Download Cell Biology and Cancer

Cell Biology and Cancer
What Is Cancer?
Cancer results from a series of molecular events that change the normal structure and function of cells. In
cancer cells, the normal control systems that prevent cell overgrowth and the invasion of other tissues are
turned off. These altered cells divide and grow in the presence of signals that normally slow or stop cell growth;
therefore, they no longer require special signals to begin cell growth and division. As these cells grow they
develop new characteristics, including changes in cell structure, decreased cell adhesion, and production of
new enzymes. These genetic changes allow the cell and its offspring to divide and grow, even in the presence
of normal cells that typically slow or stop the growth of nearby cells. Such changes allow the cancer cells to
spread and invade other tissues. This enhanced growth describes most cancer cells, which have gained
functions that are normally shut down in the normal, healthy cells. As long as these cells remain in their original
location, they are considered benign; if they invade other tissues or organs in the body, they are considered
malignant. Cancer cells in malignant tumors can often metastasize, which means sending cancer cells to
distant sites in the body where new tumors may form.
Cell Cycle
Normal cells grow and divide in an orderly fashion, in accordance with the cell cycle. Mutations in the genes
that control the cell cycle can often lead to cancer. The major events in the cell cycle are described in Fig. 1.
Figure 1. The cell cycle is an ordered
process of events that occurs in four
stages. During the two gap phases,
G1 and G2, the cell is actively
metabolizing but not dividing. In S
(synthesis) phase, the chromosomes
duplicate as a result of DNA
replication. During the M (mitosis)
phase, the chromosomes separate in
the nucleus and the division of the
cytoplasm (cytokinesis) occurs. There
are checkpoints in the cycle at the
end of G1 and G2 that can prevent
the cell form entering the S or M
phases of the cycle. Cells that are not
in the process of dividing are in the
G0 stage, which includes most adult
cells.
Cancer cells do not stop dividing, so what stops a normal cell from dividing? In terms of cell division, normal
cells differ from cancer cells in at least four ways.
 Normal cells require chemicals, called growth factors, made elsewhere in the body to divide. When
the making of these growth factors is slowed or stopped by normal cell regulation, the cells stop
dividing. Cancer cells have lost the need for being regulated by growth factors, so they divide
whether or not these factors are present. Consequently, they do not behave as part of the tissue —
they have become independent cells.
 Normal cells show contact inhibition; that is, they respond to contact with other cells by stopping cell
division. Therefore, cells can divide to fill in a gap, but they stop dividing as soon as there are
enough cells to fill the gap. This characteristic is lost in cancer cells, which continue to grow after
they touch other cells, causing a large mass of cells to form.
 Normal cells age and die, and are replaced in a controlled and orderly manner by new cells.
Apoptosis is the normal, programmed death of cells. Normal cells can divide only about fifty times
before they die. This is related to their ability to replicate (copy) DNA only a limited number of times.
Each time the chromosome replicates, the ends (telomeres) shorten. In growing cells, the enzyme
telomerase replaces these lost ends. Adult cells lack telomerase, limiting the number of times the
cell can divide. However, telomerase is activated in cancer cells, allowing an unlimited number of
cell divisions.
 Normal cells stop dividing and die when there is DNA damage or when cell division is abnormal.
Cancer cells continue to divide, even when there is a large amount of damage to DNA or when the
cells are abnormal. These offspring cancer cells contain the abnormal DNA; so, as the cancer cells
continue to divide they accumulate even more damaged DNA.
Tumor Biology
Cancer cells behave as independent cells, growing without control to form tumors. Tumors grow in a series of
steps (see Fig. 2). The first step is hyperplasia, meaning that there are too many cells resulting from
uncontrolled cell division. These cells appear normal, but changes have occurred that result in some loss of
control of growth. The second step is dysplasia, resulting from further growth, accompanied by abnormal
changes to the cells. The third step requires additional changes, which result in cells that are even more
abnormal and can now spread over a wider area of tissue. These cells begin to lose their original function;
such cells are called anaplastic. At this stage, because the tumor is still contained within its original location
(called in situ) and is not invasive, it is not considered malignant - it is potentially malignant. The last step
occurs when the cells in the tumor metastasize, which means that they can invade surrounding tissue,
including the bloodstream, and spread to other locations. This is the most serious type of tumor, but not all
tumors progress to this point. Non-invasive tumors are said to be benign.
Figure 2. The formation of a tumor
begins with the mutation of the genes in
a cell. These mutations accumulate and
the cells begin to divide uncontrollably.
As mitosis continues, the size of the
tumor continues to increase. Eventually,
if left untreated, the tumor will become
invasive and spread to other areas of the
body.
The type of tumor that forms depends on the type of cell that was initially changed. There are five types of
tumors.
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Carcinomas result from altered epithelial cells, which cover the surface of our skin and internal organs.
Most cancers are carcinomas.
Sarcomas result from changes in muscle, bone, fat, or connective tissue.
Leukemia results from malignant white blood cells.
Lymphoma is a cancer of the lymphatic system cells that derive from bone marrow.
Myelomas are cancers of specialized white blood cells that make antibodies.
Although tumor cells are no longer dependent on the control mechanisms that govern normal cells, they still
require nutrients and oxygen in order to grow just like normal cells. All living tissues are supplied with capillary
vessels, which bring nutrients and oxygen to every cell. As tumors enlarge, the cells in the center no longer
receive nutrients from the normal blood vessels. To provide a blood supply for all the cells in the tumor, it must
form new blood vessels to supply the cells in the center with nutrients and oxygen. In a process called
angiogenesis, tumor cells make growth factors, which cause the formation of new capillary blood vessels. The
cells of the blood vessels that divide to make new capillary vessels are inactive in normal tissue; however,
tumors make angiogenic factors, which activate these blood vessel cells to divide. Without the additional blood
supplied by angiogenesis, tumors can grow no larger than about half a millimeter.
Without a blood supply, tumor cells also cannot spread, or metastasize, to new tissues. Tumor cells can cross
through the walls of the capillary blood vessel at a rate of about one million cells per day. However, not all cells
in a tumor are angiogenic. Both angiogenic and non-angiogenic cells in a tumor cross into blood vessels and
spread; however, non-angiogenic cells give rise to dormant tumors when they grow in other locations. In
contrast, the angiogenic cells quickly establish themselves in new locations by growing and producing new
blood vessels, resulting in rapid growth of the tumor.
Normal, healthy cells can specialize (or differentiate), which means they take various shapes and forms to be
able to function as part of an organ. The cancerous cells do not become differentiated and they become
disorganized and have oddly shaped nuclei. Because the cancer cells do not differentiate, they do not perform
the beneficial functions of normal cells and simply take up the nutrients and resources needed by the normal
cells. This means that the normal cells lose the nutrients they need and eventually starve to death. Nearly all
types of differentiated cells may become cancer cells.
The grade of the cancer is thus established based on how well developed or mature the cancer cells look
under the microscope. If the cancer cells have a more normal look and resemble healthy, differentiated cells it
is more likely that they will behave as a normal cell. Therefore, the grade of cancer is low when the cancerous
cells look normal but the more abnormal they are, the higher the grade of cancer and the worse prognosis for
the patient. Cancer grades or stages range from 1 to 3, where 1 is the lowest grade meaning the tumor is
made of well-differentiated cells. Grade 2 is specific for moderately differentiated tumor cells and grade 3 for
poorly differentiated tumor cells. The grades associates to cancer can be useful indicators for treatment plans
and may provide information on whether treatment was effective or not. Grade 1 cancer is usually less
aggressive and unlikely to spread to other organs or tissues of the body. Grade 3 is the most aggressive type
of cancer. Poorly differentiated cells look primitive, disorganized and immature and they are more likely to
divide rapidly and spread to surrounding tissues and organs. These cells do not specialize. Unlike poorly
differentiated cells, the well-differentiated cells have a similar appearance to the normal cells. They are less
likely to spread and invade surrounding tissue and they are more responsible to cancer treatment. This is
mainly due to the fact that the cells are well differentiated in an early stage.
Traditional Treatments
Because cancer comprises many diseases, doctors use many different treatments. The course of treatment
depends on the type of cancer, its location, and its state of advancement. Surgery, often the first treatment, is
used to remove solid tumors. It may be the only treatment necessary for early stage cancers and benign
tumors.
Radiation kills cancer cells with high-energy rays targeted directly to the tumor. It acts primarily by damaging
DNA and preventing its replication; therefore, it preferentially kills cancer cells, which rapidly divide. It also kills
some normal cells, particularly those that are dividing. Surgery and radiation treatment are often used together.
Chemotherapy drugs are toxic compounds that target rapidly growing cells. Many of these drugs are designed
to interfere with the creation of the molecules needed for DNA replication; they interfere with the ability of the
cell to complete the S phase of the cell cycle. Other drugs cause extensive DNA damage, which stops
replication. A class of drugs called spindle inhibitors stops cell replication early in mitosis. During mitosis,
chromosome separation requires the function of an organelle called a centrosome. This class of drugs attacks
the spindle fibers that are created by the centrosome and cell division is slowed dramatically. Because most
adult cells don’t divide often, they are less sensitive to these drugs than are cancer cells. Chemotherapy drugs
also kill certain adult cells that divide more rapidly, such as those that line the gastrointestinal tract, bone
marrow cells, and hair follicles. This causes some of the side effects of chemotherapy, including
gastrointestinal distress, low white blood cell count, and hair loss.
Questions:
1. What is the difference between a benign tumor and a malignant tumor? ________________________
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2. What is the cell cycle? _______________________________________________________________
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3. What are growth factors and what do they have to do with cancer? ____________________________
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4. What is contact inhibition and what does it have to do with cancer? ____________________________
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5. What is apoptosis and what does it have to do with cancer? __________________________________
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6. What determines the name of the type of tumor someone has? _______________________________
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7. If tumors are masses of extra cells, why do they cause people to become sick rather than making them
stronger or better? ___________________________________________________________________
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8. What is the difference between Grade (stage) 1, 2 and 3 cancers? ____________________________
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9. How does radiation therapy get rid of cancer cells? _________________________________________
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10. How does chemotherapy get rid of cancer cells? ___________________________________________
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11. Why do patients getting chemotherapy lose their hair? ______________________________________
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12. After reading this article and answering the questions, if you had to define the word “cancer” to
someone who did not know this information, how would you define it? __________________________
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