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

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.

ASBT(SLC10A2): A promising target for treatment of diseases and drug discovery

Bile acids has gradually become a new focus in various diseases, and ASBT as a transporter responsible for the reabsorption of ileal bile acids, is a key hinge associated to the bile acids-cholesterol balance and bile acids of enterohepatic circulation. The cumulative studies have also shown that ASBT is a promising target for treatment of liver, gallbladder, intestinal and metabolic diseases. This article briefly reviewed the process of bile acids enterohepatic circulation, as well as the regulations of ASBT expression, covering transcription factors, nuclear receptors and gut microbiota. In addition, the relationship between ASBT and various diseases were discussed in this paper. According to the structural classification of ASBT inhibitors, the research status of ASBT inhibitors and potential ASBT inhibitors of traditional Chinese medicine (such resveratrol, jatrorrhizine in Coptis chinensis) were summarized. This review provides a basis for the development of ASBT inhibitors and the treatment strategy of related diseases.

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.

Mechanism of Dyslipidemia in Obesity-Unique Regulation of Ileal Villus Cell Brush Border Membrane Sodium-Bile Acid Cotransport

In obesity, increased absorption of dietary fat contributes to altered lipid homeostasis. In turn, dyslipidemia of obesity leads to many of the complications of obesity. Bile acids are necessary for the absorption of dietary fat. In the mammalian intestine, apical sodium-dependent bile acid cotransporter (ASBT; SLC10A2) is exclusively responsible for the reabsorption of bile acids in the terminal ileum. In rat and mice models of obesity and importantly in obese humans, ASBT was increased in ileal villus cells. The mechanism of stimulation of ASBT was secondary to an increase in ASBT expression in villus cell brush border membrane. The stimulation of ASBT was not secondary to the altered Na-extruding capacity of villus cells during obesity. Further, increased Farnesoid X receptor (FXR) expression in villus cells during obesity likely mediated the increase in ASBT. Moreover, enhanced FXR expression increased the expression of bile-acid-associated proteins (IBABP and OSTα) that are responsible for handling bile acids absorbed via ASBT in villus cells during obesity. Thus, this study demonstrated that in an epidemic condition, obesity, the dyslipidemia that leads to many of the complications of the condition, may, at least in part, be due to deregulation of intestinal bile acid absorption.

EPEC effector EspF promotes Crumbs3 endocytosis and disrupts epithelial cell polarity

Enteropathogenic Escherichia coli (EPEC) uses a type III secretion system to inject effector proteins into host intestinal epithelial cells causing diarrhoea. EPEC infection redistributes basolateral proteins β1-integrin and Na⁺ /K⁺ ATPase to the apical membrane of host cells. The Crumbs (Crb) polarity complex (Crb3/Pals1/Patj) is essential for epithelial cell polarisation and tight junction (TJ) assembly. Here, we demonstrate that EPEC displaces Crb3 and Pals1 from the apical membrane to the cytoplasm of cultured intestinal epithelial cells and colonocytes of infected mice. In vitro studies show that EspF, but not Map, alters Crb3, whereas both effectors modulate Pals1. EspF perturbs polarity formation in cyst morphogenesis assays and induces endocytosis and apical redistribution of Na⁺ /K⁺ ATPase. EspF binds to sorting nexin 9 (SNX9) causing membrane remodelling in host cells. Infection with ΔespF/pespFD3, a mutant strain that ablates EspF binding to SNX9, or inhibition of dynamin, attenuates Crb3 endocytosis caused by EPEC. In addition, infection with ΔespF/pespFD3 has no impact on Na⁺ /K⁺ ATPase endocytosis. These data support the hypothesis that EPEC perturbs apical-basal polarity in an EspF-dependent manner, which would contribute to EPEC-associated diarrhoea by disruption of TJ and altering the crucial positioning of membrane transporters involved in the absorption of ions and solutes.

Modulation of epithelial cell polarity by bacterial pathogens

Epithelial cells constitute a physical barrier that aids in protecting the host from microbial pathogens. Polarized epithelial cells contain distinct apical and basolateral membrane domains separated by intercellular junctions, including tight junctions (TJs), which contribute to the maintenance of apical-basal polarity. Polarity complexes also contribute to the establishment of TJ formation. Several pathogens perturb epithelial TJ barrier function and structure in addition to causing a loss of apical-basal polarity. Here, we review the impact of pathogenic bacteria on the disruption of cell-cell junctions and epithelial polarity.

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.

Molecular mechanism of regulation of villus cell Na-K-ATPase in the chronically inflamed mammalian small intestine

Na-K-ATPase located on the basolateral membrane (BLM) of intestinal epithelial cells provides a favorable intracellular Na+ gradient to promote all Na dependent co-transport processes across the brush border membrane (BBM). Down-regulation of Na-K-ATPase activity has been postulated to alter the absorption via Na-solute co-transporters in human inflammatory bowel disease (IBD). Further, the altered activity of a variety of Na-solute co-transporters in intact villus cells has been reported in animal models of chronic enteritis. But the molecular mechanism of down-regulation of Na-K-ATPase is not known. In the present study, using a rabbit model of chronic intestinal inflammation, which resembles human IBD, Na-K-ATPase in villus cells was shown to decrease. The relative mRNA abundance of α-1 and β-1 subunits was not altered in villus cells during chronic intestinal inflammation. Similarly, the protein levels of these subunits were also not altered in villus cells during chronic enteritis. However, the BLM concentration of α-1 and β-1 subunits was diminished in the chronically inflamed intestinal villus cells. An ankyrin-spectrin skeleton is necessary for the proper trafficking of Na-K-ATPase to the BLM of the cell. In the present study, ankyrin expression was markedly diminished in villus cells from the chronically inflamed intestine resulting in depolarization of ankyrin-G protein. The decrease of Na-K-ATPase activity was comparable to that seen in ankyrin knockdown IEC-18 cells. Therefore, altered localization of Na-K-ATPase as a result of transcriptional down-regulation of ankyrin-G mediates the down-regulation of Na-K-ATPase activity during chronic intestinal inflammation.

Prednisolone increases enterohepatic cycling of bile acids by induction of Asbt and promotes reverse cholesterol transport

BACKGROUND & AIMS

Glucocorticoids, produced by the adrenal gland under control of the hypothalamic-pituitary-adrenal axis, exert their metabolic actions largely via activation of the glucocorticoid receptor (GR). Synthetic glucocorticoids are widely used as anti-inflammatory and immunosuppressive drugs but their application is hampered by adverse metabolic effects. Recently, it has been shown that GR may regulate several genes involved in murine bile acid (BA) and cholesterol metabolism, yet the physiological relevance hereof is controversial. The aim of this study is to provide a mechanistic basis for effects of prednisolone on BA and cholesterol homeostasis in mice.

METHODS

Male BALB/c mice were treated with prednisolone (12.5mg/kg/day) for 7days by subcutaneous implantation of slow-release pellets, followed by extensive metabolic profiling.

RESULTS

Sustained prednisolone treatment induced the expression of the apical sodium-dependent bile acid transporter (Asbt) in the ileum, which stimulated BA absorption. This resulted in elevated plasma BA levels and enhanced biliary BA secretion. Concomitantly, both biliary cholesterol and phospholipid secretion rates were increased. Enhanced BA reabsorption suppressed hepatic BA synthesis, as evident from hepatic gene expression, reduced plasma C4 levels and reduced fecal BA loss. Plasma HDL cholesterol levels were elevated in prednisolone-treated mice, which likely contributed to the stimulated flux of cholesterol from intraperitoneally injected macrophage foam cells into feces.

CONCLUSIONS

Sustained prednisolone treatment increases enterohepatic recycling of BA, leading to elevated plasma levels and reduced synthesis in the absence of cholestasis. Under these conditions, prednisolone promotes macrophage-derived reverse cholesterol transport.

The solute carrier family 10 (SLC10): beyond bile acid transport

The solute carrier (SLC) family 10 (SLC10) comprises influx transporters of bile acids, steroidal hormones, various drugs, and several other substrates. Because the seminal transporters of this family, namely, sodium/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2), were primarily bile acid transporters, the term "sodium bile salt cotransporting family" was used for the SLC10 family. However, this notion became obsolete with the finding of other SLC10 members that do not transport bile acids. For example, the sodium-dependent organic anion transporter (SOAT; SLC10A6) transports primarily sulfated steroids. Moreover, NTCP was shown to also transport steroids and xenobiotics, including HMG-CoA inhibitors (statins). The SLC10 family contains four additional members, namely, P3 (SLC10A3; SLC10A3), P4 (SLC10A4; SLC10A4), P5 (SLC10A5; SLC10A5) and SLC10A7 (SLC10A7), several of which were unknown or considered hypothetical until approximately a decade ago. While their substrate specificity remains undetermined, great progress has been made towards their characterization in recent years. Explicitly, SLC10A4 may participate in vesicular storage or exocytosis of neurotransmitters or mastocyte mediators, whereas SLC10A5 and SLC10A7 may be involved in solute transport and SLC10A3 may have a role as a housekeeping protein. Finally, the newly found role of bile acids in glucose and energy homeostasis, via the TGR5 receptor, sheds new light on the clinical relevance of ASBT and NTCP. The present mini-review provides a brief summary of recent progress on members of the SLC10 family.

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.

Intestinal inflammation and the enterocyte transportome

Diarrhoea is a hallmark of intestinal inflammation. The mechanisms operating in acute inflammation of the intestine are well characterized and are related to regulatory changes induced by inflammatory mediators such as prostaglandins, cytokines or reactive oxygen species, along with leakage due to epithelial injury and changes in permeability. In chronic colitis, however, the mechanisms are less well known, but it is generally accepted that both secretory and absorptive processes are inhibited. These disturbances in ionic transport may be viewed as an adaptation to protracted inflammation of the intestine, since prolonged intense secretion may be physiologically unacceptable in the long term. Mechanistically, the changes in transport may be due to adjustments in the regulation of the different processes involved, to broader epithelial alterations or frank damage, or to modulation of the transportome in terms of expression. In the present review, we offer a summary of the existing evidence on the status of the transportome in chronic intestinal inflammation.