Investigating the Effects of Endothelial BMPR2 Loss in Neoangiogenesis and Growth of Lung Metastatic Lesions in Solid Tumour Models

Abstract

Background: Tumour angiogenesis is a hallmark of cancer progression, enabling tumours to access nutrients, oxygen, and routes for dissemination. Aberrant vascular growth in tumours produces tortuous, leaky, and poorly perfused vessels, contributing to therapeutic resistance and immune evasion. Conventional anti-angiogenic therapies that block pro-angiogenic pathways such as VEGF, angiopoietin, or Notch have shown limited success, often leading to resistance through vessel co-option and exacerbating hypoxia. As an alternative, vascular normalization therapies seek to restore structural integrity and function to tumour vasculature, improving perfusion, therapeutic delivery, and anti-tumour immunity. One pathway of interest is bone morphogenetic protein 9 (BMP9) signaling, which canonically promotes endothelial quiescence through BMP type-II receptor (BMPR-II), but may drive pro-proliferative endothelial signaling when BMPR-II is absent. Additionally, in the E0771 orthotopic model of metastatic breast cancer, endothelial specific Bmpr2 deletion increased the overall burden and vascularization of pulmonary metastases. However, the exact mechanism by which this occurs is unknown. Results: In this study, we investigated the role of BMPR-II in pulmonary metastases using a B16F10 melanoma model in endothelial-specific Bmpr2 knockout mice (Bmpr2EC-/-). Contrary to prior findings in the E0771 model, where Bmpr2 loss was associated with increased vascular density, B16F10 lung metastases did not exhibit heightened vascular density in the Bmpr2EC-/- mice. Single-cell RNA sequencing was performed to characterize transcriptional alterations in tumour-associated endothelial cells. This revealed a knock-out specific (KOS) cluster of cells which was significantly enriched for genes involved in VEGFC/VEGFR-3 signaling pathways. To further probe these mechanisms, a BMP-responsive reporter construct was developed to track changes in signaling dynamics following BMPR-II deletion. Conclusions: This study identified a potentially novel cell cluster in the Bmpr2EC-/- , which may play a larger role in vascularizing pulmonary metastases. The creation of the Smad1 reporter construct will allow for a more robust understanding of intracellular signaling kinetics without the BMPR-II receptor. Additionally, while Bmpr2 loss can enhance vascular proliferation in certain tumour settings, this effect may not generalize across cancer models. These results underscore the importance of tumour-specific context when considering BMP9/BMPR-II signaling as a therapeutic target for novel therapies.