title: Platelets retain high levels of active plasminogen activator inhibitor 1.
abstract: The vascular fibrinolytic system is crucial for spontaneous lysis of blood clots. Plasminogen activator inhibitor 1 (PAI-1), the principal inhibitor of the key fibrinolytic enzyme tissue-type plasminogen activator (tPA), is present in platelets at high concentrations. However, the majority of PAI-1 stored in platelets has been considered to be inactive. Our recent finding (Brogren H, et al. Blood 2004) that PAI-1 de novo synthesized in platelets remained active for over 24 h, suggested that PAI-1 stored in the α-granules might be active to a larger extent than previously reported. To re-evaluate this issue, we performed experiments where the fraction of active PAI-1 was estimated by analyzing the tPA-PAI-1 complex formation. In these experiments platelets were lysed with Triton X-100 in the presence of serial dilutions of tPA and subsequently the tPA-PAI-1 complex was evaluated by Western blot. Also, using a non-immunologic assay, tPA was labeled with (125)I, and (125)I-tPA and (125)I-tPA-PAI-1 was quantified by scintigraphy. Interestingly, both methods demonstrated that the majority (>50%) of platelet PAI-1 is active. Further analyses suggested that pre-analytical procedures used in previous studies (sonication or freezing/thawing) may have substantially reduced the activity of platelet PAI-1, which has lead to an underestimation of the proportion of active PAI-1. Our in vitro results are more compatible with the role of PAI-1 in clot stabilization as demonstrated in physiological and pathophysiological studies.
authors: Wallenberg Laboratory for Cardiovascular Research, Institute of Medicine, University of Gothenburg, Gothenburg, Sweden. helen.brogren@gu.se
authors: Helén
authors: H
authors: Brogren
authors: Karin
authors: K
authors: Wallmark
authors: Johanna
authors: J
authors: Deinum
authors: Lena
authors: L
authors: Karlsson
authors: Sverker
authors: S
authors: Jern
mesh_terms: D001792:Blood Platelets / Q000378:metabolism*; D015153:Blotting, Western; D004797:Enzyme-Linked Immunosorbent Assay; D005342:Fibrinolysis; D006801:Humans; D007457:Iodine Radioisotopes; D017830:Octoxynol; D017395:Plasminogen Activator Inhibitor 1 / Q000378:metabolism*; D015539:Platelet Activation / Q000502:physiology*; D010976:Platelet Count; D010959:Tissue Plasminogen Activator / Q000378:metabolism*
chemical_list: D007457:Iodine Radioisotopes; D017395:Plasminogen Activator Inhibitor 1; D017830:Octoxynol; D010959:Tissue Plasminogen Activator