Conjugated linoleic acid and linoleic acid are distinctive modulators of mammary carcinogenesis

Previous work by Ip and co-workers showed that mammary cancer prevention by conjugated linoleic acid (CLA) is independent of the level of fat in the diet. Because CLA is an isomer of linoleic acid, there is the question regarding whether the effect of CLA is due to a displacement of linoleic acid in cells. To further evaluate whether there might be an interaction between linoleic acid and CLA, the present study was designed to examine the dose response to CLA (at 0.5%, 1%, 1.5%, and 2%) in rats fed a 2% or a 12% linoleate diet (both basal diets contained 20% total fat by weight). The end points of investigation included the bioassay of mammary tumorigenesis in the rat dimethylbenz[a]anthracene model as well as the incorporation of CLA, linoleic acid, and arachidonic acid in mammary glands. The mammary carcinogenesis results showed that the efficacy of tumor suppression by CLA was not affected by linoleate intake. With either linoleate diet, no further protection was evident with levels of CLA > 1%. Analysis of neutral lipids and phospholipids of the mammary tissue indicated that 1) the accumulation of CLA in mammary tissue was dose dependent from 0.5% to 2%, 2) CLA concentration was 10 times higher in neutral lipids than in phospholipids, 3) the incorporation of CLA in either fraction was not affected by the availability of linoleic acid, and 4) CLA did not appear to displace linoleic acid or arachidonic acid in the mammary tissue. The above findings suggest that there may be distinctive mechanisms in the modulation of tumor development by linoleic acid and CLA.