Review
mTORC1 signaling controls multiple steps in ribosome biogenesis

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Abstract

Ribosome biogenesis is critical for cells to generate the ribosomes they need for protein synthesis in order to survive, grow and proliferate. It is a complex process, involving the coordinated production of four different RNA species and about 80 proteins, as well as their assembly into functional ribosomal subunits. Given its high demand for amino acids and nucleotides, it is also a metabolically expensive process for the cell. The mammalian target of rapamycin complex 1 (mTORC1) is a protein kinases which is activated by nutrients, anabolic hormones and oncogenic signaling pathways. mTORC1 positively regulates several steps in ribosome biogenesis, including ribosomal RNA transcription, the synthesis of ribosomal proteins and other components required for ribosome assembly. mTORC1 can thus coordinate stimuli which promote ribosome production with the various steps involved in this process. Although important advances have been made in our understanding of mTORC1 signaling, major questions remain about the molecular mechanisms by which it regulates ribosome biogenesis.

Section snippets

Overview and introduction

The ribosome content of a cell potentially imposes an upper limit on the rate of protein synthesis it can sustain. In general, dividing cells need to maintain adequate rates of protein synthesis to double their protein content each time they go through the cell cycle, and the available ribosomes need to be divided between the two daughter cells. Thus, cells undergoing cell division also need to maintain a sufficient rate of ribosome production. Similarly, cells undergoing growth (increase in

mTORC1

A major node of convergence for oncogenic signalling, as well as for responses to nutrients, is the multisubunit protein kinase termed mammalian target of rapamycin complex 1 (mTORC1). mTORC1 contains a catalytic subunit, mTOR, and additional components including mLst8, raptor (which confers substrate specificity on the complex as described below) and Rheb, a small GTPase which, in its GTP-bound form, activates mTORC1 [2]. Some, but not all, of the functions of mTORC1 are impaired by rapamycin,

rRNAs are made by RNA polymerases I and III

As noted, eukaryotic ribosomes contain 4 different rRNAs and a set of RPs. In mammals, the 4 rRNAs are termed 5S, 5.8S, 18S and 28S. The last three are transcribed by RNA polymerase I (Pol I) as a single precursor, the 47S pre-rRNA, which has to be cleaved (processed) to generate the mature rRNA species. Nucleoli are the nuclear regions where synthesis and processing of these rRNAs, as well as other steps in ribosome biogenesis (assembly) occur. Genomes contain numerous (in mammals, hundreds)

Overview of the role of mTORC1 signaling in ribosome assembly

Briefly stated, ribosome biogenesis involves the ordered assembly of the 40S and 60S subunits from the constituent components, i.e., the four rRNAs and about 80 RPs. It requires many sno RNAs, assembly factors and other components, totalling several hundred [15], [18]. For a number of reasons, ribosome production clearly requires sophisticated control mechanisms. Firstly, as described above, it is a crucially important process. Secondly, it is expensive for the cell in terms of its demands for

Regulation of Pol I by mTORC1

Pol I-mediated rDNA transcription (Fig. 1) may be regarded as the very first stage of rRNA synthesis, which is also a rate-determining step in ribosome biogenesis [30], [31]. Dysfunction of Pol I transcription or dysregulation of its control contribute to various human diseases, as reviewed in [32]. The regulation of Pol I is complex and involves several different signaling pathways such as MEK/ERK, AMPK, JNK2 and mTORC1. Early observations that depletion of amino acids, especially branched

Control of Pol III by mTORC1

In all eukaryotes, RNA polymerase III (Pol III) makes the 5S rRNA, tRNAs and other RNAs involved in the processing of rRNA or tRNAs in the nucleoplasm (Fig. 1). mTORC1 associates with Pol III-transcribed gene promoters (including 5S RNA) in mammalian cells [36], [54], [55]. The protein TFIIIC binds DNA to recruit Pol III and associated factors to target genes.

Maf1 is a repressor of Pol III function (Fig. 1). It, and its role in inhibiting Pol III-mediated transcription, are highly conserved

Control of the translation of mRNAs for cytoplasmic RPs

In mammals, the synthesis of the ribosomal proteins in the cytoplasm is tightly regulated by nutrient or oxygen availability and by hormone or growth factor stimulation. All these conditions alter the activity of the mTORC1 pathway [61], [62].

All the mammalian mRNAs for cytoplasmic RPs contain a feature known as a 5′-terminal oligopyrimidine tract (5′-TOP; Fig. 1). This comprises a C residue, immediately after the 5′-cap, followed by an uninterrupted stretch of pyrimidines of about 4–15

Feedback loops from ribosome biogenesis to mTORC1

In 2000, Amaldi and collaborators [78] showed that inhibition of rRNA synthesis by Actinomycin D caused an increase in the proportion of 5′-TOP mRNAs in polysomes. This may be due to activation of the mTORC1 pathway, as suggested by the observation that 4E-BP1 showed increased phosphorylation while pre-treatment of cells with rapamycin eliminated the increase in polysomal 5-TOP mRNAs. Interference with the PeBoW complex, and thus with ribosome biogenesis, in HEK293 cells also causes the

mTORC1 signaling promotes synthesis of precursors for RNA synthesis

Since ribosomal RNA makes up 80-90% of total RNA in cells, its synthesis is a major consumer of precursors such as nucleobases and ribose. Recent studies have revealed that mTORC1 signaling is linked to the supply of such precursors, presumably to ensure an adequate supply of them is available to support the ongoing rates of rRNA transcription and ribosome production. Recent metabolomic studies showed that mTORC1 promotes flux through the pathway by which pyrimidines are synthesised [90]. The

Feedback mechanisms that modulate ribosome biogenesis

As alluded to already, our own recent data show that expression of a truncation mutant of BOP1 (BOP1Δ), which is known to interfere with pre-rRNA processing [28], elicits the activation of mTORC1 signalling [79] and activation of mTORC1 signalling, as shown by the increased phosphorylation of S6K1 (and impaired eEF2K activity) and 4E-BP1 (leading to disinhibition of eIF4E). Thus, the activation of mTORC1 in this setting of impaired ribosome production may be an ‘attempt’ to help stimulate the

Perspectives

Recent years have seen major advances in our understanding of signaling downstream of mTORC1. Nevertheless, several important questions remain to be answered regarding the control of ribosome biogenesis by this pathway. For example, it is not known how mTORC1 and S6Ks promote the activity of Pol I and the mechanisms by which mTOR regulates 5′-TOP mRNA translation are also not understood. Furthermore, it is not clear how mTORC1 signaling promotes the processing of rRNA and whether mTORC1 also

Acknowledgements

Work on ribosome biogenesis in this laboratory is funded by a grant (to CGP) from Cancer Research UK. We thank Dr. Jianling Xie for his helpful advice.

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