In the early stages of nonalcoholic fatty liver disease (NAFLD), triglycerides accumulate in hepatocytes. Diacylglycerol acyltransferase 2 (DGAT2) catalyzes the final step in hepatocyte triglyceride biosynthesis. DGAT2 antisense oligonucleotide (ASO) treatment improved hepatic steatosis dramatically in a previous study of obese mice. According to the 2-hit hypothesis for progression of NAFLD, hepatic steatosis is a risk factor for nonalcoholic steatohepatitis (NASH) and fibrosis. To evaluate this hypothesis, we inhibited DGAT2 in a mouse model of NASH induced by a diet deficient in methionine and choline (MCD). Six-week-old genetically obese and diabetic male db/db mice were fed either the control or the MCD diet for 4 or 8 weeks. The MCD diet group was treated with either 25 mg/kg DGAT2 ASO or saline intraperitoneally twice weekly. Hepatic steatosis, injury, fibrosis, markers of lipid peroxidation/oxidant stress, and systemic insulin sensitivity were evaluated. Hepatic steatosis, necroinflammation, and fibrosis were increased in saline-treated MCD diet-fed mice compared to controls. Treating MCD diet-fed mice with DGAT2 ASO for 4 and 8 weeks decreased hepatic steatosis, but increased hepatic free fatty acids, cytochrome P4502E1, markers of lipid peroxidation/oxidant stress, lobular necroinflammation, and fibrosis. Progression of liver damage occurred despite reduced hepatic expression of tumor necrosis factor alpha, increased serum adiponectin, and striking improvement in systemic insulin sensitivity. Conclusion: Results from this mouse model would suggest accumulation of triglycerides may be a protective mechanism to prevent progressive liver damage in NAFLD. (HEPATOLOGY 2007;45:1366-1374 N onalcoholic fatty liver disease (NAFLD) is one of the most common liver diseases in the world. 1-3 Clinically, the most common disorder associated with NAFLD is insulin resistance. 4 Accumulation of triglycerides in hepatocytes is the hallmark of NAFLD. Recent studies have demonstrated that acyl-coenzyme A:diacylglycerol acyltransferase 2 (DGAT2) plays an important role in hepatocyte triglyceride synthesis and hepatic steatosis. 5,6 Triglyceride synthesis is increased in the fatty livers that accompany obesity and type 2 diabetes in humans and mice. 7 According to the 2-hit hypothesis for NAFLD progression, hepatic steatosis is a risk factor for nonalcoholic steatohepatitis (NASH) and fibrosis. 8,9 In a previous study, knocking down DGAT2 in the livers of mice with diet-induced obesity (DIO) and diabetes successfully prevented hepatic steatosis. 10 Because only mild NASH and little fibrosis develop in mice with DIO, however, that model is not helpful for determining if inhibiting steatosis prevents progression of NAFLD to more advanced stages of liver damage (i.e., NASH and liver fibrosis).To address this question, we studied a recently described model of progressive obesity-related NASH in db/db mice. 11,12 Db/db mice spontaneously develop obe- Gastroenterology, Snyderman GSRB I, Suite 1073, 595 LaSalle Street, Box 3256,...

In bile duct-ligated (BDL) rodents, as in humans with chronic cholangiopathies, biliary obstruction triggers proliferation of bile ductular cells that are surrounded by fibrosis produced by adjacent myofibroblastic cells in the hepatic mesenchyme. The proximity of the myofibroblasts and cholangiocytes suggests that mesenchymal-epithelial crosstalk promotes the fibroproliferative response to cholestatic liver injury. Studying BDL mice, we found that bile duct obstruction induces activity of the Hedgehog (Hh) pathway, a system that regulates the viability and differentiation of various progenitors during embryogenesis. After BDL, many bile ductular cells and fibroblastic-appearing cells in the portal stroma express Hh ligands, receptor and/or target genes. Transwell cocultures of an immature cholangiocyte line that expresses the Hh receptor, Patched (Ptc), with liver myofibroblastic cells demonstrated that both cell types produced Hh ligands that enhanced each other's viability and proliferation. Further support for the concept that Hh signaling modulates the response to BDL was generated by studying PtcLacZ mice, which have an impaired ability to constrain Hh signaling due to a heterozygous deficiency of Ptc. After BDL, PtcLacZ mice upregulated fibrosis gene expression earlier than wild-type controls and manifested an unusually intense ductular reaction, more expanded fibrotic portal areas, and a greater number of lobular necrotic foci. Our findings reveal that adult livers resurrect developmental signaling systems, such as the Hh pathway, to guide remodeling of the biliary epithelia and stroma after cholestatic injury.

Hedgehog signaling through its receptor, Patched, activates transcription of genes, including Patched, that regulate the fate of various progenitors. Although Hedgehog signaling is required for endodermal commitment and hepatogenesis, the possibility that it regulates liver turnover in adults had not been considered because mature liver epithelial cells lack Hedgehog signaling. Herein, we show that this pathway is essential throughout life for maintaining hepatic progenitors. Patched-expressing cells have been identified among endodermally lineage-restricted, murine embryonic stem cells as well as in livers of fetal and adult Ptc-lacZ mice. An adult-derived, murine hepatic progenitor cell line expresses Patched, and Hedgehog-responsive cells exist in stem cell compartments of fetal and adult human livers. In both species, manipulation of Hedgehog activity influences hepatic progenitor cell survival. Therefore, Hedgehog signaling is conserved in hepatic progenitors from fetal development through adulthood and may be a new therapeutic target in patients with liver damage.

Liver injury activates quiescent hepatic stellate cells (Q-HSC

The impaired regenerative capacity of fatty livers might promote the progression of nonalcoholic fatty liver disease (NAFLD). To identify mechanisms involved, regenerative responses were compared in normal mice and ob/ob mice (a model for NAFLD) after partial hepatectomy (PH). We hypothesized that the usual PH activation of oxidant-sensitive, growth-regulatory kinase cascades would be abnormal in fatty hepatocytes, which have adapted to chronic oxidant stress, and expected that this might interfere with the induction of proliferative-and stress-related genes. The normal coordinated induction of Jun N-terminal kinases (Jnks) and extracellular regulated kinases (Erks) does not occur after PH in ob/ob mice, which cannot activate Jnks but can superinduce Erks. Jnk inhibition is associated with enhanced activation of Akt, which inhibits phosphoenolpyruvate carboxykinase ( Obesity and type 2 diabetes are risk factors for nonalcoholic fatty liver disease (NAFLD), 1-3 a spectrum of liver pathology that resembles alcohol-induced liver damage. 4 Fatty liver is the earliest and most prevalent stage of NAFLD. 5 Hepatic steatosis generally has a benign outcome, 6 but some individuals with this lesion develop progressive liver injury and eventually experience significant liver-related morbidity and increased mortality. 1 The mechanisms that promote disease progression in NAFLD are poorly understood. It has been suggested that more advanced stages of NAFLD might be triggered when an acute inflammatory insult, i.e., "second hit," is superimposed on hepatic steatosis. 7 Because healthy livers typically regenerate and recover completely from acute inflammation, 8 this suggests that the normal regenerative response to injury might be impaired in fatty livers.Hepatic regenerative capacity is often assessed experimentally by monitoring the response to PH. 8 Liver regeneration after two thirds (partial) hepatectomy (PH) is inhibited in obese, diabetic Zucker fa/fa rats with fatty livers, 9 supporting the concept that the liver's ability to regenerate is decreased in NAFLD. However, the mechanism(s) that impair liver regeneration in fatty livers have not been identified. The present study uses PH as a tool to compare the regenerative activation of oxidant-sensitive, growth-regulatory protein kinase cascades in genetically obese, ob/ob mice, an animal model for NAFLD, 10 and lean control mice. After PH, these enzymes are normally activated by cytokines and growth factors, 11,12 and their interactions help to orchestrate the changes in hepatocyte gene expression [13][14][15][16][17][18][19] that are required to maintain hepatocyte viability, 20,21 induce proliferation, 19,22-24 and preserve tissue-specific functions 18,19 during the regenerative response. We hypothesize that the normal, acute oxidant-mediated, activation of the mitogen-and stress-activated protein kinases will be disturbed in fatty livers, which over-produce oxidants chronically. 25 If this concept is confirmed, then secondary objectives are to identify possible mechani...

Hepatic stellate cells (HSC) have a complex phenotype that includes both neural and myofibroblastic features. The Hedgehog (Hh) pathway has been shown to direct the fate of neural and myofibroblastic cells during embryogenesis and during tissue remodeling in adults. Therefore, we hypothesized that Hh signaling may regulate the fate of HSC in adults. In this study, we find that freshly isolated stellate cells from adult PatchedlacZ transgenic mice exhibit b-galactosidase activity, indicating Hh pathway activity. Transcripts of Hh ligands, the Hh pathway receptor, and Hh-regulated transcription factors are expressed by stellate cells from mice, rats, and humans. Transfection experiments in a cell line using a Hh-inducible luciferase reporter demonstrate constitutive Hh pathway activity. Moreover, neutralizing antibodies to Hh increase apoptosis, while viability is restored by treatment with Hh ligand. In vitro treatment of primary stellate cells with cyclopamine (Cyc), a pharmacologic inhibitor of the Hh pathway, inhibits activation and slightly decreases cell survival, while a single injection of Cyc into healthy adult mice reduces activation of HSC by more than 50% without producing obvious liver damage. Our findings reveal a novel mechanism, namely the Hh pathway, that regulates the activation and viability of HSC.

Both myofibroblastic hepatic stellate cells (HSC) and hepatic epithelial progenitors accumulate in damaged livers. In some injured organs, the ability to distinguish between fibroblastic and epithelial cells is sometimes difficult because cells undergo epithelial-mesenchymal transitions (EMT). During EMT, cells coexpress epithelial and mesenchymal cell markers. To determine whether EMT occurs in adult liver cells, we analyzed the expression profile of primary HSC, two HSC lines, and hepatic epithelial progenitors. As expected, all HSC expressed HSC markers. Surprisingly, these markers were also expressed by epithelial progenitors. In addition, one HSC line expressed typical epithelial progenitor mRNAs, and these epithelial markers were inducible in the second HSC line. In normal and damaged livers, small ductular-type cells stained positive for an HSC marker. In conclusion, HSC and hepatic epithelial progenitors both coexpress epithelial and mesenchymal markers, providing evidence that EMT occurs in adult liver cells.

Hepatic steatosis may have a generally benign prognosis, either because most hepatocytes are not significantly injured or mechanisms to replace damaged hepatocytes are induced. To determine the relative importance of these mechanisms, we compared hepatocyte damage and replication in ethanol-fed and ob/ob mice with very indolent fatty liver disease to that of healthy control mice and PARP-1 -/-mice with targeted disruption of the DNA repair enzyme, poly(ADP-ribose) polymerase. Compared to the healthy controls, both groups with fatty livers had significantly higher serum alanine aminotransferase values, hepatic mitochondrial H 2 O 2 production, and hepatocyte oxidative DNA damage. A significantly smaller proportion of the hepatocytes from fatty livers entered S phase when cultured with mitogens. Moreover, this replicative senescence was not reversed by treating cultured hepatocytes with agents (i.e., betaine or leptin) that improve liver disease in intact ethanol-fed or leptindeficient mice. Hepatocytes from PARP1 -/-mice also had more DNA damage and reduced DNA synthesis in response to mitogens. However, neither mice with fatty livers nor PARP-1-deficient mice had atrophic livers. All of the mice with senescent mature hepatocytes exhibited hepatic accumulation of liver progenitor (oval) cells and oval cell numbers increased with the demand for hepatocyte replacement. Therefore, although hepatic oxidant production and damage are generally increased in fatty livers, expansion of hepatic progenitor cell populations helps to compensate for the increased turnover of damaged mature hepatocytes. In conclusion, these results demonstrate that induction of mechanisms to replace damaged hepatocytes is important for limiting the progression of fatty liver disease. (HEPATOLOGY 2004;39:403-411.)