Sulfonation in chemical carcinogenesis--history and present status

Sulphotransferase-mediated toxification of chemicals in mouse models: effect of knockout or humanisation of SULT genes

Cytosolic sulphotransferase (SULT) enzymes catalyse reactions involved in xenobiotic elimination and hormone regulation. However, SULTs can also generate electrophilic reactive intermediates from certain substrates, including the activation of carcinogens. Here, we review toxicological studies of mouse strains with SULT status altered by genetic modification. Knockout mouse strains have been constructed for the enzymes Sult1a1, 1d1, 1e1, 2b1 and 4a1. In addition, transgenic strains are available for human SULT1A1/2. Among SULT knockout mouse strains, reduced fertility (Sult1e1) and early postnatal death (Sult4a1) were observed. In contrast, Sult1a1 or Sult1d1 knockouts and SULT1A1/2 transgenics were healthy and showed no obvious deficiencies. These strains were used in toxicological studies with 13 chemicals. Manipulation of the SULT system altered dramatically the adverse effects of many compounds; thus, very large differences in levels of DNA adducts formed in the liver or other tissues were seen with some chemicals - up to 99.2% decreases in knockouts and 83-fold increases in SULT1A1/2 transgenics. In many cases, these changes were restricted to the tissues in which the corresponding enzymes are expressed, arguing for local activation. However, with some compounds, the kidney was an important target tissue, due to the active transfer to that organ, via the circulation, of reactive sulphuric acid esters.

Urinary Excretion of Mercapturic Acids of the Rodent Carcinogen Methyleugenol after a Single Meal of Basil Pesto: A Controlled Exposure Study in Humans

Methyleugenol (ME), found in numerous plants and spices, is a rodent carcinogen and is classified as "possibly carcinogenic to humans". The hypothesis of a carcinogenic risk for humans is supported by the observation of ME-derived DNA adducts in almost all human liver and lung samples examined. Therefore, a risk assessment of ME is needed. Unfortunately, biomarkers of exposure for epidemiological studies are not yet available. We hereby present the first detection of N-acetyl-l-cysteine conjugates (mercapturic acids) of ME in human urine samples after consumption of a popular ME-containing meal, pasta with basil pesto. We synthesized mercapturic acid conjugates of ME, identified the major product as N-acetyl-S-[3'-(3,4-dimethoxyphenyl)allyl]-l-cysteine (E-3'-MEMA), and developed methods for its extraction and LC-MS/MS quantification in human urine. For conducting an exposure study in humans, a basil cultivar with a suitable ME content was grown for the preparation of basil pesto. A defined meal containing 100 g of basil pesto, corresponding to 1.7 mg ME, was served to 12 participants, who collected the complete urine at defined time intervals for 48 h. Using d6-E-3'-MEMA as an internal standard for LC-MS/MS quantification, we were able to detect E-3'-MEMA in urine samples of all participants collected after the ME-containing meal. Excretion was maximal between 2 and 6 h after the meal and was completed within about 12 h (concentrations below the limit of detection). Excreted amounts were only between 1 and 85 ppm of the ME intake, indicating that the ultimate genotoxicant, 1'-sulfooxy-ME, is formed to a subordinate extent or is not efficiently detoxified by glutathione conjugation and subsequent conversion to mercapturic acids. Both explanations may apply cumulatively, with the ubiquitous detection of ME DNA adducts in human lung and liver specimens arguing against an extremely low formation of 1'-sulfooxy-ME. Taken together, we hereby present the first noninvasive human biomarker reflecting an internal exposure toward reactive ME species.

Bioactivation and Reactivity Research Advances - 2021 year in review

This year's review on bioactivation and reactivity began as a part of the annual review on biotransformation and bioactivation led by Cyrus Khojasteh (Khojasteh et al., 2021, 2020, 2019, 2018, 2017; Baillie et al., 2016). Increased contributions from experts in the field led to the development of a stand alone edition for the first time this year focused specifically on bioactivation and reactivity. Our objective for this review is to highlight and share articles which we deem influential and significant regarding the development of covalent inhibitors, mechanisms of reactive metabolite formation, enzyme inactivation, and drug safety. Based on the selected articles, we created two sections: (1) reactivity and enzyme inactivation, and (2) bioactivation mechanisms and safety (Table 1). Several biotransformation experts have contributed to this effort from academic and industry settings.

Estrogen sulfotransferase in the metabolism of estrogenic drugs and in the pathogenesis of diseases

Biotransformation is important in the metabolism of endobiotics and xenobiotics. This process comprises the activity of phase I and phase II enzymes. Estrogen sulfotransferase (SULT1E1 or EST) is a phase II conjugating enzyme that belongs to the family of cytosolic sulfotransferases. The expression of SULT1E1 can be detected in many tissues, including the liver. SULT1E1 catalyzes the transfer of a sulfate group from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to any available hydroxyl group in estrogenic molecules. The substrates of SULT1E1 include the endogenous and synthetic estrogens. Upon SULT1E1-mediated sulfation, the hydrosolubility of estrogens increases, preventing the binding between the sulfated estrogens and the estrogen receptor (ER). This sulfated state of the estrogens is not irreversible, as the steroid sulfatase (STS) can convert sulfoconjugated estrogens to free estrogens. The expression of SULT1E1 is inducible by several diseases that involve tissue inflammation, such as type 2 diabetes, sepsis, and ischemia-reperfusion injury. Areas covered: This systematic literature review aims to summarize the role of SULT1E1 in the metabolism of estrogenic drugs and xenobiotics, and the role of SULT1E1 in the pathogenesis of several diseases, including cancer, metabolic disease, sepsis, liver injury, and cystic fibrosis. Meanwhile, ablation or pharmacological inhibition of SULT1E1 can affect the outcomes of the aforementioned diseases. Expert opinion: In addition to its role in metabolizing estrogenic drugs, SULT1E1 is unexpectedly being unveiled as a mediator for the disease effect on estrogen metabolism and homeostasis. Meanwhile, because the expression and activity of SULT1E1 can affect the outcome of diseases, the same sulfotransferase and the reversing enzymes STS can be potential therapeutic targets to prevent or manage diseases. Accumulating evidence suggest that the physiological and pathophysiological effects of SULT1E1 can be estrogen-independent and it is necessary to elucidate what other possible substrates may be recognized by the enzyme. Moreover, human studies are paramount to confirm the human relevance of the animal studies.

Effects of inhibition of hepatic sulfotransferase activity on renal genotoxicity induced by lucidin-3-O-primeveroside

Sulfotransferase 1A (SULT1A) expression is lower in the liver of humans than that of rodents. Therefore, species differences should be taken into consideration when assessing the risk of rodent hepatocarcinogens metabolically activated by SULT1A in humans. Although some renal carcinogens require SULT1A-mediated activation, it is unclear how SULT1A activity in the liver affects renal carcinogens. To explore the effects of SULT1A activity in the liver on genotoxicity induced by SULT1A-activated renal carcinogens, B6C3F₁ mice or gpt delta mice of the same strain background were given lucidin-3-O-primeveroside (LuP), a hepatic and renal carcinogen of rodents, for 4 or 13 weeks, respectively, and pentachlorophenol (PCP) as a liver-specific SULT inhibitor, was given from 1 week before LuP treatment to the end of the experiment. A 4 week exposure of LuP induced lucidin-specific DNA adduct formation. The suppression of Sult1a expression was observed only in the liver but not in the kidneys of PCP-treated mice, but co-administration of PCP suppressed LuP-induced DNA adduct formation in both organs. Thirteen-week exposure of LuP increased mutation frequencies and cotreatment with PCP suppressed these increases in both organs. Given that intact levels of SULT activity in the liver were much higher than in the kidneys of rodents, SULT1A may predominantly activate LuP in the liver, consequently leading to genotoxicity not only in the liver but also in the kidney. Thus, species differences should be considered in human risk assessment of renal carcinogens activated by SULT1A as in the case of the corresponding liver carcinogens.

Hepatic Transcript Levels for Genes Coding for Enzymes Associated with Xenobiotic Metabolism are Altered with Age

Metabolism studies are crucial for data interpretation from rodent toxicity and carcinogenicity studies. Metabolism studies are usually conducted in 6 to 8 week old rodents. Long-term studies often continue beyond 100 weeks of age. The potential for age-related changes in transcript levels of genes encoding for enzymes associated with metabolism was evaluated in the liver of male F344/N rats at 32, 58, and 84 weeks of age. Differential expression was found between the young and old rats for genes whose products are involved in both phase I and phase II metabolic pathways. Thirteen cytochrome P450 genes from CYP families 1–3 showed alterations in expression in the older rats. A marked age-related decrease in expression was found for 4 members of the Cyp3a family that are critical for drug metabolism in the rat. Immunohistochemical results confirmed a significant decrease in Cyp3a2 and Cyp2c11 protein levels with age. This indicates that the metabolic capacity of male rats changes throughout a long-term study. Conducting multiple hepatic microarray analyses during the conduct of a long-term study can provide a global view of potential metabolic changes that might occur. Alterations that are considered crucial to the interpretation of long-term study results could then be confirmed by subsequent metabolic studies.

Identification of Human Sulfotransferases Involved in Lorcaserin N-Sulfamate Formation

Lorcaserin [(R)-8-chloro-1-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine] hydrochloride hemihydrate, a selective serotonin 5-hydroxytryptamine (5-HT) 5-HT(2C) receptor agonist, is approved by the U.S. Food and Drug Administration for chronic weight management. Lorcaserin is primarily cleared by metabolism, which involves multiple enzyme systems with various metabolic pathways in humans. The major circulating metabolite is lorcaserin N-sulfamate. Both human liver and renal cytosols catalyze the formation of lorcaserin N-sulfamate, where the liver cytosol showed a higher catalytic efficiency than renal cytosol. Human sulfotransferases (SULTs) SULT1A1, SULT1A2, SULT1E1, and SULT2A1 are involved in the formation of lorcaserin N-sulfamate. The catalytic efficiency of these SULTs for lorcaserin N-sulfamate formation is widely variable, and among the SULT isoforms SULT1A1 was the most efficient. The order of intrinsic clearance for lorcaserin N-sulfamate is SULT1A1 > SULT2A1 > SULT1A2 > SULT1E1. Inhibitory effects of lorcaserin N-sulfamate on major human cytochrome P450 (P450) enzymes were not observed or minimal. Lorcaserin N-sulfamate binds to human plasma protein with high affinity (i.e., >99%). Thus, despite being the major circulating metabolite, the level of free lorcaserin N-sulfamate would be minimal at a lorcaserin therapeutic dose and unlikely be sufficient to cause drug-drug interactions. Considering its formation kinetic parameters, high plasma protein binding affinity, minimal P450 inhibition or induction potential, and stability, the potential for metabolic drug-drug interaction or toxicological effects of lorcaserin N-sulfamate is remote in a normal patient population.

Detecting reactive drug metabolites for reducing the potential for drug toxicity

INTRODUCTION

A number of withdrawn drugs are known to undergo bioactivation by a range of drug metabolizing enzymes to chemically reactive metabolites that bind covalently to protein and DNA resulting in organ toxicity and carcinogenesis, respectively. An important goal in drug discovery is to identify structural sites of bioactivation within discovery molecules for providing strategic modifications that eliminate or minimize reactive metabolite formation, while maintaining target potency, selectivity and desired pharmacokinetic properties leading to the development of efficacious and nontoxic drugs.

AREAS COVERED

This review covers experimental techniques currently used to detect reactive drug metabolites and provides recent examples where information from mechanistic in vitro studies was successfully used to redesign candidate drugs leading to blocked or minimized bioactivation. Reviewed techniques include in vitro radiolabeled drug covalent binding to protein and reactive metabolite trapping with reagents such as glutathione, cyanide, semicarbazide and DNA bases. Case studies regarding reactive metabolite detection using a combination of varied techniques, including liquid chromatography-tandem mass spectrometry and NMR analyses and subsequent structural modification are discussed.

EXPERT OPINION

Information derived from state-of-art mechanistic drug metabolism studies can be used successfully to direct medicinal chemistry towards the synthesis of candidate drugs devoid of bioactivation liabilities, while maintaining desired pharmacology and pharmacokinetic properties.

Sulfotransferase 1A1 (SULT1A1) gene expression is regulated by members of the NFI transcription factors in human breast cancer cells

Background

Sulfotransferase 1A1 (SULT1A1) gene expression is tissue specific, with little to no expression in normal breast epithelia. Expression in breast tumors has been documented, but the transcriptional regulation of SULT1A1 in human breast tissue is poorly understood. We identified Nuclear Factor I (NFI) as a transcription factor family involved in the regulation of SULT1A1 expression.

Methods

Transcription Factor Activation Profiling Plate Array assay was used to identify the possible transcription factors that regulate the gene expression of SULT1A1in normal breast MCF-10A cells and breast cancer ZR-75-1 cells. Expression levels of NFI-C and SULT1A1 were determined by real-time RT-PCR using total RNA isolated from 84 human liver samples. Expression levels of SULT1A1, NFI-A, NFI-B, NFI-C, and NFI-X were also determined in different human breast cancer cell lines (MCF-7, T-47D, ZR-75-1, and MDA-MB-231), in the transformed human epithelial cell line MCF-10A, and in ZR-75-1 cells that were transfected with siRNAs directed against NFI-A, NFI-B, NFI-C, or NFI-X for 48 h. The copy numbers of SULT1A1 in cell lines ZR-75-1, MCF-7, T-47D, MDA-MB-231, and MCF-10A were determined using a pre-designed Custom Plus TaqMan^® Copy Number kit from Life Technologies.

Results

In normal human liver samples, SULT1A1 mRNA level was positively associated with NFI-C. In different human breast cancer and normal epithelial cell lines, SULT1A1 expression was positively correlated with NFI-B and NFI-C. SULT1A1 expression was decreased 41% and 61% in ZR-75-1 cells treated with siRNAs against NFI-A and NFI-C respectively. SULT1A1 gene expression was higher in cells containing more than one SULT1A1 copy numbers.

Conclusions

Our data suggests that SULT1A1 expression is regulated by NFI, as well as SULT1A1 copy number variation in human breast cancer cell lines. These data provide a mechanistic basis for the differential expression of SULT1A1 in different tissues and different physiological states of disease.

Sulfation of dietary flavonoids by human sulfotransferases

Dietary flavonoids catechin, epicatechin, eriodictyol, and hesperetin were investigated as substrates and inhibitors of human sulfotransferases (hSULTs). Purified recombinant proteins and human intestine cytosol were used as enzyme sources. hSULT1A1 and hSULT1A3 as well as human intestine cytosol can catalyse the sulfation of the investigated flavonoids. Sulfation of catechin, epicatechin, eriodictyol, and hesperetin by recombinant hSULTs showed substrate inhibition at high flavonoid concentrations. Hesperetin and eriodictyol are potent inhibitors of purified hSULT1A1, hSULT1A3, hSULT1E1, and hSULT2A1. Catechin and epicatechin inhibited hSULT1A1 and hSULT1A3, but not hSULT1E1 and hSULT2A1. The sulfation efficacy and potency of inhibition is related to the C-ring structure of flavonoids. These results suggest that dietary flavonoids may regulate human SULT activity and, therefore, affect the regulation of hormones and neurotransmitters, detoxification of drugs, and the bioactivation of pro- carcinogens and pro-mutagens.

Inhibition of rat liver sulfotransferases SULT1A1 and SULT2A1 and glucuronosyltransferase by dietary flavonoids

1. Dietary flavonoids including kaempferol, quercetin, genistein and daidzein were tested for their ability to alter the conjugation of oestradiol (E(2)) via rat liver sulfotransferases and glucuronosyltransferase. 2. All four flavonoids inhibited the sulfonation of E(2) via phenol sulfotransferase, SULT1A1 with IC(50)s ranging from 0.29 to 4.61 micro M. Sulfonation of dehydroisoandrosterone (DHEA) via hydroxysteroid sulfotransferase, SULT2A1, was inhibited by higher amounts of the flavonoids (IC(50)s ranging from 34 to 116 micro M). 3. All flavonoids inhibited the formation of E(2)-beta-glucuronides (at carbon atoms 3 and 17) with IC(50)s ranging from 43 to 260 micro M. Glucuronidation of 4-methylumbelliferone (4-MU) was inhibited by high amounts of the flavonoids (IC(50)s ranging from 860 to 1550 micro M). 4. Hydrolysis of sulfonated oestrogens via arylsulfatase-c (ARSC) or 4-methylumbelliferone beta-glucuronidate (MUG) were not inhibited by the flavonoids. 5. It is concluded that SULT1A1 but not SULT2A1 or glucuronosyltransferase is highly sensitive to inhibition by dietary flavonoids. The potency of the inhibition for SULT1A1 (quercetin > kaempferol > genistein > daidzein) suggests a dependency on the number and position of hydroxyl radicals in the flavonoid molecule.

Interaction of the cytochrome P4501A2, SULT1A1 and NAT gene polymorphisms with smoking and dietary mutagen intake in modification of the risk of pancreatic cancer

Aromatic amines, N-nitroso compounds and heterocyclic amines are suspected human pancreatic carcinogens. Cytochrome P450 (CYP) 1A2, N-acetyltransferase (NAT) 1, NAT2 and sulfotransferase (SULT) are enzymes involved in the metabolism of these carcinogens. To test the hypothesis that genetic variations in carcinogen metabolism modify the risk of pancreatic cancer (PC), we investigated the effect of single-nucleotide polymorphisms (SNPs) of the CYP1A2, NAT1, NAT2 and SULT1A1 gene on modification of the risk of PC in a hospital-based study of 755 patients with pancreatic adenocarcinoma and 636 healthy frequency-matched controls. Smoking and dietary mutagen exposure information was collected by personal interviews. Genotypes were determined using the polymerase chain reaction–restriction fragment length polymorphism and Taqman methods. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using unconditional multivariate logistic regression analysis. We observed no significant main effects of any of these genes on the risk of PC. The CYP1A2 and NAT1 but not SULT1A1 and NAT2 genotypes showed significant interactions with heavy smoking in women not men. In contrast, a significant interaction between NAT1 genotype and dietary mutagen intake on modifying the risk of PC were observed among men but not women. The OR (95% CI) of PC was 2.23 (1.33–3.72) and 2.54 (1.51–4.25) for men having the NAT1*10 and a higher intake of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and benzo[a]pyrene, respectively, compared with individuals having no NAT1*10 or a lower intake of these dietary mutagens. These data suggest the existence of gender-specific susceptibility to tobacco carcinogen and dietary mutagen exposure in PC.

Role of biotransformation studies in minimizing metabolism-related liabilities in drug discovery

Metabolism-related liabilities continue to be a major cause of attrition for drug candidates in clinical development. Such problems may arise from the bioactivation of the parent compound to a reactive metabolite capable of modifying biological materials covalently or engaging in redox-cycling reactions leading to the formation of other toxicants. Alternatively, they may result from the formation of a major metabolite with systemic exposure and adverse pharmacological activity. To avert such problems, biotransformation studies are becoming increasingly important in guiding the refinement of a lead series during drug discovery and in characterizing lead candidates prior to clinical evaluation. This article provides an overview of the methods that are used to uncover metabolism-related liabilities in a pre-clinical setting and offers suggestions for reducing such liabilities via the modification of structural features that are used commonly in drug-like molecules.

Comparative analysis of microarray data identifies common responses to caloric restriction among mouse tissues

Caloric restriction has been extensively investigated as an intervention that both extends lifespan and delays age-related disease in mammals. In mice, much interest has centered on evaluating gene expression changes induced by caloric restriction (CR) in particular tissue types, but the overall systemic effect of CR among multiple tissues has been examined less extensively. This study presents a comparative analysis of microarray datasets that have collectively examined the effects of CR in 10 different tissue types (liver, heart, muscle, hypothalamus, hippocampus, white adipose tissue, colon, kidney, lung and cochlea). Using novel methods for comparative analysis of microarray data, detailed comparisons of the effects of CR among tissues are provided, and 28 genes for which expression response to CR is most shared among tissues are identified. These genes characterize common responses to CR, which consist of both activation and inhibition of stress-response pathways. With respect to liver tissue, transcriptional effects of CR exhibited surprisingly little overlap with those of aging, and a variable degree of overlap with the potential CR-mimetic drug resveratrol. These analyses shed light on the systemic transcriptional activity associated with CR diets, and also illustrate new approaches for comparative analysis of microarray datasets in the context of aging biology.

In vivo and in vitro metabolism of arylamine procarcinogens in acetyltransferase-deficient mice

Arylamine N-acetyltransferases (NATs) catalyze the biotransformation of a number of aromatic and heterocyclic amines, many of which are procarcinogenic agents. Interestingly, these enzymes are binary in nature, participating in both detoxification and activation reactions, and thus it is unclear what role NATs actually play in either preventing or enhancing toxic responses. The ultimate direction may be substrate-specific and dependent on its tissue-specific metabolism by competing, but genetically variable, drug-metabolizing enzymes. To investigate the effect of N-acetylation on the metabolism of some classical procarcinogenic arylamines, we have used our double knockout Nat1/2(-/-) mouse model to test both in vitro activity and the in vivo clearance of some of these agents. As expected, N-acetylation activity was undetectable in tissue cytosol preparations from Nat1/2(-/-) mice for 4-aminobiphenyl (ABP) and 2-aminofluorene (AF), whereas significant levels were measured in all wild-type tissue cytosols tested, indicating the widespread metabolism of these agents. Nat1/2(-/-) mice displayed a variable response with respect to in vivo pharmacokinetics. AF appeared to be most severely compromised, with a 3- to 4-fold increased area under the curve (AUC), whereas the clearance of ABP was found to be less dependent on N-acetylation, with no difference in ABP-AUC between wild-type and knockout animals. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine was neither N-acetylated nor was its clearance affected by NAT genotype, signifying a dependence on other drug-metabolizing enzymes. The elucidation of the role that N-acetylation plays in the clearance of procarcinogenic agents is the first step in attempting to correlate metabolism by NATs to toxic outcome prevention or augmentation.

Polychlorobiphenylols are selective inhibitors of human phenol sulfotransferase 1A1 with 4-nitrophenol as a substrate

Polychlorobiphenylols (OH-PCBs) were reported as potent inhibitors of estrogen sulfotransferase, thyroid hormone and 3-hydroxybenzo(a)pyrene sulfotransferases. The aim of this study was to examine the effects of selected OH-PCBs on SULT1A1 activity in human liver cytosol, measured with 4microM 4-nitrophenol, a concentration considered to be diagnostic for selectively detecting SULT1A1. All the OH-PCBs studied inhibited the sulfonation of 4-nitrophenol in human liver cytosol. Among the eighteen OH-PCBs studied, 3'-OH-CB3 (4-chlorobiphenyl-3'-ol) was the most potent inhibitor (IC(50): 0.73+/-0.15microM, mean+/-S.D., n=3). The least potent inhibitor studied was 6'-OH-CB35 (3,3',4-trichlorobiphenyl-6'-ol) with IC(50): 49.1+/-10.8microM. The IC(50) values of the other OH-PCBs studied ranged from 0.78 to 3.76microM. Some OH-PCBs with various inhibitory potencies with human liver cytosol were selected for study with recombinant human SULT1A1 and SULT1B1. These OH-PCBs showed more potent inhibition of 4-nitrophenol sulfonation with SULT1A1 than with human liver cytosol. The IC(50) values with human liver cytosol showed a perfect linear correlation with those found with SULT1A1 (r(2)=1), but not with SULT1B1 (r(2)=0.21). The results suggested that in these human samples SULT1A1 was predominantly responsible for the sulfonation of 4-nitrophenol, with very little or no contribution from SULT1B1. The kinetics of inhibition were studied with 4'-OH-CB165, which is similar in structure to OH-PCBs found in human blood. The 4'-OH-CB165 was a mixed noncompetitive-uncompetitive inhibitor (K(i)=1.80+/-0.2microM, K(ies)=0.16+/-0.02microM). Finally, it was demonstrated that the tested OH-PCBs were themselves only slowly sulfonated by human sulfotransferases in the presence of (35)S-PAPS, as measured by the production of (35)S-labeled metabolites. Although this series of 18 OH-PCBs was too small to draw conclusions about structure-potency relationships, this work demonstrated that several OH-PCBs were potent inhibitors of 4-nitrophenol sulfonation but poor substrates in human liver cytosol, and suggested that OH-PCBs may inhibit the sulfation rate of those xenobiotics sulfated by SULT1A1.

Expression profiling of human fetal cytosolic sulfotransferases involved in steroid and thyroid hormone metabolism and in detoxification

Protection against chemical insult is essential for normal development of the fetus, however many detoxification enzymes are poorly expressed during fetal development. A major exception is the sulfotransferase (SULT) family, which appears to be widely expressed in the developing human. These enzymes also play a key role in biosynthesis and homeostasis of a number of hormones, including estrogens and iodothyronines. We therefore examined the enzyme activity, protein and mRNA expression of SULT 1A, 1B, 1C, 1E and 2A families in a variety of human fetal and adult tissues. Our results show that these SULTs are expressed in the human fetus, with most present at levels equivalent to or higher than the adult. As there are no isoform-selective substrates for SULTs 1B1 and 1C2 we used immunoblot analysis to show for the first time expression of SULT1B1 at high levels in fetal small intestine, and expression of SULT1C2 in fetal liver, kidney and small intestine. SULT1C2 was not expressed in adult liver or colon. Sulfotransferase expression in the developing fetus is therefore more widespread than in the adult, and this has significant implication for our understanding of human developmental physiology.

Genetic polymorphism in the sulfotransferase SULT1A1 gene in cancer

Cytosolic sulfotransferases are enzymes that catalyze the conjugation of sulfate groups to a variety of xenobiotic and endogenous substrates. A mutation in the SULT1A1 gene has been associated with decreased sulfotransferase activity. We studied 125 cancer patients and 100 healthy controls from Brazil matched by age and gender. The objective of this study was to assess the impact of the SULT1A1 polymorphism on sulfotransferase activity in a population of cancer patients. Both heterozygous and homozygous individuals for the mutant allele had significantly decreased sulfotransferase enzymatic activity. This decrease was more significant in cancer patients. The frequency of the SULT1A1( *)2 allele was increased in the myeloma group (odds ratio=0.53). These data suggest a functional role for the SULT1A1 gene polymorphism in cancer.

Phylogenomic approaches to common problems encountered in the analysis of low copy repeats: the sulfotransferase 1A gene family example

Background

Blocks of duplicated genomic DNA sequence longer than 1000 base pairs are known as low copy repeats (LCRs). Identified by their sequence similarity, LCRs are abundant in the human genome, and are interesting because they may represent recent adaptive events, or potential future adaptive opportunities within the human lineage. Sequence analysis tools are needed, however, to decide whether these interpretations are likely, whether a particular set of LCRs represents nearly neutral drift creating junk DNA, or whether the appearance of LCRs reflects assembly error. Here we investigate an LCR family containing the sulfotransferase (SULT) 1A genes involved in drug metabolism, cancer, hormone regulation, and neurotransmitter biology as a first step for defining the problems that those tools must manage.

Results

Sequence analysis here identified a fourth sulfotransferase gene, which may be transcriptionally active, located on human chromosome 16. Four regions of genomic sequence containing the four human SULT1A paralogs defined a new LCR family. The stem hominoid SULT1A progenitor locus was identified by comparative genomics involving complete human and rodent genomes, and a draft chimpanzee genome. SULT1A expansion in hominoid genomes was followed by positive selection acting on specific protein sites. This episode of adaptive evolution appears to be responsible for the dopamine sulfonation function of some SULT enzymes. Each of the conclusions that this bioinformatic analysis generated using data that has uncertain reliability (such as that from the chimpanzee genome sequencing project) has been confirmed experimentally or by a "finished" chromosome 16 assembly, both of which were published after the submission of this manuscript.

Conclusion

SULT1A genes expanded from one to four copies in hominoids during intra-chromosomal LCR duplications, including (apparently) one after the divergence of chimpanzees and humans. Thus, LCRs may provide a means for amplifying genes (and other genetic elements) that are adaptively useful. Being located on and among LCRs, however, could make the human SULT1A genes susceptible to further duplications or deletions resulting in 'genomic diseases' for some individuals. Pharmacogenomic studies of SULT1Asingle nucleotide polymorphisms, therefore, should also consider examining SULT1A copy number variability when searching for genotype-phenotype associations. The latest duplication is, however, only a substantiated hypothesis; an alternative explanation, disfavored by the majority of evidence, is that the duplication is an artifact of incorrect genome assembly.

Thresholds for the effects of 2-acetylaminofluorene in rat liver

To explore for practical thresholds for DNA-reactive carcinogens in rat liver carcinogenicity, we have conducted a series of exposure-response studies using 2 well-studied hepatocarcinogens, 2-acetylaminofluorene (AAF) and diethylnitrosamine (DEN). Findings with AAF, including as yet unpublished experiments, are reviewed here and related to DEN observations. In these studies, we have administered exact intragastric doses during an initiation segment (IS) of 12-16 weeks followed in some experiments by phenobarbital (PB) as a liver tumor promoter for 24 weeks to enhance manifestation of initiation. The cumulative doses (CD) of AAF at the end of ISs ranged from 0.094 to 282.2 mg/kg. Our findings for AAF in the IS can be summarized as follows: (1) the earliest parameter to be affected with administration of low doses was the appearance of DNA adducts (around 4 weeks), followed at higher doses by cell proliferation; (2) formation of DNA adducts was nonlinear, with a no-observed effect level (NOEL) at a CD of 0.094 mg/kg and a plateau at higher doses (94.1 mg/kg); (3) cytotoxicity (necrosis) showed a NOEL at a CD of 28.2 mg/kg; (4) compensatory hepatocellular proliferation showed a NOEL at a CD of 28.2 mg/kg and was supralinear at a high CD (282.2 mg/kg); (5) formation of preneoplastic hepatocellular altered foci (HAF) showed a NOEL at a CD of 28.2 mg/kg, and was supralinear at a high CD (282.2 mg/kg); (6) a NOEL (CD 28.2 mg/kg) was found for tumor development and the exposure-response was supralinear. We interpret these findings to reflect practical thresholds for hepatocellular initiating effects of AAF and exaggerated responses at high-exposures doses, as also found for DEN. Thus, mechanisms of carcinogenesis can differ between low and high doses.

Methotrexate induction of human sulfotransferases in Hep G2 and Caco-2 cells

Methotrexate (MTX) was the first antifolate drug developed for the treatment of cancer. It is also effective in treating inflammatory and autoimmune diseases. Sulfotransferases are phase II drug-metabolizing enzymes and their induction by hormones and endogenous molecules is relatively well known, although xenobiotic drug induction of sulfotransferases has not been well studied. In the present investigation, MTX is shown to be a xenobiotic inducer of human sulfotransferases in transformed human liver (Hep G2) and intestinal (Caco-2) cells. Following MTX treatment, various sulfotransferases were induced in both cell lines. Enzyme assay, Western blot and reverse-transcription polymerase chain reaction (RT-PCR) results demonstrated that protein and mRNA expressions of human simple phenol sulfotransferase (P-PST), human monoamine sulfotransferase (M-PST), human dehydroepiandrosterone sulfotransferase (DHEA-ST) and human estrogen sulfotransferase (EST) were induced in Hep G2 cells; M-PST and DHEA-ST were induced in Caco-2 cells. Inductions in both cell lines were dose dependent. Enzyme activity and Western blot results were in good agreement with RT-PCR results, suggesting that the induction is at the gene transcription level. Folic acid had a significantly lesser effect on sulfotransferases compared with MTX. Interestingly, the induction of different sulfotransferases by MTX was inhibited by high doses of folic acid at both protein and mRNA levels in Hep G2 cells. Methotrexate is the first antifolate and apoptosis-inducing drug to show induction of sulfotransferases in Hep G2 cells and Caco-2 cells. The inhibition by folic acid suggests a possible mechanism for MTX induction.

Sulfotransferases and Acetyltransferases in Mutagenicity Testing: Technical Aspects

Sulfotransferases (SULTs) and N-acetyltransferases (NATs) mediate the terminal activation step of various mutagens and carcinogens. Target cells of standard in vitro mutagenicity tests do not express any endogenous SULTs. NATs are expressed in some cells, but may not reflect the substrate specificity of human NATs. External activating systems usually lack the cofactors for these enzymes. Upon addition of the cofactor, the ultimate mutagen may be formed, but especially sulfo conjugates--anions--may not reliably penetrate into the target cells. This chapter presents methods used to incorporate these enzyme systems into in vitro mutagenicity test systems and to identify the critical human forms. The method of choice is direct expression of the enzymes in target cells. We present procedures on how this can be reached in bacteria and in mammalian cell lines in culture. Furthermore, genetically manipulated mouse models are a very promising perspective for answering open questions.

Influence of phenylalanines 77 and 138 on the stereospecificity of aryl sulfotransferase IV

Aryl sulfotransferase (AST) IV (also named tyrosine-ester sulfotransferase and ST1A1) is a major phenol sulfotransferase in the rat, and it catalyzes the sulfation of many drugs, carcinogens, and other xenobiotics that contain phenol, benzylic alcohol, N-hydroxy arylamine, and oxime functional groups. Previous work discovered a stereospecificity of AST IV toward the enantiomers of 1,2,3,4-tetrahydro-1-naphthol and varying degrees of stereoselectivity with other chiral benzylic alcohols. The studies described here were directed toward understanding the roles of specific amino acid residues at the substrate binding site in determining the stereoselectivity of this sulfotransferase isoform. Docking experiments with a homology model of AST IV revealed three amino acid residues, Phe77, Phe138, and Tyr236, that may potentially be important for interactions with substituents on the chiral carbon of a benzylic alcohol serving as a sulfuryl acceptor, thereby imparting stereoselectivity. To test this hypothesis, mutants were constructed wherein each of the above residues was substituted with alanine. Kinetic studies on the sulfation of the enantiomers of 1,2,3,4-tetrahydro-1-naphthol indicated that the stereospecificity of the sulfotransferase was altered by the substitutions of alanine for either Phe77 or Phe138, but stereospecificity was maintained by alanine substitution at Tyr236. Molecular models of the mutant enzymes interacting with enantiomers of 1,2,3,4-tetrahydro-1-naphthols and with 2-naphthol indicate that Phe77 and Phe138 provide significant steric interactions at the active site that both enhance catalytic efficiency and impart stereospecificity in molecular recognition of substrates and inhibitors.

Sulfotransferase 1A1 (SULT1A1) polymorphism and bladder cancer risk: a case-control study

Sulfotransferases (SULT) catalyze both the bioactivation and detoxification of a wide range of promutagens and procarcinogens. SULT1A1 appears to be an important phenol SULT because of its abundance and distribution in many tissues and wide substrate specificity. The SULT1A1 gene possesses a G-->A polymorphism that results in an Arg to His amino acid substitution, and the His(213) allele has been shown to have low activity and low thermal stability. Because of its functional role and published data showing the influence of Arg213His polymorphism on the risk of some cancers, we hypothesized that the His(213) allele of the SULT1A1 gene may modify bladder cancer risk. To test this hypothesis, we determined the SULT1A1 Arg213His genotypes in 384 incident bladder cancer patients and 386 healthy frequency-matched controls. A comprehensive epidemiologic interview was conducted on all participants to collect personal information, such as demographics and smoking status. The Arg/His and His/His genotypes were more common in the controls than the cases (P=0.035), resulting in a His(213) allele frequency of 35.0% in controls and 28.8% in cases. When individuals with the His(213) allele genotypes (Arg/His+His/His) were combined and compared to individuals with the Arg/Arg genotype, we observed a statistically significant reduced risk of bladder cancer (OR=0.72; 95% CI 0.54-0.97). When we examined the data by gender, there was a statistically significant reduced risk of bladder cancer only in women (OR=0.42; 95% CI 0.23-0.78) and not in men (OR=0.84; 95% CI 0.60-1.19) with the His(213) genotypes. In addition, there was a reduced bladder cancer risk in never smokers (OR=0.59; 95% CI 0.36-0.98) with the His(213) allele genotypes, but not in former (OR=0.82; 95% CI 0.54-1.25) or current smokers (OR=0.68; 95% CI 0.29-1.58). The His(213) allele genotypes also appeared to provide some protective benefit for current and former smokers, as compared to those with the Arg/Arg genotype. In conclusion, this study provides epidemiologic evidence of a reduced bladder cancer risk associated with the SULT1A1 His(213) polymorphism. Further studies are warranted to elucidate the function of this SULT1A1 polymorphism with regard to organ specificity, gene-environment interactions, and the gender-related difference we observed.

Sulfotransferase 1A1 (SULT1A1) polymorphism, PAH-DNA adduct levels in breast tissue and breast cancer risk in a case-control study

Gene-environment interactions are hypothesized to be major contributors to susceptibility to environmental carcinogens and interindividual variability in cancer risk. We present findings on associations between genetic susceptibility due to inherited polymorphisms of the Phase II detoxification enzyme sulfotransferase 1A1 (SULT1A1), breast cancer risk, and polycyclic aromatic hydrocarbon (PAH)-DNA adducts. A hospital based case-control study was conducted at the New York-Presbyterian Medical Center (NYPMC). The study utilized two control groups: one comprised of women with benign breast disease (BBD) and the other comprised of women visiting NYPMC for routine gynecologic checkups (healthy controls). Blood samples were collected from cases and controls; and breast tissue from pathology blocks was collected from cases (tumor and non-tumor tissue) and BBD controls (benign tissue). PAH-DNA adduct levels were measured by immunohistochemistry in breast tissue samples, and the SULT1A1 (Arg/His) polymorphism at codon 213 was determined by PCR RFLP analyses using DNA from white blood cells. Increasing number of His alleles was modestly associated with breast cancer case-control status, when cases were compared to healthy controls (p for trend = 0.08), when cases were compared to BBD controls (p for trend = 0.08) and when cases were compared to both control groups combined (p for trend = 0.07). Contrary to our hypothesis PAH-DNA adduct levels in breast tissue were not associated with SULT1A1 genotype. Our findings are consistent with a prior report that the Arg/His polymorphism in SULT1A1 is associated with breast cancer risk.

Sulfonation and molecular action

The sulfonation of endogenous molecules is a pervasive biological phenomenon that is not always easily understood, and although it is increasingly recognized as a function of fundamental importance, there remain areas in which significant cognizance is still lacking or at most minimal. This is particularly true in the field of endocrinology, in which the sulfoconjugation of hormones is a widespread occurrence that is only partially, if at all, appreciated. In the realm of steroid/sterol sulfoconjugation, the discovery of a novel gene that utilizes an alternative exon 1 to encode for two sulfotransferase isoforms, one of which sulfonates cholesterol and the other pregnenolone, has been an important advance. This is significant because cholesterol sulfate plays a crucial role in physiological systems such as keratinocyte differentiation and development of the skin barrier, and pregnenolone sulfate is now acknowledged as an important neurosteroid. The sulfonation of thyroglobulin and thyroid hormones has been extensively investigated and, although this transformation is better understood, there remain areas of incomplete comprehension. The sulfonation of catecholamines is a prevalent modification that has been extensively studied but, unfortunately, remains poorly understood. The sulfonation of pituitary glycoprotein hormones, especially LH and TSH, does not affect binding to their cognate receptors; however, sulfonation does play an important role in their plasma clearance, which indirectly has a significant effect on biological activity. On the other hand, the sulfonation of distinct neuroendocrine peptides does have a profound influence on receptor binding and, thus, a direct effect on biological activity. The sulfonation of specific extracellular structures plays an essential role in the binding and signaling of a large family of extracellular growth factors. In summary, sulfonation is a ubiquitous posttranslational modification of hormones and extracellular components that can lead to dramatic structural changes in affected molecules, the biological significance of which is now beginning to be appreciated.

Sulfotransferases in the bioactivation of xenobiotics

Conjugation of xenobiotics is often associated with detoxification. However, this traditional view is one-sided. In particular, numerous compounds are known that are metabolized to chemically reactive metabolites via sulfation (O-sulfonation). This can be rationalized by the fact that the sulfate group is electron-withdrawing and may be cleaved off heterolytically in appropriate molecules, thus leading to the formation of a strongly electrophilic cation. The heterologous expression of sulfotransferases in indicator cells of standard mutagenicity tests has substantially improved the accessibility of this activation pathway. The use of this technology is important, since many reactive sulfate conjugates only show strong toxicological effects if they are generated directly within the indicator cell, due to their insufficient penetration of cell membranes. Xenobiotic-metabolizing sulfotransferases are cytosolic enzymes, which form a superfamily (SULT). Eleven distinct human SULT forms are known, which strongly differ in their tissue distribution and their substrate specificity. Common functionally relevant genetic polymorphisms of the transcribed region are known for two of the forms, SULT1A1 and 1A2. Studies using recombinant test systems demonstrate that many promutagens are activated with high selectivity by an individual SULT form. Pronounced differences in promutagen activation were detected between the different human forms, including their allelic variants, and also between orthologous SULTs from different species. Therefore, SULTs may be involved in the individual genetic disposition, species differences, and organotropisms for toxicological effects of chemicals. Activation by SULTs differs from other activation pathway in its cyclic nature: reaction of a sulfuric acid ester with water usually regenerates the hydroxylated compound, which becomes available for a new cycle of activation. SULT-mediated reactivation may even occur if another initial reactive species, e.g. an epoxide, has reacted with water.

Relationship of phenol sulfotransferase activity (SULT1A1) genotype to sulfotransferase phenotype in platelet cytosol

Sulfation catalysed by human cytosolic sulfotransferases is generally considered to be a detoxification mechanism. Recently, it has been demonstrated that sulfation of heterocyclic aromatic amines by human phenol sulfotransferase (SULT1A1) can result in a DNA binding species. Therefore, sulfation capacity has the potential to influence chemical carcinogenesis in humans. To date, one genetic polymorphism (Arg213His) has been identified that is associated with reduced platelet sulfotransferase activity. In this study, data on age, race, gender, SULT1A1 genotype and platelet SULT1A1 activity were available for 279 individuals. A simple colorimetric phenotyping assay, in conjunction with genotyping, was employed to demonstrate a significant correlation (r = 0.23, P < 0.01) of SULT1A1 genotype and platelet sulfotransferase activity towards 2-naphthol, a marker substrate for this enzyme. There was also a difference in mean sulfotransferase activity based on gender (1.28 nmol/min/mg, females; 0.94 nmol/min/mg, males, P = 0.001). DNA binding studies using recombinant SULT1A1*1 and SULT1A1*2 revealed that SULT1A1*1 catalysed N-hydroxy-aminobiphenyl (N-OH-ABP) DNA adduct formation with substantially greater efficiency (5.4 versus 0.4 pmol bound/mg DNA/20 min) than the SULT1A1*2 variant. A similar pattern was observed with 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5b]pyridine (N-OH-PhIP) (4.6 versus 1.8 pmol bound/mg DNA/20 min).

Metabolic activation capacity of neonatal mice in relation to the neonatal mouse tumorigenicity bioassay

The neonatal mouse tumorigenicity bioassay is a well-developed animal model that has recently been recommended as an alternative tumorigenicity bioassay by the International Conference on Harmonization (ICH) for Technical Requirements for the Registration of Pharmaceuticals for Human Use. There are sufficient data to conclude that this animal model is highly sensitive to genotoxic chemical carcinogens that exert their tumorigenicity through mechanisms involving the formation of covalently bound exogenous DNA adducts that lead to mutation. On the other hand, it is not sensitive to chemical carcinogens that exert tumorigenicity through a secondary mechanism. The metabolizing enzymes present in the neonatal mouse, particularly the cytochromes P450, are critical factors in determining the tumorigenic potency of a chemical tested in this bioassay. However, compared to the metabolizing enzymes of the adult mouse and rat, the study of the metabolizing enzymes in neonatal mouse tissues has been relatively limited.

Heterologous expression of xenobiotic mammalian-metabolizing enzymes in mutagenicity tester bacteria: an update and practical considerations

There is an increasing need for metabolic competent cell systems for the mechanistic studies of biotransformation of xenobiotics in toxicology in general and in genotoxicology in particular. These cell systems combine the heterologous expression of a particular mammalian biotransformation enzyme with a specific target/ end point by which a functional analysis of the expressed gene product in the (geno)toxicity of chemicals can be performed. cDNAs of an increasing number of mammalian biotransformation enzymes is being cloned. The construction of specific expression vectors permits their heterologous expression in laboratory bacteria, such as Escherichia coli strains. This development does not only allow biochemical and enzymatic studies of (pure) enzyme preparations but also facilitates the engineering of metabolically competent mutagenicity tester bacteria, thereby providing new tools for genotoxicity testing and for studying of the roles of biotransformation in chemical carcinogenesis. In this review, we describe an update as well as an evaluation of enzymes expressed in mutagenicity tester bacteria. Four types of biotransformation enzymes are now expressed in these bacteria, namely, GSTs, CYPs, NATs, and STs. The expression of these enzymes in the tester bacteria and their subsequent application in mutagenicity assays demonstrates that heterologous expression in this type of bacteria has a number implications for the functionality of the biotransformation enzymes as well as for the functioning of the tester bacteria in mutagenicity detection. We also describe here a number of practical considerations in this regard.

Mechanistic basis for nonlinearities and thresholds in rat liver carcinogenesis by the DNA-reactive carcinogens 2-acetylaminofluorene and diethylnitrosamine

To explore differences in mechanisms of carcinogenicity at low and high exposures, we have conducted a series of exposure-response studies of hepatocarcinogenesis in rats using 2 well-studied DNA-reactive carcinogens, 2-acetylaminofluorene and diethylnitrosamine. In these studies, we have used intraperitoneal injection or intragastric instillation to deliver exact doses during an initiation segment followed by phenobarbital as a liver tumor promoter to enhance manifestation of initiation. This protocol results in carcinogenicity comparable to that produced by lifetime exposure to the carcinogens. Our findings in these experiments provide evidence for the following: (a) formation of DNA adducts can be nonlinear, with a plateau at higher exposures; (b) cytotoxicity shows no-effect levels and is related to exposure; (c) compensatory hepatocyte proliferation shows no-effect levels and can be supralinear at high exposures; (d) formation of preneoplastic hepatocellular altered foci can show no-effect levels and appears supralinear at high exposures; (e) no-effect levels can exist for tumor development, and the exposure response can be supralinear. We interpret these findings to reflect thresholds for hepatocellular initiating effects of these carcinogens and exaggerated responses at high exposures attributable to cytotoxicity and compensatory hepatocyte proliferation. Such enhanced proliferation of hepatocytes harboring DNA damage likely results in an exaggerated yield of mutations in critical genes, leading to supralinear initiation of carcinogenesis. Thus, mechanisms differ between low and high exposures. Based on these observations, we suggest that linear extrapolation from high toxic exposures to postulated low-exposure effects of DNA-reactive carcinogens can yield overestimates. Such extrapolation must be supported by mechanistic information. The finding of no-effect levels provides a basis for understanding why low-level environmental exposures of humans to even DNA-reactive carcinogens may convey no cancer risk.

Human sulfotransferases SULT1C1 and SULT1C2: cDNA characterization, gene cloning, and chromosomal localization

Sulfate conjugation catalyzed by sulfotransferase (SULT) enzymes is an important pathway in the biotransformation of many drugs, other xenobiotics, neurotransmitters, and hormones. We previously described a human cDNA, SULT1C1, that encoded a protein similar in sequence to that of rat ST1C1. Subsequently, a related human cDNA, SULT1C2, was reported. In the present study, we set out to characterize further the human SULT1C1 cDNA and then to clone, structurally characterize, and map its gene. As an initial step, we performed 5'- and 3'-RACE with SULT1C1 cDNA. Those experiments demonstrated that a small number of SULT1C1 transcripts contained an "insert," which we later showed resulted from alternative splicing that involved an Alu sequence in intron 3 of SULT1C1. We then cloned and structurally characterized the SULT1C1 gene from a human genomic BAC library. Because the sequence of SULT1C2 was closely related to that of SULT1C1 and because the genes for other human SULT paralogues occur in clusters, we screened the BAC clones that had been positive for SULT1C1 to search for SULT1C2 and discovered a clone that contained both genes. That BAC was used to sequence and structurally characterize SULT1C2. SULT1C1 and SULT1C2 were approximately 21 and 10 kb in length, respectively. Both genes contained seven exons that encoded protein, and both had structures that were similar to those of other genes that encode members of the SULT1 family. Finally, human SULT1C1 and SULT1C2 mapped to 2q11.2 by fluorescence in situ hybridization. The cloning and structural characterization of SULT1C1 and SULT1C2 will now make it possible to perform molecular genetic and pharmacogenomic studies of these sulfate-conjugating enzymes in humans.

Impact of polymorphisms in xeno(endo)biotic metabolism on pattern and frequency of p53 mutations in bladder cancer

We have analyzed the tumor biopsies of 45 patients with bladder cancer for p53 mutations by direct sequencing. In addition to N-acetyltransferase-2 (NAT2) and GSTM1 allelisms, which were examined previously, we have analyzed the genetic polymorphisms of GSTT1, GSTP1, COMT, NQO1, TS-SULT and MPO in buffy coat DNA using PCR-based methods. All subjects were interviewed through a questionnaire on smoking, dietary habits and other risk factors. No specific pattern was evident for p53 mutations. Eight out of ten mutations occurred in grade 3 tumors. All p53 mutations occurred in subjects with the COMT mutated allele (p=0.03). The prevalence of cases with p53 mutations was 3.5-fold higher in subjects with wild type than in those with variant GSTP1 alleles (p=0.03). The other polymorphisms investigated were not associated with p53 mutations.

Phenol sulphotransferase SULT1A1 polymorphism: molecular diagnosis and allele frequencies in Caucasian and African populations

Sulphation, catalysed by members of the sulphotransferase (SULT) enzyme family, is an important component of the body's chemical defence mechanism, but also acts to bioactivate mutagens such as hydroxylated aryl and heterocyclic amines. A major human sulphotransferase, SULT1A1 (P-PST), metabolizes and/or bioactivates many drugs, iodothyronines and hydroxylated aromatic amines. The enzyme activity varies widely within the population and is under genetic control. We have developed an assay detecting a G-->A transition in SULT1A1 that causes an Arg213-->His substitution associated with low SULT activity and altered enzyme properties, and have used it to assess the SULT1A1 genotype in Caucasian (n=293) and African (Nigerian, n=52) populations. We show that the mutant SULT1A1*2 allele is present at frequencies of 0.321 and 0.269 in the Caucasian and African populations respectively. We also demonstrate a significant age-related difference in SULT1A1 genotype within our Caucasian population, with increasing incidence of SULT1A1*1 homozygosity and decreasing incidence of SULT1A1*2 homozygosity with increasing age, indicating a potential association of SULT1A1*1 allozyme(s) with protection against cell and/or tissue damage during aging.

Polymorphisms of N-acetyltransferases, glutathione S-transferases, microsomal epoxide hydrolase and sulfotransferases: influence on cancer susceptibility

It has become clear that several polymorphisms of human drug-metabolizing enzymes influence an individual's susceptibility for chemical carcinogenesis. This review gives an overview on relevant polymorphisms of four families of drug-metabolizing enzymes. Rapid acetylators (with respect to N-acetyltransferase NAT2) were shown to have an increased risk of colon cancer, but a decreased risk of bladder cancer. In addition an association between a NAT1 variant allele (NAT*10, due to mutations in the polyadenylation site causing approximately two fold higher activity) and colorectal cancer among NAT2 rapid acetylators was observed, suggesting a possible interaction between NAT1 and NAT2. Glutathione S-transferases M1 and T1 (GSTM1 and GSTT1) are polymorphic due to large deletions in the structural gene. Meta-analysis of 12 case-control studies demonstrated a significant association between the homozygous deletion of GSTM1 (GSTM1-0) and lung cancer (odds ratio: 1.41; 95% CI: 1.23-1.61). Combination of GSTM1-0 with two allelic variants of cytochrome P4501A1 (CYP1A1), CYP1A1 m2/m2 and CYP1A1 Val/Val further increases the risk for lung cancer. Indirect mechanisms by which deletion of GSTM1 increases risk for lung cancer may include GSTM1-0 associated decreased expression of GST M3 and increased activity of CYP1A1 and 1A2. Combination of GST M1-0 and NAT2 slow acetylation was associated with markedly increased risk for lung cancer (odds ratio: 7.8; 95% CI: 1.4-78.7). In addition GSTM1-0 is clearly associated with bladder cancer and possibly also with colorectal, hepatocellular, gastric, esophageal (interaction with CYP1A1), head and neck as well as cutaneous cancer. In individuals with the GSTT1-0 genotype more chromosomal aberrations and sister chromatid exchanges (SCEs) were observed after exposure to 1,3-butadiene or various haloalkanes or haloalkenes. Evidence for an association between GSTT1-0 and myelodysplastic syndrome and acute lymphoblastic leukemia has been presented. A polymorphic site of GSTP1 (valine to isoleucine at codon 104) decreases activity to several carcinogenic diol epoxides and was associated with testicular, bladder and lung cancer. Microsomal expoxide hydrolase (mEH) is polymorphic due to amino acid variation at residues 113 and 139. Polymorphic variants of mEH were associated with hepatocellular cancer (His-113 allele), ovarian cancer (Tyr-113 allele) and chronic obstructive pulmonary disease (His-113 allele). Three human sulfotransferases (STs) are regulated by genetic polymorphisms (hDHEAST, hM-PST, TS PST). Since a large number of environmental mutagens are activated by STs an association with human cancer risk might be expected.

Biology and function of the reversible sulfation pathway catalysed by human sulfotransferases and sulfatases

Sulfation and sulfate conjugate hydrolysis play an important role in metabolism, and are catalysed by members of the sulfotransferase and sulfatase enzyme super-families. In general, sulfation is a deactivating, detoxication pathway, but for some chemicals the sulfate conjugates are much more reactive than the parent compound. The range of compounds which are sulfated is enormous, yet we still understand relatively little of the function of this pathway. This review summarises current knowledge of the sulfation system and the enzymes involved, and illustrates how heterologous expression of sulfotransferases (SULTs) and sulfatases is aiding our appreciation of the properties of these important proteins. The role of sulfation in the bioactivation of procarcinogens and promutagens is discussed, and new data on the inhibition of the sulfotransferase(s) involved by common dietary components such as tea and coffee are presented. The genetic and environmental factors which are known to influence the activity and expression of human SULTs and sulfatases are also reviewed.

Inhibitory effect of hemin on mutagenicity of the electrophilic sulfuric acid ester of 6-hydroxymethylbenzo[a]pyrene

In the present study, we examined the effects of hemin on the mutagenicity of 6-sulfooxymethylbenzo[a]pyrene (SMBP) in Salmonella typhimurium TA98 and Chinese hamster lung fibroblast (V79) cells. The compound was tested for the possible chemoprotective activity against mutagenesis induced by SMBP and its precursor, 6-hydroxymethylbenzo[a]pyrene (HMBP), activated by hepatic cytosol and PAPS in S. typhimurium TA98. Hemin not only inhibited the mutagenic activity of SMBP in V79 cells but repressed the cytotoxicity induced by this reactive ester as demonstrated by increased cell growth. The intracellular accumulation of radioactivity in V79 cells exposed to [3H]SMBP was reduced by approximately 50% when hemin (10 microM) was added to the medium. Likewise, the formation of SMBP-DNA adducts in these cells was significantly attenuated by treatment with hemin. The covalent complex formation of hemin with SMBP was confirmed by solvent extraction and reverse-phase HPLC.

Bioactivation of benzylic and allylic alcohols via sulfo-conjugation

Although sulfo-conjugation, in general, has been regarded as a detoxification process in the xenobiotic metabolism, there is a substantial body of data supporting that the same reaction can also lead to activation of certain types of chemical carcinogens and mutagens. Examples include some aromatic amines and amides, alkenylbenzenes, methyl-substituted polyaromatic hydrocarbons, nitrotoluenes and nitrosamines. The N- or O-hydroxy derivatives of these compounds undergo sulfonation to form extremely reactive sulfuric acid esters that can play a role as ultimate carcinogenic/mutagenic metabolites. Previous studies from several laboratories have shown that hydroxymethyl polyarenes, such as hydroxymethylbenz[a]anthracenes, 6-hydroxymethylbenzo[a]pyrene, and 1-hydroxymethylpyrene, are activated to reactive benzylic sulfuric acid esters, preferentially by rat hepatic hydroxysteroid sulfotransferase. Some aromatic hydrocarbons bearing the secondary benzylic hydroxy functionality can also yield electrophilic sulfate esters in the presence of hepatic sulfotransferase activity. Thus, benzylic mono- and dihydroxy derivatives of cyclopenta[cd]pyrene form mutagenic and DNA binding species when incubated with rat liver cytosol and the sulfo-group donor, 3'-phosphoadenosine-5'-phosphosulfate. 1-Hydroxy-3-methylcholanthrene that also possesses the cyclopenta-fused ring system appears to be metabolically activated through sulfo-conjugation. Likewise, benzo[a]pyrene tetraol might be activated through sulfuric acid esterification at one of two benzylic hydroxyl groups. Methylene-bridged polyarenols represent another potential group of cyclic secondary benzylic alcohols that can be activated by sulfotransferases. Certain non-polycyclic aromatic type benzylic alcohols have also been proposed to undergo sulfotransferase-mediated activation. Besides benzylic sulfonation, sulfuric acid esterification of certain allylic alcohols can produce reactive species.

Design, production and characterization of antibodies discriminating between the phenol- and monoamine-sulphating forms of human phenol sulphotransferase

1. Phenol sulphotransferases (PSTs) are important enzymes in xenobiotic and endobiotic detoxication, and a key component of the body's chemical defence mechanism. 2. Human phenol-(P-PST) and monoamine-(M-PST) sulphating forms of PST share 93% amino acid sequence identity, and to date the various antibodies produced against PSTs all recognize both enzymes. 3. We have identified two peptides based on the cDNA-derived amino acid sequences of human P-PST and M-PST, which elicited for the first time antibodies capable of discriminating between these highly homologous enzymes. 4. These antibodies represent valuable tools for studying the expression, distribution and function of human phenol sulphotransferases.

Targeted overexpression of androgen receptor with a liver-specific promoter in transgenic mice

The rodent liver displays marked age- and sex-dependent changes in androgen sensitivity due to the sexually dimorphic and temporally programmed expression of the androgen receptor (AR) gene. We have altered this normal phenotype by constitutive overexpression of the rat AR transgene in the mouse liver by targeting it via the human phenylalanine hydroxylase (hPAH) gene promoter. These transgenic animals in their heterozygous state produce an approximately 30-fold higher level of the AR in the liver as compared with the nontransgenic control. Androgen inactivation via sulfonation of the hormone by dehydroepiandrosterone sulfotransferase (DST), an androgen-repressible enzyme, also contributes to the age- and sex-dependent regulation of hepatic androgen sensitivity. DST has a broad range of substrate specificity and is responsible for the age- and sex-specific activation of certain polycyclic aromatic hepatocarcinogens as well, by converting them to electrophilic sulfonated derivatives. In the transgenic female, the hepatic expression of DST was approximately 4-fold lower than in normal females, a level comparable to that in normal males. The hPAH-AR mice will serve as a valuable model for studying the sex- and age-invariant expression of liver-specific genes, particularly those involved in the activation of environmental hepatocarcinogens such as the aromatic hydrocarbons.

Quercetin, a potent and specific inhibitor of the human P-form phenosulfotransferase

The natural product quercetin was a potent inhibitor of the human P-form phenolsulfo-transferase with an IC50 value of 0.10 +/- 0.03 microM (mean +/- SEM; N = 5), which was three to four orders of magnitude more potent than its inhibition of other human sulfotransferases. The inhibition was noncompetitive with a Ki value of 0.10 microM. The potency and mechanism of this inhibition appear similar to those of the current standard P-form inhibitor, 2,6-dichloro-4-nitrophenol. Among other flavonoids examined, kaempferol was found to have an IC50 value of 0.39 +/- 0.07 microM, naringenin 10.6 +/- 1.6 microM and naringin 265 +/- 90 microM (N = 3). These observations suggest the potential for clinically important pharmacologic and toxicologic interactions by flavonoid-containing foods and beverages.