Phosphate homeostasis

Editor-In-Chief: Henry A. Hoff

Overview
In the extracellular region near the plasma membrane, portions of membrane-associated molecules wait to capture phosphate and transport it into the cell. The phosphate may occur as inorganic orthophosphate particles or be part of an organic molecule (organophosphate). Bringing phosphate in any form into the cell and when needed transporting phosphate out of the cell is a necessary activity of phosphate homeostasis for that cell.

Introduction
Due to its high reactivity, phosphorus is never found as a free element in nature. Phosphates are found pervasively in biology. Phosphate is a component of DNA and RNA and an essential element for all living cells. Phosphate metabolism is the complete set of phosphate chemical reactions that occur in living cells. Phosphorus (as phosphate usually) is often a limiting nutrient in many environments; i.e. the availability of phosphorus governs the rate of growth of many organisms. Living cells also use phosphate to transport cellular energy via adenosine triphosphate (ATP). Nearly every cellular process that uses energy obtains it in the form of ATP. Inside a cell, phosphate may be structural to a nucleic acid or phospholipid, form high-energy ester bonds (e.g., in adenosine triphosphate), or participate in signaling. Outside the cell, phosphate may be dissolved in extracellular fluid (ECF) or form structures such as bone and teeth.

In medicine, low phosphate syndromes are caused by malnutrition, by failure to absorb phosphate, and by metabolic syndromes which draw phosphate from the blood or pass too much of it into the urine. All are characterized by hypophosphatemia (see article for medical details). Symptoms of low phosphate include muscle and neurological dysfunction, and disruption of muscle and blood cells due to lack of ATP.

In ecosystems an excess of phosphorus can be problematic, especially in aquatic systems, see eutrophication and algal blooms.

Bringing phosphate in any form into the cell and when needed transporting phosphate out of the cell is a necessary activity of phosphate homeostasis for that cell. An organism that can regulate its internal environment so as to maintain equilibrium has the property of homeostasis. ATP is important for phosphorylation, a key regulatory event in cells.

Homeostasis
Homeostasis is a relatively stable state of equilibrium or a tendency toward such a state of an open or closed system, especially a living organism. An organism that can regulate its internal environment so as to maintain equilibrium has the property of homeostasis.

Phosphate
A phosphate can occur as a salt of phosphoric acid or an ester of phosphoric acid (an organophosphate). Phosphates are found pervasively in biology. Phosphorus (as phosphate usually) is an essential macromineral for plants, which is studied extensively in soil conservation in order to understand plant uptake from soil systems. In ecological terms, phosphorus is often a limiting nutrient in many environments; i.e. the availability of phosphorus governs the rate of growth of many organisms. In ecosystems an excess of phosphorus can be problematic, especially in aquatic systems, see eutrophication and algal blooms.

Inside a cell, phosphate may be structural to a nucleic acid or phospholipid, form high-energy ester bonds (e.g., in adenosine triphosphate), or participate in signaling.

Outside the cell, phosphate may be dissolved in extracellular fluid (ECF) or form structures such as bone and teeth.

Phosphate metabolism
With respect to the complete set of phosphate chemical reactions that occur in living cells, there are approximately 1385 such reactions. Indeed, concerning polyphosphate there are approximately 10 such reactions.

In pyrimidine metabolism, EC 3.6.1.2 (trimetaphosphatase) can convert trimetaphosphate into triphosphate per the reaction:

trimetaphosphate + H2O <=> triphosphate (ultimately leading to the production of 2'-Deoxy-5-hydroxy-methylcytidine-5'-triphosphate).

Trimetaphosphatase activity has been found to be localized in the mouse testis during acrosomal formation. Osteoclasts secrete trimetaphosphatase towards the resorbing bone surfaces.

Phosphorylation
The addition of a phosphate (PO4) group to another molecule, including any protein, is phosphorylation. Many enzymes and receptors are switched "on" or "off" by phosphorylation. Phosphorylation is catalyzed by specific protein kinases. Dephosphorylation is catalyzed by phosphatases. Phosphorylation of any amino acid having a free hydroxyl group on a given protein can change the function, association, or localization of that protein.

Oxidative phosphorylation is the process of oxidizing nutrients to produce adenosine triphosphate (ATP). Substrate-level phosphorylation forms ATP by the direct transfer of a phosphate group to adenosine diphosphate (ADP) from a reactive intermediate. Photophosphorylation uses solar energy to synthesize ATP.

Phosphorylation of sugars allows cells to accumulate sugars because the phosphate group prevents the molecules from diffusing back across their transporter.

Phosphate bioenergetics
Bioenergetics concerns energy flow through living systems. This area of biological research includes the study of cellular processes such as cellular respiration that can lead to production and utilization of energy in forms such as ATP molecules. Living cells use phosphate to transport cellular energy via nucleotides such as adenosine triphosphate (ATP). Nearly every cellular process that uses energy obtains it in the form of ATP.

Protons in aqueous solution function in the production of ATP in cell organelles such as mitochondria. Other cellular sources of ATP such as glycolysis for direct coupling of enzyme activity to ATP production are not the major source of useful chemical energy in most cells. Chemiosmotic coupling is the major energy producing process in most cells, being utilized in chloroplasts and many single celled organisms in addition to mitochondria.

Catalytic phosphate
A phosphate which affects the catalytic activity of an enzyme is a catalytic phosphate. Occasionally, an enzyme contains a structural phosphate and a catalytic phosphate.

The γ phosphate of ATP is the catalytic phosphate.

Catalytic phosphates are acid-labile and base-stable. Structural phosphates are acid-stable, base-labile.

Structural phosphate
Inside a cell, phosphate may be structural to a nucleic acid such as DNA and RNA or phospholipid. Outside the cell, phosphate may be dissolved in extracellular fluid (ECF) or form structures such as bone and teeth. Bringing phosphate in any form into the cell from a phosphate containing structure or for such a structure and when needed transporting phosphate out of the cell perhaps to a structure is a necessary activity of phosphate homeostasis for that cell.

Signaling
Lipid signaling involves G-protein coupled receptors to which lysophosphatidic acid (LPA) binds, through which sphingosine-1-phosphate (S1P) acts, and platelet-activating factor (PAF) signals. The key event of diacylglycerol (DAG) signaling is the hydrolysis of phosphatidylinositol (4,5)-bisphosphate (PIP2) to DAG and inositol triphosphate (IP3). IP3 is soluble and diffuses freely into the cytoplasm. It can be recognised by the inositol triphosphate receptor (IP3R). IP3 contributes to the activation of protein kinase C (PKC).

Extracellular fluid (ECF)
Extracellular fluid usually denotes all body fluid outside of cells. It is frequently contained within organs. The skin, for example, is an organ often referred to as the largest organ of the human body as it covers the body, appearing to have the largest surface area of all the organs. But it is a major container for ECF and other organs. ECF includes interstitial fluid (ISF) and transcellular fluid (TCF).

Cardiovascular systems are usually closed, meaning that the blood never leaves the network of blood vessels. In contrast, oxygen and nutrients diffuse across the blood vessel layers and enter interstitial fluid (ISF), which carries oxygen and nutrients to cells, and carbon dioxide and wastes in the opposite direction. Also, the digestive system, which contains TCF, works with the cardiovascular system to provide the nutrients the system needs to keep a heart, when present, pumping.

Phosphatidate
Lysophosphatidic acid (LPA) is an intermediate in the synthesis of phosphatidic acid (PA). ENPP2 functions as a phospholipase, which catalyzes the transformation of lysophosphatidylcholine into LPA in ECF. LPA has been detected in plasma, ascitic fluid, follicular fluid, and aqueous humor.

ECF Orthophosphate (Pi)
The level of inorganic orthophosphate (Pi) is tightly balanced inside the cell and in the whole vertebrate organism. The concentration of free Pi is balanced in the millimolar range in ICF and ECF. Bacteria, yeast, plants, and vertebrates have developed their own strategies to control Pi homeostasis. The daily need for Pi is covered by intestinal absorption from the diet, the major storage compartment is bone, and the metabolic and structural intracellular need is met in part by phosphate cotransporters. The ECF concentration of Pi is controlled by tightly regulating renal excretion. Principal properties of mammalian renal Pi reabsorption are pH-dependency, regulation by parathyroid hormone (PTH), and Pi availability. These properties are expressed by SLC34A1-3 (sodium-dependent phosphate transporter 2, NPT2), whereas NPT3 (SLC17A2) mediates Na+-dependent Pi transport. NPT1 (SLC17A1) does not have a prominent role in regulating body Pi homeostasis. Insulin stimulates NPT1 expression and Na+/Pi uptake which is reversed by glucagon. NPT3 may have a housekeeping role at the cellular level per its broad range of expression and its ability to adapt to changes in extracellular Pi concentration. The expression of NPT3 is compatible with the presence of other NPT. Members of the NPT1 family are linked with insulin-stimulated glucose metabolism. NPT2 in the kidney and small intestine are responsible for intracellular Pi accumulation in order to establish a transepithelial flux of Pi. NPT4 (SLC17A3) is expressed in the small intestine, kidney, liver, and testis.

ECF Pyrophosphate (PPi)
PPi occurs in synovial fluid, plasma, and urine at levels sufficient to block calcification and may be a natural inhibitor of hydroxyapatite formation in ECF.

Blood
Blood is a specialized body ECF contained within the cardiovascular system that is composed of blood cells suspended in plasma. Its phosphate content is in many forms. Per the list of human blood components: adenosine triphosphate (ATP) concentration in whole blood is 3.1-5.7 x 10-4 g/cm3, which is the equivalent of 5-10 x 10-5 g/cm3 of phosphorus; diphosphoglycerate is 8-16 x 10-5 g/cm3; cGMP is 0.6-4.4 x 10-9 g/cm3; hexosephosphate P is 1.4-5 x 10-5 g/cm3 (0-2 x 10-6 g/cm3 in plasma or serum); DNA is 0-1.6 x 10-5 g/cm3 in plasma or serum; phospholipid is 2.25-2.85 x 10-3 g/cm3 (5-12 x 10-5 g/cm3 in plasma or serum); pyrimidine nucleotide is 2.6-4.6 x 10-5 g/cm3 (2-12 x 10-7 g/cm3 in plasma or serum); and RNA is 5-8 x 10-4 g/cm3 (4-6 x 10-5 g/cm3 in plasma or serum). Pi occurs in blood, blood plasma and blood serum. The total phosphorus content is 2-3.9 x 10-5 g/cm3 as inorganic phosphate and 3.5-4.3 x 10-4 g/cm3 total (2.3-4.5 x 10-5 g/cm3 and 1-1.5 x 10-4 g/cm3 in plasma or serum, respectively).

Blood Orthophosphate
A slight increase of pH in blood plasma above 7.4 causes precipitation of calcium phosphate and resulting turbidity, whereas, in the case of blood serum (plasma without clotting proteins) of the same inorganic composition, the pH may vary fairly widely without precipitation occurring.

It is the proteins that tend to keep calcium salts in solution or at least in suspension. Blood serum is supersaturated with tricalcium phosphate from about pH 6.8 up to about pH 9.25, with a maximum dissolution at pH 7.3. The stability of calcium phosphate in suspension may be improved by reduction of phosphate ion in proportion to calcium in the mixture. With increasing alkalinity above pH 6.3 monocalcium phosphate is converted into dicalcium phosphate. At about pH 6.7 tricalcium phosphate begins to form yet remains in suspension in the presence of proteins.

Hydroxide ions and protein exert antagonistic effects on the suspension of tricalcium phosphate so that with increasing alkalinity the size of the suspension depends on the protein concentration present. Increasing serum concentration decreases turbidity. Protein exerts an inhibitory effect on the precipitation of calcium phosphate both by holding it in solution against other physical factors and by supporting it in suspension.

Blood Pyrophosphate
Blood plasma levels of PPi in normal human children can range from approximately 300-700 pmol/µg protein.

Phosphate in
Bringing phosphate in any form into a cell possibly from a phosphate containing structure, for such a structure, or whatever the need, may require phagocytosis, endocytosis, or a phosphate-importing protein or symporter.

EC 3.6.3.20 is a phosphate-importing enzyme that catalyzes the chemical reaction

ATP + H2O + glycerol-3-phosphateout <=> ADP + phosphate + glycerol-3-phosphatein.

EC 3.6.3.27 is a phosphate-importing enzyme that catalyzes the chemical reaction

ATP + H2O + phosphateout <=> ADP + phosphate + phosphatein.

Orthophosphate in
Growth hormone, at least in part mediated by insulin-like growth factor I (IGF-I), stimulates Pi cotransport. Thyroid hormone stimulates Pi absorption via a specific increase in Pi cotransport. Stanniocalcin 1 (STC1) stimulates membrane Pi cotransport.

Phosphaturic factors reduce the expression of Pi transporters or cotransporters in the cell membrane.

Insulin enhances Pi absorption by stimulation of Pi cotransport and prevents the phosphaturic action of parathyroid hormone (PTH). Calcitonin reduces membrane Pi cotransport in a PTH- and cAMP-independent manner.

Some factors that increase Pi absorption increase the number of the transporters or cotransporters in the cell membrane. Knowledge of the mechanisms that control transporter expression and membrane retrieval of the transporters is essential to understanding how Pi homeostasis is achieved.

Physiological regulation of Pi absorption involves, as far as it has been studied at the molecular level, an altered expression of cotransporter protein that is related in most cases to changes in the maximum velocity (Vmax) of Pi cotransport activity.

ISF Pi absorption
PiT1 is likely to be a major carrier of phosphate from ISF into most cell types. Vmax can go approximately from 39 to 121, with ISF phosphate to phosphate-free ISF. Mammaliam cell internal phosphate levels (75 milliequivalents per liter) are maintained at higher levels than ISF (4 milliequivalents per liter) in opposition to an electrochemical gradient that favors phosphateout.

Low ECF Pi levels can result in upregulated PiT1 and PiT2 expression in mammalian cells. PiT1 and PiT2 exhibit positve cooperative Pi uptake. PiT2 supports Na+-independent Pi uptake. Ca2+ or Ca2+ and Mg2+ increase PiT1- and PiT2-mediated NaPi import. Vmax for PiT1 and PiT2 are approximately 453 and 450 pmol·cell-1·h-1, respectively. The average Pi uptake for PiT2 and PiT1 at pH 7.5 is approximately 118 and 115 pmol·cell-1·h-1, respectively. Pi transport has similar average Pi uptake at different pH values: 5.5 to 8.5. The Na+-independent Pi uptake of PiT2 is in acidic ECF conditions. With physiological Na+ concentration in the human body in the range of 130–145 mM, there is always plenty to sustain Pi uptake via PiT1 and PiT2.

Small intestine Pi absorption
A low dietary inorganic phosphate (Pi) intake can lead to an almost 100% absorption of filtered Pi, whereas a high dietary Pi intake leads to a decreased Pi absorption. These changes can occur independent of changes in the ECF concentration of different phosphaturic hormones.

In the upper small intestine Vitamin D3 stimulates Pi cotransport. Matrix extracellular phosphoglycoprotein (MEPE), a phosphatonin, promotes phosphaturia. Short-term infusion of MEPE inhibits phosphate absorption in the jejunum but not the duodenum. The short-term inhibitory effect of MEPE on renal and intestinal phosphate handling occurs without any changes in circulating levels of parathyroid hormone (PTH), 1,25-dihydroxyvitamin D3, or fibroblast growth factor 23 (FGF23). MEPE may be involved in phosphate homeostasis, acting in both the kidney and the gastrointestinal tract.

There is evidence for a hormone/enzyme/extracellular matrix protein cascade involving fibroblastic growth factor 23 (FGF23), the X-linked phosphate regulating endopeptidase homolog (PHEX), and a matrix extracellular phosphoglycoprotein (MEPE) that regulates systemic phosphate homeostasis and mineralization.

Renal Pi reabsorption
NPT2 dominates in renal Pi reabsorption, which is exclusive under non-pathological conditions in the mammalian kidney. Na+ interacts in a cooperative way with NPT2 with a stoichiometry of 3Na+ to 1Pi. Protons (H+) decrease the affinity of NPT2 for Na+.

There is a link between the action of PTH, Pi availability, and renal cell membrane integration/retrieval of NPT2. Changes in dietary Pi content have a major regulatory effect on Pi reabsorption. Dietary Pi restriction is associated with an adaptive increase of the overall capacity for Pi uptake mediated by an increase of Vmax of sodium gradient-dependent phosphate transport. In response to chronic (8 day) dietary Pi restriction, the adaptive increase in sodium-Pi cotransport activity is associated with parallel increases in NPT2 and its mRNA abundances. However, in response to acute (2 h) dietary Pi restriction, the rapid adaptive increase in sodium-Pi cotransport activity is associated with parallel increases in NPT2, but without change in its mRNA abundance.

The role of intracellular NPT2 in the rapid adaptation to dietary Pi is correlated with the localization of the Golgi membrane protein 58 kDa and the lysosomal membrane glycoprotein Igp120, as follows : after chronic adaptation to a high Pi diet, NPT2 is in the subcell membrane portion and perinuclear region where in part it spatially coincides with the Golgi-like compartment but not with the lysosomal compartment. Acute adaptation from a high Pi diet to low Pi leads to depletion of intracellular NPT2, with NPT2 only in the Golgi-like compartment. After chronic adaptation to a low Pi diet, intracellular NPT2 is predominantly localized in the region of the Golgi-like compartment and additional sub cell membrane and central cell portions, with minimal localization to Igp120. Acute adaptation from a low Pi to a high Pi diet leads to increased abundance of intracellular NPT2 in the sub cell membrane and central portions, with considerable spatial localization to the lysosomal compartment.

The chronic adaptive response to a low Pi diet is characterized by an increase in NPT2 mRNA abundance.

Role of microtubules
In the presumable rapid translocation of NPT2 in response to acute administratiion of a low Pi diet, microtubules (tubulin) are involved with upregulation of NPT2 activity and translocation from intracellular compartments in the vicinity of the nucleus to the cell membrane, but uninvolved in NPT2 expression. Regarding response to an acute administration of a high Pi diet after chronic administration of a low Pi, microtubules are not involved in rapid downregulation of NPT2 activity or NPT2 abundance.

The rapid adaptive increase in sub cell membrane NPT2 with chronic feeding of a high Pi diet, after an acute administration of a low Pi diet, is probably mediated by translocation of presynthesized NPT2 to this portion by microtubule-dependent mechanisms.

The targeting of NPT2 to the cell membrane seems to be dependent on the microtubule network.

Role of microfilaments
Microfilaments (actin cytoskeleton) may play a central role in the rapid downregulation of Na+-Pi cotransport. The dependence of endocytosis on intact microfilaments in polarized epithelial cells has been demonstrated.

Organophosphates in
Phosphatidic acid phosphatases (PAPs) transport phosphatidic acid (PA), lysophosphatidic acid (LPA), ceramide 1-phosphate (C1P), and sphingosine 1-phosphate (S1P) from ECF through the plasma membrane at different Vmax. Once inside the cell, these PAPs (PPAP2A, PPAP2B, and PPAP2C) hydrolyze each phosphate per EC 3.1.3.4:

a 3-sn-phosphatidate + H2O <=> a 1,2-diacyl-sn-glycerol + Pi.

PPAP2A displays comparable Vmax values for all four substrates, with highest activity for LPA and PA. PPAP2B shows a similarly higher relative Vmax activity with LPA, while PPAP2C displays significantly higher activity with S1P. For some cells PPAP2A may be an ectoenzyme that dephosphorylates PA to form diacylglycerol (DG) prior to DG transport into the cell.

Phosphate exchange
Glycerol phosphate transporter (GlpT) transports glycerol-3-phosphate (G3P) into the cytoplasm and Pi into the periplasm. While GlpT transports between cellular compartments through internal membranes, it can also transport across the cell membrane. GlpT functions for G3P uptake and is driven by a Pi gradient. Periplasmic (or ISF) release of Pi allows its replacement in the substrate-binding site by G3P, which has a higher affinity; on the cytoplasmic side of the membrane, Pi replaces G3P because of its higher cytosolic concentration.

Orthophosphate out
Under normal or steady-state physiological conditions, urinary Pi excretion corresponds roughly to phosphate intake in the gastrointestinal tract, mainly via the upper small intestine. To fulfill Pi homeostasis, i.e., keeping extracellular Pi concentration within a narrow range, urinary Pi excretion is under strong physiological control. In contrast to intestinal Pi absorption, which adjusts rather slowly, renal Pi excretion can adjust very rapidly to altered physiological conditions. Specifically, membrane Pi cotransporter protein content, and thus Pi absorption, responds within hours to alterations in dietary Pi intake.

Parathyroid hormone (PTH) induces phosphaturia by inhibiting Pi cotransport activity. Glucocorticoids increase phosphate excretion by an inhibition of membrane Pi cotransport, independent of an increase in PTH.

Stanniocalcin 2 (STC2) may suppress Pi cotransport.

Pyrophosphate out
Cells may channel intracellular PPi into ECF. ANK is a nonenzymatic plasma-membrane PPi channel that supports extracellular PPi levels. Defective function of the membrane PPi channel ANK is associated with low extracellular PPi and elevated intracellular PPi. Ectonucleotide pyrophosphatase/phosphodiesterase (ENPP) may function to raise extracellular PPi.

Phosphate reserves
A well-fed adult in the industrialized world consumes and excretes about 1-3 g of phosphorus per day in the form of phosphate (2-6 x 1022 molecules). Per the elemental composition of the "standard man" of 70 kg, phosphorus is 780 g or 1.1% (as 1.52 x 1025 molecules of phosphate). Of this 1.4 g/kg (98 g, 1.9 x 1024 molecules of phosphate) are present in soft tissue with the remainder (1.33 x 1025 molecules of phosphate) in mineralized tissue such as bone and teeth. Only about 0.1% of body phosphate (about 2 x 1022 molecules) circulates in the blood, but this amount reflects the amount of phosphate available to soft tissue cells. Blood plasma contains orthophosphate (as HPO42-) and H2PO4- in the ratio of about 4:1.

The total quantity of ATP in the human body is about 0.1 mole (about 6 x 1022 molecules). This ATP is constantly being broken down into ADP, and then converted back into ATP. At any given time, the total amount of ATP + ADP remains fairly constant. The energy used by human cells requires the hydrolysis of 100 to 150 moles (6 to 9 x 1025 molecules) of ATP daily which is around 50 to 75 kg. Typically, a human will use up their body weight of ATP over the course of the day. This means that each ATP molecule is recycled 1000 to 1500 times daily, or about once every minute.

PPi-generating nucleoside triphosphate pyrophosphohydrolase (EC 3.1.4.1) activities of a group of ecto-enzymes in the phosphodiesterase nucleotide pyrophosphatase (PDNP) family have been recognized to contribute to the regulation of intracellular and extracellular PPi levels in several tissues.

Intracellular phosphate
With the number of cells in the human body of 10-100 trillion or 1013 to 1014, there are approximately 1.9-19.0 x 1010 atoms of phosphorus (1.9 to 19 x 1010 molecules of phosphate) per cell. A typical cell volume is 5 x 10-16 m3. If a typical cell was totally liquid water, there would be about 1.7 x 1013 molecules of water present. Then the phosphate concentration would be on the order of 10-3. However, a cell is about twice as dense as liquid water, then perhaps only 50% water yielding 5 x 10-3 for phosphate concentration. As computer simulation has shown the probability of finding a target by diffusion decreases rapidly with distance and becomes <1% when the starting distance exceeds the target's 10-fold radius, which by assumption of a simple spherical volume would put the target's concentration on the order of 10-3.

Although ectonucleotide pyrophosphatase/phosphodiesterase 3 (ENPP3) regulates intracellular PPi concentrations it does not seem to significantly regulate extracellular PPi.

Extracellular phosphate
PPi inhibits hydroxyapatite deposition in bone and cartilage. Many studies have shown that PPi is a potent inhibitor of calcification, bone mineralization, and bone resorption. Human defects in alkaline phosphatase, an enzyme that degrades PPi, lead to an increase in PPi levels and a severe block in skeletal mineralization. Genetic defects in a cell surface ectoenzyme that normally generates extracellular PPi from nucleotide triphosphate cause ectopic mineralization of joints and ligaments and may be associated with spinal ligament ossification in humans.

Bone
During bone resorption high levels of phosphate are released into the ECF as osteoclasts tunnel into mineralized bone, breaking it down and releasing phosphate, that results in a transfer of phosphate from bone fluid to the blood. During childhood, bone formation exceeds resorption, but as the aging process occurs, resorption exceeds formation.

Phosphate regulation
An organism that can regulate its internal environment so as to maintain equilibrium has the property of homeostasis. Those proteins which stimulate phosphate in or phosphate out regulate phosphate concentration. Specific protein kinases catalyze phosphorylation whereas phosphatases dephosphorylate. These kinases and phosphatases regulate phosphate form and phosphate availability.

ATP is important for phosphorylation, a key regulatory event in cells.

Fibroblast growth factor 23 (FGF23) is essential for maintaining phosphate homeostasis. Klotho is integral to the plasma membrane and essential for endogenous FGF23 function. FGF23 regulates NPT2a independent of parathyroid hormone (PTH) and serum 1,25-dihydroxyvitamin D level by controlling renal expressions of key enzymes of vitamin D metabolism. FGF23 inhibits renal tubular phosphate transport. The secretion of FGF23 requires O-glycosylation, which is selectively directed by GALNT3, to block processing of FGF23.

Both NPT2a and NPT2c are regulated in a similar fashion by parathyroid hormone (PTH), FGF23, and dietary phosphate. Phosphate deprivation increases NPT2a and NPT2c expression. The relative expression of NPT2a is an order of magnitude higher than that of NPT2c in kidneys of organisms maintained on a normal phosphate diet. Normal animals exhibit a more robust increase in NPT2c than in NPT2a in response to phosphate deprivation. NPT2c is regulated by FGF23 and probably downregulated by PTH. SLC34A3 (NPT2c) contributes to the maintenance of inorganic phosphate concentration at the kidney.

PHEX may regulate FGF23 expression as part of a potential hormonal axis between bone and kidney that controls systemic phosphate homeostasis and mineralization. PHEX is highly expressed in cartilage, bone, and teeth, where it is present in osteoblasts, osteocytes, and odontoblasts, especially at the transition between the maturation of pre-osteoblasts to mature osteoblasts when PHEX expression is normally up-regulated.

Phosphate transistasis
The power or tendency of a living thing to keep changing its phosphate needs according to the transformation of circumstances is phosphate transistasis; i.e., to reform its functions to maintain a meaningful condition, by means of multiple dynamic equilibrium adjustments, controlled by interrelated phosphate regulation mechanisms. Homeostasis and transistasis are needed to maintain stability and to survive. An organism has to endure, adapt, and evolve to modifications of the environment. It does this by testing which way its variables should be adjusted, by being ultraflexible.

Negative feedback is a reaction in which the system responds in such a way as to reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of homeostasis.

In positive feedback, the response is to amplify the change in the variable, allowing the maintenance of transistasis.

Cell growth and phosphate limitation
Mathematical modeling of the rate kinetics of growth and acid phosphatase formation under varying degrees of phosphate limitation is concerned with (a) the time lag for exponential growth, (b) the biphasic growth on a substrate (glucose) and its product, (c) sustained growth on conservative phosphate, and (d) the derepression of acid phosphatase. The numerical calculations using appropriate parametric constants describe the variation in the cell mass, glucose, product, and inorganic phosphate concentrations, and the enzyme activity of acid phosphatase during aerobic growth under different conditions of phosphate starvation. Study by simulation revealed that the optimum initial phosphate concentration in the medium giving a high productivity of acid phosphate is 2.0 mg phosphorus/g glucose liter.

Regulation of phosphate transporters
The regulation of phosphate transporters by nutrient-responsive signaling pathways allows cells to tailor phosphate uptake to environmental conditions. Cells starved for phosphate activate positive and negative feedback loops in an interplay that leads to bistability in phosphate transporter usage. Positive feedback can be generated by a negative regulator of low-affinity phosphate, where individual cells express predominantly either low- or high-affinity transporters and both can yield similar phosphate uptake capacity.

Coagulation cascade
Up-regulation of tissue factor (TF) is induced by the interplay between thrombin and sphingosine 1-phosphate (S1P). Expression of TF after thrombin and S1P stimulation activates the coagulation cascade leading to further generation of thrombin and S1P; hence, a positive feedback loop.

Cell proliferation
There is a positive feedback loop in which Ras signaling promotes CD44v6 splicing, and CD44v6 then sustains late Ras signaling, which is important for cell cycle progression. The production of CD44 variants is stimulated by Ras/MAPK signaling (Ras-Raf-MEK-ERK) and regulated by splicing factors which promote the inclusion of CD44 variable exons, controlled by Ras/MAPK signaling, at least in part through modification of splicing factors at the level of phosphorylation.

Growth and regrowth of phosphate reserves
From the phosphate point of view inside a cell, changes in the allocation of phosphate or phosphate needs that come from a force that can transform circumstances becomes a phosphate transistasis. The interplay of feedbacks then determines either a new equilibrium state; hence, a new homeostasis, or returns the circumstances back to the original state.

One such force internal to the cell is the autonomously replicating sequence (ARS). Eukaryotes often have multiple origins of replication on each linear chromosome, with up to 100,000 present in a single human cell. This small DNA sequence can initiate replication of itself unless kept in check by feedbacks.

In addition, there are many genomes that create phosphate reserves. Or, the particular genome needs to alter its phosphate budget, e.g., to create more structural phosphate, such as by mitosis, or more catalytic phosphate.

When any of these demands are placed on the current state of phosphate homeostasis, and the demand is not readily negated by negative feedback, a dynamic state of change occurs. A zygote, e.g., happily feeding and replicating itself in homeostatic bliss may eventually reach a polycellular situation when a phosphate reserve is needed. Endochondral ossification and intramembranous ossification are two processes resulting in the formation of normal, healthy bone tissue.

Several hypotheses have been proposed for how bone evolved as a structural element in vertebrates. One hypothesis is that bone developed from tissues that evolved to store minerals, such as hydroxyapatite. Up to fifty percent of bone is made up of a modified form of the inorganic mineral hydroxylapatite.

Or, specifically, calcium-based minerals were stored in cartilage and bone was an exaptation development from this calcified cartilage. However, other possibilities include bony tissue evolving as an osmotic barrier, or as a protective structure.

Then again, carbonated-calcium deficient hydroxyapatite is the main mineral of which bone, dental enamel and dentin are comprised.

Phosphate budget
The amount of phosphate needed or available for a purpose, including estimates of phosphate in and phosphate out, and the phosphate form, determine the phosphate budget for a cell or an entire organism.

A dynamic phosphate budget can be modeled by determining the relations between the external phosphate loading, recycling through the formation of structural phosphates, and the resulting productivity, which are complicated by feed-back mechanisms, periodic variations and trends. In a dynamic phosphate budget model the variables include inorganic and organic particulate phosphate and dissolved phosphate, in both structural phosphate and nucleosol. Structural phosphate may be aerobic or anaerobic, depending on localization, temperature (or pH) and composition. The major describeable processes are primary production, mineralization, precipitation, adsorption and diffusion. The precipitate dilution rate, the extent of anaerobic conditions and the number and character of adsorption sites are important controlling factors.

A simplified phosphate-cycling model for a periodic phosphate budget is used to examine how changes in circulation might influence phosphate export production via the supply of phosphate to the phosphate utilization zone and the preservation of organic phosphate which is controlled by the oxygen content. The model uses coupled equations: phosphate utilization, phosphate remineralization, and the oxygen concentration. The initial conditions for all the integrations of the model consist of uniform values for the concentration of each component.

For a "standard man" of 70 kg the available phosphate is ~1.52 x 1025 molecules of phosphate in some form. The available phosphate of an adult human female may differ from the "standard man".

Phosphate recycling
When a mRNA is transcribed for a particular protein ~80% of the structural phosphate is internal to the introns. As these are composed of a line of nucleotide monophosphates (NMP), once finished and spliced out of the transcript, they can be taken apart by a variety of enzymes EC 2.7.7.6, 2.7.7.8, 2.7.7.19, and 2.7.7.48. The newly formed nucleotide diphosphates and triphosphates can then be reused to make additional RNA or DNA.

Phosphate medicine
Low phosphate syndromes are caused by malnutrition, by failure to absorb phosphate, and by metabolic syndromes which draw phosphate from the blood or pass too much of it into the urine. All are characterized by hypophosphatemia (see article for medical details). Symptoms of low phosphate include muscle and neurological dysfunction, and disruption of muscle and blood cells due to lack of ATP.

Hyperphosphoric acid can be added to blood to release CO2.

Hyperphosphates (salts or esters) or oleophosphates can result from the use of hyperphosphoric acid. Hyperphosphate, when used to induce hyperphosphatemia, may directly promote calcium deposition and osteocalcin expression.

Two rare autosomal recessive metabolic disorders characterized by hyperphosphatemia: hyperphosphatemic familial tumoral calcinosis (HFTC) and hyperphosphatemia-hyperostosis syndrome (HHS) appear to be caused by mutations in GALNT3 which in turn affects FGF23.

Acknowledgements
The content on this page was first contributed by: Henry A. Hoff.

Initial content for this page in some instances came from Wikipedia.