Osteocyte secreted factors inhibit skeletal muscle differentiation.

It is generally accepted that bone and muscle possess the capacity to act in an autocrine, paracrine, or endocrine manner, with a growing body of evidence that suggests muscle can secrete muscle specific cytokines or "myokines", which influence bone metabolism. However, there has been little investigation into the identity of bone specific cytokines that modulate skeletal muscle differentiation and function. This study aimed to elucidate the influence of osteocytes on muscle progenitor cells in vitro and to identify potential bone specific cytokines or "osteokines". We treated C2C12 myoblasts with media collected from differentiated osteocytes (Ocy454 cells) grown in 3D, either under static or fluid flow culture conditions (2 dynes/cm(2)). C2C12 differentiation was significantly inhibited with a 75% reduction in the number of myofibers formed. mRNA analysis revealed a significant reduction in the expression of myogenic regulatory genes. Cytokine array analysis on the conditioned media demonstrated that osteocytes produce a significant number of cytokines "osteokines" capable of inhibiting myogenesis. Furthermore, we demonstrated that when osteocytes are mechanically activated they induce a greater inhibitory effect on myogenesis compared to a static state. Lastly, we identified the downregulation of numerous cytokines, including Il-6, Il-13, Il-1β, MIP-1α, and Cxcl9, involved in myogenesis, which may lead to future investigation of the role "osteokines" play in musculoskeletal health and pathology.