Scandinavian Journal of Clinical & Laboratory Investigation, 2014; 74: 74–78
ORIGINAL ARTICLE
Serum YKL-40 concentrations are elevated and correlated with disease severity in patients with obstructive sleep apnea syndrome
XIN WANG & GUO-HONG XING Department of Respiration, Jinan Military General Hospital, Jinan, Shandong, P. R China Abstract Inflammation plays a critical role in the pathogenesis of obstructive sleep apnea syndrome (OSAS). YKL-40 is a novel biomarker of systemic inflammation. We aim to detect serum YKL-40 concentrations in OSAS patients and to clarify their relationship with clinical severity of the disease. We enrolled 159 OSAS patients and 104 healthy controls. The presence and severity of OSAS was assessed by Apnea-hypopnea index (AHI). Serum YKL-40 concentrations were detected by enzyme-linked immunosorbent assay (ELISA). We demonstrated that serum YKL-40 concentrations were significantly elevated in OSAS patients than those in controls. Multivariate logistic regression including all variables revealed that YKL-40 was the significant and independent predictor for the present of OSAS. In OSAS patients, there is a significant positive correlation between increments in serum YKL-40 concentrations and severity of OSAS. Serum YKL-40 concentrations were independently and significantly correlated with AHI scores. These results suggest that YKL-40 could be used as a potential biomarker for predicting the development and progression of OSAS. Key Words: Chitin binding glycoprotein,YKL-40, obstructive sleep apnea, biomarker, inflammatory cytokine
Introduction Obstructive sleep apnea syndrome (OSAS) is characterized by recurrent episodes of complete or partial obstruction of the upper airway during sleep, leading to oxygen desaturation, fragmented sleep, and daytime somnolence [1]. OSAS is a highly prevalent disorder, affecting about 3–7% of adults [2]. Accelerated atherosclerosis and increased risk of cardiovascular and cerebrovascular events are frequently reported in patients with OSAS. Although the exact mechanism involved in the development and progression of OSAS remains unclear, several possible mechanisms such as increased oxidative stress, increased platelet aggregation, metabolic dysregulation, inflammation, and systemic vascular endothelial dysfunction are suggested [3]. YKL-40, also known as human cartilage glycoprotein-39(HC-gp39) and chitinase-like protein 1, is a glycoprotein consisting of 383 amino acids with a molecular mass of 40 kDa [4]. As a novel biomarker of systemic inflammation, YKL-40 is produced at sites of inflammation in many cells and is secreted from macrophages, neutrophils and vascular smooth muscle cells (VSMCs) [5,6]. Various inflammatory
and tissue remodeling conditions, including atherosclerosis, diabetes, obstructive lung disease, inflammatory bowel disease, liver fibrosis, rheumatoid arthritis, cancer and asthma are associated with elevated YKL-40 levels [4,7–12]. Previous studies have verified the presence of inflammation in the pathogenesis of OSAS. From this viewpoint, we hypothesized that YKL-40 concentrations in patients with OSAS might be greater than those in patients without OSAS and that this might be correlated with disease severity. However, little information has been obtained about YKL40 concentrations in OSAS patients. Therefore, we aimed to detect serum YKL-40 concentrations in OSAS patients, and to investigate their correlation with the presence and severity of the disease.
Materials and methods Subjects The study was conducted in the sleep unit of our hospital. From May 2011 to May 2013, 159 patients
Correspondence: Guo-Hong Xing, Department of Respiration, Jinan Military General Hospital, No. 25 Shifan Road, 250031, Jinan, Shandong, P. R China. Tel/fax: ⫹ 86 531 51666356. E-mail: [email protected] (Received 4 July 2013 ; accepted 23 October 2013) ISSN 0036-5513 print/ISSN 1502-7686 online © 2014 Informa Healthcare DOI: 10.3109/00365513.2013.859726
YKL-40 and OSAS diagnosed with OSAS by polysomnography (PSG) and 104 age, sex and body mass index (BMI) matched healthy check-up examinees were recruited. Exclusion criteria included the following: Asthma, chronic obstructive pulmonary disease, respiratory tract infection with in the following 4 weeks, auto immune disease, malignancy, advanced renal or hepatic disorder, history of psychiatric disorders, severe heart failure and history of coronary artery disease. The study protocol was approved by the ethics committee of our hospital. All patients gave written informed consent regarding participation in this study. Sleep study All subjects underwent overnight PSG using a digital polygraph (Embla-Monet 19, Embla-Monet Corp., USA) in our hospital. The subjects were advised not to consume caffeine-containing beverages and not to take medication that might affect their sleeping pattern. Polysomnographic monitoring was performed using standard technique. Apneas were defined as complete cessation of airflow ⱖ 10 sec. Hypopnea was defined as a reduction in airflow with a 50% from baseline for at least 10 sec, a 3% drop in oxygen saturation from the preceding stable saturation, and/or arousal. Apnea-hypopnea index (AHI) was defined as the number of apneas and hypopneas per hour of sleep. The diagnosis of OSAS was based on AHI ⬎ 5 which was further subdivided into mild (5 ⱕ AHI ⬍ 15), moderate (15 ⱕ AHI ⬍ 30) and severe (ⱖ 30) OSAS. Biochemical analyses Blood samples were collected from all subjects after an overnight fast. After clotting, the samples were immediately centrifuged and stored at ⫺80°C until analysis. All subjects underwent laboratory analyses, including determination of fasting blood glucose (FBG), glycated hemoglobin (HbA1c), total cholesterol (TC), triglyceride (TG), low density lipoprotein cholesterol (LDL-c) and high density lipoprotein cholesterol (HDL-c) using a routine autoanalyzer (LX20, Beckman Coulter, CA, USA). Serum YKL-40 concentrations were measured in duplicate by a commercially available enzyme-linked immunosorbent assay (ELISA) kit with an intra-assay and inter-assay CV of both ⬍ 10% (BlueGene, Shanghai, China). Preliminary data obtained in our laboratory revealed that the intra-assay and inter-assay CV were both ⬍ 5%. The ELISA was conducted in Jinan Military General Hospital using a Zenyth 200 automatic microplate reader (Biochrom Ltd, UK). Statistical analysis Data normality was analyzed using the KolmogorovSmirnov test. Continuous, normally distributed
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variables are presented as mean ⫾ SD, and nonnormally distributed variables as median (interquartile range). Categorical data are presented as number and percentage. Differences between the two groups were analyzed using unpaired t-test, Mann-Whitney U test or Chi-square test as indicated. Differences among groups were analyzed by one-way analysis of variance (ANOVA) followed by Tukey post-hoc test or the Kruskal-Wallis analysist when appropriate. Multivariate logistic analysis was performed to assess the independent predictors for OSAS. Spearman rank correlation coefficient was computed to assess the correlation between serum YKL-40 concentrations and AHI. Multivariate linear analysis was performed to determine the independent predictors of AHI. For non-normally distributed variables, log transformations values were used for multivariate linear analysis. All statistical calculations were done with SPSS software package 13.0 for windows (SPSS Inc., Chicago, Illinois, USA). A p ⬍ 0.05 (two-tailed) was considered as statistically significant.
Results Baseline clinical characteristics The baseline characteristics of the subjects are presented in Table I. The age, sex and BMI distributions did not differ significantly between the control and OSAS groups. There was also no significant difference between the two groups regarding the other clinical parameters such as waist circumference, FBG, TC, TG concentrations and smoking status. However, OSAS patients had significantly higher systolic blood pressure (SBP), diastolic blood pressure (DBP), HbA1c and LDL-c concentrations and significantly lower HDL-c concentrations compared with controls. Serum YKL-40 concentrations in controls and OSAS patients As shown in Table I, serum YKL-40 levels were significantly elevated in OSAS patients than those in controls (87.91 [range 60.92–114.33] μg/L and 35.37 [range 28.55–52.96] μg/L, respectively, p ⬍ 0.001). Multivariate logistic regression including all variables revealed that YKL-40 was the significant and independent predictor for the present of OSAS (odds ratio 1.054, 95% confidence interval 1.039–1.070; p ⬍ 0.001; (Table II). Correlation between serum YKL-40 concentrations and the severity of OSAS Serum YKL-40 concentrations in mild, moderate, and severe OSAS patients are shown in Figure 1. There is a significant positive correlation between increments in serum YKL-40 concentrations and
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X. Wang & G.-H. Xing Table I. Baseline clinical characteristics and serum YKL-40 concentrations in controls and OSAS patients. Control group (n ⫽ 104)
OSAS group (n ⫽ 159)
p value
53.46 ⫾ 13.18 64 (61.53%) 36 (34.62%) 87.63 ⫾ 7.31 26.09 ⫾ 2.45 129.12 ⫾ 16.73 81.22 ⫾ 10.46 5.37 (4.92–6.14) 5.39 ⫾ 0.81 4.48 ⫾ 1.28 1.57 (1.17–2.10) 2.55 ⫾ 0.97 1.12 ⫾ 0.28 35.37 (28.55–52.96)
52.62 ⫾ 11.96 98 (61.64%) 70 (44.03%) 87.91 ⫾ 8.39 26.14 ⫾ 2.68 137.76 ⫾ 18.22 88.87 ⫾ 11.29 5.63 (4.99–6.74) 5.89 ⫾ 1.22 4.66 ⫾ 1.08 1.76 (1.25–2.42) 2.83 ⫾ 0.84 1.05 ⫾ 0.22 87.91 (60.92–114.33)
0.591 0.987 0.129 0.776 0.886 ⬍ 0.001 ⬍ 0.001 0.161 ⬍ 0.001 0.217 0.413 0.014 0.026 ⬍ 0.001
Variables Age (years) Male n (%) Smoking, n (%) Waist circumference (cm) BMI (kg/mm2) SBP (mm Hg) DBP (mm Hg) FBG (mmol/L) HbA1c (%) TC (mmol/L) TG (mmol/L) LDL-c (mmol/L) HDL-c (mmol/L) YKL-40 (μg/L)
All values are mean ⫾ SD, median with interquartile range or n (%). BMI, body mass index; SBP, systolic blood pressure; DBP, diastolic blood pressure; FBG, fasting blood glucose; HbA1c, glycated hemoglobin; TC, total cholesterol; TG, triglycerides; LDL-c, low density lipoprotein cholesterol; HDL-c, high density lipoprotein cholesterol.
severity of OSAS (YKL-40 concentrations of 72.52 [range 47.36–97.59] μg/L in mild OSAS group, 85.58 [range 59.89–110.9] μg/L in moderate OSAS group and 101.33 [range 66.49–158.30] μg/L in severe OSAS group; p ⬍ 0.001). Spearman correlation analysis indicated that serum YKL-40 concentrations were significantly correlated with AHI (r ⫽ 0.329, p ⬍ 0.001; Figure 2). Multivariate linear regression analysis demonstrated that there was still a positive association between serum YKL-40 concentrations and log (AHI) after adjusting for all other variables as potential confounders (β ⫽ 0.309, t ⫽ 4.382, p ⬍ 0.001). Discussion Our study showed significantly higher serum YKL-40 concentrations in OSAS patients compared with Table II. Multiple logistic regression analysis for the presence of OSAS. Variables Age (per year) Male (yes) Smoking (yes) Waist circumference (per cm) BMI (per kg/mm2) SBP (per mm Hg) DBP (per mm Hg) FBG (per mmol/L) HbA1c (per 1%) TC (per mmol/L) TG (per mmol/L) LDL-c (per mmol/L) HDL-c (per mmol/L) YKL-40 (per μg/L)
OR (95% CI) 1.000 0.933 0.725 0.992 1.079 1.007 1.069 1.060 1.698 1.295 0.940 1.119 0.222 1.054
(0.971–1.029) (0.441–1.971) (0.340–1.54) (0.949–1.037) (0.933–1.249) (0.982–1.033) (1.028–1.112) (0.817–1.374) (1.146–2.516) (0.896–1.872) (0.670–1.319) (0.708–1.769) (0.048–1.020) (1.039–1.070)
controls. Multivariate logistic regression demonstrated that the elevated YKL-40 concentration was an independent predictor of the presence of OSAS. Moreover, in OSAS patients, serum YKL-40 concentrations were independently associated with disease severity. To the best of our knowledge, this is the first study to investigate the relationship between serum YKL-40 concentrations and OSAS. The identification of novel biomarkers for OSAS, which largely depended on better understanding of the pathophysiological mechanisms of the disease, has the potential to provide information related to diagnosis, severity and prognosis. There is growing evidence that systemic and upper airway inflammation is increased in OSAS [13]. Inflammatory processes occur in upper airway tissues of OSAS patients contribute to anatomic upper airway narrowing, abnormalities in upper airway reflexes, upper airway collapsibility and inspiratory pharyngeal muscle dysfunction, which is one of the major
p value 0.975 0.855 0.406 0.992 0.305 0.578 0.001 0.662 0.008 0.169 0.720 0.630 0.055 ⬍ 0.001
OR, odds ratio; CI, confidence interval; other abbreviations are as in Table I.
Figure 1. Box-and-whisker plot showing serum YKL-40 concentrations in mild (n ⫽ 42), moderate (n ⫽ 62) and severe (n ⫽ 55) obstructive sleep apnea syndrome (OSAS) patients.
YKL-40 and OSAS
Figure 2. The correlation between serum YKL-40 concentrations and Apnea-hypopnea index (AHI) scores.
pathophysiological process of OSAS [14]. Inflammatory processes are closely related to activation of neutrophils and monocytes and elaboration of proinflammatory cytokines. Therefore, pro-inflammatory cytokines are being explored as biomarkers for OSAS owing to the genesis and natural history of the disease. Serum YKL-40 concentrations are elevated in many systemic inflammatory diseases [7–12]. Its clinical importance is gradually increasing as a potential new biomarker in determining the severity and prognosis of various inflammatory diseases [15]. In the present study, we demonstrated for the first time that serum YKL-40 concentrations increased in OSAS patients. The cellular origin of the elevated serum concentrations of YKL-40 in OSAS patients in not known. OSAS is debilitating respiratory disease characterized by airway inflammation and remodeling. YKL-40 is probably secreted from activated inflammatory cells located in the upper airway or synthesized and released from differentiated VSMCs in response to tissue remodeling. Besides, prominent expression of YKL-40 could also be elicited by airflow obstruction [9]. Whether other factors that play a role in the pathogenesis of OSAS regulate YKL-40 expression is an area for further investigation. Once generated within the airways, YKL-40 may contribute to the development and progression of OSAS through intricate interactions with other cytokines and transcription factors. Exposure of macrophages to YKL-40 stimulated the release of pro-inflammatory and profibrogenic mediators such as interleukin (IL)-8, IL-18, monocyte chemotactic protein (MCP)-1, macrophage inflammatory protein (MIP)-1α and matrix metallopeptidase (MMP)-9 [16]. These unfavorable processes may promote the activation of neutrophils and monocytes and elaboration of pro-inflammatory
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cytokines, setting up a vicious cycle. Thus, YKL-40 may play a crucial role in the pathogenesis of OSAS by inducing this positive feedback. In this study, we demonstrated that the increased serum YKL-40 concentration was an independent risk factor for the presence of OSAS. These results concurred with those of earlier studies linking YKL-40 to OSAS and suggested that YKL-40 might be a potential biomarker for evaluating future OSAS risk. However, it is notable that the OR range of YKL-40 for the presence of OSAS was slightly above 1.0. Therefore, its clinical significance should be cautiously interpreted. To see whether correlations exist between serum YKL-40 concentrations and the severity of OSAS, serum YKL-40 concentrations in OSAS patients with different AHI were evaluated. We found that serum YKL-40 concentrations elevated with the increment of AHI. Moreover, serum YKL-40 concentrations were independently correlated with AHI after adjusting for potential confounders such as age and BMI in multivariate linear analysis. These results revealed that YKL-40 might also providing valuable information about risk stratification in OSAS patients. However, the association between serum YKL-40 concentrations and AHI was moderate and its clinical significance is yet to be further investigated. The following limitations of the study should be considered. First, this was a cross-sectional study with a relatively small number of Chinese patients. Therefore, prospective longitudinal studies with a larger population and a wider range of race would be desirable for further clarification of the relationship between YKL-40 and OSAS. Second, only serum YKL-40 concentrations were assessed in our study. We did not measure the concentrations other proinflammatory cytokines in the same samples, which may provide more valuable information on the disease-promoting role of YKL-40 signaling pathways in the initiation and progression of OSAS and to truly appreciate YKL-40 uniqueness. Third, the pathogenetic role of YKL-40 in OSAS should be confirmed by biochemical studies, such as immunoblotting. Therapeutic interventions by blocking YKL-40 signaling pathways to delay the pathophysiology process of OSAS warrants further investigations. In conclusion, our results indicate that serum YKL-40 concentrations were correlated with the presence and severity of OSAS. These data suggest that YKL-40 could be used as a potential biomarker for predicting the development and progression of OSAS. However, further large scale clinical studies are warranted.
Acknowledgements The authors thank the volunteers for their support to this study.
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Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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ORIGINAL ARTICLE
Serum YKL-40 concentrations are elevated and correlated with disease severity in patients with obstructive sleep apnea syndrome
XIN WANG & GUO-HONG XING Department of Respiration, Jinan Military General Hospital, Jinan, Shandong, P. R China Abstract Inflammation plays a critical role in the pathogenesis of obstructive sleep apnea syndrome (OSAS). YKL-40 is a novel biomarker of systemic inflammation. We aim to detect serum YKL-40 concentrations in OSAS patients and to clarify their relationship with clinical severity of the disease. We enrolled 159 OSAS patients and 104 healthy controls. The presence and severity of OSAS was assessed by Apnea-hypopnea index (AHI). Serum YKL-40 concentrations were detected by enzyme-linked immunosorbent assay (ELISA). We demonstrated that serum YKL-40 concentrations were significantly elevated in OSAS patients than those in controls. Multivariate logistic regression including all variables revealed that YKL-40 was the significant and independent predictor for the present of OSAS. In OSAS patients, there is a significant positive correlation between increments in serum YKL-40 concentrations and severity of OSAS. Serum YKL-40 concentrations were independently and significantly correlated with AHI scores. These results suggest that YKL-40 could be used as a potential biomarker for predicting the development and progression of OSAS. Key Words: Chitin binding glycoprotein,YKL-40, obstructive sleep apnea, biomarker, inflammatory cytokine
Introduction Obstructive sleep apnea syndrome (OSAS) is characterized by recurrent episodes of complete or partial obstruction of the upper airway during sleep, leading to oxygen desaturation, fragmented sleep, and daytime somnolence [1]. OSAS is a highly prevalent disorder, affecting about 3–7% of adults [2]. Accelerated atherosclerosis and increased risk of cardiovascular and cerebrovascular events are frequently reported in patients with OSAS. Although the exact mechanism involved in the development and progression of OSAS remains unclear, several possible mechanisms such as increased oxidative stress, increased platelet aggregation, metabolic dysregulation, inflammation, and systemic vascular endothelial dysfunction are suggested [3]. YKL-40, also known as human cartilage glycoprotein-39(HC-gp39) and chitinase-like protein 1, is a glycoprotein consisting of 383 amino acids with a molecular mass of 40 kDa [4]. As a novel biomarker of systemic inflammation, YKL-40 is produced at sites of inflammation in many cells and is secreted from macrophages, neutrophils and vascular smooth muscle cells (VSMCs) [5,6]. Various inflammatory
and tissue remodeling conditions, including atherosclerosis, diabetes, obstructive lung disease, inflammatory bowel disease, liver fibrosis, rheumatoid arthritis, cancer and asthma are associated with elevated YKL-40 levels [4,7–12]. Previous studies have verified the presence of inflammation in the pathogenesis of OSAS. From this viewpoint, we hypothesized that YKL-40 concentrations in patients with OSAS might be greater than those in patients without OSAS and that this might be correlated with disease severity. However, little information has been obtained about YKL40 concentrations in OSAS patients. Therefore, we aimed to detect serum YKL-40 concentrations in OSAS patients, and to investigate their correlation with the presence and severity of the disease.
Materials and methods Subjects The study was conducted in the sleep unit of our hospital. From May 2011 to May 2013, 159 patients
Correspondence: Guo-Hong Xing, Department of Respiration, Jinan Military General Hospital, No. 25 Shifan Road, 250031, Jinan, Shandong, P. R China. Tel/fax: ⫹ 86 531 51666356. E-mail: [email protected] (Received 4 July 2013 ; accepted 23 October 2013) ISSN 0036-5513 print/ISSN 1502-7686 online © 2014 Informa Healthcare DOI: 10.3109/00365513.2013.859726
YKL-40 and OSAS diagnosed with OSAS by polysomnography (PSG) and 104 age, sex and body mass index (BMI) matched healthy check-up examinees were recruited. Exclusion criteria included the following: Asthma, chronic obstructive pulmonary disease, respiratory tract infection with in the following 4 weeks, auto immune disease, malignancy, advanced renal or hepatic disorder, history of psychiatric disorders, severe heart failure and history of coronary artery disease. The study protocol was approved by the ethics committee of our hospital. All patients gave written informed consent regarding participation in this study. Sleep study All subjects underwent overnight PSG using a digital polygraph (Embla-Monet 19, Embla-Monet Corp., USA) in our hospital. The subjects were advised not to consume caffeine-containing beverages and not to take medication that might affect their sleeping pattern. Polysomnographic monitoring was performed using standard technique. Apneas were defined as complete cessation of airflow ⱖ 10 sec. Hypopnea was defined as a reduction in airflow with a 50% from baseline for at least 10 sec, a 3% drop in oxygen saturation from the preceding stable saturation, and/or arousal. Apnea-hypopnea index (AHI) was defined as the number of apneas and hypopneas per hour of sleep. The diagnosis of OSAS was based on AHI ⬎ 5 which was further subdivided into mild (5 ⱕ AHI ⬍ 15), moderate (15 ⱕ AHI ⬍ 30) and severe (ⱖ 30) OSAS. Biochemical analyses Blood samples were collected from all subjects after an overnight fast. After clotting, the samples were immediately centrifuged and stored at ⫺80°C until analysis. All subjects underwent laboratory analyses, including determination of fasting blood glucose (FBG), glycated hemoglobin (HbA1c), total cholesterol (TC), triglyceride (TG), low density lipoprotein cholesterol (LDL-c) and high density lipoprotein cholesterol (HDL-c) using a routine autoanalyzer (LX20, Beckman Coulter, CA, USA). Serum YKL-40 concentrations were measured in duplicate by a commercially available enzyme-linked immunosorbent assay (ELISA) kit with an intra-assay and inter-assay CV of both ⬍ 10% (BlueGene, Shanghai, China). Preliminary data obtained in our laboratory revealed that the intra-assay and inter-assay CV were both ⬍ 5%. The ELISA was conducted in Jinan Military General Hospital using a Zenyth 200 automatic microplate reader (Biochrom Ltd, UK). Statistical analysis Data normality was analyzed using the KolmogorovSmirnov test. Continuous, normally distributed
75
variables are presented as mean ⫾ SD, and nonnormally distributed variables as median (interquartile range). Categorical data are presented as number and percentage. Differences between the two groups were analyzed using unpaired t-test, Mann-Whitney U test or Chi-square test as indicated. Differences among groups were analyzed by one-way analysis of variance (ANOVA) followed by Tukey post-hoc test or the Kruskal-Wallis analysist when appropriate. Multivariate logistic analysis was performed to assess the independent predictors for OSAS. Spearman rank correlation coefficient was computed to assess the correlation between serum YKL-40 concentrations and AHI. Multivariate linear analysis was performed to determine the independent predictors of AHI. For non-normally distributed variables, log transformations values were used for multivariate linear analysis. All statistical calculations were done with SPSS software package 13.0 for windows (SPSS Inc., Chicago, Illinois, USA). A p ⬍ 0.05 (two-tailed) was considered as statistically significant.
Results Baseline clinical characteristics The baseline characteristics of the subjects are presented in Table I. The age, sex and BMI distributions did not differ significantly between the control and OSAS groups. There was also no significant difference between the two groups regarding the other clinical parameters such as waist circumference, FBG, TC, TG concentrations and smoking status. However, OSAS patients had significantly higher systolic blood pressure (SBP), diastolic blood pressure (DBP), HbA1c and LDL-c concentrations and significantly lower HDL-c concentrations compared with controls. Serum YKL-40 concentrations in controls and OSAS patients As shown in Table I, serum YKL-40 levels were significantly elevated in OSAS patients than those in controls (87.91 [range 60.92–114.33] μg/L and 35.37 [range 28.55–52.96] μg/L, respectively, p ⬍ 0.001). Multivariate logistic regression including all variables revealed that YKL-40 was the significant and independent predictor for the present of OSAS (odds ratio 1.054, 95% confidence interval 1.039–1.070; p ⬍ 0.001; (Table II). Correlation between serum YKL-40 concentrations and the severity of OSAS Serum YKL-40 concentrations in mild, moderate, and severe OSAS patients are shown in Figure 1. There is a significant positive correlation between increments in serum YKL-40 concentrations and
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X. Wang & G.-H. Xing Table I. Baseline clinical characteristics and serum YKL-40 concentrations in controls and OSAS patients. Control group (n ⫽ 104)
OSAS group (n ⫽ 159)
p value
53.46 ⫾ 13.18 64 (61.53%) 36 (34.62%) 87.63 ⫾ 7.31 26.09 ⫾ 2.45 129.12 ⫾ 16.73 81.22 ⫾ 10.46 5.37 (4.92–6.14) 5.39 ⫾ 0.81 4.48 ⫾ 1.28 1.57 (1.17–2.10) 2.55 ⫾ 0.97 1.12 ⫾ 0.28 35.37 (28.55–52.96)
52.62 ⫾ 11.96 98 (61.64%) 70 (44.03%) 87.91 ⫾ 8.39 26.14 ⫾ 2.68 137.76 ⫾ 18.22 88.87 ⫾ 11.29 5.63 (4.99–6.74) 5.89 ⫾ 1.22 4.66 ⫾ 1.08 1.76 (1.25–2.42) 2.83 ⫾ 0.84 1.05 ⫾ 0.22 87.91 (60.92–114.33)
0.591 0.987 0.129 0.776 0.886 ⬍ 0.001 ⬍ 0.001 0.161 ⬍ 0.001 0.217 0.413 0.014 0.026 ⬍ 0.001
Variables Age (years) Male n (%) Smoking, n (%) Waist circumference (cm) BMI (kg/mm2) SBP (mm Hg) DBP (mm Hg) FBG (mmol/L) HbA1c (%) TC (mmol/L) TG (mmol/L) LDL-c (mmol/L) HDL-c (mmol/L) YKL-40 (μg/L)
All values are mean ⫾ SD, median with interquartile range or n (%). BMI, body mass index; SBP, systolic blood pressure; DBP, diastolic blood pressure; FBG, fasting blood glucose; HbA1c, glycated hemoglobin; TC, total cholesterol; TG, triglycerides; LDL-c, low density lipoprotein cholesterol; HDL-c, high density lipoprotein cholesterol.
severity of OSAS (YKL-40 concentrations of 72.52 [range 47.36–97.59] μg/L in mild OSAS group, 85.58 [range 59.89–110.9] μg/L in moderate OSAS group and 101.33 [range 66.49–158.30] μg/L in severe OSAS group; p ⬍ 0.001). Spearman correlation analysis indicated that serum YKL-40 concentrations were significantly correlated with AHI (r ⫽ 0.329, p ⬍ 0.001; Figure 2). Multivariate linear regression analysis demonstrated that there was still a positive association between serum YKL-40 concentrations and log (AHI) after adjusting for all other variables as potential confounders (β ⫽ 0.309, t ⫽ 4.382, p ⬍ 0.001). Discussion Our study showed significantly higher serum YKL-40 concentrations in OSAS patients compared with Table II. Multiple logistic regression analysis for the presence of OSAS. Variables Age (per year) Male (yes) Smoking (yes) Waist circumference (per cm) BMI (per kg/mm2) SBP (per mm Hg) DBP (per mm Hg) FBG (per mmol/L) HbA1c (per 1%) TC (per mmol/L) TG (per mmol/L) LDL-c (per mmol/L) HDL-c (per mmol/L) YKL-40 (per μg/L)
OR (95% CI) 1.000 0.933 0.725 0.992 1.079 1.007 1.069 1.060 1.698 1.295 0.940 1.119 0.222 1.054
(0.971–1.029) (0.441–1.971) (0.340–1.54) (0.949–1.037) (0.933–1.249) (0.982–1.033) (1.028–1.112) (0.817–1.374) (1.146–2.516) (0.896–1.872) (0.670–1.319) (0.708–1.769) (0.048–1.020) (1.039–1.070)
controls. Multivariate logistic regression demonstrated that the elevated YKL-40 concentration was an independent predictor of the presence of OSAS. Moreover, in OSAS patients, serum YKL-40 concentrations were independently associated with disease severity. To the best of our knowledge, this is the first study to investigate the relationship between serum YKL-40 concentrations and OSAS. The identification of novel biomarkers for OSAS, which largely depended on better understanding of the pathophysiological mechanisms of the disease, has the potential to provide information related to diagnosis, severity and prognosis. There is growing evidence that systemic and upper airway inflammation is increased in OSAS [13]. Inflammatory processes occur in upper airway tissues of OSAS patients contribute to anatomic upper airway narrowing, abnormalities in upper airway reflexes, upper airway collapsibility and inspiratory pharyngeal muscle dysfunction, which is one of the major
p value 0.975 0.855 0.406 0.992 0.305 0.578 0.001 0.662 0.008 0.169 0.720 0.630 0.055 ⬍ 0.001
OR, odds ratio; CI, confidence interval; other abbreviations are as in Table I.
Figure 1. Box-and-whisker plot showing serum YKL-40 concentrations in mild (n ⫽ 42), moderate (n ⫽ 62) and severe (n ⫽ 55) obstructive sleep apnea syndrome (OSAS) patients.
YKL-40 and OSAS
Figure 2. The correlation between serum YKL-40 concentrations and Apnea-hypopnea index (AHI) scores.
pathophysiological process of OSAS [14]. Inflammatory processes are closely related to activation of neutrophils and monocytes and elaboration of proinflammatory cytokines. Therefore, pro-inflammatory cytokines are being explored as biomarkers for OSAS owing to the genesis and natural history of the disease. Serum YKL-40 concentrations are elevated in many systemic inflammatory diseases [7–12]. Its clinical importance is gradually increasing as a potential new biomarker in determining the severity and prognosis of various inflammatory diseases [15]. In the present study, we demonstrated for the first time that serum YKL-40 concentrations increased in OSAS patients. The cellular origin of the elevated serum concentrations of YKL-40 in OSAS patients in not known. OSAS is debilitating respiratory disease characterized by airway inflammation and remodeling. YKL-40 is probably secreted from activated inflammatory cells located in the upper airway or synthesized and released from differentiated VSMCs in response to tissue remodeling. Besides, prominent expression of YKL-40 could also be elicited by airflow obstruction [9]. Whether other factors that play a role in the pathogenesis of OSAS regulate YKL-40 expression is an area for further investigation. Once generated within the airways, YKL-40 may contribute to the development and progression of OSAS through intricate interactions with other cytokines and transcription factors. Exposure of macrophages to YKL-40 stimulated the release of pro-inflammatory and profibrogenic mediators such as interleukin (IL)-8, IL-18, monocyte chemotactic protein (MCP)-1, macrophage inflammatory protein (MIP)-1α and matrix metallopeptidase (MMP)-9 [16]. These unfavorable processes may promote the activation of neutrophils and monocytes and elaboration of pro-inflammatory
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cytokines, setting up a vicious cycle. Thus, YKL-40 may play a crucial role in the pathogenesis of OSAS by inducing this positive feedback. In this study, we demonstrated that the increased serum YKL-40 concentration was an independent risk factor for the presence of OSAS. These results concurred with those of earlier studies linking YKL-40 to OSAS and suggested that YKL-40 might be a potential biomarker for evaluating future OSAS risk. However, it is notable that the OR range of YKL-40 for the presence of OSAS was slightly above 1.0. Therefore, its clinical significance should be cautiously interpreted. To see whether correlations exist between serum YKL-40 concentrations and the severity of OSAS, serum YKL-40 concentrations in OSAS patients with different AHI were evaluated. We found that serum YKL-40 concentrations elevated with the increment of AHI. Moreover, serum YKL-40 concentrations were independently correlated with AHI after adjusting for potential confounders such as age and BMI in multivariate linear analysis. These results revealed that YKL-40 might also providing valuable information about risk stratification in OSAS patients. However, the association between serum YKL-40 concentrations and AHI was moderate and its clinical significance is yet to be further investigated. The following limitations of the study should be considered. First, this was a cross-sectional study with a relatively small number of Chinese patients. Therefore, prospective longitudinal studies with a larger population and a wider range of race would be desirable for further clarification of the relationship between YKL-40 and OSAS. Second, only serum YKL-40 concentrations were assessed in our study. We did not measure the concentrations other proinflammatory cytokines in the same samples, which may provide more valuable information on the disease-promoting role of YKL-40 signaling pathways in the initiation and progression of OSAS and to truly appreciate YKL-40 uniqueness. Third, the pathogenetic role of YKL-40 in OSAS should be confirmed by biochemical studies, such as immunoblotting. Therapeutic interventions by blocking YKL-40 signaling pathways to delay the pathophysiology process of OSAS warrants further investigations. In conclusion, our results indicate that serum YKL-40 concentrations were correlated with the presence and severity of OSAS. These data suggest that YKL-40 could be used as a potential biomarker for predicting the development and progression of OSAS. However, further large scale clinical studies are warranted.
Acknowledgements The authors thank the volunteers for their support to this study.
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X. Wang & G.-H. Xing
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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