Translational regulation of PIM-1 kinase

Pim-1 is an oncogenic serine-threonine kinase whose expression is regulated at the transcriptional. post-transcriptional and post-transîational levels. Here we report translational regulation which is mediated by cis-acting elements .within the O'-untranslated region (UTR) of the mRNA. HUT-78 T ceils treated with PMA/ionomycin exhibited a 20-fold increase in pim-1 mRXA levels yet Pim-1 protein was not expressed. -In contrast, U937 myeloid cells treated with IFN"_/ exhibited a concurrent increase in both mRNA and protein levels. The 5"-UTR of pim-l is long {400bp), 76% GC-rich and is predicted to form a stable stem loop structure (DELTAG=-l§3 kcal/mol); characteristics shown to inhibit translation in other systems. Deletion of the 5"-UTR of pim-1 mRNA increased translation of Pim-1 10-fold as compared to Pim-1 expressed from the full length 5'-UTR containing mRNA. Eukaryotic translation initiation factor 4E (eIF-4E) is known to overcome translational repression of mRNA's. As predicted, when pim-1 plasmid cDNA constructs are transfected into NIH-3T3 cells overexpressing eIF-4E, 6-fold higher levels of Pim-1 protein are expressed. In contrast when the o'-UTR of pim-l is deleted, eIF-4E levels do not increase the levels of Pim-1 protein. These results indicate that Pim-1 is t r an si at ion ally regulated in vivo and suggest a mechanism for translational regulation of pim-1 mediated in part, by cis-acting elements within the 5'-UTR that can be overcome by expression of eIF-4E.