In an increasingly aging and obese population, type 2 diabetes (T2DM) and osteoporotic fractures are major public health concerns. Affecting more than 1 in 3 adults in the United States, obesity is the most important risk factor for T2DM. It is now well established that obesity and T2DM have effects on fracture risk, and fractures in T2DM are associated with greater morbidity than in the general population. Although bone density is generally known to be increased in obese and T2DM patients, the underpinning causes that contribute to increased fracture risk in T2DM/obese subjects remain to be established. Therefore, understanding the interactions of obesity, T2DM and fracture is becoming a pressing need to reduce the societal and individual costs of fracture. Our efforts to identify control molecules and their signaling pathways that delineate a link between T2DM, osteoporosis and obesity led to the exciting discovery that targeted disruption of kinase suppressor of Ras 2 (Ksr2), an obesity and T2DM gene, led to a >2-fold increase in trabecular bone mass at the distal femur of 4-month-old mice besides inducing obesity and T2DM. By contrast, disruption of a closely related Ksr1 gene had no significant effect on bone or obese phenotypes. KSR1 and 2 were discovered as scaffolding proteins that orchestrate the assembly of Raf, MEK and ERK in the canonical Ras-Raf-MAPKs pathway. While a central role for Ksr2 expressed in hypothalamus has been implicated in the control of feeding behavior and energy balance and, thereby obesity, we have new exciting preliminary data that suggest that KSR2 expressed in osteoblasts acts locally in a cell autonomous fashion to regulate its functions. Based on our preliminary data and published data, we propose a novel hypothesis that KSR2 regulates osteoblast differentiation and bone formation via regulating mTORC1 signaling and its downstream hypoxia signaling. In this competitive renewal RO1 application, we will test this model of KSR2 action as follows: 1) To test the hypothesis that KSR2 acts in a cell autonomous fashion to regulate trabecular bone formation, we will generate osteoblast-specific Ksr2 conditional knockout mice and characterize its skeletal phenotype by micro-CT, histology, mechanical testing and gene expression. 2) To test the hypothesis that KSR2 effects on osteoblasts are mediated via its regulation of mTORC1 signaling, we will determine the functional consequence of KSR2/mTORC1 pathway interactions by evaluating if knockdown of Raptor in osteoblasts abolishes the increased anabolic functions of Ksr2 knockdown osteoblasts. 3) To test the hypothesis that KSR2/mTORC1 effects on osteoblasts are mediated via HIF1? signaling, we will evaluate changes in hypoxia signaling in osteoblasts with conditional disruption of Ksr2 and/or Raptor. We will also determine if disruption of Hif1? expression in osteoblasts rescues the skeletal phenotype in osteoblast-specific Ksr2 conditional knockout mice. Successful completion of our proposed studies will provide important information on the pathway/s by which KSR2 regulates osteoblast and bone marrow adipocyte functions and could lead to improved understanding of the mechanisms linking obesity and a high bone mass phenotype.
The current obesity rate of 39 percent is anticipated to climb to 55 percent by year 2045 in the United States. Since obesity is becoming a prevalent and destructive health disorder linked to some of the leading causes of preventable death including type 2 diabetes (T2DM) and osteoporosis, there is an urgent need to understand the molecular pathways driving the pathogenesis of obesity-related disorders such as T2DM and osteoporosis. Based on our novel discovery that disruption of Kinase Suppressor of Ras (Ksr) 2, a gene recently implicated in obesity and T2DM, leads to a dramatic increase in trabecular bone mass, our proposed efforts in this grant application will delineate the mechanisms by which this obesity gene regulates the skeletal phenotype. The long-term goal of this proposal is to identify novel therapeutic approaches for prevention and treatment of fractures in obesity/T2DM-related conditions, thereby reducing the burden of these chronic diseases both on individuals and on society.
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