Estrogen receptors in mitochondrial metabolism: age-related changes and implications for pregnancy complications.

Given the important role of estrogen in age-related pregnancy and maternal health, it is critical to consider the current state of pregnancy and any concerns about maternal health when evaluating treatments. Recent findings show that in frozen embryo transfer, hormone replacement therapy is associated with an increased risk of preterm birth complications, low birth weight, and hypertensive pregnancies compared to natural cycle-based methods. 198 This indicates that the full effects of pregnancy remain poorly understood and cannot be replicated through E2 treatment alone. There remains a lack of research regarding how the pregnancy process and secretion of estrogen may differ due to certain social determinants and aging. Furthermore, future treatments may consider targeting mitochondria. Also, as opposed to estrogen treatment, it may be interesting to see if it is possible to modulate the binding affinity of the ERs to reduce pregnancy-negative outcomes as a potential therapy. Finally, the role of the microbiome in maternal health, as both modulated by estrogen and as an effector, requires further study. Together, this suggests that each of these receptors plays differential, tissue-dependent roles in mitochondrial function thus conferring risk of pregnancy complications.

A further area of research that demands more research is how estrogen affects microbiomes during pregnancy. Notably, the microbiome diversifies in the postpartum period while in healthy pregnancy, there is low diversity with a dominance of Lactobacillus , which is concomitant with elevated estrogen levels. 157 Group B Streptococcus growth in the lower reproductive tract microbiome is understood to be a key marker of adverse pregnancy outcomes, yet how Lactobacillus interacts with it to protect pregnancy remains unclear. 158 While supernatants secreted by Lactobacillus can protect against biofilm formation, these were strain-dependent, and bacterial associations resulted in host cell death, in some cases. 158 Beyond this, recent therapeutic techniques for adverse perinatal outcomes have been facilitated by Lactoferrin, which inhibits the biofilm formation of Group B Streptococcus . 159 Lactoferrin is an estrogen-inducible protein that interacts with estrogen receptor-related receptor α1. 160 Thus, it is possible that certain adverse outcomes or interactions between these gut microbiomes may be modulated by certain ERs-interactions which aid in explaining the differential development in pregnancy between healthy Lactobacillus -dominant environments and the buildup of Group B Streptococcus . Given the alteration in the vaginal microbiome that occurs following the loss of estrogen levels, the role of estrogen in affecting other microbiomes is highlighted. The gut microbiome is also subject to reduced diversity during pregnancy in a progesterone-dependent manner. 161 This lowered diversity may be a defense mechanism against “estrobolomes” which are adverse health outcomes of metabolizing estrogens bacteria that may increase the likelihood of ER–positive breast cancer. 162 Conversely, a key health risk during pregnancy is gingivitis as estrogen can enhance planktonic growth. 163 Notably, clinical studies indicate that salivary estrogen levels are correlated with the likelihood of developing pregnancy gingivitis and gingival inflammation. 164 This suggests that opportunistic infections can utilize elevated estrogen levels during pregnancy, yet estrogen may serve a modulating role in reducing microbiome biodiversity in certain organ systems to prevent this. Still, the roles of microbiomes in disease states during pregnancy need further investigation to understand these multifaceted roles of estrogen.

As reviewed in the reference 134 , mitochondria dysfunction is well-linked to many pregnancy complications, including preterm birth, stillbirth, and pre-eclampsia. Targeting mitochondria, such as through antioxidants, has been shown to mitigate preeclampsia. 187 Multiple avenues must be explored to further our understanding of how the structural remodeling of mitochondria is altered in response to the modulation of ERs in a tissue-dependent manner, contributing to pregnancy complications. For example, in murine brown adipocytes, ERα was required for Drp1 -dependent mitochondrial remodeling, yet it is unclear if this is the case for other tissue types. 57 Beyond only looking at expression levels of associated genes, understanding mitochondrial ultrastructure is important. Further investigation is necessary to better understand the direct effect of estrogen on MERCs. Future directions for the field include determining the relationship between GPER on the endoplasmic reticulum and MERCs with regard to calcium signaling and characterizing MERC activity in response to an estrogen-induced drop in ROS production. E2 and estrone are widely studied, however, the different roles of estrogen types should further be investigated. For example, equine models express ring B-unsaturated equilin or equilenin which have relatively poorly defined functions. 188 A better understanding of the functional roles of species-specific estrogens can offer insight into the evolutionarily conserved mechanisms that govern estrogens. Beyond the differential roles of estrogens, while this review focused on the different types of ERs, it should also be noted that isoforms may be implicated in pathologies. For example, ERα-LBD, an isoform of ERα is apparent in breast cancer and localizes to mitochondria. 189 A greater understanding of the intersectionality between estrogens and bacterial microbiomes to both provide negative and positive birth outcomes remains an important avenue to study. Recent studies have indicated that HIV-1 transmission can be prevented in the female genital tract through a combination of elevated E2 levels and lactobacilli, 190 paralleling the results observed in pregnancy. In contrast, E2 degrading bacteria may also naturally exist in the environment and reduce E2 to estrone to reduce stress on certain organisms. 191 While common understanding shows that bacteria reduce E1 to produce ring-cleaved metabolites, the exact mechanism still requires further elucidation. 192 It is understood that different bacteria may reduce E2 levels in varied mechanisms, 193 however, the implications of these varied mechanisms on pregnancy outcomes, mitochondrial structure, and ERs remain poorly elucidated. Given the varied relationships between bacteria and E2, studies on mutualistic interactions of E2 and bacteria on host mitochondria may prove promising. Notably, a study in yeast found that the expression of the bacterial cell division protein FtsZ may lead to mitochondrial fission. 194 This suggests that certain alterations in bacteria may alter mitochondria, and if E2-modulation of its microbiome may also affect mitochondria through poorly explored mechanisms. Beyond only host cell studies, a better understanding of how ERs and E2 interact with bacteria may give insight into their modulation of mitochondrial function given mitochondria’s endosymbiotic origin. For example, while binary fission is known in bacteria, it is not clear if, in response to E2, they also use similar fusion and fission pathways to regulate their cell shape and size. Beyond this, how estrogens affect bacterial DNA replication may provide insights into the similar processes that occur in with mtDNA. Across models, the localization of ERβ in mitochondria is commonly observed, but the localization of ERα differs. 195 Notably, estrogen treatment is associated with increased ERβ localization, which in turn promotes ERE binding to reduce oxidative stress and regulate mtDNA transcription. 5 , 195 As a result, ERβ has arisen as a key target for the treatment of Alzheimer’s Disease through the inhibition of fission in an AKAP1-dependent manner. 196 However, the potential therapeutic role of targeting ERα and its effects on mitochondrial structure need further investigation. Even less understood is GPER, which past research has suggested serves protective roles against obesity, glucose intolerance, and increased blood pressure. 197 The roles of the differential ERs in cardiac cells have previously been reviewed, 197 which shows differential roles of the ERs due, in part, to different roles in cardiac Ca 2+ ion channels. 197

Estrogens are recognized to be important molecules in sexual development, which have been studied intensely ( Figure 1 ). 1 – 3 Estrogens are a class of sex hormones defined by their ability to promote female sexual development. 4 Despite this characteristic, 17β-estradiol (E2), a major product of estrogen synthesis, is produced in both men and women. 5 In reproductive-aged women, E2 is primarily synthesized in the ovaries. 5 In men, the testes are capable of producing estradiol. Estradiol production also occurs in non-gonadal tissue, such as the brain, liver, muscle, and bone. 6 The conversion of androgens to estrogens is recognized as a critical process in the body, with important implications for reproductive health, bone health, and the pathogenesis of certain diseases such as breast cancer. 6 , 7 Increasingly, the literature shows that mitochondria must be considered in the roles of estrogen and its receptors. Beyond simply considering estrogen as a steroid, the roles of its receptors have similarly arisen as important effectors, especially when it comes to maternal health. In this narrative review, we highlight the important role of mitochondria in estrogen receptors during pregnancy and aging.

In this narrative review, we sought to synthesize current knowledge on estrogen receptors and mitochondrial function. However much of the existing literature may be fragmented or focused on specific aspects, making it challenging to draw comprehensive conclusions. As this is a narrative review, we sought to highlight areas of perceived interest, but additional systematic reviews are necessary to cover all relevant pathways or conditions.

Important in the process of pregnancy is placental development, which remains heavily dependent on both ERs and mitochondrial function and structure, although few studies have looked at the interconnection of these processes. The roles of ERs in placental development and dysfunction are an active field of study. Estrogen related receptor-gamma, which shares DNA-binding domain and the ligand-binding domain with ERα and ERβ, is highly expressed in normal placenta with decreases in placental dysfunctions characteristic of fetal growth restriction (reviewed in the reference 165 ). The placenta can also play differential roles in binding ERs, with ERβ expression in chorionic villi, particularly in syncytiotrophoblasts, associated with trophoblasts differentiation, while ERα is decreased. 166 Vascular endothelial growth factor expression is important for placental angiogenesis, with E2 often playing regulatory roles. 167 As previously reviewed in the reference 167 , vascular endothelial growth factor, and associated angiopoietin-1, expression estrogenic changes caused by are primarily regulated by ERs. However, upregulated ERs expression is not always associated with improved outcomes; the upregulation of expression of ERα and gene polymorphism of ERα in placental tissues were both associated with gestational diabetes mellitus in pregnant women. 168 Placental angiogenesis is also heavily dependent on the action of GPER, with recent findings showing that E2 treatment resulted in GPER-dependent endothelial nitric oxide synthase and Akt signaling, important for maintaining endothelial cell tube formation under conditions of hypoxia and reoxygenation. 169 Additionally, recent findings have shown that E2-dependent increases in GPER activation are responsible for placenta-secreted human chorionic gonadotropin expression, which produces progesterone, trough protein kinase A (PKA)-CREB signaling pathway in human cytotrophoblast cells. 170 Together, this demonstrates differential roles of ERs in placental development, with GPER being of particular interest. Placenta parallel rapid aging in some ways, as they develop over quick periods. At around ten weeks, blood flow is reduced by trophoblasts resulting in impaired oxygen, associated with decreased respiration and compensatory increased mitochondrial content. 171 Beyond this, numerous pregnancy complications may be linked to alterations in the placenta. As past reviews have emphasized, malnutrition, hypoxia, and obesity may affect placenta mitochondrial function thus causing fetal complications. 172 Additionally, placental trophoblast cells experiencing gestational diabetes mellitus display elevated oxidative stress. 173 However, the exact mechanisms of these complications remain unclear in some contexts. For example, in pig models with excessive back fat, placental dysfunction was noted due to oxidative stress arising from mitochondrial injury. 174 ROS has remained a well-studied topic as a mediator of uteroplacental dysfunction due to the mild oxidative stress caused by pregnancy at baseline which may be exacerbated by certain conditions, as previously reviewed in the reference 153 . However, other aspects of mitochondrial function in placental dysfunction remain poorly elucidated, such as if mtDNA changes in the placenta are implicated in complications is unclear. For example, a hallmark of aging is an accumulation of mtDNA mutations which can result in deleterious heteroplasmy that interferes with mitochondrial function. 175 It is unclear if the short life span of the placenta also causes accelerated mtDNA mutations. Notably, certain conditions such as intrauterine growth restriction are marked by increased mtDNA content. 176 Beyond this, how mitochondrial structure changes across placental aging is unclear. Past results have shown that in the brain, donut-shaped 3D structures are more favorable upon hypoxic conditions, 177 yet it is unclear if certain mitochondrial phenotypes are more favorable to the hypoxic-like conditions that occur at Week ten in placental development. Similarly, several MERC proteins are known to be implicated in the ROS generation, 178 suggesting that these contact sites should be considered as a potential indicator to better understand oxidative stress in the placenta, which has previously been reviewed to be predictive of pregnancy complications. 179 Given that mitochondrial transplantation is increasingly emerging as an option to potentially restore endometrial injury, 180 this may be a valuable option. As previously reviewed in the references 181 and 182 , direct injection, systemic administration, and intranasal administration are all routes of mitochondrial transplantation through which mitochondrial diseases can be mitigated through the replacement of mitochondria. Similarly, mitochondrial replacement therapy via ooplasm injection has been performed for oocyte rejuvenation, but despite successful pregnancies, concerns over mitochondrial DNA heteroplasmy and the risk of mitochondrial diseases led to regulatory restrictions. 183 A better understanding of these dynamics in pregnancy complications and how estrogen affects them in the placenta can be important for precision medicine, especially in future clinical studies aiming to use mitochondrial transplantation or replacement for disease states. Given the functional requirements of the cholesterol side chain cleavage enzyme P450 in the inner mitochondrial membrane for steroidogenesis, not all cell types are steroidogenic. 89 In the placenta, steroidogenic metabolites and precursors of fetal glands are utilized, emphasizing the relationship and dependency between the placenta and fetus. However, the role of E2 in poor placentation is controversial 184 with some recent cohort studies finding no association and peak E2 levels during ovarian stimulation being independent of birthweight, 185 suggesting that estrogen serum levels alone may not be a primary modulator of mitochondrial function. 186 This again highlights the importance of looking at specific activation of estrogen receptors, as differential recruitment may be a factor in poor placentation.

For this review, literature published from 2000 to 2024 was searched using PubMed. The following keywords were used: “pregnancy” OR “estrogen” OR “estradiol” AND “mitochondria.” The articles included in this review were selected based on their relevance to the topic, along with additional historical articles or related articles. The results were further screened according to the title and abstract, and whether they included animal experiments, in vitro studies, clinical trials, and database or software applications.