Urinary renin and angiotensinogen in type 2 diabetes: added value beyond urinary albumin?

High glucose induces trafficking of prorenin receptor and stimulates profibrotic factors in the collecting duct

Growing evidence indicates that prorenin receptor (PRR) is upregulated in collecting duct (CD) of diabetic kidney. Prorenin is secreted by the principal CD cells, and is the natural ligand of the PRR. PRR activation stimulates fibrotic factors, including fibronectin, collagen, and transforming growth factor-β (TGF-β) contributing to tubular fibrosis. However, whether high glucose (HG) contributes to this effect is unknown. We tested the hypothesis that HG increases the abundance of PRR at the plasma membrane of the CD cells, thus contributing to the stimulation of downstream fibrotic factors, including TGF-β, collagen I, and fibronectin. We used streptozotocin (STZ) male Sprague–Dawley rats to induce hyperglycemia for 7 days. At the end of the study, STZ-induced rats showed increased prorenin, renin, and angiotensin (Ang) II in the renal inner medulla and urine, along with augmented downstream fibrotic factors TGF-β, collagen I, and fibronectin. STZ rats showed upregulation of PRR in the renal medulla and preferential distribution of PRR on the apical aspect of the CD cells. Cultured CD M-1 cells treated with HG (25 mM for 1 h) showed increased PRR in plasma membrane fractions compared to cells treated with normal glucose (5 mM). Increased apical PRR was accompanied by upregulation of TGF-β, collagen I, and fibronectin, while PRR knockdown prevented these effects. Fluorescence resonance energy transfer experiments in M-1 cells demonstrated augmented prorenin activity during HG conditions. The data indicate HG stimulates profibrotic factors by inducing PRR translocation to the plasma membrane in CD cells, which in perspective, might be a novel mechanism underlying the development of tubulointerstitial fibrosis in diabetes mellitus.

α-Ketoglutarate Upregulates Collecting Duct (Pro)renin Receptor Expression, Tubular Angiotensin II Formation, and Na+ Reabsorption During High Glucose Conditions

Diabetes mellitus (DM) causes high glucose (HG) levels in the plasma and urine. The (pro)renin receptor (PRR) is a key regulator of renal Na+ handling. PRR is expressed in intercalated (IC) cells of the collecting duct (CD) and binds renin to promote angiotensin (Ang) II formation, thereby contributing to Na+ reabsorption. In DM, the Kreb's cycle is in a state of suppression in most tissues. However, in the CD, expression of glucose transporters is augmented, boosting the Kreb's cycle and consequently causing α-ketoglutarate (αKG) accumulation. The αKG receptor 1 (OXGR1) is a Gq-coupled receptor expressed on the apical membrane of IC cells of the CD. We hypothesize that HG causes αKG secretion and activation of OXGR1, which increases PRR expression in CD cells. This effect then promotes intratubular AngII formation and Na+ reabsorption. To test this hypothesis, streptozotocin (STZ)-induced diabetic mice were treated with or without montelukast (ML), an OXGR1 antagonist, for 6 days. STZ mice had higher urinary αKG and PRR expression along with augmented urinary AngII levels and Na+ retention. Treatment with ML prevented all these effects. Similarly, primary cultured inner medullary CD cells treated with HG showed increased PRR expression, while OXGR1 antagonist prevented this effect. αKG increases PRR expression, while treatments with ML, PKC inhibition, or intracellular Ca2+ depletion impair this effect. In silico analysis suggested that αKG binds to mouse OXGR1. These results indicate that HG conditions promote increased levels of intratubular αKG and OXGR1-dependent PRR upregulation, which impact AngII formation and Na+ reabsorption.

Effect of sodium bicarbonate supplementation on the renin-angiotensin system in patients with chronic kidney disease and acidosis: a randomized clinical trial

Background

Acidosis-induced kidney injury is mediated by the intrarenal renin-angiotensin system, for which urinary renin is a potential marker. Therefore, we hypothesized that sodium bicarbonate supplementation reduces urinary renin excretion in patients with chronic kidney disease (CKD) and metabolic acidosis.

Methods

Patients with CKD stage G4 and plasma bicarbonate 15–24 mmol/l were randomized to receive sodium bicarbonate (3 × 1000 mg/day, ~ 0.5 mEq/kg), sodium chloride (2 × 1,00 mg/day), or no treatment for 4 weeks (n = 15/arm). The effects on urinary renin excretion (primary outcome), other plasma and urine parameters of the renin-angiotensin system, endothelin-1, and proteinuria were analyzed.

Results

Forty-five patients were included (62 ± 15 years, eGFR 21 ± 5 ml/min/1.73m², plasma bicarbonate 21.7 ± 3.3 mmol/l). Sodium bicarbonate supplementation increased plasma bicarbonate (20.8 to 23.8 mmol/l) and reduced urinary ammonium excretion (15 to 8 mmol/day, both P < 0.05). Furthermore, a trend towards lower plasma aldosterone (291 to 204 ng/L, P = 0.07) and potassium (5.1 to 4.8 mmol/l, P = 0.06) was observed in patients receiving sodium bicarbonate. Sodium bicarbonate did not significantly change the urinary excretion of renin, angiotensinogen, aldosterone, endothelin-1, albumin, or α1-microglobulin. Sodium chloride supplementation reduced plasma renin (166 to 122 ng/L), and increased the urinary excretions of angiotensinogen, albumin, and α1-microglobulin (all P < 0.05).

Conclusions

Despite correction of acidosis and reduction in urinary ammonium excretion, sodium bicarbonate supplementation did not improve urinary markers of the renin-angiotensin system, endothelin-1, or proteinuria. Possible explanations include bicarbonate dose, short treatment time, or the inability of urinary renin to reflect intrarenal renin-angiotensin system activity.

Graphic abstract

Urinary angiotensinogen: an indicator of active antineutrophil cytoplasmic antibody-associated glomerulonephritis

BACKGROUND

One of the major challenges in improving the management of antineutrophil cytoplasmic antibody-associated glomerulonephritis (ANCA-GN) is the lack of a disease-specific indicator for histological lesions and disease activity. Here we tested the utility of urinary angiotensinogen (UAGT) as a biomarker of renal disease activity in ANCA-GN.

METHODS

A prospective, two-stage cohort study was performed in ANCA-GN patients. In Stage I, UAGT was measured at the time of renal biopsy in 69 patients from two centers (test set) and 25 patients from two other centers (validation set). In Stage II, UAGT was monitored in 50 subjects in the test set for 24 months.

RESULTS

In Stage I, UAGT significantly increased in ANCA-GN patients, correlating well with cellular crescents formation and active interstitial inflammation. Patients with crescentic ANCA-GN exhibited the highest UAGT compared with other histopathological classes of ANCA-GN. After multivariable adjustment, the highest quartile of UAGT, compared with the lowest quartile, associated with a 6-fold increased risk of crescentic ANCA-GN. For predicting crescentic ANCA-GN, UAGT [area under the receiver operating characteristics curve (AUC) = 0.88] outperformed albuminuria (AUC = 0.73) and estimated glomerular filtration rate (AUC = 0.69). UAGT improved the performance of those clinical markers in diagnosing crescentic ANCA-GN (P < 0.034), suggesting a role of UAGT in identifying active crescentic ANCA-GN. In Stage II, UAGT decreased after immunotherapy and increased at the time of renal relapse during the 2-year follow-up, suggesting the usefulness of UAGT to monitor disease activity over time.

CONCLUSIONS

These results suggest the potential use of UAGT for assessing disease activity and renal relapse in ANCA-GN.

Megalin: a Novel Determinant of Renin-Angiotensin System Activity in the Kidney?

PURPOSE OF REVIEW

Megalin is well known for its role in the reabsorption of proteins from the ultrafiltrate. Recent studies suggest that megalin also reabsorbs renin and angiotensinogen. Indeed, without megalin urinary renin and angiotensinogen levels massively increase, and even prorenin becomes detectable in urine.

RECENT FINDINGS

Intriguingly, megalin might also contribute to renal angiotensin production, as evidenced from studies in megalin knockout mice. This review discusses these topics critically, concluding that urinary renin-angiotensin system components reflect diminished reabsorption rather than release from renal tissue sites and that alterations in renal renin levels or megalin-dependent signaling need to be ruled out before concluding that angiotensin production at renal tissue sites is truly megalin dependent. Future studies should evaluate megalin-mediated renin/angiotensinogen transcytosis (allowing interstitial angiotensin generation), and determine whether megalin prefers prorenin over renin, thus explaining why urine normally contains no prorenin.

Urinary angiotensinogen increases in the absence of overt renal injury in high fat diet-induced type 2 diabetic mice

AIM OF THE STUDY

During type 2 diabetes (T2D) and hypertension there is stimulation of renal proximal tubule angiotensinogen (AGT), but whether urinary excretion of AGT (uAGT) is an indicator of glomerular damage or intrarenal RAS activation is unclear. We tested the hypothesis that elevations in uAGT can be detected in the absence of albuminuria in a mouse model of T2D.

METHODS

Male C57BL/6 mice (N = 10) were fed a high fat (HFD; 45% Kcal from fat) for 28 weeks, and the metabolic phenotype including body weight, blood pressures, glucose, insulin, ippGTT, HOMA-IR, and cholesterol was examined. In addition, kidney Ang II content and reactive oxygen species (ROS) was measured along with urinary albumin, creatinine, Ang II, and AGT.

RESULTS

All parameters consistent with T2D were present in mice after 12-14 weeks on the HFD. Systolic BP increased after 18 weeks in HFD but not NFD mice. Intrarenal ROS and Ang II concentrations were also increased in HFD mice. Remarkably, these changes paralleled the augmentation uAGT excretion (3.66 ± 0.50 vs. 0.92 ± 0.13 ng/mg by week 29; P < 0.01), which occurred in the absence of overt albuminuria.

CONCLUSIONS

In HFD-induced T2D mice, increases in uAGT occur in the absence of overt renal injury, indicating that this biomarker accurately detects early intrarenal RAS activation.

Urinary Angiotensinogen Predicts Renal Disease Activity in Lupus Nephritis

Aims: A noninvasive indicator of renal histological lesions and disease activity in lupus nephritis (LN) is needed for timely and targeted treatment before overt renal injury. Here, we tested the utility of urinary angiotensinogen (UAGT) to predict renal disease activity in LN. Results: A prospective, three-stage study was performed in patients with LN. In stage I, UAGT was measured in 140 newly diagnosed LN patients. UAGT significantly increased in LN patients, correlating well with kidney angiotensinogen expression and histological activity. Patients with LN class IV exhibited the highest UAGT compared with other histopathological classes of LN. For identifying LN class IV, a particularly aggressive type of LN, UAGT outperformed the conventional clinical measures and improved their performance. In stage II, UAGT was monitored in 61 subjects from stage I for up to 12 months. UAGT decreased after induction therapy and remained low in patients with LN remission during follow-up. For predicting therapy success at month 12, the area under the receiver operating characteristics curve of UAGT reduction at month 4 was 0.83, outperforming that of 24-h proteinuria. In stage III, UAGT was monitored in 12 LN patients before, during, and after the onset of renal flares. An elevation in UAGT predicted recurrence of LN, and a decline in UAGT after a renal flare heralded the remission of disease before conventional clinical measures. Innovation and Conclusion: UAGT in LN is a promising indicator for dynamic surveillance of renal disease activity and prediction of renal flares. Antioxid. Redox Signal. 31, 1289-1301.

Urinary Renin in Patients and Mice With Diabetic Kidney Disease

In patients with diabetic kidney disease (DKD), plasma renin activity is usually decreased, but there is limited information on urinary renin and its origin. Urinary renin was evaluated in samples from patients with longstanding type I diabetes mellitus and mice with streptozotocin-induced diabetes mellitus. Renin-reporter mouse model (Ren1d-Cre;mT/mG) was made diabetic with streptozotocin to examine whether the distribution of cells of the renin lineage was altered in a chronic diabetic environment. Active renin was increased in urine samples from patients with DKD (n=36), compared with those without DKD (n=38; 3.2 versus 1.3 pg/mg creatinine; P<0.001). In mice with streptozotocin-induced diabetes mellitus, urine renin was also increased compared with nondiabetic controls. By immunohistochemistry, in mice with streptozotocin-induced diabetes mellitus, juxtaglomerular apparatus and proximal tubular renin staining were reduced, whereas collecting tubule staining, by contrast, was increased. To examine the role of filtration and tubular reabsorption on urinary renin, mice were either infused with either mouse or human recombinant renin and lysine (a blocker of proximal tubular protein reabsorption). Infusion of either form of renin together with lysine markedly increased urinary renin such that it was no longer different between nondiabetic and diabetic mice. Megalin mRNA was reduced in the kidney cortex of streptozotocin-treated mice (0.70±0.09 versus 1.01±0.04 in controls, P=0.01) consistent with impaired tubular reabsorption. In Ren1d-Cre;mT/mG with streptozotocin-induced diabetes mellitus, the distribution of renin lineage cells within the kidney was similar to nondiabetic renin-reporter mice. No evidence for migration of cells of renin linage to the collecting duct in diabetic mice could be found. Renin mRNA in microdissected collecting ducts from streptozotocin-treated mice, moreover, was not significantly different than in controls, whereas in kidney cortex, largely reflecting juxtaglomerular apparatus renin, it was significantly reduced. In conclusion, in urine from patients with type 1 diabetes mellitus and DKD and from mice with streptozotocin-induced diabetes mellitus, renin is elevated. This cannot be attributed to production from cells of the renin lineage migrating to the collecting duct in a chronic hyperglycemic environment. Rather, the elevated levels of urinary renin found in DKD are best attributed to altered glomerular filteration and impaired proximal tubular reabsorption.

Urinary angiotensinogen predicts progressive chronic kidney disease after an episode of experimental acute kidney injury

One of the major obstacles to prevent AKI-CKD transition is the lack of effective methods to follow and predict the ongoing kidney injury after an AKI episode. In the present study, we test the utility of urinary angiotensinogen (UAGT) for dynamically evaluating renal structural changes and predicting AKI-CKD progression by using both mild and severe bilateral renal ischemia/reperfusion injury mice. UAGT returns to pre-ischemic levels 14 days after mild AKI followed by kidney architecture restoration, whereas sustained increase in UAGT accompanies by ongoing renal fibrosis after severe AKI. UAGT at day 14-42 correlates with renal fibrosis 84 days after AKI. For predicting fibrosis at day 84, the area under receiver operating characteristics curve of UAGT at day 14 is 0.81. Persistent elevation in UAGT correlates with sustained activation of intrarenal renin-angiotensin system (RAS) during AKI-CKD transition. Abrogating RAS activation post AKI markedly reduced renal fibrosis, with early RAS intervention (from 14 days after IRI) more beneficial than late intervention (from 42 days after IRI) in alleviating fibrosis. Importantly, UAGT decreases after RAS intervention, and its level at day 14-28 correlates with the extent of renal fibrosis at day 42 post RAS blockade. A pilot study conducted in patients with acute tubular necrosis finds that compared with those recovered, patients with AKI-CKD progression exhibits elevated UAGT during the 3-month follow-up after biopsy. Our study suggests that UAGT enables the dynamical monitoring of renal structural recovery after an AKI episode and may serve as an early predictor for AKI-CKD progression and treatment response.

Urinary renin-angiotensin markers in polycystic kidney disease

In autosomal dominant polycystic kidney disease (ADPKD), activation of the renin-angiotensin aldosterone system (RAAS) may contribute to hypertension and disease progression. Although previous studies have focused on circulating RAAS components, preliminary evidence suggests that APDKD may increase urinary RAAS components. Therefore, our aim was to analyze circulating and urinary RAAS components in ADPKD. We cross-sectionally compared 60 patients with ADPKD with 57 patients with non-ADPKD chronic kidney disease (CKD). The two groups were matched by sex, estimated glomerular filtration rate (eGFR), blood pressure, and RAAS inhibitor use. Despite similar plasma levels of angiotensinogen and renin, urinary angiotensinogen and renin excretion were five- to sixfold higher in ADPKD (P < 0.001). These differences persisted when adjusting for group differences and were present regardless of RAAS inhibitor use. In multivariable analyses, ADPKD, albuminuria, and the respective plasma concentrations were independent predictors for urinary angiotensinogen and renin excretion. In ADPKD, both plasma and urinary renin correlated negatively with eGFR. Total kidney volume correlated with plasma renin and albuminuria but not with urinary renin or angiotensinogen excretions. Albuminuria correlated positively with urinary angiotensinogen and renin excretions in ADPKD and CKD. In three ADPKD patients who underwent nephrectomy, the concentrations of albumin and angiotensinogen were highest in plasma, followed by cyst fluid and urine; urinary renin concentrations were higher than cyst fluid. In conclusion, this study shows that, despite similar circulating RAAS component levels, higher urinary excretions of angiotensinogen and renin are a unique feature of ADPKD. Future studies should address the underlying mechanism and whether this may contribute to hypertension or disease progression in ADPKD.

Effects of Direct Renin Blockade on Renal & Systemic Hemodynamics and on RAAS Activity, in Weight Excess and Hypertension: A Randomized Clinical Trial

Aim

The combination of weight excess and hypertension significantly contributes to cardiovascular risk and progressive kidney damage. An unfavorable renal hemodynamic profile is thought to contribute to this increased risk and may be ameliorated by direct renin inhibition (DRI). The aim of this trial was to assess the effect of DRI on renal and systemic hemodynamics and on RAAS activity, in men with weight excess and hypertension.

Methods

A randomized, double-blind, cross-over clinical trial to determine the effect of DRI (aliskiren 300 mg/day), with angiotensin converting enzyme inhibition (ACEi; ramipril 10 mg/day) as a positive control, on renal and systemic hemodynamics, and on RAAS activity (n = 15).

Results

Mean (SEM) Glomerular filtration rate (101 (5) mL/min/1.73m²) remained unaffected by DRI or ACEi. Effective renal plasma flow (ERPF; 301 (14) mL/min/1.73m²) was increased in response to DRI (320 (14) mL/min/1.73m², P = 0.012) and ACEi (317 (15) mL/min/1.73m², P = 0.045). Filtration fraction (FF; 34 (0.8)%) was reduced by DRI only (32 (0.7)%, P = 0.044). Mean arterial pressure (109 (2) mmHg) was reduced by DRI (101 (2) mmHg, P = 0.008) and ACEi (103 (3) mmHg, P = 0.037). RAAS activity was reduced by DRI and ACEi. Albuminuria (20 [9–42] mg/d) was reduced by DRI only (12 [5–28] mg/d, P = 0.030).

Conclusions

In men with weight excess and hypertension, DRI and ACEi improved renal and systemic hemodynamics. Both DRI and ACEi reduced RAAS activity. Thus, DRI provides effective treatment in weight excess and hypertension.

Trial Registration

Dutch trial register, registration number: 2532 www.trialregister.nl

On the Origin of Urinary Renin: A Translational Approach

Urinary angiotensinogen excretion parallels albumin excretion, which is not the case for renin, while renin's precursor, prorenin, is undetectable in urine. We hypothesized that renin and prorenin, given their smaller size, are filtered through the glomerulus in larger amounts than albumin and angiotensinogen, and that differences in excretion rate are because of a difference in reabsorption in the proximal tubule. To address this, we determined the glomerular sieving coefficient of renin and prorenin and measured urinary renin/prorenin 1) after inducing prorenin in Cyp1a1-Ren2 rats and 2) in patients with Dent disease or Lowe syndrome, disorders characterized by defective proximal tubular reabsorption. Glomerular sieving coefficients followed molecular size (renin>prorenin>albumin). The induction of prorenin in rats resulted in a >300-fold increase in plasma prorenin and doubling of blood pressure but did not lead to the appearance of prorenin in urine. It did cause parallel rises in urinary renin and albumin, which losartan but not hydralazine prevented. Defective proximal tubular reabsorption increased urinary renin and albumin 20- to 40-fold, and allowed prorenin detection in urine, at ≈50% of its levels in plasma. Taken together, these data indicate that circulating renin and prorenin are filtered into urine in larger amounts than albumin. All 3 proteins are subsequently reabsorbed in the proximal tubule. For prorenin, such reabsorption is ≈100%. Minimal variation in tubular reabsorption (in the order of a few %) is sufficient to explain why urinary renin and albumin excretion do not correlate. Urinary renin does not reflect prorenin that is converted to renin in tubular fluid.

Urinary biomarkers for early diabetic nephropathy: beyond albuminuria

Diabetic nephropathy (DN) is the most common cause of end-stage kidney disease in the USA and accounts for a significant increase in morbidity and mortality in patients with diabetes. Early detection is critical in improving clinical management. Although microalbuminuria is regarded as the gold standard for diagnosing the onset of DN, its predictive powers are limited. Consequently, great efforts have been made in recent years to identify better strategies for the detection of early stages of DN and progressive kidney function decline in diabetic patients. Here, we review the various urinary biomarkers that have emerged from these studies which hold promise as more sensitive diagnostic tools for the earlier detection of diabetic kidney disease and the prediction of progression to end-stage kidney disease. A number of key biomarkers present in the urine have been identified that reflect kidney injury at specific sites along the nephron, including glomerular/podocyte damage and tubular damage, oxidative stress, inflammation and activation of the intrarenal renin-angiotensin system. We also describe newer approaches, including urinary microRNAs, which are short noncoding mRNAs that regulate gene expression, and urine proteomics, that can be used to identify potential novel biomarkers in the development and progression of diabetic kidney disease.

Association studies suggest a key role for endothelin-1 in the pathogenesis of preeclampsia and the accompanying renin-angiotensin-aldosterone system suppression

Women with preeclampsia display low renin-angiotensin-aldosterone system activity and a high antiangiogenic state, the latter characterized by high levels of soluble Fms-like tyrosine kinase (sFlt)-1 and reduced placental growth factor levels. To investigate whether renin-angiotensin-aldosterone system suppression in preeclampsia is because of this disturbed angiogenic balance, we measured mean arterial pressure, creatinine, endothelin-1 (ET-1), and renin-angiotensin-aldosterone system components in pregnant women with a high (≥85; n=38) or low (<85; n=65) soluble Fms-like tyrosine kinase-1/placental growth factor ratio. Plasma ET-1 levels were increased in women with a high ratio, whereas their plasma renin activity and plasma concentrations of renin, angiotensinogen, and aldosterone were decreased. Plasma renin activity-aldosterone relationships were identical in both the groups. Multiple regression analysis revealed that plasma renin concentration correlated independently with mean arterial pressure and plasma ET-1. Plasma ET-1 correlated positively with soluble Fms-like tyrosine kinase-1 and negatively with plasma renin concentration, and urinary protein correlated with plasma ET-1 and mean arterial pressure. Despite the lower plasma levels of renin and angiotensinogen in the high-ratio group, their urinary levels of these components were elevated. Correction for albumin revealed that this was because of increased glomerular filtration. Subcutaneous arteries obtained from patients with preeclampsia displayed an enhanced, AT2 receptor-mediated response to angiotensin II. In conclusion, a high antiangiogenic state associates with ET-1 activation, which together with the increased mean arterial pressure may underlie the parallel reductions in renin and aldosterone in preeclampsia. Because ET-1 also was a major determinant of urinary protein, our data reveal a key role for ET-1 in the pathogenesis of preeclampsia. Finally, the enhanced angiotensin responsiveness in preeclampsia involves constrictor AT2 receptors.

Sex-dependent differences in renal angiotensinogen as an early marker of diabetic nephropathy

AIM

The renal renin-angiotensin system (RAS) has been implicated in the pathogenesis of diabetic nephropathy. The aim of this study was to investigate sex differences in renal renin-angiotensin system (RAS) and the roles of androgens in diabetes-associated renal injury.

METHODS

Renal injury and fibrosis were studied in streptozotocin-induced diabetic rats by albuminuria and by gene expression of collagen I and fibronectin. RAS was investigated by analysing the plasma angiotensinogen (AOGEN) and renin activity (PRA) and their renal gene expression. Also, a group of diabetic rats was treated with the anti-androgen flutamide.

RESULTS

Albuminuria was significantly lower in diabetic females than in males (1.2 [0.8-1.5] versus 4.4 [2.2-6.1] mg/24 h, data are median [IQR] values, P < 0.05). Renal AOGEN mRNA levels were increased by diabetes in males (8.1 ± 0.8% in diabetes versus 0.8 ± 0.2% in control, P < 0.001) but not in females (1.0 ± 0.1% in diabetes versus 0.8 ± 0.1% in control, P > 0.05), as were collagen I and fibronectin mRNAs. Furthermore, AOGEN mRNA levels were strongly correlated with albuminuria (Spearman r = 0.64, 95% [CI] 0.36-0.81, P < 0.0001). Diabetes decreased PRA, renal renin mRNA and plasma AOGEN in both females and males. Anti-androgen treatment decreased albuminuria only in diabetic males without affecting the endocrine or renal RAS.

CONCLUSIONS

These data indicate that renal but not hepatic AOGEN or renin is positively associated with diabetic albuminuria and contribute to the sex-dependent differences in renal injury. Androgens may contribute to albuminuria in male independently of the RAS.

Urinary angiotensinogen as a potential biomarker of diabetic nephropathy

BACKGROUND

Activation of the renin-angiotensin-aldosterone system (RAAS) is an important mediator of diabetic nephropathy. Urinary angiotensinogen, a novel biomarker of the intrarenal RAAS, is associated with progressive kidney injury. In this study, the authors investigated the determinants of urinary angiotensinogen and its associations with staging of diabetic nephropathy.

METHODS

Random urine samples were collected from the patients with type 2 diabetes with normoalbuminuria (n = 52), microalbuminuria (n = 52) and macroalbuminuria (n = 51) for the measurement of angiotensinogen by sensitive and specific ELISAs. Control samples were collected from healthy volunteers (n = 20) who had normal albuminuria and renal function.

RESULTS

Urinary angiotensinogen was higher in microalbuminuric and macroalbuminuric diabetes than in controls [63.44 (interquartile range, IQR: 22.08, 174.8) versus 398.38 (IQR: 205.03, 673.68) versus 9.12 (IQR: 3.76, 23.82) ng/mg creatinine, respectively, P < 0.001]. In diabetes with normoalbuminuria, urinary angiotensinogen was also higher than in controls [16.42 (IQR: 7.69, 34.71) versus 9.12 (IQR: 3.76, 23.82) ng/mg creatinine, P = 0.047]. The performance of the biomarker in differentiating each stage of type 2 diabetes from controls was illustrated by receiver-operating characteristic curves. The areas under the curve for the diagnosis of established normoalbuminuric, microalbuminuric and macroalbuminuric type 2 diabetes using urine angiotensinogen (ng/mg creatinine) were 0.62 (95% CI: 0.48-0.77), 0.85 (95% CI: 0.76-0.94) and 0.96 (95% CI: 0.92-1.00), respectively. In addition, the cut-off levels were 9.30 ng/mg (sensitivity 65.4%, specificity 55.0%), 12.32 ng/mg (sensitivity 55.8%, specificity 65.0%) and 17.44 ng/mg (sensitivity 44.2%, specificity 70.0%), respectively, for distinguishing normoalbuminuric type 2 diabetes from healthy controls.

CONCLUSIONS

The authors propose that angiotensinogen could be one of the potential urinary biomarkers for diagnosis in established diabetic nephropathy. It appeared even before the significant albuminuria in diabetic nephropathy. It might be useful as an early biomarker of activation of the renin-angiotensin system in diabetic nephropathy.

Methodologic issues in the measurement of urinary renin

BACKGROUND AND OBJECTIVES

Alge et al. recently reported that urinary renin may be a prognostic biomarker for AKI after cardiac surgery. However, their urinary renin levels far exceeded published plasma renin levels, whereas normally, urinary renin is <10% of plasma renin. This result raises questions about the specificity of the new Quantikine Renin ELISA Kit used in the work by Alge et al., which is claimed to detect total renin (i.e., renin and prorenin). Therefore, this study tested this assay.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Plasma and urine from 30 patients with hypertension, diabetes, or preeclampsia and 10 healthy pregnant women (randomly selected from sample sets obtained earlier to investigate urinary renin-angiotensin system components) were used to compare the ELISA with a validated renin immunoradiometric assay and an in-house enzyme kinetic assay. Measurements were performed before and after in vitro prorenin activation, representing renin and total renin, respectively.

RESULTS

Total renin measurements by ELISA, immunoradiometric assay, and enzyme kinetic assay were highly correlated. However, ELISA results were consistently ≥10-fold higher. The ELISA standard yielded low to undetectable levels in the immunoradiometric assay and enzyme kinetic assay, except after prorenin activation, when the results were ≥10-fold lower than the ELISA results. In plasma, prorenin activation increased ELISA results by 10%-15%. Urine contained no detectable prorenin.

CONCLUSIONS

The ELISA renin kit standard is prorenin, and its immunoreactivity and enzymatic activity after conversion to renin do not match the International Reference Preparation of human renin that has been used to validate previous immunoradiometric assays and enzyme kinetic assays; in fact, they are at least 10-fold lower, and thus, any measurements obtained with this ELISA kit yield levels that are at least 10-fold too high. The ELISA antibodies detect both renin and prorenin, with a preference for the former. Given these inconsistencies, urinary renin levels should be measured by established renin assays.

The renin-angiotensin-aldosterone system in pre-eclampsia: the delicate balance between good and bad

Pregnancy demands major changes of the cardiovascular system, and this involves, among others, activation of the RAAS (renin-angiotensin-aldosterone system), allowing an aldosterone-dependent increase in volume. Remarkably, a relative resistance to the pressor response of AngII (angiotensin II) develops simultaneously to prevent the increase in blood pressure that would normally accompany RAAS activation. The increase in volume, the degree of RAAS activation and the diminished pressor response to AngII are less pronounced in pre-eclampsia. However, animal models displaying excessive RAAS activation also result in a pre-eclampsia-like syndrome, and the aldosterone/renin ratio is elevated in pre-eclampsia compared with a normal pregnancy. New insights into the pathogenesis of pre-eclampsia have revealed a major role for VEGF (vascular endothelial growth factor), VEGF-inactivating sFlt-1 (soluble fms-like tyrosine kinase-1) and AT1 (angiotensin II type 1) receptor autoantibodies. The last mentioned activate AT(1) receptors, thereby potentially suppressing circulating renin and aldosterone. VEGF, both directly and indirectly (by increasing capillary density), affects adrenal aldosterone synthesis. The present review summarizes all of the recent findings regarding RAAS regulation in pre-eclampsia compared with normal pregnancy, concluding that factors such as sFlt-1 and AT(1) receptor autoantibodies disturb the delicate balance that normally results in a volume increase and a diminished vasoconstrictor response to AngII in pregnant women. It is possible that there are non-parallel changes in the circulating and renal RAAS in pre-eclampsia, which are potentially reflected by the urinary levels of renin.

Tissue Renin-Angiotensin systems: a unifying hypothesis of metabolic disease

The actions of angiotensin peptides are diverse and locally acting tissue renin-angiotensin systems (RAS) are present in almost all tissues of the body. An activated RAS strongly correlates to metabolic disease (e.g., diabetes) and its complications and blockers of RAS have been demonstrated to prevent diabetes in humans. Hyperglycemia, obesity, hypertension, and cortisol are well-known risk factors of metabolic disease and all stimulate tissue RAS whereas glucagon-like peptide-1, vitamin D, and aerobic exercise are inhibitors of tissue RAS and to some extent can prevent metabolic disease. Furthermore, an activated tissue RAS deteriorates the same risk factors creating a system with several positive feedback pathways. The primary effector hormone of the RAS, angiotensin II, stimulates reactive oxygen species, induces tissue damage, and can be associated to most diabetic complications. Based on these observations, we hypothesize that an activated tissue RAS is the principle cause of metabolic syndrome and type 2 diabetes, and additionally is mediating the majority of the metabolic complications. The involvement of positive feedback pathways may create a self-reinforcing state and explain why metabolic disease initiate and progress. The hypothesis plausibly unifies the major predictors of metabolic disease and places tissue RAS regulation in the center of metabolic control.