Sensing phosphate across the kingdoms

Hydrogen phosphate selectively induces MDA MB 231 triple negative breast cancer cell death in vitro

Phosphate ions are the most abundant anions inside the cells, and they are increasingly gaining attention as key modulators of cellular function and gene expression. However, little is known about the effect of inorganic phosphate ions on cancer cells, particularly breast cancer cells. Here, we investigated the toxicity of different phosphate compounds to triple-negative human breast cancer cells, particularly, MDA-MB-231, and compared it to that of human monocytes, THP-1. We found that, unlike dihydrogen phosphate (H₂PO₄⁻), hydrogen phosphate (HPO₄²⁻) at 20 mM or lower concentrations induced breast cancer cell death more than immune cell death, mainly via apoptosis. We correlate this effect to the fact that phosphate in the form of HPO₄²⁻ raises pH levels to alkaline levels which are not optimum for transport of phosphate into cancer cells. The results in this study highlight the importance of further exploring hydrogen phosphate (HPO₄²⁻) as a potential therapeutic for the treatment of breast cancer.

Roles of osteocytes in phosphate metabolism

Osteocytes are dendritic cells in the mineralized bone matrix that descend from osteoblasts. They play critical roles in controlling bone mass through the production of sclerostin, an inhibitor of bone formation, and receptor activator of nuclear factor κ B ligand, an inducer of osteoblastic bone resorption. Osteocytes also govern phosphate homeostasis through the production of fibroblast growth factor 23 (FGF23), which lowers serum phosphate levels by increasing renal phosphate excretion and reducing the synthesis of 1,25-dihydroxyvitamin D (1,25(OH)₂D), an active metabolite of vitamin D. The production of FGF23 in osteocytes is regulated by various local and systemic factors. Phosphate-regulating gene homologous to endopeptidase on X chromosome (PHEX), dentin matrix protein 1 (DMP1), and family with sequence similarity 20, member C function as local negative regulators of FGF23 production in osteocytes, and their inactivation causes the overproduction of FGF23 and hypophosphatemia. Sclerostin has been suggested to regulate the production of FGF23, which may link the two functions of osteocytes, namely, the control of bone mass and regulation of phosphate homeostasis. Systemic regulators of FGF23 production include 1,25(OH)₂D, phosphate, parathyroid hormone, insulin, iron, and inflammation. Therefore, the regulation of FGF23 in osteocytes is complex and multifactorial. Recent mouse studies have suggested that decreases in serum phosphate levels from youth to adulthood are caused by growth-related increases in FGF23 production by osteocytes, which are associated with the down-regulation of Phex and Dmp1.

Physiological regulation of phosphate by vitamin D, parathyroid hormone (PTH) and phosphate (Pi)

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4- and HPO42-). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

Phosphate (Pi)-regulated heterodimerization of the high-affinity sodium-dependent Pi transporters PiT1/Slc20a1 and PiT2/Slc20a2 underlies extracellular Pi sensing independently of Pi uptake

Extracellular phosphate (P_i) can act as a signaling molecule that directly alters gene expression and cellular physiology. The ability of cells or organisms to detect changes in extracellular P_i levels implies the existence of a P_i-sensing mechanism that signals to the body or individual cell. However, unlike in prokaryotes, yeasts, and plants, the molecular players involved in P_i sensing in mammals remain unknown. In this study, we investigated the involvement of the high-affinity, sodium-dependent P_i transporters PiT1 and PiT2 in mediating P_i signaling in skeletal cells. We found that deletion of PiT1 or PiT2 blunted the P_i-dependent ERK1/2-mediated phosphorylation and subsequent gene up-regulation of the mineralization inhibitors matrix Gla protein and osteopontin. This result suggested that both PiTs are necessary for P_i signaling. Moreover, the ERK1/2 phosphorylation could be rescued by overexpressing P_i transport-deficient PiT mutants. Using cross-linking and bioluminescence resonance energy transfer approaches, we found that PiT1 and PiT2 form high-abundance homodimers and P_i-regulated low-abundance heterodimers. Interestingly, in the absence of sodium-dependent P_i transport activity, the PiT1-PiT2 heterodimerization was still regulated by extracellular P_i levels. Of note, when two putative P_i-binding residues, Ser-128 (in PiT1) and Ser-113 (in PiT2), were substituted with alanine, the PiT1-PiT2 heterodimerization was no longer regulated by extracellular P_i These observations suggested that P_i binding rather than P_i uptake may be the key factor in mediating P_i signaling through the PiT proteins. Taken together, these results demonstrate that P_i-regulated PiT1-PiT2 heterodimerization mediates P_i sensing independently of P_i uptake.

Phosphate mass removal during hemodialysis: a comparison between eKT/V-matched conventional and extended dialysis

BACKGROUND AND OBJECTIVES

The control of hyperphosphatemia is an unmet need in dialysis care. Compared to conventional hemodialysis (cHD), extended hemodialysis (eHD) appears to more easily control blood phosphate levels in chronically dialyzed patients. Here, we sought to compare eKT/V-matched cHD and eHD procedures in order to quantify the contribution of dialysis prescription and time in the mass removal of phosphate.

METHODS

Eight stable hemodialysis patients with negligible residual renal function underwent cHD and eHD sessions adjusted to provide the same eKT/V(urea). Total dialysate, total and hourly partial dialysate and blood samples were collected for comparison of mass extraction of urea, creatinine, and phosphate.

RESULTS

Mean eKT/V(urea) was similar in eHD and cHD (1.30 vs. 1.28, p = nonsignificant). Likewise, mass removal of urea and creatinine during cHD and eHD were not significantly different. Conversely, phosphate mass removal was 40% higher with eHD as compared to cHD (1,219 ± 262 vs. 858 ± 186 mg, p = 0.015). Although hourly mass removal of phosphate was higher during cHD, the prolonged period of lesser but continuous removal was responsible for higher total phosphate elimination during eHD.

CONCLUSION

In dialysis sessions matched to provide a similar eKT/V(urea), removal of phosphate increases by 40% when time is extended from 4 to 8 h. Urea-based adequacy models cannot be used to predict the amount of phosphorus removal during hemodialysis.

Phosphate and the parathyroid

The phosphate (Pi) retention in patients with chronic kidney disease leads to secondary hyperparathyroidism (2HPT). 2HPT is the physiological response of the parathyroid not only to Pi retention but also to decreased synthesis of 1,25(OH)(2) vitamin D, and the attendant hypocalcemia. 2HPT is characterized by increased PTH synthesis, secretion, and parathyroid cell proliferation. Extracellular fluid (ECF) Ca(2+) is recognized by the parathyroid calcium receptor and a small decrease in the ECF Ca(2+) results in relaxation of the calcium receptor and allows the unrestrained secretion and synthesis of PTH and in the longer term, parathyroid cell proliferation. Both 1,25(OH)(2) vitamin D and fibroblast growth factor 23 inhibit PTH gene expression and secretion. Secondary hyperparathyroidism can initially be controlled by a single therapeutic intervention, such as a Pi-restricted diet, a calcimimetic, or an active vitamin D analog. In this review we discuss the mechanisms whereby Pi regulates the parathyroid. Pi has a direct effect on the parathyroid which requires intact parathyroid tissue architecture. The effect of Pi, as of Ca(2+), on PTH gene expression is post-transcriptional and involves the regulated interaction of parathyroid cytosolic proteins to a defined cis acting sequence in the PTH mRNA. Changes in serum Ca(2+) or Pi regulate the activity of trans acting interacting proteins in the parathyroid, which alters their binding to a defined 26 nucleotide cis acting instability sequence in the PTH mRNA 3'-untranslated region. The trans factors are either stabilizing or destabilizing factors and their regulated binding to the PTH cis acting element determines the PTH mRNA half-life. The responses of the parathyroid to changes in serum Pi are now being revealed but the sensing mechanisms remain a mystery.

Regulation of PTH synthesis and secretion relevant to the management of secondary hyperparathyroidism in chronic kidney disease

Regulation of PTH synthesis and secretion relevant to the management of secondary hyperparathyroidism in chronic kidney disease. Small decreases in serum Ca(++) and more prolonged increases in serum phosphate (P) stimulate the parathyroid (PT) to secrete parathyroid hormone (PTH), while 1,25(OH)(2)-vitamin D(3) decreases PTH synthesis and secretion. A prolonged decrease in serum Ca(++) and 1,25(OH)(2)D(3), or increase in serum P, such as in patients with chronic renal failure, leads to the appropriate secondary increase in serum PTH. This secondary hyperparathyroidism involves increases in PTH gene expression, synthesis, and secretion, and, if chronic, to proliferation of the parathyroid cells. A low serum Ca(++) leads to an increase in PTH secretion, PTH mRNA stability, and parathyroid cell proliferation. Pi also regulates the parathyroid in a similar manner. The effect of Ca(++) on the parathyroid is mediated by a membrane Ca(2+) receptor (CaR). 1,25(OH)(2)D(3) decreases PTH gene transcription. Ca(2+) and P regulate the PTH gene post-transcriptionally by regulating the binding of parathyroid cytosolic proteins, trans factors, to a defined cis sequence in the PTH mRNA 3'-untranslated region (UTR), thereby determining the stability of the transcript. The parathyroid trans factors and cis elements have been defined.