The effect of X-irradiation on membrane lipids of lymphosarcoma cells in vivo and in vitro.

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Abstract
Phospholipids of the membranes of spleen lymphosarcoma cells were radioactively labelled with the radioresistant fatty acid palmitic acid (16:0) and the radiosensitive fatty acid arachidonic acid (20:4). The effect of irradiation on the specific radioactivity of the phospholipids was studied. For the in vivo experiments trace amounts of radioactive palmitate or arachidonate were injected intraperitoneally into lymphosarcoma-bearing mice. Incorporation of label into the lipids of the tumorous spleen cells was monitored in control animals and in mice that were whole body irradiated after injection of the label. In both groups of animals the label was detected in the blood within minutes after injection and was found to be efficiently incorporated into the phospholipids of the tumor cells. In the irradiated animals a marked transient increase in label incorporation was observed as compared to control animals. The radiation effect was observed in the lipids of the total homogenate, purified nuclei, the mitochondriallysosomal fraction and in the microsomal fraction. Most experiments were performed with nuclei, which are known for their high radiosensitivity. The levels of label incorporation for palmitate and arachidonate were increased to the same extents and found to be dose dependent. For a dose of 5 Gy the increased label incorporation started immediately after irradiation and lasted for a period of about 50 minutes. The increase in label incorporation into the phospholipids was preceded by an increase in the concentration of fatty acids in the cytosol of the tumors. Our experiments point to the occurrence of a transient increase in the flux of fatty acids through the plasma membrane as a result of irradiation and suggest that under normal physiological conditions fatty acid uptake through the plasma membrane is the rate-limiting step in the incorporation of acyl groups into the phospholipids. Experiments with isolated tumor cells also showed an increased incorporation of fatty acids into the phospholipids after irradiation. Again the incorporation patterns of both types of fatty acids (16:0) and (20:4) were very similar. A hypotonic treatment of the cells also resulted in a similar increase in fatty acid incorporation as irradiation did; the effects of hypotonic treatment and irradiation were not additive.