Derrick M. Van Rooyen

NASH is an Inflammatory Disorder: Pathogenic, Prognostic and Therapeutic Implications.

While non-alcoholic fatty liver disease (NAFLD) is highly prevalent (15% to 45%) in modern societies, only 10% to 25% of cases develop hepatic fibrosis leading to cirrhosis, end-stage liver disease or hepatocellular carcinoma. Apart from pre-existing fibrosis, the strongest predictor of fibrotic progression in NAFLD is steatohepatitis or non-alcoholic steatohepatitis (NASH). The critical features other than steatosis are hepatocellular degeneration (ballooning, Mallory hyaline) and mixed inflammatory cell infiltration. While much is understood about the relationship of steatosis to metabolic factors (over-nutrition, insulin resistance, hyperglycemia, metabolic syndrome, hypoadiponectinemia), less is known about inflammatory recruitment, despite its importance for the perpetuation of liver injury and fibrogenesis. In this review, we present evidence that liver inflammation has prognostic significance in NAFLD. We then consider the origins and components of liver inflammation in NASH. Hepatocytes injured by toxic lipid molecules (lipotoxicity) play a central role in the recruitment of innate immunity involving Toll-like receptors (TLRs), Kupffer cells (KCs), lymphocytes and neutrophils and possibly inflammasome. The key pro-inflammatory signaling pathways in NASH are nuclear factor-kappa B (NF-κB) and c-Jun N-terminal kinase (JNK). The downstream effectors include adhesion molecules, chemokines, cytokines and the activation of cell death pathways leading to apoptosis. The upstream activators of NF-κB and JNK are more contentious and may depend on the experimental model used. TLRs are strong contenders. It remains possible that inflammation in NASH originates outside the liver and in the gut microbiota that prime KC/TLR responses, inflamed adipose tissue and circulating inflammatory cells. We briefly review these mechanistic considerations and project their implications for the effective treatment of NASH.

Hepatic Free Cholesterol Accumulates in Obese, Diabetic Mice and Causes Nonalcoholic Steatohepatitis

BACKGROUND & AIMS Type 2 diabetes and nonalcoholic steatohepatitis (NASH) are associated with insulin resistance and disordered cholesterol homeostasis. We investigated the basis for hepatic cholesterol accumulation with insulin resistance and its relevance to the pathogenesis of NASH. METHODS Alms1 mutant (foz/foz) and wild-type NOD.B10 mice were fed high-fat diets that contained varying percentages of cholesterol; hepatic lipid pools and pathways of cholesterol turnover were determined. Hepatocytes were exposed to insulin concentrations that circulate in diabetic foz/foz mice. RESULTS Hepatic cholesterol accumulation was attributed to up-regulation of low-density lipoprotein receptor via activation of sterol regulatory element binding protein 2 (SREBP-2), reduced biotransformation to bile acids, and suppression of canalicular pathways for cholesterol and bile acid excretion in bile. Exposing primary hepatocytes to concentrations of insulin that circulate in diabetic Alms1 mice replicated the increases in SREBP-2 and low-density lipoprotein receptor and suppression of bile salt export pump. Removing cholesterol from diet prevented hepatic accumulation of free cholesterol and NASH; increasing dietary cholesterol levels exacerbated hepatic accumulation of free cholesterol, hepatocyte injury or apoptosis, macrophage recruitment, and liver fibrosis. CONCLUSIONS In obese, diabetic mice, hyperinsulinemia alters nuclear transcriptional regulators of cholesterol homeostasis, leading to hepatic accumulation of free cholesterol; the resulting cytotoxicity mediates transition of steatosis to NASH.

NLRP3 inflammasome blockade reduces liver inflammation and fibrosis in experimental NASH in mice

BACKGROUND & AIMS NOD-like receptor protein 3 (NLRP3) inflammasome activation occurs in Non-alcoholic fatty liver disease (NAFLD). We used the first small molecule NLRP3 inhibitor, MCC950, to test whether inflammasome blockade alters inflammatory recruitment and liver fibrosis in two murine models of steatohepatitis. METHODS We fed foz/foz and wild-type mice an atherogenic diet for 16weeks, gavaged MCC950 or vehicle until 24weeks, then determined NAFLD phenotype. In mice fed an methionine/choline deficient (MCD) diet, we gavaged MCC950 or vehicle for 6weeks and determined the effects on liver fibrosis. RESULTS In vehicle-treated foz/foz mice, hepatic expression of NLRP3, pro-IL-1β, active caspase-1 and IL-1β increased at 24weeks, in association with cholesterol crystal formation and NASH pathology; plasma IL-1β, IL-6, MCP-1, ALT/AST all increased. MCC950 treatment normalized hepatic caspase 1 and IL-1β expression, plasma IL-1β, MCP-1 and IL-6, lowered ALT/AST, and reduced the severity of liver inflammation including designation as NASH pathology, and liver fibrosis. In vitro, cholesterol crystals activated Kupffer cells and macrophages to release IL-1β; MCC950 abolished this, and the associated neutrophil migration. MCD diet-fed mice developed fibrotic steatohepatitis; MCC950 suppressed the increase in hepatic caspase 1 and IL-1β, lowered numbers of macrophages and neutrophils in the liver, and improved liver fibrosis. CONCLUSION MCC950, an NLRP3 selective inhibitor, improved NAFLD pathology and fibrosis in obese diabetic mice. This is potentially attributable to the blockade of cholesterol crystal-mediated NLRP3 activation in myeloid cells. MCC950 reduced liver fibrosis in MCD-fed mice. Targeting NLRP3 is a logical direction in pharmacotherapy of NASH. LAY SUMMARY Fatty liver disease caused by being overweight with diabetes and a high risk of heart attack, termed non-alcoholic steatohepatitis (NASH), is the most common serious liver disease with no current treatment. There could be several causes of inflammation in NASH, but activation of a protein scaffold within cells termed the inflammasome (NLRP3) has been suggested to play a role. Here we show that cholesterol crystals could be one pathway to activate the inflammasome in NASH. We used a drug called MCC950, which has already been shown to block NLRP3 activation, in an attempt to reduce liver injury in NASH. This drug partly reversed liver inflammation, particularly in obese diabetic mice that most closely resembles the human context of NASH. In addition, such dampening of liver inflammation in NASH achieved with MCC950 partly reversed liver scarring, the process that links NASH to the development of cirrhosis.

Roles of adipose restriction and metabolic factors in progression of steatosis to steatohepatitis in obese, diabetic mice

Background and Aims:  We previously reported that steatohepatitis develops in obese, hypercholesterolemic, diabetic foz/foz mice fed a high‐fat (HF) diet for 12 months. We now report earlier onset of steatohepatitis in relation to metabolic abnormalities, and clarify the roles of dietary fat and bodily lipid partitioning on steatosis severity, liver injury and inflammatory recruitment in this novel non‐alcoholic steatohepatitis (NASH) model.

Hepatocyte free cholesterol lipotoxicity results from JNK1-mediated mitochondrial injury and is HMGB1 and TLR4-dependent

BACKGROUND & AIMS Free cholesterol (FC) accumulates in non-alcoholic steatohepatitis (NASH) but not in simple steatosis. We sought to establish how FC causes hepatocyte injury. METHODS In NASH-affected livers from diabetic mice, subcellular FC distribution (filipin fluorescence) was established by subcellular marker co-localization. We loaded murine hepatocytes with FC by incubation with low-density lipoprotein (LDL) and studied the effects of FC on JNK1 activation, mitochondrial injury and cell death and on the amplifying roles of the high-mobility-group-box 1 (HMGB1) protein and the Toll-like receptor 4 (TLR4). RESULTS In NASH, FC localized to hepatocyte plasma membrane, mitochondria and ER. This was reproduced in FC-loaded hepatocytes. At 40 μM LDL, hepatocyte FC increased to cause LDH leakage, apoptosis and necrosis associated with JNK1 activation (c-Jun phosphorylation), mitochondrial membrane pore transition, cytochrome c release, oxidative stress (GSSG:GSH ratio) and ATP depletion. Mitochondrial swelling and crystae disarray were evident by electron microscopy. Jnk1(-/-) and Tlr4(-/-) hepatocytes were refractory to FC lipotoxicity; JNK inhibitors (1-2 μM CC-401, CC-930) blocked apoptosis and necrosis. Cyclosporine A and caspase-3 inhibitors protected FC-loaded hepatocytes, confirming mitochondrial cell death pathways; in contrast, 4-phenylbutyric acid, which improves ER folding capacity did not protect FC-loaded hepatocytes. HMGB1 was released into the culture medium of FC-loaded wild type (WT) but not Jnk1(-/-) or Tlr4(-/-) hepatocytes, while anti-HMGB1 anti-serum prevented JNK activation and FC lipotoxicity in WT hepatocytes. CONCLUSIONS These novel findings show that mitochondrial FC deposition causes hepatocyte apoptosis and necrosis by activating JNK1; inhibition of which could be a novel therapeutic approach in NASH. Further, there is a tight link between JNK1-dependent HMGB1 secretion from lipotoxic hepatocytes and a paracrine cytolytic effect on neighbouring cholesterol-loaded hepatocytes operating via TLR4.

Peroxisome proliferator‐activated receptor‐α agonist, Wy 14 643, improves metabolic indices, steatosis and ballooning in diabetic mice with non‐alcoholic steatohepatitis

Background and Aims:  Lipid accumulation precedes hepatocellular injury and liver inflammation in non‐alcoholic steatohepatitis (NASH). The peroxisome proliferator‐activated receptor (PPAR)α regulates hepatic lipid disposal. We studied whether pharmacological stimulation of PPARα reverses NASH associated with metabolic syndrome in high‐fat (HF)‐fed foz/foz obese/diabetic mice.

Cholesterol-lowering drugs cause dissolution of cholesterol crystals and disperse Kupffer cell crown-like structures during resolution of NASH

Cholesterol crystals form within hepatocyte lipid droplets in human and experimental nonalcoholic steatohepatitis (NASH) and are the focus of crown-like structures (CLSs) of activated Kupffer cells (KCs). Obese, diabetic Alms1 mutant (foz/foz) mice were a fed high-fat (23%) diet containing 0.2% cholesterol for 16 weeks and then assigned to four intervention groups for 8 weeks: a) vehicle control, b) ezetimibe (5 mg/kg/day), c) atorvastatin (20 mg/kg/day), or d) ezetimibe and atorvastatin. Livers of vehicle-treated mice developed fibrosing NASH with abundant cholesterol crystallization within lipid droplets calculated to extend over 3.3% (SD, 2.2%) of liver surface area. Hepatocyte lipid droplets with prominent cholesterol crystallization were surrounded by TNFα-positive (activated) KCs forming CLSs (≥3 per high-power field). KCs that formed CLSs stained positive for NLRP3, implicating activation of the NLRP3 inflammasome in response to cholesterol crystals. In contrast, foz/foz mice treated with ezetimibe and atorvastatin showed near-complete resolution of cholesterol crystals [0.01% (SD, 0.02%) of surface area] and CLSs (0 per high-power field), with amelioration of fibrotic NASH. Ezetimibe or atorvastatin alone had intermediate effects on cholesterol crystallization, CLSs, and NASH. These findings are consistent with a causative link between exposure of hepatocytes and KCs to cholesterol crystals and with the development of NASH possibly mediated by NLRP3 activation.

SREBP-2: a link between insulin resistance, hepatic cholesterol, and inflammation in NASH.

See article in J. Gastroenterol. Hepatol. 2011; 26: 875–883.

Strain dependence of diet-induced NASH and liver fibrosis in obese mice is linked to diabetes and inflammatory phenotype

Obese Alms1 mutant (foz/foz) NOD.B10 mice develop diabetes and fibrotic NASH when fed high‐fat(HF) diet. To establish whether diabetes or obesity is more closely associated with NASH fibrosis, we compared diabetic foz/foz C57BL6/J with non‐diabetic foz/foz BALB/c mice. We also determined hepatic cytokines, growth factors and related profibrotic pathways.

Dietary modification dampens liver inflammation and fibrosis in obesity-related fatty liver disease

Background: Alms1 mutant (foz/foz) mice develop hyperphagic obesity, diabetes, metabolic syndrome, and fatty liver (steatosis). High‐fat (HF) feeding converts pathology from bland steatosis to nonalcoholic steatohepatitis (NASH) with fibrosis, which leads to cirrhosis in humans.