MILESTONES

MILESTONE 6 Moreover, metabolism in the pentose- phosphate pathway (PPP) was found to be crucial for oncogenic KRAS-induced growth, through generating nucleotide precursors as Not a simple switch well as NADPH. PPP-derived antioxidants are also linked to promoting survival, as shown by Joan Brugge’s group, thus highlighting the importance of antioxidant defence in cancer growth. Research in the 2010s led the field further away from Warburg’s path, showing substantial heterogeneity in cancer metabolism, and high- lighting the oncogenotype and tissue environ- ment as key determinants. Eileen White’s and Alec Kimmelman’s groups revealed that onco- genic KRAS-driven cancer cells depend on autophagy, a catabolic pathway that degrades intracellular organelles and macromolecules. Autophagy maintains mitochondrial metab- olism and control, and allows cancer cells to maintain growth in nutrient-scarce

Credit: peepo Credit: environments. David M. Sabatini’s and Cantley’s groups have reported amplification of Cells respire—they consume and The link of cancer-driving gene the gene encoding the rate-limiting enzyme in glucose to produce in the form of to the Warburg effect was corroborated on the serine synthesis pathway in certain cancer ATP. When oxygen is not available, cells in the level of tumour suppressors. Cellular res- types, thus making the growth of these differentiated tissues break glucose down piration relies on the tricarboxylic acid (TCA) dependent on serine and glycine metabolism. into lactate (through ) for energy cycle as well as the electron-transport chain Crucially, J. Michael Bishop’s group has shown production. The ‘Warburg effect’ describes (ETC) coupled to oxidative phosphorylation in mice that metabolism in MYC-driven liver Otto Warburg’s observations in the 1920s (OXPHOS) in the mitochondria. Indeed, the cancer not only differs from metabolism in that malignant tumour cells mainly perform tumour-suppressor protein p53 was found to MYC-driven lung cancer, but also differs from glycolysis even in the presence of oxygen participate in controlling the balance between metabolism in liver cancer driven by the onco- (aerobic glycolysis), thus resulting in high glycolysis and OXPHOS, as reported by Paul protein MET. In humans, Ralph J. DeBerardinis lactate secretion. These observations led him M. Hwang’s and Karen Vousden’s groups in and colleagues have observed metabolic to believe that during malignant transforma- 2006 (See Mitochondrial complex II mutations heterogeneity among tumours across patients tion, initial irreversible damage in respiration found in tumours on the interactive timeline). and within the same patient as well as among was followed by upregulation of glycolysis The TCA cycle also generates molecules regions within the same tumour. to replace the loss of respiration energy. needed for biomass production. Considering This complexity signifies that the cancer According to Warburg, the origin of cancer that glycolysis is a relatively inefficient way to metabolism field has long outgrown the con- was purely based on metabolic alterations. produce energy, Craig B. Thompson’s group, fines of the hypothesis that Warburg proposed We know today that Warburg was not quite in one of the first cancer metabolism studies nearly a century ago. Since then, seminal right: oncogenic signalling fundamentally to use carbon-13 isotope tracing, explored the findings have been reported by many groups. contributes to , metabolic underpinnings of the Warburg effect. Although research in the past two decades partly by regulating metabolism. They found that cancer cells use glucose- has largely focused on primary tumours, By the 1990s, the presence of aerobic derived TCA-cycle intermediates in synthetic insights into metabolism during metastasis, glycolysis in some tumours had been clinically pathways (particularly fatty acid synthesis). in the tumour microenvironment, and in the exploited for cancer detection through fluoro- Cells also require continuous replenishment systemic context, are rapidly advancing. To deoxyglucose–PET imaging. Yet the molecular of the TCA cycle with a non-glucose carbon date, only a limited number of agents targeting basis of the Warburg effect remained ill-de- source—a process achieved through uptake cancer metabolism have been successfully fined. In 1997, Chi V. Dang and colleagues and metabolism of the amino acid glutamine. applied in the clinic (See IDH1 mutations lead reported that the glycolytic enzyme lactate These and other data led Matt G. Vander to the generation of 2-hydroxyglutarate and dehydrogenase A (LDHA), which catalyses Heiden, Lewis C. Cantley and Thompson FDA approval of enasidenib for acute myeloid the final step of glycolysis, is a transcriptional to propose a model in which metabolism in leukaemia on the interactive timeline), but this target of the oncoprotein MYC. They showed cancer cells is adapted to optimize access to is likely to change in the future. that overexpression of MYC or LDHA resulted nutrients and their incorporation into biomass, Ulrike Harjes, Nature Reviews Cancer in greater aerobic lactate production in cul- thereby generating a growth advantage. ORIGINAL ARTICLES Shim, H. et al. c-Myc transactivation of tured fibroblasts. LDHA was also necessary Further shaking Warburg’s hypothesis of LDH-A: implications for tumor metabolism and growth. Proc. for MYC-induced anchorage-independent cell irreversibly damaged mitochondria in cancer, Natl Acad. Sci. USA 94, 6658–6636 (1997) | Matoba, S. et al. p53 regulates mitochondrial respiration. Science 312, 1650–1653 growth. These findings provided a molecular Navdeep S. Chandel’s group showed that (2006) | Bensaad, K. et al. TIGAR, a p53-inducible regulator of basis for the Warburg effect, which, contrary mitochondrial metabolism and ETC function glycolysis and . Cell 126, 107–120 (2006). to Warburg’s hypothesis, involved an oncogene are required for cancer cell growth induced FURTHER READING Please visit the online article for a full list of further reading. frequently altered in cancer. by the oncoprotein KRAS in vitro and in vivo.

NATURE MILESTONES | CANCER DECEMBER 2020 | S9