Gene Therapeutic Overexpression of Mdr1 Results in Upregulation of Further Genes Involved in Detoxification and Delivers Radioprotection

Background: Gene therapeutic-delivered overexpression of Pglycoprotein (P-gp), the product of MDR1 (multidrug resistance 1) gene, might protect normal tissue during chemoand radiotherapy of P-gp-expressing tumors. However, little is known about the influence of MDR1-overexpression on the expression of other genes. Methods: Differentially gene expression in untransduced and oncoretrovirally transduced human lymphoblastoid TK6 cells were analysed by using the GeneChip Human Genome U133 Plus2.0 (Affymetrix). The expression of several genes was validated with quantitative real-time PCR (TaqMan; Applied Biosystems). Radiation-induced apoptosis was analysed by the sub-G1 DNA content using flow cytometry. Cell survival was measured by the colony formation assay. Results: Sixty-one annotated genes showed a significant change in expression (p < 10 ) in MDR1-overexpressing compared to untransduced and control virus-transduced cells. Several genes coding for proteins involved in detoxification and exocytosis (e.g. ALDH1A, UNC13) were up-regulated. Additionally, proapoptotic genes were down-regulated (e.g. CASP1, CASP4) with concomitant increased expression of the potential antiapoptotic gene AKT3. In functional assays overexpression of MDR1 conferred protection against radiation-induced clonogenic inactivation and apoptosis. Conclusion: The resistant phenotype of MDR1-mediated P-gp overexpressing cells is associated with differential expression of several genes coding for metabolic as well as proand antiapoptotic proteins. Our results could have important implications for MDR1-gene therapy in patients receiving chemotherapy regimens in combination with radiotherapy.