Decreased Quantity and Quality of the Periarticular and Nonperiarticular Bone in Patients With Rheumatoid Arthritis: A Cross‐Sectional HR‐pQCT Study

Rheumatoid arthritis (RA) is a highly bone destructive disease. Although it is well established that RA leads to bone loss and increased fracture risk, current knowledge on the microstructural changes of bone in RA is still limited. The purpose of this study was to assess the microstructure of periarticular and nonperiarticular bone in female and male RA patients and compare it with respective healthy controls. We performed two high‐resolution peripheral quantitative computed tomography (HR‐pQCT; Xtreme‐CT) scans, one of the distal radius and one of the ultradistal radius in 90 patients with RA (60 females, 30 males) and 70 healthy controls (40 females, 30 males) matched for sex, age, and body mass index. Volumetric bone mineral density (vBMD), bone geometry, and bone microstructure including trabecular bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), cortical thickness (Ct.Th) and cortical porosity (Ct.Po) were assessed. At the distal and ultradistal radius, trabecular (p = 0.005 and p < 0.001) and cortical BMD (p < 0.001 and p < 0.001) were significantly decreased in male and female patients with RA, respectively. BV/TV was also decreased at both sites, based on lower Tb.N in female RA (p < 0.001 for both sites) and lower Tb.Th (p = 0.034 and p = 0.005) in male RA patients compared with respective healthy controls. Cortical thinning (p = 0.018 and p = 0.002) but not Ct.Po (p = 0.070 and p = 0.275) was pronounced in male and female RA patients at the distal radius. Cortical perimeter was increased in male and female RA patients at both sites. Multiple regression models showed that bone geometry (cortical perimeter) is predominantly influenced by age of the RA patient, cortical thickness by both age and disease duration, and trabecular microstructure predominantly by the disease duration. In summary, these data show profound deterioration of bone microstructure in the appendicular skeleton of RA patients at both periarticular and nonperiarticular sites. © 2014 American Society for Bone and Mineral Research.     

Age‐ and Sex‐Dependent Changes of Intra‐articular Cortical and Trabecular Bone Structure and the Effects of Rheumatoid Arthritis

The objective of this cross‐sectional study was to define normal sex‐ and age‐dependent values of intra‐articular bone mass and microstructures in the metacarpal heads of healthy individuals by high‐resolution peripheral quantitative computed tomography (HR‐pQCT) and test the effect of rheumatoid arthritis (RA) on these parameters. Human cadaveric metacarpal heads were used to exactly define intra‐articular bone. Healthy individuals of different sex and age categories and RA patients with similar age and sex distribution received HR‐pQCT scans of the second metacarpal head and the radius. Total, cortical, and trabecular bone densities as well as microstructural parameters were compared between 1) the different ages and sexes in healthy individuals; 2) between metacarpal heads and the radius; and 3) between healthy individuals and RA patients. The cadaveric study allowed exact definition of the intra‐articular (intracapsular) bone margins. These data were applied in measuring intra‐articular and radial bone parameters in 214 women and men (108 healthy individuals, 106 RA patients). Correlations between intra‐articular and radial bone parameters were good (r = 0.51 to 0.62, p < 0.001). In contrast to radial bone, intra‐articular bone remained stable until age 60 years (between 297 and 312 mg HA/cm³) but decreased significantly (p < 0.001) in women thereafter (237.5 ± 44.3) with loss of both cortical and trabecular bone. Similarly, RA patients showed significant (p < 0.001) loss of intra‐articular total (263.0 ± 44.8), trabecular (171.2 ± 35.6), and cortical bone (610.2 ± 62.0) compared with sex‐ and age‐adjusted controls. Standard sex‐ and age‐dependent values for physiological intra‐articular bone were defined. Postmenopausal state and RA led to significant decrease of intra‐articular bone. © 2016 American Society for Bone and Mineral Research.     

Alterations of Bone Density,Microstructure, and Strength of the Distal Radius in Male Patients With Rheumatoid Arthritis: A Case‐Control Study With HR‐pQCT

In this cross‐sectional study, we investigated volumetric bone mineral density (vBMD), bone microstructure, and biomechanical competence of the distal radius in male patients with rheumatoid arthritis (RA). The study cohort comprised 50 male RA patients of average age of 61.1 years and 50 age‐matched healthy males. Areal BMD (aBMD) of the hip, lumbar spine, and distal radius was measured by dual‐energy X‐ray absorptiometry. High‐resolution peripheral quantitative computed tomography (HR‐pQCT) of the distal radius provided measures of cortical and trabecular vBMD, microstructure, and biomechanical indices. aBMD of the hip but not the lumbar spine or ultradistal radius was significantly lower in RA patients than controls after adjustment for body weight. Total, cortical, and trabecular vBMD at the distal radius were, on average, –3.9% to –23.2% significantly lower in RA patients, and these differences were not affected by adjustment for body weight, testosterone level, or aBMD at the ultradistal radius. Trabecular microstructure indices were, on average, –8.1% (trabecular number) to 28.7% (trabecular network inhomogeneity) significantly inferior, whereas cortical pore volume and cortical porosity index were, on average, 80.3% and 63.9%, respectively, significantly higher in RA patients. RA patients also had significantly lower whole‐bone stiffness, modulus, and failure load, with lower and more unevenly distributed cortical and trabecular stress. Density and microstructure indices significantly correlated with disease activity, severity, and levels of pro‐inflammatory cytokines (interleukin [IL] 12p70, tumor necrosis factor, IL‐6 and IL‐1β). Ten RA patients had focal periosteal bone apposition most prominent at the ulnovolar aspect of the distal radius. These patients had shorter disease duration and significantly higher cortical porosity. In conclusion, HR‐pQCT reveals significant alterations of bone density, microstructure, and strength of the distal radius in male RA patients and provides new insight into the microstructural basis of bone fragility accompanying chronic inflammation. © 2014 American Society for Bone and Mineral Research.     

Aberrant Activation of TGF‐β in Subchondral Bone at the Onset of Rheumatoid Arthritis Joint Destruction

Rheumatoid arthritis (RA) is an autoimmune disease that often leads to joint destruction. A myriad of drugs targeting the immune abnormalities and downstream inflammatory cascades have been developed, but the joint destruction is not effectively halted. Here we report that aberrant activation of TGF‐β in the subchondral bone marrow by immune response increases osteoprogenitors and uncoupled bone resorption and formation in RA mouse/rat models. Importantly, either systemic or local blockade of TGF‐β activity in the subchondral bone attenuated articular cartilage degeneration in RA. Moreover, conditional deletion of TGF‐β receptor II (Tgfbr2) in nestin‐positive cells also effectively halted progression of RA joint destruction. Our data demonstrate that aberrant activation of TGF‐β in the subchondral bone is involved at the onset of RA joint cartilage degeneration. Thus, modulation of subchondral bone TGF‐β activity could be a potential therapy for RA joint destruction. © 2015 American Society for Bone and Mineral Research     

Milk Fat Globule‐Epidermal Growth Factor 8 (MFG‐E8) Is a Novel Anti‐inflammatory Factor in Rheumatoid Arthritis in Mice and Humans

Milk fat globule‐epidermal growth factor 8 (MFG‐E8) is an anti‐inflammatory glycoprotein that mediates the clearance of apoptotic cells and is implicated in the pathogenesis of autoimmune and inflammatory diseases. Because MFG‐E8 also controls bone metabolism, we investigated its role in rheumatoid arthritis (RA), focusing on inflammation and joint destruction. The regulation of MFG‐E8 by inflammation was assessed in vitro using osteoblasts, in arthritic mice and in patients with RA. K/BxN serum transfer arthritis (STA) was applied to MFG‐E8 knock‐out mice to assess its role in the pathogenesis of arthritis. Stimulation of osteoblasts with lipopolysaccharide (LPS) and tumor necrosis factor (TNF)‐α downregulated the expression of MFG‐E8 by 30% to 35%. MFG‐E8‐deficient osteoblasts responded to LPS with a stronger production of pro‐inflammatory cytokines. In vivo, MFG‐E8 mRNA levels were 52% lower in the paws of collagen‐induced arthritic (CIA) mice and 24% to 42% lower in the serum of arthritic mice using two different arthritis models (CIA and STA). Similarly, patients with RA (n = 93) had lower serum concentrations of MFG‐E8 (–17%) compared with healthy controls (n = 140). In a subgroup of patients who had a moderate to high disease activity (n = 21), serum concentrations of MFG‐E8 rose after complete or partial remission had been achieved (+67%). Finally, MFG‐E8‐deficient mice subjected to STA exhibited a stronger disease burden, an increased number of neutrophils in the joints, and a more extensive local and systemic bone loss. This was accompanied by an increased activation of osteoclasts and a suppression of osteoblast function in MFG‐E8‐deficient mice. Thus, MFG‐E8 is a protective factor in the pathogenesis of RA and subsequent bone loss. Whether MFG‐E8 qualifies as a novel biomarker or therapeutic target for the treatment of RA is worth addressing in further studies. © 2015 American Society for Bone and Mineral Research.     

TNF-Polarized Macrophages Produce Insulin-like 6 Peptide to Stimulate Bone Formation in Rheumatoid Arthritis in Mice

The risk of osteoporosis is increased in rheumatoid arthritis (RA). Anti-tumor necrosis factor (TNF) therapy has markedly improved the outcomes of RA patients but does not improve osteoporosis in some reports. This could be a combined result of disease severity and other therapeutic agents, such as glucocorticoids that accelerate osteoporosis progression. We evaluated the effects of anti-TNF therapy on osteoporosis in an animal model of RA and explored the possible mechanisms involved. Six-week-old TNF transgenic (TNF-Tg) mice with early stage erosive arthritis were treated with TNF antibody (Ab) or control immunoglobulin (IgG) weekly for 4 weeks. We found that TNF Ab completely blocked the development of erosive arthritis in TNF-Tg mice, but only slightly increased vertebral bone mass, associated with reduction in parameters of both bone resorption and formation. Similarly, TNF Ab slightly increased trabecular bone mass in tibias of 8-month-old TNF-Tg mice with advanced erosive arthritis. Interestingly, TNFα increased osteoblast differentiation from mouse bone marrow stromal cells (BMSCs) containing large number of macrophages but not from pure mesenchymal progenitor cells (MPCs). TNFα-polarized macrophages (TPMs) did not express iNos and Arginase 1, typical markers of inflammatory and resident macrophages. Interestingly, TPMs stimulated osteoblast differentiation, unlike resident and inflammatory macrophages polarized by IL-4 and interferon-λ, respectively. RNA-seq analysis indicated that TPMs produced several anabolic factors, including Jagged1 and insulin like 6 (INSL6). Importantly, inhibition of either Jagged1 or INSL6 blocked TNFα-induced osteoblast differentiation. Furthermore, INSL6 Ab significantly decreased the expansion of TNF-induced MPCs in BMSCs, and anti-TNF Ab reduced INSL6 expression by macrophages in vitro and in TNF-Tg mice in vivo. We conclude that TPMs produce INSL6 to stimulate bone formation and anti-TNF Ab blocks not only enhanced bone resorption but also the anabolic effect of TPMs on bone, limiting its effect to increase bone mass in this model of RA. © 2021 American Society for Bone and Mineral Research (ASBMR).     

Shared Genetic Architecture Between Rheumatoid Arthritis and Varying Osteoporotic Phenotypes

Rheumatoid arthritis (RA) and low bone mineral density (BMD), an indicator of osteoporosis (OP), appear epidemiologically associated. Shared genetic factors may explain this association. This study aimed to investigate the presence of pleiotropy to clarify the potential genetic association between RA and OP. We examined BMDs at varying skeletal sites reported in UK Biobank as well as OP fracture acquired from the Genetic Factors for Osteoporosis (GEFOS) Consortium and the TwinsUK study. PRSice-2 was used to assess the potential shared genetic overlap between RA and OP. The presence of pleiotropy was examined using colocalization analysis. PRSice-2 revealed that RA was significantly associated with OP fracture (β = 351.6 ± 83.9, p value = 2.76E-05), total BMD (β = −1763.5 ± 612.8, p = 4.00E-03), spine BMD (β = −919.8 ± 264.6, p value = 5.09E-04), and forearm BMD (β = −66.09 ± 31.40, p value = 3.53E-02). Through colocalization analysis, the same causal genetic variants, associated with both RA and OP, were apparent in 12 genes: PLCL1, BOLL, AC011997.1, TNFAIP3, RP11-158I9.1, CDK6, CHCHD4P2, RP11-505C13.1, PHF19, TRAF1, C5, and C11orf49 with moderate posterior probabilities (>50%). Pleiotropy is involved in the association between RA and OP phenotypes. These findings contribute to the understanding of disease mechanisms and provide insight into possible therapeutic advancements and enhanced screening measures. © 2021 American Society for Bone and Mineral Research (ASBMR).     

Periarticular Bone Loss in Arthritis Is Induced by Autoantibodies Against Citrullinated Vimentin

Periarticular bone loss is a long known but yet insufficiently understood phenomenon in patients with rheumatoid arthritis. This study investigated whether autoimmunity against citrullinated proteins is causally involved in triggering periarticular bone loss. Periarticular bone loss was studied in the standard antigen‐induced arthritis (AIA) mouse model with methylated bovine serum albumin (mBSA) as well as a modified model with mutated citrullinated vimentin (MCV) alone or in combination with mBSA. Periarticular bone loss, subchondral osteoclastogenesis, as well as local expression of cytokines, osteoclast genes, and peptidyl‐arginine deiminase (PAD) enzymes were assessed after arthritis induction. Immune cell and osteoclast precursor infiltration were detected in the periarticular bone marrow and local lymph nodes. In addition, periarticular bone loss was assessed upon challenge of mice with purified anti‐MCV antibody. Despite inducing a milder form of arthritis than mBSA, MCV triggered significant periarticular bone loss associated with an increased infiltration of osteoclast precursors and mature osteoclasts in the periarticular bone marrow. MCV enhanced the expression of the osteoclast inducers RANKL and M‐CSF, the cytokines IL‐8, IL‐1, IL‐6, and TNF‐α, as well as PAD2 and PAD4 enzymes in the periarticular bone marrow. Furthermore, also anti‐MCV antibody challenge induced significant periarticular bone loss and local osteoclastogenesis in the mice. Autoimmunity against citrullinated vimentin triggers periarticular bone loss by osteoclast activation in the bone marrow. These findings may explain why periarticular bone loss is already found very early in the disease course of patients with rheumatoid arthritis. © 2017 American Society for Bone and Mineral Research.     

Validation of Power Doppler Versus Contrast‐Enhanced Magnetic Resonance Imaging Quantification of Joint Inflammation in Murine Inflammatory Arthritis

Contrast‐enhancement magnetic resonance imaging (CE‐MRI) of synovial volume is the radiographic gold standard to quantify joint inflammation; however, cost limits its use. Therefore, we examined if power Doppler‐ultrasound (PD‐US) outcomes of synovitis in tumor necrosis factor transgenic (TNF‐Tg) mice correlate with CE‐MRI. TNF‐Tg mice underwent PD‐US of their knees to measure the joint space volume (JSV) and power Doppler volume (PDV), and the results were correlated with synovial volume determined by CE‐MRI. Immunohistochemistry for CD31 was performed to corroborate the PD signal. Synovial volume strongly correlated with both JSV and PDV (p < 0.01). CD31⁺ blood vessels were observed in inflamed synovium proximal to the joint surface, which corresponded to areas of intense PD signals. JSV and PDV are valid measures of joint inflammation that correlate with synovial volume determined by CE‐MRI and are associated with vascularity. Given the emergence of PD‐US as a nonquantitative outcome of joint inflammation, we find JSV and PDV to be feasible and highly cost‐effective for longitudinal studies in animal models. Furthermore, given the increasing use of PD‐US in standard clinical practice, JSV and PDV could be translated to better quantify joint flare and response to therapy in patients with rheumatoid arthritis (RA). © 2014 American Society for Bone and Mineral Research.     

Combined Collagen-Induced Arthritis and Organic Dust-Induced Airway Inflammation to Model Inflammatory Lung Disease in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is characterized by extra-articular involvement including lung disease, yet the mechanisms linking the two conditions are poorly understood. The collagen-induced arthritis (CIA) model was combined with the organic dust extract (ODE) airway inflammatory model to assess bone/joint–lung inflammatory outcomes. DBA/1J mice were intranasally treated with saline or ODE daily for 5 weeks. CIA was induced on days 1 and 21. Treatment groups included sham (saline injection/saline inhalation), CIA (CIA/saline), ODE (saline/ODE), and CIA + ODE (CIA/ODE). Arthritis inflammatory scores, bones, bronchoalveolar lavage fluid, lung tissues, and serum were assessed. In DBA/1J male mice, arthritis was increased in CIA + ODE > CIA > ODE versus sham. Micro-computed tomography (µCT) demonstrated that loss of BMD and volume and deterioration of bone microarchitecture was greatest in CIA + ODE. However, ODE-induced airway neutrophil influx and inflammatory cytokine/chemokine levels in lavage fluids were increased in ODE > CIA + ODE versus sham. Activated lung CD11c⁺CD11b⁺ macrophages were increased in ODE > CIA + ODE > CIA pattern, whereas lung hyaluronan, fibronectin, and amphiregulin levels were greatest in CIA + ODE. Serum autoantibody and inflammatory marker concentrations varied among experimental groups. Compared with male mice, female mice showed less articular and pulmonary disease. The interaction of inhalation-induced airway inflammation and arthritis induction resulted in compartmentalized responses with the greatest degree of arthritis and bone loss in male mice with combined exposures. Data also support suppression of the lung inflammatory response, but increases in extracellular matrix protein deposition/interstitial disease in the setting of arthritis. This coexposure model could be exploited to better understand and treat RA–lung disease. © 2019 American Society for Bone and Mineral Research.     

Quantitative and Qualitative Changes of Bone in Psoriasis and Psoriatic Arthritis Patients

Psoriatic arthritis (PsA) is a chronic inflammatory disease characterized by periarticular bone loss and new bone formation. Current data regarding systemic bone loss and bone mineral density (BMD) in PsA are conflicting. The aim of this study was to evaluate bone microstructure and volumetric BMD (vBMD) in patients with PsA and psoriasis. We performed HR‐pQCT scans at the ultradistal and periarticular radius in 50 PsA patients, 30 psoriasis patients, and 70 healthy, age‐ and sex‐related controls assessing trabecular bone volume (BV/TV), trabecular number (Tb.N), inhomogeneity of the trabecular network, cortical thickness (Ct.Th), and cortical porosity (Ct.Po), as well as vBMD. Trabecular BMD (Tb.BMD, p = 0.021, 12.0%), BV/TV (p = 0.020, –11.9%), and Tb.N (p = 0.035, 7.1%) were significantly decreased at the ultradistal radius and the periarticular radius in PsA patients compared to controls. In contrast, bone architecture of the ultradistal radius and periarticular radius was similar in patients with psoriasis and healthy controls. Duration of skin disease was associated with low BV/TV and Tb.N in patients with PsA. These data suggest that trabecular BMD and bone microstructure are decreased in PsA patients. The observation that duration of skin disease determines bone loss in PsA supports the concept of subclinical musculoskeletal disease in psoriasis patients. © 2015 American Society for Bone and Mineral Research.     

Direct Crosstalk Between Cancer and Osteoblast Lineage Cells Fuels Metastatic Growth in Bone via Auto‐Amplification of IL‐6 and RANKL Signaling Pathways

The bone microenvironment and its modification by cancer and host cell interactions is a key driver of skeletal metastatic growth. Interleukin‐6 (IL‐6) stimulates receptor activator of NF‐κB ligand (RANKL) expression in bone cells, and serum IL‐6 levels are associated with poor clinical outcomes in cancer patients. We investigated the effects of RANKL on cancer cells and the role of tumor‐derived IL‐6 within the bone microenvironment. Using human breast cancer cell lines to induce tumors in the bone of immune‐deficient mice, we first determined whether RANKL released by cells of the osteoblast lineage directly promotes IL‐6 expression by cancer cells in vitro and in vivo. We then disrupted of IL‐6 signaling in vivo either via knockdown of IL‐6 in tumor cells or through treatment with specific anti‐human or anti‐mouse IL‐6 receptor antibodies to investigate the tumor effect. Finally, we tested the effect of RANK knockdown in cancer cells on cancer growth. We demonstrate that osteoblast lineage‐derived RANKL upregulates secretion of IL‐6 by breast cancers in vivo and in vitro. IL‐6, in turn, induces expression of RANK by cancer cells, which sensitizes the tumor to RANKL and significantly enhances cancer IL‐6 release. Disruption in vivo of this auto‐amplifying crosstalk by knockdown of IL‐6 or RANK in cancer cells, or via treatment with anti‐IL‐6 receptor antibodies, significantly reduces tumor growth in bone but not in soft tissues. RANKL and IL‐6 mediate direct paracrine‐autocrine signaling between cells of the osteoblast lineage and cancer cells, significantly enhancing the growth of metastatic breast cancers within bone. © 2014 American Society for Bone and Mineral Research.     

Structure and strength of the distal radius in female patients with rheumatoid arthritis: A case‐control study

The purpose of this work was to investigate the volumetric bone mineral density (vBMD), bone microstructure, and mechanical indices of the distal radius in female patients with rheumatoid arthritis (RA). We report a cross‐sectional study of 66 middle‐aged female RA patients and 66 age‐matched healthy females. Areal BMD (aBMD) of the hip, lumbar spine, and distal radius was measured by dual‐energy X‐ray absorptiometry (DXA). High‐resolution peripheral quantitative computed tomography (HR‐pQCT) was performed at the distal radius, yielding vBMD, bone microstructure, and mechanical indices. Cortical and trabecular vBMD were 3.5% and 10.7% lower, respectively, in RA patients than controls, despite comparable aBMD. Trabecular microstructural indices were –5.7% to –23.1% inferior, respectively, in RA patients compared to controls, with significant differences in trabecular bone volume fraction, separation, inhomogeneity, and structural model index. Cortical porosity volume and percentage were 128% and 93% higher, respectively, in RA patients, with stress being distributed more unevenly. Fourteen RA patients had exaggerated periosteal bone apposition primarily affecting the ulnovolar aspect of the distal radius. These particular patients were more likely to have chronic and severe disease and coexisting wrist deformity. The majority of the differences in density and microstructure between RA patients and controls did not depend on menstrual status. Recent exposure to glucocorticoids did not significantly affect bone density and microstructure. HR‐pQCT provides new insight into inflammation‐associated bone fragility in RA. It detects differences in vBMD, bone microstructure, and mechanical indices that are not captured by DXA. At the distal radius, deterioration in density and microstructure in RA patients involved both cortical and trabecular compartments. Excessive bone resorption appears to affect cortical more than trabecular bone at distal radius, particularly manifested as increased cortical porosity. Ulnovolar periosteal apposition of the distal radius is a feature of chronic, severe RA with wrist deformity. © 2013 American Society for Bone and Mineral Research.     

Rheumatoid Arthritis Exacerbates the Severity of Osteonecrosis of the Jaws (ONJ) in Mice. A Randomized,Prospective, Controlled Animal Study

Rheumatoid arthritis (RA), an autoimmune inflammatory disorder, results in persistent synovitis with severe bone and cartilage destruction. Bisphosphonates (BPs) are often utilized in RA patients to reduce bone destruction and manage osteoporosis. However, BPs, especially at high doses, are associated with osteonecrosis of the jaw (ONJ). Here, utilizing previously published ONJ animal models, we are exploring interactions between RA and ONJ incidence and severity. DBA1/J mice were divided into four groups: control, zoledronic acid (ZA), collagen‐induced arthritis (CIA), and CIA‐ZA. Animals were pretreated with vehicle or ZA. Bovine collagen II emulsified in Freund's adjuvant was injected to induce arthritis (CIA) and the mandibular molar crowns were drilled to induce periapical disease. Vehicle or ZA treatment continued for 8 weeks. ONJ indices were measured by micro‐CT (µCT) and histological examination of maxillae and mandibles. Arthritis development was assessed by visual scoring of paw swelling, and by µCT and histology of interphalangeal and knee joints. Maxillae and mandibles of control and CIA mice showed bone loss, periodontal ligament (PDL) space widening, lamina dura loss, and cortex thinning. ZA prevented these changes in both ZA and CIA‐ZA groups. Epithelial to alveolar crest distance was increased in the control and CIA mice. This distance was preserved in ZA and CIA‐ZA animals. Empty osteocytic lacunae and areas of osteonecrosis were present in ZA and CIA‐ZA but more extensively in CIA‐ZA animals, indicating more severe ONJ. CIA and CIA‐ZA groups developed severe arthritis in the paws and knees. Interphalangeal and knee joints of CIA mice showed advanced bone destruction with cortical erosions and trabecular bone loss, and ZA treatment reduced these effects. Importantly, no osteonecrosis was noted adjacent to areas of articular inflammation in CIA‐ZA mice. Our data suggest that ONJ burden was more pronounced in ZA treated CIA mice and that RA could be a risk factor for ONJ development. © 2016 American Society for Bone and Mineral Research.     

Class I PI‐3‐Kinase Signaling Is Critical for Bone Formation Through Regulation of SMAD1 Activity in Osteoblasts

Bone formation and homeostasis is carried out by osteoblasts, whose differentiation and activity are regulated by osteogenic signaling networks. A central mediator of these inputs is the lipid kinase phosphatidylinositol 3‐kinase (PI3K). However, at present, there are no data on the specific role of distinct class IA PI3K isoforms in bone biology. Here, we performed osteoblast‐specific deletion in mice to show that both p110α and p110β isoforms are required for survival and differentiation and function of osteoblasts and thereby control bone formation and postnatal homeostasis. Impaired osteogenesis arises from increased GSK3 activity and a depletion of SMAD1 protein levels in PI3K‐deficient osteoblasts. Accordingly, pharmacological inhibition of GSK3 activity or ectopic expression of SMAD1 or SMAD5 normalizes bone morphogenetic protein (BMP) transduction and osteoblast differentiation. Together, these results identify the PI3K‐GSK3‐SMAD1 axis as a central node integrating multiple signaling networks that govern bone formation and homeostasis. © 2016 American Society for Bone and Mineral Research.     

Increased EPO Levels Are Associated With Bone Loss in Mice Lacking PHD2 in EPO‐Producing Cells

The main oxygen sensor hypoxia inducible factor (HIF) prolyl hydroxylase 2 (PHD2) is a critical regulator of tissue homeostasis during erythropoiesis, hematopoietic stem cell maintenance, and wound healing. Recent studies point toward a role for the PHD2‐erythropoietin (EPO) axis in the modulation of bone remodeling, even though the studies produced conflicting results. Here, we used a number of mouse strains deficient of PHD2 in different cell types to address the role of PHD2 and its downstream targets HIF‐1α and HIF‐2α in bone remodeling. Mice deficient for PHD2 in several cell lineages, including EPO‐producing cells, osteoblasts, and hematopoietic cells (CD68:cre‐PHD2^f/f) displayed a severe reduction of bone density at the distal femur as well as the vertebral body due to impaired bone formation but not bone resorption. Importantly, using osteoblast‐specific (Osx:cre‐PHD2^f/f) and osteoclast‐specific PHD2 knock‐out mice (Vav:cre‐ PHD2^f/f), we show that this effect is independent of the loss of PHD2 in osteoblast and osteoclasts. Using different in vivo and in vitro approaches, we show here that this bone phenotype, including the suppression of bone formation, is directly linked to the stabilization of the α‐subunit of HIF‐2, and possibly to the subsequent moderate induction of serum EPO, which directly influenced the differentiation and mineralization of osteoblast progenitors resulting in lower bone density. Taken together, our data identify the PHD2:HIF‐2α:EPO axis as a so far unknown regulator of osteohematology by controlling bone homeostasis. Further, these data suggest that patients treated with PHD inhibitors or EPO should be monitored with respect to their bone status. © 2016 American Society for Bone and Mineral Research.