Alcoholic liver disease: pathogenesis and new therapeutic targets

Abstract

Alcoholic liver disease (ALD) is a major cause of chronic liver disease worldwide and can lead to fibrosis and cirrhosis. The latest surveillance report published by the National Institute on Alcohol Abuse and Alcoholism showed that liver cirrhosis was the 12th leading cause of death in the United States, with a total of 29,925 deaths in 2007, 48% of which were alcohol related. The spectrum of ALD includes simple steatosis, alcoholic hepatitis, fibrosis, cirrhosis, and superimposed hepatocellular carcinoma. Early work on the pathogenesis of the disease focused on ethanol metabolism-associated oxidative stress and glutathione depletion, abnormal methionine metabolism, malnutrition, and production of endotoxins that activate Kupffer cells. We review findings from recent studies that have characterized specific intracellular signaling pathways, transcriptional factors, aspects of innate immunity, chemokines, epigenetic features, microRNAs, and stem cells that are associated with ALD, improving our understanding of its pathogenesis. Despite this progress, no targeted therapies are available. The cornerstone of treatment for alcoholic hepatitis remains as it was 40 years ago: abstinence, nutritional support, and corticosteroids. There is an urgent need to develop new pathophysiology-oriented therapies. Recent translational studies of human samples and animal models have identified promising therapeutic targets.

Figure 1

Spectrum of ALD, risk factors, and comorbidity. More than 95% of heavy drinkers develop fatty liver, but only up to 35% of this population develops more severe forms of ALD, including fibrosis, alcoholic hepatitis, cirrhosis, and HCC. Many risk factors have been proposed for the severe forms of ALD. Alcohol consumption and comorbid factors act in synergy to accelerate the progression of ALD.

Figure 2

Mechanisms of alcoholic fatty liver. (1) Alcohol consumption can directly (via acetaldehyde) or indirectly (via regulation of multiple factors) up-regulate the expression of SREBP-1c and down-regulate the expression of PPAR-α, leading to the induction of fatty acid synthesis and inhibition of fatty liver β-oxidation, which results in the development of alcoholic fatty liver. Alcohol exposure also inhibits AMPK and subsequently increases ACC activity but decreases carnitine palmitoyltransferase 1 (CPT-1) activity, leading to an increase in fatty acid synthesis and a decrease in fatty acid β-oxidation. (2) Alcohol consumption can also modify many factors, including HIF-1, C3, C1qa, PKC[H9255], and iNOS, that subsequently contribute to the development of fatty liver. The mechanisms underlying the effects of these factors remain unclear.

Figure 3

Mechanisms underlying inflammation in ALD. (1) Activation of innate immunity. Parenchymal infiltration of neutrophils and macrophages is a prominent feature of ALD and is likely due to ethanol-mediated activation of innate immunity and subsequent induction of proinflammatory cytokines and chemokines. Alcohol consumption up-regulates a variety of factors that activate Kupffer cells, stellate cells, and hepatocytes, resulting in the production of cytokines and chemokines. Alcohol exposure also decreases proteasome activity and elevates IL-8 expression in hepatocytes. (2) Activation of adaptive immunity. ALD is associated with infiltration of CD4⁺ and CD8⁺ T cells in the liver. Alcohol consumption induces reactive oxygen species (ROS) and causes the formation of many protein adducts that might serve as antigens in the adaptive immune response, resulting in the accumulation of T and B cells in the liver.

Figure 4

Mechanisms of liver fibrosis in patients with ALD. (1) Alcohol consumption causes hepatocyte damage, which leads to the release of a variety of mediators and the subsequent induction of stellate cell activation. (2) Acetaldehyde directly targets stellate cells and up-regulates the expression of collagens in these cells. (3) Alcohol consumption results in elevation of LPS levels in the liver. LPS can directly enhance stellate cell activation via up-regulation of TGF-β signaling and indirectly promote stellate cell activation via activation of Kupffer cells to release profibrotic cytokines and chemokines. (4) Natural killer (NK) cells are activated during viral hepatitis or IFN-α therapy. Activated natural killer cells can kill stellate cells by releasing TRAIL and inhibit stellate cell proliferation by releasing IFN-γ; they therefore have an important role in inhibiting liver fibrosis. Alcohol consumption suppresses the antifibrotic effects of NK cells and IFN-γ, thereby promoting liver fibrosis.

Figure 5

Algorithms for treatment of compensated ALD and decompensated ALD with superimposed AH. DF, discriminant function; MELD, Model for End-Stage Liver Disease; ABIC, age/bilirubin/international normalized ratio/creatinine scoring system; OLT, orthotopic liver transplantation.