Human sulfotransferase SULT2A1 pharmacogenetics: genotype-to-phenotype studies

Complex roles for sulfation in the toxicities of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are persistent organic toxicants derived from legacy pollution sources and their formation as inadvertent byproducts of some current manufacturing processes. Metabolism of PCBs is often a critical component in their toxicity, and relevant metabolic pathways usually include their initial oxidation to form hydroxylated polychlorinated biphenyls (OH-PCBs). Subsequent sulfation of OH-PCBs was originally thought to be primarily a means of detoxication; however, there is strong evidence that it may also contribute to toxicities associated with PCBs and OH-PCBs. These contributions include either the direct interaction of PCB sulfates with receptors or their serving as a localized precursor for OH-PCBs. The formation of PCB sulfates is catalyzed by cytosolic sulfotransferases, and, when transported into the serum, these metabolites may be retained, taken up by other tissues, and subjected to hydrolysis catalyzed by intracellular sulfatase(s) to regenerate OH-PCBs. Dynamic cycling between PCB sulfates and OH-PCBs may lead to further metabolic activation of the resulting OH-PCBs. Ultimate toxic endpoints of such processes may include endocrine disruption, neurotoxicities, and many others that are associated with exposures to PCBs and OH-PCBs. This review highlights the current understanding of the complex roles that PCB sulfates can have in the toxicities of PCBs and OH-PCBs and research on the varied mechanisms that control these roles.

Sulfated bile acid is a host-derived ligand for MAIT cells

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that recognize bacterial riboflavin-based metabolites as activating antigens. Although MAIT cells are found in tissues, it is unknown whether any host tissue-derived antigens exist. Here, we report that a sulfated bile acid, cholic acid 7-sulfate (CA7S), binds the nonclassical MHC class I protein MR1 and is recognized by MAIT cells. CA7S is a host-derived metabolite whose levels were reduced by more than 98% in germ-free mice. Deletion of the sulfotransferase 2a family of enzymes (Sult2a1-8) responsible for CA7S synthesis reduced the number of thymic MAIT cells in mice. Moreover, recognition of CA7S induced MAIT cell survival and the expression of a homeostatic gene signature. By contrast, recognition of a previously described foreign antigen, 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU), drove MAIT cell proliferation and the expression of inflammatory genes. Thus, CA7S is an endogenous antigen for MAIT cells, which promotes their development and function.

Sulfation of 12-hydroxy-nevirapine by human SULTs and the effects of genetic polymorphisms of SULT1A1 and SULT2A1

Nevirapine (NVP) is an effective drug for the treatment of HIV infections, but its use is limited by a high incidence of severe skin rash and liver injury. 12-Hydroxynevirapine (12-OH-NVP) is the major metabolite of nevirapine. There is strong evidence that the sulfate of 12-OH-NVP is responsible for the skin rash. While several cytosolic sulfotransferases (SULTs) have been shown to be capable of sulfating 12-OH-NVP, the exact mechanism of sulfation in vivo is unclear. The current study aimed to clarify human SULT(s) and human organs that are capable of sulfating 12-OH-NVP and investigate the metabolic sulfation of 12-OH-NVP using cultured HepG2 human hepatoma cells. Enzymatic assays revealed that of the thirteen human SULTs, SULT1A1 and SULT2A1 displayed strong 12-OH-NVP-sulfating activity. 1-Phenyl-1-hexanol (PHHX), which applied topically prevents the skin rash in rats, inhibited 12-OH-NVP sulfation by SULT1A1 and SULT2A1, implying the involvement of these two enzymes in the sulfation of 12-OH-NVP in vivo. Among five human organ cytosols analyzed, liver cytosol displayed the strongest 12-OH-NVP-sulfating activity, while a low but significant activity was detected with skin cytosol. Cultured HepG2 cells were shown to be capable of sulfating 12-OH-NVP. The effects of genetic polymorphisms of SULT1A1 and SULT2A1 genes on the sulfation of 12-OH-NVP by SULT1A1 and SULT2A1 allozymes were investigated. Two SULT1A1 allozymes, Arg37Asp and Met223Val, showed no detectable 12-OH-NVP-sulfating activity, while a SULT2A1 allozyme, Met57Thr, displayed significantly higher 12-OH-NVP-sulfating activity compared with the wild-type enzyme. Collectively, these results contribute to a better understanding of the involvement of sulfation in NVP-induced skin rash and provide clues to the possible role of SULT genetic polymorphisms in the risk of this adverse reaction.

Quantitative Proteomics in Translational Absorption, Distribution, Metabolism, and Excretion and Precision Medicine

A reliable translation of in vitro and preclinical data on drug absorption, distribution, metabolism, and excretion (ADME) to humans is important for safe and effective drug development. Precision medicine that is expected to provide the right clinical dose for the right patient at the right time requires a comprehensive understanding of population factors affecting drug disposition and response. Characterization of drug-metabolizing enzymes and transporters for the protein abundance and their interindividual as well as differential tissue and cross-species variabilities is important for translational ADME and precision medicine. This review first provides a brief overview of quantitative proteomics principles including liquid chromatography-tandem mass spectrometry tools, data acquisition approaches, proteomics sample preparation techniques, and quality controls for ensuring rigor and reproducibility in protein quantification data. Then, potential applications of quantitative proteomics in the translation of in vitro and preclinical data as well as prediction of interindividual variability are discussed in detail with tabulated examples. The applications of quantitative proteomics data in physiologically based pharmacokinetic modeling for ADME prediction are discussed with representative case examples. Finally, various considerations for reliable quantitative proteomics analysis for translational ADME and precision medicine and the future directions are discussed. SIGNIFICANCE STATEMENT: Quantitative proteomics analysis of drug-metabolizing enzymes and transporters in humans and preclinical species provides key physiological information that assists in the translation of in vitro and preclinical data to humans. This review provides the principles and applications of quantitative proteomics in characterizing in vitro, ex vivo, and preclinical models for translational research and interindividual variability prediction. Integration of these data into physiologically based pharmacokinetic modeling is proving to be critical for safe, effective, timely, and cost-effective drug development.

Use of a Baculovirus-Mammalian Cell Expression-System for Expression of Drug-Metabolizing Enzymes: Optimization of Infection With a Focus on Cytochrome P450 3A4

Heterologous expression systems are important for analyzing the effects of genetic factors including single nucleotide polymorphisms on the functions of drug-metabolizing enzymes. In this study, we focused on a baculovirus-mammalian cell (Bac-Mam) expression system as a safer and more efficient approach for this purpose. The baculovirus-insect cell expression system is widely utilized in large-scale protein expression. Baculovirus has been shown to also infect certain mammalian cells, although the virus only replicates in insect cells. With this knowledge, baculovirus is now being applied in a mammalian expression system called the Bac-Mam system wherein a gene-modified baculovirus is used whose promotor is replaced with one that can function in mammalian cells. We subcloned open-reading frames of cytochrome P450 3A4 (CYP3A4), UDP-glucuronosyltransferase (UGT) 1A1, and UGT2B7 into a transfer plasmid for the Bac-Mam system, and prepared recombinant Bac-Mam virus. The obtained virus was amplified in insect Sf9 cells and used to infect mammalian COS-1 cells. Expression of CYP3A4, UGT1A1, and UGT2B7 in COS-1 cell homogenates were confirmed by immunoblotting. Optimum infection conditions including the amount of Bac-Mam virus, culture days before collection, and concentration of sodium butyrate, an enhancer of viral-transduction were determined by monitoring CYP3A4 expression. Expressed CYP3A4 showed appropriate activity without supplying hemin/5-aminolevulinic acid or co-expressing with NADPH-cytochrome P450 reductase. Further, we compared gene transfer efficiency between the Bac-Mam system and an established method using recombinant plasmid and transfection reagent. Our results indicate that the Bac-Mam system can be applied to introduce drug-metabolizing enzyme genes into mammalian cells that are widely used in drug metabolism research. The expressed enzymes are expected to undergo appropriate post-translational modification as they are in mammalian bodies. The Bac-Mam system may thus accelerate pharmacogenetics and pharmacogenomics research.

SULT genetic polymorphisms: physiological, pharmacological and clinical implications

INTRODUCTION

Cytosolic sulfotransferases (SULTs)-mediated sulfation is critically involved in the metabolism of key endogenous compounds, such as catecholamines and thyroid/steroid hormones, as well as a variety of drugs and other xenobiotics. Studies performed in the past three decades have yielded a good understanding about the enzymology of the SULTs and their structural biology, phylogenetic relationships, tissue/organ-specific/developmental expression, as well as the regulation of the SULT gene expression. An emerging area is related to the functional impact of the SULT genetic polymorphisms.

AREAS COVERED

The current review aims to summarize our current knowledge about the above-mentioned aspects of the SULT research. An emphasis is on the information concerning the effects of the polymorphisms of the SULT genes on the functional activity of the SULT allozymes and the associated physiological, pharmacological, and clinical implications.

EXPERT OPINION

Elucidation of how SULT SNPs may influence the drug-sulfating activity of SULT allozymes will help understand the differential drug metabolism and eventually aid in formulating personalized drug regimens. Moreover, the information concerning the differential sulfating activities of SULT allozymes toward endogenous compounds may allow for the development of strategies for mitigating anomalies in the metabolism of these endogenous compounds in individuals with certain SULT genotypes.

Integrating LCM-Based Spatio-Temporal Transcriptomics Uncovers Conceptus and Endometrial Luminal Epithelium Communication that Coordinates the Conceptus Attachment in Pigs

Attachment of conceptus to the endometrial luminal epithelium (LE) is a critical event for early placentation in Eutheria. Since the attachment occurs at a particular site within the uterus, a coordinated communication between three spatially distinct compartments (conceptus and endometrial LE from two anatomical regions of the uterus to which conceptus attaches and does not attach) is essential but remains to be fully characterized. Using the laser capture microdissection (LCM) technique, we firstly developed an approach that can allow us to pair the pig conceptus sample with its nearby endometrial epithelium sample without losing the native spatial information. Then, a comprehensive spatio-temporal transcriptomic profile without losing the original conceptus-endometrium coordinates was constructed. The analysis shows that an apparent difference in transcriptional responses to the conceptus exists between the endometrial LE from the two anatomically distinct regions in the uterus. In addition, we identified the communication pathways that link the conceptus and endometrial LE and found that these pathways have important roles in conceptus attachment. Furthermore, a number of genes whose expression is spatially restricted in the two different anatomical regions within the uterus were characterized for the first time and two of them (SULT2A1 and MEP1B) may cooperatively contribute to establish conceptus attachment in pigs. The results from our study have implications in understanding of conceptus/embryo attachment in pigs and other large polytocous species.

Cholesterol and oxysterol sulfates: Pathophysiological roles and analytical challenges

Cholesterol and oxysterol sulfates are important regulators of lipid metabolism, inflammation, cell apoptosis, and cell survival. Among the sulfate-based lipids, cholesterol sulfate (CS) is the most studied lipid both quantitatively and functionally. Despite the importance, very few studies have analysed and linked the actions of oxysterol sulfates to their physiological and pathophysiological roles. Overexpression of sulfotransferases confirmed the formation of a range of oxysterol sulfates and their antagonistic effects on liver X receptors (LXRs) prompting further investigations how are the changes to oxysterol/oxysterol sulfate homeostasis can contribute to LXR activity in the physiological milieu. Here, we aim to bring together for novel roles of oxysterol sulfates, the available techniques and the challenges associated with their analysis. Understanding the oxysterol/oxysterol sulfate levels and their pathophysiological mechanisms could lead to new therapeutic targets for metabolic diseases. LINKED ARTICLES: This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Interaction of the Brain-Selective Sulfotransferase SULT4A1 with Other Cytosolic Sulfotransferases: Effects on Protein Expression and Function

Sulfotransferase (SULT) 4A1 is a brain-selective sulfotransferase-like protein that has recently been shown to be essential for normal neuronal development in mice. In the present study, SULT4A1 was found to colocalize with SULT1A1/3 in human brain neurons. Using immunoprecipitation, SULT4A1 was shown to interact with both SULT1A1 and SULT1A3 when expressed in human cells. Mutation of the conserved dimerization motif located in the C terminus of the sulfotransferases prevented this interaction. Both ectopically expressed and endogenous SULT4A1 decreased SULT1A1/3 protein levels in neuronal cells, and this was also prevented by mutation of the dimerization motif. During differentiation of neuronal SH-SY5Y cells, there was a loss in SULT1A1/3 protein but an increase in SULT4A1 protein. This resulted in an increase in the toxicity of dopamine, a substrate for SULT1A3. Inhibition of SULT4A1 using small interference RNA abrogated the loss in SULT1A1/3 and reversed dopamine toxicity. These results show a reciprocal relationship between SULT4A1 and the other sulfotransferases, suggesting that it may act as a chaperone to control the expression of SULT1A1/3 in neuronal cells. SIGNIFICANCE STATEMENT: The catalytically inactive sulfotransferase (SULT) 4A1 may regulate the function of other SULTs by interacting with them via a conserved dimerization motif. In neuron-like cells, SULT4A1 is able to modulate dopamine toxicity by interacting with SULT1A3, potentially decreasing the metabolism of dopamine.

Effect of SULT2B1 genetic polymorphisms on the sulfation of dehydroepiandrosterone and pregnenolone by SULT2B1b allozymes

Pregnenolone and dehydroepiandrosterone (DHEA) are hydroxysteroids that serve as biosynthetic precursors for steroid hormones in human body. SULT2B1b has been reported to be critically involved in the sulfation of pregnenolone and DHEA, particularly in the sex steroid-responsive tissues. The current study was designed to investigate the impact of the genetic polymorphisms of SULT2B1 on the sulfation of DHEA and pregnenolone by SULT2B1b allozymes. Ten SULT2B1b allozymes previously prepared were shown to exhibit differential sulfating activities toward DHEA and pregnenolone in comparison to the wild-type enzyme. Kinetic studies revealed further significant changes in their substrate-binding affinity and catalytic activity toward DHEA and pregnenolone. Taken together, these results indicated clearly a profound effect of SULT2B1 genetic polymorphisms on the sulfating activity of SULT2B1b allozymes toward DHEA and pregnenolone, which may have implications in inter-individual variations in the homeostasis of these two important steroid precursors.

Effects of Human Sulfotransferase 2A1 Genetic Polymorphisms 3 on the Sulfation of Tibolone

BACKGROUND AND OBJECTIVES

Previous studies have demonstrated the metabolism of tibolone through sulfation, with the cytosolic sulfotransferase (SULT) SULT2A1 as the major responsible enzyme. The current study aimed to investigate how SULT2A1 genetic polymorphisms may affect the dehydroepiandrosterone (DHEA)- and tibolone-sulfating activity of SULT2A1.

METHODS

Site-directed mutagenesis was employed to generate cDNAs encoding ten different SULT2A1 allozymes. Recombinant SULT2A1 allozymes were expressed in BL21 E. coli cells, and purified using glutathione-sepharose affinity chromatography. An established sulfotransferase assay was used to analyze DHEA- and tibolone-sulfating activity of the purified SULT2A1 allozymes.

RESULTS

The nine human SULT2A1 allozymes plus the wild-type SULT2A1 were found to display differential sulfating activity toward DHEA and tibolone. Kinetic analysis revealed that different SULT2A1 allozymes exhibited differential substrate affinity and catalytic efficiency toward the two substrates tested.

CONCLUSION

The results obtained provided useful information concerning the differential metabolism of tibolone through sulfation in individuals with different SULT2A1 genotypes.

Effects of genetic polymorphisms on the sulfation of dehydroepiandrosterone and pregnenolone by human cytosolic sulfotransferase SULT2A1

The cytosolic sulfotransferase (SULT) SULT2A1 is known to mediate the sulfation of DHEA as well as some other hydroxysteroids such as pregnenolone. The present study was designed to investigate how genetic polymorphisms of the human SULT2A1 gene may affect the sulfation of DHEA and pregnenolone. Online databases were systematically searched to identify human SULT2A1 single nucleotide polymorphisms (SNPs). Of the 98 SULT2A1 non-synonymous coding SNPs identified, seven were selected for further investigation. Site-directed mutagenesis was used to generate cDNAs encoding these seven SULT2A1 allozymes, which were expressed in BL21 Escherichia coli cells and purified by glutathione-Sepharose affinity chromatography. Enzymatic assays revealed that purified SULT2A1 allozymes displayed differential sulfating activity toward both DHEA and pregnenolone. Kinetic analyses showed further differential catalytic efficiency and substrate affinity of the SULT2A1 allozymes, in comparison with wild-type SULT2A1. These findings provided useful information concerning the effects of genetic polymorphisms on the sulfating activity of SULT2A1 allozymes.

Polymorphisms of STS gene and SULT2A1 gene and neurosteroid levels in Han Chinese boys with attention-deficit/hyperactivity disorder: an exploratory investigation

This study examined the relationships among polymorphisms of the STS gene and SULT2A1 gene, dehydroepiandrosterone (DHEA) and its sulfated form (DHEA-S), and characteristics of attention-deficit/hyperactivity disorder (ADHD). We used cheek swabs to obtain the genomic DNA of 200 ADHD male probands (mean age: 8.7 years), 192 patients’ mothers and 157 patients’ fathers. Three SNPs in the STS gene (rs6639786, rs2270112, and rs17268988) and one SNP in the SULT2A1 gene (rs182420) were genotyped. Saliva samples were collected from the ADHD patients to analyze DHEA and DHEA-S levels. The behavioral symptoms were evaluated with the Swanson, Nolan, and Pelham, and Version IV Scale for ADHD (SNAP-IV), and the neuropsychological function was assessed using the Conners’ Continuous Performance Tests (CPT). We found the C allele of rs2270112 within the STS gene to be over-transmitted in males with ADHD. Polymorphisms of rs182420 within the SULT2A1 gene were not associated with ADHD. In addition, the C allele carriers of rs2270112 demonstrated significantly higher DHEA-S levels than the G allele carriers. Levels of DHEA were positively correlated with attention as measured by the CPT. These findings support a potential role in the underlying biological pathogenesis of ADHD with regard to STS polymorphisms and neurosteroid levels.

The Important Roles of Steroid Sulfatase and Sulfotransferases in Gynecological Diseases

Gynecological diseases such as endometriosis, adenomyosis and uterine fibroids, and gynecological cancers including endometrial cancer and ovarian cancer, affect a large proportion of women. These diseases are estrogen dependent, and their progression often depends on local estrogen formation. In peripheral tissues, estrogens can be formed from the inactive precursors dehydroepiandrosterone sulfate and estrone sulfate. Sulfatase and sulfotransferases have pivotal roles in these processes, where sulfatase hydrolyzes estrone sulfate to estrone, and dehydroepiandrosterone sulfate to dehydroepiandrosterone, and sulfotransferases catalyze the reverse reactions. Further activation of estrone to the most potent estrogen, estradiol, is catalyzed by 17-ketosteroid reductases, while estradiol can also be formed from dehydroepiandrosterone by the sequential actions of 3β-hydroxysteroid dehydrogenase-Δ⁴-isomerase, aromatase, and 17-ketosteroid reductase. This review introduces the sulfatase and sulfotransferase enzymes, in terms of their structures and reaction mechanisms, and the regulation and different transcripts of their genes, together with the importance of their currently known single nucleotide polymorphisms. Data on expression of sulfatase and sulfotransferases in gynecological diseases are also reviewed. There are often unchanged mRNA and protein levels in diseased tissue, with higher sulfatase activities in cancerous endometrium, ovarian cancer cell lines, and adenomyosis. This can be indicative of a disturbed balance between the sulfatase and sulfotransferases enzymes, defining the potential for sulfatase as a drug target for treatment of gynecological diseases. Finally, clinical trials with sulfatase inhibitors are discussed, where two inhibitors have already concluded phase II trials, although so far with no convincing clinical outcomes for patients with endometrial cancer and endometriosis.

Characterization of regio- and stereo-selective sulfation of bufadienolides: exploring the mechanism and providing insight into the structure–sulfation relationship by experimentation and molecular docking analysis

Bufadienolides are a major class of bioactive compounds derived from amphibian skin secretion.

Genetics of androgen metabolism in women with infertility and hypoandrogenism

Hypoandrogenism in women with low functional ovarian reserve (LFOR, defined as an abnormally low number of small growing follicles) adversely affects fertility. The androgen precursor dehydroepiandrosterone (DHEA) is increasingly used to supplement treatment protocols in women with LFOR undergoing in vitro fertilization. Due to differences in androgen metabolism, however, responses to DHEA supplementation vary between patients. In addition to overall declines in steroidogenic capacity with advancing age, genetic factors, which result in altered expression or enzymatic function of key steroidogenic proteins or their upstream regulators, might further exacerbate variations in the conversion of DHEA to testosterone. In this Review, we discuss in vitro studies and animal models of polymorphisms and gene mutations that affect the conversion of DHEA to testosterone and attempt to elucidate how these variations affect female hormone profiles. We also discuss treatment options that modulate levels of testosterone by targeting the expression of steroidogenic genes. Common variants in genes encoding DHEA sulphotransferase, aromatase, steroid 5α-reductase, androgen receptor, sex-hormone binding globulin, fragile X mental retardation protein and breast cancer type 1 susceptibility protein have been implicated in androgen metabolism and, therefore, can affect levels of androgens in women. Short of screening for all potential genetic variants, hormonal assessments of patients with low testosterone levels after DHEA supplementation facilitate identification of underlying genetic defects. The genetic predisposition of patients can then be used to design individualized fertility treatments.

Human mass balance, metabolite profile and identification of metabolic enzymes of [¹⁴C]ASP015K, a novel oral janus kinase inhibitor

1. The human mass balance of (14)C-labelled ASP015K ([(14)C]ASP015K), an orally bioavailable Janus kinase (JAK) inhibitor, was characterized in six healthy male subjects after a single oral dose of [(14)C]ASP015K (100 mg, 3.7 MBq) in solution. [(14)C]ASP015K was rapidly absorbed with tmax of 1.6 and 1.8 h for ASP015K and total radioactivity in plasma, respectively. Mean recovery in urine and feces amounted to 36.8% and 56.6% of the administered dose, respectively. The main components of radioactivity in plasma and urine were ASP015K and M2 (5'-O-sulfo ASP015K). In feces, ASP015K and M4 (7-N-methyl ASP015K) were the main components. 2. In vitro study of ASP015K metabolism showed that the major isozyme contributing to the formation of M2 was human sulfotransferase (SULT) 2A1 and of M4 was nicotinamide N-methyltransferase (NNMT). 3. The in vitro intrinsic clearance (CLint_in vitro) of M4 formation from ASP015K in human liver cytosol (HLC) was 11-fold higher than that of M2. The competitive inhibitory effect of nicotinamide on M4 formation in the human liver was considered the reason for high CLint_in vitro of M4 formation, while each metabolic pathway made a near equal contribution to the in vivo elimination of ASP015K. ASP015K was cleared by multiple mechanisms.

Expression of the orphan cytosolic sulfotransferase SULT4A1 and its major splice variant in human tissues and cells: dimerization, degradation and polyubiquitination

The cytosolic sulfotransferase SULT4A1 is highly conserved between mammalian species but its function remains unknown. Polymorphisms in the SULT4A1 gene have been linked to susceptibility to schizophrenia. There are 2 major SULT4A1 transcripts in humans, one that encodes full length protein (wild-type) and one that encodes a truncated protein (variant). Here, we investigated the expression of SULT4A1 in human tissues by RT-PCR and found the wild-type mRNA to be expressed mainly in the brain, gastrointestinal tract and prostate while the splice variant was more widely expressed. In human cell-lines, the wild-type transcript was found in neuronal cells, but the variant transcript was expressed in nearly all other lines examined. Western blot analysis only identified SULT4A1 protein in cells that expressed the wild-type mRNA. No variant protein was detected in cells that expressed the variant mRNA. Ectopically expressed full length SULT4A1 protein was stable while the truncated protein was not, having a half-life of approximately 3 hr. SULT4A1 was also shown to homodimerize, consistent with other SULTs that contain the consensus dimerization motif. Mutation of the dimerization motif resulted in a monomeric form of SULT4A1 that was rapidly degraded by polyubiquitination on the lysine located within the dimerization motif. These results show that SULT4A1 is widely expressed in human tissues, but mostly as a splice variant that produces a rapidly degraded protein. Dimerization protects the protein from degradation. Since many other cytosolic sulfotransferases possess the conserved lysine within the dimerization motif, homodimerization may serve, in part, to stabilize these enzymes in vivo.

Farnesoid X receptor alpha: a molecular link between bile acids and steroid signaling?

Bile acids are cholesterol metabolites that have been extensively studied in recent decades. In addition to having ancestral roles in digestion and fat solubilization, bile acids have recently been described as signaling molecules involved in many physiological functions, such as glucose and energy metabolisms. These signaling pathways involve the activation of the nuclear receptor farnesoid X receptor (FXRα) or of the G protein-coupled receptor TGR5. In this review, we will focus on the emerging role of FXRα, suggesting important functions for the receptor in steroid metabolism. It has been described that FXRα is expressed in the adrenal glands and testes, where it seems to control steroid production. FXRα also participates in steroid catabolism in the liver and interferes with the steroid signaling pathways in target tissues via crosstalk with steroid receptors. In this review, we discuss the potential impacts of bile acid (BA), through its interactions with steroid metabolism, on glucose metabolism, sexual function, and prostate and breast cancers. Although several of the published reports rely on in vitro studies, they highlight the need to understand the interactions that may affect health. This effect is important because BA levels are increased in several pathophysiological conditions related to liver injuries. Additionally, BA receptors are targeted clinically using therapeutics to treat liver diseases, diabetes, and cancers.

Polymorphisms in androgen signaling pathway predisposing to prostate cancer

Prostate cancer is the most frequent male malignancy diagnosed in western countries and androgens are known to mediate key physiological processes in prostate tissue. Since endogenous androgens have long been considered to be risk factors for prostate cancer, genes involved in androgen biosynthesis and metabolism have been extensively studied. In this review, association of androgen pathway genes, their polymorphic sites and risk of prostate cancer in different ethnic backgrounds is addressed together with their use to predict susceptibility and clinical outcomes of prostate cancer patients. The effect of the polymorphisms seems vary in different patients, populations and ethnic backgrounds. To date it is evident that the association between androgen pathway gene polymorphisms and prostate cancer risk is complex and many of the results are characterized by irreproducibility, which can be attributed to a variety of biological, statistical and technical reasons. In the future, with increasing knowledge, developing technologies and new genomic biomarkers it likely becomes possible to better estimate the risk of prostate cancer, and distinguish indolent disease from aggressive based on molecular profiling, and the analysis of gene-gene and gene-environment interactions.

Estrogen-related receptor ERRα-mediated downregulation of human hydroxysteroid sulfotransferase (SULT2A1) in Hep G2 cells

Hydroxysteroid sulfotransferase SULT2A1 catalyzes the sulfation of hydroxysteroids and xenobiotics. It plays an important role in the detoxification of hydroxyl-containing xenobiotics and in the regulation of the biological activities of hydroxysteroids. ERRα is an orphan member of the nuclear receptor superfamily that is closely related to estrogen receptor alpha (ERα). Here we report that the mRNA expression of human SULT2A1 was suppressed by ERRα in Hep G2 cells. To investigate the mechanisms of this regulation, the effects of ERRα on human SULT2A1 promoter transcription in Hep G2 cells were investigated. Reporter luciferase assay results showed that ERRα significantly represses human SULT2A1 promoter transcription in Hep G2 cells. Deletion analysis indicated that human SULT2A1 promoter region between positions -188 and -130 is necessary for its repression by ERRα in Hep G2 cells. The 5' DNA -188 to -130 region of human SULT2A1 contains IR2 and DR4 hormone response elements and two putative ERRα response elements (ERREs) (ERRE188: GCAAGCTCA and ERRE155: ATAAGTTCA). Interestingly, ERRE188 overlaps with the IR2 element and ERRE155 overlaps with the DR4 element. Our further investigation demonstrated that ERRα represses human SULT2A1 promoter transcription by competing with other nuclear receptors for binding to IR2 or DR4 elements. The interaction of ERRE188 and ERRE155 elements with ERRα was confirmed by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analysis. Our results suggest that ERRα may play an important role in regulating the metabolism of drugs and xenobiotics and in regulating endogenous hydroxysteroid activities via the regulation of SULT2A1.

Genetic polymorphism in metabolism and host defense enzymes: implications for human health risk assessment

Genetic polymorphisms in xenobiotic metabolizing enzymes can have profound influence on enzyme function, with implications for chemical clearance and internal dose. The effects of polymorphisms have been evaluated for certain therapeutic drugs but there has been relatively little investigation with environmental toxicants. Polymorphisms can also affect the function of host defense mechanisms and thus modify the pharmacodynamic response. This review and analysis explores the feasibility of using polymorphism data in human health risk assessment for four enzymes, two involved in conjugation (uridine diphosphoglucuronosyltransferases [UGTs], sulfotransferases [SULTs]), and two involved in detoxification (microsomal epoxide hydrolase [EPHX1], NADPH quinone oxidoreductase I [NQO1]). This set of evaluations complements our previous analyses with oxidative and conjugating enzymes. Of the numerous UGT and SULT enzymes, the greatest likelihood for polymorphism effect on conjugation function are for SULT1A1 (*2 polymorphism), UGT1A1 (*6, *7, *28 polymorphisms), UGT1A7 (*3 polymorphism), UGT2B15 (*2 polymorphism), and UGT2B17 (null polymorphism). The null polymorphism in NQO1 has the potential to impair host defense. These highlighted polymorphisms are of sufficient frequency to be prioritized for consideration in chemical risk assessments. In contrast, SNPs in EPHX1 are not sufficiently influential or defined for inclusion in risk models. The current analysis is an important first step in bringing the highlighted polymorphisms into a physiologically based pharmacokinetic (PBPK) modeling framework.

Genetic variation in androgen disposition: implications in clinical medicine including testosterone abuse

BACKGROUND

Testosterone replacement therapy in hypogonadal men has been used for > 60 years. The use of testosterone substitution is continuously growing and is given to aging men to improve the quality of life. Because testosterone use is associated with muscle strength enhancing effects, it has become a popular drug to abuse. Doping with anabolic steroids, such as testosterone, is a severe challenge to the vision, moral and ethics in sports and has also become a significant and increasing problem in society.

OBJECTIVE

The primary aim of this review is to summarize and discuss the contribution of genetic components to inter-individual variation in androgen disposition.

CONCLUSION

Genetic variation has a large impact on androgen disposition. This variation is of the utmost importance for the interpretation of doping test results and may modulate the effects of testosterone replacement therapy and testosterone doping.

Thiopurine S-methyltransferase pharmacogenetics: functional characterization of a novel rapidly degraded variant allozyme

A novel human thiopurine S-methyltransferase (TPMT) variant allele, (319 T>G, 107Tyr>Asp, *27), was identified in a Thai renal transplantation recipient with reduced erythrocyte TPMT activity. The TPMT*27 variant allozyme showed a striking decrease in both immunoreactive protein level and enzyme activity after transient expression in a mammalian cell line. We set out to explore the mechanism(s) responsible for decreased expression of this novel variant of an important drug-metabolizing enzyme. We observed accelerated degradation of TPMT*27 protein in a rabbit reticulocyte lysate. TPMT*27 degradation was slowed by proteasome inhibition and involved chaperone proteins-similar to observations with regard to the degradation of the common TPMT*3A variant allozyme. TPMT*27 aggresome formation was also observed in transfected mammalian cells after proteasome inhibition. Inhibition of autophagy also decreased TPMT*27 degradation. Finally, structural analysis and molecular dynamics simulation indicated that TPMT*27 was less stable than was the wild type TPMT allozyme. In summary, TPMT*27 serves to illustrate the potential importance of protein degradation - both proteasome and autophagy-mediated degradation - for the pharmacogenetic effects of nonsynonymous SNPs.

Identification of genetic variants in the human indoleamine 2,3-dioxygenase (IDO1) gene, which have altered enzyme activity

OBJECTIVES

Indoleamine 2,3-dioxygenase (IDO), a rate-limiting enzyme in tryptophan catabolism, is a key regulator of immune tolerance. We identified genetic variations in the IDO1 gene and evaluated their functional activities using in-vitro transfection studies.

METHODS

We resequenced the exons and the intron/exon borders of the IDO1 gene in 96 samples from the Coriell DNA Repository. To determine the functional effects of the coding variations that were predicted to have functional consequences, we expressed three of the variant cDNAs in COS-7 and HEK293 cells and determined their enzyme activity.

RESULTS

Seventeen variants were identified; three were nonsynonymous single nucleotide polymorphisms (Ala4Thr, Arg77His, Leu197Ile) and one was a 9 bp deletion in exon 7. Compared with the wild-type protein, the Arg77His and the 9 bp deletion resulted in significantly reduced protein expression and in nearly complete loss of enzyme activity. The allelic frequencies of these two functional variants were approximately 1% and were exclusively observed in the African-American samples.

CONCLUSION

We conclude that there are naturally occurring polymorphisms that render the human IDO1 gene nonfunctional and should result in reduced IDO activity in affected individuals.

Quantitative evaluation of the expression and activity of five major sulfotransferases (SULTs) in human tissues: the SULT "pie"

Expression levels of the major human sulfotransferases (SULTs) involved in xenobiotic detoxification in a range of human tissues (i.e., SULT "pies") are not available in a form allowing comparison between tissues and individuals. Here we have determined, by quantitative immunoblotting, expression levels for the five principal human SULTs-SULT1A1, SULT1A3/4, SULT1B1, SULT1E1, and SULT2A1-and determined the kinetic properties toward probe substrates, where available, for these enzymes in cytosol samples from a bank of adult human liver, small intestine, kidney, and lung. We produced new isoform-selective antibodies against SULT1B1 and SULT2A1, which were used alongside antibodies against SULT1A3 and SULT1A1 previously produced in our laboratory or available commercially (SULT1E1). Expression levels were derived using purified recombinant enzymes to construct standard curves for each individual isoform and immunoblot. Substantial intertissue and interindividual differences in expression were observed. SULT1A1 was the major enzyme (>50% of total, range 420-4900 ng/mg cytosol protein) in the liver, followed by SULT2A1, SULT1B1, and SULT1E1. SULT1A3 was completely absent from this tissue. In contrast, the small intestine contained the largest overall amount of SULT of any of the tissues, with SULT1B1 the major enzyme (36%), closely followed by SULT1A3 (31%), and SULT1A1, SULT1E1, and SULT2A1 more minor forms (19, 8, and 6% of total, respectively). The kidney and lung contained low levels of SULT. We provide a unique data set that will add value to the study of the role and contribution of sulfation to drug and xenobiotic metabolism in humans.

Sulfotransferase gene copy number variation: pharmacogenetics and function

Pharmacogenetics is the study of the role of inheritance in variation to drug response. Drug response phenotypes can vary from adverse drug reactions at one end of the spectrum to equally serious lack of the desired effect of drug therapy at the other. Many of the current important examples of pharmacogenetics involve inherited variation in drug metabolism. Sulfate conjugation catalyzed by cytosolic sulfotransferase (SULT) enzymes, particularly SULT1A1, is a major pathway for drug metabolism in humans. Pharmacogenetic studies of SULT1A1 began over a quarter of a century ago and have advanced from biochemical genetic experiments to include cDNA and gene cloning, gene resequencing, and functional studies of the effects of single nucleotide polymorphisms (SNPs). SNP genotyping, in turn, led to the discovery of functionally important copy number variations (CNVs) in the SULT1A1 gene. This review will briefly describe the evolution of our understanding of SULT1A1 pharmacogenetics and CNV, as well as challenges involved in utilizing both SNP and CNV data in an attempt to predict SULT1A1 function. SULT1A1 represents one example of the potential importance of CNV for the evolving disciplines of pharmacogenetics and pharmacogenomics.

SULT1E1 and ID2 genes as candidates for inherited predisposition to breast and ovarian cancer in Jewish women

Mutations in currently known genes account for only a subset of breast/ovarian cancer risk families. Three loci (2p, 4q, 22q) seemingly harbor breast cancer susceptibility genes. To explore their putative role in Jewish women, 46 affected women representing 22 high risk families were genotyped with D2S2211, D4S392, D22S278 and D22S283 and two flanking markers for each locus, and mutational analysis of ID2 (Chromosome 2) and SULT1E1 (Chromosome 4) genes was carried out in seemingly linked families. No ID2 gene mutations were detected in 8 women from the 4 families seemingly linked to D2S2211, whereas a missense mutation (His224Gln) in one affected woman from a single family was detected among 9 women from the 4 families linked to D4S392. This mutation was not found among 153 high risk, 98 sporadic breast/ovarian cancer patients, or 97 healthy controls. The SULT1E1 gene may need to be further explored as candidate breast cancer gene.

Human betaine-homocysteine methyltransferase (BHMT) and BHMT2: common gene sequence variation and functional characterization

Betaine-homocysteine methyltransferase (BHMT) catalyzes the remethylation of homocysteine. BHMT2 encodes a protein 73% identical in amino acid sequence to BHMT, but the function of BHMT2 remains unclear. We set out to identify and functionally characterize common genetic variation in BHMT and BHMT2. Specifically, we sequenced exons, exon-intron splice junctions and the 5'-flanking regions (5'-FRs) of BHMT and BHMT2 using 240 DNA samples from four ethnic groups. Twenty-five single nucleotide polymorphisms (SNPs), including 4 nonsynonymous SNPs, and 39 SNPs, including 4 nonsynonymous, were observed in BHMT and BHMT2, respectively. BHMT wild type (WT) and variant allozymes were expressed in COS-1 cells. Variant allozymes showed no significant differences from WT in levels of enzyme activity or immunoreactive protein, but there were statistically significant differences in apparent K(m) values. Luciferase reporter gene constructs were created for the three most common BHMT 5'-FR haplotypes, and significant variation was observed in the ability of these constructs to drive transcription. Although BHMT2 mRNA has been observed in human liver and kidney, expression of the protein has not been reported. We were unable to express BHMT2 in mammalian cells, and the protein aggregated after bacterial expression. Furthermore, BHMT2 was rapidly degraded in a rabbit reticulocyte lysate, but it could be stabilized by cotransfection of COS-1 cells with BHMT and, after cotransfection, it coprecipitated with BHMT. These studies have defined common genetic variation in BHMT and BHMT2 and functionally characterized BHMT SNPs. They may also help to explain why BHMT2 has not previously been defined functionally.

Androgens and the molecular epidemiology of prostate cancer

PURPOSE OF REVIEW

Despite clinical and experimental evidence that show androgens are important in prostate carcinogenesis, epidemiologic studies of serum androgens have been inconclusive. In this review, we summarize the current state of the literature and provide insights and direction for epidemiologic research on androgens and prostate cancer.

RECENT FINDINGS

To date, data on serum androgens in prostate cancer remain inconclusive. Large studies on variants in some androgen-metabolizing genes [SRD5A2, CYP17A1, and hydroxysteroid dehydrogenase (HSD)17B1] do not show a convincing links to prostate cancer, though there are insufficient data to draw conclusions on other genes related to androgen metabolism, including UDP-glycosyltransferases (UGT), sulfotransferases (SULT), CYP3A, and estrogen-related genes. There is some evidence, although controversial, suggesting that select variants may confer risk to certain subtypes of prostate cancer. The most notable finding in 2007 is the highly reproducible link between the chromosome 8q24 risk region and prostate cancer susceptibility.

SUMMARY

Besides the link between the 8q24 region and prostate cancer risk, population studies do not convincingly show that polymorphisms in androgen metabolism genes are associated with prostate cancer risk. Large epidemiologic studies with comprehensive gene coverage and reliable exposure data are needed to clarify further the role of androgens and their related genes in prostate cancer.

Human hydroxysteroid sulfotransferase SULT2B1 pharmacogenomics: gene sequence variation and functional genomics

The human hydroxysteroid sulfotransferase (SULT) 2B1 gene is a member of the cytosolic SULT gene superfamily. The two SULT2B1 isoforms, SULT2B1a and SULT2B1b, are encoded by a single gene as a result of alternative transcription initiation and alternative splicing. SULT2B1b catalyzes the sulfonation of 3beta-hydroxysteroid hormones and cholesterol, whereas SULT2B1a preferentially catalyzes pregnenolone sulfonation. We used a genotype-to-phenotype approach to identify and characterize common sequence variation in SULT2B1. Specifically, we resequenced all exons, splice junctions, and approximately 2.5 kb of the 5'-flanking regions (FRs) for each isoform using 60 DNA samples each from African-American and Caucasian-American subjects. We observed 100 polymorphisms, including four nonsynonymous coding single nucleotide polymorphisms and one 6-base pair deletion-all within the "shared" region of the open reading frame. Functional genomic studies of the wild type (WT) and five variant allozymes for each isoform performed with a mammalian expression system showed that variant allozyme activities ranged from 64 to 88% of WT for SULT2B1a and from 76 to 98% for SULT2B1b. Relative levels of immunoreactive protein were similar to those for enzyme activity. Luciferase reporter gene constructs for 2.5 kb of the SULT2B1b 5'-FR displayed a cell line-dependent pattern of variation in activity. Finally, deletion of the proline-rich SULT2B1 carboxyl terminus resulted in intracellular protein aggregate formation and accelerated degradation of the truncated protein. These studies resulted in the identification of common SULT2B1 gene sequence variation, as well as insight into the effects of that variation on the function of this important steroid-metabolizing enzyme.

New FRET primers for quantitative real-time PCR

FRET primer real-time PCR chemistry depends on internally labeled primers with FRET dyes linked to their 3' end. The best distance between the FRET dyes for obtaining the largest signal and the lowest background is six nucleotides. In this study the forward primer was labeled with FAM and the reverse primer with Texas red; the labeled primers meet in cycle two of PCR. At the end of the elongation step FAM is excited to emit fluorescence which will excite Texas red to emit new fluorescence that correlates directly with the quantity of PCR product accumulated. FRET primer techniques amplify short amplicons with unique thermal cycling steps, 0 s at 85 degrees C for denaturation, 7 s for annealing, and 2 s for elongation. The FRET primer technique was very efficient (92.6, 97.2, and 100%), correlation coefficients were high (1.0, 0.999, and 0.999), and total run time was very short (20, 45, and 40 min per 40 cycles with LightCycler, iCycler, and RotorGene 3000, respectively). When FRET-labeled primers were compared with similar but unlabeled primers it was observed that the FRET primer technique had a lower Ct value and was more efficient than use of unlabeled primers detected by use of SYBR Green I.

Pharmacogenetics and Pharmacogenomics: Development, Science, and Translation

Pharmacogenetics and pharmacogenomics involve the study of the role of inheritance in individual variation in drug response, a phenotype that varies from potentially life-threatening adverse drug reactions to equally serious lack of therapeutic efficacy. This discipline evolved from the convergence of rapid advances in molecular pharmacology and genomics. Originally, pharmacogenetic studies focused on monogenic traits, often involving genetic variation in drug metabolism. However, contemporary studies increasingly involve entire "pathways" encoding proteins that influence both pharmacokinetics--factors that influence the concentration of a drug reaching its target(s)--and pharmacodynamics, the drug target itself, as well as genome-wide approaches. Pharmacogenomics is also increasingly moving across the "translational interface" into the clinic and is being incorporated into the drug development process and the governmental regulation of that process. However, significant challenges remain to be overcome if pharmacogenetics-pharmacogenomics is to achieve its full potential as a major medical application of genomic science.

Genetic diversity and function in the human cytosolic sulfotransferases

Amino-acid substitutions, which result from common nonsynonymous (NS) polymorphisms, may dramatically alter the function of the encoded protein. Gaining insight into how these substitutions alter function is a step toward acquiring predictability. In this study, we incorporated gene resequencing, functional genomics, amino-acid characterization and crystal structure analysis for the cytosolic sulfotransferases (SULTs) to attempt to gain predictability regarding the function of variant allozymes. Previously, four SULT genes were resequenced in 118 DNA samples. With additional resequencing of the remaining eight SULT family members in the same DNA samples, a total of 217 polymorphisms were revealed. Of 64 polymorphisms identified within 8785 bp of coding regions from SULT genes examined, 25 were synonymous and 39 were NS. Overall, the proportion of synonymous changes was greater than expected from a random distribution of mutations, suggesting the presence of a selective pressure against amino-acid substitutions. Functional data for common variants of five SULT genes have been previously published. These data, together with the SULT1A1 variant allozyme data presented in this paper, showed that the major mechanism by which amino acid changes altered function in a transient expression system was through decreases in immunoreactive protein rather than changes in enzyme kinetics. Additional insight with regard to mechanisms by which NS single nucleotide polymorphisms alter function was sought by analysis of evolutionary conservation, physicochemical properties of the amino-acid substitutions and crystal structure analysis. Neither individual amino-acid characteristics nor structural models were able to accurately and reliably predict the function of variant allozymes. These results suggest that common amino-acid substitutions may not dramatically alter the protein structure, but affect interactions with the cellular environment that are currently not well understood.

Association of SULT2A1 allelic variants with plasma adrenal androgens and prostate cancer in African American men

Dehydroepiandrosterone (DHEA) sulfate which is present at micromolar levels in the plasma, can be desulfated to supply free DHEA for metabolism to androgens or estrogens in peripheral tissues. Human cytosolic sulfotransferase (SULT) 2A1 catalyzes DHEA sulfation in the adrenal cortex. Three SULT2A1 nonsynonymous coding single nucleotide polymorphisms (SNPs), identified only in African Americans (AA), are associated with decreased levels of activity and expression as compared to wild-type cDNA when expressed in COS cells. To test whether the SNPs are associated with decreased plasma androgens, 124 normal AA men were genotyped and plasma DHEA, DHEA-sulfate and testosterone levels determined. The two SNPs identified in these participants occurred at allelic frequencies of 0.044 (G187C) and 0.101 (G781A). The G187C SNP was highly linked to the G781A SNP. Although no differences in hormone levels were associated with the individual SNPs, a significant increase in the DHEA:DHEA-sulfate ratio was observed in participants with a heterozygous G187C/G781A genotype. Increased free DHEA levels may result in increased testosterone synthesis and stimulation in the prostate, therefore a group of AA prostate cancer (PC) patients and controls were genotyped. No significant association of the presence of the different SULT2A1 alleles with the occurrence of PC was detected.

Sulfation of tibolone metabolites by human postmenopausal liver and small intestinal sulfotransferases (SULTs)

Sulfation is a major pathway in humans for the biotransformation of steroid hormones and structurally related therapeutic agents. Tibolone is a synthetic steroid used for the treatment for climacteric symptoms and postmenopausal osteoporosis. Sulfation inactivates the hydroxylated metabolites, 3alpha-hydroxytibolone (3alpha-OH-tibolone) and 3beta-hydroxytibolone (3beta-OH-tibolone), and contributes to the regulation of tissue responses to tibolone. We detected SULT1A1, SULT1A3, SULT1E1 and SULT2A1 mRNA expression by RT-PCR in postmenopausal liver and small intestine. Liver pool (n=5) SULT activities measured with tibolone substrates reflected COS-1 expressed SULT2A1 and SULT1E1 activities. Liver SULT2A1 activity (1.8 +/- 0.3 units/mg protein, n = 8, mean +/- SEM), and activities with 3alpha-OH-tibolone (0.6 +/- 0.1, n = 8) and 3beta-OH-tibolone (0.9 +/- 0.2, n = 8) were higher than SULT1E1 activities (<0.05, n = 10). SULT1E1 activities were low or not detected in many samples. Mean small intestinal activities were 0.03 +/- 0.01 with 3alpha-OH-tibolone and 0.04 +/- 0.01 with 3beta-OH-tibolone (n = 3). In conclusion, SULT2A1 is the major endogenous enzyme responsible for sulfation of the tibolone metabolites in human postmenopausal tissues. The results support the occurrence of pre-receptor enzymatic regulation of hydroxytibolone metabolites and prompt further investigation of the tissue-selective regulation of tibolone effects.

GLUTATHIONES-TRANSFERASE OMEGA 1 AND OMEGA 2 PHARMACOGENOMICS

Glutathione S-transferase omega 1 and omega 2 (GSTO1 and GSTO2) catalyze monomethyl arsenate reduction, the rate-limiting reaction in arsenic biotransformation. As a step toward pharmacogenomic studies of these phase II enzymes, we resequenced human GSTO1 and GSTO2 using DNA samples from four ethnic groups. We identified 31 and 66 polymorphisms in GSTO1 and GSTO2, respectively, with four nonsynonymous-coding single nucleotide polymorphisms (cSNPs) in each gene. There were striking variations among ethnic groups in polymorphism frequencies and types. Expression constructs were created for all eight nonsynonymous cSNPs, as well as a deletion of codon 155 in GSTO1, and those constructs were used to transfect COS-1 cells. Quantitative Western blot analysis, after correction for transfection efficiency, showed a reduction in protein level of greater than 50% for the GSTO1 Tyr32 variant allozyme compared with wild type (WT), whereas levels for the Asp140, Lys208, Val236, and codon 155 deletion variant constructs were similar to that of the WT. For GSTO2, the Tyr130 and Ile158 variant allozymes showed 50 and 84% reductions in levels of expression, respectively, compared with WT, whereas the Ile41 and Asp142 allozymes displayed levels similar to that of WT GSTO2. Rabbit reticulocyte lysate degradation studies showed that the GSTO1 Tyr32 and the GSTO2 Tyr130, Ile158, and Asp142/Ile158 variant allozymes were degraded more rapidly than were their respective WT allozymes. These observations raise the possibility of functionally significant pharmacogenomic variation in the expression and function of GSTO1 and GSTO2.

Thiopurine S-methyltransferase pharmacogenetics: insights, challenges and future directions

The thiopurine S-methyltransferase (TPMT) genetic polymorphism is one of the most 'mature' examples in pharmacogenetics. That is true because of its importance clinically for the individualization of thiopurine drug therapy and also because TPMT has provided novel insights into molecular mechanisms responsible for the functional effects of common genetic polymorphisms. This review will summarize the development of our understanding of the role of inheritance in the regulation of TPMT as well as the clinical implications of that genetic regulation. It will also summarize recent studies in which TPMT pharmacogenetics has enhanced our understanding of molecular mechanisms by which common polymorphisms influence or alter function. TPMT pharmacogenetics highlights the potential clinical importance of the translation of pharmacogenetics from bench to bedside, the potential for basic pharmacogenetic research to provide insight into mechanisms by which genetic polymorphisms can alter function, and the challenges associated with the achievement of both of those goals.

Human methylenetetrahydrofolate reductase pharmacogenomics: gene resequencing and functional genomics

5,10-Methylenetetrahydrofolate reductase (MTHFR) is an important enzyme in the folate metabolic pathway. Common genetic polymorphisms in the human MTHFR gene are associated with individual variation in the efficacy and toxicity of chemotherapeutic agents, such as methotrexate and 5-fluorouracil. However, the full range of polymorphisms and intragene haplotypes in the human MTHFR gene remains unclear. Furthermore, cellular mechanisms by which common, naturally occurring nonsynonymous coding single nucleotide polymorphisms (cSNPs) might alter the function of this enzyme have not been defined. The present study focused on the systematic identification and investigation of common polymorphisms and haplotypes in the MTHFR gene using a genotype-to-phenotype strategy, followed by functional genomic studies. Specifically, we resequenced exons, splice junctions and portions of the 5'-flanking region (5'-FR) of the human MTHFR gene using 240 DNA samples from four ethnic groups. A total of 65 polymorphisms were observed, 11 of which were nonsynonymous cSNPs. We then performed functional genomic studies with constructs for wild-type and 15 variant allozymes (some with multiple alterations in amino acid sequence) using a mammalian expression system. Activity for the variant allozymes ranged from 13% to 149% of wild-type activity. Levels of immunoreactive protein for the allozymes ranged from 31% to 120% of wild-type and were significantly correlated with enzyme activity (Rp=0.85, P<0.0001), suggesting that a major mechanism by which nonsynonymous cSNPs influence the function of this gene is by alteration in the quantity of protein. These observations represent steps towards an understanding of molecular genetic mechanisms responsible for variation in MTHFR function that may contribute to individual differences in drug efficacy and toxicity, as well as disease risk.

Pharmacogenetics of human cytosolic sulfotransferases

Cytosolic sulfotransferases (SULTs) are phase II detoxification enzymes that are involved in the biotransformation of a wide variety of structurally diverse endo- and xenobiotics, including many therapeutic agents and endogenous steroids. Single-nucleotide polymorphisms (SNPs) in SULTs have functional consequences on the translated protein. For the most part, these SNPs are fairly uncommon in the population, but some, most notably for SULT isoform 1A1, are commonly found and have been associated with cancer risk for a variety of tumor sites and also with response to therapeutic agents. SNPs in the hydroxysteroid sulfotransferase, SULT2A1, have been identified in African-American subjects and influence the ratio of plasma DHEA:DHEA-S. This modification could potentially influence cancer risk in steroidogenic tissues. SNPs in many SULTs are ethnically distributed, another factor that could influence SULT pharmacogenetics. Finally, genetic variation has also been identified in 3'-phosphoadenoside 5'-phosphosulfate synthetase (PAPPS), the enzymes responsible for producing the obligatory cosubstrate for all sulfotransferases. Taken together, this variability could substantially influence the disposition of drugs metabolized by SULTs. Elucidation of the basis and effect of variability in sulfation could greatly impact individualized therapy in the future.

Transcreener™: screening enzymes involved in covalent regulation

Enzymes that catalyse group transfer reactions comprise a significant fraction of the human proteome and are a rich source of drug targets because of their role in covalent regulatory cycles. Phosphorylation, glycosylation, sulfonation, methylation and acetylation represent some of the key types of group transfer reactions that modulate the function of diverse biomolecules through covalent modification. Development of high-throughput screening methods for these enzymes has been problematic because of the diversity of acceptor substrates. Recently, the authors developed a novel assay platform called Transcreener that relies upon fluorescence detection of the invariant reaction product of a group transfer reaction, usually a nucleotide. This platform enables screening of any isoform in a family of group transfer enzymes, with any acceptor substrate, using the same assay reagents.

Thiopurine S-methyltransferase pharmacogenetics: variant allele functional and comparative genomics

Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs. Genetic polymorphisms for TPMT are a major factor responsible for large individual variations in thiopurine toxicity and therapeutic effect. The present study investigated the functional effects of human TPMT variant alleles that alter the encoded amino acid sequence of the enzyme, TPMT*2, *3A, *3B, *3C and *5 to *13. After expression in COS-1 cells and correction for transfection efficiency, allozymes encoded by these alleles displayed levels of activity that varied from virtually undetectable (*3A,*3B and *5) to 98% (*7) of that observed for the wild-type allele. Although some allozymes had significant elevations in apparent Km values for 6-mercaptopurine and S-adenosyl-L-methionine (i.e. the two cosubstrates for the reaction), the level of enzyme protein was the major factor responsible for variation in activity. Quantitative Western blot analysis demonstrated that the level of enzyme protein correlated closely with level of activity for all allozymes except TPMT*5. Furthermore, protein levels correlated with rates of TPMT degradation. TPMT amino acid sequences were then determined for 16 non-human mammalian species and those sequences (plus seven reported previously, including two nonmammalian vertebrate species) were used to determine amino acid sequence conservation. Most human TPMT variant allozymes had alterations of residues that were highly conserved during vertebrate evolution. Finally, a human TPMT homology structural model was created on the basis of a Pseudomonas structure (the only TPMT structure solved to this time), and the model was used to infer the functional consequences of variant allozyme amino acid sequence alterations. These studies indicate that a common mechanism responsible for alterations in the activity of variant TPMT allozymes involves alteration in the level of enzyme protein due, at least in part, to accelerated degradation.

Human aromatase: gene resequencing and functional genomics

Aromatase [cytochrome P450 19 (CYP19)] is a critical enzyme for estrogen biosynthesis, and aromatase inhibitors are of increasing importance in the treatment of breast cancer. We set out to identify and characterize genetic polymorphisms in the aromatase gene, CYP19, as a step toward pharmacogenomic studies of aromatase inhibitors. Specifically, we "resequenced" all coding exons, all upstream untranslated exons plus their presumed core promoter regions, all exon-intron splice junctions, and a portion of the 3'-untranslated region of CYP19 using 240 DNA samples from four ethnic groups. Eighty-eight polymorphisms were identified, resulting in 44 haplotypes. Functional genomic studies were done with the four nonsynonymous coding single nucleotide polymorphisms (cSNP) that we observed, two of which were novel. Those cSNPs altered the following amino acids: Trp39Arg, Thr201Met, Arg264Cys, and Met364Thr. The Cys264, Thr364, and double variant Arg39Cys264 allozymes showed significant decreases in levels of activity and immunoreactive protein when compared with the wild-type (WT) enzyme after transient expression in COS-1 cells. A slight decrease in protein level was also observed for the Arg39 allozyme, whereas Met201 displayed no significant changes in either activity or protein level when compared with the WT enzyme. There was also a 4-fold increase in apparent K(m) value for Thr364 with androstenedione as substrate. Of the recombinant allozymes, only the double mutant (Arg39Cys264) displayed a significant change from the WT enzyme in inhibitor constant for the aromatase inhibitors exemestane and letrozole. These observations indicate that genetic variation in CYP19 might contribute to variation in the pathophysiology of estrogen-dependent disease.

Aggresome formation and pharmacogenetics: sulfotransferase 1A3 as a model system

A common cause for pharmacogenetic alteration in drug response is genetic variation in encoded amino acid sequence. We have used the catecholamine and drug-metabolizing enzyme sulfotransferase (SULT)1A3 to create an artificial model system to study mechanisms-especially possible aggresome formation-by which genetic alteration in amino acid sequence might influence function. Specifically, we created a double variant SULT1A3 allozyme that included the naturally occurring Asn234 polymorphism plus an additional Trp172Arg mutation. Analysis of the SULT1A3 X-ray crystal structure had indicated that the Trp172Arg mutation might destabilize the protein's structure. Expression of SULT1A3 Arg172,Asn234 in COS-1 cells resulted in undetectable enzyme activity and a virtual lack of enzyme protein. Rabbit reticulocyte lysate degradation studies showed that the double variant allozyme was degraded much more rapidly than was wild type SULT1A3 by a ubiquitin-proteasome-dependent process. In addition, after expression in COS-1 cells, the double variant allozyme localized to aggresomes, a process not previously described or studied in pharmacogenetics. Therefore, the alteration of only one or two amino acids can lead to decreased levels of protein as a result of both aggresome formation and accelerated degradation. The possible role of aggresome formation in pharmacogenetics should be evaluated in naturally occurring systems with inherited alteration in encoded amino acid sequence.

Human SULT1A3 pharmacogenetics: gene duplication and functional genomic studies

Sulfotransferase (SULT) 1A3 catalyzes the sulfate conjugation of catecholamines. Inheritance is an important factor responsible for individual variation in SULT1A3 activity, and gene resequencing studies have shown the presence of one functionally significant SULT1A3 nonsynonymous cSNP. However, following completion of the Human Genome Project, it appeared that SULT1A3 might be duplicated. We used specific PCR-based assays and fluorescence in situ hybridization to verify that 2 SULT1A3 genes-SULT1A3 and SULT1A4-were present on chromosome 16 in all human DNA samples studied. Furthermore, reanalysis of previous gene resequencing data confirmed the presence of the SULT1A3 SNPs identified previously, but also revealed 11 novel polymorphisms, including 3 nonsynonymous cSNPs. Functional genomic studies showed that two of those cSNPs, C302T, and C302A, resulted in decreased enzyme activity without striking changes in substrate kinetics but with parallel changes in levels of immunoreactive protein. In addition, RT-PCR revealed that both SULT1A3 and SULT1A4 can be transcriptionally active. The duplication of SULT1A3 will have to be taken into account in future efforts to understand individual variation in SULT1A3 activity or properties.

Gene regulation for the senescence marker protein DHEA-sulfotransferase by the xenobiotic-activated nuclear pregnane X receptor (PXR)

Dehydroepiandrosterone (DHEA)-sulfotransferase (SULT2A1) is a phase II metabolizing/detoxifying enzyme with substrate preference for physiological hydroxysteroids, diverse drugs and other xenobiotics. The first-pass tissues (liver and intestine) express SULT2A1 at high levels. In senescent male rodents, Sult2A1 gene transcription in the liver is markedly enhanced and calorie restriction retards this increase. Age-associated loss of the liver expression of androgen receptor in part explains the up-regulation of Sult2A1 expression at late life, since androgen receptor is a negative regulator of this gene. In line with its role in xenobiotic metabolism, the Sult2A1 gene is induced by the pregnane X receptor (PXR). PXR is a xenosensing nuclear receptor that is activated by endobiotic (natural steroids) and xenobiotic (therapeutic drugs and environmental chemicals) molecules. An inverted-repeat arrangement (IR0) of the consensus half site binding sequence for nuclear receptors mediates the xenobiotic induction of the Sult2A1 promoter. The IR0 element is a specific binding site for PXR and its heterodimer partner retinoid X receptor (RXR-alpha) and it directs PXR-mediated induction of a heterologous promoter. In contrast to the loss of androgen receptor expression, PXR and RXR-alpha mRNA expression is invariant during aging. Repression by the androgen receptor and induction by PXR may act coordinately to cause the senescence associated and xenobiotic mediated stimulation of Sult2A1 transcription. Increased Sult2A1 expression appears to be an adaptive response to ensure optimal metabolism of Sult2A1 substrates at old age.