Reduced Adenosine Uptake and Its Contribution to Signaling that Mediates Profibrotic Activation in Renal Tubular Epithelial Cells: Implication in Diabetic Nephropathy

Abstract

Altered nucleoside levels may be linked to pathogenic signaling through adenosine receptors. We hypothesized that adenosine dysregulation contributes to fibrosis in diabetic kidney disease. Our findings indicate that high glucose levels and experimental diabetes decreased uptake activity through the equilibrative nucleoside transporter 1 (ENT1) in proximal tubule cells. In addition, a correlation between increased plasma content of adenosine and a marker of renal fibrosis in diabetic rats was evidenced. At the cellular level, exposure of HK2 cells to high glucose, TGF-β and the general adenosine receptor agonist NECA, induced the expression of profibrotic cell activation markers α-SMA and fibronectin. These effects can be avoided by using a selective antagonist of the adenosine A3 receptor subtype in vitro. Furthermore, induction of fibrosis marker α-SMA was prevented by the A3 receptor antagonist in diabetic rat kidneys. In conclusion, we evidenced the contribution of purinergic signaling to renal fibrosis in experimental diabetic nephropathy.

Adenosine contribution to normal renal physiology and chronic kidney disease

“…Interestingly, A 2B receptor expression in non-diabetic CKD exhibited a similar upregulated pattern (Zhang et al 2013). For the A 3 receptor the most remarkable feature was receptor distribution at the tubules and interstitium during the advanced state of the diabetic kidney disease, which was recently recognized by Kretschmar et al (2016).…”

“…Roa et al (2009) using this same animal model showed that isolated glomeruli from diabetic rats contained significantly higher external adenosine compared to controls (37.4 ± 3.1 v/s 6.0 ± 0.6 nM, corrected per µg of total glomeruli protein). Further, it was recently demonstrated that renal injury progression in diabetic rats develops with increased adenosine levels, having a strong correlation with the profibrotic marker α-SMA (Kretschmar et al 2016), meanwhile increased urinary adenosine excretion could be detected during early onset of kidney dysfunction (Oyarzún et al 2016). These observations highlight the fact that locally generated adenosine contributes to setting the pathological milieu of CKD progression.…”

“…At this compartment, sodium-independent uptake activity was significantly decreased in diabetic rat glomeruli, in particular ENT1 activity was inhibited to 50%; these being the main engines for increasing extracellular adenosine accumulation that drives diabetic glomerulopathy (Roa et al 2009). Recent evidence demonstrated inhibition of ENT1 activity in the proximal tubules of rat diabetic kidney (Kretschmar et al 2016), thus probably also contributing to increased external adenosine. Indeed, in ent1-/-mice or animals with pharmacologically inhibited ENT activity, the levels of interstitial and plasmatic adenosine were elevated (Li et al 2013), highlighting the critical role of ENTs in adenosine homeostasis.…”

“…ent1-/-mice revealed increased interstitial collagen deposition and α-SMA in the kidney (Guillén-Gómez et al 2012;Kretschmar et al 2016). Some studies indicated that insulin and elevated glucose levels can regulate ENT activity in some cell types (Sakowicz et al 2004;Pawelczyk et al 2003;Muñoz et al 2006;Westermeier et al 2011).…”

Pathological overproduction: the bad side of adenosine

“…This increase correlated with a marker of renal fibrosis in diabetic rats. Furthermore, the expression of profibrotic cell activation markers α‐smooth muscle actin and fibronectin was increased by stimulation of A 3 receptors (Kretschmar et al , ) (Figure ).…”