Up-regulation of the multidrug resistance genes, mrp1 and mdr1b, and down-regulation of the organic anion transporter, Mrp2, and the bile salt transporter, spgp, in endotoxemic rat liver

Endotoxin-induced cholestasis is mainly caused by an impaired canalicular s ecretion. Mrp2, the canalicular multispecific organic anion transporter, is strongly downregulated in this situation, and canalicular bile salt secret ion is also reduced. We hypothesized that other adenosine triphosphate-bind ing cassette (ABC) transporters may compensate for the decreased transport activity to protect the cell from cytokine-induced oxidative damage. Theref ore, we examined the expression of ABC-transport proteins in membrane fract ions of whole liver and of isolated hepatocytes of endotoxin-treated rats a nd performed reverse-transcriptase polymerase chain reaction (RT-PCR) on mR NA isolated from these livers. In addition, the localization of these trans porters was examined using confocal scanning laser microscopy. By 6 hours a fter endotoxin administration, we found a clear increase of mrp1 mRNA and p rotein, whereas mrp2 mRNA and protein were decreased. This was confirmed in isolated hepatocytes. In addition, mdr1b mRNA was strongly increased, wher eas mdr1a and mdr2 mRNA did not change significantly. Both the mRNA and pro tein levels of the sister of P-glycoprotein (spgp), the recently cloned bil e salt transporter, decreased. After endotoxin treatment, the normally shar ply delineated canalicular staining of mrp2 and spgp had changed to a fuzzy pattern, suggesting localization in a subapical compartment, We conclude t hat endotoxin-induced cholestasis is caused by decreased mrp2 and spgp leve ls, as well as an abnormal localization of these proteins. The simultaneous up-regulation of mrp1 and mdr1b may confer resistance to hepatocytes again st cytokine-induced metabolic stress.