Targeting breast cancer stem-like cells using chloroquine encapsulated by a triphenylphosphonium-functionalized hyperbranched polymer

Cancer stem cells (CSCs) have garnered increasing attention over the past decade, as they are believed to play a crucial role in tumor progression and drug resistance. Accumulating evidence provides insight into the function of autophagy in maintenance and survival of CSCs. Here, we studied the impact of a mitochondriotropic triphenylphosphonium-functionalized dendrimeric nanocarrier on cultured breast cancer cell lines, grown either as adherent cells or as mammospheres that mimic a stem-like phenotype. The nanocarrier manifested a substantial cytotoxicity both alone as well as after encapsulation of chloroquine, a well-known autophagy inhibitor. The cytotoxic effects of the nanocarrier could be ascribed to interference with mitochondrial function. Importantly, mammospheres were selectively sensitive to encapsulated chloroquine and this depends on the expression of the gene encoding ATM kinase. Ataxia-telangiectasia mutated (ATM) kinase is an enzyme that functions as an essential signaling mediator that enables growth of cancer stem cells through the regulation of autophagy. We noted that this ATM-dependent sensitivity of mammospheres to encapsulated chloroquine was independent of the status of the tumor suppressor gene p53. Our study suggests that breast cancer stem cells, as they are modeled by mammospheres, are sensitive to encapsulated chloroquine, depending on the expression of the ATM kinase, which is thereby characterized as a potential biomarker for sensitivity to this type of treatment.