Faculty Bibliography

We and others have shown that adenosine, acting at its receptors, is a potent modulator of inflammation and angiogenesis. To better understand the regulation of adenosine receptors during these processes we studied the effects of IL-1, TNF-alpha, and IFN-gamma on expression and function of adenosine receptors and select members of their coupling G proteins in human dermal microvascular endothelial cells (HMVEC). HMVEC expressed message and protein for A(2A) and A(2B), but not A(1) or A(3) receptors. IL-1 and TNF-alpha treatment increased message and protein expression of A(2A) and A(2B) receptor. IFN-gamma treatment also increased the expression of A(2B) receptors, but decreased expression of A(2A) receptors. Resting HMVEC and IFN-gamma-treated cells showed minimal cAMP response to the selective A(2A) receptor agonist 2-[2-(4-chlorophenyl)ethoxy]adenosine (MRE0094). In contrast, MRE0094 stimulated a dose-dependent increase in cAMP levels in TNF-alpha-treated cells that was almost completely blocked by the A(2A) receptor antagonist ZM-241385 (4-[2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino] ethyl]phenol). The nonselective adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine increased cAMP levels in both TNF-alpha- and IFN-gamma-treated cells, but not control cells, and its effect was only partially reversed by ZM-241385 in TNF-alpha-treated cells and not affected in IFN-gamma-treated cells. HMVEC expressed a higher level of G protein beta1 isoform than beta4 isoform. Although none of the cytokines tested affected G(beta1) expression, both IL-1 and TNF-alpha significantly up-regulated G(beta4) expression. These findings indicate that inflammatory cytokines modulate adenosine receptor expression and function on HMVECs and suggest that the interaction between proinflammatory cytokines and adenosine receptors may affect therapeutic responses to anti-inflammatory drugs that act via adenosine-dependent mechanisms

OBJECTIVE: Low-dose weekly methotrexate therapy remains a mainstay in the treatment of inflammatory arthritis. Results of previous studies demonstrated that adenosine, acting at one or more of its receptors, mediates the antiinflammatory effects of methotrexate in animal models of both acute and chronic inflammation. We therefore sought to establish which receptor(s) is involved in the modulation of acute inflammation by methotrexate and its nonpolyglutamated analog MX-68 (N-[[4-[(2,4-diaminopteridin-6-yl)methyl]-3,4-dihydro-2H-1,4-benzothiazin- 7-yl]-carbonyl]-L-homoglutamic acid). METHODS: We studied the effects of low-dose methotrexate (0.75 mg/kg intraperitoneally [IP] every week for 5 weeks), MX-68 (2 mg/kg IP 2 days and 1 hour before induction of inflammation), dexamethasone (1.5 mg/kg IP 1 hour before induction of inflammation), or vehicle control on acute inflammation in an air-pouch model in A(2A) and A(3) receptor knockout mice. RESULTS: Low-dose weekly methotrexate treatment increased the adenosine concentration in the exudates of all mice studied and reduced leukocyte and tumor necrosis factor alpha accumulation in the exudates of wild-type mice, but not in those of A(2A) or A(3) receptor knockout mice. Dexamethasone, an agent that suppresses inflammation by a different mechanism, was equally effective at suppressing leukocyte accumulation in A(2A) knockout, A(3) knockout, and wild-type mice, indicating that the lack of response was specific for methotrexate and MX-68. CONCLUSION: These findings confirm that adenosine, acting at A(2A) and A(3) receptors, is a potent regulator of inflammation. Moreover, these results provide strong evidence that adenosine, acting at either or both of these receptors, mediates the antiinflammatory effects of methotrexate and its analog MX-68

Recent evidence indicates that topical application of adenosine A(2A) receptor agonists, unlike growth factors, increases the rate at which wounds close in normal animals and promotes wound healing in diabetic animals as well as growth factors, yet neither the specific adenosine receptor involved nor the mechanism(s) by which adenosine receptor occupancy promotes wound healing have been fully established. To determine which adenosine receptor is involved and whether adenosine receptor-mediated stimulation of angiogenesis plays a role in promotion of wound closure we compared the effect of topical application of the adenosine receptor agonist CGS-21680 (2-p-[2-carboxyethyl]phenethyl-amino-5'-N-ethylcarboxamido-adenosine) on wound closure and angiogenesis in adenosine A(2A) receptor knockout mice and their wild-type littermates. There was no change in the rate of wound closure in the A(2A) receptor knockout mice compared to their wild-type littermates although granulation tissue formation was nonhomogeneous and there seemed to be greater inflammation at the base of the wound. Topical application of CGS-21680 increased the rate of wound closure and increased the number of microvessels in the wounds of wild-type mice but did not affect the rate of wound closure in A(2A) receptor knockout mice. Similarly, in a model of internal trauma and repair (murine air pouch model), endogenously produced adenosine released into areas of internal tissue injury stimulates angiogenesis because there was a marked reduction in blood vessels in the walls of healing air pouches of A(2A) receptor knockout mice compared to their wild-type controls. Inflammatory vascular leakage and leukocyte accumulation in the inflamed air pouch were similarly reduced in the A(2A) receptor knockout mice reflecting the reduced vascularity. Thus, targeting the adenosine A(2A) receptor is a novel approach to promoting wound healing and angiogenesis in normal individuals and those suffering from chronic wounds

Under normoxic conditions, macrophages from C57BL mice produce low levels of vascular endothelial growth factor (VEGF). Hypoxia stimulates VEGF expression by approximately 500%; interferon-gamma (IFN-gamma) with endotoxin [lipopolysaccharide (LPS)] also stimulates VEGF expression by approximately 50 to 150% in an inducible nitric oxide synthase (iNOS)-dependent manner. Treatment of normoxic macrophages with 5'-N-ethyl-carboxamido-adenosine (NECA), a nonselective adenosine A(2) receptor agonist, or with 2-[p-(2-carboxyethyl)-phenylethyl amino]-5'-N-ethyl-carboxamido-adenosine (CGS21680), a specific adenosine A(2A) receptor agonist, modestly increases VEGF expression, whereas 2-chloro-N(6)-cyclopentyl adenosine (CCPA), an adenosine A(1) agonist, does not. Treatment with LPS (0 to 1000 ng/ml), or with IFN-gamma (0 to 300 U/ml), does not affect VEGF expression. In the presence of LPS (EC(50) < 10 ng/ml), but not of IFN-gamma, both NECA and CGS21680 synergistically up-regulate VEGF expression by as much as 10-fold. This VEGF is biologically active in vivo in the rat corneal bioassay of angiogenesis. Inhibitors of iNOS do not affect this synergistic induction of VEGF, and macrophages from iNOS-/- mice produce similar levels of VEGF as wild-type mice, indicating that NO does not play a role in this induction. Under hypoxic conditions, VEGF expression is slightly increased by adenosine receptor agonists but adenosine A(2) or A(1) receptor antagonists 3,7-dimethyl-1-propargyl xanthine (DMPX), ZM241385, and 8-cyclopentyl-1,3-dipropylxanthine (DCPCX) do not modulate VEGF expression. VEGF expression is also not reduced in hypoxic macrophages from A(3)-/- and A(2A)-/- mice. Thus, VEGF expression by hypoxic macrophages does not seem to depend on endogenously released or exogenous adenosine. VEGF expression is strongly up-regulated by LPS/NECA in macrophages from A(3)-/- but not A(2A)-/- mice, confirming the role of adenosine A(2A) receptors in this pathway. LPS with NECA strongly up-regulates VEGF expression by macrophages from C(3)H/HeN mice (with intact Tlr4 receptors), but not by macrophages from C(3)H/HeJ mice (with mutated, functionally inactive Tlr4 receptors), implicating signaling through the Tlr4 pathway in this synergistic up-regulation. Finally, Western blot analysis of adenosine A(2A) receptor expression indicated that the synergistic interaction of LPS with A(2A) receptor agonists does not involve up-regulation of A(2A) receptors by LPS. These results indicate that in murine macrophages there is a novel pathway regulating VEGF production, that involves the synergistic interaction of adenosine A(2A) receptor agonists through A(2A) receptors with LPS through the Tlr4 pathway, resulting in the strong up-regulation of VEGF expression by macrophages in a hypoxia- and NO-independent manner

Animal studies of the topical application of adenosine A2A receptor agonists show that it promotes wound closure. To further confirm the efficacy of adenosine A2A receptor agonists as promoters of wound healing, we compared the effect of MRE0094, a novel selective adenosine A2A receptor agonist, to CGS-21680, a reference selective adenosine A2A receptor agonist, as well as to recombinant human platelet-derived growth factor (0.01% Becaplermin gel), an agent currently used to promote healing of diabetic ulcers, on wound closure in healthy BALB/C mice. Wounds (approximately 12 mm diameter) were created on the dorsum of mice (two per mouse) and then treated daily with vehicle, 0.01% Becaplermin gel, or different doses of the adenosine A2A receptor agonists. The wound margins were traced onto plastic sheets, and the wound areas were digitized, quantitated, and compared. We found that application of MRE0094 (1 microg/wound and 10 microg/wound) and CGS-21680 (1 microg/wound and 5 microg/wound) achieved 50% wound closure significantly more rapidly than control application (day 1.9, 1.9, 3.5, 3.2, respectively, versus control day 4, p < 0.05 ANOVA). Surprisingly, neither higher nor lower concentrations of CGS-21680 affected the rate of wound closure, as compared to control. In contrast, Becaplermin gel did not increase the rate at which wounds closed (50% closure by day 7.2, p = NS versus control). These data confirm our prior observations that adenosine A2A receptor agonists promote wound closure, and they suggest that these agents may be as effective if not more effective than Becaplermin gel for the treatment of poorly healing wounds

Despite the recent introduction of biological response modifiers and potent new small-molecule antirheumatic drugs, the efficacy of methotrexate is nearly unsurpassed in the treatment of inflammatory arthritis. Although methotrexate was first introduced as an antiproliferative agent that inhibits the synthesis of purines and pyrimidines for the therapy of malignancies, it is now clear that many of the anti-inflammatory effects of methotrexate are mediated by adenosine. This nucleoside, acting at one or more of its receptors, is a potent endogenous anti-inflammatory mediator. In confirmation of this mechanism of action, recent studies in both animals and patients suggest that adenosine-receptor antagonists, among which is caffeine, reverse or prevent the anti-inflammatory effects of methotrexate

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