Identifying transcription factors controlling the basal expression of human MRP4 highlights a substantial role for Sp1

The multidrug resistance protein 4 (MRP4/ABCC4) is a versatile efflux pump, known to transport several drugs but also signaling molecules such as cyclic nucleotides and lipid mediators. Based on this substrate spectrum and its broad tissue distribution, MRP4 plays a significant physiological and pathophysiological role in both the cardiovascular and oncological fields. However, the determinants of its gene expression are still incompletely defined. This study aimed to identify key regulatory elements and transcription factors that are essential for basal MRP4 expression. Using luciferase reporter assays with a series of 5'-deletion constructs, we identified a region upstream of the transcription start site as crucial for basal expression across diverse cell types. This region is evolutionary highly conserved and contains putative binding sites for Sp1 and Ets transcription factors. Site-directed mutagenesis of both binding elements resulted in a significant decrease in the promoter activity in HeLa and megakaryoblastic M07e cells. The binding of Sp1 to this region was further confirmed by electrophoretic mobility shift and chromatin immunoprecipitation assays. Finally, siRNA knockdown of Sp1 led to a significant decrease in MRP4 protein levels and function. In summary, we show that Sp1 binds to the MRP4 promoter and plays an essential role in the basal expression of MRP4, with Ets factors also potentially cooperating in this regulation.