We studied whether the acute-phase protein alpha1-acid glycoprotein (AGP) induces rises in [Ca2+]i in neutrophils and sought to identify the corresponding AGP receptor (or receptors). We found that AGP elicited a minimal rise in [Ca2+]i in Fura-2-loaded neutrophils, and this response was markedly enhanced by pretreatment with anti-L-selectin antibodies. (The EC50 value of the AGP-induced Ca2+ response was 9 microg/ml.) Activation of phospholipase-C, Src tyrosine kinases, and PI3 kinases proved to be essential for the AGP-mediated increase in [Ca2+]i, whereas the p38 MAPK and SYK signaling pathways were not involved. Furthermore, antibodies against sialic acid binding, immunoglobulin-like lectin 5 (Siglec-5) and oligosaccharide 3'-sialyl-lactose both antagonized the AGP-induced response and caused an immediate increase in [Ca2+]i in anti-L-selectin-treated neutrophils, which indicates a signaling capacity of Siglec-5. We used modified forms of AGP (treated with mild periodate or neuraminidase) to establish the importance of sialic acid residues. The modified forms of AGP caused a much smaller rise in [Ca2+]i than did unaltered AGP. Affinity chromatography confirmed that unchanged AGP, but not neuraminidase-treated AGP, bound to Siglec-5. Our report provides the first evidence for a signaling capacity by AGP through a defined receptor. Pre-engagement of L-selectin significantly enhanced this signaling capacity.

We recently showed that the acute‐phase protein α1‐acid glycoprotein (AGP) induces rises in cytosolic calcium concentration, [Ca2+]i, in neutrophils through sialic acid dependent interactions with the neutrophil receptors siglec‐5 and/or siglec‐14. Whereas both siglec‐5 and siglec‐14 have a relatively broad specificity for sialylated oligosaccharide structures, including both structures with terminal α2–3 or α2–6 linked sialic acid, there is a markedly reduced affinity to the fucosylated epitope sialyl Lewis x (SLex). Increased fucosylation, leading to increased expression of SLex on AGP is commonly associated with inflammatory conditions. In the present study, we investigated whether an increased SLex expression would affect the Ca2+‐mobilizing effect of AGP. AGP with elevated fucose content isolated from patients with untreated chronic joint inflammation showed a decreased [Ca2+]i modulatory effect on neutrophils compared to normally fucosylated AGP. Furthermore a hyperfucosylated AGP form produced by in vitro fucosylation, that consequently had an elevated expression of SLex, could not elicit a [Ca2+]i increase in neutrophils. The role of the carbohydrate portion of AGP in modulating neutrophil responses was further strengthened by showing that synthetic glycoconjugates carrying oligosaccharides with terminal α2–3 or α2–6 linked sialic acid were able to mimic the Ca2+‐mobilizing effect of AGP whereas a synthetic glycoconjugate carrying SLex was not. Based on these data, we conclude that increased fucosylation can alter the ability of AGP to induce neutrophil signalling and further supports an important role of the oligosaccharide chains of AGP in the modulation of leukocyte functions during an inflammatory process.

alpha(1)-acid glycoprotein (AGP) is an acute-phase protein that contributes to inflammation processes. The role of AGP in platelet activation and thrombosis is, however, largely unknown. Therefore, we thoroughly investigated the effects of AGP on human platelets. Platelets were isolated from healthy volunteers and subsequently exposed to AGP. Platelet responses were monitored as change in light transmission, intracellular calcium concentration, light microscopy and protein phosphorylation by Western blot. We found that AGP induced platelet shape change independently of a second release of adenine nucleotides or thromboxane A(2), and that effect was abolished by endothelium-derived platelet inhibitors such as nitric oxide (NO) and adenosine. Furthermore, AGP triggered a minor calcium response and a pronounced Rho/Rho-kinase-dependent increase in Thr696 phosphorylation of myosin phosphatase target subunit 1 (MYPT1). Moreover, the Rho/Rho-kinase inhibitor Y-27632 significantly decreased the AGP-induced shape change. The results also showed that the AGP-elicited shape change was antagonised by pretreatment with low doses of collagen and thrombospondin-1. Our results describe a novel mechanism by which AGP stimulates platelet shape change via activation of the Rho/Rho-kinase signalling pathway. Physiological important platelet inhibitors, such as NO, completely counterbalance the effect of AGP. Hence, the present study indicates that AGP directly contributes to platelet activation, which in turn might have an impact in physiological haemostasis and/or pathological thrombosis.

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