The Action of Epidermal Growth Factor (EGF) is Limited to Specific Phases of the Cell Cycle in an EGF Dependent Colonic Cell Line

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Abstract
In the presence of epidermal growth factor (EGF) a human colon cell line, LIM 1215, proliferates in serum-free medium. Under these culture conditions the cells are dependent on the presence of EGF for both proliferation and survival. In order to study the action of growth factors at different stages of the LIM 1215 cell cycle, pure populations of G1, S and G2/M cells were obtained by cell sorting after supravital staining of the DNA with Hoechst 33342. Conditions were established for Hoechst 33342 staining which produced satisfactory DNA histograms and greater than 80% survival of cells. The kinetics of passage for sorted S or G2/M cells into G1 were not affected by EGF or fetal calf serum. After sorting there appeared to be a 4 h delay before the cells proceeded in the cell cycle. Sorted S cells entered G2 over an 8 h period and maintained this same transition period from G2 into G1. If EGF or serum was present, these cells then re-entered the cell cycle after a variable delay and in an asynchronous manner. EGF was applied to S phase and G2/M phase LIM 1215 cells for periods of 2-10 h at various times after replating in serum-free conditions. Cells in S phase only responded to EGF as they passed from G2/M into G1. Exposure to EGF in S phase resulted in little growth stimulus once the cells returned to G1. For cells in G2/M phase, EGF was required immediately to give the maximum stimulus for re-entering the cell cycle. If the EGF was delayed for more than 8 h, the cells did not re-enter the cycle within the following 20 h. Exposure to EGF for less than 2 h failed to stimulate proliferation. These results indicate that EGF must be present as cells enter G1 from mitosis. Once the cells have entered G1, EGF is required for a 10 h period for a large number of cells to re-enter the cycle from G1.