ARTICLE ABSTRACTHsp27 is a stress-activated multifunctional chaperone that inhibits treatment-induced apoptosis and causes treatment resistance in prostate and other cancers. We previously showed that targeted suppression of Hsp27 sensitizes cancer cells to hormone and chemotherapy. However, mechanisms by which Hsp27 confers cell treatment resistance are incompletely defined. Here, we report that Hsp27 protects human prostate cancer cells against proteotoxic stress induced by proteasome inhibition, and that Hsp27 silencing using siRNA or antisense (OGX-427) induced both apoptosis and autophagy through mechanisms involving reduced proteasome activity and induction of endoplasmic reticulum (ER) stress. We found that autophagy activation protected against ER stress-induced cell death, whereas inhibition of autophagy activation following Hsp27 silencing using either pharmacologic inhibitors or atg3 silencing enhanced cell death. Importantly, cotargeting Hsp27 and autophagy by combining OGX-427 with the autophagy inhibitor, chloroquine, significantly delayed PC-3 prostate tumor growth in vivo. These findings identify autophagy as a cytoprotective, stress-induced adaptive pathway, activated following disruption of protein homeostasis and ER stress induced by Hsp27 silencing. Combinatorial cotargeting cytoprotective Hsp27 and autophagy illustrates potential benefits of blocking activation of adaptive pathways to improve treatment outcomes in cancer. Mol Cancer Ther; 11(8); 1661–71. ©2012 AACR.