Effects of Long-Term Exposure of Exogenous Advanced Glycation Endproducts on Vertebral Bone Microarchitecture; Sex-Differences on Structure Derived Mechanical Properties

Abstract

IntroductionAccumulation of advanced glycation end products (AGE) are commonly associated with diabetic complications, including increased painful intervertebral disc pathologies, osteoporosis, and bone fracture risk. Exogenous AGEs develop by processing food at high temperatures and can accumulate in bone where they crosslink collagen. A clinical study reported association of increased serum AGE levels with vertebral fracture risk in women but not in men, highlighting gender effects. The purpose of this study was to investigate the sex-dependent effects of exogenous AGEs on vertebral bone quantity, quality, and mechanical behavior. Material and MethodsC57BL/6 mice (female, n = 8/group; male, n = 4/group) were pair-fed isocaloric diets containing low AGE amounts (LAGE, 7.6 μg/mg chow) or high AGE amounts (HAGE, 40.9 μg/mg chow) for 6 months; after euthanasia, blood glucose and serum AGEs were analyzed. Lumbar spines were μCT-scanned (resolution=4.9μm; SkyScan 1172, Bruker Corp., Belgium). Hydroxyapatite phantoms were similarly scanned for density calibration. Cortical bone analysis included tissue mineral density (TMD), BMD, cross sectional thickness (Ct.Th), bone surface are (BSA) and porosity. Trabecular bone analysis included trabecular thickness, separation (Tb.Sp), number (Tb.Nm), TMD, BMD, BVF, bone surface to volume ratio (BS/BV), Structure Model Index (SMI) and fabric anisotropy. A validated analytical model was used to calculate trabecular bone anisotropic compressive and shear moduli. Male/female and HAGE/LAGE differences were analyzed via Student's t-tests (p < 0.05). All experiments were approved by the Institutional Animal Care and Use Committee. ResultsSerum AGE levels were significantly increased in female HAGE mice compared with LAGE mice (p < 0.01). No differences were observed between blood glucose levels among groups. Dietary comparisons indicated that female trabecular BS/BV and porosity were significantly increased by HAGE-diet whereas trabecular BMD, compressive and shear moduli, and cortical BMD were decreased. In males dietary comparisons revealed only a significant increase in trabecular porosity. Male cortical bone remained unchanged. Sex comparisons within LAGE-diet group revealed that female trabecular bone had significantly reduced Tb.Nm, BMD, degree of anisotropy, compressive and shear moduli, and significantly increased Tb.Sp, porosity, and SMI compared with LAGE males. LAGE females also had significantly decreased cortical BSA and significantly increased Ct.Th compared with LAGE males. Sex comparisons of female versus male HAGE-diet groups revealed no significant differences. ConclusionResults indicated more pronounced detrimental effects on cortical and trabecular bone in females on HAGE-diet than in males. These sex-dependent effects suggest that AGEs effects on vertebral bone may be hormonally mediated. This work showed for the first time significant alterations to the elastic compressive and shear properties of vertebra following variations in dietary AGE levels. Changes in trabecular architecture and density manifested as functional changes, especially in females, warranting further post-yield and fracture analysis. Importantly, this study investigated the sex dependent effects of AGEs on vertebral bone quality and quantity independent from serum glucose (diabetes). Further research of AGE effects on cell turnover and cellular modifications to bone microarchitecture and mineralization is necessary to validate the link between changes in bone quality, quantity and function.