An explanation for recent discrepancies in levels of human circulating betatrophin

To the Editor: In addition to hyperglycaemia and hyperinsulinaemia, type 2 diabetes mellitus is often associated with hypertriglyceridaemia. Recently, we and others have identified a novel circulating factor, referred to as lipasin [1], refeeding induced in fat and liver (RIFL) [2], angiopoietin-like protein 8 (ANGPTL8) [3, 4], and betatrophin [5], which may have a dual role in mediating both triacylglycerol metabolism and glucose homeostasis. Lipasin mRNA levels in liver and fat, where it is predominantly expressed, are suppressed by fasting and highly induced by feeding and insulin resistance [1–3, 5]. Serum triacylglycerol levels are increased in lipasin overexpressing mice and decreased in lipasin knockout mice, and this likely occurs through a mechanism involving regulation of the activity of lipoprotein lipase (LPL) either directly [1] or indirectly by promoting ANGPTL3 cleavage [3]. Indeed, the knockout mice exhibit higher LPL activity [4]. The discovery of betatrophin as a potent and specific stimulator of pancreatic beta cell proliferation has drawn significant attention because it represents an intriguing therapeutic target for promoting beta cell regeneration in both type 1 and type 2 diabetes [5]. As a logical next step to understanding the roles of betatrophin in human physiology and pathology, there is a growing interest in examining circulating levels of the protein in humans [6–9]. Betatrophin levels have been found to be increased in type 1 [6] and type 2 diabetes [8, 9], and to be associated with atherogenic lipid profiles [7] and BMI [9]. However, there are notable discrepancies among results of these studies. For instance, levels of fasting betatrophin (1) ranged from 0.3 ng/ml [6] to 2.2 ng/ml [9] in lean and nondiabetic individuals; (2) were uncorrelated [6, 7] or positively correlated with BMI [9]; and (3) were either unaltered [7] or increased in type 2 diabetes [8, 9]. We propose that all these results can be correct, but they reflect different betatrophin species, because different ELISA kits, although accurate and reliable, can generate different results because of betatrophin proteolytic regulation [10]. We compared the betatrophin kits manufactured by EIAAB (Catalogue no. E1164H; Wuhan, China) and Phoenix Pharmaceuticals (Catalogue no. EK-051-55; Burlingame, CA, USA). When measuring a human recombinant betatrophin (EIAAB), both kits were accurate, and the correlation coefficient of values based on the two kits was 0.99 by linear regression (Fig. 1a). We next examined circulating betatrophin levels in sera collected by Valley Biomedical (Winchester, VA, USA) from 30 human male individuals (BMI range 17.8 to 58.1 kg/m) following overnight fasting; aliquoted samples were saved at −80°C. The study participants were given informed consent and the study protocol was approved by the Institutional Review Boards of Wayne State University. Based on the Phoenix kit, the mean serum betatrophin level in lean individuals (BMI <25 kg/m) was significantly lower than that in overweight and obese individuals (BMI ≥25), (2.36±0.25 ng/ml vs 3.63±0.29 ng/ml [mean±SEM]; Z. Fu :A. B. Abou-Samra :R. Zhang (*) Center for Molecular Medicine and Genetics, School of Medicine, Wayne State University, 540 East Canfield Street, Detroit, MI 48201, USA e-mail: rzhang@med.wayne.edu