Impact of serum omentin-1 levels on cardiac prognosis in patients with heart failure

Background Various adipokines are reported to be associated with the development of heart failure (HF) through insulin resistance and chronic inflammation. Omentin-1 is a novel adipokine and is associated with incident coronary artery disease. However, it remains unclear whether serum omentin-1 levels are associated with cardiac prognosis in patients with HF. Methods We measured serum omentin-1 levels at admission in 136 consecutive patients with HF, and 20 control subjects without signs of significant heart disease. We prospectively followed patients with HF to endpoints of cardiac death or re-hospitalization for worsening HF. Results Serum omentin-1 levels were markedly lower in HF patients with cardiac events compared with to without. The patients who were in New York Heart Association (NYHA) functional class IV showed significantly lower serum omentin-1 levels compared to those in class II and III, whereas serum omentin-1 levels did not correlate with serum brain natriuretic peptide levels (r = 0.217, P = 0.011). We divided the HF patients into three groups based on the tertiles of serum omentin-1 level (low T1, middle T2, and high T3). Multivariate Cox hazard analysis showed that the lowest serum omentin-1 level (T1) was independently associated with cardiac events after adjustment for confounding factors (hazard ratio 5.78, 95% confidence interval 1.20-12.79). We divided the HF patients into two groups according to the median serum omentin-1 levels. Kaplan-Meier analysis revealed that the patients with low serum omentin-1 levels had a higher risk of cardiac events compared with those with high serum omentin-1 levels (log-rank test p < 0.001). Conclusion Decreased serum omentin-1 levels were associated with a poor cardiac outcome in patients with HF.


Background
Heart failure (HF) remain a major cause of death worldwide and has a poor prognosis despite advances in treatment [1]. Adipocytokines, such as tumor necrosis factor-alpha, interleukin-6, and plasminogen activator inhibitor-1, play a crucial role in the development of cardiovascular diseases through insulin resistance and chronic inflammation [2][3][4][5]. Adipokines, such as adiponectin, are also reported to have anti-inflammatory, anti-oxidant, and anti-apoptotic properties, and are decreased in patients with cardiovascular disease [6][7][8][9]. There has been a move to clarify the causal relationship between various adipokines and cardiovascular disease [10,11].
Omentin-1 is a novel adipokine whose serum levels are decreased in obese individuals, and is associated with insulin resistance [12][13][14][15][16]. Omentin-1 has been suggested to play a beneficial role in preventing atherosclerosis [17,18], however, it remains unclear whether serum omentin-1 levels are associated with clinical outcome in patients with HF.
The purpose of this study was to clarify the impact of serum omentin-1 levels on cardiac prognosis in patients with HF.

Study population
We enrolled 136 consecutive patients who were admitted to the Yamagata University Hospital for treatment of worsening HF, diagnosis and pathophysiological investigations, or for therapeutic evaluation of HF. We also enrolled 20 control subjects without signs of significant heart disease.
A diagnosis of HF was based on a history of dyspnea and symptoms of exercise intolerance followed by pulmonary congestion, pleural effusion, or left ventricular enlargement by chest X-ray or echocardiography [19,20]. Control subjects were excluded if they had significant coronary artery disease, systolic and diastolic dysfunction, valvular heart disease, or myocardial hypertrophy on echocardiography [21]. All patients gave written informed consent prior to their participation, and the protocol was approved by the institution's Human Investigation Committee. The procedures were performed in accordance with the Helsinki Declaration.

Measurement of serum omentin-1 and brain natriuretic peptide levels
Blood samples were drawn at admission and centrifuged at 2,500 g for 15 minutes at 4°C within 30 minutes of collection. The serum was stored at −80°C until analysis. Serum omentin-1 concentrations were measured with a sandwich enzyme-linked immunosorbent assay (ELISA, Immuno-Biological Laboratories CO., Ltd., Gunma, Japan), according to the manufacturer's instructions [22,23]. The serum omentin-1 levels were measured in duplicate by an investigator unaware of the associated patients' characteristics. Serum brain natriuretic peptide (BNP) concentrations were measured using a commercially available specific radio-immuno assay for human BNP (Shiono RIA BNP assay kit, Shionogi & Co., Ltd., Tokyo, Japan) [24].

Endpoints and follow-up
The patients were prospectively followed for a median duration of 399 ± 378 days. The end points were cardiac death, including death due to progressive HF, myocardial infarction, stroke and sudden cardiac death, and re-hospitalization for worsening HF. Sudden cardiac death was defined as death without definite premonitory symptoms or signs, and was confirmed by the attending physician. Two cardiologists who were blinded to the blood biomarker data reviewed the medical records and conducted telephone interviews to survey the incidence of cardiovascular events.

Statistical analysis
Data are presented as the mean ± standard deviation (SD). The Mann-Whitney U-test was used when the data were not distributed normally. If the data were not distributed normally, they were presented as medians with an interquartile range. The unpaired Student's t-test and the chi-square test were used for comparisons of continuous and categorical variables, respectively. Comparison of data among three groups was performed by the Kruskal-Wallis test. Uni-and multivariate analyses with Cox proportional hazard regression were used to determine significant predictors of cardiovascular events. Cumulative overall and event-free survival rates were computed using the Kaplan-Meier method and were compared using the logrank test. We calculated the net reclassification improvement (NRI) and the integrated discrimination improvement (IDI) to measure the quantity of improvement for the correct reclassification and sensitivity according to the addition of serum omentin-1 levels to the prediction model [25]. NRI and IDI are new statistical measures to assess and quantify the improvement in risk prediction offered by a new marker. A P value < 0.05 was considered statistically significant. All statistical analyses were performed with a standard statistical program package (JMP version 10; SAS Institute, Cary, North Carolina, USA), and the R-3.0.2 with additional packages (Rcmdr, Epi, pROC, and PredictABEL).

Comparison between patients with and without heart failure
The patients with HF had a lower BMI and left ventricular ejection fraction, and lower serum total cholesterol, triglyceride levels, and higher serum BNP levels compared with control subjects (Table 1).

Comparison between HF patients with and without cardiac events
There were 59 cardiac events including 17 deaths and 32 re-hospitalizations in patients with HF during the follow-up period ( Table 2). The patients who experienced cardiac events were in a more severe New York Heart Association (NYHA) functional class, and had a lower estimated glomerular filtration rate, lower left ventricular ejection fraction, higher left ventricular end-diastolic diameter, and higher serum BNP levels compared with those who did not. Moreover, patients with cardiac events showed markedly lower serum omentin-1 levels compared with those without (Figure 1). There were no significant differences in etiologies of HF between patients with and without cardiac events ( Table 2).

Serum omentin-1 levels and HF severity
The patients who were in NYHA functional class IV showed significantly lower serum omentin-1 levels compared to those in class II and III (P = 0.029 vs. class II and P = 0.041 vs. class III, Figure 2A). On the other hand, serum omentin-1 levels were not significantly different between the patients who were in NYHA functional class II and III (P = 0.582). Furthermore, there was no relationship between the serum omentin-1 levels and the serum BNP levels (r = 0.217, Figure 2B).

Association between serum omentin-1 levels and cardiac events
We divided patients with HF into three groups according to the tertiles of serum omentin-1 levels. Multivariate Cox hazard analysis showed that the lowest serum omentin-1 levels (T1) were independently associated with cardiac events after adjustment for age, gender, NYHA functional class, left ventricular ejection fraction, and serum brain natriuretic peptide levels (hazard ratio 5.65, 95% confidence interval 2.61-12.20; Figure 3, Table 3). We divided the patients into two groups according to the median serum omentin-1 levels. Kaplan-Meier analysis revealed that the patients with low serum omentin-1 levels had a  higher risk of cardiac events compared to those with high serum omentin-1 levels (log-rank test p < 0.001, Figure 4).

Net reclassification improvement and integrated discrimination improvement
To measure the quantity of improvement for the correct reclassification and sensitivity according to the addition of serum omentin-1 levels to the prediction model, we calculated the NRI and the IDI. The inclusion of serum omentin-1 levels in the prediction model (includes age, gender, NYHA functional class, left ventricular ejection fraction, and serum BNP levels) for the prediction of cardiac events, improved the NRI and IDI values, suggesting effective reclassification and discrimination (Table 4).

Discussion
The present study demonstrated that decreased serum omentin-1 levels predicted cardiac events in patients with HF. Serum omentin-1 level appears to be a novel prognostic marker for the risk stratification of patients with HF. Various types of adipocytokines are reported to be a predictor of unfavorable cardiac outcomes in patients with HF [26]. In addition to their roles as predictors of cardiac outcome, a variety of adipocytokines have been associated with the development of HF through insulin resistance and chronic inflammation [14,[27][28][29]. Serum adiponectin levels are reported to be correlated with BNP levels, and are associated with HF severity and unfavorable outcomes in patients with HF [30,31]. Adiponectin has been suggested to play a role in the prevention of cardiovascular diseases via its anti-inflammatory, anti-oxidant, and antiapoptotic properties [6][7][8][9]. Recently, reports have shown several adipokines to have beneficial effects on cardiovascular diseases [32][33][34]. However, the precise role of these adipokines remains unclear.
Omentin-1 is a 38 kDa novel adipokine identified in 2004 from visceral adipose tissue [12,13]. Shibata et al. reported Figure 1 Comparisons of serum omentin-1 levels between control subjects and HF patients with or without cardiac events. HF patients with cardiac events showed markedly lower serum omentin-1 levels compared with those without (p < 0.001). HF, heart failure. The association between serum omentin-1 levels and serum BNP levels. There was no relationship between the serum omentin-1 levels and the serum BNP levels (r = 0.217). BNP, brain natriuretic peptide; NYHA, New York Heart Association. that decreased plasma omentin-1 levels predict the prevalence of coronary artery disease [18]. Yang et al. reported that omentin-1 enhances insulin-stimulated glucose uptake in human adipocytes and may regulate insulin sensitivity [13]. Yamawaki et al. reported that omentin-1 modulates vascular function and attenuates cyclooxygenase-2 expression and c-jun N-terminal kinase (JNK) activation in cytokine-stimulated endothelial cells [35,36]. These studies all suggest that omentin-1 may improve insulin resistance and suppress vascular inflammation. Interestingly, Pan et al. suggested that omentin-1 expression and production are decreased with elevated inflammatory adipokines, such as tumor necrosis factor-alpha and interleukin-6, in patients with impaired glucose intolerance and newly diagnosed type 2 diabetes mellitus [37].
Unlike to adiponectin, serum omentin-1 was reported to decrease with chronic inflammation and oxidative stress in patients with HF. The bioactivity of omentin-1 appears multifaceted and remains to be fully defined. The present study showed no correlation between serum omentin-1 and BNP levels unlike adiponectin [30], suggesting that these markers indicate different features of the pathophysiological process of HF. Serum omentin-1 levels may represent a promising biomarker for cardiac prognosis, irrespective of serum BNP levels. The inclusion of serum omentin-1 levels in the prediction model (includes age, gender, NYHA functional class, left ventricular ejection fraction, and serum BNP levels) for the prediction of cardiac events, improved the NRI and IDI values, suggesting effective reclassification and discrimination.
The present study has certain limitations. Firstly, the sample size was relatively small and it was a single center study. Nonetheless, there was a significant relationship between serum omentin-1 levels and cardiac events. In addition, the inclusion of serum omentin-1 levels in the Figure 3 Hazard ratio of the tertiles of omentin-1 levels for cardiac events after adjustment of age, gender, body mass index, NYHA functional class, left ventricular ejection fraction, serum triglycerides, serum HDLc levels, and serum BNP levels. BNP, brain natriuretic peptide; HDLc, high density lipoprotein cholesterol; NYHA, New York Heart Association. Figure 4 Kaplan-Meier analysis. The patients with low serum omentin-1 levels had a higher risk of cardiac events compared to those with high serum omentin-1 levels (log-rank test p < 0.001).  prediction model with conventional risk factors, including serum BNP levels, for the prediction of cardiac events, improved the NRI and IDI values. Secondly, there were no data for other adipocytokines. Further study is needed to clarify the association between serum omentin-1 and other adipocytokines in a large HF population.
In conclusion, decreased serum omentin-1 levels were associated with cardiac events in patients with HF, irrespective of serum BNP levels. Serum omentin-1 level appears to represent a novel prognostic marker for the risk stratification of patients with HF.