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

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Abstract

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.

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Acknowledgments

The authors would like to express their gratitude for the strong support by Mercer University School of Medicine, where most of the study is performed. Shi-Wen Jiang is supported by the Distinguished Cancer Scholarship of Georgia Cancer Coalition (GCC). This work was partially funded by research grants from National Institute of Health (NIH) (R01 HD 41577, Shi-Wen Jiang), NIH/NCI MD Anderson Uterine Cancer SPORE (Jinping Li, Shi-Wen Jiang), and research supplements from the Department of Obstetrics and Gynecology, Mayo Clinic and Foundation (Shi-Wen Jiang, Brian Brost), and Seed Grants from the Mercer University School of Medicine (Jinping Li, Shi-Wen Jiang).

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Correspondence to Ya Gao or Shi-Wen Jiang.

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Q. Huang and H. Chen contributed equally to this work.

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Huang, Q., Chen, H., Wang, F. et al. Reduced syncytin-1 expression in choriocarcinoma BeWo cells activates the calpain1–AIF-mediated apoptosis, implication for preeclampsia. Cell. Mol. Life Sci. 71, 3151–3164 (2014). https://doi.org/10.1007/s00018-013-1533-8

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