Single-Cell Atlas of Lineage States, Tumor Microenvironment, and Subtype-Specific Expression Programs in Gastric Cancer.

Gastric cancer heterogeneity represents a barrier to disease management. We generated a comprehensive single-cell atlas of gastric cancer (>200,000 cells) comprising 48 samples from 31 patients across clinical stages and histologic subtypes. We identified 34 distinct cell-lineage states including novel rare cell populations. Many lineage states exhibited distinct cancer-associated expression profiles, individually contributing to a combined tumor-wide molecular collage. We observed increased plasma cell proportions in diffuse-type tumors associated with epithelial-resident KLF2 and stage-wise accrual of cancer-associated fibroblast subpopulations marked by high INHBA and FAP coexpression. Single-cell comparisons between patient-derived organoids (PDO) and primary tumors highlighted inter- and intralineage similarities and differences, demarcating molecular boundaries of PDOs as experimental models. We complemented these findings by spatial transcriptomics, orthogonal validation in independent bulk RNA-sequencing cohorts, and functional demonstration using in vitro and in vivo models. Our results provide a high-resolution molecular resource of intra- and interpatient lineage states across distinct gastric cancer subtypes. SIGNIFICANCE: We profiled gastric malignancies at single-cell resolution and identified increased plasma cell proportions as a novel feature of diffuse-type tumors. We also uncovered distinct cancer-associated fibroblast subtypes with INHBA-FAP-high cell populations as predictors of poor clinical prognosis. Our findings highlight potential origins of deregulated cell states in the gastric tumor ecosystem. This article is highlighted in the In This Issue feature, p. 587.

Protocol to establish cancer-associated fibroblasts from surgically resected tissues and generate senescent fibroblasts.

Accumulating evidence suggests that the malignant behavior of cancer is influenced by stromal activity in the tumor microenvironment. Cancer-associated fibroblasts (CAFs), which are the main component of the cancerous stroma, play an important role in cancer development. Here, we describe a protocol to establish CAFs from surgically resected tissues. CAFs could be a vital tool for understanding the microenvironment and its impact on tumor progression and metastasis. Moreover, we generated inflammation-induced senescent fibroblasts that more closely mimic the tumor microenvironment. For complete details on the use and execution of this protocol, please refer to Yasuda et al. (2021).

Inflammation-driven senescence-associated secretory phenotype in cancer-associated fibroblasts enhances peritoneal dissemination.

In the tumor microenvironment, senescent non-malignant cells, including cancer-associated fibroblasts (CAFs), exhibit a secretory profile under stress conditions; this senescence-associated secretory phenotype (SASP) leads to cancer progression and chemoresistance. However, the role of senescent CAFs in metastatic lesions and the molecular mechanism of inflammation-related SASP induction are not well understood. We show that pro-inflammatory cytokine-driven EZH2 downregulation maintains the SASP by demethylating H3K27me3 marks in CAFs and enhances peritoneal tumor formation of gastric cancer (GC) through JAK/STAT3 signaling in a mouse model. A JAK/STAT3 inhibitor blocks the increase in GC cell viability induced by senescent CAFs and peritoneal tumor formation. Single-cell mass cytometry revealed that fibroblasts exist in the ascites of GC patients with peritoneal dissemination, and the fibroblast population shows p16 expression and SASP factors at high levels. These findings provide insights into the inflammation-related SASP maintenance by histone modification and the role of senescent CAFs in GC peritoneal dissemination.

Gastric Cancer Stem Cells: Current Insights into the Immune Microenvironment and Therapeutic Targets.

Gastric cancer (GC) is a leading cause of cancer-related death worldwide. Cancer stem cells (CSCs) are known to be involved in chemotherapy resistance and the development of metastases. Although CSCs harbor self-renewal and tumorigenic abilities, the immune microenvironment surrounding CSCs provides various factors and supports the maintenance of CSC properties. The current review summarizes the accumulating findings regarding the relationship between the immune microenvironment and gastric CSCs (GCSCs), which will support the possibility of developing novel therapeutic strategies for targeting GCSCs.

Extracellular Vesicles from Cancer-Associated Fibroblasts Containing Annexin A6 Induces FAK-YAP Activation by Stabilizing ß1 Integrin, Enhancing Drug Resistance.

Extracellular vesicles (EV) from cancer-associated fibroblasts (CAF) are composed of diverse payloads. Although CAFs impact the aggressive characteristics of gastric cancer cells, the contribution of CAF-EV to gastric cancer progression has not been elucidated. Here, we investigated the molecular mechanism of the changes in gastric cancer characteristics induced by CAF-EV. CAF abundance in gastric cancer tissues was associated with poor prognosis of patients with gastric cancer receiving chemotherapy. Moreover, CAF-EV induced tubular network formation and drug resistance of gastric cancer cells in the extracellular matrix (ECM). Comprehensive proteomic analysis of CAF-EV identified that Annexin A6 plays a pivotal role in network formation and drug resistance of gastric cancer cells in the ECM via activation of ß1 integrin-focal adhesion kinase (FAK)-YAP. A peritoneal metastasis mouse model revealed that CAF-EV induced drug resistance in peritoneal tumors, and inhibition of FAK or YAP efficiently attenuated gastric cancer drug resistance in vitro and in vivo. These findings demonstrate that drug resistance is conferred by Annexin A6 in CAF-EV and provide a potential avenue for overcoming gastric cancer drug resistance through the inhibition of FAK-YAP signaling in combination with conventional chemotherapeutics. SIGNIFICANCE: This study elucidates a novel molecular mechanism through which Annexin A6 in CAF-EV activates FAK-YAP by stabilizing ß1 integrin at the cell surface of gastric cancer cells and subsequently induces drug resistance. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/16/3222/F1.large.jpg.

Inhibition of 15-PGDH causes Kras-driven tumor expansion through prostaglandin E2-ALDH1 signaling in the pancreas.

The accumulation of prostaglandin E2 (PGE2) during chronic inflammation has been implicated in the progression of several cancers. Cyclooxygenase is the key synthesizing enzyme of PGE2, although the degradation enzyme 15-hydroxyprostaglandin dehydrogenase (15-PGDH) has received considerable attention recently. We investigated the molecular mechanisms of pancreatic ductal adenocarcinoma (PDAC) progression via 15-PGDH downregulation. Here, we found that 15-PGDH expression was inversely correlated with ALDH1, an important cancer stem cell-associated marker indicative of poor prognosis in humans. Moreover, we demonstrated that pharmacological inhibition of 15-PGDH enhanced CYP26A1 expression, leading to depletion of all-trans retinoic acid (ATRA) and expansion of the ALDH1-positive subset in both human PDAC cells and tumor cells of KrasLSL-G12D/+; Ptf1aCre/+ (KC) mice. Furthermore, genetic deletion of 15-Pgdh in KC mice showed PGE2 accumulation and ATRA depletion in the pancreas, resulting in PDAC with high levels of Aldh1 and Ki-67. Finally, ATRA replacement suppressed 15-PGDH inhibition-induced tumor progression in KC mice, and ATRA treatment attenuated Aldh1 activity in tumor cells isolated from the pancreas of 15-Pgdh-/- KC mice. These findings provide evidence that 15-PGDH inhibition enhances KRAS-driven tumor progression via ATRA depletion in the pancreas. Therefore, ATRA replacement could be a potential strategy for PDAC treatment.