CONCOMITANT ACTIVATION OF MEK-1 AND RAC-1 INCREASES THE PROLIFERATIVEPOTENTIAL OF THYROID EPITHELIAL-CELLS, WITHOUT AFFECTING THEIR DIFFERENTIATION

Activating point mutations in the Ras oncogene occur in a large number of human tumors, especially of epithelial origin. In thyroid follicul ar cells, ectopic expression of oncogenic H-Ras results in growth fact or-independent proliferation, loss of differentiation and tumor format ion in nude mice. In fibroblasts concomitant activation of the MAP kin ase cascade and the small GTPase Rac-1 leads to full malignant transfo rmation. We have tested the effects of these key downstream mediators of Pas in thyroid epithelial cells, by stably expressing either a cons titutively active form of MEK-1 (MEKDelta N3/S218E/S222D), a constitut ively active form of Rac-1 (Val12-Rac), or both. While the activation of one molecule or the other results in a weak phenotype, concomitant activation of both MEK-1 and Rac-1 in thyroid cells leads to growth fa ctor-independent proliferation, morphological transformation and ancho rage-independent growth. However, in contrast to Pas-transformed thyro id cells, the ones expressing the constitutively active forms of MEK-1 and Rac-1 maintain their differentiate phenotype and fail to form tum ors when injected into nude mice. Thus, in thyroid epithelial cells, c oncomitant activation of MEK-1 and Rac-1 can reproduce only a subset o f the Ras-induced effects and is not sufficient to cause full malignan t transformation. Significantly, Pas-mediated increased proliferation and loss of differentiation can be dissociated in these cells.