Renin increases mesangial cell transforming growth factor-beta1 and matrix proteins through receptor-mediated, angiotensin II-independent mechanisms

(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.

Renin and prorenin have no direct effect on aldosterone synthesis in the human adrenocortical cell lines H295R and HAC15

INTRODUCTION

Transgenic rats expressing the human (pro)renin receptor (h(P)RR) have elevated plasma aldosterone levels despite unaltered levels, in plasma and adrenal, of renin and angiotensin II.

MATERIALS AND METHODS

To investigate whether renin/prorenin-(P)RR interaction underlies these elevated aldosterone levels, the effect of (pro)renin on steroidogenesis was compared with that of angiotensin II in two (P)RR-expressing human adrenocortical cell lines, H295R and HAC15. Angiotensin II rapidly induced extracellular signal-regulated kinase (ERK) phosphorylation and increased the expression of STAR, CYP21A2, CYP11B2, and CYP17A1 at 6 and 24 hours, whereas the expression of CYP11A1 and HSD3B2 remained unaltered. Incubation with renin or prorenin at nanomolar concentrations had no effect on the expression of any of the steroidogenic enzymes tested, nor resulted in ERK phosphorylation. Angiotensin II, but not renin or prorenin, induced aldosterone production.

CONCLUSION

Although the (P)RR is present in adrenocortical cells, renin and prorenin do not elicit ERK phosphorylation nor directly affect steroid production via this receptor at nanomolar concentrations. Thus, direct (pro)renin-(P)RR interaction is unlikely to contribute to the elevated aldosterone levels in human (P)RR transgenic rats. This conclusion also implies that the aldosterone rise that often occurs during prolonged renin-angiotensin system blockade is rather due to the angiotensin II 'escape' during such blockade.

Does hypokalaemia cause nephropathy? An observational study of renal function in patients with Bartter or Gitelman syndrome

BACKGROUND

Hypokalaemic nephropathy has been described in patients with chronic potassium depletion; it is a condition in which proximal tubular vacuolization and interstitial fibrosis occur, resulting in a decline in glomerular filtration rate (GFR) and, in some cases, renal failure. It has been described in patients with chronic diarrhoea, eating disorders, laxative abuse and primary hyperaldosteronism; also occasionally in Bartter syndrome (BS), in which severe hypokalaemia accompanies significant renal sodium and water losses, though rarely in Gitelman syndrome (GS), in which there is equally severe hypokalaemia, but only modest sodium losses.

AIM

We hypothesized that hypokalaemic nephropathy may not be due to potassium depletion per se, but persistently elevated circulating levels of aldosterone, possibly with superimposed episodes of renal hypoperfusion.

DESIGN AND METHODS

We searched UK and European data sets to retrospectively compare serum and urinary parameters in patients with GS and BS.

RESULTS

The patients with GS often had lower serum potassium concentrations than patients with BS, but the BS patients had significantly higher serum creatinine concentrations and lower estimated GFRs (eGFR). BS patients had significantly higher fractional excretions of sodium compared with GS patients, as well as higher plasma renin activities and serum aldosterone levels.

CONCLUSION

These findings show that in genetically confirmed cases of BS and GS, the degree of hypokalaemia (as an index of chronic potassium depletion) does not correlate with GFR, and that on-going sodium and water losses, and consequent secondary hyperaldosteronism, may play a more important role in the aetiology of hypokalaemic nephropathy.

(Pro)renin receptor: another member of the system controlled by angiotensin II?

The prorenin receptor [(P)RR] is upregulated in the diabetic kidney and has been implicated in the high glucose (HG)-induced overproduction of profibrotic molecules by mesangial cells (MCs), which is mediated by ERK1/2 phosphorylation. The regulation of (P)RR gene transcription and the mechanisms by which HG increases (P)RR gene expression are not fully understood. Because intracellular levels of angiotensin II (AngII) are increased in MCs stimulated with HG, we used this in vitro system to evaluate the possible role of AngII in (P)RR gene expression and function by comparing the effects of AT1 receptor blockers (losartan or candesartan) and (P)RR mRNA silencing (siRNA) in human MCs (HMCs) stimulated with HG. HG induced an increase in (P)RR and fibronectin expression and in ERK1/2 phosphorylation. These effects were completely reversed by (P)RR siRNA and losartan but not by candesartan (an angiotensin receptor blocker that, in contrast to losartan, blocks AT1 receptor internalization). These results suggest that (P)RR gene activity may be controlled by intracellular AngII and that HG-induced ERK1/2 phosphorylation and fibronectin overproduction are primarily induced by (P)RR activation. This relationship between AngII and (P)RR may constitute an additional pathway of MC dysfunction in response to HG stimulation.

Therapeutic perspectives in hypertension: novel means for renin-angiotensin-aldosterone system modulation and emerging device-based approaches

The conventional antihypertensive therapies including renin–angiotensin–aldosterone system antagonists (converting enzyme inhibitors, receptor blockers, renin inhibitors, and mineralocorticoid receptor blockers), diuretics, β-blockers, and calcium channel blockers are variably successful in achieving the challenging target blood pressure values in hypertensive patients. Difficult to treat hypertension is still a commonly observed problem world-wide. A number of drugs are considered to be used as novel therapies for hypertension. Renalase supplementation, vasopeptidase inhibitors, endothelin antagonists, and especially aldosterone antagonists (aldosterone synthase inhibitors and novel selective mineralocorticoid receptor blockers) are considered an option in resistant hypertension. In addition, the aldosterone antagonists as well as (pro)renin receptor blockers or AT₂ receptor agonists might attenuate end-organ damage. This array of medications has now been complemented by a number of new approaches of non-pharmacological strategies including vaccination, genomic interference, controlled breathing, baroreflex activation, and probably most successfully renal denervation techniques. However, the progress on innovative therapies seems to be slow and the problem of resistant hypertension and proper blood pressure control appears to be still persisting. Therefore the regimens of currently available drugs are being fine-tuned, resulting in the establishment of several novel fixed-dose combinations including triple combinations with the aim to facilitate proper blood pressure control. It remains an exciting question which approach will confer the best blood pressure control and risk reduction in this tricky disease.

Ascochlorin suppresses TGF-β1-induced PAI-1 expression through the inhibition of phospho-EGFR in rat kidney fibroblast cells

Fibrosis is induced by the excessive and abnormal deposition of extracellular matrix (ECM) with various growth factors in tissues. Transforming growth factor-β1 (TGF-β1), the growth factor involved in fibrosis, modulates ECM synthesis and accumulation. TGF-β1 enhances the production of stimulators of ECM synthesis such as plasminogen activator inhibitor type 1 (PAI-1). As such, PAI-1 expression directly influences the proteolysis, invasion, and accumulation of ECM. It was shown in this study that ascochlorin, a prenylpenl antiobiotic, prevents the expression of profibrotic factors, such as PAI-1 and collagen type I, and that the TGF-β1-induced PAI-1 promoter activity is inhibited by ascochlorin. Ascochlorin abolishes the phosphorylation of the EGFR-MEK-ERK signaling pathway to regulate the TGF-β1-induced expression of PAI-1 without the inhibition of TβRII phosphorylation. Furthermore, the MEK inhibitor and EGFR siRNA block PAI-1 expression, and the Raf-1, MEK, and ERK signaling pathways for the regulation of PAI-1 expression. Ascochlorin suppresses the matrix metalloproteinases (MMPs) activity to activate the heparin-binding EGF-like growth factor (HB-EGF), to induce the phosphorylation of EGFR, and the MMPs inhibitor suppresses EGFR phosphorylation and the PAI-1 mRNA levels. These results suggest that ascochlorin prevents the expression of PAI-1 via the inhibition of an EGFR-dependent signal transduction pathway activated by MMPs.

Podocyte COX-2 exacerbates diabetic nephropathy by increasing podocyte (pro)renin receptor expression

Diabetic nephropathy (DN) increases podocyte cyclooxygenase-2 (COX-2) expression, and COX-2 inhibition reduces proteinuria and glomerular injury in animal models of diabetes. To investigate the role of podocyte COX-2 in development of diabetic nephropathy, we employed a streptozotocin model of diabetic mellitus in wild-type and transgenic mice expressing COX-2 selectively in podocytes. Progressive albuminuria developed only in diabetic COX-2 transgenic mice despite hyperglycemia, BP, and GFR being similar to those in wild-type mice. Transgenic mice also manifested significant foot-process effacement, moderate mesangial expansion, and segmental thickening of the glomerular basement membrane. In cultured podocytes overexpressing COX-2, high glucose induced cell injury and increased both expression of the pro(renin) receptor and activation of the renin-angiotensin system. Downregulation of the (pro)renin receptor attenuated the injury induced by high glucose. In vivo, podocyte pro(renin) receptor expression increased in diabetic COX-2-transgenic mice, and treatment with a COX-2 inhibitor abrogated the upregulation of (pro)renin receptor and reduced albuminuria, foot-process effacement, and mesangial matrix expansion. In summary, these results demonstrate that increased expression of podocyte COX-2 predisposes to diabetic glomerular injury and that the (pro)renin receptor may be one mediator for this increased susceptibility to injury.

Rationale for combining a direct renin inhibitor with other renin- angiotensin system blockers. Focus on aliskiren and combinations

Inhibition of the renin-angiotensin system has been a highly successful therapeutic approach for the prevention of hypertension-related end organ damage. Angiotensin converting enzyme inhibitors and angiotensin II receptor blockers lower blood pressure and reduce morbidity and mortality in patients with cardiovascular and kidney disease. However, progression to end-stage disease remains common in these patient populations. A compensatory increase in plasma renin activity occurs with the use of either angiotensin converting enzyme inhibitors or angiotensin II receptor blockers, thus causing increased levels of angiotensin II, which may limit the therapeutic effectiveness of these agents. The direct renin inhibitor, aliskiren, suppresses the renin-angiotensin system by inhibiting its first and rate-limiting step. This early inhibition reduces the production of all downstream components of the system. In this review, recent clinically relevant advances in the understanding of renin-angiotensin system biology are explored as a rationale for combining aliskiren with other blockers of the renin-angiotensin system. These combinations more fully inhibit the renin-angiotensin system, with the goal of providing additional therapeutic benefits in diseases associated with chronic activation of the renin-angiotensin system.

Enrichment of (pro)renin receptor promoter with activating histone codes in the kidneys of spontaneously hypertensive rats

BACKGROUND

The (pro)renin receptor [(P)RR] non-proteolytically, through conformational change, activates prorenin which can convert angiotensinogen to angiotensin I in addition to the classic conversion of angiotensinogen to angiotensin I by circulating renin. Since renal (P)RR is upregulated in hypertension and implicated in the pathogenesis of malignant hypertension, we hypothesized that (pro)renin receptor promoter is enriched with activating histone codes in the kidney of spontaneously hypertensive rats (SHR).

METHODS

The mRNA and protein expression levels were measured by real-time polymerase chain reaction (PCR) and western blot, respectively. The DNA methylation status of (P)RR promoter region was analyzed by bisulfite sequencing. The histone modifications were determined by chromatin immunoprecipitation followed by real-time PCR.

RESULTS

The (P)RR mRNA expression in the kidney was about six times greater in SHR than in Wistar-Kyoto (WKY) rats. The (P)RR promoter was little methylated in the kidneys of both WKY and SHR. Acetylated histone H3 (H3Ac) and di-methylated histone H3 at lysine 4 (H3K4me2), activating histone codes, were about 25 and three times higher in SHR than in WKY, respectively. On the other hand, di-methylated histone H3 at lysine 9 (H3K9me2), a suppressive histone code, was 50 times lower in SHR than in WKY.

CONCLUSION

These results suggest that the (P)RR promoter is enriched with activating histone codes in the kidneys of SHR.

Renal (pro)renin receptor contributes to development of diabetic kidney disease through transforming growth factor-β1-connective tissue growth factor signalling cascade

1. Transforming growth factor-β1 (TGF-β1) and connective tissue growth factor (CTGF) are expressed in renal glomeruli, and contribute to the development of diabetic nephropathy. Recently, we showed that (pro)renin receptor (PRR) is upregulated in the kidneys of the streptozocin (STZ)-induced diabetes rat model. We hypothesized that in the presence of hyperglycaemia, increased renal PRR expression contributes to enhanced TGF-β1-CTGF signalling activity, leading to the development of diabetic kidney disease. 2. In vivo and in vitro studies were carried out in Sprague-Dawley rats and rat mesangial cells (RMC). PRR blockade was achieved in vivo by treating STZ induced diabetes rats with the handle region peptide (HRP) of prorenin and in vitro by HRP or PRR siRNA in RMC. Angiotensin AT1 receptor blockade was achieved by valsartan treatment. 3. Results showed that expression of PRR, TGF-β1 and CTGF were upregulated in diabetic kidneys and RMC exposed to high glucose. Glucose exposure also induced PRR phosphorylation, a process that was inhibited by HRP, valsartan or PRR siRNA. HRP and valsartan significantly attenuated renal TGF-β1 and CTGF expression in diabetic animals and high glucose treated RMC. Similar results were observed in high glucose exposed RMC in response to PRR siRNA. TGF-β receptor blockade decreased CTGF expression in RMC. Combined administration of valsartan and PRR siRNA showed further reduction of TGF-β1 and CTGF expression in RMC. 4. In conclusion, PRR contributes to kidney disease in diabetes through an enhanced TGF-β1-CTGF signalling cascade.

Combining angiotensin II blockade and renin receptor inhibition results in enhanced antifibrotic effect in experimental nephritis

The limited antifibrotic effect of therapeutic angiotensin blockade, the fact that angiotensin blockade dramatically elevates renin levels, and recent evidence that renin has an angiotensin-independent, receptor-mediated profibrotic action led us to hypothesize that combining renin receptor inhibition and ANG II blockade would increase the antifibrotic effect of angiotensin blockade alone. Using cultured nephritic glomeruli from rats with anti-Thy-1-induced glomerulonephritis, the maximally effective dose of enalaprilate was determined to be 10(-4) M, which reduced mRNAs for transforming growth factor (TGF)-β1, fibronectin (FN), and plasminogen activator inhibitor-1 (PAI-1) by 49, 65, and 56% and production of TGF-β1 and FN proteins by 60 and 49%, respectively. Disease alone caused 6.8-fold increases in ANG II levels that were reduced 64% with enalaprilate. In contrast, two- and threefold disease-induced increases in renin mRNA and activity were further increased 2- and 3.7-fold with 10(-4) M enalaprilate treatment. Depressing the renin receptor by 80% with small interfering (si) RNA alone reduced fibrotic markers in a manner remarkably similar to enalaprilate alone but had no effect on glomerular renin expression. Enalaprilate and siRNA combination therapy further reduced disease markers. Notably, elevated TGF-β1 and FN production was reduced by 73 and 81%, respectively. These results support the notion of a receptor-mediated profibrotic action of renin, suggest that the limited effectiveness of ANG II blockade may be due, at least in part, to the elevated renin they induce, and support our hypothesis that adding renin receptor inhibitor to ANG II blockade in patients may have therapeutic potential.

(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.

Aliskiren reduces prorenin receptor expression and activity in cultured human aortic smooth muscle cells

INTRODUCTION

The recent discovery of a specific receptor for renin/prorenin (PRR) has added new interest to the potential pharmacological actions of aliskiren, the first direct renin inhibitor.

MATERIALS AND METHODS

In the present study, to gain new insights into the pharmacological properties of aliskiren, we investigated the effect of aliskiren on PRR expression and activity in cultured human smooth muscle cells (HSMCs).

RESULTS

Co-incubation of HSMCs with angiotensinogen (ANG) (1.5 × 10(-7)M) and prorenin (10(-8)-10(-7)M) resulted in an efficient production (within 4h) of angiotensin I, almost completely inhibited by 10(-5)M aliskiren (-86.0 ± 14.0%). In HSMCs stimulated with both ANG and prorenin, a 24h incubation with aliskiren (10(-6)-10(-5)M) resulted in a concentration-dependent reduction of PRR mRNA levels (IC(50) 4.6 × 10(-6)M). The cell surface expression of PRR determined by flow cytometry analysis was also reduced after incubation with aliskiren in a concentration-dependent manner. The lower levels of PRR were associated with a reduced expression of TGF-β, PAI-1 and type I collagen mRNA.

CONCLUSIONS

These results suggest a direct pharmacological action of aliskiren on PRR expression and its signalling pathway in HSMCs. This reported action of aliskiren may reveal a new scenario of the pharmacological properties of aliskiren.

Potential benefits of aliskiren beyond blood pressure reduction

There is now clear evidence that reducing blood pressure (BP) with a broad range of agents, including angiotensin converting enzyme inhibitors and angiotensin receptor blockers, improves cardiovascular and renal outcomes. There is also evidence suggesting that these drugs have beneficial effects that are independent of BP lowering. Aliskiren is a direct renin inhibitor that interrupts the renin-angiotensin-aldosterone system (RAAS) at its rate-limiting step. Unlike angiotensin-converting enzyme inhibitors and angiotensin receptor blockers, aliskiren produces a sustained reduction in plasma renin activity and reduces plasma levels of angiotensin II and aldosterone. Preclinical data and clinical trials in high-risk patients using surrogate markers increasingly suggest that aliskiren can reduce the progression of end-organ damage beyond that afforded by BP control. With its unique mechanism of action, combining aliskiren with another RAAS-blocking agent that has a different mechanism of action may provide more comprehensive blockade of the RAAS, potentially conferring additional clinical benefits. Evaluation of these end-organ effects in humans is underway in clinical trials designed to assess the effects of aliskiren alone and in combination with other antihypertensive agents on cardiovascular and renal outcomes.

Associations of plasma renin with 10-year cardiovascular mortality, sudden cardiac death, and death due to heart failure

AIMS

Renin is the key regulator of the renin-angiotensin-aldosterone system. Previous studies have reported conflicting results on the relation of plasma renin with fatal cardiovascular events. This study in a large cohort of patients sought to evaluate the association between plasma renin concentration (PRC) and cardiovascular mortality after long-term follow-up of almost 10 years.

METHODS AND RESULTS

Plasma renin concentration [median: 11.4 (6.0-24.6) pg/mL] was measured in 3303 patients (mean age: 62.7 ± 10.6 years; 30.3% women) referred to coronary angiography. After a median follow-up of 9.9 years, 554 participants (16.8%) with PRC measurement at baseline had died due to fatal cardiovascular events. Multivariable-adjusted Cox analysis revealed that when compared with participants in the lowest PRC quartile, those in the highest quartile were at increased risk of cardiovascular mortality (hazard ratio: 1.79, 95% CI 1.28-2.48). Analyses of specific causes of cardiovascular death showed that for each standard deviation increase in log-PRC there was a 22% (P = 0.006) increase in risk of sudden cardiac death and a 23% (P = 0.033) greater risk of death due to heart failure. The association of PRC with cardiovascular mortality remained stable after adjustment for established cardiovascular risk factors, ongoing antihypertensive medication, immunoreactive angiotensin II, and aldosterone levels. Age, N-terminal pro-B-type natriuretic peptide levels, coronary artery disease, the use of angiotensin-converting enzyme-inhibitors, beta-blockers, diuretics, and kidney function were important effect modifiers.

CONCLUSIONS

Plasma renin concentration is associated with long-term cardiovascular mortality in patients referred to coronary angiography. Further intervention studies should determine whether renin is a potential therapeutic target or only a marker of mortality risk in various cardiovascular risk groups.

Renin and prorenin receptor in hypertension: what's new?

The (pro)renin receptor, PRR, was initially characterized as a component of the renin-angiotensin system (RAS). PRR-bound renin and prorenin display increased enzymatic activity, and binding activates intracellular signaling, upregulating the expression of profibrotic proteins. As a consequence, most studies set out to demonstrate a role of PRR in hypertension, cardiovascular and renal diseases, and organ damage, and to identify PRR as a therapeutic target to optimize RAS blockade. The results of animal studies were disappointing and did not convincingly establish PRR as major player in hypertension or in organ damage, although human studies suggested a link between a polymorphism in the PRR gene and blood pressure. New data now suggest that PRR is functionally linked to the vacuolar proton-ATPase and, quite unexpectedly, that PRR is necessary to Wnt signaling pathways that are essential (independently of renin) for adult and embryonic stem cell biology, embryonic development, and diseases including cancer, thereby opening new perspectives on the pathophysiological roles of PRR.

Enhanced angiotensin-converting enzyme 2 attenuates angiotensin II-induced collagen production via AT1 receptor-phosphoinositide 3-kinase-Akt pathway

Recent reports support a protective role for angiotensin-converting enzyme 2 (ACE2) against glomerular diseases, especially by decreasing of extracellular matrix (ECM) proteins. However, the mechanism regulating this effect appears to be complex and poorly understood. Our aim was to investigate whether or not ACE2 ameliorates the profibrotic effects of Ang II-mediated, Akt-dependent pathways in the mouse mesangial cell line, MES-13.Gene transfer of ACE2 suppressed Ang II-activated Akt-phosphorylation, accompanied by a decreased level of collagen type I in cells. In addition, Ang II-induced collagen type I synthesis in MES-13s by activating the Ang II/AT-1R-PI3K pathway. This transactivation was dependent on cAMP/Epac but not on PKA. TGF-βRI played a pivotal role in this signaling pathway inducing collagen deposition effects which could be reversed by ACE2 gene transfer in MES-13 cells. The results revealed that gene transfer of ACE2 regulated Ang II-mediated AT1R-TGFβRI-PI3K-Akt signaling and involved the synthesis of collagen. The beneficial effect of ACE2 overexpression appeared to result mainly from blocking phosphorylation of Akt in mesangial cells, suggesting that the ACE2 gene might be a novel therapeutic target for glomerular diseases.

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.

Role of aliskiren in blood pressure control and renoprotection

Patients with chronic renal disease are at increased risk for the development of cardiovascular disease, which is the main cause of death in this growing population. Among the risk factors involved, hypertension and proteinuria are major contributors to kidney damage and, if not controlled, may eventually lead to the progression of renal failure and end-stage renal disease. Both proteinuria and hypertension can be primary pathologic events or can appear as complications of other disease processes. Initially, these two factors may operate separately but, as progression ensues, both processes generally combine, potentiating their effects and hastening renal damage. Therefore, strategies to reduce blood pressure and proteinuria are essential in order to slow the worsening of many nephropathies. Therapies that target the renin-angiotensin system offer particular benefit, as hypertension and proteinuria can be precisely reduced with angiotensin-converting enzyme inhibitors and/or angiotensin receptor blockers. However, with this intervention, plasma renin activity remains high, and although primary endpoints may be controlled, elevated renin concentration can contribute to cardiovascular damage. Aliskiren, a direct renin inhibitor, is the first example of a novel class of antihypertensive drugs with potent antiproteinuric effects, which, alone or combined, can contribute to delaying the progression of kidney disease.

Structural analysis of the intracellular domain of (pro)renin receptor fused to maltose-binding protein

The (pro)renin receptor (PRR) is an important component of the renin-angiotensin system (RAS), which regulates blood pressure and cardiovascular function. The integral membrane protein PRR contains a large extracellular domain (∼310 amino acids), a single transmembrane domain (∼20 amino acids) and an intracellular domain (∼19 amino acids). Although short, the intracellular (IC) domain of the PRR has functionally important roles in a number of signal transduction pathways activated by (pro)renin binding. Meanwhile, together with the transmembrane domain and a small portion of the extracellular domain (∼30 amino acids), the IC domain is also involved in assembly of V(0) portion of the vacuolar proton-translocating ATPase (V-ATPase). To better understand structural and multifunctional roles of the PRR-IC, we report the crystal structure of the PRR-IC domain as maltose-binding protein (MBP) fusion proteins at 2.0Å (maltose-free) and 2.15Å (maltose-bound). In the two separate crystal forms having significantly different unit-cell dimensions and molecular packing, MBP-PRR-IC fusion protein was found to be a dimer, which is different with the natural monomer of native MBP. The PRR-IC domain appears as a relatively flexible loop and is responsible for the dimerization of MBP fusion protein. Residues in the PRR-IC domain, particularly two tyrosines, dominate the intermonomer interactions, suggesting a role for the PRR-IC domain in protein oligomerization.

(Pro)renin promotes fibrosis gene expression in HEK cells through a Nox4-dependent mechanism

The (pro)renin receptor (PRR) has recently been demonstrated to bind equally well renin and its precursor, prorenin, leading to a similar intracellular signaling independent of angiotensin II. In this study, we report that human embryonic kidney cells (HEK) exposed to renin or prorenin for 24 h in the presence of a blocking concentration of the angtiotensin-converting enzyme inhibitor perindoprilate increased superoxide anion production as measured by luminescence (lucigenin) and electron spin resonance spectroscopy (hydroxylamine radical transition). Also, both renin and prorenin increased Nox4 expression while Nox2, p47(phox), and p67(phox) remained unchanged. In an investigation of the effects of renin and prorenin on fibrosis genes, it appeared that both proteins stimulated transforming growth factor-β (TGF-β), fibronectin, and plasminogen activator inhibitor type 1 (PAI-1) expression and therefore participated to an overall switch toward a profibrotic state of the kidney cells. When the cells were transfected with a siRNA targeting the PRR, Nox4 expression was efficiently prevented as well as the increase in superoxide production, TGF-β, fibronectin, and PAI-1. Finally, we demonstrated that transfection of the cells with a Nox4-specific small interfering (si) RNA also prevented fibrosis gene expression following treatment with renin or prorenin. The results demonstrate that renin and prorenin, through their specific membrane receptor and independently of angiotensin II, promote fibrosis gene expression via a Nox4-dependent mechanism.

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.

Application of direct renin inhibition to chronic kidney disease

Purpose

Chronic kidney disease has serious implications with a high risk for progressive loss of renal function, increased cardiovascular events as well as a substantial financial burden. The renin-angiotensin-aldosterone system (RAAS) is activated in chronic kidney disease, especially in diabetes and hypertension, which are the leading causes of chronic kidney disease. Angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs) decrease the rate of progression of diabetic and non-diabetic nephropathy and are recommended therapy for chronic kidney disease.

Methods

Key clinical trials supporting the use of ACE inhibitors and ARBs in chronic kidney disease are discussed. Recent developments in our understanding of RAAS biology and the use of direct renin inhibition are reviewed in the context of their potential impact on the prevention and management of chronic kidney disease.

Results

Despite the clinical success of ACE inhibitors and ARBs the rates of mortality and progression to renal failure remain high in these patient populations. ACE inhibitor or ARB monotherapy, in doses commonly used in clinical practice does not result in complete suppression of the RAAS. Aliskiren, a direct renin inhibitor, offers a novel approach to inhibit the RAAS in chronic kidney disease.

Conclusions

High dose ARB therapy or combination therapies with ACE inhibitors and ARBs have shown beneficial effects on surrogate markers of chronic kidney disease. Early data based on urinary protein excretion rates as a surrogate marker for renal function suggest a possibly novel role for aliskiren alone or in combination with ARBs in chronic kidney disease.

Dual renin-angiotensin-aldosterone system blockade for diabetic kidney disease

Blockade of the renin-angiotensin-aldosterone system (RAAS) prevents the development and progression of diabetic kidney disease (DKD). It is controversial whether the simultaneous use of two RAAS inhibitors (ie, dual RAAS blockade) further improves renal outcomes. This review examines the scientific rationale and current clinical evidence addressing the use of dual RAAS blockade to prevent and treat DKD. It is concluded that dual RAAS blockade should not be routinely applied to patients with low or moderate risk of progressive kidney disease (normoalbuminuria or microalbuminuria with preserved glomerular filtration rate). For patients with high risk of progressive kidney disease (substantial albuminuria or impaired glomerular filtration rate), clinicians should carefully weigh the potential risks and benefits of dual RAAS blockade on an individual basis until ongoing clinical trials provide further insight.

New approaches to blockade of the renin-angiotensin-aldosterone system: characteristics and usefulness of the direct renin inhibitor aliskiren

Since renin inhibition interferes with the first and rate-limiting steps in the renin-angiotensin system, the renin step is a very attractive target for lowering blood pressure and minimizing target-organ damage. The newly developed direct renin inhibitor aliskiren has several attractive characteristics: it definitively reduces plasma renin activity among inhibitors of the renin-angiotensin system, is remarkably specific for human renin, exhibits a long half-life in plasma comparable to that of amlodipine, and has a high affinity for renal glomeruli and vasculature. Although these characteristics suggest the clinical usefulness and safety of aliskiren, several problems remain unsolved. Why does aliskiren have beneficial effects on the heart and kidneys of patients treated with angiotensin-converting enzyme (ACE) inhibitors and/or angiotensin II type 1-receptor blockers (ARBs)? Is the blood-pressure-lowering effect of aliskiren dependent on the plasma renin activity? Does aliskiren exert a possible adverse effect via (pro)renin receptor-dependent intracellular signals? Here, we review the characteristics and usefulness of aliskiren and discuss the current issues associated with this direct renin inhibitor.

Review: Therapeutic potential of plasminogen activator inhibitor-1 inhibitors

Plasminogen activator inhibitor-1 (PAI-1) is the major physiological inhibitor of fibrinolysis and regulates cell migration and fibrosis. Preclinical studies using genetically altered mice and biological or small molecule inhibitors have elucidated a role for PAI-1 in the pathogenesis of thrombosis, vascular remodeling, renal injury, and initiation of diabetes. Inhibition of PAI-1 is a potential therapeutic strategy in these diseases.

Expression of (pro)renin receptor in human kidneys with end-stage kidney disease due to diabetic nephropathy

(Pro)renin receptor ((P)RR), a specific receptor for renin and prorenin, is a 350 amino-acid protein with a single transmembrane domain and may play important pathophysiological roles in diabetic nephropathy. The aim of the present study is to clarify the expression of (P)RR in the kidney with end-stage renal disease due to diabetic nephropathy. The kidney tissues were obtained at autopsy from patients with and without Type 2 diabetes mellitus (n=5 without diabetes mellitus; and n=8 with diabetes mellitus). Immunocytochemistry showed that (P)RR was mainly expressed in the tubular cells and collecting duct cells of the kidney without diabetic nephropathy. Cells in glomeruli were very weakly and sporadically immunostained for (P)RR. Vascular smooth muscle cells and endothelial cells were very weakly or were not immunostained for (P)RR. Adipocytes in the adipose tissue around the kidney were positively immunostained for (P)RR. Immunostaining pattern of (P)RR in the kidney with diabetic nephropathy was similar to that without diabetic nephropathy. However, most notably, (P)RR immunostaining in the tubular cells and collecting duct cells was clearly and frequently more strongly observed in the kidney with diabetic nephropathy up to the end-stage renal disease. The present study has raised the possibility that (P)RR expressed in the diabetic kidney may play a pathophysiological role in angiotensin I generation and renal fibrosis found in end-stage renal disease.

Regulation of (pro)renin receptor expression by glucose-induced mitogen-activated protein kinase, nuclear factor-kappaB, and activator protein-1 signaling pathways

Renal (pro)renin receptor (PRR) expression is increased in diabetes. The exact mechanisms involved in this process are not well established. We hypothesized that high glucose up-regulates PRR through protein kinase C (PKC)-Raf-ERK and PKC-c-Jun N-terminal kinase (JNK)-c-Jun signaling pathways. Rat mesangial cells exposed to 30 mm d-glucose demonstrated significant increase in PRR mRNA and protein expression, intracellular phosphorylation of Raf-1 (Y340/341), ERK, JNK, nuclear factor-kappaB (NF-kappaB) p65 (S536) and c-Jun (S63). By chromatin immunoprecipitation assay and EMSA, high glucose induced more functional NF-kappaB and activator protein (AP)-1 dimers bound to corresponding cis-regulatory elements in the predicted PRR promoter to up-regulate PRR transcription. Conventional and novel PKC inhibitors Chelerythrine and Rottlerin, Raf-1 inhibitor GW5074, MEK1/2 inhibitor U0126, JNK inhibitor SP600125, NF-kappaB inhibitor Quinazoline, and AP-1 inhibitor Curcumin, respectively, attenuated glucose-induced PRR up-regulation. Chelerythrine and Rottlerin also inhibited glucose-induced phosphorylation of Raf-1 (Y340/341), ERK1/2, JNK, NF-kappaB p65 (S536), and c-Jun (S63). GW5074 and U0126 inhibited the phosphorylation of ERK1/2 and NF-kappaB p65 (S536). SP600125 inhibited phosphorylation of NF-kappaB p65 (S536) and c-Jun (S63). We conclude that high glucose up-regulates the expression of PRR through mechanisms dependent on both PKC-Raf-ERK and PKC-JNK-c-Jun signaling pathways. NF-kappaB and AP-1 are involved in high-glucose-induced PRR up-regulation in rat mesangial cells.

Renin inhibition for hypertension: selecting the right role for a new class of drug

With the arrival of a new class of drugs for the management of hypertension comes the need to define its role. Aliskiren, an orally administered direct renin inhibitor, has been approved by the US Food and Drug Administration for the treatment of hypertension. Currently, the recommendation for choice of agent in the treatment of uncomplicated hypertension is a thiazide diuretic, and for patients with diabetic nephropathy, heart failure, or coronary artery disease, an angiotensin-converting enzyme inhibitor. Patients for whom an angiotensin-converting enzyme inhibitor is indicated who are intolerant as a result of side effects should take an angiotensin receptor blocker. A new class of medicines that specifically inhibits renin is an exciting addition to the armamentarium in the treatment of hypertension. This article explores the role of aliskiren in treating hypertension as well as its side effects and appropriate dosing.

Recent advances in tissue (pro)renin imaging

Due to its pivotal role in blood pressure control and renal pathologies there is renewed interest in renin and its precursor prorenin. Also, the newly discovered (pro)renin receptor is a new element of the ever broadening renin-angiotensin system (RAS). The complexity of RAS including the recently recognized collecting duct site of (pro)renin (a term denoting both renin and prorenin) synthesis requires the use of advanced research techniques such as multiphoton fluorescence microscopy. With the help of this technology we have pioneered an imaging approach to directly visualize (pro)renin content, release and tissue activity in the living kidney. The use of this technology is reviewed here and exemplified by the direct visualization of (pro)renin activity in the collecting duct. New pharmacological tools, the renin inhibitor aliskiren and the handle region peptide (decoy peptide) was used to further characterize the intra-renal, collecting duct RAS.

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.

New pharmacological treatments for improving renal outcomes in diabetes

Diabetic nephropathy is the most common and most rapidly growing cause of end-stage renal failure in developed countries. Diabetic nephropathy results from complex interactions between genetic, metabolic and hemodynamic factors. Improvements in our understanding of the pathogenesis of fibrosis associated with diabetic kidney disease have led to the identification of several novel targets for the treatment of diabetic nephropathy. Albuminuria is a useful clinical marker of diabetic nephropathy, as it can be used to predict a decline in renal function. A reduction in albuminuria might not, however, be reflective of a protective effect of therapies focused on ameliorating renal fibrosis. Although new strategies for slowing down the progression of several types of renal disease have emerged, the challenge of arresting the relentless progression of diabetic nephropathy remains. In this Review, we discuss novel pharmacological approaches that aim to improve the renal outcomes of diabetic nephropathy, including the use of direct renin inhibitors and statins. We also discuss the promise of using antifibrotic agents to treat diabetic nephropathy. The need for novel biomarkers of diabetic nephropathy is also highlighted.

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

BACKGROUND

A direct renin inhibitor (DRI) had a benefit in decreasing albuminuria in type 2 diabetic patients having already been treated with angiotensin (Ang) II type 1 receptor blocker (ARB), suggesting that aliskiren may have another effect other than blockade of the traditional renin-angiotensin system (RAS). Recently, prorenin bound to (pro)renin receptor ((P)RR) was found and shown to evoke two pathways; the generation of Ang peptides and the receptor-dependent activation of extracellular signal-related protein kinase (ERK). Because (P)RR is present in the podocytes, a central component of the glomerular filtration barrier, we hypothesized that aliskiren influences the (P)RR-induced two pathways in human podocytes.

METHODS

Human podocytes were treated with 2 nmol/l prorenin in the presence and absence of an angiotensin-converting enzyme inhibitor (ACEi) imidaprilat, an ARB candesartan, a DRI aliskiren, or the siRNA knocking down the (P)RR mRNA and the intracellular AngII levels and the phosphorylation of ERK were determined.

RESULTS

The expression of (P)RR mRNA of human podocytes was unaffected by the treatment with RAS inhibitors, but decreased by 69% with the siRNA treatment. The basal levels of intracellular AngII and the prorenin-induced increase in intracellular AngII were significantly reduced by aliskiren and siRNA treatment, compared with imidaprilat and candesartan. The prorenin-induced ERK activation was reduced to control level by the siRNA treatment, but it was unaffected by imidaprilat, candesartan, or aliskiren.

CONCLUSIONS

Aliskiren is the most potent inhibitor of intracellular AngII levels of human podocytes among RAS inhibitors, although it is incapable of inhibiting the (P)RR-dependent ERK phosphorylation.

Novel neurohormonal insights with therapeutic potential in chronic heart failure

Despite considerable therapeutic advances over recent years, chronic heart failure remains associated with significant morbidity and mortality. Further improvements in the treatment of this syndrome are therefore needed and this will require advances in the understanding of its underlying pathophysiology. This article reviews the literature regarding recently identified neurohormonal pathways that are declaring themselves as potential therapeutic targets in chronic heart failure.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

New insights into the renoprotective actions of the renin inhibitor aliskiren in experimental renal disease

The renin-angiotensin-aldosterone system (RAAS) has a central function in the regulation of blood pressure. Aliskiren, the first direct renin inhibitor to be approved for the treatment of hypertension, blocks the RAAS at its point of activation. As renin inhibition acts at the top of the RAAS cascade, this mechanism has been proposed to offer advantages over existing modes of RAAS blockade. The RAAS is also considered to be a major factor in the pathogenesis of many renal diseases, especially diabetic nephropathy (DN), the main cause of end-stage renal disease. Existing therapies to block the RAAS slow the progression of DN, but they do not halt the disease. Therefore, more effective modes of interventions are needed. Studies to determine the efficacy of aliskiren in human renal disease are in progress. This review summarizes in vivo studies in which the efficacy of aliskiren was tested in experimental models of renal disease, and presents in vitro studies that provide insights into the possible mechanisms by which aliskiren confers renoprotection in animals. These works are discussed in the framework of the intrarenal RAAS and suggest that aliskiren may act by unique renoprotective mechanisms.