Cyclin D1 and c-myc Internal Ribosome Entry Site (IRES)-dependent Translation Is Regulated by AKT Activity and Enhanced by Rapamycin through a p38 MAPK- and ERK-dependent Pathway

Abstract

The macrolide antibiotic rapamycin inhibits the mammalian target of rapamycin protein (mTOR) kinase resulting in the global inhibition of cap-dependent protein synthesis, a blockade in ribosome component biosynthesis, and G 1 cell cycle arrest. G 1 arrest may occur by inhibiting the protein synthesis of critical factors required for cell cycle progression. Hypersensitivity to mTOR inhibitors has been demonstrated in cells having elevated levels of AKT kinase activity, whereas cells containing quiescent AKT activity are relatively resistant. Our previous data suggest that low AKT activity induces resistance by allowing continued cap-independent protein synthesis of cyclin D1 and c-Myc proteins. In support of this notion, the current study demonstrates that the human cyclin D1 mRNA 5 untranslated region contains an internal ribosome entry site (IRES) and that both this IRES and the c-myc IRES are negatively regulated by AKT activity. Furthermore, we show that cyclin D1 and c-myc IRES function is enhanced following exposure to rapamycin and requires both p38 MAPK and RAF/MEK/ERK signaling, as specific inhibitors of these pathways reduce IRES-mediated translation and protein levels under conditions of quiescent AKT activity. Thus, continued IRES-mediated translation initiation may permit cell cycle progression upon mTOR inactivation in cells in which AKT kinase activity is relatively low.The global regulation of cap-dependent translation is mediated via the mTOR 1 signaling cascade (1-3). Activation of mTOR results in phosphorylation of the p70 S6 kinase and the translation repressor 4E-BP1, allowing the formation of functional eIF-4F complexes resulting in cap-dependent mRNA translation initiation and ribosomal component biogenesis (4, 5). The efficiency with which a mRNA can initiate cap-dependent translation is a function of the length and degree of secondary structure present within the 5Ј-UTR as well as the sequence context of the initiation codon (6). Most eukaryotic mRNAs contain 5Ј-UTRs with relatively short and unstructured 5Ј-UTRs (Ͻ100 nucleotides), which allow efficient capdependent ribosomal scanning (6). However, some key regulators of cell proliferation and apoptosis have leaders that are quite long, highly structured, and contain many upstream AUG or CUG codons and, as a result, are inhibitory to scanning ribosomes (7). Translation initiation in a number of these mRNAs is achieved via IRES-mediated mechanisms (8). Protein synthesis via this alternative form of initiation is typically favored under conditions when the default cap-dependent pathway is inhibited (9 -12).The ability of AKT to regulate cap-dependent initiation is mediated via its inhibitory effects on the mTOR inhibitor complex TSC1/TSC2 (13-15). A direct linkage between AKT and the mTOR kinase has also been described. AKT can phosphorylate mTOR, and studies in Drosophila have demonstrated that dTOR (Drosophila target of rapamycin) is downstream and epistatic to the phosphatidylinositol 3-kinase/AKT pathway (5,16,17). However, AKT has rec...

AKT activity regulates the ability of mTOR inhibitors to prevent angiogenesis and VEGF expression in multiple myeloma cells

“…However, an alternative, IRES-regulated salvage pathway allows for RNA translation under conditions in which cap-dependent protein translation is inhibited, such as during exposure to rapamycin. In recent work in other tumor models, AKT activity has been shown to negatively regulate the IRES function of c-myc and cyclin D1 Shi et al, 2005). We now describe a third IRES, that of VEGF, whose function is also regulated by AKT activity.…”

“…A possible example of an indirect influence is the inhibitory effect of AKT on p38 and extracellular signal-regulated protein kinase (ERK) mitogen-activated protein kinase function (Berra et al, 1998). These kinases are important for myc IRES function (Shi et al, 2005), and the ability of AKT to downregulate their activity explains AKT's negative regulatory role. Experiments are underway to determine if p38 and/or ERK are also important for VEGF IRES function.…”

9-Aminoacridine-based anticancer drugs target the PI3K/AKT/mTOR, NF-κB and p53 pathways

“…One possible explanation is that p110a expression might rely on cap-independent translation initiation mechanism involving internal ribosome entry site (IRES) element (Komar and Hatzoglou, 2005). IRESs elements are generally insensitive to mTOR inhibition by rapamycin and their activity can be even enhanced under conditions of rapamycin treatment (see for example, Shi et al, 2005).…”

Synergistic growth inhibition by Iressa and Rapamycin is modulated by VHL mutations in renal cell carcinoma

“…For instance, ERK1/2 activation affects multiple translation initiation components including S6K, p90 RSK, MNK and 4E-BP1, among others (Roux and Blenis, 2004). Also, in addition to eIF4E-mediated cap-dependent initiation, there are internal ribosome entry sites present in several mRNAs important in RCC such as MYC, VEGF and Cyclin D1 (Stoneley and Willis, 2004;Shi et al, 2005), which may allow translation initiation in a cap-independent manner.…”