Mechanisms driving resistance to oxidative stress during endometrial stromal cell decidualization†

During the menstrual cycle, the endometrium undergoes cyclic changes and renewal due to progesterone and estrogen stimulation. These hormones coordinate the secretory phase and prepare the endometrium for pregnancy establishment [ 3 , 4 ]. During the mid to late secretory phase of the menstrual cycle, stromal cells transdifferentiate into epithelial-like decidual cells under the influence of progesterone [ 3 – 5 ]. These decidual cells provide a rich source of growth factors and cytokines that regulate trophoblast invasion, support embryonic development, modulate local immune responses, and promote uterine angiogenesis [ 4 , 31 ]. Decidualization happens in eutherian mammals with hemochorial placentae but does not occur in marsupials or monotremes, nor in mammals with epitheliochorial or endotheliochorial placentae [ 8 – 10 ]. It is likely that evolutionary changes, which include decidualization at the feto-maternal interface, enable certain mammals to establish placentae that invade more deeply, resulting in more prolonged pregnancies and deeper, more complex interactions between the mother and embryo. Tolerating oxidative stress associated with early pregnancy is a major challenge. When the embryo attaches to the endometrium, it triggers an inflammatory response, similar to how the body heals after an injury [ 32 , 33 ]. This inflammatory response consists of the release of cytokines along with increased ROS production [ 15 , 18 ]. In species like marsupials, this inflammatory process leads to parturition [ 34 , 35 ]. This response involves the release of specific molecules (interleukins (IL-1, IL-6), tumor necrosis factor-alpha (TNF-α), and prostaglandins) that signal the body to start the delivery process. However, in eutherians, inflammation is tightly regulated to support pregnancy rather than induce premature labor [ 34 – 36 ]. Pro-inflammatory signals are generated at implantation but are promptly restrained by the decidua, so that these mediators contribute to placental development and fetal growth without precipitating premature labor [ 35 ]. These molecules help maintain the placenta and uterine lining, supporting fetal development and protecting against infections. This regulation is important for keeping the pregnancy healthy and preventing problems that could harm both the mother and the embryo. The decidua plays a key role in managing this inflammation during early pregnancy, acting as a barrier, protecting the mother and embryo from excessive inflammation and damage caused by ROS [ 19 , 20 , 37 ]. The most important role of the decidua is to help the pregnancy advance successfully by regulating inflammation, preventing immune system dysfunction, and tissue damage [ 19 , 33 , 35 , 38 ]. In placental mammals, maintaining a balance between immune response management and tissue protection is critical for embryo growth and health. Decidualization is a vital transformation that allows stromal cells to transform into specialized decidual cells, a hallmark of many placental mammals (eutherians), but it is not universally present across all mammals. To understand how decidualization evolved, we need to look at how the process occurs in different types of mammals: monotremes (egg-laying mammals), marsupials (middle ground-limited decidualization), and eutherians (placental mammals) ( Figure 1 ). Among all mammal species, the platypus and echidna function as the most ancient mammals by laying eggs due to their reptilian ancestry [ 39 – 42 ]. In these animals, the embryo develops outside the body inside a leathery egg, meaning there is no need for uterine changes like decidualization to aid in the embryo development, as there is a lack of a placenta [ 39 – 41 ]. For example, the female platypus lays eggs that are incubated outside the maternal body [ 34 , 41 ]. Their reproductive system is designed to produce eggs with yolk, which are laid and cared for externally. In pregnancy, the platypus does not develop a placenta or a long-lasting connection between mother and fetus. However, they nourish their young with milk through specialized milk patches. Without a placenta or prolonged maternal-fetal connection, the monotreme uterus remains structurally simple. Studies suggest this reflects an early evolutionary stage, prioritizing egg production over internal embryonic development [ 42 – 44 ]. Marsupials, like kangaroos and opossums, are between egg-laying animals and fully developed mammals [ 45 ]. They give birth to very underdeveloped young after a short pregnancy, with most of their embryonic growth happening inside a maternal pouch. Marsupials have a yolk-sac placenta, which allows only limited nutrient exchange between the mother and the fetus [ 21 , 45 – 47 ]. Unlike the complex placenta of mammals that carry their young for longer, the yolk-sac placenta is not as efficient, so there’s little to no decidualization [ 21 , 45 – 47 ]. Embryo implantation elicits an acute endometrial response that resembles an inflammatory reaction rather than a sustained decidualization. The conceptus remains enclosed in a protective shell coat for much of gestation, but once this shell coat thins and the trophoblast makes direct contact with the uterine lining, a transient “attachment reaction” occurs in the endometrium [ 36 , 46 ]. This attachment reaction is characterized by increased vascular permeability and immune cell presence, analogous to a brief decidual-like response, but marsupials do not develop true decidual stromal cells or permanent decidua during pregnancy. Instead, the maternal endometrial changes are short-lived and tightly constrained to the implantation period. At cellular level, when embryo attaches, endometrial stromal fibroblasts (previously described as “paleo-ESFs” by Wagner P., et al.) experience oxidative and inflammatory stress: prostaglandin E 2 and progesterone stabilize FOXO1 and trigger NOX4-driven ROS production, eliciting an antioxidant/apoptosis rather than stable decidual differentiation [ 10 , 12 , 21 ]. Some marsupials, like the tammar wallaby, show small changes in the uterus during pregnancy, like slight swelling and changes in blood vessels, but not the full development of decidual cells seen in mammals like humans or rodents [ 36 , 46 ]. This limited development fits well with their strategy of short pregnancies followed by prolonged periods of breastfeeding. Eutherian mammals exhibit diverse reproductive strategies shaped by prolonged intrauterine gestation and intimate maternal-fetal interactions. To accommodate this, some evolved decidualization, a specialized transformation of endometrial stromal cells that supports implantation and modulates immune tolerance. However, not all eutherians undergo decidualization. Its presence and extent are closely linked to the placental subtype. In species with hemochorial placentation (e.g., humans, rodents), deep trophoblast invasion demands robust decidual responses. In contrast, species with epitheliochorial placentas (e.g., pigs, cows) exhibit minimal or no decidualization, reflecting their non-invasive implantation strategies. “Minimal decidualization” refers to a much weaker or incomplete decidual response, only partial and localized stromal cell changes, typically seen in mammals with less invasive placental types (epitheliochorial or endotheliochorial placentation). Eutherian mammals can be categorized into three major groups based on the depth of trophoblast invasion and placental structure, each with distinct implications for the evolution of decidualization: Epitheliochorial Placenta , as seen in cows, pigs, and horses, is the least invasive form of placentation. The trophoblasts do not penetrate beyond the maternal epithelium, maintaining a complete barrier between fetal and maternal tissues. In these species, decidualization is absent or minimal, as there is little to no need for maternal stromal remodeling or immune modulation in response to fetal invasion. Endotheliochorial Placenta is observed in dogs and cats, and this intermediate form involves trophoblast invasion through the maternal epithelium and stroma but stops at the maternal endothelium. Some degree of decidual response may occur, particularly to help regulate local inflammation and invasion, but it is generally less robust than in hemochorial species. Hemochorial Placenta is found in humans, rodents, many non-human primates, and some bats, and is the most invasive type of placentation. The fetal trophoblasts penetrate through all maternal tissue layers, coming into direct contact with maternal blood. This deep invasion necessitates extensive and often spontaneous decidualization, which prepares the endometrium for implantation and helps regulate immune tolerance and trophoblast behavior. Here, decidualization is indispensable. It works alongside the chorioallantoic placenta, which connects the chorion and allantois to facilitate the exchange of nutrients, gases, and waste between the mother and fetus [ 8 , 10 , 21 , 34 ]. These animals also establish hemochorial placentae, which are characterized by the deep invasion of fetal trophoblasts that place them in direct contact with maternal blood, allowing for efficient nutrient and gas exchange. Decidual cells repress IL-17-mediated neutrophil recruitment and secrete cytokines that foster maternal tolerance of the invading embryo [ 48 ]. DSCs control oxidative stress by regulating GPX3/4, SODs, PRDXs, NRF2 and HMOX1 to survive the ROS burst that accompanies trophoblast invasion [ 4 , 10 , 48 ]. The decidua supports a successful pregnancy by regulating inflammation, preventing immune rejection of the fetus, and minimizing tissue damage.[ 4 , 5 , 38 ]. In addition, DSCs are known to coordinate vascular remodeling by producing VEGF, prolactin, and IGFBP-1 that guide spiral artery transformation. These functions together create a protective, nutrient-rich, and immunologically permissive niche that supports long-term placental exchange [ 12 , 21 , 22 ]. Comparative transcriptomics show that the decidual gene-regulatory network was pieced together by rewiring an ancestral cellular-stress cascade into a stable differentiation, an example of stress-induced evolutionary innovation [ 10 ].

The evolutionary adaptation of decidualization to oxidative stress makes SLC40A1 and GPX4 potential therapeutic targets for endometrial dysfunction and pregnancy complications, along with other factors like the GPX family, FOXO1, NR2F2, SOD family, NRF2, PRDXs, and HMOX1. Dysregulated oxidative stress underlies conditions where endometrial dysfunction and pregnancy fail, often due to excessive ROS from iron overload or inflammation [ 16 , 20 , 26 , 28 , 30 , 200 ]. SLC40A1, which exports Fe 2 +, can mitigate Fenton reaction-driven ROS, thereby reducing damage from ROS in the endometrium or at implantation sites. Targeting the hepcidin–ferroportin axis locally without disturbing systemic iron can be achieved with short-acting methods such as intrauterine or vaginal delivery of glycol-split heparins, anti-hepcidin aptamers, or mini-antibodies in biodegradable nanoparticles [ 205 – 207 ]. This relieves ferroportin blockade only in DSCs and the placenta, avoiding systemic hypo-hepcidinemia and the risk of liver or spleen iron overload. Alternatively, site-directed iron chelation (e.g., topical deferoxamine) can sequester labile iron pools without altering maternal serum iron. These tissue-targeted approaches would amplify SLC40A1 activity, reduce lipid-hydroperoxide accumulation, and lower the risk of inflammatory ferroptosis [ 199 , 200 ], all while preserving maternal–fetal iron transfer [ 208 – 211 ]. At the same time, an increase in SLC40A1 activity counteracts inflammation risk by reducing DSC membranes to lipid hydroperoxides so they do not undergo peroxidation and ferroptosis [ 212 , 213 ]. Selenium or GPX4 agonists could bolster this defense [ 145 ]. Complementary mechanisms, the GPX family (e.g., GPX3) for H 2 O 2 detoxification, SOD (SOD1, SOD2) for superoxide neutralization, and PRDXs (e.g., PRDX3) for peroxide scavenging could mitigate ROS, while FOXO1, NR2F2, NRF2, and HMOX1 regulate antioxidant and anti-inflammatory responses [ 15 , 22 , 62 , 77 , 80 , 87 , 113 , 115 , 213 – 215 ]. Antioxidant therapies like N-acetylcysteine, which has been proven effective in reproductive contexts, could amplify these targeted interventions [ 216 ]. Future directions should prioritize SLC40A1 and GPX4 validation through patient-derived DSC models and dissecting how hormonal and immune signals regulate these redox pathways. Comparative multi-omics across species (e.g., human vs. marsupial stromal cells) may reveal additional stress response genes, enriching the target pool [ 10 ]. Genetic screening for polymorphisms in SLC40A1, GPX4, and related genes (e.g., TE-derived enhancers) could identify risk factors for implantation [ 9 ]. These mechanistic insights would potentially identify more targeted therapeutic candidates and lay the foundation for future pre-clinical studies targeting oxidative stress for infertility or pregnancy complications.

Based on established mechanisms of oxidative stress resistance in DSCs, the Solute Carrier Family 40 Member 1 (SLC40A1) and Glutathione Peroxidase 4 (GPX4) have recently emerged as potential regulators of oxidative stress during early pregnancy. SLC40A1, or ferroportin, is the only known iron exporter in mammalian cells, and is thus critical for maintaining iron homeostasis by transporting ferrous iron (Fe 2 +) out of the cytoplasm [ 119 ]. Iron accumulation drives oxidative stress by fueling the Fenton reaction, where Fe 2 + reacts with hydrogen peroxide (H 2 O 2 ) to produce highly reactive hydroxyl radicals (OH), exacerbating cellular damage [ 119 , 120 ]. This process also causes ferroptosis, a type of controlled cell death linked to too much iron and damage to fats in the cell ( Figure 3 ) [ 121 , 122 ]. In the endometrium, iron levels surge during menstruation and implantation due to blood exposure from tissue breakdown and vascular remodeling, increasing oxidative stress and ferroptosis risk [ 123 – 125 ]. Research shows that SLC40A1 is tightly regulated by various transcription factors, including NRF2 (nuclear factor erythroid 2-related factor 2), which influences oxidative stress responses [ 126 ]. Reduction in iron transport by SLC40A1 may also increase oxidative stress and ferroptosis in gestational complications like obesity or iron overload to impair positive pregnancy outcomes, including fetal growth restriction [ 127 ]. Inhibiting SLC40A1 leads to increased iron accumulation and reactive oxygen species (ROS), which activate stress pathways like the PERK/ATF4/CHOP pathway, ultimately causing cell dysfunction that may impair decidualization due to iron overload [ 128 ]. Limited research has investigated SLC40A1 in early pregnancy and its role in the endometrium. A list of recent studies carried out to characterize the role of SLC40A1 in several different cell types and disease states are outlined in Table 1 . Although SLC40A1 has not yet been directly studied in DSCs, compelling evidence from other systems supports its potential protective role during decidualization [ 119 , 129 ]. We observed that SLC40A1 is upregulated during decidualization [ 130 ], and we hypothesize that this upregulation of SLC40A1 could mitigate oxidative stress and ferroptosis by exporting excess iron, thus limiting ROS generation and preserving DSC viability amidst the inflammatory milieu of early pregnancy. This is consistent with studies showing its induction in oxidative stress-exposed macrophages, where TLR2/6 signaling suppresses SLC40A1 expression, promoting intracellular iron retention and reducing ferroportin-mediated export, which is an effect detrimental in inflammatory conditions [ 131 ]. In addition, overexpression of SLC40A1 in various models has been shown to limit iron-mediated ROS and reduce oxidative damage. In mice, cardiomyocyte-specific overexpression of SLC40A1 alleviated cardiac iron overload and reduced ROS levels, mitigating damage under hypoxic stress [ 132 ]. Similarly, in human neural precursor cells (NPCs), SLC40A1 overexpression protected against ferroptosis by preventing iron overload, whereas its knockdown mimicked ferroptosis, causing increased oxidative stress [ 133 ]. Furthermore, SLC40A1 expression is linked to favorable outcomes in several disease contexts. For instance, high expression correlates with better prognosis in glioblastoma, potentially via modulation of immune-related cytokines like CCL14 and IL18 [ 134 ]. In glioma stem cells, REST-mediated upregulation of SLC40A1 decreased ferroptosis, while erianin-induced degradation of SLC40A1 sensitized cells to ferroptotic death, indicating its direct involvement in cell fate under oxidative stress [ 135 ]. Supporting a reproductive context, SLC40A1 mRNA was found to be lower in granulosa and cervical cells of infertile women compared to fertile controls, which shows a link between its expression and reproductive health [ 136 ]. Additionally, a global knockout of SLC40A1 in mice resulted in embryonic lethality by E7.5 and impaired placental development, providing evidence of a critical role in early pregnancy [ 119 ]. Thus, it is plausible that SLC40A1 rises during early pregnancy, allowing DSCs to tolerate oxidative stress during differentiation. Glutathione peroxidase 4 (GPX4) is an important antioxidant enzyme that plays an essential role in maintaining cellular redox homeostasis, protecting against oxidative damage, and preventing ferroptosis, particularly by inhibiting lipid peroxidation [ 144 , 145 ]. During the early stages of pregnancy, especially during the establishment of pregnancy, the uterus undergoes a series of complex biological processes that require tight regulation of oxidative stress. Studies have shown that GPX4 is highly expressed in the uterus on days 1 and 3 of pregnancy in rats, particularly within the decidua, where it acts as a primary defense against oxidative damage [ 146 ]. There has been minimal research on GPX4 in early pregnancy and its role in the endometrium. Studies focusing on GPX4 are summarized in Table 2 . The deficiency of GPX4 in various uterine epithelial tissues leads to increased lipid peroxidation and ferroptosis, resulting in an inability to implant and infertility [ 147 ]. Furthermore, the PPARγ/NRF2/GPX4 signaling pathway has been demonstrated to play a crucial role in preventing ferroptosis in pregnancy-related conditions, such as unexplained recurrent pregnancy loss (URPL). Alpha-lipoic acid has been shown to enhance this pathway and improve pregnancy outcomes in URPL models [ 148 ]. MdESFs are Monodelphis domestica endometrial stromal fibroblasts, a type of uterine cell derived from the gray short-tailed opossum (Monodelphis domestica) [ 10 ]. These cells are homologous to human endometrial stromal fibroblasts (HsESFs) and serve as a model for studying the evolution of pregnancy-related cellular processes. GPX family upregulation has been described to occur as an oxidative response in MdESFs in Erkenbrack et al. [ 10 ], warranting further exploration of its potential in DSCs. Unlike other glutathione peroxidases (e.g., GPX3), GPX4 specifically reduces lipid hydroperoxides in cellular membranes, a critical defense against lipid peroxidation, which is a hallmark of oxidative stress and a driver of ferroptosis [ 149 ]. During early pregnancy, decidualization and implantation, elevated metabolic activity and inflammatory lipid mediators amplify oxidative stress, increasing lipid peroxidation and threatening DSC membrane integrity [ 18 , 147 , 150 , 151 ]. GPX4 has an established role in other contexts, such as cancer cells, demonstrating its capacity to suppress ferroptosis by neutralizing lipid ROS, dependent on glutathione availability [ 144 , 146 , 149 , 152 ]. Given its presence in MdESFs and significant upregulation during DSCs [ 130 ], GPX4 could serve as a key protector against oxidative stress-induced lipid damage and ferroptosis, complementing broader antioxidant systems like catalase and GPX3 in the decidual environment. To completely comprehend how SLC40A1 and GPX4 proteins work to mitigate ROS during early pregnancy, more investigation is necessary. Examining their functions in DSCs and how they are controlled (by hormones or transcription factors like NRF2 or FOXO1) may provide novel information on how they contribute to preventing ferroptosis and oxidative stress, advancing our knowledge of successful pregnancy.

Normal uterine function, like other tissues, requires that levels of ROS are balanced. In eutherians with hemochorial placentae, decidualization evolved as a mechanism to manage this stress, repurposing ancestral stress responses into a specialized cellular framework [ 10 ]. However, disturbances in this delicate balance can cause excessive ROS, which disrupt endometrial homeostasis and contribute to reproductive disorders like endometriosis and recurrent pregnancy loss (RPL) [ 26 , 28 , 30 , 49 , 165 , 166 ]. Within this review, SLC40A1 and GPX4 are suggested as key molecules allowing DSCs to tolerate oxidative stress. They work on different but complementary parts of this imbalance. SLC40A1 controls iron homeostasis to limit ROS generation, while GPX4 neutralizes lipid peroxidation to protect cellular integrity. They are important for human reproductive health because they can help tolerate the oxidative damage that is required for the processes of decidualization and implantation. In endometriosis, tissue that resembles the endometrium grows ectopically outside of the uterus [ 167 , 168 ]. Approximately 10% of women of reproductive age are affected [ 169 ]. One of the most widely accepted theories for the cause of endometriosis is retrograde menstruation, which happens when menstrual blood flows backward through the fallopian tubes into the pelvic cavity instead of leaving the body. This could lead to the development of endometriosis cysts, which have elevated iron and lead to the production of harmful molecules called ROS through a chemical process known as the Fenton reaction [ 170 – 172 ]. The production of ROS causes inflammation, growth of lesions, and cell damage, all of which play a major role in the progression of the disease [ 120 – 122 ]. SLC40A1 protein, which moves ferrous iron (Fe 2 +) out of cells, is essential for iron homeostasis and controls the contribution of excess iron to oxidative stress [ 119 , 128 , 129 ]. Menstrual reflux in an ectopic lesion causes an imbalance in iron levels in the peritoneal cavity with consequent accumulation in ectopic lesions which promotes inflammation and lesion growth [ 173 – 176 ]. Studies have also shown elevated expression of SLC40A1 in peritoneal macrophages of endometriosis patients [ 174 ]. Thus, patients with endometriosis display dysregulated iron balance in the ectopic and eutopic endometrium that may be correlated with disease and infertility. Another critical protein involved in oxidative stress regulation is GPX4, which helps prevent lipid peroxidation, a major consequence of ROS production [ 144 , 177 – 179 ]. In endometriosis, inflammatory mediators like Prostaglandin E2 (PGE2), Tumor Necrosis Factor-alpha (TNF-α), Nitric Oxide (NO), Interleukin-1 beta (IL-1β), and Interleukin-6 (IL-6) exacerbate lipid peroxidation, disrupting cellular membranes and triggering ferroptosis [ 180 – 190 ]. It has been demonstrated in research that endometriotic tissues have higher ferritin expression and are positive for lower GPX4 expression, which leads to increased lipid peroxidation and ferroptosis markers [ 191 ]. In vitro studies have demonstrated that supplementing GPX4 with activators, such as selenium, can reduce oxidative damage and inflammation [ 192 , 193 ]. Stabilizing cellular membranes and promoting decidualization of the eutopic endometrium may help protect ectopic and eutopic tissues from oxidative damage, boosting GPX4 activity and increasing the quality or quantity of surviving embryos, resulting in better fertility outcomes. Together, SLC40A1 and GPX4 represent promising targets for therapeutic strategies aimed at restoring redox balance in endometriosis and mitigating its debilitating effects. Recurrent pregnancy loss (RPL) characterized by two or more consecutive miscarriages, is seen in 1–5 % of couples [ 194 , 195 ]. While several causes have been attributed to RPL, implantation defects attributed to excessive oxidative stress at the maternal-fetal interface are partially involved [ 194 – 197 ]. Recurrent pregnancy loss (RPL) is a key factor of impaired decidualization, ectopic pregnancy, defective implantation, and placentation [ 196 , 198 ]. These deformities can be attributed to progesterone resistance [ 199 ], immune dysfunction [ 200 , 201 ], or ROS overload [ 16 , 20 , 26 , 28 , 30 ] that hinder decidualization, implantation, and placentation. Some studies document elevated ROS levels in RPL decidua that correlate with reduced antioxidant defenses [ 32 , 49 , 165 ]. During pregnancy, trophoblast invasion facilitates the exposure of the decidua to maternal iron and heme by remodeling spiral arteries, which can lead to local iron accumulation and increased oxidative stress [ 202 ]. High ROS generation has been associated with elevated iron levels in the decidua of RPL patients, resulting in impaired DSC function and disfavoring embryo support [ 11 , 29 ]. Recent studies have shown that the level of SLC40A1 is altered in the uterine tissue lining (decidua) of women with RPL, and it can cause an increase in iron and oxidative stress damage [ 203 ]. Further research is warranted to explore how increasing SLC40A1 levels in DSCs helps prevent iron-related oxidative damage, improves the conditions for decidualization and implantation, and decreases the risk of miscarriage. Excessive ROS in RPL leads to lipid peroxidation that can lead to DSC membrane disruption, which is crucial for implantation and trophoblast-DSC attachments [ 29 , 204 ]. As reported by Yang et al. (2025), iron deposition in DSCs from patients with RPL who were also found to have decreased GPX4 resulted in increased lipid peroxidation and cell death [ 11 ]. In addition, glutathione (GSH), a crucial controller of GPX4 activity, supplemented into DSCs during pregnancy, reduced oxidative damage and ferroptosis in mice [ 11 ]. Previously, studies have also shown that GPX4 activity is reduced in placental tissues in the context of preeclampsia (PE), which shares pathophysiological mechanisms with RPL. This reduction of GPX4 activity is associated with increased oxidative stress and lipid peroxidation [ 85 ]. The results of these studies indicate that lower GPX4 activity results in placental dysfunction that leads to poor pregnancy outcomes. Additionally, studies on the NRF2/GPX4 pathway in trophoblasts have demonstrated that modulation of this pathway can protect against ferroptosis in preeclampsia, indicating a promising therapeutic target for mitigating ferroptosis-induced pregnancy complications such as RPL [ 148 ]. Moreover, In RPL, SLC40A1 and GPX4 may coordinate to regulate oxidative stress and mitigate oxidative damage from ROS ( Figure 4 ). Increasing these proteins may improve the likelihood of a successful pregnancy and may lower the chance of miscarriage and recurrent pregnancy loss, making it a promising therapeutic option.

Decidualization represents an evolutionary adaptation in mammals with hemochorial placentae, co-opting primitive cellular stress responses into a sophisticated system to manage oxidative stress during pregnancy establishment. The endometrium’s ability to support early pregnancy depends on decidualization, which rewires stress-responsive pathways and recruits key genes to drive endometrial transformation. SLC40A1 and GPX4 can protect DSCs from stress and oxidative damage, offering novel therapeutic strategies to enhance endometrial health and fertility. As the genetic adaptations that contributed to the evolution of decidualization are uncovered, targeted therapeutic options will emerge that enable successful pregnancy establishment.

During implantation and early placenta formation, the decidua experiences sudden and changing oxygen levels. These changes generate reactive oxygen species (ROS), which can serve as beneficial signals but also cause damage. To survive in this stressful environment, DSCs build a layered defense system of antioxidants that is essential for a healthy pregnancy. One of the first defenses is GPX3, an antioxidant enzyme triggered by the hormone progesterone [ 75 , 76 ]. Another line of defense is SOD2, a mitochondrial enzyme that turns harmful superoxide into less dangerous hydrogen peroxide [ 77 – 79 ]. When ROS levels rise, another sensor system, NRF2 and KEAP1, steps in to boost more antioxidants like HO-1, GPXs, and other forms of SOD. At the same time, cells shift their metabolism to rely more on glycolysis and store fats, which lowers the production of ROS from mitochondria but still meets energy needs [ 80 , 81 ]. Extra protection comes from enzymes like PRDX6, which remove leftover peroxide [ 82 , 83 ]. All these defenses work together to make the uterine environment more resilient to stress. If they are disrupted, it can lead to pregnancy problems caused by unchecked oxidative damage. The GPX family, notably GPX3 is known to serve as the antioxidant defense by neutralizing hydrogen peroxide (H 2 O 2 ) and lipid hydroperoxides, which are prevalent during the oxidative stress of decidualization [ 75 , 76 ]. GPX3 is a secreted extracellular enzyme and is strongly expressed by DSCs, contributing to the detoxification of diffusible peroxides in the decidual microenvironment [ 10 , 76 ]. Exposure of human endometrial stromal cells to the environmental toxin zearalenone (an estrogenic mycotoxin) during in vitro decidualization markedly reduced GPX3 expression, which coincided with increased oxidative stress and impaired decidualization; treatment with the antioxidant resveratrol restored GPX3 levels and rescued decidual function [ 84 ]. This suggests that GPX3 upregulation is a normal part of the decidual antioxidant response and that its deficiency can predispose cells to oxidative damage. Clinically, aberrations in GPX3 have been noted in pregnancy disorders: decreased placental GPX activity or expression is associated with preeclampsia, a syndrome of oxidative stress and poor placental perfusion [ 85 ]. GPX3 likely protects decidual and trophoblastic cells from oxidative injury and modulates local redox-sensitive signaling during implantation and keeps the balance between oxidants and antioxidants in the cell [ 75 , 76 , 86 ]. The nuclear receptor NR2F2, also known as COUP-TFII (Chicken Ovalbumin Upstream Promoter Transcription Factor II), is an important transcription factor in the uterine stroma that orchestrates decidualization and modulates the tissue’s response to hormonal and metabolic changes [ 87 – 89 ]. NR2F2 is required for the proper crosstalk between progesterone-driven stromal differentiation and the suppression of estrogen-driven epithelial proliferation in early pregnancy [ 89 , 90 ]. This balancing act is essential for establishing a receptive, non-inflammatory uterine environment. A conditional deletion of COUP-TFII using PGR-Cre leads to implantation failure and increased epithelial estrogen signaling, disrupting uterine receptivity [ 89 ]. Acting as a P4-dependent “estrogen brake,” NR2F2 preserves the uterine receptivity window by restraining luminal-epithelial ERα activity, a role underscored by rescue experiments in which pharmacological ERα blockade with ICI-182,780 (fulvestrant, estrogen-receptor antagonist) restored embryo attachment and stromal BMP2/WNT4 expression in Nr2f2 -null uteri [ 90 ]. Uterine ChIP-seq and RNA-seq on Day 3.5 pregnancy show that NR2F2 binds a conserved enhancer driving Hand2, and NR2F2 loss reduces Hand2, liberates its effectors Fgf1/Fgf18, and skews the stromal transcriptome toward inflammatory, anti-decidual states, indicating an NR2F2/HAND2 axis that synchronizes stromal immune quiescence, differentiation, and redox balance [ 91 ]. Acting downstream of Wnt/β-catenin, NR2F2 directly represses PPARγ to delay full adipogenic commitment in progenitor cells, thereby restraining premature lipid accumulation and limiting fatty-acid–driven ROS production outside the uterus [ 92 ]. Regarding the control of oxidative stress, the influence of NR2F2 is somewhat indirect but significant. As a known regulator of cellular metabolism, NR2F2 helps switch stromal cells towards a metabolic profile suited for low-oxygen, high-demand conditions (favoring glycolysis and fatty acid storage over oxidative phosphorylation), an adaptation that inherently reduces mitochondrial ROS production. Overexpression studies in mice have shown that chronically high NR2F2 disrupts mitochondrial function, whereas reduced NR2F2 levels cause defects in the electron transport chain and lead to excessive ROS generation, creating a highly oxidizing cellular environment with attendant macromolecular damage [ 93 , 94 ]. This means that too little NR2F2 will fail to differentiate and manage oxidative stress, whereas too much, and metabolic imbalance, can itself become a source of oxidative stress. As an oxidative stress resistance gene, NR2F2 modulates pathways like PGC-1α to protect against ferroptosis and mitochondrial dysfunction, particularly in conditions like diabetes-induced heart failure [ 95 ]. NR2F2 helps protect against oxidative stress by controlling lipid metabolism and by reducing reactive oxygen species (ROS) accumulation by immune cells [ 96 , 97 ]. Thus, NR2F2 acts as a critical coordinator that ensures decidual cells respond appropriately to progesterone, adopt a metabolism that minimizes excess ROS, and possibly activate antioxidant genes. By balancing hormone signaling, immune milieu, and cellular metabolism, NR2F2 helps maintain redox equilibrium in the decidua, thus safeguarding the early pregnancy environment. Early pregnancy is a period of intense metabolic up-shift; higher oxygen consumption inevitably produces superoxide anions (O 2 −) that can damage lipids, proteins, and DNA [ 98 ]. Superoxide dismutase, primarily cytoplasmic SOD1 and mitochondrial SOD2, constitute the pregnancy’s primary line of defense. They convert O 2 − to hydrogen peroxide (H 2 O 2 ), which glutathione peroxidase (GPX) and catalase then reduce to water, completing a two-step detoxification loop [ 77 , 78 ]. As endometrial stromal cells transform into DSCs, the transcription factor FOXO1 sharply upregulates SOD2, boosting mitochondrial SOD2 so that DSCs can withstand the spike in oxygen tension after implantation [ 22 ]. By removing superoxide, SOD2 prevents the formation of highly reactive species (like peroxynitrite or the hydroxyl radical) and thus protects mitochondrial enzymes and DNA from oxidative damage. Human chorionic gonadotropin (hCG) provides an additional layer of protection by stimulating copper-zinc SOD activity in luteal cells, thereby stabilizing progesterone output and maintaining the hormonal milieu that supports early gestation [ 98 ]. Meanwhile, the very burst of mitochondrial reactive oxygen species (ROS) produced during implantation feeds back to accentuate SOD2 expression still further, ensuring that superoxide never accumulates unchecked [ 79 ]. Gene expression studies have identified SOD as part of the conserved “stress-coping” gene set activated during decidualization, and impaired SOD activity is linked to pregnancy complications like placental tissues from preeclampsia (a disorder characterized by chronic oxidative stress) [ 10 , 99 ]. Antioxidant supplementation in women with low SOD significantly reduced the incidence of preeclampsia compared to controls [ 99 ]. Similarly, studies have shown that SOD activity in conditions like recurrent miscarriage (RM) is significantly reduced compared to healthy pregnancy, which leads to increased oxidative stress and potentially contributes to pregnancy loss [ 100 ]. These findings show a layered antioxidant hierarchy in which SODs intercept superoxide at its source, preventing the formation of more reactive derivatives such as peroxynitrite or hydroxyl radicals, while GPX and catalase eliminate the H 2 O 2 by-product. By preserving mitochondrial function and genomic integrity in the corpus luteum, decidua, and placenta, this two-step system proves indispensable for normal placental development and successful pregnancy progression [ 98 , 100 ]. Nuclear factor erythroid 2 2-related factor 2 (NRF2, encoded by NFE2L2 ) is a transcription factor that globally coordinates cellular responses to oxidative stress. The transcription factor NRF2 activates antioxidant genes, such as heme oxygenase 1 (HO1) and NAD(P)H quinone oxidoreductase 1 (NQO1), as well as enzymes involved in glutathione production, to help the cell resist oxidative stress [ 101 , 102 ]. In DSCs, NRF2 functions as a central “sensor” and transcriptional activator of numerous antioxidants and cytoprotective genes. Under normal conditions, NRF2 is bound to its repressor KEAP1, which targets it for degradation, but under oxidative stress, ROS modify KEAP1, causing NRF2 to dissociate, translocate to the nucleus, and bind to antioxidant response elements (AREs) to activate genes encoding antioxidant enzymes like superoxide dismutase, catalase, and glutathione peroxidase [ 53 , 80 , 103 ]. This NRF2/KEAP1 axis is operative in the decidua: it provides a rapid, inducible mechanism to boost the tissue’s antioxidant capacity in the face of increasing oxidative load (for instance, as placental oxygenation rises at the end of the first trimester). Several lines of evidence illustrate NRF2’s role in decidual health. Immunohistochemical studies show that NRF2 is expressed in most decidual cell types, and that its activation (nuclear localization and target gene expression) is tuned to oxidative stress conditions [ 104 ]. During early pregnancy, NRF2 activates defense mechanisms against oxidation through cells of the decidua’s stromal layer [ 104 ]. However, in complicated pregnancies, NRF2 signaling may be dysregulated. In preeclampsia, a condition characterized by increased oxidative stress, NRF2-regulated antioxidant responses are impaired, which contributes to the exacerbation of oxidative damage and pregnancy complications like fetal growth restriction [ 105 ]. Preeclampsia with accompanying FGR showed markedly elevated oxidative stress in the decidua and a concomitant increase in NRF2-regulated gene expression, suggesting that extreme stress triggered maximal NRF2 activation as a compensatory mechanism [ 52 ]. In addition, the NRF2 pathway also interacts with other signaling networks, such as the TAZ pathway, to promote cell survival and differentiation in response to oxidative stress [ 104 , 106 ]. Absence or dysfunction of NRF2 leads to severe oxidative stress that results in fetal growth restriction, together with placental dysfunction [ 105 , 106 ]. Thus, NRF2 serves as a master switch that can amplify the decidual antioxidant network when needed. It ensures upregulation of multiple defense enzymes in a coordinated fashion, effectively future-proofing decidual cells against oxidative surges. The evolution of an efficient NRF2 response in eutherian endometrial stromal cells would have been advantageous in managing the oxidative challenges of deep trophoblast invasion. By leveraging NRF2, decidual cells gain a flexible and robust means to sense, respond to, and survive oxidative stress, thereby protecting pregnancy. Peroxiredoxins are a family of thiol-based antioxidants that reduce H 2 O 2 and organic hydroperoxides and provide added protection in DSCs [ 107 ]. Studies have shown that PRDXs such as PRDX1, PRDX2, and PRDX6 are essential to mediate trophoblast proliferation, differentiation, and apoptosis, which are all processes important for a healthy pregnancy [ 82 , 83 ]. In early pregnancy, the decidua expresses PRDXs, such as PRDX6, to prevent oxidative stress and support proper implantation and fetal development, with its deficiency leading to implantation failure due to unmitigated oxidative stress [ 108 ]. Likewise, similar regulation of ROS levels and influence on survival-related pathways as seen in the PTEN/AKT axis, which modulates PRDX1 and protects trophoblast function [ 83 ]. Disruption of these mechanisms, as seen in conditions like preeclampsia and intrauterine growth restriction, results in decreased PRDX levels, leading to impaired proliferation and invasion of cytotrophoblasts [ 109 ]. In contrast, higher PRDX levels, as seen in the follicular fluid of IVF patients, are associated with improved pregnancy outcomes [ 110 ]. Thus, PRDX family in the decidua provides a redundant, highly responsive peroxide scavenging system. PRDX1 handles diffuse cellular peroxides, PRDX3 shields mitochondria (preventing energy crisis and apoptosis from oxidative damage), and PRDX6 protects membranes and implantation sites from oxidative damage. The importance of each is evidenced by adverse outcomes when expression is altered. PRDX3 downregulation correlates with mitochondrial dysfunction and cell cycle arrest at G2M [ 111 ], and PRDX6 downregulation leads to oxidative stress to derail implantation [ 108 ]. Thus, peroxiredoxins form a vital component of the decidual antioxidant repertoire, working in concert with SODs and GPXs to maintain redox balance during the establishment of pregnancy. Heme Oxygenase-1 (HMOX1) is a protein made from the HMOX1 gene, activated by NRF2, that helps break down heme into useful byproducts and protects the body with antioxidant and anti-inflammatory effects [ 112 , 113 ]. In DSCs, the upregulation of HMOX1 is mediated by hypoxia-inducible factor 1 alpha (HIF1A), which enhances HMOX1 expression and promotes cellular survival against oxidative stress during early pregnancy [ 114 ]. HMOX1 plays a crucial role in modulating the immune microenvironment at the feto-maternal interface, promoting immune tolerance by regulating both innate and adaptive immune responses [ 115 , 116 ]. By influencing T-cell activation and supporting regulatory T cell (Treg) generation, HMOX1 helps reduce inflammation, and decreased HMOX1 expression is often due to genetic variations that are linked to adverse pregnancy outcomes like preeclampsia and placental dysfunction [ 115 , 116 ]. HMOX1 counteracts oxidative stress and inflammation, with increased levels found in both the decidua and maternal serum, which confers its protective role against oxidative stress and ROS during pregnancy [ 117 , 118 ]. Decidualization equips uterine stromal cells with a robust antioxidant defense system to manage oxidative stress during early pregnancy. From superoxide dismutase that provides the first line of ROS neutralization, to glutathione-dependent peroxidases and peroxiredoxins that eliminate peroxides, to master regulators like NRF2 and FOXO1 that amplify protective genes, and HMOX1 that removes pro-oxidant stimuli while curbing inflammation, each layer works in concert. These defenses represent an evolutionary adaptation to the unique demands of eutherian pregnancy, where maternal tissues must accommodate an invasive, semi-allogeneic embryo in an initially hypoxic environment that quickly becomes oxygenated. Decidualization reprograms the stress response of a cell, turning a liability (oxidative stress) into a signal for differentiation and a stimulus for fortifying resilience [ 12 , 21 ]. When this antioxidative network is impaired, conditions like preeclampsia, miscarriages, and implantation failure can arise due to uncontrolled oxidative injury. Conversely, a well-functioning system supports successful placentation and pregnancy maintenance.

The endometrium, or the inner-lining mucosa of the uterus, is a unique tissue with critical roles in the reproductive capacity of mammals [ 1 – 3 ]. Eutherian mammals are defined by extended gestation, with certain species developing an invasive placenta that requires specialized maternal adaptations. The dynamic nature of the endometrium allows it to perform various changes that aid eutherian mammals within the many processes governing the establishment and progression of successful pregnancies in a prolonged gestational period. One of the integral processes necessary for the successful establishment of pregnancy occurs as endometrial stromal cells differentiate into decidual stromal cells (DSCs) under the influence of pregnancy hormones [ 4 – 6 ]. Decidualization, driven by progesterone and cyclic adenosine monophosphate (cAMP), provides the endometrium with the ability to establish pregnancy, to sustain implantation, nourish the embryo, and regulate trophoblast invasion. Unlike most mammals, where decidualization is triggered by embryonic signaling [ 4 , 7 ], decidualization in human occurs spontaneously during the menstrual cycle, reflecting a unique evolutionary adaptation to the demands of invasive hemochorial placentation typical of eutherian mammals [ 8 , 9 ]. Furthermore, decidualization is not exclusive to humans; it also occurs in several non-human species, such as primates, certain bats, the elephant shrew, and the spiny mouse [ 3 – 5 ]. Recent studies suggest that decidualization evolved to protect cells from damage caused by oxidative stress and may also help prevent iron-dependent cell death (ferroptosis) during the inflammation that happens when an embryo attaches to the uterus [ 10 – 12 ]. This review summarizes how decidualization evolved not just to support pregnancy but also as an adaptation to protect against oxidative stress, which is a significant challenge during the establishment of pregnancy, implantation, and placentation. The early stages of pregnancy are marked by extensive physiological transformation and oxygen level fluctuations as the embryo moves from a hypoxic pre-implantation condition to an oxygen-demanding environment after the establishment of pregnancy and placental vascularization [ 13 , 14 ]. This shift generates ROS, which are highly reactive molecules derived from oxygen metabolism, including superoxide anions, hydrogen peroxide (H 2 O 2 ), and hydroxyl radicals [ 15 , 16 ]. While ROS serve as signaling molecules under physiological conditions, their overproduction or inadequate neutralization by antioxidants leads to oxidative stress, a state implicated in cellular damage and pregnancy complications [ 17 , 18 ]. The maternal-fetal interface is especially sensitive to oxidative stress because of high energy demand and immune system activity [ 19 , 20 ]. To protect both the offspring and the mother, the uterus needs a strong defense against damage caused by ROS. Decidualization appears to have evolved to counteract these stresses. Under the stress-induced evolutionary innovation (SIEI) model, the ancestral implantation-induced stress response was co-opted into the decidual cell differentiation program [ 12 ]. This evolutionary trajectory is illustrated by the difference between marsupials and eutherian mammals. Marsupials (e.g., the opossum, Didelphis virginiana ) give birth to altricial young and lack true decidualization. When their endometrial stromal cells are treated with progestin/camp, they activate apoptosis and oxidative stress genes [ 10 , 12 , 21 ]. In contrast, eutherian stromal cells instead activate robust antioxidant defenses [ 22 ]. This evolutionary adaptation probably provided a benefit by protecting the maternal-fetal interface from damage caused by oxidative stress, enabling prolonged gestation and deeper placentation [ 4 , 5 , 16 , 19 ]. It is likely that such an evolutionary shift in response to oxidative stress occurred not only to shape cellular behavior but also to set the stage for more profound molecular changes in the uterus. At the genomic level, transposable elements have facilitated network rewiring. Ancient DNA transposons (e.g., the primate MER20 family) inserted near endometrial genes and brought novel regulatory motifs (including progesterone receptor and CEBPβ binding sites) into the decidual gene regulatory networks [ 9 ]. The MER20 family is a primate-specific lineage of DNA transposons whose insertions dispersed enhancer and promoter motifs, such as progesterone receptor and C/EBPβ binding sites, across the genome, thereby reshaping gene regulatory networks in the endometrium. MER20 insertions are enriched near classic decidualization genes (such as PRL , WNT5A , HSD11B1 ) and provide C/EBPβ-binding sites that facilitate subsequent recruitment of FOXO1 and PR-A to those loci [ 9 , 10 , 23 ]. One possibility is convergent co-option: different lineages independently recruited lineage-specific TEs (e.g., rodent SINEs/LTRs) that created analogous hormone-responsive enhancers [ 23 ]. Alternatively, ancestral cis- or trans-regulatory features likely made these genes hormone responsive before TE insertions, with TEs such as MER20 later amplifying or fine-tuning those pre-existing network [ 24 , 25 ]. This MER20-driven gene regulatory network expanded the progesterone-mediated gene regulatory network in DSCs, linking hormone signaling to stress-defense circuits and thereby enhancing decidual function and protection against oxidative damage. In addition, decidualization activates a coordinated antioxidant event in the uterus and placenta: Progesterone-induced FOXO1 and NR2F2 increase glutathione peroxidases (GPX3/4), mitochondrial superoxide dismutase (SOD2), peroxiredoxins (PRDXs), and heme oxygenase-1 (HMOX1). These enzymes rapidly detoxify reactive oxygen species generated at implantation, prevent lipid peroxidation and ferroptosis, and maintain stromal cell viability. By linking hormone signaling to redox homeostasis, decidualization ensures both trophoblast invasion and tissue integrity under oxidative stress. As a result, decidualization now serves a dual role: supporting embryo implantation and safeguarding the maternal-fetal interface from oxidative damage. This confers the endometrium with the capacity to balance the demands of embryonic development with the need to maintain cellular homeostasis under oxidative pressure. Regardless of recent advances, challenges with the establishment of pregnancy and managing oxidative stress are connected to different reproductive diseases, such as endometriosis and recurrent pregnancy loss (RPL) [ 26 – 30 ]. Impaired decidualization and redox imbalance are characteristics of endometriosis, where progesterone resistance and elevated ROS in the endometrium prevent a normal decidual response. Recurrent miscarriage is similarly associated with excessive ROS, iron accumulation, and markers of ferroptosis in the decidua [ 26 – 30 ]. This review summarizes the current knowledge of how decidualization developed as a response to stress, explores known pathways that help protect against oxidative stress, such as the GPX family, FOXO1, NR2F2, NRF2, HMOX1, and PRDXs, while suggesting new targets for research, like SLC40A1 and GPX4.

DSCs have evolved a multilayered antioxidant defense system to manage the increased oxidative stress that accompanies deep trophoblast invasion and fluctuating oxygen tension in early pregnancy [ 7 , 12 , 21 , 22 ]. The decidua must simultaneously tolerate a burst of ROS signaling (essential for tissue remodeling and angiogenesis) and prevent excessive oxidative damage to itself and the placenta. To achieve this balance, DSCs constitute and engage numerous antioxidant enzymes and cytoprotective proteins ( Figure 2 ). These pathways, outlined below, form a robust network that detoxifies ROS, repairs oxidative damage, and maintains redox homeostasis at the maternal-fetal interface. Their importance is underscored by studies showing that pregnancy disorders like preeclampsia and recurrent pregnancy loss are often associated with dysregulated decidual antioxidant responses [ 8 , 49 – 53 ]. We summarize key antioxidant mechanisms in decidual cells, including their functions, regulation, and evidence linking them to oxidative stress control in pregnancy. Decidual stromal cells (DSCs) assimilated an ancestral stress response triggered by embryo implantation, evolving under the stress-induced evolutionary innovation to withstand the oxidative and inflammatory challenges of pregnancy. While progesterone/cAMP signaling in marsupials (lacking true decidualization) activates pro-apoptotic and oxidative stress genes, eutherian mammals repurposed this signaling to mount antioxidant defenses instead. This evolutionary shift endowed decidual cells with enhanced ROS neutralization and oxidative damage resistance, supporting prolonged gestation and invasive placentation [ 12 , 21 ]. Crucially, decidualizing cells detect cellular stress but actively suppress destructive pathways; instead of stress kinases (JNK/p38 MAPK) triggering apoptosis under ROS accumulation, they induce the phosphatase DUSP1 (MKP-1), which inactivates JNK/p38, blocking apoptosis [ 12 , 21 , 54 ]. By reducing these pro-apoptotic pathways, DSCs survive and differentiate under pro-oxidative implantation conditions, transforming an acute ancestral stress response into a controlled differentiation program that minimizes cell death. FOXO1, a forkhead transcription factor central to oxidative stress responses, is preserved and harnessed in decidual cells. In other cell types, oxidative stress often triggers FOXO1 as part of a defense mechanism only during damage however progesterone signaling during decidualization prevents FOXO1 degradation and promotes its accumulation in the nucleus upon cAMP and prostaglandin E 2 (PGE 2 ) stimulation [ 12 , 22 , 55 , 56 ]. This mimics an “internalized” oxidative stress signal. PGE 2 and hormonal cues generate a low levels of ROS, which in turn activates FOXO1 and other protective pathways even in the absence of an external stressor [ 12 , 50 , 57 ]. Consequently, decidual cells preemptively induce antioxidant and survival genes as if under stress, but without incurring damage. Notably, FOXO1 and FOXO3 (another forkhead factor) are differentially regulated: decidual stimuli induce FOXO1 but repress FOXO3a, the latter of which would typically drive stress-induced apoptosis [ 22 , 58 ]. This switch endows decidual cells with extraordinary resistance to oxidative stress-induced apoptosis. FOXO1 drives the expression of antioxidant enzymes like mitochondrial superoxide dismutase (MnSOD), silencing the pro-apoptotic signals of FOXO3a [ 22 , 55 , 59 ]. In effect, FOXO1 converts what would typically be a pro-death signal into a differentiation cue that maintains redox homeostasis. Beyond stress control, FOXO1 also serves as a gate-keeper of epithelial receptivity: its conditional deletion via progesterone-receptor-Cre renders female mice infertile because luminal epithelial cells keep high PGR levels, preserve apico-basal polarity, and block blastocyst penetration, whereas in wild-type mice, nuclear FOXO1 rises precisely as PGR declines, actively dismantling the epithelial barrier and fine-tuning progesterone signaling during the “window of receptivity” [ 60 ]. FOXO1 acts as a chromatin pioneer for the progesterone receptor in human endometrial stromal cells, co-occupying over 75 % of PGR binding sites, and its loss abolishes PGR loading at an IRF4 enhancer, where IRF4 knockdown subsequently blocks morphologic decidual transformation and suppresses key markers (IGFBP1, PRL, WNT4), demonstrating that FOXO1 is essential both for PGR function and for triggering a downstream transcription-factor cascade [ 61 ]. FOXO1 directly regulates canonical decidual genes: its over-expression elevates IGFBP1, PRL and DCN, whereas siRNA knock-down suppresses IGFBP1 and DCN but paradoxically increases PRL, TIMP3 and CNR1, over-expression alone is sufficient to induce the rounded, epithelioid morphology typical of decidual cells [ 62 ]. Through cross-talk with the progesterone receptor, FOXO1 coordinates cell-cycle exit and differentiation by repressing proliferation drivers (CCNB1, CCNB2, CDC2, MCM5, NEK2); silencing either factor removes this brake, allowing stromal cells to re-enter the cycle, whereas intact FOXO1 enforces durable quiescence required for full decidual maturation [ 63 ]. In endometrial carcinoma cells, progestin raises FOXO1 levels only when the PR-B isoform is present, and forced FOXO1 expression with progestin induces G0/G1 arrest and apoptosis specifically in PR-B–positive cells, revealing a feedback loop in which PR-B stabilizes FOXO1 while FOXO1 enforces P4-dependent growth control [ 64 ]. In addition to FOXO1, eutherian decidualization recruited other transcriptional regulators to enforce the pro-survival, differentiated state. Cyclic AMP and progesterone sharply induce C/EBPβ (CCAAT-enhancer-binding protein β), which is essential for decidual differentiation: murine uteri lacking CEBPB cannot undergo decidualization [ 65 ]. C/EBPβ integrates hormonal signals with the stress-response gene network, activating decidual genes (e.g. prolactin, IGFBP1) while also contributing to cell cycle exit and inflammatory restraint. Recent chromatin studies show that C/EBPβ recruits PGC-1α to enhancer clusters, linking oxidative-stress sensing to mitochondrial biogenesis and energy buffering in DSCs [ 66 ]. Loss-of-function in mouse and human systems shows C/EBPβ is indispensable for decidual differentiation. C/EBPβ -null uteri in mice results in severe implantation failure with >85% of transferred embryos failing to implant and no significant decidual growth even with hormonal priming [ 67 ]. Similarly knocking down of C/EBPβ using siRNA resulted in failure to acquire the characteristic enlarged, polygonal morphology of decidualized cells, instead maintaining a slender, fibroblast-like appearance and the expression of key decidual transcription factors, including FOXO1 and HOXA10, was markedly reduced [ 68 ]. E2F8 is a multifunctional transcription factor involved in diverse cellular processes, including proliferation, differentiation, DNA repair, and apoptosis, with critical roles in angiogenesis, lymphangiogenesis, and embryonic development [ 69 , 70 ]. E2F8, an atypical E2F family transcription factor, is highly expressed in decidual cells and regulated by progesterone-mediated heparin-binding EGF-like growth factor–ERK/STAT3 signaling pathway [ 71 ]. E2F8 acts as a cell cycle repressor, facilitating the transient polyploidy or permanent G0 arrest of decidual cells as an adaptation to stress during differentiation [ 71 ]. By inducing cell cycle exit, E2F8 helps redirect cellular resources toward specialized secretory and defensive functions. Its activity in mice has been linked to stress-associated decidual cell polyploidization. Individual deletion of E2F8 showed no apparent embryonic lethality; however, combined deletion of E2f7 and E2f8 leads to massive apoptosis, widespread vascular defects (dilated vessels, hemorrhages), and embryonic lethality by E11.5 [ 72 ]. Conditional and germline E2f8 deletion causes tissue-specific conditions as neonatal loss in liver promotes hepatocellular carcinoma and uterine-specific loss disrupts the decidual cell polyploidization [ 71 , 73 ]. Reviews and cancer studies report E2F8 single-loss or knockdown promotes tumorigenic phenotypes in some contexts and is being studied as a potential cancer target [ 69 , 70 ]. MER20 is a DNA transposable element (hAT superfamily Charlie subfamily) that invaded the genome of early placental mammals at approximately the same time that extended pregnancy and invasive placentation evolved [ 23 ]. The evolution of decidualization and invasive placentation in early placental mammals coincided with the insertion of MER20 into their genomes, where it contributes to 13% of the enhancers regulating genes critical for pregnancy in humans and other higher primates [ 9 , 23 ]. MER20 elements contain hormone-sensitive regulatory motifs, including progesterone receptor (PR) binding sites, allowing them to act as ancient pregnancy hormone-responsive enhancers [ 9 , 23 , 74 ]. Epigenomic profiling has shown that MER20 sequences in decidual cells bear active chromatin marks typical of functional enhancers and insulators [ 23 ]. These sequences directly bind critical transcription factors required for pregnancy (including PR itself, as well as factors like C/EBPβ) [ 23 , 74 ]. In fact, MER20 is thought to have helped recruit the cAMP/Progesterone signaling cascade into ESCs, a key step in enabling decidualization [ 23 ]. Together, factors such as FOXO1, C/EBPβ, E2F8 and MER20 form a rewired gene regulatory network that stabilizes the decidual cell phenotype, a cell that is stress-resistant, anti-apoptotic, and supportive of embryo implantation. This allowed the maternal uterine lining to actively counteract the oxidative and inflammatory stresses of early pregnancy, thereby protecting the developing embryo during the critical implantation period.