Gestation-Stage Related Changes in the IGF System Components in the Equine Placenta

The insulin-like growth factor (IGF) system regulates implantation, placental development, and angiogenesis in eutherian mammals. However, little is known about the changes in this system in equine placenta (chorioallantois; CA) and the endometrium (EN) during pregnancy, or the relationship to vascular endothelial growth factor (VEGF) expression. The current study investigated the expression of the IGF system components, namely the ligands (IGF1 and IGF2), their receptors (IGF1R, IGF2R, and INSR), and their binding proteins (IGFBPs and IGF2BPs) in equine CA at 45 days, 4, 6, 10, and 11 months of gestational age (GA) and immediately postpartum (PP), and in equine EN at 4, 6, 10, and 11 months GA. IGF1 immunolocalization and serum concentrations were also evaluated across gestation. IGF1 mRNA expression in CA increased from day 45 to peak at 6 months and then gradually declined to reach a nadir in PP samples. This profile correlated positively with the VEGF expression profile (r = 0.62, p = 0.001). In contrast, IGF2 expression in CA was not correlated with VEGF (p = 0.14). Interestingly, IGF2 mRNA was more abundant in equine CA than IGF1 (p < 0.05) throughout gestation. Among the IGFBPs investigated in CA, the expression of IGFBP2 and IGF2BP2 was highly abundant (p < 0.05) at day 45 compared to other GAs. Conversely, mRNA expression for IGFBP3 and IGFBP5 was more abundant (p < 0.05) in PP than at all investigated GAs. Immunohistochemistry revealed that IGF1 is localized in the equine chorionic epithelium (cytoplasm and nucleus). IGF1 serum concentrations peaked at 9 months and declined to their lowest levels PP. In conclusion, this study demonstrates a positive correlation between IGF1 and VEGF expression in equine CA during gestation, suggesting that the IGF system plays a crucial role in placental angiogenesis by regulating VEGF.