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Multidrug resistance (MDR) is a major obstacle in the successful application of chemotherapy. The overexpression of ABC transporters, Pgp/ABCB1, MRP1/ABCC1, and BCRP/ABCG2, has been a well-known indicator of MDR. However, the importance of ABC transporters in MDR, and the mechanism of their upregulation have not been fully elucidated. The aim of this study is to examine the role of Notch1 in the expression of the multidrug resistance protein, MRP1/ABCC1. Notch1 is an enzymatic cleavable type I transmembrane protein, which is also known as an intracellular transcriptional regulator involved in regulating cell fate, apoptosis, proliferation and migration. We evaluated the levels of transmembrane Notch1 (N1TM) in cancer cell lines, such as CEM/VLB100, CEM/VM1-5, and BeWo known to overexpress the ABC transporters, Pgp, MRP1 and BCRP, respectively. Interestingly, we found an inverse relationship between the expression of Notch1 and MRP1/ABCC1 in the etoposide (VP16)-resistant CEM/VM1-5 cells and parental CEM cells. To investigate whether this inverse expression of N1TM and MRP1 is associated with etoposide resistance, we used another cell model, VP-16-resistant MCF7/VP and its drug sensitive parents, MCF7/WT, and observed the same inverse relationship: low N1TM but high MRP1 in the resistant MCF7/VP cells, compared to the parental cells. We hypothesize that Notch1 is one of the biological factors involved in MRP1 expression in the drug resistant cells. To understand the mechanism by which Notch1 may regulate MRP1, we measured the level and activity of intracellular Notch1 (ICN1) in MCF7/WT and MCF7/VP. As opposed to N1TM, there were no detectable differences in the levels of ICN1 between MCF7/WT and MCF7/VP. However, the activity of ICN1 in MCF7/VP was about 1.5-times higher than MCF7/WT, as measured by CBF1 promoter assay. To further verify the effect of ICN1 on MRP1 expression, we blocked the generation of ICN1 in MCF7/WT and MCF7/VP with DAPT, a gama-secretase inhibitor (GSI). We observed a dose-dependent reduction of both ICN1 and MRP1 expression in the MCF7/VP cells but not in the parental cells, suggesting a possible role for ICN1 in the overexpression of MRP1 in drug resistant cells. We next measured the level of presenilin 1 (PSEN1), which is a main proteolytic enzyme involved in the generation of ICN1, and observed increased levels of PSEN1 in MCF7/VP. We conclude that etoposide-resistant cells show faster processing of ICN1, possibly via PSEN1 overexpression, and that this is associated with MRP1 upregulation. This MRP1 upregulation can be blocked by GSI, suggesting a therapeutic possibility to prevent the development of MDR. (This work was supported in part by grant CA-40570 [to WTB] and in part by the University of Illinois at Chicago)

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA