Abstract
Human mRNA DeXD/H-box helicases are ubiquitous molecular motors that are required for the majority of cellular processes that involve RNA metabolism. One of the most abundant is eIF4A, which is required during the initiation phase of protein synthesis to unwind regions of highly structured mRNA that would otherwise impede the scanning ribosome. Dysregulation of protein synthesis is associated with tumorigenesis, but little is known about the detailed relationships between RNA helicase function and the malignant phenotype in solid malignancies. Therefore, immunohistochemical analysis was performed on over 3000 breast tumors to investigate the relationship among expression of eIF4A1, the helicase-modulating proteins eIF4B, eIF4E and PDCD4, and clinical outcome. We found eIF4A1, eIF4B and eIF4E to be independent predictors of poor outcome in ER-negative disease, while in contrast, the eIF4A1 inhibitor PDCD4 was related to improved outcome in ER-positive breast cancer. Consistent with these data, modulation of eIF4A1, eIF4B and PCDC4 expression in cultured MCF7 cells all restricted breast cancer cell growth and cycling. The eIF4A1-dependent translatome of MCF7 cells was defined by polysome profiling, and was shown to be highly enriched for several classes of oncogenic genes, including G-protein constituents, cyclins and protein kinases, and for mRNAs with G/C-rich 5′UTRs with potential to form G-quadruplexes and with 3′UTRs containing microRNA target sites. Overall, our data show that dysregulation of mRNA unwinding contributes to the malignant phenotype in breast cancer via preferential translation of a class of genes involved in pro-oncogenic signaling at numerous levels. Furthermore, immunohistochemical tests are promising biomarkers for tumors sensitive to anti-helicase therapies.
Highlights
Translation initiation is rate limiting, highly regulated and dependent on the coordinated action of eukaryotic initiation factors.[3,4] The DEAD-box helicase eIF4A1 is required to unwind structured RNA elements within the 5′ untranslated region (5′UTR) to facilitate ribosome binding and scanning, its activity is stimulated by interaction with the proteins eIF4B5 and eIF4E,6 and inhibited by the tumor suppressor programmed cell death 4 (PDCD4)
EIF4E is a well-established oncogene,[10,11] and the translation of several oncogenic mRNAs with long or structured 5′UTRs, such as c-myc, cyclin D1 and ornithine decarboxylase (ODC) were shown to correlate with eIF4E expression.[12] eIF4B is required for HeLa cell growth,[13] and eIF4B overexpression was associated with poor survival of lymphoma patients, probably resulting from increased eIF4A1-dependent expression of DAXX, BCL2 and ERCC5.14 In contrast, high levels of
To assess eIF4A1 helicase activity in breast tumors, tissue microarrays (TMAs) derived from 3903 patients were scored for eIF4E, eIF4A1, eIF4B and PDCD4
Summary
Translation initiation is rate limiting, highly regulated and dependent on the coordinated action of eukaryotic initiation factors (eIFs).[3,4] The DEAD-box helicase eIF4A1 is required to unwind structured RNA elements within the 5′ untranslated region (5′UTR) to facilitate ribosome binding and scanning, its activity is stimulated by interaction with the proteins eIF4B5 and eIF4E,6 and inhibited by the tumor suppressor PDCD4.7. EIF4E is a well-established oncogene,[10,11] and the translation of several oncogenic mRNAs with long or structured 5′UTRs, such as c-myc, cyclin D1 and ornithine decarboxylase (ODC) were shown to correlate with eIF4E expression.[12] eIF4B is required for HeLa cell growth,[13] and eIF4B overexpression was associated with poor survival of lymphoma patients, probably resulting from increased eIF4A1-dependent expression of DAXX, BCL2 and ERCC5.14 In contrast, high levels of PDCD4 were associated with a reduction in the expression of growth-promoting factors such as ODC and CDK4,15 and correlate with good outcome in a small study of ER-positive breast tumors,[16] while levels were reduced in several tumor types including breast adenocarcinoma.[17].
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