Matthias Wettstein

Regulation of the multidrug resistance protein 2 in the rat liver by lipopolysaccharide and dexamethasone

BACKGROUND & AIMS Endotoxin lipopolysaccharide (LPS) induces cholestasis and down-regulates the multidrug resistance protein 2 (MRP2). This study intends to characterize the short-term effects of LPS on MRP2. METHODS The effects of LPS and dexamethasone on excretion of bromosulphalein (BSP), MRP2 messenger RNA (mRNA) levels, and subcellular MRP2 localization were studied by means of liver perfusion, Northern blots, and confocal microscopy. RESULTS LPS treatment for 3-12 hours decreased biliary BSP excretion (10 micromol/L) by 40%. Hyposmolarity stimulated BSP excretion to control levels 3 hours after LPS injection, but was ineffective after 12 hours or in saline-treated controls. LPS led to a strong decrease of MRP2 mRNA after 12 hours, but not during the first 6 hours. LPS induced the appearance of MRP2 in intracellular vesicles in the immediate vicinity of the canaliculi within 3 hours, and these vesicles were remote from the canaliculi after 6 and 12 hours. The MRP2-containing vesicles did not stain for dipeptidylpeptidase IV (DPPIV). Dexamethasone counteracted the LPS effects on MRP2 mRNA levels, subcellular distribution, and BSP excretion. CONCLUSIONS LPS induces cholestasis due to an early retrieval of MRP2 from the canalicular membrane, whereas down-regulation of MRP2 mRNA is a later event. LPS-induced MRP2 retrieval from the canalicular membrane is not associated with the retrieval of DPPIV, suggestive for selectivity of the process.

Involvement of integrins and Src in tauroursodeoxycholate-induced and swelling-induced choleresis

BACKGROUND & AIMS Stimulation of canalicular secretion by tauroursodeoxycholate (TUDC) involves dual activation of p38 mitogen-activated protein kinase (p38(MAPK)) and extracellular signal-regulated kinase (ERK). This study investigates the sensing and upstream signaling events of TUDC-induced choleresis. METHODS TUDC and hypo-osmolarity effects on protein kinase activities and taurocholate excretion were studied in perfused rat liver. RESULTS TUDC induced a rapid activation of focal adhesion kinase (FAK) and Src, as shown by an increase in Y418 phosphorylation and a decrease in Y529 phosphorylation of Src. Inhibition of Src by PP-2 abolished the TUDC-induced activation of p38(MAPK) but not of FAK and ERKs. An integrin-inhibitory peptide with an RGD motif blocked TUDC-induced FAK, Src, ERK, and p38(MAPK) activation, suggesting that integrin signaling toward FAK/Src is required for TUDC-induced MAPK activation. The RGD peptide and PP-2 also abolished the stimulation of taurocholate excretion in perfused rat liver in response to TUDC. Integrin-dependent Src activation was also identified as an upstream event in hypo-osmotic signaling toward MAPKs and choleresis. CONCLUSIONS TUDC-induced stimulation of canalicular taurocholate excretion involves integrin sensing, FAK, and Src activation as upstream events for dual MAPK activation. Integrins may also represent one long-searched sensor for cell hydration changes in response to hypo-osmolarity.

Lipopolysaccharide-induced tyrosine nitration and inactivation of hepatic glutamine synthetase in the rat.

Glutamine synthetase (GS) in the liver is restricted to a small perivenous hepatocyte population and plays an important role in the scavenging of ammonia that has escaped the periportal urea‐synthesizing compartment. We examined the effect of a single intraperitoneal injection of lipopolysaccharide (LPS) in vivo on glutamine synthesis in rat liver. LPS injection induced expression of inducible nitric oxide synthase, which was maximal after 6 to 12 hours but returned toward control levels within 24 hours. Twenty‐four hours after LPS injection, an approximately fivefold increase in tyrosine‐nitrated proteins in liver was found, and GS protein expression was decreased by approximately 20%, whereas GS activity was lowered by 40% to 50%. GS was found to be tyrosine‐nitrated in response to LPS, and immunodepletion of tyrosine‐nitrated proteins decreased GS protein by approximately 50% but had no effect on GS activity. Together with the finding via mass spectrometry that peroxynitrite‐induced inactivation of purified GS is associated with nitration of the active site tyrosine residue, our data suggest that tyrosine nitration critically contributes to inactivation of the enzyme. In line with GS inactivation, glutamine synthesis from ammonia (0.3 mmol/L) in perfused livers from 24‐hour LPS‐treated rats was decreased by approximately 50%, whereas urea synthesis was not significantly affected. In conclusion, LPS impairs hepatic ammonia detoxification by both downregulation of GS and its inactivation because of tyrosine nitration. The resulting defect of perivenous scavenger cell function with regard to ammonia elimination may contribute to sepsis‐induced development of hyperammonemia in patients who have cirrhosis. (HEPATOLOGY 2005.)

Lipodystrophy Syndrome and Self-Assessment of Well-Being and Physical Appearance in HIV-Positive Patients

The lipodystrophy syndrome (LDS) is a growing problem in human immunodeficiency virus (HIV)-positive patients treated with highly active antiretroviral therapy (HAART). It is characterized by alterations of body composition and metabolic abnormalities. The goal of the study was to investigate attitudes toward health condition, well-being, and individual appearance in relation to LDS. Outpatients between July and October 2000 in an HIV-specialized unit at the University Hospital of Düsseldorf, Germany, underwent clinical evaluation and received a standardized written questionnaire. Of 389 patients eligible for analysis, 313 patients returned completed questionnaires (response rate, 80.5%). LDS was observed in 37.7%; the predominant manifestation was lipoatrophy of the face (32.9%). Individuals with and without LDS did not differ significantly in their attitude to the quality of their health condition and the amount of disturbance of their well-being by HIV infection. Participants with LDS felt recognizable as HIV-positive by physical appearance in 30.1%, compared to 18.3% in patients without LDS (p = 0.027). This difference became more pronounced after adjustment for gender, age, stage of disease, CD4 cell count, and duration of HAART (odds ratio, 2.04, 95%-confidence interval [CI] 1.09-3.84). In conclusion, LDS does not seem to disturb the general attitude toward health condition and well-being. However, patients presenting with lipodystrophy are about twice as likely to feel recognizable as HIV-positive by their physical appearance. LDS may thus be perceived as a characteristic mark of being HIV-positive by affected persons. A stigmatizing effect and social disadvantages may be the consequences.

Clinical Efficacy of L-Ornithine–L-Aspartate in the Management of Hepatic Encephalopathy

The clinical efficacy of both oral and parenteral L-ornithine–L-aspartate (OA) was confirmed by randomized, placebo-controlled, double-blind studies in patients with manifest hepatic encephalopathy and hyperammonemia. The drug was able to reduce high blood ammonia levels induced either by ammonium chloride or protein ingestion or existing as a clinical complication of cirrhosis per se. Furthermore, OA improved performance in Number Connection Test-A as well as mental state gradation. In contrast to the positive effects observed in patients with more advanced hepatic encephalopathy, oral OA does not seem to affect minimal hepatic encephalopathy. In a recent trial, OA decreased protein breakdown and stimulated protein synthesis in muscle. The therapy had little side effects, increasing with higher intravenously administered dosages, and was well tolerated after oral and parenteral administration.

Retrieval of the mrp2 gene encoded conjugate export pump from the canalicular membrane contributes to cholestasis induced by tert-butyl hydroperoxide and chloro-dinitrobenzene.

Abstract Oxidative stress is known to induce cholestasis, but the underlying mechanisms are poorly understood. In this study we have characterized the short-term effects of tert-butyl hydroperoxide (t-BOOH)- and 1-chloro-2,4-dinitrobenzene (CDNB) on the mrp2 gene encoded canalicular export pump (Mrp2). The effects of t-BOOH and CDNB on bile formation, tissue GSH levels and subcellular Mrp2 localization were studied in perfused rat liver. Both, t-BOOH (0.5 mM) and CDNB (0.1mM) induced within 60 min a decrease of hepatic GSH levels by more than 90% and an almost complete cessation of bile flow. As revealed by confocal laser scanning microscopy, this cholestasis was accompanied by a loss of immunoreactive MRP2 from the canalicular membrane and its appearance inside the hepatocytes in putative intracellular vesicles. On the other hand, the intracellular distribution of dipeptidyl peptidase IV (DPPIV), another canalicular protein, and of zonula occludens associated polypeptide (ZO-1) remained unaffected, indicating selectivity of the Mrp2 retrieval pattern. Both, t-BOOH and CDNB induced a rapid net K+ efflux from the liver and a significant decrease of liver cell hydration. We conclude that severe glutathione depletion induces cholestasis by a retrieval of Mrp2, but not of DPPIV from the canalicular membrane. The underlying mechanism is unclear; however, a decrease in liver cell hydration, which occurs under these conditions, may contribute to this effect.

REGULATION OF BILE SALT EXPORT PUMP MRNA LEVELS BY DEXAMETHASONE AND OSMOLARITY IN CULTURED RAT HEPATOCYTES

Abstract The major canalicular bile salt export pump (Bsep) of mammalian liver is downregulated by endotoxin. This study reports on the effects of dexamethasone and osmolarity on Bsep mRNA expression in cultured rat hepatocytes and its functional relevance in rat liver. Expression of Bsep mRNA in rat hepatocytes 24 and 48 h after isolation was dependent on the presence of dexamethasone (100nm) in the culture medium. Bsep was functionally active at the pseudocanalicular membrane in cells cultured for 4 days in medium containing dexamethasone. Hypoosmolarity (205 mosmol/l) led to an induction of Bsep mRNA levels, whereas expression was decreased by hyperosmolarity (405 mosmol/l). Also the decay of Bsep mRNA following dexamethasone withdrawal was osmosensitive. In rat liver, dexamethasone counteracted the lipopolysaccharide (LPS)-induced down-regulation of Bsep mRNA levels after 12 hours and abolished the LPS-induced inhibition of taurocholate excretion. These results indicate that glucocorticoids are strong inducers of Bsep in liver. Furthermore, Bsep mRNA levels are osmosensitively regulated. The data suggest a long-term control of Bsep mRNA by osmolarity in addition to the short-term effects on canalicular bile acid excretion, which were reported recently.

Oligonucleotide microarray analysis of differential transporter regulation in the regenerating rat liver

Abstract: Aims: The aim of this study was to investigate the regulation of hepatic transport systems during liver regeneration.

Taurine supplementation induces multidrug resistance protein 2 and bile salt export pump expression in rats and prevents endotoxin-induced cholestasis

The effect of oral taurine supplementation on endotoxin-induced cholestasis was investigated in rat liver. At 12h following lipopolysaccharide (LPS) injection (4mg/kg body weight i.p.) bile flow and bromosulfophthalein (BSP) and taurocholate (TC) excretion were determined in the perfused liver and the expression of the canalicular transporters multidrug resistance protein 2 (Mrp2) and bile salt export pump (Bsep) was analyzed. Injection of LPS induced a significant decrease of bile flow ( 2.2+/-0.2 microl/g liver wet weight/min vs 3.3+/-0.1 microl/g liver wet weight in controls), biliary BSP excretion (10.8+/-2.2 nmol/g/min vs 21.0+/-3.8 nmol/g/min), and biliary TC excretion (114+/-23 nmol/g/min vs 228+/-8 nmol/g/min). These effects were due to transporter retrieval from the canalicular membrane and downregulation of Mrp2 and Bsep expression. In taurine-supplemented rats bile flow was 30% higher than that in untreated rats and the expression of Mrp2 and Bsep protein was increased two- to threefold. In taurine-supplemented rats there was no significant reduction of bile flow or of BSP and TC excretion at 12h following LPS injection. This protective effect of taurine was due to higher Mrp2 and Bsep protein levels compared to nonsupplemented LPS-treated rats, whereas relative Mrp2 retrieval from the canalicular membrane induced by LPS was not significantly different. LPS-induced tumor necrosis factor alpha and interleukin-1beta release were lower in taurine-fed rats; however, downregulation of Mrp2 and Bsep expression by LPS was delayed but not prevented. The data show that oral supplementation of taurine induces Mrp2 and Bsep expression and may prevent LPS-induced cholestasis.

Betaine is an osmolyte in RAW 264.7 mouse macrophages

Hyperosmotic (405 mosmol/l) exposure of RAW 264.7 mouse macrophages led to a stimulation of betaine uptake and an increase in betaine transporter (BGT‐1) mRNA levels. Conversely, hypoosmotic (205 mosmol/l) exposure decreased betaine uptake and diminished BGT‐1 mRNA levels. Betaine uptake was Na+‐dependent and was inhibited by about 90% by GABA, whereas inhibition by methylaminoisobutyrate and myoinositol was less than 15%. Addition of betaine strongly diminished BGT‐1 mRNA levels in cells exposed to normoosmotic or hyperosmotic media. When mouse macrophages were preloaded with betaine, lowering of the extracellular osmolarity was followed by a rapid betaine efflux from the cells. This study identifies a constitutively expressed and osmosensitive betaine transporter in RAW 264.7 macrophages and the use of betaine as an osmolyte in these cells.