Aging and Diabetes-Related Factors Compromising Bone Fracture Resistance

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DESCRIPTION (provided by applicant): There is an increase in the risk of bone fracture with aging and adult diabetes, and this increase cannot be solely explained by changes in bone mineral density (BMD). One barrier to new diagnostic tools and treatments is that the underlying cause for the disproportionate increase in fracture risk among diabetics and the elderly is currently unknown. Consequently, there is a need to identify the biophysical basis of the age- and diabetes-related changes in bone that decrease fracture resistance, not just bone strength. Addressing this, the proposal aims to determine whether increases in advanced glycation end-products (AGEs) explain the effect of diabetes and aging on the fracture resistance of bone (as characterized by fracture properties) and to determine whether an AGE inhibitor and/or an antioxidant can both improve the quality of bone structure and increase the fracture properties of bone. Aim 1 will determine the role of bone structure, BMD, and AGEs in the effect of Type 2 Diabetes (T2D) and aging on the fracture properties of bone. In the first experiment, bones will be collected from T2D rats and non-diabetic rats at 24 weeks and 32 weeks of age. In the second experiment, bones will be harvested from aging rats at 4 months (young), 12 months (adult), and 24 months (old) of age. All the bones will undergo extensive analysis in order to identify the relative contribution of compositional properties such as BMD and AGEs and structural properties such as moment of inertia to a set of biomechanical properties including traditional measurements of strength and new measurements of fracture toughness and fatigue life. Aim 2 will evaluate how exogenous glycation of collagen affects the fracture properties of bone. This aim investigates whether the direct accumulation of AGEs within the extracellular matrix of bone decreases the fracture resistance of bone and whether the AGE inhibitor pyridoxamine, a B6 vitamin, protects against such a change. With appropriate controls, both human cortical bone and rodent bone will be incubated in diabetic concentrations of glucose with and without the inclusion of pyridoxamine. After quantifying the concentration of pentosidine, a biomarker for AGEs, and BMD, each specimen will be subjected to a mechanical test. In the case of the human bone, tests will determine the effects of increasing AGE on bone strength, post-yield energy dissipation, fatigue life, and crack-initiation & crack-growth toughness. For the rodent bones, tests will determine the effect of increasing AGE on fatigue life and fracture toughness, the ability to resist crack propagation. Aim 3 will assess the efficacy of pyridoxamine and N-acetylcysteine to increase fracture resistance of bone in an aging rat model of T2D through changes in bone structure, BMD, and AGEs. In this translation aim, the role of oxidative stress and AGE accumulation in the aging and diabetic effects on bone will be investigated using these two compounds. Starting at 4 months of age, non-diabetic and T2D rats will drink water, water with pyridoxamine, or water with N-acetylcysteine, an antioxidant. After 4 months, 8 months, and 14 months (aging) of treatment, bones will be harvested for extensive analysis to determine the effects of treatment on the structural, compositional, and biomechanical properties of bone. Additionally, histological and cell culture assays will assess the biological effects of the compounds on oxidative stress and osteoblast differentiation. To achieve these aims, Micro-Computed Tomography will quantify volumetric BMD; high performance liquid chromatography will quantify the concentration of crosslinks and collagen content; and thermal gravimetric analysis will quantify the collagen and mineral fractions as well as water content. Statistical models will determine the relative contribution of the structural and the compositional properties to the fracture properties of bone. This will address the relevance of targeting oxidative stress and AGEs to improve the bone health of Veterans and prevent bone fractures. The long-term goal is to identify the factors affecting the important determinants of fracture resistance and developing accurate diagnostic assessments of fracture risk. PUBLIC HEALTH RELEVANCE: The incidence of bone fractures increases with age and diabetes, and this phenomenon cannot be attributed to a decrease in bone mineral density (BMD). Greater numbers of Veterans are living longer, and many Veterans are developing type 2 diabetes. Thus, the number of fractures will increase. This is a rather significant problem because 1) current clinical tools based on BMD measurements do not necessarily identify individuals at risk of a fracture, 2) fracture resistance is more than just a problem of low BMD and low bone strength, 3) fractures are costly to repair and are associated with a high mortality rate, and 4) current osteoporosis drugs targeted to increasing BMD may not reduce the fracture risk of diabetics. Thus, there is a need to identify new mechanisms that affect the quality of bone tissue. The proposed research therefore explores oxidative stress and the accumulation of non-enzymatic crosslinks as factors underlying the biophysical changes that reduce the fracture resistance of bone.