Transforming growth factor β1 (TGFβ1) inhibits gap junctional communication during human trophoblast differentiation

It was recently established that, during trophoblast differentiation, gap junctional communication (GJC) precedes the formation of the syncytiotrophoblast and is required for trophoblast cell fusion. Therefore, the end of cell-to-cell communication through gap junctions appears as a judicious criterion of cell fusion. Although GJC was seen to be stimulated by human Chorionic Gonadotropin (hCG), its regulation remains poorly understood. Transforming Growth Factor-β1 (TGF-β1), a multifunctional cytokine has been shown to modulate trophoblast differentiation. Therefore, the effects of TGF-β1 on intercellular dye diffusion have been investigated in cultured trophoblast of human term placenta, by means of the Fluorescence Recovery After Photobleaching (gap FRAP) technique. In parallel, trophoblast differnetiation, hCG production and human Chorionic Somatomammotropin (hCS, a specific syncytiotrophoblast hormonal product) expression were assessed. The presence of TGF-β1 (5 or 10 ng/ml) in the culture medium for two days partially inhibited syncytium formation. The percentage of coupled cells was significantly decreased (2.8 times) after two days in presence of 10 ng/ml of TGF β1. Simultaneously, hCG release in culture medium was reduced (at this concentration, to 0.65 and 0.79 after respectively two and three days). In these conditions, Western blot analysis of trophoblast cellular proteins revealed that, after two days, hCS expression was reduced by 40% compared to control. Furthermore, the stimulation of trophoblastic GJC by exogenous hCG (500 mIU/ml) was considerably reduced by simultaneous exposure to TGF-β1 (10 mg/ml). The addition of a polyclonal hCG antibody in excess decreased basal GJC. In the presence of hCG antibody, no significant additive inhibition by TGFβ1 was observed. In conclusion, TGF-β1 was found to inhibit intercellular communication and, subsequently, differentiation and concomitant placental hormone secretions.