Inhibition of proteolysis in the liver by chronic ethanol feeding

Abstract

Effects of chronic ethanol feeding on the volume density of lysosomes, the rate of protein degradation and the amount of protein were studied in livers perfused in situ at 07:00, 11:00, 17:00 and 23:00 h. Ethanol was given to the rats in drinking water for either 3 or 8-10 weeks. During week 3 of treatment and onwards, the average daily consumption of ethanol was 12.3 +0.3 g/kg body wt. Morphometric analysis revealed that the volume density of autophagosomes and autolysosomes was lower in the ethanol-fed rats than in the controls. When compared with age-matched controls, the rate of proteolysis, measured as release of valine, was 33 % and 26 % less in the ethanol-fed rats after treatment for 3 and 8-10 weeks respectively. The difference between the two groups was most pronounced at 07:00 and 11:00 h. Protein content of the liver increased significantly after the longer ethanol treatment. According to these results, chronic ethanol feeding inhibits proteolysis in the liver by preventing the sequestration of protein into lysosomes. INTRODUCTIONChronic ethanol ingestion disturbs the finely adjusted balance of protein turnover in liver and leads to accumulation of hepatocellular protein (for review, see [1]). Part of this accumulation can be explained by an increase in the amount of fatty-acid-binding protein [2] and by plasma proteins which in the presence of ethanol are retained in the liver [3]. It can, however, be calculated that the amount of plasma proteins together with the fatty-acid-binding protein account for less than 30 % of the total protein accumulation during chronic ethanol feeding [2,3], which leaves the nature of the major contributor(s) unknown.The ethanol-induced increase in hepatic protein mass can be caused by a change in the rate of either protein synthesis or degradation. Despite extensive studies, effects of chronic ethanol feeding on protein synthesis in vivo are far from clear, and the reported results range from stimulation to inhibition (for review, see [1]). Ethanol has been shown to inhibit the rate of intracellular protein degradation in perfused rat liver [4], whereas the chronic effects of ethanol have only been calculated from the changes in the amount of protein and the rate of protein synthesis [5,6].The bulk of long-lived protein in the liver is degraded intralysosomally [7]. The process has been shown to be more sensitive to nutritional factors than is protein synthesis, and there are several lines of evidence which indicate that protein degradation rather than synthesis is the dominant site for regulating the cytoplasmic protein content of the hepatocyte [7,8]. It was shown previously that in perfused rat liver ethanol inhibits the formation of autophagosomes [4], the first step of lysosomal proteolysis, whereas the effect of chronic ethanol feeding on autophagocytosis is not known. The purpose of the present study was to measure the rate of protein degradation and volume densities of lysosomal components in livers of rats that were chronically fed on an ethanol-containing die...

Ethanol-induced oxidant stress modulates hepatic autophagy and proteasome activity

“…A detailed electron microscopy study confirmed that, compared with perfused livers of control rats, autophagy was impeded in ethanol-fed rats, showing lower numbers of autolysosomes (i.e. the products of AV–lysosome fusions) and lower rates of valine release, a sign of reduced proteolysis [54] .…”

Autophagy as a Stress-Response and Quality-Control Mechanism: Implications for Cell Injury and Human Disease

“…Interestingly, chronic alcohol consumption inhibits liver cell autophagy, suggesting that decreased autophagy may underlie the pathogenesis of alcoholic liver disease. Rats chronically fed alcohol exhibit decreased numbers of hepatic autophagic vacuoles and the accumulation of abnormal lysosomes in parallel with decreased degradation of long-lived proteins and accumulation of protein and lipid (135,136). Consistent with an autophagy defect, patients with a form of alcoholic liver disease called alcoholic steatohepatitis accumulate protein aggregates called Mallory-Denk bodies (MDBs) primarily composed of keratin 8 and keratin 18 that are positive for ubiquitin and p62 (137).…”

Role of Autophagy in HIV Pathogenesis and Drug Abuse

“…Several initial studies have shown that autophagy is suppressed in alcoholic liver disease. For example, less density of autophagosome and autolysosome were found in the liver cells of rats fed with alcohol compared with control rats [68]. The lowering of the protein degradation rate in alcohol-fed rats caused increased accumulation of long-lived proteins [69].…”