Large induction of c-Myc is not essential for the mitogenic response of Swiss 3T3 fibroblasts

Quiescent Swiss 3T3 fibroblasts can be stimulated to reenter the cell cycle following stimulation with growth factors. Among these, bombesin is a potent mitogen for Swiss 3T3 cells and can act synergistically with insulin to stimulate DNA synthesis through protein kinase C-independent pathways. One of the earliest nuclear responses of quiescent cells treated with a combination of bombesin and insulin is a dramatic increase in c-Myc expression, and it has been suggested that this proto-oncogene plays a central role in the mitogenic response. In the present study, we have taken two approaches to study the relationship between c-Myc expression and the reinitiation of DNA synthesis. First, low concentrations of bombesin, in the presence of insulin, stimulated DNA synthesis in Swiss 3T3 fibroblasts in the absence of a large increase in c-myc mRNA or protein levels. Second, selective down-regulation of phorbol ester-inducible protein kinase C in Swiss 3T3 cells resulted in a 90% decrease in the induction of c-myc mRNA and an 80% reduction in Myc protein expression but did not affect the mitogenic response to bombesin and insulin. These observations were confirmed in detailed dose-response and time-course experiments. We conclude that the large induction of c-Myc is not an essential event for the entry of Swiss 3T3 fibroblasts into S phase. Quantitation of Myc protein levels using a sensitive ELISA indicated that quiescent cells could enter S phase with only 450 c-Myc molecules per cell. These results indicate that cells in the G0 phase of the cell cycle can be stimulated to reinitiate DNA synthesis with only marginal increases in Myc protein expression.