Evidence that stimulation of growth following long term density arrest of WI-38 cells proceeds via a pathway independent of protein kinase C and of cAMP-dependent protein kinase.

When cultures of WI-38 human diploid fibroblasts reach high cell densities, they cease to proliferate and enter a viable state of quiescence in which they can remain for long periods of time. However, the longer the cells remain growth arrested, the more time they require to leave G-0, to progress through G-1, and to enter S following stimulation with fresh serum. Previous results from our laboratory showed that in this system the time of expression of c-fos and c-myc was related only to the time of stimulation and not to the time of DNA synthesis [Owen et al., (1985) J. Biol. Chem. 262, 15111-15117]. It is possible that the initial response of both the short and long term quiescent WI-38 cells to growth factor stimulation was the same but that the subsequent secondary events which led to the progression of the stimulated cells through G-1 into S were different. Therefore, experiments were performed to compare the signal transduction pathway(s) utilized following serum stimulation of long term quiescent WI-38 cells to those utilized in WI-38 cells stimulated after short periods of time in quiescence. The only differences detected between the short and long term quiescent WI-38 cells involved their responses to epidermal growth factor (EGF). These results suggest that the response of quiescent WI-38 cells to fetal calf serum involves an EGF-dependent signal transduction pathway. Moreover, the EGF pathway which functions in long term quiescent WI-38 cells would appear to be defective compared to that employed by short term quiescent WI-38 cells.