Purification and characterization of angiotensin I-converting enzymes from mesangial cells in culture

Both aldosterone and spironolactone can modulate the intracellular ACE/ANG II/AT1 and ACE2/ANG (1-7)/MAS receptor axes in human mesangial cells

The kidney is an important target of the renin‐ANG‐aldosterone system (RAAS). To date, several studies have demonstrated the existence of a local RAAS in various tissues, including the renal tissue. The mineralocorticoid aldosterone is known to play a critical role in the classical RAAS; however, its effect on mesangial cells (MCs) remains to be elucidated. Based on this, our aim was to investigate whether aldosterone stimulation can modulate the intracellular RAAS of immortalized human MCs by evaluating ANG‐converting enzyme (ACE)/ANG II/ANG II receptor type 1 (AT1) and ANG‐converting enzyme 2 (ACE2)/ANG (1‐7)/MAS receptor axes. To realise this, protein expression, enzyme activity, and immunofluorescence were performed under aldosterone stimulation and in the presence of the mineralocorticoid receptor (MR) antagonist spironolactone (SPI). We observed that high doses of aldosterone increase ACE activity. The effect of aldosterone on the catalytic activity of ACE was completely abolished with the pretreatment of SPI suggesting that the aldosterone‐induced cell injuries through ANG II release were attenuated. Aldosterone treatment also decreased the expression of MAS receptor, but did not alter the expression or the catalytic activity of ACE 2 and ANG (1‐7) levels. Spironolactone modulated the localization of ANG II and AT1 receptor and decreased ANG (1‐7) and MAS receptor levels. Our data suggest that both aldosterone and the MR receptor antagonist can modulate both of these axes and that spironolactone can protect MCs from the damage induced by aldosterone.

Pharmacodynamic effects of C-domain-specific ACE inhibitors on the renin-angiotensin system in myocardial infarcted rats

INTRODUCTION

The renin-angiotensin system (RAS) is a dynamic network that plays a critical role in blood pressure regulation and fluid and electrolyte homeostasis. Modulators of the RAS, such as angiotensin-converting enzyme (ACE) inhibitors, are widely used to treat hypertension, heart failure and myocardial infarction.

METHODS

The effect of ACE inhibitors (lisinopril and C-domain-selective LisW-S) on the constituent peptides of the RAS following myocardial infarction was examined in rats. Ten angiotensin peptides were analysed using a sensitive LC-MS/MS-based assay to examine both the circulating and equilibrium levels of these peptides.

RESULTS

Administration of lisinopril or LisW-S caused a significant decrease in Ang 1-8/Ang 1-10 ratios as determined by circulating and equilibrium peptide level analysis. Furthermore, Ang 1-7 levels were elevated by both ACE inhibitors, but only lisinopril decreased the Ang 1-5/Ang 1-7 ratio. This indicates LisW-S C-domain specificity as Ang 1-5 is generated by hydrolysis of Ang 1-7 by the N-domain. Further corroboration of LisW-S C-domain specificity is that only lisinopril increased the circulating levels of the N-domain ACE substrate Ac-SDKP.

CONCLUSION

LisW-S is able to effectively block ACE in vivo by C-domain-selective inhibition. The LC-MS/MS-based assay allows the evaluation of the pharmacologic impact of RAS inhibitors in different pathophysiological conditions.

Hypertensive epigenetics: from DNA methylation to microRNAs

The major epigenetic features of mammalian cells include DNA methylation, posttranslational histone modifications and RNA-based mechanisms including those controlled by small non-coding RNAs (microRNAs (miRNAs)). An important aspect of epigenetic mechanisms is that they are potentially reversible and may be influenced by nutritional-environmental factors and through gene-environment interactions. Studies on epigenetic modulations could help us understand the mechanisms involved in essential hypertension and further prevent it's progress. This review is focused on new knowledge on the role of epigenetics, from DNA methylation to miRNAs, in essential hypertension.

Angiotensin-I converting enzyme (ACE): structure, biological roles, and molecular basis for chloride ion dependence

Somatic angiotensin-I converting enzyme (sACE) has an essential role in the regulation of blood pressure and electrolyte fluid homeostasis. It is a zinc protease that cleaves angiotensin-I (AngI), bradykinin, and a broad range of other signalling peptides. The enzyme activity is provided by two homologous domains (N- and C-), which display clear differences in substrate specificities and chloride activation. The presence of chloride ions in sACE and its unusual role in activity was identified early on in the characterisation of the enzyme. The molecular mechanisms of chloride activation have been investigated thoroughly through mutagenesis studies and shown to be substrate-dependent. Recent results from X-ray crystallography structural analysis have provided the basis for the intricate interactions between ACE, its substrate and chloride ions. Here we describe the role of chloride ions in human ACE and its physiological consequences. Insights into the chloride activation of the N- and C-domains could impact the design of improved domain-specific ACE inhibitors.

Serine proteases as candidates for proteolytic processing of angiotensin-I converting enzyme

Somatic angiotensin-I converting enzyme (sACE) is a broadly distributed peptidase which plays a role in blood pressure and electrolyte homeostasis by the conversion of angiotensin I into angiotensin II. N-domain isoforms (nACE) with 65 and 90 kDa have been described in body fluids, tissues and mesangial cells (MC), and a 90 kDa nACE has been described only in spontaneously hypertensive rats. The aim of this study was to investigate the existence of proteolytic enzymes that may act in the hydrolysis of sACE generating nACEs in MC. After the confirmation of the presence of ACE sheddases in Immortalized MC (IMC), we purified and characterized these enzymes using fluorogenic substrates specifically designed for ACE sheddases. Purified enzyme identified as a serine protease by N-terminal sequence was able to generate nACE. In the present study, we described for the first time the presence of ACE sheddases in IMC, identified as serine proteases able to hydrolyze sACE in vitro. Further investigations are necessary to elucidate the mechanisms responsible for the expression and regulation of ACE sheddases in MC and their roles in the generation of nACEs, especially the 90 kDa form possibly related to hypertension.

Subcellular characteristics of functional intracellular renin-angiotensin systems

The renin-angiotensin system (RAS) is now regarded as an integral component in not only the development of hypertension, but also in physiologic and pathophysiologic mechanisms in multiple tissues and chronic disease states. While many of the endocrine (circulating), paracrine (cell-to-different cell) and autacrine (cell-to-same cell) effects of the RAS are believed to be mediated through the canonical extracellular RAS, a complete, independent and differentially regulated intracellular RAS (iRAS) has also been proposed. Angiotensinogen, the enzymes renin and angiotensin-converting enzyme (ACE) and the angiotensin peptides can all be synthesized and retained intracellularly. Angiotensin receptors (types I and 2) are also abundant intracellularly mainly at the nuclear and mitochondrial levels. The aim of this review is to focus on the most recent information concerning the subcellular localization, distribution and functions of the iRAS and to discuss the potential consequences of activation of the subcellular RAS on different organ systems.

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

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

Spectroscopic and structural analysis of somatic and N-domain angiotensin I-converting enzyme isoforms from mesangial cells from Wistar and spontaneously hypertensive rats

Angiotensin I-converting enzyme (ACE) plays a key role in the renin-angiotesin aldosterone cascade. We analysed the secondary structure and structural organization of a purified 65kDa N-domain ACE (nACE) from Wistar rat mesangial cells, a 90 kDa nACE from spontaneously hypertensive rats and a 130 kDa somatic ACE. The C-terminal alignment of the 65 kDa nACE with rat ACE revealed that the former was truncated at Ser(482), and the sequence of the 90 kDa nACE ended at Pro(629). Protein's secondary structure consisted predominantly of alpha-helices. The 90 and 65 kDa isoforms were the most stable in guanidine and at low pH, respectively. Enzymatic activity decreased with loss in secondary structure, except in the case of guanidine HCl where the 90 kDa fragment loses its secondary structure faster than its enzymatic activity. We identified and characterized the activity and stability of these isoforms and these findings would be helpful on the understanding of the role of nACE isoforms in hypertension.

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

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

Glucose promotes the production of interleukine-1beta and cyclooxygenase-2 in mesangial cells via enhanced (Pro)renin receptor expression

(Pro)renin receptor (PRR) is present in renal glomeruli, and its expression is up-regulated in diabetes. Similarly, renal inflammation is increased in the presence of hyperglycemia. The linkage between PRR and renal inflammation is not well established. We hypothesized that glucose-induced up-regulation of PRR leads to increased production of the proinflammatory factors IL-1beta and cyclooxygenase-2 (COX-2). Studies were conducted in rat mesangial cells (RMCs) exposed to 30 mm D-glucose for 2 wk followed by PRR small interfering RNA knockdown, IL-1 receptor blockade with IL-1 receptor antagonist or angiotensin II type 1 receptor blockade with valsartan. The results showed that D-glucose treatment up-regulates prorenin, renin, angiotensin II, PRR, IL-1beta, and COX-2 mRNA and protein expression and increases phosphorylation of ERK1/2, c-Jun N-terminal kinase, c-Jun, and nuclear factor-kappaB (NF-kappaB) p65 (serine 276,468 and 536), respectively. PRR small interfering RNA attenuated PRR, IL-1beta, and COX-2 mRNA and protein expressions and significantly decreased angiotensin II production and phosphorylation of ERK1/2 and NF-kappaB p65 associated with high glucose exposure. Similarly, IL-1 receptor antagonist significantly reduced COX-2 mRNA and protein expression induced by high glucose. COX-2 inhibition reduced high-glucose-induced PRR expression. We conclude that glucose induces the up-regulation of PRR and its ligands prorenin and renin, leading to increased IL-1beta and COX-2 production via the angiotensin II-dependent pathway. It is also possible that PRR could enhance the production of these inflammatory cytokines through direct stimulation of ERK1/2-NF-kappaB signaling cascade.

Inhibition of intracellular angiotensin II formation blocks high glucose effect on mesangial matrix

High glucose causes increased matrix synthesis by glomerular mesangial cells and angiotensin II (Ang II) has been shown to mediate this effect of glucose. These studies investigate whether inhibition of Ang II formation can block high glucose-induced increase in mesangial matrix. Human mesangial cells were incubated with 25 mM glucose (HG) along with captopril, an ACE inhibitor, to block Ang II formation. In other experiments, cells were nucleofected with siRNA to knockdown angiotensinogen (Agt), the precursor of Ang II, and then exposed to high glucose. Captopril blocked high glucose-induced increase in Ang II levels in the cell media (extracellular) but failed to inhibit it in the cell lysate (intracellular). Moreover, captopril treatment did not block the stimulatory effect of high glucose on TGF-beta1 and fibronectin. In contrast, knockdown of the Agt gene prevented high glucose-induced increase in both extracellular and intracellular Ang II levels, and was accompanied by normalization of TGF-beta1 and fibronectin. These data suggest that intracellular Ang II may play an important role in the mediation of the high glucose effect on matrix and that ACE inhibitors may not be effective in blocking intracellular Ang II formation in mesangial cells.

Regulation of glucose uptake in mesangial cells stimulated by high glucose: role of angiotensin II and insulin

Mesangial cells (MCs) play a central role in the pathogenesis of diabetic nephropathy (DN). MC dysfunction arises from excessive glucose uptake through insulin-independent glucose transporter (GLUT1). The role of the insulin-dependent transporter (GLUT4) remains unknown. This study evaluated the effect of high glucose on GLUT1, GLUT4, and fibronectin expression levels. Glucose uptake was determined in the absence and presence of insulin. Angiotensin II has been implicated as a mediator of MC abnormalities in DN, and its effects on the GLUTs expression were evaluated in the presence of losartan. MCs were exposed to normal (NG, 10 mM) or high (HG, 30 mM) glucose for 1, 4, 12, 24, and 72 hrs. Glucose uptake was elevated from 1 hr up to 24 hrs of HG, but returned to NG levels after 72 hrs. HG induced an early (1-, 4-, and 12-hrs) rise in GLUT1 expression, returning to NG levels after 72 hrs, whereas GLUT4 was overexpressed at later timepoints (24 and 72 hrs). HG during 4 hrs induced a 40% rise in glucose uptake, which was unaffected by insulin. In contrast, after 72 hrs, glucose uptake was increased by 50%, only under insulin stimulus. Losartan blunted the effects of HG on GLUT1, GLUT4, and fibronectin expression and on glucose uptake. Results suggest that MCs can be highly susceptible to the HG environment since they uptake glucose in both an insulin-independent and insulin-dependent manner. The beneficial effects of angiotensin II inhibition in DN may also involve a decrease in the rate of glucose uptake by MCs.

[Molecular bases of diabetic nephropathy]

The determinant of the diabetic nephropathy is hyperglycemia, but hypertension and other genetic factors are also involved. Glomerulus is the focus of the injury, where mesangial cell proliferation and extracellular matrix occur because of the increase of the intra- and extracellular glucose concentration and overexpression of GLUT1. Sequentially, there are increases in the flow by the poliol pathway, oxidative stress, increased intracellular production of advanced glycation end products (AGEs), activation of the PKC pathway, increase of the activity of the hexosamine pathway, and activation of TGF-beta1. High glucose concentrations also increase angiotensin II (AII) levels. Therefore, glucose and AII exert similar effects in inducing extracellular matrix formation in the mesangial cells, using similar transductional signal, which increases TGF-beta1 levels. In this review we focus in the effect of glucose and AII in the mesangial cells in causing the events related to the genesis of diabetic nephropathy. The alterations in the signal pathways discussed in this review give support to the observational studies and clinical assays, where metabolic and antihypertensive controls obtained with angiotensin-converting inhibitors have shown important and additive effect in the prevention of the beginning and progression of diabetic nephropathy. New therapeutic strategies directed to the described intracellular events may give future additional benefits.

ACE-dependent and chymase-dependent angiotensin II generation in normal and glucose-stimulated human mesangial cells

High glucose (HG) increases angiotensin II (AngII) generation in mesangial cells (MC). Chymase, an alternative AngII-generating enzyme, is upregulated in the glomeruli of diabetic kidneys. In this study, we examined AngII synthesis by human MC via angiotensin-converting enzyme (ACE)-dependent and chymase-dependent pathways under normal glucose (NG, 5 mM) and HG (30 mM) conditions. NG cells expressed ACE and chymase mRNA. Under NG conditions the chymase inhibitor chymostatin reduced AngII levels in cell lysates and in the culture medium, and the ACE inhibitor captopril had no effect. HG induced a 3-fold increase in chymase mRNA and protein but not in ACE mRNA; however, HG induced a 10-fold increase in intracellular ACE activity. The increase in AngII generation induced by HG was found in the cell lysate but not in the culture medium. The rise in intracellular AngII was not prevented by captopril or by chymostatin. Moreover, captopril inhibited extracellular ACE activity but failed to block intracellular ACE activity; these results suggested that captopril was unable to reach intra-cellular ACE. Losartan did not change the intracellular AngII content in either NG or HG conditions, and this lack of change suggested that the increase in AngII was due to intracellular generation. Together these results suggest that chymase may be active in human MC and that both ACE and chymase are involved in increased AngII generation during the HG stimulus by different mechanisms, including an upregulation of chymase mRNA and a rise in intracellular ACE activity, favoring the generation and accumulation of intracellular AngII.

Angiotensin II- and glucose-stimulated extracellular matrix production: mediation by the insulin-like growth factor (IGF) axis in a murine mesangial cell line

In diabetic nephropathy, glomerular mesangial cells exhibit aberrant anabolic activity that includes excessive production of extracellular matrix (ECM) proteins, leading to crowding of filtration surface areas and possible renal failure. In the present study, a murine mesangial cell line (MES-13 cells) was studied to determine the roles of the renin-angiotensin system (RAS) and the insulin-like growth factor (IGF) axis in the anabolic response to elevated glucose levels. Culture of MES-13 cells in medium containing supra-physiological glucose concentrations (>5.5 mmol/l) resulted in increased production of ECM proteins including laminin, fibronectin, and heparan sulfate proteoglycan with concurrent increases in IGF-binding protein (IGFBP)-2 production. These responses were blocked by the angiotensin receptor antagonists saralasin and losartan, while exogenous angiotensin II (Ang II) treatment directly stimulated increases in ECM and IGFBP-2. In all experiments, IGFBP-2 levels were correlated with anabolic activity implicating IGFBP-2 as a possible mediator in cellular responses to high glucose and Ang II. Such mediation appears to involve IGFBP-2 modulation of IGF-I signaling, since all responses to high glucose or Ang II were blocked by immuno-neutralization of IGF-I. These data suggest alterations in the IGF axis as key mechanisms underlying nephropathic responses of mesangial cells to Ang II and high glucose.

Expression of angiotensin I-converting enzymes and bradykinin B2 receptors in mouse inner medullary-collecting duct cells

We described in mouse inner medullary-collecting duct cells (mIMCD-3) the somatic and the N-domain ACE synthesis and its interaction with the kallikrein-kinin system co-localized in the same cells. We purified two ACE forms from culture medium, M1 (130 kDa) and M2 (N-domain, 60 kDa), and cellular lysate, C1 (130 kDa) and C2 (N-domain, 60 kDa). Captopril and enalaprilat inhibited the purified enzymes. The immunofluorescence studies indicated that ACE is present in the membrane, cytoplasm and in the cell nucleus. Kinin B1 and B2 receptors were detected by immunofluorescence and showed to be activated by BK and DesR9 BK, increasing the acidification rate which was enhanced in the presence of enalaprilat. The presence of secreted and intracellular ACE in mIMCD-3 confirmed the hypothesis previously proposed by our group for a new site of ACE secretion in the collecting duct.

Effect of ACE inhibitors on angiotensin II in rat mesangial cells cultured in high glucose

This study investigates whether angiotensin converting enzyme (ACE) inhibitors can block high glucose-induced increase of angiotensin II (Ang II) levels in cultured rat mesangial cells. Incubation of cells with high glucose for 5 days increased Ang II in the cell media (extracellular) and cell lysates (intracellular) by approximately 1.5-fold and approximately 2.2-fold, respectively. Captopril blocked high glucose-induced increase in extracellular Ang II levels but not in intracellular Ang II levels. Since the inability of captopril to inhibit intracellular Ang II in intact cells could be explained by failure of the drug to enter the cells, further studies were performed using lysates from cells pre-treated with high glucose for 5 days. Addition of captopril to cell lysates was indeed able to inhibit Ang I conversion to Ang II but only partially. These findings suggest the involvement of non-ACE mechanisms in Ang II formation from Ang I in mesangial cells.

Expression and localization of N-domain ANG I-converting enzymes in mesangial cells in culture from spontaneously hypertensive rats

The angiotensin-converting enzyme (ACE) profile in urine of hypertensive patients and spontaneously hypertensive rats (SHR; 90- and 65-kDa N-domain ACEs) is different from that of healthy subjects and Wistar rats (190 and 65 kDa). In addition, four ACE isoforms were purified from mesangial cells (MC) of Wistar rats in the intracellular compartment (130 and 68 kDa) and as secreted forms (130 and 60 kDa). We decided to characterize ACE forms from SHR MC in culture. Analysis of the ACE gene showed that SHR MC are able to express ACE mRNA. The concentrated medium and cell homogenate were separately purified by gel filtration and then subjected to lisinopril-Sepharose chromatography. The molecular masses of purified enzymes, 90 kDa for ACEm1A and 65 kDa for ACEm2A (secreted enzymes) and 90 kDa for ACEInth1A and 65 kDa for ACEInth2A (intracellular), were different from those of Wistar MC. The purified enzymes are Cl−dependent, inhibited by enalaprilat and captopril, and able to hydrolyze AcSDKP. Immunofluorescence and cell fractionation followed by Western blotting showed predominant immunoreaction of the 9B9 antiserum for N-domain ACE in the nuclei. The N-domain ACE was localized in the glomerulus from Wistar rats and SHR. ANG II and ANG-(1–7) were localized in the cell cytoplasm and nuclei. The 90-kDa N-domain ACE, described recently as a possible genetic marker of hypertension, was found inside the cell nuclei of SHR MC colocalized with ANG II and ANG-(1–7). The presence of ANG II in the cell nuclei could suggest an important role for this peptide in the transcription of new genes.

N-domain angiotensin-converting enzyme isoform expression in tissues of Wistar and spontaneously hypertensive rats

BACKGROUND

Angiotensin I-converting enzyme (ACE) is a protein containing two active sites, called N- and C-domains, according to their position in the protein.

AIM

The aim of the present study was to verify whether the expression of the N-domain ACEs detected in the urine of Wistar and spontaneously hypertensive (SHR) rats was restricted to the kidney.

METHODS

Adrenal, aorta, heart, liver, lung, kidney and testicle tissue from Wistar rats and spontaneously hypertensive rats were homogenized in assay buffer and analyzed by gel filtration, Western blotting and radio-immunoassay.

RESULTS

Two peaks (at 136 and 69 kDa) with ACE activity upon ZPhe-His-Leu were separated by gel filtration from homogenate tissues of Wistar rats, in contrast with the tissue from hypertensive rats, which showed ACE forms of 96 and 69 kDa. The bands detected by Western blotting for all studied tissue from Wistar and spontaneously hypertensive rats showed a correspondence with the two peaks containing ACE activity detected in the polyacrylamide gel slices. Angiotensin II levels were increased in hypertensive rat tissue when compared with Wistar rat tissues. In addition, captopril 3 micromol/l inhibited the enzymic activity, where the Km was in the order of mmol/l and micromol/l using hippuryl-His-Leu and Abz-Ser-Asp-Lys(Dnp)Pro-OH as substrates, respectively. All tissues from Wistar rats presented ACE with 136 kDa, similar to somatic ACE, and N-domain ACE with 69 kDa. In the same tissue of spontaneously hypertensive rats, 96 and 69 kDa N-domain ACEs were detected.

CONCLUSIONS

Our results demonstrated that N-domain ACEs were not exclusively produced in the kidney and excreted in the urine; they were expressed in all tissue studied, suggesting that these enzymes could influence local angiotensin II production, contributing to organ-specific regulation.

A novel mechanism for angiotensin II formation in streptozotocin-diabetic rat glomeruli

Recent evidence suggests that the intrarenal renin-angiotensin system (RAS) may play an important role in the development of glomerular changes associated with diabetic nephropathy. In this study, the glomerular RAS was examined in male Sprague-Dawley rats made diabetic with streptozotocin (STZ), and the findings compared with those obtained in control nondiabetic rats. In diabetic rat glomerular extracts, angiotensinogen and angiotensin II (ANG II) levels were increased significantly by 2.2- and 1.9-fold, respectively, compared with nondiabetic controls. No significant differences in ANG I and angiotensin-converting enzyme (ACE) levels were observed between these groups. The HPLC analysis of the glomerular extracts demonstrated that exogenous ANG I was converted into various ANG peptides including ANG II, ANG1-9, and ANG1-7. A significant increase in formation of ANG II from exogenous ANG I was observed in STZ rats compared with control rats. Preincubation of glomerular extracts with captopril resulted in a 20-30% decrease in ANG II conversion from exogenous ANG I in diabetic and control rats. The possible role of ANG1-9 in formation of ANG II was examined by HPLC. Exogenous ANG1-9 in glomerular extracts was converted into ANG II, this conversion being significantly higher in STZ rats than in control rats. These findings provide new information that ANG1-9 is produced in rat glomerular extracts, can be converted to ANG II, and that this conversion is also stimulated in diabetic rat glomeruli. Thus this study demonstrates that in diabetic rats, glomerular ANG II levels are increased due to an increase in angiotensinogen and an increase in the formation of ANG II.

N-Domain angiotensin I-Converting enzyme expression in renal artery of Wistar, Wistar kyoto, and spontaneously hypertensive rats

One of the most intriguing features in kidney transplantation is the finding that kidneys from hypertensive rats can transfer arterial hypertension on transplantation into normotensive rats. Some evidence also suggest that, in humans undergoing renal transplantation, the genotype of the donor kidney determines the blood pressure in the recipient. The renin-angiotensin-aldosterone system is the major etiological candidate in hypertension because it plays an important role in the control of cardiovascular homeostasis. Angiotensin-converting enzyme (ACE) cleaves the C-terminal from angiotensin I as well as from bradykinin. Thus, by generating the potent vasoconstrictor angiotensin II and by degrading the vasodepressor bradykinin, ACE is considered to play a role in blood pressure regulation. We have previously described the presence of N-domain ACE in urine of Wistar (W), Wistar Kyoto (WKY), and spontaneously hypertensive rats (SHR), all of which can hydrolyze the vasodilator peptide Angiotensin 1-7 and also the N-Acetyl-Ser-Asp-Lys-Pro, two peptides described as specific for N-domain ACE. These findings suggest that the 90 kd ACE isoform found in urine and in tissues of SHR is a possible genetic marker of hypertension. Based on the fact that the renal artery has an important role in the control of renal blood flow, we evaluated the presence of N-domain ACE in the renal artery of hypertensive and normotensive rats. Using Western blotting techniques on the renal arteries of W and WKY rats, we detected the 190-kd ACE (similar to somatic ACE) and also the 65-kd ACE previously described in urine and renal tissue as N-domain ACE. The 65-kd and 90-kd isoforms of ACE were also detected in renal arteries in SHR rats. Further studies are required to understand the role of 90-kd enzyme described as a possible local marker of hypertension, its contribution in angiotensin catabolism, and whether this abnormal form of the enzyme has any link with the development and transfer of hypertension after transplantation.

Neutral endopeptidase expression in mesangial cells

In the kidney, neutral endopeptidase (NEP) is implicated in the metabolism of several peptides involved in blood pressure and sodium homeostasis control, such as the atrial natriuretic peptide, bradykinin and angiotensin I. Due to its physiological importance in the modulation of pressor responses, the presence of NEP in mouse mesangial cells has been investigated, since these cells control glomerular function and are able to synthesise components of the renin-angiotensin system. A NEP-like activity (NEP-like) that cleaves the fluorogenic substrates Abz-BKQ-EDDnp and Abz-DRRL-EDDnp was purified from mesangial cell lysate by ion-exchange, followed by gel filtration chromatography. The enzyme was able to hydrolyse bradykinin at the G4-F5 peptide bond and was inhibited by thiorphan. A pH study established that enzyme activity was maximal at pH 7.5 and the determined K(m) was 4.86 M using Abz-DRRL-EDDnp as substrate. NEP-like was recognised by monoclonal anti-NEP and had a molecular mass of 95 kDa. The purified enzyme was sequenced and showed similarity with human, rat, mouse and rabbit NEPs. We isolated, for the first time, NEP-like from mesangial cells. This enzyme could have an important role in the renal physiology by its action upon different peptides that are able to alter renal haemodynamics.

High glucose concentration stimulates intracellular renin activity and angiotensin II generation in rat mesangial cells

Increased intrarenal renin-angiotensin system activity contributes to diabetic nephropathy. ANG II generation in mesangial cells (MC) is increased by high-glucose (HG) exposure. This study assessed the mechanisms involved in the glucose-induced ANG II generation in rat MC. Under basal conditions, MC mainly secreted prorenin. HG decreased prorenin secretion and induced a striking 30-fold increase in intracellular renin activity. After 72 h of HG exposure, only the mRNA levels for angiotensinogen and angiotensin-converting enzyme (ACE) were significantly elevated. However, after shorter periods of 24 h of HG stimulation the mRNA levels of the enzymes prorenin and cathepsin B, besides that for ACE, were significantly increased. The results suggest that the HG-induced increase in ANG II generation in MC results from an increase in intracellular renin activity mediated by at least three factors: a time-dependent stimulation of (pro)renin gene transcription, a reduction in prorenin enzyme secretion, and an increased rate of conversion of prorenin to active renin, probably mediated by cathepsin B. The increase in angiotensinogen mRNA in parallel to increased renin activity indicates that HG also increased the availability of the renin substrate. The consistent upregulation of ACE mRNA suggests that, besides renin, ACE is directly involved in the increased mesangial ANG II generation induced by HG.

N-domain angiotensin I-converting enzyme with 80 kDa as a possible genetic marker of hypertension

We have previously described angiotensin I-converting enzyme (ACE) forms in urine of normotensive (190 and 65 kDa) and hypertensive patients (90 and 65 kDa, N-domain ACEs). Based on the results described above, experimental and genetic models of hypertension were investigated to distinguish hemodynamic and genetic influence on the generation of ACE profile in urine: Wistar-Kyoto and Brown Norway rats (WKY and BN), spontaneously and stroke-prone spontaneously hypertensive rats (SHR and SHR-SP), one kidney/one clip rats (1K1C), deoxycorticosterone acetate (DOCA) salt-treated and untreated rats, and enalapril-treated SHR (SHRen). Two peaks with ACE activity were separated from the urine of WKY and BN rats submitted to an AcA-44 column, WK-1/BN-1 (190 kDa), and WK-2/BN-2 (65 kDa), as described for urine of normotensive subjects. The same results were obtained for urine of 1K1C and DOCA salt-treated and untreated rats, analyzed to evaluate the influence of hemodynamic factors in the ACE profile in urine. The urine from SHR, SHR-SP, and SHRen presented 80 (S-1, SP-1, Sen-1) and 65 (S-2, SP-2, Sen-2) kDa ACE forms, differing from the urine profile of normotensive rats, but similar to that described for hypertensive patients. The presence of 80 kDa ACE in urine of SHR, SHR-SP, and SHRen and its absence in urine of experimental hypertensive rats (1K1C and DOCA salt) support the hypothesis that this enzyme could be a possible genetic marker of hypertension. Taken together, our results provide evidence that ACE forms with 90/80 kDa isolated from the urine of hypertensive subjects and genetic hypertensive animals behaves as a possible genetic marker of hypertension and not as a marker of high blood pressure.

Mechanism of increased angiotensin II levels in glomerular mesangial cells cultured in high glucose

Previous studies have shown that glucose increases angiotensin II (AngII) levels in rat glomerular mesangial cells and that AngII mediates the inhibitory effects of high glucose on matrix degradation in these cells. The present study addresses the following questions: (1) What are the mechanisms for the generation of AngII in mesangial cells? (2) What are the effects of glucose on AngII generation by these mechanisms? Experiments employed primary mesangial cells from normal Sprague-Dawley rats. The levels of immunoreactive angiotensinogen (AGT), angiotensin I (AngI), and angiotensin II (AngII) were measured by ELISA. AGT mRNA expression was determined by Northern blot analysis. Incubation of cells for 24 h in high glucose (30 mM) increased AGT levels by 1.5-fold and increased AGT mRNA expression; this was accompanied by a 1.5-fold increment in AngI and 1.7-fold increment in AngII levels. Renin activity (measured as AngI generation in the presence of excess AGT) and ACE levels and activity were not altered by high glucose. In further experiments, the effect of high glucose on formation of Ang peptides from exogenous AngI in mesangial cell extracts was examined using HPLC. Exogenous AngI was converted into various Ang peptides, including AngII, Ang(1-9), Ang(1-7), and Ang(3-8). A significant increase in formation of AngII from AngI was observed in cells incubated in high glucose. In addition, AngII production from exogenous Ang(1-9) in cell extracts was also stimulated by high glucose. These findings demonstrate that glucose increases mesangial AngII levels via an increase in AGT and AngI. In addition, this study provides new information that Ang(1-9) is produced by mesangial cells, can be converted to AngII, and that this conversion is also stimulated under high-glucose conditions.

Moexipril and quinapril inhibition of tissue angiotensin-converting enzyme activity in the rat: evidence for direct effects in heart, lung and kidney and stimulation of prostacyclin generation

The activation of angiotensin converting enzyme (ACE) may contribute to the development of vascular and myocardial structural changes. The level of ACE is stable in human plasma, and only limited data are available on its regulation at the tissue level. The aim of this study was to characterize the effects of two ACE inhibitors, moexipril and quinapril on tissue ACE activity. Adult male rats were treated intragastrically once daily for 6 days either with 2 mg/kg moexipril or quinapril. After single treatment, moexipril and quinapril effectively inhibited ACE activity in plasma and slightly in heart and aorta, whereas after 6 days of treatment they inhibited ACE activity in plasma (87% and 94%, respectively), lung (92% and 93%), myocardium (26% and 23%), kidney (21% and 20%), and aorta (39% and 40%), but not in skeletal muscle. Interestingly, the two ACE-inhibitors also induced a significant increase in cardiac homogenates of 6-keto-PGF1alpha levels, an important index of PGI2 generation. To test whether the reduced effects of ACE inhibitors in heart and kidney were caused by a limited availability of the drugs, 100 microl of lung, heart and kidney homogenates from control rats were incubated in vitro with moexipril and quinapril immediately before assay. Both drugs were more effective in lung than heart and kidney homogenates, with inhibition values superimposable to those obtained in vivo. These results clearly indicate that inhibition of tissue ACE activity does not depend primarily on the availability of ACE inhibitors in each organ.

Pathways for angiotensin-(1---7) metabolism in pulmonary and renal tissues

Two of the primary sites of actions for angiotensin (ANG)-(1---7) are the vasculature and the kidney. Because little information exists concerning the metabolism of ANG-(1---7) in these tissues, we investigated the hydrolysis of the peptide in rat lung and renal brush-border membrane (BBM) preparations. Radiolabeled ANG-(1---7) was hydrolyzed primarily to ANG-(1---5) by pulmonary membranes. The ANG-converting enzyme (ACE) inhibitor lisinopril abolished the generation of ANG-(1---5), as well as that of smaller metabolites. Kinetic studies of the hydrolysis of ANG-(1---7) to ANG-(1---5) by somatic (pulmonary) and germinal (testes) forms of rat ACE yielded similar values, suggesting that the COOH-domain is responsible for the hydrolysis of ANG-(1---7). Pulmonary metabolism of ANG-(1---5) yielded ANG-(3---5) and was independent of ACE but may involve peptidyl or dipeptidyl aminopeptidases. In renal cortex BBM, ANG-(1---7) was rapidly hydrolyzed to mono- and dipeptide fragments and ANG-(1---4). Aminopeptidase (AP) inhibition attenuated the hydrolysis of ANG-(1---7) and increased ANG-(1---4) formation. Combined treatment with AP and neprilysin (Nep) inhibitors abolished ANG-(1---4) formation and preserved ANG-(1---7). ACE inhibition had no effect on the rate of hydrolysis or the metabolites formed in the BBM. In conclusion, ACE was the major enzymatic activity responsible for the metabolism of ANG-(1---7) in the lung, which is consistent with the ability of ACE inhibitors to increase the half-life of circulating ANG-(1---7) and raise endogenous levels of the peptide. An alternate pathway of metabolism was revealed in the renal cortex, where increased AP and Nep activities, relative to ACE activity, promote conversion of ANG-(1---7) to ANG-(1---4) and smaller fragments.

Role of angiotensin II in diabetic nephropathy

Considerable evidence suggests that the intrarenal renin-angiotensin system plays an important role in diabetic nephropathy. Angiotensin-converting enzyme (ACE) inhibitors and angiotensin II (Ang II) receptor blockers (ARBs) can attenuate progressive glomerulosclerosis in disease models and can slow disease progression in humans. Because agents that interfere with Ang II action may decrease glomerular injury without altering glomerular pressures, it has been suggested that Ang II has direct effects on glomerular cells to induce sclerosis independent of its hemodynamic actions. To study nonhemodynamic effects of Ang II on matrix metabolism, many investigators have used cell culture systems. Glucose and Ang II have been shown to produce similar effects on renal cells in culture. For instance, incubation of mesangial cells in high-glucose media or in the presence of Ang II stimulates matrix protein synthesis and inhibits degradative enzyme (e.g., collagenase, plasmin) activity. Glucose and Ang II also can inhibit proximal tubule proteinases. Glucose increases expression of the angiotensinogen gene in proximal tubule cells and Ang II production in primary mesangial cell culture, which indicates that high glucose itself can activate the renin-angiotensin system. The effects of glucose and Ang II on mesangial matrix metabolism may be mediated by transforming growth factor-beta (TGF-beta). Exposure of mesangial cells to glucose or Ang II increases TGF-beta expression and secretion. Their effects on matrix metabolism can be blocked by anti-TGF-beta antibody or ARBs such as losartan, which also prevents the glucose-induced increment in TGF-beta secretion. Taken together, these findings support the hypothesis that the high-glucose milieu of diabetes increases Ang II production by renal, and especially, mesangial cells, which results in stimulation of TGF-beta secretion, leading to increased synthesis and decreased degradation of matrix proteins, thus producing matrix accumulation. This may be an important mechanism linking hyperglycemia and Ang II in the pathogenesis of diabetic nephropathy.

Angiotensin I-converting enzyme isoforms (high and low molecular weight) in urine of premature and full-term infants

Angiotensin I-converting enzyme (ACE) isoforms in urine from healthy and mildly hypertensive untreated patients have been described in the literature. Healthy subjects have high- and low-molecular-weight ACEs (170 and 65 kDa), whereas mildly hypertensive untreated patients have only low-molecular-weight ACEs (90 and 65 kDa), both of which resemble ACE from the N-terminal domain. Previous studies have shown that ACE is regulated during development, and renal tubules of premature human infants are not completely mature, given that nephrogenesis is not complete until the 36th week of gestation. The aim of the present study was to purify and characterize ACE isoforms from urine of premature and full-term infants and to detect the presence of the N-domain form of ACE during prenatal development. Urine from premature and full-term infants was concentrated in an Amicon concentrator, dialyzed in the same equipment against 50 mmol/L Tris-HCl buffer (pH 8.0) that contained 150 mmol/L NaCl, and submitted to gel filtration on an AcA-34 column equilibrated with the buffer described above. Two peaks (P1 and P2 for premature infants; TP1 and TP2 for full-term infants) with ACE activity on hippuryl-His-Leu (K(m), 3 mmol/L) were detected. All enzymes were Cl(-) dependent and inhibited by captopril and EDTA. The peptides angiotensin-(1-7) and N-acetyl-Ser-Asp-Lys-Pro, described as specific for N-domain ACE, were hydrolyzed by P2 and TP2, which suggests that both enzymes are N-domain ACE. In premature infants, P1 activity with hippuryl-His-Leu was 12-fold lower than P2 activity, but in full-term infants, the difference between TP1 and TP2 was 1.6-fold. Chromatography profiles of urine from premature infants were analyzed on days 1, 3, 7, 14, 21, and 30 after birth. The P1 of ACE was detected around the 21st and 30th days, whereas P2 was detected from day 1. These results suggest that ACE activity is related to renal development and that N-domain ACE as well as full-length ACE is present in urine from premature infants. This may indicate that healthy subjects produce and secrete the N-domain form of ACE even before term development.

Hydrolysis by cathepsin B of fluorescent peptides derived from human prorenin

Cathepsin B is a lysosomal thiolprotease that, because of its colocalization with renin and its ability to activate prorenin, has been proposed as a prorenin processing enzyme. To characterize the biochemical aspect of this potential cathepsin B activity in more detail, we synthesized and assayed with human cathepsin B the internally quenched fluorescent peptide Abz-FSQPMKRLTLGNTTQ-EDDnp (Abz, ortho-aminobenzoic acid fluorescent group and EDDnp, N-¿2, 4-dinitrophenyl-ethylenediamine quencher group) that contains 7 amino acids for each side of the R-L bond that is the processing site of human prorenin. Human cathepsin B hydrolyzed this peptide at the correct site (R-L bond), with k(cat)/K(m)=75 mmol/L(-1) s(-1). Analogues of this peptide obtained by Ala scanning at positions P(5) to P(5)' were also synthesized and assayed as substrates for human cathepsin B. The obtained specificity constant (k(cat)/K(m)) values have a significant parallel with the previous data of prorenin activation by AtT-20 cells and in vitro by cathepsin B. In addition, we demonstrated the presence of cathepsin B-like activity in rat mesangial cells and the ability of its whole soluble fraction lysates, as well as that of purified cloned rat cathepsin B, to hydrolyze Abz-IKKSSF-EDDnp at the K-S bond, which contains 6 amino acids of rat prorenin processing site. The specificity data of cathepsin B toward peptides derived from prorenin processing site support the view that human or rodent cathepsin B could be involved in the intracellular processing of prorenin that is locally synthesized or taken up from the extracellular compartment.

Effects of kinins upon cytosolic calcium concentrations in mouse mesangial cells

Bradykinin (BK) induces increases in cytosolic calcium concentration [Ca++]i in several cell lines. Because the role of BK in the renal system, particularly in mesangial cell (MC), is not clear, we investigated the effects of kinins on [Ca++]i in mouse-immortalized MC. [Ca++]i was evaluated by spectrofluorometry and expressed as a ratio between the obtained and basal [Ca++]i. BK (0.1 microM) induced a non-sustained increase in [Ca++]i (4.70 +/- 0.27; N = 28). A similar effect was observed with the B2 receptor agonist, Tyr8-BK (0.1 microM, 3.34 +/- 0.48; N = 7), while B1 receptor agonists, des-Arg10-Kallidin (Kal) (1 microM, N = 11) and des-Arg9-BK (1 microM, N = 8), exhibited only discrete responses (1.45 +/- 0.08 and 1.12 +/- 0.04, respectively). Cross-desensitization was seen between BK and Tyr8-BK, but not between BK and des-Arg10-Kal. The BK response was decreased (5.09 +/- 0.30, N = 6 to 1.57 +/- 0.12, N = 7, P < 0.001) by the B2 receptor antagonist HOE 140 (0.1 microM, 15 min), while the B1 receptor antagonist des-Arg9-[Leu8]-BK (1 microM, 15 min) had no effect on BK or des-Arg10-Kal actions. Incubation of cells with Escherichia coli lipopolysaccharide (100 microg/ml, 24 h) alone or in association with tumor necrosis factor-alpha (TNF-alpha) (10 ng/ml, N = 6) did not enhance B1 agonist responses. BK was inhibited by repeated cell washouts in zero Ca++ solution (2.04 +/- 0.19, N = 6 P < 0.001), and the residual response was almost abolished by thapsigargin (Thaps) a sarcoplasmic reticulum (SR) calcium-ATPase inhibitor (1 microM) (1.18 +/- 0.08, N = 5 P < 0.001). Additionally, BK was not inhibited by verapamil (50 microM), nifedipine (30 microM), Ni++ (300 microM) or La (10 microM). In conclusion, BK induces [Ca++]i in mouse MC mainly by B2 receptor activation. B1 receptors have a minor role in this phenomenon even in the presence of known B1 receptor synthesis inducers. Finally, BK mobilizes extracellular calcium sources and, to a lesser extent, intracellular Thaps-sensitive calcium stores. The ion channels involved in calcium influx remain to be detected.