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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 16  |  Issue : 1  |  Page : 32-38

Urinary level of vitamin D-binding protein as a new biomarker for diabetic nephropathy


1 Department of Internal Medicine, Faculty of Medicine, Tanta University, Tanta, Egypt
2 Department of Clinical Pathology, Faculty of Medicine, Tanta University, Tanta, Egypt

Date of Submission04-Nov-2015
Date of Acceptance15-Dec-2015
Date of Web Publication22-Mar-2016

Correspondence Address:
Kamal M Okasha
MD, Department of Internal Medicine, Faculty of Medicine, Tanta University, Tanta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1110-9165.179210

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  Abstract 

Diabetes is now the major cause of end-stage kidney failure, both in developing and developed nations. It is the primary diagnosis causing kidney diseases in 20-40% of patients starting treatment for end-stage renal diseases worldwide. The aim of the study was to evaluate the urinary level of vitamin D-binding protein (UVDBP) as a new biomarker for diabetic nephropathy (DN). Urine samples were obtained from 45 patients with type 2 diabetes mellitus and were classified into three groups (normoalbuminuric, microalbuminuric, and macroalbuminuric). Fifteen healthy participants served as the control group. The excretion levels of UVDBP were quantified with enzyme-linked immunosorbent assay. The results showed that UVDBP levels were significantly elevated in patients of the DN3 and DN4 groups compared with those of the DN2 group and normal controls. In conclusion, the current study demonstrated that UVDBP levels were significantly elevated in patients with DN. Moreover, a strong positive correlation was observed between the expression level of UVDBP and the development of DN. Thus, the findings indicate that UVDBP levels are a potential biomarker for the early detection of DN.

Keywords: Binding protein, diabetic nephropathy, vitamin D


How to cite this article:
Khodeir SA, Kotb NM, Okasha KM, Ahmed KA, Nagy HM. Urinary level of vitamin D-binding protein as a new biomarker for diabetic nephropathy. J Egypt Soc Nephrol Transplant 2016;16:32-8

How to cite this URL:
Khodeir SA, Kotb NM, Okasha KM, Ahmed KA, Nagy HM. Urinary level of vitamin D-binding protein as a new biomarker for diabetic nephropathy. J Egypt Soc Nephrol Transplant [serial online] 2016 [cited 2017 Aug 17];16:32-8. Available from: http://www.jesnt.eg.net/text.asp?2016/16/1/32/179210


  Introduction Top


Diabetes mellitus (DM) belongs to a group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both. The chronic hyperglycemia of diabetes is associated with long-term damage, dysfunction, and failure of different organs, especially the eyes, kidneys, nerves, heart, and blood vessels [1]. In 2013, according to the International Diabetes Federation, an estimated 381 million people had diabetes. Its prevalence is increasing rapidly, and by 2030 this number is estimated to almost double [2].

Diabetic nephropathy (DN) is one of the most important long-term complications of diabetes. It is characterized by the development of proteinuria with a subsequent decline in glomerular filtration rate (GFR), which progresses over a long period of time, often over 10-20 years [3,4]. Over the past 20 years, the prevalence of DN in the USA has increased in direct proportion to the prevalence of diabetes [5]. Although DN cases vary largely among countries, on average it develops in 30-40% of patients with diabetes [6].

D-binding protein (DBP), a multifunctional and highly polymorphic plasma protein synthesized primarily in the liver, was identified about half a century ago and characterized as able to bind various forms of vitamin D [7]. DBP (also referred to as Gc-globulin) is a member of the albumin gene family [8]. Vitamin D circulate bound to vitamin D-binding protein (VDBP) (85-90%) and albumin (10-15%), with less than 1% of circulating hormone in its free form. VDBP prolongs the serum half-life of 25-hydroxyvitamin D and protects against vitamin D deficiency by serving as a vitamin D reservoir [9]. The study comprised 45 patients referred to Tanta University Hospital with type 2 diabetes during the period from 13 March 2014 to 9 November 2014. They were examined at a single center (Nephrology Department, Tanta University, Egypt) and compared with 15 healthy controls. Patients were enrolled into the study after their informed written consent and approval of the Ethics Committee of Tanta University Hospitals were obtained. The patients were divided into four groups according to their urinary albumin concentration: group I, healthy controls (15 participants); group II, diabetic patients with normoalbuminuria (15 participants); group III, diabetic patients with microalbuminuria (15 participants); and group IV, diabetic patients with macroalbuminuria (15 controls). Patients with type I DM, collagen diseases (such as rheumatoid arthritis and systemic lupus erythematosus), oncogenic patients, patients with severe renal impairment (GFR < 30 ml/min), those with infections (such as urinary tract infection and chest infections), liver disease patients, and patients without any renal diseases other than diabetes and decompensated heart failure were excluded.


  Patients and methods Top


All patients and controls were subjected to full history taking, thorough clinical examination, routine laboratory investigations [estimated glomerular filtration rate (eGFR), fasting and 2-h postprandial blood sugar, glycosylated hemoglobin, serum creatinine, blood urea, serum cholesterol, triglycerides, low-density lipoprotein (LDL), high-density lipoprotein (HDL), and C-reactive protein (CRP)], and specific laboratory investigations [urinary vitamin D-binding protein (UVDBP) by enzyme-linked immunosorbent assay and albumin creatinine ratio (ACR)].

Collection and handling of blood and urine samples: Two blood samples were collected: the first after the patient had fasted for 8-10 h for determination of blood glucose, and the second under complete aseptic conditions after the patient had fasted for another 2-4 h. The collected samples were divided into two aliquots. For urine samples the second morning urine voided after rising was preferred.

Determination of UVDBP: This was carried out by means of the quantitative sandwich enzyme immunoassay technique. A monoclonal antibody specific for VDBP was precoated onto a microplate. Standards and samples were pipetted into the wells, and any VDBP present was bound by the immobilized antibody. After washing away any unbound substances, an enzyme-linked monoclonal antibody specific for VDBP was added to the wells. Following a wash to remove any unbound antibody-enzyme reagent, a substrate solution was added to the wells and color was developed in proportion to the amount of VDBP bound in the initial step. The color development was stopped and the intensity of the color was measured.

Statistical analysis

Statistical presentation and analysis was conducted using the mean, SD, and χ2 -test. A P-value less than 0.05 was considered statistically significant.


  Results Top


Comparison between studied groups with respect to ACR : ACR was significantly increased in group IV (mean ± SD = 1449.333 ± 643.456) compared with group III, group II, and group I (mean ± SD = 247.267 ± 36.654, 20.000 ± 5.720, and 16.400 ± 4.881, respectively), whereas there was no significant difference when comparing group III with group II and group I (P > 0.05). There was no significant difference between group II and group I either (P > 0.05) [Table 1] and [Figure 1].
Figure 1: Comparison between studied groups regarding albumin creatinine ratio (ACR).

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Table 1: Comparison between studied groups regarding albumin creatinine ratio

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Comparison between the studied groups regarding UVDBP: UVDBP was significantly increased in group IV (mean ± SD = 159.920 ± 19.608) compared with group III, group II, and group I (mean ± SD = 141.720 ± 7.540, 132.574 ± 8.757, and 123.347 ± 8.452, respectively). It was also significantly increased in group III compared with group I (P < 0.05), whereas there was no significant difference between group III and group II (P > 0.05), nor between group II and group I (P > 0.05) [Table 2] and [Figure 2].
Figure 2: Comparison between studied groups regarding urinary vitamin D-binding protein (UVDBP).

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Table 2: Comparison between studied groups regarding urinary vitamin D-binding protein

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Correlation between ACR and all variables in all groups: ACR correlated positively with UVDBP, systolic blood pressure, diastolic blood pressure, fasing blood glucose (FBS), HbA1c, creatinine, blood urea, cholesterol, triglycerides, LDL, and duration of diabetes but negatively with HDL and eGFR. There was no correlation between ACR and age, sex, PPBS, CRP, and smoking [Table 3].
Table 3: Correlation between albumin creatinine ratio and all variables in all groups

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Correlation between UVDBP and all variables in all groups: UVDBP correlated positively with ACR, FBS, PPBS, HbA1c, creatinine, blood urea, cholesterol, triglycerides, LDL, and duration of diabetes but negatively with HDL and eGFR. There was no correlation between UVDBP and age, sex, systolic blood pressure, diastolic blood pressure, CRP, and smoking [Table 4] and [Figure 3],[Figure 4],[Figure 5] and [Figure 6].
Figure 3: Correlation between urinary vitamin D-binding protein (UVDBP) and albumin creatinine ratio (ACR).

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Figure 4: Correlation between urinary vitamin D-binding protein (UVDBP) and estimated glomerular filtration rate (eGFR).

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Figure 5: Correlation between urinary vitamin D-binding protein (UVDBP) and high-density lipoprotein (HDL).

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Figure 6: Correlation between urinary vitamin D-binding protein (UVDBP) and HbA1c.

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Table 4: Correlation between urinary vitamin D-binding protein and all variables in all groups

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Receiver operating characteristic curves were used to assess the potential utility of UVDBP detection in our patients.

Cutoff value greater than 135 differentiated between diabetic patients and controls with a sensitivity of 90% and specificity of 76.7%.

Cutoff value greater than 151.2 differentiated between macroalbuminuric and microalbuminuric groups with a sensitivity of 66.7% and specificity of 100% [Table 5] and [Table 6] and [Figure 7] and [Figure 8].
Figure 7: The sensitivity, specificity, and accuracy of urinary vitamin D-binding protein (UVDBP) (patients and controls).

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Figure 8: The sensitivity, specificity, and accuracy of urinary vitamin D-binding protein (UVDBP) (microalbuminuric and macroalbuminuric groups).

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Table 5: The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of urinary vitamin D-binding protein (patients and controls)

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Table 6: The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of urinary vitamin D-binding protein (microalbuminuric and macroalbuminuric groups)

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  Discussion Top


About 246 million people worldwide had diabetes in 2007, which is projected to increase to 550 million in 2030 [10]. Increased risk for vascular complications, including both macrovascular and microvascular complications, accounts for the major cause of morbidity and mortality in diabetic patients [11].

VDBP is a 58-kDa glycoprotein and is present in the serum at a concentration of 300-600 mg/ml [12]. It serves as the main carrier protein for vitamin D in the bloodstream. VDBP is important for the bioavailability of active 1,25-dihydroxyvitamin D (1,25 (OH) 2 D) and its precursor 25-hydroxyvitamin D [13]. The transportation of vitamin D by VDBP is important for the functioning of a wide variety of tissues, and changes in VDBP activity result in the development of a number of diseases [14].

In addition to its transport function, VDBP is the parent molecule of VDBP-MAF (macrophage activating factor). VDBP-MAF is the product of the selectively deglycosylated form of VDBP and has been demonstrated to be a potent antiangiogenic and antitumorigenic molecule. Such functions would greatly benefit the regulation of the growth of cancer cells and protection against certain immune disorders. These important and diverse properties of VDBP have been suggested in previous studies with regard to a number of tumor types. Moreover, VDBP is important in inflammation processes [15].

The aim of the present work was to assess the UVDBP as a new biomarker for DN and evaluate its clinical significance in diagnosis as well as assessment of disease severity. In the present study, following quantitative measurements of 60 urine samples with enzyme-linked immunosorbent assay, diabetic patients were categorized into three groups depending on their ACR (normoalbuminuria, microalbuminuria, and macroalbuminuria). Receiver operating characteristic analysis rendered that an optimum cutoff value of UVDBP corresponding to 90.0% sensitivity and 76.7% specificity is appropriate for detecting DN.

The reasons underlying the enhanced excretion of UVDBP in patients with DN may be associated with renal tubular damage in DN patients [16]. It has been increasingly documented that renal tubular injury plays an integral role in the pathogenesis of diabetic kidney disease. In addition, tubulointerstitial lesions were found to be the early and independent features of diabetic kidney disease [17]. In a previous study, increased excretion of UVDBP was observed following long-term cadmium exposure (significant positive correlations between urinary levels of DBP and renal tubular dysfunction), and it was suggested that the marked loss of VDBP in the urine may be linked to renal tubular dysfunction and bone lesions in the inhabitants of cadmium-polluted areas [18]. Clinically, a study by Zoidakis and colleagues (2012) identified that the reduction in VDBP levels in the urine of patients with invasive bladder cancer was significant, which is consistent with the findings by Li and colleagues (2012). Moreover, Li and colleagues also demonstrated that the expression levels of UVDBP were positively associated with the pathological classification of bladder cancer. Their results suggested that UVDBP may be a potential noninvasive biomarker for the early diagnosis and effective surveillance of bladder cancer [19]. Mirkoviæ et al . (2013) [20] indicated that the urinary excretion of VDBP may be a novel urinary biomarker of tubulointerstitial damage. They also indicated that damaged tubular epithelial cells in areas of tubulointerstitial fibrosis may no longer be able to handle VDBP, resulting in gross VDBP loss into the urine, and that it can be modulated by antiproteinuric treatment in patients. Although the combination of the renin-angiotensin-aldosterone system blockade and dietary sodium restriction, an intervention considered optimal for renoprotection, considerably reduced VDBP excretion, they demonstrated that UVDBP excretion is increased early after renal injury and is associated with tubulointerstitial inflammation and fibrosis independently of albuminuria. In humans, UVDBP increased with increasing severity of renal damage, and responded to renoprotective therapy. Yet, persisting UVDBP above normal suggested persistent tubular interstitial damage [21].

Concerning UVDBP our results were in agreement with those of Tian and colleagues (2014), who studied 105 Chinese individuals with diabetes and 45 healthy volunteers. The patients were divided into three groups according to the value of ACR: the DM group without nephropathy and albuminuria; the early DN group with microalbuminuria; and the overt DN group with macroalbuminuria. They demonstrated that UVDBP levels were significantly elevated in patients with DN compared with patients without nephropathy. Moreover, strong positive correlation was observed between expression levels of UVDBP and the parameters of kidney dysfunction. The analysis of their result rendered an optimum cutoff value of 552.243 ng/mg corresponding to 92.86% sensitivity and 85% specificity. They concluded that UVDBP levels are a potential biomarker for early detection and prevention of DN [22].

It has been demonstrated that the presence of vitamin D deficiency or insufficiency in patients with diabetes is independently associated with the development of DN. This can be explained by decreased UVDBP secondary to tubular damage [23].

Our results revealed that eGFR, based on the MDRD study equation, was significantly lower in diabetic patients than in healthy controls. Also, serum creatinine and blood urea nitrogen (BUN) were significantly higher in microalbuminuric and macroalbuminuric patients when compared with healthy controls. This is in agreement with the findings of Tian et al. (2014) [22], who reported that higher levels of BUN and creatinine indicate a failing of GFR as a result of decreased capability of the kidney to excrete waste products.

However, in chronic kidney disease, BUN will be elevated above the normal level only when more than 60% of kidney cells are no longer functioning. Hence, a more accurate measure of renal function is generally preferred to assess the clearance. Israni and Kasiske (2011) [24] reported that the usefulness of BUN as an independent indicator of renal function is limited to the variability of its blood levels as a result of nonrenal factors such as gastrointestinal hemorrhage, mild dehydration, high protein diet, and decreased perfusion of the kidneys.

Concerning parameters of kidney function our results revealed that ACR correlated positively with creatinine and BUN and negatively with eGFR. These results were in agreement with those of Hoefield and colleagues (2011), who reported that individuals with diabetes and macroalbuminuria have an estimated 19 times more rapid decline in renal function compared with those without albuminuria (5.7% per annum vs. 0.3% per annum). Individuals with microalbuminuria also have a significantly increased rate of eGFR decline (five times greater) compared with people with normoalbuminuria [25].

Our findings are comparable to recent studies demonstrating that albuminuria is independently associated with increased risk for declining GFR in people with diabetes. In particular, data from a follow-up study (n = 1303) in a hospital-based cohort of patients with type 2 diabetes in Japan showed that those with normoalbuminuria at baseline had a relatively benign rate of progression, with less than 0.2% reaching eGFR below 30 ml/min compared with 24% of those with macroalbuminuria after 3 years [26],[27].

An important limitation of the present study regarding the specificity of this biomarker should be considered when UVDBP detection is used for early prevention of DN. It has been demonstrated that UVDBP levels are closely associated with renal dysfunction. In the present study, urine samples were collected from patients with DN only but not from patients with additional nephropathies. This may have caused an overestimation of the specificity of VDBP as a biomarker for the detection of DN. Therefore, further studies including a larger sample and analyses of patients with various types of non-DN are required to clarify this issue.


  Conclusion Top


Our findings indicate that UVDBP levels are a potential biomarker for early detection of DN, but an important limitation in terms of the specificity of this biomarker should be considered when UVDBP detection is used for early prevention of DN. In the present study, urine samples were collected from patients with DN only but not from patients with additional nephropathies. This may have caused an overestimation of the specificity of VDBP as a biomarker for the detection of DN. Therefore, we recommend that further studies including a larger sample and analyses of patients with various types of non-DN are required to clarify this issue.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
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