INTEGRIN-DEPENDENT ACTIVATION OF MAP KINASE - A LINK TO SHAPE-DEPENDENT CELL-PROLIFERATION

Adhesion to extracellular matrix mediates cell cycle progression in mi d-late G1; this effect involves an integrin-dependent organization of the cytoskeleton and a consequent change in cell shape. In an effort t o identify potential signal-transducing agents that are associated wit h integrin-dependent shape changes, we looked for kinase activities th at were stimulated by long-term adhesion of G0-synchronized NIH-3T3 ce lls to fibronectin-coated dishes. Several kinase activities were stimu lated by this procedure, two of which migrated at 42 and 44 kDa and ph osphorylated myelin basic protein in vitro. Blotting with anti-phospho tyrosine and anti-mitogen-activated protein (MAP) kinase antibodies id entified these enzymes as ERK 1 and ERK 2. In contrast to the rapid an d transient activation of these MAP kinases by platelet-derived growth factor, stimulation of MAP kinase activity by fibronectin was gradual persistent, and associated with cell spreading rather than cell attac hment itself. Cytochalasin D blocked the activation of MAP kinase acti vity that was induced by the binding of cells to fibronectin. Moreover , MAP kinase was also activated by adhesion of cells to vitronectin an d type IV collagen; these effects were also associated with cell sprea ding. These results distinguish the regulation of G1 phase MAP kinase activity by soluble mitogens and extracellular matrix. They also impli cate MAP kinase in shape-dependent cell cycle progression.