Mnk earmarks eIF4E for cancer therapy

Abstract

The PI3K/Akt/mTORC1 axis is perhaps the most frequently activated pathway in human cancers. Therefore, concerted efforts are being made to inhibit this pathway pharmacologically for cancer therapy. Although some success has been achieved, the multiple feedback loops that “litter” this pathway (ref. 1; Fig. 1), combined with the potential toxicity resulting from its inhibition, make the pharmacological targeting efforts very challenging. The ideal goal is to inhibit the functionality of the pathway in cancer cells without affecting normal cells and without eliciting feedback loops that could diminish the therapeutic efficacy. Back-to-back papers published in PNAS may provide this kind of therapeutic avenue (2, 3). Schematic illustration depicting the cellular pathways that lead to eIF4E activation and phosphorylation by Mnk1/2. The PI3K/Akt/mTORC1 pathway, which is frequently activated in human cancers, releases 4E-BPs from eIF4E, and enables eIF4E to bind eIF4G, which, in turn, assembles the eIF4F complex comprising eIF4E, eIF4G, eIF4A, and eIF3. Mnk1 and Mnk2, which are activated by Erk and by the stress inducible kinase p38, use eIF4G as a docking site to phosphorylate efficiently eIF4E. The phosphorylation of eIF4E is critical for its oncogenic activity, probably through the differential translation of proteins that are required for oncogenesis. The phosphorylation of eIF4E by Mnk1/2 provides a new avenue for cancer therapy. The inhibition of eIF4E phosphorylation could have similar consequences as the inhibition mTORC1 by rapalaogs, but with the advantage that it does not elicit the activation of Akt as a result of the inhibition of the negative feedback loops mediated by mTORC1. Previous studies suggest that the most critical target of the serine/threonine kinase Akt, required for tumorigenesis, is mTORC1 (4). The two major downstream targets of mTORC1 are S6K1 and eukaryotic initiation factor-4E binding proteins (4E-BPs), which regulate ribosomal biogenesis and mRNA translation. S6K1 …