1-O-hexadecyl-2-acetyl-sn-glycero-3-phospho (N,N,N trimethyl) hexanolamine: an analogue of platelet-activating factor with partial agonist activity

1. During studies of the role of platelet-activating factor (PAF) in macrophage superoxide anion generation (O2(-1], we identified an agonist action of the putative PAF receptor antagonist 1-O-hexadecyl-2-acetyl-sn-glycero-3-phospho (N,N,N-trimethyl) hexanolamine (hexanolamine PAF) in guinea-pig macrophages. The 1-O-octadecyl form of this compound has specific antagonist actions at PAF receptors. 2. The agonist properties of hexanolamine PAF were examined in rabbit washed platelets (aggregation) and in guinea-pig peritoneal macrophages (O2- generation). 3. Hexanolamine PAF induced significant platelet aggregation (50% of the PAF maximum). However, the omission of bovine serum albumin (BSA) from the Tyrode buffer resulted in a diminution of the response of washed platelets during storage for 24 h at 4 degrees C (7% of PAF maximum), whereas the maximum response to PAF was unaffected by storage for this period, irrespective of the presence of BSA. 4. Platelet aggregation induced by hexanolamine PAF was not accompanied by a detectable increase in intracellular calcium [Ca2+]i, whereas the aggregation response to PAF was preceded by a large rise in [Ca2+]i. 5. Hexanolamine PAF induced O2- generation in adherent macrophages, with a maximum response 45% of that to PAF. Hexanolamine PAF (100 nM), at a concentration equi-effective with PAF (1 nM) for stimulation of O2- generation in macrophages, induced an increase in [Ca2+]i which was significantly less than that induced by PAF. 6. PAF concentration-response curves were constructed in platelets or macrophages following pretreatment with hexanolamine PAF (0.1 and 1 microM). The interaction between PAF and the putative partial agonist (hexanolamine PAF) had the characteristics expected of a partial agonist interacting with a full agonist.7. Platelet aggregation induced by hexanolamine PAF was antagonized non-competitively by the PAF receptor antagonist, WEB 2086, whereas antagonism of PAF-induced aggregation by WEB 2086 was competitive. Macrophage 2- generation induced by hexanolamine PAF or PAF was antagonized by WEB 2086.8. These data indicate that hexanolamine PAF is a partial agonist at PAF receptors in macrophages and platelets. The inability of hexanolamine PAF to increase [Ca2+]i in platelets suggests that PAF receptors may be coupled to platelet aggregation by both Ca2 +-dependent and -independent pathways.

Signal transduction systems in growth hormone-releasing hormone and somatostatin release from perifused rat hypothalamic fragments

The role of signal transduction systems was examined in the secretion of GH-releasing hormone (GHRH) and somatostatin (SS) from perifused rat hypothalamic fragments. Forskolin, an adenylate cyclase activator, stimulated the release of GHRH and SS in a concentration-dependent manner (10-100 microM) with greatest stimulation for GHRH at 100 microM (mean +/- SE, 249 +/- 14%) and for SS at 30 microM (172 +/- 18%). (Bu)2cAMP also augmented GHRH and SS release. The protein kinase-C activator phorbol 12-myristate 13-acetate did not significantly stimulate basal GHRH or SS release at concentrations of 10 nM to 1 microM. The calcium ionophore A23187 enhanced the release of GHRH and SS in a concentration-dependent manner (2-20 microM), with the greatest responses of 282 +/- 50% at 10 microM and 189 +/- 24% at 20 microM, respectively. Potentiation by phorbol 12-myristate 13-acetate of forskolin-stimulated GHRH and SS release was observed. A23187 at 10 microM did not enhance forskolin-stimulated GHRH release, but did potentiate forskolin-stimulated SS release in a more than additive response. We conclude that there is 1) cAMP stimulation of hypothalamic GHRH and SS release, 2) a modulating role of protein kinase-C on cAMP-stimulated release of GHRH and SS, 3) a stimulatory role of the calcium messenger system for GHRH and SS release, 4) interaction of the signal pathways with differences in net GHRH and SS responses, and 5) a modulatory effect of protein kinase-C in perifused hypothalamic fragments which differs from the stimulation of basal GHRH and SS release reported in fetal-derived hypothalamic cell cultures. Our observations suggest an important regulatory role of interacting signal transduction systems in the hypothalamic secretion of GHRH and SS.

Intact but not truncated insulin-like growth factor binding protein-3 (IGFBP-3) blocks IGF I-induced stimulation of osteoblasts: control of IGF signalling to bone cells by IGFBP-3-specific proteolysis?

IGFBP-3 is the predominant IGFBP in serum and the major IGFBP secreted by osteoblasts. Native and recombinant IGFBP-3 and a truncated form lacking the carboxyterminal third were tested for their effects on 2 osteoblastic cell lines. Intact but not truncated IGFBP-3 blocked IGF I-stimulated DNA and glycogen synthesis. Inhibition was dose-dependent and found whenever the concentration of intact IGFBP-3 exceeded the concentration of IGF I. Truncated IGFBP-3 appears to result from proteolytic cleavage and does occur in vivo. The loss of inhibition by IGFBP-3 may be regulated at the site of IGF target cells and thus be essential for IGF I-induced osteoblast growth.

Roles of cyclic GMP and inositol trisphosphate in phototransduction of the molluscan extraocular photoreceptor

The internal messengers mediating the photocurrent of the molluscan extraocular photoreceptor, A-P-1, were examined. In the dark, pressure-injection of cGMP into the A-P-1, voltage-clamped at resting levels, produced a rapid outward current, associated with an increase in conductance. However, the cGMP-induced current and increase in conductance were suppressed by subsequent photostimulation, suggesting hydrolysis of cGMP by light. The steady-state I/V relation for the cGMP-induced current was non-linear. The I/V relation for the instantaneous cGMP-induced current, measured 50 ms after the beginning of a voltage step, was linear, and reversed at the membrane potential, -67 mV, which corresponded to the K+ equilibrium potential of A-P-1 in 10 mM K+ normal saline. These findings indicate that the internal cGMP induces a voltage- and time-dependent K+ current. Since the photocurrent results from the suppression of a voltage- and time-dependent K+ current similar to above, the photocurrent is considered to be equivalent to the suppression of the cGMP-induced current. Short pressure-injection of GDP-beta-S into A-P-1 reduced the subsequent photocurrent. The photocurrent was also suppressed after an external application of Pertussis toxin. On the other hand, the photocurrent was amplified by prior pressure-injection of inositol 1,4,5-trisphosphate (IP3). However, a short pressure-injection of neomycin into A-P-1 depressed the subsequent photocurrent. These results suggested that the cGMP-induced (dark) current is mediated by cGMP, and that hydrolysis of cGMP by light leads to the photocurrent, then being modified by another messenger, IP3, to be amplified.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of halothane on the interactions of serotonin1A and adenosine A1 receptors with G proteins in rat brain membranes

The influence of halothane on the interactions of 5-HT1A and adenosine A1 receptors with G proteins was determined by monitoring the guanine nucleotide sensitivity of agonist binding to these receptors. Halothane inhibited the binding of radiolabeled agonists to 5-HT1A and adenosine A1 receptors by up to 30%, but only at concentrations considerably greater than those necessary for the maintenance of the anesthetic state. The sensitivity of high-affinity agonist binding to a guanine nucleotide (guanylyl-5'-imidodiphosphate) was not affected by halothane, indicating no disruption of receptor-G protein coupling. Thus, it appears that the ability of halothane to disrupt receptor-mediated signal transduction by interference with receptor-G protein interactions is receptor specific.

FK-506 and cyclosporin A: selective inhibition of calcium ionophore-induced polymorphonuclear leukocyte degranulation

This paper investigates the abilities of FK-506 and cyclosporin A (CsA) to inhibit human polymorphonuclear leukocyte (PMNL) degranulation. PMNLs, purified from human blood, were stimulated in vitro with A23187, ionomycin, the complement derived peptide C5a, formylmethionylleucinylphenylalanine (FMLP) or phorbol myristate acetate (PMA). Degranulation was assessed by measuring the release of either lactoferrin or N-acetyl-beta-D-glucosaminidase (NAG). Both FK-506 and CsA produced a concentration-related inhibition of degranulation induced by either A23187 or ionomycin but did not affect C5a-, FMLP- or PMA-induced degranulation. The IC50 values for inhibition of degranulation (approximately 0.7 nM for FK-506 and 33.7 nM for CsA) are very close to the published values for inhibition of human T-cell proliferation. Removal of calcium from the incubation medium with ethyleneglycolbis(aminoethylether)tetra-acetate (EGTA) totally inhibited calcium ionophore-induced degranulation but had no effect against C5a-, FMLP- or PMA-induced degranulation. Preincubation of PMNLs with actinomycin D or cycloheximide did not affect either A23187- or PMA-induced degranulation. Non-immunosuppressive analogs of CsA were ineffective at inhibiting degranulation. Rapamycin, a macrolide structurally related to FK-506, did not inhibit degranulation but it did antagonize the inhibition produced by FK-506. Given the similar profiles of activity of FK-506 and CsA in neutrophils and T cells, we conclude that similar activation or signal transduction pathways may be present in both T cells and neutrophils. Because A23187-induced PMNL degranulation was not sensitive to either actinomycin D or cycloheximide, it is apparent that the signal transduction pathways ultimately control different cellular functions.

Suppression by staurosporine of Ca(2+)-mobilization triggered by ligation of antigen-specific receptors on t and B lymphocytes. An essential role of protein tyrosine kinase in the signal transduction

It is known that the receptor for platelet-derived growth factor (PDGF) activates phospholipase C (PLC) by phosphorylating the gamma 1 isoform of PLC with the receptor protein-tyrosine kinase (PTK), whereas a guanine nucleotide-binding protein participates as a transducer in the PLC activation through the receptors for vasopressin, bombesin and prostaglandin F2 alpha (PGF2 alpha). We have shown in a rat fibroblast line that staurosporine is a potent PTK inhibitor capable of clearly discriminating the two types of receptor-stimulated Ca2+ mobilization and, by inference, PLC activations the response triggered by PDGF was completely inhibited, whereas the responses triggered by vasopressin, bombesin and PGF2 alpha were not affected at all. The Ca2+ mobilization in human T and B cell lines induced by anti-CD3 and anti-immunoglobulins (Ig) was completely suppressed by staurosporine. The results indicate that the PTK activity plays an essential role in the PLC activation through the T cell receptor/CD3 complex and through membrane Ig.

Regulation of c-fos and c-jun protooncogene expression by the Ca(2+)-ATPase inhibitor thapsigargin

Thapsigargin, a non-phorbol-ester-type tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca(2+)-ATPase. We used this drug to analyze the involvement of Ca2+ and Ca(2+)-ATPases in the control of growth- and transformation-related genes. Here we show that treatment of mouse NIH 3T3 fibroblasts with thapsigargin induced rapid expression of the c-fos and c-jun protooncogenes. Inhibition or depletion of protein kinase C partially diminished the c-fos but not the c-jun response. Furthermore, thapsigargin could synergize with the tumor promoter phorbol 12-myristate 13-acetate to induce c-fos but not c-jun. However, thapsigargin had no effect on basal or phorbol ester-induced protein kinase C activity. Our results indicate that Ca2+ is a potent second messenger that controls expression of growth- and transformation-related genes. Since inhibition of the endoplasmic reticulum Ca(2+)-ATPase results in a strong induction of these genes, our data suggest that this Ca2+ pump may act as a negative regulator of cell growth.

The low-affinity p75 nerve growth factor (NGF) receptor mediates NGF-induced tyrosine phosphorylation

Protein tyrosine phosphorylation is a potential mechanism for initial signaling in PC12 cells during differentiation in response to nerve growth factor (NGF). NGF-induced tyrosine phosphorylation has been found to be initiated by the trk protooncogene, which participates in the formation of high-affinity NGF binding sites. In contrast to transfection of wild-type low-affinity p75 NGF receptors, transfection of p75NGFR with mutations in the cytoplasmic domain resulted in an inability of NGF to elicit tyrosine phosphorylation of intracellular substrates, indicating that p75NGFR is involved in initiating phosphorylation events by NGF. Even though the p75NGFR receptor does not possess any inherent tyrosine kinase activity, these experiments demonstrate that the p75NGFR has a potential role in NGF-induced tyrosine phosphorylation.

Lysophosphatidylcholine as a possible second messenger synergistic to diacylglycerol and calcium ion for T-lymphocyte activation

An immediate reaction product of phosphatidylcholine hydrolysis catalyzed by phospholipase A2, lysophosphatidylcholine (lysoPC), synergizes with a membrane-permeable diacylglycerol, 1,2-dioctanoylglycerol, and ionomycin to activate resting T-lymphocytes as measured by interleukin-2 alpha-receptor expression. It is suggestive that both phospholipase C and phospholipase A2 are directly involved in signal transduction in a synergistic fashion and that lysoPC acts as an additional second messenger for cellular regulation, probably for long-term responses such as cell proliferation and differentiation.

Tumor necrosis factor-induced interleukin-6 expression and cytotoxicity follow a common signal transduction pathway in L929 cells

Interleukin (IL)-6 gene induction was studied in response to treatment with Tumor Necrosis Factor (TNF) in the sensitive murine L929 cell line. Under conditions where TNF-mediated cytotoxicity was either increased or decreased, depending on addition of activators or inhibitors, we found that the TNF-induced IL6 gene expression was likewise enhanced or repressed. We conclude that the signal (or part of the signals) going to the nucleus and responsible for gene activation is conducted along the reaction mechanism leading to cellular toxicity.

Regulation of G proteins by chronic antidepressant drug treatment in rat brain: tricyclics but not clorgyline increase Go alpha subunits

The effect of long-term (3-week) administration of various antidepressant drugs on the steady-state concentrations of G protein alpha subunits, Gs alpha, Gi alpha, and Go alpha, has been investigated in rat brain using an enzyme-linked immunosorbent assay. Tricyclic antidepressants and clorgyline decreased Gs alpha and, to a lesser extent, Gi alpha in several brain regions, while Go alpha was increased by tricyclics but not clorgyline. We conclude that long-term treatment with antidepressant drugs exerts differential effects on G protein alpha subunits, and that antidepressant efficacy may potentially be based on functional modifications of signal transduction.

A cell cycle ts mutant, tsJT16, is defective in p70 synthesis through protein kinase C-dependent and -independent pathways

tsJT16 is a G0/G1 ts mutant from the Fischer rat fibroblast line. It has a ts defect in a function operating early after growth stimulation with fetal bovine serum (FBS). A primarily induced gene product, p70, was not synthesized at 40 degrees C after stimulation with serum, while c-fos and c-myc mRNAs accumulated under the same condition. This paper reports that p70 was synthesized following stimulation of G0-arrested cells with platelet-derived growth factor, epidermal growth factor (EGF), and 12-0-tetradecanoylphorbol-13-acetate (TPA) at 34 degrees C, but not at 40 degrees C. However, it was synthesized at both temperatures after addition of A23187. In protein kinase C-deprived cells, peptide growth factors and A23187 induced p70 at 34 degrees C, whereas TPA did not. Fibroblast growth factor and insulin did not induce p70. Induction of c-fos and c-myc occurred at both temperatures after the stimulation with FBS, TPA or A23187. These results indicated that the defect in tsJT16 to induce p70 is likely to be located at the common downstream of protein kinase C-dependent and -independent pathways, but is independent from the pathway of calcium mobilization.

Neutrophil CR3 expression and specific granule exocytosis are controlled by different signal transduction pathways

Neutrophils express receptors (CR3) for the complement fragment C3bi. CR3 expression can be increased by exposure of the cells to chemotactic factors such as FMLP or to the calcium ionophore A23187. It has been suggested that CR3 moieties are stored in the membrane bounding either the secondary or the tertiary (gelatinase containing) granules. To help define the mechanisms mediating CR3 up-regulation, the effects of several inhibitors upon CR3 expression and secondary granule exocytosis were investigated. Pertussis toxin inhibited FMLP-induced (but not A23187-induced) CR3 expression and exocytosis, indicating that an early step in FMLP-induced CR3 expression is activation of a pertussis toxin-sensitive G protein. However, CR3 expression and exocytosis appeared to be controlled by separate mechanisms distal to G protein activation because 1) DBcAMP and the protein kinase inhibitor H-7 inhibited or stimulated exocytosis, respectively, without affecting CR3 expression; 2) the calmodulin (chlorpromazine and trifluoperazine) and myosin L chain kinase (ML9) inhibitors had greater effects on exocytosis than on CR3 expression; and 3) the kinetics of CR3 expression and exocytosis differed markedly. Thus, although G protein activation is a common early step in both processes, there is a bifurcation of the two processes distally. The mechanisms mediating CR3 up-regulation and tertiary granule exocytosis were also investigated. Extracellular Ca2+ was essential for tertiary granule exocytosis, but not for CR3 up-regulation. We conclude that because the mechanisms controlling CR3 up-regulation and exocytosis diverge soon after the binding of a chemotactic ligand to its receptor, that at least the bulk of increased CR3 expression is not simply a by-product of secondary and tertiary granule exocytosis but is the result of the mobilization of CR3 moieties from an intracellular pool of uncertain identity.

Altered antigen receptor signaling in anergic T cells from self-tolerant T-cell receptor beta-chain transgenic mice

T-cell tolerance to the minor lymphocyte-stimulating antigen Mls-1a in a T-cell receptor (TcR) V beta 8.1 transgenic line of mice is maintained by both clonal deletion and clonal anergy. Approximately 20-50% of peripheral CD4+ (but not CD8+) T cells isolated from these mice are anergic and fail to proliferate following TcR ligation. We have examined key events in T-cell signaling in peripheral T cells isolated from these mice. In this report, we show that the anergic CD4+ T cells did not mobilize calcium or express receptors for interleukin 2 (IL-2) following TcR ligation. However, the cells retained viability and functional potential because stimulation with phorbol 12-myristate 13-acetate and ionomycin bypassed the block in receptor-mediated signaling and induced IL-2 receptor expression and proliferation of the anergic cells.

Follicle-stimulating hormone signal transduction: role of carbohydrate in aromatase induction in immature rat Sertoli cells

Receptor activated adenylate cyclase acts as a major transmembrane signalling system. It is widely accepted that upon binding to its receptor, follicle-stimulating hormone (FSH) activates the cAMP-dependent pathway which in turn mediates FSH-induced estradiol production in Sertoli cells. Studies utilizing several chemically derived variants of FSH have demonstrated that these variants bind to the FSH receptors with equal avidity but differ in their ability to activate cAMP-dependent pathways. Since cAMP is believed to be the second messenger responsible for FSH signal transduction, we tested two hypotheses: (1) that the effects of different oFSH variants on cAMP production and aromatase induction (as measured by estradiol production) would be in parallel; and (2) that deglycosylated ovine FSH (DG-oFSH) would antagonize the ability of intact oFSH to stimulate aromatase induction, similar to its reported antagonistic effect on cAMP production. Immature rat (7- to 10-day-old) Sertoli cells were cultured and the effects of several different oFSH variants on cAMP production and/or aromatase induction were tested. The variants tested were native oFSH, DG-oFSH, asialo oFSH (AS-oFSH), a recombinant of intact LH alpha and FSH beta (alpha + beta) and a recombinant of deglycosylated LH alpha and intact FSH beta (DG alpha + beta). Both native oFSH and alpha + beta recombinant at relatively large doses (10 ng) elicited a significant increase in extracellular cAMP accumulation as well as total cAMP production. In contrast, DG-oFSH did not produce an increase in cAMP even at 10-fold higher doses than native oFSH. Intracellular cAMP concentrations did not increase following stimulation with native oFSH, DG-oFSH or DG alpha + beta. In contrast to the divergent effects of oFSH and DG-oFSH on cAMP production all variants of oFSH stimulated estradiol production from Sertoli cells albeit with varying potencies. The sensitivity (minimal effective dose) and ED50 (dose at which half maximal response is achieved) of the estradiol (E2) response curve to increasing concentrations of native oFSH were 0.025 +/- 0.01 and 0.33 +/- 0.05 ng, respectively. Asialo-oFSH (AS-oFSH) increased E2 production with a potency (comparative dose required for effect) similar to that of native oFSH.(ABSTRACT TRUNCATED AT 400 WORDS)

IL-4 counteracts anti-mu-induced human B cell proliferation: involvement of a cAMP-dependent inhibitory pathway

In this report we show that IL-4 inhibits DNA synthesis induced by stimulation of human B cells with mitogenic doses of either soluble anti-mu mAb DA44 or phorbol ester. In contrast, earlier steps of anti-mu-induced B cell stimulation, such as RNA synthesis, CD23 expression and IL-6 production, were not inhibited but rather increased in the presence of IL-4. From these results, IL-4 appears therefore to exert two opposite effects on DA44 anti-mu mAb-induced human B cell activation: early steps are stimulated, and later steps inhibited. The results of kinetic analysis were consistent with this model. The inhibitory activity of IL-4 required an active cAMP-dependent pathway since IL-4-mediated inhibition of anti-mu-induced B cell proliferation was abolished in the presence of two specific inhibitors of the cAMP pathway (H8 and 2',5'-dideoxyadenosine which are specific for cAMP-dependent protein kinase and adenylate cyclase respectively). Furthermore, IL-4 induced a delayed and prolonged increase in intracellular cAMP concentrations (observed between 4 and 48 hours of culture), and this strongly suggests that the late inhibitory effects of IL-4 is cAMP-dependent. Moreover, this delayed IL-4-mediated cAMP production is probably sufficient to prevent anti-mu induced DNA synthesis since addition of the cAMP agonist forskolin on day 1 or 2 of culture also suppresses the anti-mu-mediated B cell proliferation.(ABSTRACT TRUNCATED AT 250 WORDS)

Carboxy-terminal truncations of epidermal growth factor (EGF) receptor affect diverse EGF-induced cellular responses

The binding of epidermal growth factor (EGF) to its receptor induces tyrosine phosphorylation of phospholipase C gamma (PLC gamma), which appears to be necessary for its activation leading to phosphatidyl inositol (PI) hydrolysis. Moreover, EGF-receptor (EGF-R) activation and autophosphorylation results in binding of PLC gamma to the tyrosine phosphorylated carboxy-terminus of the receptor. To gain further insights into the mechanisms and interactions regulating these processes, we have analyzed transfected NIH-3T3 cells expressing two EGF-R carboxy-terminal deletion mutants (CD63 and CD126) with reduced capacity to stimulate PI hydrolysis, Ca2+ rises, and DNA synthesis. In fact, the CD126 mutant lacking 126 carboxy-terminal amino acids, including four tyrosine autophosphorylation sites, was unable to stimulate PI hydrolysis or Ca2+ rise in response to EGF. Surprisingly, EGF binding to the cell lines expressing CD63 or CD126 mutants was followed by similar stimulation of tyrosine phosphorylation of PLC gamma. Our results suggest that although necessary, tyrosine phosphorylation of PLC gamma may not be sufficient for stimulation and PI hydrolysis. It is clear, however, that the carboxy-terminal region of EGF-R is involved in regulation of interactions with cellular targets and therefore plays a crucial role in postreceptor signaling pathways.

p42/mitogen-activated protein kinase as a converging target for different growth factor signaling pathways: use of pertussis toxin as a discrimination factor

Mitogen-activated protein (MAP) kinase is a 42-kDa serine/threonine-specific protein kinase that requires phosphorylation on both tyrosine and threonine residues for activity. This enzyme is rapidly and transiently activated in quiescent cells after addition of various agonists, including insulin, epidermal growth factor, platelet-derived growth factor, and phorbol esters. We show here that addition of the growth factors thrombin or basic fibroblast growth factor to CCL39 fibroblasts rapidly induces tyrosine phosphorylation of the p42 MAP kinase protein and concomitantly stimulates MAP kinase enzymatic activity. To elucidate the signaling pathways utilized in this activation, we took advantage of the sensitivity of CCL39 cells to the toxin of bordetella pertussis, which ADP-ribosylates two Gi proteins in this cell system. We show that pretreatment of cells with the toxin inhibited thrombin stimulation of MAP kinase by greater than 75% but had no detectable effect on the stimulation induced by basic fibroblast growth factor. We also demonstrate that these two growth factors that synergize for mitogenicity are able to cooperate in activation of MAP kinase and that this synergism is partially sensitive to pertussis toxin. Finally, we describe a 44-kDa protein, the tyrosine phosphorylation of which appears to be coregulated with p42 MAP kinase. We conclude that p42 MAP kinase (and the pp44 protein) are at or are downstream from a point of convergence of two different receptor-induced signaling pathways and might well play a key role in integrating those signals.

Signal transduction and transcriptional regulation of interferon-alpha-stimulated genes

Interferon-alpha (IFN-alpha) stimulates the expression of a number of genes in a pathway that begins with binding to specific high-affinity plasma membrane receptors. All IFN-alpha-stimulated genes cloned thus far are characterized by the presence of a DNA element, termed Interferon-Stimulated Response Element (ISRE), usually in the 5' upstream region of the genes. The ISRE binds a nuclear factor(s) following IFN-receptor triggered signal transduction and provides a convenient assay for the rapid phase of IFN-alpha signal transduction. This phase utilizes a phospholipase A2-generated second messenger which modulates ISRE-binding factors. Expression cloning has resulted in the identification of two specific ISRE-binding proteins that are candidates as signal recipients. Further advances in our understanding of the molecular mechanisms of IFN action may come through the use of yeast genetics. The human p68 kinase expressed in yeast has a growth inhibitory phenotype and provides a useful alternative system for analyzing components of the IFN-stimulated pathways.

Calcium signaling capacity of the CD11b/CD18 integrin on human neutrophils

The CD11b/CD18 integrin is a major cell adhesion molecule of myelomonocytic cells. Exposure of human neutrophils in suspension to CD11b or CD18 monoclonal antibodies (mAbs)2 does not affect the resting level of cytosolic free Ca2+ in these cells; however, a subsequent cross-linking of either of these antibodies triggers a prompt and significant cytosolic-free Ca2+ transient lasting about 10 min. The rise in cytosolic-free Ca2+ (from 130 +/- 2 to 414 +/- 12 nM or 111 +/- 12 to 331 +/- 22 nM caused by cross-linking of CD11b or CD18 subunits, respectively) is due to both mobilization of Ca2+ from intracellular stores and influx of Ca2+ across the plasma membrane. Cross-linking of the common leukocyte antigen (CD45) did not alter the basal level of cytosolic free Ca2+. In accordance with other adherence-induced phenomena and with CD11/CD18-mediated phagocytosis, these Ca2+ signals were only modestly affected by pertussis toxin. Thus, the present data clearly indicate that the CD11b/CD18 integrin on human neutrophils is capable of inducing a prompt cytosolic-free Ca2+ signal. These findings directly support the recent suggestion that the CD11b/CD18 integrin is responsible for the "spontaneous oscillations" of cytosolic-free Ca2+ observed in adherent neutrophils and, at least partially, also explain how integrin-mediated adherence can modify the functional responsiveness of neutrophils to a subsequent agonist stimulation.

Loss of the endothelin signal pathway in C6 rat glioma cells persistently infected with measles virus

Endothelin 1 causes a strong Ca2+ signal in C6 rat glioma cells as measured by fura-2 fluorescence. This endothelin 1-induced Ca2+ signal was not observed when the cells were persistently infected with a measles virus strain of subacute sclerosing panencephalitis (SSPE, strain Lec). Binding of 125I-labeled endothelin 1 to the C6/SSPE cells was less than 5% of the binding to the C6 control cells, suggesting that the impairment in signal transduction was due to a loss of binding sites for endothelin 1. Treatment of the C6/SSPE cells with measles antiserum resulted in the loss of expression of viral proteins located in the membrane as well as inside the cells (antigenic modulation), but it restored neither the endothelin 1-induced Ca2+ rise nor the 125I-endothelin 1 binding. Cocultivation of uninfected C6 cells with C6/SSPE cells (9:1 ratio) resulting in contact-mediated transmission of measles virus showed that the 125I-endothelin 1 binding activity was gradually lost as a consequence of persistent virus infection.

Involvement of prostaglandin-producing pathway in the cytotoxic action of tumor necrosis factor

To elucidate the cytotoxic mechanism of tumor necrosis factor (TNF), we isolated TNF-resistant sublines of L929 cells. As compared with L929 cells, TNF-resistant cells retained similar number and affinity of TNF-binding sites, and showed a similar growth rate. TNF stimulated arachidonate release from L929 cells, while no stimulation was observed at all in TNF-resistant cells tested. The cytotoxic action of TNF on L929 cells was inhibited by indomethacin, suggesting that prostaglandin may be involved in the action. Therefore, TNF-stimulated prostaglandin production was examined in L929 and TNF-resistant sublines. The amount of PGE2 produced by L929 cells was increased more than 5-fold after the addition of TNF, whereas the amount of PGE2 did not change in the resistant sublines following addition of the factor. TNF-stimulated arachidonate release and PGE2 production were reversed by islet-activating protein (IAP)-treatment of L929 cells. These results suggest that arachidonate release and subsequent prostaglandin production are important for the cytotoxic action of TNF and that these processes are mediated by GTP-binding protein (G protein) that is coupled to the TNF-receptor.

Isoprenoid pathway activity is required for IgE receptor-mediated, tyrosine kinase-coupled transmembrane signaling in permeabilized RBL-2H3 rat basophilic leukemia cells

Previously, we reported that the isoprenoid pathway inhibitor, lovastatin, blocks the activation by IgE receptor cross-linking of 45Ca2+ influx, 1,4,5-inositol trisphosphate production, secretion, and membrane changes (ruffling, spreading) in intact RBL-2H3 rat basophilic leukemia cells. These results indicated that an isoprenoid pathway intermediate, very likely an isoprenylated protein, is importantly involved in the control of IgE receptor-mediated signal transduction. Here, we show that 20 h of pretreatment with lovastatin also inhibits antigen-induced secretion and membrane responses in streptolysin O-(SLO)-permeabilized cells. However, lovastatin does not inhibit secretion stimulated by the nonhydrolyzable GTP analog, GTP gamma S. Furthermore, the membrane responses to GTP gamma S persist, although in an attenuated form, in lovastatin-treated permeabilized cells. The relative insensitivity of GTP gamma S-induced responses to lovastatin was one of several indications that antigen and GTP gamma S may activate separate pathways leading to transmembrane responses in permeabilized cells. Further experiments showed that the beta-thio derivative of GDP, GDPBAS, inhibits the secretory and membrane responses to GTP gamma S, as expected for a GTP-binding protein-dependent signaling pathway, while having little effect on antigen-induced responses. Conversely, genistein blocks the secretory and membrane responses to antigen, as expected for a tyrosine kinase-dependent pathway, without altering the GTP gamma S-induced responses. From these results, and from additional data from cells treated with tyrphostins and sodium orthovanadate, we propose that IgE receptor-mediated secretion from permeabilized RBL-2H3 cells occurs by a tyrosine kinase-dependent pathway that requires isoprenoid pathway activity for function. We propose further that RBL-2H3 cells contain a separate GTP-binding protein-mediated signaling pathway whose direct activation by GTP gamma S is either independent of isoprenoid pathway activity or depends on the activity of an isoprenylated protein that is not significantly depleted after 20 h of lovastatin treatment.

Acute phosphate depletion dissociates hormonal stimulated second messengers in osteoblast-like cells

The acute effect (24 h) of either phosphate depletion or phosphate surfeit on hormonal stimulated signal transduction systems was studied in the osteoblastic cell line UMR-106. Elevation of intracellular Ca2+ ([Ca2+]in), induced by different calciotropic hormones (PTH, prostaglandin E2, endothelin) was blunted by acute phosphate depletion, whereas at high inorganic phosphate (Pi) concentrations the rise in [Ca2+]in was augmented. Basal [Ca2+]in was not altered by either Pi depletion or Pi excess. The effect of acute phosphate depletion on hormonal mediated [Ca2+]in rise was not observed in the absence of extracellular Ca2+ suggesting that under these conditions, the release of Ca2+ from intracellular stores, is not affected. Also, nonhormonal calcium entry pathways such as depolarization-activated calcium channels or protein kinase C-activated Ca2+ channels were not affected by acute phosphate depletion. cAMP accumulation in the cells, either through receptor or nonreceptor-mediated mechanisms, increased under low Pi conditions and decreased as Pi concentration in the culture media was progressively increased from 0 to 2 mM during 24 h of incubation. Changes in Pi concentration had no effect on basal cAMP generation by the cells. The facilitative effect of acute Pi depletion on agonist-induced cAMP accumulation could be demonstrated in both the presence and absence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.2 mM). PTH receptor binding assessed with [Nle8 Nle18 Tyr34] bovine PTH (1-34) NH2 was not altered by phosphate depletion. We conclude that exposure of osteoblasts to different Pi environments modulates the second messenger responses to hormones in a reciprocal fashion so that acute phosphate depletion down-regulates [Ca2+]in signals while augmenting cAMP generation and vice versa. Inasmuch as bone resorption processes can be modulated by Ca2+ and cAMP the data presented herein suggest that the altered bone resorptive response to calciotropic hormones (e.g. PTH), under surfeit or deficit of phosphate, is mediated by changes in [Ca2+]in and cAMP.

Immunophilin ligands demonstrate common features of signal transduction leading to exocytosis or transcription

Investigations of the actions and interactions of the immunophilin ligands FK506, cyclosporin A (CsA), rapamycin, and 506BD suggest that complexes of FK506 with an FK506-binding protein or of CsA with a cyclophilin (CsA-binding protein) inhibit the T-cell receptor-mediated signal transduction that results in the transcription of interleukin 2. Now we report an identical spectrum of activities of FK506, CsA, rapamycin, and 506BD on IgE receptor-mediated signal transduction that results in exocytosis of secretory granules from the rat basophilic leukemia cell line RBL-2H3, a mast cell model. Both FK506 and CsA inhibit receptor-mediated exocytosis (CsA IC50 = 200 nM; FK506 IC50 = 2 nM) without affecting early receptor-associated events (hydrolysis of phosphatidylinositol, synthesis and release of eicosanoids, uptake of Ca2+). In contrast, rapamycin and 506BD, which share common structural elements with FK506, by themselves have no effect on IgE receptor-mediated exocytosis. Both compounds, however, prevent inhibition by FK506 but not by CsA. Affinity chromatography with FK506, CsA, and rapamycin matrices indicates that the same set of immunophilins present in RBL-2H3 cells have been found in Jurkat T cells and calf thymus; however, the relative amounts of these proteins differ in the two cell types. These results suggest the existence of a common step in cytoplasmic signaling in T cells and mast cells that may be part of a general signaling mechanism.

Comparison between the effects of the microsomal Ca(2+)-translocase inhibitors thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone on cellular calcium fluxes

The effects of two inhibitors of the microsomal Ca(2+)-ATPase, thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone, were compared in hepatocytes and in a T-cell line (JURKAT). Both compounds mobilized the same intracellular Ca2+ pool, which contained the Ins(1,4,5)P3-sensitive store, in hepatocytes and in JURKAT cells. The mobilization of the internal Ca2+ store with either compound activated Mn2+ entry in JURKAT cells, but not in hepatocytes. This suggests different properties of the bivalent-cation entry pathway between these cell types.

Carbachol-activated muscarinic (M1 and M3) receptors transfected into Chinese hamster ovary cells inhibit trafficking of endosomes

We examined the effects of isoproterenol and carbachol on fluid-phase endocytosis by Chinese hamster ovary (CHO) cells transfected with beta-adrenergic, M1, or M3 cholinergic receptors. Isoproterenol increased cAMP production and carbachol increased intracellular Ca, indicating successful expression of the receptor genes and coupling to typical signal transduction pathways. Carbachol inhibited the uptake of horseradish peroxidase (HRP) or Lucifer yellow (markers of fluid-phase endocytosis) in both M1- and M3-containing cells but not in wild-type cells, whereas isoproterenol did not affect pinocytosis in cells transfected with beta-adrenergic receptors. Carbachol inhibited the transit of HRP from an exchangeable pool to a nonexchangeable pool by a latent process requiring minimally 5 min of incubation. During the latent period, only one peak of low-density HRP-containing vesicles was found on Percoll gradients; after 5 min, HRP appeared in both high- and low-density vesicles. Carbachol-treated cells contained less HRP in the high-density fraction enriched in lysosomal markers. Early endosomes from CHO cells labeled for 5 min with HRP underwent fusion to make a more dense population of vesicles in the presence of ATP and KCl at 37 degrees C but not at 4 degrees C. The fused material contained increased levels of G proteins as detected either by ADP ribosylation with appropriate toxins or by immunoblotting with specific antibodies. These findings suggest that GTP binding proteins are internalized in endocytic vesicles and enter into a complex trafficking process involving fusion with other vesicular compartments. Trafficking of endosomes to these compartments is inhibited by activated M1 and M3 muscarinic receptors in CHO cells.

Sensitization enhances the adenylyl cyclase responsiveness in alveolar macrophages. Changes induced at post-receptor level

Using membrane fractions (MF) from guinea pig alveolar macrophages (AM), we investigated the effects of sensitization and antigen challenge on the stepwise activation of adenylyl cyclase considering receptor binding, G-protein coupling and direct stimulation of the enzyme. Receptor binding studies, using [125I]ICYP as the beta-adrenoceptor specific ligand, show that neither receptor number (Bmax) nor receptor affinity constants (Kd values) were affected by sensitization or antigen challenge. Using forskolin as a direct stimulant of AC, alterations in the enzymatic activity of AC could be excluded. Pretreatment of the different MF with cholera toxin (CT, a toxin which eliminates GTPase activity) and subsequent stimulation of AC with GTP, shows an increased responsiveness in MF from sensitized and antigen challenged AM. In addition, pretreatment of MF from naive AM with increasing doses of CT results in a maximal AC response at the higher concentrations used (50-100 micrograms/mL), an effect not observed in MF from sensitized and antigen challenged AM. In these MF, the AC response still increases after pretreatment with such doses of CT. These data suggest that the enhanced AC responsiveness in AM, induced by sensitization and antigen challenge, results from alterations in alpha s-subunits.

Differential amplification of antagonistic receptor pathways in neutrophils

In human neutrophils approximately 500 ligand-occupied beta-adrenergic receptors almost completely inhibit the superoxide production generated by at least 50,000 formyl peptide receptors, suggesting a massive amplification of the inhibitory receptor signals. We estimated two stages of amplification. In the first stage, we quantitated the ligand-dependent GTPase activities. For the formyl peptide receptor, the number of phosphates released from GTP in the presence of the saturating ligand is relatively modest, i.e. approximately 1/min/receptor, even though there are approximately 200 Gn (Gi type II) proteins/formyl peptide receptor in neutrophil membranes. In contrast, the number of GTPs cleaved in the presence of a beta-adrenergic agonist is approximately 100/min/beta-adrenergic receptor, and there are about 700 Gs/beta-adrenergic receptor in membranes. Thus the signal of the beta-adrenergic receptor is already massively amplified at the G protein, whereas the signal of the formyl peptide receptor is likely to be amplified at subsequent steps. New kinetic evidence from intact cells and biochemical evidence from permeabilized cells is provided that the second messenger of the inhibitory pathway is cAMP. To estimate the amplification of this step, we determined the cAMP concentration necessary to maximally inhibit superoxide anion production of formyl peptide-stimulated electropermeabilized cells, and we compare these concentrations to previously determined values of cAMP production in neutrophils. We conclude that each receptor may generate up to 10,000 molecules of cAMP.

Tyrosine mutations within the alpha platelet-derived growth factor receptor kinase insert domain abrogate receptor-associated phosphatidylinositol-3 kinase activity without affecting mitogenic or chemotactic signal transduction

A phosphatidylinositol-3 (PI-3) kinase activity of unknown biological function associates with tyrosine kinase-containing proteins, including a number of growth factor receptors after ligand stimulation. In the beta platelet-derived growth factor (beta PDGF) receptor, phosphorylation of a specific tyrosine residue within the kinase insert domain was required for its interaction with this enzyme. We show that substitutions of phenylalanine for tyrosine residue 731 or 742 within the kinase insert domain of the alpha PDGF receptor do not impair PDGF-induced tyrosine phosphorylation of the receptor or of an in vivo substrate, phospholipase C-gamma. Moreover, phosphatidylinositol turnover in response to ligand stimulation is unaffected. However, both lesions markedly impair receptor association with PI-3 kinase. Antiphosphotyrosine antibody-recoverable PI-3 kinase was also dramatically reduced in PDGF-stimulated cells expressing either mutant receptor. Since neither mutation abolished PDGF-induced mitogenesis or chemotaxis, we conclude that alpha PDGF receptor-associated PI-3 kinase activity is not required for either of these major PDGF signalling functions.

Signal transduction in human alveolar macrophages: diminished chemotactic response to FMLP correlates with a diminished density of Gi proteins and FMLP receptors

Alveolar macrophages (AM) migrate less well in response to chemotactic ligands than do monocytes and neutrophils. The response of monocytes and neutrophils to chemotactic ligands is mediated at least in part by pertussis toxin-sensitive guanine nucleotide binding proteins (Gi proteins). Whether this is also true in AM is uncertain. We hypothesized that decreased chemotaxis by AM was due in part to diminished Gi protein and/or chemotactic receptor density in AM. G proteins are heterotrimers made up of alpha, beta, and gamma subunits; the predominant pertussis toxin-sensitive Gi proteins are those containing alpha i2 or alpha i3 subunits. Pertussis toxin pretreatment (0.5 microgram/ml) significantly reduced AM, monocyte, and neutrophil chemotaxis to N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) and human zymosan-activated serum (P less than 0.05). However, as previously noted, AM chemotaxis was much less than that observed in monocytes and neutrophils. Immunoblots using antibodies that are specific for alpha i2 and alpha i3 showed that AM contained approximately 3-fold less alpha i2 and approximately 10-fold less alpha i3 per microgram of plasma membrane protein than did monocytes or neutrophils. Similar results were obtained in immunoblots made using antibodies to common alpha subunit determinants and to the beta 36 subunit. A comparable approximately 4-fold reduction in density of receptors for [3H]FMLP was found in AM compared to neutrophils. The diminished density of Gi proteins and FMLP receptors was not due to a generally decreased density of plasma membrane proteins in AM, since the density of the membrane-associated tyrosine kinase hck was similar in AM, monocytes, and neutrophils.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that glucocorticosteroid-mediated immunosuppressive effects do not involve altering second messenger function

The mechanism by which glucocorticosteroids (GCS) suppress proliferation of human peripheral blood mononuclear leukocytes (PBML) was investigated. Using the proliferative responses to immobilized anti-CD3 mAb or mitogens (PHA + PMA) as biological readouts, dexamethasone (DEX) and 6 alpha-methylprednisolone (6 alpha-MP) were shown to inhibit PBML proliferation in a concentration-dependent fashion. The mechanism by which GCS mediate immunosuppression did not involve interference with Ca2+ fluxes as: (1) DEX failed to block Ca2+ entry into anti-CD3 + PMA stimulated cells; and (2) Ca2+ ionophores (ionomycin and A23187) failed to circumfent DEX-mediated suppression. DEX also had no effect on protein kinase C (PKC) activity as: (1) inhibitors (H-7 and staurosporin) or stimulators (1,2-dihexanoyl-sn-glycerol [DiC6] and 1,2-dioctanoyl-rac-glycerol [DiC8]) of PKC did not prevent DEX-mediated suppression; (2) DEX did not affect the activation-induced upregulation of CD4 and CD8 expression, an indirect index of PKC activity; and (3) DEX did not alter the activation-associated translocation of PKC from cytosolic to membrane-bound compartments. This, in addition to previous results demonstrating that GCS directly inhibit cytokine gene transcription and that rII-1 + rIL-6 + rIFN-gamma completely abrogated GCS-mediated suppressive effects, further supports the notion that GCS exert their immunosuppressive effects through inhibition of cytokine gene expression.

Dissection of signals controlling T cell function and activation: H7, an inhibitor of protein kinase C, blocks induction of primary T cell proliferation by suppressing interleukin (IL)2 receptor expression without affecting IL2 production

T cell activation induced via cross-linking of the T cell receptor (TcR) stimulates hydrolysis of phosphatidylinositol to the second messengers diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). DAG is necessary for the activation and function of protein kinase C (PKC) which is suggested to play a key role in the cascade of signal transduction when translocated from the cytosol to the cell membrane. In this report, we investigated responses of resting vs. activated Ly-2+ and L3T4+ T lymphocytes in the presence of the PKC inhibitor H7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine]. H7 inhibited the induction of primary T cell proliferation, while interleukin 2 (IL 2) production was fully retained. The effect of the PKC inhibitor on primary T cells depended on the type of ligand interacting with the TcR: increasing doses of concanavalin A or of immobilized anti-CD3 monoclonal antibody (mAb), but not of anti-V beta 8 or of anti-TcR alpha/beta mAb, partly overcame the blockade, indicating a differential signaling compared to the former stimuli. The blockade of T cell proliferation by H7 was not due to an inhibition of PKC translocation, but occurred even 4-8 h after T cell induction and correlated with a significant reduction of IL 2 receptor (IL 2R) expression. In contrast, the mRNA levels of IL 2R and the cellular proto-oncogenes c-fos and c-myc were not affected. On activated T cells, H7 neither blocked proliferation nor IL2R expression. Consequently, H7 dissects the signal resulting in T cell proliferation from those governing the triggering of other T cell functions, i.e. IL 2 production, during primary responses of Ly-2+ or L3T4+ murine T lymphocytes.

[Lipocortins and phospholipases: new aspects in the physiology of glucocorticosteroid effect]

Glucocorticosteroids are among the most useful and most widely prescribed anti-inflammatory drugs. Despite their wide use little is yet known about their mode of action. In the last 10 years a group of proteins called lipocortins or annexins has been characterized. Those proteins exert an inhibitory effect on the synthesis of lipid mediators by way of an important proinflammatory enzyme, phospholipase A2. Phospholipases are known to be involved in cell-signal transduction and generation of inflammatory mediators like prostaglandins, leukotrienes and platelet-activating-factor. The cellular expression of lipocortins is induced by glucocorticosteroids. The inhibition of cellular phospholipases via lipocortins may account for some aspects of the action of glucocorticosteroids.

Rapid activation of phosphatidate phosphohydrolase in mesangial cells by lipid A

Knowledge of rapid events in cell signaling initiated by lipid A, the core moiety of bacterial lipopolysaccharide, is limited. In the present study we have demonstrated that cis-parinaric acid (cis-PnA) rapidly labels 1,2-sn-diacylglycerol (DAG) subsequent to labeling of phosphatidic acid (PA). Stimulation of microsomal membranes with lipid A decreased the level of PA labeled with cis-PnA within 5 s and increased the proportion of fluorescent label in DAG. Lipid A stimulation of DAG synthesis at 5-15 s was inhibited by incubation of mesangial cells with pertussis toxin prior to isolation of microsomal membranes. Inhibition of DAG formation was accompanied by an accumulation of the mass and fluorescent label in the cis-PnA-labeled phosphatidic acid pool. GTP gamma S caused a decrease in labeled PA and an increase in labeled 1,2-DAG. We conclude that the PA pool was enlarged via the lipid A sensitive lyso-PA acyl transferase (lyso-PA-AT) and was decreased by a phosphatidate phosphohydrolase to form DAG. The phosphatidate phosphohydrolase was at least partly regulated by a pertussis-sensitive G-protein. Lipid A or 1,2-dilinoleyl-PA, a product of lyso-PA-AT, induced cell activation as monitored by actin reorganization and cellular shape changes. Pretreatment of cells with pertussis toxin prevented the morphological changes normally induced by lipid A or 1,2-dilinoleyl-PA. In contrast, 1-oleoyl-2-acetylglycerol induced rapid actin reorganization and shape change, presumably bypassing the pertussis blockade. We propose that specific pools of PA and PA-derived DAG are key elements in rapid signaling in mesangial cells and are independent of the PI cycle and phospholipase C.

Mechanisms of ex vivo aortic hypocontractility in endotoxemic rat

To clarify the mechanism of the vascular hypocontractility in endotoxemia, the effect of endotoxin injection on phosphatidylinositol turnover and the contractile responses to NH4Cl and okadaic acid were investigated in aorta dissected from rats. The basal level of phosphatidylinositol hydrolysis and the phenylephrine- and 5-hydroxytryptamine-stimulated increase in hydrolysis were all markedly reduced in endotoxemic aortas as compared to in control aortas. Stimulation with KCl did not increase phosphatidylinositol hydrolysis in control or endotoxemic aortas. The NH4Cl-induced contractile response was significantly diminished in endotoxemic aorta, whereas the okadaic acid-induced contractile response was not altered. These results suggest that both transmembrane and intracellular signal transduction mechanisms are impaired in the endotoxemic artery whereas the contractile machinery remains intact.

Receptor-activated cytoplasmic Ca2+ spikes in communicating clusters of pancreatic acinar cells

The transmission of receptor-generated cytoplasmic Ca2+ signals between communicating pancreatic acinar cells has been investigated by comparing patch-clamp recordings of Ca(2+)-dependent Cl- current in internally perfused single cells and small multi-cell clusters. Acetylcholine (50 nM) generates shortlasting repetitive spikes of Ca(2+)-dependent current and these spikes are not transmitted to neighbouring cells. Cholecystokinin octapeptide (5 pM) also generates repetitive spikes, but a significant proportion of these trigger longer and larger pulses of Ca(2+)-dependent current and these waves can easily spread from cell to cell. In pancreatic acinar units it is therefore possible to observe both local Ca2+ signals confined to the cell of its origin as well as Ca2+ signals that spread through communicating junctions to all cells in the unit.

The neutrophil respiratory burst. Responses to fatty acids, N-formylmethionylleucylphenylalanine and phorbol ester suggest divergent signalling mechanisms

The oxygen-dependent respiratory burst is a key neutrophil function required for the killing of bacteria. However, despite intensive investigation, the molecular events which initiate the respiratory burst remain unclear. Recent reports have suggested the agonist-induced hydrolysis of cellular phosphatidylcholine (PtdCho) by phospholipase D may be an essential requirement for initiating or mediating the respiratory burst. We have investigated the effects of the chemotactic peptide N-formylmethionylleucylphenylalanine (fMLF), the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and the polyunsaturated fatty acids arachidonic [20:4 (n-6)] and docosahexaenoic [22:6 (n-3)] acids in light of this hypothesis. Ethanol-inhibited superoxide production in response to 20:4, 22:6 and fMLF, in a dose-dependent fashion, suggesting an involvement of phospholipase D. The phosphatidic-acid phosphohydrolase inhibitor DL-propranolol completely inhibited superoxide production induced by both 20:4 and 22:6, and partially inhibited the response to TPA. In contrast, superoxide production in response to fMLF was increased by propranolol. fMLF and TPA, but not the fatty acids, stimulated phospholipase D as indicated by the accumulation of phosphatidic acid and, in the presence of ethanol, phosphatidylethanol derived from PtdCho. Extracellular Ca2+ was found to be an essential requirement for fMLF-induced superoxide production. However, responses to the fatty acids were dramatically enhanced under Ca(2+)-free conditions. Responses to TPA were independent of the extracellular Ca2+ concentration. Both fatty acids and fMLF, but not TPA, mobilised Ca2+ from intracellular stores, a response insensitive to the effects of both ethanol and propranolol. These results show that, unlike fMLF and TPA, the fatty acids do not cause hydrolysis of PtdCho by phospholipase D. However, the data indirectly suggests that the fatty acids may initiate the phospholipase-D-catalysed hydrolysis of phospholipids other than PtdCho.

Auranofin inhibits the activation pathways of polymorphonuclear leukocytes at multiple sites

In order to characterize the mechanism by which the anti-rheumatic gold complex auranofin (AF) affects the functions of resting and activated polymorphonuclear leukocytes (PMN) the following studies were performed: (1) The effect of AF on the major processes involved in the respiratory burst of PMN: glucose transport and phosphorylation; hexose monophosphate (HMP) shunt activity in intact cells and in a cell-free system; superoxide production by particulate fractions and intact PMN measured as lucigenin-dependent chemiluminescence. (2) A comparison of the effects of AF added to the PMN before, at the time of, or subsequent to the stimulants [N-formyl-methionyl-leucyl phenylalanine (FMLP), concanavalin A (ConA), calcium ionophore (A23187) and phorbol myristate acetate (PMA)]. (3) The effect of AF on PMN activated by two stimulates (PMA, ConA) added sequentially. AF (0.1-10 microM) caused a dose-dependent inhibition of lucigenin-dependent chemiluminescence regardless of the activator (FMLP, ConA, A23187, PMA) when AF was added before the activator. In contrast, when AF was added to PMN after stimulation, it inhibited only the chemiluminescence of PMN stimulated by PMA. Furthermore, the chemiluminescence was largely unaffected by AF in sequentially activated PMN. The relative sensitivity to AF of the various processes studied indicates that blockade of the activation signal appears to be responsible for inhibition of the respiratory burst of PMN.

Signal transduction pathways in the induction of HLA class I antigen expression on Huh 6 cells by interferon-gamma

This study investigated the intracellular signal transduction regulating the appearance of HLA class I antigens on Huh 6 cells induced by interferon-gamma. The expression was blocked by a protein kinase C inhibitor, H-7, but not by a calmodulin antagonist, W-7, nor by a protein kinase A inhibitor, H-8, at low dose. The antigen expression was induced by a direct activator of protein kinase C, phorbol myristate acetate, but not by calcium ionophore A23187 nor an analog of cAMP, dbcAMP. Therefore, we concluded that protein kinase C is involved in the expression of HLA class I antigens on Huh 6 cells induced by interferon-gamma but Ca(2+)-calmodulin and cAMP are not.

Modifications of cell signalling in the cytotoxicity of metals

Many metals act on biological systems at low concentrations and recent epidemiological and experimental research indicates that toxic effects of certain metals occur at levels only marginally higher than those found in healthy subjects. Despite a large number of studies describing metal cytotoxicity, the molecular mechanisms involved are still poorly understood. However, it now seems evident that several metals can interact with enzyme functional groups and that proteins involved in signal transduction, including Ca2+ channels and pumps, may be especially sensitive to this interaction. Impairment of the ability of cells to adequately respond to the stimulation by hormones and growth factors may result in the loss of important cell functions or activation of mechanisms that compromise cell survival. In the following sections we will briefly describe the effects of various metals on cell signalling and present our recent findings on the mechanism by which inorganic mercury affects signal transduction.

Cellular processes in atherogenesis: potential targets of Ca2+ channel blockers

Atherosclerosis is characterized by increased endothelial permeability, monocyte infiltration, intimal smooth muscle cell (SMC) proliferation, platelet aggregation and the accumulation of lipids, calcium and extracellular matrix components in the vessel wall. In various animal studies and recently in humans it could be established that Ca2+ channel blockers delayed the progression of the atherosclerotic process at the stage of early lesions. This review surveys the interaction of Ca2+ channel blockers with various membrane proteins (purinergic receptors, nucleoside transporter, peripheral benzodiazepine receptors, multi-drug resistance protein) which are involved in signal transduction and their potential impact on the observed antiatherosclerotic effects. Although the precise mechanisms have yet to be fully elucidated, it has been clearly shown that these drugs inhibit smooth muscle cell proliferation and migration, improve cellular lipoprotein metabolism in vascular cells, alter phospholipid turnover, decrease platelet adhesion in the vessel wall, reduce extracellular matrix synthesis and protect against radical induced cell damage. Most of these effects are independent of Ca2+ flux across voltage-operated Ca2+ channels. However, all these processes are relevant to the pathogenesis of atherosclerosis and therefore the elucidation of the antiatherogenic mechanisms of Ca2+ channel blockers at the cellular level is of great interest. The future development of Ca2+ channel blockers with altered molecular structures optimized for their antiatherosclerotic targets may provide a useful tool in the therapy of atherosclerosis and risk factor intervention. The protective mechanisms are related to a stabilization of cell membrane integrity, the modulation of secretory activities and cell/cell communication processes rather than to a lowering of plasma lipoprotein levels.

Determinants of renin secretion and renin synthesis in isolated mouse juxtaglomerular cells

In this study we examined the effects of classic second messenger molecules on renin secretion and renin synthesis in primary cultures of mouse renal juxtaglomerular (JG) cells. Stimulation of cAMP formation by forskolin, inhibition of calmodulin by calmidazolium, and inhibition of Na+/H+ exchange by ethylisopropylamiloride enhanced renin secretion. Raising of intracellular cGMP by 8-bromo-cGMP and activation of protein kinase C by phorbol ester led to an inhibition of secretion. Renin synthesis was stimulated by forskolin. Calmidazolium, EIPA, 8-bromo-cGMP, and phorbol ester were without effect on basal renin synthesis. The data suggest that renin secretion is influenced by a number of transmembrane transduction systems which in their majority exert a negative control on renin secretion. Activation of adenylate cyclase appears to be a stimulatory control mechanism for both the secretion and the synthesis of renin. The findings suggest, moreover, that the second messenger controls of renin secretion and renin synthesis are not strictly linked in renal JG cells.

The mechanisms of action of lithium. II. Effects on adenylate cyclase activity and beta-adrenergic receptor binding in normal subjects

As part of a study of the effects of lithium carbonate on neurochemical function in man, platelet and lymphocyte adenylate cyclase activity and lymphocyte beta-adrenergic receptor binding characteristics were determined before and after 2 weeks of lithium treatment in 10 normal volunteers. Lithium had differential effects on platelet and lymphocyte adenylate cyclase activity. In platelets, basal and stimulated (guanyl imidodiphosphate [Gpp[NH]p] or cesium fluoride) adenylate cyclase activity was significantly augmented by lithium treatment. By contrast, in lymphocytes, Gpp(NH)p- and cesium fluoride-stimulated adenylate cyclase activity was unaffected, while basal activity was decreased modestly after lithium. These results are consistent with preclinical studies that suggest that lithium's effects on adenylate cyclase activity are specific with respect to tissue and brain region and that lithium may interfere with guanine nucleotide binding (G) protein function. Lithium treatment significantly increased the ratio of low- to high-affinity dissociation constants for agonist displacement of antagonist binding to lymphocyte beta-adrenergic receptors (thought to reflect coupling between the beta-adrenergic receptor and stimulatory G protein). Lithium had significant effects on measures associated with signal transduction that might be contrasted to its more subtle effects on neuronal function (norepinephrine release) and neuroendocrine systems (responses to serotoninergic challenge) in these same subjects (reported in a companion article). Lithium's primary site of action may be on signal transduction mechanisms. These effects subsequently may be manifested in changes in neurotransmitter function that may be important to lithium's mood-stabilizing actions.

Nonmutagenic mechanisms in carcinogenesis: role of protein kinase C in signal transduction and growth control

There is accumulating evidence that the multistage carcinogenic process is associated with the progressive acquisition of mutations in cellular proto-oncogenes and in growth-suppressor genes. At the same time, several types of evidence indicate that nongenotoxic agents and epigenetic events also play an important role in the evolution of tumors. One of the most intensively studied nongenotoxic agents is the phorbol ester tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and related compounds. Since TPA appears to exert its biologic effects through protein kinase C (PKC), a key enzyme in signal transduction, we have studied this enzyme in considerable detail. Our strategy has been to perturb signal transduction by developing cell lines that overexpress the beta 1 isoform of PKC. Such derivatives of rat fibroblasts display alterations in morphology and growth factors, altered expression of c-myc, ornithine decarboxylase, and phorbin, and increased susceptibility to transformation by certain oncogenes, H-ras, myc, and fos. These findings provide direct genetic evidence that PKC plays a critical role in growth control and the action of certain growth factors, tumor promoters, and oncogenes. In related studies, we have characterized the beta 1 isoform that is overproduced in the above cell systems in terms of its biochemical, kinetic, and immunologic properties. The enzyme has several properties characteristic of native PKCs. A surprising finding is that c-H-ras-transformed derivatives of the cells that overexpress PKC beta 1 display a several-fold increase in the expression of the endogenous alpha 1 isoform of PKC and a decrease in the expression of the endogenous epsilon isoform.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-steroidal anti-inflammatory drugs: effects on a GTP binding protein within the neutrophil plasma membrane

Sodium salicylate and other non-steroidal anti-inflammatory drugs (NSAIDs) inhibit neutrophil functions via unknown mechanisms. To examine their site of action in the neutrophil we have studied discrete events within the plasma membrane which depend upon the normal function of a GTP binding protein (G protein). We demonstrated that sodium salicylate and piroxicam inhibit neutrophil activation in response to stimuli which require signal transduction via a G protein (e.g. formyl-methionine-leucine-phenylalanine) but have no effect on stimuli which do not (e.g. phorbol myristate acetate, ionomycin). NSAIDs blocked the ADP-ribosylation of the pertussis toxin substrate in human neutrophils. This effect was associated with the capacity of NSAIDs to block pertussis toxin-dependent inhibition of neutrophil functions. Finally, NSAIDs inhibited the binding of GTP gamma S, a stable analog of GTP, to purified neutrophil membrane preparations. The data indicate that salicylate and other NSAIDs interact with a G protein in the neutrophil plasmalemma and thereby uncouple post-receptor signaling events.