Phenotypic modulation of arterial smooth muscle coils is associated with prolonged activation of Erk1/2

Arterial smooth muscle cells grown in primary culture on a substrate of fib ronectin in serum-free medium are converted from a contractile to a synthet ic phenotype. This process is dependent on integrin signaling and includes a major structural reorganization with loss of myofilaments and formation o f a large secretory apparatus. Functionally, the cells lose their contracti lity and become competent to migrate, secrete extracellular matrix componen ts, and proliferate in response to growth factor stimulation. Here, it is d emonstrated that the mitogen-activated protein kinases ERK1/2 play a vital role in the fibronectin-mediated modification of rat aortic smooth muscle c ells. Immunoblotting showed that phosphorylated ERK1/2 (p44/p42) were expre ssed throughout the period when the change in phenotypic properties of the cells took place. Moreover, phosphorylated ERK1/2 accumulated in the nucleu s as revealed by immunocytochemical staining. Additional support for an act ive role of ERK1/2 in the shift in smooth muscle phenotype was obtained by the finding that PD98059, an inhibitor of the upstream kinase MEK1, potentl y suppressed both the expression of phosphorylated ERK1/2 and the fine stru ctural rebuilding of the cells. In conclusion, the observations point to an important and multifaceted role of ERK1/2 in the regulation of differentia ted properties and growth of vascular smooth muscle cells.