Expression of Truncated Eukaryotic Initiation Factor 3e (eIF3e) Resulting from Integration of Mouse Mammary Tumor Virus (MMTV) Causes a Shift from Cap-dependent to Cap-independent Translation

Abstract

Integration of mouse mammary tumor virus (MMTV) at the common integration site Int6 occurs in the gene encoding eIF3e, the p48 subunit of translation initiation factor eIF3. Integration is at any of several introns of the Eif3e gene and causes the expression of truncated Eif3e mRNAs. Ectopic expression of the truncated eIF3e protein resulting from integration at intron 5 (3e5) induces malignant transformation, but by an unknown mechanism. Because eIF3e makes up at least part of the binding site for eIF4G, we examined the effects of 3e5 expression on protein synthesis. We developed an NIH3T3 cell line that contains a single copy of the 3e5 sequence at a predetermined genomic site. Co-immunoprecipitation indicated diminished binding of eIF3 to eIF4G, signifying a reduction in recruitment of the mRNA-unwinding machinery to the 43 S preinitiation complex. Cell growth and overall protein synthesis were decreased. Translation driven by the eIF4G-independent hepatitis C virus internal ribosome entry sequence (HCV IRES) in a bicistronic mRNA was increased relative to cap-dependent translation. Endogenous mRNAs encoding XIAP, c-Myc, CYR61, and Pim-1, which are translated in a cap-independent manner, were shifted to heavier polysomes whereas mRNAs encoding GAPDH, actin, L32, and L34, which are translated in a cap-dependent manner, were shifted to lighter polysomes. We propose that expression of 3e5 diminishes eIF4G interaction with eIF3 and causes abnormal gene expression at the translational level. The correlation between up-regulation of cap-independent translation and MMTV-induced tumorigenesis contrasts with the well established model for malignant transformation involving up-regulation of highly cap-dependent translation.

Translation Regulation as a Therapeutic Target in Cancer

“…In contrast, eIF3f is downregulated in melanoma and pancreatic cancer, and its overexpression inhibits proliferation and protein synthesis and induces apoptosis (33). The murine Int-6 gene, encoding eIF3e, was first identified as a mouse mammary tumor virus integration site (34,35) that results in the production of a truncated oncoprotein with transforming activity (36) capable of inducing cap-independent translation, as reported recently (37). Several subsequent studies of full-length eIF3e showed its tumor suppressor activity (38)(39)(40).…”

Tumor Suppressor PDCD4 Represses Internal Ribosome Entry Site-Mediated Translation of Antiapoptotic Proteins and Is Regulated by S6 Kinase 2

“…PCR primers specific for XIAP isoforms, Bcl-x L , Apaf-1, VEGF-A, cIAP1, or GAPDH were used to amplify messages by the use of quantitative PCR as described above. Weighted averages (F W ) were calculated for the distribution of each mRNA as described previously (8). ⌬F W represents the difference between small interfering control cells and siPDCD4-treated cells in mRNA polysome distribution.…”

“…HEK293T cells were initially transfected with PDCD4-targeting siRNA or a nonsilencing control siRNA. At 48 h later, the cells were transfected with the bicistronic reporter plasmids, and reporter protein levels were assayed after 24 h. We found that the activity of both the XIAP and Bcl-x L IRES elements was enhanced approximately (8). (E) Bicistronic DNA constructs containing XIAP or Bcl-x L IRES elements were transfected into HEK293T cells after treatment with PDCD4 siRNA or control (CTRL) siRNA.…”

Decreased eIF3e Expression Can Mediate Epithelial-to-Mesenchymal Transition through Activation of the TGFβ Signaling Pathway

“…A recent study found that expression of a truncated eIF3e protein that lacks normal eIF3e activity (i.e., the ability to bind to eIF4G) causes both a decrease in global, cap-dependent translation initiation, and a concomitant increase in an alternative initiation mechanism that relies on RNA structures called internal ribosome entry sites (IRES; ref. 3), suggesting that decreased eIF3e activity creates an environment that is favorable for mRNAs that rely on capindependent translation. Some translation initiation factors have been shown to be involved in cancer development and progression (4), most notably the cap-binding protein eIF4E (5) whose overexpression has been correlated with many types of cancer (6), including breast (7,8) and lung (9) cancers.…”