Impact of Developmental Exposures to Endocrine Disrupting Chemicals on Pituitary Gland Reproductive Function

Abstract

The pituitary gland is crucial for regulating many physiological systems, including reproduction. Clear evidence suggests pituitary function could be impaired by exposure to endocrine disrupting chemicals (EDCs). Humans and animals are exposed to EDCs throughout life but exposure during critical periods when the pituitary is developing could have more damaging consequences. In this review we summarize the development of the pituitary gland, including the impact of hormone signals, and describe how in vivo EDC exposure during development might alter pituitary function. These include changes in pituitary hormone, mRNA and protein expression levels, as well as pituitary cell number and population balance. We focus on reproductive hormone-producing cells as well as the other endocrine and pituitary stem/progenitor cells. We reveal the current gaps in knowledge and suggest future directions in terms of understanding effects of developmental EDC exposure directly on the pituitary gland.

In Brief

Endocrine disrupting chemicals can impact reproduction by affecting the hypothalamic-pituitary- gonadal axis. This review emphasizes the impact of endocrine disrupting chemicals on pituitary development and function.

Introduction

The pituitary gland is the “master gland” of endocrine system and is central to reproductive function as part of the hypothalamic-pituitary-gonadal axis. The anterior pituitary gland coordinates gonadal steroidogenesis, gamete production, and lactation as well as body growth, stress response, and energy metabolism. The pituitary gland regulates these physiological processes by taking intrinsic signals from the body and executing its functions via a group of cells that make up the anterior lobe: gonadotrophs- synthesize gonadotropin: luteinizing hormone (LH) and follicle-stimulating hormone (FSH), lactotrophs- synthesize prolactin (PRL), thyrotrophs- synthesize thyroid stimulating hormone (TSH), somatotrophs- synthesize growth hormone (GH), corticotrophs- synthesize adrenocorticotropic hormone (ACTH) and melanotrophs- synthesize melanocyte-stimulating hormone (αMSH) (Drouin 2017). Pituitary stem/progenitor cells expressing SOX2/SOX9 are responsible for populating the anterior lobe with these hormone cells (Nantie et al. 2014; Edwards and Raetzman 2018) (le Tissier and Mollard 2021). Failure of the pituitary gland to develop properly results in congenital pituitary hormone deficiencies (hypopituitarism), that can lead to reproductive disorders (de Angelis et al. 2017; Mikhael et al. 2019), growth deficiencies (Tritos and Klibanski 2016) and other endocrine dysfunctions. Genetic causes of hypopituitarism include mutations in the transcription factors SOX2, PROP1, POU1F1, HESX1, LHX3/LHX4, GLI2, and OTX2 (Castinetti et al. 2016; Fang et al. 2016; Gregory and Dattani 2020). Despite intensive studies to identify mutations that cause hypopituitarism, only 10– 16% of patients are diagnosed with known genetic abnormalities (Bosch i Ara et al. 2021; Gregory et al. 2023). Unknown causes of hypopituitarism might include environmental factors which can alter pituitary cell populations during development and interfere with proper hormone secretion.

Environmental factors that could cause pituitary dysfunctions include traumatic injuries, infections, and chemical exposures. Among all the agents associated with pituitary dysfunction, effects of endocrine-disrupting chemicals (EDCs) are attracting increasing research interest as humans are ubiquitously exposed to them. Commonly known types of EDCs include dioxins, brominated flame retardants, tributyltin (TBT), and polychlorinated biphenyls (PCBs); pesticides: dichlorodiphenyltrichloroethane (DDT), neonicotinoids, and endosulfan; plasticizers: phthalates and bisphenol A (BPA); pharmaceutical EDCs: parabens and triclosan; and heavy metals: arsenic and mercury (Lauretta et al. 2019; Yilmaz et al. 2020). EDC exposures can have multiple mechanisms of action including activating or inhibiting hormone receptors, activating of kinase signaling pathways, interfering with hormone feedback regulation, disrupting hormone production, transportation, and metabolism (Yilmaz et al. 2020), and epigenetic regulation (Alavian-Ghavanini et al. 2018; Montjean et al. 2022). Much evidence suggests that EDC exposure is correlated with decreased fertility in both woman and men (Pan et al. 2023), yet how EDC exposures impair reproductive function in the pituitary gland is understudied.

Although humans and animals are constantly exposed to EDCs, there are some critical developmental windows during which exposure can cause more serious consequences. Critical windows include gestation, the neonatal period, puberty and pregnancy/lactation (Palanza et al. 2016; Kumar et al. 2020, Carretero et al. 2003) as those are developmental periods when the pituitary goes through intensive tissue proliferation and differentiation. Many of the impacts of developmental exposure to EDCs could last into adult life (Horzmann et al. 2021) and even pass on to future generations (Brehm et al. 2018; Drobná et al. 2018; Wen et al. 2020). In this review, we describe the development and regulation of the pituitary gland and how exposure to EDCs during critical developmental periods can alter pituitary organogenesis leading to disruptions in reproductive and endocrine function. We will highlight the gonadotrophs and other specialized hormone producing cells, describing how extrinsic EDC exposures alter their developmental trajectories and functions.

Articles used in the review were selected from Web Of Science database using the keywords in Table 1. Each keyword from keyword 1, keyword 2, keyword 3 (and keyword 4 for transgenerational effects) columns were used in different combinations for the literature search. The most recent work evaluating the effects of EDCs on different animal models that reported changes in pituitary function, pituitary hormone concentrations, gene and protein expression and pituitary cell properties were selected. When available, human relevant concentrations for the chemicals discussed are cited (Table 2).

Table 1.

Terms used for literature search. Combinations of each keyword from keyword1 + keyword2 + keyword3 (+ keyword4) were used for searching.

Keyword 1	Keyword 2	Keyword 3-Gonadotroph	Keyword 3-Lactotroph	Keyword 3-Somatotroph	Keyword 3-Corticotroph	Keyword 3-Thyrotroph	Keyword 3-Stem cells	Keyword 4-Transgenerational effects
Endocrine Disrupting Chemical	Prenatal	LH	PRL	GH	ACTH	TSH	Pituitary Stem cell	Transgenerational
	Developmental	FSH	Prolactin	Growth hormone	Adrenocorticotropic hormone	Thyroid stimulating hormone
	In utero	Gonadotroph	Lactotroph	Somatotropin	Corticotroph	Thyrotropin
Endocrine Disruptor	Gestational	Gonadotroph	Lactotrope	Somatotroph	Corticotrope	Thyrotropic hormone
	Lactational	Gonadotropin		Somatotrope		Thyrotroph
	Perinatal					Thyrotrope
	Neonatal

Table 2.

EPA RfD and human ranges of exposure for the chemicals discussed in this review.

Compound	EPA (RfD)* mg/kg/day	Reported human daily intake	Reference
Bisphenol A (BPA)	0.05	<0.0006–0.0015 mg/kg/day (adult) 0.0003–0.0011 mg/kg/day (child)	Prins et al 2019
Di(2- ethylhexyl) phthalate (DEHP)	0.02	0.003–0.03 mg/kg/day 0.36–14 mg/kg/day (medical devices)	Hannon and Flaws 2015
Dibutyl phthalate DBP		0.0008–0.005 mg/kg/day	Hannon and Flaws 2015
Hexavalent chromium (Cr VI)	0.003	0.03–12.9 ppb (drinking water US) 0.0005–0.18 μg/person/day (inhalation UK)	Sutton 2010 Rowbotham et al 2000
Glyphosate	0.1	0.314–0.449 μg/L (urine) 3.3 μg/kg/day (Europe and US)	Mills et al 2017 Niemann et al 2015
Methoxychlor (MXC)	0.005	167.9 ng/g lipid (follicular fluid China) 0.31 ng/kg/day (food Nigeria)	Zhu et al 2015 Adeyi et al 2021
Polychlorinated Biphenyls (PCB)	2–7×10−5 Aroclor 1254 Aroclor 1016	PCB Aroclor 1260 0.3 μg/kg/day (Inuit breastfed infants) PCB 105 1.01 ng/kg/day PCB 138 7.28 ng/kg/day PCB 153 12.66 ng/kg/day PCB 170 4.86 ng/kg/day (Croatia breastfed infants)	Ayotte et al 1995 Romanic et al 2023
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)	7×10−10	1.2–2.2 pg/g lipid (Vietnam) 5.5–73.3 pg/g lipid (Italy Seveso) 3.6–33 pg/g lipid (US) 2.2–101 pg/g lipid (Vietnam) 2.6–6.4 pg/g lipid (Japan)	Vu et al 2023 Marques and Domingo 2019 Schecter et al 2006
Cypermethrin	0.01	1.7 ×10−4 mg/kg/day (US) 1.6×10−5-4.9×10−6 mg/kg/day (China)	Riederer et al 2010 Li et al 2018
Tributyltin (TBT)	3×10−4 (TBTO)	1.8–2.6 μg/day/person (seafood) 7–70 μg/kg/day (coastal Norway)	Kotake 2012 Antizar-Ladislao 2008
Furan	0.001	0.1–80 μg/kg/day 1.82 mg/kg/day (baby food)	Batool et al 2023
2,3,4,7,8-Pentachlorodibenzofuran (PnCDF)	N/A	5.6–50 pg/g lipid (US) 6.8–3.1 pg/g lipid (Vietnam) 7.3–122 pg/g lipid (Japan)	Schecter et al 2006
Nonylphenol	N/A	0.95 μg/kg/day	Ringbeck et al 2022

Regulation of Pituitary Gland Development

The anterior and intermediate pituitary lobes originate from Rathke’s pouch, an invagination from oral ectoderm. The posterior pituitary originates from an evagination from neural ectoderm, derived from the ventral diencephalon (Daly and Camper 2020). Together they form the pituitary gland starting from gestational week 5 in human and embryonic day 10.5 (E10.5) in mice (Alatzoglou et al. 2020). Figure 1 depicts the developmental trajectory of the pituitary gland in mice and humans. The differentiation of hormone-producing cells in the anterior and intermediate pituitary is regulated by various transcription factors which sequentially activate different signaling pathways.

Figure 1.

Pituitary development. Sagittal view of pituitary cell differentiation and lineage commitment is illustrated (blue: anterior lobe, yellow: intermediate lobe, light purple: posterior lobe, dark purple: ventral diencephalon). The developmental stages shown are from mouse embryonic day 10.5 (E10.5) to e15.5, the corresponding stages of human gestational week (GW) are noted below. Following the origination of Rathke’s pouch, corticotrophs (C) along with rostral tip thyrotrophs (RT) can be identified at around E12.5 in mouse and GW 7 in human. By the time of E15.5 in mouse and GW 12 in human, thyrotrophs (T), gonadotrophs (Gona) and Pit1 positive future Lactotrophs (Lac) and Somatotrophs (Som) can also be distinguished. Terminal differentiation and maturation of different cell types continues through the gestational period in human and postnatally in mouse.

In addition to intrinsic signals regulating pituitary gland development, the key functions of the pituitary gland are also regulated by hypothalamic hormonal input. Other than regulating the secretion of pituitary hormones through a hypothalamic-pituitary axis, hypothalamic signals can also impact the development of the pituitary gland. For example, gonadotropin releasing hormone (GnRH) which regulates the secretion of LH and FSH from the pituitary gland has also been found to induce the differentiation of gonadotrophs and thyrotrophs (Héritier and Dubois 1994). Similarly, growth hormone releasing hormone (GHRH) is also essential for postnatal expansion of the somatotroph lineage (Lin et al. 1993; Gonigam 2023). For thyrotrophs, thyrotropin-releasing hormone (TRH) is required for their postnatal maintenance but is not necessary for embryonic differentiation (Shibusawa et al. 2000). Interestingly, TRH can also induce the differentiation of gonadotrophs and lactotrophs (Heritier and Dubois 1993). Therefore, any developmental change in the number, neuropeptide expression, or function of the hypothalamic neurons that secrete these hormones could impact pituitary function. Importantly, developmental EDC exposure is known to impact hypothalamic neuron populations and/or signaling networks (Graceli et al. 2020; Lopez-Rodriguez et al. 2021; Patisaul 2021); however, the effects of EDCs on the hypothalamic inputs to the pituitary are beyond the scope of this review.

The pituitary gland can also be regulated by other pituitary-expressed hormone receptors that take feedback signals from downstream endocrine organs of the hypothalamic-pituitary axis such as estrogen receptor (ER) (Arao et al. 2019), androgen receptor (AR) (Ryan et al. 2021) and thyroid hormone receptor (THR) (Ortiga-Carvalho et al. 2014). Disruptions in hormone receptor signaling at the level of pituitary can cause impaired reproductive functions in mouse models. For example, ERα in the pituitary is found to be necessary for fertility in females as pituitary-specific ERα knockout (KO) mice presented with irregular estrous cycles and infertility (Gieske et al. 2008). Additionally, exogenous estradiol administration postnatally regulated gonadotroph gene expression (Eckstrum et al. 2016). Androgen signaling at the pituitary is crucial for repression of prolactin secretion and lactotroph development in males as pituitary-specific AR KO male mice showed hyperprolactinemia and female-like lactotroph development and distribution (O’Hara et al. 2021). Aside from KO models, androgen is also known to regulate gonadotropin expression (Kreisman et al. 2017) and gonadotropin-releasing hormone receptor (Gnrhr) in gonadotrophs (Ryan et al. 2021). As for thyroid hormone, it is essential for maintaining the population of somatotrophs and lactotrophs in mouse and chicken (Stahl et al. 1999; Liu and Porter 2004), and a congenital hypothyroidism mouse model exhibited increased numbers of thyrotrophs (Stahl et al. 1999). Importantly, hormone signaling at the pituitary could be affected by endocrine disruption, including direct effects at the pituitary and through circulating hormone level changes, and thus impact pituitary development.

Given the complicated regulation of pituitary development by different intrinsic and extrinsic factors, disruptions in any of the mechanisms mentioned above by EDCs could ultimately impact pituitary function; however, most current studies concerning EDC’s effect on developing pituitary only focused on changes in circulating pituitary hormone concentrations later in life. Direct impacts of EDCs on the pituitary including changes in pituitary gene and protein expression during development, pituitary progenitor properties, as well as alterations in transcription factor regulation and signaling pathways remain understudied. This review will detail the known effects of developmental EDC exposure on the pituitary and spotlight areas for further study.

Effects of Developmental EDC Exposure on Gonadotrophs

Among all the endocrine functions the pituitary gland regulates, reproduction is the most well- studied target of developmental EDC exposure. The pituitary cell type that mediates reproduction is the gonadotroph. EDCs affect the gonadotroph lineage and its functions by changing serum gonadotropin hormone- luteinizing hormone (LH) and follicle stimulating hormone (FSH) concentrations, gonadotropin mRNA and/or protein concentrations, Gnrhr concentrations, and gonadotroph cell numbers. Alterations in gonadotroph functions caused by EDC exposure can be observed in various animal models and the effects are diverse depending on the type of EDC.

Estrogenic and antiandrogenic EDCs like BPA and di(2- ethylhexyl) phthalate (DEHP) can often increase serum level of gonadotropins as well as induce the mRNA or protein concentrations of Lhb and Fshb. For example, prenatal and lactational exposure to BPA (3 μg/kg/day) increased serum LH concentrations in female rats at P30 (Gámez et al. 2015). Interestingly, only female mice exposed to BPA from embryonic day 10.5 (E10.5) until E18.5 were found to have increased Lhb, Fshb and Gnrhr mRNA after being exposed to 0.5 μg/kg/day BPA at P1, but mRNA levels of those three genes were decreased in mice exposed to 50 μg/kg/day of BPA (Brannick et al. 2012). Prenatal and lactational exposure to DEHP caused an increase in Lhb mRNA in female mice (0.05, 5 mg/kg/day), as well as increase in Lhb and Fshb mRNA in males (5 mg/kg/day) at P21 (Pocar et al. 2012). In female quails exposed to DEHP (500, 1000 mg/kg) from P15 to P60 (a period before sexual maturation), serum LH concentrations were increased at P60, however, FSH concentrations were decreased (Li et al. 2020). Our lab’s recent paper reported that prenatal exposure (E15.5-E18.5) to DEHP (200 μg/kg/day) in mice specifically increased Fshb mRNA levels in males, while female Fshb, and Lhb mRNA for both sexes remained unchanged at P0 (Ge et al. 2022). Additionally, prenatal exposure to DEHP (50 μg/kg) in male rats caused increases in LHβ and FSHβ protein concentrations at P35 but decreases in both proteins at P90 (Abdel-Maksoud et al. 2015). Therefore, DEHP causes an increase in gonadotropins when assayed near the exposure window. In rats, prenatal exposure to DBP (100 and 500 mg/kg/day) from E12.5 to E21.5 was reported to decrease only Lhb mRNA levels at P3, 7, 14 and 21, while 100 mg/kg/day DBP dosage increased the Lhb level at P28 and P56 (Chen et al. 2017). This suppression of Lhb mRNA could be due to the substantially higher dose of DBP relative to DEHP or that DBP might signal through different receptors or pathways than DEHP.

In terms of understanding a mechanism for phthalate disruption of gonadotropin expression, we found that exposure to DEHP in vivo and its metabolite, MEHP, in vitro was able to activate aryl hydrocarbon receptor (AhR) signaling in the pituitary. Prenatal DEHP upregulated the AhR target Cyp1b1 mRNA in both male and female offspring at P0, and MEHP treatment of embryonic pituitary explants caused an increase in Cyp1b1 mRNA which was blocked by an AhR antagonist (Ge et al. 2022). It is known that AhR signaling impacts gonadotropin synthesis (Hattori et al. 2018), although the involvement of AhR in DEHP mediated gonadotropin changes is still under investigation. Prenatal exposure to the dioxin 2,3,7,8-Tetrachlorodibenzodioxin (TCDD) (1 μg/kg), a potent AhR ligand, at E15 decreased serum LH and FSH concentrations as well as decreased Lhb and Fshb mRNA levels in rat fetuses from E18 to postnatal day 0 (P0). Mechanistically, TCDD induced increases in histone deacetylase concentrations and this correlated with a decrease in acetylated histone recruitment to Lhb’s promoter region. (Takeda et al. 2012).

Other than TCDD, early-life PCB exposure is correlated with reduced serum gonadotropin hormone concentrations: LH concentrations in male prepubertal humans (Grandjean et al. 2012), LH and FSH concentrations in male rats (PCB169, 30 μg/kg/day) (Yamamoto et al. 2005), and LH in neonatal female rats exposed prenatally to Aroclor1221 (1 mg/kg/day) (Weis et al. 2023). Interestingly, female lambs were found to have increased GnRH-induced LH and FSH concentrations after prenatal and lactational exposure to PCB (PCB153, 49 μg/bw/day) (Kraugerud et al. 2012). These differences may be due to species, congeners of PCBs examined or specifically examining GnRH induced concentrations of gonadotropins. Prenatal exposure to hydraulic fracturing chemicals (unconventional oil and natural gas (UOG) mixture at 3, 30, 3000 μg/kg/day) in female mice could reduce serum LH and FSH concentrations on postnatal day 85 (P85), which is a long-term effect of prenatal exposure (Kassotis et al. 2016). Finally, prenatal or early-life postnatal exposure to other EDCs such as pyrethroid insecticide cypermethrin (0.5, 5, 50 μg/kg), Microcystin-LR (MC-LR, 30 μg/L), Hexavalent chromium (100, 200 ppm) and TBT (10 mg/kg) are also found to increase LH and/or FSH hormone concentrations and mRNA levels in both sexes in rats or zebrafish (Kariyazono et al. 2015; Hou et al. 2016; Ye et al. 2017; Shobana et al. 2020). These studies highlight the varied sensitivity of pituitary gonadotropins to developmental EDC exposures.

Developmental exposure to EDCs also affects gonadotroph cell numbers and properties. Prenatal exposure to low-dose BPA (0.5 μg/kg/day) increased gonadotroph cell number as well as pituitary proliferation in female mice at P1 (Brannick et al. 2012). Ovines prenatally exposed to sewage sludge had decreased numbers of LHβ- and ERα- positive gonadotrophs in both sexes at E110 (Bellingham et al. 2009). Other EDCs like Di-n-butyl phthalate (DBP) and methoxychlor (MXC) have more complicated impacts on gonadotroph cell number: late embryonic through lactational exposure to DBP (10000 ppm) decreased FSH-positive cells but increased LH-positive cells in both males and females at P21, while both sexes had increased FSH-positive cells at postnatal week 11 (PNW11) (Lee et al. 2004). Exposure to MXC (1200 ppm) during a similar period decreased LH- and FSH-positive cells in males but only decreased LH-positive cells in females at PNW3. However, FSH-positive cells were increased in females at PNW11 (Masutomi et al. 2004). These studies demonstrate that gonadotroph development from stem cells or selective expansion from committed progenitors may be altered by to EDC exposure, leading to inappropriate gonadotroph populations.

In summary, looking at effects of various EDCs on reproductive functions, BPA and phthalates are two of the most well-studied types. As to impacts on gonadotroph functions after developmental exposure, EDCs were able to increase gonadotropin hormone and mRNA levels in both sexes at multiple ages in most cases. These relatively consistent outcomes indicate the most likely mechanism of their actions may be to interfere with the regulation of HPG axis. These EDCs could directly impact gonadal steroidogenesis or block the negative feedback from estrogens or androgens on the pituitary and/or hypothalamus, thus making the pituitary increase the production of gonadotropins. However, BPA and phthalates, as well as other types of EDCs mentioned in this section, can have multiple modes of action, which is reflected in the fact that differing results were dependent on sex, exposure window, dosage, and time of inspection. In addition, although numerous studies reported changes in gonadotroph functions in response to developmental EDC exposure, most of them only focused on gonadotropin hormone and/or mRNA level changes. Future studies are needed to better understand the direct impacts of developmental exposure to EDCs on gonadotrophs, including stem cell proliferation and differentiation to gonadotrophs, alterations in transcription factors regulating gonadotrophs, changes in hormone receptors on gonadotrophs, and signaling pathways possibly regulating the expression of gonadotropins.

Effects of developmental EDC exposure on lactotrophs

Like gonadatrophs, pituitary lactotrophs are another common target of developmental EDC exposure related to reproductive dysfunction, especially during pregnancy and lactation when the lactotroph cell population expands tremendously (Goluboff and Ezrin 1969; Carretero et al. 2003). Changes in serum prolactin (PRL) concentrations, pituitary Prl mRNA levels, and lactotroph numbers are reported to occur with EDC exposure.

Studies on rodents revealed prenatal exposure to hexavalent chromium (100, 200 ppm) and hydraulic fracturing chemicals (UOG mixture at 3, 30, 300, 3000 μg/kg/day) decreased serum PRL concentrations in males at P30 (Shobana et al. 2020) and females at P85 (Kassotis et al. 2016), respectively. Other EDCs caused the opposite effect on PRL concentrations in multiple animal species: prenatal and lactational exposure to glyphosate-based herbicide (350 mg/kg/day) increased PRL concentrations in adult male rats (Gomez et al. 2019). As for females, exposure to DEHP (1000 mg/kg) from P15 to P60 increased PRL concentrations in quails (Li et al. 2020). BPA and phthalates are the most well-studied EDCs in terms of lactotroph function disruptions: in rats, prenatal exposure to BPA (250 μg/kg) increased PRL concentrations in males at P30 (Ramos et al. 2003), and similar effects of BPA (0.03, 3 ppm or 100, 300 mg/kg/day) were also observed in mice and female rats with a longer exposure time (prenatal and postnatal exposure) (Kendig et al. 2012; Delclos et al. 2014). Researchers have also investigated the effects of exposure to mixtures of EDCs: prenatal and perinatal exposure to an anti-androgenic mix (AAmix450) containing different types of phthalates and pesticides decreased PRL concentrations in female rats at P22 (Johansson et al. 2016).

Prenatal exposure to EDCs also changed the population and properties of lactotrophs in rats. Late embryonic and lactational exposure to DBP (10000 ppm) decreased the number of lactotrophs in both males and females at P21 (Lee et al. 2004). Interestingly, prenatal and lactational exposure to DEHP (200 μg/kg/day) increased the lactotroph population in adult females at P75 (Pérez et al. 2020). The reason for this opposite effect on lactotroph population caused by different types of phthalates is unknown and possible explanations include differences in phthalates (DBP vs. DEHP), different dosages (10000 ppm vs. 200 μg/kg/day), various developmental stages at the time of inspection (P21 vs. P75) and different exposure windows (E15 - P21 vs. E0 - P21). Late embryonic and perinatal exposure to MXC (1200 ppm) caused more complicated effects on lactotroph populations between different sexes at different ages: MXC exposure decreased lactotroph cell number at PNW3 in males but increased lactotroph cell number at PNW11 in females (Masutomi et al. 2004). In addition to increasing lactotroph number, DEHP (200 μg/kg/day) exposure also affected the properties of lactotrophs: decreased proportions of ERα- and ERβ- positive lactotrophs were found in adult females (Pérez et al. 2020). As the most well studied EDCs, BPA and phthalates generally increase PRL concentrations and/or lactotroph population in developmentally exposed animals. These consistent outcomes could be explained by their estrogen-like properties, as estradiol is known to be able to induce lactotroph proliferation and PRL secretion (Zárate and Seilicovich 2010). However, the mechanism for the induction is still unclear. Since PRL secretion from lactotrophs is generally inhibited by dopamine secreted from the hypothalamus (Dobolyi et al. 2020), it is possible that changes in lactotrophs’ response to dopamine, such as changes in dopamine receptor expression and downstream signaling cascades caused by EDC exposure could be part of the mechanism. The downregulation of estrogen receptors by DEHP in lactotrophs (Pérez et al. 2020) also indicates that DEHP may interfere with estrogen signaling pathways, and future studies are needed for understanding the detailed mechanisms. Given the important role lactotrophs play in regulating reproductive functions, more research is needed to better understand the mechanisms of direct impacts of developmental EDC exposure on lactotrophs. Specifically, since the development of lactotrophs is sex-dependent and they’ve been known to be sensitive to estrogen and androgen regulation (Ishida et al. 2007; O’Hara et al. 2021), future studies should also consider the EDCs’ impacts on sex steroid hormone concentrations and receptor signaling in relation to lactotroph development and functions in different sexes.

Effects of developmental EDC exposure on other pituitary hormone cell types

While thyrotroph, somatotroph, and corticotroph cells do not directly regulate reproduction, alterations of hormone content or cell number in these endocrine cells can contribute to problems with fertility and pregnancy.

Thyrotroph cells in the pituitary maintain metabolic homeostasis through the hypothalamic- pituitary-thyroid (HPT) axis and proper titration of circulating thyroid hormones. There have been extensive human studies investigating the correlation between maternal exposure to EDCs and alteration of the newborn’s TSH level and thyroid function. (Sun et al. 2022). In addition to human studies looking at correlations between prenatal EDC exposure and TSH concentrations, dosing experiments using rodents have revealed more direct relationships between EDC exposure and thyrotroph functions. Prenatal exposure to hydraulic fracturing chemicals (UOG mixture at 300 μg/kg/day) in mice caused elevated TSH concentrations in adulthood (Kassotis et al. 2016). Perinatal exposure to glycophosphate based herbicide (5, 50 mg/kg/day) decreased TSH concentrations at P90 (de Souza et al. 2017). Di-n-hexyl phthalate (DnHP) and dicyclohexyl phthalate (DCHP) could also have long-term effects on both sexes in rats after prenatal exposure, but their effects are dose-dependent: DnHP (100 mg/kg/day) decreased TSH concentrations while DCHP increased TSH concentrations at a lower dosage (20, 100 mg/kg/day) and increased TSH concentrations at a higher dosage (500 mg/kg/day) (Göktekin and Barlas 2017). Similar periods of exposure to PCB-126 (20, 40 μg/kg) caused elevated fetal TSH concentrations at E20 (R.G. Ahmed et al. 2018). For longer exposure periods, prenatal and lactational exposure to DEHP (30, 300, 750 mg/kg/day) and propylthiouracil (PTU, 3 ppm) cause elevated TSH concentrations in prepubertal offspring (Bansal and Zoeller 2019; Dong et al. 2019). Effects of BPA exposure on thyrotrophs are more complicated: prenatal exposure to BPA caused males to have decreased TSH at P15 (300 mg/kg/day) and females to have increased TSH at P90 (100, 300 mg/kg/day) (Delclos et al. 2014), however, other studies showed increased TSH in young offspring caused by prenatal and/or lactational BPA exposure (R. Ahmed et al. 2018; Mahmoudi et al. 2018).

Overall, although there has been extensive research looking at TSH concentrations in relation to developmental EDC exposure, in most cases, a mechanism of action for altered TSH concentrations was not studied. Hypotheses include interference with thyroid function and thyroid hormone production. This would lead to feedback-level alterations of TRH from the hypothalamus and TSH from the pituitary, along with thyroid hormone receptor signaling interference. Studies have shown that thyrotroph development and proliferation can be regulated by thyroid hormone, TRH, and estrogens (Malaguti et al. 2004; Tonyushkina et al. 2017), which can all be impacted by developmental EDC exposure. Thyroid dysfunction is linked to polycystic ovary syndrome (PCOS) as well as endometriosis, two common pathologies that can interfere with human fertility (Janssen et al. 2004; Poppe et al. 2002). Future studies could focus on understanding the impacts of EDCs on thyrotrophs by discovering the mechanisms by which hormone signals in this pathway are impacted by EDC exposure. Also, uncovering the direct developmental impact of EDC exposure on components of the HPT axis at different exposure concentrations may yield more consistent results and allow mechanisms to be inferred more readily.

A variety of EDC exposures have been studied in the context of pituitary somatotroph cells. In many cases, developmental EDC exposures caused a decrease in GH concentrations regardless of different exposure window (prenatal or postnatal), various timepoints of inspection (embryonic, pre-pubertal, and adult), and sex differences. Exposure to both PCB-126 (20, 40 μg/kg) and dexamethasone (DEXA, 0.2 mg/kg) starting from E1 decreased rat fetal GH concentrations at e20 (Ahmed 2016; R.G. Ahmed et al. 2018). As for a slightly different exposure window, prenatal and lactational exposure to sodium valproate (50 mg/kg) also decreased GH concentrations in prepubertal rats at P14 and P21 (A-G et al. 2021). One-dose exposure to TCDD (0.05 – 30 μg/kg) and PnCDF (1–1000 μg/kg) at E15 also decreased GH concentrations, as well as Gh mRNA levels, in both fetal (e20) and peri-adolescent (PNW5) rats (Taura et al. 2014). As for long-term effects, 10-day exposure to furan (10, 20 mg/kg/day) from P1 to P10 decreased GH concentrations in adult male rats (Rehman et al. 2019).

Although multiple EDCs caused a decrease in GH concentrations, we found reports that others caused an increase in GH. Prenatal exposure to hydraulic fracturing chemical (UOG mixture at 300 μg/kg/day) caused female rats to have increased GH concentrations during adulthood (Kassotis et al. 2016). Furthermore, GH concentrations of different sexes seem to respond to BPA exposure differently: early-life postnatal exposure in male rats (20, 40 μg/kg) cause a decrease in GH concentrations (R. Ahmed et al. 2018) while an increase was seen in females (2.5–6.25, 25–62 mg/kg) (Ramirez et al. 2012). However, the opposite effect observed from these two studies could also be due to a different exposure window- P1-P10 vs. P15-P30, different dosages, as well as different timepoints of inspection- P30 (immediately after the exposure) vs. 5 months (long-term effects). Other than changes in GH concentrations, prenatal and lactational exposure DEHP (10, 100 nM) was found to alter some properties of somatotrophs: the proportion of ERα- and ERβ- positive somatotrophs was decreased in adult female rats (Pérez et al. 2020).

In sum, EDC exposure can often lead to decreased GH concentrations regardless of the type of EDCs and experimental conditions. Lack of GH can lead to growth hormone deficiency (GHD) or combined pituitary hormone deficiency (CPHD), and there are clear links between GHD and delays in puberty onset as well as infertility (de Paula et al. 2021; de Boer et al. 1997; Keene et al. 2002).

The proliferation and differentiation of somatotrophs can be mediated by hormones including GHRH, thyroid hormone, glucocorticoids, and retinoic acid (Ellsworth and Stallings 2018), all of which could potentially be disturbed by developmental EDC exposure. One direction for future studies could be looking at impacts of developmental EDC exposure on signaling pathways regulated by those hormones at somatotrophs.

The effects of developmental EDC exposure on corticotrophs have been evaluated by quantifying serum ACTH concentrations or measuring pituitary Pomc mRNA levels. However, the relationship between developmental EDC exposure and corticotroph functions is less studied compared to other cell types. Prenatal and lactational exposure to sodium valproate (50 mg/kg) caused an increase in ACTH concentrations in prepubertal rats at the ages of P14 and P21 (Neama et al. 2021). Exposure to nonylphenol (2 μg/mL in drinking water) during a similar exposure window caused a long-term increase in ACTH concentrations in adult male rats (Chang et al. 2012). Besides, a shorter period of exposure (e10 to P7) to BPA (2 μg/kg) also caused increases in ACTH concentrations in adult male rats as well (Chen et al. 2014). Interestingly, a 7- day postnatal exposure (P0 to P7) to BPA (0.5 μg/kg/day) caused a decrease in pituitary Pomc mRNA level in mice at P7 (Eckstrum et al. 2018), which is contradictory to previous findings describing the change of ACTH level in adults. However, EDCs may have multiple targets in the production of ACTH peptide as it is cleaved from its precursor protein POMC. EDC exposure might disrupt any step of this post-translational modification suggesting that more investigation is required to fully understand the impact of EDCs on ACTH and POMC expression.

Given the limited number of studies of developmental EDC exposure on corticotrophs, little is known about the mechanisms of how EDCs impact corticotrophs during development. However, it’s an important area for research since studies have shown the association between developmental EDC exposure and changes in stress response (Kitraki et al. 2016; Gore et al. 2022), which could be related to organizational hypothalamic-pituitary-adrenal axis effects. Changes in TFs expression, specifically Pitx1 and Tbx19 that regulate the differentiation of corticotrophs, alterations in corticotroph’s response to CRH and cortisol, and regulation of CRH receptor and glucocorticoid receptor expression in response to developmental EDC exposure could be directions for future study.

Effects of developmental EDC exposure on stem cells/progenitor cells

Unlike hormone-producing cells in the anterior pituitary, the impacts of developmental exposure to EDCs on pituitary stem/progenitor cells are quite understudied. So far there is only one paper focused on revealing how developmental exposure to BPA (0.5, 50 μg/kg/day) could affect pituitary stem/progenitor cell differentiation into gonadotroph cells. Prenatal exposure to BPA from E10.5 to E18.5 increased proliferation in what appeared to be the stem cell population surrounding the cleft while increasing gonadotroph number only in female offspring. This finding indicates that developmental exposure to BPA in mice could influence reproductive functions by affecting pituitary stem cell proliferation and population numbers of gonadotrophs, which could result in altered gonadotropin concentrations (Brannick et al. 2012).

Stem cells are the earliest type of cells appearing in the pituitary gland. It’s important to understand the impacts of developmental EDC exposure on the pituitary stem/progenitor cells as they are responsible for forming the gland and regulating the population size of different cell types. To understand the direct impacts of EDCs on pituitary stem/progenitor cells, future studies could investigate different aspects including the expression of key transcription factors related to stem cells maintenance and proliferation like Sox2, Sox9, and Notch signaling. Additionally, the impact on lineage driving transcription factors like Tbx19, Pou1f1 and Nr5a1 should also be explored.

Transgenerational effects of developmental EDC exposure

Increasingly studies are focusing on understanding the transgenerational effects of EDC exposure (Rissman and Adli 2014; van Cauwenbergh et al. 2020; Martini et al. 2020), specifically, inheritance of epigenetic modifications from exposed individuals (F0) to subsequent generations. The mechanisms controlling these processes include DNA modification, histone modification and non-coding RNAs (ncRNAs) (Rissman and Adli 2014). Transgenerational effects on reproductive functions are a popular research topic and multiple studies examined both male and female reproductive function alterations in the offspring (Brehm and Flaws 2019). However, we found limited studies focused on pituitary gland transgenerational effects of EDC exposure on gonadotroph cells. Embryonic exposure to DEHP (500 mg/kg/day) from E11 until birth in the F1 generation caused F3 females to have significant increase in FSH and borderline increase in LH concentrations at 1 year of age (Brehm et al. 2018). Two-dose embryonic exposure to a PCB mixture (Aroclor 1221, 0.1, 1, 10 mg/kg) at E16 and E18 in the F1 generation caused a decrease in LH concentrations in F2 females during proestrus at age of P42 (Steinberg et al. 2008). F1 embryonic exposure to TCDD (500 ng/kg/day) from E8 to E14 caused an increase in LH concentrations in F3 females during estrus at age of P70 (Yu et al. 2019). Beyond rodents, transgenerational effects of EDC exposure were also observed in other animal models: in medaka (Oryzias latipes). A 7-day embryonic exposure to BPA (100 μg/L in water) in F0 fish caused adult males from F2 generation to have increased Gnrhr2 mRNA level, and exposure to 17α- ethinylestradiol (EE) caused an increase in Lhb mRNA level (Thayil et al. 2020).

Most transgenerational EDC reports we found focused on gonads and very few of them investigated upstream changes in the pituitary gland epigenome. Thus, more explorations of EDC’s transgenerational effects on the pituitary is needed, especially regarding important mechanistic processes like epigenetic modifications, including DNA methylation and histone remodeling. Epigenetic modifications in the pituitary have been extensively studied for their association with various types of pituitary adenomas, and the epigenetically modified genes include multiple tumor suppressor genes and imprinted genes (Pease et al. 2013; Hauser et al. 2019). However, there hasn’t been any research focusing on possible changes in epigenetic modifications in the pituitary in response to developmental EDC exposure, which could be a new perspective for understanding mechanisms of EDC’s transgenerational effects on reproductive function.

Conclusions and future directions

As the master gland regulating the body’s endocrine system that is associated with multiple physiological functions, the pituitary gland is an important organ for research in understanding systemic effects of EDC exposure. In this review, we summarized the regulation of pituitary gland development, and how exposure to EDCs during critical developmental periods could impact its actions, specifically focused on reproductive function, hormone producing cells and stem/progenitor cells in the anterior pituitary. Immediate, long-term and transgenerational effects of developmental EDC exposure on different cell types in the anterior pituitary are summarized and multiple aspects of dysregulated functions are reported including changes in pituitary hormone, mRNA expression, protein concentrations, hormone receptor levels, as well as pituitary cell number and properties.

Summarizing current research on how developmental EDC exposure affects different pituitary cell types, one thing we can notice is that many studies on the same EDC reported numerous, yet often contradictory, results (Supplementary Table 1), and it is crucial to understand the reasons for those inconsistencies. Sex, exposure window, dosage, and age are four major variations in terms of understanding how the pituitary gland could respond to developmental EDC exposure differently. Sexual dimorphism is an important factor known to cause different responses to EDC exposure between males and females in the hypothalamic-pituitary axis, specifically in reproductive-related functions which have the most prominent sex differences (Patisaul 2021). Sexually dimorphic regions in the hypothalamus could differently signal to the pituitary gland in response to EDC exposure, where sex differences also exist in the developing pituitary gland transcriptome (Bjelobaba et al. 2015; Eckstrum et al. 2016; Qiao et al. 2016; Hou et al. 2017). These variations in pituitary gene expression could partially account for how developmental exposure to EDCs could directly impact the pituitary differently between sexes.

Exposure window is another factor that could cause inconsistencies in the pituitary’s response to EDC exposure. In rodents, the formation of the pituitary gland starts at around e10.5 and the major cell types of the gland are established by the age of e15.5 (Alatzoglou et al. 2020; Daly and Camper 2020). Likewise, the human pituitary gland develops from GW5 to GW12 (Figure 1). During this period a host of hormones, TFs and signaling molecules orchestrate the developmental processes and can be perturbed by EDC actions (Figure 2). Exposure windows that start earlier in development are more likely to cause structural and/or permanent changes in the pituitary while exposure later might have less of an effect on establishment of the pituitary but might fine-tune cell numbers. Also, different exposure windows may selectively impact the ability of the pituitary cells to release hormones in response to signals from the hypothalamus or positive and negative feedback from target organ hormones.

Figure 2.

Lists of intrinsic and extrinsic signals regulating the development of pituitary. Impacts of EDCs including BPA, DEHP and TCDD on different factors are indicated.

The pituitary is a well-vascularized organ, yet the exact dosage of EDCs that directly contact the pituitary versus the dosage in blood or urine samples is not correlated. Research questions focusing on EDC metabolism in human and animal models and specifically local concentration of EDCs near the pituitary need to be answered. This review details reports describing a range of EDCs and doses used in various animal models. It is important to understand how these data compare to actual human exposures. Table 2 lists the majority of EDC compounds detailed in this review along with the EPA oral reference dose (RfD) and documented human intake worldwide. Importantly, we see a wide range of human exposure in different contexts. For example, typical human intake of the phthalate DEHP is 0.003–0.03 mg/kg/day, but that level can increase to 0.36– 14 mg/kg/day in individuals exposed to medical plastics (Flaws and Hannon 2015). Moreover, geographic location can greatly vary human contact with EDCs in the environment. PCB compounds and dioxins measured in humans can differ by an order of magnitude depending on proximity to EDC contaminated sites. (Ayotte et al 1995, Marques and Domingo 2019, Schecter et al 2006). Further, while humans may appear to be exposed to low concentrations of some compounds, it is well known in vivo that EDCs often have more potent low-dose responses or nonmonotonic dose responses (Vandenberg et al. 2012). Taken together, the wide range of EDC doses used in animal experiments reviewed here have strong physiological relevance to human exposures and reproductive health.

The last factor that may contribute to pituitary’s different response to developmental exposure to EDCs is age, which can overlap with other variables including the previous three mentioned. To understand the pituitary’s different response to EDCs at different ages, questions about mechanisms of EDC’s long-term effects of developmental exposure, which could be involved with changes in TFs, signaling cascades, epigenetic modifications among others need to be answered.

Most of the current research looking at impacts of developmental EDC exposure on the pituitary only reported secondary effects- mostly changes in pituitary hormone concentrations. Pituitary hormone changes caused by developmental EDC exposure are frequently framed as dysfunctions in the end glands along the hypothalamic-pituitary-organ axis, which secrete hormones acting on the pituitary in a feedback loop. However, our lab has begun studies investigating the effects of EDC exposure directly on the developing pituitary. These effects may be reflected in alterations in pituitary cell proliferation and differentiation, proportion of different cell types that make up the pituitary, expression of TFs related to pituitary development, and changes in hormone receptors expressed in the pituitary (Figure 1,2) (Eckstrum et al. 2016; Weis and Raetzman 2016; Weis and Raetzman 2019; Ge et al. 2022). As it relates to reproduction, it is important to consider the pituitary gland as an independent organ as well as a component of the hypothalamic-pituitary-gonadal (HPG) axis when understanding its responses to developmental EDC exposure. Inappropriate hormone activation or antagonism via EDCs not only affect the gonads but disrupt feedback and proper hormone action at the level of the pituitary gland and hypothalamus as well. More detailed research investigating different mechanisms of the pituitary’s response to developmental EDC exposure is needed especially as it contributes toward human fertility and reproductive pathologies.

Supplementary Material

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Supplemental Table 1. Summary of EDCs effects on different cell types in developing pituitary.