RBP2 Biology and Pathobiology

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ABSTRACT The goal of this project has been and remains to gain understanding of the biochemical basis for our observation that retinol-binding protein 2 (RBP2) has an unsuspected role in the maintenance of body weight, normal responses to a glucose challenge, and normal fasting hepatic triglyceride levels. We found that when maintained solely on a control chow diet, 6-7-month-old Rbp2-deficient (Rbp2-/-) mice accrue significantly more body weight as white adipose tissue (WAT), respond significantly less well to a glucose challenge, and possess significantly more hepatic fat than matched wild type (WT) littermate controls. These phenotypes were fully recapitulated by younger 55-day old Rbp2-/- and matched WT mice fed a high fat diet for 6-7 weeks. Our published work establishes that both RBP2 and retinol-binding protein 4 (RBP4) are expressed in enteroendocrine cells (EECs) within the gut, including by glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) expressing cells. Rbp2-/- mice show elevated circulating GIP levels in response to an oral fat challenge. We further showed that RBP2 binds long-chain unsaturated 2- monoacylglycerol (2-MAGs) with an equally high affinity as it does retinol and is consequently a physiologically relevant 2-MAG-binding protein as well as a retinoid-binding protein. We identified that Rbp4-/- mice have significantly diminished circulating GIP concentrations both fasting and after an oral fat challenge compared to WT controls. RBP4 localizes to EEC secretory granules and is co-secreted along with GIP upon stimulation of cultured EEC-enriched primary mouse intestinal cells. Since elevated GIP levels are associated with increased body weight and adipose mass in both humans and animal models, we hypothesize that this contributes to the metabolic phenotypes seen for Rbp2-/- mice. We are proposing 3 Specific Aims exploring the actions of RBP2, RBP4, retinoid- and 2-MAG signaling in regulating EEC activities and synthesis/secretion of GIP and GLP-1. Specific Aim 1 will assess the involvement of RBP2, all-trans-retinoic acid (ATRA) and ATRA-RAR signaling, and RBP4 in modulating EEC responses. This entails both in vivo and in vitro studies. Specific Aim 2 will assess the involvement of 2-MAGs and enzymes responsible for 2-MAG synthesis (diacylglycerol lipase (DAGL)), 2-MAG degradation (monoacylglycerol lipase (MAGL) and α/β hydrolase domain 6 and 12 (ABDH6 and ADBH12)) and 2-MAG actions (the cell surface cannabinoid receptors 1 and 2 (Cb1 and Cb2) and GPR119 receptors) in modulating K-cell responses to dietary nutrient availability. These studies will make use of the same methodologies employed in Specific Aim 1. Specific Aim 3 will provide greater understanding of whether RBP2 has a direct role in GLP-1 synthesis/secretion in both the small intestine and in the colon. Although we have reported the presence of RBP2 protein in L-cells, unlike GIP and K-cells, we have not systematically studied GLP-1 or L-cell biology from the perspective of what role RBP2 may have in this biology.