Influence of acute ethanol intoxication on rat liver Golgi apparatus glycosylation activities

Influence of chronic ethanol consumption on toxic effects of 1,2-dichloroethane: glycolipoprotein retention and impairment of dolichol concentration in rat liver microsomes and Golgi apparatus

Our previous investigations demonstrated that 1,2-dichloroethane (DCE) and chronic ethanol treatment separately are able to impair glycoprotein metabolism and secretion, and reduce dolichol concentration in liver membranes. The purpose of this study was to investigate whether chronic ethanol consumption can induce potentiation of rat liver damage due to DCE haloalkane used in several chemical processes and in agriculture. Rats were given 36% of their total energy as ethanol in the Lieber-DeCarli liquid diet for 8 weeks (CH group). The pair-fed control group received an isocaloric amount of dextrine-maltose (PF group). "In vitro" experiments: the DCE (6.5 mM) treatment of isolated hepatocytes from CH rats enhanced glycoprotein retention and further reduced glycoprotein secretion and 14C-glucosamine incorporation compared to the hepatocytes from CH or from PF and DCE treated rats. "In vivo" experiments: a marked decrease of dolichol concentration in microsomes (in which dolichyl phosphate is rate-limiting for the initial glycosylation of protein) and in Golgi membranes (in which total dolichol is very important for membrane permeability, fluidity and vesicle fusion) was observed in CH rats acutely treated with 628 mg/kg bw of DCE (CH+DCE) compared with CH or PF+DCE treated rats. These data suggest that chronic ethanol consumption increases DCE liver toxicity by affecting protein glycosylation processes and impairing glycolipoprotein secretion, with a concomitant retention at the level of the Golgi apparatus.

Carbohydrate-deficient transferrin and sialidase levels in coronary heart disease

Transferrin is a N-glycosylated glycoprotein and plays an important role in iron transport from sites of absorption and storage to sites of utilization. The main component of normal serum transferrin contains two biantennary glycans, each consisting of 2 mol of sialic acid (Tetrasialo transferrin). Normal serum also contains small amounts of tri- and disialotransferrin. We have undertaken this study to investigate the levels of serum carbohydrate-deficient transferrin (Desialotransferrin) and sialidase levels in patients with coronary heart disease. In patient group, serum desialotransferrin and sialidase levels were found to be significantly higher than control group (p < 0.01 and p < 0.001, respectively). We conclude that increased activity of sialidase may be responsible for increased desialotransferrin in patients with coronary heart disease. Serum desialotransferrin levels may be useful critaria to diagnosis and pathogenesis of coronary heart disease.

Alcohol and molecular regulation of protein glycosylation and function

Chronic alcohol exposure leads to the appearance of carbohydrate-deficient transferrin (CDT), a N-glycosylated protein and sialic acid-deficient apolipoprotein E (apoE), an O-glycosylated protein. We show that chronic ethanol treatment destabilizes sialyltransferase (ST) mRNA resulting in a concomitant decreased steady-state level of ST mRNA. As a result, alcohol markedly decreases the hepatic synthetic rate of ST. This leads to impaired sialylation of transferrin and apoE. Consequently, apoE content in plasma high-density lipoproteins (HDL) is decreased. ApoE plays a significant role in the delivery of HDL cholesterol to the liver via apo B/E receptor, a process called reverse cholesterol transport (RCT). Desialylation of apoE results in its decreased association with HDL. Thus, the dissociation constant of HDL for binding to sialo-apoE is 90 +/- 35 nM, whereas that for desialo-apoE is 1010 +/- 250 nM. More importantly, the uptake of labeled cholesterol by human HepG2 cells is decreased by 30-40% from reconstituted HDL particles (rHDL)-containing desialo-apoE compared to rHDL with sialo-apoE. We conclude that chronic alcohol exposure down-regulates the expression of sialyltransferase genes resulting in impaired sialylation of apoE. This leads to its decreased binding to plasma HDL and thereby, impairs the RCT function of HDL.

Serum carbohydrate-deficient transferrin: mechanism of increase after chronic alcohol intake

Carbohydrate-deficient transferrin (CDT) is now considered to be the most sensitive and specific biological marker of alcohol abuse. However, the mechanism by which chronic alcohol consumption causes an elevation of CDT levels in serum is still not understood. Therefore, we fed eight pairs of male rats a nutritionally adequate liquid diet containing either alcohol (36% of energy) or isocaloric dextrose (control) for 4 weeks, after which blood and liver samples were obtained. Serum CDT content in alcohol-treated rats increased by 45% (P < .05) in ethanol-fed animals compared with their corresponding controls. In contrast, in rats fed ethanol, the activities of sialyltransferase (ST), galactosyltransferase (GT), and N-acetylglucosamine transferase (N-AGT), which are glycosyltransferases involved in transferrin carbohydrate side chain synthesis, were diminished by 24% and 40% (P < .05), 23% and 51% (P < .05, .001), and 20% and 26% (P < .05) in total liver homogenates and Golgi fraction (GF) 1, respectively, when expressed as units/100 g body weight. These enzymes were also significantly less active in hepatic GFs 2 and 3. The depression of the transferase activities in ethanol-fed rats appeared to be due, at least in part, to enzyme inactivation by acetaldehyde, whereas ethanol itself was without effect. Similar results were obtained in humans: five alcohol abusers were found to exhibit a 23% decrease in hepatic sialyltransferase and a 41% increase in sialidase activities, respectively, when compared with three nondrinking subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term ethanol exposure impairs glycosylation of both N- and O-glycosylated proteins in rat liver

Carbohydrate residues of glycoproteins play important roles in their functions. We have previously shown that long-term ethanol treatment in rats alters the normal glycosylation pattern of plasma transferrin and apolipoprotein (apo) E. Glycosylation of proteins is a posttranslational process that is regulated by both glycosyltransferases and glycosidases, the resident enzymes of hepatic subcellular organelles. In this investigation using rat transferrin and apo E as model N- and O-glycosylated proteins, respectively, we have explored the effects of long-term ethanol treatment on the (1) incorporation of various labeled sugar precursors into these specific glycoproteins, (2) activities of mannosyltransferase, galactosyltransferase, and sialytransferases, and (3) hepatic synthetic rate of N-acetyl glucosamine (GlcNAc) alpha 2,6-sialyltransferase (2,6-ST). The relative ratio of labeled sugar to leucine incorporation (glycosylation index) showed a 43% (P < .01) decrease for relative mannosylation of transferrin molecule at both the microsomal and Golgi level in the ethanol group (AN) versus the control group (CN). For apo E, relative mannosylation was reduced by 48.9% (P < .01) and 46.9% (P < .01), respectively, at the microsomal and Golgi level in the AN versus CN. More importantly, relative sialation of transferrin was reduced by 86% (P < .001) in AN as compared with CN. Relative sialation of apo E was reduced by 35% (P < .01) in AN as compared with CN.(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary level of L-fucose as a marker of alcoholic liver disease

The urinary levels of L-fucose were measured in 93 alcoholics; 20 of these were without liver disease, 57 with noncirrhotic alcoholic liver disease, and 16 with alcoholic liver cirrhosis. In addition, patients with cirrhosis due to viral infection, and healthy subjects were evaluated. The mean urinary L-fucose concentration showed significantly higher values in patients with alcoholic liver disease and alcoholic liver cirrhosis when compared with the healthy subjects or the chronic alcoholics without liver disease (p < 0.001). The urinary L-fucose level was also significantly higher (p < 0.001) in cases of alcoholic liver cirrhosis than in noncirrhotic alcoholic liver disease (384 +/- 97 vs. 240 +/- 95 mumol/g of creatinine). No difference was observed between the healthy subjects and chronic alcoholics without liver disease (143 +/- 29 vs. 155 +/- 60 mumol/g of creatinine). The urinary level of L-fucose was significantly higher with alcoholic cirrhosis (384 +/- 97 mumol/g of creatinine) than with viral cirrhosis (265 +/- 42 mumol/g of creatinine) (p < 0.001). The measurement of urinary L-fucose may be a useful marker of alcoholic liver disease.

Effect of acute and chronic ethanol administration on rat liver alpha 2,6-sialyltransferase activity responsible for sialylation of serum transferrin

The effect of a single administration and a 6-week treatment with ethanol on rat liver sialyltransferase activity towards asialoglycoproteins and N-acetyllactosamine (Gal beta 1,4GlcNAc) was studied. Since only the alpha 2,6-sialyltransferase is involved in the in vivo sialylation of transferrin, Gal beta 1,4GlcNAc was chosen as an acceptor and alpha 2,6-sialyl-N-acetyllactosamine was separated from the corresponding alpha 2,3-sialyl isomer present in the sialyltransferase reaction mixture by high-performance liquid chromatography. After a single ethanol administration there was a low (about 20%) but significant (p less than 0.005) reduction of sialyltransferase activity towards asialotransferrin as well as a reduced alpha 2,6-sialyltransferase activity towards N-acetyllactosamine. An opposite result was found in the chronically ethanol-treated rats: in these animals either the total or alpha 2,6-sialyltransferase activity was slightly higher than in control animals. Blood ethanol concentration was significantly high (3.3 +/- 1.2 mg/ml) only in the acute-treated animals, suggesting that the accumulation in the body of ethanol and/or its metabolites induces a reduction of liver alpha 2,6-sialyltransferase activity responsible for the transferrin sialylation. Current results are consistent with the finding (Stibler H, Hultcrantz R: Alcohol Clin Exp Res 11:468-473, 1987) that an enhanced level of hyposialylated transferrin isoforms is a marker of present but not previous alcohol abuse.

The role of lipid peroxidation in liver damage

The consequences of the peroxidative breakdown of membrane lipids have been considered in relation to both the subcellular and tissue aspects of liver injury. Mitochondrial functions can be impaired by lipid peroxidation probably through the oxidation of pyridine nucleotides and the consequent alteration in the uptake of calcium. Several enzymatic functions of the endoplasmic reticulum are also affected as a consequence of peroxidative events and among these are the activities of glucose 6-phosphatase, cytochrome P-450 and the calcium sequestration capacity. Moreover, a release of hydrolytic enzymes from lysosomes and a decrease in the fluidity of plasma membranes can contribute to the liver damage consequent to the stimulation of lipid peroxidation. Extensive studies carried out in vivo and integrated with the use of isolated hepatocytes have shown that lipid peroxidation impairs lipoprotein secretion mainly at the level of the dismission from the Golgi apparatus, rather than during their assembly. However, such an alteration appears to give a late and not essential contribution to the fat accumulation. A more critical role is played by peroxidative reactions in the pathogenesis of acute liver necrosis induced by several pro-oxidant compounds as indicated by the protective effects against hepatocyte damage exerted by antioxidants. In addition, even in the cases where lipid peroxidation has been shown not to be essential in causing cell death there is evidence that it can still act synergistically with other damaging mechanisms in the amplification of liver injury.

Carbohydrate-deficient transferrin in serum in patients with liver diseases

Carbohydrate-deficient transferrin (CDT) in serum was analyzed by isocratic microanion exchange chromatography at pH 5.65 followed by a transferrin radioimmunoassay in 102 patients with biopsy-verified liver diseases. CDT values were normal in all of the 87 nonalcohol-abusing patients irrespective of type or degree of liver disease. Thirteen of the 15 alcoholic patients (87%) with current abuse showed elevated CDT values while in abstaining alcoholics with remaining liver disease the values were normal. No correlations were found between CDT level and volume density of liver fibrosis or steatosis or values of a number of clinicochemical liver tests. The only significant correlation demonstrated was between CDT concentration and the level of present daily alcohol consumption in the alcoholic patients. These results indicate that CDT can be used as a marker of present but not previous alcohol abuse, even in patients with various liver diseases.

Alterations in the cytochemical activity of several phosphatases in hepatocytes from rats exposed prenatally to ethanol

The activities of acid phosphatase, alkaline phosphatase, glucose-6-phosphatase, uridine diphosphatase, inosine diphosphatase, thiamine pyrophosphatase and 5'-nucleotidase have been investigated cytochemically in hepatocytes of the offspring of alcohol-fed rats, using cerium ions as a capturing agent and qualitative and quantitative electron microscopy. All these enzyme activities were decreased in the experimental animals compared with controls not exposed to ethanol. The pattern of deposition of the product of glucose-6-phosphatase activity in the cisternae of the endoplasmic reticulum was also different in the two groups. The phosphatases analyzed are functional markers of different cell components, and the results suggest that prenatal exposure of rats to ethanol causes functional alterations in the endoplasmic reticulum, Golgi apparatus, lysosomes and plasma membrane of hepatocytes.

Phospholipids, vitamin A and ubiquinone of the Golgi apparatus subfractions from rat liver after acute ethanol intoxication

Previous investigations from our laboratory have shown that during acute ethanol intoxication the Golgi apparatus seems involved in impaired dismission of lipoproteins. In the present paper the phospholipid distributions of Golgi subfractions have been analyzed in livers of normal and ethanol intoxicated rats. No significant differences in the phospholipid classes have been observed in this study. On the contrary, a decrease of vitamin A and ubiquinone in Golgi subfractions is evident. The results are briefly discussed in view of the role played by these endogenous antioxidants in the Golgi membranes and in the pathogenesis of ethanol induced fatty liver.

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.

Analysis of rat liver Golgi apparatus proteins and apolipoproteins in acute ethanol intoxication

Protein patterns of rat liver secretive (F1, F2) and formative (F3) Golgi fractions and their corresponding VLDLs content have been analyzed by disc-electrophoresis in normal conditions and after acute ethanol intoxication and compared with serum proteins and isolated lipoproteins. The major bands of the normal nascent VLDLs isolated from both formative and secretive Golgi fractions migrate in the same area of serum apo-C and apo-ARP apolipoproteins; in the same density interval of VLDLs another major lipoprotein band, called U, may be separated showing a slower electrophoretic mobility of apo-ARP. After ethanol intoxication apo-C bands seem to be concentrated in secretive and decreased in formative fractions, while apo-ARP and U bands show an overlapping in both Golgi fractions. The possible mechanisms of these behaviours are discussed.