Editorial: Old dog, new tricks: proangiogenic effect of adenosine via stimulation of thrombospondin-1 in macrophages.

Almost 1 century after the original observation of Drury and Szent-Gyorgyi [1], that it exerts cardiovascular effects, adenosine remains a subject of intensive studies, not only in the cardiovascular system but in many other areas of biology and medicine [2]. The first observations that adenosine exerts proangiogenic effects were made almost 30 years ago, when Teuscher and Weidlich [2] showed in Boyden chamber experiments that adenosine exerts chemotactic effects in porcine endothelial cells [3]. Subsequent studies have demonstrated the proangiogenic effect of adenosine in various experimental models, ranging from chicken chorioallantoic membranes to mammalian systems of postischemic angiogenesis and tumor angiogenesis (overviewed in ref. [4]). The proangiogenic effects of adenosine include direct actions on endothelial cells and indirect effects via multiple cell types (such as smooth muscle cells, monocytes/ macrophages/foam cells, and mast cells; see Fig. 1A). Part of the direct angiogenic effects of adenosine are directly related to the regulation by adenosine of endothelial signaling pathways, thereby affecting cell cycle, cell division, and cell migration. However, it appears that a substantial portion of adenosine’s actions is related to its ability to induce the release of the proangiogenic hormone VEGF from macrophages/monocytes, mast cells, as well as from the endothelial cells themselves. Adenosine has also been shown to stimulate the production of other proangiogenic hormones, such as IL-8, bFGF, and IGF-1. Thus, the proangiogenic effects of adenosine are the result of a complex network of autocrine and paracrine actions in the vascular microenvironment that ultimately result in endothelial cell proliferation, migration, tube formation, and the formation of new blood vessels. All four adenosine receptor subtypes (A1, A2A, A2B, A3) have been implicated in these responses (with some speciesdependent, cell-type, and modeldependent differences that remain to be elucidated further; see Fig. 1A). The physiologic and pathophysiological significance of these processes ranges from postischemic neovascularization to tumor angiogenesis and wound healing [2, 4]. TSP-1 is generally viewed as an antiangiogenic molecule. Identified over 25 years ago, TSP-1 has been shown to antagonize the growth-promoting effects of bFGF and of VEGF and is widely regarded as an endogenous inhibitor of endothelial cell proliferation and angiogenesis (overviewed in ref. [5]). Its mode of action involves direct binding to VEGF, inhibition of endothelial cellcycle progression through CD36, and inhibition of the endothelial cell-derived vasodilator and proangiogenic mediator NO. Before the report of Ernens and colleagues [6] in the current issue of the Journal of Leukocyte Biology, the only connection between adenosine’s proangiogenic effect and TSP-1 was made by Cronstein and colleagues [7], who, working in HUVECs, reported that adenosine, as well as selective A2A receptor agonists, suppress endothelial cell TSP-1 expression and secretion. The same group demonstrated that TSP-1 antibodies and CD36 antibodies prevent the ability of A2A receptor agonists to stimulate HUVEC proliferation [7]. These data led to the conclusion that adenosine exerts proangiogenic effects through inhibiting the formation of the antiangiogenic hormone TSP-1. It may be worth mentioning that the study by Cronstein and colleagues [7] did not include positive control studies to test the effects of authentic TSP-1 on endothelial cell proliferation in their HUVEC experimental system. The current report presents a different facet to the proangiogenic effects of adenosine (as well as TSP-1). First, somewhat surprisingly, it shows that the effects of TSP-1, at lower concentrations (0.01–5 mg/ml) stimulates neovessel formation in a rat aortic ring angiogenesis assay in vitro [6]. Elevation of the concentration of TSP-1 to 10 mg/ml, on the other hand, tended to become inhibitory. It may seem surprising that after 20 years of intensive work on TSP-1, only a few studies have noticed that TSP-1 exerts proangiogenic effects at lower concentrations. Nevertheless, in a study published in 1997 in bovine endothelial cells, TSP-1, at 1–4 mg/ml, was found to stimulate the length of endothelial network formation, and the "canonical" inhibition of endothelial cell angiogenesis only ensued at higher concentrations, 10 mM and above [8]. Likewise, Motegi and colleagues [9] reported that TSP-1 stimulates endothelial cell migration at low concentrations (0.1–10 mg/ml), and it only becomes inhibitory at 25–100 mg/ml. Next, Ernens and colleagues [6] showed that adenosine stimulates the production of TSP-1 by human macrophages, an effect that was attributed to