253 Targeting tumour vascularisation through endothelial-specific Sox9 deletion reduces melanoma metastasis through reduction of vascularisation, changing melanoma gene expression pattern

Abstract

The formation of de novo vascular structures is vital for melanoma growth and metastasis. Endovascular progenitor cells (EVPs) residing in vessel walls give rise to mature endothelial cells and contribute to a new blood vessel network formation in tumours. Sox9 is a transcription factor that is highly upregulated in EVPs and is playing a crucial role in stem cell self-renewal and quiescence. In this study, we aimed to explore the role of Sox9 deletion in the endothelium on tumour vascularisation, metastasis, and melanoma gene expression. An endothelial-specific knock out mouse model, Sox9fl/fl/Cdh5CreERt2/Rosa-YFP was utilised to delete Sox9 conditionally in the endothelium. A significant reduction in tumour EVPs was observed upon Sox9 deletion in the endothelium in B16-F0 or HcMel12 melanoma tumours. Immunofluorescence staining of tumour sections demonstrated a significant reduction in the number and area of CD31+ vessels and a significant increase of pericytes coverage suggesting increased maturity of the remaining vessels. Lung metastasis significantly reduced after surgical excision of the primary Hcmel12 tumours. Of importance, intravenous delivery of B16-F10 melanoma cells engrafting and forming lung metastases was not changed by Sox9 deletion in the endothelium suggesting that Sox9 deletion acts primarily in the primary tumour. Deletion of Sox9 in the vasculature led to significant gene expression changes in flow-sorted melanoma cells in the primary tumour. Epithelial to mesenchymal transition pathway was among the downregulated pathways suggesting that deletion of Sox9 in the endothelium affected the ability of primary tumour cells to metastasize. In summary, Sox9 deletion in the endothelium resulted in the depletion of EVPs, prompting fewer de novo vessels in the tumour and reduced metastasis. This new knowledge strongly suggests EVPs as a new target in the field of anti-vascularisation treatment in cancer and metastasis.