Diabetes Care 1 Rong Huang,1,2 Songping Yin,1,2 Circulating Retinol Binding Yongxin Ye,1,2 Nixuan Chen,1,2 Protein 4 Is Inversely Associated Shiyun Luo,1,2 Min Xia,1,2 and Lina Zhao1,2 With Pancreatic b-Cell Function Across the Spectrum of Glycemia https://doi.org/10.2337/dc19-2432 OBJECTIVE The aim of this study was to examine the association of circulating retinol binding protein 4 (RBP4) levels with b-cell function across the spectrum of glucose tolerance from normal to overt type 2 diabetes. RESEARCH DESIGN AND METHODS A total of 291 subjects aged 35–60 with normal glucose tolerance (NGT), newly diagnosed impaired fasting glucose or glucose tolerance (IFG/IGT), and type 2 diabetes were screened by a standard 2-h oral glucose tolerance test (OGTT) with the use of traditional measures to evaluate b-cell function. From these participants, 74 subjects were recruited for an oral minimal model test, and b-cell function was PATHOPHYSIOLOGY/COMPLICATIONS assessed with model-derived indices. Circulating RBP4 levels were measured by a commercially available ELISA kit. RESULTS Circulating RBP4 levels were significantly and inversely correlated with b-cell function indicated by the Stumvoll first-phase and second-phase insulin secretion indexes, but not with HOMA-b, calculated from the 2-h OGTT in 291 subjects across the spectrum of glycemia. The inverse association was also observed in subjects 1Guangdong Provincial Key Laboratory of Food, involved in the oral minimal model test with b-cell function assessed by both direct Nutrition and Health, Guangdong, P.R. China 2 measures and model-derived measures, after adjustment for potential confound- Department of Nutrition, School of Public Health, Sun Yat-sen University, Guangzhou, ers. Moreover, RBP4 emerged as an independent factor of the disposition index- Guangdong Province, P.R. China total insulin secretion. Corresponding authors: Lina Zhao, zhaoln5@ mail.sysu.edu.cn, and Min Xia, xiamin@mail CONCLUSIONS .sysu.edu.cn Circulating RBP4 levels are inversely and independently correlated with b-cell Received 3 December 2019 and accepted 14 function across the spectrum of glycemia, providing another possible explanation of March 2020 the linkage between RBP4 and the pathogenesis of type 2 diabetes. This article contains supplementary material online at Figshare: https://doi.org/10.2337/dc20-1234/ suppl.11985918. Retinol binding protein 4 (RBP4), a 21-kDa protein that belongs to the lipocalin family, S.Y. is currently affiliated with the Clinical Nu- is primarily known as the principal retinol transporter in plasma. It is encoded by the trition Department, The Third People’s Hospital RBP4 gene that maps to chromosome 10q23-q24, a region that has been shown to be of Shenzhen, Shenzhen, Guangdong, P.R. China. associated with an increased risk of type 2 diabetes (1,2). Controversy still exists about © 2020 by the American Diabetes Association. the origin of the increased circulating RBP4 levels. RBP4 was originally recognized as Readers may use this article as long as the work is properly cited, the use is educational and not for an adipokine, with adipocytes being the main source expressing a considerable profit, and the work is not altered. More infor- amount of RBP4 (3). In contrast, other studies identified RBP4 as a hepatokine, with mation is available at https://www.diabetesjournals hepatocytes being the major site for the synthesis and secretion of the circulating .org/content/license. Diabetes Care Publish Ahead of Print, published online April 17, 2020 2 RBP4 and Pancreatic b-Cell Function Diabetes Care RBP4 (4,5). Cross-sectional studies re- Province, China. Subjects were invited to through a face-to-face interview. Smok- ported that circulating RBP4 levels participate in a screening examination ing was defined as at least one cigarette were significantly elevated in both obese for diabetes by the 2-h standard oral per day for .6 months. Alcohol drinking and diabetic murine models and in pa- glucose tolerance test (OGTT), which was defined as drinking any type of tients (6,7). In addition, genetic studies was conducted under overnight fasting alcoholic beverage at least once a support the inductive role for RBP4 in conditions with blood sampling at week for .6 months. Age was deter- causing type 2 diabetes as a gain-of- 0 and 120 min after oral loading of mined to the nearest year, weight and function human nucleotide polymor- 75 g glucose. Subjects with type 2 di- height were determined to the nearest phism in the RBP4 promoter correlated abetes were defined as having a fasting 0.1 kg and 0.1 cm, respectively, and with the increased risk of type 2 diabetes blood glucose level (FBG) of $7.0 resting blood pressure was measured (8). Further prospective cohort studies mmol/L (126 mg/dL) or 2-h glucose with an aneroid sphygmomanometer confirmed the predictive value of circu- level $11.1 mmol/L (200 mg/dL) during on the upper arm using an appropri- lating RBP4 for the incident type 2 di- the OGTT, impaired fasting glucose ately sized cuff. abetes in high-risk populations (9,10). (IFG)ashavinganFBGlevelof5.6– Type 2 diabetes is characterized by 6.9 mmol/L (100–125 mg/dL), and im- Clinical and Biochemical insulin resistance and pancreatic b-cell paired glucose tolerance (IGT) as having Measurements dysfunction. Previous studies found that a 2-h glucose level of 7.8–11.0 mmol/L Fasting lipids and liver enzymes and genetic or pharmacologic elevation of (140–199 mg/dL) according to the Ameri- parameters of renal function in the cir- RBP4 levels in mice resulted in the de- can Diabetes Association guidelines from culation were performed in the Health velopment of insulin resistance, whereas 2010 (20). Pathology Laboratory using an auto- lowering RBP4 levels markedly improved The inclusion criteria of the study mated autoanalyzer (BC5180; Mindray). insulin sensitivity (11,12). However, subjects were local residents aged be- Plasma glucose and HbA1c were mea- these associations are not supported tween 35 and 60 years old and stable sured by the glucose oxidase method and by all studies in humans, as several body weight (change ,610% of current anion exchange high-performance liquid studies found no correlation between body weight) for 3 months before the chromatography, respectively. The esti- circulating RBP4 levels and insulin re- study. Exclusion criteria were known di- mated glomerular filtration rate (eGFR) sistance in type 2 diabetes (13–16). This agnosed diabetes, benign or malignant was calculated using the equation of the discrepancy suggests a limited under- tumor, acute or chronic liver diseases, Chronic Kidney Disease Epidemiology standing of the relationship between history of cardiovascular and cerebrovas- Collaboration, which is based on age, RBP4 and the pathogenesis of type 2 cular diseases, regular therapy with an- sex, ethnicity, and serum creatinine con- diabetes. Although insulin resistance is tihypertensive drug, hypolipidemic agents, centrations (22). The secreted insulin and recognized as the major pathophysiolog- hormonal agents, or antidepressants within C-peptide levels during the tests were ical feature of type 2 diabetes, pancre- 3 months, and women during pregnancy measured using commercial ELISA kits atic b-cell dysfunction seems to be or lactation. (intraassay coefficient of variation of ,8%) predominant for the transition from After anthropometric measurement, (Mercodia, Uppsala, Sweden). Serum full- specific metabolic dysfunctional status inquisition of detailed medical and clinic length RBP4 levels were measured with a to type 2 diabetes (17–19). However, record, and 2-h standard OGTT test, we commercial ELISA kit (AdipoGen, Seoul, little is currently known about the asso- finally recruited 291 subjects, including Korea) according to the manufacturer’s ciation of RBP4 levels with pancreatic 105 NGT subjects, 115 IFG/IGT subjects, instructions and compared with purified b-cell function. and 71 subjects with diabetes in the human RBP4 standards. The mean intra- To gain a better understanding of the final analysis. Among these subjects, 74 assay and interassay coefficients of pathophysiological actions of RBP4 in (21NGT,26IFG/IGT,and27withdiabetes) variation were 1.363–11.41% and 2.419– the development of type 2 diabetes, volunteered to take part in the 3-h oral 13.85%, respectively. we evaluated the potential relationship minimal model tests with sampling at 0, between serum RBP4 levels and b-cell 10, 20, 30, 60, 90, 120, and 180 min after a Assessment of b-Cell Function function in subjects across glycemic sta- 75-g oral glucose challenge (17,21). This Direct Measurements tuses from normal glucose tolerance to study complied with the Declaration of The traditional surrogates of insulin sen- overt diabetes by using both traditional Helsinki and was approved by the School sitivity and b-cell function were calcu- measures and the oral minimal model– of Public Health Institutional Review lated from the OGTT data. Insulin sensitivity derivedindexestoevaluateb-cell Board of Sun Yat-sen University. All sub- was evaluated by the HOMA of insulin function. jects provided written informed consent resistance (HOMA-IR) index and the before data collection. quantitative insulinsensitivitycheck RESEARCH DESIGN AND METHODS index(QUICKI)model:1/(log[Ins0]1 Study Design and Participants Data Collection log[Gluc0]) (23). b-Cell function was Recruitment A standardized questionnaire including calculated by the HOMA of b-cell func- The study subjects were recruited be- general information on the examination tion (HOMA-b) and the Stumvoll for- tween July 2018 and November 2018 via date of demographic characteristics, mula including 1)theStumvollfirst- phase posters and advertisements in local and physical activity, smoking habits, alcohol insulin secretion index: 2,032 1 4.681 3 social media at the Community Health Ser- consumption, family history of diabetes, Ins0 2135.0 3Gluc120 10.995 3Ins120 1 vice Center in Dongguan City, Guangdong and medication history was conducted 27.99 3 BMI 2 269.1 3 Gluc0; and 2) care.diabetesjournals.org Huang and Associates 3 the Stumvoll second-phase insulin secre- ranges).