TARGETED DISRUPTION OF THE MOUSE MDR1B GENE REVEALS THAT STEROID-HORMONES ENHANCE MDR GENE-EXPRESSION

To evaluate the role of P-glycoprotein in steroid secretion in adrenal cells, we have used gene targeting to introduce a null mutation into one allele of the mdr1b gene in mouse Y1 adrenal cells. Characterizati on of both the wild-type and the mutant cell lines revealed the follow ing. 1) The expression of mdr1b is enhanced by steroid hormones, in a feedback regulatory mechanism. Inhibition of steroid biosynthesis by 2 -aminoglutethimide blocks the adrenocorticotropin (ACTH)-induced incre ase in mdr1b mRNA levels. 2) ACTH-stimulated steroid secretion is mark edly decreased in the mutant cell line. This decreased steroid secreti on in the mutant cells occurs despite an increase in the levels of mdr 1b mRNA and P-glycoprotein. Kinetic analyses of vinblastine and daunom ycin accumulation in both the wild-type and the mutant cell lines duri ng ACTH-stimulated steroidogenesis show that in the mutant cells both drugs accumulated to higher levels than in Y1 cells, suggesting that t he remaining mdr1b allele in the mutant cells is relatively inactive a s an exporter of steroids, or that the targeted disruption of the mdr1 b allele is associated with other changes in the mutant cells which bl ock ACTH-stimulated steroid secretion.