Proliferating cells require a nitrogen source for protein and nucleotide biosynthesis.Mammalian cells acquire nitrogen through the uptake and metabolism of amino acids, using mechanisms that are also highly regulated by growth factor signaling.
Myc, which is downstream from several signaling pathways, including those involving Ras and Hedgehog, plays an especially important role in regulating metabolism of the amino acid glutamine. It is perhaps surprising that glutamine metabolism is highly regulated by growth factor signaling, because it is traditionally considered a non-essential amino acid that can be synthesized from other amino acids through enzymes such as glutamine synthetase. However, observations made by Eagle and others revealed more than 50 years ago that glutamine is uniquely critical for the proliferation of mammalian cells in culture. Myc stimulates glutamine uptake by promoting the expression of glutamine transporter proteins. It can also promote the intracellular metabolism of glutamine by increasing the expression of the enzyme glutaminase, which converts glutamine to glutamate and ammonia.
Glutamine's carbon backbone is important for replenishing the supply of mitochondrial Krebs cycle intermediates (anaplerosis). This is accomplished by the conversion of glutamine to glutamate and then to the Krebs cycle intermediate a-ketoglutarate. Importantly, treating cells that are glutamine-addicted because they express oncogenic Myc with a cell-permeable form of a-ketoglutarate can maintain cell viability in the absence of glutamine. However, for the cells to proliferate, the complete glutamine molecule that includes nitrogenous groups is required. Myc-induced glutamine catabolism can provide both the amide and amino groups needed for non-essential amino acid synthesis. It can also generate precursors for the biosynthesis of nucleotides and nicotinamide.Myc provides additional stimulation of nucleotide biosynthesis by promoting the expression of several key enzymes including TS, IMPDH2,and PRPS2.
Growth factor signaling also regulates amino acid metabolism through the mTOR kinase. mTORC1, one of two protein complexes containing mTOR, is stimulated by PI3K/Akt, which phosphorylates and consequently disrupts the tuberous sclerosis complex (TSC). When disrupted, the TSC complex can no longer hydrolyze the GTP-binding protein Rheb to its GDP-bound form, thus leaving the GTP-bound Rheb free to interact with and activate mTORC1. mTORC1 inhibition can also be relieved by Akt phosphorylation of PRAS40 at T246.
mTORC1 promotes protein synthesis by phosphorylating and inhibiting the activity of 4E-BPs, which normally sequester eIF4E and thus block cap-dependent mRNA translation.mTORC1 can also phosphorylate and activate S6Ks to enhance translational elongation by relieving the suppression of eEF2. In addition, mTORC1 regulates the translation of the 5′TOP mRNAs, an abundant class of mRNAs that includes many encoding components of the translational apparatus. mTORC1 also increases ribosome synthesis byactivating RNA polymerase I to transcribe rRNAs.
Through its responsiveness to PI3K/Akt signaling, mTORC1 activity enhances protein synthesis when a cell is directed to grow. However, mTORC1 is also highly sensitive to inputs that reflect the cell's metabolic state. For example, upon depletion of cellular ATP levels, AMP activated protein kinase (AMPK) can inhibit mTORC1 through multiple mechanisms, including activation of the TSC complex and inhibition of the mTOR binding partner Raptor. This provides a means for the cell to down-regulate the energeticallycostly process of protein synthesis when ATP is limiting. In contrast, mTORC1 can be activated by the availability of essential amino acids including leucine. Essential amino acids induce the Rag GTPase complex to assume an active conformation on lysosomal membranes. Active Rag can then recruit mTORC1 to the lysosomal surface, where it may be more amenable to activation by Rheb. Interestingly, recent work shows that cellular leucine uptake is coupled with glutamine efflux, linking Myc regulated glutamine uptake with regulation of mTORC1 by essential amino acid availability.
The much less well understood mTORC2 complex also has important roles in growth factor signaling and metabolic regulation. Like mTORC1, it is activated by growth factors such as insulin. But, in contrast to mTORC1, mTORC2 lies upstream of Akt. Recent data indicate that growth-factor-stimulated activation of mTORC2 involves a direct association with ribosomes, which may ensure that mTORC2 is active only in cells that are growing and undergoing protein synthesis. Once active, mTORC2 can phosphorylate Akt at S473, which is considered important both for enhancing the strength of Akt activation downstream from PI3K and for widening the range of effective Akt substrates.
• Ward P S, Thompson C B. Signaling in control of cell growth and metabolism[J]. Cold Spring Harbor perspectives in biology, 2012, 4(7): a006783.