Release of matrix metalloproteinases-1 and -2, but not -9, from activated platelets measured by enzyme-linked immunosorbent assay.

Matrix metalloproteinases (MMPs), in particular MMP-9, have been introduced as novel biomarkers in coronary artery disease. Activated platelets are considered to be a major source of the highly elevated levels of MMPs that are detected in serum compared to plasma. The aim of this study was to clarify if activated platelets release MMPs-1, -2 and -9 as measured by enzyme-linked immunosorbent assays (ELISA). Isolated platelets (separated by several procedures) or platelet-rich plasma (PRP) were stimulated by collagen, thrombin or the TLR2 agonist Pam(3)CSK(4). The concentrations of MMPs-1,-2 and -9 in supernatants were determined by ELISA. In addition, a MMP-9 enzyme activity assay was used as well as immunofluorescent staining of MMPs-1,-2 and 9 in platelets. Isolated platelets stimulated by collagen, thrombin or Pam(3)CSK(4) released significant amounts of MMP-1 to the supernatant measured as either pro- or total-MMP-1. However, there was no detectable release of MMP-2 or -9 from isolated platelets. Collagen-stimulated platelets in PRP released MMP-2, but not -9. Before stimulation; platelets were positive for MMPs-1 and -2, but not -9, as assessed by immunofluorescence. Acting as positive controls, neutrophils were found to release significant amounts of MMP-9. Our findings indicate that activated platelets may be a major source of MMP-1 and to a minor extent MMP-2, in peripheral blood. However, in contrast to what has been argued in previous literature, platelets appear to be only negligible contributors to circulating MMP-9.