Hepatology highlights
Authors: Neil Kaplowitz
Journal: Hepatology (2006)

A brief period of ischemia/reperfusion (I/R) has been identified as a useful strategy to generate an adaptive protective response [ischemic preconditioning, (IPC)] which induces resistance to subsequent liver injury from prolonged I/R as would occur in the setting of OLT or hepatic resection. Izuishi et al. explored the contribution of the PI3 kinase/Akt (PKB) pathway, known to exert cytoprotective effects. PI3-kinase inhibitors given intravenously at the time of reperfusion abrogated the protective effect of IPC and the associated appearance of activated phospho-Akt. Downstream targets of Akt kinase such as Bad and GSK-3β, exhibited enhanced phosphorylation in the IPC followed by prolonged I/R group which was blocked by the upstream PI3-kinase inhibitor wortmannin. Bad and GSK-3β phosphorylation inactivates these mediators of cell death. Furthermore, IPC decreased sustained JNK phosphorylation and NF-κB activation, which was reversed by wortmannin. Administration of SOD before IPC inhibited Akt activation after I/R, pointing to IPC-induced oxidative stress as the upstream initiator of the protective pathway. In conclusion, this study demonstrates that IPC primes the PI3-kinase/Akt pathway to respond rapidly to the subsequent reperfusion after prolonged ischemia, and this response is critical in promoting survival. IPC-induced oxidative stress appears to prime this protective pathway (see Fig.) although the mechanism remains to be elucidated. In addition, it will be important to determine if the Akt-related down-regulation of NF-κB and JNK is important in inhibiting cytokine production by Kupffer cells, because considerable evidence supports the role of cytokines in promoting inflammation in the I/R injury. (See HEPATOLOGY 2006;44:573-580.) Alpha-1-antitrypsin deficiency leads to progressive liver injury as a consequence of accumulation of mutant α1-ATZ glycoprotein. One puzzling feature of this genetic disorder is that only 10%-15% of homozygous infants develop overt liver damage. In an attempt to identify environmental factors that modify this disease, Rudnick et al. assessed the effect of the NSAID indomethacin in male PiZ mice. Hepatocellular proliferation was increased by indomethacin in the mutant mice. This appears to be a response to increased injury as reflected in increased caspase 9 activation in PiZ mice given indomethacin. The mechanism for increased injury and proliferation appears to be related to increased expression of the mutant gene and accumulation of the mutant protein in globule-containing polymers in response to indomethacin. The possibility of NSAID-induced inhibition of proteasomal or autophagic activity was ruled out as a cause of α1-ATZ polymer accumulation. Because α1-antitrypsin is an acute phase protein, the authors considered the possibility that indomethacin blocked the prostaglandin-mediated suppression of the IL-6–STAT3 cytokine signaling pathway. Indeed, indomethacin-treated mutant mice exhibited a 10-fold increase of IL-6 in serum and an increase of activated phospho-STAT3 in liver (see Fig.). Thus, indomethacin up-regulates a cytokine-stimulated pathway which induced the expression of the α1-ATZ mutant protein. Increased accumulation of the mutant protein then promotes injury and consequent proliferation of hepatocytes. In conclusion, this is an important study that provides serious concerns about the use of NSAIDs in patients with α1-ATZ deficiency. Furthermore, it would be interesting to see if an adverse effect of NSAIDs is seen in heterozygous mice or humans. NSAIDs appear to increase the accumulation of polymeric mutant protein in hepatocytes exhibiting globules. The accumulation is due to an acute-phase-mediated increased expression of the mutant protein. No explanation is provided for the augmented IL-6 response to indomethacin in the mutant mice (perhaps primed Kupffer cells are responsible). The study illustrates how an environmental factor (i.e., a drug) can potentially influence the course or severity of this disease. (See HEPATOLOGY 2006;44:976-982.) A key issue in immune-mediated liver disease is how T lymphocytes interact with hepatocytes in the presence of a sinusoidal endothelial cell (SEC) barrier. Warren et al. studied this interaction using electron microscopy to assess whether hepatocytes or lymphocytes have projections which pass through the fenestrae or through gaps between SECs. Naïve CD8+ T cells exhibited cytoplasmic extensions which protruded through fenestrae to contact hepatocyte microvilli in the space of Disse. About 11% of intrahepatic lymphocytes found in the lumen had a mean of 5 of these extensions on single sections (see Fig.). In addition, hepatic microvilli protruded through fenestrae into the sinusoidal lumen, but no contact with lymphocytes was observed. No gaps between SECs were seen. LFA-1 was expressed on lymphocyte villae. On the other hand MHC/peptide complexes and ICAM-1, which are two molecules required for T cell activation, were distributed on the sinusoidal membrane of hepatocytes. Blocking ICAM interaction with LFA-1 using Fab fragments inhibited antigen-specific retention of T cells in liver sinusoids. Thus, intrahepatic lymphocytes and naïve T cells insert extensions through fenestrae in SECs which directly contact hepatocytes. One can speculate that hepatocytes can function as antigen-presenting cells in this scenario. The implications of transendothelial hepatocyte-lymphocyte interaction are of considerable importance. High local polarized expression of MHC-I and ICAM-1 on the basolateral surface of hepatocytes favors the role of hepatocytes as antigen-presenting cells which can activate T cells leading either to reduced survival (tolerance) or maturation into effector T cells. Furthermore, the loss of fenestrae in cirrhosis or aging could alter immune responses. (See HEPATOLOGY 2006;44:1182-1190.) Mounting evidence suggests that the liver is an immunomodulatory organ. Preliminary results from in vitro studies suggest that one mechanism for the immune system's tolerance to the liver is apoptosis of T cells induced by stellate cells. Chen et al. examined the significance of stellate cells in immune modulation. Activated mouse stellate cells in culture inhibited a mixed lymphocyte reaction. To assess the effect of stellate cells in vivo, activated or quiescent stellate cells were co-administered with pancreatic islet transplants (subcapsular kidney insertion). The activated stellate cells prolonged the survival of the insulin-expressing islet grafts and produced prolonged euglycemia of diabetic recipient mice. Nephrectomy led to return of hyperglycemia. A systemic immunosuppressive effect was excluded. Histological examination of the long-term surviving islets revealed encapsulation of the insulin-positive islets (red) by multiple layers of αSMA+ stellate cells (green) (see Fig.). CD4+ and CD8+ cells were reduced in islet grafts co-transplanted with stellate cells due to enhanced apoptosis of infiltrating cells. Previous work showed that activated, but not quiescent, stellate cells express increased B7-H1. Co-transplantation of activated stellate cells from B7-H1−/− mice lost their protective effect against graft rejection. B7-H1 ligates a receptor (PD-1) on activated T cells which suppresses cytokine production and promotes activated T cell apoptosis. Aside from the practical direct application of this work to islet cell transplantation, important questions are raised about liver tolerance. For example, if activated stellate cells protect hepatocytes from immune attack, will antifibrosis strategies directed at enhancing stellate cell apoptosis worsen immune attack on the liver. B7-H1 may not be the only stellate cell immunomodulator, as immunoneutralization of B7-H1 may activate naïve T cells. In contrast to immune tolerance of whole liver transplantation, isolated hepatocyte allografts are rapidly killed. Therefore, will combining stellate cells with hepatocytes improve the survival of hepatocyte transplants? (See HEPATOLOGY 2006;44:1171-1181.) Uncoupling protein 2 (UCP2) is an inner mitochondrial membrane protein which mediates proton leak when activated by oxidative stress, competing with ATP synthase for the electrochemical energy provided by the proton gradient, thereby decreasing ATP production. UCP2 is normally expressed in Kupffer cells but is markedly increased in hepatocytes in fatty liver. Theoretically, although increased UCP2 would decrease ATP, it can decrease superoxide production by lowering mitochondrial membrane potential. To assess whether UCP2 expression is harmful or protective, Fülöp et al. challenged ob/ob:ucp2+/+ versus ob/ob:ucp2−/− mice with agonistic anti-Fas (Jo2) monoclonal antibody. Ob/ob:ucp2−/− mice exhibited less liver injury, better survival, and less apoptosis in response to Jo2 compared to Ob/ob:ucp2+/+ mice. In addition, less necrosis was seen in ucp2-deficient mice, suggesting that less energy compromise in ucp−/− mice favors apoptosis over necrosis. Indeed, after high doses of Jo2, ATP was less depleted in the absence of UCP2 (see Fig.). The protection by knockout (KO) of ucp2 was not due to a difference in Fas expression. No difference in lipid peroxidation (MDA) was seen. Ob/ob mice expressed lower ucp2 in Kupffer cells, whereas higher expression was seen in hepatocytes. The authors speculate that low Kupffer cells UCP2 in ob/ob:ucp +/+ would minimize the impact of ucp2 KO with respect to reactive oxygen species production by Kupffer cells; this interesting hypothesis needs to be verified experimentally. This paper demonstrates that ucp2 in the steatotic hepatocytes of ob/ob makes the liver more vulnerable to Fas-induced cell death. The protection afforded by the absence of ucp2 appears to be linked to better preservation of ATP, although this is a correlation which does not prove causality. Does ucp2 KO preserve ATP leading to less injury, or does ucp2 KO decrease liver injury which preserves ATP? (See HEPATOLOGY 2006;44:592-601.)