Endotoxin leads to rapid subcellular re-localization of hepatic RXRα: A novel mechanism for reduced hepatic gene expression in inflammation

BACKGROUND: Lipopolysaccharide (LPS) treatment of animals down-regulates the expression of hepatic genes involved in a broad variety of physiological processes, collectively known as the negative hepatic acute phase response (APR). Retinoid X receptor α (RXRα), the most highly expressed RXR isoform in liver, plays a central role in regulating bile acid, cholesterol, fatty acid, steroid and xenobiotic metabolism and homeostasis. Many of the genes regulated by RXRα are repressed during the negative hepatic APR, although the underlying mechanism is not known. We hypothesized that inflammation-induced alteration of the subcellular location of RXRα was a common mechanism underlying the negative hepatic APR. RESULTS: Nuclear RXRα protein levels were significantly reduced (~50%) within 1–2 hours after low-dose LPS treatment and remained so for at least 16 hours. RXRα was never detected in cytosolic extracts from saline-treated mice, yet was rapidly and profoundly detectable in the cytosol from 1 hour, to at least 4 hours, after LPS administration. These effects were specific, since the subcellular localization of the RXRα partner, the retinoic acid receptor (RARα), was unaffected by LPS. A potential cell-signaling modulator of RXRα activity, c-Jun-N-terminal kinase (JNK) was maximally activated at 1–2 hours, coincident with maximal levels of cytoplasmic RXRα. RNA levels of RXRα were unchanged, while expression of 6 sentinel hepatic genes regulated by RXRα were all markedly repressed after LPS treatment. This is likely due to reduced nuclear binding activities of regulatory RXRα-containing heterodimer pairs. CONCLUSION: The subcellular localization of native RXRα rapidly changes in response to LPS administration, correlating with induction of cell signaling pathways. This provides a novel and broad-ranging molecular mechanism for the suppression of RXRα-regulated genes in inflammation

Rosiglitazone attenuates suppression of RXR alpha-dependent gene expression in inflamed liver

Background/Aims: A recently determined target of lipopolysaccharide (LPS) and cytokine signaling in liver is the central Type 11 nuclear receptor (NR) heterodimer partner, retinoid X receptor alpha (RXR alpha). We sought to determine if Rosiglitazone (Rosi), a peroxisome proliferator activated receptor gamma (PPAR gamma) agonist with anti-inflammatory properties, can attenuate LPS and cytokine-induced molecular suppression of RXR alpha-regulated genes. Methods: In vivo, mice were gavage-fed Rosi for 3 days, prior to intraperitoneal injection of LPS, followed by harvest of liver and serum. In vitro, HepG2 cells were treated with IL-1 beta, +/- short-term Rosi pretreatment. RNA was analyzed by quantitative RT-PCR, while nuclear and cytoplasmic proteins were analyzed by immunoblotting and gel shifts. Results: Rosi attenuated LPS-mediated suppression of RNA levels of several Type IINR-regulated genes, including bile acid transporters and the major drug metabolizing enzyme, Cyp3a11, without affecting cytokine expression, suggesting a novel, direct anti-inflammatory effect in hepatocytes. Rosi suppressed the inflammation-induced nuclear export of RXR alpha, in both LPS-injected mice and IL-1 beta-treated HepG2 cells, leading to maintenance of nuclear RXR alpha levels and heterodimer binding activity. Conclusions: Rosi directly attenuates the suppressive effects of inflammation-induced cell signaling on nuclear RXR alpha levels in liver. (c) 2006 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved

Cardiomyopathy reverses with recovery of liver injury, cholestasis and cholanemia in mouse model of biliary fibrosis

BackgroundTriggers and exacerbants of cirrhotic cardiomyopathy (CC) are poorly understood, limiting treatment options in patients with chronic liver diseases. Liver transplantation alone reverses some features of CC, but the physiology behind this effect has never been studied.AimsWe aimed to determine whether reversal of liver injury and fibrosis in mouse affects cardiac parameters. The second aim was to determine whether cardiomyopathy can be induced by specifically increasing systemic bile acid (BA) levels.Methods6-8 week old male C57BL6J mice were fed either chow (n = 5) or 3,5-diethoxycarbonyl-1,4-dihydroxychollidine (DDC) (n = 10) for 3 weeks. At the end of 3 weeks, half the mice in the DDC fed group were randomized to chow (the reversed [REV] group). Serial ECHOs and electrocardiographic analysis was conducted weekly for 6 weeks followed by liver tissue and serum studies. Hearts were analysed for key components of function and cell signalling. Cardiac physiological and molecular parameters were similarly analysed in Abcb11(-/-) mice (n = 5/grp) fed 0.5% cholic acid supplemented diet for 1 week.ResultsMice in the REV group showed normalization of biochemical markers of liver injury with resolution of electrocardiographic and ECHO aberrations. Catecholamine resistance seen in DDC group resolved in the REV group. Cardiac recovery was accompanied by normalization of cardiac troponin-T2 as well as resolution of cardiac stress response at RNA level. Cardiovascular physiological and molecular parameters correlated with degree of cholanemia. Cardiomyopathy was reproduced in cholanemic BA fed Abcb11(-/-) mice.ConclusionsCardiomyopathy resolves with resolution of liver injury, is associated with cholanaemia, and can be induced by BA feeding