Abstract
BACKGROUND: At the site of vascular injury, monocytes (MN) interacting with activated platelets (PLT) synthesize tissue factor (TF) and promote thrombus formation. Intracellular signals necessary for the expression of TF in MN, in the context of a developing thrombus, remain unknown. OBJECTIVE: The study was designed to investigate the role of the glycogen synthase kinase 3 (GSK3, a serine-threonine kinase) downstream insulin receptor pathway, in PLT-induced TF expression in MN. METHODS: To this purpose we used a well-characterized in vitro model of human MN-PLT interactions that allows detailed analysis of TF activity, TF protein and gene expression. RESULTS: The results demonstrated that, in MN interacting with activated PLT: (i) TF activity, antigen and mRNA were low until 8-10 h and dramatically increased thereafter, up to 24 h; (ii) according to the kinetics of TF expression in MN, GSK3β undergoes phosphorylation on serine 9, a process associated with down-regulation of enzyme activity; (iii) pharmacological blockade of GSK3 further increased TF expression and was accompanied by increased accumulation of NF-kB, in the nucleus; (iv) blockade of phosphoinositide-3 kinase (PI(3)K) by wortmannin inhibited PLT-induced TF expression; and (v) according to the established role of the GSK3 downstream insulin receptor, insulin increased PLT-induced TF expression in a PI(3)K-dependent manner. CONCLUSION: GSK3 acts as a molecular brake on the signaling pathway, leading to TF expression in MN interacting with activated PLT. PI(3)K, through Akt-dependent phosphorylation of GSK3, relieves this brake and allows TF gene expression. This study identifies a novel molecular link between thrombotic risk and metabolic disorders.