Functional Characterization of Aebp1 in Metabolism

The obesity pandemic and concomitant rise in type 2 diabetes incidence have underscored the urgent need for better anti-obesity and antidiabetic treatments. Obesity is the result of extended periods of positive energy balance during which caloric intake from food exceeds caloric expenditure. Currently, most therapies for obesity focus on appetite suppression and exercise to increased energy expenditure. Brown adipose tissue specializes in thermogenesis and as such, contributes significantly to energy expenditure. It also plays a major role in the regulation of triglyceride and glucose metabolism. Enteroendocrine L-cells produce and secrete the incretin hormone GLP-1, whose global action is the lowering of blood glucose levels. This peptide hormone exerts a net effect of reducing food uptake by reducing appetite and increasing satiety. Owing to their activity, L-cells and brown fat represent promising targets for the treatment of obesity and type 2 diabetes. Aebp1 (Adipocyte enhancer binding protein 1) is a secreted B-regulatory carboxypeptidase. Initially, it was reported to be a repressor of white adipocyte differentiation through gene regulatory control. Previous studies suggest an involvement of Aebp1 in adiposity and energy homeostasis, as global deletion of Aebp1 led to protection from diet-induced obesity whereas transgenic overexpression enhanced the obese phenotype. Despite these reports, the biochemical and physiological function of Aebp1 remains elusive. In this study we focused on the following aspects of Aebp1: In the first part, we analyzed the functional role of Aebp1 in intestinal L-cell activity. In the second and third part, we investigated the biochemical characteristics of the protein and its implication in white and brown adipocytes formation and their activity. In the beginning, we profiled potential regulators of L-cell activity using a proteomic approach. Aebp1 was identified as a negative regulator of GLP-1 secretion and production in loss-of-function and gain-of-function assays in intestinal L-cell lines. However, most likely the functional role of Aebp1 as a regulator of L-cell activity is dispensable since KO mice with global Aebp1 ablation did not show a phenotype. This assessment is supported by transcriptomic data showing the absence of Aebp1 expression in primary L-cells. Here, we provided strong evidence for the existence of only one full-length Aebp1 isoform (130kDa). Furthermore, we showed that full-length Aebp1 is a circulating protease in human blood. Using in vivo tracing technique, we identified the adrenal cortex and the brown adipose tissue as its preferred target location. We observed dimer/multimer formation of overexpressed Aebp1 in mouse plasma. In addition, we evaluated and confirmed homodimer/-multimer formation of Aebp1 by co-immunoprecipitation. Using standard technique, we failed to confirm a catalytic activity for Aebp1, although our data are directed towards weak activity if any existed. To promote a better understanding of the functional role of Aebp1 in vivo, we generated conditional global and tissue-specific knockout mouse models and characterized them under different metabolic conditions. Adipocyte, brown adipocyte and global inducible Aebp1 ablation revealed that Aebp1 is dispensable for lipid metabolism, glycemic control, thermogenic energy expenditure, and white and brown adipocyte differentiation. Temporally post-developmental global ablation of Aebp1 also failed to protect from diet-induced obesity, therefore disputing previous findings by Ro and colleagues. In light with its high expression in vascular smooth muscle cells and the adrenal, we also tested the functional role of Aebp1 in these two tissues. However, the assessment revealed that systemic ablation of Aebp1 in adult mice did neither affect vascular tone regulation nor cortical hormone synthesis and secretion