FEATURE ARTICLE
Metastasis
The spread of cancer cells to distant sites implies a complex series of cellular abnormalities caused, in part, by genetic aberrations
Cornelis J. Van Noorden, Linda Meade-Tollin, Fred Bosman
Remodeling
In order for a cell to make protrusions, its cytoskeleton has to be configured for motility. But as already noted, most cancer cells start out as sedentary cells and are neither programmed genetically for motility, nor is their basic skeletal structure conducive to it. Before a cancer cell can walk away from an organ, its cellular skeleton must be restructured for movement. And it is. The idea that a cell can alter its internal architecture is akin to suggesting that a fish can grow leg bones and walk off. And yet, biologists have long observed that cells do in fact change shape through the course of cancerous transformation.
In their normal state, epithelial cells come in several shapes—cylindrical, cuboid or flattened. Their cancerous counterparts become somewhat star-shaped and elongated and more closely resemble fibroblasts than they do epithelial cells. (Fibroblasts are the cells in the connective tissue that form a wall around epithelia and produce the matrix of connective-tissue proteins.) Interestingly, similar fibroblastic morphology is also observed in epithelial cells during embryonal development. This raises the interesting point that many characteristics of tumor cells resemble those normally seen only during the very early stages of embryonic development.
The internal cell structure is not the only thing that must be remodeled before the metastatic cell can leave its host organ. The matrix of proteins forming the connective tissue is like a wall without doors. Somehow, the metastatic cells must pass through this wall. This means that either the wall must be altered, or the cancer cells must acquire the ability to bore through tissue.
The latter scenario seems to take place. Cancer cells penetrate and pass through this barrier by dissolving it. For this purpose, cancer cells use proteases, enzymes that break down proteins, to degrade the mortar of the connective-tissue wall. It is probable that expression of active proteases is precisely coordinated in time as well as in extracellular location. For example, fibroblasts produce and degrade the matrix continuously, and under normal circumstances, this process is in equilibrium with other factors that rebuild the matrix. At the moment, it is not entirely clear whether the cancer cell releases its own proteases, or whether it stimulates protease release from other cells, or whether both processes take place sequentially or simultaneously. What is known is that certain messenger molecules alter the balance. It is possible that cancer cells have acquired the ability to synthesize these messenger molecules.
All proteases share a common feature: They are synthesized by cells in an inactive form, which must be chemically processed before they become active. Processing requires other proteases to chew away at a portion of the precursor protein, called a proprotein, in order to expose the active site of the protease. Complicating the balance is the fact that cells producing proteases may also produce natural protease inhibitors. Sometimes, chemicals in the environment can inhibit the protease inhibitors. So, the ability of a cancer cell to pass through its host tissue depends on some constellation of events that either promotes the synthesis and/or the activation of proteases or prohibits protease inhibition. The net effect is that proteases chew through the connective tissue, allowing the metastatic cancer cells to pass through. (Proteases secreted by endothelial cells may play a role in the angiogenesis required for the growth and metastasis of the primary tumor.)
There has been intense interest in learning more about the specific nature of these proteases, since inhibiting them would go a long way toward preventing metastasis. Much of the focus has been on the family of proteases called matrix metalloproteinases (MMPs). Members of this family can degrade various components of the connective-tissue matrix. The coordination and physiological functions of the different active forms of the MMPs are poorly understood. It is nevertheless clear that they are key to the mechanisms by which metastatic cells migrate through tissue compartments. In an in vitro model for malignant progression in human squamous cell carcinoma, one of us (Meade-Tollin), with coworkers at the University of Arizona at Tucson, has shown that three different MMPs have quite different expression patterns. We have observed that the expression of one of these, matrilysin, exists in higher concentrations in cells that form benign tumors, a presumed earlier stage in the progression to malignancy, than in cells that form invasive tumors in vivo. It is likely that different MMPs are involved at specific stages during which cancer cells become metastatic. Determining the levels of active MMPs in tissue could provide crucial information about the mechanisms of regulation of MMP activity.
Two other protease families are also under investigation for activity during metastasis. The role of these families—cysteine proteases, such as cathepsin-B, and serine proteases, such as urokinase-type plasminogen activator—in cancer-cell invasion is less clear.
It is possible that members of some or all of these families of proteases interact during metastasis. One current scenario postulates that MMPs are secreted in their proprotein form by fibroblasts, but are activated by either cathepsin-B or urokinase-type plasminogen activator secreted by cancer cells.
The secretion of proteases, such as cathepsin-B, by cancer cells demonstrates another way these cells deviate from normal. Normally, cathepsin-B is sequestered within the cell inside membrane-bound vesicles called lysosomes. Lysosomes are the cellular trash cans that do double-duty as recycling centers. These vesicles contain enzymes—cathepsin-B among them—that can break down all macromolecules into their constituent units so they can be reused to make new macromolecules. Bonnie Sloane and her colleagues at Wayne State University School of Medicine in Detroit have discovered that cathepsin-B can be found in the extracellular environment of invading cancer cells. Furthermore, it appears that the invading cells not only secrete cathepsin-B, but can subsequently bind it to their surfaces.
Recently, one of us (Van Noorden) and colleagues were able to demonstrate that the surface-bound form of the enzyme is active. Once the metastasis is established at its new site, cathepsin-B continues to be expressed on the membrane, but in an inactive form. Urokinase-type plasminogen activator, like cathepsin-B, is present and active on invading cells. Such observations suggest that these proteases are at the head of a cascade of proteases that ultimately activate MMPs, which in turn chew up proteins of the connective matrix.
Metastasis is therefore a complex process, requiring the cancer cells first to release themselves of their intercellular adhesive bonds, then leave their cellular microenvironment and migrate through the connective tissue matrix encapsulating their tissue compartment in order to gain access to the blood vessels that carry them to a new organ. Once there, they must re-enact this process in reverse. They must pass through the blood-vessel wall into the new organ and there form new connections.
» Post Comment