The effects of caffeine on bone mineral density and fracture risk.

Caffeine is a regular part of the diet of many adults (coffee, tea, soft drinks, and energy drinks). Multiple molecular effects of caffeine suggest that it may promote bone loss. Given the extensive consumption of caffeine worldwide, any impact of caffeine consumption on bone strength and/or density would have large population health implications. The most well-established pharmacological effect of caffeine is non-specific antagonism of adenosine receptors. Adenosine regulates bone metabolism in a complex manner, with in vitro studies suggesting that direct stimulation of adenosine A2A and A2B receptors induces bone formation by activating osteoblasts and suppressing osteoclast differentiation and function. Thus, competitive inhibition of adenosine A2 receptors by caffeine may inhibit bone formation and promote bone resorption. However, antagonism of adenosine A1 receptors may have opposing effects. Caffeine has also been suggested to affect bone through derangement of calcium metabolism, alteration of vitamin D responses, and other mechanisms. In clinical and population-based studies, the impact of caffeine consumption on bone metabolism offers a mixed picture, with some but not all studies suggesting a potential link between caffeine intake and reduced bone mineral density or increased fracture risk. Differences in methodology, selected populations, and duration/timing of the studies may account for study outcome discrepancies. The in vitro effects of caffeine on cells involved in bone metabolism suggest that caffeine intake may promote osteoporosis, and some but not all clinical studies support a modest adverse caffeine impact. Herein, we describe the basic biology of caffeine as it pertains to bone, review the clinical literature to date, and consider the implications of the current data on clinical practice and future studies.

Adenosine A1 receptor regulates osteoclast formation by altering TRAF6/TAK1 signaling.

Adenosine is an endogenous nucleoside that modulates many physiological processes through four receptor subtypes (A(1), A(2a), A(2b), A(3)). Previous work from our laboratory has uncovered a critical role for adenosine A(1) receptor (A(1) R) in osteoclastogenesis both in vivo and in vitro. Our current work focuses on understanding the details of how A(1) R modulates the receptor activator of NF-κB ligand (RANKL)-induced signaling in osteoclastogenesis. Osteoclasts were generated from mouse bone marrow precursors in the presence of RANKL and macrophage-colony stimulating factor. A pharmacological antagonist of A(1) R (DPCPX) inhibited RANKL-induced osteoclast differentiation, including osteoclast-specific genes (Acp5, MMP9, ß(3) Integrin, α(v) Integrin, and CTSK) and osteoclast-specific transcription factors such as c-fos and nuclear factor of activated T cells cytoplasmic 1 (NFATc1) expression in a dose-dependent manner. DPCPX also inhibited RANKL-induced activation of NF-κB and JNK/c-Jun but had little effect on other mitogen-activated protein kinases (p38 and Erk). Finally, immunoprecipitation analysis showed that blockade of A(1)R resulted in disruption of the association of tumor necrosis factor receptor-associated factor 6 (TRAF6) and transforming growth factor-ß-activated kinase 1 (TAK1), a signaling event that is important for activation of NF-κB and JNK, suggesting the participation of adenosine/A(1)R in early signaling of RANKL. Collectively, these data demonstrated an important role of adenosine, through A(1)R in RANKL-induced osteoclastogenesis.

Adenosine in inflammatory joint diseases.

Inflammatory joint diseases are a group of heterogeneous disorders with a variety of different etiologies and disease manifestations. However, there are features that are common to all of them: first, the recruitment of various inflammatory cell types that are attracted to involved tissues over the course of the disease process. Second, the treatments used in many of these diseases are commonly medications that suppress or alter immune function. The demonstration that adenosine has endogenous anti-inflammatory functions and that some of the most commonly used anti-rheumatic medications exert their therapeutic effects through stimulation of adenosine release suggest an important role for purinergic signaling in inflammatory rheumatic disorders.

Adenosine A2A receptors in diffuse dermal fibrosis: pathogenic role in human dermal fibroblasts and in a murine model of scleroderma.

OBJECTIVE: Adenosine regulates inflammation and tissue repair, and adenosine A2A receptors promote wound healing by stimulating collagen matrix production. We therefore examined whether adenosine A2A receptors contribute to the pathogenesis of dermal fibrosis. METHODS: Collagen production by primary human dermal fibroblasts was analyzed by real-time polymerase chain reaction, 14C-proline incorporation, and Sircol assay. Intracellular signaling for dermal collagen production was investigated using inhibitors of MEK-1 and by demonstration of ERK phosphorylation. In vivo effects were studied in a bleomycin-induced dermal fibrosis model using adenosine A2A receptor-deficient wild-type littermate mice, C57BL/6 mice, and mice treated with adenosine A2A receptor antagonist. Morphometric features and levels of hydroxyproline were determined as measures of dermal fibrosis. RESULTS: Adenosine A2A receptor occupancy promoted collagen production by primary human dermal fibroblasts, which was blocked by adenosine A2A, but not A1 or A2B, receptor antagonism. Adenosine A2A receptor ligation stimulated ERK phosphorylation, and A2A receptor-mediated collagen production by dermal fibroblasts was blocked by MEK-1 inhibitors. Adenosine A2A receptor-deficient and A2A receptor antagonist-treated mice were protected from developing bleomycin-induced dermal fibrosis. CONCLUSION: These results demonstrate that adenosine A2A receptors play an active role in the pathogenesis of dermal fibrosis and suggest a novel therapeutic target in the treatment and prevention of dermal fibrosis in diseases such as scleroderma.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation.

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Immune complexes and IFN-gamma decrease cholesterol 27-hydroxylase in human arterial endothelium and macrophages.

The enzyme cholesterol 27-hydroxylase, expressed by arterial endothelium and monocytes/macrophages, is one of the first lines of defense against the development of atherosclerosis. By catalyzing the hydroxylation of cholesterol to 27-hydroxycholesterol, which is more soluble in aqueous medium, the enzyme promotes the removal of cholesterol from the arterial wall. Prior studies have suggested that immune reactants play a role in the pathogenesis of atherosclerosis; we report here that immune reactants, IFN-gamma and immune complexes bound to C1q, but not interleukin-1 and tumor necrosis factor, diminish the expression of cholesterol 27-hydroxylase in human aortic endothelial cells, peripheral blood mononuclear cells, monocyte-derived macrophages, and the human monocytoid cell line THP-1. In addition, our studies demonstrate that immune complexes down-regulate cholesterol 27-hydroxylase only after complement fixation via interaction with the 126-kD C1qRp protein on endothelial cells and THP-1 cells. These results are consistent with the prior demonstration that IFN-gamma contributes to the pathogenesis of atherosclerosis and suggest a role for C1q receptors in the atherogenic process. Moreover, these observations suggest that one mechanism by which immune reactants contribute to the development of atherosclerosis is by down-regulating the expression of the enzymes required to maintain cholesterol homeostasis in the arterial wall.

Inflammatory cytokines regulate function and expression of adenosine A(2A) receptors in human monocytic THP-1 cells.

Adenosine, acting at its receptors, particularly A(2A) receptors, is a potent endogenous anti-inflammatory agent that modulates the functions and differentiation of inflammatory and immune cells. Because the inflammatory milieu abounds in proinflammatory cytokines, we investigated the effects of Th1-inflammatory cytokines on function and expression of adenosine A(2A) receptors in the human monocytic cell line THP-1. We found that, consistent with previous reports, adenosine and 2-[p-(2-carnonylethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS-21680), a selective A(2A) receptor agonist, suppress IL-12 production but increase IL-10 production in LPS-activated THP-1 cells. These effects were blocked by the A(2A) receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385). More importantly, the suppressive effect of adenosine and CGS-21680 on IL-12 production was significantly enhanced in cells pretreated with either IL-1 (10 U/ml) or TNF-alpha (100 U/ml) but markedly attenuated in cells pretreated with IFN-gamma (100 U/ml). Similarly, IL-1 and TNF-alpha treatment potentiated the stimulatory effect of adenosine and CGS-21680 on IL-10 production, whereas IFN-gamma treatment almost completely abolished this effect. CGS-21680 stimulated an increase in intracellular cAMP in a time- and dose-dependent manner in IL-1- and TNF-alpha-treated cells but not in control or IFN-gamma-treated cells. Both IL-1 and TNF-alpha increased A(2A) receptor mRNA and protein. In parallel with its effect on A(2A) receptor function, IFN-gamma down-regulated A(2A) receptor message and protein. Because adenosine mediates many of the antiinflammatory effects of drugs such as methotrexate, these observations suggest that local changes in the cytokine milieu may influence the therapeutic response to those drugs by altering the expression and function of adenosine receptors on inflammatory cells.

Reversal of the antiinflammatory effects of methotrexate by the nonselective adenosine receptor antagonists theophylline and caffeine: evidence that the antiinflammatory effects of methotrexate are mediated via multiple adenosine receptors in rat adjuvant arthritis.

OBJECTIVE: Weekly low-dose methotrexate (MTX) remains the mainstay of second-line therapy for rheumatoid arthritis (RA). We have previously reported that adenosine, acting at specific receptors on inflammatory cells, mediates the antiinflammatory effects of MTX in both in vitro and in vivo models of acute inflammation, but the mechanism by which MTX suppresses the chronic inflammation of arthritis remains controversial. The present study was undertaken to further investigate the means by which adenosine mediates the antiinflammatory effects of MTX. METHODS: The effects of 2 nonselective adenosine receptor antagonists, theophylline and caffeine, were examined, using the rat adjuvant arthritis model of RA. These agents were given alone and in conjunction with MTX, and arthritis severity was assessed clinically, radiologically, and histologically. Since rodent adenosine A3 receptors are not blocked by theophylline, selective A1, A2A, and A2B receptor antagonists were tested as well. RESULTS: Control animals developed severe arthritis, which was markedly attenuated by weekly treatment with MTX (0.75 mg/kg/week). Neither theophylline alone nor caffeine alone (each at 10 mg/kg/day) significantly affected the severity of the arthritis, but both agents markedly reversed the effect of MTX as measured by a severity index, hindpaw swelling, and hindpaw ankylosis. Radiographic and histologic analyses confirmed these observations. Neither A1, A2A, nor A2B receptor antagonists affected the capacity of MTX to ameliorate inflammation in adjuvant arthritis. CONCLUSION: These results provide strong evidence that adenosine mediates the antiinflammatory effects of MTX in this model of RA. Moreover, the findings suggest that abstinence from caffeine, a ubiquitous food additive and medication, may enhance the therapeutic effects of MTX in RA.

Sites of action for future therapy: an adenosine-dependent mechanism by which aspirin retains its antiinflammatory activity in cyclooxygenase-2 and NFkappaB knockout mice.

The antiinflammatory action of aspirin is generally attributed to inhibition of cyclooxygenases 1 and 2, but additional mechanisms are at work. These include inhibition of NFkappaB translocation to the nucleus and the capacity of aspirin to promote accumulation of adenosine, a potent antiinflammatory autocoid. We tested these hypotheses in the murine air pouch model of acute inflammation in wild type mice and in cyclooxygenase 2 or NFkappaB knockouts. The antiinflammatory effects of aspirin, sodium salicylate and indomethacin did not correlate with inhibition of cyclooxygenase in either group. Indeed, aspirin retained its antiinflammatory properties even in COX-2 knockouts. Similarly, aspirin was no less antiinflammatory in mice rendered deficient for NFkappaB (p105) than in wild type controls. In contrast, dexamethasone lost its antiinflammatory capacity in NFkappaB knockouts. Aspirin and sodium salicylate dramatically increased concentrations of adenosine in exudates, a property shared with methotrexate and sulfasalazine. Removal of adenosine by adenosine deaminase or specific antagonism of adenosine at A(2)receptors completely reversed the antiinflammatory effects of aspirin and sodium salicylate, but not those of dexamethasone. This adenosine-dependent, antiinflammatory effect of aspirin points to another target of drug development.

Salicylates and sulfasalazine, but not glucocorticoids, inhibit leukocyte accumulation by an adenosine-dependent mechanism that is independent of inhibition of prostaglandin synthesis and p105 of NFkappaB.

The antiinflammatory action of aspirin generally has been attributed to direct inhibition of cyclooxygenases (COX-1 and COX-2), but additional mechanisms are likely at work. These include aspirin's inhibition of NFkappaB translocation to the nucleus as well as the capacity of salicylates to uncouple oxidative phosphorylation (i.e., deplete ATP). At clinically relevant doses, salicylates cause cells to release micromolar concentrations of adenosine, which serves as an endogenous ligand for at least four different types of well-characterized receptors. Previously, we have shown that adenosine mediates the antiinflammatory effects of other potent and widely used antiinflammatory agents, methotrexate and sulfasalazine, both in vitro and in vivo. To determine in vivo whether clinically relevant levels of salicylate act via adenosine, via NFkappaB, or via the "inflammatory" cyclooxygenase COX-2, we studied acute inflammation in the generic murine air-pouch model by using wild-type mice and mice rendered deficient in either COX-2 or p105, the precursor of p50, one of the components of the multimeric transcription factor NFkappaB. Here, we show that the antiinflammatory effects of aspirin and sodium salicylate, but not glucocorticoids, are largely mediated by the antiinflammatory autacoid adenosine independently of inhibition of prostaglandin synthesis by COX-1 or COX-2 or of the presence of p105. Indeed, both inflammation and the antiinflammatory effects of aspirin and sodium salicylate were independent of the levels of prostaglandins at the inflammatory site. These experiments also provide in vivo confirmation that the antiinflammatory effects of glucocorticoids depend, in part, on the p105 component of NFkappaB.

Methotrexate and sulfasalazine promote adenosine release by a mechanism that requires ecto-5'-nucleotidase-mediated conversion of adenine nucleotides.

We and others have shown that an increased extracellular concentration of adenosine mediates the antiinflammatory effects of methotrexate and sulfasalazine both in vitro and in vivo, but the mechanism by which these drugs increase extracellular adenosine remains unclear. The results of the experiments reported here provide three distinct lines of evidence that adenosine results from the ecto-5'-nucleotidase- mediated conversion of adenine nucleotides to adenosine. First, pretreatment of a human microvascular endothelial cell line (HMEC-1) with methotrexate increases extracellular adenosine after exposure of the pretreated cells to activated neutrophils; the ecto-5'-nucleotidase inhibitor alpha, beta-methylene adenosine-5'-diphosphate (APCP) abrogates completely the increase in extracellular adenosine. Second, there is no methotrexate-mediated increase in extracellular adenosine concentration in the supernate of cells deficient in ecto-5'-nucleotidase, but there is a marked increase in extracellular adenosine concentration in the supernates of these cells after transfection and surface expression of the enzyme. Finally, as we have shown previously, adenosine mediates the antiinflammatory effects of methotrexate and sulfasalazine in the murine air pouch model of inflammation, and injection of APCP, the ecto-5'-nucleotidase inhibitor, abrogates completely the increase in adenosine and the decrement in inflammation in this in vivo model. These results not only show that ecto-5'-nucleotidase activity is a critical mediator of methotrexate- and sulfasalazine-induced antiinflammatory activity in vitro and in vivo but also indicate that adenine nucleotides, released from cells, are the source of extracellular adenosine.

Wound healing is accelerated by agonists of adenosine A2 (G alpha s-linked) receptors.

The complete healing of wounds is the final step in a highly regulated response to injury. Although many of the molecular mediators and cellular events of healing are known, their manipulation for the enhancement and acceleration of wound closure has not proven practical as yet. We and others have established that adenosine is a potent regulator of the inflammatory response, which is a component of wound healing. We now report that ligation of the G alpha s-linked adenosine receptors on the cells of an artificial wound dramatically alters the kinetics of wound closure. Excisional wound closure in normal, healthy mice was significantly accelerated by topical application of the specific A2A receptor agonist CGS-21680 (50% closure by day 2 in A2 receptor antagonists. In rats rendered diabetic (streptozotocin-induced diabetes mellitus) wound healing was impaired as compared to nondiabetic rats; CGS-21680 significantly increased the rate of wound healing in both nondiabetic and diabetic rats. Indeed, the rate of wound healing in the CGS-21680-treated diabetic rats was greater than or equal to that observed in untreated normal rats. These results appear to constitute the first evidence that a small molecule, such as an adenosine receptor agonist, accelerates wound healing in both normal animals and in animals with impaired wound healing.

The mechanism of action of methotrexate.

Because of methotrexate's well-documented efficacy in the treatment of rheumatoid arthritis, it is important that we understand the mechanism of action of this drug. There are two biochemical mechanisms by which methotrexate may modulate inflammation: (1) promotion of adenosine release and (2) inhibition of transmethylation reactions. Evidence is reviewed that favors the notion that the endogenous anti-inflammatory autocoid adenosine mediates the anti-inflammatory effects of methotrexate. This insight should aid in the design of new agents for the treatment of rheumatoid arthritis and other inflammatory diseases.

Adenosine A1 receptor promotion of multinucleated giant cell formation by human monocytes: a mechanism for methotrexate-induced nodulosis in rheumatoid arthritis.

OBJECTIVE: To determine why methotrexate (MTX) exacerbates rheumatoid nodules in some patients, despite the effective suppression of synovial inflammation. METHODS: Phorbol myristate acetate (PMA)-induced differentiation of monocytes into multinucleated giant cells was used as an in vitro model to study the effects of adenosine on nodulosis. RESULTS: MTX at 200-2,000 nM or the adenosine A1 agonist N5-cyclopentyl adenosine (CPA) (10(-12) to 10(-9) M) or the A2 antagonist 3,7-dimethyl-1-propargylxanthine markedly enhanced giant cell formation, whereas the adenosine A1 antagonist 8-cyclopentyl-dipropylxanthine completely reversed these effects. PMA, CPA, and MTX induced adenosine release by cultured monocytes at concentrations consistent with those associated with predominantly A1 effects. Furthermore, surface expression of A1 receptors was found to remain unchanged on the differentiating cells throughout the culture period. CONCLUSION: Agents that inhibit adenosine A1 receptors might be useful in the treatment of MTX-induced rheumatoid nodulosis, while still potentiating the A2-mediated antiinflammatory effects of MTX on synovitis.

Inhibition of methotrexate-induced rheumatoid nodulosis by colchicine: evidence from an in vitro model and regression in 7 of 14 patients.

Methotrexate is one of the most effective and widely used medications in the treatment of rheumatoid arthritis. One poorly understood side effect of methotrexate is increased rheumatoid nodule formation, a phenomenon which has been reported to occur in some patients despite suppression of synovial inflammation. Using an in vitro model of nodulosis, induction of monocyte differentiation into multinucleated giant cells, we previously found that methotrexate promotes this inflammatory response by a mechanism dependent on adenosine A1 receptor stimulation. In the current study, we tested the effects of an A1 signal inhibitor, the commonly available anti-inflammatory medication colchicine, and found that it markedly inhibited nodulosis in vitro as well as in seven of fourteen patients in a clinical series.