Individual Overexpression of Five Subunits of Human Translation Initiation Factor eIF3 Promotes Malignant Transformation of Immortal Fibroblast Cells

Abstract

Transcriptional and post-transcriptional regulatory mechanisms are commonly accepted paradigms of tumorigenesis. The view is emerging that deregulation of translation contributes importantly to cancer development, a role not generally appreciated before. Eukaryotic initiation factor eIF3 contains at least thirteen non-identical subunits, named from eIF3a to eIF3m, and plays an essential role in the rate-limiting initiation phase of translation. Increased mRNA and protein levels of the eIF3a, -3b, -3c, -3h, and -3i subunits have been detected in a wide variety of human tumors and are frequently identified as prognostic biomarkers for poor clinical outcome. However, it remains to be established whether up-regulation of eIF3 subunits is a consequence or a cause of the malignant phenotypes. Here we report that ectopic expression of eIF3a, -3b, -3c, -3h, or -3i in stably transfected NIH3T3 cells leads to a number of oncogenic properties: decreased doubling times, increased clonogenicity and viability, facilitated S-phase entry, attenuation of apoptosis, formation of transformed foci, and anchorage-independent growth. Only overexpression of the transforming subunits results in a stimulation of initiation and global protein synthesis rates and enhanced translation of poorly translated mRNAs that encode growth-regulating proteins, including cyclinD1, c-Myc, fibroblast growth factor-2, and ornithine decarboxylase, which may be responsible for oncogenic malignancy in the transformed cell lines. Based on these results, we hypothesize that eIF3 contributes to hyperactivation of the translation initiation machinery and thereby may play an important role in neoplasia. Cancer cells appear to require an aberrantly activated translational state to survive, suggesting that the initiation factors may be promising therapeutic targets for treating cancer.Cancers develop when the expression of genes involved in cell proliferation is altered. Although most studies have focused on transcriptional control of gene expression, recent discoveries indicate that the regulation of protein synthesis also may be important in the etiology of cancer (1). Overexpression of the translation initiation factor eIF4E 2 causes malignant transformation of immortal cells (2-7). eIF4E binds the m 7 G-cap structure at the 5Ј terminus of mRNAs and plays a key role in the binding of mRNAs to ribosomes (8). Malignant transformation also follows the expression of a dominant negative mutant form of protein kinase R (9), a protein kinase that phosphorylates the ␣-subunit of eIF2 and inhibits protein synthesis by reducing the binding of the initiator methionyl-tRNA (8). Malignancy caused by reducing eIF2 inhibition also occurs by overexpression of a mutant form of eIF2 that cannot be phosphorylated by protein kinase R (10, 11). These observations have led to the hypothesis that failure to down-regulate protein synthesis results in a malignant phenotype.Mechanisms leading to the regulation of protein synthesis most frequently affect the initiation phase of translat...

eIF3m expression influences the regulation of tumorigenesis-related genes in human colon cancer

“…eIF3f was reported to be lost in melanoma and pancreatic cancer (Doldan et al, 2008). Recently, it was reported that the overexpression of eIF3 subunits leads to malignant transformation of immortal fibroblast cells (Zhang et al, 2007). Thus far, there have been no reports of altered expression of any of the other eIF3 subunits in cancer and whether these eIF3 subunits are involved in tumorigenesis is not known.…”

An oncogenic role of eIF3e/INT6 in human breast cancer

“…Components of eIF3 have previously been reported to support malignant transformation and tumor growth. Individual overexpression of any of five subunits of human eIF3 (eIF3a, b, c, h or i) promoted malignant transformation of immortal fibroblasts (Ahlemann et al, 2006;Savinainen et al, 2006;Zhang et al, 2007). By contrast, studies of eIF3e/INT6 have suggested various roles for this accessory eIF3 subunit either as an oncoprotein or a tumor suppressor (Marchetti et al, 2001;Rasmussen et al, 2001;Buttitta et al, 2005;Chen et al, 2007).…”

Phage display biopanning identifies the translation initiation and elongation factors (IF1α-3 and eIF-3) as components of Hsp70–peptide complexes in breast tumour cells

“…eIF3A expression is cell cycle-regulated, peaking in the S-phase and is a key regulator of a subset of mRNAs including p27 (kip1), tyrosinated α-tubulin and ribonucleotide reductase M2 subunit, which all play central roles in the regulation of the cell cycle (Dong et al 2004;Silvera et al 2010). The over-expression of individual subunits of eIF3 appears to be sufficient to result in the malignant transformation of the immortal NIH3T3 cell line (Zhang et al 2007). Taking these data together, it appears that eIF3A is a major contributor of malignant transformation and/or cell growth control (Saletta et al 2010).…”