ENDOTOXIC LIPID A INDUCES BINDING OF FIBRINOGEN TO HUMAN PLATELETS VIA PROTEIN KINASE C PATHWAY

Abstract

Endotoxic Lipid A is the biologically active principle of lipopolysaccharide of Gram-negative bacteria, a most frequent cause of sepsis underlying Disseminated Intravascular Coagulation (DIC) and shock. We have shown that endotoxic Lipid A activates Protein Kinase C in human platelets. Phosphorylation of a 47kDa protein (P47), a marker for Protein Kinase C activation, was observed within the first minute of interaction of Lipid A with platelets. This was accompanied by gradual exposure of the receptor for 125I-labeled fibrinogen (F). Binding of 125I-F was saturable and specific. When Lipid X, a precursor of endotoxic Lipid A and its competitive inhibitor, was used, the binding of 125I-F was blocked with 50% inhibition at a 1:1 stoichiometry between Lipid X and Lipid A. At the same time, phosphorylation of P47 was prevented. Since Lipid X constitutes a "half molecule" of Lipid A, we interpret these results as indicative of competitive blocking of endotoxic Lipid A in terms of Protein Kinase C activation and exposure of platelet receptors for fibrinogen. Binding of fibrinogen is necessary for platelet aggregation and endotoxic Lipid A-induced aggregation was also blocked by Lipid X. Endotoxic Lipid A-induced exposure of fibrinogen receptors via the Protein Kinase C pathway can contribute to involvement of platelets in microcirculatory thrombosis observed in patients with DIC and Gram-negative sepsis