PDF(Pubmed)
Abstract：
Consumption of a high-fat diet (HFD) has been suggested as a contributing factor behind increased intestinal permeability in obesity, leading to increased plasma levels of microbial endotoxins and, thereby, increased systemic inflammation. We and others have shown that HFD can induce jejunal expression of the ketogenic rate-limiting enzyme mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (HMGCS). HMGCS is activated via the free fatty acid binding nuclear receptor PPAR-α, and it is a key enzyme in ketone body synthesis that was earlier believed to be expressed exclusively in the liver. The function of intestinal ketogenesis is unknown but has been described in suckling rats and mice pups, possibly in order to allow large molecules, such as immunoglobulins, to pass over the intestinal barrier. Therefore, we hypothesized that ketone bodies could regulate intestinal barrier function, e.g., via regulation of tight junction proteins. The primary aim was to compare the effects of HFD that can induce intestinal ketogenesis to an equicaloric carbohydrate diet on inflammatory responses, nutrition sensing, and intestinal permeability in human jejunal mucosa. Fifteen healthy volunteers receiving a 2-week HFD diet compared to a high-carbohydrate diet were compared. Blood samples and mixed meal tests were performed at the end of each dietary period to examine inflammation markers and postprandial endotoxemia. Jejunal biopsies were assessed for protein expression using Western blotting, immunohistochemistry, and morphometric characteristics of tight junctions by electron microscopy. Functional analyses of permeability and ketogenesis were performed in Caco-2 cells, mice, and human enteroids. Ussing chambers were used to analyze permeability. CRP and ALP values were within normal ranges and postprandial endotoxemia levels were low and did not differ between the two diets. The PPARα receptor was ketone body-dependently reduced after HFD. None of the tight junction proteins studied, nor the basal electrical parameters, were different between the two diets. However, the ketone body inhibitor hymeglusin increased resistance in mucosal biopsies. In addition, the tight junction protein claudin-3 was increased by ketone inhibition in human enteroids. The ketone body β-Hydroxybutyrate (βHB) did not, however, change the mucosal transition of the large-size molecular FD4-probe or LPS in Caco-2 and mouse experiments. We found that PPARα expression was inhibited by the ketone body βHB. As PPARα regulates HMGCS expression, the ketone bodies thus exert negative feedback signaling on their own production. Furthermore, ketone bodies were involved in the regulation of permeability on intestinal mucosal cells in vitro and ex vivo. We were not, however, able to reproduce these effects on intestinal permeability in vivo in humans when comparing two weeks of high-fat with high-carbohydrate diet in healthy volunteers. Further, neither the expression of inflammation markers nor the aggregate tight junction proteins were changed. Thus, it seems that not only HFD but also other factors are needed to permit increased intestinal permeability in vivo. This indicates that the healthy gut can adapt to extremes of macro-nutrients and increased levels of intestinally produced ketone bodies, at least during a shorter dietary challenge.
摘要：
高脂肪饮食（HFD）的消费已被认为是肥胖患者肠道通透性增加的一个促成因素。导致血浆微生物内毒素水平升高，因此，全身性炎症增加。我们和其他人已经表明HFD可以诱导生酮限速酶线粒体3-羟基-3-甲基戊二酰辅酶A合酶（HMGCS）的空肠表达。HMGCS通过游离脂肪酸结合核受体PPAR-α激活，它是酮体合成中的关键酶，早期被认为仅在肝脏中表达。肠道酮生成的功能尚不清楚，但已在乳鼠和幼鼠中进行了描述，可能是为了允许大分子，如免疫球蛋白，穿过肠道屏障。因此,我们假设酮体可以调节肠道屏障功能，例如，通过调节紧密连接蛋白。主要目的是比较可诱导肠道生酮的HFD与马尾碳水化合物饮食对炎症反应的影响。营养传感，和人空肠粘膜的肠通透性。比较了接受2周HFD饮食与高碳水化合物饮食相比的15名健康志愿者。在每个饮食期结束时进行血样和混合餐测试以检查炎症标志物和餐后内毒素血症。空肠活检用蛋白质印迹法评估蛋白质表达，免疫组织化学，通过电子显微镜观察紧密连接的形态特征。在Caco-2细胞中进行通透性和生酮的功能分析，老鼠,和人类肠样物质。使用Ussing室分析渗透率。CRP和ALP值在正常范围内，餐后内毒素血症水平较低，两种饮食之间没有差异。HFD后PPARα受体呈酮体依赖性降低。没有研究紧密连接蛋白，也不是基础电参数，两种饮食不同。然而,酮体抑制剂hymeglusin增加了粘膜活检的耐药性。此外,人类类肠样物质中的酮抑制作用增加了紧密连接蛋白claudin-3。酮体β-羟基丁酸酯（βHB）没有，然而,在Caco-2和小鼠实验中改变大分子FD4探针或LPS的粘膜转变。我们发现PPARα的表达被酮体βHB抑制。由于PPARα调节HMGCS的表达，因此,酮体对它们自己的生产施加负反馈信号。此外，酮体参与体外和离体肠粘膜细胞通透性的调节。我们不是,然而,当在健康志愿者中比较两周的高脂肪和高碳水化合物饮食时，能够再现这些对人体体内肠道通透性的影响。Further,炎症标志物和聚集紧密连接蛋白的表达均未改变。因此，似乎不仅需要HFD，还需要其他因素来增加体内肠通透性。这表明健康的肠道可以适应极端的宏观营养和增加的肠道产生的酮体水平，至少在较短的饮食挑战期间。
