Effect of Diabetes/Hyperglycemia on the Rat Retinal Adenosinergic System

Abstract

The early stages of diabetic retinopathy (DR) are characterized by alterations similar to neurodegenerative and inflammatory conditions such as increased neural apoptosis, microglial cell activation and amplified production of pro-inflammatory cytokines. Adenosine regulates several physiological functions by stimulating four subtypes of receptors, A1AR, A2AAR, A2BAR, and A3AR. Although the adenosinergic signaling system is affected by diabetes in several tissues, it is unknown whether diabetic conditions in the retina can also affect it. Adenosine delivers potent suppressive effects on virtually all cells of the immune system, but its potential role in the context of DR has yet to be studied in full. In this study, we used primary mixed cultures of rat retinal cells exposed to high glucose conditions, to mimic hyperglycemia, and a streptozotocin rat model of type 1 diabetes to determine the effect diabetes/hyperglycemia have on the expression and protein levels of adenosine receptors and of the enzymes adenosine deaminase and adenosine kinase. We found elevated mRNA and protein levels of A1AR and A2AAR, in retinal cell cultures under high glucose conditions and a transient increase in the levels of the same receptors in diabetic retinas. Adenosine deaminase and adenosine kinase expression and protein levels showed a significant decrease in diabetic retinas 30 days after diabetes induction. An enzymatic assay performed in retinal cell cultures revealed a marked decrease in the activity of adenosine deaminase under high glucose conditions. We also found an increase in extracellular adenosine levels accompanied by a decrease in intracellular levels when retinal cells were subjected to high glucose conditions. In conclusion, this study shows that several components of the retinal adenosinergic system are affected by diabetes and high glucose conditions, and the modulation observed may uncover a possible mechanism for the alleviation of the inflammatory and excitotoxic conditions observed in diabetic retinas.

Lung injury pathways: Adenosine receptor 2B signaling limits development of ischemic bronchiolitis obliterans organizing pneumonia

“…Bands at ~48 kDa and 37 kDa were observed in our immunoblots. The band just smaller than 50 kDa is consistent with immunospecific band identified by this Alomone antibody in rodent tissue (20). Both bands were competed away by blocking peptide (Alomone Labs: www.alomone.com/p/anti-A2BA_adenosine_receptor_(extracellular)/aar-003/114).…”

Adenosine signalling in diabetes mellitus—pathophysiology and therapeutic considerations

“…High levels of glucose increase expression levels of A 1 and A 2A adeno sine receptors, but not those of A 2B or A 3 adenosine receptors in both primary cultures of rat retinal cells and streptozotocin-treated diabetic rats. 141 Retinal expression and activity of adenosine kinase and adenosine deaminase were also reduced, which correlated with an increase in the adenosine concentration in the extracellular milieu. 141 Decreased levels of adenosine kinase and increased expression of adenosine deaminase 2 but not adenosine deaminase 1 have been observed both in the retina of diabetic pigs and human donors with diabetes mellitus.…”

“…141 Retinal expression and activity of adenosine kinase and adenosine deaminase were also reduced, which correlated with an increase in the adenosine concentration in the extracellular milieu. 141 Decreased levels of adenosine kinase and increased expression of adenosine deaminase 2 but not adenosine deaminase 1 have been observed both in the retina of diabetic pigs and human donors with diabetes mellitus. 142 These changes were accompanied by upregulation of the nucleoside transporter ENT1, without changes in the expression of CD73.…”

Purinergic signaling in retinal degeneration and regeneration

“…A decrease in the extracellular ATP level may lead to a resensitization of P2Y receptors that were desensitized by ATP ; the resensitization of P2Y receptors may contribute to the increased calcium responsiveness of Müller cells to P2Y activation under pathological conditions (see above). Experimental diabetic retinopathy is also associated with an upregulation of retinal adenosine A 1 and A 2A receptors, and a downregulation of retinal adenosine deaminase and adenosine kinase, resulting in increased extracellular adenosine levels under high glucose conditions (Vindeirinho et al, 2013; but see Costa et al, 2009). The data suggest that elevation of the endogenous adenosine signaling represents an neuroprotective response of retinal cells to various pathological conditions like ischemiahypoxia and diabetes.…”

“…Elevation of the intraocular pressure is associated with an increased level of extracellular ATP (Zhang et al, 2007;Resta et al, 2007). In addition, high glucose induces raised levels of extracellular ATP, via increasing the exocytotic release of ATP and decreasing its extracellular degradation (Costa et al, 2009; but see Vindeirinho et al, 2013).…”