Decreased Osteoclastogenesis and High Bone Mass in Mice with Impaired Insulin Clearance Due to Liver-specific Inactivation to CEACAM1

Type 2 diabetes is associated with normal-to-higher bone mineral density (BMD) and increased rate of fracture. Hyperinsulinemia and hyperglycemia may affect bone mass and quality in the diabetic skeleton. In order to dissect the effect of hyperinsulinemia from the hyperglycemic impact on bone homeostasis, we have analyzed L-SACC1 mice, a murine model of impaired insulin clearance in liver causing hyperinsulinemia and insulin resistance without fasting hyperglycemia. Adult L-SACC1 mice exhibit significantly higher trabecular and cortical bone mass, attenuated bone formation as measured by dynamic histomorphometry, and reduced number of osteoclasts. Serum levels of bone formation (BALP) and bone resorption markers (TRAP5b and CTX) are decreased by approximately 50%. The L-SACC1 mutation in the liver affects myeloid cell lineage allocation in the bone marrow: the (CD3(-)CD11b(-)CD45R(-)) population of osteoclast progenitors is decreased by 40% and the number of (CD3(-)CD11b(-)CD45R(+)) B-cell progenitors is increased by 60%. L-SACC1 osteoclasts express lower levels of c-fos and RANK and their differentiation is impaired. In vitro analysis corroborated a negative effect of insulin on osteoclast recruitment, maturation and the expression levels of c-fos and RANK transcripts. Although bone formation is decreased in L-SACC1 mice, the differentiation potential and expression of the osteoblast-specific gene markers in L-SACC1-derived mesenchymal stem cells (MSC) remain unchanged as compared to the WT. Interestingly, however, MSC from L-SACC1 mice exhibit increased PPARgamma2 and decreased IGF-1 transcript levels. These data suggest that high bone mass in L-SACC1 animals results, at least in part, from a negative regulatory effect of insulin on bone resorption and formation, which leads to decreased bone turnover. Because low bone turnover contributes to decreased bone quality and an increased incidence of fractures, studies on L-SACC1 mice may advance our understanding of altered bone homeostasis in type 2 diabetes.