Subcellular location of secretory proteins retained in the liver during the ethanol-induced inhibition of hepatic protein secretion in the rat

Alcohol consumption impairs hepatic protein trafficking: mechanisms and consequences

Alcoholic liver disease is a major biomedical health concern in the United States. Despite considerable research efforts aimed at understanding the progression of the disease, the specific mechanisms leading to alcohol-induced damage remain elusive. Numerous proteins are known to have alcohol-induced alterations in their dynamics. Defining these defects in protein trafficking is an active area of research. In general, two trafficking pathways are affected: transport of newly synthesized secretory or membrane glycoproteins from the Golgi to the basolateral membrane and clathrin-mediated endocytosis from the sinusoidal surface. Both impaired secretion and internalization require ethanol metabolism and are likely mediated by acetaldehyde. Although the mechanisms by which ethanol exposure impairs protein trafficking are not fully understood, recent work implicates alcohol-induced modifications on tubulin or components of the clathrin machinery as potential mediators. Furthermore, the physiological ramifications of impaired protein trafficking are not fully understood. In this review, we will list and discuss the proteins whose trafficking patterns are known to be impaired by ethanol exposure. We will then describe what is known about the possible mechanisms leading to impaired protein trafficking and how disrupted protein trafficking alters liver function and may explain clinical features of the alcoholic patient.

Ethanol selectively impairs clathrin-mediated internalization in polarized hepatic cells

Although alcoholic liver disease is clinically well-described, the molecular basis for alcohol-induced hepatotoxicity is not well understood. Previously, we determined that the clathrin-mediated internalization of asialoglycoprotein receptor was impaired in ethanol-treated WIF-B cells whereas the internalization of a glycophosphatidylinositol-anchored protein thought to be endocytosed via a caveolae/raft-mediated pathway was not changed suggesting that clathrin-mediated endocytosis is selectively impaired by ethanol. To test this possibility, we examined the internalization of a panel of proteins and compounds internalized by different mechanisms in control and ethanol-treated WIF-B cells. We determined that the internalization of markers known to be internalized via clathrin-mediated mechanisms was impaired. In contrast, the internalization of markers for caveolae/raft-mediated endocytosis, fluid phase internalization or non-vesicle-mediated uptake was not impaired in ethanol-treated cells. We further determined that clathrin heavy chain accumulated at the basolateral surface in small puncta in ethanol-treated cells while there was decreased dynamin-2 membrane association. Interestingly, the internalization of resident apical proteins that lack any known internalization signals was also disrupted by ethanol suggesting that these proteins are internalized via clathrin-mediated mechanisms. This conclusion is consistent with our findings that dominant negative dynamin-2 overexpression impaired internalization of known clathrin markers and single spanning apical residents, but not of markers of fluid phase or raft-mediated internalization. Together these results indicate that ethanol exposure selectively impairs hepatic clathrin-mediated internalization by preventing vesicle fission from the plasma membrane.

Acute and chronic ethanol consumption differentially impact pathways limiting hepatic protein synthesis

This review identifies the various pathways responsible for modulating hepatic protein synthesis following acute and chronic alcohol intoxication and describes the mechanism(s) responsible for these changes. Alcohol intoxication induces a defect in global protein synthetic rates that is localized to impaired translation of mRNA at the level of peptide-chain initiation. Translation initiation is regulated at two steps: formation of the 43S preinitiation complex [controlled by eukaryotic initiation factors 2 (eIF2) and 2B (eIF2B)] and the binding of mRNA to the 40S ribosome (controlled by the eIF4F complex). To date, alcohol-induced alterations in eIF2 and eIF2B content and activity are best investigated. Ethanol decreases eIF2B activity when ingested either acutely or chronically. The reduced eIF2B activity most likely is a consequence of twofold increased phosphorylation of the alpha-subunit of eIF2 on Ser(51) following acute intoxication. The increase in eIF2alpha phosphorylation after chronic alcohol consumption is the same as that induced by acute ethanol intoxication, and protein synthesis is not further reduced by long-term alcohol ingestion despite additional reduced expression of initiation factors and elongation factors. eIF2alpha phosphorylation alone appears sufficient to maximally inhibit hepatic protein synthesis. Indeed, pretreatment with Salubrinal, an inhibitor of eIF2alpha(P) phosphatase, before ethanol treatment does not further inhibit protein synthesis or increase eIF2alpha phosphorylation, suggesting that acute ethanol intoxication causes maximal eIF2alpha phosphorylation elevation and hepatic protein synthesis inhibition. Ethanol-induced inhibition of hepatic protein synthesis is not rapidly reversed by cessation of ethanol consumption. In conclusion, sustained eIF2alpha phosphorylation is a hallmark of excessive alcohol intake leading to inhibition of protein synthesis. Enhanced phosphorylation of eIF2alpha represents a unique response of liver to alcohol intoxication, because the ethanol-induced elevation of eIF2alpha(P) is not observed in skeletal muscle or heart.

Relationship between serum sialic acid and sialylated glycoproteins in alcoholics

AIMS

Total sialic acid (TSA) has been suggested as a marker for chronic alcohol abuse. It seems that the elevation of TSA during excessive alcohol consumption reflects the changes in sialylated glycoproteins in the sera. On the other hand, chronic ethanol consumption increases the desialylation rate of many serum glycoproteins. The aim of this study was to evaluate the relationship between the total and free form of sialic acid levels (FSA), and the concentration of sialylated glycoproteins in alcoholics.

METHODS

We determined the serum concentration of many glycoproteins (alpha1-antitrypsin, alpha1-acid glycoprotein, haptoglobin, ceruloplasmin, transferrin, complement C3 protein, fibrinogen and immunoglobulin G) in a sample of 100 alcoholics and 30 healthy controls. Total sialic acid was determined by the enzymatic method and its free form by using a modification of the thiobarbituric acid method.

RESULTS

Among alcoholics, we found increased concentrations of alpha1-antitrypsin and alpha1-acid glycoprotein but decreased levels of transferrin. The concentrations of TSA and FSA were significantly higher in alcoholics than in healthy controls. The tested glycoproteins, except for transferrin and immunoglobulin G, positively correlated with TSA and FSA. The correlations with TSA were higher than that with FSA.

CONCLUSIONS

Chronic alcohol abuse alters the concentrations of some sialylated glycoproteins in the sera. The alpha1-antitrypsin, alpha1-acid glycoprotein, and transferrin are the only affected glycoproteins. The serum level of total and free form of sialic acid in the sera of alcoholics depends on the concentration of the most sialylated glycoproteins.

Rat sinusoidal liver endothelial cells (SECs) produce pro-fibrotic factors in response to adducts formed from the metabolites of ethanol

Previous studies with alcohol-associated malondialdehyde-acetaldehyde (MAA)-modified proteins have demonstrated an increase in the expression of adhesion molecules, and the secretion of pro-inflammatory cytokines/chemokines by rat sinusoidal liver endothelial cells (SECs). However, no studies have been initiated to examine the effects of MAA-modified proteins on the expression of the extracellular matrix (ECM) protein, fibronectin and its isoforms. For these studies, SECs were isolated from the liver of normal rats, and exposed to MAA-modified bovine serum albumin (MAA-Alb). At selected time points, the total plasma and cellular fibronectin were determined by Western blot. Injection of rat liver via the mesenteric vein with MAA-Alb was performed in an effort to evaluate the potential in vivo role of MAA-modified proteins in the development of fibrosis. Expression of both plasma and cellular fibronectin was significantly increased over controls in the MAA-Alb stimulated SECs (>3-fold). Importantly, the isotype of fibronectin secreted was determined to be of the EIIIA variant and not EIIIB. These data were confirmed using RT-PCR procedures on liver tissue from; isolated SECs, and from an in vivo animal model wherein MAA-Alb was administered via the mesenteric vein. Thus, these studies demonstrate that MAA-modified proteins initiate a pro-fibrogenic response by initiating the expression of the fibronectin EIIIA isoform by SECs.

Rat hepatocyte aquaporin-8 water channels are down-regulated in extrahepatic cholestasis

Hepatocytes express the water channel aquaporin-8 (AQP8), which is mainly localized in intracellular vesicles, and its adenosine 3',5'-cyclic monophosphate (cAMP)-induced translocation to the plasma membrane facilitates osmotic water movement during canalicular bile secretion. Thus, defective expression of AQP8 may be associated with secretory dysfunction of hepatocytes caused by extrahepatic cholestasis. We studied the effect of 1, 3, and 7 days of bile duct ligation (BDL) on protein expression, subcellular localization, and messenger RNA (mRNA) levels of AQP8; this was determined in rat livers by immunoblotting in subcellular membranes, light immunohistochemistry, immunogold electron microscopy, and Northern blotting. One day of BDL did not affect expression or subcellular localization of AQP8. Three days of BDL reduced the amount of intracellular AQP8 (75%; P <.001) without affecting its plasma membrane expression. Seven days after BDL, AQP8 was markedly decreased in intracellular (67%; P <.05) and plasma (56%; P <.05) membranes. Dibutyryl cAMP failed to increase AQP8 in plasma membranes from liver slices, suggesting a defective translocation of AQP8 in 7-day BDL rats. Immunohistochemistry and immunoelectron microscopy in liver sections confirmed the BDL-induced decreased expression of hepatocyte AQP8 in intracellular vesicles and canalicular membranes. AQP8 mRNA expression was unaffected by 1-day BDL but was significantly increased by about 200% in 3- and 7-day BDL rats, indicating a posttranscriptional mechanism for protein level reduction. In conclusion, BDL-induced extrahepatic cholestasis caused posttranscriptional down-regulation of hepatocyte AQP8 protein expression. Defective expression of AQP8 water channels may contribute to bile secretory dysfunction of cholestatic hepatocytes.

Sialic acid: new potential marker of alcohol abuse

BACKGROUND

A number of laboratory markers are suggested for the detection and monitoring of alcohol abuse. However, there is still a need to find better indicators of alcohol abuse. Sialic acid (SA) is the name for a series of acyl-derivatives of neuraminic acids that occur as nonreducing terminal residues of glycoproteins or glycolipids in biological fluids and cell membranes. In this study, we investigated the diagnostic value of SA as a marker of alcohol abuse.

METHODS

Sera from social drinkers (n = 38) and alcoholics (n = 77) were analyzed for sialic acid by a colorimetric assay and for carbohydrate-deficient transferrin (CDT) by a radioimmunoassay method. Mean corpuscular volume (MCV), gamma-glutamyltransferase (GGT), aspartate aminotransferase (ASAT), and alanine aminotransferase (ALAT) were determined by using routine methods.

RESULTS

The sialic acid levels of both female and male subjects were significantly (p < 0.001) increased among alcoholic subjects when compared with social drinkers. SA levels were decreased after 3 weeks of treatment. The sensitivity and specificity for SA, respectively, were 57.7 and 95.5 for women and 47.8 and 81.3 for men. The respective values for CDT were 57.7 and 95.5 for women and 78.3 and 100.0 for men; for GGT, 60.0 and 95.5 for women and 60.9 and 87.5 for men; for MCV, 52.4 and 95.5 for women and 47.8 and 100.0 for men; for ASAT, 53.8 and 95.5 for women and 43.5 and 100.0 for men; and for ALAT, 38.5 and 90.9 for women and 39.1 and 87.5 for men. Among women, SA and GGT, and among men CDT, showed the largest area under receiver operation curve.

CONCLUSION

This study indicated that sialic acid levels were elevated by high alcohol consumption and reduced during abstinence, especially among women. Thus, sialic acid seems to be an interesting marker that needs further evaluation as a diagnostic tool for alcohol abuse.

Chronic ethanol consumption impairs receptor-mediated endocytosis of formaldehyde-treated albumin by isolated rat liver endothelial cells

Receptor-mediated endocytosis (RME) by a scavenger receptor on sinusoidal liver endothelial cells (LECs) for formaldehyde-treated bovine serum albumin (f-Alb) has previously been shown to be impaired following chronic ethanol consumption. These studies were initially performed by in situ perfusion, making it difficult to determine the point in the process at which RME is affected. Therefore, it was the purpose of this study to use isolated LECs to begin elucidating at what point in the process chronic ethanol consumption affects RME. Initial studies showed that degradation at the single-cell level were similarly decreased at levels that had been observed for in situ studies, suggesting that the ethanol effects can be repeated using isolated LECs, making them useful for in vitro studies. Binding studies with 125I-formaldehyde-treated bovine serum albumin (125I-f-Alb) demonstrated there was a slight, but significantly different, decrease in binding by LECs from ethanol-fed rats when compared with pair-fed or chow-fed rats. However, the affinity of these receptors was not different between these groups. In contrast, a defect in the initial stages of receptor-ligand internalization was indicated as less surface-bound ligand was internalized and subsequently degraded in cells from the ethanol-treated animals as compared with controls. Additionally, once the data were adjusted for the amount of ligand internalized, the degradation of the internalized ligand was only slightly impaired. These results indicate that chronic ethanol feeding impairs the process of RME by the liver; the major cause of this impairment appears to be caused by a decreased ability of these cells to internalize all of the surface-bound ligand, with a minimal defect in postinternalization events.

Monoclonal and polyclonal antibodies recognizing acetaldehyde-protein adducts

Studies have investigated the hypothesis that metabolically derived acetaldehyde (AA) is capable of complexing with liver cell proteins to form AA-protein adducts that are capable of acting as antigens and inducing an immune response, as detected by the formation of unique antibodies. In an effort to better characterize and describe these adducts, mouse monoclonal and rabbit polyclonal antibodies specific for antigens prepared with AA under non-reducing (physiologic) and reducing (presence of sodium cyanoborohydride) conditions have been prepared. Two monoclonal antibodies were developed. The first antibody was RT1.1, which is specific to N-ethyl lysine (NEL); it is of the IgG2b isotype and recognizes all proteins modified with AA under reducing conditions. The other monoclonal antibody, NR-1, was of the IgG3 isotype; it recognizes proteins modified with AA under non-reducing conditions and cannot be inhibited by NEL. Affinity-purified and/or absorbed polyclonal antibodies were also produced to these epitopes. Using this panel of monoclonal and affinity-purified polyclonal antibodies, unique antigen-antibody binding occurred that: (1) detected only NEL; (2) reacted with the alpha-amino group on proteins prepared under reducing conditions; and (3) detected adducts on proteins prepared under non-reducing conditions. However, the only antibodies that recognized antigen(s) from alcohol-fed rat livers were those that were not specific to NEL or the alpha-amino group modified under reducing conditions. These data indicate that the relevant adduct in alcohol-fed rat livers is not NEL, and that it presumably is related to proteins modified with AA under non-reducing conditions.

Detection of circulating antibodies to malondialdehyde-acetaldehyde adducts in ethanol-fed rats

BACKGROUND & AIMS

Malondialdehyde and acetaldehyde react together with proteins and form hybrid protein conjugates designated as MAA adducts, which have been detected in livers of ethanol-fed rats. The aim of this study was to examine the immune response to MAA adducts and other aldehyde adducts during long-term ethanol exposure.

METHODS

Rats were pair-fed for 7 months with a liquid diet containing either ethanol or isocaloric carbohydrate. Circulating antibody titers against MAA adducts and acetaldehyde adducts were measured and characterized in these animals.

RESULTS

A significant increase in antibody titers against MAA-adducted proteins was observed in the ethanol-fed animals. Competitive inhibitions of antibody binding indicated that the circulating antibodies against MAA-modified proteins in the ethanol-fed rats recognized mainly a specific, chemically defined MAA epitope. Antibody titers to reduced and nonreduced acetaldehyde adducts were very low, and no significant differences were observed between ethanol-fed and control animals. Significant plasma immunoreactivity to not only MAA-adducted but also unmodified rat liver proteins (cytosol, microsomes, and especially plasma membrane) were also observed in the ethanol-fed rats.

CONCLUSIONS

Long-term ethanol feeding generates circulating antibodies not only against MAA epitopes but possibly also against unmodified, native (self) protein epitopes, suggesting that MAA adducts could trigger harmful autoimmune responses.

Ethanol-induced retention of nascent proteins in rat hepatocytes is accompanied by altered distribution of the small GTP-binding protein rab2

Chronic ethanol consumption induces hepatocellular retention of nascent proteins leading to hepatomegaly. While the molecular mechanisms behind this impairment are undefined, it has been predicted that protein retention results from a disruption of vesicle-mediated secretory processes. Small GTP-binding proteins (rab proteins) have recently been implicated in the regulation of vesicular trafficking in eukaryotic cells. Our objectives were to identify intracellular sites of ethanol-induced protein retention and to determine whether the distribution of secretory rab proteins was altered by ethanol. Transport of hepatic proteins along the secretory pathway in livers from control and ethanol-fed rats was analyzed using subcellular fractionation and immunoprecipitation in the context of in vivo pulse-chase experiments. We show that pre-Golgi and Golgi compartments, as well as secretory vesicles, are sites of ethanol-induced retention of nascent soluble and transmembrane secretory proteins. These results are supported by immunofluorescence localization of hepatic proteins on liver sections. Further, immunoblot analyses of hepatic subcellular fractions from ethanol-damaged livers indicate a dramatic reduction in the association of rab2 with a Golgi compartment as compared with controls. In contrast, rab6 and alpha-mannosidase II, Golgi marker proteins, appear unchanged. These studies provide a detailed analysis of the intracellular site of ethanol-induced protein retention in the hepatocyte and lend novel insight into a potential mechanism behind this impairment. The effects of ethanol exposure on rab proteins and Golgi function are discussed.

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.

Ethanol consumption reduces the proteolytic capacity and protease activities of hepatic lysosomes

Chronic ethanol consumption causes decreased hepatic protein degradation, resulting in protein accumulation within hepatocytes. In this investigation, we sought to determine whether chronic ethanol feeding alters the degradative capacity and protease activities of isolated hepatic lysosomes. Male Sprague-Dawley-derived rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric maltose-dextrin for 1-5 wk. Hepatic lysosomes were isolated by differential centrifugation and purified through Percoll gradients. Lysosomes obtained from livers of ethanol-fed rats degraded both endogenous protein substrates and the exogenously added radioactive substrate, 125I-RNase A, 26-42% more slowly than lysosomes from pair fed controls. The ethanol-elicited reduction in proteolytic capacity appeared to result in part, from a deficiency of the lysosomal cathepsins B, L, and H. Compared with controls, the specific activities of these enzymes were 31-45% lower in lysosomes from ethanol-fed rats. Immunoblot analyses also revealed that the intralysosomal as well as the intracellular content of cathepsin B was significantly lower in ethanol-fed rats. In contrast, ethanol consumption did not affect the cellular quantity of cathepsin L but lowered its amount in isolated lysosomes. Our findings suggest that chronic ethanol consumption causes a deficiency in lysosomal cathepsins by altering their biosynthesis and/or their trafficking into lysosomes.

Effects of ethanol on hepatitis B virus Pre-S/S gene expression in the human hepatocellular carcinoma derived HEP G2 hepatitis B DNA positive cell line

BACKGROUND/AIMS

Among the reported interactions between ethanol and hepatitis B virus (HBV), studies of transgenic mice have suggested an effect of ethanol on the secretion of viral envelope proteins.

METHODS

We further investigated these interactions in vitro by determining HBs antigen levels and performing northern blots of viral mRNA in human cell culture (HepG2 HBV positive cells) exposed for 3 to 12 days to various concentrations of ethanol.

RESULTS

In cultures exposed to 200 mM ethanol, HBs antigen concentrations increased in the medium (p < 0.05) after 3 days as Pre-S1 and Pre-S2 protein concentrations. This increase was not specific, as albumin and ferritin increased in the same proportions. Ethanol also increased the HBs antigen concentration in the cells (p < 0.05), whereas levels of viral mRNA encoding surface proteins were unaffected.

CONCLUSIONS

These findings show that short-term ethanol exposure in vitro can induce HBs antigen overexpression via a post-transcriptional mechanism.

Ethanol inhibits asialoglycoprotein receptor synthesis but augments its mRNA expression in a human hepatoma cell line

The effect of ethanol on the expression of asialoglycoprotein receptor protein and its mRNA was studied in a human hepatoma cell line, HepG2. The number of asialoglycoprotein receptors on the cell surface was decreased to 60% of the control level, without a loss in affinity, by incubating the cells with 100 mM ethanol. The decrease in cell surface asialoglycoprotein receptors was paralleled by a decrease in total receptor numbers, including intracellular and surface receptors. The internalization of asialoglycoprotein was also diminished, to 44% of that in control cells. The decreases in cell surface receptors and total receptor numbers were partially restored by 2 mM 4-methylpyrazole, suggesting that ethanol metabolites participated in the diminution of asialoglycoprotein receptor expression. However, the steady-state expression of asialoglycoprotein receptor mRNA was increased in ethanol-treated cells and further augmented by a longer ethanol exposure. These paradoxical results, i.e., the decrease of asialoglycoprotein receptor protein and the increase of its mRNA expression, suggest that the reduction in the asialoglycoprotein receptor protein is a post-transcriptional event and that a possible feedback regulatory mechanism may control asialoglycoprotein receptor gene transcription and/or impair the degradation of its mRNA.

Zonal differences in ethanol-induced impairments in hepatic receptor binding

We have previously shown ethanol-induced defects in receptor-mediated endocytosis of asialoorosomucoid (ASOR), epidermal growth factor (EGF), and insulin in isolated rat hepatocytes. The present study was undertaken to compare the binding of these three ligands in both Zone 1 (periportal [PP] region) and Zone 3 (perivenule [PV] region) of rat liver. Cells from the PV region of ethanol-fed animals bound significantly less EGF (40% decrease) than did cells from the same area in control rats. EGF binding was decreased to a lesser extent (15-25%) in PP cells from ethanol-fed animals compared to controls. When binding of ASOR was examined, ethanol feeding significantly impaired binding in both PP cells (30-35% decrease) and PV cells (50-55% decrease), again showing a greater ethanol-induced impairment in the PV region. Insulin binding in ethanol animals was decreased by 20-25% in both regions compared to controls. In addition, we found that ASOR receptor recycling was impaired to a greater extent in the PV than in the PP region of liver after ethanol feeding, indicating selective impairment of receptor function in the centrilobular region of the liver.

Chronic ethanol consumption induces accumulation of proteins in the liver Golgi apparatus and decreases galactosyltransferase activity

The effect of chronic ethanol consumption on labeled glycoprotein secretion and galactosyltransferase activity has been analyzed in cis- and trans-Golgi apparatus fractions isolated from rat liver. Rats were injected intraperitoneally with 3H-leucine, after different chase periods (30, 60, 180 min), and the radioactivity of the different subcellular fractions as well as of the isolated Golgi apparatus was measured. Chronic alcohol treatment induces an increase in liver weight as well as an enhancement of total liver protein. Ethanol treatment produces a significant accumulation of labeled proteins in isolated Golgi apparatus fractions after a 60- and 180-min chase. An accumulation of labeled proteins in the cytosolic fraction was observed only after 180 min. The alcohol treatment also induces a significant decrease in the activity of galactosyltransferase in both liver homogenate and Golgi apparatus fractions. These results suggest that an impairment of Golgi apparatus functions, including glycosylation and glycoprotein trafficking, could be one of the mechanisms involved in the accumulation of hepatic protein and thus in the pathogenesis of alcohol-induced injury in the liver of chronic ethanol-consuming animals.

Liver triglyceride concentration and body protein metabolism in ethanol-treated rats: effect of energy and nutrient supplementation

The objective of this study was to compare the metabolic effects of long-term ethanol consumption with oral (Lieber-DeCarli) and enteral feeding techniques. Enteral feeding allowed administration of greater amounts of energy and nutrients. After 21 days of treatment using the Lieber-DeCarli technique, the ethanol-treated rats had the following significant (P less than 0.05) differences from pair-fed controls: lower cumulative nitrogen balance (days 5-21; 2.8 +/- 0.1 g N vs. 3.5 +/- 0.1 g N), lower protein content of gastrocnemius muscle (289 +/- 17 mg vs. 358 +/- 11 mg) and intestinal mucosa (461 +/- 19 mg vs. 577 +/- 40 mg), higher plasma leucine concentration (147 +/- 8 mumol/L vs. 102 +/- 8 mumol/L), higher liver protein content (2222 +/- 122 mg vs. 1679 +/- 58 mg), and higher liver triglyceride concentration (38.4 +/- 2.8 mg/g vs. 8.7 +/- 1.0 mg/g). When rats received the same amount of nitrogen (1.5 g.kg-1.day-1) and ethanol (13 g.kg-1.day-1) but 16.3% more energy and nutrients by a surgically implanted gastric cannula (enterally fed), the effects of ethanol on nitrogen balance, tissue protein content, plasma leucine concentration, and liver triglyceride concentration were similar to those observed in the rats fed orally. It is concluded that the metabolic effects observed using the Lieber-DeCarli feeding technique are due to ethanol per se and not the synergism of ethanol and undernutrition as recently suggested.

Effects of ethanol on the secretion of hepatic secretory protein in rat alcoholic liver injury

It has been pointed out that one of the pathogenetic causes of alcoholic liver injury is the hepatocytic accumulation of exportable proteins due to a decrease in hepatic microtubules caused by acetaldehyde. To confirm and extend this secretory protein accumulation in the hepatocytes, the effects of alcohol treatment on the intracellular transport of secretory protein in the hepatocyte was studied using radioisotope-labeled leucine and fucose. Acute ethanol administration to rats did not show any effects on intrahepatocytic transport and secretion of transferrin. In alcohol pyrazole hepatitis rats, the secretion of transferrin labeled with both radioactive leucine and fucose into the serum was significantly delayed. Delaying in the secretion of fucose-labeled transferrin was more prominent than in leucine-labeled transferrin. This secretory inhibition was accompanied by a corresponding increase in the hepatic retention of both leucine- and fucose-labeled transferrin. At the time of the maximum inhibition of secretion, radioisotope labeled transferrin mainly retained in the Golgi apparatus. These results indicated that movement of secretory proteins along the secretory pathway impaired in alcoholic liver injury and that accumulation of the secretory proteins might play an important role in the development of alcoholic liver injury.

Interactive effects of ethanol and caffeine on rat fetal hepatocyte replication and EGF receptor expression

This study reports on the interactive effects of ethanol and caffeine on growth of rat fetal hepatocytes. Exposure of cultured rat fetal hepatocytes (RFH) to ethanol in concentrations above 1 mg/ml, causes a blockade of EGF-dependent cell replication along with an overexpression of surface EGF receptors (EGF-R). However, RFHs exposed for 24 hours to ethanol at a concentration of 1 mg/ml alone had little effect on cell replication. Caffeine, when combined with this concentration of alcohol, progressively impaired RFH growth by up to 100%. Caffeine alone up to 10 micrograms/ml, on the other hand, caused a progressive increase in RFH replication associated with a 69% enhancement of DNA synthesis. Caffeine concentrations in excess of 50 micrograms/ml had no effect on replicative capacity. Concomitant caffeine exposure had no effect on the ethanol-related increase in cell DNA content, yet it caused a further enhancement of the cell protein accural induced by ethanol alone. Caffeine (10 micrograms/ml) alone had no effect on EGF-R expression, while ethanol (2 mg/ml) increased it by almost 200%. Addition of caffeine to ethanol reduced this enhanced EGF binding by 45%. Scatchard analysis indicated that no treatment altered ligand affinity for the receptor, but that the alterations in binding caused by ethanol and the caffeine/ethanol combination reflected changes in binding capacity, in both low and high affinity components. It is concluded that (1) ethanol blocks EGF-mediated replication accompanied by a reduction in DNA synthesis, (2) caffeine alone at low concentrations has the opposite effect and can actually potentiate the EGF-mediated mitogenic response, (3) caffeine in combination with ethanol acts synergistically to reduce RFH replication.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma protein catabolism in ethanol- and colchicine-treated liver slices

The present study was conducted to determine whether the antisecretory agents colchicine and ethanol affect the intracellular degradation of plasma proteins in rat liver. Plasma proteins were prelabeled in vivo with [3H]leucine and their levels were monitored immunochemically in both the medium and extracts of rat liver slices incubated alone or in the presence of 50 microM colchicine or 25 mM ethanol. Compared with those left untreated, colchicine-treated slices had a 40-55% lower secretory capacity and, at one point, showed significant hepatocellular retention of total plasma proteins. Plasma protein secretion by ethanol-treated liver slices was 22-32% lower than controls, but there was no detectable retention of unsecreted plasma proteins in the ethanol-treated liver tissue. In all experiments, the total radioactivity in plasma proteins (i.e., the immunoprecipitable radioactivity in the liver plus that in the medium) decreased with time in a manner suggestive of intracellular degradation. Regression analyses of the rates of degradation of presecretory proteins revealed that compared with controls, plasma protein catabolism was accelerated 57% in colchicine-treated slices. In ethanol-treated liver slices, there was a 50% increase in the degradation of total plasma proteins and a 46% increase in albumin catabolism. In all cases, degradation was intracellular. These findings indicate that inhibition of hepatic protein secretion by either colchicine or ethanol is associated with accelerated catabolism of unsecreted plasma proteins, suggesting that hepatocellular degradative processes are responsive to changes in the levels of presecretory proteins and/or perturbations of the secretory process.

Effect of chronic ethanol administration on the uptake and degradation of asialoglycoproteins by the perfused rat liver

We have shown previously reduced binding, internalization, degradation and receptor-ligand dissociation during receptor-mediated endocytosis (RME) of 125I-asialoorosomucoid (ASOR) by hepatocytes isolated from rats fed ethanol for 4-6 weeks. In the present study, we investigated the effect of ethanol feeding on RME by using the intact perfused liver as a model. Male, Sprague-Dawley rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric carbohydrate. Receptor-mediated endocytosis of 125I-ASOR was then examined over a time course of perfusion. In all cases, clearance of the labeled glycoprotein was followed by a slower but steady appearance of acid-soluble products in the medium. Ethanol-fed animals had a significantly (P less than 0.01) slower rate of clearance of the labeled ligand from the circulating perfusate than did control animals. Impairment of ASOR surface binding and degradation in ethanol-fed animals was also demonstrated in this model. When we examined the subcellular distribution of labeled ligand after various times of perfusion, we found that in control livers, a shift of radiolabeled ligand from the subcellular fractions containing endosomes and plasma membranes to fractions containing lysosomes occurred, while significantly less ligand was shifted to the lysosomes of ethanol-treated rats. These results show that ethanol administration inhibits RME of ASOR in the isolated perfused liver model, thus confirming our earlier reported defects in isolated hepatocytes. In addition, transport of ligand along the intracellular RME pathway was also shown to be altered by ethanol treatment as indicated by the impaired movement of ASOR from the endosomal to the lysosomal compartment.

Ethanol and soluble mediators of host response

Lymphokines serve to modulate host inflammatory response by providing a communication link among cells involved in resistance to infection. In the alcoholic, this system may be impaired due to a combination of the direct effects of ethanol on immunocompetent cells and the soluble factors involved in cell-cell interactions. In this paper, we review the literature on this subject, describe an ethanol-related impairment of migration inhibitory factor activity in the rat, and present a possible mechanism for this alteration.

Ethanol-induced impairments in receptor-mediated endocytosis of asialoorosomucoid in isolated rat hepatocytes: time course of impairments and recovery after ethanol withdrawal

Chronic ethanol administration markedly impairs the process of receptor-mediated endocytosis (RME) of a representative asialoglycoprotein, asialoorosomucoid (ASOR), by the liver. In this study, we further characterized these impairments by identifying the time of onset for ethanol-induced changes in RME as well as establishing the time course for recovery to normal endocytotic values after ethanol withdrawal. Ethanol administration for 3 days did not alter any aspect of endocytosis examined in this study. After feeding ethanol to rats for 7 days, however, significant decreases in amounts of ligand bound, internalized, and degraded were apparent. These impairments persisted throughout the 5-week feeding study although the effects were somewhat attenuated with more prolonged ethanol feeding. In addition, an accumulation of intracellular receptors was observed in ethanol-fed animals relative to controls after 7 days of ethanol feeding. In all cases, recovery of endocytotic values to control levels was partially completed after 2 to 3 days of refeeding control diet and was fully completed after 7 days of refeeding. These results indicate that ethanol feeding for as little as 7 days profoundly impairs the process of RME by the liver. These impairments can be reversed after refeeding control diet for 7 days.

Effect of chronic ethanol administration on protein catabolism in rat liver

Hepatic protein catabolism was measured in rats which were pair-fed a liquid diet containing either ethanol or isocaloric maltose-dextrin (control diet). Within 12 days after initiation of pair feeding, the level of total hepatic protein in ethanol-fed rats was 26% higher than that in pair-fed control rats. During this time interval, the catabolic rates of both short-lived [3H]puromycin-labeled proteins and long-lived native [14C]bicarbonate-labeled proteins were measured in the two groups of animals. The degradation rate of short-lived [3H]puromycinyl proteins and peptides was the same in ethanol-fed and pair-fed control rats. However, the overall catabolic rate of long-lived proteins in rats fed the ethanol liquid diet for 2-10 days was 37-40% lower than that in pair-fed controls. This difference in protein turnover was not a general phenomenon, since the time-dependent decay of [14C]proteins in the hepatic microsome fraction of ethanol-fed rats was 33% slower than that in pair-fed controls, but the apparent rate of cytosolic protein catabolism was the same in both groups of animals. The differences in protein turnover did not reflect quantitative changes in lysosomal proteases since the activities of four hepatic lysosomal cathepsins were unaffected by alcohol administration. When rats were subjected to longer periods of pair feeding (16-25 days), the difference in overall hepatic protein catabolism between ethanol-fed rats and their pair-fed controls was considerably attenuated.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic ethanol ingestion on alpha-mannosidase isoenzymes in rat liver

Identification of biochemical changes induced by ethanol ingestion would aid in the diagnosis and management of many alcohol-related problems in man. In this paper we identify a pH 5.5 alpha-mannosidase activity in the rat which is affected by chronic ethanol consumption. Chronic (16 wk) ingestion of alcohol (36% of calories) causes the activity of this alpha-mannosidase (thought to be the cytosolic alpha-mannosidase) in liver to decrease by 50%. We hypothesize that this deficiency of (pH 5.5) alpha-mannosidase activity may account for the reduced rate of secretion of glycoproteins by livers of alcohol-fed rats reported by other investigators (Volentine et al, Hepatology 1987;7:490-495).

Gangliosides, or sialic acid, antagonize ethanol intoxication

Because ethanol elicits a dose-dependent hydrolysis of brain sialogangliosides, we tested the possibility that injected gangliosides might antagonize intoxicating doses of ethanol. Clear anti-intoxication effects were seen at 24 hr post-injection of mixed mouse-brain gangliosides at 125-130 mg/kg, but not at lower or higher doses. Sleep time was reduced on the order of 50%, and roto-rod agility was significantly enhanced. Sialic acid (SA) similarly antagonized ethanol; however, the precursor of SA, N-acetyl-D-mannosamine, as well as ceramide and asialoganglioside did not.

Androgen-responsive functions of male rat liver. Effect of chronic alcohol ingestion

Many liver processes are sexually dimorphic, and in rats, testosterone is the major steroid hormone determinant of the differing patterns of hepatic function. The microsomal content of specific enzymes and the syntheses of specific proteins are dependent on serum testosterone to maintain this dimorphism. Because the liver of male rats is strikingly androgen responsive, and because chronic alcohol ingestion decreases serum testosterone, we sought to determine whether chronic alcohol feeding would alter the masculine pattern of hepatic liver function in male rats. We quantitated both the cytosolic and nuclear forms of the hepatic androgen receptor. Alcohol feeding of male rats results in a significant loss of both types of androgen receptor sites; the specific binding capacity of both cytosolic and nuclear receptor in alcohol-fed rats is reduced to about 30% of that in either isocalorically fed rats or rats fed ad libitum. This reduction in hepatic androgen receptor activity is concomitant with a 50% reduction in serum testosterone content in the alcohol-fed animals. In addition, the activities of two hepatic androgen-responsive proteins, namely a cytosolic estrogen binder and a microsomal enzyme, estrogen 2-hydroxylase, demonstrate a decrease in activity that parallels the decreases in both forms of the androgen receptor. Administration of testosterone to the alcohol-fed animals normalized both the hepatic androgen receptor and the androgen-responsive protein activities. From these results, we conclude that chronic alcohol feeding results in a decreased androgen responsiveness of the liver, a condition that most likely results from the decreased serum testosterone levels in the alcohol-fed animals.

Fatty acid-binding protein: a major contributor to the ethanol-induced increase in liver cytosolic proteins in the rat

To study the acute and chronic effects of ethanol on hepatic fatty acid-binding protein, rats were pair-fed with liquid diets containing 36% of energy either as ethanol or as additional carbohydrate for 4 to 5 weeks. Animals were killed 90 min after intragastric administration of diets with or without ethanol. Alcohol feeding markedly increased liver triglycerides, with a modest rise in nonesterified fatty acids. Alcohol-fed rats developed hepatomegaly, with a 48% increase in hepatic cytosolic proteins. Fatty acid binding was first assessed by the kinetics of [14C]palmitate binding to cytosolic proteins. The maximal binding capacity more than doubled in the cytosol of the ethanol-fed rats compared to pair-fed controls, whereas the dissociation constant increased by 64%. Acute ethanol administration (3 gm per kg body weight) either to ethanol-fed or control rats did not have a significant effect. To identify the fatty acid-binding protein, labeled cytosolic proteins were fractionated by gel filtration: most of the cytosolic fatty acids eluted as a single peak in the 12,000 to 18,000 molecular weight region corresponding to the hepatic fatty acid-binding protein. The increase in this protein, confirmed by radial immunodiffusion (27.0 +/- 1.4 mg per 100 gm body weight vs. 11.2 +/- 1.6, in controls; p less than 0.01), accounted for 22% of the total rise in cytosolic protein induced by chronic ethanol feeding.

Hepatocyte and nuclear areas in alcoholic liver cirrhosis: their relationship with the size of the nodules and the degree of fibrosis

The aim of the study in alcoholic cirrhotic patients was to determine if a relationship exists between the areas of hepatocytes and their nuclei and the area of the nodules to which these cells belong as well as the thickness of the fibrous tracts which delimit these nodules. It was found that hepatocyte and nuclear areas were enlarged the smaller the nodules and the thicker the surrounding fibrous tracts. Considering that oxygen supply in liver cirrhosis decreases with increasing fibrosis, our results permit the hypothesis that a low oxygen supply causes an increase not only in liver cell size but also in nuclear size, which is an index of nuclear activity.

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.

Stimulatory effects of ethanol on amino acid transport by rat fetal hepatocytes

Previous studies have indicated that acute, and especially chronic, maternal ethanol consumption can depress placental uptake of various amino acids. Since the fetal cell itself represents a second barrier to nutrients, one which may be altered by ethanol exposure, the effects of ethanol on amino acid net uptake by rat fetal hepatocytes was addressed. The present study determined that ethanol stimulated amino acid net uptake by fetal hepatocytes grown in monolayer culture. Fetal liver cells were grown in custom Williams' E medium (without L-arginine and with L-ornithine) and exposed to epidermal growth factor (0, 1, 2 or 5 ng per ml) and ethanol (1.7 +/- 0.1 or 3.9 +/- 0.2 mg per ml). Addition of ethanol (3.9 mg per ml) to the culture medium completely blocked measurable cell replication during a 48-hr exposure period. Fetal hepatocytes exposed to ethanol accrued both protein and water in a parallel fashion, both in excess of that by control cells. Ethanol (1.7 and 3.9 mg per ml) for 48 hr stimulated alpha-aminoisobutyric acid net uptake by fetal hepatocytes (p less than 0.05). Efflux was not affected (p greater than 0.05). The onset of this significant stimulation of net uptake was progressive and required in excess of 6 hr of contact with ethanol. This ethanol stimulation of alpha-aminoisobutyric acid net uptake persisted for at least 24 hr following ethanol withdrawal. The component(s) of alpha-aminoisobutyric acid net uptake stimulated by ethanol was independent of extracellular Na+.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane glycoconjugates as potential mediators of alcohol effects

1. Membrane glycoconjugates include glycoproteins and glycolipids that have many important functions in a wide variety of tissues, especially brain. 2. Alcohol's ability to fluidize and swell plasma membranes could be expected to alter the orientation and conformation of the embedded glycoconjugates. 3. Both kinds of glycoconjugates can contain terminal moieties of sialic acid, which has been shown to be decreased by single doses of alcohol. Chronic exposure to alcohol may have no effect on sialic acid, except in very young animals. 4. Glycolipids containing sialic acid (gangliosides) are also decreased by acute doses of alcohol, but chronic alcohol has little effect. Thus, gangliosides may have a role in the development and expression of tolerance. 5. Glycoproteins containing sialic acid may also be involved in alcohol action, but there has been less research in this area. 6. Alcohol-induced disruptions in membrane glycoconjugates could affect the important cellular functions that glycoconjugates have, and thus research on alcohol effects on glycoconjugates could lead to important discoveries of diagnostic and therapeutic value for alcohol abuse and alcoholism.

Ethanol-induced alterations of plasma membrane assembly in the liver

The effects of acute ethanol administration on the assembly of glycoproteins into the hepatic plasma membrane were studied in the rat. When [14C]fucose and N-acetyl[3H]mannosamine, a sialic acid precursor, were injected following an acute dose of ethanol, the incorporation of these precursors into the total pool of membrane glycoproteins was minimally affected. This finding indicated that ethanol treatment did not appreciably alter the glycosylation of proteins in the Golgi apparatus. However, the assembly of labeled fucoproteins and sialoproteins into the plasma membrane was markedly inhibited in the ethanol-treated animals. This inhibition of plasmalemmal glycoprotein assembly was accompanied by a corresponding accumulation of labeled glycoproteins in the cytosolic fraction of the hepatocyte. The content of labeled glycoproteins in the Golgi complex was not significantly altered by ethanol treatment. These results indicate that ethanol administration impairs the late stages of hepatic plasma membrane assembly and further suggest that ethanol administration interferes with the flow of membrane components from the Golgi apparatus to the surface membrane.