Journal of Cancer Stem Cell Research
Ovarian cancer causes the most deaths among gynecological cancers. One of the major reasons for its lethality despite
improved surgical techniques and improved chemotherapeutic agents in recent years is the recurrence due to chemoresistance.
There have been several proposed mechanisms for chemoresistance of which the existence of cancer stem cells has recently
been brought to attention. Cancer stem cells represent a small proportion of cells within a tumor with the ability to proliferate in
order to increase the number of cancer stem cells, as well as the ability to differentiate into heterogeneous-nontumorigenic cancer
cell types. Also in ovarian cancer as in other types of cancer, different populations of cancer stem cells were isolated from tumors.
In literature the cancer stem cells are represented by different cell populations expressing different phenotypes and characteristics.
The most commonly used markers for their isolation are CD44, CD133, CD24, CD117 and ALDH, which are often used in
combinations. In vitro and in vivo studies confirmed that these isolated cells behave as cancer stem cells and use different
mechanisms to minimize the effect of chemotherapeutic agents. Also proteins, which are generally known as pluripotent stem cell
markers of normal stem cells, such as NANOG, OCT-4A, SOX-2 and c-MYC, have an important role in ovarian cancer
development and chemoresistance. Additionally, it was demonstrated that the expression of the mentioned markers in ovarian
tumors can be used as a prognostic tool, where usually the over-expression of a specific marker correlates with a poor prognosis.
However, the conclusions of the prognostic value of cancer stem cell markers are still under debate. The reason for vague
conclusions may lay in the fact that stem cells expressing the same markers can be also found in "healthy" ovaries, which could
represent a bias in cancer studies. Even more, since stem cells located in "healthy" ovaries, including small stem cells resembling
very small embryonic-like stem cells, represent the most of the cells that divide and proliferate, they seem to be the best candidate
to undergo malignant transformation and could serve as tumor initiating cells. Therefore, the knowledge on stem cells, expressing
the pluripotency-related markers in "healthy" ovaries is a prerequisite to better understand the ovarian cancer.