Summary
Histones are highly alkaline proteins found in cell nuclei and they can be released
by either dying or inflammatory cells. The recent observations that histones are major
components of neutrophil extracellular traps and promote platelet aggregation and
platelet-dependent thrombin generation have shown that these proteins are potent prothrombotic
molecules. Because the mechanism(s) of platelet activation by histones are not completely
understood, we explored the ability of individual recombinant human histones H1, H2A,
H2B, H3 and H4 to induce platelet activation as well as the possible molecular mechanisms
involved. All histones were substrates for platelet adhesion and spreading and triggered
fibrinogen binding, aggregation, von Willebrand factor release, P-selectin and phosphatidylserine
(PS) exposure and the formation of platelet-leukocyte aggregates; however, H4 was
the most potent. Histone-mediated fibrinogen binding, P-selectin and PS exposure and
the formation of mixed aggregates were potentiated by thrombin. Histones induced the
activation of ERK, Akt, p38 and NFkB. Accordingly, histone-induced platelet activation
was significantly impaired by pretreatment of platelets with inhibitors of ERK (U
0126), PI3K/Akt (Ly 294002), p38 (SB 203580) and NFkB (BAY 11–7082 and Ro 106–9920).
Preincubation of platelets with either aspirin or dexamethasone markedly decreased
fibrinogen binding and the adhesion mediated by histones without affecting P-selectin
exposure. Functional platelet responses induced by H3 and H4, but not H1, H2A and
H2B, were partially mediated through interaction with Toll-like receptors –2 and –4.
Our data identify histones as important triggers of haemostatic and proinflammatory
platelet responses, and only haemostatic responses are partially inhibited by anti-inflammatory
drugs.
keywords
Histones - platelets - neutrophil extracellular traps - inflammation