One of the fundamental ideas in cancer has been that cancer and normal cells differ in their metabolism. First proposed by Nobel prize-winner Otto Warburg in 1924, the 'Warburg Effect' is the hypothesis that cancer cells generate energy by the non-oxidative (without oxygen) breakdown of glucose (a process called glycolysis). This is in contrast to normal cells, which generate energy through an oxygen-dependent pathway. Tumours use the non-oxidative pathway, which is less efficient than normal metabolism, even in conditions where there is plenty of oxygen available. We can see this greediness for glucose in PET scans, for example, where radioactively labelled sugar is sucked up by tumours and not by normal tissue.
For many years the Warburg effect has been investigated as a possible Achilles heel for tumours, and there is still lots of work being done on ways to attack tumours by interrupting this glycolytic process. Such work ranges from dietary interventions (see for example the article on diets and cancer, and the interview with Dr Gerald Krystal), to looking for drugs which disrupt the process at different points.
However, in recent years Dr Michael Lisanti and a group of colleagues have proposed a fundamental re-appraisal of the theory. They have proposed the cancer cells in advanced solid tumours are able to induce the Warburg Effect in the cells that surround them (the tumour stroma), and that they then consume the by-products of glycolysis produced by the stromal cells. In other words they cannibalise the tissues around themselves, inducing the Warburg Effect so that they can feed off the nutrients produced by the stromal cells. They have termed this new theory, which is a radical departure from what is currently accepted, the 'Reverse Warburg Effect'.
For those who wish to learn more there are a number of freely available papers by Lisanti and co available via PubMed.
Dr Anthony Howell is one of Lisanti's co-authors and a researcher and clinician based in Manchester. He has kindly agreed to an interview so that we can learn more about this new theory and what it may mean in terms of clinical practice. Although the language in this interview is more technical than much of what is published on this site, I feel it's important to bring these new theories and results to as wide an audience as possible.
PP: A recent editorial in the journal Cell Cycle described the work of Michael Lisanti, yourself and others as ‘iconoclastic’. Just how revolutionary is the theory of the ‘reverse Warburg’ effect and the idea of ‘cancer as a parasite’?
AH: Warburg was a great scientist as reflected in the award of the Nobel Prize for his pioneering work on tumour metabolism. His major contribution was to show that tumour cells showed enhanced glycolysis and he thus assumed that mitochondrial respiration was impaired in tumour cells. It certainly looks like this when you put tumour cell lines such as the breast MCF-7 cell line into culture by themselves. These cells have few mitochondria and there is increase in the sugar degradation glycolytic pathway. What Michael Lisanti and his colleagues have shown is that this may be an artefact of culturing the cells alone. When co-cultured with human fibroblasts, the MCF-7 cells synthesise mitochondria and thus use oxidative phosphorylation as their main source of energy. In the body tumour cells are always associated with what is called a supportive stroma derived from the host, so that the co-culture system is a better mimic of what might be happening in-vivo, and indeed we find up-regulation of mitochondria in most breast cancers compared with normal breast tissue. Cultured alone the fibroblasts have a large number of mitochondria. However, when co-cultured with tumour cells they degrade the mitochondria and depend upon gylcolysis. So all these observation are opposite to Warburg’s observations. It is as if the Warburg effect occurs in tumour associated fibroblasts rather than the tumour cells themselves which has lead Lisanti to call what is happening in co-culture ‘The Reverse Warburg Effect’. I think this is where the implication that these observations are iconoclastic comes from.
PP: At the moment Lisanti and the rest of the team are producing one or two papers a month – to the outsider it’s almost a production line. How much of an influence is this work having? Do you feel that it’s attracting support from within the cancer research community?
AH: Scientists are very careful and naturally sceptical people. Before accepting anything totally new they must be convinced. These data are so new that there is bound to be initial and quite natural scepticism. Indeed, the great philosopher of science, Thomas Kuhn, suggested that sometimes old paradigms are held onto so strongly that you have to wait for the next generation before something new is accepted. Scientists also need to repeat the observations of others in their own laboratories and this can be a relatively slow process. However, some of the observations made by Lisanti have already been repeated and are thus becoming accepted. Let me give you an example. In co-culture between tumour cells and fibroblasts, Lisanti has shown that the tumour cells induce a lot of changes in the fibroblasts in addition to causing down-regulation of mitochondria. The tumour cells seem to ‘activate’ the fibroblasts and another feature of this is down-regulation of a cell surface protein called caveolin-1 (Cav-1) which transmits signals into the cell. Lisanti and his pathologist Agnes Witkowietz showed that Cav-1 was also down-regulated in about half of human primary tumours. If this represents tumour cells which can activate the fibroblasts it may be thought that patients with this type of tumour fare badly. Indeed they showed this to be the case for not only were these patients more likely to relapse but also be more likely to be resistant to treatments such as tamoxifen. The important point is that these observations have now been confirmed in five other laboratories/clinics around the world.
PP: The promise of therapies targeted at tumour angiogenesis has largely failed to be translated into successful clinical use. How do your theories explain this failure? What other phenomena can be explained by the theory?
AH: I do not think we can say that anti-angiogenic therapies are dead. What we need are markers for tumours most likely to respond. You might have said the same for herceptin if we did not have the HER-2 test. However Lisanti has shown that activation of tumour associated fibroblasts in breast tumours is not associated, on average, with increased blood vessel counts and so the mechanism of the poor prognosis does not appear to be related to induction of angiogenesis in these tumours. This does not explain what you call the ‘failure’ of anti-angiogenic therapy, they simply indicate that something else is going on which does not appear to be dependent upon angiogenesis and thus activation of tumour fibroblasts will not be a marker of patients likely to respond.
PP: What else can be explained by Lisanti’s work?
AH: You mentioned that he has used the concept that the tumour is acting as a parasite and his observations seem to support such a notion. He has demonstrated, as I have said, that certain tumour cells appear to have the capacity to ‘activate’ adjacent fibroblasts. Activation appears to be associated with increased production of the reactive oxygen species, hydrogen peroxide (H2O2), since the interactions can be abrogated by catalase, (the enzyme responsible for degrading H2O2). One molecule which is upregulated in the co-cultures fibroblasts is hypoxia-inducible factor 1 alpha (HIF-1 alpha) which is a protein involved in switching cells from predominantly respiratory (oxygen using) metabolism to glycolysis. Under these circumstance mitochondria are no longer required and are degraded. This is exactly what is seen in co-culture and because the fibroblasts are now predominantly glycolytic there is a marked increase in the production of lactate within the cell, which is the end product of glycolytic metabolism. Lisanti has shown that there is up-regulation of a protein, monocarboxylate transporter-4, in the fibroblast which exports lactate out of the cell and up-regulation of monocarboxylate transporter-1 on the surface of adjacent tumour cells which imports lactate into the tumour cell. These types of experiments lead Lisanti to suggest that the tumour feeds off the host (the fibroblasts) which, in-turn leads to his suggestion that the tumour is acting like a parasite.
PP: Much of your work on these theories has been in the context of breast cancer, with some work also in pancreatic cancer. Do you think the ‘cancer parasite’ paradigm extends to other forms of cancer, including the sarcomas? For example there is evidence that calveolin-1 (a key marker according to the theory) is downregulated in osteosarcoma.
AH: It seems likely that the mechanisms outlined above may be generalisable but we just don’t have hard data. Lisanti has published that Cav-1 is reduced in prostate cancer bone metastases which support the idea that the mechanisms he has shown may be widely used by tumour to ‘feed’ themselves.
PP: It seems to me that there is something of an intersection between your work and that of Stefano Fais of the Italian National Institute of Health. His focus is on exosomes as a mechanism for what he calls ‘tumour cannibalism’. Do you feel that there is a cross-over between what you and Lisanti are doing and what he and his team are doing?
AH: As I understand it Professor Fais is interested in tumour proton pumps and the release from tumours of packets of cell material called exosomes. I think that this group believe that tumour cells export lactate - i.e. the Warburg effect - whereas Lisanti has shown that tumour cells are net importers of lactate which produces rather contradictory hypotheses. Also I think they are interested in the potential immunosuppressive effects of tumour cell released exosomes. They may possibly be involved in signalling to the adjacent stromal cells and thus this would be for us an interesting line of research but we have not explored this.
PP: There are very obvious clinical consequences to your theories. In your papers there has been a focus on the use of metformin and anti-oxidants such as quercetin. How soon can we expect to see some clinical trials based on your ideas?
AH: Some clinical trials have been completed and a number of others are in progress. Groups in Dundee, Milan and Toronto have shown that, given pre-operatively, metformin reduces primary tumour cell proliferation. We have demonstrated a possible mechanism for this effect in that our most recent paper shows that metformin down regulates mitochondrial complex I in primary tumour cells and in so doing reverses tamoxifen resistance. Thus we plan studies to see whether metformin synergises with endocrine therapy in the clinic and a number of other studies using the principles defined by Lisanti.
PP: We have seen an increased interest in therapies that target the tumour micro-environment with anti-inflammatories (e.g. celecoxib), proton pump inhibitors, metronomic chemotherapy etc. These treatments are generally less toxic than standard high dose chemotherapy. How do they fit in with the potential treatments arising from your work?
AH: Other groups besides ourselves have shown that changes in the stroma can indicate poor prognosis. This does not indicate that anti- stromal therapies will work. However the concept of interfering with tumour stromal interactions is an important one. In fact we have a trial of a proton pump inhibitor starting next year. As I explained we think an important signalling molecule from tumour to stroma is H2O2 and that catalase degrades this ROS. It is interesting that catalase deficient mice tend to develop tumour and administration of catalase reduces tumour volume in animals. In addition people with certain catalase SNPs are less likely to develop cancer.
PP: The question of anti-oxidants and cancer therapies has been a very vexed one, with no clear consensus. Do you think that things are becoming clearer?
AH: I agree with the word vexed. Initial studies are disappointing but were conceptually problematical. We need to think it through and get wiser. I am sure there is a lot of mileage in the area.
PP: In your work oxidative stress is seen as a weapon that tumours exploit to cannibalise surrounding tissues and is clearly identified with disease progression. Given that there are medical treatments that generate high rates of oxidative stress – such as anti-depressant drug amitryptiline, for example – do you think physicians should be wary of prescribing them to cancer patients?
AH: Amitryptyline has been shown to increase reactive oxygen species (ROS) and to reduce antioxidants such as catalase and thus is a sort of oxidant therapy like many forms of chemotherapy. Since mitochondria are increased in tumour cells it appears that anything which impairs their function may be therapeutic in the cancer cell. So depriving the parasitic tumour of sustenance (lactate, ketones, glutamine) may also be beneficial in addition to directly poisoning mitochondria. It seems likely we will be using combined therapy in women with overt breast tumours in the future. These approaches and standard ones may well not be mutually exclusive.