Ethanol Administration Exacerbates the Abnormalities in Hepatic Lipid Oxidation in Genetically Obese Mice

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fatty liver disease, ethanol and obesity synergism, signal transduction, lipid metabolism

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Alcohol consumption synergistically increases the risk and severity of liver damage in obese patients. To gain insight into cellular or molecular mechanisms underlying the development of fatty liver caused by ethanol-obesity synergism, we have carried out animal experiments that examine the effects of ethanol administration in genetically obese mice. Lean wild-type (WT) and obese (ob/ob) mice were subjected to ethanol feeding for 4 wk using a modified Lieber-DeCarli diet. After ethanol feeding, the ob/ob mice displayed much more pronounced changes in terms of liver steatosis and elevated plasma levels of alanine aminotransferase and aspartate aminotransferase, indicators of liver injury, compared with control mice. Mechanistic studies showed that ethanol feeding augmented the impairment of hepatic sirtuin 1 (SIRT1)-AMP-activated kinase (AMPK) signaling in the ob/ob mice. Moreover, the impairment of SIRT1-AMPK signaling was closely associated with altered hepatic functional activity of peroxisome proliferator-activated receptor γ coactivator-α and lipin-1, two vital downstream lipid regulators, which ultimately contributed to aggravated fatty liver observed in ethanol-fed ob/ob mice. Taken together, our novel findings suggest that ethanol administration to obese mice exacerbates fatty liver via impairment of the hepatic lipid metabolism pathways mediated largely by a central signaling system, the SIRT1-AMPK axis.

worldwide, alcoholic fatty liver disease (AFLD) and nonalcoholic fatty liver disease (NAFLD) are two common disorders caused by either chronic alcohol intake or obesity (3, 7, 16, 22). Clinically, although the majority of alcoholics and obese individuals will develop liver steatosis, only a minority progress into more serious forms of liver injury, namely hepatitis, fibrosis, cirrhosis, and liver failure (3, 7, 16, 22). However, alcohol consumption synergistically increases the prevalence and the severity of liver damage in obese patients. Likewise, liver disease in alcohol abusers is significantly increased by obesity (2, 7, 11, 15, 20, 28, 31, 37). Whereas the synergistic effects of alcohol and obesity in liver injury in rodents and humans are well documented, the underlying cellular and molecular mechanisms remain elusive.

The liver is one of the major organs in which sirtuin 1 (SIRT1), an NAD+-dependent protein deacetylase, plays a crucial role in the regulation of lipid metabolism by regulating the acetylation/deacetylation status of a wide range of molecules including histones and transcriptional regulators (29, 3436). AMP-activated kinase (AMPK) is another master lipid regulator controlling the pathways of hepatic lipid metabolism largely through direct phosphorylation and subsequent inhibition of acetyl-CoA carboxylase (ACC), a rate-limiting enzyme in hepatic fatty acid biosynthesis that exerts its effects through the inhibition of malonyl-CoA, a basic substrate for fatty acids (33).

SIRT1 and AMPK reciprocally regulate each other's activity and share several common target molecules (4, 17, 19). Whereas SIRT1 stimulates AMPK activity through LKB1, AMPK enhances SIRT1 activity by increasing cellular NAD+ levels (4, 17, 19). In liver, activation of the SIRT1-AMPK axis protects against the development of steatosis by turning on fatty acid oxidation pathways and inhibiting lipid synthesis through interaction with multiple transcriptional factors and cofactors such as sterol regulatory element-binding protein 1 (SREBP-1), peroxisome proliferator-activated receptor α (PPARα), and PPARγ coactivator-α (PGC-1α) (9, 24).

Lipin-1 has recently emerged as a vital lipid regulator by displaying two distinct functions in regulating lipid metabolism dependent upon its subcellular localization. In the cytosol, lipin-1 functions as a Mg2+-dependent phosphatidate phosphatase, catalyzing a key step in the synthesis of glycerolipids (10). The nuclear-localized lipin-1 acts as a cotranscriptional regulator to increase the capacity of the liver for fatty acid oxidation and reduce the lipogenesis through interacting with transcriptional regulators including PGC-1α, PPARα, and SREBP-1 (10, 24).

The development of AFLD and NAFLD have been attributed to a combination of increased fatty acid uptake, de novo lipogenesis, decreased fatty acid oxidation, and impaired fatty acid export from the liver (3, 7, 16). Considerable evidence from human and rodent studies demonstrates that both AFLD and NAFLD are associated with impairment of the hepatic the SIRT1-AMPK signaling system and subsequent derangement of PGC-1α or lipin-1 (3, 4, 7, 16, 17, 19). Nevertheless, the role of hepatic SIRT1-AMPK signaling in the development of fatty liver disease that is manifest by the synergism of ethanol and obesity is largely unknown.

In the present study, we used a murine model of obesity (ob/ob mouse model) to examine the effects of chronic ethanol administration to obese ob/ob mice and investigated the hepatic lipid metabolism pathways affected by this combination, which results in a synergistic enhancement of fatty liver.

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Citation / Publisher Attribution

American Journal of Physiology-Gastrointestinal and Liver Physiology, v. 304, issue 1, p. G38-G47