Elevated autophagic sequestration of mitochondria and lipid droplets in steatotic hepatocytes of chronic ethanol-treated rats: an immunohistochemical and electron microscopic study

Ethanol-induced hepatic steatosis may induce the progression of alcoholic liver disease. The involvement of autophagic clearance of damaged mitochondria (mitophagy) and lipid droplets (LDs) (lipophagy) in chronic ethanol-induced hepatic steatosis is not clearly understood. Adult Wistar rats were fed either 5 % ethanol in Lieber-DeCarli liquid diet or an isocaloric control diet for 10 weeks. Light microscopy showed marked steatosis in hepatocytes of ethanol-treated rats (ETRs), which was further revealed by transmission electron microscopy (TEM), where significant numbers of large LDs and damaged mitochondria were detected in steatotic hepatocytes. Moreover, TEM demonstrated that hepatocyte steatosis was associated with greatly enhanced autophagic vacuole (AV) formation compared to control hepatocytes. Mitochondria and LDs were the predominant contents of AVs in steatotic hepatocytes. Immunohistochemistry of LC3, a specific marker of early AVs (autophagosomes), demonstrated an extensive punctate pattern in hepatocytes of ETRs, while LC3 puncta were much less frequent in control hepatocytes. This was confirmed by immunoelectron microscopy (IEM), which showed localization of LC3 to autophagosomes sequestering damaged mitochondria and LDs. In addition, IEM revealed that PINK1 (a sensor of mitochondrial damage and marker of mitophagy) was overexpressed in mitochondria of ETRs. Enhanced autophagic lysosomal activity was evidenced by increased immunolabeling of LAMP-2, a marker of late AVs (autolysosomes) in hepatocytes of ETRs and colocalization of LC3 and lysosomal cathepsins using double immunofluorescence labeling. Increased AVs in hepatocytes of ETRs reflect ethanol toxicity and could represent a possible protective mechanism via stimulation of mitophagy and lipophagy.