Up-regulation of a constitutively active form of the beta2-adrenoceptor by sustained treatment with inverse agonists but not antagonists

17β-Estradiol Concentration and Direct β2-Adrenoceptor Inhibition Determine Estrogen-Mediated Reversal of Adrenergic Immunosuppression

Background:

Sympathetic innervation of lymphoid organs, and the presence of 17β-estradiol (estrogen or E2) and adrenergic receptors (ARs) on lymphocytes, suggests that sympathetic stimulation and hormonal activation may influence immune functions.

Purpose:

Modeling and simulating these pathways may help to understand the dynamics of neuroendocrine-immune modulation at the cellular and molecular levels.

Methods:

Dose- and receptor-dependent effects of E2 and AR subtype-specific agonists were established in vitro on lymphocytes from young male Sprague-Dawley rats and were modeled in silico using the MATLAB Simbiology toolbox. Kinetic principles were assigned to define receptor–ligand dynamics, and concentration/time plots were obtained using Ode15s solvers at different time intervals for key regulatory molecules. Comparisons were drawn between in silico and in vitro data for validating the constructed model with sensitivity analysis of key regulatory molecules to assess their individual impacts on the dynamics of the system. Finally, docking studies were conducted with key ligands E2 and norepinephrine (NE) to understand the mechanistic principles underlying their interactions.

Results:

Adrenergic activation triggered proapoptotic signals, while E2 enhanced survival signals, showing opposing effects as observed in vitro. Treatment of lymphocytes with E2 shows a 10-fold increase in survival signals in a dose-dependent manner. Cyclic adenosine monophosphate (cAMP) activation is crucial for the activation of survival signals through extracellular signal-regulated kinase (p-ERK) and cAMP responsive element binding (p-CREB) protein. Docking studies showed the direct inhibition of ERK by NE and β2-AR by E2 explaining how estrogen signaling overrides NE-mediated immunosuppression in vitro.

Conclusion:

The cross-talk between E2 and adrenergic signaling pathways determines lymphocyte functions in a receptor subtype and coactivation-dependent manner in health and disease.

A Systematic Review of Inverse Agonism at Adrenoceptor Subtypes

As many, if not most, ligands at G protein-coupled receptor antagonists are inverse agonists, we systematically reviewed inverse agonism at the nine adrenoceptor subtypes. Except for β3-adrenoceptors, inverse agonism has been reported for each of the adrenoceptor subtypes, most often for β2-adrenoceptors, including endogenously expressed receptors in human tissues. As with other receptors, the detection and degree of inverse agonism depend on the cells and tissues under investigation, i.e., they are greatest when the model has a high intrinsic tone/constitutive activity for the response being studied. Accordingly, they may differ between parts of a tissue, for instance, atria vs. ventricles of the heart, and within a cell type, between cellular responses. The basal tone of endogenously expressed receptors is often low, leading to less consistent detection and a lesser extent of observed inverse agonism. Extent inverse agonism depends on specific molecular properties of a compound, but inverse agonism appears to be more common in certain chemical classes. While inverse agonism is a fascinating facet in attempts to mechanistically understand observed drug effects, we are skeptical whether an a priori definition of the extent of inverse agonism in the target product profile of a developmental candidate is a meaningful option in drug discovery and development.

Expression of beta-adrenergic receptor up-regulation is mediated by two different processes

Mechanisms of up-regulation of beta-adrenergic receptors (beta-ARs) induced by sustained exposure to 10(-8) M nadolol, a non-selective beta-AR antagonist, were examined using mouse cerebrocortical neurons. Nadolol dose- and time-dependently increased [3H]CGP-12177 bindings to the particulate fractions. This increase occurred 6 h and attained its plateau 12 h after the exposure, whereas beta1- and beta2-AR mRNA significantly increased 24 h and attained their plateaus 3 days after the exposure. Scatchard analysis revealed that the increased bindings were due to increase of receptor density. The [3H]CGP-12177 bindings to beta1- and beta2-ARs increased both 12 h and 5 days after the exposure. Although cycloheximide (CHX) decreased the bindings with or without nadolol, the extent of increase of the bindings induced by nadolol was not affected by CHX. Actinomycin D (AD) with nadolol showed no affects on the bindings 12 h after nadolol exposure, while AD treated 6 h after nadolol exposure significantly reduced the bindings 48 h after nadolol exposure. During 24 h after nadolol exposure, the increase in proteins of beta1- and beta2-ARs in the neuronal membrane was due to the increased receptor protein translocation from cytosol to membrane. These results indicate that the up-regulation of beta-ARs induced by nadolol is mediated by, at least, two different processes, one is increase in translocation of receptor proteins from cytosol to membrane with no changes in synthesis of receptor proteins and their mRNA and another is dependent on receptor protein synthesis with increased synthesis of their mRNA.

Heterologous desensitization is evoked by both agonist and antagonist stimulation of the human 5-HT(7) serotonin receptor

Previously, we demonstrated that human serotonin (5-HT) 5-HT(7) receptors display marked constitutive activity. Here, we tested if the constitutive activation of adenylyl cyclase by 5-HT(7) receptors influenced both the desensitization properties of transfected 5-HT(7) receptors and the ability of endogenous G(s)-coupled receptors to activate adenylyl cyclase. Using membranes from stably transfected HEK293 cells expressing the recombinant human 5-HT(7) receptor splice variants (5-HT(7(a)), 5-HT(7(b)) and 5-HT(7(d))), we compared the effects of 1-h or 24-h preincubation of the agonist 5-HT, partial inverse agonists mesulergine and SB269970, and full inverse agonists clozapine and methiothepin on subsequent activation of adenylyl cyclase by both 5-HT through transfected 5-HT(7) receptors and the endogenous G(s)-coupled beta-adrenoceptors and prostaglandin receptors of HEK293 cells. The data show that stable expression of 5-HT(7) receptors is sufficient to attenuate adenylyl cyclase activation by endogenous G(s)-coupled receptors. Interestingly, preincubation with inverse agonists not only failed to result in the predicted resensitization of all receptor mediated adenylyl cyclase activation, but some inverse agonists further attenuated (desensitized) beta-adrenoceptor and prostaglandin-stimulated adenylyl cyclase activation similar to long-term agonist exposure by 5-HT. These effects were not correlated with inverse agonist efficacy, were not accompanied by receptor down-regulation and appear to be mediated by a protein kinase A (PKA) independent mechanism. It is concluded that the human 5-HT(7) receptor mediates heterologous desensitization of endogenous G(s)-coupled receptors through an unknown and potentially novel mechanism.

Alteration of intracellular histamine H2 receptor cycling precedes antagonist-induced upregulation

Long-term administration of a histamine H2 receptor (H2R) antagonist (inverse agonist) induces upregulation of H2R in parietal cells, which may be relevant to the rebound hypersecretion of gastric acid that occurs after withdrawal of treatment. The mechanisms underlying this effect are unknown. We hypothesized that the H2R upregulation could be related to receptor trafficking and used H2R-green fluorescent protein (H2R-GFP) to test the hypothesis. Human H2R-GFP was generated and functionally expressed in HEK-293 cells. Binding of the H2R antagonist [3H]tiotidine was performed to quantify H2R expression, and H2R-GFP was imaged in living cells by confocal and evanescent wave microscopy. The binding affinity of [3H]tiotidine was not significantly different between H2R-GFP- and wild-type H2R-expressing HEK-293 cells, both of which had constitutive activity of adenylate cyclase. Visualization of H2R-GFP revealed that the agonist-induced H2R internalization and the antagonist-induced recycling of the internalized H2R from the recycling endosome within 2 h. Long exposure to the antagonist increased GFP fluorescence in the plasma membrane and also induced upregulation of H2R-GFP estimated by the binding assay, whereas long exposure to the agonist enhanced degradative trafficking of H2R-GFP. We examined whether the upregulation reflected an increase in receptor synthesis. Treatment with antagonist did not augment H2R mRNA, and subsequent inhibition of protein synthesis by cycloheximide had no effect on H2R upregulation. These findings suggested that upon exposure to an antagonist (inverse agonist), the equilibrium between receptor endocytosis and recycling is altered before H2R upregulation, probably via suppressing H2R degradation.

Ac His1 [D-Phe2, K15, R16, L27] VIP (3-7)/GRF (8-27)--a VPAC1 receptor antagonist--is an inverse agonist on two constitutively active truncated VPAC1 receptors

C-terminally truncated human VPAC(1) receptors were constructed and stably transfected in Chinese hamster ovary (CHO) cells. Selected clones expressing comparable receptor densities were studied for ligand's binding properties, basal and stimulated adenylate cyclase activity. The wild-type (1-457) receptor served as reference. The binding properties of all the constructions were preserved. As judged by the intrinsic activity of the partial agonist Q(3)-VIP, the shortest receptors have a moderate impairment of the coupling efficacy to G(alpha s) protein. Cells expressing the VPAC(1) (1-436) and (1-441) truncated receptors had a two- to three-fold higher basal adenylate cyclase activity than those expressing the wild-type or the VPAC(1) (1-444), (1-433), (1-429), (1-421) and (1-398) receptor. The stimulatory effect of VIP and other agonist was preserved. This suggested that VPAC(1) (1-436) and (1-441) receptors had a constitutive activity. The selective VPAC(1) receptor antagonist Ac His(1) [D-Phe(2), K(15), R(16), L(27)] VIP (3-7)/GRF (8-27) reduced by 60% the basal activity with an EC(50) value of 3 nM comparable to its IC(50) value for binding. This agonist behaved thus like an inverse agonist on the constitutively active VPAC(1) receptors generated by C-terminal truncation and expressed in CHO cells.

Ligand Specific Up-Regulation of aRenilla reniformisLuciferase-Tagged, Structurally Unstable Muscarinic M3Chimeric G Protein-Coupled Receptor

The rat muscarinic acetylcholine receptor subtype 3 was modified by swapping the third intracellular loop with the corresponding region of a constitutively active mutant human beta2-adrenergic receptor and attaching Renilla reniformis luciferase to its C terminus. The chimeric fusion receptor displayed constitutive Gq- and Gs-coupled activity as demonstrated in nuclear factor of activated T cell and cAMP response element reporter gene assays. The chimeric receptor displayed a pharmacological binding profile comparable with that of the wild-type receptor for agonists, antagonists, and inverse agonists but showed a large decrease in expression in both human embryonic kidney 293 and COS-7 cells. Long-term treatment of cells expressing the chimeric receptor with agonists, antagonists, and inverse agonists resulted in a concentration-dependent up-regulation in the steady-state levels that was not observed for the wild-type receptor. The EC50 of neutral antagonists and inverse agonists was significantly correlated to their binding affinities at the wild-type receptor, whereas agonists demonstrated greater EC50 values for the chimeric receptor. To validate the approach as a means of discovering novel receptor modulators, a cell-based, high-throughput screening assay was developed and used to screen a small molecule compound collection against the chimeric fusion receptor. Several novel hits were identified and confirmed by ligand binding assay and functional assays using the wild-type rat muscarinic acetylcholine receptor subtype 3.

Constitutive activation of the opioid receptor-like (ORL1) receptor by mutation of Asn133 to tryptophan in the third transmembrane region

We have introduced a series of point mutations into the human opioid receptor-like (ORL1) receptor and characterized them for their ability to constitutively activate G protein-coupled receptor signalling pathways. Among the 12 mutants generated, mutation at Asn133 (N133W) gave increased basal signalling through three separate pathways. N133W increased the basal activity of G14- and G16-dependent pathways by two- to three-fold. The constitutive activity of the mutant was confirmed by the finding that the enhanced activity is dependent on the level of receptor expression. In HEK-293 cells stably expressing N133W, signalling through Gi/o-dependent pathways was also observed. Radioligand binding studies revealed that the affinity for nociceptin of the wild-type ORL1 receptor and the N133W mutant do not differ significantly, suggesting that the ligand binding and signalling functions of constitutively active mutants of G protein-coupled receptors are not necessarily intrinsically linked. In conclusion, our results demonstrate that a mutation in the third transmembrane domain is able to increase the basal signalling activity of the human ORL1 receptor.

Regulation of mu-opioid receptors, G-protein-coupled receptor kinases and beta-arrestin 2 in the rat brain after chronic opioid receptor antagonism

The aim of this study was to analyse the biochemical and behavioural consequences of chronic treatment with opioid receptor antagonists in rats. We have evaluated the respiratory depressant and antinociceptive effects of the mu-opioid agonist sufentanil, the density of brain mu-opioid receptors, and the expression of G-protein-coupled receptor kinases and beta-arrestin 2 in cerebral cortex and striatum, following sustained opioid receptor blockade. Our results demonstrate that 24 h after interruption of 7 days chronic infusion of naltrexone (120 microg/h), the respiratory depressant potency of the mu-opioid receptor agonist sufentanil was increased to a similar extent as the antinociceptive potency (about three-fold). This was accompanied by mu-opioid receptor up-regulation in several areas of the rat brain associated with opioid control of pain perception and breathing. Moreover, chronic treatment with either naltrexone (120 microg/h) or naloxone (120 microg/h) caused significant increases in the expression levels of G-protein-coupled receptor kinases types 2, 3, and 6, and of beta-arrestin 2 in brain cortex and striatum. Together our data suggest an increased constitutive receptor activity secondary to mu-opioid receptor up-regulation following chronic antagonist treatment.

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

Constitutive internalization of constitutively active agiotensin II AT(1A) receptor mutants is blocked by inverse agonists

As constitutively active mutants (CAMs) mimic an active conformation, they can be used to characterize the process of G protein-coupled receptor activation. Here, we used CAMs to study the link between activation and internalization of the angiotensin II AT(1A) receptor. The cellular localization of fluorescently tagged N111A, I245T, and L305Q mutants was determined by confocal microscopy. In the absence of ligand, CAMs were mostly located in intracellular vesicles, whereas the wild-type AT(1A) was found at the cell surface. After 2 h incubation with inverse agonist, losartan, CAMs were translocated to the plasma membrane. Similar observations were made in H295, a human adrenocortical cell line which expresses physiologically the AT(1) receptor. This phenomenon, which was not dependent on protein synthesis and the pharmacology and kinetics of which were similar to the recycling of the wild-type receptor, was called "externalization". After externalization and losartan removal, the L305Q CAM underwent rapid ligand-independent endocytosis, with the same kinetics and temperature sensitivity as the angiotensin II-induced internalization of the wild-type AT(1A). Moreover, the addition of a second mutation known to block internalization (Delta 329 truncation) prevented intracellular localization of the CAM. These data show that AT(1A) CAMs are constitutively and permanently internalized and recycled. This mechanism is different from the down-regulation observed for CAMs of other G protein-coupled receptors and thus defines a new paradigm for the cellular regulation of CAMs.

Functional role of a conserved motif in TM6 of the rat mu opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp

Mutations within the "X1BBX2X3B" motif or its variants in the junction of the third intracellular (i3) loop and the sixth transmembrane domain (TM6) have been shown to lead to constitutive activation of several G protein-coupled receptors (GPCRs). In this study, T6.34(279) at the X3 locus of the rat mu opioid receptor was mutated to Lys and Asp, and the mutants were examined for binding and signaling properties. The T6.34(279)K mutant was poorly expressed, and pretreatment with naloxone greatly enhanced its expression. This construct exhibited properties identified previously with constitutive activation: (1) compared with the wild type, it produced much higher agonist-independent [35S]GTPgammaS binding, which was abolished by pertussis toxin treatment; (2) it displayed an enhanced affinity for the agonist DAMGO similar to that of the high-affinity state of the wild type, which was not altered by GTPgammaS, while having unchanged affinity for the antagonist diprenorphine. The T6.34(279)K mutant displayed a higher intracellular receptor pool than the wild type. Naloxone inhibited the basal [35S]GTPgammaS binding of the T6.34(279)K mutant, demonstrating inverse agonist activity at this mutant receptor. In contrast, the T6.34(279)D substitution did not increase basal [35S]GTPgammaS binding, greatly reduced agonist-promoted [35S]GTPgammaS binding, and markedly decreased affinity for DAMGO. Thus, the T6.34(279)D mutant adopts conformations corresponding to inactive states of the receptor. The results were interpreted in the structural context of a model for the mu opioid receptor that incorporates the information from the crystal structure of rhodopsin. The interaction of T6.34(279) with R3.50(165) in the mu opioid receptor is considered to stabilize the inactive conformations. The T6.34(279)K substitution would then disrupt this interaction and support agonist-free activation, while T6.34(279)D mutation should strengthen this interaction which keeps the receptor in inactive states. T6.34(279) may, in addition, interact with the neighboring R6.35(280) to help constrain the receptor in inactive states, and T6.34(279)K and T6.34(279)D mutations would affect this interaction by disrupting or strengthening it, respectively. To the best of our knowledge, the results presented here represent the first structurally rationalized demonstration that mutations of this locus can lead to dramatically different properties of a GPCR.

Inverse agonist up-regulates the constitutively active D3.49(164)Q mutant of the rat mu-opioid receptor by stabilizing the structure and blocking constitutive internalization and down-regulation

We demonstrated previously that D3.49(164) mutations resulted in constitutive activation of the rat mu-opioid receptor and abolished receptor expression unless cells were pretreated with naloxone, an inverse agonist. In this study, we investigated the properties of the D3.49(164)Q mutant and the mechanisms underlying the effect of naloxone. Naloxone pretreatment up-regulated [(3)H]diprenorphine binding and protein expression of the D3.49(164)Q mutant in a time- and dose-dependent manner without affecting its mRNA level. After naloxone removal, binding and protein expression of the mutant declined with time with no effect on its mRNA level. Naloxone methiodide (a quaternary ammonium analog) caused a maximal up-regulation about 50% of the naloxone effect, indicating that naloxone acts extracellularly and intracellularly. Expression of the mutant was enhanced by inverse agonists, a neutral antagonist, and agonists, with inverse agonists being most effective. In membranes, the mutant was structurally less stable than the wild type upon incubation at 37 degrees C, and naloxone and [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin stabilized the mutant. Coexpression of the dominant-negative mutants GRK2-K220R, arrestin-2(319-418), dynamin I-K44A, rab5A-N133I or rab7-N125I partially prevented the decline in binding of the mutant after naloxone removal. Chloroquine or proteasome inhibitor I reduced the down-regulation of the mutant. These results indicate that the D3.49(164)Q mutant is constitutively internalized via G protein coupled-receptor kinase-, arrestin-2-, dynamin-, rab5-, and rab7-dependent pathways and probably trafficked through early and late endosomes into lysosomes and degraded by lysosomes and proteasomes. Naloxone up-regulates the D3.49(164)Q mutant by stabilizing the mutant protein and blocking its constitutive internalization and down-regulation. To the best of our knowledge, this represents the first comprehensive analysis of the mechanisms involved in up-regulation of constitutively active mutants by an inverse agonist.

Type II tumour necrosis factor-alpha receptor (TNFR2) activates c-Jun N-terminal kinase (JNK) but not mitogen-activated protein kinase (MAPK) or p38 MAPK pathways

The pleitropic actions of tumour necrosis factor-alpha (TNF) are transmitted by the type I 55 kDa TNF receptor (TNFR1) and type II 75 kDa TNF receptor (TNFR2), but the signalling mechanisms elicited by these two receptors are not fully understood. In the present study, we report for the first time subtype-specific differential kinase activation in cell models that respond to TNF by undergoing apoptotic cell death. KYM-1 human rhabdomyosarcoma cells and HeLa human cervical epithelial cells, engineered to overexpress TNFR2, displayed c-Jun N-terminal kinase (JNK) activation by wild-type TNF, a TNFR1-specific TNF mutant and a TNFR2-specific mutant TNF in combination with an agonistic TNFR2-specific monoclonal antiserum. A combination of the TNFR2-specific mutant and agonistic antiserum elicited maximal endogenous or exogenous TNFR2 responsiveness. Moreover, alternative expression of a TNFR2 deletion mutant lacking its cytoplasmic domain rendered the cells unable to activate JNK activity through this receptor subtype. The profile of JNK activation by TNFR1 was more transient than that of TNFR2, with TNFR2-induced JNK activity also being more sensitive to the caspase inhibitor, benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone. Conversely, only activation of the TNFR1 could stimulate mitogen-activated protein kinase (MAPK) or p38 MAPK activities in a time-dependent manner. The role of TNFR2 activation in enhanced apoptotic cell death was confirmed with agonistic monoclonal antisera in cells expressing high levels of TNFR2. Activation of TNFR2 alone elicited cell death, but full TNF-induced death required stimulation of both receptor types. These findings indicate that efficient activation of TNFR2 by soluble TNFs is achievable with co-stimulation by antisera, and that both receptors differentially modulate extracellular signal-regulated kinases contributing to the cytokine's cytotoxic response.

Constitutive activation of the mu opioid receptor by mutation of D3.49(164), but not D3.32(147): D3.49(164) is critical for stabilization of the inactive form of the receptor and for its expression

The roles of conserved aspartates in the third transmembrane domain of the rat mu opioid receptor (RMOR) were explored with mutations of D3.32(147) and D3.49(164). D3.49(164) in the highly conserved DRY motif was mutated to 13 amino acids. Except for the D3.49(164)E mutant, each mutant displayed little or no detectable [(3)H]diprenorphine binding, and pretreatment with naloxone greatly enhanced binding. D3.49(164)H, -Q, -Y, -M, and -E mutants were further studied. D3.32(147) was substituted with A or N. All seven mutants exhibited similar binding affinities for the antagonist [(3)H]diprenorphine as the wild-type. The D3.49(164)H, -Q, -Y, and -M mutants, but not the D3.49(164)E and D3.32(147) mutants, exhibited enhanced basal [(35)S]GTPgammaS binding which was comparable to the maximally activated level of the wild-type and was related to expression levels. Naloxone, naltrexone, and naloxone methiodide significantly inhibited the basal [(35)S]GTPgammaS binding of the D3.49(164) mutants, indicating inverse agonist activities. Treatment of the D3.49(164)Y mutant with pertussis toxin greatly reduced the basal [(35)S]GTPgammaS binding, demonstrating constitutive activation of Galpha(i)/Galpha(o). The D3.49(164)H, -Y, -M, and -Q mutants had higher affinities for DAMGO than the wild-type, which were not significantly lowered by GTPgammaS. Thus, mutation of D3.49(164) to H, Y, M, or Q in RMOR resulted in receptor assuming activated conformations. In contrast, the D3.49(164)E mutant displayed significantly lower basal [(35)S]GTPgammaS binding and reduced affinity for DAMGO. Upon incubation of membranes at 37 degrees C, the constitutively active D3.49(164)Y mutant was structurally less stable, whereas the inactivated D3.49(164)E mutant was more stable, than the wild-type. Computational simulations showed that the E3.49 side chain interacted strongly with the conserved R3.50 in the DRY motif and stabilized the inactive form of the receptor. Taken together, these results indicate that D3.49 plays an important role in constraining the receptor in inactive conformations.

Diverse signalling by 5-hydroxytryptamine (5-HT) receptors

Fourteen different receptor subtypes might be regarded as a diversity that is sufficient to accommodate the wide-ranging physiological roles of 5-hydroxytryptamine (5-HT). However, it is becoming clear that, for 5-HT as for other neurotransmitters, the concept of a receptor as a gatekeeper for a specific cellular process or event is too restrictive. Multiple receptor-mediated biochemical cascades can be activated in cells in response to an agonist by a number of mechanisms. Whereas it is well established that different agonists do not necessarily elicit the same magnitude of response, they probably also select between various possible signal transduction pathways. Receptor signalling may be diverse via a single receptor subtype as a consequence of specific agonist-receptor-G protein interactions. 5-HT receptors are even more heterogeneous when one considers that the amino acid sequence of these receptor subtypes may vary from individual to individual, and that there is an increasing number of receptor isoforms due to alternative splicing and RNA editing of 5-HT receptor transcripts. Activation, in particular constitutive, agonist-independent activation, of some of these receptor isoforms has been reported to be altered. This implies that ligands with similar binding affinities may display different pharmacological properties (partial agonist, antagonist, or inverse agonist) versus these receptor isoforms, depending on their activation state. Therefore, intervention with receptor ligands to modify hampered neurotransmission pathways is a difficult task, and one needs to consider the growing evidence of diversity in G protein-coupled receptor signalling.

Antimitotic and cytotoxic effects of theophylline in MDA-MB-231 human breast cancer cells

A variety of cancer cell lines, including MDA-MB-231 human breast cancer cells, exhibit mitotic inhibition by cAMP. In earlier work, we found that the phosphodiesterase inhibitor, theophylline, reduced the number of cells and altered cellular morphology. In the current study, we evaluated the effects of theophylline on macromolecule synthesis and indices of cell viability. Theophylline evoked a concentration- and time-dependent decrease in DNA synthesis. However, the net decrease in cell number was greater than that predicted solely from mitotic arrest. Assessment of protein synthesis indicated a second effect of theophylline separable from that on DNA synthesis. This was confirmed by decreased cell viability and adhesion. Exposure of the cells to the phosphodiesterase inhibitor, IBMX, in concentrations that produced inhibition of DNA synthesis equivalent to that seen with theophylline, elicited a smaller reduction in cell number. Theophylline also evoked specific changes in the expression or function of membrane-bound adenylyl cyclase activity, effects that are likely to contribute to sustained reactivity of these cells to other cAMP-related inhibitors of cell proliferation, such as isoproterenol. The multiple pharmacologic properties of theophylline, producing mitotic inhibition, cytotoxicity and altered signaling in MDA-MB-231 cells, may provide insight into novel therapeutic strategies.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

Non-competitive antagonism of beta(2)-agonist-mediated cyclic AMP accumulation by ICI 118551 in BC3H1 cells endogenously expressing constitutively active beta(2)-adrenoceptors

Constitutive activity of the beta(2)-adrenoceptor, which is sensitive to inhibition by an inverse agonist such as ICI 118551, has been readily demonstrated in recombinant systems expressing constitutively-active mutant receptors or over-expressing the wild-type beta(2)-adrenoceptor. Here we demonstrate the presence of constitutive beta(2)-adrenoceptor activity in BC3H1 cells which endogenously express this receptor. In BC3H1 cells, only ICI 118551 behaved as an inverse agonist at beta(2)-adrenoceptors, while propranolol, ICI 118551, atenolol and, to a lesser extent, alprenolol exhibited inverse agonism in CHO-beta(2)4 cells transfected with cDNA for the human beta(2)-adrenoceptor (310 fmol. mg protein(-1)). The level of expression of beta2-adrenoceptors in BC3H1 cells was not high (78 fmol.mg protein-1) and the efficiency of receptor - effector coupling in this cell line was much lower than in the recombinant CHO-beta(2)4 cells (as judged by the partial agonist nature of both salbutamol and clenbuterol). ICI 118551 (log K(D)-9.73+/-0.07) and propranolol (log K(D)-9.25+/-0.12) both behaved as conventional competitive antagonists of isoprenaline-stimulated cyclic AMP accumulation in high expressing CHO-beta(2)4 cells. In contrast, ICI 118551 appeared to act as a non-competitive antagonist in BC3H1 cells and in low expressing CHO-beta(2)6 cells (50 fmol.mg protein(-1)). This non-competitive effect of ICI 118551 in BC3H1 cells was also observed when either salbutamol was used as agonist, or the incubation period with isoprenaline was extended to 30 min. The possibility that these effects of ICI 118551 are due to an interaction with different affinity states (R, R* and R') of the receptor is discussed.

Selective down-regulation of the G(q)alpha/G11alpha G-protein family in tumour necrosis factor-alpha induced cell death

Investigations into the regulation of heterotrimeric GTP-binding protein alpha-subunits in models of tumour necrosis factor-alpha (TNF)-induced cell death, revealed the selective down-regulation of the G(q)alpha/G11alpha family of G-proteins. The human HeLa and murine L929 cells treated with recombinant human TNF for up to 24 h displayed down-regulated G(q)alpha/G11alpha family protein levels, but not G(s)alpha, G(i)alpha and G(o)alpha protein levels as determined by Western analyses. This effect of TNF was observed in a concentration--and time-dependent manner, consistent with the profiles of TNF-induced cell death observed. Moreover, the functioning of G(q)alpha/G11alpha family proteins were found to be impaired in TNF-treated cells, as measured by agonist-induced [Ca2+]i release. In contrast, G(s)alpha activity was unaltered by TNF-treatment, determined by measurement of agonist-induced intracellular cyclic AMP generation. These findings in TNF-induced cytotoxic models, indicate a novel 'cross-talk' mechanism by which TNF alters Ca2+-signalling mechanisms, which may contribute towards the apoptotic and necrotic cell death.

Beta-adrenoceptor signaling and its control of cell replication in MDA-MB-231 human breast cancer cells

MDA-MB-231 human breast cancer cells express high beta-adrenoceptor levels, predominantly the beta2 subtype. Receptor stimulation by isoproterenol evoked immediate reductions in DNA synthesis which were blocked completely by propranolol and were of the same magnitude as effects elicited by high concentrations of 8-Br-cAMP. Isoproterenol-induced inhibition of DNA synthesis was maintained throughout several days of exposure, resulting in a decrement in total cell number, and the effects were augmented by cotreatment with dexamethasone; an even greater effect was seen when cAMP breakdown was inhibited by theophylline, with or without addition of isoproterenol. Despite the persistent effect of isoproterenol, receptor downregulation was evident with as little as 1 h of treatment, and over 90% of the receptors were lost within 24 h. Receptor downregulation was paralleled by homologous desensitization of the adenylyl cyclase response to beta-adrenoceptor stimulation. Dexamethasone augmented the effects of isoproterenol on DNA synthesis but did not prevent receptor downregulation or desensitization. These results indicate that beta-adrenoceptors are effectively linked, through cAMP, to the termination of cell replication in MDA-MB-231 human breast cancer cells, and that activation of only a small number of receptors is sufficient for a maximal effect. Novel pharmacologic strategies that focus on cell surface receptors operating through adenylyl cyclase may offer opportunities to combat cancers that are unresponsive to hormonal agents, or that have developed multidrug resistance.

Regulation of peripheral cannabinoid receptor CB2 phosphorylation by the inverse agonist SR 144528. Implications for receptor biological responses

We recently demonstrated that the selective cannabinoid receptor antagonist SR 144528 acts as an inverse agonist that blocks constitutive mitogen-activated protein kinase activity coupled to the spontaneous autoactivated peripheral cannabinoid receptor (CB2) in the Chinese hamster ovary cell line stably transfected with human CB2. In the present report, we studied the effect of SR 144528 on CB2 phosphorylation. The CB2 phosphorylation status was monitored by immunodetection using an antibody specific to the COOH-terminal CB2 which can discriminate between phosphorylated and non-phosphorylated CB2 isoforms at serine 352. We first showed that CB2 is constitutively active, phosphorylated, and internalized at the basal level. By blocking autoactivated receptors, inverse agonist SR 144528 treatment completely inhibited this phosphorylation state, leading to an up-regulated CB2 receptor level at the cell surface, and enhanced cannabinoid agonist sensitivity for mitogen-activated protein kinase activation of Chinese hamster ovary-CB2 cells. After acute agonist treatment, serine 352 was extensively phosphorylated and maintained in this phosphorylated state for more than 8 h after agonist treatment. The cellular responses to CP-55,940 were concomitantly abolished. Surprisingly, CP-55,940-induced CB2 phosphorylation was reversed by SR 144528, paradoxically leading to a non-phosphorylated CB2 which could then be fully activated by CP-55,940. The process of CP-55,940-induced receptor phosphorylation followed by SR 144528-induced receptor dephosphorylation kept recurring many times on the same cells, indicating that the agonist switches the system off but the inverse agonist switches the system back on. Finally, we showed that autophosphorylation and CP-55, 940-induced serine 352 CB2 phosphorylation involve an acidotropic GRK kinase, which does not use Gibetagamma. In contrast, SR 144528-induced CB2 dephosphorylation was found to involve an okadaic acid and calyculin A-sensitive type 2A phosphatase.

Interactions between effectors linked to serotonin receptors

In general, there are two types of interactions between effector signaling pathways. "Homologous" interactions are those that occur within a receptor system to alter its own responsiveness, for example the loss of responsiveness (desensitization) that can occur upon agonist occupancy of a receptor. "Heterologous" interactions are those that occur between different receptor systems where the responsiveness of one receptor system is regulated (positively or negatively) by activation of another receptor system (i.e., "cross-talk"). Many, if not all receptors, couple to multiple cellular effector pathways and alterations in the responsiveness of a receptor system can be effector pathway-dependent which underscores the importance of studying each effector coupled to a receptor. Regulation of receptor system responsiveness, and consequently the efficacy of drugs, is a highly dynamic process. Perhaps by exploiting these interactions, new targets for pharmacotherapy may be uncovered which will provide for increased efficacy and specificity of drug action.

Agonist-independent regulation of constitutively active G-protein-coupled receptors

G-protein-coupled receptors constitute one of the largest protein super-families in mammals. Since the cloning of the encoding genes, these important drug targets have been subjected to thorough biochemical and pharmacological studies. It has become clear that G-protein-coupled receptors not only transmit signals after stimulation by agonists but can also spontaneously couple to signal-transduction pathways. Recent findings show that constitutively active G-protein-coupled receptors can also be regulated in an agonist-independent manner, which has important implications for the interpretation of the actions of (inverse) agonists and the results of site-directed-mutagenesis studies.

Efficacy of inverse agonists in cells overexpressing a constitutively active beta2-adrenoceptor and type II adenylyl cyclase

1 Maximal stimulant output from the adenylyl cyclase cascade in neuroblastoma x glioma hybrid, NG108-15, cells is limited by the levels of expression of isoforms of adenylyl cyclase. Stable expression in these cells of a constitutively active mutant (CAM) version of the human beta2-adrenoceptor resulted in higher basal adenylyl cyclase activity than following expression of the human wild type beta2-adrenoceptor. Isoprenaline acted as a full agonist in membranes from both wild type and CAM beta2-adrenoceptor expressing clones. 2 Expression of type II adenylyl cyclase resulted in a substantially elevated capacity of isoprenaline to stimulate [3H]-forskolin binding, whereas in CAM beta2-adrenoceptor expressing cells the basal high affinity [3H]-forskolin binding represented a markedly greater % of the maximal effect which could be produced by addition of isoprenaline, and the EC50 for isoprenaline was some 10 fold lower than in cells expressing the wild type beta2-adrenoceptor. 3 Further transfection of the CAM beta2-adrenoceptor expressing cells with type II adenylyl cyclase greatly increased both absolute basal and agonist-stimulated levels of adenylyl cyclase activity. 4 Betaxolol, ICI 118,551, sotalol and timolol acted as inverse agonists with varying degrees of efficacy, whereas propranolol functioned as a neutral antagonist and alprenolol as a partial agonist. 5 Pretreatment of the CAM beta2-adrenoceptor and type II adenylyl cyclase expressing clones with the irreversible alkylating agent BAAM (1 microM) did not reduce the efficacy of isoprenaline but eliminated efficacy from all the inverse agonist ligands. This effect was dependent upon the concentration of BAAM employed, with half-maximal effects being produced between 10 nM and 100 nM of the alkylating agent, which is similar to the concentrations required to prevent subsequent ligand access to some 50% of the CAM beta2-adrenoceptor population. 6 These data demonstrate that inverse agonist efficacy can be modulated by receptor availability and also indicate why in physiological systems, inverse agonism can be difficult to detect.

Modulation of forskolin-mediated adenylyl cyclase activation by constitutively active G(S)-coupled receptors

In transfected CHO cells constitutively active histamine H2 receptors not only increase the basal cAMP level, but also enhance forskolin-induced cAMP production. The increased forskolin response was inhibited by inverse H2 agonists with potencies similar to those determined at basal levels. The modulation of the forskolin response was also observed after H2 receptor expression in HEK-293 and Sf9 cells or TSH receptor expression in COS-7 cells. The enhancement of forskolin-induced cAMP production seems to be a general characteristic of constitutively active G(S)-coupled receptors and can be very useful to study inverse agonism at wild-type receptors.

Inverse agonism and the regulation of receptor number

Inverse agonists are ligands that preferentially stabilize inactive conformations of G protein-coupled receptors. In a range of systems, sustained treatment with inverse agonists can produce substantially greater upregulation of receptor levels than antagonists. The use of constitutively active mutant receptors can exaggerate this effect but may also allow agonists and antagonists to mimic the effect by preventing denaturation of the mutant receptor polypeptide. In this review Graeme Milligan and Richard Bond consider the basis for these effects and their therapeutic implications.

Up-regulation of the levels of expression and function of a constitutively active mutant of the hamster alpha1B-adrenoceptor by ligands that act as inverse agonists

The alpha1-adrenergic agonist phenylephrine stimulated phospholipase D (PLD) activity in Rat 1 fibroblasts transfected to express either the wild-type hamster alpha1B-adrenoceptor or a constitutively active mutant (CAM) form of this receptor. The EC50 for agonist stimulation of PLD activity was substantially lower at the CAM receptor than at the wild-type receptor as previously noted for phenylephrine stimulation of phosphoinositidase C activity. Sustained treatment of cells expressing the CAM alpha1B-adrenoceptor with phentolamine resulted in a marked up-regulation in levels of this receptor with half-maximal effects produced within 24 h and with an EC50 of approx. 40 nM. Such an up-regulation could be produced with a range of other ligands generally viewed as alpha1-adrenoceptor antagonists but equivalent treatment of cells expressing the wild-type alpha1B-adrenoceptor was unable to mimic these effects. After sustained treatment of the CAM alpha1B-adrenoceptor expressing cells with phentolamine, basal PLD activity was increased and phenylephrine was now able to stimulate PLD activity to greater levels than in vehicle-treated CAM alpha1B-adrenoceptor-expressing cells. The EC50 for phenylephrine stimulation of PLD activity was not altered, however, by phentolamine pretreatment and the associated up-regulation of the receptor. After phentolamine-induced up-regulation of basal PLD activity, a range of alpha1-antagonists were shown to possess the characteristics of inverse agonists of the CAM alpha1B-adrenoceptor as they were able to substantially decrease the elevated basal PLD activity.