Here is an essay on ‘How Cancers Spread’ ? To get the answer, we will begin with a discussion of blood vessels growth and then topics of tumor invasion and metastasis.
How Cancers Spread ?
When a doctor tells a person that he or she has cancer, one of the most important questions that need to be addressed is whether the cancer cells are still confined to their initial location. Once a tumor has invaded neighboring tissues and begins to spread to other regions of the body, it becomes more difficult to treat.
The term metastasis refers to the spread of cancer cells via the bloodstream or lymphatic system to distant sites, where they form secondary tumors—called metastases—that are not physically connected to the primary tumor. Because they can arise in almost any vital organ, metastases rather than primary tumors are responsible for most cancer deaths.
Scientists have expended much effort in studying the cellular properties responsible for metastasis with the hope that a better understanding of the mechanisms involved will eventually lead to better treatments. Such studies have revealed that metastasis is a complex process that can be subdivided into several distinct stages, each involving a different set of cellular traits and interactions.
Among the earliest steps in converting a tiny localized mass of cancer cells into an invasive, metastasizing tumor is the growth of blood vessels that penetrate into the tumor, supplying nutrients and oxygen to the cancer cells and removing waste products. In the absence of such a network of blood vessels, tumors cannot grow beyond a few millimeters in size and would not be a major health hazard.
Since the development of a sustaining network of blood vessels is therefore a crucial step in converting a tiny group of cancer cells into a larger tumor capable of spreading to distant sites.
1. Essay on Tumor Angiogenesis:
To survive and grow, all body tissues require a continual supply of oxygen and nutrients accompanied by the removal of carbon dioxide and other waste products. These needs are met by a system of blood vessels comprised of arteries that carry blood from the heart to the rest of the body, veins that carry blood from the body back toward the heart, and tiny capillaries that connect the smallest arteries and veins.
The wall of a capillary is only a single cell layer thick, so oxygen and nutrients carried in the bloodstream can easily diffuse through capillary walls and nourish the surrounding tissues, and carbon dioxide and other waste products produced by tissues diffuse back into the capillaries for removal from the body.
Like the cells of any other tissue, tumor cells require a network of blood vessels to perform these same tasks. The vessels that feed and sustain tumors are produced by angiogenesis, a term that refers to the process by which new blood vessels sprout and grow from pre-existing vessels in the surrounding normal tissues.
To understand how a tumor causes surrounding tissues to provide it with such a growing network of blood vessels, we first need to describe the process of normal angiogenesis and the factors that control it.
i. Angiogenesis is Prominent in Embryos but Relatively Infrequent in Adults:
Angiogenesis is a normal biological event that occurs at specific times for specific purposes. For example, a developing embryo in a mother’s womb must create the vast network of arteries, veins, and capillaries that are needed for a mature circulatory system. To initiate blood vessel formation, the embryo first creates a primary population of cells, called endothelial cells that form the inner lining of blood vessels.
As part of this process of vasculogenesis, the newly created endothelial cells are organized into a primitive network of channels representing the major blood vessels of the circulatory system. Once the primordial network of vessels has been created, angiogenesis takes over.
Angiogenesis involves an extensive phase of growth and proliferation of pre-existing endothelial cells, which form buds that sprout from existing vessels and develop into an interconnected network of new vessels (Figure 1).
Although vasculogenesis is restricted to early embryonic development, angiogenesis continues to occur after birth when additional blood vessels are required. In adults, who have a fully formed circulatory system, this need for new vessels is limited to a few special situations and endothelial cells rarely divide, doing so about once every three years on average. When new vessels are required, however, endothelial cell division can be stimulated and angiogenesis will take place.
For example, blood vessel growth is needed each month in the inner lining of the uterus as part of the normal menstrual cycle. Angiogenesis is therefore activated a few days each month in the uterine lining of women of reproductive age. In both males and females, angiogenesis is also called upon anytime an injury requires new blood vessels for wound healing and tissue repair.
Angiogenesis needs to be precisely regulated in such cases, turning on for a short time and then stopping. Regulation is accomplished through the use of both activator and inhibitor molecules. Normally the inhibitors predominate, blocking angiogenesis.
When a need for new blood vessels arises, angiogenesis activators increase in concentration and inhibitors decrease, triggering the proliferation of endothelial cells and the formation of new vessels. As we will see shortly, many of these regulatory molecules were first identified through the study of angiogenesis triggered by cancer cells.
ii. Angiogenesis is Required for Tumors to Grow beyond a Few Millimeters in Diameter:
For more than 100 years, scientists have known that tumors are supplied with a dense network of blood vessels. Some investigators initially believed that these blood vessels were pre-existing vessels that had expanded in response either to the increased metabolic activity of tumors or to toxic products that tumors release.
Others thought that the vessels were new structures formed as part of an inflammatory response designed to defend the host against the tumor. Then in 1971, Judah Folkman proposed a radical new idea regarding the significance of blood vessels in tumor development. He suggested that tumors release signaling molecules that trigger the growth of new blood vessels in the surrounding host tissues and that these new vessels are required to sustain tumor growth.
This concept was initially based on experiments in which cancer cells were grown in isolated organs under artificial laboratory conditions. In one such experiment, illustrated in Figure 2 (left), a normal thyroid gland was removed from a rabbit and placed in a glass chamber. A small number of cancer cells were then injected into the gland and a nutrient solution was pumped into the organ to keep it alive.
The cancer cells divided for a few days but suddenly stopped when the tumor mass reached a diameter of 1 to 2 millimeters. Virtually every tumor stopped growing at exactly the same size, suggesting that some kind of limitation allowed them to grow only so far.
When tumor cells were removed from the thyroid gland and injected back into animals, cell proliferation resumed and massive tumors developed. Why did the tumors stop growing at a tiny size in the isolated thyroid gland and yet grow in an unrestrained fashion in live animals? On closer examination, a possible explanation became apparent.
The tiny tumors, alive but dormant in the isolated thyroid gland, had failed to link up to the organs blood vessels; as a result, the tumors stopped growing when they reached a diameter of 1 to 2 mm. When injected into live animals, these same tumors became infiltrated with blood vessels and grew to an enormous size.
To test the theory that blood vessels are needed to sustain tumor growth, Folkman implanted tumor cells in the anterior chamber of a rabbit’s eye, where there is no blood supply. As shown in Figure 2 (right), cancer cells placed in this location survived and formed tiny tumors, but blood vessels from the nearby iris could not reach the cells and the tumors quickly stopped growing.
When the same cells were implanted directly on the iris tissue, blood vessels from the iris quickly infiltrated the tumor cells and the mass of each tumor grew to thousands of times its original size. Once again, it appeared that tumors must trigger development of a blood supply before they can grow beyond a tiny mass. This process by which cancer cells stimulate the development of a blood supply is called tumor angiogenesis (Figure 3).
Angiogenesis is Controlled by the Balance between Angiogenesis Activators and Inhibitors:
If tumors require blood vessels to sustain tumor growth, how do they ensure that this need is met? The first hint came from studies in which cancer cells were placed inside a chamber surrounded by a filter possessing tiny pores through which cells cannot pass (Figure 4).
When such chambers are implanted into animals, new capillaries begin to proliferate in the surrounding host tissue. In contrast, normal cells placed in the same type of chamber do not stimulate blood vessel growth. The most straightforward interpretation is that cancer cells produce molecules that diffuse through the tiny pores in the filter and activate angiogenesis in the surrounding host tissue.
The next job was to identify the molecules responsible for stimulating angiogenesis, a task that occupied many investigators over a span of several decades. This intensive effort eventually led to the identification of more than a dozen proteins, as well as several smaller molecules, that can activate angiogenesis (Table 1).
Two of the proteins, vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF), appear to be especially important for sustaining tumor growth. VEGF and FGF are produced by many kinds of cancer cells (and certain types of normal cells), and they trigger angiogenesis by binding to specific receptor proteins located on the surface of endothelial cells.
To see how this process works, let us briefly focus on VEGF (Figure 5). VEGF is produced by the majority of tumors and is secreted into the surrounding tissues. When VEGF molecules encounter an endothelial cell, they bind to and activate VEGF receptors located on the endothelial cell surface.
The signal is then relayed from the activated receptors to a sequential pathway of signal transduction proteins that trigger changes in cell behavior and gene expression. As a result, endothelial cells begin to proliferate and to produce matrix metalloproteinases (MMPs), a group of protein-degrading enzymes that are released into the surrounding tissue.
The MMPs break down components of the extracellular matrix that fills the spaces between neighboring cells, thereby allowing the endothelial cells to migrate into the surrounding tissues. As they migrate, the proliferating endothelial cells become organized into hollow tubes that evolve into new networks of blood vessels.
Although many tumors produce VEGF or FGF, they are not the sole explanation for the activation of angiogenesis. For angiogenesis to proceed, these molecules must overcome the effects of angiogenesis inhibitors that normally restrain the growth of blood vessels. More than a dozen naturally occurring inhibitors of angiogenesis have been identified (see Table 1), including the proteins angiostatin, endostatin, and thrombospondin.
A finely tuned balance between the concentration of these angiogenesis inhibitors and the concentration of activators (such as VEGF and FGF) determines whether a tumor will induce the growth of new blood vessels. When tumors trigger angiogenesis, it is usually accomplished by increasing the production of angiogenesis activators and, at the same time, decreasing the production of angiogenesis inhibitors.
iii. Inhibitors of Angiogenesis can Restrain Tumor Growth and Spread:
The discovery of angiogenesis inhibitors has caused scientists to speculate about the potential usefulness of such molecules. If sustained tumor development requires the proliferation of new blood vessels, using angiogenesis inhibitors to block vessel formation might be useful for slowing tumor growth.
This approach has been quite effective when tested in mice. In one striking study, mice with several types of cancer were injected with the angiogenesis inhibitor endostatin. After a few cycles of treatment, the primary tumors virtually disappeared (Figure 6).
Studies involving mice with mutations that hinder angiogenesis have provided additional support for the idea that tumor growth can be restrained by inhibiting angiogenesis. As shown in Figure 7 (left), injecting breast cancer cells into such angiogenesis-deficient mutant mice leads to the formation of tumors that grow for a short time and then completely regress.
In contrast, normal mice injected with the same breast cancer cells die of cancer within a few weeks. When lung cancer cells are injected into the same mutant mice, the results are slightly different. Unlike breast cancer cells, lung cancer cells do develop into tumors in the angiogenesis-deficient mice, but the tumors grow more slowly than in normal mice and fail to metastasize to other organs.
The failure of these lung cancer cells to metastasize in angiogenesis-deficient mice raises the possibility that angiogenesis-inhibiting drugs might be useful in preventing metastasis. In an experiment designed to address this issue, shown in Figure 7 (right), cancer cells were injected beneath the skin of laboratory mice and allowed to grow for two weeks.
The primary tumors were then removed and the animals were monitored for several weeks to see whether visible metastases would appear in other organs. Within a few weeks the average mouse developed almost 50 lung tumors, which arose from cancer cells that had spread to the lungs prior to removal of the primary tumor. In contrast, mice treated with angiostatin developed an average of only two or three tumors in their lungs, indicating that the angiogenesis inhibitor had reduced the rate of metastasis about 18-fold.
Such observations provide a possible explanation for the phenomenon of tumor dormancy, in which cancer cells spread from a primary tumor to another organ and form tiny clumps of cancer cells that remain dormant for prolonged periods of time. One factor that might contribute to tumor dormancy is that these tiny tumors, called micrometastases, may not have triggered the angiogenesis that is needed for tumor growth beyond a small size.
Animal studies suggest one possible reason- Some primary tumors produce large amounts of angiostain that spill over into the bloodstream and circulate throughout the body. It has been hypothesized that this circulating angiostatin can inhibit angiogenesis at other sites, thereby preventing micrometastases from growing into visible tumors.
Why wouldn’t the angiostatin also prevent the angiogenesis that is needed for growth of the primary tumor? Most likely, the inhibitory effects of angiostatin on the primary tumor are overcome by the stimulatory effects of VEGF, which is also produced in large amounts by many primary tumors.
Unlike angiostatin, however, VEGF is quickly destroyed when it enters the bloodstream. So angiostatin, but not VEGF, circulates in the bloodstream and might block angiogenesis at sites of micrometastases, which do not yet produce enough VEGF of their own to overcome the inhibitory effects of the circulating angiostatin.
Although the proposed role of angiogenesis inhibitors in tumor dormancy remains to be firmly established, the overall body of evidence strongly supports the idea that angiogenesis inhibitors can restrain the growth and spread of cancer cells in animals. Are such findings relevant to humans? To address this question, numerous angiogenesis- inhibiting drugs are being tested in cancer patients and one such drug, called Avastin, has already been approved for use in treating colon cancer.
2. Essay on Invasion and Metastasis:
Once angiogenesis has occurred at the site of a primary tumor, the stage is set for tumor cells to invade neighboring tissues and spread to distant sites. A few kinds of cancer, such as non-melanoma skin cancers, rarely invade and metastasize. About half the people who develop other forms of cancer, however, have tumors that have already begun to spread beyond the site of origin by the time the cancer is diagnosed.
Because cancer is much harder to treat after it has spread, this alarming statistic points to the importance of better procedures for early cancer detection. It might also be possible to develop improved treatments for cancer if we better understood the mechanisms that allow cancer cells to spread, the topic to which we now turn.
i. Spreading of Cancer Cells by Invasion and Metastasis is a Complex, Multistep Process:
Cancers spread through the body via two distinct mechanisms:
i. Invasion and
ii. Metastasis.
Invasion refers to the direct migration and penetration of cancer cells into neighboring tissues, whereas metastasis involves the ability of cancer cells to enter the bloodstream (or other body fluids) and travel to distant sites, where they form new tumors that are not physically contiguous with the primary tumor.
Metastasis involves a complex cascade of events, beginning with the process of angiogenesis that has already been described. The events following angiogenesis can be grouped into three main steps. First, cancer cells invade surrounding tissues and penetrate through the walls of lymphatic and blood vessels, thereby gaining access to the bloodstream.
Second, these cancer cells are then transported by the circulatory system throughout the body. And third, the cancer cells leave the bloodstream and enter particular organs, where they establish new metastatic tumors (Figure 8).
If cells from the initial tumor fail to complete any of these steps, or if any of the steps can be prevented, metastasis will not occur. It is therefore crucial to understand how the properties of cancer cells make these three steps possible.
ii. Changes in Cell Adhesion, Motility, and Protease Production allow Cancer Cells to Invade Surrounding Tissues and Vessels:
The initial step leading to metastasis is the invasion of surrounding tissues and vessels by cancer cells (see Figure 8, step ①). Thus, unlike the cells of benign tumors or most normal cells, which remain together in the location where they are formed, cancer cells are capable of leaving their original site and penetrating through surrounding tissue barriers, eventually entering the circulatory system.
Several mechanisms make this invasive behavior possible. The first involves changes in the adhesive forces between cells. In most tissues, adjoining cells are held together by binding interactions between cell-cell adhesion proteins found on the outer surface of each cell. These adhesion molecules, which normally function to keep cells in place, are often missing or deficient in cancer cells, thereby allowing cells to separate from the main tumor mass more readily.
The diminished adhesiveness of cancer cells can be readily demonstrated by a simple experiment: If you take a sample of cancer tissue, suspend it in a fluid-filled flask, and shake the flask vigorously, the cells will separate from one another more readily than cells obtained from a comparable sample of normal tissue.
In many cases, the reduced adhesiveness of cancer cells can be traced to the loss of E-cadherin, a cell-cell adhesion protein that normally binds epithelial cells to one another. Highly invasive cancers usually have less E-cadherin than normal cells, suggesting a relationship between the loss of cadherins and the ability to invade.
Support for this idea has come from studies in which noninvasive populations of cancer cells were treated with antibodies that block the function of E-cadherin. Such treatment gives cancer cells the ability to invade. Conversely, restoring E-cadherin to cancer cells lacking this molecule inhibits their ability to form invasive tumors when the cells are injected back into animals.
A second property involved in tumor invasion is cell motility, which is activated after the loss of cell-cell adhesion permits cancer cells to detach from one another. Cancer cells possess all the normal cytoplasmic machinery required for cell locomotion, but their actual movement needs to be stimulated by signaling molecules produced either by surrounding host tissues or by cancer cells themselves. Besides activating cell motility, some of these signaling molecules act as chemo attractants that guide cell movement by serving as attracting signals toward which the cancer cells will migrate.
In addition to decreased adhesiveness and activated motility, a third property involved in invasion is the production of proteases (protein-degrading enzymes). The purpose of these enzymes is to break down structures that would otherwise represent barriers to cancer cell movement.
For example, epithelial cells, the source of about 90% of all human cancers, are separated from underlying tissues by a thin, dense layer of protein-containing material called the basal lamina. Before epithelial cancers can invade adjacent tissues, the basal lamina must first be breached. Cancer cells break through this barrier by producing proteases that facilitate degradation of the proteins that form the backbone of the basal lamina.
One such protease is plasminogen activator, an enzyme that converts the inactive precursor plasminogen into the active protease plasmin (Figure 9). Because high concentrations of plasminogen are present in almost all tissues, small amounts of plasminogen activator released by cancer cells can quickly catalyze the formation of large quantities of plasmin.
The plasmin in turn performs two tasks:
(1) It degrades components of the basal lamina and the extracellular matrix, thereby facilitating tumor invasion; and
(2) It cleaves inactive precursors of matrix metalloproteinases (MMPs), produced mainly by surrounding host cells, into active enzymes that also degrade components of the basal lamina and extracellular matrix.
After proteases allow the basal lamina to be penetrated, they degrade the matrix of the underlying tissues to open up paths through which the cancer cells can move. The cancer cells migrate until they reach tiny blood or lymphatic vessels, which are also surrounded by a basal lamina.
The proteases then digest holes in this second basal lamina, allowing cancer cells to pass through it and through the layer of endothelial cells that form the vessel’s inner lining, at which point the cancer cells have finally gained entry into the circulatory system.
iii. Relatively Few Cancer Cells Survive the Voyage through the Bloodstream:
Cancer cells that penetrate through the walls of tiny blood vessels gain direct entry to the bloodstream, which then transports the cells to distant parts of the body (Figure 8, step ②). When cancer cells initially penetrate the walls of lymphatic vessels rather than blood capillaries, the cells are first carried to regional lymph nodes, where they may become lodged and grow.
For this reason, regional lymph nodes are a common site for the initial spread of cancer. Nonetheless, lymphatic vessels have numerous interconnections with blood vessels, so cancer cells that initially enter into the lymphatic system eventually arrive in the bloodstream and circulate throughout the body.
Regardless of their initial entry route, the net result is often large numbers of cancer cells in the bloodstream. Even a tiny malignant tumor weighing only a few grams can release several million cancer cells into the circulatory system each day. To produce metastases, however, the cells must survive the trip through the circulatory system, and most cancer cells do not thrive in such an environment.
Evidence for this conclusion has come from experiments in which cancer cells were radioactively labeled (to allow them to be identified) and then injected into the bloodstream of laboratory animals. After a few weeks, less than one in a thousand radioactive cells was found to be alive. Apparently the bloodstream is an inhospitable place for most cancer cells and only a tiny number survive the trip to potential sites of metastasis.
iv. The Ability to Metastasize Differ among Cancer Cells and Tumors:
Since only a tiny fraction of the cancer cells that enter the bloodstream survive and establish metastases, the question arises as to whether these cells are random members of the original tumor population or specialized cells better suited for metastasis. Figure 10 illustrates an experiment designed by Isaiah Fidler to address this question. In these studies, mouse melanoma cells were injected into the bloodstream of healthy mice to study the ability of the cells to metastasize.
A few weeks after the injections, metastases were detected in a variety of locations, but mainly the lungs. Cells from the lung metastases were removed and injected into another mouse, leading to the production of more lung metastases. By repeating the same procedure many times in succession, Fidler eventually obtained a population of cancer cells that formed greater numbers of lung metastases than did the original tumor cell population.
The most straightforward interpretation was that the initial melanoma consisted of a heterogeneous population of cells with differing metastatic capabilities and that the successive experiments had gradually selected for those cells that are especially well suited for metastasizing. To test this hypothesis, studies were subsequently performed in which single cells were isolated from a primary melanoma and each isolated cell was allowed to grow in culture to form a separate population of cells.
Such cell populations, each derived from the proliferation of a single initial cell, are referred to as clones. When the various cloned populations of cells were injected into animals, some of the clones produced few metastases, some produced numerous metastases, and some fell in between. Since each clone was derived from a different cell in the original tumor, the results support the idea that the cells in a primary tumor differ in their ability to metastasize.
It has been known for many years that human cancers of the same type can differ significantly in their ability to metastasize. In one set of experiments, investigators analyzed gene activity in lung cancers and discovered a pattern involving the expression of 17 genes that predicted whether a primary lung cancer was likely to metastasize.
This same gene-expression “signature” also predicts the metastatic behavior of other types of cancer. For example, prostate or breast cancer patients whose primary tumors exhibit the 17-gene signature are more likely to develop metastases than individuals whose tumors do not exhibit the 17-gene signature. Thus, the likelihood that metastasis will occur appears to be genetically programmed into the cells of the primary tumor.
v. Blood-Flow Patterns often Dictate Where Cancer Cells will Metastasize:
After traveling through the circulatory system to distant parts of the body, cancer cells exit from the bloodstream and invade organs that may be located far from the primary tumor (Figure 8, step ③).
Although the bloodstream carries cancer cells everywhere in the body, the final distribution of metastases is not random, nor is it the same for every type of cancer, Instead, cancers arising in each organ preferentially metastasize to particular locations. For example, stomach and colon cancers frequently metastasize to the liver, prostate and breast cancers often metastasize to bone, and many forms of cancer tend to metastasize to the lungs.
One factor underlying these distinctive relationships is the pattern of blood flow in the circulatory system, which determines where cancer cells floating in the bloodstream are likely to become lodged. Based solely on size considerations, the most probable site for circulating cancer cells to become stuck is in capillaries—the tiny vessels whose diameter is generally no larger than that of a single blood cell.
Because they are usually larger than blood cells, circulating cancer cells often become lodged in tiny capillaries as they move through the body. The arrested cancer cells may then adhere to and penetrate through the capillary walls and invade the surrounding organ, beginning the formation of a new metastatic tumor.
The preceding scenario suggests that after a cancer cell has entered the bloodstream, it is susceptible to becoming arrested in the first capillary bed it encounters. Figure 11 shows that for most primary tumors, the first such capillary bed will be in the lungs, which may help explain why the lungs are a relatively frequent site of metastasis for many kinds of cancer.
However, blood-flow patterns do not always point to the lungs. For cancers arising in the stomach and colon, cancer cells that enter the bloodstream are first carried to the liver, where the vessels break up into a bed of capillaries. As a result, the liver is a common site of metastasis for stomach and colon cancers.
Finally, when cancer arises in the lung, cancer cells entering the bloodstream will flow first to the left side of the heart and from there to capillary beds located throughout the body. It is therefore not surprising that lung cancers metastasize too many different organs, including the liver, bones, brain, kidney, adrenal gland, thyroid, and spleen.
vi. Organ-Specific Factors Play a Role in Determining where Cancer Cells will Metastasize:
Although blood-flow patterns are clearly important, they do not always explain the observed distribution of metastases. As early as 1889, Stephen Paget proposed that the nonrandom distribution of metastases arises in part because individual cancer cells have a special affinity for the environment provided by particular organs. Paget’s idea is often referred to as the “seed and soil” hypothesis, based on his analogy that when a plant produces seeds, they are carried by the wind in all directions but they only grow if they fall on congenial soil.
According to this view, cancer cells are carried to a variety of organs by the bloodstream, but only a few sites provide an optimal environment for the growth of a particular type of cell. In other words, metastasis only takes place where the seed (a cancer cell) and the soil (a particular organ) are compatible.
Support for the “seed and soil” concept has come from a systematic analysis of the sites to which various human cancers tend to metastasize. For roughly two-thirds of the cancer types examined, the rates of metastasis to each organ can be explained solely on the basis of blood-flow patterns. Of the remaining cases, some kinds of cancers metastasize to particular organs less frequently than would be expected and others metastasize to particular organs more frequently than would be expected.
Animal experiments have revealed that this non- random behavior can be traced to the properties of individual cancer cells. In one set of experiments, similar to those shown in Figure 10, mouse melanoma cells were injected into normal mice and metastases were isolated from the brain instead of the lung.
The metastatic cells were then injected into another healthy mouse, and the same cycle was repeated again and again. Even though the melanoma cells employed in this study initially metastasized more often to the lung than they did to the brain, the repeated selection of metastatic cells from the brain eventually led to the isolation of cells that preferentially metastasize to the brain rather than to the lung.
Similar experiments involving the selection of cells derived from ovarian metastases yielded cells that preferentially metastasize to the ovary rather than to the lung or brain. Hence the initial tumor must have consisted of a heterogeneous population of cells that differ in the sites to which they tend to metastasize.
Why do individual cancer cells grow best at particular sites? The general answer is that the ability to grow in different locations is affected by interactions between cancer cells and molecules present in the organs to which they are delivered. An example of this principle is provided by prostate cancer cells, which preferentially metastasize to bone (a pattern that would not be predicted based on blood flow).
To investigate the reason for this preference, experiments have been performed in which prostate cancer cells were mixed together with cells from various locations—including bone, lung, and kidney—and the cell mixtures were then injected into animals. It was found that the ability of prostate cancer cells to develop into tumors was stimulated by the presence of cells derived from bone, but not from lung or kidney.
Subsequent studies uncovered a possible explanation- Bone cells produce growth factors that stimulate the proliferation of prostate cancer cells. This example illustrates just one of several kinds of molecular interactions that influence the ability of cancer cells to grow in particular organs.
vii. Some of the Cellular Properties Involved in Invasion and Metastasis Arise during Tumor Progression:
After tumor cells have left the bloodstream and invaded an appropriate organ where conditions for their growth are favorable, the cells begin proliferating at the new site. Before a metastatic tumor can grow beyond a few millimeters in diameter, however, it must first trigger angiogenesis (as was the case for the primary tumor).
Some metastatic tumors are slow to elicit angiogenesis and remain dormant for significant periods of time; whereas others are quick to trigger angiogenesis and may grow so rapidly that they soon exceed the size of the primary tumor from which they were derived.
In the latter case, the first sign that a person has cancer may arise from metastases rather than from the primary tumor. For example, the earliest symptom noticed by a person with lung cancer could be back pain triggered by cancer cells that have metastasized to the bones of the spinal column.
Or a person might develop liver failure caused by cancer cells that metastasized to the liver from a smaller primary tumor located in the esophagus, stomach, or colon.
It may seem surprising that tumor metastases can be larger than the primary tumor from which they originated. Remember, however, that primary tumors consist of a heterogeneous population of cells that differ in their capacity to metastasize and in their ability to grow at particular sites.
Those cells that successfully manage to metastasize to a particular organ may represent a subpopulation of the original cancer cells that are especially well suited for creating metastases.
Such cells may be faster growing, more invasive, better at triggering angiogenesis, and generally more aggressive than the average cell of the initial tumor population. Metastatic tumors may therefore end up growing and invading more efficiently than the original primary tumor, shedding more cells into the bloodstream and generating further metastases that are even more aggressive.
Cancer is thus a disease whose properties change with time. In the beginning stages of tumor formation, the properties required for malignant growth are not fully developed. Instead, as a tumor begins to grow, individual cells often acquire gene mutations and alter the genes they express, turning on some genes and turning off others.
Such alterations create a population of cells whose properties, including the ability to invade and metastasize, gradually change over time. Cells acquiring traits that confer a selective advantage—such as increased growth rate, increased invasiveness, ability to survive in the bloodstream, resistance to immune attack, ability to grow in other organs, resistance to drugs used in cancer treatment, and evasion of apoptosis—will be more successful than cells lacking these traits and so will gradually tend to predominate. This gradual change in the properties of a tumor cell population, as cells acquire more and more aberrant traits and become increasingly aggressive, is known as tumor progression.
The Immune System can Inhibit the Process of Metastasis:
Given the life-threatening nature of metastasis, the question arises as to whether the body has any defenses against it. One possibility is the immune system, which has the ability to attack and destroy foreign cells.
When cancer cells circulate in the bloodstream, where cells of the immune system travel in large numbers, they are especially vulnerable to attack. Of course, cancer cells are not literally of “foreign” origin, but they often exhibit molecular changes that allow the immune system to recognize the cells as being abnormal.
Animal experiments suggest that in some cases, attack by the immune system does limit the process of metastasis. One such study involved two strains of mouse lung cancer cells; D122 cells that metastasize with high frequency and A9 cells that rarely metastasize.
In general, the ability of the immune system to recognize cells as being foreign or abnormal requires the involvement of cell surface proteins called major histocompatibility complex (MHC) molecules. The MHC molecules carried by the two lines of lung cancer cells exhibit a prominent difference- A9 cells carry two types of MHC (called H-2K and H-2D), whereas the D122 cells express only one form (H-2D).
The discovery that D122 and A9 cells carry different cell surface MHC molecules raises an important question; Is the differing metastatic behavior of the two cells related to the immune system’s ability to recognize and attack the two cell types? This issue was investigated by injecting A9 and D122 cells into separate groups of animals and monitoring the production of cytotoxic T lymphocytes (CTLs), a class of immune cells specialized for attacking foreign and abnormal cells. The animals were found to produce numerous CTLs targeted against A9 cancer cells, but few CTLs targeted against D122 cells.
Why do CTLs attack A9 cells more readily than D122 cells? The most obvious possibility is that the immune system recognizes the H-2K MHC molecules, which are carried by A9 cells but not by D122 cells. This hypothesis was tested by introducing purified DNA containing the H-2K gene into D122 cells, thereby causing the D122 cells to produce the H-2K form of MHC (Figure 12).
As predicted, the altered D122 cells expressing H-2K exhibited a reduced capacity to metastasize when injected into mice, suggesting that the presence of H-2K made the cells more susceptible to immune attack. However, the primary tumor at the site of injection grew normally, implying that tumor cells are more susceptible to immune attack when they are circulating in the bloodstream, where large numbers of immune cells reside.
viii. Invasion and Metastasis Involve a Variety of Tumor-Host Interactions:
The ability of the immune system to inhibit the process of metastasis illustrates an important principle: The behavior of malignant tumors depends not just on the traits of tumor cells, but also on interactions between tumor cells and normal cells of the surrounding host tissues.
As is summarized in Figure 13. For example, angiogenesis is triggered by growth factors released by tumor cells that act on normal endothelial cells of the surrounding host tissue, thereby stimulating the proliferation of new blood vessels.
Invasion is facilitated by both tumor- and host-derived proteases that degrade normal extracellular structures such as the basal lamina and the extracellular matrix. The motility of cancer cells and the direction in which they migrate is influenced by signaling molecules made by normal cells of the surrounding tissues.
Penetration through capillaries involves adhesion of cancer cells to molecules present in the basal lamina. And finally, the growth of metastases at distant sites is simulated by growth factors and other molecules produced by cells residing in the organs being invaded.
Normal tissues also contain cells and molecules that are capable of hindering invasion and metastasis. For example, we have already seen that immune lymphocytes are capable of attacking and destroying cancer cells, thereby limiting their ability to metastasize. In addition, normal tissues produce protease inhibitors that reduce the activity of the proteases that cancer cells require for degrading the basal lamina and extracellular matrix.
The invasiveness of cancer cells therefore reflects a competition between proteases produced by tumor cells and protease inhibitors produced by surrounding normal cells. These are just a few of the numerous examples in which tumor-host interactions influence the ability of cancer cells to invade neighboring tissues and metastasize to distant sites.
ix. Specific Genes Promote or Suppress the Ability of Cancer Cells to Metastasize:
Because metastasis is the property that makes cancer so dangerous, scientists have been trying to identify some of the key molecules involved in metastasis that might serve as useful targets for new anticancer drugs. The matrix metalloproteinases (MMPs), which facilitate destruction of the basal lamina and extracellular matrix, are one possibility. MMPs are involved in angiogenesis as well as invasion and metastasis, so drugs that inhibit MMP activity could conceivably interfere with cancer progression in multiple ways.
MMP inhibitors are therefore being tested in human cancer patients to see if they can slow down or stop the spread of cancer. Another group of attractive targets are those molecules found on the cancer cell surface that help cancer cells adhere to capillary walls.
If inhibitors could be found that block the activity of these cell surface molecules, it might be possible to hinder the interactions between cancer cells and capillaries that allow cancer cells to pass into and out of the bloodstream.
Attempts are also being made to target the growth factors that stimulate cancer cell proliferation in specific target organs, such as the growth factor produced in bone tissue that stimulates the proliferation of metastatic prostate cancer cells.
Genetic analysis is beginning to facilitate the identification of molecules that might be good targets for drugs designed to halt cancer spread. Through this approach, researchers have identified dozens of genes that influence the ability of cancer cells to metastasize, some exerting positive effects and others exerting negative effects.
The positively acting genes, called metastasis promoting genes, code for proteins that stimulate events associated with invasion and metastasis. While it might seem surprising that cells possess genes that promote invasion and metastasis, several traits exhibited by invasive cancer cells—for example, decreased adhesiveness and enhanced motility—are also important for certain kinds of normal cells, such as embryonic cells and cells of the immune system.
The negatively acting genes, called metastasis suppressor genes, code for proteins that inhibit events associated with invasion and metastasis. Acquiring the ability to metastasize is usually associated with enhanced activity of metastasis promoting genes as well as diminished activity of metastasis suppressor genes.
Of the two classes of genes, metastasis suppressors are easier to identify because it often takes the action of only one of these genes to block metastasis. To determine whether a gene is a metastasis suppressor, a normal copy of the gene is simply introduced into a population of cancer cells that already possess the ability to metastasize.
If activation of the newly introduced gene blocks the ability to metastasize without inhibiting the ability of the cells to form tumors, it is classified as a metastasis suppressor gene. Several metastasis suppressors have been identified using such approaches. One of the best understood is the CADI gene, which codes for E-cadherin.
E-cadherin is a cell-cell adhesion molecule whose loss from the cell surface contributes to tumor invasion by allowing cancer cells to detach from one another and move away from the primary tumor. A number of other metastasis suppressor genes have also been identified, including genes called NM23, KiSS1, KAI1, BRMS1, and MKK4. Some of these genes code for other proteins involved in cell adhesion and motility, but additional mechanisms appear to be involved as well.
Progress has also been made in identifying genes that promote rather than suppress metastasis. An especially interesting example is the gene coding for a protein called Twist, which regulates the activity of a specific group of genes during embryonic development. Genes activated by the Twist protein cause cells to lose their adhesive properties, become motile, and migrate from one part of the embryo to another.
After embryonic development is complete, the Twist protein is no longer needed and its production is shut down in most tissues. However, production of the Twist protein is reactivated in cancer cells, allowing them to reacquire the embryonic traits that allow cells to move throughout the body.
Recent experiments have shown that introducing an inhibitor of Twist production into mouse breast cancer cells reduces the ability of the cells to metastasize. It is therefore hoped that further study of metastasis promoting genes such as Twist will help identify those events and activities whose disruption by appropriate drugs would be most effective at preventing metastasis.