The role of the annexin A2 heterotetramer in vascular fibrinolysis

The vascular endothelial cells line the inner surface of blood vessels and function to maintain blood fluidity by producing the protease plasmin that removes blood clots from the vasculature, a process called fibrinolysis. Plasminogen receptors play a central role in the regulation of plasmin activity. The protein complex annexin A2 heterotetramer (AIIt) is an important plasminogen receptor at the surface of the endothelial cell. AIIt is composed of 2 molecules of annexin A2 (ANXA2) bound together by a dimer of the protein S100A10. Recent work performed by our laboratory allowed us to clarify the specific roles played by ANXA2 and S100A10 subunits within the AIIt complex, which has been the subject of debate for many years. The ANXA2 subunit of AIIt functions to stabilize and anchor S100A10 to the plasma membrane, whereas the S100A10 subunit initiates the fibrinolytic cascade by colocalizing with the urokinase type plasminogen activator and receptor complex and also providing a common binding site for both tissue-type plasminogen activator and plasminogen via its C-terminal lysine residue. The AIIt mediated colocalization of the plasminogen activators with plasminogen results in the rapid and localized generation of plasmin to the endothelial cell surface, thereby regulating fibrinolysis.

Identification of the annexin A2 heterotetramer as a receptor for the plasmin-induced signaling in human peripheral monocytes

We have previously demonstrated that plasmin acts as a potent proinflammatory activator of human peripheral monocytes. Here we identify the annexin A2 heterotetramer, composed of annexin A2 and S100A10, as a receptor for the plasmin-induced signaling in human monocytes. Monocytes express the annexin A2 heterotetramer on the cell surface as shown by flow cytometry, fluorescence microscopy, and coimmunoprecipitation of biotinylated cell surface proteins. Binding of plasmin to annexin A2 and S100A10 on monocytes was verified by biotin transfer from plasmin labeled with a trifunctional cross-linker. Antibodies directed against annexin A2 or S100A10 inhibited the chemotaxis elicited by plasmin, but not that induced by fMLP. Further, down-regulation of annexin A2 or S100A10 in monocytes by antisense oligodeoxynucleotides impaired the chemotactic response to plasmin, but not that to fMLP. Antisense oligodeoxynucleotides similarly decreased the TNF-α release by plasmin-stimulated, but not by LPS-stimulated, monocytes. At the molecular level, stimulation with plasmin, but not with catalytically inactivated plasmin, induced cleavage of annexin A2 and dissociation of the heterotetramer complex. Substitution of lysine to alanine in position 27 abolished the cleavage of recombinant annexin A2 in vitro. Together, these data identify the annexin A2 heterotetramer as a signaling receptor activated by plasmin via proteolysis.