SUMMARY A large proportion of colorectal cancers (CRCs) display mutational inactivation of the TGF-beta pathway yet paradoxically, they are characterized by elevated TGF-beta production. Here, we unveil a prometastatic programme induced by TGF-beta in the microenvironment that associates with a high-risk of CRC relapse upon treatment. The activity of TGF-beta on stromal cells increases the efficiency of organ colonization by CRC cells whereas mice treated with a pharmacological inhibitor of TGFBR1 are resilient to metastasis formation. Secretion of IL11 by TGF-beta-stimulated cancer-associated fibroblasts (CAFs) triggers GP130/STAT3 signalling in tumour cells. This crosstalk confers a survival advantage to metastatic cells. The dependency on the TGF-beta stromal programme for metastasis initiation could be exploited to improve the diagnosis and treatment of CRC.

Recent molecular classifications of colorectal cancer (CRC) based on global gene expression profiles have defined subtypes displaying resistance to therapy and poor prognosis. Upon evaluation of these classification systems, we discovered that their predictive power arises from genes expressed by stromal cells rather than epithelial tumor cells. Bioinformatic and immunohistochemical analyses identify stromal markers that associate robustly with disease relapse across the various classifications. Functional studies indicate that cancer-associated fibroblasts (CAFs) increase the frequency of tumor-initiating cells, an effect that is dramatically enhanced by transforming growth factor (TGF)-β signaling. Likewise, we find that all poor-prognosis CRC subtypes share a gene program induced by TGF-β in tumor stromal cells. Using patient-derived tumor organoids and xenografts, we show that the use of TGF-β signaling inhibitors to block the cross-talk between cancer cells and the microenvironment halts disease progression.

Endothelial cells (ECs) provide angiocrine factors orchestrating tumor progression. Here, we show that activated Notch1 receptors (N1ICD) are frequently observed in ECs of human carcinomas and melanoma, and in ECs of the pre-metastatic niche in mice. EC N1ICD expression in melanoma correlated with shorter progression-free survival. Sustained N1ICD activity induced EC senescence, expression of chemokines and the adhesion molecule VCAM1. This promoted neutrophil infiltration, tumor cell (TC) adhesion to the endothelium, intravasation, lung colonization, and postsurgical metastasis. Thus, sustained vascular Notch signaling facilitates metastasis by generating a senescent, pro-inflammatory endothelium. Consequently, treatment with Notch1 or VCAM1-blocking antibodies prevented Notch-driven metastasis, and genetic ablation of EC Notch signaling inhibited peritoneal neutrophil infiltration in an ovarian carcinoma mouse model.