We demonstrate that the activity of A2BAR has a pathogenic effect in the early glomerular dysfunctions observed in STZ-induced diabetic rats and it can be blocked by using a selective antagonist in vivo. These alterations mediated by A2BAR correlate with an increased expression of this receptor in rats' diabetic glomerulus. Furthermore, the pathogenic state also involves a decrease in the content of A1AR subtype in glomerular cells. This event, first described by Pawelczyk et al, probably turns off the inhibitory effects on glomerular VEGF release accompanied with an increased production of this growth factor, as a consequence of the modulation of ARs activities by differential expression in the diabetic state.
We also correlate the activity of a low-affinity AR with an increase in the ligand availability in glomeruli of diabetic rats. Interestingly, it was found elevated adenosine levels in plasma of clinically manifest DN patients compared with the basal levels of diabetic patients without DN and in healthy individuals. Probably, the measurement of adenosine levels in plasma or urine could represent a novel specific marker to evaluate the progression of DN.
Recently, the pathogenic role of adenosine in mediating renal dysfunction and fibrosis in the adenosine desaminase knockout (ADA−/−) mice model, with increased levels of the nucleoside, was described. In addition, the progression of renal fibrosis generated in the ADA−/− animals or in mice infused with angiotensin II (Ang II)or subjected to unilateral ureteral obstruction can be blocked using an A2BAR antagonist, suggesting a common pathogenic pathway involving adenosine signaling in chronic kidney disease. Our major contribution is to demonstrate that this pathogenic pathway also occurs early in the progression of the renal disease derived from diabetes thus supporting the option of a DN pharmacological A2BAR target intervention.
The pathomechanisms of DN converge in inducing a host of growth factors in the kidney. The fibrogenic cytokine transforming growth factor-beta 1 (TGF-β), through its Smad3 signaling pathway, is the etiologic agent of renal hypertrophy and the accumulation of mesangial extracellular matrix components and tubule-interstitial fibrosis in diabetes. However, there is limited evidence to support a role for TGF-β in the development of albuminuria. Podocyte-derived VEGF is a permeability and angiogenic factor whose expression is also increased in diabetic kidney diseases animal models. It appears to act in an autocrine signaling mode to induce the podocytopathy of diabetes, especially the genesis of proteinuria. Therefore, currently strategies for DN therapy may need to involve interception of both the TGF-β and the VEGF signaling pathways. In this way, the A2BAR antagonist could be a valuable tool because it was demonstrated that it is able to block the increased release of TGF-β from diabetic glomeruli in vitro and the myofibroblast transdifferentiation of masangial cells in vivo, as reported in this work. Similarly, interception of VEGF induction was a remarkable feature.
Studies have shown that the VEGF expression in renal podocytes increased upon exposure to high glucose concentration and it is intracellularly mediated by PKC-alpha and ERK1/2 signaling molecules.[34,35] In addition, cultured podocytes exposed to high glucose concentration exhibit increased levels of extracellular adenosine. Previously, we showed that the VEGF production increase in ex vivo glomeruli exposed to high glucose concentration could be blocked by MRS1754. In consequence, our study in vivo suggests that A2BAR could be a transducer of hyperglycemic conditions to mediate VEGF increase. Up to date, Ang II has been recognized as the major factor to increase VEGF expression in diabetic glomerular podocytes. Intra-renal renin and angiotensinogen levels are induced in diabetic animals,[38,39] and high glucose has been shown to stimulate renin and Ang II synthesis in mesangial cells and podocytes.[37,40,41] Further, the role of notch I signaling have also been recently associated with pathogenic VEGF induction and podocytopathy. It remains to be determined if the use of A2BAR antagonist could also intercept the activation of the renin–angiotensin system or notch I signaling.
The extracellular adenosine availability was shown to increase in diabetic glomeruli as shown by the higher urinary nucleoside levels. This effect could be linked to a decreased activity of ENT1 measured in isolated diabetic glomeruli. Similar to our finding, ENT1 activity was found to be decreased in human endothelial cells isolated from gestational diabetes pregnancy or in cells exposed to high glucose concentrations.[44,45] This decreased activity of ENT1 elicits adenosine signaling by ARs as previously described, using inhibitors of ENT1 activity or in ENT1−/− mice.[10,19,20] Interestingly, a correlation between ENT1 decreased activity with epithelium mesenchymal transition (EMT) of renal proximal tubule epithelial cells in vitro, was recently demonstrated. EMT is a requisite for progression of tubule-interstitial fibrosis in the diabetic kidney disease.
Collectively, these findings support a pathogenic role of adenosine and A2B receptor subtype in mediating early states of renal dysfunction in DN, and also suggest a possible pharmacological target for intervention. Furthermore, it has been described that A2BAR blockade reverses insulin resistance in type II diabetes animal models. Therefore, A2BAR blockers useful in humans such as CVT-6883 are being developed, and would represent a novel alternative for management of DN patients.
This work was funded by grants FONDECYT 1100484 and CONICYT-PIA ANILLO ACT73.
Lab Invest. 2013;93(1):135-144. © 2013 Nature Publishing Group