Distinct involvement of Cdc42 and RhoA GTPases in actin organization and cell shape in untransformed and Dbl oncogene transformed NIH3T3 cells

The Dbl oncogene is a putative exchange factor for the small GTPases RhoA a nd Cdc42, which are involved in actin polymerization into stress fibers and filopodia, respectively. We report here that, upon adhesion to fibronectin , Dbl-transformed NIH3T3 cells display a contracted, polygonal shape with a high number of short stress fibers. In contrast, untransformed NIH3T3 cell s acquire the characteristic fibroblast morphology and organize a regular m esh of long stress fibers. We show that in Dbl-transformed and in untransfo rmed NIH3T3 cells the different shape and actin cytoskeleton organization o bserved in the early steps of adhesion involves activation of distinct GTPa ses, Upon adhesion to fibronectin, cell morphology of Dbl-transformed NIH3T 3 cells depends on activation of RhoA and not of Cdc42, In contrast Cdc42 a ctivation is necessary to untransfected NIH3T3 cells to acquire their fibro blast shape. In both Dbl-transformed and in untransformed NIH3T3 cells a ba sal Rac activation is necessary to support stress fiber organization, while constitutive Rac activation promotes ruffles and lamellipodia formation. A s a consequence of RhoA activation, Dbl-transformed cells show high activit y of ROCK-alpha and CRIK kinases, two known RhoA effecters. In addition Dbl -transformed and NIH3T3 cells expressing the constitutive active form of Rh oA are less motile on fibronectin than cells expressing constitutive active Cdc42, We conclude that in NIH3T3 cells in response to fibronectin the exp ression of the Dbl oncogene leads to a predominant activation of RhoA which both supports the peculiar cell shape and actin cytoskeleton organization in stress fibers and regulates cell motility.