Two fatty acid‐binding proteins expressed in the intestine interact differently with endocannabinoids

Abstract

Two different members of the fatty acid‐binding protein (FABP) family are found in enterocyte cells of the gastrointestinal system, namely liver‐type and intestinal fatty acid‐binding proteins (LFABP and IFABP, also called FABP1 and FABP2, respectively). Striking phenotypic differences have been observed in knockout mice for either protein, for example, high fat‐fed IFABP‐null mice remained lean, whereas LFABP‐null mice were obese, correlating with differences in food intake. This finding prompted us to investigate the role each protein plays in directing the specificity of binding to ligands involved in appetite regulation, such as fatty acid ethanolamides and related endocannabinoids. We determined the binding affinities for nine structurally related ligands using a fluorescence competition assay, revealing tighter binding to IFABP than LFABP for all ligands tested. We found that the head group of the ligand had more impact on binding affinity than the alkyl chain, with the strongest binding observed for the carboxyl group, followed by the amide, and then the glycerol ester. These trends were confirmed using two‐dimensional 1H–15N nuclear magnetic resonance (NMR) to monitor chemical shift perturbation of the protein backbone resonances upon titration with ligand. Interestingly, the NMR data revealed that different residues of IFABP were involved in the coordination of endocannabinoids than those implicated for fatty acids, whereas the same residues of LFABP were involved for both classes of ligand. In addition, we identified residues that are uniquely affected by binding of all types of ligand to IFABP, suggesting a rationale for its tighter binding affinity compared with LFABP.

Drugs Form Ternary Complexes with Human Liver Fatty Acid Binding Protein (FABP1) and FABP1 Binding Alters Drug Metabolism

“…In the preliminary experiments, majority of the hFABP1 was observed bound with CPMs after nickel purification and before gel filtration and delipidation treatments (Figure 1A). Lipidex-5000 and butanol extraction have been reported to efficiently delipidate FABP1 (Velkov et al ., 2008; Wang et al ., 2017; Lai et al ., 2020). Various levels of CPMs were removed with individual treatments with Lipidex-5000, 1:1 (v/v) butanol and 1:3 (v/v) butanol based on the native MS analysis (Figure 1).…”

The Multifunctional Family of Mammalian Fatty Acid–Binding Proteins

“…The NMR solution structure of FABP1 with bound monoolein resembles that of the FA-liganded protein, with two bound ligands, while bile salts bind with a 1:1 stoichiometry ( 41 ). Like other members of the FABP family, FABP1 is involved in the intracellular transport of endocannabinoids (ECs) and related molecules ( 97 ). FABP1 is the predominant EC-binding protein in the liver and is proposed to be involved in the cellular uptake and transport of ECs to intracellular degradation pathways ( 120 ).…”

“…FABP2, similar to FABP1, binds hypolipidemic drugs and promotes the activation of PPARα; nevertheless, they both differentially modulate PPARα activation in a ligand selective manner ( 72 ). FABP2 also binds endocannabinoids, but with a different affinity compared with that of FABP1 ( 97 ). As a whole, these studies are suggestive of different roles for FABP1 and FABP2 in the intestine and open the possibility for the development of targeted delivery of drugs in different tissues.…”

“…FABP6 has a higher affinity for bile acids, and FABP1 as well as other family members are now shown to bind other hydrophobic molecules including lysophospholipids, fatty acyl coenzyme A, cholesterol, bile acids, heme, and monoacylglycerols. In the last decade, several studies have demonstrated that FABPs can also bind and transport endocannabinoids, cannabinoid-like molecules, and phytocannabinoids, thus expanding the spectrum of FABP functions to include endocannabinoid signaling processes ( 34 , 80 , 97 ). FABPs bind several classes of lipophilic drugs, and FABP1 and FABP2, among other FABPs, were found to bind hypolipidemic fibrates, with differential binding specificities and consequent PPAR activation ( 72 ).…”

Mechanisms of endocannabinoid transport in the brain

“…Mechanistically, FABP1 deletion elevates circulating levels of arachidonic acid, which is postulated to diffuse into the brain and increase the levels of anandamide and 2‐AG precursors leading to augmented anandamide and 2‐AG biosynthesis (Martin et al, 2016). FABP2, an FABP isoform expressed in the intestine, was likewise demonstrated to interact with anandamide and 2‐AG albeit with low affinity (Lai et al, 2020). Although not a binding protein, caveolae‐mediated endocytosis has been proposed as another mechanism that contributes to anandamide internalization (McFarland et al, 2004, 2008).…”