Rab GTPases Associate with Isolated Lipid Droplets (LDs) and Show Altered Content After Ethanol Administration: Potential Role in Alcohol-Impaired LD Metabolism

Background: Alcoholic liver disease is manifested by the presence of fatty liver, primarily due to accumulation of hepatocellular lipid droplets (LDs). The presence of membrane-trafficking proteins (e.g., Rab GTPases) with LDs indicates that LDs may be involved in trafficking pathways known to be altered in ethanol (EtOH) damaged hepatocytes. As these Rab GTPases are crucial regulators of protein trafficking, we examined the effect EtOH administration has on hepatic Rab protein content and association with LDs. Methods: Male Wistar rats were pair-fed Lieber-DeCarli diets for 5 to 8 weeks. Whole liver and isolated LD fractions were analyzed. Identification of LDs and associated Rab proteins was performed in frozen liver or paraffin-embedded sections followed by immunohistochemical analysis. Results: Lipid accumulation was characterized by larger LD vacuoles and increased total triglyceride content in EtOH-fed rats. Rabs 1, 2, 3d, 5, 7, and 18 were analyzed in postnuclear supernatant (PNS) as well as LDs. All of the Rabs were found in the PNS, and Rabs 1, 2, 5, and 7 did not show alcohol-altered content, while Rab 3d content was reduced by over 80%, and Rab 18 also showed EtOH-induced reduction in content. Rab 3d was not found to associate with LDs, while all other Rabs were found in the LD fractions, and several showed an EtOH-related decrease (Rabs 2, 5, 7, 18). Immunohistochemical analysis revealed the enhanced content of a LD-associated protein, perilipin 2 (PLIN2) that was paralleled with an associated decrease of Rab 18 in EtOH-fed rat sections. Conclusions: Chronic EtOH feeding was associated with increased PLIN2 and altered Rab GTPase content in enriched LD fractions. Although mechanisms driving these changes are not established, further studies on intracellular protein trafficking and LD biology after alcohol administration will likely contribute to our understanding of fatty liver disease.