Low Dose IGF‐I Augments the Bone‐Lengthening Effect of Targeted Heat in the Mouse Hindlimb

Abstract

Linear growth disorders have multiple underlying causes, ranging from injury or illness to genetic disease. These conditions can lead to limb length inequality and disabling musculoskeletal pain. Delivering therapeutics to avascular growth plates is challenging, and treatments are often limited to invasive limb‐lengthening surgeries and/or expensive drug regimens that are only partially effective. We previously developed a unilateral heating model to increase extremity length in growing mice using targeted heat exposure on one side of the body. We demonstrated that femora were significantly longer and tibial elongation rate was over 12% greater on the heat‐treated side. The purpose of this study was to combine heat with the growth‐stimulating drug IGF‐I. We tested the hypothesis that low dose IGF‐I augments the bone‐lengthening effect of intermittent heat application on hindlimbs of growing mice. 3‐week old female C57BL/6 mice (N=12) were injected each morning with IGF‐I (2.5mg/kg SQ) and then treated with 40C unilateral heat for 40 minutes/day for 14 days. Non‐injected, heat‐treated mice from our prior study (N=14) and saline‐injected mice (N=6) served as heat‐treated controls; samples were pooled as results did not differ between heat alone groups. Body mass and tail length were recorded. Mice were given oxytetracycline (7.5 mg/kg IP) to quantify elongation rate. Bones were measured at the 5‐week endpoint. Metatarsal homogenates were probed for phosphorylated Akt (p‐Akt) by western blotting to assess IGF‐I activation. Tibial elongation rate was nearly 19% greater on the heat‐treated side (40C) of IGF‐I injected mice compared to their non‐treated contralateral side (30C). The growth acceleration averaged 19 μm/day (paired t = 7.16, p < 0.001), 36% faster than heat‐treated controls (14 μm/day). Femoral length increased 1.9% (t = 7.64, p < 0.001). When compared to heat‐treated controls (1.2% increase in femoral length), there was a significant heat by drug interaction (ANOVA, F = 7.61, p < 0.01), indicating that heat had greater lengthening effects when combined with IGF‐I. Biomarkers of IGF‐I activation (p‐Akt levels) were similar on heat‐treated and non‐treated sides of saline‐injected mice. In contrast, p‐Akt levels were markedly increased on the heat‐treated side of the IGF‐I injected mice, suggesting that systemic IGF‐I localizes to the site of heat application. Control and IGF‐I groups did not differ in starting or ending body mass (repeated measures ANOVA, F = 0.088, p = 0.77), confirming that IGF‐I did not systemically increase growth. Together, these data support the hypothesis that low dose IGF‐I enhances the bone‐lengthening effects of heat in growing mice without impacting body mass. This research has practical relevance for treating a spectrum of linear growth disorders in children. Results could lead to new, noninvasive approaches with better outcomes by reducing costs and side effects of surgeries and high‐dose pharmaceuticals.Support or Funding InformationSupported by ASBMR Grants in Aid Program and the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health (1R15AR067451‐01).