Fibrillogenic C-terminal fragment of alpha-1-antitrypsin activates human monocytes via oxidative mechanisms

Production of alpha-1-antitrypsin by human monocytes is an important factor in controlling tissue damage by proteases in the microenvironment of infla mmation. Increases of four- to eightfold in levels of native and fragmented forms of alpha-1-antitrypsin have been detected in inflammatory loci in vi vo. In this study we have extended our previous observation that the carbox yl-terminal peptide, (C-36) of alpha-1-antitrypsin produced by specific pro teinase cleavage, when added in its fibrillar form at concentrations of 5 m uM or more to monocytes in culture, induces cytotoxic effects. Experiments with synthetic amyloid-forming peptides suggest fibril cytotoxicity to be m ediated via a common oxidative stress mechanism. We undertook to determine whether C-36 fibril. cytotoxicity also involves this common pathway. Monocy tes stimulated with C-36 fibrils for 1 h showed significant elevation in mo nocyte chemoattractant protein-1 expression, induced reduced nicotinamide-a denine dinucleotide phosphate oxidase activity, increased intracellular lip id peroxidation, altered mitochondrial membrane potential, and increased cy tosolic cytochrome c and caspase-3 activity. Treatment of monocytes with C- 36 fibrils after 24 h also resulted in increased cytosolic cathepsin D acti vity, suggesting that lysosomes may also be destabilized over longer period s of time. In contrast, native alpha-1-antitrypsin only showed concentratio n and time-dependent effects on chemoattractant protein-1 expression, and t hese appear to be independent of oxidative stress. These results indicate t hat the cytotoxicity of the fibrillar fragment is mediated via oxidative me chanisms and support important multiple roles for native and also for cleav ed forms of alpha-1-antitrypsin in monocyte recruitment and activation duri ng inflammatory processes such as atherosclerosis.