During blood vascular development, the formation of a complex network requires the precise coordination of multiple cell types and interplay between key developmental pathways, which are mediated by growth factors and transcriptional activators. Whilst examples exist of endothelial transcription factors regulating vascular endothelial growth factor (VEGF) receptor and co-receptor levels to mediate signaling outcomes, the modulation of transcription factor activity by these signaling pathways remains relatively unexplored. Therefore, I developed a small scale screen in zebrafish to assay for genetic interactions between the Vegf/Vegf-receptor pathway and SoxF transcription factors, and identified a specific genetic interaction between vegfd and sox18/sox7 during blood vessel development.
VEGFD, a member of the VEGF family, is a potent inducer of tumour lymphangiogenesis and is considered as a key therapeutic target to inhibit cancer metastasis. Despite its identified roles in pathological neo-lymphangiogenesis, the characterisation of an endogenous role for VEGFD in vascular development has remained elusive. The outcome of the screen revealed that double knockdown of Sox18/Vegfd and Sox7/Vegfd in zebrafish embryos exhibited defects in arteriovenous differentiation. Supporting this observation, we found that Sox18/Vegfd double but not single knockout mice, displayed defects in vasculature development. We examined the ability of VEGFD to influence SOX18-dependent gene expression, and found that VEGFD-induced signaling enhanced SOX18 mediated transcription in vitro and in vivo and influenced SOX18 concentration in the nucleus. This work showed the first developmental role for VEGFD in the context of endothelial cell differentiation and identified that VEGFD acts as a modulatory factor, functioning at least in part by enhancing the transcriptional activity of SOX18 in blood vascular endothelial cells. This study suggests that VEGFD mediated pathologies such as cancer metastasis may involve an underlying dysregulation of SoxF-mediated transcriptional networks.
Developmental programs can become dysregulated under pathological conditions and thus in the second part of my PhD, I have investigated the role of SOX18 in cancer metastasis. I evaluated the effect of SOX18 loss-of-function on tumour induced neo-vascular formation in a mouse melanoma model. Previous research has shown that SOX18 is re-expressed in tumour blood vessels and that impaired SOX18 function inhibits tumour-induced vascularization and subsequent cancer growth. Since SOX18 has been identified as a critical switch for lymphangiogenesis in the mouse embryo, I sought to clarify the potential effect of SOX18 on tumour-induced lymphangiogenesis and metastasis in a preclinical mouse model of melanoma.
The lymphatic vasculature provides a major route for tumour metastasis and inhibiting neo-lymphangiogenesis, induced by tumours, can reduce metastasis in animal models. Immunofluorescence analysis of murine tumour xenografts showed that SOX18 was re-expressed in tumour lymphatic vessels. Using an in vivo live-imaging approach in a melanoma mouse model, I demonstrated that genetic disruption of SOX18 function interfered with tumour lymphangiogenesis, kinetics of tumour lymphatic drainage and remarkably, cancer metastasis to a regional lymph node. Overall, these findings suggest that SOX18 induction might be an important step in mediating tumour lymphangiogenesis and metastasis, and identify SOX18 as a potential therapeutic target for metastatic blockade.
In summary, I have addressed two key points related to SOX18 biology during vascular development in health and disease: (i) VEGFD plays a role in enhancing SOX18 transcriptional activity during early blood vessel development (Blood, accepted in press 2013); and (ii) In pathological conditions, SOX18 has a critical function in tumour-induced lymphangiogenesis, and metastasis (Cancer Research, 2012).