Aliskiren inhibits intracellular angiotensin II levels without affecting (pro)renin receptor signals in human podocytes

Silencing of Central (Pro)renin Receptor Ameliorates Salt-Induced Renal Injury in Chronic Kidney Disease

Aims: A high-salt diet can aggravate oxidative stress, and renal fibrosis via the brain and renal renin-angiotensin system (RAS) axis in chronic kidney disease (CKD) rats. (Pro)renin receptor (PRR) plays a role in regulating RAS and oxidative stress locally. However, whether central PRR regulates salt-induced renal injury in CKD remains undefined. Here, we hypothesized that the reduction of central PRR expression could ameliorate central lesions and thereby ameliorate renal injury in high-salt-load CKD rats. Results: We investigated RAS, sympathetic nerve activity, oxidative stress, inflammation, and tissue injury in subfornical organs and kidneys in high-salt-load 5/6 nephrectomy CKD rats after the silencing of central PRR expression by intracerebroventricular lentivirus-RNAi. We found that the sympathetic nerve activity was reduced, and the levels of inflammation and oxidative stress were decreased in both brain and kidney. Renal injury and fibrosis were ameliorated. To explore the mechanism by which central inhibition of PRR expression ameliorates kidney damage, we blocked central MAPK/ERK1/2 and PI3K/Akt signaling pathways as well as angiotensin converting enzyme 1-angiotensin II-angiotensin type 1 receptors (ACE1-Ang II-AT1R) axis. Salt-induced overexpression of renal RAS, inflammation, oxidative stress, and fibrosis in CKD rats were prevented by central blockade of the pathways. Innovation: This study provides new insights into the mechanisms underlying salt-induced kidney damage. Targeting central PRR or PRR-mediated signaling pathway may be a novel strategy for the treatment of CKD. Conclusions: These results suggested that the silencing of central PRR expression ameliorates salt-induced renal injury in CKD through Ang II-dependent and -independent pathways.

(Pro)renin receptor promotes crescent formation via the ERK1/2 and Wnt/β-catenin pathways in glomerulonephritis

(Pro)renin receptor [(P)RR] has multiple functions, but its regulation and role in the pathogenesis in glomerulonephritis (GN) are poorly defined. The aims of the present study were to determine the effects of direct renin inhibition (DRI) and demonstrate the role of (P)RR on the progression of crescentic GN. The anti-glomerular basement membrane nephritis rat model developed progressive proteinuria (83.64 ± 10.49 mg/day) and glomerular crescent formation (percent glomerular crescent: 62.1 ± 2.3%) accompanied by increased macrophage infiltration and glomerular expression of monocyte chemoattractant protein (MCP)-1, (P)RR, phospho-extracellular signal-regulated kinase (ERK)1/2, Wnt4, and active β-catenin. Treatment with DRI ameliorated proteinuria (20.33 ± 5.88 mg/day) and markedly reduced glomerular crescent formation (20.9 ± 2.6%), induction of macrophage infiltration, (P)RR, phospho-ERK1/2, Wnt4, and active β-catenin. Furthermore, primary cultured parietal epithelial cells stimulated by recombinant prorenin showed significant increases in cell proliferation. Notably, while the ERK1/2 inhibitor PD98059 or (P)RR-specific siRNA treatment abolished the elevation in cell proliferation, DRI treatment did not abrogate this elevation. Moreover, cultured mesangial cells showed an increase in prorenin-induced MCP-1 expression. Interestingly, (P)RR or Wnt4-specific siRNA treatment or the β-catenin antagonist XAV939 inhibited the elevation of MCP-1 expression, whereas DRI did not. These results suggest that (P)RR regulates glomerular crescent formation via the ERK1/2 signaling and Wnt/β-catenin pathways during the course of anti-glomerular basement membrane nephritis and that DRI mitigates the progression of crescentic GN through the reduction of (P)RR expression but not inhibition of prorenin binding to (P)RR.

The (pro)renin receptor in health and disease

The (pro)renin receptor ((P)RR) was first identified as a single-transmembrane receptor in human kidneys and initially attracted attention owing to its potential role as a regulator of the tissue renin-angiotensin system (RAS). Subsequent studies found that the (P)RR is widely distributed in organs throughout the body, including the kidneys, heart, brain, eyes, placenta and the immune system, and has multifaceted functions in vivo. The (P)RR has roles in various physiological processes, such as the cell cycle, autophagy, acid-base balance, energy metabolism, embryonic development, T cell homeostasis, water balance, blood pressure regulation, cardiac remodelling and maintenance of podocyte structure. These roles of the (P)RR are mediated by its effects on important biological systems and pathways including the tissue RAS, vacuolar H⁺-ATPase, Wnt, partitioning defective homologue (Par) and tyrosine phosphorylation. In addition, the (P)RR has been reported to contribute to the pathogenesis of diseases such as fibrosis, hypertension, pre-eclampsia, diabetic microangiopathy, acute kidney injury, cardiovascular disease, cancer and obesity. Current evidence suggests that the (P)RR has key roles in the normal development and maintenance of vital organs and that dysfunction of the (P)RR is associated with diseases that are characterized by a disruption of the homeostasis of physiological functions.

Role of Podocyte Injury in Glomerulosclerosis

Finding new therapeutic targets of glomerulosclerosis treatment is an ongoing quest. Due to a living environment of various stresses and pathological stimuli, podocytes are prone to injuries; moreover, as a cell without proliferative potential, loss of podocytes is vital in the pathogenesis of glomerulosclerosis. Thus, sufficient understanding of factors and underlying mechanisms of podocyte injury facilitates the advancement of treating and prevention of glomerulosclerosis. The clinical symptom of podocyte injury is proteinuria, sometimes with loss of kidney functions progressing to glomerulosclerosis. Injury-induced changes in podocyte physiology and function are actually not a simple passive process, but a complex interaction of proteins that comprise the anatomical structure of podocytes at molecular levels. This chapter lists several aspects of podocyte injuries along with potential mechanisms, including glucose and lipid metabolism disorder, hypertension, RAS activation, micro-inflammation, immune disorder, and other factors. These aspects are not technically separated items, but intertwined with each other in the pathogenesis of podocyte injuries.

Prorenin independently causes hypertension and renal and cardiac fibrosis in cyp1a1-prorenin transgenic rats

Plasma prorenin is commonly elevated in diabetic patients and appears to predict the development of diabetic nephropathy. However, the pathological role of prorenin is unclear. In the present study, a transgenic, inducible, hepatic prorenin-overexpressing rat model was generated and the effect of prorenin in organ injury was examined. Four groups of rats (cyp1a1 prorenin transgenic male and female rats and non-transgenic littermates) were assigned to receive a diet containing 0.3% of the transgene inducer indole-3-carbinol (I3C) for 4 weeks. Plasma prorenin concentration was increased and mean arterial pressure (MAP) increased from 80 ± 18 to 138 ± 17 (mmHg), whereas renal prorenin/renin protein expression was unchanged, in transgenic rats fed with I3C diet. The intact prorenin, not renin, in plasma and urine samples was further observed by Western blot analysis. Importantly, transgenic rats with high levels of prorenin developed albuminuria, glomerular and tubulointerstitial fibrosis associated with increased expression of transforming growth factor β (TGFβ) 1 (TGFβ1), plasminogen activator inhibitor-1 (PAI-1), collagen, and fibronectin (FN). These rats also exhibited cardiac hypertrophy determined by echocardiography, with elevated ratio of heart weight to body weight (HW/BW). Cardiac collagen in interstitial and perivascular regions was prominent, accompanied by the increase in mRNA contents of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), β-myosin heavy chain (β-MHC), TGFβ1, PAI-1, and collagen in the heart tissue. Furthermore, renal protein levels of p-NF-κB-p65 and monocyte chemoattractant protein-1 (MCP-1), NAPDH oxidases, malondialdehyde (MDA) and 8-isoprostane (8-IP), p-ERK, p-β-catenin, and p-Akt were dramatically increased in prorenin overexpressing rats. These results indicate that prorenin, without being converted into renin, causes hypertension, renal and cardiac fibrosis via the induction of inflammation, oxidative stress and the ERK, β-catenin, and Akt-mediated signals.

Prorenin receptor in kidney development

Prorenin receptor (PRR), a receptor for renin and prorenin and an accessory subunit of the vacuolar proton pump H⁺-ATPase, is expressed in the developing kidney. Global loss of PRR is lethal in mice, and PRR mutations are associated with a high blood pressure, left ventricular hypertrophy and X-linked mental retardation in humans. With the advent of modern gene targeting techniques, including conditional knockout approaches, several recent studies have demonstrated critical roles for the PRR in several lineages of the developing kidney. PRR signaling has been shown to be essential for branching morphogenesis of the ureteric bud (UB), nephron progenitor survival and nephrogenesis. PRR regulates these developmental events through interactions with other transcription and growth factors. Several targeted PRR knockout animal models have structural defects mimicking congenital anomalies of the kidney and urinary tract observed in humans. The aim of this review, is to highlight new insights into the cellular and molecular mechanisms by which PRR may regulate UB branching, terminal differentiation and function of UB-derived collecting ducts, nephron progenitor maintenance, progression of nephrogenesis and normal structural kidney development and function.

Aliskiren directly improves endothelial progenitor cell function from Type II diabetic patients

BACKGROUND

Endothelial progenitor cell (EPC) functions are impaired in the presence of diabetes mellitus. Aliskiren is a direct renin inhibitor, which is expected to modify proangiogenic cells. This study aimed to investigate whether and how aliskiren could improve the function of EPCs from patients with type II diabetes (T2DM).

MATERIALS AND METHODS

Endothelial progenitor cells fibronectin adhesion assay, chamber assay and in vitro tube formation assay were used to estimate the degree of EPC adhesion, migration and tube formation abilities. EPC protein and mRNA expressions were evaluated by Western blot and quantitative RT-PCR, respectively. EPC vascular endothelial growth factor (VEGF) and (pro)renin receptor ((P)RR) expression was knocked down by VEGF and (P)RR siRNA.

RESULTS

Aliskiren (0·1 or 10 μM) dose-dependently improved functions and increased both VEGF and stromal cell-derived factor-1α (SDF-1α) expression of EPCs from patients with T2DM or EPCs from healthy volunteers and treated with high glucose. Transfection with VEGF siRNA significantly reduced the aliskiren-induced SDF-1α expression. Furthermore, (P)RR siRNA transfection impaired the aliskiren-induced VEGF and SDF-1 expression.

CONCLUSIONS

The results show that aliskiren improved EPC function from patients with T2DM in a dose-dependent manner probably via the (P)RR and VEGF/SDF-1α-related mechanisms.

The podocyte as a direct target for treatment of glomerular disease?

The Centers for Disease Control and Prevention estimates more than 10% of adults in the United States, over 20 million Americans, have chronic kidney disease (CKD). A failure to maintain the glomerular filtration barrier directly contributes to the onset of CKD. The visceral epithelial cells, podocytes, are integral to the maintenance of this renal filtration barrier. Direct podocyte injury contributes to the onset and progression of glomerular diseases such as minimal change disease (MCD), focal segmental glomerular sclerosis (FSGS), diabetic nephropathy, and HIV-associated nephropathy (HIVAN). Since podocytes are terminally differentiated with minimal capacity to self-replicate, they are extremely sensitive to cellular injury. In the past two decades, our understanding of the mechanism(s) by which podocyte injury occurs has greatly expanded. With this newfound knowledge, therapeutic strategies have shifted to identifying targets directed specifically at the podocyte. Although the systemic effects of these agents are important, their direct effect on the podocyte proves to be essential in ameliorating glomerular disease. In this review, we highlight the mechanisms by which these agents directly target the podocyte independent of its systemic effects.

Expression and functional role of the prorenin receptor in the human adrenocortical zona glomerulosa and in primary aldosteronism

OBJECTIVES

Prorenin can be detected in plasma of hypertensive patients. If detected in patients with primary aldosteronism could implicate prorenin in the development of primary aldosteronism. To address this issue, we measured the plasma prorenin levels in primary aldosteronism patients, the expression of the prorenin receptor (PRR) in the normal human adrenocortical zona glomerulosa and aldosterone-producing adenoma (APA), and we investigated the functional effects of PRR activation in human adrenocortical cells.

METHOD

Plasma renin activity, aldosterone, and active and total trypsin-activated renin were measured in primary aldosteronism patients, essential hypertensive patients, and healthy individuals, and then prorenin levels were calculated. Localization and functional role of PRR were investigated in human and rat tissues, and aldosterone-producing cells.

RESULTS

Primary aldosteronism patients had detectable plasma levels of prorenin. Using digital-droplet real-time PCR, we found a high PRR-to-porphobilinogen deaminase ratio in both the normal adrenal cortex and APAs. Marked expression of the PRR gene and protein was also found in HAC15 cells. Immunoblotting, confocal, and immunogold electron microscopy demonstrated PRR at the cell membrane and intracellularly. Renin and prorenin significantly triggered both CYP11B2 expression (aldosterone synthase) and ERK1/2 phosphorylation, but only CYP11B2 transcription was prevented by aliskiren.

CONCLUSION

The presence of detectable plasma prorenin in primary aldosteronism patients, and the high expression of PRR in the normal human adrenal cortex, APA tissue, CD56+ aldosterone-producing cells, along with activation of CYP11B2 synthesis and ERK1/2 phosphorylation, suggest that the circulating and locally produced prorenin may contribute to the development or maintenance of human primary aldosteronism.

Direct renin inhibitor ameliorates insulin resistance by improving insulin signaling and oxidative stress in the skeletal muscle from post-infarct heart failure in mice

Insulin resistance can occur as a consequence of heart failure (HF). Activation of the renin-angiotensin system (RAS) may play a crucial role in this phenomenon. We thus investigated the effect of a direct renin inhibitor, aliskiren, on insulin resistance in HF after myocardial infarction (MI). MI and sham operation were performed in male C57BL/6J mice. The mice were divided into 4 groups and treated with sham-operation (Sham, n=10), sham-operation and aliskiren (Sham+Aliskiren; 10mg/kg/day, n=10), MI (n=11), or MI and aliskiren (MI+Aliskiren, n=11). After 4 weeks, MI mice showed left ventricular dilation and dysfunction, which were not affected by aliskiren. The percent decrease of blood glucose after insulin load was significantly smaller in MI than in Sham (14±5% vs. 36±2%), and was ameliorated in MI+Aliskiren (34±5%) mice. Insulin-stimulated serine-phosphorylation of Akt and glucose transporter 4 translocation were decreased in the skeletal muscle of MI compared to Sham by 57% and 69%, and both changes were ameliorated in the MI+Aliskiren group (91% and 94%). Aliskiren administration in MI mice significantly inhibited plasma renin activity and angiotensin II (Ang II) levels. Moreover, (pro)renin receptor expression and local Ang II production were upregulated in skeletal muscle from MI and were attenuated in MI+Aliskiren mice, in tandem with a decrease in superoxide production and NAD(P)H oxidase activities. In conclusion, aliskiren ameliorated insulin resistance in HF by improving insulin signaling in the skeletal muscle, at least partly by inhibiting systemic and (pro)renin receptor-mediated local RAS activation, and subsequent NAD(P)H oxidase-induced oxidative stress.

Combined renin inhibition/(pro)renin receptor blockade in diabetic retinopathy--a study in transgenic (mREN2)27 rats

Dysfunction of renin-angiotensin system (RAS) contributes to the pathogenesis of diabetic retinopathy (DR). Prorenin, the precursor of renin is highly elevated in ocular fluid of diabetic patients with proliferative retinopathy. Prorenin may exert local effects in the eye by binding to the so-called (pro)renin receptor ((P)RR). Here we investigated the combined effects of the renin inhibitor aliskiren and the putative (P)RR blocker handle-region peptide (HRP) on diabetic retinopathy in streptozotocin (STZ)-induced diabetic transgenic (mRen2)27 rats (a model with high plasma prorenin levels) as well as prorenin stimulated cytokine expression in cultured Müller cells. Adult (mRen2)27 rats were randomly divided into the following groups: (1) non-diabetic; (2) diabetic treated with vehicle; (3) diabetic treated with aliskiren (10 mg/kg per day); and (4) diabetic treated with aliskiren+HRP (1 mg/kg per day). Age-matched non-diabetic wildtype Sprague-Dawley rats were used as control. Drugs were administered by osmotic minipumps for three weeks. Transgenic (mRen2)27 rat retinas showed increased apoptotic cell death of both inner retinal neurons and photoreceptors, increased loss of capillaries, as well as increased expression of inflammatory cytokines. These pathological changes were further exacerbated by diabetes. Aliskiren treatment of diabetic (mRen2)27 rats prevented retinal gliosis, and reduced retinal apoptotic cell death, acellular capillaries and the expression of inflammatory cytokines. HRP on top of aliskiren did not provide additional protection. In cultured Müller cells, prorenin significantly increased the expression levels of IL-1α and TNF-α, and this was completely blocked by aliskiren or HRP, their combination, (P)RR siRNA and the AT1R blocker losartan, suggesting that these effects entirely depended on Ang II generation by (P)RR-bound prorenin. In conclusion, the lack of effect of HRP on top of aliskiren, and the Ang II-dependency of the ocular effects of prorenin in vitro, argue against the combined application of (P)RR blockade and renin inhibition in diabetic retinopathy.

Autophagy and the (Pro)renin Receptor

The (pro)renin receptor (PRR) is a newly reported member of the renin-angiotensin system (RAS); a hormonal cascade responsible for regulating blood pressure. Originally, identification of PRR was heralded as the next drug target of the RAS, of which such therapies would have increased benefits against target-organ damage and hypertension. However, in the years since its discovery, several conditional knockout mouse models of PRR have demonstrated an essential role for this receptor unrelated to the RAS and blood pressure. Specific deletion of PRR in podocytes or cardiomyocytes resulted in the rapid onset of organ failure and subsequently animal mortality after only a matter of weeks. In both cell types, loss of PRR resulted in the intracellular accumulation of autophagosomes and misfolded proteins, indicating a disturbance in autophagy. In light of the fact that the majority of PRR is located intracellularly, this molecular function appears to be more relevant than its ability to bind to high, non-physiological concentrations of (pro)renin. This review will focus on the role of PRR in autophagy and its importance in maintaining cellular homeostasis. Understanding the link between PRR, autophagy and how its loss results in cell death will be essential for deciphering its role in physiology and pathology.

The role of the renin-angiotensin-aldosterone system in obesity-related renal diseases

Obesity is an independent risk factor for the development and progression of chronic kidney disease and one of the emerging reasons for end-stage renal disease owing to its dramatic increase worldwide. Among the potential underlying pathophysiologic mechanisms, activation of the renin-angiotensin-aldosterone-system (RAAS) plays a central role. Increased angiotensin II (AngII) levels also are central in hypertension, dyslipidemia, and insulin resistance, which, taken together with obesity, represent the metabolic syndrome. Increased AngII levels contribute to hyperfiltration, glomerulomegaly, and subsequent focal glomerulosclerosis by altering renal hemodynamics via afferent arteriolar dilation, together with efferent renal arteriolar vasoconstriction as well as by its endocrine and paracrine properties linking the intrarenal and the systemic RAAS, adipose tissue dysfunction, as well as insulin resistance and hypertension. The imbalance between increased AngII levels and the angiotensin converting enzyme 2/Ang (1-7)/Mas receptor axis additionally contributes to renal injury in obesity and its concomitant metabolic disturbances. As shown in several large trials and experimental studies, treatment of obesity by weight loss is associated with an improvement of kidney disease because it also is beneficial in dyslipidemia, hypertension, and diabetes. The most promising data have been seen by RAAS blockade, pointing to the central position of RAAS within obesity, kidney disease, and the metabolic syndrome.

[The cortical collecting duct plays a pivotal role in the kidney's local renin-angiotensin system]

The renin-angiotensin system is one of the most important hormone systems in the body, and the regulations as well as the role in the juxtaglomerular apparatus are well known. The present review focuses on renin secretion in a recently described localization, the cortical collecting duct. The authors display it in parallel of the copying strategy of an adult and a developing kidney. Furthermore, based on different animal studies it highlights the local role of renin released from the collecting duct. In chronic angiotensin II-infused, 2-kidney, 1-clip hypertensive model as well as in diabetic rats the major source of (pro)renin is indeed the collecting duct. In this localization this hormone can reach both the systemic circulation and the interstitial renin-angiotensin system components including the newly described (pro)renin receptor, by which (pro)renin is able to locally activate pro-fibrotic intracellular signal pathways. Consequently, one can postulate that in the future renin may serve either as a new therapeutic target in nephropathy associated with both hypertension and diabetes or as an early diagnostic marker in chronic diseases leading to nephropathy.

The renin-angiotensin-aldosterone system in podocytes

The renin-angiotensin-aldosterone system (RAAS) plays a critical role in kidney function and its inhibition reduces proteinuria and preserves kidney function in patients with chronic kidney disease. Recent studies have shown that podocytes generate many components of the RAAS and they express receptors of RAAS, including angiotensin II, mineralocorticoid, and prorenin receptors. Crucial functions of podocytes, such as contraction, apoptosis, autophagocytosis, and cytoskeletal organization, have been shown to be regulated by the angiotensin II type 1 receptors. An activation of the glomerular RAAS and protection from podocyte injury by RAAS inhibitors have been shown in many glomerular diseases. Exploring the interaction between the local RAAS and the signaling involved in RAAS activation in podocytes will lead to new therapeutic strategies of podocyte protection.

Role of (pro)renin receptor in Ang II-mediated EGF receptor transactivation

Prorenin-induced intracellular signaling pathway is not fully elucidated. We investigated whether the (pro)renin receptor mediates epidermal growth factor (EGF) receptor transactivation through angiotensin (Ang) II-dependent and -independent pathways in human embryo kidney 293 cells. Prorenin (2 nmol/L) caused biphasic phosphorylation of EGF receptor (Tyr992) and extracellular signal-regulated kinase (ERK) 1/2, peaking at 5 minutes followed by a decrease and a second peak at 60-120 minutes, whereas EGF receptor (Tyr1068) and Src were phosphorylated at only 120 minutes. These prorenin-induced phosphorylation processes were inhibited by (pro)renin receptor siRNA. Similarly, Ang II type 1 (AT1) receptor blocker (ARB) or AT1 receptor siRNA completely inhibited prorenin-induced phosphorylation of EGF receptor (Tyr1068) and Src, as well as the late peaks of EGF receptor (Tyr992) and ERK 1/2. However, early peaks of EGF receptor (Tyr992) and ERK 1/2 at 5 minutes were not effectively blocked by ARB or AT1 receptor siRNA. Incubation with prorenin significantly increased Ang II levels of cell lysate. These data indicate that the (pro)renin receptor mediates EGF receptor transactivation in both Ang II-dependent and -independent pathways.

Key developments in renin-angiotensin-aldosterone system inhibition

The renin-angiotensin-aldosterone system (RAAS) was initially thought to be fairly simple. However, this idea has been challenged following the development of RAAS blockers, including renin inhibitors, angiotensin-converting-enzyme (ACE) inhibitors, type 1 angiotensin II (AT(1))-receptor blockers and mineralocorticoid-receptor antagonists. Consequently, new RAAS components and pathways that might contribute to the effectiveness of these drugs and/or their adverse effects have been identified. For example, an increase in renin levels during RAAS blockade might result in harmful effects via stimulation of the prorenin receptor (PRR), and prorenin-the inactive precursor of renin-might gain enzymatic activity on PRR binding. The increase in angiotensin II levels that occurs during AT(1)-receptor blockade might result in beneficial effects via stimulation of type 2 angiotensin II receptors. Moreover, angiotensin 1-7 levels increase during ACE inhibition and AT(1)-receptor blockade, resulting in Mas receptor activation and the induction of cardioprotective and renoprotective effects, including stimulation of tissue repair by stem cells. Finally, a role of angiotensin II in sodium and potassium handling in the distal nephron has been identified. This finding is likely to have important implications for understanding the effects of RAAS inhibition on whole body sodium and potassium balance.

The intracrine renin-angiotensin system

The RAS (renin-angiotensin system) is one of the earliest and most extensively studied hormonal systems. The RAS is an atypical hormonal system in several ways. The major bioactive peptide of the system, AngII (angiotensin II), is neither synthesized in nor targets one specific organ. New research has identified additional peptides with important physiological and pathological roles. More peptides also mean newer enzymatic cascades that generate these peptides and more receptors that mediate their function. In addition, completely different roles of components that constitute the RAS have been uncovered, such as that for prorenin via the prorenin receptor. Complexity of the RAS is enhanced further by the presence of sub-systems in tissues, which act in an autocrine/paracrine manner independent of the endocrine system. The RAS seems relevant at the cellular level, wherein individual cells have a complete system, termed the intracellular RAS. Thus, from cells to tissues to the entire organism, the RAS exhibits continuity while maintaining independent control at different levels. The intracellular RAS is a relatively new concept for the RAS. The present review provides a synopsis of the literature on this system in different tissues.

The role of renin inhibition in treating the hypertensive patient with diabetes: a summary of preclinical and clinical evidence

Comorbid hypertension and diabetes is common and associated with substantially greater cardiovascular and renal risk relative to hypertension alone. Tissue renin-angiotensin system (RAS) overactivity is a hallmark of diabetes and contributes to target organ damage. Treatment guidelines recommend angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs) for reducing cardiorenal risk in patients with hypertension plus diabetes. However, these agents only partially prevent cardiovascular and renal morbidity/mortality. Further attempts to improve clinical outcomes have focused on the use of an ACE inhibitor plus an ARB, but this combination has not demonstrated a favorable risk-benefit profile. Direct renin inhibitors provide a more comprehensive blockade of the RAS compared with ACE inhibitors or ARBs, and may be of particular benefit in counteracting tissue RAS overactivity. In this article, the role of the RAS in diabetic hypertension and the preclinical and clinical effects of direct renin inhibitor therapy on target organs are reviewed.

Early treatment with olmesartan prevents juxtamedullary glomerular podocyte injury and the onset of microalbuminuria in type 2 diabetic rats

BACKGROUND

Studies were performed to determine if early treatment with an angiotensin II (Ang II) receptor blocker (ARB), olmesartan, prevents the onset of microalbuminuria by attenuating glomerular podocyte injury in Otsuka Long-Evans Tokushima Fatty (OLETF) rats with type 2 diabetes mellitus.

METHODS

OLETF rats were treated with either a vehicle, olmesartan (10 mg/kg/day) or a combination of nonspecific vasodilators (hydralazine 15 mg/kg/day, hydrochlorothiazide 6 mg/kg/day, and reserpine 0.3 mg/kg/day; HHR) from the age of 7-25 weeks.

RESULTS

OLETF rats were hypertensive and had microalbuminuria from 9 weeks of age. At 15 weeks, OLETF rats had higher Ang II levels in the kidney, larger glomerular desmin-staining areas (an index of podocyte injury), and lower gene expression of nephrin in juxtamedullary glomeruli, than nondiabetic Long-Evans Tokushima Otsuka (LETO) rats. At 25 weeks, OLETF rats showed overt albuminuria, and higher levels of Ang II in the kidney and larger glomerular desmin-staining areas in superficial and juxtamedullary glomeruli compared to LETO rats. Reductions in mRNA levels of nephrin were also observed in superficial and juxtamedullary glomeruli. Although olmesartan did not affect glucose metabolism, it decreased blood pressure and prevented the renal changes in OLETF rats. HHR treatment also reduced blood pressure, but did not affect the renal parameters.

CONCLUSIONS

This study demonstrated that podocyte injury occurs in juxtamedullary glomeruli prior to superficial glomeruli in type 2 diabetic rats with microalbuminuria. Early treatment with an ARB may prevent the onset of albuminuria through its protective effects on juxtamedullary glomerular podocytes.

Receptor-mediated nonproteolytic activation of prorenin and induction of TGF-β1 and PAI-1 expression in renal mesangial cells

While elevated plasma prorenin levels are commonly found in diabetic patients and correlate with diabetic nephropathy, the pathological role of prorenin, if any, remains unclear. Prorenin binding to the (pro)renin receptor [(p)RR] unmasks prorenin catalytic activity. We asked whether elevated prorenin could be activated at the site of renal mesangial cells (MCs) through receptor binding without being proteolytically converted to renin. Recombinant inactive rat prorenin and a mutant prorenin that is noncleavable, i.e., cannot be activated proteolytically, are produced in 293 cells. After MCs were incubated with 10(-7) M native or mutant prorenin for 6 h, cultured supernatant acquired the ability to generate angiotensin I (ANG I) from angiotensinogen, indicating both prorenins were activated. Small interfering RNA (siRNA) against the (p)RR blocked their activation. Furthermore, either native or mutant rat prorenin at 10(-7) M alone similarly and significantly induced transforming growth factor-β(1), plasminogen activator inhibitor-1 (PAI-1), and fibronectin mRNA expression, and these effects were blocked by (p)RR siRNA, but not by the ANG II receptor antagonist, saralasin. When angiotensinogen was also added to cultured MCs with inactive native or mutant prorenin, PAI-1 and fibronectin were further increased significantly compared with prorenin or mutant prorenin alone. This effect was blocked partially by treatment with (p)RR siRNA or saralasin. We conclude that prorenin binds the (p)RR on renal MCs and is activated nonproteolytically. This activation leads to increased expression of PAI-1 and transforming growth factor-β(1) via ANG II-independent and ANG II-dependent mechanisms. These data provide a mechanism by which elevated prorenin levels in diabetes may play a role in the development of diabetic nephropathy.

Contrasting effects of aliskiren versus losartan on hypertensive vascular remodeling

BACKGROUND

Hyperactivation of the renin-angiotensin system contributes to hypertension-induced upregulation of vascular matrix metalloproteinases (MMPs) and remodeling, especially in the two kidney, one clip (2K1C) hypertension model. We hypothesized that the AT1R antagonist losartan or the renin inhibitor aliskiren, given at doses allowing similar antihypertensive effects, could prevent in vivo vascular MMPs upregulation and remodeling, and collagen/elastin deposition found in 2K1C hypertension by preventing the activation of extracellular signal-regulated kinase 1/2 (ERK 1/2) and transforming growth factor-β1 (TGF-β1). We also hypothesized that aliskiren could enhance the effects of losartan.

METHODS

2K1C rats were treated with aliskiren (50mg.kg(-1).day(-1)), or losartan (10mg.kg(-1).day(-1)), or both by gavage during 4 weeks.

RESULTS

Aliskiren, losartan, or both drugs exerted similar antihypertensive effects when compared with 2K-1C rats treated with water. Aliskiren reduced plasma renin activity in both sham and 2K-1C rats. Losartan alone or combined with aliskiren, but not aliskiren alone, abolished 2K1C-induced aortic hypertrophy and hyperplasia, and prevented the increases in aortic collagen/elastin content, MMP-2 levels, gelatinolytic activity, and expression of phospho-ERK 1/2 and TGF-β1. No significant differences were found in the aortic expression of the (pro)renin receptor.

CONCLUSIONS

These findings show that although losartan and aliskiren exerted similar antihypertensive effects, only losartan prevented the activation of vascular profibrotic mechanisms and MMP upregulation associated with vascular remodeling in 2K1C hypertension. Our findings also suggest that aliskiren does not enhance the protective effects exerted by losartan.

(Pro)renin and its receptors: pathophysiological implications

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin [together denoted as (pro)renin]. Since tissue renin levels are usually somewhat higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. Several candidates have been evaluated in the past three decades, including a renin-binding protein, the mannose 6-phosphate/insulin-like growth factor II receptor and the (pro)renin receptor. Although the latter seemed the most promising, its nanomolar affinity for renin and prorenin is several orders of magnitude above their actual (picomolar) levels in blood, raising doubt on whether (pro)renin–(pro)renin receptor interaction will ever occur in vivo. A wide range of in vitro studies have now demonstrated (pro)renin-receptor-induced effects at nanomolar renin and prorenin concentrations, resulting in a profibrotic phenotype. In addition, beneficial in vivo effects of the putative (pro)renin receptor blocker HRP (handle region peptide) have been observed, particularly in diabetic animal models. Despite these encouraging results, many other studies have reported either no or even contrasting effects of HRP, and (pro)renin-receptor-knockout studies revealed lethal consequences that are (pro)renin-independent, most probably due to the fact that the (pro)renin receptor co-localizes with vacuolar H+-ATPase and possibly determines the stability of this vital enzyme. The present review summarizes all of the recent findings on the (pro)renin receptor and its blockade, and critically compares it with the other candidates that have been proposed to mediate (pro)renin uptake from blood. It ends with the conclusion that the (pro)renin–(pro)renin receptor interaction, if it occurs in vivo, is limited to (pro)renin-synthesizing organs such as the kidney.

Biphasic time course of the changes in aldosterone biosynthesis under high-salt conditions in Dahl salt-sensitive rats

OBJECTIVE

The comorbidity of excess salt and elevated plasma aldosterone has deleterious effects in cardiovascular disease. We evaluated the mechanisms behind the paradoxical increase in aldosterone biosynthesis in relation to dietary intake of salt.

METHODS AND RESULTS

Dahl salt-sensitive (Dahl-S) and salt-resistant (Dahl-R) rats were fed a high-salt diet, and plasma and tissue levels of aldosterone in the adrenal gland and heart were quantified by liquid chromatography-electrospray ionization-tandem mass spectrometry. In Dahl-S rats, we found that the delayed and paradoxical increase in aldosterone biosynthesis after the initial and appropriate response to high salt. The late rise in aldosterone biosynthesis was accompanied by upregulation of CYP11B2 expression in the zona glomerulosa and increased adrenal angiotensin II levels and renin-angiotensin system components. It preceded the appearance of left ventricular systolic dysfunction and renal insufficiency. Blockade of angiotensin AT(1) receptors reversed the paradoxical increase in aldosterone biosynthesis. In contrast, Dahl-R rats maintained the initial suppression of aldosterone biosynthesis. Aldosterone levels in the heart closely paralleled those in the plasma and adrenal gland and disappeared after bilateral adrenalectomy.

CONCLUSIONS

Chronic salt overload in Dahl-S rats stimulates aberrant aldosterone production via activation of the local renin-angiotensin system in the adrenal gland, thereby creating the comorbidity of excess salt and elevated plasma aldosterone.

Prorenin receptor is essential for normal podocyte structure and function

The prorenin receptor is an accessory subunit of the vacuolar H(+)-ATPase, suggesting that it has fundamental functions beyond activation of the local renin-angiotensin system. Podocytes express the prorenin receptor, but its function in these cells is unknown. Here, podocyte-specific, conditional, prorenin receptor-knockout mice died of kidney failure and severe proteinuria within 4 weeks of birth. The podocytes of these mice exhibited foot process effacement with reduced and altered localization of the slit-diaphragm proteins nephrin and podocin. Furthermore, the podocytes contained numerous autophagic vacuoles, confirmed by enhanced accumulation of microtubule-associated protein 1 light chain 3-positive intracellular vesicles. Ablation of the prorenin receptor selectively suppressed expression of the V(0) c-subunit of the vacuolar H(+)-ATPase in podocytes, resulting in deacidification of intracellular vesicles. In conclusion, the prorenin receptor is important for the maintenance of normal podocyte structure and function.

A new look at the renin-angiotensin system--focusing on the vascular system

The renin-angiotensin system (RAS), critically involved in the control of blood pressure and volume homeostasis, is a dual system comprising a circulating component and a local tissue component. The rate limiting enzyme is renin, which in the circulating RAS derives from the kidney to generate Ang II, which in turn regulates cardiovascular function by binding to AT(1) and AT(2) receptors on cardiac, renal and vascular cells. The tissue RAS can operate independently of the circulating RAS and may be activated even when the circulating RAS is suppressed or normal. A functional tissue RAS has been identified in brain, kidney, heart, adipose tissue, hematopoietic tissue, gastrointestinal tract, liver, endocrine system and blood vessels. Whereas angiotensinsinogen, angiotensin converting enzyme (ACE), Ang I and Ang II are synthesized within these tissues, there is still controversy as to whether renin is produced locally or whether it is taken up from the circulation, possibly by the (pro)renin receptor. This is particularly true in the vascular wall, where expression of renin is very low. The exact function of the vascular RAS remains elusive, but may contribute to fine-tuning of vascular tone and arterial structure and may amplify vascular effects of the circulating RAS, particularly in pathological conditions, such as in hypertension, atherosclerosis and diabetes. New concepts relating to the vascular RAS have recently been elucidated including: (1) the presence of functionally active Ang-(1-7)-Mas axis in the vascular system, (2) the importance of the RAS in perivascular adipose tissue and cross talk with vessels, and (3) the contribution to vascular RAS of Ang II derived from immune and inflammatory cells within the vascular wall. The present review highlights recent progress in the RAS field, focusing on the tissue system and particularly on the vascular RAS.

Podocyte biology for the bedside

The explosion of podocyte biology during the last decade has radically altered our views on the pathophysiologic process of proteinuria, glomerular disease, and progressive kidney disease. In this review, we highlight some of these landmark findings, but focus on recent advances in the field and implications for translating this biology into therapy for podocyte diseases.

Podocytopathy in diabetes: a metabolic and endocrine disorder

Diabetic nephropathy (DN) represents a major public health cost. Tight glycemic and blood pressure control can dramatically slow, but not stop, the progression of the disease, and a large number of patients progress toward end-stage renal disease despite currently available interventions. An early and key event in the development of DN is loss of podocyte function (or glomerular visceral epithelial cells) from the kidney glomerulus, where they contribute to the integrity of the glomerular filtration barrier. Recent evidence suggests that podocytes can be the direct target of circulating hormones, lipids, and adipokines that are affected in diabetes. We review the clinical and experimental evidence implicating novel endocrine and metabolic pathways in the pathogenesis of podocyte dysfunction and the development of DN.

(Pro)renin Receptor in Kidney Development and Disease

The renin-angiotensin system (RAS), a key regulator of the blood pressure and fluid/electrolyte homeostasis, also plays a critical role in kidney development. All the components of the RAS are expressed in the developing metanephros. Moreover, mutations in the genes encoding components of the RAS in mice or humans are associated with a broad spectrum of congenital anomalies of the kidney and urinary tract (CAKUT). These forms of CAKUT include renal papillary hypoplasia, hydronephrosis, duplicated collecting system, renal tubular dysgenesis, renal vascular abnormalities, and aberrant glomerulogenesis. Emerging evidence indicates that (pro)renin receptor (PRR), a novel component of the RAS, is essential for proper kidney development and that aberrant PRR signaling is causally linked to cardiovascular and renal disease. This paper describes the role of the RAS in kidney development and highlights emerging insights into the cellular and molecular mechanisms by which the PRR may regulate this critical morphogenetic process.

Aliskiren ameliorates renal inflammation and fibrosis induced by unilateral ureteral obstruction in mice

PURPOSE

Renin-angiotensin system activation is involved in inflammation and fibrosis in the kidney. Aliskiren, a direct renin inhibitor, decreases renin-angiotensin system activation, including plasma renin activity and angiotensin II, but increases the prorenin level, which may promote inflammation and fibrosis in renal tissue. Thus, we evaluated whether inhibiting the renin-angiotensin system by aliskiren would decrease renal inflammation and fibrosis in a mouse model of unilateral ureteral obstruction.

MATERIALS AND METHODS

Ten-week-old male C57BL/6 mice (Samtako, Kyoung Gi-Do, Korea) weighing 30 to 33 gm were divided into 4 groups, including vehicle or aliskiren treated sham operated and vehicle or aliskiren treated unilateral ureteral obstruction groups. We evaluated plasma renin activity, and plasma renin and renal mRNA expression levels of renin and (pro)renin receptor. To evaluate inflammation and fibrosis renal mRNA expression of monocyte chemotactic protein-1, osteopontin and transforming growth factor-β was measured. Hematoxylin and eosin, Masson's trichrome staining, and immunohistochemical staining for CD68, transforming growth factor-β and α-smooth muscle actin were performed.

RESULTS

Plasma renin activity was significantly lower in the aliskiren treated obstruction group than in the vehicle treated obstruction group. Aliskiren treatment increased renal mRNA expression of renin. The number of CD68 positive cells, and renal monocyte chemotactic protein-1 and osteopontin mRNA levels were significantly higher in mice with unilateral ureteral obstruction than in sham operated mice. Aliskiren decreased the increased levels of these inflammation markers. Aliskiren also decreased renal transforming growth factor-β mRNA expression, transforming growth factor-β and α-smooth muscle actin immunostaining, and Masson's trichrome stained areas of unilateral ureteral obstruction kidneys.

CONCLUSIONS

Aliskiren has anti-inflammatory and antifibrotic effects in an experimental unilateral ureteral obstruction mouse model.

Retinoic acid receptor-mediated signaling protects cardiomyocytes from hyperglycemia induced apoptosis: role of the renin-angiotensin system

Diabetes mellitus (DM) is a primary risk factor for cardiovascular diseases and heart failure. Activation of the retinoic acid receptor (RAR) and retinoid X receptor (RXR) has an anti-diabetic effect; but, a role in diabetic cardiomyopathy remains unclear. Using neonatal and adult cardiomyocytes, we determined the role of RAR and RXR in hyperglycemia-induced apoptosis and expression of renin-angiotensin system (RAS) components. Decreased nuclear expression of RARα and RXRα, activation of apoptotic signaling and cell apoptosis was observed in high glucose (HG) treated neonatal and adult cardiomyocytes and diabetic hearts in Zucker diabetic fatty (ZDF) rats. HG-induced apoptosis and reactive oxygen species (ROS) generation was prevented by both RAR and RXR agonists. Silencing expression of RARα and RXRα, by small interference RNA, promoted apoptosis under normal conditions and significantly enhanced HG-induced apoptosis, indicating that RARα and RXRα are required in regulating cell apoptotic signaling. Blocking angiotensin type 1 receptor (AT(1) R); but, not AT(2) R, attenuated HG-induced apoptosis and ROS generation. Moreover, HG induced gene expression of angiotensinogen, renin, AT(1) R, and angiotensin II (Ang II) synthesis were inhibited by RARα agonists and promoted by silencing RARα. Activation of RXRα, downregulated the expression of AT(1) R; and RXRα silencing accelerated HG induced expression of angiotensinogen and Ang II synthesis, whereas there was no significant effect on renin gene expression. These results indicate that reduction in the expression of RARα and RXRα has an important role in hyperglycemia mediated apoptosis and expression of RAS components. Activation of RAR/RXR signaling protects cardiomyocytes from hyperglycemia, by reducing oxidative stress and inhibition of the RAS.

Renin and cardiovascular disease: Worn-out path, or new direction

Inhibition of the renin angiotensin system has beneficial effects in cardiovascular prevention and treatment. The advent of orally active direct renin inhibitors adds a novel approach to antagonism of the renin-angiotensin system. Inhibition of the first and rate-limiting step of the renin angiotensin cascade offers theoretical advantages over downstream blockade. However, the recent discovery of the (pro)renin receptor which binds both renin and prorenin, and which can not only augment catalytic activity of both renin and prorenin in converting angiotensinogen to angiotensin I, but also signal intracellularly via various pathways to modulate gene expression, adds a significant level of complexity to the field. In this review, we will examine the basic and clinical data on renin and its inhibition in the context of cardiovascular pathophysiology.

Renin, (pro)renin and receptor: an update

PRR [(pro)renin receptor] was named after its biological characteristics, namely the binding of renin and of its inactive precursor prorenin, that triggers intracellular signalling involving ERK (extracellular-signal-regulated kinase) 1/2. However the gene encoding for PRR is named ATP6ap2 (ATPase 6 accessory protein 2) because PRR was initially found as a truncated form co-purifying with V-ATPase (vacuolar H+-ATPase). There are now data showing that this interaction is not only physical, but also functional in the kidney and the heart. However, the newest and most fascinating development of PRR is its involvement in both the canonical Wnt/β-catenin and non-canonical Wnt/PCP (planar cell polarity) pathways, which are essential for adult and embryonic stem cell biology, embryonic development and disease, including cancer. In the Wnt/β-catenin pathway, it has been shown that PRR acts as an adaptor between the Wnt receptor LRP5/6 (low-density lipoprotein receptor-related protein 5/6) and Fz (frizzled) and that the proton gradient generated by the V-ATPase in endosomes is necessary for LRP5/6 phosphorylation and β-catenin activation. In the Wnt/PCP pathway, PRR binds to Fz and controls its asymetrical subcellular distribution and therefore the polarization of the cells in a plane of a tissue. These essential cellular functions of PRR are independent of renin and open new avenues on the pathophysiological role of PRR. The present review will summarize our knowledge of (pro)renin-dependent functions of PRR and will discuss the newly recognized functions of PRR related to the V-ATPase and to Wnt signalling.

Role of the intrarenal renin-angiotensin-aldosterone system in chronic kidney disease

The existence of local or tissue-based renin-angiotensin-aldosterone systems (RAAS) is well documented and has been implicated as a key player in the pathogenesis of cardiovascular and renal diseases. The kidney contains all elements of the RAAS, and intrarenal formation of angiotensin II not only controls glomerular hemodynamics and tubule sodium transport, but also activates a number of inflammatory and fibrotic pathways. Experimental and clinical studies have shown that the intrarenal RAAS is activated early in diabetic nephropathy, the leading cause of chronic kidney disease (CKD). Although angiotensin-converting enzyme inhibitors and angiotensin receptor blockers decrease the rate of decline in kidney function in patients with diabetic and non-diabetic nephropathy, many patients still progress to end-stage renal disease or die from cardiovascular events. There is still a clear need for additional strategies to block the RAAS more effectively to reduce progression of CKD. The focus of this paper is to review the importance of the intrarenal RAAS in CKD and recent findings in renin-angiotensin biology pertinent to the kidney. We also discuss additional strategies to inhibit the RAAS more effectively and the potential impact of direct renin inhibition on the prevention and management of CKD.