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

Prenatal alcohol exposure affects galactosyltransferase activity and glycoconjugates in the Golgi apparatus of fetal rat hepatocytes

Prenatal exposure to alcohol affects the morphological, structural, and functional features of the Golgi apparatus (GA), thus altering the glycosylation process in fetal hepatocytes. To elucidate the cellular mechanisms underlying these alterations, we have studied the effect of alcohol exposure in utero on the activity of liver galactosyltransferase, an enzyme involved in the glycosylation process, and on the hepatic glycoprotein sugar composition. For this, livers from 21-day-old fetuses obtained from control and ethanol-fed rats were used. Galactosyltransferase (GT) activity was determined in isolated GA cis and trans fractions. Colloidal gold-labeled lectin cytochemistry was used to analyze sugar residues in hepatocytes at the subcellular level. Finally, the integrity of the GA after alcohol treatment was assessed by electron microscopy and by evaluating the distribution of the Golgi beta-COP, a protein involved in vesicular trafficking. Prenatal alcohol exposure induces a significant increase in both liver weight and total protein content in the trans Golgi. Moreover, this treatment decreases the activity of galactosyltransferase, increases alpha-L-Fuc residues, and reduces the number of alpha-Man, GlcNAc(beta1,4,GlcNAc)1,2, GalNAc alpha1,3GalNAc, alpha-GalNAc, and a-Gal residues. Alcohol exposure also causes the Golgi cisternae to disappear in about 30% of the hepatocytes, and reduces 75% the number of anti-Golgi beta-COP protein binding sites. Our results suggest that the decrease in galactosyltransferase activity, the alterations in the oligosaccharide chain composition, and the reduction in the amount of Golgi beta-COP protein could be involved in the alterations in the glycosylation process, as well as in the accumulation of hepatic proteins observed after prenatal alcohol exposure. These alterations could contribute, therefore, to the alcohol-induced injury in the developing 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.

Prenatal exposure to ethanol alters the synthesis and glycosylation of proteins in fetal hepatocytes

In the present work we have studied the effect of prenatal exposure to alcohol on the synthesis, glycosylation, and transport of proteins in fetal hepatocytes isolated from 21-day-old fetuses derived from control and chronic alcoholic rats. Protein synthesis was evaluated both in a cell-free system and in hepatocytes after (35S)methionine and (3H)leucine incorporation, respectively. Glycosylation was assessed using (3H)mannose and (3H)galactose as precursors. Protein synthesis was significantly decreased in treated hepatocytes. In control hepatocytes, quantitative electron microscope autoradiography showed that both (3H)leucine and (3H)mannose incorporation occur first in the rough endoplasmic reticulum (rER). Later the silver grains appeared over the Golgi apparatus, and, finally there was a transport towards the cell periphery. After pulse, silver grains corresponding to (3H)galactose incorporation appeared over the Golgi apparatus. The label then moved to the hepatocyte periphery. Alcohol treated hepatocytes showed a retention of grains over the Golgi apparatus with a diminution in the label at the cell periphery. These results indicate that prenatal exposure to alcohol induces a decrease in the synthesis of proteins in the hepatocyte as well as an alteration in the process of glycosylation and/or transport of secretory proteins.

A biochemical and stereological study of neonatal rat hepatocyte subpopulations. Effect of pre- and postnatal exposure to ethanol

Hepatocytes from 12-day-old rats, pre- and post-natally exposed to alcohol, together with those from pair-fed controls, were isolated and subfractionated in six cell subpopulations on Percoll density gradients. These cells were characterized using a combination of biochemical and stereological methods. The low density cells (F2) mainly showed biochemical and stereological features of perivenous hepatocytes, whereas the heavier cells (F6) were primarily periportal hepatocytes. The alcohol-metabolizing enzymes, alcohol dehydrogenase and aldehyde dehydrogenase (high and low Km) showed more activity in the F2 fraction. Alcohol-altered mitochondria and Golgi apparatus occurred mainly in F2 cells, whereas the endoplasmic reticulum and lysosomes appeared to be more altered in the F6 hepatocytes. Alcohol also induced the appearance of some small hepatocytes, with a well-developed rough endoplasmic reticulum and an increased number of mitochondria. Biochemical data indicated that glutamate dehydrogenase and alanine aminotransferase were more affected in F2 cells from alcohol-treated rats, and that the activity of the ethanol-metabolizing enzymes was alos reduced in these hepatocytes. Our results indicate that alcohol exposure during zonal development in the liver could have a selective effect on specific cell components depending on the acinar zone, and that the perivenous hepatocytes appear to be more damaged under these conditions.

Effects of prolonged exposure to ammonia on fluid-phase, receptor-mediated, and adsorptive (non specific) endocytosis in cultured neuroblastoma cells. A flow-cytometry and cytochemical study

The effect of prolonged exposure to ammonia on fluid-phase, receptor-mediated, and adsorptive (non specific) endocytosis in cultured neuroblastoma (Neuro-2a) cells were studied using fluorescein-labeled dextran, concanavalin A conjugated with fluorescein isothiocyanate, and cationized ferritin as tracers. Ammonia treatment increased the rate of endocytosis of cationized ferritin as well as the number of cell elements involved in the process. Moreover, the number of cytoplasmic components containing acid phosphatase activity was also found to increase following ammonia treatment. In contrast, flow-cytometric analyses showed that, under experimental conditions, exposure to ammonia did not alter the intralysosomal pH and had little effect on the fluid-phase and receptor-mediated endocytosis of fluorescein-labeled dextran and concanavalin-A fluorocrome, respectively.