Role of branched-chain amino catabolism in lymphopoiesis and lymphomagenesis

 描述(由申请人提供)癌细胞重新连接代谢途径，以驱动营养物质的吸收和/或分解代谢，以维持其生长(质量)和快速增殖所需的高能量需求。MYC癌蛋白作为转录因子发挥作用，在超过一半的人类癌症中被激活。MYC通过LAT1/SLC7A5诱导糖酵解和谷氨酰胺分解相关基因的转录，包括编码葡萄糖、谷氨酰胺、乳酸和支链氨基酸(支链氨基酸：缬氨酸、亮氨酸和异亮氨酸)转运体的基因。由Myc驱动的肿瘤上瘾并利用L-谷氨酰胺将碳中间体输送到Krebs循环中。我们推测，Myc还可以控制支链氨基酸的分解代谢，提供关键的代谢中间产物，特别是乙酰乙酸酯、乙酰辅酶A、丙酰辅酶A/琥珀酰辅酶A、NADH和FADH2。为了支持这一观点，我们对Eμ-Myc转基因小鼠(一个有效的人类B细胞Burkitt淋巴瘤(BL)的Myc/Ig易位模型)和BL细胞的新研究表明：(I)癌前和肿瘤Myc表达的B细胞表达高水平的BCAT1mRNA和蛋白，它是一个真正的Myc转录靶点和支链氨基酸分解代谢的第一酶；(Ii)在Eμ-Myc B细胞和BL中明显地增加了其他几种指导BCAA分解代谢的酶的水平；(Iii)BCAT1基因敲除损害了BL细胞的生长、存活和克隆形成；(4)加巴喷丁，一种已知的BCAT1抑制剂，可损害Eμ-Myc淋巴瘤和BL的生长和存活，但不损害正常B细胞的生长和存活。这些发现支持这样的假设，即BCAT1对于Myc驱动的淋巴瘤的发生和维持是必要的。在具体目标1中，我们将使用小鼠模型来测试BCAT1是否在B淋巴细胞的生成和动态平衡中发挥必要的、内在的作用，以及BCAT1缺失是否影响B细胞的增殖和存活。在特定的目标2中，我们将测试BCAT1是否对Myc驱动的淋巴增生症的发展和维持是必要的。在这里，我们将评估BCAT1缺失的影响：(I)对癌前E-μ-Myc B细胞的增殖和存活；(Ii)对淋巴瘤的发生和总体生存；以及(Iii)对恶性状态的维持。在特定的目标3中，我们将解决在正常B细胞、Eμ-Myc淋巴瘤和BL细胞系中BCAT1慢性或急性丢失或耗竭后的代谢扰动。在这里，我们将评估来自支链氨基酸的代谢物的稳态水平和流量的变化，以及线粒体呼吸、糖酵解和营养吸收的可能变化。此外，还将确定BCAT1缺失对小鼠和人类淋巴瘤对糖酵解、OXPHOS和乳酸运输抑制剂敏感性的影响。我们认为，这些研究将使支链氨基酸分解代谢成为参与MYC/MYCN参与的人类肿瘤的一种新的治疗易感性，并将为癌症预防和治疗提供新的途径。

 DESCRIPTION (provided by applicant) Cancer cells rewire metabolic pathways to drive the uptake and/or catabolism of nutrients that sustain the high energetic demands needed for their growth (mass) and rapid rates of proliferation. Myc oncoproteins function as transcription factors and are activated in over half of all human cancers. Myc induces the transcription of several genes involved in glycolysis and glutaminolysis, including those encoding transporters of glucose, glutamine, lactate and branched-chain amino acids (BCAA: valine, leucine and isoleucine, via LAT1/SLC7A5). Tumors driven by Myc are addicted to and utilize L-glutamine to feed carbon intermediates into the Krebs cycle. We reasoned that Myc would also control the catabolism of BCAAs, which also provides key metabolic intermediates, specifically acetoacetate, acetyl-CoA, propionyl-CoA/succinyl-CoA, NADH and FADH2. In support of this notion, our new studies of Eμ-Myc transgenic mice, a validated model of human B cell Burkitt lymphoma (BL) that bear MYC/Ig translocations, and of BL cells, demonstrate that: (i) premalignant and neoplastic Myc-expressing B cells express elevated mRNA and protein levels of Bcat1, which is a bona fide Myc transcription target and the first enzyme in BCAA catabolism; (ii) elevated levels of several other enzymes that direct BCAA catabolism are evident in Eμ-Myc B cells and BL; (iii) BCAT1 knockdown impairs the growth, survival and clonogenicity of BL cells; (iv) treatment with Gabapentin, a known BCAT1 inhibitor, impairs the growth and survival of Eμ-Myc lymphoma and of BL, but not of normal B cells. These findings support the hypothesis that BCAT1 is necessary for the development and maintenance of Myc-driven lymphoma. In Specific Aim 1, we will use mouse models to test if Bcat1 plays essential, intrinsic roles in B lymphopoiesis and homeostasis, and if Bcat1 loss affects B cell proliferation and survival. In Specific Aim 2, we will test if Bcat1 is necessary for the development and maintenance of Myc-driven lymphomagenesis. Here we will assess the effects of Bcat1 loss: (i) on the proliferation and survival of pre-malignant Eμ-Myc B cells; (ii) on lymphoma onset and overall survival; and (iii) on maintenance of the malignant state. In Specific Aim 3, we will defin the metabolic perturbations that ensue following chronic versus acute Bcat1 loss or depletion in normal B cells, Eμ-Myc lymphomas and BL cell lines. Here we will assess changes in the steady-state levels and flux of metabolites derived from BCAAs, and possible alterations in mitochondrial respiration, glycolysis and nutrient uptake. In addition, the effects of Bcat1 loss o depletion on the sensitivity of mouse and human lymphomas to glycolytic, OXPHOS and lactate transport inhibitors will be determined. We submit that these studies will establish BCAA catabolism as a novel therapeutic vulnerability for human tumors with MYC/MYCN involvement and that they will offer new avenues for cancer prevention and treatment.