IL-4 abrogates osteoclastogenesis through STAT6-dependent inhibition of NF-κB

Osteoclast differentiation and function are required for normal bone resorption and remodeling (1, 2). The mature osteoclast polykaryon arises from marrowresiding precursor cells of hematopoietic origin, believed to be monocytes/macrophages (1–3). Early studies have shown that cocultures of these precursor cells with stromal/osteoblastic cells in the presence of hormonal steroids led to differentiation of mature osteoclast-like cells capable of resorbing bone (4–6). Recent advances revealed that factors produced by osteoblastic cells, such as M-CSF and receptor activator of NF-κB ligand (RANKL), are the factors required for macrophage differentiation into mature boneresorbing osteoclasts (7–10). Although M-CSF is a prerequisite for macrophage and osteoclast survival, RANKL appears to be the sole factor essential for acquiring the osteoclastic phenotype. RANKL, a transmembrane protein, and its cleaved soluble form have been shown to be effective in inducing osteoclast differentiation through binding to its receptor RANK in osteoclast progenitors (11, 12). Although details of the intracellular signal transduction pathway after RANK/RANKL interaction are not entirely in hand, activation of the transcription factors NF-κB and AP-1 has been proposed (7, 12–15). NF-κB is essential for osteoclastogenesis, and inactivation of components of this transcription factor led to osteopetrosis due to impaired osteoclast formation (16, 17). Inactive NF-κB is normally present in the cytoplasm complexed with its inhibitory protein IκB (18–20). A variety of extracellular stimuli activate a cascade of kinases including IκB kinases (IKKs), the MAP kinase MEKK-1, and c-src, leading to phosphorylation of IκB and to its dissociation from NF-κB (21–26). Liberated NF-κB translocates in a dimeric form to the nucleus, binds to DNA response elements, and activates basal transcription of target genes (18, 19). The osteoclastogenic process is regulated by immune cell–derived cytokines. The role of lymphocyte-derived cytokines that impact bone metabolism under physiological and pathological conditions has been steadily investigated in the past few years (27–29). Although cytokines secreted by Th1-type lymphocytes, such as RANKL and TNF, induce osteoclastogenesis, those produced by Th2 cells, such as IL-4, IL-10, and IL-13, have been shown as potent inhibitors of the osteoclastogenic process (6, 27, 28, 30–32). However, the molecular mechanisms by which these anti-inflammatory cytokines curtail osteoclast differentiation remain obscure. IL-4 is a pleiotropic immunomodulatory cytokine produced by Th2 lymphocytes, mast cells, and eosinophils (33, 34). The cytokine promotes immunological responses, and its levels are elevated in tissues succumb to chronic inflammatory diseases (35–37). IL-4 ligation to its receptor activates protein tyrosine kinases of the Janus kinase (JAK) family, which in turn leads to phosphorylation, dimerization, and nuclear