Apical plasma membrane vesicles formed from organ donor colon demonstrate Na+ and H+ conductances and Na+/H+ exchange

S-acylation modulates the function of the apical sodium-dependent bile acid transporter in human cells

The ileal apical sodium-dependent bile acid transporter (ASBT) is crucial for the enterohepatic circulation of bile acids. ASBT function is rapidly regulated by several posttranslational modifications. One reversible posttranslational modification is S-acylation, involving the covalent attachment of fatty acids to cysteine residues in proteins. However, whether S-acylation affects ASBT function and membrane expression has not been determined. Using the acyl resin-assisted capture method, we found that the majority of ASBT (∼80%) was S-acylated in ileal brush border membrane vesicles from human organ donors, as well as in HEK293 cells stably transfected with ASBT (2BT cells). Metabolic labeling with alkyne-palmitic acid (100 μm for 15 h) also showed that ASBT is S-acylated in 2BT cells. Incubation with the acyltransferase inhibitor 2-bromopalmitate (25 μm for 15 h) significantly reduced ASBT S-acylation, function, and levels on the plasma membrane. Treatment of 2BT cells with saturated palmitic acid (100 μm for 15 h) increased ASBT function, whereas treatment with unsaturated oleic acid significantly reduced ASBT function. Metabolic labeling with alkyne-oleic acid (100 μm for 15 h) revealed that oleic acid attaches to ASBT, suggesting that unsaturated fatty acids may decrease ASBT's function via a direct covalent interaction with ASBT. We also identified Cys-314 as a potential S-acylation site. In conclusion, these results provide evidence that S-acylation is involved in the modulation of ASBT function. These findings underscore the potential for unsaturated fatty acids to reduce ASBT function, which may be useful in disorders in which bile acid toxicity is implicated.

Modulation of ileal bile acid transporter (ASBT) activity by depletion of plasma membrane cholesterol: association with lipid rafts

Apical sodium-dependent bile acid transporter (ASBT) represents a highly efficient conservation mechanism of bile acids via mediation of their active transport across the luminal membrane of terminal ileum. To gain insight into the cellular regulation of ASBT, we investigated the association of ASBT with cholesterol and sphingolipid-enriched specialized plasma membrane microdomains known as lipid rafts and examined the role of membrane cholesterol in maintaining ASBT function. Human embryonic kidney (HEK)-293 cells stably transfected with human ASBT, human ileal brush-border membrane vesicles, and human intestinal epithelial Caco-2 cells were utilized for these studies. Floatation experiments on Optiprep density gradients demonstrated the association of ASBT protein with lipid rafts. Disruption of lipid rafts by depletion of membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) significantly reduced the association of ASBT with lipid rafts, which was paralleled by a decrease in ASBT activity in Caco-2 and HEK-293 cells treated with MbetaCD. The inhibition in ASBT activity by MbetaCD was blocked in the cells treated with MbetaCD-cholesterol complexes. Kinetic analysis revealed that MbetaCD treatment decreased the V(max) of the transporter, which was not associated with alteration in the plasma membrane expression of ASBT. Our study illustrates that cholesterol content of lipid rafts is essential for the optimal activity of ASBT and support the association of ASBT with lipid rafts. These findings suggest a novel mechanism by which ASBT activity may be rapidly modulated by alterations in cholesterol content of plasma membrane and thus have important implications in processes related to maintenance of bile acid and cholesterol homeostasis.

Function, expression, and characterization of the serotonin transporter in the native human intestine

The enteric serotonin transporter (SERT) plays a critical role in modulating serotonin availability and thus has been implicated in the pathogenesis of various intestinal disorders. To date, SERT expression and function in the human intestine have not been investigated. Current studies were designed to characterize the function, expression, distribution, and membrane localization of SERT in the native human intestine. Real-time PCR studies showed relatively higher SERT mRNA expression in the human small intestine compared with colon (ileum >> duodenum >> jejunum). Northern blot analysis revealed three mRNA hybridizing species encoding SERT (3.0, 4.9, and 6.8 kb) in the human ileum. Consistent with SERT mRNA expression, SERT immunostaining was mainly detected in the epithelial cells of human duodenal and ileal resected tissues. Notably, SERT expression was localized predominantly to the apical and intracellular compartments and was distributed throughout the crypt-villus axis. Immunoblotting studies detected a prominent protein band ( approximately 70 kDa) in the ileal apical plasma membrane vesicles (AMVs) isolated from mucosa obtained from organ-donor intestine. Functional studies showed that uptake of [(3)H]serotonin (150 nM) in human ileal AMVs was 1) significantly increased in the presence of both Na(+) and Cl(-); 2) inhibited ( approximately 50%) by the neuronal SERT inhibitor, fluoxetine (10 microM) and by unlabeled 5-HT; and 3) exhibited saturation kinetics indicating the presence of a carrier-mediated process. Our studies demonstrated differential expression of SERT across various regions of the human intestine and provide evidence for the existence of a functional SERT capable of removing intraluminal serotonin in human ileal epithelial cells.

Expression and membrane localization of MCT isoforms along the length of the human intestine

Recent studies from our laboratory and others have demonstrated the involvement of monocarboxylate transporter (MCT)1 in the luminal uptake of short-chain fatty acids (SCFAs) in the human intestine. Functional studies from our laboratory previously demonstrated kinetically distinct SCFA transporters on the apical and basolateral membranes of human colonocytes. Although apical SCFA uptake is mediated by the MCT1 isoform, the molecular identity of the basolateral membrane SCFA transporter(s) and whether this transporter is encoded by another MCT isoform is not known. The present studies were designed to assess the expression and membrane localization of different MCT isoforms in human small intestine and colon. Immunoblotting was performed with the purified apical and basolateral membranes from human intestinal mucosa obtained from organ donor intestine. Immunohistochemistry studies were done on paraffin-embedded sections of human colonic biopsy samples. Immunoblotting studies detected a protein band of approximately 39 kDa for MCT1, predominantly in the apical membranes. The relative abundance of MCT1 mRNA and protein increased along the length of the human intestine. MCT4 (54 kDa) and MCT5 (54 kDa) isoforms showed basolateral localization and were highly expressed in the distal colon. Immunohistochemical studies confirmed that human MCT1 antibody labeling was confined to the apical membranes, whereas MCT5 antibody staining was restricted to the basolateral membranes of the colonocytes. We speculate that distinct MCT isoforms may be involved in SCFA transport across the apical or basolateral membranes in polarized colonic epithelial cells.

Evidence for the existence of a distinct SO(4)(--)-OH(-) exchange mechanism in the human proximal colonic apical membrane vesicles and its possible role in chloride transport

Recent studies have demonstrated that mutations in human downregulated in adenoma gene (DRA) result in congenital chloride diarrhea (CLD), and that DRA may be involved in chloride transport across the intestinal apical domains. DRA is highly homologous to sulfate transporters, but not to any member of the anion exchanger gene family (AEs). Our previous studies have characterized the existence of a distinct Cl(-)-OH(-) (HCO(3)(-)) exchanger, with minimal affinity for sulfate in the human colonic apical membrane vesicles (AMV). However, the mechanism(s) of sulfate movement across the colonocyte plasma membranes in the human colon is not well understood. Current studies were undertaken to elucidate sulfate transport pathways in AMVs of human proximal colon. Purified AMV and rapid filtration (35)SO(4)(--) uptake techniques were used. Our results demonstrate the presence of a pH gradient-driven carrier-mediated SO(4)(--)-OH(-) exchange process in the human proximal colonic luminal membranes based on the following: a marked increase in the SO(4)(--) uptake in the presence of an outwardly directed OH(-) gradient; a significant inhibition of SO(4)(--) uptake by the membrane anion transport inhibitor, DIDS; demonstration of saturation kinetics (K(m) for SO(4)(--): 0.80 +/- 0.17 mM and Vmax 649 +/- 74 pmol/mg protein/10 sec); competitive inhibition of SO(4)(--)-OH(-) exchange by oxalate; SO(4)(--) uptake was insensitive to alterations in the membrane potential; and inwardly directed Na(+) gradient under non-pH gradient conditions did not stimulate SO(4)(--) uptake. SO(4)(--) uptake was significantly inhibited by increasing concentrations of chloride (1-10 mM) in the incubation media with a K(i) for Cl(-) of 9.3 +/- 1.4 mM. In contrast, OH(-)/HCO(3)(-) gradient-driven (36)Cl(-) uptake into these vesicles was unaffected by increasing concentrations of sulfate (10-50 mM). The above data indicate that two distinct transporters may be involved in SO(4)(--) and Cl(-) transport in the human intestinal apical membranes: an anion exchanger with high affinity for SO(4)(--) and oxalate but low affinity for Cl(-), and a distinct Cl(-)-OH(-) (HCO(3)(-)) exchanger with low affinity for SO(4)(--).

Human intestinal anion exchanger isoforms: expression, distribution, and membrane localization

A family of anion exchangers (AEs) including AE1, AE2 and AE3 has been described. AE3 gene has been shown to encode two alternatively spliced isoforms termed as bAE3 (brain subtype) and cAE3 (cardiac subtype). The identity of the AE(s) involved in the human intestinal NaCl absorption is not fully understood. Current studies were undertaken to identify the AE isoforms expressed in the human intestine, to define their regional and vertical axis (crypt vs. surface cells) distribution, and to elucidate their membrane localization in the epithelial cells along the entire length of the human intestine. Our studies utilizing reverse transcription (RT)-PCR with total RNA extracted from pinch biopsies from various regions of the human intestine demonstrate that AE2 and bAE3 but not AE1 or cAE3 were expressed in all the regions of the human intestine. Utilizing in situ RT-PCR, we demonstrated that the message of AE2 was expressed throughout the vertical surface--crypt axis of the colon. Our Western blotting studies demonstrated that AE2 and bAE3 are localized to the basolateral but not the apical membranes of the intestinal epithelial cells from the human ileum and colon. In conclusion, our results demonstrated that in the human intestine, AE2 and bAE3, but not AE1 or cAE3, are expressed throughout the tract with the highest expression in the colon compared to the ileum and jejunum. Both the isoforms were found to be localized to the basolateral but not the apical membranes of the epithelial cells. We speculate that, in the human intestine, AE2 and bAE3 may be the 'housekeeping' isoforms, and the apical AE, the potential candidate for chloride absorption, remains to be identified.

Hormonal regulation of chicken intestinal NHE and SGLT-1 activities

The effects of aldosterone and arginine vasotocin (AVT) on intestinal Na(+)/H(+) exchange (NHE) and Na(+)-sugar cotransport (SGLT-1) activities have been investigated using brush-border membrane vesicles isolated from Hubbard chicken small and large intestines, and they were compared with those induced by either Na(+) depletion or dehydration. Na(+) depletion was induced by feeding the chickens with either a low- or a high-Na(+) diet for either 0.5, 1, 2, 4, or 8 days. Ileal and colonic NHE2 activity increased with the duration of the Na(+) depletion, whereas that of intestinal SGLT-1 decreased, reaching a plateau after 2 days of treatment. Three-hour incubation of the intestine with aldosterone produced the same effects on NHE activity as does Na(+) depletion, without altering SGLT-1 activity. However, 3-h incubation of the intestine with AVT increased intestinal SGLT-1 activity, without affecting intestinal NHE activity. It is concluded that aldosterone regulates apical ileal and colonic NHE2 activity, whereas that of SGLT-1 is regulated by AVT.

The characterization of butyrate transport across pig and human colonic luminal membrane

1. Luminal membrane vesicles (LMV) were isolated from human and pig colonic tissues. They were characterized in terms of purity and ability to transport [14C]butyrate. 2. The activity of cysteine-sensitive alkaline phosphatase, and the abundance of villin, NHE2 and NHE3 proteins, markers of the colonic luminal membrane, were significantly enriched in the LMV compared with the original cellular homogenate. The LMV were free from contamination by other cellular organelles and basolateral membranes, as revealed by the negligible presence of either specific marker enzyme activity or characteristic immunogenic protein. 3. The transport of butyrate into the luminal membrane vesicles was enhanced 5-fold at pH 5.5 compared with pH 8.0. Butyrate transport was temperature dependent, and was stimulated in the presence of an outward-directed anion gradient in the order of butyrate > bicarbonate > propionate > chloride. Kinetic analysis of increasing substrate concentration showed saturation kinetics with an apparent Km value of 14.8 +/- 3.6 mM and a Vmax of 54 +/- 14 nmol min-1 (mg protein)-1. 4. Butyrate transport was significantly reduced in the presence of short chain fatty acids (SCFA), acetate, propionate and other monocarboxylates (pyruvate and L-lactate). Butyrate uptake was inhibited by several cysteine group modifying reagents such as p-chloromercuribenzosulphonic acid (pCMBS), p-chloromercuribenzoate (pCMB), mersalyl acid and HgCl2, but not by the stilbene anion exchange inhibitors, 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS) and 4,4'-dinitrostilbene-2,2'-disulphonate (SITS). 5. The described properties of butyrate transport across the luminal pole of the colon suggest the involvement of a carrier protein, in the form of a pH-activated anion exchange process. The transporter is distinct from the erythrocyte band-3 type anion exchanger and may belong to the monocarboxylate-type transport proteins (MCT1).

Na+-H+ exchange and intracellular pH regulation in colonocytes from the chick

The involvement of Na(+)-H+ exchange in chicken colonocyte homeostasis was investigated. Colonocyte pH (pH(i)) was measured with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). The proton ionophore FCCP reduced basal pH(i), indicating that cytosolic [H+] is not at electrochemical equilibrium across the membrane. External Na+ removal decreased pH(i) and subsequent addition of Na+ returns pH(i) towards its control value. The rate of pH(i) recovery from an acid load was Na(+)-dependent (K(m) for Na+, 24 mM) and inhibited by EIPA (IC50, 0.18 microM). The initial rate of Na(+)-dependent cell alkalization increased as the pH(i) decreased from 7.2 to 6.6 (Hill coefficient, 1.88). Radioisotope flux studies revealed that an outwardly directed proton gradient transiently stimulated Na+ uptake into BBMV isolated from the chick colon. EIPA and amiloride inhibited pH gradient-driven Na+ uptake (IC50 of 4 microM and 32 microM, respectively). The K(m) for Na+ of pH gradient-driven Na+ uptake was 6.8 mM. The Hill coefficient of the relationship between the initial rate of pH-driven Na+ uptake and the intravesicular pH was 0.70. It is concluded that a Na(+)-H+ exchanger is involved in pH(i) homeostasis in chicken colonocytes and that these cells possess at least two types of Na(+)-H+ antiporters with different sensitivity to EIPA and different kinetic parameters.

Chloride transport in human proximal colonic apical membrane vesicles

The mechanism(s) of Cl- transport across the human colonic apical membranes are not well understood. Apical membrane vesicles (AMV) purified from organ donor proximal colonic mucosa and a rapid millipore filtration technique were utilized to study 36Cl- uptake into these vesicles. Outwardly directed OH- and HCO3- gradient stimulated 36Cl- uptake into these vesicles demonstrating a transient accumulation over equilibrium uptake. Voltage clamping the membrane potential of the vesicles or making them inside positive with K+/valinomycin failed to influence chloride uptake, indicating that the conductive Cl- uptake pathway is minimal in proximal colonic AMV. Anion exchange inhibitors, DIDS and SITS (1 mM) inhibited OH- and HCO3- stimulated 36Cl- uptake by approximately 60%. Furosemide also demonstrated a small but significant inhibition of chloride uptake. Amiloride, bumetanide and acetazolamide (1 mM) failed to inhibit 36Cl uptake. HCO3- and pH gradient stimulated 36Cl- uptake exhibited saturation kinetics with an apparent Km for chloride of 4.0 +/- 0.7 mM and Vmax of 17.8 +/- 3.9 nmol/mg per min. Bromide, chloride, nitrate and acetate (50 mM each) inhibited 5 mM 36Cl uptake. Inwardly directed gradients of Na+, K+, or Na+ and K+ did not stimulate 36Cl- uptake into these vesicles, indicating that uptake of Na+ and Cl- in human proximal colonic AMV does not involve Na-Cl or Na-K-2Cl cotransport. The above findings indicate that chloride transport in human proximal colonic AMV involves an electroneutral Cl-HCO3- (OH-) exchange process. In view of the previous demonstration of Na+-H+ antiporter in these vesicles, dual ion exchange mechanism of Na+-H+ and Cl-HCO3- in apical membrane domain of human colonocytes is postulated to be the primary mechanism for NaCl absorption in the human proximal colon.

Calcium transport mechanism in human colonic apical membrane vesicles

BACKGROUND & AIMS

Recent studies have shown the role of human colon in the absorption of calcium, especially in the presence of severe disease or resection of the small bowel. The aim of the current study was to explore the mechanism(s) of calcium uptake by the purified human colonic apical membrane vesicles.

METHODS

Apical membrane vesicles were purified from mucosal scrapings of colons from organ donors, and 45Ca uptake was measured using a rapid filtration technique.

RESULTS

The majority of the 45Ca associated with vesicles represented uptake into closed intravesicular space, whereas the remaining 45Ca uptake represented binding to the vesicles. Ca2+ uptake was found to be dependent on time, pH, temperature, and ionic strength of the incubation medium and inhibitable by ruthenium red, La3+, and ethylene glycol-bis(beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid. Experiments of the effects of membrane potential generated by K+/valinomycin or anion substitutions on Ca2+ uptake showed that the Ca2+ uptake process was potential insensitive. Calcium uptake was unaffected by outwardly directed H+, K+, and Na+ gradients. Ca2+ uptake showed saturation kinetics with no significant differences in Michaelis constant and maximum velocity values of this transporter between proximal and distal colonic segments.

CONCLUSIONS

The uptake of Ca2+ by human colonic apical membranes involves predominantly a carrier-mediated transport mechanism.

Mercaptopropionate inhibits butyrate uptake in isolated apical membrane vesicles of the rat distal colon

BACKGROUND/AIMS

Previous observations have shown that mercapto- and bromo- short-chain fatty acids diminish fatty acid use in colonic epithelium. The aim of this study was to investigate whether this effect is attributable to the inhibition of short-chain fatty acid uptake.

METHODS

Apical membrane vesicles of rat colonocytes were prepared by a discontinuous sucrose gradient after isolation of membrane caps. [14C]butyrate uptake was measured by rapid filtration technique.

RESULTS

Preloading of isolated apical membrane vesicles with bicarbonate or butyrate stimulated [14C]butyrate uptake and resulted in up to fivefold overshoots. Increasing extravesicular butyrate concentrations saturated the bicarbonate-stimulated butyrate uptake with a binding constant of 44.7 +/- 5.9 mmol/L and a maximum velocity of 33.2 +/- 2.7 nmol.mg protein-1.3 s-1. Intravesicular butyrate uptake was inhibited by addition of 20 mmol/L 3-mercaptopropionate (43.0% +/- 5.6%), whereas 2-bromo-propionate (13.9% +/- 4.1%) and 4-bromobutyrate (22.6% +/- 5.3%) did not significantly alter butyrate uptake. Increasing concentrations of 3-mercaptopropionate had a competitive inhibitory effect on butyrate uptake with a binding constant following inhibition of 6.25 +/- 0.87 mmol/L and a maximum velocity of 5.82 +/- 1.01 nmol.mg protein-1.3 s-1.

CONCLUSIONS

Butyrate uptake in apical membrane vesicles of rat distal colon is mediated by a low-affinity anion transport system, which can be competitively inhibited by 3-mercaptopropionate but not by 2-bromopropionate and 4-bromobutyrate.

Mechanisms of Na+ transport in human distal colonic apical membrane vesicles

Apical membrane vesicles purified from mucosal scrapings obtained from distal segments of organ donor colons and a 22Na-uptake technique were used to characterize the mechanism(s) of Na+ transport into these vesicles. An outwardly directed H+ gradient (pH 5.5in/7.5out) markedly increased uptake of 22Na into these vesicles. Osmolarity studies demonstrated that 22Na was taken up into the intravesicular space with minimal binding observed to the surface of the vesicles. Voltage clamping in the presence of K+/valinomycin reduced the H+ gradient-dependent 22Na uptake into these vesicles by approximately 45% and generation of an inside negative membrane potential significantly increased 22Na uptake. Under non voltage clamped conditions, H+ gradient-dependent 22Na uptake into these vesicles was significantly inhibited by specific inhibitors of Na(+)-H+ exchange (DMA, HMA and EIPA) as well as by inhibitor of epithelial Na+ channels (phenamil). Under voltage clamped conditions, H+ gradient-dependent 22Na uptake, however, was unaffected by phenamil (20 microM), but was almost completely inhibited by DMA, HMA and EIPA (20 microM each). The mechanism of amiloride inhibition of electroneutral Na(+)-H+ exchange was noncompetitive with a Ki for amiloride of 340 microM. Electroneutral 22Na uptake exhibited saturation kinetics with an apparent Km for Na+ of 8.7 +/- 1.7 mM and a Vmax of 2.02 +/- 0.45 nmol/mg per 5 s. The Na(+)-H+ exchange demonstrated cation specificity similar to the Na(+)-H+ exchangers described in other epithelia. These studies demonstrate for the first time that Na+ transport across the apical membranes of human distal colon involves both conductive Na+ uptake and an electroneutral Na(+)-H+ exchange process.

Active transport of benzo[a]pyrene in apical membrane vesicles from normal human intestinal epithelium

Transport of the carcinogen benzo[a]pyrene in apical membrane vesicles (AMV) from normal human intestine, was investigated. Benzo[a]pyrene transport was found in AMV throughout the small intestine, but was greatest in colon. Evidence suggesting involvement of P-Glycoprotein (P-Gp), included (1) comparable transport of P-Gp substrate doxorubicin, (2) transport stimulation by ATP and (3) transport suppression by the P-Gp inhibitor, verapamil.

Na+ transport in human proximal colonic apical membrane vesicles

BACKGROUND/AIMS

The mechanisms of Na+ movement across colonocyte plasma membranes in the human colon are not well understood. Current studies were undertaken to investigate Na+ transport pathways in apical membranes of proximal organ donor colons.

METHODS

Purified apical membrane vesicles and rapid filtration 22Na-uptake techniques were used.

RESULTS

An outwardly directed H(+)-gradient (pH 5.5 in/7.5 out) increased 22Na uptake into these vesicles. H+ gradient-driven 22Na uptake was significantly reduced by voltage clamping with K+/valinomycin, but was significantly stimulated by creation of an inside-negative potential. Potential sensitive 22Na uptake was inhibited by Na+ channel inhibitors phenamil and benzamil. Electroneutral 22Na uptake was insensitive to phenamil and benzamil, but was inhibited by amiloride, 5-(N,N-dimethyl)amiloride, 5-(N,N-hexamethylene)amiloride, and 5-(N-ethyl-N-isopropyl)amiloride. Electroneutral 22Na uptake showed saturation kinetics with an apparent Michaelis constant for Na+ of 11.8 +/- 2.4 mmol/L and a maximal velocity of 2.5 +/- 0.6 nmol.mg protein-1 x 5 s-1. The mechanism of amiloride inhibition was noncompetitive with an inhibitor constant for amiloride of 325 mumol/L. Acetazolamide, furosemide, bumetanide, 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stibene, and 4,4'-di-isothiocyanatostilbene-2,2'-disulfonic acid (1 mmol/L each) failed to inhibit 22Na uptake. Li+ and NH4+ (but not Cs+, K+, or choline+) inhibited H(+)-gradient driven 22Na uptake.

CONCLUSIONS

Na+ transport in human proximal colonic apical membrane vesicles involves both conductive Na+ transport and an electroneutral Na(+)-H+ exchange.