Almost all types of differentiated cells can become neoplastic ,or cancerous. 'The process of cell change in which ,a cell loses its ability to control its rate of division, and thus becomes a tumour cell, is called cell transformation.
The cancerous cell generally retains the structural and functional characteristics of the normal cell type from, which it is derived. Thus cancerous cells of the thyroid gland continue to secrete thyroxin. Neoplastic cells, however, differ from their normal counterparts in several respects.
1. Immortalization. Normal cell cultures do not survive indefinitely. For example human cell cultures die after about 50 generations, and chicken cell cultures have a much shorter life expectancy. On the other hand transformed cell cultures are immortal and can grow indefinitely. Cell cultures infected with mouse sarcoma virus can be maintained as long as nutrition is provided and overcrowding avoided.
2. Loss of contact inhibitions. Normal cells in a culture stop growing when their plasma membranes come into contact with one another. When two normal cells come into contact, one or both will stop moving and then begin to move in another direction. This inhibition of growth after contact is caned contact inhibition.
If mouse cells are grown in a culture medium the adhesive properties of the cell membranes cause the cells to stick to the walls of the glass vessel. As long as the cells are few, they go on dividing regularly at 24 hour intervals, until they form a single layer (monolayer). After this their rate of division slows down. Apparently contact of the plasma membranes of the cells inhibits growth and division.
Transformed cells on the other hand usually do not stop dividing after forming a monolayer. Division continues until several layers of cells are formed. Thus transformed cells are unable to go into a quiescent stage, and will grow continuously until they kill themselves.
Transformed cells apparently, undergo a change in the property of their cell membranes which become less adhesive. This change enables the cells to dissociate from neighboring cells and to infiltrate other organs, where they form metastatic tumours. Cancer cells apparently lack proper recognition and communication.
3. Reduced cellular adhesion. When normal cells become cancerous there is a change in the 'stickiness' of their cell membranes. Normal cells show stickiness or adhesiveness. If grown in a nutrient medium kept in a glass vessel, the cells stick to the glass rather than float in the medium. Transformed cells show a decreased adhesiveness, and if grown in solid media stick to each other less than do normal cells. However, examples of increased adhesiveness following malignant transformation have also been reported
Adhesiveness shows considerable specificity. Thus liver cells tend to stick to other liver cells but not to other cell types, E.g, kidney cells, If the Cells of the liver and the pancreas are separated by the enzyme trypsin and incubated together, they aggregate to form small pieces of liver tissue and kidney tissue.
Thus kidney cells stick to kidney cells and liver cells to liver cells, Cancerous cells do not show this property. H malignant skin cancer cells are mixed with normal kidney cells the aggregates formed contain both kidney and skin cells mixed together. This probably explains why malignant cells can invade several normal organs
4 Invasiveness. One of the most important characteristics of transformed cells is their invasiveness, i.e. the ability to invade other tissues. In contrast to normal cells, transformed cells can penetrate the chorioaallantoic membrane of the hen's egg. This invasiveness could be the result of changes in the plasma membrane and or proteases released by the cells
6. Loss of anchorage dependence. Most normal cells must be attached to a rigid substratum (i.e, they must be anchored) in order to grow. Transformed cells can grow even when they are not attached to the substratum, as for example when they are suspended in a semisolid medium containing agar or methyl cellulose. This loss of anchorage is the most striking characteristic of transformed cells which form malignant tumours. It is used to select transformed cells from a normal cell population.
6. Lower serum requirements. Growth of normal cel1s in a tissue culture medium requires a high concentration of serum. Some serum growth factors (somatomedins) resemble insulin in interacting with external receptors of the cell membrane to regulate biochemical activities within the cell. Transformed cells can grow in a culture medium containing much less serum than required by normal cells.
For example normal 3T3 cells (established fibroblasts of the mouse, line commonly used in tissue culture) grow optimally in 10% foetal calf serum, while cel1s transformed by SV40 (simian virus number 40) can grow equally well in 1% or 10 % serum. It has been suggested that the lower serum requirement of transformed cells is because of their lesser requirement of outside substances to lower their intracellular cAMP (cyclic- AMP) level to trigger mitosis
7. Selective agglutination by lectins - Lectins are proteins widely distributed in plants, particularly legumes, but are also found in some animals. They have the ability to bind to receptors, which are branched chain sugar molecules (oligosaccharides), on the surface of the cell membrane. As a result of this binding lectins cause aggluti*nation or clumping of cells. They are therefore, agglutinins.
In normal cells the receptors or agglutinin binding sites for lectins lie in a diffuse manner on the cell surface and are immobile. Lectins make Jew intercellular bridges, and therefore agglutination is not possible. In transformed cel1s the, receptors are more mobile within the membrane. Local regions of high binding site concentration are formed. Lectins are thus able to form enough intercellular bridges to result in agglutination.
8. Molecular changes in cell membrane components. There are several differences between the surface cell membranes of normal and transformed cells. The cell membrane consists of four main types of phospholipids, which form the lipid bilayer, with glycolipids and glyco proteins inserted into this bilayer. Cancerous cells apparently do not differ from normal cells in their relative amounts of phospholipids
However, gangliosides (glycolipids which contain sialic acids) become reduced in certain mouse cancer cells. Enzymes involved in their biosynthesis are also reduced. Normal cells possess four types of gangliosides, GMla, GM1, GM2 and GM3. Tumours cell predominantly contain the simplest type, GM3. It is however, not certain whether these changes are primarily responsible for the cancerous condition, since some transformed mouse cell lines contain normal amount of gangliosides.
Certain changes have been found in the glycoproteins of cancerous cells. The surface glycoprotein of MW 46,000 disappears early in transformation to the cancerous condition. There is also a slow disappearance of a major protein, called LETS (large, external, transforming sensitive) protein (MW -240,0.00).
Probably the most important protein to, disappear after transformation is the One having MW 200,000. As mentioned previously, the mobility of the surface proteins increases in transformed cells, thus permitting easier agglutination of tumour cells by lectins.
9 Disorganisation of the Cytoskeleton - Normal cells have a cytoskeleton (very much like. muscle fibres) which consists of microtubules and microfilaments. These fibres have a regular arrangement and bring about coordinated cell movement. In transformed cancer cells the fibres are much fewer in number and usually much thinner. It has been suggested that in transformed cells the cytoskeleton undergoes depolymerization. The microtubules disaggregate.
The microfilaments (actin) fibres undergo depolymerization and disappear, but diffuse actin remains. The myosin-like filaments also disappear. Thus in transformed cells the cytoskeleton proteins become less organised than in normal cells. It has been suggested that it is this disorganisation of the cytoskeleton, and not the changed fluidity of lipids, that results in increased mobility of cell membrane proteins.
The disorganisation of the cytoskeleton also affects the cell surface in another way. When cancer cells touch there is a constant and unco ordinated throwing out and retraction of blebs, microvilli and ruffles from the cell surface. Tumour cells have a more ruffled surface than normal cells, with many more surface processes.
10.Increase in negative surface charge of cell membrane: Comparisons of surface membrane charge by microlectrophoresis have been made between normal and malignant cells. In malignant cells anodic mobility is usually higher, indicating increase in negative surface charge
11. Increased sugar transport: Tumour cells consume much more glucose than normal cells because they have to grow and multiply. There is a great increase in the rate of sugar transport across the surface cell membrane after transformation. This increases sugar intake by malignant cells
12. Appearance of virus specific transplantation rejection antigens: Plasma membranes of most transformed cells contain antigens which are not present in normal cells. Thus in cell transformed by adenoviruses and papovaviruses the T antigen is always present. Similarly all cells transformed by the Epstein-Barr virus (EB. virus) contain an antigen called the EB nuclear antigen (EBNA). Tumour antigens can bring about an immunity response against themselves in. genetically similar hosts, This is in contrast to normal histocompatibility antigens.
The immunity response brought about by the antigens results in recognition and destruction of newly formed cancer cells and their descendants. Such a defence mechanism is called immunological surveillance, and can lead to the elimination of cancer cells under favorable conditions. It has been suggested that only in the rare cases when this defense mechanism fails that tumours are formed
13. Defective electrical communication: Electrical connections normally occur between individual cells. In some cancer cells, however, It has been reported that such connections are defective
14. Increased secretion of proteolytic enzymes. Large amounts of proteolytic enzymes are secreted by all types of cancer cells, except those of blood-forming tissues. The cancer cell secretes a protease called the cell factor (MWI , 40,00O), to form a plasmin, a proteolytic enzyme (MW 76,000). It has been suggested that plasmin removes many proteins projecting from the cell surface by enzymatic digestion and signals the cell into division
If normal cells are treated with proteases they show many of the characteristics of transformed cells, It has been speculated that viral proteins cause the release of extracellular protease. There is no direct evidence for this, however
15. Aldolases: In most mammalian tissues the enzyme aldolase exists in the form of three isozymes A, Band C. Isozymes A and C predominate in embryonic tissues, while in adult differentiated tissues the B isozyme is predominant. In some tumours, especially in poorly differentiated and rapidly growing cancers like certain hepatomas, isozyme B is replaced by isozyme A, the embryonic form
16. Increased rate of glycolysis: In the 1920s War burg pointed out the oxidative (aerobic) respiration is depressed in tumour cells, and that glycolysis (anaerobic respiration) increases. This has been demonstrated by an increase in lactic acid production in cells of solid tumours. There is a corresponding increase in the uptake of glucose, It has been suggested that increase in glycolysis is the result of injury to the normal oxidation mechanism of the cell.
Studies On respiration in tumour cells have, however, yielded conflicting results. Both oxidation and ATP synthesis have remained unchanged in isolated tumour mitochondria. On the other hand, increase in these activities has also been observed.
Moreover, it is not clear whether the changes in respiratory metabolism are the causes of cancer or the results of cancer. It is possible that the high energy requirements of actively dividing cancer cells may result in the cell adapting anaerobic glycolysis as a supplement to normal aerobic respiration