Radionuclide toxicity in cultured mammalian cells: elucidation of the primary site of radiation damage.

Synchronized suspension cultures of Chinese hamster ovary cells (CHO) were labeled with various doses of 3H-thymidine or 125I-iododeoxyuridine to evaluate the cytocidal effects of intranuclear radionuclide decay. Damage produced by radionuclide decay outside the cell nucleus was studied on cells exposed to 125I labeled, monovalent concanavalin A. After labeling, the cells were resynchronized in G1-phase and incubated for 36 h at 4 degrees C to permit dose accumulation. Cell lethality was evaluated by the standard colony assay. Based on radionuclide incorporation data, cellular dimensions, and subcellular radionuclide distributions, the cumulative dose to whole cells, cell nuclei, and cellular cytoplasm was calculated from the known decay properties of 3H and 125I. As expected, DNA associated 125I (LD50: 60 decays/cell; 45 rad) was much more toxic to CHO cells than 3H (LD50: 1350 decays/cell; 380 rad) 380 rad) or external X-irradiation (LD50: 330 rad). In contrast, membrane associated 125I was surprisingly non-toxic (LD50: 19 600 decays/cell). At 19 600 decays/cell the dose to the cell membrane was approximately 52 krad and the overlap dose into the cytoplasm was about 2470 rad. Even at these high dose levels, membrane damage or cytoplasmic damage apparently did not contribute significantly to radiation induced cell death. With 19 600 decays on the plasma membrane the CHO nuclei received an overlap dose of about 410 rad. As can be seen from the LD50 data for 3H and X-rays, a nuclear dose of 410 rad should be sufficient to account for 50% cell death. These findings indicate that, although intranuclear decay by electron capture is extremely destructive, identical decay events in the plasma membrane cause only minimal cell damage. This parallels our earlier studies on 67Ga labeled leukemia cells which showed that electron capture decay in the cytoplasm is also highly ineffective in killing mammalian cells. It therefore appears that radiation-induced cell lethality in dividing mammalian cells results primarily from nuclear damage. Cytoplasmic or membrane contributions to radiation-induced cell death, if any, must be minimal. By implication, these findings refute the enzyme release hypothesis and similar theories designed to explain mitotic death in terms of cytoplasmic or membrane damage rather than nuclear damage.