Ethanol impairs monosaccharide uptake and glycosylation in cultured rat astrocytes

Abstract

Astrocyte and glial-neuron interactions have a critical role in brain development, which is partially mediated by glycoproteins, including adhesion molecules and growth factors. Ethanol affects the synthesis, intracellular transport, subcellular distribution and secretion of these glycoproteins, suggesting alterations in glycosylation. We analyzed the effect of long-term exposure to low doses of ethanol (30 mM) on glycosylation process in growing cultured astrocytes in vitro. Cells were incubated for short (5 min) and long (90 min) periods with several radioactively labeled carbohydrate precursors. The uptake, kinetics and metabolism of these precursors, as well as the radioactivity distribution in protein gels were analyzed. The levels of GLUT1 and mannosidase II were also determined. Ethanol increased the uptake of monosaccharides and the protein levels of GLUT1 but decreased those of mannosidase II. It altered the carbohydrate moiety of proteins and increased cell surface glycoproteins containing terminal non-reduced mannose. These results indicate that ethanol impairs glycosylation in rat astrocytes, thus disrupting brain development.

Alcohol abuse and glycoconjugate metabolism

“…During the translocation process from cytoplasm, through the aqueous pores of endoplasmic reticulum membrane (ER), to endoplasmatic reticulum, pre-propolypeptide chains undergo modifications including N-or O-glycosylation and folding assisted by chaperones [27]. The biosynthesis of glycans is a complex process that occurs along the secretory pathway [28].…”

“…After heavy alcohol drinking, an abnormal glycosylation of glycoproteins and glycolipids (due to decreased activity of glycosyltransferases including galactosyltransferase, sialyltransferase, N-acetylglucosaminyltransferase, and increased activity of sialidase) and decreased secretion (due to inhibited polymerization of tubulin) of glycoproteins and glycolipids, have been reported [28,40,41]. Carbohydrate-deficient transferrin (CDT) has been widely investigated as one of the most valuable markers for monitoring chronic alcohol abuse [9,10,30].…”

“…On the other hand, ethanol treatment inhibits activity of mannosidase II (most likely by acetaldehyde adduct formation), resulting in the synthesis of cell surface glycoproteins with high mannose structures [28]. The increase in the uptake of several monosaccharides depends on the type of monosaccharide, and on the cell type, as well as on the dose and the duration of alcohol consumption.…”

“…Given that long-term ethanol exposure modulates the function of carbohydrate transporters located in the plasma membrane e.g. decreases uptake of glucose in astrocytes or increases in myotubes (decrease or increase in the levels of glucose transporter -GLUT), so a decrease in monosaccharide availability to glycosylation may also participate in inhibition of the glycosylation process [28].…”

“…Glycosylation of proteins is required for their proper folding, secretion, and stability [28,44]. Thus, aforementioned ethanol-induced glycosylation disturbances (as increased deglycosylation), may impair folding of the glycoconjugates, their transport through the secretory pathway, and may contribute to a decrease in their life-span, subsequent distribution or subcellular localization [28,44].…”