Novel endothelin B receptor transcripts with the potential of generating a new receptor

EDNRB isoform 3 confers Temozolomide resistance in A375 melanoma cells by modulating membrane potential, reactive oxygen species and mitochondrial Ca2

Background

The role of endothelin receptor type B (EDNRB) isoform 3 involved in Temozolomide (TMZ)-induced melanoma cell death has not yet been elucidated.

Methods

The subcellular localization of EDNRB isoform 3 was determined by confocal and immunoblotting assays. Silencing EDNRB isoform 3 was performed by CRISPR/Cas9. Apoptosis was assessed by annexin V/propium iodide staining and caspases 3/7/9 activity. Mitochondrial membrane potential, reactive oxygen species and mitochondrial Ca²⁺ were measured by flow cytometry. Apoptosis protein array was applied.

Results

Confocal and immunoblot analyses indicate mitochondrial localization of EDNRB isoform 3 and the first N-terminal (1–22) amino acids are sufficient for its mitochondrial targeting. EDNRB isoform 3 depleted A375 cells significantly confers chemoresistance with mitochondrial depolarization, reduced reactive oxygen species, enhanced mitochondrial Ca²⁺ uptake and decreased caspase 9 activation. Additionally, apoptosis array shows that lack of EDNRB isoform 3 has relatively lower expression of phosphorylation of p53 at S392 and a slightly higher expression of Paraoxonase 2.

Conclusion

Our findings raise the possibility of targeting EDNRB isoform 3 as a new therapeutic strategy in combination with TMZ for melanoma treatment.

Endothelin-1 (ET-1) induces resistance to bortezomib in human multiple myeloma cells via a pathway involving the ETB receptor and upregulation of proteasomal activity

PURPOSE

Bortezomib (BTZ) is used for the treatment of multiple myeloma (MM). However, a significant proportion of patients may be refractory to the drug. This study aimed to investigate whether the endothelin (ET-1) axis may act as an escape mechanism to treatment with bortezomib in MM cells.

METHODS

NCI-H929 and RPMI-8226 (human MM cell lines) were cultured with or without ET-1, BTZ, and inhibitors of the endothelin receptors. ET-1 levels were determined by ELISA, while the protein levels of its receptors and of the PI3K and MAPK pathways' components by western blot. Effects of ET-1 on cell proliferation were studied by MTT and on the ubiquitin proteasome pathway by assessing the chymotryptic activity of the 20S proteasome in cell lysates.

RESULTS

Endothelin receptors A and B (ETAR and ETBR, respectively) were found to be expressed in both cell lines, with the RPMI-8226 cells that are considered resistant to BTZ, expressing higher levels of ETBR and in addition secreting ET-1. Treatment of the NCI-H929 cells with ET-1 increased proliferation, while co-incubation of these cells with ET-1 and BTZ decreased BTZ efficacy with concomitant upregulation of 20S proteasomal activity. Si-RNA silencing or chemical blockade of ETBR abrogated the protective effects of ET-1. Finally, data suggest that the predominant signaling pathway involved in ET-1/ETBR-induced BTZ resistance in MM cells may be the MAPK pathway.

CONCLUSION

Our data suggest a possible role of the ET-1/ETBR axis in regulating the sensitivity of MM cells to BTZ. Thus, combining bortezomib with strategies to target the ET-1 axis could prove to be a novel promising therapeutic approach in MM.

Endothelin-Receptor Antagonists beyond Pulmonary Arterial Hypertension: Cancer and Fibrosis

The endothelin axis and in particular the two endothelin receptors, ETA and ETB, are targets for therapeutic intervention in human diseases. Endothelin-receptor antagonists are in clinical use to treat pulmonary arterial hypertension and have been under clinical investigation for the treatment of several other diseases, such as systemic hypertension, cancer, vasospasm, and fibrogenic diseases. In this Perspective, we review the molecules that have been evaluated in human clinical trials for the treatment of pulmonary arterial hypertension, as well as other cardiovascular diseases, cancer, and fibrosis. We will also discuss the therapeutic consequences of receptor selectivity with regard to ETA-selective, ETB-selective, or dual ETA/ETB antagonists. We will also consider which chemical characteristics are relevant to clinical use and the properties of molecules necessary for efficacy in treating diseases against which known molecules displayed suboptimal efficacy.

Genetic Analyses of a Three Generation Family Segregating Hirschsprung Disease and Iris Heterochromia

We present the genetic analyses conducted on a three-generation family (14 individuals) with three members affected with isolated-Hirschsprung disease (HSCR) and one with HSCR and heterochromia iridum (syndromic-HSCR), a phenotype reminiscent of Waardenburg-Shah syndrome (WS4). WS4 is characterized by pigmentary abnormalities of the skin, eyes and/or hair, sensorineural deafness and HSCR. None of the members had sensorineural deafness. The family was screened for copy number variations (CNVs) using Illumina-HumanOmni2.5-Beadchip and for coding sequence mutations in WS4 genes (EDN3, EDNRB, or SOX10) and in the main HSCR gene (RET). Confocal microscopy and immunoblotting were used to assess the functional impact of the mutations. A heterozygous A/G transition in EDNRB was identified in 4 affected and 3 unaffected individuals. While in EDNRB isoforms 1 and 2 (cellular receptor) the transition results in the abolishment of translation initiation (M1V), in isoform 3 (only in the cytosol) the replacement occurs at Met91 (M91V) and is predicted benign. Another heterozygous transition (c.-248G/A; -predicted to affect translation efficiency-) in the 5'-untranslated region of EDN3 (EDNRB ligand) was detected in all affected individuals but not in healthy carriers of the EDNRB mutation. Also, a de novo CNVs encompassing DACH1 was identified in the patient with heterochromia iridum and HSCR Since the EDNRB and EDN3 variants only coexist in affected individuals, HSCR could be due to the joint effect of mutations in genes of the same pathway. Iris heterochromia could be due to an independent genetic event and would account for the additional phenotype within the family.

Generation and characterization of rendomab-B1, a monoclonal antibody displaying potent and specific antagonism of the human endothelin B receptor

Endothelin B receptor (ETBR) is a G protein-coupled receptor able to bind equally to the three identified human endothelin peptides. It is expressed primarily on vascular endothelial cells and involved in various physiological processes including vascular tone homeostasis, enteric nervous system development, melanogenesis and angiogenesis. Furthermore, overactivation or overexpression of ETBR have been associated with the development of various diseases such as cardiovascular disorders and cancers. Therefore, ETBR appears to be relevant target for the therapy or diagnosis of highly prevalent human diseases. In this study, we report the in vitro characterization of rendomab-B1, a monoclonal antibody (mAb) obtained by genetic immunization, which selectively recognizes the native form of human ETBR (hETBR). Rendomab-B1 is the first-reported mAb that behaves as a potent antagonist of hETBR. It recognizes an original extracellular conformational epitope on the receptor, distinct from the endothelin-1 (ET-1) binding site. Rendomab-B1 not only blocks ET-1-induced calcium signaling pathway and triggers rapid receptor internalization on recombinant hETBR-expressing cells, but also exerts pharmacological activities on human vascular endothelial cells, reducing both cell viability and ET-1-induced hETBR synthesis. In addition, binding experiments using rendomab-B1 on different melanoma cell lines reveal the structural and functional heterogeneity of hETBR expressed at the surface of these cancer cells, strongly suggesting the existence of tumor-specific receptors. Collectively, our results underscore the value of rendomab-B1 for research, therapeutic and diagnostic applications dealing with hETBR.

Vascular endothelin receptor type B: structure, function and dysregulation in vascular disease

Endothelin-1 (ET-1) is a major regulator of vascular function, acting via both endothelin receptor type A (ET(A)R) and type B (ET(B)R). Although the role of ET(A)R in vascular smooth muscle (VSM) contraction has been studied, little is known about ET(B)R. ET(B)R is a G-protein coupled receptor with a molecular mass of ~50 kDa and 442 amino acids arranged in seven transmembrane domains. Alternative splice variants of ET(B)R and heterodimerization and cross-talk with ET(A)R may affect the receptor function. ET(B)R has been identified in numerous blood vessels with substantial effects in the systemic, renal, pulmonary, coronary and cerebral circulation. ET(B)R in the endothelium mediates the release of relaxing factors such as nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor, and could also play a role in ET-1 clearance. ET(B)R in VSM mediates increases in [Ca(2+)](i), protein kinase C, mitogen-activated protein kinase and other pathways of VSM contraction and cell growth. ET-1/ET(A)R signaling has been associated with salt-sensitive hypertension (HTN) and pulmonary arterial hypertension (PAH), and ET(A)R antagonists have shown some benefits in these conditions. In search for other pathogenetic factors and more effective approaches, the role of alterations in endothelial ET(B)R and VSM ET(B)R in vascular dysfunction, and the potential benefits of modulators of ET(B)R in treatment of HTN and PAH are being examined. Combined ET(A)R/ET(B)R antagonists could be more efficacious in the management of conditions involving upregulation of ET(A)R and ET(B)R in VSM. Combined ET(A)R antagonist with ET(B)R agonist may need to be evaluated in conditions associated with decreased endothelial ET(B)R expression/activity.

New roles of EDNRB and EDN3 in the pathogenesis of Hirschsprung disease

PURPOSE

Hirschsprung disease is characterized by the absence of intramural ganglion cells in the myenteric and submucosal plexuses within distal intestine, because of a fail in the enteric nervous system formations process. Endothelin-3-endothelin receptor B signaling pathway is known to play an essential role in this process. The aim of this study was to evaluate the implication of the EDN3 and EDNRB genes in a series of patients with Hirschsprung disease from Spain and determinate their mutational spectrum.

METHODS

We performed the mutational screening of both genes in 196 patients with Hirschsprung disease using denaturing high-performance liquid chromatography technology. A case-control study using TaqMan Technology was also carried out to evaluate some common polymorphisms and haplotypes as susceptibility factors for Hirschsprung disease.

RESULTS

Besides several novel mutations in both genes, we found a truncating mutation in an alternative isoform of EDNRB. Interestingly, we obtained an overrepresentation of a specific EDN3 haplotype in cases versus controls.

CONCLUSIONS

Our results suggest that the isoform EDNRB Delta 3 might be playing an essential role in the formation of enteric nervous system. In addition, based on the haplotype distribution, EDN3 might be considered as a common susceptibility gene for sporadic Hirschsprung disease in a low-penetrance fashion.

Therapeutic potential of endothelin receptor modulators: lessons from human clinical trials

The endothelin system, and in particular endothelin receptors, are targets for therapeutic intervention in human diseases. Endothelin receptor antagonists have reached clinical use for treating pulmonary arterial hypertension, and are under clinical investigation for several other diseases, such as cancer, vasospasm or fibrogenic diseases. We review the molecules that have been evaluated in the main clinical trials, from the point of view of receptor selectivity and of their chemical characteristics which were important for efficacy in pulmonary hypertension. We will also discuss future use of antagonists to endothelin receptor(s) in several human diseases and what should be the necessary properties of the future molecules for efficacy in diseases where the presently tested molecules displayed suboptimal efficacy.

Epigenetic and genetic alterations of the EDNRB gene in nasopharyngeal carcinoma

BACKGROUND

Loss of heterozygosity (LOH) at 13q22 is a common event in nasopharyngeal carcinoma (NPC). EDNRB gene located at 13q22 has been demonstrated to be hypermethylated in some kinds of tumors. In the current study, we focused on the epigenetic and genetic alterations of EDNRB in NPC.

METHODS

The mRNA expression of EDNRB was detected by semiquantitative RT-PCR and real-time quantitative PCR in 49 NPC and 12 chronic nasopharyngitis biopsies. The methylation and LOH status of EDNRB were examined by methylation-specific polymerase chain reaction, microsatellite PCR and sequencing. We also examined the mRNA expression of EDNRB in four NPC cell lines after 5-aza-2'-deoxycytidine treatment.

RESULTS

EDNRB was downregulated in primary NPC tissues and NPC cell lines, and a relatively higher methylation level of EDNRB was found in NPC biopsies (84%) compared to that in chronic nasopharyngitis biopsies (42%). Treatment of NPC cell lines with 5-aza-2'-deoxycytidine activated EDNRB expression. LOH of EDNRB gene was also found at two microsatellite sites with ratios of 6.25 and 16.67% in NPC.

CONCLUSION

Our results suggested that EDNRB expression may be affected by aberrant promoter methylation and gene deletion and may play a role in the development of NPC.

Functional Difference of the SOX10 Mutant Proteins Responsible for the Phenotypic Variability in Auditory-Pigmentary Disorders

Waardenburg syndrome (WS) is an inherited disorder, characterized by auditory-pigmentary abnormalities. SOX10 transcription factor and endothelin receptor type B (EDNRB) are responsible for WS type 4 (WS4), which also exhibits megacolon, while microphthalmia-associated transcription factor (MITF) is responsible for WS2, which is not associated with megacolon. Here, we investigated the functions of SOX10 mutant proteins using the target promoters, EDNRB and MITF. The SOX10 mutations chosen were E189X, Q377X, and 482ins6, which are associated with WS4, and S135T that is associated with Yemenite deaf-blind hypopigmentation syndrome (YDBS), which does not manifest megacolon. These SOX10 mutant proteins showed impaired transactivation activity on the MITF promoter. In contrast, E189X and Q377X proteins, each of which lacks its C-terminal portion, activated the EDNRB promoter, whereas no activation was detected with the SOX10 proteins mutated at the DNA-binding domain, 482ins6 and S135T. However, unlike 482ins6 protein, S135T protein synergistically activated EDNRB promoter with a transcription factor Sp1, indicating that Sp1 could compensate the impaired function of a SOX10 mutant protein. We suggest that the variability in transactivation ability of SOX10 mutant proteins may account for the different phenotypes between WS4 and YDBS and that Sp1 is a potential modifier gene of WS4.

SOX10, in combination with Sp1, regulates the endothelin receptor type B gene in human melanocyte lineage cells

Waardenburg syndrome (WS) is an auditory-pigmentary disorder that exhibits varying combinations of sensorineural hearing loss and abnormal pigmentation of the hair and skin. WS type 4 (WS4), a subtype of WS, is characterized by the presence of the aganglionic megacolon and is associated with mutations in the gene encoding either endothelin 3, endothelin receptor type B (EDNRB), or Sry-box 10 (SOX10). Here, we provide evidence that SOX10 regulates the expression of EDNRB gene in human melanocyte-lineage cells, as judged by RNA interference and chromatin immunoprecipitation analyses. Human melanocytes preferentially express the EDNRB transcripts derived from the conventional EDNRB promoter. SOX10 transactivates the EDNRB promoter through the cis-acting elements, the two CA-rich sequences and the GC box. Moreover, a transcription factor Sp1 enhances the degree of the SOX10-mediated transactivation of the EDNRB promoter through these cis-acting elements. Furthermore, we have shown that the EDNRB promoter is heavily methylated in HeLa human cervical cancer cells, lacking EDNRB expression, but not in melanocytes and HMV-II melanoma cells. The expression of EDNRB became detectable in HeLa cells after treatment with a demethylating reagent, 5'-aza-2'-deoxycytidine, which was further enhanced in the transformed cells over-expressing SOX10. We therefore suggest that SOX10, alone or in combination with Sp1, regulates transcription of the EDNRB gene, thereby ensuring appropriate expression level of EDNRB in human melanocytes.

Hypermethylation of CpG islands in primary and metastatic human prostate cancer

Aberrant DNA methylation patterns may be the earliest somatic genome changes in prostate cancer. Using real-time methylation-specific PCR, we assessed the extent of hypermethylation at 16 CpG islands in DNA from seven prostate cancer cell lines (LNCaP, PC-3, DU-145, LAPC-4, CWR22Rv1, VCaP, and C42B), normal prostate epithelial cells, normal prostate stromal cells, 73 primary prostate cancers, 91 metastatic prostate cancers, and 25 noncancerous prostate tissues. We found that CpG islands at GSTP1, APC, RASSF1a, PTGS2, and MDR1 were hypermethylated in >85% of prostate cancers and cancer cell lines but not in normal prostate cells and tissues; CpG islands at EDNRB, ESR1, CDKN2a, and hMLH1 exhibited low to moderate rates of hypermethylation in prostate cancer tissues and cancer cell lines but were entirely unmethylated in normal tissues; and CpG islands at DAPK1, TIMP3, MGMT, CDKN2b, p14/ARF, and CDH1 were not abnormally hypermethylated in prostate cancers. Receiver operator characteristic curve analyses suggested that CpG island hypermethylation changes at GSTP1, APC, RASSF1a, PTGS2, and MDR1 in various combinations can distinguish primary prostate cancer from benign prostate tissues with sensitivities of 97.3-100% and specificities of 92-100%. Hypermethylation of the CpG island at EDNRB was correlated with the grade and stage of the primary prostate cancers. PTGS2 CpG island hypermethylation portended an increased risk of recurrence. Furthermore, CpG island hypermethylation patterns in prostate cancer metastases were very similar to the primary prostate cancers and tended to show greater differences between cases than between anatomical sites of metastasis.

Functional analysis of the endogenous retroviral promoter of the human endothelin B receptor gene

We previously reported that the long terminal repeats (LTRs) of retroviral elements belonging to the HERV-E family contribute to the expression of the human apolipoprotein C1 (APOC1) and endothelin B receptor (EDNRB) genes by providing alternative promoters. While both LTRs were shown to promote transcription in vivo and in vitro, their respective activity and tissue specificity appeared to differ even though they shared a high degree of sequence identity. In the present study, we further characterized the promoter of the EDNRB LTR and delineated the regions and motifs required for strong activity. We confirmed the placenta-restricted expression of the LTR by transient transfections and quantitative real-time PCR and determined that the retroviral promoter contributes significantly to the level of EDNRB transcripts in placenta, where chimeric mRNAs were found to represent 15% of overall EDNRB mRNAs. Transient transfection of 5' deletion constructs in cells of placental origin identified a motif, named LPE1, between positions 111 and 122 of the EDNRB LTR necessary for transcriptional activity. Removal of this region, which contains a putative SP1 binding site, abolished promoter activity. A second enhancing region resides between positions 175 and 215 of the LTR and was termed LPE2. Interestingly, this section contained three binding sites that were not present in the APOC1 LTR due to minor nucleotide differences. The predicted motifs in the EDNRB LTR were found to likely act in symbiosis as modifications to any of the three sites reduced transcription by one-third while alterations to all three eliminated promoter activity. The results from this study illustrate how slight variations in transcriptional regulatory sequences can have a profound effect on promoter activity and demonstrate the complex regulatory effects of human endogenous retrovirus elements on human gene expression.

Unique endothelin receptor binding in kidneys of ETB receptor deficient rats

Gariepy and colleagues (Gariepy CE, Williams SC, Richardson JA, Hammer RE, and Yanagisawa M. J Clin Invest 102: 1092-1101, 1998.) developed rescued spotting-lethal rats that carry a naturally occurring deletion of the endothelin (ET) type B receptor gene resulting in a lack of functional renal ETB receptor expression. It has been shown that rats homozygous (sl/sl) for the deletion have elevated plasma ET-1 levels; thus, the purpose of this study was to determine whether this deletion would result in a downregulation of ETA receptors in renal tissue. ET-1 and ET-3 binding experiments were performed with cortex, outer medullary, and inner medullary membranes of heterozygous (sl/+) and sl/sl ETB receptor-deficient rats. 125I-labeled ET-1 binding in sl/sl cortex and outer medulla was significantly lower than cortex and outer medulla from sl/+ rats. In contrast to sl/+ rats, [125I]ET-3 binding was not detected in the cortex and outer medulla of sl/sl rats, indicating a lack of ETB receptor expression. The inner medulla of sl/+ rats also demonstrated an abundance of ETB receptors. Surprisingly, however, we also observed significant [125I]ET-3 binding in the sl/sl inner medulla. Furthermore, ET-3 binding in the inner medulla could be blocked with an ETA receptor antagonist in sl/sl rats but not in tissue from sl/+ rats. These studies indicate that rats deficient in ETB receptors have decreased renal cortical and outer medullary ETA receptor number, most likely in response to elevated plasma ET-1 levels. In addition, homozygous ETB-deficient rats express a novel inner medullary ET-3 binding site.

Significance of novel endothelin-B receptor gene polymorphisms in Hirschsprung's disease: predominance of a novel variant (561C/T) in patients with co-existing Down's syndrome

Several genes have been implicated in the pathogenesis of Hirschsprung's disease (HSCR). In a previous study performed, five novel (V202M, E480K, IVS10-2A/G, D771N, IVS19-9C/T) mutations and one previously described mutation (P937L) have been identified in the RET proto-oncogene in 20% of the study population. To further investigate the involvement of other genes, mutation analysis of the endothelin-B receptor (EDNRB) gene was performed in 52 unrelated sporadic HSCR patients, including 38 non-syndromic and 14 patients with HSCR and Down's syndrome. Six novel (178G/A, 552C/T, 561C/T, 702C/T, IVS3-6C/T and IVS4 + 3A/G) sequence variants and one previously described (831G/A) polymorphism were identified. Statistically significant differences were achieved for six (178G/A, 552C/T, 561C/T, 702C/T, IVS3-6C/T and 831G/A) of these variants. The T-allele of the 561C/T polymorphism was over represented in the HSCR/Down's syndrome patient group (36% representing 5 of 14) compared to normal controls (6% representing 5 of 84) (p < 0.002, chi(2) with Yates correction = 12.14), suggesting that the 561C/T variant is associated with a low penetrance effect in patients with this complex phenotype. Detection of the 178G/A polymorphism in only non-syndromic HSCR patients, provide further support for an important role of specific sequence variants in the EDNRB gene in the HSCR/Down's syndrome phenotype.

Promoter hypermethylation of the EDNRB gene in nasopharyngeal carcinoma

To identify the epigenetic changes in nasopharyngeal carcinoma (NPC), we performed methylation-sensitive restriction fingerprinting (MSRF) analysis on NPC cell lines and xenografts. A 190 bp sequence methylated in NPC tumors was isolated and showed high homology to the 5' CpG island of the endothelin receptor B (EDNRB) gene. Since the EDNRB gene is commonly inactivated in prostate and bladder cancers, it may be a candidate target gene involved in NPC tumorigenesis. By bisulfite sequencing, we have confirmed that hypermethylation of the 5' CpG island of EDNRB occurred in both xenografts and all 4 cell lines but not in 2 normal nasopharyngeal outgrowths. RT-PCR demonstrated that only original EDNRB transcripts, but not the splicing transcripts, were expressed in normal nasopharyngeal epithelial cells. Loss of the original EDNRB expression was consistently found in 2 xenografts and 3 cell lines with dense methylation patterns. Treatment of these 3 cell lines with 5'-aza-2'-deoxycytidine led to re-expression of the EDNRB transcript and demethylation of its promoter regions. Our results demonstrate that silencing of EDNRB gene expression in NPC is associated with promoter hypermethylation. Using methylation-specific PCR, we also detected methylation of the 5' CpG island of EDNRB in 19/21 (90.5%) primary tumors, while no methylation was found in all 6 normal nasopharyngeal epithelia. The high frequencies of promoter hypermethylation suggest that repression of the EDNRB gene may play a role in the development of NPC.

Long terminal repeats are used as alternative promoters for the endothelin B receptor and apolipoprotein C-I genes in humans

To examine the potential regulatory involvement of retroelements in the human genome, we screened the transcribed sequences of GenBank and expressed sequence tag data bases with long terminal repeat (LTR) elements derived from different human endogenous retroviruses. These screenings detected human transcripts containing LTRs belonging to the human endogenous retrovirus-E family fused to the apolipoprotein CI (apoC-I) and the endothelin B receptor (EBR) genes. However, both genes are known to have non-LTR (native) promoters. Initial reverse transcription-polymerase chain reaction experiments confirmed and authenticated the presence of transcripts from both the native and LTR promoters. Using a 5'-rapid amplification of cDNA ends protocol, we showed that the alternative transcripts of apoC-I and EBR are initiated and promoted by the LTRs. The LTR-apoC-I fusion and native apoC-I transcripts are present in many of the tissues tested. As expected, we found apoC-I preferentially expressed in liver, where about 15% of the transcripts are derived from the LTR promoter. Transient transfections suggest that the expression is not dependent on the LTR itself, but the presence of the LTR increases activity of the apoC-I promoter from both humans and baboons. The native EBR-driven transcripts were also detected in many tissues, whereas the LTR-driven transcripts appear limited to placenta. In contrast to the LTR of apoC-I, the EBR LTR promotes a significant proportion of the total EBR transcripts, and transient transfection results indicate that the LTR acts as a strong promoter and enhancer in a placental cell line. This investigation reports two examples where LTR sequences contribute to increased transcription of human genes and illustrates the impact of mobile elements on gene and genome evolution.