Cancer: a disease of stem cells?Cancer is the subject of intensive research worldwide, but many questions about the causes of this disease have not yet been answered. What causes cancer. How tumors grow? The cancer stem cell theory may provide some explanations about what causes certain cancers and why some patients relapse.
What do we know? ▼
Mutations in genes that control cell division can cause healthy cells to become cancer cells. But it is still unclear how cancer cells grow in a tumor. Only some of the cells in tumors divide. Continue to multiply. This observation has led to two theories about how tumors grow.
The "cancer stem cell model" ames that cancer stem cells generate all other cells in a tumor. Some tumor cells can divide for a short time, but only cancer stem cells can produce new cells indefinitely.
The "stochastic model" ames that many cells in a tumor multiply, differentiate and contribute equally to tumor growth.
With regard to cancer stem cells, much is still unknown, including whether they are present at all in all types of cancer. So far, neither of the two theories has been definitively proven. Different theories may apply to different types of cancer. Researchers are looking for more concrete evidence that cancer stem cells are present in tumors.
Researchers are trying to determine which cells become cancer cells. Can any cell become a cancer stem cell or do cancer stem cells only arise from gene mutations in natural stem cells?
Researchers want to know how different genes, mutations, signals and environments affect cancer stem cell behavior. Cancer stem cells are often difficult to study because cells have to be isolated in a lab. There, they may behave quite differently than they do in the body.
In the healthy body, we find many different types of cells that can divide to produce new cells. This is a carefully controlled process that allows the body to grow and replace lost or damaged cells throughout life. In cancer, however, cells divide in an uncontrolled manner, eventually forming an abnormal collection of cells called a tumor. Cancer cells grow out of control due to mutations (changes) in their genetic material. These mutations accumulate as cells divide and age, with certain combinations of genetic changes subsequently causing cancer.
Despite intensive research, it is still unclear how exactly tumors grow. Malignant growths are often made up of a mixture of different cell types. Some of these cells divide repeatedly, while others develop into mature, specialized cell types that can no longer proliferate. Currently, there are two primary theories to explain tumorigenesis: the cancer stem cell theory and the stochastic model of cancer growth.
carcinogenesis: Cancer stem cell theory proposes a clear hierarchy of cells within a tumor. The stochastic model states that tumor growth is a random process to which all cells can contribute.
Cancer stem cell theory Suggests that tumors grow just like normal ties in the body, with stem cells being the starting point of an organized system that produces new cells to grow a tie. According to this theory, a tumor includes :
– Cancer stem cells dividing and driving tumor growth. These cells are capable of extensive self-renewal (copying). Also produce more mature cells in intermediate stages which are called transit amplifying cells. – Transit-amplifying cells, which have a limited ability to divide, and then differentiate (mature) into specialized tumor cells. – Specialized tumor cells that stop dividing and contribute to tumor growth.
According to this idea, all cells in a tumor are part of a tightly organized system – with cancer stem cells as the initial starting point that can produce all other cell types in a tumor.
The stochastic model in turn, offers a different explanation for tumor growth. This theory argues that basically all cancer cells have the same potential to grow and divide, with each cell choosing randomly between self-renewal or differentiation. The cells in a tumor are thus not part of an organized system but each individual cell has the same intrinsic potential to contribute to tumor growth.
different types of cancers may have different characteristics – therefore it is possible that both theories are true. It is conceivable that they could be applied to different types of cancer or even different stages of development.
There is no definitive evidence in favor of either theory of cancer growth. A growing body of new research, however, suggests that the cancer stem cell theory appears to hold true in many cases. The first evidence in favor of cancer stem cells came from studies on human leukemia. Scientists found that only some of the leukemia cells were capable of actually causing leukemia when transplanted into a healthy body – a key property of stem cells.
Since this discovery, many scientists have detected cells with the characteristics of cancer stem cells in other human and mouse cancers as well. These include breast cancer, but also brain, skin, prostate and colon tumors. Cancer stem cells appear to be less common in some cancers, such as colorectal cancer. Other types, such as melanoma, have an enormously increased number of tumor cells with cancer stem cell characteristics.
Cancer stem cells are controversial
The cancer stem cell theory remains controversial, because to prove the existence of cancer stem cells, scientists currently rely solely on experiments in which the tumor is first dissected into its component parts and then individual cells are isolated and transplanted. This method does not accurately reflect natural tumor growth. In order to prove that the transplanted cells are indeed cancer stem cells, scientists in the future will need to focus on the individual cells to find direct evidence that these cells actually contribute to tumor growth within the natural environment.
The cancer stem cell concept has important implications for cancer therapy. If cancer stem cells really are responsible for tumor growth, their elimination could potentially cure the patient.
Cancer stem cells and therapy: if cancer treatment destroys most of the tumor but leaves the cancer stem cells untouched, then these stem cells can renew themselves and the tumor comes back.
It has also been suggested that cancer stem cells are more resistant to chemotherapy or radiotherapy than other cells in a tumor. This could be one of the reasons why some patients relapse after successful cancer therapy. A better understanding of how cancer stem cells fight back against therapy could potentially lead to the development of new, more effective cures.
Many more questions about cancer need to be answered. Further studies are urgently needed to determine more precisely how different types of cancer grow and why they are resistant to some medical therapies. Are stem cells a component of all tumors? Which cell is the original source of the cancer? Scientists continue to study how exactly cancer stem cells are controlled. What genes, proteins or other molecules are involved in their development and behavior? Does the immediate environment of the tumor also determine how cancer stem cells behave? The answers to these questions will be critical for future therapeutic strategies.