Ghosh P, Ender I, Hale E A
Department of Surgery, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
Alcohol Clin Exp Res. 1998 Sep;22(6):1220-6.
Gangliosides are sialo-glycosphingolipids that play important roles in the interaction of cells with their environment and are thus involved in the regulation of many cellular events. Sialic acid residues are important for the conformation of a glycomolecule, their structural stability and their functions. Although decreased brain ganglioside sialic acid has been previously reported as a result of chronic ethanol treatment in rats, no reports are available on the sialylation of specific gangliosides and/or the mechanism leading to depletion of their sialic acid residues. Therefore, in this investigation, we have examined the effects of chronic ethanol treatment on (1) incorporation of [4,5-3H]N-acetylmannosamine (ManNAc) into specific rat brain gangliosides, GD3, GD1a, GT1a, and GT1b; and (2) enzymatic activities of brain sialyltransferase and sialidase at specific subcellular levels. The experiments were done in male Wistar rats pair-fed with either ethanol or control liquid diets for a period of 8 weeks. The rats were intracerebroventricularly injected with labeled ManNAc (30 microCi/rat) and killed after 90 min. Radioactivity was determined in respective ganglioside bands separated on a thin layer chromatography system. Specific activities of sialyltransferase and sialidase were assessed using GM3 and GD3 as substrates, respectively. The results showed significant decreases of 57.7% (p < 0.001) and 68.9% (p < 0.001), respectively, in the labeled ManNAc incorporation into GD3 and GD1a fractions in rats of the ethanol group, compared with rats of the control group. No significant changes were noted in the incorporation of labeled ManNAc into GT1a or GT1b ganglioside fractions between the ethanol and control groups. Concomitantly, compared with control rats, a decrease of 18.9% (p < 0.05), 20.6% (p < 0.05), and 15.8% (p < 0.001) was found in the sialyltransferase activity, respectively, at the whole brain, and brain Golgi and synaptosomal levels. However, dramatic increases of 32.4% (p < 0.05), 105% (p < 0.001), and 150% (p < 0.001) in sialidase activity were found, respectively, at the whole brain and brain cytosol and synaptosomal fractions of rat treated chronically with ethanol. Thus, we conclude that the deleterious actions of ethanol on the sialylation of rat brain gangliosides is specific, and the reduced sialic acid label found in GD3 and GD1a in this study is mainly due to increased activity of brain sialidase. Furthermore, the study reaffirms our tenet that, regardless of whether it is the liver or the brain, glycosylation cascade is one of the main target of the deleterious attacks of ethanol.
神经节苷脂是唾液酸糖鞘脂，在细胞与周围环境的相互作用中发挥重要作用，因此参与许多细胞活动的调节。唾液酸残基对于糖分子的构象、其结构稳定性及其功能很重要。虽然先前已有报道称，慢性乙醇处理会导致大鼠脑内神经节苷脂唾液酸减少，但关于特定神经节苷脂的唾液酸化和/或导致其唾液酸残基耗竭的机制尚无相关报道。因此，在本研究中，我们研究了慢性乙醇处理对以下方面的影响：（1）[4,5-³H]N-乙酰甘露糖胺（ManNAc）掺入大鼠脑内特定神经节苷脂GD3、GD1a、GT1a和GT1b中的情况；（2）脑唾液酸转移酶和唾液酸酶在特定亚细胞水平的酶活性。实验选用雄性Wistar大鼠，分别给予乙醇或对照液体饲料进行配对喂养，为期8周。给大鼠脑室内注射标记的ManNAc（30微居里/只），90分钟后处死。在薄层色谱系统上分离的各个神经节苷脂条带中测定放射性。分别以GM3和GD3为底物评估唾液酸转移酶和唾液酸酶的比活性。结果显示，与对照组大鼠相比，乙醇组大鼠GD3和GD1a组分中标记的ManNAc掺入量分别显著降低了57.7%（p<0.001）和68.9%（p<0.001）。乙醇组和对照组之间，标记的ManNAc掺入GT1a或GT1b神经节苷脂组分中未发现显著变化。同时，与对照大鼠相比，全脑、脑高尔基体和突触体水平的唾液酸转移酶活性分别降低了18.9%（p<0.05）、20.6%（p<0.05）和15.8%（p<0.001）。然而，在慢性乙醇处理的大鼠的全脑、脑细胞质和突触体组分中，唾液酸酶活性分别显著增加了32.4%（p<0.05）、105%（p<0.001）和150%（p<0.001）。因此，我们得出结论，乙醇对大鼠脑神经节苷脂唾液酸化的有害作用具有特异性，本研究中在GD3和GD1a中发现的唾液酸标记减少主要是由于脑唾液酸酶活性增加。此外，该研究再次证实了我们的观点，即无论对肝脏还是大脑，糖基化级联反应都是乙醇有害攻击的主要靶点之一。
