The mechanism of regulation of P-glycoprotein (Analysis of multidrug-resistant mutants that do not express P-glycoprotein)

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1) We have isolated human epidermoid KB cell lines resistant to high levels of adriamycin, C-A90, C-A120, C-A500 and C-A1000. They were isolated in selection medium containing increasing concentrations of adriamycin, 1mug/ml of cepharanthine, a multidrug-resistance (MDR) reversing agent, and 100nM mezerein, a protein kinas C activating agent. One of the adriamycin-resistant KB cell lines, C-A500, was cross-resistant to drugs that typify the classical multidrug resistance phenotype, such as vincristine, actinomycin D,VP-16 and colchicine.2) The accumulation of adriamycine and vincristine was decreased in C-A500 cells and the efflux of adriamycine from C-A 500 was enhanced compared with parental KB-3-1 cells.These adriamycin-resistant KB cells did not contain detectable levels of P-glycoprotein or overexpress MDR1. Multidrug-resistance-associated protein (MRP) and MRP mRNA were expressed in the adriamycin-resistant KB-cells, C-A120, C-A500 and C-A1000, but not in parental KB-3-1 and reve … More rtant C-AR cells. The MRP gene was amplified in these cells that overexpressed MRP mRNA.4) DNA topoisomerase II levels were markedly decreased in C-A500 and C-A1000 cells but only slightly decreased in C-A120 cells. These results indicate that MRP overexpressed in the resistant cells may be responsible for the reduced accumulation of adriamycin and vincrisrine and that both the increased expression of MRP and decreasd levels of topoisomerase II underlie te drug resistance in C-A120, C-A500 and C-A1000 cell lines.5) We investigated the expression of MRP mRNA in multidrug-resistant KB cell line (KB-8-5, KB-C2, C-A40 and C-A120), human non-small-cell lung carcinomas (NSCLC), gastric and colorectal carcinomas and compared it with that in drug-sensitive human KB cells. MRP gene expression was elavated in 8 of 9 (89%) squamous-cell carcinomas of the lung. Furthermore, MRP expression in 4 squamous-cell carcinomas (L13,18,19 and 20) was more than 3.6 times higher than in KB-3-1 cells, and the average MRP mRNA expression level of all squamous-cell carcinomas was significantly higher than that of adenocarcinoma of the lung and of colorectal and gastric carcinomas. Less