Increased intestinal permeability to macromolecules and endotoxemia in patients with chronic alcohol abuse in different stages of alcohol-induced liver disease

The effects of alcohol consumption upon the gastrointestinal tract.

Previous studies have shown that high concentrations of ethanol (>/=40%) cause functional damage of the gastrointestinal epithelial barrier by direct cytotoxic effect on the epithelial cells. The effects of lower noncytotoxic doses of ethanol on epithelial barrier function are unknown. A major function of gastrointestinal epithelial cells is to provide a barrier against the hostile substances in the gastrointestinal lumen. The apicolaterally located tight junctions (TJs) form a paracellular seal between the lateral membranes of adjacent cells and act as a paracellular barrier. In this study, we investigated the effects of lower doses of ethanol on intestinal epithelial TJ barrier function using filter-grown Caco-2 intestinal epithelial monolayers. The Caco-2 TJ barrier function was assessed by measuring epithelial resistance or paracellular permeability of the filter-grown monolayers. Ethanol (0, 1, 2.5, 5, 7.5, and 10%) produced a dose-related drop in Caco-2 epithelial resistance and increase in paracellular permeability. Ethanol also produced a progressive disruption of TJ protein (ZO-1) with separation of ZO-1 proteins from the cellular junctions and formation of large gaps between the adjacent cells. Ethanol, at the doses used (</=10%), did not cause cytotoxicity (lactate dehydrogenase release) to the Caco-2 cells. Ethanol produced a disassembly and displacement of perijunctional actin and myosin filaments from the perijunctional areas. On ethanol removal, actin and myosin filaments rapidly reassembled at the cellular borders. Ethanol stimulated the Caco-2 myosin light chain kinase (MLCK) activity but did not affect the MLCK protein levels. Specific MLCK inhibitor ML-7 inhibited both ethanol increases in MLCK activity and TJ permeability without affecting the MLCK protein levels. Consistent with these findings, metabolic inhibitors sodium azide and 2,4-dinitrophenol significantly prevented ethanol-induced increase in Caco-2 TJ permeability, whereas cycloheximide or actinomycin D had no effect. The results of this study indicate that ethanol at low noncytotoxic doses causes a functional and structural opening of the Caco-2 intestinal epithelial TJ barrier by activating MLCK.

Ethanol modulation of intestinal epithelial tight junction barrier

Disruption of the intestinal barrier is a causal factor in the development of alcoholic endotoxemia and hepatitis. This study was undertaken to determine whether zinc deficiency is related to the deleterious effects of alcohol on the intestinal barrier. Mice were pair fed an alcohol or isocaloric liquid diet for 4 wk, and hepatitis was detected in association with elevated blood endotoxin level. Alcohol exposure significantly increased the permeability of the ileum but did not affect the barrier function of the duodenum or jejunum. Reduction of tight-junction proteins at the ileal epithelium was detected in alcohol-fed mice although alcohol exposure did not cause apparent histopathological changes. Alcohol exposure significantly reduced the ileal zinc concentration in association with accumulation of reactive oxygen species. Caco-2 cell culture demonstrated that alcohol exposure increases the intracellular free zinc because of oxidative stress. Zinc deprivation caused epithelial barrier disruption in association with disassembling of tight junction proteins in the Caco-2 monolayer cells. Furthermore, minor zinc deprivation exaggerated the deleterious effect of alcohol on the epithelial barrier. In conclusion, epithelial barrier dysfunction in the distal small intestine plays an important role in alcohol-induced gut leakiness, and zinc deficiency attributable to oxidative stress may interfere with the intestinal barrier function by a direct action on tight junction proteins or by sensitizing to the effects of alcohol.

https://www.physiology.org/doi/pdf/10.1152/ajpgi.00350.2009

The role of zinc deficiency in alcohol-induced intestinal barrier dysfunction

Ethanol is widely consumed and is associated with an increasing global health burden. Several reviews have addressed the effects of ethanol and its oxidative metabolite, acetaldehyde, on the gastrointestinal (GI) tract, focusing on carcinogenic effects or alcoholic liver disease. However, both the oxidative and the nonoxidative metabolites of ethanol can affect the epithelial barrier of the small and large intestines, thereby contributing to GI and liver diseases. This review outlines the possible mechanisms of ethanol metabolism as well as the effects of ethanol and its metabolites on the intestinal barrier. Limited studies in humans and supporting in vitro data have indicated that ethanol as well as mainly acetaldehyde can increase small intestinal permeability. Limited evidence also points to increased colon permeability following exposure to ethanol or acetaldehyde. In vitro studies have provided several mechanisms for disruption of the epithelial barrier, including activation of different cell-signaling pathways, oxidative stress, and remodeling of the cytoskeleton. Modulation via intestinal microbiota, however, should also be considered. In conclusion, ethanol and its metabolites may act additively or even synergistically in vivo. Therefore, in vivo studies investigating the effects of ethanol and its byproducts on permeability of the small and large intestines are warranted.

Ethanol metabolism and its effects on the intestinal epithelial barrier

Previous studies by us indicated that ethanol in concentrations of 2.0–4.8% produced subepithelial blebs in the jejunum of the hamster. In the rat, due to rupture of the blebs, there was denudation of the villus tip epithelium. There are no similar data on humans. Ethanol, in quantities equivalent to 4.8–6.4 ounces of 80 proof whiskey (diluted to 20% w/v), was infused into stomachs of 20 normal human volunteers. Subjects were divided into groups (Gr) according to the amount or type of alcohol given, and the site of biopsies (SB). Gr 1∶60 g ethanol, SB=jejunum. Gr 2∶45 g ethanol, SB=jejunum. Gr 3∶45 g ethanol, SB=duodenum. Gr 4∶45 g ethanol as 4.8 oz 80 proof whiskey, SB=duodenum. To compare the morphology in the absence and presence of ethanol, biopsies were obtained from each volunteer before ethanol administration (control period). immediately after peak ethanol concentration in the duodenum or jejunum (ethanol period), and when intraluminal ethanol concentration fell towards zero (recovery period). The mean peak intraluminal ethanol concentrations in the four groups varied between 5.69% and 9.37% (w/v). Ethanol-induced damage was evaluated using strict preset criteria. Coded slides were evaluated by two observers. Suction biopsy damaged 18% of the villi even in biopsies obtained during the control period. Ethanol produced a statistically significant increase in the number of damaged villi (mean of all groups 45%, range∶32% in Gr 2 to 62% in Gr 3). During the recovery period the number of damaged villi fell to that seen in control period biopsies. Ethanol, in quantities equivalent to those ingested during moderate drinking, may produce transient damage to the upper small intestine of man. Conversely, ethanol may simply increase the susceptibility of the mucosa to the unavoidable trauma of suction biopsy. However, the histological and ultrastructural changes were similar to those induced by ethanol in small laboratory animals.

M. S. Millan

Papers

Villous damage induced by suction biopsy and by acute ethanol intake in normal human small intestine.