Activation of autocrine and paracrine signalling by endothelin 1 (ET1) binding to its receptors elicits pleiotropic effects on tumour cells and on the host microenvironment. This activation modulates cell proliferation, apoptosis, migration, epithelial-to-mesenchymal transition, chemoresistance and neovascularization, thus providing a strong rationale for targeting ET1 receptors in cancer. In this Review, we discuss the advances in our understanding of the diverse biological roles of ET1 in cancer and describe the latest preclinical and clinical progress that has been made using small-molecule antagonists of ET1 receptors that inhibit ET1-driven signalling.

The connexins are the protein subunits of the gap junction intercellular channels. In the present study a new rat connexin was cloned by degenerate reverse transcription-polymerase chain reaction and its gene isolated from a mouse genomic library. The nucleotide sequence encodes a protein of 321 amino acids (called Cx36) with highly significant homology to the members of the connexin family. In situ hybridization analysis of rat brain and retina showed the strongest expression in neurons of the inferior olive, the olfactory bulb, the CA3/CA4 hippocampal subfields and several brain-stem nuclei. An intense expression was also found in the pineal gland and in the retinal ganglion cell and inner nuclear layers. Experiments with neurotoxins, locally injected in the hippocampus or specifically acting on inferior olivary neurons, confirmed the neuronal localization of Cx36. It is the first connexin to be expressed predominantly in mammalian neurons and its identification paves the way for a molecular approach in the study of the role played by gap junctions in the physiology and the pathology of the mammalian brain.

No specific funding was obtained for this study. None of the authors have any competing interests to declare.

Despite considerable efforts to improve early detection and advances in chemotherapy, metastatic relapses remain a major challenge in the management of ovarian cancer. The endothelin A receptor (ET A R)/endothelin-1 (ET-1) axis has been shown to have a significant role in ovarian carcinoma by promoting tumorigenesis. Here we show that the ET-1/ET A R autocrine pathway drives epithelial-to-mesenchymal transition (EMT) in ovarian tumor cells by inducing a fibroblastoid and invasive phenotype, down-regulation of E-cadherin, increased levels of B-catenin, Snail, and other mesenchymal markers, and suppression of E-cadherin promoter activity. Activation of ET A R by ET-1 triggers an integrin-linked kinase (ILK)-mediated signaling pathway leading to glycogen synthase kinase-3B (GSK-3B) inhibition, Snail and B-catenin stabilization, and regulation of transcriptional programs that control EMT. Transfection of dominant negative ILK or exposure to an ILK inhibitor suppresses the ET-1-induced phosphorylation of GSK-3B as well as Snail and B-catenin protein stability, activity, and invasiveness, indicating that ET-1/ ET A R-induced EMT-promoting effects depend on ILK. ET A R blockade by specific antagonists or reduction by ET A R RNA interference reverses EMT and cell invasion by inhibiting autocrine signaling pathways. In ovarian carcinoma xenografts, ABT-627, a specific ET A R antagonist, suppresses EMT determinants and tumor growth. In human ovarian cancers, ET A R expression is associated with E-cadherin downregulation, N-cadherin expression, and tumor grade. Collectively, these findings provide evidence of a critical role for the ET-1/ET A R axis during distinct steps of ovarian carcinoma progression and identify novel targets of therapeutic intervention. (Cancer Res 2005; 65(24): 11649-57)

The activation of endothelin-A receptor (ETAR) by endothelin-1 (ET-1) has a critical role in ovarian tumorigenesis and progression. To define the molecular mechanism in ET-1-induced tumor invasion and metastasis, we focused on ␤-arrestins as scaffold and signaling proteins of G protein-coupled receptors. Here, we demonstrate that, in ovarian cancer cells, ␤-arrestin is recruited to ETAR to form two trimeric complexes: one through the interaction with Src leading to epithelial growth factor receptor (EGFR) transactivation and ␤-catenin Tyr phosphorylation, and the second through the physical association with axin, contributing to release and inactivation of glycogen synthase kinase (GSK)-3␤ and ␤-catenin stabilization. The engagement of ␤-arrestin in these two signaling complexes concurs to activate ␤-catenin signaling pathways. We then demonstrate that silencing of both ␤-arrestin-1 and ␤-arrestin-2 inhibits ETAR-driven signaling, causing suppression of Src, mitogen-activated protein kinase (MAPK), AKT activation, as well as EGFR transactivation and a complete inhibition of ET-1-induced ␤-catenin/TCF transcriptional activity and cell invasion. ETAR blockade with the specific ETAR antagonist ZD4054 abrogates the engagement of ␤-arrestin in the interplay between ETAR and the ␤-catenin pathway in the invasive program. Finally, ETAR is expressed in 85% of human ovarian cancers and is preferentially co-expressed with ␤-arrestin-1 in the advanced tumors. In a xenograft model of ovarian metastasis, HEY cancer cells expressing ␤-arrestin-1 mutant metastasize at a reduced rate, highlighting the importance of this molecule in promoting metastases. ZD4054 treatment significantly inhibits metastases, suggesting that specific ETAR antagonists, by disabling multiple signaling activated by ETAR/␤-arrestin, may represent new therapeutic opportunities for ovarian cancer.beta-arrestin ͉ beta-catenin ͉ endothelin A receptor ͉ metastasis ͉ ovarian cancer