LPAM-1 (integrin α4β7)-ligand binding: overlapping binding sites recognizing VCAM-1, MAdCAM-1 and CS-1 are blocked by fibrinogen,a fibronectin-like polymer and RGD-like cyclic peptides

The α4 integrin LPAM-1 (α4β7) mediates lymphocyte attachment within the extracellular matrix (ECM) by adhering to the connecting segment (CS)-1 site of fibronectin (FN). Here we reveal that very late antigen (VLA)-4⁻ LPAM-1⁺ T cell lymphoma TK-1 cells bind via LPAM-1 to multiple copies of the RGD sequence engineered within an FN-like polymer. Further, the small conformationally restrained RGD-like cyclic peptides 1-adamantaneacetyl-Cys-Gly-Arg-Gly-Asp-Ser-Pro-Cys and Arg-Cys-Asp-thioproline-Cys inhibit the adhesion of TK-1 cells to immobilized CS-1 peptide, and to endothelial counterreceptors for LPAM-1, namely mucosal addressin cell adhesion molecule (MAdCAM)-1 and vascular cell adhesion molecule (VCAM)-1. Spontaneous adhesion of the VLA-4⁻ LPAM-1⁺ B lymphoma cell line RPMI 8866 to CS-1 was likewise inhibited, confirming a previously undocumented ability of LPAM-1 to recognize the RGD tripeptide. The RGD-binding site in LPAM-1 either overlaps or is identical to sites required for interaction with MAdCAM-1, VCAM-1, and the CS-1. The binding of LPAM-1 and VLA-4 to RGD-containing ligands may have relevance in vivo given that fibrinogen at physiological concentrations is able to partially block the binding of TK-1 cells to MAdCAM-1. Hence fibrinogen and other vascular RGD-containing proteins may have mild anti-inflammatory activity required for maintaining effective homeostasis, analogous to the anti-thrombogenic activity of the vascular endothelium.