A Signaling Pathway to Translational Control

Growth factors however are not the only mechanism available to activate this rapamycin-sensitve signaling pathway. It has been known for over 20 years that small molecule inhibitors of protein translation, like cycloheximide and puromycin, also cause the phosphorylation of S6 in vivo (Gressner and Wool 1974xGressner, A.M and Wool, I.G. Biochem. and Biophys. Res. Com. 1974; 60: 1482–1490Crossref | PubMed | Scopus (48)See all ReferencesGressner and Wool 1974). More recently, anisomycin and cycloheximide treatment have been shown to activate p70S6k in a rapamycin-sensitive fashion (references withinNielsen et al. 1995xNielsen, F.C, Ostergaard, L, Nielson, J, and Christiansen, J. Nature. 1995; 377: 358–362Crossref | PubMedSee all ReferencesNielsen et al. 1995Chou and Blenis, 1995) and cause the phosphorylation of 4E-BP1 (von Manteuffel et al. 1996xvon Manteuffel, S.R, Gingras, A.-C, Sonenberg, N, and Thomas, G. Proc. Natl. Acad. Sci. USA. 1996; 93: 4076–4080Crossref | PubMedSee all Referencesvon Manteuffel et al. 1996). Interestingly, treatment of chick embryo fibroblasts with cycloheximide at concentrations that only partially inhibit elongation causes an overall increase in the rate of synthesis of ribosomal proteins, some of the same proteins whose translation is activated by growth factors and inhibited by rapamycin (Ignotz et al. 1981xIgnotz, G.G, Hokari, S, DePhilip, R.M, Tsukada, K, and Lieberman, I. Biochemistry. 1981; 20: 2550–2558Crossref | PubMedSee all ReferencesIgnotz et al. 1981). In addition, the small molecule anisomycin causes a 2-3 fold increase in the association of eEF1A and IGF-II (6.0 kb variant) mRNAs with polyribosomes (Nielsen et al. 1995xNielsen, F.C, Ostergaard, L, Nielson, J, and Christiansen, J. Nature. 1995; 377: 358–362Crossref | PubMedSee all ReferencesNielsen et al. 1995). Overall, these results suggest that, in addition to a pathway initiated by growth factor receptors, an intracellular signaling cascade emanating from the protein synthesis machinery itself induces translational events that are rapamycin-sensitive. Other than FRAP, p70S6k, and 4E-BP1, the molecules involved in this intracellular signaling pathway remain to be identified. Nevertheless, some clues may be obtained through an analysis of a signal transduction pathway propagated by another member of the PIK-related kinase family, MEC1.MEC1 is an S. cerevisiae gene product that is homologous to FRAP and regulates the cell cycle progression in response to conditions that retard DNA synthesis, such as depletion of deoxyribonucleotides (Sanchez et al. 1996xSanchez, Y, Desany, B.A, Jones, W.J, Liu, Q, Wang, B, and Elledge, S.J. Science. 1996; 271: 357–360Crossref | PubMedSee all ReferencesSanchez et al. 1996references therein). Other than regulating S phase progression, MEC1 responds to the depletion of deoxyribonucleotides (via hydroxyurea treatment) by mediating a signaling pathway that induces the expression of ribonucleotide reductase, an enzyme involved in the biosynthesis of deoxyribonucleic acids. This signaling pathway, which is just beginning to be understood, is strikingly similar to the activation of the FRAP/p70S6k pathway that is initiated by inhibitors of translation elongation (Figure 2Figure 2). In analogy to the MEC1-mediated upregulation of ribonucleotide reductase in response to depletion of deoxyribonucleotides, FRAP appears to mediate an increase in the translation of ribosomal proteins in response to inhibition of the 60S ribosomal subunit. Thus, in addition to an involvement in growth factor–regulated events, these feedback pathways suggest a sensory role for FRAP analogous to that of MEC1. The implication of FRAP in this process and in the control of translation and cell cycle progression provides opportunities for further research in areas that are just beginning to emerge.Figure 2A Comparison of the Currently Understood Signaling Pathways Induced by Treatment with Inhibitors of DNA and Protein SynthesisHydroxyurea (HU) inhibits the activity of ribonucleotide reductase (RNR), an enzyme involved in the synthesis of deoxyribonucleotides. The transcriptional increase in RNR that occurs in response to HU treatment is dependent on the PIK-related kinase MEC1. Genetic studies indicate that the signals generated by MEC1 are mediated by two protein kinases, RAD53 and DUN1 (Sanchez et al. 1996xSanchez, Y, Desany, B.A, Jones, W.J, Liu, Q, Wang, B, and Elledge, S.J. Science. 1996; 271: 357–360Crossref | PubMedSee all ReferencesSanchez et al. 1996). In analogy to the MEC1 pathway, inhibitors of 60S ribosomal subunit increase the translation of mRNA transcripts encoding ribosomal proteins and the eukaryotic elongation factor 1A (eEF1A). These inhibitors also induce the activity of p70S6k and the phosphorylation of 4E-BP1 in a manner that is dependent on FRAP. Arrows in this figure do not indicate direct interactions but rather illustrate the known sequence of events. Other molecules that have not yet been defined certainly may function in this pathway.View Large Image | View Hi-Res Image | Download PowerPoint Slide