Over-expression of stomatin causes syncytium formation in nonfusogenic JEG-3 choriocarcinoma placental cells

Placental trophoblast differentiation involves the continuous fusion of mononuclear cytotrophoblasts. However, except for syncytin, little is known about the detailed mechanisms underlying trophoblast fusion. A previous study indicated that lipid rafts play an important role in HTLV‐1 syncytium formation. To identify proteins that may be involved in placental trophoblast differentiation, we examined stomatin, an important lipid‐raft protein that localizes to detergent‐resistant membrane domains. The syncytium and human chorionic gonadotropin (β‐hCG; a marker of placental trophoblast differentiation) were visualized by immunofluorescence staining. We found that overexpression of stomatin in the nonfusogenic JEG‐3 cell line caused syncytium formation and increased the fusion index of cells. Treating these cells with N6,2′‐O‐dibutyryladenosine 3′,5′‐cyclic monophosphate further increased cell fusion by stomatin. β‐hCG was found in a few JEG‐3 cells overexpressing stomatin at 48 h, and its levels increased dramatically at 72 h along with the formation of the multinuclear syncytium. RNA interference was used to decrease stomatin expression in BeWo cells, a fusogenic human choriocarcinoma cell line. After knockdown for 72 h, stomatin levels decreased by almost 95%. The fusion indexes of control and stomatin‐knockdown cells at 72 h were 9.4 and 6.5%, respectively. Our data indicated that stomatin could trigger syncytium formation and upregulate β‐hCG for cell fusion in nonfusogenic JEG‐3 cells. Downregulation of stomatin slightly inhibited the fusion index of fusogenic BeWo cells. Thus, these data suggested that stomatin plays an important role in trophoblast differentiation.