SEQUENTIAL METABOLISM OF 2,3,7-TRICHLORODIBENZO-P-DIOXIN (2,3,7-triCDD) BY CYTOCHROME P450 AND UDP-GLUCURONOSYLTRANSFERASE IN HUMAN LIVER MICROSOMES

Duplication, Loss, and Evolutionary Features of Specific UDP-Glucuronosyltransferase Genes in Carnivora (Mammalia, Laurasiatheria)

UDP-glucuronosyltransferases (UGTs) are one of the most important enzymes for xenobiotic metabolism or detoxification. Through duplication and loss of genes, mammals evolved the species-specific variety of UGT isoforms. Among mammals, Carnivora is one of the orders that includes various carnivorous species, yet there is huge variation of food habitat. Recently, lower activity of UGT1A and 2B were shown in Felidae and pinnipeds, suggesting evolutional loss of these isoforms. However, comprehensive analysis for genetic or evolutional features are still missing. This study was conducted to reveal evolutional history of UGTs in Carnivoran species. We found specific gene expansion of UGT1As in Canidae, brown bear and black bear. We also found similar genetic duplication in UGT2Bs in Canidae, and some Mustelidae and Ursidae. In addition, we discovered contraction or complete loss of UGT1A7–12 in phocids, some otariids, felids, and some Mustelids. These studies indicate that even closely related species have completely different evolution of UGTs and further imply the difficulty of extrapolation of the pharmacokinetics and toxicokinetic result of experimental animals into wildlife carnivorans.

Uridine Diphosphate-Glucuronosyltransferase (UGT) 2B Subfamily Interspecies Differences in Carnivores

UDP-glucuronosyltransferases (UGTs) are among the most important xenobiotic metabolizing enzymes that conjugate a wide range of chemicals. Previous studies showed that Felidae and Pinnipedia species have very low UGT activities toward some phenolic compounds because of the UGT1A6 pseudogene and small numbers of UGT1A isozymes. In addition to the UGT1As, UGT2Bs isozymes also conjugate various endogenous (eg, estrogens, androgens, and bile acids) and exogenous compounds (opioids, non-steroidal anti-inflammatory drugs, and environmental pollutants). However UGT2B activity and genetic background are unknown in carnivore species. Therefore, this study was performed to elucidate the species differences of UGT2Bs. Using typical substrates for UGT2Bs, UGT activity was measured in vitro. In addition, UGT2B genetic features are analyzed in silico. Results of UGT activity measurement indicate marked species differences between dogs and other carnivores (cats, Northern fur seals, Steller sea lions, Harbor seals, and Caspian seals). Dogs have very high Vmax/Km toward estradiol (17-glucuronide), estrone, lorazepam, oxazepam, and temazepam. Conversely, cats and pinniped species (especially Caspian seals and Harbor seals) have very low activities toward these substrates. The results of genetic synteny analysis indicate that Felidae and pinniped species have very small numbers of UGT2B isozymes (one or none) compared with dogs, rodents, and humans. Furthermore, Felidae species have the same nonsense mutation in UGT2B, which suggests that Felidae UGT2B31-like is also a pseudogene in addition to UGT1A6. These findings of lower activity of UGT2B suggest that Felidae and some pinniped species have very low UGT activity toward a wide range of chemicals. These results are important for Felidae and Pinnipedia species that are frequently exposed to drugs and environmental pollutants.

DNA damage by 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced p53-mediated apoptosis through activation of cytochrome P450/aryl hydrocarbon receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; polycyclic aromatic hydrocarbon) is a persistent and ubiquitous environmental contaminant that causes a wide variety of deleterious effects. In this study, the DNA damage and apoptotic activity induced by TCDD was examined using in silico and in vitro approaches. In silico study showed that conformational changes and energies involved in the binding of TCDD to cytochrome P450 1B1 (CYP1B1) were crucial for its target proteins. Moreover, activated TCDD had high affinity to bind with aryl hydrocarbon receptor (AhR), with a binding energy of -564.7 Kcal/mol. Further, TCDD-CYP1B1 complex showed strong binding affinity for caspase 3, showing a binding energy of -518.5 Kcal/mol, and the docking of caspase inhibitors in the complex showed weak interaction with low binding energy as compared to TCDD-CYP1B1 caspase complexes. Interestingly, TCDD-induced apoptosis was significantly suppressed in Ac-DEVD-CMK-pretreated cells. The DNA damage activity of TCDD was quantified by comet tail formation and γ-H2AX foci formation in HaCaT cells. The role of CYP1B1 and AhR in DNA damage and apoptosis was demonstrated, and clotrimazole as well as knockdown of CYP1B1 and AhR could inhibit TCDD activation and suppress DNA damage followed by apoptosis in HaCaT cells. Moreover, TCDD increased expression of p53 and PUMA and our data showed that TCDD induced DNA damage followed by p53-mediated apoptosis. This study highlights the critical role of CYP1B1 and AhR in TCDD activity and proposes that inhibition of these key molecules might serve as a potential therapeutic approach for treatment of allergy and cancer.

Comprehensive Characterization of Mouse UDP-Glucuronosyltransferase (Ugt) Belonging to the Ugt2b Subfamily: Identification of Ugt2b36 as the Predominant Isoform Involved in Morphine Glucuronidation

UDP-Glucuronosyltransferases (UGTs) are classified into three subfamilies in mice: Ugt1a, 2b, and 2a. In the Ugt1a subfamily, Ugt1a1 and 1a6 appear to correspond to human UGT1A1 and 1A6 The mouse is an important animal for its use in investigations, but the substrate specificities of Ugt isoforms belonging to the 2b subfamily in mice remain largely unknown. To address this issue, we characterized the substrate specificity of all isoforms of the Ugt2b subfamily expressed in the mouse liver. The cDNAs of Ugt1a1, Ugt2a3, and all the Ugt2b isoforms expressed in the liver were reverse-transcribed from the total RNA of male FVB-mouse livers and then amplified. A baculovirus-Sf9 cell system for expressing each Ugt was established. Of all the Ugts examined, Ugt2b34, 2b36, and 2b37 exhibited the ability to glucuronidate morphine with Ugt2b36, the most active in this regard. Ugt1a1, but also Ugt2b34, 2b36, and 2b37 to a lesser extent, preferentially catalyzed the glucuronidation of 17β-estradiol on the 3-hydroxyl group (E3G). With these isoforms, E3G formation by Ugt1a1 was efficient; however, Ugt2b5 exhibited a preference for the 17β-hydroxyl group (E17G). Ugt2b1 and Ugt2a3 formed comparable levels of E3G and E17G. Ugt2b1 and 2b5 were the only isoforms involved in chloramphenicol glucuronidation. As Ugt2b36 is highly expressed in the liver, it is most likely that Ugt2b36 is a major morphine Ugt in mouse liver. Regarding E3G formation, Ugt1a1, like the human homolog, seems to play an important role in the liver.

Structural-functional adaptations of porcine CYP1A1 to metabolize polychlorinated dibenzo-p-dioxins

Polychlorinated dibenzo-p-dioxins (PCDDs) are widespread by-products of human industrial activity. They accumulate in tissues of animals and humans, exerting numerous adverse effects on different systems. In living organisms, dioxins are metabolized by enzymes of the cytochrome P450 family, including CYP1A1. Particular dioxin congeners differ in their toxicity level and ability to undergo biodegradation. Since the molecular mechanisms underlying dioxin susceptibility or resistance to biodegradation are unknown, in the present study the molecular interactions between five selected dioxins and porcine CYP1A1 protein were investigated. It was found that the ability of a dioxin to undergo CYP1A1-mediated degradation is associated mainly with the number and position of chlorine atoms in the dioxin molecule. Among all examined congeners, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) demonstrated the highest affinity to CYP1A1 and, at the same time, the greatest distance to the active site of the enzyme. Interestingly, in contrast to other dioxins, the binding of the TCDD molecule to the porcine CYP1A1 active site resulted in a rapid and continuous closure of substrate channels. All the information may help to explain the extended half-life of TCDD in living organisms as well as its high toxicity.

Mammalian cytochrome P450-dependent metabolism of polychlorinated dibenzo-p-dioxins and coplanar polychlorinated biphenyls

Polychlorinated dibenzo-p-dioxins (PCDDs) and coplanar polychlorinated biphenyls (PCBs) contribute to dioxin toxicity in humans and wildlife after bioaccumulation through the food chain from the environment. The authors examined human and rat cytochrome P450 (CYP)-dependent metabolism of PCDDs and PCBs. A number of human CYP isoforms belonging to the CYP1 and CYP2 families showed remarkable activities toward low-chlorinated PCDDs. In particular, human CYP1A1, CYP1A2, and CYP1B1 showed high activities toward monoCDDs, diCDDs, and triCDDs but no detectable activity toward 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-tetraCDD). Large amino acids located at putative substrate-recognition sites and the F-G loop in rat CYP1A1 contributed to the successful metabolism of 2,3,7,8-tetraCDD. Rat, but not human, CYP1A1 metabolized 3,3',4,4',5-pentachlorobiphenyl (CB126) to two hydroxylated metabolites. These metabolites are probably less toxic than is CB126, due to their higher solubility. Homology models of human and rat CYP1A1s and CB126 docking studies indicated that two amino acid differences in the CB126-binding cavity were important for CB126 metabolism. In this review, the importance of CYPs in the metabolism of dioxins and PCBs in mammals and the species-based differences between humans and rats are described. In addition, the authors reveal the molecular mechanism behind the binding modes of dioxins and PCBs in the heme pocket of CYPs.

Dioxin suppresses benzo[a]pyrene-induced mutations and DNA adduct formation through cytochrome P450 1A1 induction and (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide inactivation in human hepatoma cells

Benzo[a]pyrene (BaP) is metabolically activated by cytochrome P450 enzymes, and forms DNA adduct leading to mutations. Cytochrome P450 1A1 plays a central role in this activation step, and this enzyme is strongly induced by chemical agents that bind to the aryl hydrocarbon receptor (AhR), which is also known as a dioxin receptor. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a potent AhR ligand has not been shown to form any DNA adduct, but has a possibility to aggravate the toxicity of precarcinogenic polycyclic hydrocarbons through the induction of metabolic enzymes. We treated human hepatoma cells (HepG2) with TCDD, and subsequently exposed them to BaP to elucidate the synergistic effects on mutations. Surprisingly, mutant frequency induced by BaP at the hypoxanthine-guanine phosphribosyltransferase (HPRT) locus was decreased by pretreatment with TCDD. In correlation with decrease in the mutant frequencies, BaP-DNA adduct formation was also decreased by TCDD pretreatment. This suppressive effect of TCDD was more potent when the cells were exposed to (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), a reactive metabolic intermediate of BaP. Among the enzymes catalyzing BaP oxidation and conjugation, cytochrome P450 1A1, 1A2, 3A4 and UDP-glucuronosyltransferase 1A1 mRNAs were induced by the exposure to TCDD. In cytochrome P450 1A1-deficient murine cells and cytochrome P450 1A1-uninducible human cells, TCDD could not suppress BPDE-DNA adduct formation. Further experiments using "Tet-On" cytochrome P450 1A1-overexpressing cells and a recombinant cytochrome P450 1A1 enzyme demonstrated that this is the key enzyme involved in the biotransformation of BaP, that is, both production and inactivation of BPDE. We conclude that TCDD-induced cytochrome P450 catalyzes the metabolism of BPDE to as yet-unidentified products that are not apparently DNA-reactive, thereby reducing mutations in hepatoma cells.

Comparison of serotonin glucuronidation activity of UDP-glucuronosyltransferase 1a6a (Ugt1a6a) and Ugt1a6b: evidence for the preferential expression of Ugt1a6a in the mouse brain

Mouse UDP-glucuronosyltransferase (Ugt) 1a6a and Ugt1a6b share 98% sequence homology, but there have been no reports to date that compare their expression levels or enzymatic activities in serotonin glucuronidation. Thus, we designed specific primers for Ugt1a6a and Ugt1a6b to compare their expression in mouse brain regions and livers. Ugt1a6a was dominantly expressed in mouse brains, especially the hippocampus, while both Ugt1a6a and Ugt1a6b were highly expressed in mouse livers, indicating that there are significant differences in the expression patterns of Ugt1a6a and Ugt1a6b among mouse tissues. Glucuronidation of endogenous neurotransmitter serotonin was catalyzed by Ugt1a6b with k(cat)/K(m) (4.5 M(-1)·s(-1)) slightly higher than that of Ugt1a6a (2.4 M(-1)·s(-1)). However, the difference in expression levels between Ugt1a6a and Ugt1a6b in the hippocampus led us to speculate that Ugt1a6a is likely the predominant catalyst of serotonin glucuronidation in the mouse brain. In conclusion, we successfully elucidated the differences between Ugt1a6a and Ugt1a6b expression in the mouse brain. Our new findings indicate that Ugt1a6a and Ugt1a6b play different roles in mice, driven by differences in expression and kinetic properties for serotonin glucuronidation.

Metabolism of 26,26,26,27,27,27-F6-1 alpha,23S,25-trihydroxyvitamin D3 by human UDP-glucuronosyltransferase 1A3

26,26,26,27,27,27-Hexafluoro-1alpha,25-dihydroxyvitamin D(3) [F(6)-1alpha, 25(OH)(2)D(3)], which is now clinically used as a drug for secondary hyperparathyroidism, is a hexafluorinated analog of the active form of vitamin D(3). Our previous studies demonstrated that CYP24A1 is responsible for the metabolism of F(6)-1alpha,25(OH)(2)D(3) in the target tissues and that F(6)-1alpha,25(OH)(2)D(3) was successively converted to F(6)-1alpha,23S,25(OH)(3)D(3) and F(6)-23-oxo-1alpha,25(OH)(2)D(3). In this study, we examined the metabolism of F(6)-1alpha,25(OH)(2)D(3),F(6)-1alpha,23S,25(OH)(3)D(3), and F(6)-23-oxo-1alpha,25(OH)(2)D(3) by human UDP-glucuronosyltransferases (UGTs). Of these compounds, F(6)-1alpha,23S,25(OH)(3)D(3) was remarkably glucuronidated both in human liver microsomes and in the recombinant system expressing human UGT. No significant interindividual differences were observed among 10 human liver samples. The recombinant system for 12 species of human UGTs revealed that F(6)-1alpha,23S,25(OH)(3)D(3) glucuronidation was specifically catalyzed by UGT1A3. The information obtained in this study seems very useful to predict the metabolism and efficacy of vitamin D analogs in human bodies before clinical trials. In addition, note that for the first time a possible probe substrate for UGT1A3 has been found.