PGE2 combined with IL-6 promote non-tenogenic and inhibit tenogenic differentiation of tendon stem cell via the SMAD pathways

As vital healing cells, inappropriate differentiation of tendon stem cells (TSCs) in persistent inflammation wound cause injured tendons to difficult healing, eventually developing into tendinopathy. The key inflammation cytokines Prostaglandin E2 (PGE2) and Interleukin-(IL-)6 are increased in injured tendons and the SMAD signaling pathways play key role in regulating stem cell activity. However the combined effects of PGE2 and IL-6 on TSC differentiation and the molecular mechanism in it have not been unexplored. In the current study, we found that PGE2 combined with IL-6 promote osteogenic, chondrogenic, and adipogenic differentiation of TSCs, inhibited tenogenic differentiation of TSCs, and activated the SMAD1/5/8 and SMAD2/3 signaling, while the SMAD inhibitors LDN 193,189 combined with SB431542 reversed those effects. SB431542 inhibited IL-6-induced SMAD2/3 signaling and up-regulated the phosphorylation level of SMAD1/5/8 induced by PGE2, which was consistent with the results that IL-6 inhibited PGE2-induced adipogenic differentiation of TSC. In vivo, LDN 193,189 combined with SB431542 prevented tendon injury induced by treadmill running in mice, and suppressed the expression of non-tenogenic differentiation markers and enhanced the tenogenic marker expression in injured tendons. Collectively, the present study demonstrated that PGE2 combined with IL-6 promoted non-tenogenic differentiation and inhibited tenogenic differentiation of TSCs via the SMAD1/5/8 and SMAD2/3 pathway. Additionally, we confirmed the positive effects of the SMAD inhibitors LDN193189 and SB431542 in preventing tendon injury in vivo and provide a novel strategy for preventing and treating tendon injuries.