A 1 Adenosine Receptor Activation Promotes Angiogenesis and Release of VEGF From Monocytes

Abstract

Adenosine is a proangiogenic purine nucleoside released from ischemic and hypoxic tissues. Of the 4 adenosine receptor (AR) subtypes (A 1 , A 2A , A 2B , and A 3 ), the A 2 and A 3 have been previously linked to the modulation of angiogenesis. We used the chicken chorioallantoic membrane (CAM) model to determine whether A 1 AR activation affects angiogenesis. We cloned and pharmacologically characterized chicken AR subtypes to evaluate the selectivity of various agonists and antagonists. Application of the A 1 AR-selective agonist N 6 -cyclopentyladenosine (CPA; 100 nmol/L) to the CAM resulted in a 40% increase in blood vessel number ( P <0.01), which was blocked by the A 1 AR-selective antagonist C 8 -(N-methylisopropyl)-amino-N 6 -(5′-endohydroxy)-endonorbornan-2-yl-9-methyladenine (WRC-0571; 1 μmol/L). Selective A 2A AR agonists did not stimulate angiogenesis in the CAM. In an ex vivo rat aortic ring model of angiogenesis that includes cocultured endothelial cells, fibroblasts, and smooth muscle cells, 50 nmol/L CPA did not directly stimulate capillary formation; however, medium from human mononuclear cells pretreated with CPA, but not vehicle, increased capillary formation by 48% ( P <0.05). This effect was blocked by WRC-0571 (1.5 μmol/L) or anti-VEGF antibody (1 μg/mL). CPA (5 nmol/L) stimulated a 1.7-fold increase in VEGF release from the mononuclear cells. This is the first study to show that A 1 AR activation induces angiogenesis. Stimulation of A 2 ARs on endothelial cells results in proliferation and tube formation, and A 2 and A 3 ARs on inflammatory cells modulate release of angiogenic factors. We conclude that adenosine promotes a coordinated angiogenic response through its interactions with multiple receptors on multiple cell types.