Microarray Analysis of Differentially Expressed Genes in Response to Retrovirally Mediated Overexpression of MDR1.

An important strategy for improving cancer therapy is the protection of healthy cells against therapy-induced damage. In this context gene therapy can complement radio- and chemotherapy in the treatment of tumours. Retroviral overexpression of P-glycoprotein (P-gp), the product of the MDR1 (multidrug resistance 1) gene, can deliver such a protective effect. P-gp possesses two relevant functions: its detoxifying effect acts as an efflux pump and could be important in high-dose chemotherapy, the anti-apoptotic effect of overexpressed P-gp could protect normal cells in tumour radio- or chemotherapy. The protection of haematopoietic stem cells in lymphoma, myeloma or solid tumor therapy is a promising candidate for this approach. However, little is known which other gene expressions might be influenced by MDR1-overexpression. Therefore we analyzed differentially expressed genes in the human lymphoblastoid cell line TK6 transduced with onco-retrovirus SF91m3 encoding MDR1 using the GeneChip Human Genome U133 Plus2.0 (Affymetrix). 61 annotated genes showed a significant change in expression (p<10⁻⁴) in MDR1 overexpressing compared to control transduced (overexpression the neomycin resistance gene) and untransduced cells. We found that genes coding for detoxifying and exocytotis proteins (e.g. ABCB4, ALDH1A, unc13) were up-regulated. Furthermore several proapoptotic genes were down-regulated (e.g. Casp1, Casp4) with concomitant increased expression of antiapoptotic genes (e.g. Akt3). The differential expressions for several genes were verified with real-time PCR. The results confirmed the dependence of these gene regulations on MDR1 overexpression. The influence of MDR1/P-gp overexpression on apoptotic signalling and cell survival was further corroborated by reduced apoptosis rates in response to irradiation in SF91m3 transduced cells compared to the control cell lines. Our genomics results support the findings from other studies that overexpression of MDR1 confers protection against apoptotic stimuli. The multidrug resistance phenotype does not seem to be only due to high P-gp expression but also to other metabolic proteins. Our results could have important implications for MDR1-gene therapy in patients receiving chemotherapy regimens in combination with radiotherapy.