Bone remodeling in physiological and pathological conditions represents a balance between bone resorption mediated by osteoclasts and bone formation by osteoblasts. Bone resorption is tightly and dynamically regulated by multiple mediators, including cytokines that act directly on osteoclasts and their precursors, or indirectly by modulating osteoblast lineage cells that in turn regulate osteoclast differentiation. The critical role of cytokines in inducing and promoting osteoclast differentiation, function and survival is covered by the accompanying review by Zwerina and colleagues. Recently, it has become clear that negative regulation of osteoclastogenesis and bone resorption by inflammatory factors and cytokines, downstream signaling pathways, and a newly described network of transcriptional repressors plays a key role in bone homeostasis by fine tuning bone remodeling and restraining excessive bone resorption in inflammatory settings. In this review we discuss negative regulators of osteoclastogenesis and mechanisms by which these factors suppress bone resorption.

It is well-established that receptor activator of NF-κB ligand (RANKL) is the inducer of physiological osteoclast differentiation. However, the specific drivers and mechanisms driving inflammatory osteoclast differentiation under pathological conditions remain obscure. This is especially true given that inflammatory cytokines such as tumor necrosis factor (TNF) demonstrate little to no ability to directly drive osteoclast differentiation. Here, we found that transforming growth factor β (TGFβ) priming enables TNF to effectively induce osteoclastogenesis, independently of the canonical RANKL pathway. Lack of TGFβ signaling in macrophages suppresses inflammatory, but not basal, osteoclastogenesis and bone resorption in vivo. Mechanistically, TGFβ priming reprograms the macrophage response to TNF by remodeling chromatin accessibility and histone modifications, and enables TNF to induce a previously unrecognized non-canonical osteoclastogenic program, which includes suppression of the TNF-induced IRF1-IFNβ-IFN-stimulated-gene axis, IRF8 degradation and B-Myb induction. These mechanisms are active in rheumatoid arthritis, in which TGFβ level is elevated and correlates with osteoclast activity. Our findings identify a TGFβ/TNF-driven inflammatory osteoclastogenic program, and may lead to development of selective treatments for inflammatory osteolysis.

Targeting microRNAs recently shows significant therapeutic promise; however, such progress is underdeveloped in treatment of skeletal diseases with osteolysis, such as osteoporosis and rheumatoid arthritis (RA). Here, we identified miR-182 as a key osteoclastogenic regulator in bone homeostasis and diseases. Myeloid-specific deletion of miR-182 protects mice against excessive osteoclastogenesis and bone resorption in disease models of ovariectomy-induced osteoporosis and inflammatory arthritis. Pharmacological treatment of these diseases with miR-182 inhibitors completely suppresses pathologic bone erosion. Mechanistically, we identify protein kinase double-stranded RNA-dependent (PKR) as a new and essential miR-182 target that is a novel inhibitor of osteoclastogenesis via regulation of the endogenous interferon (IFN)-β-mediated autocrine feedback loop. The expression levels of miR-182, PKR, and IFN-β are altered in RA and are significantly correlated with the osteoclastogenic capacity of RA monocytes. Our findings reveal a previously unrecognized regulatory network mediated by miR-182-PKR-IFN-β axis in osteoclastogenesis, and highlight the therapeutic implications of miR-182 inhibition in osteoprotection.

M-CSF is a critical growth factor for myeloid lineage cells, including monocytes, macrophages, and osteoclasts. Tissue-resident macrophages in most organs rely on local M-CSF. However, it is unclear what specific cells in the bone marrow produce M-CSF to maintain myeloid homeostasis. Here, we found that Adipoq-lineage progenitors but not mature adipocytes in bone marrow or in peripheral adipose tissue, are a major cellular source of M-CSF, with these Adipoq-lineage progenitors producing M-CSF at levels much higher than those produced by osteoblast lineage cells. The Adipoq-lineage progenitors with high CSF1 expression also exist in human bone marrow. Deficiency of M-CSF in bone marrow Adipoq-lineage progenitors drastically reduces the generation of bone marrow macrophages and osteoclasts, leading to severe osteopetrosis in mice. Furthermore, the osteoporosis in ovariectomized mice can be significantly alleviated by the absence of M-CSF in bone marrow Adipoq-lineage progenitors. Our findings identify bone marrow Adipoq-lineage progenitors as a major cellular source of M-CSF in bone marrow and reveal their crucial contribution to bone marrow macrophage development, osteoclastogenesis, bone homeostasis, and pathological bone loss.

Increased osteoclastogenesis is responsible for osteolysis, which is a severe consequence of inflammatory diseases associated with bone destruction, such as rheumatoid arthritis and periodontitis. The mechanisms that limit osteoclastogenesis under inflammatory conditions are largely unknown. We previously identified transcription factor RBP-J as a key negative regulator that restrains TNF-α–induced osteoclastogenesis and inflammatory bone resorption. In this study, we tested whether RBP-J suppresses inflammatory osteoclastogenesis by regulating the expression of microRNAs (miRNAs) important for this process. Using high-throughput sequencing of miRNAs, we obtained the first, to our knowledge, genome-wide profile of miRNA expression induced by TNF-α in mouse bone marrow–derived macrophages/osteoclast precursors during inflammatory osteoclastogenesis. Furthermore, we identified miR-182 as a novel miRNA that promotes inflammatory osteoclastogenesis driven by TNF-α and whose expression is suppressed by RBP-J. Downregulation of miR-182 dramatically suppressed the enhanced osteoclastogenesis program induced by TNF-α in RBP-J–deficient cells. Complementary loss- and gain-of-function approaches showed that miR-182 is a positive regulator of osteoclastogenic transcription factors NFATc1 and B lymphocyte–induced maturation protein-1. Moreover, we identified that direct miR-182 targets, Foxo3 and Maml1, play important inhibitory roles in TNF-α–mediated osteoclastogenesis. Thus, RBP-J–regulated miR-182 promotes TNF-α–induced osteoclastogenesis via inhibition of Foxo3 and Maml1. Suppression of miR-182 by RBP-J serves as an important mechanism that restrains TNF-α–induced osteoclastogenesis. Our results provide a novel miRNA-mediated mechanism by which RBP-J inhibits osteoclastogenesis and suggest that targeting of the newly described RBP-J–miR-182–Foxo3/Maml1 axis may represent an effective therapeutic approach to suppress inflammatory osteoclastogenesis and bone resorption.