Nuevas estrategias terapéuticas para tratar la hiperfosfatemia en la IRC: foco en la absorción intestinal / New therapeutic strategies to treat hyperphosphatemia in CRF: focus on intestinal absorption

RESUMEN El manejo de la hiperfosfatemia de los pacientes con insuficiencia renal crónica en diálisis permanece como un desafío. A pesar de utilizar un enfoque multifacético que incluye la restricción dietética, la remoción de fósforo por la diálisis y el uso de quelantes de fósforo, esta estrategia múltiple no logra reducir los niveles de fósforo en más de 2 mg/dl. El control de fósforo de los pacientes en diálisis es fundamental en razón de la relación monotónica entre los niveles séricos de fosfato y el incremento del riesgo cardiovascular. Por lo tanto, hay una necesidad de explorar nuevas estrategias para reducir los niveles séricos de fosfato a niveles normales. Recientes avances en nuestra compresión de los mecanismos que subyacen a la homeostasis del fósforo sugieren que el transporte gastrointestinal del fósforo podría ser un objetivo. Recientemente se han desarrollado inhibidores de los cotransportadores sodio fosfato del intestino y se ha revalorizado el uso de la nicotinamida, en su formulación de liberación prolongada, que también actuaria por ese mecanismo. También se han drogas como el tenapanor, que inhibiendo el intercambiador sodio/hidrogeno isoforma 3 del enterocito, disminuyen la absorción paracelular de fósforo.

ABSTRACT Management of hyperphosphatemia in patients with chronic renal failure on dialysis remains challenging. Despite using a multifaceted approach that includes dietary restriction, phosphorus removal by dialysis, and phosphate binders, these multiple strategies fail to reduce phosphorus levels by more than 2 mg/dL. Phosphorus control in dialysis patients is essential due to the monotonic relationship between serum phosphate levels and increased cardiovascular risk. Therefore, there is a need to explore new strategies to reduce serum phosphate levels to normal levels. Recent advances in understanding the mechanisms underlying phosphorus homeostasis suggest that the gastrointestinal transport of phosphorus could be a target. Inhibitors of intestinal sodium phosphate cotransporters recently developed, and using of nicotinamide, in its prolonged release formulation, which would also act by this mechanism, has been revalued. There have also been drugs such as tenapanor, which, by inhibiting the isoform three sodium/hydrogen exchanger of the enterocyte, decreases the paracellular absorption of phosphorus.

Growth hormone therapy in HHRH.

Background: Hereditary Hypophosphatemic Rickets with Hypercalciuria (HHRH) (SLC34A3 gene, OMIM 241530) is an autosomal recessive disorder that results in a loss of function of the sodium-phosphate NPT2c channel at the proximal tubule. Phosphate supplementation rarely improves serum phosphate, hypercalciuria, nephrocalcinosis, 1,25(OH)2 vitamin D (1,25(OH)2D) levels or short stature. Methods: We describe 23Na MRI and the successful use of recombinant human growth hormone (rhGH) and Fluconazole to improve growth (possibly confounded by puberty) and hypercalciuria in a now 12-year-old male with HHRH (novel homozygous SLC34A3 mutation, c.835_846 + 10del.T). Results: The patient had chronic bone pain, hypophosphatemia (0.65 mmol/L[reference interval 1.1-1.9]), pathological fractures and medullary nephrocalcinosis/hypercalciuria (urinary calcium/creatinine ratio 1.66 mol/mmol[<0.6]). TmP/GFR was 0.65 mmol/L[0.97-1.64]; 1,25(OH)2D was >480 pmol/L[60-208]. Rickets Severity Score was 4. Treatment with 65 mg/kg/day of sodium phosphate and potassium citrate 10 mmol TID failed to correct the abnormalities.Adding rhGH at 0.35 mg/kg/week to the phosphate therapy, improved bone pain, height z-score from -2.09 to -1.42 over 6 months, without a sustained effect on TmP/GFR. Fluconazole was titrated to 100 mg once daily, resulting for the first time in a reduction of the 1,25(OH)2D to 462 and 426 pmol/L; serum phosphate 0.87 mmol/L, and calcium/creatinine ratio of 0.73.23Na MRI showed normal skin (z-score + 0.68) and triceps surae muscle (z-score + 1.5) Na+ levels; despite a defect in a sodium transporter, hence providing a rationale for a low sodium diet to improve hypercalciuria. Conclusions: The addition of rhGH, Fluconazole and salt restriction to phosphate/potassium supplementation improved the conventional therapy. Larger studies are needed to confirm our findings.

Clonal osteoblastic cell lines with CRISPR/Cas9-mediated ablation of Pit1 or Pit2 show enhanced mineralization despite reduced osteogenic gene expression.

Multiple actions of extracellular Pi on the skeletal cells are likely to be partly mediated by type III sodium/phosphate (Na+/Pi) cotransporters Pit1 and Pit2, although the details are not fully understood. In the current study, to determine the roles of Pit1 and Pit2 in osteoblasts, we generated Pit1-knockout (KO) and Pit2-KO osteoblastic cells by applying CRISPR/Cas9 genome editing to an osteoblastic cell line MC3T3-E1 subclone 4. The extracellular Pi level was increased in the Pit1-KO and Pit2-KO clones due to the reduced Pi uptake. Interestingly, in vitro mineralization was accelerated in the Pit1-KO and Pit2-KO clones, although the induction of the expression of osteogenic marker genes was suppressed. In the cells before mineralization, extracellular levels of pyrophosphate (PPi) and adenosine triphosphate (ATP) were increased in the Pit1-KO and Pit2-KO clones, which might be attributable to the reduced expression and activity of tissue-nonspecific alkaline phosphatase (TNSALP). A 24-h treatment with high Pi reduced the expression and activity of TNSALP, suggesting that the suppression of TNSALP in the Pit1-KO and Pit2-KO clones was caused by the increased availability of extracellular Pi. Lentiviral gene transfer of Pit1 and Pit2 restored the changes observed in Pit1-KO and Pit2-KO clones, respectively. The expressions of P2Y2 and P2X7 which encode receptors for extracellular ATP were altered in the Pit1-KO and Pit2-KO clones, suggesting an influence on purinergic signaling. In mineralized cells after long-term culture, intracellular levels of PPi and ATP were higher in the Pit1-KO and Pit2-KO clones. Taken together, ablation of Pit1 or Pit2 in this osteoblastic cell model led to accelerated mineralization, suppressed TNSALP and altered the levels of extracellular and intracellular PPi and ATP, which might be partly mediated by changes in the availability of extracellular Pi.

Expression of Phosphatonin-Related Genes in Sheep, Dog and Horse Kidneys Using Quantitative Reverse Transcriptase PCR.

The aim of this preliminary study was to determine the relative expression of phosphatonin pathway-related genes in normal dog, sheep and horse kidneys and to explore the relationships between the different genes. Kidneys were collected post-mortem from 10 sheep, 10 horses and 8 dogs. RNA was extracted, followed by reverse transcriptase quantitative polymerase chain reaction for fibroblast growth factor receptor 1 IIIc (FGFR1IIIC), sodium-phosphate co-transporter (NPT) 1 (SLC17A1), NPT2a (SLC34A1), NPT2c (SLC34A3), parathyroid hormone 1 receptor (PTH1R), klotho (KL), vitamin D receptor (VDR), 1a-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24A1). NPT2a was highly expressed in the dog kidneys, compared with those of the horses and sheep. NPT1 had greatest expression in horses and sheep, although the three different NPTs all had relatively similar expression in sheep. There was little variability in FGFR1IIIc expression, particularly in the dogs and horses. FGFR1IIIc expression was negatively correlated with NPT genes (except NPT2a in sheep), while NPT genes were all positively correlated with each other. Unexpectedly, klotho was positively correlated with NPT genes in all three species. These results provide the basis for further research into this important regulatory system. In particular, species differences in phosphatonin gene expression should be considered when considering the pathogenesis of chronic kidney disease.

Efficacy of 1-α-Hydroxycholecalciferol Supplementation in Young Broiler Feed Suggests Reducing Calcium Levels at Grower Phase.

Increasing biopotency of cholecalciferol (D3) from vitamin sources is essential in the poultry industry to meet nutritional demands and counter stressors. D3 exhibits hormonal traits and is responsible for calcium (Ca) absorption. 1-α-Hydroxycholecalciferol (1α) is a synthetic form of D3 that has equal efficacy and is cheaper to synthesize than 1,25-dihydroxycholecalciferol (active form of D3), on broilers. However, 1α bypasses a critical regulatory point, the kidney, and may consequently lead to toxicity levels of Ca via Ca absorption. This study examined 1α supplementation in broiler diets with different Ca inclusion levels to determine if 1α at higher Ca levels caused Ca toxicity at starter and grower phases with Ross 708 male broiler chicks. In Experiment 1 (1-15 days of age), chicks were assigned to one of 10 treatment starter diets with five levels of Ca inclusion (0.80, 0.95, 1.10, 1.25, and 1.40%) with or without 1α supplementation (5 µg 1α/kg in feed) and eight replicate cages per treatment. In Experiment 2, chicks were fed common starter diet until 16 days of age, and then they were assigned to one of eight treatment diets with four levels of Ca inclusion (0.54, 0.76, 0.98, or 1.20%) with or without 1α supplementation (5 µg 1α/kg in feed). At the end of both experiments, blood was collected from broilers to determine blood chemistry, including concentrations of vitamin D metabolites. Intestinal tissues were also collected to examine gene expression. In Experiment 1, broilers not fed 1α exhibited a quadratic effect in ionized blood Ca (iCa) as dietary Ca inclusion levels increased; 1α-fed broilers displayed an increase in iCa as Ca inclusion levels increased (p = 0.0002). For Experiment 2, 1α-fed broilers displayed a decrease in 25-hydroxycholecalciferol plasma concentration as dietary Ca inclusion levels increased (p = 0.035); also, increasing Ca inclusion in diets increased expression of duodenal sodium phosphate cotransporter type II b (NPTIIb, p = 0.03). Our findings imply that inclusion of 1α was beneficial because 1α enhanced Ca absorption during the starter phase; however, to avoid potential Ca toxicity or antagonism while using 1α during the grower phase, there should be consideration with reducing dietary Ca levels.

Optimal Dietary Levels of 1α-Hydroxycholecalciferol in Broiler Chickens from 1 to 42 Days of Age.

1α-Hydroxycholecalciferol (1α-OH-D3) is an active vitamin D derivative. In this study, three experiments were conducted to evaluate the optimal dietary levels of 1α-OH-D3 in broiler chickens from 1 to 42 days of age. 1α-OH-D3 levels used were 0, 1.25, 2.5, 5, and 10 µg/kg in experiment 1, 0.625, 1.25, 2.5, 5, 7.5, and 10 µg/kg in experiment 2, and 2, 2.5, 3, 3.5, 4, 4.5, and 5 µg/kg in experiment 3. In experiment 1, the addition of 0 to 10 µg/kg of 1α-OH-D3 quadratically improved growth performance, tibia development, and mRNA expression levels of nuclear vitamin D receptor (nVDR), membrane vitamin D receptor (mVDR), and type IIb sodium-phosphate cotransporter (NaPi-IIb) in the duodenum of broiler chickens from 1 to 12 days of age. Body weight gain (BWG), the weight and ash weight of the tibia, and mRNA expression levels of mVDR and NaPi-IIb of broilers fed with 0 and 10 µg/kg of 1α-OH-D3 were lower than those of birds fed with 2.5 µg/kg of 1α-OH-D3. In experiment 2, 1α-OH-D3 levels were quadratically related to BWG and to weight and ash weight of the femur and the tibia of broiler chickens at 42 days of age. The highest values of growth performance and bone mineralization were recorded in broilers fed with 2.5 to 5 µg/kg of 1α-OH-D3. In experiment 3, there was no difference observed in BWG and the weight and ash weight of the femur and the tibia of the 42-day-old broilers fed with 2 to 5 µg/kg of 1α-OH-D3. These data suggest that the optimal dietary levels of 1α-OH-D3 were 2 to 5 µg/kg for broiler chickens from 1 to 42 days of age.

Supplemental Nicotinamide Dose-Dependently Regulates Body Phosphorus Excretion via Altering Type II Sodium-Phosphate Co-Transporter Expressions in Laying Hens.

BACKGROUND: Dietary supplemental nicotinamide is used to treat hyperphosphatemia in humans. However, the mechanisms of its impact on body phosphorus homeostasis remain unclear. OBJECTIVE: This study was to determine effects and molecular mechanisms of 3 dietary nicotinamide concentrations on body phosphorus homeostasis in laying hens. METHODS: Hy-Line Brown layers (total = 21; 40 wk old; body weight: 1,876 ± 24 g) were individually housed (n = 7) and fed a corn-soybean meal-based diet supplemented with nicotinamide at 20 (N20), 140 (N140), and 1000 (N1000) mg/kg for 21 d. Serum phosphorus and fibroblast growth factor 23 (FGF23) concentrations, phosphorus and calcium excretion, and mRNA and/or protein of type II sodium-phosphate co-transporters (NPt2a, NPt2ab) and FGF23 and FGF23 receptors were measured in the intestines, calvaria, kidney, and liver. RESULTS: Hens in the N1000 group had a 16% lower serum phosphorus concentration and 22% greater phosphorus excretion than those in the N20 or N140 group (P ≤ 0.05). Compared with hens in the N20 group, hens in the N140 and N1000 groups, which did not differ, had 15-21% lower serum FGF23 concentrations, 19-22% greater calcium excretion, 43-56% lower ileum NPT2b protein production, and 1.5- to 1.6-fold greater kidney NPT2a protein production, respectively (all differences at P ≤ 0.05). CONCLUSIONS: Supplementing high concentrations of nicotinamide in diets for laying hens led to accelerated phosphorus and calcium excretions and decreased serum phosphorus and FGF23 concentrations, which were associated with downregulated intestinal NPt2b protein production. Our findings exclude kidney NPt2a protein production as a primary mechanism for the nicotinamide-induced body phosphorus loss.

Receptor for advanced glycation end products: a key molecule in the genesis of chronic kidney disease vascular calcification and a potential modulator of sodium phosphate co-transporter PIT-1 expression.

BACKGROUND: Chronic kidney disease (CKD) is associated with increased cardiovascular mortality, frequent vascular calcification (VC) and accumulation of uraemic toxins. Advanced glycation end products and S100 proteins interact with the receptor for advanced glycation end products (RAGE). In the present work, we aimed to investigate the role(s) of RAGE in the CKD-VC process. METHODS: Apoe-/- or Apoe-/-Ager (RAGE)-/- male mice were assigned to CKD or sham-operated groups. A high-phosphate diet was given to a subgroup of Apoe-/-and Apoe-/-Ager-/- CKD mice. Primary cultures of Ager+/+ and Ager-/- vascular smooth muscle cells (VSMCs) were established and stimulated with either vehicle, inorganic phosphate (Pi) or RAGE ligands (S100A12; 20 µM). RESULTS: After 12 weeks of CKD we observed a significant increase in RAGE ligand (AGE and S100 proteins) concentrations in the serum of CKD Apoe-/- mice. Ager messenger RNA (mRNA) levels were 4-fold higher in CKD vessels of Apoe-/- mice. CKD Apoe-/- but not CKD Apoe-/- or Ager-/- mice displayed a marked increase in the VC surface area. Similar trends were found in the high-phosphate diet condition. mRNA levels of Runx2 significantly increased in the Apoe-/- CKD group. In vitro, stimulation of Ager+/+VSMCs with Pi or S100A12 induced mineralization and osteoblast transformation, and this was inhibited by phosphonoformic acid (Pi co-transporters inhibitor) and Ager deletion. In vivo and in vitro RAGE was necessary for regulation of the expression of Pit-1, at least in part through production of reactive oxygen species. CONCLUSION: RAGE, through the modulation of Pit-1 expression, is a key molecule in the genesis of VC.

Daidzein upregulates anti-aging protein Klotho and NaPi 2a cotransporter in a rat model of the andropause.

In a rat model of the andropause we aimed to examine the influence of daidzein, soy isoflavone, on the structure and function of parathyroid glands (PTG) and the expression levels of some of the crucial regulators of Ca2+ and Pi homeostasis in the kidney, and to compare these effects with the effects of estradiol, serving as a positive control. Middle-aged (16-month-old) male Wistar rats were divided into the following groups: sham-operated (SO), orchidectomized (Orx), orchidectomized and estradiol-treated (Orx+E; 0.625mg/kg b.w./day, s.c.) as well as orchidectomized and daidzein-treated (Orx+D; 30mg/kg b.w./day, s.c.) group. Every treated group had a corresponding control group. PTH serum concentration was decreased in Orx+E and Orx+D groups by 10% and 21% (p<0.05) respectively, in comparison with the Orx. PTG volume was decreased in Orx+E group by 16% (p<0.05), when compared to the Orx. In Orx+E group expression of NaPi 2a was lower (p<0.05), while NaPi 2a abundance in Orx+D animals was increased (p<0.05), when compared to Orx. Expression of PTH1R was increased (p<0.05) in Orx+E group, while in Orx+D animals the same parameter was decreased (p<0.05), in comparison with Orx. Klotho expression was elevated (p<0.05) in Orx+D rats, in regard to Orx. Orx+D induced reduction in Ca2+/creatinine and Pi/creatinine ratio in urine by 32% and 16% (p<0.05) respectively, in comparison with Orx. In conclusion, presented results indicate the more coherent beneficial effects of daidzein compared to estradiol, on disturbed Ca2+ and Pi homeostasis, and presumably on bone health, in the aging male rats.

Nonlinear disposition of lithium in rats and saturation of its tubular reabsorption by the sodium-phosphate cotransporter as a cause.

We previously reported the contribution of sodium-phosphate cotransporter to the tubular reabsorption of lithium in rats. In the present study, the dose dependency of the renal handling of lithium was examined in rats. When lithium chloride at 1.25 mg/kg, 2.5 mg/kg and 25 mg/kg was intravenously injected as a bolus, the areas under the plasma concentration-time curve of lithium until 60 minutes were calculated to be 6.23 mEq·min/l, 8.77 mEq·min/l and 64.6 mEq·min/l, respectively. The renal clearance of lithium and its fractional excretion increased with increments in the dose administered. The renal clearance of lithium strongly correlated with the urinary excretion rate of phosphate in the 1.25 mg/kg group (r = 0.840) and 2.5 mg/kg group (r = 0.773), whereas this correlation was weak in the 25 mg/kg group (r = 0.306). The infusion of foscarnet, a typical inhibitor of sodium-phosphate cotransporter, decreased the fractional reabsorption of lithium in rats administered lithium chloride at 2.5 mg/kg, but did not affect it in rats administered 25 mg/kg. These results demonstrate the nonlinearity of the renal excretion of lithium in rats, with the saturation of lithium reabsorption by the sodium-phosphate cotransporter potentially being involved.

Effects of sodium, 1,25-dihydroxyvitamin D3 and parathyroid hormone fragment on inorganic P absorption and Type IIb sodium-phosphate cotransporter expression in ligated duodenal loops of broilers.

Three experiments were conducted with 22-day-old Arbor Acres male broilers to study the effects of Na+, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and parathyroid hormone fragment [PTH (1-34)] on inorganic P absorption and Type IIb sodium-phosphate cotransporter (NaP-IIb) mRNA and protein expression levels in ligated duodenal loops. The duodenal loops were perfused with solutions (pH = 6) containing zero, 50, or 150 mmol/L of Na+ as NaCl in Exp. 1, containing zero, 30, or 300 pmol/L of 1,25-(OH)2D3 in Exp. 2, or containing zero, 65, or 650 pmol/L of PTH (1-34) in Exp. 3, respectively. Compared with the control, additions of 50 and 150 mmol/L of Na+, 30 and 300 pmol/L of 1,25-(OH)2D3, or 65 and 650 pmol/L of PTH (1-34) to the perfusates promoted (P < 0.02) the P absorption percentages and rates, respectively. Additions of the above-mentioned concentrations of Na+ or 1,25-(OH)2D3 to the perfusates increased (P < 0.003) NaP-IIb mRNA level in the duodenum of broilers, and a similar trend (P = 0.08) was observed for PTH (1-34). The Na+, 1,25-(OH)2D3, and PTH (1-34) had no effects (P > 0.15) on NaP-IIb protein level in the duodenum of broilers. The results indicate that increased P absorption due to perfusions of Na+, 1,25-(OH)2D3 or PTH (1-34) might be attributed to enhanced NaP-IIb expression in the duodenum of broilers.

Changes in circulating levels of fibroblast growth factor 23 induced by short-term dietary magnesium deficiency in rats.

Fibroblast growth factor 23 (FGF23) is a potent regulator of phosphorus (P) and vitamin D metabolism. Long-term dietary magnesium (Mg) deficiency increases circulating levels of FGF23, whereas the effects of short-term dietary Mg deficiency are unclear. Thus, the present study investigated whether short-term dietary Mg deficiency affects circulating levels of FGF23. We also assessed changes in renal mRNA expression of vitamin D metabolizing enzymes and type II sodium-phosphate (Na/Pi) cotransporters, since these are regulated by FGF23. Rats were fed a control diet (control group) or an Mg-deficient diet (Mg-deficient group) for 2, 4 or 7 days. Serum Mg levels were significantly lower in the Mg-deficient group than in the control group at all time points. Serum FGF23 levels were significantly higher in the Mg-deficient group than in the control group at day 7. The 25-hydroxyvitamin D-24-hydroxylase (24(OH)ase) mRNA levels were significantly higher in the Mg-deficient group than in the control group at day 7 . No significant differences in types IIa and IIc Na/Pi cotransporter mRNA levels were observed between the control and Mg-deficient groups. These results suggest that dietary Mg deficiency causes a rapid increase in circulating levels of FGF23 and renal 24(OH)ase mRNA levels.

Pulmonary Alveolar Microlithiasis.

Pulmonary alveolar microlithiasis (PAM) is a genetic lung disorder that is characterized by the accumulation of calcium phosphate deposits in the alveolar spaces of the lung. Mutations in the type II sodium phosphate cotransporter, NPT2b, have been reported in patients with PAM. PAM progresses gradually, often producing incremental dyspnea on exertion, desaturation in young adulthood, and respiratory insufficiency by late middle age. Treatment remains supportive, including supplemental oxygen therapy. For patients with end-stage disease, lung transplantation is available as a last resort. The recent development of a laboratory animal model has revealed several promising treatment approaches for future trials.

Identification of an RNA-binding protein that is phosphorylated by PTH and potentially mediates PTH-induced destabilization of Npt2a mRNA.

Parathyroid hormone (PTH) is a key regulator of the expression and function of the type IIa sodium-phosphate cotransporter (Npt2a), the protein responsible for regulated renal phosphate reabsorption. We previously showed that PTH induces rapid decay of Npt2a mRNA through posttranscriptional mechanisms. We hypothesized that PTH-induced changes in RNA-binding protein (RBP) activity mediate the degradation of Npt2a mRNA. To address this aim, we treated opossum kidney (OK) cells, a PTH-sensitive proximal tubule cell culture model, with 100 nM PTH for 30 min and 2 h, followed by mass spectrometry characterization of the PTH-stimulated phosphoproteome. We identified 1,182 proteins differentially phosphorylated in response to PTH, including 68 RBPs. Preliminary analysis identified a phospho-RBP, hnRNPK-homology-type-splicing regulatory protein (KSRP), with predicted binding sites for the 3'-untranslated region (UTR) of Npt2a mRNA. Western blot analysis confirmed expression of KSRP in OK cells and showed PTH-dependent translocation to the nucleus. Immunoprecipitation of KSRP from control and PTH-treated cells followed by RNA isolation and RT-quantitative PCR analysis identified Npt2a mRNA from both control and PTH-treated KSRP pulldowns. Knockdown of KSRP followed by PTH treatment showed that KSRP is required for mediating PTH-stimulated reduction in sodium/hydrogen exchanger 3 mRNA, but not Npt2a mRNA. We conclude that 1) PTH is a major regulator of both transcription and translation, and 2) KSRP binds Npt2a mRNA but its role in PTH regulation of Npt2a mRNA is not clear.

Phosphorus Regulation in Chronic Kidney Disease.

Serum phosphorus levels stay relatively constant through the influence of multiple factors-such as parathyroid hormone, fibroblast growth factor 23, and vitamin D-on the kidney, bone, and digestive system. Whereas normal serum phosphorus ranges between 3 mg/dL to 4.5 mg/dL, large cross-sectional studies have shown that even people with normal kidney function are sometimes found to have levels ranging between 1.6 mg/dL and 6.2 mg/dL. While this may partially be due to diet and the factors mentioned above, total understanding of these atypical ranges of serum phosphorus remains uncertain. Risks for bone disease are high in people aged 50 and older, and this group comprises a large proportion of people who also have chronic kidney disease. Consuming diets low in calcium and high in phosphorus, especially foods with phosphate additives, further exacerbates bone turnover. Existing bone disease increases the risk for high serum phosphorus, and higher serum phosphorus has been associated with increased adverse events and cardiovascular-related mortality both in people with chronic kidney disease and in those with no evidence of disease. Once kidney function has deteriorated to end-stage disease (Stage 5), maintaining normal serum phosphorus requires dietary restrictions, phosphate-binding medications, and dialysis. Even so, normal serum phosphorus remains elusive in many patients with Stage 5 kidney disease, and researchers are testing novel targets that may inhibit intestinal transport of phosphorus to achieve better phosphate control. Protecting and monitoring bone health should also aid in controlling serum phosphorus as kidney disease advances.

Sodium-phosphate cotransporter mediates reabsorption of lithium in rat kidney.

Lithium, used for the treatment of bipolar disorders, is reabsorbed via sodium-transport system in the proximal tubule. This step causes intra-/inter-individual difference of lithium disposition, and it has not been unclear which transporter contributes. In this study, we examined effect of foscarnet and parathyroid hormone (PTH), inactivators for sodium-phosphate cotransporter, and phlorizin, a typical inhibitor for sodium-glucose cotransporter, on the disposition of lithium in rats. Their intravenous administration stimulated urinary excretion of phosphate or glucose. After the intravenous injection of lithium chloride as a bolus, plasma concentration of lithium decreased time-dependently. The renal clearance of lithium was calculated to be 0.740 ml/min/kg in control rats, and this was 26.7% of creatinine clearance. Foscarnet and PTH significantly increased the renal clearance of lithium and its ratio to creatinine clearance, suggesting that they prevented the reabsorption of lithium. No effect of phlorizin on the renal handling of lithium was recognized. In control rats, the renal clearance of lithium showed a strong correlation with the renal excretion rate of phosphate, compared with creatinine clearance. These findings suggest that sodium-phosphate cotransporter reabsorbs lithium in the rat kidney. Furthermore, its contribution was estimated to be more than 65.9% in the lithium reabsorption. And, this study raised the possibility that therapeutic outcome of lithium is related with the functional expression of sodium-phosphate cotransporter in the kidney.

Kidney and phosphate metabolism.

The serum phosphorus level is maintained through a complex interplay between intestinal absorption, exchange intracellular and bone storage pools, and renal tubular reabsorption. The kidney plays a major role in regulation of phosphorus homeostasis by renal tubular reabsorption. Type IIa and type IIc Na(+)/Pi transporters are important renal Na(+)-dependent inorganic phosphate (Pi) transporters, which are expressed in the brush border membrane of proximal tubular cells. Both are regulated by dietary Pi intake, vitamin D, fibroblast growth factor 23 (FGF23) and parathyroid hormone. The expression of type IIa Na(+)/Pi transporter result from hypophosphatemia quickly. However, type IIc appears to act more slowly. Physiological and pathophysiological alteration in renal Pi reabsorption are related to altered brush border membrane expression/content of the type II Na(+)/Pi cotransporter. Many studies of genetic and acquired renal phosphate wasting disorders have led to the identification of novel genes. Two novel Pi regulating genes, PHEX and FGF23, play a role in the pathophysiology of genetic and acquired renal phosphate wasting disorders and studies are underway to define their mechanism on renal Pi regulation. In recent studies, sodium-hydrogen exchanger regulatory factor 1 (NHERF1) is reported as another new regulator for Pi reabsorption mechanism.

Phosphatonins: new hormones that control phosphorus homeostasis.

Phosphorus (Pi) plays an important role in nucleic acid synthesis, energy metabolism, bone mineralization and cell signaling, and is also present in sugars, phospholipids and phosphoproteins. Phosphate homeostasis is controlled by processes that regulate the intestinal absorption and renal excretion of Pi, and bone turnover. These processes are influenced by peptide and sterol hormones, such as parathyroid hormone and 1α,25-dihydroxyvitamin D (1α,25[OH]2D3). Recently, a new class of phosphate-regulating peptides has been discovered: phosphatonins. These factors, such as FGF-23, secreted frizzled-related protein-4, matrix extracellular phosphoglycoprotein and FGF-7, are circulating peptides with potent phosphaturic activity. These peptides inhibit Na/Pi transporters in renal epithelial cells and, therefore, increase renal Pi excretion. In addition, FGF-23 and secreted frizzled-related protein-4 inhibit 25-hydroxyvitamin D 1α-hydroxylase activity, reducing 1α,25(OH)2D3 synthesis and, thus, intestinal Pi absorption. Phosphatonins have been associated with hypophosphatemic diseases, such as tumor-induced osteomalacia, X-linked hypophosphatemic rickets, autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets and hyperphosphatemic disease (e.g., tumoral calcinosis). The aim of this article is to review the role of phosphatonins in Pi metabolism in normal and pathologic conditions and also to investigate the correlations among the various phosphatonins.

Kidney and Phosphate Metabolism

The serum phosphorus level is maintained through a complex interplay between intestinal absorption, exchange intracellular and bone storage pools, and renal tubular reabsorption. The kidney plays a major role in regulation of phosphorus homeostasis by renal tubular reabsorption. Type IIa and type IIc Na+/Pi transporters are important renal Na+-dependent inorganic phosphate (Pi) transporters, which are expressed in the brush border membrane of proximal tubular cells. Both are regulated by dietary Pi intake, vitamin D, fibroblast growth factor 23 (FGF23) and parathyroid hormone. The expression of type IIa Na+/Pi transporter result from hypophosphatemia quickly. However, type IIc appears to act more slowly. Physiological and pathophysiological alteration in renal Pi reabsorption are related to altered brush-border membrane expression/content of the type II Na+/Pi cotransporter. Many studies of genetic and acquired renal phosphate wasting disorders have led to the identification of novel genes. Two novel Pi regulating genes, PHEX and FGF23, play a role in the pathophysiology of genetic and acquired renal phosphate wasting disorders and studies are underway to define their mechanism on renal Pi regulation. In recent studies, sodium-hydrogen exchanger regulatory factor 1 (NHERF1) is reported as another new regulator for Pi reabsorption mechanism.

Preparation and characterization of type III sodium-phosphate cotransporter siRNA-loaded chitosan nanoparticles / 第二军医大学学报

Objective: To prepare type III sodium-phosphate cotransporter (NaPi-III) siRNA-loaded chitosan microspheres and to evaluate their physico-chemical properties and siRNA release in vitro. Methods: NaPi- III siRNA was loaded into chitosan microspheres by using the complex coacervation method. The structure of the nanoparticles was observed under scanning electron microscope and their diameter distribution was measured by a laser grain analyzer. RNase assay was used to detect the efficacy of anoparticles in prevention of siRNA from degradation. Ultraviolet spectrophotometry and HPLC technique were used to determine the entrapment efficiency, loading capacity, and siRNA releasing rate in vitro. Results: The chitosan nanoparticles loaded with NaPi-III siRNA were successfully prepared. The nanoparticles were spherical in shape and were well distributed, with an average diameter of 173 nm. After RNase treatment, D260 rose more slowly in chitosan nanoparticles-based siRNA suspension than in simple NaPi-III siRNA solution(P<0.05, t = 4.32). Loading capacity of the nanoparticles was 28.1 %, the encapsulation efficiency was 73.07 %, and the total releasing rate of NaPi-III siRNA was less than 20% within 12 hours. Conclusion: Chitosan nanoparticles loaded with NaPi-III siRNA have been prepared through a complex coacervation method. The particles have high encapsulation efficency and good stability and are homogeneous in size. They can obviously delay the release of NaPi-III siRNA in vitro and therefore prevent NaPi-III siRNA from degradation.