Precision-cut liver slices from diet-induced obese rats exposed to ethanol are susceptible to oxidative stress and increased fatty acid synthesis

Oxidative stress from fat accumulation in the liver has many deleterious effects. Many believe that there is a second hit that causes relatively benign fat accumulation to transform into liver failure. Therefore, we evaluated the effects of ethanol on ex vivo precision-cut liver slice cultures (PCLS) from rats fed a high-fat diet resulting in fatty liver. Age-matched male Sprague-Dawley rats were fed either high-fat (obese) (45% calories from fat, 4.73 kcal/g) or control diet for 13 mo. PCLS were prepared, incubated with 25 mM ethanol for 24, 48, and 72 h, harvested, and evaluated for ethanol metabolism, triglyceride production, oxidative stress, and cytokine expression. Ethanol metabolism and acetaldehyde production decreased in PCLS from obese rats compared with age-matched controls (AMC). Increased triglyceride and smooth muscle actin production was observed in PCLS from obese rats compared with AMC, which further increased following ethanol incubation. Lipid peroxidation, measured by thiobarbituric acid reactive substances assay, increased in response to ethanol, whereas GSH and heme oxygenase I levels were decreased. TNF-α and IL-6 levels were increased in the PCLS from obese rats and increased further with ethanol incubation. Diet-induced fatty liver increases the susceptibility of the liver to toxins such as ethanol, possibly by the increased oxidative stress and cytokine production. These findings support the concept that the development of fatty liver sensitizes the liver to the effects of ethanol and leads to the start of liver failure, necrosis, and eventually cirrhosis.

Role of MGST1 in reactive intermediate-induced injury

Microsomal glutathione transferase (MGST1, EC 2.5.1.18) is a membrane bound glutathione transferase extensively studied for its ability to detoxify reactive intermediates, including metabolic electrophile intermediates and lipophilic hydroperoxides through its glutathione dependent transferase and peroxidase activities. It is expressed in high amounts in the liver, located both in the endoplasmic reticulum and the inner and outer mitochondrial membranes. This enzyme is activated by oxidative stress. Binding of GSH and modification of cysteine 49 (the oxidative stress sensor) has been shown to increase activation and induce conformational changes in the enzyme. These changes have either been shown to enhance the protective effect ascribed to this enzyme or have been shown to contribute to cell death through mitochondrial permeability transition pore formation. The purpose of this review is to elucidate how one enzyme found in two places in the cell subjected to the same conditions of oxidative stress could both help protect against and contribute to reactive oxygen species-induced liver injury.

Malondialdehyde-acetaldehyde adduct is the dominant epitope after MDA modification of proteins in atherosclerosis

Antibodies to malondialdehyde (MDA)-modified macromolecules (adducts) have been detected in the serum of patients with atherosclerosis and correlate with the progression of this disease. However, the epitope and its formation have not been characterized. Studies have shown that excess MDA can be degraded to acetaldehyde, which combines with proteins to from a stable dihydropyridine adduct. To investigate, mice were immunized with MDA adducts in the absence of adjuvant and showed an increase in antibodies to MDA adducts and the carrier protein as the concentration of MDA was increased. In fact, a number of the commercially available antibodies to MDA-modified proteins were able to be inhibited by a chemical analogue, hexyl-MAA. Also, MDA-MAA adducts were detected in the serum and aortic tissue of JCR diabetic/atherosclerotic rats. These studies determined that commercially available antibodies to MDA predominantly react with the MAA adduct and are present in the JCR model of atherosclerosis in both the serum and the aortic tissue. Therefore, the immune response to MDA-modified proteins is most probably to the dihydropyridine structure (predominant epitope in MAA), which suggests that MAA adducts may play a role in the development and/or progression of atherosclerosis.

Autoimmune hepatitis induced by syngeneic liver cytosolic proteins biotransformed by alcohol metabolites

BACKGROUND AND AIMS

Aldehydes that are produced following the breakdown of ethanol (acetaldehyde) and lipid peroxidation of membranes (malondialdehyde) have been shown to bind (adduct) proteins. Additionally, these two aldehydes can combine (MAA) on nonsyngeneic and syngeneic proteins to initiate numerous immune responses to the unmodified part of the protein in the absence of an adjuvant. Therefore, these studies provide a potential mechanism for the development of antigen-specific immune responses resulting in liver damage should syngeneic liver proteins be adducted with MAA.

METHODS

This study sought to test whether MAA-modified syngeneic liver cytosolic proteins administered daily in the absence of adjuvant into C57BL/6 mice abrogates tolerance to initiate a MAA-induced autoimmune-like hepatitis.

RESULTS

In mice immunized with MAA-modified cytosols, there was an increase in liver damage as assessed by aspartate aminotransferase/alanine aminotransferase levels that correlated with liver pathology scores and the presence of the pro-fibrotic factors, smooth muscle actin, TGF-β, and collagen. IgG antibodies and T-cell proliferative responses specific for cytosolic proteins were also detected. Pro-inflammatory cytokines were produced in the livers of animals exposed to MAA-modified cytosols. Finally, transfer of immunized T cells to naïve animals caused biochemical and histological evidence of liver damage.

CONCLUSIONS

These data demonstrate that a disease with an autoimmune-like pathophysiology can be generated in this animal model using soluble MAA-modified syngeneic liver cytosols as the immunogen. These studies provide insight into potential mechanism(s) that the metabolites of alcohol may play in contributing to the onset of an autoimmune-like disease in patients with alcoholic liver disease.

Exposure of precision-cut rat liver slices to ethanol accelerates fibrogenesis

Ethanol metabolism in the liver induces oxidative stress and altered cytokine production preceding myofibroblast activation and fibrogenic responses. The purpose of this study was to determine how ethanol affects the fibrogenic response in precision-cut liver slices (PCLS). PCLS were obtained from chow-fed male Wistar rats (200-300 g) and were cultured up to 96 h in medium, 25 mM ethanol, or 25 mM ethanol and 0.5 mM 4-methylpyrazole (4-MP), an inhibitor of ethanol metabolism. Slices from every time point (24, 48, 72, and 96 h) were examined for glutathione (GSH) levels, lipid peroxidation [thiobarbituric acid-reactive substance (TBARS) assay], cytokine production (ELISA and RT-PCR), and myofibroblast activation [immunoblotting and immunohistochemistry for smooth muscle actin (SMA) and collagen]. Treatment of PCLS with 25 mM ethanol induced significant oxidative stress within 24 h, including depletion of cellular GSH and increased lipid peroxidation compared with controls (P < 0.05). Ethanol treatment also elicited a significant and sustained increase in interleukin-6 (IL-6) production (P < 0.05). Importantly, ethanol treatment accelerates a fibrogenic response after 48 h, represented by significant increases in SMA and collagen 1alpha(I) production (P < 0.05). These ethanol-induced effects were prevented by the addition of 4-MP. Ethanol metabolism induces oxidative stress (GSH depletion and increased lipid peroxidation) and sustained IL-6 expression in rat PCLS. These phenomena precede and coincide with myofibroblast activation, which occurs within 48 h of treatment. These results indicate the PCLS can be used as in vitro model for studying multicellular interactions during the early stages of ethanol-induced liver injury and fibrogenesis.

Alcohol metabolites and lipopolysaccharide: roles in the development and/or progression of alcoholic liver disease

The onset of alcoholic liver disease (ALD) is initiated by different cell types in the liver and a number of different factors including: products derived from ethanol-induced inflammation, ethanol metabolites, and the indirect reactions from those metabolites. Ethanol oxidation results in the production of metabolites that have been shown to bind and form protein adducts, and to increase inflammatory, fibrotic and cirrhotic responses. Lipopolysaccharide (LPS) has many deleterious effects and plays a significant role in a number of disease processes by increasing inflammatory cytokine release. In ALD, LPS is thought to be derived from a breakdown in the intestinal wall enabling LPS from resident gut bacterial cell walls to leak into the blood stream. The ability of adducts and LPS to independently stimulate the various cells of the liver provides for a two-hit mechanism by which various biological responses are induced and result in liver injury. Therefore, the purpose of this article is to evaluate the effects of a two-hit combination of ethanol metabolites and LPS on the cells of the liver to increase inflammation and fibrosis, and play a role in the development and/or progression of ALD.

An in vitro method of alcoholic liver injury using precision-cut liver slices from rats

Alcohol abuse results in liver injury, but investigations into the mechanism(s) for this injury have been hampered by the lack of appropriate in vitro culture models in which to conduct in depth and specific studies. In order to overcome these shortcomings, we have developed the use of precision-cut liver slices (PCLS) as an in vitro culture model in which to investigate how ethanol causes alcohol-induced liver injury. In these studies, it was shown that the PCLS retained excellent viability as determined by lactate dehydrogenase and adenosine triphosphate (ATP) levels over a 96-h period of incubation. More importantly, the major enzymes of ethanol detoxification; alcohol dehydrogenase, aldehyde dehydrogenase, and cytochrome P4502E1, remained active and PCLS readily metabolized ethanol and produced acetaldehyde. Within 24 h and continuing up to 96h the PCLS developed fatty livers and demonstrated an increase in the redox state. These PCLS secreted albumin, and albumin secretion was decreased by ethanol treatment. All of these impairments were reversed following the addition of 4-methylpyrazole, which is an inhibitor of ethanol metabolism. Therefore, this model system appears to mimic the ethanol-induced changes in the liver that have been previously reported in human and animal studies, and may be a useful model for the study of alcoholic liver disease.

Chronic ethanol treatment impairs Rac and Cdc42 activation in rat hepatocytes

BACKGROUND

The effects of chronic ethanol feeding on rat hepatocytes have been shown to include impaired cell-extracellular matrix (ECM) adhesion events, such as decreased attachment and spreading as well as increased integrin-actin cytoskeleton association. These results, observed previously by this laboratory, are highly suggestive of impaired actin cytoskeleton reorganization, an event mediated by differential activation of the Rho family GTPases Rac, Cdc42, and RhoA. Therefore, the purpose of this study was to examine the effects of chronic ethanol administration on these GTPases.

METHODS

Male Wistar rats were pair-fed 4 to 5 weeks with a liquid diet containing either ethanol (as 36% of total calories) or isocaloric carbohydrate. Hepatocytes were isolated and plated on collagen IV up to 24 hours. At specific times, the hepatocytes were lysed and these lysates were analyzed for RhoA, Cdc42, and Rac activation.

RESULTS

In freshly isolated hepatocytes from ethanol-fed rats, the GTP-bound (active) forms of Rac and Cdc42 were significantly decreased compared with pair-fed control rats, while the GTP-bound form of RhoA was not significantly altered. These ethanol-induced impairments in Rac and Cdc42 activation persisted even after plating the hepatocytes on collagen IV. Additionally, chronic ethanol treatment did not directly affect GTP binding of Cdc42 and Rac, as incorporation of GTPgammaS was not affected.

CONCLUSIONS

Chronic ethanol administration selectively impairs Rac and Cdc42 activation in rat hepatocytes. As activation of these 2 GTPases is crucial for efficient cell attachment and spreading on ECM substrates, the results from this study suggest that the ethanol-induced impairments in Rac and Cdc42 activation are responsible for the impaired hepatocyte-ECM adhesion events observed previously by our laboratory. Furthermore, these results raise the intriguing possibility that these GTPases are involved in other ethanol-induced functional impairments, such as protein trafficking and receptor-mediated endocytosis.

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.

Amylin receptor blockade stimulates food intake in rats

Amylin is postulated to act as a hormonal signal from the pancreas to the brain to inhibit food intake and regulate energy reserves. Amylin potently reduces food intake, body weight, and adiposity when administered systemically or into the brain. Whether selective blockade of endogenous amylin action increases food intake and adiposity remains to be clearly established. In the present study, the amylin receptor antagonist acetyl-[Asn(30), Tyr(32)] sCT-(8-32) (AC187) was used to assess whether action of endogenous amylin is essential for normal satiation to occur. Non-food-deprived rats received a 3- to 4-h intravenous infusion of AC187 (60-2,000 pmol.kg(-1).min(-1)), either alone or coadministered with a 3-h intravenous infusion of amylin (2.5 or 5 pmol.kg(-1).min(-1)) or a 2-h intragastric infusion of an elemental liquid diet (4 kcal/h). Infusions began just before dark onset. Food intake and meal patterns during the first 4 h of the dark period were determined from continuous computer recordings of changes in food bowl weight. Amylin inhibited food intake by approximately 50%, and AC187 attenuated this response by approximately 50%. AC187 dose-dependently stimulated food intake (maximal increases from 76 to 171%), whether administered alone or with an intragastric infusion of liquid diet. Amylin reduced mean meal size and meal frequency, AC187 attenuated these responses, and AC187 administration alone increased mean meal size and meal frequency. These results support the hypothesis that endogenous amylin plays an essential role in reducing meal size and increasing the postmeal interval of satiety.

Expression and cytoskeletal association of integrin subunits is selectively increased in rat perivenous hepatocytes after chronic ethanol administration

BACKGROUND

For normal function and survival, hepatocytes require proper cell-extracellular matrix (ECM) contacts mediated by integrin receptors and focal adhesions. Previous studies have shown that chronic ethanol consumption selectively impairs perivenous (PV) hepatocyte attachment and spreading on various ECM substrates but increases expression of the beta1 integrin subunit, the common beta subunit for two major hepatocyte-ECM receptors, alpha1beta1 and alpha5beta1 integrins. This study examined the effects of ethanol treatment on the expression and cytoskeletal distribution of alpha1, alpha5, and beta1 integrin subunits, the epidermal growth factor receptor (EGF-R), and the cytoskeletal proteins focal adhesion kinase, paxillin, vinculin, and actin in periportal and PV hepatocytes.

METHODS

Periportal and PV hepatocytes were isolated from control and ethanol-fed rats. For expression analysis, lysates were examined by SDS-PAGE and immunoblotting procedures. For cytoskeletal distribution studies, Triton-soluble and -insoluble (cytoskeletal) fractions from hepatocytes cultured on collagen IV were analyzed by SDS-PAGE and immunoblotting.

RESULTS

Chronic ethanol administration caused PV-specific increases in expression and cytoskeletal association of the integrin subunits. Although ethanol treatment did not affect expression of the EGF-R in either cell type, it did increase the association of the EGF-R with the cytoskeleton selectively in PV hepatocytes. Ethanol treatment had no significant effect on either the expression or the cytoskeletal distribution of focal adhesion kinase, paxillin, vinculin, or actin in either cell type.

CONCLUSIONS

The increases in integrin expression and cytoskeletal association observed after chronic ethanol administration suggest that a process downstream of integrin-ECM interactions is impaired selectively in PV hepatocytes, possibly involving altered focal adhesion assembly or turnover, processes essential for efficient cell-ECM adhesion. Alterations in these processes could contribute to the impaired hepatocyte function and structure observed after chronic ethanol administration.

Ethanol feeding selectively impairs the spreading of rat perivenous hepatocytes on extracellular matrix substrates

BACKGROUND

Hepatocytes require attachment and subsequent spreading on an extracellular matrix for their proper growth, function and survival. Our previous studies have shown that ethanol feeding selectively impairs perivenule hepatocyte attachment to various extracellular matrices. This study was undertaken to determine whether zonal differences in hepatocyte spreading in response to ethanol feeding occurs and to ascertain the influence of ethanol consumption on the zonal expression of the beta1 subunit of integrins, which are the major surface receptors responsible for matrix binding and subsequent interactions.

METHODS

Hepatocytes from the perivenous and periportal regions of the liver were isolated by digitonin/collagenase perfusion from rats that were pair-fed for 2 to 3 weeks with a liquid diet containing either ethanol or isocaloric carbohydrate. The ability of perivenous and periportal hepatocytes to spread on plates coated with either type IV collagen, laminin, fibronectin or polylysine was determined. In addition, the isolated cells were used for the analysis of total cellular and surface beta1 integrin expression.

RESULTS

With all of the matrix substrates tested, the spreading of perivenous hepatocytes isolated from the ethanol-fed animals was markedly impaired, while the spreading of periportal hepatocytes was essentially unaffected by ethanol feeding. Both the total cellular as well as the surface expression of the beta1 integrin subunit in perivenous cells from the ethanol-fed rats were significantly higher than from the perivenous control cells, whereas the total and surface expression of the beta1 integrin in periportal cells isolated from ethanol-fed and control rats were not significantly different.

CONCLUSIONS

The results indicated that in addition to impairing hepatocyte attachment, ethanol feeding also impairs another critical step of the adhesion process, that of hepatocyte spreading on extracellular matrix substrates. This defect occurred preferentially in perivenous cells and not periportal cells and was associated with an increase in beta1 integrin expression, suggesting that a compensatory mechanism occurs as an attempt by the perivenous cells to overcome impaired cell-matrix interactions caused by ethanol. Overall, these alterations in extracellular matrix-hepatocyte interactions could lead to alterations of hepatocyte structure and function and potentially play a role in alcoholic liver injury.