Lipids and Oxidative Stress Associated with Ethanol-Induced Neurological Damage

The excessive intake of alcohol is a serious public health problem, especially given the severe damage provoked by chronic or prenatal exposure to alcohol that affects many physiological processes, such as memory, motor function, and cognitive abilities. This damage is related to the ethanol oxidation in the brain. The metabolism of ethanol to acetaldehyde and then to acetate is associated with the production of reactive oxygen species that accentuate the oxidative state of cells. This metabolism of ethanol can induce the oxidation of the fatty acids in phospholipids, and the bioactive aldehydes produced are known to be associated with neurotoxicity and neurodegeneration. As such, here we will review the role of lipids in the neuronal damage induced by ethanol-related oxidative stress and the role that lipids play in the related compensatory or defense mechanisms.

Enzymatic mechanisms of ethanol oxidation in the brain

BACKGROUND

The exact enzymatic mechanisms of ethanol oxidation in the brain are still unclear. The catalase-mediated oxidation of ethanol was demonstrated in rat brain using incubation of brain homogenates with catalase inhibitors. The role of the alcohol dehydrogenase (ADH) or cytochrome P450-dependent system in this process is possible, but has not been confirmed. The objective of the study was to determine the contribution of the different enzymatic pathways to ethanol oxidation in brain homogenates from mice and rats.

METHODS

Three approaches were used to investigate the enzymatic mechanisms of ethanol oxidation in the brain of rats and mice: (1) preincubation of brain homogenates with inhibitors of the ethanol-metabolizing enzymes (catalase, CYP2E1, ADH, and ALDH); (2) utilization of mice with genetic deficiency in ethanol-metabolizing enzymes (catalase, CYP2E1, or both enzymes); and (3) determination of ethanol oxidation in brain subcellular fractions known to have differential activity of ethanol-metabolizing enzymes. The ethanol-derived acetaldehyde (AC) and acetate were determined in brain samples by gas chromatography.

RESULTS

The catalase inhibitors sodium azide (5 mM) and aminotriazole (5 mM) as well as CYP2E1 inhibitors diallyl sulfide (2 mM) and beta-phenethyl isothiocyanate (0.1 mM) lowered significantly the accumulation of the ethanol-derived AC and acetate in brain homogenates. The ADH inhibitor 4-methyl pyrazole (5 mM) significantly decreased the acetate but not the AC accumulation. Ethanol-derived AC accumulation in brain homogenates of acatalasemic mice was 47% of the control value, 91% in CYP2E1-null mice, and 24% in double mutants (with deficiency of both catalase and CYP2E1). The highest levels of ethanol oxidation were found in microsomal and peroxisomal subcellular brain fractions, where CYP2E1 and catalase are located, respectively.

CONCLUSIONS

Catalase is the key enzyme of ethanol oxidation in the brain of rodents: it may be responsible for about 60% of the process. CYP2E1 plays an important role in ethanol oxidation in the rodent brains. Alcohol dehydrogenase plays a minor role, if any, in this process. Aldehyde dehydrogenase plays the crucial role in the further oxidation of ethanol-derived AC in the brain homogenates.

Is ethanol a pro-drug? Acetaldehyde contribution to brain ethanol effects

This article presents the proceedings of a symposium at the 2004 meeting of the International Society for Biomedical Research on Alcoholism, held in Mannheim, Germany. The symposium was organized by Etienne Quertemont and chaired by C. J. Peter Eriksson. The presentations were (1) Brain ethanol metabolism and its behavior consequences, by Sergey M. Zimatkin and P. S. Pronko; (2) Acetaldehyde increases dopaminergic neuronal activity: a possible mechanism for acetaldehyde reinforcing effects, by Marco Diana and Milena Pisano; (3) Contrasting the reinforcing actions of acetaldehyde and ethanol within the ventral tegmental area (VTA) of alcohol-preferring (P) rats, by Zachary A. Rodd and Richard R. Bell; (4) Molecular and biochemical changes associated with acetaldehyde in human alcoholism and alcohol abuse, by C. J. Peter Eriksson.

Chronic alcohol exposure alters transcription broadly in a key integrative brain nucleus for homeostasis: the nucleus tractus solitarius

Chronic exposure to alcohol modifies physiological processes in the brain, and the severe symptoms resulting from sudden removal of alcohol from the diet indicate that these modifications are functionally important. We investigated the gene expression patterns in response to chronic alcohol exposure (21–28 wk) in the rat nucleus tractus solitarius (NTS), a brain nucleus with a key integrative role in homeostasis and cardiorespiratory function. Using methods and an experimental design optimized for detecting transcriptional changes less than twofold, we found 575 differentially expressed genes. We tested these genes for significant associations with physiological functions and signaling pathways using Gene Ontology terms and the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, respectively. Chronic alcohol exposure resulted in significant NTS gene regulation related to the general processes of synaptic transmission, intracellular signaling, and cation transport as well as specific neuronal functions including plasticity and seizure behavior that could be related to alcohol withdrawal symptoms. The differentially expressed genes were also significantly enriched for enzymes of lipid metabolism, glucose metabolism, oxidative phosphorylation, MAP kinase signaling, and calcium signaling pathways from KEGG. Intriguingly, many of the genes we found to be differentially expressed in the NTS are known to be involved in alcohol-induced oxidative stress and/or cell death. The study provides evidence of very extensive alterations of physiological gene expression in the NTS in the adapted state to chronic alcohol exposure.

The Choroid Plexus‐Cerebrospinal Fluid System: From Development to Aging

The function of the cerebrospinal fluid (CSF) and the tissue that secretes it, the choroid plexus (CP), has traditionally been thought of as both providing physical protection to the brain through buoyancy and facilitating the removal of brain metabolites through the bulk drainage of CSF. More recent studies suggest, however, that the CP-CSF system plays a much more active role in the development, homeostasis, and repair of the central nervous system (CNS). The highly specialized choroidal tissue synthesizes trophic and angiogenic factors, chemorepellents, and carrier proteins, and is strategically positioned within the ventricular cavities to supply the CNS with these biologically active substances. Through polarized transport systems and receptor-mediated transcytosis across the choroidal epithelium, the CP, a part of the blood-CSF barrier (BCSFB), controls the entry of nutrients, such as amino acids and nucleosides, and peptide hormones, such as leptin and prolactin, from the periphery into the brain. The CP also plays an important role in the clearance of toxins and drugs. During CNS development, CP-derived growth factors, such as members of the transforming growth factor-beta superfamily and retinoic acid, play an important role in controlling the patterning of neuronal differentiation in various brain regions. In the adult CNS, the CP appears to be critically involved in neuronal repair processes and the restoration of the brain microenvironment after traumatic and ischemic brain injury. Furthermore, recent studies suggest that the CP acts as a nursery for neuronal and astrocytic progenitor cells. The advancement of our knowledge of the neuroprotective capabilities of the CP may therefore facilitate the development of novel therapies for ischemic stroke and traumatic brain injury. In the later stages of life, the CP-CSF axis shows a decline in all aspects of its function, including CSF secretion and protein synthesis, which may in themselves increase the risk for development of late-life diseases, such as normal pressure hydrocephalus and Alzheimer's disease. The understanding of the mechanisms that underlie the dysfunction of the CP-CSF system in the elderly may help discover the treatments needed to reverse the negative effects of aging that lead to global CNS failure.

Ethanol inhibits gap-junctional coupling between P19 cells

BACKGROUND

Gap junctions are plaques of multiple intercellular channels that connect the cytoplasm of adjacent cells. They provide both electrical and metabolic coupling and are an essential element in normal growth, development, and physiology. Little research exists on the relationship between alcohol administration and gap-junctional function or expression. This study looks at the function and expression of gap junctions after incubation and withdrawal of ethanol with P19 cell cultures.

METHODS

Gap-junctional communication was assessed after 24 and 48 hr of exposure to 20 and 40 mM ethanol and after a 24-hr withdrawal period. The seeding technique was used, and diacyl-3,3'-indocarbocyanine iodide/calcein-stained donor cells were seeded on an unstained monolayer and then reviewed by confocal microscope and counted by flow cytometry. Analysis of connexin (Cx) proteins was performed by Western blot, gel electrophoresis, and immunoblots with antibodies for Cx26 and Cx43.

RESULTS

All treatment regimens produced similar results, reducing dye coupling by more than 50% without recovery after a 24-hr withdrawal period. Exposing the cells to 20 mM ethanol for 48 hr did not significantly change the levels of Cx26 protein, but ethanol significantly decreased the levels of Cx43 in cultured P19 cells.

CONCLUSIONS

This study illustrates that ethanol can inhibit gap-junction function in the P19 cell line. Chronic exposure to 20 mM ethanol selectively decreased the levels of Cx43 protein in the membrane fraction of the cell cultures.

Expression, localization and potential physiological significance of alcohol dehydrogenase in the gastrointestinal tract

ADH1 and ADH4 are the major alcohol dehydrogenases (ADH) in ethanol and retinol oxidation. ADH activity and protein expression were investigated in rat gastrointestinal tissue homogenates by enzymatic and Western blot analyses. In addition, sections of adult rat gastrointestinal tract were examined by in situ hybridization and immunohistochemistry. ADH1 and ADH4 were detected along the whole tract, changing their localization and relative content as a function of the area studied. While ADH4 was more abundant in the upper (esophagus and stomach) and lower (colorectal) regions, ADH1 was predominant in the intestine but also present in stomach. Both enzymes were detected in mucosa but, in general, ADH4 was found in outer cell layers, lining the lumen, while ADH1 was detected in the inner cell layers. Of interest were the sharp discontinuities in the expression found in the pyloric region (ADH1) and the gastroduodenal junction (ADH4), reflecting functional changes. The precise localization of ADH in the gut reveals the cell types where active alcohol oxidation occurs during ethanol ingestion, providing a molecular basis for the gastrointestinal alcohol pathology. Localization of ADH, acting as retinol dehydrogenase/retinal reductase, also indicates sites of active retinoid metabolism in the gut, essential for mucosa function and vitamin A absorption.

Distribution of class I, III and IV alcohol dehydrogenase mRNAs in the adult rat, mouse and human brain

The localization of different classes of alcohol dehydrogenases (ADH) in the brain is of great interest because of their role in both ethanol and retinoic acid metabolism. Conflicting data have been reported in the literature. By Northern blot and enzyme activity analyses only class III ADH has been detected in adult brain specimens, while results from riboprobe in situ hybridization indicate class I as well as class IV ADH expression in different regions of the rat brain. Here we have studied the expression patterns of three ADH classes in adult rat, mouse and human tissues using radioactive oligonucleotide in situ hybridization. Specificity of probes was tested on liver and stomach control tissue, as well as tissue from class IV ADH knock-out mice. Only class III ADH mRNA was found to be expressed in brain tissue of all three investigated species. Particularly high expression levels were found in neurons of the red nucleus in human tissue, while cortical neurons, pyramidal and granule cells of the hippocampus and dopamine neurons of substantia nigra showed moderate expression levels. Purkinje cells of cerebellum were positive for class III ADH mRNA in all species investigated, whereas granular layer neurons were positive only in rodents. The choroid plexus was highly positive for class III ADH, while no specific signal for class I or class IV ADH was detected. Our results thus support the notion that the only ADH expressed in adult mouse, rat and human brain is class III ADH.