Metabolism of the biologically active inositol phosphates Ins(1,4,5)P3 and Ins(1,3,4,5)P4 by ovarian follicles of Xenopus laevis

Metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate by the oocytes of Xenopus laevis

The pathway and kinetics of inositol 1,4,5-trisphosphate (IP3) metabolism were measured in Xenopus laevis oocytes and cytoplasmic extracts of oocytes. Degradation of microinjected IP3 in intact oocytes was similar to that in the extracts containing comparable concentrations of IP3 ([IP3]). The rate and route of metabolism of IP3 depended on the [IP3] and the intracellular free Ca2+ concentration ([Ca2+]). At low [IP3] (100 nM) and high [Ca2+] (>/=1 microM), IP3 was metabolized predominantly by inositol 1,4, 5-trisphosphate 3-kinase (3-kinase) with a half-life of 60 s. As the [IP3] was increased, inositol polyphosphate 5-phosphatase (5-phosphatase) degraded progressively more IP3. At a [IP3] of 8 microM or greater, the dephosphorylation of IP3 was the dominant mode of IP3 removal irrespective of the [Ca2+]. At low [IP3] and low [Ca2+] (both </=400 nM), the activities of the 5-phosphatase and 3-kinase were comparable. The calculated range of action of IP3 in the oocyte was approximately 300 micron suggesting that IP3 acts as a global messenger in oocytes. In contrast to IP3, inositol 1,3,4, 5-tetrakisphosphate (IP4) was metabolized very slowly. The half-life of IP4 (100 nM) was 30 min and independent of the [Ca2+]. IP4 may act to sustain Ca2+ signals initiated by IP3. The half-life of both IP3 and IP4 in Xenopus oocytes was an order of magnitude or greater than that in small mammalian cells.

Differential induction of inositol phosphate metabolism by three adipokinetic hormones

Many (in)vertebrates simultaneously release several structurally and functionally related hormones; however, the relevance of this phenomenon is poorly understood. In the locust e.g. each of three adipokinetic hormones (AKHs) is capable of controlling mobilization of carbohydrate and lipid from fat body stores, but it is unclear why three AKHs coexist. We now demonstrate disparities in the signal transduction of these hormones. Massive doses of the AKHs stimulated total inositol phosphate (InsPn) production in the fat body biphasicly, but time courses were different. Inhibition of phospholipase C (PLC) resulted in attenuation of both InsPn synthesis and glycogen phosphorylase activation. The AKHs evoked differential formation of individual [3H]InsPn isomers (InsP(1-6)), the effect being most pronounced for InsP3. 40 nM of AKH-I and -III induced a substantial rise in total InsPn and [3H]InsP3 at short incubations, whereas the AKH-II effect was negligible. At a more physiological dose of 4 nM, the AKHs equally enhanced Ins(1,4,5)P3 levels. The InsP3 effect was most prolonged for AKH-III. These subtle differences in InsPn metabolism, together with earlier findings on differences between the AKHs, support the hypothesis that each AKH exerts specific biological functions in the overall syndrome of energy mobilization during flight.

Insect adipokinetic hormone stimulates inositol phosphate metabolism: roles for both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in signal transduction?

Adipokinetic hormones (AKHs) control the mobilization of energy reserves from the insect fat body as fuels for flight activity. As a part of our investigations on AKH signal transduction, we demonstrate in this study that the inositol lipid cycle may be involved in the action of AKH-I on fat body of the migratory locust. We show that [3H]inositol is incorporated into fat body phosphoinositides in vitro, whose hydrolysis leads to the formation of the following inositol phosphates (InsPs): Ins(1 and/or 3)P, Ins(4)P, Ins(1,3)P2, Ins(1,4)P2, Ins(3,4)P3, Ins(1,3,4)P3, Ins(1,4,5)P3 and Ins(1,3,4,5)P4. AKH stimulates the formation of these isomers, eliciting an increase in radioactivity of total InsPs already after 1 min. Mass measurements show that Ins(1,4,5)P3 levels are substantially enhanced by AKH, which is indicative of hormonal activation of phospholipase C. In cell-free tissue preparations, Ins(1,4,5)P3 is metabolized through dephosphorylation as well as further phosphorylation. Ins(1,3,4,5)P4 is dephosphorylated primarily to Ins(1,3,4)P3, although the ability for its reconversion to Ins(1,4,5)P3 suggests that in vivo Ins(1,3,4,5)P4 may function as a rapidly mobilizable pool for Ins(1,4,5)P3 generation. Metabolic pathways for the conversion of InsPs to inositol in the locust fat body are proposed.

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.

Bovine testis and human erythrocytes contain different subtypes of membrane-associated Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphomonoesterases

1. We have purified membrane-associated Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatases from bovine testis and human erythrocytes by chromatography on several media, including a novel 2,3-bisphosphoglycerate affinity column. 2. The enzymes have apparent molecular masses of 42 kDa (testis) and 70 kDa (erythrocyte), as determined by SDS/PAGE, and affinities for Ins(1,4,5)P3 of 14 microM and 22 microM respectively. 3. The two enzymes hydrolyse both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 and are therefore type I Ins(1,4,5)P3 5-phosphatases [nomenclature of Hansen, Johanson, Williamson and Williamson (1987) J. Biol. Chem. 262, 17319-17326]. 4. On chromatofocusing, the partially purified testicular enzyme migrates as two peaks of activity, with pI values of about 5.8 and 5.5. The erythrocyte enzyme exhibits only the latter peak. 5. The testis 5-phosphatase is labile at 37 degrees C, but its activity can be maintained in the presence of 50 mM phorbol dibutyrate (PdBu). After PdBu treatment, a third form of the enzyme, with pI about 6.2, appears on chromatofocusing, but without change in its Km or Vmax. 6. Consideration of the properties of these enzymes and of the 5-phosphatases from other tissues suggests that type I Ins(1,4,5)P3 5-phosphatases are of two well-defined subtypes. We propose that these be termed type Ia [typified by the testis enzyme: approximately 40 kDa, higher affinity for Ins(1,4,5)P3] and Type Ib [typified by the erythrocyte enzyme: approximately 70 kDa, lower affinity for Ins(1,4,5)P3].

Differential activation of inositol 1,4,5-trisphosphate-sensitive calcium pools by muscarinic receptors in Xenopus laevis oocytes

Muscarinic acetylcholine (ACh) receptors activate the phospholipase C signal transduction pathway to promote the formation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and the consequent elevation of cytoplasmic calcium (Ca2+). The inositol phosphate and Ca(2+)-mobilization responses to ACh were analyzed in Xenopus oocytes possessing endogenous receptors, and in oocytes expressing exogenous receptors from injected muscarinic RNA transcripts, to evaluate the patterns of signal transduction mediated by native and expressed receptors. Activation of native ACh receptors elicited dose- and time-dependent increases in Ins(1,4,5)P3 and inositol bisphosphate (InsP2) production. ACh-induced Ins(1,4,5)P3 production increased rapidly within the first 2 min and continued to rise over the next 20 min. ACh was a much more effective stimulus of inositol phosphate production at native (up to 35-fold) than at expressed receptors (less than 2-fold). In contrast, measurements of Ca(2+)-mobilization in oocytes injected with the Ca(2+)-specific photoprotein, aequorin, revealed that ACh stimulation of expressed receptors evoked up to 200-fold increase in light emission, whereas ACh stimulation of native receptors elicited less than a 2-fold response. These observations indicate that the oocyte possesses functionally distinct agonist-sensitive Ca2+ pools which differ markedly in their sensitivity to Ins(1,4,5)P3 production and suggest that these pools are mobilized by different effector mechanisms. The finding that the magnitude of the intra-oocyte Ca2+ response is not necessarily determined by the degree of Ins(1,4,5)P3 production, but rather by another aspect of the signal transduction pathway (e.g. the nature and/or location of the Ins(1,4,5)P3 releasable Ca2+ pool), reveals an additional level of complexity in the transduction mechanisms responsible for intracellular Ca2+ signaling.

Metabolism of inositol-1,4,5-trisphosphate in the taste organ of the channel catfish, Ictalurus punctatus

1. The metabolism of inositol-1,4,5-trisphosphate was studied in the taste organ (barbel) of the channel catfish, Ictalurus punctatus. 2. Homogenates of epithelial barbel scrapings were incubated with [3H]-1,4,5-IP3, whose dephosphorylation or phosphorylation was assayed under first-order conditions by measuring the production of either [3H]-1,4-IP2 (representing the activity of IP3-5-phosphatase) or [3H]-1,3,4,5-IP4 (representing the activity of IP3-3-kinase). 3. Both enzymes were predominantly cytosolic, magnesium-dependent and maximally active at pH 6.4. For IP3-phosphatase, Km = 6 microM and Vmax = 10.5 nmol/min/mg. For IP3-kinase, Km = 0.23 microM and Vmax = 0.05 nmol/min/mg. 4. Neither enzyme was significantly affected by the presence of taste stimuli (amino acids), GTP gamma S, cAMP or phorbol esters. 5. In the presence of physiological levels of free calcium (0.05-12 microM) IP3-phosphatase was moderately activated whereas IP3-kinase was moderately inhibited. 6. IP3-phosphatase was moderately activated by Mn2+, unaffected by LiCl, and strongly inhibited by 2,3-diphosphoglycerate, Na-pyrophosphate, CdCl2, HgCl2, CuCl2, FeCl3 and ZnSO4 7. IP3-kinase was strongly activated by 2,3-diphosphoglycerate, Na-pyrophosphate, CdCl2, HgCl2, FeCl3 and LiCl and inhibited by ZnSO4 and Mn2+. 8. IP3-kinase was significantly activated in a calcium-dependent manner by exogenously-added phosphatidylcholine and sphingomyelin, and to a lesser extent by diacylglycerol. IP3-phosphatase was unaffected by exogenously-added lipids. 9. IP3-phosphatase may participate in taste transduction since calculations based on the first-order rate constant (6.9 sec-1) indicate that it is capable of dephosphorylating basal levels of IP3 with a half-life of 0.1 sec.

Intercellular communication between follicular angiotensin receptors and Xenopus laevis oocytes: medication by an inositol 1,4,5-trisphosphate-dependent mechanism

In Xenopus laevis oocytes, activation of angiotensin II (AII) receptors on the surrounding follicular cells sends a signal through gap junctions to elevate cytoplasmic calcium concentration ([Ca2+]i) within the oocyte. The two major candidates for signal transfer through gap junctions into the oocyte during AII receptor stimulation are Ins(1,4,5)P3 and Ca2+. In [3H]inositol-injected follicular oocytes, AII stimulated two- to fourfold increases in phosphoinositide hydrolysis and production of inositol phosphates. Injection of the glycosaminoglycan, heparin, which selectively blocks Ins(1,4,5)P3 receptors, prevented both AII-stimulated and Ins(1,4,5)P3-induced Ca2+ mobilization in Xenopus follicular oocytes but did not affect mobilization of Ca2+ by ionomycin or GTP. These results indicate that the AII-regulated process of gap junction communication between follicular cells and the oocyte operates through an Ins(1,4,5)P3-dependent mechanism rather than through transfer of Ca2+ into the ooplasm and subsequent Ca(2+)-induced Ca2+ release.

The metabolism of microinjected inositol trisphosphate in Xenopus oocytes

Microinjection of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) into Xenopus oocytes evokes a complex physiological response composed of a transient and a slow depolarizing chloride current. We investigated the relationship between intracellular levels of Ins(1,4,5)P3 and the kinetics of the physiological response. Microinjected Ins(1,4,5)P3 was slowly degraded following first order kinetics of disappearance (t1/2 = 10 min). The degradation products were inositol bisphosphate (InsP2), inositol monophosphate (InsP) and inositol, as well as inositol tetrakisphosphate (InsP4). The rate of degradation of injected 3[H]-Ins(1,4)P2 was much greater (t1/2 = 3 min), indicating that the conversion of InsP3 to InsP2 may be the rate-limiting step in the degradation process. The slow degradation of 3[H]-Ins(1,4,5)P3 was not a result of its conversion to Ins(1,3,4)P3 since no accumulation of InsP3 was observed within 10 min of microinjection of 3[H]-Ins(1,3,4,5)P4. Activation of protein kinase C (PK-C) with a phorbol ester transiently increased the rate of conversion of 3[H]-Ins(1,4,5)P3 to InsP2. This, however, did not significantly affect the overall kinetics of 3[H]-Ins(1,4,5)P3 disappearance. Our results indicate that the kinetics of Ins(1,4,5)P3 degradation do not correlate well with the termination of both the rapid and the slow components of the physiological response. The termination of the slow component of the response, however, may be related to the decay of Ins(1,4,5)P3-induced 45Ca efflux, which lasted about 10 min.

Angiotensin II and acetylcholine differentially activate mobilization of inositol phosphates in Xenopus laevis ovarian follicles

Angiotensin II (AII) evokes a Ca(2+)-dependent Cl- current in Xenopus laevis ovarian follicles that appears to involve a pertussis-toxin-sensitive G protein mediating phosphoinositide hydrolysis and Ca2+ mobilization from intracellular stores. Follicle responses to AII closely resemble the two-component response stimulated by acetylcholine (ACh) in this tissue. Intraoocyte injections of phytic acid, heparin, and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], acting as inhibitors of Ins(1,4,5)P3-induced Ca(2+)-release, resulted in loss of responsiveness to AII and ACh. As previously reported for ACh [Moriarty et al. (1988) Proc Natl Acad Sci USA 85: 8865-8869], pertussis toxin and microinjected GTP[gammaS] were found to inhibit follicle responses to AII, implying the involvement of a G protein. However, ACh and AII responses differ strikingly in the way they mobilize inositol phosphates and in densitization characteristics. We have previously been unable to find significant increases in inositol phosphates after 60 min stimulation (with Li+) by AII, although ACh potently activated increases in these [McIntosh and McIntosh (1990) Arch Biochem Biophys 283: 135-140]. In the present paper, AII was found to activate rapid increases in inositol bis- and trisphosphates after 1 min stimulation without Li+. ACh and AII also exerted different actions on follicle adenylate-cyclase-dependent responses. We conclude that at least two separate inositol-phosphate-linked receptor mechanisms may exist in ovarian follicles, resulting from involvement of one or more pertussis-toxin-sensitive G protein(s).

Inositol tetrakisphosphates as second messengers induce Ca(++)-dependent chloride currents in Xenopus laevis oocytes

Microinjection of inositol 1,3,4,5-tetrakisphosphate or inositol 1,4,5-trisphosphate induced distinct chloride membrane currents in defolliculated Xenopus laevis oocytes. To decide whether these Cl(-)-currents were due to the injected compounds or their metabolic products, [3H]Ins(1,3,4,5)P4 or [3H]Ins(1,4,5)P3 were injected into oocytes and their metabolites were analyzed by HPLC. Our results indicate that Ins(1,3,4,5)P4 itself or its metabolite Ins(1,3,4,6)P4 is able to induce Cl(-)-membrane currents, most likely by increasing the cytosolic Ca(++)-concentration.

A salt-activated inositol 1,3,4,5-tetrakisphosphate 3-phosphatase at the inner surface of the human erythrocyte membrane

The localization of the human erythrocyte membrane Ins(1,3,4,5)P4 3-phosphatase was investigated by saponin permeabilization of resealed 'isoionic' erythrocyte ghosts. This enzyme is active at the inner face of the plasma membrane, at the same site as a specific 5-phosphatase that degrades both Ins (1,4,5)P3 and Ins(1,3,4,5)P4. In the presence of EDTA, Ins(1,4,5)P3 was the only product of Ins(1,3,4,5)P4 metabolism. However, when Mg2+ was present both the 5-phosphatase and the 3-phosphatase attacked Ins (1,3,4,5)P4, directly forming Ins(1,3,4)P3 and Ins(1,4,5)P3;some Ins(1,4)P2 was also formed as a product of 5-phosphatase attack on the liberated Ins(1,4,5)P3. The Ins(1,3,4,5)P4 3-phosphatase was potently activated by KCl, thus making the route of metabolism of Ins(1,3,4,5)P4 by erythrocyte ghosts strikingly sensitive to variations in ionic strength: at 'cytosolic' K+ and Mg2+ levels, 3-phosphatase activity slightly predominated over 5-phosphatase. Ins(1,3,4,5)P4 3-phosphatase was potently inhibited by Ins-(1,3,4,5,6)P5 and InsP6 at levels lower than those often observed within cells. This leaves open the question as to whether the cellular function of inositol polyphosphate 3-phosphatase is to participate in a physiological cycle that interconverts Ins(1,3,4,5)P4 and Ins(1,4,5)P3 or to metabolize other inositol polyphosphates in the cytosol compartment of cells.

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.