Activated human neutrophils rapidly release the chemotactically active D2D3 form of the urokinase-type plasminogen activator receptor (uPAR/CD87)

The urokinase-type plasminogen activator receptor (uPAR/CD87) exists both in cell-bound and soluble forms. Neutrophils contain extensive intracellular pools of uPAR that are translocated to the plasma membrane upon activation. In the present study, we investigated the ability of human neutrophils to shed uPAR from cell surface following activation and addressed the possible involvement of the released receptor in the inflammatory response. We first observed that the spontaneous release of suPAR by resting neutrophils was strongly and rapidly (within minutes) enhanced by calcium ionophore ionomycin and to a lesser extent when cells were primed with TNF-α and then stimulated with fMLP or IL-8. We demonstrated that suPAR is produced by resting and activated neutrophils predominantly as a truncated form devoid of N-terminal D1 domain (D2D3 form) that lacks GPI anchor. Migration of formyl peptide receptor-like 1 (FPRL1)-transfected human embryonic kidney (HEK) 293 cells toward the supernatants harvested from activated neutrophils was significantly diminished when D2D3 form of suPAR was immunodepleted from the supernatants. We conclude that activated neutrophils release the chemotactically active D2D3 form of suPAR that acts as a ligand of FPRL1. Interestingly, we present evidence that GPI-specific phospholipase D (GPI-PLD) that has previously been shown to shed uPAR in cancer cells is not involved in suPAR release from human neutrophils. We suggest that production of the chemotactically active D2D3 form of suPAR by activated human neutrophils in vivo could contribute to the recruitment of monocytes and other formyl peptide receptors-expressing cells to the sites of acute inflammation where neutrophil accumulation and activation occur.