Glutamine synthetase in cancer cell metabolism and oncogenesis

摘要 长期以来，代谢失调一直被认为是肿瘤疾病的一个关键标志。其中一个主要的 肿瘤细胞的代谢变化是通过谷氨酰胺分解增加谷氨酰胺(Gln)的使用：Gln是通过 谷氨酰胺酶(GLS)转化为谷氨酸(Glu)，由谷氨酸脱氢酶(Glud)转化 使α-酮戊二酸(AKG)进入三羧酸循环以恢复(代谢的补充 用于能源生产或生物合成的中间体)。众所周知，致癌的c-Myc(以下简称c-Myc 称为Myc)通过直接反式激活Gln转运体SLC1A5的表达来提高Gln的使用 和SLC7A5/SLC3A2，并通过转录抑制GLS1增加GLS1的表达 抑制物microRNAs(MiR)-23a/b。以GLS1为靶点的药理学研究正在积极地作为一种抗 癌症方法，尽管到目前为止收效甚微。另一方面，最近的几项研究，包括 我们实验室的那些人指出了谷氨酰胺合成的重要性，至少在某些细胞/组织类型中是这样。Gln是 谷氨酰胺合成酶(GS，又称谷氨酰胺合成酶)催化谷氨酸和氨缩合生成从头合成 称为谷氨酸氨连接酶，GLUL)。使用基于稳定同位素的代谢物追踪，我们最近 报道称，这种合成的谷氨酰胺不是通过谷氨酰胺分解来为三氯乙酸循环提供燃料的，而是用于 几个不依赖TCA的合成代谢过程，包括核苷酸的生物合成和必需物质的运输 氨基酸。重要的是，GS的高表达促进了谷氨酰胺限制下的细胞存活；抑制 GS抑制Gln诱导细胞增殖，增加细胞死亡，延缓移植瘤生长 成长。此外，我们最近报道Myc可以诱导一些癌细胞中GS的表达 台词。我们还发现Myc的激活与GS的表达在几个小鼠模型中呈正相关 以及在人类患者样本中。这些发现导致我们提出以下假设：致癌Myc， 至少在某些细胞/组织类型中，上调GS的表达以促进Gln的产生和其 使用合成代谢(远离三氯乙酸循环)，从而促进肿瘤的发生。我们提出了两个具体的 目的是研究这一假说：1)研究代谢和细胞生物学后果，以及对 在Myc激活的背景下增加GS的表达；以及2)确定GS在Myc-1中的体内作用。 驱动新陈代谢重新编程和肿瘤发生。如果完成，这项研究将提供一个新的维度 了解Myc基因在肿瘤细胞中的功能及治疗肿瘤的新靶点 由MYC驱动的肿瘤。

SUMMARY Dysregulated metabolism has long been recognized as a key hallmark of neoplastic disease. One of the major metabolic changes in tumor cells is increased glutamine (Gln) usage via glutaminolysis: Gln is deaminated by glutaminase (GLS) to glutamate (glutamic acid, Glu), which is converted by glutamate dehydrogenase (GLUD) to α-ketoglutarate (aKG) to enter the tricarboxylic acid (TCA) cycle for anaplerosis (replenishment of metabolic intermediates for energy production or biosynthesis). It is well recognized that oncogenic c-Myc (hereafter referred to as Myc) enhances Gln usage by directly transactivating the expression of Gln transporters SLC1A5 and SLC7A5/SLC3A2, and by increasing GLS1 expression via transcriptional suppression of the GLS1 repressor micro RNAs (miR)-23a/b. Pharmacologically targeting GLS1 is being actively pursued as an anti- cancer approach, although thus far with little success. On the other hand, several recent studies, including those from our laboratory, point to the importance of Gln synthesis, at least in certain cell/tissue types. Gln is synthesized de novo by condensation of Glu and ammonia, catalyzed by the enzyme Gln synthetase (GS, also known as glutamate ammonia ligase, GLUL). Using stable isotope-based metabolite tracing, we recently reported that this synthesized Gln is not used via glutaminolysis to fuel the TCA cycle; rather it is used for several TCA-independent anabolic processes including biosynthesis of nucleotides and transport of essential amino acids. Importantly, elevated expression of GS promoted cell survival under Gln limitation; inhibition of GS led to decreased cell proliferation and increased cell death upon Gln limitation, and slowed xenograft tumor growth. Moreover, we recently reported that Myc can induce the expression of GS in a number of cancer cell lines. We also found a positive correlation between Myc activation and GS expression several mouse models and in human patient samples. These findings lead us to propose the following hypothesis: oncogenic Myc, at least in certain cell/tissue types, upregulates GS expression to promote Gln production and its anabolic usage (away from the TCA cycle), thereby facilitating oncogenesis. We propose two Specific Aims to study this hypothesis: 1) Study the metabolic and cell biological consequences, and the regulation of increased GS expression in the context of Myc activation; and 2) Determine the in vivo role of GS in Myc- driven metabolic reprogramming and oncogenesis. If accomplished, this study will provide a novel dimension to understanding the functions of dysregulated Myc in cancer cells and a potential new target for treatment of Myc-driven tumors.