Differentiation therapy: targeting breast cancer stem cells to reduce resistance to radiotherapy and chemotherapy

Studies have shown that cancer stem-like cells (CSCs) from solid cancers are resistant to both radiotherapy and chemotherapy. We have shown that primary breast cancers (n = 8) and a breast cancer cell line (MCF7) enriched for breast cancer stem cells (BCSC) using mammosphere (MS) clonogenic culture can preferentially survive radiotherapy and chemotherapy treatment in vitro, showing ≥ 50% increase in MS survival compared with non-BCSC enriched cells. The BCSC enriched population, defined by the cell surface markers ESA⁺/CD44⁺/CD24^-/low, had reduced levels of DNA damage (measured by γH2AX) after 4 Gy irradiation or doxorubicin (1 μM) treatment. This suggests that the BCSC enriched population avoids or repairs the DNA damage significantly more than the whole population.

Differentiating agents have been used to re-sensitise breast cancers to endocrine treatment but effects on BCSC are unknown. All-trans-retinoic acid (ATRA), tricostatin A and vorinostat caused a dose-dependent decrease in the BCSC population using MS culture and FACS analysis after 72 hours of treatment in a monolayer. Vorinostat (100 nM) showed the greatest effect, with 80% reduction in the ESA⁺/CD44⁺/CD24^-/low population and a 50% reduction in MS formation. Our data suggest that in vitro treatment with differentiating agents reduces the number of the BCSC within the MCF7 cell line.

Combination of ATRA (2 μM) or vorinostat with 6 Gy irradiation caused a significant reduction in MS survival showing a 30% and 70% decrease compared with an irradiated control. Similarly, in combination with paclitaxel (0.5 μM) ATRA and vorinostat caused a significant reduction in MS survival, showing 70% and 60% decrease compared with paclitaxel alone. In primary breast cancers (n = 3), combination of ATRA and 6 Gy irradiation significantly decreased MS formation by ≥ 25% respectively compared with irradiation alone.

These observations suggest that targeting BCSC with agents that eliminate or differentiate BCSC is a promising strategy to overcome resistance to radiotherapy and chemotherapy in the clinic.