Bradykinin (BK) stimulates bone resorption in vitro and synergistically potentiates interleukin-1 (IL-1)–induced bone resorption and prostaglandin (PG) formation, suggesting that kinins are important in inflammation-induced bone loss. The present study was undertaken to study 1) the role of the kinin B1 and B2 receptors in the synergistic interaction with IL-1 and tumor necrosis factor α (TNFα), 2) the molecular mechanisms involved in synergistic enhancement of PG formation, and 3) the effects of kinins on cytokine-induced expression of RANKL, RANK, and osteoprotegerin (OPG) (the latter being crucial molecules in osteoclast differentiation).Methods
Formation of PGs, expression of enzymes involved in arachidonic acid metabolism, and expression of RANKL, RANK, and OPG were assessed in the human osteoblastic cell line MG-63 and in mouse calvarial bones. The role of NF-κB and MAP kinases was studied using pharmacologic inhibitors.Results
PGE2 formation and cyclooxygenase 2 (COX-2) protein expression were induced by IL-1β and potentiated by kinins with affinity for the B1 or B2 receptors, resulting in PGE2-dependent enhancement of RANKL. The enhancements of PGE2 formation and COX-2 were markedly decreased by inhibition of p38 and JNK MAP kinases, whereas inhibition of NF-κB resulted in abolishment of the PGE2 response with only slight inhibition of COX-2.Conclusion
Kinin B1 and B2 receptors synergistically potentiate IL-1– and TNFα-induced PG biosynthesis in osteoblasts by a mechanism involving increased levels of COX-2, resulting in increased RANKL. The synergistic stimulation is dependent on NF-κB and MAP kinases. These mechanisms might help to explain the enhanced bone resorption associated with inflammatory disorders, including that in rheumatoid arthritis.