Compromised autophagy and mitochondrial dysfunction downregulate chondrocytic activity, accelerating the development of osteoarthritis (OA). Irisin, a cleaved form of fibronectin type III domain containing 5 (FNDC5), regulates bone turnover and muscle homeostasis. Little is known about the effect of Irisin on chondrocytes and the development of osteoarthritis. This study revealed that human osteoarthritic articular chondrocytes express decreased level of FNDC5 and autophagosome marker LC3-II but upregulated levels of oxidative DNA damage marker 8-hydroxydeoxyguanosine (8-OHdG) and apoptosis. Intra-articular administration of Irisin further alleviated symptoms of medial meniscus destabilization, like cartilage erosion and synovitis, while improved the gait profiles of the injured legs. Irisin treatment upregulated autophagy, 8-OHdG and apoptosis in chondrocytes of the injured cartilage. In vitro, Irisin improved IL-1β-mediated growth inhibition, loss of specific cartilage markers and glycosaminoglycan production by chondrocytes. Irisin also reversed Sirt3 and UCP-1 pathways, thereby improving mitochondrial membrane potential, ATP production, and catalase to attenuated IL-1β-mediated reactive oxygen radical production, mitochondrial fusion, mitophagy, and autophagosome formation. Taken together, FNDC5 loss in chondrocytes is correlated with human knee OA. Irisin repressed inflammation-mediated oxidative stress and extracellular matrix underproduction through retaining mitochondrial biogenesis, dynamics and autophagic program. Our analyses shed new light on the chondroprotective actions of this myokine, and highlight the remedial effects of Irisin on OA development.

Long-term glucocorticoid treatment impairs the survival and bone formation of osteogenic cells, leading to bone mass loss. The Wnt inhibitor Dickkopf-1 (DKK1) acts as a potent bone-remodeling factor that mediates several types of skeletal disorders. Whereas excess glucocorticoid is known to disturb Wnt signaling in osteogenic cells, modulation of the skeletally deleterious effects of DKK1 to alleviate glucocorticoid induction of bone loss has not been tested. In this study, knockdown of DKK1 expression by end-capped phosphorothioate DKK1 antisense oligonucleotide (DKK1-AS) abrogated dexamethasone suppression of alkaline phosphatase activity and osteocalcin expression in MC3T3-E1 preosteoblasts. Exogenous DKK1-AS treatment alleviated dexamethasone suppression of mineral density, trabecular bone volume, osteoblast surface, and bone formation rate in bone tissue and ex vivo osteogenesis of primary bone-marrow mesenchymal cells. The DKK1-AS inhibited adipocyte volume in the marrow cavity of steroid-treated bone tissue. Immunohistochemical observation revealed that DKK1-AS abrogated dexamethasone-induced DKK1 expression and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling of osteoblasts adjacent to trabecular bone. Knocking down DKK1 abrogated dexamethasone-modulated expression of nuclear β-catenin and phosphorylated Ser473-Akt and survival of osteoblasts and adipocytic differentiation of mesenchymal progenitor cell cultures. Taken together, knocking down DKK1 alleviated the deleterious effect of glucocorticoid on bone microstructure. The DKK1-AS treatment appeared to protect bone tissue by modulating β-catenin and Akt-mediated survival as well as the osteogenic and adipogenic activities of glucocorticoid-stressed osteoprogenitor cells. Interference with the osteogenesis-inhibitory action of DKK1 has therapeutic potential for preventing glucocorticoid induction of osteopenia.

Objective: Excessive glucocorticoid treatment increases the incidence of osteopenia and osteonecrosis. MicroRNAs (miRNAs) reportedly target messenger RNA expression and regulate osteoblastogenesis and skeletal development. We undertook this study to investigate whether miR‐29a regulates glucocorticoid‐mediated bone loss.Methods: Rats were given methylprednisolone, lentivirus‐mediated miR‐29a precursor, or lentivirus‐mediated miR‐29a inhibitor. Dual x‐ray absorptiometry, micro–computed tomography, material testing, and enzyme‐linked immunosorbent assay were performed to quantify bone mass, microarchitecture, peak load, and serum Dkk‐1 levels. Differential miRNA expression profiles were detected using polymerase chain reaction arrays. The abundance of signaling molecules was assessed using immunoblotting.Results: Glucocorticoid treatment induced loss of bone mineral density and trabecular microstructure in association with reduced miR‐29a expression. Treatment with miR‐29a precursor attenuated the adverse effects of glucocorticoid on bone mass, trabecular bone volume fraction, and biomechanical load‐bearing capacity of bone tissue. Gain of miR‐29a function alleviated the detrimental effects of glucocorticoid treatment on mineral acquisition and ex vivo osteoblast differentiation, and also reduced osteoclast surface, ex vivo osteoclast differentiation, and RANKL expression in bone microenvironments. Knockdown of miR‐29a accelerated osteoclast resorption, cortical bone porosity, bone fragility, and loss of ex vivo osteogenic differentiation capacity. MicroRNA‐29a regulated the abundance of Wnt signaling components (Wnt‐3a, glycogen synthase kinase 3β, and β‐catenin), the Wnt inhibitor Dkk‐1, Akt, and phosphorylated ERK, and the expression of the osteogenic factors RUNX‐2 and insulin‐like growth factor 1 in bone tissue.Conclusion: MicroRNA‐29a signaling protected against glucocorticoid‐induced disturbance of Wnt and Dkk‐1 actions and improved osteoblast differentiation and mineral acquisition. Promotion of miR‐29a signaling is an alternative strategy for alleviating glucocorticoid‐induced bone deterioration.

We evaluated whether proinflammatory cytokine expression and myofibroblast recruitment in subacromial bursa was linked to rotator cuff lesions with shoulder stiffness. We analyzed expressions of IL‐1β, IL‐6, and TNF‐α in subacromial bursa and joint fluid collected from 14 patients with cuff tears with stiffness as a study group (Group I) and 14 patients with rotator cuff tears without shoulder stiffness as a control group (Group II) using real‐time RT‐PCR, immunohistochemistry, and ELISA. Myofibroblast apoptosis in subacromial bursa was analyzed using terminal deoxynucleotidyl transferase ‐mediated deoxyuridine triphosphate‐biotin nick end‐labeling (TUNEL) and α‐smooth muscle actin immunofluorescence staining. Shoulder function was evaluated using the Constant score. Group I had higher mRNA expression (p < 0.001) and immunoreactivities (p < 0.001) of IL‐1β. They also had higher levels of IL‐1β, IL‐6, and TNF‐α in joint fluid. Increased IL‐1β mRNA expression in the subacromial bursa and IL‐1β levels in joint fluid were correlated with a preoperative deficit in shoulder motion (p < 0.001) and preoperative Constant scores (p < 0.001). Immunofluorescence observations showed that Group I subjects had more myofibroblasts (p < 0.001) than Group II. In Group II, a significant correlation was found between apoptotic myofibroblasts and total myofibroblasts (p = 0.002), but not in Group I (p = 0.510). Increased expression of IL‐1β and myofibroblast recruitment in the subacromial bursa in rotator cuff lesions are linked to shoulder stiffness. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:1090–1097, 2008

Bone-forming cells build mineralized microstructure and couple with bone-resorbing cells, harmonizing bone mineral acquisition, and remodeling to maintain bone mass homeostasis. Mitochondrial glycolysis and oxidative phosphorylation pathways together with ROS generation meet the energy requirement for bone-forming cell growth and differentiation, respectively. Moderate mechanical stimulations, such as weight loading, physical activity, ultrasound, vibration, and electromagnetic field stimulation, etc., are advantageous to bone-forming cell activity, promoting bone anabolism to compromise osteoporosis development. A plethora of molecules, including ion channels, integrins, focal adhesion kinases, and myokines, are mechanosensitive and transduce mechanical stimuli into intercellular signaling, regulating growth, mineralized extracellular matrix biosynthesis, and resorption. Mechanical stimulation changes mitochondrial respiration, biogenesis, dynamics, calcium influx, and redox, whereas mechanical disuse induces mitochondrial dysfunction and oxidative stress, which aggravates bone-forming cell apoptosis, senescence, and dysfunction. The control of the mitochondrial biogenesis activator PGC-1α by NAD+-dependent deacetylase sirtuins or myokine FNDC/irisin or repression of oxidative stress by mitochondrial antioxidant Nrf2 modulates the biophysical stimulation for the promotion of bone integrity. This review sheds light onto the roles of mechanosensitive signaling, mitochondrial dynamics, and antioxidants in mediating the anabolic effects of biophysical stimulation to bone tissue and highlights the remedial potential of mitochondrial biogenesis regulators for osteoporosis.