EXISTENCE OF A COMMITMENT PROGRAM FOR MITOSIS IN EARLY G(1) IN TUMOR-CELLS

The proliferation of normal non-tumourigenic mouse fibroblasts is stringently controlled by regulatory mechanisms located in the postmitotic stage of G(1) (which we have designated G(1)pm). Upon exposure to growth factor depletion or a lowered de novo protein synthesis, the normal cells leave the cell cycle from G(1)pm and enter G(0). The G(1)pm phase is characterized by a remarkably constant length (the duration of which is 3 h in Swiss 3T3 cells), whereas the intercellular variability of intermitotic time is mainly ascribable to late G(1) or pre S phase (G(1)ps) (Zetterberg and Larsson (1985) Proc. Nail. Acad. Sci. USA 82, 5365). As shown in the present study two tumour-transformed derivatives of mouse fibroblasts, i.e. BPA31 and SVA31, did not respond at all, or only responded partially, respectively, to serum depletion and inhibition of protein synthesis. If the tumour cells instead were subjected to 25-hydroxycholesterol (an inhibitor of 3-hydroxy-3 methyglutaryl coenzyme A reductase activity), their growth was blocked as measured by growth curves and [H-3]-thymidine uptake. Time-lapse analysis revealed that the cells were blocked specifically in early G(1) (3-4h after mitosis), and DNA cytometry confirmed that the arrested cells contained a G(1) amount of DNA. Closer kinetic analysis revealed that the duration of the postmitotic phase containing cells responsive to 25-hydroxycholesterol was constant. These data suggest that transformed 3T3 cells also contain a 'G(1)pm program', which has to be completed before commitment to mitosis. By repeating the experiments on a large number of tumour-transformed cells, including human carcinoma cells and glioma cells, it was demonstrated that all of them possessed a G(1)pm-like stage. Our conclusion is that G(1)pm is a general phenomenon in mammalian cells, independent of whether the cells are normal or neoplastic.