We reported distinct roles for three different PKC isozymes, α, δ, and ϵ, in integrin-mediated muscle cell spreading ( Disatnik et al., 2002). The activation of PKC has been shown to be an important intermediate step in integrin-mediated cell spreading and focal adhesion formation in different cell types ( Woods and Couchman, 1992 Vuori and Ruoslahti, 1993 Schlaepfer et al., 1994 Haimovich et al., 1996).
Myoblasts deficient in α5β1 integrin or myoblasts in which PKC has been inhibited fail to spread on fibronectin ( Disatnik and Rando, 1999). These studies have shown that the binding of α5β1 integrin to its receptor, fibronectin, leads to the activation of protein kinase C (PKC), which is necessary for the cells to adhere and spread. We have studied the mechanisms by which integrin activation promotes adhesion and spreading of muscle cells ( Disatnik and Rando, 1999 Disatnik et al., 2002). These results demonstrate a direct biochemical pathway linking α5β1 integrin signaling to cytoskeletal dynamics and involving bi-directional translocation of MARCKS during the dramatic changes in cellular morphology that occur during cell migration and tissue morphogenesis. All of these processes are directly dependent on the binding of α5β1 integrin to its extracellular matrix receptor, fibronectin.
The dephosphorylation of MARCKS and its translocation back to the membrane permits the later stages of cell spreading during the polymerization and cross-linking of actin and the maturation of the cytoskeleton. Using MARCKS mutants that are defective in membrane association or responsiveness to PKC-dependent phosphorylation, we demonstrate that the translocation of MARCKS from the membrane to the cytosol in a PKC-dependent manner permits the initial phases of cell adhesion. Here we show that myristoylated alanine-rich C-kinase substrate (MARCKS) a specific substrate of protein kinase C (PKC), is regulated by α5β1 integrin-mediated activation of PKC and is critical to the regulation of actin stress fiber formation during muscle cell spreading. Cell-matrix interactions mediated by integrins regulate cytoskeletal dynamics, but the signaling cascades that control these processes remain largely unknown. The regulation of the cytoskeleton is critical to normal cell function during tissue morphogenesis.
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