Translational control and metastatic progression: Enhanced activity of the mRNA cap-binding protein eIF-4E selectively enhances translation of metastasis-related mRNAs

To form metastases, tumors must break from the primary tumor site, invade surrounding tissues, enter and survive within the circulation and ultimately colonize a distal tissue. Each of these steps requires the cooperative function of numerous proteins - proteins that facilitate angiogenesis ( e. g., VEGF), cell survival ( e. g., Bcl-2), invasion ( e. g., MMPs), and autocrine growth stimulation ( e. g., c-myc, cyclin D1). Although expression of these proteins is regulated at many levels by disparate stimuli, translation of these key malignancy-related proteins is regulated primarily by the activity of the mRNA cap-binding protein eIF-4E, the rate-limiting member of the eIF-4F translation initiation complex. By binding the cap structure at the 5' terminus of cellular mRNAs, eIF-4E recruits mRNAs to the eIF-4F complex, which then scans from the 5' cap through the untranslated region (5' UTR), unwinding secondary structure to reveal the translation initiation codon and to enable ribosome loading. Messenger RNAs with short unstructured 5' UTRs are more easily translated than mRNAs harboring lengthy, highly structured 5' UTRs, as these prohibit efficient scanning and start codon recognition. As such, the translation of these mRNAs, which typically encode proteins involved in angiogenesis ( e. g., VEGF), tumor growth ( cyclin D1) and survival (Bcl-2), is suppressed except when eIF-4E is engaged with the eIF-4F complex - a common event in many human and experimental cancers. This review focuses on the hypothesis that enhanced eIF-4E function contributes to metastatic progression by selectively upregulating the translation of key malignancy-related proteins that together conspire to drive the metastatic process.