Human group II 14 kDa phospholipase A2 activates human platelets

Area of Science:

• Biochemistry
• Cell Biology
• Hematology

Background:

• Previous studies indicated phospholipase A2 (sPLA2) could not activate platelets.
• Platelet activation is crucial in hemostasis and thrombosis.
• Understanding novel platelet agonists is vital for cardiovascular research.

Purpose of the Study:

• To investigate the direct effects of recombinant human group II phospholipase A2 (sPLA2) on human platelet activation.
• To elucidate the mechanisms and requirements for sPLA2-mediated platelet responses.

Main Methods:

• Measurement of platelet aggregation, thromboxane A2 generation, intracellular calcium (Ca2+) influx, and protein tyrosine phosphorylation.
• Testing the effects of Ca2+, Mg2+, albumin, indomethacin, heparitinase, and phosphatidylinositol-specific phospholipase C on sPLA2-induced platelet activation.

Main Results:

• Low concentrations of sPLA2 induced human platelet activation, including aggregation, thromboxane A2 release, Ca2+ influx, and tyrosine phosphorylation.
• Platelet activation by sPLA2 was dependent on extracellular Ca2+ and inhibited by Mg2+.
• sPLA2-induced aggregation was blocked by pre-exposure to sPLA2, albumin, or indomethacin.
• Platelet responsiveness to sPLA2 was abolished by heparitinase or phosphatidylinositol-specific phospholipase C, suggesting a glycophosphatidylinositol-anchored heparan sulfate proteoglycan as the binding site.

Conclusions:

• Recombinant human group II phospholipase A2 (sPLA2) is a potent activator of human platelets.
• A specific platelet membrane heparan sulfate proteoglycan mediates sPLA2 binding and subsequent activation.
• The inhibitory effects of albumin and Mg2+ in previous studies may have obscured the platelet-activating capacity of sPLA2.