G-protein beta gamma-subunits activate the cardiac muscarinic K+-channel via phospholipase A2

Functionally selective and biased agonists of muscarinic receptors

Disruption of cholinergic signalling via muscarinic receptors is associated with various pathologies, like Alzheimer's disease or schizophrenia. Selective muscarinic agonists possess therapeutic potential in the treatment of diabetes, pain or Sjögren's syndrome. The orthosteric binding site of all subtypes of the muscarinic receptor is structurally identical, making the development of affinity-based selective agonists virtually impossible. Some agonists, however, are functionally selective; they activate only a subset of receptors or signalling pathways. Others may stabilise specific conformations of the receptor leading to non-uniform modulation of individual signalling pathways (biased agonists). Functionally selective and biased agonists represent a promising approach for selective activation of individual subtypes of muscarinic receptors. In this work we review chemical structures, receptor binding and agonist-specific conformations of currently known functionally selective and biased muscarinic agonists in the context of their intricate intracellular signalling. Further, we take a perspective on the possible use of biased agonists for tissue and organ-specific activation of muscarinic receptors.

Profiling of Differential Expression of Genes in Mice Carrying Both Mutant Presenilin 1 and Amyloid Precursor Protein Transgenes with or without Knockout of B2 Adrenergic Receptor Gene

Alzheimer's disease (AD) is a lifelong progressive neurodegenerativa disease related with accumulation of amyloid β peptide (Aβ) produced by processing of amyloid precursor protein (APP) in the brain. In spite of several-decades effort on AD, there is still no medicine used to intervene with its pathological processes. Our previous studies made in transgenic animal models harboring familial AD genes of mutant presenilin 1 and amyloid precursor protein (APP) showed that β₂AR gene knock-out (β₂AR-KO) is beneficial in senile AD animals. Consistently, an epidemiological study lasted for two decades showed that the sole usage of β blockers as antihypertensive medicines is associated with fewer brain lesions and less brain shrinkage seen in senile AD patients. In order to understand why senile β₂AR-KO AD mice had better learning and memory, genomic effects of β₂AR-KO in the double transgenic AD mice were investigated. In the analysis, major genomic significance of β₂AR-KO was directed to influence protein-processing and presentation involving membrane structure and MHC class I and II protein complex, and lysosome and hydrolase activity for protein degradation, which are critical for accumulation of amyloid β peptide, the hallmark of AD.

Sensory TRP channel interactions with endogenous lipids and their biological outcomes

Lipids have long been studied as constituents of the cellular architecture and energy stores in the body. Evidence is now rapidly growing that particular lipid species are also important for molecular and cellular signaling. Here we review the current information on interactions between lipids and transient receptor potential (TRP) ion channels in nociceptive sensory afferents that mediate pain signaling. Sensory neuronal TRP channels play a crucial role in the detection of a variety of external and internal changes, particularly with damaging or pain-eliciting potentials that include noxiously high or low temperatures, stretching, and harmful substances. In addition, recent findings suggest that TRPs also contribute to altering synaptic plasticity that deteriorates chronic pain states. In both of these processes, specific lipids are often generated and have been found to strongly modulate TRP activities, resulting primarily in pain exacerbation. This review summarizes three standpoints viewing those lipid functions for TRP modulations as second messengers, intercellular transmitters, or bilayer building blocks. Based on these hypotheses, we discuss perspectives that account for how the TRP-lipid interaction contributes to the peripheral pain mechanism. Still a number of blurred aspects remain to be examined, which will be answered by future efforts and may help to better control pain states.

Arachidonic acid and ion channels: an update

Arachidonic acid (AA), a polyunsaturated fatty acid with four double bonds, has multiple actions on living cells. Many of these effects are mediated by an action of AA or its metabolites on ion channels. During the last 10 years, new types of ion channels, transient receptor potential (TRP) channels, store-operated calcium entry (SOCE) channels and non-SOCE channels have been studied. This review summarizes our current knowledge about the effects of AA on TRP and non-SOCE channels as well as classical ion channels. It aims to distinguish between effects of AA itself and effects of AA metabolites. Lipid mediators are of clinical interest because some of them (for example, leukotrienes) play a role in various diseases, others (such as prostaglandins) are targets for pharmacological therapeutic intervention.

Diverse Kir modulators act in close proximity to residues implicated in phosphoinositide binding

Inwardly rectifying potassium (Kir) channels were the first shown to be directly activated by phosphoinositides in general and phosphatidylinositol bisphosphate (PIP(2)) in particular. Atomic resolution structures have been determined for several mammalian and bacterial Kir channels. Basic residues, identified through mutagenesis studies to contribute to the sensitivity of the channel to PIP(2), have been mapped onto the three dimensional channel structure and their localization has given rise to a plausible model that can explain channel activation by PIP(2). Moreover, mapping onto the three-dimensional channel structure sites involved in the modulation of Kir channel activity by a diverse group of regulatory molecules, revealed a striking proximity to residues implicated in phosphoinositide binding. These observations support the hypothesis that the observed dependence of diverse modulators on channel-PIP(2) interactions stems from their localization within distances that can affect PIP(2)-interacting residues.

Immunohistochemical localization of G protein betagamma subunits in the lateral wall of the rat cochlea

The role of G protein-mediated signal transduction in the production of endolymph, an extracellular fluid of unusual ionic composition, is beginning to be understood. The identity of Galpha subunits in the stria vascularis and the spiral ligament of the lateral wall of the cochlear duct is well established. However, little is known about the presence of betagamma subunits. This study used immunohistochemistry to investigate the distribution of G protein betagamma subunits in the lateral wall of the cochlea. Temporal bones of 6- to 8-week-old rats were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for embedding in paraffin wax. The dewaxed, midmodiolar sections of the cochlea were incubated with subunit-specific polyclonal antibodies. The results show that the pattern of immunoreactivity varies for the G protein beta1-4 and gamma1-3, 5 and 7 subunits in the stria vascularis and spiral ligament. In the stria vascularis, immunoreactivity was detected for beta2, beta3, beta4, gamma1, gamma2 and gamma7 subunits. All five types of fibrocytes in the spiral ligament exhibited positive staining for gamma2 and gamma7. However, immunoreactivity for beta1-4 subunits was variable. Immunoreactivity for gamma3 and gamma5 subunits was not detected in the lateral cochlear wall. The expression pattern of G protein betagamma subunits in lateral wall provides a basis for interpreting the functions of G protein-coupled receptors in cochlear fluid homeostasis.

Somatostatin increases phospholipase D activity and phosphatidylinositol 4,5-bisphosphate synthesis in clonal beta cells HIT-T15

In the presence of arginine vasopressin (AVP), somatostatin increases [Ca(2+)](i), leading to a transient increase in insulin release from clonal beta cells HIT-T15 via G(i/o) and phospholipase C (PLC) pathway (Cheng et al., 2002a). The present study was to elucidate the mechanisms underlying somatostatin-induced [Ca(2+)](i) increase in the presence of AVP. We found that the effect of somatostatin was mediated by betagamma subunits but not by the alpha subunit of G(i/o). Because somatostatin alone failed to increase [Ca(2+)](i), we hypothesized that somatostatin increases phosphatidylinositol 4,5-bisphosphate (PIP(2)) synthesis, providing extra substrate for preactivated PLC-beta to generate inositol 1,4,5-trisphosphate (IP(3)). Somatostatin alone did not increase IP(3) levels, but AVP + somatostatin did. Somatostatin increased PIP(2) levels but decreased phosphatidylinositol 4-phosphate levels. We further hypothesized that PLD mediates somatostatin-induced changes in PIP(2) levels. Both the phospholipase D (PLD) inhibitors and antibody versus PLD1 antagonized AVP-somatostatin-induced increases in [Ca(2+)](i). PLD inhibitor also antagonized somatostatin-induced increase in PIP(2) levels. In addition, somatostatin increased PLD activity. These results suggest that activation of somatostatin receptors that are coupled to the betagamma dimer of G(i/o) led to PLD1 activation, thus promoting the synthesis of phosphatidic acid. Phosphatidic acid activates PIP-5 kinase, which evokes an increase in PIP(2) synthesis. The PIP(2) generated by somatostatin administration increases substrate for preactivated phospholipase C-beta, which hydrolyzes PIP(2) to form IP(3), leading to an increase in [Ca(2+)](i). The regulation of PIP(2) synthesis by G(i/o)-coupled receptors via PLD activation represents a novel signaling mechanism for somatostatin and a novel concept in the cross-talk between G(q)- and G(i/o)-coupled receptors in beta cells.

The cardiac beta2-adrenergic signalling a new role for the cPLA2

The cardiac actions of catecholamines have long been attributed to the predominant beta(1)-AR subtype that couples to the classical Gs/AC/cAMP pathway. Recent research clearly indicates that cardiac beta(2)-ARs play a functional role in healthy heart and assume increasing importance in failing and aged heart. beta(2)-ARs are primarily coupled to an atypical compartmentalized cAMP pathway, regulated by phosphorylation and/or oligomerization of beta(2)-ARs, and under the control of additional beta(2)-AR/Gi-coupled lipidic pathways, the impact of which seems to vary depending on the animal species, the developmental and pathophysiological state. This review focuses, more especially, on one of the last identified beta(2)-AR/Gi pathway, namely the cPLA(2).

Nitric oxide triggers the toxicity due to glutathione depletion in midbrain cultures through 12-lipoxygenase

Glutathione (GSH) depletion is the earliest biochemical alteration shown to date in brains of Parkinson's disease patients. However, data from animal models show that GSH depletion by itself is not sufficient to induce nigral degeneration. We have previously shown that non-toxic inhibition of GSH synthesis with l-buthionine-(S,R)-sulfoximine in primary midbrain cultures transforms a nitric oxide (NO) neurotrophic effect, selective for dopamine neurons, into a toxic effect with participation of guanylate cyclase (GC) and cGMP-dependent protein kinase (PKG) (Canals, S., Casarejos, M. J., de Bernardo, S., Rodríguez-Martín, E., and Mena, M. A. (2001) J. Neurochem. 79, 1183-1195). Here we demonstrate that arachidonic acid (AA) metabolism through the 12-lipoxygenase (12-LOX) pathway is also central for this GSH-NO interaction. LOX inhibitors (nordihydroguaiaretic acid and baicalein), but not cyclooxygenase (indomethacin) or epoxygenase (clotrimazole) ones, prevent cell death in the culture, even when added 10 h after NO treatment. Furthermore, the addition of AA to GSH-depleted cultures precipitates a cell death process that is indistinguishable from that initiated by NO in its morphology, time course, and 12-LOX, GC, and PKG dependence. The first AA metabolite through the 12-LOX enzyme, 12-hydroperoxyeicosatetraenoic acid, induces cell death in the culture, and its toxicity is greatly enhanced by GSH depletion. In addition we show that if GSH synthesis inhibition persists for up to 4 days without any additional treatment, it will induce a cell death process that also depends on 12-LOX, GC, and PKG activation. In this study, therefore, we show that the signaling pathway AA/12-LOX/12-HPETE/GC/PKG may be important in several pathologies in which GSH decrease has been documented, such as Parkinson's disease. The potentiating effect of NO over such a signaling pathway may be of relevance as part of the cascade of events leading to and sustaining nerve cell death.

PKC-independent inhibition of glutamate exocytosis by arachidonic acid in rat cerebrocortical synaptosomes

In rat cerebrocortical synaptosomes, the addition of 4 beta-phorbol dibutyrate (4 beta-PDBu) and arachidonic acid enhances and decreases, respectively, the glutamate release evoked by 4-aminopyridine. Pretreatment of synaptosomes with 12-O-tetradecanoylphorbol 13-acetate (TPA) or pre-incubation with staurosporine, prevent the stimulatory effect of 4 beta-PDBu, but are without effect on the inhibitory action of arachidonic acid. Moreover, methyl arachidonate, which is not effective as a PKC activator, also strongly inhibits glutamate exocytosis. These results suggest that PKC is not involved in the inhibition of glutamate release by arachidonic acid.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

The association of heterotrimeric GTP-binding protein (Go) with microtubules

The heterotrimeric GTP-binding regulatory proteins (G proteins) play an important role in the regulation of membrane signal transduction. Recently, we identified the association of Go protein with mitotic spindles. Here we have investigated the relationship between Go protein and microtubules. We used temperature-dependent reversible assembly and taxol methods to purify microtubules from bovine brains. Goalpha and Gbeta proteins were identified in the microtubular fraction by both methods. The Goalpha subunit in the microtubular fraction could be ADP ribosylated by pertussis toxin. Co-immunoprecipitation data also revealed that Go protein can interact with microtubules. Exogenous Go protein could be incorporated into the assembled microtubular fraction, and 5 microg/ml (60 nM) of Go protein inhibited 40% of microtubule assembly. Western blot analysis of Goalpha-1 and Goalpha-2 in microtubular fractions showed that only Goalpha-1 is associated with microtubules. We conclude that the Goalpha-1betagamma proteins are associated with microtubules and may play some role in regulating the assembly and disassembly of microtubules.

A new pathway for glucose-dependent insulinotropic polypeptide (GIP) receptor signaling: evidence for the involvement of phospholipase A2 in GIP-stimulated insulin secretion

The hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of insulin secretion. GIP has been shown to increase adenylyl cyclase activity, elevate intracellular Ca(2+) levels, and stimulate a mitogen-activated protein kinase pathway in the pancreatic beta-cell. In the current study we demonstrate a role for arachidonic acid in GIP-mediated signal transduction. Static incubations revealed that both GIP (100 nm) and ATP (5 microm) significantly increased [(3)H]arachidonic acid ([(3)H]AA) efflux from transfected Chinese hamster ovary K1 cells expressing the GIP receptor (basal, 128 +/- 11 cpm/well; GIP, 212 +/- 32 cpm/well; ATP, 263 +/- 35 cpm/well; n = 4; p < 0.05). In addition, GIP receptors were shown for the first time to be capable of functionally coupling to AA production through Gbetagamma dimers in Chinese hamster ovary K1 cells. In a beta-cell model (betaTC-3), GIP was found to elicit [(3)H]AA release, independent of glucose, in a concentration-dependent manner (EC(50) value of 1.4 +/- 0.62 nm; n = 3). Although GIP did not potentiate insulin release under extracellular Ca(2+)-free conditions, it was still capable of elevating intracellular cAMP and stimulating [(3)H]AA release. Our data suggest that cAMP is the proximal signaling intermediate responsible for GIP-stimulated AA release. Finally, stimulation of GIP-mediated AA production was shown to be mediated via a Ca(2+)-independent phospholipase A(2). Arachidonic acid is therefore a new component of GIP-mediated signal transduction in the beta-cell.

G protein beta2 subunit antisense oligonucleotides inhibit cell proliferation and disorganize microtubule and mitotic spindle organization

The association of G protein beta2 subunit (Gbeta2) with mitotic spindles in various mammalian cells has been demonstrated previously. Recently, we have identified the association of Gbeta2 protein with microtubules (Wu et al., [1998] J. Cell. Biochem. 70: 552-562). In the present experiment we have demonstrated the possible functional role of Gbeta2 in microtubule and mitotic spindle organization in mammalian cells. When Gbeta2 antisense phosphorothioate oligonucleotides were transfected into mammalian cells, inhibition of cell proliferation with cell death after a 4-day treatment was observed. If the transfected cells were incubated for two days and their Gbeta2 and microtubules were examined by Western blotting and immunofluorescence localization, marked reduction of the Gbeta2 protein, fragmentation and disassembly of cytoplasmic microtubules, and disorganized mitotic spindles were found. We conclude that the Gbeta2 protein is closely associated with microtubule assembly and may play a potential role in the regulation of cell proliferation and microtubule and mitotic spindle organization in mammalian cells.

Human bone cell hyperpolarization response to cyclical mechanical strain is mediated by an interleukin-1beta autocrine/paracrine loop

Mechanical stimuli imparted by stretch, pressure, tension, fluid flow, and shear stress result in a variety of biochemical responses important in bone (re)modeling. The molecules involved in the recognition and transduction of mechanical stimuli that lead to modulation of bone cell function are not yet fully characterized. Cyclical pressure-induced strain (PIS) induces a rapid change in membrane potential of human bone cells (HBC) because of opening of membrane ion channels. This response is mediated via integrins and requires tyrosine kinase activity and an intact actin cytoskeleton. We have used this electrophysiological response to further study the signaling events occurring early after mechanical stimulation of HBC. Stimulation of HBC at 0.33 Hz PIS, but not 0.104 Hz PIS, results in the production of a transferable factor that induces membrane hyperpolarization of unstimulated HBC. The production of this factor is inhibited by antibodies to beta1-integrin. Interleukin-1beta (IL-1beta and prostaglandin E2 (PGE2) were identified as candidate molecules for the transferable factor as both were shown to induce HBC hyperpolarization by opening of small conductance calcium-activated potassium channels, the means by which 0.33 Hz PIS causes HBC hyperpolarization. Antibodies to IL-1beta, but not other cytokines studied, inhibit the hyperpolarization response of HBC to 0.33 Hz PIS. Comparison of the signaling pathways required for 0.33 Hz PIS and IL-1beta-induced membrane hyperpolarization shows that both involve the phospholipase C/inositol triphosphate pathway, protein kinase C (PKC), and prostaglandin synthesis. Unlike 0.33 Hz PIS-induced membrane hyperpolarization, IL-1beta-induced hyperpolarization does not require tyrosine kinase activity or an intact actin cytoskeleton. These studies suggest that 0.33 Hz PIS of HBC induces a rapid, integrin-mediated, release of IL-1beta with a subsequent autocrine/paracrine loop resulting in membrane hyperpolarization. IL-1beta production in response to mechanical stimuli is potentially of importance in regulation of bone (re)modeling.

Altered electrophysiological responses to mechanical stimulation and abnormal signalling through alpha5beta1 integrin in chondrocytes from osteoarthritic cartilage

OBJECTIVE

To establish whether chondrocytes from normal and osteoarthritic human articular cartilage recognize and respond to pressure induced mechanical strain in a similar manner.

DESIGN

Chondrocytes, extracted from macroscopically normal and osteoarthritic human articular cartilage obtained from knee joints at autopsy, were grown in monolayer culture and subjected to cyclical pressure-induced strain (PIS) in the absence or presence of anti-integrin antibodies, agents known to block ion channels and inhibitors of key molecules involved in the integrin-associated signalling pathways. The response of the cells to mechanical stimulation was assessed by measuring changes in membrane potential.

RESULTS

Unlike chondrocytes from normal articular cartilage, which showed a membrane hyperpolarization response to PIS, chondrocytes from osteoarthritic cartilage responded by membrane depolarization. The mechanotransduction pathway involves alpha5beta1 integrins, stretch-activated ion channels, tyrosine kinases and phospholipase C but the actin cytoskeleton and protein kinase C, which are important in the membrane hyperpolarization response in normal chondrocytes, are not necessary for membrane depolarization in osteoarthritic chondrocytes in response to PIS.

CONCLUSION

Chondrocytes derived from osteoarthritic cartilage show a different signalling pathway via alpha5beta1 integrin in response to mechanical stimulation which may be of importance in the production of phenotypic changes recognized to be present in diseased cartilage.

Cytosolic phospholipase A2 and the distinct transcriptional programs of astrocytoma cells

Astrocytes constitute the most abundant cell type in the nervous system. Under physiological conditions, they respond to the stimuli to which neurons are also responsive. The use of astrocytoma cell lines with well-defined morphological and functional markers has been helpful for addressing the mechanisms of signal transduction that operate in the nervous system. On the basis of the effects produced by agonists of different types of receptor (muscarinic ACh receptors, thrombin receptors, phospholipases A2 receptors and tumor necrosis factor alpha receptors), several different transcriptional programs that involve the MAP kinase-cytosolic phospholipase A2 system and the transcription factor NF-kappaB have been described.

Inhibition of cystic fibrosis transmembrane conductance regulator chloride channel currents by arachidonic acid

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is inhibited by a number of different classes of organic anions which are able to enter and block the channel pore from its cytoplasmic end. Here I show, using patch clamp recording from CFTR-transfected baby hamster kidney cell lines, that the cis-unsaturated fatty acid arachidonic acid also inhibits CFTR Cl- currents when applied to the cytoplasmic face of excised membrane patches. This inhibition was of a relatively high affinity compared with other known CFTR inhibitors, with an apparent Kd of 6.5 ± 0.9 µM. However, in contrast with known CFTR pore blockers, inhibition by arachidonic acid was only very weakly voltage dependent, and was insensitive to the extracellular Cl- concentration. Arachidonic acid-mediated inhibition of CFTR Cl- currents was not abrogated by inhibitors of lipoxygenases, cyclooxygenases or cytochrome P450, suggesting that arachidonic acid itself, rather than some metabolite, directly affects CFTR. Similar inhibition of CFTR Cl- currents was seen with other fatty acids, with the rank order of potency linoleic [Formula: see text] arachidonic [Formula: see text] oleic > elaidic [Formula: see text] palmitic [Formula: see text] myristic. These results identify fatty acids as novel high affinity modulators of the CFTR Cl- channel.Key words: CFTR, chloride channel, fatty acid, channel block, cystic fibrosis.

Direct activation of capsaicin receptors by products of lipoxygenases: endogenous capsaicin-like substances

Capsaicin, a pungent ingredient of hot peppers, causes excitation of small sensory neurons, and thereby produces severe pain. A nonselective cation channel activated by capsaicin has been identified in sensory neurons and a cDNA encoding the channel has been cloned recently. However, an endogenous activator of the receptor has not yet been found. In this study, we show that several products of lipoxygenases directly activate the capsaicin-activated channel in isolated membrane patches of sensory neurons. Among them, 12- and 15-(S)-hydroperoxyeicosatetraenoic acids, 5- and 15-(S)-hydroxyeicosatetraenoic acids, and leukotriene B(4) possessed the highest potency. The eicosanoids also activated the cloned capsaicin receptor (VR1) expressed in HEK cells. Prostaglandins and unsaturated fatty acids failed to activate the channel. These results suggest a novel signaling mechanism underlying the pain sensory transduction.

Cytoplasmic unsaturated free fatty acids inhibit ATP-dependent gating of the G protein-gated K(+) channel

This study reports the identification of an endogenous inhibitor of the G protein-gated (K(ACh)) channel and its effect on the K(ACh) channel kinetics. In the presence of acetylcholine in the pipette, K(ACh) channels in inside-out atrial patches were activated by applying GTP to the cytoplasmic side of the membrane. In these patches, addition of physiological concentration of intracellular ATP (4 mM) upregulated K(ACh) channel activity approximately fivefold and induced long-lived openings. However, such ATP-dependent gating is normally not observed in cell-attached patches, indicating that an endogenous substance that inhibits the ATP effect is present in the cell. We searched for such an inhibitor in the cell. ATP-dependent gating of the K(ACh) channel was inhibited by the addition of the cytosolic fraction of rat atrial or brain tissues. The lipid component of the cytosolic fraction was found to contain the inhibitory activity. To identify the lipid inhibitor, we tested the effect of approximately 40 different lipid molecules. Among the lipids tested, only unsaturated free fatty acids such as oleic, linoleic, and arachidonic acids (0.2-2 microM) reversibly inhibited the ATP-dependent gating of native K(ACh) channels in atrial cells and hippocampal neurons, and of recombinant K(ACh) channels (GIRK1/4 and GIRK1/2) expressed in oocytes. Unsaturated free fatty acids also inhibited phosphatidylinositol-4, 5-bisphosphate (PIP(2))-induced changes in K(ACh) channel kinetics but were ineffective against ATP-activated background K(1) channels and PIP(2)-activated K(ATP) channels. These results show that during agonist-induced activation, unsaturated free fatty acids in the cytoplasm help to keep the cardiac and neuronal K(ACh) channels downregulated by antagonizing their ATP-dependent gating. The opposing effects of ATP and free fatty acids represent a novel regulatory mechanism for the G protein-gated K(+) channel.

Phospholipase A(2) is involved in thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that emptying of intracellular Ca(2+) pools with endoplasmatic Ca(2+)-ATP-ase inhibitor thapsigargin leads to the Na(+) influx in human lymphocytes (M. Tepel et al., 1994, J. Biol. Chem. 269, 26239-26242). In the present study we examined the mechanism underlying the thapsigargin-induced Na(+) entry. We found that the thapsigargin-induced increase in Na(+) concentration was effectively inhibited by three structurally unrelated phospholipase A(2) (PLA(2)) inhibitors, p-bromophenacyl bromide, 3-(4-octadecyl)-benzoylacrylic acid (OBAA), and bromoenol lactone (BEL). The thapsigargin-induced Na(+) influx could be mimicked by PLA(2) exogenously added to the lymphocyte suspension. In addition, thapsigargin stimulated formation of arachidonic acid (AA), the physiological PLA(2) product. AA induced Na(+) entry in a time- and concentration-dependent fashion. Both, thapsigargin-induced Na(+) influx and AA liberation were completely inhibited in the presence of tyrosine kinase inhibitor genistein but not in the absence of extracellular Ca(2+). Collectively, these data show that thapsigargin-induced Na(+) entry is associated with tyrosine kinase-dependent stimulation of PLA(2).

Structure-activity relationships of G protein-coupled receptors

The primary function of cell-surface receptors is to discriminate the specific signaling molecule or ligand from a large array of chemically diverse extracellular substances and to activate an effector signaling cascade that triggers an intracellular response and eventually a biological effect. G protein-coupled cell-surface receptors (GPCRs) mediate their intracellular actions through the activation of guanine nucleotide-binding signal-transducing proteins (G proteins), which form a diverse family of regulatory GTPases that, in the GTP-bound state, bind and activate downstream membrane-localized effectors. Hundreds of GPCRs signal through one or more of these G proteins in response to a large variety of stimuli including photons, neurotransmitters, and hormones of variable molecular structure. The mechanisms by which these ligands provoke activation of the receptor/G-protein system are highly complex and multifactorial. Knowledge and mapping of the structural determinants and requirements for optimal GPCR function are of paramount importance, not only for a better and more detailed understanding of the molecular basis of ligand action and receptor function in normal and abnormal conditions, but also for a rational design of early diagnostic and therapeutic tools that may allow exogenous regulation of receptor and G protein function in disease processes.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Oxidative downmodulation of the transient K-current IA by intracellular arachidonic acid in rat hippocampal neurons

Membrane-permeable arachidonic acid (AA) is liberated in a Ca2+-dependent way inside cells. By using whole cell patch clamp we show that intracellular AA (1 pM) selectively reduces IA in rat hippocampal neurons, whereas extracellular application requires a 10(6)-fold concentration. The nonmetabolized AA analogue ETYA mimics the effect of AA that is blocked by ascorbic acid or intracellular glutathione, suggesting an intracellular oxidative mechanism. We conclude that intracellular AA is extremely potent in reducing IA by an oxidative mechanism, particularly during oxidative stress.

ADP-induced platelet aggregation and inhibition of adenylyl cyclase activity stimulated by prostaglandins: signal transduction mechanisms

ADP is the oldest and one of the most important agonists of platelet activation. ADP induces platelet shape change, exposure of fibrinogen binding sites, aggregation, and influx and intracellular mobilization of Ca2+. ADP-induced platelet aggregation is important for maintaining normal hemostasis, but aberrant platelet aggregation manifests itself pathophysiologically in myocardial ischemia, stroke, and atherosclerosis. Another important aspect of ADP-induced platelet activation is the ability of ADP to antagonize adenylyl cyclase activated by prostaglandins. ADP-induced inhibition of the stimulated adenylyl cyclase activity does not appear to play a role in ADP-induced platelet aggregation in vitro or in vivo. It is believed that a single ADP receptor mediates the above two ADP-induced platelet responses in platelets. The ADP receptor mediating ADP-induced platelet aggregation and inhibition of the stimulated adenylyl cyclase activity has not been purified. Therefore, the nature of molecular mechanisms underlying the two seemingly unrelated ADP-induced platelet responses remains either unclear or less well understood. The purpose of this commentary is to examine and make suggestions concerning the role of phospholipases and G-proteins in the molecular mechanisms of signal transduction underlying the two ADP-induced platelet responses. It is hoped that such discussion would stimulate thinking and invite future debates on this subject, and energize investigators in their efforts to advance our knowledge of the details of the molecular mechanisms of ADP-induced platelet activation.

Integrin-regulated secretion of interleukin 4: A novel pathway of mechanotransduction in human articular chondrocytes

Chondrocyte function is regulated partly by mechanical stimulation. Optimal mechanical stimulation maintains articular cartilage integrity, whereas abnormal mechanical stimulation results in development and progression of osteoarthritis (OA). The responses of signal transduction pathways in human articular chondrocytes (HAC) to mechanical stimuli remain unclear. Previous work has shown the involvement of integrins and integrin-associated signaling pathways in activation of plasma membrane apamin-sensitive Ca2+-activated K+ channels that results in membrane hyperpolarization of HAC after 0. 33 Hz cyclical mechanical stimulation. To further investigate mechanotransduction pathways in HAC and show that the hyperpolarization response to mechanical stimulation is a result of an integrin-dependent release of a transferable secreted factor, we used this response. Neutralizing antibodies to interleukin 4 (IL-4) and IL-4 receptor alpha inhibit mechanically induced membrane hyperpolarization and anti-IL-4 antibodies neutralize the hyperpolarizing activity of medium from mechanically stimulated cells. Antibodies to interleukin 1beta (IL-1beta) and cytokine receptors, interleukin 1 receptor type I and the common gamma chain/CD132 (gamma) have no effect on me- chanically induced membrane hyperpolarization. Chondrocytes from IL-4 knockout mice fail to show a membrane hyperpolarization response to cyclical mechanical stimulation. Mechanically induced release of the chondroprotective cytokine IL-4 from HAC with subsequent autocrine/paracrine activity is likely to be an important regulatory pathway in the maintenance of articular cartilage structure and function. Finally, dysfunction of this pathway may be implicated in OA.

G protein beta subunit is closely associated with microtubules

Previously, we have identified the association of G protein beta subunit (Gbeta) with mitotic spindles in various mammalian cells. Since microtubules are the main component of mitotic spindles, here we have isolated bovine brain microtubules and purified Gbeta subunit to identify the close association of Gbeta subunit with purified brain microtubules and have shown the direct incorporation of Gbeta subunit into the microtubules both in vitro and in vivo. It was found that: (1) microtubular fraction isolated from bovine brain contained Gbeta subunit, (2) coimmunoprecipitation demonstrated that Gbeta subunit could be coprecipitated with tubulin, (3) addition of purified Gbeta subunit into cytosolic extract for microtubule assembly caused direct incorporation of Gbeta subunit into assembled microtubules and increased the association of microtubule-associated proteins with microtubules, and (4) incubation of exogenous Gbeta subunit with detergent-permeabilized cells resulted in direct incorporation of Gbeta subunit into microtubule fibers and depolymerized tubulin molecules. We conclude that G protein beta subunit is closely associated with microtubules and may play an important role in the regulation of microtubule formation in addition to its regulatory role in cellular signal transduction.

Omega-3 polyunsaturated fatty acids--modulation of voltage-dependent L-type Ca2+ current in guinea-pig tracheal smooth muscle cells

Omega-3 polyunsaturated fatty acids have been reported to be associated with favorable changes in the respiratory system. To determine one of the mechanisms for this effect, membrane currents were recorded in guinea-pig tracheal myocytes by using the whole-cell voltage clamp technique. Without EGTA in the patch pipette containing the Cs-internal solution, command voltage pulses positive to +0 mV from a holding potential of -60 mV elicited a voltage-dependent L-type Ca2+ current (I(Ca x L)) and a subsequent outward current. Upon repolarization, slowly decaying inward tail currents were recorded. The outward currents and the inward tail current were enhanced by methyl-1,4,-dihydro-2,6-dimethyl-3-nitro-4-(2-trigluromethylphenyl )-pyridine-5-carboxylate, and blocked by Cd2+ or nifedipine. Inclusion of EGTA (5 mM) in the patch pipette also abolished these currents, indicating that they were Ca2+-dependent. When [Cl-]o or [Cl-]i was changed, the reversal potential of these currents shifted, thus behaving like a Cl(-)-sensitive ion channel. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonic acid. a Cl- channel blocker, inhibited the currents. The omega-3 polyunsaturated fatty acids eicosapentaenoic acid (3-30 microM) and docosahexaenoic acid (30 microM) suppressed I(Ca x L) and then inhibited I(Ca x Cl) in a reversible manner. Similar inhibitory effects of eicosapentaenoic acid on I(Ca x L) were observed with 5 mM EGTA in the patch pipette. Neurokinin A (1 microM) and caffeine (10 mM) also transiently activated I(Cl x Ca), probably due to Ca2+ release from Ca2+ storage sites. Pretreatment of the cells with eicosapentaenoic acid markedly suppressed the activation of I(Cl x Ca) by neurokinin A or caffeine. These results suggest that omega-3 polyunsaturated fatty acids inhibit voltage-dependent L-type Ca2+ currents and also Ca2+-activated Cl- currents in tracheal smooth muscle cells from the guinea-pig, which may play a role in modulation of tracheal smooth muscle tone.

CD4/GP120mac251 interaction induces phospholipase A2 (PLA2) activation in cynomolgus monkey lymphocytes

Cynomolgus monkey are susceptible to infection with select simian immunodeficiency virus (SIV). We investigated the early interactions between SIV envelope glycoproteins (gp120mac251) and macaque lymphocytes. Our results demonstrate that the soluble viral glycoprotein induce a specific phospholipase A2 (PLA2) activation in lymphocytes through CD4. This PLA2 activation, induced after envelope glycoprotein-CD4 interaction, because of its locally destabilizing membrane effect, may have important implications for preparing the lymphocyte membrane for fusion with the viral particle. However, this effect is not sufficient to accomplish fusion. These data indicate that the specific step of fusion may be downstream from PLA2 activation.

Effects of the 5-lipoxygenase activating protein inhibitor MK886 on voltage-gated and Ca2+-activated K+ currents in rat arterial myocytes

1. The effects on the voltage-gated (IK) and Ca2+ activated (I(K,Ca)) K+ currents in rat arterial myocytes of the 5-lipoxygenase activating protein (FLAP) inhibitor MK886, and its inactive analogue L583,916 were evaluated. 2. In rat pulmonary arterial myocytes (RPAMs), MK886 caused a concentration-dependent reduction of the IK, with little obvious change in the kinetics of the current. Half maximal current block was observed at 75 nM MK886. 3. MK886 application led to a concentration-dependent increase in the amplitude of the TEA-sensitive I(K,Ca) current and single channel activity in RPAMs in whole cell and inside-out configurations, respectively. The threshold concentration for this effect was approximately 300 nM and a maximal 4-5 fold increase was observed at 10 microM MK886. MK886 also increased I(K,Ca) in rat mesenteric arterial myocytes (RMAMs). 4. L538,916, an analogue of MK886 which does not block FLAP, had no effect on either IK or I(K,Ca) at a concentration of 10 microM. 5. Leukotriene C4 (100 nM) had no effect on either IK or I(K,Ca) in RPAMs. MK886 produced its usual increase in I(K,Ca) and also blocked IK, in the presence of leukotriene C4. Similarly, leukotriene E4 (100 nM) did not alter the amplitude of IK. Also, the nonselective leukotriene receptor antagonist ICI 198,615 (3 microM) did not affect IK in RPAMs, and did not affect the response to MK886. 6. Arachidonic acid (10 microM) enhanced I(K,Ca) in both RPAMs and RMAMs. 7. The results show that MK886 markedly affects both IK and I(K,Ca) in a manner similar to that of arachidonic acid and independent of the endogenous production of leukotrienes. It is therefore possible that MK886, which is thought to compete with arachidonic acid for its binding to FLAP, may similarly occupy arachidonic acid binding sites on these K+ channels, and mimic its effects. Alternatively, MK886 might act via non-selective effects on other arachidonic acid metabolites which could modify K+ channel function.

Neuronal ion channel signalling pathways: modulation by angiotensin II

The brain contains both angiotensin II (Ang II) type 1 (AT1) and Ang II type 2 (AT2) receptors. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. The physiological actions of Ang II in the brain, whether mediated by AT1 or AT2 receptors, involve changes in neuronal activity that are initiated by changes in the activity of membrane ionic currents and channels. This review focusses on the intracellular signalling pathways that couple neuronal AT1 and AT2 receptors to changes in the activity of membrane K+ and Ca2+ currents and channels. As will become clear from our discussion, the signalling pathways that are modulated by neuronal AT1 and AT2 receptors are quite distinct.

Phospholipases A2 of rod outer segment-free bovine retinae are different from well-known phospholipases A2

We have recently demonstrated the presence of phospholipase A2 (PLA2) activity in a rod outer segment-free retinal fraction which we called P200 and which contains neuronal cells, Müller cells and rod inner segments. We report here our results on the characterization of this P200-PLA2 activity. We show that P200 probably contains more than one type of PLA2, as indicated by the results obtained with different chromatographically eluted PLA2-active fractions which were treated with either Ca2+, EGTA, dithiothreitol (DTT) or p-bromophenacyl bromide (pBPB), or heated. Moreover, the results from PLA2 assays using different substrates, as well as those obtained after treatment of the homogenate with H2SO4, guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) and ATP, suggest that P200-PLA2 are different from well-known secretory PLA2, cytosolic PLA2 and Ca2+-independent PLA2. Control experiments using our 'back-and-forth'-thin layer chromatography (bf-TLC) technique allowed us to confirm that, in our assay conditions, the release of fatty acids was due to PLA2 enzymes. These results, which constitute the first characterization of PLA2 of the neural retina, thus suggest that it contains novel types of PLA2 enzyme, in contrast to well-known PLA2.

Activation of the atrial KACh channel by the betagamma subunits of G proteins or intracellular Na+ ions depends on the presence of phosphatidylinositol phosphates

The betagamma subunits of GTP-binding proteins (Gbetagamma) activate the muscarinic K+ channel (KACh) in heart by direct binding to both of its component subunits. KACh channels can also be gated by internal Na+ ions. Both activation mechanisms show dependence on hydrolysis of intracellular ATP. We report that phosphatidylinositol 4,5-bisphosphate (PIP2) mimics the ATP effects and that depletion or block of PIP2 retards the stimulatory effects of Gbetagamma subunits or Na+ ions on channel activity, effects that can be reversed by restoring PIP2. Thus, regulation of KACh channel activity may be crucially dependent on PIP2 and phosphatidylinositol signaling. These striking functional results are in agreement with in vitro biochemical studies on the PIP2 requirement for Gbetagamma stimulation of G protein receptor kinase activity, thus implicating phosphatidylinositol phospholipids as a potential control point for Gbetagamma-mediated signal transduction.

Alterations in beta-adrenoceptor mechanisms in the aging heart. Relationship with heart failure

In chronic heart failure substantial and characteristic changes occur in the function of the adrenergic nervous system. Studies in isolated left ventricular muscle and in single cardiomyocytes from experimental models of aging and, recently, from humans show an age-related reduced contractile response to beta-adrenoceptor stimulation. "beta-adrenoceptor desensitization" is thought to be a general and common mechanism to explain the age- and heart failure-related decrease in beta-adrenoceptor response. The aim of this review is to compare alterations in beta-adrenoceptor mechanisms in physiological cardiovascular aging and chronic heart failure. From an analysis of the overall data on the role of aging in beta-adrenoceptor regulation in human and animal hearts, it is possible to conclude that the reduced response to beta-agonists is common to all species and all cardiac tissues. Moreover, the age-related changes are limited to beta-adrenoceptor-G-protein (s)-adenylyl cyclase system abnormalities, while the type and level of abnormalities change with species and tissues. The modifications shown in the aging heart are not very different from some observed in heart failure. In particular, both in aged and failing hearts we may see that the decrease in beta-adrenoceptor responsiveness is related to changes in G-protein function.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

Bovine retinal pigment epithelium contains novel types of phospholipase A2

We have recently demonstrated the presence of phospholipase A2 (PLA2) activity in cells from bovine retinal pigment epithelium (RPE) [Jacob et al. (1996) J. Biol. Chem. 271, 19209-19218]. We report here our results on the characterization of this RPE-PLA2 activity. We show that RPE probably contains two types of PLA2 enzyme, as indicated by the results obtained with different PLA2-active fractions eluted from cation-exchange columns and treated with Ca2+/EGTA, dithiothreitol, p-bromophenacyl bromide or heat. These results, in addition to those from PLA2 assays using different substrates, also suggest that RPE-PLA2 enzymes are different from the well-known secretory, cytoplasmic and Ca2+-independent forms. Sequential extraction of RPE with (1) isotonic, (2) hypertonic and (3) detergent-containing PBS argues for the presence of weakly membrane-associated enzymes. Control experiments using 'back and forth' TLC allowed us to discriminate between PLA2 and phospholipase C/diacylglycerol lipase activity and confirmed that, in our assay conditions, the release of fatty acids was indeed due to PLA2 enzymes. These results, together with those obtained by treating RPE homogenates with H2SO4, guanosine 5'-[gamma-thio]triphosphate, ATP and different protease inhibitors, permitted us to make the first characterization of these RPE-PLA2 enzymes. We conclude that RPE contains novel types of PLA2 that are different from the secretory, cytoplasmic and Ca2+-independent forms.

HIV and SIV envelope glycoproteins induce phospholipase A2 activation in human and macaque lymphocytes

We investigated the early interactions between HIV-1, HIV-2, and simian immunodeficiency virus (SIV) envelope glycoproteins gp120(IIIB), gp105(ROD), and gp120(mac251), and human and macaque cells of the lymphocytic series. Our results demonstrate that the soluble viral glycoproteins induce a specific phospholipase A2 (PLA2) activation in lymphocytes through CD4. This PLA2 activation was induced after envelope glycoprotein-CD4 interaction and, because of its local membrane-destabilizing effect, may have important implications for preparing the lymphocyte membrane for fusion with the viral particle. However, this effect is not sufficient to accomplish fusion. These data indicate that the specific step of fusion may be downstream from PLA2 activation.

Structure and analysis of the transducin beta3-subunit gene, a candidate for inherited cone degeneration (cd) in the dog

The cDNA for the beta3-subunit of cone-specific transducin (Tbeta3) was cloned and characterized from wild type dogs, and used in linkage studies as a candidate gene for cone degeneration. Sequence analysis of the Tbeta3 cDNA revealed an open reading frame of 1020 bp, potentially coding for a protein of 340 amino acids (aa). The deduced aa sequence of canine Tbeta3 shares 97% identity with the previously identified human Tbeta3, and 82% identity with bovine rod-specific transducin (Tbeta1). RT-PCR and sequencing of the amplified products demonstrated that the retinal canine Tbeta3 gene is expressed in two different transcripts which can be generated by alternative splicing of the intron in the 3'-untranslated region (UTR). The short and the long mRNAs differ in the length of their 3'-UTR by 456 nt. We have also determined the genomic organization of the canine Tbeta3 gene; it consists of ten exons and the first exon is in the 5'-UTR. The cDNA encoding Tbeta3 from cd-affected dogs was also cloned and sequenced. We found no differences at the nucleotide level between the cDNAs isolated from normal and diseased retinas. The level of transcription of Tbeta3 mRNA in the cd dog retina appeared to be normal. Linkage analysis of a crossbred informative pedigree showed five obligate recombinants out of nine informative offspring. These results suggest that Tbeta3 is not a candidate gene for the cone degeneration of the cd mutant.

Leukotriene D4 activates a chloride conductance in hepatocytes from lipopolysaccharide-treated rats

Endotoxin (LPS) can cause hepatocellular injury under several circumstances, and leukotrienes have been implicated as a contributing factor. Since ion channel activation has been associated with cytotoxicity, the aim of this study was to determine the circumstances under which LPS and/or leukotrienes activate ionic conductances in hepatocytes. LPS treatment of rats increased Cl- conductance in hepatocytes from 232+/-42 to 1236+/-134 pS/pF. Voltage dependence and inhibitor specificity of this conductance were similar to that of a swelling-activated Cl- conductance, and internal dialysis with nucleoside analogues suggested control by an inhibitory G protein. The lipoxygenase inhibitor nordihydroguaiaretic acid, the specific leukotriene D4 (LTD4) receptor antagonist MK-571, and the 5-lipoxygenase activating protein inhibitor MK-886 all significantly inhibited the conductance. Intracellular dialysis with LTD4 (1.5 microM) elevated intracellular Ca2+ from 143+/-6.5 to 388+/-114 nM within 6 min and stimulated an outwardly rectifying conductance from 642+/-159 to 1669+/-224 pS/pF (n = 9, P < 0.001). In hepatocytes prepared from untreated rats, this concentration of intracellular LTD4 neither raised intracellular Ca2+ nor activated the conductance. The LTD4 response could be induced in normal hepatocytes by culture with either conditioned medium from LPS-treated macrophages or purified TNF-alpha. In conclusion, intracellular LTD4 activates a chloride conductance in hepatocytes isolated from rats treated with LPS or primed in vitro with TNF-alpha. Changes in the hepatocellular accumulation of leukotrienes therefore mediate channel activation and may contribute to liver injury during sepsis and other inflammatory conditions.

Inhibitory effects of omega-3 polyunsaturated fatty acids on receptor-mediated non-selective cation currents in rat A7r5 vascular smooth muscle cells

1. The effects of omega-3 polyunsaturated fatty acids on receptor-mediated non-selective cation current (Icat) and K+ current were investigated in aortic smooth muscle cells from foetal rat aorta (A7r5 cells). The whole-cell voltage clamp technique was employed. 2. With a K(+)-containing solution, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA, 30 microM) produced an outward current at a holding potential of -40 mV. This response was inhibited by tetraethylammonium (20 mM) or Cs+ in the patch pipette solution, and the reversal potential of the EPA-induced current followed the K+ equilibrium potential in a near Nernstian manner. 3. Under conditions with a Cs(+)-containing pipette solution, both vasopressin and endothelin-1 (100 nM) induced a long-lasting inward current at a holding potential of -60 mV. The reversal potential of these agonist-induced currents was about +0 mV, and was not significantly altered by the replacement of the extracellular or intracellular Cl+ concentration, suggesting that the induced current was a cation-selective current (Icat). 4. La3+ and Cd2+ (1 mM) completely abolished these agonist-induced Icat, but nifedipine (10 microM) failed to inhibit it significantly. 5. omega-3 polyunsaturated fatty acids (3-100 microM), EPA, DHA and docosapentaenoic acids (DPA), inhibited the agonist-induced Icat in a concentration-dependent manner. The potency of the inhibitory effect was EPA > DHA > DPA, and the half maximal inhibitory concentration (IC50) of EPA was about 7 microM. 6. Arachidonic and linoleic acids (10, 30 microM) showed a smaller inhibitory effect compared to omega-3 fatty acids. Also, oleic and stearic acids (30 microM) did not show a significant inhibitory effect on Icat. 7. A similar inhibitory action of EPA was observed when Icat was activated by intracellularly applied GTP gamma S in the absence of agonists, suggesting that the site of action of omega-3 fatty acids is not located on the receptor. 8. These results demonstrate that omega-3 polyunsaturated fatty acids can activate a K+ current and also effectively inhibit receptor-mediated non-selective cation currents in rat A7r5 vascular smooth muscle cells. Thus, the data suggest that omega-3 fatty acids may play an important role in the regulation of vascular tone.

Priming effects of lipopolysaccharide on UTP-induced arachidonic acid release in RAW 264.7 macrophages

Stimulation of mouse RAW 264.7 macrophages with UTP activates both the inositol phosphate signal transduction pathway and the phospholipase A2 pathway. In the present study, we investigated the interactions between bacterial lipopolysaccharide and UTP in these two systems and the underlying mechanisms involved. While the UTP-induced release of arachidonic acid was only 2.9-fold that in controls, priming the cells with 1 microgram/ml lipopolysaccharide for 1 h before UTP treatment resulted in 9.2-fold arachidonic acid release upon stimulation with UTP. Lipopolysaccharide priming was both concentration- and time-dependent with a peak effect after 1 h treatment at a concentration of 1 microgram/ml. Lipopolysaccharide treatment affect neither the basal nor the UTP-stimulated inositol phosphate formation and [Ca2+]i rise. Pretreatment of the cells with staurosporine, calphostin, N-(2-aminoethyl)-5-isoquinolinesulfonamide H-7), genistein or K-252a led marked inhibition of the priming effect, suggesting that both protein kinase C and tyrosine kinase are involved in the lipopolysaccharide effect. Buffering intracellular Ca2+ levels using [1,2-bis-(o-aminophenoxyl)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester] (BAPTA/AM) or pretreatment with either N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide (H-89), 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD098059) or {1-N,O-bis-(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl] -4-phenyl-piperazine (KN-62) did not affect the lipopolysaccharide-induced priming effect. Primed UTP stimulation was inhibited by actinomycin D and cycloheximide, indicating a requirement for both gene expression and protein translation. To further examine whether the stimulatory effects of lipopolysaccharide on phospholipase A2 activity were independent of [Ca2+]i levels but dependent on protein phosphorylation, a fixed Ca2+ concentration and inhibitors of protein phosphatases were used in primed permeabilized cells. Arachidonic acid release from permeabilized cells containing 100 nM Ca2+ was high in lipopolysaccharide-primed cells and potentiated by addition of microcystin, orthovanadate or FK 506. These results that the Ser/Thr and tyrosine phosphorylation cascades induced by protein kinase C and tyrosine kinase, respectively, are required for the arachidonic acid potentiation effect of lipopolysaccharide, which was independent of modulation of the upper stream signaling pathways of UTP.

Functional dissociation between glucocorticoid-induced decrease in arachidonic acid release and inhibition of adrenocorticotropic hormone secretion in AtT-20 corticotrophs

The biochemical basis of the short-term inhibitory effects of glucocorticoids on corticotropin release from pituitary corticotrophs is still obscure. A well-characterized effect of glucocorticoids in several cell types is the inhibition of arachidonic acid (AA) generation by phospholipase A2 (PLA2). Arachidonic acid and its metabolites have been implicated in the secretory process from a number of pituitary cells, such as the corticotrophs. We have thus examined the role of AA in the anti-secretagogue effects of glucocorticoids in a corticotropin-secreting clonal corticotroph line (AtT-20 D16/16). Glucocorticoids decreased AA release induced by melittin, a bee venom protein related to extracellular PLA2. When a possible role of AA in corticotropin release was studied, the following results were obtained: (a) all corticotropin secretagogues tested, including corticotropin-releasing factor (CRF), did not alter AA generation; (b) calcium and guanine nucleotides, which stimulate corticotropin release in permeabilized cells, inhibited the release of AA under the same conditions; (c) administration of melittin or of exogenous AA had no effect on basal and CRF-stimulated corticotropin release; (d) administration of large amounts of exogenous AA was unable to restore the ability to secrete corticotropin under suppression by glucocorticoids. Altogether, the data suggest that whereas glucocorticoids can inhibit both AA generation and corticotropin release, these two effects appear to be causally unrelated.