PIM-1 PROTEIN EXPRESSION IS REGULATED BY ITS 5'-UNTRANSLATED REGION AND TRANSLATION INITIATION-FACTOR EIF-4E

Expression of Pim-1, an oncogenic serine/threonine kinase, is highly r egulated at the transcriptional, posttranscriptional, and posttranslat ional levels. Here, we report that expression of Pim-l kinase is addit ionally regulated at the translational level. Pim-l protein expression did not increase in Hut-78 lymphocytes in response to PMA1/ionomycin stimulation despite similar to 20-fold increases in mRNA levels, sugge sting that translation was repressed. Sequence analysis of the 5'-untr anslated region (UTR) indicated a long (400 nucleotide), 76% G+C-rich region, characteristics known to inhibit translation. Deletion of the 5'-UTR of pim-l increased translation of the Pim-l protein similar to 10-fold in vitro in reticulocyte lysates and similar to 1.6-fold in vi vo in NIH-3T3 cells. When full-length 5'-UTR-containing pim-l cDNA con structs were transfected into NIH-3T3 cells overexpressing eukaryotic translation initiation factor 4E (eIF-4E), similar to 6-fold higher le vels of Pim-l protein were produced, as compared to that produced in c ontrol NIH-3T3 cells. Moreover, elF-4E overexpression had little effec t in the absence of the 5'-UTR, suggesting that it relieved 5'-UTR-med iated inhibition of Pim-l expression.