The mTORC1 Signaling Network Senses Changes in Cellular Purine Nucleotide Levels

Abstract

mTORC1 integrates signals from growth factors and nutrients to control biosynthetic processes, including protein, lipid and nucleic acid synthesis. We find that the mTORC1 pathway is responsive to changes in purine nucleotides in a manner analogous to its sensing of amino acids. Depletion of cellular purines, but not pyrimidines, inhibits mTORC1, and restoration of intracellular adenine nucleotides via addition of exogenous purine nucleobases or nucleosides acutely reactivates mTORC1. Adenylate sensing by mTORC1 is dependent on the tuberous sclerosis complex (TSC) protein complex and its regulation of Rheb upstream of mTORC1, but independent of energy stress and AMPK. While mTORC1 signaling is not acutely sensitive to changes in intracellular guanylates, long-term depletion of guanylates decreases Rheb protein levels. Our findings suggest that nucleotide sensing, like amino acid sensing, enables mTORC1 to tightly coordinate nutrient availability with the synthesis of macromolecules, such as protein and nucleic acids, produced from those nutrients.

IMPDH inhibitors for antitumor therapy in tuberous sclerosis complex

“…Mizoribine also exhibited greater selectivity than MPA in 3 isogenic pairs of TSC2-deficient or -expressing cell lines: a murine Tsc2 -/renal tumor-derived cell line (105K cell line) and a human TSC2 -/renal angiomyolipoma-derived cell line (621-101 cell line), both stably reconstituted with either wild-type TSC2 or empty vector, and HeLa cells with stable shRNA-mediated knockdown of TSC2 or nontargeting control ( Figure 1B Figure 2A). Consistent with previous reports (27,28), higher doses of MPA and AVN-944 reduced mTORC1 signaling in wild-type cells, as measured by phosphorylation of the mTORC1 substrate S6K, likely due to their reported effects on the protein levels of Rheb 36 , whereas mizoribine did not affect Rheb levels or mTORC1 activity ( Figure 1C).…”

“…The greater antitumor efficacy of mizoribine compared with MMF likely results from a greater depletion of tumor guanylate nucleotides in response to mizoribine ( Figure 5C). Unlike mizoribine, MMF was found to partially inhibit mTORC1 signaling, possibly resulting from the partial depletion of tumor adenylates, which are sensed by mTORC1 and required for its activity (27). mTORC1 inhibition could limit the rate of guanylate nucleotide depletion, and thus the antitumor efficacy of MMF, by reducing rRNA synthesis and thus nucleotide demand.…”

Genetics, genomics, and genotype–phenotype correlations of TSC: Insights for clinical practice

“…The black line with an arrow represents the activation process and the red line with a rectangular bar at the end represents the suppression process. (Ballif et al, 2005;Campos et al, 2016;Corradetti, Inoki, Bardeesy, DePinho, & Guan, 2004;Ellisen, 2005;Gao et al, 2015;Hoxhaj et al, 2017;Huang, Wu, Wu, & Manning, 2009;Inoki et al, 2006;Inoki, Li, Zhu, Wu, & Guan, 2002;Natarajan, Trivedi-Vyas, & Wairkar, 2013;Sofer, Lei, Johannessen, & Ellisen, 2005;Zhang et al, 2003) variants have been sought in DNA from blood. However, when the analysis is negative and the patient has a clinical diagnosis but a mild phenotype, clinicians should consider mosaicism and may try to analyze a different tissue, such as buccal smear, skin biopsy of a facial angiofibroma or of a hypomelanotic macule, brain or renal tissue if surgery is going to be performed.…”

T Cell Activation Triggers Reversible Inosine-5'-Monophosphate Dehydrogenase Assembly

“…Recently, mTOR was shown to promote purine biosynthesis (Ben-Sahra et al, 2016), in part as a mechanism to support ribosomal biogenesis (Valvezan et al, 2017). Conversely, purine levels have been shown to regulate mTORC1 activity (Emmanuel et al, 2017;Hoxhaj et al), highlighting an intimate relationship between mTOR and purine nucleotides. Furthermore, mTOR is reported to act downstream of store operated calcium entry to promote metabolic alterations required for T cell activation (Vaeth et al, 2017).…”