Retinal Renin-angiotensin System Research Articles

Systemic ß adrenergic stimulation/ sympathetic nerve system stimulation influences intraocular RAS through cAMP in the RPE

Several lines of evidence support the existence of a renin-angiotensin system (RAS) in the retina that is separated from the blood stream by the retinal pigment epithelium (RPE). Under physiological conditions, increased activity of intraretinal RAS regulates neuronal activity of the retina but patho-physiologically participates in retinal degeneration such as hypertensive or diabetic retinopathy. Interestingly, the RPE appears to be a modulator of intraretinal RAS in response to changes in systemic RAS. As increased systemic RAS activity is associated with increased sympathetic tonus, we investigated whether systemic β-adrenergic stimulation of the RPE also modulates renin expression in the RPE. In vivo, the mouse RPE expresses the β-adrenergic receptor subtypes 1 and 2. Staining of retina sagittal sections showed tyrosine hydroxylase positive nerve endings in the choroid indicating adrenaline/noradrenaline production sites in close proximity to the RPE. Systemic infusion of isoproterenol increased renin expression in the RPE but not in the retina. This increase was sensitive to concomitant systemic application of the angiotensin-2 receptor-type-1 blocker losartan. In vitro analysis of renin gene expression using polarized porcine RPE showed that the activity of the renin promoter can be increased by cAMP stimulation (IBMX/forskolin) but was not influenced by angiotensin-2. Thus, with the identification of the β-adrenergic system we added a new regulator of the retinal RAS with relevance for retinal function and pathology. Furthermore, it appears that the RPE is not only a close interaction partner of the photoreceptors but also a regulator or retinal activity in general.

Renal iron accelerates the progression of diabetic nephropathy in the HFE gene knockout mouse model of iron overload.

Diabetic nephropathy (DN) is the most common cause of end-stage renal disease associated with high mortality worldwide. Increases in iron levels have been reported in diabetic rat kidneys as well as in human urine of patients with diabetes. In addition, a low-iron diet or iron chelators delay the progression of DN in patients with diabetes and in animal models of diabetes. Possible maladaptive mechanisms of organ damage by tissue iron accumulation have not been well studied. We recently reported that iron induced the retinal renin-angiotensin system (RAS) and accelerated the progression of diabetic retinopathy. However, whether iron regulates the systemic RAS is unknown. To explore if iron alters the expression of intrarenal RAS and its role in the progression of DN, we used the high Fe iron (HFE) knockout mouse, a genetic model of systemic iron overload. We found that diabetes upregulated the expression of iron regulatory proteins and augmented tissue iron accumulation in the kidneys of both type 1 and type 2 diabetic mouse models. Iron accumulation in the kidneys of HFE knockout mice was associated with increase in serum and intrarenal renin expression. Induction of diabetes in HFE knockout mice using streptozotocin caused a much higher accumulation of renal iron and accelerated the progression of nephropathy compared with diabetic wild-type mice. Treatment of diabetic mice with the iron chelator deferiprone reversed the renin upregulation and reduced kidney injury. Thus, our results establish a new link between renal iron and RAS activity. Exploring the mechanisms of iron-induced RAS activation further may have a significant therapeutic impact on hypertension and DN.

(Pro)renin receptor: Involvement in diabetic retinopathy and development of molecular targeted therapy.

The renin–angiotensin system (RAS), a crucial regulator of systemic blood pressure (circulatory RAS), plays distinct roles in pathological angiogenesis and inflammation in various organs (tissue RAS), such as diabetic microvascular complications. Using ocular clinical samples and animal disease models, we elucidated molecular mechanisms in which tissue RAS excites the expression of vascular endothelial growth factor (VEGF)‐A responsible for retinal inflammation and angiogenesis, the two major pathological events in diabetic retinopathy (DR). Furthermore, we showed the involvement of (pro)renin receptor [(P)RR] in retinal RAS activation and its concurrent intracellular signal transduction (e.g., extracellular signal‐regulated kinase); namely, the (P)RR‐induced dual pathogenic bioactivity referred to as the receptor‐associated prorenin system. Indeed, neovascular endothelial cells in the fibrovascular tissue collected from eyes with proliferative DR were immunoreactive for the receptor‐associated prorenin system components including prorenin, (P)RR, phosphorylated extracellular signal‐regulated kinase and VEGF‐A. Protein levels of soluble (P)RR increased with its positive correlations with prorenin, renin enzymatic activity and VEGF in the vitreous of proliferative DR eyes, suggesting a close link between (P)RR and VEGF‐A‐driven angiogenic activity. Furthermore, we revealed an unsuspected, PAPS‐independent role of (P)RR in glucose‐induced oxidative stress. Recently, we developed an innovative single‐strand ribonucleic acid interference molecule selectively targeting human and mouse (P)RR, and confirmed its efficacy in suppressing diabetes‐induced retinal inflammation in mice. Our data using clinical samples and animal models suggested the significant implication of (P)RR in the pathogenesis of DR, and the potential usefulness of the ribonucleic acid interference molecule as a therapeutic agent to attenuate ocular inflammation and angiogenesis.

Iron Overload Accelerates the Progression of Diabetic Retinopathy in Association with Increased Retinal Renin Expression

Diabetic retinopathy (DR) is a leading cause of blindness among working-age adults. Increased iron accumulation is associated with several degenerative diseases. However, there are no reports on the status of retinal iron or its implications in the pathogenesis of DR. In the present study, we found that retinas of type-1 and type-2 mouse models of diabetes have increased iron accumulation compared to non-diabetic retinas. We found similar iron accumulation in postmortem retinal samples from human diabetic patients. Further, we induced diabetes in HFE knockout (KO) mice model of genetic iron overload to understand the role of iron in the pathogenesis of DR. We found increased neuronal cell death, vascular alterations and loss of retinal barrier integrity in diabetic HFE KO mice compared to diabetic wildtype mice. Diabetic HFE KO mouse retinas also exhibited increased expression of inflammation and oxidative stress markers. Severity in the pathogenesis of DR in HFE KO mice was accompanied by increase in retinal renin expression mediated by G-protein-coupled succinate receptor GPR91. In light of previous reports implicating retinal renin-angiotensin system in DR pathogenesis, our results reveal a novel relationship between diabetes, iron and renin-angiotensin system, thereby unraveling new therapeutic targets for the treatment of DR.

Involvement of Renin-Angiotensin System in Retinopathy of Prematurity - A Possible Target for Therapeutic Intervention.

ObjectiveExamining the Retinal Renin Angiotensin System (RRAS) in the ROP neonates and analyzing the possibility of modulating the RRAS to prevent the progression in Oxygen Induced Retinopathy (OIR) model.MethodVitreous of ROP patients (n = 44, median age 5.5 months) was quantified for RRAS components, VEGF, HIF-1α and compared with age matched control. The involvement of RRAS in ROP was tested in the rat model of OIR and compared with normoxia. Expressions of RAS components, VEGF and HIF-1α in retina were analyzed using qPCR and retinal structure and function was also analyzed. Effect of Angiotensin Converting Enzyme Inhibitor (ACEI) and Angiotensin Receptor Blocker (ARB) was evaluated and compared with Bevacizumab which served as a positive control. Drug penetration into retina was confirmed by liquid chromatography coupled ESI-tandem mass spectroscopy (LC-MS/MS).ResultsMultifold increase in the expression of RAS components in human vitreous and rat retina showed their involvement in ROP. ERG & fundus studies in OIR revealed the altered function of retina and were successfully prevented by ARB (telmisartan), ACEI (lisinopril) and bevacizumab. Retinal analysis revealed the presence of ACEI and ARB in their therapeutic levels.ConclusionThis study for the first time demonstrates the upregulated level of RAS components in human ROP vitreous and further that the pharmacological intervention in RRAS can functionally and structurally preserve retina against the progression of ROP in the OIR model.

Aliskiren decreases oxidative stress and angiogenic markers in retinal pigment epithelium cells.

There is growing evidence on the role of ocular renin-angiotensin system (RAS) in the development of diabetic retinopathy (DR), particularly due to the trigger of oxidative stress and angiogenesis. Despite this there is no effective RAS-based therapy in DR capable of preventing retinal damage induced by RAS activation. We recently described that retinal pigment epithelium (RPE) cells express the main components of the RAS. We here propose to investigate the role of glucose upon the retinal RAS and whether aliskiren, a direct renin inhibitor, protects RPE cells from angiogenesis and oxidative stress. RPE cells were chosen as target since one of the first events in DR is the dysfunction of the RPE retinal layer, which as a key function in maintaining the integrity of the retina. We found that the RAS present in the RPE cells was deregulated by hyperglycemic glucose concentrations. Exposure of RPE cells to angiotensin II increased the levels of the main pro-angiogenic factor, vascular endothelial growth factor (VEGF) in a concentration-dependent manner. Additionally, angiotensin II also stimulated the production of reactive oxygen species in RPE cells. Treatment of RPE cells with aliskiren decreased the levels of oxidative stress and promoted the expression of anti-angiogenic factors such as the pigment epithelium-derived factor and the VEGF165b isoform. Our findings demonstrate that the RAS is deregulated in hyperglycemic conditions and that aliskiren successfully protected RPE cells from RAS over activation. These anti-angiogenic and antioxidant properties described for aliskiren over RPE cells suggest that this drug has potential to be used in the treatment of diabetic retinopathy.

Receptor-associated prorenin system contributes to development of inflammation and angiogenesis in proliferative diabetic retinopathy.

The renin-angiotensin system (RAS) plays a potential role in the development of end-organ damage, and tissue RAS activation has been suggested as a risk factor of several diseases including diabetes. So far, using animal disease models, we have shown molecular mechanisms, in which tissue RAS stimulates retinal angiogenesis, and the critical roles of (pro)renin receptor [(P)RR] in retinal RAS activation and its concurrent intracellular signal transduction, referred to as the receptor-associated prorenin system (RAPS). Moreover, we recently reported that the protein levels of prorenin and soluble (P)RR increased in the vitreous fluids obtained from patients with proliferative diabetic retinopathy (PDR), suggesting the association of (P)RR with vascular endothelial growth factor (VEGF)-driven angiogenic activity in human PDR, and also showed a close relationship between the vitreous renin activity and VEGF-induced pathogenesis of diabetic retinopathy. Our data using animal disease models and human clinical samples suggest that both vitreous RAS and retinal RAPS play critical roles in the molecular pathogenesis of diabetic retinopathy.

Angiotensin-converting enzyme gene variants interact with the renin-angiotensin system pathway to confer risk and protection against type 2 diabetic retinopathy.

The angiotensin‐converting enzyme (ACE) gene has been found to be associated with pathogenesis and progression of diabetic retinopathy (DR). A recent meta‐analysis comprising 2,224 Chinese patients showed moderate evidence of a relationship between the ACE insertion/deletion (I/D) polymorphism and proliferative diabetic retinopathy (PDR)1; however, the role of other renin–angiotensin system (RAS) polymorphisms and their possible interactions with ACE I/D genotypes are less clearly defined. Thus, we investigated the association and interaction between RAS gene polymorphisms and the development and progression of DR in 1,446 north Indian type 2 diabetic patients. DR patients were divided into two groups, non‐proliferative diabetic retinopathy (NPDR) and PDR. DR was diagnosed by dilating the pupils with mydriatics and then carefully examining the retina. Retinal photography or fluorescein angiography tests were also carried out. A total of 1,720 type 2 diabetic patients were screened for the presence of DR, of which, 1,446 patients were finally included for the present study. Patients receiving antihypertensive drug treatment, with any retinopathy other than DR, cataracts and cataract surgery, and age‐related macular degeneration were excluded from the study. All the participants in both the cohorts were age (≥35 years), sex and ethnicity matched. Significant deviation from the Hardy–Weinberg equilibrium of genotype distribution in the present population in ACE I/D and angiotensinogen (AGT) variant rs5050 might be a result of the moderate population size or the rare allele, which can cause a random change in allele frequencies. Furthermore, we excluded the possibility of a typing error (logarithmic odds ratio score >0). The present sample size had the power of 90% at a small effect size (0.1) and alpha level (0.05). In both the cohorts, we observed a consistently higher prevalence and increased risk of PDR in patients with the DD genotype of ACE I/D, TT genotype of AGTrs699 and the CC genotype of angiotensin II receptor type 1 A1166C (Table 1). Table 1 Genotype frequencies of the renin–angiotensin system variants in type 2 diabetes patients (diabetes mellitus vs non‐proliferative diabetic retinopathy vs proliferative diabetic retinopathy) The biological mechanism through which the ACE I/D polymorphism might be related to an increased risk of PDR is unclear. Serum ACE concentrations are significantly higher in those carrying the DD genotype2. Also, the final component of RAS, which is converted by ACE, has a vasoconstrictive effect, promotes the accumulation of extracellular matrix and induces plasminogen activator inhibitor‐1 production in endothelial cells3. Furthermore, DR is associated with impaired balance of retinal RAS. Increased expression of ACE/AGT overcomes this imbalance and confers protection against DR4. The present results suggest RAS polymorphism as a significant risk factor for DR in Asian Indians.

ACE2 and Ang-(1-7) Confer Protection Against Development of Diabetic Retinopathy

Despite evidence that hyperactivity of the vasodeleterious axis (ACE/angiotensin II (Ang II)/AT1 receptor) of the renin-angiotensin system (RAS) is associated with the pathogenesis of diabetic retinopathy (DR) use of the inhibitors of this axis has met with limited success in the control of this pathophysiology. We investigated the hypothesis that enhancing the local activity of the recently established protective axis of the RAS, ACE2/Ang-(1-7), using adeno-associated virus (AAV)-mediated gene delivery of ACE2 or Ang-(1-7) would confer protection against diabetes-induced retinopathy. Genes expressing ACE2 and Ang-(1-7) were cloned in AAV vector. The effects of ocular AAV-ACE2/Ang-(1-7) gene transfer on DR in diabetic eNOS(-/-) mice and Sprague-Dawley (SD) rats were examined. Diabetes was associated with approximately tenfold and greater than threefold increases in the ratios of ACE/ACE2 and AT1R/Mas mRNA levels in the retina respectively. Intraocular administration of AAV-ACE2/Ang-(1-7) resulted in significant reduction in diabetes-induced retinal vascular leakage, acellular capillaries, infiltrating inflammatory cells and oxidative damage in both diabetic mice and rats. Our results demonstrate that DR is associated with impaired balance of retinal RAS. Increased expression of ACE2/Ang-(1-7) overcomes this imbalance and confers protection against DR. Thus, strategies enhancing the protective ACE2/Ang-(1-7) axis of RAS in the eye could serve as a novel therapeutic target for DR.

DIRECT study: a commentary

This commentary reviews the recent reports of the DIRECT study. 2 This study has explored the role of the Ang II receptor blocker, candesartan as a prevention or a treatment strategy in type 1 and type 2 diabetes subjects with, or at risk of, diabetic retinopathy. The findings from these trials extend our understanding of the role of medical treatments in diabetic retinopathy. This condition is currently managed when the retinal lesion is already advanced with essentially destructive, albeit useful, treatments such as laser photocoagulation. Diabetic retinopathy remains the major cause of blindness in the working age population although the prevalence of vision-threatening retinopathy appears to be decreasing and in general progression rates have been on the decline. The underlying explanation for the changes in the ‘natural history’ is not clear but may be as a result of improvement management of key risk factors such as glycaemic control and blood pressure, based on the results of studies such as the DCCT and UKPDS. 4 Indeed, as recently confirmed in the follow-up of the Steno-2 trial, multi-factorial intervention including blood pressure, glucose and lipid control does reduce the development and progression of retinopathy. The rationale for the DIRECT study was partly based on a range of experimental studies that had first identified a retinal renin-angiotensin system and, second, demonstrated, using both ACE inhibitors and Ang II antagonists including candesartan, functional and structural effects on various retinal disorders including diabetic retinopathy as well as a model of retinal neovascularisation, retinopathy of prematurity. 7 Furthermore, with increasing evidence that certain growth factors implicated in enhanced retinal permeability to proteins and angiogenesis such as VEGF are Ang II dependent, it was considered that interruption of the reninangiotensin system would have direct effects on pathological processes implicated in diabetic retinopathy. The DIRECT study was essentially three separate placebo controlled trials, recruiting a total of over 5,000 subjects with type 1 and type 2 diabetes. It included an arm known as Prevent 1, which studied the role of candesartan in preventing or delaying the onset of retinopathy in normotensive type 1 diabetes subjects. Other arms included Protect 1 and Protect 2, which examined the role of the Ang II antagonist in retarding or reversing progression of retinopathy in type 1 and type 2 diabetes subjects, respectively, with evidence of early retinopathy. 2 It should be noted that only in a subgroup of subjects in the Protect 2 arm who had concomitant hypertension could additional antihypertensive drugs be added. This multi-national study with over 300 sites worldwide, involving a follow-up of subjects for four years, demonstrated some benefits of candesartan treatment in the various studies. However, some effects observed were borderline and of debatable clinical significance. With respect to Prevent 1, there was an 18% reduction in two-step progression of retinopathy of borderline statistical significance. However, although only a post-hoc analysis, there was a 35% reduction in three-step progression of retinopathy, a parameter that has been previously considered the ‘gold standard’ of diabetic retinopathy studies. The findings from Protect 1 were not particularly impressive with only modest benefits of doubtful clinical significance observed. However, in Protect 2, a 34% increase in regression of retinopathy was observed. With this complex series of findings it is difficult to see how these results would translate to clinical practice. Without achieving the pre-specified primary endpoint, it may be difficult, based on this trial, for candesartan to be registered by major regulatory agencies as a retino-protective agent. Nevertheless, with increasing use of agents which interrupt the renin-angiotensin system in diabetic individuals for other indications such as hypertension and nephropathy, this study provides the clinician with a lower threshold for commencing such agents in the diabetic population. Furthermore, the positive findings from the DIRECT study build on previous, much smaller, less well designed studies such as the EUCLID study, which demonstrated clear effects on retinopathy progression with the ACE inhibitor lisinopril. The issue of blocking the renin-angiotensin system in type 1 diabetic subjects has been further explored in the RASS study which reported in a smaller number of subjects a reduction in the development and progression of diabetic retinopathy with either the ACE inhibitor, enalapril or the Ang II antagonist, losartan. This additional study, although performed in a relatively small cohort of type 1 diabetic subjects builds on the underlying postulate that the renin-angiotensin system plays a key role in the diabetic retina and that retinal protection may be afforded by various different approaches to interrupt the renin-angiotensin system.

Diabetes and retinal vascular disorders: role of the renin-angiotensin system.

Angiotensin II, the effector peptide of the renin-angiotensin system (RAS), has potent growth factor properties in a variety of organs. In the retina, a complete RAS exists, with components residing in the vasculature, neurons and glia. There is increasing interest in a pathogenetic role for angiotensin II in ischaemic retinopathies such as diabetic retinopathy and retinopathy of prematurity. In these situations, the retinal RAS becomes activated and stimulates growth factors such as vascular endothelial growth factor, which contribute to vascular leakage, pericyte migration, angiogenesis and fibrosis. Blockade of the RAS, with either angiotensin-converting enzyme (ACE) inhibitors or antagonists selective for angiotensin type 1 (AT1) and angiotensin type 2 (AT2) receptors, attenuates many of the vascular abnormalities that develop in diabetic retinopathy and retinopathy of prematurity. Eagerly awaited are the findings of the Diabetic Retinopathy Candesartan Trial (DIRECT), evaluating the effects of AT1 receptor antagonism in patients with different stages of diabetic retinopathy. This review examines the role of the RAS in diabetic retinopathy and retinopathy of prematurity, and the potential of RAS blockade as a treatment strategy for these vision-threatening diseases.

The retinal renin-angiotensin system: implications for therapy in diabetic retinopathy.

Retinopathy is the most common complication of diabetes, and a leading cause of blindness in people of working age. Optimal blood pressure and metabolic control can reduce the risk of diabetic retinopathy, but are difficult to achieve in clinical practice. In the EUCLID Study, the angiotensin converting enzyme (ACE) inhibitor lisinopril reduced the risk of progression of retinopathy by approximately 50%, and also significantly reduced the risk of progression to proliferative retinopathy. These findings are consistent with extensive evidence that the renin-angiotensin system is expressed in the eye, and that adverse effects of angiotensin II on retinal angiogenesis and function can be inhibited by ACE inhibitors or angiotensin II-receptor blockers. However, in the EUCLID Study retinopathy was not a primary end-point and the study was not sufficiently powered for the eye-related outcomes. Hence, the Diabetic Retinopathy Candesartan Trials (DIRECT) programme has been established to determine whether AT(1)-receptor blockade with candesartan can prevent the incidence and progression of diabetic retinopathy. This programme comprises three studies, involving a total of 4500 patients recruited from about 300 centres worldwide. The patients are normotensive or treated hypertensive individuals, and so the DIRECT programme should assess the potential of an AT(1)-receptor blocker to protect against the pathological changes in the eye following diabetes.