Insulin-like Growth Factor II (IGF-II) in Adipocyte Regulation: Depot-Specific Actions Suggest a Potential Role Limiting Excess Visceral Adiposity

The IGF system has an important role in growth and development. IGF-II is a recognized fetal growth promoter. However, its physiological post-natal role remains uncertain although it is maintained in the circulation at a substantially high level throughout life. IGF-II has been strongly linked to obesity in genetic studies and more recent evidence suggests a metabolic role. We examined fat depot differences in IGF-II's action on differentiation and metabolism. We speculate a specific effect on visceral adipocytes in relation to the differential distribution of insulin receptors between visceral and subcutaneous fat depots. Using a previously established adipocyte, cell culture system of matched pairs of visceral and subcutaneous fat biopsies from 20 normal weight children undergoing routine surgery for non-malignant, non-septic conditions. Preadipocytes were differentiated for 14 days in the presence or absence of IGF-II. Oil Red O staining, western blotting and reverse transcription polymerase chain reaction techniques were employed to assess levels of adipogenesis markers and levels of the insulin receptor and insulin receptor isoforms. Our data indicate that IGF-II promotes preadipocyte differentiation in subcutaneous preadipocytes but showed a protective effect restricting visceral preadipocyte differentiation, confirmed by reductions in the differentiation markers PPARγ and adiponectin and in triglyceride staining. Additionally, IGF-II reduced mRNA expression of the insulin receptor in adipocytes, and downregulated IR-A and GLUT4 abundance and corresponding glucose uptake in visceral adipocytes. In conclusion, IGF-II is a regulator of preadipocyte differentiation and metabolism by acting as a differential modulator of fat accumulation favoring less visceral fat deposition in children.