STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY/SPL DC/LatinstockRecent studies, some of which were carried out in Brazil or with the collaboration of Brazilians, are changing the way in which physicians and researchers regard and treat cancer. Gradually, experts are ceasing to see it merely as a set of cells that reproduce uncontrollably within an organ and are embracing a broader view,that values the interaction of the tumor cells with healthy neighboring cells. This improved understanding results from the accumulated knowledge about the continuous adaptations of tumor cells – which enable them to live in environments that are damaging to normal cells – and from the detailed mapping of the chemical interactions among molecules that cause them to produce energy within the tumors. The result is that one is now able to understand better how the usual cancer drugs work (unexpectedly, sometimes) and to pursue more effective, less aggressive treatments. At present, some 700 compounds to fight cancer are undergoing trials, with a mean success rate of 7 percent.
The mapping of biochemical interaction of tumor cells raises the possibility that drugs currently used for other diseases, such as diabetes, might block the development of tumor cells and even kill them. Years of tests are still needed to check whether this strategy, which combines traditional chemotherapy drugs with other ones, will work on humans. Even if it does, it is unlikely that it will promptly eliminate the need for conventional treatments such as chemotherapy and radiotherapy, given the seriousness and reach of this disease. Every year, almost 8 million die of cancer worldwide. In Brazil, it is the second most common cause of death (cardiovascular diseases rank first), killing some 130 thousand people a year and giving rise to almost 500 new cases, mainly of prostate cancer and lung cancer among men, and breast cancer and cervical cancer among women, according to estimates from Inca, the National Cancer Institute.
Now, an improved understanding of the biochemical interactions that occur within tumor cells – and between the latter and the healthy cells in the surrounding tissues – suggests that, rather than thinking about destroying the tumors completely, it might be possible to control their growth, transforming cancer into a chronic disease, such as Aids or even certain types of leukemia. “The current cancer treatments are generally too radical,” says Fernando Soares, a researcher from the A. C. Camargo Cancer Hospital and coordinator of the Cancer Cepid (Center for Research, Innovation and Dissemination) financed by FAPESP. “We can accept that an aggressive tissue is present and learn to live with it.”
Roger Chammas, a research from the Medical School of the University of São Paulo (USP), observes: “We have moved away from reductionism, centered on tumor cells, to a view that values the interaction between tumor cells and other cells and molecules near them.” His team, one that adopts this approach in Brazil, studies the mechanisms whereby the defense cells known as macrophages benefit from the abnormal cells that form the tumors, instead of fighting them. In another laboratory on the same floor of the impeccably conserved historical building, Maria Aparecida Koike Folgueira and her group found that the tissue support cells called fibroblasts can also favor the multiplication of tumor cells, while the tumor cells stimulate the growth of the fibroblasts, according to a study by Patrícia Rozenchan recently published in the International Journal of Cancer.
At the School of Medical Sciences at the State University of Campinas (Unicamp), José Barreto Carvalheira tests a combination of two drugs – metformin, normally used to control diabetes, and paclitaxel, used to eliminate tumors – to contain tumor growth, by reducing the amount of glucose that the tumor cells receive. The strategy had yielded good results in laboratory-cultivated lung tumor cells when he attended the annual meeting of Asco, the American Society of Clinical Oncology, held in late May in Orlando, Florida. There he experienced a situation that made him feel both gratified and overtaken: gratified because he found that Unicamp’s research with metformin really was innovative, but overtaken in that he saw other researchers willing to embrace the same approach in their research but who would probably make faster progress by having a bigger team than his.
The challenge of understanding and modifying the cellular environment that enables tumors to grow recovers and integrates studies published decades ago, going into more depth than they did, however. Soares, from the Cancer Cepid, first heard about tumor ecology some ten years ago, when we worked with the Spanish physician José Costa, a professor from Yale University in the United States. Costa compared tumors to the trees in a forest, which would not grow if they were isolated from each other or might grow freely in the absence of competition. “At the time,” he recalls, “the problem was how to make use of such concepts.” Now the concepts and results are converging and uncovering new working strategies.
“We can now see the entire elephant, and not merely its parts,” celebrated Bert Vogelstein, director of a cancer research center at Johns Hopkins University, USA, at the May congress. “We have discovered all the genes that undergo mutations and the tumor’s main metabolic signaling paths.” According to him, a tumor cell has 50 to 100 genetic changes (or mutations), although we do not yet know which of them appears first and triggers the others. At USP, Koike identified certain causes and consequences of these mutations: “The tumor cells interfere in the expression of the genes that stimulate the growth of fibroblasts, which, in turn, also make the tumor cells grow faster,” she says. Normally, many genes act at the same time, with greater or lower activity than normal. In breast tumor cells, for instance, the activity of the gene SP/int2 is lower than in normal cells. Consequently, the cell is more easily able to migrate to other body tissues.
“For a tumor to go ahead, a lot of things have to go wrong,” comments Luiz Fernando Lima Reis, director of research at the Sirio-Libanes Hospital. According to him, a tumor’s capacity to interact with its normal neighboring cells – the stroma – is what determines its capacity to invade other tissues, i.e., to metastasize, along with its affinity to distant organs. For instance, prostate cancers often metastasize into bone tissues, whereas breast tumors may generate proliferation points in the liver, lungs, bones and brain. “The tumor cell has to communicate with the external milieu as part of its survival strategy,” says Lima Reis, who applies these concepts to find molecules that can indicate how stomach and esophagus lesions can evolve into tumors. The interface between the tumor and the normal stroma cells, according to him, may further this evolution and contribute even more to the tumor’s behavior. “The tumor is a disaster that can die from the large number of errors in its cells’ DNA,” he says. “Recent data indicate that it is the stroma that causes certain cells on the border of the tumor to remain less changed than the rest of the tumor cells, as a means of survival. I have always felt that the stroma was part of the tumor.”
Carvalheira is confident. “A new treatment is going to come out of here,” he says in his Unicamp office, as he appreciates a scheme of biochemical reactions that are part of an article published in Science in May. Coordinated by Matthew Heiden, from the Dana-Farber Cancer Institute in Boston, USA, this study details a phenomenon that the German physiologist Otto Warburg presented back in 1924: the capacity of tumor cells to produce the energy that enables them to survive by consuming the free glucose in the cytoplasm, the cell region that lies between the cell membrane and its nucleus. Normal cells generally break down the glucose molecules found in the mitochondria, which are organelles within the cytoplasm, although in specific situations they may also use the cytoplasm’s glucose to generate energy. Healthy cells work like this, for instance, when we run and the need to produce energy to keep up the movement is greater than the oxygen intake.
One of the residues of this sequence of reactions that transform glucose into energy is lactate. This is also fragmented, releasing hydrogen ions (H+) that accumulate within tumor cells. As a result, the tumor becomes slightly acid, with a pH (hydrogen ions potential, which measures the abundance of H+) ranging from 6.5 to 6.9, close to the pH of milk (6.3 to 6.6). The difference versus normal cells, which exist under an alkaline pH (7.2 to 7.5), may seem small, but each pH point means an amount of H+ that is ten times greater or smaller inside the cell. “Acidity is the result of the abnormal glucose metabolism observed in virtually any tumor,” says the mathematical oncologist Robert Gatenby, who heads a research group at the Moffitt Cancer Center, in Florida. “In turn,” says Gatenby, “acidity enables tumors to invade normal tissues.”
Based on this rationale, Gatenby resorted to an acidity neutralizing substance, sodium bicarbonate, normally used against heartburn and poor digestion, in order to reduce acidity and keep the tumor from metastasizing in mice. It worked. The animals that were given a sodium bicarbonate solution had fewer metastases and of smaller size in the lungs, bowels and diaphragm, as compared to the mice that were fed on acid foods or that were given nothing. According to the study published in June in the journal Cancer Research, 80% of the animals treated were still alive after 120 days, versus only 40% in the control group.
But merely experimental results are not enough. Seeking explanations, Ariosto Silva, an engineer who graduated from ITA, the Technological Aeronautics Institute, with a doctorate in biology from Unicamp and a member of Gatenby’s team since last year, built a computer program that reproduces the biochemical paths whereby tumor cells and normal cells use their glucose. The results he obtained, which were published in the same Cancer Research issue, mathematically confirmed the results obtained in animals.
Together, the two studies strengthen Gatenby’s argument to see whether bicarbonate might work on humans as it does on mice. Ariosto points out one of the advantages of this strategy: “Bicarbonate is produced by the body in any event and is not toxic for other cells, contrary to synthetic medication.” However, there are limits. According to his simulations, the extra dose of bicarbonate must not exceed 40% of the amount that is circulating in the organism. “In higher concentrations, bicarbonate can generate dehydration and weight loss,” he warns.
“The tumor became slightly more predictable,” states José Andrés Yunes, a researcher from the Boldrini Children’s Center, a hospital in the city of Campinas that cares for children with leukemia, regarding the results that he helped to build by having been Ariosto’s PhD advisor. More predictable, but not necessarily controllable. Sodium bicarbonate is already used by people with leukemia to speed up the elimination of the cell residues left behind by the drugs that kill the cells that are multiplying too fast, but the new results do not yet indicate, with certainty, that it is truly a useful substance for treating cancer.
“We must now examine whether bicarbonate doesn’t diminish the efficacy or enhance the toxicity of the cancer treatment drugs,” says Yunes. Chammas imagines that controlling acidity might, in principle, help to hold back tumors that are surrounded by healthy cells, but this is unlikely when it comes to cells that are furthest away from blood vessels: “Bicarbonate might annihilate the population of acidity-sensitive tumor cells, but not control the metastases, because the tumor cell populations are very different and may resort to different survival mechanisms.”
The possibility of resorting to a substance used to fight heartburn, sometimes due to having too much coffee, in order to contain the growth of tumors, though it may seem too simple to work, is the result of a long scientific argument.
In 1995, Gatenby released two articles, one published in Cancer Research and the other in the Journal of Theoretical Biology, in which he explained his hypothesis that the intensification of glucose in tumor cells might generate acidity. This, in turn, might modify the tumor’s environment to the point of selecting tumor cells, leaving only the most resistant ones. This might also be decisive in determining tumor development, by causing the death of neighboring cells and allowing tumor cells to migrate to other body areas. “The initial hypothesis met with skepticism and lack of interest,” commented Gatenby.
His subsequent work also took into account the six typical features of tumor cells that Douglas Hanahan, from the University of California at San Francisco, and Robert Weinberg, from the Massachusetts Institute of Technology (MIT) presented in a review article in a special issue of the journal Cell in January 2000. These six characteristics, which are common to more than one hundred types of cancer, correspond to the successive adaptations of a normal cell until it becomes a tumor cell capable of migrating and lodging itself in other tissues. The first is the ability to independently produce the molecules that stimulate cell growth. The second, to escape from the action of the molecules that inhibit cell proliferation. The third, to multiply indefinitely, blocking the mechanisms that normally limit cell division. The fourth, the capacity to escape programmed cell death, a mechanism that cells trigger whenever they detect something abnormal, such as accelerated division. The fifth, the ability to induce the formation of blood vessels, which carry the blood with the nutrients and oxygen that are indispensable for a growing tumor. And, finally, the sixth, the ability to invade other tissues.
The accumulated knowledge about tumor cells and the environment in which they live reveals other possibilities for action. Carvalheira is considering the possibility of selecting the treatments that are most effective for cancer patients: those who are resistant to insulin may also be more resistant to the anti-tumor drugs currently available. From this, one might derive specific diets, currently undergoing trials, with fewer carbohydrates and more protein, so as to strengthen normal cells and weaken tumor cells. Or, alternatively, diets that could enhance the efficiency of cancer treatments already used, such as radiotherapy.
The latest discoveries have led Chammas to think again about the very drugs that are currently used to treat cancer. “If oxygen, which is a relatively small molecule, doesn’t get to tumor cells, antibodies and medication, which are much larger, may also fail to get to them,” he says. “We must do more studies about how and if the drugs reach the tumors.”
More precise and earlier diagnoses may emerge from these studies about tumoral ecology. Carlos Alberto Buchpiguel, the director of the nuclear medicine center of the Clinicas Hospital of the USP Medical School tells us that, for the time being, it is impossible to detect areas of the organism with low acidity, but those with low oxygenation – which are more likely to harbor tumors – can be identified by applying glucose molecules with fluoride in PET scans (positron emission scans). “If we could carry out this kind of exam on a broader basis, we could identify the focuses of new tumors and target treatments better,” he says. The problem is that such exams are expensive, costing about R$3.5 thousand each, and the public health system will not pay for them yet. Another challenge, worldwide, is the long path from the discovery and trials of new molecules capable of precisely identifying tumors without harming the organism. “We will only advance through the integration of experts from different areas.”
Soares, from the Cancer Hospital, recommends: “At this time, one must keep one’s feet planted on the ground. Experimental results may take 10 to 15 years to actually turn into new treatments.” The quest for new saving drugs has already caused a number of disappointments and has raised hopes that went unfulfilled. In 1998, through a New York Times article, Judah Folkman, a researcher at a Boston hospital, announced that angiostatin and endostatin, two proteins naturally produced by the body, had blocked, in mice, the formation of the blood vessels tumors need to grow.
Folkman had warned that the results were preliminary, but James Watson, one of the discoverers of the DNA molecule structure and Nobel Prize laureate commented, in the same New York Times article, that Folkman would cure cancer within two years. As we know, he did not. However, today, some 1.2 million people take approximately ten drugs that are based on the possibility of blocking the delivery of blood to growing tumors and at least 50 compounds based on the same principle are undergoing trials.
One of the chief cancer research challenges discussed at the Oncology Congress in the United States is precisely the issue of how to transform this scientific knowledge into applications that can help people. Richard Schilsky, the congress’s chairman and a University of Chicago professor, emphasized the need to change the current new drug development models. According to him, it will be difficult to advance without taking into account that animal tests are not very effective, that human populations are heterogeneous, and that there is no consensus on the meaning of the expression “clinical benefits.” Chammas suggests: “We must learn to think in a different way and accept the challenges to our creative capacity.”
The projects
1. Antonio Prudente Cancer Research Center; Type Cepids (Research, Innovation and Dissemination Center); Coordinator Fernando Augusto Soares – AC Camargo Hospital; Investment R$ 2,375,938.62.
2. Role of the path Irs/Pi 3-quinase/akt/mtor in tumor development (04/06064-1); Type Young Researchers Program; Coordinator José Barreto Carvalheira- Unicamp; Investment: 2, R$ 485,435.69.
3. Genic expression in stomach and esophagus tumors: from the biology to the diagnosis (06/03227-2); Type Thematic project; Coordinator Luiz Fernando Lima Reis – Sírio-Libanês Hospital; Investment R$ 1,039,696.62
4. Molecular characterization of fibroblasts from neoplastic mammary tissue (05/51593-5); Type Post-doctoral Grant; Coordinator Maria Mitzi Brentani – USP (grant holder: Patrícia Rozenchan); Investment R$ 154,362.64.
