Transcriptional regulation of the human NaPi-IIb cotransporter by EGF in Caco-2 cells involves c-myb

Mechanisms of Epidermal Growth Factor Effect on Animal Intestinal Phosphate Absorption: A Review

Phosphorus is one of the essential mineral elements of animals that plays an important role in animal growth and development, bone formation, energy metabolism, nucleic acid synthesis, cell signal transduction, and blood acid–base balance. It has been established that the Type IIb sodium-dependent phosphate cotransporters (NaPi-IIb) protein is the major sodium-dependent phosphate (Pi) transporter, which plays an important role in Pi uptake across the apical membrane of epithelial cells in the small intestine. Previous studies have demonstrated that epidermal growth factor (EGF) is involved in regulating intestinal Pi absorption. Here we summarize the effects of EGF on active Pi transport of NaPi-IIb under different conditions. Under normal conditions, EGF inhibits the active transport of Pi by inhibiting the expression of NaPi-IIb, while, under intestinal injury condition, EGF promotes the active absorption of Pi through upregulating the expression of NaPi-IIb. This review provides a reference for information about EGF-regulatory functions in Pi absorption in the animal intestine.

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

Inorganic phosphate (Pi) plays a critical function in many tissues of the body: for example, as part of the hydroxyapatite in the skeleton and as a substrate for ATP synthesis. Pi is the main source of dietary phosphorus. Reduced bioavailability of Pi or excessive losses in the urine causes rickets and osteomalacia. While critical for health in normal amounts, dietary phosphorus is plentiful in the Western diet and is often added to foods as a preservative. This abundance of phosphorus may reduce longevity due to metabolic changes and tissue calcifications. In this review, we examine how dietary phosphorus is absorbed in the gut, current knowledge about Pi sensing, and endocrine regulation of Pi levels. Moreover, we also examine the roles of Pi in different tissues, the consequences of low and high dietary phosphorus in these tissues, and the implications for healthy aging.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Mechanisms and Regulation of Intestinal Phosphate Absorption

States of hypo- and hyperphosphatemia have deleterious consequences including rickets/osteomalacia and renal/cardiovascular disease, respectively. Therefore, the maintenance of appropriate plasma levels of phosphate is an essential requirement for health. This control is executed by the collaborative action of intestine and kidney whose capacities to (re)absorb phosphate are regulated by a number of hormonal and metabolic factors, among them parathyroid hormone, fibroblast growth factor 23, 1,25(OH)₂ vitamin D₃ , and dietary phosphate. The molecular mechanisms responsible for the transepithelial transport of phosphate across enterocytes are only partially understood. Indeed, whereas renal reabsorption entirely relies on well-characterized active transport mechanisms of phosphate across the renal proximal epithelia, intestinal absorption proceeds via active and passive mechanisms, with the molecular identity of the passive component still unknown. The active absorption of phosphate depends mostly on the activity and expression of the sodium-dependent phosphate cotransporter NaPi-IIb (SLC34A2), which is highly regulated by many of the factors, mentioned earlier. Physiologically, the contribution of NaPi-IIb to the maintenance of phosphate balance appears to be mostly relevant during periods of low phosphate availability. Therefore, its role in individuals living in industrialized societies with high phosphate intake is probably less relevant. Importantly, small increases in plasma phosphate, even within normal range, associate with higher risk of cardiovascular disease. Therefore, therapeutic approaches to treat hyperphosphatemia, including dietary phosphate restriction and phosphate binders, aim at reducing intestinal absorption. Here we review the current state of research in the field. © 2017 American Physiological Society. Compr Physiol 8:1065-1090, 2018.

Effect of early dietary energy restriction and phosphorus level on subsequent growth performance, intestinal phosphate transport, and AMPK activity in young broilers

We aimed to determine the effect of low dietary energy on intestinal phosphate transport and the possible underlying mechanism to explain the long-term effects of early dietary energy restriction and non-phytate phosphorus (NPP). A 2 × 3 factorial experiment, consisting of 2 energy levels and 3 NPP levels, was conducted. Broiler growth performance, intestinal morphology in 0–21 days and 22–35 days, type IIb sodium-phosphate co-transporter (NaP_i-IIb) mRNA expression, adenylate purine concentrations in the duodenum, and phosphorylated adenosine monophosphate-activated protein kinase (AMPK-α) activity in 0–21 days were determined. The following results were obtained. (1) Low dietary energy (LE) induced a high feed conversion ratio (FCR) and significantly decreased body weight gain in young broilers, but LE induced significantly higher compensatory growth in low NPP (LP) groups than in the high or medium NPP groups (HP and MP). (2) LE decreased the villus height (VH) in the intestine, and LE-HP resulted in the lowest crypt depth (CD) and the highest VH:CD ratio in the initial phase. However, in the later period, the LE-LP group showed an increased VH:CD ratio and decreased CD in the intestine. (3) LE increased ATP synthesis and decreased AMP:ATP ratio in the duodenal mucosa of chickens in 0–21 days, and LP diet increased ATP synthesis and adenylate energy charges but decreased AMP production and AMP:ATP ratio. (4) LE led to weaker AMPK phosphorylation, higher mTOR phosphorylation, and higher NaP_i-IIb mRNA expression. Thus, LE and LP in the early growth phase had significant compensatory and interactive effect on later growth and intestinal development in broilers. The effect might be relevant to energy status that LE leads to weaker AMPK phosphorylation, causing a lower inhibitory action toward mTOR phosphorylation. This series of events stimulates NaP_i-IIb mRNA expression. Our findings provide a theoretical basis and a new perspective on intestinal phosphate transport regulation, with potential applications in broiler production.

Tumour-induced osteomalacia

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic disorder caused by tumours that secrete fibroblast growth factor 23 (FGF23). Owing to the role of FGF23 in renal phosphate handling and vitamin D synthesis, TIO is characterized by decreased renal tubular reabsorption of phosphate, by hypophosphataemia and by low levels of active vitamin D. Chronic hypophosphataemia ultimately results in osteomalacia (that is, inadequate bone mineralization). The diagnosis of TIO is usually suspected when serum phosphate levels are chronically low in the setting of bone pain, fragility fractures and muscle weakness. Locating the offending tumour can be very difficult, as the tumour is often very small and can be anywhere in the body. Surgical removal of the tumour is the only definitive treatment. When the tumour cannot be located or when complete resection is not possible, medical treatment with phosphate salts or active vitamin D is necessary. One of the most promising emerging treatments for unresectable tumours that cause TIO is the anti-FGF23 monoclonal antibody KRN23. The recent identification of a fusion of fibronectin and fibroblast growth factor receptor 1 (FGFR1) as a molecular driver in some tumours not only sheds light on the pathophysiology of TIO but also opens the door to a better understanding of the transcription, translocation, post-translational modification and secretion of FGF23, as well as suggesting approaches to targeted therapy. Further study will reveal if the FGFR1 pathway is also involved in tumours that do not harbour the translocation.

Plasma Triglyceride Levels May Be Modulated by Gene Expression of IQCJ, NXPH1, PHF17 and MYB in Humans

A genome-wide association study (GWAS) by our group identified loci associated with the plasma triglyceride (TG) response to ω-3 fatty acid (FA) supplementation in IQCJ, NXPH1, PHF17 and MYB. Our aim is to investigate potential mechanisms underlying the associations between single nucleotide polymorphisms (SNPs) in the four genes and TG levels following ω-3 FA supplementation. 208 subjects received 3 g/day of ω-3 FA (1.9–2.2 g of EPA and 1.1 g of docosahexaenoic acid (DHA)) for six weeks. Plasma TG were measured before and after the intervention. 67 SNPs were selected to increase the density of markers near GWAS hits. Genome-wide expression and methylation analyses were conducted on respectively 30 and 35 participants’ blood sample together with in silico analyses. Two SNPs of IQCJ showed different affinities to splice sites depending on alleles. Expression levels were influenced by genotype for one SNP in NXPH1 and one in MYB. Associations between 12 tagged SNPs of IQCJ, 26 of NXPH1, seven of PHF17 and four of MYB and gene-specific CpG site methylation levels were found. The response of plasma TG to ω-3 FA supplementation may be modulated by the effect of DNA methylation on expression levels of genes revealed by GWAS.

Identifying the location of epidermal growth factor-responsive element involved in the regulation of type IIb sodium-phosphate cotransporter expression in porcine intestinal epithelial cells

Phosphate is an important mineral nutrient for both human and animals in growth and physiological functions; thus, much effort in the past has been made to clarify the mechanisms governing its absorption. Previous studies have found that epidermal growth factor (EGF) inhibits phosphate absorption in human intestinal cells via modulating the interaction of transcriptional factor c-myb with sodium-phosphate cotransporter (NaPi-IIb) gene promoter. This finding provoked our interest in determining the effect of EGF on NaPi-IIb gene expression in intestinal cells of pigs and the location of EGF-responsive element in the gene promoter. Using quantitative PCR, it was observed that EGF significantly reduced NaPi-IIb gene expression in porcine intestinal epithelial IPEC-J2 cells. Transfection with a series of constructs that contain different lengths of the 5'-flanking promoter region of the NaPi-IIb gene manifested that EGF-responsive element is located in the -1200 to -800 region. Further, c-myb was extracted from the cell nucleus of IPEC cells that were exposed to EGF or not via immunoprecipitation. The electrophoretic mobility shift assay showed a specific binding of transcription factor c-myb to labelled probes encompassing DNA sequence from -1092 to -1085 (-TCCAGTTG-). This protein-DNA complex was decreased with cells exposed to EGF and abrogated when c-myb was pre-incubated with excessive unlabelled competitive probes. Results from mutagenesis studies demonstrated that the c-myb-binding site is the EGF-responsive element involved in the regulation of NaPi-IIb expression. Identifying the location of EGF-responsive element contributes to understanding mechanisms underlying EGF down-regulated NaPi-IIb gene expression and provides a foundation for further investigating EGF-regulatory functions in phosphate absorption in pig intestine.

Small molecule inhibition of cAMP response element binding protein in human acute myeloid leukemia cells

The transcription factor CREB (cAMP Response-Element Binding Protein) is overexpressed in the majority of acute myeloid leukemia (AML) patients, and this is associated with a worse prognosis. Previous work revealed that CREB overexpression augmented AML cell growth, while CREB knockdown disrupted key AML cell functions in vitro. In contrast, CREB knockdown had no effect on long-term hematopoietic stem cell activity in mouse transduction/transplantation assays. Together, these studies position CREB as a promising drug target for AML. To test this concept, a small molecule inhibitor of CREB, XX-650-23, was developed. This molecule blocks a critical interaction between CREB and its required co-activator CBP (CREB Binding Protein), leading to disruption of CREB-driven gene expression. Inhibition of CBP-CREB interaction induced apoptosis and cell-cycle arrest in AML cells, and prolonged survival in vivo in mice injected with human AML cells. XX-650-23 had little toxicity on normal human hematopoietic cells and tissues in mice. To understand the mechanism of XX-650-23, we performed RNA-seq, ChIP-seq and Cytometry Time of Flight with human AML cells. Our results demonstrate that small molecule inhibition of CBP-CREB interaction mostly affects apoptotic, cell-cycle and survival pathways, which may represent a novel approach for AML therapy.

Novel Genetic Loci Associated with the Plasma Triglyceride Response to an Omega-3 Fatty Acid Supplementation

BACKGROUND

A recent genome-wide association study (GWAS) by our group identified 13 loci associated with the plasma triglyceride (TG) response to omega-3 (n-3) fatty acid (FA) supplementation. This study aimed to test whether single-nucleotide polymorphisms (SNPs) within the IQCJ, NXPH1, PHF17 and MYB genes are associated with the plasma TG response to an n-3 FA supplementation.

METHODS

A total of 208 subjects followed a 6-week n-3 FA supplementation of 5 g/day of fish oil (1.9-2.2 g of eicosapentaenoic acid and 1.1 g of docosahexaenoic acid). Measurements of plasma lipids were made before and after the supplementation. Sixty-seven tagged SNPs were selected to increase the density of markers near GWAS hits.

RESULTS

In a repeated model, independent effects of the genotype and the gene-supplementation interaction were associated with plasma TG. Genotype effects were observed with two SNPs of NXPH1, and gene-diet interactions were observed with ten SNPs of IQCJ, four SNPs of NXPH1 and three SNPs of MYB. Positive and negative responders showed different genotype frequencies with nine SNPs of IQCJ, two SNPs of NXPH1 and two SNPs of MYB.

CONCLUSION

Fine mapping in GWAS-associated loci allowed the identification of SNPs partly explaining the large interindividual variability observed in plasma TG levels in response to an n-3 FA supplementation.

Effect of Dietary Nutrient Density on Small Intestinal Phosphate Transport and Bone Mineralization of Broilers during the Growing Period

A 2 × 4 factorial experiment was conducted to determine the effects of dietary nutrient density on growth performance, small intestinal epithelial phosphate transporter expression, and bone mineralization of broiler chicks fed with diets with different nutrient densities and nonphytate phosphorus (NPP) levels. The broilers were fed with the same starter diets from 0 to 21 days of age. In the grower phase (day 22 to 42), the broilers were randomly divided into eight groups according to body weight. Relatively high dietary nutrient density (HDND) and low dietary nutrient density (LDND) diets were assigned metabolic energy (ME) values of 3,150 and 2,950 kcal/kg, respectively. Crude protein and essential amino acid levels were maintained in the same proportion as ME to prepare the two diet types. NPP levels were 0.25%, 0.30%, 0.35%, and 0.40% of the diets. Results showed that a HDND diet significantly increased the body weight gain (BWG) of broilers and significantly decreased the feed conversion ratio and NPP consumed per BWG. HDND significantly decreased tibial P content of the broilers. Conversely, mRNA expression of NaPi-IIb and protein expression of calbindin were significantly increased in the intestine of broilers fed a HDND diet. HDND also increased vitamin D receptor (VDR) expression, especially at a relatively low dietary NPP level (0.25%). The mRNA expression of NaPi-IIa in the kidneys was significantly increased at a relatively low dietary NPP level (0.25%) to maintain P balance. Tibial P, calcium, and ash content were significantly decreased, as were calbindin and VDR expression levels in the intestine at a low NPP level. Therefore, HDND improved the growth rate of broilers and increased the expression of phosphate and calcium transporter in the small intestine, but adversely affected bone mineralization.

Intestinal NHE8 is highly expressed in goblet cells and its expression is subject to TNF-α regulation

While the intestine plays an important role in digestion and absorption, the mucus lining the epithelium represents a pivotal function in mucosal protection. Goblet cells are scattered in both the crypts and among enterocytes, and they secrete an important component of mucus, mucin. We have reported that sodium/hydrogen exchanger (NHE) 8 is a novel player in mucosal protection, since loss of NHE8 function resulted in reduced mucin production and increased bacterial adhesion. While NHE8 has been shown to be expressed in enterocytes and its expression is reduced during intestinal inflammation, nothing is known about the role of NHE8 in goblet cells. This current study is designed to define the expression of NHE8 and the role of TNF-α in the regulation of NHE8 in goblet cells. Using HT29-MTX cells as an in vitro model, we detected abundant NHE8 mRNA in goblet cells. Immunohistochemical staining localized NHE8 protein on the plasma membrane and in the intracellular compartments in goblet cells. Furthermore, NHE8 expression in goblet cells is regulated by the proinflammatory cytokine TNF-α. The expression of NHE8 in HT29-MTX cells was significantly reduced at both mRNA and protein levels in the presence of TNF-α. This inhibition of NHE8 mRNA expression could be blocked by the transcriptional inhibitor actinomycin D. Promoter reporter assay showed that NHE8 promoter activity was indeed reduced by TNF-α. Mechanistically, TNF-α reduced Sp3 protein binding to the human NHE8 basal promoter region. Therefore, NHE8 is expressed in goblet cells, and the inflammatory cytokine TNF-α downregulates NHE8 expression by a transcriptional mechanism.

Sodium butyrate stimulates NHE8 expression via its role on activating NHE8 basal promoter activity

Butyrate is a major metabolite in colonic lumen. It is produced from bacterial fermentation of dietary fiber. Butyrate has been shown to stimulate electroneutral sodium absorption through its regulation on sodium/hydrogen exchanger 3 (NHE3). Although NHE8, the newest addition of intestinal NHE family, is involved in sodium absorption in the intestinal tract, whether butyrate modulates NHE8 expression in the intestinal epithelial cells is not known. In the current study, we showed that butyrate treatment strongly induced NHE8 protein and NHE8 mRNA expression in human intestinal epithelial cells. Transfection with the human NHE8 promoter reporter constructs showed that butyrate treatment stimulated reporter gene expression at an amount comparable with its stimulation of NHE8 mRNA expression. Interestingly, a similar result was also observed in human NHE8 promoter transfected cells after trichostatin (TSA) treatment. Gel mobility shift assay identified an enhanced Sp3 protein binding on the human NHE8 basal promoter region upon butyrate stimulation. Furthermore, Sp3 acetylation modification is involved in butyrate-mediated NHE8 activation in Caco-2 cells. Our findings suggest that the mechanism of butyrate action on NHE8 expression involves enhanced Sp3 interaction at the basal promoter region of the human NHE8 gene promoter to activate NHE8 gene transcription. Thus butyrate is involved in intestinal regulation of NHE8 resulting enhanced sodium absorption.

Upregulation of the Na⁺-coupled phosphate cotransporters NaPi-IIa and NaPi-IIb by B-RAF

B-RAF, a serine/threonine protein kinase, contributes to signaling of insulin-like growth factor IGF1. Effects of IGF1 include stimulation of proximal renal tubular phosphate transport, accomplished in large part by Na⁺-coupled phosphate cotransporter NaPi-IIa. The related Na⁺-coupled phosphate cotransporter NaPi-IIb accomplishes phosphate transport in intestine and tumor cells. The present study explored whether B-RAF influences protein abundance and/or activity of type II Na⁺-coupled phosphate cotransporters NaPi-IIa and NaPi-IIb. cRNA encoding wild-type NaPi-IIa and wild-type NaPi-IIb was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild-type B-RAF, and electrogenic phosphate transport determined by dual-electrode voltage clamp. NaPi-IIa protein abundance in Xenopus oocyte cell membrane was visualized by confocal microscopy and quantified by chemiluminescence. Moreover, in HEK293 cells, the effect of B-RAF inhibitor PLX-4720 on NaPi-IIa cell surface protein abundance was quantified utilizing biotinylation of cell surface proteins and western blotting. In NaPi-IIa-expressing Xenopus oocytes, but not in oocytes injected with water, addition of phosphate to extracellular bath generated a current (I P), which was significantly increased following coexpression of B-RAF. According to kinetic analysis, coexpression of B-RAF enhanced the maximal IP. Coexpression of B-RAF further enhanced NaPi-IIa protein abundance in the Xenopus oocyte cell membrane. Treatment of HEK293 cells for 24 h with PLX-4720 significantly decreased NaPi-IIa cell membrane protein abundance. Coexpression of B-RAF, further significantly increased IP in NaPi-IIb-expressing Xenopus oocytes. Again, B-RAF coexpression enhanced the maximal IP. In conclusion, B-RAF is a powerful stimulator of the renal and intestinal type II Na⁺-coupled phosphate cotransporters NaPi-IIa and NaPi-IIb, respectively.

Downregulation of NaPi-IIa and NaPi-IIb Na-coupled phosphate transporters by coexpression of Klotho

Klotho, a transmembrane protein, protease and hormone has been shown to exert a profound effect on phosphate metabolism. Klotho overexpression lowers and Klotho deficiency increases the plasma phosphate concentration, effects in part attributed to an inhibitory effect of Klotho on the formation of 1,25-dihydroxycholecalciferol (1,25(OH) (2)D(3)), the active form of Vitamin D. Beyond that Klotho has been shown to decrease renal tubular phosphate transport more directly. The influence of Klotho on the plasma phosphate concentration contributes to the profound effect of Klotho on ageing and life span. The present study explored whether Klotho influences the major renal tubular (NaPi-IIa) and the major intestinal (NaPi-IIb) phosphate transporters. For functional analysis NaPi-IIa or NaPi-IIb were expressed in Xenopus oocytes both, without or with additional coexpression of Klotho and electrogenic phosphate transport was estimated from the phosphate-induced current (Ip). According to RT-PCR Klotho is expressed in the murine kidney and intestine. Coexpression of Klotho decreased Ip in both NaPi-IIa- and NaPi-IIb-expressing oocytes. Klotho decreased the maximal Ip without appreciably affecting the concentration required for halfmaximal Ip. Treatment of NaPi-IIa- or NaPi-IIb-expressing oocytes with Klotho protein similarly decreased Ip. In conclusion, Klotho down regulates both, renal (NaPi-IIa) and intestinal (NaPi-IIb) phosphate transporters.

Epidermal growth factor upregulates serotonin transporter in human intestinal epithelial cells via transcriptional mechanisms

Serotonin transporter (SERT) regulates extracellular availability of serotonin and is a potential pharmacological target for gastrointestinal disorders. A decrease in SERT has been implicated in intestinal inflammatory and diarrheal disorders. However, little is known regarding regulation of SERT in the intestine. Epidermal growth factor (EGF) is known to influence intestinal electrolyte and nutrient transport processes and has protective effects on intestinal mucosa. Whether EGF regulates SERT in the human intestine is not known. The present studies examined the regulation of SERT by EGF, utilizing Caco-2 cells grown on Transwell inserts as an in vitro model. Treatment with EGF from the basolateral side (10 ng/ml, 24 h) significantly stimulated SERT activity (∼2-fold, P < 0.01) and mRNA levels compared with control. EGF increased the activities of the two alternate promoter constructs for human SERT gene: SERT promoter 1 (hSERTp1, upstream of exon 1a) and SERT promoter 2 (hSERTp2, upstream of exon 2). Inhibition of EGF receptor (EGFR) tyrosine kinase activity by PD168393 (1 nM) blocked the stimulatory effects of EGF on SERT promoters. Progressive deletions of the SERT promoter indicated that the putative EGF-responsive elements are present in the -672/-472 region of the hSERTp1 and regions spanning -1195/-738 and -152/+123 of hSERTp2. EGF markedly increased the binding of Caco-2 nuclear proteins to the potential AP-1 cis-elements present in EGF-responsive regions of hSERTp1 and p2. Overexpression of c-jun but not c-fos specifically transactivated hSERTp2, with no effects on hSERTp1. Our findings define novel mechanisms of transcriptional regulation of SERT by EGF via EGFR at the promoter level that may contribute to the beneficial effects of EGF in gut disorders.

Intestinal phosphate transport

Phosphate is absorbed in the small intestine by a minimum of 2 distinct mechanisms: paracellular phosphate transport which is dependent on passive diffusion, and active transport which occurs through the sodium-dependent phosphate cotransporters. Despite evidence emerging for other ions, regulation of the phosphate-specific paracellular pathways remains largely unexplored. In contrast, there is a growing body of evidence that active transport through the sodium-dependent phosphate cotransporter, Npt2b, is highly regulated by a diverse set of hormones and dietary conditions. Furthermore, conditional knockout of Npt2b suggests that it plays an important role in maintenance of phosphate homeostasis by coordinating intestinal phosphate absorption with renal phosphate reabsorption. The knockout mouse also suggests that Npt2b is responsible for the majority of sodium-dependent phosphate uptake. The type-III sodium-dependent phosphate transporters, Pit1 and Pit2, contribute to a minor role in total phosphate uptake. Despite coexpression along the apical membrane, differential responses of Pit1 and Npt2b regulation to chronic versus dietary changes illustrates another layer of phosphate transport control. Finally, a major problem in patients with CKD is management of hyperphosphatemia. The present evidence suggests that targeting key regulatory pathways of intestinal phosphate transport may provide novel therapeutic approaches for patients with CKD.

Transcriptional inhibition of intestinal NHE8 expression by glucocorticoids involves Pax5

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

Phosphate homeostasis and the renal-gastrointestinal axis

Transport of phosphate across intestinal and renal epithelia is essential for normal phosphate balance, yet we know less about the mechanisms and regulation of intestinal phosphate absorption than we do about phosphate handling by the kidney. Recent studies have provided strong evidence that the sodium-phosphate cotransporter NaPi-IIb is responsible for sodium-dependent phosphate absorption by the small intestine, and it might be that this protein can link changes in dietary phosphate to altered renal phosphate excretion to maintain phosphate balance. Evidence is also emerging that specific regions of the small intestine adapt differently to acute or chronic changes in dietary phosphate load and that phosphatonins inhibit both renal and intestinal phosphate transport. This review summarizes our current understanding of the mechanisms and control of intestinal phosphate absorption and how it may be related to renal phosphate reabsorption; it also considers the ways in which the gut could be targeted to prevent, or limit, hyperphosphatemia in chronic and end-stage renal failure.

Epidermal growth factor inhibits intestinal NHE8 expression via reducing its basal transcription

Sodium/hydrogen exchangers (NHEs) play a major role in Na(+) absorption, cell volume regulation, and intracellular pH regulation. Of the nine identified mammalian NHEs, three (NHE2, NHE3, and NHE8) are localized on the apical membrane of epithelial cells in the small intestine and the kidney. Although the regulation of NHE2 and NHE3 expression has been extensively studied in the past decade, little is known about the regulation of NHE8 gene expression under physiological conditions. The current studies were performed to explore the role of epidermal growth factor (EGF) on NHE8 expression during intestinal maturation. Brush-border membrane vesicles (BBMV) were isolated from intestinal epithelia, and Western blot analysis was performed to determine NHE8 protein expression of sucking male rats treated with EGF. Real-time PCR was used to quantitate NHE8 mRNA expression in rats and Caco-2 cells. Human NHE8 promoter activity was characterized through transfection of Caco-2 cells. Gel mobility shift assays (GMSAs) were used to identify the promoter sequences and the transcriptional factors involved in EGF-mediated regulation. Our results showed that intestinal NHE8 mRNA expression was decreased in EGF-treated rats and Caco-2 cells, and NHE8 protein abundance was also decreased in EGF-treated rats. The activity of the human NHE8 gene promoter transfected in Caco-2 cells was also reduced by EGF treatment. This could be explained by reduced binding of transcription factor Sp3 on the NHE8 basal promoter region in the presence of EGF. Pretreatment with MEK1/2 inhibitor UO-126 could prevent EGF-mediated inhibition of NHE8 gene expression. In conclusion, this study showed that EGF inhibits NHE8 gene expression through reducing its basal transcription, suggesting an important role of EGF in regulating NHE expression during intestinal maturation.

Tumor necrosis factor-alpha impairs intestinal phosphate absorption in colitis

Phosphate homeostasis is critical for many physiological functions. Up to 85% of phosphate is stored in bone and teeth. The remaining 15% is distributed in cells. Phosphate absorption across the brush-border membrane (BBM) of enterocytes occurs mainly via a sodium-dependent pathway, which is mediated by type IIb sodium-phosphate cotransporters (NaPi-IIb). Patients of inflammatory bowel diseases (IBDs) suffer not only from diarrhea and nutrient malabsorption but also from bone loss. About 31-59% of patients with IBD develop bone disorders. Since the intestine is a primary location for dietary phosphate absorption, it is logical to postulate that there is an inverse relationship between gastrointestinal disorders and phosphate transport, which, in turn, contributes to bone disorders observed in patients with IBD. Phosphate absorption and NaPi-IIb expression was studied with BBM vesicles isolated from trinitrobenzene sulphonic acid (TNBS) animals as well as in Caco-2 cells. The mechanism of TNF-alpha downregulation of NaPi-IIb expression was investigated by luciferase assay, gel mobility shift assay (GMSA), and coimmunoprecipitation. Intestinal phosphate absorption mediated by NaPi-IIb was reduced both in TNBS colitis and in TNF-alpha-treated cells. Transient transfection indicated that TNF-alpha inhibits NaPi-IIb expression by reducing NaPi-IIb basal promoter activity. GMSAs identified NF1 protein as an important factor in TNF-alpha-mediated NaPi-IIb downregulation. Signaling transduction study and coimmunoprecipitation suggested that TNF-alpha interacts with EGF receptor to activate ERK1/2 pathway. Intestinal phosphate absorption mediated by NaPi-IIb protein is reduced in colitis. This inhibition is mediated by the proinflammatory cytokine TNF-alpha through a novel molecular mechanism involving TNF-alpha/EGF receptor interaction.

Tumor necrosis factor-{alpha} downregulates intestinal NHE8 expression by reducing basal promoter activity

NHE8 transporter is a member of the sodium/hydrogen exchanger (NHE) family. This transporter protein is expressed at the apical membrane of epithelial cells of kidney and intestine and contributes to vectorial Na(+) transport in both tissues. Although NaCl absorption has been shown to be reduced in diarrhea associated with colitis and enteritis, little is known about the role of Na(+)/H(+) exchange and the involvement of NHE isoforms in the pathogenesis of inflammatory disorders and the mechanism of inflammation-associated diarrhea. This study investigated the role of NHE8 in the setting of inflammatory states. Jejunal mucosa was harvested from trinitrobenzene sulfonic acid (TNBS) colitis rats or lipopolysaccharide (LPS) rats for RNA extraction and brush-border membrane protein purification. The human NHE8 gene promoter was cloned from human genomic DNA and characterized in Caco-2 cells. The promoter was further used to study the mechanisms of TNF-alpha-mediated NHE8 expression downregulation in Caco-2 cells. Results from Western blot and real-time PCR indicated that NHE8 protein and mRNA were significantly reduced in TNBS rats and LPS rats. In Caco-2 cells, TNF-alpha produces similar reduction levels in the endogenous NHE8 mRNA expression observed in our in vivo studies. The downregulation of NHE8 expression mediated by TNF-alpha could be blocked by transcription inhibitor actinomycin D, suggesting the involvement of transcriptional regulation. Further studies indicated that the human NHE8 gene transcription could be activated by Sp3 transcriptional factor, and TNF-alpha inhibits human NHE8 expression by reducing Sp3 interaction at the minimal promoter region of the human NHE8 gene. In conclusion, our studies suggest that TNF-alpha decreases NHE8 expression in inflammation induced by TNBS and LPS, which may contribute to the diarrhea associated with inflammation.

Induction of Na+/K+/2Cl- cotransporter expression mediates chronic potentiation of intestinal epithelial Cl- secretion by EGF

Alterations in EGF receptor (EGFR) signaling occur in intestinal disorders associated with dysregulated epithelial transport. In the present study, we investigated a role for the EGFR in the chronic regulation of intestinal epithelial secretory function. Epithelial Cl(-) secretion was measured as changes in short-circuit current (Isc) across voltage-clamped monolayers of T84 cells in Ussing chambers. Acute treatment of T84 cells with EGF (100 ng/ml, 15 min) chronically enhanced Isc responses to a broad range of secretagogues. This effect was apparent within 3 h, maximal by 6 h, and sustained for 24 h after treatment with EGF. The Na+/K+/2Cl(-) cotransporter (NKCC1) inhibitor bumetanide (100 microM) abolished the effect of EGF, indicating increased responses are due to potentiated Cl(-) secretion. Neither basal nor agonist-stimulated levels of intracellular Ca2+ or PKA activity were altered by EGF, implying that the effects of the growth factor are not due to chronic alterations in levels of second messengers. EGF increased the expression of NKCC1 with a time course similar to that of its effects on Cl(-) secretion. This effect of EGF was maximal after 6 h, at which time NKCC1 expression in EGF-treated cells was 199.9 +/- 21.9% of that in control cells (n = 21, P < 0.005). EGF-induced NKCC1 expression was abolished by actinomycin D, and RT-PCR analysis demonstrated EGF increased expression of NKCC1 mRNA. These data increase our understanding of mechanisms regulating intestinal fluid and electrolyte transport and reveal a novel role for the EGFR in the chronic regulation of epithelial secretory capacity through upregulation of NKCC1 expression.

Sp1 and Sp3 mediate NHE2 gene transcription in the intestinal epithelial cells

Our previous studies have identified a minimal Sp1-driven promoter region (nt -36/+116) directing NHE2 expression in mouse renal epithelial cells. However, this minimal promoter region was not sufficient to support active transcription of NHE2 gene in the intestinal epithelial cells, suggesting the need for additional upstream regulatory elements. In the present study, we used nontransformed rat intestinal epithelial (RIE) cells as a model to identify the minimal promoter region and transcription factors necessary for the basal transcription of rat NHE2 gene in the intestinal epithelial cells. We identified a region within the rat NHE2 gene promoter located within nt -67/-43 upstream of transcription initiation site as indispensable for the promoter function in intestinal epithelial cells. Mutations at nt -56/-51 not only abolished the DNA-protein interaction in this region, but also completely abolished NHE2 gene promoter activity in RIE cells. Supershift assays revealed that Sp1 and Sp3 interact with this promoter region, but, contrary to the minimal promoter indispensable for renal expression of NHE2, both transcription factors expressed individually in Drosophila SL2 cells activated rat NHE2 gene promoter. Moreover, Sp1 was a weaker transactivator and when coexpressed in SL2 cells it reduced Sp3-mediated NHE2 basal promoter activity. Furthermore, DNase I footprinting confirmed that nt -58/-51 is protected by nuclear protein from RIE cells. We conclude that the mechanism of basal control of rat NHE2 gene promoter activity is different in the renal and intestinal epithelium, with Sp3 being the major transcriptional activator of NHE2 gene transcription in the intestinal epithelial cells.

Remote control of gene expression

The elucidation of a growing number of species' genomes heralds an unprecedented opportunity to ascertain functional attributes of non-coding sequences. In particular, cis regulatory modules (CRMs) controlling gene expression constitute a rich treasure trove of data to be defined and experimentally validated. Such information will provide insight into cell lineage determination and differentiation and the genetic basis of heritable diseases as well as the development of novel tools for restricting the inactivation of genes to specific cell types or conditions. Historically, the study of CRMs and their individual transcription factor binding sites has been limited to proximal regions around gene loci. Two important by-products of the genomics revolution, artificial chromosome vectors and comparative genomics, have fueled efforts to define an increasing number of CRMs acting remotely to control gene expression. Such regulation from a distance has challenged our perspectives of gene expression control and perhaps the very definition of a gene. This review summarizes current approaches to characterize remote control of gene expression in transgenic mice and inherent limitations for accurately interpreting the essential nature of CRM activity.

Intestinal phosphate absorption and the effect of vitamin D: a comparison of rats with mice

Previously, it was thought that intestinal phosphate transport occurred exclusively in the proximal small intestine of rodents and humans. However, a recent study has demonstrated that the ileum of mice contributes significantly to the absorption of dietary phosphate, but it is not known whether this region is also an important site of phosphate absorption in the rat. In the present study, we have investigated the mRNA and protein levels of the sodium-phosphate cotransporter, NaPi-IIb, in three regions of rat and mouse small intestine, and related its expression levels to the rate of net phosphate absorption, as measured using the in situ intestinal loop technique. 1,25-Dihydroxyvitamin D3 is an important physiological regulator of intestinal phosphate absorption that increases phosphate transport in both the duodenum and jejunum of the rat. Based on the recently proposed regional profile of phosphate absorption along the mouse small intestine, we have re-evaluated the effects of 1,25-dihydroxyvitamin D3 using three distinct regions of the mouse and rat small intestine. Our studies have revealed important differences in the intestinal handling of phosphate between mice and rats. In mice, maximal phosphate absorption occurs in the ileum, which is paralleled by the highest expression levels of NaPi-IIb mRNA and protein. In contrast, in rats maximal absorption occurs in the duodenum with very little absorption occurring in the ileum, which is similar to the pattern reported in humans. However, in both rodent species only the jejunum shows an increase in phosphate absorption in response to treatment with 1,25-dihydroxyvitamin D3.

Regulation of smooth muscle-specific gene expression by homeodomain proteins, Hoxa10 and Hoxb8

Smooth muscle cells arise from different populations of precursor cells during embryonic development. The mechanisms that specify the smooth muscle cell phenotype in each of these populations of cells are largely unknown. In many tissues and organs, homeodomain transcription factors play a key role in directing cell specification. However, little is known about how these proteins regulate smooth muscle differentiation. Using degenerate reverse transcription-PCR coupled to cDNA library screening we identified two homeodomain proteins, Hoxa10 and Hoxb8, which are expressed in adult mouse smooth muscle tissues. All three of the previously described transcripts of the Hoxa10 gene, Hoxa10-1, Hoxa10-2, and Hoxa10-3, were identified. Hoxa10-1 directly activated the smooth muscle-specific telokin promoter but did not activate the SM22alpha, smooth muscle alpha-actin, or smooth muscle myosin heavy chain promoters. Small interfering RNA-mediated knock-down of Hoxa10-1 demonstrated that Hoxa10-1 is required for high levels of telokin expression in smooth muscle cells from uterus and colon. On the other hand, Hoxb8 inhibited the activity of the telokin, SM22alpha, and smooth muscle alpha-actin promoters. Cotransfection of Hoxa10-1 together with Hoxa10-2 or Hoxb8 suggested that Hoxa10-2 and Hoxb8 act as competitive inhibitors of Hoxa10-1. Results from gel mobility shift assays demonstrated that Hoxa10-1, Hoxa10-2, and Hoxb8 bind directly to multiple sites in the telokin promoter. Mutational analysis of telokin promoter reporter genes demonstrated that the three homeodomain protein binding sites located between -80 and -75, +2 and +6, and +14 and +17 were required for maximal promoter activation by Hoxa10-1 and maximal inhibition by Hoxb8. Together these data demonstrate that the genes encoding smooth muscle-restricted proteins are direct transcriptional targets of clustered homeodomain proteins and that different homeodomain proteins have distinct effects on the promoters of these genes.

NF1 transcriptional factor(s) is required for basal promoter activation of the human intestinal NaPi-IIb cotransporter gene

The human intestinal type IIb Na+-P(i) cotransporter (hNaPi-IIb) gene promoter lacks a TATA box and has a high GC content in the 5'-flanking region. To understand the mechanism of hNaPi-IIb gene transcription, the current study was performed to characterize the minimal promoter region and transcriptional factor(s) necessary to activate gene expression in human intestinal cells (Caco-2). With the use of progressively shorter promoter constructs, a minimal promoter extending from bp -58 to +15 was identified and shown to direct high levels of hNaPi-IIb cotransporter expression in Caco-2 cells. Gel mobility shift assays (GMSAs) indicated that two regions could be bound by nuclear proteins from Caco-2 cells: region A at bp -26/-23 and region B at bp -44/-35. The introduction of mutations in region A abolished promoter activity, whereas mutations in region B had no effect. Deletion mutants of the same regions showed identical results. Furthermore, DNase I footprinting experiments confirmed the observation made by GMSAs. Additional studies, which used a specific nuclear factor 1 (NF1) antiserum, demonstrated that NF1 protein(s) binds to the minimal promoter at region A. These results indicated that the NF1 protein(s) is required to activate the basal transcription of hNaPi-IIb gene under normal growth conditions. This study has thus identified a new target gene in the small intestinal epithelium that is directly regulated by NF1 transcriptional factor(s).

Identification of a carboxyl-terminal motif essential for the targeting of Na+-HCO-3 cotransporter NBC1 to the basolateral membrane

The Na+-HCO3- cotransporter NBC1 is located exclusively on the basolateral membrane and mediates vectorial transport of bicarbonate in a number of epithelia, including kidney and pancreas. To identify the motifs that direct the targeting of kidney NBC1 to basolateral membrane, wild type and various carboxyl-terminally truncated kidney NBC1 mutants were generated, fused translationally in-frame to GFP, and transiently expressed in kidney epithelial cells. GFP was linked to the NH2 terminus of NBC1, and labeling was examined by confocal microscopy. Full-length (1035 aa) and mutants with the deletion of 3 or 20 amino acids from the COOH-terminal end of NBC1 (lengths 1032 and 1015 aa, respectively) showed strong and exclusive targeting on the basolateral membrane. However, the deletion of 26 amino acid residues from the COOH-terminal end (length 1010 aa) resulted in retargeting of NBC1 to the apical membrane. Expression studies in oocytes demonstrated that the NBC1 mutant with the deletion of 26 amino acid residues from the COOH-terminal end is functional. Additionally, the deletion of the last 23 amino acids or mutation in the conserved residue Phe at position 1013 on the COOH-terminal end demonstrated retargeting to the apical membrane. We propose that a carboxyl-terminal motif with the sequence QQPFLS, which spans amino acid residues 1010-1015, and specifically the amino acid residue Phe (position 1013) are essential for the exclusive targeting of NBC1 to the basolateral membrane.

Cell and molecular biology of the small intestine: new insights into differentiation, growth and repair

PURPOSE OF REVIEW

This paper will discuss recent research that has provided new insights into the molecular and cell biology of the small intestine.

RECENT FINDINGS

Differentiation of the epithelial cell lineages, including the enterocytes, enteroendocrine, Goblet and Paneth cells, from the stem cells is better understood. Important interactions have been demonstrated between these cells, luminal bacteria, and underlying mesenchymal tissue. Intestine-specific gene expression is regulated by transcription factors that are becoming well characterized, including CDX1, CDX2 and HNF1. The actions of growth factors such as GLP-2 and EGF are now known to be complex, demonstrating multiple effects in this tissue at a number of levels.

SUMMARY

Progress in the cellular and molecular biology of the small intestine is producing many intriguing new findings.