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What Abnormality of cancer cells makes them invasive?It's not that normal body cells can't crawl, because almost every differentiated cell type will crawl if put in tissue culture, on glass or plastic (or a fibrin clot, or a collagen gel). So the real question is: what allows cancer cells to move without whatever normal control mechanisms keep cells in place, once embryonic development is complete? It's not even known how much "to-and-fro" locomotion continues among body cells throughout life. The following abnormalities have been proposed as explanations for the abnormal invasiveness of cancer cells: a) Secretion of proteolytic enzymes by the cancer cells. b) Secretion of enzymes that activate pro-enzymes that are already present in tissues (that digest fibrin). c) Abnormally reduced amounts of adhesion molecules.
Originally it was named Large Extracellular Transformation Sensitive Protein L.E.T.S. (it was separately discovered as "cold insoluble globulin" and as "spreading factor" Adding higher concentrations of fibronectin to cancer cells can make them look more normal. Fibronectin binds to integrin in the plasma membrane, and to fibrin and collagen.
If you culture non-cancerous cells on less adhesive solid substrata, like untreated polystyrene, then their shapes become similar to the shapes than cancer cells have on adhesive substrata, like glass. d) Weakened force of traction. You have seen the video of phorbol ester tumor promoter causing a very dramatic reduction of wrinkling of rubber substrata. Treatment of certain cancerous cells with dibutyryl cyclic AMP causes certain cancerous cells to increase the force of their traction, and to develop more acto-myosin stress fibers (Sort of the reverse of the tumor promoter effect, but slower and using different cells). I wonder if the vimentin "knock out" mouse cells, used by those researchers in Germany, would be any more invasive than equivalent cells in normal mice. Either a negative or a positive result would be interesting. I regret that those authors did not pick up on those parts of our research that found a correlation between weakened traction and increased invasiveness. I hope they will also "challenge" this "dogma". e) Disorganized acto-myosin stress fibers. Not only can this difference simply be observed by fluorescent staining, but much research has been done using mutant forms of small GTPase oncogene proteins rho, rac and cdc-42). These have similar amino acid sequences and folding structures to the ras gene, which about the most deadly of all oncogenes. ("R.HO." originally stood for "Ras Homology"). Small GTPases work by having different folded shapes depending on whether they are currently bound to guanosine tri-phosphate, or guanosine di-phosphate (which are the guanine equivalents of ATP and ADP). Except the purpose is not to transmit energy, but to control behaviors and properties of other molecules, and to transmit and amplify intracellular signals. For example, a signal from the cell surface might cause a ras-GDP to exchange its GDP for a GTP. Then the ras protein itself would fold into a slightly different shape, and remain in that different shape until the GTP that it is holding gets hydrolyzed into a GDP. Until that happens, the protein will stimulate some other proteins to do something in particular, that is the point of the whole system. One initial signal can produce changes in many of the eventual target molecules. This conversion of GTP to GDP may not occur for a fraction of a second, or even for several seconds; these are very slow enzymes, and their point is to be slow. The slower that are, the greater is the amplification effect. Consider them analogous to dated passes that allow you to be in a National Park for a week, or a month. Another analogy is to fancy door-bells that ring some tune, that lasts for several seconds after you push the button. A closer, but more far-fetched analogy would be if pushing a door-bell released a dog biscuit to a watch-dog, who was trained to bark for as long as it took him to chew up the dog biscuit. (I realize that it would be difficult to bark and chew biscuits at the same time, as in the President Ford joke.) When a ras gene gets mutated in the active site, then the protein may not be able to cleave GTP to GTP, at all! So then it would be like a door-bell that rings forever once pushed even one time. That's the kind of mutation in the ras gene that causes a high percentage of human cancer deaths. Usually, people consider the mutant program over-active, in the sense of transmitting too much signal, although they are underactive in the sense of hydrolyzing the GTP. Incidentally, tubulin is a small GTPase, as well as being the structural subunit of microtubules. Some of the implications of this fact have been noticed, but there may be more that no one has yet thought of, so please keep it in mind. Experiments have shown that when you put the permanently GTP versions of rho, rac and cdc-42 into tissue culture cells, then they become very over-active in certain ways related to cell locomotion (not that they actually go faster, or anything so straightforward as that!). Therefore, researchers have concluded that these small GTPases must be what normally controls cell locomotion. From that point of view, it makes a lot of sense that cell locomotion would go out of control if, for example, a rho gene were mutated in such a way that the rho protein would get stuck in the GTP-bound state. In a public lecture, I once compared this to joke in "The Hitch-Hiker's Guide to the Universe", in which the giant computer announces that the secret to life is 42. It was too much to pass up the coincidence of cdc42 being an oncogene. The discoverer of rho, rac and cdc-42 was sitting in the audience, and smiled; but she didn't seem to be amused. f) Contact inhibition. In the 1950s, the British scientists Prof. Michael Abercrombie and his assistant Joan Heaysman published a series of papers in the Swedish "Experimental Cell Research", in which they developed statistical criteria for measuring the extent to which cell locomotion in a given direction is inhibited by touching another cell. For example, using time lapse films of chicken embryo heart mesenchymal cells crawling in tissue culture, they made graphs of speeds of individual cells plotted on the Y axis with the number of other cells that cell was touching (during a given time period) plotted on the X axis. The more other cells touched, the slower cells moved, on the average, and to a very high statistical significance. But they still moved about half as fast when touching 6 cells as when touching none. Zero and six were the minimum and maximum numbers of other cells touched. There were several other statistical tests. Abercrombie was a genius for thinking of how to get numerical measures out of complicated behaviors, and was a famously excellent statistician and designer of experiments. In addition, if you simply watch what happens when one crawling tissue culture cells comes into contact with another one, you can easily see that they usually either stop and reverse directions, or at least slow down and/or change turn off to a new direction. Based on having watched many instances of such behavior, my belief is that something is "turned off" about the "motor" that pulls a leading edge of a cell forward in a given direction. The surface protrusions called lamellipodia and blebs stop forming, as if turned off, in the area where the cells are touching each other, and there is also a local inhibition of the formation of new adhesions to the glass or plastic substratum. The previously advancing edge of the cell often forms an adhesion to the other cell, at least temporarily, and somehow lets go of its adhesion to the glass or plastic. Often, there is an abrupt retraction of one cell away from the one it has touched. Abercrombie and Dunn interpreted this as evidence that cell-cell contact stimulates a strengthening of the cells' contraction. I couldn't find evidence of this using the rubber substrata, however. My best guess is that actin polymerization is reduced along the contacted parts of the cell margin. It would be a major discovery if someone could prove conclusively whether this is true or not. A positive result would be more publishable than a negative one; but you should never let that influence you, except possibly at the stage where you decide how competitive a journal you decide to submit the paper to, first. Around the early 1960s, after Abercrombie and Heaysman had discovered and named contact inhibition (of cell locomotion), many people guessed that there ought also to be an inhibition of cell growth and division, also caused by contact. But nobody developed the kind of statistical criteria needed to prove whether this was true or not. Nevertheless, even scientific papers began to confuse this issue, and talked as if an inhibition of cell growth had been demonstrated, and as if that's what "contact inhibition" meant. Take a look at Wikipedia's definition of contact inhibition, for an example of how far this misunderstanding has degenerated. Incidentally, the word "lamellipodium" was also coined by Abercrombie and Heaysman, in an ECR paper in the early 1970s. And look how Wikipedia defines it! Cancer cells are somewhat less sensitive to contact inhibition (of both kinds, if there is another kind). On the other hand, watch the collision between the sarcoma cell and the mesenchymal cell on my web site about amoeboid locomotion. This is a perfect example of the kind of cell response that "contact inhibition" was meant to refer to. For over a decade, average researchers ("the biologist in the street") uncritically accepted the idea that the invasiveness and excess growth of cancer cells were because of loss of contact inhibition. They went far beyond what Abercrombie had actually written. Then average opinions swung in the opposite direction. For lack of evidence that cell growth is inhibited by cell-cell contact (which nobody had ever claimed to have discovered, anyway), the majority swung to another unjustified extreme. They would tell you that contact inhibition had been shown not really to exist. They were thinking of contact inhibition of cell growth, which had never been claimed to exist, anyway. Also, it may really occur. That just hasn't been studied carefully enough, and is difficult to distinguish from inhibition of cell growth in crowded tissue cultures caused by depletion of nutrients and/or growth factors (which definitely does occur). Anyone would expect that scientists working on cancer would be a lot more careful in how they use words. Abercrombie was very careful, and I try to be. But there is a tendency for sloppy thinking to drown out careful thinking. It's like people shouting in a crowd drowning out the voices of people who talk calmly. Remember the question whether microtubules push cell margins outward, and whether that's why anti-microtubule drugs cause a doubling of the strength of cell traction. Experiments with taxol (and other experiments, too) proved that couldn't be the correct explanation, but do a web search on the word "tensegrity", or that word combined with "microtubule". See what opinion has drowned out others. Despite having made a small effort to get Wikipedia to change how it defines "contact inhibition", it's more educational the way it is. The anonymous, self-appointed people who write Wikipedia are doing a public service even when they get something really, badly, ridiculously wrong, as in this case. The guy who wrote it meant well, and doesn't want people to die of cancer, and hopes that better treatments will be found. I assume he does. But progress is held back by sloppiness about facts and logic. I regret that people, including most scientists, tend to be such sloppy thinkers, and follow trends. But given that they actually are that way, you students should notice it. It's also good that you don't just have to take my word for it. The good news is that you can do a lot better. g) Anaerobic metabolism (including secretion of lactic acid, getting energy from glycolysis instead of mitochondrial respiration, defective mitochondria, "The Warburg Phenomenon" etc.). This became very popular in the 1930s, and there is just as much or more evidence for it now as there was then, & maybe more. Cancer cells really do tend to secrete lots of lactic acid. But nobody knows why they do this, nor does it make much sense for increased growth to be caused by abnormalities in the metabolism that result in food energy being wasted. It would have seemed to me that defective or non-functional mitochondria would cause cells to grow more slowly, and maybe move more slowly. I would be very interested to know whether these anaerobic cells have a lower-than-normal ratio of ATP to ADP in their cytoplasm (normal is about 100 to one). Or maybe they have a smaller-than-normal resting potential, given that between 1/3 and 1/2 of an average cells ATP gets used pumping out sodium, and not nerve and muscle cells have resting potentials (which is not realized by one in ten cell biologists). Another question is whether cancer-like behavior of cultured cells can be mimicked by moderate concentrations of cyanide (which kills you by blocking function of cytochromes in mitochondria). Cyanide does decrease the ratio of ATP to ADP, which causes each ATP to be worth slightly less energy. [Did you know that? Do you see why it's true? The textbooks rubbish about "high energy bonds" tends to submerge actual understanding of questions like that. But you can rise above it.] There seems to be a vague idea that anaerobic metabolism occurs because something is out of control, and that same something is also failing to control cell growth, and that cancer is the result of that same loss of control. If this makes sense to you, then please explain it to me. At first thought, it seemed to make sense to me, but then it sort of fell apart in my hands. ____________________________________________________________________________________ Treatment of CancerPlease try not to forget that the goal is to invent drugs or conditions that will kill all cancer cells without killing too many normal cells. The goal is NOT to find drugs that will reverse the abnormalities of the cancer cells. Repeat: NOT. Because then the cure would only be temporary.But in your future reading, in this and other courses, notice how often medical science tries to make cancer cells less abnormal in their behavior. Do they grow too fast without control? Then develop drugs that inhibit either DNA synthesis or mitosis! Is their abnormality caused by a "rouge enzyme", such as an over-active protein kinase that binds too many phosphates to tyrosines, etc. then develop a drug (Gleevek) that selectively fits in the binding site of that abnormal enzyme. Take a minute or two to do a web search on Gleevek. It really has delayed death by as much as several years in many patients with a certain kind of leukemia. The question to ask yourself is "Why should it kill cancer cells to prevent them from doing something that's abnormal anyway?" The next question to ask yourself is "Why should so many very good and dedicated MDs and other cancer researchers would ever have expected that cancer cells will be killed by slowing down or stopping their cell cycle?" A possible answer to the first of these two questions depends on cell cycle checkpoints. I first read a version of this idea in a paper by the excellent scientist Arthur Pardee, who I never met but greatly admire. A more up to date version of the key idea is that non-cancerous cells (and maybe cancerous one, too) can detect when their DNA synthesis and/or their mitoses are being interfered with, and that the non-cancerous cells respond to this inhibition by slowing down or halting their cell cycle, until the poison goes away. The cancerous cells, lacking normal control, can't slow down, and are therefore much more damaged by the poisons. If this were true, it might make sense to try to stimulate or speed up cell growth during chemotherapy, if you could do that without weakening the normal cells' cell cycle controls. Consider whether it would be a good way to prevent speeding: to cut people's brake lines? This wouldn't explain the ability of Gleevek to cause selective death of cancerous cells, without killing too many normal white blood cells. It really does have that ability, to a surprising extent (or at least what ought to be a surprising extent). Remember: it selectively blocks an abnormal enzyme, that cells aren't supposed to have anyway (It's a fusion protein, coded for by a certain genetic translocation, called "The Philadelphia Chromosome" because of where it was discovered, not its shape,). I can see why blocking that enzyme would be likely to slow down the growth of the cancerous cells. But why would it KILL the cancer cells to slow down their growth. Why did anyone expect that it would? Why aren't people surprised that it does? Why are chapters about such drugs titled "Rational Drug Design". The answer may be as simple as that physicians are focused on making things normal again. If a bone is broken, help repair it. If metabolism is abnormally slow, then speed it up. And if cancer cells divide too much...? (Slow them down? Really?) My suggestion to students is that they might discover a cure for science by asking (and finding out) why the current treatments work as well as they do, and then making them work better. People who don't wonder why Gleevec kills cancer cells at all are not going to be the ones to figure out how to make at kill ALL a patient's cancerous cells, and really cure them rather than just delay their death a year or two. As in "defensive driving", as you go through the world, notice how incorrigibly foolish people can be. They can't help it. But it can help you to notice it. Another good question is why textbooks and courses on the biology of cancer have less than one percent (a rough estimate) of their content on the subject of invasiveness of cancer cells. The various forms of leukemia and lymphoma are able to spread through the body as part of their normal original differentiation as some form of white blood cell. But for all "solid tumors", they would not be malignant unless the cells lose their normal control of cell locomotion, in addition to their lack of control of cell growth.
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