Response of HEK293 and CHO cells overexpressing fusiogenic syncytin-1 to mitochondrion-mediated apoptosis induced by antimycin A

A Replication-Competent Retroviral Vector Expressing the HERV-W Envelope Glycoprotein is a Potential Tool for Cancer Gene Therapy

The fusogenic membrane glycoprotein (FMG) derived from the human endogenous retrovirus-W (HERV-W) exhibits fusogenic properties, making it a promising candidate for cancer gene therapy. When cells are transfected with HERV-W FMG, they can fuse with neighboring cells expressing the receptor, resulting in the formation of syncytia. These syncytia eventually undergo cell death within a few days. In addition, it has been observed that an HERV-W env mutant, which is truncated after amino acid 483, displays increased fusogenicity compared to the wild-type HERV-W env. In this study, we observed syncytium formation upon transfection of HeLa and TE671 human cancer cells with plasmids containing the HERV-W 483 gene. To explore the potential of a semi-replication-competent retroviral (s-RCR) vector encoding HERV-W 483 for FMG-mediated cancer gene therapy, we developed two replication-defective retroviral vectors: a gag-pol vector encoding HERV-W 483 (MoMLV-HERV-W 483) and an env vector encoding VSV-G (pCLXSN-VSV-G-EGFP). When MoMLV-HERV-W 483 and pCLXSN-VSV-G-EGFP were co-transfected into HEK293T cells to produce the s-RCR vector, gradual syncytium formation was observed. However, the titers of the s-RCR virus remained consistently low. To enhance gene transfer efficiency, we constructed an RCR vector encoding HERV-W 483 (MoMLV-10A1-HERV-W 483), which demonstrated replication ability in HEK293T cells. Infection of A549 and HT1080 human cancer cell lines with this RCR vector induced syncytium formation and subsequent cell death. Consequently, both the s-RCR vector and RCR encoding HERV-W 483 hold promise as valuable tools for cancer gene therapy.

The important biological roles of Syncytin-1 of human endogenous retrovirus W (HERV-W) and Syncytin-2 of HERV-FRD in the human placenta development

BACKGROUND

 Human endogenous retroviruses (HERVs) entered the germ line by retroviral infection from a distant ancestor over 30 million years ago and constitute 8% of the human genome. The majorities of HERVs are non-protein coding and lack function because of the accumulation of mutations, insertions, deletions, and/or truncations. However, a small number of HERV genes carried ORFs with beneficial functions for the host.

METHODS & RESULTS

In this review, we summarize the structural and important biological roles of two HERV gene products termed Syncytin-1 and Syncytin-2 in human placenta development. Indeed, two retroviral gene products that have important roles in mammalian development, Syncytin-1 (HERV-W) and Syncytin-2 (HERV-FRD), are prime examples encoded by env genes and expressed in the placental trophoblasts. Several pivotal studies revealed that Syncytins are fundamental genes implicated in regulating trophoblast fusion and placenta morphogenesis.

CONCLUSION

 Interestingly, it has been suggested that syncytins may also be implicated in non-fusogenic activities leading to apoptosis, proliferation, and immunosuppressive activities.

Decreased expression and DNA hypermethylation of syncytin-1 in human villus tissues with unexplained recurrent spontaneous abortion

Recurrent spontaneous abortion (RSA) affects approximately 5 % of women of reproductive age worldwide. The etiology and pathogenesis of approximately 50 % of RSA cases currently remain unclear, which known as unexplained RSA (URSA). Syncytin-1, an envelope protein encoded by HERV-W gene, is essential for human embryonic development. The purpose of this study was to explore the correlation between syncytin-1 expression and URSA occurrence. The villi tissues of URSA patients and patients with voluntary termination of pregnancy for non-medical reasons in early pregnancy (Control group) were collected. Compared with the Control group, syncytin-1 was abnormally low expressed in URSA villus tissues, and the HERV-W gene promoter was hypermethylated. Compared with the control group, the global DNA methylation level and the expression level of DNA methylases in the villus tissues of the URSA group had no significant difference. In addition, compared with the Control group, URSA villus tissue showed obviously abnormal apoptosis. Overexpression of syncytin-1 promoted the proliferation of HTR-8 cells and inhibited their apoptosis; while knockdown of syncytin-1 inhibited cell proliferation and promoted cell apoptosis. URSA villus tissue exhibited hypermethylation of the HERV-W gene and down-regulation of syncytin-1 expression. Syncytin-1 has the potential to be a predictive and diagnostic biomarker for URSA.

Artesunate alleviates schistosomiasis-induced liver fibrosis by downregulation of mitochondrial complex Ⅰ subunit NDUFB8 and complex Ⅲ subunit UQCRC2 in hepatic stellate cells

Hepatic stellate cells (HSCs) play a key role in the pathogenesis of hepatic fibrosis. Inhibition of the HSCs activity is an ideal strategy in the treatment of fibrosis, but there is no drug yet for this strategy. Artesunate (ART) has been shown to protect liver from fibrosis through inhibition of HSCs activity. However, the mechanism of ART activity remains to be fully uncovered. In this study, we tested ART in a mouse model of hepatic fibrosis established in the schistosomiasis-infected mice. The mechanism of ART action was investigated in the HSC cell line LX-2. ART significantly inhibited hepatic fibrosis. In LX-2 cells, ART efficiently inhibited the cell activity in proliferation and mRNA expression of fibrosis marker genes including Col1a1 and Col3a1. An impact of ART on mitochondria was observed for suppression of enzymes in the citric acid cycle (TCA), such as citrate synthase (CS), isocitrate dehydrogenase (IDH₂), and alpha ketoglutarate dehydrogenase (OGDH) in a dose-dependent manner. ART decreased the mitochondrial oxygen consumption rate (OCR) and the protein levels of mitochondrial complex Ⅰ subunit NDUFB8 and complex Ⅲ subunit UQCRC2 in HSCs. All of these alterations were observed with an increase in HSC apoptosis. This study suggests that ART may alleviate liver fibrosis by downregulation of HSC activity through suppression of NDUFB8 and UQCRC2 in mitochondria. This study provides a new insight into the mechanism of the ART activity in the inhibition of schistosomiasis-induced liver fibrosis.

Galectin-1 interacts with the human endogenous retroviral envelope protein syncytin-2 and potentiates trophoblast fusion in humans

Syncytin (Syn)-2 is an important fusogenic protein that contributes to the formation of the placental syncytiotrophoblast. Galectin (Gal)-1, a soluble lectin, is also involved in trophoblast cell fusion and modulates the interaction of certain retroviral envelopes with their cellular receptor. This study aimed to investigate the association between Syn-2 and Gal-1 during human trophoblast cell fusion. This association was evaluated in vitro on primary villous cytotrophoblasts (vCTBs) and cell lines using recombinant Gal-1 and Syn-2-pseudotyped viruses. Using lactose, a Gal antagonist, and Gal-1-specific small interfering RNA (siRNA) transfections, we confirmed the implication of Gal-1 in vCTBs and BeWo cell fusion, although RT-PCR and ELISA analyses suggested that Gal-1 alone did not induce syncytialization. Infection assays showed a specific and significant effect of Gal-1 on the infectivity of Syn-2-pseudotyped viruses that depended on the expression of major facilitator superfamily domain-containing 2A (MFSD2a). Moreover, Gal-3, another placental Gal, did not modulate the infectivity of Syn-2-positive viruses, strengthening the specific association between Gal-1 and Syn-2. Interestingly, Gal-1 significantly reduced the infectivity of Syn-1-pseudotyped viruses, suggesting the opposite effects of Gal-1 on Syn-1 and -2. Finally, coimmunoprecipitation experiments showed a glycan-dependent interaction between Syn-2-bearing virions and Gal-1. We conclude that Gal-1 specifically interacts with Syn-2 and possibly regulates Syn-2/MFSD2a interaction during syncytialization of trophoblastic cells.-Toudic, C., Vargas, A., Xiao, Y., St-Pierre, G., Bannert, N., Lafond, J., Rassart, É., Sato, S., Barbeau, B. Galectin-1 interacts with the human endogenous retroviral envelope protein syncytin-2 and potentiates trophoblast fusion in humans.

The role of syncytins in human reproduction and reproductive organ cancers

Human life begins with sperm and oocyte fusion. After fertilization, various fusion events occur during human embryogenesis and morphogenesis. For example, the fusion of trophoblastic cells constitutes a key process for normal placental development. Fusion in the placenta is facilitated by syncytin 1 and syncytin 2. These syncytins arose from retroviral sequences that entered the primate genome 25 million and more than 40 million years ago respectively. About 8% of the human genome consists of similar human endogenous retroviral (HERVs) sequences. Many are inactive because of mutations or deletions. However, the role of the few that remain transcriptionally active has not been fully elucidated. Syncytin proteins maintain cell-cell fusogenic activity based on ENV: gene-mediated viral cell entry. In this review, we summarize how syncytins and their receptors are involved in fusion events during human reproduction. The significance of syncytins in tumorigenesis is also discussed.

Promoter Hypermethylation and Decreased Expression of Syncytin-1 in Pancreatic Adenocarcinomas

Syncytin-1 is a member of human endogenous retroviral W gene family (HERVW1). Known to be expressed in human placental trophoblast, syncytin-1 protein mediates the fusion of cytotrophoblasts for the formation of syncytiotrophoblasts, the terminally differentiated form of trophoblast lineage. In addition, in vitro studies indicate that syncytin-1 possessed nonfusogenic functions such as those for immune suppression, cell cycle regulation and anti-apoptotic activities. Overexpression of syncytin-1 has been observed in various malignant tissues including breast, endometrial and ovarian cancers. It was reported that syncytin-1 gene expression is associated with dynamic changes of DNA hypomethylation in the 5' LTR. In this study, applying the real-time PCR, Western blot analysis and immunohistochemistry methods, we demonstrate a constitutive expression of syncytin-1 in normal pancreas tissues as well as normal tissues adjacent to cancer lesions. Moreover, a reduced expression is found in the pancreatic adenocarcinoma tissues. The expression levels of syncytin-1 are not correlated with the stage, historical grade and gender, but inversely correlated with patients' age. Furthermore, COBRA and bisulfite sequencing results indicated that the lower expression of syncytin-1 is correlated with the hypermethylation of two CpG dinucleotides in the 5' LTR of syncytin-1 gene. The nonfusogenic function of syncytin-1 in normal pancreas as well as its role(s) in the pathogenesis and progression of pancreatic cancers remains to be investigated. Identification of the two CpG dinucleotides around transcription start site as key epigenetic elements has provided valuable information for further studies on the epigenetic regulation of syncytin-1 in pancreatic cancer cells.

Reduced syncytin-1 expression in choriocarcinoma BeWo cells activates the calpain1-AIF-mediated apoptosis, implication for preeclampsia

Placentas associated with preeclampsia are characterized by extensive apoptosis in trophoblast lineages. Syncytin-1 (HERVWE1) mediates the fusion of cytotrophoblasts to form syncytiotrophoblasts, which assume the placental barrier, fetal-maternal exchange and endocrine functions. While decreased syncytin-1 expression has been observed in preeclamptic placentas, it is not clear if this alteration is involved in trophoblast apoptosis. In the current study, we found that siRNA-mediated knockdown of syncytin-1 led to apoptosis in choriocarcinoma BeWo, a cell line of trophoblastic origin. Characterization of the apoptotic pathways indicated that this effect does not rely on the activation of caspases. Rather, decreased syncytin-1 levels activated the apoptosis inducing factor (AIF) apoptotic pathway by inducing the expression, cleavage, and nuclear translocation of AIF. Moreover, calpain1, the cysteine protease capable of cleaving AIF, was upregulated by syncytin-1 knockdown. Furthermore, treatment with calpain1 inhibitor MDL28170 effectively reversed AIF cleavage, AIF nuclear translocation, and cell apoptosis triggered by syncytin-1 downregulation, verifying the specific action of calpain1-AIF pathway in trophoblast apoptosis. We confirmed that preeclamptic placentas express lower levels of syncytin-1 than normal placentas, and observed an inverse correlation between syncytin-1 and AIF/calpain1 mRNA levels, a result consistent with the in vitro findings. Immunohistochemistry analyses indicated decreased syncytin-1 and increased AIF and calpain1 protein levels in apoptotic cells of preeclamptic placentas. These findings have for the first time revealed that decreased levels of syncytin-1 can trigger the AIF-mediated apoptosis pathway in BeWo cells. This novel mechanism may contribute to the structural and functional deficiencies of syncytium frequently observed in preeclamptic placentas.

Epigenetic and non-epigenetic regulation of syncytin-1 expression in human placenta and cancer tissues

Syncytin-1 is a human endogenous retroviral envelope gene (HERVW1) product specifically expressed in placental trophoblasts. By mediating the formation of syncytiotrophoblasts through cell-cell fusion, syncytin-1 plays a critical role for the placental barrier, endocrine and exchange functions. During pregnancy, syncytin-1 expression is dynamically regulated by various pathophysiological factors and pathways. This review summarizes and examines published data on epigenetic and non-epigenetic regulation of syncytin-1 gene expression, with a focus on the changes of syncytin-1 DNA methylation and expression in placental trophoblasts under preeclamptic and hypoxic conditions. The functions of syncytiotrophoblasts, the fusogenic and non-fusogenic activities of syncytin-1, and aberrant activation of syncytin-1 expression in cancer cells are also discussed. New findings on the epigenetic regulation of syncytin-1 in placentas from monozygotic/dichorionic discordant twins are analyzed. The close correlation among changes of DNMTs expression, syncytin-1 gene methylation, and syncytin-1 mRNA levels, in placentas associated with discordant fetal growth indicated a dynamic nature of syncytin-1 regulation.

Syncytin-1 modulates placental trophoblast cell proliferation by promoting G1/S transition

Placental syncytiotrophoblasts formed by the fusion of cytotrophoblasts constitute the interface between maternal and fetal circulations. The syncytium, composed of a continuous layer of syncytiotrophoblasts, assumes the fetal-maternal nutrient exchange, placental barrier, and endocrine functions important for the maintenance of normal pregnancy. Syncytin-1, an endogenous retroviral gene product, mediates the fusion of cytotrophoblasts. While the fusogenic function of syncytin-1 has been well established, little is known regarding its nonfusogenic activities. This study investigates the role of syncytin-1 in trophoblast proliferation. We found that syncytin-1 knockdown significantly inhibited BeWo cell growth and DNA synthesis. Moreover, time course studies on key cell cycle regulators demonstrated an upregulation of p15 and downregulation of CDK4, E2F1, PCNA, and c-Myc, which consequently led to a reduced level of CDK1. These results, together with those from flow cytometry analysis, indicated that syncytin-1 knockdown blocked the G1/S transition phase of the cell cycle. Moreover, syncytin-1 overexpression promoted CHO cell proliferation and led to changes opposite to those observed in syncytin-1 knockdown experiments, confirming the critical role of syncytin-1 for G1/S transition. Thus, syncytin-1, through both nonfusogenic and fusogenic, functions, may co-regulate the input (proliferation) and output (fusion) of the cytotrophoblast "pool". Such co-regulation could be an efficient way to achieve the balance between these two opposing processes, which is required for syncytium homeostasis. Since decreased syncytin-1 expression has been shown to be associated with preeclamptic and hypoxic condition, insufficient replenishing of the cytotrophoblast "pool" may contribute to syncytium deficiency, a critical pathological change frequently found in preeclamptic placentas.