4E‐BP1 is a target of Smad4 essential for TGFβ‐mediated inhibition of cell proliferation

Assembly of the multi‐subunit eukaryotic translation initiation factor‐4F (eIF4F) is critical for protein synthesis and cell growth and proliferation. eIF4F formation is regulated by the translation‐inhibitory protein 4E‐BP1. While proliferation factors and intracellular pathways that impinge upon 4E‐BP1 phosphorylation have been extensively studied, how they control 4E‐BP1 expression remains unknown. Here, we show that Smad4, a transcription factor normally required for TGFβ‐mediated inhibition of normal cell proliferation, enhances 4E‐BP1 gene‐promoter activity through binding to a conserved element. 4E‐BP1 expression is specifically modulated by treatment with TGFβ and by manipulations of the natural Smad4 regulators (co‐Smads) in cells isolated from Smad4 +/+ human tumours, whereas no response is observed in cells isolated from Smad4 −/− human tumours or in cells where Smad4 has been knocked down by specific siRNAs. In addition, cells where 4E‐BP1 has been knocked down (inducible shRNAs in human pancreatic cancer cells or siRNAs in non‐malignant human keratinocytes) or has been knocked out (mouse embryonic fibroblasts isolated from 4E‐BP1 −/− mice) proliferate faster and are resistant to the antiproliferative effect of TGFβ. Thus, 4E‐BP1 gene appears critical for TGFβ/Smad4‐mediated inhibition of cell proliferation.