The Action of Caffeine on X-Irradiated HeLa Cells: III. Enhancement of X-Ray-Induced Killing during G 2 Arrest

Abstract

The ability of caffeine to enhance the expression of potentially lethal X-ray damage in HeLa S3 cells was examined as a function of the age of the cells in the generation cycle. Synchronous populations were irradiated at different times after mitotic collection and treated for various intervals with 1 mM caffeine, which causes negligible killing of unirradiated cells. The response was thereby determined as a function of cell age at both the time of irradiation and the time of exposure to caffeine. The amount of cell killing depends strongly on when in the cycle caffeine is present and only weakly on when the cells are irradiated. If cells are irradiated in early G1, caffeine treatment enhances killing for 2 to 3 hr. No additional enhancement is observed until 16 to 17 hr postcollection, corresponding to G2, here they enter a second period of much greater sensitivity. Similarly, fluorodeoxyuridine resynchronized cells irradiated during S and treated with caffeine suffer no enhanced killing until they pass into this sensitive phase in G2, approximately 7 hr after release from the fluorodeoxyuridine block. The sensitive period appears to coincide with G2 arrest. The rate and extent of killing during this period are dependent upon the X-ray dose and the caffeine concentration. In the absence of caffeine, cells irradiated in G1 lose sensitivity to caffeine in about 9 hr; they do so faster in G2. It is concluded that the potentially lethal X-ray damage expressed on treatment with caffeine is retained for many hours in the presence of caffeine and is maximally manifested by ${\rm G}_{2}\text{-arrested}$ cells.