MBP-1 is efficiently encoded by an alternative transcript of the ENO1 gene but post-translationally regulated by proteasome-dependent protein turnover

The c-myc promoter-binding protein-1 (MBP-1) is a transcriptional suppressor of tumorigenesis and thought to be the product of alternative translation initiation of the -enolase (ENO1) transcript. In the present study, we cloned a 2552-bp novel cDNA with a putative coding sequence of MBP-1 and functionally examined its ability to encode the MBP-1 protein. Similarly to ENO1, the obtained MBP-1 was widely and differentially expressed in a variety of normal tissues and cancer cells. Experiments using MBP-1 promoter-driven luciferase reporter assays, biochemical cell fractionation followed by RT-PCR detection of the cytoplasmic mRNA, and transcriptiontranslation-coupled reactions, consistently demonstrated that this novel transcript was alternatively transcribed from intron III of the ENO1 gene and was feasible for MBP-1 production. Hypoxia treatments significantly increased the transcriptional activation of the MBP-1 gene. Blocking the proteasomal degradation by MG132 stabilized the MBP-1 protein in cells. Compared with the translation efficiency for production of the MBP-1 protein, the MBP-1 transcript was 17.8 times more efficient than the ENO1 transcript. Thus, we suggest that this newly discovered transcript is a genuine template for the protein synthesis of MBP-1 in cells, and optimal expression of this gene in tumors may lead to effective clinical therapies for cancers.