利用植化素調控骨骼肌汲取葡萄糖以改善胰島素阻抗性

Abstract

許多植化素被認為具有抗糖尿病的藥理活性，但是作用機制不明，而將多種植化素組合起來的理論基礎和應用研究更是闕如。本研究採用脂肪酸誘導而呈現胰島素阻抗症狀之骨骼肌作為模型，探討骨骼肌中訊息傳遞路徑和囊泡運輸機制，以了解植化素如何激活葡萄糖汲取能力。本模型於胰島素處理下，細胞膜上葡萄糖轉運蛋白所介導之葡萄糖汲取活性顯著低落。然而，許多植化素對葡萄糖汲取能力卻具有正面提升的效果。進而，藉由比較骨骼肌中訊息分子抑制和囊泡化學消融的結果，將這些植化素刺激下訊息傳遞路徑和囊泡運輸的現象加以量化和分類，結果顯示利用多種植化素組合來提升骨骼肌之葡萄糖汲取能力，似乎可行。根據試驗結果，在呈胰島素阻抗之肌管細胞中，胰島素所激發的內質體循環機制很低，僅約占16％，表示仍有極大的增進空間。胰島素和白藜蘆醇所激發的效應該是第四號葡萄糖轉運蛋白儲存囊泡（glucose transporter 4 storage vesicle, GSV）分別佔各自總體的84％與94％；相反地，兒茶素和阿魏酸作用下所刺激之內質體循環，佔了各自總效應中相當大的比例，分別為78％與71％。在訊息傳遞鏈方面，胰島素與阿魏酸的作用主要仰賴磷脂醯肌醇3激酶（phosphatidyl inositol-3 kinase, PI3K）訊息分子，然而白藜蘆醇和兒茶素的作用卻主要仰賴腺苷酸活化蛋白激酶（AMP-activated proteinkinase, AMPK）訊息分子。上述結果顯示肌肉細胞汲取葡萄糖的能力受到至少四種方式進行調控，亦即經由兩種含葡萄糖轉運蛋白的囊泡和兩種訊息分子的交錯作用的結果。於是，本研究接續進行複合植化素組合物的探討。而結果顯示，各類作用機制之植化素間可協調性加成提升骨骼肌汲取葡萄糖的能力，進而發現在訊息傳遞路徑PI3K和AMPK與胞內囊泡GSV和內質體之間，可能存在一個匯流的結點。綜而言之，本研究結果證實，多種植化素具有強化骨骼肌的葡萄糖汲取能力，並且這些植化素的作用具互補性質。因此，可以應用植化素的組合物，藉由骨骼肌細胞內的替代途徑，繞開胰島素阻抗性細胞功能缺陷，對代謝症候群疾病如糖尿病與高血糖發揮治療的效果。

Many phytochemicals have been reported to have the ability of confronting type-2 diabetes, but guidelines for their uses is quite lacking. Signaling and vesicle transport in skeletal muscle have been investigated to explain how phytochemicals stimulate glucose transport. Glucose uptake and utilization (glycogen and lipid synthesis) were assessed using a free fatty acid induced insulin resistant cell model in this study. It was found that the glucose uptake activity of the cell model was restored in response to certain phytochemicals. By comparing the results of signaling inhibition and vesicles ablation we found that quantification and classification of these phytochemicals in terms of their functions in mediating signaling and vesicular transport phenomena seemed to be feasible. The results also showed that insulin-mediated endosomal recycling pathway was low in this insulin resistant cell model. Insulin and resveratrol affected the glucose transporter 4 (GLUT4) storage vesicle. Conversely, (−)-epigallocatechin-3-gallate (EGCG) and ferulic acid majorly stimulated endosomal compartment. Insulin and ferulic acid mainly activated phosphoinositide 3-kinase (PI3K) signaling molecules, however, resveratrol and catechin primarily activated AMP-activated protein kinase (AMPK) signaling molecules. These findings suggested that GLUT4 is regulated by at least four ways, i.e. cross-linking of two kinds of vesicles and two signaling molecules. Thus, the combination of EGCG, resveratrol and ferulic acid can drive insulin-independent GLUT4 translocation in muscle cells synergistically. In conclusion, this study revealed the specificity and complementarity of these phytochemicals and indicated that their combination may possess a therapeutic effect on metabolic disorders by modulating glucose metabolism. These findings provide a strategy for the understanding of the anti-diabetic potential of phytochemical combination and might be exploited to bypass intracellular trafficking defects of insulin resistance.