title: Loss of Nrf2 accelerates ionizing radiation-induced bone loss by upregulating RANKL.
abstract: Radiation therapy is an integral part of treatment for cancer patients; however, major side effects of this modality include aberrant bone remodeling and bone loss. Ionizing radiation (IR) is a major external factor that contributes to a significant increase in oxidative stress such as reactive oxygen species (ROS), has been implicated in osteoporotic phenotypes, and has been implicated in osteoporotic phenotypes, bone loss, and fracture risk. One of the major cellular defenses against heightened oxidative stress is mediated by nuclear factor (erythroid-derived 2)-like 2 (Nrf2), a master transcription factor that regulates induction of antioxidant gene expression and phase II antioxidant enzymes. Our objective was to test the hypothesis that loss of functional Nrf2 increases radiation-induced bone loss. We irradiated (single dose, 20Gy) the hindlegs of age- and sex-matched Nrf2(+/+) and Nrf2(-/-) mice. After 1 month, microCT analysis and histology revealed a drastic overall decrease in the bone volume after irradiation of mice lacking Nrf2. Although radiation exposure led to bone loss in mice with intact Nrf2, it was dramatically enhanced by loss of Nrf2. Furthermore, in the absence of Nrf2, a decrease in osteoblast mineralization was noted in calvarial osteoblasts compared with wild-type controls, and treatment with a common antioxidant, N-acetyl-l-cysteine (NAC), was able to rescue the mineralization. As expected, we observed a higher number of osteoclasts in Nrf2(-/-) mice compared to Nrf2(+/+) mice, and after irradiation, the trend remained the same. RT-PCR analysis of calvarial osteoblasts revealed that in the absence of Nrf2, the expression of RANKL was increased after irradiation. Interestingly, RANKL expression was suppressed when the calvarial osteoblasts were treated with NAC before IR exposure. Taken together, our data suggest that loss of Nrf2 leads to heightened oxidative stress and increased susceptibility to radiation-induced bone loss.
authors.affiliation: Department of Radiation Oncology 1161 21st Avenue S MCN DD1218 Nashville, TN 37232 United States.
authors.forename: Tapasi
authors.initials: T
authors.lastname: Rana
authors.affiliation: Department of Radiation Oncology 1161 21st Avenue S MCN DD1218 Nashville, TN 37232 United States.
authors.forename: Michelle A
authors.initials: MA
authors.lastname: Schultz
authors.affiliation: Department of Radiation Oncology 1161 21st Avenue S MCN DD1218 Nashville, TN 37232 United States; Cancer Biology, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Freeman
authors.affiliation: Department of Radiation Oncology 1161 21st Avenue S MCN DD1218 Nashville, TN 37232 United States; Cancer Biology, Vanderbilt University Medical Center, Nashville, TN 37232, USA. Electronic address: swati.biswas@vanderbilt.edu.
authors.forename: Swati
authors.initials: S
authors.lastname: Biswas
mesh_terms: D000818:Animals; D001854:Bone Marrow Cells / Q000502:physiology / Q000528:radiation effects; D002470:Cell Survival / Q000528:radiation effects; D051379:Mice; D018345:Mice, Knockout; D051267:NF-E2-Related Factor 2 / Q000172:deficiency / Q000235:genetics*; D010006:Osteoblasts / Q000502:physiology / Q000528:radiation effects; D010010:Osteoclasts / Q000502:physiology / Q000528:radiation effects; D010024:Osteoporosis / Q000235:genetics* / Q000378:metabolism; D018384:Oxidative Stress; D053245:RANK Ligand / Q000235:genetics / Q000378:metabolism*; D011833:Radiation Injuries, Experimental / Q000235:genetics* / Q000378:metabolism; D011836:Radiation Tolerance; D012886:Skull / Q000378:metabolism / Q000473:pathology / Q000528:radiation effects; D013977:Tibia / Q000378:metabolism / Q000473:pathology / Q000528:radiation effects; D015854:Up-Regulation / Q000528:radiation effects
chemical_list: D051267:NF-E2-Related Factor 2; C495636:Nfe2l2 protein, mouse; D053245:RANK Ligand; C505689:Tnfsf11 protein, mouse
keywords: Bone loss; Free radicals; Ionizing radiation; N-acetylcysteine; Nrf2; Osteoblast differentiation; Oxidative stress; RANKL; Reactive oxygen species