The endogenous cannabinoid anandamide is a lipid messenger activating cell growth via a cannabinoid receptor-independent pathway in hematopoietic cell lines

Modulatory role of the endocannabinoidome in the pathophysiology of the gastrointestinal tract

Originating from Eastern Asia, the plant Cannabis sativa has been used for centuries as a medicinal treatment. The unwanted psychotropic effects of one of its major components, Δ⁹-tetrahydrocannabinol, discouraged its therapeutic employment until, recently, the discovery of cannabinoids receptors and their endogenous ligands endocannabinoids reignited the interest. The endocannabinoid system has lately been found to play an important role in the maintenance of human health, both centrally and peripherally. However, the initial idea of the endocannabinoid system structure has been quickly understood to be too simplistic and, as new receptors, mediators, and enzymes have been discovered to participate in a complex relationship, the new, more comprehensive term "expanded endocannabinoid system" or "endocannabinoidome", has taken over. The discovery of other endocannabinoid-like receptors, such as the G protein-coupled receptor 119 and G protein-coupled receptor 55, has opened the way to the development of potential therapeutic targets for the treatment of various metabolic disorders. In addition, recent findings have also provided evidence suggesting the potential therapeutic link between the endocannabinoidome and various inflammatory-based gut diseases, such as inflammatory bowel disease and cancer. This review will provide an introduction to the endocannabinoidome, focusing on its modulatory role in the gastrointestinal tract and on the interest generated by the link between gut microbiota, the endocannabinoid system and metabolic diseases such as inflammatory bowel disease, type-2 diabetes and obesity. In addition, we will look at the potential novel aspects and benefits of drugs targeting the endocannabinoid system.

The Interplay between the Immune and the Endocannabinoid Systems in Cancer

The therapeutic potential of Cannabis sativa has been recognized since ancient times. Phytocannabinoids, endocannabinoids and synthetic cannabinoids activate two major G protein-coupled receptors, subtype 1 and 2 (CB1 and CB2). Cannabinoids (CBs) modulate several aspects of cancer cells, such as apoptosis, autophagy, proliferation, migration, epithelial-to-mesenchymal transition and stemness. Moreover, agonists of CB1 and CB2 receptors inhibit angiogenesis and lymphangiogenesis in vitro and in vivo. Low-grade inflammation is a hallmark of cancer in the tumor microenvironment (TME), which contains a plethora of innate and adaptive immune cells. These cells play a central role in tumor initiation and growth and the formation of metastasis. CB2 and, to a lesser extent, CB1 receptors are expressed on a variety of immune cells present in TME (e.g., T cells, macrophages, mast cells, neutrophils, NK cells, dendritic cells, monocytes, eosinophils). The activation of CB receptors modulates a variety of biological effects on cells of the adaptive and innate immune system. The expression of CB2 and CB1 on different subsets of immune cells in TME and hence in tumor development is incompletely characterized. The recent characterization of the human cannabinoid receptor CB2-G_i signaling complex will likely aid to design potent and specific CB2/CB1 ligands with therapeutic potential in cancer.

Endocannabinoid signaling at the periphery: 50 years after THC

In 1964, the psychoactive ingredient of Cannabis sativa, Δ(9)-tetrahydrocannabinol (THC), was isolated. Nearly 30 years later the endogenous counterparts of THC, collectively termed endocannabinoids (eCBs), were discovered: N-arachidonoylethanolamine (anandamide) (AEA) in 1992 and 2-arachidonoylglycerol (2-AG) in 1995. Since then, considerable research has shed light on the impact of eCBs on human health and disease, identifying an ensemble of proteins that bind, synthesize, and degrade them and that together form the eCB system (ECS). eCBs control basic biological processes including cell choice between survival and death and progenitor/stem cell proliferation and differentiation. Unsurprisingly, in the past two decades eCBs have been recognized as key mediators of several aspects of human pathophysiology and thus have emerged to be among the most widespread and versatile signaling molecules ever discovered. Here some of the pioneers of this research field review the state of the art of critical eCB functions in peripheral organs. Our community effort is aimed at establishing consensus views on the relevance of the peripheral ECS for human health and disease pathogenesis, as well as highlighting emerging challenges and therapeutic hopes.

Endocannabinoids and the Immune System in Health and Disease

Endocannabinoids are bioactive lipids that have the potential to signal through cannabinoid receptors to modulate the functional activities of a variety of immune cells. Their activation of these seven-transmembranal, G protein-coupled receptors sets in motion a series of signal transductional events that converge at the transcriptional level to regulate cell migration and the production of cytokines and chemokines. There is a large body of data that supports a functional relevance for 2-arachidonoylglycerol (2-AG) as acting through the cannabinoid receptor type 2 (CB2R) to inhibit migratory activities for a diverse array of immune cell types. However, unequivocal data that supports a functional linkage of anandamide (AEA) to a cannabinoid receptor in immune modulation remains to be obtained. Endocannabinoids, as typical bioactive lipids, have a short half-life and appear to act in an autocrine and paracrine fashion. Their immediate effective action on immune function may be at localized sites in the periphery and within the central nervous system. It is speculated that endocannabinoids play an important role in maintaining the overall "fine-tuning" of the immune homeostatic balance within the host.

GPR55: from orphan to metabolic regulator?

GPR55 belongs to the class A family of G-protein coupled receptor (GPCRs) and its activity is regulated by a range of synthetic and endogenous cannabinoids, and by lipid-derived ligands. Cannabinoids are known to be important in controlling appetite and metabolic balance, and it is now emerging that GPR55 may have a role to play in energy homeostasis through the regulation of food intake, fuel storage in adipocytes, gut motility and insulin secretion. This review summarises our current knowledge of expression and function of GPR55 in tissues involved in metabolic regulation, the signalling cascades through which GPR55 is reported to act to exert its effects, and it comments on the difficulties in reaching firm conclusions when using GPR55 ligands of poor specificity. Understanding the role of GPR55 in energy homeostasis may provide a novel target for therapeutic intervention in obesity and type 2 diabetes mellitus.

The endocannabinoid system: a revolving plate in neuro-immune interaction in health and disease

Studies of the last 40 years have brought to light an important physiological network, the endocannabinoid system. Endogenous and exogenous cannabinoids mediate their effects through activation of specific cannabinoid receptors. This modulatory homoeostatic system operates in the regulation of brain function and also in the periphery. The cannabinoid system has been shown to be involved in regulating the immune system. Studies examining the effect of cannabinoid-based drugs on immunity have shown that many cellular and cytokine mechanisms are modulated by these agents, thus raising the hypothesis that these compounds may be of value in the management of chronic inflammatory diseases. The special properties of endocannabinoids as neurotransmitters, their pleiotropic effects and the impact on immune function show that the endocannabinoid system represents a revolving plate of neural and immune interactions. In this paper, we outline current information on immune effects of cannabinoids in health and disease.

Anandamide induces cell death through lipid rafts in hepatic stellate cells

BACKGROUND AND AIMS

Anandamide (AEA), the most extensively studied endocannabinoid, and its putative cannabinoid receptors, CB1 and CB2, exert a variety of physiological and pharmacological effects in chronic liver diseases, such as hyperdynamic circulation. Anandamide selectively blocks proliferation and induces cell death in hepatic stellate cells (HSC), the key cell type of liver fibrogenesis. However, its precise molecular mechanism in rat HSC has not been fully elucidated.

METHODS

CB1 and CB2 mRNA transcriptions were evaluated by reverse transcription polymerase chain reaction; CB1, CB2, phosphoinositide 3-kinases (PI3K) and protein kinase B (PKB) protein expressions were investigated by western blot and/or immunofluorescence. Cell death was detected by Annexin V-PE/7AAD flow cytometry, lipid raft content by confocal microscopic analysis, cell viability by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, nuclear morphological changes by Hoechst 33258 fluorochrome, and inflammatory cytokines interleukin (IL)-2 and IL-6, and tumor necrosis factor-alpha (TNF-alpha) by enzyme-linked immunosorbent assay.

RESULTS

CB1 and CB2 receptors were detectable in HSC. AEA caused HSC growth inhibition in a concentration-dependent manner. Furthermore, a high concentration of AEA (20 micromol/L) triggered potent cell death-induced necrosis but not apoptosis. None of these effects were blocked by CB1 or CB2 receptor antagonist, but by methyl-beta-cyclodextrin (MCD; 10 mmol/L), a cholesterol depletory agent. AEA significantly inhibited PI3K/PKB activity, and increased IL-2, IL-6 and TNF-alpha release.

CONCLUSION

These results demonstrated that AEA induced HSC necrosis through lipid rafts: a possible role of PI3K/PKB signaling pathway downregulation and inflammatory factors production. Cholesterol depletion abolished the effects of AEA on HSC necrosis.

Cannabinoids and the immune system: an overview

Cannabinoids can influence the immune network. Data on the impact of exogenous cannabinoid ligands on immune function serve not only to understand how the endocannabinoid system modulates immune phenomena associated with infection or inflammation, but also to identify therapeutic targets for immune diseases. Cannabinoids can modulate immune reactions in the periphery but also in the brain, influence T cell subset balance and cytokine expression and play a role in the balance between neuroinflammation and neurodegeneration. Immune cells can synthesize endocannabinoids and also be influenced by cannabinoid analogues. Cannabinoid receptors show different expression on immune cells depending on activation status and stimuli. The complexity of relation between cannabinoid ligands of various classes and cannabinoid receptors brought the need to refine the simple conceptual frame of agonist-antagonists and offered potential implications for understanding interactions in pathological conditions. The immune influence of cannabinoid ligands is not fully elucidated. However, aspects of their immunomodulatory effects provide the basis for a context-dependent targeted therapeutic approach, thus leading to the possibility for the use of cannabinoids in the treatment of inflammatory disease.

Endocannabinoids and immune regulation

Cannabinoid pharmacology has made important advances in recent years after the discovery of the cannabinoid receptors. These discoveries have added to our understanding of exogenous and endogenous cannabinoid signaling along with exploring the various pathways of their biosynthesis, molecular structure, inactivation, and anatomical distribution of their receptors throughout the body. The endocannabinoid system is involved in immunoregulation and neuroprotection. In this article, we have reviewed the possible mechanisms of the regulation of the immune response by endocannabinoids which include modulation of immune response in different cell types, effect on cytokine network, induction of apoptosis in immune cells and downregulation of innate and adaptive immune response. Studies from our laboratory have suggested that administration of endocannabinoids or use of inhibitors of enzymes that breakdown the endocannabinoids, leads to immunosuppression and recovery from immune-mediated injury to organs such as the liver. Thus, manipulation of endocannabinoids in vivo may constitute a novel treatment modality against inflammatory disorders.

The endocannabinoid system in cancer-potential therapeutic target?

Endogenous arachidonic acid metabolites with properties similar to compounds of Cannabis sativa Linnaeus, the so-called endocannabinoids, have effects on various types of cancer. Although endocannabinoids and synthetic cannabinoids may have pro-proliferative effects, predominantly inhibitory effects on tumor growth, angiogenesis, migration and metastasis have been described. Remarkably, these effects may be selective for the cancer cells, while normal cells and tissues are spared. Such apparent tumor cell selectivity makes the endocannabinoid system an attractive potential target for cancer therapy. In this review we discuss various means by which the endocannabinoid system may be targeted in cancer and the current knowledge considering the regulation of the endocannabinoid system in malignancy.

5-Lipoxygenase and anandamide hydrolase (FAAH) mediate the antitumor activity of cannabidiol, a non-psychoactive cannabinoid

It has been recently reported that cannabidiol (CBD), a non-psychoactive cannabinoid, is able to kill glioma cells, both in vivo and in vitro, independently of cannabinoid receptor stimulation. However, the underlying biochemical mechanisms were not clarified. In the present study, we performed biochemical analysis of the effect of CBD both in vivo, by using glioma tumor tissues excised from nude mice, and in vitro, by using U87 glioma cells. In vivo exposure of tumor tissues to CBD significantly decreased the activity and content of 5-lipoxygenase (LOX, by approximately 40%), and of its end product leukotriene B4 ( approximately 25%). In contrast cyclooxygenase (COX)-2 activity and content, and the amount of its end product prostaglandin E2, were not affected by CBD. In addition, in vivo treatment with CBD markedly stimulated ( approximately 175%) the activity of fatty acid amide hydrolase (FAAH), the main anandamide-degrading enzyme, while decreasing anandamide content ( approximately 30%) and binding to CB1 cannabinoid receptors ( approximately 25%). In vitro pre-treatment of U87 glioma cells with MK-886, a specific 5-LOX inhibitor, significantly enhanced the antimitotic effect of CBD, whereas the pre-treatment with indomethacin (pan-COX inhibitor) or celecoxib (COX-2 inhibitor), did not alter CBD effect. The study of the endocannabinoid system revealed that CBD was able to induce a concentration-dependent increase of FAAH activity in U87 cells. Moreover, a significantly reduced growth rate was observed in FAAH-over-expressing U87 cells, compared to wild-type controls. In conclusion, the present investigation indicates that CBD exerts its antitumoral effects through modulation of the LOX pathway and of the endocannabinoid system, suggesting a possible interaction of these routes in the control of tumor growth.

Anandamide and Delta9-tetrahydrocannabinol directly inhibit cells of the immune system via CB2 receptors

This study shows that two cannabinoids, Delta(9)-tetrahydrocannabinol (THC) and anandamide, induce dose-related immunosuppression in both the primary and secondary in vitro plaque-forming cell assays of antibody formation. The immunosuppression induced by both compounds could be blocked by SR144528, an antagonist specific for the CB(2) receptor, but not by SR141716, a CB(1) antagonist. These studies are novel in that they show that both anandamide and THC are active in the nanomolar to picomolar (for anandamide) range in these assays of immune function, and that both mediate their effects directly on cells of the immune system through the CB(2) receptor.

Temporal variation in CB2R levels following T lymphocyte activation: Evidence that cannabinoids modulate CXCL12-induced chemotaxis

Cannabinoids have long been proposed to affect the immune system, especially as one of the cannabinoid receptors, the cannabinoid receptor-2 (CB(2)R) has been found almost exclusively on immune cells. Here, using human in vitro activated peripheral blood-derived T lymphocytes we investigated the long-term changes in cannabinoid receptor protein expression following cellular activation and the effects of cannabinoids on migration. We report that resting T lymphocytes do not detectably express either the cannabinoid receptor-1 (CB(1)R) or CB(2)R at the protein level. However, CB(2)R protein expression is upregulated in a biphasic manner in T lymphocytes following activation by superantigen. The cannabinoids 2-AG and JWH-133 were found to elicit activation of downstream biochemical effectors (as assessed by the phosphorylation of the ERK1/2 MAP kinases). Neither 2-AG nor JWH-133 induced chemotaxis in day 5 activated T lymphocytes, when receptor expression was at its highest. Interestingly, both 2-AG and JWH-133 inhibited CXCL12-induced chemotaxis, suggesting a modulatory role for cannabinoids in activated T lymphocytes.

Rimonabant: Just an Antiobesity Drug? Current Evidence on Its Pleiotropic Effects

The advent of the highly selective cannabinoid receptor (CB1) antagonist, rimonabant (SR141716; Acomplia) can revolutionize the ability of the clinicians to manage obesity. Large-scale clinical trials have demonstrated that rimonabant therapy can reduce obesity. Although, the precise mechanisms of action of rimonabant have to be further dissected, it is emerging, from both preclinical and clinical research, that not only is rimonabant an antiobesity drug, but also its pleiotropic functions affect a broad range of diseases, from obesity-related comorbidities to drug dependence and cancer. Here we review recent data from the literature and discuss the full pharmacological potential of this drug.

Anandamide protects from low serum-induced apoptosis via its degradation to ethanolamine

Anandamide (AEA) is a lipid molecule belonging to the family of endocannabinoids. Various studies report neuroprotective activity of AEA against toxic insults, such as ischemic conditions and excitotoxicity, whereas some show that AEA has pro-apoptotic effects. Here we have shown that AEA confers a protective activity in N18TG2 murine neuroblastoma cells subjected to low serum-induced apoptosis. We have demonstrated that the protection from apoptosis by AEA is not mediated via the CB1 receptor, the CB2 receptor, or the vanilloid receptor 1. Interestingly, breakdown of AEA by fatty acid amide hydrolase is required for the protective effect of AEA. Furthermore, the ethanolamine (EA) generated in this reaction is the metabolite responsible for the protective response. The elevation in the levels of reactive oxygen species during low serum-induced apoptosis is not affected by AEA or EA. On the other hand, AEA and EA reduce caspase 3/7 activity, and AEA attenuates the cleavage of PARP-1. Taken together, our results demonstrate a role for AEA and EA in the protection against low serum-induced apoptosis.

Characterization of Peripheral Human Cannabinoid Receptor (hCB2) Expression and Pharmacology Using a Novel Radioligand, [35S]Sch225336

Studies to characterize the endogenous expression and pharmacology of peripheral human cannabinoid receptor (hCB2) have been hampered by the dearth of authentic anti-hCB2 antibodies and the lack of radioligands with CB2 selectivity. We recently described a novel CB2 inverse agonist, N-[1(S)-[4-[[4-methoxy-2-[(4methoxyphenyl)sulfonyl] phenyl]sulfonyl] phenyl]ethyl]methane-sulfonamide (Sch225336), that binds hCB2 with high affinity and excellent selectivity versus hCB1. The precursor primary amine of Sch225336 was prepared and reacted directly with [(35)S]mesyl chloride (synthesized from commercially obtained [(35)S]methane sulfonic acid) to generate [(35)S]Sch225336. [(35)S]Sch225336 has high specific activity (>1,400 Ci/mmol) and affinity for hCB2 (65 pm). Using [(35)S]Sch225336, we assayed hemopoietic cells and cell lines to quantitate the expression and pharmacology of hCB2. Lastly, we used [(35)S]Sch225336 for detailed autoradiographic analysis of CB2 in lymphoid tissues. Based on these data, we conclude that [(35)S]Sch225336 represents a unique radioligand for the study of CB2 endogenously expressed in blood cells and tissues.

Cannabinoids attenuate norepinephrine-induced melatonin biosynthesis in the rat pineal gland by reducing arylalkylamine N-acetyltransferase activity without involvement of cannabinoid receptors

Cannabinoids modulate neuronal and neuroendocrine circuits by binding to cannabinoid receptors acting upon cAMP/Ca(2+)-mediated intracellular signaling cascades. The rat pineal represents an established model to investigate intracellular signaling processes because a well defined input, the neurotransmitter norepinephrine, is transformed via cAMP/Ca(2+)-dependent mechanisms into an easily detectable output signal, the biosynthesis of melatonin. Here we investigated the impact of cannabinoids on norepinephrine-regulated melatonin biosynthesis in the rat pineal. We demonstrated that treatment of cultured rat pineals with 9-carboxy-11-nor-delta-9-tetrahydrocannabinol (THC), cannabidiol or cannabinol significantly reduced norepinephrine-induced arylalkylamine N-acetyltransferase (AANAT) activity and melatonin biosynthesis. These effects were not mimicked by the cannabinoid receptor agonist WIN55,212-2 and were not blocked by cannabinoid 1 and 2 receptor antagonists. The cannabinoids used did not affect norepinephrine-induced increases in cAMP/Ca(2+) levels. Notably, cannabinoids were found to directly inhibit AANAT activity in lysates of the pineal gland. This effect was specific in so far as cannabinoids did not influence the activity of hydroxyindole-O-methyltransferase (HIOMT), the last enzyme in melatonin biosynthesis. Taken together, our data strongly suggest that cannabinoids inhibit AANAT activity and attenuate melatonin biosynthesis through intracellular actions without involvement of classical cannabinoid receptor-dependent signaling cascades.

Cannabinoid receptor ligands mediate growth inhibition and cell death in mantle cell lymphoma

We have earlier reported overexpression of the central and peripheral cannabinoid receptors CB1 and CB2 in mantle cell lymphoma (MCL), a B cell non-Hodgkin lymphoma. In this study, treatment with cannabinoid receptor ligands caused a decrease in viability of MCL cells, while control cells lacking CB1 were not affected. Interestingly, equipotent doses of the CB1 antagonist SR141716A and the CB1/CB2 agonist anandamide inflicted additive negative effects on viability. Moreover, treatment with the CB1/CB2 agonist Win-55,212-2 caused a decrease in long-term growth of MCL cells in culture. Induction of apoptosis, as measured by FACS/Annexin V-FITC, contributed to the growth suppressive effect of Win-55,212-2. Our data suggest that cannabinoid receptors may be considered as potential therapeutic targets in MCL.

Effects on the immune system

Marijuana and other exogenous cannabinoids alter immune function and decrease host resistance to microbial infections in experimental animal models and in vitro. Two modes of action by which delta9-tetrahydrocannabinol (THC) and other cannabinoids affect immune responses have been proposed. First, cannabinoids may signal through the cannabinoid receptors CB1 and CB2. Second, at sites of direct exposure to high concentrations of cannabinoids, such as the lung, membrane perturbation may be involved. In addition, endogenous cannabinoids or endocannabinoids have been identified and have been proposed as native modulators of immune functions through cannabinoid receptors. Exogenously introduced cannabinoids may disturb this homoeostatic immune balance. A mode by which cannabinoids may affect immune responses and host resistance maybe by perturbing the balance of T helper (Th)1 pro-inflammatory versus Th2 anti-inflammatory cytokines. While marijuana and various cannabinoids have been documented to alter immune functions in vitro and in experimental animals, no controlled longitudinal epidemiological studies have yet definitively correlated immunosuppressive effects with increased incidence of infections or immune disorders in humans. However, cannabinoids by virtue of their immunomodulatory properties have the potential to serve as therapeutic agents for ablation of untoward immune responses.

A role for the anandamide membrane transporter in TRPV1-mediated neurosecretion from trigeminal sensory neurons

Many n-acylethanolamines utilize the anandamide membrane transporter (AMT) to gain facilitated access to the intracellular compartment, hence, we hypothesized that this mechanism might be important for anandamide (AEA)- and N-arachidonoyl-dopamine (NADA)-evoked CGRP release from cultured trigeminal ganglion (TG) neurons. Using [14C]AEA we demonstrated that TG neurons transported AEA in a FAAH- and AMT-inhibitable fashion. Although TRPV1-positive TG neurons were found to express fatty acid amide hydrolase, the application of FAAH inhibitors had no effect on AEA-evoked CGRP release. In contrast, application of the AMT inhibitors OMDM-2 or VDM-11 significantly reduced the potency and efficacy of AEA-, NADA- and capsaicin-evoked CGRP release. Moreover OMDM-2 (IC50 values ranging from 6.4-9.6 microM) and VDM-11 (IC50 values ranging from 5.3-11 microM) inhibited CGRP release evoked by EC80 concentrations of AEA, NADA and CAP and these values were consistent with IC50s obtained for inhibition of uptake. OMDM-2 had no effect on CGRP release per se while VDM-11 evoked CGRP release on its own (EC50 approximately 35 microM) in a CPZ-insensitive, but ruthenium red (RR)-sensitive fashion. This is the first demonstration that TG sensory neurons possess an AMT-like mechanism suggesting that this mechanism is important for the pharmacological action of AEA and NADA at native TRPV1 channels.

Role of cannabinoid receptors in inhibiting macrophage costimulatory activity

Delta(9)-tetrahydrocannabinol (THC) inhibits several immunologic functions of macrophages. THC's impact on peritoneal macrophages to deliver costimulatory signals to a helper T cell hybridoma was investigated by T cell interleukin-2 production stimulated with immobilized anti-CD3 antibody. The drug's inhibition of costimulatory activity depended on the macrophages. THC decreased costimulation provided by peritoneal cells elicited with polystyrene beads and thioglycollate, but the drug had no influence with macrophages elicited with thioglycollate alone. Bead administration induced CB2 mRNA expression in macrophages, while CB1 mRNA was not detected. Although inhibition was associated with functional heat-stable antigen, a costimulatory molecule, on macrophages, THC exposure did not alter cell surface heat-stable antigen expression. Inhibition by THC and anti-heat-stable antigen antibody was not additive suggesting the inhibitory mechanisms may overlap. Cannabinoid suppression was stereoselective; low affinity synthetic isomer CP56,667 did not diminish the T cell response. CB1-selective antagonist SR141716A completely reversed, and CB2-selective antagonist SR144528 partially blocked THC's inhibition. Both antagonists appeared to behave as inverse agonists in a receptor-selective manner. Although T cells expressed a low level of CB2 mRNA, neither THC nor SR141716A affected T cell activation in a system independent of macrophages, while SR144528 was inhibitory. High affinity synthetic agonist CP55,940, but not partial agonist THC, impaired costimulation by macrophages from mice lacking CB2 receptor. Although CB1 mRNA was not detected in CB2 null macrophages, CP55,940 reversed the inverse agonist activity of SR141716A. Hence, CB2 and possibly another receptor subtype may be involved in mediating cannabinoid suppression of macrophage costimulation.

R(+)-methanandamide-induced cyclooxygenase-2 expression in H4 human neuroglioma cells: possible involvement of membrane lipid rafts

Cannabinoids induce the expression of the cyclooxygenase-2 (COX-2) isoenzyme in H4 human neuroglioma cells via a pathway independent of cannabinoid- or vanilloid receptor activation. The underlying mechanism was recently shown to involve increased synthesis of ceramide, which in turn leads to activation of p38 and p42/44 mitogen-activated protein kinases (MAPKs). The present study investigates a possible contribution of membrane lipid rafts to cannabinoid-induced COX-2 expression. To address this issue, we tested the influence of methyl-beta-cyclodextrin (MCD), a membrane cholesterol depletor, on COX-2 expression by the endocannabinoid analogue R(+)-methanandamide (R(+)-MA). Incubation of H4 cells with MCD was associated with a loss of lipid raft integrity and a substantial inhibition of R(+)-MA-induced COX-2 expression and subsequent formation of prostaglandin E2. Moreover, MCD was shown to suppress signal transduction steps upstream to COX-2 induction by R(+)-MA. Accordingly, the cholesterol depletor suppressed R(+)-MA-induced formation of ceramide as well as phosphorylation of p38 and p42/44 MAPKs. Together, our results suggest that R(+)-MA induces COX-2 expression in human neuroglioma cells via a pathway linked to lipid raft microdomains.

Arachidonylethanolamide Induces Apoptosis of Human Glioma Cells through Vanilloid Receptor-1

The anti-tumor properties of cannabinoids have recently been evidenced, mainly with delta9-tetrahydrocannabinol (THC). However, the clinical application of this drug is limited by possible undesirable side effects due to a broad expression of cannabinoid receptors (CB1 and CB2). An attractive field of research therefore is to identify molecules with more selective tumor targeting. This is particularly important for malignant gliomas, considering their poor prognosis and their location in the brain. Here we investigated whether the most potent endogenous cannabinoid, arachidonylethanolamide (AEA), could be a candidate. We observed that AEA induced apoptosis in long-term and recently established glioma cell lines via aberrantly expressed vanilloid receptor-1 (VR1). In contrast with their role in THC-mediated death, both CB1 and CB2 partially protected glioma against AEA-induced apoptosis. These data show that the selective targeting of VR1 by AEA or more stable analogues is an attractive research area for the treatment of glioma.

Membrane cholesterol but not putative receptors mediates anandamide-induced hepatocyte apoptosis

The endogenous cannabinoid anandamide, a lipid mediator, induces various physiologic events such as vascular relaxation, inhibition of gap-junctions formation, tumor proliferation, neurologic analgesia, and apoptosis. Although increased concentration of anandamide in plasma has been implicated in pathophysiologic states including endotoxin-induced hypotension, the effects of anandamide on hepatocytes still remain unclear. In this study, we present evidence that plasma anandamide concentration is highly increased in severe hepatitis and cirrhosis patients. In addition, concentrations of anandamide within the pathophysiologic range potently induced apoptosis of hepatoma cell line (Hep G2) and primary hepatocytes, suggesting a possible link between increased anandamide level and hepatocyte damage. Anandamide-induced cell death was preceded by G0/G1 cell-cycle arrest, activation of proapoptotic signaling (i.e., p38 MAPK and JNK), and inhibition of antiapoptotic signaling (i.e., PKB/Akt) pathways. Moreover, anandamide increased susceptibility to oxidative stress-induced hepatocyte damage. In this context, methyl-beta-cyclodextrin (MCD), a membrane cholesterol depletor, or mevastatin, an HMG-CoA reductase inhibitor, or N-acetyl cysteine, an antioxidant, potently inhibited the anandamide-induced proapoptotic events and cell death, whereas putative cannabinoid receptor antagonists did not exhibit an inhibitory effect on anandamide-induced cell death. Furthermore, binding assay using polymyxin beads revealed that anandamide could interact with cholesterol. In conclusion, our data suggest that cholesterol present in the cell membrane determines the fate of hepatocytes exposed to anandamide, possibly functioning as an anandamide receptor.

The endocannabinoid system, anandamide and the regulation of mammalian cell apoptosis

Endocannabinoids are a new class of lipid mediators, which include amides, esters and ethers of long-chain polyunsaturated fatty acids. Anandamide (N-arachidonoylethanolamine; AEA) and 2-arachidonoylglycerol (2-AG) are the main endogenous agonists of cannabinoid receptors able to mimic several pharmacological effects of Delta-9-tetrahydrocannabinol, the active principle of Cannabis sativa preparations like hashish and marijuana. The pathways leading to the synthesis and release of AEA and 2-AG from neuronal and non-neuronal cells are still rather uncertain. Instead, it is known that the activity of AEA is limited by cellular uptake through a specific membrane transporter, followed by intracellular degradation by a fatty acid amide hydrolase. Together with AEA and congeners these proteins form the 'endocannabinoid system'. Here, the involvement of AEA in apoptosis and the underlying signal transduction pathways will be reviewed, along with the metabolic routes and the molecular targets of this endocannabinoid. Also, recent findings on the apoptotic potential of AEA for neuronal cell differentiation and brain development will be discussed.

ASK1-p38 MAPK/JNK signaling cascade mediates anandamide-induced PC12 cell death

Anandamide is a neuroimmunoregulatory molecule that triggers apoptosis in a number of cell types including PC12 cells. Here, we investigated the molecular mechanisms underlying anandamide-induced cell death in PC12 cells. Anandamide treatment resulted in the activation of p38 mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase (JNK), and p44/42 MAPK in apoptosing cells. A selective p38 MAPK inhibitor, SB203580, or dn-JNK, JNK1(A-F) or SAPKbeta(K-R), blocked anandamide-induced cell death, whereas a specific inhibitor of MEK-1/2, U0126, had no effect, indicating that activation of p38 MAPK and JNK is critical in anandamide-induced cell death. An important role for apoptosis signal-regulating kinase 1 (ASK1) in this event was also demonstrated by the inhibition of p38 MAPK/JNK activation and death in cells overexpressing dn-ASK1, ASK1 (K709M). Conversely, the constitutively active ASK1, ASK1DeltaN, caused prolonged p38 MAPK/JNK activation and increased cell death. These indicate that ASK1 mediates anandamide-induced cell death via p38 MAPK and JNK activation. Here, we also found that activation of p38 MAPK/JNK is accompanied by cytochrome c release from the mitochondria and caspase activation (which can be inhibited by SB203580), suggesting that anandamide triggers a mitochondrial dependent apoptotic pathway. The caspase inhibitor, zVAD, and the mitochondrial pore opening inhibitor, cyclosporine A, blocked anandamide-induced cell death but not p38 MAPK/JNK activation, suggesting that activation of these kinases may occur upstream of mitochondrial associated events.

Cell signaling by endocannabinoids and their congeners: questions of selectivity and other challenges

The major endocannabinoids, anandamide (N-arachidonoylethanolamide, 20:4n-6 N-acylethanolamine) and 2-arachidonoylglycerol (2-AG) are structurally and functionally similar, but they are produced by different metabolic pathways and their levels must therefore be regulated by different mechanisms. Both endocannabinoids are accompanied by cannabinoid receptor-inactive, saturated and mono- or di-unsaturated congeners which can influence their metabolism and function. Here we review published data on the presence and production of anandamide and 2-AG and their congeners in mammalian cells and discuss this information in terms of their proposed signaling functions.

Arachidonic acid release and prostaglandin F(2alpha) formation induced by anandamide and capsaicin in PC12 cells

Anandamide, an endogenous agonist of cannabinoid receptors, activates various signal transduction pathways. Anandamide also activates vanilloid VR(1) receptor, which was a nonselective cation channel with high Ca(2+) permeability and had sensitivity to capsaicin, a pungent principle in hot pepper. The effects of anandamide and capsaicin on arachidonic acid metabolism in neuronal cells have not been well established. We examined the effects of anandamide and capsaicin on arachidonic acid release in rat pheochromocytoma PC12 cells. Both agents stimulated [3H]arachidonic acid release in a concentration-dependent manner from the prelabeled PC12 cells even in the absence of extracellular CaCl(2). The effect of anandamide was neither mimicked by an agonist nor inhibited by an antagonist for cannabinoid receptors. The effects of anandamide and capsaicin were inhibited by phospholipase A(2) inhibitors, but not by an antagonist for vanilloid VR(1) receptor. In PC12 cells preincubated with anandamide or capsaicin, [3H]arachidonic acid release was marked and both agents were no more effective. Co-addition of anandamide or capsaicin synergistically enhanced [3H]arachidonic acid release by mastoparan in the absence of CaCl(2). Anandamide stimulated prostaglandin F(2alpha) formation. These findings suggest that anandamide and capsaicin stimulated arachidonic acid metabolism in cannabinoid receptors- and vanilloid VR(1) receptor-independent manner in PC12 cells. The possible mechanisms are also discussed.

Cannabinoids and cell fate

Cannabinoids recently have been shown to control the cell survival/death decision. Thus, cannabinoids induce growth arrest or apoptosis in a number of transformed neural and non-neural cells in culture. In addition, cannabinoid administration induces regression of malignant gliomas in rodents by a mechanism that may involve sustained ceramide generation and extracellular signal-regulated kinase activation. In contrast, most of the experimental evidence indicates that cannabinoids may protect normal neurons from toxic insults, such as glutamatergic overstimulation, ischaemia, and oxidative damage. Regarding immune cells, low doses of cannabinoids may enhance proliferation, whereas high doses of cannabinoids usually induce growth arrest or apoptosis. The potential therapeutic applications of these findings are discussed.

Cloning and molecular characterization of the rat CB2 cannabinoid receptor

The rat peripheral cannabinoid receptor (rCB2) was cloned from a Sprague-Dawley rat spleen cDNA library and when translated, encodes a protein of 410 amino acids. Alignment of rCB2 with mouse (mCB2) and human (hCB2) peripheral cannabinoid receptors reveals a high degree of homology except in the carboxy terminus where rCB2 is 50 and 63 residues longer than hCB2 and mCB2, respectively. PCR screening and sequencing of rat genomic DNA showed that rCB2 is encoded by three exons interrupted by two introns, one of which is polymorphic and contains a 209 base pair B2 (SINE) element. By Northern hybridization and ribonuclease protection assay (RPA), rCB2 mRNA was detected in rat spleen, testis, thymus and lung but not in rat brain, heart, kidney or liver. Like hCB2 and mCB2 receptors, rCB2 activates mitogen-activated protein kinase when it is stably expressed in Chinese Hamster Ovary (CHO) cells. The importance of the carboxy terminus in regulating CB2 receptor desensitization and internalization is well-established. Thus, the profound differences identified in this region of the CB2 receptor between species mandates caution when extrapolating experimental results from non-human models to the effects of chronic CB2 receptor stimulation in humans.

Hematopoietic cells expressing the peripheral cannabinoid receptor migrate in response to the endocannabinoid 2-arachidonoylglycerol

Cb2 is a novel protooncogene encoding the peripheral cannabinoid receptor. Previous studies demonstrated that 2 distinct noncoding first exons exist: exon-1A and exon-1B, which both splice to protein-coding exon-2. We demonstrate that in retrovirally induced murine myeloid leukemia cells with proviral insertion in Cb2, exon-1B/exon-2 Cb2 messenger RNA levels have been increased, resulting in high receptor numbers. In myeloid leukemia cells without virus insertion in this locus, low levels of only exon-1A/exon-2 Cb2 transcripts were present and receptors could not be detected. To elucidate the function of Cb2 in myeloid leukemia cells, a set of in vitro experiments was carried out using 32D/G-CSF-R (granulocyte colony-stimulating factor receptor) cells transfected with exon-1B/exon-2 Cb2 complementary DNA and a myeloid cell line carrying a virus insertion in Cb2 (ie, NFS 78). We demonstrate that a major function of the Cb2 receptor is stimulation of migration as determined in a transwell assay. Exposure of Cb2-expressing cells to different cannabinoids showed that the true ligand for Cb2 is 2-arachidonoylglycerol (2-AG), which may act as chemoattractant and as a chemokinetic agent. Furthermore, we observed a significant synergistic activity between 2-AG and interleukin-3 or G-CSF, suggesting cross-talk between the different receptor systems. Radioactive-ligand binding studies revealed significant numbers of Cb2 receptors in normal spleen. Transwell experiments carried out with normal mouse spleen cells showed 2-AG-induced migration of B220-, CD19-, immunoglobulin M-, and immunoglobulin D-expressing B lymphocytes. Our study demonstrates that a major function of Cb2 receptor expressed on myeloid leukemia cells or normal splenocytes is stimulation of migration.

Anandamide receptors

Anandamide (N -arachidonoyl-ethanolamine, AEA) was the first endogenous ligand of cannabinoid receptors to be discovered. Yet, since early studies, AEA appeared to exhibit also some effects that were not mediated by cannabinoid CB(1) or CB(2) receptors. Indeed, AEA exerts some behavioral actions also in mice with genetically disrupted CB(1) receptors, whereas in vitro it is usually a partial agonist at these receptors and a weak activator of CB(2) receptors. Nevertheless, several pharmacological effects of AEA are mediated by CB(1) receptors, which, by being coupled to G-proteins, can be seen as AEA "metabotropic" receptors. Furthermore, at least two different, and as yet uncharacterized, G-protein-coupled AEA receptors have been suggested to exist in the brain and vascular endothelium, respectively. AEA is also capable of directly inhibiting ion currents mediated by L-type Ca(2+) channels and TASK-1 K(+) channels. However, to date the only reasonably well characterized, non-cannabinoid site of action for AEA is the vanilloid receptor type 1 (VR1), a non-selective cation channel gated also by capsaicin, protons and heat. VR1 might be considered as an AEA "ionotropic" receptor and, under certain conditions, mediates effects ranging from vasodilation, broncho-constriction, smooth muscle tone modulation and nociception to stimulation of hippocampal pair-pulse depression, inhibition of tumor cell growth and induction of apoptosis.

Biosynthesis and degradation of anandamide and 2-arachidonoylglycerol and their possible physiological significance

N -arachidonoylethanolamine (anandamide) was the first endogenous cannabinoid receptor ligand to be discovered. Dual synthetic pathways for anandamide have been proposed. One is the formation from free arachidonic acid and ethanolamine, and the other is the formation from N -arachidonoyl phosphatidylethanolamine (PE) through the action of a phosphodiesterase. These pathways, however, do not appear to be able to generate a large amount of anandamide, at least under physiological conditions. The generation of anandamide from free arachidonic acid and ethanolamine is catalyzed by a degrading enzyme anandamide amidohydrolase/fatty acid amide hydrolase operating in reverse and requires large amounts of substrates. As for the second pathway, arachidonic acids esterified at the 1-position of glycerophospholipids, which are mostly esterified at the 2-position, are utilized for the formation of N -arachidonoyl PE, a stored precursor form of anandamide. In fact, the actual levels of anandamide in various tissues are generally low except in a few cases. 2-Arachidonoylglycerol (2-AG) was the second endogenous cannabinoid receptor ligand to be discovered. 2-AG is a degradation product of arachidonic acid-containing glycerophospholipids such as inositol phospholipids. Several investigators have demonstrated that 2-AG is produced in a variety of tissues and cells upon stimulation. 2-AG acts as a full agonist at the cannabinoid receptors (CB1 and CB2). Evidence is gradually accumulating and indicates that 2-AG is the most efficacious endogenous natural ligand for the cannabinoid receptors. In this review, we summarize the tissue levels, biosynthesis, degradation and possible physiological significance of two endogenous cannabimimetic molecules, anandamide and 2-AG.

Endocannabinoids in the immune system and cancer

The present review focuses on the role of the endogenous cannabinoid system in the modulation of immune response and control of cancer cell proliferation. The involvement of cannabinoid receptors, endogenous ligands and enzymes for their biosynthesis and degradation, as well as of cannabinoid receptor-independent events is discussed. The picture arising from the recent literature appears very complex, indicating that the effects elicited by the stimulation of the endocannabinoid system are strictly dependent on the specific compounds and cell types considered. Both the endocannabinoid anandamide and its congener palmitoylethanolamide, exert a negative action in the onset of a variety of parameters of the immune response. However, 2-arachidonoylglycerol appears to be the true endogenous ligand for peripheral cannabinoid receptors, although its action as an immunomodulatory molecule requires further characterization. Modulation of the endocannabinoid system interferes with cancer cell proliferation either by inhibiting mitogenic autocrine/paracrine loops or by directly inducing apoptosis; however, the proapoptotic effect of anandamide is not shared by other endocannabinoids and suggests the involvement of non-cannabinoid receptors, namely the VR1 class of vanilloid receptors. In conclusion, further investigations are needed to elucidate the function of endocannabinoids as immunosuppressant and antiproliferative/cytotoxic agents. The experimental evidence reviewed in this article argues in favor of the therapeutic potential of these compounds in immune disorders and cancer.

Cannabinoid-receptor-independent cell signalling by N-acylethanolamines

Anandamide and other polyunsaturated N-acylethanolamines (NAEs) exert biological activity by binding to cannabinoid receptors. These receptors are linked to G(i/o) proteins and their activation leads to extracellular-signal-regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase (MAP kinase) activation, inhibition of cAMP-dependent signalling and complex changes in the expression of various genes. Saturated and monounsaturated NAEs cannot bind to cannabinoid receptors and may thus mediate cell signalling through other targets. Here we report that both saturated/monounsaturated NAEs and anandamide (20:4(n-6) NAE) stimulate cannabinoid-receptor-independent ERK phosphorylation and activator protein-1 (AP-1)-dependent transcriptional activity in mouse epidermal JB6 cells. Using a clone of JB6 P(+) cells with an AP-1 collagen-luciferase reporter construct, we found that 16:0, 18:1(n-9), 18:1(n-7), 18:2(n-6) and 20:4(n-6) NAEs stimulated AP-1-dependent transcriptional activity up to 2-fold, with maximal stimulation at approx. 10-15 microM. Higher NAE concentrations had toxic effects mediated by alterations in mitochondrial energy metabolism. The AP-1 stimulation appeared to be mediated by ERK but not JNK or p38 signalling pathways, because all NAEs stimulated ERK1/ERK2 phosphorylation without having any effect on JNK or p38 kinases. Also, overexpression of dominant negative ERK1/ERK2 kinases completely abolished NAE-induced AP-1 activation. In contrast with 18:1(n-9) NAE and anandamide, the cannabinoid receptor agonist WIN 55,212-2 did not stimulate AP-1 activity and inhibited ERK phosphorylation. The NAE-mediated effects were not attenuated by pertussis toxin and appeared to be NAE-specific, as a close structural analogue, oleyl alcohol, failed to induce ERK phosphorylation. The data support our hypothesis that the major saturated and monounsaturated NAEs are signalling molecules acting through intracellular targets without participation of cannabinoid receptors.

R(+)-methanandamide induces cyclooxygenase-2 expression in human neuroglioma cells via a non-cannabinoid receptor-mediated mechanism

Cannabinoids affect prostaglandin (PG) formation in the central nervous system through as yet unidentified mechanisms. Using H4 human neuroglioma cells, the present study investigates the effect of R(+)-methanandamide (metabolically stable analogue of the endocannabinoid anandamide) on the expression of the cyclooxygenase-2 (COX-2) enzyme. Incubation of cells with R(+)-methanandamide was accompanied by concentration-dependent increases in COX-2 mRNA, COX-2 protein, and COX-2-dependent PGE(2) synthesis. Moreover, treatment of cells with R(+)-methanandamide in the presence of interleukin-1beta led to an overadditive induction of COX-2 expression. The stimulatory effect of R(+)-methanandamide on COX-2 expression was mimicked by the structurally unrelated cannabinoid Delta(9)-tetrahydrocannabinol. Stimulation of both COX-2 mRNA expression and subsequent PGE(2) synthesis by R(+)-methanandamide was not affected by the selective CB(1) receptor antagonist AM-251 or the G(i/o) protein inactivator pertussis toxin. Enhancement of COX-2 expression by R(+)-methanandamide was paralleled by time-dependent phosphorylations of p38 mitogen-activated protein kinase (MAPK) and p42/44 MAPK. Consistent with the activation of both kinases, R(+)-methanandamide-induced COX-2 mRNA expression and PGE(2) formation were abrogated in the presence of specific inhibitors of p38 MAPK (SB203580) and p42/44 MAPK activation (PD98059). Together, our results demonstrate that R(+)-methanandamide induces COX-2 expression in human neuroglioma cells via a cannabinoid receptor-independent mechanism involving activation of the MAPK pathway. In conclusion, induction of COX-2 expression may represent a novel mechanism by which cannabinoids mediate PG-dependent effects within the central nervous system.

Effects of cannabinoids on LPS-stimulated inflammatory mediator release from macrophages: involvement of eicosanoids

Delta(9)-Tetrahydrocannabinol (Delta(9)-THC) is the major psychoactive component of marijuana and elicits pharmacological actions via cannabinoid receptors. Anandamide (AEA) and 2-arachidonoyl-glycerol (2-AG) are endogenous ligands for cannabinoid receptors, which because of their structural similarities to arachidonic acid (AA), AEA, and 2-AG could serve as substrates for lipoxygenases and cyclooxygenases (COXs) that metabolize polyunsaturated fatty acids to potent bioactive molecules. In this study, we have compared the effects of Delta(9)-THC, AEA, 2-AG, and another cannabinoid agonist, indomethacin morpholinylamide (IMMA), on lipopolysaccharide (LPS)-induced NO, IL-6, and PGE(2) release from J774 macrophages. Delta(9)-THC, IMMA, and AEA diminish LPS-induced NO and IL-6 production in a concentration-dependent manner. 2-AG inhibits the production of IL-6 but slightly increases iNOS-dependent NO production. Delta(9)-THC and IMMA also inhibit LPS-induced PGE(2) production and COX-2 induction, while AEA and 2-AG have no effects. These discrepant results of 2-AG on iNOS and COX-2 induction might be due to its bioactive metabolites, AA and PGE(2), whose incubation cause the potentiation of both iNOS and COX-2 induction. On the contrary, the AEA metabolite, PGE(2)-ethanolamide, influences neither the LPS-induced NO nor IL-6 production. Taken together, direct cannabinoid receptor activation leads to anti-inflammatory action via inhibition of macrophage function. The endogenous cannabinoid, 2-AG, also serves as a substrate for COX-catalyzing PGE(2) production, which in turn modulates the action of CB2.

Receptor-independent effects of natural cannabinoids in rat peritoneal mast cells in vitro

Cannabinoids can activate CB(1) and CB(2) receptors. Since a CB(2) mRNA has been described in rat peritoneal mast cells (RPMC), we investigated a series of cannabinoids and derivatives for their capacity to stimulate RPMC. Effects of natural cannabinoids Delta(9)-tetrahydrocannabinol (Delta(9)-THC), Delta(8)-THC, endocannabinoids (anandamide, palmitoylethanolamide) and related compounds (N-decanoyl-, N-lauroyl-, N-myristoyl-, N-stearoyl- and N-oleoyl-ethanolamines; N-palmitoyl derivatives (-butylamine, -cyclohexylamine, -isopropylamine); and N-palmitoyl, O-palmitoylethanolamine), and synthetic cannabinoids including WIN 55,212-2, SR141716A and SR144528 were assessed for their capacity to induce histamine release or prime RPMC stimulated by compound 48/80. Only Delta(9)-THC and Delta(8)-THC could induce non-lytic, energy- and concentration-dependent histamine releases from RPMC (respective EC(50) values: 23.5+/-1.2; 53.4+/-20.6 microM, and maxima: 71.2+/-5.5; 55.7+/-2.7% of the total RPMC histamine content). These were not blocked by CB(1) (SR141716A) or CB(2) (SR144528) antagonists, but reduced by pertussis toxin (100 ng/ml). Endocannabinoids and analogues did neither induce histamine secretion, nor prime secretion induced by compound 48/80 (0.2 microg/ml). Delta(9)-THC and Delta(8)-THC induced in vitro histamine secretion from RPMC through CB receptor-independent interactions, partly involving G(i/o) protein activation.

Cannabinoid receptors and the regulation of immune response

Cannabinoid research underwent a tremendous increase during the last 10 years. This progress was made possible by the discovery of cannabinoid receptors and the endogenous ligands for these receptors. Cannabinoid research is developing in two major directions: neurobehavioral properties of cannabinoids and the impact of cannabinoids on the immune system. Recent studies characterized the cannabinoid-induced response as a very complex process because of the involvement of multiple signalling pathways linked to cannabinoid receptors or effects elicited by cannabinoids without receptor participation. The objective of this review is to present this complexity as it applies to immune response. The functional properties of cannabinoid receptors, signalling pathways linked to cannabinoid receptors and the modulation of immune response by cannabinoid receptor ligands are discussed. Special attention is given to 'endocannabinoids' as immunomodulatory molecules.

Pathways and mechanisms of N-acylethanolamine biosynthesis: can anandamide be generated selectively?

Long-chain N-acylethanolamines (NAEs) and their precursors, N-acylethanolamine phospholipids, are ubiquitous trace constituents of animal and human cells, tissues and body fluids. Their cellular levels appear to be tightly regulated and they accumulate as the result of injury. Saturated and monounsaturated congeners which represent the vast majority of cellular NAEs can have cytoprotective effects while polyunsaturated NAEs, especially 20:4n-6 NAE (anandamide), elicit physiological effects by binding to and activating cannabinoid receptors. It is the purpose of this article to review published data on the pathways and mechanisms of NAE biosynthesis in mammals and to evaluate this information for its physiological significance. The generation and turnover of NAE via N-acyl PE through the transacylation-phosphodiesterase pathway may represent a novel cannabinoid receptor-independent signalling system, analogous to and possibly related to ceramide-mediated cell signalling.

Cellular signal transduction by anandamide and 2-arachidonoylglycerol

Anandamide (arachidonylethanolamide) and 2-arachidonoylglycerol mediate many of their actions via either CB(1) or CB(2) cannabinoid receptor subtypes. These agonist-receptor interactions result in activation of G proteins, particularly those of the G(i/o) family. Signal transduction pathways that are regulated by these G proteins include inhibition of adenylyl cyclase, regulation of ion currents (inhibition of voltage-gated L, N and P/Q Ca(2+)-currents; activation of K(+) currents); activation of focal adhesion kinase (FAK), mitogen activated protein kinase (MAPK) and induction of immediate early genes; and stimulation of nitric oxide synthase (NOS). Other effects of anandamide and/or 2-arachidonoylglycerol that are not mediated via cannabinoid receptors include inhibition of L-type Ca(2+) channels, stimulation of VR(1) vanilloid receptors, transient changes in intracellular Ca(2+), and disruption of gap junction function. Cardiovascular regulation by anandamide appears to occur by a variety of receptor-mediated and non-receptor-mediated mechanisms. This review will describe and evaluate each of these signal transduction pathways and mechanisms.

Anandamide induces apoptosis in human cells via vanilloid receptors. Evidence for a protective role of cannabinoid receptors

The endocannabinoid anandamide (AEA) is shown to induce apoptotic bodies formation and DNA fragmentation, hallmarks of programmed cell death, in human neuroblastoma CHP100 and lymphoma U937 cells. RNA and protein synthesis inhibitors like actinomycin D and cycloheximide reduced to one-fifth the number of apoptotic bodies induced by AEA, whereas the AEA transporter inhibitor AM404 or the AEA hydrolase inhibitor ATFMK significantly increased the number of dying cells. Furthermore, specific antagonists of cannabinoid or vanilloid receptors potentiated or inhibited cell death induced by AEA, respectively. Other endocannabinoids such as 2-arachidonoylglycerol, linoleoylethanolamide, oleoylethanolamide, and palmitoylethanolamide did not promote cell death under the same experimental conditions. The formation of apoptotic bodies induced by AEA was paralleled by increases in intracellular calcium (3-fold over the controls), mitochondrial uncoupling (6-fold), and cytochrome c release (3-fold). The intracellular calcium chelator EGTA-AM reduced the number of apoptotic bodies to 40% of the controls, and electrotransferred anti-cytochrome c monoclonal antibodies fully prevented apoptosis induced by AEA. Moreover, 5-lipoxygenase inhibitors 5,8,11,14-eicosatetraynoic acid and MK886, cyclooxygenase inhibitor indomethacin, caspase-3 and caspase-9 inhibitors Z-DEVD-FMK and Z-LEHD-FMK, but not nitric oxide synthase inhibitor Nomega-nitro-l-arginine methyl ester, significantly reduced the cell death-inducing effect of AEA. The data presented indicate a protective role of cannabinoid receptors against apoptosis induced by AEA via vanilloid receptors.

The cannabinoid system and cytokine network

Many advances have been made in the last few years concerning our understanding of the receptors and ligands composing the cannabinoid system. Likewise, the science surrounding cytokine biology has advanced enabling us to measure these proteins more precisely as well as understand and interpret the meaning of changes in their levels. Scientists wishing to study the health consequences of smoking marijuana as well as understand the possible role of endogenous cannabimimetic ligands in immune regulation have continued to study the influence of these substances on the regulation and development of the cytokine network. Research has shown that two major cannabinoid receptor subtypes exist and that subtype 1 (CB1) is expressed primarily in the brain whereas subtype 2 (CB2) is expressed primarily in the periphery. A variety of ligands for these receptors based on the cannabinoid structure have been synthesized and studied as well as low affinity compounds, noncannabinoid ligands, and endogenous ligands derived from fatty acid eicosanoids. Highly selective receptor antagonists have also been introduced and studied. Synthetic, low affinity ligands such as (+)-HU-211 and DMH-11C have been shown to cause anti-inflammatory effects possibly through inhibiting the production and action of TNF-alpha and other acute phase cytokines. In addition, suppression of TNF and other cytokines such as GM-CSF, IL-6, IFNgamma, and IL-12 has also been seen following exposure to high affinity and psychoactive ligands such as marijuana and THC. However, some of these ligands have also been shown to increase rather than decrease interleukins such as IL-1, IL-4, IL-10, and IL-6, cytokines such as TNF-alpha, and chemokines such as IL-8, MIP-1, and RANTES. The endogenous ligand, anandamide, has been shown in culture to either suppress the proliferation response to prolactin or enhance the response to cytokines such as IL-3 and IL-6. This eicosanoid has also been shown to increase the production of interleukins and other cytokines. Cannabinoid receptors have been shown to be involved in some but not all of these effects. It is clear that psychoactive and nonpsychoactive compounds have demonstrated effects in vivo and in vitro on the production and function of a variety of cytokines. Depending upon the model system, these effects are often conflicting, and the involvement of cannabinoid receptors is unclear. However, enough evidence exists to suggest that the cannabinoid system significantly impacts the functioning of the cytokine network, and this association may provide clues to the mechanisms of certain immune diseases and form the basis for new immunotherapies.

Inhibition of nitric oxide production in RAW264.7 macrophages by cannabinoids and palmitoylethanolamide

We have investigated the inhibition of lipopolysaccharide stimulated nitric oxide production in RAW264.7 macrophages by the cannabinoids and the putative cannabinoid CB(2)-like receptor ligand, palmitoylethanolamide. (R)-(+)-[2, 3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo-[1,2,3-de]-1, 4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate ((+)-WIN55212) and, to a lesser extent (-)-cis-3-[2-hydroxy-4-(1, 1-dimethylheptyl)phenyl]-trans-4-(3-hydroxy-propyl)cyclohexan++ +-1-ol (CP55940), significantly inhibited lipopolysaccharide stimulated nitric oxide production. The level of inhibition was found to be dependent on the concentration of lipopolysaccharide used to induce nitric oxide production. Palmitoylethanolamide significantly inhibited nitric oxide production induced by lipopolysaccharide. The inhibition of nitric oxide production by (+)-WIN55212 but not palmitoylethanolamide was significantly attenuated in the presence of the cannabinoid CB(2) receptor antagonist, N-[(1S)-endo-1,3, 3-trimethyl bicyclo [2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazo le- 3-carboxamide (SR144528). (+)-WIN55212 produced a pertussis toxin-sensitive parallel rightward shift in the log concentration-response curve for lipopolysaccharide, causing a fivefold increase in the EC(50) value for lipopolysaccharide with no change in the E(max) value. (-)-WIN55212 had no effect on the log concentration-response curve for lipopolysaccharide. Palmitoylethanolamide did not produce a rightward shift in the lipopolysaccharide concentration-response curve. However, it did produce a pertussis toxin-insensitive reduction in the E(max) value. The results suggest that the inhibition of lipopolysaccharide mediated nitric oxide release by (+)-WIN55212 in murine macrophages is mediated by cannabinoid CB(2) receptors. In contrast, the inhibition by palmitoylethanolamide does not appear to be mediated by cannabinoid receptors.

Functional CB1 cannabinoid receptors in human vascular endothelial cells

Cannabinoid CB1 receptor mRNA was detected using reverse transcription-polymerase chain reaction (RT-PCR) in endothelial cells from human aorta and hepatic artery and in the ECV304 cell line derived from human umbilical vein endothelial cells. CB1 receptor-binding sites were detected by the high-affinity antagonist radioligand [(125)I]AM-251. In ECV304 cells, both the highly potent synthetic cannabinoid agonist HU-210 and the endogenous ligand anandamide induce activation of mitogen-activated protein (MAP) kinase, and the effect of HU-210 was completely blocked, whereas the effect of anandamide was partially inhibited by SR141716A, a selective CB1 receptor antagonist. Transfection of ECV304 cells with CB1 receptor antisense, but not sense, oligonucleotides caused the same pattern of inhibition as SR141716A. This provides more definitive evidence for the involvement of CB1 receptors in MAP kinase activation and suggests that anandamide may also activate MAP kinase via an additional, CB1 receptor-independent, SR141716A-resistant mechanism. The MAP kinase activation by anandamide in ECV304 cells requires genistein-sensitive tyrosine kinases and protein kinase C (PKC), and anandamide also activates p38 kinase and c-Jun kinase. These findings indicate that CB1 receptors located in human vascular endothelium are functionally coupled to the MAP kinase cascade. Activation of protein kinase cascades by anandamide may be involved in the modulation of endothelial cell growth and proliferation.

The CB1 cannabinoid receptor is coupled to the activation of protein kinase B/Akt

Cannabinoids exert most of their effects in the central nervous system through the CB(1) cannabinoid receptor. This G-protein-coupled receptor has been shown to be functionally coupled to inhibition of adenylate cyclase, modulation of ion channels and activation of extracellular-signal-regulated kinase. Using Chinese hamster ovary cells stably transfected with the CB(1) receptor cDNA we show here that Delta(9)-tetrahydrocannabinol (THC), the major active component of marijuana, induces the activation of protein kinase B/Akt (PKB). This effect of THC was also exerted by the endogenous cannabinoid anandamide and the synthetic cannabinoids CP-55940 and HU-210, and was prevented by the selective CB(1) antagonist SR141716. Pertussis toxin and wortmannin blocked the CB(1) receptor-evoked activation of PKB, pointing to the sequential involvement of a G(i)/G(o) protein and phosphoinositide 3'-kinase. The functionality of the cannabinoid-induced stimulation of PKB was proved by the increased phosphorylation of glycogen synthase kinase-3 serine 21 observed in cannabinoid-treated cells and its prevention by SR141716 and wortmannin. Cannabinoids activated PKB in the human astrocytoma cell line U373 MG, which expresses the CB(1) receptor, but not in the human promyelocytic cell line HL-60, which expresses the CB(2) receptor. Data indicate that activation of PKB may be responsible for some of the effects of cannabinoids in cells expressing the CB(1) receptor.

Delta9-tetrahydrocannabinol induces apoptosis in human prostate PC-3 cells via a receptor-independent mechanism

The effect of delta9-tetrahydrocannabinol (THC), the major psycho-active component of marijuana, in human prostate cancer cells PC-3 was investigated. THC caused apoptosis in a dose-dependent manner. Morphological and biochemical changes induced by THC in prostate PC-3 cells shared the characteristics of an apoptotic phenomenon. First, loss of plasma membrane asymmetry determined by fluorescent anexin V binding. Second, presence of apoptotic bodies and nuclear fragmentation observed by DNA staining with 4',6-diamino-2-phenylindole (DAPI). Third, presence of typical 'ladder-patterned' DNA fragmentation. Central cannabinoid receptor expression was observed in PC-3 cells by immunofluorescence studies. However, several results indicated that the apoptotic effect was cannabinoid receptor-independent, such as lack of an effect of the potent cannabinoid agonist WIN 55,212-2, inability of cannabinoid antagonist AM 251 to prevent cellular death caused by THC and absence of an effect of pertussis toxin pre-treatment.

Effects of cannabinoids on energy metabolism

The present review summarizes the recent work carried out by our group on the link between signal transduction pathways and metabolic regulation systems as affected by cannabinoids. In cells such as astrocytes and lymphocytes, which express cannabinoid receptors, physiologically relevant doses of cannabinoids induce a remarkable metabolic stimulation as determined e.g. by enhanced glucose utilization. Studies performed in astrocytes show that the cannabinoid-evoked stimulation of glucose metabolism is independent of adenylyl cyclase inhibition, and seems to rely on the cascade CB1 cannabinoid receptor --> Sphingomyelin breakdown --> Ceramide --> Raf-1 --> Mitogen-activated protein kinase (MAPK) --> Glucose utilization. A role for phosphoinositide 3'-kinase in the stimulation of glucose utilization by cannabinoids is also put forward. In addition, ceramide generated upon CB1 cannabinoid receptor activation may enhance ketone body production by astrocytes independently of MAPK. Anandamide has also been shown to exert metabolic effects in hepatocytes, cells that do not express cannabinoid receptors. The biological role of cannabinoids as modulators of metabolism is as yet unclear.