Functional expression of P-glycoprotein and multidrug resistance-associated protein (Mrp1) in primary cultures of rat astrocytes.
Although it has been well established that the drug efflux pump P-glycoprotein (P-gp) protects the brain against the entry of cytotoxic drugs, its real in situ localization, i.e., at brain capillary endothelial cells or on astrocyte foot processes, is still controversial. The aim of this study was to compare the expression of P-gp and of multidrug resistance-associated protein (Mrp1), another drug efflux pump, in cultured neonatal rat brain astrocytes and in cultured brain capillary endothelial cells. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed that the mdr1b gene was preferentially expressed in astrocytes, whereas both mdr1a and mdr1b mRNA were detected in endothelial cells. Moreover, the mrp1 gene encoding Mrp1 was expressed in both cell types. Western blotting analysis revealed higher expression of P-gp in endothelial cells as compared with astrocytes, but higher expression of Mrp1 in astrocytes. Moreover, P-gp and Mrp1 expression was not modified in more differentiated astrocytes obtained when cultured with db-cAMP for 48 hr. Our functional analysis of P-gp showed a modest effect of P-gp modulators (CsA, verapamil, PSC 833) on the uptake of colchicine (a substrate of P-gp) by astrocytes, whereas they increased by about 50% the uptake of vincristine (a common substrate of P-gp and MRP) by astrocytes. MRP modulators (genistein, probenecid, and sulfinpyrazone) did not modify the uptake of colchicine but increased that of vincristine with a major effect found for sulfinpyrazone. Moreover, indomethacin, probenecid, and sulfinpyrazone increased the uptake of fluorescein (a substrate of MRP but not of P-gp). Taken together, our results provide the first biochemical and functional evidence supporting the expression of P-gp and Mrp1 in rat cultured astrocytes.