Contrasting morphological changes in PC12 flat cells expressing two different forms of exogenous oncogenic ras

Oncogenic ras is known to transform certain cells, whereas it induces terminal differentiation of others, e.g., neuronal differentiation of PC12 cells. MPT1 is a PC12 flat cell variant that extends glial-like processes and exhibits some properties of noncancer cells in culture, e.g., absence of anchorage-independent growth. Expression of oncogenic ras by MPT1 cells failed to result in neuronal differentiation, but such cells exhibited two contrasting morphological changes under certain conditions. First, they retained their extended processes in the presence of dexamethasone, unlike MPT1 cells not expressing oncogenic ras. Second, confluent cultures of ras-expressing MPT1 cells contained foci of transformed-looking cells that were refractile and grew in multiple layers. Thus, ras seemed to induce both a kind of differentiation and transformation of MPT1 cells. MPT1 cells were transfected with a plasmid carrying an oncogenic Harvey ras gene under transcriptional control of the metallothionein promoter. Two subclones of the transfected cells exhibited different responses to the induction of ras and expressed two different forms of the ras gene product. One clone extended dexamethasone-resistant processes and the second clone exhibited a more transformed phenotype. The ras gene product expressed in these two clones differed in migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and in the extent of phosphorylation. These results suggest that ras protein phosphorylation may be important in determining whether a ras-mediated response is differentiation or transformation.