High Dietary Pi Research Articles

High-phosphate diet causes atrial remodeling and increases atrial fibrillation vulnerability via STAT3/NF-κB signaling and oxidative stress.

Hyperphosphatemia is associated with adverse cardiovascular outcomes in both the general population and patients with end-stage renal disease. We evaluated whether high inorganic phosphate (Pi) intake causes atrial remodeling and increased atrial fibrillation (AF) risk. The 5/6 nephrectomized chronic kidney disease (CKD) mice were fed a high-Pi (2%) diet for 10 weeks. AF vulnerability was evaluated through transesophageal burst atrial pacing. Phosphoproteomic, Western blotting, and immunohistochemistry were used to evaluate the effects of high Pi in atrial fibroblasts, atrial myocytes, and HL-1 myocytes. CKD and sham mice fed a high-Pi diet exhibited increased AF vulnerability, atrial fibrosis, and oxidative stress compared with mice fed a normal diet. Compared with normal (1 mM) Pi, high (2 mM) Pi significantly increased the activity of atrial fibroblasts and mitochondrial oxidative stress. Phosphoproteomic analysis revealed that compared with normal Pi, high Pi considerably increased the phosphorylation of intracellular proteins in atrial fibroblasts, including proteins related to NF-κB signaling and STAT3. Inhibition of NF-κB, STAT3, and Nox4 by small interfering RNA reduced the high-Pi-induced expression of collagen. In HL-1 myocytes, the high Pi induced the degradation of myofibril proteins and hyperphosphorylation of RyR2, which was abolished by Nox4 and CaMKII inhibition. Switching back to a normal-Pi diet improved the atrial abnormalities induced by high-Pi diet. High-Pi intake causes atrial structural and electrical remodeling and increases AF vulnerability, which is mediated through STAT3/NF-κB signaling and oxidative stress. High dietary Pi intake can exert detrimental effects on atria and may increase AF risk.

Abstract 2408: Elevated phosphate promotes a pro-migration phenotype in lung cancer

Abstract Despite advances in treatment, metastasis remains a major cause of cancer-associated death in the US. Thus, identification of intrinsic and extrinsic factors that promote or inhibit tumor migration and invasion are critical. Increasingly, the dysregulation of cellular metabolism is being linked not only to tumor growth but tumor cell migration. Phosphorus (Pi) is an essential micronutrient necessary for energy metabolism, cell signaling via ATP, formation of nucleic acids and the cell membrane, and mineralization of the skeleton. Unsurprisingly, high levels of Pi are observed in tumor tissue compared to normal tissue, consistent with a demand for this metabolite in rapidly growing cells. Further, our lab has reported that high extracellular Pi increases cell growth, alters gene expression and protein translation, and promotes angiogenesis. High dietary Pi levels has also been shown to increase tumorigenesis in both lung and skin cancer models. However, it remains unknown how phosphate metabolism affects tumor migration and invasion. Here we report that high levels of Pi increases gene expression of EMT-related genes such as Twist, Vimentin, CD44, and OPN in the lung cancer cell line, A549. Further, increased Pi promotes tumor cell migration in a transwell assay and decreased Pi levels or inhibition of phosphate transport by phosphonoformic acid (PFA) impaired migration. Suggesting that high Pi can promote a pro-migration phenotype by upregulating EMT in cancer cells and by increasing directed cell migration. Ongoing research will focus on how intracellular Pi flux changes during cell invasion using a novel FRET-based phosphate sensor in vitro and the effect of Pi inhibition on metastasis in vivo. These findings provide further evidence for the role dysregulated Pi metabolism in promoting cancer progression and the potential for therapeutic intervention with broad impacts on patient outcomes. Citation Format: Jamie Arnst, George Beck. Elevated phosphate promotes a pro-migration phenotype in lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2408.

Increased phosphate absorption in mice lacking NHE3 in intestinal epithelial cells

The kidneys play a major role in maintaining phosphate (Pi) homeostasis. Patients in later stages of CKD develop hyperphosphatemia. One novel treatment option is tenapanor, an intestinal‐specific NHE3 inhibitor. To gain mechanistic insight into the role of intestinal NHE3 in Pi homeostasis, we studied tamoxifen (Tam) ‐inducible intestinal epithelial cell‐specific NHE3 knockout (NHE3IEC‐KO) mice with the hypothesis that their phenotype should reflect inhibitory effects of tenapanor on intestinal Pi uptake. Mice were challenged by high dietary Pi, and hormones as well as plasma Pi were determined. The in vivo effects of tenapanor were studied via 33P uptake from the intestine. Pi transport was measured ex vivo in everted gut sacs (EGS) and brush border membrane vesicles (BBMV). Intestinal and renal protein expression of Pi transporters were determined. No differences were found between genotypes in terms of Pi homeostasis before Tam. Two weeks after Tam, NHE3IEC‐KO mice had similar Pi homeostasis, but a ~25% reduction in FGF23 compared with control mice on standard diet. Two‐fold greater intestinal Npt2b expression in NHE3IEC‐KO mice (P<0.05) was associated with significantly enhanced Pi uptake in EGS and BBMV. One hour after acute oral Pi loading (0.5 mol/L NaH2PO4), plasma Pi levels increased to a significantly greater extent in NHE3IEC‐KO compared to control mice (2.0±0.2 vs 1.1±0.1 mmol L‐1, P<0.05). Tenapanor treatment caused ~67% lower 33P uptake (P<0.05) after 5 minutes compared with vehicle‐treated control mice. At later time points, tenapanor‐treated control mice showed an increase in intestinal Pi uptake, reaching a maximum of 33P in plasma after 60 minutes. In response to high dietary Pi, plasma Pi slightly but significantly decreased (‐0.25±0.1 mmol/L, P<0.05) in control mice, possibly the consequence of the combination of significantly elevated PTH and FGF23. In contrast to control mice, high dietary Pi significantly increased plasma Pi in NHE3IEC‐KO mice (0.5±0.1 mmol/L, P<0.05), which was associated with significantly increased plasma PTH levels (P<0.05). FGF23 levels also increased to a significantly greater extent in NHE3IEC‐KO vs control mice (552±66 vs 244±14 pg/mL, P<0.05). This was associated with ~2.25‐fold greater Npt2b expression (P<0.05). Fifty percent lower renal Npt2c expression (P<0.05) and a trend for lower Npt2a expression (~25%) were unable to correct for higher plasma Pi. In summary, intestinal NHE3 significantly contributes to Pi homeostasis. In contrast to effects described for tenapanor on Pi homeostasis, NHE3IEC‐KO mice show higher, rather than lower, intestinal Pi uptake.

Enhanced phosphate absorption in intestinal epithelial cell-specific NHE3 knockout mice.

AimsThe kidneys play a major role in maintaining Pi homeostasis. Patients in later stages of CKD develop hyperphosphatemia. One novel treatment option is tenapanor, an intestinal‐specific NHE3 inhibitor. To gain mechanistic insight into the role of intestinal NHE3 in Pi homeostasis, we studied tamoxifen‐inducible intestinal epithelial cell‐specific NHE3 knockout (NHE3IEC‐KO) mice.MethodsMice underwent dietary Pi challenges, and hormones as well as urinary/plasma Pi were determined. Intestinal 33P uptake studies were conducted in vivo to compare the effects of tenapanor and NHE3IEC‐KO. Ex vivo Pi transport was measured in everted gut sacs and brush border membrane vesicles. Intestinal and renal protein expression of Pi transporters were determined.ResultsOn the control diet, NHE3IEC‐KO mice had similar Pi homeostasis, but a ~25% reduction in FGF23 compared with control mice. Everted gut sacs and brush border membrane vesicles showed enhanced Pi uptake associated with increased Npt2b expression in NHE3IEC‐KO mice. Acute oral Pi loading resulted in higher plasma Pi in NHE3IEC‐KO mice. Tenapanor inhibited intestinal 33P uptake acutely but then led to hyper‐absorption at later time points compared to vehicle. In response to high dietary Pi, plasma Pi and FGF23 increased to higher levels in NHE3IEC‐KO mice which was associated with greater Npt2b expression. Reduced renal Npt2c and a trend for reduced Npt2a expression were unable to correct for higher plasma Pi.ConclusionIntestinal NHE3 has a significant contribution to Pi homeostasis. In contrast to effects described for tenapanor on Pi homeostasis, NHE3IEC‐KO mice show enhanced, rather than reduced, intestinal Pi uptake.

Central Calcineurin Plays a Role in Skeletal Muscle Reflex Overactivity Induced by High Dietary Phosphate Intake in Rats

Inorganic phosphate (Pi) has emerged as an important dietary risk factor for hypertension. Our laboratory has previously shown that high dietary Pi intake augments blood pressure and sympathetic nerve activity during muscle contraction in otherwise normal rats primarily due to an overactive exercise pressor reflex (EPR). However, the mechanism underlying the abnormal responsiveness induced by dietary Pi excess remains poorly understood. Evidence suggests that a high Pi diet-induced increase in the phosphaturic hormone fibroblast growth factor 23 activates calcineurin in cardiomyocytes, potentially contributing to left ventricular hypertrophy. Calcineurin has been shown to exert direct central sympathoexcitatory action in rodents but its role in blood pressure control during dietary Pi excess is unknown. Accordingly, we conducted studies to examine the role of brain calcineurin in generating high Pi diet-induced EPR overactivation in the Sprague-Dawley rats fed either a normal 0.6% Pi diet (NP) or a high 1.2% Pi diet (HP) for 12 weeks. In decerebrate NP and HP animals, we measured mean arterial pressure (MAP) response to hindlimb muscle contraction (i.e. activation of the EPR) or hindlimb intra-arterial capsaicin injection (i.e. activation of the metabolically-sensitive component of the EPR known as the metaboreflex) before and after intracerebroventricular (ICV) administration of a calcineurin inhibitor cyclosporin A. We also assessed levels of regulator of calcineurin 1.4 (Rcan1.4), a protein that is upregulated in response to increased calcineurin activity in the brainstem. We found that brainstem Rcan1.4 expression was significantly increased by 60% in HP rats compared to NP rats (P<0.05), suggesting activation of calcineurin. Calcineurin expression in the brainstem was also greater in the HP group than in the NP group (9.2±0.6 vs. 1.0±0.6 arbitrary unit, p<0.05). ICV injection of cyclosporin A significantly attenuated the MAP (∆=28±13 vs. 7±3 mmHg, P<0.05) response to EPR activation in HP animals, but did not alter the response in NP animals (∆MAP=12±6 vs. 9±2 mmHg, P>0.05). Similarly, the potentiated MAP response to stimulation of the metaboreflex was significantly decreased by ICV cyclosporin A in HP rats (∆=49±7 vs. 25±5 mmHg, P<0.05), but remained unchanged in NP rats (∆MAP=26±3 vs. 20±8 mmHg, P>0.05). In conclusion, the results implicate brain calcineurin activity in the abnormal skeletal muscle reflex-mediated hypertensive responsiveness induced by a high Pi diet.

High Phosphate Induces and Klotho Attenuates Kidney Epithelial Senescence and Fibrosis

Cellular senescence is an irreversible cell growth arrest and is associated with aging and age-related diseases. High plasma phosphate (Pi) and deficiency of Klotho contribute to aging and kidney fibrosis, a pathological feature in the aging kidney and chronic kidney disease. This study examined the interactive role of Pi and Klotho in kidney senescence and fibrosis. Homozygous Klotho hypomorphic mice had high plasma Pi, undetectable Klotho in plasma and kidney, high senescence with massive collagen accumulation in kidney tubules, and fibrin deposits in peritubular capillaries. To examine the Pi effect on kidney senescence, a high (2%) Pi diet was given to wild-type mice. One week of high dietary Pi mildly increased plasma Pi, and upregulated kidney p16/p21 expression, but did not significantly decrease Klotho. Two weeks of high Pi intake led to increase in plasminogen activator inhibitor (PAI)-1, and decrease in kidney Klotho, but still without detectable increase in kidney fibrosis. More prolonged dietary Pi for 12 weeks exacerbated kidney senescence and fibrosis; more so in heterozygous Klotho hypomorphic mice compared to wild-type mice, and in mice with chronic kidney disease (CKD) on high Pi diet compared to CKD mice fed a normal Pi diet. In cultured kidney tubular cells, high Pi directly induced cellular senescence, injury and epithelial-mesenchymal transition, and enhanced H2O2-induced cellular senescence and injury, which were abrogated by Klotho. Fucoidan, a bioactive molecule with multiple biologic functions including senescence inhibition, blunted Pi-induced cellular senescence, oxidation, injury, epithelial-mesenchymal transition, and senescence-associated secretary phenotype. In conclusion, high Pi activates senescence through distinct but interconnected mechanisms: upregulating p16/p21 (early), and elevating plasminogen activator inhibitor-1 and downregulating Klotho (late). Klotho may be a promising agent to attenuate senescence and ameliorate age-associated, and Pi-induced kidney degeneration such as kidney fibrosis.

Phosphate-sensing and regulatory mechanism of FGF23 production.

Inorganic phosphate (Pi) is an essential mineral for human. Hypophosphatemia and hyperphosphatemia cause rickets/osteomalacia and ectopic calcification, respectively, indicating that serum Pi level needs to be regulated. Fibroblast growth factor (FGF) 23 is a principal hormone to regulate serum Pi level. FGF23 is produced by the bone, especially by the osteoblasts and osteocytes, and works by binding to FGF receptor (FGFR) 1c and α-Klotho complex in the kidney. FGF23 reduces serum Pi level by inhibiting both renal phosphate reabsorption and intestinal phosphate absorption via reduction of serum 1,25-dihydroxyvitamin D level. It has been unclear how the bone senses changes of serum Pi level and how the bone regulates the production of FGF23. Our recent results indicate that the post-translational modification of FGF23 protein through a gene product of GALNT3 is the main regulatory mechanism of enhanced FGF23 production by high dietary Pi. Furthermore, high extracellular Pi directly activates FGFR1 and its downstream intracellular signaling pathway regulates the expression level of GALNT3. We propose that FGFR1 works as a Pi-sensing receptor in the regulation of FGF23 production and serum Pi level. There is a negative feedback system, which is a basic mechanism of endocrine regulation, in the regulation of serum Pi involving FGFR1, and FGF23. These findings may lead to the development of new therapeutic methods to treat diseases caused by abnormal Pi level.

Dietary vitamin D interacts with high phosphate-induced cardiac remodeling in rats with normal renal function.

Vitamin D (VD) and phosphate (Pi) load are considered as contributors to cardiovascular disease in chronic kidney disease and the general population, but interactive effects of VD and Pi intake on the heart are not clearly illustrated. We fed normal male rats with three levels of dietary VD (100, 1100 or 5000 IU/kg chow) and Pi (0.2, 0.6 or 1.6%) (3X3 design) for 8 weeks and examined renal and cardiac function and histology. High dietary Pi decreased plasma and renal Klotho and plasma 25-hydroxyvitamin D, and increased plasma Pi, fibroblast growth factor 23 and parathyroid hormone without affecting renal function, while low Pi increased plasma and renal Klotho. Both low and high VD diets enhanced high Pi-reduced Klotho expression. Low dietary VD reduced-plasma Klotho was rescued by a low Pi diet. High dietary Pi reduced-cardiac ejection fraction was not modified by a low or high VD diet, but the dietary VD effects on cardiac pathologic changes were more complex. High dietary Pi-induced cardiac hypertrophy was attenuated by a low VD and exacerbated by a high VD diet. In contrast, high dietary Pi -induced cardiac fibrosis was magnified by a low VD and attenuated by a high VD diet. High Pi diet induces hypertrophy and fibrosis in left ventricles, a low VD diet accelerates high Pi-induced fibrosis, and a high VD diet exacerbated high Pi -induced hypertrophy. Therefore, cardiac phosphotoxicity is exacerbated by either high or low dietary VD in rats with normal kidney function.

Effect of acute high-phosphate intake on muscle metaboreflex activation and vascular function.

Increased consumption of inorganic phosphate (Pi), an abundant ingredient in processed foods, has been associated with elevated cardiovascular disease risk; however, studies investigating underlying mechanisms are limited. Recently, high dietary Pi was shown to exaggerate the pressor response to static muscle contraction in rodents in part because of overactivation of metabolically sensitive skeletal muscle afferents. Whether acute high Pi consumption affects muscle metaboreflex activation in humans remains unknown. Furthermore, although acute high Pi consumption has been shown to impair vascular function in young healthy men, equivocal results have been reported. Therefore, we hypothesized that acute high Pi consumption augments mean arterial pressure (MAP) responses during muscle metaboreflex activation, impairs endothelial function, and increases arterial stiffness in young healthy men. Subjects performed 35% maximal voluntary contraction static handgrip (HG), followed by postexercise ischemia (PEI) to isolate muscle metaboreflex activation. Resting flow-mediated dilation (FMD) and arterial stiffness were assessed. Measures were made before (pre) and 60 min after (post) subjects consumed either a high-phosphate drink (2,000 mg phosphorus and 1,520 mg sodium) or a sodium drink (1,520 mg sodium; control). MAP responses during HG (preΔ = +23 ± 3 mmHg; postΔ = +21 ± 2 mmHg; P = 0.101) and PEI (preΔ = +21 ± 4 mmHg; postΔ = +18 ± 3 mmHg; P = 0.184) were similar before and after Pi consumption. In contrast, FMD was significantly attenuated following Pi (pre = 5.1 ± 0.5%; post = 3.5 ± 0.5%; P = 0.010), whereas arterial stiffness remained unchanged. There were no changes in any measured variable after control drink consumption. In summary, although the muscle metaboreflex remains unaffected following acute high Pi consumption in young healthy men, endothelial function is impaired. NEW & NOTEWORTHY This study was the first to investigate the influence of acute high-phosphate consumption on the pressor response during isometric handgrip and isolated muscle metaboreflex activation during postexercise ischemia in young healthy humans. We demonstrated that a single high dose of phosphate (2,000 mg) did not augment blood pressure in response to exercise or isolated muscle metaboreflex activation, but endothelial function was blunted in young healthy men.

Regulation of vitamin D metabolizing enzymes in murine renal and extrarenal tissues by dietary phosphate, FGF23, and 1,25(OH)2D3.

BackgroundThe 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) together with parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) regulates calcium (Ca2+) and phosphate (Pi) homeostasis, 1,25(OH)2D3 synthesis is mediated by hydroxylases of the cytochrome P450 (Cyp) family. Vitamin D is first modified in the liver by the 25-hydroxylases CYP2R1 and CYP27A1 and further activated in the kidney by the 1α-hydroxylase CYP27B1, while the renal 24-hydroxylase CYP24A1 catalyzes the first step of its inactivation. While the kidney is the main organ responsible for circulating levels of active 1,25(OH)2D3, other organs also express some of these enzymes. Their regulation, however, has been studied less.Methods and resultsHere we investigated the effect of several Pi-regulating factors including dietary Pi, PTH and FGF23 on the expression of the vitamin D hydroxylases and the vitamin D receptor VDR in renal and extrarenal tissues of mice. We found that with the exception of Cyp24a1, all the other analyzed mRNAs show a wide tissue distribution. High dietary Pi mainly upregulated the hepatic expression of Cyp27a1 and Cyp2r1 without changing plasma 1,25(OH)2D3. FGF23 failed to regulate the expression of any of the studied hydroxylases at the used dosage and treatment length. As expected, renal mRNA expression of Cyp27b1 was reduced and Cyp24a1 was increased in response to 1,25(OH)2D3 treatment. However, the 25-hydroxylases were rather unaffected by 1,25(OH)2D3 treatment.ConclusionsThe analyzed vitamin D hydroxylases are regulated in a tissue and treatment-specific manner.

High dietary phosphate intake induces hypertension and augments exercise pressor reflex function in rats.

An increasing number of studies have linked high dietary phosphate (Pi) intake to hypertension. It is well established that the rise in sympathetic nerve activity (SNA) and blood pressure (BP) during physical exertion is exaggerated in many forms of hypertension, which are primarily mediated by an overactive skeletal muscle exercise pressor reflex (EPR). However, it remains unknown whether high dietary Pi intake potentiates the EPR-mediated SNA and BP response to exercise. Accordingly, we measured renal SNA (RSNA) and mean BP (MBP) in normotensive Sprague-Dawley rats fed a normal Pi diet (0.6%, n = 13) or high Pi diet (1.2%, n = 13) for 3 mo. As previously reported, we found that resting BP was significantly increased by 1.2% Pi diet in both conscious and anesthetized animals. Activation of the EPR by electrically induced hindlimb contraction triggered greater increases in ΔRSNA and ΔMBP in the 1.2% compared with 0.6% Pi group (126 ± 25 vs. 42 ± 9%; 44 ± 5 vs. 14 ± 2 mmHg, respectively, P < 0.01). Activation of the muscle mechanoreflex, a component of the EPR, by passively stretching hindlimb muscle also evoked greater increases in ΔRSNA and ΔMBP in the 1.2% compared with 0.6% Pi group (109 ± 27 vs. 24 ± 7%, 38 ± 7 vs. 8 ± 2 mmHg, respectively, P < 0.01). A similar response was produced by hindlimb intra-arterial capsaicin administration to stimulate the metaboreflex arm of the EPR. Thus, our data demonstrate a novel action of dietary Pi loading in augmenting EPR function through overactivation of both the muscle mechanoreflex and metaboreflex.

High inorganic phosphate diet augments the sympathetic response to exercise pressor reflex activation in rats

Inorganic phosphates are widely used in the food industry as preservatives, flavor enhancers, and color stabilizers. Evidence suggests high inorganic phosphate (Pi) diets induce hypertension in normal rats. Whether alterations in sympathetic function contribute to this manifestation of high blood pressure is unknown. Accordingly, we measured changes in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) during stimulation of the muscle exercise pressor reflex (EPR) in decerebrated Sprague‐Dawley rats fed a normal (0.6%) or high (1.2%) Pi diet for 12 weeks. Compared to normal Pi diet‐fed rats, a high Pi diet significantly increased 24‐hour urine excretion of phosphate (12±1 vs. 77±5 mg/day, P&lt;0.05) as well as resting MAP (94±5 vs. 120±5 mmHg, P&lt;0.05). There were no differences in sodium and potassium excretion between groups. Comparing control rats to those fed a high Pi diet, contraction‐induced stimulation of the EPR evoked significantly greater increases in RSNA (Δ = 46±9 vs. 126±25 %, P&lt;0.05) and MAP (Δ = 14±2 vs. 44±5 mmHg, P&lt;0.05) in rats fed a 1.2% Pi diet. The data provide the first evidence that excess phosphorus intake induces sympathetic overactivity in response to EPR activation. This finding suggests that the high dietary Pi commonly observed in the normal American diet may lead to sympathetic dysfunction contributing to the pathogenesis of hypertension.Support or Funding InformationSupported by NIH HL‐113738

Identification of Phosphate Regulated Transcripts in Ileum and Renal Proximal Tubules

Phosphate (Pi) is a vital mineral for every cell in order to maintain energy metabolism, cell signaling and structure. The kidney and intestine are two major organs in maintaining Pi homeostasis by their role in absorbing Pi from the diet and by the reabsorption of filtered Pi. Apical uptake of Pi into intestinal and proximal tubular epithelial cells is well characterized, whereas basolateral efflux mechanisms are poorly understood. The amount of Na‐Pi cotransporters (members of the Slc34 family) expressed at the apical membrane is regulated by changes in dietary Pi intake through mechanisms, which are not fully understood. The aim of this project is to identify transcripts in ileum and renal proximal tubules regulated by dietary Pi. Identified genes may be involved in the transcellular transport of Pi like the unknown basolateral transport pathway or contribute to adaption of apical Pi uptake and metabolism. C57BL/6 male mice (8 weeks old) were fed a high (1.2%) or low (0.1%) Pi diet for 3 days. Epithelial scrapings were prepared from ileum and micro‐dissected renal proximal tubules were collected. Renal and intestinal adaption was confirmed by qPCR for known Pi regulated transcripts. RNA was then analysed using the RNA sequencing service provided by the Functional Genomics Center Zurich. Comparison of the transcriptome of low and high dietary Pi conditions revealed 2029 genes to be differently (at least 2 fold changes) expressed in ileum and 327 in renal proximal tubules. Regulation of selected transcripts was confirmed by qPCR. Pathway analysis is currently being performed and selected candidates for basolateral Pi transporters will be expressed in Xenopus laevis oocytes.Funded by the Swiss National Science Foundation.

Dietary phosphate: what do we know about its toxicity

Inorganic phosphate (Pi) is an essential mineral required for diverse cellular processes. Recent genetic and dietary experiments in animal models indicate that Pi may be toxic to a variety of biological processes. High dietary Pi load in such animal models resulted in an increase in oxidative stress, DNA damage that resulted in phenotypic expression of premature aging, and short life span. Further, high Pi load was reported to induce carcinogenesis in lung and skin cancer animal models. So far, translational research is limited to observational studies that show an independent association of Pi intake with morbidity and mortality across all strata of kidney function. Fast-food and processed-food consumption in an average American diet leads to large increases in daily phosphate intake. Whether such dietary patterns explain the epidemiology of disease processes in humans remains to be further investigated. Phosphotoxicity is a novel concept with a potential for a large public health impact in future, but urgent studies are needed to prove reducing Pi intake to a certain target generates better clinical outcomes.

Inorganic Phosphate Modulates the Expression of the NaPi-2a Transporter in thetrans-Golgi Network and the Interaction with PIST in the Proximal Tubule

Inorganic phosphate (Pi) homeostasis is maintained by the tight regulation of renal Pi excretion versus reabsorption rates that are in turn modulated by adjusting the number of Pi transporters (mainly NaPi-2a) in the proximal tubules. In response to some hormones and a high dietary Pi content, NaPi-2a is endocytosed and degraded in the lysosomes; however, we show here that some NaPi-2a molecules are targeted to the trans-Golgi network (TGN) during the endocytosis. In the TGN, NaPi-2a interacts with PIST (PDZ-domain protein interacting specifically with TC10), a TGN-resident PDZ-domain-containing protein. The extension of the interaction is proportional to the expression of NaPi-2a in the TGN, and, consistent with that, it is increased with a high Pi diet. When overexpressed in opossum kidney (OK) cells, PIST retains NaPi-2a in the TGN and inhibits Na-dependent Pi transport. Overexpression of PIST also prevents the adaptation of OK cells to a low Pi culture medium. Our data supports the view that NaPi-2a is subjected to retrograde trafficking from the plasma membrane to the TGN using one of the machineries involved in endosomal transport and explains the reported expression of NaPi-2a in the TGN.

Calcium pros and cons significance and risk of phosphorus supplementation. The risk of dietary phosphorus intake

Dietary intake of phosphorus (Pi) is an important determinant of Pi balance in patients who have chronic kidney disease (CKD) and a reduced GFR. High dietary Pi burden may promote vascular calcification and cardiovascular events. Recently, Ohnishi and Razzaque suggest that phosphate toxicity accelerates the mammalian aging process and that reducing the phosphate burden can delay the aging (FASEB J 24, 3562, 2010) . Dietary Pi is derived largely from foods with high protein content or food additives. Accurate information on the Pi content of foods is needed to achieve a low Pi intake and effectively manage CKD and the aging. In this review, we discuss the risk of dietary Pi intake in CKD and the aging.

Low Dietary Inorganic Phosphate Stimulates Lung Tumorigenesis Through Altering Protein Translation and Cell Cycle in K-ras LA1 Mice

Recent surveys indicate that Pi intake has increased steadily as Pi-containing foods have increased. Our previous study demonstrated that high dietary Pi strongly stimulated lung tumorigeneis. In order to answer the issue whether low Pi may be chemopreventive, we examined the effects of low Pi on lung cancer. Eighteen 5-wk-old male K-ras LA1 lung cancer model mice were randomly allocated to 2 groups. One group was fed a normal diet (0.5% Pi) and other group was fed low Pi (0.1% Pi) diet for 4 wk. Lung cancer development was evaluated by histopathological examination, Western blot, kinase assay, and immunohistochemistry. Low Pi increased the expression of sodium-dependent phosphate co-transporter 2b, and activated Akt signal with decreased PTEN expression in the lungs of K-ras LA1 mice. Low Pi increased the Akt/mTOR-mediated protein translation through upregulating the phosphorylation of p70S6K and 4E-BP1. In addition, low Pi stimulated cell cycling as evidenced by altered cell cycle regulators such as cyclin D1 and D3. Finally, low Pi increased lung tumorigenesis in K-ras LA1 mice compared to the normal diet group. Our results clearly demonstrated that low Pi also promoted lung tumorigenesis, thus suggesting that an appropriate intake of dietary Pi may be critical for lung cancer prevention as well as treatment.

High dietary inorganic phosphate increases lung tumorigenesis and alters Akt signaling.

Phosphate (Pi) is an essential nutrient to living organisms. Recent surveys indicate that the intake of Pi has increased steadily. Our previous studies have indicated that elevated Pi activates the Akt signaling pathway. An increased knowledge of the response of lung cancer tissue to high dietary Pi may provide an important link between diet and lung tumorigenesis. The current study was performed to elucidate the potential effects of high dietary Pi on lung cancer development. Experiments were performed on 5-week-old male K-ras(LA1) lung cancer model mice and 6-week-old male urethane-induced lung cancer model mice. Mice were fed a diet containing 0.5% Pi (normal Pi) and 1.0% Pi (high Pi) for 4 weeks. At the end of the experiment, all mice were killed. Lung cancer development was evaluated by diverse methods. A diet high in Pi increased lung tumor progression and growth compared with normal diet. High dietary Pi increased the sodium-dependent inorganic phosphate transporter-2b protein levels in the lungs. High dietary consumption of Pi stimulated pulmonary Akt activity while suppressing the protein levels of tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 as well as Akt binding partner carboxyl-terminal modulator protein, resulting in facilitated cap-dependent protein translation. In addition, high dietary Pi significantly stimulated cell proliferation in the lungs of K-ras(LA1) mice. Our results showed that high dietary Pi promoted tumorigenesis and altered Akt signaling, thus suggesting that careful regulation of dietary Pi may be critical for lung cancer prevention as well as treatment.

High Dietary Inorganic Phosphate Affects Lung through Altering Protein Translation, Cell Cycle, and Angiogenesis in Developing Mice

Inorganic phosphate (Pi) plays a key role in diverse physiological functions. Several studies indicate that Pi may affect lung cell development through Na/Pi cotransporter (NPT). Several NPT subtypes have been identified in mammalian lung, and considerable progress has been made in our understanding of their function and regulation. Therefore, current study was performed to elucidate the potential effects of high dietary Pi on lungs of developing mice. Our results clearly demonstrate that high dietary Pi may affect the lung of developing mice through Akt-related cap-dependent protein translation, cell cycle regulation, and angiogenesis. Our results support the hypothesis that Pi works as a critical signal molecule for normal lung growth and suggest that careful restriction of Pi consumption may be important in maintaining a normal development.

A High Inorganic Phosphate Diet Perturbs Brain Growth, Alters Akt-ERK Signaling, and Results in Changes in Cap-Dependent Translation

Inorganic phosphate (Pi) plays a key role in diverse physiological functions. Recently, considerable progress has been made in our understanding of the function and regulation of the brain-specific sodium-dependent inorganic phosphate transporter 1 (NPT1), which is found to exist principally in cerebrum and cerebellum. The potential importance of Pi as a novel signaling molecule and the poor prognosis of diverse neurodegenerative diseases that involve brain-specific NPT1 have prompted us to define the pathways by which Pi affects mouse brain growth. A high phosphate diet caused an increase in serum Pi accompanied by a decrease in calcium, and a decrease in body weight coupled with a decreased relative weight of cerebellum. A high phosphate diet caused a significant increase in protein expression of NPT1, both in cerebrum and cerebellum. Additionally, the high phosphate diet increased Homo sapiens v-akt murine thymoma viral oncogene homolog 1 (Akt) phosphorylation at Ser473 in cerebrum and cerebellum, whereas suppression of Akt phosphorylation at Thr308 was observed only in cerebellum. Selective suppression of eukaryotic translation initiation factor-binding protein (eIF4E-BP1) in cerebrum was induced by high levels of Pi, which induced cap-dependent and cap-independent protein translation in cerebrum and cerebellum, respectively. Phosphorylation of extracellular regulated kinase 1 (ERK1) in comparison with that of ERK2 was significantly reduced in both cerebrum and cerebellum. High levels of Pi reduced protein expressions of proliferating cell nuclear antigen (PCNA) and cyclin D1 in cerebrum and cerebellum. In conclusion, the results indicate that high dietary Pi can perturb normal brain growth, possibly through Akt-ERK signaling in developing mice.