A role for plasmin in platelet aggregation: Differential regulation of IGF release from IGF–IGFBP complexes?

Abstract Objectives To determine if plasmin differentially augments platelet aggregation through variable efficiencies of IGF–IGFBP complex cleavage. Methods We utilized ADP-triggered platelet aggregation assays to test the effects of IGF-I versus IGF-II in complex with IGFBP-2 or IGFBP-3 upon the efficiency of plasmin (a known IGFBP protease) as a pro-aggregatory stimulus. In vitro proteolysis assays were performed as controls. Results We found that IGF-I complexes augmented platelet-mediated aggregation whereas IGF-II either had no effect (IGFBP-2) or inhibited platelet-mediated aggregation (IGFBP-3). In vitro proteolysis assays of IGFBP-2 and IGFBP-3 using plasmin revealed that three of the four aggregation findings were explained by the disparate efficiencies of IGFBP proteolysis associated with each IGF. Only IGF-II–IGFBP-2 complex resulted in a finding that could not be explained by the concept of differential regulation of plasmin’s proteolysis efficiency by the two IGF ligands. Conclusions Our findings demonstrate that the plasmin can differentially modulate platelet aggregation in response to intrinsic heterogeneities within the IGF axis.