Sharon E. Rickard

Antitumorigenic effect of a mammalian lignan precursor from flaxseed.

Secoisolariciresinol diglycoside (SD), a mammalian lignan precursor found in high-fiber foods, was isolated from flaxseed and tested for effects on mammary tumorigenesis in rats fed a high-fat (20%) diet. Ingestion of purified SD at 1.5 mg/day for 20 weeks starting 1 week after treatment with the carcinogen dimethylbenzanthracene resulted in a 37% reduction (p < 0.05) in the number of tumors per tumor-bearing rat and a 46% reduction (p < 0.05) in the number of tumors per tumor-bearing rat and a 46% reduction (p < 0.05) in the number of tumors per number of rats in each group. Urinary mammalian lignan excretion significantly increased (p < 0.0001) with SD treatment, indicating the conversion of SD to mammalian lignans. No enlargement or gross abnormalities of the major organs were observed in the SD-treated rats. This study showed, for the first time, that SD has an antitumor effect when provided at the early promotion stage of tumorigenesis and may contribute to the health benefits of high-fiber foods.

Variability in anticancer lignan levels in flaxseed

Flaxseed and its major mammalian lignan precursor secoisolariciresinol diglycoside have been shown to be protective against chemically induced carcinogenesis in animal models. Although flaxseed is the richest source of mammalian lignan precursors, it is not known whether these levels vary with source. Thus the objective of this study was to determine how lignan levels in flaxseed are affected by variety, growing location, harvest year, and seeding time. Ten varieties of flaxseed (AC Linora, Andro, Flanders, Linott, McGregor, Noralta, NorLin, NorMan, Somme, and Vimy) were subjected to 1) in vitro fermentation with human fecal inoculum for 24 hours under anaerobic conditions to assess mammalian lignan production and 2) high-performance liquid chromatography (HPLC) analysis for secoisolariciresinol levels. Three of these varieties (Linott, McGregor, and NorLin) were grown in four locations, seeded early (May) for three different years, and, in one year, seeded early (May) or late (June). Significant differences in lignan production were observed among the different varieties, ranging from 0.96 mumol/g for Linott to 3.15 mumol/g for Somme flaxseed (p < 0.05). Growing location had significant effects on lignan production from all three varieties. Harvest year significantly affected only the Linott variety (p < 0.05), whereas seeding time had no effect. A significant correlation (r = 0.572, p < 0.003) was observed between lignan values obtained from HPLC and in vitro fermentation methods, indicating that HPLC analysis of flaxseed may be used as a predictor of its lignan production levels. Differences due to variety, harvest location, and harvest year of flaxseed should be taken into consideration when tumorigenesis studies are designed.

Plasma insulin-like growth factor I levels in rats are reduced by dietary supplementation of flaxseed or its lignan secoisolariciresinol diglycoside

Flaxseed and its lignan secoisolariciresinol diglycoside (SDG) inhibit mammary tumor development in rats. Increased plasma insulin-like growth factor I (IGF-I) concentrations are associated with increased breast cancer risk. Therefore, the effect of flaxseed (5%) or SDG (1.5 mg/day) supplementation on plasma IGF-I levels was examined in rats treated with or without N-methyl-N-nitrosourea (MNU). In MNU-free rats, flaxseed and SDG reduced plasma IGF-I levels, which were inversely related to urinary lignan excretion. Only flaxseed significantly reduced plasma IGF-I concentrations in MNU-treated rats. The anticancer effect of flaxseed and SDG may be related, in part, to reductions in plasma IGF-I.

Dose Effects of Flaxseed and Its Lignan on N-Methyl-N-Nitrosourea-Induced Mammary Tumorigenesis in Rats

Dietary supplementation with flaxseed or its lignan secoisolariciresinol diglycoside (SDG) has reduced dimethylbenz[a]anthracene-induced mammary tumor size and number in rats. The objective of this study was to determine whether flaxseed has a dose-dependent effect on N-methyl-N-nitrosourea (MNU)-induced mammary tumor promotion and whether this effect can be attributed to its SDG. Two days after injection with MNU (50 mg/kg body wt i.p.), female Sprague-Dawley rats were fed a high-fat (20% soybean oil) AIN-93G basal diet alone (BD) or supplemented with flaxseed (2.5% F and 5% F) or SDG by gavage [SDG in 2.5% F (LSDG) and SDG in 5% F (HSDG)] for 22 weeks. Although tumors tended to be smallest in the 5% F group throughout the experimental period, flaxseed feeding did not significantly affect tumor size, multiplicity, or incidence in comparison to BD. However, there was a dose-dependent effect of SDG on tumor multiplicity. Tumor multiplicity was lowest in the HSDG group and highest in the LSDG group throughout treatment (p < 0.05), indicating that HSDG inhibited, whereas LSDG promoted, MNU-induced mammary tumor development. Tumor invasiveness and grade were decreased in all treatment groups compared with the BD (p < 0.032). Thus, although flaxseed feeding had no significant effect on tumor growth indexes, flaxseed and SDG treatment, regardless of dose, appeared to delay the progression of MNU-induced mammary tumorigenesis. Disparities between this study and previous studies on flaxseed may be related to differences in experimental design, the use and dose of a different carcinogen, and protective effects by the alpha-linolenic acid present in the BD.

Flaxseed and its mammalian lignan precursor cause a lengthening or cessation of estrous cycling in rats

Flaxseed and its mammalian lignan precursor secoisolariciresinol diglycoside (SDG) have been shown to be mammary cancer-protective in rats. Thus, the antiestrogenic effects of flaxseed and SDG were compared with tamoxifen, an antiestrogen, by monitoring rat estrous cycling. Four-week supplementation of a high-fat diet with flaxseed (2.5, 5, or 10%) or SDG (0.75, 1.5 or 3.0 mg/day) produced a dose-related cessation or lengthening (by 18-39%) of estrous cycles in up to 66% of rats. With tamoxifen (1 mg/kg body weight/day), 83% of the animals had irregular cycles or were in persistent diestrus. Flaxseed and SDG were antiestrogenic without gross tissue toxicity.

Short-term feeding of flaxseed or its lignan has minor influence on in vivo hepatic antioxidant status in young rats

A proposed mechanism underlying the anticarcinogenic and antiatherogenic effects of flaxseed (flax) and its lignan secoisolariciresinol diglycoside (SDG) is the potential antioxidant activity of SDG and its mammalian lignan metabolites enterodiol (ED) and enterolactone (EL). Recent evidence indicates that SDG, ED and EL scavenge hydroxyl radicals in vitro. This study evaluated the effect of dietary flax on specific antioxidant enzymes and nonenzymatic antioxidant molecules in young rats. After 3 days acclimatization on an AIN-93G basal diet (BD), female Sprague Dawley rats (24 days of age) were randomized into three dietary groups: BD, BD supplemented with 10% flax or 3 mg SDG gavage. At 50 days of age, rats were sacrificed and livers removed for antioxidant enzyme determinations. Body weight gain, feed efficiency ratio and liver weights were not different between treatment groups indicating that animals grew well on the flax diet. Hepatic reactive oxygen species metabolizing enzymes catalase (CAT) and glutathione peroxidase (GSH-Px) were not different between treatment groups. Hepatic glutathione reductase (GSSG-Red) activity of SDG and flax-fed rats, when combined, were lower (P = 0.037) compared to BD-fed rats. Hepatic acid-soluble sulfhydryl groups as GSH were not influenced by flax or SDG treatment. Total urinary lignan excretion was higher in SDG- and flax-fed groups than in the BD group (P < 0.001). Reduced activity of GSSG-Red and unchanged GSH levels may relate to an antioxidant sparing effect of ED and EL in tissues of flax and SDG-fed rats.

Flaxseed and lignans increase cecal beta-glucuronidase activity in rats.

Flaxseed has been shown in previous studies to decrease some early markers of colon cancer risk in part because of its lignans. This study determined whether the intake of flaxseed and lignans is related to the activity of bacterial beta-glucuronidase, an enzyme suggested to increase colon cancer risk. Seven groups of six female rats each were fed, for four weeks, a basal high-fat (20%) diet (BD), BD supplemented with 2.5%, 5.0%, or 10.0% flaxseed, or BD with daily gavage of 0.75, 1.5, or 3.0 mg of secoisolariciresinol diglycoside (SDG), the major mammalian lignan precursor. The specific and total activities of beta-glucuronidase in the cecum were significantly related to the levels of flaxseed (r = 0.539, p < 0.008 and r = 0.599, p < 0.002, respectively) and SDG (r = 0.567, p < 0.007 and r = 0.435, p < 0.04, respectively). The urinary mammalian lignan excretion also increased with increasing flaxseed or SDG levels and thus was significantly related to the specific activity (r = 0.38, p < 0.017) and total activity (r = 0.429, p < 0.007) of beta-glucuronidase. Because flaxseed and lignans are colon cancer protective, it is concluded that, in contrast to other studies, beta-glucuronidase activity may play a beneficial role in their presence by increasing mammalian lignan absorption and enterohepatic circulation.