Targeting the self-renewal capacity of cancer stem cells with the telomere eradicating agent KML001
Tumor tissue is of heterogeneous nature containing a cancer stem cell (CSC) population which is believed to be responsible
for failure of current anticancer therapies and which may get further enriched under selective pressure enabling the
recurrence of more aggressive forms of the tumor. Therefore, understanding the CSC’s in comparison to normal stem cells is
necessary for developing therapies which target the CSC population effectively. CSC’s share some key characteristics with
normal stem cells like self-renewal potential, proliferative quiescence and protection from cytotoxicinsult. The activation of
stemnessgenes like telomerase in more than 90 % of all cancers ensures endless self-renewal potential. Unlike normal stem
cells which have long telomeres, telomeres of cancer cells shorten rapidly during early tumorigenesisand are maintained at a
stable length by reactivation of telomerase. We hypothesize that cancer stem cells have a higher telomerase activity than the
bulk tumor cell mass for maintaining a critical minimal telomere length necessary for their survival and targeting telomeres
and telomerase with the telomere eradicating agent KML001 would specifically erode the CSC subpopulation and would not
affect normal stem cells which have long telomeres and slower proliferation rates. We analyzed various prostate cancer cell
lines reflecting different prostate cancer progression stages (hormone responsive, hormone unresponsive and drug resistant)
for their telomerase and telomere length characteristics as well as for the existence of a CSC subpopulation using side
population (SP) assay which is based on the ability of stem cells to express drug and xenobiotics efflux pumps responsible
for the resistance to standard chemotherapies. Furthermore, we analyzed the expression of stem cell markers and drug efflux
pumps. We have tested the effect of KML001 on cell growth in a standard MTT and tested whether the compound could
specifically eradicate the CSC population in the SP assay. Cell growth curves showed that all of the tested prostate cancer
cell lines respond with similar sensitivity to KML001 regardless of their drug resistance and cancer stem cell potential.
Telomere lengths were shorter in the more progressed prostate cancer cell line DU145 and the taxane resistant cell lines.
The taxaneresistant cell line had the highest percentage of SP (CSC population) of about 8 and 50% respectively,
accompanied with a high level of expression of the drug efflux pump P-glycoprotein. Higher level of telomerase activity was
found in the isolated SP (CSC) population. The compound reduced the telomerase expression level and the SP (CSC)
population significantly at the IC50 and IC100 concentrations determined before in a standard MTT. We found that the
cancer stem cell population is enriched in the drug resistant cell lines and that this population in particular showed the
highest telomerase activity necessary for the maintenance of short telomeres in this subpopulation. We suggest that we will
detect a decrease in telomere length with KML001 treatment along with the observed reduction in telomerase level which
we believe is responsible for the erosion of the cancer stem cell fraction as seen in the SP assay and that this compound
may be useful for other types of cancer.
2010 World Stem Cell Summit, Detroit, Michigan