Possible role of cyclic AMP in the receptor-mediated regulation of glycosyltransferase activities in neurotumor cell lines

Protein Kinases and Cross-talk between Post-translational Modifications in the Regulation of Drug Transporters

Drug transporters are modulators for drug absorption, distribution, and excretion. Key drug transporters including P-glycoprotein and breast cancer resistance protein of the ABC superfamily; organic anion transporting polypeptide 1B1 and 1B3, organic anion transporter 1 and 3, and organic cation transporter 2, as well as multidrug and toxin extrusion 1 and 2 of the SLC superfamily have been recommended by regulatory agencies to be investigated and evaluated in drug-drug interaction (DDI) studies due to their important roles in determining the efficacy, toxicity and DDI of various drugs. Drug transporters are subjected to multiple levels of control and post-translational modifications (PTMs) provide rapid and versatile ways of regulation. Under pathologic and/or pharmacological conditions, PTMs may be altered in the cellular system, leading to functional changes of transporter proteins. Phosphorylation is by far the most actively investigated form of PTMs in the regulation of transporters. Further, studies in recent years also found that protein kinases coordinate with other PTMs for the dynamic control of these membrane proteins. Here we summarized the regulation of major drug transporters by protein kinases and their cross-talking with other PTMs that may generate a complex regulatory network for fine-tuning the function of these important drug processing modulators. SIGNIFICANCE STATEMENT: Kinases regulate drug transporters in versatile manners; Kinase regulation cross-talks with other PTMs, forming a complex network for transporter regulation; Pathological and/or pharmacological conditions may alter PTMs and affect transporter function with different molecular mechanisms.

GM1 ganglioside-induced modulation of opioid receptor-mediated functions

Electrophysiologic studies of dorsal-root ganglion (DRG) neurons in culture have demonstrated both excitatory (Gs-coupled) as well as inhibitory (Gi/Go-coupled) opioid receptor-mediated actions. Brief treatment of DRG neurons with cholera toxin-beta which binds specifically to GM1 sites on neuronal membranes, selectively blocks opioid excitatory but not inhibitory effects. Conversely, after brief treatment of DRG neurons with GM1, but not with GM2, GM3, or other related gangliosides, the threshold concentration of opioid agonists for eliciting excitatory effects is markedly decreased from nM to pM-fM levels and opioid antagonists, for example, naloxone (NLX), at low concentrations paradoxically elicit excitatory effects. These studies suggest that the excitatory opioid supersensitivity of GM1-treated DRG neurons is due primarily to increased efficacy of excitatory opioid-receptor activation of Gs. Recent studies of cloned delta opioid receptors transfected into CHO cells suggest that this supersensitivity of GM1-treated DRG neurons may be further augmented by rapid conversion of many opioid receptors from a Gi/Go-coupled inhibitory mode to a Gs-coupled excitatory mode. The opioid excitatory supersensitivity elicited in DRG neurons by acute elevation of exogenous GM1 provides novel insights into mechanisms underlying opioid tolerance and dependence, since remarkably similar supersensitivity occurs in DRG and other neurons after chronic treatment with morphine or other opioid agonists that upregulate endogenous GM1.

Cell-specific regulation of the stably expressed serotonin 5-HT1A receptor and altered ganglioside synthesis

Neurotransmission is dependent on the presence of neuronal receptors at the synapses, and important cell surface molecules such as gangliosides are pivotal in the maintenance of synaptic contacts. To study the interrelationship between these two classes of molecules, we achieved stable expression of the hippocampus- and CNS-localized serotonin 1A receptor (5-HT1A-R) in three 5-HT1A-R-deficient neuronal cell lines and also the control, non-neural CHO cells. A strong passage dependence of 5-HT1A-R expression, as measured by mRNA levels as well as membrane binding to the selective agonist [3H]8-OH-DPAT, was observed only in the HN2 (hippocampal) and NCB-20 (CNS) cells which are derived from tissues of natural occurrence of the 5-HT1A-R. A paradigm of stress was obtained by carrying out continuous culture of cells without feeding. During this time a dramatic increase in 5-HT1A-R mRNA and [3H]8-OH-DPAT binding was observed only in the neuronal cells after confluence and during decreased cell viability (days 10/11). This was not due to differentiation, since deliberate serum deprivation and differentiation of cells did not result in any dramatic increase in 5-HT1A-R expression. Analysis of ganglioside synthesis by pulse labeling of the transfected cells produced striking results. In the dorsal root of the ganglion (DRG) derived F-11 cells which show low but significant levels of complex gangliosides before transfection, the mere presence of the serotonin 1A receptor resulted in a dramatic increase in synthesis of gangliosides comigrating with GM2, GD1a, GD1b, and GT1b (20-fold by densitometry). In contrast, there was only a 2-fold increase in the overall content of complex gangliosides in the presence of the 5-HT1A-R. In the NCB-20 cells which contain only GD1a but no GD1b or GT1b before transfection, a decrease in GD1a synthesis was observed following transfection. Also agonist (8-OH-DPAT) binding to the serotonin 1A receptor in NCB-20 cells produced a 3-fold increase in synthesis of a ganglioside comigrating with GM3. Thus, our neuroblastoma transfectants help demonstrate stress-induced regulation of the 5-HT1A-R, which in turn exerts a strong and cell type-specific control over such essential cell-surface determinants like gangliosides.

Chronic opioid treatment of neuroblastoma x dorsal root ganglion neuron hybrid F11 cells results in elevated GM1 ganglioside and cyclic adenosine monophosphate levels and onset of naloxone-evoked decreases in membrane K+ currents

Prolongation of the action potential duration of dorsal root ganglion (DRG) neurons by low (nM) concentrations of opioids occurs through activation of excitatory opioid receptors that are positively coupled via Gs regulatory protein to adenylate cyclase. Previous results suggested GM1 ganglioside to have an essential role in regulating this excitatory response, but not the inhibitory (APD-shortening) response to higher (microM) opioid concentrations. Furthermore, it was proposed that synthesis of GM1 is upregulated by prolonged activation of excitatory opioid receptor functions. To explore this possibility we have utilized cultures of hybrid F11 cells to carry out closely correlated electrophysiological and biochemical analyses of the effects of chronic opioid treatment on a homogeneous population of clonal cells which express many functions characteristic of DRG neurons. We show that chronic opioid exposure of F11 cells does, in fact, result in elevated levels of GM1 as well as cyclic adenosine monophosphate (AMP), concomitant with the onset of opioid excitatory supersensitivity as manifested by naloxone-evoked decreases in voltage-dependent membrane K+ currents. Such elevation of GM1 would be expected to enhance the efficacy of excitatory opioid receptor activation of the Gs/adenylate cyclase/cyclic AMP system, thereby providing a positive feedback mechanism that may account for the remarkable supersensitivity of chronic opioid-treated neurons to the excitatory effects of opioid agonists as well as antagonists. These in vitro findings may provide novel insights into the mechanisms underlying naloxone-precipitated withdrawal syndromes and opioid-induced hyperalgesia after chronic opiate addiction in vivo.

Chronic selective activation of excitatory opioid receptor functions in sensory neurons results in opioid 'dependence' without tolerance

We previously showed that mouse sensory dorsal root ganglion (DRG) neurons chronically exposed to 1 microM D-ala2-D-leu5-enkephalin (DADLE) or morphine for > 2-3 days in culture become tolerant to the usual opioid inhibitory receptor-mediated effects, i.e. shortening of the duration of the calcium-dependent component of the action potential (APD), and supersensitive to opioid excitatory APD-prolonging effects elicited by low opioid concentrations. Whereas nanomolar concentrations of dynorphin(1-13) or morphine are generally required to prolong the APD of naive DRG neurons (by activating excitatory opioid receptors), femtomolar levels become effective after chronic opioid treatment. Whereas 1-30 nM naloxone or diprenorphine prevent both excitatory and inhibitory opioid effects but do not alter the APD of native DRG neurons, both opioid antagonists unexpectedly prolong the APD of most of the chronic opioid-treated cells. In the present study, chronic exposure of DRG neurons to 1 microM DADLE together with cholera toxin-B subunit (which selectively blocks GM1 ganglioside-regulated opioid excitatory, but not inhibitory, receptor functions) prevented the development of opioid excitatory supersensitivity and markedly attenuated tolerance to opioid inhibitory effects. Conversely, sustained exposure of DRG neurons to 1 nM DADLE, which selectively activates excitatory opioid receptor functions, resulted in characteristic opioid excitatory supersensitivity but no tolerance. These results suggest that 'dependence'-like properties can be induced in chronic opioid-treated sensory neurons in the absence of tolerance. On the other hand, development of some components of tolerance in these cells may require sustained activation of both excitatory, as well as inhibitory, opioid receptor functions.

After chronic opioid exposure sensory neurons become supersensitive to the excitatory effects of opioid agonists and antagonists as occurs after acute elevation of GM1 ganglioside

Mouse sensory dorsal-root ganglion (DRG) neurons chronically exposed to 1 microM D-Ala2-D-Leu5-enkephalin (DADLE) for greater than 1 week in culture become tolerant to opioid inhibitory effects, i.e. shortening of the duration of the calcium-dependent component of the action potential (APD). Acute application of higher concentrations of DADLE (ca. 10 microM) to these treated neurons not only fails to shorten the APD but, instead, generally elicits excitatory effects, i.e. prolongation of the APD. The present study shows that chronic DADLE- or morphine-treated DRG neurons also become supersensitive to the excitatory effects of opioids. Whereas nM concentrations of dynorphin(1-13) are generally required to prolong the APD of naive DRG neurons, fM levels become effective after chronic opioid treatment. Whereas 1-30 nM naloxone or diprenorphine do not alter the APD of naive DRG neurons, both opioid antagonists unexpectedly prolong the APD of most of the treated cells. Similar supersensitivity to the excitatory effects of opioid agonists and antagonists was previously observed after acute treatment of naive DRG neurons with GM1 ganglioside. Our results suggest that both chronic opioid and acute GM1 treatments of DRG neurons greatly enhance the efficacy of opioid excitatory receptor functions so that even the extremely weak agonist properties of naloxone and diprenorphine become effective in prolonging the APD of these treated cells when tested at low concentrations, whereas their antagonist properties at inhibitory opioid receptors do not appear to be altered. Furthermore, whereas cholera toxin-B subunit (CTX-B; 1-10 nM) blocks opioid-induced APD prolongation in naive DRG neurons (presumably by interfering with endogenous GM1 modulation of excitatory opioid receptors functions), even much higher concentrations of CTX-B were ineffective in chronic opioid-treated as well as acute GM1-elevated neurons. These and related data suggest that opioid excitatory supersensitivity in chronic opioid-treated DRG neurons may be due to a cyclic AMP-dependent increase in GM1 ganglioside levels. Our results may clarify mechanisms of opioid dependence and the paradoxical supersensitivity to naloxone which triggers withdrawal symptoms after opiate addiction.

Coordinate regulation of ganglioside glycosyltransferases in differentiating NG108-15 neuroblastoma x glioma cells

The enzymatic basis for ganglioside regulation during differentiation of NG108-15 mouse neuroblastoma x rat glioma hybrid cells was studied. This cell line contains four gangliosides that lie along the same biosynthetic pathway: GM3, GM2, GM1, and GD1a. Chemically induced neuronal differentiation of NG108-15 cells led to an 80% drop in the steady-state level of their major ganglioside, GM3, a sixfold increase in the level of a minor ganglioside, GM2 (which became the predominant ganglioside of differentiated cells); and relatively little change in the levels of GM1 and GD1a, which lie further along the same biosynthetic pathway. The enzymatic basis for this selective change in ganglioside expression was investigated by measuring the activity of two glycosyltransferases involved in ganglioside biosynthesis. UDP-N-acetylgalactosamine: GM3 N-acetylgalactosaminyltransferase (GM2-synthetase) activity increased fivefold during butyrate-induced differentiation, whereas UDP-galactose: GM2 galactosyltransferase (GM1-synthetase) activity decreased to 10% of its control level. Coordinate regulation of these two glycosyltransferases appears to be primarily responsible for the selective increase of GM2 expression during NG108-15 differentiation.

Kappa-opiate agonists inhibit adenylate cyclase and produce heterologous desensitization in rat spinal cord

The nature of the opiate modulation of adenylate cyclase following acute and chronic agonist exposure has been investigated in rat spinal cord. Using membranes of both adult rat spinal cord and spinal cord-dorsal root ganglion cocultures, we found that kappa-opiate receptors are negatively coupled to adenylate cyclase. The kappa-opiate agonists (e.g., U50488) inhibit significantly and dose-dependently the basal and the forskolin-stimulated cyclase activities, whereas mu and delta agonists are ineffective. The regulatory action is stereospecific and requires the presence of GTP. EGTA treatment of the plasma membranes abolished the effect of kappa-opiate agonists on the basal cyclase activity, and this inhibitory effect could not be restored by subsequent addition of Ca2+. The EGTA treatment did not affect the kappa agonist inhibition of the forskolin-stimulated cyclase. The results also show that following chronic exposure of cultured cells to etorphine or U50488, there is a loss of kappa agonist inhibition of the cyclase. Moreover, this desensitization process appears to be heterologous, because alpha 2-adrenergic agonists (e.g., clonidine or norepinephrine) and the muscarinic agonist (carbachol) exhibited significantly lower potency for inhibiting cyclase activity when compared to untreated cultures. This pattern of heterologous desensitization suggests that chronic exposure to kappa opiates leads to alterations in postreceptor regulatory components, possibly GTP-binding proteins.

Modulation of adenylate cyclase activity of mouse spinal cord-ganglion explants by opioids, serotonin and pertussis toxin

Organotypic cultures of fetal mouse spinal cord-ganglion explants (2-4 weeks in vitro) contain forskolin-stimulated adenylate cyclase (AC) activity that is inhibited by levorphanol and other opioid agonists in a dose-dependent manner. Inhibition by levorphanol no longer occurs if sodium is omitted from the incubation and the levorphanol inhibition is blocked by the opioid antagonist, naloxone. These findings together with the ineffectiveness of dextrorphan indicate that the opioid inhibition of forskolin-stimulated AC is receptor mediated. Both the delta- and kappa-receptor subtypes appear to be involved since the selective delta-opioid agonist, [D-Pen2, D-Pen5]enkephalin, and the selective kappa-opioid agonist, t-3,4-dichloro-N-methyl-N[2-(1-pyrrolidinyl)cyclohexyl]-benzene acetamide (U-50,488H) are both effective at nanomolar concentrations. In contrast, the selective mu-opioid agonist, Tyr-D-Ala-Gly-N-MePhe-Gly-ol, has no significant effect even at micromolar concentrations. Both cord and ganglion components of the explants contain opioid-sensitive AC. Forskolin-stimulated AC of the explants is also inhibited by serotonin and carbachol. The serotonin effect appears to be mediated by 5-HT1A receptors, based on relative agonist and antagonist selectivity. Chronic exposure of cultures to morphine results in enhanced basal and forskolin-stimulated AC as well as attenuation of opioid-inhibition of AC assayed in the presence of forskolin; treatment of explants with pertussis toxin causes similar changes in the AC system. The inhibitory effect of serotonin is also attenuated by the pertussis toxin treatment. Basal AC activity of the explants (assayed without forskolin present) is stimulated to a small but significant extent by opioids and by serotonin. The opioid stimulatory effect is markedly enhanced following either morphine or pertussis toxin treatment of the explants. The attenuation of opioid- and serotonin-inhibition of AC produced by chronic exposure to pertussis toxin and the attenuation of opioid inhibition produced by exposure to morphine are consonant with the attenuation of opioid and monoaminergic depression of sensory evoked dorsal horn network responses after similar chronic treatments. It is proposed that the inhibitory effects of opioids and serotonin on these neurons are mediated by receptors that are negatively coupled via a pertussis toxin sensitive Gi protein to AC. Furthermore, alterations of AC with chronic morphine treatment may be involved in the development of physiologic tolerance to opioids.

Evidence for [D-Ala2,D-Leu5]enkephalin-induced supersensitivity to 5-hydroxytryptamine in a neurotumor x brain hybrid cell line (NCB-20)

A neuroblastoma X Chinese hamster embryonic brain explant hybrid cell line (NCB-20) expressed 5-hydroxytryptamine (5-HT1) receptors, linked to adenylate cyclase, which closely resembled 5-HT1 receptors previously characterized in central nervous tissue. However, the affinity of the receptors for 5-HT was only 150 nM compared to 5 nM in membranes prepared from cerebral cortex. The elevation of cyclic AMP levels in NCB-20 cells produced by 5-HT was found additive to that produced by cholera toxin but synergistic with that produced by either prostaglandin E1 (PGE1) or forskolin, suggesting that these latter two agents elevate cyclic AMP levels by a different mechanism than 5-HT. The elevation of cyclic AMP levels by either 5-HT or PGE1 was reversed by [D-Ala2,D-Leu5]enkephalin (DADLE), morphine, clonidine, and 3,4-dihydroxyphenylethylamine (dopamine) on a short (30 min) time scale. However, continued exposure to DADLE resulted in loss of the initial inhibitory effects of DADLE after 6 h and return of cyclic AMP levels to that seen with either 5-HT or PGE1 alone. When the DADLE exposure time was increased to 48 h, 5-HT produced a further twofold increase in cyclic AMP levels, but there was no increase in the responsiveness of the cells to PGE1 unless naloxone was added 1 h prior to treatment with PGE1. Scatchard analysis showed that the increased potency of 5-HT resulted from an increase in receptor affinity for 5-HT (from a KD of 150 +/- 20 nM to one of 20 +/- 7 nM), with a reduction in the number of apparent binding sites. The 5-HT supersensitivity observed in NCB-20 cells may be a good model for neurotransmitter interactions that produce desensitization or facilitation in the intact nervous system.

Cell-cycle dependence of a ganglioside glycosyltransferase activity and its inhibition by enkephalin in a neurotumor cell line

Rat glioma X mouse neuroblastoma hybrid neurotumor cells (NG108-15), synchronized by amino acid deprivation, showed a cell-cycle-dependent peak of activity of a ganglioside N-acetylgalactosaminyl transferase 14-24 h following release from the cell cycle block (S/G2 phase). Maximal expression of two typical lysosomal hydrolases, N-acetyl-beta-hexosaminidase and beta-galactosidase, occurred between 18 and 21 h following release (S phase), declining to G1 phase levels during the peak of N-acetylgalactosamine (GalNAc) transferase activity. In addition, glycosyltransferase activity in G2 phase cells showed an increase in apparent Vmax (suggesting the presence of more enzyme/mg of cell protein) and apparent binding affinity for uridine diphosphate N-acetylgalactosamine (UDP-GalNAc) (32 versus 14 microM) when compared to transferase activity in the G1 phase. However, the opioid peptide enkephalin [D-Ala2, D-Leu5], which inhibits ganglioside GalNAc transferase activity in unsynchronized NG108-15 cultures, was much more inhibitory in whole cells 8 h after release from the cell cycle block (G1 phase) than in cells 20 h after release (G2 phase), with 50% inhibition occurring at 2 X 10(-9) M and 2 X 10(-7) M, respectively. These results suggest that the GalNAc transferase activity is regulated in more than one way during the cell cycle, since both Vmax and Km changes are observed, and that the cyclic AMP-dependent mechanism by which opiates reduce transferase activity is receptor mediated and cell cycle dependent.

Gangliosides as modulators of the coupling of neurotransmitters to adenylate cyclase

Cultured NCB-20 mouse neuroblastoma X Chinese hamster brain clonal hybrid cells express an adenylate cyclase-coupled receptor for serotonin (5HT) which corresponds pharmacologically to the 5HT1 receptor in whole brain, except for its much lower affinity for serotonin. Studies showed that the affinity of the NCB-20 receptor could be increased to near that of the whole brain receptor and the potency of 5HT in elevating cyclic AMP levels increased by pre-incubating NCB-20 cells for at least 3 hours with submicromolar concentrations of brain gangliosides. Tetrasialoganglioside (GQ1b) was found to be the most potent ganglioside tested, producing a ten-fold increase in affinity. However, the actual 5HT binding site is a protein and we have obtained no evidence that serotonin binds directly to gangliosides at the concentrations at which it labels the receptor. The receptor-mediated inhibition of adenylate cyclase by biogenic amines such as dopamine and clonidine through dopamine (D2) and alpha-adrenoreceptors was unaffected by pre-incubation of the NCB-20 cells with gangliosides. Enkephalin was also found to acutely supress both the ability of 5HT to stimulate adenylate cyclase activity and the synthesis of polysialogangliosides in NCB-20 cells. After 6 hours of exposure, the cells became tolerant to enkephalin and after 36 hours the cells became supersensitive to 5HT in terms of adenylate cyclase activation and 5HT binding. The affinity of the receptor for 5HT increased the same 10-fold magnitude as achieved by GQ1b pre-incubation in comparison with untreated cells. This increase in receptor affinity appeared to coincide chronologically with the increase in ganglioside synthesis observed in enkephalin tolerant cells, further suggesting an important role of polysialogangliosides in the function of the serotonin (5HT1) receptor.

Glycosylation-dependent regulation of opiate (enkephalin) receptors in neurotumor cells

Electron inactivation analysis revealed that the opiate (enkephalin) binding site in neurotumor cell lines NG108-15 and NCB-20 had an apparent target size of 200,000 daltons. Expression of functional opiate receptors in neurotumor cells appeared to require glycosylation, as treatment of such cells with tunicamycin (TM; under conditions where de novo glycosylation of asparagine residues in protein was reduced by 80%, but overall protein and DNA synthesis were inhibited by less than 10%) resulted in the loss of 50% of the opiate binding sites. The loss of binding sites could not be prevented by addition of protease inhibitors to cell cultures, but binding sites were partially restored 48-60 h after removal of the TM. In addition, the number of enkephalin binding sites in TM-treated cells was also restored to near-normal levels by addition of physiological concentrations (1-10 mM) of manganese ions to the in vitro receptor binding incubation mixture. TM treatment resulted in receptor supersensitivity to manganese ions for both opiate agonists and antagonists, no change in the sodium effect for either agonists or antagonists, and subsensitivity to GTP for both agonists and antagonists. However, opiate binding to cell membranes was not substantially inhibited by either neuraminidase treatment or short-term incubation with lectins such as wheat germ agglutinin, ricin, or concanavalin A. Thus, the data suggest that oligosaccharide units are not directly involved in opiate receptor-ligand interactions, but protein glycosylation is required for functional expression of receptors.

Effect of methylmercuric chloride on gangliosides of mouse neuroblastoma cells in culture

The effect of methylmercuric chloride (CH3HgCl) on the levels of gangliosides in mouse neuroblastoma cells (NBP2) in culture was studied. The treatment of NB cells with low concentrations (0.1 microM and 0.2 microM) of CH3HgCl, which did not affect the growth rate or morphology, caused an increase in the level of the GM3 ganglioside without changing the level of other gangliosides. The treatment of NB cells with higher concentrations (0.5 microM and 1 microM) of CH3HgCl, which inhibited the growth of NB cells, caused a decrease in the level of GM3 and an increase in the level of GM2. These results show that alterations in the levels of specific gangliosides can be observed in cells which do not exhibit any detectable change in growth rate or morphology. This change may be associated with subtle changes in brain functions, including behavioral and psychological changes, after exposure to low concentrations of organic mercury.