Alpha 1-acid glycoprotein (alpha 1-AGP) on the membrane of human lymphocytes: possible involvement in cellular activation

Flow cytometric detection of alpha-1-acid glycoprotein on feline circulating leucocytes

OBJECTIVE

To assess whether alpha-1-acid glycoprotein (AGP) can be detected on the membrane of feline circulating leucocytes.

DESIGN

The presence of AGP on circulating leucocytes was investigated in both clinically healthy cats and cats with different diseases. A group of feline coronavirus (FCoV)-positive cats, comprising cats with feline infectious peritonitis (FIP) and cats not affected by FIP but seropositive for FCoV, were included in this study because the serum concentration of AGP increases during FCoV infection.

PROCEDURE

Flow cytometry (using an anti-feline AGP antibody), serum protein electrophoresis, routine haematology and measurement of the serum AGP concentration were performed using blood samples from 32 healthy cats (19 FCoV-seropositive), 13 cats with FIP and 12 with other diseases (6 FCoV-seropositive). The proportion of cats with AGP-positive leucocytes in the different groups (e.g. controls vs sick; FIP vs other diseases, etc.) or in cats with different intensities of inflammatory response was compared using a Chi-square test.

RESULTS

AGP-positive leucocytes were found in 23% of cats. Compared with controls, the proportion of patients with positive granulocytes and monocytes was higher among sick cats (especially cats with diseases other than FIP) and cats with high serum AGP concentration, but not in cats with leucocytosis or that were FCoV-seropositive.

CONCLUSION

AGP-positive leucocytes can be found in feline blood, especially during inflammation. Conversely, no association between AGP-positive leucocytes and FIP was found. Further studies are needed to elucidate the mechanism responsible for this finding and its diagnostic role in cats with inflammation.

Quantification of protein expression changes in the aging left ventricle of Rattus norvegicus

As the heart ages, electrophysiological and biochemical changes can occur, and the ventricle in many cases loses distensibility, impairing diastolic function. How the proteomic signature of the aged ventricle is unique in comparison to young hearts is still under active investigation. We have undertaken a quantitative proteomics study of aging left ventricles (LVs) utilizing the isobaric Tagging for Relative and Absolute Quantification (iTRAQ) methodology. Differential protein expression was observed for 117 proteins including proteins involved in cell signaling, the immune response, structural proteins, and proteins mediating responses to oxidative stress. For many of these proteins, this is the first report of an association with the aged myocardium. Additionally, two proteins of unknown function were identified. This work serves as the basis for making future comparisons of the aged left ventricle proteome to that of left ventricles obtained from other models of disease and heart failure.

Alpha-1-acid Glycoprotein Inhibits Phorbol Ester-induced but not Fc-Receptor-induced Generation of Reactive Oxygen Species in Bovine Peripheral Blood Neutrophils

Alpha-1-acid glycoprotein (AGP) is an acute-phase protein with anti-inflammatory and immunomodulating properties. AGP is described as a potent inhibitor of the production of reactive oxygen species (ROS) in human neutrophils. However, published reports about the mechanism of inhibition are conflicting. The influence of bovine AGP on the production of ROS by bovine peripheral blood polymorphonuclear leucocytes (PMN) was studied using a highly sensitive method approaching its inhibitory mechanism. ROS production in PMN was induced with phorbol 12-myristate 13-acetate (PMA) or opsonized Staphylococcus aureus bacteria. ROS generation was quantified and evaluated by flow cytometry. AGP efficiently suppressed PMA, but did not opsonize bacteria-induced ROS generation in vitro. The suppressive effect was concentration-dependent and adversely proportional to PMA concentration. The selective inhibitory potential of AGP in comparison with ovalbumin (OVA) and bovine serum albumin (BSA) showed that ROS inhibition was not a mere protein effect. ROS production was suppressed only if AGP and PMA were simultaneously present with PMN. Pre-incubation of PMN with AGP did not alter the PMN response to PMA. Moreover, AGP could not suppress ROS production after pre-stimulation of PMN with PMA. Human and bovine AGP did not differ in their inhibitory potential to the PMA-induced ROS production in bovine, human and equine PMN. The results show that AGP does not modulate bovine neutrophil functions directly, but acts as a scavenger of PMA.

Alpha(1)-acid glycoprotein: an acute phase protein with inflammatory and immunomodulating properties

alpha(1)-Acid glycoprotein (AGP) is a protein with a molecular weight of 41-43 kDa and is heavily glycosylated (45%). Due to the presence of sialic acids, it is negatively charged (pI=2.7-3.2). AGP is an acute phase protein in all mammals investigated to date. The serum concentration of AGP rises several fold during an acute phase response, the systemic answer to a local inflammatory stimulus. Also, its glycosylation pattern can change depending on the type of inflammation. The biological function of this protein is not clear. A number of activities on different type of blood cells have been described. In vivo, AGP clearly has protective effects in several models of inflammation. Here we review the data supporting an anti-inflammatory and immunomodulating role of AGP.

Immunocalins: a lipocalin subfamily that modulates immune and inflammatory responses

A subset of the lipocalins, notably alpha(1)-acid glycoprotein, alpha(1)-microglobulin, and glycodelin, exert significant immunomodulatory effects in vitro. Interestingly, all three are encoded from the q32-34 region of human chromosome 9, together with at least four other lipocalins (neutrophil gelatinase-associated lipocalin, complement factor gamma-subunit, tear prealbumin, and prostaglandin D synthase) that also may have anti-inflammatory and/or antimicrobial activity. This review addresses important features of this genetically linked subfamily of lipocalins (involvement with the acute phase response, immunomodulatory and anti-inflammatory properties, the tissue localization, complex formation with other proteins and receptors, etc.). It is likely that these proteins have evolved to be an integrated part of the body's defense system as part of the extended cytokine network. Its members exert a regulatory, dampening influence on the inflammatory cascade, thereby protecting against tissue damage from excessive inflammation. That most major mammalian allergens are lipocalins may reflect this connection of lipocalins with the immune system. We propose that this immunologically active lipocalin subset be named the 'immunocalins', signifying not only the structural homology and close genetic linkage of its members, but also their protective involvement with immunological and inflammatory processes. As immune mediators, immunocalins appear to use at least three interactive sites: the lipocalin 'pocket', binding sites for other plasma proteins, and binding sites for cell surface receptors.

Effects of alpha 1-acid glycoprotein on tissue factor expression and tumor necrosis factor secretion in human monocytes

Activated monocytes express tissue factor (TF) and secrete tumor necrosis factor alpha (TNF alpha), which are important in the initiation of blood coagulation and inflammation. We investigated the effect of alpha 1-acid glycoprotein (alpha 1-AGP), an acute phase protein, on the induction of the expression of TF and the secretion of TNF alpha in human monocytes in vitro. The TF activity of both fresh human monocytes and human monocytic cell line U937 significantly increased in a dose-dependent manner after a 6 h incubation with human or bovine alpha 1-AGP. The activity of TF gradually tailed off after 24 h. RT-PCR and Southern blot analysis revealed that TF mRNA synthesis was induced in monocytes. Inhibition of alpha 1-AGP induced TF expression by actinomycin D (ActD) further support that de novo TF mRNA synthesis was required. The specificity of the alpha 1-AGP-induced TF activity was demonstrated by anti-alpha 1-AGP antibody inhibition. TNF alpha secretion in alpha 1-AGP stimulated monocytes was also increased; this could be blocked by pentoxifylline (PTX). The possible contamination of lipopolysaccharide (LPS) in the alpha 1-AGP was excluded by limulus amoebocyte lysate. Therefore, these results indicate that alpha 1-AGP may contribute to the cellular initiation of coagulation and inflammation by increasing TF expression and TNF alpha secretion of monocytes.

Interleukin-6-type cytokine-induced changes in acute phase protein glycosylation

The plasma levels and the glycosylation of acute-phase proteins (APP) are subject to marked changes during acute and chronic inflammation. The pathophysiological variations in different glycoforms of APP in serum most likely result from changes in the glycosylation process during their biosynthesis in the parenchymal cells of the liver. This is suggested from in vitro studies with isolated hepatocytes and hepatoma cell lines. Inflammatory cytokines appear to regulate the changes in glycosylation independent from the rate of synthesis of the APP. In addition, other humoral factors like corticosteroids and growth factors are involved. The interplay of these factors is determined by the stage of the disease (as in rheumatoid arthritis) or the physiological situation (as in pregnancy). The changes in glycosylation of specific APP might affect the operation of the immune system.

Alpha 1-acid glycoprotein (orosomucoid): pathophysiological changes in glycosylation in relation to its function

The aim of this review is to summarize the research efforts of the last two decades with respect to (i) the determination and characterization of the changes in glycosylation of AGP under various physiological and pathological states; and (ii) the effects of such changes on its possible anti-inflammatory functions. It will become clear that the heterogeneity observed in the glycosylation of AGP in serum, represents various so-called glycoforms of AGP, of which the relative amounts are strictly determined by the (patho) physiological conditions.

Effects of alpha-1 acid glycoprotein on human polymorphonuclear neutrophils: influence of glycan microheterogeneity

The biological functions of alpha-1 acid glycoprotein (AGP) are poorly understood but appear to depend on glycan microheterogeneity. Variations of AGP glycan structure (in terms of concanavalin A (ConA) reactivity) have been observed during the inflammatory process. We studied these modifications in AGP from patients with chronic renal impairment and investigated the effects of AGP microheterogeneity on healthy polymorphonuclear leukocyte (PMN) chemotaxis and oxidative metabolism. AGP was extracted by a two-step procedure from sera from ten patients with various degrees of renal impairment, selected according to AGP glycan heterogeneity determined by crossed immunoaffinity electrophoresis with ConA. AGP (0.5 g/l) significantly inhibited the chemotactic response of PMN to formyl-methionyl-leucyl-phenylalanine (10(-7) mol/l) and complement fraction C5a, regardless of ConA reactivity. AGP also inhibited superoxide anion generation in response to phorbol myristate acetate (10(-7) mol/l). After stimulation by opsonized zymosan (1 g/l), the effect of AGP appeared to depend on its glycan structure (r = 0.70, P < 0.05), decreasing with ConA non-reactivity. These data suggest that AGP can down-regulate neutrophil responsiveness, an effect that depends in part on its glycan microheterogeneity. Alterations of AGP microheterogeneity in various pathological states, particularly renal failure, may be related to the inflammatory process.

Modulation of human polymorphonuclear neutrophil functions by alpha 1-acid glycoprotein

alpha 1-Acid glycoprotein (alpha 1-AGP), a naturally occurring human plasma protein and acute-phase reactant, was extracted by a two-step procedure from sera collected from four healthy men. Its activity was tested in vitro on human polymorphonuclear (PMN) functions (migration, aggregation, O2- generation). alpha 1-AGP was not chemoattractant but inhibited the PMN response to the chemoattractant formylmethionyl-leucyl-phenylalanine without affecting spontaneous migration (Boyden and agarose methods of assessment). At concentrations between 0.15 and 0.45 mg/ml, alpha 1-AGP exerted an aggregating effect with a maximal effective concentration of 0.3 mg/ml. alpha 1-AGP inhibited superoxide generation by PMNs stimulated either by opsonized zymosan or phorbol myristate acetate. This inhibition varied according to the intensity of the stimulation. At low stimulus concentrations, a dose-dependent inhibition of membrane-associated PMN responsiveness to soluble or particulate stimuli was observed. These findings suggest that alpha 1-AGP may be able to prevent PMN activation in the course of inflammatory processes in vivo.