Recent studies have shown that low-grade inflammation plays an important role in the development of diabetes vascular complications (1). Interleukin-18 (IL-18) is well known to play a key role in various inflammatory and autoimmune diseases (2). IL-18 produces proinflammatory cytokines and upregulates various adhesion molecule expression (3,4). These processes could lead to the development of diabetes vascular complications including atherosclerosis. Indeed, serum IL-18 levels can be a strong predictor of death in patients with cardiovascular diseases (5,6) and is associated with nephropathy and atherosclerosis in type 2 diabetic patients (7).

It was also reported (8) that serum IL-18 levels were increased during the early developing stage of type 1 diabetes, but we cannot exclude the possibility that these results were simply influenced by inflammation at the onset of type 1 diabetes. In this study, we examined serum IL-18 levels in the chronic stage of type 1 diabetes and the possible association between serum IL-18 and various soluble adhesion molecule levels.

A total of 77 Japanese type 1 diabetic patients (25 men and 52 women, mean ± SD age 23.3 ± 4.4 years, duration of diabetes 13.2 ± 6.3 years, and daily urine C-peptide 5.2 ± 8.8 μg/day) undergoing periodic follow-up examinations at the Diabetes Clinic of Osaka University Hospital and the Osaka Police Hospital were enrolled. All patients were treated with insulin alone and performed at least three or four daily insulin injections. The daily insulin dose was 0.88 ± 0.25 U/kg. Urinary albumin excretion was 16.4 ± 40.7 mg/day; 17 patients had simple diabetic retinopathy, and 4 had proliferative retinopathy. As control subjects, we enrolled 22 age-matched healthy nondiabetic individuals (11 men and 11 women aged 25.0 ± 2.5 years). None of the subjects had any clinical evidence of infection, connective tissue disease, liver dysfunction, or cardiovascular diseases. None of the subjects were taking any oral hypoglycemic drugs or antihypertensive, antiplatelet, or lipid-lowering medication. After a full explanation of the study, written informed consent was obtained from each subject. The study was approved by the Ethical Committee for Human Studies at Osaka University Graduate School of Medicine.

Fasting serum total cholesterol and HDL cholesterol, triglyceride, A1C, high-sensitivity C-reactive protein (hs-CRP), soluble intracellular adhesion molecule-1 (sICAM-1), soluble vascular cell adhesion molecule-1 (sVCAM-1), and soluble E-selectin (sE-selectin) levels were measured by SRL (Tokyo, Japan). To measure serum IL-18 levels, we used an enzyme-linked immunosorbent assay (MBL, Nagoya, Japan).

Data between two groups were compared by two-tailed unpaired Student’s t test, χ2 test, or Mann-Whitney U test. Single linear univariate correlations and forward and backward stepwise multivariate regression analyses (F value for the inclusion and exclusion of variables was set at 4.0) were performed to evaluate the relationship between IL-18 and the variables listed in Table 1. Statistical significance was defined as a P value <0.01.

Clinical characteristics of the study subjects are presented in Table 1. A1C, hs-CRP levels, sICAM-1, sVCAM-1, and sE-selectin levels were significantly higher in type 1 diabetic patients compared with nondiabetic subjects (P < 0.01), as previously reported (9–11). There was no significant difference between the two groups regarding the other clinical parameters.

Serum IL-18 levels were significantly higher in type 1 diabetic subjects compared with nondiabetic subjects (192 ± 80 vs. 122 ± 69 ng/ml, P = 0.0005) and were closely associated with A1C levels (r = 0.351, P = 0.0005). Furthermore, a stepwise multivariate regression analysis demonstrated that A1C was an independent risk factor for a high IL-18 value (F = 13.1, P = 0.0005), suggesting that hyperglycemia per se influences IL-18 levels. Similarly, serum hs-CRP, sICAM-1, sVCAM-1, and sE-selectin levels were positively associated with A1C levels (data not shown). There was no statistically significant association between IL-18 levels and the duration of diabetes.

Serum IL-18 levels were also closely associated with various soluble adhesion molecule levels such as sICAM-1 (r = 0.313, P = 0.0019) and sE-selectin levels (r = 0.451, P < 0.0001), although there was no correlation between IL-18 and sVCAM-1 levels (data not shown). Furthermore, a stepwise multivariate regression analysis demonstrated that IL-18 levels were an independent risk factor for a high sE-selectin value (F = 14.1). These results suggest that serum IL-18 levels influence various soluble adhesion molecule levels, although there was no significant association between serum IL-18 levels and diabetic retinopathy or nephropathy (data not shown).

In this study, we found that circulating IL-18 levels were significantly higher in type 1 diabetic subjects and that IL-18 levels were positively correlated with A1C levels. Furthermore, A1C levels were an independent determinant of IL-18 levels, suggesting that hyperglycemia itself affects IL-18 levels.

Serum IL-18 levels were also closely associated with serum levels of several soluble adhesion molecules such as sICAM-1 and sE-selectin. It has been shown that such adhesion molecule expression in endothelium leads to vascular dysfunction (12,13) and that increased soluble adhesion molecule levels in type 1 diabetic patients are related to the progression of diabetic vascular complications (14–16). Therefore, increased IL-18 levels might be, at least in part, involved in the development of diabetic vascular complications, although we failed to detect a significant association between serum IL-18 levels and the severity of diabetic microangiopathy. A prospective study with a larger number of subjects would be necessary to prove whether diabetic patients with higher circulating IL-18 levels are more susceptible to diabetic vascular diseases.

Table 1—

Characteristics of the study subjects

Control subjectsType 1 diabetic subjectsP value
n 22 77  
Sex (male/female) 11/11 25/52  
Age (years) 25.0 ± 2.5 23.3 ± 4.4 NS 
Duration of diabetes (years) — 13.2 ± 6.3 — 
Smoking (yes/no) 3/19 9/68 NS* 
BMI (kg/m2) 20.6 ± 1.9 21.4 ± 1.8 NS 
Systolic BP (mmHg) 113 ± 11 116 ± 13 NS 
Diastolic BP (mmHg) 66 ± 8 71 ± 9 NS 
A1C (%) 4.6 ± 0.3 7.8 ± 1.5 <0.0001 
Total cholesterol (mmol/l) 4.66 ± 0.73 4.87 ± 0.80 NS 
HDL cholesterol (mmol/l) 1.68 ± 0.34 1.86 ± 0.36 NS 
Triglyceride (mmol/l) 0.81 ± 0.31 1.08 ± 0.68 NS 
hs-CRP (mg/l) 0.223 ± 0.225 0.977 ± 1.799 0.0081† 
sICAM-1 (ng/ml) 166 ± 72 215 ± 63 0.0025 
sVCAM-1 (ng/ml) 499 ± 200 630 ± 142 0.0008 
sE-selectin (ng/ml) 30 ± 10 46 ± 22 0.0009 
IL-18 (ng/ml) 122 ± 69 192 ± 80 0.0005 
Microalbuminuria (yes/no) — 63/6 — 
Urinary albumin excretion (mg/day) — 16.2 ± 43.8 — 
Retinopathy (NDR/SDR/PDR) — 55/17/4 — 
Control subjectsType 1 diabetic subjectsP value
n 22 77  
Sex (male/female) 11/11 25/52  
Age (years) 25.0 ± 2.5 23.3 ± 4.4 NS 
Duration of diabetes (years) — 13.2 ± 6.3 — 
Smoking (yes/no) 3/19 9/68 NS* 
BMI (kg/m2) 20.6 ± 1.9 21.4 ± 1.8 NS 
Systolic BP (mmHg) 113 ± 11 116 ± 13 NS 
Diastolic BP (mmHg) 66 ± 8 71 ± 9 NS 
A1C (%) 4.6 ± 0.3 7.8 ± 1.5 <0.0001 
Total cholesterol (mmol/l) 4.66 ± 0.73 4.87 ± 0.80 NS 
HDL cholesterol (mmol/l) 1.68 ± 0.34 1.86 ± 0.36 NS 
Triglyceride (mmol/l) 0.81 ± 0.31 1.08 ± 0.68 NS 
hs-CRP (mg/l) 0.223 ± 0.225 0.977 ± 1.799 0.0081† 
sICAM-1 (ng/ml) 166 ± 72 215 ± 63 0.0025 
sVCAM-1 (ng/ml) 499 ± 200 630 ± 142 0.0008 
sE-selectin (ng/ml) 30 ± 10 46 ± 22 0.0009 
IL-18 (ng/ml) 122 ± 69 192 ± 80 0.0005 
Microalbuminuria (yes/no) — 63/6 — 
Urinary albumin excretion (mg/day) — 16.2 ± 43.8 — 
Retinopathy (NDR/SDR/PDR) — 55/17/4 — 

Data are means ± SD. Student’s t test was performed.

*

χ2 test.

†

Mann-Whitney U test. BP, blood pressure; NDR, no diabetic retinopathy; PDR, proliferative diabetic retinopathy; SDR, simple diabetic retinopathy.

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A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.

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