Isolation of a homolog of the mammalian multidrug resistance gene from Xenopus laevis

Abstract

P-glycoprotein (pgp) is a membrane-associated protein that functions in multidrug resistance (MDR) cells as a drug efflux pump to maintain intracellular concentrations of hydrophobic compounds below cytotoxic levels. We have identified a homolog of the mammalian mdr gene in Xenopus laevis (Xe-mdr). The gene was isolated from a tadpole cDNA library using as probe the full length mouse mdr1b cDNA. The Xe-mdr encodes a protein which is 66% identical to the mouse mdr1b gene and 68% identical to the human mdr1 gene. The predicted structure of the Xe-mdr gene product identifies twelve membrane spanning domains and two ATP binding sites both of which are the hallmark of the ABC (ATP binding cassette) transporters. The Xe-mdr mRNA is expressed as a single message of 4.4 kb and is confined to the intestine. Xe-mdr message is increased three to four fold in the ileum compared to the rest of the intestine. In situ hybridization of sequential sections from the small intestine localized the expression of the Xe-mdr to the cells lining the lumenal epithelium. A dual color flow cytometric assay for Rhodamine 123, a substrate for P-glycoprotein, and alkaline phosphatase, a marker for brush border cells, demonstrated that alkaline phosphatase positive cells isolated from the ileum effluxed Rhodamine 123 to a greater extent than cells from the duodenum. Treatment with verapamil, a known inhibitor of pgp function, resulted in a decreased efflux of Rhodamine 123 from intestinal cells. Brush border membrane vesicles prepared from the small intestine of Xenopus laevis effluxed vinblastine in an ATP dependent manner and the efflux was inhibited by verapamil.