Autophagy positively regulates the CD44⁺CD24^-/low breast cancer stem-like phenotype

Abstract

The molecular mechanisms used by breast cancer stem cells (BCSCs) to survive and/or maintain their undifferentiated CD44⁺CD24^-/low mesenchymal-like antigenic state remains largely unexplored.  Autophagy, a key homeostatic process of cytoplasmic degradation and recycling evolved to respond to stress conditions, might be causally fundamental in the biology of BCSCs.  Stable & specific knockdown of autophagy-regulatory genes by lentiviral-delivered small hairpin (sh) RNA drastically decreased the number of JIMT-1 epithelial BC cells bearing CD44⁺CD24^-/low cell-surface antigens from ~75% in parental and control (-) shRNA-transduced cells to 26% and 7% in ATG8/LC3 shRNA- and ATG12 shRNA-transduced cells, respectively.  Autophagy inhibition notably enhanced transcriptional activation of CD24 gene, potentiating the epithelial-like phenotype of CD44⁺CD24⁺ cells versus the mesenchymal CD44⁺CD24^-/low progeny. EMT-focused Real Time RT-PCR profiling revealed that genetic ablation of autophagy transcriptionally repressed the gene coding for the mesenchymal filament vimentin (VIM).  shRNA-driven silencing of the ATG12 gene and disabling the final step in the autophagy pathway by the antimalarial drug chloroquine both prevented TGFb1-induced accumulation of vimentin in JIMT-1 cells.  Knockdown of autophagy-specific genes was sufficient also to increase by up to 11-times the number of CD24⁺ cells in MDA-MB-231 cells, a BC model of mesenchymal origin that is virtually composed of CD44⁺CD24^-/low cells. Chloroquine treatment augmented the number of CD24⁺ cells and concomitantly reduced constitutive overexpression of vimentin in MDA-MB-231 cells. This is the first report demonstrating that autophagy is mechanistically linked to the maintenance of tumor cells expressing high levels of CD44 and low levels of CD24, which are typical of BCSCs .