Taking Food Away: Targeting Nutrient Scavenging by RAS-driven Cancers

August 25, 2015, by Eileen White

It has long been known that cancers commonly have an altered metabolism that favors aerobic glycolysis¹. Indeed, the increased glucose uptake of cancers is the basis for fluorodeoxyglucose positron emission tomography (FDG-PET) imaging in widespread use for cancer detection and monitoring. We now know that oncogenic events, including oncogenic activation of Ras genes, can produce this and many other metabolic changes^2,3. We also now know that these metabolic changes favor the production of building blocks, energy and redox homeostasis for the generation of new cancer cells, providing an explanation for their occurrence. This has suggested that targeting the altered metabolic pathways in cancer may be a viable therapeutic option.

It has recently emerged that cancers may not only use food differently in their reprogrammed metabolism, but they also obtain food differently. This is a particular characteristic of Ras-driven cancers, which engage in nutrient scavenging. Simply put, Ras promotes the uptake and metabolic consumption of extracellular proteins and fatty acids by macropinocytosis^4-6 and of intracellular components by macroautophagy (autophagy)^7-11. Why do Ras-driven cancers scavenge extracellular and intracellular nutrients, a property not apparently shared extensively with most normal cells? While we are only beginning to answer this question, we know that nutrient scavenging provides an alternative and readily accessible food supply, can circumvent the requirement for de novo macromolecular synthesis, and confers stress tolerance^9,12. As inhibition of nutrient scavenging compromises the growth, proliferation, survival and tumorigenesis of Ras-driven cancers, this reveals a selective metabolic vulnerability potentially targetable for cancer therapy.

Extracellular Nutrient Scavenging by Macropinocytosis

How does macropinocytosis work and what does it specifically do? Ras activation induces plasma membrane ruffling and endocytic uptake of extracellular fluid into vesicles called macropinosomes⁴. Extracellular fluid contains many potential nutrient sources including lipids and proteins, particularly albumin. Albumin is taken up by macropinocytosis and delivered to lysosomes where it is degraded, providing amino acids for central carbon metabolism^5,13. Indeed, albumin can support the growth of pancreatic cancer cell lines in the absence of essential amino acids¹³. Pharmacologic inhibition of macropinocytosis with the amiloride derivative EIPA, which inhibits sodium-proton exchange, suppresses the growth of Ras-driven pancreatic xenographs⁵. These findings suggest that scavenging of extracellular protein is important for the metabolism, growth and survival of Ras-driven pancreatic cancers and that targeting this mechanism may provide therapeutic benefit^9,12.

Ras-driven cancers also scavenge unsaturated fatty acids from extracellular serum lysophopholipids, bypassing the need for de novo lipogenesis⁶. Lipid synthesis consumes cellular resources, thus scavenging of fatty acids promotes metabolic robustness. While it is not yet known if fatty acid scavenging occurs via macropinocytosis, it is highly probable. Scavenging of other macromolecules beyond albumin and fatty acids may also contribute to the enhancement of the metabolism of Ras-driven cancers^9,12.

Intracellular Nutrient Scavenging by Autophagy

How and why is autophagy upregulated in Ras-driven cancers and what does it do? Autophagy captures intracellular proteins and organelles, degrades them in lysosomes, and recycles their building blocks into metabolic and biosynthetic pathways¹⁴. Autophagy normally functions at a low level in normal tissues but is upregulated by stress and starvation. In contrast, Ras-driven cancers often have high levels of basal autophagy that promotes metabolism, growth and survival^7,8.

Ras upregulates nuclear localization and activity of members of the Microphthalmia-associated transcription factor (MiT) family of basic helix-loop-helix-leucine zipper (bHLH-Zip) transcription factors (MiTF, TFE3, TEFB and TFEC)¹⁵. MiT family members are the master regulators of autophagy and lysosomal biogenesis, and their activation by Ras is one mechanism of autophagy upregulation. Interestingly, non-Ras cancers activate MiT family members by chromosomal translocation and other mechanisms¹⁶, suggesting a more general requirement for lysosomal-based nutrient scavenging beyond cancers with Ras activation. Increased lysosomal biogenesis may enhance nutrient scavenging by macropinocytosis as well as autophagy.

Autophagy maintains mitochondrial metabolism possibly by supplying catabolic substrates to metabolic pathways and by maintaining mitochondrial quality control^9,11,14,17. Loss of autophagy in autochthonous Ras lung cancer models causes the striking accumulation of defective mitochondria and other autophagy substrates, suppresses proliferation, promotes apoptosis and reduces tumor burden¹⁸. Moreover, autophagy deficiency renders Ras-driven lung tumors benign indicating that autophagy is required for manifestation of the more aggressive carcinoma fate¹⁸. Where Trp53 is intact, autophagy inactivation accentuates p53 activation that contributes to tumor suppression, although autophagy deficiency still suppresses tumorigenesis even in the absence of p53 (refs 18,19,20). In sum, autophagy inactivation potentiates p53 activity and compromises mitochondrial metabolism and energy and redox homeostasis. Collectively this reduces the stress tolerance, proliferation and survival of Ras-driven lung tumors in preclinical models¹¹.

A similar requirement for autophagy is found in autochthonous mouse models for Ras-driven pancreatic cancer with both or one allele of Trp53 intact^21,22. Note that while activation of Ras and deletion of both alleles of Trp53 in the embryonic pancreas does not show autophagy dependence²¹, the physiologic model with Ras activation and spontaneous allelic loss of Trp53 in the adult pancreas does display autophagy-dependence^22,23. Moreover, human pancreatic cancer cell lines and tumors grown in mice are sensitive to genetic and pharmacologic (hydroxychloroquine [HCQ]) inhibition of autophagy^8,22. Thus, substantial preclinical data supports the concept of autophagy addiction of Ras-driven cancers and suggests that targeting autophagy in human cancers with activating Ras mutations may provide clinical benefit.

Targeting Nutrient Scavenging in RAS-Driven Cancers

As inhibition of all oncogenic signaling downstream of Ras may be ultimately limited by toxicity, combining inhibition of a signaling pathway component with other dependent properties such as nutrient scavenging is an alternative approach. In the case of macropinocytosis, nutrient uptake, transport, degradation and use are all potential targets. Currently these processes are molecularly poorly defined, but clearly approachable. In the case of autophagy, targets for drug discovery are the enzymes that control the initiation process, machinery, trafficking, degradation and recycling of autophagic cargo. There is also evidence for a therapeutic window since conditional deletion of an essential autophagy gene throughout an adult mouse is not immediately lethal while having potent anti-tumor activity against established Ras-driven lung cancers²⁰. Targeting the lysosome may have the additional benefit of compromising both autophagy and macropinocytosis. The lysosomal inhibitor HCQ is currently in clinical trials with early encouraging results, but whether it will be sufficiently potent remains in question, prompting the quest for superior agents²⁴.

About the Author

Eileen White obtained her Ph.D. In biology at SUNY Stony Brook, and did her postdoctoral work with Bruce Stillman at Cold Spring Harbor Laboratory. She then went on to Rutgers University where she is currently Distinguished Professor of Molecular Biology and Biochemistry and Associate Director for Basic Science at the Rutgers Cancer Institute of New Jersey.

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