Regulation of hepatic steroid receptors and enzymes by the 3beta-hydroxysteroid dehydrogenase inhibitor trilostane

Expression of 11β-hydroxysteroid dehydrogenase isoforms in canine adrenal glands treated with trilostane

Trilostane, a competitive inhibitor of 3β-hydroxysteroid dehydrogenase, is often used to treat canine hyperadrenocorticism. In some species, trilostane has been shown to have additional effects on steroid biosynthesis, and it has been postulated that trilostane might have effects on 11β-hydroxysteroid dehydrogenase (11β-HSD) in dogs. To investigate the effect of trilostane on 11β-HSD in canine adrenal glands, healthy Beagle dogs were treated with trilostane for 8 weeks. Trilostane treatment resulted in a significant decrease of the cortisol/cortisone ratio in the serum. The adrenal gland mRNA and protein expression levels of 11β-HSD type 1 and 11β-HSD type 2 were significantly higher and significantly lower respectively in dogs treated with trilostane compared to those in control healthy Beagle dogs. These findings suggest that trilostane may have an effect on 11β-HSD activity in canine adrenal glands.

Transcriptional regulatory dynamics of the hypothalamic-pituitary-gonadal axis and its peripheral pathways as impacted by the 3-beta HSD inhibitor trilostane in zebrafish (Danio rerio)

To study mechanisms underlying generalized effects of 3β hydroxysteroid dehydrogenase (HSD3B) inhibition, reproductively mature zebrafish (Danio rerio) were exposed to trilostane at two dosages for 24, 48, or 96 h and their gonadal RNA samples profiled with Agilent zebrafish microarrays. Trilostane had substantial impact on the transcriptional dynamics of zebrafish, as reflected by a number of differentially expressed genes (DEGs) including transcription factors (TFs), altered TF networks, signaling pathways, and Gene Ontology (GO) biological processes. Changes in gene expression between a treatment and its control were mostly moderate, ranging from 1.3 to 2.0 fold. Expression of genes coding for HSD3B and many of its transcriptional regulators remained unchanged, suggesting transcriptional up-regulation is not a primary compensatory mechanism for HSD3B enzyme inhibition. While some trilostane-responsive TFs appear to share cellular functions linked to endocrine disruption, there are also many other DEGs not directly linked to steroidogenesis. Of the 65 significant TF networks, little similarity, and therefore little cross-talk, existed between them and the hypothalamic-pituitary-gonadal (HPG) axis. The most enriched GO biological processes are regulations of transcription, phosphorylation, and protein kinase activity. Most of the impacted TFs and TF networks are involved in cellular proliferation, differentiation, migration, and apoptosis. While these functions are fairly broad, their underlying TF networks may be useful to development of generalized toxicological screening methods. These findings suggest that trilostane-induced effects on fish endocrine functions are not confined to the HPG-axis alone. Its impact on corticosteroid synthesis could also have contributed to some system wide transcriptional changes in zebrafish observed in this study.

Temporal evaluation of effects of a model 3β-hydroxysteroid dehydrogenase inhibitor on endocrine function in the fathead minnow

Inhibition of enzymes involved in the synthesis of sex steroids can substantially impact developmental and reproductive processes controlled by the hypothalmic-pituitary-gonadal (HPG) axis. A key steroidogenic enzyme that has received little attention from a toxicological perspective is 3β-hydroxysteroid dehydrogenase (3β-HSD). In these studies, we exposed reproductively-active fathead minnows (Pimephales promelas) to the model 3β-HSD inhibitor trilostane at two test concentrations (300 and 1,500 µg/L) over a 16-d period that included both 8-d exposure and 8-d recovery phases. Plasma concentrations of 17β-estradiol (E2) in females were depressed within hours of exposure to the drug and remained decreased at the highest trilostane concentration throughout the 8-d exposure. Reductions in E2 were accompanied by decreases in plasma concentrations of the estrogen-responsive protein vitellogenin (VTG). During the recovery phase of the test, plasma E2 and VTG concentrations returned to levels comparable to those of controls, in the case of E2 within 1 d. Up-regulation of ovarian expression of gene products for follicle-stimulating hormone receptor (fshr) and aromatase (cyp19a1a) suggested active compensation in trilostane-exposed animals. Effects of trilostane on HPG-related endpoints in exposed males were less pronounced, although, as in females, up-regulation of gonadal fshr was seen. Data from these time-course studies provide insights as to direct impacts, compensatory responses, and recovery from effects associated with perturbation of a comparatively poorly characterized enzyme/pathway critical to sex steroid synthesis. This information is important to the design and interpretation of approaches for assessing the occurrence and effects of HPG-active chemicals in both the laboratory and the field.

Multiple Routes to Oestrogen Antagonism

Several lines of evidence attest to the existence of alternative ligand binding sites on the oestrogen receptor (ER), including non-competitive inhibition by trilostane or tamoxifen. It is possible that the inhibitory action of conventional oestrogen agonists at high concentrations may indicate that they too interact at alternative ER sites, albeit at low affinity. To test this possibility an oestrogen reporter assay was used to compare the activity of different oestrogens and antagonists in breast cancer and prostate cell lines. All four cell lines tested contained different amounts of oestrogen receptor α (ERα), ERβ, progesterone receptor and coregulator mRNA. Though differences were observed in response to stimulation and inhibition, these correlated only with the presence or absence of ERα, and not with the other components. Thus stimulation of the reporter by oestradiol and oestrone was biphasic in the breast cancer cells, while prostate cells were unable to respond. Only T47D cells were stimulated by oestriol or diethylstilboestrol, however reporter activity of all the cell lines was repressed by 10μM diethylstilboestrol. Reporter activity of MCF-7 cells was inhibited by tamoxifen, raloxifene and ICI 182,780, but stimulated by trilostane, yet all these antioestrogens inhibited agonist-stimulated activity. Trilostane also inhibited the agonism seen in cells co-treated with E2 and tamoxifen. It is clear that several of the compounds tested may have either agonist or antagonist effects under different conditions and at different concentrations, acting through ERα alone. Though biphasic dose response curves, or hormesis, have been attributed to various mechanisms, we here provide evidence that alternative ligand binding sites may contribute to this phenomenon.

Classification of chemicals based on concentration-dependent toxicological data using ToxClust

Concentration-dependent response relationships provide essential information on the characteristics of chemical-induced effects on toxicological end points, which include effect (inhibition or induction), potency, and efficacy of the chemical. Recent developments in systems biology and high throughputtechnologies have allowed simultaneous examination of many chemicals at multiple end point levels. While this increase in the quantity of information generated offers great potential, it also poses a significant challenge to environmental scientists to efficiently manage and interpret these large data sets. Here we present a novel method, ToxClust, that allows clustering of chemicals on the basis of concentration-response data derived with single or multiple end points. This method utilizes a least distance-searching algorithm (LDSA) to measure the pattern dissimilarity of concentration-response curves between chemicals and their relative toxic potency. ToxClust was tested using simulated data and chemical test data collected from the human H295R cell-based in vitro steroidogenesis assay. ToxClust effectively identified similar patterns of simulated data and responses to the exposure with the five model chemicals and separated them into different groups on the basis of their dissimilarities. These observations demonstrate that ToxClust not only provides an effective data analysis and visualization tool, but also has value in hypothesis generation and mechanism-based chemical classification.