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-beta 1)-induced anabolic signaling in chondrocytes. The level of 4E-BP1 expression was significantly higher in human OA cartilage than normal cartilage. TGF-beta 1 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-beta 1-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-beta 1 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-beta 1-modulated phosphorylation of 4EBP1 plays a role in the expression of Col II and ACAN through differential alteration of Smad signaling pathway.