Hyperthermia inhibits platelet hemostatic functions and selectively regulates the release of alpha-granule proteins

Background: Hyperthermia is one of the main disturbances of homeostasis occurring during sepsis or hyper-metabolic states such as cancer. Platelets are important mediators of the inflammation that accompanies these processes, but very little is known about the changes in platelet function that occur at different temperatures. Objectives: To explore the effect of higher temperatures on platelet physiology. Methods: Platelet responses including adhesion, spreading (fluorescence microscopy), alpha(IIb)beta(3) activation (flow cytometry), aggregation (turbidimetry), ATP release (luminescence), thromboxane A(2) generation, alpha-granule protein secretion (ELISA) and protein phosphorylation from different signaling pathways (immunoblotting) were studied. Results: Preincubation of platelets at temperatures higher than 37 degrees C (38.5-42 degrees C) inhibited thrombin-induced hemostasis, including platelet adhesion, aggregation, ATP release and thromboxane A(2) generation. The expression of P-selectin and CD63, as well as vascular endothelial growth factor (VEGF) release, was completely inhibited by hyperthermia, whereas von Willebrand factor (VWF) and endostatin levels remained substantially increased at high temperatures. This suggested that release of proteins from platelet granules is modulated not only by classical platelet agonists but also by microenvironmental factors. The observed gradation of response involved not only antiangiogenesis regulators, but also other cargo proteins. Some signaling pathways were more stable than others. While ERK1/2 and AKT phosphorylation were resistant to changes in temperature, Src, Syk, p38 phosphorylation and IkappaB degradation were decreased in a temperature-dependent fashion. Conclusions: Higher temperatures, such as those observed with fever or tissue invasion, inhibit the hemostatic functions of platelets and selectively regulate the release of alpha-granule proteins.