Altered Localization of Retinoid X Receptor α Coincides with Loss of Retinoid Responsiveness in Human Breast Cancer MDA-MB-231 Cells

To understand the mechanism of retinoid resistance, we studied the subcellular localization and function of retinoid receptors in human breast cancer cell lines. Retinoid X receptor α (RXRα) localized throughout the nucleoplasm in retinoid-sensitive normal human mammary epithelial cells and in retinoid-responsive breast cancer cell line (MCF-7), whereas it was found in the splicing factor compartment (SFC) of the retinoid-resistant MDA-MB-231 breast cancer cell line and in human breast carcinoma tissue. In MDA-MB-231 cells, RXRα was not associated with active transcription site in the presence of ligand. Similarly, ligand-dependent RXR homo- or heterodimer-mediated transactivation on RXR response element or RARE showed minimal response to ligand in MDA-MB-231 cells. Infecting MDA-MB-231 cells with adenoviral RXRα induced nucleoplasmic overexpression of RXRα and resulted in apoptosis upon treatment with an RXR ligand. This suggests that nucleoplasmic RXRα restores retinoid sensitivity. Epitope-tagged RXRα and a C-terminus deletion mutant failed to localize to the SFC. Moreover, RXRα localization to the SFC was inhibited with RXRα C-terminus peptide. This peptide also induced ligand-dependent transactivation on RXRE. Therefore, the RXRα C terminus may play a role in the intranuclear localization of RXRα. Our results provide evidence that altered localization of RXRα to the SFC may be an important factor for the loss of retinoid responsiveness in MDA-MB-231 breast cancer cells.