Briaud I, Harmon J S, Kelpe C L, Segu V B, Poitout V
Pacific Northwest Research Institute, University of Washington, Seattle 98122, USA.
Diabetes. 2001 Feb;50(2):315-21. doi: 10.2337/diabetes.50.2.315.
Prolonged exposure of isolated islets to supraphysiologic concentrations of palmitate decreases insulin gene expression in the presence of elevated glucose levels. This study was designed to determine whether or not this phenomenon is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids. Gene expression of sn-glycerol-3-phosphate acyltransferase (GPAT), diacylglycerol acyltransferase (DGAT), and hormone-sensitive lipase (HSL), three key enzymes of lipid metabolism, was detected in isolated rat islets. Their levels of expression were not affected after a 72-h exposure to elevated glucose and palmitate. To determine the effects of glucose on palmitate-induced neutral lipid synthesis, isolated rat islets were cultured for 72 h with trace amounts of [14C]palmitate with or without 0.5 mmol/l unlabeled palmitate, at 2.8 or 16.7 mmol/l glucose. Glucose increased incorporation of [14C]palmitate into complex lipids. Addition of exogenous palmitate directed lipid metabolism toward neutral lipid synthesis. As a result, neutral lipid mass was increased upon prolonged incubation with elevated palmitate only in the presence of high glucose. The ability of palmitate to increase neutral lipid synthesis in the presence of high glucose was concentration-dependent in HIT cells and was inversely correlated to insulin mRNA levels. 2-Bromopalmitate, an inhibitor of fatty acid mitochondrial beta-oxidation, reproduced the inhibitory effect of palmitate on insulin mRNA levels. In contrast, palmitate methyl ester, which is not metabolized, and the medium-chain fatty acid octanoate, which is readily oxidized, did not affect insulin gene expression, suggesting that fatty-acid inhibition of insulin gene expression requires activation of the esterification pathway. These results demonstrate that inhibition of insulin gene expression upon prolonged exposure of islets to palmitate is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids.
在葡萄糖水平升高的情况下，将分离的胰岛长时间暴露于超生理浓度的棕榈酸会降低胰岛素基因表达。本研究旨在确定这种现象是否与脂肪酸酯化生成中性脂质的葡萄糖依赖性增加有关。在分离的大鼠胰岛中检测了脂质代谢的三种关键酶，即甘油-3-磷酸酰基转移酶（GPAT）、二酰甘油酰基转移酶（DGAT）和激素敏感性脂肪酶（HSL）的基因表达。在暴露于高葡萄糖和棕榈酸72小时后，它们的表达水平未受影响。为了确定葡萄糖对棕榈酸诱导的中性脂质合成的影响，将分离的大鼠胰岛在2.8或16.7 mmol/l葡萄糖条件下，用微量的[14C]棕榈酸培养72小时，添加或不添加0.5 mmol/l未标记的棕榈酸。葡萄糖增加了[14C]棕榈酸掺入复合脂质中的量。添加外源性棕榈酸使脂质代谢导向中性脂质合成。结果，仅在高葡萄糖存在的情况下，长时间与升高的棕榈酸孵育会增加中性脂质总量。在HIT细胞中，棕榈酸在高葡萄糖存在下增加中性脂质合成的能力是浓度依赖性的，并且与胰岛素mRNA水平呈负相关。脂肪酸线粒体β氧化抑制剂2-溴棕榈酸重现了棕榈酸对胰岛素mRNA水平的抑制作用。相比之下，未代谢的棕榈酸甲酯和易于氧化的中链脂肪酸辛酸不影响胰岛素基因表达，这表明脂肪酸对胰岛素基因表达的抑制需要酯化途径的激活。这些结果表明，胰岛长时间暴露于棕榈酸后胰岛素基因表达的抑制与脂肪酸酯化生成中性脂质的葡萄糖依赖性增加有关。
