Novel approaches to obesity: modulation of intestinal glucose absorption in humans

解释 糖尿病和肥胖是新出现的世界性健康问题。针对这两种情况的新的预防和治疗选择可以基于抑制或抑制营养吸收的战略。类似的策略是目前临床上用于抑制脂肪吸收、胆固醇吸收和复杂碳水化合物的肠道分解代谢的药物的基础。一类延缓或抑制葡萄糖吸收的新型药物可能会在控制糖尿病和肥胖症方面产生重大影响。新的证据表明，顶端或腔面的GLUT2是糖吸收的主要途径，因此是这类潜在药物的一个有吸引力的靶点。 类黄酮是广泛存在于食物中的多酚类物质，尤其是水果和蔬菜。黄酮类化合物的营养功能尚不清楚。槲皮素是一种常见的摄入类黄酮，每天通过饮食摄入20-100毫克。摄取后，血浆中黄酮类化合物的峰值浓度不会超过每升1-2微克分子，但肠腔浓度据信会高出约50倍。基于这些高的腔内浓度，我们提出了腔内黄酮类化合物的一个新的作用可能是抑制、重新分配或坦率地抑制候选营养物质的肠道吸收。无论是作为食物成分的黄酮类化合物，还是与食物联合使用的黄酮类化合物，都可能具有这些作用，而且这些作用不需要类黄酮本身被吸收。数据显示，食物中发现的一些黄酮类化合物抑制了维生素C和葡萄糖的肠道运输和吸收，这部分支持了这一建议。这一发现表明，人类摄取的主要类黄酮类化合物--栎素可能通过钠非依赖性促进葡萄糖转运体GLUT2调节葡萄糖在肠道的吸收。 尽管这些发现很有希望，但仍有许多不确定因素。一些研究人员认为，类黄酮通过钠依赖的葡萄糖转运体SGLT1，通过钠依赖的途径减少葡萄糖的摄取。这一结论是基于使用肠道细胞、刷状边缘膜小泡或表达SGLT1的非洲爪哇卵母细胞的实验得出的。在细胞和囊泡制剂中，很难区分哪些转运蛋白(S)被抑制，而且所有这些实验中使用的底物和类黄酮类化合物的浓度与体内条件无关。在SGLT1在细胞或非洲爪哇卵母细胞中过表达的系统中，黄酮类化合物对葡萄糖运输的影响不是很大，就是没有直接测试。其他研究人员认为，根据类黄酮在分离的酶系统中的行为，它们可能是非特异性的抑制剂。目前尚不确定哪些肠道葡萄糖转运蛋白被不同的黄酮类化合物抑制，哪些黄酮类化合物是最有效的葡萄糖转运蛋白抑制剂，不同底物对转运蛋白的抑制是否有选择性，是否必须首先脱糖或转运才能发生抑制作用，以及合适的转运蛋白是否与黄酮类化合物的抑制浓度处于同一位置。 解决这些问题将提供黄酮类作用的明确数据基础，可作为类黄酮类对糖吸收影响的试点临床研究的基础。为了测试转运体的特异性，我们研究了注射CRNAs的卵母细胞表达特定的肠糖转运体，并将这些卵母细胞与不同的类黄酮类化合物孵育以确定其效力。为了了解类黄酮对糖转运的抑制是否是仅在卵母细胞中表达的葡萄糖转运蛋白所特有的，我们还对细胞系统进行了研究。黄酮类化合物、类黄酮浓度和糖底物浓度都被选为具有体内相关性的物质。我们发现，几种黄酮类化合物是GLUT2介导的葡萄糖和果糖转运的有效抑制剂，但对其他主要的肠道糖转运蛋白没有影响；类黄酮结构和糖基化影响抑制作用；GLUT2在细胞中过表达并存在于肠道细胞模型中，可被相关的黄酮类化合物抑制；GLUT2存在于适当的位置，可以被管腔类黄酮抑制；并且有效的抑制剂--栎素本身不是由GLUT2运输的。我们的发现为先导性临床研究的推进提供了必要的基础，现在可以继续进行了。

Explanation Diabetes and obesity are emerging worldwide health problems. New prevention and treatment options for both conditions could be based on strategies to dampen or inhibit nutrient absorption. Similar strategies are the basis of agents currently used clinically to inhibit fat absorption, cholesterol absorption, and intestinal catabolism of complex carbohydrates. A new class of agents that delayed or inhibited glucose absorption could have substantial impact in managing diabetes and obesity. Emerging evidence indicates that apical, or luminal facing, GLUT2 is a major pathway of sugar absorption, and is therefore an attractive target of such potential agents. Flavonoids are polyphenols that are widely distributed in foods, especially fruits and vegetables. Nutritive functions of flavonoids are unknown. Quercetin is a commonly ingested flavonoid, and 20-100 mg daily is ingested by dietary intake. Peak plasma concentrations of flavonoids such as quercetin do not exceed 1-2 umoles per liter after ingestion, but intestinal luminal concentrations are believed to be approximately 50 fold higher. Based on these high intraluminal concentrations, we proposed that a novel action of intraluminal flavonoids may be to dampen, re-distribute, or frankly inhibit intestinal absorption of candidate nutrients. Flavonoids either as food components or co-administered with foods could potentially have these actions, and such actions would not require flavonoids themselves to be absorbed. Partial support for this proposal was provided by data showing that some flavonoids found in foods inhibited vitamin C and glucose intestinal transport and absorption. The findings suggested that quercetin, the dominant flavonoid ingested by humans, may modulate glucose intestinal absorption by the sodium-independent facilitative glucose transporter GLUT2. Although these findings are promising, a number of uncertainties remain. Some investigators suggested that flavonoids decreased glucose uptake by a sodium-dependent pathway, via the sodium-dependent glucose transporter SGLT1. This conclusion was based on experiments using intestinal cells, brush border membrane vesicles, or Xenopus laevis oocytes expressing SGLT1. In cell and vesicle preparations it is difficult to distinguish which transporter(s) are inhibited, and the concentrations of substrates and flavonoids used in all of these experiments were not relevant to in vivo conditions. In systems where SGLT1 was either overexpressed in cells or in Xenopus oocytes, flavonoid effects on glucose transport were either modest or not directly tested. Other investigators suggested that flavonoids could be non-specific inhibitors, based on their behavior in isolated enzyme systems. It is uncertain which intestinal glucose transporters are inhibited by different flavonoids, which flavonoids are the most potent inhibitors of glucose transport, whether there is selectivity for inhibition of transport of different substrates, whether flavonoids must first be deglycosylated or transported for inhibition to occur, and whether the appropriate transporters are in the same location as the inhibitory concentrations of flavonoids. Addressing these issues would provide a clear data base of flavonoid action that could serve as the foundation of a pilot clinical study of flavonoid effects on sugar absorption. To test transporter specificity, we studied oocytes that were injected with cRNAs to express specific intestinal sugar transporters, and incubated these oocytes with a variety of flavonoids to determine potency. To learn whether flavonoid inhibition of sugar transport was specific for glucose transporters expressed only in oocytes, cell systems were also studied. Flavonoids, flavonoid concentrations, and sugar substrate concentrations were all selected to have in vivo relevance. We showed that several flavonoids were potent inhibitors of GLUT2-mediated glucose and fructose transport but had no effect on other major intestinal sugar transporters; that flavonoid structure and glycosylation affected inhibition; that GLUT2 overexpressed in cells and present in an intestinal cell model was inhibited by the relevant flavonoids; that GLUT2 was present in the proper location to be inhibited by luminal flavonoids; and that the potent inhibitor, quercetin, was not itself transported by GLUT2. Our findings provide the essential foundation for a pilot clinical study to move forward, and this can now proceed.