On the Origin of Cancer Cells

On the Origin of Cancer Cells

Otto Warburg

Our principal experimental object for the measurement of the metabolism of cancer cells is today no longer the tumor but the ascites cancer cells (1) living free in the abdominal cavity, which are almost pure cultures of cancer cells with which one can work quantitatively as in chemical analysis. Formerly, it could be said of tumors with their varying cancer cell content, that they ferment more strongly the more cancer cells they con­tain, but today we can determine the absolute fermentation values of the can­cer cells and find such high values that we come very close to the fermentation values of wildly proliferating Torula yeasts.

What was formerly only qualitative has now become quantitative. What was formerly only probable has now become certain. The era in which the fermenta­tion of the cancer cells or its importance could be disputed is over, and no one today can doubt that we understand the origin of cancer cells if we know how their large fermentation originates, or, to express it more fully, if we know how the damaged respiration and the exces­sile fermentation of the cancer cells originate.

Energy of Respiration and Fermentation

We now understand the chemical mechanism of respiration and fermenta­tion almost completely, but we do not need this knowledge for what follows, since energy alone will be the center of our considerations. We need to know no more of respiration and fermentation here than that they are energy-produc­ing reactions and that they synthesize the energy-rich adenosine triphosphate, through which the energy of respiration and fermentation is then made available for life. Since it is known how much adenosine triphosphate can be synthe­sized by respiration and how much by fermentation, we can write immediately the potential, biologically utilizable en­ergy production of any cells if we have measured their respiration and fermen­tation. With the ascites cancer cells of the mouse, for example, we find an aver­age respiration of 7 cubic, millimeters of oxygen consumed per milligram, per hour, and fermentation of 60 cubic milli­meters of lactic acid produced per milli­gram, per hour. This, converted to en­ergy equivalents, means that the cancer cells can obtain approximately the same amount of energy from fermentation as from respiration, whereas the normal body cells obtain much more energy from respiration than from fermenta­tion. For example, the liver and kidney of an adult animal obtain about 100 times as much energy from respiration as from fermentation.

I shall not consider aerobic fermenta­tion, which is a result of the interaction of respiration and fermentation, because aerobic fermentation is too labile and too dependent on external conditions. Of importance for the considerations that follow are only the two stable inde­pendent metabolic processes, respiration and anaerobic fermentation-respiration, which is measured by the oxygen con­sumption of cells that are saturated with oxygen, and fermentation, which is measured by the formation of lactic acid in the absence of oxygen.

Injuring of Respiration

Since the respiration of all cancer cells is damaged, our first question is, How can the respiration of body cells be injured? Of this damage to respira­tion, it can be said at the outset that it must be irreversible, since the respira­tion of cancer cells never returns to nor­mal. Second, the injury to respiration must not be so great that the cells are killed, for then no cancer cells could re­sult. If respiration is damaged when it forms too little adenosine triphosphate, it may be either that the oxygen con­sumption has been decreased or that, with undiminished oxygen consumption, the coupling between respiration and the formation of adenosine triphosphate has been broken, as was first pointed out by Feodor Lynen (2).

One method for the destruction of the respiration of body cells is removal of oxygen. If, for example, embryonal tis­sue is exposed to an oxygen deficiency for some hours and then is placed in oxygen again, 50 percent or more of the respiration is usually destroyed. The cause of this destruction of respiration is lack of energy. As a matter of fact, the cells need their respiratory energy to preserve their structure, and if respira­tion is inhibited, both structure and res­piration disappear.

Another method for destroying respi­ration is to use respiratory poisons. From the standpoint of energy, this method comes to the same result as the first method. No matter whether oxygen is withdrawn from the cell or whether the oxygen is prevented from reacting by a poison, the result is the same in both cases-namely, impairment of respira­tion from lack of energy. I may mention a few respiratory poi­sons. A strong, specific respiratory poi­son is arsenious acid, which, as every clinician knows, may produce cancer. Hydrogen sulfide and many of its deriv­atives are also strong, specific respira­tory poisons. We know today that cer­tain hydrogen sulfide derivatives, thio­urea and thioacetamide, with which citrus fruit juices have been preserved in recent times, induce cancer of the liver and gall bladder in rats.

Urethane is a nonspecific respiratory poison. It inhibits respiration as a chem­ically indifferent narcotic, since it dis­places metabolites from cell structures. In recent years it has been recognized that subnarcotic doses of urethane cause lung cancer in mice in 100 percent of treatments. Urethane is particularly suit­able as a carcinogen, because, in con­trast to alcohol, it is not itself burned up on the respiring surfaces and, unlike ether or chloroform, it does not cyto­lyze the cells. Any narcotic that has these properties may cause cancer upon chronic administration in small doses.

The first notable experimental induc­tion of cancer by oxygen deficiency was described by Goldblatt and Cameron (3), who exposed heart fibroblasts in tissue culture to intermittent oxygen deficiency for long periods and finally obtained transplantable cancer cells, whereas in the control cultures that were maintained without oxygen deficiency, no cancer cells resulted. Clinical experi­ences along these lines are innumerable: the production of cancer by intermittent irritation of the outer skin and of, the mucosa of internal organs, by the plug­ging of excretory ducts of glands, by cirrhoses of tissues, and so forth. In all these cases, the intermittent irritations lead to intermittent circulatory disturb­ances. Probably chronic intermittent oxygen deficiency plays a greater role in the formation of cancer in the body than does the chronic administration of respi­ratory poisons.

Any respiratory injury due to lack of energy, however, whether it is produced by oxygen deficiency or by respiratory poisons, must be cumulative, since it is irreversible. Frequent small doses of res­piratory poisons are therefore more dan­gerous than a single large dose, where there is always the chance that the cells will be killed rather than that they will become carcinogenic.

Grana

If an injury of respiration is to pro­duce cancer, this injury must, as already mentioned, be irreversible. We under­stand by this not only that the inhibi­tion of respiration remains after removal of the respiratory poison but, even more, that the inhibition of respiration also continues through all the following cell divisions, for measurements of metabo­lism in transplanted tumors have shown that cancer cells cannot regain normal respiration, even in the course of many decades, once they have lost it.

This originally mysterious phenome­non has been explained by a discovery that comes from the early years of cell physiology (4). When liver cells were cytolyzed by infusion of water and the cytolyzate was centrifuged, it was found that the greater part of the respiration sank to the bottom with the cell grana. It was also shown that the respiration of the centrifuged grana was inhibited by narcotics at concentrations affecting cell structures, from which it was concluded already in 1914 that the respiring grana are not insoluble cell particles but autonomous organisms, a result that has been extended in recent years by the English botanist Darlington (5) and particularly by Mark Woods and H. G. du Buy (6) of the National Cancer In­stitute in Bethesda, Md. Woods and du Buy have experimentally expanded our concepts concerning the self-perpetua­ting nature of mitochondrial elements (grana) and have demonstrated the he­reditary role of extranuclear aberrant forms of these in the causation of neo­plasia. The autonomy of the respiring grana, both biochemically and geneti­cally, can hardly be doubted today.

If the principle Omne granum e grano is valid for the respiring grana, we un­derstand why the respiration connected with the grana remains damaged when it has once been damaged; it is for the same reason that properties linked with genes remain damaged when the genes have been damaged.

Furthermore, the connection of respi­ration with the grana (7) also explains a carcinogenesis that I have not men­tioned previously, the carcinogenesis by x-rays. Rajewsky and Pauly have re­cently shown that the respiration linked with the grana can be destroyed with strong doses of x-rays, while the small part of the respiration that takes place in the fluid protoplasm can be inhibited very little by irradiation. Carcinogenesis by x-rays is obviously nothing else than a destruction of respiration by elimina­tion of the respiring grana.

It should also be mentioned here that grana, as Graffi has shown (8), fluoresce brightly if carcinogenic hydrocarbons are brought into their surroundings, be­cause the grana accumulate the carcino­genic substances. Probably this accumu­lation is the explanation for the fact that carcinogenic hydrocarbons, although almost insoluble in water, can inhibit respiration and therefore have a carcino­genic effect.

Increase of Fermentation by no means immediately result. For cancer formation there is necessary not only an irreversible damaging of the respiration but also an increase in the fermentation-indeed, such an increase of the fermentation that the failure of respiration is compensated for energetically. But how does this increase of fermentation come about?

The most important fact in this field is that there is no physical or chemical agent with which the fermentation of cells in the body can be increased di­rectly; for increasing fermentation, a long time and many cell divisions are always necessary. The temporal course of this increase of fermentation in carcinogenesis has been measured in many interesting works, among which I should like to make special mention of those of Dean Burk (9).

Burk first cut out part of the liver of healthy rats and investigated the me­tabolism of the liver cells in the course of the ensuing regeneration, in which, as is well known, the liver grows more rapidly than a rapidly growing tumor. No increase of fermentation was found. Burk then fed rats for 200 days on butter yellow, whereupon liver carcinomas were produced, and he found that the fer­mentation slowly increased in the course of 200 days toward values characteristic of tumors.

The mysterious latency period of the production of cancer is, therefore, noth­ing more than the time in which the fer­mentation increases after a damaging of the respiration. This time differs in vari­ous animals; it is especially long in man and here often amounts to several decades, as can be determined in the cases in which the time of the respiratory dam­age is known - for example, in arsenic cancer and irradiation cancer.

The driving force of the increase of fermentation, however, is the energy de­ficiency under which the cells operate after destruction of their respiration, which forces the cells to replace the irre­trievably lost respiration energy in some way. They are able to do this by a selec­tive process that makes use of the fer­mentation of the normal body cells. The more weakly fermenting body cells perish, but the more strongly fermenting ones remain alive, and this selective process continues until the respiratory failure is compensated for energetically by the increase in fermentation. Only then has a cancer cell resulted from the normal body cell.

Now we understand why the increase in fermentation takes such a long time and why it is possible only with the help of many cell divisions. We also under­stand why the latency period is different in rats and in man. Since the average fermentation of normal rat cells is much greater than the average fermentation of normal human cells, the selective process begins at a higher fermentation level in the rat and, hence, is completed more quickly than it is in man.

It follows from this that there would be no cancers if there were no fermenta­tion of normal body cells, and hence we should like to know, naturally, from where the fermentation of the normal body cells stems and what its significance is in the body. Since, as Burk has shown, the fermentation remains almost zero in the regenerating liver growth, we must conclude that the fermentation of the body cells has nothing to do with normal growth. On the other hand, we have found that the fermentation of the body cells is greatest in the very earliest stages of embryonal development and that it then decreases gradually in the course of embryonal development. Under these conditions, it is obvious, since ontogeny is the repetition of phylogeny that the fermentation of body cells is the inheri­tance of undifferentiated ancestors that have lived in the past at the expense of fermentation energy.

Structure and Energy

But why-and this is our last ques­tion-are the body cells dedifferentiated when their respiration energy is replaced by fermentation energy? At first, one would think that it is immaterial to the cells whether they obtain their energy from respiration or from fermentation, since the energy of both reactions is transformed into the energy of adeno­sine triphosphate, and yet adenosine triphosphate = adenosine triphosphate. This equation is certainly correct chemically and energetically, but it is incorrect morphologically, because, although res­piration takes place for the most part in the structure of the grana, the fermen­tation' enzymes are found for a greater part in the fluid protoplasm. The adeno­sine triphosphate synthesized by respira­tion therefore involves more structure than the adenosine triphosphate synthe­sized by fermentation. Thus, it is as if one reduced the same amount of silver on a photographic plate by the same amount of light, but in one case with diffused light and in the other with pat­terned light. In the first case, a diffuse blackening appears on the plate, but in the second case, a picture appears; how­ever, the same thing happens chemically and energetically in both cases. Just as the one type of light energy involves more structure than the other type, the adenosine triphosphate energy involves more structure when it is formed by respiration than it does when it is formed by fermentation.

In any event, it is one of the funda­mental facts of present-day biochemis­ try that adenosine triphosphate can be synthesized in homogeneous solutions with crystallized fermentation enzymes, whereas so far no one has succeeded in synthesizing adenosine triphosphate in homogeneous solutions with dissolved respiratory enzymes, and the structure always goes with oxidative phosphorylation.

Moreover, it was known for a long time before the advent of crystallized fer­mentation enzymes and oxidative phosphorylation that fermentation-the en­ergy-supplying reaction of the lower or­ganisms-is morphologically inferior to respiration. Not even yeast, which is one of the lowest forms of life, can maintain its structure permanently by fermentation alone; it degenerates to bizarre forms. However, as Pasteur showed, it is re­juvenated in a wonderful manner, if it comes in contact with oxygen for a short time. "I should not be surprised," Pas­teur said in 1876 (10) in the description of these experiments, "if there should arise in the mind of an attentive hearer a presentiment about the causes of those great mysteries of life which we conceal under the words youth and age of cells." Today, after 80 years, the explanation is as follows: the firmer connection of respiration with structure and the looser connection of fermentation with struc­ture.

This, therefore, is the physicochemi­cal explanation of the dedifferentiation of cancer cells. If the structure of yeast cannot be maintained by fermentation alone, one need not wonder that highly differentiated body cells lose their dif­ferentiation upon continuous replace­ment of their respiration with fermen­tation.

I would like at this point to draw at­tention to a consequence of practical importance. When one irradiates a tis­sue that contains cancer cells as well as normal cells, the respiration of the cancer cells, already too small, will decline further. If the respiration falls below a certain minimum that the cells need un­conditionally, despite their increased fer­mentation, they die; whereas the normal cells, where respiration may be harmed by the same amount, will survive because, with a greater initial respiration, they will still possess a higher residual respi­ration after irradiation. This explains the selective killing action of x-rays on can­cer cells. But still further: the descend­ants of the surviving normal cells may in the course of the latent period compen­sate the respiration decrease by fermen­tation increase and, thence, become can­cer cells. Thus it happens that radiation, which kills cancer cells can also at the same time produce cancer or that ure­thane, which kills cancer cells, can also at the same time produce cancer. Both events take place from harming respiration: the killing, by harming an already harmed respiration; the carcinogenesis by the harming of a not yet harmed respi­ration.

Maintenance Energy

When dedifferentiation of the body cells has occurred and cancer cells have thereby developed, there appears a phe­nomenon to which our attention has been called by the special living conditions of the ascites cancer cells. In extensively progressed ascites cancer of the mouse, the abdominal cavity contains so many cancer cells that the latter cannot utilize thier full capacity to respire and ferment because of the lack of oxygen and sugar. Nevertheless, the cancer cells remain alive in the abdominal cavity, as the re­sult of transplantation proves.

Recently we have confirmed this re­sult by direct experiments in which we placed varying amounts of energy at the disposal of the ascites outside the body, in vitro, and then transplanted it. This investigation showed that all cancer cells were killed when no energy at all was supplied for 24 hours at 38°C but that one-fifth of the growth energy was suffi­cient to preserve the transplantability of the ascites. This result can also be ex­pressed by saying that cancer cells re­quire much less energy to keep them alive than they do for growth. In this they resemble other lower cells, such as yeast cells, which remain alive for a long time in densely packed packets-almost without respiration and fermentation.

In any case, the ability of cancer cells to survive with little energy, if they are not growing, will be of great importance for the behavior of the cancer cells in the body.

Sleeping Cancer Cells

Since the increase in fermentation in the development of cancer cells takes place gradually, there must be a transi­tional phase between normal body cells and fully formed cancer cells. Thus, for example, when fermentation has become so great that dedifferentiation has com­menced, but not so great that the respira­tory defect has been fully compensated for energetically by fermentation, we may have cells which indeed look like cancer cells but are still energetically insuffi­cient. Such cells, which are clinically not cancer cells, have lately been found, not only in the prostate, but also in the lungs, kidney, and stomach of elderly persons. Such cells have been referred to as "sleeping cancer cells" (II, 12).

The sleeping cancer cells will possibly play a role in chemotherapy. >From energy considerations, I could think that sleeping cancer cells could be killed more readily than growing cancer cells in the body and that the most suitable test ob­jects for finding effective killing agents would be the sleeping cancer cells of skin-that is, precancerous skin.

Summary

Cancer cells originate from normal body cells in two phases. The first phase is the irreversible injuring of respiration. Just as there are many remote causes of plague-heat, insects, rats-but only one common cause, the plague bacillus, there are a great many remote causes of cancer-tar, rays, arsenic, pressure, ure­thane-but there is only one common cause into which all other causes of can­cer merge, the irreversible injuring of respiration.

The irreversible injuring of respira­tion is followed, as the second phase of cancer formation, by a long struggle for existence by the injured cells to maintain their structure, in which a part of the cells perish from lack of energy, while another part succeed in replacing the ir­retrievably lost respiration energy by fer­mentation energy. Because of the mor­phological inferiority of fermentation energy, the highly differentiated body cells are converted by this into undiffer­entiated cells that grow wildly-the can­cer cells.

To the thousands of quantitative ex­periments on which these results are based, I should like to add, as a further argument, the fact that there is no alter­native today. If the explanation of a vital process is its reduction to physics and chemistry, there is today no other ex­planation for the origin of cancer cells, either special or general. From this point of view, mutation and carcinogenic agent are not alternatives, but empty words, unless metabolically specified. Even more harmful in the struggle against cancer can be the continual discovery of miscel­laneous cancer agents and cancer viruses, which, by obscuring the underlying phe­nomena, may hinder necessary preventive measures and thereby become respon­sible for cancer cases.