Abstract
Tumor necrosis factor-α (TNF) enhances osteoclast formation and activity leading to bone loss in various pathological conditions, but its precise role in osteoclastogenesis remains controversial. Although several groups showed that TNF can promote osteoclastogenesis independently of the receptor activator of NF-κB (RANK) ligand (RANKL), others demonstrated that TNF-mediated osteoclastogenesis needs permissive levels of RANKL. Here, we independently reveal that although TNF cannot stimulate osteoclastogenesis on bone slices, it can induce the formation of functional osteoclasts on bone slices in the presence of permissive levels of RANKL or from bone marrow macrophages (BMMs) pretreated by RANKL. TNF can still promote the formation of functional osteoclasts 2 days after transient RANKL pretreatment. These data have confirmed that TNF-mediated osteoclastogenesis requires priming of BMMs by RANKL. Moreover, we investigated the molecular mechanism underlying the dependence of TNF-mediated osteoclastogenesis on RANKL. RANK, the receptor for RANKL, contains an IVVY(535-538) motif that has been shown to play a vital role in osteoclastogenesis by committing BMMs to the osteoclast lineage. We show that TNF-induced osteoclastogenesis depends on RANKL to commit BMMs to the osteoclast lineage and RANKL regulates the lineage commitment through the IVVY motif. Mechanistically, the IVVY motif controls the lineage commitment by reprogramming osteoclast genes into an inducible state in which they can be activated by TNF. Our findings not only provide important mechanistic insights into the action of RANKL in TNF-mediated osteoclastogenesis but also establish that the IVVY motif may serve as an attractive therapeutic target for bone loss in various bone disorders.
Highlights
Tumor necrosis factor-␣ (TNF), a proinflammatory cytokine produced by many cells including fibroblasts, endothelial cells, and macrophages, plays a critical role in immune and inflammatory responses [1]
2 The abbreviations used are: M-CSF, macrophage/monocyte colony-stimulating factor; bone marrow macrophages (BMMs), bone marrow macrophage; Car2, carbonic anydrase II; Ch1, chimeric receptor with normal RANK cytoplasmic domain; Ch2, chimeric receptor with mutated RANK IVVY535–538 motif (VY is replaced by AF); Ctsk, cathepsin K; Fas-AB, human Fas-activating antibody; hFas, human Fas external domain; MMP9, matrix metalloproteinase 9; OPG, osteoprotegerin; RANK, receptor activator of nuclear factor-B; RANKL, receptor activator of nuclear factor-B ligand; TNFR, TNF receptor; TNF receptor-associated factors (TRAFs), TNFR-associated factor; TRAP, tartrate resistant acid phosphatase; UAB, University of Alabama at Birmingham
BMMs treated with M-CSF and RANKL formed numerous osteoclasts in tissue culture dish, those treated with M-CSF and TNF did not form any osteoclast (Fig. 1A), replicating a finding from the previous study that TNF cannot replace RANKL in mediating osteoclastogenesis in tissue culture dish [26]
Summary
M-CSF, macrophage/monocyte colony-stimulating factor; BMM, bone marrow macrophage; Car, carbonic anydrase II; Ch1, chimeric receptor with normal RANK cytoplasmic domain; Ch2, chimeric receptor with mutated RANK IVVY535–538 motif (VY is replaced by AF); Ctsk, cathepsin K; Fas-AB, human Fas-activating antibody; hFas, human Fas external domain; MMP9, matrix metalloproteinase 9; OPG, osteoprotegerin; RANK, receptor activator of nuclear factor-B; RANKL, receptor activator of nuclear factor-B ligand; TNFR, TNF receptor; TRAF, TNFR-associated factor; TRAP, tartrate resistant acid phosphatase; UAB, University of Alabama at Birmingham. RANK IVVY Motif Is Vital for TNF-induced Osteoclastogenesis that RANK utilizes TRAFs to activate intracellular signaling pathways (19 –21) have led to a hypothesis that TNF can directly target osteoclast precursors to stimulate osteoclastogenesis in vitro independently of the RANKL/RANK system [22, 23]. OPG prevents inflammation-induced bone loss in animal model of arthritis [25]. These in vivo studies have raised concerns about the reported ability of TNF to mediate osteoclastogenesis in vitro independently of RANKL.
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
