Although best known for its role in T lymphocyte activation, the calcineurin/nuclear factor of activated T cells (NFAT) signaling pathway is also known to be involved in a wide range of other biological responses in a variety of different cell types. Here we have investigated the role of the calcineurin/NFAT signaling pathway in the regulation of osteoclast differentiation. Osteoclasts are bone-resorbing multinucleated cells that are derived from the monocyte/macrophage cell lineage after stimulation with a member of the tumor necrosis factor family of ligands known as receptor activator of nuclear factor-B ligand (RANKL). We now report that inhibition of calcineurin with either the immunosuppressant drugs cyclosporin A and FK506, or the retrovirally mediated ectopic expression of a specific calcineurin inhibitory peptide, all potently inhibit the RANKL-induced differentiation of the RAW264.7 monocyte/macrophage cell line into mature multinucleated osteoclasts. In addition, we find that NFAT family members are expressed in RAW264.7 cells and that their expression is up-regulated in response to RANKL stimulation. Most importantly, we find that ectopic expression of a constitutively active, calcineurin-independent NFATc1 mutant in RAW264.7 cells is sufficient to induce these cells to express an osteoclast-specific pattern of gene expression and differentiate into morphologically distinct, multinucleated osteoclasts capable of inducing the resorption of a physiological mineralized matrix substrate. Taken together, these data define calcineurin as an essential downstream effector of the RANKL-induced signal transduction pathway leading toward the induction of osteoclast differentiation and furthermore, indicate that the activation of the NFATc1 transcription factor is sufficient to initiate a genetic program that results in the specification of the mature functional osteoclast cell phenotype.Bone is a dynamic tissue that is under a constant state of remodeling or homeostasis. This remodeling process is a delicate balance between the activities of osteoblasts, the cells that deposit bone, and osteoclasts, the cell type that is responsible for bone resorption (1, 2). Interference with this delicate balance can result in very serious human pathologies that affect bone integrity, such as osteoporosis and osteopetrosis. Accordingly, the molecular signaling pathways that regulate osteoblast and osteoclast differentiation and function have come under intense scrutiny.Osteoclasts, the cells that resorb bone, are hematopoietically derived, multinucleated cells that arise from the monocyte/ macrophage lineage (3). It is now clear that osteoclast differentiation is dependent upon the intimate cellular interaction of myeloid preosteoclast precursors with either osteoblasts or stromal cells and is influenced by a wide range of local factors (4, 5). In fact, a wealth of data has indicated that a member of the tumor necrosis factor family of ligands known as receptor activator of nuclear factor-B (NF-B) 1 ligand (RANKL; also known...

Mature bone-resorbing osteoclasts (OCs) mediate excessive bone loss seen in several bone disorders, including osteoporosis. Here, we showed that reveromycin A (RM-A), a small natural product with three carboxylic groups in its structure, induced apoptosis specifically in OCs, but not in OC progenitors, nonfunctional osteoclasts, or osteoblasts. RM-A inhibited protein synthesis in OCs by selectively blocking enzymatic activity of isoleucyl-tRNA synthetase. The proapoptotic effect of RM-A was inhibited by neutralization or disruption of the acidic microenvironment, a prominent characteristic of OCs. RM-A was incorporated in OCs but not in nonfunctional osteoclasts and OC progenitors in neutral culture medium. Effects of RM-A on OC apoptosis increased under acidic culture conditions. RM-A not only was incorporated, but also induced apoptosis in OC progenitors in acidic culture medium. RM-A inhibited osteoclastic pit formation, decreased prelabeled 45 Ca release in organ cultures, and antagonized increased bone resorption in ovariectomized mice. These results suggested that preventive effects of RM-A on bone resorption in vitro and in vivo were caused by apoptosis through inhibition of isoleucyl-tRNA synthetase in OCs and that specific sensitivity of OCs to RM-A was due to the acidic microenvironment, which increased cell permeability of RM-A by suppressing dissociation of protons from carboxylic acid moieties, making them less polar. This unique mechanism suggested that RM-A might represent a type of therapeutic agent for treating bone disorders associated with increased bone loss.isoleucyl-tRNA synthetase ͉ inhibitor ͉ antiresorptive agent M ature osteoclasts resorb bone and mediate excessive bone loss seen in several bone disorders, including osteoporosis, arthritis, periodontitis, bone metastasis, and corticosteroidinduced bone loss (1-4). Mature bone-resorbing osteoclasts (OCs) exhibit highly specialized morphological structures, such as actin rings, clear zones, and ruffled borders, which are functional markers of OCs (2-5). These structures permit OCs to establish an isolated acidic microenvironment between themselves and the bone surface. The acidic microenvironment formed by V-ATPase on the ruffled borders of OCs allows dissolution of bone minerals and degradation of the bone matrix by proteases (6, 7). Therefore, these specialized features in OCs represent major potential targets for reducing OC activity and, consequently, could be useful for treatment of bone disorders. In addition, recent studies showed that receptor activator of NF-B ligand (RANKL), which is an essential factor for OC differentiation and activation, was one of the major targets of antiresorptive agents (2).Two types of antiresorptive drugs targeting OCs, i.e., bisphosphonates (BPPs) and calcitonin (CT), are used for clinical treatment of osteoporosis. BPPs, the most effective class of antiresorptive drugs available, inhibit bone resorption by suppressing functions of OCs and by shortening their lifespan (8 -10). Because of targeting of ...

HON and MAG have potential as novel agents for amelioration of adiposity and associated insulin resistance and inflammation.

No abstract