Apolipoprotein synthesis in nonalcoholic steatohepatitis

Abstract

The pathophysiology of hepatic steatosis, a prerequisite of nonalcoholic fatty liver disease, is poorly understood. Because very-low-density lipoprotein (VLDL) formation is the chief route of hepatic lipid export, we hypothesized that the synthesis of apoB-100, a rate-determining step in hepatic VLDL formation, may be altered in patients with nonalcoholic steatohepatitis (NASH). This study evaluated the relative synthesis rates of apolipoprotein B-100 (apoB-100) in patients with NASH and in lean and body mass index (BMI)-matched (obese) controls without NASH. A primed continuous infusion of L-[1-13 C] leucine was used to measure the absolute synthesis rates (ASR) of apoB-100 and fibrinogen in 7 patients with NASH and compared them with 7 lean and 7 obese (BMI-matched) controls without NASH. The ASRs of fibrinogen and albumin also were measured. The mean ASR of apoB-100 in patients with NASH was lower (31.5 ؎ 3.4 mg/kg/d) than that of obese (115.2 ؎ 7.2 mg/kg/d, P < .001) and lean controls (82.4 ؎ 4.1 mg/kg/d, P ‫؍‬ .002). In contrast, the mean ASR of fibrinogen was greater in subjects with NASH than in both control groups. These data indicate that NASH is associated with markedly altered hepatic synthesis of apoB-100. The finding that albumin synthesis was not similarly decreased in patients with NASH shows that the attenuation of apoB-100 synthesis is not on the basis of globally impaired hepatic protein synthesis. In conclusion, because apoB-100 synthesis is a rate-determining step in hepatocyte lipid export, decreased synthesis of this protein may be an important factor in the development of hepatic steatosis, a prerequisite for NASH. (HEPATOLOGY 2002;35:898-904.) N on-alcoholic steatohepatitis (NASH; part of the spectrum of nonalcoholic fatty liver disease) is characterized by macrovesicular steatosis, nuclear glycogenation, lobular and portal inflammation, and, occasionally, Mallory's hyaline, in the absence of excessive alcohol ingestion. 1 NASH is most frequently associated with obesity 2-9 and type 2 diabetes mellitus. 3,[7][8][9] Despite the high prevalence and the increasingly recognized morbidity associated with NASH, little is known about the pathophysiology of this condition. The liver is central to the regulation of fat metabolism, and the net accumulation of fat within hepatocytes, a prerequisite for steatosis and steatohepatitis, could potentially result from alterations in the uptake, synthesis, degradation, or secretory pathways of hepatic lipid metabolism.Apolipoprotein B-100 (apoB-100) is a large protein (512 kd) involved in the transport of triglycerides and cholesterol from the liver to peripheral tissues. Lipids are chiefly exported from the liver in the form of very-lowdensity lipoproteins (VLDL), a complex of protein (apoB-100), lipids (triglycerides or cholesteryl esters), and phospholipids. Although the mechanisms regulating the synthesis of apoB-100 are incompletely understood, the production of apoB-100 messenger RNA (mRNA) has been shown to be altered by insulin and unaffected by...

Steatosis and progression of fibrosis in untreated patients with chronic hepatitis C infection

“…Insulin resistance is associated with an increase in adipocyte lipolysis and elevated circulating free fatty acids as well as inhibition of hepatic lipid oxidation and export, all of which serve to promote triglyceride deposition in the liver. 30,31 We observed an inverse association between age and degree of steatosis: milder steatosis is associated with older age. This discrepancy may be due to type II error because of the small number of patients older than 60 years of age relative to the younger age groups.…”

Insulin resistance in non-diabetic patients with non-alcoholic fatty liver disease: sites and mechanisms

“…Reesterification is dependent on the availability of glycerol, and the packaging of triglycerides into VLDL is dependent upon apolipoprotein B synthesis. Since glycerol-3-phosphate can be diverted into the overactive gluconeogenic pathway, and apolipoprotein B production may be reduced in NAFLD (as recently described [51]), the liver retains excessive amounts of lipids, and steatosis ensues. Our finding of a correlation between whole-body lipid oxidation and amount of steatosis (Fig.…”

Binge Alcohol Consumption Aggravates Oxidative Stress and Promotes Pathogenesis of NASH from Obesity-Induced Simple Steatosis

“…The decreased glucose absorption promotes influx of free fatty acids into the liver, increased de novo lipogenesis and a decrease in hepatic clearance of triglycerides due to the impaired β-oxidation system (37,38). This scenario causes a marked elevation of hepatic triglycerides along with hepatic inflammation, influx of mononuclear cells, hepatocyte ballooning and steatosis.…”