Diabetes causes disorders in the performance of endothelial progenitor cells, and obesity and vitamin D deficiency are associated with endothelial dysfunction and cardiovascular disease. This case-control study investigated the relationship between serum CD34 antigen and vitamin D levels and insulin resistance in type 2 diabetes. The results showed that CD34 has a significant inverse relationship with BMI, A1C, fasting blood glucose, insulin resistance, and insulin levels and has a significant direct relationship with vitamin D levels. Both CD34 and vitamin D were found to be significantly associated with type 2 diabetes. The association between reduced CD34 and vitamin D levels with type 2 diabetes and increased insulin resistance suggests that these parameters may be helpful in assessing diabetes and predicting its complications.

Diabetes is a metabolic disease that is diagnosed by high blood glucose and causes problems in insulin secretion, insulin function, or both. One of the dangers of chronic high blood glucose is its potential to cause damage to various organs, including the eyes, kidneys, nerves, heart, and blood vessels.

The prevalence of diabetes has been increased dramatically in the past two decades. The number of people with diabetes >20 years of age worldwide was estimated to be 171 million in 2000, and that number is expected to increase to 366 million by 2030 (1).

Type 1 diabetes is caused by autoimmune destruction of pancreatic β-cells in the islets of Langerhans, leading to absolute insulin deficiency. Type 2 diabetes is caused by a slow reduction in insulin secretion, which can be associated with cellular insulin resistance. Although the incidence of both type 1 and type 2 diabetes is increasing worldwide, it is expected that the acceleration will be greater for type 2 diabetes. The incidence of type 2 diabetes is variable because of the combination of associated environmental and genetic factors within the population (2), but this form of the disease accounts for 90–95% of all diabetes cases. Most patients with type 2 diabetes have obesity, which itself leads to some degree of insulin resistance. There are probably many causes for type 2 diabetes, although the specific etiologies are unknown.

Vitamin D has a potential protective effect on the vascular endothelium, but vitamin D deficiency is common not only in the elderly, but also in adolescents and children (3,4). Various factors such as age, sex, geographical location, nutritional status, and physical fitness affect a person’s vitamin D status; diabetes, kidney function, hypoalbuminemia, and albuminuria are also risk factors for vitamin D deficiency (5). This deficiency is associated with insulin secretion, insulin resistance, and dysfunction of pancreatic β-cells (6). However, it has not been proven that vitamin D supplementation improves glycemic control or prevents type 2 diabetes.

Endothelial progenitor cells (EPCs) are specialized cells for endothelial repair and angiogenesis in the body and are mainly used in vascular damage repair. The particular marker of these cells is CD34 antigen, which is used as a measure of the activity level of EPCs (7). These cells have a significant impact on cardiovascular disease (CVD) and vascular complications, and they are one of the most crucial factors in the body to repair cardiovascular damage (8). However, CD34 is a membrane-type phosphoglycoprotein encoded by the CD34 gene in humans, mice, rats, and other species (9). It acts as a cell surface glycoprotein and as a cell adhesion agent. It may also mediate the connection of hematopoietic stem cells to the extracellular matrix of bone marrow or directly to stromal cells. Clinically, it is used along with the selection and enrichment of hematopoietic stem cells for bone marrow transplantation. Given these historical and clinical connections, CD34 expression is associated with hematopoietic cells almost everywhere, but it is also present in many other cells (10).

Patients diagnosed with diabetes experience a high rate of CVD, but this risk varies significantly even within the population with diabetes (11). Some researchers have shown that people with diabetes and vascular disease have lower levels of EPCs in blood circulation (12,13). However, this logical and convincing concept suggests the use of these biomarkers for improving individual risk prediction.

Human CD34 cells have vascular regenerative capacity and the potential to be angiogenesis precursors in vivo (14), and the reduction in their power is now considered an essential factor in cardiovascular homeostasis disorder in diabetes. The latest meta-analysis has shown that diabetes can cause long-term dysfunction of the bone marrow and EPC production (15). One of the proposed causes for dysfunction in making EPCs are the sudden and severe changes in blood glucose level that can lead to severe cross-sectional stress in the body and prevent the formation of EPCs in some cell lines in the bone marrow (16). However, the exact relationship between EPCs and diabetes and insulin resistance is not apparent. Some studies have shown conflicting results, with some considering insulin resistance to be separate from diabetes and unrelated to the level of EPCs (17).

Endothelial disorder predicts cardiovascular events and represents an underlying condition for vascular abnormalities found in people with type 2 diabetes (18). The level of circulating EPCs has also been proposed as a marker of vascular dysfunction and is decreased in patients with various cardiovascular risk factors (19). The relationship between vitamin D status and circulating EPCs in patients with type 2 diabetes has not yet been fully recognized. The association between vitamin D deficiency, impaired EPC formation, and insulin resistance in type 2 diabetes is mainly unknown. This study was designed to investigate the relationship between serum CD34 level and vitamin D and insulin resistance in people with type 2 diabetes.

This case-control study included 30 people with type 2 diabetes and 30 healthy individuals referred to Mashhad University Hospitals in 2020.

Participant Selection

People with type 2 diabetes who were >25 years of age, had a duration of diabetes of 1–5 years, and had no known nephropathy or liver disease, heart problems, hypothyroidism, hyperthyroidism, stroke, or cancer were eligible. Participants had to have not taken vitamin supplements or medications and not had a fever for at least 1 week for any reason, including viral infection, and pregnant women were also excluded. The control group included people determined by two blood glucose tests to have no diabetes who were matched with the diabetes group in terms of age and sex.

Data Collection

Data collected included participants’ age, sex, duration of diabetes, fasting blood glucose (FBG), A1C, blood pressure, serum creatinine, serum uric acid, lipid profile (total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides), presence of other diseases, history of drug use, and BMI. These data were obtained from participants’ medical records.

Preparation of Samples and Procedures

Venous blood, 5 mL, was drawn from all participants after 8 hours of fasting. After transfer to the blood laboratory, it was centrifuged, and its serum was separated and stored at a temperature of −70°C until the test day. Serum levels of insulin, CD34, and vitamin D for all subjects were measured by the ELISA (enzyme-linked immunosorbent assay) method. To determine the degree of insulin resistance, we used the HOMA-IR formula, as follows:
formula

The vitamin D kit used was the 25(OH)-Vitamin D (TR) ELISA Kit (PishtazTEB Diagnostics). The CD34 kit used was the human cluster of differentiation 34 (CD34) ELISA kit (ZellBio). The procedures used in this study were based on the brochures from each kit manufacturer.

We used the sandwich ELISA method, in which an antigen is placed between two particular antibodies. In this method, one antibody is used to trap the antigen on ELISA wells, and the second antibody is labeled with an enzyme acting as a marker. It should be noted that, in this method, the antigen must have at least two different antigenic regions to bind to both antibodies.

Data Analysis

Appropriate tables and statistical indicators (such as mean and SD) have been used to report data. First, normal distribution was examined by the Shapiro-Wilk test. When abnormal data were observed, the Mann-Whitney U test was used, and for confirmation of normality, appropriate parametric methods such as the Student t test were used. A χ2 test was used to analyze the data with a nominal scale. Pearson correlation coefficient and a linear model were used to investigate the relationship between variables. Analyses were performed using SPSS v.26 statistical software, and statistical significance was set at P <0.05.

Ethical Considerations

The Ethical Committee of the Mashhad Medical Sciences Branch, Islamic Azad University, Mashhad, Iran, approved the study protocol (registration no. IR.IAU.MSHD.REC.1399. 132).

Patients provided written informed consent before participating in the study.

Study Population

In this case-control study, the group with type 2 diabetes and the healthy control group each included 30 individuals, of whom 47% were male and 53% female.

Demographic Data

The mean age was 48.53 ± 10.36 years in the control group and 52.90 ± 10.38 years in the case group (P = 0.18). The mean BMI was 25.99 ± 3.77 kg/m2 in the control group and 29.55 ± 4.80 kg/m2 in the case group (P = 0.002). The mean systolic blood pressure was 130.67 ± 16.17 mmHg in the control group and 138.77 ± 12.47 mmHg in the case group (P = 0.034), and the mean diastolic blood pressure was 89.00 ± 14.70 mmHg in the control group and 91.67 ± 13.41 mmHg in the case group (P = 0.466).

Distribution of Blood Indices

The results of the distribution of blood indices in the two groups are shown in Table 1. As can be seen, the mean of FBG, A1C, and insulin showed a statistically significant difference between the two groups (P <0.05), but the mean of serum creatinine, serum uric acid, and lipid profile (total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides), had no statistically significant differences between the two groups (P >0.05).

TABLE 1

Distribution of Blood Indices in the Case Group (Patient With Diabetes) and the Control Group

VariableNormal, n (%)Abnormal, n (%)Mean ± SDTest Statistics
FBG, (mg/dL)
Control
Diabetes
Total 

29 (96.7)
9 (30.0)
38 (63.3) 

1 (3.3)
21 (70.0)
22 (36.7) 

95.23 ± 15.47
128.33 ± 17.92
111.78 ± 23.54 
t = −7.65, P = 0.0001 
A1C, %
Control
Diabetes
Total 

30 (100.0)
0 (0.0)
30 (50.0) 

5.00 (0.0)
7.42 (100.0)
6.21 (50.0) 

5.00 ± 0.53
742 ± 0.60
6.21 ± 1.34 
t = −16.34, P = 0.0001 
Total cholesterol, mg/dL
Control
Diabetes
Total 

23 (76.7)
20 (66.7)
43 (71.7) 

177.60 (23.3)
180.37 (33.3)
178.98 (28.3) 

177.60 ± 50.27
180.37 ± 49.59
178.98 ± 49.53 
t = −0.21, P = 0.831 
Triglycerides, mg/dL
Control
Diabetes
Total 

21 (70.0)
23 (76.7)
44 (73.3) 

163.43 (30.0)
165.87 (23.3)
164.65 (26.7) 

163.43 ± 66.27
165.87 ± 85.06
164.65 ± 75.61 
t = −0.12, P = 0.902 
HDL cholesterol, mg/dL
Control
Diabetes
Total 

9 (30.0)
13 (43.3)
22 (36.7) 

43.37 (70.0)
42.77 (56.7)
43.07 (63.3) 

43.37 ± 14.48
42.77 ± 13.42
43.07 ± 13.84 
t = 0.17, P = 0.868 
LDL cholesterol, mg/dL
Control
Diabetes
Total 

14 (46.7)
15 (50.0)
29 (48.3) 

102.93 (53.3)
98.50 (50.0)
100.72 (51.7) 

102.93 ± 33.44
98.50 ± 33.10
100.72 ± 33.06 
t = 0.52, P = 0.608 
Creatinine, mg/dL
Control
Diabetes
Total 

9 (30.0)
4 (13.3)
13 (21.7) 

1.38 (70.0)
1.68 (86.7)
1.53 (78.3) 

1.38 ± 0.61
1.68 ± 0.81
1.53 ± 0.72 
t = −1.59, P = 0.116 
Uric acid, mg/dL
Control
Diabetes
Total 

23 (76.7)
24 (80.0)
47 (78.3) 

5.77 (23.3)
5.92 (20.0)
5.84 (21.7) 

5.77 ± 0.94
5.92 ± 0.59
5.84 ± 0.78 
t = 0.74, P = 0.464 
Insulin, µU/mL
Control
Diabetes
Total 

30 (100.0)
24 (80.0)
54 (90.0) 

3.49 (0.0)
11.43 (20.0)
7.46 (10.0) 

3.49 ± 2.15
11.43 ± 12.35
7.46 ± 9.66 
t = −3.47, P = 0.002 
VariableNormal, n (%)Abnormal, n (%)Mean ± SDTest Statistics
FBG, (mg/dL)
Control
Diabetes
Total 

29 (96.7)
9 (30.0)
38 (63.3) 

1 (3.3)
21 (70.0)
22 (36.7) 

95.23 ± 15.47
128.33 ± 17.92
111.78 ± 23.54 
t = −7.65, P = 0.0001 
A1C, %
Control
Diabetes
Total 

30 (100.0)
0 (0.0)
30 (50.0) 

5.00 (0.0)
7.42 (100.0)
6.21 (50.0) 

5.00 ± 0.53
742 ± 0.60
6.21 ± 1.34 
t = −16.34, P = 0.0001 
Total cholesterol, mg/dL
Control
Diabetes
Total 

23 (76.7)
20 (66.7)
43 (71.7) 

177.60 (23.3)
180.37 (33.3)
178.98 (28.3) 

177.60 ± 50.27
180.37 ± 49.59
178.98 ± 49.53 
t = −0.21, P = 0.831 
Triglycerides, mg/dL
Control
Diabetes
Total 

21 (70.0)
23 (76.7)
44 (73.3) 

163.43 (30.0)
165.87 (23.3)
164.65 (26.7) 

163.43 ± 66.27
165.87 ± 85.06
164.65 ± 75.61 
t = −0.12, P = 0.902 
HDL cholesterol, mg/dL
Control
Diabetes
Total 

9 (30.0)
13 (43.3)
22 (36.7) 

43.37 (70.0)
42.77 (56.7)
43.07 (63.3) 

43.37 ± 14.48
42.77 ± 13.42
43.07 ± 13.84 
t = 0.17, P = 0.868 
LDL cholesterol, mg/dL
Control
Diabetes
Total 

14 (46.7)
15 (50.0)
29 (48.3) 

102.93 (53.3)
98.50 (50.0)
100.72 (51.7) 

102.93 ± 33.44
98.50 ± 33.10
100.72 ± 33.06 
t = 0.52, P = 0.608 
Creatinine, mg/dL
Control
Diabetes
Total 

9 (30.0)
4 (13.3)
13 (21.7) 

1.38 (70.0)
1.68 (86.7)
1.53 (78.3) 

1.38 ± 0.61
1.68 ± 0.81
1.53 ± 0.72 
t = −1.59, P = 0.116 
Uric acid, mg/dL
Control
Diabetes
Total 

23 (76.7)
24 (80.0)
47 (78.3) 

5.77 (23.3)
5.92 (20.0)
5.84 (21.7) 

5.77 ± 0.94
5.92 ± 0.59
5.84 ± 0.78 
t = 0.74, P = 0.464 
Insulin, µU/mL
Control
Diabetes
Total 

30 (100.0)
24 (80.0)
54 (90.0) 

3.49 (0.0)
11.43 (20.0)
7.46 (10.0) 

3.49 ± 2.15
11.43 ± 12.35
7.46 ± 9.66 
t = −3.47, P = 0.002 

Main Results

The mean vitamin D level was 47.47 ± 22.54 ng/mL in the control group and 29.51 ± 9.98 ng/mL in the group with diabetes. The distribution of vitamin D in Figure 1 showed a significant difference between the two groups in terms of vitamin D level (P = 0.0001).

FIGURE 1

Distribution of vitamin D (ng/mL) in patients with type 2 diabetes and control group (P = 0.0001).

FIGURE 1

Distribution of vitamin D (ng/mL) in patients with type 2 diabetes and control group (P = 0.0001).

Close modal

The mean insulin resistance calculated by the HOMA-IR formula was 0.57 ± 0.84 in the control group and 3.86 ± 3.56 in the case group, which showed a significant difference between the two groups (P = 0.001) (Figure 2).

FIGURE 2

Distribution of insulin resistance in patients with type 2 diabetes and control group (P = 0.001).

FIGURE 2

Distribution of insulin resistance in patients with type 2 diabetes and control group (P = 0.001).

Close modal

The mean CD34 level was 4.80 ± 0.78 ng/mL in the control group and 2.35 ± 1.04 ng/mL in the case group, which was a significant difference between the two groups (P = 0.0001) (Figure 3).

FIGURE 3

Distribution of CD34 (ng/mL) in patients with type 2 diabetes and the control group (P = 0.0001).

FIGURE 3

Distribution of CD34 (ng/mL) in patients with type 2 diabetes and the control group (P = 0.0001).

Close modal

Relationships Between CD34 and Other Variables

Table 2 shows the relationships between CD34 and other variables. CD34 had a significant inverse relationship with BMI, A1C, FBG (P = 0.0001), insulin resistance (P = 0.005), and insulin level (P = 0.009). CD34 had a significant direct relationship with vitamin D (P = 0.002).

TABLE 2

Association Between CD34 and Other Variables

VariableControlDiabetesTotal
RPRPRP
Age −0.243 0.196 −0.213 0.259 −0.036 0.787 
BMI −0.371 0.044 −0.305 0.101 −0.290 0.025 
Systolic blood pressure −0.295 0.113 −0.106 0.579 0.091 0.488 
Diastolic blood pressure −0.226 0.229 −0.130 0.495 −0.033 0.802 
FBG 0.111 0.559 −0.234 0.213 −0.549 0.0001 
A1C 0.117 0.537 −0.105 0.583 −0.702 0.0001 
Total cholesterol −0.097 0.609 0.006 0.973 −0.009 0.948 
Triglycerides 0.224 0.234 −0.158 0.405 0.034 0.797 
HDL cholesterol −0.016 0.933 0.006 0.976 −0.022 0.869 
LDL cholesterol −0.094 0.622 −0.095 0.616 −0.110 0.403 
Creatinine −0.121 0.523 −0.103 0.587 0.102 0.439 
Uric acid −0.299 0.108 0.113 0.553 −0.021 0.871 
Insulin 0.013 0.944 0.004 0.983 −0.335 0.009 
Insulin resistance 0.040 0.832 −0.023 0.905 −0.356 0.005 
Vitamin D 0.092 0.629 0.179 0.343 0.385 0.002 
VariableControlDiabetesTotal
RPRPRP
Age −0.243 0.196 −0.213 0.259 −0.036 0.787 
BMI −0.371 0.044 −0.305 0.101 −0.290 0.025 
Systolic blood pressure −0.295 0.113 −0.106 0.579 0.091 0.488 
Diastolic blood pressure −0.226 0.229 −0.130 0.495 −0.033 0.802 
FBG 0.111 0.559 −0.234 0.213 −0.549 0.0001 
A1C 0.117 0.537 −0.105 0.583 −0.702 0.0001 
Total cholesterol −0.097 0.609 0.006 0.973 −0.009 0.948 
Triglycerides 0.224 0.234 −0.158 0.405 0.034 0.797 
HDL cholesterol −0.016 0.933 0.006 0.976 −0.022 0.869 
LDL cholesterol −0.094 0.622 −0.095 0.616 −0.110 0.403 
Creatinine −0.121 0.523 −0.103 0.587 0.102 0.439 
Uric acid −0.299 0.108 0.113 0.553 −0.021 0.871 
Insulin 0.013 0.944 0.004 0.983 −0.335 0.009 
Insulin resistance 0.040 0.832 −0.023 0.905 −0.356 0.005 
Vitamin D 0.092 0.629 0.179 0.343 0.385 0.002 

Bold type signifies statistical significance.

Moreover, the results of the linear model to investigate the relationship between CD34 or vitamin D with different variables showed that CD34 and vitamin D were significantly associated with type 2 diabetes (P <0.05).

Accelerated atherogenesis is an important complication of diabetes, which increases the risk of atherothrombotic events compared with people without diabetes. Despite various treatments and preventive measures, there is an urgent need to identify new strategies for managing the residual risk of cardiovascular events.

In people with type 2 diabetes, progenitor cells modify their proliferation, adhesion, and incorporation into vascular structures, and in patients at high risk for CVD, EPCs may be reduced. CD34 is a standard marker for stem cells, including vascular endothelial components, heart cells, and smooth muscle cells (10,19).

On the other hand, the primary physiological function of vitamin D is to regulate the metabolism of calcium and bones. Vitamin D deficiency is prevalent in autoimmune diseases and can affect the function of the vascular endothelium and lead to the onset or progression of atherosclerosis (20). Recent studies have shown a direct link between vitamin D and EPCs.

Because the relationships between the above factors in diabetes were unclear, we designed a study to examine the relationship between serum CD34 and vitamin D levels with insulin resistance in people with type 2 diabetes by comparing their results with those of a control group.

In this study, the mean age and sex proportions in the control group were not significantly different from those in the case group, but mean BMI and systolic blood pressure in the control group were significantly lower than in the case group. Also, there were statistically significant differences in mean FBG, A1C, and insulin between the two groups, but mean serum creatinine, serum uric acid, and lipid profile were not statistically different between the two groups. With regard to vitamin D, there was a significant difference between the two groups, such that the level of vitamin D was lower in people with diabetes than in the control group. In addition, a significant increase in insulin resistance was observed in the case group, and mean CD34 level in the control group was significantly higher than in the case group. We showed a significant inverse relationship between CD34 and BMI, A1C, FBG, insulin resistance, and insulin level. Interestingly, CD34 had a significant direct relationship with vitamin D. We also showed that vitamin D had a significant inverse relationship with A1C, FBG, and BMI. Consequently, our results showed that CD34 and vitamin D are significantly associated with type 2 diabetes.

In a study conducted in Slovenia by Kresnik et al. (21), cell grafts from 100 adult patients scheduled for autologous CD34+ cell transplantation were investigated. The number of CD34+ cells and their viability were determined by flow cytometry. The researchers found that CD34+ cell levels significantly decreased with increasing age. Also, they concluded that there was a tendency for an inverse relationship between diabetes and CD34+ cell count; however, this relationship was not significant.

A 2006 Italian study by Fadini et al. (22) examined the relationship between CD34 and metabolic syndrome in patients with cardiovascular risk. In this study, different types of CD34 were studied in 200 patients at different levels of cardiovascular risk. Results indicated a close negative correlation between CD34 and cardiovascular risk, as well as a synergistic detrimental effect of clustered metabolic components.

Another study by Fadini et al. (13) examined the role of EPCs in peripheral vascular disease in patients with type 2 diabetes. By analyzing flow cytometry, the researchers evaluated circulating progenitor cells (CPCs) and EPCs in 51 patients in a case group and 17 patients in a control group. They found that CPC and EPC levels in people with diabetes were decreased by 33 and 40%, respectively, compared with the control group.

A study conducted in California by Vrtovec et al. (17) examined the effectiveness of CD34 cell therapy in nonischemic cardiomyopathy in 45 patients, including 12 people with type 2 diabetes, 17 people without diabetes but with insulin resistance based on HOMA-IR, and 16 people without insulin resistance or diabetes. All of the patients had heart disease. All patients were treated with granulocyte-colony stimulating factor ampoules 5 mg/kg for 5 days, and then CD34 cell levels were measured. The patients were then treated with CD34 cells injected into the left ventricle. They were evaluated for CD34, blood glucose, insulin resistance, and response to cell therapy 1, 3, and 6 months after cell therapy. In both groups without diabetes, the level of the N-terminal pro-brain natriuretic peptide marker, which indicates heart damage, decreased significantly after 3 months, but a reduction was not observed in the group with diabetes. This study indicates that diabetes alone affects the function of CD34 cells, but insulin resistance without diabetes has no effect on the function of CD34 cells. This study found no significant relationship between insulin resistance and diabetes and CD34, which contradicts the findings of our study. More research is therefore needed to clarify the role of insulin resistance.

A 2014 Ukrainian study by Berezin (23) examined the relationship between circulating EPCs and insulin resistance in patients with chronic heart failure but without diabetes. The study included 300 patients who were 48–62 years of age and diagnosed with coronary artery disease by angiography. Insulin resistance was determined using the HOMA-IR formula. Also, CD45, CD34, CD14, and CD309 cell populations were measured as markers expressing EPCs. In this study, the EPC count was significantly lower in the group with insulin resistance than in the group without insulin resistance. This study indicates that insulin resistance may be an essential factor in reducing EPC function and EPC number in blood circulation, and insulin resistance and EPCs may have a significant relationship with each other, which is in line with the results of our study.

Previous studies have shown that a reduction in circulating CD34 in the early stages of type 2 diabetes may be apparent in individuals with glucose intolerance and that this decrease persists over time and may be worse in patients with advanced complications (8,24).

Also, Cubbon et al. (25) believe that the effects of insulin resistance on EPCs could be caused indirectly by the effects of insulin (e.g., oxidative stress, inflammation, and increased free fatty acids) or directly in a mechanism related to cells in the bone marrow.

To date, it is not clear by what mechanisms diabetes affects EPCs. Previous studies have suggested that a reduction of EPC effects by nitric oxide (NO) may be the result of altered mobilization from the bone marrow returned to damaged vessels or tissues (26,27), as well as an increase in apoptosis and reduced cell survival (28). These disturbances in EPC levels and functional properties may contribute to the endothelial dysfunction, progression of atherosclerotic disease, and reduced scarring seen in people with diabetes (29). More studies are needed to clarify the complex interplay among insulin resistance, EPCs, and endothelial damage and repair.

Many studies have also shown that vitamin D deficiency is associated with endothelial dysfunction and an increased risk of CVD (3033). Also, previous studies have shown that vitamin D may have a protective effect on EPCs and that a depletion of EPCs is induced by oxidative stress (3436).

The vitamin D receptor is present in vascular tissues such as endothelial cells, vascular smooth muscle cells, and cardiomyocytes. Indeed, vitamin D has several beneficial effects in the vascular wall via this receptor, including reduction in thrombogenicity, decrease in vasoconstrictors, inhibition of oxidative stress and atherogenesis, improvement of endothelial repair, reduction in foam cell formation, and vascular dilatation (37). A recent cohort study demonstrated that higher vitamin D concentrations could be associated with a lower risk of CVD-related mortality (38).

Hammer et al. (29) investigated the effect of vitamin D on the functional aspects of EPCs isolated from healthy subjects and from people with diabetes, who are known to have attenuated EPC activity. Their results showed that, in people with diabetes, vitamin D supplementation (in vitro) improved EPCs’ capacity to form colonies and viability. A recent study also showed that vitamin D is independently associated with insulin resistance in people with type 2 diabetes (39).

Today we know that diabetes negatively affects EPCs and that insulin resistance can cause endothelial dysfunction, leading to vascular complications (40). Diabetes leads to multiple areas of vascular damage. EPC count and vitamin D status can be used as biomarkers in people with diabetes to predict vascular complications, and their reduction can worsen the situation for these patients.

Limitations

One of the main limitations of this study was its relatively low sample size, which was the result of time limitations and the coronavirus disease 2019 pandemic. The obtained results could be confirmed in a study with a similar scheme performed with a larger participant sample. In addition, a lack of financial resources meant that it was not possible to measure more serum markers in this study, which could have provided valuable additional information.

Our findings suggest that vitamin D deficiency is common in type 2 diabetes and associated with endothelial dysfunction and a reduced number of EPCs. The reduced CD34 and vitamin D levels simultaneously associated with an increase in insulin resistance in people with type 2 diabetes may have an impact on the progression of this disease, as controlling these factors may prevent or slow disease progression. Hence, these factors can be helpful in providing a more accurate assessment of diabetes status and to predict the development of its complications, especially CVD.

Acknowledgments

The authors thank all of their colleagues at Islamic Azad University, Mashhad branch, for their help and support, especially the colleagues at the Innovation Research Center of Shahinfar Medical School, as well as those at Imam Reza Hospital in Mashhad.

Duality of Interest

No potential conflicts of interest relevant to this article were reported.

Author Contributions

V.P. conceptualized the study. V.P. and S.E. wrote the manuscript and collected/analyzed data. V.P. and M.S.Y. reviewed/edited the manuscript and contributed to the discussion. V.P. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

1.
Dokken
B
.
The pathophysiology of cardiovascular disease and diabetes: beyond blood pressure and lipids
.
Diabetes Spectr
2008
;
21
:
160
165
2.
de Souza
BM
,
Brondani
LA
,
Bouças
AP
, et al
.
Associations between UCP1 -3826A/G, UCP2 -866G/A, Ala55Val and Ins/Del, and UCP3 -55C/T polymorphisms and susceptibility to type 2 diabetes mellitus: case-control study and meta-analysis
.
PLoS One
2013
;
8
:
e54259
3.
Bischoff-Ferrari
HA
,
Giovannucci
E
,
Willett
WC
,
Dietrich
T
,
Dawson-Hughes
B
.
Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes
.
Am J Clin Nutr
2006
;
84
:
18
28
4.
Sullivan
SS
,
Rosen
CJ
,
Halteman
WA
,
Chen
TC
,
Holick
MF
.
Adolescent girls in Maine are at risk for vitamin D insufficiency
.
J Am Diet Assoc
2005
;
105
:
971
974
5.
Tanaka
H
,
Hamano
T
,
Fujii
N
, et al
.
The impact of diabetes mellitus on vitamin D metabolism in predialysis patients
.
Bone
2009
;
45
:
949
955
6.
Chiu
KC
,
Chu
A
,
Go
VLW
,
Saad
MF
.
Hypovitaminosis D is associated with insulin resistance and β cell dysfunction
.
Am J Clin Nutr
2004
;
79
:
820
825
7.
Dore
FJ
,
Domingues
CC
,
Ahmadi
N
, et al
.
The synergistic effects of saxagliptin and metformin on CD34+ endothelial progenitor cells in early type 2 diabetes patients: a randomized clinical trial
.
Cardiovasc Diabetol
2018
;
17
:
65
8.
Aragona
CO
,
Imbalzano
E
,
Mamone
F
, et al
.
Endothelial progenitor cells for diagnosis and prognosis in cardiovascular disease
.
Stem Cells Int
2016
;
2016
:
8043792
9.
Satterthwaite
AB
,
Burn
TC
,
Le Beau
MM
,
Tenen
DG
.
Structure of the gene encoding CD34, a human hematopoietic stem cell antigen
.
Genomics
1992
;
12
:
788
794
10.
Sidney
LE
,
Branch
MJ
,
Dunphy
SE
,
Dua
HS
,
Hopkinson
A
.
Concise review: evidence for CD34 as a common marker for diverse progenitors
.
Stem Cells
2014
;
32
:
1380
1389
11.
Barkoudah
E
,
Skali
H
,
Uno
H
,
Solomon
SD
,
Pfeffer
MA
.
Mortality rates in trials of subjects with type 2 diabetes
.
J Am Heart Assoc
2012
;
1
:
8
15
12.
Egan
CG
,
Lavery
R
,
Caporali
F
, et al
.
Generalised reduction of putative endothelial progenitors and CXCR4-positive peripheral blood cells in type 2 diabetes
.
Diabetologia
2008
;
51
:
1296
1305
13.
Fadini
GP
,
Miorin
M
,
Facco
M
, et al
.
Circulating endothelial progenitor cells are reduced in peripheral vascular complications of type 2 diabetes mellitus
.
J Am Coll Cardiol
2005
;
45
:
1449
1457
14.
Pozzoli
O
,
Vella
P
,
Iaffaldano
G
, et al
.
Endothelial fate and angiogenic properties of human CD34+ progenitor cells in zebrafish
.
Arterioscler Thromb Vasc Biol
2011
;
31
:
1589
1597
15.
Fadini
GP
,
Ciciliot
S
,
Albiero
M
.
Concise review: perspectives and clinical implications of bone marrow and circulating stem cell defects in diabetes
.
Stem Cells
2017
;
35
:
106
116
16.
Kitade
M
,
Yoshiji
H
,
Noguchi
R
, et al
.
Crosstalk between angiogenesis, cytokeratin-18, and insulin resistance in the progression of non-alcoholic steatohepatitis
.
World J Gastroenterol
2009
;
15
:
5193
5199
17.
Vrtovec
B
,
Sever
M
,
Jensterle
M
, et al
.
Efficacy of CD34+ stem cell therapy in nonischemic dilated cardiomyopathy is absent in patients with diabetes but preserved in patients with insulin resistance
.
Stem Cells Transl Med
2016
;
5
:
632
638
18.
Kim
JA
,
Montagnani
M
,
Koh
KK
,
Quon
MJ
.
Reciprocal relationships between insulin resistance and endothelial dysfunction: molecular and pathophysiological mechanisms
.
Circulation
2006
;
113
:
1888
1904
19.
Hill
JM
,
Zalos
G
,
Halcox
JP
, et al
.
Circulating endothelial progenitor cells, vascular function, and cardiovascular risk
.
N Engl J Med
2003
;
348
:
593
600
20.
Huang
Z
,
Liu
L
,
Huang
S
, et al
.
Vitamin D (1, 25-(OH)2D3) improves endothelial progenitor cells function via enhanced NO secretion in systemic lupus erythematosus
.
Cardiol Res Pract
2020
;
2020
:
6802562
21.
Kresnik
PK
,
Krasna
M
,
Rozman
P
,
Vrtovec
B
,
Malicev
E
.
Collection and immunoselection of CD34+ cells: the impact of age, sex, and diabetes in patients with chronic heart failure
.
Transfusion
2016
;
56
:
1792
1800
22.
Fadini
GP
,
de Kreutzenberg
SV
,
Coracina
A
, et al
.
Circulating CD34+ cells, metabolic syndrome, and cardiovascular risk
.
Eur Heart J
2006
;
27
:
2247
2255
23.
Berezin
A
.
Serum uric acid as a metabolic regulator of endothelial reparative processes in heart failure patients
.
Stem Cell Transl Investig
2014
;
1
:
e432
24.
Fadini
GP
.
A reappraisal of the role of circulating (progenitor) cells in the pathobiology of diabetic complications
.
Diabetologia
2014
;
57
:
4
15
25.
Cubbon
RM
,
Kahn
MB
,
Wheatcroft
SB
.
Effects of insulin resistance on endothelial progenitor cells and vascular repair
.
Clin Sci (Lond)
2009
;
117
:
173
190
26.
Fadini
GP
,
Sartore
S
,
Schiavon
M
, et al
.
Diabetes impairs progenitor cell mobilisation after hindlimb ischaemia-reperfusion injury in rats
.
Diabetologia
2006
;
49
:
3075
3084
27.
Gallagher
KA
,
Liu
Z-J
,
Xiao
M
, et al
.
Diabetic impairments in NO-mediated endothelial progenitor cell mobilization and homing are reversed by hyperoxia and SDF-1 α
.
J Clin Invest
2007
;
117
:
1249
1259
28.
Fadini
GP
,
Sartore
S
,
Agostini
C
,
Avogaro
A
.
Significance of endothelial progenitor cells in subjects with diabetes
.
Diabetes Care
2007
;
30
:
1305
1313
29.
Hammer
Y
,
Soudry
A
,
Levi
A
, et al
.
Effect of vitamin D on endothelial progenitor cells function
.
PLoS One
2017
;
12
:
e0178057
30.
Giulietti
A
,
van Etten
E
,
Overbergh
L
,
Stoffels
K
,
Bouillon
R
,
Mathieu
C
.
Monocytes from type 2 diabetic patients have a pro-inflammatory profile. 1,25-Dihydroxyvitamin D(3) works as anti-inflammatory
.
Diabetes Res Clin Pract
2007
;
77
:
47
57
31.
Kosch
M
,
Hausberg
M
,
Vormbrock
K
, et al
.
Impaired flow-mediated vasodilation of the brachial artery in patients with primary hyperparathyroidism improves after parathyroidectomy
.
Cardiovasc Res
2000
;
47
:
813
818
32.
Oh
J
,
Weng
S
,
Felton
SK
, et al
.
1,25(OH)2 vitamin D inhibits foam cell formation and suppresses macrophage cholesterol uptake in patients with type 2 diabetes mellitus
.
Circulation
2009
;
120
:
687
698
33.
Talmor
Y
,
Golan
E
,
Benchetrit
S
, et al
.
Calcitriol blunts the deleterious impact of advanced glycation end products on endothelial cells
.
Am J Physiol Renal Physiol
2008
;
294
:
F1059
F1064
34.
Lin
AM
,
Chen
KB
,
Chao
PL
.
Antioxidative effect of vitamin D3 on zinc-induced oxidative stress in CNS
.
Ann N Y Acad Sci
2005
;
1053
:
319
329
35.
Talmor
Y
,
Bernheim
J
,
Klein
O
,
Green
J
,
Rashid
G
.
Calcitriol blunts pro-atherosclerotic parameters through NFkappaB and p38 in vitro
.
Eur J Clin Invest
2008
;
38
:
548
554
36.
Yiu
Y-F
,
Chan
Y-H
,
Yiu
K-H
, et al
.
Vitamin D deficiency is associated with depletion of circulating endothelial progenitor cells and endothelial dysfunction in patients with type 2 diabetes
.
J Clin Endocrinol Metab
2011
;
96
:
E830
E835
37.
Zittermann
A
,
Trummer
C
,
Theiler-Schwetz
V
,
Lerchbaum
E
,
März
W
,
Pilz
S
.
Vitamin D and cardiovascular disease: an updated narrative review
.
Int J Mol Sci
2021
;
22
:
2896
38.
Fan
X
,
Wang
J
,
Song
M
, et al
.
Vitamin D status and risk of all-cause and cause-specific mortality in a large cohort: results from the UK Biobank
.
J Clin Endocrinol Metab
2020
;
105
:
e3606
e3619
39.
Zhao
H
,
Tang
Y
,
Zheng
C
,
Ren
L
,
Song
G
.
Vitamin D status is independently associated with insulin resistance in patients with type 2 diabetes mellitus
.
Risk Manag Healthc Policy
2021
;
14
:
1393
1399
40.
Hu
L
,
Dai
S-C
,
Luan
X
,
Chen
J
,
Cannavicci
A
.
Dysfunction and therapeutic potential of endothelial progenitor cells in diabetes mellitus
.
J Clin Med Res
2018
;
10
:
752
757
Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. More information is available at https://www.diabetesjournals.org/journals/pages/license.