Rev Osteoporos Metab Miner. 2016; 8 (4): 138-46
1 Área de Reumatología y Metabolismo Óseo – Instituto de Investigación Sanitaria-Fundación Jiménez Díaz – UAM – Madrid (España)
2 Departamento de Fisiología Animal II – Universidad Complutense – Madrid (España)
3 Hospital del Mar-IMIM-Universidad Autónoma de Barcelona – Barcelona (España)
4 Red Temática de Investigación Cooperativa en Envejecimiento y Fragilidad (RETICEF) – Instituto de Salud Carlos III – Madrid (España)
Senile or involutional osteoporosis is a major problem in the developed world. Recent studies point to increased oxidative stress associated with aging, whether biological or chronological, as an important factor in its development. In this review paper, we focus on bone tissue disorders related to aging, the source of oxidative stress and negative influence on bone tissue. Finally, we consider the potential oxidative stress therapies currently being developed for this disease.
The aging population in developed nations has led to an increase in the prevalence and incidence of osteoporosis. An estimated 200 million people suffer with this condition worldwide1.
Defined as a decrease in bone mass and quality that increases the risk of fracture2, osteoporosis is closely related to aging. Although the factors involved have not been fully identified, those associated with involutional osteoporosis include estrogen after menopause3, glucocorticoid deficit therapy4, diabetes mellitus (DM), primarily type 25; renal failure6 (which causes secondary hyperparathyroidism) and, more recently, increased oxidative stress associated with many of these conditions7. In this review paper, we consider the role of oxidative stress in bone metabolism as well as possible alternative drug therapy to mitigate harmful effects in cases of osteoporosis.
Bone disorders associated with aging
Bone tissue undergoes a continuous remodeling process, with considerable regenerative capacity and adaptation to physiological changes. This process takes place in so-called bone remodeling units, consisting of different cell types: osteoclasts, osteoblasts and osteocytes (fully differentiated osteoblasts embedded in the mineralized matrix and actual orchestrator of remodeling process)8. Bone remodeling is highly regulated by genetic, mechanical, hormonal and local factors which determine the outcome of bone balance.
Peak bone mass is reached during puberty in women and somewhat later in males. The latter group develop a higher bone mass and present larger, wider bones, while the female bone structure tend to be smaller in diameter and cortical thickness. From about 30 years of age, a negative bone balance is observed in both sexes (with a predominance of bone resorption) which leads to a gradual loss of bone mass similar in both sexes, initially in the trabecular bone and later in the cortical3. This decline is accelerated after menopause in women due to loss of estrogens, agents with proven antioxidant properties, which maintain lower bone mass than in the case of men during aging. With age, metabolic disorders that affect the bones occur: neuromuscular changes related to lack of mobility; increased endogenous glucocorticoid production and renal failure with decreased synthesis of calcitriol. Moreover, with aging, bone collagen fibers undergo structural changes and the bone loses the ability to repair microfractures9. All this contributes to the increased incidence of fractures.
Most current concepts on the development of senile osteoporosis have been obtained from studies in experimental models, mainly in rodents. However, when interpreting these results, some bone peculiarities in rodents compared to humans must be taken into account, such as continuous modeling bone from the growth plate, the absence of menopause, as well as a lack of Haversian cortical bone system. However, as in humans, rodents have shown bone mass loss and a deterioration of structure and of long bone regenerative capacity associated with aging10,11. The bone loss in aged rats is related to a decrease in osteoblast maturation and the increased number of osteoclasts compared to osteoblasts in the trabecular bone12. Also, in inbred mice in which bone mass is regulated primarily by genetic factors, bone loss associated with age may assume up to 10% of the total bone mass, which is attributed to decreased bone remodeling13-16.
As observed in rodents, humans initially tend to lose trabecular bone with age, especially in women17, related in part to a decrease in physical activity and, therefore, the mechanical stimuli in the tissue18. From 70 years, decreased cortical thickness is more pronounced with a concomitant increase in the intra-cortical porosity of the femur. The medullar area increases both in men and women19. These changes are associated with increased risk of osteoporotic fractures. However, in both mice and humans, the mechanical properties of bone are relatively conserved through a sustained increase in sub-periosteal mineral, which increases inertia time20.
Mechanisms associated with bone aging
The underlying molecular mechanisms of involutional osteoporosis have begun to be elucidated in recent years. Associated with age, there has been a decrease in the osteoprotegerin (OPG) ratio/ligand receptor activator of nuclear factor (NF). This ratio is an important modulator of the remodeled bone21. Both OPG and RANKL are produced and secreted into the extracellular medium by osteoblastic cells and osteocytes. In fact, studies in mice models indicate that osteocytes produce most RANKL, thus directly influencing bone remodeling22,23. OPG is a soluble decoy receptor that captures RANKL in the extracellular medium (or on the surface of osteoblasts) and prevents it from binding to its receptor (RANK) in cells of osteoclastic lineage, thereby preventing the maturation and activation of osteoclasts. Thus, the OPG/RANKL relationship is an important anabolic/catabolic balance factor during bone remodeling24. Thus, the decreased OPG/RANKL relationship with age is consistent with increased osteoclast precursors in the bone marrow of old mice25. Osteocyte apoptosis plays an important role in bone loss associated with age and to immobilization or lack of stimuli26-28 and also associated with an increased RANKL expression21. Moreover, in old mice of the C57BL/6 strain, an increase in the production of endogenous glucocorticoids has been observed through the activation of the enzyme 11 beta-hydroxysteroid dehydrogenase type 1. This is related to reduced viability of bone cells (osteoblasts and osteoclasts) and angiogenesis, a key process in bone formation29.
Several factors may affect the rate of fracture repair with age30. With aging, there is a decrease in bone marrow osteoprogenitor, which occurs in parallel with increased adipognesis31. Both osteoblasts and adipocytes share a mesenchymal precursor cell differentiable either lineage depending on the microenvironment which are exposed these cells. Furthermore, osteoblasts from old mice RANKL production increase parallel to the decrease in expression of OPG. This alteration results in increased osteoclastogenesis and osteoclast activity21,25. It is noteworthy that there are a decreased number of endothelial cells and angiogenesis, which may contribute negatively to the process of bone repair in older people32.
Recently an increase in bone mass and reduced risk of fractures have been observed in elderly subjects who undergo angiotensin II receptor antagonist treatment33. The drug’s apparent beneficial effect on the bone is attributed to the inhibitory action of angiotensin II on various osteoblast differentiation markers, such as runt-related transcription factor 2 (Runx2), essential for osteoblast differentiation, osteocalcin34 and the increase of RANKL, which favors osteoclast differentiatio35. These data suggest that high blood pressure which is prevalent in the elderly could also contribute to involutional osteoporosis.
Sclerostin, the osteocyte-derived product of the Sost gene, is a potent inhibitor of bone formation through the binding to receptors associated with low density lipoprotein 5 and 6, inhibiting the canonical Wnt. Recent studies have shown that circulating sclerostin increases in post-menopausal women and with age in both sexes, which could have a negative influence on bone mass36,37.
Currently, the product of the klotho gene is known to be an important modulator of cellular aging38, a transmembrane protein acting as fibroblast growth factor (FGF) co-receptor 23 produced by the osteocytes and inducer of phosphaturia. Mice deficient in the Klotho gene suffer accelerated aging and osteopenia characterized by a decrease (20-40%) of cortical thickness in the femur, tibia and vertebrae, and low bone remodeling with a very sharp decline in cortical bone formation. Stromal cells from the bone marrow of these mice have a reduced ability mineralized nodule formation and phosphatase alkaline activity39. Paradoxically, these Klotho deficient mice have increased trabecular bone in the vertebrae and the metaphysis of long bones; an effect which the authors attribute to a selective activation of the Wnt pathway on the trabecular component. Klotho interacts with the Wnt pathway through its secreted product, which binds to ligands of this pathway by inhibiting its action, hence the absence of Klotho could lead to activation of the pathway Wnt39. Furthermore, mice without telomerase have been shown to exhibit increased cellular senescence and a decrease in bone mass 3 months from birth, associated with a reduction in bone formation and osteoblastogenesis40. Apparently, this reduction is because mice without telomerase have poorly differentiated osteoblasts and the pro-inflammatory environment that promotes osteoclast activity.
Oxidative stress as a pathogenic factor in involutional osteoporosis
Aging can be seen as a consequence of the imbalance between oxidizing agents produced naturally in cell metabolism and antioxidant defenses, with a predominance of the first. This is known as oxidative stress, which involves the oxidation of biomolecules and functional loss of cells41,42. Increased oxidative stress, carried out primarily in the mitochondria, is based on the overproduction of reactive oxygen species (ROS) such as superoxide anion (O2.–), hydroxyl radicals (OH) and hydrogen peroxide (H2O2).
This increase cannot be properly balanced by antioxidants systems such as superoxide dismutase (SOD), catalase (CAT) enzymes glutathione cycle (glutathione reductase and glutathione peroxidase) and thioredoxin, among others. Excess ROS with chronological (and/or biological) age oxidizes DNA, proteins and lipids and induces the phosphorylation of mitochondria p66shc protein, leading to cell death7,43-45 (Figure 1). Recently, oxidative stress has been found to have important functions in cell signaling46,47. In this context, ROS can be considered second messengers of inflammatory response. In fact, oxidation and inflammation are two closely related processes that increase with age48.
Although some researchers have raised questions about whether oxidative stress is a cause or consequence of aging, in recent years it has been implicated in the bone deterioration49. Using various animal models: premature aging, osteoporosis due to estrogen deficit (after ovariectomy) or diabetes, increased oxidative stress markers was found to decrease bone formation mechanisms50-54. The effects of oxidative stress to induce deleterious effects on bone tissue are not yet well known. Increased ROS may stabilize forkhead box O (FoxO) transcription, an important family of transcription regulators of many genes. Its functions include control of glucose metabolism, tumorigenesis and cell defense against oxidative stress55. FoxO 1 and 3 are expressed in the bone 56, where they seem to play a key role in maintaining bone formation56. It has been shown that genetic deletion of FoxOs in mice increases oxidative stress in bone and induces bone loss trabecular and cortical, associated with increased osteoblast/osteocytic apoptosis and a decrease bone formation57. The activation involves FoxO phosphorylation engagement with the beta-catenin57 causing gene induction of oxidative stress response, as GADD45 and CAT58. In fact, the protective action of oxidative stress of Klotho protein appears aforementioned mediated activation FoxOs39. Furthermore, activation of the FoxO prevents beta-catenin to act as transcription factor in stimulating the proliferation and differentiation of osteoblasts56.
Increased ROS in bone cells causes damage and apoptosis genomic DNA of osteoblasts and osteocytes. In addition, lipid peroxidation dependent lipoxygenase activated by oxidative stress plays an important role in bone loss associated with aging. This is evidenced by analyzing the expression of the lipoxygenase and ALOX12 and formation Alox15 4-hydroxynonenal, a product of lipid peroxidation, increased bone in older mice59. It has also been shown that products of lipid oxidation inhibiting action osteogenic factors60.
Furthermore, the increase of ROS has been linked to an increase of osteoclastogenesis and osteoclast activity61,62.
It has recently been shown that the enzyme nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX 4) plays a key role in osteoclastogenesis. Mice deficient of this enzyme, which produces constitutively ROS have a high bone mass and osteoclast markers deficit; also in human bone samples high osteoclast activity is correlated with increased activity of NOX 463. Furthermore, it is noted that in situations of increased ROS associated with experimental DM, are mixed results. While some authors have observed an increase in osteoclast activity , it has been suggested that could be related to the greater severity of DM65, however, other DM models, osteoclastic activity is reduced66. In fact, studies using murine osteoclasts pre-incubated in the presence of high glucose appear to confirm their inhibitory effect on osteoclasts67. Thus, differences in the degree of DM, strain and age of the animal, could contribute to the varying levels of bone resorption observed in different models65,68.
Possible oxidative stress therapies in senile osteoporosis
The development of new anabolic therapies for osteoporosis that combine increased bone mass with its ability to neutralize the harmful effects of oxidative stress is of great interest. An intuitive approach to prevent bone loss with age would be based on the antioxidant administration. However, it pointed out that classic antioxidants, such as the CAT or N-acetylcysteine, exert undesirable effects on bone tissue as authentic anti-osteoclastogenic act as agents interfering with bone remodeling69. In addition, such agents inhibit the canonical Wnt/beta-catenin whose activation is vitally important for maintaining bone formation, partly by inducing the seizure of activating the protein disheveled by the regulatory protein redox balance, nucleoredoxin70.
Recently, the bone anabolic effect has been associated with intermittent administration of parathyroid hormone (PTH) with its stress oxidative properties, such as the decrease in the amount of ROS, inhibition of phosphorylation of p66shc adaptor protein and increasing the amount of total glutathione69. The advantage of this treatment with PTH versus the classic antioxidants determines its stimulatory action of bone remodeling, with a predominance of bone formation in part through its interaction with the Wnt/beta-catenin (Figure 2). In this context, in vitro testing has been shown that the N-terminal (1-36) (homologous with PTH) and C-terminal (107-109) of the PTH-related protein (PTHrP) fragments are able to counteract oxidative stress induced by H2O2 in osteoprogenitor cells relative to their osteogenic action52,71.
In vitro studies and animal models suggest that resveratrol, a compound bifenilic group of polyphenolic antioxidants present in the skin of grapes and other fruits72,73, could be a potential anti-osteoporotic agent. This compound increases the proliferation and differentiation of osteoblast in the pre-MC3T3-E1 mouse in vitro73. Furthermore, administering resveratrol to mesenchymal cells derived from human embryonic stem cells has been shown to induce the expression of mature Runx274 differentiation75 and osteoblasts. This mechanism of action of resveratrol appears to be mediated by SIRT1 deacetylation activation which increases FoxO3a expression and complex formation with resveratrol, increasing Runx2 expression (Figure 3). SIRT1 could also increase the activity of Runx2 directly by deacetylation of this transcription factor in pre-osteoblast cells. In recent research into older rats, administering resveratrol (10 mg/kg daily for 10 weeks) has been shown to improve bone quality and bone biomechanical properties of the osteoporotic bone76. Although these pre-clinical results are promising, there are still no hard data to confirm the efficacy of resveratrol in senile osteoporosis in humans. However, of note is a recent study conducted in obese and osteopenic patients, in which oral administration of resveratrol (1 g daily for 16 weeks) significantly increased bone mass, and the amount of bone alkaline phosphatase, compared to the placebo group77. Recent reports indicate that mice deficient in SIRT6, another deacetylase related to the response to oxidative stress, present an osteoporotic phenotype at an early age. The absence of SIRT6 is associated with overexpression of Runx2, osterix and OPG as well as the increased Wnt pathway inhibitor, Dickkopf 1, which leads to a deficit of osteoblast and osteoclast maturation78. These data suggest that SIRT6 could be a therapeutic target in involutional osteoporosis.
Furthermore, glucocorticoid excess also induces oxidative stress. In this situation, the oxidative stress observed in plasma reticulum can be reversed by translation initiation factor 2α phosphorylation, which disrupts protein translation. A dephosphorylation inhibitor compound, salubrinal, has recently been shown to prevent deficit mineralization of osteoblasts treated with glucocorticoids in vitro as well as osteoblast and osteocyte apoptosis in an osteoporotic mouse model by prednisolone administration79.
The progressive aging of the population in the developed world leads to increased musculoskeletal disorders, including osteoporosis. Osteoporosis and increased fragility of the elderly population are a socio-economic challenge of the first magnitude. Different factors contribute to bone loss in the elderly, among which stands out as a common element increased oxidative stress (Figure 4). Thus, reducing oxidative stress could be a useful tool to combat involutional osteoporosis. However, the fact that oxidative stress compounds could interfere with the bone remodeling or key anabolic pathways for bone formation, such as the Wnt signaling pathway, requires certain considerations prior to therapeutic use. We must also take into account the physiological role of ROS, which act as secondary messengers of many metabolic pathways; therefore its uncontrolled inhibition could lead to unwanted side effects in bone cells. Further research is needed to determine the true effect of antioxidant therapies and appropriate dosing schedules to avoid deleterious action on bone remodeling. Taking into account these considerations, therapies aimed at neutralizing oxidative stress to prevent or alter the course of involutional osteoporosis would represent an obvious medical breakthrough.
Competing interests: The authors declare no conflicts of interest.
Financing: This work has been funded by aid from the Spanish Foundation for Bone Research and Mineral Metabolism (Grant FEIOMM Translational Research 2015), the Institute Carlos III (RD12/0043/0022, PI11/00449, RD06/0013/1002 Health, RD12/0043/0018 and RD12/0043/0008). SP-N enjoyed by a RETICEF contract (RD06/0013/1002 and RD12/0043/0008).
1. Cooper C, Campion G, Melton LJ. Hip fractures in the elderly: a world-wide projection. Osteoporos Int. 1992;2:285-9.
2. Reginster J-Y, Burlet N. Osteoporosis: a still increasing prevalence. Bone. 2006;38:S4-9.
3. Khosla S, Riggs BL. Pathophysiology of age-related bone loss and osteoporosis. Endocrinol Metab Clin North Am. 2005;34:1015-30, xi.
4. Van Staa TP, Laan RF, Barton IP, Cohen S, Reid DM, Cooper C. Bone density threshold and other predictors of vertebral fracture in patients receiving oral glucocorticoid therapy. Arthritis Rheum. 2003;48:3224-9.
5. Vestergaard P, Rejnmark L, Mosekilde L. Diabetes and its complications and their relationship with risk of fractures in type 1 and 2 diabetes. Calcif Tissue Int. 2009;84:45-55.
6. Miller PD. Bone disease in CKD: a focus on osteoporosis diagnosis and management. Am J Kidney Dis. 2014;64:290-304.
7. Manolagas SC. From estrogen-centric to aging and oxidative stress: a revised perspective of the pathogenesis of osteoporosis. Endocr Rev. 2010;31:266-300.
8. Eriksen EF. Cellular mechanisms of bone remodeling. Rev. Endocr. Metab Disord. 2010;11:219-27.
9. Bailey AJ, Knott L. Molecular changes in bone collagen in osteoporosis and osteoarthritis in the elderly. Exp Gerontol. 1999;34:337-51.
10. Wang L, Banu J, McMahan CA, Kalu DN. Male rodent model of age-related bone loss in men. Bone. 2001;29:141-8.
11. Liang CT, Barnes J, Seedor JG, Quartuccio HA, Bolander M, Jeffrey JJ, et al. Impaired bone activity in aged rats: alterations at the cellular and molecular levels. Bone. 1992;13:435-41.
12. Roholl PJ, Blauw E, Zurcher C, Dormans JA, Theuns HM. Evidence for a diminished maturation of preosteoblasts into osteoblasts during aging in rats: an ultrastructural analysis. J Bone Miner Res. 1994;9:355-66.
13. Kobayashi Y, Goto S, Tanno T, Yamazaki M, Moriya H. Regional variations in the progression of bone loss in two different mouse osteopenia models. Calcif Tissue Int. 1998;62:426-36.
14. Ferguson VL, Ayers RA, Bateman TA, Simske SJ. Bone development and age-related bone loss in male C57BL/6J mice. Bone. 2003;33:387-98.
15. Turner CH, Hsieh Y-F, Müller R, Bouxsein ML, Baylink DJ, Rosen CJ, et al. Genetic Regulation of Cortical and Trabecular Bone Strength and Microstructure in Inbred Strains of Mice. J Bone Miner Res. 2000;15:1126-31.
16. Weiss A, Arbell I, Steinhagen-Thiessen E, Silbermann M. Structural changes in aging bone: osteopenia in the proximal femurs of female mice. Bone. 1991;12:165-72.
17. Schaadt O, Bohr H. Different trends of age-related diminution of bone mineral content in the lumbar spine, femoral neck, and femoral shaft in women. Calcif Tissue Int. 1988;42:71-6.
18. Hamrick MW, Ding K-H, Pennington C, Chao YJ, Wu Y-D, Howard B, et al. Age-related loss of muscle mass and bone strength in mice is associated with a decline in physical activity and serum leptin. Bone. 2006;39:845-53.
19. Feik SA, Thomas CD, Clement JG. Age-related changes in cortical porosity of the midshaft of the human femur. J Anat. 1997;191:407-16.
20. Stein MS, Thomas CD, Feik SA, Wark JD, Clement JG. Bone size and mechanics at the femoral diaphysis across age and sex. J Biomech. 1998;31:1101-10.
21. Cao J, Venton L, Sakata T, Halloran BP. Expression of RANKL and OPG Correlates With Age-Related Bone Loss in Male C57BL/6 Mice. J Bone Miner Res. 2003;18:270-7.
22. Nakashima T, Hayashi M, Fukunaga T, Kurata K, Oh-Hora M, Feng JQ, et al. Evidence for osteocyte regulation of bone homeostasis through RANKL expression. Nat Med. 2011;17:1231-4.
23. Xiong J, Onal M, Jilka RL, Weinstein RS, Manolagas SC, O’Brien CA. Matrix-embedded cells control osteoclast formation. Nat Med. 2011;17:1235-41.
24. Kearns AE, Khosla S, Kostenuik PJ. Receptor activator of nuclear factor kappaB ligand and osteoprotegerin regulation of bone remodeling in health and disease. Endocr Rev. 2008;29:155-92.
25. Cao JJ, Wronski TJ, Iwaniec U, Phleger L, Kurimoto P, Boudignon B, et al. Aging Increases Stromal/ Osteoblastic Cell-Induced Osteoclastogenesis and Alters the Osteoclast Precursor Pool in the Mouse. J Bone Miner Res. 2005;20:1659-68.
26. Jilka RL, O’Brien CA. The Role of Osteocytes in Age-Related Bone Loss. Curr Osteoporos Rep. 2016;14:16-25.
27. Jilka RL, Noble B, Weinstein RS. Osteocyte apoptosis. Bone. 2013;54:264-71.
28. Bikle DD, Sakata T, Halloran BP. The impact of skeletal unloading on bone formation. Gravit Space Biol Bull. 2003;16:45-54.
29. Weinstein RS, Wan C, Liu Q, Wang Y, Almeida M, O’Brien CA, et al. Endogenous glucocorticoids decrease skeletal angiogenesis, vascularity, hydration, and strength in aged mice. Aging Cell. 2010;9:147-61.
30. Gruber R, Koch H, Doll BA, Tegtmeier F, Einhorn TA, Hollinger JO. Fracture healing in the elderly patient. Exp Gerontol. 2006;41:1080-93.
31. Gimble JM, Zvonic S, Floyd ZE, Kassem M, Nuttall ME. Playing with bone and fat. J Cell Biochem. 2006;98:251-66.
32. Edelberg JM, Reed MJ. Aging and angiogenesis. Front. Biosci. 2003;8:s1199-209.
33. Rejnmark L, Vestergaard P, Mosekilde L. Treatment with beta-blockers, ACE inhibitors, and calcium-channel blockers is associated with a reduced fracture risk: a nationwide case-control study. J Hypertens. 2006;24:581-9.
34. Franceschi RT. The developmental control of osteoblast-specific gene expression: role of specific transcription factors and the extracellular matrix environment. Crit Rev Oral Biol Med. 1999;10:40-57.
35. Shimizu H, Nakagami H, Osako MK, Hanayama R, Kunugiza Y, Kizawa T, et al. Angiotensin II accelerates osteoporosis by activating osteoclasts. FASEB J. 2008;22:2465-75.
36. Ardawi M-SM, Al-Kadi HA, Rouzi AA, Qari MH. Determinants of serum sclerostin in healthy pre- and postmenopausal women. J Bone Miner Res. 2011;26:2812-22.
37. Mödder UI, Hoey KA, Amin S, McCready LK, Achenbach SJ, Riggs BL, et al. Relation of age, gender, and bone mass to circulating sclerostin levels in women and men. J Bone Miner Res. 2011;26:373-9.
38. Kuro-o M, Matsumura Y, Aizawa H, Kawaguchi H, Suga T, Utsugi T, et al. Mutation of the mouse klotho gene leads to a syndrome resembling ageing. Nature. 1997;390:45-51.
39. Kuro-o M. Klotho and aging. Biochim. Biophys. Acta. 2009;1790:1049-58.
40. Saeed H, Abdallah BM, Ditzel N, Catala-Lehnen P, Qiu W, Amling M, et al. Telomerase-deficient mice exhibit bone loss owing to defects in osteoblasts and increased osteoclastogenesis by inflammatory microenvironment. J Bone Miner Res. 2011;26:1494-505.
41. Harman D. About “Origin and evolution of the free radical theory of aging: a brief personal history, 1954-2009”. Biogerontology. 2009;10:783.
42. Harman D. Aging: a theory based on free radical and radiation chemistry. J Gerontol. 1956;11:298-300.
43. De la Fuente M, Miquel J. An update of the oxidation-inflammation theory of aging: the involvement of the immune system in oxi-inflamm-aging. Curr Pharm Des. 2009;15:3003-26.
44. Almeida M, Ambrogini E, Han L, Manolagas SC, Jilka RL. Increased lipid oxidation causes oxidative stress, increased peroxisome proliferator-activated receptor-gamma expression, and diminished pro-osteogenic Wnt signaling in the skeleton. J. Biol Chem. 2009;284:27438-48.
45. Almeida M, Han L, Martin-Millan M, O’Brien CA, Manolagas SC. Oxidative stress antagonizes Wnt signaling in osteoblast precursors by diverting beta-catenin from T cell factor- to forkhead box O-mediated transcription. J Biol Chem. 2007;282:27298-305.
46. Bindoli A, Rigobello MP. Principles in redox signaling: from chemistry to functional significance. Antioxid Redox Signal. 2013;18:1557-93.
47. Lushchak VI. Free radicals, reactive oxygen species, oxidative stress and its classification. Chem Biol Interact. 2014;224:164-75.
48. Vida C, González EM, De la Fuente M. Increase of oxidation and inflammation in nervous and immune systems with aging and anxiety. Curr Pharm Des. 2014;20:4656-78.
49. Hamada Y, Kitazawa S, Kitazawa R, Fujii H, Kasuga M, Fukagawa M. Histomorphometric analysis of diabetic osteopenia in streptozotocin-induced diabetic mice: a possible role of oxidative stress. Bone. 2007;40:1408-14.
50. Almeida M, Han L, Martin-Millan M, Plotkin LI, Stewart SA, Roberson PK, et al. Skeletal involution by age-associated oxidative stress and its acceleration by loss of sex steroids. J Biol Chem. 2007;282:27285-97.
51. Brunet A. [Aging and the control of the insulin-FOXO signaling pathway]. Médecine Sci. M/S. 2012;28:316-20.
52. de Castro LF, Lozano D, Portal-Núñez S, Maycas M, De la Fuente M, Caeiro JR, et al. Comparison of the skeletal effects induced by daily administration of PTHrP (1-36) and PTHrP (107-139) to ovariectomized mice. J Cell Physiol. 2012;227:1752-60.
53. Portal-Núñez S, Manassra R, Lozano D, Acitores A, Mulero F, Villanueva-Peñacarrillo ML, et al. Characterization of skeletal alterations in a model of prematurely aging mice. Age (Dordr). 2013;35:383-93.
54. Portal-Núñez S, Cruces J, Gutiérrez-Rojas I, Lozano D, Ardura JA, Villanueva-Peñacarrillo ML, et al. The vertebrae of prematurely aging mice as a skeletal model of involutional osteoporosis. Histol Histopathol. 2013;28:1473-81.
55. Greer EL, Brunet A. FOXO transcription factors at the interface between longevity and tumor suppression. Oncogene. 2005;24:7410-25.
56. Ambrogini E, Almeida M, Martin-Millan M, Paik J-H, Depinho RA, Han L, et al. FoxO-mediated defense against oxidative stress in osteoblasts is indispensable for skeletal homeostasis in mice. Cell Metab. 2010;11:136-46.
57. Essers MAG, de Vries-Smits LMM, Barker N, Polderman PE, Burgering BMT, Korswagen HC. Functional interaction between beta-catenin and FOXO in oxidative stress signaling. Science. 2005;308:1181-4.
58. Katoh M, Katoh M. Human FOX gene family (Review). Int J Oncol. 2004;25:1495-500.
59. Huang MS, Morony S, Lu J, Zhang Z, Bezouglaia O, Tseng W, et al. Atherogenic phospholipids attenuate osteogenic signaling by BMP-2 and parathyroid hormone in osteoblasts. J Biol Chem. 2007;282:21237-43.
60. Lean JM, Davies JT, Fuller K, Jagger CJ, Kirstein B, Partington GA, et al. A crucial role for thiol antioxidants in estrogen-deficiency bone loss. J Clin Invest. 2003;112:915-23.
61. Garrett IR, Boyce BF, Oreffo RO, Bonewald L, Poser J, Mundy GR. Oxygen-derived free radicals stimulate osteoclastic bone resorption in rodent bone in vitro and in vivo. J Clin Invest. 1990;85:632-9.
62. Lee NK, Choi YG, Baik JY, Han SY, Jeong D-W, Bae YS,