Pim-1 protein expression is regulated by its 5'-untranslated region and translation initiation factor elF-4E

Expression of Pim-1, an oncogenic serine/threonine kinase, is highly regulated at the transcriptional, posttranscriptional, and posttranslational levels. Here, we report that expression of Pim-1 kinase is additionally regulated at the translational level. Pim-1 protein expression did not increase in Hut-78 lymphocytes in response to PMA1/ionomycin stimulation despite approximately 20-fold increases in mRNA levels, suggesting that translation was repressed. Sequence analysis of the 5'-untranslated region (UTR) indicated a long (400 nucleotide), 76% G + C-rich region, characteristics known to inhibit translation. Deletion of the 5'-UTR of pim-1 increased translation of the Pim-1 protein approximately 10-fold in vitro in reticulocyte lysates and approximately 1.6-fold in vivo in NIH-3T3 cells. When full-length 5'-UTR-containing pim-1 cDNA constructs were transfected into NIH-3T3 cells overexpressing eukaryotic translation initiation factor 4E (eIF-4E), approximately 6-fold higher levels of Pim-1 protein were produced, as compared to that produced in control NIH-3T3 cells. Moreover, eIF-4E overexpression had little effect in the absence of the 5'-UTR, suggesting that it relieved 5'-UTR-mediated inhibition of Pim-1 expression.