Lysosomal traffic of liganded endothelin B receptor

An overview of receptor endocytosis and signaling

Endocytosis is a cellular process which mediates receptor internalization, nutrient uptake, and the regulation of cell signaling. Microorganisms (many bacteria and viruses) and toxins also use the same process and enter the cells. Generally, endocytosis is considered in the three forms such as phagocytosis (cell eating), pinocytosis (cell drinking), and highly selective receptor-mediated endocytosis (clathrin-dependent and independent). Several endocytic routes exist in an analogous, achieving diverse functions. Most studies on endocytosis have used transformed cells in culture. To visualize the receptor internalization, trafficking, and signaling in subcellular organelles, a green fluorescent protein-tagged receptor has been utilized. It also helps to visualize the endocytosis effects in live-cell imaging. Confocal laser microscopy increases our understanding of receptor endocytosis and signaling. Site-directed mutagenesis studies demonstrated that many short-sequence motifs of the cytoplasmic domain of receptors significantly play a vital role in receptor internalization, subcellular trafficking, and signaling. However, other factors also regulate receptor internalization through clathrin-coated vesicles. Receptor endocytosis can occur through clathrin-dependent and clathrin-independent pathways. This chapter briefly discusses the internalization, trafficking, and signaling of various receptors in normal conditions. In addition, it also highlights the malfunction of the receptor in disease conditions.

Emerging tools for studying receptor endocytosis and signaling

Ligands, agonists, or antagonists use receptor-mediated endocytosis (RME) to reach their intracellular targets. After the internalization of ligand-receptor complexes, it traffics through different subcellular organelles such as early endosome, recycling endosome, lysosome, etc. Further, after the ligand binding to the receptor, different second messengers are generated, such as cGMP, cAMP, IP₃, etc. Several methods have been used, such as radioligand binding assay, western blotting, co-immunoprecipitation (co-IP), qRT-PCR, immunofluorescence and confocal microscopy, microRNA/siRNA, and bioassays to understand the various events, such as internalization, subcellular trafficking, signaling, metabolic degradation, etc. This chapter briefly discusses the key principles and methods used to study internalization, subcellular trafficking, signaling, and metabolic degradation of numerous receptors.

Endocytosis and signaling of 5-HT1A receptor

The neurotransmitter serotonin (also known as 5-hydroxytryptamine, 5-HT) regulates many important physiological as well as pathological functions in the body like psychoemotional, sensation, blood circulation, food intake, autonomic, memory, sleep, pain, etc. 5-HT binds to its receptor 5-HT1A to initiate GTP exchange at the Gi/o protein, which activates the receptor G protein complex. G protein subunits attach to different effectors and generate various responses, such as inhibition of adenyl cyclase enzyme and regulates the opening of Ca⁺⁺ and K⁺ ion channels. Activated signalling cascades activate protein kinase C (PKC) (a second messenger), which further induces the detachment of Gβγ-dependent receptor signaling and leads to 5-HT1A internalization. After internalization, 5-HT1A receptor attaches to the Ras-ERK1/2 pathway. The receptor further trafficks to the lysosome for degradation. Receptor skips the trafficking to the lysosomal compartments and undergoes dephosphorylation. Dephosphorylated receptors now recycled back to the cell membrane. In this chapter, we have discussed the internalization, trafficking and signaling of the 5-HT1A receptor.

Receptor biology: Challenges and opportunities

Receptor biology provides a great opportunity to understand the ligand-receptor signaling involved in health and disease processes. Receptor endocytosis and signaling play a vital role in health conditions. Receptor-based signaling is the main form of communication between cells and cells with the environment. However, if any irregularities happen during these events, the consequences of pathophysiological conditions occur. Various methods are utilized to know structure, function, and regulation of receptor proteins. Further, live-cell imaging and genetic manipulations have aided in the understanding of receptor internalization, subcellular trafficking, signaling, metabolic degradation, etc. Understanding the genetics, biochemistry, and physiology of receptors and ligands is very helpful to explore various aspects such as prognosis, diagnosis, and treatment of disease. However, there are enormous challenges that exist to explore receptor biology further. This chapter briefly discusses the current challenges and emerging opportunities of receptor biology.

The ring size of monocyclic ET-1 controls selectivity and signaling efficiency at both endothelin receptor subtypes

Cardiovascular diseases (CVDs) like hypertension are a major cause for death worldwide. In the cardiovascular tissue, the endothelin system-consisting of the receptor subtypes A (ET_A R) and B (ET_B R) and the mixed agonist endothelin 1 (ET-1)-is a major key player in the regulation of vascular tone and blood pressure. Tight control of this system is required to maintain homeostasis; otherwise, the endothelin system can cause severe CVDs like pulmonary artery hypertension. The high sequence homology between both receptor subtypes limits the development of novel and selective ligands. Identification of small differences in receptor-ligand interactions and determination of selectivity constraints are crucial to fine-tune ligand properties and subsequent signaling events. Here, we report on novel ET-1 analogs and their detailed pharmacological characterization. We generated simplified ET-1-derived monocyclic peptides to provide an accessible synthesis route. By detailed in vitro characterization, we demonstrated that both G protein signaling and the subsequent arrestin recruitment of activated ET_B R remain intact, whereas activation of the ET_A R depends on the intramolecular ring size. Increasing of the intramolecular ring structure reduces activity at the ET_A R and shifts the peptide toward ET_B R selectivity. All ET-1 analogs displayed efficient ET_B R-mediated signaling by G protein activation and arrestin 3 recruitment. Our study provides in-depth characterization of the ET-1/ET_A R and ET-1/ET_B R interactions, which has the potential for future development of endothelin-based drugs for CVD treatment. By identification of Lys⁹ for selective labeling, novel analogs for peptide-mediated shuttling by ET-1 are proposed.

Mechanisms of Renal Vasodilation and Hyperfiltration During Pregnancy

Glomerular filtration rate (GFR) and renal plasma flow (RPF) increase by 40-65% and 50-85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration appears to be almost completely due to the increase in RPF, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The major pregnancy hormone involved is relaxin. The mediators downstream from relaxin include endothelin (ET) and nitric oxide (NO). New evidence indicates that relaxin increases vascular gelatinase activity during pregnancy, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration, and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and NO. Whether the chronic volume expansion characteristic of pregnancy contributes to the maintenance of gestational renal changes requires further investigation. Additional studies are also needed to further delineate the molecular basis of these mechanisms and, importantly, to investigate whether they apply to women.

Validation of endothelin B receptor antibodies reveals two distinct receptor-related bands on Western blot

Antibodies are important tools for the study of protein expression but are often used without full validation. In this study, we used Western blots to characterize antibodies targeted to the N or C terminal (NT or CT, respectively) and the second or third intracellular loop (IL2 or IL3, respectively) of the endothelin B receptor (ETB). The IL2-targeted antibody accurately detected endogenous ETB expression in rat brain and cultured rat astrocytes by labeling a 50-kDa band, the expected weight of full-length ETB. However, this antibody failed to detect transfected ETB in HEK293 cultures. In contrast, the NT-targeted antibody accurately detected endogenous ETB in rat astrocyte cultures and transfected ETB in HEK293 cultures by labeling a 37-kDa band but failed to detect endogenous ETB in rat brain. Bands detected by the CT- or IL3-targeted antibody were found to be unrelated to ETB. Our findings show that functional ETB can be detected at 50 or 37kDa on Western blot, with drastic differences in antibody affinity for these bands. The 37-kDa band likely reflects ETB processing, which appears to be dependent on cell type and/or culture condition.

Intracellular endothelin type B receptor-driven Ca2+ signal elicits nitric oxide production in endothelial cells

Endothelin-1 exerts its actions via activation of ET(A) and ET(B) G(q/11) protein-coupled receptors, located in the plasmalemma, cytoplasm, and nucleus. Although the autocrine/paracrine nature of endothelin-1 signaling has been extensively studied, its intracrine role has been largely attributed to interaction with receptors located on nuclear membranes and the nucleoplasm. Because ET(B) receptors have been shown to be targeted to endolysosomes, we used intracellular microinjection and concurrent imaging methods to test their involvement in Ca(2+) signaling and subsequential NO production. We provide evidence that microinjected endothelin-1 produces a dose-dependent elevation in cytosolic calcium concentration in ET(B)-transfected cells and endothelial cells; this response is sensitive to ET(B) but not ET(A) receptor blockade. In endothelial cells, the endothelin-1-induced Ca(2+) response is abolished upon endolysosomal but not Golgi disruption. Moreover, the effect is prevented by inhibition of microautophagy and is sensitive to inhibitors of the phospholipase C and inositol 1,4,5-trisphosphate receptor. Furthermore, intracellular endothelin-1 increases nitric oxide via an ET(B)-dependent mechanism. Our results indicate for the first time that intracellular endothelin-1 activates endolysosomal ET(B) receptors and increase cytosolic Ca(2+) and nitric oxide production. Endothelin-1 acts in an intracrine fashion on endolysosomal ET(B) to induce nitric oxide formation, thus modulating endothelial function.

Anti-endothelin drugs in solid tumors

IMPORTANCE OF THE FIELD

The endothelin (ET) axis, which includes the biological functions of ETs and their receptors, has played a physiological role in normal tissue, acting as a modulator of vasomotor tone, tissue differentiation and development, cell proliferation and hormone production. Interestingly, it also functions in the growth and progression of various tumors. Several researchers have identified the blockade of the ET-1 receptor as a promising therapeutic approach.

AREAS COVERED IN THIS REVIEW

The clinical investigation of an orally bioavailable ET antagonist, atrasentan, in prostate cancer, is encouraging. In this neoplasia, it has shown antitumor activity, bone metastasis control and amelioration of cancer-related pain but improvement in time to progression and overall survival has still not been demonstrated. The clinical trials of other ET antagonists are reported. Literature research was performed by Pubmed and Pharmaprojects.

WHAT THE READER WILL GAIN

A comprehensive view about the use of atrasentan in the treatment of castration-resistant prostate cancer (CRPC) is provided together with the scientific rationale based on the function of ET and its receptor in various cancer development mechanisms.

TAKE HOME MESSAGE

Atrasentan seems to be active in CRPC, although strong scientific evidence is still to be found. Interesting clinical findings regard zibotentan.

Endothelin (ET)-1 and ET-3 promote expression of c-fos and c-jun in human choriocarcinoma via ET(B) receptor-mediated G(i)- and G(q)-pathways and MAP kinase activation

BACKGROUND AND PURPOSE

Endothelins (ETs) and their G protein-coupled receptors exert key physiological functions during normal and aberrant placental development. Trophoblast cells mediate the contact between the embryo and the mother, by establishing a transient organ, the placenta. Choriocarcinoma cells display many of the biochemical and morphological characteristics of in utero invasive trophoblast cells and may therefore be used as a suitable model to study epithelial tumour progression of foetal-derived cells.

EXPERIMENTAL APPROACH

The present study aimed at investigating ET receptor-mediated activation of the mitogen-activated protein kinase (MAPK) pathway in human choriocarcinoma.

KEY RESULTS

Both JAR and Jeg-3 choriocarcinoma cell lines expressed ET receptor subtype B (ET(B)) but not ET(A) receptor transcripts. ET(B) receptor engagement by ET-1 and ET-3 resulted in a similar time- and concentration-dependent phosphorylation of p42/44 MAPK, also known as extracellular regulated kinase 1/2. Using specific pharmacological antagonists/inhibitors, we showed that ET-1/-3-mediated signal transduction by the ET(B) receptor is transmitted via G(i)- and G(q)-dependent pathways through activation of the Src (G(i)) and protein kinase C (G(q)) axis that converge at Ras/Raf, leading to downstream activation of p42/44. On a functional level, ET(B) engagement and subsequent phosphorylation of p42/44 resulted in enhanced transcription of the immediate early response genes c-fos and c-jun, a process commonly assumed to be mediated by the ET(A) receptor, and increased cell growth and relative cell area.

CONCLUSIONS AND IMPLICATIONS

As human choriocarcinoma cells secrete ETs, pharmacological antagonism of ETs and/or ET(B) receptor-mediated signal transduction could represent a likely target therapy for choriocarcinoma.

The ovarian endothelin network: an evolving story

The endothelin (ET) system consists of three ET isopeptides, several converting enzyme isoforms and two G-protein-coupled receptors, ETA and ETB, which are linked to multiple signaling pathways. Less than 20 years after the initial detection of ET-1 in granulosa cells, the ovarian ET network continues to expand with the discovery of new members and functions. ETs influence a broad range of essential reproductive processes, such as ovulation, steroidogenesis and luteolysis. Therefore, a more comprehensive understanding of the ovarian ET network might provide new strategies for controlling reproduction. This review presents up-to-date findings on the ET network in the ovary.

Role of relaxin in maternal renal vasodilation of pregnancy

The remarkable hemodynamic changes of normal pregnancy are briefly reviewed. In addition, new findings and current concepts related to the underlying hormonal and molecular mechanisms are presented. Finally, work that is in progress as well as future directions is briefly discussed.

Localization of endothelin-converting enzyme in bovine optic nerve and retina

A significant loss and remodeling of the lamina cribrosa tissue leading to the excavation of the optic nerve is seen in glaucoma. Elevated endothelin-1 (ET-1) levels are detected in the aqueous humor of patients of open-angle glaucoma and in the plasma of patients with normal- tension glaucoma. Optic nerve damage, including axonal loss, can be mimicked by ET-1 injection near the optic nerve. ET-1 is produced from its precursor Big ET-1 (38 amino acids) by endothelin-converting enzyme (ECE). Although ET-1 and its receptors have been identified in the retina, little is known of the distribution of ECE at the optic nerve. Presently, ET-1 receptors and Big ET-1 converting activities were characterized in bovine optic nerve and the retina. The ET(B) receptor was detected in both the optic nerve and retina by immunoblotting and cross-linking, using 125I-ET-1. However, the ET(A) receptor was detected only in the retina. Big ET-1 conversion activities were detected in the plasma membrane (PM) of bovine retina, but not in the PM of the optic nerve. The retinal PM Big ET-1 converting activity was inhibited by phosphoramidon, thiorphan, and acidification. Furthermore, ECE cytosolic activities were detected in both the optic nerve and retina. Unlike the PM-ECE, cytosolic Big ET-1 converting activities were activated by acidification (pH 6.4), suggesting the involvement of ECE-2-like activity and/or cathepsin activity. Pepstatin, a potent inhibitor of cathepsins, inhibited the optic nerve (ON) cytosolic conversion of Big ET-1 peptide by 50%, and the combination of pepstatin and phosphoramidon, a potent inhibitor of ECE, inhibited the ON cytosolic activity by 86%. By contrast, the combination of both inhibitors weakly inhibited the cytosolic retinal Big ET-1 converting activity. Western blotting revealed the presence of ECE-1 at the PM of the retina not the ON. ECE-2 and cathpesins B, D, and L were detected only in the cytosol of both the retina and ON. In summary, it appears that ET-1 could be produced in the retina and optic nerve by at least two ECE subtypes and, perhaps, cathepsins. Big ET-1 converting activity may be an important target in preventing ET-1-induced optic nerve pathology.

Mechanisms of renal vasodilation and hyperfiltration during pregnancy: current perspectives and potential implications for preeclampsia

A thorough understanding of the renal and cardiovascular adaptations to normal gestation is essential for proper diagnosis and management of hypertensive disorders and renal diseases during pregnancy. Here, we briefly review the renal hemodynamic changes of normal pregnancy. In addition, we present new findings and current concepts related to the underlying hormonal and molecular mechanisms. Finally, we speculate on the potential contribution of these insights from normal pregnancy to the pathogenesis of preeclampsia.