Clinical Impact of Circulating Irisin on Classified Coronary Plaque Characteristics

Abstract

Background

Myokines are hormones secreted by skeletal muscles during physical activity. Low myokine levels may contribute to metabolic dysfunction and cardiovascular disorders. Irisin, a newly identified myokine, has been the focus of recent research. The aim of the present study was to analyze the association between circulating irisin levels and tissue characteristics of nonculprit left main coronary artery (LMCA) plaques with the use of integrated backscatter (IB) intravascular ultrasound (IVUS).

Methods

This observational study enrolled 55 Japanese patients following successful percutaneous coronary intervention for lesions in the left anterior descending arteries or left circumflex arteries. Circulating myokine levels, including myostatin, brain-derived neurotrophic factor, and irisin, were measured by an enzyme-linked immunosorbent assay. Tissue characteristics of LMCA plaque were evaluated by IB-IVUS.

Results

Circulating irisin levels were negatively associated with percent lipid volume (%LV) [r = −0.31 (95% CI, −2.52 to −0.21), P = 0.02] and positively associated with percent fibrous volume (%FV) [r = 0.32 (95% CI, 0.22–2.20), P = 0.02]. The optimal cutoff value of circulating irisin for the prediction of lipid-rich LMCA plaques was 6.02 μg/mL [area under the curve = 0.713, P < 0.01 (95% CI, 0.58–0.85)]. Multivariate linear regression analysis identified circulating irisin levels as independent predictors for %LV and %FV of the LMCA [β = −0.29 (95% CI, −2.53 to −0.07), P = 0.04 and β = 0.30 (95% CI, 0.10–2.23), P = 0.03, respectively].

Conclusions

Circulating irisin levels are significantly associated with tissue characteristics of nonculprit LMCA plaques.

Impact Statement

Acute coronary syndrome (ACS) of left main coronary arteries (LMCA) is associated with very poor prognosis owing to severe pump failure and ventricular arrhythmia. Therefore, it is important to evaluate the tissue characteristics of LMCA plaques. Recently, research studies have suggested the association between irisin and atherosclerosis. In the study, we found that circulating irisin levels are significantly associated with tissue characteristics of LMCA plaques. Irisin will be a novel biomarker for the classification of tissue characteristics of coronary plaques and contribute toward detecting the high-risk population, to whom much careful attention should be paid.

A range of myokines, hormones secreted by skeletal muscles during physical activity, have been identified; these include myostatin or brain-derived neurotrophic factor (BDNF) (1, 2). It has been postulated that low myokine levels play a role in metabolic dysfunction and cardiovascular disorders (3, 4). Myostatin, a transforming growth factor-β superfamily protein, regulates myoblast proliferation and is upregulated in cardiomyocytes surrounding infarct areas after myocardial infarction (5, 6). Studies have shown that BDNF, which belongs to the neurotropin family, is expressed in nonneurogenic tissues, including skeletal muscles (2), and it regulates glucose metabolism in diabetic mice (7). However, recent research has focused on irisin, a newly identified myokine.

Irisin is a peroxisome proliferator-activated receptor-γ (PPAR-γ) coactivator-1α (PGC-1α)-dependent myokine (8). During exercise, PGC-1α expression in skeletal muscle stimulates the expression of fibronectin type 3 domain containing 5 (FNDC5). Irisin is secreted into the circulation after cleavage from the membrane protein FNDC5 and activates expression of mitochondrial uncoupling protein-1 (UCP-1) and “browning” of white adipose tissue (8), leading to increased insulin sensitivity, improved glucose tolerance, lower body weight, and decreased fat mass (9, 10). Several studies have shown that irisin also promotes endothelial function and inhibits atherosclerosis (11, 12). However, to the best of our knowledge, the association between myokines, especially irisin, and the characteristics of coronary plaques is still unclear. We hypothesized that high levels of circulating irisin were associated with protective effects on coronary plaque vulnerability. The aim of the present study was to analyze the association between circulating irisin levels and tissue characteristics of nonculprit left main coronary artery (LMCA)³ plaques with the use of the integrated backscatter (IB) intravascular ultrasound (IVUS).

MATERIALS AND METHODS

Study population

A total of 167 Japanese patients underwent percutaneous coronary intervention (PCI) for stable angina pectoris (AP) or acute coronary syndrome (ACS), between February 2015 and December 2015, at Nagoya University Hospital. Of these, 55 patients with culprit lesions in the left anterior descending arteries or left circumflex arteries were included in this observational study. We excluded patients with culprit lesions in the LMCA or right coronary arteries, and those with severe stenosis of the LMCA, defined as a stenosis >50% on coronary angiography. Dual antiplatelet therapy with aspirin and thienopyridine derivatives was administered to all patients before PCI. Written informed consent was obtained from all patients. This study was submitted to the Institutional Review Board/Ethics Committee of the Nagoya University Graduate School of Medicine. After the deliberation, the protocol was approved by the ethical committee of the University with the clinical trial number 1139-3, 2016, and was conducted in accordance with the ethical principles outlined in the 1975 Declaration of Helsinki.

Preparation of blood samples and measurement of serum myokine levels

In elective settings, all patients were admitted the day before PCI, and fasting venous blood samples were obtained on the morning of the PCI. In urgent settings such as ACS, venous blood samples were obtained immediately before PCI. For myokines' measurement, blood samples were promptly centrifuged at 4 °C at 1500 g for 5 min and stored in a deep freezer (−80 °C) until performance of an ELISA. Serum irisin levels were measured with an irisin ELISA kit (RAG018R, BioVendor, Karasek; sensitivity of 1 ng/mL; interassay and intraassay CV were <10%). Levels of serum myostatin were measured by Quantikine^® ELISA kit (DGDF80, R&D systems; sensitivity of 2.25 pg/mL; interassay and intraassay CV were <10%); serum BDNF levels were measured by Quantikine^® ELISA kit (DBD00, R&D systems; sensitivity <20 pg/mL; interassay CV 8.1%–10.3%; intraassay CV <5%).

Clinical definition and laboratory examination

Hypertension was defined as high blood pressure (systolic blood pressure ≥140 mmHg or diastolic blood pressure ≥90 mmHg) or use of antihypertensive medication. Diabetes mellitus was defined as a history of diabetes, a fasting plasma glucose concentration >126 mg/dL, a random plasma glucose concentration >200 mg/dL, and/or glycohemoglobin (HbA1c) levels ≥6.5%. Dyslipidemia was defined as medication-dependent, previously known dyslipidemia or LDL-cholesterol ≥140 mg/dL, triglyceride ≥150 mg/dL, or HDL-cholesterol <40 mg/dL. The left ventricular ejection fraction was calculated by echocardiography with the Teichholz formula before the PCI. The estimated glomerular filtration rate (eGFR), based on serum creatinine, was calculated with the 3-variable equation, proposed by the Japanese Society of Nephrology: eGFR (mL · min⁻¹ · 1.73 m⁻²) = 194 × (serum creatinine)^−1.094 × (age)^−0.287 × (0.739 in female participants) (13).

Human plasma levels of creatinine, LDL-cholesterol, HDL-cholesterol, and C-reactive protein were measured at our clinical laboratory.

Conventional and IB-IVUS analysis

A commercially available IVUS imaging system (View It, Terumo Co.) was used for conventional IVUS analysis. The images were obtained at a rate of 0.5 mm/s with a motored pull-back device and a commercial scanner. The cross-sectional area (CSA) of the external elastic membrane (EEM) was measured by tracing the leading edge of the adventitia. The plaque plus media CSA was defined as EEM CSA – lumen CSA. The percent plaque area (volume) was calculated as [EEM area (volume) – luminal area (volume)]/EEM area (volume) × 100 (14). IVUS imaging from the bifurcation of the left anterior descending or left circumflex artery to the LMCA ostium and of the culprit lesion was performed. We assessed the entire length of the LMCA, which contained plaques with more than 30% of percent plaque volume at minimum luminal area detected with IVUS, as previously reported (15).

IB signals were obtained with a commercially available system connected to the IVUS imaging system (VISIWAVE, Terumo Co.). The IB value for each component was calculated as an average power of the ultrasound back-scattered signal from a small volume of tissue with a fast Fourier transform, measured in decibels. The definition of the IB value was determined for each of 3 histological categories: lipid, fibrous, and high-signal (calcification on the inner surface) area. The analysis for 3-dimensional IVUS images including lipid volume (LV), fibrous volume (FV), and high-signal volume was determined by the sum of lipid, fibrous, and high-signal areas in each CSA, at 1-mm axis intervals, respectively. The percentages of lipid volume (%LV; LV/plaque volume), fibrous volume (%FV; FV/plaque volume), and high-signal volume (high-signal volume/plaque volume) were calculated automatically. The lipid-rich plaque was defined as %LV > 60% (16, 17).

Statistical methods

Statistical analysis was performed with the SPSS 24.0 software program. We used linear regression analysis to examine the correlation between myokines and baseline variables. The optimal cutoff value of circulating irisin for the prediction of lipid-rich plaques of LMCA was determined by ROC curve analysis. The patients were then divided into 2 groups, according to the cutoff value of circulating irisin: a low irisin group (circulating irisin levels < the cutoff value) and a high irisin group (circulating irisin levels ≥ the cutoff point). Data are expressed as mean (SD) or median and interquartile ranges for continuous variables and as percentages for discrete variables. Student unpaired t test or the Mann–Whitney U-test was used to compare continuous variables between groups. Categorical variables were compared with the χ² test or Fisher exact test, as appropriate. Linear regression analysis was performed to examine the association between %LV or %FV and circulating irisin levels or different risk factors. Multivariate analysis also used univariate predictors of %LV or %FV with a P value <0.2 to determine independent predictors. A two-sided P value of <0.05 was considered statistically significant.

RESULTS

Study population

The baseline characteristics of the patients are shown in the left side of Table 1. The mean levels of circulating irisin, myostatin, and BDNF were 7.11 ± 3.16 μg/mL, 1.74 ± 1.20 ng/mL, and 12.8 ± 7.4 ng/mL, respectively. The mean age of the patients was approximately 70 years. The mean body mass index (BMI) of the patients was 24.0 ± 3.2 kg/m².

Table 1.

Baseline clinical characteristics.^a

Variable	All (n = 55)	Low irisin (n = 25)	High irisin (n = 30)	P value
Male (%)	44 (80.0)	22 (88.0)	22 (73.3)	0.18
Age, years	70.2 ± 10.5	70.6 ± 9.8	69.8 ± 11.3	0.78
Body mass index, kg/m2	24.0 ± 3.2	24.3 ± 3.5	23.7 ± 3.0	0.46
Hypertension (%)	42 (76.4)	19 (76.0)	23 (76.7)	0.95
Diabetes (%)	24 (43.6)	9 (36.0)	15 (50.0)	0.30
Dyslipidemia (%)	39 (70.9)	19 (76.0)	20 (66.7)	0.45
Current or past smoking (%)	37 (67.3)	16 (64.0)	21 (70.0)	0.64
Previous AMIb (%)	14 (25.9)	7 (28.0)	7 (24.1)	0.75
LVEF (%)	59.5 ± 13.1	62.6 ± 10.6	57 ± 14.6	0.11
Laboratory data
LDL cholesterol, mg/dL	99.9 ± 30.9	100.3 ± 27.8	99.5 ± 33.8	0.92
HDL cholesterol, mg/dL	44.2 ± 11.5	43.5 ± 9.8	44.8 ± 13.0	0.68
Triglyceride, mg/dL	126.8 ± 77.0	131.5 ± 75.5	122.8 ± 79.4	0.69
eGFR, mL/min/1.73m2	67.3 (50.0–78.5)	62.6 (47.4–76.2)	55.3 (46.8–76.5)	0.24
HbA1c, %	6.3 ± 0.8	6.1 ± 0.7	6.4 ± 0.9	0.19
CRP, mg/L	0.10 (0.05–0.29)	0.10 (0.06–0.36)	0.09 (0.04–0.30)	0.79
BNP, pg/mL	40.7 (10.5–123.9)	28.8 (10.9–106.4)	44.4 (9.4–255.1)	0.77
Clinical diagnosis
Stable AP (%)	37 (67.3)	16 (64.0)	21 (70.0)	0.64
AMI (%)	10 (18.2)	5 (20.0)	5 (16.7)	0.51
Unstable AP (%)	8 (14.5)	4 (16.0)	4 (13.3)	0.54
Culprit vessel
Left anterior descending (%)	38 (69.1)	15 (60.0)	23 (76.7)	0.18
Circumflex (%)	17 (30.9)	10 (40.0)	7 (23.3)	0.18
AHA type B2 or C (%)	48 (87.3)	22 (88.0)	26 (86.7)	0.60
Medication
ARB or ACE inhibitor (%)	31 (73.8)	12 (66.7)	19 (79.2)	0.29
Ca-blocker (%)	22 (40.0)	10 (40.0)	12 (40.0)	1.00
β-blocker (%)	17 (30.9)	8 (32.0)	9 (30.0)	0.87
Statin (%)	34 (61.8)	15 (60.0)	19 (63.3)	0.80
Myokine
Irisin, μg/mL	7.11 ± 3.16	4.58 ± 0.88	9.23 ± 2.80	<0.01
Myostatin, ng/mL	1.74 ± 1.20	1.87 ± 1.18	1.63 ± 1.23	0.47
BDNF, ng/mL	12.8 ± 7.4	13.4 ± 8.3	12.3 ± 6.8	0.61

a

Values are expressed as mean ± SD, median (interquartile), or percentages.

b

AMI, acute myocardial infarction; LVEF, left ventricular ejection fraction; HbA1c, glycated hemoglobin; CRP, C-reactive protein; BNP, brain natriuretic peptide; AHA, American Heart Association; ARB, angiotensin II receptor blocker; ACE, angiotensin converting enzyme; BDNF, brain-derived neurotrophic factor.

Table 2 shows the correlations between circulating myokines levels and risk factors. Circulating irisin levels correlated with %LV and %FV of LMCA [r = −0.31 (95% CI, −2.52 to −0.21), P = 0.02 and r = 0.32 (95% CI, 0.22–2.20), P = 0.02]. Conversely, there were no significant correlations between levels of circulating myostatin or BDNF and %LV or %FV of LMCA. Representative LMCA plaque images analyzed by conventional and IB-IVUS are shown in Fig. 1. A ROC curve analysis determined 6.02 μg/mL as the cutoff value [area under the curve = 0.713, P < 0.01 (95% CI, 0.58–0.85)] to maximize the predictive power of circulating irisin levels for lipid-rich plaques of LMCA.

Table 2.

Correlation between myokines and variables.

Variable	Irisin		Myostatin		BDNF
	r	P Value	r	P Value	r	P Value
Male	−0.24	0.08	0.14	0.31	0.27	0.05
Age	0.02	0.90	−0.07	0.61	−0.24	0.07
Body mass index	−0.03	0.84	−0.02	0.89	0.21	0.12
Hypertension	0.06	0.67	−0.03	0.81	−0.10	0.49
Diabetes	0.16	0.26	−0.002	0.99	0.05	0.70
Dyslipidemia	−0.02	0.89	0.11	0.42	0.06	0.67
Current or past smoking	−0.06	0.65	0.12	0.38	−0.05	0.74
LVEFa	−0.28	0.05	0.21	0.12	0.12	0.38
Laboratory data
LDL cholesterol	−0.01	0.48	0.15	0.28	0.20	0.15
HDL cholesterol	0.14	0.32	−0.03	0.84	−0.17	0.22
LDL/HDL ratio	−0.17	0.24	0.13	0.38	0.30	0.03
eGFR	−0.12	0.40	0.18	0.20	0.20	0.16
C-reactive protein	−0.002	0.99	−0.24	0.10	0.001	0.99
Clinical diagnosis
ACS	−0.01	0.94	−0.37	<0.01	0.08	0.56
Plaque characteristics of LMCA
%LV	−0.31	0.02	0.24	0.08	0.05	0.74
%FV	0.32	0.02	−0.22	0.10	−0.05	0.73
Myokine
Irisin	–	–	−0.14	0.31	−0.02	0.88
Myostatin	−0.14	0.31	–	–	−0.24	0.08
BDNF	−0.02	0.88	−0.24	0.08	–	–

a

LVEF, left ventricular ejection fraction; ACS, acute coronary syndrome.

Representative LMCA Plaque Images Analyzed by Conventional and IB-IVUS

Fig. 1.

The circulating irisin level, the percentage of lipid volume, and the percentage of fibrous volume were 4.73 μg/mL, 73.1%, and 26.4% (A); and 10.47 μg/mL, 31.8%, and 61.3% (B). Blue, lipid plaque; green and yellow, fibrous plaque; red, high-signal (calcification on the inner surface) area.

We divided the 55 patients into 2 groups, according to the cutoff value of circulating irisin: the low irisin group, including 25 patients with circulating irisin levels <6.02 μg/mL, and the high irisin group, including 30 patients with circulating irisin levels ≥6.02 μg/mL. There were no significant differences in patient characteristics including eGFR between the 2 groups, except for circulating irisin levels (Table 1).

Conventional and IB-IVUS data of left main coronary arteries

Intravascular imaging data of the LMCA are shown in Table 3. The low irisin group had a higher %LV of the LMCA and a lower %FV of the LMCA than the high irisin group (%LV, 64.9 ± 12.4% vs 55.5 ± 13.9%, P = 0.01; %FV, 31.5 ± 10.4% vs 39.8 ± 12.1%, P = 0.01), although there were no significant differences in percent plaque volume of the LMCA.

Table 3.

Data on intravascular imaging of left main coronary artery.^a

Variable	Low irisin (n = 25)	High irisin (n = 30)	P Value
Conventional IVUS
EEM area of MLAb, mm2	24.9 ± 6.6	21.8 ± 5.4	0.06
Plaque area of MLA, mm2	12.3 ± 6.0	10.4 ± 3.9	0.17
Percent plaque area of MLA, %	48.8 ± 16.1	47.7 ± 11.7	0.76
EEM volume, mm3	165.9 ± 80.5	146.4 ± 68.5	0.34
Plaque volume, mm3	75.6 ± 45.1	58.1 ± 23.8	0.09
Percent plaque volume, %	44.8 ± 12.7	41.4 ± 9.5	0.26
IB-IVUS
Lipid area of MLA, %	63.8 ± 14.2	59.2 ± 14.8	0.24
Fibrous area of MLA, %	33.3 ± 12.6	37.6 ± 13.2	0.22
High signal area of MLAa, %	0.5 ± 0.6	0.7 ± 1.2	0.54
Lipid volume, %	64.9 ± 12.4	55.5 ± 13.9	0.01
Fibrous volume, %	31.5 ± 10.4	39.8 ± 12.1	0.01
High signal volume, %	0.8 ± 0.7	0.9 ± 0.9	0.42

a

Values are expressed as mean ± SD.

b

MLA, minimum luminal area.

Table 4 shows the factors associated with %LV and %FV of the LMCA. In univariate linear regression analysis, circulating irisin levels were significantly associated with %LV and %FV of the LMCA [r = −0.31 (95% CI, −2.52 to −0.21), P = 0.02, and r = 0.32 (95% CI, 0.22–2.20), P = 0.02, respectively].

Table 4.

Univariate linear regression analysis.

Variable	%LV of LMCA		%FV of LMCA
	r	P Value	r	P Value
Male	0.21	0.13	−0.20	0.14
Age	−0.11	0.41	0.01	0.51
Body mass index	−0.06	0.66	0.06	0.69
Hypertension	−0.02	0.86	0.01	0.94
Diabetes	−0.15	0.29	0.17	0.21
Dyslipidemia	0.10	0.49	−0.07	0.62
Current or past smoking	0.15	0.26	−0.14	0.31
LVEF	−0.22	0.87	0.04	0.79
Laboratory data
LDL cholesterol	0.18	0.21	−0.17	0.24
HDL cholesterol	−0.12	0.40	0.11	0.43
LDL/HDL ratio	0.20	0.15	−0.19	0.18
eGFR	0.13	0.35	−0.12	0.38
C-reactive protein	0.03	0.85	−0.05	0.76
Clinical diagnosis
ACS	0.07	0.63	−0.06	0.65
Myokine
Irisin	−0.31	0.02	0.32	0.02

Multivariate linear regression analysis

Male sex and a higher LDL/HDL ratio were likely to be associated positively with %LV of the LMCA and negatively with %FV of the LMCA (P < 0.2). Multivariate linear regression analysis revealed that circulating irisin levels were independent predictors of %LV and %FV of the LMCA [β = −0.29 (95% CI, −2.53 to −0.07), P = 0.04 and β = 0.30 (95% CI, 0.10–2.23), P = 0.03, respectively] (Table 5).

Table 5.

Multivariate linear regression analysis.

Variable	%LV of LMCA		%FV of LMCA
	β	P Value	β	P Value
Irisin	−0.29	0.04	0.30	0.03
Male	0.13	0.37	−0.14	0.35
LDL/HDL ratio	0.11	0.43	−0.10	0.49

DISCUSSION

The major findings of this study were as follows. First, we found that circulating irisin levels were negatively associated with %LV of LMCA plaques and positively associated with %FV of LMCA plaques, whereas myostatin and BDNF levels were not associated with coronary plaque characteristics. Second, we showed that circulating irisin levels were more strongly associated with coronary plaque characteristics than conventional risk factors. Finally, we demonstrated the definitive cutoff value of circulating irisin for predicting lipid-rich plaques of the LMCA. In a previous study, it was shown that plasma myostatin levels were not associated with the prevalence of cardiovascular risk factors such as hypertension and hyperlipidemia (18). Other study showed that there was no significant difference in the circulating BDNF levels among the patients with unstable AP, those with stable AP, and those without cardiovascular disease (19). These results were consistent with our results. To the best of our knowledge, this is the first study to investigate the potential association between circulating myokines levels and tissue characteristics of LMCA plaques with the use of IB-IVUS. From the results of our study, we speculate that irisin has the most protective effects against atherosclerosis by causing the stabilization of coronary plaques.

A number of studies have examined the association between irisin and atherosclerosis. Lee et al. (20) showed that circulating irisin levels were negatively associated with carotid intima–media thickness in peritoneal dialysis patients. In some rodent studies, the mechanisms underlying the protective effects of irisin on atherosclerosis are clarifying (11, 12, 21). However, our findings are in conflict with those of a number of studies showing that circulating irisin may negatively affect cardiovascular disease or risk factors. Aronis et al. (22) demonstrated that increased irisin levels were associated with the development of major adverse cardiovascular events in patients with established coronary artery disease after PCI. Park et al. (23) reported that high circulating irisin levels were frequently seen in patients with metabolic syndrome, cardiometabolic variables, and cardiovascular disease. These studies suggest that a compensatory increase in irisin levels may occur to overcome underlying irisin resistance. The different associations between circulating irisin and cardiovascular disease or risk factors may be due to differences in participant characteristics, such as BMI. In previous studies, the majority of subjects were overweight or obese and the mean BMI was greater than that of the subjects in our study. In the present study, there are no significant differences in BMI or prevalence of other cardiovascular risk factors between the 2 groups. Hence, we believe that the association between circulating irisin levels and tissue characteristics of LMCA plaques found in this study provides useful information on the antiatherosclerosis effect of irisin.

Some studies have shown that circulating irisin levels may be positively associated with eGFR (24–26). In the present study, renal function of the patients was relatively conserved, and there was no significant difference in eGFR between the 2 groups. Hence, renal function might less influence the results of the present study.

It is well established that exercise has many benefits, such as decreasing the risk of death and improving longevity (27). Recently, studies have demonstrated the association between slow gait speed and an increased risk of cardiovascular disease or death (28, 29). Circulating irisin may explain the association between exercise and lower incidence of cardiovascular disease. Irisin secreted by muscles during exercise may exert pleiotropic effects, preventing atherosclerosis through plaque stabilization, increasing insulin sensitivity, improving glucose tolerance, lowering body weight, and decreasing fat mass, resulting in a lower risk of cardiovascular events. However, little is known about how the circulating irisin levels can be continuously increased. Several studies have demonstrated that circulating irisin levels increase immediately in response to exercise and then decline over a few hours (30, 31). Other studies have shown that there are no differences in circulating irisin levels between baseline and a few weeks or months of continuous physical activities (32, 33). Conversely, circulating irisin levels are reported to increase after 1-year lifestyle intervention (34). Further studies are required to investigate the association between circulating irisin and exercise.

ACS often occurs following the rupture or erosion of vulnerable plaques and subsequent thrombosis (35, 36). ACS of the LMCA results in a very poor prognosis due to severe complications, such as pump failure and ventricular arrhythmia (37–39). Therefore, this study evaluated the tissue characteristics of nonculprit LMCA plaques. Although IB-IVUS allows the analysis of coronary plaque tissue components (40), IB-IVUS analysis is an invasive technique. If irisin is established as a biomarker for classifying coronary plaque characteristics in the future, it will be both beneficial and less invasive than IB-IVUS analysis.

The present study has several limitations. First, we enrolled a small number of Japanese patients. Second, the present study did not include data on muscle mass or muscle strength, such as midarm muscle circumference, hand grip strength, or distance walked in the 6-min walking test. Myokines are secreted by muscles; therefore, it is possible that circulating irisin levels were affected by muscle mass or strength. Finally, due to the cross-sectional study design, there were no available data on clinical outcomes. It would be useful to follow-up the patients and assess the data on future cardiovascular events; changes in laboratory or anthropometric data; and lifestyle, such as exercise habits.

In conclusion, the present study demonstrates, for the first time, that circulating irisin levels are significantly associated with tissue characteristics of nonculprit LMCA plaques. Irisin may be a novel biomarker for the classification of tissue characteristics of coronary plaques and may elucidate the association between exercise and the reduction in the risk of cardiovascular disease and death.

3 Nonstandard abbreviations

 
• LMCA

  left main coronary artery

 
• IB

  integrated backscatter

 
• IVUS

  intravascular ultrasound

 
• LV

  lipid volume

 
• FV

  fibrous volume

 
• PCI

  percutaneous coronary intervention

 
• AP

  angina pectoris

 
• ACS

  acute coronary syndrome

 
• eGFR

  estimated glomerular filtration rate

 
• CSA

  cross-sectional area

 
• EEM

  external elastic membrane

 
• BMI

  body mass index.

Author Contributions:  All authors confirm they have contributed to the intellectual content of this paper and have met the following 4 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved. K. Hirayama, H. Ishii, R. Kikuchi, S. Suzuki, and T. Murohara participated in the protocol design, inclusion of the candidates, and were involved in editing the manuscript. T. Aoki, K. Harada, T. Sumi, Y. Negishi, Y. Shibata, Y. Tatami, and A. Tanaka participated in the collection of clinical information and statistical analysis.

Authors' Disclosures or Potential Conflicts of Interest:  Upon manuscript submission, all authors completed the author disclosure form.

Employment or Leadership: None declared.

Consultant or Advisory Role: None declared.

Stock Ownership: None declared.

Honoraria: H. Ishii, Astellas Pharma, Astrazeneca Inc., Daiichi-Sankyo Pharma Inc., Inc. MSD K.K.; T. Murohara, Bayel Pharmaceutical Co., Ltd., Daiichi-Sankyo Co., Ltd., Dainippon Sumitomo Pharma Co., Ltd., Kowa Co., Ltd., MSD K.K., Mitsubishi Tanabe Pharma Co., Nippon Boehringer Ingelheim Co., Ltd., Novartis Pharma K.K., Pfizer Japan Inc., Sanofi-Aventis K.K., and Takeda Pharmaceutical Co., Ltd.

Research Funding: This study was supported by a Grant-in-Aid for Scientific Research (KAKENHI) (No. 17 KO9493) from the Japanese Ministry of Education, Culture Sports, Scientific and Technology (MEYT) and a grant from the Japanese Society for the promotion of Science (JSPA). T. Murohara, Astellas Pharma Inc., Daiichi-Sankyo Co., Ltd., Dainippon Sumitomo Pharma Co., Ltd., Kowa Co., Ltd., MSD K. K., Mitsubishi Tanabe Pharma Co., Nippon Boehringer Ingelheim Co., Ltd., Novartis Pharma K.K., Otsuka Pharma Ltd., Pfizer Japan Inc., Sanofi-Aventis K.K., Takeda Pharmaceutical Co., Ltd., and Teijin Pharma Ltd.

Expert Testimony: None declared.

Patents: None declared.

Role of Sponsor: No sponsor was declared.