Accelerated aging of intervertebral discs in a mouse model of progeria

Intervertebral disc degeneration (IDD) is a common and debilitating disorder that results in reduced flexibility of the spine, pain, and reduced mobility. Risk factors for IDD include age, genetic predisposition, injury, and other environmental factors such as smoking. Loss of proteoglycans (PGs) contributes to IDD with advancing age. Currently there is a lack of a model for rapid investigation of disc aging and evaluation of therapeutic interventions. Here we examined progression of disc aging in a murine model of a human progeroid syndrome caused by deficiency of the DNA repair endonuclease, ERCC1–XPF (Ercc1^?/Δ mice). The ERCC1‐deficient mice showed loss of disc height and degenerative structural changes in their vertebral bodies similar to those reported for old rodents. Compared to their wild‐type littermates, Ercc1^?/Δ mice also exhibit other age‐related IDD characteristics, including premature loss of disc PG, reduced matrix PG synthesis, and enhanced apoptosis and cell senescence. Finally, the onset of age‐associated disc pathologies was further accelerated in Ercc1^?/Δ mice following chronic treatment with the chemotherapeutic agent mechlorethamine. These results demonstrate that Ercc1^?/Δ mice represent an accurate and rapid model of disc aging and provide novel evidence that DNA damage negatively impacts PG synthesis. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:1600–1607, 2010     

mTOR signaling plays a critical role in the defects observed in muscle‐derived stem/progenitor cells isolated from a murine model of accelerated aging

Mice expressing reduced levels of ERCC1‐XPF (Ercc1 ^−/Δ mice) demonstrate premature onset of age‐related changes due to decreased repair of DNA damage. Muscle‐derived stem/progenitor cells (MDSPCs) isolated from Ercc1 ^−/Δ mice have an impaired capacity for cell differentiation. The mammalian target of rapamycin (mTOR) is a critical regulator of cell growth in response to nutrient, hormone, and oxygen levels. Inhibition of the mTOR pathway extends the lifespan of several species. Here, we examined the role of mTOR in regulating the MDSPC dysfunction that occurs with accelerated aging. We show that mTOR signaling pathways are activated in Ercc1 ^−/Δ MDSPCs compared with wild‐type (WT) MDSPCs. Additionally, inhibiting mTOR with rapamycin promoted autophagy and improved the myogenic differentiation capacity of the Ercc1 ^−/Δ MDSPCs. The percent of apoptotic and senescent cells in Ercc1 ^−/Δ MDSPC cultures was decreased upon mTOR inhibition. These results establish that mTOR signaling contributes to stem cell dysfunction and cell fate decisions in response to endogenous DNA damage. Therefore, mTOR represents a potential therapeutic target for improving defective, aged stem cells. © 2016 The Authors. Journal of Orthopaedic Research Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 35:1375–1382, 2017.

     

Silent information regulator (Sir)T1 inhibits NF‐κB signaling to maintain normal skeletal remodeling

Silent information regulator T1 (SirT1) is linked to longevity and negatively controls NF‐κB signaling, a crucial mediator of survival and regulator of both osteoclasts and osteoblasts. Here we show that NF‐κB repression by SirT1 in both osteoclasts and osteoblasts is necessary for proper bone remodeling and may contribute to the mechanisms linking aging and bone loss. Osteoclast‐ or osteoblast‐specific SirT1 deletion using the Sirt^flox/flox mice crossed to lysozyme M‐cre and the 2.3 kb col1a1‐cre transgenic mice, respectively, resulted in decreased bone mass caused by increased resorption and reduced bone formation. In osteoclasts, lack of SirT1 promoted osteoclastogenesis in vitro and activated NF‐κB by increasing acetylation of Lysine 310. Importantly, this increase in osteoclastogenesis was blocked by pharmacological inhibition of NF‐κB. In osteoblasts, decreased SirT1 reduced osteoblast differentiation, which could also be rescued by inhibition of NF‐κB. In further support of the critical role of NF‐κB signaling in bone remodeling, elevated NF‐κB activity in IκBα^+/? mice uncoupled bone resorption and formation, leading to reduced bone mass. These findings support the notion that SirT1 is a genetic determinant of bone mass, acting in a cell‐autonomous manner in both osteoblasts and osteoclasts, through control of NF‐κB and bone cell differentiation. © 2013 American Society for Bone and Mineral Research.     

Maslinic acid suppresses osteoclastogenesis and prevents ovariectomy‐induced bone loss by regulating RANKL‐mediated NF‐κB and MAPK signaling pathways

Activation of NF‐κB and MAPK/activator protein 1 (AP‐1) signaling pathways by receptor activator NF‐κB ligand (RANKL) is essential for osteoclast activity. Targeting NF‐κB and MAPK/AP‐1 signaling to modulate osteoclast activity has been a promising strategy for osteoclast‐related diseases. In this study we examined the effects of maslinic acid (MA), a pentacyclic triterpene acid that is widely present in dietary plants, on RANKL‐induced osteoclastogenesis, osteoclast function, and signaling pathways by in vitro and in vivo assay systems. In mouse bone marrow monocytes (BMMs) and RAW264.7 cells, MA inhibited RANKL‐induced osteoclastogenesis in a dose‐dependent manner within nongrowth inhibitory concentration, and MA decreased osteoclastogenesis‐related marker gene expression, including TRACP, MMP9, c‐Src, CTR, and cathepsin K. Specifically, MA suppressed osteoclastogenesis and actin ring formation at early stage. In ovariectomized mice, administration of MA prevented ovariectomy‐induced bone loss by inhibiting osteoclast activity. At molecular levels, MA abrogated the phosphorylation of MAPKs and AP‐1 activity, inhibited the IκBα phosphorylation and degradation, blocked NF‐κB/p65 phosphorylation, nuclear translocation, and DNA‐binding activity by downregulating RANK expression and blocking RANK interaction with TRAF6. Together our data demonstrate that MA suppresses RANKL‐induced osteoclastogenesis through NF‐κB and MAPK/AP‐1 signaling pathways and that MA is a promising agent in the treatment of osteoclast‐related diseases such as osteoporosis. © 2011 American Society for Bone and Mineral Research.     

Processing of the NF‐κB2 precursor p100 to p52 is critical for RANKL‐induced osteoclast differentiation

Gene targeting of the p50 and p52 subunits of NF‐κB has shown that NF‐κB plays a critical role in osteoclast differentiation. However, the molecular mechanism by which NF‐κB regulates osteoclast differentiation is still unclear. To address this issue, we analyzed alymphoplasia (aly/aly) mice in which the processing of p100 to p52 does not occur owing to an inactive form of NF‐κB‐inducing kinase (NIK). Aly/aly mice showed a mild osteopetrosis with significantly reduced osteoclast numbers. RANKL‐induced osteoclastogenesis from bone marrow cells of aly/aly mice also was suppressed. RANKL still induced the degradation of IκBα and activated classical NF‐κB, whereas processing of p100 to p52 was abolished by the aly/aly mutation. Moreover, RANKL‐induced expression of NFATc1 was impaired in aly/aly bone marrow. Overexpression of constitutively active IKKα or p52 restored osteoclastogenesis in aly/aly cells. Finally, transfection of either wild‐type p100, p100ΔGRR that cannot be processed to p52, or p52 into NF‐κB2‐deficient cells followed by RANKL treatment revealed a strong correlation between the number of osteoclasts induced by RANKL and the ratio of p52 to p100 expression. Our data provide a new finding for a previously unappreciated role for NF‐κB in osteoclast differentiation. © 2010 American Society for Bone and Mineral Research     

The pivotal role of the alternative NF‐κB pathway in maintenance of basal bone homeostasis and osteoclastogenesis

The alternative NF‐κB pathway consists predominantly of NF‐κB‐inducing kinase (NIK), IκB kinase α (IKKα), p100/p52, and RelB. The hallmark of the alternative NF‐κB signaling is the processing of p100 into p52 through NIK, thus allowing the binding of p52 and RelB. The physiologic relevance of alternative NF‐κB activation in bone biology, however, is not well understood. To elucidate the role of the alternative pathway in bone homeostasis, we first analyzed alymphoplasic (aly/aly) mice, which have a defective NIK and are unable to process p100, resulting in the absence of p52. We observed increased bone mineral density (BMD) and bone volume, indicating an osteopetrotic phenotype. These mice also have a significant defect in RANKL‐induced osteoclastogenesis in vitro and in vivo. NF‐κB DNA‐binding assays revealed reduced activity of RelA, RelB, and p50 and no binding activity of p52 in aly/aly osteoclast nuclear extracts after RANKL stimulation. To determine the role of p100 itself without the influence of a concomitant lack of p52, we used p100^?/? mice, which specifically lack the p100 inhibitor but still express p52. p100^?/? mice have an osteopenic phenotype owing to the increased osteoclast and decreased osteoblast numbers that was rescued by the deletion of one allele of the relB gene. Deletion of both allele of relB resulted in a significantly increased bone mass owing to decreased osteoclast activity and increased osteoblast numbers compared with wild‐type (WT) controls, revealing a hitherto unknown role for RelB in bone formation. Our data suggest a pivotal role of the alternative NF‐κB pathway, especially of the inhibitory role of p100, in both basal and stimulated osteoclastogenesis and the importance of RelB in both bone formation and resorption. © 2010 American Society for Bone and Mineral Research     

Mitochondrial‐derived reactive oxygen species (ROS) play a causal role in aging‐related intervertebral disc degeneration

Oxidative damage is a well‐established driver of aging. Evidence of oxidative stress exists in aged and degenerated discs, but it is unclear how it affects disc metabolism. In this study, we first determined whether oxidative stress negatively impacts disc matrix metabolism using disc organotypic and cell cultures. Mouse disc organotypic culture grown at atmospheric oxygen (20% O₂) exhibited perturbed disc matrix homeostasis, including reduced proteoglycan synthesis and enhanced expression of matrix metalloproteinases, compared to discs grown at low oxygen levels (5% O₂). Human disc cells grown at 20% O₂ showed increased levels of mitochondrial‐derived superoxide anions and perturbed matrix homeostasis. Treatment of disc cells with the mitochondria‐targeted reactive oxygen species (ROS) scavenger XJB‐5‐131 blunted the adverse effects caused by 20% O₂. Importantly, we demonstrated that treatment of accelerated aging Ercc1^?/Δ mice, previously established to be a useful in vivo model to study age‐related intervertebral disc degeneration (IDD), also resulted in improved disc total glycosaminoglycan content and proteoglycan synthesis. This demonstrates that mitochondrial‐derived ROS contributes to age‐associated IDD in Ercc1^?/Δ mice. Collectively, these data provide strong experimental evidence that mitochondrial‐derived ROS play a causal role in driving changes linked to aging‐related IDD and a potentially important role for radical scavengers in preventing IDD. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 31:1150–1157, 2013

     

Alternative NF‐κB Regulates RANKL‐Induced Osteoclast Differentiation and Mitochondrial Biogenesis via Independent Mechanisms

Mitochondrial biogenesis, the generation of new mitochondrial DNA and proteins, has been linked to osteoclast (OC) differentiation and function. In this study we used mice with mutations in key alternative NF‐κB pathway proteins, RelB and NF‐κB–inducing kinase (NIK), to dissect the complex relationship between mitochondrial biogenesis and osteoclastogenesis. In OC precursors lacking either NIK or RelB, receptor activator of NF‐κB ligand (RANKL) was unable to increase mitochondrial DNA or oxidative phosphorylation (OxPhos) protein expression, which was associated with lower oxygen consumption rates. Transgenic OC precursors expressing constitutively active NIK showed normal RANKL‐induced mitochondrial biogenesis (OxPhos expression and mitochondria copy number) compared to controls, but larger mitochondrial dimensions and increased oxygen consumption rates, suggesting increased mitochondrial function. To deduce the mechanism for mitochondrial biogenesis defects in NIK‐deficient and RelB‐deficient precursors, we examined expression of genes known to control this process. PGC‐1β (Ppargc1b) expression, but not PGC‐1α, PPRC1, or ERRα, was significantly reduced in RelB^–/– and NIK^–/– OCs. Because PGC‐1β has been reported to positively regulate both mitochondrial biogenesis and differentiation in OCs, we retrovirally overexpressed PGC‐1β in RelB^–/– cells, but surprisingly found that it did not affect differentiation, nor did it restore RANKL‐induced mitochondrial biogenesis. To determine whether the blockade in osteoclastogenesis in RelB‐deficient cells precludes mitochondrial biogenesis, we rescued RelB^–/– differentiation via overexpression of NFATc1. Mitochondrial parameters in neither WT nor RelB‐deficient cultures were affected by NFATc1 overexpression, and bone resorption in RelB^–/– was not restored. Furthermore, NFATc1 co‐overexpression with PGC‐1β, although allowing OC differentiation, did not rescue mitochondrial biogenesis or bone resorption in RelB^–/– OCs, by CTX‐I levels. Thus, our results indicate that the alternative NF‐κB pathway plays dual, but distinct, roles in controlling the independent processes of OC differentiation and OC mitochondrial biogenesis. Furthermore, the inability of PGC‐1β to drive mitochondrial biogenesis in OCs without RelB indicates a cell‐type specificity in mitochondria regulation. © 2015 American Society for Bone and Mineral Research.     

Investigating the role of DNA damage in tobacco smoking-induced spine degeneration

Background contextTobacco smoking is a key risk factor for spine degeneration. However, the underlying mechanism by which smoking induces degeneration is not known. Recent studies implicate DNA damage as a cause of spine and intervertebral disc degeneration. Because tobacco smoke contains many genotoxins, we hypothesized that tobacco smoking promotes spine degeneration by inducing cellular DNA damage.PurposeTo determine if DNA damage plays a causal role in smoking-induced spine degeneration.Study designTo compare the effect of chronic tobacco smoke inhalation on intervertebral disc and vertebral bone in normal and DNA repair-deficient mice to determine the contribution of DNA damage to degenerative changes.MethodsTwo-month-old wild-type (C57BL/6) and DNA repair-deficient Ercc1^?/Δ mice were exposed to tobacco smoke by direct inhalation (4 cigarettes/day, 5 days/week for 7 weeks) to model first-hand smoking in humans. Total disc proteoglycan (PG) content (1,9-dimethylmethylene blue assay), PG synthesis (³⁵S-sulfate incorporation assay), aggrecan proteolysis (immunoblotting analysis), and vertebral bone morphology (microcomputed tomography) were measured.ResultsExposure of wild-type mice to tobacco smoke led to a 19% increase in vertebral porosity and a 61% decrease in trabecular bone volume. Intervertebral discs of smoke-exposed animals also showed a 2.6-fold decrease in GAG content and an 8.1-fold decrease in new PG synthesis. These smoking-induced degenerative changes were similar but not worse in Ercc1^?/Δ mice.ConclusionsShort-term exposure to high levels of primary tobacco smoke inhalation promotes degeneration of vertebral bone and discs. Disc degeneration is primarily driven by reduced synthesis of proteoglycans needed for vertebral cushioning. Degeneration was not exacerbated in congenic DNA repair-deficient mice, indicating that DNA damage per se does not have a significant causal role in driving smoke-induced spine degeneration.     

Osteoblast‐Derived TGF‐β1 Stimulates IL‐8 Release Through AP‐1 and NF‐κB in Human Cancer Cells

Introduction: The bone marrow microenvironment is further enriched by growth factors released during osteoclastic bone resorption. It has been reported that the chemokine interleukin (IL)‐8 is a potent and direct activator of osteoclastic differentiation and bone resorption. However, the effect of bone‐derived growth factors on the IL‐8 production in human cancer cells and the promotion of osteoclastogenesis are largely unknown. The aim of this study was to investigate whether osteoblast‐derived TGF‐β1 is associated with osteolytic bone diseases. Materials and Methods: IL‐8 mRNA levels were measured using RT‐PCR analysis. MAPK phosphorylation was examined using the Western blot method. siRNA was used to inhibit the expression of TGF‐β1, BMP‐2, and IGF‐1. DNA affinity protein‐binding assay and chromatin immunoprecipitation assays were used to study in vitro and in vivo binding of c‐fos, c‐jun, p65, and p50 to the IL‐8 promoter. A transient transfection protocol was used to examine IL‐8, NF‐κB, and activator protein (AP)‐1 activity. Results: Osteoblast conditioned medium (OBCM) induced activation of IL‐8, AP‐1, and NF‐κB promoter in human cancer cells. Osteoblasts were transfected with TGF‐β1, BMP‐2, or IGF‐1 small interfering RNA, and the medium was collected after 48 h. TGF‐β1 but not BMP‐2 or IGF‐1 siRNA inhibited OBCM‐induced IL‐8 release in human cancer cells. In addition, TGF‐β1 also directly induced IL‐8 release in human cancer cells. Activation of AP‐1 and NF‐κB DNA‐protein binding and MAPKs after TGF‐β1 treatment was shown, and TGF‐β1–induced IL‐8 promoter activity was inhibited by the specific inhibitors of MAPK cascades. Conclusions: In this study, we provide evidence to show that the osteoblasts release growth factors, including TGF‐β1, BMP‐2, and IGF‐1. TGF‐β1 is the major contributor to the activation of extracellular signal‐related kinase (ERK), p38, and c‐Jun N‐terminal kinase (JNK), leading to the activation of AP‐1 and NF‐κB on the IL‐8 promoter and initiation of IL‐8 mRNA and protein release, thereby promoting osteoclastogenesis.     

JNK/c‐Jun Signaling Mediates an Anti‐Apoptotic Effect of RANKL in Osteoclasts

Introduction: RANKL is known to be important not only for differentiation and activation of osteoclasts but also for their survival. Experimentally, apoptosis of osteoclasts is rapidly induced by the deprivation of RANKL. RANKL activates Elk‐related tyrosine kinase (ERK), p38, c‐Jun N‐terminal kinase (JNK), and NF‐κB pathways through TRAF6 in osteoclasts and the precursor cells. It has been shown that ERK is critical for regulation of osteoclast survival. However, an involvement of other RANKL signaling pathways such as JNK signaling in survival of osteoclasts has not been fully understood yet. Materials and Methods: Osteoclasts derived from primary mouse bone marrow cells by soluble RANKL (sRANKL) were treated with a JNK inhibitor, SP600125, or infected with adenovirus carrying dominant‐negative (DN)‐c‐jun, DN‐c‐fos, mitogen‐activated protein kinase kinase 1 (MEKK1), I‐κBα mutant, or NF‐κB components, p50 and p65. Osteoclasts were cultured with or without sRANKL, and apoptotic phenotype was determined by TUNEL assay, DAPI staining, and expression of cleaved caspase 3 followed by TRACP staining. Results: Overexpression of TRAF6 activated JNK and NF‐κB signaling pathways and clearly prevented osteoclasts from apoptosis caused by abrogation of sRANKL. An anti‐apoptotic effect of RANKL/RANK/TRAF6 signaling on osteoclast was inhibited by JNK‐specific inhibitor SP600125 and by overexpression of dominant‐negative JNK1, c‐jun, and c‐fos. Also, overexpression of MEKK1 inhibited apoptosis of osteoclasts even in the absence of sRANKL along with activation of JNK/c‐jun signaling. On the other hand, blockade of NF‐κB signaling by I‐κBα mutant or overexpression of NF‐κB components showed a marginal effect on apoptosis of osteoclasts. Conclusions: An important role of RANKL‐induced activation of MEKK1/JNK/c‐jun signaling in the regulation of apoptosis in osteoclasts was shown. Our study suggests that c‐fos plays a role as a partner of activator protein‐1 factor, c‐jun, during the regulation of apoptosis in osteoclasts.     

IKKβ activation is sufficient for RANK‐independent osteoclast differentiation and osteolysis

Monocytes differentiate into osteoclasts through stimulation of receptor activator of NF‐κB (RANK). Many downstream effectors of RANK play a positive role in osteoclastogenesis, but their relative importance in osteoclast differentiation is unclear. We report the discovery that activation of a single pathway downstream of RANK is sufficient for osteoclast differentiation. In this regard, introduction of constitutively activated IKKβ (IKKβ^SSEE) but not wild‐type IKKβ into monocytes stimulates differentiation of bona fide osteoclasts in the absence of RANK ligand (RANKL). This phenomenon is independent of upstream signals because IKKβ^SSEE induced the development of bone‐resorbing osteoclasts from RANK and IKKα knockout monocytes and in conditions in which NEMO‐IKKβ association was inhibited. NF‐κB p100 and p105, but not RelB, were critical mediators of this effect. Inflammatory autocrine signaling by tumor necrosis factor α (TNF‐α) and interleukin 1 (IL‐1) were dispensable for the spontaneous osteoclastogenesis driven by IKKβ^SSEE. More important, adenoviral gene transfer of IKKβ^SSEE induced osteoclasts and osteolysis in calvariae and knees of mice. Our data establish the sufficiency of IKKβ activation for osteolysis and suggest that IKKβ hyperactivation may play a role in conditions of pathologic bone destruction refractory to RANK/RANKL proximal therapeutic interventions. © 2010 American Society for Bone and Mineral Research     

Decreased osteogenesis in mesenchymal stem cells derived from the aged mouse is associated with enhanced NF‐κB activity

Aging is associated with significant bone loss and delayed fracture healing. NF‐κB activation is highly correlated with inflammatory‐associated bone diseases including infection, wear particle exposure, and chronic inflammation during natural aging processes. The critical roles of NF‐κB in both the pro‐inflammatory response and osteoclast‐mediated bone resorption have been well defined. However, the biological effects of NF‐κB activation in mesenchymal stem cell (MSC)‐mediated bone formation remain largely unknown. In the current study, bone marrow‐MSCs were isolated from young (8 weeks old) and aged (72 weeks old) mice. NF‐κB activity in MSCs at basal levels and under different biological conditions were determined by our recently established lentiviral vector‐based luciferase reporter assay. We found that NF‐κB activity was increased in aged MSCs at basal levels or when exposed to low dose (10 or 100 ng/ml) lipopolysaccharide (LPS); this effect was not seen when the cells were exposed to higher dose (1 μg/ml) LPS. During osteogenesis, NF‐κB activity was increased in aged MSCs at weeks 1 and 2, but showed no significant difference at week 3. Both Smurf2 and TAZ, the NF‐κB target genes that regulate osteogenic differentiation, were increased in aged MSCs. In addition, the expression of RANKL was dramatically increased, and OPG was decreased in aged MSCs. Our findings suggest that targeting NF‐κB activity in MSCs has the potential to modulate aging‐associated bone loss, or enhance bone‐healing in aged patients. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:281–288, 2017.

     

Regulation of bone mass and osteoclast function depend on the F-actin modulator SWAP-70

Bone remodeling involves tightly regulated bone‐resorbing osteoclasts and bone‐forming osteoblasts. Determining osteoclast function is central to understanding bone diseases such as osteoporosis and osteopetrosis. Here, we report a novel function of the F‐actin binding and regulatory protein SWAP‐70 in osteoclast biology. F‐actin ring formation, cell morphology, and bone resorption are impaired in Swap‐70^?/? osteoclasts, whereas the expression of osteoclast differentiation markers induced in vitro by macrophage colony‐stimulating factor (M‐CSF) and receptor activator of NF‐κB ligand (RANKL) remains unaffected. Swap‐70^?/? mice develop osteopetrosis with increased bone mass, abnormally dense bone, and impaired osteoclast function. Ectopic expression of SWAP‐70 in Swap‐70^?/? osteoclasts in vitro rescues their deficiencies in bone resorption and F‐actin ring formation. Rescue requires a functional pleckstrin homology (PH) domain, known to support membrane localization of SWAP‐70, and the F‐actin binding domain. Transplantation of SWAP‐70–proficient bone marrow into Swap‐70^?/? mice restores osteoclast resorption capacity in vivo. The identification of the role of SWAP‐70 in promoting osteoclast function through modulating membrane‐proximal F‐actin rearrangements reveals a new pathway to control osteoclasts and bone homeostasis. © 2012 American Society for Bone and Mineral Research.     

Cherubism Mice Also Deficient in c‐Fos Exhibit Inflammatory Bone Destruction Executed by Macrophages That Express MMP14 Despite the Absence of TRAP+ Osteoclasts

Currently, it is believed that osteoclasts positive for tartrate‐resistant acid phosphatase (TRAP+) are the exclusive bone‐resorbing cells responsible for focal bone destruction in inflammatory arthritis. Recently, a mouse model of cherubism (Sh3bp2^KI/KI) with a homozygous gain‐of‐function mutation in the SH3‐domain binding protein 2 (SH3BP2) was shown to develop auto‐inflammatory joint destruction. Here, we demonstrate that Sh3bp2^KI/KI mice also deficient in the FBJ osteosarcoma oncogene (c‐Fos) still exhibit noticeable bone erosion at the distal tibia even in the absence of osteoclasts at 12 weeks old. Levels of serum collagen I C‐terminal telopeptide (ICTP), a marker of bone resorption generated by matrix metalloproteinases (MMPs), were elevated, whereas levels of serum cross‐linked C‐telopeptide (CTX), another resorption marker produced by cathepsin K, were not increased. Collagenolytic MMP levels were increased in the inflamed joints of the Sh3bp2^KI/KI mice deficient in c‐Fos. Resorption pits contained a large number of F4/80+ macrophages and genetic depletion of macrophages rescued these erosive changes. Importantly, administration of NSC405020, an MMP14 inhibitor targeted to the hemopexin (PEX) domain, suppressed bone erosion in c‐Fos‐deficient Sh3bp2^KI/KI mice. After activation of the NF‐κB pathway, macrophage colony‐stimulating factor (M‐CSF)‐dependent macrophages from c‐Fos‐deficient Sh3bp2^KI/KI mice expressed increased amounts of MMP14 compared with wild‐type macrophages. Interestingly, receptor activator of NF‐κB ligand (RANKL)‐deficient Sh3bp2^KI/KI mice failed to show notable bone erosion, whereas c‐Fos deletion did restore bone erosion to the RANKL‐deficient Sh3bp2^KI/KI mice, suggesting that osteolytic transformation of macrophages requires both loss‐of‐function of c‐Fos and gain‐of‐function of SH3BP2 in this model. These data provide the first genetic evidence that cells other than osteoclasts can cause focal bone destruction in inflammatory bone disease and suggest that MMP14 is a key mediator conferring pathological bone‐resorbing capacity on c‐Fos‐deficient Sh3bp2^KI/KI macrophages. In summary, the paradigm that osteoclasts are the exclusive cells executing inflammatory bone destruction may need to be reevaluated based on our findings with c‐Fos‐deficient cherubism mice lacking osteoclasts. © 2017 American Society for Bone and Mineral Research.