ENO is present in almost all mature tissues of the human body and coexists in the cytoplasm, cell membrane and nucleus[6]. ENO1, also known as α-enolase, is one of the most important enzymes in enolase. Recent studies have shown that ENOl plays a critical regulatory role in biological processes such as plasminogen activation, plasmin activity, myogenesis, apoptosis and tumor development[7]. ENO1 can also be used as an autoantigen to induce an immune inflammatory response[8]. The autoimmune response against ENO1 may be an important mechanism in diseases such as Hashimoto encephalopathy and glomerulonephritis[9]. ENO1 may also play a crucial role in the stress response, which is important for cell survival. For example, myocardial cell damage could activate ENO1 in the cytoplasm, and then ENO1 is transported to the cytoskeleton and contractile filaments[10]. As a molecular companion of ENO1, the above structures are stabilized, and the normal function of the myocardium is maintained. Recently, substantial evidence has shown that ENO1 is related to the occurrence and metastasis of malignant tumors, such as metastatic breast cancer, pancreatic cancer, liver cancer and other cells with high malignancy. In pancreatic ductal adenocarcinoma (PDA) cells, ENO1 can act as a plasminogen receptor and promote cell invasion and migration. In addition, ENO1 silencing can induce oxidative stress and alter the metabolism of PDA cells. The results of the study suggest that ENO1 promotes the survival, migration, and transfer of PDA through a synergistic effect with integrin and urokinase plasminogen activator receptor (uPAR)[11]. The α-enolase on the surface of lung cancer cells promotes the degradation of extracellular matrix and the invasion of cancer cells. Targeting surface α-enolase is a promising method for inhibiting tumor metastasis[12]. In addition, overexpression of ENO1 will increase the secretion of progesterone in granulosa cells cultured in vitro in goose follicles, suggesting that it may be related to corpus luteum function[13].
The role of ENO1 in RM and its mechanism of action in trophoblasts is unclear. The related functions of ENO1 that are known, including plasminogen activity, endocrine activity, autoimmune response, and oncogene promoting cell migration, invasion and metastasis, are closely related and highly similar to the molecular mechanisms of RM.
Using real-time RT-PCR and Western blot experiments, we found that the mRNA and protein levels of ENO1 in the villous tissue of patients with RM were significantly lower than those of normal abortions. Through immunohistochemical experiments, we found that ENO1 was expressed in the cytoplasm, cell membrane and nucleus of trophoblast cells. The villous trophoblast cell membrane coloration in the control group was significantly darker than that in the RM group, suggesting that ENO1 is expressed at low levels on the cell membrane of trophoblast cells of patients with RM. The villous trophoblast cell membrane coloration was significantly deeper than that in the recurrent abortion group, suggesting that ENO1 is expressed at low levels on the cell membrane of villous trophoblast cells. Previous studies have found that ENO1 located on the surface of cell membranes plays an important role in the plasmin system and is a plasminogen-binding molecule[7]. The interaction between plasmin and ENO1 promotes the pathophysiological process of cell migration, such as inflammation, cell invasion and cancer metastasis.
The implantation process of the embryo depends on the dynamic balance between the synthesis of the placental trophoblast cells and the secretion of a large number of fibrinolysis/anti-fibrinolysis-related protein factors, which can effectively dissolve the endometrial tissue to facilitate the invasion of trophoblast cells and the formation of the placenta while causing local bleeding of decidual tissue and leading to adverse pregnancy outcomes. We found that the expression of ENO1 is downregulated on the cell membrane of trophoblast cells in RM. According to the previous function of ENO1, ENO1 may reduce the invasion and migration of trophoblast cells by inhibiting the action of the fibrinolytic system, thereby promoting the occurrence of RM, which needs further experimental verification.
Trophoblast cell dysfunction can lead to pregnancy-related diseases such as intrauterine growth restriction, preeclampsia, eclampsia, spontaneous abortion, premature delivery, and choriocarcinoma[14]. We detected changes in the proliferation and invasion function of HTR-8/SVneo cells after knocking down ENO1 through CCK8 and Transwell chamber assays. We found that the proliferation and invasion function of EVT cells decreased significantly with the downregulation of ENO1, which is consistent with the biological behavior of tumor cells. Fu QF et al. found that upregulation of ENO1 activates FAK/PI3K/AKT and its downstream signals, regulating glycolysis, the cell cycle and EMT-related genes to promote the proliferation and metastasis of non-small cell lung cancer (NSCLC)[15]. Overexpression of ENO1 is also related to the progression of gliomas, and inhibiting ENO1 expression can inhibit the process of cell growth, migration and invasion by inactivating the PI3K/Akt pathway in glioma cells[16]. In previous studies on RM, cell proliferation, invasion, migration, and angiogenesis-related signaling pathways, such as the Wnt/β-catenin signaling pathway[17], the PI13K signaling pathway[18], and the ERK and other signaling pathways[19], participated in the occurrence and development of RM. Further research is needed to verify whether ENO1 affects the function of trophoblast cells through the above pathways.
In summary, the low expression of ENO1 in tissues of RM and the effect on the function of EVT cells in vitro suggest that it may play an important role in RM, and further studies of the specific mechanism are needed. Whether ENO1 leads to trophoblast cell proliferation and invasion by activating plasminogen and whether ENO1 is involved in the occurrence of recurrent miscarriage through the autoimmune response and affects the secretion of progesterone and the related mechanisms are worthy of further discussion.