Influence of a Water-Soluble Extract from Culture Medium of Ganoderma lucidum Mycelia (WER) on Carbohydrate Metabolism in the Liver of Type 2 Diabetic Mice

摘要

【目的】霊芝菌糸体培養培地抽出物 (WER) の長期摂取による 2 型糖尿病マウスの血糖上昇抑制作用を明らかにするために，本研究では肝臓での糖質代謝について検討した．【方法】KK-Ay マウスに WER を混合した飼料を自由摂取させ，8 週間飼育した．飼育終了後の肝臓における糖代謝関連酵素の発現量および酵素活性を解析した．さらに，グリコーゲン量を測定するとともに，GLUT2 発現量および AMP キナーゼ (AMPK)，グリコーゲン合成酵素キナーゼ 3β の活性化の有無を検討した．【結果】WER は，糖新生酵素のグルコース-6-ホスファターゼ，ホスホエノールピルビン酸カルボキシキナーゼ mRNA 量および酵素活性をともに減少させ，解糖系酵素であるグルコキナーゼを増加させた．さらに，GLUT2 の発現量を亢進させるとともに，グリコーゲン合成酵素を活性化してグリコーゲン量を増加させ，加えて AMPK を活性化することから，WER は肝臓での糖新生を抑制するとともに解糖系およびグリコーゲン合成を活性化させ，さらに肝臓への糖の輸送能を亢進して血糖上昇を抑制するものと考えられた．Objective: Recently, we reported that long-term intake of a water-soluble extract from culture medium of Ganoderma lucidum mycelia (WER) reduced hyperglycemia and enhanced glucose transporter-4 (GLUT4) translocation to the plasma membrane in skeletal muscles and adipose tissue in KK-Ay mice, a type 2 diabetic animal model with obesity. In the present study, we investigated the effect of WER on hepatic carbohydrate metabolism. Methods: Female KK-Ay mice were given free access to water and high-fat food containing 0.5% WER for 8 weeks, and blood glucose levels were assessed every week. At the end of the experimental period, the expression and activities of sugar metabolic enzymes in the liver were determined by Real Time RT-PCR and each activity measurement method. Also, the amount of glycogen was measured by anthrone-sulfuric acid method. Furthermore, the expression level of GLUT2 and activation of AMP kinase (AMPK) and glycogen synthase kinase 3β (GSk3β) was also determined by western blot analysis. Results: The mice with the high-fat ingestion showed a gradual increase in the levels of blood glucose and body weight. In the WER-treated mice, the blood glucose level was suppressed after 2 weeks of intake. The gene expression and enzyme activities of both glucose-6-phosphatase and phosphoenolpyruvate carboxykinase were suppressed, whereas those of glucokinase were increased in the mice with WER intake and pioglitazone administration. The accumulation of glycogen was increased. Moreover the expression of GLUT2 and phosphorylation levels of AMPK and GSk3β were also increased in the mice with WER intake. Conclusion: These results indicate that WER affects hepatic carbohydrate metabolism, which may derive from the suppression of gluconeogenesis through the modulation of related enzymes and enhancement of glucose uptake, glycolysis and glycogen synthesis.