Lim Chun-Yan, Hong Wanjin, Han Weiping
Laboratory of Metabolic Medicine, Singapore Bioimaging Consortium, 11 Biopolis Way, 138667, Singapore.
Institute of Molecular and Cell Biology, Agency for Science, Technology and Research (A*STAR), Singapore.
Biochem J. 2015 Nov 1;471(3):381-9. doi: 10.1042/BJ20150301. Epub 2015 Sep 1.
Adiponectin, a hormone secreted from adipocytes and released at a high rate into the circulation, plays a pivotal role in maintaining insulin sensitivity at the whole-body level. Despite the importance of this adipokine in metabolic homoeostasis, the mechanism of its secretion from adipocytes remains largely unclear. In the present study, we investigated the subcellular localization of adiponectin, and its secretion regulation in 3T3-L1-differentiated adipocytes, using biochemical methods and fluorescence microscopic imaging. We show that adiponectin is localized in vesicular compartments with no apparent overlap with the Golgi apparatus or endosomes. Moreover, adiponectin-containing vesicles are enriched in two distinct pools: one at the plasma membrane (PM) and the other co-fractionating with endoplasmic reticulum membranes. When viewed under a total internal refection fluorescence microscope, a subset of adiponectin-Venus vesicles is readily observed in proximity to PMs, and could be released in response to insulin. Insulin-stimulated adiponectin release appears to be from a pre-existing pool of vesicles, and is not dependent on new protein synthesis, because adiponectin mRNA levels remain unchanged over a 6-h period of insulin treatment, and inhibition of protein synthesis has no effect on adiponectin release. Disruption of insulin signalling by inhibitors of phosphoinositide 3-kinase and protein kinase B (Akt)-1/2 abrogates the stimulated release of adiponectin. Taken together, our results show that adiponectin is stored in a unique vesicular compartment, and released through a regulated exocytosis pathway that is dependent on insulin signalling.
脂联素是一种由脂肪细胞分泌并以高速率释放到循环系统中的激素，在维持全身水平的胰岛素敏感性方面发挥着关键作用。尽管这种脂肪因子在代谢稳态中很重要，但其从脂肪细胞分泌的机制仍不清楚。在本研究中，我们使用生化方法和荧光显微镜成像技术，研究了脂联素在3T3-L1分化脂肪细胞中的亚细胞定位及其分泌调节。我们发现脂联素定位于囊泡区室，与高尔基体或内体无明显重叠。此外，含有脂联素的囊泡富集在两个不同的池中：一个位于质膜（PM），另一个与内质网共分离。在全内反射荧光显微镜下观察时，很容易在质膜附近观察到一部分脂联素-维纳斯囊泡，并且它们可响应胰岛素而释放。胰岛素刺激的脂联素释放似乎来自预先存在的囊泡池，并且不依赖于新的蛋白质合成，因为在胰岛素处理的6小时内脂联素mRNA水平保持不变，并且蛋白质合成的抑制对脂联素释放没有影响。磷酸肌醇3激酶和蛋白激酶B（Akt）-1/2抑制剂对胰岛素信号的破坏消除了脂联素的刺激释放。综上所述，我们的结果表明脂联素储存在独特的囊泡区室中，并通过依赖于胰岛素信号的调节性胞吐途径释放。
