Long-term oral administration of osteocalcin induces insulin resistance in male mice fed a high-fat, highsucrose diet. Am J Physiol Endocrinol Metab 310: E662-E675, 2016. First published February 16, 2016 doi:10.1152/ajpendo.00334.2015.-Uncarboxylated osteocalcin (GluOC), a bone-derived hormone, regulates energy metabolism by stimulating insulin secretion, pancreatic -cell proliferation, and adiponectin expression in adipocytes. Previously, we showed that long-term intermittent or daily oral administration of GluOC reduced the fasting blood glucose level, improved glucose tolerance, and increased the fasting serum insulin concentration as well as pancreatic -cell area in female mice fed a normal or high-fat, high-sucrose diet. We have now performed similar experiments with male mice and found that such GluOC administration induced glucose intolerance, insulin resistance, and adipocyte hypertrophy in those fed a high-fat, high-sucrose diet. In addition, GluOC increased the circulating concentration of testosterone and reduced that of adiponectin in such mice. These phenotypes were not observed in male mice fed a high-fat, high-sucrose diet after orchidectomy, but they were apparent in orchidectomized male mice or intact female mice that were fed such a diet and subjected to continuous testosterone supplementation. Our results thus reveal a sex difference in the effects of GluOC on glucose homeostasis. Given that oral administration of GluOC has been considered a potentially safe and convenient option for the treatment or prevention of metabolic disorders, this sex difference will need to be taken into account in further investigations. adiponectin; insulin resistance; osteocalcin; sex difference; testosterone OSTEOCALCIN IS A BONE MATRIX PROTEIN synthesized by osteoblasts that also functions as a hormone in the uncarboxylated state to regulate various aspects of physiology, including glucose metabolism, brain function, and male fertility (14,17,30,39,40). Uncarboxylated osteocalcin (GluOC) constitutes ϳ10% of total circulating osteocalcin in adult mice under normal physiological conditions (2, 10). However, the serum concentration of GluOC is increased in pathological conditions such as vitamin K deficiency and osteoporosis (2,26,31). Animal studies have shown that an increase in the circulating concentration of GluOC prevents obesity and glucose intolerance (10,13,28,30). Mice genetically deficient in Esp (a protein tyrosine phosphatase expressed in osteoblasts), a gainof-function model for osteocalcin, were thus protected from obesity, glucose intolerance, and insulin resistance induced by a high-fat diet (14, 30). Continuous administration of GluOC via a subcutaneous osmotic pump lowered blood glucose levels and increased pancreatic -cell mass, insulin secretion, and insulin sensitivity in mice with high-fat diet-or gold thioglucose-induced obesity, with this latter effect being achieved via upregulation of expression of the gene for the insulin-sensitizing adipokine adiponectin in adipocytes (10). Moreove...