Differential response of rapidly and slowly proliferating human cells to X irradiation.

Abstract

MOST STUDIES of the effects of ionizing radiations on mitosis and on the progression of mammalian cells through the replicative cycle in vitro have been carried out with rapidly proliferating cell systems, as have in vivo investigations in normal and tumor tissue. Very little is known specifically about these effects in slowly proliferating cells, although many tissue complexes are comprised of populations of slowly turning-over cells. For this reason, we have been studying the effects of x irradiation on progression through the life cycle in primary human amnion cell cultures in vitro (1, 2). These cultures are derived from amnionic membranes of normal human placentas delivered by cesarean section. They consist of morphologically pure diploid cells which, owing to their slow rate of proliferation, may be maintained in the same culture tube for several months without wide changes in cell count (3). In various experiments, the average generation time of the amnion cells during the first month, as calculated from the mitotic index and time, ranged from sixteen to thirty-one days; at any given time, more than 90 per cent of the cells in a culture were in the G1 or postmitotic resting phase. When primary human amnion cell cultures were exposed to 10–1,000 R of x rays, the effects on progress through the cell cycle and on the first postirradiation mitosis appeared similar to those observed in rapidly proliferating human cells, with one important exception: the initiation of DNA synthesis, usually considered a relatively radioresistant process. The rate of passage of G1 cells into the DNA-synthetic (S) phase was significantly depressed by exposures as low as lOR and completely blocked for five to ten days by 300–1,000 R (4). If such an effect occurred in slowly turning-over cell populations in vivo, it could be of considerable importance in determining their net response to fractionated or continuous irradiation. The object of the present investigation was to determine whether this effect on the initiation of DNA synthesis occurs in other slowly proliferating human cell systems or is peculiar to the primary amnion cells grown under the particular cultural conditions we have employed. Recently, Hahn and his co-workers (5) have shown that when monolayer cultures of Chinese hamster cells are allowed to become confluent, they enter a plateau phase of growth in which the cell concentration shows no further increase, despite regular replenishment of nutrient media. During this time the average cell generation time more than doubles, while the rate of DNA synthesis per cell falls to about 10 per cent of its former value. The latter finding has also been observed in other mammalian cells allowed to reach this growth stage (6, 7).