Phosphorylation of initiation factor-2α is required for activation of internal translation initiation during cell differentiation

The long uORF-burdened 5′UTRs of many genes encoding regulatory proteins involved in cell growth and differentiation contain internal ribosomal entry site (IRES) elements. In a previous study we showed that utilization of the weak IRES of platelet-derived growth factor (PDGF2) is activated during megakaryocytic differentiation. The establishment of permissive conditions for IRES-mediated translation during differentiation has been confirmed by our demonstration of the enhanced activity of vascular endothelial growth factor, c-Myc and encephalomyocarditis virus IRES elements under these conditions, although their mRNAs are not naturally expressed in differentiated K562 cells. In contrast with the enhancement of IRES-mediated protein synthesis during differentiation, global protein synthesis is reduced, as judged by polysomal profiles and radiolabelled amino acid incorporation rate. The reduction in protein synthesis rate correlates with increased phosphorylation of the translation initiation factor eIF2α. Furthermore, IRES use is decreased by over-expression of the dominant-negative form of the eIF2α kinase, PKR, the vaccinia virus K3L gene, or the eIF2α-S51A variant which result in decreased eIF2α phosphorylation. These data demonstrate a connection between eIF2α phosphorylation and activation of cellular IRES elements. It suggests that phosphorylation of eIF2α, known to be important for cap-dependent transaltional control, serves to fine-tune the translation efficiency of different mRNA subsets during the course of differentiation and has the potential to regulate expression of IRES-containing mRNAs under a range of physiological circumstances.