Long-term ethanol administration alters the degradation of acetaldehyde adducts by liver endothelial cells

Immunological response in alcoholic liver disease

The development of alcoholic liver disease (ALD) can be attributed to many factors that cause damage to the liver and alter its functions. Data collected over the last 30 years strongly suggests that an immune component may be involved in the onset of this disease. This is best evidenced by the detection of circulating autoantibodies, infiltration of immune cells in the liver, and the detection of hepatic aldehyde modified proteins in patients with ALD. Experimentally, there are numerous immune responses that occur when proteins are modified with the metabolites of ethanol. These products are formed in response to the high oxidative state of the liver during ethanol metabolism, causing the release of many inflammatory processes and potential of necrosis or apoptosis of liver cells. Should cellular proteins become modified with these reactive alcohol metabolites and be recognized by the immune system, then immune responses may be initiated. Therefore, it was the purpose of this article to shed some insight into how the immune system is involved in the development and/or progression of ALD.

Scavenger receptors on sinusoidal liver endothelial cells are involved in the uptake of aldehyde-modified proteins

Scavenger receptors on sinusoidal liver endothelial cells (SECs) eliminate potentially harmful modified proteins circulating through the liver. It was shown recently that aldehyde-modified proteins bind to scavenger receptors and are associated with the development/progression of alcoholic liver diseases. For these studies, rat livers were perfused in situ with 125I-formaldehyde-bovine serum albumin (f-Alb) or 125I-malondialdehyde-acetaldehyde-bovine serum albumin (MAA-Alb) in the presence of known scavenger receptor ligands as inhibitors. Reverse transcription-polymerase chain reaction (RT-PCR) analysis and scavenger receptor Type A (SRA) knock-out mice were used to assess the role of these receptors in mediating immune responses. The degradation of 125I-f-Alb or 125I-MAA-Alb in whole livers and isolated SECs can be inhibited by known scavenger receptor ligands, including f-Alb, maleylated bovine albumin, and fucoidan. 125I-f-Alb could not be completely inhibited by MAA-Alb. In contrast, 125I-MAA-Alb was only partially inhibited with advanced glycosylated endproduct albumin. RT-PCR data show the presence of a number of scavenger receptors on SECs that may be responsible for the binding of MAA-modified proteins. SRA seems to be one of these receptors involved in the effects mediated by MAA-modified proteins. In a study using SRA knockout mice, it was shown that a decreased antibody response to MAA-Alb resulted. By RT-PCR, CD36, LOX-1, and SR-AI are the scavenger receptors most likely involved in the degradation of MAA-Alb.

Lipopolysaccharide is a cofactor for malondialdehyde-acetaldehyde adduct-mediated cytokine/chemokine release by rat sinusoidal liver endothelial and Kupffer cells

BACKGROUND

The nonparenchymal cells of the liver have been suggested to play a significant role in the inflammatory processes observed in the development and/or progression of alcoholic liver disease. Our laboratories have shown that malondialdehyde-acetaldehyde (MAA)-modified proteins can induce immune responses, cytokine/chemokine secretion, and antigen processing and presentation by liver sinusoidal endothelial cells (SECs). Another molecule that has been shown to induce similar types of responses in Kupffer cells (KCs) is lipopolysaccharide (LPS). Because these materials induce similar responses, it was the purpose of this study to investigate the relationship between LPS and MAA-modified proteins in the development of proinflammatory responses by SECs and KCs.

METHODS

For these studies, SECs and KCs were isolated from chow-fed, pair-fed, and ethanol-fed rats. Cells were stimulated with media alone, bovine serum albumin (Alb), or MAA-modified Alb (MAA-Alb) in the presence or absence of LPS 1 ng/ml, and the supernatants were assayed by enzyme-linked immunosorbent assay for tumor necrosis factor alpha, macrophage chemotactic protein 1, and macrophage inhibitory protein.

RESULTS

All three cytokines/chemokines were 3 to 5 times higher when SECs or KCs were stimulated by MAA-Alb in the presence of LPS, in contrast to cells stimulated with Alb or media in the presence of LPS. Chronic ethanol consumption (6 weeks) had variable effects on the secretion of these cytokines/chemokines but in general did not alter the increased secretion in response to MAA-Alb in the presence of LPS.

CONCLUSIONS

These studies strongly suggest that the sensitization of SECs and KCs by LPS plays a significant role in the development and/or progression of alcoholic liver disease, and the subsequent activation by MAA-modified proteins may be a mechanism by which proinflammatory processes are initiated.

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.

Adduction of soluble proteins with malondialdehyde-acetaldehyde (MAA) induces antibody production and enhances T-cell proliferation

BACKGROUND

The alcohol metabolites malondialdehyde and acetaldehyde can combine to form stable adducts (MAA) which are found in the livers of humans and rats after significant alcohol ingestion. While adducted proteins induce antibody responses in the absence of adjuvants, the mechanisms by which these responses occur are unknown. Thus, it was the purpose of these studies to investigate how MAA modification stimulates antibody and T-cell responses in the absence of adjuvants.

METHODS

Hen egg lysozyme (HEL) was modified with increasing levels of MAA and was used as an immunogen, and antibody and T-cell responses were determined. The role of scavenger receptors in the immunogenicity of MAA-adducted proteins was also investigated.

RESULTS

Maximum antibody response was induced after immunization with 1.8 nM MAA/nM HEL, and was primarily an IgG1 response to HEL as determined by inhibition ELISAs. T-cell proliferative responses after immunization with HEL-MAA were solely to HEL. Immunization with a scavenger receptor ligand in conjunction with HEL-MAA increased the predominant IgG1 response and sharply decreased the IgG2a response by approximately 50%. Binding of HEL-MAA by splenocytes was determined by flow cytometry to be approximately 15% greater than HEL alone, showing a doubling of the geometric mean fluorescence. Also, most of the cells that bound HEL-MAA were class II positive, indicating that antigen-presenting cells can bind the MAA-adducted HEL, and potentially initiate immune responses.

CONCLUSIONS

MAA modification of proteins induces antibody and T-cell proliferative responses in vivo. Initial studies suggest that these responses may be mediated by scavenger receptors that recognize MAA-adducted proteins. This suggests a mechanism by which proteins modified with oxidative products associated with chronic ethanol consumption may alter immune responses that may play an active role in the development and/or progression of alcoholic liver disease.

Malondialdehyde-acetaldehyde-haptenated protein induces cell death by induction of necrosis and apoptosis in immune cells

Recent studies have demonstrated that circulating antibodies against malondialdehyde-acetaldehyde (MAA)-haptenated proteins are significantly increased in patients with alcohol-induced cirrhosis and hepatitis and correlate with the severity of liver damage. Additionally, when proteins are haptenated with MAA, they become highly immunogenic in vivo in the absence of adjuvants. However, the mechanism(s) of this immunogenicity are currently unknown. Initial in vitro studies on the effects of MAA-modified proteins on cells demonstrated an increase in cell death at concentrations that were cell type specific and time-dependent. Since immunogenicity due to cell death has been described, we investigated the mechanism(s) by which cell death was occurring. Assessment of cell death in splenocytes after 1 h found significant levels of apoptosis as compared to controls. After 5 h, a significant and dose-dependent necrosis occurred in which cells were exposed to >62.5 microg/ml (43.1 mM) MAA-haptenated protein. After 24 h, exposure to >31.3 microg/ml (21.6 mM) MAA-haptenated protein resulted in significant levels of necrosis, although DNA laddering studies found apoptosis was occurring as well. Morphological changes in the cells were observed by light microscopy that correlated with a "low" forward scatter population by flow cytometry. Since necrosis has been implicated in enhancing both primary and secondary immune responses, and necrosis was predominantly occurring in response to MAA-haptenated proteins, a possible mechanism by which the immunogenicity of MAA modification of proteins in vivo may occur is suggested. Specifically, MAA modification of self proteins may result in the death of various cell types, most likely those in the liver. These necrotic materials may induce anti-MAA antibodies and other auto antibodies, whose levels may then correlate with the severity of ALD.

The chemistry and biological effects of malondialdehyde-acetaldehyde adducts

This article represents the proceedings of a workshop at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Geoffrey M. Thiele and Simon Worrall. The presentations were (1) The chemistry of malondialdehyde-acetaldehyde (MAA) adducts, by Dean J. Tuma; (2) The formation and clearance of MAA adducts in ethanol-fed rats, by Simon Worrall; (3) Immune responses to MAA adducts may play a role in the development of alcoholic liver disease, by Lynell W. Klassen; (4) Unique biological responses to MAA-modified proteins that may play a role in the development and/or progression of alcoholic liver disease, by Geoffrey M. Thiele; (5) MAA-adducted bovine serum albumin activates protein kinase C and stimulates interleukin-8 release in bovine bronchial epithelial cells, by Todd A. Wyatt; and (6) An enzyme immune assay for serum antiacetaldehyde adduct antibody using low-density lipoprotein-adduct and its significance in alcoholic liver injury and ALDH2 heterozygotes, by Naruhiko Nagata.

Covalent adducts of proteins with acetaldehyde in the liver as a result of acetaldehyde administration in drinking water

BACKGROUND/AIMS

Acetaldehyde, the first metabolic product of ethanol, has been suggested to be responsible for several adverse effects of ethanol through its ability to form covalent adducts with proteins and cellular constituents. It has recently been suggested that acetaldehyde derived from microbial ethanol oxidation in the gut could also contribute to the effects of ethanol in the liver. The present work aimed to examine whether modification of proteins by acetaldehyde occurs in rat liver as a result of acetaldehyde administration in drinking water.

METHODS

Rats were fed with either 0.7% acetaldehyde (n=10) or water (n=10) for 11 weeks. At the end of the feeding period, liver specimens were processed for immunohistochemistry for protein adducts with acetaldehyde and for hepatic cell type-specific protein markers.

RESULTS

Mild fatty change was found in the liver of the acetaldehyde-treated animals but not in the control animals. Immunohistochemical stainings for acetaldehyde adducts revealed intensive positive staining for acetaldehyde adducts in eight (80%) of the animals fed with acetaldehyde. The adducts were predominantly perivenular, although positive staining also occurred along the sinusoids and in the periportal area. Double immunofluorescence staining experiments revealed that hepatocytes were the primary targets of acetaldehyde adduct deposition, although stellate cells and Kupffer cells also showed weak positive reactions.

CONCLUSIONS

The present data indicate that acetaldehyde-protein adducts are formed in the liver of animals following acetaldehyde administration in drinking water, which may contribute to the hepatotoxicity of extrahepatic acetaldehyde. These findings should be implicated in studies on the extrahepatic pathways of ethanol oxidation.

Coagulation protein function VII: diametric effects of acetaldehyde on factor VII and factor IX function

The first metabolite of ethanol, acetaldehyde, has the ability to form adducts with proteins and alter their function. It has been shown that acetaldehyde reacts with various proteins of the blood coagulation pathway and, subsequently, produces a prolongation of the clotting time. This study evaluated the function of clotting proteins from the extrinsic coagulation pathway (factor VII) and the intrinsic coagulation pathway (factor IX) when preincubated with acetaldehyde as compared to a control and compared to preincubation with ethanol. Prior to use in a clotting assay, incubation times with acetaldehyde, ethanol, and the control were the same for both factors VII and IX. An automatic fibrometer measured the clotting times. Factor VII preincubated with acetaldehyde prolonged the clotting time. However, factor IX preincubated with acetaldehyde actually decreased the clotting time. Of interest, both factors VII and IX preincubated with acetaldehyde produced statistically significant results when compared to the control and ethanol. This experiment indicates that acetaldehyde, in forming an adduct with proteins of the blood coagulation pathway, may induce a conformational change of factors VII and IX so as to either increase or decrease the clotting time. Therefore, it is possible that some of the deranged coagulation in alcohol abusers may be a final net result of the interaction of acetaldehyde and proteins of the coagulation pathway.

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.

Soluble proteins modified with acetaldehyde and malondialdehyde are immunogenic in the absence of adjuvant

Recent studies have shown that the alcohol metabolites malondialdehyde and acetaldehyde can combine to form a stable adduct (MAA) on proteins. This adduct has been detected in the livers of rats chronically consuming ethanol, and serum antibodies to MAA have been observed at significantly higher concentrations in ethanol-fed when compared with pair-fed or chow-fed control rats. More recently, preliminary studies have strongly suggested that the MAA adduct is capable of stimulating antibody responses to soluble proteins in the absence of adjuvants. The antibodies produced recognize either the MAA epitope or the carrier protein itself. Therefore, it was the purpose of this study to examine the potential immunogenicity of MAA-modified exogenous proteins in the absence of adjuvants. Balb/c mice were immunized in the presence or absence of adjuvant with different concentrations of unmodified or MAA-modified proteins. The antibody response to both the MAA epitope and unmodified protein epitopes were determined by ELISA. In the absence of adjuvant, significant antibody responses were induced to both the MAA epitope and nonmodified protein epitopes. Smaller immunizing doses of MAA-protein conjugate favored the production of antibodies to nonmodified proteins, whereas larger doses induced a strong anti-MAA response. In studies to begin determining a mechanism for the specificity of the response in the absence of adjuvants, peritoneal macrophages were found to bind and degrade MAA-adducted proteins through the use of a scavenger receptor. This indicated that MAA-adducted proteins may be specifically taken up and epitopes presented to the humoral immune system in the absence of adjuvants. Importantly, these are the first data showing that an alcohol-related metabolite can induce an antibody response in the absence of adjuvant and suggesting a mechanism by which antibody to the MAA adduct or its carrier (exogenous or endogenous) proteins may be generated in vivo.

Acetaldehyde inhibits NF-kappaB activation through IkappaBalpha preservation in rat Kupffer cells

BACKGROUND AND AIMS

Treatment with acetaldehyde dehydrogenase inhibitors leads to increased liver acetaldehyde levels and prevents hepatic inflammation and necrosis in ethanol-fed rats. This is accompanied by IkappaBa preservation and decreased activation of nuclear factor (NF)-kappaB. The present in vitro study was aimed to clarify whether acetaldehyde has an effect on degradation of IkappaBalpha and activation of NF-kappaB in LPS-stimulated rat Kupffer cells.

METHODS

Kupffer cells were isolated from male Sprague-Dawley rats and preincubated with various concentrations of acetaldehyde (25-100 microM). Thereafter the cells were stimulated with LPS, and cytosolic and nuclear fractions were prepared. IkappaBalpha and p65 proteins and activation of NF-kappaB were evaluated.

RESULTS

In LPS-stimulated rat Kupffer cells, acetaldehyde diminished proteolytic degradation of IkappaBalpha, inhibited nuclear translocation of cytosolic p65 protein, and, accordingly, markedly decreased NF-kappaB activation.

CONCLUSIONS

Acetaldehyde is clearly involved in the stabilization of IkappaBalpha protein and suppression of NF-kappaB activation in rat Kupffer cells. Acetaldehyde may form an adduct with IkappaBalpha, thus making the protein less susceptible to degradation.