The labile nature of the insulin signal(s) for the stimulation of DNA synthesis in mouse lens epithelial and 3T3 cells

A kinetic study was carried out to assess the stability of the intracellular signal(s) generated by insulin in quiescent cells for the stimulation of DNA synthesis. Using murine lens epithelial cells and Swiss 3T3 cells in culture, it was found that insulin stimulated DNA synthesis after a lag of 14.5 h. If, however, 6 h after the addition of insulin to the cells, the insulin-containing media were totally removed, followed by the addition of fresh media (even if insulin was returned to the medium within approximately 10 min), a 14.5-h lag still remained after insulin readdition before DNA synthesis started. In another set of experiments, the insulin was removed after 6 h by diluting its concentration approximately 60,000-fold. In this case, if insulin was at the diluted concentration for approximately 60 min before being added back, a full 14.5 h was necessary for the start of DNA synthesis. The half-time for loss of signal was 2 +/- 1 min for total washout and 18.4 +/- 0.5 min for the dilution experiment. These results indicate that the intracellular signal(s) for DNA synthesis produced by the binding of insulin to its cellular receptor are extremely transitory in nature. The signal disappears at approximately the same rate that insulin dissociates from the receptor. Thus, insulin must be constantly binding to the membrane receptor in order to keep the key signal(s) at a high enough level for the cell to progress on to S phase. Early events, such as specific protein synthesis, changes in ion flux, changes in cellular metabolism, and changes in cellular pH, may be essential, but they are not sufficient to cause a cell to progress on to S phase. Addition of sodium vanadate to the cell is found to stabilize the messenger such that there is no loss of signal when insulin is removed. These data are consistent with the tyrosine-phosphorylated insulin receptor or a product of its action being the signal.