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Cadmium Exposure and Toxicity
Published in Debasis Bagchi, Manashi Bagchi, Metal Toxicology Handbook, 2020
Soisungwan Satarug, Kenneth R. Phelps
Retinol-binding protein 4 (RBP4) is synthesized in the liver. Its molecular weight is approximately 21,000, and most circulating RBP4 is bound to transthyretin. This complex is minimally filterable because its molecular weight exceeds 70,000. In contrast, unbound RPB4 is filtered, reabsorbed, and metabolized by proximal tubular cells, and its excessive appearance in urine is an early indicator of reabsorptive dysfunction. RBP4 is more stable in urine than β2MG, and it survives freezing and thawing. Both RBP4 and metabolic products are measured with the urine assay (Norden et al., 2014).
The association between retinol-binding protein 4 and risk of type 2 diabetes: A systematic review and meta-analysis
Published in International Journal of Environmental Health Research, 2023
Xiaomeng Tan, Han Zhang, Limin Liu, Zengli Yu, Xinxin Liu, Lingling Cui, Yao Chen, Huanhuan Zhang, Zhan Gao, Zijian Zhao
Retinol-binding protein 4 (RBP4) was initially considered as a hormone secreted by the liver and a transporter that transports retinol from the liver to peripheral tissues (Quadro et al. 2003). However, a meta-analysis reported that nonalcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), and simple steatosis (SS) patients have not altered RBP4 levels, although there was substantial unexplained between-study heterogeneity (Zhou et al. 2017). Recent research has found that adipocyte is another important source of RBP4, RBP4 was therefore considered as an adipokine (Newcomer and Ong 2000). Animal experiments have proven that RBP4 could change insulin sensitivity. Transgenic overexpression of human RBP4 or injection of purified RBP4 can lead to insulin resistance in mice (Yang et al. 2005). Meanwhile, an increased level of serum RBP4 level was reported to induce the expression of the gluconeogenic enzyme phosphoenol pyruvate carboxykinase and impaire insulin signal transmission in muscles and liver of mouse (Yang et al. 2005), that is the characteristic of diabetes. Conversely, some studies have also observed an opposite correlation between RBP4 and insulin sensitivity in humans (Suh et al. 2010; Meisinger et al. 2011). In addition to insulin resistance, insulin deficiency is also one of the mechanisms leading to diabetes, and islet β cell dysfunction is the main reason for insulin secretion reduction. Researches have reported that RBP4 is related to β cell dysfunction (Broch et al. 2007; Ribel-Madsen et al. 2009). A systematic review and meta-analysis indicated that the levels of circulating RBP4 of T2DM patients with micro/macroalbuminuria or declined estimated glomerular filtration rate (eGFR) were significantly higher, and the levels of circulating RBP4 were positively correlated with albumin-to-createnine ratio (ACR) but negatively related to eGFR (Zhang et al. 2020). Furthermore, epidemiological studies have indicated that elevated RBP4 is positively correlated to T2DM incidence (Jagannathan et al. 2015; Fan et al. 2019; Cho et al. 2020). In contrast, some cross-sectional studies showed that there was no association between RBP4 and T2DM (Chavez et al. 2009; Ulgen et al. 2010; Kaess et al. 2012). Therefore, conclusions about the relationship between RBP4 levels and the risk of T2DM are inconclusive.