Endothelin-2 synthesis is stimulated by the type-1 tumour necrosis factor receptor and cAMP: comparison with endothelin-converting enzyme-1 expression

Endothelin-2-mediated protection of mutant photoreceptors in inherited photoreceptor degeneration

Expression of the Endothelin-2 (Edn2) mRNA is greatly increased in the photoreceptors (PRs) of mouse models of inherited PR degeneration (IPD). To examine the role of Edn2 in mutant PR survival, we generated Edn2^−/− mice carrying homozygous Pde6b^rd1 alleles or the Tg(RHO P347S) transgene. In the Edn2^−/− background, PR survival increased 110% in Pde6b^rd1/rd1 mice at post-natal (PN) day 15, and 60% in Tg(RHO P347S) mice at PN40. In contrast, PR survival was not increased in retinal explants of Pde6b^rd1/rd1; Edn2^−/− mice. This finding, together with systemic abnormalities in Edn2^−/− mice, suggested that the increased survival of mutant PRs in the Edn2^−/− background resulted at least partly from the systemic EDN2 loss of function. To examine directly the role of EDN2 in mutant PRs, we used a scAAV5-Edn2 cDNA vector to restore Edn2 expression in Pde6b^rd1/rd1; Edn2^−/− PRs and observed an 18% increase in PR survival at PN14. Importantly, PR survival was also increased after injection of scAAV5-Edn2 into Pde6b^rd1/rd1 retinas, by 31% at PN15. Together, these findings suggest that increased Edn2 expression is protective to mutant PRs. To begin to elucidate Edn2-mediated mechanisms that contribute to PR survival, we used microarray analysis and identified a cohort of 20 genes with >4-fold increased expression in Tg(RHO P347S) retinas, including Fgf2. Notably, increased expression of the FGF2 protein in Tg(RHO P347S) PRs was ablated in Tg(RHO P347S); Edn2^−/− retinas. Our findings indicate that the increased expression of PR Edn2 increases PR survival, and suggest that the Edn2-dependent increase in PR expression of FGF2 may contribute to the augmented survival.

Endothelin-2, the forgotten isoform: emerging role in the cardiovascular system, ovarian development, immunology and cancer

Endothelin-2 [ET-2; also known as vasoactive intestinal contractor (VIC), in rodents] differs from endothelin-1 (ET-1) by only two amino acids, and unlike the third isoform, endothelin-3 (ET-3), it has the same affinity as ET-1 for both ET(A) and ET(B) receptors. It is often assumed that ET-2 would mimic the actions of the more abundant ET-1 and current pharmacological interventions used to inhibit the ET system would also block the actions of ET-2. These assumptions have focused research on ET-1 with ET-2 studied in much less detail. Recent research suggests that our understanding of the ET family requires re-evaluation. Although ET-2 is very similar in structure as well as pharmacology to ET-1, and may co-exist in the same tissue compartments, there is converging evidence for an important and distinct ET-2 pathway. Specifically is has been demonstrated that ET-2 has a key role in ovarian physiology, with ET-2-mediated contraction proposed as a final signal facilitating ovulation. Furthermore, ET-2 may also have a pathophysiological role in heart failure, immunology and cancer. Comparison of ET-2 versus ET-1 mRNA expression suggests this may be accomplished at the level of gene expression but differences may also exist in peptide synthesis by enzymes such as endothelin converting enzymes (ECEs) and chymase, which may allow the two pathways to be distinguished pharmacologically and become separate drug targets. LINKED ARTICLES This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Mechanical stress triggers selective release of fibrotic mediators from bronchial epithelium

Transforming growth factor-beta (TGF-beta) and endothelin (ET) are found in elevated amounts in the airways of individuals with asthma. The cellular source of these peptides and their role in mediating the airway fibrosis of chronic asthma are unknown. In response to mechanical stresses similar to those occurring in vivo during airway constriction, bronchial epithelial cells increase the steady-state level of mRNA for both ET-1 and ET-2, followed by increased release of ET protein. Mechanical stress also enhances release of TGF-beta2 from a preformed cell-associated pool. TGF-beta2 and ET act individually and, more importantly, synergistically to promote fibrotic protein synthesis in reporter fibroblasts. To confirm the role of these intermediates in stress-induced fibrosis, conditioned medium from mechanically stressed bronchial epithelial cells was shown to elicit fibrotic protein synthesis in reporter fibroblasts; this effect was significantly inhibited by combined treatment with ET receptor antagonists and a neutralizing antibody to TGF-beta2. These data are consistent with a primary pathogenic role for mechanical stress-induced release of both TGF-beta2 and ET in the subepithelial fibrosis that characterizes chronic asthma.

Inhibitory effect of adrenomedullin on basal and tumour necrosis factor alpha-stimulated endothelin-1 synthesis in bovine aortic endothelial cells is independent of cyclic AMP

Adrenomedullin (ADM) is a potent vasodilator and reverses the vasoconstrictor action of endothelin-1 (ET-1). These studies aimed to determine the effect of ADM on ET-1 synthesis in bovine aortic endothelial cells (BAEC) and to identify the possible mechanisms involved. In this cell model, ADM increased cyclic AMP production by BAEC with threshold concentrations of 100 pM and an EC(50) of 1 nM. This effect was not blocked by co-treatment with the CGRP type 1 receptor antagonist CGRP(8--37). ADM caused a potent concentration-dependent inhibition of ET-1 release that was correlated with reduced preproET-1 mRNA levels. This reached a maximal reduction of 70% compared to basal levels after 2 and 6 hr exposure of BAEC to 1 nM ADM, with significant decreases at concentrations as low as 10 pM. However, a 100-fold discrepancy between the threshold ADM concentration for cyclic AMP production and inhibition of ET-1 release was observed. Treatment of BAEC with tumour necrosis factor alpha (TNFalpha; 10 ng/mL) caused a 2-fold increase over basal ET-1 release. ADM caused a more marked reduction in stimulated ET-1 synthesis with a threshold of 1 pM, and suppression of ET-1 release to basal levels at 100 nM. 8-Bromo cyclic AMP, showed no concentration-dependent inhibition of ET-1 release, yet caused a 50% reduction in TNFalpha-stimulated intercellular adhesion molecule-1 (ICAM-1) mRNA levels. Thus, physiological ADM concentrations inhibit ET-1 synthesis independently of cyclic AMP in BAEC at the level of preproET-1 mRNA expression. The high sensitivity of this inhibition implicates ADM as an important physiological regulator of endothelial ET-1 production.