The inflammatory process of gout and its treatment.

Gouty arthritis is a characteristically intense acute inflammatory reaction that erupts in response to articular deposits of monosodium urate (MSU) crystals. Important recent molecular biologic advances in this field have given us a clear picture of the mechanistic basis of gouty inflammation. The innate immune inflammatory response is critically involved in the pathology of gout. Specifically, MSU crystals promote inflammation directly by stimulating cells via Toll-like receptor signaling and by providing a surface for cleavage of C5 and formation of complement membrane attack complex (C5b-9), culminating in secretion of cytokines, chemokines, and other inflammatory mediators with a dramatic influx of neutrophils into the joint. Despite the detailed mechanistic picture for gouty inflammation, there are no placebo-controlled, randomized clinical studies for any of the therapies commonly used, although comparative studies have demonstrated that many nonsteroidal anti-inflammatory drugs are equivalent to indomethacin with respect to controlling acute gouty attacks. In general, the first line of anti-inflammatory therapy for acute gout is nonsteroidal anti-inflammatory drugs, and the selective cyclo-oxygenase-2 inhibitor celecoxib can be used where appropriate. The second line of treatment is glucocorticosteroids, given systemically (oral, intravenous, or intramuscular) or intra-articularly. Alternatively, synthetic adrenocorticotropic hormone is effective, partly via induction of adrenal glucocorticosteroids and partly via rapid peripheral suppression of leukocyte activation by melatonin receptor 3 signaling. The third line of treatment is oral colchicine, which is highly effective when given early in an acute gouty attack, but it is poorly tolerated because of predictable gastrointestinal side effects

Adenosine receptors and fibrosis: a translational review

Adenosine—a purine nucleoside generated extracellularly from adenine nucleotides released by cells as a result of direct stimulation, hypoxia, trauma, or metabolic stress—is a well-known physiologic and pharmacologic agent. Recent studies demonstrate that adenosine, acting at its receptors, promotes wound healing by stimulating both angiogenesis and matrix production. Subsequently, adenosine and its receptors have also been found to promote fibrosis (excess matrix production) in the skin, lungs, and liver, but to diminish cardiac fibrosis. A commonly ingested adenosine receptor antagonist, caffeine, blocks the development of hepatic fibrosis, an effect that likely explains the epidemiologic finding that coffee drinking, in a dose-dependent fashion, reduces the likelihood of death from liver disease. Accordingly, adenosine may be a good target for therapies that prevent fibrosis of the lungs, liver, and skin

Suppression of inflammation by low-dose methotrexate is mediated by adenosine A(2A )receptor but not A(3 )receptor activation in thioglycollate-induced peritonitis

Prior studies demonstrate that adenosine, acting at one or more of its receptors, mediates the anti-inflammatory effects of methotrexate in animal models of both acute and chronic inflammation. Both adenosine A(2A )and A(3 )receptors contribute to the anti-inflammatory effects of methotrexate treatment in the air pouch model of inflammation, and the regulation of inflammation by these two receptors differs at the cellular level. Because different factors may regulate inflammation at different sites we examined the effect of low-dose weekly methotrexate treatment (0.75 mg/kg/week) in a model of acute peritoneal inflammation in adenosine A(2A )receptor knockout mice and A(3 )receptor knockout mice and their wild-type littermates. Following intraperitoneal injection of thioglycollate there was no significant difference in the number or type of leukocytes, tumor necrosis factor alpha (TNF-α) and IL-10 levels that accumulated in the thioglycollate-induced peritoneal exudates in adenosine A(2A )knockout mice or wild-type control mice. In contrast, there were more leukocytes, TNF-α and IL-10 in the exudates of the adenosine A(3 )receptor-deficient mice. Low-dose, weekly methotrexate treatment increased the adenosine concentration in the peritoneal exudates of all mice studied, and reduced the leukocyte accumulation in the wild-type mice and A(3 )receptor knockout mice but not in the A(2A )receptor knockout mice. Methotrexate reduced exudate levels of TNF-α in the wild-type mice and A(3 )receptor knockout mice but not the A(2A )receptor knockout mice. More strikingly, IL-10, a critical regulator of peritoneal inflammation, was increased in the methotrexate-treated wild-type mice and A(3 )knockout mice but decreased in the A(2A )knockout mice. Dexamethasone, an agent that suppresses inflammation by a different mechanism, was similarly effective in wild-type mice, A(2A )mice and A(3 )knockout mice. These findings provide further evidence that adenosine is a potent regulator of inflammation that mediates the anti-inflammatory effects of methotrexate. Moreover, these data provide strong evidence that the anti-inflammatory effects of methotrexate and adenosine are mediated by different receptors in different inflammatory loci, an observation that may explain why inflammatory diseases of some organs but not of other organs respond to methotrexate therapy

Adenosine A2A Receptor Ligation Inhibits Osteoclast Formation

Adenosine is generated in increased concentrations at sites of injury/hypoxia and mediates a variety of physiological and pharmacological effects via G protein–coupled receptors (A1, A2A, A2B, and A3). Because all adenosine receptors are expressed on osteoclasts, we determined the role of A2A receptor in the regulation of osteoclast differentiation. Differentiation and bone resorption were studied as the macrophage colony-stimulating factor-1–receptor activator of NF-κB ligand formation of multinucleated tartrate-resistant acid phosphatase (TRAP)–positive cells from primary murine bone marrow–derived precursors. A2A receptor and osteoclast marker expression levels were studied by RT-PCR. Cytokine secretion was assayed by enzyme-linked immunosorbent assay. In vivo examination of A2A knockout (KO)/control bones was determined by TRAP staining, micro–computed tomography, and electron microscopy. The A2A receptor agonist, CGS21680, inhibited osteoclast differentiation and function (half maximal inhibitory concentration, 50 nmol/L), increased the percentage of immature osteoclast precursors, and decreased IL-1β and tumor necrosis factor-α secretion, an effect that was reversed by the A2A antagonist, ZM241385. Cathepsin K and osteopontin mRNA expression increased in control and ZM241385-pretreated osteoclasts, and this was blocked by CGS21680. Micro–computed tomography of A2AKO mouse femurs showed a significantly decreased bone volume/trabecular bone volume ratio, decreased trabecular number, and increased trabecular space. A2AKO femurs showed an increased TRAP-positive osteoclast. Electron microscopy in A2AKO femurs showed marked osteoclast membrane folding and increased bone resorption. Thus, adenosine, acting via the A2A receptor, inhibits macrophage colony-stimulating factor-1–receptor activator of NF-κB ligand–stimulated osteoclast differentiation and may regulate bone turnover under conditions in which adenosine levels are elevated

Adenosine A 2A Receptor Occupancy Stimulates Collagen Expression by Hepatic Stellate Cells via Pathways Involving Protein Kinase A, Src, and Extracellular Signal-Regulated Kinases 1/2 Signaling Cascade or p38 Mitogen-Activated Protein Kinase Signaling Pat

ABSTRACT Prior studies indicate that adenosine and the adenosine A 2A receptor play a role in hepatic fibrosis by a mechanism that has been proposed to involve direct stimulation of hepatic stellate cells (HSCs). The objective of this study was to determine whether primary hepatic stellate cells produce collagen in response to adenosine (via activation of adenosine A 2A receptors) and to further determine the signaling mechanisms involved in adenosine A 2A receptor-mediated promotion of collagen production. Cultured primary HSCs increase their collagen production after stimulation of the adenosine A 2A receptor in a dose-dependent fashion. Likewise, LX-2 cells, a human HSC line, increases expression of procollagen ␣I and procollagen ␣III mRNA and their translational proteins, collagen type I and type III, in response to pharmacological stimulation of adenosine A 2A receptors. Based on the use of pharmacological inhibitors of signal transduction, adenosine A 2A receptor-mediated stimulation of procollagen ␣I mRNA and collagen type I collagen expression were regulated by signal transduction involving protein kinase A, src, and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (erk), but surprisingly, adenosine A 2A receptor-mediated stimulation of procollagen ␣III mRNA and collagen type III protein expression depend on the activation of p38 mitogen-activated protein kinase (MAPK), findings confirmed by small interfering RNA-mediated knockdown of src, erk1, erk2, and p38 MAPK. These results indicate that adenosine A 2A receptors signal for increased collagen production by multiple signaling pathways. These results provide strong evidence in support of the hypothesis that adenosine receptors promote hepatic fibrosis, at least in part, via direct stimulation of collagen expression and that signaling for collagen production proceeds via multiple pathways

Methotrexate Prevents Wear Particle-induced Inflammatory Osteolysis in Mice via Activation of Adenosine A2A Receptor

Objective. Adenosine, acting at the A2A receptor, mediates the antiinflammatory effects of methotrexate (MTX) in models of inflammation. We previously reported that A2A receptor ligation diminishes wear particle–driven osteolysis. The aim of this study was to investigate whether MTX treatment could prevent bone resorption caused by inflammatory osteolysis. Methods. C57BL/6 mice (6–8 weeks old) received intraperitoneal injections of 1 mg/kg MTX (n 10) or 0.9% saline (n 5), starting 2 weeks prior to surgical implantation of 3 mg of wear particles (ultrahigh molecular weight polyethylene [UHMWPE] particles). The MTX-treated mice received daily injections of vehicle or ZM241385 at the surgical site until they were killed, 14 days later. XenoLight RediJect Bone Probe was injected intravenously, and fluorescence analysis of the calvaria using an IVIS imaging system was performed to assess bone formation. Micro–computed tomography (micro- CT) and immunostaining for osteoclast and osteoblast markers were performed. Results. Implantation of wear particles induced bone pitting and thinning, as shown by micro-CT. MTX treatment markedly reduced osteolysis, and this effect was abrogated by treatment with the A2A receptor antagonist ZM241385. Implantation of UHMWPE reduced new bone formation, and MTX treatment restored new bone formation, an effect that was completely reversed by treatment with ZM241385. Histologic examination of particle-exposed calvarias demonstrated that MTX prevented accumulation of an inflammatory infiltrate at the site of particle implantation, increased the number of osteoblasts, and reduced the number of osteoclasts at the site of inflammation, an effect that was reversed by treatment with ZM241385. Conclusion. MTX reduces inflammatory osteolysis indirectly via stimulation of A2A receptor and may represent a novel approach to enhance orthopedic implant survival, delaying or eliminating the need for revision arthroplasty surgery.NI