1 Introduction
Type 2 diabetes mellitus (T2DM) is a chronic metabolic disease characterised by peripheral insulin resistance (IR) and inadequate relative insulin secretion, resulting in hyperglycaemia (
1). T2DM encompasses approximately 90% of all cases of diabetes, with the worldwide prevalence of diabetes expected to rise from 415 million (2015) to 642 million by 2040, equivalent to more than 5% of the world’s population (
2).
Excess adipose tissue, as seen in obesity, plays a central role in promoting IR and increasing the risk of developing T2DM. Adipose tissue acts as a powerful endocrine organ secreting a range of cytokines (termed adipokines) that affect whole-body physiology (
3). Ectopic fat is associated with elevated levels of pro-inflammatory adipokines (e.g. tumour-necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6) and leptin) and lower levels of anti-inflammatory adipokines (e.g. adiponectin) which have been postulated to contribute to the development of IR and T2DM (
4‐
7).
One adipokine that has come under recent scrutiny for its association with T2DM is Zinc-alpha2-glycoprotein (ZAG). ZAG is a 40–43 kDa single chain polypeptide (
8) that is typically found in the majority of body fluids. Due to ZAG’s presence in a high number of body components, it has been suggested to play many roles in the body, including: fertilization, ribonuclease activity, cell adhesion, the regulation of melanin production, hindering tumour proliferation and promoting lipolysis (the hydrolysis of lipids into non-esterified fatty acids) (
8). ZAG acts as an initiator of the catalytic conversion of adenosine triphosphate to cyclic adenosine monophosphate (c-AMP), which is the initial step of lipolysis (
9) by binding to beta-3-adrenoreceptors on adipocyte surfaces. For this reason, ZAG has recently been categorised as a lipid-mobilising factor (
10).
Although ZAG has been identified as an adipokine, its physiological effects on the risk and development of obesity-related diseases has not been fully determined. Findings have indicated that increased macrophage infiltration and the subsequent release of inflammatory cytokines, including TNF-alpha and IL-6, supresses ZAG release (
11), which could be a contributor to reduced lipolysis and greater levels of ectopic fat. In line with these findings, ZAG has been shown to be positively associated with adiponectin (
12,
13).
Expanding upon this evidence, elevated ZAG concentrations have been observed in cancer cachexia (
14,
15), which could be a result of increased glucocorticoid levels (
16). However, cancer cachexia has been associated with increased IR (
17), which suggests a complex interaction of inflammatory markers.
The expanding evidence demonstrating the role of ZAG in lipid and glucose homeostasis has led to ZAG becoming a protein of increasing interest for anti-obesity and obesity-related disease therapy (
18). In particular, lower levels of ZAG have been reported to be associated with the development and severity of T2DM, however the literature is contradictory, and no summary evidence has been published to date to quantify the magnitude of the association. This study aims to systematically review the current literature investigating the association between ZAG and metabolic dysfunction, identified as chronic diseases or conditions classified by hyperglycaemia or IR, referred to as dysglycaemia from here on. This includes reporting on the relationship between ZAG, insulin and glucose. Additionally, a meta-analysis will estimate the extent to which ZAG differs between individuals with or without dysglycaemia. In order to examine the potential influence of adiposity, the meta-analysis will also be restricted to only look at individuals considered overweight or obese.
4 Discussion
The findings of our meta-analyses and narrative review suggest that increased ZAG is negatively associated with the development of metabolic dysfunction, particularly that related to dysglycaemia. This effect may be mediated by differences in adiposity, as there were no associations between ZAG and dysglycaemia in studies where BMI was taken into account. However, only three studies (encompassing four cohorts) were included in the latter analysis, which suggests that further research is needed in this area.
ZAG has been suggested to be an adipokine that acts as a lipid-mobilising factor playing a role in lipolysis in adipose tissue (
9,
10). Reduction in the levels of ZAG could lead to lower rates of lipolysis and subsequent accumulation of lipids. This resulting lipotoxicity is recognised to result in cellular dysfunction, including impaired glucose uptake and pancreatic beta-cell dysfunction (
40,
41) thus increasing the likelihood of metabolic diseases such as T2DM. The role of lipotoxicity in T2DM development is also attributed to increased inflammation through changes in cytokine profiles, which extends to adipokines (
42). Therefore, if declining ZAG levels are associated with the onset of lipotoxicity, this could suggest a possible anti-inflammatory role of ZAG, which is supported by its positive association with adiponectin (
13,
14,
31,
43,
44) and negative association with TNF-alpha (
12,
45). Furthermore, ZAG mRNA expression in both subcutaneous and visceral adipose tissue is largely found to be predicted by adiponectin mRNA expression (
33).
Along with being modulated by levels of adiposity, ZAG has also been directly implicated in the metabolic action of adipose tissue (
18). For example, ZAG has been associated with reduced insulin sensitivity in visceral adipose tissue, suggesting a feedback loop whereby increased visceral adiposity reduces ZAG expression, which in turn may increase adverse metabolic functioning of visceral adipose tissue (
31). A potential link been ZAG and the metabolic activity of visceral adipose tissue is important given the deleterious impact of visceral fat to metabolic health (
46) and the risk of T2DM. In the opposite direction, higher levels and greater expression of ZAG have been associated with the browning of adipose tissue, which could have positive implications to metabolic health (
47).
The role of ZAG in adipose tissue metabolism and the results from this meta-analyses raise the question as to whether the relationship between ZAG and dysglycaemia is in fact mediated by, or independent of, adiposity. In contrast to the findings of this meta-analysis, Yeung and colleagues investigated the difference in ZAG messenger ribonucleic acid (mRNA) and protein expression in individuals with normal glucose tolerance and T2DM patients, all of whom were overweight or obese. The mRNA expression and protein content of ZAG in adipose tissue was found to be 1.5-fold and 2.4-fold lower in those with T2DM (
35), indicating that the relationship between ZAG and dysglycaemia may be independent of adiposity.
Treating rats with recombinant human ZAG has also been shown to decrease circulating levels of glucose and triglycerides, as well as increasing the expression of glucose transporter 4 (GLUT4), without a decrease in food or water intake (
48). These findings are consistent with previous observations demonstrating a positive association between subcutaneous ZAG mRNA and GLUT4 mRNA expression in adipose tissue (
32). However, whether ZAG's mechanistic role in GLUT4 translocation is mediated by adiposity independent pathways needs to be clarified and this may be better understood through mRNA analysis rather than circulating concentrations.
The magnitude of the influence of adiposity on other adipokines in MetS and subsequent T2DM also varies in the literature. Comparison of normal weight, overweight, obese and morbidly obese T2DM patients with metabolically healthy normal weight controls showed a difference in adipokine markers that did not exist in normal weight T2DM subjects, but the difference was exacerbated by being overweight or (morbidly) obese (
49). Adiponectin, a commonly measured anti-inflammatory adipokine, has been shown to be lower in obese subjects compared to metabolically healthy normal weight controls, but there was no difference between obese subjects and T2DM patients (
50). A previous systematic review and meta-analysis on circulating adiponectin and T2DM did reveal that across 15 prospective studies (14,598 participants), higher levels of adiponectin were associated with a lower risk of T2DM (
51). With 13 of these studies stratified for BMI (<25 kg
.m
-2 and ≥ 25 kg
.m
-2), it was found that the relationship remained significant.
As the first study to summarise the literature surrounding the association between circulating ZAG and dysglycaemia, it identifies key limitations in the literature. Firstly, substantial heterogeneity exists, meaning more research is needed to establish whether ZAG is a valuable biomarker in the development of dysglycaemia. Secondly, there is a need to determine if the alterations in ZAG occur before the onset of dysglycaemia or vice versa. Furthermore, the summarised data shows a lack of studies isolating individuals by adiposity status to further determine if the relationship between ZAG and dysglycaemia is primarily driven by adiposity, or whether there are adiposity independent pathways linking ZAG to dysglycaemia.
This meta-analysis also has important limitations. Mainly, the inability to undertake a meta-analysis on correlation or association statistics between ZAG and markers of glucose control and IR. Additionally, there has been some investigation into the relationship between other measures of ZAG (i.e. adipose tissue mRNA and protein content) and dysglycaemia which warrants investigation.
In conclusion, this is the first study to review the current findings on circulating concentrations of ZAG and dysglycaemia. More investigations into changes in circulating ZAG with the development of dysglycaemia, with the removal of adiposity as a confounder, are needed to provide a clearer understanding.
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