Activation of protein kinase C precedes alpha 5 beta 1 integrin-mediated cell spreading on fibronectin

Increasing evidence indicates that the integrin family of cell adhesion receptors can transduce biochemical signals from the extracellular matrix to the cell interior to modulate cell behavior. We have investigated the role of protein kinase C in alpha 5 beta 1 integrin-mediated signal transduction in Chinese hamster ovary cells. Up-regulation of protein kinase C activity by phorbol esters was found to enhance cell adhesion, spreading, and migration on a fibronectin substrate, without affecting the cell surface expression or fibronectin binding of the alpha 5 beta 1 integrin, whereas inhibition of protein kinase C activity by calphostin C inhibited these functions as well in unstimulated cells. In addition, we observed that protein kinase C activity in the cell membrane fraction transiently increases preceding cell spreading on fibronectin, but not on polylysine, additionally implying a specific role of protein kinase C in alpha 5 beta 1 integrin-mediated spreading on fibronectin. Experiments with phorbol esters and calphostin C also suggested that protein kinase C activity is required for enhanced phosphorylation of the focal adhesion kinase, pp125fak, in cells plated on fibronectin. Protein kinase C-alpha was not, however, found to directly act on pp125fak, suggesting that other mechanisms are involved in this phenomenon.