Gestational and Lactational Exposure to an Environmentally Relevant Mixture of Brominated Flame Retardants Downregulates Junctional Proteins, Thyroid Hormone Receptor α1 Expression, and the Proliferation-Apoptosis Balance in Mammary Glands Post Puberty

Exposure to brominated flame retardants during pregnancy and lactation increases the prevalence of breast lesions and cancer-associated pathways in sprague-dawley rats

The mammary gland undergoes significant changes during pregnancy, lactation, and involution, making it highly susceptible to endocrine-disrupting chemicals such as brominated flame retardants (BFRs). Despite being restricted in many countries, some BFRs persist in the environment and accumulate in human tissues, including the mammary gland and human milk. This study investigates the effects of BFRs exposure during pregnancy and lactation on mammary gland development and breast cancer risk in a rat model. Dams were exposed to a mixture of polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCDD), formulated based on relative congener levels found in house dust. Post-weaning, dams were treated with 7,12-dimethylbenz[a]anthracene (DMBA) to initiate tumor formation. The results revealed that both low and high doses of BFRs induced lesions in mammary epithelium, with an increase in total lesion number in low dose. Molecular analysis revealed disruptions in the Wnt/β-catenin signaling pathway, leading to an increase in oncogene expression, including c-Myc and c-Jun. RNA sequencing also indicated dysregulation in calcium signaling and glucose metabolism pathways. Our findings suggest that BFR exposure during the critical window of mammary gland involution compromises the cancer-protective effects of pregnancy and lactation. These effects are particularly significant at low exposure levels, demonstrating a non-monotonic dose-response. The study underscores the potential long-term health risks associated with environmental BFR exposure and highlights the need for further research on its implications on the risks of developing breast cancer later in life.

The rat mammary gland undergoes dynamic transcriptomic and lipidomic modifications from pre-puberty to adulthood

Mammary gland development is a complex process involving dynamic interaction between the epithelial and stromal components at different critical stages, particularly around puberty. While epithelial tissue changes are well-documented, stromal mechanisms are less understood. To address this gap, this study employed histology, lipidomic, and transcriptomic analyses to investigate molecular and cellular dynamics in the mammary gland during pre-puberty (Post Natal Day (PND21)), peri-puberty (PND46), and adulthood (PND90) in rats. The epithelial area was significantly smaller at PND21 than at PND46 and PND90, with a higher complexity at PND21 compared to PND46. Significant differences in adipocyte number and size were observed between PND21, PND46, and PND90. Transcriptomic analysis revealed that 1563 genes changed significantly between PND21 and PND46, with only 14 genes altered between PND46 and PND90. Enrichment analyses indicated dynamic regulation of pathways related to proliferation, differentiation, lipid metabolism, and immune responses. In lipidomic analysis, 29/43 and 7/43 fatty acids differed significantly between PND21 - PND46 and PND46 - PND90, respectively. These results suggest that mammary gland development involves complex interactions between metabolic demands, hormonal regulation, and immune responses.

Exposure to brominated flame retardants in utero and through lactation delays the development of DMBA-induced mammary cancer: potential effects on subtypes?

Introduction

Brominated flame retardants (BFRs) are chemical compounds used to reduce the flammability of various products; some BFRs exhibit endocrine-disrupting properties and can leach into the environment leading to human and wildlife exposure. The mammary gland has specific vulnerability windows during which it is more sensitive to the effects of endocrine disrupting compounds (EDCs), such as the in utero life, puberty and pregnancy. Our previous studies revealed precocious mammary gland development, disruptions in junctional proteins, and altered proliferation-apoptosis balance during puberty in rats exposed to BFRs in utero and through lactation. Such effects have been associated with increased mammary cancer risk.

Objective

The current study aimed to determine if in utero and lactational exposure to BFRs renders the mammary gland more susceptible to 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary cancer.

Methods

Dams were exposed to a BFRs mixture (0. 0.06 or 60 mg/kg/day), and mammary cancer was induced in pups using DMBA at post-natal day 46. Tumors onset and growth were monitored, and tumors were characterized using histology and molecular biology.

Results

Although BFRs exposure did not significantly affect mammary tumor number or burden, it showed significant delay in mammary tumor onset and growth in BFR-exposed animal. These effects could potentially be due to BFRs' impact on cellular responses, DMBA metabolism, or mammary gland shift of the sensitivity window. Molecular analysis of mammary tumors showed a shift in the ratio of luminal A, luminal B, and (HER2)-enriched tumors, and an increase in triple-negative breast cancer (TNBC) subtypes in BFR-exposed animals. Additionally, BFRs exposure showed lung lesions indicative of inflammation, independent of mammary cancer development.

Conclusion

Our study highlights the complex relationship between BFRs exposure and mammary cancer risk, emphasizing the need for further investigation into underlying mechanisms and long-term effects of BFRs on mammary gland development and carcinogenesis.

Adverse outcome pathways of PBDEs inducing male reproductive toxicity

Polybrominated diphenyl ethers (PBDEs) are widely used brominated flame retardants, they are easily released into environment and causing adverse effects to the ecosystem and human health. This review aims to summarize the research status of PBDEs-induced male reproductive toxicity and its mechanisms at various levels such as molecular/cellular, tissue/organ and individual/population. The Adverse Outcome Pathways (AOPs) diagram showed that PBDEs-induced reactive oxygen species (ROS) production, disruptions of estrogen receptor-α (ERα) and antagonism of androgen receptor (AR) were defined as critical molecular initiating events (MIEs). They caused key events (KEs) at the molecular and cellular levels, including oxidative stress, increased DNA damage, damaging mitochondria, increased glycolipid levels and apoptosis, depletion of ectoplasmic specialization and decreased Leydig cells numbers. These in turn lead to followed KEs at the tissue or organ levels, such as the impaired spermatogenesis, impaired blood-testis barrier and reduced testosterone synthesis and function. As a result, reproductive system-related adverse outcomes (AOs) were reported, such as the decreased sperm quantity or quality, shorten male anogenital distance and cryptorchidism in individual and reduced reproduction of the population. This review assembled information on the mechanisms of male reproductive toxicity induced by PBDEs, and constructed a causal mechanism relationship diagram from different levels using the an AOP framework to provide theoretical basis for ecological risk assessment and environmental management of PBDEs. The AOP framework makes it possible to develop risk management strategies based on toxicity mechanisms and support for development of Integrated Approach to Testing and Assessment (IATA) which are available for regulatory purposes.

Mechanisms of Male Reproductive Toxicity of Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers (PBDE) are a group of flame retardants used in a variety of artificial materials. Despite being phased out in most industrial countries, they remain in the environment and human tissues due to their persistence, lipophilicity, and bioaccumulation. Populational and experimental studies demonstrate the male reproductive toxicity of PBDEs including increased incidence of genital malformations (hypospadias and cryptorchidism), altered weight of testes and other reproductive tissues, altered testes histology and transcriptome, decreased sperm production and sperm quality, altered epigenetic regulation of developmental genes in spermatozoa, and altered secretion of reproductive hormones. A broad range of mechanistic hypotheses of PBDE reproductive toxicity has been suggested. Among these hypotheses, oxidative stress, the disruption of estrogenic signaling, and mitochondria disruption are affected by PBDE concentrations much higher than concentrations found in human tissues, making them unlikely links between exposures and adverse reproductive outcomes in the general population. Robust evidence suggests that at environmentally relevant doses, PBDEs and their metabolites may affect male reproductive health via mechanisms including AR antagonism and the disruption of a complex network of metabolic signaling.

Developmental exposure to the DE-71 mixture of polybrominated diphenyl ether (PBDE) flame retardants induce a complex pattern of endocrine disrupting effects in rats

Polybrominated diphenyl ethers (PBDEs) are legacy compounds with continued widespread human exposure. Despite this, developmental toxicity studies of DE-71, a mixture of PBDEs, are scarce and its potential for endocrine disrupting effects in vivo is not well covered. To address this knowledge gap, we carried out a developmental exposure study with DE-71. Pregnant Wistar rat dams were exposed to 0, 40 or 60 mg/kg bodyweight/day from gestation day 7 to postnatal day 16, and both sexes were examined. Developmental exposure affected a range of reproductive toxicity endpoints. Effects were seen for both male and female anogenital distances (AGD), with exposed offspring of either sex displaying around 10% shorter AGD compared to controls. Both absolute and relative prostate weights were markedly reduced in exposed male offspring, with about 40% relative to controls. DE-71 reduced mammary gland outgrowth, especially in male offspring. These developmental in vivo effects suggest a complex effect pattern involving anti-androgenic, anti-estrogenic and maybe estrogenic mechanisms depending on tissues and developmental stages. Irrespective of the specific underlying mechanisms, these in vivo results corroborate that DE-71 causes endocrine disrupting effects and raises concern for the effects of PBDE-exposure on human reproductive health, including any potential long-term consequences of disrupted mammary gland development.

Modern, exogenous exposures associated with altered mammary gland development: A systematic review

BACKGROUND

Many women report low milk supply as the reason for premature breastfeeding cessation. Altered mammary gland development may impact a woman's lactation ability.

OBJECTIVE

This review identifies modern exogenous exposures which alter mammary gland development during embryonic life, puberty and pregnancy.

METHODS

A systematic review was undertaken whereby Medline, CINAHL and Embase articles published from January 1, 2005 to November 20, 2020 were searched using the keywords puberty or embry* or fetal or foetal or foetus or fetus or pregnan* or gestation* AND "mammary gland development" or "breast development" or "mammary development" or "mammary gland function" or "mammary function" or "insufficient glandular tissue" or "mammary hypoplasia" or "breast hypoplasia" or "mammary gland hypoplasia" or "tubular breast*" or "tuberous breast*" or "glandular tissue" or "breast composition" or "mammary composition" or "mammary gland composition". After initial screening of 1207 records, 60 full texts were assessed for eligibility; 6 were excluded due to lack of information about exposure or outcome, leaving 54 studies.

RESULTS

The review included results from 52 animal (rats and mice, monkeys, rabbits, sheep, goats pigs and cows) and 2 human studies. Various endocrine disrupting chemicals and an obesogenic diet were found to be associated with altered mammary gland morphology during key development stages.

CONCLUSIONS

To improve lactation outcomes, future studies need to focus on lactation as the endpoint and be conducted in a standardised manner to allow for a more significant contribution to the literature that allows for better comparison across studies.

In Utero and Lactational Exposure to an Environmentally Relevant Mixture of Brominated Flame Retardants Induces a Premature Development of the Mammary Glands

In utero and prepubertal development of the mammary glands occurs minimally in a hormone independent manner until puberty where maturation of the hypothalamic-pituitary-gonadal axis drives an extensive remodeling. Nevertheless, because the immature glands contain functional hormone receptors, they are especially vulnerable to the effects of endocrine disruptors, such as brominated flame retardants (BFRs). BFRs are widespread chemicals added to household objects to reduce their flammability, and to which humans are ubiquitously exposed. We previously reported that in utero and lactational exposure to BFRs resulted in an impaired mammary gland development in peripubertal animals. Here, we assessed whether BFR-induced disruption of mammary gland development could manifest earlier in life. Dams were exposed prior to mating until pups' weaning to a BFR mixture (0, 0.06, 20, or 60 mg/kg/day) formulated according to levels found in house dust. The mammary glands of female offspring were collected at weaning. Histo-morphological analyses showed that exposure to 0.06 mg/kg/day accelerates global epithelial development as demonstrated by a significant increase in total epithelial surface area, associated with a tendency to increase of the ductal area and thickness, and of lumen area. Significant increases of the Ki67 cell proliferation index and of the early apoptotic marker cleaved caspase-9 were also observed, as well as an upward trend in the number of thyroid hormone receptor α1 positive cells. These molecular, histologic, and morphometric changes are suggestive of accelerated pubertal development. Thus, our results suggest that exposure to an environmentally relevant mixture of BFRs induces precocious development of the mammary gland.