PPT1 is a negative regulator of STING signaling in cancer cells and its inhibition reactivates immune surveillance in cold tumors

Immunotherapy modalities have revolutionized cancer treatment for a number of metastatic and treatment-refractory tumor types. Still, many malignancies that are lacking in T cell infiltrates and immunologically “cold” fail to respond to these modalities. One approach to increase tumor immunogenicity has been to induce STING and downstream interferon signaling that is often dysregulated in “cold” tumors. Despite some early success of STING agonists in preclinical cancer models, these approaches have not been successful in the clinic due to poor tumor penetrance and systemic toxicities. Here we performed a genome-wide CRISPR screen to uncover novel therapeutic targets to activate STING expression in human tumors. We identified Palmitoyl Protein Thioesterase1 (PPT1) as a negative regulator of STING highly expressed in “cold” ovarian and prostate tumors. Genetic or pharmacological PPT1 suppression increased STING protein stability and its downstream activation of interferon and inflammatory cytokine signaling to enhance T cell migration. Treatment of preclinical prostate and ovarian cancer models expressing low levels of STING with the small molecule PPT1 inhibitor GNS561 enhanced STING expression and activation, leading to infiltration and activation of cytotoxic T cells that turned these tumors “hot” and reduced tumor growth, fibrosis, and dissemination without toxicity. Further analysis demonstrated that PPT1 is associated with reduced STING expression, CD8+ T cell numbers, overall survival, and immunotherapy outcomes in ovarian and prostate cancer patients. Thus, PPT1 inhibition may be a promising approach to activate STING and potentiate the effects of immunotherapy in “cold” tumors. RNA-seq profiling of A1847, PC3, and MycCAP;p53KO cells treated with vehicle (DMSO) or 5 µM GNS561 for 24 hrs

免疫治疗手段已革新了多种转移性及治疗抵抗性肿瘤的癌症治疗方案。然而，诸多缺乏T细胞浸润、免疫学上呈“冷”表型的恶性肿瘤无法对这类治疗手段产生应答。提升肿瘤免疫原性的一种策略是诱导干扰素基因刺激因子（STING）及其下游的干扰素信号通路，而这类通路在“冷”肿瘤中通常存在失调情况。尽管STING激动剂在临床前癌症模型中已取得部分早期成果，但由于肿瘤穿透性较差且存在全身毒性，这类策略在临床应用中并未获得成功。本研究通过全基因组CRISPR筛选，旨在挖掘可在人类肿瘤中激活STING表达的新型治疗靶点。我们鉴定出棕榈酰蛋白硫酯酶1（PPT1）是STING的负向调控因子，其在“冷”性卵巢癌与前列腺癌中呈高表达状态。通过遗传学或药理学手段抑制PPT1，可提升STING蛋白的稳定性，并增强其下游的干扰素与炎症细胞因子信号通路激活，进而促进T细胞迁移。使用小分子PPT1抑制剂GNS561处理STING低表达的临床前前列腺癌与卵巢癌模型，可增强STING的表达与激活，促使细胞毒性T细胞浸润并活化，从而将肿瘤转化为“热”表型，同时在无毒性的前提下抑制肿瘤生长、纤维化及播散。进一步分析显示，在卵巢癌与前列腺癌患者中，PPT1的高表达与STING表达降低、CD8+ T细胞数量减少、总生存期缩短以及免疫治疗应答不佳存在关联。因此，抑制PPT1或许是一种极具前景的策略，可在“冷”肿瘤中激活STING并增强免疫治疗的疗效。本数据集包含经溶剂对照（二甲基亚砜，DMSO）或5 µM GNS561处理24小时的A1847、PC3以及MycCAP;p53KO细胞的RNA测序分析数据。