Adenosine inhibits growth of human aortic smooth muscle cells via A2B receptors

Adenosine inhibits rat vascular smooth muscle cell (SMC) growth. However, the effects of adenosine on human vascular SMC proliferation and synthesis of extracellular matrix proteins, such as collagen, are unknown. The objective of this study was to characterize the effects of exogenous and endogenous (SMC-derived) adenosine on human aortic SMC proliferation and collagen synthesis. Growth-arrested SMCs were stimulated with 2.5% fetal calf serum (FCS) in the presence and absence of adenosine, 2-chloroadenosine (stable adenosine analogue), and with agents that increase endogenous adenosine levels, including erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), dipyridamole, and iodotubericidin. All of these agents inhibited in a concentration-dependent manner FCS-induced SMC proliferation as assessed by DNA synthesis (H-3-thymidine incorporation) and cell counting, as well as collagen synthesis (H-3-proline incorporation). EHNA, dipyridamole, and iodotubericidin increased extracellular levels of adenosine by 1.7-fold to 18-fold when added separately to SMCs, and EHNA+iodotubericidin and EHNA+iodotubericidin+dipyridamole increased extracellular adenosine levels by more than 392-fold. Both KF17837 (selective A(2) antagonist) and DPSPX (A(1)/A(2) antagonist), but not DPCPX (selective A(1) antagonist), blocked the antimitogenic effects of 2-chloroadenosine, EHNA, and dipyridamole on DNA and collagen synthesis, suggesting the involvement of A(2A) and/or A(2B), but excluding the participation of A(1), receptors. The lack of effect of CGS21680 (selective A(2A) agonist), excluded involvement of A(2A) receptors and suggested a major role for A(2B) receptors. A comparison of the inhibitory potencies of 2-chloroadenosine, N-6-cyclopentyladenosine (selective A(1) agonist), NECA (A(1)/A(2) agonist), and MECA (A(1)/A(2) agonist) were consistent with an A(2B) receptor subtype mediating the inhibitory effects of adenosine on human aortic SMC proliferation. In conclusion, human aortic SMCs synthesize adenosine, and exogenous as well as endogenous (SMC-derived) adenosine inhibits SMC proliferation and collagen synthesis via activation of A(2B) receptors.