Isoflavone Aglycones and Oligopeptides in Lactic Acid-Fermented Soy Milk Differentially Regulate Lipid Metabolism-Related Gene Expression

Abstract

We previously investigated the physiological effect of an ethanol extract of fermented soymilk on rats and clarified that this extract modulated their hepatic lipid metabolism. Although the soy isoflavones and oligopeptides are representative functional components of the ethanol extract, it remained unclear whether these substances share a role in lipid metabolism modulation. Therefore, we attempted to clarify the effects of isoflavones and oligopeptides in lactic acid-fermented soymilk on lipid metabolism-related gene expression in rats and HepG2 cells. The fermented soymilk extract had a higher isoflavone aglycone content than the soymilk extract. Sevenweek-old male Sprague-Dawley rats were fed an AIN-93G diet, a diet plus 70% soymilk ethanol extract, or a diet plus 70% fermented soymilk ethanol extract for 5 weeks. Although both the soymilk and fermented soymilk ethanol extracts did not significantly affect plasma and hepatic lipid profiles, the expression levels of the genes encoding CYP7a1 and SREBP-2 were significantly upregulated in the livers of rats fed with the fermented soymilk extract. Whereas isoflavone aglycones upregulated CYP7a1-encoding gene expression in HepG2, oligopeptides in soymilk and fermented soymilk downregulated this expression. Oligopeptides in fermented soymilk downregulated the expression stronger than that observed with soymilk. On the other hand, no significant change in FAS expression was observed in the livers of rats fed the fermented soymilk extract. Although isoflavone aglycones did not affect FAS expression in HepG2 cells, oligopeptides in fermented soymilk downregulated FAS expression. The downregulation of FAS with oligopeptides from fermented soymilk was stronger than that from soymilk. In the present animal experiment, the effect on reduction of fat synthesis was not found because of insufficient amount of peptides derived from digestion of soy protein. These results suggest that isoflavone aglycones increase CYP7a1 gene expression, whereas oligopeptides decrease FAS expression. Isoflavone glycosides and proteins in soymilk were converted to isoflavone aglycones and oligopeptides by lactic acid fermentation, respectively, and these functional components independently improved the lipid metabolism. In the present study, it was found that isoflavone aglycones and oligopeptides in fermented soymilk differentially regulate hepatic lipid metabolism-related gene expression. Therefore, the consumption of fermented soymilk containing isoflavone aglycones and soy oligopeptides might prevent dyslipidemia more effectively than that of any other soy food. Fermented soymilk is a superior functional food modulating lipid metabolism.