A fatty liver is the accumulation of triglycerides and other fats in the liver cells. The amount of fatty acid in the liver depends on the balance between the processes of delivery and removal. In some patients, fatty liver may be accompanied by hepatic inflammation and liver cell death (steatohepatitis). Potential pathophysiologic mechanisms for fatty liver include the following:
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Decreased mitochondrial fatty acid beta-oxidation
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Increased endogenous fatty acid synthesis or enhanced delivery of fatty acids to the liver
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Deficient incorporation or export of triglycerides as very low-density lipoprotein (VLDL)
No single pathway of cause and effect has been found. However, some studies show higher levels of activation of Hedgehog pathways in patients with the most advanced fatty liver disease. [1] Tripodi et al reported that in nonalcoholic fatty liver disease (NAFLD), a procoagulant imbalance progresses from steatosis to metabolic cirrhosis, which may be caused by an increase in factor VIII and a reduction of protein C. [2] The investigators speculated that this imbalance could play a role in the risk for cardiovascular disease and liver fibrosis, conditions commonly associated with NAFLD.
The American Association for the Study of Liver Diseases (AASLD) defines NAFLD as an overarching term that encompasses all disease grades and stages as well as refers to individuals in whom 5% or more of their hepatocytes demonstrate macrovesicular steatosis when there is no easily identified alternative cause of steatosis, such as medications, starvation, monogenic conditions, in those who drink little to no alcohol (women: < 20 g/d; men: < 30 g/d). [3] NAFLD includes NAFL (nonalcoholic fatty liver) (macrovesicular hepatic steatosis with or without mild inflammation) and NASH (nonalcoholic steatohepatitis) (additionally characterized by inflammation and cellular injury [ballooning], with/without fibrosis, cirrhosis.) [3]
Pathologic changes observed in patients with alcoholic liver disease (ALD) can be divided into the following three groups:
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Alcoholic fatty liver (simple steatosis)
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Alcoholic hepatitis
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Alcohol-related cirrhosis
Alcoholic fatty liver is an early and reversible consequence of excessive alcohol consumption. Fatty liver develops in every individual who consumes more than 60 g of alcohol per day. Many mechanisms of ethanol-induced fatty liver have been proposed. Increased hepatic levels of glycerol 3-phosphate (3-GP) after ethanol ingestion are related to an increase in the ratio of the oxidized form of nicotinamide adenine dinucleotide (NAD+) to the reduced form (NADH) in the liver. A higher concentration of 3-GP results in enhanced esterification of fatty acids.
An increase in free fatty acids has also been incriminated in the pathogenesis. Large amounts of alcohol enhance lipolysis through direct stimulation of the adrenal-pituitary axis. In addition, chronic ethanol ingestion inhibits the oxidation of fatty acids in the liver and the release of VLDL into the blood. All of these mechanisms favor steatosis. Centrilobular localization of steatosis results from decreased energy stores caused by relative hypoxia and a shift in lipid metabolism, along with a shift in the redox reaction as a result of preferential oxidation of alcohol in the central zone.
Advances in the understanding of the pathogenesis of alcoholic steatosis have provided some useful insights, including the role of peroxisome proliferator-activated receptor alpha, which is crucial for the regulation of hepatic fatty acid metabolism. Its blockade, in animal models, along with ethanol consumption, contributes to the development of alcoholic fatty liver. In addition, the induction of adiponectin, a hormone secreted by adipocytes, has been implicated in the protective effect of saturated fat against the development of alcoholic fatty liver in mice.
The role of the early growth response-1 (EGr-1) transcription factor is thought to be essential for ethanol-induced fatty liver injury in mice. Hepatocyte death by apoptosis occurs in alcoholic fatty liver and has been demonstrated in rats and mice after ethanol feeding. This may be related to mitochondrial proteins that regulate apoptosis and necrosis and that are shown to be induced in mouse fatty liver models.
Serum leptin, a cytokine-type peptide hormone mainly produced by adipocytes, may play an important role in the pathogenesis of steatosis. Steatosis occurs with decreased leptin action, whether due to leptin deficiency or resistance. In patients with alcoholic liver disease, the serum leptin level appears to be independently correlated with the grade of steatosis.
Data from animal studies and clinical studies support the role of proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) in the early stages of fatty liver, as well as in alcoholic steatohepatitis.
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