Osteocytes regulate osteoblast differentiation and osteoclast activity through Interleukin-6 under mechanical loading

Osteocytes are the major mechanosensors that respond to mechanical strain and regulate bone formation and resorption. Previous studies have indicated that IL-6 is a mechano-sensitive cytokine. However, there are few data describing the effect of IL-6 on osteoblasts and osteoclasts under mechanical loading. The purpose of this study was to evaluate whether osteocytes affect osteoblast differentiation and osteoclast activity via IL-6 production. Murine primary calvarial osteoblasts and RAW264.7 macrophage cells were cultured in conditioned medium collected from murine long bone osteocytes Y4 (MLO-Y4) subjected to compressive cyclic force (CCF) with or without IL-6 and soluble IL-6 receptor (sIL-6R). The results showed that MLO-Y4 osteocytes increased ALP activity and osteoblast markers expression in murine calvarial osteoblasts and inhibited osteoclastogenesis and osteoclast differentiation in osteoclast precursors via soluble factors in the CCF-induced conditioned medium (CCF CM). Typically, the osteocyte-induced effects were significantly greater in the presence of exogenous IL-6/sIL-6R. However, when the CCF CM was treated with the IL-6 neutralizing antibody, the stimulating effect was significantly blocked. Activation of Janus protein tyrosine kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) and extracellular signal-regulated kinase (ERK) signal transduction pathways were involved in IL-6-induced osteoblasts and osteoclast differentiation and activities. In conclusion, osteocytes regulate osteogenic response and osteoclast activity under mechanical loading through IL-6 production via the STAT3 and ERK signaling pathways. These pathways not only induce distinct biological responses, but also coordinate with each other to have detrimental effects on bone homeostasis.