Cellular signaling mechanisms in alcohol-induced liver damage

Chronic excessive alcohol intake is associated with multiple liver defects ranging from mild steatosis to advanced cirrhosis. However, the mechanisms by which chronic ethanol intake affects liver function remain a matter of intense debate and investigation. The liver is the major site of ethanol metabolism in the body, and a wide range of metabolic alterations is associated with ethanol intake. As a result, the liver is exposed to dramatic changes in redox state, transient hypoxia, episodes of oxidative stress, and the products of ethanol metabolism, such as acetaldehyde, acetate, and fatty acid ethyl esters. Chronic ethanol consumption is associated with increased levels of circulating endotoxins and proinflammatory cytokines that affect liver function. A major source of the increase in circulating proinflammatory cytokines is the Kupffer cells, which are sensitized to generate tumor necrosis factor alpha (TNF-alpha) through multiple mechanisms. In addition, the hepatocytes themselves are more susceptible to external stress. In isolated hepatocytes, this effect of chronic ethanol is evident in a greater sensitivity to proapoptotic challenges and, more specifically, to the cytotoxic actions of TNF-alpha. The mechanism by which hepatocytes are sensitized to external stress remains poorly characterized but may involve defects in mitochondrial function and oxidative defense mechanisms, the activation of death-promoting signaling pathways, and the inactivation of survival pathways. In this article, we emphasize the role of the stress-activated mitogen-activated protein kinase (MAPK) cascades in the onset of cell injury and their regulation by the phosphoinositide-3-kinase/Akt signaling cascade, which appears to function as the central integrating module of the stress-signaling machinery in the cell. We also discuss the complications and challenges of extrapolating these findings to the conditions in vivo and what we can learn from these studies regarding the nature of the liver defects associated with chronic alcohol consumption.