Effects of dietary fat and dimethylhydrazine on rat colonic antioxidant status: implications for carcinogenesis : a dissertation in home economics



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Texas Tech University


Previous studies have shown that the composition of membrane phospholipids reflects the fatty acid composition of the diet. Membranes that contain a high proportion of polyunsaturated fatty acids (PUFA) are potentially more susceptible to attack by free radicals. Radicals are reactive chemical species that contain one or more unpaired electrons. Oxygen and some of its metabolites are radicals that are highly reactive, potentially toxic to the cell, and have been implicated in a variety of disease processes. Double bonds within PUFA participate in reactions with these radicals and propagate their formation. Cells contain protective antioxidant mechanisms including superoxide dismutase (SOD), glutathione peroxidase (GSHPx), catalase (CAT), glutathione (GSH), glutathione reductase (GSSGRx), glutathione-S-transferase (GST) among others. Cellular damage results as these mechanisms become depleted. The effects of dietary fat on these antioxidant mechanisms in colon mucosa were examined using male Sprague-Dawley rats. Animals were fed one of four AIN 76A-based test diets differing in amount and type of lipid. The basal diet (BD) contained 5% corn oil; the menhaden oil diet (MO) contained 1% corn oil and 19% menhaden oil; the corn oil diet (CO) contained 20% corn oil; and the beef tallow diet (BT) contained 1% corn oil and 19% beef tallow; all were adjusted to provide equal amounts of other nutrients. Homogenates of colon mucosa were assayed after 2 weeks, 1, 3, 6 and 9 months for activities of CAT, GSHPx, SOD, GST, GSSGRx, and total GSH content.

In the first objective of this study, measurements were made at the various time points in rats fed test diets to determine whether diets high in polyunsaturated fatty acids (PUFA) would produce an increased demand on antioxidant activity in colon mucosa. Beginning at the 2-week time point GSH, GSSGRx, and SOD showed differences due to diet. At 1 month only GST showed differences. Then, at the 3-month point, GSH, GSSGRx, GSHPx, and at the 6-month point, GST, SOD, GSHPx and CAT, respectively, were affected. By the 9-month time point, only CAT showed differences due to diet. Both the type and amount of fat in the <ii-&t affected these antioxidant mechanisms. However, the greatest number of significant differences were seen between the low-fat and high-fat diets, with the low-fat diet generally showing greater antioxidant capability. None of the high-fat diets were consistently more protective with regard to antioxidant capability, and there were no detectable differences in antioxidant activity according to the polyunsaturated fatty acid composition of the diet. When compared to the high-fat diets, the BD group showed greater GSHPx activity at the 3- and 6-month time points, higher GSH concentration and GSSGRx activity at 2 weeks, and higher GST activity at 6 months. BD and MO-fed animals had higher levels of CAT and GSSGRx activity as compared to the CO and BT diets at various time points, while BT and CO-fed animals showed greater GST activity. The age of the animal also affected these parameters, as SOD, GSSGRx, and GSHPx activities, as well as GSH, decreased over time. Ornithine decarboxylase activity was not induced by the diets and malondialdehyde was virtually undetectable at any time point in any of the diets. Therefore, this tissue appeared capable of compensating for all of the changes in antioxidant activity resulting from dietary treatment alone.

In a second objective of this study, the colonic antioxidant response to 1,2-dimethylhydrazine (DMH) was determined. Again, rats were fed one of the test diets as used in the first objective and given one or two intraperitoneal injections of either DMH (20 mg/kg) or an equal volume of 1 mmol/L EDTA. Colon homogenates were measured for antioxidant activity as in the first objective and nuclear aberrations were quantitated. DMH modulated antioxidant mechanisms in the colon primarily through increases in GSH content in all diets. EDTA treatment alone also caused unexplained changes in these mechanisms. (Diets high in PUFA did not consistently differ from diets high in saturated fatty acids in response to DMH. MnSOD, an enzyme found to have very low or absent activity in a wide variety of tumors and transformed cell lines, decreased in this study in all diets as the result of DMH treatment. Colon mucosal membranes were apparently resistant to peroxidation as malondialdehyde formation continued to be low after DMH challenge. The MO group showed the greatest lipid peroxidation, among the diets, while the CO group resulted in the greatest number of nuclear aberrations. The significance of lipid peroxidation and its relationship to carcinogenesis in this tissue remains uncertain.

The fatty acid composition of colon mucosal microsomes varied according to the fatty acid composition of the diet. Variation occurred primarily in the type of unsaturated fatty acids that became incorporated, with the MO-fed animals showing a higher content of long chain, unsaturated n-3 fatty acids and lower concentrations of linoleic and arachidonic acids. Linoleic acid concentration was highest in the CO group and monounsaturated fatty acid incorporation was highest in the BT group.

Results from both objectives of this study demonstrated that dietary fat intake influences antioxidant activity in the colon. The low-fat BD appeared to have some advantages over the high-fat diets regarding antioxidant protection. However, none of the high-fat diets appeared more protective and there were no detectable differences in antioxidant mechanisms according to PUFA content of the diet. Implications are that a high-fat intake, regardless of the source, may increase oxidative stress in the colon.