Plasma sialic-acid index of apolipoprotein J (SIJ): a new alcohol intake marker

Old and New Biomarkers of Alcohol Abuse: Narrative Review

The harmful use of alcohol is responsible for 5.1% of the global burden of disease, and the early detection of alcohol problems may prevent its development and progression. Therefore, the aim of the study is to review traditional and new biomarkers associated with alcohol use. The nature and practical application and limitations of alcohol biomarkers in the diagnosis and monitoring of drinking are reviewed. Despite the limited specificity and sensitivity in alcohol drinking detection, traditional biomarkers are useful in clinical practice, and new generations of biomarkers, e.g., proteomic markers, are in need of further investigation. Traditional biomarkers are broadly available and cost-efficient, providing valuable data on the complications of drinking and prognosis, as well as on concurrent conditions affected by drinking. The most important challenge in the future will be to translate methodically advanced methods of detecting alcohol markers into simpler and cheaper methods. Larger population studies are also needed to test the usefulness of these potential markers of alcohol use.

Biomarkers of alcohol misuse: recent advances and future prospects

Alcohol abuse and dependence are highly prevalent in many cultures and contribute considerably to the global burden of health and social issues. The current inability to accurately characterise long-term drinking behaviours is a major obstacle to alcoholism diagnosis and treatment. Therefore, it is of great importance to develop objective diagnostic tools to discern subjects with excessive alcohol use and alcoholism or to confirm abstinence. Research over past years has revealed several biochemical compounds with considerable potential for accurate reflection of alcohol intake. This review will address the issue of alcohol biomarker definition, the types of molecules used as so-called traditional biomarkers, and the compounds that can serve as novel biomarker candidates or components of biomarker panels.

Protein biomarkers of alcohol abuse

Alcohol abuse can lead to a number of health and social issues. Our current inability to accurately assess long-term drinking behaviors is an important obstacle to its diagnosis and treatment. Biomarkers for chronic alcohol consumption have made a number of important advances but have yet to become highly accurate and as accepted as objective tests for other diseases. Thus, there is a crucial need for the development of more sensitive and specific markers of alcohol abuse. Recent advancements in proteomic technologies have greatly increased the potential for alcohol abuse biomarker discovery. Here, the authors review established and novel protein biomarkers for long-term alcohol consumption and the proteomic technologies that have been used in their study.

CYP2E1, oxidative stress, post-translational modifications and lipid metabolism

Chronic alcohol-mediated down-regulation of hepatic ST6Gal1 gene leads to defective glycosylation of lipid-carrying apolipoproteins such as apo E and apo J, resulting in defective VLDL assembly and intracellular lipid and lipoprotein transport, which in turn is responsible for alcoholic hepatosteatosis and ALD. The mechanism of ethanol action involves thedepletion of a unique RNA binding protein that specifically interacts with its 3'-UTR region of ST6Gal1 mRNA resulting in its destabilization and consequent appearance of asialoconjugates as alcohol biomarkers. With respect to ETOH effects on Cardio-Vascular Diseases, we conclude that CYP2E1 and ETOH mediated oxidative stress significantly down regulates not only the hepatic PON1 gene expression, but also serum PON1 and HCTLase activities accompanied by depletion of hepatic GSH, the endogenous antioxidant. These results strongly implicate the susceptibility of PON1 to increased ROS production. In contrast, betaine seems to be both hepatoprotective and atheroprotective by reducing hepatosteatosis and restoring not only liver GSH that quenches free radicals, but also the antiatherogenic PON1 gene expression and activity.

Plasma proteomic alterations in non-human primates and humans after chronic alcohol self-administration

Objective diagnostics of excessive alcohol use are valuable tools in the identification and monitoring of subjects with alcohol use disorders. A number of potential biomarkers of alcohol intake have been proposed, but none have reached widespread clinical usage, often due to limited diagnostic sensitivity and specificity. In order to identify novel potential biomarkers, we performed proteomic biomarker target discovery in plasma samples from non-human primates that chronically self-administer high levels of ethanol. Two-dimensional difference in-gel electrophoresis (2D-DIGE) was used to quantify plasma proteins from within-subject samples collected before exposure to ethanol and after 3 months of excessive ethanol self-administration. Highly abundant plasma proteins were depleted from plasma samples to increase proteomic coverage. Altered plasma levels of serum amyloid A4 (SAA4), retinol-binding protein, inter-alpha inhibitor H4, clusterin, and fibronectin, identified by 2D-DIGE analysis, were confirmed in unmanipulated, whole plasma from these animals by immunoblotting. Examination of these target plasma proteins in human subjects with excessive alcohol consumption (and control subjects) revealed increased levels of SAA4 and clusterin and decreased levels of fibronectin compared to controls. These proteins not only serve as targets for further development as biomarker candidates or components of biomarker panels, but also add to the growing understanding of dysregulated immune function and lipoprotein metabolism with chronic, excessive alcohol consumption.

Glycoconjugates in the detection of alcohol abuse

Up to 30% of all hospital admissions and health-care costs may be attributable to alcohol abuse. Ethanol, its oxidative metabolites, acetaldehyde and ROS (reactive oxygen species), non-oxidative metabolites of alcohol [e.g. FAEEs (fatty acid ethyl esters)] and the ethanol-water competition mechanism are all involved in the deregulation of glycoconjugate (glycoprotein, glycolipid and proteoglycan) metabolic processes including biosynthesis, modification, transport, secretion, elimination and catabolism. An increasing number of new alcohol biomarkers that are the result of alcohol-induced glycoconjugate metabolic errors have appeared in the literature. Glycoconjugate-related alcohol markers are involved in, or are a product of, altered glycoconjugate metabolism, e.g. CDT (carbohydrate-deficient transferrin), SA (sialic acid), plasma SIJ (SA index of apolipoprotein J), CETP (cholesteryl ester transfer protein), β-HEX (β-hexosaminidase), dolichol, EtG (ethyl glucuronide) etc. Laboratory tests based on changes in glycoconjugate metabolism are useful in settings where the co-operativeness of the patient is impaired (e.g. driving while intoxicated) or when a history of alcohol use is not available (e.g. after trauma). In clinical practice, glycoconjugate markers of alcohol use/abuse let us distinguish alcoholic from non-alcoholic tissue damage, having important implications for the treatment and management of diseases.

Future prospects for biomarkers of alcohol consumption and alcohol-induced disorders

The lack of reliable measures of alcohol intake is a major obstacle to the diagnosis, treatment, and research of alcohol abuse and alcoholism. Successful development of a biomarker that allows for accurate assessment of alcohol intake and drinking patterns would not only be a major advance in clinical care but also a valuable research tool. A number of advances have been made in testing the validity of proposed biomarkers as well as in identifying potential new biomarkers through systems biology approaches. This commentary will examine the definition of a biomarker of heavy drinking, the types of potential biomarkers, the steps in biomarker development, the current state of biomarker development, and critical obstacles for the field. The challenges in developing biomarkers for alcohol treatment and research are similar to those found in other fields. However, the alcohol research field must reach a competitive level of rigor and organization. We recommend that NIAAA consider taking a leadership role in organizing investigators in the field and providing a common set of clinical specimens for biomarker validation studies.

Circulating cytokines as biomarkers of alcohol abuse and alcoholism

There are currently no consistent objective biochemical markers of alcohol abuse and alcoholism. Development of reliable diagnostic biomarkers that permit accurate assessment of alcohol intake and patterns of drinking is of prime importance to treatment and research fields. Diagnostic biomarker development in other diseases has demonstrated the utility of both open, systems biology, screening for biomarkers and more rational focused efforts on specific biomolecules or families of biomolecules. Long-term alcohol consumption leads to altered inflammatory cell and adaptive immune responses with associated pathologies and increased incidence of infections. This has led researchers to focus attention on identifying cytokine biomarkers in models of alcohol abuse. Alcohol is known to alter cytokine levels in plasma and a variety of tissues including lung, liver, and very importantly brain. A number of cytokine biomarker candidates have been identified, including: tumor necrosis factor-alpha, interleukin (IL)-1-alpha, IL-1-beta, IL-6, IL-8, IL-12, and monocyte chemoattractant protein-1. This is an emerging and potentially exciting avenue of research in that circulating cytokines may contribute to diagnostic biomarker panels, and a combination of multiple biomarkers may significantly increase the sensitivity and specificity of the biochemical tests aiding reliable and accurate detection of excessive alcohol intake.

Liver Galbeta1,4GlcNAc alpha2,6-sialyltransferase is down-regulated in human alcoholics: possible cause for the appearance of asialoconjugates

Galbetal,4GlcNAc alpha2,6-sialyltransferase (ST6GalI) mediates the glycosylation of proteins and lipids to form functionally important glycoproteins and glycolipids in the Golgi compartment. Our previous work demonstrated that long-term ethanol feeding in rats caused a marked 59% decrease in ST6GalI activity as well as ST6GalI messenger RNA (mRNA) level in the liver that was due to decreased stability of the mRNA. Clinical observations show that down-regulation of ST6GalI gene and consequent impaired activity of ST6GalI seems to be the major cause for the appearance of asialoconjugates in the blood of long-term alcoholics. The plasma carbohydrate-deficient transferrin (CDT) and sialic acid index of plasma apolipoprotein J were also altered in the alcoholic group compared with the nondrinkers. We have now investigated how alcohol affects the gene regulation of ST6GalI and the possible mechanism in postmortem human liver specimens taken from nondrinkers, moderate alcohol drinkers, and heavy alcohol drinkers. Real-time polymerase chain reaction analyses of the liver RNA extract showed that ST6GalI mRNA level was progressively decreased by 49% in moderate drinkers (P < .01) and by 69% in heavy drinkers (P < .01) compared with nondrinkers. Western blot analysis showed that liver ST6GalI protein level was negligibly decreased in moderate drinkers but decreased by 30% (P < .05) in heavy drinkers compared with nondrinkers. We further demonstrated a single ST6GalI mRNA-binding protein complex in the normal human liver extract, which progressively decreased in the liver extracts of moderate and heavy alcohol drinkers. Thus, it is concluded that the appearance of asialoconjugates in alcoholics is possibly due to the down-regulation of ST6GalI gene expression.

Biochemical markers of alcoholism

Alcohol and alcohol-related diseases have become a major cause of death in Western countries. The most sensitive and specific of the commonly used biomarkers of alcohol intake are carbohydrate-deficient transferrin (CDT), and the combination of gamma-glutamyltransferase (GGT) and CDT. Other widely used laboratory markers are GGT, mean corpuscular volume of erythrocytes and the ratio of aspartate aminotransferase to alanine aminotransferase. Blood ethanol levels reveal recent alcohol use. However, more specific and sensitive biomarkers to improve the detection of excessive alcohol use at an early stage are needed. New biomarkers, not yet used in routine clinical work, include phosphatidylethanol, fatty acid ethyl esters, ethyl glucuronide, sialic acid, and acetaldehyde adducts.

Differential Expression of Serum Clusterin Isoforms in Colorectal Cancer

Clusterin is an enigmatic protein altered in tumors of colorectal cancer patients. Because there is no information available about serum clusterin regarding this pathology, we applied proteomic techniques to analyze its isoforms in donors and patients. First we separated serum proteins through concanavalin A, obtaining a fraction with non- and O-glycosylated proteins (FI) and a second fraction enriched in N-glycoproteins (FII) wherein clusterin was supposed to elute on the basis of its glycosylation. Surprisingly analysis of the FI fraction revealed the existence of an unexpected and aberrantly N-glycosylated clusterin that was overexpressed in patients and comprised at least five isoforms with different isoelectric points. On the other hand, two-dimensional electrophoretic analysis of the clusterin eluted in FII detected one isoform that was increased and 15 isoforms that were decreased or absent in serum of patients. Finally immunoquantification by slot blot showed that in total serum and in FI the clusterin levels were significantly increased in patients, whereas in FII there was no significant variation. Therefore, serum clusterin and some of its isoforms could have a potential value as colorectal tumor markers and are interesting subjects for biomarker studies.

Chronic ethanol consumption upregulates the cytosolic and plasma membrane sialidase genes, but down regulates lysosomal membrane sialidase gene in rat liver

We have previously shown that chronic ethanol feeding stimulates liver cytosolic sialidase (CS) and plasma membrane sialidase (PMS), whereas it decreases lysosomal membrane sialidase (LMS) activities with concomitant alterations in their relative synthetic rate in rat. To understand the molecular mechanism(s) for these changes, we have evaluated the effect of ethanol administration in male Wistar rats as a function of increasing dietary ethanol concentration after 8 weeks of pair-feeding on (i) the expression of CS, PMS, and LMS genes by real-time quantitative polymerase chain reaction method; (ii) their relative transcription rates by nuclear run-on assay; and (iii) the actual amount of these sialidase proteins in the liver fractions of the respective groups by Western blot method. We have demonstrated that the animals fed with 10.6%, 20.8%, and 36% of total calories as ethanol showed a 20% (P<.05), 34% (P<.01), and 69% (P<.01) increase in CS mRNA level, and 22% (P<.05), 26% (P<.01), and 47% (P<.01) increase in PMS mRNA level, but a decrease in LMS mRNA level by 35% (P<.05), 50% (P<.01), and 80% (P<.01), respectively, as compared to controls. Western blot analyses of CS, PMS, and LMS in the liver subfractions showed that changes in protein levels of CS, LMS, and PMS were consistent with the corresponding changes in the respective mRNA levels. Thus, the upregulation of CS and PMS, but not LMS which is down regulated by chronic ethanol, may account for the appearance of asialoconjugates in alcoholics.

Ethanol destabilizes liver Gal beta l, 4GlcNAc alpha2,6-sialyltransferase, mRNA by depleting a 3'-untranslated region-specific binding protein

Asialoconjugates are viable biomarkers for alcohol abuse. We previously showed that chronic ethanol feeding down-regulated liver Gal beta l, 4GlcNAc alpha2,6-sialyltransferase (ST6Gal l) mRNA by destabilizing it. Since RNA-binding proteins are known to stabilize many eukaryotic mRNAs by interacting with the 3'-untranslated region (UTR), we have delineated the possible mechanism by which ethanol destabilizes ST6Gal l mRNA. Using (32)P-labeled RNA probes generated from a 2.7-kb 3'-UTR of ST6Gal l mRNA, we identified a liver cytosolic 41-kDa specific binding protein that interacts with its 3'-UTR domain and protects it from degradation in normal rat liver but disappears after chronic ethanol treatment. Mapping of the binding region revealed that four RNA probes of 80-base pair (bp) length spanning the 304 bp of the 3'-UTR of ST6Gal l mRNA showed equal binding intensity. The corresponding cDNA sequences for the four 80-bp RNA probes share the 13-bp consensus sequence. Mutagenesis analysis identified that four nucleotides, AG and TC, among the consensus sequences were critical for the RNA-protein interaction. Therefore, 5'-CAGCCTCCTCCCT-3' serves as a cis-element critically involved in this interaction. The RNA-protein complex formation progressively decreased with increasing dietary ethanol, resulting in its virtual disappearance with 36% of the dietary calories as ethanol. Concomitantly, the same ethanol diet decreased sialic acid index of plasma apolipoprotein J by 45% (p < 0.05). Thus, depletion of a binding protein that specifically interacts with its 3'-UTR region of ST6Gal l mRNA may account for its destabilization and consequent appearance of asialoconjugates as alcohol biomarkers.

Chronic ethanol feeding controls the activities of various sialidases by regulating their relative synthetic rates in the rat liver

We have determined the concentration effects of feeding for 8 weeks 10.8%, 21.6%, and 36% dietary ethanol calories on the activities and relative synthetic rates (RSRs) of various subcellular sialidases of rat liver. The hepatic RSRs of each species of sialidase was determined based on the ratio of 1-hour incorporation of [35S]-methionine into immunoprecipitable sialidase as percent of the incorporation into total protein in each subcellular fraction. Ganglioside sialidase activities in the hepatic subcellular fractions were also determined. Ethanol feeding at 36% dietary calories caused an increase in the ganglioside sialidase activity of the plasma membrane sialidase (PMS) by 232% (P < .01) and that of cytosolic sialidase (CS) by 184% (P < .05), but decreased the lysosomal membrane sialidase (LMS) by 54% (P < .01) when compared with the control animals. The specificity of each antisialidase antibody was verified by immunoblots. The RSR of PMS was increased by 40% (P < .01), 67% (P < .01), and 220% (P < .01) in the 10.8%, 21.6%, and 36% ethanol groups, respectively. Similarly, the RSR of CS was increased by 17% (P < .01), 19% (P < .01), and 63% (P < .01), respectively, in these ethanol groups. In contrast, the RSR of LMS was inhibited by 36% (P < .01), 34% (P < .01), and 69% (P < .01), respectively, in these ethanol groups. Intralysosomal sialidase failed to hydrolyze gangliosides. Thus, PMS and CS, but not LMS or intralysosomal sialidase, may play important roles in ethanol-modulated desialylation of gangliosides and consequent liver injury and behavioral alterations.

Mechanism of action of ethanol in the down-regulation of Gal(beta)1, 4GlcNAc alpha2,6-sialyltransferase messenger RNA in human liver cell lines

Gal beta l, 4GlcNAc alpha 2,6-sialyltransferase (2,6-ST) mediates the addition of alpha 2,6-linked sialic acid to glycoproteins in the Golgi compartment. Down-regulation of its gene and consequent impaired activity of 2,6-ST seems to be the major cause for the appearance of asialoconjugates in the blood of long-term alcoholics. Therefore, mechanism(s) involved in the regulation of 2,6-ST gene is important and clinically relevant. Our previous work showed that long-term ethanol feeding in rats caused a marked 59% decrease of 2,6-ST activity as well as 2,6-ST messenger RNA (mRNA) level in liver that were due to the decreased stability of its mRNA. We now mimic these actions of ethanol using ( a ) human liver HepG2 cells stably transfected with ethanol-inducible human cytochrome P4502E1 (CYP2E1 cells), or ( b ) with high alcohol dehydrogenase (HAD cells) but not in wild-type HepG2 cells lacking either of the above 2 enzymes as models. Incubation of these cells for 72 hours with 100 mmol/L ethanol caused decreases (up to 76%, P < .05) of 2,6-ST mRNA levels in CYP2E1 and HAD cells but not in the wild type. However, incubation of wild-type cells with acetaldehyde at concentrations of 50 and 100 micro mol/L showed a dramatic decrease (up to 69%, P < .02) in the 2,6-ST mRNA levels. Furthermore, exposure of CYP2E1 cells to 4-hydroxy-2-nonenal, an endogenous lipid peroxidation product of reactive oxygen species, strongly decreased 2,6-ST mRNA level by 61% ( P < .02). These results demonstrate that 2,6-ST gene is highly sensitive to ethanol action in human liver cells either via its oxidation product, acetaldehyde, or via reactive oxygen species leading to the generation of a more reactive aldehyde such as 4-hydroxy-2-nonenal. Thus, this study assumes major importance and clinical relevance because 2,6-ST gene regulation in a human liver cell model is demonstrated within a few days of ethanol exposure, whereas its in vivo regulation in liver generally takes prolonged period of ethanol exposure.

The Potential Beneficial Effect of Glycine on the Carbohydrate Moieties of Glycoproteins in an Experimental Model of Alcohol-Induced Hepatotoxicity

Glycine is known to have a protective role against alcohol-induced liver damage. The aim of our study was to evaluate the effect of glycine on liver and brain glycoproteins in alcohol-fed rats. Administering ethanol (7.9 g/kg of body of weight) every day to Wistar rats for 60 days resulted in significantly elevated levels of liver and brain hexosamine, fucose, and sialic acid and significantly reduced levels of total hexoses as compared with those of the control rats. Simultaneous glycine supplementation (0.6 g/kg of body weight) during the last 30 days of the experiment to rats given alcohol normalized the levels of hexosamine, fucose, and sialic acid and elevated the levels of total hexoses in the liver and brain significantly as compared with unsupplemented alcohol-treated rats. Microscopic examination of alcohol-fed rat liver showed inflammatory cell infiltrates and fatty changes, which were reversed on treatment with glycine. Similarly, alcohol-treated rat brain demonstrated edema, which was markedly reduced on treatment with glycine. Thus glycine administration plays a significant role in reducing the toxicity of ethanol.

Current status of carbohydrate deficient transferrin, total serum sialic acid, sialic acid index of apolipoprotein J and serum beta-hexosaminidase as markers for alcohol consumption

AIMS

The purpose of this paper is to present a brief review of the literature and to summarize the current status of four biochemical markers for alcohol consumption, carbohydrate deficient transferrin (CDT), total serum sialic acid (TSA), sialic acid index of apolipoprotein J (SIJ) and serum beta-hexosaminidase (beta-HEX).

FINDINGS

Of these markers, CDT has been the most widely studied, is currently thought to be the most accurate predictor of alcohol consumption, is most readily available and is the only test approved by the FDA for the identification of heavy alcohol use. TSA and SIJ have the potential to be useful markers, but have only recently been discovered, are not readily available and have not yet been studied comprehensively. Finally, the relationship between serum beta-HEX and heavy alcohol consumption has been studied for about 20 years, but the test is not readily available and has not been widely accepted or used as a marker for heavy alcohol consumption.

CONCLUSIONS

These markers have the potential to be included in a combination of measurements to provide an accurate, more exact assessment of alcohol consumption in a variety of clinical and research settings.

Separation methods for sialic acids and critical evaluation of their biologic relevance

Sialic acids are biosynthesized by almost all organisms as a 9-carbon carboxylated monosaccharide and are integral components of glycoconjugates. More than 40 naturally occurring sialic acid derivatives of the three main forms of sialic acids, the N-acetyl- and N-glycolylneuraminic acid and 2-keto-3-deoxy-nonulosonic acid have been identified. Due to the great importance of sialic acids as key mediators in a plethora of cellular events, including cell-cell recognition and cell-matrix interactions, their analysis in biologic samples is useful for a deeper understanding of the various (patho)physiological processes and of value in disease diagnosis and monitoring. In this review we summarize the methodology developed to isolate and liberate sialic acids from biologic samples as well as the chromatographic, electromigration and hyphenated techniques available for their separation and analysis. A critical evaluation of the biological relevance of the results obtained by analyzing sialic acids in biologic samples is also presented.