Oncogenic Ras has previously been shown to promote macropinocytosis; here it is demonstrated that this process allows tumour cells to use extracellular proteins to generate amino acids necessary to support tumour growth. This paper describes a previously unrecognized pathway of nutrient supply to cancer cells. Oncogenic Ras proteins are known to promote macropinocytosis, an endocytic process by which extracellular fluid and its contents are internalized into cells through vesicles known as macropinosomes. Dafna Bar-Sagi and colleagues now demonstrate that Ras-transformed cells can use this process to 'feed' on extracellular protein. The macropinocytosed protein undergoes degradation, giving rise to free amino acids necessary to support tumour growth. This finding suggests that inhibition of macropinocytosis may be effective against a subset of cancers. Macropinocytosis is a highly conserved endocytic process by which extracellular fluid and its contents are internalized into cells through large, heterogeneous vesicles known as macropinosomes. Oncogenic Ras proteins have been shown to stimulate macropinocytosis but the functional contribution of this uptake mechanism to the transformed phenotype remains unknown1,2,3. Here we show that Ras-transformed cells use macropinocytosis to transport extracellular protein into the cell. The internalized protein undergoes proteolytic degradation, yielding amino acids including glutamine that can enter central carbon metabolism. Accordingly, the dependence of Ras-transformed cells on free extracellular glutamine for growth can be suppressed by the macropinocytic uptake of protein. Consistent with macropinocytosis representing an important route of nutrient uptake in tumours, its pharmacological inhibition compromises the growth of Ras-transformed pancreatic tumour xenografts. These results identify macropinocytosis as a mechanism by which cancer cells support their unique metabolic needs and point to the possible exploitation of this process in the design of anticancer therapies.

Malignant cells are able to survive and grow in detached conditions, despite the associated increase in reactive oxygen species; here a novel metabolic pathway used by cancer cells as they adapt to anchorage-independent growth is described. The paper describes a previously unknown mechanism by which cancer cells reprogram their metabolism to enable growth in three-dimensional spheroids. Detachment of cells from the extracellular matrix is normally inhibited by reactive oxygen species (ROS) that are released by detachment, but malignant cells can acquire the ability to survive and grow in detached conditions. Ralph DeBerardinis and colleagues demonstrate that cells in spheroids mitigate oxidative stress by generating citrate in the cytosol by IDH1-driven reductive glutamine metabolism. Cytosolic citrate is then taken up by the mitochondria where it participates in oxidative metabolism, leading to NADPH generation and suppression of mitochondrial ROS generation. Cells receive growth and survival stimuli through their attachment to an extracellular matrix (ECM)1. Overcoming the addiction to ECM-induced signals is required for anchorage-independent growth, a property of most malignant cells2. Detachment from ECM is associated with enhanced production of reactive oxygen species (ROS) owing to altered glucose metabolism2. Here we identify an unconventional pathway that supports redox homeostasis and growth during adaptation to anchorage independence. We observed that detachment from monolayer culture and growth as anchorage-independent tumour spheroids was accompanied by changes in both glucose and glutamine metabolism. Specifically, oxidation of both nutrients was suppressed in spheroids, whereas reductive formation of citrate from glutamine was enhanced. Reductive glutamine metabolism was highly dependent on cytosolic isocitrate dehydrogenase-1 (IDH1), because the activity was suppressed in cells homozygous null for IDH1 or treated with an IDH1 inhibitor. This activity occurred in absence of hypoxia, a well-known inducer of reductive metabolism. Rather, IDH1 mitigated mitochondrial ROS in spheroids, and suppressing IDH1 reduced spheroid growth through a mechanism requiring mitochondrial ROS. Isotope tracing revealed that in spheroids, isocitrate/citrate produced reductively in the cytosol could enter the mitochondria and participate in oxidative metabolism, including oxidation by IDH2. This generates NADPH in the mitochondria, enabling cells to mitigate mitochondrial ROS and maximize growth. Neither IDH1 nor IDH2 was necessary for monolayer growth, but deleting either one enhanced mitochondrial ROS and reduced spheroid size, as did deletion of the mitochondrial citrate transporter protein. Together, the data indicate that adaptation to anchorage independence requires a fundamental change in citrate metabolism, initiated by IDH1-dependent reductive carboxylation and culminating in suppression of mitochondrial ROS.

An allosteric inhibitor of acetyl-CoA carboxylase reveals a metabolic liability of non-small-cell lung cancer and slows tumor growth alone and in combination with chemotherapy in mouse models. Continuous de novo fatty acid synthesis is a common feature of cancer that is required to meet the biosynthetic demands of a growing tumor. This process is controlled by the rate-limiting enzyme acetyl-CoA carboxylase (ACC), an attractive but traditionally intractable drug target. Here we provide genetic and pharmacological evidence that in preclinical models ACC is required to maintain the de novo fatty acid synthesis needed for growth and viability of non-small-cell lung cancer (NSCLC) cells. We describe the ability of ND-646—an allosteric inhibitor of the ACC enzymes ACC1 and ACC2 that prevents ACC subunit dimerization—to suppress fatty acid synthesis in vitro and in vivo. Chronic ND-646 treatment of xenograft and genetically engineered mouse models of NSCLC inhibited tumor growth. When administered as a single agent or in combination with the standard-of-care drug carboplatin, ND-646 markedly suppressed lung tumor growth in the Kras;Trp53−/− (also known as KRAS p53) and Kras;Stk11−/− (also known as KRAS Lkb1) mouse models of NSCLC. These findings demonstrate that ACC mediates a metabolic liability of NSCLC and that ACC inhibition by ND-646 is detrimental to NSCLC growth, supporting further examination of the use of ACC inhibitors in oncology.