In the current study, we found that serum apelin-12 was significantly higher in the MIS group than that in the control group and was independently correlated with MIS. To the best of our knowledge, this is the first study to evaluate the correlation of apelin-12 with MIS in elderly Chinese, thus indicating the potential role of apelin-12 in the pathogenesis and the development of MIS.
As a ubiquitously expressed peptide, apelin was identified as a novel kind of adipokine expressed and secreted by adipocytes in both murine and human [16]. Due to the wide expression of apelin and its receptor APJ, it has been reported that apelin participated in various physiological processes, including energy metabolism such as glucose and lipid metabolism [17], endocrine response to stress, homeostasis and so on [18]. Previous clinical studies reported increased concentrations of apelin in T2DM and obesity in humans and animals [19, 20], which further confirms its essential role in metabolic disorders.
There are several isomers produced by enzymatic cleavage of proapelin that are bioactive, such as apelin-12 [21], which also has a variety of physiological and biological functions. Few studies have explored the levels of apelin-12 in patients with metabolic disorders [22]. Significantly elevated apelin-12 was found in obese children with metabolic syndrome (MetS) compared to those without MetS and was determined to be the independent risk factor for MetS [9]. Similar findings were reported by Helske et al. in middle-aged individuals [10]. This was also verified in our study that apelin-12 was increased in elderly patients with MIS in comparison with the control group. When divided by the number of four components, a general increasing trend of apelin-12 was found as the number of components increased, although no difference was found between the third group and other groups, which might be attributed to the small number of individuals.
To determine the predictive power of apelin-12 for MIS, Firth's bias-reduced logistic regression analysis has been performed. As a result, we identified apelin-12 as a risk predictor for MIS, even after the adjustment for several variates, which was also in accordance with the previous study in obese children [9]. In addition to apelin-12, BMI, HbA1c and SBP were also determined as independent factors for the presence of MIS, indicating the involvement of obesity, blood glucose and blood pressure in MIS.
Several studies described MIS as a state characterized by chronic low-grade inflammation (CLGI) [1, 14, 23, 24]. It had been widely accepted that CRP plays a huge part in the process of inflammation and the response to infection. During inflammatory conditions, the expression of CRP increases. Hence, it is considered to be a traditional marker of inflammation [25]. As one of the most abundant adipocytokines, adiponectin has been proved to be a classic adipocytokine with anti-inflammatory properties, as well as antidiabetic and anti-atherogenic activity [26, 27]. Various reports suggest the decline of plasma adiponectin concentration in obesity, diabetes and MetS and can be considered a potent predictor for MetS [28, 29]. Recently, circulating adiponectin was observed lower in elderly patients with MIS at the cross-sectional population level [24]. In our study, the MIS group exhibited higher levels of CRP and lower levels of adiponectin compared to controls, revealing the relatively inflammatory conditions in patients. Most importantly, apelin-12 showed a significant positive correlation with CRP and a negative correlation with adiponectin in our research, which further confirmed the potential role of apelin-12 in MIS.
It is well-known that insulin resistance is a common mechanism of obesity, diabetes [30], NAFLD [31] and atherosclerosis [32], the four components of MIS. Insulin sensitivity is compromised in metabolic deterioration. The current study found that apelin-12 was positively related to fasting insulin and HOMA-IR after age-, sex- and BMI-adjustment, suggesting the underlying role of apelin-12 in insulin resistance. Besides, the expression of apelin has been reported to be directly up-regulated during adipocyte differentiation stage. This may partly explain the possible link between apelin and insulin [4]. In our study, positive associations of apelin-12 with liver function indexes (ALT and AST) and a negative correlation with HDL-C were also observed.
Both clinical and experimental studies illustrated that UA was a contributory causal factor in metabolic disorders [33]. We found an apparent increase in UA in the MIS group, whereas no significant association was found between apelin-12 and UA. Contrary to our expectations, lower levels of TC and LDL-C were found in the MIS group, which could be explained by the use of lipid-lowering drugs in these patients.
There are several limitations of our study to be noted. First, this is a cross-sectional study with no follow-up data. Therefore, the causal role of apelin-12 in MIS remains unclear. Second, since the participants in our study were over 60, it is unknown whether our results can be generalized to middle-aged or younger subjects. Moreover, the number of subjects was not large enough in this study, resulting in the wide width of the confidence interval of OR of apelin-12 by Firth's bias-reduced logistic regression analysis. Subsequent studies with a larger population should be conducted to follow up on the changes in serum apelin-12 over a few years both in healthy controls and MIS patients.