HEPATOLOGY HIGHLIGHTS *
Authors: Neil Kaplowitz
Journal: Hepatology (2005)

An imbalance of the action of vasoconstrictors and vasodilators is believed to determine the hemodynamic complications of cirrhosis. Three vasodilator gases are known: NO, CO and H2S. The role of the latter rotten-egg, foul-smelling gas, produced from cysteine by transulfuration enzymes, is ill-defined. Fiorucci et al. confirmed that normal liver homogenate produces H2S from cysteine mainly by cystathionine-γ-lyase (CSE) which is inhibited by propargylglycine. In the perfused liver, the increased perfusion pressure in response to norepinephrine was inhibited by 100μmol/L H2S or 100μmol/L L-cysteine. The vasorelaxant effect of H2S was not mediated by NO. CSE was expressed in stellate cells, but not endothelial cells whereas both CBS and CSE were expressed in hepatocytes (See Fig.). Liver CSE expression and H2S levels were decreased in the liver with cirrhosis (bile duct ligation or CC14). These livers showed heightened sensitivity to norepinephrine vasoconstriction which was reduced by H2S, but not by L-cysteine (presumably due to decreased CSE). Activated stellate cells downregulated CSE. Contraction of these cells was relaxed by H2S but not cysteine, reflecting loss of CSE. It is believed that H2S opens KATP channels in vascular smooth muscle and thus relaxation by H2S was inhibited by KATP channel blocker, glibenclamide. A picture emerges in which NO is released from endothelial cells by shear stress and H2S is released by stellate cells. Downregulation of CSE in activated stellate cells in cirrhosis may lead to enhanced contraction of stellate cells around sinusoids, contributing to portal hypertension. Like NO and CO, H2S is both a good gas and a bad gas (excess); further work in this exciting area is anticipated to define the regulation and dysregulation of H2S. (See HEPATOLOGY 2005;42:539-548.) HIP/PAP is a C-type lectin gene product which has been suggested to promote liver regeneration and inhibit apoptosis. Lieu et al. assessed the role of this lectin in several models. HIP/PAP-expressing and wildtype hepatocytes were implanted in the spleen; partial hepatectomy (PH) 1 month later was associated with greater rate of regeneration in mice implanted with HIP/PAP cells. Direct injection of HIP/PAP into the spleen also accelerated regeneration. HIP/PAP accelerated the time course (activation/inactivation) of STAT3 and accelerated the time course of TNF production after PH. HIP/PAP transgenic mice and wildtype pretreated with HIP/PAP were resistant to lethal acetaminophen (APAP) toxicity (See Fig.). HIP/PAP transgenic mice given a sublethal dose of APAP exhibited less injury, accompanied by less GSH depletion, less lipid peroxidation, greater expression of SOD, and less cytochrome c release and caspase 3 activation. This work supports a paracrine effect of HIP/PAP on liver regeneration. The mechanism of protection against APAP is less certain. The authors propose an antioxidant role which needs direct confirmation. Furthermore, the effect of HIP/PAP on cyp2e1 and covalent binding needs to be addressed to exclude an effect on APAP activation. Finally, the role of apoptosis is APAP toxicity is controversial and it is not clear that protection by HIP/PAP is mediated by its anti-apoptotic effect. (See HEPATOLOGY 2005;42:618-626.) The accumulation of fat in the liver is a key step in the pathogenesis of alcoholic liver disease. Previous work has demonstrated that a diet high in saturated fat prevents alcohol-induced fatty liver. You et al. studied the mechanism of this protection by comparing mice fed a Leiber–DiCarli diet enriched in unsaturated fat (PUFA) versus saturated fat (HSF). As expected HSF diet prevented ethanol-induced triglyceride accumulation and increased ALT. Plasma adiponectin levels were slightly decreased by PUFA ethanol diet but nearly doubled in HSF-ethanol fed mice. The HSF + ethanol diet doubled phosphorylation of AMP kinase-α (activation) and acetyl CoA carboxylase (ACC) (inhibition) which were prevented by the HSF replacement. At the same time PUFA-ethanol decreased expression of PPARα genes and fatty acid oxidation which were enhanced by HSF-ethanol. Since PGC-1α is a co-activator of PPARα, they examined its expression which correlated with the changes in fatty acid oxidation. Then, in hepatoma cells they demonstrated that adiponectin regulates expression of PGC-1α, PPARα and prevents the inhibition of phosphorylation of AMPK by ethanol. Using adipocytes, PUFA + ethanol decreased whereas HSF + ethanol stimulated adiponectin expression. This elegant work strongly suggests that the lipid composition of the diet influences ethanol's effect on adiponectin production. Increased adiponectin in the HSF-ethanol condition allows the liver to enhance fatty acid oxidation by stimulating AMPK and PPARα pathways, thereby preventing ethanol induced fatty liver. Thus, this work provides a likely explanation for the protection afforded by HSF. However, the primary mechanism for the development of fatty liver is not addressed in this study, although other work by these authors in rats fed ethanol suggests a role for impaired AMPK action, although in the present mouse studies the decreases in P-AMPK, P-ACC, adiponectin and fatty acid oxidation in the PUFA-ethanol fed mice (vs. PUFA fed) were modest. In addition, it will be of interest to see the effect of HSF on ethanol induced increased expression and processing of SREBP-1c which regulates lipid synthesis. (See HEPATOLOGY 2005;42:568-577.) Mice with fatty liver are more susceptible to LPS-induced injury. Previous work in leptin-deficient ob/ob mice has related this enhanced sensitivity to LPS to depletion of NKT cells in the liver with concomitant imbalance in Th-1 (pro-) and Th-2 (anti-inflammatory) cytokines favoring Th-1. Li et al. have now extended this work to a more clinically relevant model, overfed hyperleptinemic obese wild type mice which develop fatty liver. High fat and/or high sucrose diet induced a selective decrease in NKT cells (not NK cells) in the liver but not in the spleen. T cells and NKT cells from high-fat fed mice produced more TNF and IFNγ; serum IFNγ was increased in mice fed the high-fat diet. The high fat/high sucrose diet sensitized to LPS hepatotoxicity. The decrease in NKT cells correlated with increase IL-12 which is known to decrease NKT cells viability and indeed the NKT cells isolated from the fatty liver exhibited significant increased apoptosis, although modest in magnitude. This work clearly shows that experimental diet-induced NAFLD is associated with a disturbance in Th1-Th2 balance which is associated with increased susceptibility to LPS induced inflammation and injury. Since NKT cell depletion was accompanied by increased TNF and IFNγ production by these cells it is not clear whether their depletion or activation is critical. The findings in this report should provide a useful model to verify how the associated changes in IL-12, NKT cells and cytokines are mechanistically linked. In addition, further work is needed to prove that the depletion of NKT cells causes the increased susceptibility to LPS. (See HEPATOLOGY 2005;42:880-885.) The major metabolite of acetaminophen (APAP) is the nontoxic glucuronide. Since MRP3 in the basolateral membrane of hepatocytes preferentially transports glucuronide, Manautou et al. examined its role in APAP disposition and toxicity by comparing wild type and Mrp3 null mice. No difference was seen in biliary excretion of APAP and sulfate and GSH conjugates but null mice markedly accumulated the glucuronide in the liver in conjunction with low plasma levels. Increased biliary excretion of the glucuronide could not compensate for the decreased sinusoidal excretion. Interestingly, Mrp3−/− mice were more resistant to APAP toxicity (Fig.) which correlated with faster repletion of hepatic GSH. This work clearly demonstrates that Mrp3 is responsible for APAP-glucuronide secretion into plasma. In addition, it illustrates a new and important concept, i.e., alteration of the export of drug metabolites formed in the liver (so called phase 3 of drug metabolism) can alter the susceptibility to drug-induced liver injury. Although complete absence of Mrp3 protected against APAP toxicity it will be of interest to know if lesser variations can influence susceptibility (e.g., heterozygote +/− mice). In addition, more work is needed to address the mechanism for apparent increased GSH synthesis in null mice. Does GSH recover faster in response to other GSH depletors (e.g., diethyl maleate) or is this specifically related to APAP-glucuronide accumulation? In either case, which step in GSH synthesis is affected and how? (See HEPATOLOGY 2005;42:1091-1098.) CAR is a nuclear hormone family transcription factor which responds to phenobarbital. CAR agonists have been shown to induce hepatocyte hyperplasia. One immediate early response gene induced by both CAR agonists and partial hepatectomy is Gadd45β. Gadd45β is known to be induced by TNF/NF-κB pathway. Therefore, Columbano et al. examined whether the response to CAR ligands involves the latter pathway. They examined NF-κB and Gadd45β response to the direct CAR agonist, TCPOBOP, in wild type, TNFR-1 null and TNFR-1/TNFR-2 null mice. TCPOBOP did not activate NF-κB in any of these mouse genotypes but strongly induced Gadd45β in all three. Gadd45β expression and late phase of hepatocyte proliferation was more greatly induced by TCPOBOP in both TNFR null mice. CAR null mice did not respond to TCPOBOP with respect to Gadd45β expression or proliferation, verifying that the response to TCPOBOP is CAR-specific. Thus, two distinct pathways for Gadd45β induction were identified: a regenerative pathway involving TNF and NF-κB and a proliferative (hyperplasia) pathway involving CAR independent of TNF and NF-κB. More work is anticipated on how CAR regulates Gadd45β (directly or indirectly) and on the specific role of Gadd45β (e.g., known effect of turning off JNK) in regeneration and proliferation. (See HEPATOLOGY 2005;42:1118-1126.)