Mechanism of agonist-induced down-regulation and subsequent recovery of muscarinic acetylcholine receptors in a clonal neuroblastoma x glioma hybrid cell line

Evidence of nigericin as a potential therapeutic candidate for cancers: A review

Emerging studies have shown that nigericin, an H⁺, K⁺ and Pb²⁺ ionophore, has exhibited a promising anti-cancer activity in various cancers. However, its anti-cancer mechanisms have not been fully elucidated. In this review, the recent progresses on the use of nigericin in human cancers have been summarized. By exchanging H⁺ and K⁺ across cell membranes, nigericin shows promising anti-cancer activities in in vitro and in vivo as a single agent or in combination with other anti-cancer drugs through decreasing intracellular pH (pHi). The underlying mechanisms of nigericin also include the inactivation of Wnt/β-catenin signals, blockade of Androgen Receptor (AR) signaling, and activation of Stress-Activated Protein Kinase/c-Jun N-terminal Kinase (SAPK/JNK) signaling pathways. In many cancers, nigericin is proved to specifically target putative Cancer Stem Cells (CSCs), and its synergistic effects on photodynamic therapy are also reported. Other mechanisms of nigericin including influencing the mitochondrial membrane potentials, inducing an increase in drug accumulation and autophagy, controlling insulin accumulation in nuclei, and increasing the cytotoxic activity of liposome-entrapped drugs, are also discussed. Notably, the potential adverse effects such as teratogenic effects, insulin resistance and eryptosis shall not be ignored. Taken together, these reports suggest that treatment of cancer cells with nigericin may offer a novel therapeutic strategy and future potential of translation to clinics.

Impaired cholinergic mechanisms following exposure to monocrotophos in young rats

Studies on the neurobehavioral toxicity of monocrotophos, an organophosphate, have been carried out on rats following their exposure from postnatal day (PD) 22 to PD 49 to investigate whether neurobehavioral changes are transient or persistent. Exposure of rats to monocrotophos (0.50 or 1.0 mg/kg body weight, p.o.) decreased body weight (10% and 30%) and impaired grip strength (28% and 32%) and learning ability (65% and 68%) at both the doses, respectively in comparison to controls. A trend of recovery was observed in body weight and learning, while decrease in grip strength persisted in rats 15 days after withdrawal. Activity of acetylcholinesterase was decreased in frontal cortex (36% and 67%), hippocampus (21% and 49%) and cerebellum (29% and 51%) in monocrotophos-treated rats at both the doses. The decrease in the activity of acetylcholinesterase persisted in frontal cortex and hippocampus; however, a trend of recovery was observed in cerebellum 15 days after withdrawal. Binding of 3 H-quinuclidinyl benzilate ( 3 H-QNB) to frontocortical (19% and 35%), hippocampal (32% and 39%) and cerebellar (19% and 28%) membranes was decreased in monocrotophos-treated rats compared to controls. The decrease in the binding of 3 H-QNB persisted in frontocortical, hippocampal and cerebellar membranes 15 days after withdrawal. The results suggest that repeated exposure to monocrotophos in rats may cause behavioral and neurochemical modifications which may persist even after withdrawal. The findings are of concern in view of the high consumption of monocrotophos in many countries.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

Inhibition of G protein-coupled receptor trafficking in neuroblastoma cells by MAP 4 decoration of microtubules

One mechanism for the reappearance of G protein-coupled receptors after agonist activation is microtubule-based transport. In pressure-overload cardiac hypertrophy, there is downregulation of G protein-coupled receptors and the appearance of a densified microtubule network extensively decorated by a microtubule-associated protein, MAP 4. Our hypothesis is that overdecoration of a dense microtubule network with this structural protein, as in hypertrophied myocardium, would impede receptor recovery. We tested this hypothesis by studying muscarinic acetylcholine receptor (mAChR) internalization and recovery after agonist stimulation in neuroblastoma cells. Exposure of cells to carbachol, a muscarinic receptor agonist, decreased membrane receptor binding activity. After carbachol withdrawal, receptor binding recovered toward the initial value. When microtubules were depolymerized before carbachol withdrawal, mAChR recovery was only 44% of that in intact cells. Cells were then infected with an adenovirus containing MAP 4 cDNA. MAP 4 protein decorated the microtubules extensively, and receptor recovery upon carbachol withdrawal was reduced to 54% of control. Thus muscarinic receptor recovery after agonist exposure is microtubule dependent, and MAP 4 decoration of microtubules inhibits receptor recovery.

Muscarinic M2 receptor synthesis: study of receptor turnover with propylbenzilylcholine mustard

We have investigated the rate and the functional responsiveness of the newly synthesised M2 muscarinic receptors in HEL 299 cells following propylbenzilylcholine mustard treatment at 37 degrees C. Propylbenzilylcholine mustard induced a dose-dependent loss of the hydrophilic ligand [3H]N-methylscopolamine binding sites with 80% inactivation at 0.1 microM. The rate of muscarinic receptor synthesis in these cells, estimated from wash-out experiments following propylbenzilylcholine mustard treatment, was very slow and returned to control values after 36 h of propylbenzilylcholine mustard removal. The recovery of muscarinic receptors was blocked by the cycloheximide pre-treatment, indicating the synthetic pathway for the new receptors. In control cells as well as in cells treated with propylbenzilylcholine mustard and allowed to recover for 12 h, carbachol still inhibited forskolin-induced cAMP accumulation. These results show that (i) the rate of M2 muscarinic receptor synthesis is slow (ii) the recovery of receptors is mainly through increased synthesis and (iii) the newly synthesised receptors retain their full functional activity.

Characterization of a triple opioid system in the human neuroblastoma NMB cell line

The human neuroblastoma NMB cell line was found to contain the three types of opioid receptors (60% delta 25% kappa and 15% mu). The opioid receptors were negatively coupled to adenylyl-cyclase. Maximal reduction in cAMP content was achieved by selectively activating single receptor types, indicating the co-presence of the various opioid receptors in the same cells. The opioid receptors in NMB cells were up-regulated following prolonged exposure to the opioid antagonist naloxone and down-regulated following chronic treatment with the opioid agonist etorphine. Down-regulation was time-, dose- and temperature-dependent and was inhibited by colchicine and sodium azide. The NMB culture is presented as an excellent experimental model for studying the selective activation and regulation of the different opioid receptor types when they are co-expressed in the same neuron, as well as for studying interactions between the various opioid receptors.

Neurotoxicity of artemisinin analogs in vitro

The sesquiterpene endoperoxide antimalarial agents arteether and artemether have been reported to cause neurotoxicity with a discrete distribution in the brain stems of rats and dogs after multiple doses. The nature and distribution of the brain lesions suggest a specific neuronal target, the identity of which is unknown. In order to further investigate artemisinin analog-induced neurotoxicity, we evaluated several in vitro models: fetal rat primary neuronal cultures, fetal rat secondary astrocyte cultures, and transformed neuronal cultures (rat-derived neuroblastoma NG108-15 and mouse-derived neuroblastoma Neuro-2a). Results indicate that toxicity was specific for neuronal cell types but not glial cells. Neurotoxicity, as indexed by liberation of lactate dehydrogenase and/or inhibition of radiolabelled-leucine uptake, was seen in all three neuronal culture types, implicating a common target. In vitro neurotoxicity was dose and time dependent. Acute exposure to drug results in delayed, but not immediate, manifestations of cell toxicity. Structure-activity comparisons indicate that substitutions at positions 9 and 10 and stereoisomerism at position 10 of the artemisinin backbone influence the degree of toxicity. The endoperoxide is necessary but not sufficient for toxicity. Sodium artesunate and dihydroartemisinin, a metabolite common to all artemisinin analogs currently being developed for clinical use, are the most potent of all analogs tested. These results are consistent with a specific neuronal target, but the identity of the target(s) remains unknown.

Routes of delivery of muscarinic acetylcholine receptors to the plasma membrane in NG108-15 cells

1. We have examined the routes of delivery of muscarinic acetylcholine receptors to the plasma membrane in unstimulated and agonist-stimulated NG108-15 cells. Delivery of receptors to the plasma membrane was measured by irreversible alkylating receptors already at the cell surface with propylbenzilylcholine mustard (PrBCM) and then following the recovery of binding of the polar radioligand [3H]-N-methylscopolamine ([3H]-NMS) in intact cells. 2. In unstimulated cells, recovery of [3H]-NMS binding after 2 h of incubation at 37 degrees C was 30% of binding in control cells. Binding affinity of [3H]-NMS was unchanged. In cells that had been pre-exposed to carbachol (0.5 mM) for 30 min, initial [3H]-NMS binding was reduced by 38% but recovery of binding was increased from 30% to 43% of control binding. 3. When the cells were pre-incubated for 1 h with the protein synthesis inhibitor cycloheximide (20 micrograms ml-1), recovery of [3H]-NMS binding was reduced by similar extents in unstimulated (30% to 9%) and carbachol-stimulated (43% to 19%) cells. Incubation of the cells at 20 degrees C instead of 37 degrees C reduced recovery of [3H]-NMS binding from 30% to 9% in unstimulated cells and from 43% to 23% in carbachol-stimulated cells. 4. Depletion of cellular ATP by addition of antimycin (50 nM) and deoxyglucose (50 mM), reduced recovery of binding to 12% in unstimulated cells and to 6% in carbachol-stimulated cells. 5. These results indicate that in unstimulated NG108-15 cells, delivery of muscarinic receptors to the plasma membrane is almost exclusively through the synthetic pathway. In agonist-stimulated cells,receptor sequestration into an intracellular compartment (probably endosomes) occurs. Under these circumstances, delivery of receptors to the plasma membrane along the synthetic route is unaffected but an additional route of delivery (recycling) now operates.

Inhibition of bioenergetics alters intracellular calcium, membrane composition, and fluidity in a neuronal cell line

The effect of inhibited bioenergetics and ATP depletion on membrane composition and fluidity was examined in cultured neuroblastoma-glioma hybrid NG108-15 cells. Sodium cyanide (CN) and 2-deoxyglucose (2-DG) were used to block, oxidative phosphorylation and anaerobic glycolysis, respectively. Endoplasmic reticulum (ER) Ca(2+)-pump activity measured by 45Ca2+ uptake was > 92% inhibited in intact cells incubated with CN (1 mM) and 2-DG (20 mM) for 30 min. In addition, exposure of cells to CN and 2-DG caused a 134% increased release of isotopically labeled arachidonic acid (3H-AA) or arachidonate-derived metabolites from membranes. Removal of Ca2+ from the incubation medium ablated the CN/2-DG induced release of 3H-AA or its metabolites. Membrane fluidity of intact cells was measured by electron spin resonance spectroscopy using the spin label 12-doxyl stearic acid. The mean rotational correlation time (tau c) of the spin label increased 49% in CN/2-DG exposed cells compared to controls, indicating a decrease in membrane fluidity. These results show that depletion of cellular ATP results in inhibition of the ER Ca(2+)-pump, loss of AA from membranes, and decreased membrane fluidity. We propose that impaired bioenergetics can increase intracellular Ca2+ as a result of Ca(2+)-pump inhibition and thereby activate Ca(2+)-dependent phospholipases causing membrane effects. Since neurons derive energy predominantly from oxidative metabolism, ATP depletion during brain hypoxia may initiate a similar cytotoxic mechanism.

Rapid agonist-induced decrease of neurotensin receptors from the cell surface in rat cultured neurons

The regulation of neurotensin receptors was studied in vitro in primary cultures of neuronal cells. High affinity receptors for [3H]neurotensin were found in homogenates and at the cell surface of intact neurons cultured from the brain of rat embryos. When intact cells were incubated with 3 nM neurotensin (1-13), a rapid decrease in [3H]neurotensin binding was observed; about 60% of neurotensin receptors disappeared from the cell surface in less than 15 min. This corresponded to a reduction of the Bmax value without a change in the binding affinity. The decrease in neurotensin receptor number was also induced by the active fragment (8-13) of neurotensin but not by its inactive fragment (1-8). It was partially inhibited by bacitracin, at concentrations which are known to interact with receptor internalization, and was not detected when intact cells were incubated at 0-4 degrees with the unlabeled peptide. When intact neurons were incubated with [3H]neurotensin, there was a rapid ligand uptake and the kinetics of endocytosis were similar to those of the cell surface receptor disappearance. Once endocytosed, [3H]neurotensin could not be released (or displaced) from either intact neurons or homogenates, suggesting the sequestration of the labeled peptide in vesicles or other subcellular structures. Therefore, the present results suggest that the rapid agonist-induced decrease in the number of neurotensin receptors from the cell surface corresponds to an internalization process which involves a simultaneous receptor-mediated peptide endocytosis.

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.

Entry and replication of Japanese encephalitis virus in cultured neurogenic cells

The entry mode and growth pattern of Japanese encephalitis (JE) virus in mouse neuroblastoma N18TG2 cells and mouse neuroblastoma x rat glioma NG108-15 hybrid cells were studied by electron microscopy. At two minutes after inoculation, JE virions adsorbed onto and directly penetrated through the plasma membrane of the hybrid cells, whereas virions did not adsorb nor entered the neuroblastoma cells. Correspondingly, the hybrid cells showed assembling progeny JE virions in the cisternae of rough endoplasmic reticulum (RER) 1 day postinoculation (p.i.) although virions were rarely found on the following days during the experiment. On the other hand, progeny virions did not assemble in the RER cisternae of the neuroblastoma cells throughout the experiment. The morphologic observations, therefore, suggest that (a) the hybrid cells express JE-virus receptors which facilitate the viral attachment onto and entry into the cells, while the neuroblastoma cells do not and (b) JE virus replicates very poorly after the entry into the hybrid cells while it does not replicate at all in the neuroblastoma cells. The virus titrations of the media of the neuroblastoma and hybrid cell cultures showed only titers indicative of residual virus of the inoculum that progressively decreased during the experiment. The present results show therefore that of the two neurogenic cell culture lines studied only the hybrid cell line can be used for the study of viral entry and replication, although it is not suited for virus production. Possible reasons for the poor replication of JE virus in the hybrid cells are discussed.

Prostaglandin E2 Receptors in the Chicken Spinal Cord: 1. Biochemical Characterization

Prostaglandins (PGs) are neuroactive substances which act in the vicinity of their site of synthesis through receptors coupled to G-proteins. Since large amounts of PGE2 can be synthesized by chicken spinal cord, binding sites for PGE2 were looked for in various cell fractions of spinal cord. In the 17 000 g pellet incubated with 0.3 nM [3H]PGE2, 70% of ligand was specifically bound. Two types of PGE2 binding site were characterized (i) high affinity, low capacity binding sites (KD1 1.34 nM, Bmax1 34.5 fmol/mg prot); (ii) low affinity, high capacity binding sites (KD2 2.23 microM, Bmax2 13.2 pmol/mg prot). The high affinity binding sites fulfil several requirements for being receptors to PGE2: (i) since the KD1 is increased in the presence of the GTP analogue, Gpp(NH)p, these binding sites would be regulated by a G-protein; (ii) a desensitization was obtained by an excess of unlabelled PGE2 and reversed by Gpp(NH)p; (iii) the competition experiments between PGE2 and various prostanoids pointed to PGE2 receptors such as EP2 or EP3. The receptor characteristics of the low-affinity binding sites were not investigated. Hence, our results support the presence of two types of PGE2 binding site in the chicken spinal cord; a high affinity site, which corresponds to a PGE2 receptor responding to nanomolar concentrations and a low affinity site sensitive to micromolar concentrations of PGE2.

Molecular mechanisms of regulation of neuronal muscarinic receptor sensitivity

Like other neurotransmitter receptors, muscarinic acetylcholine receptors are subject to regulation by the state of receptor activation. Prolonged increases in the concentration of muscarinic agonists result in a decrease in receptor density and loss of receptor sensitivity, both in vivo and in vitro. On the other hand, when the receptor is deprived of acetylcholine for a long duration in vivo, the receptor becomes more sensitive in responding to muscarinic agonists. However, it has been more difficult to demonstrate increases in receptor concentration that accompany this supersensitive state. The purpose of this review is to provide current information related to the characteristics of muscarinic receptor regulation and the molecular mechanisms underlying this phenomenon, regarding both the density of receptors and their transduction mechanisms. Furthermore, possible feedback regulatory roles of different second messenger signals are discussed. Particular emphasis is dedicated to molecular mechanisms of regulation of neuronal muscarinic receptors.

In vitro and in vivo modulation of cholinergic muscarinic receptors in rat lymphocytes and brain by cholinergic agents

A binding site for 3H-quinuclidinyl benzylate (QNB) has been identified in rat lymphocytes which has the characteristics of a cholinergic muscarinic receptor (Costa, L. G., Kaylor, G. & Murphy, S. D. (1988). Muscarinic cholinergic binding sites on rat lymphocytes. Immunopharmacology, 16, 139-149.) Here we show that prolonged exposures to cholinergic compounds in vitro and in vivo modulate muscarinic receptor binding in lymphocytes as well as in brain tissue. Exposure of rat splenic lymphocytes in vitro to oxotremorine caused a time- and concentration-dependent decrease in the density of 3H-QNB binding sites. This decrease occurred only when incubation with oxotremorine was carried out at 37 degrees C and not at 0-4 degrees C, suggesting that it was not an artifact due to residual, unwashed, oxotremorine. The effect of oxotremorine was mimicked by two other cholinergic agonists, acetylcholine and carbachol, and was antagonized by atropine, which, when present alone, caused an increase in 3H-QNB binding. In vivo exposures to oxotremorine or atropine (both at 20 mg/kg/day for 14 days via an ALZA minipump) caused a significant decrease (20-30%) and increase (13-30%), respectively, of 3H-QNB binding in various brain areas as well as circulating lymphocytes. Repeated administrations of the organophosphorus insecticide disulfoton (2 mg/kg/day for 14 days, i.p.) caused significant reductions (59-88%) of acetylcholinesterase activity in brain, lymphocytes, plasma and red blood cells, as well as a 23-39% decrease of 3H-QNB binding in brain areas and circulating lymphocytes.(ABSTRACT TRUNCATED AT 250 WORDS)