Neutral Na-amino acid cotransport is differentially regulated by glucocorticoids in the normal and chronically inflamed rabbit small intestine

Molecular Mechanism of Stimulation of Na-K-ATPase by Leukotriene D4 in Intestinal Epithelial Cells

Na-K-ATPase provides a favorable transcellular Na gradient required for the functioning of Na-dependent nutrient transporters in intestinal epithelial cells. The primary metabolite for enterocytes is glutamine, which is absorbed via Na-glutamine co-transporter (SN2; SLC38A5) in intestinal crypt cells. SN2 activity is stimulated during chronic intestinal inflammation, at least in part, secondarily to the stimulation of Na-K-ATPase activity. Leukotriene D4 (LTD4) is known to be elevated in the mucosa during chronic enteritis, but the way in which it may regulate Na-K-ATPase is not known. In an in vitro model of rat intestinal epithelial cells (IEC-18), Na-K-ATPase activity was significantly stimulated by LTD4. As LTD4 mediates its action via Ca-dependent protein kinase C (PKC), Ca levels were measured and were found to be increased. Phorbol 12-myristate 13-acetate (PMA), an activator of PKC, also mediated stimulation of Na-K-ATPase like LTD4, while BAPTA-AM (Ca chelator) and calphostin-C (Cal-C; PKC inhibitor) prevented the stimulation of Na-K-ATPase activity. LTD4 caused a significant increase in mRNA and plasma membrane protein expression of Na-K-ATPase α1 and β1 subunits, which was prevented by calphostin-C. These data demonstrate that LTD4 stimulates Na-K-ATPase in intestinal crypt cells secondarily to the transcriptional increase of Na-K-ATPase α1 and β1 subunits, mediated via the Ca-activated PKC pathway.

Inducible Nitric Oxide Regulates Na-Glucose Co-transport in a Spontaneous SAMP1/YitFc Mouse Model of Chronic Ileitis

In mammalian small intestine, glucose is primarily absorbed via Na-dependent glucose co-transporter (SGLT1) on the brush border membrane (BBM) of absorptive villus cells. Malabsorption of nutrients (e.g., glucose) leads to malnutrition, a common symptom of inflammatory bowel disease (IBD), where the mucosa is characterized by chronic inflammation. Inducible nitric oxide (iNO) is known to be elevated in IBD mucosa. SAMP1/YitFc (SAMP1) mouse is a spontaneous model of chronic ileitis that develops lesions in its terminal ileum, very similar to human IBD. How SGLT1 may be affected in SAMP1 model of chronic ileitis is unknown. Ten-week-old SAMP1 mice with AKR mice as control were treated with N6-(1-iminoethyl)-L-lysine dihydrochloride (L-NIL) to inhibit iNO production. Intracellular NO levels were found to be increased in villus cells from SAMP1 mice. Moreover, SGLT1 and Na^+/K⁺-ATPase activities and BBM SGLT1 expression were significantly decreased. However, L-NIL treatment reduced the intracellular iNO production, and reversed both downregulated SGLT1 and Na⁺/K⁺-ATPase activities in SAMP1 mice. Inhibition of iNO by L-NIL treatment also significantly reversed the BBM SGLT1 protein expression in SAMP1 mice. L-NIL reversed the inflammation mediated downregulation of SGLT1 activity by restoring the BBM SGLT1 expression. Thus, regulation of SGLT1 in chronic ileitis is likely mediated by iNO.

Mechanisms of Regulation of Transporters of Amino Acid Absorption in Inflammatory Bowel Diseases

Intestinal absorption of dietary amino acids/peptides is essential for protein homeostasis, which in turn is crucial for maintaining health as well as restoration of health from significant diseases. Dietary amino acids/peptides are absorbed by unique transporter processes present in the brush border membrane of absorptive villus cells, which line the entire length of the intestine. To date, the only nutrient absorptive system described in the secretory crypt cells in the mammalian intestine is the one that absorbs the amino acid glutamine. Majority of the amino acid transporters are sodium dependent and therefore require basolateral membrane Na-K-ATPase to maintain an efficient transcellular Na gradient for their activity. These transport processes are tightly regulated by various cellular and molecular mechanisms that facilitate their optimal activity during normal physiological processes. Malabsorption of amino acids, recently described in pathophysiological states such as in inflammatory bowel disease (IBD), is undoubtedly responsible for the debilitating symptoms of IBD such as malnutrition, weight loss and ultimately a failure to thrive. Also recently, in vivo models of IBD and in vitro studies have demonstrated that specific immune-inflammatory mediators/pathways regulate specific amino acid transporters. This provides possibilities to derive novel nutrition and immune-based treatment options for conditions such as IBD. © 2020 American Physiological Society. Compr Physiol 10:673-686, 2020.

Effects of glucocorticoids on the gene expression of nutrient transporters in different rabbit intestinal segments

Glucocorticoids (GCs) are counterregulatory hormones with broad effects on the digestion and absorption of dietary carbohydrates, lipids and proteins, but the underlying molecular mechanisms of these effects remain unclear. The present experiment was conducted to investigate the main expression sites of nutrient transporters and the effects of GCs on the gene expression of these transporters in the rabbit small intestine. The results showed that peptide transporter 1 (PepT1), facultative amino acid transporter (rBAT), neutral amino acid transporter (B0AT), excitatory amino acid transporter 3 (EAAT3), sodium-glucose transporter 1 (SGLT1) and glucose transporter 5 (GLUT5) were mainly expressed in the distal segment, glucose transporter 2 (GLUT2) and fatty-acid-binding protein 4 (FATP4) were mainly expressed in the proximal segment and cationic amino acid transporter 1 (CAT1) was mainly expressed in the middle segment of the rabbit small intestine. In addition, we analysed the effects of 3 h (short-term) or 7 days (long-term) dexamethasone (DEX) treatment on the gene expression of most nutrient transporters. The results showed that short-term DEX treatment significantly decreased PepT1, B0AT, EAAT3, rBAT and SGLT1 expressions in all small intestinal segments, while it significantly decreased GLUT2 in the duodenum and FATP4 in the duodenum and ileum (P < 0.05). Long-term DEX treatment also significantly decreased PepT1, CAT1, B0AT, EAAT3, rBAT and SGLT1 in all small intestinal segments and significantly decreased GLUT2 in the jejunum and FATP4 in the ileum (P < 0.05). In conclusion, DEX could decrease the gene expression of most nutrient transporters (except GLUT5) and affect the transport of intestinal amino acids, monosaccharides and fatty acids.

Unique Regulation of Enterocyte Brush Border Membrane Na-Glutamine and Na-Alanine Co-Transport by Peroxynitrite during Chronic Intestinal Inflammation

Na-amino acid co-transporters (NaAAcT) are uniquely affected in rabbit intestinal villus cell brush border membrane (BBM) during chronic intestinal inflammation. Specifically, Na-alanine co-transport (ASCT1) is inhibited secondary to a reduction in the affinity of the co-transporter for alanine, whereas Na-glutamine co-transport (B0AT1) is inhibited secondary to a reduction in BBM co-transporter numbers. During chronic intestinal inflammation, there is abundant production of the potent oxidant peroxynitrite (OONO). However, whether OONO mediates the unique alteration in NaAAcT in intestinal epithelial cells during chronic intestinal inflammation is unknown. In this study, ASCT1 and B0AT1 were inhibited by OONO in vitro. The mechanism of inhibition of ASCT1 by OONO was secondary to a reduction in the affinity of the co-transporter for alanine, and secondary to a reduction in the number of co-transporters for B0AT1, which were further confirmed by Western blot analyses. In conclusion, peroxynitrite inhibited both BBM ASCT1 and B0AT1 in intestinal epithelial cells but by different mechanisms. These alterations in the villus cells are similar to those seen in the rabbit model of chronic enteritis. Therefore, this study indicates that peroxynitrite may mediate the inhibition of ASCT1 and B0AT1 during inflammation, when OONO levels are known to be elevated in the mucosa.

Unique regulation of Na-glutamine cotransporter SN2/SNAT5 in rabbit intestinal crypt cells during chronic enteritis

The only Na‐nutrient cotransporter described in mammalian small intestinal crypt cells is SN2/SNAT5, which facilitates glutamine uptake. In a rabbit model of chronic intestinal inflammation, SN2 stimulation is secondary to an increase in affinity of the cotransporter for glutamine. However, the immune regulation of SN2 in the crypt cells during chronic intestinal inflammation is unknown. We sought to determine the mechanism of regulation of Na‐nutrient cotransporter SN2 by arachidonic acid metabolites in crypt cells. The small intestines of New Zealand white male rabbits were inflamed via inoculation with Eimeria magna oocytes. After 2‐week incubation, control and inflamed rabbits were subjected to intramuscular injections of arachidonyl trifluoromethyl ketone (ATK), piroxicam and MK886 for 48 hrs. After injections, the rabbits were euthanized and crypt cells from small intestines were harvested and used. Results: Treatment of rabbits with ATK prevented the release of AA and reversed stimulation of SN2. Inhibition of cyclooxygenase (COX) with piroxicam did not affect stimulation of SN2. However, inhibition of lipoxygenase (LOX) with MK886, thus reducing leukotriene formation during chronic enteritis, reversed the stimulation of SN2. Kinetic studies showed that the mechanism of restoration of SN2 by ATK or MK886 was secondary to the restoration of the affinity of the cotransporter for glutamine. For all treatment conditions, Western blot analysis revealed no change in SN2 protein levels. COX inhibition proved ineffective at reversing the stimulation of SN2. Thus, this study provides evidence that SN2 stimulation in crypt cells is mediated by the leukotriene pathway during chronic intestinal inflammation.

Milk Intolerance, Beta-Casein and Lactose

True lactose intolerance (symptoms stemming from lactose malabsorption) is less common than is widely perceived, and should be viewed as just one potential cause of cows’ milk intolerance. There is increasing evidence that A1 beta-casein, a protein produced by a major proportion of European-origin cattle but not purebred Asian or African cattle, is also associated with cows’ milk intolerance. In humans, digestion of bovine A1 beta-casein, but not the alternative A2 beta-casein, releases beta-casomorphin-7, which activates μ-opioid receptors expressed throughout the gastrointestinal tract and body. Studies in rodents show that milk containing A1 beta-casein significantly increases gastrointestinal transit time, production of dipeptidyl peptidase-4 and the inflammatory marker myeloperoxidase compared with milk containing A2 beta-casein. Co-administration of the opioid receptor antagonist naloxone blocks the myeloperoxidase and gastrointestinal motility effects, indicating opioid signaling pathway involvement. In humans, a double-blind, randomized cross-over study showed that participants consuming A1 beta-casein type cows’ milk experienced statistically significantly higher Bristol stool values compared with those receiving A2 beta-casein milk. Additionally, a statistically significant positive association between abdominal pain and stool consistency was observed when participants consumed the A1 but not the A2 diet. Further studies of the role of A1 beta-casein in milk intolerance are needed.

Mast cell regulation of Na-glutamine co-transporters B0AT1 in villus and SN2 in crypt cells during chronic intestinal inflammation

Background

In the chronically inflamed rabbit small intestine, brush border membrane (BBM) Na-glutamine co-transport is inhibited in villus cells (mediated by B0AT1), while it is stimulated in crypt cells (mediated by SN2/SNAT5). How mast cells, known to be enhanced in the chronically inflamed intestine, may regulate B0AT1 in villus and SN2/SNAT5 in crypt cell is unknown. Thus, the aim of the present study is to determine the regulation of B0AT1 and SN2/SNAT5 by mast cells during chronic enteritis.

Methods

Chronic intestinal inflammation was induced in male rabbits with intra-gastric inoculation of Eimeria magna oocytes. Rabbits with chronic inflammation were treated with ketotifen (10 mg/day) or saline (Placebo) for 2 days. Villus and crypts cells were isolated from the rabbit intestine using the Ca++ chelation technique. Na/K-ATPase activity was measured as Pi from cellular homogenate. BBM vesicles (BBMV) were prepared from villus and crypt cells and uptake studies were performed using rapid filtration technique with ³H-Glutamine. Western blot analyses were done using B0AT1 and SN2 specific antibodies.

Results

In villus cells, Na-glutamine co-transport inhibition observed during inflammation was completely reversed by ketotifen, a mast cell stabilizer. In contrast, in crypt cells, Na-glutamine co-transport stimulation was reversed to normal levels by ketotifen. Kinetic studies demonstrated that ketotifen reversed the inhibition of B0AT1 in villus cells by restoring co-transporter numbers in the BBM, whereas the stimulation of SN2/SNAT5 in crypts cells was reversed secondary to restoration of affinity of the co-transporter. Western blot analysis showed that ketotifen restored immune-reactive levels of B0AT1 in villus cells, while SN2/SNAT5 levels from crypts cell remained unchanged.

Conclusion

In the present study we demonstrate that mast cells likely function as a common upstream immune pathway regulator of the Na-dependent glutamine co-transporters, B0AT1 in villus cells and SN2 in crypts cells that are uniquely altered in the chronically inflamed small intestine.

Chronic and selective inhibition of basolateral membrane Na-K-ATPase uniquely regulates brush border membrane Na absorption in intestinal epithelial cells

Na-K-ATPase, an integral membrane protein in mammalian cells, is responsible for maintaining the favorable intracellular Na gradient necessary to promote Na-coupled solute cotransport processes [e.g., Na-glucose cotransport (SGLT1)]. Inhibition of brush border membrane (BBM) SGLT1 is, at least in part, due to the diminished Na-K-ATPase in villus cells from chronically inflamed rabbit intestine. The aim of the present study was to determine the effect of Na-K-ATPase inhibition on the two major BBM Na absorptive pathways, specifically Na-glucose cotransport and Na/H exchange (NHE), in intestinal epithelial (IEC-18) cells. Na-K-ATPase was inhibited using 1 mM ouabain or siRNA for Na-K-ATPase-α1 in IEC-18 cells. SGLT1 activity was determined as 3-O-methyl-D-[(3)H]glucose uptake. Na-K-ATPase activity was measured as the amount of inorganic phosphate released. Treatment with ouabain resulted in SGLT1 inhibition at 1 h but stimulation at 24 h. To further characterize this unexpected stimulation of SGLT1, siRNA silencing was utilized to inhibit Na-K-ATPase-α1. SGLT1 activity was significantly upregulated by Na-K-ATPase silencing, while NHE3 activity remained unaltered. Kinetics showed that the mechanism of stimulation of SGLT1 activity was secondary to an increase in affinity of the cotransporter for glucose without a change in the number of cotransporters. Molecular studies demonstrated that the mechanism of stimulation was not secondary to altered BBM SGLT1 protein levels. Chronic and direct silencing of basolateral Na-K-ATPase uniquely regulates BBM Na absorptive pathways in intestinal epithelial cells. Specifically, while BBM NHE3 is unaffected, SGLT1 is stimulated secondary to enhanced affinity of the cotransporter.

Inducible nitric oxide regulates intestinal glutamine assimilation during chronic intestinal inflammation

To facilitate assimilation of glutamine, different Na-dependent glutamine absorptive pathways are present in the rabbit small intestine, specifically B0AT1 in villus and SN2 in crypt cell brush border membrane. Further, both are uniquely regulated in the chronically inflamed intestine. B0AT1 is inhibited secondary to reduced number of brush border membrane (BBM) co-transporters while SN2 is stimulated secondary to an increased affinity for glutamine. These unique changes are reversible by treatment with a broad spectrum immune modulator such as glucocorticoids. However, whether inducible nitric oxide (iNO), known to be elevated in the mucosa of the chronically inflamed intestine, may be responsible for these co-transporter alterations is not known. In the present study, treatment of chronically inflamed rabbits with L-NIL, a selective inhibitor of iNO synthase, reversed the inhibition of B0AT1 in villus and the stimulation of SN2 in crypt cells. At the level of the co-transporter in the brush border membrane, inhibition of iNO production reversed the inhibition of villus B0AT1 by restoring the co-transporter numbers while the stimulation of crypt SN2 was reversed back to normal by restoring its affinity for glutamine. Western blot analyses of BBM proteins also confirmed the kinetic studies. Thus, L-NIL treatment restores the uniquely altered Na-glutamine co-transporters in the enterocytes of chronically inflamed intestine. All these data indicate that iNO functions as an upstream immune modulator directly regulating glutamine assimilation during chronic intestinal inflammation.

Protein kinase C-mediated phosphorylation of RKIP regulates inhibition of Na-alanine cotransport by leukotriene D(4) in intestinal epithelial cells

Leukotriene D4 (LTD4) is an important immune inflammatory mediator that is known to be elevated in the mucosa of chronically inflamed intestine and alter nutrient absorption. LTD4 inhibits Na-alanine cotransport in intestinal epithelial cells by decreasing the affinity of the cotransporter ASCT1. LTD4 is known to increase intracellular Ca(++) and cAMP concentrations. However, the intracellular signaling mechanism of LTD4-mediated ASCT1 inhibition is unknown. In the present study, pretreatment with calcium chelator BAPTA-AM or inhibition of Ca(++)-dependent protein kinase C (PKC), specifically PKCα, resulted in the reversal of LTD4-mediated inhibition of ASCT1, revealing the involvement of the Ca(++)-activated PKC pathway. PKCα is known to phosphorylate Raf kinase inhibitor protein (RKIP), thus activating its downstream signaling pathway. Immunoblotting with anti-RKIP-Ser(153) antibody showed an increase in phosphorylation levels of RKIP in LTD4-treated cells. Downregulation of endogenous RKIP showed no decrease in ASCT1 activity by LTD4, thus confirming its involvement in ASCT1 regulation. Phosphorylation of RKIP by PKC is known to activate different signaling pathways, and in this study it was found to activate cAMP-activated protein kinase A (PKA) pathway. Although protein abundance of ASCT1 was not altered in any of the experimental conditions, there was an increase in the levels of phosphothreonine in ASCT1 protein, thus showing that phosphorylation changes were responsible for the altered affinity of ASCT1 by LTD4. In conclusion, LTD4 inhibits ASCT1 through PKC-mediated phosphorylation of RKIP, leading to the subsequent activation of PKA pathway, possibly through β2-andrenergic receptor activation.

Regulation of sodium-glutamine cotransport in villus and crypt cells by glucocorticoids during chronic enteritis

BACKGROUND

Assimilation of the preferred nutrient of enterocytes is mediated primarily by sodium (Na)-dependent cotransport (NGct) in the intestine. The predominant NGcT in villus cells, B0AT1, is inhibited secondary to a decrease in cotransporter numbers during chronic intestinal inflammation. In contrast, NGcT mediated by SN2 in crypt cells is stimulated secondary to increased affinity of the cotransporter for glutamine during chronic ileitis. Glucocorticoid is a mainstay of treatment for inflammatory bowel disease. However, its effect on NGcT is not known.

METHODS

The inhibition of B0AT1 in villus cells during chronic intestinal inflammation was reversed back to normal by methylprednisolone (MP). This was secondary to the restoration of the cotransporter numbers in the brush border membrane rather than an alteration in the affinity. The stimulation of NGcT in crypt cells during chronic ileitis was also restored back to its normal levels by MP treatment. This reversal was secondary to the restoration of the altered affinity of the cotransporter SN2 for glutamine.

RESULTS

Kinetic studies and western blot analysis were consistent with functional studies for both cotransporters. Thus, glucocorticoids restore two uniquely altered Na-glutamine cotransporters, B0AT1 in villus and SN2 in crypt cells during chronic enteritis.

CONCLUSIONS

These data indicate that glucocorticoids function as an upstream broad spectrum immune modulator in the chronically inflamed intestine.

Prostaglandins, not the leukotrienes, regulate Cl(-)/HCO(3)(-) exchange (DRA, SLC26A3) in villus cells in the chronically inflamed rabbit ileum

Previously studies have demonstrated that Cl(-)/HCO(3)(-) exchange was inhibited during chronic intestinal inflammation secondary to decrease in the affinity of the exchanger for Cl(-) rather than the number of transporters. Arachidonic acid metabolites (AAM) are elevated in the mucosa of the chronically inflamed small intestine. However, their role in the alteration of Cl(-)/HCO(3)(-) during chronic enteritis was unknown. Inhibition of AAM formation with arachidonyl trifluoro methylketone (ATMK) in chronically inflamed rabbit intestine reversed the diminished Cl(-)/HCO(3)(-) exchange activity. Kinetics studies showed that the reversal was secondary to restoration of the altered affinity of transporter. Downstream regulation of Cl(-)/HCO(3)(-) inhibition by AAM was determined to be by the cyclooxygenase pathway since only inhibition of cyclooxygenase with piroxicam treatment reversed the inhibited Cl(-)/HCO(3)(-) exchange. Further, DRA was shown to be the primary Cl(-)/HCO(3)(-) exchanger in villus cells. Kinetics and molecular studies indicated that the mechanism of inhibition of Cl(-)/HCO(3)(-) exchange by cyclooxygenase pathway metabolites was secondary to diminished affinity of the transporter for Cl(-) without a change in DRA BBM expression. Thus our data indicated that cyclooxygenase pathway metabolites mediate the inhibition of DRA during chronic intestinal inflammation.

Transcriptional regulation of the intestinal luminal Na⁺ and Cl⁻ transporters

The epithelial apical membrane Na+/H+ exchangers [NHE (sodium hydrogen exchanger)2 and NHE3] and Cl-/HCO3- exchangers [DRA (down-regulated in adenoma) and PAT-1 (putative anion transporter 1)] are key luminal membrane transporters involved in electroneutral NaCl absorption in the mammalian intestine. During the last decade, there has been a surge of studies focusing on the short-term regulation of these electrolyte transporters, particularly for NHE3 regulation. However, the long-term regulation of the electrolyte transporters, involving transcriptional mechanisms and transcription factors that govern their basal regulation or dysregulation in diseased states, has only now started to unfold with the cloning and characterization of their gene promoters. The present review provides a detailed analysis of the core promoters of NHE2, NHE3, DRA and PAT-1 and outlines the transcription factors involved in their basal regulation as well as in response to both physiological (butyrate, protein kinases and probiotics) and pathophysiological (cytokines and high levels of serotonin) stimuli. The information available on the transcriptional regulation of the recently identified NHE8 isoform is also highlighted. Therefore the present review bridges a gap in our knowledge of the transcriptional mechanisms underlying the alterations in the gene expression of intestinal epithelial luminal membrane Na+ and Cl- transporters involved in electroneutral NaCl absorption. An understanding of the mechanisms of the modulation of gene expression of these transporters is important for a better assessment of the pathophysiology of diarrhoea associated with inflammatory and infectious diseases and may aid in designing better management protocols.

Transcriptional inhibition of intestinal NHE8 expression by glucocorticoids involves Pax5

Sodium/hydrogen exchangers (NHEs) are a family of proteins that transport sodium ions into the cells by moving protons out of the cells. They play a major role in sodium absorption, cell volume regulation, and intracellular pH regulation. Three out of nine identified NHEs (NHE2, NHE3, and NHE8) are expressed on the apical membrane of intestinal epithelial cells. Glucocorticoids have been found to regulate NHE3 function in the intestine, but it is unknown if they have a similar function on NHE8 expression. Interestingly, high glucocorticoid levels in the intestine coincide chronologically with the change from high expression of NHE8 to high expression of NHE3. Studies were performed to explore the role of glucocorticoids on NHE8 expression during intestinal maturation. Brush-border membrane vesicles were isolated from intestinal epithelia, and Western blotting was performed to determine NHE8 protein expression of suckling male rats treated with methylpredisolone. Real-time PCR was used to quantitate NHE8 mRNA expression in rats and Caco-2 cells. Human NHE8 promoter activity was characterized through transfection of Caco-2 cells. Gel mobility shift assays (GMSAs) were used to identify the promoter sequences and the transcription factors involved in glucocorticoid-mediated regulation. Our results showed that the expression of NHE8 mRNA and protein was decreased in glucocorticoid-treated rats and human intestinal epithelial cells (Caco-2). The activity of the human NHE8 gene promoter transfected in Caco-2 cells was also reduced by glucocorticoid treatment. GMSAs suggested that the reduction in promoter activity in the presence of glucocorticoids was due to enhanced transcription factor Pax5 binding on the NHE8 proximal promoter region. In conclusion, this study showed that glucocorticoids inhibit NHE8 gene expression by increasing Pax5 binding on NHE8 gene promoter, suggesting an important role for Pax5 during intestinal maturation.

Glucocorticoids differentially regulate Na-bile acid cotransport in normal and chronically inflamed rabbit ileal villus cells

Previous studies have demonstrated that apical Na-bile acid cotransport (ASBT) is inhibited during chronic ileitis by both a decrease in the affinity as well as a decrease in the number of cotransporters. Methylprednisolone (MP), a commonly used treatment for inflammatory bowel disease (IBD, e.g., Crohn's disease), has been shown to reverse the inhibition of several other Na-solute cotransporters during chronic enteritis. However, the effect of MP on ASBT in the chronically inflamed ileum is not known. MP stimulated ASBT in villus cells from the normal rabbit ileum by increasing the cotransporter expression without a change in the affinity of the cotransporter for bile acid. Western blot studies demonstrated an increase in cotransporter expression. MP reversed the inhibition of ASBT in villus cells from the chronically inflamed ileum. Kinetic studies demonstrated that the mechanism of MP-mediated reversal of ASBT inhibition was secondary to a restoration of both affinity as well as cotransporter numbers. Western blot analysis demonstrated restoration of cotransporter numbers after MP treatment of rabbits with chronic ileitis. Thus MP stimulates ASBT in the normal ileum by increasing cotransporter numbers. MP reverses the inhibition of ASBT during chronic ileitis. However, MP restores the diminished affinity as well as cotransporter expression levels during chronic ileitis. Thus MP differentially regulates ASBT in the normal and in the chronically inflamed ileum.

Even low-grade inflammation impacts on small intestinal function

Independent of the cause and location, inflammation - even when minimal - has clear effects on gastrointestinal morphology and function. These result in altered digestion, absorption and barrier function. There is evidence of reduced villus height and crypt depth, increased permeability, as well as altered sugar and peptide absorption in the small intestine after induction of inflammation in experimental models, which is supported by some clinical data. Identification of inflammatory factors which may promote the process of gastrointestinal dysfunction as well as clinical research to verify experimental observations of inflammatory modulation of gastrointestinal function are required. Moreover, nutritional strategies to support functional restitution are needed.

Molecular bases of impaired water and ion movements in inflammatory bowel diseases

The intestine is dedicated to the absorption of water and nutrients. Fine tuning of this process is necessary to maintain an adequate balance and inflammation disrupts the equilibrium. This review summarizes the current evidence in this field. Classical mechanisms proposed include alteration of epithelial integrity, augmented secretion, and reduced absorption. In addition, intestinal inflammation is associated with defects in epithelial barrier function. However, our understanding of the phenomenon has been complicated by the fact that ionic secretion is in fact diminished in vivo, even after inflammation has subsided. Inhibited ionic secretion can be reversed partially or totally in vitro by maneuvers such as blockade of inducible nitric oxide synthase or removal of the submucosal layer. Disturbances in ionic absorption are less well characterized but clearly involve both electroneutral and electrogenic Na(+) absorption. Altered ionic transport is associated with changes in the expression and function of the transporters, including the Na(+)/K(+) ATPase, the sodium/potassium/chloride cotransporter 1 (NKCC1), the sodium/hydrogen exchanger 3 (NHE3), and the epithelial sodium channel (ENaC), as well as to the modulation of intracellular signaling. Further investigation is needed in this area in order to provide an integrated paradigm of ionic transport in the inflamed intestine. In particular, we do not know exactly how diarrhea ensues in inflammation and, consequently, we do not have specific pharmacological tools to combat this condition effectively and without side effects. Moreover, whether transport disturbances are reversible independently of inflammatory control is unknown.

Identification and characterization of rabbit small intestinal villus cell brush border membrane Na-glutamine cotransporter

Glutamine, the primary metabolic fuel for the mammalian small intestinal enterocytes, is primarily assimilated by Na-amino acid cotransporters. Although Na-solute cotransport has been shown to exist in the brush border membrane (BBM) of the absorptive villus cells, the identity of Na-glutamine cotransport in rabbit small intestinal villus cells was unknown. Na-dependent glutamine uptake is present in villus BBM vesicles. An intravesicular proton gradient did not stimulate this Na-dependent glutamine uptake, whereas Li+ did not significantly suppress this uptake. These observations in concert with amino acid substitution studies suggested that Na-glutamine cotransporter in the villus cell BBM was the newly identified cotransporter B0AT1 (SLC6A19). Quantitative real-time PCR identified the message for this cotransporter in villus cells. Thus a full-length cDNA of B0AT1 was cloned and expressed in MDA-MB-231 cells. This expressed cotransporter exhibited characteristics similar to those observed in villus cells from the rabbit small intestine. Antibody was generated for B0AT1 that demonstrated the presence of this cotransporter protein in the villus cell BBM. Kinetic studies defined the kinetic parameters of this cotransporter. Thus this study describes the identification, cloning, and characterization of the Na-amino acid cotransporter responsible for the assimilation of a critical amino acid by the absorptive villus cells in the mammalian small intestine.

Constitutive nitric oxide differentially regulates Na-H and Na-glucose cotransport in intestinal epithelial cells

Previous in vivo studies suggest that constitutive nitric oxide (cNO) can regulate Na- glucose cotransport (SGLT1) and Na-H exchange (NHE3) in rabbit intestinal villus cells. Whether these two primary Na absorbing pathways are directly regulated by cNO and the mechanisms of this regulation in the enterocyte is not known. Thus nontransformed rat small intestinal epithelial cells (IEC-18) were treated with N(G)-nitro-l-arginine methyl ester (l-NAME), which directly decreased cNO in these cells. l-NAME treatment decreased SGLT1 in IEC-18 cells. Kinetic studies demonstrated that the mechanism of inhibition was secondary to a decrease in the affinity of the cotransporter for glucose without a change in the number of cotransporters. In contrast, l-NAME treatment increased NHE3 in IEC-18 cells. Kinetic studies demonstrated that the mechanism of stimulation was by increasing the number of the exchangers without a change in the affinity for Na. Quantitative RT-PCR (RTQ-PCR) and Western blot analysis of SGLT1 demonstrated no change in mRNA and protein, respectively. RTQ-PCR and Western blot analysis of NHE3 indicated that NHE3 was increased by l-NAME treatment by an increase in mRNA and protein, respectively. These results indicate that decreased cNO levels directly mediate the inhibition of SGLT1 and stimulation of NHE3 in intestinal epithelial cells. Thus cNO directly but uniquely regulates the two primary Na-absorptive pathways in the mammalian small intestine.

Transport of butyryl-L-carnitine, a potential prodrug, via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+)

L-carnitine is absorbed in the intestinal tract via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+). Loss-of-function mutations in OCTN2 may be associated with inflammatory bowel disease (IBD), suggesting a role for carnitine in intestinal/colonic health. In contrast, ATB(0,+) is upregulated in bowel inflammation. Butyrate, a bacterial fermentation product, is beneficial for prevention/treatment of ulcerative colitis. Butyryl-L-carnitine (BC), a butyrate ester of carnitine, may have potential for treatment of gut inflammation, since BC would supply both butyrate and carnitine. We examined the transport of BC via ATB(0,+) to determine if this transporter could serve as a delivery system for BC. We also examined the transport of BC via OCTN2. Studies were done with cloned ATB(0,+) and OCTN2 in heterologous expression systems. BC inhibited ATB(0,+)-mediated glycine transport in mammalian cells (IC(50), 4.6 +/- 0.7 mM). In Xenopus laevis oocytes expressing human ATB(0,+), BC induced Na(+) -dependent inward currents under voltage-clamp conditions. The currents were saturable with a K(0.5) of 1.4 +/- 0.1 mM. Na(+) activation kinetics of BC-induced currents suggested involvement of two Na(+) per transport cycle. BC also inhibited OCTN2-mediated carnitine uptake (IC(50), 1.5 +/- 0.3 microM). Transport of BC via OCTN2 is electrogenic, as evidenced from BC-induced inward currents. These currents were Na(+) dependent and saturable (K(0.5), 0.40 +/- 0.02 microM). We conclude that ATB(0,+) is a low-affinity/high-capacity transporter for BC, whereas OCTN2 is a high-affinity/low-capacity transporter. ATB(0,+) may mediate intestinal absorption of BC when OCTN2 is defective.