Endometriosis (EMs) is a medical condition characterized by the growth of functional endometrial tissues (glands and stroma) outside the uterine cavity, affecting approximately 10% of women of reproductive age globally (As-Sanie et al., 2019, Saunders and Horne, 2021). The etiology of EMs is multifaceted, and numerous research theories have been proposed (Kajiyama et al., 2019). Despite extensive investigations in both fundamental and clinical domains, the current hypotheses fail to fully comprehend the intricate nature of EMs. Currently, EMs is acknowledged as an estrogen-dependent disorder, and its clinical management entails surgical excision and/or pharmacotherapy. Additionally, EMs has a tendency to recur, and medication therapy may have a deleterious effect on a variety of elements of female reproductive health, including bone metabolism and reproductive physiology (Burney and Giudice, 2019). As a result, it is imperative to explore the occurrence and development mechanisms of EMs and find a new therapeutic target to avoid the adverse effects of hormone therapy.

Ovarian endometriosis is one of the most prevalent endometriosis lesions. In addition to dysmenorrhea, it can also result in infertility and ovarian cancer (Yachida et al., 2021, Tan et al., 2022). Typically, chocolate cysts are caused by ovarian endometriosis, and ovarian endometriosis cysts have higher levels of oxidative stress than other benign cysts. The presence of periodic and repetitive bleeding from ectopic focus inside the focal micro-environment has a crucial effect in increasing iron concentrations (Dixon and Stockwell, 2014, Mori et al., 2015, Zhou et al., 2018). Following periodic and repeated bleeding of ectopic lesions, numerous macrophages phagocytose hemoglobin in the local micro-environment, resulting in iron overload, increased oxidative stress, an inflammatory response, and then increased tissue repair, all of which contribute to the occurrence and progression of endometriosis (Clower et al., 2022). A significant influx of iron ions into the cytosol, on the other hand, might amplify the Fenton reaction, increasing oxidative stress, lipid oxidation, and disrupting the delicate balance between oxidative and antioxidant activities. As a result, this chain of events results in cellular DNA damage and the consequent induction of cell ferroptosis (Henning et al., 2022). In such a high concentration of iron micro-environment, are ectopic endometrium cells more likely to occur ferroptosis?

Ferroptosis is a recently found cell death cascade that differs from previous kinds of controlled cell death (Dixon et al., 2012). It is primarily characterized by iron dependency, lipid peroxidation accumulation and plasma membrane damage. Fundamental research on ferroptosis is rapidly expanding due to the discovery of a variety of regulators and pathways, therefore offering new opportunities to treat a broad variety of pathological conditions (Li et al., 2020a, Liang et al., 2022). According to recent studies, erastin, a class I ferroptosis inducer, induces ferroptosis in ectopic endometrial stromal cells (EESCs) by regulating iron transporters, and ectopic lesions were regressed after erastin administration (Li et al., 2021).

Enhancer of zeste 2 polycomb repressive complex 2 subunit (EZH2) is the core catalytic subunit of the polycomb repressive complex 2 (PRC2), which also contains embryonic ectoderm development (EED) and suppressor of zeste 12 (SUZ12) and RbAp46/48. EZH2 mediate the silencing of hundreds of target genes mainly through tri-methylation of histone H3 lysine 27 (H3K27me3) (Duan et al., 2020, Wang et al., 2022). Existing studies have confirmed that the elevated level of EZH2 has been found to be correlated with various tumor cells, including breast cancer, prostate cancer and gastric cancer (Li et al., 2020b, Pan et al., 2016, Yuan et al., 2020). Other studies have found that EZH2 regulates the expression of many genes related to cell proliferation, apoptosis and cell cycle through methyltransferase, thus promoting tumor growth and metastasis (Kim and Roberts, 2016). Furthermore, western blot results displayed that EZH2 protein level considerably elevated after lentivirus infection, increased evidence indicates that EZH2 has an PRC2-independent function as a transcriptional activator or coactivator in various diseases (Kim and Roberts, 2016, Huang et al., 2021). However, the role of EZH2 in EMs was rarely studied. Recent studies have reported that EZH2 is highly expressed in ectopic tissues, and inhibiting the expression of EZH2 will reduce the migration and proliferation ability of EESCs (Brunty et al., 2021).

In the present study, it was observed that the micro-environment of ovarian endometriosis exhibits a high concentration of iron, thereby increasing the likelihood of inducing ferroptosis in ectopic endometrial stromal cells (EESCs). Furthermore, the essential role of EZH2 as a suppressor gene in ferroptosis was discovered. Interestingly, the methyltransferase inhibitor GSK343, also targets EZH2, exhibited a notable decrease in the expression of EZH2 and decreased the tri-methylation of Histone 3 at Lysine 27, leading to an enhanced susceptibility of EESCs to undergo ferroptosis. GSK126, another methyltransferase inhibitor, also decreased tri-methylation of Histone 3 at Lysine 27, but did not affect EZH2 expression, leading to no change in ferroptosis sensitivity. These findings suggest that the suppressive effect of EZH2 on cell ferroptosis may not rely on its methyltransferase activity.