Alpha1-antitrypsin binds hemin and prevents oxidative activation of human neutrophils: putative pathophysiological significance

Abstract

Heme is a ubiquitous compound of human tissues, and it is involved in cellular physiology and metabolism. Once released from the cell, free heme oxidizes to the ferric state (hemin). High levels of hemin can cause oxidative stress and inflammation if not neutralized immediately by specialized scavenger proteins. Human alpha1-antitrypsin (A1AT), an acute-phase glycoprotein and important inhibitor of neutrophil proteases, is also a hemin-binding protein. A short-term exposure of freshly isolated human blood neutrophils to 4 µM hemin results in cell spreading, surface expression of filament protein, vimentin, free radical production, expression of heme oxygenase-1 (HO-1), release of IL-8, and enhanced neutrophil adhesion to human endothelial cells. Consequently, the phosphorylation of protein kinase C (PKC) occurs after 25 min. Under the same experimental conditions, addition of 1 mg/ml A1AT markedly reduces or abolishes neutrophil-activating effects of hemin and prevents PKC phosphorylation. In a mouse model of acute kidney injury (AKI) plus injection of hemin, monotherapy with 4 mg/mouse A1AT significantly lowered serum levels of free hemin at 2 h after surgery. Moreover, a tendency toward lower AKI scores, reduced infiltration of neutrophils, and lower levels of serum chemokine [CXCL1/keratinocyte-derived chemokine (KC)] was observed. Our findings highlight A1AT as a potential serum scavenger of hemin and suggest that the commercial preparations of human plasma A1AT might prove to be useful therapeutics in conditions associated with hemolysis.

Vimentin as a Multifaceted Player and Potential Therapeutic Target in Viral Infections

“…In turn, phosphorylation of the head domain at S72 has been shown to associate with vimentin recruitment to the cell surface upon treatment with N-acetyl-glucosamine (N-AcGln) polymers [49], thus strengthening the importance of PTM for vimentin exposure. Interestingly, extracellular exposure of the protein is frequently associated with oxidative stress [42]. Consistent with this, oxidative modifications and electrophile addition have also been reported on extracellular vimentin [33].…”

“…Vimentin can be exposed at the surface or secreted by several cell types, including activated macrophages [14] and neutrophils [42], endothelial cells (HUVEC and brain microvascular endothelial cells) [43,44], and platelets [45]. Extracellular vimentin has been involved in a variety of processes, including cell-cell interaction, hemostasis, immune activation, interaction with pathogens and tissue repair (see below).…”

The Multifaceted Effects of Alpha1-Antitrypsin on Neutrophil Functions

“…Hemoglobin-derived free heme is a cytotoxic molecule that mediates oxidative stress, cell activation, and amplifies inflammatory responses. Our in vitro data provide novel evidence that AAT, as a free heme scavenger, markedly reduces or abolishes the neutrophil-activating effects of heme including neutrophil spreading, surface expression of vimentin, ROS production, and release of IL-8, and neutrophil adhesion to human endothelial cells ( Janciauskiene et al, 2017 ). It is important to point out that the oxidized form of AAT (without anti-elastase activity), still binds heme.…”

TLR4 Signaling by Heme and the Role of Heme-Binding Blood Proteins

“…Notably, albumin is a negative acute phase protein in humans and it is conceivable that during severe inflammatory conditions, when the heme-neutralizing capacity of albumin decreases, other acute-phase proteins such as AAT will participate. AAT is a HBP with binding affinity similar to albumin ( 59 ) and it has previously been demonstrated that AAT markedly reduces free heme neutrophil-activating effects, including the production of ROS ( 63 ). Serum HBPs not only bind and neutralize free heme with different binding affinities, but may also acquire novel biological activities via specific interactions with heme ( 64 – 66 ).…”