SIGNALING PATHWAYS IN RAS-MEDIATED TUMORIGENICITY AND METASTASIS

The effector domain mutants of oncogenic Ras, V12S35 Ras, V12G37 Ras, and V12C40 Ras were tested for their abilities to mediate tumorigenic and metastatic phenotypes in athymic nude mice when expressed in NIH 3 T3 fibroblasts. All mutants displayed comparable tumorigenic propertie s, but only the mutant that activates the Raf mitogen-activated protei n kinase kinase (MEK)-extracellular regulated kinase (ERK) 1/2 pathway , V12S35 Ras, induced tumors in the experimental metastasis assay. Fur thermore, direct activation of the MEK-ERK1/2 pathway in NIH 3T3 cells by mos or a constitutively active form of MEK was sufficient to induc e metastasis whereas R-Ras, which fails to activate the ERK1/2 pathway , is tumorigenic but nonmetastatic, The subcutaneous tumors and lung m etastases derived from V12S35 Ras-transformed NIH 3T3 cells expressed higher levels of activated ERK1/2 in culture when compared with the pa rental cellular pool before injection, indicating that selection for c ells with higher levels of activated ERK1/2 occurred during tumor grow th and metastasis. By contrast, cells explanted from V12G37-Ras or V12 C40-Ras-induced tumors did not show changes in the level of ERK1/2 act ivation when compared with the parental cells. When tumor-explanted ce ll lines derived from each of the effector domain mutants were passage d one additional time in vivo, all mediated rapid tumor growth, but, a gain, only cells derived from V12S35 Ras-tumors formed numerous metast atic lesions within the lung. These results show that the metastatic p roperties of the Ras effector domain mutants segregate, and that, wher eas Ras-mediated tumorigenicity can arise independently of ERK1/2 acti vation, experimental metastasis appears to require constitutive activa tion of the ERK1/2 pathway.