Chronic Ethanol Induced Impairment of Hepatic Glycosylation Machinery in Rat Is Independent of Dietary Carbohydrate

Adverse signaling of scavenger receptor class B1 and PGC1s in alcoholic hepatosteatosis and steatohepatitis and protection by betaine in rat

Because scavenger receptor class B type 1 is the cholesterol uptake liver receptor, whereas peroxisome proliferator-activated receptor γ coactivator-1β (PGC-1β) and PGC-1α are critical for lipid synthesis and degradation, we investigated the roles of these signaling molecules in the actions of ethanol-polyunsaturated fatty acids and betaine on hepatosteatosis and steatohepatitis. Ethanol-polyunsaturated fatty acid treatment caused the following: i) hepatosteatosis, as evidenced by increased liver cholesterol and triglycerides, lipid score, and decreased serum adiponectin; ii) marked inhibition of scavenger receptor class B type 1 glycosylation, its plasma membrane localization, and its hepatic cholesterol uptake function; and iii) moderate steatohepatitis, as evidenced by histopathological characteristics, increased liver tumor necrosis factor α and IL-6, decreased glutathione, and elevated serum alanine aminotransferase. These actions of ethanol involved up-regulated PGC-1β, sterol regulatory element-binding proteins 1c and 2, acetyl-CoA carboxylase, and HMG-CoA reductase mRNAs/proteins and inactive non-phosphorylated AMP kinase; and down-regulated silence regulator gene 1 and PGC-1α mRNA/proteins and hepatic fatty acid oxidation. Betaine markedly blunted all these actions of ethanol on hepatosteatosis and steatohepatitis. Therefore, we conclude that ethanol-mediated impaired post-translational modification, trafficking, and function of scavenger receptor class B type 1 may account for alcoholic hyperlipidemia. Up-regulation of PGC-1β and lipid synthetic genes and down-regulation of silence regulator gene 1, PGC-1α, adiponectin, and lipid degradation genes account for alcoholic hepatosteatosis. Induction of proinflammatory cytokines and depletion of endogenous antioxidant, glutathione, account for alcoholic steatohepatitis. We suggest betaine as a potential therapeutic agent because it effectively protects against adverse actions of ethanol.

CYP2E1, oxidative stress, post-translational modifications and lipid metabolism

Chronic alcohol-mediated down-regulation of hepatic ST6Gal1 gene leads to defective glycosylation of lipid-carrying apolipoproteins such as apo E and apo J, resulting in defective VLDL assembly and intracellular lipid and lipoprotein transport, which in turn is responsible for alcoholic hepatosteatosis and ALD. The mechanism of ethanol action involves thedepletion of a unique RNA binding protein that specifically interacts with its 3'-UTR region of ST6Gal1 mRNA resulting in its destabilization and consequent appearance of asialoconjugates as alcohol biomarkers. With respect to ETOH effects on Cardio-Vascular Diseases, we conclude that CYP2E1 and ETOH mediated oxidative stress significantly down regulates not only the hepatic PON1 gene expression, but also serum PON1 and HCTLase activities accompanied by depletion of hepatic GSH, the endogenous antioxidant. These results strongly implicate the susceptibility of PON1 to increased ROS production. In contrast, betaine seems to be both hepatoprotective and atheroprotective by reducing hepatosteatosis and restoring not only liver GSH that quenches free radicals, but also the antiatherogenic PON1 gene expression and activity.

Ethanol destabilizes liver Gal beta l, 4GlcNAc alpha2,6-sialyltransferase, mRNA by depleting a 3'-untranslated region-specific binding protein

Asialoconjugates are viable biomarkers for alcohol abuse. We previously showed that chronic ethanol feeding down-regulated liver Gal beta l, 4GlcNAc alpha2,6-sialyltransferase (ST6Gal l) mRNA by destabilizing it. Since RNA-binding proteins are known to stabilize many eukaryotic mRNAs by interacting with the 3'-untranslated region (UTR), we have delineated the possible mechanism by which ethanol destabilizes ST6Gal l mRNA. Using (32)P-labeled RNA probes generated from a 2.7-kb 3'-UTR of ST6Gal l mRNA, we identified a liver cytosolic 41-kDa specific binding protein that interacts with its 3'-UTR domain and protects it from degradation in normal rat liver but disappears after chronic ethanol treatment. Mapping of the binding region revealed that four RNA probes of 80-base pair (bp) length spanning the 304 bp of the 3'-UTR of ST6Gal l mRNA showed equal binding intensity. The corresponding cDNA sequences for the four 80-bp RNA probes share the 13-bp consensus sequence. Mutagenesis analysis identified that four nucleotides, AG and TC, among the consensus sequences were critical for the RNA-protein interaction. Therefore, 5'-CAGCCTCCTCCCT-3' serves as a cis-element critically involved in this interaction. The RNA-protein complex formation progressively decreased with increasing dietary ethanol, resulting in its virtual disappearance with 36% of the dietary calories as ethanol. Concomitantly, the same ethanol diet decreased sialic acid index of plasma apolipoprotein J by 45% (p < 0.05). Thus, depletion of a binding protein that specifically interacts with its 3'-UTR region of ST6Gal l mRNA may account for its destabilization and consequent appearance of asialoconjugates as alcohol biomarkers.

Galactosyltransferase--still up and running

The following review on galactosyltransferase (gal-T1) intends to cover genetic, biochemical, structural, biotechnological, cell biological and medical aspects of this enzyme in a comprehensive manner from discovery to the present day which have brought to light a genetic defect of this enzyme. Early work has only been included if it appeared relevant to ongoing issues. Following the evolution of a research topic over 40 years is in itself a fascinating endeavor as it permits to observe the ins and outs of hypotheses, fashions and errors. Gal-T1 is a beautiful example as it has been involved in almost every aspect of life science. Importantly, there is a future to this enzyme as a research topic, since many questions still remain unanswered: to which extent is it a representative Golgi protein? What is the role of the gene family of gal-Ts? Does gal-T1 exert any functions other than a catalytic one? Why is it phosphorylated? Does it form homodimers in vivo? Surely, there is room for further work, which is likely to reveal further insights into cellular trafficking and signaling and, in the context of the gene family, shall contribute to understanding development and morphogenesis.

Chronic ethanol exposure in rats affects rabs-dependent hepatic trafficking of apolipoprotein E and transferrin

Because of the important roles of rabs in protein trafficking, we tested whether chronic ethanol exposure affected the trafficking of newly synthesized apolipoprotein E (apoE) or transferrin (O-glycosylated and N-glycosylated proteins, respectively) attached to acylated or prenylated rabs. The in vivo 30-min incorporation ratios of [3H]palmitate:[35S]methionine or [3H]mevalonate:[35S]methionine (relative ratios of rabs acylation or prenylation to total protein or to immunoisolated apoE or transferrin) were measured in various hepatic subcellular organelles of 8 week-ethanol-fed (E) and pair-fed control (C) Wistar-Furth rats. With respect to total protein trafficking, ethanol increased rabs acylation ratio by 136% (P <.01), 69% (P <.05), and 64% (P <.01) in the endoplasmic reticulum (ER), Golgi light fraction (GLF), and Golgi heavy fraction (GHF), respectively, and decreased this ratio by 76% (P <.01) in carrier vesicle fraction 2 (CV2). With respect to apoE trafficking, ethanol increased rabs acylation ratio by 121% in GHF and decreased this ratio by 27% in CV2. Rabs prenylation ratio increased by 21% and 53% in GHF and CV2, respectively, and decreased by 42% in GLF. With respect to transferrin trafficking, ethanol increased rabs acylation ratio by 53% (P <.01) in GHF, with no significant effect in ER, whereas rabs prenylation ratio increased by 26% (P <.05) in ER, with no significant effect in GHF. Therefore, we conclude that ethanol-induced impaired trafficking of newly synthesized O- and N-glycosylated proteins occurs primarily in ER and Golgi and is due to altered lipidation of rabs, possibly rabs 1, 2, or 6 or combinations of these three rabs.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Increased serum concentrations of carbohydrate-deficient transferrin (CDT) in patients with cystic fibrosis

Carbohydrate-deficient transferrin (CDT) has been reported to be one of the best laboratory markers in serum (S) for detection of alcohol abuse. We have studied S-CDT values in cystic fibrosis (CF) patients and show that CF patients have increased S-CDT values without high alcohol consumption. CF patients have abnormalities in their protein glycosylation and sialylation, which may explain the increased S-CDT values.