Is RANKL inhibition both anti-resorptive and anabolic in rheumatoid arthritis?

Interleukin-21 knockout reduces bone loss in ovariectomized mice by inhibiting osteoclastogenesis

Estrogen deficiency accelerates osteoporosis in elderly women. However, the role of IL-21 in postmenopausal osteoporosis remains unclear. Female wild-type (WT) C57BL/6 and IL-21 knockout (KO) mice were used for ovariectomy (OVX). Here, IL-21 levels were significantly increased in the serum and bone tissues of WT-OVX mice. The trabecular bone space of the femur was significantly increased, and the bone mass was reduced in OVX mice, accompanied by a significant decrease in the maximum load, energy absorption, and elastic modulus indices. In contrast, IL-21 knockout effectively alleviated the effects of OVX on bone mass. Serum TRACP-5b and receptor activator of nuclear factor kappa B ligand (RANKL) levels and osteoclastogenesis were significantly higher in OVX mice than in sham mice, while serum TRACP-5b and RANKL levels and osteoclastogenesis were significantly decreased in IL-21 KO + OVX mice compared to WT + OVX mice. IL-21 knockdown reduces TRACP-5b, RANKL, and osteoclastogenesis, effectively preventing bone resorption and alleviating the progression of OVX-induced osteoporosis.

Personalized Therapeutic Strategies in the Management of Osteoporosis in Patients with Autoantibody-Positive Rheumatoid Arthritis

Bone mineral density (BMD) reduction and fragility fractures still represent a major source of morbidity in rheumatoid arthritis (RA) patients, despite adequate control of the disease. An increasing number of clinical and experimental evidence supports the role of autoantibodies, especially anti-citrullinated protein antibodies (ACPAs), in causing localized and generalised bone loss in ways that are both dependent on and independent of inflammation and disease activity. The human receptor activator of nuclear factor kappa B and its ligand—the so-called RANK-RANKL pathway—is known to play a key role in promoting osteoclasts’ activation and bone depletion, and RANKL levels were shown to be higher in ACPA-positive early untreated RA patients. Thus, ACPA-positivity can be considered a specific risk factor for systemic and periarticular bone loss. Through the inhibition of the RANK-RANKL system, denosumab is the only antiresorptive drug currently available that exhibits both a systemic anti-osteoporotic activity and a disease-modifying effect when combined with conventional synthetic or biologic disease-modifying anti-rheumatic drugs (DMARDs). Thus, the combination of DMARD and anti-RANKL therapy could be beneficial in the prevention of fragility fractures and structural damage in the subset of RA patients at risk of radiographic progression, as in the presence of ACPAs.

The polypeptide OP3-4 induced osteogenic differentiation of bone marrow mesenchymal stem cells via protein kinase B/glycogen synthase kinase 3β/β-catenin pathway and promoted mandibular defect bone regeneration

OBJECTIVES

The aims of this study were to explore: (ⅰ) the effect of the polypeptide OP 3-4 on bone regeneration in vivo; (ⅱ) the effect of OP 3-4 on osteogenic differentiation of bone marrow mesenchymal stem cells in vitro; and (ⅲ) the potential mechanism of OP 3-4 in promoting osteogenic differentiation of bone marrow mesenchymal stem cells.

DESIGNS

30 Wistar rats (8-week, male) were randomly divided into Control group (n = 5), Hydrogel group (n = 5), and Hydrogel loaded OP 3-4 group (n = 5). Hematoxylin and eosin staining was used to evaluate the level of bone regeneration in mandibular defect. Immunohistochemistry staining was used to evaluate the expression of alkaline phosphatase, runt-related transcription factor 2, and type Ⅰ collagen. Flow cytometry was applied to identify the phenotype of bone marrow mesenchymal stem cells. Furthermore, LY294002, the inhibitor of protein kinase B, was applied to verify the role of OP 3-4 in promoting osteogenic differentiation via protein kinase B/glycogen synthase kinase 3β/β-catenin pathway through western blot.

RESULTS

OP 3-4 promoted bone regeneration of rat mandibular defect. The expression of osteogenic differentiation related markers were increased after adding OP 3-4 to bone marrow mesenchymal stem cells. OP 3-4 promoted osteogenic differentiation of bone marrow mesenchymal stem cells via protein kinase B/glycogen synthase kinase 3β/β-catenin pathway.

CONCLUSION

OP 3-4 could promote bone regeneration of mandibular defect and improve osteogenic differentiation through protein kinase B/glycogen synthase kinase 3β/β-catenin pathway.

uPA/uPAR signaling in rheumatoid arthritis: Shedding light on its mechanism of action

Rheumatoid arthritis (RA) is a systemic and chronic autoimmune inflammatory disorder affecting multiple joints. Various cytokines, chemokines and growth factors synergistically modulate the joint physiology leading to bone erosion and cartilage degradation. Other than these conventional mediators that are well established in the past, the newly identified plasminogen activator (PA) family of proteins have been witnessed to possess a multifactorial approach in mediating RA pathogenesis. One such family of proteins comprises of the urokinase-type plasminogen activator (uPA) and its receptor (uPAR)/soluble-type plasminogen activator receptor (suPAR). PA family of proteins are classified into two types namely: uPA and tissue type plasminogen activator (tPA). Both these subtypes have been implicated to play a key role in RA disease progression. However during RA pathogenesis, uPA secreted by neutrophils, chondrocytes, and monocytes are designated to interact with uPAR expressed on macrophages, fibroblast-like synoviocytes (FLS), chondrocytes and endothelial cells. Interaction of uPA/uPAR promotes the disease progression of RA through secretion of several cytokines, chemokines, growth factors and matrix metalloproteinases (MMPs). Moreover, uPA/uPAR initiates inflammatory responses in macrophages and FLS through activation of PI3K/Akt signaling pathways. Furthermore, uPAR plays a dual role in osteoclastogenesis under the presence/absence of growth factors like monocyte-colony stimulating factor (M-CSF). Overall, this review emphasizes the role of uPA/uPAR on various immune cells, signaling pathways and osteoclastogenesis involved in RA pathogenesis.

The role of osteoblasts in bone metastasis

The primary role of osteoblasts is to lay down new bone during skeletal development and remodelling. Throughout this process osteoblasts directly interact with other cell types within bone, including osteocytes and haematopoietic stem cells. Osteoblastic cells also signal indirectly to bone-resorbing osteoclasts via the secretion of RANKL. Through these mechanisms, cells of the osteoblast lineage help retain the homeostatic balance between bone formation and bone resorption. When tumour cells disseminate in the bone microenvironment, they hijack these mechanisms, homing to osteoblasts and disrupting bone homeostasis. This review describes the role of osteoblasts in normal bone physiology, as well as interactions between tumour cells and osteoblasts during the processes of tumour cell homing to bone, colonisation of this metastatic site and development of overt bone metastases.

Peptide drugs accelerate BMP-2-induced calvarial bone regeneration and stimulate osteoblast differentiation through mTORC1 signaling

Both W9 and OP3‐4 were known to bind the receptor activator of NF‐κB ligand (RANKL), inhibiting osteoclastogenesis. Recently, both peptides were shown to stimulate osteoblast differentiation; however, the mechanism underlying the activity of these peptides remains to be clarified. A primary osteoblast culture showed that rapamycin, an mTORC1 inhibitor, which was recently demonstrated to be an important serine/threonine kinase for bone formation, inhibited the peptide‐induced alkaline phosphatase activity. Furthermore, both peptides promoted the phosphorylation of Akt and S6K1, an upstream molecule of mTORC1 and the effector molecule of mTORC1, respectively. In the in vivo calvarial defect model, W9 and OP3‐4 accelerated BMP‐2‐induced bone formation to a similar extent, which was confirmed by histomorphometric analyses using fluorescence images of undecalcified sections. Our data suggest that these RANKL‐binding peptides could stimulate the mTORC1 activity, which might play a role in the acceleration of BMP‐2‐induced bone regeneration by the RANKL‐binding peptides.