Endothelin: a new vasoactive ocular peptide

Diabetic retinopathy and endothelin system: microangiopathy versus endothelial dysfunction

In the face of the global epidemic of diabetes, it is critical that we update our knowledge about the pathogenesis of diabetes and the related micro alterations on the vascular network in the body. This may ultimately lead to early diagnosis and novel treatment options for delaying the progression of diabetic complications. Research has recently revealed the pivotal role of endothelin in the pathogenesis of diabetic complications, particularly in the regulation of the capillary flow, which is affected in the course of retinopathy. Although there are several reviews on various approaches to the treatment of diabetes, including normalization of glucose and fat metabolism, no reviews in literature have focused on the endothelin system as a therapeutic target or early indicator of diabetic microangiopathy. In this review, we summarize some of the experimental and clinical evidence suggesting that current therapeutic approaches to diabetes may include the modulation of the blood concentration of compounds of the endothelin system. In addition, we will briefly discuss the beneficial effects produced by the inhibition of the production of high levels of endothelin in vasculopathy, with focus on diabetic retinopathy. The cutting-edge technology currently widely used in opththalmology, such as the OCT angiography, allows us to detect very early retinal morphological changes alongside alterations in choroidal and retinal vascular network. Combination of such changes with highly sensitive measurements of alterations in serum concentrations of endothelin may lead to more efficient early detection and treatment of diabetes and related macro/microvascular complications.

Inhibition of endothelin-1 and KCL-induced increase of [CA2+]i by antiglaucoma drugs in cultured A7r5 vascular smooth-muscle cells

Over contraction of vascular smooth muscle may result in ischemia to ocular neuronal cells and deteriorate the glaucoma. The purpose of this study was to investigate the inhibitory effects of various commercial antiglaucoma drugs including brimonidine, dipivefrin, betaxolol, timolol, levobunolol, carteolol, brinzolamide, dorzolamide, unoprostone, latanoprost, pilocarpine, and preservative benzalkonium chloride on endothelin-1(ET-1) and KCl-induced increase of intracellular free Ca2+ ([Ca2+]i) in cultured rat A7r5 vascular smooth muscle cells. These drugs were diluted from original concentrations to 1/100, 1/1000, and 1/10000. [Ca2+]i mobility was analyzed by spectrofluorometry after loading with fura-2-AM. Betaxolol, timolol, levobunolol, and carteolol were found to inhibit KCl-induced release of [Ca2+]i in a dose-dependent manner. High concentrations of betaxolol, timolol, levobunolol, carteolol, and unoprostone also inhibited ET-1-induced increase of [Ca2+]i in A7r5 cells. However, ET-1- and KCl-induced increase of [Ca2+]i was not diminished by other drugs including brimonidine, dipivefrin, brinzolamide, dorzolamide, latanoprost, pilocarpine, and benzalkonium chloride. These results indicate that high concentrations of unoprostone and beta-adrenergic blocking agents including betaxolol, timolol, levobunolol, and carteolol may inhibit ET-1-induced increase of [Ca2+]i. The mechanism may be mediated by inhibition of extracellular calcium influx via blocking of L-type voltage-dependent Ca2+ channel in A7r5 cells.

Change of cytosolic Ca(2+) mobility in cultured bovine corneal endothelial cells by endothelin-1

The effect of endothelin-1 (ET-1) on the intracellular free Ca(2+) ([Ca(2+)](i)) in cultured bovine corneal endothelial cells was studied after loading with fura-2-AM. In Ca(2+)-containing buffer and Ca(2+)-free buffer, ET-1 induced a significant rise in [Ca(2+)](i) at concentrations from 10(-9) to 10(-7) M. In Ca(2+)-free buffer, pretreatment of the cells with ET-1 inhibited thapsigargin-induced [Ca(2+)](i) increase and carbonylcyanide m-chlorophenylhydrazone (CCCP)-induced Ca(2+) release by 99% and 62%, respectively. Pretreatment of the cells with thapsigargin or CCCP also inhibited ET-1-induced [Ca(2+)](i) rise by 36% and 92%, respectively. In Ca(2+)-containing buffer, the ET(A) receptor antagonist (BQ123) and ET(B) receptor antagonist (BQ788) partially inhibited ET-1-induced [Ca(2+)](i) by 92% and 98%, respectively. Nifedipine and La(3+) also inhibited ET-1-induced [Ca(2+)](i) increase by 26% and 91%, respectively. The intracellular calcium release caused by ET-1 was partially inhibited by phospholipase C inhibitor (U73122). After incubation of the cells with ET-1 in Ca(2+)-free buffer, the addition of 5 mM CaCl(2) increased Ca(2+) influx, implying that release of Ca(2+) from internal stores caused by ET-1 further induced capacitative Ca(2+) entry. These data suggest that ET-1-induced [Ca(2+)](i) rise in bovine corneal endothelial cells are mediated by ET(A) receptor, ET(B) receptor, La(3+)-sensitive Ca(2+) pump and L-type voltage-operated Ca(2+) channel, leading to Ca(2+) influx. ET-1 also increased the internal Ca(2+) release from endoplasmic reticulum and mitochondria Ca(2+) stores followed by capacitative Ca(2+) entry. ET-1-induced intracellular Ca(2+) release was modulated by phospholipase C-coupled events.

Induction of cellular toxicity in cultured porcine corneal keratocytes by endothelin-1

The effect of endothelin-1 (ET-1) on corneal cells is not well understood. We investigated the biochemical changes of cultured porcine corneal keratocytes under exposure to ET-1. The results indicate that ET-1 has remarkable effects to inhibit corneal keratocytes on 3H-thymidine, 3H-leucine, 3H-uridine uptakes and cellular migration. It is in a dose-dependent manner at concentrations ranging from 10(-7) M to 10(-9) M. The 50% inhibitory dose (ID50) for ET-1, as measured by 3H-thymidine uptake, 3H-uridine uptake and 3H-leucine uptake, were 10(-7) M, 10(-0.52) M and 10(-11.8) M, respectively. The dead and living cells were estimated with MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) assay that was converted tetrazolium dye of living cells only into insoluble purple formazan crystals within mitochondria. In the presence of ET-1, the cellular MTT values were also decreased. The ID50 for ET-1 with cell migration assay and MTT assay were measured at 10(-7.86) M and 10(-5.1) M. Endothelin-1 (10(-6) M) promptly changed cellular morphology and attenuated adhesion observed with laser scanning cytometer. Endothelin-1-induced characteristic apoptosis cells were observed using a TUNEL assay that detected fragmented DNA of apoptosis. Western blot assay revealed that endothelin-1 induced proteolysis and decreased in fibronectin protein. These findings indicate that endothelin-1 may lead keratocytes to death resulting from induction of apoptosis and functional loss.

Endothelin-induced changes of secondary messengers in cultured corneal endothelial cells

The effect of endothelins on corneal endothelial cells is not well understood. We have investigated the effects of endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3) on bovine corneal endothelial cellular proliferation and the secondary messenger changes in cells in the presence of ET-1. It was found that the 3H-thymidine uptake was enhanced by ET-1 significantly, whereas ET-2 and ET-3 had no effect. ET-1 remarkably affects the increase of corneal endothelial cells on 3H-thymidine, 3H-leucine, and 3H-uridine uptakes in a dose-dependent manner. The 50% effective concentrations (EC50) for ET-1, as measured by 3H-thymidine uptake, 3H-uridine uptake, and 3H-leucine uptake were 10(-8.78) M, 10(-8.53) M and 10(-8.04) M, respectively. It was found that endothelin-1 increased intracellular calcium concentration by using the method of preloading with Fura-2-AM and assaying with spectrophotometry. The cellular IP1, IP2, and IP3 were also stimulated in the presence of ET-1. Moreover, ET-1 enhanced the basal cellular cAMP and cGMP concentrations in corneal endothelial cells in a dose-dependent manner. Immunofluorescent staining revealed that ET-1 increased the fibronectin protein concentration and changed protein distribution in corneal endothelial cells. These findings indicate that endothelin-1 increases in cell proliferation and biological changes may be involved in changing intracellular calcium mobility, increasing intracellular phosphoinositides, enhancing intracellular cGMP and cAMP accumulation, and fibronectin protein synthesis.

Endothelin as a therapeutic target in the treatment of cardiovascular disease

Endothelins, a family of peptides derived from the vascular endothelium and smooth muscle cells possess vasoconstrictor and mitogenic properties. By acting predominantly in a paracrine fashion, these peptides activate specific receptors and have protean effects in normal and diseased organ systems. The wide distribution of these receptors in various tissues mediate the multiplicity of physiologic actions attributed to endothelins. Much of our understanding about endothelins has come from the development of an array of receptor-specific and mixed receptor antagonists. Based on the promising results from animal studies, active research and drug development programs are under way to investigate the clinical potential of endothelin antagonism for treatment of cardiovascular disease.

Role of endothelin in diabetic vascular complications

Endothelin-1 (ET-1), a 21 amino acid peptide originally purified from conditioned medium of cultures of porcine aortic endothelial cells, is recognized as a product of many other cells as well. It is now known that there are three endothelin genes in the human genome (ET-1, ET-2, and ET-3). ET-1 and ET-2 are both strong vasoconstrictors, whereas ET-3 is a potentially weaker vasoconstrictor than the other two isoforms. Besides being the most potent vasoconstrictor yet known, ET-1 also acts as a mitogen on the vascular smooth muscle, and, thus, it may play a role in the development of vascular diseases. It is well known that accelerated angiopathy is a major complication in diabetes mellitus. As generalized endothelial cell damage is thought to occur in diabetic patients, ET-1, being released from the damaged endothelial cells, is able to make contact with the underlying vascular smooth muscle cells and thus could be one important cause of diabetic angiopathy. This article summarizes the reported literature of the role of ET-1 in the development of diabetic complications, with particular focus on the possible role of ET-1 in mediating the effects of angiotensin-converting enzyme inhibitors.

Pre-enrichment of modified low density lipoproteins with alpha-tocopherol mitigates adverse effects on cultured retinal capillary cells

PURPOSE

We determined whether pre-enrichment of low density lipoproteins (LDL) with alpha-tocopherol mitigates their adverse effects, following in vitro glycation, oxidation or glycoxidation, towards cultured bovine retinal capillary endothelial cells (RCEC) and pericytes.

METHODS

LDL, while still in plasma obtained and pooled from non-diabetic humans, was supplemented in vitro with alpha-tocopherol. It was then isolated and modified in vitro by glycation, minimal oxidation, and glycoxidation. Bovine RCEC and pericytes were exposed to LDL (100mg protein/ ml) for three days. Cell count was determined by cell counting, supernatant levels of plasminogen activator inhibitor-1 (PAI-1) and endothelin-1 (ET-1) by ELISA, and nitrite levels by spectroscopic colorimetric assay.

RESULTS

While pre-enrichment of LDL with alpha-tocopherol did not reduce the measured extent of lipoprotein modification, it abolished the reduction in cell count observed with glycated, oxidized and glycoxidized LDL v. normal LDL. Pre-enrichment of LDL with alpha-tocopherol also reduced RCEC supernatant PAI-1 and ET-1 (corrected for cell counts) and increased RCEC and pericyte-associated supernatant nitrite levels: such effects of alpha-tocopherol may inhibit clot formation and favor vasodilatation.

CONCLUSIONS

Enrichment of LDL with alpha-tocopherol abolishes adverse effects of glycated, mildly oxidized, and glycoxidized LDL on cultured retinal cell count, and mitigates adverse effects on modulators of fibrinolysis and vascular tone. Direct evidence is required before Vitamin E supplementation is recommended for people with diabetes.

Increased vascular endothelial growth factor (VEGF) and transforming growth factor beta (TGFbeta) in experimental autoimmune uveoretinitis: upregulation of VEGF without neovascularization

Experimental autoimmune uveoretinitis (EAU) was induced in Lewis rats and B10.A mice by immunization with S-antigen (S-Ag) to study the potential roles of vascular endothelial growth factor (VEGF) and the beta1 and beta2 isoforms of transforming growth factor (TGFbeta1 and TGFbeta2) during the progression of the disease. VEGF has been implicated as an angiogenic factor in ischemic retinopathies; however, Lewis rats developing EAU have high levels of VEGF in the retina, but no neovascularization. In the present study, immunohistochemical staining for VEGF, TGFbeta1 and TGFbeta2 was performed on the retinas of Lewis rats developing EAU or with oxygen-induced ischemic retinopathy. In rats immunized with S-antigen, a marked upregulation of VEGF was immunohistochemically visualized from the inner nuclear layer to the inner limiting membrane prior to blood-retinal barrier (BRB) failure and lymphocytic infiltration. VEGF is normally induced by hypoxia and its induction leads to neovascularization. Coincident with the increase in VEGF, there was increased immunoreactivity for TGFbeta1 and TGFbeta2 within the same layers of the retina. In contrast, rats with ischemic retinopathy and retinal neovascularization showed only a modest increase in VEGF immunoreactivity, which is largely confined to retinal ganglion cells and inner retinal vessels, and little or no increase in TGFbeta1 or TGFbeta2. In addition, in mice developing EAU, which does not have an abrupt onset as it does in rats and may involve neovascularization, a comparable upregulation of VEGF in the inner retina to that seen in rats developing EAU occurs with no increase in TGFbeta1 or TGFbeta2. Since TGFbeta can inhibit endothelial cell proliferation, it is likely that an increase in TGFbeta may prevent VEGF from exerting its endothelial growth activity in the rat EAU model, but VEGF may be operative in inducing BRB failure. These data suggest that there is a complex interaction among growth factors in the retina and that retinal neovascularization may require an imbalance between stimulatory and inhibitory factors.

Inflammatory reaction via arachidonic acid cascade after intravitreal injection of endothelin-1

PURPOSE

To investigate the characteristics of anterior chamber inflammatory reaction induced by intravitreal injection of endothelin-1 (ET-1).

METHODS

The time course of changes in aqueous protein concentration (APC) after intravitreal injection of 10(-4), 10(-5), 10(-6) and 10(-7) M ET-1 into rabbit eyes was measured with a laser flare-cell meter. The influence of a topical diclofenac sodium (DFNa) pre- and post-treatment was assessed. Aqueous prostaglandin E2 and leukotriene B4 concentration was quantified using a radioimmunoassay technique.

RESULTS

Intravitreal injection of 10(-4) and 10(-5) M ET-1 significantly increased APC, while 10(-6) and 10(-7) M ET-1 did not induce anterior chamber inflammation. After 10(-5) M ET-1 injection, APC reached a maximum at 4 h post-treatment and returned to a normal level 48 h after injection. Eyes treated with 10(-4) M ET-1 displayed a bi-phasic time course, with peak values observed 4 to 8 h as well as 48 h after administration. Pre- and post-treatment with topical DFNa completely suppressed the APC increase in the 10(-5) M ET-1 preparation, and considerably inhibited it in the 10(-4) M ET-1 preparation. After ET-1 injection, aqueous prostaglandin E2 concentration increased significantly, followed by an increase in APC. There were no changes in leukotriene B4 concentration.

CONCLUSIONS

ET-1 induces anterior chamber inflammation via the cyclooxygenase pathway of the arachidonic acid cascade. The lipoxygenase pathway is not involved in this reaction.

Endothelin and endothelin antagonism: roles in cardiovascular health and disease

Endothelin is the most potent mammalian vasoconstrictor yet discovered. Its three isoforms play leading roles in regulating vascular tone and causing mitogenesis. The isoforms bind to two major receptor subtypes (ETA and ETB), which mediate a wide variety of physiologic actions in several organ systems. Endothelin may also be a disease marker or an etiologic factor in ischemic heart disease, atherosclerosis, congestive heart failure, renal failure, myocardial and vascular wall hypertrophy, systemic hypertension, pulmonary hypertension, and subarachnoid hemorrhage. Specific and nonspecific receptor antagonists and ECE inhibitors that have been developed interfere with endothelin's function. Many available cardiovascular therapeutic agents, such as angiotensin-converting-enzyme inhibitors, calcium-entry blocking drugs, and nitroglycerin, also may interfere with endothelin release or may modify its activity. The endothelin antagonists have great potential as agents for use in the treatment of a wide spectrum of disease entities and as biologic probes for understanding the actions of endothelin in human beings.

Intraocular pressure response to intravitreal injection of endothelin-1 and the mediatory role of ETA receptor, ETB receptor, and cyclooxygenase products in rabbits

Endothelin-1 (ET-1) affects intraocular pressure (IOP) in rabbits. First, we studied IOP responses to the intravitreal injection of various doses of ET-1 ranging from 5 ng to 5 micrograms in unanesthetized rabbits, and observed a transient rise in IOP, from 0.5 to 2 h in duration, invariably followed by a prolonged IOP reduction, lasting for more than 72 h in rabbits treated with 0.5 microgram and 5 micrograms of ET-1. ET-1 (0.05 microgram and 0.15 microgram) resulted in a prolonged IOP reduction without an early IOP rise. Both IOP rise and reduction were significantly related to the dose of ET-1. A masked, randomized, study revealed that the intraperitoneal administration of indomethacin (50 mg/kg) prior to ET-1 injection significantly reduced the ocular hypertensive response, but not th ocular hypotensive response, to ET-1. The ETA receptor selective antagonist, 97-139 (155 micrograms) had no effect on IOP when used alone. However, when used in combination with 0.5 microgram of ET-1, 97-139 significantly inhibited both the IOP rise (0.5-2 h) and reduction (8-96 h) caused by ET-1. The ETB receptor selective agonist, sarafotoxin S6c (0.5 microgram), caused a sustained IOP reduction of 2 to 96 h in duration without the initial IOP rise. We also determined the concentration of prostaglandin (PG) E2 in the aqueous humor using radioimmunoassay techniques on samples obtained at 1 and 24 h after ET-1 injection, and examined the effects of pretreatment with indomethacin or 97-139 on PGE2 concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Sarafotoxins and endothelins: evolution, structure and function

The venom of the burrowing asp Atractaspis engaddensis contains several 21 amino acid residue peptides known as sarafotoxins. The sarafotoxins are homologous to the mammalian endothelin family, and they have similar biological activities. This review covers recent advances in the study of the chemical and biological properties of the sarafotoxins and endothelins.