Potential role and mechanism of IFN-gamma inducible protein-10 on receptor activator of nuclear factor kappa-B ligand (RANKL) expression in rheumatoid arthritis

Introduction: IFN-gamma inducible protein-10 (CXCL10), a member of the CXC chemokine family, and its receptor CXCR3 contribute to the recruitment of T cells from the blood stream into the inflamed joints and have a crucial role in perpetuating inflammation in rheumatoid arthritis (RA) synovial joints. Recently we showed the role of CXCL10 on receptor activator of nuclear factor kappa-B ligand (RANKL) expression in an animal model of RA and suggested the contribution to osteoclastogenesis. We tested the effects of CXCL10 on the expression of RANKL in RA synoviocytes and T cells, and we investigated which subunit of CXCR3 contributes to RANKL expression by CXCL10. Methods: Synoviocytes derived from RA patients were kept in culture for 24 hours in the presence or absence of TNF-alpha. CXCL10 expression was measured by reverse transcriptase polymerase chain reaction (RT-PCR) of cultured synoviocytes. Expression of RANKL was measured by RT-PCR and western blot in cultured synoviocytes with or without CXCL10 and also measured in Jurkat/Hut 78 T cells and CD4+ T cells in the presence of CXCL10 or dexamethasone. CXCL10 induced RANKL expression in Jurkat T cells was tested upon the pertussis toxin (PTX), an inhibitor of Gi subunit of G protein coupled receptor (GPCR). The synthetic siRNA for G alpha i(2) was used to knock down gene expression of respective proteins. Results: CXCL10 expression in RA synoviocytes was increased by TNF-alpha. CXCL10 slightly increased RANKL expression in RA synoviocytes, but markedly increased RANKL expression in Jurkat/Hut 78 T cell or CD4+ T cell. CXCL10 augmented the expression of RANKL by 62.6%, and PTX inhibited both basal level of RANKL (from 37.4 +/- 16.0 to 18.9 +/- 13.0%) and CXCL10-induced RANKL expression in Jurkat T cells (from 100% to 48.6 +/- 27.3%). Knock down of G alpha i(2) by siRNA transfection, which suppressed the basal level of RANKL (from 61.8 +/- 17.9% to 31.1 +/- 15.9%) and CXCL10-induced RANKL expression (from 100% to 53.1 +/- 27.1%) in Jurkat T cells, is consistent with PTX, which inhibited RANKL expression. Conclusions: CXCL10 increased RANKL expression in CD4+ T cells and it was mediated by G alpha(i) subunits of CXCR3. These results indicate that CXCL10 may have a potential role in osteoclastogenesis of RA synovial tissue and subsequent joint erosion.