Abstract
Purpose: :
Recent evidence has indicated that adenosine can regulate retinal immune cell function and barrier function of retinal vasculature. Therefore, understanding the mechanism of this regulation may provide a use for targeted intervention in diabetic retinopathy. Ischemic stress in the diabetic retina is known to activate microglia, cause neuronal cell death, and breakdown of the blood-retinal barrier. Ischemic stress also causes the release of adenosine through the bi-directional equilibrative nucleoside transporters (ENT). Extracellular adenosine is known to attenuate inflammation by activating A2a receptor, and A2a receptor-selective agonists have been shown to be effective in anti-inflammation in acute or chronic inflammatory diseases. In high glucose-treated cardiac fibroblasts, however, decreased adenosine release and uptake were measured in comparison to low glucose-treated cells. This suggests impairment in the function of ENT and the subsequent adenosine-A2a receptor signaling in diabetes. This study is the first step to test the hypothesis that the anti-inflammatory function of adenosine-A2a receptor signaling is impaired in the diabetic retina.
Methods: :
Primary cultures of rat retinal microglial cells were treated with or without Amadori-glycated albumin (AGA), which has been implicated as a causal factor in diabetic complications. TNF-α levels in the media were measured by ELISA. Adenosine uptake by microglial cells was determined within one minute after the addition of [2-3H] adenosine. Washed cells were solubilized and radioactivity quantified. For adenosine release, cellular ATP was labeled with [8-14C] adenine, and ATP was depleted in the presence of 2-deoxyglucose and antimycin A. At appropriate time points within 15 minutes of ATP depletion, medium aliquots containing released adenosine was separated on silica gel plate and quantified.
Results: :
Retinal microglial cells showed a significant increase in TNF-α release after AGA treatment. Adenosine uptake and release by these cells were inhibited by AGA treatment.
Conclusions: :
These results suggest that cell protection mechanisms effected by adenosine are impaired in the diabetic retina. Activation of A2a receptor by a selective agonist, as shown in other organ systems, may be effective in anti-inflammation in the diabetic retina.
Keywords: diabetic retinopathy • adenosine • inflammation