Identification of LRRc17 as a negative regulator of receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast differentiation.

Osteoblasts are the primary cells responsible for bone formation. They also support osteoclast formation from bone marrow precursors in response to osteotropic factors by inducing receptor activator of NF-kappaB ligand (RANKL) expression and down-regulating osteoprotegerin (OPG) production. In addition to the RANKL-RANK-OPG signaling axis, other factors produced by osteoblasts/stromal cells are involved in osteoclastogenesis. Here, we describe the identification and characterization of leucine-rich repeat-containing 17 (LRRc17), a member of the LRR superfamily that acts as a negative regulator of RANKL-induced osteoclast differentiation. Osteoblasts showed high levels of LRRc17 expression, which was down-regulated in response to the pro-osteoclastogenic factor 1,25-dihydroxyvitamin D(3). Recombinant LRRc17 protein inhibited RANKL-induced osteoclast differentiation from bone marrow precursors, whereas it did not affect the differentiation or activation of macrophages and dendritic cells. These results suggest that among the cell types derived from common myeloid precursors, LRRc17 specifically regulates osteoclasts. Further analysis revealed that LRRc17 attenuated RANKL-induced expression of NFATc1 by blocking phospholipase C-gamma signaling, which, in turn, inhibited RANKL-mediated osteoclast differentiation. Taken together, our results demonstrated a novel inhibitory activity of LRRc17 in RANKL-induced osteoclastogenesis.