IGF-1 Combined with OPN Promotes Neuronal Axon Growth in Vitro Through the IGF-1R/Akt/mTOR Signaling Pathway in Lipid Rafts

This study aims to investigate the effect of insulin-like growth factor 1 (IGF-1) combined with osteopontin (OPN) on the protein expression levels and growth of neuronal axons and its possible mechanism. In this study, IGF-1 combined with OPN promoted neuronal axon growth through the IGF-1R/Akt/mTOR signaling pathway in lipid rafts, and the effect was better than that of either agent alone. This effect was suppressed when given the mTOR inhibitor rapamycin or the lipid raft cholesterol extraction agent methyl-beta-cyclodextrin (M-beta-CD). Rapamycin could inhibit the expression of phosphorylated ribosomal S6 protein (p-S6) and phosphorylated protein kinase B (p-Akt) and limit axon growth. In addition to the above effects, M-beta-CD significantly downregulated the expression of phosphorylated insulin-like growth factor 1 receptor (p-IR). To further investigate the changes in lipid rafts when stimulated by different recombinant proteins, membrane lipid rafts were isolated to observe the changes by western blot. The expression levels of insulin-like growth factor 1 receptor (IR) and P-IR in the IGF-1 combined with OPN group were the highest. When M-beta-CD was administered to the lipid rafts of neurons, the enrichment of IR by IGF-1 combined with OPN was weakened, and the p-IR was decreased. Our study found that IGF-1 combined with OPN could promote axon growth by activating the IGF-1R/Akt/mTOR signaling pathway in neuronal lipid rafts.

Automated summary

This study investigated the effect of IGF-1 combined with OPN on neuronal axon growth and protein expression levels, and its underlying mechanism. The results showed that IGF-1 combined with OPN promoted axon growth through the IGF-1R/Akt/mTOR signaling pathway in lipid rafts, and the effect was superior to either agent alone. However, this effect was suppressed by the mTOR inhibitor rapamycin or the lipid raft cholesterol extraction agent M-beta-CD. Additionally, M-beta-CD significantly decreased the expression of p-IR. Western blot analysis revealed that the IGF-1 combined with OPN group had the highest expression levels of IR and p-IR in the neuronal lipid rafts. The enrichment of IR by IGF-1 combined with OPN was weakened, and p-IR was reduced when M-beta-CD was administered to the lipid rafts of neurons. This study demonstrated that IGF-1 combined with OPN promoted axon growth by activating the IGF-1R/Akt/mTOR signaling pathway in lipid rafts.