Enteropathogenic Escherichia coli inhibits ileal sodium-dependent bile acid transporter ASBT

Bile acid-induced metabolic changes in the colon promote Enterobacteriaceae expansion and associate with dysbiosis in Crohn's disease

Bile acids (BAs) affect the growth of potentially pathogenic commensals, including those from the Enterobacteriaceae family, which are frequently overrepresented in inflammatory bowel disease (IBD). BAs are normally reabsorbed in the ileum for recycling and are often increased in the colonic lumina of patients with IBD, including those with Crohn's disease (CD). Here, we investigated the influence of BAs on gut colonization by Enterobacteriaceae. We found increased abundance of Enterobacteriaceae in the colonic mucosae of patients with CD with a concomitant decrease in the transporters that resorb BAs in the ileum. The increase in Enterobacteriaceae colonization was greater in the colons of patients who had undergone terminal ileum resection compared with those with intact ileum, leading us to hypothesize that BAs promote intestinal colonization by Enterobacteriaceae. Exposure of human colonic epithelial cell lines to BAs reduced mitochondrial respiration, increased oxygen availability, and enhanced the epithelial adherence of several Enterobacteriaceae members. In a publicly available human dataset, mucosal Enterobacteriaceae was negatively associated with the expression of genes related to mitochondrial function. In a murine model, increased intestinal BA availability enhanced colonization by Escherichia coli in a manner that depended on bacterial respiration. Together, our findings demonstrate that BAs reduce mitochondrial respiration in the colon, leading to an increase in oxygen availability that facilitates Enterobacteriaceae colonization. This identification of BAs as facilitators of host-commensal interactions may be relevant to multiple intestinal diseases.

Interleukin 1 β suppresses bile acid-induced BSEP expression via a CXCR2-dependent feedback mechanism

Inflammation-induced cholestasis is a common problem in septic patients and results from cytokine-mediated inhibition of bile acid export including impaired expression of the bile salt export pump (BSEP) with a consecutive increase in intracellular bile acids mediating cell damage. The present study focuses on the mechanisms by which interleukin 1 β (IL-1β), as a critical mediator of sepsis-induced cholestasis, controls the expression of BSEP in hepatocytes. Notably, the treatment of hepatocytes with IL-1β leads to the upregulation of a broad chemokine pattern. Thereby, the IL-1β -induced expression of in particular the CXCR2 ligands CXCL1 and 2 is further enhanced by bile acids, whereas the FXR-mediated upregulation of BSEP induced by bile acids is inhibited by IL-1β. In this context, it is interesting to note that inhibitor studies indicate that IL-1β mediates its inhibitory effects on bile acid-induced expression of BSEP indirectly via CXCR2 ligands. Consistently, inhibition of CXCR2 with the inhibitor SB225002 significantly attenuated of the inhibitory effect of IL-1β on BSEP expression. These data suggest that part of the cholestasis-inducing effect of IL-1β is mediated via a CXCR2-dependent feedback mechanism.

Novel Expression of Apical Bile Acid Transport (ASBT) More Proximally Than Distal Ileum Contributing to Enhanced Intestinal Bile Acid Absorption in Obesity

Dietary lipid absorption is facilitated by bile acids. In the Zucker rat (ZR) model of obesity, bile acid absorption, mediated by the apical sodium bile acid transporter (ASBT), was increased in villus cells from the distal ileum. However, whether ASBT may be de novo expressed more proximally in the small intestine during obesity to facilitate additional bile acid absorption is not known. For this, starting from the end of the ileum to the mid jejunum, caudal-orally, five intestinal segments of equal length (S1-S5) were separated from lean and obese ZRs (LZR and OZR). Intestinal mucosa obtained from these segments were used for total RNA extraction, RT-qPCR and 3H-TCA uptake. The results showed that bile acid absorption along with the mRNA expression of ASBT and FXR progressively decreased caudal-orally in both LZRs and OZRs but was significantly higher in all small intestinal segments in OZRs. The expression of GATA4 was absent in the distal ileum (S1) in both LZRs and OZRs, but steadily increased along the proximal length in both. However, this steady increase was significantly reduced in the comparative obese proximal intestinal segments S2, S3, S4 and S5. The expressions of bile acid-activated G-protein-coupled bile acid receptor TGR5 and S1PR2 were unaltered in segments S1-S4 but were significantly increased in OZR S5. The paradigm changing observation of this study is that ASBT is expressed more proximally in the small intestine in obesity. This likely increases overall bile acid absorption and thereby lipid absorption in the proximal small intestine in obesity.

Intestinal flora and bile acid interactions impact the progression of diabetic kidney disease

In recent years, with the rapid development of omics technologies, researchers have shown that interactions between the intestinal flora and bile acids are closely related to the progression of diabetic kidney disease (DKD). By regulating bile acid metabolism and receptor expression, the intestinal flora affects host metabolism, impacts the immune system, and exacerbates kidney injury in DKD patients. To explore interactions among the gut flora, bile acids and DKD, as well as the related mechanisms, in depth, in this paper, we review the existing literature on correlations among the gut flora, bile acids and DKD. This review also summarizes the efficacy of bile acids and their receptors as well as traditional Chinese medicines in the treatment of DKD and highlights the unique advantages of bile acid receptors in DKD treatment. This paper is expected to reveal a new and important potential strategy for the clinical treatment of DKD.

Determination of Site-Specific Phosphorylation Occupancy Using Targeted Mass Spectrometry Reveals the Regulation of Human Apical Bile Acid Transporter, ASBT

The human apical bile acid transporter (hASBT, SLC10A2) reabsorbs bile acids in the distal ileum, facilitating their recycling to the liver and resecretion. Its activity has been implicated in various disease states, including Crohn's disease, hypercholesterolemia, cholestasis, and type-2 diabetes. Post-translational modifications such as N-glycosylation, ubiquitination, and S-acylation regulate ASBT function by controlling its translocation and stability. However, the precise role of phosphorylation and its relationship with activity remains unknown. Here, we employed parallel reaction monitoring targeted mass spectrometry to investigate ASBT phosphorylation in the presence of various kinase inhibitors and activators. Our study ascertains phosphorylation at multiple sites (Thr330, Ser334, and Ser335), with Ser335 being the predominant phosphosite. We further demonstrate the critical involvement of PKC in regulating ASBT activity by phosphorylation at Ser335. Importantly, we establish a proportional relationship between the phosphorylation level of Ser335 and ASBT bile acid uptake activity. Collectively, our findings shed light on the molecular mechanisms underlying phosphorylation-mediated regulation of ASBT.

Anaplastic Lymphoma Kinase signaling stabilizes SLC3A2 expression via MARCH11 to promote neuroblastoma cell growth

Solute Carrier Family 3, Member 2 (SLC3A2 or 4F2hc) is a multifunctional glycoprotein that mediates integrin-dependent signaling, acts as a trafficking chaperone for amino acid transporters, and is involved in polyamine transportation. We identified SLC3A2 as a potential Anaplastic Lymphoma Kinase (ALK) interacting partner in a BioID-proximity labeling screen in neuroblastoma (NB) cells. In this work we show that endogenous SLC3A2 and ALK interact in NB cells and that this SLC3A2:ALK interaction was abrogated upon treatment with the ALK inhibitor lorlatinib. We show here that loss of ALK activity leads to decreased SLC3A2 expression and reduced SLC3A2 protein stability in a panel of NB cell lines, while stimulation of ALK with ALKAL2 ligand resulted in increased SLC3A2 protein levels. We further identified MARCH11, an E3 ligase, as a regulator of SLC3A2 ubiquitination downstream of ALK. Further, knockdown of SLC3A2 resulted in inhibition of NB cell growth. To investigate the therapeutic potential of SLC3A2 targeting, we performed monotreatment of NB cells with AMXT-1501 (a polyamine transport inhibitor), which showed only moderate effects in NB cells. In contrast, a combination lorlatinib/AMXT-1501 treatment resulted in synergistic inhibition of cell growth in ALK-driven NB cell lines. Taken together, our results identify a novel role for the ALK receptor tyrosine kinase (RTK), working in concert with the MARCH11 E3 ligase, in regulating SLC3A2 protein stability and function in NB cells. The synergistic effect of combined ALK and polyamine transport inhibition shows that ALK/MARCH11/SLC3A2 regulation of amino acid transport is important for oncogenic growth and survival in NB cells.

Pathophysiology of Enteropathogenic Escherichia coli-induced Diarrhea

Enteropathogenic Escherichia coli (EPEC) are important diarrheal pathogens of infants and young children. Since the availability of molecular diagnosis methods, we now have new insights into the incidence and prevalence of these infections. Recent epidemiological studies indicate that atypical EPEC (aEPEC) are seen more frequently than typical EPEC (tEPEC) worldwide, including in both endemic diarrhea and diarrhea outbreaks. Therefore, it is important to further characterize the pathogenicity of these emerging strains. The virulence mechanisms and pathophysiology of the attaching and effacing lesion (A/E) and the type-three-secretion-system (T3SS) are complex but well-studied. A/E strains use their pool of locus of enterocyte effacement (LEE)-encoded and non-LEE-encoded effector proteins to subvert and modulate cellular and barrier properties of the host. However, the exact mechanisms of diarrhea in EPEC infection are not completely understood. From the clinical perspective, there is a need for fast, easy, and inexpensive diagnostic methods to define optimal treatment and prevention for children in endemic areas. In this article, we present a review of the classification of EPEC, epidemiology, pathogenesis of the disease caused by these bacteria, determinants of virulence, alterations in signaling, determinants of colonization vs. those of disease, and the limited information we have on the pathophysiology of EPEC-induced diarrhea. This article combines peer-reviewed evidence from our own studies and the results of an extensive literature search in the databases PubMed, EMBASE, and Scopus.

Structural Rearrangement of the Serotonin Transporter Intracellular Gate Induced by Thr276 Phosphorylation

The reuptake of the neurotransmitter serotonin from the synaptic cleft by the serotonin transporter, SERT, is essential for proper neurological signaling. Biochemical studies have shown that Thr276 of transmembrane helix 5 is a site of PKG-mediated SERT phosphorylation, which has been proposed to shift the SERT conformational equilibria to promote inward-facing states, thus enhancing 5-HT transport. Recent structural and simulation studies have provided insights into the conformation transitions during substrate transport but have not shed light on SERT regulation via post-translational modifications. Using molecular dynamics simulations and Markov state models, we investigate how Thr276 phosphorylation impacts the SERT mechanism and its role in enhancing transporter stability and function. Our simulations show that Thr276 phosphorylation alters the hydrogen-bonding network involving residues on transmembrane helix 5. This in turn decreases the free energy barriers for SERT to transition to the inward-facing state, thus facilitating 5-HT import. The results provide atomistic insights into in vivo SERT regulation and can be extended to other pharmacologically important transporters in the solute carrier family.

Intestinal and Mucosal Microbiome Response to Oral Challenge of Enterotoxigenic Escherichia coli in Weaned Pigs

Enterotoxigenic Escherichia coli (ETEC) is closely associated with diarrhoea in children in resource-limited countries. This study aims to investigate the change of the mucosal microbiome and protein expression in the ileum induced by E. coli K88 (ETEC) using pigs as a model. Seven weaned male pigs were orally given ETEC (1 × 10⁹ CFU, n = 7), and the other seven received saline (CON, n = 7). Ileal tissues were obtained 48 hours after the ETEC challenge for both proteomic and mucosal microbiome analyses. Nine proteins were found with altered abundance between the two groups, including a decrease in FABP1 and FABP6, involved in bile acid circulation. The TLR-9 mediated pathway was also affected showing increased transcription of genes SIGIRR and MyD88. Correlations between the ileal proteins and mucosal bacterial taxa found included a positive correlation between Lactobacilllus and PPP3CA (r = 0.9, p < 0.001) and a negative correlation between Prevotella with CTNND1 (r = −0.7, p < 0.01). In conclusion, ETEC infection caused inflammation and impaired the circulation of bile acids and the mucosal microbiome may affect the expression of intestinal proteins. Further studies are needed to explain the exact roles of these affected processes in the pathogenesis of ETEC-triggered diarrhoea.

Ileal Bile Acid Transporter Inhibitor Improves Hepatic Steatosis by Ameliorating Gut Microbiota Dysbiosis in NAFLD Model Mice

Nonalcoholic fatty liver disease (NAFLD), characterized by excessive fat deposition in the liver unrelated to alcohol consumption, is highly prevalent worldwide. However, effective therapeutic agents approved for NAFLD treatment are lacking. An ileal bile acid transporter inhibitor (IBATi), which represents a new mode of treatment of chronic idiopathic constipation, leads to increased delivery of bile acids to the colon. We investigated the effect of IBATi against NAFLD through modification of the gut microbiota in mice. IBATi treatment significantly suppressed body weight gain, liver dysfunction, and serum low-density lipoprotein levels and significantly decreased NAFLD activity scores in high-fat diet (HFD) mice. Treatment with IBATi ameliorated the decreased hepatic cholesterol 7-a-monooxygenase (Cyp7a1) and increased ileal fibroblast growth factor 15 (Fgf15) mRNA expression in HFD mice. Further, IBATi treatment changed the α-diversity in the gut microbiota reduced by HFD, which was analyzed in feces using 16S rRNA sequencing. To establish the mechanism underlying improvement in NAFLD induced by IBATi, we recolonized antibiotic solution-treated mice by fecal microbiome transplantation (FMT) using stool from HFD or HFD plus IBATi mice. This is the first report that fecally transplanted gut microbiota from HFD plus IBATi mice prevented hepatic steatosis caused by HFD. In conclusion, IBATi improved hepatic steatosis by ameliorating gut microbiota dysbiosis in NAFLD model mice, suggesting a potential therapeutic agent for NAFLD treatment. IMPORTANCE NAFLD is an increasingly recognized condition that may progress to liver cirrhosis and hepatocellular carcinoma, and community surveys have assessed that the prevalence is 14 to 32% worldwide. The first line of treatment for NAFLD is lifestyle modification to achieve weight reduction, particularly through diet and exercise. However, weight reduction is difficult to achieve and maintain, and pharmacological agents approved for the treatment of NAFLD are lacking. This study investigated the influence of the gut microbiota and the effect of an IBATi on NAFLD using a murine model. Treatment with IBATi significantly improved NAFLD in HFD mice. Further, fecal microbiome transplantation using stool from HFD plus IBATi mice prevented hepatic steatosis caused by HFD. Our study makes a significant contribution to the literature because the study findings suggest a potential treatment strategy for NAFLD patients by ameliorating gut microbiota dysbiosis.

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.

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.

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.

A novel bioluminescence-based method to investigate uptake of bile acids in living cells

Bile acid transporters, including the ileal apical sodium-dependent bile acid transporter (ASBT) and the hepatic sodium-taurocholate cotransporting polypeptide (NTCP), are crucial for the enterohepatic circulation of bile acids. Our objective was to develop a method for measuring bile acid transporter activity in real time to precisely evaluate rapid changes in their function. We designed a reporter system relying on a novel probe: cholic acid attached to luciferin via a disulfide-containing, self-immolating linker (CA-SS-Luc). Incubation of human embryonic kidney-293 cells coexpressing luciferase and ASBT with different concentrations of CA-SS-Luc (0.01-1 μM) resulted in bioluminescence with an intensity that was concentration- and time-dependent. The bioluminescence measured during incubation with 1 μM CA-SS-Luc was dependent on the levels of ASBT or NTCP expressed in the cells. Coincubation of CA-SS-Luc with natural bile acids enhanced the bioluminescence in a concentration-dependent manner with kinetic parameters for ASBT similar to those previously reported using conventional methods. These findings suggest that this method faithfully assesses ASBT function. Further, incubation with tyrosine phosphatase inhibitor III (PTPIII) led to significantly increased bioluminescence in cells expressing ASBT, consistent with previous studies showing an increase in ASBT function by PTPIII. We then investigated CA-SS-Luc in isolated mouse intestinal epithelial cells. Ileal enterocytes displayed significantly higher luminescence compared with jejunal enterocytes, indicating a transport process mediated by ileal ASBT. In conclusion, we have developed a novel method to monitor the activity of bile acid transporters in real time that has potential applications both for in vitro and in vivo studies. NEW & NOTEWORTHY This article reports the development of a real-time method for measuring the uptake of bile acids using a bioluminescent bile acid-based probe. This method has been validated for measuring uptake via the apical sodium-dependent bile acid transporter and the sodium-taurocholate cotransporting polypeptide in cell culture and ex vivo intestinal models.

Post-translational modifications of transporters

Drug transporter proteins are critical to the distribution of a wide range of endogenous compounds and xenobiotics such as hormones, bile acids, peptides, lipids, sugars, and drugs. There are two classes of drug transporters- the solute carrier (SLC) transporters and ATP-binding cassette (ABC) transporters -which predominantly differ in the energy source utilized to transport substrates across a membrane barrier. Despite their hydrophobic nature and residence in the membrane bilayer, drug transporters have dynamic structures and adopt many conformations during the translocation process. Whereas there is significant literature evidence for the substrate specificity and structure-function relationship for clinically relevant drug transporters proteins, there is less of an understanding in the regulatory mechanisms that contribute to the functional expression of these proteins. Post-translational modifications have been shown to modulate drug transporter functional expression via a wide range of molecular mechanisms. These modifications commonly occur through the addition of a functional group (e.g. phosphorylation), a small protein (e.g. ubiquitination), sugar chains (e.g. glycosylation), or lipids (e.g. palmitoylation) on solvent accessible amino acid residues. These covalent additions often occur as a result of a signaling cascade and may be reversible depending on the type of modification and the intended fate of the signaling event. Here, we review the significant role in which post-translational modifications contribute to the dynamic regulation and functional consequences of SLC and ABC drug transporters and highlight recent progress in understanding their roles in transporter structure, function, and regulation.

An important intestinal transporter that regulates the enterohepatic circulation of bile acids and cholesterol homeostasis: The apical sodium-dependent bile acid transporter (SLC10A2/ASBT)

The enterohepatic circulation of bile acids (BAs) is governed by specific transporters expressed in the liver and the intestine and plays a critical role in the digestion of fats and oils. During this process, the majority of the BAs secreted from the liver is reabsorbed in intestinal epithelial cells via the apical sodium-dependent bile acid transporter (ASBT/SLC10A2) and then transported into the portal vein. Previous studies revealed that regulation of the ASBT involves BAs and cholesterol. In addition, abnormal ASBT expression and function might lead to some diseases associated with disorders in the enterohepatic circulation of BAs and cholesterol homeostasis, such as diarrhoea and gallstones. However, decreasing cholesterol or BAs by partly inhibiting ASBT-mediated transport might be used for treatments of hypercholesterolemia, cholestasis and diabetes. This review mainly discusses the regulation of the ASBT by BAs and cholesterol and its relevance to diseases and treatment.

Colonization, resistance to bile, and virulence properties of Escherichia coli strains: Unusual characteristics associated with biliary tract diseases

Escherichia coli is the species that is most frequently isolated from bile of patients with biliary tract diseases. This study was aimed to investigate any association between resistance and virulence properties of these isolates with occurrence of the diseases. A total of 102 bile samples were obtained from patients subjected to endoscopic retrograde cholangiopancreatography for different biliary diseases. Clinical data were collected and culture of the bile samples was done on selective media. Resistance of characterized Escherichia coli isolates to deoxycholate sodium (0-7%) and nineteen antibiotics was determined and PCR using 16 pairs of primers targeting stx1, stx2, exhA, eae, bfp, agg, pcvd432, lt, st, ipaH, pic, pet, ast, set, sen, and cdtB genes was done. Our results showed a statistically significant association between E. coli colonization and existence of common bile duct and gallbladder stones (p value 0.028). Out of the 22 E. coli strains (22/102) multidrug resistance phenotype was present in 95.45%. None of the strains belonged to common E. coli pathotypes. However, bfp + EhxA-hly, bfp + astA, bfp + EhxA-hly + pic, and EhxA-hly + pic + astA, bfp, and astA genotypes were detected in these strains. bfp (7/22, 31.8%) and astA (5/22, 22.7%) were among most frequent virulence factors in these strains. Results of this study showed significant association between colonization of E. coli and choledocholithiasis. Unusual existence of virulence gene combinations in these strains and their resistance to DOC and multiple classes of antibiotics could be considered as possible causes of their persistence in this harsh microenvironment.

Repression of intestinal transporters and FXR-FGF15 signaling explains bile acids dysregulation in experimental colitis-associated colon cancer

Bile acids (BAs) are important endogenous signaling molecules that play vital roles in the pathological development of various diseases including colitis-associated cancer (CAC). BAs were previously found dysregulated under conditions of CAC; however, the exact patterns and underlying molecular mechanisms remain largely elusive. Based on the development of a method for comprehensive analysis of BAs, this study aims to elucidate the dysregulation patterns and involved mechanisms in a typical CAC model induced by azoxymethane (AOM)/dextran sodium sulfate (DSS). CAC mice showed decreased BAs transformation in gut and glucuronidation in colon, leading to accumulation of primary BAs but reduction of secondary BAs in colon. CAC mice were characterized by an accumulation of BAs in various compartments except ileum, which is in line with repressed ileal FXR-FGF15 feedback signaling and the increased expression of hepatic CYP7A1. The compromised ileal FXR-FGF15 signaling was caused in part by the reduced absorption of FXR ligands including free and tauro-conjungated BAs due to the downregulation of various transporters of BAs in the ileum of CAC mice.

Survival of the Fittest: How Bacterial Pathogens Utilize Bile To Enhance Infection

Bacterial pathogens have coevolved with humans in order to efficiently infect, replicate within, and be transmitted to new hosts to ensure survival and a continual infection cycle. For enteric pathogens, the ability to adapt to numerous host factors under the harsh conditions of the gastrointestinal tract is critical for establishing infection. One such host factor readily encountered by enteric bacteria is bile, an innately antimicrobial detergent-like compound essential for digestion and nutrient absorption. Not only have enteric pathogens evolved to resist the bactericidal conditions of bile, but these bacteria also utilize bile as a signal to enhance virulence regulation for efficient infection. This review provides a comprehensive and up-to-date analysis of bile-related research with enteric pathogens. From common responses to the unique expression of specific virulence factors, each pathogen has overcome significant challenges to establish infection in the gastrointestinal tract. Utilization of bile as a signal to modulate virulence factor expression has led to important insights for our understanding of virulence mechanisms for many pathogens. Further research on enteric pathogens exposed to this in vivo signal will benefit therapeutic and vaccine development and ultimately enhance our success at combating such elite pathogens.

Gut microbiota inhibit Asbt-dependent intestinal bile acid reabsorption via Gata4

BACKGROUND & AIMS

Regulation of bile acid homeostasis in mammals is a complex process regulated via extensive cross-talk between liver, intestine and intestinal microbiota. Here we studied the effects of gut microbiota on bile acid homeostasis in mice.

METHODS

Bile acid homeostasis was assessed in four mouse models. Germfree mice, conventionally-raised mice, Asbt-KO mice and intestinal-specific Gata4-iKO mice were treated with antibiotics (bacitracin, neomycin and vancomycin; 100 mg/kg) for five days and subsequently compared with untreated mice.

RESULTS

Attenuation of the bacterial flora by antibiotics strongly reduced fecal excretion and synthesis of bile acids, but increased the expression of the bile acid synthesis enzyme CYP7A1. Similar effects were seen in germfree mice. Intestinal bile acid absorption was increased and accompanied by increases in plasma bile acid levels, biliary bile acid secretion and enterohepatic cycling of bile acids. In the absence of microbiota, the expression of the intestinal bile salt transporter Asbt was strongly increased in the ileum and was also expressed in more proximal parts of the small intestine. Most of the effects of antibiotic treatment on bile acid homeostasis could be prevented by genetic inactivation of either Asbt or the transcription factor Gata4.

CONCLUSIONS

Attenuation of gut microbiota alters Gata4-controlled expression of Asbt, increasing absorption and decreasing synthesis of bile acids. Our data support the concept that under physiological conditions microbiota stimulate Gata4, which suppresses Asbt expression, limiting the expression of this transporter to the terminal ileum. Our studies expand current knowledge on the bacterial control of bile acid homeostasis.

N-glycosylation is essential for ileal ASBT function and protection against proteases

The bile acid transporter ASBT is a glycoprotein responsible for active absorption of bile acids. Inhibiting ASBT function and bile acid absorption is an attractive approach to lower plasma cholesterol and improve glucose imbalance in diabetic patients. Deglycosylation of ASBT was shown to decrease its function. However, the exact roles of N-glycosylation of ASBT, and how it affects its function, is not known. Current studies investigated the roles of N-glycosylation in ASBT protein stability and protection against proteases utilizing HEK-293 cells stably transfected with ASBT-V5 fusion protein. ASBT-V5 protein was detected as two bands with molecular mass of ~41 and ~35 kDa. Inhibition of glycosylation by tunicamycin significantly decreased ASBT activity and shifted ASBT bands to ~30 kDa, representing a deglycosylated protein. Treatment of total cellular lysates with PNGase F or Endo H glycosidases showed that the upper 41-kDa band represents a fully mature N-acetylglucosamine-rich glycoprotein and the lower 35-kDa band represents a mannose-rich core glycoprotein. Studies with the glycosylation deficient ASBT mutant (N10Q) showed that the N-glycosylation is not essential for ASBT targeting to plasma membrane. However, mature glycosylation significantly increased the half-life and protected ASBT protein from digestion with trypsin. Incubating the cells with high glucose (25 mM) for 48 h increased mature glycosylated ASBT along with an increase in its function. These results unravel novel roles for N-glycosylation of ASBT and suggest that high levels of glucose alter the composition of the glycan and may contribute to the increase in ASBT function in diabetes mellitus.

HEK-293 cells expressing the cystic fibrosis transmembrane conductance regulator (CFTR): a model for studying regulation of Cl- transport

The Human Embryonic Kidney 293 cell line (HEK‐293) readily lends itself to genetic manipulation and is a common tool for biologists to overexpress proteins of interest and study their function and molecular regulation. Although these cells have some limitations, such as an inability to form resistive monolayers necessary for studying transepithelial ion transport, they are nevertheless valuable in studying individual epithelial ion transporters. We report the use of HEK‐293 cells to study the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel. While HEK‐293 cells endogenously express mRNA for the Cl⁻ channels, ClC‐2 and TMEM16A, they neither express CFTR mRNA nor protein. Therefore, we stably transfected HEK‐293 cells with EGFP‐CFTR (HEK‐CFTR) and demonstrated CFTR function by measuring forskolin‐stimulated iodide efflux. This efflux was inhibited by CFTR_inh172, and the protein kinase A inhibitor H89, but not by Ca²⁺ chelation. In contrast to intestinal epithelia, forskolin stimulation does not increase surface CFTR expression and does not require intact microtubules in HEK‐CFTR. To investigate the role of an endogenous Gα_S‐coupled receptor, we examined the bile acid receptor, TGR5. Although HEK‐CFTR cells express TGR5, the potent TGR5 agonist lithocholic acid (LCA; 5–500 μmol/L) did not activate CFTR. Furthermore, forskolin, but not LCA, increased [cAMP]_i in HEK‐CFTR suggesting that endogenous TGR5 may not be functionally linked to Gα_S. However, LCA did increase [Ca²⁺]_i and interestingly, abolished forskolin‐stimulated iodide efflux. Thus, we propose that the stable HEK‐CFTR cell line is a useful model to study the multiple signaling pathways that regulate CFTR.

In this study, we characterize a HEK‐293 cell line, stably transfected with EGFP‐CFTR (HEK‐CFTR). We examined its regulation by endogenously expressed signaling pathways, in particular the cAMP and the GαS‐coupled bile acid receptor, TGR5, signaling pathways.

Bile acid dysregulation, gut dysbiosis, and gastrointestinal cancer

Because of increasingly widespread sedentary lifestyles and diets high in fat and sugar, the global diabetes and obesity epidemic continues to grow unabated. A substantial body of evidence has been accumulated which associates diabetes and obesity to dramatically higher risk of cancer development, particularly in the liver and gastrointestinal tract. Additionally, diabetic and obese individuals have been shown to suffer from dysregulation of bile acid (BA) homeostasis and dysbiosis of the intestinal microbiome. Abnormally elevated levels of cytotoxic secondary BAs and a pro-inflammatory shift in gut microbial profile have individually been linked to numerous enterohepatic diseases including cancer. However, recent findings have implicated a detrimental interplay between BA dysregulation and intestinal dysbiosis that promotes carcinogenesis along the gut-liver axis. This review seeks to examine the currently investigated interactions between the regulation of BA metabolism and activity of the intestinal microbiota and how these interactions can drive cancer formation in the context of diabesity. The precarcinogenic effects of BA dysregulation and gut dysbiosis including excessive inflammation, heightened oxidative DNA damage, and increased cell proliferation are discussed. Furthermore, by focusing on the mediatory roles of BA nuclear receptor farnesoid x receptor, ileal transporter apical sodium dependent BA transporter, and G-coupled protein receptor TGR5, this review attempts to connect BA dysregulation, gut dysbiosis, and enterohepatic carcinogenesis at a mechanistic level. A better understanding of the intricate interplay between BA homeostasis and gut microbiome can yield novel avenues to combat the impending rise in diabesity-related cancers.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Functional transformations of bile acid transporters induced by high-affinity macromolecules

Apical sodium-dependent bile acid transporters (ASBT) are the intestinal transporters that form intermediate complexes with substrates and its conformational change drives the movement of substrates across the cell membrane. However, membrane-based intestinal transporters are confined to the transport of only small molecular substrates. Here, we propose a new strategy that uses high-affinity binding macromolecular substrates to functionally transform the membrane transporters so that they behave like receptors, ultimately allowing the apical-basal transport of bound macromolecules. Bile acid based macromolecular substrates were synthesized and allowed to interact with ASBT. ASBT/macromolecular substrate complexes were rapidly internalized in vesicles, localized in early endosomes, dissociated and escaped the vesicular transport while binding of cytoplasmic ileal bile acid binding proteins cause exocytosis of macromolecules and prevented entry into lysosomes. This newly found transformation process of ASBT suggests a new transport mechanism that could aid in further utilization of ASBT to mediate oral macromolecular drug delivery.

On the fiftieth anniversary. Postinfectious irritable bowel syndrome: mechanisms related to pathogens

BACKGROUND

Gastrointestinal (GI) infections resulting from bacterial, viral, and parasitic pathogens predispose to postinfectious irritable bowel syndrome (PI-IBS) and other functional GI disorders. Existing literature supports the role of enterochromaffin cell hyperplasia, serotonin synthesis and reuptake, impaired barrier function, altered immune activation, and potentially mast cell activation in the pathophysiology of PI-IBS.

PURPOSE

The objective of this review was to summarize from the literature the characteristics of the pathogens commonly implicated in PI-IBS, their acute enteritis phases, and the changes seen in the postinfectious phase that may contribute toward development of IBS. A limitation of our current understanding is that the postinfectious GI sequelae reported in prior studies followed epidemic diarrheal outbreaks often involving more than one pathogen, or the studies focused on highly selected, tertiary referral patients. Understanding the mechanisms, natural history, and optimized management of individuals suffering PI-IBS following the more typical sporadic infection requires larger studies of PI-IBS following GI infections encountered in community settings. These studies should include genetic, physiological, and molecular studies to provide more generalizable information that can ultimately be used to diagnose, manage, and potentially prevent the development of PI-IBS.