Circulating levels of adipose tissue-derived inflammatory factors in elderly diabetes patients with carotid atherosclerosis: a retrospective study

Atherosclerosis has been recognized as a fundamental pathologic change in many cardiovascular diseases [1], and thus, represents an important threat to the health of the global population. Indeed, the primary pathophysiological feature of both coronary heart disease (CHD) and stroke is atherosclerosis, and these diseases have become major contributors to morbidity and mortality worldwide [2, 3], particularly among the elderly population [4]. Conventionally, many risk factors have been identified for the pathogenesis of atherosclerosis, including male gender, family history of early incidence of cardiovascular diseases, smoking, hypertension, dyslipidemia, obesity, and diabetes mellitus (DM) [5]. Atherosclerosis is now considered an inflammatory disease, because overactivated inflammation has been demonstrated to be a common pathway that mediates the pathogenesis of atherosclerosis [6, 7]. Therefore, identification of key inflammatory mediators involved in the initiation and progression of atherosclerosis may be of important clinical significance for the prevention and treatment of atherosclerosis-related cardiovascular diseases [8].

Type 2 DM (T2DM) is characterized by insulin resistance and has been confirmed to be a strong risk factor for the pathogenesis of atherosclerosis [9]. More than 400 million people globally are estimated to DM, and data from a recent epidemiological study suggest that the age-standardized prevalence of total DM is nearly 10% and the prevalence in people over 60 years old exceeds 20% [10]. Development of vascular disease has become a major cause of morbidity and mortality in patients with T2DM. Indeed, a previous collaborative meta-analysis of 102 prospective studies showed that compared with controls without T2DM, patients with T2DM have a 73% increased risk for total vascular diseases, including a 100% increased risk for CHD, a 127% increased risk for ischemic stroke, and a 54% increased risk for hemorrhagic stroke [11]. However, the potential mechanisms underlying the association between T2DM and atherosclerosis have not been fully determined, and the activated inflammatory response has been considered an important common pathophysiological feature of both diseases [12, 13]. Indeed, classical inflammatory factors, such as the high-sensitivity C-reactive protein (hs-CRP) and tumor necrosis factor alpha are involved in the pathogenesis of both T2DM and CHD [14, 15]. Moreover, adiponectin, an inflammatory peptide secreted by adipocytes, and neutrophil gelatinase-associated lipocalin (NGAL), an acute phase protein released by neutrophils, are involved in the interrelationship between T2DM and CHD [16, 17]. These findings raised the hypothesis that anti-inflammatory treatment may be effective for the prevention of both T2DM and CHD [18]. Importantly, the recently published Canakinumab Anti-inflammatory Thrombosis Outcomes Study (CANTOS) showed that anti-inflammatory treatment with Canakinumab targeting the interleukin-1β innate immunity pathway reduced the risk of recurrent cardiovascular events in patients with previous myocardial infarction [19]. However, the incidence of T2DM was not significantly affected [20], indicating the importance of identifying novel inflammatory factors that mediate the association between T2DM and CHD. Recent evidence suggests that in addition to storing energy, adipose tissue may secrete several inflammatory factors, such as resistin, vaspin and visfatin, which may be important mediators of obesity, atherosclerosis, and DM [21, 22]. Resistin was initially discovered as an adipocyte-secreted hormone mediating obesity and insulin resistance in animal studies, and a recently published study in humans suggests its potential role in the pathogenesis of atherosclerosis [23]. Vaspin, a visceral adipose tissue-derived serine protease inhibitor that is upregulated in animal models of obesity and insulin resistance, has also been hypothesized to participate in the development of atherosclerosis [24]. Moreover, visfatin, also known as the enzyme nicotinamide phosphoribosyltransferase (Nampt), which was initially identified as a molecule with insulin-like properties in 2005, also has been suggested to be involved in the pathogenesis of atherosclerosis primarily by mediating the inflammatory response [25, 26]. However, the circulating levels of these inflammatory factors in T2DM patients according to the severity of atherosclerosis have not been determined. More importantly, the associations between these adipose-derived inflammatory factors and the severity of atherosclerosis in elderly patients with T2DM have not been reported. The aim of the current retrospective cohort study was to compare the levels of resistin, vaspin, and visfatin in T2DM patients according to the severity of carotid atherosclerosis and to explore the potential relationships of these factors and the conventional risk factors for T2DM and atherosclerosis, such as body mass index (BMI) and indicators of insulin resistance.

This retrospective cross-sectional study included 256 elderly patients (> 60 years) with T2DM who were admitted to the Department of Geriatrics or Department of Endocrinology of Xuanwu Hospital affiliated to the Capital Medical University between July 2013 and July 2017. Written informed consent was obtained from each included patient before enrollment. The study protocol was approved by the Ethics Committee of Xuanwu Hospital affiliated to the Capital Medical University before the performance of the study.

In this cross-sectional study of elderly patients with T2DM, we found that circulating levels of novel adipose tissue-derived inflammatory factors showed a trend of significant change according to the severity of systemic atherosclerosis. Specifically, with the worsening of carotid atherosclerosis, circulating levels of resistin and visfatin gradually increased, while the level of vaspin decreased. Subsequent results of Pearson coefficient analyses indicated that levels of resistin and visfatin were positively correlated, and all the above adipose tissue-derived inflammatory factors were positively correlated with the BMI and HOMA-IR of the patients, even after adjustment for hs-CRP. These results suggest that resistin and visfatin, both as the proinflammatory factors, may exert a synergetic effect during the pathogenesis of atherosclerosis in these patients, and the circulating levels of resistin, vaspin, and visfatin may parallel the severity of systemic atherosclerosis, despite their roles as indicators of the extent of insulin resistance. Based on above findings, we hypothesize that resistin, vaspin and visfatin may be involved in the pathogenesis of atherosclerosis in elderly patients with T2DM.

Resistin was initially identified in white adipocytes of mice [28], and the serum level of resistin was found to be more remarkable in animal models of obesity and insulin resistance [29]. Subsequent experimental studies showed that resistin may be a mediator of insulin resistance, likely via a decrease in the phosphorylation of 5′ adenosine monophosphate-activated protein kinase (AMPK) in the liver [30]. This is consistent with our results, which showed a positive correlation between the circulating resistin level and the indicator of insulin resistance HOMA-IR in elderly patients with T2DM. Moreover, some evidence from experimental studies also suggests that resistin may accelerate the pathogenesis of atherosclerosis by promoting endothelial dysfunction, vascular smooth muscle cell proliferation, arterial inflammation, and the formation of foam cells [23]. Indeed, accumulating evidence from human epidemiological studies indicates that an increased circulating level of resistin may be related to increased risks of many cardiovascular diseases, including CHD and stroke [31]. A recently published prospective cohort study showed that higher resistin is a significant predictor of cardiovascular diseases independent of conventional risk factors in individuals over 70 years [32]. However, the predictive efficacy of resistin for cardiovascular risk was significantly attenuated by adjustment for inflammation [32]. We extended these findings by showing that the circulating resistin level increased in parallel with the severity of systemic atherosclerosis in elderly patients with T2DM, indicating that resistin may be an important contributor to vascular complication in these patients. In addition, visfatin/Nampt was initially identified as a substance with insulin-like properties in mice in 2005 [33]. Subsequent studies revealed that an important role of visfatin/Nampt is the regulation of the inflammatory response, likely via induction of other inflammatory factors, including interleukin-1, tumor necrosis factor alpha, interleukin-6, etc. [34]. The pro-inflammation efficacy of visfatin/Nampt has been considered to be the major mechanism underlying the induction of insulin resistance by visfatin. This was further confirmed by our findings, which showed a positive correlation between circulating visfatin and HOMA-IR. Interestingly, recently published studies in patients with T2DM showed that circulating visfatin is correlated with an increased intima-media thickness (IMT) of carotid arteries [35]. These findings were extended by our study, which showed that the circulating visfatin level increased gradually with the increasing severity of carotid atherosclerosis in elderly patients with T2DM. Moreover, a recent study in high-risk patients with ST segment elevated myocardial infarction suggested that the circulating level of visfatin in these patients can independently predict mortality risk [36]. The results of our study showed that circulating resistin and visfatin were positively associated with the severity of atherosclerosis in elderly diabetes patients. Interestingly, some recent prospective cohort studies also indicated that higher resistin and visfatin levels at baseline are associated with an increased risk of major cardiovascular adverse events (MACEs). In a prospective cohort study of 150 patients with diabetic nephropathy, increased resistin and visfatin levels at baseline were found to be independent predictors of cardiovascular mortality [37]. Moreover, the prospective association between baseline circulating resistin and the risk of MACEs has also been indicated in recent large cohort studies and a meta-analysis, particularly in T2DM patients [38, 39]. In summary, the inflammation-inducing efficacy of resistin and visfatin may be important for the development of atherosclerosis in patients with T2DM.

As a visceral adipose tissue-derived serine protease inhibitor, vaspin is also confirmed to be upregulated in animal models of obesity and insulin resistance [24]. Although the potential mechanisms underlying the effect of vaspin on insulin resistance remain to be determined, it has been suggested that the circulating level of vaspin correlates with the extent of insulin resistance in certain populations, such as overweight female patients with polycystic ovary syndrome [40]. However, no correlation between the circulating level of vaspin and HOMA-IR was observed in 108 subjects with normal glucose tolerance [41]. Our results were consistent with the previous study in that the circulating level of vaspin was correlated with HOMA-IR in elderly patients with T2DM. The discrepancy of the other results may be explained by the different study population included. However, whether other potential factors exist that confound the association between the level of vaspin and extent of insulin resistance deserves further investigation. Moreover, we found that the circulating level of vaspin decreased significantly in elderly T2DM patients increasing severity of atherosclerosis. This is inconsistent with the majority of previous findings, which showed that the circulating level of vaspin was positively associated with the severity of coronary stenosis in females with metabolic syndrome [42] and positively associated with the severity of carotid atherosclerosis in patients who underwent carotid endarterectomy [43]. Also, vaspin was shown to correlate with CAD in T2DM [44]. However, a previous cross-sectional study of T2DM patients showed an inverse association between serum vaspin and the presence of carotid plaque, which is similar to our findings [45]. Again, differences in the populations studied may contribute to the heterogeneity of the results, although other factors that may confound the association between the circulating level of vaspin and severity of atherosclerosis deserve investigation in future studies.

The strengths of our study include enrollment of elderly patients with T2DM who had rarely participated in previous similar studies. Moreover, we analyzed the changes in circulating levels of three novel adipose tissue-derived inflammatory factors to comprehensively evaluate their association with the severity of atherosclerosis in these elderly patients. Despite the above strengths, our study also has limitations that should be considered when interpreting the results. First, as a cross-sectional study, we could not examine the causative relationships of resistin, vaspin and visfatin with atherosclerosis or insulin resistance. Moreover, we did not perform multivariate adjusted analyses for the potential associations of resistin, vaspin and visfatin with atherosclerosis or insulin resistance. Therefore, we cannot not exclude the possibility that confounding factors, such as differences in lifestyle or medication used in the included patients, may affect the associations. In addition, the sample size of the study was relatively small. Due to the small number of patients included in the groups with differing severity of atherosclerosis, the study may be statistically underpowered to evaluate the correlations in the three different groups according to the severity of atherosclerosis. Therefore, the correlations of the three adipose tissue-derived inflammatory factors with the parameters of obesity, insulin resistance, etc. were not evaluated. Finally, as a single-center study, the generalizing of the conclusion to patients from other centers should be done cautiously.

In conclusion, we found that circulating levels of novel adipose tissue-derived inflammatory factors, including resistin, vaspin and visfatin, may change according to the severity of atherosclerosis in elderly patients of T2DM, and these factors were correlated with the degree of insulin resistance. These results suggest that resistin, vaspin and visfatin may be involved in the pathogenesis of atherosclerosis in elderly patients with T2DM. The potential predictive efficacy and treatment significance of resistin, vaspin and visfatin for vascular complications in elderly T2DM patients warrant further investigation.