Lack of Lrp5 Signaling in Osteoblasts Sensitizes Male Mice to Diet-Induced Disturbances in Glucose Metabolism

Abstract

Wnt signaling through the low-density lipoprotein-related receptor 5 (Lrp5) coreceptor regulates osteoblast maturation, matrix mineralization, and intermediary metabolism. In the mature osteoblast, signals downstream of Lrp5 are required for normal long-chain fatty acid β-oxidation. Mice rendered deficient for this coreceptor in osteoblasts and osteocytes accumulate body fat with elevated serum lipid levels but retain normal insulin sensitivity. In the present study, we challenged Lrp5-mutant mice with a high-fat diet (HFD) to determine whether they were more susceptible to diet-induced disturbances in glucose homeostasis. The HFD-fed Lrp5 mutant mice maintained a low bone mass phenotype with an increase in adipose tissue mass and hypertriglyceridemia and hypercholesterolemia. Examination of glucose metabolism revealed that Lrp5 deficiency in the osteoblast also resulted in hyperglycemia and hyperinsulinemia, with reductions in glucose tolerance, insulin sensitivity, and serum undercarboxylated osteocalcin. The results from in vivo genetic epistasis and in vitro studies suggest that this phenotype proceeds via the accumulation of diacylglycerol species and impaired insulin signaling in Lrp5-deficient osteoblasts. In turn, glucose uptake and osteocalcin production are diminished in mutant osteoblasts. Taken together, these data identify a link between Wnt-Lrp5 signaling and insulin signaling in the osteoblast that has the potential to influence energy balance and compound the detrimental effects of a HFD on whole-body metabolism.