The transcription factor Cdx2 regulates the intestine-specific expression of human peptide transporter 1 through functional interaction with Sp1

In vitro gastrointestinal stability and intestinal absorption of ACE-1 and DPP4 inhibitory peptides from poultry by-product hydrolysates

Bioactive peptides derived from food are promising health-promoting ingredients that can be used in functional foods and nutraceutical formulations. In addition to the potency towards the selected therapeutic target, the bioavailability of bioactive peptides is a major factor regarding clinical efficacy. We have previously shown that a low molecular weight peptide fraction (LMWPF) from poultry by-product hydrolysates possesses angiotensin-1-converting enzyme (ACE-1) and dipeptidyl-peptidase 4 (DPP4) inhibitory activities. The present study aimed to investigate the bioavailability of the bioactive peptides in the LMWPF. Prior to the investigation of bioavailability, a dipeptide YA was identified from this fraction as a dual inhibitor of ACE-1 and DPP4. Gastrointestinal (GI) stability and intestinal absorption of the bioactive peptides (i.e., YA as well as two previously reported bioactive dipeptides (VL and IY)) in the LMWPF were evaluated using the INFOGEST static in vitro digestion model and intestinal Caco-2 cell monolayer, respectively. Analysis of peptides after in vitro digestion confirmed that the dipeptides were resistant to the simulated GI conditions. After 4 hours of incubation, the concentration of the peptide from the apical side of the Caco-2 cell monolayer showed a significant decrease. However, the corresponding absorbed peptides were not detected on the basolateral side, suggesting that the peptides were not transported across the intestinal monolayer but rather taken up or metabolized by the Caco2 cells. Furthermore, when analyzing the gene expression of the Caco-2 cells upon peptide stimulation, a down-regulation of peptide transporters, the transcription factor CDX2, and the tight junction protein-1 (TJP1) was observed, suggesting the specific effects of the peptides on the Caco-2 cells. The study demonstrated that bioactive dipeptides found in the LMWPF were stable through in vitro GI digestion; however, the overall bioavailability may be hindered by inadequate uptake across the intestinal barrier.

Molecular regulation after mucosal injury and regeneration in ulcerative colitis

Ulcerative colitis (UC) is a chronic nonspecific inflammatory disease with a complex etiology. Intestinal mucosal injury is an important pathological change in individuals with UC. Leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5+) intestinal stem cells (ISCs) exhibit self-renewal and high differentiation potential and play important roles in the repair of intestinal mucosal injury. Moreover, LGR5+ ISCs are intricately regulated by both the Wnt/β-catenin and Notch signaling pathways, which jointly maintain the function of LGR5+ ISCs. Combination therapy targeting multiple signaling pathways and transplantation of LGR5+ ISCs may lead to the development of new clinical therapies for UC.

Effect of Tributyrin on Growth Performance and Pathway by which Tributyrin Regulates Oligopeptide Transporter 1 in Juvenile Grass Carp (Ctenopharyngodon idellus)

Simple Summary

Oligopeptide transporter 1 (PepT1) plays a role in the transportation and absorption of oligopeptides, which is an important part of protein nutrition and affects the growth of animals. Tributyrin (TB), the precursor of butyrate, exhibits similar functions to those of the butyrate in intestinal nutrients absorption. The analysis of TB on the growth of grass carp and its regulation pathway on PepT1 may help us to better understand the functions of TB and oligopeptide transportation via PepT1, which can be modulated by diet. In this study, we demonstrated that an appropriate level of tributyrin supplementation in the diet promoted the growth of juvenile grass carp and elevated the expressions of caudal type homeobox 2 (CDX2), specificity protein 1 (SP1), and PepT1 in the grass carp intestine and primary intestine cell. In addition, CDX2 and SP1 regulating the expression of PepT1 was investigated. Finally, CDX2/SP1-mediating tributyrin regulation on PepT1 was elucidated. This study verified the effect of tributyrin on the growth of juvenile grass carp and clarified the tributyrin regulation pathway on CDX2/SP1-PepT1.

Abstract

The nutritional functions of tributyrin (TB) have been extensively studied, but questions remain regarding its influence on the growth of juvenile grass carp (Ctenopharyngodon idellus) and the regulation pathway to PepT1 in the intestine of grass carp. To answer the remaining questions, feeding trials, cell trials, and peritoneal injection trials were conducted in this study. The results showed that an appropriate level of TB (0.5 g/kg and 1.0 g/kg) supplementation in feed significantly promoted the growth performance of juvenile grass carp. The expressions of intestine genes (CDX2, SP1 and PepT1) related to oligopeptide transportation increased in the 0.5 g/kg TB group of feeding trials and both the 5 mM and 10 mM TB groups of the intestine cell trials, respectively. Subsequently, the injection trials of inhibitors CDX2 and SP1 demonstrated that the inhibition of CDX2 or SP1 decreased the mRNA expression of PepT1. Finally, the results of independent or combined treatments of TB and the inhibitors suggested that CDX2/SP1 mediated TB regulation on PepT1. These findings may help us to better understand the functions of TB on growth and PepT1 oligopeptide transportation, which could be modulated by dietary TB through the CDX2/SP1-PepT1 pathway in juvenile grass carp.

Epigenetic regulation of intestinal peptide transporter PEPT1 as a potential strategy for colorectal cancer sensitization

Human intestinal peptide transporter PEPT1 is commonly repressed in human colorectal cancer (CRC), yet its relationship with sensitivity to the common CRC treatment ubenimex has not previously been elucidated. In this study, we confirmed PEPT1 suppression in CRC using real-time quantitative polymerase chain reaction and western blotting and then investigated the underlying epigenetic pathways involved using bisulfite sequencing, chromatin immunoprecipitation, siRNA knockdown, and reporter gene assays. We found that PEPT1 transcriptional repression was due to both DNMT1-mediated DNA methylation of the proximal promoter region and HDAC1-mediated histone deacetylation, which blocked P300-mediated H3K18/27Ac at the PEPT1 distal promoter. Finally, the effects of the epigenetic activation of PEPT1 on the CRC response to ubenimex were evaluated using sequential combination therapy of decitabine and ubenimex both in vitro and in xenografts. In conclusion, epigenetic silencing of PEPT1 due to increased DNMT1 and HDAC1 expression plays a vital role in the poor response of CRC to ubenimex.

Properties and regulation of Gly-Sar uptake and transport in bovine intestinal epithelial cells

Intestinal absorption of peptides is vital for the overall health and productivity of dairy cows. This study investigated the regulation, uptake and transport of dipeptides in bovine intestinal epithelial cells (BIECs). We also evaluated the effects of time, pH, concentration of the dipeptides, temperature, presence of diethylpyrocarbonate (DEPC)-an inhibitor of PepT1, and other dipeptides (Met-Met, Lys-Lys or Met-Lys), on the uptake and transport of Gly-Sar-FITC, which was a common fluorophore-labelled dipeptide. Under controlled experiments, BIECs were treated with 25 μM LY294002 (a phosphatidylinositol 3-kinase (PI3K) inhibitor) and 25 μM Perifosine (a protein kinase B (AKT) inhibitor). The subsequent expression of PepT1 in the BIECs was assessed by reverse transcriptase polymerase chain reaction (RT-PCR) and Western blotting. It was found that the uptake and transport of Gly-Sar-FITC were significant high at 37℃ than that at 4℃. The optimal pH for transport and uptake of Gly-Sar-FITC was 6.0-6.5, whereas the two properties decreased significantly in the presence of DEPC, Met-Met, Lys-Lys and Met-Lys (p < 0.05). The apical-to-basolateral transport was also found to be significantly higher than the reverse transport (p < 0.05). PI3K and AKT inhibitors were found to significantly suppress the expression of PepT1, thus impairing uptake and transport of Gly-Sar-FITC. Findings of this study thus suggest that the uptake and transport of Gly-Sar-FITC in BIECs are mediated by PepT1, and the PI3K/AKT signalling pathway regulates the absorption of small peptides.

JAK2 Mediates the Regulation of Pept1 Expression by Leptin in the Grass Carp (Ctenopharyngodon idella) Intestine

Oligopeptide transporter 1 (Pept1) is located on the brush border membrane of the intestinal epithelium and plays an important role in dipeptide and tripeptide absorption from protein digestion. In this study, we cloned and characterized the cDNA sequence of Janus kinase 2 (JAK2) from Ctenopharyngodon idella. The expression patterns of JAK2 in various tissues and developmental stages were characterized by quantitative real-time PCR (qRT-PCR). The mRNA expression levels of JAK2 and Pept1 regulated by leptin in the intestine were also analyzed in vitro and in vivo. The cDNA sequence of JAK2 is 3378 bp in length, and the mRNA of JAK2 was broadly expressed in all tissues and embryonic stages of C. idella analyzed. In addition, we found that leptin regulated expression of JAK2 and Pept1 in the intestine; Pept1 expression was down-regulated by the JAK2 inhibitor AG490 in vivo and in vitro. Furthermore, luciferase experiments showed that overexpression of the JAK2 gene significantly upregulated the activity of the Pept1 5′ regulatory sequence in C. idella. In conclusion, these results may help in elucidating the regulatory effect of the leptin-mediated JAK2 pathway on intestinal Pept1 expression in C. idella and the molecular mechanism of peptide transport by the intestinal transporter Pept1 in fishes.

The clinical significance of CDX2 in leukemia: A new perspective for leukemia research

CDX2 gene encodes a transcription factor involved in primary embryogenesis and hematopoietic development; however, the expression of CDX2 in adults is restricted to intestine and is not observed in blood tissues. The ectopic expression of CDX2 has been frequently observed in acute myeloid and lymphoid leukemia which in most cases is concomitant with poor prognosis. Induction of CDX2 in mice leads to hematologic complications, showing the leukemogenic origin of this gene. CDX2 plays significant role in the most critical pathways as the regulator of important transcription factors targeting cell proliferation, multi-drug resistance and survival. On the whole, the results indicate that CDX2 has the potential to be suggested as the diagnostic marker in hematologic malignancies. This review discusses the role of aberrant expression of CDX2 in the prognosis and the response to treatment in patients with different leukemia in clinical reports in the recent decades. The improvement in this regard could be of high importance in diagnosis and treatment methods.

Regulation and biological role of the peptide/histidine transporter SLC15A3 in Toll-like receptor-mediated inflammatory responses in macrophage

The peptide/histidine transporter SLC15A3 is responsible for transporting histidine, certain dipeptide and peptidomimetics from inside the lysosome to cytosol. Previous studies have indicated that SLC15A3 transcripts are mainly expressed in the lymphatic system, however, its regulation and biological role in innate immune responses and inflammatory diseases are as yet unknown. In this study, mouse peritoneal macrophages (PMs), mouse bone marrow-derived macrophages (BMDMs), the human acute monocytic leukemia cell line THP-1 and the human lung epithelial carcinoma cell line A549 were used to investigate the regulation and biological role of SLC15A3 in TLR-mediated inflammatory responses. Our results showed that SLC15A3 was upregulated by TLR2, TLR4, TLR7 and TLR9 ligands in macrophages at both the mRNA and protein levels via activation of NF-κB (nuclear factor-kappa-B), MAPK (mitogen-activated protein kinase) and IRF3 (interferon regulatory factor 3). Furthermore, knockdown or overexpression of SLC15A3 influenced the TLR4-triggered expression of proinflammatory cytokines. A reporter gene assay showed that the SLC15A3 promotor contained potential NF-κB binding sites, which were reasonable for regulating SLC15A3 by TLR-activation through NF-κB signaling. Additionally, SLC15A3 expression was increased and positively related to inflammation in mice with bacterial peritonitis. The collective findings suggest that SLC15A3 is regulated by various TLRs, and that it plays an important role in regulating TLR4-mediated inflammatory responses.

Integrative multi-omics analysis of intestinal organoid differentiation

Intestinal organoids accurately recapitulate epithelial homeostasis in vivo, thereby representing a powerful in vitro system to investigate lineage specification and cellular differentiation. Here, we applied a multi-omics framework on stem cell-enriched and stem cell-depleted mouse intestinal organoids to obtain a holistic view of the molecular mechanisms that drive differential gene expression during adult intestinal stem cell differentiation. Our data revealed a global rewiring of the transcriptome and proteome between intestinal stem cells and enterocytes, with the majority of dynamic protein expression being transcription-driven. Integrating absolute mRNA and protein copy numbers revealed post-transcriptional regulation of gene expression. Probing the epigenetic landscape identified a large number of cell-type-specific regulatory elements, which revealed Hnf4g as a major driver of enterocyte differentiation. In summary, by applying an integrative systems biology approach, we uncovered multiple layers of gene expression regulation, which contribute to lineage specification and plasticity of the mouse small intestinal epithelium.

Function, Regulation, and Pathophysiological Relevance of the POT Superfamily, Specifically PepT1 in Inflammatory Bowel Disease

Mammalian members of the proton-coupled oligopeptide transporter family are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs and couple substrate translocation to the movement of H⁺ , with the transmembrane electrochemical proton gradient providing the driving force. Peptide transporters are responsible for the (re)absorption of dietary and/or bacterial di- and tripeptides in the intestine and kidney and maintaining homeostasis of neuropeptides in the brain. These proteins additionally contribute to absorption of a number of pharmacologically important compounds. In this overview article, we have provided updated information on the structure, function, expression, localization, and activities of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4), and PhT2 (SLC15A3). Peptide transporters, in particular, PepT1 are discussed as drug-delivery systems in addition to their implications in health and disease. Particular emphasis has been placed on the involvement of PepT1 in the physiopathology of the gastrointestinal tract, specifically, its role in inflammatory bowel diseases. © 2018 American Physiological Society. Compr Physiol 8:731-760, 2018.

Regulation profile of the intestinal peptide transporter 1 (PepT1)

The intestinal peptide transporter 1 (PepT1) was first identified in 1994. It plays a crucial role in the absorption of small peptides including not only >400 different dipeptides and 8,000 tripeptides digested from dietary proteins but also a repertoire of structurally related compounds and drugs. Owing to its critical role in the bioavailability of peptide-like drugs, such as the anti-cancer agents and anti-virus drug, PepT1 is increasingly becoming a striking prodrug-designing target. Therefore, the understanding of PepT1 gene regulation is of great importance both for dietary adaptation and for clinical drug treatment. After decades of research, it has been recognized that PepT1 could be regulated at the transcriptional and post-transcriptional levels by numerous factors. Therefore, the present review intends to summarize the progress made in the regulation of PepT1 and provide insights into the PepT1's potential in clinical aspects of nutritional and drug therapies.

Di- and tripeptide transport in vertebrates: the contribution of teleost fish models

Solute Carrier 15 (SLC15) family, alias H⁺-coupled oligopeptide cotransporter family, is a group of membrane transporters known for their role in the cellular uptake of di- and tripeptides (di/tripeptides) and peptide-like molecules. Of its members, SLC15A1 (PEPT1) chiefly mediates intestinal absorption of luminal di/tripeptides from dietary protein digestion, while SLC15A2 (PEPT2) mainly allows renal tubular reabsorption of di/tripeptides from ultrafiltration, SLC15A3 (PHT2) and SLC15A4 (PHT1) possibly interact with di/tripeptides and histidine in certain immune cells, and SLC15A5 has unknown function. Our understanding of this family in vertebrates has steadily increased, also due to the surge of genomic-to-functional information from 'non-conventional' animal models, livestock, poultry, and aquaculture fish species. Here, we review the literature on the SLC15 transporters in teleost fish with emphasis on SLC15A1 (PEPT1), one of the solute carriers better studied amongst teleost fish because of its relevance in animal nutrition. We report on the operativity of the transporter, the molecular diversity, and multiplicity of structural-functional solutions of the teleost fish orthologs with respect to higher vertebrates, its relevance at the intersection of the alimentary and osmoregulative functions of the gut, its response under various physiological states and dietary solicitations, and its possible involvement in examples of total body plasticity, such as growth and compensatory growth. By a comparative approach, we also review the few studies in teleost fish on SLC15A2 (PEPT2), SLC15A4 (PHT1), and SLC15A3 (PHT2). By representing the contribution of teleost fish to the knowledge of the physiology of di/tripeptide transport and transporters, we aim to fill the gap between higher and lower vertebrates.

Intestinal barrier dysfunction: implications for chronic inflammatory conditions of the bowel

The intestinal epithelium of adult humans acts as a differentially permeable barrier that separates the potentially harmful contents of the lumen from the underlying tissues. Any dysfunction of this boundary layer that disturbs the homeostatic equilibrium between the internal and external environments may initiate and sustain a biochemical cascade that results in inflammation of the intestine. Key to such dysfunction are genetic, microbial and other environmental factors that, singularly or in combination, result in chronic inflammation that is symptomatic of inflammatory bowel disease (IBD). The aim of the present review is to assess the scientific evidence to support the hypothesis that defective transepithelial transport mechanisms and the heightened absorption of intact antigenic proinflammatory oligopeptides are important contributing factors in the pathogenesis of IBD.

CDX2 homeoprotein is involved in the regulation of ST6GalNAc-I gene in intestinal metaplasia

De novo expression of Sialyl-Tn (STn) antigen is one of the most common features of intestinal metaplasia (IM) and gastric carcinomas, and its biosynthesis has been mostly attributed to ST6GalNAc-I activity. However, the regulation of this glycosyltransferase expression is not elucidated. In IM lesions and in the intestine, CDX2 homeobox transcription factor is co-expressed with STn and ST6GalNAc-I. We therefore hypothesized that CDX2 might induce STn expression by positive regulation of ST6GalNAc-I. We showed that ST6GalNAc-I transcript levels and CDX2 have a coordinated expression upon Caco-2 in vitro differentiation, and overexpression of CDX2 in MKN45 gastric cells increases ST6GalNAc-I transcript levels. Nine putative CDX-binding sites in the ST6GalNAc-I-regulatory sequence were identified and analyzed by chromatin immunoprecipitation in Caco-2 cells and in IM. The results showed that CDX2 protein is recruited to all regions, being the most proximal sites preferentially occupied in vivo. Luciferase assays demonstrated that CDX2 is able to transactivate ST6GalNac-I-regulatory region. The induction was stronger for the regions mapped in the neighbourhood of ATG start codon and site-directed mutagenesis of these sites confirmed their importance. In conclusion, we show that CDX2 transcriptionally regulates ST6GalNAc-I gene expression, specifically in the preneoplastic IM lesion.

Clinical relevance of intestinal peptide uptake

AIM: To determine available information on an independent peptide transporter 1 (PepT1) and its potential relevance to treatment, this evaluation was completed.

METHODS: Fully published English language literature articles sourced through PubMed related to protein digestion and absorption, specifically human peptide and amino acid transport, were accessed and reviewed. Papers from 1970 to the present, with particular emphasis on the past decade, were examined. In addition, abstracted information translated to English in PubMed was also included. Finally, studies and reviews relevant to nutrient or drug uptake, particularly in human intestine were included for evaluation. This work represents a summary of all of these studies with particular reference to peptide transporter mediated assimilation of nutrients and pharmacologically active medications.

RESULTS: Assimilation of dietary protein in humans involves gastric and pancreatic enzyme hydrolysis to luminal oligopeptides and free amino acids. During the ensuing intestinal phase, these hydrolytic products are transported into the epithelial cell and, eventually, the portal vein. A critical component of this process is the uptake of intact di-peptides and tri-peptides by an independent PepT1. A number of “peptide-mimetic” pharmaceutical agents may also be transported through this carrier, important for uptake of different antibiotics, antiviral agents and angiotensin-converting enzyme inhibitors. In addition, specific peptide products of intestinal bacteria may also be transported by PepT1, with initiation and persistence of an immune response including increased cytokine production and associated intestinal inflammatory changes. Interestingly, these inflammatory changes may also be attenuated with orally-administered anti-inflammatory tripeptides administered as site-specific nanoparticles and taken up by this PepT1 transport protein.

CONCLUSION: Further evaluation of the role of this transporter in treatment of intestinal disorders, including inflammatory bowel disease is needed.

Effect of free thymol on differential gene expression in gastric mucosa of the young pig

Thymol is the most common molecule in thyme and has been proposed as an oral alternative to antibiotics in the feed of pigs and broilers. The knowledge of the in vivo physiological effects of thymol on tissues is limited, particularly its impact on the gastric mucosa, where it is primarily absorbed when it is orally supplied. In this study, thymol (TH, 50 mg/ kg BW) or a placebo (CO) was introduced directly into the stomach of 8 weaned pigs that were slaughtered 12 h later and sampled for gastric oxyntic and pyloric mucosa. The analysis of whole transcript expression was performed using Affymetrix© Porcine Gene 1.1 ST array strips. Affymetrix Transcripts IDs were associated with 13 406 human gene names based on Sus scrofa Ensemble. Gene Set Enrichment Analysis was performed, comparing TH and CO pigs. For each gene set, the normalized enrichment score (NES) was defined as significant when the false discovery rate % was <25 and the P-value of NES was <0.05. In response to TH, 72 and 19 gene sets were significantly enriched in the oxyntic and pyloric mucosa, respectively. Several gene sets involved in mitosis and its regulation ranked near the top, primarily in the oxyntic mucosa; the gene set DIGESTION ranked first and ninth in the pyloric and oxyntic mucosa, respectively. Within this group, somatostatin (SST), SST receptors, peptide transporter 1 (SLC15A1) and calpain 9 (gastrointestinal tract-specific calpain) were the most strongly upregulated genes. Thymol reduced the enrichment of 120 and 59 gene sets in the oxyntic and pyloric mucosa, respectively. Several gene sets related to ion transport and channeling and aqueous pores across membranes, including short transient receptor potential (TRP) channel 4, potassium voltage-gated channel members 1 and 2, and ryanodine receptors 2 and 3, were less enriched. The downregulation of these genes sensitive to thymol in vitro could depend on the thymol dose and contact with the gastric tissues that causes an adaptive response with their reduced activation. Conversely, the activation of the TRPA1 gene (ranked 1072 and 128 among all the genes in the oxyntic and pyloric mucosa, respectively) indicates the involvement of another TRP-regulating cellular calcium storage. In conclusion, the stimulation of gastric proliferative activity and the control of digestive activity by thymol can influence positively gastric maturation and function in the weaned pigs. These properties should be considered in addition to thymol's antimicrobial properties when supplementation of this molecule in feed is evaluated.

Characterization and dietary regulation of oligopeptide transporter (PepT1) in different ploidy fishes

The oligopeptide transporter (PepT1) is located on the brush-border membrane of the intestinal epithelium which has been regarded as a mediator of protein absorption. Here, we cloned and characterized PepT1 genes from diploid (red crucian carp), triploid and tetraploid fish. Then, the PepT1 expression pattern in different tissues and embryogenesis were assayed. Meanwhile, using real-time PCR and western blotting, we showed the expression profiles of diets with different protein levels, protein sources and additives (sodium butyrate) in triploids. The cDNAs of the three different ploidy fishes have a high sequence similarity of PepT1 among vertebrates. PepT1 mRNA expression was also developmentally regulated and showed the strongest expression around the 2-cell and 4-cell stage in all three kinds of fishes. The maternal transcripts were first detected in eggs and dropped from blastula stage to muscle contraction stage. Tissue expression studies showed higher expression of PepT1 genes in the intestines of fishes compared with other tissues. In adults, triploids showed significantly higher expression levels of PepT1 in the intestines of the three kinds of ploidy fishes during breeding season and non-breeding season. In addition, high or low protein level diets both promote PepT1 expression in the intestine. We also confirmed that fish meal showed a significant increase in PepT1 expression than soybean meal in triploid intestines. Furthermore, sodium butyrate additives induce PepT1 expression that may be mediated by CDX2 and CREB. This research provides a new insight into protein absorption and its regulation in triploid fish.

Transcriptional and functional regulation of the intestinal peptide transporter PEPT1

Dietary proteins are cleaved within the intestinal lumen to oligopeptides which are further processed to small peptides (di- and tripeptides) and free amino acids. Although the transport of amino acids is mediated by several specific amino acid transporters, the proton-coupled uptake of the more than 8000 different di- and tripeptides is performed by the high-capacity/low-affinity peptide transporter isoform PEPT1 (SLC15A1). Its wide substrate tolerance also allows the transport of a repertoire of structurally closely related compounds and drugs, which explains their high oral bioavailability and brings PEPT1 into focus for medical and pharmaceutical approaches. Although the first evidence for the interplay of nutrient supply and PEPT1 expression and function was described over 20 years ago, many aspects of the molecular processes controlling its transcription and translation and modifying its transporter properties are still awaiting discovery. The present review summarizes the recent knowledge on the factors modulating PEPT1 expression and function in Caenorhabditis elegans, Danio rerio, Mus musculus and Homo sapiens, with focus on dietary ingredients, transcription factors and functional modulators, such as the sodium-proton exchanger NHE3 and selected scaffold proteins.

Influence of overexpression of efflux proteins on the function and gene expression of endogenous peptide transporters in MDR-transfected MDCKII cell lines

The objective of this study is to delineate whether overexpression of human efflux transporters (P-gp, MRP2, and BCRP) in transfected MDCK cells affect the functional activities, and gene and protein expression of endogenous influx peptide transporter system (PepT). Real-time PCR, immunoblotting, uptake and permeability studies of [(3)H]Gly-Sar were conducted on transfected MDCKII and wild-type cells to investigate functional differences. Cellular [(3)H]Gly-Sar accumulation was significantly lower in transfected MDCKII cell lines compared to wild-type cells. Transport efficiency of apical peptide transporters was markedly reduced to around 25%, 30%, and 40% in P-gp-, MRP2-, and BCRP-overexpressed MDCK cell lines, respectively. With ascending cell-passage, transport efficiency was enhanced. A significantly higher Gly-Sar permeability was observed across parental cell-monolayers over transfected cells at all pHs. Levels of mRNA for both canine PepT1 and PepT2 were substantially reduced when efflux transporters overexpressed but enhanced when mRNA-levels of efflux genes diminished with ascending cell-passage of transfected cells. An inverse correlation was evident between endogenous PepT and exogenous efflux transporters in transfected MDCKII cells. Results of protein expression also supported these findings. Overexpression of MDR genes can affect endogenous PepT function which might be due to the phenomenon of transporter-compensation resulting in down-regulation of endogenous genes.

Constitutive expression of an antioxidant enzyme, glutathione S-transferase P1, during differentiation of human intestinal Caco-2 cells

In this study, we investigated the molecular mechanism of the constitutive expression of an antioxidant enzyme, glutathione S-transferase (GST), during differentiation of human intestinal epithelial Caco-2 cells. We observed that the class π GST isozyme (GST P1) expression correlated with the increased expression of caudal-related homeodomain protein 2 (CDX2), a member of the mammalian homeobox family of transcription factors. In addition, transfection of Caco-2 cells with the human CDX2 cDNA resulted in enhanced expression of the GST P1 gene and protein. Chromatin immunoprecipitation assay demonstrated that CDX2 binds to the GST P1 promoter containing the putative consensus CDX-binding element, TTTAC, located at -247 upstream from the established site for transcription initiation. Using the dsDNA pull-down assay, it was revealed that CDX2 recognized and bound to the putative consensus CDX-binding element within the human GST P1 promoter region and that the amount of the CDX2 bound to the putative consensus CDX-binding element increased during Caco-2 cell differentiation. Furthermore, we demonstrated that CDX2 formed the transcriptional complex with Sp1 and bound to the putative consensus CDX-binding element within the human GST P1 promoter region. These data suggest that CDX2 binds to the human GST P1 promoter via complex formation with Sp1 and controls the constitutive expression of GST P1 during Caco-2 cell differentiation.

TNF-α-induced down-regulation of CDX2 suppresses MEP1A expression in colitis

BACKGROUND/AIMS

High levels of pro-inflammatory cytokines are linked to inflammatory bowel disease (IBD). The transcription factor Caudal-related homeobox transcription factor 2 (CDX2) plays a crucial role in differentiation of intestinal epithelium and regulates IBD-susceptibility genes, including meprin 1A (MEP1A). The aim was to investigate the expression of CDX2 and MEP1A in colitis; to assess if they are regulated by tumor necrosis factor-α (TNF-α), and finally to reveal if CDX2 is involved in a TNF-α-induced down-regulation of MEP1A.

METHODS

Expression of CDX2 and MEP1A was investigated in colonic biopsies of ulcerative colitis (UC) patients and in dextran sodium sulfate (DSS)-induced colitis. CDX2 protein expression was investigated by immunoblotting and immunohistochemical procedures. CDX2 and MEP1A regulation was examined in TNF-α-treated Caco-2 cells by reverse transcription-polymerase chain reaction and with reporter gene assays, and the effect of anti-TNF-α treatment was assessed using infliximab. Finally, in vivo CDX2-DNA interactions were investigated by chromatin immunoprecipitation.

RESULTS

The CDX2 and MEP1A mRNA expression was significantly decreased in active UC patients and in DSS-colitis. Colonic biopsy specimens from active UC showed markedly decreased CDX2 staining. TNF-α treatment diminished the CDX2 and MEP1A mRNA levels, a decrease which, was counteracted by infliximab treatment. Reporter gene assays showed significantly reduced CDX2 and MEP1A activity upon TNF-α stimulation. Finally, TNF-α impaired the ability of CDX2 to interact and activate its own, as well as the MEP1A expression.

CONCLUSIONS

The present results indicate that a TNF-α-mediated down-regulation of CDX2 can be related to suppressed expression of MEP1A during intestinal inflammation.

Regulation of the gene encoding the intestinal bile acid transporter ASBT by the caudal-type homeobox proteins CDX1 and CDX2

The apical sodium-dependent bile acid transporter (ASBT) is expressed abundantly in the ileum and mediates bile acid absorption across the apical membranes. Caudal-type homeobox proteins CDX1 and CDX2 are transcription factors that regulate genes involved in intestinal epithelial differentiation and proliferation. Aberrant expression of both ASBT and CDXs in Barrett's esophagus (BE) prompted us to study, whether the expression of the ASBT gene is regulated by CDXs. Short interfering RNA-mediated knockdown of CDXs resulted in reduced ASBT mRNA expression in intestinal cells. CDXs strongly induced the activity of the ASBT promoter in reporter assays in esophageal and intestinal cells. Nine CDX binding sites were predicted in silico within the ASBT promoter, and binding of CDXs to six of them was verified in vitro and within living cells by electrophoretic mobility shift assays and chromatin immunoprecipitation assays, respectively. RNAs were extracted from esophageal biopsies from 20 BE patients and analyzed by real-time PCR. Correlation with ASBT expression was found for CDX1, CDX2, and HNF-1α in BE biopsies. In conclusion, the human ASBT promoter is activated transcriptionally by CDX1 and CDX2. Our finding provides a possible explanation for the reported observation that ASBT is aberrantly expressed in esophageal metaplasia that also expresses CDX transcription factors.

Influence of fed-fasted state on intestinal PEPT1 expression and in vivo pharmacokinetics of glycylsarcosine in wild-type and Pept1 knockout mice

PURPOSE

To determine if fasting would affect the intestinal expression and in vivo functional activity of PEPT1 as determined after oral dosing of the dipeptide glycylsarcosine (GlySar).

METHODS

Systemic exposure and tissue distribution studies were performed in wild-type and Pept1 knockout mice, under fed and fasted conditions, following both intravenous and oral doses of [(14)C]GlySar at 5 nmol/g body weight. Intestinal PEPT1 expression was evaluated by real-time PCR and immunoblot analyses.

RESULTS

We found that expression of PEPT1 protein in the small intestine was increased ~2-fold in wild-type mice during fasted as compared to fed conditions. In agreement, systemic exposure and peak plasma concentrations of orally administered GlySar were 40 and 65% greater, respectively, in wild-type mice during fasted vs. fed state. No significant differences were observed between fed and fasted animals during PEPT1 ablation. Tissue distribution of GlySar was unchanged after oral dosing for all four treatment groups.

CONCLUSIONS

As little as 16 h of fasting can cause significant upregulation of PEPT1 protein expression in the small intestine, which then translates into a significant increase in in vivo oral absorption of GlySar.

Functionally significant nicotine acetylcholine receptor subunit α5 promoter haplotypes are associated with susceptibility to lung cancer in Chinese

BACKGROUND

Although recent genome-wide association studies have been conducted to reveal the relation between variations in the α5 nicotinic receptor subunit and lung cancer in European and American populations, to the authors' knowledge, no definite information on the role of nicotine acetylcholine receptor subunit α5 (CHRNA5) in lung cancer risk has been obtained in a Chinese population.

METHODS

To test this possible association, a case-control study was conducted in 505 patients with lung cancer (cases) and a control group of 498 cancer-free individuals.

RESULTS

Participants were screened for variations in the CHRNA5 promoter region by sequencing, and 2 common polymorphisms were selected at -1640 (reference single nucleotide polymorphism identifier rs3829787 cytosine to thymine [C→T]) and at -62 (rs3841324 insertion→deletion [ins→del]) from the transcription start site of the CHRNA5 gene. Haplotype analysis revealed that the 2 least frequent haplotypes (T/ins and C/del) were statistically protective against lung cancer (P = .0002 and P = .0094, respectively). Unexpectedly, the luciferase results indicated that these 2 protective haplotype constructs had the extremely opposite promoter activity in various cells: the T/ins haplotype had the highest activity and the C/del haplotype had the lowest activity. Surface plasmon resonance demonstrated that both minor alleles (T and del) decreased DNA binding affinity to nuclear extracts, which the authors presumed was responsible for the disparity in promoter activity.

CONCLUSIONS

The current results indicated that the CHRNA5 gene with under-activated or over-activated promoter activity may be protective against lung cancer. These results indicated a new associated risk pattern between CHRNA5 promoter activity and susceptibility to lung cancer that implies a complex role of the CHRNA5 gene in lung cancer.

MicroRNA-92b regulates expression of the oligopeptide transporter PepT1 in intestinal epithelial cells

MicroRNAs (miRNAs), which are noncoding RNAs that posttranscriptionally inhibit expression of target genes, have recently emerged as important regulators of many cellular functions such as cell differentiation. The epithelial di/tripeptide membrane transporter PepT1 is expressed in highly differentiated cells (the villous tip) but not in undifferentiated cells (the crypt) of the small intestine. Here, we investigated the regulation of PepT1 expression by miRNAs and its functional consequences. We observed a reverse correlation between the expression levels of PepT1 and mature miRNA-92b (miR-92b) during the differentiation of intestinal epithelial Caco2-BBE cells, suggesting a miR-92b-mediated regulation of PepT1 expression. We demonstrate that miR-92b suppressed PepT1 expression at both mRNA and protein levels, with subsequent reduced PepT1 transport activity, in Caco2-BBE cells by directly targeting the PepT1 3'-untranslated region. In addition, miR-92b suppresses bacterial peptide-induced proinflammatory responses in intestinal epithelial cells by inhibiting PepT1 expression. Altogether, our study provides for the first time evidence for the regulation of PepT1 expression at a posttranscriptional level by miRNAs in intestinal epithelial cells during pathophysiological states.

The role of CDX2 in intestinal homeostasis and inflammation

Many transcription factors are known to control transcription at several promoters, while others are only active at a few places. However, due to their importance in controlling cellular functions, aberrant transcription factor function and inappropriate gene regulation have been shown to play a causal role in a large number of diseases and developmental disorders. Inflammatory bowel disease (IBD) is characterized by a chronically inflamed mucosa caused by dysregulation of the intestinal immune homeostasis. The aetiology of IBD is thought to be a combination of genetic and environmental factors, including luminal bacteria. The Caudal-related homeobox transcription factor 2 (CDX2) is critical in early intestinal differentiation and has been implicated as a master regulator of the intestinal homeostasis and permeability in adults. When expressed, CDX2 modulates a diverse set of processes including cell proliferation, differentiation, cell adhesion, migration, and tumorigenesis. In addition to these critical cellular processes, there is increasing evidence for linking CDX2 to intestinal inflammation. The aim of the present paper was to review the current knowledge of CDX2 in regulation of the intestinal homeostasis and further to reveal its potential role in inflammation.

PepT1 mediates transport of the proinflammatory bacterial tripeptide L-Ala-{gamma}-D-Glu-meso-DAP in intestinal epithelial cells

PepT1 is a di/tripeptide transporter highly expressed in the small intestine, but poorly or not expressed in the colon. However, during chronic inflammation, such as inflammatory bowel disease, PepT1 expression is induced in the colon. Commensal bacteria that colonize the human colon produce a large amount of di/tripeptides. To date, two bacterial peptides (N-formylmethionyl-leucyl-phenylalanine and muramyl dipeptide) have been identified as substrates of PepT1. We hypothesized that the proinflammatory tripeptide l-Ala-gamma-d-Glu-meso-DAP (Tri-DAP), a breakdown product of bacterial peptidoglycan, is transported into intestinal epithelial cells via PepT1. We found that uptake of glycine-sarcosine, a specific substrate of PepT1, in intestinal epithelial Caco2-BBE cells was inhibited by Tri-DAP in a dose-dependent manner. Tri-DAP induced activation of NF-kappaB and MAP kinases, consequently leading to production of the proinflammatory cytokine interleukin-8. Tri-DAP-induced inflammatory response in Caco2-BBE cells was significantly suppressed by silencing of PepT1 expression by using PepT1-shRNAs in a tetracycline-regulated expression (Tet-off) system. Colonic epithelial HT29-Cl.19A cells, which do not express PepT1 under basal condition, were mostly insensitive to Tri-DAP-induced inflammation. However, HT29-Cl.19A cells exhibited proinflammatory response to Tri-DAP upon stable transfection with a plasmid encoding PepT1. Accordingly, Tri-DAP significantly increased keratinocyte-derived chemokine production in colonic tissues from transgenic mice expressing PepT1 in intestinal epithelial cells. Finally, Tri-DAP induced a significant drop in intracellular pH in intestinal epithelial cells expressing PepT1, but not in cells that did not express PepT1. Our data collectively support the classification of Tri-DAP as a novel substrate of PepT1. Given that PepT1 is highly expressed in the colon during inflammation, PepT1-mediated Tri-DAP transport may occur more effectively during such conditions, further contributing to intestinal inflammation.

Control of intestinal promoter activity of the cellular migratory regulator gene ELMO3 by CDX2 and SP1

An important aspect of the cellular differentiation in the intestine is the migration of epithelial cells from the crypt to the villus tip. As homeodomaine transcription factor CDX2 has been suggested to influence cell migration, we performed a genome-wide promoter analysis for CDX2 binding in the differentiated human intestinal cancer cell line Caco-2 in order to identify CDX2-regulated genes involved in cellular migration. The engulfment and cell motility 3 (ELMO3) gene was identified as a potential CDX2 target gene. ELMO3 is an essential upstream regulator of the GTP-binding protein RAC during cell migration. However, no information is available about the transcriptional regulation of the ELMO3 gene. The aim of this study was to investigate the potential role of CDX2 in the regulation of the ELMO3 promoter activity. Electrophoretic mobility shift assays showed that CDX2 bound to conserved CDX2 sequences and mutations of the CDX2-binding sites, significantly reduced the promoter activity. Reporter gene assays demonstrated that the region mediating ELMO3 basal transcriptional activity to be located between -270 and -31 bp. Sequence analysis revealed no typical TATA-box, but four GC-rich sequences. In vitro analyses (electrophoretic mobility shift assays and promoter analyses) demonstrate that the SP1-binding sites are likely to play an important role in regulating the ELMO3 promoter activity. Furthermore, we showed here that CDX2 and SP1 can activate the ELMO3 promoter. Taken together, the present study reports the first characterization of the ELMO3 promoter and suggests a significant role of CDX2 in the basal transcriptional regulation of the intestine-specific expression of ELMO3, possibly through interaction with SP1.

The nicotinic acetylcholine receptor CHRNA5/A3/B4 gene cluster: dual role in nicotine addiction and lung cancer

More than 1 billion people around the world smoke, with 10 million cigarettes sold every minute. Cigarettes contain thousands of harmful chemicals including the psychoactive compound, nicotine. Nicotine addiction is initiated by the binding of nicotine to nicotinic acetylcholine receptors, ligand-gated cation channels activated by the endogenous neurotransmitter, acetylcholine. These receptors serve as prototypes for all ligand-gated ion channels and have been extensively studied in an attempt to elucidate their role in nicotine addiction. Many of these studies have focused on heteromeric nicotinic acetylcholine receptors containing α4 and β2 subunits and homomeric nicotinic acetylcholine receptors containing the α7 subunit, two of the most abundant subtypes expressed in the brain. Recently however, a series of linkage analyses, candidate-gene analyses and genome-wide association studies have brought attention to three other members of the nicotinic acetylcholine receptor family: the α5, α3 and β4 subunits. The genes encoding these subunits lie in a genomic cluster that contains variants associated with increased risk for several diseases including nicotine dependence and lung cancer. The underlying mechanisms for these associations have not yet been elucidated but decades of research on the nicotinic receptor gene family as well as emerging data provide insight on how these receptors may function in pathological states. Here, we review this body of work, focusing on the clustered nicotinic acetylcholine receptor genes and evaluating their role in nicotine addiction and lung cancer.

Pathogenic bacteria induce colonic PepT1 expression: an implication in host defense response

BACKGROUND & AIMS

Expression of the di/tripeptide transporter PepT1 has been observed in the colon under inflammatory conditions; however, the inducing factors and underlying mechanisms remain unknown. Here, we address the effects of pathogenic bacteria on colonic PepT1 expression together with its functional consequences.

METHODS

Human colonic HT29-Cl.19A cells were infected with the attaching and effacing enteropathogenic Escherichia coli (EPEC). Wild-type and PepT1 transgenic mice or cultured colonic tissues derived from these mice were infected with Citrobacter rodentium, a murine attaching and effacing pathogen related to EPEC.

RESULTS

EPEC induced PepT1 expression and activity in HT29-Cl.19A cells by intimately attaching to host cells through lipid rafts. Induction of PepT1 expression by EPEC required the transcription factor Cdx2. PepT1 expression reduced binding of EPEC to lipid rafts, as well as activation of nuclear factor-kappaB and mitogen-activated protein kinase and production of interleukin-8. Accordingly, ex vivo and in vivo experiments revealed that C rodentium induced colonic PepT1 expression and that, compared with their wild-type counterparts, PepT1 transgenic mice infected with C rodentium exhibited decreased bacterial colonization, production of proinflammatory cytokines, and neutrophil infiltration into the colon.

CONCLUSIONS

Our findings demonstrate a molecular mechanism underlying the regulation of colonic PepT1 expression under pathologic conditions and reveal a novel role for PepT1 in host defense via its capacity to modulate bacterial-epithelial interactions and intestinal inflammation.

Reduced intestinal absorption of dipeptides via PepT1 in mice with diet-induced obesity is associated with leptin receptor down-regulation

Leptin is a major determinant of energy homeostasis, acting both centrally and in the gastrointestinal tract. We previously reported that acute leptin treatment enhances the absorption of di- and tripeptides via the proton-dependent PepT1 transporter. In this study, we investigated the long term effect of leptin on PepT1 levels and activity in Caco2 cell monolayers in vitro. We then assessed the significance of the regulation of PepT1 in vivo in a model of diet-induced obesity. We demonstrated that 1) leptin regulated PepT1 at the transcriptional level, via the MAPK pathway, and at the translational level, via ribosomal protein S6 activation, in Caco2 cells and 2) this activation was systematically followed by a time- and concentration-dependent loss of leptin action reflecting desensitization. Deciphering this desensitization, we demonstrated that leptin induced a down-regulation of its own receptor protein and mRNA expression. More importantly, we showed, in mice with diet-induced obesity, that a 4-week hypercaloric diet resulted in a 46% decrease in PepT1-specific transport, because of a 30% decrease in PepT1 protein and a 50% decrease in PepT1 mRNA levels. As shown in Caco2 cells, these changes in PepT1 were supported by a parallel 2-fold decrease in leptin receptor expression in mice. Taken together, these results indicate that during induction of obesity, leptin resistance may also occur peripherally in the gastrointestinal tract, disrupting the absorption of oligopeptides and peptidomimetic drugs.

Peptide transporters and their roles in physiological processes and drug disposition

1. The peptide transporters belong to the peptide transporter (PTR) family and serve as integral membrane proteins for the cellular uptake of di- and tripeptides in the organism. By their ability also to transport peptidomimetics and other substrates with therapeutic activities or precursors of pharmacologically active agents, they are of considerable importance in pharmacology. 2. PEPT1 is the low-affinity, high-capacity transporter and is mainly expressed in the small intestine, whereas PEPT2 is the high-affinity, low-capacity transporter and has a broader distribution in the organism. 3. Targeted mouse models have revealed PEPT2 to be the dominant transporter for the reabsorption of di- and tripeptides and its pharmacological substrates in the organism, and for the removal of these substrates from the cerebrospinal fluid. Moreover, the peptide transporters undergo physiological and pharmacological regulation and, of great interest, are present in disease states where PEPT1 exhibits ectopic expression in colonic inflammation. 4. The paper reviews the structural characteristics of the peptide transporters, the structural requirements for substrates, the distribution of the peptide transporters in the organism, and finally their regulation in the organism in healthy and pathological situations.

Comparison of drug transporter gene expression and functionality in Caco-2 cells from 10 different laboratories

Caco-2 cells, widely used to study carrier mediated uptake and efflux mechanisms, are known to have different properties when cultured under different conditions. In this study, Caco-2 cells from 10 different laboratories were compared in terms of mRNA expression levels of 72 drug and nutrient transporters, and 17 other target genes, including drug metabolising enzymes, using real-time PCR. The rank order of the top five expressed genes was: HPT1>GLUT3>GLUT5>GST1A>OATP-B. Rank correlation showed that for most of the samples, the gene ranking was not significantly different. Functionality of transporters and the permeability of passive transport markers metoprolol (transcellular) and atenolol (paracellular) were also compared. MDR1 and PepT1 function was investigated using talinolol and Gly-Sar transport, respectively. Sulfobromophthalein (BSP) was used as a marker for MRP2 and OATP-B functionality. Atenolol permeability was more variable across laboratories than metoprolol permeability. Talinolol efflux was observed by all the laboratories, whereas only five laboratories observed significant apical uptake of Gly-Sar. Three laboratories observed significant efflux of BSP. MDR1 expression significantly correlated to the efflux ratio and net active efflux of talinolol. PepT1 mRNA levels showed significant correlation to the uptake ratio and net active uptake of Gly-Sar. MRP2 and OATP-B showed no correlation to BSP transport parameters. Heterogeneity in transporter activity may thus be due to differences in transporter expression as shown for PepT1 and MDR1 which in turn is determined by the culture conditions. Absolute expression of genes was variable indicating that small differences in culture conditions have a significant impact on gene expression, although the overall expression patterns were similar.

Regulatory mechanism governing the diurnal rhythm of intestinal H+/peptide cotransporter 1 (PEPT1)

The intestinal H(+)/peptide cotransporter 1 (PEPT1) plays important roles as a nutrient and drug transporter. Previously, we reported that rat intestinal PEPT1 showed a diurnal rhythm and that this rhythm is closely related to the feeding schedule. Furthermore, we also demonstrated that transcription factors, Sp1, Cdx2, and peroxisome proliferator-activated receptor-alpha (PPAR-alpha) contribute to the basal, intestine-specific, and fasting-induced expression of PEPT1, respectively. In this study, to clarify the molecular mechanism governing the diurnal rhythm of PEPT1 expression, we compared expression profiles of these transcription factors under two kinds of feeding schedules. The intestinal Sp1 and Cdx2 did not show a circadian accumulation of mRNA or response to the daytime feeding regimen. Plasma free fatty acids, endogenous PPAR-alpha ligands, exhibited a robust circadian fluctuation in phase with that of PEPT1. However, subsequent experiments using PPAR-alpha-null mice revealed the absence of any association between the circadian rhythm of PEPT1 and PPAR-alpha. We then focused on the clock genes (Clock, Bmal1, Per1-2, and Cry1) and clock-controlled gene, albumin D site-binding protein (DBP). A robust and coordinated circadian expression of the clock genes was observed, and daytime feeding entirely inverted the phase except for Clock. The expression of DBP was in phase with that of PEPT1 in both groups. Electrophoretic mobility shift assays and reporter assays revealed that DBP has the ability to bind the DBP binding site located in the distal promoter region of the rat PEPT1 gene and induce the transcriptional activity. These findings indicate that DBP plays pivotal roles in the circadian oscillation of PEPT1.

Kidney-specific expression of human organic cation transporter 2 (OCT2/SLC22A2) is regulated by DNA methylation

Human organic cation transporter 2 (OCT2/SLC22A2), which is specifically expressed in the kidney, plays critical roles in the renal secretion of cationic compounds. Tissue expression and membrane localization of OCT2 are closely related to the tissue distribution, pharmacological effects, and/or adverse effects of its substrate drugs. However, the molecular mechanisms underlying the kidney-specific expression of OCT2 have not been elucidated. In the present study, therefore, we examined the contribution of DNA methylation of the promoter region for the OCT2 gene to its tissue-specific expression using human tissue samples. In vivo methylation status of the proximal promoter region of OCT2 and that of OCT1, a liver-specific organic cation transporter, were investigated by bisulfite sequencing using human genomic DNA extracted from the kidney and liver. All CpG sites in the OCT2 proximal promoter were hypermethylated in the liver, while hypomethylated in the kidney. On the other hand, the promoter region of OCT1 was hypermethylated in both the kidney and liver. The level of methylation of the OCT2 promoter was especially low at the CpG site in the E-box, the binding site of the basal transcription factor upstream stimulating factor (USF) 1. In vitro methylation of the OCT2 proximal promoter dramatically reduced the transcriptional activity, and an electrophoretic mobility shift assay showed that methylation at the E-box inhibited the binding of USF1. These results indicate that kidney-specific expression of human OCT2 is regulated by methylation of the proximal promoter region, interfering with the transactivation by USF1.

Butyrate transcriptionally enhances peptide transporter PepT1 expression and activity

Background

PepT1, an intestinal epithelial apical di/tripeptide transporter, is normally expressed in the small intestine and induced in colon during chronic inflammation. This study aimed at investigating PepT1 regulation by butyrate, a short-chain fatty acid produced by commensal bacteria and accumulated inside inflamed colonocyte.

Results

We found that butyrate treatment of human intestinal epithelial Caco2-BBE cells increased human PepT1 (hPepT1) promoter activity in a dose- and time-dependent manner, with maximal activity observed in cells treated with 5 mM butyrate for 24 h. Under this condition, hPepT1 promoter activity, mRNA and protein expression levels were increased as assessed by luciferase assay, real-time RT-PCR and Western blot, respectively. hPepT1 transport activity was accordingly increased by ∼2.5-fold. Butyrate did not alter hPepT1 mRNA half-life indicating that butyrate acts at the transcriptional level. Molecular analyses revealed that Cdx2 is the most important transcription factor for butyrate-induced increase of hPepT1 expression and activity in Caco2-BBE cells. Butyrate-activated Cdx2 binding to hPepT1 promoter was confirmed by gel shift and chromatin immunoprecipitation. Moreover, Caco2-BBE cells overexpressing Cdx2 exhibited greater hPepT1 expression level than wild-type cells. Finally, treatment of mice with 5 mM butyrate added to drinking water for 24 h increased colonic PepT1 mRNA and protein expression levels, as well as enhanced PepT1 transport activity in colonic apical membranes vesicles.

Conclusions

Collectively, our results demonstrate that butyrate increases PepT1 expression and activity in colonic epithelial cells, which provides a new understanding of PepT1 regulation during chronic inflammation.

Board-invited review: Peptide absorption and utilization: Implications for animal nutrition and health

Over the last 50 yr, the study of intestinal peptide transport has rapidly evolved into a field with exciting nutritional and biomedical applications. In this review, we describe from a historical and current perspective intestinal peptide transport, the importance of peptides to whole-body nutrition, and the cloning and characterization of the intestinal peptide transporter, PepT1. We focus on the nutritional significance of peptide transport and relate these findings to livestock and poultry. Amino acids are transported into the enterocyte as free AA by a variety of AA transporters that vary in substrate specificity or as di- and tripeptides by the peptide transporter, PepT1. Expression of PepT1 is largely restricted to the small intestine in most species; however, in ruminants, peptide transport and activity is observed in the rumen and omasum. The extent to which peptides are absorbed and utilized is still unclear. In ruminants, peptides make a contribution to the portal-drained visceral flux of total AA and are detected in circulating plasma. Peptides can be utilized by the mammary gland for milk protein synthesis and by a variety of other tissues. We discuss the factors known to regulate expression of PepT1 including development, diet, hormones, diurnal rhythm, and disease. Expression of PepT1 is detected during embryological stages in both birds and mammals and increases with age, a strategic event that allows for the immediate uptake of nutrients after hatch or birth. Both increasing levels of protein in the diet and dietary protein deficiencies are found to upregulate the peptide transporter. We also include in this review a discussion of the use of dietary peptides and potential alternate routes of nutrient delivery to the cell. Our goal is to impart to the reader the nutritional implications of peptide transport and dietary peptides and share discoveries that shed light on various biological processes, including rapid establishment of intestinal function in early neonates and maintenance of intestinal function during fasting, starvation, and disease states.

PPARalpha agonists positively and negatively regulate the expression of several nutrient/drug transporters in mouse small intestine

A systematic analysis to examine the effects of peroxisome proliferator-activated receptor (PPAR)alpha agonists on the expression levels of all the nutrient/drug plasma-membrane transporters in the mouse small intestine was performed. Transporter mRNAs that were induced or repressed by two independent PPARalpha-specific agonists were identified by a genome-wide microarray method, and the changes were confirmed by real-time PCR using RNA isolated from the intestines and livers of wild-type and PPARalpha-null mice. Expression levels of seven nutrient/drug transporters (Abcd3, Octn2/Slc22a5, FATP2/Slc27a2, Slc22a21, Mct13/Slc16a13, Slc23a1 and Bcrp/Abcg2) in the intestine were up-regulated and the expression level of one (Mrp1/Abcc1) was down-regulated by PPARalpha; although the previously report that the H(+)/peptide co-transporter 1 (Pept1) is up-regulated by PPARalpha was not replicated in our study. We propose that the transport processes can be coordinately regulated with intracellular metabolism by nutrient nuclear receptors.

Critical roles of Sp1 in gene expression of human and rat H+/organic cation antiporter MATE1

A H+/organic cation antiporter (multidrug and toxin extrusion 1: MATE1/SLC47A1) plays important roles in the tubular secretion of various clinically important cationic drugs such as cimetidine. We have recently found that the regulation of this transporter greatly affects the pharmacokinetic properties of cationic drugs in vivo. No information is available about the regulatory mechanisms for the MATE1 gene. In the present study, therefore, we examined the gene regulation of human (h) and rat (r) MATE1, focusing on basal expression. A deletion analysis suggested that the regions spanning -65/-25 and -146/-38 were essential for the basal transcriptional activity of the hMATE1 and rMATE1 promoter, respectively, and that both regions contained putative Sp1-binding sites. Functional involvement of Sp1 was confirmed by Sp1 overexpression, a mutational analysis of Sp1-binding sites, mithramycin A treatment, and an electrophoretic mobility shift assay. Furthermore, we found a single nucleotide polymorphism (SNP) in the promoter region of hMATE1 (G-32A), which belongs to a Sp1-binding site. The allelic frequency of this rSNP was 3.7%, and Sp1-binding and promoter activity were significantly decreased. This is the first study to clarify the transcriptional mechanisms of the MATE1 gene and to identify a SNP affecting the promoter activity of hMATE1.

The human mucin MUC4 is transcriptionally regulated by caudal-related homeobox, hepatocyte nuclear factors, forkhead box A, and GATA endodermal transcription factors in epithelial cancer cells

The human gene MUC4 encodes a large transmembrane mucin that is developmentally regulated and expressed along the undifferentiated pseudostratified epithelium, as early as 6.5 weeks during fetal development. Immunohistochemical analysis of Muc4 expression in developing mouse lung and gastrointestinal tract showed a different spatio-temporal pattern of expression before and after cytodifferentiation. The molecular mechanisms governing MUC4 expression during development are, however, unknown. Hepatocyte nuclear factors (HNF), forkhead box A (FOXA), GATA, and caudal-related homeobox transcription factors (TFs) are known to control cell differentiation of gut endoderm derived-tissues during embryonic development. They also control the expression of cell- and tissue-specific genes and may thus control MUC4 expression. To test this hypothesis, we studied and deciphered the molecular mechanisms responsible for MUC4 transcriptional regulation by these TFs. Experiments using small interfering RNA, cell co-transfection, and site-directed mutagenesis indicated that MUC4 is regulated at the transcriptional level by CDX-1 and -2, HNF-1 alpha and -1 beta, FOXA1/A2, HNF-4 alpha and -4 gamma, and GATA-4, -5, and -6 factors in a cell-specific manner. Binding of TFs was assessed by chromatin immunoprecipitation, and gel-shift assays. Altogether, these results demonstrate that MUC4 is a target gene of endodermal TFs and thus point out an important role for these TFs in regulating MUC4 expression during epithelial differentiation during development, cancer, and repair.

Transcription Factor PHOX2A Regulates the Human α3 Nicotinic Receptor Subunit Gene Promoter

PHOX2A is a paired-like homeodomain transcription factor that participates in specifying the autonomic nervous system. It is also involved in the transcriptional control of the noradrenergic neurotransmitter phenotype as it regulates the gene expression of tyrosine hydroxylase and dopamine-beta-hydroxylase. The results of this study show that the human orthologue of PHOX2A is also capable of regulating the transcription of the human alpha3 nicotinic acetylcholine receptor gene, which encodes the ligand-binding subunit of the ganglionic type nicotinic receptor. In particular, we demonstrated by chromatin immunoprecipitation and DNA pulldown assays that PHOX2A assembles on the SacI-NcoI region of alpha3 promoter and, by co-transfection experiments, that it exerts its transcriptional effects by acting through the 60-bp minimal promoter. PHOX2A does not seem to bind to DNA directly, and its DNA binding domain seems to be partially dispensable for the regulation of alpha3 gene transcription. However, as suggested by the findings of our co-immunoprecipitation assays, it may establish direct or indirect protein-protein interactions with Sp1, thus regulating the expression of alpha3 through a DNA-independent mechanism. As the alpha3 subunit is expressed in every terminally differentiated ganglionic cell, this is the first example of a "pan-autonomic" gene whose expression is regulated by PHOX2 proteins.

Hepatocyte nuclear factor-4{alpha} regulates the human organic anion transporter 1 gene in the kidney

Human organic anion transporter 1 (OAT1, SLC22A6), which is localized to the basolateral membranes of renal tubular epithelial cells, plays a critical role in the excretion of anionic compounds. OAT1 is regulated by various pathophysiological conditions, but little is known about the molecular mechanisms regulating the expression of OAT1. In the present study, we investigated the transcriptional regulation of OAT1 and found that hepatocyte nuclear factor (HNF)-4alpha markedly transactivated the OAT1 promoter. A deletion analysis of the OAT1 promoter suggested that the regions spanning -1191 to -700 base pairs (bp) and -140 to -79 bp were essential for the transactivation by HNF-4alpha. These regions contained a direct repeat separated by two nucleotides (DR-2), which is one of the consensus sequences binding to HNF-4alpha, and an inverted repeat separated by eight nucleotides (IR-8), which was recently identified as a novel element for HNF-4alpha, respectively. An electrophoretic mobility shift assay showed that HNF-4alpha bound to DR-2 and IR-8 under the conditions of HNF-4alpha overexpression. Furthermore, under normal conditions, HNF-4alpha bound to IR-8, and a mutation in IR-8 markedly reduced the OAT1 promoter activity, indicating that HNF-4alpha regulates the basal transcription of OAT1 via IR-8. This paper reports the first characterization of the human OAT1 promoter and the first gene in the kidney whose promoter activity is regulated by HNF-4alpha.

Gene expression and regulation of drug transporters in the intestine and kidney

Intestinal absorption and renal secretion of ionic drugs are controlled by a number of drug transporters expressed at the brush-border and basolateral membranes of epithelial cells. Over the last several years, considerable progress has been made regarding the molecular identification and functional characterization of drug transporters. Under some physiological and pathophysiological conditions, the expression and transport activity of drug transporters are changed, affecting the pharmacokinetics of substrate drugs. The regulation of transport activity in response to endogenous and exogenous signals can occur at various levels such as transcription, mRNA stability, translation, and posttranslational modification. Transcriptional regulation is of particular interest, because changes in transport activity are dynamically regulated by increases or decreases in levels of mRNA expression. The tissue-specific expression of drug transporters is also under transcriptional control, and recent studies using clinical samples from human tissues have revealed the expression profiles of drug transporters in the human body. The purpose of this research updates is to review the recent progress in the study of the gene expression and regulation of intestinal and renal drug transporters.

Induction of intestinal peptide transporter 1 expression during fasting is mediated via peroxisome proliferator-activated receptor alpha

We previously demonstrated that starvation markedly increased the amount of mRNA and protein levels of the intestinal H+/peptide cotransporter (PEPT1) in rats, leading to altered pharmacokinetics of the PEPT1 substrates. In the present study, the mechanism underlying this augmentation was investigated. We focused on peroxisome proliferator-activated receptor alpha (PPARalpha), which plays a pivotal role in the adaptive response to fasting in the liver and other tissues. In 48-h fasted rats, the expression level of PPARalpha mRNA in the small intestine markedly increased, accompanied by the elevation of serum free fatty acids, which are endogenous PPARalpha ligands. Oral administration of the synthetic PPARalpha ligand WY-14643 to fed rats increased the mRNA level of intestinal PEPT1. Furthermore, treatment of the human intestinal model, Caco-2 cells, with WY-14643 resulted in enhanced PEPT1 mRNA expression and uptake activity of glycylsarcosine. In the small intestine of PPARalpha-null mice, augmentation of PEPT1 mRNA during fasting was completely abolished. In the kidney, fasting did not induce PEPT1 expression in either PPARalpha-null or wild-type mice. Together, these results indicate that PPARalpha plays critical roles in fasting-induced intestinal PEPT1 expression. In addition to the well-established roles of PPARalpha, we propose a novel function of PPARalpha in the small intestine, that is, the regulation of nitrogen absorption through PEPT1 during fasting.