ABSTRACT
Evasion from apoptosis is one of the hallmarks of cancer. X-linked inhibitor of apoptosis protein (XIAP) is known to modulate apoptosis by inhibiting caspases and ubiquitinating target proteins. XIAP is mainly found at the cytoplasm, but recent data link nuclear XIAP to poor prognosis in breast cancer. Here, we generated a mutant form of XIAP with a nuclear localization signal (XIAPNLS-C-term) and investigated the oncogenic mechanisms associated with nuclear XIAP in breast cancer. Our results show that cells overexpressing XIAPΔRING (RING deletion) and XIAPNLS-C-term exhibited XIAP nuclear localization more abundantly than XIAPwild-type. Remarkably, overexpression of XIAPNLS-C-term, but not XIAPΔRING, conferred resistance to doxorubicin and increased cellular proliferative capacity. Interestingly, Survivin and c-IAP1 expression were not associated with XIAP oncogenic effects. However, NFκB expression and ubiquitination of K63, but not K48 chains, were increased following XIAPNLS-C-term overexpression, pointing to nuclear signaling transduction. Consistently, multivariate analysis revealed nuclear, but not cytoplasmic XIAP, as an independent prognostic factor in hormone receptor-negative breast cancer patients. Altogether, our findings suggest that nuclear XIAP confers poor outcome and RING-associated breast cancer growth and chemoresistance.
Subject(s)
Breast Neoplasms/genetics , Breast Neoplasms/pathology , Cell Nucleus/metabolism , Drug Resistance, Neoplasm , Gene Expression Regulation, Neoplastic , X-Linked Inhibitor of Apoptosis Protein/metabolism , Apoptosis/drug effects , Carcinoma, Ductal, Breast/genetics , Carcinoma, Ductal, Breast/pathology , Cell Line, Tumor , Cell Nucleus/drug effects , Cell Proliferation/drug effects , Cell Proliferation/genetics , Doxorubicin/pharmacology , Drug Resistance, Neoplasm/drug effects , Drug Resistance, Neoplasm/genetics , Female , Gene Expression Regulation, Neoplastic/drug effects , HEK293 Cells , Humans , Lysine/metabolism , Multivariate Analysis , Mutant Proteins/metabolism , Mutation/genetics , NF-kappa B/metabolism , Polyubiquitin/metabolism , Prognosis , Protein Domains , Receptors, Cell Surface/metabolism , Survival Analysis , Ubiquitination/drug effects , X-Linked Inhibitor of Apoptosis Protein/chemistryABSTRACT
Drug resistance is a major hurdle for successful treatment of breast cancer, the leading cause of deaths in women throughout the world. The FOXM1 transcription factor is a potent oncogene that transcriptionally regulates a wide range of target genes involved in DNA repair, metastasis, cell invasion, and migration. However, little is known about the role of FOXM1 in cell survival and the gene targets involved. Here, we show that FOXM1-overexpressing breast cancer cells display an apoptosis-resistant phenotype, which associates with the upregulation of expression of XIAP and Survivin antiapoptotic genes. Conversely, FOXM1 knockdown results in XIAP and Survivin downregulation as well as decreased binding of FOXM1 to the promoter regions of XIAP and Survivin. Consistently, FOXM1, XIAP, and Survivin expression levels were higher in taxane and anthracycline-resistant cell lines when compared to their sensitive counterparts and could not be downregulated in response to drug treatment. In agreement with our in vitro findings, we found that FOXM1 expression is significantly associated with Survivin and XIAP expression in samples from patients with IIIa stage breast invasive ductal carcinoma. Importantly, patients co-expressing FOXM1, Survivin, and nuclear XIAP had significantly worst overall survival, further confirming the physiological relevance of the regulation of Survivin and XIAP by FOXM1. Together, these findings suggest that the overexpression of FOXM1, XIAP, and Survivin contributes to the development of drug-resistance and is associated with poor clinical outcome in breast cancer patients.
