Soluble jagged 1 represses the function of its transmembrane form to induce the formation of the Src-dependent chord-like phenotype

We have previously demonstrated that the expression of the soluble extracel lular domain of the transmembrane ligand for Notch receptors, Jagged 1 (sJ1 ), in NIH 3T3 cells results in the formation of a matrix-dependent chord-li ke phenotype, the loss of contact inhibition of growth, and an inhibition o f pro-al(I) collagen expression. In an effort to define the mechanism by wh ich sJ1 induces this phenotype, we report that sJ1 transfectants display bi ochemical and cytoskeletal alterations consistent with the activation of Sr c. Indeed, cotransfection of sJ1 transfectants with a dominant-negative mut ant of Src resulted in the loss of matrix-dependent chord formation and cor related with the restoration of type I collagen expression and contact inhi bition of growth. We also report that the sJ1-mediated induction of Src act ivity and related phenotypes, including chord formation, may result from. t he inhibition of endogenous Jagged 1-mediated Notch signaling since it was not possible to detect an sJ1-dependent induction of CSL-dependent transcri ption in these cells. Interestingly, NIH 3T3 cells transfected with dominan t-negative (but not constitutively active) mutants of either Notch I or Not ch 2 displayed a similar Src-related phenotype as the sJ1 transfectants. Th ese data suggest that the ability, of sJ1 to mediate chord formation is Src -dependent and requires the repression of endogenous Jagged 1-mediated Notc h signaling, which is tolerant to the destabilization of the actin cytoskel eton, a mediator of cell migration.