Disruption of Retinol (Vitamin A) Signaling by Phthalate Esters: SAR and Mechanism Studies

Reproductive toxicity following in utero and lactational exposure to a human-relevant phthalate mixture in rats

This rodent (Wistar rats) study examined reproductive effects of in utero/lactational exposure to a mixture of 6 antiandrogenic phthalates (PMix): diisobutyl phthalate, di-n-butyl phthalate, diisopentyl phthalate, butylbenzyl phthalate, di-2-ethylhexyl phthalate, and diisononyl phthalate. The PMix was defined based on exposure data from pregnant women in Brazil. Experimental groups were established by extrapolating the estimated human dose to rats (0.1 mg/kg/day), followed by up to 3 additional doses corresponding to 5, 1000, and 5000 times the starting rat dose: 0 (control), 0.1, 0.5, 100, and 500 mg/kg/day. The fetal experiment assessed gestational exposure effects on fetal gonads, whereas the postnatal experiment evaluated reproductive parameters in males and females after in utero and lactational exposure. Prenatal exposure decreased fetal testicular testosterone production at 0.5 and 500 mg/kg/day. PMix 500 also reduced mRNA expression of steroidogenesis-related genes, upregulated transcript expression of the retinoic acid-degrading enzyme Cyp26b1, and increased multinucleated gonocytes incidence in fetal testes. Postnatal assessment revealed antiandrogenic effects at the highest dose, including reduced anogenital distance, nipple retention, and decreased weight of reproductive organs. Early puberty onset (preputial separation) was observed at the lowest dose in males. In contrast, females did not show significant changes in fetal and adult endpoints. Overall, the PMix recapitulated early and late male rat phthalate syndrome phenotypes at the highest dose, but also induced some subtle changes at lower doses, which warrant confirmation and mechanistic assessments. Our data support the use of epidemiologically defined mixtures for exposure risk assessments over traditional toxicological approaches.

Interaction between mono-(2-ethylhexyl) phthalate and retinoic acid alters Sertoli cell development during fetal mouse testis cord morphogenesis

Phthalic acid esters (phthalates) are a class of industrial chemicals that cause developmental and reproductive toxicity, but there are significant gaps in knowledge of phthalate toxicity mechanisms. There is evidence that phthalates disrupt retinoic acid signaling in the fetal testis, potentially disrupting control of spatial and temporal patterns of testis development. Our goal was to determine how a phthalate would interact with retinoic acid signaling during fetal mouse testis development. We hypothesized that mono-(2-ethylhexyl) phthalate (MEHP) would exacerbate the adverse effect of all-trans retinoic acid (ATRA) on seminiferous cord development in the mouse fetal testis. To test this hypothesis, gestational day (GD) 14 C57BL/6 mouse testes were isolated and cultured on media containing MEHP, ATRA, or a combination of both compounds. Cultured testes were collected for global transcriptome analysis after one day in culture and for histology and immunofluorescent analysis of Sertoli cell differentiation after three days in culture. ATRA disrupted seminiferous cord morphogenesis and induced aberrant FOXL2 expression. MEHP alone had no significant effect on cord development, but combined exposure to MEHP and ATRA increased the number of FOXL2-positive cells, reduced seminiferous cord number, and increased testosterone levels, beyond the effect of ATRA alone. In RNA-seq analysis, ATRA treatment and MEHP treatment resulted in differential expression of genes 510 and 134 genes, respectively, including 70 common differentially expressed genes (DEGs) between the two treatments, including genes with known roles in fetal testis development. MEHP DEGs included RAR target genes, genes involved in angiogenesis, and developmental patterning genes, including members of the homeobox superfamily. These results support the hypothesis that MEHP modulates retinoic acid signaling in the mouse fetal testis and provide insight into potential mechanisms by which phthalates disrupt seminiferous cord morphogenesis.

Identifying candidate reference chemicals for in vitro testing of the retinoid pathway for predictive developmental toxicity

Evaluating chemicals for potential in vivo toxicity based on their in vitro bioactivity profile is an important step toward animal- free testing. A compendium of reference chemicals and data describing their bioactivity on specific molecular targets, cellular pathways, and biological processes is needed to bolster confidence in the predictive value of in vitro hazard detection. Endogenous signaling by all-trans retinoic acid (ATRA) is an important pathway in developmental processes and toxicities. Employing data extraction methods and advanced literature extraction tools, we assembled a set of candidate reference chemicals with demonstrated activity on ten protein family targets in the retinoid system. The compendium was culled from Protein Data Bank, ChEMBL, ToxCast/Tox21, and the biomedical literature in PubMed. Finally, we performed a case study on one chemical in our collection, citral, an inhibitor of endogenous ATRA production, to determine whether the literature supports an adverse outcome pathway explaining the compound’s developmental toxicity initiated by disruption of the retinoid pathway. We also deliver an updated Abstract Sifter tool populated with these reference compounds and complex search terms designed to query the literature for the downstream consequences to support concordance with targeted retinoid pathway disruption.

Retinoic acid can improve autophagy through depression of the PI3K-Akt-mTOR signaling pathway via RARα to restore spermatogenesis in cryptorchid infertile rats

Cryptorchidism-caused adult infertility is a common component of idiopathic reasons for male infertility. Retinoic acid (RA) has a vital effect on the spermatogenesis process. Here, we found that the expression of c-Kit, Stra8, and Sycp3 could be up-regulated via the activation of retinoic acid receptor α (RARα) after RA supplementation in neonatal cryptorchid infertile rats. We also demonstrated that the protein expression of PI3K, p-Akt/pan-Akt, and p-mTOR/mTOR was higher in cryptorchid than in normal testes, and could be suppressed with RA in vivo. After RA treatment in infertile cryptorchid testis in vivo, the levels of the autophagy proteins LC3 and Beclin1 increased and those of P62 decreased. Biotin tracer indicated that the permeability of blood-testis barrier (BTB) in cryptorchid rats decreased after RA administration. Additionally, after blocking the RARα with AR7 (an RARα antagonist) in testicle culture in vitro, we observed that compared with normal testes, the PI3K-Akt-mTOR signaling pathway and the autophagy pathway was increased and decreased, respectively, which were coincident with cryptorchisd testes in vivo. Additionally, the appropriate concentrations of RA treatment could depress the PI3K-Akt-mTOR signaling pathway and improve the autophagy pathway. The results confirmed that RA can rehabilitate BTB function and drive key protein levels in spermatogonial differentiation through depressing the PI3K-Akt-mTOR signaling pathway via RARα.

Paternal exposure to di-n-butyl-phthalate induced developmental toxicity in zebrafish (Danio rerio)

BACKGROUND

Dibutyl phthalate (DBP) is an environmental endocrine disruptor detected in water, soil, and other environmental media frequently. Growing concerns regarding DBP exposure focus on toxicity to male reproduction. Reports about the developmental toxicity of paternal DBP exposure are rare. In this study, we investigated the developmental toxicity of paternal exposure to DBP on offspring in zebrafish.

METHODS

Adult male zebrafish with normal reproductive function were exposed to 0.2, 0.6, 1.8 mg/L of DBP or acetone solvent control for 30 days, and then mated with females. Thirty embryos per group were randomly selected to be observed, and malformations were recorded and photographed. The mating and observations were repeated three times, for a total of 90 embryos per group.

RESULTS

The results showed that the percentage of malformations, such as edema and a bent trunk, was increased in the 0.6 and 1.8 mg/L DBP exposure groups, the heart rate and spontaneous contraction decreased in the 0.6 and 1.8 mg/L DBP exposure groups and migration of primordial germ cells was disrupted in some F1 embryos in all DBP exposure group after paternal exposure. The axial skeleton was affected in some F1 adults in the 1.8 mg/L DBP exposure group.

CONCLUSIONS

Our findings demonstrate the developmental toxicity of paternal DBP exposure in zebrafish.

Development of an adverse outcome pathway for cranio-facial malformations: A contribution from in silico simulations and in vitro data

Mixtures of substances sharing the same molecular initiating event (MIE) are supposed to induce additive effects. The proposed MIE for azole fungicides is CYP26 inhibition with retinoic acid (RA) local increase, triggering key events leading to craniofacial defects. Valproic acid (VPA) is supposed to imbalance RA-regulated gene expression trough histone deacetylases (HDACs) inhibition. The aim was to evaluate effects of molecules sharing the same MIE (azoles) and of such having (hypothetically) different MIEs but which are eventually involved in the same adverse outcome pathway (AOP). An in silico approach (molecular docking) investigated the suggested MIEs. Teratogenicity was evaluated in vitro (WEC). Abnormalities were modelled by PROAST software. The common target was the branchial apparatus. In silico results confirmed azole-related CYP26 inhibition and a weak general VPA inhibition on the tested HDACs. Unexpectedly, VPA showed also a weak, but not marginal, capability to enter the CYP 26A1 and CYP 26C1 catalytic sites, suggesting a possible role of VPA in decreasing RA catabolism, acting as an additional MIE. Our findings suggest a new more complex picture. Consequently two different AOPs, leading to the same AO, can be described. VPA MIEs (HDAC and CYP26 inhibition) impinge on the two converging AOPs.

All-trans Retinoic Acid Disrupts Development in Ex Vivo Cultured Fetal Rat Testes. II: Modulation of Mono-(2-ethylhexyl) Phthalate Toxicity

Humans are universally exposed to low levels of phthalate esters (phthalates), which are used to plasticize polyvinyl chloride. Phthalates exert adverse effects on the development of seminiferous cords in the fetal testis through unknown toxicity pathways. To investigate the hypothesis that phthalates alter seminiferous cord development by disrupting retinoic acid (RA) signaling in the fetal testis, gestational day 15 fetal rat testes were exposed for 1-3 days to 10-6 M all-trans retinoic acid (ATRA) alone or in combination with 10-6-10-4 M mono-(2-ethylhexyl) phthalate (MEHP) in ex vivo culture. As previously reported, exogenous ATRA reduced seminiferous cord number. This effect was attenuated in a concentration-dependent fashion by MEHP co-exposure. ATRA and MEHP-exposed testes were depleted of DDX4-positive germ cells but not Sertoli cells. MEHP alone enhanced the expression of the RA receptor target Rbp1 and the ovary development-associated genes Wnt4 and Nr0b1, and suppressed expression of the Leydig cell marker, Star, and the germ cell markers, Ddx4 and Pou5f1. In co-exposures, MEHP predominantly enhanced the gene expression effects of ATRA, but the Wnt4 and Nr0b1 concentration-responses were nonlinear. Similarly, ATRA increased the number of cells expressing the granulosa cell marker FOXL2 in testis cultures, but this induction was attenuated by addition of MEHP. These results indicate that MEHP can both enhance and inhibit actions of ATRA during fetal testis development and provide evidence that RA signaling is a target for phthalate toxicity in the fetal testis.

Bioinformatic identification of key genes and molecular pathways in the spermatogenic process of cryptorchidism

This study aims to determine key genes and pathways that could play important roles in the spermatogenic process of patients with cryptorchidism. The gene expression profile data of GSE25518 was obtained from the Gene Expression Omnibus (GEO) database. Microarray data were analyzed using BRB-Array Tools to identify differentially expressed genes (DEGs) between high azoospermia risk (HAZR) patients and controls. In addition, other analytical methods were deployed, including hierarchical clustering analysis, class comparison between patients with HAZR and the normal control group, gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, and the construction of a protein–protein interaction (PPI) network. In total, 1015 upregulated genes and 1650 downregulated genes were identified. GO and KEGG analysis revealed enrichment in terms of changes in the endoplasmic reticulum cellular component and the endoplasmic reticulum protein synthetic process in the HAZR group. Furthermore, the arachidonic acid pathway and mTOR pathway were also identified as important pathways, while RICTOR and GPX8 were indentified as key genes involved in the spermatogenic process of patients with cryptorchidism. In present study, we found that changes in the synthesis of endoplasmic reticulum proteins, arachidonic acid and the mTOR pathway are important in the incidence and spermatogenic process of cryptorchidism. GPX8 and RICTOR were also identified as key genes associated with cryptorchidism. Collectively, these data may provide novel clues with which to explore the precise etiology and mechanism underlying cryptorchidism and cryptorchidism-induced human infertility.

Effect of subacute and prenatal DINCH plasticizer exposure on rat dams and male offspring hepatic function: The role of PPAR-α

Plasticizers are compounds added to plastics to modify their physical proprieties. The most well-known class of plasticizers, the phthalates, has been shown to possess antiandrogenic and tumor promoting activities. 1,2-Cyclohexane dicarboxylic acid diisononyl ester (DINCH) was approved for use in food contact containers in 2006 and has been used as a replacement for phthalates in toys and children products. However, we reported previously that the DINCH metabolite MINCH acts on primary rat adipocytes through the peroxisome proliferator activated receptor (PPAR)-α pathway in a manner similar to phthalates. Evidence from our studies, as well as from the current bibliography on DINCH, suggests that the liver might be one of its target organs. In the present study, we collected tissues from dams exposed subacutely and progeny at postnatal day (PND) 3 and 60 exposed in utero to DINCH (1, 10 and 100 mg/kg bw/day). Exposure to DINCH drastically affected liver gene expression in all 3 age groups tested and in particular at the dose of 1 mg/kg bw/day. The PPAR-α pathway along with other metabolic and DNA replication pathways were affected by DINCH. Modifications in PPAR-α and superoxide dismutase (SOD)-1 protein levels were observed in dams at PND21, as well as male progeny at PND3 and 60. No sign of fibrosis or direct liver toxicity was observed after 8 days of stimulus with low doses of DINCH. This study provides evidence that DINCH is not a biologically inert molecule in the rat, and in the liver its actions are mediated, at least in part, by PPAR-α.

The role of ANXA5 in DBP-induced oxidative stress through ERK/Nrf2 pathway

Di-N-butylphthalate (DBP) have given rise to more and more attention due to its unique endocrine toxicity to male reproductive system. Our previous studies have demonstrated antioxidative Nrf2 (nuclear factor erythroid related factor 2) pathway play a vital role in DBP induced oxidative stress injury. ANXA5 (annexin A5), which is highly expressed in testicular Leydig and Sertoli cells, was found upregulated after DBP stimulation. Mouse Leydig and Sertoli cells were exposed to different concentration of DBP for 24 h to examine the ROS (Reactive oxygen species), MDA (Malondialdehyde), SOD (superoxide dismutase) level and ANXA5, Nrf2, NQO1 (NAD(P)H-quinone oxidoreductase 1), HO-1 (heme oxygenase 1) and ERK/P-ERK protein expression by DHE (Dihydroethidium) staining, ELISA (enzyme-linked immunosorbent assay) and Western blot respectively. Firstly, the oxidative stress injury induced by DBP was re-validated. Then, we confirmed the change of Nrf2 pathway and ANXA5 level after DBP exposure to testicular cells. Additionally, overexpressed ANXA5 could activate Nrf2/HO-1/NQO1 antioxidant pathway and significantly attenuate DBP-induced oxidative stress. Ultimately, we demonstrated ANXA5 could increase ERK phosphorylated level and the activated role of ANXA5 on ERK/Nrf2 pathway could be reversed by ERK inhibitor. Overall, this study illuminated that ANXA5 could defend testicle Leydig and Sertoli cells against DBP-induced oxidative stress injury through ERK/Nrf2 pathway.

[Di (2-ethylhexyl) phthalate-induced hypospadias in SD rats is related with Mafb expression: a transcriptome profiling-based study]

OBJECTIVE

To investigate the transcriptome profile of genital tubercles (GTs) in male SD rats and explore the mechanism of hypospadias induced by Di (2-ethylhexyl) phthalate (DEHP).

METHODS

Forty time-pregnant SD rats were randomly divided into 4 equal groups, namely GD16 group and GD19 group (in which the male GTs were collected on gestation day[GD]16 and GD19 for RNA-seq, respectively), control group and DEHP exposure group (with administration of oil and 750 mg/kg DEHP by gavage from GD12 to GD19, respectively).In the control and DEHP exposure groups, the GTs were collected from the male fetuses on GD19.5, and scanning electron microscopy and HE staining were used to observe the morphological changes.The differentially expressed genes (DEGs) in the GTs were screened using lllumina HiSeq 2000 followed by GO and KEGG enrichment analyses to characterize the transcriptome profile.Immunofluorescence assay was performed to verify the DEGs (Mafb) identified by RNA-seq results.Immunofluorescence assay and Western blotting were used to examine the expression levels of Mafb in the penile tissue.

RESULTS

A total of 1360 DEGs were detected in the GTs between GD16 group and GD19 group by RNA-seq.Among these genes, 797 were up-regulated and 563 were down-regulated.These DEGs were mainly enriched in the cell adhesion plaque signaling pathway, axon guidance signaling pathway, and extracellular matrix receptor signaling pathway.Compared with that in GD16 group, Mafb was significantly up-regulated in GD19 group, which was consistent with the sequencing results.Mafb and β-catenin were significantly down-regulated in DEHP-exposed group compared with the control group (P < 0.01).

CONCLUSIONS

Mafb expression increases progressively with the development of GTs in male SD rats.DEHP exposure causes significant down-regulation of Mafb and β-catenin, suggesting that β-catenin signaling pathway that affects Mafb is related to DEHP-induced hypospadias in SD rats.