Tumorigenesis is dependent on the reprogramming of cellular metabolism as both direct and indirect consequence of oncogenic mutations. A common feature of cancer cell metabolism is the ability to acquire necessary nutrients from a frequently nutrient-poor environment and utilize these nutrients to both maintain viability and build new biomass. The alterations in intracellular and extracellular metabolites that can accompany cancer-associated metabolic reprogramming have profound effects on gene expression, cellular differentiation and the tumor microenvironment. In this Review, we have organized known cancer-associated metabolic changes into six hallmarks: (1) deregulated uptake of glucose and amino acids, (2) use of opportunistic modes of nutrient acquisition, (3) use of glycolysis/TCA cycle intermediates for biosynthesis and NADPH production, (4) increased demand for nitrogen, (5) alterations in metabolite-driven gene regulation, and (6) metabolic interactions with the microenvironment. While few tumors display all six hallmarks, most display several. The specific hallmarks exhibited by an individual tumor may ultimately contribute to better tumor classification and aid in directing treatment.

Summary Despite being surrounded by diverse nutrients, mammalian cells preferentially metabolize glucose and free amino acids. Recently, Ras-induced macropinocytosis of extracellular proteins was shown to reduce a transformed cell’s dependence on extracellular glutamine. Here, we demonstrate that protein macropinocytosis can also serve as an essential amino acid source. Lysosomal degradation of extracellular proteins can sustain cell survival and induce activation of mTORC1, but fails to elicit significant cell accumulation. Unlike its growth-promoting activity under amino acid-replete conditions, we discovered that mTORC1 activation suppresses proliferation when cells rely on extracellular proteins as an amino acid source. Inhibiting mTORC1 results in increased catabolism of endocytosed proteins and enhances cell proliferation during nutrient-depleted conditions in vitro and within vascularly compromised tumors in vivo. Thus, by preventing nutritional consumption of extracellular proteins, mTORC1 couples growth to availability of free amino acids. These results may have important implications for the use of mTOR inhibitors as therapeutics.

Biochemistry textbooks and cell culture experiments seem to be telling us two different things about the significance of external glutamine supply for mammalian cell growth and proliferation. Despite the fact that glutamine is a nonessential amino acid that can be synthesized by cells from glucose-derived carbons and amino acid-derived ammonia, most mammalian cells in tissue culture cannot proliferate or even survive in an environment that does not contain millimolar levels of glutamine. Not only are the levels of glutamine in standard tissue culture media at least ten-fold higher than other amino acids, but glutamine is also the most abundant amino acid in the human bloodstream, where it is assiduously maintained at approximately 0.5 mM through a combination of dietary uptake, synthesis, and muscle protein catabolism. The complex metabolic logic of the proliferating cancer cells' appetite for glutamine-which goes far beyond satisfying their protein synthesis requirements-has only recently come into focus. In this review, we examine the diversity of biosynthetic and regulatory uses of glutamine and their role in proliferation, stress resistance, and cellular identity, as well as discuss the mechanisms that cells utilize in order to adapt to glutamine limitation.

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The transcription factor REST/NSRF (RE1-Silencing Transcription Factor) is a master repressor of neuronal gene expression and neuronal programs in non-neuronal lineages 1 − 3 . Recently, REST was identified as a human tumor suppressor in epithelial tissues 4 , suggesting that REST regulation may have important physiologic and pathologic consequences. However, the pathways controlling REST have yet to be elucidated. Here, we demonstrate that REST is regulated by ubiquitin-mediated proteolysis, and use an RNAi screen to identify SCF βTRCP as an E3 ubiquitin ligase responsible for REST degradation. βTRCP binds and ubiquitinates REST and controls its stability through a conserved phosphodegron. During neural differentiation REST is degraded in a βTRCP-dependent manner. βTRCP is required for proper neural differentiation only in the presence of REST, indicating that βTRCP facilitates this process through degradation of REST. Conversely, failure to degrade REST attenuates differentiation. Furthermore, we find that βTRCP overexpression, which is common in human epithelial cancers, causes oncogenic transformation of human mammary epithelial cells and this pathogenic function requires REST degradation. Thus, REST is a key target in βTRCP-driven transformation and the βTRCP-REST axis is a new regulatory pathway controlling neurogenesis.REST levels decline during differentiation of embryonic stem cells to neural stem and progenitor cells 5 , consistent with a role for REST in restraining neuronal gene expression programs. This decrease results from a 3-fold reduction in REST half-life (Fig. 1a), suggesting that a regulatory pathway controls REST degradation during early neural differentiation. To determine whether ubiquitination is involved, REST was evaluated for ubiquitin-modification in vivo. Immunoprecipitation of HA-ubiquitin revealed slower migrating species of REST suggestive of polyubiquitination (Fig. 1b, lane 3 (Fig. 1b, lane 4), suggesting REST is K48 polyubiquitinated which promotes degradation.To search for the E3 ubiquitin ligase for REST, we began with the SCF superfamily of ligases 6 . Each SCF family contains a common Cullin scaffold that is required for ligase function. Notably, coexpression of a dominant negative Cullin-1 (Cul1) mutant resulted in a dramatic increase (11-fold) in REST levels (Supp. Fig. 1b), indicating that one or more Cul1-containing ligases negatively regulate REST abundance.F-box proteins act as substrate receptors for the SCF 7,8 . To determine which F-box proteins are required for REST turnover, we established a system for monitoring REST abundance in a high-throughput manner using an mRFP-REST fusion protein. Similar to endogenous REST, mRFP-REST was unstable, and its abundance increased upon inhibition of Cul-1 (Supp. Fig. 2a). To identify the F-box proteins regulating REST, individual siRNAs targeting each F-box protein (4 siRNAs/gene) were cotransfected with a plasmid encoding mRFP-REST, and changes in cellular fluorescence were monitored by flow cytometry (Supp. Fig. 2...