IL-17 expression and its effect on osteoclastogenesis under mechanical loading

Abstract

OBJECTIVES: The molecular mechanisms of bone remodeling under mechanical loading is crucial for understanding alveolar bone metabolism. Interleukin (IL)-17 plays a key role in orchestrate osteoclast activation and differentiation. This study investigated the expression of IL-17 under mechanical force and its effect on osteoclastogenesis in osteocyte-like cells. METHODS: MLO-Y4 osteocytes were loaded with flow shear stress driven by Streamer® Shear Stress Device (4 dyn/cm2 and 16 dyn/cm2 respectively, 0.5 Hz, 2h). IL-17 at 0.1, 1 and 10ng/ml stimulated the osteocytes for 24h. The IL-17, IL-17 receptor (IL-17R), osteoprotegerin (OPG) and the receptor activator of NF-κB ligand (RANKL) mRNA expressions were analyzed by real-time polymerase chain reaction (qPCR). Co-culture of osteocytes and bone marrow-derived macrophage cells (BMMs) was undertaken to investigate the effect of IL-17 on osteoclastogenesis. Conditional medium (CM) from MLO-Y4 was collected 24h after force loading. Cells were co-cultured in normal medium or CM with or without IL-17 for 7 days. Tartrate-resistant acid phosphatase (TRAP) staining cells were quantified. RESULTS: The basal expression of IL-17 and IL-17R in MLO-Y4 osteocytes was up-regulated under shear stress loading. IL-17 significantly enhanced the mRNA expression of RANKL while decreased the ratio of OPG/RANKL. In co-culture of MLO-Y4 and BMMs, IL-17 significantly increased the formation of TRAP+ osteoclasts, while the CM from shear stress loaded MLO-Y4 inhibited the stimulatory effect of IL-17 on osteoclastogenesis. CONCLUSIONS: Mechanical loading enhances the expression of IL-17 and IL-17R in MLO-Y4 osteocytes. IL-17 promotes osteoclastogenisis, whereas such effect seems to be weakened under shear stress stimulation. Further study on the exact underlying mechanisms is warranted.