The role of NF-kappaB activation in stress-induced resistance of EMT6 cells to anticancer therapy

Clinical success of breast cancer chemotherapy is often limited by the development of drug resistance. Here we show that treatment of EMT6 mouse mammary tumor cells with the physiologic stress, hypoxia (HYX) or the chemical stress agents, brefeldin A (BFA) or okadaic acid (OA) results in resistance to the topoisomerase II (topoII) inhibitors etoposide and doxorubicin. The mechanism of physiologic-induced drug resistance may involve the activation of stress-responsive proteins and we show here that BFA, HYX or OA treatment results in activation of the stress-responsive nuclear transcription factor, NF-κB. Pretreatment with the synthetic cyclopentenone prostaglandin A₁ (PGA₁) inhibits stress-induced NF-κB activation and prevents BFA- and HYX-induced resistance to etoposide, suggesting that NF-κB activation may play an important role in the development of stress-induced drug resistance.; To test specifically whether NF-κB mediates stress-induced drug resistance, a phosphorylation site-deficient mutant of IκBα (IκBαM, S32/36A) was introduced into EMT6 cells. IκBαM expression inhibits stress-induced NF-κB activation and prevents BFA-, hypoxia- and OA-induced resistance to etoposide and doxorubicin. Transgenic expression of either of the two predominant subunits of NF-κB, p65 and p50, enhances NF-κB transactivation and is sufficient to cause resistance to etoposide and doxorubicin.; To identify a common mechanism of physiological and chemical stress-induced drug resistance, we compared gene expression profiles of cells treated with either BFA or hypoxia. Our data show BFA or hypoxia treatment results in enhanced expression of the transforming growth factor-β (TGF-β) and decreased expression of the platelet-derived growth factor receptor, PDGFRα and the mitogen-activated protein kinase (MAPK), MEK1. Western blot analysis shows BFA or hypoxia treatment results in increased TGF-β protein levels and reduced PDGFRα and phosphorylated MEK protein levels. Furthermore, treatment with TGF-β, inhibition of PDGFRα with blocking antibodies, or inhibition of MEK phosphorylation with U0126 treatment is sufficient to cause etoposide resistance.; Taken together, these data support the key involvement of NF-κB activation in the development of chemical and physiologic stress-induced drug resistance to topoII inhibitors and suggest that co-administration of agents which inhibit NF-κB, TGF-β or the PDGFRα/MAPK pathway may improve the efficacy of topoII inhibitors in the clinical treatment of solid tumors.