NIK Stabilization in Osteoclasts Results in Osteoporosis and Enhanced Inflammatory Osteolysis

Maintenance of healthy bone requires the balanced activities of osteoclasts (OCs), which resorb bone, and osteoblasts, which build bone. Disproportionate action of OCs is responsible for the bone loss associated with postmenopausal osteoporosis and rheumatoid arthritis. NF-κB inducing kinase (NIK) controls activation of the alternative NF-κB pathway, a critical pathway for OC differentiation. Under basal conditions, TRAF3-mediated NIK degradation prevents downstream signaling, and disruption of the NIK:TRAF3 interaction stabilizes NIK leading to constitutive activation of the alternative NF-κB pathway.Using transgenic mice with OC-lineage expression of NIK lacking its TRAF3 binding domain (NT3), we now find that alternative NF-κB activation enhances not only OC differentiation but also OC function. Activating NT3 with either lysozyme M Cre or cathepsinK Cre causes high turnover osteoporosis with increased activity of OCs and osteoblasts. In vitro, NT3-expressing precursors form OCs more quickly and at lower doses of RANKL. When cultured on bone, they exhibit larger actin rings and increased resorptive activity. OC-specific NT3 transgenic mice also have an exaggerated osteolytic response to the serum transfer model of arthritis.Constitutive activation of NIK drives enhanced osteoclastogenesis and bone resorption, both in basal conditions and in response to inflammatory stimuli

Geraniol attenuates osteoclast differentiation by suppressingNF-kB activity and expression of osteoclastogenic genes

Osteoporotic patients have lower bone mass due to increased bone resorption by osteoclasts. The aim of this study was to investigate the cytotoxic and antiosteoclastogenic effects of geraniol, a natural monoterpene on human CD14+ monocytes (ex vivo) and murine RAW264.7 macrophages (in vitro) using alamar blue and tartrate resistant acid phosphatase staining respectively. The anti-osteoclastogenic activity of geraniol was further explored by analyzing its effects on actin ring formation and bone resorptive function of osteoclasts. Geraniol significantly (p < 0.001) inhibited osteoclast formation in CD14+ monocytes and RAW264.7 macrophages without cytotoxicity. Moreover, reduced osteoclastogenesis in these cells led to an arrest in actin ring formation and diminished bone resorption. Analysis of underlying molecular mechanisms revealed that geraniol alleviated NF-kB activity, an indispensable upstream modulator of osteoclast formation. Furthermore, expression of key osteoclastogenic genes such as dendritic cell-specific transmembrane protein (DC-STAMP) involved in cell-cell fusion and nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1), a master transcription factor essential for osteoclast differentiation was downregulated by geraniol. These observations indicate that inhibition of osteoclast differentiation is presumably one of the pharmacological properties of geraniol.Grants from the University of Pretoria Vice Chancellor’s Postdoctoral Research Fellowship; RESCOM, University of Pretoria and the University of Pretoria’s Strategic Institutional Research Theme in Food, Nutrition and Well-being.http://link.springer.com/journal/442017-12-31hb2016Food ScienceHuman NutritionPhysiolog

PLCγ2 regulates osteoclastogenesis via its interaction with ITAM proteins and GAB2

Excessive bone loss in arthritic diseases is mostly due to abnormal activation of the immune system leading to stimulation of osteoclasts. While phospholipase Cγ (PLCγ) isoforms are known modulators of T and B lymphocyte–mediated immune responses, we found that blockade of PLCγ enzymatic activity also blocks early osteoclast development and function. Importantly, targeted deletion of Plcg2 in mice led to an osteopetrotic phenotype. PLCγ2, independent of PLCγ1, was required for receptor activator of NF-κB ligand–induced (RANKL-induced) osteoclastogenesis by differentially regulating nuclear factor of activated T cells c1 (NFATc1), activator protein–1 (AP1), and NF-κB. Specifically, we show that NFATc1 upregulation is dependent on RANKL-mediated phosphorylation of PLCγ2 downstream of Dap12/Fc receptor γ (Dap12/FcRγ) receptors and is blocked by the PLCγ inhibitor U73122. In contrast, activation of JNK and NF-κB was not affected by U73122 or Dap12/FcRγ deletion. Interestingly, we found that in osteoclasts, PLCγ2 formed a complex with the regulatory adapter molecule GAB2, was required for GAB2 phosphorylation, and modulated GAB2 recruitment to RANK. Thus, PLCγ2 mediates RANKL-induced osteoclastogenesis and is a potential candidate for antiresorptive therapy