The osteoblast: an insulin target cell controlling glucose homeostasis

The past five years have witnessed the emergence and discovery of unexpected functions played by the skeleton in whole-organism physiology. Among these newly described tasks is the role of bone in the control of energy metabolism, which is achieved through the secretion of osteocalcin, an osteoblasts-derived hormone regulating insulin secretion, insulin sensitivity, and energy expenditure. These initial findings raised several fundamental questions on the nature of insulin action in bone. Discoveries made independently by our two groups have provided answers recently to some of these questions. Through the analysis of mice lacking insulin receptor (InsR) only in osteoblasts, we found that insulin signaling in these cells favors whole-body glucose homeostasis. Importantly, this function of insulin signaling in osteoblasts was achieved through the negative regulation of osteocalcin carboxylation and bioavailability. Our studies also established that insulin signaling in osteoblasts was a positive regulator not only of postnatal bone acquisition but also of bone resorption. Interestingly, it appears that insulin signaling in osteoblasts induced osteocalcin activation by stimulating osteoclast activity. Indeed, the low pH generated during bone resorption is a sufficient means to decarboxylate osteocalcin. Our findings establish that the osteoblast is an important target used by insulin to control whole-body glucose homeostasis and identify bone resorption as the mechanism regulating osteocalcin activation.