Isolation and characterization of the inositol cyclic phosphate products of polyphosphoinositide cleavage by phospholipase C. Physiological effects in permeabilized platelets and Limulus photoreceptor cells

DNA Damage and Repair in Migraine: Oxidative Stress and Beyond

Energy generation in the brain to ameliorate energy deficit in migraine leads to oxidative stress as it is associated with reactive oxygen species (ROS) that may damage DNA and show a pronociceptive action in meninges mediated by transient receptor potential cation channel subfamily A member 1 (TRPA1). Recent studies show high levels of single-strand breaks (SSBs) at specific sites in the genome of postmitotic neurons and point at SSB repair (SSBR) as an important element of homeostasis of the central nervous system. DNA topoisomerase 1 (TOP1) is stabilized in the DNA damage-inducing state by neuronal stimulation, including cortical spreading depression. Impairment in poly (ADP-ribose) polymerase 1 (PARP-1) and X-ray repair cross complementing 1 (XRCC1), key SSBR proteins, may be linked with migraine by transient receptor potential melastatin 2 (TRPM2). TRPM2 may also mediate the involvement of migraine-related neuroinflammation with PARP-1 activated by oxidative stress-related SSBs. In conclusion, aberrant activity of SSBR evoked by compromised PARP-1 and XRCC1 may contribute to pathological phenomena in the migraine brain. Such aberrant SSBR results in the lack of repair or misrepair of SSBs induced by ROS or resulting from impaired TOP1. Therefore, components of SSBR may be considered a prospective druggable target in migraine.

Inositol phosphate metabolism and platelet activation

Platelet activation by thrombin and most other agonists appears to require two second messenger systems that are both initiated by phospholipase C-catalysed cleavage of phosphatidylinositol phosphates leading to: 1. formation of inositol phosphates with a subsequent rise in intracellular calcium from intracellular stores and from outside the cell; 2. formation of diacylglycerol with subsequent activation of protein kinase C. This review examines inositol phosphate metabolism in platelets and its involvement in calcium metabolism.

Identification of cellular target proteins for signaling cyclic phosphates

Cyclic glycerophosphates and their deoxy analogs were previously found to induce intracellular tyrosine and threonine phosphorylation in Chinese hamster ovary (CHO) cells. Further studies have indicated that these compounds induce neuronal outgrowth in PC-12 cells, as well as elevation of the state of cellular differentiation in human breast cancer cell lines. The mechanism by which these cyclic phosphates operate is not yet fully delineated. Using an affinity labeling approach we probed for possible cyclic phosphate target proteins in CHO cells. A 170 kDa protein that was labeled by an affinity cyclic phosphate reagent was identified by mass spectrometry as the largest subunit of the eukaryotic initiation factor 3 (eIF3). Using In-Gel kinase assays allowed the detection of a approximately 70 kDa target kinase directly activated by cyclic phosphates. Identification of these proteins may provide a basis for deciphering the mechanisms, by which cyclic phosphates exert their effects.

Identification of myo-inositol 1,2-cyclic monophosphate by electrospray tandem mass spectrometry, a major constituent of EGF-stimulated phosphoinositide turnover in MDA 468 cells

Epidermal growth factor (EGF) caused an increase in phosphoinositide (PI) turnover in MDA 468 cells. This EGF-stimulated effect was inhibited by the protein tyrosine kinase inhibitor lavendustin A (LA). MDA 468 cells generated an atypical PI turnover profile. Examination and quantitation of the PI metabolite profile showed that even control cells produced a metabolite which was acid-labile and which formed about 60% of the total PI metabolites. By using the technique of electrospray ionization tandem mass spectrometry, we were able to confirm the identity of this acid-labile metabolite through the specific fragmentation as compared with the standard. The precursor molecule fragmented into two distinct productions with molar masses identical to that of the standard myo-inositol 1,2-cyclic monophosphate (cInsP). Changes in the PI turnover profile could be accounted for by the alterations in myo-inositol 1,2-cyclic monophosphate generated in these cells. We thus conclude that, by some as-yet-unidentified mechanism, cyclic inositol monophosphate forms a major constituent of EGF-stimulated PI turnover in MDA 468 cells.

Inositol phosphates in the environment

The inositol phosphates are a group of organic phosphorus compounds found widely in the natural environment, but that represent the greatest gap in our understanding of the global phosphorus cycle. They exist as inositols in various states of phosphorylation (bound to between one and six phosphate groups) and isomeric forms (e.g. myo, D-chiro, scyllo, neo), although myo-inositol hexakisphosphate is by far the most prevalent form in nature. In terrestrial environments, inositol phosphates are principally derived from plants and accumulate in soils to become the dominant class of organic phosphorus compounds. Inositol phosphates are also present in large amounts in aquatic environments, where they may contribute to eutrophication. Despite the prevalence of inositol phosphates in the environment, their cycling, mobility and bioavailability are poorly understood. This is largely related to analytical difficulties associated with the extraction, separation and detection of inositol phosphates in environmental samples. This review summarizes the current knowledge of inositol phosphates in the environment and the analytical techniques currently available for their detection in environmental samples. Recent advances in technology, such as the development of suitable chromatographic and capillary electrophoresis separation techniques, should help to elucidate some of the more pertinent questions regarding inositol phosphates in the natural environment.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Inositol 1,4,5-trisphosphate accumulation in brain of lithium-treated rats

The mechanism of action of lithium as a drug for patients with affective disorders was investigated. Three-week-old male rats were orally administered 2.7 mEq Li2CO3/kg/d for 1 or 3 wk, and phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), inositol phosphate (IP), inositol diphosphate (IP2) and inositol triphosphate (IP3) levels in brain were measured. The levels of IP were increased 1.7 and 2.4 times after 1 wk and 3 wk of lithium administration, respectively, while PI, PIP, PIP2, IP2 and IP3 levels were not altered. IP3 was further fractionated by high-performance liquid chromatography into I-1,3,4-P3 and I-1,4,5-P3. In the control rat brain, the relative percentages of I-1,3,4-P3 and I-1,4,5-P3 were 95.8 and 4.2, respectively. However, after 3 wk of lithium administration, the values were changed to 69.6 and 30.3%, respectively. This increase in the neurotransducer I-1,4,5-P3 in the brain may be relevant to the mechanism of action in the lithium treatment of patients with manic-depressive disorders.

Oncogenicity of AKR mink cell focus-inducing murine leukemia virus correlates with induction of chronic phosphatidylinositol signal transduction

Naturally occurring recombinant murine leukemia viruses (MuLVs), termed mink cell focus-inducing (MCF) viruses, are the proximal leukemogens in spontaneous thymic lymphomas of AKR mice. The mechanism by which these viruses transform lymphocytes is not clear. Previous studies have implicated either integrational activation of proto-oncogenes, chronic autocrine immune stimulation, and/or autocrine stimulation of growth factor receptors (e.g., interleukin 2 receptors) via binding of the viral env glycoprotein (gp70) to these receptors. Any one of these events could also involve activation of second messenger signaling pathways in the cell. We examined whether infection with oncogenic AKR-247 MCF MuLV induced transmembrane signaling cascades in thymocytes of AKR mice. Cyclic AMP levels were not changed, but there was enhanced turnover of phosphatidylinositol phosphates, with concomitant increases in diacyglycerol and inositol 1,4,5-triphosphate. Thus, phospholipase C activity was increased. Protein kinase C activity was also elevated in comparison to that in uninfected thymocytes. The above events occurred in parallel with MCF expression in the thymus and were chronically maintained thereafter. No changes in phospholipid turnover occurred in an organ which did not replicate the MCF virus (spleen) or in thymocytes of AKR mice infected with a thymotropic, nononcogenic MCF virus (AKV-1-C36). Therefore, only the oncogenic MCF virus induced phosphatidylinositol signal transduction. Flow cytometric comparison of cell surface gp70 revealed that AKR-247 MCF virus-infected thymocytes expressed more MCF virus gp70 than did thymocytes from AKV-1-C36 MCF virus-infected mice, suggesting that certain threshold quantities of MCF virus env glycoproteins may be involved in this signaling. This type of signal transduction is not induced by stimulation of the interleukin 2 receptor but is involved in certain oncogene systems (e.g., ras and fms). Its chronic induction by oncogenic MCF MuLV may thus initiate thymocyte transformation.

Phospholipids chiral at phosphorus. Stereochemical mechanism for the formation of inositol 1-phosphate catalyzed by phosphatidylinositol-specific phospholipase C

The phosphatidylinositol-specific phospholipase C (PI-PLC) from mammalian sources catalyzes the simultaneous formation of both inositol 1,2-cyclic phosphate (IcP) and inositol 1-phosphate (IP). It has not been established whether the two products are formed in sequential or parallel reactions, even though the latter has been favored in previous reports. This problem was investigated by using a stereochemical approach. Diastereomers of 1,2-dipalmitoyl-sn-glycero-3-(1D- [16O,17O]phosphoinositol) ([16O,17O]DPPI) and 1,2-dipalmitoyl-sn-glycero-3-(1D-thiophosphoinositol) (DPPsI) were synthesized, the latter with known configuration. Desulfurization of the DPPsI isomers of known configurations in H2(18)O gave [16O,18O]DPPI with known configurations, which allowed assignment of the configurations of [16O,17O]DPPI on the basis of 31P NMR analyses of silylated [16O,18O]DPPI and [16O,17O]DPPI (the inositol moiety was fully protected in this operation). (Rp)- and (Sp)-[16O,17O]DPPI were then converted into trans- and cis-[16O,17O]IcP, respectively, by PI-PLC from Bacillus cereus, which had been shown to proceed with inversion of configuration at phosphorus [Lin, G., Bennett, F. C., & Tsai, M.-D. (1990) Biochemistry 29, 2747-2757]. 31P NMR analysis was again used to differentiate the silylated products of the two isomers of IcP, which then permitted assignments of IcP with unknown configuration derived from transesterification of (Rp)- and (Sp)-[16O,17O]DPPI by bovine brain PI-PLC-beta 1. The results indicated inversion of configuration, in agreement with the steric course of the same reaction catalyzed by PI-PLCs from B. cereus and guinea pig uterus reported previously. For the steric course of the formation of inositol 1-phosphate catalyzed by PI-PLC, (Rp)- and (Sp)-[16O,17O]DPPI were hydrolyzed in H2(18)O to afford 1-[16O,17O,18O]IP, which was then converted to IcP chemically and analyzed by 31P NMR. The results indicated that both B. cereus PI-PLC and the PI-PLC-beta 1 from bovine brain catalyze conversion of DPPI to IP with overall retention of configuration at phosphorus. These results suggest that both bacterial and mammalian PI-PLCs catalyze the formation of IcP and IP by a sequential mechanism. However, the conversion of IcP to IP was detectable by 31P NMR only for the bacterial enzyme. Thus an alternative mechanism in which IcP and IP are formed by totally independent pathways, with formation of IP involving a covalent enzyme-phosphoinositol intermediate, cannot be ruled out for the mammalian enzyme. It was also found that both PI-PLCs displayed lack of stereo-specifically toward the 1,2-diacylglycerol moiety, which suggests that the hydrophobic part of phosphatidylinositol is not recognized by PI-PLC.

Determination of endogenous levels of cyclic ADP-ribose in rat tissues

Cyclic ADP-ribose (cADPR) is a potent mediator of calcium mobilization in sea urchin eggs. The cADPR synthesizing enzyme is present not only in the eggs but also in various mammalian tissue extracts. The purpose of this study was to ascertain whether cADPR is a naturally occurring nucleotide in mammalian tissues. Rat tissues were frozen and powdered in liquid N2, followed by extraction with perchloric acid at -10 degrees C. [32P]cADPR was prepared and used as a tracer. The acid extracts were chromatographed on a Mono-Q column and cADPR in the fractions were determined by its ability to release Ca2+ from egg homogenates. That the release was mediated by cADPR and not inositol trisphosphate (IP3) in the extracts was shown by the fact that the homogenates, subsequent to Ca2+ release induced by active fractions, were desensitized to authentic cADPR but not to IP3. Furthermore, the Ca2+ release activity was shown to co-elute with [32P]cADPR. The endogenous level of cADPR determined in rat liver is 3.37 +/- 0.64 pmol/mg, in heart is 1.04 +/- 0.08 pmol/mg and in brain is 2.75 +/- 0.35 pmol/mg. These results indicate cADPR is a naturally occurring nucleotide and suggest that it may be a general second messenger for mobilizing intracellular Ca2+.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Phosphatidylinositol-specific phospholipase C from Bacillus cereus combines intrinsic phosphotransferase and cyclic phosphodiesterase activities: a 31P NMR study

The inositol phosphate products formed during the cleavage of phosphatidylinositol by phosphatidylinositol-specific phospholipase C from Bacillus cereus were analyzed by 31P NMR. 31P NMR spectroscopy can distinguish between the inositol phosphate species and phosphatidylinositol. Chemical shift values (with reference to phosphoric acid) observed are 0.41, 3.62, 4.45, and 16.30 ppm for phosphatidylinositol, myo-inositol 1-monophosphate, myo-inositol 2-monophosphate, and myo-inositol 1,2-cyclic monophosphate, respectively. It is shown that under a variety of experimental conditions this phospholipase C cleaves phosphatidylinositol via an intramolecular phosphotransfer reaction producing diacylglycerol and D-myo-inositol 1,2-cyclic monophosphate. We also report the new and unexpected observation that the phosphatidylinositol-specific phospholipase C from B. cereus is able to hydrolyze the inositol cyclic phosphate to form D-myo-inositol 1-monophosphate. The enzyme, therefore, possesses phosphotransferase and cyclic phosphodiesterase activities. The second reaction requires thousandfold higher enzyme concentrations to be observed by 31P NMR. This reaction was shown to be regiospecific in that only the 1-phosphate was produced and stereospecific in that only D-myo-inositol 1,2-cyclic monophosphate was hydrolyzed. Inhibition with a monoclonal antibody specific for the B. cereus phospholipase C showed that the cyclic phosphodiesterase activity is intrinsic to the bacterial enzyme. We propose a two-step mechanism for the phosphatidyl-inositol-specific phospholipase C from B. cereus involving sequential phosphotransferase and cyclic phosphodiesterase activities. This mechanism bears a resemblance to the well-known two-step mechanism of pancreatic ribonuclease, RNase A.

Mass measurement of inositol phosphates

This review summarises the methods available for the mass measurement of inositol phosphates, i.e., use of radioactive inositol lipid precursors, optical techniques, gas chromatography, mass spectrometry, nuclear magnetic resonance, fast atom bombardment and assays specific for Ins(1,4,5)P3. Examples of the use of each method, its sensitivity, advantages and drawbacks are given.

Methods for the analysis of inositol phosphates

Interest in the inositol phospholipids was stimulated by the simultaneous discoveries that the products of hydrolysis of these lipids could serve as messengers to activate to synergistic signaling pathways in hormonally responsive cells, namely, inositol 1,4,5-trisphosphate which causes the release of Ca2+ from intracellular stores and diacylglycerol which promotes the activation of protein kinase C. At the same time, Berridge and co-workers introduced relatively simple approaches to study the inositol phospholipid cycle. These included the use of [3H]inositol to label the inositol metabolites, all of which are confined to this cycle, and of Li+ to decrease the rate of degradation of the inositol phosphates. Water-soluble inositol phosphates and chloroform-soluble inositol phospholipids could then be separated by solvent partition and the inositol phosphates further separated by use of an anion-exchange resin. However, the subsequent application of high-performance liquid chromatography as a separation technique indicated the existence of many isomers of the inositol phosphates formed by different pathways of dephosphorylation and phosphorylation. Mapping of these metabolic pathways may be substantially complete, but novel pathways may still be discovered. We review both old and new methods of analysis of the inositol phosphates for the measurement of mass and radioactivity. Although the complexity of the cycle sometimes demands the use of sophisticated methods of separation and rigorous identification, older and inexpensive methods may still be useful for some purposes.

The partial chemical synthesis of inositol 1,2-cyclic 4,5-trisphosphate

Appreciable amounts of inositol 1,2-cyclic 4,5-trisphosphate (cIP3) are formed on agonist stimulation of secretory cells, e.g., pancreas (1,2) and parotid (3,4). However, the physiological role of this compound is unknown. To obtain sufficient amounts of cIP3, we have developed a synthetic method to produce cIP3 from inositol 1,4,5-trisphosphate (I(1,4,5)P3). The method is an adaptation of the dicyclohexylcarbodiimide (DCCD) method of Khorana et al. (5), which was originally developed to synthesize 2',3'-cyclic ribonucleotides. The method involves treatment of the pyridinium salt of I(1,4,5)P3 with DCCD in pyridine water, which cyclizes part of the 1-phosphate on the inositol ring to the 1,2-cyclic phosphate. The compound identified as cIP3 cochromatographed with authentic cIP3 in two HPLC systems and on ionophoresis. It was converted to I(1,4,5)P3 on mild acid treatment--a characteristic of cyclic inositol phosphates. Inositol 1,2-cyclic 4,5-trisphosphate is then purified by HPLC. Sufficient amounts of cIP3 can be prepared by this method to carry out numerous experiments on its possible cellular role.

Inositol phosphates and cell signalling

Inositol 1,4,5-trisphosphate is a second messenger which regulates intracellular calcium both by mobilizing calcium from internal stores and, perhaps indirectly, by stimulating calcium entry. In these actions it may function with its phosphorylated metabolite, inositol 1,3,4,5-tetrakisphosphate. The subtlety of calcium regulation by inositol phosphates is emphasized by recent studies that have revealed oscillations in calcium concentration which are perhaps part of a frequency-encoded second-messenger system.

Cyclic and noncyclic inositol phosphates are formed at different ratios by phospholipase C isozymes

The cyclic inositol phosphate content in the product of PLC-beta, gamma, and delta mediated cleavage of three phosphoinositides, PtdIns, PtdIns(4)p, and PtdIns(4,5)P2, was measured under several different experimental conditions. The ratio of cyclic to noncyclic product generally decreased in the order PLC-beta greater than PLC-delta greater than PLC-gamma. For all three enzymes the ratio decreased in the order PtdIns greater than PtdIns(4)P greater than PtdIns(4,5)P2. For all combinations of the three enzymes and three substrates cyclic product content was always higher at pH 5.5 than at pH 7.0. The effect of Ca2+ on the ratio of cyclic to noncyclic was also measured. The ratio remained constant between 0.5 microM and 2 mM for PtdIns. For PtdIns(4)P and PtdIns(4,5)P2, the ratios were unchanged between 0.5 and 500 microM, but increased abruptly at millimolar Ca2+ concentrations.

Modulation of intracellular free Ca2+ concentration by IP3-sensitive and IP3-insensitive nonmitochondrial Ca2+ pools

Intracellular Ca2+ pools play an important role in the adjustment of cytosolic free Ca2+ concentrations. This review summarizes the recent knowledge on receptor-mediated Ca2+ release and Ca2+ uptake mechanisms in Ca2+ stores of exocrine cells taking the exocrine pancreas and the parotid gland as an example. The intracellular mediator for agonist-induced Ca2+ release is inositol 1,4,5-trisphosphate (IP3) which acts by opening Ca2+ channels from the endoplasmic reticulum or a more specialized organelle called 'calciosome'. This Ca2+ release is the major event to increase cytosolic free Ca2+ concentrations of exocrine glands from a resting level of approximately 10(-7) mol/l to approximately 10(-6) mol/l. Subsequently also Ca2+ influx from the extracellular fluid into the cell is increased which involves the action of inositol 1,3,4,5-tetrakisphosphate (IP4). Intracellular nonmitochondrial Ca2+ reuptake occurs into IP3-sensitive (IsCaP) as well as into IP3-insensitive Ca2+ pools Ca2+ pools (IisCaP). While Ca2+ uptake into the IisCaP is mediated by a vanadate-sensitive Ca2+ pump, Ca2+ uptake into the IsCaP is mediated by a Ca2+/H+ exchanger at the expense of an H+ gradient which is established by a vacuolar type H+ pump present in the same Ca2+ pool. During stimulation both Ca2+ pools, IsCaP and IisCaP, are probably connected, the nature of which has not yet been clarified. It is suggested that GTP and/or IP4 control Ca2+ conveyance between intracellular Ca2+ pools by forming Ca2+-carrying junctions between membranes. Other models propose that Ca2+, which is released by IP3, induces Ca2+ release from another Ca2+ pool. Taking into account that H+ transport is present in IP3-sensitive Ca2+ pools the possibility of pH-regulated Ca2+ channels in the IisCaP, located in close neighbourhood to the IsCaP, is also considered.

Microinjection of inositol 1,2-(cyclic)-4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, and inositol 1,4,5-trisphosphate into intact Xenopus oocytes can induce membrane currents independent of extracellular calcium

Inositol phosphate action in an intact cell has been investigated by intracellular microinjection of eight inositol phosphate derivatives into Xenopus laevis oocytes. These cells have calcium-regulated chloride channels but do not have a calcium-induced calcium release system. Microinjection of inositol 1,3,4,5-tetrakisphosphate (IP4), inositol 1,2-(cyclic)-4,5-trisphosphate (cIP3), inositol 1,4,5-trisphosphate (IP3), or inositol 4,5-bisphosphate [(4,5)IP2], open chloride channels to induce a membrane depolarization. However, inositol 1-phosphate (IP1), inositol 1,3,4,5,6-pentakisphosphate (IP5), inositol 1,4-bisphosphate, or inositol 3,4-bisphosphate are unable to induce this depolarization. The depolarization is mimicked by calcium microinjection, inhibited by EGTA coinjection, and is insensitive to removal of extracellular calcium. By means of the depolarization response, the efficacy of various inositol phosphate derivatives are compared. IP3 and cIP3 induce similar half-maximal, biphasic depolarization responses at an intracellular concentration of approximately 90 nM, whereas IP4 induces a mono- or biphasic depolarization at approximately 3400 nM. At concentrations similar to that required for IP3 and cIP3, (4,5)IP2 induces a long-term (greater than 40 min) depolarization. The efficacy (cIP3 = IP3 = (4,5)IP2 much greater than IP4) and action of the various inositol phosphates in an intact cell and their inability to induce meiotic cell division are discussed.

Inositol 1,2-cyclic 4,5-trisphosphate is an order of magnitude less potent than inositol 1,4,5-trisphosphate in mobilizing intracellular stores of calcium in mouse pancreatic acinar cells

Semi-synthetic inositol 1,2-cyclic 4,5-trisphosphate is 1/16th as potent as inositol 1,4,5-trisphosphate in releasing Ca2+ from intracellular stores in permeabilized mouse pancreatic acinar cells. Competitive displacement studies in mouse pancreatic microsomes show that the affinity of inositol 1,2-cyclic 4,5-trisphosphate is 1/20th of that of inositol 1,4,5-trisphosphate at the latter's receptor, indicating that the lower potency of inositol 1,2-cyclic 4,5-trisphosphate in releasing Ca2+ can be accounted for by a weaker affinity at the receptor. These results suggest that inositol 1,2-cyclic 4,5-trisphosphate is unlikely to play any significant role in Ca2+ mobilization, at least in mouse pancreatic acinar cells.

[Inositol trisphosphate, a new "second messenger" for positive inotropic effects on the heart?]

Myocardial alpha 1-adrenoceptors mediate a positive inotropic effect and influence the inositol phosphate cycle. The receptor-stimulated, phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate (PIP2) results in the generation of two novel second messengers, inositol trisphosphate (IP3) and diacylglycerol (DG). This effect is concentration-dependent and precedes the increase in force of contraction. Recently, it has been shown that the alpha 1-adrenoceptor-mediated increase in IP3 and force of contraction exists in the human heart as well. Possible mechanisms for an inositol phosphate-mediated positive inotropic effect are: (i) release of Ca2+ from the sarcoplasmic reticulum, elicited by IP3, (ii) increase in Ca2+ sensitivity of the contractile proteins, elicited by IP3, inositol tetrakisphosphate (IP4) and/or DG, (iii) increase in slow Ca2+ inward current, elicited directly by IP4 and/or indirectly by DG through a phosphorylation of the protein kinase C substrate in the sarcolemma. In ventricular cardiac preparations muscarinic agonists have a weak positive inotropic effect, but in cardiac atrial preparations they have a negative inotropic effect. In both preparations, these different effects coincide with a concentration-dependent increase in IP3. Thus, the possible positive inotropic effect in atrial preparations is probably masked by an activation of a K+ outward current. The relationship between the inositol phosphate cycle and the positive inotropic effect is in some points still speculative because not all of the mechanisms discussed are well settled yet. However, the stimulation of myocardial phosphoinositide breakdown resulting in an increased IP3 may be involved in the mechanism(s) whereby alpha1-adrenergic and muscarinic receptor stimulation exert an increase in myocardial force of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide hormones, cytosolic calcium and renal epithelial response

We have reviewed the evidence that a number of hormones interact with renal tubular epithelial cells. The evidence suggests that in the mammalian renal tubule bradykinin and parathyroid hormone interact with cell surface receptors to initiate the hydrolysis of PIP2 leading to the formation of I 1,4,5P3 and diacylglycerol in the distal and proximal tubule, respectively. The activation of this second messenger system leads to the mobilization of Ca2+ from intracellular stores. Vasopressin does not activate this second messenger system in mammalian renal epithelial cells, and we cannot demonstrate I 1,4,5P3 formation and Ca2+ mobilization either in the rabbit papillary collecting tubules or in MDCK cells. There is evidence emerging, but not discussed here, that angiotensin II may also mediate some of its effects on the mammalian proximal tubule via the inositol polyphosphate second messenger system.

Development of a novel, Ins(1,4,5)P3-specific binding assay. Its use to determine the intracellular concentration of Ins(1,4,5)P3 in unstimulated and vasopressin-stimulated rat hepatocytes

The binding of [3H]Ins(1,4,5)P3 to bovine adrenocortical microsomes has been shown to be rapid, reversible and saturable. The microsomal preparation contained a single population of high affinity sites (KD = 6.82+/-2.3 nM, Bmax = 370+/-38 fmol/mg protein). The binding site was shown to exhibit positional specificity with respect to inositol trisphosphate binding, i.e. Ins(2,4,5)P3 was able to compete with [3H]Ins(1,4,5)P3 whereas Ins(1,3,4)P3 was not. Ins(1,3,4,5)P4 showed a similar affinity for the receptor as Ins(2,4,5)P3 whereas the other inositol phosphates tested, ATP, GTP and 2,3-DPG, were poor competitors. [3H]Ins(1,4,5)P3-binding was independent of free Ca2+ concentrations. The adrenocortical microsomal preparation has been incorporated into an assay which has been used to determine the basal and vasopressin-stimulated content of neutralised acid extracts of rat hepatocytes. Intracellular concentrations of Ins(1,4,5)P3 were calculated to be 0.22+/-0.15 microM basal and 2.53+/-1.8 microM at peak stimulation. This assay provides a simple, specific and quantitative method for the measurement of Ins(1,4,5)P3 concentrations in the picomolar range.

Lymphocyte activation by HIV-1 envelope glycoprotein

Cell activation by phytohaemagglutinin, phorbol ester and by the supernatant of phytohaemagglutinin-stimulated peripheral blood mononuclear cells induces the expression and cytopathic effects of latent human immunodeficiency virus type-1 (HIV-1) in vitro. The lymphocyte surface protein CD4 has been identified as a receptor for HIV-1 and binds the viral envelope glycoprotein (gp120). In the light of evidence indicating that one natural function of CD4 is as a growth factor receptor, we examined the ability of native gp120 to activate resting CD4-bearing lymphocytes. Our results indicate that gp120 has innate biological activity as a result of a specific interaction with CD4, inducing increases in intracellular levels of inositol trisphosphate and of calcium, and in interleukin-2 receptor expression and cell motility.

Persistence of early crosslink-dependent signal transduction events in human basophils after desensitization

Desensitization of human basophils with anti-IgE antibody or antigen induced an increased sensitivity to the phorbol ester TPA, evidenced as 2-5 fold increase in the potency of TPA to induce histamine release. As noted in previous publications the magnitude of the change in sensitivity to TPA was a function of the extent of cell surface IgE crosslinking. Thus, the density of cell surface antigen-specific IgE determined the magnitude of the curve shift and the multivalent antigen, BPO21-HSA was found to produce a greater curve shift than the simpler bivalent hapten, BPO2, in accord with previous studies which demonstrated that BPO2 was a "weak" stimulus compared to BPO21-HSA. Basophils which had been fully desensitized by prior treatment with anti-IgE or antigen in the absence of calcium also displayed the curve shift for at least 24 h after desensitization. However, the change induced by crosslinking which altered the cells' sensitivity to TPA required the continued presence of crosslinks and was therefore not the result of a permanent alteration induced by desensitization. Basophils which were fully desensitized to BPO7-HSA demonstrated the curve shift provided that the antigen-induced crosslinks were maintained: treatment with monovalent hapten, BPO-EACA, rapidly returned the cell response to TPA to control, non-desensitized, levels. Since TPA is thought to act by activation of protein kinase C (PKC) we demonstrated that BPO-HSA induced crosslinking increased PKC activity and that the increased activity persisted unless antigen-induced crosslinks were dissociated by the addition of monovalent hapten.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenaline stimulation of the phosphoinositide system: evidence for a novel hydrophobic inositol-containing compound in resistance arterioles

1. Five inositol phosphates were extracted from adult rat resistance arterioles and separated by ion-exchange high performance liquid chromatography. 2. By use of this technique, inositol phosphates liberated were identified as inositol 1-phosphate, inositol 1,4-bisphosphate, inositol 1,3,4-trisphosphate, inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Stimulation of phosphoinositide hydrolysis with noradrenaline produced increases in inositol phosphate production. 3. Three inositol-containing phospholipids extracted from resistance arterioles were measured as their glycerol esters following deacylation, thereby permitting an analysis of both membrane and cytosolic components of the phosphoinositide signalling system. 4. A substantial agonist-sensitive pool of a previously undescribed inositol but not glycerol-containing lipid extract component was also identified in this tissue. 5. These experiments for the first time allow a precise description of phosphoinositide metabolism in resting and agonist-stimulated resistance arterioles and provide data on a novel compound possibly similar to that recently described in other tissues.