Combination of carbon-ion beam and dual tyrosine kinase inhibitor, lapatinib, effectively destroys HER2 positive breast cancer stem-like cells

To investigate whether carbon-ion beam alone, or in combination with lapatinib, has a beneficial effect in targeting HER2-positive breast cancer stem-like cells (CSCs) compared to that of X-rays, human breast CSCs derived from BT474 and SKBR3 cell lines were treated with a carbon-ion beam or X-rays irradiation alone or in combination with lapatinib, and then cell viability, spheroid formation assays, apoptotic analyses, gene expression analysis of related genes, and immunofluorescent gamma-H2AX foci assays were performed. Spheroid formation assays confirmed that ESA+/CD24- cells have CSC properties compared to ESA-/CD24+ cells. CSCs were more highly enriched after X-ray irradiation combined with lapatinib, whereas carbon-ion beam combined with lapatinib significantly decreased the proportion of CSCs. Carbon-ion beam combined with lapatinib significantly suppressed spheroid formation compared to X-rays combined with lapatinib or carbon ion beam alone. Cell cycle analysis showed that carbon ion beam combined with lapatinib predominantly enhanced sub-G1 and G2/M arrested population compared to that of carbon-ion beam, X-ray treatments alone. Carbon-ion beam combined with lapatinib significantly enhanced apoptosis and carbon-ion beam alone dose-dependently increased autophagy-related expression of Beclin1 and in combination with lapatinib greatly enhanced ATG7 expression at protein levels. In addition, a large sized gamma H2AX foci in CSCs were induced by carbon ion beam combined with lapatinib treatment in CSCs compared to cells receiving X-rays or carbon-ion beam alone. Altogether, combination of carbon-ion beam irradiation and lapatinib has a high potential to kill HER2-positive breast CSCs, causing severe irreparable DNA damage, enhanced autophagy, and apoptosis.