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Food Lipids: Chemistry, Nutrition, and Biotechnology

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—both evidence of apoptosis—in a rat mammary tumor cell line, indicating a potential<br />

mechanism of action of CLA. By inducing apoptosis in mammary gl<strong>and</strong><br />

epithelial cells, CLA could prevent breast cancer by reducing mammary epithelial<br />

cell density <strong>and</strong> inhibiting the outgrowth of initiated MEO.<br />

Park et al. (116) showed that LA stimulated MCF-7 breast cancer cell growth,<br />

whereas CLA was inhibitory. The LA stimulated phospholipase C (PLC) activity <strong>and</strong><br />

tended to increase membrane potein kinase C (PKC) activity. However, CLA supplementation<br />

did not modify membrane PLC or PKC activity. PGE 2 production was<br />

not influenced by LA or CLA addition in this experiment.<br />

Isomers of CLA may exert their biochemical <strong>and</strong> physiological effects by modulating<br />

tissue or cellular eicosanoid metabolism (61,62,67,68,73,74,104–106). In<br />

studies to determine the physiological action of CLA, investigators found reduced<br />

PGE 2 [a cyclooxygenase (COX)-catalyzed product of arachidonic acid] concentration<br />

in rat serum <strong>and</strong> spleen (67,104). The amount of PGE 2 in cultured keratinocytes<br />

(106,117) <strong>and</strong> in ex vivo bone organ culture (73) were lowered with CLA treatment<br />

in cells <strong>and</strong> rats, respectively. Igarashi <strong>and</strong> Miyazawa (118) studied the inhibitory<br />

effect of CLA on the growth of a human hepatoma cell line, HepG2, <strong>and</strong> found that<br />

CLA’s effect was interfered significantly by addition of LA or arachidonic acid.<br />

Addition of antioxidants, such �-tocopherol <strong>and</strong> BHT, did not diminish CLA’s inhibitory<br />

effect on cell proliferation, indicating that CLA’s effect was not mediated<br />

by induction of lipid peroxidation but rather by changes in fatty acid metabolism.<br />

Moreover, CLA reduced the level of leukotriene B 4 (LTB 4) from the exudates of<br />

cells (67) <strong>and</strong> thromboxane A 2 (TXA 2) (another cyclooxygenase-catalyzed product<br />

of arachidonic acid) in platelet suspension (119). Indirectly, CLA was found to suppress<br />

the growth of human MCF-7 breast cancer cells in culture synergistically with<br />

NDGA (a lipoxygenase inhibitor) (61), suggesting an inhibitory effect of CLA on<br />

the lipoxygenase pathway.<br />

The mixed isomers of CLA were detected in numerous tissues examined in<br />

animals given a dietary supplement of CLA (73,104). In one of our experiments,<br />

four groups of male rats were given a basal semipurified diet (AIN-93-G) containing<br />

70 g/kg of added fat for 42 days (73,74). The fat treatments were formulated to<br />

contain two levels of CLA (0% <strong>and</strong> 1% diet) <strong>and</strong> either n-6 (soybean oil having a<br />

ratio of n-6:n-3 fatty acids of 7.3) or n-3 fatty acids (menhaden oil � safflower oil<br />

having a ratio of n-6 to n-3 fatty acids of 1.8) following a 2 � 2 factorial design.<br />

The CLA isomers analyzed by GC were found in all rat tissues analyzed although<br />

their concentrations varied with brain exhibiting the lowest concentration of CLA<br />

isomers but bone tissues (periosteum <strong>and</strong> marrow) containing the highest amounts.<br />

Dietary CLA decreased the concentrations of 16:1n-7, 18:1, total monounsaturates,<br />

<strong>and</strong> n-6 fatty acids, but increased the concentrations of n-3 fatty acids (22:5n-3 <strong>and</strong><br />

22:6n-3), total n-3, <strong>and</strong> saturates in the tissues analyzed. Ex vivo PGE 2 production<br />

in bone organ culture was decreased by the n-3 fatty acid <strong>and</strong> CLA treatments. In a<br />

subsequent study with rats, feeding 0.5% CLA resulted in total CLA concentrations<br />

ranging from 0.27% to 0.43% in the polar lipid fraction <strong>and</strong> from 2.02% to 3.37%<br />

in the neutral lipids in liver, bone marrow, <strong>and</strong> bone periosteum.<br />

When CLA is incorporated into membrane phospholipids (induced by feeding<br />

a dietary supplement), it may compete with arachidonic acid <strong>and</strong> is likely to inhibit<br />

eicosanoid biosynthesis (74,120,121). The reduction in the amount of n-6 fatty acids<br />

in peritoneal exudate cells <strong>and</strong> splenic lymphocyte total lipids by CLA seemed to be<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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