IGF-I and -II influence cytotrophoblast proliferation by activating the type-I IGF receptor (IGF1R) in first trimester human placenta. Ligand access to receptors is regulated by IGF binding proteins (IGFBPs) 1-6. In humans, the most abundant IGFBPs at the maternal-fetal interface are IGFBP-1 and IGFBP-3; we hypothesised that these IGFBPs function to regulate the effects of IGFs in the placenta and used our placental explant model, in which proliferation and differentiation are maintained, to investigate their effect on IGF-induced cytotrophoblast proliferation.
First trimester placental explants were cultured for 24 h in serum-free DMEM/F12 before the addition of 100 μM BrdU and a 5:1 or 1:1 molar ratio of IGFBP (IGFBP-1 or -3) to ligand (10 nM IGF-I or II). 24 hours later, cell proliferation was assessed by immunohistochemical analysis of BrdU incorporation.
Both IGF-I and II enhanced cytotrophoblast proliferation (P<0.01; n=9). IGFBP-1 had no significant effect on the activity of either ligand, whereas inclusion of IGFBP-3 at either a 1:1 or 5:1 molar ratio to ligand reduced cytotrophoblast proliferation in response to both IGF-I and II (P<0.05; n=3). Incubation of tissue with the IGF1R inhibitor PPP (5 μM) for 30 minutes prior to addition of IGF/IGFBP-3 caused further inhibition of IGF-I and II induced proliferation, suggesting that both Iigands act via IGF1R.
IGFBP-3 also inhibited basal cytotrophoblast proliferation (from 34.6±3.7% to 16.3±4.6% and 12.2±2.6% respectively; P<0.05); to examine if this effect was due to inhibition of endogenous IGF activity, tissue was pre-incubated with PPP (5 μM; 30 minutes). This had no affect on IGFBP-3 activity, thereby suggesting that IGFBP-3 has a cognate receptor in the placenta.
This study is the first to report the contrasting roles of IGFBP-1 and IGFBP-3 in regulating IGF-I and II induced cytotrophoblast proliferation. Furthermore our study demonstrates an IGF-independent role for IGFBP-3 in the placenta.