Phosphorylated Pleckstrin Induces Cell Spreading via an Integrin-Dependent Pathway

Pleckstrin is a 40-kD phosphoprotein containing NH₂- and COOH-terminal pleckstrin homology (PH) domains separated by a disheveled-egl 10-pleckstrin (DEP) domain. After platelet activation, pleckstrin is rapidly phosphorylated by protein kinase C. We reported previously that expressed phosphorylated pleckstrin induces cytoskeletal reorganization and localizes in microvilli along with glycoproteins, such as integrins. Given the role of integrins in cytoskeletal organization and cell spreading, we investigated whether signaling from pleckstrin cooperated with signaling pathways involving the platelet integrin, αIIbβ3. Pleckstrin induced cell spreading in both transformed (COS-1 & CHO) and nontransformed (REF52) cell lines, and this spreading was regulated by pleckstrin phosphorylation. In REF52 cells, pleckstrin-induced spreading was matrix dependent, as evidenced by spreading of these cells on fibrinogen but not on fibronectin. Coexpression with αIIbβ3 did not enhance pleckstrin-mediated cell spreading in either REF52 or CHO cells. However, coexpression of the inactive variant αIIbβ3 Ser753Pro, or β3 Ser753Pro alone, completely blocked pleckstrin-induced spreading. This implies that αIIbβ3 Ser753Pro functions as a competitive inhibitor by blocking the effects of an endogenous receptor that is used in the signaling pathway involved in pleckstrin-induced cell spreading. Expression of a chimeric protein composed of the extracellular and transmembrane portion of Tac fused to the cytoplasmic tail of β3 completely blocked pleckstrin-mediated spreading, whereas chimeras containing the cytoplasmic tail of β3 Ser753Pro or αIIb had no effect. This suggests that the association of an unknown signaling protein with the cytoplasmic tail of an endogenous integrin β-chain is also required for pleckstrin-induced spreading. Thus, expressed phosphorylated pleckstrin promotes cell spreading that is both matrix and integrin dependent. To our knowledge, this is the first example of a mutated integrin functioning as a dominant negative inhibitor.