title: Bortezomib blocks the catabolic process  of autophagy via a cathepsin-dependent  mechanism, affects endoplasmic reticulum stress  and induces caspase-dependent cell death in  antiestrogen-sensitive and resistant ER+ breast  cancer cells.
abstract: In recent studies, we and others showed that autophagy is critical to estrogen receptor positive (ER+) breast cancer cell survival and the development of antiestrogen resistance. Consequently, new approaches are warranted for targeting autophagy in breast cancer cells undergoing antiestrogen therapy. Because crosstalk has been demonstrated between the autophagy- and proteasome-mediated pathways of protein degradation, this study investigated how the proteasome inhibitor bortezomib affects autophagy and cell survival in antiestrogen-treated ER+ breast cancer cells. Bortezomib, at clinically achievable doses, induced a robust death response in ER+, antiestrogen-sensitive and antiestrogen-resistant breast cancer cells undergoing hormonal therapy. Cleavage of PARP and lamin A was detectable as a read-out of cell death, following bortezomib-induced mitochondrial dysfunction. Prior to induction of cell death, bortezomib-treated cells showed high levels of light chain 3 (LC3) and p62, two protein markers for autophagy. The accumulation of these proteins was due to bortezomib-mediated blockade of long-lived protein turnover during macroautophagy. This novel action of bortezomib was linked to its blockade of cathepsin-L activity, which is required for autolysosomal-mediated protein turnover in ER+ breast cancer cells. Further, bortezomib-treated breast cancer cells showed induction of the unfolded protein response, with upregulation of CH OP and GRP78. Bortezomib also induced high levels of the pro-apoptotic protein BNIP3. Knockdown of CH OP and/or BNIP3 expression via RNAi targeting significantly attenuated the death-promoting effects of bortezomib. Thus, bortezomib inhibits prosurvival autophagy, in addition to its known function in blocking the proteasome, and is cytotoxic to hormonally treated ER+ breast cancer cells. These findings indicate that combining a proteasome inhibitor like bortezomib with antiestrogen therapy may have therapeutic advantage in the management of early-stage breast cancer.
authors.affiliation: Department of Cellular Biology and Anatomy, Medical College of Georgia, Augusta, GA, USA.
authors.forename: Sudharsan
authors.initials: S
authors.lastname: Periyasamy-Thandavan
authors.forename: William H
authors.initials: WH
authors.lastname: Jackson
authors.forename: Julia S
authors.initials: JS
authors.lastname: Samaddar
authors.forename: Brian
authors.initials: B
authors.lastname: Erickson
authors.forename: John R
authors.initials: JR
authors.lastname: Barrett
authors.forename: Lauren
authors.initials: L
authors.lastname: Raney
authors.forename: Elangovan
authors.initials: E
authors.lastname: Gopal
authors.forename: Vadivel
authors.initials: V
authors.lastname: Ganapathy
authors.forename: William D
authors.initials: WD
authors.lastname: Hill
authors.forename: Kapil N
authors.initials: KN
authors.lastname: Bhalla
authors.forename: Patricia V
authors.initials: PV
authors.lastname: Schoenlein
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology; D001343:Autophagy / Q000187:drug effects* / Q000235:genetics / Q000502:physiology; D001897:Boronic Acids / Q000494:pharmacology*; D000069286:Bortezomib; D001943:Breast Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D020169:Caspases / Q000378:metabolism / Q000502:physiology*; D002403:Cathepsins / Q000502:physiology*; D016923:Cell Death / Q000187:drug effects / Q000235:genetics; D015536:Down-Regulation / Q000187:drug effects / Q000235:genetics; D004353:Drug Evaluation, Preclinical; D019008:Drug Resistance, Neoplasm / Q000187:drug effects*; D004721:Endoplasmic Reticulum / Q000187:drug effects* / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D020847:Estrogen Receptor Modulators / Q000627:therapeutic use; D005260:Female; D006801:Humans; D008660:Metabolism / Q000187:drug effects / Q000235:genetics; D011719:Pyrazines / Q000494:pharmacology*; D011960:Receptors, Estrogen / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects; D013312:Stress, Physiological / Q000187:drug effects; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D001897:Boronic Acids; D000091342:Endoplasmic Reticulum Chaperone BiP; D020847:Estrogen Receptor Modulators; C000717247:HSPA5 protein, human; D011719:Pyrazines; D011960:Receptors, Estrogen; D000069286:Bortezomib; D002403:Cathepsins; D020169:Caspases