Dominant negative action of SXR AF-2 mutant for multidrug-resistant cancer gene therapy.

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The multidrug resistance (MDR) is a major cause of failure of cancer chemotherapy. The overexpression of the MDR1 gene that encodes P-glycoprotein is responsible for the development of drug-resistant tumor cells. Recent study revealed that the orphan nuclear receptor SXR regulates MDR1 gene expression. A variety of compounds such as rifampicin (RFP) have been shown to bind to SXR as ligands and stimulate transcription. We speculate that the inhibition of SXR-mediated transcription may be an effective approach to suppress MDR1 gene expression, whereby tumor cells may turn to response to the chemotherapy.We created artificial deletion mutant of human SXR (SXRΔAF2), lacking 15 residues in the helix 12 of the C-terminus. We examined transcriptional activity and dominant negative activity of SXRΔAF2, and found that SXRΔAF2 had strong dominant negative activity. A mammalian two-hybrid assay revealed that the dominant negative activity by SXRΔAF2 is likely due to lack of coactivator binding and constitutive recruitment of the corepressors.In real time PCR analysis, mRNA expression of MDR1 in colon cancer cell line, LS 174T cells, was in creased in RFP-dependent manner. Then, we created stable cell line, which express SXRΔAF2 mutant to examine whether SXRΔAF2 can inhibit RFP-stimulated MDR1 mRNA expression in this cell line. Currently, we are under investigation.The use of SXRΔAF2 may be a novel therapeutic approach using mutant nuclear receptor for the treatment of multidrug-resistant cancer.