Physiology and Physical Chemistry of Bile Acids

Abstract

Bile acids (BAs) are facial amphiphiles synthesized in the body of all vertebrates. They undergo the enterohepatic circulation: they are produced in the liver, stored in the gallbladder, released in the intestine, taken into the bloodstream and lastly re-absorbed in the liver. During this pathway, BAs are modified in their molecular structure by the action of enzymes and bacteria. Such transformations allow them to acquire the chemical-physical properties needed for fulling several activities including metabolic regulation, antimicrobial functions and solubilization of lipids in digestion. The versatility of BAs in the physiological functions has inspired their use in many bio-applications, making them important tools for active molecule delivery, metabolic disease treatments and emulsification processes in food and drug industries. Moreover, moving over the borders of the biological field, BAs have been largely investigated as building blocks for the construction of supramolecular aggregates having peculiar structural, mechanical, chemical and optical properties. The review starts with a biological analysis of the BAs functions before progressively switching to a general overview of BAs in pharmacology and medicine applications. Lastly the focus moves to the BAs use in material science.

Keywords: bile acid derivatives; bile acids; material science applications; pharmacological application; physiological functions; self-assembly; surfactants.

Figure 1

(a) Planar representation of the general molecular structure of bile acids (Bas). Letters, numbers and labels R_i indicate the rings of the steroid skeleton, the carbon atoms and the functional groups, respectively. (b) Chair representation of the general molecular structure of BAs. Brackets indicate the hydrophobic and hydrophilic faces. (c) Molecular structures of BAs showing different hydroxyl groups on the steroid backbone. (d) Molecular structures of the aminoacid conjugated BAs.

Figure 2

(a) Schematic representation of the BA functions and self-assembly during the lipid digestion and transport in the intestine (adapted from Macierzanka, A.; Torcello-Gómez, A.; Jungnickel, C.; Maldonado-Valderrama, J. Bile Salts in Digestion and Transport of Lipids. Adv. Colloid Interface Sci. 2019, 274, 102045. Ref. [46] with permission from (2019) Elsevier). (b) Transmission Electron Microscopy images of oil-in-water emulsions stabilized by two surfactants with interest in food and drug industry, namely Pluronic F68 and Lecithin (left top). Transmission Electron Microscopy images of the surfactant-emulsion transformation upon BA (left center) and BA + lipase addition (left bottom). Scheme representing the disposition of surfactant-BA-lipase at the oil/water interface (right). Reproduced from Torcello-Gómez, A.; Maldonado-Valderrama, J.; Martín-Rodríguez, A.; McClements, D.J. Physicochemical Properties and Digestibility of Emulsified Lipids in Simulated Intestinal Fluids: Influence of Interfacial Characteristics. Soft Matter 2011, 7, 6167–6177. Ref. [53] with permission from The Royal Society of Chemistry.

Figure 3

(a) Direct antimicrobial mechanism of natural BA: detergent effect of BA on the bacterial membrane. Transmission Electron Microscopy images of S. aureus before (b) and after (c) the interaction with glycocholic acid (GCA) (Sannasiddappa, T.H.; Lund P.A.; Clarke S.R. In Vitro Antibacterial Activity of Unconjugated and Conjugated Bile Salts on Staphylococcus aureus. Front. Microbiol. 2017, 8, 1581 [99] copyright © 2017 Sannasiddappa, Lund, Clarke (CCBY)) (d) Indirect antimicrobial mechanism of natural BA: BAs activate the farnesoid X receptor (FXR) that in turn induces the expression of genes producing toxic molecules for bacteria (Inagaki, T.; Moschetta, A.; Lee, Y.K.; Peng, L.; Zhao, G.; Downes, M.; Yu, R.T.; Shelton, J.M.; Richardson, J.A.; Repa, J.J.; et al. Regulation of Antibacterial Defense in the Small Intestine by the Nuclear Bile Acid Receptor. Proc. Natl. Acad. Sci. USA 2006, 103, 3920–3925. Ref. [64], Copyright (2006) National Academy of Sciences, U.S.A.). (e) Transverse sections of terminal ileum of mice immunostained with anti-occludin antisera (top) and hematoxylin and eosin (H&E, bottom), scale bar 50 µm. Micrographs of control mice (left) are contraposed to micrographs of FXR knockout mice (right). (f) Lymphatic vessel section of FXR knockout mice, scale bar 2 µm. Arrows point to traces of edema and dilated lymphatic vessels that are induced by bacteria. (g) Copolymers of cholic acid (CA) and polyethylene glycol self-assemble into rods that are able to penetrate the bacterial membrane Scanning Electron Microscopy images of E.coli without (h) and with (i) BA polymer treatment (adapted with permission from Rahman, M.A.; Jui, M.S.; Bam, M.; Cha, Y.; Luat, E.; Alabresm, A.; Nagarkatti, M.; Decho, A.W.; Tang, C. Facial Amphiphilicity-Induced Polymer Nanostructures for Antimicrobial Applications ACS Appl. Mater. Interfaces 2020, 12, 21221–21230 [105]. Copyright (2020) American Chemical Society).

Figure 4

(a) Star-shaped block copolymers are able to load the drug Doxorubicin (Dox) through hydrophobic interaction (yellow box) or electrostatic interaction with (blue frame) or without (red frame) oleic acid (OA) as cosurfactant. (b) TEM micrographs of the star-polymer micelles before (upper panel) and after (lower panel) Dox loading. Adapted with permission from Cunningham, A.J.; Robinson, M.; Banquy, X.; Leblond, J.; Zhu, X.X. Bile Acid-Based Drug Delivery Systems for Enhanced Doxorubicin Encapsulation: Comparing Hydrophobic and Ionic Interactions in Drug Loading and Release Mol. Pharm. 2018, 15, 1266–1276. Ref. [118] Copyright 2018 American Chemical Society(c) Block-copolymer formed by cholic acid and b-cyclodextrin residues assembling into a self-healing gel; schematic representation showing the interplay among the residues in the gel matrix (top). (d) Optical (left) and rheological (right) evidence of the gel break and self-healing process (adapted with permission from Jia, Y.G.; Zhu, X.X. Self-Healing Supramolecular Hydrogel Made of Polymers Bearing Cholic Acid and β-Cyclodextrin Pendants. Chem Mater 2015, 27, 1, 387–393. [120] Copyright (2015) American Chemical Society). (e) The block copolymer PNIPAM120-b-PAMPTMA(+)30 when mixed with CA self-assembles into tape-like complexes where single stripes present recurring spacing. (f,g) Cryo TEM images of the tape-like aggregates (adapted from Schillén, K.; Galantini, L.; Du, G.; Del Giudice, A.; Alfredsson, V.; Carnerup, A.M.; Pavel, N.V.; Masci, G.; Nyström, B. Block Copolymers as Bile Salt Sequestrants: Intriguing Structures Formed in a Mixture of an Oppositely Charged Amphiphilic Block Copolymer and Bile Salt. Phys. Chem. Chem. Phys. 2019, 21, 12518–12529. Ref. [141] Published by the PCCP Owner Societies).

Figure 5

(a) Co-assembled lithocholate/taurolithocholate tubes (optical microscopy image, top left) are longitudinally unzipped into flat structures (Atomic Force Microscopy image, top right) by capillary force upon dehydration on substrates. The process of unzipping is proved by optical microscopy images (center) and schematized (bottom) (adapted with permission from Zhang, X.; Bera, T.; Liang, W.; Fang, J. Longitudinal Zipping/Unzipping of Self-Assembled Organic Tubes J. Phys. Chem. B, 2011, 115, 14445–14449. [175] Copyright (2011) American Chemical Society). (b) BA or BA derivatives facially interact with carbon nanotubes’ surface, enabling their dispersion in water solution (scheme, top). Atomic Force Microscopy images of BA derivative dispersed carbon nanotubes in water (bottom right). Graph reporting the carbon nanotubes dispersion efficiency upon addition of BA derivatives, natural BA and conventional head–tail surfactant SDS (dark grey, light grey, black bars, respectively, bottom right) (adapted with permission from Gubitosi, M.; Trillo, J.V.; Alfaro Vargas, A.; Pavel, N.V.; Gazzoli, D.; Sennato, S.; Jover, A.; Meijide, F.; Galantini, L. Characterization of Carbon Nanotube Dispersions in Solutions of Bile Salts and Derivatives Containing Aromatic Substituents J. Phys. Chem. B 2014, 118, 1012–1021. [181] Copyright (2014) American Chemical Society). (c) BA derivative-based scrolls having negative (top left, red frame) and positive charge (bottom left, blue frame) interact with positive and negatively charged microgels, respectively, giving rise to electrostatically stabilized supracolloidal aggregates (right) (from Cautela, J.; Lattanzi, V.; Månsson, L.; Galantini, L.; Crassous, J.J. Sphere–Tubule Superstructures through Supramolecular and Supracolloidal Assembly Pathways Small 2018, 14, 1803215. [199] Copyright (2018) Wiley). (d) Thermoresponsive cholic acid derivatives functionalized with a tert-butyl phenyl amide residue on C-3 (top left) and C-12 (top right). Microscopy images of the structures forming at lower and higher temperatures than the critical transition temperatures for the C-3 (bottom left) and C-12 (bottom right) derivatives (from Galantini, L.; Leggio, C.; Jover, A.; Meijide, F.; Pavel, N.V.; Soto Tellini, V.H.; Vázquez Tato, J.; Di Leonardo, R.; Ruocco, G. Kinetics of Formation of Supramolecular Tubules of a Sodium Cholate Derivative. Soft Matter 2009, 5, 3018–3025. [184] permission conveyed through Copyright Clearance Center, Inc. Cautela, J.; Severoni, E.; Redondo-Gómez, C.; di Gregorio, M.C.; Del Giudice, A.; Sennato, S.; Angelini, R.; D’Abramo, M.; Schillén, K.; Galantini, L. C-12 vs. C-3 Substituted Bile Salts: An Example of the Effects of Substituent Position and Orientation on the Self-Assembly of Steroid Surfactant Isomers. Colloids Surf. B. 2020, 185, 110556. [201] Copyright (2019), with permission from Elsevier.