Estrogen receptor (ER) positive breast cancer is successfully treated with antiestrogenic drugs, yet primary or acquired pharmacological resistance represents a serious clinical issue. The most widely used antiestrogenic drug, Tamoxifen (Tam), binds to ERα, inducing a conformation that interacts with corepressors (NCoR/SMRT), rather than coactivators, at estrogen target genes. When resistance develops, corepressor dismissal is observed and coactivator recruitment to ER switches the effect of the drug from antagonistic to agonistic. The protein TAB2, a facultative component of the NCoR complex, was shown to shuttle NCoR to the cytoplasm when phosphorylated in response to inflammatory signals, causing resistance to antiandrogen in prostate and antiestrogen in breast cancer cells (Zhu et al., 2006, Cell 124: 615-29). We addressed the question whether TAB2 has a more general role in tamoxifen resistance in breast cancer. First, we studied the localization of TAB2 and NCoR in TAM-resistant cell clones derived from MCF7 by continuous exposure to the drug (TAMR). In all studied clones that retained ERα expression, NCoR was aberrantly localized in the cytoplasm, similarly to what is observed in wild-type, Tamoxifen-sensitive cells, after treatment with IL-1β. Following these observations, we directly assessed the role of TAB2 in NCoR delocalization, by down-regulating TAB2 in TAMR cells using specific siRNA. Reduction of TAB2 protein was indeed sufficient to recover the correct nuclear localization of NCoR to cell nuclei, as demonstrated by immunofluorescence and cell fractionation studies and, importantly, restored tamoxifen sensitivity in TAMR cells. Similar results were obtained using the BT474 cell line. Gene expression profiling was then carried out on one TAMR cell clone, after TAB2 downregulation by siRNA, using the 44K Agilent oligo-glass microarray system. The data obtained suggest that TAB2 may be involved in the regulation of a number of genes important for cell proliferation and estrogen response.
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