Date on Master's Thesis/Doctoral Dissertation

12-2014

Document Type

Master's Thesis

Degree Name

M.S.

Department

Pharmacology and Toxicology

Committee Chair

Barve, Shirish Shrikrishna

Committee Co-Chair (if applicable)

Gobejishvili, Leila

Committee Member

McClain, Craig

Committee Member

Arteel, Gavin E.

Committee Member

Clark, Geoffrey J.

Subject

Liver--Diseases; Alcoholic liver diseases; Alcohol--Physiological effect; Phosphodiesterases

Abstract

Alcohol induced hepatic steatosis is a significant risk factor for progressive liver disease. Steatotic hepatocytes have increased sensitivity to injury produced by inflammatory cytokines, particularly TNF. Cyclic adenosine monophosphate (cAMP) has been shown to play a significant role in the regulation of both TNF production and lipid metabolism. However, the role of altered cAMP homeostasis in alcohol mediated hepatic steatosis and injury has not been studied. cAMP levels are tightly regulated by phosphodiesterase family of enzymes. Our recent work demonstrated that increased expression of hepatic PDE4, which specifically hydrolyzes and decreases cAMP levels, plays a pathogenic role in the development of liver injury. Hence, the aim of this study was to examine the effect of alcohol on PDE4 expression in the liver and its potential role in the development of alcoholic steatosis. Methods: C57Bl/6 wild type and Pde4b knockout (Pde4b-/-) mice were pair-fed control or ethanol liquid diets for 4 weeks. One group of wild type mice received rolipram, a PDE4 specific inhibitor, during alcohol feeding. Wild type mice fed alcohol with and without rolipram treatment were sacrificed after 2 and 4 weeks. Liver steatosis was evaluated by Oil-Red-O staining and documented by biochemical assessment of hepatic triglycerides and free fatty acids. Expression of hepatic PDE4 and the effect of PDE4 inhibition on protein expression and activity of key enzymes involved in lipid metabolism were evaluated at both mRNA and protein levels. Results: We demonstrate for the first time that an early increase in lipogenesis mediated by acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN) in alcohol fed wild type mice coincides with the significant up-regulation of hepatic PDE4 expression. Notably, after 4 weeks of alcohol feeding, Pde4b-/- mice and mice treated with rolipram had significantly lower hepatic free fatty acid content compared to wild type mice. PDE4 inhibition did not affect alcohol metabolism as demonstrated by unaltered CYP2E1 expression in bothPde4b-/- mice as well as mice treated with rolipram. Importantly, PDE4 inhibition in alcohol fed mice (i) prevented the decrease in hepatic sirtuin 1 (SIRT-1) levels, (ii) decreased hepatic ACC activity and (iii) increased hepatic CPT1A expression. Conclusion: These results demonstrate that alcohol feeding induced increase in hepatic PDE4 expression is a significant pathogenic mechanism underlying dysregulated lipid metabolism and development of hepatic steatosis. Moreover, these data also suggest that hepatic PDE4 is a clinically relevant therapeutic target for the treatment of alcohol induced hepatic steatosis.

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