Nrf2 的SUMO1修饰应答丝氨酸饥饿通过ROS-PHGDH轴调控肝癌生长

Nuclear factor erythroid-2 related factor 2 (Nrf2) that belongs to the Cap’n’collar(CNC) transcription factor family, is a pivotal activator of genes encoding cytoprotective and detoxifying enzymes that protect against oxidative and electrophilic stress, to maintain cellular redox homeostasis. Abundant expression of Nrf2 has been found in various cancers, facilitating malignant phenotypes of cancer including aggressive proliferation and resistance to anticancer therapy. The role of Nrf2 in tumor progression has been focused, with increasing evidences being found in the tumor metabolism modulation. It has been reported that Nrf2 could be modified by SUMO1 and SUMO2/3. But the biological role of Nrf2 SUMOylation in cancers, is largely unknown. Our previous work has identified Nrf2 being modified by SUMO1 at K110 which possesses the ability of promoting heptocellular carcinoma (HCC) growth. SUMO1 modification of Nrf2 decreased intracellular ROS level in HCCs which enhances the expression of PHGDH, the key controlling enzyme in de novo serine synthesis. Furthermore, serine starvation increases the SUMO1 modification of Nrf2 in HCCs. Hence, we proposes that SUMO1 modification of Nrf2 at K110 could promote cellular serine synthesis and HCCs growth in response to environmental metabolic stress. In this study, three scientific questions will be addressed. 1. Does Nrf2 SUMOylation at K110 promote HCCs growth in vitro and in vivo? 2. Does Nrf2 SUMOylation at K110 decrease ROS level, which upregulates PHGDH expression and enhances de novo serine synthesis in HCCs? 3. How nutrient deprivation signal, like serine starvation, regulate Nrf2 SUMOylation in HCCs? The answers for these questions will help us understand how the critical stress sensor, Nrf2, adapts to environmental metabolic stress through altering its SUMOylation status, to reprogram cellular metabolism and maintain tumorigenesis in HCCs via ROS-PHGDH axis, during nutrient deprivation condition. Thus, this study will provide us a potential strategy for HCCs treatment by using Nrf2 SUMOylation as a target in the clinic.

我们前期研究鉴定了K110是Nrf2的SUMO1修饰位点，该位点修饰促进肝癌生长。Nrf2的SUMO1修饰降低了肝癌细胞内ROS，提高丝氨酸合成酶PHGDH的表达。同时，丝氨酸饥饿上调了Nrf2的SUMO1修饰。因此，我们提出Nrf2的SUMO1修饰能够应答丝氨酸饥饿，增强丝氨酸从头合成，促进肝癌生长。在本课题中，将对以下科学问题进行回答：1、Nrf2的SUMO1修饰在体内外促进肝癌生长；2、Nrf2的SUMO1修饰降低了肝癌细胞内ROS，上调了PHGDH表达及丝氨酸合成；3、丝氨酸饥饿诱导Nrf2的SUMO1修饰的信号通路。对上述问题的回答，将使我们理解，通过改变Nrf2的SUMO1修饰水平，肝癌细胞可感知代谢压力，通过ROS-PHGDH轴调控丝氨酸合成，在营养缺乏时维持肿瘤生长，并为将Nrf2的SUMO1修饰作为治疗靶标在临床应用提供线索。

作为一种非营养必需氨基酸，丝氨酸除了作为蛋白质合成的原料，还可通过分解代谢为细胞提供 NADPH、一碳单位、谷胱甘肽和磷脂酰丝氨酸等小分子物质，因此对细胞的生长和防御至关重要。细胞可以通过从外界摄取丝氨酸，或者通过丝氨酸从头合成（de novo serine synthesis）途径获取丝氨酸，满足细胞生长的需求。肿瘤细胞往往处于营养物质供应不稳定的环境中，丝氨酸从头合成途径的活跃，对于保证肿瘤细胞的快速增长是非常关键的。作为一种重要的抗氧化应激分子，Nrf2能够调控非小细胞肺癌（non-small cell lung cancer, NSCLC）内丝氨酸的合成，促进肿瘤的生长和进展。本项目的前期工作基础鉴定了Nrf2的SUMO1修饰位点在K110位，Nrf2 的 SUMO1 修饰会促进肝癌细胞的体内外成瘤和生长；并且发现 Nrf2 的 SUMO1 修饰降低了肝癌细胞内 ROS 的水平，提高了丝氨酸从头合成关键酶 PHGDH（phosphoglycerate dehydrogenase，磷酸甘油酸脱氢酶）蛋白的表达；而丝氨酸饥饿（serine starvation）能上调肝癌细胞内 Nrf2 的 SUMO1 修饰。在本项目的资助下，我们整合了分子、细胞和动物水平的实验研究系统和策略，获得了如下科学发现：1. 丝氨酸饥饿通过下调肝癌细胞中SENP1的表达，升高细胞内Nrf2的SUMO1修饰水平；2. SUMO1修饰提高了Nrf2的转录活性，转录激活抗氧化酶GPX2的表达，降低肝癌细胞内ROS水平；3. NRF2的SUMO1修饰通过降低ROS，间接升高了PHGDH的表达，通过促进丝氨酸从头合成和嘌呤核苷酸的合成，促进肝癌细胞的体内外生长。本项目的研究结果能够为深入理解代谢压力条件下肿瘤细胞行为方式的改变增加新认知，并且为靶向Nrf2的SUMO化联合丝氨酸剥夺治疗肝癌提供新思路。