Identification of Two Nuclear Protein Binding Sites and Their Role in the Regulation of the Murine Multidrug Resistance mdrla Promoter

Multidrug resistance genes (mdr) that encode P-glycoproteins (P-gp) are transcriptionally regulated in normal tissues and in some multidrug-resistant (MDR) cells. Several lines of evidence suggest that regulation of P-gp overexpression at the transcriptional level is also important in human tumors. In murine MDR cells, mdrla and/or mdrlb genes are overexpressed and P-gp isoforms are overproduced. To identify the mdrla promoter regions that are required for transcription, the promoter has been linked to the chloramphenicol acetyltransferase (CAT) gene in transient expression vectors. 5′-Deletions of the promoter sequences have demonstrated that the region between −155 to +89 bp is crucial for basal activity of the mdrla gene. DNase I footprinting, methylation interference, and gel retardation assays identified two nuclear protein binding sites within these sequences. One of the nuclear protein binding sites contains an 11-bp DNA sequence that interacts with nuclear protein(s) and is conserved in the promoters of the murine mdrla and mdrlb, hamster pgpl, and human MDR1 genes. The conserved SP1 site (5′-GGGCGGG-3′) that is present further downstream was shown to interact with its nuclear factor. These observations suggest that at least part of mdr gene transcriptional regulation is mediated by conserved mdr cis-regulalory elements and common nuclear factors.