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The Golgi apparatus (GA) is a dynamic store of Ca(2+) that can be released into the cell cytosol. It can thus participate in the regulation of the Ca(2+) concentration in the cytosol ([Ca(2+) ](cyt) ), which might be critical for intra-Golgi transport. Secretory pathway Ca(2+) -ATPase pump type 1 (SPCA1) is important in Golgi homeostasis of Ca(2+) . The subcellular localization of SPCA1 appears to be GA specific, although its precise location within the GA is not known. Here, we show that SPCA1 is mostly excluded from the cores of the Golgi cisternae and is instead located mainly on the lateral rims of Golgi stacks, in tubular noncompact zones that interconnect different Golgi stacks, and within tubular parts of the trans Golgi network, suggesting a role in regulation of the local [Ca(2+) ](cyt) that is crucial for membrane fusion. SPCA1 knockdown by RNA interference induces GA fragmentation. These Golgi fragments lack the cis-most and trans-most cisternae and remain within the perinuclear region. This SPCA1 knockdown inhibits exit of vesicular stomatitis virus G-protein from the GA and delays retrograde redistribution of the GA glycosylation enzymes into the endoplasmic reticulum caused by brefeldin A; however, exit of these enzymes from the endoplasmic reticulum is not affected. Thus, correct SPCA1 functioning is crucial to intra-Golgi transport and maintenance of the Golgi ribbon.

The Phormidium sp. ETS-05 thermophile blue-green alga is one of the most typical and widespread species of cyanobacteria of the thermal muds of the Euganean hot springs, the therapeutic properties of which have been known since ancient times. The polar diacylglycerolipids of this cyanobacterium consists of monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol. We have isolated and purified these four diacylglycerolipids from ETS-05, and then analysed them for their quantitative and structural features and fatty acid contents. The monogalactosyldiacylglycerol and digalactosyldiacylglycerol show a marked presence of polyunsaturated fatty acids, of which C18 : 4 is the most common. We propose that these glycoglycerolipids can be used as markers for monitoring the thermal mud colonisation process.

Glycerophosphoinositol 4-phosphate (GroPIns-4P) is a biologically active, water-soluble phospholipase A metabolite derived from phosphatidylinositol 4-phosphate, whose cellular concentrations have been reported to increase in Ras-transformed cells. It is therefore important to understand its biological activities. Herein, we have examined whether GroPIns-4P can regulate the organization of the actin cytoskeleton, because this could be a Ras-related function involved in cell motility and metastatic invasion. We find that in serum-starved Swiss 3T3 cells, exogenously added GroPIns-4P rapidly and potently induces the formation of membrane ruffles, and, later, the formation of stress fibers. These actin structures can be regulated by the small GTPases Cdc42, Rac, and Rho. To analyze the mechanism of action of GroPIns-4P, we selectively inactivated each of these GTPases. GroPIns-4P requires active Rac and Rho, but not Cdc42, for ruffle and stress fiber formation, respectively. Moreover, GroPIns-4P induces a rapid translocation of the green fluorescent protein-tagged Rac into ruffles, and increases the fraction of GTP-bound Rac, in intact cells. The activation of Rac by GroPIns-4P was near maximal and long-lasting. Interestingly, this feature seems to be critical in the induction of actin ruffles by GroPIns-4P.

The lysophospholipids are integral components of the plasma membrane that have often been considered as side products of the phospholipase A2 (PLA2)-dependent production of arachidonic acid and the deacylation/reacylation processes involved in phospholipid homeostasis. Data indicating roles of these lipid derivatives in hormone responses and cell transformation have now led to a different view, and the understanding of their involvement in the modulation of cell function is building up. Here, we will summarise the current knowledge concerning the biological roles of the lysophosphoinositides and the glycerophosphoinositols (their fully deacylated counterparts) in the framework of their known effects, and those of the other lysophospholipids and glycerophospholipids.

Signaling pathways involving the inositol polyphosphates and the polyphosphoinositides have become intricately linked with a number of disease states. More recently, this has principally involved the 3-phosphorylated products of phosphoinositide 3-kinase, an enzyme that itself shows oncogenic activity and has hence become of interest in the design of antitumorigenic drugs. The downstream effectors of phosphoinositide 3-kinase are involved in different aspects of cellular signaling and cytoskeleton and trafficking events that are linked to specific polyphosphoinositide binding properties of specific protein domains, which themselves have emerging roles in specific disease states. Our recent findings have demonstrated that there is a selectivity of the intracellular effects of extracellularly applied inositol polyphosphates in their abilities to inhibit a range of growth-related in vivo assay conditions, and that these can themselves be linked to the inhibition of the membrane localization of a green fluorescent protein (GFP) -tagged PH domain. We propose that GFP fusions of the polyphosphoinositides binding domains of specific proteins of interest can be used in high-throughput investigations of the therapeutic value of specific inositol polyphosphates analogs. Inhibition of in vivo membrane targeting of these domains from proteins involved in cell growth and tumorigenesis can thus be used in the search for new anticancer drugs.

The glycerophosphoinositols, phosphoinositide metabolites formed by Ras-dependent activation of phospholipase A2 and a lysophospholipase, have been proposed to be markers of Ras-induced cell transformation. These compounds can have important cellular effects; GroPIns4P is an inhibitor of G protein-stimulated adenylate cyclase and is transiently produced in several cell types after growth factor receptor stimulation of phosphatidylinositol 3-kinase and the small G protein Rac, indicating the importance of defining further its cellular actions and metabolism. We show here that, in postnuclear membranes from Swiss 3T3 cells, there is no high-affinity 'receptor' binding of GroPIns4P. Instead, possibly through the interaction with a transporter, GroPIns4P rapidly equilibrates between medium and cell cytosol, and, at higher concentrations, can concentrate in the cell cytosol. GroPIns4P can be dephosphorylated to GroPIns in vitro by an enzyme that is membrane-associated, Ca2+-dependent, GroPIns4P-selective and has a specific pH profile. Under in vitro phosphorylating conditions, there is production of GroPIns(4,5)P2 and other inositol phosphates. As these in vitro enzyme activities do not fully correlate with the in vivo handling of GroPIns4P, the intracellular GroPIns4P levels may be controlled by its direct physical removal from the cells.

In order to investigate the cellular function of the mammalian phosphatidylinositol transfer protein alpha (PI-TPalpha), NIH3T3 fibroblast cells were transfected with the cDNA encoding mouse PI-TPalpha. Two stable cell lines, i.e. SPI6 and SPI8, were isolated, which showed a 2- and 3-fold increase, respectively, in the level of PI-TPalpha. Overexpression of PI-TPalpha resulted in a decrease in the duration of the cell cycle from 21 h for the wild type (nontransfected) NIH3T3 (wtNIH3T3) cells and mock-transfected cells to 13-14 h for SPI6 and SPI8 cells. Analysis of exponentially growing cultures by fluorescence-activated cell sorting showed that a shorter G(1) phase is mainly responsible for this decrease. The saturation density of the cells increased from 0.20 x 10(5) cells/cm(2) for wtNIH3T3 cells to 0.53 x 10(5) cells/cm(2) for SPI6 and SPI8 cells. However, anchorage-dependent growth was maintained as shown by the inability of the cells to grow in soft agar. Upon equilibrium labeling of the cells with myo-[(3)H] inositol, the relative incorporation of radioactivity in the total inositol phosphate fraction was 2-3-fold increased in SPI6 and SPI8 cells when compared with wtNIH3T3 cells. A detailed analysis of the inositol metabolites showed increased levels of glycerophosphoinositol, Ins(1)P, Ins(2)P, and lysophosphatidylinositol (lyso-PtdIns) in SPI8 cells, whereas the levels of phosphatidylinositol (PtdIns) and phosphatidylinositol 4, 5-bisphosphate were the same as those in control cells. The addition of PI-TPalpha to a total lysate of myo-[(3)H]inositol-labeled wtNIH3T3 cells stimulated the formation of lyso-PtdIns. The addition of Ca(2+) further increased this formation. Based on these observations, we propose that PI-TPalpha is involved in the production of lyso-PtdIns by activating a phospholipase A acting on PtdIns. The increased level of lyso-PtdIns that is produced in this reaction could be responsible for the increased growth rate and the partial loss of contact inhibition in SPI8 and SPI6 cells. The addition of growth factors (platelet-derived growth factor, bombesin) to these overexpressers did not activate the phospholipase C-dependent degradation of phosphatidylinositol 4,5-bisphosphate.

Many and various experimental techniques have been developed to fully analyze the intracellular signaling pathways of membrane phosphoinositides and their water‐soluble derivatives. This chapter concentrates mainly on the range of lipid‐derived, water‐soluble signaling molecules that can be produced in cells from these membrane phosphoinositides, for which we and others have proposed biological roles. These include lysophosphatidylinositol, produced via phospholipase A 1/2 activities on phosphatidylinositol; cyclic inositol phosphates, produced via phosphatidylinositol/lysophosphatidylinositol‐specific phospholipase C activities; and glycerophosphoinositols, produced via lysophospholipase A 2/1 activities on their corresponding lysophosphoinositides. While the methodologies described in this chapter are aimed more specifically at the separation, identification, and quantification of monophosphorylated glyceropho sphoinositols and other similarly charged inositol‐containing products of the membrane phosphoinositides in cell extracts, they can be equally well applied tothe full range of lysophosphoinositides, glycerophosphoinositols, inositol phosphates, and further inositol‐containing water‐soluble products of the phosphoinositides (e.g., cyclic inositol phosphates, methylphosphoinositol phosphates).

The thermophilic blue-green alga ETS-05 colonises the therapeutic thermal muds of Abano and Montegrotto, Italy. Following the isolation, purification and identification of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulphoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol from ETS-05, we here examine their in vivo anti-inflammatory activities. MGDG, DGDG and SQDG inhibit croton-oil-induced ear oedema in the mouse in a dose-dependent manner. Inhibition by MGDG is greater than that of the reference drug, betamethasone 17,21-dipropionate, and is largely abrogated following acyl group saturation. SQDG is the least potent of these glycoglycerolipids, and shows an early transient effect. In the in vivo carrageenan-induced paw oedema model in the mouse, the inhibitory effects are again dose dependent, with an enhanced efficacy of MGDG over DGDG, SQDG and the reference drug, indomethacin. These compounds are all less toxic than indomethacin. The selective and enhanced inhibitory effects of MGDG over DGDG indicate the mechanisms behind these in vivo anti-inflammatory actions.

Glycerophosphoinositol (GroPIns) has been demonstrated to have important roles in many intracellular regulatory processes. GroPIns has been analysed for many years by anion-exchange HPLC after radiolabelling of cells in culture, but no method has been developed, to our knowledge, for the direct detection and quantitation of the unlabelled compound in such biological samples. Here is reported a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method for the direct quantitative analysis of GroPIns that can indeed be applied to cell extracts. Analyses were performed on a β-cyclodextrin-bonded HPLC column using a binary mobile phase of acetonitrile and 20 mM ammonium formate in water, which allowed direct on-line detection by tandem mass spectrometry in negative electrospray ionisation (ESI) mode. The method was applied to the quantitative analysis of GroPIns in selected rat cell lines after a two-phase acid extraction of cultured cells using external calibration. The potential matrix signal suppression effects were investigated by the parallel quantitation of GroPIns in extracts of selected cultured cell lines with both external calibration and the standard additions method. The accuracy data obtained demonstrated the feasibility of external calibration, so allowing a simpler and less time-consuming approach than that of the standard additions method.

Inositol-containing molecules are involved in important cellular functions, including signalling, membrane transport and secretion. Our interest is in lysophosphatidylinositol and the glycerophosphoinositols, which modulate cell proliferation and G-protein-dependent activities such as adenylyl cyclase and phospholipase A(2). To investigate the role of glycerophosphoinositol (GroPIns) in the modulation of Ras-dependent pathways and its correlation to Ras transformation, we employed a novel liquid chromatography-tandem mass spectrometry technique to directly measure GroPIns in cell extracts. The cellular levels of GroPIns in selected parental and Ras-transformed cells, and in some carcinoma cells, ranged from 44 to 925 microM, with no consistent correlation to Ras transformation across all cell lines. Moreover, the derived cellular inositol concentrations revealed a wide range ( approximately 150 microM to approximately 100 mM) under standard [(3)H]-inositol-loading, suggesting a complex relationship between the inositol pool and the phosphoinositides and their derivatives. We have investigated these pools under specific loading conditions, designing a further HPLC analysis for GroPIns, combined with mass determinations of cellular phosphatidylinositol 4,5-bisphosphate. The data demonstrate that limiting inositol conditions identify a preferred pathway of inositol incorporation and retention into the polyphosphoinositides pool. Thus, under conditions of increased metabolic activity, such as receptor stimulation or cellular transformation, the polyphosphoinositide levels will be maintained at the expense of phosphatidylinositol and the turnover of its aqueous derivatives.

Over the past ten years, our knowledge of the integral role that the phospho-inositide 3-kinases (PI3Ks) and their 3'-phosphorylated lipid products (3'-phosphorylated phosphoinositides; 3P-PIs) play in the mediation of signal transduction, cytoskeletal rearrangements and membrane trafficking has expanded considerably. They are now known to be involved in the regulation of cell growth, differentiation, mobility, proliferation and survival and hence they have become a potential target for the control of the growth and spread of cancer cells. More recently, the correlation of the multiplicity of isomers (both catalytic and regulatory) within the different classes of the PI3Ks with their functional relevance has become possible. This, combined with our further understanding of the protein recognition patterns for their different 3P-PIs and the newly-described pathways in the control of the levels of these by dephosphorylation, has provided new aspects and areas for interference in these multiple PI3K signalling pathways. However, in the search for effective, non-toxic, drugs for use in the treatment of cancers, these individual targets for PI3K inhibition need to be further correlated with the specific in vivo effects on cell survival, invasivity and metastatic potential. Here, the range of PI3K inhibition targets are discussed in the light of recent experimental findings, with a view to the exploitation of their specificities in new approaches to effective cancer treatments based on PI3K activity inhibition.

Upon engagement of TCR with peptide-MHC complexes displayed on the surface of antigen-presenting cells, T lymphocytes undergo a sustained elevation of intracellular Ca(2+) concentration([Ca(2+)](i)), which is required for cytokine production. In the present work, we investigate how inositol lipid metabolism can be activated for a prolonged time to ensure a sustained link between receptor triggering and downstream signaling effectors. Four lines of evidence indicate that an extensive phosphoinositide turnover induced by TCR and CD28 engagement allows this task to be accomplished: (i) continuous phosphoinositide breakdown is required for a sustained [Ca(2+)](i )increase in antigen-stimulated T cells; (ii) TCR triggering results in a continuous release of inositol phosphates from the cell membrane paralleled by a massive and sustained phosphoinositide re-synthesis due to free inositol re-incorporation; (iii) TCR-induced phosphoinositide turnover is strongly increased by CD28 ligation; and (iv) CD28 engagement augments and sustains the TCR-induced [Ca(2+)](i )increase. Our results show that the T cell pool of phosphoinositides is continuously re-formed during T cell-APC cognate interaction, thereby explaining how sustained receptor triggering can transduce an equally sustained [Ca(2+)](i) increase. Importantly, our data identify a novel step in the signaling cascade where co-stimulation converges with TCR-generated signals to sustain and amplify the activation process.

Previously it has been shown that injecting a cytosolic sperm protein factor into mammalian eggs induces sustained repetitive transients of cytosolic free Ca2+ ([Ca2+]i), or [Ca2+]i oscillations [Swann (1990) Development 110, 1295-1302]. These sperm-factor (SF)-induced [Ca2+]i oscillations are similar to those seen at fertilization. Here we demonstrate that injecting the same cytosolic extracts of mammalian sperm into single rat hepatocytes induces a series of [Ca2+]i oscillations, as measured by aequorin luminescence. SF injection into hepatocytes induced [Ca2+]i oscillations that were of longer duration, lower frequency and greater amplitude than those seen with the Ins(1,4,5)P3-generating agonist phenylephrine. The SF-induced [Ca2+]i responses appeared to be due to internal release of Ca2+, since transients could occur in Ca2+-free media. Addition of the phorbol ester phorbol 12,13-dibutyrate (PDBu) at low concentrations did not inhibit the SF-induced [Ca2+]i oscillations; high concentrations of PDBu led to a sustained increase in [Ca2+]i concentrations. These data demonstrate that sperm contain a protein factor capable of inducing a characteristic series of [Ca2+]i oscillations in a somatic cell, the hepatocyte. Along with previous observations in dorsal root ganglion neurons, the data suggest a widespread efficacy of the factor in triggering Ca2+ oscillations.

Cell Membrane Lipids and Signal Transduction Over the past two decades, the build-up of a complex understanding of the metabolism of cell membrane lipids has seen the development of the roles of these lipids from solely ‘passive’ structural components of the membrane bilayer to ‘active’ regulators of cell function. These roles have included the fields of lipid-protein interaction, membrane biogenesis and membrane transport (for reviews see Casey, 1995; Liscovitch and Cantley, 1995; Milligan et al., 1995), but it is in that of signal transduction (for reviews see Bell and Burns, 1991; Nishizuka, 1992, 1995; Billah, 1993; Divecha and Irvine, 1995; Liscovitch and Cantley, 1995; Milligan et al., 1995; Corda and Falasca, 1996) where novel lipid second messengers continue to be described (Panayotou and Waterfield, 1992; Cuadrado et al., 1993; Hannun, 1994; Su et al., 1994; Moolenaar, 1995; Pushkareva et al., 1995; Falasca et al., 1996a). Thus membrane lipids are acted upon by intracellular phospholipases (phospholipase A2 (PLA2), phospholipase C (PLC) and phospholipase D), leading to the production of lipid second messengers that are involved in the modulation of different functional cellular responses (Exton, 1990; Dennis et al., 1991; Cockcroft and Thomas, 1992; Nishizuka, 1992; Berridge, 1993; Billah, 1993; Glaser et al., 1993; Iacovelli et al., 1993; Mayer and Marshall, 1993; Dennis, 1994; Divecha and Irvine, 1995; Lee and Rhee, 1995). Activation of one of these phospholipases, PLA2, leads to the mobilisation of arachidonic acid from an sn-2 linkage of phosphatidylcholine (PtdCho), phosphatidyl-ethanolamine (PtdEth) and phosphatidylinositol (PtdIns) in numerous mammalian cell types (Van den Bosch, 1980; Irvine, 1982), which, in turn, serves as a substrate for the production of other cellular second messengers, such as the prostaglandins and leukotrienes (Piomelli and Greengard, 1990; Piomelli, 1993; Khan et al., 1995). In the case of PLA2 activation, this is also accompanied by the formation of biologically active lysolipids that have been shown to be mitogens in several cell lines, including lysophosphatidic acid (LysoPtdCOOH; Moolenaar, 1995), lysophosphatidylinositol (LysoPtdIns; Falasca and Corda, 1994) and lysophosphatidylcholine (LysoPtdCho; Asaoka et al., 1991). These lysolipids themselves are the substrates for lysophospholipases that release the fatty acid from the sn-1 linkage, thus producing glycerophospholipids.

We describe the reconstitution and purification of a membrane-associated phosphoinositide-specific phospholipase C (PIC) from turkey erythrocyte ghosts. This PIC is responsive to a G-protein coupled to P2y purinergic receptors which are expressed in turkey erythrocytes. Reconstitution is achieved by adding partially purified PIC to [3H]inositol-prelabelled turkey erythrocyte membranes depleted of their endogenous PIC (acceptor membranes). PIC activity is associated with a 52 kDa polypeptide on SDS-polyacrylamide gel electrophoresis. Addition of a 307-fold purified enzyme to the acceptor membranes has no effect on basal PIC activity, but markedly increases the response to GTPγS and P2y-purinergic receptor activation.

1. A screen for agonists capable of stimulating the formation of inositol phosphates in erythrocytes from 5-day-old chickens revealed the presence of a population of phosphoinositidase C-linked purinergic receptors. 2. If chicken erythrocytes prelabelled with [3H]Ins were exposed to a maximal effective dose of adenosine 5′-[beta-thio]diphosphate for 30 s, the agonist-stimulated increment in total [3H]inositol phosphates was confined to [3H]Ins(1,4,5)P3, Ins(1,3,4,5)P4 and InsP2. After 40 min stimulation, the radiolabelling of nearly all of the [3H]inositol phosphates that have been detected in these extracts [Stephens, Hawkins & Downes (1989) Biochem. J. 262, 727-737] had risen. However, some of these increases [especially those in Ins(3,4,5,6)P4 and Ins(1,3,4,5,6)P5] were accountable for almost entirely by increases in specific radioactivity rather than in mass. 3. The effect of purinergic stimulation on the rate of incorporation of [32P]Pi in the medium into the gamma-phosphate group of ATP and InsP4 and InsP5 was also measured. After 40 min stimulation, the incorporation of 32P into Ins(1,3,4,6)P4, Ins(1,3,4,5)P4, Ins(3,4,5,6)P4 and Ins(1,3,4,5,6)P5 was significantly elevated, whereas the mass of the last two and the specific radioactivity of the gamma-phosphate of ATP were unchanged compared with control erythrocyte suspensions. 4. In control suspensions of avian erythrocytes, the specific radioactivity of the individual phosphate moieties of Ins(1,3,4,6)P4 increased through the series 1, 6, 4 and 3 [Stephens & Downes (1990) Biochem. J. 265, 435-452]. This pattern of 32P incorporation is not the anticipated outcome of 6-hydroxy phosphorylation of Ins(1,3,4)P3 [the assumed route of synthesis of Ins(1,3,4,6)P4]. Although adenosine [beta-thio]diphosphate significantly stimulated the accumulation of [3H]Ins(1,3,4)P3, and despite the fact that avian erythrocyte lysates were shown to possess a chromatographically distinct, soluble, ATP-dependent, Ins(1,3,4)P3 6-hydroxykinase activity, purinergic stimulation of intact cells did not significantly alter the pattern of incorporation of [32P]Pi into the individual phosphate moieties of Ins(1,3,4,6)P4. These results suggest that the route of synthesis of this inositol phosphate species is not changed during the presence of an agonist.

Stable, soluble preparations of rat brain muscarinic receptors can be prepared by extracting membranes with digitonin, or with combinations of sodium cholate and sodium chloride. The stability of the cholate/NaCl extract is enhanced by the addition of egg phosphatidylcholine, which, at the same time, suppresses the considerable dispersity apparent in the hydrodynamic behaviour of the solubilised receptor. The Stokes radius of the brain muscarinic receptor in cholate/NaCl/lecithin extracts is 6.7 nm, with very similar values in other detergents, including digitonin and sodium dodecyl sulphate. Its sedimentation coefficient is 3.78s, and its molecular weight approximately 110,000 after correction for detergent binding. The isoelectric point of the digitonin - solubilised receptor is approximately 4.5.

An apparently monomeric form of the digitonin-solubilized muscarinic acetylcholine receptor from the rat cerebral cortex retains a high affinity of 7 X 10(7) M-1 for pirenzepine. Muscarinic receptor binding sites in the rat cerebral cortex with a low affinity for pirenzepine are solubilized with relatively little change in affinity. The ability of pirenzepine to distinguish between subtypes of muscarinic binding site in the cerebral cortex is manifest in both the membrane-bound and soluble state.

Summary The binding properties of muscarinic receptors in homogenates of the rat myocardium are altered in the presence of guanine nucleotides. There is a small increase in the affinity constant of the antagonist, [ 3 H]-N-methylscopolamine, and a large decrease in agonist binding. The most potent nucleotide is Gpp(NH)p, with GTP and GDP both being about ten times less active. The effect of GTP on agonist binding is transient whereas that of Gpp(NH)p is more persistent.

Some antagonists exhibit tissue selectivity in their pharmacological antagonism of muscarinic responses. However, the affinity constants for equilibrium binding of classical antagonists to muscarinic receptors in subcellular preparations have shown only small variations in different peripheral tissues and regions of the brain. The binding curves do not deviate significantly from the simple Langmuir isotherm, indicating apparent homogeneity of the receptor population in any given region. In contrast, heterogeneity has been detected by agonist binding studies but this may arise from different environmental or coupling restraints on the agonist-induced conformational change and cannot be taken as evidence for different receptor subtypes. We report here binding studies using a new anti-muscarinic drug, pirenzepine, in which we found heterogeneity of binding that correlates well with the pharmacological activity.

EDTA treatment of membrane preparations from rat brain and myocardium reduced the relative proportion of superhigh and high affinity binding sites for muscarinic agonists by up to 60%. This effect was partially or completely reversed by millimolar concentrations of Mg2+ or Mn2+. A number of multivalent metal cations gave qualitatively similar effects, yielding stimulation of agonist binding at low concentrations but inhibition at higher concentrations. The divalent cation-linked subpopulation of muscarinic agonist binding sites identified in this study appears to be the primary target for guanine nucleotide inhibition.

Muscarinic receptors from rat myocardial membranes may be solubilized by digitonin in good yield at low temperatures in the presence of Mg2+. Under these conditions, up to 60% of the soluble receptors show high affinity binding for the potent agonist [3H]-oxotremorine-M (KA = 10(9)M-1), which is inhibited by 5'-guanylylimidodiphosphate. The muscarinic binding site labelled with [3H]-oxotremorine-M has a higher sedimentation coefficient (13.4 s) than sites labelled with a 3H antagonist in the presence of guanylylimidodiphosphate (11.6 s) and probably represents a complex between the ligand binding subunit of the receptor and a guanine nucleotide binding protein.

An examination of the binding of antagonists to the receptors in rat cerebral cortex has shown that this occurs to an apparently uniform set of sites (8). Furthermore, the binding constants so obtained are in close agreement with those estimated by antagonism of the contraction of the longitudinal smooth muscle of guinea pig ileum. However, it has been reported by Barlow et al. (1) that the activity of some antagonists on guinea pig heart was not identical to that on the ileum.

Three distinct types of detergent have been found to be capable of solubilizing muscarinic binding activity from the rat forebrain. These are: (a) the alkylpolyoxyethylenes such as Lubrol PX [1]; (b) steroids such as cholate [2, 3], digitonin [4, 5] and the cholate sulphobetaine derivative CHAPS [6]; and (c) lysophosphatidylcholine [7]. In such studies, it has proved convenient to compare the overall yield of solubilized receptors from brain membranes (P2 and P3) pre-labelled with the irreversible alkylating affinity label [3H]propylbenzilylcholine (3H-PrBCM) [8] with the yield of active binding sites which can be assayed in the supernatant after extraction of non-pre-labelled membranes. Solubilized binding sites were conveniently measured by incubating 600,000 g-min supernatants with receptor-saturating concentrations (3 × 10-8 M) of (-)-N-[Me-3H]- scopolamine (3H-NMS) or (-)-[3H]quinuclidinyl-benzilate (3H-QNB), followed by separation of bound from free ligand by rapid gel filtration on small columns of Sephadex G-50 M [3].

The muscarinic acetylcholine receptor (mAChR) is a member of a class of neurotransmitter and hormone receptors which can activate more than one type of transmembrane signalling system. In the case of the mAChR, established mechanisms include (a) GTP-binding-proteinmediated inhibition of adenylate cyclase (Jakobs et al., 1979), (b) stimulation of the breakdown of triphosphoinositides (Jacobson et al., 1985), and (c) the opening of a certain category of K+ channels (Soejima & Noma, 1984). In each of these cases, there is evidence for the activation of distinct membrane-bound effector species by the agonistreceptor complex. The mAChR thus provides an example of a divergence of biochemical action at the level of effectuation of the response. The converse situation is also common, an example being the convergence of mAChR, crl -adrenoceptors and substance P receptors on the same pool of mobilizable intracellular Ca2+ in parotid gland cells (Putney , 1979). A likely implication of such functional diversity is that receptors such as the mAChR are, essentially, modular constructs, in which ligand-binding domains of different specificities exposed on the extracellular surface are linked to distinct effector recognition domains, or catalytic sites exposed to the intracellular milieu. Appropriate structural analogies may be provided by tyrosine kinases such as the epidermal growth factor and insulin receptors (Ullrich et al., 1984, 1985), by proteins of the major histocompatibility complex (Hood et al., 1982) or by immunoglobulins (Tonegawa, 1983). Conformational differences in the ligand-binding site induced by linkage to a variety of effector recognition sites might provide an explanation for the origin of the 'subtypes' detectable by selective ligands, in the case of the mAChR, by the selective antagonist pirenzepine (Pz) (Hammer et al., 1980; Birdsall & Hulme, 1983). However, conformational effects on the ligandbinding site could equally result from differences in effector coupling, or tissue-specific post-translational modifications, thus leading to the appearance of pharmacological subtypes without recourse to differences in the primary structure of the receptor. Convincing differentiation between these possibilities requires structure determination, and in particular the primary amino acid sequence of the receptor. This provides a major impetus for the purification, partial sequencing and cloning of receptors such as the mAChR. Further stimulus is provided by the prospect of obtaining monoclonal antibodies directed against different parts of the receptor sequence which may be used to detect structural homologies between different receptors and their subtypes (Andre et al., 1984; Venter et al., 1984), and which hold out the prospect of detailed immunohistochemical studies of receptor localization. To this end, we have purified the mAChR from rat forebrain by a straightforward procedure which exploits the properties of the o-atninobenzhydry

New efforts in cancer therapy are being focused at various levels of signaling pathways. With phosphoinositide 3-kinase (PI3-K) potentially being necessary for a range of cancer-related functions, we have investigated the influence of selected inositol tris- to hexakisphosphates on cell growth and tumorigenicity. We show that micromolar concentrations of inositol 1,3,4,5,6-pentakisphosphate and inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P(4)] inhibit IGF-1-induced [(3)H]-thymidine incorporation in human breast cancer (MCF-7) cells and the ability to grow in liquid medium and form colonies in agarose semisolid medium by small cell lung cancer (SCLC) cells, a human cancer cell line containing a constitutively active PI3-K. In an ovarian cancer cell line that also contains a constitutively active PI3-K (SKOV-3 cells), Ins(1,4,5,6)P(4) again inhibited liquid medium growth. Furthermore, when applied extracellularly, inositol 1,3,4,5-tetrakisphosphate was shown indeed to enter SCLC cells. These effects appeared specifically related to PH domains known to bind to phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P(2)] and phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)], indicating involvement of the PI3-K downstream target protein kinase B (PKB/Akt). This was further supported by inhibition of PKB/Akt PH domain membrane targeting in COS-7 cells by Ins(1,4,5,6)P(4). Thus, we propose that specific inositol polyphosphates inhibit PI3-K by competing with PtdIns(3,4, 5)P(3)-binding PH domains and that this occurs mainly at the level of the downstream PI3-K target, PKB/Akt.