Effects of Advanced Glycation Endproducts on Type 2 Diabetes and Fragility Fractures

Abstract Bone fractures, due to aging and disease, contribute significantly to healthcare cost affecting the quality of life of 32.6 million adults in the US. Clinically, fracture risk can be predicted by dual x-ray absorptiometry (DXA) or the fracture risk assessment (FRAX) tool. Because type 2 diabetes (T2D) patients exhibit high bone mineral density, both of these tools

fail to correctly predict fracture risk, leading to a significant increase of fragility fractures in diabetic subjects. Therefore, there is a need to investigate how modifications in collagen and other organic components in bone can forecast diabetic fractures. Pentosidine (PEN), a fluorescent Advanced Glycation Endproduct (AGE) that forms in bone by reaction between

sugars and proteins, is the only established marker of bone fragility. However, it does not consistently predict T2D and fragility fractures. Here, for the first time in bone, we demonstrate the formation of carboxymethyl-lysine (CML), a non-fluorescent AGE associated with glycoxidative damage and hyperglycemia. We further show that it forms in abundance in bone

and is highly correlated to loss of bone toughness. Preliminary data presented in the proposal demonstrate that, in contrast to other AGEs, CML is upregulated >60% in T2D human bone compared to their age-matched controls. We then provide evidence that CML promotes formation and growth of additional hydroxyapatite (HA) crystals, similar to human T2D condition,

and forms a ‘molecular link’ between the organic and inorganic components of bone (collagen- HA interface) impairing bone quality. CML could therefore be a ‘new and relevant’ biomarker of T2D fracture that captures the effects of hyperglycemia and oxidative stress in bone and explains why diabetic bone is susceptible to fracture during overt T2D. Using an

obese and a non-obese mouse model of T2D, that mimic both causality and impact of human T2D on bone, we provide evidence that T2D increases AGEs, with CML explaining bone fragility. Similarly, we show that higher serum CML levels are associated with increased risk of incident clinical and prevalent vertebral fractures in T2D, independent of BMD. Thus, our overall

goal is to establish CML as a new and relevant biomarker of bone fragility and determine how it contributes to bone fragility in T2D. Using a combination of in vitro, ex vivo and in vivo models we will pursue the following three aims: Aim 1: Evaluate CML as a putative new biomarker of bone fragility and determine the mechanism(s) by which it reduces energy dissipation in bone;

Aim 2: Determine the contribution of CML and other AGEs to alterations in bone matrix and energy dissipation in human T2D vertebrae and cortical and cancellous bone from hip fracture patients.; Aim 3: Validate CML as a biomarker of T2D bone fragility using obese and non-obese mouse models of T2D and data from the Health, Aging and Body Composition (ABC) study. Our

findings will provide a new understanding of the mechanism and the effects of CML and other AGEs on bone fractures leading to new strategies to predict, manage and mitigate T2D and fragility fractures.