Multidrug resistance-associated proteins are crucial for the viability of activated rat hepatic stellate cells

Abstract

Hepatic stellate cells (HSCs) survive and proliferate in the chronically injured liver. ATP-binding cassette (ABC) transporters play a crucial role in cell viability by transporting toxic metabolites or xenobiotics out of the cell. ABC transporter expression in HSCs and its relevance to cell viability and/or activation have not been reported so far. The aim of this study was to investigate the expression, regulation, and function of multidrug resistance-associated protein (Mrp)-type and multidrug resistance protein (Mdr)-type ABC transporters in activated rat HSCs. Rat HSCs were exposed to cytokines or oxidative stress. ABC transporter expression was determined by quantitative polymerase chain reaction and immunohistochemistry. HSCs were exposed to the Mdr inhibitors verapamil and PSC-833 and the Mrp inhibitor MK571. Mdr and Mrp transporter function was evaluated with flow cytometry. Apoptosis was determined by activated caspase-3 and acridine orange staining, and necrosis was determined by Sytox green nuclear staining. An in vivo model of carbon tetrachloride (CCl(4))-induced liver fibrosis was used. With respect to hepatocytes, activated HSCs expressed high levels of Mrp1 and comparable levels of Mrp3, Mrp4, Mdr1a, and Mdr1b but not the hepatocyte-specific transporters bile salt export pump, Mrp2, and Mrp6. Mrp1 protein staining correlated with desmin staining in livers from CCl(4)-treated rats. Mrp1 expression increased upon activation of HSCs. Cytokines induced Mdr1b expression only. Oxidative stress was not a major regulator of Mdr and Mrp transporter expression. Activated HSCs became necrotic when exposed to the Mrp inhibitors. Activated HSCs contain relatively high levels of Mrp1. Mrp-type transporters are required for the viability of activated HSCs. Mrp-dependent export of endogenous metabolites is important for the survival of activated HSCs in chronic liver diseases.