A chronic signaling TGFb zebrafish reporter identifies immune response in melanoma

Developmental signaling pathways associated with growth factors such as TGFb are commonly dysregulated in melanoma. Here we identified a human TGFb enhancer specifically activated in melanoma cells treated with TGFB1 ligand. We generated stable transgenic zebrafish with this TGFb Induced Enhancer driving green fluorescent protein (TIE:EGFP). TIE:EGFP was not expressed in normal melanocytes or early melanomas but was expressed in spatially distinct regions of advanced melanomas. Single cell RNA- sequencing revealed that TIE:EGFP⁺ melanoma cells down-regulated interferon response, while up-regulating a novel set of chronic TGFb target genes. ChIP-sequencing demonstrated that AP-1 factor binding is required for activation of chronic TGFb response. Overexpression of SATB2, a chromatin remodeler associated with tumor spreading, showed activation of TGFb signaling in melanoma precursor zones and early melanomas. Confocal imaging and flow cytometric analysis showed that macrophages are recruited to TIE:EGFP⁺regions and preferentially phagocytose TIE:EGFP⁺ cells. This work identifies a TGFb induced immune response and demonstrates the need for the development of chronic TGFb biomarkers to predict patient response to TGFb inhibitors.