Hypoxic postconditioning attenuates apoptosis via activation of adenosine A_2a receptors on dermal microvascular endothelial cells of human flaps

Background: In recent years, many studies have demonstrated that endogenous adenosine induced by ischemia postconditioning reduces apoptosis in various models, but no study has clearly elucidated the effects of hypoxic postconditioning (HPC) in human dermal microvascular endothelial cells (HDMECs) of flaps, and the subtype of adenosine receptors involved remains unknown. In our study, we sought to identify the role of adenosine A_2a receptor in the antiapoptotic effects of HPC.

Materials and methods: The HDMECs were isolated from human upper eyelid tissue. After hypoxia for 6 h, the HDMECs were either abruptly reperfused with medium for 12 h (reoxygenation) or postconditioned by three cycles of 5 min of transitory reoxygenation and 5 min of rehypoxia followed by 11.5 h of reoxygenation. CGS-21680 was used as an adenosine A_2a agonist.

Results: Adenosine A_2a receptors were found in the cytoplasm of HDMECs. Hypoxia/reoxygenation (H/R) resulted in severe injury in HDMECs as evidenced by an increase in apoptosis percentage and an increase in expressions of apoptosis-related proteins (the ratio of Bax/Bcl-2 and caspase-3), which can be significantly attenuated by HPC treatment or exposure to CGS-21680, a selective adenosine A_2a receptor agonist (all P values < 0.05). Meanwhile, HPC treatment or CGS-21680 significantly activated A_2a receptors compared with the hypoxia/reoxygenation group (P < 0.05 versus P < 0.05, respectively). Statistical analysis showed that the increase of cell apoptosis closely correlated inversely with the increase of relative protein level of adenosine A_2a receptors (r² = 0.8177, P < 0.0001).

Conclusions: HPC protects against apoptosis induced by reoxygenation via activation of adenosine A_2a receptors on HDMECs.