Mutational analysis and molecular modelling of the antagonist SR 144528 binding site on the human cannabinoid CB(2) receptor

We have investigated the binding site of the subtype specific antagonist SR 144528, (N-[(1S)-endo-1,3,3-trimethyl bicyclo [2.2. 1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methoxybenzyl)- pyrazo le-3-carboxamide) on the human cannabinoid CB(2) receptor based on functional studies with mutated receptors. Two serine residues in the fourth transmembrane region, Ser(161) and Ser(165), were singly mutated to the cognate cannabinoid CB(1) receptor residue, alanine, and each gave receptors with wild-type properties for the cannabinoid agonists CP 55,940 (1R,3R,4R)-3-[2-hydroxy-4-(1, 1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexan-1-ol) and WIN 55212-2 (R)-(+)[2, 3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1, 4-benzoxazin-6-yl](1-naphthalenyl) methanone, which SR 144528 completely failed to antagonise. Molecular modelling studies show that SR 144528 interacts with residues in transmembrane domains 3, 4, and 5 of the cannabinoid CB(2) receptor through a combination of hydrogen bonds and aromatic and hydrophobic interactions. In addition, the replacement by serine of a nearby cannabinoid CB(2) receptor-specific residue, Cys(175) resulted in wild-type receptor properties with CP 55,940, loss of SR 144528 binding and eight-fold reduced binding and activity of WIN 55212-2, a result compatible with a recently-proposed binding site model for WIN 55212-2.

Cannabinoid receptor interactions with the antagonists SR 141716A and SR 144528

The G protein-coupled cannabinoid receptor subtypes CB1 and CB2 have been cloned from several species. The CB1 receptor is highly conserved across species, whereas the CB2 receptor shows considerable cross-species variations. The two human receptors share only 44% overall identity, ranging from 35% to 82% in the transmembrane regions. Despite this structural disparity, the most potent cannabinoid agonists currently available are largely undiscriminating and are therefore unsatisfactory tools for investigating the architecture of ligand binding sites. However, the availability of two highly specific antagonists, SR 141716A for the CB1 receptor and SR 144528 for the CB2 receptor, has allowed us to adopt a systematic approach to defining their respective binding sites through the use of chimeric CB1 receptor/CB2 receptor constructs, coupled with site-directed mutagenesis. We identified the region encompassed by the fourth and fifth transmembrane helices as being critical for antagonist specificity. Both the wild type human receptors overexpressed in heterologous systems are autoactivated; SR 141716A and SR 144528 exhibit classical inverse agonist properties with their respective target receptors. In addition, through its interaction with the CB1 receptor SR 141716A blocks the Gi protein-mediated activation of mitogen-activated protein kinase stimulated by insulin or insulin-like growth factor I. An in-depth analysis of this discovery has led to a modified three-state model for the CB1 receptor, one of which implicates the SR 141716A-mediated sequestration of Gi proteins, with the result that the growth factor-stimulated intracellular pathways are effectively impeded.

Dual intracellular signaling pathways mediated by the human cannabinoid CB1 receptor

It has long been established that the cannabinoid CB1 receptor transduces signals through a pertussis toxin-sensitive Gi/Go inhibitory pathway. Although there have been reports that the cannabinoid CB1 receptor can also mediate an increase in cyclic AMP levels, in most cases the presence of an adenylyl cyclase costimulant or the use of very high amounts of agonist was necessary. Here, we present evidence for dual coupling of the cannabinoid CB receptor to the classical pathway and to a pertussis toxin-insensitive adenylyl cyclase stimulatory pathway initiated with low quantities of agonist in the absence of any costimulant. Treatment of Chinese hamster ovary (CHO) cells expressing the cannabinoid CB1 receptor with the cannabinoid CP 55,940, {(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hyd roxypropyl) cyclohexan-1-ol} resulted in cyclic AMP accumulation in a dose-response manner, an accumulation blocked by the cannabinoid CB1 receptor-specific antagonist SR 141716A, {N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1H-pyrazole-3-carboxamide hydrochloride}. In CHO cells coexpressing the cannabinoid CB1 receptor and a cyclic AMP response element (CRE)-luciferase reporter gene system, CP 55,940 induced luciferase expression by a pathway blocked by the protein kinase A inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide hydrochloride (H-89). Under the same conditions the peripheral cannabinoid CB2 receptor proved to be incapable of inducing cAMP accumulation or luciferase activity. This incapacity allowed us to study the luciferase activation mediated by CB /CB2 chimeric constructs, from which we determined that the first and second internal loop regions of the cannabinoid CB1 receptor were involved in transducing the pathway leading to luciferase gene expression.

SR 144528, an antagonist for the peripheral cannabinoid receptor that behaves as an inverse agonist

In the present report, we investigated in detail the effects of SR 144528, a selective antagonist of the peripheral cannabinoid receptor (CB2), on two well-characterized functions mediated by CB2: the induction of the early response gene krox24 and the inhibition of adenylyl cyclase. We generated Chinese hamster ovary cells doubly transfected with human CB2 and a luciferase reporter gene linked to either the murine krox24 regulatory sequence or multiple cAMP responsive elements. Our results show that (1) SR 144528 antagonizes the effect of receptor agonists-it inhibits the krox24 reporter activity and prevents the inhibition of forskolin-induced cAMP reporter activity mediated by CP 55,940; (2) CB2 is autoactivated-CB2 mediates signaling in the absence of ligand, and this basal activity is reduced by pretreating the cells with pertussis toxin; (3) SR 144528 is an inverse agonist-it reproduces the effects of pertussis toxin; and (4) inhibition of precoupled CB2 by a long-term pretreatment of cells with SR 144528 potentiates krox24 response to cannabinoid receptor agonists and restores activation of adenylyl cyclase. Taken together, these data provide evidences for the inverse agonist property of SR 144528 and the constitutive activation of CB2 in Chinese hamster ovary-expressing cells.

SR 144528, the first potent and selective antagonist of the CB2 cannabinoid receptor

Based on both binding and functional data, this study introduces SR 144528 as the first, highly potent, selective and orally active antagonist for the CB2 receptor. This compound which displays subnanomolar affinity (Ki = 0.6 nM) for both the rat spleen and cloned human CB2 receptors has a 700-fold lower affinity (Ki = 400 nM) for both the rat brain and cloned human CB1 receptors. Furthermore it shows no affinity for any of the more than 70 receptors, ion channels or enzymes investigated (IC50 > 10 microM). In vitro, SR 144528 antagonizes the inhibitory effects of the cannabinoid receptor agonist CP 55,940 on forskolin-stimulated adenylyl cyclase activity in cell lines permanently expressing the h CB2 receptor (EC50 = 10 nM) but not in cells expressing the h CB1 (no effect at 10 microM). Furthermore, SR 144528 is able to selectively block the mitogen-activated protein kinase activity induced by CP 55,940 in cell lines expressing h CB2 (IC50 = 39 nM) whereas in cells expressing h CB1 an IC50 value of more than 1 microM is found. In addition, SR 144528 is shown to antagonize the stimulating effects of CP 55,940 on human tonsillar B-cell activation evoked by cross-linking of surface Igs (IC50 = 20 nM). In vivo, after oral administration SR 144528 totally displaced the ex vivo [3H]-CP 55,940 binding to mouse spleen membranes (ED50 = 0.35 mg/kg) with a long duration of action. In contrast, after the oral route it does not interact with the cannabinoid receptor expressed in the mouse brain (CB1). It is expected that SR 144528 will provide a powerful tool to investigate the in vivo functions of the cannabinoid system in the immune response.

A selective inverse agonist for central cannabinoid receptor inhibits mitogen-activated protein kinase activation stimulated by insulin or insulin-like growth factor 1. Evidence for a new model of receptor/ligand interactions

In the present study, we showed that Chinese hamster ovary (CHO) cells transfected with human central cannabinoid receptor (CB1) exhibit high constitutive activity at both levels of mitogen-activated protein kinase (MAPK) and adenylyl cyclase. These activities could be blocked by the CB1-selective ligand, SR 141716A, that functions as an inverse agonist. Moreover, binding studies showed that guanine nucleotides decreased the binding of the agonist CP-55,940, an effect usually observed with agonists, whereas it enhanced the binding of SR 141716A, a property of inverse agonists. Unexpectedly, we found that CB1-mediated effects of SR 141716A included inhibition of MAPK activation by pertussis toxin-sensitive receptor-tyrosine kinase such as insulin or insulin-like growth factor 1 receptors but not by pertussis toxin-insensitive receptor-tyrosine kinase such as the fibroblast growth factor receptor. We also observed similar results when cells were stimulated with Mas-7, a mastoparan analog, that directly activates the Gi protein. Furthermore, SR 141716A inhibited guanosine 5'-0-(thiotriphosphate) uptake induced by CP-55,940 or Mas-7 in CHO-CB1 cell membranes. This indicates that, in addition to the inhibition of autoactivated CB1, SR 141716A can deliver a biological signal that blocks the Gi protein and consequently abrogates most of the Gi-mediated responses. By contrast, SR 141716A had no effect on MAPK activation by insulin or IGF1 in CHO cells lacking CB1 receptors, ruling out the possibility of a direct interaction of SR 141716A with the Gi protein. This supports the notion that the Gi protein may act as a negative intracellular signaling cross-talk molecule. From these original results, which considerably enlarge the biological properties of the inverse agonist, we propose a novel model for receptor/ligand interactions.

Characterization of two cloned human CB1 cannabinoid receptor isoforms

We have investigated the pharmacology of two central human cannabinoid receptor isoforms, designated CB1 and CB1A, stably expressed in Chinese hamster ovary cell lines, designated as CHO-CB1 and CHO-CB1A, respectively. In direct binding assays on isolated membranes the agonist [3H]CP 55,940 bound in a saturable and highly specific manner to both cannabinoid receptor isoforms. Competition binding experiments performed with other commonly used receptor agonists showed the following rank order of potency: CP 55,940 > tetrahydrocannabinol > WIN 55212-2 > anandamide. Except for the endogenous ligand anandamide (CB1, Ki = 359.6 nM vs. CB1A, Ki = 298 nM), these agonists bound to CB1A (CP 55,940, WIN 55212-2 and delta 9-THC, Ki = 7.24,345 and 26.7 nM, respectively) with about 3-fold less affinity than to CB1 (CP 55,940, WIN 55212-2 and delta 9-THC, Ki = 2.26, 93 and 7.1 nM, respectively). The cannabinoid receptor antagonist SR 141716A also bound to CB1A (Ki = 43.3 nM) with slightly less affinity than to CB1 (Ki = 4.9 nM). Cannabinoid receptor-linked second messenger system studies performed in the CHO-CB1 and CHO-CB1A cells showed that both receptors mediated their action through the agonist-induced inhibition of forskolin-stimulated cAMP accumulation. This activity was totally blocked by pretreatment with PTX. Additionally, both isoforms activated mitogen-activated protein kinase. The selective antagonist SR 141716A was able to selectively block these responses in both cell lines, to an extent that reflected its binding characteristics. Our results show that the amino-truncated and -modified CB1 isoform CB1A exhibits all the properties of CB1 to a slightly attenuated extent.

Molecular cloning, expression and function of the murine CB2 peripheral cannabinoid receptor

We have cloned the peripheral cannabinoid receptor, mCB2, from a mouse splenocyte cDNA library. The 3.7 kb sequence contains an open reading frame encoding a protein of 347 residues sharing 82% overall identity with the only other known peripheral receptor, human CB2 (hCB2) and shorter than hCB2 by 13 amino acids at the carboxyl terminus. Binding experiments with membranes from COS-3 cells transiently expressing mCB2 showed that the synthetic cannabinoid WIN 55212-2 had a 6-fold lower affinity for mCB2 than for hCB2, whereas both receptors showed similar affinities for the agonists CP 55,940, delta(9)-THC and anandamide and almost no affinity for the central receptor- (CB1) specific antagonist SR 141716A. Both hCB2 and mCB2 mediate agonist-stimulated inhibition of forskolin-induced cAMP production in CHO cell lines permanently expressing the receptors. SR 141716A failed to antagonize this activity in either cell line, confirming its specificity for CB1.

Signaling pathway associated with stimulation of CB2 peripheral cannabinoid receptor. Involvement of both mitogen-activated protein kinase and induction of Krox-24 expression

Cannabinoids, known for their psychoactive effects, also possess immunomodulatory properties. The recent isolation and cloning of the G-protein-coupled peripheral cannabinoid receptor (CB2), mainly expressed in immune tissues, have provided molecular tools to determine how cannabinoid compounds may mediate immunomodulation. We here investigated the CB2 signaling properties using stably transfected Chinese hamster ovary cells expressing human CB2. First, we showed that stimulation by a cannabinoid agonist activated mitogen-activated protein (MAP) kinase in time- and dose-dependent manners. The rank order of potency for MAP kinase activation of cannabinoid agonists correlated well with their binding capacities. Second, we demonstrated that, following MAP kinase activation, cannabinoids induced the expression of the growth-related gene Krox-24, also known as NGFI-A, zif/268, and egr-1. Pertussis toxin completely prevented both MAP kinase activation and Krox-24 induction, even more these responses appeared to be dependent of specific protein kinase C isoforms and independent of inhibition of adenylyl cyclase. A similar coupling of CB2 to a mitogenic pathway and to the regulation of Krox-24 expression was also observed in human promyelocytic cells HL60. Taken together, these findings provide evidence for a functional role of the CB2 receptor in gene induction mediated by the MAP kinase network.

Structural features of the central cannabinoid CB1 receptor involved in the binding of the specific CB1 antagonist SR 141716A

The antagonist SR 141716A has a high specificity for the central CB1 cannabinoid receptor and negligeable affinity for the peripheral CB2 receptor, making it an excellent tool for probing receptor structure-activity relationships. From binding experiments with mutated CB1 and with chimeric CB1/CB2 receptors we have begun to identify the domains of CB1 implicated in the recognition of SR 141716A. Receptors were transiently expressed in COS-3 cells, and their binding characteristics were studied with SR 141716A and with CP 55,940, an agonist recognized equally well by the two receptors. The region delineated by the fourth and fifth transmembrane helices of CB1 proved to be crucial for high affinity binding of SR 141716A. The CB1 and CB2 second extracellular loops, e2, were exchanged, modifications that had no effect on SR 141716A binding in the CB1 variant but that eliminated CP 55,940 binding in both mutants. The replacement of the conserved cysteine residues in e2 of CB2 by serine also eliminated CP 55,940 binding, but replacement of those in CB1 resulted in the sequestration of the mutated receptors in the cell cytoplasm. The e2 domain thus plays some role in CP 55,940 binding but none in SR 141716A recognition, binding of the latter clearly implicating residues in the adjoining transmembrane helices.

Activation of mitogen-activated protein kinases by stimulation of the central cannabinoid receptor CB1

The G-protein-coupled central cannabinoid receptor (CB1) has been shown to be functionally associated with several biological responses including inhibition of adenylate cyclase, modulation of ion channels and induction of the immediate-early gene Krox-24. Using stably transfected Chinese Hamster Ovary cells expressing human CB1 we show here that cannabinoid treatment induces both phosphorylation and activation of mitogen-activated protein (MAP) kinases, and that these effects are inhibited by SR 141716A, a selective CB1 antagonist. The two p42 and p44 kDa MAP kinases are activated in a time- and dose-dependent manner. The rank order of potency for the activation of MAP kinases with various cannabinoid agonists is CP-55940 > delta 9-tetrahydrocannabinol > WIN 55212.2, in agreement with the pharmacological profile of CB1. The activation of MAP kinases is blocked by pertussis toxin but not by treatment with hydrolysis-resistant cyclic AMP analogues. This suggests that the signal transduction pathway between CB1 and MAP kinases involves a pertussis-toxin-sensitive GTP-binding protein and is independent of cyclic AMP metabolism. This coupling of CB1 subtype and mitogenic signal pathway, also observed in the human astrocytoma cell line U373 MG, may explain the mechanism of action underlying cannabinoid-induced Krox-24 induction.

An amino-terminal variant of the central cannabinoid receptor resulting from alternative splicing

The cDNA sequences encoding the central cannabinoid receptor, CB1, are known for two species, rat and human. However, little information concerning the flanking, noncoding regions is presently available. We have isolated two overlapping clones from a human lung cDNA library with CB1 cDNA inserts. One of these, cann7, contains a short stretch of the CB1 coding region and 4 kilobase pairs (kb) of the 3'-untranslated region (UTR), including two polyadenylation signals. The other, cann6, is identical to cann7 upstream from the first polyadenylation signal, and in addition, it contains the whole coding region and extends for 1.8 kb into the 5'-UTR. Comparison of cann6 with the published sequence (Gérard, C. M., Mollereau, C., Vassart, G., and Parmentier, M. (1991) Biochem. J. 279, 129-134) shows the coding regions to be identical, but reveals important differences in the flanking regions. Notably, the cann6 sequence appears to be that of an immature transcript, containing 1.8 kb of an intronic sequence in the 5'-UTR. In addition, polymerase chain reaction amplification of the CB1 coding region in the IM-9 cell line cDNA resulted in two fragments, one containing the whole CB1 coding region and the second lacking a 167-base pair intron within the sequence encoding the amino-terminal tail of the receptor. This alternatively spliced form would translate to an NH2-terminal modified isoform (CB1A) of the receptor, shorter than CB1 by 61 amino acids. In addition, the first 28 amino acids of the putative truncated receptor are completely different from those of CB1, containing more hydrophobic residues. Rat CB1 mRNA is similarly alternatively spliced. A study of the distribution of the human CB1 and CB1A mRNAs by reverse transcription-polymerase chain reaction analysis showed the presence of both CB1 and CB1A throughout the brain and in all the peripheral tissues examined, with CB1A being present in amounts of up to 20% of CB1.

SR141716A, a potent and selective antagonist of the brain cannabinoid receptor

SR141716A is the first selective and orally active antagonist of the brain cannabinoid receptor. This compound displays nanomolar affinity for the central cannabinoid receptor but is not active on the peripheral cannabinoid receptor. In vitro, SR141716A antagonises the inhibitory effects of cannabinoid receptor agonists on both mouse vas deferens contractions and adenylyl cyclase activity in rat brain membranes. After intraperitoneal or oral administration SR141716A antagonises classical pharmacological and behavioural effects of cannabinoid receptor agonists. This compound should prove to be a powerful tool for investigating the in vivo functions of the anandamide/cannabinoid system.