Determining the role of lipid droplets in glioblastoma and their therapeutic potential

摘要 胶质母细胞瘤(GBM)是最具侵袭性的脑肿瘤，中位生存期仅为12-15个月 强化治疗，表明迫切需要确定有效的方法来治疗GBM和规避 抵抗治疗，以显著提高患者的存活率。我们最近的研究表明 GBM的脂质合成和摄取显著增强，并促进肿瘤生长。众所周知，增加了 游离脂肪酸(FFA)和胆固醇可导致内质网(ER)应激和脂毒性，从而导致 细胞死亡，这提出了一个有趣的问题，即GBM细胞如何防止潜在的 增加脂肪代谢。我们最近发现来自GBM患者的肿瘤组织中含有大量的 脂滴(LDS)、甘油三酯(TG)和胆固醇酯(CE)，这表明GBM细胞可能储存了过量的 游离脂肪酸和胆固醇进入低密度脂蛋白，以避免毒性和维持肿瘤生长。进一步的分析表明，GBM 肿瘤组织中LDS和DGAT1(二甘油酰基转移酶)水平较高的患者或 SOAT1(甾醇O-酰基转移酶)，两种内质网结合的酶，催化过剩的转化 FFA和胆固醇进入甘油三酯和CE形成低密度脂蛋白，存活率较差，提示低密度脂蛋白可能有 前列腺癌的功能。与我们的发现一致的是，几个小组最近报告说，抑制胆固醇 抑制前列腺癌、胰腺癌细胞和肾癌细胞的酯化或抑制LD的形成显著增强ER 压力。然而，许多重要的LD在癌细胞中的功能仍未被研究，例如它们的 在支持肿瘤耐药性方面的能量作用和潜力。我们的初步数据表明，LDS可能 为营养减少或辐射/替莫唑胺(TMZ)下的GBM存活提供关键能源 治疗，是GBM的标准治疗方法。基于目前对癌细胞中LDS的理解和我们的新发现 初步数据，我们假设在GBM中形成LD可以防止内质网应激和脂毒性，还可以 作为能量储存库，在能量挑战时支持肿瘤存活。我们进一步假设，抑制 LD的形成会导致内质网应激、脂肪毒性和能量短缺，这可能与 放射/TMZ诱导GBM细胞死亡。在这项研究中，我们将：(1)描述潜在的保护 LD的形成在GBM细胞中的作用；(2)确定LDS是否在GBM细胞中发挥重要的能量作用；(3) 检查基因或药物抑制LD形成是否有效抑制肿瘤生长 并在GBM原位小鼠模型中使GBM对放射/TMZ治疗增敏。这项研究将揭示 先前未知的LDS在GBM中的作用和分子调控。重要的是，它还将展示 抑制LD的形成可能是一种非常有效的靶向GBM的方法，对 未检测到LDS的正常脑组织。这项研究的完成将大大推进我们的 了解脂代谢的重新编程，并可能带来拮抗GBM的新方法。

ABSTRACT Glioblastoma (GBM) is the most aggressive brain tumor, and has a median survival of only 12-15 months despite intensive therapies, indicating the urgent need to identify effective approaches to treat GBM and circumvent resistance to therapies in order to significantly improve patient survival. Our recent studies demonstrated that lipid synthesis and uptake are greatly enhanced in GBM and promote tumor growth. It is known that increased free fatty acids (FFA) and cholesterol can cause endoplasmic reticulum (ER) stress and lipotoxicity that lead to cell death, which raises the intriguing question of how GBM cells can prevent the toxicity potentially induced by increased lipid metabolism. We recently found that tumor tissues from GBM patients contain large amount of lipid droplets (LDs), triglycerides (TG) and cholesteryl esters (CE), suggesting that GBM cells may store excess FFA and cholesterol into LDs to avoid toxicity and maintain tumor growth. Further analysis showed that GBM patients whose tumor tissues contained higher levels of LDs and of DGAT1 (diglyceride acyltransferase) or SOAT1 (sterol O-acyltransferase), two ER membrane-bound enzymes that catalyze the conversion of excess FFA and cholesterol into TG and CE to form LDs, had the worse survival, suggesting that LDs may have a protumoral function. Consistent with our findings, several groups have recently reported that inhibiting cholesterol esterification or suppressing LD formation in prostate, pancreatic or renal cancer cells significantly enhanced ER stress. Nevertheless, many important LD functions in cancer cells remain unexplored, such as their energetic role and potential for supporting tumor resistance. Our preliminary data suggest that LDs may provide a critical energy source for GBM survival under nutrient reduction or radiation/temozolomide (TMZ) treatment, the standard therapy for GBM. Based on current understanding of LDs in cancer cells and our novel preliminary data, we hypothesize that LD formation in GBM prevents ER stress and lipotoxicity, and also serves as energy reservoir to support tumor survival upon energy challenges. We further hypothesize that inhibiting LD formation will cause ER stress, lipotoxicity and energy shortage, which may strongly synergize with radiation/TMZ treatment to induce GBM cell death. In this study, we will: (1) delineate the underlying protective role of LD formation in GBM cells; (2) determine whether LDs play an important energetic role in GBM cells; (3) examine whether genetically or pharmacologically inhibiting LD formation effectively suppresses tumor growth and sensitizes GBM to radiation/TMZ treatment in GBM orthotopic mouse models. This study will reveal the previously uncharacterized role and molecular regulation of LDs in GBM. Importantly, it will also demonstrate that inhibiting LD formation may be a very effective approach to specifically target GBM with little toxicity on normal brain tissues where no LDs could be detected. Completion of this study will significantly advance our understanding of lipid metabolism reprogramming and may bring about new approaches to antagonize GBM.