Cancer Metabolism: Partners in the Warburg effect

  1. Joshua D Rabinowitz  Is a corresponding author
  2. Hilary A Coller
  1. Princeton University, United States
  2. University of California, Los Angeles, United States
  3. David Geffen School of Medicine, United States

In 1918 Albert Einstein convinced Otto Warburg to leave the German infantry and fulfill his patriotic requirements in the first World War by performing research instead (Koppenol et al., 2011). Back in the lab at the Kaiser Wilhelm Institute, Warburg discovered that thin slices of tumors produced lactate much more rapidly than normal tissue. This rapid fermentation of glucose by tumors, even in the presence of ample oxygen, was the first biochemical trait assigned to cancer and is known as the Warburg effect.

When oxygen is present, most human cells rely on a process called oxidative phosphorylation inside mitochondria to convert lactate into carbon dioxide and usable energy. Warburg proposed that the rapid glucose fermentation and associated lactate secretion by the cancer cells was due to mitochondrial dysfunction. However, subsequent studies have shown that most cancer cells do have working mitochondria and, moreover, depend heavily upon them to produce energy (Zu and Guppy, 2004Moreno-Sánchez et al., 2007). Instead of causing mitochondrial dysfunction, it was found that the mutations that cause cancer also promote the breakdown of glucose in a process called glycolysis. The most striking example involves the PI3K-Akt signaling pathway, which both transduces the signal from the hormone insulin to drive glucose uptake, and is one of the most frequently mutated pathways in cancer. One way this pathway can be activated is by the loss of a tumor suppressing enzyme called PTEN (Shaw and Cantley, 2006). The observation of oncogene-driven glucose uptake seemed to neatly explain the Warburg effect.

Over the past few decades, evidence has steadily accumulated that cancer cells also hijack surrounding cells (Cirri and Chiarugi, 2012). For example, cancer cells secrete growth factors to promote the formation of new blood vessels (Orimo et al., 2005), which are required to supply tumors with nutrients. Moreover, they co-opt surrounding connective tissue cells, including fibroblasts, which exchange signals with the cancer cells in a manner that ultimately drives tumor growth and likely helps to suppress immune responses to the tumor (Cirri and Chiarugi, 2012). However, both the mechanism of this exchange and its role in tumor growth remain poorly understood.

Fibroblasts may exchange both signaling molecules and metabolic fuels with the cancer cells, either by secreting individual molecules (e.g. lactate; Martinez-Outschoorn et al., 2014) or by releasing membrane-bound vesicles known as exosomes (Castellana et al., 2009). For example, recent work has shown that the spread of cancer in the brain is promoted by the exosomes that are released by a particular type of brain cell. These exosomes contain small RNA molecules known as microRNAs that can silence the gene that encodes the PTEN enzyme, whose loss drives an increase in glycolysis (Zhang et al., 2015).

Now, in eLife, Deepak Nagrath at Rice University and colleagues – including Hongyun Zhao as first author – show that cancer-associated fibroblasts release exosomes that both deliver nutrients to cancer cells and inhibit oxidative phosphorylation (Zhao et al., 2016; Figure 1). Zhao et al. use isotope-labelled carbon compounds to provide compelling evidence that exosomes from fibroblasts can supply an amino acid called glutamine and other nutrients to cancer cells. A shortage of glutamine can limit the growth of pancreatic and perhaps other cancers (Kamphorst et al., 2015). Importantly, although the exosomes contribute modest amounts of nutrients, they can protect cancer cells from starvation, hinting at one potential role for such metabolic exchange in tumors.

More striking and surprising is the role of the exosomes in causing the Warburg effect. Adding exosomes to prostate or pancreatic cancer cells both promotes glycolysis and blocks oxidative metabolism. It is likely that the increase in glycolysis is caused by the reduction in oxidative phosphorylation so, in this respect, the exosomes trigger glycolysis in the way initially envisioned by Warburg. These results call for a re-examination of the contributions of both processes to energy generation in cancer cells that are still associated with their neighbors.

Such re-examination is particularly important given that oxidative phosphorylation is reduced so dramatically in cancer cells, with oxygen consumption lowered by up to 80% within 24 hours of receiving exosomes from fibroblasts. Zhao et al. – who are based at Rice University, Baylor College of Medicine, the University of Texas MD Anderson Cancer Center and Stanford University – propose that the exosomes may deliver microRNAs that silence oxidative metabolism genes, but this is hard to reconcile with the timing. Since the proteins involved in oxidative phosphorylation are generally long-lived, even complete inhibition of their production seems unlikely to produce such drastic effects so quickly. Nor can the decreased oxidative phosphorylation be explained by the delivery of nutrients by exosomes, because increasing the access to such nutrients would be expected to promote, not inhibit, the use of oxygen. Thus, understanding how the exosomes inhibit oxidative phosphorylation is a key challenge going forward. Such work holds the potential to illuminate not only the Warburg effect, but also the regulation of oxidative phosphorylation in cells more generally.

Fibroblasts supply nutrients to cancer cells and inhibit oxidative phosphorylation in cancer cells.

Fibroblasts (pink cells) associate with epithelial cancer cells (blue cells) and release exosomes (circles) that transfer nutrients to epithelial cancer cells (orange lines). In addition, they inhibit mitochondrial oxidative phosphorylation in the cancer cells (black blunt arrows), perhaps via microRNAs that silence particular genes.

References

    1. Zu XL
    2. Guppy M
    (2004) Cancer metabolism: Facts, fantasy, and fiction
    Biochemical and Biophysical Research Communications 313:459–465.
    https://doi.org/10.1016/j.bbrc.2003.11.136

Article and author information

Author details

  1. Joshua D Rabinowitz

    Department of Chemistry and the Lewis Sigler Institute for Integrative Genomics, Princeton University, Princeton, United States
    For correspondence
    joshr@princeton.edu
    Competing interests
    The authors declare that no competing interests exist.
  2. Hilary A Coller

    1. Department of Molecular, Cell and Developmental Biology, University of California, Los Angeles, Los Angeles, United States
    2. Department of Biological Chemistry, David Geffen School of Medicine, Los Angeles, United States
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-0992-6494

Publication history

  1. Version of Record published: April 13, 2016 (version 1)

Copyright

© 2016, Rabinowitz et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 4,060
    Page views
  • 785
    Downloads
  • 9
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Joshua D Rabinowitz
  2. Hilary A Coller
(2016)
Cancer Metabolism: Partners in the Warburg effect
eLife 5:e15938.
https://doi.org/10.7554/eLife.15938

Further reading

    1. Cell Biology
    2. Medicine
    Thao DV Le, Dianxin Liu ... Julio E Ayala
    Research Article Updated

    The canonical target of the glucagon-like peptide-1 receptor (GLP-1R), Protein Kinase A (PKA), has been shown to stimulate mechanistic Target of Rapamycin Complex 1 (mTORC1) by phosphorylating the mTOR-regulating protein Raptor at Ser791 following β-adrenergic stimulation. The objective of these studies is to test whether GLP-1R agonists similarly stimulate mTORC1 via PKA phosphorylation of Raptor at Ser791 and whether this contributes to the weight loss effect of the therapeutic GLP-1R agonist liraglutide. We measured phosphorylation of the mTORC1 signaling target ribosomal protein S6 in Chinese Hamster Ovary cells expressing GLP-1R (CHO-Glp1r) treated with liraglutide in combination with PKA inhibitors. We also assessed liraglutide-mediated phosphorylation of the PKA substrate RRXS*/T* motif in CHO-Glp1r cells expressing Myc-tagged wild-type (WT) Raptor or a PKA-resistant (Ser791Ala) Raptor mutant. Finally, we measured the body weight response to liraglutide in WT mice and mice with a targeted knock-in of PKA-resistant Ser791Ala Raptor. Liraglutide increased phosphorylation of S6 and the PKA motif in WT Raptor in a PKA-dependent manner but failed to stimulate phosphorylation of the PKA motif in Ser791Ala Raptor in CHO-Glp1r cells. Lean Ser791Ala Raptor knock-in mice were resistant to liraglutide-induced weight loss but not setmelanotide-induced (melanocortin-4 receptor-dependent) weight loss. Diet-induced obese Ser791Ala Raptor knock-in mice were not resistant to liraglutide-induced weight loss; however, there was weight-dependent variation such that there was a tendency for obese Ser791Ala Raptor knock-in mice of lower relative body weight to be resistant to liraglutide-induced weight loss compared to weight-matched controls. Together, these findings suggest that PKA-mediated phosphorylation of Raptor at Ser791 contributes to liraglutide-induced weight loss.

    1. Cell Biology
    2. Developmental Biology
    Simon Schneider, Andjela Kovacevic ... Hubert Schorle
    Research Article

    Cylicins are testis-specific proteins, which are exclusively expressed during spermiogenesis. In mice and humans, two Cylicins, the gonosomal X-linked Cylicin 1 (Cylc1/CYLC1) and the autosomal Cylicin 2 (Cylc2/CYLC2) genes, have been identified. Cylicins are cytoskeletal proteins with an overall positive charge due to lysine-rich repeats. While Cylicins have been localized in the acrosomal region of round spermatids, they resemble a major component of the calyx within the perinuclear theca at the posterior part of mature sperm nuclei. However, the role of Cylicins during spermiogenesis has not yet been investigated. Here, we applied CRISPR/Cas9-mediated gene editing in zygotes to establish Cylc1- and Cylc2-deficient mouse lines as a model to study the function of these proteins. Cylc1 deficiency resulted in male subfertility, whereas Cylc2-/-, Cylc1-/yCylc2+/-, and Cylc1-/yCylc2-/- males were infertile. Phenotypical characterization revealed that loss of Cylicins prevents proper calyx assembly during spermiogenesis. This results in decreased epididymal sperm counts, impaired shedding of excess cytoplasm, and severe structural malformations, ultimately resulting in impaired sperm motility. Furthermore, exome sequencing identified an infertile man with a hemizygous variant in CYLC1 and a heterozygous variant in CYLC2, displaying morphological abnormalities of the sperm including the absence of the acrosome. Thus, our study highlights the relevance and importance of Cylicins for spermiogenic remodeling and male fertility in human and mouse, and provides the basis for further studies on unraveling the complex molecular interactions between perinuclear theca proteins required during spermiogenesis.