MRP1 MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN AND P-GLYCOPROTEIN EXPRESSION IN RAT-BRAIN MICROVESSEL ENDOTHELIAL-CELLS

Two membrane glycoproteins acting as energy-dependent efflux pumps, md r-encoded P-glycoprotein (P-gp) and the more recently described multid rug resistance-associated protein (MRP), are known to confer cellular resistance to many cytotoxic hydrophobic drugs. In the brain, P-gp has been shown to be expressed specifically in the capillary endothelial cells forming the blood-brain barrier, but localization of MRP has not been well characterized yet. Using RT-PCR and immunoblot analysis, we have compared the expression of P-gp and Mrp1 in homogenates, isolate d capillaries, primary cultured endothelial cells, and RBE4 immortaliz ed endothelial cells from rat brain. Whereas the mdr1a P-gp-encoding m RNA was specifically detected in brain microvessels and mdr1b mRNA in brain parenchyma, mrp1 mRNA was present both in microvessels and in pa renchyma. However, Mrp1 was weakly expressed in microvessels. Mrp1 exp ression was higher in brain parenchyma, as well as in primary cultured brain endothelial cells and in immortalized RBE4 cells. This Mrp1 ove rexpression in cultured brain endothelial cells was less pronounced wh en the cells were cocultured with astrocytes. A low Mrp activity could be demonstrated in the endothelial cell primary monocultures, because the intracellular [H-3]vincristine accumulation was increased by seve ral MRP modulators. No Mrp activity was found in the cocultures or in the RBE4 cells. We suggest that in rat brain, Mrp1, unlike P-gp, is no t predominantly expressed in the blood-brain barrier endothelial cells and that Mrp1 and the mdr1b P-gp isoform may be present in other cere bral cells.