The deadly deviousness of the cancer cell, or how dichloroacetate (DCA) might fail

One byproduct of blogging that I had never anticipated when I started is how it sometimes gets me interested in scientific questions that I would never have paid much attention to before or looked into other than superficially. One such scientific question is whether dichloroacetate (DCA), the small molecule that was shown to have significant anti-tumor activity against human tumor xenografts implanted in rats, media reports about which caused a blogospheric hysteria in late January representing DCA as a “cure” for cancer that “big pharma” doesn’t want you to know about, mainly because it’s relatively cheap and unpatentable in its present form. I gathered some minor notoriety by pointing out that the hype was excessive and that the drug had not even been tested against cancer in humans yet, adding that most drugs that show promise in cell culture and against experimental tumors end up failing to show efficacy in humans. Unfortunately, none of that stopped unscrupulous “entrepreneurs” from selling DCA as “Pet-DCA” supposedly intended for use “in pets only,” even though the most cursory reading of the discussion boards revealed that desperate cancer patients are trying to buy it to use themselves, nor did it stop ignorant dupes like DaveScot from cheering them on in doing so or credulous bloggers who think far more of their scientific knowledge than is warranted from blathering about DCA as an allegedly “suppressed” or “ignored” cure like vitamin C.

It is not my purpose today to rehash all of this or to rail yet again against the dubious marketing of a “cure” that hasn’t even been shown to be a cure yet. I’m more interested in discussing an interesting bit of science related to DCA and the whole concept that altered bioenergetics are important to the development of cancer.

The entire concept behind the use of DCA is to target a phenomenon known as the Warburg Effect. This effect was first observed by a biochemist named Otto Warburg back in the late 1920’s in tumor cells. In brief, Dr. Warburg noted that tumor cells avidly consumed glucose and produced what is normally the byproduct of the anaerobic metabolism of glucose for energy (glycolysis) even in an aerobic (oxygen-containing) environment, conditions under which normal cells usually use a process that requires oxygen, oxidative phosphorylation. Normal cells usually use oxidative phosphorylation, which takes place in the mitochondria, when oxygen is available and only switch over to anaerobic glycolysis in conditions of low or no oxygen (anaerobic conditions), producing lactate as a byproduct. (Normally, the end product of glycolysis, pyruvate, is then used in the Kreb’s cycle and oxidative phosphorylation. In the absence of oxygen, the pyruvate is used for energy and turned into lactate. Lactate buildup makes your muscles sore after intense exercise, when the energy demand of the muscles can exceed the amount of oxygen available.) The problem in normal cells is that glycolysis produces much less usable chemical energy per molecule of glucose than oxidative phosphorylation, and normal cells normally cannot survive on anaerobic glycolysis alone for very long. However, many tumor cells can. Indeed, many tumor cells continue to use glycolysis and produce lactate even in aerobic conditions, an observation that led Dr. Warburg to postulate that in tumor cells the mitochondria (which is where oxidative phosphorylation takes place) are reduced or functionally impaired. Indeed, he postulated more than that, namely that impaired mitochondrial function contributes to tumorigenesis.

The reason that I became more interested in DCA is because my main research interest is tumor angiogenesis. Because blocking tumor angiogenesis works by decreasing oxygen and nutrient delivery to tumors, in essence, “starving” them, one might imagine that one way in which tumors could be or become resistant to antiangiogenic therapy might conceivably be through cranking up the Warburg Effect, allowing tumor cells. As it turns out, DCA targets the Warburg Effect. It also turns out that the enzyme that DCA happens to inhibit to accomplish this targeting, pyruvate dehydrogenase kinase (PDK), is activated by a gene called HIF-1, which itself is activated by hypoxia. PDK inactivates an enzyme complex called the pyruvate dehydrogenase complex (PDH), which, when turned off attenuates mitochondrial respiration and oxidative phosphorylation.

Consequently, I’ve been checking out papers about the bioenergetics of tumors, and I found a doozy of one last week in the February 15 issue of Cancer Research, entitled Adaptation of Energy Metabolism in Breast Cancer Brain Metastases. Basically, the investigators found a fascinating (and disturbing) adaptation that occurs in breast cancer cells when they metastasize to the brain that shows just how unbelievably complex and difficult a foe cancer can be.

The investigators at the Scripps Research Institute led by Brunhilde Felding-Haberman asked the question: What are the changes in the amounts and types of proteins made by breast cancer cells that metastasize to the brain that make them able to grow there? To attack this question, they isolated tumor cells from the blood of a patient with stage IV breast cancer, cultured them, and then grew them into SCID mice (a strain of immune deficient mice in which human tumors can grow as xenografts). The tumors grew and, even more than that, they metastasized to brain and bone. Metastases from brain and bone were isolated, injected again into new mice, and then the metastases were isolated again. It turns out that the cells from the brain metastasis became much more likely to metastasize to to and avid for growing in the brain than the parental cell line from which they were derived, as the cells from bone metastases became more avid for bone. This is a common technique used to study metastasis and why certain tumors tend to metastasize to certain organs. Basically, tumor cells are subjected to one or more rounds of selection for clones that are able to grow in the organ desired.

Now here’s where things get interesting. They next did a technique called multidimensional chromatography and tandem mass spectroscopy. There’s no need to sweat the details other than to understand that this is a proteomics technique by which it is possible to simultaneously compare the levels of hundreds of proteins between cell types. Basically, the idea was to see which proteins were expressed at higher or lower levels in the brain metastases than in the parental cell line from which the metastatic cells were involved. The results were startling. In essence, the brain metastases made lots more of the proteins involved in oxidative phosphorylation.

In other words, they underwent what might well be characterized as the anti-Warburg effect. Although they had increased levels of glycolysis, they also cranked up their oxidative phosphorylation, as well as and had increased activation of pathways that minimize the production of or damage from reactive oxygen species (a.k.a. free radicals, the production of which was stimulated by treatment with DCA in the Michelakis experiments and contributed to tumor cell apoptosis in response to the drug). The overall effect of these changes in gene expression leading to increases in the enzymes responsible for oxidative phosphorylation is that the brain metastatic cell line became resistant to drugs that affect the cellular oxidation-reduction balance.

Drugs like DCA.

It’s rather disappointing that they didn’t actually test DCA, but, then, the work on this paper and the work on Michelakis’ paper were likely going on at the same time. The drug they did test is 2-deoxyglucose (2-DG) a drug that is being tested because of its ability to inhibit glycolysis and shift the balance of energy production towards aerobic oxidative phosphorylation by a mechanism different from that of DCA. Consistent with the increased levels of proteins involved in oxidative phosphorylation, the cells derived from brain metastases were over two-fold less sensitive to 2-DG than the parental cell line, probably because the cells were no longer exhibiting the Warburg Effect, making them not nearly as dependent upon glycolysis for their energy. More than likely, these cells would also be as resistant to the effect of DCA.

The authors speculate that breast cancer cells that successfully metastasize to and colonize the brain take on characteristics that allow them to thrive in the environment found in the brain, and that their observations imply a link between a preference for oxidative phosphorylation and “homing” to the brain. The reason for this might well be that the brain is has a high blood flow and high oxygen tension, with the surrounding brain tissue always operating at a high oxidative metabolic level. As the authors state:

Our experimental metastasis data and proteomic analyses indicate that the brain metastatic cells, selected in vivo for their ability to establish brain metastases, possess a phenotype distinct from the parental circulating tumor cells and their bone metastatic counterparts. The protein expression profile of the brain metastatic cells and its functional validation imply a predisposition or bioenergetic adaptation of the tumor cells to the energy metabolism of the brain, conferring an advantage for tumor cell survival and proliferation in the brain microenvironment.

What these results suggest is something that those of us studying cancer have known for a long time. Cancer is an unbelievably devious and resourceful foe; if it weren’t we’d be far better at curing it now than we are. As much as we would like to wish it to be so, there will almost certainly never be a “magic bullet” that will cure all cancers. Antiangiogenic therapy was touted as one eight years ago and, in the time since then, has shown only modest success against cancer. Certainly it was no magic bullet. My guess is that DCA (and drugs designed to target the Warburg Effect) will similarly show modest success against cancer in humans. My guess (and remember, it is just an educated guess) is that it may well be ineffective against many forms of brain metastases and against some forms of brain tumors, while being most effective against tumors that are most avid in taking up glucose, which happen to be the tumors that show up the most brightly on PET scans. However, more work needs to be done, as one glaring weakness of this study (and probably the reason that it wasn’t accepted to a journal like Cancer Cell) is that the 2-DG experiments were all done in vitro. There were no experiments in which mice bearing brain metastases created by direct injection of either the parental cell line or the cell line derived from brain metastases were treated with 2-DG to see if in vivo results recapitulate in vitro sensitivities; so the tumor microenvironment could conceivably be sufficiently different than cell culture that these results might not hold up.

The bottom line is that cancer is always more complex than we think it is, and there are always wrinkles that we don’t think of. Moreover, cancer cells are incredibly adaptable and–dare I say it?–evolve rapidly to infiltrate and colonize new environments. (Indeed one depressing possibility raised by these experiments is that drugs designed to target the Warburg Effect might actually select for cells able to metastasize to the brain.) An adaptation that allows tumor cells to grow in the brain appears to have the byproduct of eliminating the Warburg Effect and rendering them resistant to attempts to manipulate the energy balance.

Sadly, all too often, cancer is like that.

ADDENDUM: Walnut has posted his critique on Daily Kos as well.

All Orac posts on DCA:

Posts by fellow ScienceBlogger Abel Pharmboy:

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