Unfolded protein response signaling PERK supports circulating tumor cell clusters survival and metastasis via SAM synthesis and H3K4me3-dependent PDGFB signaling

Background: Metastasis is the leading cause of cancer-related mortality, with circulating tumor cell (CTC) clusters serving as highly efficient precursors of distant metastasis. Survival of CTC clusters in the bloodstream is the primary contributor to tumor metastasis. However, the underlying mechanisms of how CTC clusters respond to the blood environment and drive metastasis remain to be elusive. This study aimed to elucidate the potential mechanisms by which CTC clusters adapt to survive in the bloodstream. Methods: CTC clusters were detected using a microfluidic system in various cancer patients, as well as in patient-derived xenograft model (PDX), cell line-derived xenograft model (CDX) and syngeneic model. The key molecules responsible for the adaptive survival of CTC clusters were characterized using RNA sequencing (RNA-seq), gene interference, and flow cytometry. To investigate the underlying mechanisms of adaptive survival, RNA-seq, targeted metabolomics, and isotope tracing experiments, chromatin immunoprecipitation sequencing (ChIP-seq), and immunofluorescence (IF) staining were employed. Finally, the potential therapeutic effects of special inhibitors against the survival signaling of CTC clusters combined with chemotherapy drugs (e.g. imatinib) were evaluated in patient-derived CTCs and PDX models. Results: CTC clusters exhibited superior survival and metastatic capacities compared to single CTCs, and were associated with adverse clinical outcomes. The unfolded protein response (UPR) meditator PKR-like endoplasmic reticulum kinase (PERK) was activated in CTC clusters and maintained S-adenosylmethionine (SAM) availability, facilitating their adaptive survival in the bloodstream. Mechanistically, PERK mediated the upregulation of activating transcription factor 4 (ATF4), which subsequently enhanced methionine adenosyltransferase 2A (MAT2A) expression, thereby contributing to SAM synthesis. Enhanced SAM, serving as a methyl donor, increased H3K4me3 modification of the platelet-derived growth factor B (PDGFB) promoter, leading to high levels of secreted PDGFB and its accumulation in the intercellular area within the CTC cluster. PDGFB functioned as a shared survival signal among cells in CTC cluster to trigger the phosphoinositide 3-kinase (PI3K)-protein kinase B (AKT) signaling via Platelet-derived growth factor receptor beta (PDGFRB), which supported CTC cluster survival in bloodstream. The inhibition of PERK and PDGFRB profoundly impaired the survival signaling and suppressed the metastatic dissemination of CTC clusters. Conclusions: Our findings revealed a novel PERK-MAT2A-PDGFB axis that confered adaptive survival capabilities to CTC clusters in the bloodstream. Targeting this survival signaling pathway represented a promising therapeutic strategy for metastatic cancer.