Activated mouse N‐ras gene transfected into PC12 rat pheochromocytoma cells suppressed proliferation and promoted neuronal differentiation. Normal mouse N‐ras in a LTR‐containing vector caused differentiation with a reduced efficiency, but normal N‐ras in a vector lacking LTR sequences failed to alter the PC12 phenotype. Cultures of NGF‐resistant PC12 variant subline U7 also showed outgrowth of neurites and cessation of cell division following transfection with the mutated ras gene. The present findings suggest that ras genes can, in certain cells, play a role in promoting differentiation and suppressing proliferation, in contrast to their established oncogenic neoplasiapromoting activity in other cells.

NGF is a protein with many profound, well-described (Levi-Montalcini 1966) actions on developing sensory and sympathetic neurons. To study the mechanism of action of NGF, we have employed an NGF-responsive clonal cell line — designated PC12 — which was isolated from a transplantable rat adrenal pheochromocytoma (Greene and Tischler 1976). The properties and experimental advantages of the PC12 line have been reviewed (Greene and Tischler 1982). In the absence of NGF, PC12 cells resemble their presumed non-neoplastic counterparts, noradrenergic adrenal chromaffin cells. After exposure to NGF, the cells slowly (i.e., over a time course of days) take on the phenotypic properties of sympathetic-like neurons so that, for instance, they cease proliferation, extend long neurites and become electrically excitable (Greene and Tischler 1982). Since, unlike sympathetic neurons, PC12 cells respond to, but do not require NGF for survival, they have the experimental advantage that they can be compared before and after various times of exposure to NGF.