The contribution of the mRNA cap-binding protein, eIF-4E, to malignant transformation and progression has been illuminated over the past decade. eIF-4E overexpression has been demonstrated in human tumors of the breast, head and neck, colon, prostate, bladder, cervix and lung, and has been related to disease progression. Overexpression of eIF-4E in experimental models dramatically alters cellular morphology, enhances proliferation and induces cellular transformation, tumorigenesis and metastasis. Conversely, blocking eIF-4E function by expression of antisense RNA, or overexpression of the inhibitory eIF-4E binding proteins (4E-BPs), suppresses cellular transformation, tumor growth, tumor invasiveness and metastasis. Although eIF-4E regulates the recruitment of mRNA to ribosomes, and thereby globally regulates cap-dependent protein synthesis, eIF-4E contributes to malignancy by selectively enabling the translation of a limited pool of mRNAs--those that generally encode key proteins involved in cellular growth, angiogenesis, survival and malignancy (e.g. cyclin D1, c-myc, vascular endothelial growth factor, matrix metalloprotease 9). A deeper understanding of the role of eIF-4E in regulating the translation of the diverse gene products involved in all aspects of malignancy will improve the capacity to exploit eIF-4E as a therapeutic target and as a marker for human cancer progression.

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Eukaryotic protein synthesis initiation factor 4E (eIF-4E) is a 25-kDa polypeptide that binds to the 7-methylguanosine-containing cap of mRNA and participates in the transfer of mRNA to the 40S ribosomal subunit, a step that is rate-limiting for protein synthesis under most cellular conditions. eIF-4E is the least abundant of the initiation factors, is present at 10% of molar concentration of mRNA, and thus may serve as a site of regulation for the recruitment of mRNA into polysomes. Previous studies have indicated that phosphorylation of eIF-4E at Ser-53 is correlated with an increased rate of protein synthesis in a variety of systems in vivo and is required for eIF-4E to become bound to the 48S initiation complex. In this study we show that overexpression of eIF4E in HeLa cells using an episomally replicating, BK virus-based vector leads to an unusual phenotype: cells grow rapidly, forming densely packed, multilayered foci. They progressively form syncytia, some containing as many as six nuclei, and ultimately lyse 1 month after transfection. Some of these properties are reminiscent of oncogenically transformed cells. Cells transfected with the identical vector expressing a variant of eIF-4E, which contains alanine at position 53 and thus cannot be phosphorylated at the major in vivo site, grow normally. Estimations using the Ala-53 variant or a bacterial chloramphenicol acetyltransferase reporter gene in the same vector indicate that the degree of eIF-4E overexpression is 3-to 9-fold more than the endogenous level. These results suggest that eIF-4E may play a key role in cell cycle progression.The initiation of translation in eukaryotes can be regulated at the level of 43S complex formation (binding of initiator Met-tRNAIpet to the 40S ribosomal subunit) and at the level of 48S complex formation (binding of mRNA to the 43S complex). The former occurs during virus infection, after interferon treatment, and in other severe and stressful circumstances (1). Under more normal cellular conditions, the second step, formation of the 48S complex, is rate limiting (2), and regulation by mitogens, growth factors, and serum or during mitosis appears to occur at this step (ref. 3 and references therein). mRNA binding to 43S complexes is catalyzed by the eIF-4 group factors, which collectively recognize the 7-methylguanosine-containing cap, melt secondary structure beginning from the 5' end, and facilitate the scanning of the mRNA sequence for the initiation codon by the 40S subunit (4, 5).How mRNA recruitment into 48S initiation complexes is regulated is not completely understood, but a factor that is likely to be involved is eIF-4E, a 25-kDa polypeptide that binds to the cap (presumably the first step in mRNA recruitment) and accompanies mRNA transfer to the 48S complex (6). Whether eIF-4E acts initially as a free polypeptide or in a complex with other polypeptides remains to be established. eIF-4E is the least abundant of the initiation factors and is present at =10% of the molar concentration of mRNA (6) and ri...

The gene Tousled of Arabidopsis Thaliana encodes a protein kinase which, when mutated, results in abnormal ower development. From a library of mRNAs that are translationally upregulated by overexpression of the translation initiation factor 4E, we identi®ed a mammalian Tousled Like kinase (TLK1B). The human TLK1B mRNA contains a 5'UTR 1088-nt-long with two upstream AUG codons, and was found to be very inhibitory for translation. The TLK1B protein localizes almost exclusively to the nuclei. TLK1B overexpression in mammalian cells rendered them more resistant to ionizing radiation (IR). Puri®ed TLK1B phosphorylated histone H3 at S 10 with high speci®city both in a mix of core histones and in isolated chromatin, suggesting that histone H3 is a physiological substrate for TLK1B. Moreover, overexpression of TLK1B in transfected cells resulted in a higher degree of H3 phosphorylation. Expression of TLK1B in a yeast strain that harbors a temperature-sensitive mutation of the major H3 kinase, Ipl1, complemented the growth defect; restored normal levels of histone H3 phosphorylation; and increased their resistance to IR. Phosphorylation of H3 has been linked to the activation of the immediate-early genes upon mitogenic stimulation, and to chromatin condensation during mitotic/meiotic events. A possible role for TLK1B in radioprotection is discussed. Oncogene (2001) 20, 726 ± 738.