Abstract
Protein synthesis is important for maintaining cellular homeostasis under various stress responses. In this study, we screened an anticancer drug library to select compounds with translational repression functions. AZD8055, an ATP-competitive mechanistic target of rapamycin complex 1/2 (mTORC1/2) inhibitor, was selected as a translational suppressor. AZD8055 inhibited protein synthesis in mouse embryonic fibroblasts and hepatocellular carcinoma HepG2 cells. Extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) were activated during the early phase of mTORC1/2 inhibition by AZD8055 treatment. Combined treatment of AZD8055 with the MAPK kinase1/2 (MEK1/2) inhibitor refametinib or the p38 inhibitor SB203580 markedly decreased translation in HepG2 cells. Thus, the inhibition of ERK1/2 or p38 may enhance the efficacy of AZD8055-mediated inhibition of protein synthesis. In addition, AZD8055 down-regulated the phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), and AZD8055-induced phosphorylation of ERK1/2 and p38 had no effect on phosphorylation status of 4E-BP1. Interestingly, AZD8055 modulated the 4E-BP1 mRNA pool by up-regulating ERK1/2 and p38 pathways. Together, these results suggest that AZD8055-induced activation of MAPKs interferes with inhibition of protein synthesis at an early stage of mTORC1/2 inhibition, and that it may contribute to the development of resistance to mTORC1/2 inhibitors.
Highlights
Regulation of protein synthesis is an energy-requiring cellular process
As eIF2α is a well-known translational regulator that responds to various stress stimuli [36,37], we investigated whether AZD8055-mediated translational inhibition is dependent on eIF2α
Combined treatment of 2 μM AZD8055 with 100 nM refametinib and 10 μM SB203580, did not significantly down‐regulate the degree of protein synthesis compared to the use of 2 μM AZD80558wofi1t5h refametinib or SB203580 (Figure 5d). These results suggest that AZD8055‐ mediated up‐regulation of ERK1/2 and p38 activation eventually interrupts with bArZaiDn8[04565,4‐7m],ewdiaastendoitnahltiebrietidonafotefrpAroZteDin80s5y5ntrheeastims.eInta(dFdigiutiroen6, dac).tiTvhaetisoenroesfuElRtsKs1u/g2gaensdt tph3a8t AbyZDA8Z0D5580r5e5gumlaatyesb4eEi-nBtPegtratnesdcriinpttoiounnlkevneolws vniasiEgRnaKl1in/g2 panatdhpw3a8y, sw. hich are involved in the inhibition of protein synthesis pathways
Summary
Regulation of protein synthesis is an energy-requiring cellular process. Several eukaryotic initiation factors (eIFs) tightly regulated by various signals are involved in translation initiation. As dysregulation of translation machinery in cancer cells is associated with poor cancer prognosis, it is imperative to investigate the role of anticancer agents in the regulation of protein synthesis. EIF4E-binding proteins (4E-BPs) are wellknown suppressors of protein synthesis. Hypophosphorylated 4E-BP1 competes with eIF4G and binds to eIF4E, interfering with the formation of the 43S pre-initiation complex and disrupting protein synthesis [1,2]. Hyperphosphorylated 4E-BP1 fails to bind to eIF4E and is released, which restarts the translation machinery [3,4,5,6,7]
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