Effects of ethanol on glycoprotein synthesis and secretion during inflammation-induced stimulation of hepatic glycoprotein secretion

Ethanol consumption alters trafficking of lysosomal enzymes and affects the processing of procathepsin L in rat liver

In order to determine whether ethanol consumption alters the targeting of hepatic lysosomal enzymes to their organelles, we examined the sedimentation properties of lysosomal hydrolases in ethanol-fed rats and their pair-fed controls. Rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric maltose dextrin for one to five wk. Liver extracts were fractionated by Percoll density gradient centrifugation and fractions obtained were analyzed for the distribution of lysosomal marker enzymes. Heavy lysosomes were further purified from these gradients and the activity of specific hydrolases was determined. Compared with those from controls, isolated lysosomes from ethanol-fed rats showed a 20-50% reduction in the activity of lysosomal acid phosphatase and beta-galactosidase. Decreased intralysosomal hydrolase activity in ethanol-fed rats was associated with a significant redistribution of these enzymes as well as those of cathepsins B and L to lighter fractions of Percoll density gradients. This indicated an ethanol-elicited shift of these enzymes to lower density cellular compartments. In order to determine whether ethanol administration affects the synthesis and proteolytic maturation of hepatic procathepsin L, we conducted immunoblot analyses to quantify the steady-state levels of precursor and mature forms of cathepsin L in hepatic post-nuclear fractions. Ethanol administration caused a significant elevation in the steady-state level of the 39 kDa cathepsin L precursor relative to its 30 kDa intermediate and 25 kDa mature product. These results were confirmed by pulse-chase experiments using isolated hepatocytes exposed to [35S]methionine. Hepatocytes from both control and ethanol-fed rats incorporated equal levels of radioactivity into procathepsin L. However, during the chase period, the ratios of the 39 kDa procathepsin L to its 30 kDa intermediate and 25 kDa mature product in cells from ethanol-fed rats were 1.5-3-fold higher than those in controls. These results demonstrate that ethanol consumption caused a marked impairment in the processing of procathepsin L to mature enzyme, without affecting its synthesis. Taken together, our findings suggest that chronic ethanol consumption caused a deficiency in intralysosomal enzyme content by altering the trafficking and processing of these hydrolases into lysosomes.

Role of acetaldehyde in the ethanol-induced impairment of hepatic glycoprotein secretion in the rat in vivo

Ethanol administration inhibits hepatic protein and glycoprotein secretion. Previous studies have shown that the metabolism of ethanol is required for this effect. Experiments were designed to determine whether acetaldehyde, the first metabolite of ethanol oxidation, mediated the ethanol-induced secretory defect in rats with normal and stimulated (inflammation-induced) rates of hepatic protein secretion. This study used cyanamide, an aldehyde dehydrogenase inhibitor, to correlate enhanced acetaldehyde levels with an increased ethanol-induced inhibition of hepatic protein secretion. Inflammation was induced by turpentine 24 hr prior to cyanamide (5 mg per kg body weight) or saline pretreatment. Nonfasted rats were intragastrically gavaged with ethanol (4 to 6 gm per kg body weight) or isocaloric glucose 1 hr following pretreatment. [3H]Fucose and/or [14C]leucine were injected intravenously 2 hr following intubation. With elevated levels of acetaldehyde, the ethanol-induced impairment of secretion of labeled proteins and their parallel retention in the liver were markedly potentiated. During inflammation, this inhibition of secretion by ethanol was maintained and further increased with cyanamide pretreatment. These results indicate that the ethanol-induced impairment of hepatic glycoprotein secretion is mediated by acetaldehyde in both normal and inflammation-stimulated animals.

Effect of ethanol on hepatic secretory proteins

Both acute and chronic ethanol administration inhibit the secretion of albumin and glycoproteins from the liver. Impairment of posttranslational steps of the secretory process are mainly involved in this secretory defect, although in some instances altered synthesis of the protein moiety may be a factor. Decreased secretion following ethanol administration results in the intrahepatic retention of export proteins. The secretory defect is a consequence of the metabolism of ethanol and is likely mediated via acetaldehyde, although more conclusive proof is still required. The manner by which acetaldehyde impairs the secretory process is unknown, but may be related to its high reactivity with hepatocellular proteins. The specific posttranslational steps or processes involved in the secretory defect are still unclear; however, it appears that the final steps of secretion (post-Golgi events) may be the primary site of impairment. Impaired secretion of proteins from the liver could contribute to altered levels of plasma proteins and hepatomegaly as well as to the liver injury observed in the alcoholic.