Angiogenesis is a tightly regulated multi-step process in which new blood vessels form from the preexisting vasculature. Due to extensive research in the field of vascular biology, a plethora of knowledge exists about the growth factors, receptors, and signaling pathways that drive angiogenesis. However, our understanding of the transcription factors that bridge the gap between these signals and new gene expression remains incomplete.

Members of the Snail superfamily of zinc-finger transcription factors are expressed and required during both embryonic development and progression of malignant epithelial tumors. Specifically, this protein family drives developmental and pathological events by inducing epithelial to mesenchymal transitions (EMT). Snail (Snai1) and Slug (Snai2) are transcriptional repressors that belong to the Snail family of zinc-finger proteins. Importantly, expression of Snail or Slug in tumor cells is known to contribute to invasion and metastasis, and these genes have also been observed in angiogenic endothelial cells (EC) in vivo. Based on these observations, we asked whether angiogenic sprouting might share common attributes with EMT.

Using our in vitro angiogenesis model, we show that sprouting EC express and regulate Slug and Snail. To demonstrate a functional role for these genes, we utilized small interfering RNA (siRNA) to inhibit Slug or Snail expression and examine phenotypic alterations in EC undergoing angiogenesis. Interestingly, both Slug and Snail appear to be required for angiogenic sprouting as siRNA-mediated knockdown of either gene inhibits EC sprouting. In addition, we demonstrate that Slug regulates expression of membrane-type 1 matrix metalloproteinase (MT1-MMP; itself a regulator of vascular morphogenic events), but not vascular endothelial-cadherin (VE-Cadherin). Furthermore, lentiviral-mediated re-expression of MT1-MMP rescues the loss of sprouting induced by Slug knockdown, confirming that MT1-MMP is a crucial downstream target of Slug during sprouting angiogenesis. Importantly, we also observe enhanced Slug and Snail expression in tumor-associated blood vessels in multiple cancers. Taken together, these data identify a critical role for Slug expression in regulating pathological angiogenesis.