Effect of fibroblastic growth factors (FGF) on steroid UDP-glucuronosyltransferase expression and activity in the LNCaP cell line

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Effects of dietary physical or nutritional factors on morphology of rumen papillae and transcriptome changes in lactating dairy cows based on three different forage-based diets

BACKGROUND

Rumen epithelial tissue plays an important role in nutrient absorption and rumen health. However, whether forage quality and particle size impact the rumen epithelial morphology is unclear. The current study was conducted to elucidate the effects of forage quality and forage particle size on rumen epithelial morphology and to identify potential underlying molecular mechanisms by analyzing the transcriptome of the rumen epithelium (RE). To achieve these objectives, 18 mid-lactation dairy cows were allocated to three groups (6 cows per group), and were fed with one of three different forage-based diets, alfalfa hay (AH), corn stover (CS), and rice straw (RS) for 14 weeks, respectively. Ruminal volatile fatty acids (VFAs) and epithelial thickness were determined, and RNA-sequencing was conducted to identify the transcriptomic changes of rumen epithelial under different forage-based diets.

RESULTS

The RS diet exhibited greater particle size but low quality, the AH diet was high nutritional value but small particle size, and CS diet was low quality and small particle size. The ruminal total VFA concentration was greater in AH compared with those in CS or RS. The width of the rumen papillae was greater in RS-fed cows than in cows fed AH or CS. In total, 31, 40, and 28 differentially expressed (DE, fold change > 2, FDR < 0.05) genes were identified via pair-wise comparisons including AH vs. CS, AH vs. RS, and RS vs. CS, respectively. Functional classification analysis of DE genes revealed dynamic changes in ion binding (such as DSG1) between AH and CS, proliferation and apoptotic processes (such as BAG3, HLA-DQA1, and UGT2B17) and complement activation (such as C7) between AH or RS and CS. The expression of HLA-DQA1 was down-regulated in RS compared with AH and CS, and the expression of UGT2B17 was down-regulated in RS compared with CS, with positive (R = 0.94) and negative (R = -0.96) correlation with the width of rumen epithelial papillae (P < 0.05), respectively.

CONCLUSION

Our results suggest that both nutrients (VFAs) and particle sizes can alter expression of genes involved in cell proliferation/apoptosis process and complement complex. Our results suggest that particle size may be more important in regulating rumen epithelial morphology when animals are fed with low-quality forage diets and the identified DE genes may affect the RE nutrient absorption or morphology of RE. Our findings provide insights into the effects of the dietary particle size in the future management of dairy cow feeding, that when cows were fed with low-quality forage (such as rice straw), smaller particle size may be beneficial for nutrients absorption and milk production.

Nuclear receptors and endobiotics glucuronidation: the good, the bad, and the UGT

The recent progresses in molecular biology and pharmacology approaches allowed the characterization of a series of nuclear receptors (NRs) as efficient regulators of uridine diphosphate glucuronosyltransferase (UGT) genes activity. These regulatory processes ensure an optimized UGT expression in response to specific endo- and/or exogenous stimuli. Many of these NRs are activated by endobiotics that also are substrates for UGTs. Thus, by activating their receptors, these endogenous substances control their own conjugation, leading to the concept that glucuronidation is an important part of feed-forward/feedback mechanisms by which bioactive molecules control their own concentrations. On the other hand, numerous studies have established the pharmacological relevance of NR-UGT regulatory pathways in the response to therapeutic ligands. The present review article aims at providing a comprehensive view of the physiological and pharmacological importance of the NR regulation of the expression and activity of endobiotics-conjugating UGT enzymes. Selected examples will illustrate how the organism profits from the feed-forward/feedback mechanisms involving NR-UGT pathways, but also how such regulatory processes are involved in the initiation and/or progression of several pathological situations. Finally, we will discuss how the present pharmacopeia involves NR-dependent regulation of endobiotics glucuronidation, and whether the unexploited NR-UGT axes could serve as pharmacological targets for novel therapeutics to restore endobiotics homeostasis.

Modulation of UDP-glucuronosyltransferase activity by endogenous compounds

Glucuronidation is one of the major pathways of metabolism of endo- and xenobiotics. UDP-Glucuronosyltransferase (UGT)-catalyzed glucuronidation accounts for up to 35% of phase II reactions. The expression and function of UGT is modulated by gene regulation, post-translational modifications and protein-protein association. Many studies have focused on drug-drug interactions involving UGT, and there are a number of reports describing the inhibition of UGT by xenobiotics. However, studies about the role of endogenous compounds as an inhibitor or activator of UGT are limited, and it is important to understand any change in the function and regulation of UGT by endogenous compounds. Recent studies in our laboratory have shown that fatty acyl-CoAs are endogenous activators of UGT, although fatty acyl-CoAs had been considered as inhibitors of UGT. Further, we have also suggested that adenine and related compounds are endogenous allosteric inhibitors of UGT. In this review, we summarize the endogenous modulators of UGT and discuss their relevance to UGT function.

Regulation of endobiotics glucuronidation by ligand-activated transcription factors: physiological function and therapeutic potential

Recent progresses in molecular pharmacology approaches have allowed the identification and characterization of a series of nuclear receptors (NR) which efficiently control the level UDP-glucuronosyltransferase (UGT) genes expression. These regulatory processes ensure optimized UGT expression in response to specific endogenous and/or exogenous stimuli. Interestingly, numerous endogenous activators of these NRs are conjugated by the UGT enzymes they regulate. In such a case, the NR-dependent regulation of UGT genes corresponds to a feedforward/feedback mechanism by which a bioactive molecule controls its own concentrations. In the present review, we will discuss i) how bilirubin reduces its circulating levels by activating AhR in the liver; ii) how bile acids modulate their hepatic glucuronidation via PXR- and FXR-dependent processes in enterohepatic tissues; and iii) how androgens inhibit their cellular metabolism in prostate cancer cells through an AR-dependent mechanism. Subsequently, with further discussion of the same examples (bilirubin and bile acids), we will illustrate how NR-dependent regulation of UGT enzymes may contribute to the beneficial effects of pharmacological activators of nuclear receptors, such as CAR and PPARa.

Association of the polymorphisms of genes involved in androgen metabolism and signaling pathways with familial prostate cancer risk in a Japanese population

BACKGROUND

Androgen plays a central role in the normal and malignant development of prostate glands. Genetic polymorphisms of genes involved in androgen metabolism and signaling might be associated with the risk of prostate cancer.

METHODS

One hundred and two patients with prostate cancer with a family history and 117 healthy age- and residence-matched male controls were enrolled. Genotypes of the CAG repeat length of androgen receptor (AR), CYP17, 5alpha-reductase type II (SRD5A2), UDG-glucuronosyltransferase (UGT) 2B15, PSA promoter genes were analyzed.

RESULTS

For single polymorphisms, the presence of Y alleles showed a significantly lower risk of prostate cancer in comparison with the D/D genotype in UGT2B15 (odds ratio [OR]=0.41, 95% confidence interval [CI]=1.40-4.28, p=0.0015), and the presence of A2 alleles showed a weak tendency to decrease prostate cancer risk in comparison with the A1/A1 genotype in CYP17 (OR=0.69, 95% CI=0.39-1.23, p=0.21). The stratification of cases according to clinical stage and pathological grade showed that the A2/A2 genotype was significantly associated with localized stage cancer in comparison with metastatic stage cancer (OR=5.18, 95% CI=1.49-17.95, p=0.007). The combination of UGT2B15 and CYP17 genotypes could identify higher risk subjects even in subjects with low-risk UGT2B15 genotypes, i.e., Y/Y+D/Y genotypes (OR=1.97, 95% CI=0.92-4.22, p=0.079).

CONCLUSION

Genetic polymorphisms of the genes involved in androgen metabolism and signaling were significantly associated with familial prostate cancer risk. Single nucleotide polymorphisms of low-penetrance genes could be targets to understand genetic susceptibility to familial prostate cancer.

Steroid hormone biotransformation and xenobiotic induction of hepatic steroid metabolizing enzymes

Normal reproductive development depends on the interplay of steroid hormones with their receptors at specific tissue sites. The concentrations of hormone ligands in the circulation and at target sites are maintained through coordinated regulation on steroid biosynthesis and degradation. Changed bioavailability of steroids, through alteration of steroidogenesis or biotransformation rates, leads to changes in endocrine function. Steroid hormones lose their receptor reactivity in most cases when they are bound to binding proteins, while metabolic conversion can result in either active or inactive metabolites. Hydroxylation by cytochrome P450 (CYP) enzymes and conjugation with glucuronide and sulfate are among the major hepatic pathways of steroid inactivation. The expression of these biotransformation enzymes can be induced by many xenobiotics. The barbiturate phenobarbital and the environmental toxicant 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) are among the well characterized inducers for the CYP 2B and 3A enzymes and selected conjugation enzymes. The induction of the steroid biotransformation enzymes is partly mediated through the activation of a group of nuclear receptors including the glucocorticoid receptor, the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), and the peroxisome proliferator activated receptors (PPAR). Drug or chemical-induced increases in hepatic enzyme activities are often a basis for drug-drug interactions that lead to enhanced elimination and reduced therapeutic efficacy of steroidal drugs. The effects of enzyme induction on endogenous steroid clearance, along with its possible consequence, are less well understood. While enzyme induction by xenobiotics may increase clearance of the endogenous steroid, regulatory mechanisms for steroid homeostasis may adapt and compensate for altered clearance.

Inactivation of androgens by UDP-glucuronosyltransferase enzymes in humans

In humans, 3beta-hydroxysteroid dehydrogenase (3beta-HSD), 17beta-HSD and 5alpha-reductase activities in androgen target tissues, such as the prostate and skin, convert dehydroepiandrosterone, androstenedione and testosterone into the most potent natural androgen dihydrotestosterone (DHT). This androgen is converted mainly in situ into two phase I metabolites, androsterone (ADT) and androstane-3alpha,17beta-diol (3alpha-DIOL), which might be back converted to DHT. Here, we discuss the recent findings regarding the characterization of specific UDP-glucuronosyltransferases (UGTs), UGT2B7, B15 and B17, responsible for the glucuronidation of these metabolites. The tissue distribution and cellular localization of the UGT2B transcripts and proteins in humans clearly indicate that these enzymes are synthesized in androgen-sensitive tissues. It is postulated that the conjugating activity of UGT enzymes is the main mechanism for modulating the action of steroids and protecting the androgen-sensitive tissues from deleteriously high concentrations of DHT, ADT and 3alpha-DIOL.

Glucuronidation of arachidonic and linoleic acid metabolites by human UDP-glucuronosyltransferases

Arachidonic acids (AA) and linoleic acids (LAs) are metabolized, in several tissues, to hydroxylated metabolites that are important mediators of many physiological and pathophysiological processes. The conjugation of leukotriene B4 (LTB4), 5-hydroxyeicosatetraenoic acid (HETE), 12-HETE, 15-HETE, and 13-hydroxyoctadecadienoic acid (HODE) by the human UDP-glucuronosyltransferase (UGT) enzymes was investigated. All substrates tested were efficiently conjugated by human liver microsomes to polar derivatives containing the glucuronyl moiety as assessed by mass spectrometry. The screening analyses with stably expressed UGT enzymes in HK293 showed that glucuronidation of LTB4 was observed with UGT1A1, UGT1A3, UGT1A8, and UGT2B7, whereas UGT1A1, UGT1A3, UGT1A4, and UGT1A9 also conjugated most of the HETEs and 13-HODE. LA and AA metabolites also appear to be good substrates for the UGT2B subfamily members, especially for UGT2B4 and UGT2B7 that conjugate all HETE and 13-HODE. Interestingly, UGT2B10 and UGT2B11, which are considered as orphan enzymes since no conjugation activity has so far been demonstrated with these enzymes, conjugated 12-HETE, 15-HETE, and 13-HODE. In summary, our data showed that several members of UGT1A and UGT2B families are capable of converting LA and AA metabolites into glucuronide derivatives, which is considered an irreversible step to inactivation and elimination of endogenous substances from the body.

Glucuronidation activity of the UGT2B17 enzyme toward xenobiotics

UDP-glucuronosyltransferase (UGT) 2B17 is one of the most important conjugating enzymes in androgen metabolism and shares more than 95% homology with UGT2B15. Although UGT2B15 has been fully characterized for its ability to conjugate drugs, environmental pollutants, and dietary components, UGT2B17 received less attention for its capacity to glucuronidate xenobiotics. In the present study, more than 55 exogenous compounds belonging to several categories of compounds were analyzed as potential substrates for UGT2B17. Glucuronidation activity was observed with several coumarins, anthraquinones, and flavonoids. The higher glucuronidation activity was measured with alizarin (125 pmol x min(-1) x mg protein(-1)), whereas UGT2B17 conjugated eugenol, scopoletin, and galangin with glucuronidation rates of 102.5, 102, and 58 pmol x min(-1) x mg protein(-1), respectively. The characterization of UGT2B17 as a xenobiotics-conjugating enzyme demonstrates that its role is not limited to androgen metabolism and that its specificity for exogenous substrates is different from other UGT2B isoforms. Taken together, these data suggest a role of UGT2B17 for the hepatic detoxification.

Relative enzymatic activity, protein stability, and tissue distribution of human steroid-metabolizing UGT2B subfamily members

Androgens and estrogens play major roles in cell differentiation, cell growth, and peptide secretion in steroid target tissues. In addition to the binding of these hormones to their receptors, formation and metabolism are important in the action of steroids. Metabolism of the potent steroid hormones includes glucuronidation, a major pathway of steroid elimination in liver and several steroid target tissues. Glucuronidation is catalyzed by UDP-glucuronosyltransferases (UGTs), which transfer the polar moiety from UDP-glucuronic acid to a wide variety of endogenous compounds, including steroid hormones. The UGT superfamily of enzymes is subdivided into two families, UGT1 and UGT2, on the basis of sequence homology. To date, six UGT2B proteins have been isolated, namely UGT2B4, UGT2B7, UGT2B10, UGT2B11, UGT2B15, and UGT2B17, all of which have been demonstrated to be active on steroid molecules, except for UGT2B10 and UGT2B11, for which no substrate was found. The relative activity of these enzymes on steroidal compounds remains unknown due to variable levels of UGT2B expression in different in vitro cell line models and various conditions of the enzymatic assays. Comparison of the glucuronidation rates of these enzymes requires a unique system for UGT2B protein expression, protein normalization, and enzymatic assays. In this study we have stably expressed UGT2B4, UGT2B7, UGT2B15, and UGT2B17 in the HK293 cell line, which is devoid of steroid UGT activity; characterized their kinetic properties relative to UGT protein expression; determined their transcript and protein stabilities; and established extensively their tissular distributions. UGT2B7 was demonstrated to glucuronidate estrogens, catechol estrogens, and androstane-3alpha,17beta-diol more efficiently than any other human UGTB isoform. UGT2B15 and UGT2B17 showed similar glucuronidation activity for androstane-3alpha,17beta-diol (30% lower than that of UGT2B7), whereas UGT2B17 demonstrated the highest activity for androsterone, testosterone, and dihydrotestosterone. UGT2B4 demonstrates reactivity toward 5alpha-reduced androgens and catechol estrogens, but at a significantly lower level than UGT2B7, 2B15, and 2B17. Cycloheximide treatment of stably transfected HK293 cells demonstrated that the UGT2B17 protein is more labile than the other enzymes; the protein levels decrease after 1 h of treatment, whereas other UGT2B proteins were stable for at least 12 h. Treatment of stable cells with actinomycin D reveals that UGT2B transcripts are stable for 12 h, except for the UGT2B4 transcript, which was decreased by 50% after the 12-h incubation period. Tissue distribution of the UGT2B enzymes demonstrated that UGT2B isoforms are expressed in the liver as well as in several extrahepatic steroid target tissues, namely, kidney, breast, lung, and prostate. This study clearly demonstrates the relative activities and the major substrates of human steroid-metabolizing UGT2B enzymes, which are expressed in a wide variety of steroid target tissues.

N-glycosylation and residue 96 are involved in the functional properties of UDP-glucuronosyltransferase enzymes

The recent cloning of several human and monkey UDP-glucuronosyltransferase (UGT) 2B proteins has allowed the characterization of these steroid metabolic enzymes. However, relatively little is known about the structure-function relationship, and the potential post-translational modifications of these proteins. The mammalian UGT2B proteins contain at least one consensus asparagine-linked glycosylation site NX(S/T). Endoglycosidase H digestion of the human and monkey UGT2B proteins demonstrates that only UGT2B7, UGT2B15, UGT2B17, and UGT2B20 are glycosylated. Although UGT2B15 and UGT2B20 contain three and four potential glycosylation sites, respectively, site-directed mutagenesis revealed that both proteins are glycosylated at the same first site. In both proteins, abolishing glycosylation decreased glucuronidation activity; however, the K(m) values and the substrate specificities were not affected. Despite the similarities between UGT2B15 and UGT2B20, UGT2B20 is largely more labile than UGT2B15. Treating HK293 cells stably expressing UGT2B20 with cycloheximide for 2 h decreased the enzyme activity by more than 50%, whereas the activity of UGT2B15 remained unchanged after 24 h. The UGT2B20 protein is unique in having an isoleucine at position 96 instead of an arginine as found in all the other UGT2B enzymes. Changing the isoleucine in UGT2B20 to an arginine stabilized enzyme activity, while the reciprocal mutation in UGT2B15 R96I produced a more labile enzyme. Secondary structure predictions of UGT2B proteins revealed a putative alpha-helix in this region in all the human and monkey proteins. This alpha-helix is shortest in UGT2B20; however, the helix is lengthened in UGT2B20 I96R. Thus, it is apparent that the length of the putative alpha-helix between residues 84 and 100 is a determining factor in the stability of UGT2B enzyme activity. This study reveals the extent and importance of protein glycosylation on UGT2B enzyme activity and that the effect of residue 96 on UGT2B enzyme stability is correlated to the length of a putative alpha-helix.

Isolation and characterization of the human UGT2B15 gene, localized within a cluster of UGT2B genes and pseudogenes on chromosome 4

Glucuronidation is a major pathway of androgen metabolism and is catalyzed by UDP-glucuronosyltransferase (UGT) enzymes. UGT2B15 and UGT2B17 are 95% identical in primary structure, and are expressed in steroid target tissues where they conjugate C19 steroids. Despite the similarities, their regulation of expression are different; however, the promoter region and genomic structure of only the UGT2B17 gene have been characterizedX to date. To isolate the UGT2B15 gene and other novel steroid-conjugating UGT2B genes, eight P-1-derived artificial chromosomes (PAC) clones varying in length from 30 kb to 165 kb were isolated. The entire UGT2B15 gene was isolated and characterized from the PAC clone 21598 of 165 kb. The UGT2B15 and UGT2B17 genes are highly conserved, are both composed of six exons spanning approximately 25 kb, have identical exon sizes and have identical exon-intron boundaries. The homology between the two genes extend into the 5'-flanking region, and contain several conserved putative cis-acting elements including Pbx-1, C/EBP, AP-1, Oct-1 and NF/kappaB. However, transfection studies revealed differences in basal promoter activity between the two genes, which correspond to regions containing non-conserved potential elements. The high degree of homology in the 5'-flanking region between the two genes is lost upstream of -1662 in UGT2B15, and suggests a site of genetic recombination involved in duplication of UGT2B genes. Fluorescence in situ hybridization mapped the UGT2B15 gene to chromosome 4q13.3-21.1. The other PAC clones isolated contain exons from the UGT2B4, UGT2B11 and UGT2B17 genes. Five novel exons, which are highly homologous to the exon 1 of known UGT2B genes, were also identified; however, these exons contain premature stop codons and represent the first recognized pseudogenes of the UGT2B family. The localization of highly homologous UGT2B genes and pseudogenes as a cluster on chromosome 4q13 reveals the complex nature of this gene locus, and other novel homologous UGT2B genes encoding steroid conjugating enzymes are likely to be found in this region of the genome.

Identification and cloning of a novel androgen-responsive gene, uridine diphosphoglucose dehydrogenase, in human breast cancer cells

Androgens inhibit the growth of breast cancer cells, but the mechanism of androgen-induced growth inhibition has not yet been elucidated, and few androgen-responsive genes have been identified. We, therefore, used differential display PCR to identify novel androgen-responsive genes in ZR-75-1 human breast cancer cells. The human UDP-glucose dehydrogenase gene (UDPGDH), which was not known to be androgen regulated, was detected and cloned by complementary DNA library screening. The UDPGDH open reading frame codes for a protein of 494 amino acids that migrates at an apparent molecular mass of approximately 54 kDa. Northern blot analysis revealed the existence of two messenger RNA species of approximately 3.5 and 2.7 kb in all of the human breast cancer cell lines examined. The major UDPGDH transcript was induced rapidly (within 6 h) by dihydrotestosterone in ZR-75-1 cells, and a maximal 13-fold induction was observed after 24 h of treatment. The increase in UDPGDH messenger RNA was completely prevented by coincubation with the pure antiandrogen hydroxyflutamide, but not by cycloheximide, indicating that UDPGDH is directly regulated by the androgen receptor. As UDPGDH is required for the production of uridine 5'-diphosphoglucuronic acid, a substrate for the steroid-conjugating uridine diphospho-glucuronosyltransferase enzymes, up-regulation of UDPGDH expression by androgens might play an important role in the control of sex steroid inactivation via glucuronidation in breast cancer cells.