Buller Carolyn L, Loberg Robert D, Fan Ming-Hui, Zhu Qihong, Park James L, Vesely Eileen, Inoki Ken, Guan Kun-Liang, Brosius Frank C
Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, Michigan 48109-0680, USA.
Am J Physiol Cell Physiol. 2008 Sep;295(3):C836-43. doi: 10.1152/ajpcell.00554.2007. Epub 2008 Jul 23.
Glucose transport is a highly regulated process and is dependent on a variety of signaling events. Glycogen synthase kinase-3 (GSK-3) has been implicated in various aspects of the regulation of glucose transport, but the mechanisms by which GSK-3 activity affects glucose uptake have not been well defined. We report that basal glycogen synthase kinase-3 (GSK-3) activity regulates glucose transport in several cell types. Chronic inhibition of basal GSK-3 activity (8-24 h) in several cell types, including vascular smooth muscle cells, resulted in an approximately twofold increase in glucose uptake due to a similar increase in protein expression of the facilitative glucose transporter 1 (GLUT1). Conversely, expression of a constitutively active form of GSK-3beta resulted in at least a twofold decrease in GLUT1 expression and glucose uptake. Since GSK-3 can inhibit mammalian target of rapamycin (mTOR) signaling via phosphorylation of the tuberous sclerosis complex subunit 2 (TSC2) tumor suppressor, we investigated whether chronic GSK-3 effects on glucose uptake and GLUT1 expression depended on TSC2 phosphorylation and TSC inhibition of mTOR. We found that absence of functional TSC2 resulted in a 1.5-to 3-fold increase in glucose uptake and GLUT1 expression in multiple cell types. These increases in glucose uptake and GLUT1 levels were prevented by inhibition of mTOR with rapamycin. GSK-3 inhibition had no effect on glucose uptake or GLUT1 expression in TSC2 mutant cells, indicating that GSK-3 effects on GLUT1 and glucose uptake were mediated by a TSC2/mTOR-dependent pathway. The effect of GSK-3 inhibition on GLUT1 expression and glucose uptake was restored in TSC2 mutant cells by transfection of a wild-type TSC2 vector, but not by a TSC2 construct with mutated GSK-3 phosphorylation sites. Thus, TSC2 and rapamycin-sensitive mTOR function downstream of GSK-3 to modulate effects of GSK-3 on glucose uptake and GLUT1 expression. GSK-3 therefore suppresses glucose uptake via TSC2 and mTOR and may serve to match energy substrate utilization to cellular growth.
葡萄糖转运是一个受到高度调控的过程，并且依赖于多种信号事件。糖原合酶激酶-3（GSK-3）参与了葡萄糖转运调控的多个方面，但GSK-3活性影响葡萄糖摄取的机制尚未完全明确。我们报告称，基础糖原合酶激酶-3（GSK-3）活性在几种细胞类型中调节葡萄糖转运。在包括血管平滑肌细胞在内的几种细胞类型中，对基础GSK-3活性进行慢性抑制（8 - 24小时），由于易化性葡萄糖转运蛋白1（GLUT1）的蛋白质表达有类似程度的增加，导致葡萄糖摄取增加了约两倍。相反，组成型活性形式的GSK-3β的表达导致GLUT1表达和葡萄糖摄取至少降低两倍。由于GSK-3可通过磷酸化结节性硬化复合物亚基2（TSC2）肿瘤抑制因子来抑制哺乳动物雷帕霉素靶蛋白（mTOR）信号传导，我们研究了GSK-3对葡萄糖摄取和GLUT1表达的慢性影响是否依赖于TSC2磷酸化以及TSC对mTOR的抑制作用。我们发现，缺乏功能性TSC2会导致多种细胞类型中的葡萄糖摄取和GLUT1表达增加1.5至3倍。用雷帕霉素抑制mTOR可阻止葡萄糖摄取和GLUT1水平的这些增加。GSK-3抑制对TSC2突变细胞中的葡萄糖摄取或GLUT1表达没有影响，这表明GSK-3对GLUT1和葡萄糖摄取的影响是由TSC2 / mTOR依赖性途径介导的。通过转染野生型TSC2载体，可恢复GSK-3抑制对TSC2突变细胞中GLUT1表达和葡萄糖摄取的影响，但转染具有突变的GSK-3磷酸化位点的TSC2构建体则不能恢复。因此，TSC2和对雷帕霉素敏感的mTOR在GSK-3的下游发挥作用，以调节GSK-3对葡萄糖摄取和GLUT1表达的影响。因此，GSK-3通过TSC2和mTOR抑制葡萄糖摄取，并可能有助于使能量底物利用与细胞生长相匹配。
