Binding of insulin-like growth factor I to human trophoblast cells during differentiation in vitro

The binding of insulin-like growth factor I (IGF-I) to its receptor stimulates cell proliferation and differentiation in several cell systems. In the present studies, we have characterized the expression of IGF-I receptors during trophoblast differentiation in vitro. During differentiation of cytotrophoblast cells to syncytiotrophoblasts, IGF-I binding capacity increased 40 per cent from 80 to 130 fmol/mg protein on day 1 to day 2, then decreased by 70 per cent from 130 to 40 fmol/mg protein on day 2 to day 3. IGF-I binding affinity increased non-significantly with a change in kDa from 6.5 to 3.5 mm between day 1 and day 3 in culture. In contrast, when cells were maintained in an undifferentiated state, there was no change in either binding capacity or affinity over three days in culture. Down-regulation of the IGF-I receptor by pre-incubation with IGF-I was demonstrated for syncytiotrophoblasts with a maximum decrease in IGF-I binding of 67 per cent after 72 h pre-incubation. Minimal down-regulation was seen for the cytotrophoblast cells (maximum decrease in IGF-I binding of 22 per cent after 48 h pre-incubation). In conclusion, these results demonstrated that IGF-I binding to the trophoblast cell surface changes during trophoblast differentiation in vitro and suggest involvement of IGF-I in this process.