Ethanol and Membrane Protein Trafficking: Diverse Mechanisms of Ethanol Action

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.

Ethanol metabolism alters major histocompatibility complex class I-restricted antigen presentation in liver cells

The proteasome is a major enzyme that cleaves proteins for antigen presentation. Cleaved peptides traffic to the cell surface, where they are presented in the context of major histocompatibility complex (MHC) class I. Recognition of these complexes by cytotoxic T lymphocytes is crucial for elimination of cells bearing "nonself" proteins. Our previous studies revealed that ethanol suppresses proteasome function in ethanol-metabolizing liver cells. We hypothesized that proteasome suppression reduces the hydrolysis of antigenic peptides, thereby decreasing the presentation of the peptide MHC class I complexes on the cell surface. To test this we used the mouse hepatocyte cell line (CYP2E1/ADH-transfected HepB5 cells) or primary mouse hepatocytes, both derived from livers of C57Bl/6 mice, which present the ovalbumin peptide, SIINFEKL, complexed with H2Kb. To induce H2Kb expression, HepB5 cells were treated with interferon gamma (IFNgamma) and then exposed to ethanol. In these cells, ethanol metabolism decreased not only proteasome activity, but also hydrolysis of the C-extended peptide, SIINFEKL-TE, and the presentation of SIINFEKL-H2Kb complexes measured after the delivery of SIINFEKL-TE to cytoplasm. The suppressive effects of ethanol were, in part, attributed to ethanol-elicited impairment of IFNgamma signaling. However, in primary hepatocytes, even in the absence of IFNgamma, we observed a similar decline in proteasome activity and antigen presentation after ethanol exposure.

CONCLUSION

Proteasome function is directly suppressed by ethanol metabolism and indirectly by preventing the activating effects of IFNgamma. Ethanol-elicited reduction in proteasome activity contributes to the suppression of SIINFEKL-H2Kb presentation on the surface of liver cells.

Rab4 facilitates cyclic adenosine monophosphate-stimulated bile acid uptake and Na+-taurocholate cotransporting polypeptide translocation

Cyclic adenosine monophosphate (cAMP) stimulates hepatic bile acid uptake by translocating sodium-taurocholate (TC) cotransporting polypeptide (Ntcp) from an endosomal compartment to the plasma membrane. Rab4 is associated with early endosomes and involved in vesicular trafficking. This study was designed to determine the role of Rab4 in cAMP-induced TC uptake and Ntcp translocation. HuH-Ntcp cells transiently transfected with empty vector, guanosine triphosphate (GTP) locked dominant active Rab4 (Rab4(GTP)), or guanosine diphosphate (GDP) locked dominant inactive Rab4 (Rab4(GDP)) were used to study the role of Rab4. Neither Rab4(GTP) nor Rab4(GDP) affected either basal TC uptake or plasma membrane Ntcp level. However, cAMP-induced increases in TC uptake and Ntcp translocation were enhanced by Rab4(GTP) and inhibited by Rab4(GDP). In addition, cAMP increased GTP binding to endogenous Rab4 in a time-dependent, but phosphoinositide-3-kinase-independent manner.

CONCLUSION

Taken together, these results suggest that cAMP-mediated phosphoinositide-3-kinase-independent activation of Rab4 facilitates Ntcp translocation in HuH-Ntcp cells.

Ethanol increases fetal human neurosphere size and alters adhesion molecule gene expression

BACKGROUND

Ethanol (ETOH) consumption by pregnant women can result in Fetal Alcohol Spectrum Disorder (FASD). To date, the cellular targets and mechanisms responsible for FASD are not fully characterized. Our aim was to determine if ETOH can affect fetal human brain-derived neural progenitor cells (NPC).

METHODS

Neural progenitor cells were isolated by positive selection from normal second trimester fetal human brains (n = 4) and cultured, for up to 72 hours, in mitogenic media containing 0, 1, 10, or 100 mM ETOH. From 48 to 72 hours in culture, neurospheres generated in these conditions were filmed using time-lapse video microscopy. At the end of 72 hours, neurosphere diameter and roundness were measured using videographic software. Mitotic phase analysis of cell-cycle activity and apoptotic cell count were also performed at this time, by flow cytometry using propidium iodide (PI) staining. Real-time PCR was used to estimate expression of genes associated with cell adhesion pathways.

RESULTS

Neurosphere diameter correlated positively (r = 0.87) with increasing ETOH concentrations. There was no significant difference in cell-cycle activity and no significant increase in apoptosis with increasing ETOH concentrations. Time-lapse video microscopy showed that ETOH (100 mM) reduced the time for neurosphere coalescence. Real-time PCR analysis showed that ETOH significantly altered the expression of genes involved in cell adhesion. There was an increase in the expression of alpha and beta Laminins 1, beta Integrins 3 and 5, Secreted phosphoprotein1 and Sarcoglycan epsilon. No change in the expression of beta Actin was observed while the expression of beta Integrin 2 was significantly suppressed.

CONCLUSIONS

ETOH had no effect on NPC apoptosis but, resulted in more rapid coalescence and increased volume of neurospheres. Additionally, the expression of genes associated with cell adhesion was significantly altered. ETOH induced changes in NPC surface adhesion interactions may underlie aspects of neurodevelopmental abnormalities in FASD.

Chronic ethanol feeding to rats decreases adiponectin secretion by subcutaneous adipocytes

Chronic ethanol feeding to mice and rats decreases serum adiponectin concentration and adiponectin treatment attenuates chronic ethanol-induced liver injury. Although it is clear that lowered adiponectin has pathophysiological importance, the mechanisms by which chronic ethanol decreases adiponectin are not known. Here, we have investigated the impact of chronic ethanol feeding on adiponectin expression and secretion by adipose tissue. Rats were fed a 36% Lieber-DeCarli ethanol-containing liquid diet or pair-fed control diet for 4 wk. Chronic ethanol feeding decreased adiponectin mRNA but had no effect on adiponectin protein in subcutaneous adipose tissue. Chronic ethanol feeding also reduced adiponectin secretion by isolated subcutaneous and retroperitoneal adipocytes despite the maintenance of equivalent intracellular concentrations of adiponectin between subcutaneous adipocytes from ethanol- and pair-fed rats. Treatment with brefeldin A suppressed adiponectin secretion by subcutaneous adipocytes from pair-fed rats but had little effect after ethanol feeding. In subcutaneous adipocytes from pair-fed rats, adiponectin was enriched in endoplasmic reticulum (ER)/Golgi relative to plasma membrane; however, after chronic ethanol feeding, adiponectin was equally distributed between plasma membrane and ER/Golgi fractions. In conclusion, chronic ethanol feeding impaired adiponectin secretion by subcutaneous and retroperitoneal adipocytes; impaired secretion likely contributes to decreased adiponectin concentrations after chronic ethanol feeding.

Integrative strategies to identify candidate genes in rodent models of human alcoholism

The search for genes underlying alcohol-related behaviours in rodent models of human alcoholism has been ongoing for many years with only limited success. Recently, new strategies that integrate several of the traditional approaches have provided new insights into the molecular mechanisms underlying ethanol's actions in the brain. We have used alcohol-preferring C57BL/6J (B6) and alcohol-avoiding DBA/2J (D2) genetic strains of mice in an integrative strategy combining high-throughput gene expression screening, genetic segregation analysis, and mapping to previously published quantitative trait loci to uncover candidate genes for the ethanol-preference phenotype. In our study, 2 genes, retinaldehyde binding protein 1 (Rlbp1) and syntaxin 12 (Stx12), were found to be strong candidates for ethanol preference. Such experimental approaches have the power and the potential to greatly speed up the laborious process of identifying candidate genes for the animal models of human alcoholism.

Decreased insulin-dependent glucose transport by chronic ethanol feeding is associated with dysregulation of the Cbl/TC10 pathway in rat adipocytes

Heavy alcohol consumption is an independent risk factor for type 2 diabetes. Although the exact mechanism by which alcohol contributes to the increased risk is unknown, impaired glucose disposal is a likely target. Insulin-stimulated glucose disposal in adipocytes is regulated by two separate and independent pathways, the PI3K pathway and the Cbl/TC10 pathway. Previous studies suggest that chronic ethanol feeding impairs insulin-stimulated glucose transport in adipocytes in a PI3K-independent manner. In search of potential targets of ethanol that would affect insulin-stimulated glucose transport, we investigated the effects of 4-wk ethanol feeding to male Wistar rats on the Cbl/TC10 pathway in isolated adipocytes. Chronic ethanol feeding inhibited insulin-stimulated cCbl phosphorylation compared with pair feeding. Insulin receptor and Akt/PKB phosphorylation were not affected by ethanol feeding. Chronic ethanol exposure also impaired cCbl and TC10 recruitment to a lipid raft fraction isolated from adipocytes by detergent extraction. Furthermore, chronic ethanol feeding increased the amount of activated TC10 and filamentous actin in adipocytes at baseline and abrogated the ability of insulin to further activate TC10 or polymerize actin. These results demonstrate that the impairment in insulin-stimulated glucose transport observed in adipocytes after chronic ethanol feeding to rats is associated with a disruption of insulin-mediated Cbl/TC10 signaling and actin polymerization.

Asialoglycoprotein receptor facilitates hemolysis in patients with alcoholic liver cirrhosis

Hemolysis in patients with advanced alcoholic liver disease is a common clinical problem and indicates an unfavorable prognosis. In many cases, the etiology of the hemolysis remains unknown. We observed three patients with alcoholic liver disease, suffering from severe hemolytic anemia, requiring multiple blood transfusions. Steroid therapy was ineffective and two of the patients died. All patients had a soluble variant of the human asialoglycoprotein receptor (s-ASGP-R) in their serum, as well as high titers of autoantibodies against this receptor (anti-ASGP-R). Consecutively, examination of 60 patients with alcoholic liver disease revealed a high incidence for s-ASGP-R (36%) and anti-ASGP-R (27%) in patients with alcoholic liver cirrhosis (ALC) compared to patients with cirrhosis due to viral hepatitis. The potential etiology of hemolysis was studied in vitro on erythrocytes from patients with ALC and from healthy donors. Isolated ASGP-R but not anti-ASGP-R bound to the surface of erythrocytes preferentially of blood group A1 and caused dose-dependent agglutination and hemolysis, while this phenomenon was much lower using erythrocytes of the blood group B and almost absent with blood group O-erythrocytes. Furthermore, agglutination and hemolysis only occurred in erythrocytes from ALC-patients or after the pre-treatment of cells with neuraminidase. ASGP-R induced agglutination and hemolysis was blocked by the competitive ASGP-R inhibitor asialofetuin. In conclusion, our results indicate a new, non-immunological mechanism for hemolysis in patients with alcoholic liver disease, mediated through agglutination by a soluble variant of the human asialoglycoprotein receptor and mechanical shear stress.

Whole body insulin resistance in rat offspring of mothers consuming alcohol during pregnancy or lactation: comparing prenatal and postnatal exposure

This study examined the effects of maternal ethanol (EtOH) consumption during pregnancy or lactation on glucose homeostasis in the adult rat offspring. Glucose disposal was determined by minimal model during an intravenous glucose tolerance test in rats that had a small or normal birth weight after EtOH exposure in utero and in rats whose mothers were given EtOH during lactation only. All three EtOH groups had decreased glucose tolerance index and insulin sensitivity index, but their glucose effectiveness was not different from that of controls. In addition, EtOH rat offspring that were small at birth had elevated plasma, liver, and muscle triglyceride levels. The data show that EtOH exposure during pregnancy programs the body to insulin resistance later in life, regardless of birth weight, but that this effect also results in dyslipidemia in growth-restricted rats. In addition, insulin resistance is also evident after EtOH exposure during lactation.

Protective effects of lysophosphatidic acid (LPA) on chronic ethanol-induced injuries to the cytoskeleton and on glucose uptake in rat astrocytes

Ethanol induces severe alterations in membrane trafficking in hepatocytes and astrocytes, the molecular basis of which is unclear. One of the main candidates is the cytoskeleton and the molecular components that regulate its organization and dynamics. Here, we examine the effect of chronic exposure to ethanol on the organization and dynamics of actin and microtubule cytoskeletons and glucose uptake in rat astrocytes. Ethanol-treated cells cultured in either the presence or absence of fetal calf serum showed a significant increase in 2-deoxyglucose uptake. Ethanol also caused alterations in actin organization, consisting of the dissolution of stress fibres and the appearance of circular filaments beneath the plasma membrane. When lysophosphatidic acid (LPA), which is a normal constituent of serum and a potent intercellular lipid mediator with growth factor and actin rearrangement activities, was added to ethanol-treated astrocytes cultured without fetal calf serum, it induced the re-appearance of actin stress fibres and the normalization of 2-deoxyglucose uptake. Furthermore, ethanol also perturbed the microtubule dynamics, which delayed the recovery of the normal microtubule organization following removal of the microtubule-disrupting agent nocodazole. Again, pre-treatment with LPA prevented this alteration. Ethanol-treated rodent fibroblast NIH3T3 cells that constitutively express an activated Rho mutant protein (GTP-bound form) were insensitive to ethanol, as they showed no alteration either in actin stress-fibre organization or in 2-deoxyglucose uptake. We discuss the putative signalling targets by which ethanol could alter the cytoskeleton and hexose uptake and the cytoprotective effect of LPA against ethanol-induced damages. The latter opens the possibility that LPA or a similar non-hydrolysable lipid derivative could be used as a cytoprotective agent against the noxious effects of ethanol.