Estrogen sulfotransferase of the rat liver: complementary DNA cloning and age- and sex-specific regulation of messenger RNA

Reduced dosing and liability in methadone maintenance treatment by targeting oestrogen signal for morphine addiction

Methadone maintenance treatment (MMT) is the major tapering therapy for morphine addictive patients. There have gender differences reported in response to MMT. This study discovered that the estrogen‐response element single nucleotide polymorphism (ERE‐SNP; rs16974799, C/T) of cytochrome 2B6 gene (cyp2b6; methadone catabolic enzyme) responded differently to MMT dosing. Oestradiol was associated with high MMT dosing, high enantiomer (R‐ or S‐) of 2‐ethylidene‐1,5‐dimethyl‐3,3‐dipheny‐pyrrolidine (EDDP; methadone metabolite) to methadone ratio and increased drug‐seeking behaviour, implicating oestradiol‐CYP‐EDDP/methadone axis decreasing MMT efficacy. In mouse model, oestrogen mitigates methadone antinociceptive response, facilitates methadone catabolism and up‐regulates methadone‐associated metabolizing enzymes. Oestrogen also ablates chronic methadone administration‐induced rewarding response. Mechanism dissection revealed the CC genotype of CYP2B6‐ERE‐SNP exerts higher ERE sequence alignment score, higher estrogenic response as compared to TT genotype. At last, preclinical study via targeting estrogen signal that tamoxifen (TMX; selective estrogen receptor modulator, SERM) could facilitate the tolerance phase rewarding response of methadone. Strikingly, TMX also reduces tapering/abstinence phases methadone liability in mice. In conclusion, this study demonstrates altering methadone metabolism through targeting estrogen signals might be able to free morphine addictive patients from the addiction of opioid replacement therapy. Therefore, the add‐on therapy clinical trial introducing SERM in MMT regimen is suggested.

Hepatocellular exposure of troglitazone metabolites in rat sandwich-cultured hepatocytes lacking Bcrp and Mrp2: interplay between formation and excretion

Inhibition of bile acid transport by troglitazone (TGZ) and its major metabolite, TGZ sulfate (TS), may lead to hepatocellular accumulation of toxic bile acids; TS accumulation and hepatotoxicity may be associated with impaired TS biliary excretion. This study evaluated the impact of impaired transport of breast cancer resistance protein (Bcrp) and multidrug resistance-associated protein 2 (Mrp2) on the hepatobiliary disposition of generated metabolites, TS and TGZ glucuronide (TG). Sandwich-cultured hepatocytes (SCH) from Mrp2-deficient (TR(-)) rats in combination with Bcrp knockdown using RNA interference were employed. The biliary excretion index (BEI) of generated TS was not significantly altered by impaired Bcrp (20.9 to 21.1%) and/or Mrp2 function (24.4% and 17.5% in WT and TR(-) rat SCH, respectively). Thus, loss-of-function of Mrp2 and/or Bcrp do not appear to be risk factors for increased hepatocellular TS accumulation in rats, potentially because of a compensatory transporter(s) that excretes TS into bile. Further investigations revealed that the compensatory TS biliary transporter was not the bile salt export pump (Bsep) or P-glycoprotein (P-gp). Interestingly, TGZ sulfation was significantly decreased in TR(-) compared with WT rat SCH (total recovery: 2.8 versus 5.0% of TGZ dose), resulting in decreased hepatocellular TS accumulation, even though sulfotransferase activity in TR(-) rat hepatocyte S9 fraction was similar. Hepatocellular TG accumulation was significantly increased in TR(-) compared with WT rat SCH due to increased glucuronidation and negligible TG biliary excretion. These data emphasize that the interplay between metabolite formation and excretion determines hepatocellular exposure to generated metabolites such as TS and TG.

Crystal structure of human dehydroepiandrosterone sulphotransferase in complex with substrate

Dehydroepiandrosterone sulphotransferase (DHEA-ST) is an enzyme that converts dehydroepiandrosterone (DHEA), and some other steroids, into their sulphonated forms. The enzyme catalyses the sulphonation of DHEA on the 3alpha-oxygen, with 3'-phosphoadenosine-5'-phosphosulphate contributing the sulphate. The structure of human DHEA-ST in complex with its preferred substrate DHEA has been solved here to 1.99 A using molecular replacement with oestradiol sulphotransferase (37% sequence identity) as a model. Two alternative substrate-binding orientations have been identified. The primary, catalytic, orientation has the DHEA 3alpha-oxygen and the highly conserved catalytic histidine in nearly identical positions as are seen for the related oestradiol sulphotransferase. The substrate, however, shows rotations of up to 30 degrees, and there is a corresponding rearrangement of the protein loops contributing to the active site. This may also reflect the low identity between the two enzymes. The second orientation penetrates further into the active site and can form a potential hydrogen bond with the desulphonated cofactor 3',5'-phosphoadenosine (PAP). This second site contains more van der Waal interactions with hydrophobic residues than the catalytic site and may also reflect the substrate-inhibition site. The PAP position was obtained from the previously solved structure of DHEA-ST co-crystallized with PAP. This latter structure, due to the arrangement of loops within the active site and monomer interactions, cannot bind substrate. The results presented here describe details of substrate binding to DHEA-ST and the potential relationship to substrate inhibition.

Pharmacogenetics of sulfotransferase

Cytosolic sulfotransferase catalyzes sulfoconjugation of relatively small lipophilic endobiotics and xenobiotics. At least 44 cytosolic sulfotransferases have been identified from mammals, and based on their amino acid sequences, these forms are shown to constitute five different families. In humans, 10 sulfotransferase genes have been identified and shown to localize on at least five different chromosomes. The enzymatic properties characterized in the recombinant forms indicate the association of their substrate specificity with metabolisms of such nonpeptide hormones as estrogen, corticoid, and thyroxine, although most forms are also active on the sulfation of various xenobiotics. Genetic polymorphisms are observed on such human sulfotransferases as ST1A2, ST1A3, and ST2A3.

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.

The 3'-terminal exon of the family of steroid and phenol sulfotransferase genes is spliced at the N-terminal glycine of the universally conserved GXXGXXK motif that forms the sulfonate donor binding site

The guinea pig estrogen sulfotransferase gene has been cloned and compared to three other cloned steroid and phenol sulfotransferase genes (human estrogen sulfotransferase, human phenol sulfotransferase, and guinea pig 3 alpha-hydroxysteroid sulfotransferase). The four sulfotransferase genes demonstrate a common outstanding feature: the splice sites for their 3'-terminal exons are identically located. That is, the 3'-terminal exon splice sites involve a glycine that constitutes the N-terminal glycine of an invariably conserved GXXGXXK motif present in all steroid and phenol sulfotransferases for which primary structures are known. This consistency strongly suggests that all steroid and phenol sulfotransferase genes will be similarly spliced. The GXXGXXK motif forms the active binding site for the universal sulfonate donor 3'-phosphoadenosine 5'-phosphosulfate. Amino acid sequence alignment of 19 cloned steroid and phenol sulfotransferases starting with the GXXGXXK motif indicates that the 3'-terminal exon for each steroid and phenol sulfotransferase gene encodes a similarly sized C-terminal fragment of the protein. Interestingly, on further analysis of the alignment, three distinct amino acid sequence patterns emerge. The presence of the conserved functional GXXGXXK motif suggests that the protein domains encoded by steroid and phenol sulfotransferase 3'-terminal exons have evolved from a common ancestor. Furthermore, it is hypothesized that during the course of evolution, the 3'-terminal exon further diverged into at least three sulfotransferase subdivisions: a phenol or aryl group, an estrogen or phenolic steroid group, and a neutral steroid group.

Obesity-induced diabetes (diabesity) in C57BL/KsJ mice produces aberrant trans-regulation of sex steroid sulfotransferase genes

The diabetes (db) gene is a recessive obesity mutation in the mouse capable of producing diabetes only through interaction with heretofore undefined modifiers in the genetic background of certain inbred strains. Here we identify the genetic map locations of androgen and estrogen sulfotransferase genes important in maintaining the balance of active sex steroids in the liver. The Std locus encoding dehydroepiandrosterone sulfotransferase was mapped to proximal Chromosome 7, and the Ste locus encoding estrogen sulfotransferase was mapped to Chromosome 5. The db mutation in the diabetes-susceptible C57BL/KsJ strain aberrantly regulated mRNA transcript levels from these two loci. Hepatic Ste mRNA transcripts were increased from undetectable levels in normal males and females to high levels in db/db mice of both sexes. An anomalous suppression of Std transcription was observed in db/db females, but not in normal females. These reciprocal changes in mRNA concentrations in mutant females were reflected by an induction of a high affinity estrogen sulfotransferase activity and a concomitant loss of dehydroepiandrosterone sulfotransferase activity. These db gene-elicited effects were specific for the sex steroid sulfotransferases since other potential sex steroid metabolizing enzymes (phenol sulfotransferase, sex steroid sulfohydrolase, and UDP-glucuronyltransferase) were unaffected. These aberrant changes would virilize hepatic metabolism in females by increasing the ratio of active androgens to estrogens. In human females, non-insulin-dependent diabetes mellitus often develops when visceral obesity and hyperinsulinemia are associated with hyperandrogenization. This study demonstrates that background modifier genes interacting deleteriously with an obesity mutation are not necessarily defective alleles. Rather, some are functional genes whose regulation has been altered by pleiotropic effects of the obesity gene.

Developmental changes in the isoelectric variants of rat hepatic hydroxysteroid sulphotransferase

Major isoenzymes of androsterone-sulphating sulphotransferase (AD-ST) were isolated from liver cytosols of weanling and young adult female rats and their isoelectric properties were compared. On chromatofocusing the enzyme activity of young adults was eluted over a wider range of pH than was that of weanling rats. The activity at pH 7.8-7.2 (fraction I) is obvious at both ages, whereas the activity eluted over the pH 6.6-5.5 range (fraction II) is much lower in weanlings than in young adults. The AD-ST activities eluted in fractions I and II were separately purified by 3'-phosphoadenosine 5'-phosphate-agarose affinity chromatography at both ages. Two-dimensional gel electrophoresis of the isolated enzyme revealed several subunits with distinct pI values, but with the same molecular mass, namely 30 kDa. The relative levels of the pI 6.7 and pI 7.2 subunits are high and the relative level of the pI 6.1 is low in fraction I. In fraction II, the levels of pI 6.1 and pI 6.7 subunits are high and the level of the pI 7.2 subunit is low. There is no significant difference in the relative levels of the pI variants in each fraction between weanlings and young adults. The N-terminal amino acid sequences of the pI variants are identical within the area determined, irrespective of animal age or pI values. These results demonstrate that the pI variants of AD-ST are derived from the same precursor by post-translational modification or that they are products of closely related, but distinct, genes. The pI 6.1 and 6.7 subunits presumably increased during the development from the weanling stage to adulthood, resulting in the increase in acidic form(s) of AD-ST (fraction II) in adult females.

Purification and immunochemical characterization of a male-specific rat liver oestrogen sulphotransferase

Sulphation of oestrogens represents an important regulatory mechanism for these biologically active compounds. We have characterized and purified a form of rat liver sulphotransferase (ST), existing as a 32,500 Da monomer, which sulphates oestrogens, and have used this preparation to produce antibodies against oestrogen ST. The enzyme was active against oestrone, oestriol and beta-oestradiol, but not towards androgens. Using the antibody as a probe for immunoblotting, it was determined that the enzyme is expressed solely in male rats, and predominantly in the liver. Of the tissues examined, the only major extrahepatic tissue found to have any oestrogen ST was the brain (although the levels were very low), indicating that there might be a role for the sulphation of oestrogens in the brain. Examination of human liver and platelet cytosols by immunoblotting showed that the antibody recognized two major proteins of 32 and 34 kDa, which were presumed to correspond to the two principal phenol ST isoenzymes present in man.