Analysis of combined effects of nonylphenol and Monobutyl phthalate on rat Sertoli cells applying two mathematical models

Impact of endocrine disrupting chemicals and pharmaceuticals on Sertoli cell development and functions

Sertoli cells play essential roles in male reproduction, from supporting fetal testis development to nurturing male germ cells from fetal life to adulthood. Dysregulating Sertoli cell functions can have lifelong adverse effects by jeopardizing early processes such as testis organogenesis, and long-lasting processes such as spermatogenesis. Exposure to endocrine disrupting chemicals (EDCs) is recognized as contributing to the rising incidence of male reproductive disorders and decreasing sperm counts and quality in humans. Some drugs also act as endocrine disruptors by exerting off-target effects on endocrine tissues. However, the mechanisms of toxicity of these compounds on male reproduction at doses compatible with human exposure are still not fully resolved, especially in the case of mixtures, which remain understudied. This review presents first an overview of the mechanisms regulating Sertoli cell development, maintenance, and functions, and then surveys what is known on the impact of EDCs and drugs on immature Sertoli cells, including individual compounds and mixtures, and pinpointing at knowledge gaps. Performing more studies on the impact of mixtures of EDCs and drugs at all ages is crucial to fully understand the adverse outcomes these chemicals may induce on the reproductive system.

Dibutyl phthalate promotes juvenile Sertoli cell proliferation by decreasing the levels of the E3 ubiquitin ligase Pellino 2

BACKGROUND

A previous study showed that dibutyl phthalate (DBP) exposure disrupted the growth of testicular Sertoli cells (SCs). In the present study, we aimed to investigate the potential mechanism by which DBP promotes juvenile SC proliferation in vivo and in vitro.

METHODS

Timed pregnant BALB/c mice were exposed to vehicle, or DBP (50, 250, and 500 mg/kg/day) from 12.5 days of gestation until delivery. In vitro, CCK-8 and EdU incorporation assays were performed to determine the effect of monobutyl phthalate (MBP), the active metabolite of DBP, on the proliferation of TM4 cells, which are a juvenile testicular SC cell line. Western blotting analysis, quantitative PCR (q-PCR), and flow cytometry were performed to analyse the expression of genes and proteins related to the proliferation and apoptosis of TM4 cells. Coimmunoprecipitation was used to determine the relationship between the ubiquitination of interleukin 1 receptor-associated kinase 1 (IRAK1) and the effect of MBP on promoting the proliferation of TM4 cells.

RESULTS

In the 50 mg/kg/day DBP-exposed male mice offspring, the number of SCs was significantly increased. Consistent with the in vivo results, in vitro experiments revealed that 0.1 mM MBP treatment promoted the proliferation of TM4 cells. Furthermore, the data showed that 0.1 mM MBP-mediated downregulation of the E3 ubiquitin ligase Pellino 2 (Peli2) increased ubiquitination of IRAK1 by K63, which activated MAPK/JNK signalling, leading to the proliferation of TM4 cells.

CONCLUSIONS

Prenatal exposure to DBP led to abnormal proliferation of SCs in prepubertal mice by affecting ubiquitination of the key proliferation-related protein IRAK1 via downregulation of Peli2.

Di-n-butyl phthalate, butylbenzyl phthalate, and their metabolites exhibit different apoptotic potential in human peripheral blood mononuclear cells

Human peripheral blood mononuclear cells (PBMCs) are one of the main cell models used in studies concerning the exposure of humans (in vitro) to various chemical substances. Changes in PBMCs may reflect the general reaction of the organism regarding the effect of xenobiotics. The aim of this work was to evaluate the effect of di-n-butyl phthalate (DBP), butylbenzyl phthalate (BBP) and their metabolites: mono-n-butylphthalate (MBP), mono-benzylphthalate (MBzP) upon the induction of apoptosis in human peripheral blood mononuclear cells in vitro. PBMCs were incubated with the studied compounds at concentrations from 1 to 100 μg/mL for 12 h and/or 24 h. In order to clarify the mechanism of phthalates-induced programmed cell death, the changes in the calcium ions (Ca²⁺) level, alterations in the transmembrane mitochondrial potential (ΔѰm) and caspase-8, -9, -3 activity as well as externalization of phosphatidylserine have been determined. An increased Ca²⁺ level and a reduction of the ΔѰm were observed in PBMCs incubated with all of the studied compounds, and particularly with DBP and BBP. Phthalates caused an increase of caspases activity. The most pronounced increase was observed for caspase -9. The most pronounced pro-apoptotic changes were caused by DBP followed by BBP and then by their metabolites.

Use of the Adverse Outcome Pathway (AOP) framework to evaluate species concordance and human relevance of Dibutyl phthalate (DBP)-induced male reproductive toxicity

Dibutyl phthalate (DBP) is a phthalate ester used as a plasticizer, and solvent. Studies using rats consistently report that DBP exposure disrupts normal development of the male reproductive system in part via inhibition of androgen synthesis. However, studies using xenograft models report that in human fetal testis DBP exposure is unlikely to impair testosterone synthesis. These results question the validity of the rat model for assessment of male reproductive effects caused by DBP. The Adverse Outcome Pathway (AOP) framework was used to evaluate the available evidence for DBP-induced toxicity to the male reproductive system. Three relevant biological elements were identified: 1) fetal rats are more sensitive than other rodents and human fetal xenografts to DBP-induced anti-androgenic effects, 2) DBP-induced androgen-independent adverse outcomes are conserved amongst different mammalian models and human fetal testis xenografts, and 3) DBP-induced anti-androgenic effects are conserved in different mammalian species when exposure occurs during postnatal life stages.

Effects of combined exposure of adult male mice to di-(2-ethylexyl)phthalate and nonylphenol on behavioral and neuroendocrine responses

The present study evaluates the effects of adult exposure to low doses of a mixture of di-(2-ethylexyl)phthalate (DEHP) and nonylphenol (NP) on reproductive neuroendocrine function and behavior. The neural circuitry that processes male sexual behavior is tightly regulated by testosterone and its neural metabolite estradiol. In previous studies, we showed that adult exposure of mice to low doses of each of these widespread environmental contaminants resulted in altered sexual behavior, without any effect on the regulation of the gonadotropic axis. Here, adult C57BL/6J male mice were exposed to DEHP/NP (0.5 or 5 μg/kg body weight/day) for 4 weeks before starting the analyses. Mice treated with DEHP/NP at 0.5 μg/kg/day show altered olfactory preference, and fewer of them emit ultrasonic vocalization compared to the other treatment groups. These mice also exhibit a lower number of mounts and thrusts, increased locomotor activity and unaffected anxiety-state level, along with unaltered testosterone levels and kisspeptin system, a key regulator of the gonadotropic axis. Analysis of the neural circuitry that underlies sexual behavior showed that the number of cells expressing androgen and estrogen receptors is comparable between control and DEHP/NP-exposed males. The comparison of these data with those obtained in males exposed to each molecule separately highlights synergistic effects at the lower dose of contaminants of 0.5 μg/kg/day. In contrast, the effects previously observed for each molecule at 5 μg/kg/day were not detected. A detailed comparison of the effects triggered by separate or combined exposure to DEHP and NP is discussed.

Endocrine-disrupting metabolites of alkylphenol ethoxylates - A critical review of analytical methods, environmental occurrences, toxicity, and regulation

Despite the fact that metabolites of alkylphenol ethoxylates (APEO) are classified as hazardous substances, they continue to be released into the environment from a variety of sources and are not usually monitored. Their wide use has led to an increase in the possible exposure pathways for humans, which is cause for alarm. Moreover, there is a lack of knowledge about the behaviour of these metabolites with respect to the environment and toxicity, and their biological effects on human health. The aim of this work is to give an overview of the APEO metabolites and their analysis, occurrences and toxicity in various environmental and human samples. APEO metabolites have adverse effects on humans, wildlife, and the environment through their release into the environment. Currently, there are some reviews available on the behaviour of alkylphenols in soil, sediments, groundwater, surface water and food. However, none of these articles consider their toxicity in humans and especially their effect on the nervous and immune system. This work summarises the environmental occurrences of metabolites of APEOs in matrices, e.g. water, food and biological matrices, their effect on the immune and nervous systems, and isomer-specific issues. With that emphasis we are able to cover most common occurrences of human exposure, whether direct or indirect.

MiR-301b-3p/3584-5p enhances low-dose mono-n-butyl phthalate (MBP)-induced proliferation by targeting Rasd1 in Sertoli cells

To investigate the possible molecular mechanism of low concentration plasticizer mono-n-butyl phthalate (MBP) -induced juvenile Sertoli cells (SCs) proliferation, we evaluated global alterations of miRNA and mRNA expression in rat SCs treated with 0.1mM MBP. Microarray analysis revealed that miR-3584-5p and miR-301b-3p were up-regulated and their common target gene Dexamethasone-induced Ras-related protein 1 (Rasd1) was down-regulated. Further work suggested that SCs proliferation induced by low concentration MBP in vitro might be mediated by Rasd1 regulating ERK1/2 signaling pathway. The present study is first to investigate the effect of low-dose MBP on SCs proliferation and may enhance our understanding on the modes of action of low concentration MBP on male reproductive system. We hope the results will contribute to explain the causes of precocious puberty and testicular tumors induced by exogenous chemicals.

New insights into cytotoxicity induced by microcystin-LR, estradiol, and ractopamine with mathematical models: Individual and combined effects

Humans are most likely to be exposed to microcystins (MCs) combined with other water pollutants rather than to individual compounds through the consumption of contaminated drinking water or through recreational activities, such as swimming. However, the combined effects of MC-LR, estradiol (EST), and ractopamine (RAC) have not been extensively researched. The goal of this study was to investigate the combined effects of these compounds. For this purpose, cytotoxicity was evaluated in HepG2 cells treated with single or combined doses of MC-LR, EST, and RAC based on concentration addition (CA), independent action (IA), and Chou-Talalay's combination-index (CI) methods. Singly applied MC-LR and EST induced HepG2 cellular proliferation at low-concentration levels (1 × 10^-12-1 × 10^-9 M), and decreased viability at higher doses of exposure (1 × 10^-9-1 × 10^-6 M). Exposure to binary or ternary mixtures of MC-LR, EST, and RAC exhibited synergistic effects at high concentrations, irrespective of the models used. In contrast, antagonism was observed for the mixture of MC-LR and EST at relatively low concentrations. A synergistic effect on reactive oxygen species (ROS) generation was observed for the combined drugs at high concentrations. Additionally, the ratio of apoptotic cells was increased more by the combined drugs than the single drugs, consistent with the inhibition of cell viability. The ROS increase after treatment with the combined drugs may enhance cytotoxicity and subsequently lead to cell apoptosis. Given the interactions between MC-LR, EST, and RAC, government regulatory standards for MC-LR should consider the toxicological interactions between MC-LR and other environment pollutions.

Effects of the interaction of single-walled carbon nanotubes with 4-nonylphenol on their in vitro toxicity

The aim of this study was to assess the toxicological risks arising from the coexistence of polyethylene glycol coated single-walled carbon nanotubes (SWCNTs-PEG) and a known environmental contaminant: 4-nonylphenol (NP). To this end, in vitro toxicity assays involving the exposure of 3T3-L1 cells (mouse embryonic fibroblasts) to SWCNTs-PEG alone or in combination with NP for 24 or 48 h were performed. Experimental treatments were conducted in both presence (10%) and absence of serum in order to evaluate its influence on the toxicity of SWCNTs-PEG. Although the results provided no unambiguous evidences of synergistic toxicity between SWCNTs-PEG and NP, some specific treatments with mixtures (SWCNTs-PEG+NP) resulted in an unexpected combined toxicity in relation to the individual treatments. Only in those cases the interaction between SWCNTs-PEG and NP could have a synergistic effect on the resulting toxicity. The addition of 10% serum increased the stability of SWCNTs-PEG in the culture medium-possibly by steric repulsions-and reduced the toxicity of nanoparticles as a result. Overall, the serum had a "protective effect" on cells against all treatments: SWCNTs-PEG, NP or their mixtures (SWCNTs-PEG+NP). Raman spectroscopy allowed the intracellular distribution of SWCNTs-PEG to be elucidated.

Regulation of microcystin-LR-induced toxicity in mouse spermatogonia by miR-96

Microcystin (MC)-LR is a cyclic heptapeptide that acts as a potent reproductive system toxin, especially by decreasing sperm quality through affecting spermatogonia. However, the molecular mechanisms of MC-induced spermatogonial cytotoxicity still remain unclear. The present study was designed to investigate changes in microRNA (miRNA) profiles and their potential functions in spermatogonia (GC-1 cell line) following treatment with MC-LR. With microarray analysis, 101 miRNAs were identified to be significantly altered in GC-1 cells treated with MC-LR. Among the 25 miRNAs associated with spermatogenesis, miR-96 was down-regulated most dramatically and thus selected for further functional analysis. Deleted-in azoospermia-associated protein 2 (DAZAP2) was predicted to have a binding sequence for miR-96 within its 3'-untranslated region. Fluorescent reporter assay confirmed that DAZAP2 was the target gene of miR-96. The expression of DAZAP2 decreased significantly when miR-96 was up-regulated. Consistently, down-regulation of miR-96 significantly increased the level of DAZAP2. Up-regulation of miR-96 promoted cell viability in GC-1 cells as a result of exposure to MC-LR. Our study suggested a crucial role for miR-96 in the regulation of cytotoxic effects of MC-LR in spermatogonia, which provides new perspectives in the diagnosis and treatment strategies for MC-induced male infertility.

Antagonistic effects of a mixture of low-dose nonylphenol and di-n-butyl phthalate (monobutyl phthalate) on the Sertoli cells and serum reproductive hormones in prepubertal male rats in vitro and in vivo

The estrogenic chemical nonylphenol (NP) and the antiandrogenic agent di-n-butyl phthalate (DBP) are regarded as widespread environmental endocrine disruptors (EDCs) which at high doses in some species of laboratory animals, such as mice and rats, have adverse effects on male reproduction and development. Given the ubiquitous coexistence of various classes of EDCs in the environment, their combined effects warrant clarification. In this study, we attempted to determine the mixture effects of NP and DBP on the testicular Sertoli cells and reproductive endocrine hormones in serum in male rats based on quantitative data analysis by a mathematical model. In the in vitro experiment, monobutyl phthalate (MBP), the active metabolite of DBP, was used instead of DBP. Sertoli cells were isolated from 9-day-old Sprague-Dawley rats followed by treatment with NP and MBP, singly or combined. Cell viability, apoptosis, necrosis, membrane integrity and inhibin-B concentration were tested. In the in vivo experiment, rats were gavaged on postnatal days 23-35 with a single or combined NP and DBP treatment. Serum reproductive hormone levels were recorded. Next, Bliss Independence model was employed to analyze the quantitative data obtained from the in vitro and in vivo investigation. Antagonism was identified as the mixture effects of NP and DBP (MBP). In this study, we demonstrate the potential of Bliss Independence model for the prediction of interactions between estrogenic and antiandrogenic agents.

Analysis of individual and combined effects of ochratoxin A and zearalenone on HepG2 and KK-1 cells with mathematical models

Ochratoxin A (OTA) and Zearalenone (ZEA) are widespread mycotoxins that contaminate foodstuffs simultaneously, but sufficient data regarding their mixed toxicities are lacking. This study aims to analyze the style of combined effects of OTA and ZEA on cells of their target organs. For this purpose, cytotoxicity was determined in HepG2 and KK-1 cells treated with single and combined forms of OTA and ZEA. Furthermore, we have analyzed the data using two mathematical models based on the concepts of concentration addition (CA) and independent addition (IA). By analyzing data with nonlinear regression, toxins applied singly showed classic sigmoid dose-response curves in HepG2 cells whereas in KK-1 cells hormetic responses were observed. Exposure to equieffective mixtures of OTA and ZEA showed additive effects, irrespective of different nonlinear regression models used. Our results demonstrate that IA is an appropriate concept to account for mixture effects of OTA and ZEA. The results in ROS generation indicate a departure from additivity to antagonism or synergism at different concentrations, probably due to potential interaction during ROS production. This study shows that a risk assessment of mycotoxins should account for mixture effects, and prediction models are valuable tools for mixture assessment.

Effects of nonylphenol exposure on expression of cell receptors and secretory function in mouse Sertoli TM4 cells

The aim of this study was to investigate the effects of nonylphenol (NP) exposure on the expression of cell receptors and secretory function in mouse Sertoli TM4 cells. There were no significant changes in mRNA expression of estrogen receptor (ER)-α and toll like receptor (TLR)-4 in the cells exposed to NP for 24h. However, the mRNA expression levels of ER-β, progesterone receptor (PR) and androgen receptor (AR) were down-regulated in NP groups. Furthermore, NP treatment evoked significant changes in protein expression levels of ER-β and follicle-stimulating hormone receptor (FSHR). There were significant changes in the mRNA expression levels of vinculin, N-cadherin and occludin, but not vimentin. Levels of inhibin B, androgen binding protein (ABP) and transferrin (Trf) were found to change significantly in NP challenged cells. Additionally, the decrease of nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) mRNA expression and increase of cytokine levels were simultaneously found in NP stimulated TM4 cells. In conclusion, these findings have shown that NP exposure affected expression of cell receptors and may damage specific secretory function of Sertoli TM4 cells, which may be associated with the male-specific reproductive toxicity of NP.

Molecular mechanism(s) of endocrine-disrupting chemicals and their potent oestrogenicity in diverse cells and tissues that express oestrogen receptors

Endocrine-disrupting chemicals (EDCs) are natural or synthetic compounds present in the environment which can interfere with hormone synthesis and normal physiological functions of male and female reproductive organs. Most EDCs tend to bind to steroid hormone receptors including the oestrogen receptor (ER), progesterone receptor (PR) and androgen receptor (AR). As EDCs disrupt the actions of endogenous hormones, they may induce abnormal reproduction, stimulation of cancer growth, dysfunction of neuronal and immune system. Although EDCs represent a significant public health concern, there are no standard methods to determine effect of EDCs on human beings. The mechanisms underlying adverse actions of EDC exposure are not clearly understood. In this review, we highlighted the toxicology of EDCs and its effect on human health, including reproductive development in males and females as shown in in vitro and in vivo models. In addition, this review brings attention to the toxicity of EDCs via interaction of genomic and non-genomic signalling pathways through hormone receptors.