[show abstract][hide abstract] ABSTRACT: Human CYP2A6 is responsible for the metabolism of nicotine and coumarin as well as the metabolic activation of tobacco-related nitrosamines. Earlier studies revealed that CYP2A6 activity was increased by dietary cadmium or cruciferous vegetables, but the underlying mechanisms remain to be clarified. In the present study, we investigated the possibility that Nrf2 might be involved in the regulation of CYP2A6. Real-time RT-PCR analysis revealed that the CYP2A6 mRNA level in human hepatocytes was significantly (P<0.01, 1.4-fold) induced by 10μM sulforaphane (SFN), a typical activator of Nrf2. A computer-based search identified three putative antioxidant response elements (AREs) in the 5'-flanking region of the CYP2A6 gene at positions -1212, -2444, and -3441, termed ARE1, ARE2, and ARE3, respectively. Electrophoretic mobility shift assays demonstrated that Nrf2 bound only to ARE1. Luciferase assays using HepG2 cells revealed that the overexpression of Nrf2 significantly increased the reporter activities of the constructs containing a 30-bp fragment that included ARE1. However, the activity of the construct containing the intact 5'-flanking region (-1 to -1395) including ARE1 was not increased by the overexpression of Nrf2. In contrast, when the reporter construct was injected into mice via the tail vein, the reporter activity in the liver was significantly (P<0.05, 1.9-fold) increased by SFN (1mg/head) administration. In conclusion, we found that human CYP2A6 is regulated via Nrf2, suggesting that CYP2A6 is induced under oxidative stress.
[show abstract][hide abstract] ABSTRACT: Human uridine diphosphate-glucuronosyltransferase 2B7 (UGT2B7) plays important roles in the metabolism of some clinical drugs, carcinogens, and steroid hormones. The molecular mechanisms of the inducible expression of UGT2B7 in response to xenobiotics have not been fully clarified. We sought to investigate whether the UGT2B7 is under the control of NF-E2 p45-related factor 2 (Nrf2), a key transcriptional factor regulating the expression of cytoprotective enzymes.
HepG2, HuH7, HLE, and Caco-2 cells were treated with sulforaphane (SFN), and the UGT2B7 mRNA levels were determined by real-time reverse transcriptase PCR. These cells were genotyped for the UGT2B7*2 (H268Y) allele using the PCR-restriction fragment length polymorphism method. Luciferase analyses and gel shift analyses were performed to identify the responsive regions for Nrf2 signaling.
The UGT2B7 mRNA was induced by SFN in HepG2 and HuH7 genotyped as UGT2B7*1/*1, but not in HLE and Caco-2 cells genotyped as UGT2B7*2/*2. In HepG2 cells, the UGT2B7 protein level and morphine glucuronosyltransferase activity were also significantly induced by SFN. The induction was prominently decreased with small interfering RNA for Nrf2. In the 5'-flanking region (-2.5 kb) of the UGT2B7*2 allele, a 324-base pair insertion at -2067 and 12 single nucleotide polymorphisms simultaneously existed. Luciferase analyses and gel shift analyses revealed that an antioxidant responsive element at -1170 was responsible for the transactivation by Nrf2. In addition, a region from -990 to -858 on the UGT2B7*1 allele was also responsible for the transactivation by Nrf2. Abrogation of the Nrf2-dependent transactivation of the UGT2B7*2 allele was owing to the single nucleotide polymorphism -900A>G.
UGT2B7 is transcriptionally regulated by Nrf2, but the mechanism is hindered by polymorphisms in the promoter region of UGT2B7*2. The allele-specific mechanism may cause variability of the glucuronidation in response to oxidative stress.
Pharmacogenetics and Genomics 08/2008; 18(8):709-20. · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: Human CYP2A6, which is predominantly expressed in liver, is a key enzyme responsible for the metabolism of nicotine, coumarin, and some pharmaceutical drugs. CYP2A6 is also expressed in sex steroid-responsive tissues such as breast, ovary, uterus, testis, and adrenal grand. In this study, we examined the regulation of CYP2A6 gene by estrogen. Reverse transcription-polymerase chain reaction (RT-PCR) assays revealed that CYP2A6 mRNA was induced by estradiol in estrogen receptor (ER)-positive MCF-7 (2.9-fold) and HepG2 (1.3-fold) cells, but not in ER-negative MDA-MB-435 cells. Real-time RT-PCR assays revealed the CYP2A6 induction by estradiol in human hepatocytes (1.2- to 1.5-fold). Computer-assisted homology search identified a putative estrogen response element (ERE) at -2436 on the CYP2A6 gene. Electrophoretic mobility shift assays demonstrated specific binding of ERalpha to this element. Luciferase assays using MCF-7 cells revealed that the transcriptional activity of the CYP2A6 promoter was significantly activated by estradiol in an ERalpha-dependent manner, in which ERE was responsible for the activation. Chromatin immunoprecipitation assays verified the in vivo association of ERalpha with the ERE on the CYP2A6 gene. Immunohistochemical analyses using human endometrial tissues indicated that the CYP2A6 protein level in glandular cells was significantly higher in the proliferative phase than in the secretory phase, concomitant with local estrogen secretion during the menstrual cycle. These findings clearly demonstrated that CYP2A6 is directly induced by estrogen in an ERalpha-dependent manner, implying a biological role of CYP2A6 in estrogen-responsive tissues. Furthermore, this mechanism can also explain clinical aspects of increased nicotine metabolism under estrogen-rich environments.
Drug Metabolism and Disposition 11/2007; 35(10):1935-41. · 3.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: Human CYP2A6 catalyzes the metabolism of nicotine, cotinine, and coumarin as well as some pharmaceutical drugs. CYP2A6 is highly expressed in liver and, also, in brain and steroid-related tissues. In this study, we investigated the inhibitory effects of neurotransmitters and steroid hormones on CYP2A6 activity. We found that coumarin 7-hydroxylation and cotinine 3'-hydroxylation by recombinant CYP2A6 expressed in baculovirus-infected insect cells were competitively inhibited by tryptamine (both K(i) = 0.2 microM), serotonin (K(i) = 252 microM and 167 microM), dopamine (K(i) = 49 microM and 22 microM), and histamine (K(i) = 428 microM and 359 microM). Cotinine formation from nicotine was inhibited by tryptamine (K(i) = 0.7 microM, competitive), serotonin (K(i) = 272 microM, noncompetitive), dopamine, noradrenaline, and adrenaline (K(i) = 11 microM, 54 microM, and 81 microM, uncompetitive). Estrogens (K(i) = 0.6-3.8 microM), androgens (K(i) = 60-149 microM), and corticosterone (K(i) = 36 microM) also inhibited cotinine formation, but coumarin 7-hydroxylation and cotinine 3'-hydroxylation did not. Nicotine-Delta(5'(1'))-iminium ion formation from nicotine was not affected by these steroid hormones, indicating that the inhibition of cotinine formation was due to the inhibitory effects on aldehyde oxidase. The nicotine-Delta(5'(1'))-iminium ion formation was competitively inhibited by tryptamine (K(i) = 0.3 microM), serotonin (K(i) = 316 microM), dopamine (K(i) = 66 microM), and histamine (K(i) = 209 microM). Thus, we found that some neurotransmitters inhibit CYP2A6 activity, being related with inter- and intraindividual differences in CYP2A6-dependent metabolism. The inhibitory effects of steroid hormones on aldehyde oxidase may also contribute to interindividual differences in nicotine metabolism.
Drug Metabolism and Disposition 05/2007; 35(4):508-14. · 3.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: CYP2A6 plays important roles in the metabolism of nicotine and some clinically used drugs. Interindividual variability in the CYP2A6 expression level in human liver might be caused by an inducible property, but the molecular mechanism of induction is unclear. Rifampicin, phenobarbital, and 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime, which are activators of pregnane X receptor (PXR) and constitutive androstane receptor (CAR), induced CYP2A6 mRNA in human hepatocytes. We identified three direct repeat separated by four nucleotides (DR4)-like elements at -6698, -5476, and -4618 in the CYP2A6 gene, to which PXR and CAR could bind after dimerization with retinoid X receptor (RXR)-alpha. In luciferase assays, overexpression of PXR or CAR could not activate the transcriptional activity of CYP2A6 promoter constructs (-6754 to -1) in HepG2 cells. Cotransfection of hepatocyte nuclear factor-4alpha did not affect the transcriptional activities in the absence or presence of PXR or CAR. Interestingly, cotransfection of peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha) as well as PXR significantly enhanced the transcriptional activity (3.9-fold of control). By the deletion of a possible suppresser region (-4533 to -185), the effects of PXR/PGC-1alpha on the transcriptional activity were increased (6.9-fold of control). Deletion or mutation analyses revealed that two DR4-like elements at -5476 and -4618 are essential for transactivation by PXR/PGC-1alpha. Chromatin immunoprecipitation assay revealed that PXR and PGC-1alpha bind to CYP2A6 chromatin. In conclusion, we found that CYP2A6 is induced via PXR and PGC-1alpha through the DR4-like element at the distal response region. This is the first study to report the molecular mechanism of the induction of CYP2A6.
Journal of Pharmacology and Experimental Therapeutics 12/2006; 319(2):693-702. · 3.89 Impact Factor
[show abstract][hide abstract] ABSTRACT: Human cytochrome P450 (CYP) 2A6 metabolizes nicotine to cotinine and is a possible modulator of nicotine addiction. Quantitative and qualitative differences in nicotine addiction have been observed between ethnic groups. However, there are few data on the ethnic influences of the CYP2A6-nicotine metabolism relationship, particularly with regard to black subjects. We determined the nicotine metabolism and CYP2A6 genotype in 176 white subjects and 160 black subjects, comparing them with our previous data from 209 Korean subjects and 92 Japanese subjects. Large interindividual differences were observed in the cotinine/nicotine ratios in plasma calculated as an index of nicotine metabolism in white subjects (range, 0.6-36.5) and in black subjects (range, 0.9-30.4). No ethnic difference in the metabolic ratio was observed among white subjects (mean, 7.2 +/- 5.0), black subjects (mean, 7.1 +/- 4.7), and Korean subjects (mean, 8.7 +/- 11.9), whereas Japanese subjects showed a significantly (P < .005) lower metabolic ratio (mean, 3.8 +/- 3.1) compared with the other populations. Women showed significantly (P < .05) higher metabolic ratios than men in the black population (8.0 +/- 5.3 versus 6.0 +/- 3.7). Obvious ethnic differences in the CYP2A6 alleles were observed among these 4 populations. The combined frequencies of the alleles lacking or showing reduced enzymatic activity (CYP2A6*2, CYP2A6*4, CYP2A6*5, CYP2A6*7, CYP2A6*9, CYP2A6*10, CYP2A6*11, CYP2A6*17, CYP2A6*19, and CYP2A6*20) were 9.1%, 21.9%, 42.9%, and 50.5% in white, black, Korean, and Japanese subjects, respectively. These CYP2A6 alleles were associated with reduced nicotine metabolism. Among the homozygotes of CYP2A6*1, interindividual and ethnic differences in the metabolic ratio were still observed. Thus some factors other than genetic ones might also contribute to the interindividual and ethnic differences. This comprehensive study of 4 populations extends our understanding of nicotine metabolism and the impact of genetic polymorphisms of the CYP2A6 gene.
[show abstract][hide abstract] ABSTRACT: Human CYP2A6 is a cytochrome P450 (CYP) isoform responsible for the metabolism of nicotine, coumarin, tegafur, and valproic acid, and metabolic activation of nitrosamines. Genetic polymorphisms of the CYP2A6 gene are a major causal factor of the large interindividual differences in nicotine metabolism. In the present study, we identified a novel allele, termed CYP2A6*20, in an African-American population. The allele possesses the deletion of two nucleotides in exon 4 resulting in a frame-shift from codon 196 and an early stop codon at 220 (exon 5) as well as three synonymous SNPs of G51A (G51A in cDNA), T5684C (T1191C), and C6692G (C1546G, 3'-untranslated region). The allele frequency in the African-American population (n=96) was 1.6% (95% confidence interval, 0.6-4.5%). In contrast, the CYP2A6*20 allele was not found in Caucasians (European-American) (n=185), Japanese (n=184) and Korean (n=209) populations. To investigate the effects of the polymorphism on the enzymatic activities, we expressed a wild type or variant (deletion of two nucleotides) CYP2A6 together with NADPH-CYP reductase in Escherichia coli. SDS-PAGE and immunoblot analyses demonstrated that truncated CYP2A6 protein was produced from the variant allele, although detected mRNA was the predicted size by reverse transcriptional-polymerase chain reaction. Coumarin 7-hydroxylation and nicotine C-oxidation, which are typical CYP2A6 activities, were completely abolished in the E. coli membrane expressing the variant allele. In vivo nicotine metabolism was evaluated using the cotinine/nicotine ratio 2 h after the chewing of one piece of nicotine gum. Two CYP2A6*1/CYP2A6*20 heterozygotes and a single CYP2A6*17/CYP2A6*20 heterozygote revealed lower cotinine/nicotine ratios compared with CYP2A6*1/CYP2A6*1 subjects (1.6 and 4.5, and 1.8 versus 9.5+/-5.4, n=52, respectively). We found a novel CYP2A6*20 allele in African-American subjects which codes a truncated protein lacking enzymatic activity.
[show abstract][hide abstract] ABSTRACT: Genetic polymorphisms of CYP2A6 gene are known as a causal factor of the interindividual differences in nicotine metabolism. We found three novel CYP2A6 alleles. The CYP2A6(*)18A allele has a single nucleotide polymorphism (SNP) of A5668T (A1175T, Y392F) in exon 8. The CYP2A6(*)18B allele has synonymous SNPs of G51A (G51A), T5684C (T1191C), and T5702C (T1209C) in addition to A5668T (A1175T, Y392F). The CYP2A6(*)19 allele has the SNPs of A5668T (A1175T, Y392F), T6354C (intron 8), and T6558C (T1412C, I471T) as well as the conversion with the CYP2A7 sequence in the 3'-untranslated region, in which the latter two changes correspond to CYP2A6(*)7. Ethnic differences in the frequencies of these alleles were observed between whites, African-Americans, Japanese, and Koreans. Wild or variant CYP2A6 (CYP2A6(*)18, CYP2A6(*)19, and CYP2A6(*)7) were expressed in Escherichia coli. For coumarin 7-hydroxylation and 5-fluorouracil formation from tegafur, the K(m) values were increased, and V(max) values were decreased in CYP2A6.18 compared with those in CYP2A6.1, resulting in decreased clearance to 50 and 35% of that of the wild type, respectively. The K(m) and V(max) values for nicotine C-oxidation were both increased, resulting in no change of clearance. In CYP2A6.19, the effects on the coumarin 7-hydroxylation and 5-fluorouracil formation (increased K(m) and decreased V(max)) were prominent, resulting in decreased clearance to 8% of those of the wild type. For nicotine C-oxidation, the K(m) and V(max) values were both decreased, resulting in decreased clearance to 30% of that of the wild type. The changes of the kinetics in CYP2A6.19 were similar to those in CYP2A6.7. In vivo nicotine metabolism was evaluated in whites (n = 56) and Koreans (n = 40). Although the CYP2A6(*)18 and CYP2A6(*)19 alleles were found only heterozygously, a subject with CYP2A6(*)7/CYP2A6(*)19 showed a lower cotinine/nicotine ratio of the plasma concentration compared with homozygotes of the CYP2A6(*)1A, supporting the in vitro results that the CYP2A6(*)19 allele leads to decreased enzymatic activity.
Drug Metabolism and Disposition 09/2005; 33(8):1202-10. · 3.36 Impact Factor