Weanling, but not adult, rabbit colon absorbs bile acids: flux is linked to expression of putative bile acid transporters

Hepatic X-Box Binding Protein 1 and Unfolded Protein Response Is Impaired in Weanling Mice With Resultant Hepatic Injury

BACKGROUND AND AIMS

The unfolded protein response (UPR) is a coordinated cellular response to endoplasmic reticulum (ER) stress that functions to maintain cellular homeostasis. When ER stress is unresolved, the UPR can trigger apoptosis. Pathways within the UPR influence bile acid metabolism in adult animal models and adult human liver diseases, however, the UPR has not been studied in young animal models or pediatric liver diseases. In this study we sought to determine whether weanling age mice had altered UPR activation compared with adult mice, which could lead to increased bile acid-induced hepatic injury.

APPROACH AND RESULTS

We demonstrate that after 7 days of cholic acid (CA) feeding to wild-type animals, weanling age mice have a 2-fold greater serum alanine aminotransferase (ALT) levels compared with adult mice, with increased hepatic apoptosis. Weanling mice fed CA have increased hepatic nuclear X-box binding protein 1 spliced (XBP1s) expression, but cannot increase expression of its protective downstream target's ER DNA J domain-containing protein 4 and ER degradation enhancing α-mannoside. In response to tunicamycin induced ER stress, young mice have blunted expression of several UPR pathways compared with adult mice. CA feeding to adult liver-specific XBP1 knockout (LS-XBP1^-/- ) mice, which are unable to resolve hepatic ER stress, leads to increased serum ALT and CCAAT/enhancer binding homologous protein, a proapoptotic UPR molecule, expression to levels similar to CA-fed LS-XBP1^-/- weanlings.

CONCLUSIONS

Weanling mice have attenuated hepatic XBP1 signaling and impaired UPR activation with resultant increased susceptibility to bile acid-induced injury.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Lithocholic acid attenuates cAMP-dependent Cl- secretion in human colonic epithelial T84 cells

Bile acids (BAs) play a complex role in colonic fluid secretion. We showed that dihydroxy BAs, but not the monohydroxy BA lithocholic acid (LCA), stimulate Cl(-) secretion in human colonic T84 cells (Ao M, Sarathy J, Domingue J, Alrefai WA, Rao MC. Am J Physiol Cell Physiol 305: C447-C456, 2013). In this study, we explored the effect of LCA on the action of other secretagogues in T84 cells. While LCA (50 μM, 15 min) drastically (>90%) inhibited FSK-stimulated short-circuit current (Isc), it did not alter carbachol-stimulated Isc LCA did not alter basal Isc, transepithelial resistance, cell viability, or cytotoxicity. LCA's inhibitory effect was dose dependent, acted faster from the apical membrane, rapid, and not immediately reversible. LCA also prevented the Isc stimulated by the cAMP-dependent secretagogues 8-bromo-cAMP, lubiprostone, or chenodeoxycholic acid (CDCA). The LCA inhibitory effect was BA specific, since CDCA, cholic acid, or taurodeoxycholic acid did not alter FSK or carbachol action. While LCA alone had no effect on intracellular cAMP concentration ([cAMP]i), it decreased FSK-stimulated [cAMP]i by 90%. Although LCA caused a small increase in intracellular Ca(2+) concentration ([Ca(2+)]i), chelation by BAPTA-AM did not reverse LCA's effect on Isc LCA action does not appear to involve known BA receptors, farnesoid X receptor, vitamin D receptor, muscarinic acetylcholine receptor M3, or bile acid-specific transmembrane G protein-coupled receptor 5. LCA significantly increased ERK1/2 phosphorylation, which was completely abolished by the MEK inhibitor PD-98059. Surprisingly PD-98059 did not reverse LCA's effect on Isc Finally, although LCA had no effect on basal Isc, nystatin permeabilization studies showed that LCA both stimulates an apical cystic fibrosis transmembrane conductance regulator Cl(-) current and inhibits a basolateral K(+) current. In summary, 50 μM LCA greatly inhibits cAMP-stimulated Cl(-) secretion, making low doses of LCA of potential therapeutic interest for diarrheal diseases.

Human Ontogeny of Drug Transporters: Review and Recommendations of the Pediatric Transporter Working Group

The critical importance of membrane-bound transporters in pharmacotherapy is widely recognized, but little is known about drug transporter activity in children. In this white paper, the Pediatric Transporter Working Group presents a systematic review of the ontogeny of clinically relevant membrane transporters (e.g., SLC, ABC superfamilies) in intestine, liver, and kidney. Different developmental patterns for individual transporters emerge, but much remains unknown. Recommendations to increase our understanding of membrane transporters in pediatric pharmacotherapy are presented.

Characterization of primary structure and tissue expression profile of the chicken apical sodium-dependent bile acid transporter mRNA

The ileal apical sodium-dependent bile acid cotransporter (ASBT) plays an essential role in the absorption of bile acids from intestinal lumina. ASBT cDNA has been cloned from mammalian and fish species, and the primary structure of the protein and expression properties of the mRNA have been characterized. In this study, we identified chicken ASBT mRNA by cDNA cloning. Chicken ASBT cDNA consisted of 91 bp of the 5'-untranslated region, 1,083 bp of the coding region, and 1,896 bp of the 3'-untranslated region. The cDNA encoded a protein of 360 amino acids showing significant sequence identity with mammalian and fish ASBT. The amino acid residues known to participate in the functions of mammalian ASBT were conserved in chicken ASBT. Real-time polymerase chain reaction analysis revealed that chicken ASBT mRNA was expressed at markedly higher levels in the ileum and proximal colon/rectum, relatively lower levels in the kidney, and very low levels in the jejunum and cecum. Expression levels in the ileum markedly increased after hatching, reached the highest levels on day 7 posthatching, and then decreased to adult levels. A similar expression pattern was observed in the proximal colon/rectum except for the significant decrease from day 7 posthatching to day 21 posthatching. These results suggest that chicken ASBT functions as a bile acid transporter in the ileum and proximal colon/rectum, particularly during the early posthatching period.

Chenodeoxycholic acid stimulates Cl(-) secretion via cAMP signaling and increases cystic fibrosis transmembrane conductance regulator phosphorylation in T84 cells

High levels of chenodeoxycholic acid (CDCA) and deoxycholic acid stimulate Cl(-) secretion in mammalian colonic epithelia. While different second messengers have been implicated in this action, the specific signaling pathway has not been fully delineated. Using human colon carcinoma T84 cells, we elucidated this cascade assessing Cl(-) transport by measuring I(-) efflux and short-circuit current (Isc). CDCA (500 μM) rapidly increases I(-) efflux, and we confirmed by Isc that it elicits a larger response when added to the basolateral vs. apical surface. However, preincubation with cytokines increases the monolayer responsiveness to apical addition by 55%. Nystatin permeabilization studies demonstrate that CDCA stimulates an eletrogenic apical Cl(-) but not a basolateral K(+) current. Furthermore, CDCA-induced Isc was inhibited (≥67%) by bumetanide, BaCl2, and the cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor CFTRinh-172. CDCA-stimulated Isc was decreased 43% by the adenylate cyclase inhibitor MDL12330A and CDCA increases intracellular cAMP concentration. The protein kinase A inhibitor H89 and the microtubule disrupting agent nocodazole, respectively, cause 94 and 47% reductions in CDCA-stimulated Isc. Immunoprecipitation with CFTR antibodies, followed by sequential immunoblotting with Pan-phospho and CFTR antibodies, shows that CDCA increases CFTR phosphorylation by approximately twofold. The rapidity and side specificity of the response to CDCA imply a membrane-mediated process. While CDCA effects are not blocked by the muscarinic receptor antagonist atropine, T84 cells possess transcript and protein for the bile acid G protein-coupled receptor TGR5. These results demonstrate for the first time that CDCA activates CFTR via a cAMP-PKA pathway involving microtubules and imply that this occurs via a basolateral membrane receptor.

Ursodeoxycholic acid attenuates colonic epithelial secretory function

Dihydroxy bile acids, such as chenodeoxycholic acid (CDCA), are well known to promote colonic fluid and electrolyte secretion, thereby causing diarrhoea associated with bile acid malabsorption. However, CDCA is rapidly metabolised by colonic bacteria to ursodeoxycholic acid (UDCA), the effects of which on epithelial transport are poorly characterised. Here, we investigated the role of UDCA in the regulation of colonic epithelial secretion. Cl(-) secretion was measured across voltage-clamped monolayers of T84 cells and muscle-stripped sections of mouse or human colon. Cell surface biotinylation was used to assess abundance/surface expression of transport proteins. Acute (15 min) treatment of T84 cells with bilateral UDCA attenuated Cl(-) secretory responses to the Ca(2+) and cAMP-dependent secretagogues carbachol (CCh) and forskolin (FSK) to 14.0 ± 3.8 and 40.2 ± 7.4% of controls, respectively (n = 18, P < 0.001). Investigation of the molecular targets involved revealed that UDCA acts by inhibiting Na(+)/K(+)-ATPase activity and basolateral K(+) channel currents, without altering their cell surface expression. In contrast, intraperitoneal administration of UDCA (25 mg kg(-1)) to mice enhanced agonist-induced colonic secretory responses, an effect we hypothesised to be due to bacterial metabolism of UDCA to lithocholic acid (LCA). Accordingly, LCA (50-200 μm) enhanced agonist-induced secretory responses in vitro and a metabolically stable UDCA analogue, 6α-methyl-UDCA, exerted anti-secretory actions in vitro and in vivo. In conclusion, UDCA exerts direct anti-secretory actions on colonic epithelial cells and metabolically stable derivatives of the bile acid may offer a new approach for treating intestinal diseases associated with diarrhoea.

Lubiprostone activates Cl- secretion via cAMP signaling and increases membrane CFTR in the human colon carcinoma cell line, T84

BACKGROUND

Lubiprostone, used clinically (b.i.d.) to treat constipation, has been reported to increase transepithelial Cl(-) transport in T84 cells by activating ClC-2 channels.

AIM

To identify the underlying signaling pathway, we explored the effects of short-term and overnight lubiprostone treatment on second messenger signaling and Cl(-) transport.

METHODS

Cl(-) transport was assessed either as I(sc) across T84 monolayers grown on Transwells and mounted in Ussing chambers or by the iodide efflux assay. [cAMP](i) was measured by enzyme immunoassay, and [Ca(2+)](i) by Fluo-3 fluorescence. Quantitation of apical cell surface CFTR protein levels was assessed by Western blotting and biotinylation with the EZ-Link Sulfo-NHS-LC-LC-Biotin. ClC-2 mRNA level was studied by RT-PCR.

RESULTS

Lubiprostone and the cAMP stimulator, forskolin, caused comparable and maximal increases of I(sc) in T84 cells. The I(sc) effects of lubiprostone and forskolin were each suppressed if the tissue had previously been treated with the other agent. These responses were unaltered even if the monolayers were treated with lubiprostone overnight. Lubiprostone-induced increases in iodide efflux were ~80% of those obtained with forskolin. Lubiprostone increased [cAMP](i). H89, bumetanide, or CFTR(inh)-172 greatly attenuated lubiprostone-stimulated Cl(-) secretion, whereas the ClC-2 inhibitor CdCl(2) did not. Compared to controls, FSK-treatment increased membrane-associated CFTR by 1.9 fold, and lubiprostone caused a 2.6-fold increase in apical membrane CFTR as seen by immunoblotting following cell surface biotinylation.

CONCLUSIONS

Lubiprostone activates Cl(-) secretion in T84 cells via cAMP, protein kinase A, and by increasing apical membrane CFTR protein.

Physiological concentrations of bile acids down-regulate agonist induced secretion in colonic epithelial cells

In patients with bile acid malabsorption, high concentrations of bile acids enter the colon and stimulate Cl(-) and fluid secretion, thereby causing diarrhoea. However, deoxycholic acid (DCA), the predominant colonic bile acid, is normally present at lower concentrations where its role in regulating transport is unclear. Thus, the current study set out to investigate the effects of physiologically relevant DCA concentrations on colonic epithelial secretory function. Cl(-) secretion was measured as changes in short-circuit current across voltage-clamped T(84) cell monolayers. At high concentrations (0.5-1 mM), DCA acutely stimulated Cl(-) secretion but this effect was associated with cell injury, as evidenced by decreased transepithelial resistance (TER) and increased lactate dehydrogenase (LDH) release. In contrast, chronic (24 hrs) exposure to lower DCA concentrations (10-200 microM) inhibited responses to Ca(2+) and cAMP-dependent secretagogues without altering TER, LDH release, or secretagogue-induced increases in intracellular second messengers. Other bile acids - taurodeoxycholic acid, chenodeoxycholic acid and cholic acid - had similar antisecretory effects. DCA (50 microM) rapidly stimulated phosphorylation of the epidermal growth factor receptor (EGFr) and both ERK and p38 MAPKs (mitogen-activated protein kinases). The EGFr inhibitor, AG1478, and the protein synthesis inhibitor, cycloheximide, reversed the antisecretory effects of DCA, while the MAPK inhibitors, PD98059 and SB203580, did not. In summary, our studies suggest that, in contrast to its acute prosecretory effects at pathophysiological concentrations, lower, physiologically relevant, levels of DCA chronically down-regulate colonic epithelial secretory function. On the basis of these data, we propose a novel role for bile acids as physiological regulators of colonic secretory capacity.

NHE8 is an intracellular cation/H+ exchanger in renal tubules of the yellow fever mosquito Aedes aegypti

The goal of this study was to identify and characterize the hypothesized apical cation/H(+) exchanger responsible for K(+) and/or Na(+) secretion in the renal (Malpighian) tubules of the yellow fever mosquito Aedes aegypti. From Aedes Malpighian tubules, we cloned "AeNHE8," a full-length cDNA encoding an ortholog of mammalian Na(+)/H(+) exchanger 8 (NHE8). The expression of AeNHE8 transcripts is ubiquitous among mosquito tissues and is not enriched in Malpighian tubules. Western blots of Malpighian tubules suggest that AeNHE8 is expressed primarily as an intracellular protein, which was confirmed by immunohistochemical localizations in Malpighian tubules. AeNHE8 immunoreactivity is expressed in principal cells of the secretory, distal segments, where it localizes to a subapical compartment (e.g., vesicles or endosomes), but not in the apical brush border. Furthermore, feeding mosquitoes a blood meal or treating isolated tubules with dibutyryl-cAMP, both of which stimulate a natriuresis by Malpighian tubules, do not influence the intracellular localization of AeNHE8 in principal cells. When expressed heterologously in Xenopus laevis oocytes, AeNHE8 mediates EIPA-sensitive Na/H exchange, in which Li(+) partially and K(+) poorly replace Na(+). The expression of AeNHE8 in Xenopus oocytes is associated with the development of a conductive pathway that closely resembles the known endogenous nonselective cation conductances of Xenopus oocytes. In conclusion, AeNHE8 does not mediate cation/H(+) exchange in the apical membrane of Aedes Malpighian tubules; it is more likely involved with an intracellular function.

Intestinal bile acid physiology and pathophysiology

Bile acids (BAs) have a long established role in fat digestion in the intestine by acting as tensioactives, due to their amphipathic characteristics. BAs are reabsorbed very efficiently by the intestinal epithelium and recycled back to the liver via transport mechanisms that have been largely elucidated. The transport and synthesis of BAs are tightly regulated in part by specific plasma membrane receptors and nuclear receptors. In addition to their primary effect, BAs have been claimed to play a role in gastrointestinal cancer, intestinal inflammation and intestinal ionic transport. BAs are not equivalent in any of these biological activities, and structural requirements have been generally identified. In particular, some BAs may be useful for cancer chemoprevention and perhaps in inflammatory bowel disease, although further research is necessary in this field. This review covers the most recent developments in these aspects of BA intestinal biology.

Role of protein kinase C-delta in the age-dependent secretagogue action of bile acids in mammalian colon

The role of specific PKC isoforms in the regulation of epithelial Cl(-) secretion by Ca(2+)-dependent secretagogues remains controversial. In the developing rabbit distal colon, the bile acid taurodeoxycholate (TDC) acts via intracellular calcium to stimulate Cl(-) transport in adult, but not in young, animals, whereas the PKC activator phorbol dibutyrate (PDB) stimulates Cl(-) transport at all ages. We tested the hypothesis that specific PKC isoforms account for the age-specific effects of TDC. The effects of conventional (cPKC) and novel (nPKC) PKC-specific inhibitors on TDC- and PDB-stimulated Cl(-) transport in adult and weanling colonocytes were assessed by using 6-methoxy-quinolyl acetoethyl ester. In adult colonocytes, the cPKC inhibitor Gö-6976 inhibited PDB action but not TDC action, whereas the cPKC and nPKC inhibitor Gö-6850 blocked both TDC and PDB actions. Additionally, rottlerin and the PKC-delta-specific inhibitor peptide (deltaV1-1) inhibited TDC- and PDB-stimulated Cl(-) transport in adult colonocytes. Rottlerin also decreased TDC-stimulated short-circuit current in intact colonic epithelia. Only Gö-6976, but neither rottlerin nor deltaV1-1, inhibited PDB-stimulated transport in weanling colonocytes. Colonic lysates express PKC-alpha, -lambda, and -iota protein equally at all ages, but they do not express PKC-gamma or -theta at any age. Expression of PKC-beta and PKC-epsilon protein was newborn>adult>weanling, whereas PKC-delta was expressed in adult but not in weanling or newborn colonocytes. TDC (1.6-fold) and PDB (2.0-fold) stimulated PKC-delta enzymatic activity in adult colonocytes but failed to do so in weanling colonocytes. PKC-delta mRNA expression showed age dependence. Thus PKC-delta appears critical for the action of TDC in the adult colon, and its low expression in young animals may account for their inability to secrete in response to bile acids.

Bile acid-induced secretion in polarized monolayers of T84 colonic epithelial cells: Structure-activity relationships

Bile acid epimers and side-chain homologues are present in the human colon. To test whether such bile acids possess secretory activity, cultured T84 colonic epithelial cells were used to quantify the secretory properties of synthetic epimers and homologues of deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA). In our study, chloride secretion was measured as changes in short-circuit current (DeltaI(sc), in microA/cm2) with the use of voltage-clamped monolayers of T84 cells mounted in Ussing chambers. Bile acids were added at 0.5 mM, a concentration that did not alter transepithelial resistance. Data were expressed as peak DeltaI(sc) (means +/- SD). When added bilaterally, DCA stimulated a DeltaI(sc) response of 15.7 +/- 12.5 microA/cm2. The 12beta-OH epimer of DCA was less potent (DeltaI(sc) = 8.0 +/- 1.7 microA/cm2), whereas its 3beta-OH epimer had no effect. CDCA stimulated secretion (DeltaI(sc) = 8.2 +/- 5.5 microA/cm2), whereas both its 7beta-OH and 3beta-OH epimers were inactive, as was lithocholic acid. HomoDCA (1 additional side-chain carbon) was active (DeltaI(sc) = 7.8 +/- 4.8 microA/cm2), whereas norDCA (1 fewer carbon) and dinorDCA (2 fewer carbons) were not. Taurine conjugates of DCA and CDCA stimulated secretion (DeltaI(sc) = 12.3 +/- 7.5 and 8.8 +/- 4.8 microA/cm2, respectively) from the basolateral side but not the apical side. Uptake of taurine conjugates from the basolateral but not the apical side was shown by mass spectrometry. These studies indicate marked structural specificity for bile acid-induced chloride secretion and show that modification of bile acid structure by colonic bacteria modulates the secretory properties of these endogenous secretagogues.