Vascular Endothelial Growth Factor (VEGF) is a potent endothelial cytokine and mitogen that directs the development of vasculature. Canonical VEGF signaling is a well described phenomenon whereby hypoxic tissues secrete VEGF, which then diffuses to surrounding VEGF- specific receptors. Interaction of VEGF and its receptors stimulates growth of the vascular system, through vasculogenic and angiogenic processes. Once formed, the circulatory system allows for the delivery of oxygen and nutrients to all tissues in the body. Blockade of VEGF signaling is used in the clinic to impede angiogenesis in some cancer settings, which suppresses tumor load by limiting the available blood supply required for survival and metastasis. However, patients treated with anti-VEGF therapy are beset by unexpected cardiovascular side-effects that suggest other non-canonical functions of VEGF protein. Here we identify several novel non-canonical roles for VEGF in the developing embryo heart and in the adult endothelium.
In embryogenesis, VEGF receptors are expressed in diverse tissues apart from the inner layer of the vasculature, which indicates that these tissues respond to VEGF in a non-canonical manner. Using a chimera approach, we found evidence that VEGF takes part in cardiac chamber specification of the ventricles, which was further demonstrated with VEGF gain- and loss-of-function genetic models. Because VEGF is implicated in early cardiomyocyte differentiation, we investigated the expression patterns of VEGF's major receptor (VEGFR2) throughout cardiogenesis with a reporter mouse model. We identified subsets of cardiomyocytes that express VEGFR2 in the atrial septum and the primordial conduction system.
VEGF is expressed by the adult endothelium, and is essential for a non-canonical pathway that promotes homeostasis via autocrine signaling. Using mouse models and primary human cell culture, we were able to demonstrate that endothelial VEGF maintains cell survival through constitutive suppression of the transcription factor Foxo1. In the absence of VEGF, cellular Foxo1 levels increase and causes deregulation of cell metabolism and autophagy, which ultimately induces cell death.
Together, this work describes several novel non-canonical roles for VEGF in the heart, and further elucidates the molecular mechanisms behind autocrine VEGF signaling in the endothelium. These findings expand our understanding of the basic biology of VEGF and expose potential limitations for the use of anti-VEGF blockade in human patients.