Titia E. Vrenken

Tauroursodeoxycholic acid protects rat hepatocytes from bile acid-induced apoptosis via activation of survival pathways.

Ursodeoxycholic acid (UDCA) is used in the treatment of cholestatic liver diseases, but its mechanism of action is not yet well defined. The aim of this study was to explore the protective mechanisms of the taurine‐conjugate of UDCA (tauroursodeoxycholic acid [TUDCA]) against glycochenodeoxycholic acid (GCDCA)‐induced apoptosis in primary cultures of rat hepatocytes. Hepatocytes were exposed to GCDCA, TUDCA, the glyco‐conjugate of UDCA (GUDCA), and TCDCA. The phosphatidylinositol‐3 kinase pathway (PI3K) and nuclear factor‐κB were inhibited using LY 294002 and adenoviral overexpression of dominant‐negative IκB, respectively. The role of p38 and extracellular signal‐regulated protein kinase mitogen‐activated protein kinase (MAPK) pathways were investigated using the inhibitors SB 203580 and U0 126 and Western blot analysis. Transcription was blocked by actinomycin‐D. Apoptosis was determined by measuring caspase‐3, ‐9, and ‐8 activity using fluorimetric enzyme detection, Western blot analysis, immunocytochemistry, and nuclear morphological analysis. Our results demonstrated that uptake of GCDCA is needed for apoptosis induction. TUDCA, but not TCDCA and GUDCA, rapidly inhibited, but did not delay, apoptosis at all time points tested. However, the protective effect of TUDCA was independent of its inhibition of caspase‐8. Up to 6 hours of preincubation with TUDCA before addition of GCDCA clearly decreased GCDCA‐induced apoptosis. At up to 1.5 hours after exposure with GCDCA, the addition of TUDCA was still protective. This protection was dependent on activation of p38, ERK MAPK, and PI3K pathways, but independent of competition on the cell membrane, NF‐κB activation, and transcription. In conclusion, TUDCA contributes to the protection against GCDCA‐induced mitochondria‐controlled apoptosis by activating survival pathways. Supplemental material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270‐9139/supplmat/index.html). (HEPATOLOGY 2004;39:1563–1573.)

Directed Evolution of Bacillus subtilis Lipase A by Use of Enantiomeric Phosphonate Inhibitors: Crystal Structures and Phage Display Selection

Phage display can be used as a protein‐engineering tool for the selection of proteins with desirable binding properties from a library of mutants. Here we describe the application of this method for the directed evolution of Bacillus subtilis lipase A, an enzyme that has important properties for the preparation of the pharmaceutically relevant chiral compound 1,2‐O‐isopropylidene‐sn‐glycerol (IPG). PCR mutagenesis with spiked oligonucleotides was employed for saturation mutagenesis of a stretch of amino acids near the active site. After expression of these mutants on bacteriophages, dual selection with (S)‐(+)‐ and (R)‐(−)‐IPG stereoisomers covalently coupled to enantiomeric phosphonate suicide inhibitors (SIRAN Sc and Rc inhibitors, respectively) was used for the isolation of variants with inverted enantioselectivity. The mutants were further characterised by determination of their Michaelis–Menten parameters. The 3D structures of the Sc and Rc inhibitor–lipase complexes were determined and provided structural insight into the mechanism of enantioselectivity of the enzyme. In conclusion, we have used phage display as a fast and reproducible method for the selection of Bacillus lipase A mutant enzymes with inverted enantioselectivity.

Carbon monoxide blocks oxidative stress-induced hepatocyte apoptosis via inhibition of the p54 JNK isoform.

Most chronic liver diseases are accompanied by oxidative stress, which may induce apoptosis in hepatocytes and liver injury. Oxidative stress induces heme oxygenase-1 (HO-1) expression. This stress-responsive cytoprotective protein is responsible for heme degradation into carbon monoxide (CO), free iron, and biliverdin. CO is an important intracellular messenger; however, the exact mechanisms responsible for its cytoprotective effect are not yet elucidated. Thus, we investigated whether HO-1 and CO protect primary hepatocytes against oxidative-stress-induced apoptosis. In vivo, bile duct ligation was used as model of chronic liver disease. In vitro, primary hepatocytes were exposed to the superoxide anion donor menadione in a normal and in a CO-- containing atmosphere. Apoptosis was determined by measuring caspase-9, -6, -3 activity and poly(ADP-ribose) polymerase cleavage, and necrosis was determined by Sytox green staining. The results showed that (1) HO-1 is induced in chronic cholestatic liver disease, (2) superoxide anions time- and dose-dependently induce HO-1 activity, (3) HO-1 overexpression inhibits superoxide-anions-induced apoptosis, and (4) CO blocks superoxide-anions-induced JNK phosphorylation and caspase-9, -6, -3 activation and abolishes apoptosis but does not increase necrosis. We conclude that HO-1 and CO protect primary hepatocytes against superoxide-anions-induced apoptosis partially via inhibition of JNK activity. CO could represent an important candidate for the treatment of liver diseases.

Metformin protects primary rat hepatocytes against oxidative stress-induced apoptosis

The majority of chronic liver diseases are accompanied by oxidative stress, which induces apoptosis in hepatocytes and liver injury. Recent studies suggest that oxidative stress and insulin resistance are important in the pathogenesis of nonalcoholic fatty liver disease (NAFLD) and the pathophysiology of diabetes complications. Metformin has been shown to be hepatoprotective in the insulin‐resistant and leptin‐deficient ob/ob mouse model of NAFLD. However, the mechanism involved in the protective effects of metformin has not been elucidated yet. Therefore, we investigated the protective effect of metformin against oxidative stress‐induced apoptosis. Primary rat hepatocytes were exposed to the oxidative stress‐generating compound menadione in the presence and absence of metformin. Apoptosis was determined by measuring caspase activity and poly(ADP‐ribose) polymerase (PARP)‐cleavage, and necrosis was measured by Sytox Green nuclear staining. We demonstrate that (1) Metformin inhibits menadione‐induced caspase‐9,‐6,‐3 activation and PARP‐cleavage in a concentration‐dependent manner. (2) Metformin increases menadione‐induced heme oxygenase‐1 (HO‐1) expression and inhibits c‐Jun N‐terminal kinase (JNK)‐phosphorylation. (3) Metformin does not induce necrosis in primary hepatocytes. Metformin protects hepatocytes against oxidative stress‐induced caspase activation, PARP‐cleavage and apoptosis. The anti‐apoptotic effect of metformin is in part dependent on HO‐1 and bcl‐xl induction and inhibition of JNK activation and independent of insulin signaling. Our results elucidate novel protective mechanisms of metformin and indicate that metformin could be investigated as a novel therapeutic agent for the treatment of oxidative stress‐related liver diseases.