Nacksung Kim

Signaling by receptor activator of NF-kappaB (nuclear factor-kappaB) ligand (RANKL) is essential for differentiation of bone marrow monocyte-macrophage lineage (BMM) cells into osteoclasts. Here, we show RANKL stimulation of BMM cells transiently increased the intracellular level of reactive oxygen species (ROS) through a signaling cascade involving TNF (tumor necrosis factor) receptor-associated factor (TRAF) 6, Rac1, and NADPH (nicotinamide adenine dinucleotide phosphate) oxidase (Nox) 1. A deficiency in TRAF6 or expression of a dominant-interfering mutant of TRAF6 blocks RANKL-mediated ROS production. Application of N-acetylcysteine (NAC) or blocking the activity of Nox, a protein leading to the formation of ROS, with diphenylene iodonium (DPI) inhibits the responses of BMM cells to RANKL, including ROS production, activation of c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein (MAP) kinase, and extracellular signal-regulated kinase (ERK), and osteoclast differentiation. Moreover, both RANKL-mediated ROS production and osteoclast differentiation were completely blocked in precursors depleted of Nox1 activity by RNA interference or by expressing a dominant-negative mutant of Rac1. Together, these results indicate that ROSs act as an intracellular signal mediator for osteoclast differentiation.

Studies of bone and the immune system have converged in recent years under the banner of osteoimmunology. The immune system is spawned in the bone marrow reservoir, and investigators now recognize that important niches also exist there for memory lymphocytes. At the same time, various factors produced during immune responses are capable of profoundly affecting regulation of bone. Mechanisms have evolved to prevent excessive interference by the immune system with bone homeostasis, yet pathologic bone loss is a common sequela associated with autoimmunity and cancer. There are also developmental links, or parallels, between bone and the immune system. Cells that regulate bone turnover share a common precursor with inflammatory immune cells and may restrict themselves anatomically, in part by utilizing a signaling network analogous to lymphocyte costimulation. Efforts are currently under way to further characterize how these two organ systems overlap and to develop therapeutic strategies that benefit from this understanding.

Osteoclasts are unique cells that degrade the bone matrix. These large multinucleated cells differentiate from the monocyte/macrophage lineage upon stimulation by two essential cytokines, macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-kappa B (NF-κB) ligand (RANKL). Activation of transcription factors such as microphthalmia transcription factor (MITF), c-Fos, NF-κB, and nuclear factor-activated T cells c1 (NFATc1) is required for sufficient osteoclast differentiation. In particular, NFATc1 plays the role of a master transcription regulator of osteoclast differentiation. To date, several mechanisms, including transcription, methylation, ubiquitination, acetylation, and non-coding RNAs, have been shown to regulate expression and activation of NFATc1. In this review, we have summarized the various mechanisms that control NFATc1 regulation during osteoclast differentiation.