Renalase might be associated with hypertension and insulin resistance in Type 2 diabetes

Abstract

Background and objectives: Renalase, a monoamine oxidase derived from the kidney, can degrade catecholamine (CA) and regulate blood pressure as well as cardiac function. To investigate the changes of serum renalase levels in patients with Type 2 diabetes mellitus (DM) and examine the correlation with other features of T2DM. Methods: Seventy-five patients with T2DM and 13 healthy volunteers were studied. The levels of serum renalase and CA were measured by enzyme linked immunosorbent assay. Several other biochemical and clinical parameters were measured. Results: Serum levels of CA and renalase as well as renalase/CA (R/C) ratio in the T2DM group were significantly higher than those of the control group (p < 0.05). There was a highly positive correlation between the levels of serum renalase and CA (r = 0.795, p < 0.001). The levels of serum renalase were positively correlated with systolic blood pressure (r = 0.217, p = 0.042) and serum creatinine (SCr) (r = 0.295, p = 0.007), and negatively correlated with eGFR (r = −0.222, p = 0.044). The R/C ratio was positively correlated with SCr (r = 0.347, p = 0.001) as well as homeostasis model assessment for insulin resistance (IR) HOMA2-% S (r = 0.340, p = 0.037). Conclusion: Serum levels of renalase and CA were highly correlated in patients with T2DM. The levels of serum renalase and R/C ratio of T2DM patients were significantly higher than those of healthy subjects and appeared correlated with changes in blood pressure, glomerular filtration rate and IR.

• Blood pressure
• diabetes mellitus
• insulin resistance
• renalase

Introduction

Renalase, a recently discovered monoamine oxidase derived from the kidney, can degrade catecholamines (CA) and regulate blood pressure as well as heart function.1–3 As a biological macromolecule strongly associated with the sympathetic nervous system, renalase has been intensively studied since its discovery.4–6

Insulin resistance (IR) is a key feature of type 2 diabetes mellitus (T2DM).7 In some T2DM patients, CA levels in blood circulation are increased. In addition, hypertension is the most common complication of T2DM.8 Currently, renalase is the only monoamine oxidase known to be able to degrade circulating CA. It is not known whether hypercatecholaminemia, hypertension, or IR in patients with T2DM is associated with changes of renalase levels.

In this context, we investigated the levels of circulating renalase in T2DM patients and analyzed its relationship with several features of T2DM.

Materials and methods

Subjects

Patients treated in Shanghai Jiao Tong University Affiliated Sixth People’s Hospital from January 2011 to February 2012 and healthy volunteers were enrolled as subjects of the study. This study adhered to the Declaration of Helsinki and was approved by the Ethics Committee of Shanghai Jiao Tong University Affiliated Sixth People's Hospital.

Inclusion criteria: (1) ≥18 years of age. (2) T2DM patients or healthy volunteers. (3) Patients with T2DM were divided into two groups according to the presence and absence of chronic kidney disease (CKD). Exclusion criteria: (1) age <18 years of age. (2) Acute kidney injury, dialysis and organ transplantation. (3) Malignant tumors, severe trauma, acute phase of infection, shock, severe hepatic dysfunction, acute heart failure and respiratory failure. (4) Primary endocrine disorders and metabolic diseases apart from DM.

Methods

Blood samples of all subjects were collected from the cubital vein after 10 h of fasting with a supine position. Serum specimens were preserved in a refrigerator at minus 80 °C.

Omron HEM-8021 electronic sphygmomanometer (Omron Dalian, Dalian, China) was used to measure sitting blood pressure. eGFR was calculated by a modified formula of modification of diet in renal disease (MDRD)9 and IR index by HOMA2-IR formula.10

Blood urea nitrogen (BUN), serum creatinine (SCr), fasting plasma glucose (FPG), fasting insulin (Fins), C-peptide, total cholesterol (TC), triglyceride (TG), high-density lipoprotein (HDL) and low-density lipoprotein (LDL) were detected by automatic biochemical analyzer (Hitachi7600, Ichige, Japan). Hemoglobin A1c (HbA1c) and glycated albumin (GA) were measured by High Performance Liquid Chromatography (HPLC).

ELISA was used to determine the level of serum renalase and the commercial kits were provided by Y-J Biological (Shanghai, China). ELISA was also employed to evaluate the total amount of serum CA and the commercial kits were provided by R&D Systems China (Shanghai, China). The measurement was performed in accordance with the instructions. R/C is the ratio of serum renalase level to CA level.

Statistical analysis

All results were presented as mean ± standard error (± SEM) and SPSS18.0 software (Chicago, IL) was used for data processing. p-Value < 0.05 was statistically significant. The difference between groups was analyzed by one-way ANOVA. And the association between parameters was analyzed via Pearson correlation analysis.

Results

Clinical information

In the present study, 75 patients with T2DM (DM group) including 38 males and 37 females, aged between 39 and 90, were recruited. Among these patients, 27 cases (DM1 group) of T2DM were not complicated with renal injury, while 48 cases (DM2 group) were associated with CKD. Oral anti-diabetic drugs and insulin were used to lower blood glucose. In the DM group, 60 cases (80%) had hypertension and received antihypertensive drugs including ACEI, ARB, β-blockers, calcium channel blockers and α-blockers. There were 13 healthy subjects in the control group. Biochemical and metabolic characteristics of the subjects are shown in Table 1.

Table 1. Metabolic and biochemical characteristics of the patients with T2DM and the control subjects.

Parameter	Control (n = 13)	DM1 (n = 27)	DM2 (n = 48)
BUN (mmol/L)	4.9 ± 0.3	5.8 ± 0.3	11.2 ± 1.1^a
SCr (μmol/L)	76.7 ± 2.6	59.1 ± 2.6	312.8 ± 50.9^a
BMI (kg/m^2)	21.4 ± 1.6	24.1 ± 3.1^a	24.9 ± 5.3^a
TC (mmol/L)	4.7 ± 0.2	4.4 ± 0.2	5.2 ± 0.3
TG (mmol/L)	1.4 ± 0.3	1.9 ± 0.4	2.1 ± 0.3
HDL (mmol/L)	1.3 ± 0.1	1.2 ± 0.1	1.2 ± 0.1
LDL (mmol/L)	3.2 ± 0.2	2.7 ± 0.2	2.9 ± 0.2
FPG (mmol/L)	5.3 ± 0.1	8.6 ± 1.0^a	8.3 ± 0.7^a
HbA1c (%)	4.8 ± 0.9	9.3 ± 1.8^a	7.5 ± 1.9^a

Notes: ^aCompared with control group, p < 0.05.

DM1, patients with T2DM not complicated with renal injury. DM2, patients with T2DM complicated with CKD. DM, patients with T2DM, composed of DM1 and DM2. BUN, blood urea nitrogen; SCr, serum creatinine; TC, serum total cholesterol; TG, serum triglyceride; HDL, high-density lipoprotein; LDL, low density lipoprotein; FPG, fasting plasma glucose.

Levels of serum renalase and CA in each group

As shown in Table 2, circulating levels of renalase in the DM group, DM1 group and DM2 group were significantly higher than that of the control group (p < 0.05). R/C ratios of the DM group and DM1 group were significantly higher than that of the control group (p < 0.05). CA levels of the DM group and DM2 group were significantly higher than that of the control group (p < 0.05).

Table 2. Serum renalase and CA in patients with T2DM.

Group	N	Renalase (ng/L)	CA (ng/L)	R/C
Control	13	137.0 ± 8.6	164.5 ± 8.3	0.83 ± 0.03
DM1	27	202.5 ± 9.2^a	191.9 ± 3.8	1.05 ± 0.03^a
DM2	48	201.7 ± 14.1^a	212.7 ± 11.5^a	0.94 ± 0.03
DM	75	202.0 ± 9.6^a	205.2 ± 7.5^a	0.96 ± 0.02^a

Notes: ^aCompared with control, p < 0.05.

CA, serum catecholamine. R/C, ratio of serum renalase to serum catecholamine. DM1, patients with T2DM not complicated with renal injury. DM2, patients with T2DM complicated with CKD. DM, patients with T2DM, composed of DM1 and DM2.

Correlation analysis between serum renalase levels and other parameters

Levels of serum renalase strongly correlated with CA levels (r = 0.795, p < 0.001), positively correlated with SBP and SCr (p < 0.05), and negatively correlated with eGFR (p < 0.05), as shown in Table 3 and Figure 1. However, there was no significant correlation between the levels of serum renalase and age, gender, diastolic blood pressure (DBP), hip circumference, waistline, FPG, body mass index (BMI), C-peptide, HbA1c or GA (p > 0.05).

Figure 1. Significant correlations between serum renalase and serum CA, SBP, SCr, and eGFR. Levels of serum renalase strongly correlated with CA levels (r = 0.795, p < 0.001), positively correlated with SBP (r = 0.217, p = 0.042) and SCr (r = 0.295, p = 0.007), and negatively correlated with eGFR (r = 0.222, p = 0.044).

Table 3. Correlation analysis between serum renalase and other parameters.

	Renalase		R/C
Parameters	r	p	r	p
CA	0.795**	0.000	0.139	0.198
Age	0.130	0.227	0.120	0.264
Gender	−0.006	0.953	0.018	0.866
SBP (DM group)	0.217*	0.042	0.067	0.535
DBP (DM group)	0.165	0.125	0.108	0.318
Waistline	−0.004	0.987	−0.199	0.429
Hipline	0.102	0.687	−0.237	0.344
Scr	0.295**	0.007	0.347**	0.001
BMI	0.147	0.247	−0.030	0.814
HOMA2-%S	0.019	0.908	0.340*	0.037
C-peptide	0.054	0.719	−0.233	0.120
FPG	0.109	0.338	0.099	0.388
eGFR	−0.222*	0.044	−0.126	0.257
HbA1c	−0.093	0.492	0.055	0.682
GA	−0.051	0.712	0.006	0.963

Notes: *p < 0.05, **p < 0.01.

CA, serum catecholamine. R/C, ratio of serum renalase to catecholamine. SBP, systolic blood pressure. DBP, diastolic blood pressure. SCr, serum creatinine. BMI, body mass index. eGFR, estimated glomerular filtration rate. HbA1c, hemoglobin A1c. GA, glycated albumin.

R/C ratio was positively correlated with SCr and HOMAIR-% S (p < 0.05), as shown in Table 3 and Figure 2.

Figure 2. Significant correlation between serum renalase, CA ratio and HOMA2-%S R/C ratio was positively correlated with SCr (r = 0.347, p = 0.001) and HOMA2-%S (r = 0.340, p = 0.037).

Discussion

The results of this study showed that the levels of CA in DM group and DM1 group were significantly higher than that of the control group (p < 0.05), which is consistent with previous reports showing that T2DM patients have hypercatecholaminemia.8 Levels of renalase in DM group, DM1 group and DM2 group were significantly higher than that of the control group (p < 0.05). R/C ratios of the DM group and DM1 group were significantly higher than that of the control group (p < 0.05). The levels of serum renalase were strongly and positively correlated to CA levels (r = 0.795, p = 0.000), which reflects the strong association between renalase level and serum CA and is consistent CA acting as a potent inducer of renalase.

The results showed that the level of R/C ratio was positively correlated with SCr and HOMA2-%S. IR is one of the common pathogenic mechanisms of diabetes and hypertension.11 Renalase, a monoamine oxidase derived from the kidneys, can directly degrade circulating CA, lower blood pressure and regulate cardiovascular functions.12 There have been evidences suggesting that renalase is not only closely related to the cardiovascular complications of CKD, but also possibly associated with essential hypertension, stroke, heart failure, and T2DM.13–16 Metabolic Syndrome, also known as IR syndrome, is a disease state in which a variety of metabolic disorders and cardiovascular disease, especially obesity, hypertension, hyperglycemia and blood lipid abnormalities are involved.12,17 Reaven et al. proposed that sympathetic activation is the bridge between IR and hypertension caused by hyperinsulinemia.18

The discovery of renalase is an important progress in renal and cardiovascular fields.19–23 In 5/6 nephrectomized rats, the decreased expression of renalase is associated with increased plasma norepinephrine, SBP and proteinuria.24 Gene polymorphism of renalase was also reported as a potentially new genetic susceptibility marker of T2DM and essential hypertension.13–16 Currently, renalase is the only enzyme found to be able to degrade circulating CA and it displays a protective effect on the heart.25–27 It is significant to carry out researches on renalase for the prevention and treatment of CKD, diabetes and hypertension.28–30

The role of CA and the sympathetic nervous system in the pathogenesis of kidney disease and hypertension is an important field of research.31 The kidney is one of the main organs participating in the regulation of blood pressure and metabolism.32–36 Renal denervation can lower blood pressure, improve cardiac function and IR.34

It is recognized that the sample amount in the present study was relatively small. In addition, enzymatic activity of renalase, which requires further technical optimization, was not measured. This study showed that serum renalase levels in patients with renal injury were higher than normal control, which is consistent to the report of Przybylowski et al.,37 but inconsistent with some other reports.2 This may be related to different determination methods. Besides, other organs (e.g. heart) beyond the kidney may compensatory secrete renalase under renal failure condition.

The study of renalase is rapidly evolving.38,39 Whether renalase participates in the development of hypercatecholaminemia or hypertension in T2DM patients and whether it is associated with IR still need further investigation.

Declaration of interest

This work was sponsored by the National Natural Science Foundation of China (81100528 and 81270824), Science and Technology Commission of Shanghai Municipality (114119a6100), the New 100-talent Plan of Shanghai Jiao Tong University School of Medicine and the development fund for Shanghai talents.

Acknowledgements

The authors thank Dr. Mingyu Liang of Medical College of Wisconsin for his revision advice.