Emerging role of endothelin-1 in tumor angiogenesis Anna Bagnato and Francesca Spinella Molecular Pathology Laboratory, Regina Elena Cancer Institute, Via delle Messi d’Oro 156, 00158 Rome, Italy Tumor vessels express distinct molecular markers that are functionally relevant in the angiogenic process. Although tyrosine kinase receptor agonists are the major mediators of angiogenesis, several G-protein- coupled receptor agonists have also been shown to have a role. Among these, endothelin-1 (ET-1), by acting directly on endothelial cells via the ET B receptor, modu- lates different stages of neovascularization, including proliferation, migration, invasion, protease production and morphogenesis, and also stimulates neovasculari- zation in vivo. ET-1 can also modulate tumor angio- genesis indirectly through the induction of vascular endothelial growth factor (VEGF). Engagement of the ET A receptor by ET-1 induces VEGF production by increasing levels of hypoxia-inducible factor 1a. More- over, tumor cells themselves, predominantly expres- sing the ET A receptor, might form vessel-like channels within the tumors. The role of ET-1 and its signaling network in tumor angiogenesis suggests that new therapeutic strategies using specific ET A -receptor antagonists could improve antitumor treatment by inhibiting both neovascularization and tumor cell growth. Angiogenesis – the formation of new vessels from existing vasculature – is an important early event in tumor progression that begins in premalignant lesions [1]. Initiation of angiogenesis (the angiogenic switch) is controlled by different regulators including local hypoxia, which induces the synthesis of angiogenic factors that can stimulate endothelial cell growth. As the new vessels mature, the endothelial tubes acquire supporting cells, such as pericytes, in addition to ECM (see Glossary). Tumor vasculature synthesizes various cell surface and ECM proteins that are not present in normal vasculature. Many of these proteins are functionally important in promoting tumor angiogenesis [2,3]. Recently, tumors have also been shown to develop lymphatic vessels, which are important routes of tumor metastasis. Several new markers of lymphatic endo- thelium have been identified, along with factors that induce lymphatic vessel growth [4,5]. Some tumors also contain vascular channels comprising cancer cells and their ECM but no endothelial cells, a phenomenon called vasculogenic mimicry [6]. Tumor vessels are tortuous and leaky, their diameter is irregular and their walls are thin, features that could be explained by the relative paucity of pericytes, or reduced pericyte function [7,8]. In addition, many molecular differences related to the synthesis of angiogenic molecules exist between tumoral and normal vasculature [9]. VEGF and bFGF are the principal regulators of neovasculariza- tion [10]. Although growth factors, such as VEGF, and their cognate tyrosine kinase receptors are the best- characterized mediators of angiogenesis, several GPCRs also play a role in angiogenesis [11]. Among the GPCR agonists, ET-1 is produced by endothelial cells and VSMC, and its concentration is raised in many tumors. Three distinct members of the ET family, ET-1, ET-2 and ET-3, have been cloned. The effects of ETs are mediated by their binding to two GPCRs, termed ET A and ET B , which have different affinities for the individual ETs. The ET B receptor binds the three peptides with equal affinity. By contrast, the ET A receptor binds ET-1 and ET-2 with higher affinity than ET-3 [12]. In blood vessels, endothelial cells express the ET B receptor [13–15], whereas VSMC express the ET A receptor [16,17]. Ligand engagement of the ET A receptor leads to phospholipase C activation and subsequent phosphorylation cascades, leading to the activation of protein kinases, MAPK and FAK [18]. Among downstream events after ET A receptor activation, ET-1 induces a rapid increase in EGF receptor phosphorylation, termed ‘trans- activation’ [19,20]. Furthermore, ET-1 triggers activation of antiapoptotic signaling through PI 3K-mediated Akt pathways [21]. These findings demonstrate the existence of multiple signal transduction pathways in ET-1-stimu- lated cells. Angiogenesis is a complex, multistep cascade that is initiated by the activation of quiescent endothelial cells in response to angiogenic factors. However, the relative synthesis of the individual factors that contribute to this process has yet to be clarified. Here, we discuss the role of ET-1, as a potential autocrine regulator of endothelial cells in the different steps of neovascularization. Endothelin-1 as an angiogenic factor Direct modulation of endothelial cells HUVECs actively produce and secrete ET-1 and simul- taneously express the ET B receptor (K d ¼ 17 pM) as the major receptor population, indicating a potential autocrine role for endogenous ET-1 [13,15]. Moreover, ET-1, acting via positive autocrine feedback action on the ET B receptor, stimulates its own synthesis in Huvec, acting via the ET B receptor [14]. Previous studies demonstrated that ET-1 Corresponding author: A. Bagnato (bagnato@ifo.it). Review TRENDS in Endocrinology and Metabolism Vol.14 No.1 January 2002 44 http://tem.trends.com 1043-2760/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S1043-2760(02)00010-3