Luo Bai, Parker Glendon J, Cooksey Robert C, Soesanto Yudi, Evans Mark, Jones Deborah, McClain Donald A
Division of Endocrinology, University of Utah School of Medicine and Veterans Affairs Medical Center, Salt Lake City, Utah 84132, USA.
J Biol Chem. 2007 Mar 9;282(10):7172-80. doi: 10.1074/jbc.M607362200. Epub 2007 Jan 16.
The hexosamine biosynthesis pathway (HBP) serves as a nutrient sensor and has been implicated in the development of type 2 diabetes. We previously demonstrated that fatty acid oxidation was enhanced in transgenic mouse adipocytes, wherein the rate-limiting enzyme of the HBP, glutamine:fructose-6-phosphate amidotransferase (GFA), was overexpressed. To explore the molecular mechanism of the HBP-induced fatty acid oxidation in adipocytes, we studied AMP-activated protein kinase (AMPK), an energy sensor that stimulates fatty acid oxidation by regulating acetyl-CoA carboxylase (ACC) activity. Phosphorylation and activity of AMPK were increased in transgenic fat pads and in 3T3L1 adipocytes treated with glucosamine to stimulate hexosamine flux. Glucosamine also stimulated phosphorylation of ACC and fatty acid oxidation in 3T3L1 adipocytes, and these stimulatory effects were diminished by adenovirus-mediated expression of a dominant negative AMPK in 3T3L1 adipocytes. Conversely, blocking the HBP with a GFA inhibitor reduced AMPK activity, ACC phosphorylation, and fatty acid oxidation. These changes are not explained by alterations in the cellular AMP/ATP ratio. Further demonstrating that AMPK is regulated by the HBP, we found that AMPK was recognized by succinylated wheat germ agglutinin, which specifically binds O-GlcNAc. The levels of AMPK in succinylated wheat germ agglutinin precipitates correlated with hexosamine flux in mouse fat pads and 3T3L1 adipocytes. Moreover, removal of O-GlcNAc by hexosaminidase reduced AMPK activity. We conclude that chronically high hexosamine flux stimulates fatty acid oxidation by activating AMPK in adipocytes, in part through O-linked glycosylation.
己糖胺生物合成途径（HBP）作为一种营养传感器，与2型糖尿病的发生发展有关。我们之前证明，在转基因小鼠脂肪细胞中脂肪酸氧化增强，其中HBP的限速酶谷氨酰胺：果糖-6-磷酸酰胺转移酶（GFA）过表达。为了探究HBP诱导脂肪细胞脂肪酸氧化的分子机制，我们研究了AMP激活的蛋白激酶（AMPK），这是一种能量传感器，通过调节乙酰辅酶A羧化酶（ACC）的活性来刺激脂肪酸氧化。在转基因脂肪垫以及用葡糖胺处理以刺激己糖胺通量的3T3L1脂肪细胞中，AMPK的磷酸化和活性增加。葡糖胺还刺激3T3L1脂肪细胞中ACC的磷酸化和脂肪酸氧化，而腺病毒介导的显性负性AMPK在3T3L1脂肪细胞中的表达减弱了这些刺激作用。相反，用GFA抑制剂阻断HBP会降低AMPK活性、ACC磷酸化和脂肪酸氧化。这些变化不能用细胞内AMP/ATP比值的改变来解释。进一步证明AMPK受HBP调节，我们发现AMPK可被琥珀酰化麦胚凝集素识别，该凝集素特异性结合O-连接的N-乙酰葡糖胺。琥珀酰化麦胚凝集素沉淀中AMPK的水平与小鼠脂肪垫和3T3L1脂肪细胞中的己糖胺通量相关。此外，用氨基己糖苷酶去除O-连接的N-乙酰葡糖胺会降低AMPK活性。我们得出结论，长期高己糖胺通量通过激活脂肪细胞中的AMPK来刺激脂肪酸氧化，部分是通过O-连接糖基化实现的。
