Insulin-like growth factor 1 synergizes with bone morphogenetic protein 7-mediated anabolism in bovine intervertebral disc cells

Objective: We undertook this study to assess the therapeutic benefits of intervertebral disc matrix repair and regeneration by evaluating the potential synergistic effect of insulin-like growth factor 1 (IGF-1) and bone morphogenetic protein 7 (BMP-7) on bovine spine discs and by elucidating the relevant molecular/cellular mechanisms. Methods Bovine nucleus pulposus (NP) cells were treated with BMP-7 and IGF-1. The subsequent anabolic effects driven by NP cells were assessed for proteoglycan (PG) synthesis by ³⁵S-sulfate incorporation and for PG accumulation by dimethylmethylene blue assays. Matrix formation was visualized by particle exclusion assay. Key matrix components and transcription factors were analyzed by real-time reverse transcription-polymerase chain reaction to determine the signaling pathways by which IGF-1 suppresses noggin, a potent inhibitor of BMP-7. Western blotting and nuclear translocation experiments were performed to assess the activation of Smad proteins. Results Stimulation of bovine NP cells by both IGF-1 and BMP-7 greatly potentiated anabolism through complementary and synergistic mechanisms on matrix formation compared with treatment with either growth factor alone. The exogenously added decoy ligand, noggin, attenuated the anabolic effects of BMP-7, and noggin was substantially increased by BMP-7, suggesting a negative feedback regulatory mechanism. In contrast, IGF-1 significantly suppressed noggin expression via the phosphatidylinositol 3-kinase/Akt pathway and thus potentiated BMP-7 signaling in bovine NP cells. Upon combination treatment, IGF-1 activated Smad2, while BMP-7 activated Smad1/5/8 and Smad3, thus inducing all Smad signaling pathways and mimicking the effects of the combination of transforming growth factor β and BMP-7 Conclusion Combination growth factor therapy using BMP-7 and IGF-1 may have considerable promise in the treatment of spine disc degeneration.