Regulation of phase I and phase II steroid metabolism enzymes by PPAR alpha activators

Profiles of Two Glycaemia Modifying Drugs on the Expression of Rat and Human Sulfotransferases

AIMS

To study the effects of blood glucose regulating compounds on human and rat sulfotransferases (SULTs) expressions.

BACKGROUND

Phase-II enzymes, sulfotransferases catalyze the sulfuryl-group-transfer to endogenous/exogenous compounds. The alteration of expressions of SULTs may have influence on the sulfation of its substrate and other biomolecules.

OBJECTIVES

The influence of the altered biotransformation might alter different biochemical events, drug-drug interactions and bioaccumulation or excretion pattern of certain drug.

METHODS

In this brief study, diabetes-inducing drug streptozotocin (STZ; 10 or 50 mg/kg to male Sprague Dawley rat for 2 weeks) or hyperglycemia controlling drug tolbutamide (TLB 0.1 or 10μM to human hepato-carcinoma cells, HepG2 for 10 days) was applied and the SULTs expressions were verified. Extensive protein-protein (STa, SULT2A1/DHEAST) interactions were studied by the STRING (Search-Tool-for-the-Retrieval-of-Interacting Genes/Proteins) Bioinformatics-software.

RESULTS

Present result suggests that while STZ increased the STa (in rat) (dehydroepiandrosterone catalyzing SULT; DHEAST in human HepG2), tolbutamide decreased PPST (phenol catalyzing SULT) and DHEAST activity in human HepG2 cells. Moderate decreases of MPST (monoamine catalyzing SULT) and EST (estrogen catalyzing) activities are noticed in this case. STa/DHEAST was found to be highly interactive to SHBG/- sex-hormone-binding-globulin; PPARα/lipid-metabolism-regulator; FABP1/fatty-acid-binding-protein.

CONCLUSION

Streptozotocin and tolbutamide, these two glycaemia-modifying drugs demonstrated regulation of rat and human SULTs activities. The reciprocal nature of these two drugs on SULTs expression may be associated with their contrasting abilities in influencing glucose-homeostasis. Possible association of certain SULT-isoform with hepatic fat-regulations may indicate an unfocused link between calorie-metabolism and the glycemic-state of an individual. Explorations of this work may uncover the role of sulfation metabolism of specific biomolecule on cellular glycemic regulation.

Interaction of glucocorticoids with FXR/FGF19/FGF21-mediated ileum-liver crosstalk

At high doses, glucocorticoids (GC) have been associated with enhanced serum bile acids and liver injury. We have evaluated the effect of GC, in the absence of hepatotoxicity, on FXR/FGF91(Fgf15)/FGF21-mediated ileum-liver crosstalk. Rats and mice (wild type and Fxr^-/-, Fgf15^-/- and int-Gr^-/- strains; the latter with GC receptor (Gr) knockout selective for intestinal epithelial cells), were treated (i.p.) with dexamethasone, prednisolone or budesonide. In both species, high doses of GC caused hepatotoxicity. At a non-hepatotoxic dose, GC induced ileal Fgf15 down-regulation and liver Fgf21 up-regulation, without affecting Fxr expression. Fgf21 mRNA levels correlated with those of several genes involved in glucose and bile acid metabolism. Surprisingly, liver Cyp7a1 was not up-regulated. The expression of factors involved in transcriptional modulation by Fxr and Gr (p300, Drip205, CBP and Smrt) was not affected. Pxr target genes Cyp3a11 and Mrp2 were not up-regulated in liver or intestine. In contrast, the expression of some Pparα target genes in liver (Fgf21, Cyp4a14 and Vanin-1) and intestine (Vanin-1 and Cyp3a11) was altered. In mice with experimental colitis, liver Fgf21 was up-regulated (4.4-fold). HepG2 cells transfection with FGF21 inhibited CYP7A1 promoter (prCYP7A1-Luc2). This was mimicked by pure human FGF21 protein or culture in medium previously conditioned by cells over-expressing FGF21. This response was not abolished by deletion of a putative response element for phosphorylated FGF21 effectors present in prCYP7A1. In conclusion, GC interfere with FXR/FGF19-mediated intestinal control of CYP7A1 expression by the liver and stimulate hepatic secretion of FGF21, which inhibits CYP7A1 promoter through an autocrine mechanism.

Mediating Roles of PPARs in the Effects of Environmental Chemicals on Sex Steroids

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear receptors that are widely involved in various physiological functions. They are widely expressed through the reproductive system. Their roles in the metabolism and function of sex steroids and thus the etiology of reproductive disorders receive great concern. Various kinds of exogenous chemicals, especially environmental pollutants, exert their adverse impact on the reproductive system through disturbing the PPAR signaling pathway. Chemicals could bind to PPARs and modulate the transcription of downstream genes containing PPRE (peroxisome proliferator response element). This will lead to altered expression of genes related to metabolism of sex steroids and thus the abnormal physiological function of sex steroids. In this review, various kinds of environmental ligands are summarized and discussed. Their interactions with three types of PPARs are classified by various data from transcript profiles, PPRE reporter in cell line, in silico docking, and gene silencing. The review will contribute to the understanding of the roles of PPARs in the reproductive toxicology of environmental chemicals.

Gemfibrozil modulates cytochrome P450 and peroxisome proliferation-inducible enzymes in the liver of the yellow European eel (Anguilla anguilla)

The human lipid regulator gemfibrozil (GEM) has been shown to induce peroxisome proliferation in rodents leading to hepatocarcinogenesis. Since GEM is found at biological active concentrations in the aquatic environment, the present study investigates the effects of this drug on the yellow European eel (Anguilla anguilla). Eels were injected with different concentrations of GEM (0.1 to 200 μg/g) and sampled 24- and 96-h post-injection. GEM was shown to inhibit CYP1A, CYP3A and CYP2K-like catalytic activities 24-h post-injection, but at 96-h post-injection, only CYP1A was significantly altered in fish injected with the highest GEM dose. On the contrary, GEM had little effect on the phase II enzymes examined (UDP-glucuronyltransferase and glutathione-S-transferase). Peroxisome proliferation inducible enzymes (liver peroxisomal acyl-CoA oxidase and catalase) were very weakly induced. No evidence of a significant effect on the endocrine system of eels was observed in terms of plasmatic steroid levels or testosterone esterification in the liver.

Regulation of the cytosolic sulfotransferases by nuclear receptors

The cytosolic sulfotransferases (SULTs) are a multigene family of enzymes that catalyze the transfer of a sulfonate group from the physiologic sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate, to a nucleophilic substrate to generate a polar product that is more amenable to elimination from the body. As catalysts of both xenobiotic and endogenous metabolism, the SULTs are major points of contact between the external and physiological environments, and modulation of SULT-catalyzed metabolism can not only affect xenobiotic disposition, but it can also alter endogenous metabolic processes. Therefore, it is not surprising that SULT expression is regulated by numerous members of the nuclear receptor (NR) superfamily that function as sensors of xenobiotics as well as endogenous molecules, such as fatty acids, bile acids, and oxysterols. These NRs include the peroxisome proliferator-activated receptors, pregnane X receptor, constitutive androstane receptor, vitamin D receptor, liver X receptors, farnesoid X receptor, retinoid-related orphan receptors, and estrogen-related receptors. This review summarizes current information about NR regulation of SULT expression. Because species differences in SULT subfamily composition and tissue-, sex-, development-, and inducer-dependent regulation are prominent, these differences will be emphasized throughout the review. In addition, because of the central role of the SULTs in cellular physiology, the effect of NR-mediated SULT regulation on physiological and pathophysiological processes will be discussed. Gaps in current knowledge that require further investigation are also highlighted.

A mixture of an environmentally realistic concentration of a phthalate and herbicide reduces testosterone in male fathead minnow (Pimephales promelas) through a novel mechanism of action

Several chemicals that are used by humans, such as pesticides and plastics, are released into the aquatic environment through wastewater and runoff and have been shown to be potent disruptors of androgen synthesis at high concentrations. Although many of these chemicals have been studied in isolation, a large amount of uncertainty remains over how fish respond to low concentrations of anti-androgenic mixtures, which more accurately reflects how such chemicals are present in the aquatic environment. In this study male fathead minnows (FHM) (Pimephales promelas) were exposed to environmentally relevant concentrations of two anti-androgens, the herbicide linuron, and the plasticizer di(2-ethylhexyl) phthalate (DEHP) individually and as part of a mixture of the two for a 28-day period. At the end of this period there was a reduction in plasma testosterone (T) concentrations in male FHM exposed to the mixture, but not in FHM exposed individually to linuron or DEHP or the control FHM. There was also a significant reduction in 17β-estradiol (E2) in the DEHP-only and mixture exposed groups as compared to the control. Contrary to what has been previously published for these two chemicals in mammals, the lower plasma T concentrations in male FHM exposed to the mixture was not a result of the inhibition of genes involved in steroidogenesis; nor due to an increase in the expression of genes associated with peroxisome proliferation. Rather, an increase in relative transcript abundance for CYP3A4 in the liver and androgen- and estrogen-specific SULT2A1 and SULT1st2 in the testes provides evidence that the decrease in plasma T and E2 may be linked to increased steroid catabolism. Feedback from the pituitary is not repressed as the relative expression of follicle stimulating hormone β-subunit mRNA transcript levels in the brain was significantly higher in both DEHP and mixture exposed FHM. In addition, luteinizing hormone β-subunit mRNA transcript levels increased but were not significant in the mixture as compared to the control. Hormone receptor mRNA transcript levels in the liver and testes were not significantly different across all four exposure groups. This study highlights the importance of assessing environmentally relevant concentrations of mixtures when determining risk to aquatic organisms.

Mechanistic considerations for human relevance of cancer hazard of di(2-ethylhexyl) phthalate

Di(2-ethylhexyl) phthalate (DEHP) is a peroxisome proliferator agent that is widely used as a plasticizer to soften polyvinylchloride plastics and non-polymers. Both occupational (e.g., by inhalation during its manufacture and use as a plasticizer of polyvinylchloride) and environmental (medical devices, contamination of food, or intake from air, water and soil) routes of exposure to DEHP are of concern for human health. There is sufficient evidence for carcinogenicity of DEHP in the liver in both rats and mice; however, there is little epidemiological evidence on possible associations between exposure to DEHP and liver cancer in humans. Data are available to suggest that liver is not the only target tissue for DEHP-associated toxicity and carcinogenicity in both humans and rodents. The debate regarding human relevance of the findings in rats or mice has been informed by studies on the mechanisms of carcinogenesis of the peroxisome proliferator class of chemicals, including DEHP. Important additional mechanistic information became available in the past decade, including, but not limited to, sub-acute, sub-chronic and chronic studies with DEHP in peroxisome proliferator-activated receptor (PPAR) α-null mice, as well as experiments utilizing several transgenic mouse lines. Activation of PPARα and the subsequent downstream events mediated by this transcription factor represent an important mechanism of action for DEHP in rats and mice. However, additional data from animal models and studies in humans exposed to DEHP from the environment suggest that multiple molecular signals and pathways in several cell types in the liver, rather than a single molecular event, contribute to the cancer in rats and mice. In addition, the toxic and carcinogenic effects of DEHP are not limited to liver. The International Agency for Research on Cancer working group concluded that the human relevance of the molecular events leading to cancer elicited by DEHP in several target tissues (e.g., liver and testis) in rats and mice can not be ruled out and DEHP was classified as possibly carcinogenic to humans (Group 2B).

Cytochrome P450 CYP2 genes in the common cormorant: Evolutionary relationships with 130 diapsid CYP2 clan sequences and chemical effects on their expression

Cytochrome P450 CYP2 family enzymes are important in a variety of physiological and toxicological processes. CYP2 genes are highly diverse and orthologous relationships remain clouded among CYP2s in different taxa. Sequence and expression analyses of CYP2 genes in diapsids including birds and reptiles may improve understanding of this CYP family. We sought CYP2 genes in a liver cDNA library of the common cormorant (Phalacrocorax carbo), and in the genomes of other diapsids, chicken (Gallus gallus), zebra finch (Taeniopygia guttata), and anole lizard (Anolis carolinensis), for phylogenetic and/or syntenic analyses. Screening of the cDNA library yielded four CYP2 cDNA clones that were phylogenetically classified as CYP2C45, CYP2J25, CYP2AC1, and CYP2AF1. There are numerous newly identified diapsid CYP2 genes that include genes related to the human CYP2Cs, CYP2D6, CYP2G2P, CYP2J2, CYP2R1, CYP2U1, CYP2W1, CYP2AB1P, and CYP2AC1P. Syntenic relationships show that avian CYP2Hs are orthologous to CYP2C62P in humans, CYP2C23 in rats, and Cyp2c44 in mice, and suggest that avian CYP2Hs, along with human CYP2C62P and mouse Cyp2c44, could be renamed as CYP2C23, based upon the nomenclature rules. Analysis of sequence and synteny identifies cormorant and finch CYPs that are apparent orthologs of phenobarbital-inducible chicken CYP2C45. Transcripts of all four cormorant CYP2 genes were detected in the liver of birds from Lake Biwa, Japan. The transcript levels bore no significant relationship to levels of chlorinated organic pollutants in the liver, including polychlorinated biphenyls and dichlorodiphenyltrichloroethane and its metabolites. In contrast, concentrations of perfluorooctane sulfonate and perfluorononanoic acid were negatively correlated with levels of CYP2C45 and/or CYP2J25, suggesting down-regulation of expression by these environmental pollutants. This study expands our view of the phylogeny and evolution of CYP2s, and provides evolutionary insight into the chemical regulation of CYP2 gene expression in diapsids including birds.

Systematic genetic and genomic analysis of cytochrome P450 enzyme activities in human liver

Liver cytochrome P450s (P450s) play critical roles in drug metabolism, toxicology, and metabolic processes. Despite rapid progress in the understanding of these enzymes, a systematic investigation of the full spectrum of functionality of individual P450s, the interrelationship or networks connecting them, and the genetic control of each gene/enzyme is lacking. To this end, we genotyped, expression-profiled, and measured P450 activities of 466 human liver samples and applied a systems biology approach via the integration of genetics, gene expression, and enzyme activity measurements. We found that most P450s were positively correlated among themselves and were highly correlated with known regulators as well as thousands of other genes enriched for pathways relevant to the metabolism of drugs, fatty acids, amino acids, and steroids. Genome-wide association analyses between genetic polymorphisms and P450 expression or enzyme activities revealed sets of SNPs associated with P450 traits, and suggested the existence of both cis-regulation of P450 expression (especially for CYP2D6) and more complex trans-regulation of P450 activity. Several novel SNPs associated with CYP2D6 expression and enzyme activity were validated in an independent human cohort. By constructing a weighted coexpression network and a Bayesian regulatory network, we defined the human liver transcriptional network structure, uncovered subnetworks representative of the P450 regulatory system, and identified novel candidate regulatory genes, namely, EHHADH, SLC10A1, and AKR1D1. The P450 subnetworks were then validated using gene signatures responsive to ligands of known P450 regulators in mouse and rat. This systematic survey provides a comprehensive view of the functionality, genetic control, and interactions of P450s.

High-dose dibutyl phthalate improves performance of F1 generation male rats in spatial learning and increases hippocampal BDNF expression independent on p-CREB immunocontent

Dibutyl phthalate (DBP), an important representative of endocrine disrupting chemical, is suspected of affecting the cognitive function of humans and animals. In this study, effects of DBP on maze performance in male rats were evaluated by spatial learning tasks; the effects of DBP on the expression of brain-derived neurotrophic factor (BDNF) were also analyzed in both mRNA and mature protein levels in the hippocampus, with intent to investigate the possible mechanism underlying the behavioral findings. Pregnant Wistar rats were treated orally by gavage with 0, 25, 75, 225 and 675mgDBP/kgBW/day from gestational day (GD) 6 to postnatal day (PND) 21, and then the weaned offspring continued receiving the same treatment till PND 28. We found that male pups treated with high-dose DBP showed enhancement in spatial acquisition in a Morris water maze during PNDs 30-33, and displayed better retention of spatial memory in a probe trial after a reverse trail during PNDs 60-62. Real-time PCR and western blotting analysis of the hippocampus from DBP-treated male rats on PND 21 revealed an increase in BDNF expression, compared to the vehicle-matched control. BDNF variant III, a transcription promoted by active CREB (i.e. p-CREB), as well as the immunocontent of p-CREB, was scarcely altered by the treatment. Our results suggest that developmental treatment with high-dose DBP improves spatial memory in male rats, and this effect may be related to an increase in BDNF expression in the hippocampus in a p-CREB independent route.

Characterization of peroxisome proliferator-activated receptor alpha--independent effects of PPARalpha activators in the rodent liver: di-(2-ethylhexyl) phthalate also activates the constitutive-activated receptor

Peroxisome proliferator chemicals (PPC) are thought to mediate their effects in rodents on hepatocyte growth and liver cancer through the nuclear receptor peroxisome proliferator-activated receptor (PPAR) alpha. Recent studies indicate that the plasticizer di-(2-ethylhexyl) phthalate (DEHP) increased the incidence of liver tumors in PPARalpha-null mice. We hypothesized that some PPC, including DEHP, induce transcriptional changes independent of PPARalpha but dependent on other nuclear receptors, including the constitutive-activated receptor (CAR) that mediates phenobarbital (PB) effects on hepatocyte growth and liver tumor induction. To determine the potential role of CAR in mediating effects of PPC, a meta-analysis was performed on transcript profiles from published studies in which rats and mice were exposed to PPC and compared the profiles to those produced by exposure to PB. Valproic acid, clofibrate, and DEHP in rat liver and DEHP in mouse liver induced genes, including Cyp2b family members that are known to be regulated by CAR. Examination of transcript changes by Affymetrix ST 1.0 arrays and reverse transcription-PCR in the livers of DEHP-treated wild-type, PPARalpha-null, and CAR-null mice demonstrated that (1) most (approximately 94%) of the transcriptional changes induced by DEHP were PPARalpha-dependent, (2) many PPARalpha-independent genes overlapped with those regulated by PB, (3) induction of genes Cyp2b10, Cyp3a11, and metallothionine-1 by DEHP was CAR dependent but PPARalpha-independent, and (4) induction of a number of genes (Cyp8b1, Gstm4, and Gstm7) was independent of both CAR and PPARalpha. Our results indicate that exposure to PPARalpha activators including DEHP leads to activation of multiple nuclear receptors in the rodent liver.

Regulation of sulfotransferase and UDP-glucuronosyltransferase gene expression by the PPARs

During phase II metabolism, a substrate is rendered more hydrophilic through the covalent attachment of an endogenous molecule. The cytosolic sulfotransferase (SULT) and UDP-glucuronosyltransferase (UGT) families of enzymes account for the majority of phase II metabolism in humans and animals. In general, phase II metabolism is considered to be a detoxication process, as sulfate and glucuronide conjugates are more amenable to excretion and elimination than are the parent substrates. However, certain products of phase II metabolism (e.g., unstable sulfate conjugates) are genotoxic. Members of the nuclear receptor superfamily are particularly important regulators of SULT and UGT gene transcription. In metabolically active tissues, increasing evidence supports a major role for lipid-sensing transcription factors, such as peroxisome proliferator-activated receptors (PPARs), in the regulation of rodent and human SULT and UGT gene expression. This review summarizes current information regarding the regulation of these two major classes of phase II metabolizing enzyme by PPARs.

A critical analysis of the biological impacts of plasticizers on wildlife

This review provides a critical analysis of the biological effects of the most widely used plasticizers, including dibutyl phthalate, diethylhexyl phthalate, dimethyl phthalate, butyl benzyl phthalate and bisphenol A (BPA), on wildlife, with a focus on annelids (both aquatic and terrestrial), molluscs, crustaceans, insects, fish and amphibians. Moreover, the paper provides novel data on the biological effects of some of these plasticizers in invertebrates, fish and amphibians. Phthalates and BPA have been shown to affect reproduction in all studied animal groups, to impair development in crustaceans and amphibians and to induce genetic aberrations. Molluscs, crustaceans and amphibians appear to be especially sensitive to these compounds, and biological effects are observed at environmentally relevant exposures in the low ng l(-1) to microg l(-1) range. In contrast, most effects in fish (except for disturbance in spermatogenesis) occur at higher concentrations. Most plasticizers appear to act by interfering with the functioning of various hormone systems, but some phthalates have wider pathways of disruption. Effect concentrations of plasticizers in laboratory experiments coincide with measured environmental concentrations, and thus there is a very real potential for effects of these chemicals on some wildlife populations. The most striking gaps in our current knowledge on the impacts of plasticizers on wildlife are the lack of data for long-term exposures to environmentally relevant concentrations and their ecotoxicity when part of complex mixtures. Furthermore, the hazard of plasticizers has been investigated in annelids, molluscs and arthropods only, and given the sensitivity of some invertebrates, effects assessments are warranted in other invertebrate phyla.

Gene Profiling in the Livers of Wild-type and PPARα-Null Mice Exposed to Perfluorooctanoic Acid

Health concerns have been raised because perfluorooctanoic acid (PFOA) is commonly found in the environment and can be detected in humans. In rodents, PFOA is a carcinogen and a developmental toxicant. PFOA is a peroxisome proliferator-activated receptor α (PPARα) activator; however, PFOA is capable of inducing heptomegaly in the PPARα-null mouse. To study the mechanism associated with PFOA toxicity, wild-type and PPARα-null mice were orally dosed for 7 days with PFOA (1 or 3 mg/kg) or the PPARα agonist Wy14,643 (50 mg/kg). Gene expression was evaluated using commercial microarrays. In wild-type mice, PFOA and Wy14,643 induced changes consistent with activation of PPARα. PFOA-treated wild-type mice deviated from Wy14,643-exposed mice with respect to genes involved in xenobiotic metabolism. In PFOA-treated null mice, changes were observed in transcripts related to fatty acid metabolism, inflammation, xenobiotic metabolism, and cell cycle regulation. Hence, a component of the PFOA response was found to be independent of PPARα. Although the signaling pathways responsible for these effects are not readily apparent, overlapping gene regulation by additional PPAR isoforms could account for changes related to fatty acid metabolism and inflammation, whereas regulation of xenobiotic metabolizing genes is suggestive of constitutive androstane receptor activation.

Key issues in the role of peroxisome proliferator-activated receptor agonism and cell signaling in trichloroethylene toxicity

Peroxisome proliferator-activated receptor alpha (PPARalpha) is thought to be involved in several different diseases, toxic responses, and receptor pathways. The U.S. Environmental Protection Agency 2001 draft trichloroethylene (TCE) risk assessment concluded that although PPAR may play a role in liver tumor induction, the role of its activation and the sequence of subsequent events important to tumorigenesis are not well defined, particularly because of uncertainties concerning the extraperoxisomal effects. In this article, which is part of a mini-monograph on key issues in the health risk assessment of TCE, we summarize some of the scientific literature published since that time on the effects and actions of PPARalpha that help inform and illustrate the key scientific questions relevant to TCE risk assessment. Recent analyses of the role of PPARalpha in gene expression changes caused by TCE and its metabolites provide only limited data for comparison with other PPARalpha agonists, particularly given the difficulties in interpreting results involving PPARalpha knockout mice. Moreover, the increase in data over the last 5 years from the broader literature on PPARalpha agonists presents a more complex array of extraperoxisomal effects and actions, suggesting the possibility that PPARalpha may be involved in modes of action (MOAs) not only for liver tumors but also for other effects of TCE and its metabolites. In summary, recent studies support the conclusion that determinations of the human relevance and susceptibility to PPARalpha-related MOA(s) of TCE-induced effects cannot rely on inferences regarding peroxisome proliferation per se and require a better understanding of the interplay of extraperoxisomal events after PPARalpha agonism.

Sex dimorphism in cardiac pathophysiology: experimental findings, hormonal mechanisms, and molecular mechanisms

The higher cardiovascular risk in men and post-menopausal women implies a protective action of estrogen. A large number of experimental studies have provided strong support to this concept. However, the recent clinical trials with negative outcomes regarding hormone replacement therapy call for "post hoc" reassessment of existing information, models, and research strategies as well as a summary of recent findings. Sex steroid hormones, in particular estrogen, regulate numerous processes that are related to the development and progression of cardiovascular disease through a variety of signaling pathways. Use of genetically modified models has resulted in interesting information on diverse actions mediated by steroid receptors. By focusing on experimental findings, we have reviewed hormonal, cellular, and signaling mechanisms responsible for sex dimorphism and actions of hormone replacement therapy and addressed current limitations and future directions of experimental research.

Di-n-butyl phthalate activates constitutive androstane receptor and pregnane X receptor and enhances the expression of steroid-metabolizing enzymes in the liver of rat fetuses

The plasticizer di-n-butyl phthalate (DBP) is a reproductive toxicant in rodents. Exposure to DBP in utero at high doses alters early reproductive development in male rats. Di-n-butyl phthalate also affects hepatic and extrahepatic enzymes. The objectives of this study were to determine the responsiveness of steroid-metabolizing enzymes in fetal liver to DBP and to investigate the potential of DBP to activate nuclear receptors that regulate the expression of liver enzymes. Pregnant Sprague-Dawley rats were orally dosed with DBP at levels of 10, 50, or 500 mg/kg/day from gestation days 12 to 19; maternal and fetal liver samples were collected on day 19 for analyses. Increased protein and mRNA levels of CYP 2B1, CYP 3A1, and CYP 4A1 were found in both maternal and fetal liver in the 500-mg dose group. Di-n-butyl phthalate at high doses also caused an increase in the mRNA of hepatic estrogen sulfotransferase and UDP-glucuronosyltransferase 2B1 in the dams but not in the fetuses. Xenobiotic induction of CYP3A1 and 2B1 is known to be mediated by the nuclear hormone receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR). In vitro transcriptional activation assays showed that DBP activates both PXR and CAR. The main DBP metabolite, mono-butyl-phthalate (MBP) did not interact strongly with either CAR or PXR. These data indicate that hepatic steroid- and xenobiotic-metabolizing enzymes are susceptible to DBP induction at the fetal stage; such effects on enzyme expression are likely mediated by xenobiotic-responsive transcriptional factors, including CAR and PXR. Our study shows that DBP is broadly reactive with multiple pathways involved in maintaining steroid and lipid homeostasis.

PPARalpha activators down-regulate CYP2C7, a retinoic acid and testosterone hydroxylase

Peroxisome proliferators (PP) are a large class of structurally diverse chemicals that mediate their effects in the liver mainly through the peroxisome proliferator-activated receptor alpha (PPARalpha). Exposure to PP results in down-regulation of CYP2C family members under control of growth hormone and sex steroids including CYP2C11 and CYP2C12. We hypothesized that PP exposure would also lead to similar changes in CYP2C7, a retinoic acid and testosterone hydroxylase. CYP2C7 gene expression was dramatically down-regulated in the livers of rats treated for 13 weeks by WY-14,643 (WY; 500 ppm) or gemfibrozil (GEM; 8000 ppm). In the same tissues, exposure to WY and GEM and to a lesser extent di-n-butyl phthalate (20,000 ppm) led to decreases in CYP2C7 protein levels in both male and female rats. An examination of the time and dose dependence of CYP2C7 protein changes after PP exposure revealed that CYP2C7 was more sensitive to compound exposure compared to other CYP2C family members. Protein expression was decreased after 1, 5 and 13 weeks of PP treatment. CYP2C7 protein expression was completely abolished at 5 ppm WY, the lowest dose tested. GEM and DBP exhibited dose-dependent decreases in CYP2C7 protein expression, becoming significant at 1000 ppm or 5000 ppm and above, respectively. These results show that PP exposure leads to changes in CYP2C7 mRNA and protein levels. Thus, in addition to known effects on steroid metabolism, exposure to PP may alter retinoic acid metabolism.

Peroxisome Proliferator-Activated Receptors: Mediators of Phthalate Ester-Induced Effects in the Male Reproductive Tract?

Many phthalate ester plasticizers are classified as peroxisome proliferators (PP), a large group of industrial and pharmaceutical chemicals. Like PP, exposure to some phthalates increases hepatocyte peroxisome and cellular proliferation, as well as the incidence of hepatocellular adenomas in mice and rats. Most effects of PP are mediated by three nuclear receptors called peroxisome proliferator-activated receptors (PPARalpha,beta,gamma). An obligate role for PPARalpha in PP-induced events leading to liver cancer is well-established. Exposure of rats in utero or in the neonate to a subset of phthalate esters causes profound, sometimes irreversible malformations in the male reproductive tract. We review here the data that supports or discounts roles for PPARs in phthalate-induced testis toxicity including (1) toxic effects of phthalates on the male reproductive tract, (2) expression of PPARs in the testis, (3) activation of PPARs by phthalates, (4) role of PPARalpha in testis toxicity, (5) gene targets of phthalates involved in steroid biosynthesis and catabolism, and (6) interactions between PPARs and other nuclear receptors that play roles in testis development and homeostasis. Critical research needs are identified that will help determine the significance of PPARs in phthalate-induced effects in the rat male reproductive tract and the relevance of toxicity to humans.