A cis-acting element in the BCL-2 gene controls expression through translational mechanisms

The bcl-2 gene becomes activated in many types of human cancers and contributes to neoplastic cell expansion, as well as to resistance to radiation and chemotherapy, by blocking programmed cell death or apoptosis. The expression of this proto-oncogene is regulated at both the transcriptional and post-transcriptional levels. DNA sequence comparisons of human, mouse, rat and chicken bcl-2 cDNAs revealed the presence of an open reading frame (ORF) [correction of (OFR)] located upstream of the normal coding region. Because upstream ORFs (uORFs) have been associated with translational repression, we analysed the functional significance of the 11 amino-acid uORF in the human BCL-2 gene (-119 to -84 bp). Deletion of this uORF from chloramphenicol acetyltransferase (CAT) reporter gene constructs that contained the bcl-2 promoter and entire 5'-untranslated region (5'-UTR), as well as introduction of an A-->T mutation at position -119 bp that destroyed the AUG-initiation codon, significantly increased CAT activity in HeLa, CEM, and other cell lines, without producing a corresponding elevation in CAT mRNA levels. Positioning this uORF, together with its accompanying Kozak sequences, between a heterologous promoter from SV40 and a CAT reporter gene resulted in marked inhibition of CAT protein production without a decrease in CAT mRNA. Mutation of the start codon (ATG-->TTG) of this uORF completely abolished its inhibitory activity, consistent with a translational mechanism. Taken together, these findings suggest that the uORF located within the 5'UTR of the bcl-2 gene is necessary and sufficient for translational regulation of bcl-2 gene expression.