Peroxisome proliferator-activated receptor γ (PPARγ) is a ligand-dependent transcription factor that directly activates the expression of adipocyte-specific genes, and is universally accepted as the master regulator for adipocyte differentiation. Using a PPARγ luciferase reporter assay system, we showed that 4',6-dimethoxyisoflavone-7-O-β-D-glucopyranoside (wistin) dose-dependently activates PPARγ. Treatment with wistin enhanced the marker of adipocyte differentiation, such as triglyceride accumulation in 3T3-L1 cells. Real-time quantitative polymerase chain reaction showed that wistin increased the expression of PPARγ2 messenger RNA. Moreover, the addition of wistin upregulated the expression of PPARγ-target genes, aP2 and adiponectin in 3T3-L1 cells. To our knowledge, wistin is the first isoflavonoid O-glycoside that exhibits PPARγ agonist activity.
Adipose tissue, which is conserved in higher eukaryotes, plays central roles in controlling the body’s energy balance, including excess energy storage and energy expenditure during starvation. In adipogenesis, intranuclear receptor, peroxisome proliferator–activated receptor gamma (PPARγ) is a key molecule, and PPARγ agonists can promote adipogenesis. Many studies on the in vitro screening of PPARγ agonists with compounds derived from various materials have been reported; however, in vivo assays for quick examination of these feeding effects have not been established. In this study, we developed a technique using a lipophilic fluorescent reagent, Nile red to quantitatively estimate the adipose tissue volumes by using Japanese rice fish, medaka (Oryzias latipes) and studied effects of dietary soy sauce oil (SSO), which is a discarded by-product from Japanese traditional food and is known to have PPARγ-agonistic activity, on adipogenesis. We found that SSO feeding increased the adipose tissue volumes, and the expression levels of adipogenesis-related genes increased in these medaka larvae. These results suggest that SSO feeding increases the adipose tissue volumes through adipogenesis promotion by PPARγ-agonistic activity in medaka, and medaka is a powerful model for studying adipogenesis. Furthermore, our study also demonstrates the availability of SSO as a dietary additive for farmed fish.
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