Enhanced killing of chemo-resistant breast cancer cells via controlled aggravation of ER stress

Moderate activity of the endoplasmic reticulum (ER) stress response system exerts anti-apoptotic function and supports tumor cell survival and chemoresistance, whereas its more severe aggravation may exceed the protective capacity of this system and turn on its pro-apoptotic module. In this study, we investigated whether the combination of two pharmacologic agents with known ability to trigger ER stress via different mechanisms would synergize and lead to enhanced tumor cell death. We combined the HIV protease inhibitor nelfinavir (Viracept®) and the cyclooxygenase 2 (COX-2) inhibitor celecoxib (Celebrex®) and investigated their combined effect on ER stress and on the viability of breast cancer cells. We found that this drug combination aggravated ER stress and caused pronounced toxicity in human breast cancer cell lines, inclusive of variants that were highly resistant to other therapeutic treatments, such as doxorubicin, paclitaxel, or trastuzumab. The anti-tumor effects of celecoxib were mimicked at increased potency by its non-coxib analog, 2,5-dimethyl-celecoxib (DMC), but were substantially weaker in the case of unmethylated-celecoxib (UMC), a derivative with superior COX-2 inhibitory efficacy. We conclude that the anti-tumor effects of nelfinavir can be enhanced by celecoxib analogs in a COX-2 independent fashion via the aggravation of ER stress, and such drug combinations should be considered as a beneficial adjunct to the treatment of drug-resistant breast cancers.