Developing ovarian cancer stem cell models: laying the pipeline from discovery to clinical intervention
Ffrench B, Gasch C, OLeary JJ, Gallagher MF. Developing ovarian cancer stem cell models: laying the pipeline from discovery to clinical intervention. Mol Cancer. 2014 Dec 11;13:262. doi: 10.1186/1476-4598-13-262. PMID: 25495823; PMCID: PMC4295405.
Ovarian cancer shows signs of having a cancer stem cell (CSC) hierarchy where a stem cell can self-renew and differentiate into progenitor cells that go on to differentiate further into the cells that make up the tumor. The apex stem cell can stay in a quiescent state for long periods making them difficult to target with anti-mitotic agents. Despite the evidence, isolating, identifying, and growing CSCs is difficult. Ovarian cancer in particular has a serious chemoresistant recurrence problem that CSCs likely contribute significantly to.
One of the main ways to identify CSCs is by cell surface proteins (like CD proteins) and stemness markers (like ALDH). These markers can be used in flow cytometry to isolate the cells of interest and then assay them for their self-renewal and differentiation capabilities.
Isolation
Ovarian CSCs can be isolated by their expression of ALDH1 in combination with CD markers like CD133. ALDH+/CD133+ cells are the most stem like at the top of the hierarchy. ALDH+/CD133- cells are less stem-like than the double positive cells, but more than ALDH-/CD133- cells. ALDH is, however, not required for the stemness characteristics of CSCs and its loss does not impact tumorigenicity.
The use of CD proteins requires prior knowledge of the cell surface expression of the protein an dare not required for CSC properties. CD133 is one of the best methods for identifying ovarian CSCs. CD24 is another marker that can be used, but has inconsistent results in classifying cells as stem-like between patient samples and cell lines. While CD44 is one of the better known markers, it cannot delineate between the different stem and progenitor cells in the hierarchy. Both CD133 and ALDH expression are linked to poor prognosis making them diagnostic markers as well.
The ability to form spheroids is yet another method to isolate stem cells since there is evidence that the spheroid-forming ability characterizes CSCs. Isolated putative stem cells can be validated using assays like single-cell asymmetric division.
Targeting CSCs
CSCs are known to be chemoresistant. Therapies aimed at CSCs need to treat with chemotherapies in addition to anti-CSC measures. Unilateral therapy methods are predicted to fail by a Clonal Cancer Stemness model. Inhibiting Notch signalling, which is necessary to maintain stemness, decreases metastasis in mouse models when combined with cisplatin. CSCs can adapt their expression profiles to form recurrent chemoresistant tumors.
However, we do not know whether CSCs are inherently resistant to therapy or are able to adapt and develop resistance. One way to combat the quiescence-based resistance to chemotherapies is to force the CSCs to exit quiescence and proliferate. The creation of a stable model that can remain in a self-renewal state is necessary to identify therapeutic targets. They are currently not stable enough because the necessary culture conditions have not been identified. Spheroid conditions hold promise in maintaining the stem-like state and work on signalling will reveal the signalling factors necessary to maintain it.