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

The (pro)renin receptor: pathophysiological roles in cardiovascular and renal pathology

PURPOSE OF REVIEW

The pathophysiological role of the (pro)renin receptor is yet to be established. The present review summarizes the findings, suggesting that it may play pathological role in cardiac and renal fibrosis, and in hypertensive and diabetic nephropathy.

RECENT FINDINGS

In-vitro and animal studies have shown that increased receptor expression could be linked to high blood pressure and to cardiac and glomerular fibrosis by activating mitogen-activated protein kinases and by upregulating gene expression of profibrotic molecules. Studies also suggest that the receptor is involved in diabetic nephropathy by activating receptor-bound prorenin, thereby increasing angiotensin II tissue generation. Moreover, in diabetic mice, a peptide able to block prorenin binding to the receptor was claimed to be more effective for renal protection than angiotensin-converting enzyme inhibitor.

SUMMARY

The experimental data confirmed the pivotal role of the receptor in cell surface generation of angiotensin and suggested its potential role in tissue fibrosis via receptor activation and intracellular signaling. The data also questioned the ability of soon available renin inhibitors to inhibit the activity of receptor-bound renin and prorenin, and the benefit of a new class of drug--(pro)renin receptor blockers--to prevent tissue damage.

Renin-stimulated TGF-beta1 expression is regulated by a mitogen-activated protein kinase in mesangial cells

Recent evidence indicates that renin itself might be profibrotic, independent of angiotensin II; however, the signaling system by which renin exerts a direct effect is not known. We tested the hypothesis that renin receptor activation, in turn, activates the extracellular-signal regulated kinase 1 and 2 (ERK1/2) of the mitogen-activated protein kinase system in mesangial cells. Recombinant rat renin induced a rapid phosphorylation of ERK1/2 and subsequent cell proliferation in a dose- and time-dependent manner. ERK1/2 activation by renin addition was not altered by angiotensin-converting enzyme inhibition or angiotensin receptor blockade. An ERK kinase inhibitor significantly reduced the renin-induced ERK1/2 phosphorylation and the subsequent increase in transforming growth factor-beta1 (TGF-beta1) and plasminogen activator inhibitor-1 mRNA expression. A small-inhibiting RNA, siRNA, against the renin receptor completely blocked ERK1/2 activation by rat renin. We conclude that renin induces ERK1/2 activation though a receptor-mediated, angiotensin II-independent mechanism in mesangial cells. This renin-activated pathway triggers cell proliferation along with TGF-beta1 and plasminogen activator inhibitor-1 gene expression. This system may play an important role in the overall profibrotic actions of renin.

Novel therapies blocking the renin-angiotensin-aldosterone system in the management of hypertension and related disorders

Although significant advances have been made in the therapeutic blockade of the renin-angiotensin-aldosterone system (RAAS) using angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers and non-selective aldosterone receptor antagonists, there is a clear need for both additional blocking strategies and enhancements of current therapeutic approaches. Vasopeptidase inhibition may still find a role despite the small incremental value of this approach and the obvious issue of kinin-mediated adverse effects still to be fully addressed. Blockade of the RAAS upstream using renin inhibitors as well as the greater selectivity of aldosterone blockade using selective aldosterone blockers such as eplerenone are also novel approaches. Not yet in clinical use but certainly an attractive therapeutic target is angiotensin II growth factor receptor transactivation, with selective inhibitors having been developed for various specific kinase pathways. Finally, ACE2 augmentation, antisense gene strategies, and vaccination against the renin-angiotensin system should still be considered experimental, but have significant appeal as additional approaches to the blockade of this system.

Prorenin receptor blockers: effects on cardiovascular complications of diabetes and hypertension

When the 'handle region' of the prorenin prosegment interacts with the (pro)renin receptor, the prorenin molecule partially changes the conformation to an enzymatically active state. On the other hand, the receptor triggers its own intracellular signalling pathways independent of the renin-angiotensin system (RAS). The 'handle region' peptide competitively binds to the receptor as a decoy peptide and inhibits both the non-proteolytic activation of prorenin and the RAS-independent intracellular signals. Therefore, prorenin receptor blockers including the decoy peptide may have superior benefits on end-organ damage in diabetes and hypertension compared with conventional RAS inhibitors.

A novel signal transduction cascade involving direct physical interaction of the renin/prorenin receptor with the transcription factor promyelocytic zinc finger protein

A human renin/prorenin receptor (RER) has recently been cloned. To gain insight into the molecular function of the RER, we studied its signal transduction mechanisms. Initially, we found a ubiquitous and intracellular expression pattern of the human RER. Consistently, we observed several transcriptional start sites and a high promoter activity of the human RER. We could identify the transcription factor promyelocytic zinc finger (PLZF) protein as a direct protein interaction partner of the C-terminal domain of the RER by yeast 2-hybrid screening and coimmunoprecipitation. Coimmunoprecipitation experiments also indicated homodimerization of the RER. On activation of the RER by renin, PLZF is translocated into the nucleus and represses transcription of the RER itself, thereby creating a very short negative feedback loop, but activates transcription of the p85alpha subunit of the phosphatidylinositol-3 kinase (PI3K-p85alpha). Small interfering RNA against the RER abolished these effects. A PLZF cis-element in the RER promoter was identified by site-directed mutagenesis and electrophoretic mobility-shift assay. Renin stimulation caused a 6-fold recruitment of PLZF to this promoter region as shown by chromatin immunoprecipitation. Moreover, renin stimulation of rat H9c2 cardiomyoblasts induced an increase of cell number and a decrease of apoptosis. These effects were partly abolished by PI3K inhibition and completely abrogated by small interfering RNA against PLZF. Finally, experiments in PLZF knockout mice confirmed the role of PLZF as an upstream regulator of RER and PI3K-p85alpha. Our data demonstrate the existence of a novel signal transduction pathway involving the ligand renin, RER, and the transcription factor PLZF, which is of physiological and putative pathophysiological relevance.

Renin-angiotensin-aldosterone system and progression of renal disease

Inhibition of the renin-angiotensin-aldosterone system (RAAS) is one of the most powerful maneuvers to slow progression of renal disease. Angiotensin II (AngII) has emerged in the past decade as a multifunctional cytokine that exhibits many nonhemodynamic properties, such as acting as a growth factor and profibrogenic cytokine, and even having proinflammatory properties. Many of these deleterious functions are mediated by other factors, such as TGF-beta and chemoattractants that are induced in the kidney by AngII. Moreover, understanding of the RAAS has become much more complex in recent years with the identification of novel peptides (e.g., AngIV) that could bind to specific receptors, elucidating deleterious effects, and non-angiotensin-converting enzyme (ACE)-mediated generation of AngII. The ability of renal cells to produce AngII in a concentration that is much higher than what is found in the systemic circulation and the observation that aldosterone may be engaged directly in profibrogenic processes independent of hypertension have added to the complexity of the RAAS. Even renin has now been identified to have a "life on its own" and mediates profibrotic effects via binding to specific receptors. Finally, drugs that are used to block the RAAS, such as ACE inhibitors or certain AngII type 1 receptor antagonists, may have properties on cells independent of AngII (ACE inhibitor-mediated outside-inside signaling and peroxisome proliferator-activated receptor-gamma stimulatory effects of certain sartanes). Although blockade of the RAAS with ACE inhibitors, AngII type 1 receptor antagonists, or the combination of both should be part of every strategy to slow progression of renal disease, a better understanding of the novel aspects of the RAAS should contribute to the development of innovative strategies not only to completely halt progression but also to induce regression of human renal disease.

The (pro)renin receptor: therapeutic consequences

It is generally assumed that the beneficial effects of renin-angiotensin system blockers in cardiovascular disease are due to blockade of the generation or action of angiotensin at tissue sites. Such generation depends on the uptake of renin and/or its inactive precursor prorenin from the circulation. Recently, a (pro)renin receptor has been cloned that might perform this task. Unexpectedly, this receptor also induced angiotensin-independent effects, suggesting that renin and/or prorenin may act as agonists for this receptor. Ultimately, this could lead to the development of (pro)renin receptor blockers (i.e., drugs that not only prevent tissue angiotensin generation but also inhibit renin- or prorenin-induced effects).

Renal injury due to renin-angiotensin-aldosterone system activation of the transforming growth factor-beta pathway

Glomerulosclerosis, interstitial fibrosis, and tubular atrophy occur with end-stage kidney failure, irrespective of the primary etiology. The transforming growth factor-beta (TGF-beta) is a key factor in these alterations either directly, by stimulating synthesis of extracellular matrix components and reducing collagenase production, or indirectly through other profibrogenic factors such as connective tissue growth factor (CTGF). TGF-beta is important for the proliferation of intrarenal fibroblasts and the epithelial-mesenchymal transition through which tubular cells become fibroblasts. Although several factors induce TGF-beta expression in the kidney, one very interesting aspect is the link between the renin-angiotensin-aldosterone (Aldo) system (RAAS) and TGF-beta. Angiotensin II (ANG II) stimulates TGF-beta expression in the kidney by various mechanisms and upregulates receptors for TGF-beta. ANG II can directly phosphorylate Smads without inducing TGF-beta. Recent data provide compelling evidence that other components of the RAAS including ANG III, renin, and Aldo also activate the TGF-beta system. As direct modulation of the TGF-beta system is not yet feasible in humans, angiotensin-converting enzyme (ACE) inhibitors and angiotensin type 1 (AT1)-receptor blockers are currently the most potential drugs to interfere with this ANG II-mediated TGF-beta expression. This review highlights some current aspects of the interaction between the RAAS and the TGF-beta axis.

Long-term angiotensin II AT1 receptor inhibition produces adipose tissue hypotrophy accompanied by increased expression of adiponectin and PPARgamma

To clarify the mechanism of the effects of angiotensin II AT(1) receptor antagonists on adipose tissue, we treated 8 week-old male Wistar Kyoto rats with the angiotensin II AT(1) receptor antagonist Candesartan cilexetil (10 mg/kg/day) for 18 weeks. Candesartan cilexetil reduced body weight gain, decreased fat tissue mass due to hypotrophy of epididymal and retroperitoneal adipose tissue and decreased adipocyte size without changing the number of adipocytes. Candesartan cilexetil decreased serum leptin levels and epididymal leptin mRNA, increased serum adiponectin levels and epididymal adiponectin mRNA, decreased epididymal tumor necrosis factor alpha (TNFalpha) mRNA, and increased fatty acid synthase mRNA. Considered free of peroxisome proliferator-activated receptor gamma (PPARgamma) agonist activity, Candesartan cilexetil increased epididymal expression of PPARgamma mRNA. The effects of Candesartan cilexetil on adipokine production and release may be attributable to PPARgamma activation and/or decrease in adipocyte cell size. In addition, Candesartan cilexetil treatment increased the expression of epididymal angiotensin II AT(2) receptor mRNA and protein and decreased the expression of renin receptor mRNA. These results suggest that Candesartan cilexetil influences lipid metabolism in adipose tissue by promoting adipose tissue rearrangement and modulating adipokine expression and release. These effects are probably consequences of local angiotensin II AT(1) receptor inhibition, angiotensin II AT(2) receptor stimulation, and perhaps additional angiotensin II-independent mechanisms. Our results indicate that the activity of local renin-angiotensin system plays an important role in adipose tissue metabolism. The decrease in the pro-inflammatory cytokine TNFalpha and the increase in the anti-inflammatory adipokine adiponectin indicate that Candesartan cilexetil may exert significant anti-inflammatory properties.

Renin inhibition

PURPOSE OF REVIEW

Initial attempts to inhibit renin in humans have faced numerous difficulties. Molecular modeling and X-ray crystallography of the active site of renin have led to the development of new orally active renin inhibitors, such as aliskiren. Recent preclinical and clinical data suggest that this drug may be of value for treating patients with cardiovascular and renal disorders.

RECENT FINDINGS

The once-daily administration of aliskiren to hypertensive patients lowers blood pressure as strongly as, or more strongly than, standard doses of established angiotensin II type 1 receptor blockers. It further decreases blood pressure in combination with hydrochlorothiazide. The biochemical consequences of renin inhibition differ from those of angiotensin I-converting enzyme inhibition and angiotensin II antagonism, particularly in terms of angiotensin profiles and interactions with the bradykinin-nitric oxide-cGMP pathway and possibly the (pro)renin receptor.

SUMMARY

Blockade of the renin-angiotensin system with angiotensin I-converting enzyme inhibitors, angiotensin II type 1 receptor blockers or a combination of these drugs has become one of the most successful therapeutic approaches in medicine. It remains unclear, however, as to how to optimize the renin-angiotensin system blockade to maximize cardiovascular and renal benefits. In this context, renin inhibition to render the renin-angiotensin system fully quiescent is a new possibility requiring further study.

Contribution of nonproteolytically activated prorenin in glomeruli to hypertensive renal damage

Prorenin is activated without proteolysis by binding of prorenin receptor to the pentameric "handle region" (HR) of prorenin prosegment. It was hypothesized that such activation occurs in the kidneys of hypertensive rats and causes tissue renin-angiotensin system (RAS) activation and end-organ damage. Because the HR's binding to its binding protein made the adjacent tetrameric "gate region" (GR) accessible to its specific antibody, immunohistochemistry of the GR was performed to test the hypothesis. Methods also were devised specifically to inhibit the nonproteolytic activation by the decapeptide corresponding to the HR as a decoy. Immunohistochemistry of the GR demonstrated that the majority of nonproteolytically activated prorenin is present in podocytes of the kidneys from stroke-prone spontaneously hypertensive rats, in which activation of renal tissue RAS, proteinuria, and glomerulosclerosis occurred. Continuous subcutaneous administration of the HR decoy peptide completely inhibited both nonproteolytic activation of tissue prorenin and activation of tissue RAS without affecting circulating RAS or arterial pressure and significantly attenuated the development and progression of proteinuria and glomerulosclerosis. These studies clearly demonstrated that nonproteolytic activation of prorenin in glomeruli is critically involved in renal tissue RAS activation, leading to renal damage in hypertensive animals.

Increased cyclooxygenase-2, hyperfiltration, glomerulosclerosis, and diabetic nephropathy: put the blame on the (pro)renin receptor?

Interaction between the renin-angiotensin system and cyclooxygenases in the kidney regulates renal microcirculation. Activation of the (pro)renin receptor has profibrotic effects, and now Kaneshiroet al. show that it also increases COX-2 synthesis. These results may have therapeutic implications, as blocking (pro)renin-receptor interaction would prevent the increase of angiotensin generation and prostaglandin synthesis, two phenomena underlying the pathogenesis of diabetic nephropathy.

Renin/prorenin receptors

The existence of a tissue renin-angiotensin (RAS) system independent of the circulating RAS has prompted the search for cellular binding sites for angiotensinogen and for renin in order to explain their tissue uptake. Two receptors that bind with similar affinity mature renin and prorenin were identified, the mannose-6-phosphate receptor (M6P-R) and a specific receptor. The M6P-R is a clearance receptor that binds exclusively the glycosylated forms of renin and prorenin. Binding of renin and prorenin to the M6P-R is followed by internalization and degradation, and the intracellular proteolysis of prorenin in mature renin did not provoke any generation of intracellular angiotensins. In contrast to the M6P-R, (pro)renin bound to the specific receptor was not degraded. Instead, receptor-bound renin showed increased catalytic activity, and receptor-bound prorenin exhibited full catalytic activity. This 'gain of activity' was explained by a conformational change of the (pro)renin molecule upon binding. Furthermore, (pro)renin binding provoked a rapid activation of the mitogen-activated protein kinases p44/p42, indicating that the receptor has mediated specific, angiotensin II-independent effects of (pro)renin. This receptor represents an elegant concept to explain the existence of active prorenin in vivo, and it provides a pathological role for prorenin in situations with paradoxical low renin and high prorenin concentrations such as in diabetes. Experimental models of rats overexpressing the receptor either in vascular smooth muscle cells and developing high blood pressure or with ubiquitous expression associated with glomerulosclerosis and proteinuria confirm a role for the receptor in cardiovascular and renal diseases.

Receptor-mediated actions of renin and prorenin

Renin can induce renal disease by generating angiotensin II and, thereby, increasing fibrosis. Huang et al describe a new mechanism of action whereby the renin-angiotensin system can also exert this effect. Direct activation of the renin/prorenin receptor in mesangial cells induced synthesis of TGF-beta and profibrotic proteins. Hence, like other proteases such as thrombin, renin and prorenin are capable of receptor-mediated cellular signaling.

Fetal deaths in Alabama, 1974-1983: a birth weight-specific analysis

Objective. Although angiotensin II-mediated inflammation and extracellular matrix accumulation are considered to be associated with the progression of diabetic nephropathy, these processes have not yet been sufficiently clarified. The objective of this study was to determine whether the correction of the abnormal renal expression of MMPs and its inhibitors (MMPs/TIMPs) and cytokines following the administration of aliskiren to KK-A^y mice results in a renoprotective effect. Methods. KK-A^y mice were divided into two groups, that is, untreated (saline) and treated (aliskiren) groups. Systolic BP, HbA1c levels, and the albumin-creatinine ratio (ACR) were measured. The renal expression of MMPs/TIMPs, fibronectin, type IV collagen, MCP-1, and (pro)renin receptor ((P)RR) was examined using real-time PCR and/or immunohistochemical staining. Renal MAPK and NF-κB activity were also examined by Western blot analyses and ELISA, respectively. Results. Significant decreases in systolic BP and ACR levels were observed in treated KK-A^y mice compared with the findings in untreated KK-A^y mice. Furthermore, increases in MMPs/TIMPs, fibronectin, type IV collagen, MCP-1, and (P)RR expression, in addition to MAPK and NF-κB activity, were significantly attenuated by aliskiren administration. Conclusions. It appears that aliskiren improves albuminuria and renal fibrosis by regulating inflammation and the alteration of collagen synthesis and degradation.