Human monocyte activation by cleaved form of alpha‐1‐antitrypsin

Abstract

Production of alpha-1-antitrypsin (AAT) by human monocytes is an important factor in controlling tissue damage by proteases in the microenvironment of inflammation. Increases, of four-to eightfold, in numbers of macrophages and levels of AAT and its cleavage fragments have been found in various inflammatory loci. We have found that the C-terminal peptide (C-36) of AAT, produced by specific proteinase cleavage when added in its fibrillar form at concentrations $5 mm to monocytes in culture for 24 h, significantly increases low density lipoprotein (LDL) binding and uptake, up-regulates levels of LDL receptors and also induces proinflammatory cytokine (interleukin-1, interleukin-6 and tumour necrosis factor a) production and glutathione reductase activity. Because it is known that various cells selectively internalize surface receptors and their ligands through receptor-mediated endocytosis via clathrin-coated pits, we tested whether antibodies raised against the clathrin heavy chain would block the effects of the fibrillar form of C-36 on human monocytes in culture. Addition of excess anti-(clathrin HC) with 10 mm fibrillar C-36 diminished the stimulatory effects of the latter on LDL binding, uptake and LDL receptor levels. In contrast, however, in the presence of anti-(clathrin HC), the potentially cytotoxic effects of fibrils, such as induction of cytokines, free radicals and cytosolic activity of cathepsin D, were much greater than those observed when cells were treated with fibrils alone. These results suggest that endocytosis is the pathway by which C-36 fibrils upregulate LDL receptors, and may be the natural mechanism for fibril clearance. We infer that human monocytes clear C-36 fibrils by a clathrin-dependent pathway, presumably endocytotic, and that loss of this pathway amplifies the cytotoxic effects of the fibrils by increasing their availability to other specific or nonspecific sites through which they exert their cytotoxic effects.Keywords: alpha-1-antitrypsin; C-terminal peptide C-36; cytotoxicity; monocytes; lipid catabolism.Alpha-1-antitrypsin (AAT) is one of the most abundant proteinase inhibitors of human plasma and has been strongly linked to increases in acute phase processes of around three to fourfold that of normal (1.34 mg´mL 21) [1]. The known sources of AAT production in humans are liver, peripheral blood monocytes and alveolar macrophages [2,3], the latter two of which increase expression of AAT in response to inflammatory mediators, such as IL-6 and lipopolysaccharide [4±6]. Despite the fact that the production of AAT in phagocytic monocytes has been shown to be much lower than in hepatocytes, the local regulation of AAT may be important in maintaining the protease/antiprotease balance and preventing the tissue damage induced by proteinases in the microenvironment of injury or inflammation where monocytes accumulate. Induction of connective tissue breakdown in rats leads to increases of around four to eightfold in macrophage and AAT levels, with some of the AAT appearing as protease cl...

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