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Keywords:

  • osteoclasts;
  • VEGF;
  • inflammation;
  • arthritis

Abstract

Large osteoclasts (10+ nuclei), predominant in rheumatoid arthritis and periodontal disease, have higher expression of proteases and activating receptors and also have increased resorptive activity when compared to small (2–5 nuclei) osteoclasts. We hypothesized that large and small osteoclasts activate different signaling pathways. A Signal Transduction Pathway Finder Array™ was used to compare gene expression of large and small osteoclasts in RAW 264.7-derived osteoclasts. Expression of vascular endothelial growth factor A (Vegfa) was higher in large osteoclasts and this result was confirmed by RT-PCR. RT-PCR further showed that RANKL treatment of RAW cells induced Vegfa expression in a time-dependent manner. Moreover, VEGF-A secretion in conditioned media was also increased in cultures with a higher proportion of large osteoclasts. To investigate the mechanism of Vegfa induction, specific inhibitors for the transcription factors NF-κB, AP-1, NFATc1, and HIF-1 were used. Dimethyl bisphenol A, the HIF-1α inhibitor, decreased Vegfa mRNA expression, whereas blocking NF-κB, AP-1, and NFATc1 had no effect. Furthermore, the NF-κB inhibitor gliotoxin inhibited Hif1α mRNA expression. In conclusion, VEGF-A gene and protein expression are elevated in large osteoclasts compared to small osteoclasts and this increase is regulated by HIF-1. In turn, Hif1α mRNA levels are induced by RANKL-mediated activation of NF-κB. These findings reveal further differences in signaling between large and small osteoclasts and thereby identify novel therapeutic targets for highly resorptive osteoclasts in inflammatory bone loss. J. Cell. Biochem. 110: 343–351, 2010. © 2010 Wiley-Liss, Inc.