High-Level Expression of c-H-ras1 Fails to Fully Transform Rat-1 Cells

Abstract

Rat-1 cells were transfected with plasmids encoding normal (Gly-12), nonactivated (Pro-12), and activated (Val-12 and Ile-12) p21^H-ras in the presence of an amplifiable dihydrofolate reductase marker. The introduced DNA was amplified by selection in methotrexate to establish the relationship between p21^H-ras expression and various hallmarks of cellular transformation. The maximum level of p21^H-ras (Gly-12) consistent with cell viability was approximately 0.13% of total cell protein (~60,000 molecules per cell); this is 44-fold greater than the level of the endogenous protein. The maximum tolerated level of a second nontransforming form of p21^H-ras (pro-12) was about half of this. Amplification in Rat-1 cells of H-ras genes encoding the highly oncogenic Val-12 and Ile-12 forms of p21^H-ras could not be achieved by methotrexate selection, providing strong evidence that synthesis of activated p21^H-ras above a certain threshold (about 0.02% of total protein) in Rat-1 cells is incompatible with cell viability. Individual cell lines were isolated and their morphology, anchorage-independent growth, tumorigenicity, and response to and production of growth factors were studied. We report that cell lines expressing near-maximum tolerated levels of either the normal or pro-12 form of p21^H-ras were not as transformed as cells expressing much more modest levels of the highly oncogenic (Val-12) form, suggesting that the complete elaboration of the transformed phenotype by ras depends, at least in part, on mutations that distinguish the cellular and viral proteins. We found that cells expressing elevated levels of the normal p21^H-ras could be fully transformed by the activated (Val-12) form and that such cells continued to overexpress p21^H-ras (Gly-12), arguing against a role for normal ras genes in suppression of the oncogenic potential of their mutationally activated counterparts.