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Dietary fish oils rich in n-3 fatty acids have been shown to inhibit the development of carcinogen-induced and transplanted mammary tumours (Karmali et al., 1984, 1987). Kort et al. (1987) have shown that fish oil inhibits the growth of mammary adenocarcinoma BN472 in BN/Bi female rats. A mechanism proposed for the anti-tumour effects of n-3 fatty acids is inhibition of arachidonic acid metabolism. Since previous studies suggested that prostaglandin E2 (Rolland et al., 1980) and thromboxane B2 (Karmali et al., 1983) were elevated in certain types of metastasis, the present studies were undertaken to determine if dietary fish oil would inhibit the metastasis of 13762NF (spontaneous model) and 13762MAT:B (experimental model) in Fischer 344 rats. Weanling Fischer 344 female rats (Charles River Breeding Laboratory, Kingston, NJ, USA) were maintained in a temperature and humidity controlled facility with a 12-h light/dark cycle. Fifteen rats were used for each treatment group. The composition of low-fat and high-fat diets and fatty acid profiles of corn oil (CO) and fish oil (FO) (MaxEPA) have been described previously (Cohen et al., 1986; Karmali et al., 1987). Rats were placed on one of four diets: (1) 23.52% corn oil, (2) 8% corn oil + 15.52% fish oil; (3) 3% corn oil + 20.52% fish oil, or (4) 5% corn oil. Diets were mixed fresh weekly and fed fresh daily to prevent autooxidation of unsaturated fatty acids (Karmali et al., 1987). Since rats were housed five per cage, only a rough estimate for daily feed intake per rat could be made. Diets were fed for 8 weeks before tumour injections and continued until killing (5 weeks spontaneous model; 3 weeks experimental model). The spontaneous metastasis studies were carried out with the 13762NF mammary adenocarcinoma subline (Mason Research Institute, Worcester, MA, USA). A pool of tumours from four rats was minced, filtered through 20 im Nytex nylon mesh, and 105 cells in 0.05 ml saline were injected i.m. in the thigh. Tumour size was measured weekly, and tumour volume was estimated using the formula +(4i/3)(length/2)(width/2)(height) (Rockwell et al., 1972). The 13762MAT:B subline was chosen for the experimental metastasis studies. A pool of ascites tumour cells from four rats was washed in phosphate buffered saline, and 105 cells in 0.2 ml saline were injected i.v. via the lateral tail vein. At the time of killing under anaesthesia in both model systems, the lungs were fixed in situ and processed by a lung clearing technique that allows the enumeration and sizing of subsurface as well as surface tumour foci in the right superior lobe (Yuhas & Walker, 1973). Metastatic tumour volumes were calculated by the formula V = 4/3nr3 in both metastatic models. All data were analysed by Dunnett's multiple comparison test. In both experiments, the animals on the low-fat corn oil diet consumed the largest number of grams of food per day (14.1), followed by those on fish oil diets (12.6 and 12.3) and the high-fat corn oil diet (11.6).

Effect of n-3 fatty acids on spontaneous and experimental metastasis of rat mammary tumour 13762