|dc.description.abstract||Bone is a complex hierarchical material that depends on both its quantity and quality to provide the mechanical, endocrinological and immunological functions in vivo. As such, deleterious conditions leading to impaired bone quality and quantity stemming from age or associated disease states have a severe impact on individual health. Diabetes mellitus (both the type 1 and 2 variants) is a disease state in which there is an increase in skeletal fragility. Notably, bone mineral density (BMD) a marker for clinical diagnosis of osteoporosis and incorporated into the fracture risk assessment tool (FRAX) to predict risk, is unchanged in type 2 diabetes mellitus (T2D). Consequently, in the absence of decreased bone quantity, the quality and composition of the bone matrix may explain the poorly understood mechanisms underlying diabetic skeletal fragility. The three major constituents of bone are the mineral phase (hydroxyapatite), the organic phase (collagen, non-collagenous proteins) and water. As such, the loss and/or modifications in these submicron components may explain and contribute to our understanding of the causes of skeletal fragility. Type 1 diabetes mellitus (T1D) and T2D, an autoimmune disease and metabolic disorder respectively, are both characterized by hyperglycemia. Arising from different etiology, determining the variation in the alterations in the organic and inorganic phases on bone which contribute to strength is integral to understanding diabetic fracture. To this end, three animal models reflecting the T1D, T2D and the effect of select therapeutics were investigated here: (a) A non-obese transgenic murine model of T2D provides a vehicle of assessing diabetic skeletal fragility in the absence of the confounding influence of obesity; (b) a drug-induced rat model of T1D treated with a sodium-glucose cotransporter-2 inhibitor provides a mean to understand the role of stabilizing glycemic control on the bone structure and matrix composition with T1D; and (c) an ovariectomized rat model with both metformin and strength exercise regimens yields insight into the effect of common diabetic interventions on bone matrix quality. Each of these models was evaluated for skeletal structure, fracture resistance and matrix composition through mechanical (strength and toughness), biochemical protocols (“in bulk” fluorescent advanced glycation end-products (AGEs)), and imaging techniques (microcomputed tomography, small-angle x-ray scattering, confocal Raman spectroscopy).
To applying the findings observed in the animal models of diabetes to the human diabetic condition, a retrospective cohort study was performed on a T1D population. Extracted from the OptumLabs Data Warehouse, a T1D population was garnered to assess the efficacy of longitudinal HbA1c as means of assessing fracture risk when accounting for various covariates, including medications, comorbidities, and patient demographics. To this end, univariate Kaplan-Meier survival modeling was used to estimate fracture risk and multivariate Cox proportional hazards modeling was used to assess the independent hazard ratios of longitudinal HbA1c and medications when adjusting for the covariates. Notably, elevated longitudinal HbA1c was associated with a significant increase in T1D fracture risk, though it was not an independent predictor of fracture risk when accounting for other covariates. Furthermore, metformin and bisphosphonates, both medications affecting bone turnover, were independent predictors of T1D fracture, imparting beneficial and deleterious effects, respectively. The findings from human T1D cohort are consistent with the rodent findings describing T1D as skeletal fragility in T1D occurred through changes in bone quantity rather than the changes in bone quality observed in the rodent models of T2D.
The evaluation of these models illuminated variations in the impact of the disease states on the bone quality and structure. The non-obese murine model revealed altered mineralization correlating with glycoxidation products leading to reduced resistance to fracture within the bone. The rat model of T1D treated with SGLT-2 inhibitor demonstrated that structural changes observed with T1D occur despite reducing diabetic hyperglycemia and that the reduction in blood glucose improves bone strength and alters the accumulation of AGEs without impacting the mineral phase. The rodent model treated with metformin and exercise saw the greatest improvement in fracture resistance with dual intervention when compared to other ovariectomized groups. Ovariectomy provided a more significant change in matrix turnover than any intervention groups without causing changes in bone mineral or modifications of collagen by AGEs – possibly due to changes in non-collagenous proteins or enzymatic processes – requiring further investigation. The combination of these studies provides insight into how the diabetic condition and ovariectomy impacts bone health leading to the observed fragility characteristics. Identifying aspects of bone quality affected by the disease state creates an avenue to evaluate the efficacy and potential targets for therapeutic interventions.||