IntroductionThe endogenous adenine nucleotides and adenosine are normally present at low concentrations in the extracellular milieu. However, metabolically stressful conditions, including inflammation and hypoxia characteristic of asthma, solid tumors, and other pathologic conditions, result in dramatic increases in extracellular concentrations of adenosine. [1][2][3] There are also mechanisms of nonlytic secretion of adenosine during hypoxic conditions.There is growing evidence that adenosine can actively modulate differentiation and function of myeloid cells. 4 Circulating cells of myeloid lineage, including monocytes and dendritic cell (DC) precursors, migrate to tissues where they differentiate into macrophages or DCs. DCs show impressive interaction with the adjacent microenvironment, 5,6 which regulates formation of DC subtypes and their functional properties, including expression of cytokines and growth factors. Because of rapid growth, solid tumors routinely experience severe hypoxia and necrosis, which causes adenine nucleotide degradation and adenosine release. Therefore, high levels of extracellular adenosine contribute to the local tumor microenvironment and may greatly influence differentiation of DCs from monocyte/macrophages and DC precursors migrating into tumor tissue. Adenosine acts through 4 subtypes of adenosine receptors, A 1 , A 2A , A 2B , and A 3 , which are members of the G-protein-coupled family of receptors. 7,8 A 2A adenosine receptors are generally anti-inflammatory, whereas A 2B and A 3 receptors are implicated in proinflammatory action of adenosine. Adenosine receptors are expressed abundantly on monocytes, and through these receptors adenosine exerts substantial modulatory effects on monocyte function and further differentiation. A 1 receptors were shown to stimulate formation of giant multinucleated cells from monocytes, whereas A 2 receptors inhibited this process. 9 A 2B receptors were implicated in mediating the inhibitory effect of adenosine on macrophage proliferation induced by M-CSF. 10 Exogenous adenosine can prevent monocytes from differentiating into macrophages, leading them to an intermediate differentiation stage between immature DCs and monocytes. 11 Cyclic nucleotides, including cAMP, which intracellular level increases in response to stimulation of adenosine A 2 receptors, regulate certain steps of monocyte differentiation and promote their differentiation toward a CD1a low CD14 ϩ/low CD209 ϩ intermediate cell but impair differentiation into functional DCs. 12 Up-regulation of DC-specific ICAM-3-grabbing nonintegrin (CD209) was not affected by cyclic nucleotides, 12 indicating that DC development was not blocked at the monocyte stage. The expression of all 4 adenosine receptor subtypes has been reported in human monocytes and myeloid DCs. 9,13-15 However, the effects of adenosine on differentiation of myeloid DCs from monocytes, macrophages, and hematopoietic progenitor cells (HPCs) and the roles of specific adenosine receptor subtypes involved in this process hav...

SUMMARYDespite available therapies, myocardial infarction (MI) remains a leading cause of death worldwide. Better understanding of the molecular and cellular mechanisms that regulate cardiac repair should help to improve the clinical outcome of MI patients. Using the reporter mouse line TOPGAL, we show that canonical (β-catenin-dependent) Wnt signaling is induced 4 days after experimental MI in subepicardial endothelial cells and perivascular smooth muscle actin (SMA)-positive (SMA+) cells. At 1 week after ischemic injury, a large number of canonical-Wnt-positive cells accumulated in the infarct area during granulation tissue formation. Coincidently with canonical Wnt activation, endothelial-to-mesenchymal transition (EndMT) was also triggered after MI. Using cell lineage tracing, we show that a significant portion of the canonical-Wnt-marked SMA+ mesenchymal cells is derived from endothelial cells. Canonical Wnt signaling induces mesenchymal characteristics in cultured endothelial cells, suggesting a direct role in EndMT. In conclusion, our study demonstrates that canonical Wnt activation and EndMT are molecular and cellular responses to MI and that canonical Wnt signaling activity is a characteristic property of EndMT-derived mesenchymal cells that take part in cardiac tissue repair after MI. These findings could lead to new strategies to improve the course of cardiac repair by temporal and cell-type-specific manipulation of canonical Wnt signaling.

Adenosine potentiates mast cell activation, but the receptor type and molecular mechanisms involved have not been defined. We, therefore, investigated the effects of adenosine on the human mast cell line HMC-1. Both the A2. selective agonist CGS21680 and the A2./Anb nonselective agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased cAMP, but NECA was fourfold more efficacious and had a Hill coefficient of 0.55, suggesting the presence of both A2. and Azb receptors. NECA 10 ,uM evoked IL-8 release from HMC-1, but CGS21680 10 ,uM had no effect. In separate studies we found that enprofylline, an antiasthmatic previously thought to lack adenosine antagonistic properties, is as effective as theophylline as an antagonist of An receptors at concentrations achieved clinically. Both theophylline and enprofylline 300 ,uM completely blocked the release of IL-8 by NECA. NECA, but not CGS21680, increases inositol phosphate formation and intracellular calcium mobilization through a cholera and pertussis toxin-insensitive mechanism. In conclusion, both A2, and A2b receptors are present in HMC-1 cells and are coupled to adenylate cyclase. In addition, Azb receptors are coupled to phospholipase C and evoke IL-8 release. This effect is blocked by theophylline and enprofylline, raising the possibility that this mechanism contributes to their antiasthmatic effects. (J. Clin. Invest. 1995. 96:1979

Abstract-Adenosine has been reported to stimulate or inhibit the release of angiogenic factors depending on the cell type examined. To test the hypothesis that differential expression of adenosine receptor subtypes contributes to endothelial cell heterogeneity, we studied microvascular (HMEC-1) and umbilical vein (HUVEC) human endothelial cells. Based on mRNA level and stimulation of adenylate cyclase, we found that HUVECs preferentially express A 2A adenosine receptors and HMEC-1 preferentially express A 2B receptors. Neither cells expressed A 1 or A 3 receptors. The nonselective adenosine agonist 5Ј-N-ethylcarboxamidoadenosine (NECA) increased expression of interleukin-8 (IL-8), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) in HMEC-1, but had no effect in HUVECs. In contrast, the selective A 2A agonist 2-p-(2-carboxyethyl)phenylethylamino-NECA (CGS 21680) had no effect on expression of these angiogenic factors. Key Words: adenosine receptors Ⅲ vascular endothelium Ⅲ angiogenesis Ⅲ vascular endothelial growth factor Ⅲ interleukin-8 T he purine nucleoside adenosine is an intermediate catabolite of adenine nucleotides. Adenosine serves as an autocoid in situations when oxygen supply is decreased or energy consumption is increased. Under these conditions, adenosine is released into the extracellular space and signals to restore the balance between local energy requirements and energy supply. Endothelial cells interact with adenosine mechanisms in many different ways. Endothelial cells are known to have a very active adenosine metabolism, characterized by a large capacity for uptake and release of the nucleoside, 1,2 and can be an important source of adenosine released during ischemia. 3 Conversely, adenosine may modulate endothelial function via activation of cell membrane receptors. The precise nature of the interaction between adenosine receptor subtypes and endothelial cells and their role in the regulation of endothelial function is not completely understood.Adenosine receptors belong to the G protein-coupled 7 transmembrane superfamily of cell surface receptors and include A 1 , A 2A , A 2B , and A 3 subtypes. Endothelial cells are known to express adenosine receptors, but there are conflicting reports on the presence and the role of specific adenosine receptor subtypes. For example, human umbilical vein endothelial cells (HUVECs) were reported to express either A 1 , 4 A 2A , 5,6 A 2B , 7 or A 3 6 adenosine receptors, depending on the functional end-point studied and pharmacological tools used. Coexpression of more than one adenosine receptor subtype has been reported also in endothelial cells 8,9 ; it is not clear, however, if and how coexpressed receptors interact. Furthermore, endothelial cells from different blood vessels are heterogenous, and it is possible that diverse endothelial cells show differential expression of adenosine receptor subtypes.The functional role of adenosine receptors in endothelial cells also remains unclear. Adenosine-induced vasodilation h...

Serotonin (5-hydroxytryptamine; 5-HT) transporters (SERTs) are critical determinants of synaptic 5-HT inactivation and the targets for multiple drugs used to treat psychiatric disorders. In support of prior studies, we found that short-term (5-30 min) application of the adenosine receptor (AR) agonist 5Ј-N-ethylcarboxamidoadenosine (NECA) induces an increase in 5-HT uptake V max in rat basophilic leukemia 2H3 cells that is enhanced by pretreatment with the cGMP phosphodiesterase inhibitor sildenafil. NECA stimulation is blocked by the A 3 AR antagonist 3-ethyl-5-benzyl-2-methyl-phenylethynyl-6-phenyl-1,4(Ϯ)dihydropyridine-3,5-dicarboxylate