Human monocyte activation by cleaved form of alpha-1-antitrypsin - Involvement of the phagocytic pathway

Production of alpha-l-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 inf lammatory loci. We have found that the C-terminal peptide (C-36) of AAT, pr oduced by specific proteinase cleavage when added in its fibrillar form at concentrations greater than or equal to 5 mu M to monocytes in culture for 24 h, significantly increases low density lipoprotein (LDL) binding and upt ake, up-regulates levels of LDL receptors and also induces proinflammatory cytokine (interleukin-l, interleukin-6 and tumour necrosis factor alpha) pr oduction and glutathione reductase activity. Because it is known that vario us cells selectively internalize surface receptors and their ligands throug h 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 e xcess anti-(clathrin HC) with 10 mu M fibrillar C-36 diminished the stimula tory effects of the latter on LDL binding, uptake and LDL receptor levels. In contrast, however, in the presence of anti-(clathrin HC), the potentiall y cytotoxic effects of fibrils, such as induction of cytokines, free radica ls and cytosolic activity of cathepsin D, were much greater than those obse rved 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 e ndocytotic, and that loss of this pathway amplifies the cytotoxic effects o f the fibrils by increasing their availability to other specific or nonspec ific sites through which they exert their cytotoxic effects.