Thyroid gangliosides with high affinity for thyrotropin: potential role in thyroid regulation

Thyroid cell membranes contain a multiplicity of gangliosides, some of which inhibit thyrotropin binding to thyroid membranes. The most potent inhibitor is a ganglioside which is present in only trace amounts and appears to have a novel structure. Thyroid gangliosides may play a role in relaying the hormonal message to the thyroid cell.

Protein kinase C-mediated down-regulation of beta(2)-adrenergic receptor and gene expression in rat C6 glioma cells

We investigated the regulation of beta(2)-adrenergic receptors (beta(2)AR) by protein kinase C (PKC) in rat C6 glioma cells at the levels of receptor activity, protein expression and gene expression. Cells exposed to 4beta-phorbol-12-myristate-13-acetate (PMA), a potent activator of PKC, exhibited a time- and concentration-dependent decrease in beta(2)AR binding activity. Maximum down-regulation was approximately 50% by 24 h and western blot analysis revealed a parallel decrease in beta(2)AR protein. In addition, PMA treatment resulted in an acute desensitization of beta(2)AR-stimulated cyclic AMP response prior to any reduction in receptor levels. PMA exposure also affected steady-state beta(2)AR mRNA levels in a time-dependent, biphasic manner. During the first 4 h, levels decreased by approximately 60% and then slowly recovered to approximately 75% of control by 24 h. As the reduction in receptor mRNA was not due to a decrease in its stability, we examined beta(2)AR gene transcription by nuclear run-on assays. Transcriptional activity in nuclei from C6 cells treated with PMA for 2 h was reduced by 70% compared to controls. Thus PKC can regulate beta(2)AR at least two levels: the first being an acute desensitization of receptor function, and the second being a more prolonged repression of receptor gene transcription that in turn results in decreased receptor expression.

Orbital ganglioglioma arising from ectopic neural tissue

PURPOSE

To report a case of neonatal orbital ganglioglioma originating from ectopic neural tissue.

METHOD

Case report.

RESULTS

An African-American male presented at birth with proptosis and expansion of the left orbit. A discrete soft-tissue mass was located inferolateral to the globe, which proved to be a ganglioglioma.

CONCLUSION

The tumor is presumed to have originated from ectopic neural tissue in the orbit. Although infrequent, this tumor should be included in the differential diagnosis of neonatal orbital neoplasms.

Protein kinase C-mediated down-regulation of beta1-adrenergic receptor gene expression in rat C6 glioma cells

In the current study, we investigated the mechanism by which protein kinase C (PKC) regulates the expression of beta1-adrenergic receptor (beta1AR) mRNA in rat C6 glioma cells. Exposure of the cells to 4beta-phorbol-12-myristate-13-acetate (PMA), an activator PKC, resulted in a down-regulation of both beta1AR binding sites and mRNA levels in a time- and concentration-dependent manner. This effect was not observed with phorbol esters that do not activate PKC and was blocked by bisindolylmaleimide, a specific PKC inhibitor. Activation of PKC did not reduce the half-life of beta1AR mRNA but significantly decreased the activity of the beta1AR promoter, as determined by reporter analysis. A putative response element, with partial homology to a consensus cAMP response element, was identified by mutation analysis of the promoter at positions -343 to -336, relative to the translational start site. Mutation of this putative regulatory element, referred to as a beta1AR-PKC response element, completely blocked the PKC-mediated down-regulation of beta1AR promoter activity. Gel mobility shift analysis detected two specific bands when C6 cell extracts were incubated with a labeled DNA probe containing the beta1AR-PKC response element sequence. Formation of one of these bands was inhibited by an oligonucleotide probe containing a consensus CRE and disrupted by an antibody for cAMP response element binding protein. Based on these studies, we propose that the PKC-induced down-regulation of beta1AR gene transcription in C6 cells is mediated in part by a cAMP response element binding protein-dependent mechanism acting on a novel response element.

Filipin-dependent inhibition of cholera toxin: evidence for toxin internalization and activation through caveolae-like domains

The mechanism by which cholera toxin (CT) is internalized from the plasma membrane before its intracellular reduction and subsequent activation of adenylyl cyclase is not well understood. Ganglioside GM1, the receptor for CT, is predominantly clustered in detergent-insoluble glycolipid rafts and in caveolae, noncoated, cholesterol-rich invaginations on the plasma membrane. In this study, we used filipin, a sterol-binding agent that disrupts caveolae and caveolae-like structures, to explore their role in the internalization and activation of CT in CaCo-2 human intestinal epithelial cells. When toxin internalization was quantified, only 33% of surface-bound toxin was internalized by filipin-treated cells within 1 h compared with 79% in untreated cells. However, CT activation as determined by its reduction to form the A1 peptide and CT activity as measured by cyclic AMP accumulation were inhibited in filipin-treated cells. Another sterol-binding agent, 2-hydroxy-beta-cyclodextrin, gave comparable results. The cationic amphiphilic drug chlorpromazine, an inhibitor of clathrin-dependent, receptor-mediated endocytosis, however, affected neither CT internalization, activation, nor activity in contrast to its inhibitory effects on diphtheria toxin cytotoxicity. As filipin did not inhibit the latter, the two drugs appeared to distinguish between caveolae- and coated pit-mediated processes. In addition to its effects in CaCo-2 cells that express low levels of caveolin, filipin also inhibited CT activity in human epidermoid carcinoma A431 and Jurkat T lymphoma cells that are, respectively, rich in or lack caveolin. Thus, filipin inhibition correlated more closely with alterations in the biochemical characteristics of CT-bound membranes due to the interactions of filipin with cholesterol rather than with the expressed levels of caveolin and caveolar structure. Our results indicated that the internalization and activation of CT was dependent on and mediated through cholesterol- and glycolipid-rich microdomains at the plasma membrane rather than through a specific morphological structure and that these glycolipid microdomains have the necessary components required to mediate endocytosis.

Endogenous beta 3- but not beta 1-adrenergic receptors are resistant to agonist-mediated regulation in human SK-N-MC neurotumor cells

Although there is considerable interest in the regulation of the different beta-adrenergic receptor (AR) subtypes, most previous studies have utilized stably transfected cells expressing recombinant receptors under the control of viral promoters. Human SK-N-MC neurotumor cells appear to be novel, since they express both endogenous beta 1AR and beta 3AR based on radioligand binding and on functional response. Saturation binding of either the hydrophilic ligand (-)-[3H]CGP-12177 or the more hydrophobic (-)-[125I]iodocyanopindolol indicated the presence of two populations of binding sites with high and low affinities. With either ligand, the beta 1AR antagonist CGP-20712A preferentially inhibited binding to the high-affinity sites. This is consistent with the latter representing beta 1AR whereas the low-affinity sites represent beta 3AR. Both subtypes appeared to be functional on the basis of isoproterenol stimulation of cyclic adenosine monophosphate (cAMP) in intact cells and adenylyl cyclase activity in cell membranes in the absence and presence of CGP-20712A. SK-N-MC-IXC cells, derived by twice subcloning the parental cells, also expressed both beta AR subtypes, indicating that they co-exist in the same cell. SK-N-MC cells exposed to isoproterenol exhibited a rapid sequestration and a slower downregulation of beta 1AR. The latter subtype also underwent desensitization, as indicated by a rightward shift to less sensitivity in the EC50 for isoproterenol stimulation of adenylyl cyclase activity. In contrast, the beta 3AR subtype was resistant to agonist-mediated sequestration, downregulation, and desensitization. Thus, when endogenously expressed in the same cell line, human beta 1AR and beta 3AR display differences in their ability to be regulated by agonist.

Adrenergic regulation of ICER (inducible cyclic AMP early repressor) and beta1-adrenergic receptor gene expression in C6 glioma cells

ICER (inducible cyclic AMP early repressor), a member of the cyclic AMP response element (CRE) modulator (CREM) family of transcription factors, is a powerful repressor of cyclic AMP-mediated transactivation. Our studies characterize the regulation of ICER in C6 glioma cells and investigate its role in repressing transcription of the beta1-adrenergic receptor (beta1AR) gene. Incubation with isoproterenol (100 nM) results in a rapid induction in levels of mRNA for ICER and its splice variant ICERgamma, with maximal induction occurring after 2 h of treatment. Incubation with isoproterenol also increased levels of CREM isoforms within 1 h; this was unexpected given previous reports that these isoforms are not rapidly induced. Increased expression of ICER and CREM was accompanied by induction of two CRE-binding complexes. The presence of ICER in these two CRE-binding complexes is demonstrated by their disruption with CREM antibody and by their comigration with recombinant ICER. Because the time course for isoproterenol induction of ICER mRNA and CRE binding corresponds to that for down-regulation of beta1AR mRNA levels in C6 glioma cells, the influence of ICER beta1AR transcription was directly examined. Coexpression of ICER significantly decreased transcriptional activity of a rat beta1AR promoter-luciferase reporter construct that contains a CRE. In contrast, coexpression of ICER did not influence two truncated rat beta1AR promoter constructs that lack the CRE site. These data demonstrate that ICER can interact at the beta1AR promoter to repress transcription.

Anomalous behavior of CGP 12177A on beta 1-adrenergic receptors

CGP 12177A originally was developed as a hydrophilic antagonist to detect cell surface beta 1- and beta 2-adrenergic receptors, and subsequently was found to be a partial agonist for the atypical or beta 3-adrenergic receptor. Using hamster cells stably expressing either the human beta 1-, human beta 2- or rat beta 1-adrenergic receptor, we found that CGP 12177A behaved as an agonist of beta 1-adrenergic receptors. Whereas at low concentrations, CGP 12177a behaved as an antagonist and inhibited isoproterenol stimulation of adenylyl cyclase activity, at higher concentrations, it stimulated a response even in the absence of isoproterenol. The agonistic properties of CGP 12177A were positively correlated with the level of beta 1-adrenergic receptor expression. Thus, at low receptor of densities, CGP 12177A behaved as a weak, partial agonist whereas as high receptor densities, the drug was a full agonist. At similar high densities of the beta 2-adrenergic receptor, CGP 12177A acted only as a partial agonist. Competition binding studies to membranes from cells expressing beta 1-adrenergic receptors indicated that approximately 90% of the receptors were in a high affinity, guanine nucleotide-insensitive state for CGP 12177A whereas approximately 10% of the receptors were in a lower affinity, guanine nucleotide-sensitive state for CGP 12177A. We propose that the latter receptors are precoupled to stimulatory G proteins and recognize CGP 12177A as an agonist whereas the high affinity, uncoupled receptors recognize CGP 12177A as an antagonist.

Regulation of beta 2-adrenergic receptor mRNA and gene transcription in rat C6 glioma cells: effects of agonist, forskolin, and protein synthesis inhibition

Incubation of rat C6 glioma cells with beta-adrenergic receptor (beta AR) agonist or with agents that increase cAMP levels results in down-regulation of the beta 2AR, as measured by the loss of radioligand binding sites. In the present study, the role of beta 2AR mRNA expression and stability in the down-regulation of beta 2AR sites in C6 cells was examined. Isoproterenol or forskolin treatment decreased beta 2AR mRNA levels in a time-dependent manner, with maximal loss of approximately 50% being observed after 2 hr. Pretreatment of the cells with a potent protein synthesis inhibitor, Pseudomonas exotoxin A, completely blocked isoproterenol- and forskolin-mediated down-regulation of beta 2AR mRNA. Exposure to agonist did not significantly influence the half-life of beta 2AR mRNA, which was approximately 60 min. In contrast, isoproterenol treatment for 2 hr significantly decreased the rate of beta 2AR gene transcription, as determined by nuclear run-on analysis. Based on these results, we propose that agonist regulation of beta 2AR mRNA in C6 cells is mediated by activation of the cAMP system and occurs at the level of beta 2AR gene transcription, not mRNA stability. In addition, the observed requirement for protein synthesis indicates that down-regulation of beta 2AR mRNA may be mediated by expression of a repressor of beta 2AR gene transcription.

Differences in desensitization between human beta 1- and beta 2-adrenergic receptors stably expressed in transfected hamster cells

It is well-established that agonist-mediated desensitization of the beta 2-adrenergic receptor (beta 2AR) involves its phosphorylation by protein kinase A (PKA) and the beta AR kinase (beta ARK). The phosphorylated receptor is less efficient at mediating agonist stimulation of adenylyl cyclase activity. The result is an increase in the concentration of agonist required for half-maximal stimulation (EC50) and a reduction in maximal stimulation (Vmax). As less is known about desentization of the human beta 1 AR, we compared the desensitization pattern of human beta 1 AR and beta 2AR stably expressed in two different hamster cell lines: Chinese hamster ovary (CHO), and Chinese hamster fibroblast (CHW). Following agonist treatment, all of the cell lines exhibited an increase in EC50, and a reduction in Vmax was observed in CHO-beta 2 but not beta 1 cells. CHW-beta 1 cells were resistant to acute agonist-mediated reduction in Vmax compared to CHW-beta 2 cells. More prolonged agonist exposure produced a modest reduction in Vmax and this effect was more noticeable when the CHW cells expressed lower levels of beta 1AR. To explore the role of protein kinases in these effects, digitonin-permeabilized CHW cells were loaded either with heparin (a beta ARK inhibitor) or a peptide inhibitor of PKA and exposed to agonist. In both beta 2AR- and beta 1AR-expressing cells, heparin inhibited the reduction in Vmax and the PKA inhibitor blocked the increase in EC50. Finally, exposing CHW cells expressing either subtype to a permeable cyclic AMP derivative caused an increase in EC50 similar to that observed in agonist-treated cells, but without any reduction in maximal activity. Our data suggest that whereas PKA-mediated desensitization is not subtype-specific, human beta 1AR is more resistant to beta ARK-mediated desensitization compared to the human beta 2AR.

Agonist and cyclic AMP-mediated regulation of beta 1-adrenergic receptor mRNA and gene transcription in rat C6 glioma cells

Exposure of rat C6 glioma cells to either agonists or agents that increase cyclic AMP levels leads to down-regulation of beta 1-adrenergic receptors (beta 1 AR) as measured by loss of radioligand binding sites. The present study examines the influence of isoproterenol and forskolin treatment on levels of beta 1 AR mRNA, mRNA stability, and gene transcription rate. Isoproterenol treatment of C6 cells altered beta 1 AR mRNA levels in a biphasic manner; i.e., short-term exposure (30-60 min) increased by 50%, whereas longer exposure (2-6 h) decreased by 50% the levels of beta 1 AR mRNA. The extent of both the up- and down-regulation was dependent on agonist concentration. Similar regulation of beta 1 AR mRNA was observed in forskolin-treated cells. Pretreatment of the cells with Pseudomonas exotoxin A, a potent inhibitor of protein synthesis, completely blocked isoproterenol- and forskolin-mediated down-regulation of beta 1 AR mRNA, and thereby potentiated the increase in receptor mRNA up to fourfold over the 6-h time course. The mechanisms underlying beta 1 AR mRNA down-regulation were examined. The half-life of beta 1 AR mRNA was slightly increased (from 61 to 77 min) after a 2-h exposure of the cells to either isoproterenol or forskolin. Nuclear run-on analysis demonstrated that the rate of beta 1 AR gene transcription was increased after isoproterenol incubation for 60 min, but then decreased after 90-240 min, consistent with the time course for up- and down-regulation of beta 1 AR mRNA. Isoproterenol treatment (120 min) also decreased the level of beta 1 AR nascent transcripts, purified by affinity chromatography of RNA isolated from 4-thiouridine-pulsed cells. The results demonstrate that beta 1 AR mRNA has a relatively short half-life and that agonist regulation of beta 1 AR mRNA is mediated by activation of the cyclic AMP system. Moreover, the results indicate that agonist regulation of beta 1 AR mRNA occurs at the level of beta 1 AR gene transcription, not mRNA stability. Finally, the observed requirement for protein synthesis indicates that beta 1 AR mRNA down-regulation may be mediated by the induction of a repressor of the beta 1 AR gene.

The heat-labile enterotoxin of Escherichia coli binds to polylactosaminoglycan-containing receptors in CaCo-2 human intestinal epithelial cells

The E. coli type I heat-labile enterotoxin (LT-I) shares considerable functional, structural, and immunological homology with cholera toxin (CT). Although the ganglioside GM1 is the sole receptor for CT, LT-I also appears to utilize additional, unique receptors on intestinal cells not recognized by CT. We characterized this second class of LT-I receptors using the human intestinal epithelial cell line, CaCo-2. CaCo-2 cells bound 8-fold more LT-I than CT, and some of these additional LT-I receptors appeared to be functional, as CT-B only partially inhibited LT-I activity at concentrations that completely inhibited CT activity. Membranes from unlabeled or [3H]galactose-labeled cells were incubated with toxin B subunits and extracted with Triton X-100, and the solubilized toxin B-receptor complexes were immunoabsorbed with anti-B bound to protein A-Sepharose. When organic extracts of the complexes were separated by thin-layer chromatography and overlayed with [125I]toxin, both toxins were found to bind only GM1. Separation of the complexes from [3H]galactose-labeled membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a series of galactoproteins specifically recognized by LT-I but not by CT. Similar proteins were detected on Western blots probed with [125I]toxin. LT-I activity on intact cells and binding to membranes and the above galactoproteins were enhanced by neuraminidase treatment even in the presence of CT-B. beta-1,4-Galactosidase and endo-beta-1,4-galactosidase, but not beta-1,3-galactosidase, significantly reduced LT-I binding. LT-I binding to fetuin and transferrin exhibited a similar glycosidase sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of adenylylcyclase by protein kinase C in human neurotumor SK-N-MC cells: evidence that the alpha isozyme mediates both potentiation and desensitization

Exposure of human SK-N-MC neurotumor cells to 4 beta-phorbol 12-myristate 13-acetate (PMA) increased isoproterenol stimulation of cyclic AMP levels by severalfold. This potentiation was blocked by inhibitors of protein kinase C (PKC) and did not occur in cells in which PKC had been down-regulated. PMA treatment also enhanced the stimulation by dopamine, cholera toxin, and forskolin. Thus, the effect of PMA on the adenylylcyclase system was postreceptor and involved either the guanine nucleotide binding regulatory (G) proteins or the cyclase itself. As PMA treatment did not impair the inhibition of isoproterenol stimulation by neuropeptide Y, an involvement of the inhibitory G protein Gi was unlikely. Cholate extracts of membranes from control and PMA-treated cells were equally effective in the reconstitution of adenylylcyclase activity in S49 cyc- membranes, which lack the stimulatory G protein subunit Gs alpha; thus, Gs did not appear to be the target of PMA action. Membranes from PMA-treated cells exhibited increased adenylylcyclase activity to all stimulators including Mn2+ and Mn2+ plus forskolin. In addition, activity was increased when control membranes were incubated with ATP and purified PKC from rat brain. This is consistent with a direct effect of PKC on the adenylylcyclase catalyst in SK-N-MC cells. PMA treatment also resulted in a shift to less sensitivity in the K(act) for isoproterenol but not for dopamine or CGP-12177 (a beta 3-adrenergic agonist) stimulation. Thus, the beta 1 but not the D1 or beta 3 receptors were being desensitized by PKC activation. Analysis of SK-N-MC cells by western blotting with antibodies against different PKC isozymes revealed that both the alpha and zeta isozymes were present in these cells. Whereas PKC-alpha was activated and translocated from cytosol to membrane by phorbol esters, the zeta isozyme was not. Thus, PKC-alpha, which has been implicated in desensitization in other cell lines, also appears to potentiate adenylylcyclase activity.

Independent and coordinate regulation of beta 1- and beta 2-adrenergic receptors in rat C6 glioma cells

Rat C6 glioma cells have both beta 1- and beta 2-adrenergic receptors in approximately 7:3 ratio. When the cells were exposed to the beta-adrenergic agonist isoproterenol, there was a rapid sequestration of up to 50% of the surface receptor population over a 30-min period as measured by the loss of binding of the hydrophilic ligand [3H] CGP-12177 to intact cells. Using the beta 1-selective antagonist CGP 20712A to quantify the proportion of the two subtypes, it was found that although both beta 1 and beta 2 receptors were sequestered, the latter were sequestered initially twice as fast as the former. More prolonged agonist exposure led to a down-regulation of approximately 90% of the total receptor population by 6 h as measured by the loss of binding of the more hydrophobic ligand [125I]iodocyanopindolol to cell lysates. The two subtypes, however, underwent down-regulation with similar kinetics. Treatment of the cells with agents that raise cyclic AMP levels such as cholera toxin and forskolin resulted in a slower, but still coordinated down-regulation of both subtypes. Thus, there appears to be both independent and coordinate regulation of endogenous beta 1-and beta 2-adrenergic receptors in the same cell line.

Induction of beta 2-adrenergic receptor mRNA and ligand binding in HeLa cells

HeLa cells express low levels of beta-adrenergic receptor (beta AR) of the beta 2-subtype. When exposed to sodium butyrate, receptor levels increased up to 4-fold in a time dependent manner, reaching a maximum after 12 to 15 h of treatment. Sodium butyrate treatment also caused a 3 to 4 fold increase in levels of beta 2AR mRNA determined by hybridization blot analysis. The induction of beta 2AR mRNA temporally preceded the increase in receptor binding activity, reaching a maximum after 4 to 6 h of treatment, and remaining elevated for up to 24 h. Prior exposure of the cells to the protein synthesis inhibitor cycloheximide prevented the butyrate-induced increase in receptor binding but had no effect on the increase in receptor mRNA. Blocking DNA synthesis and cell growth by excess thymidine did not increase beta 2AR mRNA or binding or prevent the effects of sodium butyrate. Thus, butyrate appears to induce beta 2AR mRNA by a mechanism independent of DNA and protein synthesis.

Orientation of cholera toxin bound to target cells

Cholera toxin (CT) consists of a pentameric B subunit that binds to specific cell surface receptors identified as ganglioside GM1 and an A subunit that activates adenylylcyclase. The A subunit consists of A1 and A2 peptides linked by a disulfide bond; A2 acts to connect A to B, whereas A1 is an ADP-ribosyltransferase that modifies the alpha subunit of the stimulatory G protein (Gs). How the toxin is oriented when it binds to the cell surface and the related issue of the mechanism by which A1 gains access to Gs alpha are not known. In the present study, we used subunit-specific antibodies and their corresponding Fab fragments to assess their affects on holotoxin binding to target cells and their immunoreactivity to cell-bound toxin. Our results suggest that CT binds with A1 facing away from the membrane. Our hypothesis is further supported by the ability to assemble active CT on the cell surface of cultured human intestinal and neurotumor cells by the sequential addition of purified B and A subunits. We also observed that when cells containing bound CT were incubated at 37 degrees C, both subunits rapidly became inaccessible to their respective antibodies. We propose that the holotoxin binds with its A subunit facing away from the membrane and must enter the cell in order for A1 to be released, gain access to Gs alpha, and activate adenylylcyclase.

Brefeldin A blocks the response of cultured cells to cholera toxin. Implications for intracellular trafficking in toxin action

Cholera toxin (CT) consists of a pentameric B subunit which binds to ganglioside GM1 on the cell surface and an A subunit which activates adenylylcyclase. The latter process involves the reduction of A to the A1 peptide which ADP-ribosylates the stimulatory G protein, Gs of adenylylcyclase. There is a distinct lag phase between toxin binding and activation of adenylylcyclase. Little is known about the events during this lag including where A1 is generated and how it gains access to Gs on the cytoplasmic side of the plasma membrane. We explored the effects of several inhibitors of intracellular trafficking on the response of human SK-N-MC neurotumor and Caco-2 intestinal tumor cells to CT. Whereas chloroquine or monensin had little or no effect on CT stimulation of cyclic AMP accumulation, brefeldin A (BFA) totally inhibited the response to CT in a time- and dose-dependent and reversible manner. BFA was effective when added at the same time as CT and had an IC50 of 30 ng/ml. BFA did not alter cell surface GM1 as cells treated with BFA for 30 min bound as much 125I-CT as control cells. Furthermore, BFA inhibited CT stimulation of GM1-treated rat glioma C6 cells. BFA treatment did not affect beta-adrenergic agonist stimulation of cyclic AMP. In addition, adenylylcyclase was activated by A1 peptide and NAD+ to the same extent in membranes from control and BFA-treated cells, or when BFA was added directly to the assay. Whereas control cells generated small amounts of A1 from bound CT with time, no A1 was detected in BFA-treated cells. BFA treatment did not prevent the internalization of CT but did inhibit its degradation. BFA is known to disrupt the organization of the Golgi complex, resulting in inhibition of protein transport from the endoplasmic reticulum and redistribution of Golgi enzymes to the endoplasmic reticulum. BFA also prevents the formation of non-clathrin-coated vesicles from Golgi membranes and thus vesicular transport between Golgi cisternae. We confirmed that BFA caused the morphological disruption of the Golgi apparatus in Caco-2 cells. The data support a role for a functional Golgi apparatus with its associated vesicular routing in CT action.

Brefeldin A inhibits protein synthesis in cultured cells

The fungal metabolite brefeldin A (BFA) is known to disrupt the Golgi apparatus resulting in redistribution of Golgi proteins to the endoplasmic reticulum and inhibition of protein secretion. BFA was found to inhibit protein synthesis in rat glioma C6 cells by up to 70% between 0.1 and 1 microgram/ml. Inhibition was both time-dependent and reversible. BFA inhibited protein synthesis to varying degrees in a number of other cell lines but not in BFA-resistant marsupial kidney cells. The same concentrations of BFA which inhibited protein synthesis, also blocked the inhibitory effects of Pseudomonas exotoxin and ricin on BFA-sensitive cells. BFA, however, was unable to block the inhibition of protein synthesis by the toxins in the resistant marsupial kidney cells.

Synthesis of a photoreactive, radiolabelled derivative of the oligosaccharide of GM1 ganglioside

A photoreactive, radioiodinatable derivative of the oligosaccharide (GM1OS) of ganglioside GM1 was synthesized as follows: GM1OS was generated from GM1 by ozonolysis and alkaline fragmentation, and reductively aminated to GM1OSNH2 (1-amino-1-deoxymonosialogangliotetraitol). The latter compound was then reacted with N-hydroxysuccinimidyl-4-azidosalicylic acid (NHS-ASA) to form GM1OSNH-ASA [1-(4-azidosalicoylamido)-1-deoxymonosialogangliotetraitol], which was radioiodinated and further purified. To test the [125I]GM1OSNH-IASA [1-(4-iodoazidosalicoylamido)-1-deoxymonosialogangliotetraitol+ ++] as a probe for ganglioside-binding proteins, the derivative was incubated with cholera toxin, which specifically binds GM1, followed by photolysis and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The probe only labelled the B or binding subunit of cholera toxin, but not the A or adenylyl cyclase activating subunit. Labelling was inhibited by excess GM1OS, but not by the oligosaccharides from gangliosides GD1a and GD1b. [125I]GM1OSNH-IASA and analogous oligosaccharide derivatives may be valuable probes for detecting ganglioside-binding proteins.

Intoxication of cultured cells by cholera toxin: evidence for different pathways when bound to ganglioside GM1 or neoganglioproteins

We previously reported that when the oligosaccharide of ganglioside GM1 is covalently attached to cell surface proteins of GM1-deficient rat glioma C6 cells, the cells bind large amounts of cholera toxin (CT) but their cAMP response to CT is not enhanced [Pacuszka, T., & Fishman, P. H. (1990) J. Biol. Chem. 265, 7673-7668]. We now report that when such cells were exposed to CT in the presence of chloroquine, an acidotropic agent, they accumulated cAMP. This raised the possibility that CT bound to cell surface "neoganglioproteins" may be entering the cells through a different pathway from that of CT-bound GM1. To further explore this phenomenon, we covalently attached GM1 oligosaccharide to human transferrin (Tf). The modified protein (GM1OS-Tf) bound with high affinity to Tf receptors on HeLa cells and increased the binding of CT to the cells. The bound CT, however, was unable to activate adenylyl cyclase as measured by cyclic AMP accumulation. By contrast, treatment of HeLa cells with GM1 increased both CT binding and stimulation of cyclic AMP accumulation. Control cells and cells treated with either GM1 or GM1OS-Tf were exposed to CT in the presence of chloroquine. Whereas chloroquine had little or no effect on the response of control or GM1-treated cells to CT, it made the cells treated with GM1OS-Tf responsive to the toxin. Our results indicate that CT bound to its natural receptor GM1 enters the cells through a pathway different from that of toxin bound to neoganglioproteins.

D1 dopamine receptors can interact with both stimulatory and inhibitory guanine nucleotide binding proteins

Pretreatment of striatal membranes with N-ethylmaleimide in the presence of a D1-specific agonist inactivated endogenous guanine nucleotide binding proteins (G proteins), but not D1 dopamine receptors, resulting in a loss of high-affinity agonist binding sites. Such D1 receptors were solubilized, mixed with exogenous G proteins from cells not containing D1 receptors, and reconstituted into phospholipid vesicles. These reconstituted receptors were able to couple to the exogenous G proteins, and the proportion of agonist high-affinity sites of the receptor (40-57%) was similar to levels obtained with naive receptors coupling to endogenous G proteins (40%) upon solubilization and reconstitution. These hybrid high-affinity sites were fully modulated by guanine nucleotides. Pretreatment of cells with pertussis toxin prior to extraction of G proteins resulted in a 50% decrease in the proportion of high-affinity sites; these sites remained sensitive to guanine nucleotides. When D1 receptors were reconstituted with extracts of cyc- cells, which lack stimulatory G proteins, the proportion of high-affinity sites was reduced to 31% of the total. Pertussis toxin treatment of the cyc- cells completely abolished the formation of high-affinity sites. These results demonstrate that D1-dopaminergic receptors are able to couple to not only stimulatory G proteins (Gs), but also to inhibitory G proteins (Gi).

Metabolism of cholesterol, phosphatidylethanolamine and stearylamine analogues of GM1 ganglioside by rat glioma C6 cells

Tritium-labeled neoglycolipids consisting of the oligosaccharide of ganglioside GM1 attached to cholesterol (GM1OSNH-X-CHOL), phosphatidylethanolamine (GM1OS-PE) and stearylamine (GM1OSNHC18) were synthesized and their uptake and metabolism by GM1-deficient rat glioma C6 cells were determined. When the neoglycolipids were added to serum-free culture medium, all three were rapidly taken up by the cells and initially inserted into the plasma membrane based on their resistance to trypsin and their ability to bind cholera toxin. With time, the neoglycolipids underwent internalization as the ratio of cell-associated radioactivity to cell surface toxin binding increased; this process was slow for GM1OSNH-X-CHOL and GM1OS-PE and rapid for GM1OSNHC18. Analysis of lipids extracted from the cells indicated that the neoglycolipids also underwent metabolism to GD1aOS-based analogues. In addition, GM1OSNH-X-CHOL and GM1OSNHC18 were degraded to their GM2OS-based analogues, whereas GM2OS-PE was not detected. In contrast, large amounts of 3H were recovered in the medium from cells treated with GM1OS-PE and the label was associated with material that behaved neither as an oligosaccharide or a neoglycolipid. In the presence of monensin or chloroquine, metabolism of the three neoglycolipids was inhibited. Thus, GM1OS-based neoglycolipids were taken up by the cells, internalized and sorted both to the Golgi apparatus (sialylated to GD1aOS-based analogues) and to lysosomes (hydrolyzed to GM2OS-based analogues). The rate and extent of these processes, however, were strongly influenced by the nature of lipid moiety.

Desensitization of the human beta 1-adrenergic receptor. Involvement of the cyclic AMP-dependent but not a receptor-specific protein kinase

Human SK-N-MC neurotumor cells express beta 1- but not beta 2-adrenergic receptors. Following exposure of the cells to isoproterenol, there was no reduction in the maximum response of adenylyl cyclase to the agonist but a 3-fold shift to less sensitivity in the concentration response. This desensitization was very rapid and dose dependent; half-maximal effects occurred at 10 nM isoproterenol. A similar shift was observed when membranes from control cells were incubated with ATP and the catalytic subunit of cyclic AMP-dependent protein kinase (PKA). No shift, however, was observed in intact cells exposed to either dibutyryl cyclic AMP or dopamine, which stimulates adenylyl cyclase in these cells through D1 dopamine receptors. To pursue the role of protein kinases in the desensitization process, cells were made permeable, loaded with a PKA inhibitor or with heparin, an inhibitor of the beta-adrenergic receptor kinase (beta ARK), and exposed to isoproterenol. The PKA inhibitor but not heparin blocked the agonist-mediated desensitization. In contrast, desensitized human tumor cells (HeLa and A431), which express beta 2-adrenergic receptors, exhibited both a shift in concentration response and a reduction in maximum response; the former was blocked by the PKA inhibitor and the latter by heparin. Our results indicated that whereas both human beta 1- and beta 2-adrenergic receptors are susceptible to PKA, only the beta 2 receptors are susceptible to beta ARK. These differences in desensitization may be due to differences in receptor structure as the human beta 1 receptor has fewer potential phosphorylation sites for beta ARK in the carboxyl terminus than the human beta 2 receptor.

Neoglycolipid analogues of ganglioside GM1 as functional receptors of cholera toxin

We synthesized several lipid analogues of ganglioside GM1 by attaching its oligosaccharide moiety (GM1OS) to aminophospholipids, aliphatic amines, and cholesteryl hemisuccinate. We incubated GM1-deficient rat glioma C6 cells with each of the derivatives as well as native GM1 and assayed the cells for their ability to bind and respond to cholera toxin. On the basis of the observed increase in binding of 125I-labeled cholera toxin, it was apparent that the cells took up and initially incorporated most of the derivatives into the plasma membrane. In the case of the aliphatic amine derivatives, the ability to generate new toxin binding sites was dependent on chain length; whereas the C10 derivative was ineffective, C12 and higher analogues were effective. Increased binding was dependent on both the concentration of the neoglycolipid in the medium and the time of exposure. Cells pretreated with the various derivatives accumulated cyclic AMP in response to cholera toxin, but there were differences in their effectiveness. The cholesterol and long-chain aliphatic amine derivatives were more effective than native GM1, whereas the phospholipid derivatives were less effective. The distance between GM1OS and the phospholipid also appeared to influence its functional activity. The neoglycolipid formed by cross-linking the amine of GM1OS to phosphatidylethanolamine (PE) with disuccinimidyl suberate was less effective than the neoglycolipid formed by directly attaching GM1OS to PE by reductive amination. Furthermore, insertion of a C8 spacer in the former neoglycolipid rendered it even less effective.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and regulation of beta 1-adrenergic receptors in a human neuroepithelioma cell line

Intact human neuroepithelioma SK-N-MC cells bound the beta-adrenergic antagonist (-)-[3H]-CGP 12177 with a KD of 0.13 nM and a Bmax of 17,500 sites/cell. When the cells were exposed to beta-adrenergic agonists, they accumulated cyclic AMP in the following order of potency: isoproterenol much greater than norepinephrine greater than epinephrine, which is indicative of a beta 1-subtype receptor. Membranes prepared from the cells bound (-)-3-[125I]iodocyanopindolol with a KD of 11.5 pM. Inhibition of agonist-stimulated cyclic AMP production and competition binding experiments indicated that the beta 1-selective antagonists CGP 20712A and ICI 89,406 were much more potent than the beta 2-selective antagonist ICI 118,551. Analysis of the displacement curves indicated that the cells contained only beta 1-adrenergic receptors. Northern blot analysis of SK-N-MC mRNA using cDNA probes for the beta 1- and beta 2-adrenergic receptors revealed the presence of a very strong beta 1-adrenergic receptor mRNA signal, while under the same conditions no beta 2-adrenergic receptor mRNA was observed. Thus, SK-N-MC cells appear to express a pure population of beta 1-adrenergic receptors. When the cells were exposed to isoproterenol, there was no observable desensitization during the first hour. After longer exposure, desensitization slowly occurred and the receptors slowly down-regulated to 50% of control levels by 24 h. Other agents that elevate cyclic AMP levels, such as forskolin, cholera toxin, and cyclic AMP analogues, caused no or little substantial receptor loss.

Centrally truncated neuropeptide Y analog acts as an agonist for Y1 receptors on SK-N-MC cells

The similarity of neuropeptide Y (NPY) to pancreatic polypeptide (PP), whose X-ray crystallographic structure is known, has allowed computer-assisted molecular modelling of NPY and predictions of its three-dimensional structure. Utilizing these techniques, Krstenansky et al. (Proc. Natl. Acad. Sci. U.S.A., 86 (1989) 4377-4381) reported that a centrally truncated analog of porcine NPY, [D-Cys7-Aoc8-17-Cys20]pNPY, which was designed to maintain the tertiary structure of the native molecule, bound to sites on membranes from mouse brain with even higher affinity than native NPY. As brain membranes may represent a heterogeneous mixture of receptor subtypes, we decided to characterize the activity of this analog on a defined cell line. SK-N-MC cells are a human epithelioma cell line with high-affinity receptors of the Y1 subtype which are coupled to inhibition of adenylate cyclase. (D-Cys7-Aoc8-17-Cys20]pNPY bound to receptors on SK-N-MC cells, but in contrast to membranes from mouse brain, with a lower affinity than pNPY. Furthermore, [D-Cys7-Aoc8-17-Cys20]pNPY was able to inhibit isoproterenol-stimulated cAMP production in these cells. Therefore, it appears that the central amino acids deleted from this analog are not involved in NPY binding, and biological activity can be maintained by conservation of the tertiary structure of NPY around the binding surface.

Characterization of functional neuropeptide Y receptors in a human neuroblastoma cell line

We identified receptors for neuropeptide Y (NPY) on an established human neuroblastoma cell line, SK-N-MC, which are functionally coupled to adenylate cyclase through the inhibitory guanine nucleotide-binding protein of adenylate cyclase, Gi. Intact SK-N-MC cells bound radiolabeled NPY with a KD of 2 nM and contained approximately 83,000 receptors/cell. Unlabeled porcine and human NPY and structurally related porcine peptide YY (PYY) competed with labeled NPY for binding to the receptors. NPY inhibited cyclic AMP accumulation in SK-N-MC cells stimulated by isoproterenol, dopamine, vasoactive intestinal peptide, cholera toxin, and forskolin. NPY inhibited isoproterenol-stimulated cyclic AMP production in a dose-dependent manner, with half-maximal inhibition at 0.5 nM NPY. Porcine and human NPY and porcine PYY gave similar dose-response curves. NPY also inhibited basal and isoproterenol-stimulated adenylate cyclase activity in disrupted cells. Pertussis toxin treatment of the cells completely blocked the ability of NPY to inhibit cyclic AMP production and adenylate cyclase activity. The toxin catalyzed the ADP-ribosylation of a 41-kDa protein in SK-N-MC cells that corresponds to Gi. The receptors on SK-N-MC cells appeared to be specific for NPY, as other neurotransmitter drugs, such as alpha-adrenergic, dopaminergic, muscarinic, and serotonergic antagonists, did not compete for either NPY binding or NPY inhibition of adenylate cyclase. Thus, SK-N-MC cells may be a useful model for investigating NPY receptors and NPY-mediated signal transduction.

Differential activation of the stimulatory and inhibitory guanine nucleotide-binding proteins by fluoroaluminate in cells and in membranes

Fluoroaluminate had no effect on cAMP levels in cells but inhibited agonist-stimulated cAMP accumulation. In membranes, fluoroaluminate stimulated adenylate cyclase activity between 1 and 10 mM but not at higher concentrations, and it inhibited agonist-stimulated adenylate cyclase activity at concentrations greater than 1 mM. Fluoroaluminate is known to activate Gs and Gi, the guanine nucleotide-binding (G) proteins that stimulate and inhibit adenylate cyclase. G proteins are heterotrimeric, with unique alpha and common beta gamma subunits, and activation involves dissociation of alpha from beta gamma. Pertussis toxin catalyzes ADP-ribosylation of alpha i of heterotrimeric Gi but not free alpha i. Fluoroaluminate prevented pertussis toxin-catalyzed ADP-ribosylation of Gi in cells and membranes, suggesting that Gi is activated by fluoroaluminate in both. Cholera toxin catalyzes ADP-ribosylation of the alpha s subunit of Gs. In cells, agonist often increased cholera toxin-catalyzed ADP-ribosylation of Gs, but fluoroaluminate decreased ADP-ribosylation even in the presence of agonist, suggesting that Gs cannot be activated in the presence of fluoroaluminate. In membranes, both agonist and fluoroaluminate increased cholera toxin-catalyzed ADP-ribosylation, suggesting that Gs is activated by these agents. We conclude that fluoroaluminate activates Gi but not Gs in cells and activates both G proteins in membranes. The value of bacterial toxins in assessing the state of G protein in cells and membranes is demonstrated.

Comparison of the central and renal dopamine-1 receptor

The dopamine D-1 receptor from striatal membranes was compared with the dopamine DA-1 receptor from renal proximal tubules. The dopamine-1 receptors were solubilized with 1% sodium cholate and phospholipids after pretreatment with the dopamine-1 agonist, SKF R-38393. The soluble receptors were reconstituted into phospholipid vesicles after removal of sodium cholate. The receptors were studied by radioligand binding using the dopamine-1 antagonist [125I]-SCH 23982. The reconstituted dopamine D-1 and DA-1 receptor densities were similar. However, the affinity of the solubilized D-1 receptor was 17-fold greater than the solubilized DA-1 receptor. The affinity of membrane bound D-1 receptor to the radioligand was also greater than that noted for membrane bound DA-1 receptor. The mechanism for this difference remains to be determined.

Generation of cell surface neoganglioproteins. GM1-neoganglioproteins are non-functional receptors for cholera toxin

GM1 (II3Neu5Ac-GgOse4Cer)-oligosaccharide was prepared from the ganglioside by ozonolysis and alkaline fragmentation, reductively aminated and coupled to the heterobifunctional cross-linker succinimidyl 4-(N-maleimidomethyl) cyclohexane-1-carboxylate. The resulting derivative reacted with free sulfhydryl groups and readily cross-linked to cell surface components on rat glioma C6 cells which are GM1-deficient. Attachment of the GM1-oligosaccharide derivative, which was monitored by increased binding of 125I-cholera toxin to the cells, was both time- and concentration-dependent. Prior treatment of the cells with dithiothreitol enhanced the attachment by generating additional free sulfhydryl groups. The affinity of cholera toxin for cells treated with the GM1-oligosaccharide derivative or with GM1 was similar. The nature of the newly generated toxin receptors was determined by Western blotting. Membranes from derivatized cells were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the resolved components were electrophoretically transferred to a nitrocellulose sheet which was overlain with 125I-cholera toxin. The toxin bound to a wide variety of membrane proteins, most of which were trypsin-sensitive. No such binding was observed using membranes from control cells. Although the GM1-neoganglioproteins newly generated on the surface of rat glioma C6 cells readily bound cholera toxin, the cells did not become more responsive to the toxin as measured by increased production of cyclic AMP or activation of adenylate cyclase. In contrast, cells exposed to GM1 became highly responsive to the toxin. Thus, neoganglioproteins on the cell surface appear to behave as nonfunctional receptors for cholera toxin.

A primate model of anterior segment ischemia after strabismus surgery. The role of the conjunctival circulation

Fornix conjunctival incisions in strabismus surgery may provide partial protection against anterior segment ischemia by preserving the perilimbal conjunctival-Tenon's circulation, which is disrupted with limbal conjunctival incisions. Six adult cynomolgus monkeys underwent tenotomies of three or four rectus muscles by making limbal conjunctival incisions in one eye and fornix incisions in the other. Iris fluorescein angiography and slit-lamp biomicroscopy were used to monitor changes. The eyes that had limbal incisions exhibited more severe anterior segment ischemic changes than the eyes that had fornix incisions in every instance of four rectus muscle surgery.

Identification and characterization of functional D1 dopamine receptors in a human neuroblastoma cell line

Dopamine stimulated human neuroblastoma SK-N-MC cells to accumulated cyclic AMP. The D1 agonist SKF (R)-38393 also stimulated cyclic AMP production whereas the response to dopamine was inhibited by the D1 antagonist SCH (R)-23390. Membranes from SK-N-MC cells bound the D1 ligand [125I]SCH 23982 with a Kd of 2.1 nM and a Bmax of 102 fmol/mg protein. Binding was displaced by dopamine, SKF 38393, and SCH 23390. Up to 40% of the receptors were in an agonist high affinity, guanine nucleotide-sensitive state, compared to only 6% in rat striatum. A D1 photoaffinity probe labeled a 72 kDa protein in both SK-N-MC and rat striatal membranes. Thus, SK-N-MC human neuroblastoma cells contain D1 dopamine receptors which are similar to those found in mammalian striatum, but which are more tightly coupled to adenylate cyclase. SK-N-MC cells may be a useful model to investigate the properties and regulation of D1 dopamine receptors.

Interaction of the B subunit of cholera toxin with endogenous ganglioside GM1 causes changes in membrane potential of rat thymocytes

The fluorescent anionic dye, bisoxonol, and flow cytometry have been used to monitor changes in the membrane potential of rat thymocytes exposed to the B subunit of cholera toxin. The B subunit induced a rapid hyperpolarization, which was due to activation of a Ca2+-sensitive K+ channel. Reduction of extracellular Ca2+ to less than 1 microM by the addition of [ethylene-bis(oxyethylenenitrilo)]tetraacetic acid immediately abolished the hyperpolarization caused by the B subunit. Cells treated with quinine and tetraethylammonium lost their ability to respond to the B subunit, whereas 4-aminopyridine did not have any effect. Thus, calcium-sensitive and not voltage-gated K+ channels appeared to be responsible for the hyperpolarization. The results of ion substitution experiments indicated that extracellular Na+ was not essential for changes in membrane potential. Further studies with ouabain, amiloride and furosemide demonstrated that electrogenic Na+/K+ ATPase, Na+/H+ antiporter and Na+/K+/Cl- cotransporter, respectively, were not involved in the hyperpolarization process induced by the B subunit. Thus, crosslinking of several molecules of ganglioside GM1 on the cell surface of rat thymocytes by the pentavalent B subunit of cholera toxin modulated plasma membrane permeability to K+ by triggering the opening of Ca2+-sensitive K+ channels. A role for gangliosides in regulating ion permeability would have important implications for the function of gangliosides in various cellular phenomena.

Lipid signalling pathways in normal and ras-transfected NIH/3T3 cells

The role of ras oncogenes in cellular signalling pathways involving phospholipid breakdown was studied in untransfected and proto-H-ras and mutated H-, K- and N-ras transfected NIH/3T3 cells. When the cells were grown at low cell densities, all of the ras transfected cells had 2-4 fold higher diacylglycerol (DAG) levels compared to growing NIH/3T3 cells. At high cell densities, DAG levels decreased in the former and increased in contact inhibited NIH/3T3 cells. In this regard, only cells transformed by mutated cellular and viral H-ras oncogenes (but not by the H-ras proto-oncogene) had elevated DAG levels compared to contact inhibited NIH/3T3 cells. The basal levels of inositol phosphates in ras transfected cells were not significantly different from NIH/3T3 cells and did not vary with cell density. Thus, the elevated DAG levels are not a consequence of increased phosphoinositide hydrolysis. The latter was stimulated by serum and bombesin only in normal and proto-H-ras transfected cells. In contrast, stimulation by bradykinin was observed only in cells transformed by mutated cellular ras oncogenes. Furthermore, aluminum fluoride stimulated phosphoinositide breakdown in the latter cells indicating that there was no uncoupling of the G protein from phospholipase C. Treatment of ras transfected cells with dibutyryl cyclic AMP (DB-cAMP), which causes an inhibition of growth and a reversal of the transformed morphology, did not alter the basal levels of inositol phosphates, DB-cAMP, however, did lower DAG levels in some of the transformed cell lines, but elevated DAG levels in low density NIH/3T3 cells. These findings indicate that the ras gene product p21 is not involved in phosphoinositide hydrolysis and that DAG levels do not correlate with cell growth in either normal or ras transfected NIH/3T3 cells. Thus, p21 appears to alter cell growth through mechanism(s) independent of lipid signalling pathways.

Muscarinic receptor-mediated increase in cAMP levels in SK-N-SH human neuroblastoma cells

Stimulation of muscarinic cholinergic receptors in SK-N-SH human neuroblastoma cells resulted in a 1.5-4 fold increase in intracellular cAMP levels. This unusual response was sensitive to atropine and pirenzepine but insensitive to pertussis toxin. It was observable regardless of whether basal, PGE1- or forskolin-stimulated cAMP levels were measured. The half-maximal concentration for carbachol-stimulation of cAMP levels (6 microM) was similar to that for the previously determined carbachol-induced stimulation of phosphoinositide turnover in these cells, suggesting that the former is mediated by the latter. These data indicate that cross-talk between the phosphoinositide turnover system and the adenylate cyclase system results in increased cAMP levels in SK-N-SH cells in response to muscarinic receptor stimulation.

Modulation of the beta-adrenergic receptor-coupled adenylate cyclase by chemical inducers of differentiation: effects on beta receptors and the inhibitory regulatory protein Gi

Several drugs known to induce differentiation in tumor cells were analyzed for their effects on the beta-adrenergic receptor-coupled adenylate cyclase system in two human carcinoma cell lines, HeLa and A431. Each of the drugs was tested alone or in combination with sodium butyrate (NaBu), a known inducer of this signal transduction system. Puromycine amino nucleoside (PMAN) caused the largest increase in beta-adrenergic receptors in HeLa cells followed by hexamethylenebisacetamide (HMBA) whereas 5'-azacytidine (5AZC) was ineffective. In addition, PMAN but not the others acted together with NaBu to elevate receptor levels 12-fold over control values. In contrast, HMBA and 5AZC were much more effective on A431 cells, PMAN caused only a slight increase in beta receptors and none of the drugs acted in concert with NaBu. The increase in beta receptors was usually accompanied by a corresponding increase in isoproterenol-stimulated adenylate cyclase activity. These effects of the drugs appeared to require protein synthesis as they were blocked by cycloheximide. In addition, some of the drugs caused a substantial decrease in basal adenylate cyclase activity. This effect on basal activity was abolished in cells treated with pertussis toxin, which ADP-ribosylates the inhibitory GTP-binding protein, Gi. Both HeLa and A431 cells contained a 41 kDalton substrate for the toxin which corresponds to the alpha subunit of Gi. The Gi subunit was ADP-ribosylated by the toxin to a similar extent in membranes from control and drug-treated cells. Thus, the drugs appear to induce quantitative changes in beta-adrenergic receptors and qualitative changes in Gi which results in a highly responsive beta-adrenergic-stimulated adenylate cyclase.

Duration of therapeutic levels of intravitreally injected liposome-encapsulated clindamycin in the rabbit

We investigated an intravitreal preparation of liposome-encapsulated clindamycin phosphate to determine the duration of therapeutic levels of the drug in the vitreous cavity. Twenty New Zealand albino rabbits were given an intravitreal injection of 750 micrograms/0.1 mL of encapsulated clindamycin (10 animals) or 800 micrograms/0.1 mL of nonencapsulated clindamycin (10 animals) and then were killed immediately or 6, 12, 24 or 48 hours later. The mean concentration of encapsulated clindamycin in the vitreous at 48 hours was 28.4 micrograms/mL, while that of nonencapsulated clindamycin at 24 hours was 2.3 micrograms/mL. The estimated elimination rate of nonencapsulated clindamycin was 3 hours, compared with approximately 10 hours for the encapsulated preparation. This drug delivery system warrants further investigation for possible use in humans.

Alteration of glycolipids in ras-transfected NIH 3T3 cells

Glycosphingolipid alterations upon viral transformation are well documented. Transformation of mouse 3T3 cells with murine sarcoma viruses results in marked decreases in the levels of gangliosides GM1 and GD1a and an increase in gangliotriaosylceramide. The transforming oncogenes of these viruses have been identified as members of the ras gene family. We analyzed NIH 3T3 cells transfected with human H-, K- and N-ras oncogenes for their glycolipid composition and expression of cell surface gangliosides. Using conventional thin-layer chromatographic analysis, we found that the level of GM3 was increased and that of GD1a was slightly decreased or unchanged, and GM1 was present but not in quantifiable levels. Cell surface levels of GM1 were determined by 125I-labeled cholera toxin binding to intact cells. GD1a was determined by cholera toxin binding to cells treated with sialidase prior to toxin binding. All ras-transfected cells had decreased levels of surface GM1 and GD1a as compared to logarithmically growing normal NIH 3T3 cells. Levels of GM1 and, to a lesser extent, GD1a increased as the latter cells became confluent. Using a monoclonal antibody assay, we found that gangliotriaosylceramide was present in all ras-transfected cells studied but not in logarithmically growing untransfected cells. Interestingly, gangliotriaosylceramide appeared when the latter cells became confluent. These results indicated that ras oncogenes derived from human tumors are capable of inducing alterations in glycolipid composition.

Effect of the tricyclic antidepressant desipramine on beta-adrenergic receptors in cultured rat glioma C6 cells

Rat glioma C6 cells, cultured in the presence of the tricyclic antidepressant desipramine, lost a significant number of beta-adrenergic receptors in a time- and dose-dependent manner. A similar loss was observed whether binding was determined on intact cells with the hydrophilic beta-adrenergic antagonist (+/-)-[3H]4-(3-tert-butylamino-2-hydroxypropoxyl)benzimidazole-2-o n HCl ([3H]CGP-12177) or on cell lysates with the more hydrophobic antagonists [125I]iodocyanopindolol or [3H]dihydroalprenolol. When stimulated with the agonist isoproterenol, desipramine-treated cells accumulated less cyclic AMP than control cells. The affinity of the beta-adrenergic receptors for either antagonist or agonist was unchanged after desipramine treatment. Desipramine interacted only weakly with the receptors and competed for [125I]iodocyanopindolol binding with a Ki of 30 microM. The presence in the culture medium of alprenolol or propranolol, potent beta-adrenergic antagonists, however, did not prevent the reduction in receptors by desipramine. Desipramine also caused a loss of beta-adrenergic receptors from cells maintained in serum-free medium and the cells themselves did not contain or secrete endogenous catecholamines. Although desipramine is a potent inhibitor of catecholamine uptake, it appears unlikely that the observed loss of beta-adrenergic receptors in rat glioma C6 cells exposed to the drug is due to an increase in extracellular catecholamine levels or to a direct interaction with the receptors.

Down-regulation of protein kinase C in rat glioma C6 cells: effects on the beta-adrenergic receptor-coupled adenylate cyclase

Continuous exposure of rat glioma C6 cells to 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a time and dose dependent loss of [3H]phorbol dibutyrate binding sites and protein kinase C activity. Thus, by 24 h, the cells were essentially depleted of protein kinase C activity. In agreement with previous studies, TPA treatment caused a reduction in isoproterenol-stimulated adenylate cyclase activity and a sequestration of beta-adrenergic receptors. Cells were treated with TPA for 24-48 h to completely down-regulate protein kinase C and then exposed to isoproterenol. Agonist-mediated desensitization of adenylate cyclase and sequestration of beta-adrenergic receptors occurred at similar rates in control and TPA-treated cells. In addition, agonist-mediated down-regulation of beta-adrenergic receptors was not impaired by the absence of protein kinase C activity. Although both agonists and phorbol esters cause desensitization of the beta-adrenergic receptor-coupled adenylate cyclase, agonist-mediated events can occur independently of protein kinase C.

Neuropeptide Y inhibits cardiac adenylate cyclase through a pertussis toxin-sensitive G protein

Neuropeptide Y, a major neuropeptide and potent vasoconstrictor, inhibited isoproterenol-stimulated adenylate cyclase activity in cultured rat atrial cells as well as in atrial membranes. Prior treatment of the cells with pertussis toxin blocked the inhibitory action of neuropeptide Y. Pertussis toxin is known to uncouple the receptors for other inhibitors of adenylate cyclase by ADP-ribosylation of the alpha-subunit of Gi, the inhibitory guanine nucleotide binding component of adenylate cyclase. The toxin specifically catalyzed the ADP-ribosylation of a 41-kilodalton atrial membrane protein which corresponded to the Gi subunit. These results suggest that neuropeptide Y may mediate some of its physiological effects through specific receptors linked to the inhibitory pathway of adenylate cyclase.

Gangliosides as bimodal regulators of cell growth

The B subunit of cholera toxin, which binds specifically to several molecules of ganglioside galactosyl-(beta 1----3)-N-acetylgalactosyminyl(beta 1----4)-[N- acetylneuraminyl(alpha 2----3)]-galactosyl(beta 1----4)glucosyl(beta 1----1) ceramide (GM1) on the cell surface, stimulated DNA synthesis and cell division in quiescent, nontransformed mouse 3T3 cells in a dose-dependent manner. In addition, the B subunit potentiated the response of the 3T3 cells to other mitogens, such as epidermal growth factor, platelet-derived growth factor, and insulin. This synergistic effect indicates that the B subunit does not act identically to any of these growth factors but probably modulates a common effector system crucial for cell proliferation. In distinct contrast, the B subunit inhibited the growth of ras-transformed 3T3 cells as well as rapidly dividing normal 3T3 cells. Thus, the same cells, depending on their state of growth, exhibited a bimodal response to the B subunit. We conclude that endogenous gangliosides may be bimodal regulators of positive and negative signals for cell growth.

Recent advances in identifying the functions of gangliosides

The recent development of several new approaches has proven extremely useful in identifying functions for gangliosides, the sialic-acid containing glycosphingolipids. The first is the incorporation of exogenous gangliosides into the plasma membrane of ganglioside-deficient cells. Using this approach, specific gangliosides have been identified as the receptors for certain bacterial toxins and viruses and as important factors in the organization of fibronectin into an extracellular matrix. The second approach has been a ligand blotting technique which allows detection of ganglioside-binding proteins such as toxins and antibodies. Gangliosides are separated by thin-layer chromatography and overlain with the protein of interest. Specific binding of the ligand to gangliosides can then be detected by either direct or indirect methods. The third approach is the use of the B or binding subunit of cholera toxin as a specific probe for endogenous plasma membrane ganglioside function. The ability of the B subunit to alter the growth of cells directly demonstrates a role for gangliosides as biotransducers of signals for the regulation of cell growth.

Expression of gangliosides as receptors at the cell surface controls infection of NCTC 2071 cells by Sendai virus

The involvement of gangliosides as receptors for Sendai virus was established previously using experimentally produced receptor-deficient cells. In the search for a naturally occurring counterpart, NCTC 2071 cells emerged as a likely candidate. These cells in their native state were not agglutinated nor infected by Sendai virus, but were infected by the virus when the gangliosides GD1a, GT1b, or GQ1b were supplied in the culturing medium. Preliminary analysis indicated that NCTC 2071 cells contained an unusually high ratio of sialoglycoproteins to gangliosides. A brief treatment of the cell surface with the protease trypsin made greater than 99% of the native monolayer susceptible to infection by the wild-type virus which contains the viral attachment protein HN. (Incubation of the trypsin-treated cells with a temperature-sensitive mutant missing HN produced no detectable infection.) The increased binding of cholera toxin, a ganglioside-specific probe, after incubation of the cells with trypsin and sialidase, was consistent with the hypothesis that gangliosides more complex than GM1 are on the surface of NCTC 2071 cells and that trypsin treatment increases their accessibility. The presence of receptor gangliosides in lipid extracts of NCTC 2071 cells was confirmed by thin-layer chromatography of the ganglioside fraction and by the binding of cholera toxin. These results demonstrate that cells containing receptor gangliosides may still be resistant to infection because these are not expressed properly at the cell surface as receptors for interaction with the HN protein of Sendai virus.