Thrombin-induced shedding of tumour endothelial marker 5 and exposure of its RGD motif are regulated by cell-surface protein disulfide-isomerase

Abstract

TEM5 (tumour endothelial marker 5; also known as GPR124) is an adhesion G-protein-coupled receptor containing a cryptic RGD motif in its extracellular domain. TEM5 is expressed in endothelial cells and pericytes during angiogenesis. In the present paper, we report that thrombin mediates shedding of an N-terminal TEM5 fragment of 60 kDa (termed N60) containing the RGD motif in an open conformation. Thrombin directly cleaved rsTEM5 (recombinant soluble TEM5) 5 and 34 residues downstream of the RGD motif, resulting in formation of N60 and its C-terminal counterpart (termed C50). Interestingly, N60 derived from thrombin cleavage of rsTEM5 was covalently linked to C50 by disulfide bonds, whereas N60 shed from thrombin-treated cells was not associated with its membrane-bound C-terminal counterpart. Inhibition of the reducing function of cell-surface PDI (protein disulfide-isomerase) abrogated thrombin-induced N60 shedding. Conversely, addition of reduced PDI enhanced N60 shedding. Furthermore, thrombin cleavage of rsTEM5 was increased by reduced PDI and resulted in dissociation of the N60–C50 heterodimer. We conclude that PDI regulates thrombin-induced shedding of N60 and exposure of the TEM5 RGD motif by catalysing the reduction of crucial disulfide bonds of TEM5 on the cell surface. Binding of N60 to RGD-dependent integrins may modulate cellular functions such as adhesion and migration during angiogenesis.