Time course of the carbon tetrachloride-induced decrease in mitochondrial aldehyde dehydrogenase activity

Post-translational modifications of mitochondrial aldehyde dehydrogenase and biomedical implications

Aldehyde dehydrogenases (ALDHs) represent large family members of NAD(P)+-dependent dehydrogenases responsible for the irreversible metabolism of many endogenous and exogenous aldehydes to the corresponding acids. Among 19 ALDH isozymes, mitochondrial ALDH2 is a low Km enzyme responsible for the metabolism of acetaldehyde and lipid peroxides such as malondialdehyde and 4-hydroxynonenal, both of which are highly reactive and toxic. Consequently, inhibition of ALDH2 would lead to elevated levels of acetaldehyde and other reactive lipid peroxides following ethanol intake and/or exposure to toxic chemicals. In addition, many East Asian people with a dominant negative mutation in ALDH2 gene possess a decreased ALDH2 activity with increased risks for various types of cancer, myocardial infarct, alcoholic liver disease, and other pathological conditions. The aim of this review is to briefly describe the multiple post-translational modifications of mitochondrial ALDH2, as an example, after exposure to toxic chemicals or under different disease states and their pathophysiological roles in promoting alcohol/drug-mediated tissue damage. We also briefly mention exciting preclinical translational research opportunities to identify small molecule activators of ALDH2 and its isozymes as potentially therapeutic/preventive agents against various disease states where the expression or activity of ALDH enzymes is altered or inactivated.

Inhibition of hepatic mitochondrial aldehyde dehydrogenase by carbon tetrachloride through JNK-mediated phosphorylation

The aim of this study was to investigate the mechanism of inhibition of mitochondrial aldehyde dehydrogenase (ALDH2) by carbon tetrachloride (CCl(4)). CCl(4) administration caused marked hepatocyte ballooning and necrosis in the pericentral region. CCl(4) also inhibited hepatic ALDH2 activity in a time-dependent manner without altering the protein level, suggesting ALDH2 inhibition through covalent modifications such as phosphorylation by JNK. To demonstrate phosphorylation, the isoelectric point (pI) of ALDH2 in CCl(4)-exposed rats was compared to that of untreated controls. Immunoblot analysis revealed that immunoreactive ALDH2 bands in CCl(4)-exposed rats were shifted to acidic pI ranges on two-dimensional electrophoresis (2-DE) gels. Incubation with alkaline phosphatase significantly restored the suppressed ALDH2 activity with a concurrent alkaline pI shift of the ALDH2 spots. Both JNK and activated JNK were translocated to mitochondria after CCl(4) exposure. In addition, incubation with catalytically active JNK led to significant inhibition of ALDH2 activity, with an acidic pI shift on 2-DE gels. Furthermore, immunoprecipitation followed by immunoblot analysis with anti-phospho-Ser-Pro antibody revealed phosphorylation of a Ser residue(s) of ALDH2. These results collectively indicate a novel underlying mechanism by which CCl(4) exposure activates JNK, which translocates to mitochondria and phosphorylates ALDH2, contributing to inhibition of ALDH2 activity accompanied by decreased cellular defense capacity and increased lipid peroxidation.

Inactivation of cytosolic aldehyde dehydrogenase via S-nitrosylation in ethanol-exposed rat liver

Aldehyde dehydrogenase (ALDH) isozymes are critically important in the metabolism of acetaldehyde, thus preventing its accumulation after ethanol-exposure. We previously reported that mitochondrial ALDH2 could be inactivated via S-nitrosylation in ethanol-exposed rats. This study was aimed at investigating whether cytosolic ALDH1, with a relatively-low-Km value (11-18 microM) for acetaldehyde, could be also inhibited in ethanol-exposed rats. Chronic or binge ethanol-exposure significantly decreased ALDH1 activity, which was restored by addition of dithiothreitol. Immunoblot analysis with the anti-S-nitroso-Cys antibody showed one immunoreactive band in the immunoprecipitated ALDH1 only from ethanol-exposed rats, but not from pair-fed controls, suggesting S-nitrosylation of ALDH1. Therefore inactivation of ALDH1 via S-nitrosylation can result in accumulation of acetaldehyde upon ethanol-exposure.

Inhibition of mitochondrial aldehyde dehydrogenase by nitric oxide-mediated S-nitrosylation

Mitochondrial aldehyde dehydrogenase (ALDH2) is responsible for the metabolism of acetaldehyde and other toxic lipid aldehydes. Despite many reports about the inhibition of ALDH2 by toxic chemicals, it is unknown whether nitric oxide (NO) can alter the ALDH2 activity in intact cells or in vivo animals. The aim of this study was to investigate the effects of NO on ALDH2 activity in H4IIE-C3 rat hepatoma cells. NO donors such as S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine, and 3-morpholinosydnonimine significantly increased the nitrite concentration while they inhibited the ALDH2 activity. Addition of GSH-ethylester (GSH-EE) completely blocked the GSNO-mediated ALDH2 inhibition and increased nitrite concentration. To directly demonstrate the NO-mediated S-nitrosylation and inactivation, ALDH2 was immunopurified from control or GSNO-treated cells and subjected to immunoblot analysis. The anti-nitrosocysteine antibody recognized the immunopurified ALDH2 only from the GSNO-treated samples. All these results indicate that S-nitrosylation of ALDH2 in intact cells leads to reversible inhibition of ALDH2 activity.

Activation of Kupffer cells during the course of carbon tetrachloride-induced liver injury and fibrosis in rats

Kupffer cells are involved in the pathogenesis of chemically mediated liver injury through release of biologically active mediators that promote the pathogenic process. The purpose of this study was to elucidate specific biochemical and molecular changes occurring in Kupffer cells throughout a time course of carbon tetrachloride (CCl(4))-mediated liver injury and fibrosis. Rats were administered 1 ml/kg of CCl(4) (10% v/v olive oil) twice weekly for up to 6 weeks. Plasma alanine aminotransferase values and hematoxylin-and-eosin- and trichrome-stained liver sections indicated minor liver damage at 2 weeks followed by increased damage and collagen deposition by 4 and 6 weeks. Additionally, mRNA levels in Kupffer cells isolated from CCl(4)-treated rats demonstrated significant increases in tumor necrosis factor alpha (TNF alpha); tumor growth factor beta; interleukin-6 (IL-6); interleukin 1 beta; cyclooxygenase 2; CD14, and I kappa B alpha transcripts after 2 and 4 weeks of treatment. However, the expression of these genes at 6 weeks was similar to that of controls. Increased gene expression of cytokines in Kupffer cells isolated from CCl(4)-treated rats was accompanied by increases in protein production of TNF alpha, IL-6, IL-1 beta, and interleukin 10 following lipopolysaccharide stimulation. Further, liver sections stained for ED2-positive cells demonstrated an increase in the number of resident macrophages at 2 and 4 weeks with a slight decrease in ED2-positive cells by week 6 but still significantly more than control. Analysis of reduced glutathione (GSH) and oxidized glutathione (GSSG) indicated that Kupffer cells from CCl(4)-treated animals exhibited a 50% decrease in GSH at 2 and 4 weeks, whereas no significant changes were observed for GSSG. In conclusion, these data implicate Kupffer cells as a critical mediator of the inflammatory and fibrogenic responses during CCl(4)-mediated liver damage and provide new insight into the temporal molecular and biochemical changes associated with the ability of these resident macrophages to modulate liver injury.

4-Hydroxynonenal and malondialdehyde hepatic protein adducts in rats treated with carbon tetrachloride: immunochemical detection and lobular localization

The metabolism of CCl(4) initiates the peroxidation of polyunsaturated fatty acids producing alpha,beta-unsaturated aldehydes, such as 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA). The facile reactivity of these electrophilic aldehydic products suggests they play a role in the toxicity of compounds like CCl(4). To determine the rate at which CCl(4)-initiated lipid peroxidation results in the formation of 4-HNE and/or MDA hepatic protein adducts, rats were given an intragastric dose of CCl(4) (1.0 ml/kg) and euthanized 0-72 h after administration. Rabbit polyclonal antisera directed toward 4-HNE- or MDA-protein epitopes were employed in immuno-histochemical and immuno-precipitation/Western analyses to detect 4-HNE and MDA-protein adducts in paraffin-embedded liver sections and liver homogenates. As early as 6 h post CCl(4) exposure, 4-HNE and MDA adducts were detected immuno-histochemically in hepatocytes localized to zone 2 of the hepatic acinus. Liver injury was progressive to 24 h as lipid peroxidation and hepatocellular necrosis increased. The hallmark of CCl(4) hepatotoxicity, zone 3 necrosis, was observed 24 h after CCl(4) administration and immuno-positive hepatocytes were observed in zone 2 as well as zone 3. Immuno-positive cells were no longer visible by 36 to 72 h post CCl(4) administration. From 6 to 48 h after CCl(4) administration, at least four adducted proteins were immuno-precipitated from liver homogenates with the anti-MDA or anti-4HNE serum, which corresponded to molecular weights of 80, 150, 205, and greater than 205 kDa. These results demonstrate that 4-HNE and MDA alkylate specific hepatic proteins in a time-dependent manner, which appears to be associated with hepatocellular injury following CCl(4) exposure.

Pharmacokinetic analysis of 123I-labeled medium chain fatty acid as a radiopharmaceutical for hepatic function based on beta-oxidation

Beta-oxidation is the most important pathway to provide energy for the liver. Our recent findings indicated that radiolabeled medium chain fatty acid analogs could be used as radiopharmaceuticals in the liver, allowing us to monitor alterations in energy metabolism on the cellular level. In the present study, pharmacokinetical analysis of a radioiodinated medium chain fatty acid analog, 6-[123I]iodophenylenanthic acid ([123I]IPEA), was carried out in normal and hepatitis model rats to investigate the index for the measurement of beta-oxidation activity in hepatocytes. The rate constant for metabolism of [123I]IPEA in the liver showed a strong correlation with the ATP level, which was determined as an indicator of beta-oxidation activity in hepatocytes. The radioactivity profile in the liver after [123I]IPEA administration provided important information regarding hepatic viability, and the metabolic rate constant of [123I]IPEA calculated by a pharmacokinetic method was a useful criterion for hepatic diagnosis based on hepatic cellular energy metabolism.

Evaluation of [1-11C]octanoate as a new radiopharmaceutical for assessing liver function using positron emission tomography

[1-11C]Octanoate was evaluated as a new radiopharmaceutical for the evaluation of liver function using positron emission tomography (PET). In biodistribution studies with normal mice, [1-11C]octanoate was rapidly taken up by the liver. [1-11C]Octanoate in the liver was present in the parenchymal cells and was predominantly metabolized via beta-oxidation followed by its rapid clearance. In the CCl4-treated mice, [1-11C]octanoate showed significantly slower hepatic clearance than that in the controls. In PET studies using rats, the time-radioactivity curves in the liver showed a two-phase decrease, and compared with the normal rat, the CCl4-treated rat showed a slower hepatic half-clearance time for the first phase, which is related to beta-oxidation metabolism. A preliminary PET study of [1-11C]octanoate metabolism in a normal volunteer was consistent with these animal studies. The present study showed that metabolism of [1-11C]octanoate in the liver was influenced by beta-oxidation, and it is advantageous to use [1-11C]octanoate clinically as a regional liver-function diagnostic agent in conjunction with PET.

Structure-toxicity relationship of ethylene glycol ethers

The ultimate purpose of the present study was to evaluate correlations between acute in vivo and in vitro toxicity and log P (P is n-octanol-water partition coefficient). The in vitro toxicity to cloned cells (neuroblastoma N18TG-2 and glioma C6) in culture (ED50) and the in vivo toxicity to mice (LD50) of ethylene glycol ethers were studied in terms of the structure-activity relationship. The test ethers showed a wide range of ED50 values in both cells. LD50 was determined under two conditions: LD50-cont. was estimated in mice pretreated with olive oil and LD50-CCl4 in CCl4-pretreated mice. Multiple regression analyses revealed a significant correlation between log 1/LD50 and log P as follows: log (1/LD50-cont.) = -0.120 (log P)2+0.487log P-1.182, and log (1/LD50-CCl4) = -0.128 (log P)2+0.566log P-1.157. There was no significant correlation either between ED50 and LD50 or between ED50 for N18TG-2 and ED50 for C6. The results suggest that metabolic activation might not occur during acute toxicity from the ethers, and that hydrophobicity, expressed as log P, plays an important role in acute toxicity.

Effect of carbon tetrachloride on hamster tracheal epithelial cells

This study was designed to assess effects of carbon tetrachloride (CCl4) in hamster tracheal epithelium. Adult, male, Syrian golden hamsters were treated with 2.5 ml/kg CCl4 ip, and controls received only the vehicle (peanut oil). Animals were sacrificed after 1, 4, 12, and 24 h. Tissue samples from upper and lower tracheal levels were fixed and embedded in glycol methacrylate for light microscopy. Some tracheal rings were also fixed in formaldehyde/glutaraldehyde cacodylate buffer for transmission electron microscopy. For histopathologic evaluation of the tracheal epithelial cells, each tracheal level was cut transversely at 3 microns and stained with toluidine blue. CCl4 produced injury to ciliated and nonciliated cells in all portions of hamster trachea, although the severity of CCl4-induced injury differed in various levels and regions. The number of damaged cells increased markedly after 1 h in the lower trachea, but not until after 4 h in the upper trachea. By 24 h, the number of injured cells had decreased so that no significant difference from control was evident. The ultrastructural alterations in epithelial cells were obvious as early as 1 h after CCl4 administration. Intracellular organelles, including smooth and rough endoplasmic reticulum, mitochondria, and Golgi apparatuses, were damaged by this chemical. Since CCl4-induced cell injury is dependent on metabolism by intracellular NADPH-dependent cytochrome P450 monooxygenases, these results suggest that hamster tracheal epithelial cells have the potential to activate CCl4 metabolically.

The oxidation of alpha-beta unsaturated aldehydic products of lipid peroxidation by rat liver aldehyde dehydrogenases

Lipid peroxidation of microsomal membranes produces a large number of aldehydes, alcohols, and ketones, of which some are cytotoxic. trans-4-Hydroxy-2-nonenal (4HN) and trans-2-hexenal (HX) are two alpha-beta unsaturated aldehydes which are major and minor lipid peroxidation products, respectively. The role of aldehyde dehydrogenase (ALDH) in the oxidation of 4HN and HX was examined using semipurified mitochondrial, cytosolic, and microsomal ALDH isozymes prepared from male Sprague-Dawley rat liver. High- and low- affinity mitochondrial and high-affinity cytosolic ALDH isozymes were able to oxidize 4HN. The affinities of the three isozymes for 4HN, reported as the V/K values, are 0.258, 0.032 and 0.030 nmol NADH formed/min/mg protein/mumol 4HN/liter, respectively. The low-affinity cytosolic and microsomal forms of ALDH are unable to oxidize 4HN. The high-affinity mitochondrial, low-affinity cytosolic, and microsomal ALDH isozymes oxidized HX, displaying V/K values of 0.600, 0.058, and 0.058 nmol NADH formed/min/mg protein/mumol HX/liter, respectively. Oxidation of HX by the low-affinity mitochondrial and high-affinity cytosolic isozyme was not detected. This study indicates that ALDH may participate in the in vivo metabolism of cytotoxic aldehydic products formed during lipid peroxidation.

Ethanol potentiation of carbon tetrachloride hepatotoxicity: possible role for the in vivo inhibition of aldehyde dehydrogenase

A potentiation of CCl4-induced hepatotoxicity was observed in rats pretreated with ethanol 18 hr prior to CCl4 exposure. Hepatic microsomal aldehyde dehydrogenase (ALDH) was significantly inhibited in animals sacrificed 1 hr following the sequential exposure, however, no more so than in those animals receiving CCl4 alone. The animals receiving ethanol alone had ALDH activity similar to vehicle treated controls. Twenty-four hours following a potentiating dose of ethanol and CCl4 an 81 and 57% decline in NAD+-dependent microsomal and mitochondrial ALDH activity was observed, respectively. Similar results were observed for microsomal and mitochondrial NADP+-dependent ALDH activity. The decline in membrane-bound ALDH was greater in potentiated animals than in those receiving CCl4 alone. A relatively smaller decline in cytosolic ALDH activity was observed in CCl4 treated rats with or without ethanol pre-exposure. The data suggest that inhibition of membrane bound ALDH may be one of the major mechanisms of in vivo potentiation of CCl4-induced hepatotoxicity by ethanol.

Inhibition of hepatic aldehyde dehydrogenase by carbon tetrachloride: an in vitro study

In vitro inhibition of rat liver mitochondrial and microsomal aldehyde dehydrogenase (ALDH) under conditions of active CCl4 metabolism was investigated. Incubation of microsomes or mitochondria in the presence of NADPH alone caused significant, time-dependent inhibition of mitochondrial and microsomal ALDH. EDTA partially protected ALDH from inhibition. Incubation of microsomes or microsomes plus mitochondria in the presence of NADPH and CCl4 resulted in marked inhibition of microsomal and mitochondrial ALDH activity. The inhibition was both dose- and time-dependent and was relatively less in the presence of EDTA. It is proposed that the inhibition of membrane-bound ALDH may be one of the early events responsible for the genesis of CCl4-hepatotoxicity.

Hepatic aldehyde dehydrogenases and lipid peroxidation

Recent findings have shown that microsomal membrane lipid peroxidation generates a variety of reactive aldehydic products. The interaction of lipid peroxidation products with hepatic aldehyde dehydrogenases (ALDH) was studied using rat liver subcellular fractions. The well-documented membrane peroxidation product malondialdehyde (MDA) was studied to determine if ALDH isozymes play a role in metabolism of this aldehyde. The cytosolic and mitochondrial hepatic subcellular fractions were found to contain ALDH isozymes capable of oxidizing MDA. The kinetic properties of a cytosolic ALDH (Km of approximately 16 microM) suggest that this enzyme may be involved in the metabolism of MDA in vivo. Both the cytosolic and mitochondrial fractions also contained an ALDH isozyme with Km values in the millimolar range. Addition of the cytosolic fraction of rat liver produced a significant decrease in the accumulation of MDA during CCl4-induced microsomal membrane lipid peroxidation but did not protect cytochrome P-450 from destruction. The mitochondrial low Km ALDH isozyme was found to be a target enzyme for inhibition during in vitro microsomal lipid peroxidation. These studies show that a select ALDH isozyme is sensitive to inhibition during membrane lipid peroxidation whereas other isozymes may be involved in the metabolism of aldehydic peroxidation products.