Abstract
Hageman factor (FXIIa) initiates the intrinsic coagulation pathway and triggers the kallikrein-kinin and the complement systems. In addition, it functions as a growth factor by expressing promitogenic activities toward several cell types. FXIIa binds to the cell surface via a number of structurally unrelated surface receptors; however, the underlying mechanisms are not yet fully understood. Here, we demonstrate that FXIIa utilizes cell membrane-bound glycosaminoglycans to interact with the cell surface of human lung fibroblasts (HLF). The combination of enzymatic, inhibitory, and overexpression approaches identified a heparan sulfate (HS) component of proteoglycans as an important determinant of the FXIIa binding capacity of HLF. Moreover, cell-free assays and competition experiments revealed preferential binding of FXIIa to HS and heparin over dextran sulfate, dermatan sulfate, and chondroitin sulfate A and C. Finally, we demonstrate that fibroblasts isolated from the lungs of the patients suffering from idiopathic pulmonary fibrosis (IPF) exhibit enhanced FXIIa binding capacity. Increased sulfation of HS resulting from elevated HS 6-O-sulfotransferase-1 expression in IPF HLF accounted, in part, for this phenomenon. Application of RNA interference technology and inhibitors of intracellular sulfation revealed the cooperative action of cell surface-associated HS and urokinase-type plasminogen activator receptor in the accumulation of FXIIa on the cell surface of IPF HLF. Moreover, FXIIa stimulated IPF HLF migration, which was abrogated by pretreatment of cells with heparinase I. Collectively, our study uncovers a novel role of HS-type glycosaminoglycans in a local accumulation of FXIIa on the cell membrane. The enhanced association of FXIIa with IPF HLF suggests its contribution to fibrogenesis.
Highlights
Factor XIIa (FXIIa) binds to the cell surface; the underlying mechanism remains underexplored
PGs Mediate FXIIa Binding to the human lung fibroblasts (HLF) Surface—Because FXII receptors do not share a common protein sequence motif that could serve as docking sites on the cell surface, we set out to evaluate whether their carbohydrate moieties may play a role in the FXII binding to the cell membrane
Employing a cell-based ELISA, we found that HLF robustly accommodated FXIIa on the cell surface (Fig. 1A)
Summary
Factor XIIa (FXIIa) binds to the cell surface; the underlying mechanism remains underexplored. It initiates the classical complement pathway and converts prekallikrein to kallikrein, which in turn reciprocally activates FXII and liberates bradykinin from high molecular weight kininogen [3] Both FXII and FXIIa bind to the plethora of negatively charged artificial and physiologically relevant substances, including extracellular matrix components, glycosaminoglycans (GAGs), polyphosphate, extracellular RNA, or misfolded proteins [1, 3]. Putative N-glycosylation sites have been described in gC1qR [2] In view of these findings and taking into account the important role of GAGs in the modulation of FXIIa activity, we speculated that GAG may contribute to the binding of FXIIa to the cell surface. Our data reveal a novel role of cell surface-associated HS in a local accumulation of FXIIa on the cell membrane
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