Microsomal steroid sulfatase: interactions with cytosolic steroid sulfotransferases

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.

Occurrence of sulfonated steroids and ovarian expression of steroid sulfatase and SULT1E1 in cyclic cows

Historically sulfonated steroids were primarily considered as inactive metabolites destined for elimination. However, more recently they have been increasingly recognized as precursors for the production of bioactive steroids in target tissues and as functional molecules without preceding hydrolysis. In order to comprehensively characterize their occurrence in cyclic cows and their formation and hydrolysis in bovine ovarian steroidogenesis, ovaries from cyclic cows were screened for the expression of oestrogen sulfotransferase (SULTE1) and steroid sulfatase (STS) by Western blot and immunohistochemistry. Moreover, a broad spectrum of 13 sulfonated steroids was measured applying liquid chromatography-tandem mass spectrometry (LC-MS/MS) in blood samples collected from three cycling heifers during defined stages of the ovarian cycle and in fluid obtained from ovarian follicles of different size. SULT1E1 was undetectable in ovarian tissues. For STS only a weak immunostaining was found predominantly in granulosa cells of larger follicles. However, no specific band occurred in Western blot. In blood, concentrations of all sulfonated steroids investigated were below the limit of quantification (LOQ). In follicular fluid, only cholesterol sulfate was measured in considerable concentrations (328.3 ± 63.8 ng/ml). However, the role of cholesterol sulfate in bovine follicular steroidogenesis remains unclear as concentrations were obviously unrelated to follicular size. The remaining sulfonated steroids investigated were undetectable or only slightly exceeded LOQ in a minor proportion of samples. The results are clearly contrary to a role of sulfonated steroids as important precursors, intermediates or products of bovine ovarian steroidogenesis.

Involvement of UDP-glucuronosyltransferases in higenamine glucuronidation and the gender and species differences in liver

OBJECTIVES

Higenamine (HG), an active ingredient of Aconite root in Chinese herbal medicine, is mainly metabolized by UDP-glucuronosyltransferases (UGT). However, the systematic glucuronidation of HG in humans remains unclear. The purpose of this study was to investigate the glucuronidation of HG.

METHODS

12 recombinant human UGT (rUGT) isozymes were used to characterize the HG glucuronidation. Liver microsomes from male and female mice, rats, guinea pigs, dogs, and humans were used to determine the species and gender differences using liquid chromatography-mass spectrometry.

KEY FINDINGS

One monoglucuronide was detected in reactions catalyzed by rUGT1A6, rUGT1A8, rUGT1A9, also human and dog liver microsomes. UGT1A9 is the most important glucuronosyltransferase that metabolizes HG. Because carvacrol, a specific inhibitor of UGT1A9, can significantly decrease the glucuronidation of HG in Human liver microsomes and UGT1A9. HG metabolism by UGT1A9 described in Michaelis-Menten kinetics (K_m=15.4 mM,V_max=2.2 nmol/mg/min) and glucuronidation in liver microsomes were species dependent. Gender did not affect the kinetic parameters among species except in rats.

CONCLUSIONS

UGT1A9 is a major isoenzyme responsible for the glucuronidation of HG in Human liver microsomes (HLMs). Dog may be an appropriate animal model to evaluate HG metabolism.

Roles of sigma-1 receptors on mitochondrial functions relevant to neurodegenerative diseases

The sigma-1 receptor (Sig-1R) is a chaperone that resides mainly at the mitochondrion-associated endoplasmic reticulum (ER) membrane (called the MAMs) and acts as a dynamic pluripotent modulator in living systems. At the MAM, the Sig-1R is known to play a role in regulating the Ca2+ signaling between ER and mitochondria and in maintaining the structural integrity of the MAM. The MAM serves as bridges between ER and mitochondria regulating multiple functions such as Ca2+ transfer, energy exchange, lipid synthesis and transports, and protein folding that are pivotal to cell survival and defense. Recently, emerging evidences indicate that the MAM is critical in maintaining neuronal homeostasis. Thus, given the specific localization of the Sig-1R at the MAM, we highlight and propose that the direct or indirect regulations of the Sig-1R on mitochondrial functions may relate to neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). In addition, the promising use of Sig-1R ligands to rescue mitochondrial dysfunction-induced neurodegeneration is addressed.

Compilation of a comprehensive gene panel for systematic assessment of genes that govern an individual’s drug responses

Aims: Polymorphisms of genes involved in the pharmacokinetic and pharmacodynamic processes underlie the divergent drug responses among individuals. Despite some successes in identifying these polymorphisms, the candidate gene approach suffers from insufficient gene coverage whereas the genome-wide association approach is limited by less than ideal coverage of SNPs in some important genes. To expand the potential of the candidate approach, we aim to delineate a comprehensive network of drug-response genes for in-depth genetic studies. Materials & methods: Pharmacologically important genes were extracted from various sources including literatures and web resources. These genes, along with their homologs and regulatory miRNAs, were organized based on their pharmacological functions and weighted by literature evidence and confidence levels. Their coverage was evaluated by analyzing three commercial SNP chips commonly used for genome-wide association studies: Affymetrix SNP array 6.0, Illumina HumanHap1M and Illumina Omni. Results: A panel of drug-response genes was constructed, which contains 923 pharmacokinetic genes, 703 pharmacodynamic genes and 720 miRNAs. There are only 16.7% of these genes whose all known SNPs can be directly or indirectly (r2 > 0.8) captured by the SNP chips with coverage of more than 80%. This is possibly because these SNPs chips have notably poor performance over rare SNPs and miRNA genes. Conclusion: We have compiled a panel of candidate genes that may be pharmacologically important. Using this knowledgebase, we are able to systematically evaluate genes and their variants that govern an individual’s response to a given pharmaceutical therapy. This approach can serve as a necessary complement to genome-wide associations.

Interplay of phase II enzymes and transporters in futile cycling: influence of multidrug resistance-associated protein 2-mediated excretion of estradiol 17beta-D-glucuronide and its 3-sulfate metabolite on net sulfation in perfused TR(-) and Wistar rat liver preparations

The hepatic disposition of estradiol 17beta-D-glucuronide (E(2)17G), a substrate of the organic anion-transporting polypeptides Oatp1a1, Oatp1a4, and Oatp1b2, was investigated in Wistar and TR(-) [multidrug resistance-associated protein (Mrp) 2-mutant] rats to elucidate how absence of Mrp2, the major excretory transporter for both E(2)17G and its 3-sulfate metabolite (E(2)3S17G), affected the net sulfation. With absence of Mrp2, lower microsomal desulfation activity and higher Mrp3 but unchanged immunoreactive protein expression of other transporters (Oatps and Mrp4) and estrogen sulfotransferase were found in TR(-) rats. In recirculating, perfused liver preparations, the rapid decay of E(2)17G and sluggish appearance of low levels of E(2)3S17G in perfusate for Wistar livers were replaced by a protracted, biexponential decay of E(2)17G and greater accumulation of E(2)3S17G, whose levels reached plateaus upon the almost complete obliteration of biliary excretion of E(2)17G and E(2)3S17G in the TR(-) liver. Much higher amounts of E(2)17G (28x) and E(2)3S17G (11x) in liver and reduced net sulfation (40 +/- 6 from 77 +/- 6% dose, P < 0.05) were observed at 2 h for the TR(-) versus the Wistar rats. With use of a physiologically based pharmacokinetic model, analytical solutions for the areas under the curve for the precursor and metabolite were obtained to reveal how enzyme- and transporter-mediated processes affected the hepatic disposition of the precursor and metabolite in futile cycling. The analytical solutions were useful to explain transporter-enzyme interplay in futile cycling and predicted that a shutdown of Mrp2 function led to decreased net sulfation of E(2)17G by raising the intracellular concentration of the metabolite, E(2)3S17G, which readily refurnished E(2)17G via desulfation.

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.

Sigma-1 receptor chaperones at the ER-mitochondrion interface regulate Ca(2+) signaling and cell survival

Communication between the endoplasmic reticulum (ER) and mitochondrion is important for bioenergetics and cellular survival. The ER supplies Ca(2+) directly to mitochondria via inositol 1,4,5-trisphosphate receptors (IP3Rs) at close contacts between the two organelles referred to as mitochondrion-associated ER membrane (MAM). We found here that the ER protein sigma-1 receptor (Sig-1R), which is implicated in neuroprotection, carcinogenesis, and neuroplasticity, is a Ca(2+)-sensitive and ligand-operated receptor chaperone at MAM. Normally, Sig-1Rs form a complex at MAM with another chaperone, BiP. Upon ER Ca(2+) depletion or via ligand stimulation, Sig-1Rs dissociate from BiP, leading to a prolonged Ca(2+) signaling into mitochondria via IP3Rs. Sig-1Rs can translocate under chronic ER stress. Increasing Sig-1Rs in cells counteracts ER stress response, whereas decreasing them enhances apoptosis. These results reveal that the orchestrated ER chaperone machinery at MAM, by sensing ER Ca(2+) concentrations, regulates ER-mitochondrial interorganellar Ca(2+) signaling and cell survival.

The reported in vitro anti-estrogen pentachloronitrobenzene enhances the estrogenic activity of estradiol in vivo in the rat

Pentachloronitrobenzene (PCNB) has been shown to inhibit foci-formation for MCF-7 cells in vitro (Zou, E., Hatakeyama, M., Matsumra, F., 2002. Foci-formation of MCF-7 cells as an in vitro screening method for estrogenic chemicals. Environ. Toxicol. Pharmacol. 11, 71) This effect was referred to as representing an anti-estrogenic property of PCNB. However, we have found no evidence that PCNB acts as either an estrogen or an anti-estrogen, either in vitro or in vivo. The assays conducted were binding to human and rat estrogen receptors (ER), a hER yeast trans-activation assay, the immature rat uterotrophic assay and a pubertal female rat assay. Nonetheless, when PCNB was evaluated as a possible anti-estrogen against estradiol in the immature rat uterotrophic assay, it enhanced, rather than reduced the activity of estradiol. Absence of an effect by PCNB on the uterotrophic activity of diethylstilbestrol suggests that the effect with estradiol was related to alteration of its metabolism. However, PCNB was not hepatotoxic and failed to inhibit cytochrome P450 or estradiol sulphotransferase. Pentachlorophenol, a major metabolite of PCNB, was inactive as an estrogen and failed to enhance the uterotrophic activity of estradiol.

Metabolic Labeling of Mammalian Organisms with Stable Isotopes for Quantitative Proteomic Analysis

To quantify proteins on a global level from mammalian tissue, a method was developed to metabolically introduce 15N stable isotopes into the proteins of Rattus norvegicus for use as internal standards. The long-term metabolic labeling of rats with a diet enriched in 15N did not result in adverse health consequences. The average 15N amino acid enrichments reflected the relative turnover rates in the different tissues and ranged from 74.3 mpe in brain to 92.2 mpe in plasma. Using the 15N-enriched liver as a quantitative internal standard, changes in individual protein levels in response to cycloheximide treatment were measured for 310 proteins. These measurements revealed 127 proteins with altered protein level (p < 0.05). Most proteins with altered level have previously reported functions involving xenobiotic metabolism and protein-folding machinery of the endoplasmic reticulum. This approach is a powerful tool for the global quantitation of proteins, is capable of measuring proteome-wide changes in response to a drug, and will be useful for studying animal models of disease.

Structural commonalities among integral membrane enzymes

The X-ray crystal structures of five distinct enzymes (prostaglandin H(2) synthase, squalene cyclase, fatty acid amide hydrolase, microsomal cytochrome P450, and estrone sulfatase) challenge contemporary descriptions of integral membrane proteins. This structurally divergent group represents an important component of the integral membrane proteome that lies at the bilayer's aqueous interface. We summarize here what is collectively understood about the membrane insertion of these proteins, what roles they may play in lipid biology, and their relationship to soluble structural homologs.

Estrogen metabolism by conjugation

The involvement of estrogens in carcinogenic processes within estrogen-responsive tissues has been recognized for a number of years. Classically, mitogenicity associated with estrogen receptor-mediated cellular events was believed to be the mechanism by which estrogens contributed to carcinogenesis. Recently, the possibility that estrogens might contribute directly to mutagenesis resulting from DNA damage has been investigated. That damage is apparently a result of the formation of catechol estrogens that can be further oxidized to semiquinones and quinones. Those molecules represent reactive oxygen species and electrophilic molecules that can form depurinating DNA adducts, thus having the potential to result in permanent nucleotide mutation. Conjugation of parent estrogens to sulfate and glucuronide moieties; of catechol estrogens to methyl, sulfate, and glucuronide conjugates; and of catechol estrogen quinones to glutathione conjugates all represent potential "detoxification" reactions that may protect the cell from estrogen-mediated mitogenicity and mutagenesis. In this chapter, the biochemistry and molecular genetics of those conjugative reaction pathways are discussed. When applicable, the involvement of specific enzymatic isoforms is presented. Finally, the activity of many of these conjugative biotransformation reactions is subject to large interindividual variation--often due to the presence of common nucleotide polymorphisms within the genes encoding those enzymes. Functionally significant genetic polymorphisms that might contribute to variable conjugation of estrogens and catechol estrogens are also discussed.

Steroid sulfatase activity in the rat ovary, cultured granulosa cells, and a granulosa cell line

Direct production of gonadal steroids from sulfated adrenal androgens may be an important alternative or complementary pathway for ovarian steroidogenesis. The conversion of sulfated adrenal androgens, present in serum at micromolar concentrations in adult women, into unconjugated androgens or estrogens requires steroid sulfatase (STS) activity. STS activity has not been characterized in the rat ovary. Substantial STS activity was present in homogenates of rat ovaries, primary cultures of rat granulosa cells, and a granulosa cell line, as determined by conversion of radiolabeled estrone sulfate (E1S) to unconjugated estrone. The potent inhibitor estrone sulfamate eliminated the STS activity. Using E1S as a substrate with microsomes prepared from a granulosa cell line, the K(m) of STS activity was approximately 72 microM, a value in agreement with previously published data for rat STS. Therefore, ovarian cells possess STS and can remove the sulfate from adrenal androgens such as dehydroepiandrosterone sulfate (DHEA-S). Using DHEA-S as a steroidogenic substrate represents an alternative model for the production of ovarian steroids versus the "two cell, two gonadotropin" model of ovarian estrogen synthesis, whereby thecal cells produce androgens from substrate cholesterol and granulosa cells convert the androgens into estrogens. The relative contribution of STS activity to ovarian steroidogenesis remains unclear but may have important physiological and pathophysiological implications.