Molecular mechanism of autophagy and apoptosis in endometriosis: Current understanding and future research directions

Increasing attention is now being paid to the deregulation of mitochondrial dynamics in endometriosis. 17 , 40 , 41 The maintenance of mitochondrial biogenesis and homeostasis is achieved by mitochondrial quantity and quality control through continual fusion and fission, i.e., mitochondrial dynamics. 40 , 41 Mitochondrial fusion is driven by the mitochondrial outer membrane dynamin like GTPase fusion proteins, mitofusins 1 and 2 (MFN1 and MFN2), and the mitochondrial inner membrane dynamin like GTPase fusion protein, optic atrophy 1 (OPA1). 42 Enhanced mitochondrial fusion facilitates mitochondrial genomic repair and promotes oxidative phosphorylation and generation of ATP, allowing cells to survive even under stressful environments such as starvation. 41 On the other hand, eukaryotic cells have evolved mechanisms to maintain cell survival by eliminating dysfunctional mitochondria themselves due to enhanced production of ROS, decreased rates of oxidative phosphorylation, depletion of cell ATP pool, and increased numbers of mitochondrial DNA mutations. 41 Mitochondrial fission is crucial for mitochondrial quality control mechanism that eliminates damaged mitochondria via mitophagy. 40 , 41 Mitochondrial fission is primarily mediated by the large GTPase dynamin‐related protein 1 (DRP1). 43 Regulation of the homeostatic balance between mitochondrial fusion and fission is critical for determining mitophagy‐mediated cell survival and death. Impaired mitochondrial dynamics or dysfunctional mitophagy is known to be associated with many diseases, including neurodegenerative disorders, 44 metabolic disorders, 45 cancer, 46 and endometriosis. 17 Therefore, a detailed understanding of the molecular mechanism and function underlying mitophagy is crucial for elucidating the pathogenesis of endometriosis. Next, Figure  2 provides an overview of the molecular mechanism of mitophagy. Mitophagy is regulated by the ubiquitin‐dependent and ‐independent pathways. 47 An overview of the molecular mechanism of mitophagy in endometriosis. Route 1, ubiquitin‐dependent mitophagy pathway; Route 2, Inhibition of PINK1 degradation by Bnip3; Route 3, Parkin translocation to mitochondria by Bnip3; Route 4, The ubiquitin‐independent pathway orchestrated by a receptor protein, BNIP3; Route 5, The interrelationship between molecules, Bcl‐2, Bnip3, and Beclin1: Bnip3 competes with Beclin1 for the binding site of Bcl‐2 protein and releases Beclin1; Route 6, Beclin1 is involved in autophagosome elongation, maturation, and autophagolysosome formation in the ubiquitin‐dependent pathway; Route 6′, Beclin1 is involved in autophagosome elongation, maturation, and autophagolysosome formation in the ubiquitin‐independent pathway; Route 7, The interrelationship between molecules, Bcl‐2, Bnip3, and Beclin1: Beclin1 competes with Bnip3 for the binding site of Bcl‐2 protein and releases Bnip3; Route 8, Excessive Bcl‐2 downregulates mitophagy through binding to Bnip3; and Route 9, Apoptosis induction by Bnip3 via inhibition of Bcl‐2. The pathways indicated by the black arrow has been proven to contribute to the process of mitophagy in endometriosis. The gray arrow indicates that despite the demonstrated effects of mitophagy on some other cells, there is no direct evidence for this reasoning in endometriotic cells. We summarize ubiquitin‐dependent mitophagy in endometriosis. 15 , 48 , 49 , 50 , 51 Ubiquitin‐dependent mitophagy is regulated by phosphatase and tensin homolog (PTEN)‐induced putative kinase 1 (PINK1) and E3 ubiquitin ligase Parkin (PRKN) (Figure  2 , ①). 52 , 53 Once mitochondria are damaged, stabilization of PINK1 promotes mitochondrial recruitment of Parkin and ubiquitin, which in turn further accumulates autophagy adapter proteins (e.g., p62 and Ambra1) and LC3. 54 , 55 , 56 Phagophores expand, sequester portions of the organelles, close, and create autophagosomes. Finally, fusion of autophagosomes and lysosomes generates autolysosomes where impaired mitochondria and p62 are degraded. Bnip3 inhibits PINK1 degradation and promotes its accumulation in the outer mitochondrial membrane (Figure  2 , ②) 54 , 56 and also promotes Parkin translocation to mitochondria (Figure  2 , ③). 57 In fact, ubiquitin‐dependent (i.e., Parkin‐required) mitophagy has been shown to regulate endometriosis apoptosis via upregulation of macrophage stimulating 1 (Mst1) 51 or Prohibitin2 (PHB2) 48 (see Figure  1 ). This suggests that mitophagy regulates apoptosis during endometriosis development. In contrast, the ubiquitin‐independent pathway is mediated by tail‐anchored proteins in the outer mitochondrial membrane. 18 , 47 These mitophagy receptors consist of at least five proteins, including BNIP3, BCL2 interacting protein 3 like (BNIP3L), FUN14 domain containing 1 (FUNDC1), BCL2 like 13 (BCL2L13), FKBP prolyl isomerase 8 (FKBP8). 47 , 56 , 58 , 59 Among these mitophagy receptors, the function of Bnip3 is shown in Figure  2 . Bnip3 interacts with LC3, linking the phagophore to the targeted mitochondria to mediate mitophagy 55 , 60 (Figure  2 , ④). Moreover, coordination between autophagy and apoptosis is regulated by Bcl‐2 family proteins. 21 Both Beclin1 and Bnip3 interact with Bcl‐2 in a mutually exclusive manner. 36 , 61 Bnip3 competes with Beclin1 for the binding site of Bcl‐2 protein and releases Beclin1 55 (Figure  2 , ⑤). Beclin1 forms a high‐affinity complex with Atg14, Ambra1, p150, and PI3K Class III and is involved in autophagosome elongation, maturation, fusion with lysosomes, and autophagolysosome formation, which progresses mitophagy 55 (Figure  2 , ⑥, ⑥’). Since Beclin1 and Bnip3 compete for the same binding sites in the Bcl‐2 molecule (Figure  2 , ⑦), Beclin1 promotes the Bnip3‐mediated mitophagy 62 (Figure  2 , ②, ③, and ④). Additionally, Bnip3 disrupts interaction between the Beclin1 and Bcl‐2 proteins to promote the Beclin1‐mediated autophagy 61 (Figure  1 , ①). Excessive Bcl‐2 protein downregulates autophagy and mitophagy through binding to the autophagy modulators Beclin1 (Figure  1 , ①) and Bnip3 35 , 36 (Figure  2 , ⑧). Furthermore, Bcl‐2 family proteins suppress PINK1‐PRKN‐dependent mitophagy through inhibiting the Bnip3‐mediated autophagy 63 (Figure  1 , ①). On the other hand, Bnip3 triggers apoptosis through suppressing Bcl‐2 function and promoting BCL2 associated X apoptosis regulator (Bax)‐dependent apoptosis 64 (Figure  2 , ⑨). Therefore, Bnip3 is known as a dual‐function regulator of apoptosis and autophagy/mitophagy. 37 In addition, Bcl‐2 exerts antiapoptotic and antiautophagic functions. Collectively, the balance between Bcl‐2 and Beclin1/Bnip3 largely affects mitophagy. These proteins serve as important mediators that mediate coordination between autophagy/mitophagy and apoptosis. 63 Although there are several reports on the role of autophagy‐related markers Beclin‐1 and LC3 in endometriosis, 65 , 66 , 67 , 68 , 69 , 70 , 71 only a few studies have exploited in vitro and animal models to study the function of the Bnip3 molecule. 15 , 49 Evidence in endometriosis is still scarce regarding the pathways ④, ⑥’, and ⑧ (Figure  2 ).

Ethics Approval The submitted paper is a review article and has not been approved by the Institutional Review Board and the Research and Ethical Committee of Nara Medical University Graduate School of Medicine, Kashihara, Japan.

Endometriotic cells can fine‐tune autophagy pathway to survive in their ever‐changing environments. Currently, it is difficult to monitor the expression of autophagy‐related genes or proteins in individual endometriotic cells in real time. Genomic profiles of circulating cell‐free DNA or exosomes based on liquid biopsy may provide repeated transcriptomic snapshots of lesion heterogeneity. A custom panel needs to be generated for several genes related to autophagy and apoptosis (e.g., mTOR , HIF1A , BCL2 , BECN1 , BNIP3 , ULK1 , LC3I , LC3II , p62 , PINK1 , PRKN , AMPK , SIRT1 , and p53 ) to allow diagnosis in a single acquisition. For example, altered expression of genes such as BCL2 , BECN1 , and BNIP3 may help identify endometriosis patients who would benefit from antiautophagic or proapoptotic drugs. Furthermore, the degree of overexpression of mTOR or HIF1A may provide different therapeutic options on autophagy regulation. Such panel‐based genetic diagnosis may improve clinical management of endometriosis patients. Therefore, research focusing on the regulation of autophagy will emerge as a promising therapeutic strategy for endometriosis.

Not applicable.

No funding was received.

In this section, we summarize how autophagy and mitophagy control apoptosis and the underlying molecular mechanisms, focusing on the intrinsic (Subsections  5.2 and 5.3 ) and extrinsic (Subsections  5.1 , 5.4 , 5.5 , 5.6 , and 5.7 ) pathways in endometriosis. Studies have shown that autophagy is associated with the regulation of menstruation and the pathogenesis of endometriosis. 7 The autophagy level in the secretory phase was significantly higher than that in the proliferative phase in the stromal cells of the normal endometrium. 72 , 73 However, the ectopic endometrial epithelial and stromal cells had markedly lower autophagy levels during the proliferative and secretory phases compared with the normal endometrium. 66 , 73 , 74 , 75 Also, the autophagy levels quantitatively differed among distinct endometriotic lesions (ovaries, fallopian tubes, peritoneal, gastrointestinal, and skin). 75 Normal endometrial cells with reduced autophagic function may reflux into the peritoneal cavity and proliferate as ectopic endometriotic cells by restricting apoptosis. 4 , 5 , 7 , 74 In contrast, other researchers have reported upregulation of autophagy levels in the ectopic endometrium of patients with ovarian endometriosis. 76 , 77 Upregulation of Beclin1 and LC3II expression and downregulation of p62 expression were detected in tissue samples and endometriotic cells from patients with endometriosis. 76 , 77 Therefore, previous studies have yielded mixed or inconsistent results regarding the levels of autophagy in endometriosis. To explore reasons for inconsistent reports, we then focus on how various types of stressors (e.g., signal transduction, oxidative stress, hypoxia, or energy starvation) regulate autophagy‐mediated apoptosis in endometriotic cells. As mentioned in section  3 , the PI3K/AKT/mTOR signaling pathway is a major pathway involved in the initiation and regulation of autophagy, 78 , 79 , 80 , 81 , 82 and upregulation of PI3K and Akt expression impairs autophagy in endometriotic tissues through mTOR activation. 83 In endometriosis, the genes and pathways involved in autophagy initiation and regulation (e.g., mTOR, HIF‐1α, C‐X‐C motif chemokine receptor 4 (CXCR4), and estrogen receptor 1 (ESR1)) are upregulated, whereas the downstream ATG‐related genes (e.g., ULK1 and BECN1 65 , 66 , 84 ) and microtubule‐associated proteins (e.g., LC3‐I, LC3‐II, and LC3II/LC3I ratio) are often downregulated. 74 , 85 , 86 Indeed, it is often characterized by decreased number of autophagosomes, decreased conversion of LC3‐I to LC3‐II, decreased BECN1 expression, and increased p62 expression compared with the normal endometrium. 5 Autophagy is downregulated in endometriosis through AKT/mTOR signaling, which inhibits apoptosis 83 (Figure  1 , ①). This suggests that autophagy is a key positive regulator of endometriotic cell apoptosis. Furthermore, the interlinked connections between autophagy and mitophagy have been reported in endometriosis. A decrease in autophagy activity restores the oxidative imbalance by increasing the expression of nuclear factor (erythroid‐derived) 2‐like (NRF2) and the antioxidant proteins NAD(P)H quinone dehydrogenase 1 (NQO1) and heme oxygenase 1 (HO1). 50 Inhibiting autophagy also restores mitophagy homeostasis and significantly increases the levels of Parkin in a rat model of endometriosis. 50 Therefore, well‐coordinated quality control mechanisms are essential for mitochondrial homeostasis and adaptation to stress, such as impaired autophagy and apoptosis. When endometrial cells are shed into the peritoneal cavity during menstruation, they face hypoxia. Autophagy is initiated under such stressful conditions and are particularly common in cancer lesions. 87 , 88 In patients with endometriosis, autophagic vacuoles and autophagosomes accumulate extensively within ectopic endometrial cells under hypoxic conditions. 3 Hypoxia leads to stabilization of HIF‐1α, which upregulates autophagy through activation of the downstream gene BNIP3 58 (Figure  1 , ②; Figure  2 , ②, ③, and ④). HIF‐1α promotes autophagy and attenuates apoptosis. 3 , 89 HIF‐1α also upregulates Bcl‐2 expression. 90 Beclin1 is downregulated by competing with Bcl‐2 for interaction with Bnip3. HIF‐1α was shown to enhance the migration and invasion of human endometriotic stromal cells through upregulation of autophagy and Bcl‐2 expression 3 , 89 (Figure  1 , ③), which may contribute to the pathogenesis of endometriosis by reducing the apoptosis of endometriotic cells. Thus, autophagy is thought of as a survival mechanism that can prevent cell death or apoptosis under hypoxia. 3 Conversely, excessive autophagy can accelerate cell death. Indeed, ectopic endometrial tissues also show increased LC3 levels and decreased p62 levels, which lead to apoptosis 3 (Figure  1 , ④). Therefore, autophagy may switch from a survival to a cell death program or vice versa based on the concentrations of Bcl‐2, Beclin1, and Bnip3 in a hypoxic environment. The accumulation of high levels of hemoglobin, heme, and iron is caused by cyclic bleeding found in patients with endometriosis. 91 Heme is catabolized to biliverdin, carbon monoxide, and iron by the heme oxygenase enzyme system. 92 In fact, the concentrations of iron within endometriosis cysts have been reported to vary between 65.3 and 1046.3 mg/L, demonstrating that the degree of iron‐induced oxidative stress also varies depending on endometriotic foci. 90 Excess iron accumulated in ovarian endometriotic lesions is strongly associated with an oxidative stress‐induced autophagic stimulus 93 (Figure  1 , ⑤). Iron excess is believed to generate high levels of ROS and oxidative stress through the Fenton reaction, which significantly inhibits cell proliferation and causes cell death. 94 Also, iron overload causes extensive apoptosis and induces cytoplasmic vacuolization as the morphological changes along with increased LC3‐II levels (Figure  1 , ④). 95 Therefore, the overactivation of autophagy and apoptosis under oxidative stress may negatively impact normal endometrial growth and the development of endometriosis. On the other hand, activation of autophagy may also exert cytoprotective properties in endometriosis. 67 Indeed, oxygen‐derived‐free radicals formed by excess labile iron are important modulators of Nrf2, nuclear factor kappa‐light‐chain‐enhancer of activated B cells (NF‐κB), AMP‐activated protein kinase (AMPK), Akt, and p53. 94 For example, activation of the Nrf2 pathway upregulates the expression of cytoprotective genes and antioxidant genes, reduces the level of ROS, protects cells from ROS, and blocks apoptosis 94 , 96 (Figure  1 , ⑥). Furthermore, autophagy was shown to promote the survival of eutopic endometrial stromal cells by reducing ROS generation possibly through mediating mitochondrial quality control 93 (Figure  1 , ⑥ and ⑦). In addition, iron overload causes activation of protective autophagy and inhibition of apoptosis in eutopic endometrial stromal cells through upregulation of sirtuin 1 (SIRT1) expression 93 (Figure  1 , ⑦ and ③). SIRT1 participates in the regulation of autophagy through deacetylation of specific autophagy‐related proteins (e.g., Beclin1 and LC3). 97 Altered expression of autophagy‐related molecules (e.g., Bcl‐2, Bax, Beclin1, Bnip3, Parkin, LC3, and p62) in endometriotic cells can switch the signaling from pro‐apoptosis to antiapoptosis (Figure  1 , ④ vs ③). Therefore, contradictory findings may exist in endometriosis, as iron‐induced oxidative stress exhibits a dual role in cell‐fate determination, i.e., cell death and cytoprotection, based on the concentration of iron in endometriotic cysts. Estrogen plays a crucial role in the pathogenesis of endometriosis. Genes involved in the initiation and regulation of autophagy (e.g., mTOR, CXCR4, and ESR1) are upregulated in endometriosis. 73 Estrogen receptor signaling activates signaling pathways such as PI3K/AKT/mTOR and C‐X‐C motif chemokine ligand 12 (CXCL12)‐CXCR4 axis. 73 Therefore, estrogen suppresses autophagy through mTOR signal activation and CXCL12/CXCR4 interaction and promotes endometrial stromal cell proliferation 73 (Figure  1 , ①). Mechanistically, similar to mTOR, CXCL12 may inhibit autophagy by reducing Beclin1 expression, reducing LC3B‐I to LC3B‐II conversion, increasing p62 levels, and downregulating autophagosome. 98 High estrogen levels and progesterone resistance, a hallmark of endometriosis, may be involved in the downregulation of autophagy. 5 Therefore, estrogen could decrease the apoptosis through the inhibition of autophagy. Endometriotic cells adapt to various environmental stressors and survive under nutrient deprivation and starvation. AMPK is a metabolic sensor that responds to low cellular energy stores, thereby allowing the cell to maintain energy homeostasis. 99 Activation of AMPK inhibits mTOR via inactivation of the PI3K/Akt signaling pathway 99 , 100 (Figure  1 , ①). AMPK induces autophagy by inhibiting mechanistic target of rapamycin complex 1 (MTORC1), activating the ULK1 complex, and phosphorylating Beclin1. 101 , 102 AMPK‐mediated autophagy is a cytoprotective mechanism that increases nutrient and energy demand. 99 p53 is thought to inhibit mTOR activity, leading to the induction of autophagic and apoptotic pathways (Figure  1 , ① and ②). 11 , 103 , 104 Decreased p53 expression in ovarian endometrioma suggests inhibition of apoptosis. 76 In addition to these pathways or stressors, many genes that control autophagy and apoptosis in endometriosis have been reported. For example, Mst1 is known as a major growth suppressor involved in cancer invasion, proliferation, and apoptosis. 51 Mst1 activates mitochondrial fission and inhibits mitophagy through enhancing Drp1 activation and repressing p53‐mediated Parkin transcription activity. 51 Downregulation of Mst1 expression in endometriosis has been reported to promote endometriotic cell proliferation through activation of mitophagy. 51 Yes‐associated protein (YAP), a core effector component of the Hippo signaling pathway, is associated with organogenesis, malignancy, and endometriosis. 105 The Hippo‐YAP pathway promotes cell proliferation in endometriotic stromal cells through inhibition of autophagy and apoptosis. 86 The cyclic GMP‐AMP synthase (cGAS)‐stimulator of interferon genes (STING) pathway promotes immune effector responses associated with tumorigenesis. 106 Activation of the cGAS‐STING signaling pathway in endometriosis causes cell proliferation through induction of autophagy. 107 The tumor suppressor DIRAS family GTPase 3 (DIRAS3) is a physiological autophagy inducer. 108 Upregulation of DIRAS3 expression in endometriosis promotes proliferation of endometriotic epithelial cells through activating autophagy and inhibiting apoptosis. 109 The tumor suppressor programmed cell death 4 (PDCD4) suppresses tumor progression. 110 PDCD4 partially suppresses endometriosis cell proliferation and invasion through inhibition of autophagy. 110 Animal models are essential to better understand the molecular mechanisms involved in autophagy and apoptosis in endometriosis. 15 , 50 , 75 , 77 , 107 , 111 , 112 In this section, rather than providing a detailed overview of animal models of endometriosis, we explain the contradictory role of autophagy. Most experimental endometriosis was induced by transplantation of normal uterine tissue into the peritoneal cavity. 15 , 75 The autophagic pathway was altered in the endometriosis‐like lesions as compared with eutopic endometrium and normal endometrium. 75 In a rat model of endometriosis, the expression of mTOR was increased, and the expression of Beclin1, Bnip3, Ambra1, LC3II, and Parkin was decreased, 15 suggesting that the autophagy level is decreased in ectopic endometrium compared to eutopic endometrium and normal endometrium. Conversely, there were also reports that autophagy markers (e.g., Beclin1 and LC3B) were elevated in the ectopic endometrium and further increased in the eutopic endometrium from a mouse model of endometriosis compared to controls. 75 This model suggests that activation of autophagy in endometriotic cells may favor apoptosis inhibition. Collectively, similar to in vitro and clinical data, the resulting data in animal models of endometriosis are also inconsistent. Interestingly, changes in Beclin1 and Bnip3 expression were also identified in animal models.

We highlighted studies that have evaluated alterations in autophagy/mitophagy and apoptosis in endometriosis and provided an overview regarding its pathogenesis and personalized treatment strategies. Autophagy and mitophagy have been reported to control apoptosis through various pathways and contribute to promoting the development and progression of endometriosis. 5 Since endometriosis is characterized by iron‐mediated oxidative stress, mitophagy has been studied in the context of regulatory networks that coordinate mitochondrial quality control and antioxidant capacity. However, research on mitophagy in endometriosis is limited, and only a few studies have addressed the ubiquitin‐independent mitophagy pathway. Although there is evidence suggesting that ubiquitin‐independent pathway may be a critical factor in mitophagy, their role in the development, progression, and pathogenesis of endometriosis remains unclear. Therefore, this review mainly focuses on autophagy. Previous studies on the contradictory outcomes of the autophagy‐mediated apoptosis have facilitated an elucidation of the underlying mechanisms regulating autophagy and apoptosis. Endometriotic cells can switch their autophagic responses from inhibitory to promoting mechanism or vice versa to adapt to ever‐changing environments, such as the regulation of signal pathway and oxygen, iron, and nutrient concentrations (Figure  1 , ① and ⑤). Upregulation of mTOR expression can suppress autophagy, 15 , 33 , 50 , 61 , 73 , 74 , 100 , 112 whereas hypoxia and iron‐mediated oxidative stress often promote autophagy. 3 , 25 , 62 For example, inappropriate activation of the PI3K/Akt/ERK1/2/mTOR pathway leads to inhibition of autophagy and subsequent suppression of apoptosis (Figure  1 , ①). Inhibition of autophagy suppresses apoptosis and promotes endometriotic cell proliferation. Indeed, some drugs, such as dienogest, 83 rapamycin, 15 , 33 , 74 Açai Berry, 50 and HCQ, 75 have been reported to exhibit apoptosis promoting and growth suppressive effects on endometriosis by promoting autophagy. On the other hand, previous studies have also shown that ovarian endometriotic cells upregulate autophagy to survive and promote growth. 76 , 119 In fact, activation of the HIF‐1α pathway and excess ROS induces autophagy and avoids cell death through antiapoptotic effects (Figure  1 , ②, ⑤, ⑦, and ③). The autophagy pathways that regulate cell homeostasis are crucial for scavenging damaged organelles, lipids, proteins, and DNA. 26 , 27 , 28 Therefore, endometriotic cells must rely on autophagy to survive external challenges such as hypoxia and oxidative stress. However, HIF‐1α‐ or ROS‐induced overactivation of autophagy has also been reported to induce cell death via promoting apoptosis (Figure  1 , ④). Therefore, autophagy may play a dual role in suppressing and promoting the development of endometriosis. We discuss why these seemingly contradictory results occur. First, regarding the mechanism by which autophagy controls apoptosis, alternative or compensatory mechanisms need to be considered. For example, oxidative stress and nutrient starvation induce the expression of SIRT1 120 and AMPK 121 as compensatory mechanisms as a means of coping with harsh environments. These molecules have key roles in the regulation of cellular metabolism and homeostasis. Key signaling pathways and downstream target molecules involved in autophagy are significantly altered in each endometriosis to deal with ever‐changing environments. 5 Second, autophagy and apoptosis are tightly regulated processes that share a common signal. 6 , 15 , 29 , 122 Bcl‐2, Bax, Beclin1, and Bnip3 are important molecules that regulate two distinct cellular processes, autophagy and apoptosis. 35 , 36 , 37 , 55 , 61 , 62 , 63 , 64 Beclin1 and Bnip3 can bind Bcl‐2 in a mutually exclusive manner and are involved in determining autophagy and apoptosis (see Section  4 ). Therefore, apoptosis may be promoted or inhibited depending on the concentrations of Bcl‐2, Bax, Beclin1, and Bnip3 proteins in endometriotic lesions (Figure  2 ). These divergent outcomes are likely due to the unique combination of the altered expression of multiple downstream molecules and associated pathways in each endometriotic cell. Finally, studies using endometriosis samples are important for the development of new therapeutic strategies for targeting autophagy. At present, when and how cells choose cytoprotection or apoptosis remains unclear. In some lesions, endometriotic cells can be eliminated by autophagy, so autophagy‐promoting drugs may be helpful in the treatment (Figure  1 , ①). In other lesions, antiautophagic drugs may result in therapeutic effects against endometriosis possibly through inhibiting the autophagy‐mediated quality control and restoring apoptosis (Figure  1 , ④). In particular, strategies that modulate the expression level of autophagy (i.e., the transition from cytoprotection to apoptosis) could be a potential target for endometriosis. Therefore, there is an urgent need to develop novel therapies targeting autophagy that respond to changing conditions in real time. In conclusion, this review summarizes the current understanding of the molecular mechanisms involved in autophagy and apoptosis in the pathogenesis of endometriosis and also discusses the challenges and future therapeutic directions targeting autophagy in endometriosis.

Only one paper was found that discussed pharmacological modulation of mitophagy and the therapeutic potential of targeting mitochondrial dynamics in endometriosis. 51 Therefore, this section summarizes therapeutic potential of targeting autophagy in the treatment of endometriosis. Accumulating evidence indicated that pharmacological modulation of autophagy attenuated the progression of endometriosis in both in vitro and in vivo settings. However, as shown below, research has shown varied results on the autophagy‐mediated apoptosis properties, including that autophagy‐mediated apoptosis induction inhibited the development and progression of endometriotic lesions, 15 , 50 , 74 , 112 whereas autophagy inhibition significantly reduced the proliferation of endometriotic cells through promoting apoptosis. 3 , 67 , 75 Several papers have reported that autophagy results in apoptosis induction in endometriotic cells. 15 , 50 , 74 , 112 First, rapamycin, a macrocyclic antibiotic isolated from Streptomyces hygroscopicus , is an immunosuppressant, antifungal, and antitumor drug that blocks mTOR protein kinase. 113 Induction of autophagy by rapamycin affects apoptosis in a variety of cell types. 33 Choi et al reported that rapamycin induces autophagy and further promotes apoptosis in endometriotic cyst stromal cells. 74 Mechanistically, the mTOR inhibition acts as a hub to activate autophagy mechanisms through increased expression of Beclin1, LC3II, Bnip3, Ambra1, and Parkin in a rat model of endometriosis. 15 Furthermore, rapamycin‐mediated activation of mitophagy induces apoptosis by activating proapoptotic Bcl‐2 family proteins (e.g., Bax), inducing cytochrome c release, and then promoting caspase activation. 30 , 74 Thus, upregulated autophagy triggered by rapamycin can promote apoptosis. 74 , 81 Although rapamycin can provide a therapeutic treatment for endometriosis, its efficacy may be limited because of toxicity, side effects, and a ubiquitous expression of mTOR. Second, the search for and development of novel mTOR modulators with fewer side effects to treat endometriosis remains a challenge. In recent years, plant‐based natural products have attracted attention. Açai Berry is an Amazon's popular functional food produced by the Euterpe oleracea palm and has been reported as a molecule that modulates the autophagy pathway. 114 Açai Berry has demonstrated its efficacy in animal models of endometriosis. 50 Açai Berry inhibits PI3K/Akt/extracellular‐regulated MAP kinase (ERK)1/2/mTOR signals, promotes the activity of ULK1/Beclin1/Ambra1 molecules, and enhances the processes of the autophagosome nucleation, expansion, and maturation. 50 Furthermore, it promotes apoptosis through upregulating Bax expression and downregulating Bcl‐2 expression. 50 Third, increased estrogen receptor expression and decreased progesterone receptor isoform B expression are negative regulators of apoptosis and autophagy in endometriotic cells. 112 For example, SCM‐198 is the synthetic compound of leonurine, an alkaloid found in Herba leonuri, and has an antiestrogenic property. 112 SCM‐198 promotes endometriosis cell death via upregulation of apoptosis. 112 Furthermore, dienogest treatment induces autophagy and promotes apoptosis through inhibition of Akt/ERK1/2/mTOR signaling in endometriotic cells. 83 Thus, autophagy induction via mTOR inhibition is emerging as a promising therapeutic strategy for endometriosis. Conversely, autophagy inhibition may also promise to be an effective strategy for treatment of endometriosis. Hydroxychloroquine (HCQ), an autophagy inhibitor, is an alkalinizing lysosomotropic agent that have been used for the treatment of malaria and various autoimmune diseases such as systemic lupus erythematosus and rheumatic and dermatologic diseases. 75 , 115 Bafilomycin A1, a chemical inhibitor of lysosomal proton pump vacuolar‐type ATPase (V‐ATPase), prevents the formation of autophagosomes, blocks autophagosome‐lysosome fusion, inhibits autolysosome acidification, and disrupts autophagic flux, leading to autophagy inhibition. 116 Treatment with hydroxychloroquine 75 significantly decreased endometriotic cell growth through promoting apoptosis. Additionally, bafilomycin A1 inhibited autophagy in endometriotic stromal cells. 117 In addition, HIF‐1α promotes the migration and invasion of endometrial stromal cells through upregulation of the expression of autophagy‐related molecules. 3 Paeonol, 2′‐hydroxy‐4′‐methoxyacetophenone, is a bioactive phenol present in the root bark of the Moutan Cortex 118 and downregulates the HIF‐1α‐mediated pathway proteins. 67 HIF‐1α‐mediated autophagy inhibition by Paeonol suppresses both migration and invasion in endometriotic cells. 67 Collectively, HIF‐1α and mTOR act as a positive and negative regulator of autophagy, respectively, in endometriosis. The inactivation of the PI3K/Akt/mTOR pathway can induce autophagy and subsequently promotes apoptosis. Conversely, the inactivation of the HIF‐1α pathway can promote apoptosis through inhibiting autophagy. These findings support the view that fine‐tuning the induction and inhibition of autophagy can be used as an effective intervention strategy for endometriosis treatment.

Endometriosis is a common estrogen‐dependent gynecological disease that causes pelvic pain, dysmenorrhea, and infertility. 1 It affects 10% of women during their reproductive period. Endometriosis is characterized by abnormal growth of endometrium‐like glandular and stromal cells outside of the uterine cavity, usually in the peritoneum, ovaries, and pelvic cavity. The most commonly accepted theory is the retrograde menstrual reflux (i.e., Sampson's hypothesis). 2 The normal endometrium retrogrades into the peritoneal cavity and is exposed to severe hypoxic stress. 3 , 4 How regurgitated endometrial tissues survive, implant, and grow as endometriotic lesions under harsh conditions is not well understood. Endometriosis is characterized by enhanced proliferation and diminished apoptosis of endometrial and stromal cells. 4 Apoptosis has been shown to be regulated by autophagy through several mediators. 5 The difference between autophagy and apoptosis can be summarized in two words: “self‐eating” and “self‐killing,” and autophagy is an efficient regulator of apoptosis. 6 Autophagy in eukaryotic cells promotes the degradation of subcellular elements to maintain cellular homeostasis via the autophagosome (double‐membrane vesicles)‐lysosome system. 5 , 7 Intracellular materials degraded by autophagy contain the cargo composed of cytoplasmic components including mitochondria, lipids, oxidized proteins, macromolecules, abnormal protein aggregates, and damaged or aged organelles. 5 , 8 This process induces an alternative source of bioenergetic metabolites and recycles nutrients for survival and cellular protection in response to environmental stress. 3 , 9 For example, it is well accepted that autophagy has been linked to a range of physiological and pathological processes, including reproduction, embryo implantation, 10 myopathy, metabolic disorder, neurodegenerative disease, 11 , 12 cardiovascular disease, and cancer. 7 , 9 , 13 Autophagy is classified into different categories: macroautophagy, selective autophagy, chaperone‐mediated autophagy, and microautophagy. 14 Macroautophagy consists of several steps: initiation, induction, phagophore elongation, autophagosome formation and maturation, autolysosome formation, and proteolytic degradation of the contents. 14 , 15 Selective autophagy is the elimination of specific cellular components such as mitochondria (i.e., mitophagy) and requires recognition of injured mitochondria and subsequent activation of autophagy. 14 Mitochondria of eukaryotic cells stem from a bacterium 1.5 billion years ago, regulate oxidative phosphorylation, and maintain cellular functions. 16 Until now, we have been elucidating the molecular and cellular mechanisms underlying endometriosis pathogenesis, focusing on energy metabolism, mitochondrial dynamics, and cellular redox homeostasis. 17 Mitochondria supply adenosine triphosphate (ATP) from aerobic respiration and orchestrate cell proliferation and development, but they also form reactive oxygen species (ROS) as by‐products in the electron transport chain. 16 , 17 Increased ROS production causes impairment of mitochondria, so mitophagy has evolved to eliminate the dysfunctional mitochondria. 16 , 18 Thus, mitochondria have evolved mechanisms for quality control and cellular homeostasis, 19 but they also serve as a hub for the apoptosis signaling pathways. 20 This is because mitophagy and apoptosis are often induced in response to common stimuli (e.g., mitochondrial dysfunction, oxidative stress, and calcium ion concentration 11 ). 20 , 21 Although autophagy/mitophagy and apoptosis are closely related to each other and share some common signals, they are distinct processes. 21 The signal transmission between autophagy/mitophagy and apoptosis in endometriosis is complex and still not fully understood. 22 Importantly, autophagy and mitophagy exert opposite functions, protective and lethal, so they can inhibit or promote apoptosis. 21 , 23 Several intrinsic (e.g., genetic predisposition, certain signal transduction pathways, female hormonal stimuli, ATP content, and p53 status 24 ) and extrinsic factors (e.g., hypoxia, oxidative stress, iron concentration, reduced nutrient supply, metabolic stress, and endoplasmic reticulum stress 25 , 26 , 27 ) have been reported to be involved in the regulation of autophagy/mitophagy in endometriotic tissues. 5 , 28 Endometriosis cells are thought to control apoptosis by activating or suppressing autophagy/mitophagy in response to both stimuli. In this review, we summarize our current understanding of how autophagy/mitophagy and apoptosis modulate endometriosis development and progression and discuss future directions for research.

The authors declare no competing interests.

We conducted a narrative review of the literature that focuses on autophagic, mitophagic, and apoptotic function in endometriosis. Electronic databases including PubMed and Google Scholar were searched for literature published up to the October 31, 2023, combining the following keywords: “Apoptosis,” “Autophagy,” “Endometriosis,” “Mitochondria,” and “Mitophagy.” The search strategy using the keyword string and combination of Boolean operators is shown in Table  1 . The search strategy. Papers reporting patients' data and in vitro and animal studies conducted to investigate the potential effect and underlying molecular mechanism were also included.