The lipogenic regulator SREBF2 induces Transferrin in circulating melanoma cells and suppresses ferroptosis [ChIP-Seq]

Circulating tumor cells (CTCs) are shed by cancer into the bloodstream, where a viable subset overcomes oxidative stress to initiate metastatic outgrowth. Here we show that CTCs from patients with BRAF-mutant melanoma coordinately upregulate both lipogenesis and iron homeostasis pathways. In clonally-derived cultures of melanoma CTCs these pathways are correlated with both intrinsic and acquired resistance to BRAF inhibitors. The lipogenesis regulator SREBF2 directly induces transcription of the iron carrier Transferrin (TF), reducing intracellular iron pools, reactive oxygen species (ROS) and lipid peroxidation, and conferring resistance to both BRAF inhibitors and inducers of ferroptosis. Knockdown of endogenous TF impairs tumor formation by melanoma CTCs, and their tumorigenic defects are partially rescued by the lipophilic anti-oxidants Ferrostatin-1 and Vitamin E. In a cohort of patient-derived melanoma CTCs, single cell RNA-seq identifies a subset with high lipogenic, iron metabolic and proliferative signatures, which are correlated with adverse clinical outcome, irrespective of treatment regimen. Thus, SREBF2-driven iron homeostatic pathways contribute to cancer progression, drug resistance and metastasis. To identify SREBF2-regulated genes mediating tumor-enhancing effects of SREBF2 expression, we undertook chromatin immunoprecipitation followed by Next Generation Sequencing (ChIP-Seq), defining direct SREBF2 transcriptional targets in cultured melanoma CTCs.

循环肿瘤细胞（Circulating Tumor Cells, CTCs）是由肿瘤脱落进入血液循环的细胞，其中具有存活能力的亚群可克服氧化应激，启动转移性定植与增殖。本研究证实，BRAF突变型黑色素瘤患者的CTCs可协同激活脂肪生成与铁稳态两条通路。在黑色素瘤CTCs的克隆培养体系中，上述通路与肿瘤细胞对BRAF抑制剂的固有及获得性耐药均呈相关性。脂肪生成调控因子SREBF2可直接转录激活铁转运蛋白转铁蛋白（Transferrin, TF），降低细胞内铁池、活性氧（Reactive Oxygen Species, ROS）水平与脂质过氧化程度，同时赋予肿瘤细胞对BRAF抑制剂及铁死亡诱导剂的耐药性。敲低内源性转铁蛋白会削弱黑色素瘤CTCs的成瘤能力，而亲脂性抗氧化剂铁抑素-1（Ferrostatin-1）与维生素E可部分挽救其致瘤缺陷。在一组患者来源的黑色素瘤CTCs队列中，单细胞RNA测序（single cell RNA Sequencing, scRNA-seq）鉴定出一类高表达脂肪生成、铁代谢与增殖特征谱的细胞亚群，该亚群与不良临床结局显著相关，且不受治疗方案影响。综上，SREBF2介导的铁稳态通路可促进癌症进展、耐药与转移。为鉴定介导SREBF2促瘤效应的转录调控靶基因，本研究通过染色质免疫共沉淀结合高通量测序（Chromatin Immunoprecipitation followed by Next Generation Sequencing, ChIP-Seq），在培养的黑色素瘤CTCs中明确了SREBF2的直接转录靶基因。