Cytoplasmic tails of integrin αIIbβ3 in the regulation of integrin activation, cell adhesion and spreading

Integrins are major adhesion receptors for the extracellular matrix (ECM). This thesis focuses on the motifs and interactions within integrin cytoplasmic tails during integrin-mediated cell adhesion and spreading. The present study investigated the significance of the skelemin-αIIbβ3 interaction using Chinese Hamster Ovary (CHO) cells expressing wild-type or mutant αIIbβ3 receptors defective in skelemin binding. Most mutant cells displayed unimpaired adhesive capacity and spreading on immobilized fibrinogen at the early stages of cell spreading. In addition, they formed normal focal adhesions and stress fibers with no indication of impaired cell spreading. K716A and H722A mutant cells exhibited the greatest cell spreading, which was associated with enhanced p-Src activation. The K716 residue appeared to be the most important for skelemin binding in previous in vitro studies. Here, the protrusions of the leading edge of K716A cells showed strong colocalization of talin with αIIbβ3 which was associated with a loss in skelemin binding. These data suggest that the binding of skelemin to αIIbβ3 is not essential for normal cell spreading, but may act to exert contractile forces on cell spreading and coordinate the binding of talin to the membrane proximal region of integrin tails. The functional mode of peptides corresponding to the central motifs of the αIIb and αV tail, KRNRPPLEED (αIIb peptide) and KRVRPPQEEQ (αV peptide) was also investigated. Both peptides inhibited Mn2+-activated αIIbβ3 binding to soluble fibrinogen as well as the binding of αIIbβ3-expressing CHO cells to immobilized fibrinogen. Breast cancer progression has been linked to tumor cell interaction with ECM. Our αIIb and αV peptides also inhibited adhesion of two breast cancer cell lines (MDA-MB-435 and MCF7) to αV integrin ECM ligand vitronectin. Replacement of RPP with AAA significantly attenuated the inhibitory activity of the αIIb peptide. β-tubulin was identified as a potential αIIb peptide-binding partner, suggesting that microtubule cytoskeleton may participate in the regulation of integrin functions. These results provide insights into the mechanisms by which the central motifs of αIIb and αV tails regulate integrin activation and integrin-mediated cell adhesio