Kinetics of beta-glucuronidase induction by androgen. Genetic variation in the first order rate constant

Association of ß-glucuronidase activity with menopausal status, ethnicity, adiposity, and inflammation in women

OBJECTIVES

Many dietary polyphenols with potential health-promoting benefits undergo hepatic conjugation and circulate as inactive glucuronides that can be cleaved by ß-glucuronidase to reform the bioactive aglycone. Although indirect evidence suggests estrogen may induce ß-glucuronidase, little is known about ß-glucuronidase regulation across women's reproductive lifespan. Correlates of serum ß-glucuronidase activity in healthy premenopausal versus postmenopausal women were therefore examined.

METHODS

ß-Glucuronidase activity and C-reactive protein (CRP) were assayed in stored serum from the Women's Breast and Bone Density Study, and dual-energy x-ray absorptiometry and anthropometry assessed body composition. Participants were premenopausal (n = 133) or postmenopausal (n = 89), and Hispanic (37%) or non-Hispanic White (63%). Multivariate linear regression models tested associations between ß-glucuronidase and menopausal status, ethnicity, CRP, and body composition metrics, overall and stratified by menopausal status.

RESULTS

Postmenopausal (vs premenopausal) women were older (60.4 ± 3.7 vs 44.8 ± 2.4 y) with a lower Hispanic ethnicity prevalence (27% vs 44%), and higher serum ß-glucuronidase activity (1.5 ± 0.8 vs 1.3 ± 0.5 U/L) and CRP (4.2 ± 4.4 vs 3.3 ± 4.7 mg/L). Adjusting for confounders, ß-glucuronidase was positively associated with Hispanic ethnicity, CRP, body mass index, and total fat mass (all, P < 0.01), but not menopausal status nor lean mass. Central adiposity measures were also positively associated with ß-glucuronidase with the same covariates.

CONCLUSIONS

ß-Glucuronidase enzyme activity, upon which polyphenol health-related benefits may depend, is not associated with menopausal status. Future studies are required to determine clinical significance and mechanisms driving ß-glucuronidase associations with ethnicity, inflammation, and adiposity in women.

Growth hormone and insulin-like growth factor-I enhance beta-glucuronidase gene activation by androgen in mouse kidney

Beta-glucuronidase (GUS) is a lysosomal enzyme that, in mouse kidney, is subject to control by multiple hormones: androgen, which increases GUS transcription; estrogen, which antagonizes androgen-mediated stimulation of GUS; and growth hormone (GH), which appears to be necessary for the full androgen effect. Neither estrogen nor GH affects GUS in the absence of androgen. In hypophysectomized or pituitary dwarf mice the reduced androgen stimulation of GUS can be partially restored with GH treatment. Androgen-induced GUS mRNA increased significantly with intermittent GH, compared to no GH or continuous GH. Intact mice subjected to continuous infusion of GH showed a depressed androgen effect on GUS similar to that seen in GH-deficient mice. Thus, pulsatile GH is required for the full androgen response. Insulin-like growth factor-I (IGF-I) also restored GUS induction by androgen in GH-deficient mice. We conclude that GH enhances the effect of androgen on the GUS gene via IGF-I. Using transgenic mice, we have also identified a genetic variant of the GUS gene that is insensitive to GH enhancement of the androgen effect.

The Gus-e locus regulates estrogen repression of androgen-induced beta-glucuronidase expression in mouse kidney

Both enzyme activity and mRNA concentration of beta-glucuronidase were measured in kidneys of mice treated with testosterone and the synthetic estrogen, diethylstilbestrol. Six congenic strains, all having a C57BL6/J genetic background but each having a different haplotype of the beta-glucuronidase gene complex, were compared. In each strain the induction caused by androgen was partially repressed by estrogen. The extent of this antagonism varied among the six haplotypes and was not coordinate with the extent of induction by androgen alone. Antagonism appears to be regulated by at least two alleles of a new locus, Gus-e, within the beta-glucuronidase gene complex. Repression by estrogen, like induction by androgen, appears to take place primarily at the transcriptional level. Kinetic studies revealed that estrogen causes the androgen response curve to plateau earlier and at a lower level. This suggests that estrogen increases the rate of gene deactivation rather than decreasing the rate of gene activation. Isoelectric focusing of beta-glucuronidase from Gus-ea and Gus-eb mice and their F1 progeny revealed that the genes are regulated in cis. Together, these findings support a model in which both sex hormones exert their effects on separate DNA response elements located in close proximity to the gene or within the gene itself.

Long-term effects of bone marrow transplantation on lysosomal enzyme replacement in beta-glucuronidase-deficient mice

This study uses bone marrow transplantation (BMT) between congenic strains of mice as an experimental model to examine enzyme replacement therapy of lysosomal storage diseases. Bone marrow cells from donor mice which have normal levels of the lysosomal enzyme beta-glucuronidase (Gus), which is heat-stable, rapidly repopulated the haematopoietic compartment of irradiated recipient mice which have only low levels of a thermolabile form of this enzyme. Gus activity was found to increase progressively in the tissues of the recipients, including the liver, heart and skeletal muscle. Elevated levels were also observed in the kidney and brain. The increase in enzyme activity in the host tissues was not due to the presence of contaminating blood cells, but rather to the acquisition of new, heat-stable enzyme from the donor bone marrow cells. High levels of Gus activity persisted for at least 72 weeks, showing the potential therapeutic value of BMT for enzyme deficiency diseases.

The kinetics of mammalian gene expression

When rates of transcription from specific genes change, delays of variable length intervene before the corresponding mRNAs and proteins attain new levels. For most mammalian genes, the time required to complete transcription, processing, and transport of mRNA is much shorter than the period needed to achieve a new, steady-state level of protein. Studies of inducible genes have shown that the period required to attain new levels of individual mRNAs and proteins is related to their unique half-lives. The basis for this is a physical principle that predicts rates of accumulation of particles in compartmental systems. The minimum period required to achieve a new level is directly proportional to product half-lives because rates of decay control the ratio between the rate of synthesis and the concentration of gene products at steady state. This kinetic model suggests that sensitivity of gene products to degradation by ribonucleases and proteinases is an important determinant of the time scale of gene expression.

Pineal lysosomal enzymes in the Syrian hamster: circadian rhythm and effects of castration or short photoperiod treatment

A circadian rhythm in acid phosphatase and hexosaminidase was found in adult male hamsters exposed to a long photoperiod (14:10 h light/dark [LD]; lights on 06.00 h) and killed at 08.00, 14.00, 20.00, 02.00, 04.00, 05.50 and 0.615 h. Hexosaminidase and beta-glucuronidase activity at 02.00, 04.00 and 05.50 h (values pooled for these times before lights on) were significantly elevated compared to enzyme activity at 06.15 and 08.00 h (pooled values after lights on), suggesting a fall in activity associated with lights on. Hypogonadism was induced in female Syrian hamsters by exposure to a short photoperiod (10:14 h LD) until a majority of them were vaginally acyclic. Pineal lysosomal enzyme activities (acid phosphatase, beta-glucuronidase, hexosaminidase, alpha-arabinosidase and beta-galactosidase) were significantly elevated in short photoperiod-exposed animals compared to animals in 14:10 LD, when measured near the middle of the light phase. In the third experiment, castrated animals were used to determine if lowered androgen levels might also affect pineal lysosomal enzyme activity. The results indicated that light phase beta-glucuronidase, hexosaminidase and beta-glucosidase activities were lower in castrated males compared to their intact controls. In summary, these results demonstrate that (1) lysosomal enzyme activity is present in the Syrian hamster pineal, (2) changes can be observed which suggest involvement of this activity in pineal function and, (3) a circadian rhythm in enzyme activity is present with peak activity occurring during the night. In the short photoperiod and castration experiments, the changes in lysosomal enzyme activity could reflect either a hormonal manipulation or a change in circadian regulation of enzyme activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The N haplotype of the murine beta-glucuronidase gene is altered in both its systemic regulation and its response to androgen induction

A new haplotype of the beta-glucuronidase gene complex, [Gus]N, has been characterized following its transfer from the PAC/Cr strain to the standard strain C57BL/6J. The N haplotype contains a novel structural gene allele which encodes an allozyme differing from all previously characterized allozymes in both size and charge. Altered systemic regulation is exhibited by the [Gus]N haplotype. Multiple tissues contain levels of GUS protein that are 60 +/- 15% those found in the standard B haplotype. The regulatory mechanism for reduction is complex, involving tissue-specific changes in both enzyme synthesis and enzyme turnover. The changes in GUS protein synthesis do not result from changes in GUS mRNA levels. Instead, the amount of mature enzyme formed per mRNA molecule, or translational yield, is altered. These regulatory changes parallel those seen in other systemic regulatory variants of GUS which are also altered in translational yield. A commonality of mechanism among systemic regulatory variants of this gene is suggested. The N haplotype is also exceptional in the nature of its response to androgenic induction in kidney proximal tubule epithelial cells. The time course for GUS induction consists of a lag period followed by a progressive increase in mRNA, rate of enzyme synthesis, and enzyme activity. For the [Gus]N haplotype the lag is of an exceptionally short duration and the plateau is of a greater magnitude than for any haplotype previously described.

Chemical kinetics of induced gene expression: activation of transcription by noncooperative binding of multiple regulatory molecules

A chemical kinetics model is described for the regulation of gene expression by the progressive binding of regulatory molecules to specific binding sites on DNA. Chemical rate equations are formulated and solved for the accumulation of regulatory molecules on DNA, the change in the level of induced mRNA, and the change in the level of the encoded protein in the activated tissue. Some special cases are examined, including that of an activation threshold created by a requirement for the binding of a minimum number of regulatory molecules prior to gene activation. Experimental data for several hormone-activated genetic systems are analyzed in the frame of the proposed model, and kinetic parameters are predicted. The model accounts for a number of experimental characteristics of hormone-inducible genetic systems, including the existence of a lag in the time course of mRNA accumulation, the sigmoidal curve of induced mRNA kinetics, the effect of hormone on mRNA stabilization, and the induction parameters observed when hormone analogues are used. The model also provides an explanation for the phenotypes of genetic variants with altered inducibility as changes in the molecular kinetic parameters of gene activity.

Genome organization and polymorphism of the murine beta-glucuronidase region

Thirty-eight kilobases of mouse genomic DNA which surround and include the coding sequences for beta-glucuronidase has been mapped. Intron-exon arrangements were determined by hybridization of genomic sequences with cDNA clones, and minimum estimates of gene length (11-17 kb) and intron number were obtained. Only a single gene was observed when genomic DNA was probed with subclones containing beta-glucuronidase coding sequence; there was no evidence of duplicated or pseudogenes. However, sequences distal to the 3' end of the gene are present elsewhere in the genome in a limited number of copies. Eight haplotypes of the beta-glucuronidase region with differing regulatory genotypes were compared for restriction fragment polymorphisms. Surprisingly little was found, considering the diverse origin of the haplotypes. Two of the polymorphisms that were found may be correlated with regulatory phenotypes. A BamHI site is missing from the CS and CL haplotypes that share regulatory properties, and a 0.2-kb insertion is consistently present in haplotypes showing increased response to induction by androgens in kidney.

Variation in ten lysosomal hydrolase enzyme activities in inbred mouse strains

Activities of 10 lysosomal hydrolase enzymes (beta-hexosaminidase, beta-galactosidase, alpha-galactosidase, alpha-mannosidase, beta-mannosidase, alpha-L-fucosidase, beta-glucuronidase, alpha-glucosidase, alpha-N-acetylgalactosaminidase, and acid phosphatase) were determined in eight organs (brain, liver, kidney, spleen, heart, skeletal muscle, lung, and testis) in males and females of six inbred mouse strains (C57BL/6J, C3H/HeJ, DBA/2J, BALB/cJ, P/J, and 129/J). Examples of enzyme-specific variation, organ-specific variation, and enzyme- and organ-specific variation were found. New enzyme-specific variants with the features of systemic regulators for alpha-L-fucosidase and beta-mannosidase were found. Known variants were detected. Organ-specific variants had some of the properties expected for a new class of genes affecting multiple enzymes: organ-specific regulators.

Kinetic models of induction: II. Decreased turnover of a product or its precursor

Reduction of the turnover rate constant for a substance or its metabolic precursor is a mechanism for induction which differs markedly in its kinetic characteristics from induction resulting from a direct stimulation of synthesis. The unique aspect of induction following inhibited turnover is that the time course of recovery from the induced to the basal state is more rapid than the time course for induction itself. A general set of kinetic equations is presented here which represents the complete chain of events for such an induction; namely the kinetics of the inducer, the kinetics of a synthesis precursor to the induced product, and the kinetics of the induced product itself. These equations have been used to simulate examples of this type of induction and resemble induction experiments from the literature.

Weak androgen reduces the rate constant of beta-glucuronidase induction

Administration of androgen to mice induces kidney beta-glucuronidase. Measuring beta-glucuronidase activity, rate of beta-glucuronidase synthesis, beta-glucuronidase mRNA activity and beta-glucuronidase mRNA concentration, the time course of induction was compared using a strong androgen, dihydrotestosterone (DHT), and a weakly androgenic progestin, medroxyprogesterone acetate (MPA). Using MPA resulted in a longer lag, a 3-4-fold slower rate of induction as defined by the forward rate constant, ka, a lower final extent of induction, and a slightly lower turnover constant, kb. Differences in kinetics of induction were consistent for all 4 measured parameters, and mimicked previously described genetic differences in these rate constants. The coordinate induction of beta-glucuronidase protein and beta-glucuronidase mRNA indicates that the response to androgen is regulated at a pre-translational level. That substitution of MPA for DHT decreases ka, rather than increasing kb, suggests that induction of beta-glucuronidase follows an increased rate of mRNA synthesis rather than a decreased rate of mRNA turnover. Finally, the results are consistent with a model in which the kinetic constants for beta-glucuronidase induction are dependent on the concentration of receptor molecules in the active conformational state.

Regulation of growth hormone messenger RNA synthesis by dexamethasone and triiodothyronine. Transcriptional rate and mRNA stability changes in pituitary tumor cells

We have characterized the process by which the growth hormone (GH) gene is stimulated in rat pituitary tumor cells (GC or GH3) by the steroid hormone dexamethasone (Dex) and the thyroid hormone, L-triiodothyronine (T3). A primary transcriptional response is detected within 60 minutes of addition of T3 or Dex + T3 to GH-producing cells (GC or GH3). A fivefold transcriptional stimulation of GH nuclear RNA occurs in cells cultured with serum substitute medium and induced with Dex + T3, while T3 alone induces a modest two- to threefold stimulation. The absence of fetal calf serum from the cell culture medium does not decrease the level of transcriptional activity of the GH gene during hormone stimulation. Twenty-four hours after addition of Dex + T3 the cytoplasmic GH mRNA shows a 50-fold increase, as measured by S1 nuclease analysis. This large accumulation of cytoplasmic GH mRNA in contrast to the relatively small changes in GH gene activity is inconsistent with solely a transcriptional mechanism of hormone induction. We suggest that a change in specific GH mRNA stability also takes place in response to Dex + T3. In contrast to other reports, transcriptional stimulation of the GH gene by Dex is insignificant except in the presence of T3.

Properties of rat and mouse beta-glucuronidase mRNA and cDNA, including evidence for sequence polymorphism and genetic regulation of mRNA levels

cDNA clones containing partial sequences for beta-glucuronidase (beta G) were constructed from rat preputial gland RNA and identified by their ability to selectively hybridize beta G mRNA. One such rat clone was used to isolate several cross-hybridizing clones from a mouse-cDNA library prepared from kidney RNA from androgen-treated animals. Together, the set of mouse clones spans about 2.0 kb of the 2.6-kb beta G mRNA. Using these cDNA clones as probes, a genomic polymorphism for DNA restriction fragment size was found that proved to be genetically linked to the beta G gene complex. A fragment of beta G cDNA was subcloned into a vector carrying an SP6 polymerase promoter to provide a template for the in vitro synthesis of single-stranded RNA complementary to beta G mRNA. This provided an extremely sensitive probe for the assay of beta G mRNA sequences. Using either nick-translated cDNA or transcribed RNA as a hybridization probe, we found that mouse beta G RNA levels are strongly induced by testosterone, and that induction by testosterone is pituitary-dependent. During the lag period preceding induction, during the induction period itself, and during deinduction following removal of testosterone, beta G mRNA levels paralleled rates of beta G synthesis previously measured by in vivo pulse-labelling experiments. Genetic variation in the extent of induction affected either the level of beta G mRNA or its efficiency of translation depending on the strain of mice tested.