An element of the transforming growth factor-β1 5́-untranslated region represses translation and specifically binds a cytosolic factor

In many cell types, there is a discrepancy between transforming growth factor-β1 (TGF-β1) mRNA and TGF-β1 protein, suggesting that expression of TGF-β1 is regulated posttranscriptionally. We have previously shown that a 137-nucleotide (nt) region of the TGF-β1 5′-untranslated region (UTR) potently inhibits the expression of a heterologous reporter gene, suggesting a role for this region in the post-transcriptional inhibition of TGF-β1 expression. To study the mechanism of inhibition, a chimeric plasmid containing this region of the TGF-β1 5′-UTR and the reading frame of the human GH gene was stably transfected into C₂C₁₂ myoblastic cells. Our results show that the TGF-β1 5′-UTR inhibits GH expression by inhibiting GH mRNA translation. In vitro gel retardation and cross-linking assays using a radiolabelled RNA probe transcribed from this region of the TGF-β1 5′-UTR demonstrate the specific binding of a cytosolic factor. Deletion of a potential stem-loopforming region abolishes binding of this factor and partially restores GH production. These results suggest that posttranscriptional inhibition of TGF-β1 expression is at the level of mRNA translation and that a cytosolic factor may regulate TGF-β1 mRNA translation.