Agonist-stimulated inositol phosphate metabolism in avian erythrocytes

Effect of phytase supplementation on plasma and organ myo-inositol content and erythrocyte inositol phosphates as pertaining to breast meat quality issues in chickens

‘Woody breast’ (WB) and ‘white striping’ in broiler meat is a global problem. With unknown etiology, WB negatively impacts bird health, welfare and is a significant economic burden to the poultry industry. New evidence has shown that WB is associated with dysregulation in systemic and breast muscle-oxygen homeostasis, resulting in hypoxia and anaemia. However, it has been observed that phytase (Quantum Blue (QB) a modified, E. coli-derived 6-phytase) super dosing can reverse dysregulation of muscle-oxygen homeostasis and reduces WB severity by ~5%. The objective of this study was to assess whether levels of Ins(1,3,4,5,6)P5, the main allosteric regulator of haemoglobin, are influenced by changes in plasma myo-inositol arising from super dosing with phytase. To enable this, methods suitable for measurement of myo-inositol in tissues and inositol phosphates in blood were developed. Data were collected from independent trials, including male Ross 308 broilers fed low and adequate calcium/available phosphate (Ca/AvP) diets supplemented with QB at 1,500 phytase units (FTU)/kg, which simultaneously decreased gizzard InsP6 (P<0.001) and increased gizzard myo-inositol (P<0.001). Similarly, male Cobb 500 broiler chicks fed a negative control (NC) diet deficient in AvP, Ca and sodium or diet supplemented with the QB phytase at 500, 1000 or 2,000 FTU/kg increased plasma (P<0.001) and liver (P=0.007) myo-inositol of 18d-old birds at 2,000 FTU/kg. Finally, QB supplementation of Cobb 500 breeder flock diet at 1,250 FTU/kg increased blood myo-inositol (P<0.001) and erythrocyte Ins(1,3,4,5,6)P5 (P=0.011) of their 1d-old hatchlings. These data confirmed the ability of phytase to modulate inositol phosphate pathways by provision of metabolic precursors of important signalling molecules. The ameliorations of WB afforded by super doses of phytase may include modulation of hypoxia pathways that also involve inositol signalling molecules. Elevations of erythrocyte Ins(1,3,4,5,6)P5 by phytase supplementation may enhance systemic oxygen carrying capacity, an important factor in the amelioration of WB and WS myopathy.

HPLC separation of inositol polyphosphates

High performance liquid chromatography (HPLC) is an essential analytical tool in the study of the large number of inositol phosphate isomers. This chapter focuses on the separation of inositol polyphosphates from [(3)H]myo-inositol labeled tissues and cells. We review the different HPLC columns that have been used to separate inositol phosphates and their advantages and disadvantages. We describe important elements of sample preparation for effective separations and give examples of how changing factors, such as pH, can considerably improve the resolving ability of the HPLC chromatogram.

The synthesis of inositol hexakisphosphate. Characterization of human inositol 1,3,4,5,6-pentakisphosphate 2-kinase

The enzyme(s) responsible for the production of inositol hexakisphosphate (InsP(6)) in vertebrate cells are unknown. In fungal cells, a 2-kinase designated Ipk1 is responsible for synthesis of InsP(6) by phosphorylation of inositol 1,3,4,5,6-pentakisphosphate (InsP(5)). Based on limited conserved sequence motifs among five Ipk1 proteins from different fungal species, we have identified a human genomic DNA sequence on chromosome 9 that encodes human inositol 1,3,4,5,6-pentakisphosphate 2-kinase (InsP(5) 2-kinase). Recombinant human enzyme was produced in Sf21 cells, purified, and shown to catalyze the synthesis of InsP(6) or phytic acid in vitro. The recombinant protein converted 31 nmol of InsP(5) to InsP(6)/min/mg of protein (V(max)). The Michaelis-Menten constant for InsP(5) was 0.4 microM and for ATP was 21 microM. Saccharomyces cerevisiae lacking IPK1 do not produce InsP(6) and show lethality in combination with a gle1 mutant allele. Here we show that expression of the human InsP(5) 2-kinase in a yeast ipk1 null strain restored the synthesis of InsP(6) and rescued the gle1-2 ipk1-4 lethal phenotype. Northern analysis on human tissues showed expression of the human InsP(5) 2-kinase mRNA predominantly in brain, heart, placenta, and testis. The isolation of the gene responsible for InsP(6) synthesis in mammalian cells will allow for further studies of the InsP(6) signaling functions.

Inositol 1,3,4-trisphosphate acts in vivo as a specific regulator of cellular signaling by inositol 3,4,5,6-tetrakisphosphate

Ca2+-activated Cl- channels are inhibited by inositol 3,4,5, 6-tetrakisphosphate (Ins(3,4,5,6)P4) (Xie, W., Kaetzel, M. A., Bruzik, K. S., Dedman, J. R., Shears, S. B., and Nelson, D. J. (1996) J. Biol. Chem. 271, 14092-14097), a novel second messenger that is formed after stimulus-dependent activation of phospholipase C (PLC). In this study, we show that inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) is the specific signal that ties increased cellular levels of Ins(3,4,5,6)P4 to changes in PLC activity. We first demonstrated that Ins(1,3,4)P3 inhibited Ins(3,4,5,6)P4 1-kinase activity that was either (i) in lysates of AR4-2J pancreatoma cells or (ii) purified 22,500-fold (yield = 13%) from bovine aorta. Next, we incubated [3H]inositol-labeled AR4-2J cells with cell permeant and non-radiolabeled 2,5,6-tri-O-butyryl-myo-inositol 1,3, 4-trisphosphate-hexakis(acetoxymethyl) ester. This treatment increased cellular levels of Ins(1,3,4)P3 2.7-fold, while [3H]Ins(3, 4,5,6)P4 levels increased 2-fold; there were no changes to levels of other 3H-labeled inositol phosphates. This experiment provides the first direct evidence that levels of Ins(3,4,5,6)P4 are regulated by Ins(1,3,4)P3 in vivo, independently of Ins(1,3,4)P3 being metabolized to Ins(3,4,5,6)P4. In addition, we found that the Ins(1, 3,4)P3 metabolites, namely Ins(1,3)P2 and Ins(3,4)P2, were >100-fold weaker inhibitors of the 1-kinase compared with Ins(1,3,4)P3 itself (IC50 = 0.17 microM). This result shows that dephosphorylation of Ins(1,3,4)P3 in vivo is an efficient mechanism to "switch-off" the cellular regulation of Ins(3,4,5,6)P4 levels that comes from Ins(1,3, 4)P3-mediated inhibition of the 1-kinase. We also found that Ins(1,3, 6)P3 and Ins(1,4,6)P3 were poor inhibitors of the 1-kinase (IC50 = 17 and >30 microM, respectively). The non-physiological trisphosphates, D/L-Ins(1,2,4)P3, inhibited 1-kinase relatively potently (IC50 = 0.7 microM), thereby suggesting a new strategy for the rational design of therapeutically useful kinase inhibitors. Overall, our data provide new information to support the idea that Ins(1,3,4)P3 acts in an important signaling cascade.

Expression of recombinant rat myo-inositol 1,4,5-trisphosphate 3-kinase B suggests a regulatory role for its N-terminus

We have expressed rat myo-inositol 1,4,5-trisphosphate (IP3) 3-kinase B as both a full-length, recombinant, non-fusion protein and a full-length, recombinant, fusion protein with maltose-binding protein (MBP) in Escherichia coli. The fusion protein with MBP is soluble, binds calmodulin and is enzymically active whereas the non-fusion protein is insoluble and does not bind calmodulin unless co-expressed with bacterial chaperone proteins (either GroES and GroEL, or DnaK, DnaJ and GrpE). However, soluble, calmodulin-binding non-fusion IP3 3-kinase B is enzymically inactive. The catalytic domain of the enzyme has previously been shown to reside near the C-terminus; the results we present suggest an auto-regulatory role for the N-terminus.

Inositol phosphates in the duckweed Spirodela polyrhiza L

We have undertaken an analysis of the inositol phosphates of Spirodela polyrhiza at a developmental stage when massive accumulation of InsP6 indicates that a large net synthesis is occurring. We have identified Ins3P, Ins(1,4)P2, Ins(3,4)P2 and possibly Ins(4,6)P2, Ins(3,4,6)P3, Ins(3,4,5,6)P4, Ins (1,3,4,5,6)P5, D- and/or L-Ins(1,2,4,5,6)P5 and InsP6 and revealed the likely presence of a second InsP3 with chromatographic properties similar to Ins(1,4,5)P3. The higher inositol phosphates identified show no obvious direct link to pathways of metabolism of second messengers purported to operate in higher plants, nor do they resemble the immediate products of plant phytase action on InsP6.

Nucleus-associated phosphorylation of Ins(1,4,5)P3 to InsP6 in Dictyostelium

Although many cells contain large amounts of InsP6, its metabolism and function is still largely unknown. In Dictyostelium lysates, the formation of InsP6 by sequential phosphorylation of inositol via Ins(3,4,6)P3 has been described [Stevens and Irvine (1990) Nature (London) 346, 580-583]; the second messenger Ins(1,4,5)P3 was excluded as a potential substrate or intermediate for InsP6 formation. However, we observed that mutant cells labelled in vivo with [3H]inositol showed altered labelling of both [3H]Ins(1,4,5)P3 and [3H]InsP6. In this report we demonstrate that Ins(1,4,5)P3 is converted into InsP6 in vitro by nucleus-associated enzymes, in addition to the previously described stepwise phosphorylation of inositol to InsP6 that occurs in the cytosol. HPLC analysis indicates that Ins(1,4,5)P3 is converted into InsP6 via sequential phosphorylation at the 3-, 6- and 2-positions. Ins[32P]P6, isolated from cells briefly labelled with [32P]Pi, was analysed using Paramecium phytase, which removes the phosphates of InsP6 in a specific sequence. The 6-position contained significantly more 32P radioactivity than the 4- or 5-positions, indicating that the 6-position is phosphorylated after the other two positions. The results from these in vivo and in vitro experiments demonstrate a metabolic route involving the phosphorylation of Ins(1,4,5)P3 via Ins(1,3,4,5)P4 and Ins(1,3,4,5,6)P5 to InsP6 in a nucleus-associated fraction of Dictyostelium cells.

Inhibition of porcine brain inositol 1,3,4-trisphosphate kinase by inositol polyphosphates, other polyol phosphates, polyanions and polycations

We have partially purified an enzyme activity that phosphorylates inositol 1,3,4-trisphosphate from porcine brain, rat liver and bovine testis by FPLC chromatography on Q-Sepharose anion-exchange resin and Heparin-agarose. The products of this reaction were inositol 1,3,4,6-tetrakisphosphate and inositol 1,3,4,5-tetrakisphosphate. The same enzyme appears to be responsible for both 6-kinase and 5-kinase activities against inositol 1,3,4-trisphosphate (the 6-kinase: 5-kinase activity ratio is approximately 4 to 1), has a pH optimum of approximately 6.8 and requires Mg2+ for activity. The Km values of the enzyme for inositol 1,3,4-trisphosphate and ATP were approximately 0.5 microM and approximately 100 microM, respectively. Inositol 3,4,5,6-tetrakisphosphate, inositol 1,3,4,6-tetrakisphosphate and inositol 1,3,4,5-tetrakisphosphate are all competitive inhibitors with K(i) values of 0.4 microM, 3 microM and 5 microM, respectively, well within their likely intracellular concentration ranges: they inhibited 6-kinase and 5-kinase activities equally. 2,3-Bisphosphoglycerate and spermine were also competitive inhibitors, with K(i) values of 0.8 mM an 12 mM, respectively. Dextran sulphate was a non-competitive inhibitor with a Ki of approximately 15 microM, and poly-L-lysine (IC50 approximately 200 microM), polyvinylsulphate (IC50 approximately 250 microM) and heparin (IC50 approximately 2 mg/ml) also inhibited. Inhibition by these compounds suggests that inositol 3,4,5,6-tetrakisphosphate (and to a lesser extent inositol 1,3,4,5-tetrakisphosphate and other naturally occurring intracellular ions) may restrict the synthesis of inositol 1,3,4,6-tetrakisphosphate and hence regulate the rate of inositol penta- and hexakisphosphate synthesis from receptor-generated inositol phosphates.

Activation of phosphatidylinositol 4,5-bisphosphate supply by agonists and non-hydrolysable GTP analogues

PtdIns(4,5)P2 serves as a precursor of a diverse family of signalling molecules, including diacylglycerol (and hence phosphatidic acid), Ins(1,4,5)P3 [and hence Ins(1,3,4,5)P4] and PtdIns(3,4,5)P3. The production of these messengers can be activated by agonists, and therefore the rate of utilization of PtdIns(4,5)P2 can vary dramatically. Although cells can only meet these large changes in demand for PtdIns(4,5)P2 by increasing its synthesis and/or by continuously cycling it at a rate that exceeds its potential consumption (avoiding the need for a co-ordinated activation mechanism), no satisfactory explanation for how this is achieved in agonist-stimulated cells has yet been provided. We show here that, in streptolysin-O-permeabilized neutrophils, N-formylmethionyl-leucyl-phenylalanine (FMLP), platelet-activating factor (PAF) and non-hydrolysable GTP analogues can cause large activations of PtdIns4P 5-kinase, suggesting that cells can accommodate agonist-activated rates of consumption of PtdIns(4,5)P2 without having to sustain continuous, comparably rapid and energetically expensive 'futile cycling' reactions.

Inositol phosphates and cell signaling: new views of InsP5 and InsP6

Ins(1,3,4,5,6)P5 and InsP6 comprise the bulk of the inositol phosphate content of mammalian cells, but their intracellular functions are unknown. Until recently it seemed that these compounds were metabolically lethargic; this has diverted attention away from their possible role in short-term regulation of physiological processes. Interest in the idea that these polyphosphates play more dynamic roles is now increasing, following recent demonstrations that they are precursors of several inositol phosphates that turnover rapidly.

The interrelationships of the inositol phosphates formed in vasopressin-stimulated WRK-1 rat mammary tumour cells

1. Temporal changes in the levels of many inositol phosphates, whose structural characterization is presented in the preceding paper [Wong, Barker, Morris, Craxton, Kirk & Michell (1991) Biochem. J. 286, 459-468], have been monitored in vasopressin-stimulated WRK-1 cells. 2. Upon stimulation, Ins(1,4,5)P3 accumulated within 1 s, consistent with its role as a rapidly acting second messenger produced by receptor activation of phosphoinositidase C. Ins(1,4)P2 and Ins(1,3,4,5)P4, both of which are immediate products of Ins(1,4,5)P3 metabolism, also accumulated quickly. Ins4P, Ins(1,3,4)P3, Ins(3,4)P2, Ins(1,3)P2, Ins1P and Ins3P, which are intermediates in the metabolism of Ins(1,4)P2 and Ins(1,3,4,5)P4 to inositol, accumulated after seconds or within a few minutes, and in a temporal sequence consistent with their known metabolic interrelationships. 3. The stimulated accumulation of Ins(1,3,4,6)P4 was delayed, as expected if it is formed by phosphorylation of Ins(1,3,4)P3. 4. Ins(3,4,5,6)P4 accumulated 2-3-fold in a few minutes, and mainly before Ins(1,3,4,6)P4. 5. Using a [3H]-/[14C]-inositol double-labelling protocol, we obtained evidence that all of the compounds that accumulated upon stimulation, except Ins(3,4,5,6)P4, originated from lipid-derived Ins(1,4,5)P3, but that the newly formed Ins(3,4,5,6)P4 came from a different source. 6. There were no consistent changes in the levels of Ins(1,3,4,5,6)P5 and InsP6 during stimulation. 7. Alongside the gradual accumulation of Ins(1:2-cyclic,4,5)P3 during stimulation [Wong, Barker, Shears, Kirk & Michell (1988) Biochem. J. 252, 1-5], there was an accumulation of Ins(1:2-cyclic,4)P2 and Ins(1:2-cyclic)P, probably as either minor side products of phosphoinositidase C action or metabolites of Ins(1:2-cyclic,4,5)P3. 8. When Li+ was present during stimulation, it redirected the dephosphorylation pathways downstream of Ins(1,4,5)P3 in the manner expected from its inhibition of inositol monophosphatase and Ins(1,4)P2/Ins(1,3,4)P3 1-phosphatase: there were marked increases in the accumulation of Ins(1,4)P2 and Ins(1,3,4)P3 and of monophosphates. Moreover, Li+ shifted the Ins1P/Ins3P balance in favour of Ins1P, thus demonstrating redirection of the metabolism of the accumulated Ins(1,3,4)P3 towards Ins(1,3)P2 rather than Ins(3,4)P2.

Inositol lipids in cell signalling

In the past year, major advances have been made in our understanding of the regulation of phosphoinositidase C, and of the action of the inositol trisphosphate receptor and how it may generate 'quantal' Ca2+ release. The functions of inositol tetrakisphosphate and of the 3-phosphorylated inositol lipids continue to generate controversy, but both may be well on the way towards some clarification. Finally, we may have to extend our concept of the inositide cycle to include an intranuclear signalling function.

Inositol polyphosphate metabolism and inositol lipids in a green alga, Chlamydomonas eugametos

Swimming suspensions of Chlamydomonas eugametos were pelleted and homogenized, and the metabolism of inositol polyphosphates by cellular homogenates or supernatants was investigated. Ins(1,4,5)P3 was dephosphorylated under physiological conditions to yield a single InsP2, Ins(1,4]2. In the presence of ATP it was phosphorylated to give Ins(1,3,4,5)P3 as the only InsP4. The Ins(1,4,5)P3 3-kinase activity was predominantly soluble, was not detectably affected by calmodulin or Ca2+, and had a Km for Ins(1,4,5)P3 of 50 microM (two orders of magnitude higher than its mammalian counterpart). Ins(1,3,4,5)P4 was dephosphorylated by the cellular supernatants to Ins(1,3,4)P3 and Ins(1,4,5)P3, and could be phosphorylated to Ins(1,3,4,5,6)P4. No Ins(1,3,4)P3 6-kinase activity could be detected, and experiments with [3H]Ins(1,4,[32P]5)P3 revealed that Ins(1,3,4,5,6)P5 is formed from Ins(1,4,5)P3 with little loss of the 5-phosphate, i.e. the predominant route of synthesis is probably by a direct 6-phosphorylation of Ins(1,3,4,5)P4. Similar experiments with an (NH4)2SO4 fraction of turkey erythrocyte cytosol gave essentially the same result, i.e. direct phosphorylation of Ins(1,3,4,5)P4 in the 6 position is the predominant route of synthesis of InsP5 from that InsP4 in vitro. No InsP6 formation was detected in any of these experiments, but labelling of intact C. eugametos with [3H]inositol revealed that the cells do synthesize InsP6. The lipids of C. eugametos cells contain PtdIns, PtdIns(4)P and PtdIns(4,5)P2 [Irvine, Letcher, Lander, Drøbak, Dawson & Musgrave (1989) Plant Physiol. 64, 888-892]. Further examination of 32P-labelled lipids revealed that about 20% of the PtdInsP was the PtdIns(3)P isomer, and about 1% or less of the PtdInsP2 was the PtdIns(3,4)P2 isomer. The overall inositide metabolism of C. eugametos resembles that of a mammalian cell more closely than it does that of a plant cell or slime mould, and this suggests firstly that the known metabolism of inositol polyphosphates arose at an early time in eukaryotic evolution, and secondly that Chlamydomonas might prove a useful organism for genetic and comparative studies of inositide enzymology.

Pathway of phosphatidylinositol(3,4,5)-trisphosphate synthesis in activated neutrophils

Neutrophils activated by the formyl peptide f-Met-Leu-Phe transiently accumulate a small subset of highly polar inositol lipids. A similar family of lipids also appear in many other cells in response to a range of growth factors and activated oncogenes, and are presumed to be the direct or indirect products of 3-phosphatidylinositol kinase. The structures of these lipids are shown to be phosphatidylinositol 3-phosphate, phosphatidylinositol-(3,4)bisphosphate and phosphatidylinositol-(3,4,5)trisphosphate, and we present evidence that in intact neutrophils a phosphatidyl-inositol-(4,5)bisphosphate-3-kinase seems to be the focal point through which agonists stimulate the formation of 3-phosphorylated inositol lipids.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.