Activation of PPARalpha and PPARgamma by environmental phthalate monoesters

Effect of di(n-butyl) phthalate on testicular oxidative damage and antioxidant enzymes in hyperthyroid rats

This study compared the effects of di(n-butyl) phthalate (DBP) on the oxidative damage and antioxidant enzymes activity in testes of hyperthyroid rats. Hyperthyroidism was induced in pubertal male rats by intraperitoneal injection of triiodothyronine (T3, 10 microg/kg body weight) for 30 days. An oral dose of DBP (750 mg/kg) was administered simultaneously to normal or hyperthyroid (T3) rats over a 30-day period. No changes in body weight were observed in the hyperthyroid groups (T3, T3 + DBP) compared with controls. There were significantly higher serum T3 levels observed in the hyperthyroid rats than in the control, but the serum thyroid stimulating hormone levels were markedly lower in the hyperthyroid rats. DBP significantly decreased the weight of the testes in the normal (DBP) and hyperthyroid (T3 + DBP) groups. The serum testosterone concentrations were significantly lower in only DBP group. DBP significantly increased the 8-hydroxy-2-deoxyguanosine (8-OHdG) level in the testes, whereas the DBP-induced 8-OHdG levels were slightly higher in T3 + DBP group. Superoxide dismutase and glutathione peroxidase activities were significantly higher in the testes of the DBP or T3 + DBP groups. Catalase (CAT) activity was significantly higher in the DBP treatment group, but the T3 + DBP group showed slightly lower DBP-induced CAT activity. The testicular expression of thyroid hormone receptor alpha-1 (TRalpha-1) was significantly higher in the DBP groups, and androgen receptor (AR) expression was not detected in the DBP treatment group. In addition, DBP significantly increased the peroxisome proliferator-activated receptor-r (PPAR-r) levels in the testis. These results suggest that hyperthyroidism can cause a change in the expression level of PPAR-r in testes, and may increase the levels of oxidative damage induced by the metabolic activation of DBP.

Perturbed nuclear receptor signaling by environmental obesogens as emerging factors in the obesity crisis

The modern world is plagued with expanding epidemics of diseases related to metabolic dysfunction. The factors that are driving obesity, diabetes, cardiovascular disease, hypertension, and dyslipidemias (collectively termed metabolic syndrome) are usually ascribed to a mismatch between the body's homeostatic nutrient requirements and dietary excess, coupled with insufficient exercise. The environmental obesogen hypothesis proposes that exposure to a toxic chemical burden is superimposed on these conditions to initiate or exacerbate the development of obesity and its associated health consequences. Recent studies have proposed a first set of candidate obesogens (diethylstilbestrol, bisphenol A, phthalates and organotins among others) that target nuclear hormone receptor signaling pathways (sex steroid, RXR-PPARgamma and GR) with relevance to adipocyte biology and the developmental origins of health and disease (DOHaD). Perturbed nuclear receptor signaling can alter adipocyte proliferation, differentiation or modulate systemic homeostatic controls, leading to long-term consequences that may be magnified if disruption occurs during sensitive periods during fetal or early childhood development.

Chronic toxicity of diethyl phthalate-A three generation lactational and gestational exposure study on male Wistar rats

Diethyl phthalate (DEP) is widely used in the perfume industry as a vehicle for fragrances and in personal care products making human exposure of DEP significant to adults as well as neonatals, as confirmed by levels recorded in blood as well as breast milk samples of human populations in some parts of the world. Therefore, a study was undertaken to understand the toxic effect of DEP over three generations in male Wistar rats. Healthy male and female albino rats of Wistar strain weighing 75-100g (6-7 weeks old) were randomly assigned to two groups of six each. Group I (Control) male and female rats were fed on normal diet and water ad libitum. Group II (DEP) male and female rats were given DEP dissolved in corn oil mixed with the diet at 50mg/kg of the diet/day. Hundred days after the treatment, females were mated with males for 10 days. Exposure to DEP was continued throughout mating, gestation until termination at weaning, which was 150 days of total treatment period of the parental generation. The F1 and F2 generation pups were then segregated on the basis of their sex and six male and female pups of both generations were allowed to grow till they were 75-100g in weight. The treatment was then carried out similar to the parental generation but with reduced dose of 25mg/kg of the diet/day for F1 generation and 10mg/kg of the diet/day for F2 generation. Hundred days after the treatment, females were mated with males for 10 days. Exposure to DEP was continued throughout mating, gestation (21 days) until termination at weaning (21 days), which was 150 days of total treatment period of the F1 and F2 generation. Liver and serum ALT, AST and triglycerides were significantly increased over the three generations, which was much more significant in the F2 generation DEP treated group. The serum cholesterol and liver glutathione and glutathione reductase showed a significant decrease over the three generations, which was much more significant in the F2 generation DEP treated group as compared to the parental and F1 generation DEP treated rats. Histology of the liver showed remarkably enhanced fatty degeneration in the F2 generation DEP treated rats as compared to parental and F1 generation DEP treated rats. Vacuolations were much more significant in the F1 generation DEP treated rats as compared to the controls and F2 generation DEP treated rats. It can be concluded from this study, that continuous exposure through food, gestation and lactation over three generation's inspite of dose reduction of DEP leads to an enhanced toxic effect in the latter generations.

The endocrine disruptor monoethyl-hexyl-phthalate is a selective peroxisome proliferator-activated receptor gamma modulator that promotes adipogenesis

The ability of pollutants to affect human health is a major concern, justified by the wide demonstration that reproductive functions are altered by endocrine disrupting chemicals. The definition of endocrine disruption is today extended to broader endocrine regulations, and includes activation of metabolic sensors, such as the peroxisome proliferator-activated receptors (PPARs). Toxicology approaches have demonstrated that phthalate plasticizers can directly influence PPAR activity. What is now missing is a detailed molecular understanding of the fundamental basis of endocrine disrupting chemical interference with PPAR signaling. We thus performed structural and functional analyses that demonstrate how monoethyl-hexyl-phthalate (MEHP) directly activates PPARgamma and promotes adipogenesis, albeit to a lower extent than the full agonist rosiglitazone. Importantly, we demonstrate that MEHP induces a selective activation of different PPARgamma target genes. Chromatin immunoprecipitation and fluorescence microscopy in living cells reveal that this selective activity correlates with the recruitment of a specific subset of PPARgamma coregulators that includes Med1 and PGC-1alpha, but not p300 and SRC-1. These results highlight some key mechanisms in metabolic disruption but are also instrumental in the context of selective PPAR modulation, a promising field for new therapeutic development based on PPAR modulation.

Adjuvant effects of inhaled mono-2-ethylhexyl phthalate in BALB/cJ mice

Phthalates, including di(2-ethylhexyl) phthalate (DEHP), are widely used and have been linked with the development of wheezing and asthma. The main metabolite of DEHP, mono-2-ethylhexyl phthalate (MEHP), was investigated for adjuvant effects in a mouse inhalation model. BALB/cJ mice were exposed to aerosols of 0.03 or 0.4 mg/m(3) MEHP 5 days/week for 2 weeks and thereafter weekly for 12 weeks together with a low dose of ovalbumin (OVA) as a model allergen. Mice exposed to OVA alone or OVA+Al(OH)(3) served as negative and positive controls, respectively. Finally, all groups were exposed to a nebulized 1% OVA solution on 3 consecutive days to investigate the development of an inflammatory response. Serum, bronchoalveolar lavage (BAL) fluid, and draining lymph nodes were collected 24h later. In the OVA+Al(OH)(3) group, significantly increased levels of OVA-specific IgE and IgG1 in serum as well as of eosinophils in BAL fluid were observed. OVA-specific IgG1 production in both MEHP groups was significantly increased. OVA-specific IgE and IgG2a were not increased significantly. A dose-dependent increase in inflammatory cells was observed in BAL fluid, leading to significantly higher lymphocyte and eosinophil numbers in the OVA+0.4 mg/m(3) MEHP group. Ex vivo cytokine secretion by cultures of draining lymph nodes suggested a T(H)2 profile of MEHP. In conclusion, MEHP acted as a T(H)2 adjuvant after inhalation. However, it is suggested that the inflammation in the MEHP groups was primarily mediated by an IgG1-dependent mechanism. To address implications for humans, a margin-of-exposure was estimated based on the lack of significant effects on IgE production and inflammation after exposures to 0.03 mg/m(3) MEHP observed in the present study and estimated human exposure levels.

Dietary di(2-ethylhexyl)phthalate-impaired glucose metabolism in experimental animals

The effects of chronic intake of di(2-ethylhexyl)phthalate (DEHP) on the main intermediate glycolytic metabolites in liver and gastrocnemius muscle were investigated in experimental animals. Male Wistar rats (90-100 g) were fed for 21 days either with a standard chow or the same diet supplemented with 2% (w/w) of DEHP. The DEHP-fed rats had an altered in vivo glucose tolerance associated with abnormal glucose intermediate metabolite contents in liver and skeletal muscle. In these rats, the hepatic content of glucose-6-phosphate (G-6-P), fructose-6-phosphate, pyruvate, lactate, glucose-1-phosphate and glycogen decreased. At the same time, the G-6-P content decreased while the pyruvate and lactate levels increased in skeletal muscle. These data, along with the high plasma glucose concentration and the normal lactate blood levels of this group, could indicate that DEHP-fed rats could present a deficiency in muscle glucose and lactate transport, a reduction of the flux through muscle hexokinase and hepatic glucokinase, and a reduction in glycogen synth-

Modes of action and species-specific effects of di-(2-ethylhexyl)phthalate in the liver

The industrial plasticizer di-(2-ethylhexyl)phthalate (DEHP) is used in manufacturing of a wide variety of polyvinyl chloride (PVC)-containing medical and consumer products. DEHP belongs to a class of chemicals known as peroxisome proliferators (PPs). PPs are a structurally diverse group of compounds that share many (but perhaps not all) biological effects and are characterized as non-genotoxic rodent carcinogens. This review focuses on the effect of DEHP in liver, a primary target organ for the pleiotropic effects of DEHP and other PPs. Specifically, liver parenchymal cells, identified herein as hepatocytes, are a major cell type that are responsive to exposure to PPs, including DEHP; however, other cell types in the liver may also play a role. The PP-induced increase in the number and size of peroxisomes in hepatocytes, so called 'peroxisome proliferation' that results in elevation of fatty acid metabolism, is a hallmark response to these compounds in the liver. A link between peroxisome proliferation and tumor formation has been a predominant, albeit questioned, theory to explain the cause of a hepatocarcinogenic effect of PPs. Other molecular events, such as induction of cell proliferation, decreased apoptosis, oxidative DNA damage, and selective clonal expansion of the initiated cells have been also been proposed to be critically involved in PP-induced carcinogenesis in liver. Considerable differences in the metabolism and molecular changes induced by DEHP in the liver, most predominantly the activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)alpha, have been identified between species. Both sexes of rats and mice develop adenomas and carcinomas after prolonged feeding with DEHP; however, limited DEHP-specific human data are available, even though exposure to DEHP and other phthalates is common in the general population. This likely constitutes the largest gap in our knowledge on the potential for DEHP to cause liver cancer in humans. Overall, it is believed that the sequence of key events that are relevant to DEHP-induced liver carcinogenesis in rodents involves the following events whereby the combination of the molecular signals and multiple pathways, rather than a single hallmark event (such as induction of PPARalpha and peroxisomal genes, or cell proliferation) contribute to the formation of tumors: (i) rapid metabolism of the parental compound to primary and secondary bioactive metabolites that are readily absorbed and distributed throughout the body; (ii) receptor-independent activation of hepatic macrophages and production of oxidants; (iii) activation of PPARalpha in hepatocytes and sustained increase in expression of peroxisomal and non-peroxisomal metabolism-related genes; (iv) enlargement of many hepatocellular organelles (peroxisomes, mitochondria, etc.); (v) rapid but transient increase in cell proliferation, and a decrease in apoptosis; (vi) sustained hepatomegaly; (vii) chronic low-level oxidative stress and accumulation of DNA damage; (viii) selective clonal expansion of the initiated cells; (ix) appearance of the pre-neoplastic nodules; (x) development of adenomas and carcinomas.

Phthalate exposure and male infertility

Phthalates have been used as additives in industrial products since the 1930s, and are universally considered to be ubiquitous environmental contaminants. The general population is exposed to phthalates through consumer products, as well as diet and medical treatments. Animal studies showing the existence of an association between some phthalates and testicular toxicity have generated public and scientific concern about the potential adverse effects of environmental changes on male reproductive health. In particular, prenatal exposure to phthalates seems to play a relevant role in determining these adverse effects given that human exposure has been demonstrated to begin during the intrauterine life. Unprecedented declines in fertility rates and semen quality of antenatal origin have been reported during the last half of the 20th century in developed countries and increasing interest exists on the potential relationship between exposure to environmental contaminants, including phthalates, and human male reproductive health. Here we review the data that support or discounts the evidence existing to date linking phthalate exposure and the decline of human male fertility, especially in developed countries.

Chronic toxicity of diethyl phthalate in male Wistar rats—A dose–response study

Diethyl phthalate (DEP) is widely used in personal care products, plastics and medical devices at various concentrations, but its information is limited on its toxicity associated with exposure at high as well as low doses for a prolonged period. Therefore, a study was undertaken to understand the dose-response toxic effect of DEP in male Wistar rats. Control rats were fed on normal diet and water ad libitum. Rats were given DEP dissolved individually in corn oil mixed with the diet at 10, 25 and 50 mg/kg of the diet/day, which is equal to 0.57, 1.425 and 2.85 mg/kg body wt/day. After 5 months of treatment animals were sacrificed, enzymes and other biochemical parameters in the serum and liver were assessed. Liver weight to body weight ratio showed a significant increase only in 10 ppm DEP treated rats. A significant increase was observed in the serum ACP, LDH, ALT enzyme levels of 10 mg/kg treated rats as compared to control, 25 and 50 mg/kg treated rats. Other biochemical parameters like glycogen, total cholesterol, total triglycerides and lipid peroxidation were also increased in the liver of all the three treated groups. In the 10 and 50 mg/kg diet/day treated rats, there was a significant decrease in liver total GSH as compared to controls and 25 mg/kg treated rats. Histology of liver showed severe vacuolations, fatty degeneration and loss of hepatic architecture in the 10mg/kg treated rats, whereas in the 25 and 50 mg/kg treated rats only loss of hepatic architecture and granular deposits in the hepatocytes was predominant. Histology of liver by electron micrographs showed a significant dose-dependent proliferation of mitochondria in the hepatocytes, while the 10mg/kg treated rats showed increased number of peroxisomes in the hepatocytes. It is evident from this study that treatment with higher concentrations of DEP results in mitochondrial proliferation as well as accumulation of glycogen, cholesterol and triglycerides within the liver, but exposure to lower concentrations for longer periods results in increase in peroxisome numbers leading to severe hepatocellular changes which can be confirmed by significantly increased liver weights, elevated enzyme levels in the serum and liver and impaired metabolism of glycogen, cholesterol and triglyceride as well as altered liver histology.

The biological actions of dehydroepiandrosterone involves multiple receptors

Dehydroepiandrosterone has been thought to have physiological functions other than as an androgen precursor. The previous studies performed have demonstrated a number of biological effects in rodents, such as amelioration of disease in diabetic, chemical carcinogenesis, and obesity models. To date, activation of the peroxisome proliferators activated receptor alpha, pregnane X receptor, and estrogen receptor by DHEA and its metabolites have been demonstrated. Several membrane-associated receptors have also been elucidated leading to additional mechanisms by which DHEA may exert its biological effects. This review will provide an overview of the receptor multiplicity involved in the biological activity of this sterol.

Development of a toxicogenomics in vitro assay for the efficient characterization of compounds

In vitro toxicogenomics represents a useful approach for evaluating the toxic properties of new drug candidates early in the drug discovery process using minimal amounts of compounds. The aim of this study was to develop in vitro-based gene expression assays for two prototypical toxicological classes: aryl hydrocarbon receptor (AhR) agonists and peroxisome proliferator activated receptor alpha (PPAR alpha) agonists. Primary rat hepatocytes were exposed to a number of class-specific compounds, including 3-methylcholanthrene, aroclor, and beta-napthoflavone as AhR agonists, bezafibrate, clofibrate, and Wy-14643 as peroxisome proliferators, and chlorpheniramine, penicillin and spectinomycin as negative controls. Global gene expression profiles were generated with microarrays for each class of compounds. Using linear discriminant analysis coupled with permutation-based t-test, gene signatures were established to classify compounds according to a discriminant score. The final gene signatures consist of eight genes for AhR agonism and 11 genes for PPAR alpha agonism, and were further validated using additional compounds. The assay was initially developed using a microarray platform. The authors then evaluated whether it could be transferred to a more cost-effective platform with higher throughput. The results indicate that a small set of genes can be used to quantitatively assess the degree to which a compound falls into a certain mechanistic toxicological class. While this study only focused on two classes, it could be expanded to encompass other toxicological mechanistic classes as well. Furthermore, by adapting this type of assay to a higher throughput platform, in vitro toxicogenomics can represent an effective approach to generate robust toxicological data early in the drug discovery process.

Possible impact of phthalates on infant reproductive health

Phthalates adversely affect the male reproductive system in animals, inducing hypospadias, cryptorchidism, reduced testosterone production and decreased sperm counts. Phthalate effects are much more severe after in utero than adult exposure. Little is known about human health effects. This study discusses two recent studies on perinatal phthalate exposure, which indicated that human testicular development might be susceptible to phthalates. One study analysed phthalate monoesters in breast milk and reproductive hormone levels in infants. Five of six phthalates [monoethyl-(MEP), monobutyl- (MBP), monomethyl- (MMP), mono-2-ethylhexyl- (MEHP) and mono-isononyl phthalate (MiNP)] showed correlation with hormone levels in healthy boys, which were indicative of lower androgen activity and reduced Leydig cell function. MEP and MBP were positively correlated with serum sex hormone-binding globulin (SHBG) levels. MMP, MEP, MBP, MEHP and MiNP were positively correlated with the LH/testosterone ratio. Another study found a reduction of the anogenital index (AGI) in infant boys with increasing levels of MBP, MEP, monobenzyl- and mono-isobutyl phthalate in maternal urine samples during late-pregnancy. Boys with small AGI showed a high prevalence of cryptorchidism and small genital size. Taken together these studies suggest an antivirilizing effect of phthalates in infants. Most of these findings are in line with animal observations. However, the possible effects of MEP appear to be limited to humans. This may be due to differences in exposure routes (inhalation and dermal absorption which circumvents liver detoxification in addition to oral) and metabolism, or this association could be spurious. As phthalates are produced as bulk chemicals worldwide, these new findings raise concern about the safety of phthalate exposure for pregnant women and infants.

The Toxicology of Ligands for Peroxisome Proliferator-Activated Receptors (PPAR)

Peroxisome proliferator-activated receptors (PPARs) are ligand activated transcription factors that modulate target gene expression in response to endogenous and exogenous ligands. Ligands for the PPARs have been widely developed for the treatment of various diseases including dyslipidemias and diabetes. While targeting selective receptor activation is an established therapeutic approach for the treatment of various diseases, a variety of toxicities are known to occur in response to ligand administration. Whether PPAR ligands produce toxicity via a receptor-dependent and/or off-target-mediated mechanism(s) is not always known. Extrapolation of data derived from animal models and/or in vitro models, to humans, is also questionable. The different toxicities and mechanisms associated with administration of ligands for the three PPARs will be discussed, and important data gaps that could increase our current understanding of how PPAR ligands lead to toxicity will be highlighted.

Mono(2-ethylhexyl)phthalate and mono-n-butyl phthalate activation of peroxisome proliferator activated-receptors alpha and gamma in breast

The phthalates di(2-ethylhexyl)phthalate (DEHP) and di-n-butyl phthalate (DBP) are environmental contaminants with significant human exposures. Both compounds are known reproductive toxins in rodents and DEHP also induces rodent hepatocarcinogenesis in a process believed to be mediated via the peroxisome proliferator-activated receptor alpha (PPARalpha). DEHP and DBP are metabolised to their respective monoesters, mono-(2-ethylhexyl)phthalate (MEHP) and mono-n-butyl phthalate (MBP), which are the active metabolites. MEHP also activates another member of the PPAR subfamily, PPARgamma. The effects of PPARalpha and PPARgamma activation in human breast cells appears to be opposing; PPARalpha activators in breast cells cause an increase in proliferation, while PPARgamma activation in breast cells is associated with differentiation and an inhibition of cell proliferation. Further to this the activation of the PPARs is cell and ligand specific, suggesting the importance of examining the effect of MEHP and MBP on the activation of PPARalpha, PPARbeta and PPARgamma in human breast. We used the common model of human breast cancer MCF-7 and examined the ability of MEHP and MBP to activate human PPARs in this system. The ability of MBP and MEHP to block PPAR responses was also assessed. We found that both human PPARalpha and PPARgamma were activated by MEHP whereas MEHP could not activate PPARbeta. MBP was unable to activate any PPAR isoforms in this breast model, despite being a weak peroxisome proliferator in liver, although MBP was an antagonist for both PPARgamma and PPARbeta. Our results suggest that the toxicological consequences of MEHP in the breast could be complex given the opposing effects of PPARalpha and PPARgamma in human breast cells.

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.

Role of PPARα in mediating the effects of phthalates and metabolites in the liver

Phthalate esters belong to a large class of compounds known as peroxisome proliferators (PP). PP include chemicals that activate different subtypes of the peroxisome proliferator-activated receptor (PPAR) family. The ability of phthalate esters and their metabolites to activate responses through different PPAR subtypes is not fully characterized. We investigated the ability of two phthalate esters di-(2-ethylhexyl) phthalate (DEHP) and di-n-butyl phthalate (DBP) and selected metabolites to activate PPAR (alpha, beta/delta, gamma) using a transient transfection assay. The monoester of DEHP, mono-(2-ethylhexyl) phthalate (MEHP) activated all three subtypes of PPAR, but preferentially activated PPARalpha. A second metabolite of DEHP, 2-ethylhexanoic acid (2-EHXA) was a weaker activator of all three subtypes. DBP, but not the primary metabolite mono-n-butyl phthalate weakly activated all three PPAR subtypes. MEHP and DBP but not DEHP and MBP interacted directly with human PPARalpha and PPARgamma as determined by scintillation proximity assays. Both DEHP and DBP activated expression of PP-inducible gene products in wild-type but not PPARalpha-null mice suggesting that both of these phthalates exert their effects by activation of PPARalpha in vivo. The preferential activation of PPARalpha by phthalate ester metabolites suggests that these phthalates mediate their toxic effects in rodent liver in a manner indistinguishable from other PP.

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.

Environmental and Endogenous Peroxisome Proliferator-Activated Receptor γ Agonists Induce Bone Marrow B Cell Growth Arrest and Apoptosis: Interactions between Mono(2-ethylhexyl)phthalate, 9-cis-Retinoic Acid, and 15-Deoxy-Δ12,14-prostaglandin J2

The common commercial use of phthalate esters has resulted in significant human exposure to these bioactive compounds. The facts that phthalate ester metabolites, like endogenous PGs, are peroxisome proliferator-activated receptor (PPAR) agonists, and that PPARgamma agonists induce lymphocyte apoptosis suggest that phthalate esters are immunosuppressants that could act together with PGs to modulate early B cell development. In this study we examined the effects of a metabolite of one environmental phthalate, mono(2-ethylhexyl)phthalate (MEHP), and 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)), on developing B cells. MEHP inhibited [(3)H]thymidine incorporation by primary murine bone marrow B cells and a nontransformed murine pro/pre-B cell line (BU-11). Cotreatment with a retinoid X receptor alpha ligand, 9-cis-retinoic acid, decreased [(3)H]thymidine incorporation synergistically, thereby implicating activation of a PPARgamma-retinoid X receptor alpha complex. These results were similar to those obtained with the natural PPARgamma ligand 15d-PGJ(2). At moderate MEHP concentrations (25 or 100 microM for primary pro-B cells and a pro/pre-B cell line, respectively), inhibition of [(3)H]thymidine incorporation resulted primarily from apoptosis induction, whereas at lower concentrations, the inhibition probably reflected growth arrest without apoptosis. Cotreatment of bone marrow B cells with 15d-PGJ(2) and MEHP significantly enhanced the inhibition of [(3)H]thymidine incorporation seen with MEHP alone, potentially mimicking exposure in the bone marrow microenvironment where PG concentrations are high. Finally, MEHP- and 15d-PGJ(2)-induced death does not result from a decrease in NF-kappaB activation. These data demonstrate that environmental phthalates can cooperate with an endogenous ligand, 15d-PGJ(2), to inhibit proliferation of and induce apoptosis in developing bone marrow B cells, potentially via PPARgamma activation.

Suppression by phthalates of the calcium signaling of human nicotinic acetylcholine receptors in human neuroblastoma SH-SY5Y cells

Phthalates are widely used in industry and cause public concern since they have genomic estrogenic-like effects via estrogen receptors. We previously found that some phthalates have nongenomic effects, exerting inhibitory effects on the functional activities of nicotinic acetylcholine receptors (nAChRs) in bovine chromaffin cells. In this study, we investigated the effects of eight phthalates on the calcium signaling of human nAChR by using human neuroblastoma SH-SY5Y cells. All eight phthalates, with different potency, have inhibitory roles on the calcium signaling coupled with human nAChR, but not muscarinic acetylcholine receptors (mAChRs). For inhibition of human nAChR, the strongest to weakest potencies were observed as di-n-pentyl phthalate (DPP) --> butyl benzyl phthalate (BBP) --> di-n-butyl phthalate (DBP) --> dicyclohexyl phthalate (DCHP) --> di-n-hexyl phthalate (DHP) --> di-(2-ethyl hexyl) phthalate (DEHP) --> di-n-propyl phthalate (DPrP) --> diethyl phthalate (DEP). The potencies of phthalates were associated with their structures such that the most effective ones had dialkyl group carbon numbers of C4 or C5, with shorter or longer numbers resulting in decreased potency. At as low as 0.1 microM, DPP, DBP, BBP, DCHP and DHP significantly inhibited the calcium signaling of human nAChR. The IC50 of phthalates on human nAChR, ranging from 0.32 to 7.96 microM, were 10-50 lower than those for bovine nAChR. We suggest that some phthalates effectively inhibit the calcium signaling of human nAChR, and these nongenomic effects are cause for concern.

Down-regulation of STAT5b transcriptional activity by ligand-activated peroxisome proliferator-activated receptor (PPAR) alpha and PPARgamma

The nuclear receptor peroxisome proliferator-activated receptor (PPAR) is activated by a diverse group of acidic ligands, including many peroxisome proliferator chemicals present in the environment. Janus tyrosine kinase-signal transducer and activator of transcription (JAK-STAT) signaling is activated by multiple cytokines and hormones and leads to the translocation of dimerized STAT proteins to the nucleus where they activate transcription of target genes. Previous studies have shown that growth hormone (GH)-activated STAT5b can inhibit PPAR-regulated transcription. Here, we show that this inhibitory cross-talk is mutual, and that GH-induced, STAT5b-dependent beta-casein promoter-luciferase reporter gene transcription can be inhibited up to approximately 80% by ligand-activated PPARalpha or PPARgamma. Dose-response experiments showed a direct relationship between the extent of PPAR activation and the degree of inhibition of STAT5-regulated transcription. PPAR did not block STAT5b tyrosine phosphorylation or inhibit DNA-binding activity. Both PPARs inhibited the transcriptional activity of a constitutively active STAT5b mutant, indicating that inhibition occurs downstream of the GH-stimulated STAT5 activation step. Transcriptionally inactive, dominant-negative PPAR mutants did not block STAT5b inhibition by wild-type PPAR, indicating that PPAR target gene transcription is not required. PPARalpha retained its STAT5b inhibitory activity in the presence of the histone deacetylase inhibitor trichostatin, indicating that enhanced histone deacetylase recruitment does not contribute to STAT5b inhibition. PPARalpha lacking the ligand-independent AF-1 trans-activation domain failed to inhibit STAT5b, highlighting the importance of the AF-1 region in STAT5-PPAR inhibitory cross-talk. These findings demonstrate the bidirectionality of cross-talk between the PPAR and STAT pathways and provide a mechanism whereby exposure to environmental chemical activators of PPAR can suppress expression of GH target genes.