New insights into regulation of αIIbβ3 integrin signaling by filamin A

Background: Filamin (FLN) regulates many cell functions through its scaffolding activity cross-linking cytoskeleton and integrins. FLN was shown to inhibit integrin activity, but the exact mechanism remains unclear. Objectives: The aim of this study was to evaluate the role of filamin A (FLNa) subdomains on the regulation of integrin αIIbβ3 signaling. Methods: Three FLNa deletion mutants were overexpressed in the erythro-megakaryocytic leukemic cell line HEL: Del1, which lacks the N-terminal CH1-CH2 domains mediating the FLNa-actin interaction; Del2, lacking the Ig-like repeat 21, which mediates the FLNa-β3 interaction; and Del3, lacking the C-terminal Ig repeat 24, responsible for FLNa dimerization and interaction with the small Rho guanosine triphosphatase involved in actin cytoskeleton reorganisation. Fibrinogen binding to HEL cells in suspension and talin-β3 proximity in cells adherent to immobilized fibrinogen were assessed before and after αIIbβ3 activation by the protein kinase C agonist phorbol 12-myristate 13-acetate. Results: Our results show that FLNa-actin and FLNa-β3 interactions negatively regulate αIIbβ3 activation. Moreover, FLNa-actin interaction represses Rac activation, contributing to the negative regulation of αIIbβ3 activation. In contrast, the FLNa dimerization domain, which maintains Rho inactive, was found to negatively regulate αIIbβ3 outside-in signaling. Conclusion: We conclude that FLNa negatively controls αIIbβ3 activation by regulating actin polymerization and restraining activation of Rac, as well as outside-in signaling by repressing Rho.