Chronic ethanol administration impairs receptor-mediated endocytosis of epidermal growth factor by rat hepatocytes

Role of epidermal growth factor receptor in liver injury and lipid metabolism: Emerging new roles for an old receptor

Epidermal growth factor receptor (EGFR) is conventionally known to play a crucial role in hepatocyte proliferation, liver regeneration and is also associated with hepatocellular carcinogenesis. In addition to these proliferative roles, EGFR has also implicated in apoptotic cell death signaling in various hepatic cells, mitochondrial dysfunction and acute liver necrosis in a clinically relevant murine model of acetaminophen overdose, warranting further comprehensive exploration of this paradoxical role of EGFR in hepatotoxicity. Apart from ligand dependent activation, EGFR can also be activated in ligand-independent manner, which is mainly associated to liver injury. Recent evidence has also emerged demonstrating important role of EGFR in lipid and fatty acid metabolism in quiescent and regenerating liver. Based on these findings, EGFR has also been shown to play an important role in steatosis in clinically relevant murine NAFLD models via regulating master transcription factors governing fatty acid synthesis and lipolysis. Moreover, several lines of evidences indicate that EGFR is also involved in hepatocellular injury, oxidative stress, inflammation, direct stellate cell activation and fibrosis in chronic liver injury models, including repeated CCl₄ exposure, high-fat diet and fast-food diet models. In addition to briefly summarizing role of EGFR in liver regeneration, this review comprehensively discusses all these non-conventional emerging roles of EGFR. Considering evidences of multi-facet role of EGFR at various levels in these pathophysiological process, EGFR can be a promising therapeutic target for various liver diseases, including acute liver failure and NAFLD, requiring further exploration. These roles of EGFR are relevant for alcoholic liver diseases (ALD) as well, thus providing a valid rationale for future investigations exploring a role of EGFR in ALD.

A Mechanistic Review of Cell Death in Alcohol-Induced Liver Injury

Alcoholic liver disease (ALD) is a major health problem in the United States and worldwide without successful treatments. Chronic alcohol consumption can lead to ALD, which is characterized by steatosis, inflammation, fibrosis, cirrhosis, and even liver cancer. Recent studies suggest that alcohol induces both cell death and adaptive cell survival pathways in the liver, and the balance of cell death and cell survival ultimately decides the pathogenesis of ALD. This review summarizes the recent progress on the role and mechanisms of apoptosis, necroptosis, and autophagy in the pathogenesis of ALD. Understanding the complex regulation of apoptosis, necrosis, and autophagy may help to develop novel therapeutic strategies by targeting all 3 pathways simultaneously.

Cyanamide potentiates the ethanol-induced impairment of receptor-mediated endocytosis in a recombinant hepatic cell line expressing alcohol dehydrogenase activity

Ethanol administration has been shown to alter receptor-mediated endocytosis in the liver. We have developed a recombinant hepatic cell line stably transfected with murine alcohol dehydrogenase cDNA to serve as an in vitro model to investigate these ethanol-induced impairments. In the present study, transfected cells were maintained in the absence or presence of 25 mM ethanol for 7 days, and alterations in endocytosis by the asialoglycoprotein receptor were determined. The role of acetaldehyde in this dysfunction was also examined by inclusion of the aldehyde dehydrogenase inhibitor, cyanamide. Our results showed that ethanol metabolism impaired internalization of asialoorosomucoid, a ligand for the asialoglycoprotein receptor. The addition of cyanamide potentiated the ethanol-induced defect in internalization and also impaired degradation of the ligand in the presence of ethanol. These results indicate that the ethanol-induced impairment in endocytosis is exacerbated by the inhibition of aldehyde dehydrogenase, suggesting the involvement of acetaldehyde in this dysfunction.

Pathogenesis of alcoholic liver disease: interactions between parenchymal and non-parenchymal cells

The development of alcoholic liver disease (ALD) is a complex process involving both the parenchymal and non-parenchymal cells in the liver. The impact of ethanol on hepatocytes can be characterized as a condition of organelle stress with multifactorial changes in hepatocellular function accumulating during ethanol exposure. These changes include oxidative stress, mitochondrial dysfunction, decreased methylation capacity, endoplasmic reticulum stress, impaired vesicular trafficking and altered proteasome function. Injury to hepatocytes is attributed, in part, to ethanol metabolism by the hepatocytes. Changes in the structural integrity of hepatic sinusoidal endothelial cells, as well as enhanced inflammation in the liver during ethanol exposure are also important contributors to injury. Activation of hepatic stellate cells initiates the deposition of extracellular matrix proteins characteristic of fibrosis. Kupffer cells, the resident macrophages in the liver, are particularly critical to the onset of ethanol-induced liver injury. Chronic ethanol exposure sensitizes Kupffer cells to activation by lipopolysaccharides via toll-like receptor 4. This sensitization enhances the production of inflammatory mediators, such as tumor necrosis factor-α and reactive oxygen species that contribute to hepatocyte dysfunction, necrosis and apoptosis of hepatocytes and the generation of extracellular matrix proteins leading to fibrosis. In this review we provide an overview of the complex interactions between parenchymal and non-parenchymal cells in the liver during the progression of ethanol-induced liver injury.

Alcohol consumption impairs hepatic protein trafficking: mechanisms and consequences

Alcoholic liver disease is a major biomedical health concern in the United States. Despite considerable research efforts aimed at understanding the progression of the disease, the specific mechanisms leading to alcohol-induced damage remain elusive. Numerous proteins are known to have alcohol-induced alterations in their dynamics. Defining these defects in protein trafficking is an active area of research. In general, two trafficking pathways are affected: transport of newly synthesized secretory or membrane glycoproteins from the Golgi to the basolateral membrane and clathrin-mediated endocytosis from the sinusoidal surface. Both impaired secretion and internalization require ethanol metabolism and are likely mediated by acetaldehyde. Although the mechanisms by which ethanol exposure impairs protein trafficking are not fully understood, recent work implicates alcohol-induced modifications on tubulin or components of the clathrin machinery as potential mediators. Furthermore, the physiological ramifications of impaired protein trafficking are not fully understood. In this review, we will list and discuss the proteins whose trafficking patterns are known to be impaired by ethanol exposure. We will then describe what is known about the possible mechanisms leading to impaired protein trafficking and how disrupted protein trafficking alters liver function and may explain clinical features of the alcoholic patient.

Ethanol selectively impairs clathrin-mediated internalization in polarized hepatic cells

Although alcoholic liver disease is clinically well-described, the molecular basis for alcohol-induced hepatotoxicity is not well understood. Previously, we determined that the clathrin-mediated internalization of asialoglycoprotein receptor was impaired in ethanol-treated WIF-B cells whereas the internalization of a glycophosphatidylinositol-anchored protein thought to be endocytosed via a caveolae/raft-mediated pathway was not changed suggesting that clathrin-mediated endocytosis is selectively impaired by ethanol. To test this possibility, we examined the internalization of a panel of proteins and compounds internalized by different mechanisms in control and ethanol-treated WIF-B cells. We determined that the internalization of markers known to be internalized via clathrin-mediated mechanisms was impaired. In contrast, the internalization of markers for caveolae/raft-mediated endocytosis, fluid phase internalization or non-vesicle-mediated uptake was not impaired in ethanol-treated cells. We further determined that clathrin heavy chain accumulated at the basolateral surface in small puncta in ethanol-treated cells while there was decreased dynamin-2 membrane association. Interestingly, the internalization of resident apical proteins that lack any known internalization signals was also disrupted by ethanol suggesting that these proteins are internalized via clathrin-mediated mechanisms. This conclusion is consistent with our findings that dominant negative dynamin-2 overexpression impaired internalization of known clathrin markers and single spanning apical residents, but not of markers of fluid phase or raft-mediated internalization. Together these results indicate that ethanol exposure selectively impairs hepatic clathrin-mediated internalization by preventing vesicle fission from the plasma membrane.

Alcohol-induced protein hyperacetylation: Mechanisms and consequences

Although the clinical manifestations of alcoholic liver disease are well-described, little is known about the molecular basis of liver injury. Recent studies have indicated that ethanol exposure induces global protein hyperacetylation. This reversible, post-translational modification on the epsilon-amino groups of lysine residues has been shown to modulate multiple, diverse cellular processes ranging from transcriptional activation to microtubule stability. Thus, alcohol-induced protein hyperacetylation likely leads to major physiological consequences that contribute to alcohol-induced hepatotoxicity. Lysine acetylation is controlled by the activities of two opposing enzymes, histone acetyltransferases and histone deacetylases. Currently, efforts are aimed at determining which enzymes are responsible for the increased acetylation of specific substrates. However, the greater challenge will be to determine the physiological ramifications of protein hyperacetylation and how they might contribute to the progression of liver disease. In this review, we will first list and discuss the proteins known to be hyperacetylated in the presence of ethanol. We will then describe what is known about the mechanisms leading to increased protein acetylation and how hyperacetylation may perturb hepatic function.

Microtubule acetylation and stability may explain alcohol-induced alterations in hepatic protein trafficking

We have been using polarized hepatic WIF-B cells to examine ethanol-induced liver injury. Previously, we determined microtubules were more highly acetylated and more stable in ethanol-treated WIF-B cells. We proposed that the ethanol-induced alterations in microtubule dynamics may explain the ethanol-induced defects in membrane trafficking that have been previously documented. To test this, we compared the trafficking of selected proteins in control cells and cells treated with ethanol or with the histone deacetylase 6 inhibitor trichostatin A (TSA). We determined that exposure to 50 nM TSA for 30 minutes induced microtubule acetylation ( approximately 3-fold increase) and stability to the same extent as did ethanol. As shown previously in situ, the endocytic trafficking of the asialoglycoprotein receptor (ASGP-R) was impaired in ethanol-treated WIF-B cells. This impairment required ethanol metabolism and was likely mediated by acetaldehyde. TSA also impaired ASGP-R endocytic trafficking, but to a lesser extent. Similarly, both ethanol and TSA impaired transcytosis of the single-spanning apical resident aminopeptidase N (APN). For both ASGP-R and APN and for both treatments, the block in trafficking was internalization from the basolateral membrane. Interestingly, no changes in transcytosis of the glycophosphatidylinositol-anchored protein, 5'-nucleotidase, were observed, suggesting that increased microtubule acetylation and stability differentially regulate internalization. We further determined that albumin secretion was impaired in both ethanol-treated and TSA-treated cells, indicating that increased microtubule acetylation and stability also disrupted this transport step.

CONCLUSION

These results indicate that altered microtubule dynamics explain in part alcohol-induced defects in membrane trafficking.

WIF-B cells as a model for alcohol-induced hepatocyte injury

A potential in vitro model for studying the mechanisms of alcohol-induced hepatocyte injury is the WIF-B cell line. It has many hepatocyte-like features, including a differentiated, polarized phenotype resulting in formation of bile canaliculi. The aim of this study was to examine the effects of ethanol treatment on this cell line. WIF-B cells were cultured up to 96 h in the absence or presence of 25 mM ethanol and subsequently were analyzed for ethanol-induced physiological and morphological changes. Initial studies revealed WIF-B cells exhibited alcohol dehydrogenase (ADH) activity, expressed cytochrome p4502E1 (CYP2E1), and efficiently metabolized ethanol in culture. This cell line also produced the ethanol metabolite acetaldehyde and exhibited low K(m) aldehyde dehydrogenase (ALDH) activity, comparable to hepatocytes. Ethanol treatment of the WIF-B cells for 48 h led to significant increases in the lactate/pyruvate redox ratio and cellular triglyceride levels. Ethanol treatment also significantly altered WIF-B morphology, decreasing the number of bile canaliculi, increasing the number of cells exhibiting finger-like projections, and increasing cell diameter. The ethanol-induced changes occurring in this cell line were negated by addition of the ADH inhibitor, 4-methylpyrazole (4-MP), indicating the effects were due to ethanol metabolism. In summary, the WIF-B cell line metabolizes ethanol and exhibits many ethanol-induced changes similar to those found in hepatocytes. Because of these similarities, WIF-B cells appear to be a suitable model for studying ethanol-induced hepatocyte injury.

Large-scale gene profiling of the liver in a mouse model of chronic, intragastric ethanol infusion

BACKGROUND/AIMS

The mechanisms underlying alcohol-induced liver injury are not fully elucidated. An approach in this direction would consist of an all-inclusive assessment of gene expression in the liver. The purpose of this study was to perform a comprehensive analysis of gene expression in the livers of mice treated with ethanol by means of intragastric infusion.

METHODS

An ethanol- or glucose-enriched liquid diet was fed to animals for 4 weeks via a long-term gastrostomy catheter. The animals were killed and plasma alanine:2-oxoglutarate aminotransferase (ALT) assay, liver histology and total RNA analysis by microarray gene technology were performed.

RESULTS

Alcohol increased ALT, induced steatosis, necrosis and inflammation. A total of 12,423 genes were analyzed for expression out of which 4867 were expressed by the liver. Alcohol repressed expression of 11 genes, induced expression of 13 genes, and up- or down-regulated expression of 44 and 42 genes >2-fold, respectively. Gene expression analysis identified several genes that have not previously been tested for alcohol effects.

CONCLUSIONS

This study: (i) expands the knowledge of mechanism(s) of action of ethanol; (ii) indicates novel pathways of ethanol action on the liver, and (iii) illustrates the utility of microarray gene analysis in hepatology research.

Chronic ethanol consumption impairs receptor-mediated endocytosis of MAA-modified albumin by liver endothelial cells

Alcoholic liver disease has been associated with abnormalities in receptor-mediated endocytosis (RME) which results in abnormal degradation of metabolically altered proteins. Model systems using formaldehyde-modified albumin (f-Alb) have shown an impairment in RME following chronic alcohol consumption utilizing both in situ perfused rat livers and isolated rat liver endothelial cells (LECs). The discovery that alcohol metabolite derived aldehydes can modify proteins prompted a study to determine if malondialdehyde-acetaldehyde-modified albumin (MAA-Alb) would be degraded similar to that reported for f-Alb, and whether ethanol-fed rats would demonstrate an impaired RME with respect to this ligand which occurs as a consequence of chronic ethanol consumption. MAA-Alb was degraded slightly more than f-Alb in both in situ perfused livers and at the single cell level. This degradation was completely inhibited with 100x unlabeled f-Alb, which suggests the use of a similar receptor. Following alcohol consumption there was a 50-60% decrease in MAA-Alb degradation in whole livers and isolated LECs. Utilizing isolated LECs it was determined that impairment in internalization was the most likely mechanism for the decrease in the amount of MAA-Alb that was degraded. These data show that chronic alcohol consumption by rats does in fact impair RME of alcohol metabolite-derived adducted proteins, and this impairment is due to a defect in the post-internalization step rather than the binding or degradation of the modified protein.

Alcohol-induced liver disease

Hepatic changes resulting from the regular ingestion of alcohol are many and include fat infiltration, alcoholic hepatitis, and cirrhosis. Only 10% to 15% of chronic alcoholics develop liver disease. Women are more susceptible. An area of considerable importance is the high prevalence of concomitant infection with hepatitis C virus in chronic alcoholics. Patients who have hepatitis C and alcohol-induced liver injury are much more likely to develop progressive liver disease and cirrhosis. Corticosteroid therapy has proven useful in the treatment of patients with severe acute alcoholic hepatitis.

Effects of ethanol on receptor-mediated endocytosis in the liver

Ethanol administration impairs multiple aspects in the process of receptor-mediated endocytosis (RME) in the liver. Studies from our laboratory over the last 10 years have carefully examined RME by the hepatocyte-specific asialoglycoprotein receptor (ASGP-R). We have identified a time course for ethanol-induced defects in RME and established that many of the impairments occur initially in the centrilobular region of the liver and as early as one week after ethanol administration. Impaired intravesicular acidification in ethanol-fed animals has been identified, and these defects in acidification could alter multiple protein trafficking pathways including RME. In addition to altered acidification, altered receptor function (including receptor inactivation) could also contribute to impaired trafficking. Current studies in our laboratory are aimed at an examination of posttranslational modifications in the receptor (acylation and phosphorylation) that are known to affect its function. A role for the ASGP-R in the process of alcoholic apoptosis is also being examined because proper functioning of the ASGP-R is thought to be important in clearance of apoptotic cells.

Ethanol-impaired hepatic protein trafficking: concepts from the asialoglycoprotein receptor system

OBJECTIVES

Alcohol abuse with its resulting liver injury is a major health problem worldwide. Recent studies have shown that the process of receptor-mediated endocytosis (RME) is especially susceptible to the deleterious effects of ethanol.

DESIGN AND METHODS

In our laboratory, we have shown that after as early as 1 week of ethanol administration, binding, internalization and degradation of asialoorosomucoid, a ligand for the asialoglycoprotein receptor (ASGP-R), is significantly impaired. We have also demonstrated that ethanol administration impairs ATP-dependent acidification of prelysosomal endosomes.

RESULTS

These impairments are seen using ligands internalized by the non-specific process of fluid phase endocytosis as well as those internalized by coated pit endocytosis. In addition, we have identified ethanol-induced alterations in post-translational modifications of the receptor including phosphorylation and fatty acid modification (palmitoylation).

CONCLUSIONS

Impaired function of this receptor could lead to alterations of membrane internalization events after ethanol administration and contribute to ethanol-induced alterations in protein trafficking and signaling in the liver.

Chronic ethanol consumption impairs receptor-mediated endocytosis of formaldehyde-treated albumin by isolated rat liver endothelial cells

Receptor-mediated endocytosis (RME) by a scavenger receptor on sinusoidal liver endothelial cells (LECs) for formaldehyde-treated bovine serum albumin (f-Alb) has previously been shown to be impaired following chronic ethanol consumption. These studies were initially performed by in situ perfusion, making it difficult to determine the point in the process at which RME is affected. Therefore, it was the purpose of this study to use isolated LECs to begin elucidating at what point in the process chronic ethanol consumption affects RME. Initial studies showed that degradation at the single-cell level were similarly decreased at levels that had been observed for in situ studies, suggesting that the ethanol effects can be repeated using isolated LECs, making them useful for in vitro studies. Binding studies with 125I-formaldehyde-treated bovine serum albumin (125I-f-Alb) demonstrated there was a slight, but significantly different, decrease in binding by LECs from ethanol-fed rats when compared with pair-fed or chow-fed rats. However, the affinity of these receptors was not different between these groups. In contrast, a defect in the initial stages of receptor-ligand internalization was indicated as less surface-bound ligand was internalized and subsequently degraded in cells from the ethanol-treated animals as compared with controls. Additionally, once the data were adjusted for the amount of ligand internalized, the degradation of the internalized ligand was only slightly impaired. These results indicate that chronic ethanol feeding impairs the process of RME by the liver; the major cause of this impairment appears to be caused by a decreased ability of these cells to internalize all of the surface-bound ligand, with a minimal defect in postinternalization events.

Tumor necrosis factor-alpha internalization and degradation by isolated hepatocytes of rats exposed to ethanol

Internalization and degradation of human recombinant [125I]TNF-alpha was studied in hepatocytes isolated from rats exposed to ethanol (EtOH) either acutely (i.p. injection, 2.2 g kg(-1) b.wt.) or chronically (14-16 weeks of EtOH feeding in liquid diet). Both acute and chronic EtOH exposure diminished cytokine binding to the cell-surface receptors. In the acute group, EtOH increased internalization of the cytokine, accelerated its disappearance from the cell surface, and markedly reduced its conversion into acid-soluble 125I-containing compounds. In the chronic group, EtOH did not markedly affect these parameters. Internalization and degradation of the cytokine in the chronic group was much lower than in the acute group. It is concluded that EtOH interferes not only with the cytokine binding to the cell-surface receptors, as demonstrated in previous studies, but also with postbinding events, such as internalization and intracellular degradation of TNF-alpha. Possible mechanisms of action of EtOH are discussed.

Chronic ethanol ingestion impairs TGF-alpha-stimulated receptor autophosphorylation

The effects of chronic ethanol feeding on the binding of transforming growth factor-alpha (TGF-alpha) and TGF-alpha-stimulated receptor autophosphorylation were investigated in isolated rat hepatocytes. When hepatocytes were isolated from rats that were fed an ethanol liquid diet for 6-8 weeks, these cells exhibited a marked impairment of TGF-alpha-stimulated autophosphorylation of the receptor that binds this growth factor compared with hepatocytes from the pair-fed controls. This impaired autophosphorylation of receptor tyrosine residues was accompanied by significant decreases in the amount of surface-bound TGF-alpha. Immunoanalysis indicated no changes in receptor number, indicating that decreased receptor content was not responsible for decreased TGF-alpha binding in the hepatocytes from the ethanol-fed rats. In conclusion, chronic ethanol feeding reduced TGF-alpha binding to hepatocytes with a concomitant decrease in the ability of the receptor tyrosine kinase to autophosphorylate its tyrosine residues. These changes were not accompanied by decreased receptor protein content. These defects could lead to altered signal transduction and to impaired reparative and regenerative processes in the liver.

Ethanol-induced alterations of the microtubule cytoskeleton in hepatocytes

Ethanol has been predicted to alter vesicle-based protein traffic in hepatocytes, in part, via a disruption of the microtubule (MT) cytoskeleton. However, information on the effects of chronic ethanol exposure on MT function in vivo is sparse. Therefore the goal of this study was to test for ethanol-induced changes in rat liver tubulin expression, assembly, and cellular organization, using molecular, biochemical and morphological methods. The results of this study showed that tubulin mRNA and protein levels were not altered by ethanol. Tubulin, isolated from control and ethanol-fed rats, showed similar polymerization characteristics as assessed by calculation of the critical concentration for assembly and morphological structure. In contrast, the total amount of assembly-competent tubulin was reduced in livers from ethanol-fed rats compared with control rats when assessed by quantitative immunoblot analysis using a tubulin antibody. In addition, we observed that MT regrowth and organization in cultured hepatocytes treated with cold and nocodazole was markedly impaired by chronic ethanol exposure. In summary, these results indicate that tubulin levels in liver are not reduced by ethanol exposure. While there is a substantial amount of tubulin protein capable of assembling into functional MTs in ethanol-damaged livers, a marked portion of this tubulin is polymerization incompetent. This may explain why these hepatocytes exhibit a reduced number of MTs with an altered organization.

Alcohol and cytokine networks

Experimental and clinical data pertaining to alcohol (ethanol, EtOH) interference with cytokine networks are analyzed from the viewpoint of their importance for understanding the deleterious effects of EtOH on various functions. The data are grouped and analysed according to several major directions that have emerged in the last decade since the research on EtOH interference with cytokine networks started. These are: cytokine secretion, including the effects of the drug on in vivo cytokine levels and on in vitro cytokine secretion by various cells; cytokine-receptor interaction; intracellular fate of cytokines; and cellular responses to cytokines. Correspondingly, the data are presented in four tables. Attention is paid to the fact that the effects of EtOH on various aspects of cytokine biology (e.g., secretion by various cells) are diversified. Contradictory data have been reported but the cause for discrepancy is poorly understood. EtOH effects vary with the cytokine studied, the method of EtOH administration (i.e., acute or chronic), the species used, or with other experimental conditions. It is important to note, however, that the vast majority of experimental and clinical data show that EtOH interferes with cytokine networks and that research on such interference may lead to important steps in understanding the mechanisms of action of EtOH. An attempt is made herein to select aspects of cytokine biology, as affected by EtOH, that have been studied to a lesser extent, thus calling for more research efforts in those areas.

Decreased binding and autophosphorylation of the epidermal growth factor receptor in ethanol-fed rats

We have shown previously that binding and processing of epidermal growth factor are impaired in livers of ethanol-fed rats. In the current study, we examined these ethanol-induced alterations in greater detail by studying both high and low affinity epidermal growth factor binding as well as the ability of added ligand to stimulate receptor autophosphorylation. We also measured the binding of anti-receptor antibody to intact and permeabilized cells in order to determine more accurately the levels of receptor protein. Hepatocytes were isolated from ethanol-fed and pair-fed control rats. Ligand binding, antibody binding, and ligand-induced receptor autrophosphorylation were measured in the respective cell populations. In ethanol-fed animals, binding to both high and low affinity states of the hepatic epidermal growth factor receptor was decreased by 40-50% (P < 0.01). This ethanol-induced decrease in ligand binding was accompanied by a reduced ability of epidermal growth factor to stimulate receptor autophosphorylation (32% decrease, P < 0.01). In contrast, binding of anti-receptor antibody was not altered in ethanol-fed animals. In conclusion, chronic ethanol feeding decreased epidermal growth factor binding with a concomitant decrease in the ability of the receptor tyrosine kinase to phosphorylate tyrosine residues. These changes were not accompanied by an actual decrease in receptor protein content. These findings could be relevant to modified responses to this growth factor in the livers of chronic ethanol-fed animals.

Impairment of the asialoglycoprotein receptor by ethanol oxidation

It is well established that ethanol exposure impairs the process of receptor-mediated endocytosis in hepatic cells, although the molecular mechanism(s) and the physiological consequence(s) of this impairment are unclear. Because addressing these mechanistic questions is difficult in vivo, we have developed a recombinant cell line of hepatic origin capable of metabolizing ethanol. In this study, we have used these recombinant cells, designated HAD cells, to investigate the ethanol-induced impairment to the receptor-mediated endocytosis of the hepatic asialoglycoprotein receptor. Comparing the binding of the ligand asialoorosomucoid in both the parental Hep G2 cells and the recombinant HAD cells, maintained in the presence and absence of ethanol, revealed decreased ligand binding in the HAD cells. This impairment was accentuated by prolonging the ethanol exposure, reaching approximately 40% in both surface and total receptor populations by 7 days. Addition of the alcohol dehydrogenase inhibitor pyrazole to the ethanol-containing medium abolished this impairment, indicating that the decreased binding was a result of the alcohol dehydrogenase-mediated oxidation of ethanol. Furthermore, using antibody specific to the asialoglycoprotein receptor, it was demonstrated that the ethanol-induced impairment in ligand binding was a consequence of decreased ligand binding and not a result of diminished receptor numbers. These results indicated that ethanol oxidation was required for the ethanol-induced impairment in ligand binding, and that the reduced ligand binding was a result of a decrease in the ability of the ligand to bind to the receptor.

Ethanol exposure reduces the density of the low-affinity nerve growth factor receptor (p75) on pheochromocytoma (PC12) cells

Although ethanol is detrimental to the developing nervous system, the mechanism(s) by which ethanol produces neuronal damage is (are) not clear. One potential mechanism is ethanol-induced inhibition of neurotrophic support. This study utilized an in vitro model, pheochromocytoma PC12 cells, to examine the effect of ethanol on the nerve growth factor (NGF) receptor. NGF binding studies indicated that ethanol exposure (400 mg/dl for 4 days) reduced the density of the low-affinity (p75) NGF receptor on PC12 cells, but had no effect on the density of the high-affinity NGF receptor. The equilibrium dissociation constants (Kd) for both the low-affinity and high-affinity NGF receptors were unaffected by ethanol. Low-affinity NGF binding is mediated by the p75 component of the NGF receptor. Quantification of p75 by immunoprecipitation revealed that ethanol reduced the level of p75 in PC12 cells. However, Northern analysis indicated that the p75 mRNA was not reduced by ethanol exposure, raising the possibilities that ethanol inhibited translation of p75 or incorporation of the p75 protein into the plasma membrane. This work is consistent with the hypothesis that ethanol's detrimental effects may be produced in part by inhibition of neurotrophic support at the receptor level.

Zonal differences in ethanol-induced impairments in fluid-phase endocytosis in rat hepatocytes

We have previously shown that ethanol administration impairs the processes of fluid-phase endocytosis (FPE) and receptor-mediated endocytosis (RME) in isolated rat hepatocytes after as early as 1 week of ethanol administration. The defects in RME were most prominent in the perivenule (PV) region of the liver lobule, the area wherein alcoholic liver disease has been shown to start and predominate. We undertook the present study to see if changes in FPE were likewise more apparent in the PV versus the periportal (PP) region of the liver. For these studies, we fed male Sprague-Dawley rats with an ethanol-supplemented liquid diet or an isocaloric control diet for 1 or 5 weeks. PV and PP hepatocytes were isolated using a digitonin-collagenase perfusion method. Internalization and efflux of the marker dye, Lucifer Yellow, was then examined in the cell populations. After as early as 1 week of feeding, cells from the PV region in ethanol-fed animals showed dramatic impairments in the net internalization of dye, compared with PV cells from the pair-fed controls, and these changes persisted throughout the 5-week feeding period. In contrast, internalization of Lucifer Yellow in cells from the PP region of the liver were not different between control and ethanol animals. Because net internalization represents the balance between uptake into the cells versus efflux from the cells, we examined these components individually. Early uptake of the dye into the cell was not altered by ethanol treatment. The decreased net internalization seemed to be caused by enhanced efflux of the dye, which was significantly increased in PV cells, compared with the same cell type in control animals. Cells from the PP region of the ethanol-fed animals did not exhibit altered efflux after either 1 or 5 weeks of feeding. These results indicate that ethanol-induced impairments in FPE are more dramatic in the PV region of the liver, and these impairments seem to result from an ethanol-induced enhancement of efflux.

Effects of chronic ethanol administration on the endocytosis of cytokines by rat hepatocytes

The effects of chronic ethanol administration on the endocytosis of three representative cytokines were investigated in isolated rat hepatocytes. When hepatocytes were isolated from rats that were fed an ethanol liquid diet for 12 to 13 weeks, these cells exhibited a decreased ability to internalize and degrade transforming growth factor-alpha, tumor necrosis factor-alpha and interleukin-6, compared with hepatocytes from the pair-fed controls. This impaired endocytosis of all three cytokines was accompanied by significant decreases in the amount of hepatocyte surface-bound cytokine. Changes in cytokine binding to surface receptors and reduced rates of receptor-cytokine complex internalization into the cells seem to be major contributors to defective endocytosis in hepatocytes from the ethanol-fed rats. Impaired hepatocyte endocytosis could lead to altered steady-state levels of cytokines in the liver and modified physiological responses to cytokines. These changes could affect homeostasis among the various cell types in the liver and could contribute to liver dysfunction and injury.

Effect of hyperosmolarity on both receptor-mediated and fluid-phase endocytosis in ethanol-fed animals

We have shown previously that chronic ethanol administration impairs hepatic receptor-mediated endocytosis (RME) of asialoorosomucoid (ASOR), epidermal growth factor and insulin, whereas early uptake by fluid-phase endocytosis (FPE) of a fluorescent dye, Lucifer Yellow (LY), is not altered. Results of these studies suggested that ethanol-induced injury was primarily affecting endocytosis in coated pit areas of the plasma membrane while internalization in noncoated membrane areas was unaffected. In the present study, we investigated the effects of blocking clathrin-coated pit mediated endocytosis by hyperosmolarity on FPE of LY and on RME of ASOR. We also examined the effects of hyperosmolarity on the binding and internalization of insulin, a ligand endocytosed by both RME and FPE. Uptake of LY by noncoated regions of the membrane was not altered in control animals, whereas in hepatocytes from ethanol-fed animals uptake of LY was decreased by 35-40% in the presence of 0.12 M sucrose (P < 0.05). These hyperosmolar conditions almost completely inhibited (> 85%) the endocytosis of 125I-ASOR by RME in both ethanol and control cells. Results with insulin showed slight effects (20-30% impairment) on uptake of the ligand in the presence of sucrose. These results are consistent with previous reports that in normal cells the coated pit pathway is impaired by hyperosmolarity, whereas endocytosis in noncoated regions is unaltered. It appears, however, that both FPE and RME in hepatocytes from ethanol-fed animals are susceptible to perturbation by hyperosmolarity. These results indicate that the noncoated pit pathway may be sensitive to stressful conditions such as hyperosmolarity after ethanol treatment.

Hepatocyte proliferation as an indicator of outcome in acute alcoholic hepatitis

To determine whether clinical outcome in patients with acute alcoholic hepatitis is related to the regenerative capability of the liver, liver biopsy specimens from 25 prospectively recruited patients with acute alcoholic hepatitis were studied for hepatic expression of mRNA for two proliferation markers, proliferating cell nuclear antigen and human histone, and for transforming growth factor alpha (TGF alpha) and TGF beta 1 and hepatocyte growth factor (HGF), which regulate hepatocyte proliferation. Proliferation markers were detected to varying degrees in 0-80% of hepatocytes and occasionally in sinusoidal cells and bile-duct epithelium in 19 patients (76%). Patients who survived for 6 months had greater expression of proliferation markers than those who did not survive (p < 0.01). TGF alpha was detected in hepatocytes and bile-duct epithelium, whereas TGF beta 1 was detected mainly in sinusoidal cells and was associated with perivenular fibrosis. Patients who survived for 6 months had greater expression of TGFs than non-survivors (p < 0.02). HGF was detected in sinusoidal cells in 7 patients and correlated with survival (p < 0.01). These data indicate that hepatocyte proliferation, which is possibly related to the pattern of hepatotrophic factor expression, is a good indicator of outcome in acute alcoholic hepatitis.

Zonal differences in ethanol-induced impairments in hepatic receptor binding

We have previously shown ethanol-induced defects in receptor-mediated endocytosis of asialoorosomucoid (ASOR), epidermal growth factor (EGF), and insulin in isolated rat hepatocytes. The present study was undertaken to compare the binding of these three ligands in both Zone 1 (periportal [PP] region) and Zone 3 (perivenule [PV] region) of rat liver. Cells from the PV region of ethanol-fed animals bound significantly less EGF (40% decrease) than did cells from the same area in control rats. EGF binding was decreased to a lesser extent (15-25%) in PP cells from ethanol-fed animals compared to controls. When binding of ASOR was examined, ethanol feeding significantly impaired binding in both PP cells (30-35% decrease) and PV cells (50-55% decrease), again showing a greater ethanol-induced impairment in the PV region. Insulin binding in ethanol animals was decreased by 20-25% in both regions compared to controls. In addition, we found that ASOR receptor recycling was impaired to a greater extent in the PV than in the PP region of liver after ethanol feeding, indicating selective impairment of receptor function in the centrilobular region of the liver.

Effect of chronic alcohol feeding on hepatic iron status and ferritin uptake by rat hepatocytes

Alcohol abuse is known to cause disturbances to iron homeostasis in man and is associated with elevated serum ferritin levels. We have previously shown that ethanol metabolism in the rat hepatocyte is associated with an immediate reduction in ferritin uptake by this cell. In this study we have examined the effect of pair-feeding the Lieber-DeCarli liquid alcohol diet on ferritin uptake by rat hepatocytes. Rat liver ferritin was radiolabeled with 59Fe in vivo and isolated by conventional techniques. Rats were pair-fed the Lieber-DeCarli liquid alcoholic diet for 4-6 weeks. Hepatocytes, isolated from their livers by collagenase perfusion, were incubated with [59Fe]ferritin in L-15 medium at 37 degrees C and 4 degrees to measure ferritin uptake and binding. The in vitro effect of ethanol on these hepatocytes was also studied. Ferritin and iron parameters were measured in the sera and hepatocytes of these animals and a comparable group of normal chow-fed rats. The rate of ferritin uptake by hepatocytes from alcohol-fed rats was significantly faster than that of their pair-fed controls (0.743 +/- 0.061 vs. 0.540 +/- 0.042 ng/min/10(6) cells, p < 0.05). However, the rats on Lieber-DeCarli control diet exhibited a lower hepatocyte ferritin uptake rate than chow-fed animals (79.3 +/- 8.1% of the control values, p < 0.01). In vitro incubation of cells in 100 mM ethanol resulted in less inhibition of ferritin uptake by hepatocytes from alcoholic rats than from their pair-fed controls (11 +/- 7.1% inhibition vs. 43.6 +/- 10.7% for controls, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Time course of ethanol-induced impairment in fluid-phase endocytosis in isolated rat hepatocytes

The time-course effects of long-term ethanol administration on fluid-phase endocytosis were studied in isolated rat hepatocytes. Rats were pair-fed an ethanol-supplemented liquid diet or an isocaloric control diet for 3 days, 1 wk, 2 wk or 5 wk. Hepatocytes were isolated and incubated at 37 degrees C with various concentrations of the fluid-phase marker Lucifer yellow. Net internalization of the marker dye was determined. After as little as 1 wk, ethanol-fed rats demonstrated marked decreases in the net internalization of dye compared with pair-fed controls; these changes persisted throughout 5 wk of feeding. Because net internalization is the balance between uptake into the cells vs. efflux from the cells, these components were examined individually. Early uptake was not significantly decreased by ethanol feeding; however, efflux of preloaded Lucifer yellow from cells from the ethanol-fed animals was markedly faster than efflux from pair-fed controls. This increased efflux was more prominent in the longer preload time (90 min) compared with a shorter preload time (15 min), indicating an alteration in dye distribution among various intracellular pools. These ethanol-induced changes in fluid-phase endocytosis were apparent for 1 wk through 5 wk of feeding and were similar for all Lucifer yellow concentrations examined. These results indicate that the decreased net internalization of Lucifer yellow through fluid-phase endocytosis is mainly a result of an ethanol-induced increase in efflux possibly caused by altered intracellular trafficking rather than by reduction in uptake.

Ethanol increases receptor-dependent cyclic AMP production in cultured hepatocytes by decreasing G(i)-mediated inhibition

Increasing evidence suggests that ethanol-induced changes in cyclic AMP (cAMP) signal transduction play a critical role in the acute and chronic effects of ethanol. Here we have investigated the effects of ethanol on cAMP signal transduction in primary cultures of rat hepatocytes. Acute exposure to ethanol had a biphasic effect on glucagon-receptor-dependent cAMP production in intact cells: 25-50 mM-ethanol decreased cAMP, whereas treatment with 100-200 mM-ethanol increased cAMP. After chronic exposure to 50-200 mM-ethanol for 48 h in culture, glucagon-receptor-dependent cAMP levels were increased, but no change in glucagon receptor number was observed. These effects of ethanol were independent of ethanol oxidation. Chronic ethanol treatment also increased adenosine-receptor- and forskolin-stimulated cAMP production. Increased cAMP production was also observed upon stimulation of adenylate cyclase with glucagon, forskolin and F- in membranes isolated from cells cultured with 100 mM-ethanol for 48 h. However, no differences were observed in basal and MnCl2-stimulated adenylate cyclase activity. The quantity of alpha i protein was decreased by 35% after chronic ethanol treatment, but no change in the quantity of alpha s protein was detected. Decreased alpha i protein was associated with a decrease in G(i) function, as assessed by the ability of 0.1 nM-guanosine 5'-[beta gamma-imido]triphosphate and 1 microM-somatostatin to inhibit forskolin-stimulated adenylate cyclase activity. Taken together, these results suggest that chronic exposure to ethanol increases receptor-dependent cAMP production in hepatocytes by decreasing the quantity of alpha i protein at the plasma membrane and thereby decreasing the inhibitory effects of G(i) on adenylate cyclase activity.

Effect of acute alcohol administration on TNF-alpha binding to neutrophils and isolated liver plasma membranes

The mechanisms underlying the effects of alcohol (ethanol, ETOH) on host defense are poorly understood. ETOH modulation of the cytokine regulatory network is one possible way by which ETOH could alter nonspecific immune function. In this study we examined the ability of acute alcohol intoxication to alter lipopolysaccharide (LPS)-induced changes in tumor necrosis factor (TNF)-alpha binding to neutrophils and isolated liver plasma membranes. Rats were injected intravenously with a primed constant infusion of ETOH for 7 hr to maintain blood ETOH concentration at approximately 35 mM. Four hours after the start of ETOH infusion, the animals received intravenously either sterile saline or LPS (100 micrograms/100 g body weight) and were sacrificed at the end of ETOH infusion. Blood neutrophils and liver plasma membranes were isolated, and TNF-alpha binding characteristics determined using recombinant human [125I]TNF-alpha. ETOH treatment alone induced a significant decrease (51%) of neutrophil Bmax for TNF-alpha, without affecting the cytokine binding to plasma membranes. LPS, with or without ETOH, significantly decreased (61%) neutrophil Bmax for TNF-alpha and increased (115%) its binding to liver plasma membranes. The KD values of binding to either neutrophils or liver plasma membranes were not altered by ETOH or LPS treatment of animals. By decreasing the cytokine binding to neutrophils, ETOH may impair the control exerted by TNF-alpha on cell function, thus damaging host defense.

The effect of ethanol metabolism on ferritin uptake by freshly isolated rat hepatocytes: is acetaldehyde responsible for this alteration?

Alcohol abuse is associated with disturbances to iron metabolism in man, ranging from anemia to siderosis. Also seen in these patients are increased serum ferritin levels. Since the liver not only stores iron in cytosolic ferritin, but has also been shown to take up this molecule from the plasma by an active transport mechanism, it has been suggested that the iron in this circulating ferritin may contribute to the increased incidence of siderosis seen in alcoholics. As part of an ongoing study of these disturbances, using a rat model, we have examined the uptake of ferritin by freshly isolated hepatocyte suspension to test the hypothesis that increased hepatocyte uptake of ferritin iron contributes to the siderosis seen in some alcoholics. Incubation of hepatocytes in the presence of ethanol resulted in a progressive reduction in uptake with increasing alcohol concentration, from 1.23 +/- 0.05 ng of ferritin/10(6) cells/min to 0.65 +/- 0.02 ng/10(6) cells/min (mean +/- SD) at an ethanol concentration of 100 mM. 4-Methylpyrazole (0.1 mM) restored 70% of this activity, but higher concentrations also decreased ferritin uptake in the absence of ethanol. The addition of 5 microM cyanamide decreased ferritin uptake slightly in the presence of ethanol (0.82 +/- 0.04 ng of ferritin/10(6) hepatocytes/min vs. 0.86 +/- 0.03 ng/10(6) cells/min for ethanol alone), while having no effect in the absence of ethanol (1.01 +/- 0.04 vs. 1.12 +/- 0.05 ng/10(6) cells/min). Preincubation of the hepatocytes with acetaldehyde resulted in a dose-dependent reduction to a maximum reduction of approximately 25% at 300 microM acetaldehyde.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of chronic ethanol administration on the rates of internalization of various ligands during hepatic endocytosis

The purpose of the present study was to further characterize the ethanol-induced impairments in hepatic endocytosis. Specifically, we examined the effects of ethanol treatment on receptor-ligand internalization via the coated and noncoated pit pathways. Insulin, epidermal growth factor (EGF) and asialoorosomucoid (ASOR) were used as model ligands to study internalization by isolated hepatocytes. ASOR and EGF are thought to be internalized strictly in coated pit regions of the cell membrane, while insulin may be internalized in both coated and uncoated membrane regions. Ethanol administration for 5-7 weeks decreased internalization of ASOR and EGF while internalization of insulin was unchanged during a single round of endocytosis of surface-bound ligand. Similarly, a more quantitative measure of endocytosis, the endocytic rate constant, was decreased for EGF and ASOR but not for insulin in livers of experimental rats. When endocytosis of Lucifer yellow, a fluorescent dye known to be internalized in the cell by fluid-phase endocytosis was examined, the initial rates of dye uptake were not significantly altered by alcohol administration. These results indicate that ethanol may selectively impair internalization occurring by coated pits while it has a minimal effect on initial uptake of molecules which are internalized by noncoated membrane regions.

Insulin and glucagon infusion in acute alcoholic hepatitis: a prospective randomized controlled trial

In a randomized, controlled trial to investigate the possible benefit of insulin and glucagon therapy in severe acute alcoholic hepatitis, 86 patients were randomized to receive 30 U insulin and 3 mg glucagon in 250 ml 5% dextrose over 12 hr each day for 3 wk or a similar regime of identical placebo. No significant differences were seen in patients' clinical characteristics and disease severity in the treated and placebo groups. Of the 43 patients receiving insulin and glucagon, 15 (35%) died within 4 wk of randomization, compared with 14 deaths (33%) in the control patients (p = not significant). When the patients surviving the first 4 wk were examined there were five more deaths in the treatment group, compared with one death in the control group at 6-mo follow-up (p = not significant). No significant differences in the frequency of short-term or long-term complications of alcoholic liver disease or relapse to alcohol were seen when the two groups were compared, although hypoglycemia was seen in six patients during infusion of insulin and glucagon. Similarly, no significant differences were seen in the improvement in clinical or biochemical features at 4 wk and at 6 mo in survivors when the insulin and glucagon-treated patients were compared with patients in the placebo group. This study does not confirm previous reports that insulin and glucagon infusion improves the outcome of severe acute alcoholic hepatitis.

Chronic ethanol-induced impairments in receptor-mediated endocytosis of insulin in rat hepatocytes

The effects of chronic ethanol administration on the receptor-mediated endocytosis of insulin were investigated in isolated hepatocytes. When hepatocytes were isolated from rats that were fed an ethanol liquid diet for 5-6 weeks, these cells bound 25% less insulin to their surface membrane than did cells from the chow-fed or pair-fed controls. This decreased binding was likely a result of reduced surface receptor number rather than changes in receptor affinity. Rates of insulin degradation were also reduced by 25-30% in hepatocytes from the ethanol-fed animals. In addition, chronic ethanol feeding induced a defect in the internalization of the receptor-insulin complex and altered the hepatocellular processing of the internalized insulin. These results indicate that chronic ethanol administration impairs both the surface binding and the endocytosis of insulin by the liver.