The Relationship Between Receptor For Advanced Glycation End Products And Obesity: A Systematic Review And Meta-Analysis

With the improvement of nutritional status, the prevalence of obesity is increasing. The occurrence and development of obesity is closely related to the oxidative stress pathway of AGEs production. However, the current research on the correlation between obesity and ages is controversial. Thus, we evaluated the association between obesity and sRAGE. We searched 1,424 articles from PubMed, EMBASE, Web of Science, the Cochrane Library and other resources. Among the 1,424 articles identied, 9 met the inclusion criteria. After the data analysis of research results, it is concluded that the sRAGE of obese patients is lower than normal weight people(MD [95%CI] =-247.65 [-319.63,-175.67], Z = 6.74, P <0.00001). In conclusion, the results of this meta-analysis revealed a negative correlation between obesity and sRAGE.


Introduction
Obesity is a global epidemic and has become a serious public health problem. A survey which is published in the lancet, shows that the number of obese adult women worldwide increased from 69 million in 1975 to 390 million in 2016, while the number of obese men increased from 31 million in 1975 to 281 million in 2016 1 . Hippocrates wrote "Corpulence is not only a disease itself, but the harbinger of other". Its health risks is well documented, and its prevalence has increased the incidence of cardiovascular disease, type 2 diabetes and many types of cancer worldwide 2 .
Advanced glycation end products (AGEs) are non-enzymatic and oxidative modi cations of proteins, lipids or nucleic acids after contact with reducing sugars, also known as Maillard reaction 3 . Serum soluble receptor for advanced glycation end products (RAGE) is a multi-ligand receptor in the immunoglobulin superfamily, which was rst isolated from bovine lung in 1992. Many subtypes of rage have been identi ed, but the most prominent two are the lack of transmembrane and cytoplasmic domains, called truncated or soluble (sRAGE); And endogenous secretion (esRAGE), formed by alternative RNA splicing 4 . Its interaction with AGEs stimulates oxidative stress and stimulates proin ammatory or procoagulant cell responses, including increased expression of vascular cell adhesion molecule-1 and tumor necrosis factor -α 5 . A growing number of clinical trials have demonstrated AGEs-RAGE axis is widely involved in the development of many diseases, including physiological aging and age-related diseases 6 , diabetes 7 , autoimmune/in ammatory rheumatic diseases 8 (including systemic lupus erythematosus, rheumatoid arthritis, systemic sclerosis), adult-onset Still's disease 9 , kidney disease 10 , and multiple cancers 11 .
The occurrence and development of obesity is closely related to the oxidative stress pathway of AGEs production, as well as the resulting in ammation and endothelial dysfunction 12 . However, the research on obesity and ages RAGE axis is controversial.Some trials showed that RAGE was associated with obesity 13 , while others showed the opposite 14 .Due to the small sample size, these studies cannot provide su cient evidence to support, Therefore, we conducted this meta-analysis and systematic review to provide more su cient evidence in this eld.

Protocol:
We conducted this meta-analysis strictly according to the MOOSE list, and the contents of the system review and meta-analysis have been well reported in the complete MOOSE list (S1 Table).

Search strategy:
Two investigators (HD,SZD) used advanced "glycation end products" OR "AGEs" OR "advanced glycosylation end products" OR "glycosylation end products, advanced" [MeSH] OR "glycation end products receptor" OR "RAGE" AND "obesity"[MeSH] OR "overweight" as the search strategy. Since we retrieve the following database from the database to the data on May 20, 2021: PubMed, EMBASE, Web of Science, the Cochrane Library. Furthermore, references of identi ed citations and Google Scholar were also searched to identify additional studies. Truncation was used when appropriate to ne-tune the search and increase the number of relevant ndings.

Selection of articles:
The retrieval data of different expression formats were imported into Endnote version X9 software (Thomson Reuters, Stamford, CT, USA) for integration, and the same research from different database sources was initially deleted through the " nd duplicates" function. The study will be included according to the following criteria: RAGE was detected by blood serum in study; the study should be case-control study; the article was in English in a peer-reviewed journal.
The study will be included according to the following criteria: The study designed was animal experiments reviews, conference articles and the study was not divided into obesity group and control group. Two authors (DH, SZD) excluded studies obviously unrelated to the purpose of the article by reading the title and abstract, which did not study "obesity" and "RAGE". Then screening eligible study independently selected for full-text review. Divergences were solved by the consensus that was reached in advance. If >1 article was published on the same clinical trial, we chose the article with the most complete data. When the research data is incomplete, we try to contact the author of the article to acquire the whole data.

Data extraction:
Two researchers (DH, SZD) independently extracted the relevant data of each study, including: the rst author of the study, the year of publication, Country of publication, Publication type, the number of participants, age, body mass index and gender in obesity group and non-obesity group were compared, diagnostic criteria of obesity, matching principle, sRAGE value ( Table 1). The form of conversion of RAGE is different in these articles. We convert units to make them consistent by using this formula "1 ng/ml=1000 pg/ml". Some of the data are expressed by Inter-quartile range, while others are showed by the sample mean and standard deviation. In order to keep the consistency of data, we used the calculation method of Professor Tong Tiejun to estimate the sample mean and standard deviation from the sample size, median, range and/or interquartile range (http://www.math.hkbu.edu.hk/~tongt/papers/median2mean.html).

Literature quality evaluation:
The Newcastle-Ottawa Scale (NOS) 15 will be used to evaluate biases in studies included in this meta-analysis. This scale is evaluated from three aspects, including object selection, comparability, outcome, and exposure. By comparing this scale, all studies will be divided into three quality grades based on score: high, and low, and high-quality articles score more than 5 stars, and the lower is 0-5 stars.

Data analysis:
The main outcome of our study is the relationship between serum advanced glycation end product receptor and obesity. According to the characteristics of the subjects, we will conduct subgroup analysis based on age, study area, obesity degree, obesity type, complications, etc.
Rev-man 5.3 software (Cochrane organization, England) will be used for data analysis of the study. In this meta-analysis, we used means and SDs to measure the continuous results of changes in sRAGE. The random effects model will be used for combination analysis of data and reported as mean difference (MD) and 95% con dence interval. I 2 statistics is used to evaluate the heterogeneity of meta-analysis, and when it is more than 50%, there may be substantial heterogeneity. "leave one out" is used for sensitivity analysis to evaluate the robustness of the overall results. When more than 10 studies were included in the meta-analysis, publication bias was necessary. Funnel visualization analysis and egger's regression test were used to test publication bias. In all analyses, P values (two sides) < 0.05 showed statistical signi cance.

Risk of bias assessment
Since this meta-analysis only involved observational studies, we adopted the Newcastle-Ottawa Scale to critically evaluate the quality of the included articles.
The scale includes three aspects of evaluation: the selection method of case group and control group, the comparability of case group and control group, and the exposure evaluation method. The more stars after evaluation, the better the quality, preferably 10 stars. If the number of stars is greater than 5, it will be included in quality research and systematic evaluation and articles with more than six stars are considered of high quality. Fortunately, after an independent evaluation by two researchers, all included studies ful lled the minimum criteria and retained for systematic review and meta-analysis. A total of six articles were considered high quality and three were considered low quality. (Table 2)

Meta-analysis of outcome measures
The aim of these studies was to explore whether RAGE is associated with obesity. These studies were grouped comparing the value of SRAGE among obese vs normal weight individuals. After the data analysis of research results, it is concluded that the sRAGE of obese patients is lower than normal weight people MD [95 CI] =-247.65 [-319.63,-175.67] Z = 6.74 P <0.00001 (Figure 2) .The result from the inconsistency test showed that there was high heterogeneity among the studies analysed (I2=69%). Thus, the random-effects model was used to calculate the summary measurement.  (Figure 4). At the same time, we evaluated the quality of articles through Newcastle-Ottawa Scale, and conducted a subgroup analysis of the quality of the articles. The results showed that articles in high quality groups had lower heterogeneity (I 2 =5%) ( Figure 5)

Sensitivity analysis
We removed each trial from the analysis, step by step, in order to discover the impact of each single study on the combine effect size. We observed no signi cant effect of any individual study on the effect sizes of obesity and sRAGE.

Publication bias
No more than 10 studies were included in this meta-analysis, so there was no study of publication bias.

Dicussion
Our study shows that the level of sRAGE in obese patients is lower than that in normal weight people, which is consistent with most previous research conclusions. Obesity, as a hotbed of metabolic diseases, is an important risk factor for the occurrence and development of multiple metabolic diseases. The AGE-RAGE axis, which represents the oxidative stress process, is re ected in many metabolic diseases. It can be regarded as a bridge between obesity and other metabolic diseases, such as diabetes and metabolic syndrome, and our research con rms this assumption.
The formation of ages is a naturally occurring process and the result of normal metabolism. Under the physiological conditions of aging, hyperglycemia, oxidative stress and hyperlipidemia, the formation of ages will increase. So, hyperlipidemia oxidative stress and other features of obese individuals also play a great hole in the production of advanced sugar end products. RAGE is a multi ligand cell surface receptor expressed in three variants. The full-length RAGE and NH2 truncated variants are retained in the plasma membrane, and the COOH truncated variant called esRAGE is secreted outside the cell 19 . Enzymatic digestion of full-length cell surface receptors produces another form of full-length RAGE, called sRAGE 19,20 . Due to the lack of cytoplasmic tail and transmembrane domain, it cannot activate intracellular signals. Therefore, it can be used as a competitive receptor of RAGE. It can eliminate the adverse reactions between AGE and RAGE axis by competitive binding with AGE. AGEs can lead to changes in protein function and intracellular glycosylation with impaired cell function. AGEs modi es extracellular matrix proteins, resulting in abnormal interaction between proteins and cells; Circulating AGEs bind to age receptors of different cell types, resulting in the activation of intracellular signaling pathways. These may be the mechanisms by which AGEs can play a role 21 .
A large amount of evidence shows that the AGE-RAGE axis leads to the production of pro-in ammatory cytokines, the expression of adhesion molecules and oxidative stress. So far, studies for RAGE with obesity have identi ed the main downstream effects as high in ammatory response and intracellular oxidative stress 22 . For example, a recent study showed that with the ROS dependent pathway, AGE increased the expression of progesterone / in ammatory regulator and plasminogen activator inhibitor-1 in rat white adipocytes 23 . In addition, glycosylated BSA increases the fat potential of aging upgrading (in vitro and in vitro) through the Aga RAGE axis and the damage of p53 function 24 . Here, RAGE is directly combined with RAGE inhibition at the p53 transcript level by direct binding of RAGE to cytosolic p53. This in turn enhances the adipogenic potential of ordinary adipocytes and has adverse long-term effects.
Under pathological conditions, AGE gradually accumulates, leading to oxidative stress and vascular in ammation. In our study obesity seems to exacerbate the reduction in sRAGE, suggesting that greater fat causes the deterioration of its metabolic status SRAGE can be used as a biomarker to predict obesity and other metabolic diseases.Elena et al. Found that sRAGE is closely related to EAT(epicardial visceral adipose tissues), this may indicate that the relationship between sRAGE and obesity is not only re ected in systemic obesity, but also in local obesity. Since EAT is an important risk factor for cardiovascular disease and a consistent correlation with sRAGE has been observed, sRAGE may be considered as an early marker of potential cardiac metabolic risk. in addition, another study further shows that RAGE can modulate atherosclerosis through obesity, and can participate in the progression of atherosclerosis in non-diabetic states 25 . This also requires further research. At present, some studies focus on the possibility of sRAGE signal as a treatment for obesity and its complications, targeting sRAGE and e sRAGE signal transduction, and believe that it is promised as a bene cial drug treatment in the future, or it may be an important strategy for the treatment of obesity and its complications. Our meta-analysis provides further theoretical support.

Conclusion
In conclusion, the results of this meta-analysis revealed a negative correlation between obesity and sRAGE. However, due to the limited number of articles, further study is needed to provide more support for the relationship between obesity and AGE-RAGE axis. The results may represent a start for more future studies upon the clinical use of sRAGE as an effective medication. Idea conception: HD. Literature retrieval: HD, SZD. Methodology: SZD. Writing-initial manuscript: HD. Review guarantor: QC. All authors participated in the revision of the manuscript and agreed to publish it.   Figure 1 ow diagram. Forest plots for sRAGE and obesity. Regional subgroup analysis