M. Sitahar et al., THE PHOSPHOLIPASE-C PROTEIN-KINASE-C PATHWAY IS INVOLVED IN CATHEPSING-INDUCED HUMAN PLATELET ACTIVATION - COMPARISON WITH THROMBIN, Biochemical journal, 313, 1996, pp. 401-408
Cathepsin G, an enzyme released by stimulated polymorphonuclear neutro
phils, and thrombin are two human proteinases which potently trigger p
latelet activation. Unlike thrombin, the mechanisms by which cathepsin
G initiates platelet activation have yet to be elucidated. The involv
ement of the phospholipase C (PLC)/protein kinase C (PKC) pathway in c
athepsin G-induced activation was investigated and compared with stimu
lation by thrombin. Exposure of 5-[C-14]hydroxytryptamine-labelled pla
telets to cathepsin G, in the presence of acetylsalicylic acid and pho
sphocreatine/creatine kinase, induced platelet aggregation and degranu
lation in a concentration-dependent manner (0.1-3.0 mu M). Time-course
studies (0-180 s) comparing equivalent concentrations of cathepsin G
(3 mu M) and thrombin (0.5 unit/ml) resulted in very similar transient
hydrolysis of phosphatidylinositol 4,5-bisphosphate and steady accumu
lation of phosphatidic acid. In addition cathepsin G, like thrombin, i
nitiated the production of inositol phosphates. The neutrophil-derived
proteinase also induced phosphorylation of both the myosin light chai
n and pleckstrin, a substrate for PKC, to levels similar to those obse
rved in platelets challenged with thrombin. Inhibition of PKC by GF 10
9203X, a specific inhibitor, suppressed platelet aggregation and degra
nulation to the same extent for both proteinases. Using fura 2-loaded
platelets, the rise in the cytosolic free Ca2+ concentration induced b
y cathepsin G was shown to result, as for thrombin, from both mobiliza
tion of internal stores and Ca2+ entry across the plasma membrane. The
se findings provide evidence that cathepsin G stimulates the PLC/PKC p
athway as potently as does thrombin, independently of thromboxane A(2)
formation and ADP release, and that this pathway is required for plat
elet functional responses.