.

Polymorphisms in the human AH receptor

The AH receptor (AHR) mediates toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as well as induction of three cytochrome P450 enzymes and certain Phase II enzymes. In laboratory animals, genetic variations in the AHR lead to substantial differences in sensitivity to biochemical and toxic effects of TCDD and related compounds. Relatively few polymorphisms have been discovered in the human AHR gene; these occur predominantly in exon 10, a region that encodes a major portion of the transactivation domain of the receptor that is responsible for regulating expression of other genes. In human populations there is a wide range of variation in responses regulated by the AHR for example, induction of CYP1A1. Some variation in human responsiveness likely is due to genetically based variations in AHR structure. Thus far, however, only one pair of polymorphisms, those at codons 517 and 570, has been shown to have a clear cut and strong effect on the phenotype of an AHR-mediated response. The search continues for polymorphisms that alter AHR function because this receptor is a central factor in determining responses to important environmental contaminants and also plays a physiologic role in early development in mammals.

cDNA cloning and characterization of an aryl hydrocarbon receptor from the harbor seal (Phoca vitulina): a biomarker of dioxin susceptibility?

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAHs) are found at high concentrations in some marine mammals. Species differences in sensitivity to TCDD and PHAHs are a major limitation in assessing the ecological risk to these animals. Harbor seals accumulate high levels of PHAHs and are thought to be highly sensitive to the toxic effects of these compounds. To investigate the mechanistic basis for PHAH toxicity in harbor seals (Phoca vitulina), we sought to characterize the aryl hydrocarbon receptor (AHR), an intracellular protein that is responsible for PHAH effects. Here we report the cDNA cloning and characterization of a harbor seal AHR. The harbor seal AHR cDNA has an open reading frame of 2529 nucleotides that encodes a protein of 843 amino acids with a predicted molecular mass of 94.6 kDa. The harbor seal AHR protein possesses basic helix-loop-helix (bHLH) and Per-ARNT-Sim (PAS) domains. It is most closely related to the beluga AHR (82%) and human AHR (79%) in overall amino acid identity, indicating a high degree of conservation of AHR structure between terrestrial and some marine mammals. The ligand binding properties of the harbor seal AHR were determined using protein synthesized by in vitro transcription and translation from the cloned cDNA. Velocity sedimentation analysis on sucrose gradients showed that the harbor seal AHR exhibits specific binding of [(3)H]TCDD. The [(3)H]TCDD-binding affinity of the harbor seal AHR was compared with that of the AHR from a dioxin-sensitive mouse strain (C57BL/6) using a hydroxylapatite assay. The equilibrium dissociation constants of seal and mouse AHRs were 0.93+/-0.19 and 1.70+/-0.26 nM, respectively. Thus, the harbor seal AHR bound TCDD with an affinity that was at least as high as that of the mouse AHR, suggesting that this seal species may be sensitive to PHAH effects. The characteristics of the AHR potentially can be used as a biomarker of susceptibility to dioxin-like compounds, contributing to the assessment of the risk of these compounds to marine mammals and other protected animals.

Induction of CYP1A1 and CYP1A2 expressions by prototypic and atypical inducers in the human lung

The inducibility of cytochrome P4501A1 gene (CYP1A1) expression was examined in human lung samples from 27 subjects, using an explant culture system and semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. CYP1A1 transcripts were present in all of the lung specimens and were induced by the prototypic inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene (B[a]P), and by the atypical inducers pyridine, nicotine, and omeprazole. 2-Hydroxypyridine was a better inducer than pyridine, implicating metabolites in CYP1A1 induction by the parent compound. The prototypical inducers were the most effective inducers in many samples but were ineffective in some samples in which the atypical compounds were effective inducers. Cytochrome P4501A2 (CYP1A2) transcripts were also detected in most of the lung specimens and were inducible in some specimens. The results show the suitability of the explant culture system for examining the inducibility of human pulmonary CYP1A1 and CYP1A2, indicate the heterogeneity in individual sensitivity to the induction, and underscore the need to include atypical inducers in studies of CYP1A inducibility in humans.

Genetic Polymorphism of Human Cytochrome P450 Involved in Drug Metabolism

Recent advances in human gene analysis promoted by the human genome project have brought us a massive amount of information. These data can be seen and analyzed by personal computer through individual Web sites. As a result, the best use of bioinformatic is essential for recent molecular biology research. Genetic polymorphism of drug-metabolizing enzymes influences individual drug efficacy and safety through the alteration of pharmacokinetics and disposition of drugs. Considerable amounts of data have now accumulated as allelic differences of various drug metabolizing enzymes. Current understanding of genotype information on cytochrome P450 is hereby summarized, based on the Web site for their use in individual optimization of drug therapy.

Ethnic and genetic differences in metabolism genes and risk of toxicity and cancer

Individual risk of toxicity or cancer can be affected by one's exposure to sufficiently high doses of particular environmental agents (or mixtures), combined with each person's underlying genetic predisposition. The development of unequivocal DNA tests for genetic susceptibility to toxicity and cancer and the identification of individuals at increased risk, would revolutionize the fields of public health and preventive medicine. A growing number of human genetic polymorphisms in drug-metabolizing enzymes (DMEs) and the receptors controlling DME expression, are being characterized, some of these have been shown to be correlated with risk of toxicity or cancer, whereas, others presently remain equivocal and require further study. 'Phase I' DMEs, many of which represent cytochromes P450, sometimes metabolically activate pro-carcinogens to genotoxic electrophilic intermediates and other times are involved in detoxification. 'Phase II' DMEs are sometimes activating, but usually they conjugate Phase I intermediates to water-soluble derivatives, to complete the detoxification cycle. Genetic differences in the regulation, expression and activity of genes coding for Phase I and Phase II DMEs and DME receptors that control DME activity levels, can be crucial factors in defining cancer susceptibility and the toxic or carcinogenic power of environmental chemicals. In this review, our current knowledge about polymorphisms in several of these genes is summarized.

Ethnic variability in the allelic distribution of human aryl hydrocarbon receptor codon 554 and assessment of variant receptor function in vitro

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcriptional regulator of several genes including the cytochrome P4501 (CYP1) family as well as genes encoding factors involved in cell growth and differentiation. In mice, several polymorphic forms of the AHR are known, some of which have altered affinity for toxic and carcinogenic ligands. Remarkably little genetic variation has been detected in the human AHR gene. In studies on human AHR, Kawajiri et al. (Pharmacogenetics 1995; 5:151-158) reported a variation at codon 554 that results in an amino acid change from arginine to lysine; the frequency of the variant allele in a Japanese population (n = 277) was 0.43. We investigated the Lys554 allele in 386 individuals of various ethnic origins and found the frequency to be: 0.58 in Ivory Coast Africans (n = 58); 0.53 in a mixed African group (n = 20); 0.39 in Caribbean-Africans (n = 55); 0.32 in Canadian Chinese (n = 41); 0.14 in North American Indians (n = 47); 0.12 in French Canadian Caucasians (n = 20); 0.11 in a mixed ethnicity North American group (n = 45); 0.09 in Canadian Inuits (n = 22); and 0.07 in German Caucasians (n = 78). We expressed the human Lys554 allele in an in-vitro transcription-translation system and found that the receptor bearing the R554L substitution had an equivalent ability to that of the wild-type receptor to bind to a dioxin-responsive element following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The Lys554 allele also was equivalent to the wild-type receptor at stimulating CYP1A1 mRNA expression when transfected into TCDD-treated receptor-deficient mouse Hepa-1 cells. It is not yet known if any of the wide variations in allele frequency at codon 554 are related to ethnic differences in susceptibility to adverse effects of environmental chemicals.

Variation in induced CYP1A1 levels: relationship to CYP1A1, Ah receptor and GSTM1 polymorphisms

The genotypic basis of interindividual variation in levels of induced CYP1A1 activity has been investigated by screening both the CYP1A1 gene and the Ah receptor gene (AhR) for both previously described and novel polymorphisms. A 103-fold level of interindividual variation in induced CYPlA1 activity [ethoxyresorufin O-deethylase (EROD)] was observed in lymphocytes from a group of 30 Caucasian volunteers. High levels of induced EROD activity did not correlate with the presence of CYP1A1*2 or CYP1A1*4 alleles or with the GSTM1 null genotype. Novel CYP1A1 alleles with the base substitutions C4151T, G-469A and C-459T respectively, were detected by screening the coding exons and approximately 1 kb of upstream sequence in 20 individuals by single-strand conformational polymorphism (SSCP) analysis but none of the three novel alleles appeared to be associated with high induced CYP1A1 activity in the study group. Screening of the 11 exons of the AhR gene by SSCP analysis confirmed the existence of the previously described G1721A polymorphism in a Caucasian population and a novel allele (G1768A which results in the amino acid substitution V5701) was also detected. The novel allele was very rare in Caucasians though more common in African-Americans. Individuals with at least one copy of the G1721A AhR variant allele showed a significantly higher level of induced CYP1A1 activity compared with individuals negative for the polymorphism (P = 0.0001). A similar finding was obtained for induced CYP1A1 protein levels determined by immunoblotting. Levels of induced CYP1A1 activity were also found to show a sex difference with women showing a significantly lower induced activity compared with men. We conclude that genotypes for the G1721A AhR polymorphism and gender appear to be determinants of levels of induced CYP1A1 activity and that interindividual variation in levels of induced CYP1A1 activity appears to be associated more with regulatory factors than polymorphism in the CYP1A1 gene.

Activation of procarcinogens by human cytochrome P450 enzymes

Enzymatic transformation of most chemical carcinogens is requisite to the formation of electrophiles that cause genotoxicity, and the cytochrome P450 (P450) enzymes are the most prominent enzymes involved in such activation reactions. During the past 15 years the human P450 enzymes have been extensively characterized. Considerable evidence exists that the variation in activity of these enzymes can have important consequences in the actions of drugs. Other studies have been concerned with the activation of procarcinogens by human P450s. Assignments of roles of particular P450s in the metabolism of chemical carcinogens are discussed, along with the current state of evidence for relationships of particular P450s with human cancer.