Expression of c-myc and induction of DNA synthesis by platelet-poor plasma in human diploid fibroblasts.

When WI-38 human diploid fibroblasts become confluent, they stop synthesizing DNA and dividing. Addition of serum causes the quiescent cell to reenter the cell cycle. Prolonged quiescence after confluence decreases and delays the response to serum. For a few days after reaching confluence, WI-38 cells also respond to platelet-poor plasma. During this period, although not cycling, WI-38 cells still express c-myc and other growth-regulated genes, as measured by steady-state RNA levels. If the quiescence is prolonged further, c-myc expression (and that of two other growth-regulated genes) is no longer detectable, and its disappearance coincides with a loss of response to platelet-poor plasma. These results suggest that, also under physiological conditions, the expression of c-myc and other growth-regulated genes can cooperate with platelet-poor plasma in inducing cellular DNA synthesis in human diploid fibroblasts.