The Insulin/IGF-I Axis in Diabetic Osteopathy

描述（由申请人提供）：1型糖尿病（T1 D）与几种骨骼健康疾病相关，包括骨矿物质密度（BMD）降低和骨质疏松症和脆性骨折风险增加。此外，临床证据表明，T1 D中的骨骼异常是由T1 D对骨形成的不利影响引起的。虽然2型糖尿病（T2 D）的BMD通常不会降低，但骨质量受损，骨折风险增加。T1 D和T2 D糖尿病性骨病的共同特征和独特特征目前尚不清楚;然而，研究表明，胰岛素产生、胰岛素作用、胰岛素样生长因子-I（IGF-I）作用受损以及胰岛素促进胰岛素样生长因子-I（IGF-I）产生的间接作用可能是促进糖尿病性骨病的中心事件。我们实验室检查糖尿病动物模型的最新数据表明：1）在T1 D小鼠模型中，在胰岛素缺乏的背景下发生骨形成的严重缺陷; 2）全身胰岛素水平的正常化通过糖尿病动物中的RUNX 2和RUNX 2靶基因刺激新骨形成; 3）胰岛素和IGF-I可能通过类似的下游途径发出信号，以促进成骨细胞生成。为了阐明胰岛素和/或IGF-I调节成骨的机制，以及各自的缺陷或受损信号传导如何导致糖尿病骨病，我们将研究胰岛素通过哪些信号传导途径调节RUNX 2和RUNX 2靶基因。我们还将研究我们实验室中产生的成骨细胞胰岛素受体（IR）条件性敲除小鼠（OIRKO小鼠）在子宫内骨骼发育、出生后对骨骼生长和发育的影响以及从头骨形成方面的情况。此外，我们将通过比较对照成骨细胞与缺乏IR、IGF-1受体（IGFR）或IR和IGFR的成骨细胞在胰岛素、IGF-1或胰岛素和IGF-1刺激的增殖、分化和成骨基因表达方面的作用，来剖析胰岛素和IGF-1及其同源受体在成骨细胞生成过程中的具体作用。为了明确胰岛素和IGF-I在糖尿病性骨病的发病机制和治疗中可能发挥的作用，我们将在成骨细胞特异性胰岛素受体敲除（OIRKO）小鼠、IGFR敲除（OIGFRKO）小鼠或IR/IGFR双敲除（DKO）小鼠中研究糖尿病对骨稳态和骨完整性的影响。然后，我们将评估胰岛素或IGF-I治疗对糖尿病OIRKO、OIGFRKO和DKO小鼠骨完整性的影响。最后，我们将确定FoxO转录因子，胰岛素和IGF-I信号传导的主要下游靶点，是否是糖尿病骨表型的组成部分。我们将研究成骨细胞特异性缺陷FoxO 1、FoxO 3和FoxO 4在STZ诱导的糖尿病中的作用，作为保持T1 D骨骼完整性的一种方法。总之，这些研究将提供更好地了解糖尿病中胰岛素和/或IGF-I的校正如何有益于预防和治疗糖尿病骨病所需的关键机制和临床前信息。

DESCRIPTION (provided by applicant): 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. 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 insulin receptor (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 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 effects 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.