Zonal distribution of protein-acetaldehyde adducts in the liver of rats fed alcohol for long periods

Accumulation of proteins bearing atypical isoaspartyl residues in livers of alcohol-fed rats is prevented by betaine administration: effects on protein-L-isoaspartyl methyltransferase activity

BACKGROUND/AIMS

Protein-L-isoaspartyl methyltransferase (PIMT) is a methyltransferase that plays a crucial role in the repair of damaged proteins. In this study, we investigated whether ethanol exposure causes an accumulation of modified proteins bearing atypical isoaspartyl residues that may be related to impaired PIMT activity. We further sought to determine whether betaine administration could prevent the accumulation of these types of damaged proteins.

METHODS

Livers of male Wistar rats, fed the Lieber DeCarli control, ethanol or 1% betaine-supplemented diets for 4 weeks, were processed for PIMT-related analyses.

RESULTS

We observed a significant increase in the accumulation of modified proteins bearing isoaspartyl residues, i.e. the substrates for PIMT, in homogenate samples and various subcellular fractions of livers from ethanol-fed rats. Betaine supplementation prevented this accumulation of damaged proteins. In contrast, ethanol exposure induced no changes in the PIMT enzyme activity levels as compared to controls. The accumulation of damaged proteins negatively correlated with hepatic S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) ratios.

CONCLUSIONS

Ethanol consumption results in the accumulation of modified proteins bearing atypical isoaspartyl residues via impaired in vivo PIMT activity. Betaine administration prevents the ethanol-induced accumulation of isoaspartyl-containing proteins by restoring the PIMT-catalyzed protein repair reaction through normalizing the hepatocellular SAM:SAH ratios.

Oxidative stress as a trigger for cellular immune responses in patients with alcoholic liver disease

Serum antibodies reactive with neo-antigens generated during ethanol metabolism have been identified in patients with alcoholic liver disease (ALD), although their role in the pathogenesis of disease remains unclear. In this study, we characterized peripheral blood mononuclear cell (PBMC) T-cell and antibody responses to human serum albumin (HAS) adducted with acetaldehyde under reducing conditions (AcA-HSA) or with malondialdehyde (MDA-HSA) in patients with advanced ALD (AALD, n = 28), heavy drinkers with no liver disease (NALD, n = 14), and mild/moderate drinking controls (n = 22). Peak proliferative responses of PBMC were assessed in vitro by tritiated thymidine incorporation after the addition of optimized concentrations of antigen or OKT3. Antibody titers were determined by enzyme-linked immunosorbent assay (ELISA). MDA-HSA induced PBMC T-cell proliferation was significantly higher in ALD than in NALD or control patients. Moreover, 10 of 28 (36%) of ALD patients had significant T-cell proliferative responses to MDA-HSA compared to 0 of 14 (0%, P =.02) of the NALD group and 2 of 22 (9%, P <.05) of controls. No significant difference in PBMC T-cell response to Aca-HSA was seen between subject groups. Patients with positive cellular responses to MDA had higher serum anti-MDA antibody titers than those not exhibiting a positive cellular response (P <.005). In conclusion, the pattern of cellular and humoral responses to MDA adducts suggests that the development of these responses may be a susceptibility factor for the development of advanced alcoholic liver disease. The apparent importance of T-cell responses to MDA adducts suggests that oxidative stress may represent an important stimulus for the development of cellular immune responses associated with advanced ALD.

Acute and chronic effects of alcohol exposure on skeletal muscle c-myc, p53, and Bcl-2 mRNA expression

Skeletal muscle atrophy is a common feature in alcoholism that affects up to two-thirds of alcohol misusers, and women appear to be particularly susceptible. There is also some evidence to suggest that malnutrition exacerbates the effects of alcohol on muscle. However, the mechanisms responsible for the myopathy remain elusive, and some studies suggest that acetaldehyde, rather than alcohol, is the principal pathogenic perturbant. Previous reports on rats dosed acutely with ethanol (<24 h) have suggested that increased proto-oncogene expression (i.e., c-myc) may be a causative process, possibly via activating preapoptotic or transcriptional pathways. We hypothesized that 1) increases in c-myc mRNA levels also occur in muscle exposed chronically to alcohol, 2) muscle of female rats is more sensitive than that from male rats, 3) raising acetaldehyde will also increase c-myc, 4) prior starvation will cause further increases in c-myc mRNA expression in response to ethanol, and 5) other genes involved in apoptosis (i.e., p53 and Bcl-2) would also be affected by alcohol. To test this, we measured c-myc mRNA levels in skeletal muscle of rats dosed either chronically (6-7 wk; ethanol as 35% of total dietary energy) or acutely (2.5 h; ethanol as 75 mmol/kg body wt ip) with ethanol. All experiments were carried out in male Wistar rats (approximately 0.1-0.15 kg body wt) except the study that examined gender susceptibility in male and female rats. At the end of the studies, rats were killed, and c-myc, p53, and Bcl-2 mRNA was analyzed in skeletal muscle by RT-PCR with an endogenous internal standard, GAPDH. The results showed that 1) in male rats fed ethanol chronically, there were no increases in c-myc mRNA; 2) increases, however, occurred in c-myc mRNA in muscle from female rats fed ethanol chronically; 3) raising endogenous acetaldehyde with cyanamide increased c-myc mRNA in acute studies; 4) starvation per se increased c-myc mRNA levels and at 1 day potentiated the acute effects of ethanol, indicative of a sensitization response; 5) the only effect seen with p53 mRNA levels was a decrease in muscle of rats starved for 1 day compared with fed rats, and there was no statistically significant effect on Bcl-2 mRNA in any of the experimental conditions. The increases in c-myc may well represent a preapoptotic effect, or even a nonspecific cellular stress response to alcohol and/or acetaldehyde. These data are important in our understanding of a common muscle pathology induced by alcohol.

Cytochrome P450 2E1 (CYP2E1)-dependent production of a 37-kDa acetaldehyde-protein adduct in the rat liver

Ethanol-inducible cytochrome P450 2E1 (CYP2E1) has been shown to be involved in the metabolism of both ethanol and acetaldehyde. Acetaldehyde, produced from ethanol metabolism, is highly reactive and can form various protein adducts. In this study, we investigated the role of CYP2E1 in the production of a 37-kDa acetaldehyde-protein adduct. Rats were pairfed an isocaloric control or an alcohol liquid diet with and without cotreatment of YH439, an inhibitor of CYP2E1 gene transcription, for 4 weeks. The soluble proteins from rat livers of each group were separated on SDS-polyacrylamide gels followed by immunoblot analysis using specific antibodies against the 37-kDa protein acetaldehyde adduct. In addition, catalytic activities of the enzymes involved in alcohol and acetaldehyde metabolism were measured and compared with the adduct level. Immunoblot analysis revealed that the 37-kDa adduct, absent in the pair-fed control, was evident in alcohol-fed rats but markedly reduced by YH439 treatment. Immunohistochemical analysis also showed that the 37-kDa adduct is predominantly localized in the pericentral region of the liver where CYP2E1 protein is mainly expressed. This staining disappeared in the pericentral region after YH439 treatment. The levels of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase isozymes were unchanged after YH439 treatment. However, the level of the 37-kDa protein adduct positively correlated with the hepatic content of P4502E1. These data indicate that the 37-kDa adduct could be produced by CYP2E1-mediated ethanol metabolism in addition to the ADH-dependent formation.

Peptidase activities of the multicatalytic protease in rat liver after voluntary and intragastric ethanol administration

Ethanol consumption slows down the rate of hepatic protein catabolism. The present study was conducted to determine whether ethanol consumption, given by voluntary (pair) feeding or by intragastric administration, affected the peptidase activities of the proteasome in rat liver. Rats were pair-fed liquid diets containing either ethanol or isocaloric maltose-dextrin. A separate group of animals was intragastrically infused continuously with similar liquid diets containing either ethanol or isocaloric dextrose. Crude liver homogenates and their cytosolic fractions were assayed for their chymotrypsin-like (Cht-L), trypsin-like (T-L), and peptidyl-glutamyl-peptide hydrolase (PGPH) activities, using specific fluorogenic peptides as substrates. Voluntary ethanol feeding did not affect the three peptidase activities of the proteasome. However, intragastric ethanol administration caused a 35% to 40% decline in the Cht-L and the T-L activities, but did not significantly change the PGPH activity. The lower peptidase activities in cytosol samples from intragastrically ethanol-fed rats were not restored to control levels by overnight dialysis, nor by the inclusion of low levels of sodium dodecyl sulfate (SDS) or of 0.5 mmol/L adenosine triphosphate (ATP) in the proteasome assay mixture. Immunoblot analyses using anti-rat liver proteaseome exhibited equal levels of immunoreactive proteasome subunits in livers of control and ethanol-fed rats. Similar results were obtained when blots were probed with antibody made specifically against the proteasome subunit, LMP-7. The results indicate that intragastric, but not voluntary, ethanol consumption differentially affects the separate catalytic activities of the proteasome without affecting its steady-state levels. Such changes may be related to the degree of ethanol-induced oxidative stress.

An enzyme immune assay for serum anti-acetaldehyde adduct antibody using low-density lipoprotein adduct and its significance in alcoholic liver injury

An acetaldehyde (AcH) adduct was prepared using rabbit low-density lipoprotein as carrier proteins. An antibody against this adduct was raised in Watanabe heritable hyperlipidemic rabbits and cross-reacted with human low-density lipoprotein and bovine serum albumin adducts. Using this antibody, serum anti-AcH-adduct antibody levels were measured by a direct ELISA method in 56 Japanese adults (healthy adults and patients with nonalcoholic gastrointestinal diseases, alcoholic liver injury, or alcoholic pancreatitis). The antibody level (mean +/- SD) was 22 +/- 10 microg/ml in healthy adults, 22 +/- 11 microg/ml in nonalcoholic gastrointestinal diseases, and 16 +/- 13 microg/ml in alcoholic pancreatitis. These antibody levels tended to increase with the progression of alcoholic liver injury, starting from fatty liver via hepatitis to cirrhosis, 29 +/- 24 microg/ml in fatty liver, 35 +/- 29 microg/ml in alcoholic hepatitis, and 46 +/- 54 microg/ml in alcoholic cirrhosis. The antibody level in patients taking 100 g or more of ethanol per day tended to be higher, compared with those in people taking less ethanol. A follow-up observation revealed that alcohol abstinence after hospitalization raised serum anti-AcH-adduct antibody level in some patients and kept it constantly low in other patients. The immunohistochemical study using the anti-AcH-adduct antibody revealed the presence of adduct-like substance in hepatocytes of liver biopsy specimens obtained from patients with alcoholic liver disease. The results indicate that the anti-AcH-adduct antibody may be associated with the progress of alcoholic liver diseases.

Cytotoxic effect of 7alpha-hydroxy-4-cholesten-3-one on HepG2 cells: hypothetical role of acetaldehyde-modified delta4-3-ketosteroid-5beta-reductase (the 37-kd-liver protein) in the pathogenesis of alcoholic liver injury in the rat

We recently identified delta4-3-ketosteroid-5beta-reductase as the 37 kd liver protein which is highly susceptible to acetaldehyde modification in rats continuously fed alcohol. The 5beta-reductase is a key enzyme involved in bile acid synthesis. We report here that the ability to degrade 7alpha-hydroxy-4-cholesten-3-one (HCO) was lower in the liver cytosol of alcohol-fed rats than in control animals, suggesting an inhibition of the 5beta-reductase enzyme activity by acetaldehyde modification. We also showed that HCO exhibited a time- and concentration-dependent cytotoxicity to HepG2 cells. HCO cytotoxicity was noticeable at a concentration of 2.5 microg/mL. When 10 microg/mL of HCO was added to confluent cell monolayers, 57% and 37% of cells remained viable after 24 and 48 hours of treatment. The decrease in cell viability was accompanied by an increased lactic dehydrogenase activity in the culture medium. DNA extracted from HCO-treated cells showed no evidence of DNA fragmentation when analyzed by agarose gel electrophoresis. Staining with propidium iodide showed no nuclear condensation in cells. Thus, cell death by HCO treatment was caused by necrosis and not by apoptosis. Various agents, including, serum proteins, hormones, bile acids, antioxidants, Ca++-chelators, Fe++-chelator, CYP450 inhibitor, adenylate cyclase inhibitor, protease inhibitors, and nitric oxide synthase inhibitor, did not protect against HCO cytotoxicity. We speculate that HCO concentrations may be elevated around the pericentral area in the liver after chronic alcohol ingestion, causing local cell necrosis. The release of cellular contents and protein-acetaldehyde adducts (PAAs) may activate nonparenchymal cells and provoke autoimmune reaction. Thus, the formation of the 37 kd-PAA may play an important role in the initiation of alcoholic liver injury.

Collagen-acetaldehyde adducts in alcoholic and nonalcoholic liver diseases

Alcoholic and, to a lesser extent, nonalcoholic patients with liver disease have serum antibodies to acetaldehyde-protein adducts produced in vitro. These antibodies presumably reflect the presence of adducts in the liver, but the protein that triggers this immune response has not been identified. To study this, we measured the reactivity of cytosolic proteins to rabbit IgG developed against a P-450 2E1-acetaldehyde adduct, isolated from alcohol-fed rats, that recognizes acetaldehyde-modified epitopes in proteins. Adducts were determined on Western blots by scanning densitometry of antibody-linked alkaline phosphatase activity in 4 normal livers and in needle biopsy specimens from subjects with liver disease, 17 alcoholic and 14 nonalcoholic. In all livers, except for a normal one, we found a reactive protein of at least 200 kD, similar to the collagen-acetaldehyde adduct we reported to be markedly increased in rats with experimentally induced cirrhosis. The immunostaining intensity in the alcoholic patients with liver disease was eightfold (p < 0.01) and that in nonalcoholic patients with liver disease was fourfold, greater (p < 0.02) than the weak staining in normal livers; it correlated with the degree of inflammation and serum AST or gamma-glutamyl transpeptidase activities. The adduct was reproduced on incubation of normal cytosolic proteins with 2.5 mmol/L acetaldehyde, whereas higher concentrations yielded many additional adducts; the adduct also reacted with IgG antibody to rat collagen type I and disappeared after digestion with collagenase, suggesting that the target protein is a form of collagen.(ABSTRACT TRUNCATED AT 250 WORDS)