Paclitaxel-induced transcriptional regulation of Fas signaling pathway is antagonized by dexamethasone. 
Chemotherapy agents like paclitaxel are often a first line of therapy for triple-negative breast cancer patients and a last line of therapy for other aggressive breast cancers.  While such agents are effective pro-apoptotic treatments for breast cancer cells, they produce many unwanted side effects.  Synthetic glucocorticoids like dexamethasone are commonly prescribed during chemotherapy treatment of breast cancer patients for anti-emetic and anti-hypersensitivity purposes.  Dexamethasone, however, is known to enhance cell survival in solid tumors.  The prevalent use of dexamethasone in conjunction with paclitaxel in the treatment of breast cancer patients merits concern.  Previous studies demonstrate dexamethasone-mediated survival in a subset of breast cancer cells in the presence of paclitaxel.  This work expanded the types of cell lines studied and explored the molecular mechanism driving this phenotype.  Human breast cancer cell lines representative of several subtypes of breast cancer including triple-negative breast cancer, luminal, and HER2 were treated in the presence and absence of paclitaxel, dexamethasone, and under co-treatment conditions.  Cell survival was monitored under differing conditions.  We then manipulated aspects of the nuclear factor kappa B (NFkappaB) signaling pathway to elucidate the importance of this pathway in overcoming chemotherapy treatment.  All cell lines tested were sensitive to paclitaxel and showed dexamethasone-mediated rescue from paclitaxel-induced apoptosis, but the rescue of overall cell numbers was most pronounced in triple-negative breast cancer cell lines.  Dexamethasone and paclitaxel inversely regulated the activity of NFkappaB, which is essential to both paclitaxel-mediated apoptosis and dexamethasone-mediated rescue.  The transcriptional target of NFkappaB, Fas receptor, is inversely regulated by paclitaxel and dexamethasone and is a downstream target of paclitaxel-activated NFkappaB. Dexamethasone antagonizes paclitaxel-mediated apoptosis through inhibition of NFkappaB transcription of Fas receptor.  Pre-treatment of breast cancer patients with dexamethasone may greatly reduce patient response to paclitaxel.  Our study elucidates a novel mechanism of paclitaxel-induced apoptosis in breast cancer cell lines and explicates dexamethasone’s antagonism of paclitaxel.