Dll1-mediated Notch signaling drives tumor cell crosstalk with cancer-associated fibroblasts to promote radioresistance in breast cancer

Resistance to radiotherapy is a major obstacle for effective cancer treatment. Both cancer-associated fibroblasts (CAF) within the tumor microenvironment (TME) and Notch signaling are implicated in radioresistance, but their potential interrelationship is unclear. Here, we report that irradiated samples obtained from luminal breast cancer patient tumors express higher levels of the Notch ligand Dll1 and have a greater number of αSMA- and FAP-expressing activated CAFs. Single cell transcriptomic profiles further revealed enrichment of an αSMA + myofibroblastic subpopulation of CAF in Dll1 + tumors. In murine and human PDX models, Dll1 + tumor cells were more radioresistant than Dll1 − tumor cells, and genetic and pharmacological blocking of Dll1-mediated Notch signaling decreased the number of Dll1 + cancer stem cells (CSC) and CAFs and increased Dll1 + tumor cell radiosensitivity. Dll1 + cells recruited CAFs in an IL-6-dependent fashion and promoted Wnt ligand secretion by Notch2/3-expressing CAFs, thereby driving Wnt/β-catenin-dependent increases in Dll1 + CSC function to promote metastasis. Treatment with the porcupine inhibitor LGK-974 that inhibits Wnt ligand secretion or pharmacological blockade of IL-6 or Dll1 suppressed CAF-dependent enhancement of Dll1 + CSC function and metastasis in radioresistant tumors. Together, these findings reveal an essential crosstalk between Dll1 + cancer cells and CAFs that promotes metastasis and radioresistance, which could be therapeutically exploited to improve the outcome of breast cancer patients. Dll1 expression on tumor cells drives Notch-activated Wnt secretion by CAFs to promote radioresistance in breast cancer.