HER2-mTOR signaling–driven breast cancer cells require ER-associated degradation to survive

Targeting non-oncogenic vulnerabilities may provide additional therapeutic approaches in tumors that are resistant to oncogene-targeted therapy. Using a computational pathway-based approach, we interrogated clinical breast cancer genomic data sets for candidate non-oncogenic vulnerabilities in breast cancers that have genomic amplification of ERBB2, which encodes human epidermal growth factor 2 (HER2). HER2-positive (HER2(+)) breast cancers showed increased expression of genes encoding proteins in the endoplasmic reticulum (ER)-associated degradation (ERAD) pathway. Genetic ablation or pharmacological inhibition of ERAD led to irrecoverable ER stress and selectively killed HER2(+) breast cancer cells. Cell death caused by ERAD inhibition partially depended on increased HER2-mTOR signaling, which imposed an increased proteotoxic burden on the ER. Cell death in response to ER stress required the IRE1α-JNK pathway, which was selectively suppressed in HER2(+) breast cancers by phosphatases that inactivate JNK. Accordingly, the cytotoxicity of inhibiting ERAD as well as JNK phosphatases was synergistic in HER2(+) but not in HER2-negative breast cancer cells. Therefore, our study suggests that reactivation of oncogene-induced stress by targeting stress-adaptive pathways may be a beneficial approach for therapy-resistant breast cancers.