Biochemical and Biophysical Research Communications
Nrf2 counteracts cholestatic liver injury via stimulation of hepatic defense systems
Introduction
Cholestasis leads to hepatic and systemic accumulation of potentially toxic biliary compounds; e.g., bile acids and bilirubin; which in turn results in liver injury and jaundice [1]. Spontaneous defense mechanisms of hepatocytes against cholestasis include down-regulation of influx pumps and adaptive induction of export pumps; e.g., Mrp3 and Mrp4 [2], [3]. This adaptive response may prevent hepatocellular accumulation of bile acids and bilirubin and thereby limit cholestatic liver injury. Moreover, detoxification of biliary compounds accumulated in the liver through phase I hydroxylation and phase-II conjugation may counteract cholestatic liver injury by rendering hydrophobic substrates less toxic and better soluble for biliary and urinary excretion routes [4].
Under cholestatic conditions, oxidative stress plays an important role in the cell death of hepatocytes and links to the development of liver injury. Hydrophobic bile acids stimulate the generation of reactive oxygen species (ROS) in hepatocytes and in liver mitochondria [5]. Evidence of oxidative stress-induced liver injury has been demonstrated in rats receiving intravenously infused hydrophobic bile acids [6] and in the bile duct-ligated (BDL) rat model of obstructive cholestasis [7], [8].
One transcription factor that serves as a cellular sensor for oxidative stress is termed nuclear factor-E2-related factor-2 (Nrf2). Nrf2 is sequestered in the cytosol by Kelch-like Ech-associated protein (Keap1). However, during oxidative challenge, modification of Keap1 sulfhydryl groups results in release and nuclear translocation on Nrf2 [9]. Nrf2 is essential for antioxidant responsive element/electrophile-responsive element (ARE/EpRE)-mediated gene induction of detoxifying enzymes [10] and antioxidative stress genes, and can transcriptionally activate several enzymes involved in cellular protection [9], [10], [11], [12], [13], [14], [15]. Activation of this gene battery serves to decrease the oxidative burden of the cell by increasing GSH concentrations, reducing reactive intermediates, and increasing general detoxification of chemicals via phase-II conjugation reactions [11]. Moreover, the recent studies of our group [16], [17] have shown that the Nrf2 activators induce Mrp efflux transporters in rodent livers.
In this study, we aimed at determining whether Nrf2 activation counteracts liver injury in obstructive cholestasis caused by BDL in Keap1-kd mice. The results indicate that BDL-induced liver injury is significantly attenuated in the Keap1-kd mice compared with that in wild-type (WT) mice, through a stimulation of detoxification and antioxidative stress systems, along with Mrp efflux transport.
Section snippets
Materials and methods
Animals. Pair-matched littermate wild-type and Keap1 knockdown (Keap1-kd) mice on an C57/B6 background, in which cytoplasmic protein Keap1, a repressive factor of Nrf2, is markedly reduced and thereby results in constitutive activation of Nrf2, were engineered and bred at the Center for Tsukuba Advanced Research Alliance and Institute of Basic Medical Science, the University of Tsukuba [16]. All experiments were conducted according to the institution’s guidelines for the care and use of
Effects of constitutive activation of Nrf2 on hepatoprotection of the mouse liver in obstructive cholestasis
Effects of constitutive activation of Nrf2 on hepatoprotection of the mouse liver were tested in Keap1-kd mice with BDL (Keap1-kd BDL mice), a particularly severe model of obstructive cholestasis.
Histological analysis of liver tissue sections to determine the levels of hepatic parenchymal necrosis were conducted in WT mice with BDL (WT BDL mice) and Keap1-kd BDL mice. Keap1-kd BDL mice had significantly decreased numbers of necrotic foci on the sections (Fig. 1A).
Nuclear levels of Nrf2 were
Discussion
The results of this study and those of other recent studies from our laboratories [17], [18] have demonstrated that expression levels of Mrp efflux transporters, phase I detoxifying enzymes, phase II conjugating enzymes and antioxidative stress genes are markedly up-regulated by Nrf2 activation. Especially, since Mrp efflux transporters play a key role in cellular protection by removing xenobics, metabolites, and endogenous substrates that can accumulate in tissues and lead to toxicity,
References (29)
Nuclear receptor ligands: rational and effective therapy for chronic cholestatic liver disease
Gastroenterology
(2005)- et al.
Vitamin E reduces oxidant injury to mitochondria and the hepatotoxicity of taurochenodeoxycholic acid in rat
Gastroenterology
(1998) - et al.
Evidence for accelerated generation of hydroxyl radicals in experimental obstructive jaundice of rats
Free Radic. Biol. Med.
(1998) - et al.
An Nrf2/small Maf heterodimer mediates the induction of phase II detoxifying enzyme genes through antioxidant response elements
Biochem. Biophys. Res. Commun.
(1997) - et al.
Transcriptional factor Nrf2 coordinately regulates a group of oxidative stress-inducible genes in macrophages
J. Biol. Chem.
(2000) - et al.
Simultaneous determination of bile acids in rat liver tissue by high-performance liquid chromatography
J. Chromatogr. B Biomed. Sci. Appl.
(1998) - et al.
Determination of malonaldehyde in oxidized lipids by the hantzsch fluorometric method
Anal. Biochem.
(1988) - et al.
Cellular localization and up-regulation of multidrug resistance-associated protein 3 in hepatocytes and cholangiocytes during obstructive cholestasis in rat liver
Hepatology
(2001) - et al.
Mice lacking Mrp3 (Abcc3) have normal bile salt transport, but altered hepatic transport of endogenous glucuronides
J. Hepatol.
(2006) - et al.
Effects of ursodeoxycholic and cholic acid feeding on hepatocellular transporter expression in mouse liver
Gastroenterology
(2001)
Molecular pathogenesis of cholestasis
N. Engl. J. Med.
Mrp4−/− mice have an impaired cytoprotective response in obstructive cholestasis
Hepatology
Role of farnesoid X receptor in determining hepatic ABC transporter expression and liver injury in bile duct-ligated mice
Gastroenterology
Ursodeoxycholic acid protects against secondary biliary cirrhosis in rats by preventing mitochondrial oxidative stress
Hepatology
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