Inhibiting triglyceride synthesis improves hepatic steatosis but exacerbates liver damage and fibrosis in obese mice with nonalcoholic steatohepatitis

Abstract

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,...

Mitochondrial dysfunction and oxidative stress in the pathogenesis of alcohol- and obesity-induced fatty liver diseases

“…More interesting, however, is the observation by Pritchard et al showing that even though the C3 − /− mice were protective against ethanol-mediated steatosis, mice still had elevated serum ALT and hepatic TNFα levels following alcohol exposure indicating hepatic inflammatory injury in the absence of steatosis [36]. In support of this, Diehl and colleagues demonstrated that inhibition of hepatocyte triglyceride synthesis and steatosis increased liver injury and fibrosis in obese mice with NASH [38]. Moreover, oleate-or palmitate-loaded HepG2 spheroids (hepatocyte-derived cells) were less susceptible to cytokine or peroxide induced cell death [39].…”

“…Taken together, these results are intriguing because they indicate that steatosis may not be the "first-hit" or a pre-requisite in the pathogenesis of ASH or NASH. In fact, steatosis may even be protective against hepatotoxicity [38]. Clearly, additional research is required to determine the role of steatosis in inflammatory and/or fibrotic liver diseases.…”

Therapeutic trials in nonalcoholic steatohepatitis: Insulin sensitizers and related methodological issues

“…Experimental studies in rodents have challenged the concept that all liver fat is bad. 36 Blocking esterification of fatty acids into triglycerides in animals fed an methionine-choline-deficient (MCD) diet resulted in a higher level of hepatic oxidative stress, inflammation, cell injury, and fibrosis. 36 A tempting interpretation is that when fatty acids are stored in triglycerides they are diverted from pathways that result in cellular damage.…”

“…36 Blocking esterification of fatty acids into triglycerides in animals fed an methionine-choline-deficient (MCD) diet resulted in a higher level of hepatic oxidative stress, inflammation, cell injury, and fibrosis. 36 A tempting interpretation is that when fatty acids are stored in triglycerides they are diverted from pathways that result in cellular damage. 37 Theoretically, such an effect could be explained by the accumulation of intermediate products along the chain of de novo lipogenesis, rather than by the reduction in the endproduct (here steatosis).…”

Risk of type 2 diabetes in patients with non-alcoholic fatty liver disease: Causal association or epiphenomenon?

“…Many of these lipid intermediates promote hepatic inflammation [39][40][41][42] and increasing risk of progressive liver disease that occurs with NASH. Hepatic lipids that are not esterified also induce endoplasmic reticulum stress, leading to the activation of c-Jun N-terminal kinases and nuclear factor kappa (NF-κB)-light-chain-enhancer of activated B cells [43], which are two major regulators of inflammatory pathways that also inhibit 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8 phosphorylation of insulin receptor substrate (IRS)-1 [44], potentially further aggravating hepatic insulin resistance and increasing intra-hepatic cytokine production.…”