Genevieve Tan

Identification of a Retinal Aldosterone System and the Protective Effects of Mineralocorticoid Receptor Antagonism on Retinal Vascular Pathology

Blockade of the renin–angiotensin–aldosterone system (RAAS) is being evaluated as a treatment for diabetic retinopathy; however, whether the mineralocorticoid receptor (MR) and aldosterone influence retinal vascular pathology is unknown. We examined the effect of MR antagonism on pathological angiogenesis in rats with oxygen-induced retinopathy (OIR). To determine the mechanisms by which the MR and aldosterone may influence retinal angiogenesis; inflammation and glucose-6-phosphate dehydrogenase (G6PD) were evaluated in OIR and cultured bovine retinal endothelial cells (BRECs) and bovine retinal pericytes (BRPs). In OIR, MR antagonism (spironolactone) was antiangiogenic. Aldosterone may mediate the pathogenic actions of MR in the retina, with 11&bgr;-hydroxysteroid dehydrogenase type 2 mRNA being detected and with aldosterone stimulating proliferation and tubulogenesis in BRECs and exacerbating angiogenesis in OIR, which was attenuated with spironolactone. The MR and aldosterone modulated retinal inflammation, with leukostasis and monocyte chemoattractant protein-1 mRNA and protein in OIR being reduced by spironolactone and increased by aldosterone. A reduction in G6PD may be an early response to aldosterone. In BRECs, BRPs, and early OIR, aldosterone reduced G6PD mRNA, and in late OIR, aldosterone increased mRNA for the NAD(P)H oxidase subunit Nox4. A functional retinal MR–aldosterone system was evident with MR expression, translocation of nuclear MR, and aldosterone synthase expression, which was modulated by RAAS blockade. We make the first report that MR and aldosterone influence retinal vasculopathy, which may involve inflammatory and G6PD mechanisms. MR antagonism may be relevant when developing treatments for retinopathies that target the RAAS.

Candesartan attenuates diabetic retinal vascular pathology by restoring glyoxalase-I function.

OBJECTIVE Advanced glycation end products (AGEs) and the renin-angiotensin system (RAS) are both implicated in the development of diabetic retinopathy. How these pathways interact to promote retinal vasculopathy is not fully understood. Glyoxalase-I (GLO-I) is an enzyme critical for the detoxification of AGEs and retinal vascular cell survival. We hypothesized that, in retina, angiotensin II (Ang II) downregulates GLO-I, which leads to an increase in methylglyoxal-AGE formation. The angiotensin type 1 receptor blocker, candesartan, rectifies this imbalance and protects against retinal vasculopathy. RESEARCH DESIGN AND METHODS Cultured bovine retinal endothelial cells (BREC) and bovine retinal pericytes (BRP) were incubated with Ang II (100 nmol/l) or Ang II+candesartan (1 μmol/l). Transgenic Ren-2 rats that overexpress the RAS were randomized to be nondiabetic, diabetic, or diabetic+candesartan (5 mg/kg/day) and studied over 20 weeks. Comparisons were made with diabetic Sprague-Dawley rats. RESULTS In BREC and BRP, Ang II induced apoptosis and reduced GLO-I activity and mRNA, with a concomitant increase in nitric oxide (NO•), the latter being a known negative regulator of GLO-I in BRP. In BREC and BRP, candesartan restored GLO-I and reduced NO•. Similar events occurred in vivo, with the elevated RAS of the diabetic Ren-2 rat, but not the diabetic Sprague-Dawley rat, reducing retinal GLO-I. In diabetic Ren-2 rats, candesartan reduced retinal acellular capillaries, inflammation, and inducible nitric oxide synthase and NO•, and restored GLO-I. CONCLUSIONS We have identified a novel mechanism by which candesartan improves diabetic retinopathy through the restoration of GLO-I.

RILLKKMPSV Influences the Vasculature, Neurons and Glia, and (Pro)Renin Receptor Expression in the Retina

The (pro)renin receptor [(P)RR] is implicated in organ pathology. We examined the cellular location of the (P)RR and whether a putative (P)RR antagonist, RILLKKMPSV, corresponding to the handle region of the prorenin prosegment (handle region peptide [HRP]) influences angiogenesis, inflammation, and neuronal and glial function in rat retina. The (P)RR was localized to retinal vessels, endothelial cells, and pericytes, but most immunolabeling was in ganglion cells and glia. HRP (1 mg/kg per day by IP injection) reduced physiological angiogenesis in developing retina. Moreover, HRP (0.1 mg/kg per day by subcutaneous minipump) reduced pathological retinal angiogenesis, inflammation, and vascular endothelial growth factor and intercellular adhesion molecule-1 mRNA in rats with oxygen-induced retinopathy (OIR) to an extent similar to valsartan (10 mg/kg per day, IP). In contrast to its effects on vasculature, HRP compromised the electroretinogram in shams and OIR and increased phosphorylated extracellular-signal–related protein kinase 1/2 immunolabeling in shams but not in OIR, whereas valsartan did not affect the electroretinogram and reduced extracellular-signal–related protein kinase 1/2 immunolabeling in OIR. Retinal (P)RR mRNA levels were increased in OIR; HRP, but not valsartan, increased (P)RR mRNA levels in shams, whereas both HRP and valsartan reduced (P)RR mRNA levels in OIR. A control peptide (VSPMKKLLIR, 0.1 mg/kg per day) did not influence retinal vasculopathy or function. Circulating HRP levels in rats administered 1 mg/kg per day HRP were undetectable (<3 pmol/L). We conclude that HRP had protective effects on the retinal vasculature similar to those of valsartan; however, unlike valsartan, HRP injured neuro-glia, which may involve the (P)RR, although the undetectable circulating HRP level makes a direct effect of HRP on retinal (P)RR function unlikely.

Neovascularization Is Attenuated With Aldosterone Synthase Inhibition in Rats With Retinopathy

Neovascularization is a hallmark feature of retinopathy of prematurity and diabetic retinopathy. Type 1 angiotensin receptor blockade reduces neovascularization in experimental retinopathy of prematurity, known as oxygen-induced retinopathy (OIR). We investigated in OIR whether inhibiting aldosterone with the aldosterone synthase inhibitor FAD286 reduced neovascularization as effectively as angiotensin receptor blockade (valsartan). OIR was induced in neonatal Sprague-Dawley rats, and they were treated with FAD286 (30 mg/kg per day), valsartan (10 mg/kg per day), or FAD286+valsartan. The cellular sources of aldosterone synthase, the mineralocorticoid receptor, and 11&bgr;-hydroxysteroid dehydrogenase 2 were evaluated in retinal cells involved in neovascularization (primary endothelial cells, pericytes, microglia, ganglion cells, and glia). In OIR, FAD286 reduced neovascularization and neovascular tufts by 89% and 67%, respectively, and normalized the increase in vascular endothelial growth factor mRNA (1.74-fold) and protein (4.74-fold) and was as effective as valsartan and FAD286+valsartan. In retina, aldosterone synthase mRNA was reduced with FAD286 but not valsartan. Aldosterone synthase was detected in microglia, ganglion cells, and glia, whereas mineralocorticoid receptor and 11&bgr;-hydroxysteroid dehydrogenase 2 were present in all of the cell types studied. Given the location of aldosterone synthase in microglia and their contribution to retinal inflammation and neovascularization in OIR, the effects of FAD286 on microglial density were studied. The increase in microglial density (ionized calcium binding adaptor protein 1 immunolabeling) in OIR was reduced with all of the treatments. In OIR, FAD286 reduced the increase in mRNA for tumor necrosis factor-&agr;, intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and monocyte chemoattractant molecule 1. These findings indicate that aldosterone inhibition may be a potential treatment for retinal neovascularization.

Lack of the antioxidant glutathione peroxidase-1 (GPx1) exacerbates retinopathy of prematurity in mice

PURPOSE Glutathione peroxidase-1 (GPx1) is highly expressed during normal retinal maturation; however, its role in retinopathy of prematurity (ROP) is not fully understood. We postulated that GPx1 plays an important role in protecting the premature retina from oxidative injury in a mouse model of ROP. METHODS ROP was induced in wild-type (WT) and GPx1 knockout (KO) mice by exposing neonatal mice to 75% oxygen from postnatal days 7 to 11, followed by 1 week of room air. Structural effects of ROP were evaluated by retinal histology, and gene expression of retinal pro-angiogenic factors was measured by qRT-PCR. RESULTS Retinas from ROP GPx1 KO mice had a significantly larger central avascular area compared to those from ROP WT mice (P < 0.001), indicative of a more severe vaso-obliteration. In ROP GPx1 KO mice, retinas also displayed increased preretinal neovascularization (P = 0.05) with a concurrent increase in the expression of vascular endothelial growth factor (P < 0.05) compared to values in ROP WT mice. Elevated oxidative stress was observed in ROP GPx1 KO retinas as evidenced by increased nitrotyrosine immunolabeling (P < 0.01) and superoxide (P < 0.05) in vessels compared to ROP WT retinas. In contrast to these findings of exacerbated retinal vascular injury in GPx1 KO mice, Müller cell gliosis and microglial density were similar in ROP GPx1 KO and ROP WT mice. CONCLUSIONS GPx1, an important antioxidant enzyme of the premature retina, afforded protection against oxidative stress and oxidative injury in ROP. Lack of GPx1 was associated with increased oxidative stress, an increase in retinal avascular area, upregulation of retinal VEGF, and increased neovascularization in a mouse model of ROP.