Human and murine hepatic sterol-12-alpha-hydroxylase and other xenobiotic metabolism mRNA are upregulated by soy isoflavones.

The transport and metabolism of xenobiotics is controlled by the drug transporters and drug-metabolizing enzymes in the liver and small intestine. Expression of these genes is 1 factor affecting the half-life of drugs and xenobiotics. Isoflavone-containing soyfood products and supplements are promoted to treat several different health conditions, including improvement of blood lipid profiles. Because relatively high isoflavone intake may be possible via use of supplements, we tested the hypothesis that isoflavones regulate the expression of genes critical to drug transport and metabolism. Using a gene array screening method, 2 drug transporters, Multidrug restistant-1 and Multidrug-related protein-2; 3 phase I enzymes, cytochrome 1A1, 3A4, and 8B1; and 2 phase II enzymes, carbohydrate sulfotransferase-5 and glutathione-sulfotransferase-2, were upregulated 3-fold or more of the initial expression levels in primary human hepatocytes exposed to soy isoflavones for 48 h. Isoflavone-related induction of 12-alpha-hydroxylase (CYP8B1) was further studied in other in vitro and murine in vivo models. Transfection studies suggest that isoflavones may act as a weak activating ligand for hepatocyte nuclear factor 4alpha, which in turn may activate the transcription of CYP8B1. The action of soy isoflavones on CYP8B1 may increase the conversion of cholesterol into bile acids and enhance synthesis of cholic acid. These isoflavone-induced changes in gene expression may help explain how isoflavones modulate cholesterol metabolism.

Dietary isoflavone supplementation modulates lipid metabolism via PPARalpha-dependent and -independent mechanisms.

Intake of soy protein has been associated with improvements in lipid metabolism, with much attention being focused on the serum cholesterol-lowering property of soy. The component or components of soy that are responsible for improvements in lipid metabolism have been investigated and their specific actions debated. One component, the isoflavones, has been shown to have weak estrogenic activity, and recently, several research groups have suggested that isoflavones are activating peroxisome proliferator-activated receptors (PPARs). The three different isoforms of PPARs (alpha, gamma, and delta) have overlapping tissue distributions and functions associated with lipid metabolism. The goal of the present study was to investigate the hypothesis that the effect of isoflavones is mediated through the PPARalpha receptor. Male and female 129/Sv mice were obtained, including both wild-type and genetically altered PPARalpha knockout mice. Groups of mice were fed high-fat atherogenic diets containing soy protein +/- isoflavones and PPARalpha agonist fenofibrate for 6 wk. At the end of 6 wk, serum and tissue lipid levels were measured along with hepatic gene expression. Most notably, serum triglycerides were reduced by isoflavone consumption. Compared with intake of a low-isoflavone basal diet, isoflavone intake reduced serum triglyceride levels by 36 and 52% in female and male wild-type mice, respectively, compared with 55 and 52% in fenofibrate-treated mice. Isoflavones also improved serum triglyceride levels in knockout mice, whereas fenofibrate did not, suggesting that two different regulatory mechanisms may be affected by isoflavone intake. Isoflavone intake resembled action of fenofibrate on PPARalpha-regulated gene expression, although less robustly compared with fenofibrate. We suggest that, at the levels consumed in this study, isoflavone intake is altering lipid metabolism in a manner consistent with activation of PPARalpha and also via a PPARalpha-independent mechanism as well.

Molecular mechanisms of action of the soy isoflavones includes activation of promiscuous nuclear receptors. A review.

Consumption of soy has been demonstrated to reduce circulating cholesterol levels, most notably reducing low-density lipoprotein (LDL) cholesterol levels in hypercholesterolemic individuals. The component or components that might be responsible for this effect is still a matter of debate or controversy among many researchers. Candidate agents include an activity of soy protein itself, bioactive peptides produced during the digestive process, or the soy isoflavones. Although soy intake may provide other health benefits including preventative or remediative effects on cancer, osteoporosis and symptoms of menopause, this review will focus on isoflavones as agents affecting lipid metabolism. Isoflavones were first discovered as a bioactive agent disrupting estrogen action in female sheep, thereby earning the often-used term 'phytoestrogens'. Subsequent work confirmed the ability of isoflavones to bind to estrogen receptors. Along with the cholesterol-lowering effect of soy intake, research that is more recent has pointed to a beneficial antidiabetic effect of soy intake, perhaps mediated by soy isoflavones. The two common categories of antidiabetic drugs acting on nuclear receptors known as peroxisome proliferator activated receptors (PPARs) are the fibrates and glitazones. We and others have recently asked the research question 'do the soy isoflavones have activities as either "phytofibrates" or "phytoglitazones"?' Such an activity should be able to be confirmed both in vivo and in vitro. In both the in vivo and in vitro cases, this action has indeed been confirmed. Further work suggests a possible action of isoflavones similar to the nonestrogenic ligands that bind the estrogen-related receptors (ERRs). Recently, these receptors have been demonstrated to contribute to lipolytic processes. Finally, evaluation of receptor activation studies suggests that thyroid receptor activation may provide additional clues explaining the metabolic action of isoflavones. The recent advances in the discovery and evaluation of the promiscuous nuclear receptors that bind many different chemical ligands should prove to help explain some of the biological effects of soy isoflavones and other phytochemicals.

Soy isoflavones exert antidiabetic and hypolipidemic effects through the PPAR pathways in obese Zucker rats and murine RAW 264.7 cells.

The hypocholesterolemic and anti-atherosclerotic mechanism by which soy may exert a beneficial effect remains unclear. Peroxisome-proliferator activated receptors (PPAR) are promiscuous nuclear receptors that regulate the transcription of genes involved in lipid and glucose homeostasis and lipid metabolism within the cell. We hypothesize that the isoflavones improve lipid and glucose metabolism by acting as an antidiabetic PPAR agonist. Male and female obese Zucker rats (OZR) were used as a model of Type 2 diabetes, and OZR fed a high isoflavone soy protein diet displayed improvements in lipid metabolism consistent with results in humans treated with antidiabetic PPAR agonists such as the fibrates or glitazones. Liver triglyceride and cholesterol concentrations were lower in all OZR fed high-isoflavone soy protein diets than in rats fed low-isoflavone and casein diets (P < 0.05). Concurrently, PPAR-directed gene expression was evaluated in a cell culture model. An isoflavone-containing soy extract doubled PPAR-directed gene expression (P < 0.05) in RAW 264.7 cells containing either a PPARalpha or PPARgamma expression plasmid. A similar induction was observed when the soy isoflavones genistein or daidzein were used to treat cells. Both isoflavones doubled PPARalpha-directed gene expression (P < 0.05), whereas they increased PPARgamma-directed gene expression 200-400% (P < 0.05). This study suggests that soy isoflavones improve lipid metabolism, produce an antidiabetic effect, and activate PPAR receptors.

Hepatic cytochrome p450-2A and phosphoribosylpyrophosphate synthetase-associated protein mRNA are induced in gerbils after consumption of isoflavone-containing protein.

Soy intake reduces cholesterol levels, but neither the exact component in soy causing this reduction nor the mechanism by which cholesterol is reduced is known with certainty. In this study, a genetic screen was performed to identify hepatic mRNA in gerbils regulated by soy or soy isoflavones. Gerbils were fed casein, an alcohol-washed soy-based diet (containing low levels of isoflavones), and the soy-based diet supplemented with an isoflavone-containing soy extract. After feeding for 28 d, gerbils were killed, hepatic RNA was isolated, and genes that were differentially expressed in any of the three dietary conditions were identified. Fifteen different mRNA were originally selected, including two mRNA that were studied further and shown to be highly regulated. Messenger RNA levels for both cytochrome P450-2A and phosphoribosylpyrophosphate synthetase-associated protein were up-regulated in a dose-dependent manner when soy replaced casein in the diet at 0, 33, 67 and 100% of original casein levels. A subsequent experiment used purified amino acid mixtures resembling the percentage amino acid composition of soy and casein to ensure that isoflavone-free protein sources could be tested. Using these mixtures, a 2 x 2 x 2 design tested: natural vs. synthetic protein sources, casein- vs. soy-based diets, and isoflavone extract-supplemented or supplement-free diets. This design demonstrated that these two mRNA were again significantly up-regulated more than twofold (P < 0.05) in gerbils fed all diets containing isoflavones. Induction of these two mRNA by soy may be due to the aryl hydrocarbon receptor element in the promoter region of both genes.