Have you ever seen a tanning bed? Tanning beds are used by some individuals to get the perfect tan when the idea of staying in sun for too long is not appealing or if the weather is not cooperation.
Although tanning beds seem like a great idea, the high exposure to ultraviolet (UV) radiation can cause genetic gene mutations, forming cancer cells that might lead to melanoma, a type of malignant skin cancer that arises from melanocytes. So, if you are interested in learning more about cancer cells, keep reading!
Cancer definition
First, let's review the basics of eukaryotic cell structure (Figure 1). Eukaryotic cells have a plasma membrane, cytoplasm, peroxisome, and membrane-bound organelles such as the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes/endosomes, and mitochondria.
So, what is cancer? Cancer is a disease that arises from uncontrollable cell division, creating tumors. There are two types of tumors: malignant and benign.
Malignant tumors are tumors that invade tissues and spread throughout the body. In other words, they can reproduce and metastasize.
Metastasis is defined as the process of tumor cells spreading to other places and forming secondary tumors.
Benign tumors, on the other hand, are tumors that reproduce without restraint but are noninvasive (stay within their original location). Figure 2 shows the difference between how a normal cell and a cancer cell develop.
Characteristics of cancer cells
Now, let's look at the definition of a cancer cell.
Cancer cells are cells that are capable of evading normal controls on cell growth due to mutations on genes that are involved in cell cycle regulation.
These mutations can be found within two types of genes: proto-oncogenes and tumor-suppressor genes. Proto-oncogenes are genes that provide signals for appropriate cell division to occur (stimulates normal cell growth), whereas tumor-suppressor genes give signals to inhibit cell growth and division. When a proto-oncogene gets mutated, it turns into an oncogene.
An oncogene is a mutated gene that promotes uncontrolled cell growth (cancer).
When it comes to tumor-suppressor genes, a decrease in their function can lead to cancer. Because of these mutations, cancer cells have many characteristics have made them different from normal cells. These are:
Cancer cells are self-sufficient, meaning that they can grow without the need for external growth signals. Instead, they promote their own self-replication. The second characteristic is the ability to evade/ignore growth suppressors in the cell cycle (we will learn more about this in a bit!).
Cancer cells can evade apoptosis and survive despite having DNA damage. They also have genetic instability due to the accumulation of mutations at a fast rate and unlimited replication. By undergoing angiogenesis, cancer cells are able to form new blood vessels within the tumor.
Cancer cell cycle
To better understand how cancer cells are capable of passing through the cell cycle without checking for DNA damage, let's take a look at Figure 3.
Here, we have the phases of the cell cycle. In the G0 phase, a cell is metabolically active but not proliferating. At the G1 phase, the cell prepares itself for division. The S phase is the stage where DNA synthesis occurs, while the G2 phase is where the cell prepares for mitosis. Mitosis happens in the M phase.
The cell cycle also has checkpoints, which are control mechanisms that ensure that cell division in eukaryotic cells happens properly. So, if they catch something abnormal such as damage to the DNA genome, they stop the cell from going to the next phase. If the damage is too hard to repair, a normal cell will undergo apoptosis, which is programmed cell death.
However, if a mutation in a proto-oncogene or in a tumor-suppressor gene occurs, then the cell loses control of the cell cycle, causing unregulated cell division. So, as cancer cells move through the cell cycle, they do it at an accelerated rate especially. The cancer cells do not stop in the G1 checkpoint, so it moves to the S phase without stopping for DNA repair.
Basically, for a cell to turn into a cancer cell, all it needs to do is override the cell cycle and start dividing uncontrollably!
Cancer cells function
The main function of cancer cells in that of growing in an unregulated way to generate tumors. And, as we learned before, cancers are generated from an accumulation of mutations.
Carcinogenesis occurs in three key steps:
Initiation
Promotion
Progression
Step 1: Initiation - Initiation happens when carcinogens, or initiating agents cause permanent non-lethal damage to DNA. Examples of carcinogens include chemical initiators, microbes, and even UV rays.
- Chemical initiators are the most common form of carcinogen.
Step 2: Promotion - In this stage, we have clonal expansion. Clonal expansion happens when promoters induce the proliferation of the DNA-damaged cell. This proliferation causes the formation of tumors that can either be benign or pre-neoplastic (pre-malignant).
- Promoters are agents that promote cell division. Promoters can be hormones, growth factors, and also chronic inflammation products.
Then, continued proliferation exposes the pre-neoplastic cells to additional mutations.
Step 3: Progression - During progression, we have genetic evolution and selection for aggressive cancer cell phenotypes that lead to the production of a malignant tumor!
Types of cancer cells
There are three types of cancer that you should be familiar with: carcinomas, leukemias/lymphomas and sarcomas. These cancer cells are considered malignant.
Carcinomas are cancer cells derived from epithelial cells. Cancers that arise from carcinomas are called adenocarcinomas. One of the most common types of skin cancer is called basal-cell carcinoma. This cancer is mostly caused by UV radiation and goblin syndrome.
Leukemias and lymphomas are cancer cells that arise in blood cells. Leukemia tends to affect the bone marrow. Lymphomas, on the other hand, affect the lymph node cells.
Did you know that cancer might also originate from infections by a virus that causes acquired immunodeficiency syndrome (AIDS)? The HIV virus is a type of human papillomavirus (HPV), and some strains of HPV that can lead to cervical cancer are sexually transmitted!
Sarcomas are cancer cells derived from connective tissues, such as muscles, tendons, and blood vessels. They are very rare compared to carcinomas.
Signs that cancer cells are dying
Sometimes, tumors might be removed by surgery. However, there are other types of treatment that are used to kill cancer cells, such as chemotherapy, radiation therapy, and immunotherapy. However, keep in mind that, on occasion, these treatments might induce the growth of new tumors.
In radiation therapy, a high dose of radiation is used to either destroy cancer cells (damage their DNA) or reduce tumor size. In chemotherapy, cancer cells are destroyed by using pharmaceutical drugs that damage DNA.
Immunotherapy helps the immune system in the combat against cancer. Passive immunotherapy uses antibodies against tumors to kill them. Active immunotherapy stimulates the patient's immune system to kill the tumor.
When cancer cells are killed, the signs can be different, and sometimes it might take a long time for the cancer cells to start dying. So, to know if the treatment is working and that cancer cells are indeed dying, doctors can use:
Blood tests to check for the levels of substances that a tumor secretes in the blood.
X-rays, CT scans or MRI to see if the the cancer has spread or shrunk in size.
Mammograms for breast cancer.
Cancer Cells - Key takeaways
- Cancer cells are cells that are capable of evading normal controls on cell growth due to mutations on genes that are involved in cell cycle regulation.
- Carcinogenesis occurs in three key steps: initiation, promotion, and progression.
- Cancer cells are divided into three types: carcinomas, leukemias/lymphomas and sarcomas. These cancer cells considered malignant.
References
- Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., Orr, R. B., & Campbell, N. A. (2020). Campbell biology (12th ed.). Pearson.
- Pezzella, F. (2019). Oxford textbook of cancer biology. Oxford University Press.
- Hesketh, R. (2022). Understanding cancer. Cambridge University Press.
- Mendelsohn, J., Howley, P. M., Israel, M. A., Gray, J. W., Thompson, C. B., & Al, E. (2015). The molecular basis of cancer. Saunders/Elsevier, Cop.
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