Activation of protease-activated receptor-2 leads to inhibition of osteoclast differentiation.
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.


BACKGROUND
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.


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.


CONCLUSIONS
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.
Smith
Rosealee
R
Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria, Australia.
Ransjö
Maria
M
Tatarczuch
Liliana
L
Song
Shu-Jun
SJ
Pagel
Charles
C
Morrison
John R
JR
Pike
Robert N
RN
Mackie
Eleanor J
EJ
D000818:Animals; D001854:Bone Marrow Cells / Q000166:cytology; D002352:Carrier Proteins / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects; D002460:Cell Line; D002478:Cells, Cultured; D051545:Cyclooxygenase 1; D051546:Cyclooxygenase 2; D015850:Interleukin-6 / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism; D008264:Macrophages / Q000166:cytology / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000378:metabolism; D008565:Membrane Proteins; D051379:Mice; D010006:Osteoblasts / Q000166:cytology; D010010:Osteoclasts / Q000166:cytology / Q000378:metabolism*; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D044464:Receptor, PAR-2 / Q000378:metabolism*
D002352:Carrier Proteins; D015850:Interleukin-6; D007527:Isoenzymes; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D053245:RANK Ligand; D053246:Receptor Activator of Nuclear Factor-kappa B; D044464:Receptor, PAR-2; C505687:Tnfrsf11a protein, mouse; C505689:Tnfsf11 protein, mouse; D051545:Cyclooxygenase 1; D051546:Cyclooxygenase 2; D011451:Prostaglandin-Endoperoxide Synthases; C496512:Ptgs1 protein, mouse