Short Report: Association of IL-15 with Peripheral and Hepatic Insulin Sensitivity in Healthy Middle-Aged Men

Yuichiro Nishida, Kumpei Tokuyama, Shoichiro Nagasaka, Akira Kiyonaga, Yasuki Higaki, Megumi Hara, Keitaro Tanaka, Hiroaki Tanaka

Department of Preventive Medicine, Faculty of Medicine, Saga University, Saga, Japan.
Division of Endocrinology and Metabolism, Department of Medicine, Jichi Medical University, Tochigi, Japan.
Laboratory of Biochemistry of Exercise and Nutrition, Institute of Health and Sport Sciences, University of Tsukuba, Ibaraki, Japan.
Laboratory of Exercise Physiology, Faculty of Health and Sports Science, Fukuoka University, Fukuoka, Japan.

DOI: 10.4236/ojemd.2014.43007                

Abstract

Rodent study suggests that interleukin (IL)-15 administration enhances insulin sensitivity. Although it is well known that circulating levels of typical inflammatory markers (C-reactive protein [CRP] and IL-6) are positively associated with homeostasis model assessment-insulin resistance (HOMA-IR), there are no studies investigating the associations of other inflammatory markers including IL-15 with peripheral/hepatic insulin sensitivity in humans. The current study aimed to examine the relationship between the levels of adiopokines or inflammatory cytokines and insulin sensitivity in 8 healthy middle-aged men. Circulating levels of 10 insulin sensitizing adipokines or inflammatory cytokines (total adiponectin [APN], high molecular weight adiponectin [HMW-APN], IL-4, IL-5, IL-6, IL-8, IL-15, interferon [IFN]-γ, tumor necrosis factor [TNF]-α, and TNF-β) were measured. A stable-labeled frequently sampled intravenous glucose tolerance test was performed to assess peripheral () and hepatic () insulin sensitivity estimated by 2-compartment minimal model. The levels of 3 inflammatory cytokines (IL-4, IL-6, and IL-15) were significantly and inversely correlated with either and . The association between IL-15 and either or was significant even after adjusting for age and percent body fat (p < 0.01). The current study showed a possible inverse association between serum IL-15 level and peripheral/hepatic insulin sensitivity in healthy middle-aged males, independent of percent body fat; this association in humans warrants further study.

Keywords

Inflammatory Cytokine; Interleukin; Adipokine; Insulin Sensitivity

1. Introduction

Low-grade chronic systemic inflammation plays an important role in the pathogenesis of obesity and type 2 diabetes [1] [2] . Inflammatory cytokines such as interleukin (IL)-6 and tumor necrosis factor (TNF)-α induce insulin resistance and suppress molecules involved in insulin signaling [2] . Intravenous administration of TNF-α impairs peripheral insulin sensitivity assessed by euglycemic hyperinsulinemic clamp in healthy humans [3] [4] . Consistently, epidemiological studies have shown that higher levels of serum IL-6 and C-reactive protein (CRP) were associated with the increased risk of type 2 diabetes [5] [6] .

Studies have reported a positive relationship between the circulating levels of representative inflammatory markers (e.g., CRP, IL-6, and TNF-α) and homeostasis model assessment-insulin resistance (HOMA-IR) [7] [8] . Recently, other cytokines such as IL-4, IL-8, and IL-15 have emerged as risk factors for adverse health. Inflammatory cytokines such as IL-4, IL-8, and IL-15 are involved in the process of atherogenesis [9] -[11] . For instance, an epidemiological study showed that serum IL-15 concentration in hypertension patients with severe organ damage (88.7 pg/mL) was significantly higher than that in those with no organ damage (55.2 pg/mL) [12] . On the other hand, rodent studies suggest that IL-15 has beneficial effects in the regulation of glucose metabolism. It has been demonstrated that IL-15 administration increased insulin sensitivity in obese mice [13] . There is, however, no information on the associations between the levels of inflammatory cytokines such as IL-4, IL-8, and IL-15 and insulin sensitivity in humans.

A stable-labeled 2-compartment minimal model can provide indices of peripheral insulin sensitivity (), as well as glucose effectiveness () (i.e., ability of glucose to stimulate its own uptake), insulin secretion, and endogenous glucose production (EGP) in patients and healthy individuals [14] -[16] . Additionally, this model enabled us to assess hepatic insulin sensitivity (i.e., the ability of insulin to inhibit EGP,) and hepatic glucose effectiveness (i.e., the ability of glucose to inhibit EGP,) [17] . Therefore, we examined the relationships between the circulating levels of 10 adipokines or cytokines including IL-15 and peripheral/hepatic insulin sensitivity indices (and) assessed by a stable-labeled 2-compartment minimal model in healthy middle-aged men.

2. Materials and Methods

Subjects: The study subjects were 8 healthy middle-aged men with normal glucose tolerance. We included healthy participants with a wide range of insulin sensitivity to examine the associations between serum cytokines and insulin sensitivity indices. Of these 8 subjects, 4 men were sedentary, defined as not engaged in any habitual exercise for at least 2 years, and 4 were runners, defined as those who run 79 ± 27 km/week. None of the subjects were taking any medications or supplements at the time of study enrollment. Before beginning the study, the nature, purpose, and the risks of the study were explained to all subjects, and written informed consent was obtained. The protocol was approved by the local ethical committee of the Jichi Medical University and was conducted in accordance with the Helsinki Declaration.

Anthropometric indices, oral glucose tolerance test, and stable-labeled frequently sampled intravenous glucose tolerance test: Body mass index (BMI) was determined by dividing body weight in kilograms by the square of height in meters. Each subject’s percent body fat was measured by hydrostatic weighing and was estimated based on the hydrostatic density with a correction for the residual lung volume [18] . An oral glucose tolerance test (OGTT; 75 g glucose) was performed at least 7 days prior to the frequently sampled intravenous glucose tolerance tests (FSIGTTs) to confirm normal glucose tolerance in all subjects. Stable-labeled FSIGTTs were performed, as previously described [14] -[16] . Briefly, after overnight fasting, glucose (300 mg/kg body weight) (isotopically labeled with [6, 6-²H₂] glucose [Aldrich, Milwaukee, WI]) was administered in the contralateral antecubital vein within 1 min. Regular insulin (Humalin; Shionogi, Osaka, Japan) was infused (20 mU/kg) into the antecubital vein from 20 to 25 min after the glucose bolus. Blood samples for glucose and insulin were frequently obtained up to 180 min. On the day before undergoing the FSIGTT, all subjects were provided with an evening meal consisting of ≥140 g carbohydrate, ≥30 g fat, and ≥33 g protein.

Deuterated glucose was analyzed as a penta-acetate derivative, and and specific to glucose uptake, and EGP were estimated by a 2-compartment minimal model [14] -[16] . Hepatic insulin sensitivity (i.e., the ability of insulin to enhance glucose suppression of EGP,) and hepatic glucose effectiveness (i.e., the ability of glucose to inhibit EGP,) were also analyzed as previously described [17] . To assess endogenous insulin secretion, insulin area above the basal level between 0 and 20 min after the administration of glucose was calculated using the conventional trapezoid rule. The glucose disappearance constant (K_G) was calculated as the slope of the least squares regression line related to the natural logarithm of the glucose concentration to the time from samples drawn between 10 and 19 min. The HOMA-IR was calculated as the product of the fasting serum insulin concentration (µU/mL) and fasting plasma glucose concentration (mg/dL) divided by 405 [19] .

Measurement of serum cytokine: Fasting levels of serum total-APN and HMW-APN were measured by enzyme-linked immunosorbent assay (ELISA) (Human adiponectin ELISA kit, Human HMW-adiponectin ELISA kit, Otsuka Pharmaceutical Company, Tokushima, Japan). Both intraand inter-assay coefficients of variation for total-APN and HMW-APN were <15%. Fasting levels of serum IL-4, IL-5, IL-6, IL-8, IL-15, IFN- γ, TNF-α, and TNF-β were measured by multiplex sandwich ELISA kit (Q-Plex™ Human Cytokine, Quansys Bioscience Inc., Utah, USA), according to the manufacturer’s instructions. Chemiluminescence was quantified with a Lumino-Image Analyzer LAS-3000 System (Fujifilm Inc., Tokyo). Lower detectable limits for IL-4, IL-6, IL-8, IL-15, and TNF-α were 0.39, 0.93, 0.24, 0.60, and 1.37 pg/mL, respectively. A good agreement between signal intensity and cytokine level was observed within the assay range (r² ≥ 0.99). Intraand inter-assay coefficients of variation were 10.6% and 17.4% for IL-4, respectively; 7.6% and 18.2% for IL-5, respectively; 6.5% and 11.7% for IL-6, respectively; 4.1% and 20.5% for IL-8, respectively; 5.4% and 14.7% for IL-15, respectively; 8.9% and 15.9% for IFN-γ, respectively; 6.1% and 18.8% for TNF-α, respectively; and 4.3% and 14.4% for TNF-β, respectively. All cytokines were measured 3 times, in duplicate, and the average of the 3 measurements was used for data analyses. Free fatty acid (FFA) levels were measured by standard method.

Statistical analysis: Values are shown as the mean ± SD. The significance of the correlation between variables was assessed by the Pearson correlation coefficient. Multiple regression analyses with adjustment for age and percent body fat were performed to examine if cytokine levels (independent variables) could be associated with insulin sensitivity indices (dependent variables), independent of percent body fat. A p value of < 0.05 was considered statistically significant. All analyses were performed using SAS software (version 9.3 for Windows, SAS Institute, Cary, NC, USA).

3. Results

General characteristics of the study subjects are shown in Table 1. The percent body fat tended to be positively correlated with IL-4 (r = 0.68, p = 0.06), and it was significantly and positively correlated with IL-6 and IL-15 (Figure 1). Likewise, BMI was positively correlated with the levels of IL-4 and IL-6 (IL-4: r = 0.84, p < 0.01; IL-6: r = 0.80, p < 0.05; IL-15: r = 0.65, p = 0.08). Insignificant correlations were observed between either percent body fat or BMI and the other 7 cytokines (total-APN, HMW-APN, IL-5, IL-8, IFN-γ, TNF-α, and TNF-β) (data not shown). Three cytokines (IL-4, IL-6, and IL-15) were inversely correlated with (Figure 2), while correlations between these cytokines and insulin secretion, insulin-independent glucose uptake (), or basal EGP were not statistically significant (data not shown). Additionally, the 3 cytokines (IL-4, IL-6, and IL-15) showed significant inverse correlations with (Figure 3), whereas among the 3 cytokines but only IL-6 was significantly and inversely correlated with (r = −0.73, p < 0.05)_.

When the analyses were performed separately for sedentary men (n = 4) and runners (n = 4), the correlation between percent body fat and IL-15 became non-significant in sedentary men (r = −0.71, p = 0.29) and runners (r = 0.86, p = 0.14). On the other hand, the relationship between IL-15 and tended to be significant in sedentary men (r = −0.93, p = 0.07) and remained significant in runners (r = −0.995, p < 0.01). Additionally, the correlation between IL-15 and hS_I² remained significant in sedentary men (r = −0.97, p < 0.05) and runners (r = −0.993, p < 0.01).

In the multiple regression analysis using data from all subjects (n = 8), the associations between either IL-4 or IL-6 and became insignificant, whereas that between IL-15 and remained statistically significant after the adjustment for age and percent body fat (Table 2). The inverse relationship between either IL-4 or IL-15 and were also significant even after adjusting for age and percent body fat (Table 2). Additionally, among

(a)(b)(c)

Figure 1. Correlations between percent body fat and the levels of 3 inflammatory cytokines (IL-4, IL-6, and IL-15) in healthy middle-aged individuals.

(a)(b)(c)

Figure 2. Relationships between the levels of 3 inflammatory cytokines (IL-4, IL-6, and IL-15) and in healthy middle-aged men.

(a)(b)(c)

Figure 3. Associations between the levels of 3 inflammatory cytokines (IL-4, IL-6, and IL-15) and in healthy middle-aged males.

cytokines that were measured, only IL-6 was significantly and positively correlated with HOMA-IR (r = 0.71, p < 0.05), but this association became insignificant after adjusting for age and percent body fat.

4. Discussion

In the current study, we observed inverse associations between the circulating level of IL-15 and either peri

Table 1. General characteristics of the study subjects.

Conflicts of Interest

The authors declare no conflicts of interest.

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