Retinoids and retinoid-binding proteins: Unexpected roles in metabolic disease

Abstract

Alterations in tissue expression levels of both retinol-binding protein 2 (RBP2) and retinol-binding protein 4 (RBP4) have been associated with metabolic disease, specifically with obesity, glucose intolerance and hepatic steatosis. Our laboratories have shown that this involves novel pathways not previously considered as possible linkages between impaired retinoid metabolism and metabolic disease development. We have established both biochemically and structurally that RBP2 binds with very high affinity to very long-chain unsaturated 2-monoacylglycerols like the canonical endocannabinoid 2-arachidonoyl glycerol (2-AG) and other endocannabinoid-like substances. Binding of retinol or 2-MAGs involves the same binding pocket and 2-MAGs are able to displace retinol binding. Consequently, RBP2 is a physiologically relevant binding protein for endocannabinoids and endocannabinoid-like substances and is a nexus where the very potent retinoid and endocannabinoid signaling pathways converge. When Rbp2-null mice are challenged orally with fat, this gives rise to elevated levels in the proximal small intestine of both 2-AG and the incretin hormone glucose-dependent insulinotropic polypeptide (GIP) in the proximal small intestine. We propose that elevation of GIP concentrations upon high fat diet feeding gives rise to obesity and the other elements of metabolic disease seen in Rbp2-null mice. Unexpectedly, we observed that RBP4 is present in secretory granules of the GIP-secreting intestinal K-cells and is co-secreted with GIP in response to a stimulus that provokes GIP secretion. Moreover, RBP4 is co-secreted along with glucagon from pancreatic alpha-cells in response to a secretory stimulus. The association during the secretory process of RBP4 with potent hormones that regulate metabolism (GIP and glucagon) accounts for at least some of the metabolic disease seen upon overexpression of Rbp4.

Keywords: 2-monoacylglycerol; Glucagon-like peptide-1 (GLP-1); Glucose-dependent insulinotropic polypeptide (GIP); Incretin; Obesity; Retinoic acid; Retinoids; Retinol-binding protein; Retinol-binding protein 2 (RBP2); Retinol-binding protein 4 (RBP4 or RBP); Vitamin A.

Fig. 1

The molecular architecture and ligand specificity of RBPs. (A) Structural topology comparison of cellular RBP1, RBP2, and serum RBP4. The lipocalin fold, a universal protein scaffold, features a hydrophobic binding cavity within the β-barrel (depicted in orange mesh). This cavity can be adapted to accommodate different lipid ligands. (B) Spatial orientation of endogenous ligands within the RBP binding pockets. RBP1 exhibits binding specificity exclusively for retinoids (shown in orange), while RBP2 and RBP4 have the ability to interact with 2-AG and fatty acids (both depicted in blue), respectively.

Fig. 2

RBP2 is expressed mainly in colonocytes (A) while L-cells appear deeper in the mucosa and adjacent to goblet cells (B). Anti-RBP2 was obtained from Abcam (ab180494, 1:500) and anti-GLP-1 from Invitrogen (ma5–42868, 1:100).

Fig. 3

RBP2 is a point of convergence inking two potent signaling pathways: retinoid signaling and endocannabinoid or endocannabinoid-like substance signaling.

Fig. 4

Effects of treatment of cultured mouse pancreatic islets obtained from chow fed wild type mice with either 2 μM retinol (ROL) or 10 μM all-trans-retinoic acid (RA) on glucagon release into the medium. The cultured islets were treated for 2 h after isolation/overnight culture in control medium. Significance: * p < 0.05; ** p < 0.01.