digitonin and Glioma

Regulators of G protein signaling (RGS) proteins act as GTPase-accelerating protein to negatively modulate G protein signaling and are defined by a conserved RGS domain with considerable amino acid diversity. To determine the effects of specific, purified RGS proteins on mu-opioid signaling, C6 cells stably expressing a mu-opioid receptor were rendered permeable to proteins by treatment with digitonin. Mu-opioid inhibition of forskolin-stimulated adenylyl cyclase by [D-Ala(2),N-Me-Phe(4),Gly-ol]-enkephalin (DAMGO), a mu-specific opioid peptide, remained fully intact in permeabilized cells. Purified RGS domain of RGS4 added to permeabilized cells resulted in a twofold loss in DAMGO potency but had no effect in cells expressing RGS-insensitive G proteins. The inhibitory effect of DAMGO was reduced to the same extent by purified RGS4 and RGS8. In contrast, the RGS domain of RGS7 had no effect and inhibited the action of RGS8 as a result of weak physical association with Galphai2 and minimal GTPase-accelerating protein activity in C6 cell membranes. These data suggest that differences in conserved RGS domains of specific RGS proteins contribute to differential regulation of opioid signaling to adenylyl cyclase and that a permeabilized cell model is useful for studying the effects of specific RGS proteins on aspects of G protein-coupled receptor signaling.

Topics: Adenylyl Cyclases; Analgesics, Opioid; Animals; Cell Line, Tumor; Colforsin; Cyclic AMP; Digitonin; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Flow Cytometry; Gene Expression Regulation, Neoplastic; Glioma; Guanosine 5'-O-(3-Thiotriphosphate); Protein Binding; Rats; Receptors, Opioid, mu; RGS Proteins; Signal Transduction

The guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding assay for the determination of relative opioid efficacy has been adapted to measure G protein activation in digitonin-permeabilized C6 rat glioma cells expressing a cloned mu-opioid receptor. The mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) caused a 3-fold increase in [35S]GTPgammaS binding over basal in a naloxone-sensitive manner. Relative mu-agonist efficacy was DAMGO > fentanyl > or = morphine > buprenorphine. Nalbuphine showed no efficacy. G protein activation by receptors has been predicted to occur by random encounter. In this model a reduction in the number of receptors will decrease the rate of G protein activation but not the maximum number of G proteins activated. To test this model C6 mu cells were treated with the irreversible mu-antagonist beta-funaltrexamine (10 nM) prior to permeabilization. This reduced the number of mu-opioid receptors determined with [3H]diprenorphine to 23 +/- 3% of control with no change in affinity. A commensurate reduction (to 29 +/- 10% of control) in the level of [35S]GTPgammaS binding stimulated by DAMGO was observed, but the t(1/2) for [35S]GTPgammaS binding remained unchanged. Thus, random encounters of receptor and G protein failed to occur in this permeabilized cell preparation. A model that assumes an organized association of G proteins with receptors better describes the activation of G proteins by opioid mu-receptors.

Topics: Analgesics, Opioid; Animals; Digitonin; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Glioma; GTP-Binding Proteins; Guanosine 5'-O-(3-Thiotriphosphate); Indicators and Reagents; Naloxone; Naltrexone; Narcotic Antagonists; Rats; Receptors, Opioid, mu; Tumor Cells, Cultured

We examined the effects of dissolved nitric oxide (NO) gas on cytoplasmic calcium levels ([Ca(2+)](i)) in C6 glioma cells under anoxic conditions. The maximum elevation (27 +/- 3 nM) of [Ca(2+)](i) was reached at 10 microM NO. A second application of NO was ineffective if the first was >0.5 microM. The NO donor diethylamine/NO mimicked the effects of NO. Acute exposure of the cells to low calcium levels was without effect on the NO-evoked response. Thapsigargin (TG) increased [Ca(2+)](i) and was less effective if cells were pretreated with NO. Hemoglobin inhibited the effects of NO at a molar ratio of 10:1. 8-Bromo-cGMP was without effect on the NO-evoked response. If cells were pretreated with TG or exposed chronically to nominal amounts of calcium, NO decreased [Ca(2+)](i). The results suggest that C6 glioma cells have two receptors for NO. One receptor (NO(A)) elevates [Ca(2+)](i) and resides on the endoplasmic reticulum (ER). The other receptor (NO(B)) decreases [Ca(2+)](i) and resides on the plasmalemma or the ER. The latter receptor dominates when the level of calcium within intracellular stores is diminished.

Topics: Aerobiosis; Animals; Calcium; Cell Hypoxia; Cytoplasm; Cytosol; Digitonin; Egtazic Acid; Glioma; Hemoglobins; Homeostasis; Hydrazines; Kinetics; Nitric Oxide; Nitric Oxide Donors; Nitrogen Oxides; Thapsigargin; Tumor Cells, Cultured

To investigate the role of calmodulin (CaM)-dependent pathways in agonist-induced phosphoinositide (PI) turnover, the influence of several CaM antagonists on PI-phospholipase C (PLC) activation in intact and permeabilized C6 glioma cells was examined. The extent of PI turnover was assessed by measuring the accumulation of inositol phosphates (IPs) in the presence of LiCl in C6 glioma cells prelabelled with myo-[3H]inositol. Trifluoperazine, N-(6-aminohexyl)-5-chloro-1- naphthalenesulphonamide (W-7), fendiline and calmidazolium themselves had no effect on basal IP formation, but concentration-dependently (1-30 microM) potentiated ATP-, NaF- and A23187-stimulated IP formation. The maximal response to ATP (1 mM) was increased by up to 50%, while the concentration for half-maximal effect (EC50, 60 microM) was unaffected by trifluoperazine. In digitonin-permeabilized C6 glioma cells, the concentration-dependent increase of PI-PLC activation elicited by free Ca2+ was potentiated by the GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), with an EC50 of 6 microM. Trifluoperazine (1-30 microM) enhanced the Ca(2+)-stimulated IP formation concentration dependently and this potentiation was counteracted by the addition of CaM. In the combined presence of each CaM antagonist studied and GTP gamma S, an additive increase in IP formation was observed. The results indicate that CaM antagonists enhance stimulus-induced IP formation in C6 glioma cells primarily by increasing the Ca(2+)-dependent activation of PI-PLC.

Topics: Adenosine Triphosphate; Calcimycin; Calmodulin; Digitonin; Dose-Response Relationship, Drug; Enzyme Activation; Glioma; Guanosine 5'-O-(3-Thiotriphosphate); Hydrolysis; Phosphatidylinositols; Signal Transduction; Sodium Fluoride; Trifluoperazine; Tumor Cells, Cultured; Type C Phospholipases

CGP-12177, like isoproterenol, has a lower affinity for desensitized receptors. Experiments were performed to study whether this property of CGP-12177 is due to its partial agonist activity or its impermeability for membranes. Reduced binding of [3H]DHA to desensitized receptors at 0 degrees C as well as a reversal of the reduced binding in the presence of digitonin indicate a permeability barrier. A partial agonist effect of CGP-12177 was only found in intact C6 cells and neither in C6 membranes nor in S49 cells. This peculiar effect in intact C6 cells is stereospecific and not due to an inhibition of phosphodiesterases.

Topics: Adenylyl Cyclases; Animals; Cell Line; Cyclic AMP; Digitonin; Dihydroalprenolol; Glioma; Isoproterenol; Propanolamines; Rats; Receptors, Adrenergic, beta; Stereoisomerism