Regulation of NIH-3T3 cell G1 phase transit by serum during exponential growth.

The proliferation rate of mammalian cells is regulated normally in the G1 phase of the cell cycle. During this phase, it is convenient to assign positive and negative roles to the molecular programs that regulate the duration of G1 and the phase transition from G1 to S phase. Density-dependent inhibition of cellular proliferation results in an increase in the duration of G1. This form of regulation is due to both secreted factors and cell-cell contact. Serum is mitogenic to a variety of mammalian cell types. Because quiescent cells enter S phase as a result of serum addition to culture media, serum is usually regarded as a source of positive regulatory growth factors. We have measured the length of the G1, S and G2+M phases of NIH 3T3 cells during exponential growth as a function of cell density and serum concentration. The G1 length increases during exponential growth as a function of density while S and G2+M are relatively constant. Further, this increase in G1 phase time, or density mediated negative regulation, is inhibited by increasing serum concentration. This phenotype is saturable between 10% to 20% serum. Serum concentrations above 2.5% are able to increase the rate of cell cycling (decrease the G1 phase time) by inhibiting density dependent negative regulation of NIH 3T3.