Laura E. Wright Ph.D., Jeroen T. Buijs Ph.D., Hun-Soo Kim M.D., Laura E. Coats M.D., Anne M. Scheidler M.D., Sutha K. John M.S., Yun She B.A., Sreemala Murthy M.S., Ning Ma M.D., Helen J. Chin- Sinex B.S., Teresita M. Bellido Ph.D., Ted A. Bateman Ph.D., Marc S. Mendonca Ph.D., Khalid S. Mohammad M.D., Ph.D. and Theresa A. Guise M.D.
Increased fracture risk is commonly reported in cancer patients receiving radiotherapy, particularly at sites within the field of treatment. The direct and systemic effects of ionizing radiation on bone at a therapeutic dose are not well characterized in clinically relevant animal models. Using twenty-week male C57Bl/6 mice, effects of irradiation (right hindlimb; 2 Gy) on bone volume and microarchitecture were evaluated prospectively by microcomputed tomography and histomorphometry and compared to contralateral-shielded bone (left hindlimb) and non-irradiated control bone. One-week post-irradiation, trabecular bone volume declined in irradiated tibiae (-22%; p < 0.0001) and femora (-14%; p = 0.0586) and microarchitectural parameters were compromised. Trabecular bone volume declined in contralateral tibiae (-17%; p = 0.003), and no loss was detected at the femur. Osteoclast number, apoptotic osteocyte number and marrow adiposity were increased in irradiated bone relative to contralateral and non-irradiated bone, while osteoblast number was unchanged. Despite no change in osteoblast number one-week post-irradiation, dynamic bone formation indices revealed a reduction in mineralized bone surface and a concomitant increase in unmineralized osteoid surface area in irradiated bone relative to contralateral and non-irradiated control bone. Further, dose- and time-dependent calvarial culture and in vitro assays confirmed that calvarial osteoblasts and osteoblast-like MC3T3 cells were relatively radioresistant, while calvarial osteocyte and osteocyte-like MLO-Y4 cell apoptosis was induced as early as 48h post-irradiation (4 Gy). In osteoclastogenesis assays, radiation exposure (8 Gy) stimulated murine macrophage RAW264.7 cell differentiation and co-culture of irradiated RAW264.7 cells with MLO-Y4 or murine bone marrow cells enhanced this effect. These studies highlight the multi-faceted nature of radiation-induced bone loss by demonstrating direct and systemic effects on bone and its many cell types using clinically relevant doses and have important implications for bone health in patients treated with radiation therapy.