Heart Emma, Cline Gary W, Collis Leon P, Pongratz Rebecca L, Gray Joshua P, Smith Peter J S
BioCurrents Research Center, Marine Biological Laboratory, 7 MBL St., Lillie 219, Woods Hole, MA 02543, USA.
Am J Physiol Endocrinol Metab. 2009 Jun;296(6):E1354-62. doi: 10.1152/ajpendo.90836.2008. Epub 2009 Mar 17.
Pyruvate cycling has been implicated in glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells. The operation of some pyruvate cycling pathways is proposed to necessitate malate export from the mitochondria and NADP(+)-dependent decarboxylation of malate to pyruvate by cytosolic malic enzyme (ME1). Evidence in favor of and against a role of ME1 in GSIS has been presented by others using small interfering RNA-mediated suppression of ME1. ME1 was also proposed to account for methyl succinate-stimulated insulin secretion (MSSIS), which has been hypothesized to occur via succinate entry into the mitochondria in exchange for malate and subsequent malate conversion to pyruvate. In contrast to rat, mouse beta-cells lack ME1 activity, which was suggested to explain their lack of MSSIS. However, this hypothesis was not tested. In this report, we demonstrate that although adenoviral-mediated overexpression of ME1 greatly augments GSIS in rat insulinoma INS-1 832/13 cells, it does not restore MSSIS, nor does it significantly affect GSIS in mouse islets. The increase in GSIS following ME1 overexpression in INS-1 832/13 cells did not alter the ATP-to-ADP ratio but was accompanied by increases in malate and citrate levels. Increased malate and citrate levels were also observed after INS-1 832/13 cells were treated with the malate-permeable analog dimethyl malate. These data suggest that although ME1 overexpression augments anaplerosis and GSIS in INS-1 832/13 cells, it is not likely involved in MSSIS and GSIS in pancreatic islets.
丙酮酸循环与胰腺β细胞的葡萄糖刺激胰岛素分泌（GSIS）有关。有人提出，一些丙酮酸循环途径的运作需要苹果酸从线粒体输出，并由胞质苹果酸酶（ME1）将苹果酸进行NADP(+)依赖性脱羧生成丙酮酸。其他人使用小干扰RNA介导的ME1抑制作用，提出了支持和反对ME1在GSIS中作用的证据。也有人提出ME1可解释甲基琥珀酸刺激的胰岛素分泌（MSSIS），据推测，这是通过琥珀酸进入线粒体以交换苹果酸，随后苹果酸转化为丙酮酸而发生的。与大鼠不同，小鼠β细胞缺乏ME1活性，这被认为可以解释它们缺乏MSSIS。然而，这一假设并未得到验证。在本报告中，我们证明，尽管腺病毒介导的ME1过表达极大地增强了大鼠胰岛素瘤INS-1 832/13细胞中的GSIS，但它并不能恢复MSSIS，对小鼠胰岛中的GSIS也没有显著影响。INS-1 细胞中ME1过表达后GSIS的增加并没有改变ATP与ADP的比率，但伴随着苹果酸和柠檬酸水平的升高。在用苹果酸可渗透类似物苹果酸二甲酯处理INS-1 832/13细胞后，也观察到苹果酸和柠檬酸水平升高。这些数据表明，尽管ME1过表达增强了INS-1 832/13细胞中的回补反应和GSIS，但它不太可能参与胰腺胰岛中的MSSIS和GSIS。
