Toxicant-induced hypospadias in the male rat

Endocrine disrupting chemicals in the pathogenesis of hypospadias; developmental and toxicological perspectives

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Penis development is regulated by a tight balance of androgens and estrogens.

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EDCs that impact androgen/estrogen balance during development cause hypospadias.

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Cross-disciplinary collaborations are needed to define a mechanistic link.

Hypospadias is a defect in penile urethral closure that occurs in approximately 1/150 live male births in developed nations, making it one of the most common congenital abnormalities worldwide. Alarmingly, the frequency of hypospadias has increased rapidly over recent decades and is continuing to rise. Recent research reviewed herein suggests that the rise in hypospadias rates can be directly linked to our increasing exposure to endocrine disrupting chemicals (EDCs), especially those that affect estrogen and androgen signalling. Understanding the mechanistic links between endocrine disruptors and hypospadias requires toxicologists and developmental biologists to define exposures and biological impacts on penis development. In this review we examine recent insights from toxicological, developmental and epidemiological studies on the hormonal control of normal penis development and describe the rationale and evidence for EDC exposures that impact these pathways to cause hypospadias. Continued collaboration across these fields is imperative to understand the full impact of endocrine disrupting chemicals on the increasing rates of hypospadias.

Endocrine disrupting chemicals and reproductive disorders in women, men, and animal models

This chapter covers the known effects of endocrine disrupting chemicals (EDCs) on reproductive disorders. The EDCs represented are highly studied, including plasticizers (bisphenols and phthalates), chemicals in personal care products (parabens), persistent environmental contaminants (polychlorinated biphenyls), and chemicals in pesticides or herbicides. Both female and male reproductive disorders are reviewed in the chapter. Female disorders include infertility/subfertility, irregular reproductive cycles, early menopause, premature ovarian insufficiency, polycystic ovarian syndrome, endometriosis, and uterine fibroids. Male disorders include infertility/subfertility, cryptorchidism, and hypospadias. Findings from both human and animal studies are represented.

Comparative Aspects of Pre- and Postnatal Development of the Male Reproductive System

This review describes pre- and postnatal development of the male reproductive system in humans and laboratory animals, and highlights species differences in the timing and control of hormonal and morphologic events. Major differences are that the fetal testis is dependent on gonadotropins in humans, but is independent of such in rats; humans have an extended postnatal quiescent period, whereas rats exhibit no quiescence; and events such as secretion by the prostate and seminal vesicles, testicular descent, and the appearance of spermatogonia are all prenatal events in humans, but are postnatal events in rats. Major differences in the timing of the developmental sequence between rats and humans include: gonocyte transformation period (rat: postnatal day 0-9; human: includes gestational week 22 to 9 months of age); masculinization programming window (rat: gestational day 15.5-17.5; human: gestational week 9-14); and mini-puberty (rat: 0-6 hr after birth; human: 3-6 months of age). Endocrine disruptors can cause unique lesions in the prenatal and early postnatal testis; therefore, it is important to consider the differences in the timing of the developmental sequence when designing preclinical studies as identification of windows of sensitivity for endocrine disruption or toxicants will aid in interpretation of results and provide clues to a mode of action. Birth Defects Research 110:190-227, 2018. © 2017 Wiley Periodicals, Inc.

Joint effects of genetic variants and residential proximity to pesticide applications on hypospadias risk

BACKGROUND

We examined risks associated with joint exposure of gene variants and pesticides.

METHODS

Analyses included 189 cases and 390 male controls born from 1991 to 2003 in California's San Joaquin Valley. We used logistic regression to examine risks associated with joint exposures of gene variants and pesticides that our previous work identified as associated with hypospadias. Genetic variables were based on variants in DGKK, genes involved in sex steroid synthesis/metabolism, and genes involved in genital tubercle development. Pesticide exposure was based on residential proximity to commercial agricultural pesticide applications.

RESULTS

Odds ratios (ORs) were highest among babies with joint exposures, who had two- to fourfold increased risks; for example, the OR was 3.7 (95% confidence interval [CI], 0.8-16.5) among subjects with the risk-associated DGKK haplotype and pesticide exposure; OR, 1.5 (95% CI, 0.7-3.1) among subjects with the haplotype and no pesticide exposure; and OR, 0.9 (95% CI, 0.5-1.6) among subjects without the haplotype but with pesticide exposure, relative to subjects with neither. However, results did not provide statistical evidence that these risks were significantly greater than expected on an additive scale, relative to risks associated with one exposure at a time.

CONCLUSION

We observed elevated risks associated with joint exposures to selected pesticides and genetic variants but no statistical evidence for interaction. Birth Defects Research (Part A) 106:653-658, 2016. © 2016 Wiley Periodicals, Inc.

Geographic analysis of individual and environmental risk factors for hypospadias births

BACKGROUND

Hypospadias is a relatively common birth defect affecting the male urinary tract. We explored the etiology of hypospadias by examining its spatial distribution in North Carolina and the spatial clustering of residuals from individual and environmental risk factors.

METHODS

We used data collected by the North Carolina Birth Defects Monitoring Program from 2003 to 2005 to estimate local Moran's I statistics to identify geographic clustering of overall and severe hypospadias, using 995 overall cases and 16,013 controls. We conducted logistic regression and local Moran's I statistics on standardized residuals to consider the contribution of individual variables (maternal age, maternal race/ethnicity, maternal education, smoking, parity, and diabetes) and environmental variables (block group land cover) to this clustering.

RESULTS

Local Moran's I statistics indicated significant clustering of overall and severe hypospadias in eastern central North Carolina. Spatial clustering of hypospadias persisted when controlling for individual factors, but diminished somewhat when controlling for environmental factors. In adjusted models, maternal residence in a block group with more than 5% crop cover was associated with overall hypospadias (odds ratio = 1.22; 95% confidence interval = 1.04-1.43); that is living in a block group with greater than 5% crop cover was associated with a 22% increase in the odds of having a baby with hypospadias. Land cover was not associated with severe hypospadias.

CONCLUSION

This study illustrates the potential contribution of mapping in generating hypotheses about disease etiology. Results suggest that environmental factors including proximity to agriculture may play some role in the spatial distribution of hypospadias. Birth Defects Research (Part A) 100:887-894, 2014. © 2014 Wiley Periodicals, Inc.

Hypospadias and residential proximity to pesticide applications

BACKGROUND

Experimental evidence suggests pesticides may be associated with hypospadias.

OBJECTIVE

Examine the association of hypospadias with residential proximity to commercial agricultural pesticide applications.

METHODS

The study population included male infants born from 1991 to 2004 to mothers residing in 8 California counties. Cases (n = 690) were ascertained by the California Birth Defects Monitoring Program; controls were selected randomly from the birth population (n = 2195). We determined early pregnancy exposure to pesticide applications within a 500-m radius of mother's residential address, using detailed data on applications and land use. Associations with exposures to physicochemical groups of pesticides and specific chemicals were assessed using logistic regression adjusted for maternal race or ethnicity and age and infant birth year.

RESULTS

Forty-one percent of cases and controls were classified as exposed to 57 chemical groups and 292 chemicals. Despite >500 statistical comparisons, there were few elevated odds ratios with confidence intervals that excluded 1 for chemical groups or specific chemicals. Those that did were for monochlorophenoxy acid or ester herbicides; the insecticides aldicarb, dimethoate, phorate, and petroleum oils; and adjuvant polyoxyethylene sorbitol among all cases; 2,6-dinitroaniline herbicides, the herbicide oxyfluorfen, and the fungicide copper sulfate among mild cases; and chloroacetanilide herbicides, polyalkyloxy compounds used as adjuvants, the insecticides aldicarb and acephate, and the adjuvant nonyl-phenoxy-poly(ethylene oxy)ethanol among moderate and severe cases. Odds ratios ranged from 1.9 to 2.9.

CONCLUSIONS

Most pesticides were not associated with elevated hypospadias risk. For the few that were associated, results should be interpreted with caution until replicated in other study populations.

Hypospadias and maternal intake of phytoestrogens

Experimental data indicate that gestational exposures to estrogenic compounds impact risk of hypospadias. We examined whether risk of hypospadias (i.e., a congenital malformation in which the opening of the penile urethra occurs on the ventral side of the penis) was associated with maternal intake of phytoestrogens, given their potential impact on estrogen metabolism. The analysis included data on mothers of 1,250 hypospadias cases and 3,118 controls who delivered their infants from 1997 to 2005 and participated in the National Birth Defects Prevention Study, a multistate, population-based, case-control study. After adjustment for several covariates, high intakes of daidzein, genistein, glycetin, secoisolariciresinol, total isoflavones, total lignans, and total phytoestrogens were associated with reduced risks; odds ratios comparing intakes ≥90th percentile with intakes between the 11th and 89th percentiles ranged from 0.6 to 0.8. For example, the odds ratio for total phytoestrogen intake was 0.7 (95% confidence interval: 0.5, 1.0). This study represents the first large-scale analysis of phytoestrogen intake and hypospadias. The observed associations merit investigation in additional populations before firm conclusions can be reached.

Environmental and genetic contributors to hypospadias: a review of the epidemiologic evidence

This review evaluates current knowledge related to trends in the prevalence of hypospadias, the association of hypospadias with endocrine-disrupting exposures, and the potential contribution of genetic susceptibility to its etiology. The review focuses on epidemiologic evidence. Increasing prevalence of hypospadias has been observed, but such increases tend to be localized to specific regions or time periods. Thus, generalized statements that hypospadias is increasing are unsupported. Owing to the limitations of study designs and inconsistent results, firm conclusions cannot be made regarding the association of endocrine-disrupting exposures with hypospadias. Studies with more rigorous study designs (e.g., larger and more detailed phenotypes) and exposure assessment that encompasses more breadth and depth (e.g., specific endocrine-related chemicals) will be critical to make better inferences about these important environmental exposures. Many candidate genes for hypospadias have been identified, but few of them have been examined to an extent that enables solid conclusions. Further study is needed that includes larger sample sizes, comparison groups that are more representative of the populations from which the cases were derived, phenotype-specific analyses, and more extensive exploration of variants. In conclusion, examining the associations of environmental and genetic factors with hypospadias remain important areas of inquiry, although our actual understanding of their contribution to hypospadias risk in humans is currently limited.

Cumulative effects of in utero administration of mixtures of reproductive toxicants that disrupt common target tissues via diverse mechanisms of toxicity

Although risk assessments are typically conducted on a chemical-by-chemical basis, the 1996 Food Quality Protection Act required the US Environmental Protection Agency to consider cumulative risk of chemicals that act via a common mechanism of toxicity. To this end, we are conducting studies with mixtures of chemicals to elucidate mechanisms of joint action at the systemic level with the goal of providing a framework for assessing the cumulative effects of reproductive toxicants. Previous mixture studies conducted with antiandrogenic chemicals are reviewed briefly and two new studies are described. In all binary mixture studies, rats were dosed during pregnancy with chemicals, singly or in pairs, at dosage levels equivalent to approximately one-half of the ED50 for hypospadias or epididymal agenesis. The binary mixtures included androgen receptor (AR) antagonists (vinclozolin plus procymidone), phthalate esters [di(n-butyl) phthalate (DBP) plus benzyl n-butyl phthalate (BBP) and diethyl hexyl phthalate (DEHP) plus DBP], a phthalate ester plus an AR antagonist (DBP plus procymidone), a mixed mechanism androgen signalling disruptor (linuron) plus BBP, and two chemicals which disrupt epididymal differentiation through entirely different toxicity pathways: DBP (AR pathway) plus 2,3,7,8 TCDD (AhR pathway). We also conducted multi-component mixture studies combining several 'antiandrogens'. In the first study, seven chemicals (four pesticides and three phthalates) that elicit antiandrogenic effects at two different sites in the androgen signalling pathway (i.e. AR antagonist or inhibition of androgen synthesis) were combined. In the second study, three additional phthalates were added to make a 10 chemical mixture. In both the binary mixture studies and the multi-component mixture studies, chemicals that targeted male reproductive tract development displayed cumulative effects that exceeded predictions based on a response-addition model and most often were in accordance with predictions based on dose-addition models. In summary, our results indicate that compounds that act by disparate mechanisms of toxicity to disrupt the dynamic interactions among the interconnected signalling pathways in differentiating tissues produce cumulative dose-additive effects, regardless of the mechanism or mode of action of the individual mixture component.

Hypospadias and halogenated organic pollutant levels in maternal mid-pregnancy serum samples

BACKGROUND

Environmental contaminants that disrupt endocrine function may contribute to hypospadias etiology.

OBJECTIVE

To compare levels of selected halogenated organic pollutants in women delivering infants with and without hypospadias.

METHODS

This study examined levels of nine polybrominated flame retardants (PBDEs), 30 polychlorinated biphenyls (PCBs) and nine persistent pesticides in mid-pregnancy serum samples from 20 women who delivered infants with hypospadias and 28 women who delivered unaffected infants, in California. Analytes were measured using isotope dilution high-resolution mass spectrometry. Values below individual limits of detection (LOD) for each analyte were imputed based on a truncated multivariate normal distribution. Levels of 17 analytes for which at least 50% of cases and controls had values above the LOD were compared using t-tests and by generating odds ratios from logistic regression analyses.

RESULTS

Means were greater for cases than controls for 11 of the 17 reported analytes (4 of 5 PBDEs, 7 of 9 PCBs, and 0 of 3 other persistent pesticides), but none of the differences were statistically significant. Eleven of the 17 odds ratios exceeded one (the same analytes that had greater means), but none of the confidence intervals excluded one. After adjustment for sample processing time and foreign-born Hispanic race-ethnicity, only four of the odds ratios exceeded one.

CONCLUSIONS

Levels of the PBDEs and PCBs were not statistically significantly different, but the sample size was small. The current study adds to a relatively limited knowledge base regarding the potential association of specific contaminants with hypospadias or other birth defects.

Pesticides and hypospadias: a meta-analysis

OBJECTIVE

To use meta-analytic techniques to synthesize the findings of the current body of published literature regarding the risk of hypospadias resulting from parental exposure to pesticides.

MATERIALS AND METHODS

A search of Pub Med for original research published in English from January 1966 through March 2008 identified 552 studies, 90 of which were reviewed in detail. Nine studies met all study inclusion criteria. Two reviewers independently abstracted data from each included study. Any disagreements were resolved by consensus. Pooled risk ratios (PRRs) and confidence intervals (CIs) were calculated using both random and fixed effects models, along with statistical tests of homogeneity.

RESULTS

Elevated but marginally significant risks of hypospadias were associated with maternal occupational exposure (PRR of 1.36, CI=1.04-1.77), and paternal occupational exposure (PRR of 1.19, CI=1.00-1.41). Subgroup analyses provided insights into needed designs for future studies. Notably, exposure assessment using a job-exposure matrix resulted in slightly higher estimated risk than agricultural occupation in fathers; but this effect was reversed in mothers, suggesting the importance of indirect and residential pesticide exposures in this group.

CONCLUSIONS

Despite potential exposure misclassification, which would tend to diminish observed associations, the previous literature indicates a modestly increased risk of hypospadias associated with pesticide exposure.

Xenoandrogenic activity in serum differs across European and Inuit populations

BACKGROUND

Animal and in vitro studies have indicated that human male reproductive disorders can arise as a result of disrupted androgen receptor (AR) signalling by persistent organic pollutants (POPs). Our aim in the present study was to compare serum xenoandrogenic activity between study groups with different POP exposures and to evaluate correlations to the POP proxy markers 2,2',4,4',5,5'-hexachlorobiphenyl (CB-153) and 1,1-dichloro-2,2-bis(p-chlorophenyl)-ethylene (p,p'-DDE).

METHODS

We determined xenoandrogenic activity in the serum fraction containing the lipophilic POPs but free of endogenous hormones. Adult male serum (n = 261) from Greenland, Sweden, Warsaw (Poland), and Kharkiv (Ukraine) was analyzed. Xenoandrogenic activity was determined as the effect of serum extract alone (XAR) and in the presence of the synthetic AR agonist R1881 (XARcomp) on AR transactivated luciferase activity.

RESULTS

The study groups differed significantly with respect to XARcomp activity, which was increased in the Inuits and decreased in the European study groups; we observed no difference for XAR activity. We found the highest level of the AR antagonist p,p'-DDE in Kharkiv, and accordingly, this study group showed the highest percent of serum samples with decreased XARcomp activities. Furthermore, the percentage of serum samples with decreased XARcomp activities followed the p,p'-DDE serum level for the European study groups. No correlations between serum XAR or XARcomp activities and the two POP markers were revealed.

CONCLUSIONS

The differences in XARcomp serum activity between the study groups suggest differences in chemical exposure profiles, genetics, and/or lifestyle factors.

Risk Assessment of Endocrine Disrupters: The Role of Toxicological Studies

Endocrine disrupters (ED) represent a good challenge for experimental toxicology. In order to deal with several critical points relevant to risk assessment: (a) ED may induce long-term effects upon exposure in susceptible developmental phases, including postnatal life up to puberty; thus, efforts are required to refine testing strategies, for example, by supporting the two-generation rodent study with a comprehensive in vitro/in vivo screening battery; (b) due to the regulatory role of endocrine homeostasis, mechanisms of endocrine disruption may impact on immune, neurobehavioral, and reproductive development, as well as on susceptibility to cancer; (c) the potential multiple exposure to ED with common targets through diet and/or living environment calls for the development of models to understand mechanisms of interactions and effects of mixtures; and (d) last but not least, ED may interact with a number of factors related to differential vulnerability of individuals or population subgroups, including the intake of nutrients or bioactive food components. Besides reducing the chance for noxious chemicals to enter our life, toxicological research on mechanisms may also lead to the definition of possible biomarkers of exposure, effect, and susceptibility that may be further exploited in human health surveillance.

Persistent pesticides in human breast milk and cryptorchidism

INTRODUCTION

Prenatal exposure to some pesticides can adversely affect male reproductive health in animals. We investigated a possible human association between maternal exposure to 27 organochlorine compounds used as pesticides and cryptorchidism among male children.

DESIGN

Within a prospective birth cohort, we performed a case-control study; 62 milk samples from mothers of cryptorchid boys and 68 from mothers of healthy boys were selected. Milk was collected as individual pools between 1 and 3 months postpartum and analyzed for 27 organochlorine pesticides.

RESULTS

Eight organochlorine pesticides were measurable in all samples (medians; nanograms per gram lipid) for cases/controls: 1,1-dichloro-2,2-bis(4-chlorophenyl) ethylene (p,p -DDE) : 97.3/83.8; beta-hexachlorocyclohexane (beta-HCH) : 13.6/12.3; hexachlorobenzene (HCB) : 10.6/8.8; alpha-endosulfan: 7.0/6.7; oxychlordane: 4.5/4.1; 1,1,1-trichloro-2,2-bis(4-chlorophenyl) ethane (p,p -DDT) : 4.6/4.0; dieldrin: 4.1/3.1 ; cis-heptachloroepoxide (cis-HE) : 2.5/2.2. Five compounds [octachlorostyrene (OCS); pentachlorobenzene, 1,1-dichloro-2,2-bis(4-chlorophenyl) ethane(p,p -DDD) ; o,p -DDT ; mirex] were measurable in most samples (detection rates 90.8-99.2%) but in lower concentrations. For methoxychlor, cis-chlordane, pentachloroanisole (PCA), gamma-HCH, 1,1-dichloro-2-(2-chlorophenyl) -2,2(4-chlorophenyl) ethane, trans-chlordane, alpha-HCH, and o,p -DDE, both concentrations and detection rates were low (26.5-71.5%). Heptachlor, HCH (lc delta, epsilon), aldrin, beta-endosulfan and trans-heptachloroepoxide were detected at negligible concentrations and low detection rates and were not analyzed further. Seventeen of 21 organochlorine pesticides [p,p -DDT, p,p-DDE, p,p-DDD, o,p-DDT, HCH (alpha, beta, gamma), HCB, PCA, alpha-endosulfan, cis-HE, chlordane (cis-, trans-) oxychlordane, methoxychlor, OCS, and dieldrin] were measured in higher median concentrations in case milk than in control milk. Apart from trans-chlordane (p = 0.012), there were no significant differences between cryptorchid and healthy boys for individual chemicals. However, combined statistical analysis of the eight most abundant persistent pesticides showed that pesticide levels in breast milk were significantly higher in boys with cryptorchidism (p = 0.032).

CONCLUSION

The association between congenital cryptorchidism and some persistent pesticides in breast milk as a proxy for maternal exposure suggests that testicular descent in the fetus may be adversely affected.

Pathways of endocrine disruption during male sexual differentiation and masculinization

After testis formation, further development of a male phenotype (masculinization) is driven by three hormones from the foetal testis: anti-Müllerian hormone, insulin-like factor 3, and testosterone. These hormones divert the development of reproductive and other organs from female to male and also play a role in testis development. The hormone dependence of masculinization renders this process inherently susceptible to disruption by factors that interfere with hormone production, bioavailability, metabolism, or action. This susceptibility is illustrated by the high prevalence of congenital masculinization disorders (cryptorchidism, hypospadias) and disorders in young adult men (low sperm counts, testis cancer), which may also stem from maldevelopment (dysgenesis) of the foetal testis. Testicular dysgenesis occurring in humans, or which is induced in animal models by foetal exposure to certain phthalates, is associated with impaired hormone production by the foetal testis. There is currently no definitive evidence that exposure of humans to environmental chemicals can induce testicular dysgenesis and/or impair masculinization, though pathways via which this could potentially occur are established.

Adverse effects of environmental antiandrogens and androgens on reproductive development in mammals1

Within the last decade, several classes of chemicals have been shown in laboratory studies to disrupt reproductive development by acting as androgen receptor (AR) antagonists and/or inhibitors of fetal Leydig cell testosterone production. Some phthalate esters alter gubernacular differentiation by reducing insulin-like 3 (insl3) mRNA levels. We have found that AR antagonists and inhibitors of fetal testis hormone production generally induce cumulative, apparently dose-additive adverse effects when administered in mixtures. New research has also revealed the presence of androgens in the environment. Effluents from pulp and paper mills display androgenic activity of sufficient potency to masculinize and/or sex-reverse female fish. Effluent from beef cattle concentrated animal feedlot operations from the United States also displays androgenic activity in vitro, due, in part, to the presence of a steroid used to promote growth in beef cattle. In summary, we are only beginning to identify the classes of chemicals that have the potential to alter the androgen signalling pathway in utero. This review will (i) present information on the classes of environmental chemicals that display antiandrogenic and androgenic activities in vitro and in vivo, and (ii) provide an insight into how exposure to mixtures these chemicals might behave in utero.