Rationale: Cancer stem cells (CSCs) are known to cause
tumor recurrence and drug resistance. The
heat shock protein (HSP) system plays a major role in preserving expression and function of numerous
oncoproteins, including those involved in the CSC activities. We explored novel anticancer drugs, especially those targeting HSP components required for the functional role of CSCs. Methods: Investigation of the role of the HSP system in CSCs and screening of a natural product chemical library were performed by utilizing
cancer cell lines, primary cultures of patient-derived xenografts (PDXs), and their putative CSC subpopulations (i.e., those grown under sphere-forming conditions, stably transfected with reporter vectors carrying NANOG or POUSF1 promoters, or carrying high ALDH activity) in vitro and PDX and KrasG12D/+-driven
tumor models in vivo. Regulation of the HSP system was investigated by immunoprecipitation, drug affinity responsive target stability assay, binding experiments using
ATP-agarose beads and biotinylated drug, and docking analysis. Results: The HSP system was activated in CSCs via transcriptional upregulation of the HSP system components, especially HSP70.
Evodiamine (Evo) was identified to induce apoptosis in both CSC and bulk non-CSC populations in human lung, colon, and
breast cancer cells and their sublines with chemoresistance. Evo administration decreased the multiplicity, volume, and load of lung
tumors in KrasG12D/+ transgenic mice and the growth of
cancer cell line- and PDX-derived
tumors without detectable toxicity. Mechanistically, Evo disrupted the HSP system by binding the N-terminal
ATP-binding pocket of HSP70 and causing its
ubiquitin-mediated degradation. Conclusions: Our findings illustrate HSP70 as a potential target for eliminating CSCs and Evo as an effective HSP70-targeting anticancer drug eradicating both CSCs and non-CSCs with a minimal toxicity.