Substrates and inhibitors of phosphate transporters: from experimental tools to pathophysiological relevance

Mechanistic characterization of waterborne selenite uptake in the water flea, Daphnia magna, indicates water chemistry affects toxicity in coal mine-impacted waters

Concentrations of selenium that exceed regulatory guidelines have been associated with coal mining activities and have been linked to detrimental effects on aquatic ecosystems and the organisms therein. Although the major route of selenium uptake in macroinvertebrates is via the diet, the uptake of waterborne selenite (HSeO3-), the prominent form at circumneutral pH, can be an important contributor to selenium body burden and thus selenium toxicity. In the current study, radiolabelled selenite (Se75) was used to characterize the mechanism of selenite uptake in the water flea, Daphnia magna. The concentration dependence (1-32 μM) of selenite uptake was determined in 1-hour uptake assays in artificial waters that independently varied in bicarbonate, chloride, sulphate, phosphate and selenate concentrations. At concentrations representative of those found in highly contaminated waters, selenite uptake was phosphate-dependent and inhibited by foscarnet, a phosphate transport inhibitor. At higher concentrations, selenite uptake was dependent on waterborne bicarbonate concentration and inhibited by the bicarbonate transporter inhibitor DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid). These findings suggest that concentrations of phosphate in coal mining-affected waters could alter selenite uptake in aquatic organisms and could ultimately affect the toxic impacts of selenium in such waters.

Intestinal and Renal Adaptations to Changes of Dietary Phosphate Concentrations in Rat

We have studied the role of the intestine, kidney, and several hormones when adapting to changes in dietary P concentration. Normal and parathyroidectomized (PTX) rats were fed pH-matched diets containing 0.1%, 0.6%, and 1.2% P concentrations. 32Pi uptake was determined in the jejunum and kidney cortex brush border membrane vesicles. Several hormone and ion concentrations were determined in the blood and urine of rats. Both jejunum and kidney cortex Pi transport was regulated with 5 d of chronic feeding of P diets in normal rats. Acute adaptation was determined by switching foods on day 6, which was only clearly observed in the kidney cortex of normal rats, with more statistical variability in the jejunum. However, no paradoxical increase of Pi uptake in the jejunum was reproduced after the acute switch to the 1.2% P diet. Pi uptake in the jejunum was parathyroid hormone (PTH)-independent, but in the kidney, the chronic adaptation was reduced, and no acute dietary adaptations were observed. The NaPi2a protein was more abundant in the PTX than the sham kidneys, but contrary to the modest or absent changes in Pi uptake adaptation, the transporter was similarly regulated by dietary P, as in the sham rats. PTH and fibroblast growth factor 23 (FGF23) were the only hormones regulated by all diet changes, even in fasting animals, which exhibited regulated Pi transport despite similar phosphatemia. Evidence of Pi appetite effects was also observed. In brief, our results show new characteristics of Pi adaptations, including a lack of correlation between Pi transport, NaPi2a expression, and PTH/FGF23 concentrations.

Strategies to lower fibroblast growth factor 23 bioactivity

Fibroblast growth factor 23 (FGF23) is a circulating hormone derived from the bone whose release is controlled by many factors and exerts a multitude of systemic actions. There are congenital and acquired disorders of increased and decreased FGF23 levels. In chronic kidney disease (CKD), elevations of FGF23 levels can be 1000-fold above the upper physiological limit. It is still debated whether this high FGF23 in CKD is a biomarker or causally related to morbidity and mortality. Data from human association studies support pathogenicity, while experimental data are less robust. Knowledge of the biology and pathobiology of FGF23 has generated a plethora of means to reduce FGF23 bioactivity at many levels that will be useful for therapeutic translations. This article summarizes these approaches and addresses several critical questions that still need to be answered.

Inhibition of epithelial phosphate transport by NAD+/NADH

Nicotinamide is an important regulator of Pi homeostasis after conversion into NAD⁺/NADH. In this work, we have studied the classical inhibition of Pi transport by these compounds in the brush border membrane vesicles (BBMV) of rat kidney and rat intestine, and we examined the effects in Opossum Kidney (OK) cells and in phosphate transporter-expressing Xenopus laevis oocytes. In BBMV, NAD⁺ required preincubation at either room temperature or on ice to inhibit Pi uptake in BBMV. However, no effects were observed in the known Slc34 or Slc20 Pi transporters expressed in Xenopus oocytes, in OK cells, or in isolated rat cortical nephron segments. In BBMV from jejunum or kidney cortex, the inhibition of Pi transport was specific, dose-related, and followed a competitive inhibition pattern, as shown by linear transformation and non-linear regression analyses. A K_i value of 538 µM NAD⁺ in kidney BBMV was obtained. Ribosylation inhibitors and ribosylation assays revealed no evidence that this reaction was responsible for inhibiting Pi transport. An analysis of the persistence of NAD⁺/NADH revealed a half-life of just 2 minutes during preincubation. Out of several metabolites of NAD degradation, only ADP-ribose was able to inhibit Pi uptake. Pi concentration also increased during 30 minutes of preincubation, up to 0.67mM, most likely as a metabolic end-product. In conclusion, the classical inhibition of Pi transport by NAD⁺/NADH in BBMV seems to be caused by the degradation metabolites of these compounds during the preincubation time.

The emerging role of phosphorus in human health

Phosphorus, an essential nutrient, performs vital functions in skeletal and non-skeletal tissues and is pivotal for energy production. The last two decades of research on the physiological importance of phosphorus have provided several novel insights about its dynamic nature as a nutrient performing functions as a phosphate ion. Phosphorous also acts as a signaling molecule and induces complex physiological responses. It is recognized that phosphorus homeostasis is critical for health. The intake of phosphorus by the general population world-wide is almost double the amount required to maintain health. This increase is attributed to the incorporation of phosphate containing food additives in processed foods purchased by consumers. Research findings assessed the impact of excessive phosphorus intake on cells' and organs' responses, and highlighted the potential pathogenic consequences. Research also identified a new class of bioactive phosphates composed of polymers of phosphate molecules varying in chain length. These polymers are involved in metabolic responses including hemostasis, brain and bone health, via complex mechanism(s) with positive or negative health effects, depending on their chain length. It is amazing, that phosphorus, a simple element, is capable of exerting multiple and powerful effects. The role of phosphorus and its polymers in the renal and cardiovascular system as well as on brain health appear to be important and promising future research directions.

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.

An expert update on novel therapeutic targets for hyperphosphatemia in chronic kidney disease: preclinical and clinical innovations

Introduction: The management of hyperphosphatemia in patients with chronic kidney disease (CKD) is complicated, requiring a multidisciplinary approach that includes dietary phosphate restriction, dialysis, and phosphate binders.Areas covered: We describe key players involved in regulating inorganic phosphate homeostasis and their differential role in healthy people and different stages of CKD. The contribution of paracellular and transcellular intestinal absorptive mechanisms are also examined. Finally, we illuminate recent therapeutic approaches for hyperphosphatemia in CKD. We searched PubMed/Medline (up to November 2019) using the following terms: chronic kidney disease, dialysis, diet, hyperphosphatemia, NaPi2b, nicotinamide, phosphate binder, secondary hyperparathyroidism, tenapanor and vascular calcification.Expert opinion: The precise mechanisms regulating intestinal phosphate absorption in humans is not completely understood. However, it is now established that this process involves two independent pathways: a) active transport (i.e. transcellular route, via specific ion transporters) and inactive transport (i.e. paracellular route across tight junctions). Dietary phosphate restriction and phosphate-binder use can lead to an undesirable maladaptive increase in phosphate uptake and promote active phosphate transport by increased expression of the gastrointestinal sodium-dependent phosphate transporter, NaPi2b. Nicotinamide may overcome these limitations through the inhibition of NaPi2b, by improved efficacy and reduced phosphate binder use and better compliance.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

Several phosphate transport processes are present in vascular smooth muscle cells

We have studied inorganic phosphate (P_i) handling in rat aortic vascular smooth muscle cells (VSMC) using ³²P-radiotracer assays. Our results have revealed a complex set of mechanisms consisting of 1) well-known PiT1/PiT2-mediated sodium-dependent P_i transport; 2) Slc20-unrelated sodium-dependent P_i transport that is sensitive to the stilbene derivatives 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS); 3) a sodium-independent P_i uptake system that is competitively inhibited by sulfate, bicarbonate, and arsenate and is weakly inhibited by DIDS, SITS, and phosphonoformate; and 4) an exit pathway from the cell that is partially chloride dependent and unrelated to the known anion-exchangers expressed in VSMC. The inhibitions of sodium-independent P_i transport by sulfate and of sodium-dependent transport by SITS were studied in greater detail. The maximal inhibition by sulfate was similar to that of P_i itself, with a very high inhibition constant (212 mM). SITS only partially inhibited sodium-dependent P_i transport, but the K_i was very low (14 µM). Nevertheless, SITS and DIDS did not inhibit P_i transport in Xenopus laevis oocytes expressing PiT1 or PiT2. Both the sodium-dependent and sodium-independent transport systems were highly dependent on VSMC confluence and on the differentiation state, but they were not modified by incubating VSMC for 7 days with 2 mM P_i under nonprecipitating conditions. This work not only shows that the P_i handling by cells is highly complex but also that the transport systems are shared with other ions such as bicarbonate or sulfate.NEW & NOTEWORTHY In addition to the inorganic phosphate (P_i) transporters PiT1 and PiT2, rat vascular smooth muscle cells show a sodium-dependent P_i transport system that is inhibited by DIDS and SITS. A sodium-independent P_i uptake system of high affinity is also expressed, which is inhibited by sulfate, bicarbonate, and arsenate. The exit of excess P_i is through an exchange with extracellular chloride. Whereas the metabolic effects of the inhibitors, if any, cannot be discarded, kinetic analysis during initial velocity suggests competitive inhibition.