HMGB1 binds to activated platelets via the receptor for advanced glycation end products and is present in platelet rich human coronary artery thrombi.

Abstract

High mobility group box 1 (HMGB1) acts as both a nuclear protein that regulates gene expression, as well as a pro-inflammatory alarmin that is released from necrotic or activated cells. Recently, HMGB1-expression in human atherosclerotic plaques was identified. Therapeutic blockade of HMGB1 reduced the development of diet-induced atherosclerosis in ApoE knockout mice. Thus, we hypothesised an interaction between HMGB1 and activated platelets. Binding of recombinant HMGB1 to platelets was assessed by flow cytometry. HMGB1 bound to thrombin-activated human platelets (MFI 2.49 vs 25.01, p=0.0079). Blood from wild-type, TLR4 and RAGE knockout mice was used to determine potential HMGB1 receptors on platelets. HMGB1 bound to platelets from wild type C57Bl6 (MFI 2.64 vs 20.3, p< 0.05), and TLR4-/- mice (MFI 2.11 vs 25.65, p< 0.05) but failed to show binding to platelets from RAGE-/- mice (p > 0.05). RAGE expression on human platelets was detected by RT-PCR with mRNA extracted from highly purified platelets and confirmed by Western blot and immunofluorescence microscopy. Platelet activation increased RAGE surface expression (MFI 4.85 vs 6.74, p< 0.05). Expression of HMGB1 in human coronary artery thrombi was demonstrated by immunohistochemistry and revealed high expression levels. Platelets bind HMGB1 upon thrombin-induced activation. Platelet specific expression of RAGE could be detected at the mRNA and protein level and is involved in the binding of HMGB1. Furthermore, platelet activation up-regulates platelet surface expression of RAGE. HMGB1 is highly expressed in platelet-rich human coronary artery thrombi pointing towards a central role for HMGB1 in atherothrombosis, thereby suggesting the possibility of platelet targeted anti-inflammatory therapies for atherothrombosis.