Summary
α1-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 A2, and that effect was abolished by endotheliumderived 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 whichAGP stimulates
platelet shape change via activation of the Rho/Rhokinase 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.
Keywords
α
1-acid glycoprotein - platelets - Rho-kinase - shape change