Renalase, Stroke, and Hypertension in Hemodialyzed Patients

Abstract

Introduction: Hypertension and kidney disease have been associated with increased incidence of stroke. Renalase, a newly discovered hormone, is secreted by the kidney and circulates in blood. The aim of this study was to assess possible correlations between renalase, blood pressure, stroke, and cardiovascular status in prevalent hemodialyzed patients. Methods: Renalase was assessed using commercially available assay. Echocardiography was performed in each patient. Results: Serum renalase was significantly lower in patients with a history of stroke (21%) than in patients without it. Similarly, renalase was significantly lower in hypertensive patients (82%) when compared with normotensives. Serum renalase correlated with creatinine, residual renal function, and transferrin saturation. The only predictor of renalase in multiple regression analysis was the presence of hypertension explaining 90% of the renalase variations. Conclusions: Our preliminary results suggest that renalase, probably due to the sympathetic nervous system hyperactivity, could be associated with hypertension and cardiovascular complications, including stroke in hemodialyzed patients. However, further studies are needed to establish the possible role of renalase in these complications. Renalase is “a new postulated therapeutic target.”

• hemodialysis
• stroke
• hypertension
• renalase
• residual renal function

INTRODUCTION

Hypertension and kidney disease have been associated with increased incidence of stroke.¹ Hypertension is one of the important risk factors of stroke and it contributes significantly to the onset of disease.² These two conditions might share the same etiopathogenesis. Dialytic therapy has been shown to be a risk factor for stroke.³ In one report of an hemodialysis (HD) population, the prevalence rate of cerebrovascular disease was almost 20%.⁴ It has recently been identified that renalase, a novel flavin adenine dinucleotide-dependent amine oxidase that is secreted into the blood by the kidney, metabolizes circulating catecholamines.⁵ It lowers blood pressure in vivo by decreasing cardiac contractility and heart rate and by preventing the expected compensatory increase in peripheral vascular tone.⁶ Recently, Stec et al.⁷ described polymorphisms of renalase gene in hemodialyzed patients, as well as their associations with hypertension in this particular population. They found that both the G allele frequencies of rs2576178 and of rs10887800 showed a significantly higher incidence in hypertensive hemodialyzed patients. However, they did not report the prevalence of both polymorphisms in the same population while the prevalence of hypertension was 54% and 66% in these two populations, respectively. In addition, Buraczynska et al.⁸ reported the strong association between rs10887800 polymorphism and stroke in patients with and without diabetes, whereas the C allele of rs2296545 single nucleotide polymorphism (SNP) was associated with hypertension (p < 0.01). The authors concluded that there was a possibility that observed association of renalase gene with stroke could be related to more severe hypertension. The aim of this study was to assess the possible correlations between renalase, blood pressure, stroke, and cardiovascular status in prevalent hemodialyzed patients.

MATERIALS AND METHODS

In this study, we included 34 prevalent HD patients, including 16 males. The clinical and biochemical characteristics are presented in Table 1. The causes of renal failure among HD patients included chronic glomerulonephritis (n = 9), diabetic nephropathy (n = 10), hypertensive nephropathy (n = 6), others, and unknown (n = 9). None of the patients studied underwent nephrectomy. Hypertension was diagnosed in 27 patients, coronary artery disease in 16, while 7 patients had a history of stroke. Hypotensive drugs used in this population were as follows: Angiotensin converting enzyme (ACE) inhibitors in 22, beta-blockers in 15, alpha-blockers in 10, and diuretics (furosemide) in 7. Three patients were administered four drugs, seven were given three drugs, eight were given two drugs, and five were on monotherapy. Vascular access was either proximal arterio-venous (a-v) fistula in 10 patients or distal a-v fistula in 24 patients. This study was approved by the Medical University Ethics Committee. Echocardiography b-mode was performed in each patient by the same physician. Dysfunction of each valve was described in a semi-quantitative manner (+, ++, +++, ++++ with +++ and ++++ were considered as hemodynamically significant). Blood was drawn from all patients, after an overnight fast, in the morning between 8.00 and 9.00 am before the onset of the midweek dialysis session and before heparin administration. Blood for urea and creatinine measurements for Kt/V calculation was taken after HD from the arterial line of the HD system, immediately before discontinuation of the extracorporeal circulation. Enoxaparin (Clexane, Aventis) or unfractionated heparin was used as an anticoagulant during HD. The blood for other measurements was taken once before the HD session in the middle of three dialysis sessions. Hemoglobin, albumin, serum lipids, serum iron, total iron binding capacity (TIBC), and ferritin were measured by standard laboratory methods in a central laboratory. Serum renalase was assessed using commercially available enzyme-linked immunosorbent assay (ELISA) kit from Uscn Life Science Inc., Wuhan, China, using a monoclonal antibody specific to renalase. To obtain the normal ranges for renalase in this ELISA assay, 22 healthy volunteers from the medical staff and families were studied. Statistical 8.0 Poland program (Tulsa, OK, USA) was used for statistical analysis, the Shapiro–Wilk test was used to determine the normal distribution and U Mann–Whitney test was used for comparisons.

Table 1.  Clinical and biochemical characteristics of 34 HD patients (16 males).

	HD
Age (years)	60.74 ± 12.83
Diabetes mellitus (%)	25
Ischemic heart disease (%)	40
Hypertension (%)	68
RRF (mL)	110 (0.2620)
Time on HD (months)	38 (1–174)
Kt/V	1.15 ± 0.23
Hemoglobin (g/dL)	11.61 ± 1.13
Cholesterol (mg/dL)	187.9 ± 42.5
HDL (mg/dL)	44.2 ± 14.5
LDL (mg/dL)	110.8 ± 36.4
Triglycerides (mg/dL)	163.3 ± 88.0
TSAT (%)	37.09 ± 12.51
Ferritin (ng/mL)	813.6 ± 317.6
Albumin (g/dL)	4.03 ± 0.46
EF (%)	60.7 ± 10.4
Renalase (μg/mL)	17.51 ± 6.73

Notes: Data are given as means ± SD or medians and ranges. TSAT, transferrin saturation; HD, hemodialysis; RRF, residual renal function; HDL, high-density lipoprotein; LDL, low-density lipoprotein; EF, ejection fraction.

Table 2.  Renalase (μg/mL) in different settings in hemodialyzed patients.

Proximal a-v fistula	Distal a-v fistula
14.84 (12.66–19.00)	18.00 (12.39–23.15), p < 0.05
Anticoagulation with unfractionated heparin	Anticoagulation with enoxaparin
18.83 (14.06–22.76)	12.66 (12.09–14.44), p < 0.05
Glomerulonephritis as ESRD etiology	Other etiologies
21.29 (18.83–24.92)	14.06 (12.09–19.00), p < 0.05
Diabetics	Nondiabetics
16.54 (12.09–22.76)	16.66 (12.93–21.78), NS
Male	Female
17.87 (12.66–20.14)	15.48 (12.64–24.23), NS

Notes: Data are given as medians and 25–75 percentiles. NS, not statistically significant; ESRD, end-stage renal disease.

RESULTS

The mean serum renalase concentration in the study cohort was 17.51 ± 6.73 μg/mL and it was significantly higher when compared with the healthy volunteers—3.99 ± 1.73 μg/mL (p < 0.001). In 22 healthy volunteers, renalase was related to both serum creatinine (0.49, p < 0.05) and age (r = 0.63, p < 0.01). Serum renalase was significantly lower in hemodialyzed patients with a history of stroke (Figure 1) than in patients without it. Similarly, renalase was significantly lower in hypertensive patients when compared with normotensives (Figure 2). Renalase levels in regard to gender, presence of diabetes, type a-v fistula, etiology of end-stage renal disease (ESRD), and anticoagulation during HD are given in Table 2. Serum renalase correlated with creatinine (r = 0.43, p < 0.05), residual renal function (r = −0.39, p < 0.05), transferrin saturation (r = 0.37, p < 0.05), presence of hypertension (r = −0.35, p < 0.05), and presence of mitral valve dysfunction (r = −0.40, p < 0.05). The only predictor of renalase in multiple regression analysis was the presence of hypertension (beta value was −0.63, p = 0.043) explaining 90% of the renalase variations.

Figure 1.  Serum renalase level in HD patients with and without history of stroke (p < 0.05).

Figure 2.  Serum renalase level in HD patients with and without hypertension (p < 0.05).

DISCUSSION

In our pilot study, we found that renalase was related to the presence of stroke and hypertension in hemodialyzed patients. So far there have been no data available on the possible relations between cardiovascular disease and renalase in hemodialyzed patients. In the recent study, associations between polymorphisms of renalase gene and hypertension were described in hemodialyzed patients.⁷ This is the first molecular analysis of renalase variants in hemodialyzed patients, which we suppose will pave a new way to gain a deeper insight into the hypertension origin and its complications. In another report from the same group a strong association between stroke and renalase gene polymorphism was reported. In our study, we did not observe association between renalase and blood pressure values in our population; however, hypertensive patients had lower serum renalase. Farzaneh-Far et al.⁹ reported that a functional missense polymorphism (C allele) in renalase (Glu37Asp) was associated with left ventricular hypertrophy, both systolic and diastolic dysfunction, but not with blood pressure in the Caucasians with stable coronary artery disease. In our previous studies, we found that renalase was mainly dependent on kidney function in kidney allograft recipients¹⁰ or residual renal function in peritoneally dialyzed patients.¹¹ A recent study by Wu et al.¹² reported no difference between wild-type and renalase knock out (KO) mouse in regard to renal function, but KO mice were hypertensive with tachycardia and left ventricular hypertrophy. These authors concluded that cardiac renalase deficiency may contribute to the increased susceptibility to myocardial injury due to ischemia and rhythm disturbances frequently found in chronic kidney disease (CKD). Renalase is supposed to metabolize catecholamines⁵; there are almost no data available up to date on the possible relations between catecholamines and renalase in humans. In the only one clinical study presented as an abstract, Schlaich et al.¹³ found that in patients with resistant hypertension (n = 22) arterial renalase was significantly higher in 4 normotensive control subjects relative to 22 hypertensive patients (p < 0.05), whereas whole body noradrenaline spillover tended to be higher in hypertensive patients, but without reaching statistical significance (p = 0.12).

In CKD and ESRD, sympathetic nervous system hyperactivity is observed.¹⁴ Elevated sympathetic activity is of prognostic relevance in that plasma norepinephrine concentrations predict survival and the incidence of cardiovascular events in patients with ESRD. On the other hand, dialyzed patients exhibit a higher risk of cerebrovascular events as compared with the general population. In 283 ESRD patients followed up for 10 years, Tripepi et al.¹⁵ investigated the long-term predictive value for stroke and transient ischemic attacks of traditional and nontraditional risk factors. They found that nontraditional risk factors in ESRD, such as norepinephrine, homocysteine, interleukin-6, and asymmetric dimethylarginine, failed to predict cerebrovascular events. In addition, Yano et al.¹⁶ reported that hypertensive patients with CKD had higher levels of sleep systolic blood pressure (125 mmHg vs. 129 mmHg), circulatory high-sensitivity C-reactive protein (hs-CRP), and norepinephrine (332.2 pg/mL vs. 372.8 pg/mL; all p < 0.05) when compared with those without CKD. Among hypertensive patients with CKD, those within the highest quartiles of norepinephrine had a greater stroke risk compared with those who were in the lower quartiles of norepinephrine [HR (95% CI): 2.2 (1.0–4.5); p = 0.045]. In the earlier studies of Zoccali et al.,¹⁷ norepinephrine levels represented an independent significant predictor of both death and cardiovascular events in Cox models not including asymmetric dimethyloarginine (ADMA). However, when ADMA was introduced into the models, norepinephrine levels lost substantial predictive power. In another study from this group, plasma norepinephrine was significantly higher in patients who died during follow-up than in those who survived.¹⁸ Similarly, plasma norepinephrine was higher in patients with incident cardiovascular events than in those who did not have such events. However, no relationship was found between plasma norepinephrine and Kt/V and serum creatinine. Correlations between norepinephrine and blood pressure were either not studied or reported.

In summary, renalase, a novel hormone probably associated with hypertension and cardiovascular complications, may be an attractive therapeutic target in the most vulnerable population such as dialyzed patients. It may provide a missing link between hypertension and cardiovascular complications in patients with kidney diseases. Renalase is “a new postulated therapeutic target.”

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.