Modulation of human platelet protein kinase C by endotoxic lipid A.
Lipid A is the toxic principle of lipopolysaccharide of gram-negative bacteria, which causes a spectrum of changes in blood cells and vascular cells. We now report that human platelets are directly stimulated by endotoxic lipid A that activates protein kinase C. Rapid phosphorylation of a human platelet protein of Mr 47,000, a marker of protein kinase C activation, accompanies secretion of [14C]serotonin and aggregation triggered by endotoxic lipid A. These events are time and concentration dependent, with phosphorylation reaching maximum in 2 min and the concentration of lipid A causing a 50% effect (EC50) between 12 and 15 microM. Phospholipase C activation in lipid A-stimulated platelets was not observed as judged by a lack of generation of [3H]diacylglycerol in [3H]arachidonic acid-labeled platelets and a lack of generation of [32P]-phosphatidic acid in 32PO4-labeled platelets. Lipid A did not induce formation of TXA2 as measured by radioimmunoassay for TXB2. The stimulation of human platelets and activation of protein kinase C by endotoxic lipid A was blocked by lipid X, a structural precursor of lipid A. Lipid X also blocked the stimulation of human platelets by phorbol 12-myristate 13-acetate, suggesting that lipid A, lipid X and phorbol ester share reactive site(s) on the human platelet membrane. Although lipid X inhibited thrombin-induced phosphorylation of P47 it did not suppress secretion of [14C]serotonin, indicating the role of protein kinase C-independent pathways in platelet stimulation by thrombin. The inhibitory effect of lipid X did not involve generation of cyclic AMP in human platelet membrane preparations. These results indicate that human platelets are stimulated by endotoxic lipid A, a naturally occurring biologic modifier of protein kinase C. Due to the widespread presence of this enzyme in blood cells, vascular cells, and neurons, its modulation by lipid A may represent a significant mechanism underlying hematologic and circulatory derangements observed in endotoxic shock in humans.
D001792:Blood Platelets / Q000201:enzymology* / Q000378:metabolism; D001798:Blood Proteins / Q000378:metabolism; D004731:Endotoxins / Q000494:pharmacology*; D004789:Enzyme Activation; D004926:Escherichia coli / Q000502:physiology; D006017:Glycolipids / Q000494:pharmacology; D006801:Humans; D008050:Lipid A / Q000494:pharmacology*; D010750:Phosphoproteins; D010766:Phosphorylation; D010974:Platelet Aggregation / Q000187:drug effects; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D011493:Protein Kinase C / Q000097:blood*; D012701:Serotonin / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013930:Thromboxane-A Synthase; D010738:Type C Phospholipases
D001798:Blood Proteins; D004731:Endotoxins; D006017:Glycolipids; D008050:Lipid A; D010750:Phosphoproteins; D010975:Platelet Aggregation Inhibitors; C039077:platelet protein P47; D012701:Serotonin; C044283:lipid X; D011493:Protein Kinase C; D010738:Type C Phospholipases; D013930:Thromboxane-A Synthase; D013755:Tetradecanoylphorbol Acetate