Genomic and Metabolomic Responses to Alcohol-induced Liver Damage
Albert J. Fornace, Jr.
Alcohol consumption contributes to four percent of the global disease burden and is the third leading lifestyle-related cause of death in the United States due in part to complications arising from alcohol-induced liver disease (ALD). In addition to obesity, chronic alcohol consumption leads to excessive hepatic free fatty acid (FFA) levels that inhibit β-oxidation pathways and ultimately cause liver disease (steatosis, inflammation, hepatomegaly, fibrosis, and cirrhosis). Interestingly, the adverse effects of alcohol on the liver, in humans and in mouse models, appear to be due, in part, to attenuation of the peroxisome-proliferator activated receptor alpha (PPARα), a key regulator of β-oxidation.
The alcohol-fed Ppara-null mouse serves as an excellent model for ALD observed in humans and underscores the importance of PPARα in protecting against ALD. Additionally, this mouse model has been cited in over 850 publications supporting its significant utility in understanding the role of PPARα. In this project, the mechanism of the influence of PPARα is being determined for potential therapeutic intervention strategies on ALD and for the development of biomarkers for early detection of this disease. To this end, our overall objective has been to correlate alcohol-induced liver damage with metabolomic and gene expression biomarkers identified in alcohol-fed PPARα-null mice for the purpose of developing specific ALD biomarkers. The project seeks to understand and integrate the histopathological, metabolomic, and genomic alterations associated with ALD for the purpose of identifying early biomarkers associated with ALD pathogenesis.
The effort to identify biomarkers of alcohol exposure and early ALD through metabolomics has yielded two published reports and substantial additional preliminary data for future reporting as analyses are completed. Namely, Manna and colleagues (2010 and 2011) found a panel of urine metabolites for use in reliable ethanol exposure assessment, including ethylsulfate and ethyl-ß-D-glucuronide. Markers of chronic alcohol consumption common to two strains of mice, C57Bl/6 and 129/SvJ, and wildtype as well as Ppara-null genotypes, include 4-hydroxyphenylacetic acid, 4-hydroxyphenylacetic acid sulfate, adipic acid, pimelic acid, xanthurenic acid, and taurine.
Examination of liver samples revealed presence of steatosis in Ppara-null mice on alcohol diet as early as one month. In parallel, elevated urine excretion of indole-3-lactic acid and phenyllactic acid correlated with the alcohol-fed PPARα-null genotype. Given the link between PPARα and ALD and the plausible mechanism of indole-3-lactic acid formation from excess tryptophan resulting from impaired NAD+ biosynthesis in PPARα-null mice, we speculate that indole-3-lactic acid—and phenyllactic acid too—is mechanistically linked to early ALD. These findings together form a foundation for continued development of early ALD biomarkers in humans by providing promising targets that are likely in the causal pathway of ALD.
At the RNA level, transcriptomics studies of liver tissue from alcohol-fed mice showed that expression of genes in fatty acid metabolism changed significantly. PPARα and SRBEF1 are two transcription factors involved in expression of these genes and are related to alcohol-induced steatosis. Our transcriptomics results indicate various alterations in regulation of PPARα and SREBP1 activity at early and late time points during chronic alcohol exposure. In order to investigate the in-depth changes in liver tissue lipids after alcohol exposure, we performed a mass spectrometry lipidomics approach. Lipidomics is a branch of metabolomics that studies the complete lipid profile of biological samples. As expected, we observed significant changes in multiple lipid species. We are still assessing the complicated alcohol-induced changes and differences between wild type mice with that of Ppara-null mice to determine the roles of this key nuclear receptor in alcohol-induced lipid changes. Studies are ongoing at the transcriptional level in conjunction with the above studies.
Overall, a variety of promising biomarkers have been identified at early times, e.g. one month of alcohol exposure, prior to major histologic changes. These biomarkers in some cases show progressive changes as gross alcoholic liver injury becomes apparent. Modulation of PPARα function may have utility in attenuating ALD, and our studies provides a variety of criteria that should have utility in assessing modulation of liver injury by alcohol.