TGF‐β1‐induced expression of collagen type II and ACAN is regulated by 4E‐BP1, a repressor of translation

Eukaryotic initiation factor 4E (eIF4E)‐binding protein 1 (4E‐BP1) binds eIF4E and represses protein translation by displacing it from the mRNA. In this study, we investigated the influence of 4E‐BP1 translational apparatus on the regulation of transforming growth factor‐beta 1 (TGF‐β1)‐induced anabolic signaling in chondrocytes. The level of 4E‐BP1 expression was significantly higher in human OA cartilage than normal cartilage. TGF‐β1 increased total protein synthesis, including aggrecan (ACAN) and collagen type II (Col II), together with activation of Akt/mTOR signaling pathway. mTOR silencing significantly suppressed ACAN and Col II expressions through decreasing TGF‐β1‐induced phosphorylation of 4E‐BP1. On the contrary, 4E‐BP1 knockdown promoted total protein synthesis but suppressed Col II and ACAN expressions with decreased expression of Smad2/3 and Smad4 and increased expression of inhibitory Smad6 and Smad7. TGF‐β1 suppressed the interaction of 4E‐BP1 and eIF4E and subsequently enhanced protein synthesis. Furthermore, 4E‐BP1 regulated translation levels of inhibitory Smads, which decreased the accumulation of nuclear Smad2/3 complexes on the promoter of ACAN and Col II genes, subsequently affecting transcription of ACAN and Col II. These results demonstrated that TGF‐β1‐modulated phosphorylation of 4EBP1 plays a role in the expression of Col II and ACAN through differential alteration of Smad signaling pathway.