MECHANISM OF ENDOTOXIN ABSORPTION IN ALCOHOLISM

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DESCRIPTION (provided by applicant): Evidence from clinical and experimental studies indicates that elevated intestinal permeability to endotoxins and the resulting endotoxemia play a crucial role in the pathogenesis of alcoholic liver disease. Our studies conducted so far have shown that acetaldehyde, the metabolic product of ethanol, is accumulated in the intestine of alcoholics and disrupts the intestinal epithelial barrier function and increases the permeability to endotoxins. The mechanism of this acetaldehyde-induced disruption of epithelial barrier function involves inhibition of a protein tyrosine phosphatase, PTP1B, tyrosine phosphorylation of junctional proteins, disruption of the interactions among the junctional proteins (that determine the barrier function), and loss of integrity of the junctional complexes. Our preliminary studies also indicated that epidermal growth factor (EGF) and L-glutamine prevent acetaldehyde-mediated increase in permeability to endotoxins. On the basis of our results it is further hypothesized that: a) acetaldehyde-induced phosphorylation of PTP1B and beta-catenin disrupts E-Cadherin-beta-Catenin-PTP1B complex, b) EGF and L-glutamine prevent acetaldehyde induced disruption of junctions by PLCgamma and MAP kinase mediated signals, and c) L-glutamine prevents acetaldehyde-induced disruption of junctions by EGF receptor-dependent mechanism. Using a cell culture model of the intestinal epithelium and human colonic biopsies we will to determine that: 1) interaction between E-cadherin and PTP1B is reduced by Thr-phosphorylation of PTP1B, 2) phosphorylation of beta-catenin on tyrosine residues prevents its binding to E-cadherin, 3) mutation of specific threonine residues on PTP1B and tyrosine residues in beta-catenin prevents the acetaldehyde-induced disruption of junctions, 4) EGF and glutamine prevent acetaldehyde-induced phosphorylation of PTP1B and beta-catenin, and disruption of E-cadherin/beta-catenin/PTP1B complex, 5) PLCgamma and MAP kinase signaling pathways mediate EGF- and glutamine-induced prevention of phosphorylation of PTP1B and beta-catenin, and disruption of E-cadherin/beta-catenin/PTP1B complex, 6) L-glutamine prevents acetaldehyde-induced phosphorylation of PTP1B and beta-catenin, and disruption of E-cadherin/beta-catenin/PTP1B complex by EGF receptor-dependent mechanism, 7) EGF receptor activation is involved in glutamine-mediated prevention of acetaldehyde-induced disruption of AJ and TJ, 9) glutamine prevents acetaldehyde-induced disruption of AJ and TJ by Src kinase dependent mechanism, and 10) EGF receptor activation by glutamine is mediated by extra cellular release of metalloproteinase and TGFalpha. Information derived from this study will expand our understanding of acetaldehyde-mediated injury in intestinal epithelium and endotoxin absorption in alcoholics. The studies on the protection of barrier function by epidermal growth factor and L-glutamine have the potential to provide the basis for the development of preventive and therapeutic strategies for alcoholic liver disease.