Recognition of the N-terminal modules of thrombospondin-1 and thrombospondin-2 by α6β1 integrin

In addition to its recognition by alpha(3)beta(1) and alpha(4)beta(1) integrins, the N-terminal pentraxin module of thrombospondin-1 is a ligand for alpha(6)beta(1) integrin. alpha(6)beta(1) integrin mediates adhesion of human microvascular endothelial and HT-1080 fibrosarcoma cells to immobilized thrombospondin-1 and recombinant N-terminal regions of thrombospondin-1 and thrombospondin-2. alpha(6)beta(1) also mediates chemotaxis of microvascular cells to thrombospondin-1 and thrombospondin-2. Using synthetic peptides, LALERKDHSG was identified as an alpha(6)beta(1)-binding sequence in thrombospondin-1. This peptide inhibited alpha(6)beta(1)-dependent cell adhesion to thrombospondin-1, thrombospondin-2, and the E8 fragment of murine laminin-1. The Glu residue in this peptide was required for activity, and the corresponding residue (Glu(90)) in the N-terminal module of thrombospondin-1 was required for its recognition by alpha(6)beta(1), but not by alpha(4)beta(1). alpha(6)beta(1) was also expressed in human umbilical vein endothelial cells; but in these cells, only certain agonists could activate the integrin to recognize thrombospondins. Selective activation of alpha(6)beta(1) integrin in microvascular endothelial cells by the anti-beta(1) antibody TS2/16 therefore accounts for their adhesion responses to thrombospondins and explains the distinct functions of alpha(4)beta(1) and alpha(6)beta(1) integrins as thrombospondin receptors in microvascular and large vessel endothelial cells.