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.

Evidence of an intestinal phosphate transporter alternative to type IIb sodium-dependent phosphate transporter in rats with chronic kidney disease

Background

Phosphate is absorbed in the small intestine via passive flow and active transport.NaPi-IIb, a type II sodium-dependent phosphate transporter, is considered to mediate active phosphate transport in rodents. To study the regulation of intestinal phosphate transport in chronic kidney disease (CKD), we analyzed the expression levels of NaPi-IIb, pituitary-specific transcription factor 1 (PiT-1) and PiT-2 and the kinetics of intestinal phosphate transport using two CKD models.

Methods

CKD was induced in rats via adenine orThy1 antibody injection. Phosphate uptake by intestinal brush border membrane vesicles (BBMV) and the messenger RNA (mRNA) expression of NaPi-IIb, PiT-1 and PiT-2 were analyzed. The protein expression level of NaPi-IIb was measured by mass spectrometry (e.g. liquid chromatography tandem mass spectrometry).

Results

In normal rats, phosphate uptake into BBMV consisted of a single saturable component and its Michaelis constant (K_m) was comparable to that of NaPi-IIb. The maximum velocity (V_max) correlated with mRNA and protein levels of NaPi-IIb. In the CKD models, intestinal phosphate uptake consisted of two saturable components. The V_max of the higher-affinity transport, which is thought to be responsible for NaPi-IIb, significantly decreased and the decrease correlated with reduced NaPi-IIb expression. The K_m of the lower-affinity transport was comparable to that of PiT-1 and -2. PiT-1 mRNA expression was much higher than that of PiT-2, suggesting that PiT-1 was mostly responsible for phosphate transport.

Conclusions

This study suggests that the contribution of NaPi-IIb to intestinal phosphate absorption dramatically decreases in rats with CKD and that a low-affinity alternative to NaPi-IIb, in particular PiT-1, is upregulated in a compensatory manner in CKD.

Significant Species Differences in Intestinal Phosphate Absorption between Dogs, Rats, and Monkeys

A treatment for hyperphosphatemia would be expected to reduce mortality rates for CKD and dialysis patients. Although rodent studies have suggested sodium-dependent phosphate transporter type IIb (NaPi-IIb) as a potential target for hyperphosphatemia, NaPi-IIb selective inhibitors failed to achieve efficacy in human clinical trials. In this study, we analyzed phosphate metabolism in rats, dogs, and monkeys to confirm the species differences. Factors related to phosphate metabolism were measured and intestinal phosphate absorption rate was calculated from fecal excretion in each species. Phosphate uptake by intestinal brush border membrane vesicles (BBMV) and the mRNA expression of NaPi-IIb, PiT-1, and PiT-2 were analyzed. In addition, alkaline phosphatase (ALP) activity was evaluated. The intestinal phosphate absorption rate, including phosphate uptake by BBMV and NaPi-IIb expression, was the highest in dogs. Notably, urinary phosphate excretion was the lowest in monkeys, and their intestinal phosphate absorption rate was by far the lowest. Dogs and rats showed positive correlations between V_max/K_m of phosphate uptake in BBMV and NaPi-IIb expression. Although phosphate uptake was observed in the BBMV of monkeys, NaPi-IIb expression was not detected and ALP activity was low. This study revealed significant species differences in intestinal phosphate absorption. NaPi-IIb contributes to intestinal phosphate uptake in rats and dogs. However, in monkeys, phosphate is poorly absorbed due to the slight degradation of organic phosphate in the intestine.

EOS789, a novel pan-phosphate transporter inhibitor, is effective for the treatment of chronic kidney disease-mineral bone disorder

Chronic kidney disease is characterized as impaired renal function along with the imbalance and dysregulation of mineral metabolism; recognized as chronic kidney disease-mineral and bone disorder. Hyperphosphatemia, characterized by altered phosphate homeostasis along with elevated fibroblast growth factor-23 and intact parathyroid hormone, is such an alteration of mineral metabolism. We discovered a novel inhibitor, EOS789, that interacts with several sodium-dependent phosphate transporters (NaPi-IIb, PiT-1, and PiT-2) known to contribute to intestinal phosphate absorption. This inhibitor dose-dependently increased the fecal phosphorus excretion rate and inversely decreased the urinary phosphorus excretion rate in normal rats, suggesting inhibition of intestinal phosphorus absorption. In rats with adenine-induced hyperphosphatemia, EOS789 markedly decreased the serum phosphate, fibroblast growth factor-23, and intact parathyroid hormone below values found in normal control rats. Notably, this pan-phosphate transporter inhibitor exhibited a more potent effect on serum phosphate than a NaPi-IIb-selective inhibitor in rats with hyperphosphatemia indicating that PiT-1 and PiT-2 play important roles in intestinal phosphate absorption. Moreover, in a long-term study, EOS789 sustained the suppression of serum phosphorus in parallel with fibroblast growth factor-23 and intact parathyroid hormone and ameliorated ectopic calcification of the thoracic aorta. Additionally, EOS789 treatment also ameliorated kidney deterioration in rats with progressive kidney injury, probably due to the strict phosphate control. Thus, EOS789 has potent efficacy against hyperphosphatemia and its complications and could provide a significant benefit to patients who are ineffectively treated with phosphate binders.

Paracellular transport of phosphate along the intestine

Inorganic phosphate (P_i) is crucial for many biological functions, such as energy metabolism, signal transduction, and pH buffering. Efficient systems must exist to ensure sufficient supply for the body of P_i from diet. Previous experiments in humans and rodents suggest that two pathways for the absorption of P_i exist, an active transcellular P_i transport and a second paracellular pathway. Whereas the identity, role, and regulation of active P_i transport have been extensively studied, much less is known about the properties of the paracellular pathway. In Ussing chamber experiments, we characterized paracellular intestinal P_i permeabilities and fluxes. Dilution potential measurements in intestinal cell culture models demonstrated that the tight junction is permeable to P_i, with monovalent P_i having a higher permeability than divalent P_i. These findings were confirmed in rat and mouse intestinal segments by use of Ussing chambers and a combination of dilution potential measurements and fluxes of radiolabeled ³²P_i. Both techniques yielded very similar results, showing that paracellular P_i fluxes were bidirectional and that P_i permeability was ~50% of the permeability for Na⁺ or Cl^-. P_i fluxes were a function of the concentration gradient and P_i species (mono- vs. divalent P_i). In mice lacking the active transcellular P_i transport component sodium-dependent P_i transporter NaPi-IIb, the paracellular pathway was not upregulated. In summary, the small and large intestines have a very high paracellular P_i permeability, which may favor monovalent P_i fluxes and allow efficient uptake of P_i even in the absence of active transcellular P_i uptake.NEW & NOTEWORTHY The paracellular permeability for phosphate is high along the entire axis of the small and large intestine. There is a slight preference for monovalent phosphate. Paracellular phosphate fluxes do not increase when transcellular phosphate transport is genetically abolished. Paracellular phosphate transport may be an important target for therapies aiming to reduce intestinal phosphate absorption.

The role of SLC34A2 in intestinal phosphate absorption and phosphate homeostasis

There has recently been significant interest in the concept of directly targeting intestinal phosphate transport to control hyperphosphatemia in patients with chronic kidney disease. However, we do not have a complete understanding of the cellular mechanisms that govern dietary phosphate absorption. Studies in the 1970s documented both active and passive pathways for intestinal phosphate absorption. However, following the cloning of the intestinal SLC34 cotransporter, NaPi-IIb, much of the research focused on the role of this protein in active transcellular phosphate absorption and the factors involved in its regulation. Generation of a conditional NaPi-IIb knockout mouse has demonstrated that this protein is critical for the maintenance of skeletal integrity during periods of phosphate restriction and that under normal physiological conditions, the passive sodium-independent pathway is likely be the more dominant pathway for intestinal phosphate absorption. The review aims to summarise the most recent developments in our understanding of the role of the intestine in phosphate homeostasis, including the acute and chronic renal adaptations that occur in response to dietary phosphate intake. Evidence regarding the overall contribution of the transcellular and paracellular pathways for phosphate absorption will be discussed, together with the clinical benefit of inhibiting these pathways for the treatment of hyperphosphatemia in chronic kidney disease.

Inorganic phosphate transporters in cancer: Functions, molecular mechanisms and possible clinical applications

Inorganic phosphate is one of the most essential nutrients for the maintenance of cell life. Because of its essential role in nutrient supplementation, the study of plasma membrane inorganic phosphate transporters in cancer biology has received much attention in recent years. Several studies suggest that these transporters are up-regulated in tumor cells and thus have been considered to be important promoters of tumor progression. Altered expression levels of inorganic phosphate transporters, such as NaPi-IIb (SLC34A2) and PiT-1 (SLC20A1), have been demonstrated. The purpose of this review article was to gather the relevant experimental records on inorganic phosphate transporters in tumors and to demonstrate the importance of these proteins in clinical applications. In this work, we demonstrate that for decades, the potential use of the inorganic phosphate transporter as an antigen for the diagnosis of tumor subtypes remained unknown. With the advancement in molecular biology techniques, phosphate transporters have been identified as being associated with cancer. In addition to their altered expression in cancer, several studies have demonstrated other functions of inorganic phosphate transporters, such as transceptors, rearrangements with oncogenes and modifications in the expression of ABC transporters, aiding in the process of proliferation and drug resistance.

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.

The sodium-phosphate co-transporter SLC34A2, and pulmonary alveolar microlithiasis: Presentation of an inbred family and a novel truncating mutation in exon 3

Pulmonary alveolar microlithiasis is a disorder in which many tiny fragments (microliths) of calcium phosphate gradually accumulate in alveoli. Loss of function mutations in the gene SLC34A2 coding for the sodium phosphate co-transporter (NaPi-IIb) are responsible for genetic forms of alveolar microlithiasis. We now report a consanguineous Italian family from Calabria with two affected members segregating alveolar microlithiasis in a recessive fashion. We describe, for the first time, a novel loss of function mutation in the gene coding for NaPi-IIb. A careful description of the clinical phenotype is provided together with technical details for direct sequencing of the gene.

Different phosphate transport in the duodenum and jejunum of chicken response to dietary phosphate adaptation

Intestinal phosphate (Pi) absorption across the apical membrane of small intestinal epithelial cells is mainly mediated by the type IIb Na-coupled phosphate co-transporter (NaPi-IIb), but its expression and regulation in the chicken remain unclear. In the present study, we investigated the mRNA and protein levels of NaPi-IIb in three regions of chicken small intestine, and related their expression levels to the rate of net phosphate absorption. Our results showed that maximal phosphate absorption occurs in the jejunum, however the highest expression levels of NaPi-IIb mRNA and protein occurs in the duodenum. In response to a low-Pi diet (TP 0.2%), there is an adaptive response restricted to the duodenum, with increased brush border membrane (BBM) Na-Pi transport activity and NaPi-IIb protein and mRNA abundance. However, when switched from a low- (TP 0.2%) to a normal diet (TP 0.6%) for 4 h, there is an increase in BBM NaPi-IIb protein abundance in the jejunum, but no changes in BBM NaPi-IIb mRNA. Therefore, our study indicates that Na-Pi transport activity and NaPi-IIb protein expression are differentially regulated in the duodenum vs the jejunum in the chicken.

The Influence of Dietary Calcium and Phosphorus Imbalance on Intestinal NaPi-IIb and Calbindin mRNA Expression and Tibia Parameters of Broilers

A 2×2 factorial experiment was conducted to study the effect of dietary calcium and non-phytate phosphorus (nPP) imbalance on calbindin and NaPi-IIb mRNA levels in the small intestine and tibia parameters of broiler chicks. One hundred and forty four 1-d-old Arbor Acres male broiler chicks were divided into four treatments consisted of six replicates with six chicks each. The two dietary calcium levels were 1.10% and 0.60%, and two dietary nPP levels were 0.50% and 0.27%. Results showed that a high Ca/nPP ratio diet (4.07:1) significantly depressed feed intake and weight gain of broilers (p<0.05), but a lower Ca:nPP ratio (1.2:1) had no influence (p>0.05). Low-Ca with low-P diet resulted in low tibia minerals and tibia breaking strength of broilers, and all the tibia parameters were further decreased when the dietary ratio of Ca to P was relative higher. Low dietary Ca or P up-regulated the calbindin and NaPi-IIb mRNA expression levels. Low Ca with normal P diet up-regulated duodenal calbindin mRNA expression level to the greatest extent. Low P with a normal Ca diet significantly enhanced NaPi-IIb mRNA expression level to the highest extent. These results suggest that the calbindin and NaPi-IIb mRNA expression were enhanced by the imbalance between dietary Ca and nPP, and their expression were not only influenced by Ca or nPP level, but also the ratio of Ca:nPP.