Renalase, hypertension, and kidney - the discussion continues

Renalase: a novel regulator of cardiometabolic and renal diseases

Renalase is a ~38 kDa flavin-adenine dinucleotide (FAD) domain-containing protein that can function as a cytokine and an anomerase. It is emerging as a novel regulator of cardiometabolic diseases. Expressed mainly in the kidneys, renalase has been reported to have a hypotensive effect and may control blood pressure through regulation of sympathetic tone. Furthermore, genetic variations in the renalase gene, such as a functional missense polymorphism (Glu37Asp), have implications in the cardiovascular and renal systems and can potentially increase the risk of cardiometabolic disorders. Research on the physiological functions and biochemical actions of renalase over the years has indicated a role for renalase as one of the key proteins involved in various disease states, such as diabetes, impaired lipid metabolism, and cancer. Recent studies have identified three transcription factors (viz., Sp1, STAT3, and ZBP89) as key positive regulators in modulating the expression of the human renalase gene. Moreover, renalase is under the post-transcriptional regulation of two microRNAs (viz., miR-29b, and miR-146a), which downregulate renalase expression. While renalase supplementation may be useful for treating hypertension, inhibition of renalase signaling may be beneficial to patients with cancerous tumors. However, more incisive investigations are required to unravel the potential therapeutic applications of renalase. Based on the literature pertaining to the function and physiology of renalase, this review attempts to consolidate and comprehend the role of renalase in regulating cardiometabolic and renal disorders.

Associations of Renalase With Blood Pressure and Hypertension in Chinese Adults

Objective

Renalase, a novel secretory flavoprotein with amine oxidase activity, is secreted into the blood by the kidneys and is hypothesized to participate in blood pressure (BP) regulation. We investigated the associations of renalase with BP and the risk of hypertension by examining renalase single nucleopeptide polymorphism (SNPs), serum renalase levels, and renal expression of renalase in humans.

Methods

① Subjects (n = 514) from the original Baoji Salt-Sensitive Study cohort were genotyped to investigate the association of renalase SNPs with longitudinal BP changes and the risk of hypertension during 14 years of follow-up. ② Two thousand three hundred and ninety two participants from the Hanzhong Adolescent Hypertension Study cohort were used to examine the association of serum renalase levels with hypertension. Renalase expression in renal biopsy specimens from 193 patients were measured by immunohistochemistry. ③ Renalase expression was compared in hypertensive vs. normotensive patients.

Results

① SNP rs7922058 was associated with 14-year change in systolic BP, and rs10887800, rs796945, rs1935582, rs2296545, and rs2576178 were significantly associated with 14-year change in diastolic BP while rs1935582 and rs2576178 were associated with mean arterial pressure change over 14 years. In addition, SNPs rs796945, rs1935582, and rs2576178 were significantly associated with hypertension incidence. Gene-based analysis found that renalase gene was significantly associated with hypertension incidence over 14-year follow-up after adjustment for multiple measurements. ② Hypertensive subjects had higher serum renalase levels than normotensive subjects (27.2 ± 0.4 vs. 25.1 ± 0.2 μg/mL). Serum renalase levels and BPs showed a linear correlation. In addition, serum renalase was significantly associated with the risk of hypertension [OR = 1.018 (1.006–1.030)]. ③ The expression of renalase in human renal biopsy specimens significantly decreased in hypertensive patients compared to non-hypertensive patients (0.030 ± 0.001 vs. 0.038 ± 0.004).

Conclusions

These findings indicate that renalase may play an important role in BP progression and development of hypertension.

A case report of renal calyceal diverticulum with hypertension in children and review of literature

Background

Renal calyx diverticulum refers to a cystic lesion covered with the transitional epithelium in the renal parenchyma. Although there is no clear evidence that calyx diverticulum can cause hypertension, there exists a close association between the two, and there are few related reports. Herein, we reported the case of a child with renal calyx diverticulum complicated with hypertension and summarized the diagnosis and treatment.

Case presentation

Physical examination of the patient, an 11-year-old child, revealed a left renal cyst with hypertension (155/116 mmHg). There were no related symptoms. Routine urine and blood biochemical examinations showed no abnormalities. Imaging revealed left renal cyst compression causing the hypertension. She underwent renal cyst fluid aspiration and injection of a sclerosing agent into the capsule, but her blood pressure increased again 3 days postoperatively. Color Doppler ultrasonography showed that the size of the left renal cyst was the same as that preoperatively. To further confirm the diagnosis, cystoscopic retrograde ureteropyelography was performed to confirm the diagnosis of renal calyx diverticulum. Subsequently, renal calyceal diverticulum resection and calyx neck enlargement were performed. The operation went smoothly and the blood pressure returned to normal postoperatively. No abnormalities were noted at the 7-month postoperative follow-up.

Conclusion

There exists an association between renal calyx diverticulum and hypertension. Therefore, hypertension can be considered a surgical indication for renal calyx diverticulum. Moreover, renal calyceal diverticulum in children can be easily misdiagnosed as a renal cyst. Therefore, it is important to be vigilant to prevent a series of complications, such as postoperative urine leakage, in such cases.

The Effect of Bilateral Nephrectomy on Renalase and Catecholamines in Hemodialysis Patients

Background/Aims: Renalase is an enzyme with monoamine oxidase activity that metabolizes catecholamines; therefore, it has a significant influence on arterial blood pressure regulation and the development of cardiovascular diseases. Renalase is mainly produced in the kidneys. Nephrectomy and hemodialysis (HD) may alter the production and metabolism of renalase. The aim of this study was to examine the effect of bilateral nephrectomy on renalase levels in the serum and erythrocytes of hemodialysis patients. Methods: This study included 27 hemodialysis patients post-bilateral nephrectomy, 46 hemodialysis patients without nephrectomy but with chronic kidney disease and anuria and 30 healthy subjects with normal kidney function. Renalase levels in the serum and erythrocytes were measured using an ELISA kit. Results: Serum concentrations of renalase were significantly higher in post-bilateral nephrectomy patients when compared with those of control subjects (101.1 ± 65.5 vs. 19.6 ± 5.0; p < 0.01). Additionally, renalase concentrations, calculated per gram of hemoglobin, were significantly higher in patients after bilateral nephrectomy in comparison with those of healthy subjects (994.9 ± 345.5 vs. 697.6 ± 273.4, p = 0.015). There were no statistically significant differences in plasma concentrations of noradrenaline or adrenaline. In contrast, the concentration of dopamine was significantly lower in post-nephrectomy patients when compared with those of healthy subjects (116.8 ± 147.7 vs. 440.9 ± 343.2, p < 0.01). Conclusions: Increased serum levels of renalase in post-bilateral nephrectomy hemodialysis patients are likely related to production in extra-renal organs as a result of changes in the cardiovascular system and hypertension.

Genome-wide association study suggests impact of chromosome 10 rs139401390 on kidney function in patients with coronary artery disease

Chronic kidney disease (CKD) is an independent risk factor for onset and progression of coronary artery disease (CAD). Discovery of predisposing loci for kidney function in CAD patients was performed using a genome-wide association approach. Inclusion criteria were CAD with ≥50% stenosis (≥1 coronary artery) and a creatinine-based estimated glomerular filtration rate (eGFR) of 30–75 ml/min/1.73 m². An association of rs139401390 located to a region 58.8 kb upstream of renalase (RNLS) with eGFR was detected in the Ludwigshafen Risk and Cardiovascular Health (LURIC) study (n = 499, p = 7.88 × 10⁻⁹, mean eGFR 60.7 ml/min/1.73 m²). Direct genotyping of rs139401390A > G suggested increased eGFR by 12.0 ml/min/1.73 m² per A allele (p = 0.000004). Genome-wide replication of rs139401390A > G in the Coronary Artery Disease and Renal Failure (CAD-REF) registry with a mean eGFR of 47.8 ml/min/1.73 m² (n = 574, p = 0.033) was only nominally significant. Comparison of rs139401390 genotypes for risk of reduced kidney function in the overall LURIC study revealed higher adjusted odds ratios (OR) for eGFR <60 ml/min/1.73 m² for CAD patients (n = 1992, OR = 2.36, p = 0.008, G/A + G/G vs A/A) compared to patients with/without CAD (n = 2908, OR = 1.97, p = 0.014, G/A + G/G vs A/A). No significant risk elevation was detected in patients without CAD (n = 948, p = 0.571). rs139401390 may affect kidney function in CAD patients with mild reduction in eGFR.

The enzyme: Renalase

Within the last two years catalytic substrates for renalase have been identified, some 10 years after its initial discovery. 2- and 6-dihydronicotinamide (2- and 6-DHNAD) isomers of β-NAD(P)H (4-dihydroNAD(P)) are rapidly oxidized by renalase to form β-NAD(P)⁺. The two electrons liberated are then passed to molecular oxygen by the renalase FAD cofactor forming hydrogen peroxide. This activity would appear to serve an intracellular detoxification/metabolite repair function that alleviates inhibition of primary metabolism dehydrogenases by 2- and 6-DHNAD molecules. This activity is supported by the complete structural assignment of the substrates, comprehensive kinetic analyses, defined species specific substrate specificity profiles and X-ray crystal structures that reveal ligand complexation consistent with this activity. This apparently intracellular function for the renalase enzyme is not allied with the majority of the renalase research that holds renalase to be a secreted mammalian protein that functions in blood to elicit a broad array of profound physiological changes. In this review a description of renalase as an enzyme is presented and an argument is offered that its enzymatic function can now reasonably be assumed to be uncoupled from whole organism physiological influences.

[The history of renalase from amine oxidase to a a-NAD(P)H-oxidase/anomerase]

Renalase is a recently discovered secretory protein, which plays a certain (still poorly understood) role in regulation of blood pressure. The review summarizes own and literature data accumulated since the first publication on relanase (2005). Initial reports on FAD-dependent amine oxidase activity of this protein were not confirmed in independent experiments performed in different laboratories. In addition, proposed amine oxidase activity of circulating extracellular renalase requires the presence of FAD, which has not been detected either in blood or urinary renalase. Moreover, renalase excreted into urine lacks its N-terminal peptide, which is ultimately needed for accommodation of the FAD cofactor. Results of the Aliverti's group on NAD(P)H binding by renalase and weak diaphorase activity of this protein stimulated further studies of renalase as NAD(P)H oxidase catalyzing reaction of catecholamine co-oxidation. However, physiological importance of such extracellular catecholamine-metabolizing activity (demonstrated in one laboratory and not detected in another laboratory) remains unclear due to existence of much more active enzymatic systems (e.g. neutrophil NAD(P)H oxidase, xanthine oxidase/xanthine) in circulation, which can perform such co-oxidation reactions. Recently a-NAD(P)H oxidase/anomerase activity of renalase, which also pomotes oxidative conversion of b-NADH isomers inhibiting activity of NAD-dependent dehydrogenases, has been described. However, its possible contribution to the antihypertensive effect of renalase remains unclear. Thus, the antihypertensive effect of renalase still remains a phenomenon with unclear biochemical mechanim(s) and functions of intracellular and extracellular (circulating) renalases obviously differ.

Bacterial Renalase: Structure and Kinetics of an Enzyme with 2- and 6-Dihydro-β-NAD(P) Oxidase Activity from Pseudomonas phaseolicola

Despite a lack of convincing in vitro evidence and a number of sound refutations, it is widely accepted that renalase is an enzyme unique to animals that catalyzes the oxidative degradation of catecholamines in blood in order to lower vascular tone. Very recently, we identified isomers of β-NAD(P)H as substrates for renalase (Beaupre, B. A. et al. (2015) Biochemistry, 54, 795-806). These molecules carry the hydride equivalent on the 2 or 6 position of the nicotinamide base and presumably arise in nonspecific redox reactions of nicotinamide dinucleotides. Renalase serves to rapidly oxidize these isomers to form β-NAD(P)⁺ and then pass the electrons to dioxygen, forming H₂O₂. We have also shown that these substrate molecules are highly inhibitory to dehydrogenase enzymes and thus have proposed an intracellular metabolic role for this enzyme. Here, we identify a renalase from an organism without a circulatory system. This bacterial form of renalase has the same substrate specificity profile as that of human renalase but, in terms of binding constant (K(d)), shows a marked preference for substrates derived from β-NAD⁺. 2-dihydroNAD(P) substrates reduce the enzyme with rate constants (k(red)) that greatly exceed those for 6-dihydroNAD(P) substrates. Taken together, k(red)/K(d) values indicate a minimum 20-fold preference for 2DHNAD. We also offer the first structures of a renalase in complex with catalytically relevant ligands β-NAD⁺ and β-NADH (the latter being an analogue of the substrate(s)). These structures show potential electrostatic repulsion interactions with the product and a unique binding orientation for the substrate nicotinamide base that is consistent with the identified activity.

Markers of increased cardiovascular risk in patients with chronic kidney disease

BACKGROUND

Epidemiological studies have shown that chronic kidney disease (CKD) is an important risk factor for atherosclerosis and cardiovascular disease (CAD). The aim of the study was to determine markers of increased risk of CAD and to achieve a better understanding of agents implicated in the process of atherosclerosis in CKD patients.

METHODS

The study group consisted of a total of 139 patients with CKD while the control group comprised 45 healthy volunteers. Concentrations of osteoprotegerin, osteopontin, osteocalcin, matrix γ-carboxyglutamic acid (Gla) protein (MGP), fetuin A, matrix metalloproteinase-2 (MMP-2), matrix metalloproteinase-9 (MMP-9), tissue inhibitor of metalloproteinase-1 (TIMP-1), tissue inhibitor of metalloproteinase-2 (TIMP-2), ATP binding cassette transporter A1 (ABCA1), ATP binding cassette transporter G1 (ABCG1) and renalase were measured by the ELISA method.

RESULTS

We observed decreased levels of fetuin A (control vs. CKD group: 37.5 vs. 33.2 ng/ml, p = 0.018), and increased concentrations of osteocalcin (control vs. CKD group: 9.1 ± 6.0 vs. 13.6 ± 10.3 ng/ml, p = 0.05), MMP-2 (113.1 ± 75.0 vs. 166.0 ± 129.9 ng/ml, p = 0.045), TIMP-2 (22.1 ± 5.1 vs. 25.4 ± 7,0 ng/ml, p = 0.005) and renalase (251.0 ± 157 vs. 316.1 ± 155.3 ng/ml, p = 0.026). In patients with CKD (in comparison to control group), left ventricle ejection fraction: 53.0 ± 3,5% vs. 48.5%, p = 0.012) and calcification of the aortic valve (9.5% vs. 39.8%, p = 0.008) were observed more frequently.

CONCLUSIONS

Decreased levels of fetuin A and increased concentration of osteocalcin, renalase, MMP-2 and TIMP-2 suggest that these factors may be involved in the pathogenesis of CAD in patients with CKD. Significantly increased indices of cardiac hypertrophy and its dysfunction in patients with CKD are indicators of pathological mechanisms occurring in cardiovascular system in this group of patients.

Renalase mRNA levels in the brain, heart, and kidneys of spontaneously hypertensive rats with moderate and high hypertension

Background

Renalase is a recently discovered secretory protein involved in regulation of arterial blood pressure in humans and animals. Results of animal experiments from independent laboratories indicate that administration of human recombinant renalase decreases blood pressure and some genetically predisposed hypertensive rats have lowered renalase levels.

Material/Methods

The levels of renalase mRNA expression in brain hemispheres, heart, and kidneys of spontaneously hypertensive rats (SHR) with moderate (140–180 mm Hg) or high (>180 mm Hg) hypertension and of control Wistar-Kyoto (WKY) rats were analyzed using real-time PCR.

Results

Spontaneously hypertensive rats with high hypertension (>180 mm Hg) had a lower renalase mRNA level in brain hemispheres, and higher heart and kidney renalase mRNA levels compared with control WKY rats. In SHR with a moderate increase in arterial blood pressure (140–180 mm Hg), the tissue renalase mRNA changed in the same direction but did not reach the level of statistical significance as compared with control rats.

Conclusions

The results indicate that the development of hypertension in SHR is accompanied by altered expression of the renalase gene in the examined organs as compared with control WKY rats. The brain and peripheral tissues renalase mRNA levels demonstrate opposite trends, which are obviously crucial for impaired regulation of blood pressure in SHR.

Construction of the coding sequence of the transcription variant 2 of the human Renalase gene and its expression in the prokaryotic system

Renalase is a recently discovered protein, involved in regulation of blood pressure in humans and animals. Although several splice variants of human renalase mRNA transcripts have been recognized, only one protein product, hRenalase1, has been found so far. In this study, we have used polymerase chain reaction (PCR)-based amplification of individual exons of the renalase gene and their joining for construction of full-length hRenalase2 coding sequence followed by expression of hRenalase2 as a polyHis recombinant protein in Escherichia coli cells. To date this is the first report on synthesis and purification of hRenalase2. Applicability of this approach was verified by constructing hRenalase1 coding sequence, its sequencing and expression in E. coli cells. hRenalase1 was used for generation of polyclonal antiserum in sheep. Western blot analysis has shown that polyclonal anti-renalase1 antibodies effectively interact with the hRenalase2 protein. The latter suggests that some functions and expression patterns of hRenalase1 documented by antibody-based data may be attributed to the presence of hRenalase2. The realized approach may be also used for construction of coding sequences of various (especially weakly expressible) genes, their transcript variants, etc.

Vascular adhesion protein-1 and renalase in regard to diabetes in hemodialysis patients

INTRODUCTION

Vascular adhesion protein-1 (VAP-1) is a copper-containing semicarbazide-sensitive amine oxidase (SSAO) secreted by vascular smooth muscle cells, adipocytes, and endothelial cells with functional monoamine oxidase activity. Renalase, with possible monoamine oxidase activity, which breaks down catecholamines like SSAO, is also expressed in the endothelium as well as in the kidney. The aim of the study was to assess VAP-1 level and its correlation with renalase level in 60 hemodialyzed (HD) patients.

MATERIAL AND METHODS

Complete blood count, urea, serum lipids, fasting glucose and creatinine were studied by the standard laboratory method in the hospital central laboratory. We assessed VAP-1 and renalase with commercially available assays.

RESULTS

The mean level of VAP-1 as well as renalase was significantly higher in HD patients when compared to the control group (291.01 ±94.91 ng/ml vs. 158.34 ±56.89 ng/ml, p < 0.01; 27.53 ±9.394.91 µg/ml vs. 4.00 ±1.37 µg/ml, p < 0.001, respectively). In hemodialysis patients VAP-1 correlated with presence of diabetes (r = 0.27, p < 0.05), presence of hypertension (r = 0.32, p < 0.05), use of calcium channel blockers (r = 0.30, p < 0.05), use of β-blockers (r = 0.25, p < 0.05), ejection fraction (r = -0.38, p < 0.01), systolic blood pressure before (r = 0.52, p < 0.001) and after hemodialysis (r = 0.30, p < 0.01), and weight gain (r = 0.41, p < 0.01). Renalase was not significantly different in diabetic and non-diabetic patients or between hypertensive and normotensive patients. In multiple regression analysis VAP-1 was predicted 77% by serum ejection fraction and fibrinogen.

CONCLUSIONS

Vascular adhesion protein-1, elevated in patients on hemodialysis, was predominantly dependent on blood pressure and diabetes, both factors associated with endothelial damage and promoting cardiovascular complications. Renalase appeared to be unrelated to VAP, at least in the HD population.