Adhesion of Bovine Airway Smooth Muscle Cells Activates Extracellular Signal-regulated Kinases

Extracellular signal-regulated kinases (ERKs) phosphorylate and regulate cytoskeletal components of contractile cells and have been implicated in integrin-mediated adhesion. In this study, we examined the contributions of adherence, cell flattening, and cytoskeletal reorganization to adhesion-induced ERK activation in cultured bovine tracheal myocytes. We found, as evidenced by a reduction in electrophoretic mobility, that adhesion to fibronectin induced phosphorylation of both p44ERK1 and p42ERK2. In-gel kinase assays confirmed activation of both p44ERK1 and p42ERK2 in fibronectin-adherent cells, consistent with the notion that ligand-integrin binding is required for adhesion-induced ERK activation. However, ERK activation was maximal 2-4 h after plating, and adherence to either polystyrene or poly-L-lysine also caused ERK activation (fold increase 4 h after plating: fibronectin, 3.75 +/- 0.33; polystyrene, 3.95 +/- 0.78; poly-L-lysine, 2.14 +/- 0.36). Inspection of myocytes following passage onto fibronectin showed near 100% adhesion and cell spreading after 4 h, whereas cells plated onto poly-L-lysine demonstrated adherence but minimal spreading. To test whether the cytoskeletal reorganization accompanying cell spreading is required for adhesion-induced ERK activation, we assessed ERK activity following pretreatment with cytochalasin D, an inhibitor of actin polymerization. Cytochalasin inhibited both cell spreading and ERK activation following adhesion to fibronectin, but had no effect on growth factor-induced ERK activation in adherent cells. We conclude that adhesion-induced ERK activation in bovine tracheal myocytes may occur independently of ligand-integrin binding and is primarily related to the cell spreading that follows adhesion.