Identifying SREBP-1 activation mechanism in glioblastoma and its new role in regulating glutamine metabolism

摘要 胶质母细胞瘤（GBM）是最致命的原发性脑肿瘤，其预后在10年内没有明显改善。 过去的二十年迫切需要找到有效的治疗GBM的策略，但我们对GBM的认识还不够全面， GBM生物学的研究严重阻碍了这一进展。我们的实验室长期致力于了解GBM 生物学和最近证明GBM重新编程脂质代谢，以支持其快速增长。我们 确定了固醇调节元件结合蛋白-1（SREBP-1），一种调节 脂质合成在GBM中高度活化，并且对于肿瘤生长是必需的。然而，分子 在癌细胞中激活SREBP-1和脂肪生成的机制仍不清楚。此外，无论 高度活化的SREBP-1在恶性肿瘤中发挥的新作用是未知的。在正常细胞中， 强烈抑制胰岛素诱导基因（Insig），内质网（ER）锚定蛋白。其结合 SREBP转运蛋白，SREBP-裂解蛋白（SCAP），阻止SREBP ER-到-高尔基体易位， 后续激活。因此，SREBP-1激活的第一个关键步骤是SCAP与Insig的解离。如何 癌细胞能够释放Insig对SCAP的紧密抑制，以促进高水平的SREBP。 1个激活仍然未应答。我们最近发现，葡萄糖介导的N-糖基化稳定SCAP。 有趣的是，我们新的初步数据显示，在没有谷氨酰胺的情况下，SREBP-1不能被切割， 尽管SCAP仍保持糖基化。抑制谷氨酰胺转化为谷氨酸和氨， 抑制谷氨酰胺酶（GLS）显著降低SREBP-1切割。这些数据有力地表明， 谷氨酰胺在SREBP-1活化中起关键作用。有趣的是，表达活性N-末端形式的 GBM细胞中的SREBP-1显著增强谷氨酰胺摄取。对GBM患者样本的分析显示， 谷氨酰胺转运蛋白SLC 1A 5在肿瘤组织中高度表达， 生存启动子分析显示，SLC 1A 5启动子区域存在多个推定的SREBP-1结合位点。 基于这些新的初步数据和我们已发表的研究，我们假设SCAP从 Insig被谷氨酰胺衍生的代谢物激活，导致SREBP-1激活;反过来， SREBP-1转录上调SLC 1A 5的表达，导致前馈环， 增加谷氨酰胺摄取和脂质合成以促进GBM生长。在这项研究中，我们将确定 目的1中谷氨酰胺激活GBM中SREBP-1和脂肪生成的分子机制，并揭示新的 SREBP-1在Aim 2中调节谷氨酰胺代谢的作用。我们将进一步研究是否结合 SREBP-1抑制与SLC 1A 5或GLS抑制一起提供了抑制GBM生长的强协同作用 (Aim 2）。这项研究的完成将大大推进我们对GBM生物学和代谢的理解 重新布线，并为GBM治疗提供有希望的方法。

ABSTRACT Glioblastoma (GBM) is the most lethal primary brain tumor, and its prognosis has no significantly improved in the past two decades. It is urgent to identify effective strategies to treat GBM, but our only partial understanding of GBM biology significantly hinders this progress. Our laboratory has long focused on understanding GBM biology and recently demonstrated that GBM reprograms lipid metabolism to support its rapid growth. We identified that sterol regulatory element-binding protein-1 (SREBP-1), a master transcription factor regulating lipid synthesis, is highly activated in GBM and is essential for tumor growth. However, the molecular mechanism activating SREBP-1 and lipogenesis in cancer cells remains unclear. Moreover, whether the highly activated SREBP-1 plays new roles in malignancy is unknown. In normal cells, SREBP activation is strongly repressed by Insulin-induced gene (Insig), an endoplasmic reticulum (ER)-anchored protein. It binds to the SREBP transporter, SREBP-cleavage protein (SCAP), preventing SREBP ER-to-Golgi translocation and subsequent activation. Thus, the first key step for SREBP-1 activation is SCAP dissociation from Insig. How cancer cells are able to release the tight repression of Insig on SCAP to promote high levels of SREBP- 1 activation remains unanswered. We recently found that glucose-mediated N-glycosylation stabilizes SCAP. Interestingly, our new preliminary data showed that in the absence of glutamine, SREBP-1 failed to be cleaved, although SCAP remained glycosylated. Inhibiting glutamine conversion to glutamate and ammonia by suppressing glutaminase (GLS) markedly reduced SREBP-1 cleavage. These data strongly suggest that glutamine plays a critical role in SREBP-1 activation. Interestingly, expressing the active N-terminal form of SREBP-1 in GBM cells significantly enhanced glutamine uptake. Analysis of GBM patient samples showed that the glutamine transporter, SLC1A5, is highly expressed in tumor tissues and is inversely correlated with patient survival. Promoter analysis showed multiple putative SREBP-1 binding sites in the SLC1A5 promoter region. Based on these new preliminary data and our published studies, we hypothesize that SCAP dissociation from Insig is activated by glutamine-derived metabolites, leading to SREBP-1 activation; in turn, activated SREBP-1 transcriptionally upregulate the expression of SLC1A5, resulting in a feedforward loop to increase glutamine uptake and lipid synthesis to promote GBM growth. In this study, we will identify the molecular mechanism activating SREBP-1 and lipogenesis by glutamine in GBM in Aim 1, and reveal the new role of SREBP-1 in regulating glutamine metabolism in Aim 2. We will further examine whether combining inhibition of SREBP-1 together with SLC1A5 or GLS suppression provides strong synergy to inhibit GBM growth (Aim 2). Completion of this study will significantly advance our understanding of GBM biology and metabolism rewiring, and provide promising approaches for GBM therapy.