No abstract

Muscle is one of the tissues located in close proximity to cartilage tissue. Although it has been suggested that muscle could influence skeletal development through generating mechanical forces by means of contraction, very little is known regarding whether muscle cells release biochemical signals to regulate cartilage gene expression. We tested the hypothesis that muscle cells directly regulate cartilage matrix production by analyzing chondrocytes cocultured with muscle cells in 2D or 3D conditions. We found that chondrocytes cultured with C2C12 muscle cells exhibited enhanced alcian blue staining and elevated expression of collagen II and collagen IX proteins. Although nonmuscle cells did not promote cartilage matrix production, converting them into muscle cells enhanced their pro-chondrogenic activity. Furthermore, muscle cell-conditioned medium led to increased cartilage matrix production, suggesting that muscle cells secrete prochondrogenic factors. Taken together, our study suggests that muscle cells may play an important role in regulating cartilage gene expression. This result may ultimately lead to the discovery of novel factors that regulate cartilage formation and homeostasis, and provide insights into improving the strategies for regenerating cartilage. ß

Pro-inflammatory cytokines IL-1β and TNFα play important roles in the manifestation of arthritis by disrupting the anabolic and catabolic activities of the chondrocytes. We observed a novel mechanism of cartilage regulation by which muscle cells diminish the response of chondrocytes to IL-1β and TNFα. We found that chondrocytes cocultured with muscle cells or cultured in muscle cell-conditioned medium significantly enhanced the expression of cartilage matrix proteins (collagen II and collagen IX) and resisted IL-1β and TNFα-induced cartilage damage. Our data suggest that this effect is achieved by inhibiting the expression of key components of the signaling pathways of pro-inflammatory cytokines (including NFκB, ESE-1, Cox-2 and GADD45β), leading to attenuated expression of cartilage degrading enzymes (MMPs and ADAMTS4). Therefore, our work unveils a potential role of muscle in regulating cartilage homeostasis and response to pro-inflammatory stimuli, and provides insights on designing treatment strategies for joint degenerative diseases such as arthritis.