Manganese-induced integrin affinity maturation promotes recruitment of αVβ3 integrin to focal adhesions in endothelial cells: evidence for a role of phosphatidylinositol 3-kinase and Src

Integrin activity is controlled by changes in affinity (i.e. ligand binding) and avidity (i.e. receptor clustering). Little is known, however, about the effect of affinity maturation on integrin avidity and on the associated signaling pathways.To study the effect of affinity maturation on integrin avidity, we stimulated human umbilical vein endothelial cells (HUVEC) with MnCl2 to increase integrin affinity and monitored clustering of β1 and β3 integrins. In unstimulated HUVEC, s1 integrins were present in fibrillar adhesions, while αVβ3 was detected in peripheral focal adhesions. Clustered β1 and β3 integrins expressed high affinity/ ligand-induced binding site (LIBS) epitopes. MnCl2-stimulation promoted focal adhesion and actin stress fiber formation at the basal surface of the cells, and strongly enhanced mAb LM609 staining and expression of β3 high affinity/LIBS epitopes at focal adhesions.MnCl2-induced αVβ3 clustering was blocked by a soluble RGD peptide, by wortmannin and LY294002, two parmacological inhibitors of phosphatidylinositol 3-kinase (PI 3-K), and by over-expressing a dominant negative PI 3-K mutant protein. Conversely, over-expression of active PI 3-K and pharmacological inhibiton of Src with PP2 and CGP77675, enhanced basal and manganese-induced αVβ3 clustering. Transient increased phosphorylation of protein kinase B/Akt, a direct target of PI 3K, occurred upon manganese stimulation. MnCl2 did not alter β1 integrin distribution or β1 high-affinity/ LIBS epitope expression. Based on these results, we conclude that MnCl2-induced αVβ3 integrin affinity maturation stimulates focal adhesion and actin stress fiber formation, and promotes recruitment of high affinity αVβ3 to focal adhesions. Affinity-modulated αVβ3 clustering requires PI3-K signaling and is negatively regulate by Src.