Transcriptomic alterations upon nuclear XIAP overexpression reveal IGFBP6/Wnt as a regulatory axis in breast cancer...

X-linked inhibitor of apoptosis protein (XIAP) is an antiapoptotic protein which plays canonical functions in the cytoplasm. However, we have previously demonstrated that nuclear XIAP is associated with increased cell growth and drug resistance as well as unfavorable outcomes in breast cancer. Therefore, this work aimed to investigate the non-canonical molecular functions associated with abnormal XIAP nuclear localization in breast cancer. To address this, we have performed both transcriptomic and proteomic large-scale approaches with breast cancer cell line models overexpressing HA-tagged ectopic XIAP (WT, H467A, ΔRING and NLS mutants) in distinct subcellular locations. Nuclear XIAP overexpressing cells (XIAP^NLS) have been shown enriched for genes implicated in cellular processes other than apoptosis inhibition, such as proliferation, transport, locomotion, migration, cell motility, and protein phosphorylation. Interestingly, the validation analysis has shown differential expression of genes associated with the Wingless-related integration site (Wnt)/β-catenin pathway, as well as the insulin-like growth factor-binding protein 6 (IGFBP6) tumor suppressor gene. IGFBP6 transcript levels have been found reduced in XIAP^NLS-overexpressing cells, as well as invasive breast cancer patients. Notably, IGFBP6 knockdown modulated long-term cell proliferation and doxorubicin sensitivity in cells overexpressing nuclear XIAP, further suggesting that nuclear XIAP might indirectly target the IGFBP6 pathway to promote chemoresistance. Finally, Wnt signaling effectors were identified in nuclear XIAP interactomes, closely linked to activation of β-catenin transcriptional activity in XIAP^NLS-overexpressing cells. Our data establishes a non-canonical role for XIAP in the regulation of pathways related to cell growth in the nucleus, with Wnt/IGFBP6 emerging as a potential regulatory node associated with nuclear XIAP effects in breast cancer.