Diabetic bone disease is a condition in which bones become weak, resulting in diminished mobility, increased risk of foot and other fractures, more severe periodontal diseases, and a generally diminished quality of life. This condition is known to occur mainly in Type I diabetics, although poor bone quality is now understood to occur in Type II diabetics as well. Diabetic bone disease is characterized by diminished osteoblastic or bone synthetic activity. Advanced glycation endproducts (AGEs) accumulate at high levels in diabetic bone, and result from non-enzymatic reactions. TNF-a is a cytokine present at elevated levels in mineralized and non-mineralized tissues in diabetes. AGEs and TNF-a each contribute to complications of diabetes, and each interacts with osteoblast cell surface receptors resulting in increased levels of reactive oxygen species (ROS). Increased ROS stimulates synthesis and nuclear localization of FOXO1, a transcription factor that leads to cell cycle arrest and increased apoptosis. FOXO1 can bind to ?-catenin, a transcription cofactor that is regulated by the canonical Wnt pathway. The Wnt pathway promotes osteoblast proliferation and differentiation. The major hypothesis to be tested here is that AGEs and TNF-a each stimulate increased levels of active nuclear FOXO1, which competes with TCF/LEF transcription factors for the available pool of ?-catenin. This is proposed to effectively inhibit Wnt stimulated osteoblast differentiation, and contribute to diabetic osteopenia. Preliminary data supports that deficiency of the extracellular matrix enzyme lysyl oxidase results in poor bone structure that resembles osteopenic diabetic bone, and that lysyl oxidase is up-regulated by the Wnt canonical pathway in osteoblasts.
Two specific aims are proposed.
Aim 1 will investigate in vitro and in vivo the hypothesis that AGEs and TNF-a each inhibit the canonical Wnt pathway by competing for a limited ?-catenin pool.
Aim 2 will determine the mechanism of Wnt regulation of lysyl oxidase, and the mechanism by which AGE's and TNF-a inhibit Wnt-stimulated lysyl oxidase production in vitro.
Aim 2 will further determine in calvaria defects of diabetic mice the degree of down-regulation of the canonical Wnt pathway and LOX expression, and up-regulation of FOXO dependent gene expressions. In vivo studies will utilize analyses of calvaria defects made in non-diabetic and diabetic transgenic mice that express a canonical Wnt pathway responsive promoter that drives the expression of ?-galactosidase (TOPGAL mouse). This unique combination of calvaria defects made in the TOPGAL reporter mouse model with diabetes induction will provide a novel direct analysis of canonical Wnt pathway activity as a function of diabetes, and as a function of AGE and TNF-a treatments. In vitro studies will be performed using MC3T3 osteoblasts and primary calvaria osteoblasts. Results will provide information that may be ultimately be useful in preventing or reversing effects of diabetes on bone quality.
Osteopenia, or weak bones, occurs as a complication of diabetes. Advanced glycation endproducts (AGE's) and the cytokine TNF-a are elevated in diabetic tissues, and contribute to many of the complications of diabetes. The proposed research investigates the hypothesis that AGE's and TNF-a each inhibit the Wnt canonical pathway in osteoblasts (bone synthetic cells), and thereby inhibit differentiation and development of osteoblasts, and synthesis of lysyl oxidase, an important extracellular matrix enzyme needed for bone formation.
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