Cell-cycle regulation of human diploid fibroblasts (HDF) is located in the proximal half of G I , designated G1-pm (G1-postmitosis). In order to traverse this subphase, cells require serum factors or PDGF. However, when cells have traversed into the distal half of G I , designated G1-ps (G1-pre-DNA synthesis), they become independent of serum or PDGF and progress through the remainder of the cell cycle at an invariable rate. From this, it follows that a specific G1-pm block can be induced by serum depletion. A similar G1-pm block could also be induced by a moderate inhibition of overall protein synthesis following treatment with CHM. Even this block could be prevented by the addition of PDGF, suggesting that a high level of protein synthesis in itself is not necessary for sustaining cell-cycle traverse. Nevertheless, a critical accumulation of some specific proteins might be required for the G1-pm/G1-ps-transition. However, the underlying mechanisms of modulation of the accumulation of such proteins by PDGF must involve alternative regulatory events (e.g., gene expression, protein stabilization) rather than protein synthesis. Among the possible cell cycleregulatory proteins, the present study focused on 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase. This enzyme is regulated by various kinds of control mechanisms and regulates the biosynthesis of sterols and nonsterol isoprenes, some of which are proposed to be necessary for mammalian cell growth (Brown and Goldstein, 1980). The present results suggest that regulation of HMG CoA reductase may be involved in the control of the G1-pm/ GI-ps-progression in HDF.
In a recent cell-cycle-kinetic study carried out on Swiss 3T3-cells, the authors demonstrated that the GI period is subdivided into a postmitotic growth factordependent phase (G1-pm) of constant length and a pre-DNA-synthetic growth factor-independent phase (G1-ps) of variable length (Zetterberg and Larsson, 1985). The experiments were performed by exposing time-lapse filmed cells to serum-free medium for various periods. This enabled an accurate determination of cell-cycle position (cell age) and generation time of individual cells. In brief, the data showed that cells located in G1-pm were inhibited from traversing into G1-ps as a result of serum starvation. Upon serum repletion, cells reentered the cell cycle, but these were considerably delayed in division time. These data suggest that the G1-pm cells were temporarily arrested in a quiescent state (Go) from which they could be stimulated back into the cell cycle if serum was readded.
However, data derived from experiments of a heteroploid cell line may be criticized as being influenced by the transformed phenotype, which is not necessarily representative of normal cells. Therefore, a similar analysis was done on human diploid fibroblasts (HDF). The role of de novo protein synthesis in the cell-cycle control of HDF was also investigated. Special interest was focused on the influence of platelet-derived growth factor (PDGF).
MATERIALS AND METHODS Chemicals
Radiochemicals were obtained from Amersham (Sweden). PDGF was kindly donated by Carl-