Reprogramming of Endothelial Cells in Pulmonary Metastasis

Metastatic niche provides signals for tumor cells extravasation, survival, dormancy or proliferation, ultimately leading to the formation of metastasis. Endothelial cells of metastatic niche become activated and play a key role in tumor cells extravasation and establishing inflammatory microenvironment that supports metastatic outgrowth. Although multiple signaling pathways have been implicated in formation of metastatic niche, the transcriptional network which controls reprogramming of normal endothelial cells (EC) into activated endothelial cells of metastatic niche (aEC) are poorly understood. Screening for regulators of endothelial reprogramming, we identified Forkhead Box F1 (Foxf1) transcription factor, which is highly expressed in normal pulmonary ECs and is down-regulated in aECs within mouse and human lung metastases. Endothelial-specific inactivation of both Foxf1 alleles in adult Pdgfb-CreER/Foxf1^fl/fl mice caused lung inflammation and edema, leading to respiratory insufficiency and uniform mortality. Deletion of single Foxf1 allele from ECs was sufficient to increase number and size of spontaneous pulmonary metastases in orthotopic mouse model of breast cancer. Increased lung metastasis in Foxf1-deficient mice was associated with increased recruitment of macrophages to the lungs. RNA-Seq analysis using purified lung EC/aEC demonstrated increased expression of cell adhesion molecules and pro-inflammatory chemokines, as well as reduced expression of genes critical for maintenance of adherence junctions. Foxf1 knockdown in vitro and in vivo disrupted adherence junctions, increased endothelial permeability and decreased mRNA and protein levels of VE-cadherin, a key regulator of endothelial barrier function. Foxf1 directly bound to and induced transcriptional activity of the VE-cadherin promoter. Altogether, Foxf1 controls reprogramming of normal pulmonary ECs into aECs of the metastatic niche by regulating expression of genes critical for recruitment of macrophages and extravasation of tumor cells through endothelial barrier.