The role of insulin like growth factor II in pre-adipocyte regulation

Abstract

The IGF system has an important role in growth and development. IGF-II is a recognised fetal growth promoter. However, its physiological role in post-natal life remains uncertain, although it is maintained in the circulation at a substantially high level throughout life, exceeding IGF-I levels. IGF-II has been strongly linked to body weight and obesity in genetic studies and more recent evidence suggests a metabolic role. We examined fat depot differences in the actions of IGF-II in respect to growth and metabolism. We used an 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. Multiple cell culture techniques were used to examine the role of IGF-II; cell counting and tritiated thymidine incorporation were used to assess the effect on proliferation. Oil Red O staining was used to assess preadipocyte differentiation, western blotting and reverse transcription polymerase chain reaction techniques were employed to assess the levels of adipogenesis markers and levels of the receptors and insulin receptor isoforms. Metabolic function was assessed by radioactive glucose uptake assay. We speculated a specific effect of IGF-II on visceral adipocytes in relation to the differential distribution of insulin receptor isoforms between visceral and subcutaneous fat depots. Initial characterisation of receptor levels indicated that visceral preadipocytes have higher levels of insulin receptor isoform A than those of subcutaneous preadipocytes. With differentiation, insulin receptor isoform B was increased; however, visceral adipocytes still maintained a higher expression of IR-A than that of subcutaneous adipocytes. A difference in IGF-IIR/M6P and IGF-IR was also seen between the fat depots. IGF-II promoted preadipocyte proliferation, and when preadipocytes were differentiated for 14 days in the presence or absence of IGF-II it prompted preadipocyte differentiation in subcutaneous preadipocytes but showed an opposing effect restricting visceral preadipocyte differentiation, which was confirmed by reductions in differentiation markers: PPARγ, 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. To further elaborate whether IGF-II plasma levels can be used as a potential biomarker of fat distribution, data from a lifecourse cohort, ALSPAC, were used to correlate early IGF-II levels with fat distribution measured by DXA scans during puberty. Interestingly, and in broad agreement with our cell culture data, cord blood IGF-II was negatively correlated with total mass and, in particular, trunk fat mass and showed a positive association with subcutaneous fat mass measured in the legs and arms, with adjustment of age and sex confounders but no association with lean, fat-free mass. In conclusion, IGF-II is a regulator of preadipocyte differentiation and metabolism by acting in a depot-specific manner as a differential modulator of fat accumulation favouring less visceral fat deposition in children, as well as being a suggested early-life predictor of later fat distribution.