Induction of beta 3-integrin gene expression by sustained activation of the Ras-regulated Raf-MEK-extracellular signal-regulated kinase signaling pathway

Alterations in the expression of integrin receptors for extracellular matri x (ECM) proteins are strongly associated with the acquisition of invasive a nd/or metastatic properties by human cancer cells. Despite this, comparativ ely little is known of the biochemical mechanisms that regulate the express ion of integrin genes in cells. Here we demonstrate that the Ras-activated Raf-MEK-extracellular signal-regulated kinase (ERK) signaling pathway can s pecifically control the expression of individual integrin subunits in a var iety of human and mouse cell lines. Pharmacological inhibition of MEK1 in a number of human melanoma and pancreatic carcinoma cell lines led to reduce d cell surface expression of alpha6- and beta3-integrin. Consistent with th is, conditional activation of the Raf-MEK-ERK pathway in NIH 3T3 cells led to a 5 to 20-fold induction of cell surface alpha6- and beta3-integrin expr ession, Induced beta3-integrin was expressed on the cell surface as a heter odimer with alphav-integrin; however, the overall level of alphav-integrin expression was not altered by Ras or Raf. Raf-induced beta3-integrin was ob served in primary and established mouse fibroblast lines and in mouse and h uman endothelial cells. Consistent with previous reports of the ability of the Raf-MEK-ERK signaling pathway to induce beta3-integrin gene transcripti on in human K-562 erythroleukemia cells, Raf activation in NIH 3T3 cells le d to elevated beta3-integrin mRNA. However, unlike immediate-early Raf targ ets such as heparin binding epidermal growth factor and Mdm2, beta3-integri n mRNA was induced by Raf in a manner that was cycloheximide sensitive. Sur prisingly, activation of the Raf-MEK-ERK signaling pathway by growth factor s and mitogens had little or no effect on beta3-integrin expression, sugges ting that the expression of this gene requires sustained activation of this signaling pathway. In addition, despite the robust induction of cell surfa ce alphav beta3-integrin expression by Raf in NIH 3T3 cells, such cells dis play decreased spreading and adhesion, with a loss of focal adhesions and a ctin stress fibers. These data suggest that oncogene induced alterations in integrin gene expression may participate in the changes in cell adhesion a nd migration that accompany the process of oncogenic transformation.