Homozygous PiZZ alpha1-antitrypsin deficiency is the most common genetic liver disease in children and is associated with chronic liver injury and hepatocellular carcinoma in adults. Liver injury results from the hepatotoxic effect of abnormally folded alpha1-AT Z which is unable to traverse the secretory pathway and accumulates in the ER of liver cells. It is unknown, however, why only 15% of the deficient population develop liver injury. In this project we have examined the hypothesis that this subset of the deficient population is more susceptible to liver injury by virtue of inherited traits or environmental factors which exaggerate the intracellular accumulation of alpha1-AT Z, or exaggerate the cellular pathologic sequelae of alpha1-AT Z accumulation. Two potential mechanisms have been identified and characterized. In the first, elastase-alpha1-AT complexes have been shown to mediate increases in synthesis of alpha1-AT and greater net accumulation of alpha1-AT Z in cells from PiZZ individuals. A specific pentapeptide neo-domain of alpha1-AT and a specific 8OkD cell surface polypeptide called the SEC receptor have been identified as the ligand and receptor responsible for this effect. Further detailed molecular/biochemical characterization of ligand and receptor as outlined in the current proposal will enhance the likelihood of pharmacologically blocking this receptor in PiZZ individuals, especially during alpha1-AT replacement therapy (protein or gene) for emphysema. Further characterization of this ligand-receptor interaction will also enhance our understanding of the local tissue response to inflammation for several reasons; 1) its regulatory effect allows for highly focussed and tightly controlled activity of limited proteolytic cascade pathways at sites of injury, for orderly initiation of tissue repair and for prevention of excessive connective tissue destruction around migrating cells and sprouting cell processes; 2) it mediates chemotaxis of neutrophils; 3) it may be involved in clearance/catabolism of serpin- enzyme complexes in vivo; 4) it may mediate some of the biologic effects of tachykinins and pathobiologic effects of amyloid-beta peptide. The second potential mechanism for susceptibility to liver injury in alpha1-AT deficiency was identified by examining the fate of the alpha1-AT Z in transfected skin fibroblast cell lines from PiZZ individuals with liver disease, lung disease or no apparent disease. The results indicate that there is a decrease in the rate of degradation of alpha1-AT Z in cells of PiZZ individuals with liver disease. Moreover, there is a decrease in interaction of this mutant alpha1-AT protein with a resident ER molecular chaperone, calnexin, In cells from PiZZ individuals with liver disease. More detailed molecular/biochemical studies of this defect in degradation as outlined in the current proposal will enhance the likelihood of predicting susceptibility to liver disease and developing specific pharmacological strategies for prevention of liver injury.
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