UNLABELLED: PAR-2 is expressed by osteoblasts and activated by proteases present during inflammation. PAR-2 activation inhibited osteoclast differentiation induced by hormones and cytokines in mouse bone marrow cultures and may protect bone from uncontrolled resorption. INTRODUCTION: Protease-activated receptor-2 (PAR-2), which is expressed by osteoblasts, is activated specifically by a small number of proteases, including mast cell tryptase and factor Xa. PAR-2 is also activated by a peptide (RAP) that corresponds to the "tethered ligand" created by cleavage of the receptor's extracellular domain. The effect of activating PAR-2 on osteoclast differentiation was investigated. MATERIALS AND METHODS: Mouse bone marrow cultures have been used to investigate the effect of PAR-2 activation on osteoclast differentiation induced by parathyroid hormone (PTH), 1,25 dihydroxyvitamin D3 [1,25(OH)2D3], and interleukin-11 (IL-11). Expression of PAR-2 by mouse bone marrow, mouse bone marrow stromal cell-enriched cultures, and the RAW264.7 osteoclastogenic cell line was demonstrated by RT-PCR. RESULTS: RAP was shown to inhibit osteoclast differentiation induced by PTH, 1,25(OH)2D3, or IL-11. Semiquantitative RT-PCR was used to investigate expression of mediators of osteoclast differentiation induced by PTH, 1,25(OH)2D3, or IL-11 in mouse bone marrow cultures and primary calvarial osteoblast cultures treated simultaneously with RAP. In bone marrow and osteoblast cultures treated with PTH, 1,25(OH)2D3, or IL-11, RAP inhibited expression of RANKL and significantly suppressed the ratio of RANKL:osteoprotegerin expression. Activation of PAR-2 led to reduced expression of prostaglandin G/H synthase-2 in bone marrow cultures treated with PTH, 1,25(OH)2D3, or IL-11. RAP inhibited PTH- or 1,25(OH)2D3-induced expression of IL-6 in bone marrow cultures. RAP had no effect on osteoclast differentiation in RANKL-treated RAW264.7 cells. CONCLUSION: These observations indicate that PAR-2 activation inhibits osteoclast differentiation by acting on cells of the osteoblast lineage to modulate multiple mediators of the effects of PTH, 1,25(OH)2D3, and IL-11. Therefore, the role of PAR-2 in bone may be to protect it from uncontrolled resorption by limiting levels of osteoclast differentiation.