Examining the Impact of Endothelial Bmpr2 Loss on the Growth and Vascularization of Lung Metastases in a Mouse Model of Metastatic Breast Cancer

Abstract

Rationale: Bone morphogenic protein-9 (BMP9) signaling has been shown to play a role in tumour angiogenesis. In a mouse model of metastatic breast cancer, mice lacking the gene encoding BMP9 demonstrated increases in both the number and size of secondary lung metastases, suggesting that BMP9 inhibits both metastatic engraftment and tumour angiogenesis. However, BMP9 can also promote angiogenesis when its type-II receptor, Bmpr2, is lost. We explored the impact of endothelial Bmpr2 loss on the engraftment and growth mammary tumour metastases in the lung. Methods: L1-Cre mice were crossed with animals possessing the Ai6 transgenic reporter and a Bmpr2-floxed allele to produce offspring that expressed both ZsGreen fluorescent protein and either wildtype (Bmpr2EC+/+), heterozygous (Bmpr2EC+/-) or homozygous null (Bmpr2EC-/-) levels of Bmpr2 selectively in the Cre+ endothelium of their lungs, brain, spleen, and retina. Red fluorescent protein (RFP)-expressing E0771 cancer cells were injected into the lateral tail vein or implanted orthotopically into the mammary fat pad of these animals. For the orthotopic model, mammary tumours were resected at a volume of ~500mm3 and mice were maintained for an additional four weeks to establish secondary lung metastases. Tumour burden in the lungs, liver and brain were quantified by whole organ RFP fluorescence and fluorescence microscopy. For the tail vein model, E0771 engraftment in the lung was assessed 72 hours post injection. Results: The proportion of mice exhibiting lung metastases did not differ across genotypes. However, Bmpr2EC+/- and Bmpr2EC-/- mice exhibited increased lung tumour burden and vascular density when compared to Bmpr2EC+/+ controls. Similar increases were not observed in the liver or brain, indicating that findings were specific to endothelial Bmpr2 loss in the lung. In the tail-vein model, Bmpr2EC+/- mice had significantly increased engraftment events relative to Bmpr2EC+/+ controls. Conclusion: Endothelial Bmpr2 loss enhances the engraftment, growth, and vascularization of secondary lung metastases.