Mode of estrogen action on cell proliferative kinetics in CAMA-1 cells. I. Effect of serum and estrogen.

Both serum and estrogen affect cell proliferation in CAMA-1 cells. Their effects on cell cycle kinetics are being investigated with partially synchronized cells following 48 hours of serum deprivation. By comparing the growth kinetics of synchronized (serum-deprived) cells with asynchronized (normal-fed) cells, we observed that there was a delay of cell proliferation for approximately 25 hr for synchronized cells and that estrogen only induced cell proliferation in serum-supplemented culture. A serum protein (RPF), precipitated by ammonium sulfate, dialyzed in 3,500 daltons cut off membrane and reconstituted in culture medium, was shown to stimulate cell proliferation in a dose-related manner. In the absence of RPF, estrogen had no effect on cell growth and S-phase formation in serum-free medium, but significantly induced cell growth in the presence of RPF. Experiments conducted with a synchronized cell population showed that estrogen increased the proportion of G1 phase cells to enter S phase, shortened the G1 phase duration, and ultimately increased the proportion of mitotic cells per cycle. Collectively, these results show that (a) estrogen effects of cell proliferation in vitro requires a serum component, a "G1/S-promoting factor" and (b) estrogen-induced tumor growth is a result of an accelerated rate of G1/S transition and an increased number of dividing cells per cycle. The knowledge of the interaction of estrogen with the G1/S promoting factor on cell growth is of both fundamental and therapeutic importance.