INTRODUCTION AND OBJECTIVE: Evasion from apoptosis is related with the drug resistance phenotype in many tumor types. X-linked inhibitor of apoptosis protein (XIAP) is an antiapoptotic protein mainly found at the cytoplasm. However, recent evidence points to a role of nuclear XIAP in poor prognosis in breast cancer. Data from our group demonstrated that nuclear XIAP conferred unfavorable clinical outcome to breast cancer patients as well as increased in vitro proliferative capacity and drug resistance in a RING domain-associated manner. Here, we aimed to understand the mechanisms involved with nuclear XIAP-associated aggressive features in breast cancer. MATERIAL AND METHOD: MCF-7 human breast cancer-derived cells were transfected with XIAP variants: pEBB (empty vector), XIAPwild type, XIAPH467A (lack of ubiquitin ligase activity), XIAPΔRING (RING deletion) and XIAPNLS C-term (Insertion of a Nuclear Localization Signal at the C-terminal end). MCF-7 cells stably overexpressing XIAP variants were generated by doxycycline-inducible Tet-On system. XIAP activity on NF-kB promoter was investigated by the luciferase assay. The modulation of nuclear import via karyoferrin β1 was assessed by INI-43 pharmacological inhibitor. The expression and localization of XIAP and potential interaction partners were investigated by subcellular fractionation, Western blotting and immunofluorescence (confocal analysis). RESULTS AND CONCLUSION: Our results show that expression of Survivin, c-IAP1 and c-Myc, proteins known to interact with XIAP, was not increased in XIAPNLS C-term overexpressing cells. Although c-Myc global expression remained unaffected, XIAP overexpression at both nucleus and cytoplasm led to nuclear exclusion of c-Myc in a RING-independent manner. We also found that p50, but not p65 subunit of NF-kB, had expression increased in XIAPNLS C-term overexpressing cells. Remarkably, NF-kB luciferase activity was significantly reduced following transfections with XIAPH467A and XIAPΔRING vectors, an effect partially reversed by transfection with XIAPNLS C-term. Interestingly, the pattern of ubiquitination in K63, but not K48 ubiquitin chains, was increased following overexpression of XIAPNLS C-term, pointing to nuclear signaling transduction. Preliminary results revealed that INI-43 treatment increases XIAP levels, indicating that inhibition of karyoferrin β1 might modulate XIAP stability in our model. In conclusion, our data suggests that the oncogenic effects mediated by nuclear XIAP are not associated with Survivin, cIAP-1 and c-Myc expression, but might involve the NF-kB pathway and trigger signaling pathways instead of proteasomal degradation of target proteins. Experiments involving the analysis of transcriptomic changes induced by XIAP variants by RNA microarray and validation of XIAP stable transfectants are currently being performed and will enable a better understanding of nuclear XIAP-mediated mechanisms in breast cancer growth and chemoresistance.