Antihypertensive therapy upregulates renin and (pro)renin receptor in the clipped kidney of Goldblatt hypertensive rats

Furin-mediated modification is required for epithelial sodium channel-activating activity of soluble (pro)renin receptor in cultured collecting duct cells

(Pro)renin receptor (PRR) contains an overlapping cleavage site for site-1 protease (S1P) and furin for the generation of soluble PRR (sPRR). Although S1P-mediated cleavage mediates the release of sPRR, the functional implication of furin-mediated cleavage is unclear. Here, we tested whether furin-mediated cleavage was required for the activity of sPRR in activating epithelial Na+ channel (ENaC) in cultured M-1 cells. M-1 cells were transfected with pcDNA3.4 containing full-length PRR with (Furin-site Mut) or without (WT) mutagenesis of the furin cleavage site. As compared with empty vector (EM) control, Furin-site Mut showed the attenuation effect on WT-induced α-ENaC expression and amiloride-sensitive short-circuit current. In a separate experiment, M-1 cells were transfected with pcDNA3.4 containing cDNA for sPRR with S1P cleavage (AA 1-282) (sPRR-S1P) or with furin cleavage (AA 1-279) (sPRR-furin), indicating overexpression of the two types of sPRR induced a significant and comparable increase in the release of sPRR, but only sPRR-furin showed an increase of ENaC activity. Single-channel analysis of ENaC activity in Xenopus A6-2F3 cells confirms sPRR-furin activation of ENaC open probability. At last, HEK-293 cells were pretreated with furin inhibitor α1-antitrypsin Portland (α1-PDX) followed by transfection with EM, WT PRR. sPRR in the conditioned medium was enriched by using protein centrifugal filter devices and applied to M-1 cells followed by measurement of ENaC activity, demonstrating that pretreatment with α1-PDX attenuated ENaC-acting activity induced by overexpression of WT PRR. In summary, we conclude that furin-mediated modification is required for the activity of sPRR to increase ENaC-mediated Na+ transport in the collecting duct cells.NEW & NOTEWORTHY The present study for the first time examined the functional implication of furin-dependent cleavage in the activation of sPRR during ENaC regulation in cultured CD cells. We found that sPRR with the initial S1P-dependent cleavage remained silent and only became active following furin-dependent cleavage in terms of enhancement of ENaC activity and expression of α-ENaC. These results offer novel insight into the sPRR maturation process during ENaC regulation.

(Pro)renin Receptor Blockade Prevents Increases in Systolic Blood Pressure, Sodium Retention, and αENaC Protein Expression in the Kidney of 2K1C Goldblatt Mice

Physiological control of blood pressure (BP) and extracellular fluid volume is mediated by the action of the renin-angiotensin system (RAS). The presence of RAS components throughout the nephron has been widely discussed. The (pro)renin receptor (PRR) is a member of the RAS widely expressed in the body of humans and rodents. In the kidney, PRR is expressed in mesangial cells, renal vasculature, and tubules of the proximal and distal nephron. Binding of the PRR to renin and prorenin promotes angiotensin (Ang) I-mediated sodium (Na+) reabsorption via the epithelial sodium channel (ENaC). The Goldblatt 2-kidney 1-clip (2K1C) is a model of experimental renovascular hypertension that displays activation of systemic and intrarenal RAS. We use the 2K1C hypertension mouse model for 7 days to evaluate the role of the PRR on renal αENaC expression, Na+ reabsorption, and BP using pharmacological systemic blockade of the PRR with PRO20 peptide. Mice undergoing or not to 2K1C surgery (0.13 mm clip internal gap) were chronically infused with PRO20 and compared to sham (control) mice to assess changes in systolic BP (SBP) and diastolic BP (DBP), intrarenal angiotensin-converting enzyme (ACE) activity, Ang II, and renal αENaC expression and natriuretic responses after a saline challenge. Renal artery obstruction increased SBP and DBP, intrarenal ACE activity, Ang II levels, Na+ retention, and αENaC expression in both kidneys. PRO20 prevented the increases in SBP, DBP, attenuated Na+ retention, and blunted intrarenal Ang II and αENaC levels in non-clipped kidneys of 2K1C mice. Chronic infusion of the PRR for 7 days prevents hypertensive responses in part due to impaired αENaC upregulation and intrarenal Ang II formation in the early phase of the development of renovascular hypertension in 2K1C Goldblatt mice.

Investigating the Interplay between Cardiovascular and Neurodegenerative Disease

Neurological diseases, including neurodegenerative diseases (NDDs), are the primary cause of disability worldwide and the second leading cause of death. The chronic nature of these conditions and the lack of disease-modifying therapies highlight the urgent need for developing effective therapies. To accomplish this, effective models of NDDs are required to increase our understanding of underlying pathophysiology and for evaluating treatment efficacy. Traditionally, models of NDDs have focused on the central nervous system (CNS). However, evidence points to a relationship between systemic factors and the development of NDDs. Cardiovascular disease and related risk factors have been shown to modify the cerebral vasculature and the risk of developing Alzheimer's disease. These findings, combined with reports of changes to vascular density and blood-brain barrier integrity in other NDDs, such as Huntington's disease and Parkinson's disease, suggest that cardiovascular health may be predictive of brain function. To evaluate this, we explore evidence for disruptions to the circulatory system in murine models of NDDs, evidence of disruptions to the CNS in cardiovascular disease models and summarize models combining cardiovascular disruption with models of NDDs. In this study, we aim to increase our understanding of cardiovascular disease and neurodegeneration interactions across multiple disease states and evaluate the utility of combining model systems.

Influence of carnosine and carnosinase-1 on diabetes-induced afferent arteriole vasodilation: implications for glomerular hemodynamics

BACKGROUND

Dysregulation in glomerular hemodynamics favors hyperfiltration in diabetic kidney disease (DKD). Although carnosine supplementation ameliorates features of DKD, its effect on glomerular vasoregulation is not known. We assessed the influence of carnosine and carnosinase-1 (CN1) on afferent glomerular arteriole vasodilation and its association with glomerular size, hypertrophy and nephrin expression in diabetic BTBR^ob/ob mice.

METHODS

Two cohorts of mice including appropriate controls were studied i.e., diabetic mice receiving oral carnosine supplementation (cohort 1) and human CN1 (hCN1) transgenic (TG) diabetic mice (cohort 2). Lumen area ratio (LAR) of the afferent arterioles and glomerular parameters were measured by conventional histology. Three-dimensional analysis using a tissue clearing strategy was also employed.

RESULTS

In both cohorts, LAR was significantly larger in diabetic BTBR^ob/ob vs non-diabetic BTBR^wt/ob mice (0.41±0.05 vs 0.26±0.07; p<0.0001) and (0.42±0.06 vs 0.29±0.04; p<0.0001), and associated with glomerular size (cohort 1: r =0.55, p=0.001; cohort 2: r=0.89, p<0.0001). LAR was partially normalized by oral carnosine supplementation (0.34±0.05 vs 0.41±0.05; p=0.004), but did not differ between hCN1 TG and wild type (WT) BTBR^ob/ob mice. In hCN1 TG mice, serum CN1 concentrations correlated with LAR (r=0.90; p=0.006). Diabetic mice displayed decreased nephrin expression and increased glomerular hypertrophy. This was not significantly different in hCN! TG BTBR^ob/ob mice (p=0,06 and p=0,08, respectively).

CONCLUSION

Carnosine and CN1 may affect intra-glomerular pressure in an opposing manner through regulation of afferent arteriolar tone. This study corroborates previous findings on the role of carnosine in the progression of DKD.

Mildly Increased Renin Expression in the Absence of Kidney Injury in the Murine Transverse Aortic Constriction Model

Cardiorenal syndrome type 2 is characterized by kidney failure as a consequence of heart failure that affects >50% of heart failure patients. Murine transverse aortic constriction (TAC) is a heart failure model, where pressure overload is induced on the heart without any systemic hypertension or its consequences. Whether renal function is altered in this model is debated, and if so, at which time post-TAC renal dysfunction starts to contribute to worsening of cardiac function. We therefore studied the effects of progressive heart failure development on kidney function in the absence of chronically elevated systemic blood pressure and renal perfusion pressure. C57BL/6J mice (N = 129) were exposed to TAC using a minimally invasive technique and followed from 3 to 70 days post-TAC. Cardiac function was determined with 3D ultrasound and showed a gradual decrease in stroke volume over time. Renal renin expression and plasma renin concentration increased with progressive heart failure, suggesting hypoperfusion of the kidney. In addition, plasma urea concentration, a surrogate marker for renal dysfunction, was increased post-TAC. However, no structural abnormalities in the kidney, nor albuminuria were present at any time-point post-TAC. Progressive heart failure is associated with increased renin expression, but only mildly affected renal function without inducing structural injury. In combination, these data suggest that heart failure alone does not contribute to kidney dysfunction in mice.

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.

Prorenin receptor mediates inflammation in renal ischemia

We hypothesized that PRR contributes to renal inflammation in the 2-kidney, 1-clip (2K1C) renal ischaemia model. Male Sprague-Dawley rats were fed normal sodium diet. Blood pressure (BP) was obtained on days 0 and 28 after left renal artery clipping that reduced renal blood flow by 40%. Renal expression of TNF-α, COX-2, NF-κB, IL-1β, MCP-1 and collagen type I were assessed in sham and 2K1C rats with or without left renal administration of scramble or PRR shRNA. At baseline, there were no differences in BP. Compared to sham, MAP significantly increased in clipped animals (sham 102 ± 1.9 vs 2K1C 131.8 ± 3.09 mmHg, P < .05) and was not influenced by scramble or PRR shRNA treatment. Compared to sham and contra lateral (non-clipped) kidney, there was upregulation in mRNA and protein expression of PRR (99% and 45%, P < .01), TNF-α (72% and 50%, P < .05), COX-2 (72% and 39%, P < .05), p-NF-κB (92%, P < .05), MCP-1 (87%, P < .05) and immunostaining of collagen type I in the clipped kidney. These increases were not influenced by scramble shRNA. Compared to 2K1C and scramble shRNA, PRR shRNA treatment in the clipped kidney significantly reduced the expression of PRR (62% and 57%, P < .01), TNF-α (51% and 50%, P < .05), COX-2 (50% and 56%, P < .05), p-NF-κB by 68% (P < .05), MCP-1 by 73% (P < .05) and collagen type I respectively. Ang II was increased in both kidneys and did not change in response to scramble or PRR shRNA treatments. We conclude that PRR mediates renal inflammation in renal ischaemia independent of blood pressure and Ang II.

Fibroblast growth factor 23 modifies the pharmacological effects of angiotensin receptor blockade in experimental renal fibrosis

Background

Blockade of the renin-angiotensin-aldosterone system (RAAS) retards progression of chronic kidney disease. Yet, in many patients, the renoprotective effect is incomplete. A high circulating level of the phosphaturic hormone fibroblast growth factor 23 is associated with an impaired response to RAAS blockade-based therapy in clinical studies. Therefore, we addressed whether administration of recombinant fibroblast growth factor 23 (FGF23) interferes with the efficacy of angiotensin receptor blocker (ARB) treatment in a mouse model of renal fibrosis [unilateral ureteral obstruction (UUO)].

Methods

UUO mice were treated with losartan (100 mg/L in drinking water), recombinant FGF23 (160 ng/kg i.p. twice daily), their combination or vehicle ( n = 10 per group). Seven days after the UUO procedure, kidney tissue was analyzed for markers of RAAS activity, inflammation and fibrosis using real-time PCR and immunohistochemistry.

Results

In the contralateral (non-affected) kidneys of ARB-treated UUO mice, administration of FGF23 reversed the induction of renin, ACE, ACE2 and AT1 receptor mRNA expression, suggesting interference with the physiological response to RAAS blockade by FGF23. Furthermore, recombinant FGF23 infusion prevented ARB-induced klotho upregulation in contralateral kidneys. In the UUO kidneys, klotho was majorly reduced in all groups. Pro-inflammatory gene expression (MCP-1, TNF-α) induced in UUO kidneys was reduced by ARB treatment; this anti-inflammatory effect was reversed by FGF23. In contrast, ARB-induced reduction of (pre-)fibrotic gene expression was not reversed by FGF23.

Conclusions

Our findings show pharmacological interaction between exogenous FGF23 and losartan, thus serving as a proof of principle for crosstalk between the FGF23-klotho axis and RAAS.

Site-1 protease is required for the generation of soluble (pro)renin receptor

The extracellular domain of the (pro)renin receptor [(P)RR] is cleaved to generate the soluble form of (P)RR [s(P)RR]. Multiple clinical studies have revealed the association between serum/plasma s(P)RR levels and certain diseases, thereby suggesting a potential role for s(P)RR as a disease biomarker. Here, we investigated whether site-1 protease (S1P) is responsible for cleaving (P)RR to generate s(P)RR. Reduction of endogenous S1P with siRNA attenuated s(P)RR generation in Chinese hamster ovary (CHO) cells exogenously expressing human (P)RR with a C-terminal decahistidine tag [CHO/h(P)RR-10His cells]; conversely, overexpression of S1P by transient transfection increased s(P)RR generation. The S1P inhibitor PF429242 suppressed s(P)RR generation in CHO/h(P)RR-10His and human cervical carcinoma HeLa cells; however, the ADAM inhibitor GM6001 had no effect. The furin inhibitor Dec-RVKR-CMK had no effect on the amount of s(P)RR, but caused a slight increase in the size of the s(P)RR. Moreover, the reversible vesicle-trafficking inhibitor brefeldin A (BFA) enhanced the generation of large-sized s(P)RR; PF429242, but not Dec-RVKR-CMK, suppressed this BFA-induced s(P)RR formation. The size of s(P)RR generated during BFA treatment was reduced after removal of BFA; Dec-RVKR-CMK, but not PF429242, suppressed this conversion. Together, these results suggest that s(P)RR is generated by sequential processing by S1P and furin.

Pathophysiological mechanisms of calcineurin inhibitor-induced nephrotoxicity and arterial hypertension

Solid organ transplantation is an established treatment modality in patients with end-stage organ damage in cases where other therapeutic options fail. The long-term outcomes of solid organ transplant recipients have improved considerably since the introduction of the first calcineurin inhibitor (CNI) - cyclosporine. In 1984, the potent immunosuppressive properties of another CNI, tacrolimus, were discovered. The immunosuppressive effects of CNIs result from the inhibition of interleukin-2 synthesis and reduced proliferation of T cells due to calcineurin blockade. The considerable side effects that are associated with CNIs therapy include arterial hypertension and nephrotoxicity. The focus of this article was to review the available literature on the pathophysiological mechanisms of CNIs that induce chronic nephrotoxicity and arterial hypertension. CNIs lead to activation of the major vasoconstriction systems, such as the renin-angiotensin and endothelin systems, and increase sympathetic nerve activity. On the other hand, CNIs are known to inhibit NO synthesis and NO-mediated vasodilation and to increase free radical formation. Altogether, these processes cause endothelial dysfunction and contribute to the impairment of organ function. A better insight into the mechanisms underlying CNI nephrotoxicity could assist in developing more targeted therapies of arterial hypertension or preventing CNI nephrotoxicity in organ transplant recipients, including heart transplantation.

Interaction between V-ATPase B2 and (Pro) renin Receptors in Promoting the progression of Renal Tubulointerstitial Fibrosis

To investigate the levels of (Pro) renin receptor [(P) RR], α-smooth muscle actin (α-SMA), fibronectin (FN), and vacuolar H(+)-ATPase (V-ATPase) subunits (B2, E, and c) in rat unilateral ureteral obstruction (UUO) models and rat proximal tubular epithelial cells (NRK-52E) treated with prorenin to elucidate the role of V-ATPase in these processes by activating the (P) RR. UUO significantly upregulated (P) RR, V-ATPase subunits, α-SMA and FN expression in tubulointerstitium or tubular epithelial cells. A marked colocalization of (P) RR and the B2 subunit was also observed. Prorenin treatment upregulated α-SMA, FN, (P) RR, and V-ATPase subunits and activity in NRK52E cell in a dose- and time-dependent manner. The V-ATPase inhibitor bafilomycin A1 partially blocked prorenin-induced (P) RR, FN, and α-SMA expression. Co-immunoprecipitate and immunofluorescence results demonstrated that the V-ATPase B2 subunit bound to the (P) RR, which was upregulated after prorenin stimulation. Either siRNA-mediated (P) RR or B2 subunit knockdown partially reduced V-ATPase activity and attenuated prorenin-induced FN and α-SMA expression. From the data we can assume that activation of (P) RR and V-ATPase may play an important role in tubulointerstitial fibrosis with possible involvement of interaction of V-ATPase B2 subunit and (P)RR.

Direct inhibition of plasmatic renin activity with aliskiren: a promising but under-investigated therapeutic option for non-diabetic glomerulonephritis

Non-diabetic glomerulonephritis is a frequent cause of end-stage renal disease. The use of renin-angiotensin-aldosterone system blockers is a fundamental therapeutic approach. However, converting enzyme inhibitors (ACE-is) and angiotensin receptor blockers do not always achieve the desired target of proteinuria. The induction of the prorenin and renin up-regulation is a possible explanation. Aliskiren is the first drug acting as direct inhibitor of plasmatic renin activity, also able to interfere with the prorenin and renin profibrotic escape. We aimed at reviewing the literature for the assessment of potential efficacy and safety of aliskiren in the treatment of non-diabetic glomerulonephritis. The data on this topic are limited; however, we concluded for a possible usefulness of aliskiren. The renal safety profile appears potentially acceptable in non-diabetic patients although extreme carefulness, particularly with respect to long-term renal and cardiovascular tolerability, is recommended.

Renin and the (pro)renin receptor in the renal collecting duct: Role in the pathogenesis of hypertension

The intrarenal renin-angiotensin system (RAS) plays a critical role in the pathogenesis and progression of hypertension and kidney disease. In angiotensin (Ang) II-dependent hypertension, collecting duct renin synthesis and secretion are stimulated despite suppression of juxtaglomerular (JG) renin. This effect is mediated by the AngII type I receptor (AT1 R), independent of blood pressure. Although the regulation of JG renin has been extensively studied, the mechanisms by which renin is regulated in the collecting duct remain unclear. The augmentation of renin synthesis and activity in the collecting duct may provide a pathway for additional generation of intrarenal and intratubular AngII formation due to the presence of angiotensinogen substrate and angiotensin-converting enzyme in the nephron. The recently described (pro)renin receptor ((P)RR) binds renin or prorenin, enhancing renin activity and fully activating the biologically inactive prorenin peptide. Stimulation of (P)RR also activates intracellular pathways related to fibrosis. Renin and the (P)RR are augmented in renal tissues of AngII-dependent hypertensive rats. However, the functional contribution of the (P)RR to enhanced renin activity in the collecting duct and its contribution to the development of hypertension and kidney disease have not been well elucidated. This review focuses on recent evidence demonstrating the mechanism of renin regulation in the collecting ducts and its interaction with the (P)RR. The data suggest that renin-(P)RR interactions may induce stimulation of intracellular pathways associated with the development of hypertension and kidney disease.

Regulation of nonclassical renin-angiotensin system receptor gene expression in the adrenal medulla by acute and repeated immobilization stress

The involvement of the nonclassical renin-angiotensin system (RAS) in the adrenomedullary response to stress is unclear. Therefore, we examined basal and immobilization stress (IMO)-triggered changes in gene expression of the classical and nonclassical RAS receptors in the rat adrenal medulla, specifically the angiotensin II type 2 (AT2) and type 4 (AT4) receptors, (pro)renin receptor [(P)RR], and Mas receptor (MasR). All RAS receptors were identified, with AT2 receptor mRNA levels being the most abundant, followed by the (P)RR, AT1A receptor, AT4 receptor, and MasR. Following a single IMO, AT2 and AT4 receptor mRNA levels decreased by 90 and 50%, respectively. Their mRNA levels were also transiently decreased by repeated IMO. MasR mRNA levels displayed a 75% transient decrease as well. Conversely, (P)RR mRNA levels were increased by 50% following single or repeated IMO. Because of its abundance, the function of the (P)RR was explored in PC-12 cells. Prorenin activation of the (P)RR increased phosphorylation of extracellular signal-regulated kinase 1/2 and tyrosine hydroxylase at Ser31, likely increasing its enzymatic activity and catecholamine biosynthesis. Together, the broad and dynamic changes in gene expression of the nonclassical RAS receptors implicate their role in the intricate response of the adrenomedullary catecholaminergic system to stress.

Characterization of the intrarenal renin-angiotensin system in experimental alport syndrome

Blockade of the renin-angiotensin system attenuates the progression of experimental and clinical Alport syndrome (AS); however, the underlying mechanism(s) remains largely unknown. We evaluated the renin-angiotensin system in 4- and 7-week-old homozygous for collagen, type IV, α3 gene (Col4A3(-/-)) and wild-type mice, a model of AS characterized by proteinuria and progressive renal injury. Renal angiotensin (Ang) II levels increased, whereas renal Ang-(1-7) levels decreased in 7-week-old Col4a3(-/-) mice compared with age-matched controls; these changes were partially reversed by recombinant angiotensin-converting enzyme 2 (ACE2) treatment. The expression of both the angiotensinogen and renin protein increased in Col4a3(-/-) compared with wild-type mice. Consistent with the Ang-(1-7) levels, the expression and activity of kidney ACE2 decreased in 7-week-old Col4a3(-/-) mice. The urinary excretion rate of ACE2 paralleled the decline in tissue expression. Expression of an Ang II-induced gene, heme oxygenase-1, was up-regulated in the kidneys of 7-week-old Col4a3(-/-) mice compared with wild-type mice by microarray analysis. Heme oxygenase-1 (HO-1) protein expression was increased in kidneys of Col4a3(-/-) mice and normalized by treatment with ACE inhibitor. Urinary HO-1 excretion paralleled renal HO-1 expression. In conclusion, progressive kidney injury in AS is associated with changes in expression of intrarenal renin Ang system components and Ang peptides. HO-1 and ACE2 may represent novel markers of AS-associated kidney injury, whereas administration of recombinant ACE2 and/or Ang-(1-7) may represent novel therapeutic approaches in AS.

Increased expression of (pro)renin receptor does not cause hypertension or cardiac and renal fibrosis in mice

Binding of renin and prorenin to the (pro)renin receptor (PRR) increases their enzymatic activity and upregulates the expression of pro-fibrotic genes in vitro. Expression of PRR is increased in the heart and kidney of hypertensive and diabetic animals, but its causative role in organ damage is still unclear. To determine whether increased expression of PRR is sufficient to induce cardiac or renal injury, we generated a mouse that constitutively overexpresses PRR by knocking-in the Atp6ap2/PRR gene in the hprt locus under the control of a CMV immediate early enhancer/chicken beta-actin promoter. Mice were backcrossed in the C57Bl/6 and FVB/N strain and studied at the age of 12 months. In spite of a 25- to 80-fold renal and up to 400-fold cardiac increase in Atp6ap2/PRR expression, we found no differences in systolic blood pressure or albuminuria between wild-type and PRR overexpressing littermates. Histological examination did not show any renal or cardiac fibrosis in mutant mice. This was supported by real-time PCR analysis of inflammatory markers as well as of pro-fibrotic genes in the kidney and collagen in cardiac tissue. To determine whether the concomitant increase of renin would trigger fibrosis, we treated PRR overexpressing mice with the angiotensin receptor-1 blocker losartan over a period of 6 weeks. Renin expression increased eightfold in the kidney but no renal injury could be detected. In conclusion, our results suggest no major role for PRR in organ damage per se or related to its function as a receptor of renin.

Aliskiren accumulation in the kidney: no major role for binding to renin or prorenin

BACKGROUND AND OBJECTIVE

The antihypertensive effects of the direct renin inhibitor aliskiren last substantially longer after treatment withdrawal than expected based upon its plasma half-life. This may be attributable to drug accumulation in the kidney as recently shown in rats and mice. Since aliskiren binds to renin we examined in the present study whether this accumulation depends on the renin content of the kidney.

METHODS

For this we measured the aliskiren concentration in the kidney of wild-type as well as AT1a receptor(-/-) and Ren1c(-/-) mice. AT1a receptor(-/-) mice overexpress renin due to the lack of angiotensin II-mediated negative feedback, whereas Ren1c(-/-) mice lack renal renin expression.

RESULTS

Accumulation of aliskiren was found in the kidney of wild-type mice. However, renal accumulation was neither influenced by the overexpression nor by the absence of renin in the kidney. It was recently shown that the effects of aliskiren can be blocked by a handle region peptide, which inhibits the nonproteolytic activation of prorenin bound to the (pro)renin receptor. To investigate whether this putative (pro)renin receptor blocker influences renal aliskiren accumulation, we administered the blocker in addition to aliskiren. No influence on renal aliskiren accumulation was observed.

CONCLUSION

These data confirm accumulation of aliskiren in the murine kidney and demonstrate that neither renin nor (pro)renin receptor-bound prorenin are major players in this process.

The (pro)renin receptor blocker handle region peptide upregulates endothelium-derived contractile factors in aliskiren-treated diabetic transgenic (mREN2)27 rats

BACKGROUND

Elevated prorenin levels associate with microvascular complications in patients with diabetes mellitus, possibly because prorenin affects vascular function in diabetes mellitus, for example by generating angiotensins following its binding to the (pro)renin receptor [(P)RR]. Here we evaluated whether the renin inhibitor aliskiren, with or without the putative (P)RR antagonist handle region peptide (HRP) improved the disturbed vascular function in diabetic TGR(mREN2)27 rats, a high-prorenin, high-(P)RR hypertensive model.

METHODS

Telemetry transmitters were implanted to monitor blood pressure. After 3 weeks of treatment, rats were sacrificed, and iliac and mesenteric arteries were removed to evaluate vascular reactivity.

RESULTS

Diabetes mellitus enhanced the contractile response to nitric oxide synthase (NOS) blockade, potentiated the response to phenylephrine, diminished the effectiveness of endothelin type A (ETA) receptor blockade and allowed acetylcholine to display constrictor, cyclo-oxygenase-2 mediated, endothelium-dependent responses in the presence of NOS inhibition and blockers of endothelium-derived hyperpolarizing factors. Aliskiren normalized blood pressure, suppressed renin activity, and reversed the above vascular effects, with the exception of the altered effectiveness of ETA receptor blockade. Remarkably, when adding HRP on top of aliskiren, its beneficial vascular effects either disappeared or were greatly diminished, although HRP did not alter the effect of aliskiren on blood pressure and renin activity.

CONCLUSIONS

Renin inhibition improves vascular dysfunction in diabetic hypertensive rats, and HRP counteracts this effect independently of blood pressure and angiotensin. (P)RR blockade therefore is unlikely to be a new tool to further suppress the renin-angiotensin system (RAS) on top of existing RAS blockers.

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.

Dimethylarginine dimethylaminohydrolase1 is an organ-specific mediator of end organ damage in a murine model of hypertension

BACKGROUND

The endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) is an independent predictor of cardiovascular and overall mortality. Moreover, elevated ADMA plasma concentrations are associated with the extent of hypertension. However, data from small-sized clinical trials and experimental approaches using murine transgenic models have revealed conflicting results regarding the impact of ADMA and its metabolizing enzyme dimethylarginine dimethylaminohydrolase (DDAH) in the pathogenesis of hypertension.

METHODOLOGY/PRINCIPAL FINDINGS

Therefore, we investigated the role of ADMA and DDAH1 in hypertension-induced end organ damage using the uninephrectomized, deoxycorticosterone actetate salt, and angiotensin II-induced hypertension model in human DDAH1 (hDDAH1) overexpressing and wild-type (WT) mice. ADMA plasma concentrations differed significantly between hDDAH1 and WT mice at baseline, but did not significantly change during the induction of hypertension. hDDAH1 overexpression did not protect against hypertension-induced cardiac fibrosis and hypertrophy. In addition, the hypertension-induced impairment of the endothelium-dependent vasorelaxation of aortic segments ex vivo was not significantly attenuated by hDDAH1 overexpression. However, hDDAH1 mice displayed an attenuated hypertensive inflammatory response in renal tissue, resulting in less hypertensive renal injury.

CONCLUSION/SIGNIFICANCE

Our data reveal that hDDAH1 organ-specifically modulates the inflammatory response in this murine model of hypertension. The lack of protection in cardiac and aortic tissues may be due to DDAH1 tissue selectivity and/or the extent of hypertension by the used combined model. However, our study underlines the potency of hDDAH1 overexpression in modulating inflammatory processes as a crucial step in the pathogenesis of hypertension, which needs further experimental and clinical investigation.

Regulation of the (pro)renin-renin receptor in cardiac remodelling

The (pro)renin-renin receptor [(P)RR] was discovered as an important novel component of the renin-angiotensin system (RAS). The functional significance of (P)RR is widely studied in renal and vascular pathologies and has sparked interest for a potential role in cardiovascular disease. To investigate the role of (P)RR in cardiac pathophysiology, we aimed to assess (P)RR regulation in adverse cardiac remodelling of the failing heart. In particular, we evaluated the expression of (P)RR in different models of heart failure and across different species. Significantly increased levels of (P)RR mRNA were found in post-myocardial infarcted (MI) hearts of rats (1.6-fold, P < 0.05) and mice (5-fold, P < 0.01), as well as in transgenic rats with overexpression of the mouse renin gene (Ren2) (2.2-fold, P < 0.01). Moreover, we observed a strong increase of (P)RR expression in hearts of dilated cardiomyopathy (DCM) patients (5.3-fold, P < 0.001). Because none of the tested commercially available antibodies appeared to detect endogenous (P)RR, a (P)RR-specific polyclonal antibody was generated to study (P)RR protein levels. (P)RR protein levels were significantly increased in the post-MI rat heart (1.4-fold, P < 0.05) as compared to controls. Most interestingly in DCM patients, a significant 8.7-fold (P < 0.05) increase was observed. Thus, protein expression paralleled gene expression. These results demonstrate that (P)RR expression is strongly up-regulated both in rodent models of heart failure and in the failing human heart, hinting to a potential role for (P)RR in cardiac pathophysiology.

Critical roles of (pro)renin receptor-bound prorenin in diabetes and hypertension: sallies into therapeutic approach

High plasma prorenin levels in diabetic patients predict microvascular complications, but the mechanism of the connection between them has remained unclear. (Pro)renin receptors were recently found in the human kidney, and their distribution in various organs, including the heart, has been identified. Binding of prorenin to the (pro)renin receptor triggers two major pathways: the angiotensin II-dependent pathway as a result of conversion of prorenin to the active form of prorenin by a conformational change, and the angiotensin II-independent intracellular pathway via the (pro)renin receptor. To investigate whether the (pro)renin-receptor-dependent pathways contribute to the pathophysiology of the end-organ damage that occurs in diabetes and hypertension, a (pro)renin receptor blocker (PRRB), which binds to the receptor and competitively inhibits prorenin binding to the receptor, was administered to rats with streptozotocin-induced diabetes and to stroke-prone spontaneously hypertensive rats. PRRB significantly inhibited the development and progression of end-organ damage in these animal models of diabetes and hypertension, and it was of greater benefit than conventional inhibitors in relation to the renin-angiotensin system in diabetic angiotensin II-type 1a-receptor-deficient mice. The (pro)renin receptor may prove useful as an important therapeutic target for the prevention and regression of end-organ damage in diabetes and hypertension.

Collecting Duct Renin: A major player in Angiotensin II-dependent Hypertension

Recently, the focus of interest on the role of the renin angiotensin system in the pathophysiology of hypertension has shifted towards greater emphasis on new developments in local renin angiotensin systems in specific tissues. We have focused our recent investigations on the role of the intrarenal-intratubular RAS in hypertension. All of the components needed for angiotensin II generation are present within the various compartments in the kidney. This brief review is focused on recent evidence that inappropriate activation of renin in distal nephron segments, by acting on angiotensinogen generated in the proximal tubule cells and delivered to the distal nephron may contribute to increased distal intrarenal angiotensin II formation, sodium retention and development and progression of hypertension.

Evolving concepts on regulation and function of renin in distal nephron

Sustained stimulation of the intrarenal/intratubular renin-angiotensin system in a setting of elevated arterial pressure elicits renal vasoconstriction, increased sodium reabsorption, proliferation, fibrosis, and eventual renal injury. Activation of luminal AT(1) receptors in proximal and distal nephron segments by local Ang II formation stimulates various transport systems. Augmented angiotensinogen (AGT) production by proximal tubule cells increases AGT secretion contributing to increased proximal Ang II levels and leading to spillover of AGT into the distal nephron segments, as reflected by increased urinary AGT excretion. The increased distal delivery of AGT provides substrate for renin, which is expressed in principal cells of the collecting tubule and collecting ducts, and is also stimulated by AT(1) receptor activation. Renin and prorenin are secreted into the tubular lumen and act on the AGT delivered from the proximal tubule to form more Ang I. The catalytic actions of renin and or prorenin may be enhanced by binding to prorenin receptors on the intercalated cells or soluble prorenin receptor secreted into the tubular fluid. There is also increased luminal angiotensin converting enzyme in collecting ducts facilitating Ang II formation leading to stimulation of sodium reabsorption via sodium channel and sodium/chloride co-transporter. Thus, increased collecting duct renin contributes to Ang II-dependent hypertension by augmenting distal nephron intratubular Ang II formation leading to sustained stimulation of sodium reabsorption and progression of hypertension.

AT1 antagonism and renin inhibition in mice: pivotal role of targeting angiotensin II in chronic kidney disease

The role of the renin-angiotensin system in chronic kidney disease involves multiple peptides and receptors. Exerting antipodal pathophysiological mechanisms, renin inhibition and AT(1) antagonism ameliorate renal damage. However, it is unclear which mechanism exerts better nephroprotection. We compared the renin inhibitor aliskiren with the AT(1) antagonist losartan in mice with chronic kidney disease due to renal ablation. Doses were adjusted to equipotent inhibition of the renin-angiotensin system, determined via a dose-response quantifying plasma and renal renin expression. Six-week treatment with either 500 mg/l drinking water losartan or 50 mg·kg(-1)·day(-1) aliskiren significantly decreased albuminuria, glomerular damage, and transcription rates of renal injury markers to a similar extent. An array analysis comparing renal gene expression of losartan- and aliskiren-treated mice evaluating >34,000 transcripts demonstrated regulation for 14 genes only, with small differences. No superior nephroprotection was found by combining losartan and aliskiren. Compared with plasma concentrations, aliskiren accumulated ∼7- to 29-fold in the heart, liver, lung, and spleen and ∼156-fold in the kidney. After withdrawal, plasma concentrations dropped to zero within 24 h, whereas renal tissue concentrations declined slowly over days. Withdrawal of aliskiren in mice with chronic kidney disease revealed a significantly delayed re-increase in albuminuria compared with withdrawal of losartan. This study demonstrates equieffective nephroprotection of renin inhibition and AT(1) antagonism in mice with chronic kidney disease without additional benefit of combination therapy. These observations underscore the pivotal role of targeting ANG II to reduce renal injury.

Inhibition of renin and the (pro)renin receptor system

Renin is the rate-limiting step of the renin-angiotensin system (RAS) and can induce hypertension and cardiovascular diseases (CVDs) through the over-activated renin-angiotensin-converting enzyme (ACE)-angiotensin (Ang) II-Ang II type 1 receptor (AT(1)R) axis. Prorenin and renin bound to the (pro)renin receptor [(P)RR] not only increase the catalytic conversion of angiotensinogen (AGT) to Ang I, but also upregulate the expression of profibrotic genes. This review will discuss the inhibition of renin and the (P)RR system pharmacologically and nutritionally.

The (pro)renin receptor. A decade of research: what have we learned?

The discovery of a (pro)renin receptor ((P)RR) in 2002 provided a long-sought explanation for tissue renin–angiotensin system (RAS) activity and a function for circulating prorenin, the inactive precursor of renin, in end-organ damage. Binding of renin and prorenin (referred to as (pro)renin) to the (P)RR increases angiotensin I formation and induces intracellular signalling, resulting in the production of profibrotic factors. However, the (pro)renin concentrations required for intracellular signalling in vitro are several orders of magnitude above (patho)physiological plasma levels. Moreover, the phenotype of prorenin-overexpressing animals could be completely attributed to angiotensin generation, possibly even without the need for a receptor. The efficacy of the only available putative (pro)renin receptor blocker handle region peptide remains doubtful, leading to inconclusive results. The fact that, in contrast to other RAS components, (P)RR knock-outs, even tissue-specific, are lethal, points to an important, (pro)renin-independent, function of the (P)RR. Indeed, recent research has highlighted ancillary functions of the (P)RR as an essential accessory protein of the vacuolar-type H⁺-ATPase (V-ATPase), and in this role, it acts as an intermediate in Wnt signalling independent of (pro)renin. In conclusion, (pro)renin-dependent signalling is unlikely in non-(pro)renin synthesizing organs, and the (P)RR role in V-ATPase integrity and Wnt signalling may explain some, if not all of the phenotypes previously associated with (pro)renin-(P)RR interaction.

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.

Aliskiren reduced renal fibrosis in mice with chronic ischemic kidney injury--beyond the direct renin inhibition

Chronic renal ischemia leads to renal fibrosis and atrophy. Activation of the renin-angiotensin-aldosterone system is one of the main mechanisms driving chronic renal ischemic injury. The aim of the present study was to define the effect of aliskiren in chronic ischemia of the kidney. Two-kidney, one-clip mice were used to study chronic renal ischemia. Aliskiren significantly lowered the blood pressure in mice with renal artery constriction (92.1±1.1 vs. 81.0±1.8 mm Hg, P<0.05). Renin expression was significantly increased in ischemic kidneys when treated with aliskiren. In addition, (Pro)renin receptor expression was decreased by aliskiren in ischemic kidneys. Aliskiren treatment significantly increased klotho expression and reduced the expression of fibrogenic cystokines, caspase-3 and Bax in ischemic kidneys. Histological examination revealed that aliskiren significantly reduced the nephrosclerosis score (4.5±1.9 vs. 7.3±0.4, P<0.05). Immunofluorescence staining also showed that aliskiren decreased the deposition of interstitial collagen I in ischemic kidneys. In conclusion, direct renin inhibition significantly reduced renal fibrosis and apoptosis following chronic renal ischemia.

Molecular evidence of a (pro)renin/ (pro)renin receptor system in human intrauterine tissues in pregnancy and its association with PGHS-2

Prorenin stimulates decidual prostaglandin (PG) production in vitro, the (pro)renin receptor ((P)RR) may mediate this action. The role of prorenin in amnion PG synthesis has not been examined, despite this being the key site of PG synthesis. To determine if (P)RR, prorenin and PGHS-2 are co-localized in gestational tissues and if expression is altered by labour, term amnion, chorion, decidua and placenta were collected during elective caesarean section or after spontaneous labour. Prorenin, (P)RR and PGHS-2 mRNA abundance was determined by real-time RT-PCR. (P)RR protein was examined by immunohistochemistry. The effect of recombinant human (rh) prorenin on PGHS-2 mRNA abundance in amnion explants was determined. Prorenin and (P)RR mRNA were highest in decidua and placenta, respectively. Decidual prorenin, (P)RR and placental (P)RR mRNA abundance decreased with labour. (P)RR protein was present in all gestational tissues. After labour, decidual prorenin was positively correlated with amnion PGHS-2 mRNA and rh-prorenin significantly increased PGHS-2 mRNA abundance in amnion explants. We conclude that the decidua is the principal source of prorenin and is downregulated with labour. All gestational tissues are targets for prorenin. Decidual prorenin may be involved in the labour-associated increase in amnion PGHS-2 abundance via the (P)RR.

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.

Increased expression of (pro)renin receptor in the remnant kidneys of 5/6 nephrectomized rats

Recent studies have revealed that (pro)renin receptor ((P)RR), a newly identified member of the renin-angiotensin system, is associated with renal organ damage. However, there is little information regarding the regulation of (P)RR expression in various pathophysiological conditions. We therefore examined the expression of (P)RR in the remnant kidneys of rats with renal mass ablation due to 5/6 nephrectomy by quantitative RT-PCR, Western blot analysis and immunohistochemistry. Expression levels of (P)RR mRNA were significantly increased in the remnant kidneys at day 56 after nephrectomy, when compared with sham operation (about 1.6-fold, P=0.001). Western blot analysis showed that expression levels of (P)RR protein were greatly increased in the remnant kidneys at day 56, compared with sham operation (about 7.9-fold, P=0.02). The renal tubular cells were immunostained with anti-(P)RR antibody in both 5/6 nephrectomized rats and sham operated rats. The glomeruli were sporadically immunostained in 5/6 nephrectomized rats, but not in sham operated rats. These findings indicate that the intra-renal (P)RR expression is increased in the remnant kidneys of 5/6 nephrectomized rats, and suggest that (P)RR may contribute to the renal injury.

[The prorenin receptor]

The renin-angiotensin system (RAS) is one of the most important systems in physiology and in pathology. The (pro)renin receptor [(P)RR] is a new component of the system that has attracted much attention, being potentially a new therapeutic target, because the binding of renin and of prorenin triggers the activation of the mitogen-activated protein kinase p42/p44 followed by up-regulation of the expression of profibrotic genes. and because prorenin bound to (P)RR becomes catalytically active. The introduction of a renin inhibitor in the treatment of hypertension and of organ damages, together with the discovery of (P)RR, has revived the interest for the RAS and for potential new RAS blockers, in order to optimize RAS blockade in tissues.

Telomere length and outcome in heart failure

BACKGROUND

Telomeres are causally involved in senescence. Senescence is a potential factor in the pathogenesis and progression of heart failure. In heart failure telomeres are shorter, but the prognostic value associated with telomere length has not been defined.

METHODS

Telomere length was prospectively determined by quantitative polymerase chain reaction in 890 patients with New York Heart Association (NYHA) functional class II to IV heart failure. After 18 months, we examined the association between telomere length and the predefined primary end-point: time to death or hospitalization for heart failure.

RESULTS

Mean age of the patients was 71 years, 39% were women, 51% were in NYHA class II, and 49% were in class III/IV. A total of 344 patients reached the primary end-point (130 deaths and 214 hospitalizations). Patients with shorter telomeres were at an increased risk of reaching the primary end-point (hazard ratio 1.79; 95% confidence interval (CI) 1.21-2.63). In multivariate analysis shorter telomere length remained associated with a higher risk for death or hospitalization (hazard ratio, 1.74; 95% CI 1.07-2.95) after adjustment for age of heart failure onset, gender, hemoglobin, renal function, and N-terminal pro-B-type natriuretic peptide level, a history of stroke, atrial fibrillation, and diabetes.

CONCLUSIONS

Shorter length of telomeres predicts the occurrence of death or hospitalization in patients with chronic heart failure.

Possible roles of human (pro)renin receptor suggested by recent clinical and experimental findings

Numerous in vitro and in vivo animal studies using the (pro)renin receptor (P)RR blocker handle region peptide have suggested an important role of (P)RR in the pathogenesis of end-stage organ damage in patients with diabetes and hypertension. In addition, a limited number of clinical studies have suggested an association between (P)RR gene polymorphisms and blood pressure levels and between (P)RR mRNA levels and angiotensin-converting enzyme mRNA levels in human arteries. However, recent studies have shown that the (P)RR is divided into its soluble form and a residual hydrophobic part, which includes ATPase 6 associated protein 2, within cells. Therefore, the (P)RR may have a more complex function than previously thought. In addition, the physiological roles of the (P)RR remain undetermined, because the construction of (P)RR null mice has not been successful. As a next step for research in this area, a method for determining the soluble (P)RR levels in plasma and urine and the construction of tissue-specific (P)RR-knockout mice are needed to elucidate the roles of the (P)RR in physiology and pathophysiology.

Signal transduction of the (pro)renin receptor as a novel therapeutic target for preventing end-organ damage

The (pro)renin receptor ((P)RR) not only represents a novel component of the renin-angiotensin system but is also a promising novel drug target because of its crucial involvement in the pathogenesis of renal and cardiac end-organ damage. This review discusses the signal transduction of the (P)RR with its adapter protein promyelocytic zinc-finger protein, the impact of this receptor, especially on cardiovascular disease, and its putative interaction with renin inhibitors such as aliskiren. Furthermore, the increasing complexity regarding the cellular function of the (P)RR is addressed, which arises by the intimate link with proton pumps and the phosphatase PRL-1, as well as by the presence of different subcellular localizations and of a soluble isoform of the (P)RR. Finally, the rationale and strategy for the development of small-molecule antagonists of the (P)RR, called renin/prorenin receptor blockers, are presented.

The biology of the (pro)renin receptor

The (pro)renin receptor (PRR) binds renin and prorenin, its proenzyme inactive form. Receptor-bound prorenin becomes enzymatically active and binding then activates the MAP kinases ERK1/2 and p38 pathways, leading to upregulation of profibrotic and cyclooxygenase-2 genes independent of angiotensin II generation. These characteristics explain the interest in the potential role of PRR in organ damage in diseases associated with activation of the renin-angiotensin system (RAS), in particular hypertension and diabetes. Although identification of PRR has improved our understanding of the physiology of the tissue RAS, its role in pathology is far from clear. Transgenic animals overexpressing PRR ubiquitously or selectively in smooth-muscle cells develop high BP or glomerulosclerosis, and increased expression of PRR is reported in models of hypertension or kidney damage. However, definitive proof is still lacking for a role for PRR in disease, or by showing improvement of disease by tissue-specific ablation of PRR or by administration of a specific PRR antagonist. Furthermore, the early embryonic lethality seen in PRR-null mice suggests PRR has additional essential cellular functions we do not understand.

The increase in renin during renin inhibition: does it result in harmful effects by the (pro)renin receptor?

Renin inhibitors, similar to all renin-angiotensin system (RAS) blockers, increase the plasma concentration of renin because they attenuate the negative feedback effect of angiotensin (Ang) II on renin release. The increase in renin has been suggested to be higher than that during other types of RAS blockade. This could potentially limit the effectiveness of renin inhibition, either because Ang II generation might occur again ('Ang II escape'), possibly even at the levels above baseline, as has been described before for angiotensin-converting enzyme inhibitors, or because high levels of renin will stimulate the recently discovered (pro)renin receptor, and thus induce effects in an Ang-independent manner. This review shows first that the cause(s) of the renin increase during treatment with the renin inhibitor aliskiren is the consequence of a combination of factors, including an assay artifact, allowing the detection of prorenin as renin, and a change in renin half-life. When correcting for these phenomena the increase is unlikely to be as excessive as originally thought. The review then critically describes the consequence(s) of such a increase, concluding (i) that an Ang II escape is highly unlikely, given the [aliskiren]/[renin] stoichiometry, and (ii) that renin and prorenin downregulate their receptor (similar to many agonists). On the basis of the latter, one could even speculate that this will be more substantial when the renin and prorenin levels are higher. Thus, from this point of view the larger increase in renin during renin inhibition will cause a stronger reduction in (pro)renin receptor expression, and a greater suppression of (pro)renin receptor-mediated effects than other renin-Ang blockers.

Prorenin contributes to angiotensin peptide formation in transgenic rats with rat prorenin expression targeted to the liver

We reported previously that targeted expression of rat prorenin to the liver under the control of the human alpha1-antitrypsin promoter increased plasma prorenin levels by several-hundred-fold in male transgenic rats and caused cardiac hypertrophy, severe renal lesions, and myocardial fibrosis by 20 weeks of age, despite normal blood pressure. We examined the evolution of the phenotype of male transgenic rats over 12 months and the effects of binephrectomy on the renin-angiotensin (Ang) system. Plasma prorenin levels were >1000-fold higher than in wild type littermates, whereas plasma and renal Ang II levels were no different from wild-type (WT) levels, and kidney renin levels were suppressed in transgenic rats. In contrast to our earlier report, transgenic rats had increased systolic blood pressure at 3 to 12 months of age, and only modest renal lesions and myocardial fibrosis were evident after 6 months of age. Binephrectomy reduced plasma renin activity and concentration and prorenin levels by 50% to 80% and Ang II levels by 90% in WT rats. By contrast, binephrectomy increased plasma renin activity and concentration and prorenin levels by 52.0-, 13.0-, and 5.8-fold, respectively, without change in Ang II levels in transgenic rats. We conclude that, in the animals studied in this report, elevated prorenin levels did not cause renal lesions or myocardial fibrosis during the first 6 months of age. Ang peptide formation consequent to the increased prorenin levels prevented reduction of Ang II levels after binephrectomy and was likely to have contributed to hypertension, cardiac hypertrophy, and suppression of kidney renin levels in these transgenic rats.

Diabetic complications: a role for the prorenin-(pro)renin receptor-TGF-beta1 axis?

Morbidity and mortality of diabetes mellitus are strongly associated with cardiovascular disease including nephropathy. A discordant tissue renin-angiotensin system (RAS) might be a mediator of the endothelial dysfunction leading to both micro- and macrovascular complications of diabetes. The elevated plasma levels of prorenin in diabetic subjects with microvascular complications might be part of this discordant RAS, especially since the plasma renin levels in diabetes are low. Prorenin, previously thought of as an inactive precursor of renin, is now known to bind to a (pro)renin receptor, thus activating locally angiotensin-dependent and -independent pathways. In particular, the stimulation of the transforming growth factor-beta (TGF-beta) system by prorenin could be an important contributor to diabetic disease complications. This review discusses the concept of the prorenin-(pro)renin receptor-TGF-beta(1) axis, concluding that interference with this pathway might be a next logical step in the search for new therapeutic regimens to reduce diabetes-related morbidity and mortality.

(Pro)renin receptors: are they biologically relevant?

PURPOSE OF REVIEW

The recent introduction of a renin inhibitor, aliskiren, into the clinical arena has revived interest in renin and its precursor prorenin. In addition, a renin-binding and prorenin-binding receptor has been found, which not only activates prorenin but also induces angiotensin-independent signaling. This review addresses the question of whether this receptor has any biological relevance.

RECENT FINDINGS

Prorenin is the preferred agonist of the (pro)renin receptor. When bound to the receptor, prorenin undergoes a conformational change allowing it to display full enzymatic activity. Receptor activation by renin/prorenin triggers the mitogen-activated protein kinase-extracellular signal-regulated kinase 1/2 signaling pathway, and human (pro)renin receptor transgenic rats develop glomerulosclerosis and hypertension in the absence of changes in renin or angiotensin. Aliskiren prevents angiotensin I generation by receptor-bound prorenin but does not block signaling. Conflicting results have been obtained with the putative (pro)renin receptor antagonist 'handle region peptide', suggesting that its efficacy depends on experimental conditions.

SUMMARY

Although it is tempting to speculate that the (pro)renin receptor is the missing link providing a role for prorenin in tissue angiotensin generation, the discrepant results with handle region peptide and the lack of clinical studies with (pro)renin receptor blockers do not yet firmly support such a role.