This project investigates the structure, function and regulation of the multidrug resistance (mdr) genes. We have continued to extend our previous investigations on the regulation of these genes by xenobiotics both in vivo and in vitro. The requirement for metabolic activation of xenobiotics and the reactive compounds which are critical to the regulation of mdr is being investigated. Mallory body formation has recently been demonstrated to be associated with dysregulation of P-glycoprotein. Therefore, we are investigating the possible role of alterations in mdr expression in this model. Preliminary evidence has indicated that mdr2 but not mdr1b expression is altered in these experiments. Because of its location on the bile canalicular domain of the hepatocyte, we and others have hypothesized that P-glycoprotein may have a physiological role as a biliary transporter of xenobiotics and endobiotics and that its expression may therefore be altered in cholestasis. Both obstructive and alpha-naphthylisothiocyanate-induced cholestasis increased mdr1a and 1b gene expression in rat liver. Alpha- naphthylisothiocyanate-induced cholestasis in cynomolgus monkeys increased hepatic expression of both the mdr1 and 2 genes. These findings suggest that P-glyco-protein functions as a biliary efflux pump for xenobiotics and, possibly, for unidentified physiological inducers. A full-length cDNA clone for the rat mdr2 gene was isolated and sequenced. The regulation of expression of this gene is distinct from the mdr1 genes. The highest level of expression in control tissues is in the muscle, heart, liver and spleen. Expression of both mdr1b and mdr2 increased after surgical and partial hepatectomy but the time course of induction of each gene differed. The promoter of the mdr1b gene was further characterized. A combination of structural and functional assays have indicated that sequences between -250 and -138 are sufficient to confer basal activity and that the regions between -941 to -814 and -680 to -589 contain binding sites for transcriptional repressors. Genomic clones for the promoter regions of both the mdr1a and mdr2 genes have been isolated and are currently being characterized.
