Characterisation of the cannabinoid receptor system in synovial tissue and fluid in patients with osteoarthritis and rheumatoid arthritis
- Denise Richardson†1,
- Richard G Pearson†2,
- Nisha Kurian3,
- M Liaque Latif3,
- Michael J Garle3,
- David A Barrett1,
- David A Kendall3,
- Brigitte E Scammell2,
- Alison J Reeve4 and
- Victoria Chapman3Email author
© Richardson et al.; licensee BioMed Central Ltd. 2008
Received: 18 September 2007
Accepted: 16 April 2008
Published: 16 April 2008
Cannabis-based medicines have a number of therapeutic indications, including anti-inflammatory and analgesic effects. The endocannabinoid receptor system, including the cannabinoid receptor 1 (CB1) and receptor 2 (CB2) and the endocannabinoids, are implicated in a wide range of physiological and pathophysiological processes. Pre-clinical and clinical studies have demonstrated that cannabis-based drugs have therapeutic potential in inflammatory diseases, including rheumatoid arthritis (RA) and multiple sclerosis. The aim of this study was to determine whether the key elements of the endocannabinoid signalling system, which produces immunosuppression and analgesia, are expressed in the synovia of patients with osteoarthritis (OA) or RA.
Thirty-two OA and 13 RA patients undergoing total knee arthroplasty were included in this study. Clinical staging was conducted from x-rays scored according to Kellgren-Lawrence and Larsen scales, and synovitis of synovial biopsies was graded. Endocannabinoid levels were quantified in synovial fluid by liquid chromatography-mass spectrometry. The expression of CB1 and CB2 protein and RNA in synovial biopsies was investigated. Functional activity of these receptors was determined with mitogen-activated protein kinase assays. To assess the impact of OA and RA on this receptor system, levels of endocannabinoids in the synovial fluid of patients and non-inflamed healthy volunteers were compared. The activity of fatty acid amide hydrolase (FAAH), the predominant catabolic endocannabinoid enzyme, was measured in synovium.
CB1 and CB2 protein and RNA were present in the synovia of OA and RA patients. Cannabinoid receptor stimulation of fibroblast-like cells from OA and RA patients produced a time-dependent phosphorylation of extracellular signal-regulated kinase (ERK)-1 and ERK-2 which was significantly blocked by the CB1 antagonist SR141716A. The endocannabinoids anandamide (AEA) and 2-arachidonyl glycerol (2-AG) were identified in the synovial fluid of OA and RA patients. However, neither AEA nor 2-AG was detected in synovial fluid from normal volunteers. FAAH was active in the synovia of OA and RA patients and was sensitive to inhibition by URB597 (3'-(aminocarbonyl) [1,1'-biphenyl]-3-yl)-cyclohexylcarbamate).
Our data predict that the cannabinoid receptor system present in the synovium may be an important therapeutic target for the treatment of pain and inflammation associated with OA and RA.
Osteoarthritis (OA) is the most common form of arthritis affecting synovial joints . The aetiology of OA is poorly understood, with mechanical, metabolic, and inflammatory causes. Inflammation and angiogenesis and their possible role in disease progression and pain are increasingly being recognised as important aetiological factors [2–5]. Rheumatoid arthritis (RA) is a systemic, autoimmune-mediated, inflammatory arthritis . Although the pathogenesis remains incompletely understood, it is characterised by severe, progressive synovitis with rapid destruction of the joint. Pro-inflammatory cytokines such as tumour necrosis factor (TNF)-α, interleukin (IL)-1, IL-6, and chemokines such as IL-8 are abundant in RA tissue, which is compensated to some degree by the increased production of anti-inflammatory cytokines such as IL-10 and transforming growth factor-β . The accepted therapeutic approach to RA is to use disease-modifying anti-rheumatic drugs at an early stage, and the recent introduction of cytokine inhibitor drugs has increased the effectiveness of treatment considerably. However, an effective remission-inducing drug has yet to be discovered, and the vast majority of RA patients are dependent on lifelong treatment in order to suppress joint damage and functional impairment . There are no proven disease-modifying OA drugs, and current non-steroidal anti-inflammatory drug (NSAID) treatments do not always provide adequate pain relief and have detrimental side effects. Thus, there is a strong rationale for the development of novel drug treatments for arthritis. This can be achieved only by an improved mechanistic understanding of the functional cellular changes associated with this disease.
The cannabinoid receptor system has been implicated in a wide range of physiological and pathophysiological processes . Recent pre-clinical and clinical studies have demonstrated that cannabis-based drugs have therapeutic potential in inflammatory diseases, including RA and multiple sclerosis . Animal studies have demonstrated that activation of cannabinoid receptors attenuates inflammation and nociceptive processing in models of cutaneous and joint inflammation [10–14]. The cannabis-based medicine Sativex (GW Pharmaceuticals plc, Salisbury, Wiltshire, UK) has been reported to produce a significant analgesic effect and to suppress disease activity in patients with RA .
Two cannabinoid receptors (CB1 and CB2), both of which are inhibitory G protein-coupled receptors, have been cloned . CB1 receptors are expressed predominantly by peripheral nerves, spinal cord, and the nervous system as well as peripheral immune cells . CB2 receptors are expressed mainly in peripheral tissue, in particular by immune cells . Activation of CB1 receptors is associated predominantly with a dampening down of neuronal excitability, whereas activation of CB2 receptors is associated with decreases in immune cell function, including attenuated cytokine release [9, 17]. A number of endocannabinoids with activity at the CB1 and CB2 cannabinoid receptors, including N-arachidonyl ethanolamide (anandamide, AEA) and 2-arachidonyl glycerol (2-AG), have been identified [18, 19]. Other structurally related endogenous fatty acid compounds such as oleoyl ethanolamide (OEA) and palmitoyl ethanolamide (PEA) have been identified in biological tissues. These compounds do not bind to cannabinoid receptors but might be involved in facilitating the actions of directly acting endocannabinoids and thus are commonly termed 'entourage' compounds due to their ability to modulate the endocannabinoid system [20, 21]. The endocannabinoids and PEA are synthesised on demand, and AEA, PEA, and OEA are metabolised predominantly by fatty acid amide hydrolase (FAAH) [22, 23].
Although the therapeutic benefits of Sativex in RA patients are significant, the mechanisms mediating these effects are unclear. Indeed, the impact of arthritis on the endocannabinoid receptor system, both in terms of receptor expression and levels of endocannabinoids and entourage compounds, is unknown. The endocannabinoid system appears to regulate bone mass by signalling via peripheral CB2 receptors in both osteoblasts and osteoclasts . In a separate study, CB1 knockout mice had significantly increased bone mineral density compared with wild-type mice and were protected from ovariectomy-induced bone loss and CB1- and CB2-selective cannabinoid receptor antagonists inhibited osteoclastogenesis in vivo . Thus, the role of the cannabinoid receptor system in bone remodelling and aspects of pathological conditions such as periarticular bony erosions in RA and subchondral bony sclerosis in OA remains unclear.
Several NSAIDs, including ibuprofen, ketorolac, indometacin, and niflumic acid, which act via the inhibition of cyclooxygenase (COX), have been shown to inhibit FAAH [26, 27]. This suggests that current treatment of inflammatory pain in OA and RA patients using NSAIDs may be targeting endocannabinoid metabolism in addition to arachidonic acid metabolism. These interactions may be of great clinical importance in terms of multiple-target drug development as synergistic actions of the COX-2 inhibitor rofecoxib and the endocannabinoid AEA have been observed in an animal model of pain .
The aim of the present study was to provide evidence of a role for the cannabinoid receptor system in OA and RA. Here, we report the presence of an active endocannabinoid system, including endocannabinoids, entourage compounds, CB1 and CB2 receptors, and FAAH, in the knee synovia of patients with end-stage OA and RA.
Materials and methods
Patient information and tissue collection
The Nottingham Regional Ethical Committee approved the study, and after informed consent synovial biopsies and fluid were sampled from patients undergoing total knee arthroplasty (TKA). All x-rays were scored according to Kellgren and Lawrence  (OA) and Larsen  (RA) scales.
The synovial fluid and biopsies were collected under tourniquet control at the onset of the TKA from 32 OA and 13 RA patients. The synovial fluid samples were centrifuged at 1,000 g for 40 minutes at 4°C, and the supernatants were retained for analysis. Samples of synovial fluid from non-inflamed normal volunteers (n = 6) were kindly provided by Michael Doherty, Academic Rheumatology, Nottingham University Hospitals.
Synovium histology and analysis
Quantification of inflammatory cytokines in synovial fluid
Cytokine profiles in synovial fluid were determined using a BD™ cytometric bead array (BD Biosciences, San Jose, CA, USA), which quantified IL-8, IL-1β, IL-6, IL-10, TNF, and IL-12p70. Analysis was performed using a Beckman Coulter Epics™ Altra® flow cytometer (Beckman Coulter, Fullerton, CA, USA) according to the manufacturer's protocol for measurements in serum or plasma.
Measurement of endocannabinoids
A lipid extraction method was used as previously described . In brief, tissue or fluid was homogenised in an ethyl acetate/hexane mixture with internal standards (0.42 nmol AEA-d8, 1.5 nmol 2-AG-d8, and 0.2 nmol heptadecanoyl ethanolamide) and left in extraction solvent for 2 hours with intermittent mixing. Repeated centrifugation and supernatant collection were then undertaken, followed by purification of samples by solid-phase extraction.
Simultaneous measurement of AEA, 2-AG, OEA, and PEA was then performed using liquid chromatography-tandem mass spectrometry. A triple quadrupole Quattro Ultima mass spectrometer (Waters Ltd, Manchester, UK) was used in electrospray-positive mode and coupled to an Agilent 1100 LC system (Agilent Technologies, Böblingen, Germany) for analysis. Analytes were chromatographically separated on a HyPurity Advance C8 column (Fisher Scientific UK., Loughborough, UK) with gradient elution. Individual compounds were then identified and quantified with multiple reaction monitoring, using (m/z [mass-to-charge] ratios of specific precursor and product ions) on the mass spectrometer.
Western blotting for measurement of cannabinoid receptor expression
Human synovium samples were homogenised in lysis buffer (20 mM Tris, 1 mM ethylene glycol tetraacetic acid [EGTA], 320 mM sucrose, 0.1% Triton X100, 1 mM sodium fluoride, and 10 mM β-glycerophosphate) containing a protease inhibitor cocktail (Roche Applied Sciences, Burgess Hill, UK). Homogenates were centrifuged at 5,000 g for 10 minutes at 4°C and the resulting supernatants were collected. Estimation of protein content was carried out using the Lowry method . Aliquots of the homogenate supernatant were diluted in Laemmli sample buffer, and proteins were separated using 10% SDS-PAGE and blotted onto nitrocellulose membranes. Anti-cannabinoid receptor 1 (Calbiochem, now part of EMD Biosciences, Inc., San Diego, CA, USA), anti-cannabinoid receptor 2 (Cayman Chemical Company, Ann Arbor, MI, USA), or anti-β-actin antibody (Sigma-Aldrich) was incubated overnight at 4°C with nitrocellulose membranes and visualisation using horseradish peroxidase-conjugated secondary antibodies (1:2,000 dilution; Dako Denmark A/S, Glostrup, Denmark), enhanced chemiluminescence detection (Amersham Biosciences, now part of GE Healthcare, Little Chalfont, Buckinghamshire, UK), and autoradiography. Data were quantified using a Bio-Rad GS 710 imaging densitometer (Bio-Rad Laboratories, Inc., Hercules, CA, USA).
Fatty acid amide hydrolase activity assay
Tissues were homogenised (50 mM Tris-HCl, 1 mM EDTA [ethylenediaminetetraacetic acid], 3 mM MgCl2, pH 7.4) and centrifuged at 500 g for 5 minutes at 4°C, and the supernatant was subsequently centrifuged for 30 minutes at 35,000 g at 4°C. The pellet obtained was re-suspended in Tris-HCl buffer, and protein content was determined by the method of Lowry.
The FAAH activity of each sample was measured by monitoring the release of [3H]-ethanolamine after incubation of homogenate with radiolabelled AEA ([3H]-AEA). Protein contents per assay were chosen on the basis of preliminary experiments using some of the samples to establish optimal conditions (~88 μg/mL). Homogenised tissue in assay buffer (116 mM NaCl, 5.4 mM KCl, 1.8 mM CaCl2, 25 mM HEPES, 1 mM NaH2PO4, 0.8 mM MgSO4, pH 7.0) was incubated at 37°C with 40 μM [3H]-AEA (American Radiolabeled Chemicals, Inc., St. Louis, MO, USA) in the presence of 1 mg/mL fatty acid-free bovine serum albumin (200 μL total assay volume), and the reaction was stopped by the addition of 0.4 mL activated charcoal (4% wt/vol in 0.5 M HCl) [33, 34]. A sample without homogenate was processed to determine the extent of non-enzymatic [3H]-AEA hydrolysis. The samples were vortexed and left to sediment for 30 minutes before centrifugation at 11,000 g for 5 minutes to pellet the charcoal and homogenate protein with un-hydrolysed [3H]-AEA. A 200-μL aliquot of the supernatant was counted for tritium content by liquid scintillation spectroscopy. For obtaining standards, an aliquot of the homogenate was incubated without [3H]-AEA and stopped with charcoal as for other samples. After centrifugation, 190 μL of supernatant was added into scintillation vials with 40 μM [3H]-AEA (equivalent to 2 nmol [3H]-AEA/mL) and activity was determined as before.
Preparation and culture of human synovial fibroblast cells
Human synovial samples from both OA and RA patients were chopped and finely digested for 2 hours at 37°C with 2 mg/mL collagenase type H (Sigma-Aldrich) in Dulbecco's modified Eagle's medium (Sigma-Aldrich) supplemented with 10% foetal calf serum (PAA Laboratories Ltd, Yeovil, UK, Pasching, Austria), 2 mM L-glutamine (Sigma-Aldrich), 50 U/mL penicillin, and 50 μg/mL streptomycin and fungizone (Sigma-Aldrich). Samples were occasionally agitated to aid digestion. At the end of the digest, the samples were pipetted up and down to disrupt the tissue and passed through a 100-μm cell strainer (BD Falcon, a trademark of BD Biosciences). The cell suspension was centrifuged at 500 g for 5 minutes at room temperature, and the pellet was re-suspended in complete media, plated into flasks, and allowed to become adherent. Media was replaced the following day to remove any non-adherent cells. Adherent cells were cultured and used between passages 3 and 12.
Immunoblotting of synovial fibroblast for mitogen-activated protein kinase activation
To analyse mitogen-activated protein kinase (MAPK) activation, synovial fibroblast-like cells were stimulated with the CB1/CB2 receptor agonist HU210 (1 μM) in the presence and absence of a 20-hour pre-incubation with pertussis toxin (PTX) for 5, 10, 20, and 40 minutes before analysis of MAPK phosphorylation (described below) to determine a maximum time-dependent effect of HU210 stimulation on MAPK phosphorylation compared with basal, unstimulated levels. In subsequent experiments, synovial fibroblast-like cells were stimulated with HU210 (0.1 μM) in the presence and absence of the CB1 antagonist SR141716A (1 μM) or CB2 antagonist SR144528 (1 μM). Cells were washed with phosphate-buffered saline (Sigma-Aldrich) and lysed (20 mM Tris, 1 mM EGTA, 320 m sucrose, 0.1% Triton X100, 1 mM sodium fluoride, and 80 mM β-glycerophosphate containing protease inhibitor cocktail). After removal of a sample for a protein assay, the homogenate was diluted in Laemmli sample buffer and heated at 95°C for 5 minutes. Equal amounts of protein from each sample were separated on 10% SDS-PAGE gels and then transferred onto nitrocellulose membranes for Western blotting. Nitrocellulose blots were incubated overnight at 4°C with an antibody that recognises the double-phosphorylated (activated) forms of both isoforms of extracellular signal-regulated kinase (ERK) (p44 ERK1 MAPK and p42 ERK2 MAPK) and p38 MAPK (New England Biolabs, Inc., Ipswich, MA, USA). Proteins were subsequently visualised using the ECL system (Amersham Life Sciences, now part of GE Healthcare). Blots were then stripped of antibodies using Restore Western Blot Stripping Buffer (Pierce, Rockford, IL, USA) according to the manufacturer's instructions. These blots were subsequently re-probed with an antibody against total ERK and p38 (New England Biolabs, Inc.). Bands were visualised as before. Data were quantified using the Bio-Rad GS 710 imaging densitometer and represented as a percentage of the unstimulated control.
Reverse transcription-polymerase chain reaction for CB1 and CB2receptors
Total RNA was isolated from cultured human synovial-like fibroblasts using TRiPure Isolation reagent (Roche Applied Sciences, Burgess Hill, UK) according to the manufacturer's instructions. As the open reading frame for CB1 and CB2 cannabinoid receptors for humans contains a single exon, the RNA used was treated with recombinant RNase-free DNase-1 (Roche) to remove any genomic DNA contamination and was purified using a standard phenol-chloroform extraction methodology. RNA was reverse-transcribed into cDNA using the Transcriptor first-strand cDNA synthesis kit (Roche Applied Sciences, Burgess Hill, UK) according to the manufacturer's instructions. Amplification of CB1 and CB2 cannabinoid receptor cDNA was achieved by using 'touchdown' polymerase chain reaction (PCR) with a progressive decrease in annealing temperatures from 60°C until touchdown at 55°C . Primers were designed using the Primer Express software package version 3.0 (Applied Biosystems, Foster City, CA, USA). Using bioinformatics, the specificities of all primers were confirmed to the desired mRNA of human samples. Primers used were as follows: CB1 forward: 5'-CTGAACTCCACCGTGAACC-3' and reverse: 5'-GTGCTCTTGATGCAGCT-5' and CB2 forward: 5'-CATCTATGCTCTACGGAGTGGAGAG-3' and reverse: 5'-TTGAGTTGTTTAAATTGGGAAGAGG-3' (MWG-Biotech AG, Ebersberg, Germany). The amplified products were separated on a 1.8% low-melting agarose gel (Invitrogen Corporation, Carlsbad, CA, USA) stained with ethidium bromide and documented using GeneSnap imaging software (Syngene, Cambridge, UK).
Statistical comparisons of levels of cytokines between normal, OA, and RA samples were performed with a non-parametric Kruskal-Wallis test. Comparisons of endocannabinoid (EC) levels between normal, OA, and RA samples were performed with a non-parametric Mann-Whitney test. Comparison of FAAH activity between OA and RA synovial tissue was performed using unpaired Student t tests. Comparisons between drug treatment groups in cultured fibroblast-like cell immunoblots were performed using one-way analysis of variance followed by Bonferroni multiple comparison post hoc test. A P value of less than 0.05 was considered a significant difference.
Summary of patient histories of osteoarthritis and rheumatoid arthritis patients included in the study
Mean age ± standard deviation, years
72 ± 7
70 ± 8
61 ± 11
NSAIDs, number (percentage)
Paracetamol, number (percentage)
Corticosteroid, number (percentage)
NSAIDs, paracetamol or corticosteroid, number (percentage)
Anti-cytokine therapy, number (percentage)
Histology of synovial biopsies and levels of cytokines
Haematoxylin and eosin histology was performed on 26 of the OA and 9 of the RA synovial biopsies to assess the degree of inflammation (Figure 1). None of the OA synovia was characterised as having a normal histological appearance, 4 exhibited mild (score = 1), 12 moderate (score = 2), and 10 severe (score = 3) synovial inflammation. In the case of the RA synovial biopsies, one sample had become fibrotic and devoid of a cellular component and therefore could not be scored. Six of the RA biopsies were described as exhibiting severe synovitis (score = 3) and two as having moderate inflammation (score = 2).
Cannabinoid receptors are expressed in human synovial tissue from osteoarthritis and rheumatoid arthritis patients
Determination of fatty acid amide hydrolase activity in human synovial tissue
Fatty acid amide hydrolase (FAAH) activity in synovial tissue from osteoarthritis (n = 5) and rheumatoid arthritis (n = 4) patients
FAAH activity (pmol/minute/mg protein)
2.8 ± 0.46
Rheumatoid arthritis synovium
1.6 ± 0.42
988 ± 84
1.28 ± 0.51
Endocannabinoid levels in synovium tissue and synovial fluid in normal, osteoarthritis, and rheumatoid arthritis samples
Effects of HU-210 on ERK1, ERK2, and p38 MAPK activation in fibroblast-like cells
The novel finding of the present study is the identification of the key components of the cannabinoid receptor system in the knee synovia of patients with end-stage OA and RA. We have demonstrated, for the first time, the presence of cannabinoid CB1 and CB2 receptor message and protein. The functional relevance of the presence of these receptors has been confirmed by pharmacological studies demonstrating cannabinoid agonist-induced phosphorylation of the downstream signalling targets (ERK1 and ERK2) in fibroblast-like cells derived from OA and RA patients. The endocannabinoids, plus associated entourage compounds and FAAH activity, were present in the synovia of both OA and RA patients. In addition, we have demonstrated that AEA and 2-AG are also present in the synovial fluid of OA and RA patients but are not detectable in synovial fluid taken from volunteers with no joint symptoms. Our data provide evidence for a functional endocannabinoid receptor system in OA and RA patients.
All synovia used in the present study were collected from RA and OA patients with end-stage disease undergoing TKA for severe pain. Histological analysis verified that the synovia were not normal. Both the OA and RA synovia exhibited either moderate or severe inflammation. Moderate or severe synovitis was classified as the intima layer being more than four cells deep, plus dense cellularity of subintimal tissue due to increased numbers of fibroblastic cells and inflammatory cells, including lymphoid aggregates [3, 44, 45]. In general, the number of lymphoid aggregates and cell depth of the synovial intima are greater, or more extensive, in RA than OA synovium [46, 47]. All of the RA and OA patients whose samples were used in this study exhibited severe disease and there were no significant differences between levels of cytokines in RA and OA samples studied. Levels of IL-6, however, were significantly higher in OA and RA samples compared with volunteers with no joint symptoms. IL-6 is an important driver of inflammation in RA  and all of the synovia, whether RA or OA, were inflamed in our study. IL-6 is also implicated in the induction of osteoclast differentiation and bone resorption , and all of our patients had bone-on-bone changes somewhere within their arthritic knees, reflecting the severity of end-stage disease requiring joint replacement surgery. Reported levels of IL-6 [50, 51] and IL-8 [51, 52] are in keeping with earlier reports in OA and RA.
Although it may be expected that levels of pro-inflammatory cytokines would be higher in RA samples, all of the patients had taken anti-arthritis medicines, which could affect levels of cytokines and possibly endocannabinoids. As a result of the different requirements for sample preparation and the amount of synovia available, not all of the synovia could be used for all of the experimental studies. Given the wide range of cytokine levels present in OA and RA samples, we have studied the cannabinoid receptor system in groups of OA and RA samples which represent a cross-section of the population in terms of levels of cytokines, ensuring that our data were not subject to bias. Due to difficulties in recruiting male RA subjects, only one was included in the study, but similarities between the extent of disease in the male and female subjects and the lack of significant difference between cytokine levels in RA and OA samples suggest that this should not confound our data.
Here, we report the presence of both the CB1 and CB2 receptors in the synovia of patients with end-stage OA and RA, suggesting that this system may play a role in these pathological conditions. Our pharmacological study demonstrating that the potent cannabinoid agonist HU210 phosphorylates ERK1 and ERK2 in fibroblast-like synovial cells in a PTX-dependent manner via the CB1 receptor lends further support to a functional role of this receptor system in OA and RA synovia. Although there was a trend toward an attenuation of the effects of HU210 by the CB2 receptor antagonist, significance was not reached. Pre-clinical studies have demonstrated that activation of CB1 receptors, both on peripheral nerves and at spinal and supraspinal sites, produces analgesic effects in models of acute and inflammatory pain [53–55]. By contrast, CB2 receptors are associated predominantly with immune cells (see Introduction). Although, in the present study, the cellular location of the cannabinoid receptors has not been identified, the demonstration that cannabinoid receptors are coupled to the MAPK signalling pathway in fibroblast-like cells prepared from OA and RA synovia indicates that these cells are a likely location for the cannabinoid receptors identified.
The two main endocannabinoids, AEA and 2-AG, were present in the synovia of OA and RA patients at levels in keeping with those previously reported in other biological tissues. The fatty acid amides PEA and OEA  were also detected in both OA and RA synovia. PEA is of particular interest since it has anti-inflammatory activity  via nuclear PPAR-α (peroxisome proliferator activating receptor-α) activation and possibly endocannabinoid entourage effects. Unfortunately, it was not possible to obtain non-diseased synovia and, therefore, a comparison of levels of ECs in normal synovium with OA and RA samples was not possible. However, we were able to compare levels of endocannabinoids in the synovial fluid, which contains immune cells that are capable of releasing endocannabinoids , of OA and RA patients compared with normal volunteers. AEA and 2-AG were present in the synovial fluid of OA and RA patients, but not in normal controls. Levels of 2-AG were significantly lower in the RA group compared with the OA group. Levels of PEA were significantly lower in the synovial fluid of OA and RA patients compared with that of non-inflamed normal volunteers. Since PEA has a well-described anti-inflammatory role, the reported lower levels of PEA in the synovial fluid of OA and RA patients may contribute to the disease process associated with these conditions. Given that AEA, PEA, and OEA are all substrates for FAAH, the opposing impact of OA/RA on levels of these compounds suggests that these changes are not due merely to alterations in FAAH-mediated metabolism and argues against an important contribution of the entourage effect. Levels of FAAH activity were comparable in OA and RA samples. Although activity of FAAH in the synovium was low relative to the liver, it was comparable to levels previously described in rat hindpaw . FAAH activity was undetectable in the presence of pharmacological blockade of FAAH by the well-characterised inhibitor of URB597 , indicating the functional relevance of this activity. On the basis of our FAAH activity data, changes in rates of synthesis or release of AEA and 2-AG, versus PEA and OEA, in OA and RA patients compared with non-inflamed normal volunteers are more likely to account for our data.
In some cases, the relative levels of endocannabinoids and related fatty acid amides in the synovial fluid did not mirror levels in the synovia of OA and RA patients. Levels of 2-AG in the synovial fluid of RA patients were significantly lower than levels in OA patients, whereas there were no differences in levels of 2-AG in the synovia of OA and RA patients. In addition, levels of PEA were non-significantly higher in RA synovium compared with OA synovium, but levels of PEA were similar in the synovial fluid of OA and RA patients. Thus, levels in the synovial fluid do not simply reflect the level of synthesis/release and catabolism of endocannabinoids and entourage compounds in the synovium. The source of the endocannabinoids present in the synovium and synovial fluid is an important consideration. Endocannabinoids are synthesised by numerous different cell types, including immune cells such as T cells and macrophages, which are the major immune cells present in OA and RA [47, 59, 60]. Since endothelial cells can synthesise AEA  and 2-AG , another possible source of endocannabinoids in the synovium is the vasculature. The role of vascular elements in the progression of arthritic disease is important, particularly since neovascularisation is one of the early changes in the synovium and it is thought that bone and cartilage destruction is closely linked to angiogenesis  and cannabinoids inhibit angiogenesis in tumours [37, 64].
In summary, cannabinoid CB1 and CB2 receptor protein and RNA and the endocannabinoids AEA and 2-AG are present in the synovia of patients with end-stage OA and RA. The presence of increased levels of AEA and 2-AG in the synovial fluid of OA and RA patients, compared with non-inflamed normal volunteers, suggests a greater functional role of the endocannabinoid receptor system in these patients. Importantly, levels of the anti-inflammatory substance PEA were higher in the synovial fluid of normal volunteers compared with OA and RA patients and, therefore, the loss of PEA may contribute to arthritic disease. Our data predict that the cannabinoid receptor system may be an important therapeutic target for the treatment of pain and inflammation associated with these conditions.
cannabinoid receptor 1
cannabinoid receptor 2
ethylene glycol tetraacetic acid
extracellular signal-regulated kinase
fatty acid amide hydrolase
mitogen-activated protein kinase
non-steroidal anti-inflammatory drug
polymerase chain reaction
total knee arthroplasty
tumour necrosis factor
- URB597 = 3'-(aminocarbonyl) [1:
This study was supported by the University of Nottingham Queen's Medical Centre Special Trustees Fund and GlaxoSmithKline (Uxbridge, Middlesex, UK). DR is supported by a Biotechnology and Biological Sciences Research Council studentship. The authors thank Michael Doherty for providing the synovial fluid specimens originating from volunteers with normal knee joints, Adrian Robins for the use of flow cytometry, Julie Swales for performing the cytometric bead array assay (Immunology, School of Molecular Medical Sciences, University of Nottingham), and Mauro Dionisi (School of Biomedical Sciences, University of Nottingham) for assistance with reverse transcription-polymerase chain reaction.
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