The Insulin/IGF-I Axis in Diabetic Osteopathy

Type 1 diabetes mellitus (T1D) is associated with several disorders of skeletal health, including decreased bone mineral density (BMD) and an increased risk for osteoporosis and fragility fracture. Moreover, clinical evidence suggests that skeletal abnormalities in T1D result from the detrimental effects of T1D on bone formation. While BMD is typically not diminished in Type 2 diabetes mellitus (T2D), bone quality is impaired and fracture risk is increased. The shared as well as distinct features underlying diabetic osteopathy in T1D and T2D are not well understood at this time; however, studies have suggested that impaired insulin production, insulin action, insulin-like growth factor-I (IGF-I) action, as well as indirect effects of insulin to promote insulin-like growth factor-I (IGF-I) production, may be central events in promoting diabetic bone disease. Recent data from our laboratory examining animal models of diabetes have demonstrated that: 1) severe deficits in bone formation occur in the context of insulin-deficiency in mouse models of T1D; 2) normalization of systemic insulin levels stimulates new bone formation through RUNX2 and RUNX2 target genes in diabetic animals; and 3) insulin and IGF-I may signal via similar down-stream pathways to promote osteoblastogenesis. However, overlap and compensation by each ligand for the other has not been established in the diabetic state. To clarify mechanisms by which insulin and/or IGF-I modulate osteogenesis, and how deficiencies or impaired signaling of each may contribute to diabetic osteopathy, we will examine through which signaling pathway(s) insulin regulates RUNX2 and RUNX2 target genes. We also will study mice generated in our laboratory with conditional knock-out of the IR in osteoblasts (OIRKO mice) in regards to in utero skeletal development, postnatal affects on skeletal growth and development, and de novo bone formation. Furthermore, we will dissect the specific roles of insulin and IGF-I and their cognate receptors in the osteoblastogenic process by comparing control osteoblasts to osteoblasts lacking the insulin receptor (IR), the IGF-1 receptor (IGFR), or both the IR and IGFR in regards to insulin, IGF-I or insulin and IGF-I stimulated proliferation, differentiation, and osteogenic gene expression. To specify the roles that insulin and IGF-I may play in the pathogenesis and treatment of diabetic osteopathy, we will investigate the effects of diabetes on bone homeostasis and integrity in the osteoblast-specific insulin receptor knock-out (OIRKO) mouse, the IGFR knock-out (OIGFRKO) mouse, or the IR/IGFR double-knock out (DKO) mouse. We then will evaluate the impact of insulin or IGF-I therapy to affect bone integrity in the diabetic OIRKO, OIGFRKO, and DKO mouse. Finally, we will determine if FoxO transcription factors, major down-stream targets of insulin and IGF-I signaling, are integral to the diabetic bone phenotype. We will study the affects of osteoblast-specific deficiencies in FoxO1, FoxO3 and FoxO4 in STZ-induced diabetes as a way to preserve skeletal integrity in T1D. Together, these studies will provide critical mechanistic and pre-clinical information needed to better understand how corrections in insulin and/or IGF-I in diabetes may be beneficial in preventing and treating diabetic osteopathy.