Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup 580-185, Republic of Korea.
Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup 580-185, Republic of Korea.
J Ethnopharmacol. 2019 Apr 24;234:172-179. doi: 10.1016/j.jep.2019.01.014. Epub 2019 Jan 17.
Mistletoe (Viscum album), an evergreen parasitic plant, has been widely used as an oriental phytomedicine to treat diabetes mellitus. However, it is unknown which mistletoe constituent exerts the beneficial effect against the disease. In this study, we examined the hypoglycemic activity of mistletoe and investigated whether the polypeptide viscothionin, purified from mistletoe, was responsible for the activity.
Mistletoe extracts were prepared by heating mistletoe powder made of leaves and twigs in water for 3, 6, 9, and 12 h. Rat insulinoma RINm5F cells were used to test the cytotoxicity of the extracts and their effects on the secretion of insulin and its precursor, C-peptide. The inhibitory effects of a mistletoe extract on glucose absorption were measured using an α-glucosidase inhibition assay. To determine the component of mistletoe responsible for the observed effects, the mistletoe extract was precipitated with ethanol or hydrolyzed with a protease for further testing. A potential active constituent of mistletoe was isolated by chromatography and molecular weight cut-off fractionation, and its ability to induce insulin secretion was investigated.
A 12-h heat-treated mistletoe extract, showing no cytotoxicity, significantly increased the secretion of insulin and C-peptide by RINm5F cells and enhanced the expression of glucose transporter type 4 (GLUT-4), insulin receptor substrate 1 (IRS-1), and protein kinase B (also known as AKT) in differentiated C2C12 cells. The extract also inhibited α-glucosidase activity. After ethanol precipitation, the extract showed much stronger effects on insulin- and C-peptide-secreting activities of cells, whereas the enzyme-hydrolyzed extract was less effective than the original extract, suggesting that the effect was mediated by a proteinaceous constituent of mistletoe. Subsequent analysis showed that viscothionin, a heat-stable 6-kDa polypeptide isolated from mistletoe, increased the level of insulin secretion by more than 20-fold compared to that induced by the extract.
Our study indicates that the hypoglycemic effect of mistletoe is mediated by its insulinotropic action and α-glucosidase inhibitory activity, and the effect is due to viscothionin, one of the major bioactive constituents of mistletoe.
槲寄生（Viscum album）是一种常绿寄生植物，已被广泛用作东方植物药来治疗糖尿病。然而，哪种槲寄生成分具有治疗该疾病的有益作用尚不清楚。在这项研究中，我们检测了槲寄生的降血糖活性，并研究了是否从槲寄生中分离得到的多肽 viscothionin 对活性负责。
通过将叶和小枝制成的槲寄生粉末在水中加热 3、6、9 和 12 小时来制备槲寄生提取物。使用大鼠胰岛素瘤 RINm5F 细胞测试提取物的细胞毒性及其对胰岛素及其前体 C 肽分泌的影响。使用α-葡萄糖苷酶抑制测定法测量槲寄生提取物对葡萄糖吸收的抑制作用。为了确定槲寄生中负责观察到的作用的成分，用乙醇沉淀槲寄生提取物或用蛋白酶水解进行进一步测试。通过色谱和分子量截止分级分离分离槲寄生的潜在活性成分，并研究其诱导胰岛素分泌的能力。
12 小时热处理的槲寄生提取物无细胞毒性，可显著增加 RINm5F 细胞胰岛素和 C 肽的分泌，并增强分化的 C2C12 细胞中葡萄糖转运蛋白 4（GLUT-4）、胰岛素受体底物 1（IRS-1）和蛋白激酶 B（也称为 AKT）的表达。提取物还抑制α-葡萄糖苷酶活性。乙醇沉淀后，提取物对细胞胰岛素和 C 肽分泌活性的作用更强，而酶水解提取物的作用不如原始提取物，这表明该作用是由槲寄生的蛋白质成分介导的。随后的分析表明，从槲寄生中分离得到的热稳定 6 kDa 多肽 viscothionin 可使胰岛素分泌水平比提取物诱导的水平增加 20 倍以上。
我们的研究表明，槲寄生的降血糖作用是通过其胰岛素促分泌作用和α-葡萄糖苷酶抑制活性介导的，作用是由于 viscothionin，这是槲寄生的主要生物活性成分之一。
