The role of protein kinase C in cell surface signal transduction and tumour promotion

Interdependent effects of the ionophore A23187 and serum on the serotonin N-acetyltransferase activity of cultured chick pineal glands

Marked effects of the ionophore A23187 on the cycle of N-acetyltransferase (NAT) activity in cultured chick pineal glands were observed under three conditions of illumination. However, the effects were qualitatively and quantitatively dependent on the batch of fetal calf serum used in the medium and time of explanation into culture. Ionophore increased the level of NAT activity remaining in glands exposed prematurely to light regardless of the serum used. The ionophore suppressed the "spike" in cyclic GMP content of glands cultured in the dark, and extended the period of maximum cyclic GMP content of glands under diurnal illumination.

Phosphorylation of a tropomyosin-like (30 KD) protein during platelet activation

In this study, we have used the tumor promoter 12-o-tetradecanoylphorbol-13-acetate (TPA), as well as its biologically inactive analogue 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), to investigate platelet protein phosphorylation and its possible correlation with platelet activation. Our data show that TPA, but not 4 alpha-PDD, induces a preferential phosphorylation of a 30,000 dalton (30 KD) protein. This phosphoprotein is found to be physically associated with an actomyosin-containing platelet cytoskeleton complex. Further analysis using both standard two-dimensional gel electrophoresis and one-dimensional urea-SDS gel electrophoresis reveals that this 30 KD protein has several tropomyosin-like properties. Most importantly, the degree of TPA-induced phosphorylation of the 30 KD protein is directly proportional to the extent of platelet granule release and the shape change of the platelet, as well as to the degree of aggregation. We speculate that this phosphorylated tropomyosinlike protein may play a pivotal role in the regulation of actomyosin-mediated platelet contractility, which has been previously implicated in a variety of platelet functions.

Activators of protein kinase C and 5-azacytidine induce IL-2 receptor expression on human T lymphocytes

Resting human T lymphocytes do not express receptors for interleukin-2, but expression is rapidly induced by exposure to PHA. After maximal expression 2-3 days after stimulation, a progressive decline in receptor number is observed. Receptor expression can be augmented by reexposure to PHA. In this study we show that activators of protein kinase C including phorbol diester, phospholipase C, and the diacylglycerol congener diC8 also increase IL-2 receptor expression. Moreover, 5-azacytidine, which inhibits cytosine methyltransferase, and hydroxyurea, which inhibits ribonucleotide reductase, also increased receptor number. These studies demonstrate that IL-2 receptor expression can be altered in vitro, and that IL-2 receptor number, in combination with IL-2 secretion, may contribute to the regulation of IL-2-dependent immune responses.

Platelet activation

Platelets are discoidal cytoplasmic particles that respond to a variety of stimuli by developing filopodia and rounding up (shape change), developing the ability to bind fibrinogen from the medium, and, with strong stimuli such as thrombin and PAF-acether, secreting the contents of several types of granules. Arachidonic acid is cleaved from phospholipids by phospholipase A2 and converted by the platelets to endoperoxides, and then to thromboxane A2. The bound dimeric fibrinogen molecules probably cause aggregation by forming bridges between platelets. Aggregation is reinforced by secreted fibrinogen and thrombospondin, which binds the platelets, and by thromboxane A2 and endoperoxides, as well as secreted ADP, which cause additional receptor-mediated activation. The responses to these stimuli are initiated when the agonists bind to specific receptors on the plasma membrane. Subsequent steps resemble those in other types of responsive cells: breakdown of phosphatidylinositol bisphosphate into diacylglycerol, a stimulator of protein kinase C, and inositol-1,4,5-trisphosphate, recently shown to be a potent calcium ionophore. The response of shape change results from increased cytoplasmic Ca2+ which permits phosphorylation of one of the light chains of myosin by a calcium-calmodulin-dependent kinase, with resulting enhanced actin-myosin interaction. Secretion is associated with phosphorylation of a 40,000 to 47,000 dalton protein by the diacylglycerol-activated protein kinase C. These recent findings have increased our understanding of the mechanisms of platelet activation, but much remains to be learned. How do agonist-receptor complexes influence PIP2 breakdown? Is this indeed the first step in activation? What mediates adhesion of platelets to the injured blood vessel wall? Does transduction of this stimulus occur by the same mechanism as transduction of commonly used soluble stimuli? What is the role of the phosphorylated 40-47 K protein in secretion? What change in GP IIb-IIIa promotes their ability to bind fibrinogen? What is the role of calcium-activated protease? Of the phosphorylation of actin-binding protein? Progress is being made rapidly, and these questions may be answered within a few years.

Phosphorylation of ribosomal protein S6 on serine after microinjection of the Abelson murine leukemia virus tyrosine-specific protein kinase into Xenopus oocytes

Phosphorylation of ribosomal protein S6 in NIH 3T3 fibroblasts is dependent on the presence of serum, but after transformation of these cells by Abelson murine leukemia virus (Ab-MuLV), S6 remained highly phosphorylated on serine residues either in the absence or the presence of serum. To investigate whether S6 phosphorylation in this system was a consequence of the action of the Ab-MuLV tyrosine-specific protein kinase, purified Ab-MuLV kinase made in Escherichia coli was microinjected into Xenopus oocytes and was observed to cause a 7- to 15-fold increase in the phosphorylation of S6 on serine residues. Two-dimensional phosphopeptide maps of S6 phosphorylated in Ab-MuLV-transformed NIH cells in the absence of serum were identical to those of S6 isolated from normal cells grown in the presence of serum. In addition, S6 from oocytes injected with Ab-MuLV kinase yielded an S6 phosphopeptide map indistinguishable from that of serum-stimulated NIH 3T3 cells, whereas S6 from control oocytes lacked several phosphopeptides. Ab-MuLV kinase did not phosphorylate S6 directly in vitro, and microinjection of a mutant Ab-MuLV protein lacking kinase activity had no effect. These results indicate that the Ab-MuLV kinase interacts with a cellular pathway to enhance S6 phosphorylation by directly or indirectly activating an S6 protein kinase and/or inactivating an S6 protein phosphatase.

Converging adrenergic and cholinergic mechanisms in the inhibition of Cl secretion in fish opercular epithelium

The rate of Cl secretion (Isc) by the opercular epithelium of Fundulus heteroclitus is stimulated by elevations in cyclic AMP (cAMP) levels elicited via beta 1-adrenergic receptor activation, and inhibited by both alpha 2-adrenergic and muscarinic cholinergic receptor activation via mechanisms presently unknown. A comparison of these two inhibitory responses was made using clonidine, an alpha 2-adrenergic agonist, and acetylcholine (ACh), a cholinergic agonist. The dose required for maximum inhibition was 100 times greater for ACh, but in all other respects the responses elicited by both agonists were statistically indistinguishable. Adrenergic antagonists did not diminish the ACh inhibition, and cholinergic antagonists did not diminish the clonidine inhibition, indicating that the two receptor types were distinct from each other. In control tissues and tissues pretreated with agents that increase cAMP levels (isoproterenol, IBMX, forskolin), both ACh and clonidine had no effects on cyclic AMP levels, indicating an inhibitory mechanism independent of adenylate cyclase. Neither Ca-free media nor a variety of calcium antagonists diminished the ACh or clonidine inhibitions. These results suggest that the alpha 2-adrenergic and muscarinic cholinergic pathways converge into a common pathway to inhibit Cl secretion by a mechanism not involving adenylate cyclase or the mobilization of either extracellular or intracellular calcium stores.

Phorbol ester-induced contraction of arterial smooth muscle and inhibition of alpha-adrenergic response

The influence of the phorbol ester, 12-o-tetradecanoylphorbol-13 acetate (TPA) on the contractility of rat and rabbit aorta was assessed. At 10(-8) to 10(-5)M TPA produced a gradual contraction of rat aorta, while rabbit aorta gave only a small contraction at 10(-5)M. Contractions were irreversible and dependent on extracellular Ca2+. Associated with TPA-induced contraction, alpha-adrenergic receptor responses of the rat were inhibited in a noncompetitive manner, while rabbit responses were little affected. Since TPA activates phospholipid-dependent protein kinase C in a manner analogous to endogenous diacylglycerol, a critical role for the latter in alpha-adrenergic events is suggested.

Ca2+-dependent and phorbol ester activating phosphorylation of a 36K-dalton protein of rat basophilic leukemia cell membranes and immunoprecipitation of the phosphorylated protein with IgE-anti IgE system

Endogeneous phosphorylation of rat basophilic leukemia cell membranes was investigated. EGTA specifically inhibited the phosphorylation of a protein having an approximate molecular weight of 36,000 dalton (36K-Da protein). Phosphorylation of this protein was enhanced by phorbol-12-myristate-13-acetate in the presence of phosphatidylserine. The phosphorylated 36K-Da protein was specifically immunoprecipitated with IgE and anti IgE antibody. These results suggest that the phosphorylated 36K-Da protein is the beta-chain of the receptor for IgE and that protein kinase C is involved in the phosphorylation mechanism.

Phospholipase activity and plasma membrane homeostasis

Cyclic changes in the physiochemical state of the plasma membrane appear to be necessary for the normal functioning of the cell, especially with respect to division and differentiation. Such changes require a flexible membrane lipid composition to permit the necessary sequence of physicochemical changes to occur during these cellular events. This flexibility can be lost as a result of peroxidation-induced cross linking of membrane constituents, which prevents the normal physico-chemical membrane changes from occurring, resulting in abnormal cellular function. It is proposed that phospholipase A2 and C form a mutually regulatory enzyme system playing an important role with respect to the maintenance of membrane composition and flexibility.

Long-chain unsaturated diacylglycerols cause a perturbation in the structure of phospholipid bilayers rendering them susceptible to phospholipase attack

Intracellular phospholipases (A2 and C types) can hydrolyse bilayer-forming phospholipids much more rapidly when diacylglycerol is added. Unsaturated long-chain diacylglycerols are much more effective than short-chain saturated diacylglycerols or 1-oleoyl,2-acetylglycerol. Diacylglycerol does not change the electrokinetic properties of the phospholipid water interface, nor does it enhance enzymic digestion of monolayers. 31P-NMR of phosphatidylcholine indicates that diacylglycerol causes an isotropic component to develop in the spectrum of the bilayers which correlates approximately with the enhancement of phospholipase A2 attack. Addition of further diacylglycerol converts this transitional stage of unknown structural origin to the hexagonal II phase with a total loss of enzyme activity.

Adrenocortical cytochrome P-450 responsible for cholesterol side chain cleavage (P-450scc) is phosphorylated by the calcium-activated, phospholipid-sensitive protein kinase (protein kinase C)

Purified bovine adrenocortical cytochrome P-450scc (specific for cholesterol side chain cleavage in the inner mitochondrial membrane) was selectively phosphorylated in vitro by a Ca2+-activated, phospholipid-sensitive protein kinase (protein kinase C) preparation, whereas cyclic AMP dependent and two cyclic nucleotide independent kinases were ineffective. Cytochrome P-450scc incorporated a maximum of 4 mol of phosphate in the presence of protein kinase C within 15 min at 30 degrees C, with apparent Km and Vmax of 0.14 mumol and 0.76 pmol/min, respectively. Serine and threonine were the two target aminoacids phosphorylated in a ratio of about 1:1. In the presence of 1 microM Ca2+, a mixture of phosphatidylserine and diolein (or a potent tumor promoter phorbol ester) was required for optimal cytochrome P-450scc phosphorylation. In addition, purified inner mitochondrial membrane preparations from adrenocortical mitochondria were found to contain protein kinase C activity. These findings, together with the previous demonstration that activators of protein kinase C such as a potent phorbol ester activates steroidogenesis of intact adrenocortical cells, suggest that phosphorylation of P-450scc should be examined for its possible role in the regulation of adrenocortical functions.

Neurotensin stimulates inositol phospholipid hydrolysis in rat brain slices

Neurotensin stimulated inositol phospholipid hydrolysis in matched coronal vibratome sections through the rat brain. This effect was tetrodotoxin-resistant and a good correlation was noticed between the magnitude of neurotensin-stimulated inositol phospholipid hydrolysis and the number of specific [3H]neurotensin binding sites in various brain regions. Neurotensin produced no significant effect on either basal or stimulated cAMP levels.

The mouse ear edema: a quantitatively evaluable assay for tumor promoting compounds and for inhibitors of tumor promotion

The induction of edema in the mouse ear has been established as a reliable in vivo assay for tumor promoting compounds and for inhibitors of tumor promotion. Apparently all skin tumor promoting compounds induce an edema, and all inhibitors of a tumor promoter-induced edema are most likely inhibitors of skin tumor promotion. No exception to this rule has been found as yet. Besides the application of this assay in the screening of compounds, the assay allows comparison of various compounds (e.g. complete and incomplete tumor promoters) with respect to their kinetics and dosage response in the induction of edema.

Priming of neutrophils for enhanced release of oxygen metabolites by bacterial lipopolysaccharide. Evidence for increased activity of the superoxide-producing enzyme

We investigated the capacity of bacterial endotoxin (lipopolysaccharide, LPS) to modify the oxidative metabolic response to membrane stimulation of human neutrophils. Neutrophils were pretreated for 60 min with LPS, 10 ng/ml, then stimulated by exposure to fixed immune complexes, the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP), or phorbol myristate acetate. Release of superoxide anion (O-2) was up to 7-times greater in cells preincubated with LPS, depending upon the stimulus used. Consumption of oxygen and release of hydrogen peroxide (H2O2) were similarly increased, using FMLP as stimulus. The enhancement was accompanied by a reduction in lag time and an increase in the rate of the response, but the duration of the oxidative events was not changed. The molecular basis for the augmented oxidative response of LPS-pretreated cells was investigated. Preincubation with LPS at 0 degrees C prevented priming, but preincubation in the presence of cycloheximide or chelation of extracellular calcium ion did not. Neutrophils preincubated with LPS had slightly decreased numbers of binding sites and equivalent binding affinity for radiolabeled FMLP. Possible changes in the enzyme responsible for the oxidative burst were analyzed by studying NADPH-dependent generation of O-2 by particulate fractions from cells preincubated with LPS or buffer, then stimulated before cell disruption. The fraction prepared from LPS-pretreated neutrophils exhibited greater release of O-2 over a wide range of concentrations of NADPH. The calculated apparent Km for NADPH was equivalent in the two fractions, but the Vmax was increased 2.5-fold in the subcellular fraction from LPS-pretreated cells. These results suggest that LPS could increase neutrophil-mediated host defense or the tissue damage associated with endotoxemia by enhancing the generation of oxygen metabolites by neutrophils. These results also support the concept that the neutrophil is not an end-stage cell in regard to function or metabolic activity.

Phorbol ester stimulation of insulin release and secretory-granule protein phosphorylation in a transplantable rat insulinoma

The effects of tumour-promoting phorbol esters on protein-phosphorylation reactions and secretion in rat insulinoma tissue were investigated with the objective of assessing the possible role of Ca2+- and phospholipid-dependent protein kinases (protein kinase C) in insulin release. 4 beta-Phorbol 12-myristate 13-acetate (TPA) was a potent secretagogue at concentrations above 0.1 microM. TPA-induced release was inhibited by adrenaline or omission of Ca2+ from the extracellular medium and was augmented by theophylline. These findings suggested that TPA activated an exocytotic process. TPA enhanced the Ca2+- and phospholipid-dependent phosphorylation of histone III-S by a soluble protein fraction of the tissue. Endogenous phosphorylation reactions involving soluble and secretory-granule membrane proteins were also stimulated by TPA in tissue homogenates and reconstituted subcellular fractions. Histone phosphorylation and the granule-protein phosphorylation reactions showed similar concentration-dependencies for activation by both Ca2+ and TPA, thus indicating that the same enzyme was involved. It is concluded that the phosphorylation of cytosolic and membrane protein substrates by protein kinase C may be important in the stimulus-secretion coupling mechanism of insulin release.

Cyclic AMP inhibition of fMet-Leu-Phe-dependent metabolic responses in human neutrophils is not due to its effects on cytosolic Ca2+

Cyclic AMP powerfully inhibits the fMet-Leu-Phe-dependent respiratory burst and exocytosis of azurophilic and specific granules without affecting Ca2+ release from intracellular stores. The elevation of [Ca2+]i induced by fMet-Leu-Phe is short-lived in cyclic AMP-treated cells and similar to that of untreated cells stimulated in the absence of external Ca2+. Nevertheless, in these latter cells fMet-Leu-Phe induces metabolic activation. We therefore suggest that the inhibitory action of cyclic AMP on neutrophil responses is not due to its effects on [Ca2+]i homoeostasis.

Synergism between A23187 and 1-oleoyl-2-acetyl-glycerol in superoxide production by human neutrophils

Concentrations of the calcium ionophore A23187 and 1-oleoyl-2-acetyl-glycerol, which on their own are minimally effective in stimulating superoxide release from human neutrophils, show marked mutual potentiation when given simultaneously. The potentiating effect of the diacylglycerol can be shown to be dose-related. These results support the hypothesis that synergism between cytosolic calcium and protein kinase C is involved in signal transduction for the respiratory burst in the human neutrophil.

1-(5-Isoquinolinesulfonyl)-2-methylpiperazine (H-7) is a selective inhibitor of protein kinase C in rabbit platelets

Effects of 1-(5-isoquinolinesulfonyl)-2-methylpeperazine (H-7), a potent inhibitor of protein kinase C in vitro (1), were investigated with regard to stimulus-induced protein phosphorylation of rabbit platelets. While H-7 inhibited the protein kinase C-mediated phosphorylation in 12-0-tetradecanoylphorbol-13-acetate (TPA)-stimulated platelets, this compound did not block the Ca2+-calmodulin-dependent phosphorylation in Ca2+ ionophore A23187-stimulated cells. This selective inhibitor of protein kinase C, in intact cells, will facilitate studies on the biological functions of protein kinase C.

Phorbol esters as probes of the regulation of thyrotropin secretion

12-0-Tetradecanoylphorbol 13-acetate and thyrotropin releasing hormone were compared as thyrotropin secretagogues in cultured rat pituitary cells. The maximal secretion evoked by the former was only half that evoked by the latter. A23187 plus 12-0-tetradecanoylphorbol 13-acetate together produced as large a response as thyrotropin releasing hormone. Triiodothyronine inhibited phorbol ester-induced thyrotropin secretion, and this inhibition required protein synthesis.

Chlorpromazine: a potential anticancer agent?

The antipsychotic drug chlorpromazine causes scission of the DNA in PY815 mouse mastocytoma cells or isolated PY815 cell nuclei and the broken DNA reseals when chlorpromazine is removed from nuclei. These properties suggest that chlorpromazine interferes with topoisomerase action as do several other DNA-intercalating anti-cancer drugs. However, protein is not associated with the broken DNA after chlorpromazine treatment suggesting a different mode of action on the topoisomerase. Reasons why chlorpromazine may have potential as anti-cancer agent are considered.

Inositol trisphosphate, a novel second messenger in cellular signal transduction

There has recently been rapid progress in understanding receptors that generate intracellular signals from inositol lipids. One of these lipids, phosphatidylinositol 4,5-bisphosphate, is hydrolysed to diacylglycerol and inositol trisphosphate as part of a signal transduction mechanism for controlling a variety of cellular processes including secretion, metabolism, phototransduction and cell proliferation. Diacylglycerol operates within the plane of the membrane to activate protein kinase C, whereas inositol trisphosphate is released into the cytoplasm to function as a second messenger for mobilizing intracellular calcium.

Phorbol ester and diacylglycerol mimic growth factors in raising cytoplasmic pH

There is now good evidence that cytoplasmic pH (pHi) may have an important role in the metabolic activation of quiescent cells. In particular, growth stimulation of mammalian fibroblasts leads to a rapid increase in pHi (refs 3-6), due to activation of a Na+/H+ exchanger in the plasma membrane, and this alkalinization is necessary for the initiation of DNA synthesis. However, the mechanism by which mitogens activate the Na+/H+ exchanger to raise pHi is not known, although an increase in cytoplasmic free Ca2+ ([Ca2+]i) has been postulated as the primary trigger. We now present data suggesting that the Na+/H+ exchanger is set in motion through protein kinase C, a phospholipid- and Ca2+-dependent enzyme normally activated by diacylglycerol produced from inositol phospholipids in response to external stimuli. Using newly developed pH microelectrodes and fluorimetric techniques, we show that a tumour promoting phorbol ester and synthetic diacylglycerol, both potent activators of kinase C (refs 12-15), mimic the action of mitogens in rapidly elevating pHi in different cell types. Furthermore, we demonstrate that, contrary to previous views, an early rise in [Ca2+]i is not essential for the activation of Na+/H+ exchange and the resultant increase in pHi. Finally, we suggest that an alkaline pHi shift, mediated by Na+/H+ exchange, may be a common signal in the action of those hormones which elicit the breakdown of inositol phospholipids.

The interaction of lithium with thyrotropin-releasing hormone-stimulated lipid metabolism in GH3 pituitary tumour cells. Enhancement of stimulated 1,2-diacylglycerol formation

Treatment of GH3 cells with thyrotropin-releasing hormone (TRH) for periods up to 60 min resulted in a prolonged reduction in the cellular content of phosphatidylinositol (PtdIns) with no lasting change in the levels of the other inositol-containing phospholipids. Accompanying this was a maintained increase in the GH3 cell 1,2-diacylglycerol content and a slower decline in the level of cellular triacylglycerol. When the cells were suspended in lithium-containing balanced salt solution for 30 min (in the absence of exogenous myo-inositol), there was a 15% decrease in GH3 cell inositol levels. This was associated with a small, but significant, increase in the cellular content of phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) and 1,2-diacylglycerol. Addition of TRH to cells suspended in lithium-containing medium depleted cellular inositol levels by around 65% within 30 min. By this time, there was also a 50% reduction in the cellular content of PtdIns and a 20% reduction in phosphatidylinositol 4-phosphate (PtdIns4P). Control levels of PtdIns4,5P2 were maintained in the combined presence of TRH and lithium. Under those conditions, TRH no longer depleted cellular triacylglycerol and there was a marked increase in the ability of TRH to elevate the GH3 cell content of 1,2-diacylglycerol. The effect of TRH on the cellular content of phosphatidic acid was not altered by the presence of lithium. The results show, firstly, that when PtdIns resynthesis is inhibited by lithium-induced inositol depletion, its glycerol backbone accumulates, at least in part, in 1,2-diacylglycerol and, secondly, that GH3 cells preserve their cellular levels of PtdIns4,5P2 in the face of a considerable reduction in the cellular content of PtdIns.

Carbachol causes rapid phosphodiesteratic cleavage of phosphatidylinositol 4,5-bisphosphate and accumulation of inositol phosphates in rabbit iris smooth muscle; prazosin inhibits noradrenaline- and ionophore A23187-stimulated accumulation of inositol phosphates

Rabbit iris smooth muscle was prelabelled with myo-[3H]inositol for 90 min and the effect of carbachol on the accumulation of inositol phosphates from phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol (PtdIns) was monitored with anion-exchange chromatography. Carbachol stimulated the accumulation of inositol phosphates and this was blocked by atropine, a muscarinic antagonist, and it was unaffected by 2-deoxyglucose. The data presented demonstrate that, in the iris, carbachol (50 microM) stimulates the rapid breakdown of PtdIns(4,5)P2 into [3H]inositol trisphosphate (InsP3) and diacylglycerol, measured as phosphatidate, and that the accumulation of InsP3 precedes that of [3H]inositol bisphosphate (InsP2) and [3H]inositol phosphate (InsP). This conclusion is based on the following findings. Time course experiments with myo-[3H]inositol revealed that carbachol increased the accumulation of InsP3 by 12% in 15s and by 23% in 30s; in contrast, a significant increase in InsP release was not observed until about 2 min. Time-course experiments with 32P revealed a 10% loss of radioactivity from PtdIns(4,5)P2 and a corresponding 10% increase in phosphatidate labelling by carbachol in 15s; in contrast a significant increase in PtdIns labelling occurred in 5 min. Dose-response studies revealed that 5 microM-carbachol significantly increased (16%) the accumulation of InsP3 whereas a significant increase in accumulation of InsP2 and InsP was observed only at agonist concentrations greater than 10 microM. Studies on the involvement of Ca2+ in the agonist-stimulated breakdown of PtdIns(4,5)P2 in the iris revealed the following. Marked stimulation (58-78%) of inositol phosphates accumulation by carbachol in 10 min was observed in the absence of extracellular Ca2+. Like the stimulatory effect of noradrenaline, the ionophore A23187-stimulated accumulation of InsP3 was inhibited by prazosin, an alpha 1-adrenergic blocker, thus suggesting that the ionophore stimulation of PtdIns(4,5)P2 breakdown we reported previously [Akhtar & Abdel-Latif (1978) J. Pharmacol. Exp. Ther. 204, 655-688; Akhtar & Abdel-Latif (1980) Biochem. J. 192, 783-791] was secondary to the release of noradrenaline by the ionophore. The carbachol-stimulated accumulation of inositol phosphates was inhibited by EGTA (0.25 mM) and this inhibition was reversed by excess Ca2+ (1.5 mM), suggesting that EGTA treatment of the tissue chelates extracellular Ca2+ required for polyphosphoinositide phosphodiesterase activity. K+ depolarization, which causes influx of extracellular Ca2+ in smooth muscle, did not change the level of InsP3.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction by N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), a calmodulin antagonist, in the number of phorbol ester receptors in mouse skin

A calmodulin antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7), reduced the number of phorbol ester receptors in mouse skin in a dose- and time-dependent manner. The reduction occurred immediately after topical application of 30 mumoles of W-7, reaching a maximum of 86% after 5 min. Reduction in specific binding of 12-O-tetra-decanoylphorbol-13-acetate can explain the antitumor promoting activity of W-7 in mouse skin.

Cyclic AMP-dependent protein kinase stimulates the phosphorylation of phosphatidylinositol to phosphatidylinositol-4-monophosphate in a plasma membrane preparation from pig granulocytes

Plasma membranes prepared from pig granulocytes were incubated in the presence of [gamma-32P]ATP. The dissociated catalytic subunit of cyclic AMP-dependent protein kinase stimulated the incorporation of 32P into both the protein and lipid fractions of the membrane. The SDS gel-electrophoretic analysis of the 32P-labelled proteins showed that the protein kinase phosphorylated preferentially a 24000-Mr protein, though other 32P-labelled proteins were also detected. 32P-labelled membrane lipids were analysed in two different thin layer chromatographic systems. 32P-labelling was found exclusively in polyphosphoinositides. On addition of the protein kinase the 32P-labelling of both polyphosphoinositides was increased but a higher amount of phosphate was incorporated into phosphatidylinositol-4-phosphate than into phosphatidylinositol-4,5-bisphosphate.

Phorbol myristate acetate stimulates formation of phosphatidyl inositol 4-phosphate and phosphatidyl inositol 4,5-bisphosphate in human platelets

The tumor-promoting phorbol ester PMA increased the incorporation of 32P-phosphate into PIP (150%) and PIP2 (50%) in human platelets over the same range of concentrations that stimulate protein kinase C activity (i.e. 1-10 ng/ml). PMA also increased the total content of PIP (2.5-fold) and PIP2 (1.5-fold). The increase in 32P-PIP and 32P-PIP2 was 50% completed at 2 min after 10 ng/ml PMA, and was maximal by 20 min. The increase in PIP and PIP2 was accompanied by a fall of 32P-PI and PI mass over the same time period and concentration range of PMA, but no 32P-PA was formed, indicating that phosphoinositide hydrolysis by phospholipase C was not stimulated. Inhibition of phospholipase C activity by increasing platelet cyclic AMP did not duplicate the effects of PMA. We conclude that PMA may directly affect inositol lipid kinases and/or phosphatases, or that PMA stimulation of protein kinase C provides feedback regulation of the enzymes that determine the levels of polyphosphoinositides involved in transmembrane stimulus-response coupling.

TMB-8 inhibits secretion evoked by phorbol ester at basal cytoplasmic free calcium in quin2-loaded platelets much more effectively than it inhibits thrombin-induced calcium mobilisation

TMB-8 is widely regarded as an 'intracellular calcium antagonist', supposedly inhibiting the mobilisation of intracellular calcium. Rarely, however, have the effects of this compound on Ca2+ movements been measured. We report here that TMB-8 is not very effective in inhibiting thrombin-induced Ca2+ influx or internal release in human platelets judged from the fluorescent signal of cytoplasmic quin2. Only approx. 40% inhibition was seen at 500 microns TMB-8. Somewhat lower concentrations blocked the secretory response to thrombin and also the secretion evoked at basal [Ca2+]i by phorbol ester and collagen. It is suggested that one target for TMB-8 may be the C-kinase pathway.

Protein kinase C phosphorylates the synthetic peptide Arg-Arg-Lys-Ala-Ser-Gly-Pro-Pro-Val in the presence of phospholipid plus either Ca2+ or a phorbol ester tumor promoter

The synthetic nonapeptide Arg-Arg-Lys-Ala-Ser-Gly-Pro-Pro-Val is a substrate for in vitro phosphorylation by a partially purified preparation of rat brain protein kinase C, with Kmapp of about 130 microM. The closely related peptide kemptide was a much weaker substrate, bovine serum albumin was not a substrate and the peptide Arg-Arg-Lys-Ala-Ala-Gly-Pro-Pro-Val was a weak inhibitor of the enzyme. Protein kinase C-catalyzed phosphorylation of histone III-S and the nonapeptide are regulated by identical mechanisms since with both substrates the reaction required added phospholipid and either Ca2+ (1mM) or TPA (200 nM TPA). Our findings show that polypeptides containing multiple basic residues followed by the sequence Ala-Ser can be substrates for TPA-stimulated phosphorylation by protein kinase C.

Lack of a direct role for cyclic AMP in parathyrin action on phosphate reabsorption by the kidney

Isolated chick kidney proximal tubule cells have been used in a study of the mechanism by which PTH inhibits Na+-dependent Pi transport in the kidney. Treatment with PTH inhibits Pi uptake by the cells by 13% and stimulates cyclic AMP production by 77%. Forskolin, a potent activator of adenyl cyclase, brought about an 11-fold stimulation of cyclic AMP production by the cells, but in contrast to PTH, the drug had no effect on Na+-dependent Pi uptake. These results provide evidence that PTH action on phosphate transport is not mediated by cyclic AMP.

Fc-receptor mediated protein phosphorylation in murine peritoneal macrophages

The effect of Fc receptor engagement on protein phosphorylation in murine peritoneal macrophages has been investigated. Treatment of macrophage cultures with insoluble immune complexes resulted in enhanced phosphorylation of six proteins at 73, 66, 53, 37, 31 and 25 kD. Comparison of the protein phosphorylation patterns induced by immune complexes with those induced by agents which mimic the actions of well known intracellular second messengers (i.e., A23187, dibutyryl cAMP, or phorbol myristate acetate) revealed substantial similarity between Fc receptor induced events and those induced in response to phorbol diesters. There were, however, two phosphorylated proteins which were only seen following stimulation with immune complexes. Thus, more than one kind of protein kinase activity appears to be involved in Fc receptor mediated stimulation of macrophage function.

Cytosolic free Ca2+ in insulin secreting cells and its regulation by isolated organelles

The role of Ca2+ in secretagogue-induced insulin release is documented not only by the measurements of 45Ca fluxes in pancreatic islets, but also, by direct monitoring of cytosolic free Ca2+, [Ca2+]i. As demonstrated, using the fluorescent indicator quin 2, glyceraldehyde, carbamylcholine and alanine raise [Ca2+]i in the insulin secreting cell line RINm5F, whereas glucose has a similar effect in pancreatic islet cells. The regulation of cellular Ca2+ homeostasis by organelles from a rat insulinoma, was investigated with a Ca2+ selective electrode. The results suggest that both the endoplasmic reticulum and the mitochondria participate in this regulation, albeit at different Ca2+ concentrations. By contrast, the secretory granules do not appear to be involved in the short-term regulation of [Ca2+]i. Evidence is presented that inositol 1,4,5-trisphosphate, which is shown to mobilize Ca2+ from the endoplasmic reticulum, is acting as an intracellular mediator in the stimulation of insulin release.

Inhibition of metabolic cooperation in Chinese hamster V79 cells by various organic solvents and simple compounds

Gap-junctional intercellular communication is a biological process implicated in the regulation of cell proliferation and differentiation. Metabolic cooperation between 6-thioguanine-sensitive and resistant Chinese hamster cells, in vitro, has been used as a means to detect chemicals which can inhibit this form of intercellular communication. To further characterize this in vitro system as a potential screening assay for potential teratogens, tumor promoters and reproductive toxicants, a series of common solvents as well as other chemicals representing eight different functional groups, i.e., alcohols with straight or side chains, glycols, ketones, esters, ethers, phenols, aldehydes, amines and amino compounds and oxygen-heterocyclic compounds, were tested for their ability to inhibit colony-formation and to inhibit metabolic cooperation. A wide range of effects were observed which suggested a structure/activity relationship between a chemical's ability to inhibit gap junction-mediated intercellular communication and the cytotoxicity of a chemical. Possible mechanisms affecting the modulation of gap junctional communication by these chemicals are discussed.

Phorbol 12-myristate 13-acetate, ionomycin or ouabain, and raised extracellular magnesium induce proliferation of chicken heart mesenchymal cells

Cultured chicken heart mesenchymal cells are proliferatively quiescent at low densities in medium containing plasma at 10%. Mitogenic hormones like epidermal growth factor and insulin-like growth factors cause these cells to proliferate very actively, as does infection with avian sarcoma viruses, erythroblastosis virus, or myelocytomatosis virus. We have found that the combination of phorbol 12-myristate 13-acetate (PMA), ionomycin or ouabain, and raised extracellular magnesium, likewise, causes these cells to proliferate very actively. Although these agents have no significant effect when acting singly, the combination of PMA at 100 ng/ml and 0.5 microM ionomycin induces a 6-fold increase in cell number at 4 days, and the combination of PMA, ionomycin, and 5.6 mM magnesium induces 12-fold multiplication. Likewise, PMA plus 1 microM ouabain induces 3-fold multiplication, whereas the combination of PMA, ouabain, and magnesium induces 6-fold multiplication. The tumor promoter PMA, like diacylglycerol released by breakdown of plasma membrane phosphatidylinositol diphosphate, is known to activate the serine- and threonine-specific intracellular enzyme kinase C. The divalent cation ionophore ionomycin is known to carry calcium into cells down an electrochemical gradient, and the Na+,K+-ATPase inhibitor ouabain appears to elevate intracellular calcium by means of a sodium-mediated exchange mechanism. Magnesium, like calcium, is known to enter cells passively down an electrochemical gradient and to be involved in the regulation of many key intracellular reactions. Our findings with PMA, ionotropes, and magnesium support a hypothesis that diacylglycerol-mediated activation of kinase C plus cellular divalent cation influx and/or mobilization, caused by the action of mitogenic hormones or the protein products of onc genes, are key events in the initiation of cell replication.