Interleukin-6 transiently promotes proliferation of osteoclast precursors and stimulates the production of inflammatory mediators.

Abstract

Clinical data and phenotypes of several in vivo models demonstrated that interleukin-6 (IL-6) is an essential positive regulator in inflammation-induced bone loss. However, how IL-6 affect bone resorption and the osteoclast differentiation remains in debate. In this study we elucidate the cellular responses of receptor activator of nuclear factor kappa-Β ligand (RANKL)-stimulated RAW254.7 macrophage, the process mimicking osteoclast differentiation, upon IL-6 co-stimulation. IL-6 is a pleiotropic cytokine triggering various cellular responses, ranging from pro-inflammatory responses, differentiation to proliferation or apoptosis in different cell types. Those cellular events in the RANKL-stimulated RAW cells were examined to understand how differentiating monocytic cells respond to IL-6 exposure. Proliferation, apoptosis, differentiation and Pro-inflammatory responses of RANKL-stimulated RAW254.7 macrophage treated with or without IL-6 were measured by MTT assay, quantitative PCR assay of the expression of apoptotic genes, osteoclast differentiation markers, and pro-inflammatory genes, respectively. The results were collected from different time points in a 6-day differentiation period. Also, western blot on STAT3, ERK and AKT were also performed to investigate the IL-6 signaling in those cells. IL-6 triggered transient proliferation, but not apoptosis, in RANKL-stimulated RAW cells. Osteoclastogenesis was disrupted as the expression of essential genes for bone resorption were inhibited, and the osteoclast precursors maintained their undifferentiated phenotypes, with pro-inflammatory genes upregulated. Our results suggested that IL-6 interferes osteoclastogenesis. Additionally, IL-6 promote pro-inflammatory responses of monocytic cells and aggravate inflammation.