Rolling and adhesion of apoptotic monocytes is impaired by loss of functional cell surface-expressed P-selectin glycoprotein ligand-1.

Abstract

Apoptosis induces cellular membrane changes that are thought to be linked to thrombotic processes, for example, surface exposure of procoagulant phosphatidylserine (PtdSer), upregulation of tissue factor (TF), and microvesicle formation. The latter, though, could downregulate this cellular response by shedding prothrombotic membrane elements, for example, integrins and TF. To test this hypothesis, etoposide-treated, apoptotic, monocytic cells (human monocytic leukemia cell line [THP-1]) were examined for rolling and adhesion on adherent platelets and for TF expression. Etoposide treatment did not result in a significant change in TF antigen expression. However, TF activity, measured in a continuous factor Xa generation assay, was increased fivefold concomitantly with increased exposure of PtdSer. Laminar flow adhesion assays specific for interaction between P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) revealed that in contrast to non-treated cells, apoptotic cells did not roll or firmly attach on adherent platelets. Lack of apoptotic THP-1 platelet interaction could be attributed to both a loss of cell surface-expressed PSGL-1 and loss of functional PSGL-1 as a result of disruption of the binding of PSGL-1 with the cytoskeleton. Etoposide-induced apoptosis in THP-1 cells evokes a procoagulant response by increasing TF activity associated with an increased PtdSer exposure. However, in contrast to TF, PSGL-1 shedding and loss of function, makes that apoptotic monocytes are unlikely involved in a thrombotic action because of their inability to adhere to an injured vessel wall or developing thrombus.