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

The proliferation of normal non-tumourigenic mouse fibroblasts is stri ngently controlled by regulatory mechanisms located in the postmitotic stage of G(1) (which we have designated G(1)pm). Upon exposure to gro wth factor depletion or a lowered de novo protein synthesis, the norma l cells leave the cell cycle from G(1)pm and enter G(0). The G(1)pm ph ase is characterized by a remarkably constant length (the duration of which is 3 h in Swiss 3T3 cells), whereas the intercellular variabilit y of intermitotic time is mainly ascribable to late G(1) or pre S phas e (G(1)ps) (Zetterberg and Larsson (1985) Proc. Nail. Acad. Sci. USA 8 2, 5365). As shown in the present study two tumour-transformed derivat ives of mouse fibroblasts, i.e. BPA31 and SVA31, did not respond at al l, or only responded partially, respectively, to serum depletion and i nhibition of protein synthesis. If the tumour cells instead were subje cted to 25-hydroxycholesterol (an inhibitor of 3-hydroxy-3 methyglutar yl coenzyme A reductase activity), their growth was blocked as measure d by growth curves and [H-3]-thymidine uptake. Time-lapse analysis rev ealed that the cells were blocked specifically in early G(1) (3-4h aft er mitosis), and DNA cytometry confirmed that the arrested cells conta ined a G(1) amount of DNA. Closer kinetic analysis revealed that the d uration of the postmitotic phase containing cells responsive to 25-hyd roxycholesterol was constant. These data suggest that transformed 3T3 cells also contain a 'G(1)pm program', which has to be completed befor e commitment to mitosis. By repeating the experiments on a large numbe r of tumour-transformed cells, including human carcinoma cells and gli oma cells, it was demonstrated that all of them possessed a G(1)pm-lik e stage. Our conclusion is that G(1)pm is a general phenomenon in mamm alian cells, independent of whether the cells are normal or neoplastic .