Effect of Canagliflozin Pretreatment on the Efficacy of Insulin Therapy to Rescue Type 1 Diabetes-Related Bone Fragility in Male Mice

Humans with type 1 diabetes (T1D) have a higher-than-expected fracture risk, and bone is weak in mouse models of T1D. Since hyperglycemia is thought to deleteriously affect tissue-level sensitivity of insulin action, we hypothesized that glucose-lowering by canagliflozin (Cana) improves the ability of insulin treatment to rescue the T1D-related decrease in bone strength. To test this hypothesis, male DBA/2 mice with and without streptozotocin (STZ)-induced hyperglycemia at 8 weeks of age were fed a diet with and without Cana (50 ppm) at 10 weeks of age and then the T1D mice, regardless of placebo diet or Cana diet, were treated at 14 weeks of age with a subcutaneous implant (Palm) that does or does not elute insulin. Non-diabetic (ND) mice and T1D mice were euthanized at 22 weeks of age following weekly measurements of body mass and non-fasting blood glucose. Cana pre-treatment significantly lowered circulating glucose, but not to non-diabetic levels. It did not affect the ability of insulin therapy to reduce glucose. However, pre-treatment significantly affected serum markers of bone resorption (CTX) and bone formation in ways that favored lower CTX across the 3 glucose groups (ND-Palm, T1D-Palm, and T1D-Insulin) and P1NP levels in T1D-Insulin mice that were consistent with P1NP levels in ND mice. Cana pre-treatment did not affect bone strength (compression of lumbar vertebra and bending of femur mid-diaphysis) nor the ability of insulin therapy to improve bone strength. Cana and glucose group did not affect advanced glycation end-products in the organic matrix of bone, despite glycated hemoglobin A1c being much higher in the T1D mice. Cana and glucose group both independently affected crack initiation toughness such that insulin rescued the T1D-related decline of cortical bone to resist crack growth and Cana modestly added to this effect. Lastly, T1D lowered bound water in the bone matrix and insulin treatment partially reversed the decline. Thus, glycemic control prior to insulin treatment does not appear to affect the ability of insulin to prevent bone fragility in T1D mice.