Enhanced expression of an endothelin ETA receptor in capillaries from human glioblastoma: a quantitative receptor autoradiographic analysis using a radioluminographic imaging plate system

Autocrine endothelin-3/endothelin receptor B signaling maintains cellular and molecular properties of glioblastoma stem cells

Glioblastoma stem cells (GSC) express both radial glial cell and neural crest cell (NCC)-associated genes. We report that endothelin 3 (EDN3), an essential mitogen for NCC development and migration, is highly produced by GSCs. Serum-induced proliferative differentiation rapidly decreased EDN3 production and downregulated the expression of stemness-associated genes, and reciprocally, two glioblastoma markers, EDN1 and YKL-40 transcripts, were induced. Correspondingly, patient glioblastoma tissues express low levels of EDN3 mRNA and high levels of EDN1 and YKL-40 mRNA. Blocking EDN3/EDN receptor B (EDNRB) signaling by an EDNRB antagonist (BQ788), or EDN3 RNA interference (siRNA), leads to cell apoptosis and functional impairment of tumor sphere formation and cell spreading/migration in culture and loss of tumorigenic capacity in animals. Using exogenous EDN3 as the sole mitogen in culture does not support GSC propagation, but it can rescue GSCs from undergoing cell apoptosis. Molecular analysis by gene expression profiling revealed that most genes downregulated by EDN3/EDNRB blockade were those involved in cytoskeleton organization, pause of growth and differentiation, and DNA damage response, implicating the involvement of EDN3/EDNRB signaling in maintaining GSC migration, undifferentiation, and survival. These data suggest that autocrine EDN3/EDNRB signaling is essential for maintaining GSCs. Incorporating END3/EDNRB-targeted therapies into conventional cancer treatments may have clinical implication for the prevention of tumor recurrence.

Phase I safety study of escalating doses of atrasentan in adults with recurrent malignant glioma

Atrasentan is an oral selective endothelin-A receptor antagonist that may inhibit cell proliferation and interfere with angiogenesis during glioma growth. We conducted a dose-finding study to assess atrasentan's safety and toxicity and to gather preliminary evidence of efficacy. Patients with recurrent malignant glioma received oral atrasentan at >or=10 mg/day. We increased the dose among cohorts until the maximum tolerated dose (MTD) was defined. Patients were evaluated for response every 8 weeks and remained on the study until the tumor progressed or toxicities occurred. Twenty-five patients were enrolled, with a median age of 53 years (range, 25-70) and a median KPS of 90% (range, 60-100%). Twenty-two patients had glioblastoma multiforme (GBM), 2 had anaplastic astrocytoma, and 1 had an anaplastic oliogodendroglioma; 24 patients had received one prior chemo therapy regimen before being enrolled in the study. The most common atrasentan-related toxicities were grade 1 or 2 rhinitis, fatigue, and edema. One patient developed grade 3 hypoxia and peripheral edema at a dose of 90 mg/day. We observed no dose-limiting toxicities in an expanded cohort of 10 patients at 70 mg/day, which was declared the MTD. Two partial responses (8%) were seen in patients with GBM at the 70- and 90-mg/day dose levels, and 4 patients had stable disease before progressing. Nineteen patients have died, and median survival was 6.0 months (95% confidence interval, 4.2-9.5 months). We conclude that the MTD of daily oral atrasentan in patients with recurrent malignant glioma is 70 mg/day. Further study of atrasentan with radiation therapy and temozolomide in newly diagnosed GBM is warranted to evaluate the efficacy of this novel agent.

Localization of the endothelin system in human diffuse astrocytomas

BACKGROUND

Endothelin-1 (ET-1), a vasoconstrictor and mitogen, has recently been implicated in the pathogenesis of human glioblastoma, neuroblastoma, and meningioma. ET-1, formed by proteolysis of the propeptide big ET-1 by endothelin-converting enzyme-1 (ECE-1), mediates its cellular actions through ETA and ETB receptors. Because only immunoreactive ET-1 has been observed within human astrocytic tumor cells, the authors investigated the localization of the entire ET-1 system (ET-1 mRNA, ET-1, ECE-1, ETA and ETB receptors) in surgical samples of human diffuse astrocytomas WHO Grade II (n = 6).

METHODS

ET-1 mRNA expression was elucidated by in situ reverse transcriptase polymerase chain reaction (RT-PCR) using synthetic primers. Polyclonal antibodies were used to localize ET-1, ECE-1, ETA and ETB receptors by immunocytochemistry.

RESULTS

All ET components were detected in the six tumor samples. Intense (3+) cytoplasmic ET-1 mRNA labeling was observed in more than 75% of cells in all 6 astrocytomas. Up to 75% of tumor cells displayed intense ET-1 and ECE-1 immunolabeling distributed throughout their cytoplasm. Immunoreactive ETA and ETB receptors, observed in 25% to 75% of astrocytic tumor cells, were of moderate intensity. In addition, all components of the ET system were seen within endothelial cells of tumor blood vessels.

CONCLUSIONS

The presence of ET-1 mRNA, ECE-1, and ET-1 within tumor astrocytes suggests local ET synthesis and processing. The mitogenic and antiapoptotic properties of ET-1, as well as the vasodilatory signaling of ETB receptors, may promote tumorigenesis.

Endothelin-1 binding to endothelin receptors in the rat anterior pituitary gland: possible formation of an ETA-ETB receptor heterodimer

1. Interaction in the recognition of endothelin-1 (ET-1), a typical bivalent ET receptor-ligand, between ETA and ETB receptors was investigated in the rat anterior pituitary gland, using our quantitative receptor autoradiographic method with tissue sections preserving the cell-membrane structure and ET receptor-related compounds. 2. In saturation binding studies with increasing concentrations (0.77-200 pM) of 125I-ET-1 (nonselective bivalent radioligand), 125I-ET-1 binding to the rat anterior pituitary gland was saturable and single with a KD of 71 pM and a Bmax of 120 fmol mg(-1). When 1.0 microM BQ-123 (ETA antagonist) was added to the incubation buffer, binding parameters were 8.3 pM of KD and 8.0 fmol mg(-1) of Bmax, whereas 10 nM sarafotoxin S6c (ETB agonist) exerted little change in these binding parameters (KD, 72 pM; Bmax, 110 fmol mg(-1)). 3. Competition binding studies with a fixed amount (3.8 pM) of 125 I-ET-1 revealed that when 1.0 microM BQ-123 was present in the incubation buffer, ETB receptor-related compounds such as sarafotoxin S6c, ET-3, IRL1620 (ETB agonist), and BQ-788 (ETB antagonist) competitively inhibited 125I-ET-1 binding with K(i)s of 140,18,350 pM, and 14 nM, respectively, however, these compounds were not significant competitors for 125I-ET-1 binding in the case of absence of BQ-123. 4. In cold-ligand saturation studies with a fixed amount (390 pM) of 125I-IRL 1620 (ETB radioligand), IRL1620 bound to a single population of the ETB receptor, and no change was observed in binding characteristics in the presence of 1.0 microM BQ-123. 125I-IRL1620 binding was competitively inhibited by ET-1 and ET-3 in the absence of BQ-123, with K(i)s of 20 and 29 pM, respectively, the affinities being much the same as those of 29 nM, in the presence of 1.0 microM BQ-123. 5. Two nonbivalent ETA antagonists, BQ-123 and PD151242, were highly sensitive and full competitors for 125I-ET-1 binding (5.0 pM), in the presence of 10 nM sarafotoxin S6c. 6. Taken together with the present finding that mRNAs encoding the rat ETA and the ETB receptors are expressed in the anterior pituitary gland, we tentatively conclude that although there are ETA and ETB receptors with a functional binding capability for ET receptor-ligands, the ETB receptor does not independently recognize ET-1 without the aid of the ETA receptor. If this thesis is tenable, then ET-1 can bridge between the two receptors to form an ETA-ETB receptor heterodimer.

The endothelin system in human glioblastoma

Endothelin-1 (ET-1) is a powerful mitogenic and/or anti-apoptotic peptide produced by many cancer cells. To evaluate the potential role of the endothelin system in glioblastoma we first determined the cellular distribution of the mRNA and proteins of the components of the endothelin system, preproendothelin-1 (PPET-1), endothelin-converting enzyme-1 (ECE-1), and ET(A) and ET(B) receptors in human glioblastoma tissue and glioblastoma cell lines. PPET-1, ECE-1, and ET(A) receptor were highly expressed in glioblastoma vessels and in some scattered glioblastoma areas whereas ET(B) receptor was mainly found in cancer cells. This suggests that glioblastoma vessels constitute an important source of ET-1 that acts on cancer cells via the ET(B) receptor. Four human glioblastoma cell lines expressed mRNA for all of the components of the ET-1 pathway. Bosentan, a mixed ET(A) and ET(B) receptor antagonist, induced apoptosis in these cell lines in a dose-dependent manner. Apoptosis was potentiated by Fas Ligand (APO-1L, CD95L), a pro-apoptotic peptide, only in LNZ308 cells, corresponding to the known functional Fas expression in these cell lines. LNZ308 cells also expressed the long and short forms of the cellular FLICE/caspase-8 inhibitory protein (FLIP). Bosentan and a protein kinase C inhibitor down-regulated short FLIP in these cells. ET-1 induced transient phosphorylation of extracellular signal-regulated kinase but did not induce long-term thymidine incorporation in LNZ308 glioblastoma cells. These results suggest that, in glioblastoma cells, ET-1, mainly acting via the ET(B) receptor, is a survival/antiapoptotic factor produced by tumor vasculature, but not a proliferation factor, involving protein kinase C and extracellular signal-regulated kinase pathways, and stabilization of the short form of FLIP.

Endothelin receptors in kainic acid-induced neural lesions of rat brain

Seven days after an intracerebroventricular injection of 0.8 microgram kainic acid, a time of neural tissue-repair after damage, we applied our receptor autoradiographic method to examine changes in the endothelin receptors in kainic acid-induced neural lesions of the rat brain. There were belt-shaped areas with the de novo expressed [125I]endothelin-1 binding sites in the damaged hippocampus CA1, CA3, and CA4 subfields. We also noted a homogeneous zone with a low binding-density, the area sandwiched by the belt-shaped areas. In a "remote" area corresponding anatomically to the deep soma layer of the piriform cortex plus lateral parts of amygdaloid complex we noted a well-defined area with "punched hole-figure" of low density [125I]endothelin-1 binding sites. The lesion was surrounded by areas rich in binding sites. The de novo expressed [125I]endothelin-1 binding sites were characterized endothelin B receptor. Microglia were present in the area with "punched hole-figure" and in the hippocampus pyramidal cell layer with neuronal death. In contrast to microglia, astrocytes were rich with hypertrophia in kainic acid-induced neural lesions anatomically corresponding to areas with the de novo endothelin B receptor. Taken together with the present observations of microscopic evidence of cellular distribution, we suggest that the de novo expressed endothelin B receptor was carried by astrocytes aggregating in neural lesions. In light of our findings, the possibility that astrocytes can be activated by the endothelin B receptor in response to neural tissue repair after damage to neurons would have to be considered.

Endothelin receptors in rat pituitary gland

1. We used the quantitative receptor autoradiographic method plus 125I-endothelin-1 (125I-ET-1), BQ-123, a specific antagonist for the endothelin ETA receptor, and sarafotoxin S6c, a selective agonist for the ETB receptor to investigate the ET receptor in the rat pituitary gland. 2. The method revealed that the BQ-123-sensitive ETA receptor was present predominantly in the anterior lobe and Rathke's pouch. 3. The posterior lobe contained BQ-123-sensitive ETA and sarafotoxin S6c-sensitive ETB receptors, in almost the same proportion. There was no significant 125I-ET-1 binding to the intermediate lobe. 4. Knowledge of the heterogeneous distribution of ET receptor subtypes in the pituitary gland supplies information that will be pertinent to physiological investigations of the gland.

The endothelin ETA receptor exists in the caudal solitary tract nucleus of the rat brain

1. The receptor autoradiographic method done on the rat lower brain stem and cerebellum plus 125I-endothelin-1, BQ-123, an antagonist for the endothelin ETA receptor, and sarafotoxin S6c, an agonist for the ETB receptor, revealed minute amounts of the ETA receptor coexisting with the ETB receptor in the caudal solitary tract nucleus of the rat lower brain stem. 2. The ETB receptor is present predominantly in other parts of the lower brain stem. 3. Knowledge of the heterogeneous distribution of the central endothelin receptor subtypes aids in understanding the neurophysiology of endothelins.

Endothelin receptor in microvessels isolated from human meningiomas: quantification with radioluminography

1. We characterized specific 125I-endothelin-1 (125I-ET-1) binding sites in microvessels isolated from human meningiomas, using an in vitro quantitative receptor autoradiographic technique coupled to a radioluminographic imaging plate system. 2. This newly developed and highly sensitive method revealed high-affinity ET receptors present in pellet sections of the microvessels from all the meningiomas studied, regardless of histological subtypes (dissociation constant, 1.2 +/- 0.3 nM; maximum binding capacity, 185 +/- 56 fmol/mg; means +/- SE for nine tumors). 3. In five cases of meningiomas, ET-3 competed for 125I-ET-1 binding to microvessels from those tumors with a low affinity [50% inhibiting concentration (IC50) of 1.6 +/- 0.4 x 10(-6) M], and a selective ETB receptor agonist, sarafotoxin S6c, up to 10(-6) M, did not displace ET binding from the sections. 4. In the sections of microvessels from four other tumors, biphasic competition curves were obtained in the case of incubation in the presence of increasing concentrations of ET-3, with an IC50 of 1.1 +/- 0.2 x 10(-9) M for the high-affinity component and 1.6 +/- 0.3 x 10(-6) M for the low-affinity component, respectively. In addition, S6c competed for ET binding to those sections (IC50 = 2.3 +/- 0.2 x 10(-10) M) and 10(-6) M S6c displaced 30% of the control, corresponding to the high-affinity component of competition curves obtained in the presence of ET-3. 5. Our results suggest that (a) capillaries in human meningiomas express a large number of high-affinity ETA (non-ETB) receptors with a small proportion of ETB receptors, and (b) ET may have a role in neovascularization, tumor blood flow, and/or function of the blood-tumor barrier in meningioma tissues by interacting with specific receptors present on the surface of the endothelium.