2).ROS leads to autophagy up-regulation of ovarian endometrioma through the TFEB-mediated mTOR independent pathway (figure).
Ovarian endometriosis is widely accepted as the direct precursor of clear-cell and endometrioid ovarian carcinomas.(44, 45) However, when the researchers studied the level of autophagy in ovarian endometriosis, the results were different from endometriosis. Giulia Allavena et al. compared LC3 and P62 expression levels in normal endometrial tissues (NE), ovarian endometriotic cysts tissues(OMA) and eutopic endometrium (EEOMA) groups, they found that the LC3-II and LC3-II/LC3-I ratios in OMA were significantly higher than that in NE and EEOMA, while p62 was significantly lower in OMA than that in NE and EEOMA.(46) Ying Ding et al’s study shows Beclin-1 expression of ovarian granule cells was increased and autophagy was enhanced in patients with ovarian endometrioma.(47) researchers also found that LC3B significantly increased expression in oviduct and ovarian endometrial epithelial lesions compared with secretory endometrial epithelium of the control group.(48) Serum CA125, which for the diagnosis and staging of ovarian endometriosis, was moderately positively correlated with LC3B protein expression.(49) All evidence is biased towards autophagy is up-regulation in ovarian endometriosis. However, this does not conflict with the down-regulation of autophagy observed in other areas of endometriosis. Because the increase of autophagy level may be considered as an integral part of the progression of endometriosis, which may contribute to the survival and damage maintenance of ectopic endometrial cells. Many studies indicate that oxidative stress positively associated with the proliferation and migration of endometrial cells in the peritoneal cavity, promoting endometriosis and infertility. Mitochondria is major source for ROS generation.(50) It is a class of molecules formed by incomplete reduction of oxygen, including superoxide anion (O2•−), hydrogen peroxide (H2O2), hydroxyl radical (•OH), etc.(51) In endometriotic cysts, bleeding can occur inside the cyst during every menstruation cycle, so the cysts contain large amounts of free iron or non-protein-bound iron. Subsequently, the hemoglobin from an oxyhemoglobin state to the methemoglobin state through autoxidation and the Fenton reaction can produce excess ROS .(52) Oxidative stress caused by excess iron leads to uncontrolled cell growth and aberrant intracellular signaling.(53) It has been found that TFEB is activated under oxidative stress induced by tert-butyl peroxide or hydrogen peroxide in nematodes.(54, 55) On the contrary, ROS elimination inhibits TFEB activity and lysosomal function. (56-58) So the relation between ROS and TFEB may be a mechanism for ectopic endometriosis cell proliferation. ROS can regulate autophagy via TFEB in different ways. ROS overproduction can lead to the activation of autophagy by suppressing the PI3K/AKT/mTOR signaling pathway, which plays a critical role in regulating autophagy.(59) mTOR inactivation can dissociate TFEB from 14-3-3 and promote TFEB nuclear transposition to improve autophagy level. However ovarian endometriotic lesions have been shown to exhibit enhanced activation of mTOR compared with the normal endometrium.(60) José A et al reveal the ROS inducer sodium arsenite can activate TFEB and TFE3 without Inhibiting mTORC1 activity. This mechanism, they say, may enable some cell types that need both TFEB/TFE3 and mTORC1 signalings, such as immune cells or cancer cells, to survive and achieve robust cell growth in stressful conditions.(61) So there may be other mechanisms in ovarian endometriosis that do not depend on mTOR to control TFEB to upregulate autophagy. It was shown that under oxidative stress situations, AMPK is activated and regulated mitophagy to relieve intestinal ischemia/reperfusion-induced intestine disorder.(62) Abnormal AMPK expression was observed in the endometrial microenvironment,(63) phosphate-AMPK can activates SIRT1, SIRT1 deacetylation TFEB at K116, enhances the expression of autophagy/lysosomal related genes, and regulates the formation of autophagic vacuoles,(28, 64) study has found that SIRT1 was overexpressed in endometriosis patients and may be involved in the pathogenesis of endometriosis.(65) Moreover, Jung-yoon Yoo et al. found that in the baboon endometriosis model, SIRT1 protein was not significantly expressed in the endometrium of pre-inoculation (control) baboon. However, during the progression of endometriosis, SIRT1 protein levels increased significantly at 9 and 15 months after inoculation.(66) Therefore, the ROS-AMPK-SIRT1-TFEB pathway may be a mechanism for the progression of endometriosis. Besides, Izumi Suganuma et al. discovered PGC-1a was more highly expressed in ovarian endometrioma than in endometrium with endometriosis and normal endometrium, and PGC-1a can promote the proliferation of ovarian endometrioma stromal cells , whereas PGC-1a knockdown reduced the proliferation,(67) This may be due to SIRT1-mediated deacetylation of PGC-1a, which activates TFEB and thus induces autophagy and promotes ectopic endometrial cell proliferation. Besides, Recent studies suggest that the increasing level of ROS can directly activate lysosomal MCOLN1 to induce lysosomal Ca2+ release, establish a Ca2+ microdomain near the lysosome. This leads to calcineurin activation and TFEB dephosphorylation, which is no longer bind 14-3-3 proteins and can be freely translocated into the nucleus, where transcriptional activation of the lysosome/autophagy pathway occurs .(26, 68) what’s more,Hongfeng Wang et al showed that ROS-dependent TFEB fast nuclear translocation could occur due to oxidation of TFEB on C212 without inhibiting the activity MTORC1.(69) Vitamin C is an antioxidant, with different doses of vitamin C (0.5 mg, 1.25 mg and 2.5 mg), treatment of endometrial cyst model, Durak et al Found a significant reduction in weight and volume of cystic lesions in the 2.5 mg vitamin C group at the end of treatment, suggesting that oxidative stress is involved in the progression of endometriosis(70). therefore, antioxidants may be a mechanism for the treatment of endometriosis, TFEB can be regulated by ROS through different mechanisms, so TFEB may also be a potential target for endometriosis treatment. What’s more, compared with both NE and EEOMA, p53 is significantly down-regulated in OMA,(46) Study has shown that p53 deletion or the use of pifithrin-αchemical inhibition of p53 promotes the transfer of TFEB from cytoplasm to nucleus, thereby increasing the biogenesis of TFEB-mediated lysosomes and autophagosomes. Moreover, p53 re-expression down-regulates the TFEB target gene involved in autophagy-lysosome pathway, while TFEB silencing eliminates the regulatory effect of p53 on autophagy-lysosome pathway regulatory gene transcription.(71) So TFEB inhibitors may be more effective than antioxidants in treating endometriosis. Summary, there may be a variety of mechanisms in ovarian endometriosis cyst to regulate TFEB. TFEB activation and translocate into the nucleus acts on target genes, which can promote the expression of lysosome-related proteins, thus improving the level of autophagy of ectopic cells. High expression of TFEB and biogenesis of lysosome are markers of poor prognosis in a quarter of early breast cancer.(72) The expression of TFEB was positively correlated with the malignant progression of colorectal cancer. the tumor cells with high expression of TFEB had deeper invasion and higher lymph node metastasis rate.(43) Whether the level of TFEB associated with the severity of endometriosis? It requires further study.