Abstract
The oxidation of methionine residues in many proteins, including the serine proteinase inhibitor alpha1-antitrypsin (AAT), can result in functional inactivation. In this study we investigated the pro-inflammatory properties of oxidized AAT (oxAAT), specifically its ability to activate human monocytes in culture. Monocytes stimulated with oxAAT at concentrations up to 0.2 mg/ml for 24 h showed significant elevation in monocyte chemoattractant protein-1, cytokine interleukin-6, and tumor necrosis factor-alpha expression and increased NADPH oxidase activity. Monocytes activated with oxAAT showed surprising effects on lipid metabolism. Expression of low density lipoprotein (LDL) receptors increased by up to 76% compared with controls but was not accompanied by any changes in (125)I-labeled LDL binding and, paradoxically, decreased LDL uptake, degradation, and intracellular cholesterol synthesis. oxAAT also down-regulated the scavenger receptor CD36, which takes up and is up-regulated by oxidized LDL and is down-regulated by cholesterol efflux. As a by-product of oxidative events accompanying inflammation, oxAAT has multiple effects on cytokine expression, generation of reactive oxygen species, and on intracellular lipid metabolism. The up-regulation of monocyte-derived reactive oxygen by oxAAT could potentially result in self-amplification of AAT oxidation and, thereby, the other effects deriving from it. This implies that there are as yet unidentified regulatory processes that control this cycle.
Highlights
Preventing tissue damage induced by proteases in the microenvironment of injury or inflammation
No complex formation between oxidized AAT (oxAAT) and elastase is observed under the same conditions, and only low molecular weight bands are visible on the gels
We examined the levels of pro-inflammatory cytokines in medium from monocytes cultured in the presence of oxAAT and low density lipoprotein (LDL) separately or simultaneously
Summary
Preventing tissue damage induced by proteases in the microenvironment of injury or inflammation. We examined the effects of the proteolytically modified, cleaved form of AAT on HepG2 cells and the effects of the amyloidogenic C-terminal fragment (C-36, corresponding to amino acid sequence 358 –396) of AAT on human monocyte culture We showed that these forms of AAT induce significant changes in lipid catabolism in both cell types and a remarkable stimulation in pro-inflammatory cytokine and free radical production and up-regulate scavenger receptor CD36 in primary human monocyte cultures [23,24,25,26]. We show that oxidized AAT induces monocyte chemoattractant protein-1 (MCP-1) and pro-inflammatory cytokine expression, activates NADPH oxidase, decreases LDL uptake and degradation and intracellular cholesterol synthesis, increases LDL receptor number, and decreases scavenger receptor CD36 expression
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