Lysophosphatidic acid is the unique platelet-activating substance in human malignant ascites

Pathological blood platelet activation promotes thrombosis in cancer patients, but the specific substances involved are still under investigation. Tumor exudates have been described to contain lysophosphatidic acid (LPA), a known platelet-activating substance, and the concentration of mediators present in malignant ascites are constantly equilibrated with the concentration in plasma. We hypothesized that the ascites of cancer patients might activate platelets, and that this may be caused by LPA. Indeed, ascites samples from 15 different patients with cancer induced shape change and an increase of cytosolic Ca2+ of isolated platelets; both responses were cross-desensitized by lysophosphatidic acid (LPA), but not by other platelet stimuli. Moreover shape change, Ca2+ mobilization and aggregation induced by ascites could be completely blocked by pretreatment of platelets with specific LPA-receptor antagonists. Phospholipids were extracted from ascites, separated by thin layer chromatography, and individual fractions were tested for activity on platelets. The platelet activating substance co-migrated with LPA, whereas other fractions were inactive. Notably, ascites induced through LPA-receptor activation platelet aggregation in whole blood. Our results suggest that LPA plays an essential role in the pathological platelet activation in cancer patients. We propose that LPA receptor antagonists could be effective in blocking cancer-associated platelet activation and thus preventing thrombosis.