The effect of a high-calorie diet on bone growth is mediated by the insulin receptor

Obese children grow faster than their normal-weight peers. Insulin resistance and hyperinsulinemia have been associated with obesity-related growth acceleration. To determine whether obesity-associated hyperinsulinemia promotes bone growth by activating the insulin receptor in the growth plate, we generated (IRflox/flox)-I-TamCart mice. The injection of 4 doses of tamoxifen in these mice (beginning at postnatal day 5th with 2 days interval between injections) resulted in the Insulin Receptor (IR) gene excision exclusively in the cartilage. (IRflox/flox)-I-TamCart tamoxifen-treated mice (KO mice) and their IRflox/flox control littermates (C mice) at 3 weeks of age were exposed to a standard or hypercaloric (high-fat) diet for 4 weeks. At the end of study, C and KO mice fed with a high-fat diet exhibited greater weight gain than the respective strains fed with a standard diet. Body and tibial growth and growth plate height of C mice fed with high-fat diet were greater than those of standard-diet-fed C mice; however, no difference was observed between KO mice fed with standard or high-fat diet with respect to body and tibial growth and growth plate height. Circulating levels of insulin, IGF-1 and leptin were significantly higher in C and KO mice exposed to high-fat diet compared to those in the same strain exposed to standard diet. Increased phosphorylation of Akt (one of the intracellular mediators of insulin action in bone) in the growth plate of C mice on high-fat diet (compared to those on standard diet) suggests that high-fat-mediated increased circulating insulin levels may directly affect growth plate function and bone growth. High-fat diet was not associated with any change of Akt phosphorylation in KO mice. In addition, in vitro studies in cultured primary chondrocytes revealed that Akt mediates the stimulatory effects of insulin on chondrocyte proliferation and differentiation. In conclusion, the activation of the insulin receptor in the growth plate of mice fed with a hypercaloric diet stimulates skeletal growth and growth plate chondrogenesis.