Radiation-induced Bone Loss Research Articles

Radiation-induced bone loss in mice is ameliorated by inhibition of HIF-2α in skeletal progenitor cells.

Radiotherapy remains a common treatment modality for cancer despite skeletal complications. However, there are currently no effective treatments for radiation-induced bone loss, and the consequences of radiotherapy on skeletal progenitor cell (SPC) survival and function remain unclear. After radiation, leptin receptor-expressing cells, which include a population of SPCs, become localized to hypoxic regions of the bone and stabilize the transcription factor hypoxia-inducible factor-2α (HIF-2α), thus suggesting a role for HIF-2α in the skeletal response to radiation. Here, we conditionally knocked out HIF-2α in leptin receptor-expressing cells and their descendants in mice. Radiation therapy in littermate control mice reduced bone mass; however, HIF-2α conditional knockout mice maintained bone mass comparable to nonirradiated control animals. HIF-2α negatively regulated the number of SPCs, bone formation, and bone mineralization. To test whether blocking HIF-2α pharmacologically could reduce bone loss during radiation, we administered a selective HIF-2α inhibitor called PT2399 (a structural analog of which was recently FDA-approved) to wild-type mice before radiation exposure. Pharmacological inhibition of HIF-2α was sufficient to prevent radiation-induced bone loss in a single-limb irradiation mouse model. Given that ~90% of patients who receive a HIF-2α inhibitor develop anemia because of off-target effects, we developed a bone-targeting nanocarrier formulation to deliver the HIF-2α inhibitor to mouse bone, to increase on-target efficacy and reduce off-target toxicities. Nanocarrier-loaded PT2399 prevented radiation-induced bone loss in mice while reducing drug accumulation in the kidney. Targeted inhibition of HIF-2α may represent a therapeutic approach for protecting bone during radiation therapy.

CR6-Interacting Factor-1 Promotes Osteoclastogenesis Through the NF-κB Signaling Pathway after Irradiation.

Radiation exposure arising from radiotherapy may induce rapid bone loss and an increase in the extent of bone resorption. Reactive oxygen species (ROS) caused by radiation exposure play a crucial role during the process of osteoclastogenesis. However, the pathological mechanisms underlying radiation-induced osteoclastogenesis have yet to be fully elucidated. CR6-interacting factor-1 (Crif1) as a multifunctional protein is involved in regulating multiple biological functions in cells. Here, we investigated the role of Crif1 in radiation-induced osteoclastogenesis and found that radiation exposure induced an increase in the expression level of Crif1 and enhanced osteoclastogenesis in osteoclast progenitors. Crif1 and NF-κB p65 co-localized in the cytoplasm after radiation exposure. Crif1 knockdown did not affect the phosphorylation and total protein levels of extracellular signal-regulated kinases (ERK), c-Jun amino (N)-terminal kinases (JNK), p38, and IκB-α before and after irradiation. However, Crif1 knockdown did lead to the reduced phosphorylation and nuclear translocation of NF-κB p65 after irradiation and resulted in a reduced level of osteoclastogenesis in RAW264.7 cells after irradiation. In vivo studies involving Lyz2Cre;Crif1fl/fl mice possessing the myeloid-specific deletion of Crif1 demonstrated the alleviation of bone loss after irradiation when compared with Crif1fl/fl mice. Our findings demonstrate that Crif1 mediated the phosphorylation and nuclear translocation of NF-κB p65 and promoted osteoclastogenesis via the NF-κB signaling pathway after radiation exposure. Thus, our analysis revealed a specific role for Crif1 in the mediation of radiation-induced bone loss and may provide new insight into potential therapeutic strategies for radiation-induced bone loss.

A novel multifunctional radioprotective strategy using P7C3 as a countermeasure against ionizing radiation-induced bone loss

Radiotherapy is a critical component of cancer care but can cause osteoporosis and pathological insufficiency fractures in surrounding and otherwise healthy bone. Presently, no effective countermeasure exists, and ionizing radiation-induced bone damage continues to be a substantial source of pain and morbidity. The purpose of this study was to investigate a small molecule aminopropyl carbazole named P7C3 as a novel radioprotective strategy. Our studies revealed that P7C3 repressed ionizing radiation (IR)-induced osteoclastic activity, inhibited adipogenesis, and promoted osteoblastogenesis and mineral deposition in vitro. We also demonstrated that rodents exposed to clinically equivalent hypofractionated levels of IR in vivo develop weakened, osteoporotic bone. However, the administration of P7C3 significantly inhibited osteoclastic activity, lipid formation and bone marrow adiposity and mitigated tissue loss such that bone maintained its area, architecture, and mechanical strength. Our findings revealed significant enhancement of cellular macromolecule metabolic processes, myeloid cell differentiation, and the proteins LRP-4, TAGLN, ILK, and Tollip, with downregulation of GDF-3, SH2B1, and CD200. These proteins are key in favoring osteoblast over adipogenic progenitor differentiation, cell matrix interactions, and shape and motility, facilitating inflammatory resolution, and suppressing osteoclastogenesis, potentially via Wnt/β-catenin signaling. A concern was whether P7C3 afforded similar protection to cancer cells. Preliminarily, and remarkably, at the same protective P7C3 dose, a significant reduction in triple-negative breast cancer and osteosarcoma cell metabolic activity was found in vitro. Together, these results indicate that P7C3 is a previously undiscovered key regulator of adipo-osteogenic progenitor lineage commitment and may serve as a novel multifunctional therapeutic strategy, leaving IR an effective clinical tool while diminishing the risk of adverse post-IR complications. Our data uncover a new approach for the prevention of radiation-induced bone damage, and further work is needed to investigate its ability to selectively drive cancer cell death.

Type H vessel/platelet-derived growth factor receptor β+ perivascular cell disintegration is involved in vascular injury and bone loss in radiation-induced bone damage.

Collapse of the microvascular system is a prerequisite for radiation-induced bone loss. Since type H vessels, a specific bone vessel subtype surrounded by platelet-derived growth factor receptor β+ (PDGFRβ+ ) perivascular cells (PVCs),has been recently identified to couple angiogenesis and osteogenesis, we hypothesize that type H vessel injury initiates PDGFRβ+ PVC dysfunction, which contributes to the abnormal angiogenesis and osteogenesis after irradiation. In this study, we found that radiation led to the decrease of both type H endothelial cell (EC)and PDGFRβ+ PVC numbers. Remarkably, results from lineage tracing showed that PDGFRβ+ PVCs detached from microvesselsand converted the lineage commitment from osteoblasts to adipocytes, leading to vascular injury and bone loss after irradiation. These phenotype transitions above were further verified to beassociated with the decrease in hypoxia-inducible factor-1α (HIF-1α)/PDGF-BB/PDGFRβ signalling between type H ECs and PDGFRβ+ PVCs. Pharmacological blockade of HIF-1α/PDGF-BB/PDGFRβ signalling induced a phenotype similar to radiation-induced bone damage, while the rescue of this signalling significantly alleviated radiation-induced bone injury. Our findings show that thedecrease in HIF-1α/PDGF-BB/PDGFRβ signalling between type H ECs and PDGFRβ+ PVCs after irradiation affects the homeostasis of EC-PVC coupling and plays a part in vascular damage and bone loss, which has broad implications for effective translational therapies.

A novel approach for the prevention of ionizing radiation-induced bone loss using a designer multifunctional cerium oxide nanozyme.

The disability, mortality and costs due to ionizing radiation (IR)-induced osteoporotic bone fractures are substantial and no effective therapy exists. Ionizing radiation increases cellular oxidative damage, causing an imbalance in bone turnover that is primarily driven via heightened activity of the bone-resorbing osteoclast. We demonstrate that rats exposed to sublethal levels of IR develop fragile, osteoporotic bone. At reactive surface sites, cerium ions have the ability to easily undergo redox cycling: drastically adjusting their electronic configurations and versatile catalytic activities. These properties make cerium oxide nanomaterials fascinating. We show that an engineered artificial nanozyme composed of cerium oxide, and designed to possess a higher fraction of trivalent (Ce3+) surface sites, mitigates the IR-induced loss in bone area, bone architecture, and strength. These investigations also demonstrate that our nanozyme furnishes several mechanistic avenues of protection and selectively targets highly damaging reactive oxygen species, protecting the rats against IR-induced DNA damage, cellular senescence, and elevated osteoclastic activity in vitro and in vivo. Further, we reveal that our nanozyme is a previously unreported key regulator of osteoclast formation derived from macrophages while also directly targeting bone progenitor cells, favoring new bone formation despite its exposure to harmful levels of IR in vitro. These findings open a new approach for the specific prevention of IR-induced bone loss using synthesis-mediated designer multifunctional nanomaterials.

High-specificity protection against radiation-induced bone loss by a pulsed electromagnetic field.

Radiotherapy increases tumor cure and survival rates; however, radiotherapy-induced bone damage remains a common issue for which effective countermeasures are lacking, especially considering tumor recurrence risks. We report a high-specificity protection technique based on noninvasive electromagnetic field (EMF). A unique pulsed-burst EMF (PEMF) at 15 Hz and 2 mT induces notable Ca2+ oscillations with robust Ca2+ spikes in osteoblasts in contrast to other waveforms. This waveform parameter substantially inhibits radiotherapy-induced bone loss by specifically modulating osteoblasts without affecting other bone cell types or tumor cells. Mechanistically, primary cilia are identified as major PEMF sensors in osteoblasts, and the differentiated ciliary expression dominates distinct PEMF sensitivity between osteoblasts and tumor cells. PEMF-induced unique Ca2+ oscillations depend on interactions between ciliary polycystins-1/2 and endoplasmic reticulum, which activates the Ras/MAPK/AP-1 axis and subsequent DNA repair Ku70 transcription. Our study introduces a previously unidentified method against radiation-induced bone damage in a noninvasive, cost-effective, and highly specific manner.

Particle Radiation Side-Effects: Intestinal Microbiota Composition Shapes Interferon-γ-Induced Osteo-Immunogenicity.

Microbiota can both negatively and positively impact radiation-induced bone loss. Our prior research showed that compared to mice with conventional gut microbiota (CM), mice with restricted gut microbiota (RM) reduced inflammatory tumor necrosis factor (TNF) in bone marrow, interleukin (IL)-17 in blood, and chemokine (C-C motif) ligand 20 (CCL20) in bone marrow under anti-IL-17 treatment. We showed that Muribaculum intestinale was more abundant in intestinal epithelial cells (IECs) from the small intestine of female RM mice and positively associated with augmented skeletal bone structure. Female C57BL/6J pun RM mice, which were injected with anti-IL-17 antibody one day before exposure to 1.5 Gy 28Si ions of 850 MeV/u, showed high trabecular numbers in tibiae at 6 weeks postirradiation. Irradiated CM mice were investigated for lower interferon-γ and IL-17 levels in the small intestine than RM mice. IL-17 blockage resulted in bacterial indicator phylotypes being different between both microbiota groups before and after irradiation. Analysis of the fecal bacteria were performed in relation to bone quality and body weight, showing reduced tibia cortical thickness in irradiated CM mice (-15%) vs. irradiated RM mice (-9.2%). Correlation analyses identified relationships among trabecular bone parameters (TRI-BV/TV, Tb.N, Tb.Th, Tb.Sp) and Bacteroides massiliensis, Muribaculum sp. and Prevotella denticola. Turicibacter sp. was found directly correlated with trabecular separation in anti-IL-17 treated mice, whereas an unidentified Bacteroidetes correlated with trabecular thickness in anti-IL-17 neutralized and radiation-exposed mice. We demonstrated radiation-induced osteolytic damage to correlate with bacterial indicator phylotypes of the intestinal microbiota composition, and these relationships were determined from the previously discovered dose-dependent particle radiation effects on cell proliferation in bone tissue. New translational approaches were designed to investigate dynamic changes of gut microbiota in correlation with conditions of treatment and disease as well as mechanisms of systemic side-effects in radiotherapy.

Radiation-Induced Osteocyte Senescence Alters Bone Marrow Mesenchymal Stem Cell Differentiation Potential via Paracrine Signaling.

Cellular senescence and its senescence-associated secretory phenotype (SASP) are widely regarded as promising therapeutic targets for aging-related diseases, such as osteoporosis. However, the expression pattern of cellular senescence and multiple SASP secretion remains unclear, thus leaving a large gap in the knowledge for a desirable intervention targeting cellular senescence. Therefore, there is a critical need to understand the molecular mechanism of SASP secretion in the bone microenvironment that can ameliorate aging-related degenerative pathologies including osteoporosis. In this study, osteocyte-like cells (MLO-Y4) were induced to cellular senescence by 2 Gy γ-rays; then, senescence phenotype changes and adverse effects of SASP on bone marrow mesenchymal stem cell (BMSC) differentiation potential were investigated. The results revealed that 2 Gy irradiation could hinder cell viability, shorten cell dendrites, and induce cellular senescence, as evidenced by the higher expression of senescence markers p16 and p21 and the elevated formation of senescence-associated heterochromatin foci (SAHF), which was accompanied by the enhanced secretion of SASP markers such as IL-1α, IL-6, MMP-3, IGFBP-6, resistin, and adiponectin. When 0.8 μM JAK1 inhibitors were added to block SASP secretion, the higher expression of SASP was blunted, but the inhibition in osteogenic and adipogenic differentiation potential of BMSCs co-cultured with irradiated MLO-Y4 cell conditioned medium (CM- 2 Gy) was alleviated. These results suggest that senescent osteocytes can perturb BMSCs’ differential potential via the paracrine signaling of SASP, which was also demonstrated by in vivo experiments. In conclusion, we identified the SASP factor partially responsible for the degenerative differentiation of BMSCs, which allowed us to hypothesize that senescent osteocytes and their SASPs may contribute to radiation-induced bone loss.

Bone Aging, Cellular Senescence, and Osteoporosis.

ABSTRACTChanges in aging bone that lead to osteoporosis are mediated at multiple levels, including hormonal alterations, skeletal unloading, and accumulation of senescent cells. This pathological interplay is superimposed upon medical conditions, potentially bone‐wasting medications, modifiable and unmodifiable personal risk factors, and genetic predisposition that accelerate bone loss with aging. In this study, the focus is on bone hemostasis and its dysregulation with aging. The major physiological changes with aging in bone and the role of cellular senescence in contributing to age‐related osteoporosis are summarized. The aspects of bone aging are reviewed including remodeling deficits, uncoupling phenomena, inducers of cellular senescence related to bone aging, roles of the senescence‐associated secretory phenotype, radiation‐induced bone loss as a model for bone aging, and the accumulation of senescent cells in the bone microenvironment as a predominant mechanism for age‐related osteoporosis. The study also addresses the rationale and potential for therapeutic interventions based on the clearance of senescent cells or suppression of the senescence‐associated secretory phenotype. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The effect of pentoxifylline on radiation-induced bone loss in C3H/HeN mice

Developments in cancer therapies and diagnostic techniques have improved the long-term survival of cancer patients. Certain cancer treatments, such as radiotherapy, often harm normal tissue as well as the specifically targeted cancer cells. High doses of radiation induce bone loss. This study investigated the effects of pentoxifylline (PTX) on radiation-induced bone loss in C3H/HeN mice. C3H/HeN mice were divided into sham and irradiation (3 Gy, gamma-ray, IR) groups. The irradiated mice were treated for 12 weeks with vehicle, PTX (p.o.) or PTX (s.c.). Grip strength, uterus weight, serum alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) level were measured. Tibiae were analyzed using micro-computed tomography. There were no significant differences in the degree of grip strength, body weight and uterine weight between IR group and PTX-treated group. Treatment of PTX significantly preserved trabecular bone volume, trabecular number, trabecular separation and bone mineral density of proximal tibia metaphysic. The administration of PTX lowered serum TRAP in IR mice, suggesting that PTX can reduce the bone resorptive rate in mice. Our experimental data support the protective role of PTX against bone loss in irradiated mice. Based on the findings of this study, development of PTX-based treatments is anticipated to address bone loss after radiotherapy. Prospective dose escalation studies are required to determine the appropriate dosage of PTX.

The effect of pentoxifylline on radiation-induced bone loss in C3H/HeN mice

Developments in cancer therapies and diagnostic techniques have improved the long-term survival of cancer patients. Certain cancer treatments, such as radiotherapy, often harm normal tissue as well as the specifically targeted cancer cells. High doses of radiation induce bone loss. This study investigated the effects of pentoxifylline (PTX) on radiation-induced bone loss in C3H/HeN mice. C3H/HeN mice were divided into sham and irradiation (3 Gy, gamma-ray, IR) groups. The irradiated mice were treated for 12 weeks with vehicle, PTX (p.o.) or PTX (s.c.). Grip strength, uterus weight, serum alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) level were measured. Tibiae were analyzed using micro-computed tomography. There were no significant differences in the degree of grip strength, body weight and uterine weight between IR group and PTX-treated group. Treatment of PTX significantly preserved trabecular bone volume, trabecular number, trabecular separation and bone mineral density of proximal tibia metaphysic. The administration of PTX lowered serum TRAP in IR mice, suggesting that PTX can reduce the bone resorptive rate in mice. Our experimental data support the protective role of PTX against bone loss in irradiated mice. Based on the findings of this study, development of PTX-based treatments is anticipated to address bone loss after radiotherapy. Prospective dose escalation studies are required to determine the appropriate dosage of PTX.

Rspo1-LGR4 axis in BMSCs protects bone against radiation-induced injury through the mTOR-dependent autophagy pathway.

While mesenchymal stem cells (MSCs) have been widely used to repair radiation-induced bone damage, the molecular mechanism underlying the effects of MSCs in the maintenance of bone homeostasis under radiation stress remains largely unknown. In this study, the role and mechanisms of R-spondin 1 (Rspo1)-leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4) axis on the initiation of self-defense of bone mesenchymal stem cells (BMSCs) and maintenance of bone homeostasis under radiation stress were investigated. Interestingly, radiation increased levels of Rspo1 and LGR4 in BMSCs. siRNA knockdown of Rspo1 or LGR4 aggravated radiation-induced impairment of self-renewal ability and osteogenic differentiation potential of BMSCs. However, exogenous Rspo1 significantly attenuated radiation-induced depletion of BMSCs, and promoted the lineage shift towards osteoblasts. This alteration was associated with the reversal of mammalian target of rapamycin (mTOR) activation and autophagy decrement. Pharmacological and genetic blockade of autophagy attenuated the radio-protective effects of Rspo1, rendering BMSCs more vulnerable to radiation-induced injury. Then bone radiation injury was induced in C57BL6J mice to further determine the radio-protective effects of Rspo1. In mice, administration of Rspo1 recombinant protein alleviated radiation-induced bone loss. Our results uncover that Rspo1-LGR4-mTOR-autophagy axis are key mechanisms by which BMSCs initiate self-defense against radiation and maintain bone homeostasis. Targeting Rspo1-LGR4 may provide a novel strategy for the intervention of radiation-induced bone damage.

Radiation alters osteoclastogenesis by regulating the cytoskeleton and lytic enzymes in RAW 264.7 cells and mouse bone marrow-derived macrophages

Purpose The aim of the present study was to investigate the duality of irradiation effect on osteoclastogenesis, particularly on the cytoskeleton and expression of lytic enzymes in osteoclast precursors. Therefore, the present study may serve as a useful reference for the prevention and treatment of radiation-induced bone loss in the clinic. Materials and methods Two typical osteoclast precursors, murine RAW 264.7 macrophage cells and mouse bone marrow-derived macrophages (BMMs), were exposed to radiation in the order of 0.25–8 Gy, and the effects on cell viability, TRAP activity and bone resorption were subsequently investigated. Furthermore, changes in the cytoskeleton, cell apoptosis, and expression of lytic enzymes in osteoclasts were examined to elucidate the molecular mechanism of the duality of irradiation on osteoclastogenesis. Results Morphological changes and impaired viability were observed in RAW 264.7 cells and BMMs treated with 1–8 Gy irradiation with or without RANKL. However, the cell fusion tendency of osteoclasts was enhanced after 2 Gy irradiation, and an increased number of fused giant osteoclasts and enhanced F-actin ring formation were observed. Consistently, the bone resorption activity and the enzyme expression of TRAP, cathepsin K, matrix metalloproteinase 9, activator protein 1, and Caspase 9 were increased following irradiation with 2 Gy. Furthermore, intracellular ROS production and apoptosis of osteoclast precursors were increased. Conclusions Irradiation with 2 Gy inhibited the viability of osteoclast precursors, but increased osteoclastogenesis by enhancing cell fusion and increasing the secretion of lytic enzymes, which may be an important mechanism of radiation-induced bone loss.

Dietary countermeasure mitigates simulated spaceflight-induced osteopenia in mice

Spaceflight is a unique environment that includes at least two factors which can negatively impact skeletal health: microgravity and ionizing radiation. We have previously shown that a diet supplemented with dried plum powder (DP) prevented radiation-induced bone loss in mice. In this study, we investigated the capacity of the DP diet to prevent bone loss in mice following exposure to simulated spaceflight, combining microgravity (by hindlimb unloading) and radiation exposure. The DP diet was effective at preventing most decrements in bone micro-architectural and mechanical properties due to hindlimb unloading alone and simulated spaceflight. Furthermore, we show that the DP diet can protect osteoprogenitors from impairments resulting from simulated microgravity. Based on our findings, a dietary supplementation with DP could be an effective countermeasure against the skeletal deficits observed in astronauts during spaceflight.

Can Acute Galactic Cosmic Radiation-induced Bone Loss Be Mitigated By Dietary Modulation Of Inflammatory Cytokines?

Can Acute Galactic Cosmic Radiation-induced Bone Loss Be Mitigated By Dietary Modulation Of Inflammatory Cytokines?

BM-MSC-derived exosomes alleviate radiation-induced bone loss by restoring the function of recipient BM-MSCs and activating Wnt/\u03b2-catenin signaling

BackgroundRadiotherapy to cancer patients is inevitably accompanied by normal tissue injury, and the bone is one of the most commonly damaged tissues. Damage to bone marrow mesenchymal stem cells (BM-MSCs) induced by radiation is thought to be a major cause of radiation-induced bone loss. Exosomes exhibit great therapeutic potential in the treatment of osteoporosis, but whether exosomes are involved in radiation-induced bone loss has not been thoroughly elucidated to date. The main purpose of this study is to investigate the role of exosomes derived from BM-MSCs in restoring recipient BM-MSC function and alleviating radiation-induced bone loss.MethodsBM-MSC-derived exosomes were intravenously injected to rats immediately after irradiation. After 28 days, the left tibiae were harvested for micro-CT and histomorphometric analysis. The effects of exosomes on antioxidant capacity, DNA damage repair, proliferation, and cell senescence of recipient BM-MSCs were determined. Osteogenic and adipogenic differentiation assays were used to detect the effects of exosomes on the differentiation potential of recipient BM-MSCs, and related genes were measured by qRT-PCR and Western blot analysis. β-Catenin expression was detected at histological and cytological levels.ResultsBM-MSC-derived exosomes can attenuate radiation-induced bone loss in a rat model that is similar to mesenchymal stem cell transplantation. Exosome-treated BM-MSCs exhibit reduced oxidative stress, accelerated DNA damage repair, and reduced proliferation inhibition and cell senescence-associate protein expression compared with BM-MSCs that exclusively received irradiation. Following irradiation, exosomes promote β-catenin expression in BM-MSCs and restore the balance between adipogenic and osteogenic differentiation.ConclusionsOur findings indicate that BM-MSC-derived exosomes take effects by restoring the function of recipient BM-MSCs. Therefore, exosomes may represent a promising cell-free therapeutic approach for the treatment of radiation-induced bone loss.

Lowering iron level protects against bone loss in focally irradiated and contralateral femurs through distinct mechanisms.

Radiation therapy leads to increased risk of late-onset fragility and bone fracture due to the loss of bone mass. On the other hand, iron overloading causes osteoporosis by enhancing bone resorption. It has been shown that total body irradiation increases iron level, but whether the systemic bone loss is related to the changes in iron level and hepcidin regulation following bone irradiation remains unknown. To investigate the potential link between them, we first created an animal model of radiation-induced systemic bone loss by targeting the mid-shaft femur with a single 2 Gy dose of X-rays. We found that mid-shaft femur focal irradiation led to structural deterioration in the distal region of the trabecular bone with increased osteoclasts surface and expressions of bone resorption markers in both irradiated and contralateral femurs relative to non-irradiated controls. Following irradiation, reduced hepcidin activity of the liver contributed to elevated iron levels in the serum and liver. By injecting hepcidin or deferoxamine (an iron chelator) to reduce iron level, deterioration of trabecular bone microarchitecture in irradiated mice was abrogated. The ability of iron chelation to inhibit radiation-induced osteoclast differentiation was observed in vitro as well. We further showed that ionizing radiation (IR) directly stimulated osteoclast differentiation and bone resorption in bone marrow cells isolated not from contralateral femurs but from directly irradiated femurs. These results suggest that increased iron levels after focal radiation is at least one of the main reasons for systemic bone loss. Furthermore, bone loss in directly irradiated bones is not only due to the elevated iron level, but also from increased osteoclast differentiation. In contrast, the bone loss in the contralateral femurs is mainly due to the elevated iron level induced by IR alone. These novel findings provide proof-of-principle evidence for the use of iron chelation or hepcidin as therapeutic treatments for IR-induced osteoporosis.

Suppressing angiogenesis regulates the irradiation-induced stimulation on osteoclastogenesis in vitro.

Ionizing radiation-induced bone loss is a potential health concern in radiotherapy, occupational exposure, and astronauts. Although impaired bone vasculature and reduced proliferation of bone-forming osteoblasts has been implicated in this process, it has not been clearly characterized that whether radiation affects the growth of bone-resorbing osteoclasts. The molecular crosstalk between different cell populations in the skeletal system has not yet been elucidated in detail, especially between the increased bone resorption at early stage of post-irradiation and bone marrow-derived endothelial progenitor cells (BM-EPCs). In order to further understand the mechanisms involved in radiation-induced bone loss at the cellular level, we assessed the effects of irradiation on angiogenesis of BM-EPCs and osteoclastogenesis of receptor activator for nuclear factor-κB ligand (RANKL)-stimulated RAW 264.7 cells and crosstalk between these cell populations. We herein found significantly dysfunction of BM-EPCs in response to irradiation at a dose of 2 Gy, including inhibited proliferation, migration, tube-forming abilities, and downregulated expression of pro-angiogenesis vascular endothelial growth factors A (VEGF A). Meanwhile, we observed that irradiation promoted osteoclastogenesis of RANKL-stimulated RAW 264.7 cells directly or indirectly. These results provide quantitative evidences of irradiation induced osteoclastogenesis at a cellular level, and strongly suggest the involvement of osteoclastogenesis, angiogenesis and crosstalk between bone marrow cells in the radiation-induced bone loss. This study may provide new insights for the early diagnosis and intervention of bone loss post-irradiation.

Simulated space radiation sensitizes bone but not muscle to the catabolic effects of mechanical unloading.

Deep space travel exposes astronauts to extended periods of space radiation and mechanical unloading, both of which may induce significant muscle and bone loss. Astronauts are exposed to space radiation from solar particle events (SPE) and background radiation referred to as galactic cosmic radiation (GCR). To explore interactions between skeletal muscle and bone under these conditions, we hypothesized that decreased mechanical load, as in the microgravity of space, would lead to increased susceptibility to space radiation-induced bone and muscle loss. We evaluated changes in bone and muscle of mice exposed to hind limb suspension (HLS) unloading alone or in addition to proton and high (H) atomic number (Z) and energy (E) (HZE) (16O) radiation. Adult male C57Bl/6J mice were randomly assigned to six groups: No radiation ± HLS, 50 cGy proton radiation ± HLS, and 50 cGy proton radiation + 10 cGy 16O radiation ± HLS. Radiation alone did not induce bone or muscle loss, whereas HLS alone resulted in both bone and muscle loss. Absolute trabecular and cortical bone volume fraction (BV/TV) was decreased 24% and 6% in HLS-no radiation vs the normally loaded no-radiation group. Trabecular thickness and mineral density also decreased with HLS. For some outcomes, such as BV/TV, trabecular number and tissue mineral density, additional bone loss was observed in the HLS+proton+HZE radiation group compared to HLS alone. In contrast, whereas HLS alone decreased muscle mass (19% gastrocnemius, 35% quadriceps), protein synthesis, and increased proteasome activity, radiation did not exacerbate these catabolic outcomes. Our results suggest that combining simulated space radiation with HLS results in additional bone loss that may not be experienced by muscle.

Differences in responses to X-ray exposure between osteoclast and osteoblast cells.

Radiation-induced bone loss is a potential health concern for cancer patients undergoing radiotherapy. Enhanced bone resorption by osteoclasts and decreased bone formation by osteoblasts were thought to be the main reasons. In this study, we showed that both pre-differentiating and differentiating osteoclasts were relatively sensitive to X-rays compared with osteoblasts. X-rays decreased cell viability to a greater degree in RAW264.7 cells and in differentiating cells than than in osteoblastic MC3T3-E1 cells. X-rays at up to 8 Gy had little effects on osteoblast mineralization. In contrast, X-rays at 1 Gy induced enhanced osteoclastogenesis by enhanced cell fusion, but had no effects on bone resorption. A higher dose of X-rays at 8 Gy, however, had an inhibitory effect on bone resorption. In addition, actin ring formation was disrupted by 8 Gy of X-rays and reorganized into clusters. An increased activity of Caspase 3 was found after X-ray exposure. Actin disorganization and increased apoptosis may be the potential effects of X-rays at high doses, by inhibiting osteoclast differentiation. Taken together, our data indicate high radiosensitivity of osteoclasts. X-ray irradiation at relatively low doses can activate osteoclastogenesis, but not osteogenic differentiation. The radiosensitive osteoclasts are the potentially responsive cells for X-ray-induced bone loss.