Abstract
1. Matrix metalloproteinase-2 (MMP-2) released during activation of human platelets by aggregating agents and cancer cells is known to stimulate platelet aggregation. 2. The expression, activity and role of tissue inhibitors of metalloproteinases (TIMPs), natural inhibitors of MMPs, in isolated human platelets were investigated. 3. Western blot, reverse zymography, immunogold electron microscopy, aggregometry (collagen-, thrombin and HT-1080 human fibrosarcoma cells-induced aggregation), flow cytometry and the release of (14)C-serotonin from labelled platelets recruited to the aggregate were used to characterize the presence and function of platelet TIMPs. 4. TIMP-4 (23 kDa) has been identified as the major MMP inhibitor (12-16 ng per 10(8) platelets) in human platelets. Platelets expressed lower (<1 ng per 10(8) platelets) amounts of TIMP-1. No other TIMPs were detected using Western blot analysis. 5. TIMP-4 co-localized with MMP-2 in resting platelets and was released upon platelet aggregation induced by collagen and thrombin. 6. Collagen resulted also in the release of higher molecular weight (60 kDa) complexes of TIMP-4. 7. The release of TIMP-4 was reduced by prostacyclin and S-nitroso-glutathione (GSNO), an NO donor. 8. Human recombinant TIMP-4 (rTIMP-4), but not human rTIMP-1, inhibited partially both platelet aggregation and recruitment. 9. The recombinant TIMP-4 potentiated the recruitment inhibitor effects of GSNO. 10. TIMP-4 was not released during platelet aggregation induced by HT-1080 cells. 11. Human rTIMP-4 exerted a biphasic effect on HT-1080 cells-induced aggregation. 12. Thus, TIMP-4 is the major intraplatelet MMP inhibitor and it is involved in regulation of platelet aggregation and recruitment.