Short-Term Effects of Whole-Body Exposure to⁵⁶Fe Ions in Combination with Musculoskeletal Disuse on Bone Cells

Abstract

Space travel and prolonged bed rest cause bone loss due to musculoskeletal disuse. In space, radiation fields may also have detrimental consequences because charged particles traversing the tissues of the body can elicit a wide range of cytotoxic and genotoxic lesions. The effects of heavy-ion radiation exposure in combination with musculoskeletal disuse on bone cells and tissue are not known. To explore this, normally loaded 16-week-old male C57BL/6 mice were exposed to (56)Fe ions (1 GeV/nucleon) at doses of 0 cGy (sham), 10 cGy, 50 cGy or 2 Gy 3 days before tissue harvest. Additional mice were hindlimb unloaded by tail traction continuously for 1 week to simulate weightlessness and exposed to (56)Fe-ion radiation (0 cGy, 50 cGy, 2 Gy) 3 days before tissue harvest. Despite the short duration of this study, low-dose (10, 50 cGy) irradiation of normally loaded mice reduced trabecular volume fraction (BV/TV) in the proximal tibiae by 18% relative to sham-irradiated controls. Hindlimb unloading together with 50 cGy radiation caused a 126% increase in the number of TRAP(+) osteoclasts on cancellous bone surfaces relative to normally loaded, sham-irradiated controls. Together, radiation and hindlimb unloading had a greater effect on suppressing osteoblastogenesis ex vivo than either treatment alone. In sum, low-dose exposure to heavy ions (50 cGy) caused rapid cancellous bone loss in normally loaded mice and increased osteoclast numbers in hindlimb unloaded mice. In vitro irradiation also was more detrimental to osteoblastogenesis in bone marrow cells that were recovered from hindlimb unloaded mice compared to cells from normally loaded mice. Furthermore, irradiation in vitro stimulated osteoclast formation in a macrophage cell line (RAW264.7) in the presence of RANKL (25 ng/ml), showing that heavy-ion radiation can stimulate osteoclast differentiation even in the absence of osteoblasts. Thus heavy-ion radiation can acutely increase osteoclast numbers in cancellous tissue and, under conditions of musculoskeletal disuse, can enhance the sensitivity of bone cells, in particular osteoprogenitors, to the effects of radiation.

Mechanical loading causes site-specific anabolic effects on bone following exposure to ionizing radiation

“…At space-relevant total doses (1-2 Gy), ionizing radiation exposure rapidly induces cancellous bone loss in rodents and acutely increases the number of osteoclasts that line cancellous surfaces contributing to skeletal fragility [25][26][27][28], but also may cause persistent damage to stem and progenitor cells for osteoblasts [29,30]. Exposure to low LET ( 137 Cs gamma, 2 Gy) radiation increases osteoclast surface by 46%, increases apoptosis of marrow cells, causes oxidative damage to lipids within mineralized tissue in 4-mo old male C57BL/6 mice, and decreases tibial cancellous bone volume fraction by 16% [31].…”

“…Short duration experiments demonstrate that HZE particles may impair or lead to depletion of stem cells and osteoprogenitor cells in the osteoblast lineage [29], although low energy species such as gamma, protons and X-rays, also may exert detrimental effects and reduce osteoblast numbers if exposed to sufficiently high doses (2-20 Gy) [27]. High LET radiation ( 56 Fe, 1 GeV/nm, ≤2 Gy) inhibits ex vivo osteoblastogenesis from bone marrow precursors by 25%, increases osteoclast activity, and causes a decrement in tibial bone volume fraction after only 3 days in 4-mo old male C57BL/6 mice [29].…”

“…High LET radiation ( 56 Fe, 1 GeV/nm, ≤2 Gy) inhibits ex vivo osteoblastogenesis from bone marrow precursors by 25%, increases osteoclast activity, and causes a decrement in tibial bone volume fraction after only 3 days in 4-mo old male C57BL/6 mice [29].…”

“…Irradiation of cultured osteoblasts and cell lines reduces collagen production, proliferation and differentiation and also increases sensitivity to apoptotic agents [36][37][38][39]. Furthermore, total body irradiation with high-LET 56 Fe or low-LET 137 Cs (gamma) reduces marrow cellularity and marrow-derived osteoprogenitors [29,31].…”

Protein array profiling of mouse serum, six months post whole body radiation with 56Fe

“…Mice were housed in animal rooms under controlled conditions of temperature (24°C), humidity (55%) and 12 hr light-dark cycle. Conscious mice were subjected to uniform whole body irradiation with 56 Fe (dose 0.5Gy at 0.26-0.41 Gy/ min) as previously described (Yumoto et al, 2010) at the Brookhaven National Laboratory (BNL, Upton, NY, USA) with 1 GeV/amu (Giga electron volt/atomic mass unit) 56 (1 kiloelectron volt or keV = 1.6 X 10 -16 joules of energy).…”