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Effect of 3 and 6 Fatty Acids on Transformation of Cultured Cells by Irradiation and Transfection¹

Graduate Department of Biochemistry [M. T., M. P., J. M. L.], Brandeis University, Waltham, Massachusetts 02254, and Division of Radiation and Cancer Biology [C. B.], Department of Radiation Oncology, Tufts University School of Medicine, New England Medical Center, Boston, Massachusetts 02111

Mouse embryo fibroblasts (C3H 10T) were exposed to Gy of -rays. The cells yielded 5–8 trasformed foci per 10⁴ surviving cells. Addition of 100 M of either eicosapentaenoate or docosahexaenoate to the tissue culture medium reduced the number of transformed foci to 0–1.4. C3H 10T and NIH 3T3 cells were transfected with plasmid T24 containing the Harvey ras oncogene. C3H 10T cells yielded 0.85–1.1 foci/ng DNA, while NIH 3T3 cells yielded 0.12–0.14 foci/ng DNA. Foci formation was suppressed 65% in C3H 10T cells and 93% in NIH 3T3 cells when 100 µM eicosapentaenoate was present in the culture medium. Docosahexaenoate had a similar but somewhat weaker effect. Addition of arachidonate to the medium had little or no effect.

Cells grown in the presence of added eicosapentaenoate or docosahexaenoate produced much less prostaglandin E when challenged with calcium ionophore A23187. This is a reflection of changes in arachidonate production or utilization that occur during transformation which are suppressed by the added 3 fatty acids.

Addition of eicosapentaenoate or docosahexaenoate to the culture medium resulted in extensive remodeling of the molecular species of the four major phospholipid classes that were examined. In its simplest form, 3-fatty acid-containing species substantially replaced 6-fatty acid-containing species. However, many more subtle changes occurred, and the different phospholipids responded differently to different polyunsaturated fatty acids. A feature of C3H 10T cells was their preferential accumulation of molecular species of 22-carbon fatty acids such as docosapentaenoate (22:53) and docosatetraenoate (22:46) in preference to eicosapentaenoate (20:53) and eicosatetraenoate (arachidonate, 20:46). It is proposed that the protective effect of eicosapentaenoate and docosahexaenoate arises out of the changes in the composition of the fatty acids that are released from one or more phospholipids by the action of phospholipases. The changes consist of a reduced release of arachidonate, the normal substrate of cyclooxygenase and lipoxygenases, and a greatly increased release of eicosapentaenoate and docosahexaenoate, which inhibit one or more of these enzymes, or form oxygenated products which are not as active as the arachidonate-derived products. Other mechanisms are also considered.

¹ Supported by grants from the American Cancer Society (BC-692) to J. M. L. and from the National Foundation for Cancer Research to C. B.

Received 6/27/91. Accepted 10/22/91.

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