Cell growth and renewal depend upon two separate events; the induction of proliferation in resting cells, and controls over the continued proliferation of these cells once in cycle. Most previous studies have focused upon stimulation of resting cells. Our recent studies have utilized time-lapse and quantitative fluorescence techniques to study growth control in actively cycling cultures. Our work demonstrates previously unrecognized, fundamental differences in these two aspects of growth control. Ras induces cyclin D1 levels during the G1 phase of resting NIH3T3 cells, but our studies demonstrate that Ras stimulates cyclin D1 levels only during G2 phase when these cells are proliferating. Thus, the regulation of proliferation occurs during different cell cycle phases in resting verses cycling cells. It is our goal to determine the biological significance of the Ras-induced elevation of cyclin D1 during G2 phase of actively cycling cells and to learn the molecular details of how it takes place. This will provide information essential to our understanding of cell growth. To do so, the signaling pathway from cellular Ras proteins to cyclin D1 induction will be determined. Parallel studies of the molecular mechanism by which cyclin D1 levels are elevated during G2 phase will also be undertaken. These studies will involve our techniques to study asynchronous cultures and will be extended and verified with more traditional biochemical studies in synchronized cultures. Finally, the model proposed to explain our previous studies will be tested by determining if the induction of cyclin D1 during G2 phase plays a central role in determining the proliferative fate of an actively cycling cell. Studies of related molecules will also be performed. Eventually, it is our goal to understand in detail the role of G2 events in the control of cell growth.
