Interleukin-6 induces immunometabolic reprogramming in human rheumatoid arthritis synovial fibroblasts to promote osteoclastogenesis

Abstract

Abstract Interleukin-6 (IL-6) plays an important role in progressive bone loss in rheumatoid arthritis (RA). However, the molecular mechanisms through which IL-6 propels RA synovial fibroblasts (RASFs) to contribute to bone loss are not fully understood. Treatment of human RASFs with IL-6 and IL-6 receptor (IL-6/IL-6R, 100 ng/ml each) alone or in combination with M-CSF (2 ng/ml) and RANKL (5 ng/ml) for 9 days resulted in the phenotypic changes to osteoclast-like features as determined by increase in TRAP staining and Dentin pit resorption area, which were comparable to M-CSF/RANKL group. IL-6/IL-6R induced a dose-dependent increase in the expression of transcription factor Ets2 and enhanced its nuclear translocation alone or combinatorially in M-CSF/RANKL-differentiated human RASFs. Chromatin immunoprecipitation (ChIP) analysis of Cathepsin K and B promoters showed Ets2 binding and transcriptional activation upon IL-6/IL-6R stimulation, which was further increased with M-CSF/RANKL. Interestingly, the untargeted proteomics analysis of the conditioned media from activated RASFs using Mass-Spectrometry (MS) technique and Trans-Proteomic Pipeline tool showed the production of 113 analytes unique to IL-6/IL-6R stimulation. Further analysis of the proteomics data using STRING database for pathway analysis showed the impact of IL-6/IL-6R on immunometabolic reprogramming of human RASFs towards osteoclast-like phenotype, which was partly mediated through Ets2. These preliminary studies identified a novel role of IL-6/IL-6R-mediated transsignaling in the metabolic reprogramming in RASFs that could potentially be targeted by inhibiting Ets2 to limit their role in bone resorption in RA.