ORNITHINE DECARBOXYLASE - AN INDICATOR FOR GROWTH OF NIH 3T3 FIBROBLASTS AND THEIR C-HA-RAS TRANSFORMANTS

Growth rates of different clones, all derived from NM 3T3 mouse fibrob lasts, were determined. Four different types of cells were studied: (1 ) Normal NIH 3T3 fibroblasts; (2) a fast-growing NIH 3T3 clone obtaine d by repeated passages; (3) transformed clones (obtained by transfecti ng NIH 3T3 with the oncogene c-Ha-ras); (4) a slow-growing revertant o btained by repeated passages of the transformed line. Growth rates wer e determined by the following markers of proliferation: thymidine inco rporation, protein accumulation and cell number. In parallel experimen ts growth rates were determined by a new approach based on measuring o rnithine decarboxylase (ODC) activity. Transformed cells, which were c haracterized by phase-contrast microscopy and by electron microscopy g rew rapidly and showed high ODC activity. Similarly, a high-passage NI H 3T3 variant, which grew rapidly, also possesed high ODC activity. On the other hand, high-passage of a transformed clone revealed phenotyp ic changes confirmed by electron microscopy. These cells exhibited red uced growth rates and their ODC activities were similar to those of th e normal NIH 3T3 cells. A confident correlation was found among each o f the three conventional parameters of growth and between them and ODC activity. However, in all the cases studied ODC activity appeared ear ly in the cell cycle before the expression of the other markers of pro liferation. It has been suggested that ODC is a reliable early marker of cell proliferation and might also serve as an important tool for de termining the arrest of growth.