P-selectin glycoprotein ligand-1 (PSGL-1), a glycoprotein expressed on the cell surface of leukocytes, binds to selectins and mediates leukocyte rolling on the vascular endothelium. Here we report that PSGL-1 binds to the C-terminal (G3 domain) of the extracellular proteoglycan PG-M/versican. Cells transfected with PSGL-1 or a shorter form containing the binding site, or cells expressing endogenous PSGL-1 aggregate in the presence of versican or G3 product. The aggregation appears to be induced by G3 multimers that bind to PSGL-1 and form a network. Endogenous versican and/or G3-containing fragments also bind to PSGL-1 in human plasma. Removal of the endogenous G3-containing fragments reduces the effect of plasma on leukocyte aggregation. Finally, the roles of G3-containing fragments in leukocyte aggregation were confirmed in a mouse model. Taken together, our results strongly support a physiologically relevant role for PSGL-1/versican binding and may have implications in the immunoresponse.

Versican is highly expressed in many types of tumors. In a previous study, we found that a G3 mutant [G3ΔEGF; a versican G3 domain lacking two epidermal growth factor (EGF)-like motifs] exerted a dominant-negative effect on versican secretion and binding. Here, we report that astrocytoma U87 cells expressing the versican G3 mutant lost the hallmark of cell transformation and tumorigenesis in vitro and in vivo. U87 cells expressing G3ΔEGF had enhanced cell adhesion and spreading, but lost the tumor characteristic of anchorage-independent growth. When U87 cells were deprived of serum, FAK was quickly dephosphorylated, integrin/EGF-receptor (EGFR) complexes dissociated and the cells retained an appropriate level of EGFR phosphorylation. These cells quickly detached, migrated, rounded, reorganized and survived. However, after serum withdrawal from G3ΔEGF-transfected U87 cells, sustained FAK phosphorylation and integrin-EGFR association were observed, but a greatly reduced EGFR phosphorylation. These cells remained spread and continued to grow before undergoing massive apoptosis. The addition of EGF promoted U87 cell rounding but had little effect on G3ΔEGF-transfected cells owing to reduced EGFR phosphorylation. Our study sheds light on the question of how the matrix molecule versican modulates tumorigenesis by affecting integrin and EGFR signals.