Functional renin receptors in renal mesangial cells

Pathomechanisms of Diabetic Kidney Disease

The worldwide occurrence of diabetic kidney disease (DKD) is swiftly rising, primarily attributed to the growing population of individuals affected by type 2 diabetes. This surge has been transformed into a substantial global concern, placing additional strain on healthcare systems already grappling with significant demands. The pathogenesis of DKD is intricate, originating with hyperglycemia, which triggers various mechanisms and pathways: metabolic, hemodynamic, inflammatory, and fibrotic which ultimately lead to renal damage. Within each pathway, several mediators contribute to the development of renal structural and functional changes. Some of these mediators, such as inflammatory cytokines, reactive oxygen species, and transforming growth factor β are shared among the different pathways, leading to significant overlap and interaction between them. While current treatment options for DKD have shown advancement over previous strategies, their effectiveness remains somewhat constrained as patients still experience residual risk of disease progression. Therefore, a comprehensive grasp of the molecular mechanisms underlying the onset and progression of DKD is imperative for the continued creation of novel and groundbreaking therapies for this condition. In this review, we discuss the current achievements in fundamental research, with a particular emphasis on individual factors and recent developments in DKD treatment.

Successful Use of Aliskiren in a Case of IgA- Mesangial Glomerulonephritis Unresponsive to Conventional Therapies

INTRODUCTION

The early suspension of Altitude trial in recent years has induced most nephrologists and cardiologists to abandon Aliskiren use. Consequently, the potential usefulness of the direct renin inhibition in IgA glomerulonephritis remained an under-investigated therapeutic option.

CASE REPORT

We report the case of a 53 years old IgA GMN patient unresponsive to all conventional anti-angiotensin-2 agents, steroids and immunosuppressants, in which the administration of Aliskiren permitted to achieve and maintain a complete proteinuria remission in the absence of any adverse event.

CONCLUSION

Aliskiren might represent a valid and safe therapeutic option in IgA GMN, although further investigations would be needed to confirm this conclusion.

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

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

Glioblastoma Multiforme Cancer Stem Cells Express Components of the Renin-Angiotensin System

AIM

To investigate the expression of the renin-angiotensin system (RAS) in cancer stem cells (CSCs), we have previously characterized in glioblastoma multiforme (GBM).

METHODS

3,3-Diaminobenzidine (DAB) immunohistochemical (IHC) staining for the stem cell marker, SOX2, and components of the RAS: angiotensin converting enzyme (ACE), (pro)renin receptor (PRR), angiotensin II receptor 1 (ATIIR1), and angiotensin II receptor 2 (ATIIR2) on 4 μm-thick formalin-fixed paraffin-embedded sections of previously characterized GBM samples in six patients was undertaken. Immunofluorescent (IF) IHC staining was performed to demonstrate expression of GFAP, SOX2, PRR, ACE, ATIIR1, and ATIIR2. The protein expression and the transcriptional activities of the genes encoding for ACE, PRR, ATIIR1, and ATIIR2 were studied using Western blotting (WB) and NanoString gene expression analysis, respectively.

RESULTS

DAB and IF IHC staining demonstrated the expression SOX2 on the GFAP+ GBM CSCs. Cytoplasmic expression of PRR by the GFAP+ CSCs and the endothelium of the microvessels was observed. ACE was expressed on the endothelium of the microvessels only, while nuclear and cytoplasmic expression of ATIIR1 and ATIIR2 was observed on the endothelium of the microvessels and the CSCs. ATIIR1 was expressed on the GFAP+ CSCs cells, and ATIIR2 was expressed by the SOX2+ CSCs. The expression of ACE, PRR, and ATIIR1, but not ATIIR2, was confirmed by WB. NanoString gene analysis demonstrated transcriptional activation of ACE, PRR, and ATIIR1, but not ATIIR2.

CONCLUSION

This study demonstrated the expression of PRR, ATIIR1, and ATIIR2 by the SOX2 CSC population, and ACE on the endothelium of the microvessels, within GBM. ACE, PRR, and ATIIR1 were expressed at the protein and mRNA levels, with ATIIR2 detectable only by IHC staining. This novel finding suggests that the CSCs may be a novel therapeutic target for GBM by modulation of the RAS.

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.

Inhibition of RAS in diabetic nephropathy

Diabetic kidney disease (DKD) is a progressive proteinuric renal disorder in patients with type 1 or type 2 diabetes mellitus. It is a common cause of end-stage kidney disease worldwide, particularly in developed countries. Therapeutic targeting of the renin-angiotensin system (RAS) is the most validated clinical strategy for slowing disease progression. DKD is paradoxically a low systematic renin state with an increased intrarenal RAS activity implicated in its pathogenesis. Angiotensin II (AngII), the main peptide of RAS, is not only a vasoactive peptide but functions as a growth factor, activating interstitial fibroblasts and mesangial and tubular cells, while promoting the synthesis of extracellular matrix proteins. AngII also promotes podocyte injury through increased calcium influx and the generation of reactive oxygen species. Blockade of the RAS using either angiotensin converting enzyme inhibitors, or angiotensin receptor blockers can attenuate progressive glomerulosclerosis in animal models, and slows disease progression in humans with DKD. In this review, we summarize the role of intrarenal RAS activation in the pathogenesis and progression of DKD and the rationale for RAS inhibition in this population.

Protective effects of aliskiren and valsartan in mice with diabetic nephropathy

AIM

We investigated whether aliskiren, a direct renin inhibitor, provided protection in a model of diabetic nephropathy in mice and compared its protective effects to valsartan, an angiotensin II type 1 receptor blocker.

MATERIALS AND METHODS

Hyperglycemia was induced with streptozotocin (STZ, 40 mg/kg/day × 5 days) injection in DBA/2J mice fed on a high fat diet. Mice were treated with either aliskiren (25 mg/kg/day) or valsartan (8 mg/kg/day) for 6 weeks.

RESULTS

Aliskiren and/or valsartan treatment significantly attenuated albuminuria, urinary nephrin excretion and glomerulosclerosis. Aliskiren and/or valsartan prevented reduction of podocin and WT1 protein abundance in diabetic mice. Aliskiren and/or valsartan significantly prevented increased expression of profibrotic growth factors (TGFβ, CTGF and PAI-1), proinflammatory cytokines (MCP-1, TNFα and IL-1β), endoplasmic reticulum (ER) stress markers (CHOP and XBP-1) and lipid accumulation in the kidney of diabetic animals. Aliskiren showed similar efficacy compared to valsartan therapy and dual treatment in some aspects has synergistic protective effects.

CONCLUSION

Our study indicates that aliskiren and/or valsartan protects against diabetic kidney disease through multiple mechanisms, including decreasing podocyte injury, activation of profibrotic growth factors and proinflammatory cytokines, ER stress and accumulation of lipids.

Multiple genes of the renin-angiotensin system are novel targets of Wnt/β-catenin signaling

Activation of the renin-angiotensin system (RAS) plays an essential role in the pathogenesis of CKD and cardiovascular disease. However, current anti-RAS therapy only has limited efficacy, partly because of compensatory upregulation of renin expression. Therefore, a treatment strategy to simultaneously target multiple RAS genes is necessary to achieve greater efficacy. By bioinformatics analyses, we discovered that the promoter regions of all RAS genes contained putative T-cell factor (TCF)/lymphoid enhancer factor (LEF)-binding sites, and β-catenin induced the binding of LEF-1 to these sites in kidney tubular cells. Overexpression of either β-catenin or different Wnt ligands induced the expression of all RAS genes. Conversely, a small-molecule β-catenin inhibitor ICG-001 abolished RAS induction. In a mouse model of nephropathy induced by adriamycin, either transient therapy or late administration of ICG-001 abolished established proteinuria and kidney lesions. ICG-001 inhibited renal expression of multiple RAS genes in vivo and abolished the expression of other Wnt/β-catenin target genes. Moreover, ICG-001 therapy restored expression of nephrin, podocin, and Wilms' tumor 1, attenuated interstitial myofibroblast activation, repressed matrix expression, and inhibited renal inflammation and fibrosis. Collectively, these studies identify all RAS genes as novel downstream targets of Wnt/β-catenin. Our results indicate that blockade of Wnt/β-catenin signaling can simultaneously repress multiple RAS genes, thereby leading to the reversal of established proteinuria and kidney injury.

New roles for renin and prorenin in heart failure and cardiorenal crosstalk

The renin-angiotensin-aldosterone-system (RAAS) plays a central role in the pathophysiology of heart failure and cardiorenal interaction. Drugs interfering in the RAAS form the pillars in treatment of heart failure and cardiorenal syndrome. Although RAAS inhibitors improve prognosis, heart failure–associated morbidity and mortality remain high, especially in the presence of kidney disease. The effect of RAAS blockade may be limited due to the loss of an inhibitory feedback of angiotensin II on renin production. The subsequent increase in prorenin and renin may activate several alternative pathways. These include the recently discovered (pro-) renin receptor, angiotensin II escape via chymase and cathepsin, and the formation of various angiotensin subforms upstream from the blockade, including angiotensin 1–7, angiotensin III, and angiotensin IV. Recently, the direct renin inhibitor aliskiren has been proven effective in reducing plasma renin activity (PRA) and appears to provide additional (tissue) RAAS blockade on top of angiotensin-converting enzyme and angiotensin receptor blockers, underscoring the important role of renin, even (or more so) under adequate RAAS blockade. Reducing PRA however occurs at the expense of an increase plasma renin concentration (PRC). PRC may exert direct effects independent of PRA through the recently discovered (pro-) renin receptor. Additional novel possibilities to interfere in the RAAS, for instance using vitamin D receptor activation, as well as the increased knowledge on alternative pathways, have revived the question on how ideal RAAS-guided therapy should be implemented. Renin and prorenin are pivotal since these are at the base of all of these pathways.

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

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

Rab23 plays a role in the pathophysiology of mesangial cells--a proteomic analysis

Rab23, a novel member of the Rab family of small GTPases, has recently been identified in mesangial cells (MCs). Although Rab23 levels in MCs are associated with glomerular nephropathies, the exact physiological and pathological roles of Rab23 in MCs are unknown. In the present study, its roles in MCs were explored by performing proteomics and systems biology analyses in MCs after knockdown or overexpression of Rab23. Knockdown of Rab23 was achieved by transfecting MCs with a plasmid expressing short hairpin RNA against Rab23, while overexpression of Rab23 was accomplished by transfection with the wild-type, dominant negative, and constitutively active Rab23 gene constructs. The effects of different levels of Rab23 activity on proteome of various biological pathways were investigated. Gel-based proteomic approaches and systems biology tools, respectively, were used to identify the Rab23-regulated proteins and the functional pathways. Proteomic analysis revealed the potential roles for Rab23 in multiple processes, including G-protein signal transduction, transcription modulation, RNA stabilization, protein synthesis and degradation, cytoskeleton reorganization, anti-oxidation and detoxification, circadian rhythm regulation and phagocytosis. Bioinformatics analyses showed that Rab23 impacts on multiple biological networks in MCs. These data may shed light on the roles of Rab23 in mesangiopathy or MC damage.

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.

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.

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.

A role of the (pro)renin receptor in neuronal cell differentiation

The (pro)renin receptor [(P)RR] plays a pivotal role in the renin-angiotensin system. Experimental models emphasize the role of (P)RR in organ damage associated with hypertension and diabetes. However, a mutation of the (P)RR gene, resulting in frame deletion of exon 4 [Delta4-(P)RR] is associated with X-linked mental retardation (XLMR) and epilepsy pointing to a novel role of (P)RR in brain development and cognitive function. We have studied (P)RR expression in mouse brain, as well as the effect of transfection of Delta4-(P)RR on neuronal differentiation of rat neuroendocrine PC-12 cells induced by nerve growth factor (NGF). In situ hybridization showed a wide distribution of (P)RR, including in key regions involved in the regulation of blood pressure and body fluid homeostasis. In mouse neurons, the receptor is on the plasma membrane and in synaptic vesicles, and stimulation by renin provokes ERK1/2 phosphorylation. In PC-12 cells, (P)RR localized mainly in the Golgi and in endoplasmic reticulum and redistributed to neurite projections during NGF-induced differentiation. In contrast, Delta4-(P)RR remained cytosolic and inhibited NGF-induced neuronal differentiation and ERK1/2 activation. Cotransfection of PC-12 cells with (P)RR and Delta4-(P)RR cDNA resulted in altered localization of (P)RR and inhibited (P)RR redistribution to neurite projections upon NGF stimulation. Furthermore, (P)RR dimerized with itself and with Delta4-(P)RR, suggesting that the XLMR and epilepsy phenotype resulted from a dominant-negative effect of Delta4-(P)RR, which coexists with normal transcript in affected males. In conclusion, our results show that (P)RR is expressed in mouse brain and suggest that the XLMR and epilepsy phenotype might result from a dominant-negative effect of the Delta4-(P)RR protein.

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.

Receptor-dependent prorenin activation and induction of PAI-1 expression in vascular smooth muscle cells

Although elevated plasma prorenin levels are commonly found in diabetic patients and correlate with microvascular complications, the pathological role of these increases, if any, remains unclear. Prorenin/renin binding to the prorenin/renin receptor [(p)RR] enhances the efficiency of angiotensinogen cleavage by renin and unmasks prorenin catalytic activity. We asked whether plasma prorenin could be activated in local vascular tissue through receptor binding. Immunohistochemical staining showing localization of the (p)RR in the aorta to vascular smooth muscle cells (VSMCs). After cultured rat VSMCs were incubated with 10(-7) M inactive prorenin, cultured supernatant acquired the ability to generate ANG I from angiotensinogen, indicating that prorenin had been activated. Activated prorenin facilitated angiotensin generation in cultured VSMCs when exogenous angiotensinogen was added. Small interfering RNA (siRNA) against the (p)RR blocked this activation and subsequent angiotensin generation. Prorenin alone induced dose- and time-dependent increases in mRNA and protein for the profibrotic molecule plasminogen activator inhibitor (PAI)-1, effects that were blocked by siRNA, but not by the ANG II receptor antagonist saralasin. When inactive prorenin and angiotensinogen were incubated with cells, PAI-1 mRNA increased a striking 54-fold, 8-fold higher than the increase seen with prorenin alone. PAI-1 protein increased 2.75-fold. These effects were blocked by treatment with siRNA + saralasin. We conclude that prorenin at high concentration binds the (p)RR on VSMCs and is activated. This activation leads to increased expression of PAI-1 via ANG II-independent and -dependent mechanisms. These data provide a mechanism by which elevated prorenin levels in diabetes may contribute to the progression of fibrotic disease.