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

Spatial proteomics of human diabetic kidney disease, from health to class III

AIMS/HYPOTHESIS

Diabetic kidney disease (DKD) is the leading cause of chronic and end-stage kidney disease in the USA and worldwide. Animal models have taught us much about DKD mechanisms, but translation of this knowledge into treatments for human disease has been slowed by the lag in our molecular understanding of human DKD.

METHODS

Using our Spatial TissuE Proteomics (STEP) pipeline (comprising curated human kidney tissues, multiplexed immunofluorescence and powerful analysis tools), we imaged and analysed the expression of 21 proteins in 23 tissue sections from individuals with diabetes and healthy kidneys (n=5), compared to those with DKDIIA, IIA-B and IIB (n=2 each) and DKDIII (n=1).

RESULTS

These analyses revealed the existence of 11 cellular clusters (kidney compartments/cell types): podocytes, glomerular endothelial cells, proximal tubules, distal nephron, peritubular capillaries, blood vessels (endothelial cells and vascular smooth muscle cells), macrophages, myeloid cells, other CD45+ inflammatory cells, basement membrane and the interstitium. DKD progression was associated with co-localised increases in inflammatory cells and collagen IV deposition, with concomitant loss of native proteins of each nephron segment. Cell-type frequency and neighbourhood analyses highlighted a significant increase in inflammatory cells and their adjacency to tubular and αSMA+ (α-smooth muscle actin-positive) cells in DKD. Finally, DKD progression showed marked regional variability within single tissue sections, as well as inter-individual variability within each DKD class.

CONCLUSIONS/INTERPRETATION

Using the STEP pipeline, we found alterations in protein expression, cellular phenotypic composition and microenvironment structure with DKD progression, demonstrating the power of this pipeline to reveal the pathophysiology of human DKD.

Pharmacological Blockade of the Adenosine A2B Receptor Is Protective of Proteinuria in Diabetic Rats, through Affecting Focal Adhesion Kinase Activation and the Adhesion Dynamics of Podocytes

Induction of the adenosine receptor A2B (A2BAR) expression in diabetic glomeruli correlates with an increased abundance of its endogenous ligand adenosine and the progression of kidney dysfunction. Remarkably, A2BAR antagonism protects from proteinuria in experimental diabetic nephropathy. We found that A2BAR antagonism preserves the arrangement of podocytes on the glomerular filtration barrier, reduces diabetes-induced focal adhesion kinase (FAK) activation, and attenuates podocyte foot processes effacement. In spreading assays using human podocytes in vitro, adenosine enhanced the rate of cell body expansion on laminin-coated glass and promoted peripheral pY397-FAK subcellular distribution, while selective A2BAR antagonism impeded these effects and attenuated the migratory capability of podocytes. Increased phosphorylation of the Myosin2A light chain accompanied the effects of adenosine. Furthermore, when the A2BAR was stimulated, the cells expanded more broadly and more staining of pS19 myosin was detected which co-localized with actin cables, suggesting increased contractility potential in cells planted onto a matrix with a stiffness similar to of the glomerular basement membrane. We conclude that A2BAR is involved in adhesion dynamics and contractile actin bundle formation, leading to podocyte foot processes effacement. The antagonism of this receptor may be an alternative to the intervention of glomerular barrier deterioration and proteinuria in the diabetic kidney disease.

Defective insulin-stimulated equilibrative nucleoside transporter-2 activity and altered subcellular transporter distribution drive the loss of adenosine homeostasis in diabetic kidney disease progression

AIM

Progression of diabetic nephropathy (DN) is linked to the dysregulated increase of adenosine and altered signaling properties. A major contribution to the maintenance of physiological extracellular adenosine levels relies on cellular uptake activity through plasma membrane nucleoside transporters. Because kidney cells are responsive to insulin, this study aims to determine how DN affects insulin regulation of the equilibrative nucleoside transporter-2 (ENT2).

METHODS

Human Podocytes and rat glomeruli were used to study ENT2 regulation. The effects of diabetes and insulin on ENT2 mediated transport activity were determined measuring the fraction of total adenosine uptake in sodium-free medium which is inhibitable by hypoxanthine. Alterations in ENT2 subcellular distribution were assessed in the kidney of people affected with DN and diabetic rats. The consequences of impaired ENT2 activity on the kidney were evaluated using dipyridamole in an animal model.

RESULTS

Insulin upregulates ENT2 uptake activity by increasing the Vmax, thus counteracting decreased adenosine uptake due to high d-glucose and achieving extracellular adenosine homeostasis. Insulin promoted ENT2 translocation to the plasma membrane dependent on PI3-kinase/Akt signaling and actin cytoskeleton integrity. However, in diabetic rats, the insulin-mediated induction of ENT2 activity was lost. Additionally, reduced Akt activation in response to insulin correlated with decreased ENT2 distribution at the plasma membrane. Kidney tissues from diabetic rats and human DN biopsies showed ENT2 redistribution to an intracellular pattern, evidencing dysfunctional adenosine uptake. Through ENT inhibition, we evidenced increased proteinuria and induced alpha-smooth muscle actin as a result of profibrotic activation of cells in the kidney.

CONCLUSION

Deficient insulin regulation of ENT2 activity contributes to chronically high adenosine levels and glomerular alterations that underline diabetic kidney disease progression.

Adenosine reuptake inhibition reduces diabetes-induced glomerular hyperfiltration via the adenosine A2a receptor

Diabetes-induced glomerular hyperfiltration is an early alteration in kidney function in diabetes. Previous studies have shown that reduced adenosine A2a receptor signaling contributes to diabetes-induced glomerular hyperfiltration. The present study investigated the effects of enhanced interstitial adenosine concentration by inhibition of cellular adenosine reuptake, thereby promoting endogenous adenosine signaling. Insulinopenic diabetes was induced by streptozotocin in adult male Sprague-Dawley rats. Two weeks after diabetes induction, kidney function in terms of glomerular filtration rate, and total, cortical, and medullary renal blood flows were evaluated under thiobutabarbital anesthesia during baseline and after renal artery infusion of two doses of the adenosine reuptake inhibitor dilazep. Dilazep did not affect mean arterial pressure indicating that the effects of the interventions were intrarenal. Diabetics had increased glomerular filtration rate compared with controls and dilazep dose-dependently decreased glomerular filtration rate in diabetics, whereas it had no significant effect in controls. Dilazep increased cortical renal blood flows in controls, whereas medullary blood flow was not significantly changed. Dilazep did not affect total renal blood flow in any of the groups but decreased cortical blood flow in diabetics, resulting in decreased filtration fraction by dilazep in diabetics. Pretreatment with the adenosine A2a antagonist ZM241385 prevented intrarenal dilazep-mediated effects on glomerular filtration rate and filtration fraction in diabetics. In conclusion, enhancing intrarenal adenosine signaling by dilazep normalizes diabetes-induced glomerular hyperfiltration at least in part by activation of adenosine A2a receptors.

Adenosine receptor signalling as a driver of pulmonary fibrosis

Pulmonary fibrosis is a debilitating and life-limiting lung condition in which the damage- response mechanisms of mixed-population cells within the lungs go awry. The tissue microenvironment is drastically remodelled by aberrantly activated fibroblasts which deposit ECM components into the surrounding lung tissue, detrimentally affecting lung function and capacity for gas exchange. Growing evidence suggests a role for adenosine signalling in the pathology of tissue fibrosis in a variety of organs, including the lung, but the molecular pathways through which this occurs remain largely unknown. This review explores the role of adenosine in fibrosis and evaluates the contribution of the different adenosine receptors to fibrogenesis. Therapeutic targeting of the adenosine receptors is also considered, along with clinical observations pointing towards a role for adenosine in fibrosis. In addition, the interaction between adenosine signalling and other profibrotic signalling pathways, such as TGFβ1 signalling, is discussed.

The Interplay of Autophagy and Oxidative Stress in the Kidney: What Do We Know?

BACKGROUND

Autophagy, as an indispensable metabolism, plays pivotal roles in maintaining intracellular homeostasis. Nutritional stress, amino acid deficiency, oxidative stress, and hypoxia can trigger its initiation. Oxidative stress in the kidney activates essential signal molecules, like mammalian target of rapamycin (mTOR), adenosine monophosphate-activated protein kinase (AMPK), and silent mating-type information regulation 2 homolog-1 (SIRT1), to stimulate autophagy, ultimately leading to degradation of intracellular oxidative substances and damaged organelles. Growing evidence suggests that autophagy protects the kidney from oxidative stress during acute ischemic kidney injury, chronic kidney disease, and even aging.

SUMMARY

This review emphasizes the cross talk between reactive oxygen species (ROS) signaling pathways and autophagy during renal homeostasis and chronic kidney disease according to the current latest research and provides therapeutic targets during kidney disorders by adjusting autophagy and suppressing oxidative stress.

KEY MESSAGES

ROS arise through an imbalance of oxidation and antioxidant defense mechanisms, leading to impaired cellular and organ function. Targeting the overproduction of ROS and reactive nitrogen species, reducing the antioxidant enzyme activity and the recovery of the prooxidative-antioxidative balance provide novel therapeutic regimens to contribute to recovery in acute and chronic renal failure. Although, in recent years, great progress has been made in understanding the molecular mechanisms of oxidative stress and autophagy in acute and chronic renal failure, the focus on clinical therapies is still in its infancy. The growing number of studies on the interactive mechanisms of oxidative stress-mediated autophagy will be of great importance for the future treatment and prevention of kidney diseases.

Adenosine receptors as emerging therapeutic targets for diabetic kidney disease

Diabetic kidney disease (DKD) is now a pandemic worldwide, and novel therapeutic options are urgently required. Adenosine, an adenosine triphosphate metabolite, plays a role in kidney homeostasis through interacting with four types of adenosine receptors (ARs): A1AR, A2AAR, A2BAR, and A3AR. Increasing evidence highlights the role of adenosine and ARs in the development and progression of DKD: 1) increased adenosine in the plasma and urine of diabetics with kidney injury, 2) increased expression of each of the ARs in diabetic kidneys, 3) the protective effect of coffee, a commonly ingested nonselective AR antagonist, on DKD, and 4) the protective effect of AR modulators in experimental DKD models. We propose AR modulators as a new therapeutic option to treat DKD. Detailed mechanistic studies on the pharmacology of AR modulators will help us to develop effective first-in-class AR modulators against DKD.

Role of abnormal energy metabolism in the progression of chronic kidney disease and drug intervention

Chronic kidney disease (CKD) is a severe clinical syndrome with significant socioeconomic impact worldwide. Orderly energy metabolism is essential for normal kidney function and energy metabolism disorders are increasingly recognized as an important player in CKD. Energy metabolism disorders are characterized by ATP deficits and reactive oxygen species increase. Oxygen and mitochondria are essential for ATP production, hypoxia and mitochondrial dysfunction both affect the energy production process. Renin-angiotensin and adenine signaling pathway also play important regulatory roles in energy metabolism. In addition, disturbance of energy metabolism is a key factor in the development of hereditary nephropathy such as autosomal dominant polycystic kidney disease. Currently, drugs with clinically clear renal function protection, such as Angiotensin II Type 1 receptor blockers and fenofibrate, have been proven to improve energy metabolism disorders. The sodium-glucose co-transporter inhibitors 2 that can mediate glucose metabolism disorders not only delay the progress of diabetic nephropathy, but also have significant protective effects in non-diabetic nephropathy. Hypoxia-inducible factor enhances ATP production to the kidney by improving renal oxygen supply and increasing glycolysis, and the mitochondria targeted peptides (SS-31) plays a protective role by stabilizing the mitochondrial inner membrane. Moreover, several drugs are being studied and are predicted to have potential renal protective properties. We propose that the regulation of energy metabolism represents a promising strategy to delay the progression of CKD.

The cGAS-STING pathway: more than fighting against viruses and cancer

In the classic Cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS)-stimulator of interferon genes (STING) pathway, downstream signals can control the production of type I interferon and nuclear factor kappa-light-chain-enhancer of activated B cells to promote the activation of pro-inflammatory molecules, which are mainly induced during antiviral responses. However, with progress in this area of research, studies focused on autoimmune diseases and chronic inflammatory conditions that may be relevant to cGAS-STING pathways have been conducted. This review mainly highlights the functions of the cGAS-STING pathway in chronic inflammatory diseases. Importantly, the cGAS-STING pathway has a major impact on lipid metabolism. Different research groups have confirmed that the cGAS-STING pathway plays an important role in the chronic inflammatory status in various organs. However, this pathway has not been studied in depth in diabetes and diabetes-related complications. Current research on the cGAS-STING pathway has shown that the targeted therapy of diseases that may be caused by inflammation via the cGAS-STING pathway has promising outcomes.

Dual Actions of A2A and A3 Adenosine Receptor Ligand Prevents Obstruction-Induced Kidney Fibrosis in Mice

Kidney fibrosis is the final outcome of chronic kidney disease (CKD). Adenosine plays a significant role in protection against cellular damage by activating four subtypes of adenosine receptors (ARs), A₁AR, A_2AAR, A_2BAR, and A₃AR. A_2AAR agonists protect against inflammation, and A₃AR antagonists effectively inhibit the formation of fibrosis. Here, we showed for the first time that LJ-4459, a newly synthesized dual-acting ligand that is an A_2AAR agonist and an A₃AR antagonist, prevents the progression of tubulointerstitial fibrosis. Unilateral ureteral obstruction (UUO) surgery was performed on 6-week-old male C57BL/6 mice. LJ-4459 (1 and 10 mg/kg) was orally administered for 7 days, started at 1 day before UUO surgery. Pretreatment with LJ-4459 improved kidney morphology and prevented the progression of tubular injury as shown by decreases in urinary kidney injury molecular-1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) excretion. Obstruction-induced tubulointerstitial fibrosis was attenuated by LJ-4459, as shown by a decrease in fibrotic protein expression in the kidney. LJ-4459 also inhibited inflammation and oxidative stress in the obstructed kidney, with reduced macrophage infiltration, reduced levels of pro-inflammatory cytokines, as well as reduced levels of reactive oxygen species (ROS). These data demonstrate that LJ-4459 has potential as a therapeutic agent against the progression of tubulointerstitial fibrosis.

Protective mechanism of apigenin in diabetic nephropathy is related to its regulation of miR-423-5P-USF2 axis

Apigenin (APG), a natural flavonoid with anti-inflammatory and anti-fibrosis properties, has been shown to play a protective role in diabetic nephropathy (DN), but their molecular protection mechanism for miRNA has not been elucidated in detail. This study was designed to focus on exploring its protective role in DN and whether miR-423-5p-upstream stimulating factor 2 (USF2) axis was involved in its protective mechanism. The in vivo model of rat was induced by streptozotocin (STZ) and the in vitro model of renal tubular epithelial cell (RTEC) was induced by high glucose (HG). Our in vivo study revealed that APG had different protective effects on inflammation, renal fibrosis and epithelial mesenchymal transition (EMT) in DN rats, which is mainly reflected in that the inflammatory factors (IL-6, IFN-γ, TNF-α) were obviously down-regulated, the renal fibrosis markers (IV-C, FN, Col I) were significantly inhibited, the E-cadherin (EMT factors) was significantly up-regulated, while the vimentin and α-SMA (EMT factors) were significantly down-regulated, and the renal function indexes (serum Cr, BUN) were significantly improved. In terms of mechanism, the protective effect of APG was related to the regulation of the expression of miR-423-5p-USF2 axis, and there was a targeted relationship between miR-423-5p and USF2. Down-regulating miR-423-5p or up-regulating USF2 could significantly aggravate the disease progression of in vitro model and eliminate DN resistance under APG intervention. The above results revealed that the protective role of APG on DN was mediated by miR-423-5p-USF2 axis.

Adenosine A1 Receptor Deficiency Aggravates Extracellular Matrix Accumulation in Diabetic Nephropathy through Disturbance of Peritubular Microenvironment

BACKGROUND

We previously observed that adenosine A1 receptor (A1AR) had a protective role in proximal tubular megalin loss associated with albuminuria in diabetic nephropathy (DN). In this study, we aimed to explore the role of A1AR in the fibrosis progression of DN.

METHODS

We collected DN patients' samples and established a streptozotocin-induced diabetes model in wild-type (WT) and A1AR-deficient (A1AR^-/-) mice. The location and expression of CD34, PDGFRβ, and A1AR were detected in kidney tissue samples from DN patients by immunofluorescent and immunohistochemical staining. We also analyzed the expression of TGFβ, collagen (I, III, and IV), α-SMA, and PDGFRβ using immunohistochemistry in WT and A1AR^-/- mice. CD34 and podoplanin expression were analyzed by Western blotting and immunohistochemical staining in mice, respectively. Human renal proximal tubular epithelial cells (HK2) were cultured in medium containing high glucose and A1AR agonist as well as antagonist.

RESULTS

In DN patients, the expression of PDGFRβ was higher with the loss of CD34. The location of PDGFRβ and TGFβ was near to each other. The A1AR, which was colocalized with CD34 partly, was also upregulated in DN patients. In WT-DN mice, obvious albuminuria and renal pathological leisure were observed. In A1AR^-/- DN mice, more severe renal tubular interstitial fibrosis and more extracellular matrix deposition were observed, with lower CD34 expression and pronounced increase of PDGFRβ. In HK2 cells, high glucose stimulated the epithelial-mesenchymal transition (EMT) process, which was inhibited by A1AR agonist.

CONCLUSION

A1AR played a critical role in protecting the tubulointerstitial fibrosis process in DN by regulation of the peritubular microenvironment.

Decreased Equilibrative Nucleoside Transporter 1 (ENT1) Activity Contributes to the High Extracellular Adenosine Levels in Mesenchymal Glioblastoma Stem-Like Cells

Glioblastoma multiforme is one of the most malignant types of cancer. This is mainly due to a cell subpopulation with an extremely aggressive potential, called glioblastoma stem-like cells (GSCs). These cells produce high levels of extracellular adenosine which has been associated with increased chemoresistance, migration, and invasion in glioblastoma. In this study, we attempted to elucidate the mechanisms that control extracellular adenosine levels in GSC subtypes. By using primary and U87MG-derived GSCs, we associated increased extracellular adenosine with the mesenchymal phenotype. [3H]-adenosine uptake occurred mainly through the equilibrative nucleoside transporters (ENTs) in GSCs, but mesenchymal GSCs have lower expression and ENT1-mediated uptake activity than proneural GSCs. By analyzing expression and enzymatic activity, we determined that ecto-5'-nucleotidase (CD73) is predominantly expressed in proneural GSCs, driving AMPase activity. While in mesenchymal GSCs, both CD73 and Prostatic Acid Phosphatase (PAP) contribute to the AMP (adenosine monophosphate) hydrolysis. We did not observe significant differences between the expression of proteins involved in the metabolization of adenosine among the GCSs subtypes. In conclusion, the lower expression and activity of the ENT1 transporter in mesenchymal GSCs contributes to the high level of extracellular adenosine that these GSCs present.

Blockade of the Adenosine A3 Receptor Attenuates Caspase 1 Activation in Renal Tubule Epithelial Cells and Decreases Interleukins IL-1β and IL-18 in Diabetic Rats

Diabetic nephropathy (DN) is the main cause of end-stage renal disease, which remains incurable. The progression of DN is associated with progressive and irreversible renal fibrosis and also high levels of adenosine. Our aim was to evaluate the effects of ADORA3 antagonism on renal injury in streptozotocin-induced diabetic rats. An ADORA3 antagonist that was administered in diabetic rats greatly inhibited the levels of inflammatory interleukins IL-1β and IL-18, meanwhile when adenosine deaminase was administered, there was a non-selective attenuation of the inflammatory mediators IL-1β, IL-18, IL-6, and induction of IL-10. The ADORA3 antagonist attenuated the high glucose-induced activation of caspase 1 in HK2 cells in vitro. Additionally, ADORA3 antagonisms blocked the increase in caspase 1 and the nuclear localization of NFκB in the renal tubular epithelium of diabetic rats, both events that are involved in regulating the production and activation of IL-1β and IL-18. The effects of the A3 receptor antagonist resulted in the attenuation of kidney injury, as evidenced by decreased levels of the pro-fibrotic marker α-SMA at histological levels and the restoration of proteinuria in diabetic rats. We conclude that ADORA3 antagonism represents a potential therapeutic target that mechanistically works through the selective blockade of the NLRP3 inflammasome.

Mitochondrial dysfunction in diabetic kidney disease

Although diabetic kidney disease (DKD) is the most common cause of end-stage kidney disease worldwide, the pathogenic mechanisms are poorly understood. There is increasing evidence that mitochondrial dysfunction contributes to the development and progression of DKD. Because the kidney is the organ with the second highest oxygen consumption in our body, it is distinctly sensitive to mitochondrial dysfunction. Mitochondrial dysfunction contributes to the progression of chronic kidney disease irrespective of underlying cause. More importantly, high plasma glucose directly damages renal tubular cells, resulting in a wide range of metabolic and cellular dysfunction. Overproduction of reactive oxygen species (ROS), activation of apoptotic pathway, and defective mitophagy are interlinked mechanisms that play pivotal roles in the progression of DKD. Although renal tubular cells have the highest mitochondrial content, podocytes, mesangial cells, and glomerular endothelial cells may all be affected by diabetes-induced mitochondrial injury. Urinary mitochondrial DNA (mtDNA) is readily detectable and may serve as a marker of mitochondrial damage in DKD. Unfortunately, pharmacologic modulation of mitochondrial dysfunction for the treatment of DKD is still in its infancy. Nonetheless, understanding the pathobiology of mitochondrial dysfunction in DKD would facilitate the development of novel therapeutic strategies.

Diabetes downregulates renal adenosine A2A receptors in an experimental model of hypertension

Studies on diabetic nephropathy rarely take into account that the co-existence of diabetes and hypertension is frequent and further aggravates the prognosis of renal dysfunction. Adenosine can activate four subtypes of adenosine receptors (A₁, A_2A, A_2B and A₃) and has been implicated in diabetic nephropathy. However, it is not known if, in hypertensive conditions, diabetes alters the presence/distribution profile of renal adenosine receptors. The aim of this work was to describe the presence/distribution profile of the four adenosine receptors in six renal structures (superficial/deep glomeruli, proximal/distal tubules, loop of Henle, collecting tubule) of the hypertensive kidney and to evaluate whether it is altered by diabetes. Immunoreactivities against the adenosine receptors were analyzed in six renal structures from spontaneously hypertensive rats (SHR, the control group) and from SHR rats with diabetes induced by streptozotocyin (SHR-STZ group). Data showed, for the first time, that all adenosine receptors were present in the kidney of SHR rats, although the distribution pattern was specific for each adenosine receptor subtype. Also, induction of diabetes in the SHR was associated with downregulation of adenosine A_2A receptors, which might be relevant for the development of hypertensive diabetic nephropathy. This study highlights the adenosine A_2A receptors as a potential target to explore to prevent and/or treat early diabetes-induced hyperfiltration, at least in hypertensive conditions.

Overcoming Resistance to Combination Radiation-Immunotherapy: A Focus on Contributing Pathways Within the Tumor Microenvironment

Radiation therapy has been used for many years to treat tumors based on its DNA-damage-mediated ability to kill cells. More recently, RT has been shown to exert beneficial modulatory effects on immune responses, such as triggering immunogenic cell death, enhancing antigen presentation, and activating cytotoxic T cells. Consequently, combining radiation therapy with immunotherapy represents an important area of research. Thus far, immune-checkpoint inhibitors targeting programmed death-ligand 1 (PD-L1), programmed cell death protein 1 (PD-1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) have been the focus of many research studies and clinical trials. The available data suggest that such immunotherapies are enhanced when combined with radiation therapy. However, treatment resistance, intrinsic or acquired, is still prevalent. Various theories as to how to enhance these combination therapies to overcome treatment resistance have been proposed. In this review, we focus on the principles surrounding radiation therapy's positive and negative effects on the tumor microenvironment. We explore mechanisms underlying radiation therapy's synergistic and antagonistic effects on immune responses and provide a base of knowledge for radio-immunology combination therapies to overcome treatment resistance. We provide evidence for targeting regulatory T cells, tumor-associated macrophages, and cancer-associated fibroblasts in combination radio-immunotherapies to improve cancer treatment.

Who Is Who in Adenosine Transport

Extracellular adenosine concentrations are regulated by a panel of membrane transporters which, in most cases, mediate its uptake into cells. Adenosine transporters belong to two gene families encoding Equilibrative and Concentrative Nucleoside Transporter proteins (ENTs and CNTs, respectively). The lack of appropriate pharmacological tools targeting every transporter subtype has introduced some bias on the current knowledge of the role of these transporters in modulating adenosine levels. In this regard, ENT1, for which pharmacology is relatively well-developed, has often been identified as a major player in purinergic signaling. Nevertheless, other transporters such as CNT2 and CNT3 can also contribute to purinergic modulation based on their high affinity for adenosine and concentrative capacity. Moreover, both transporter proteins have also been shown to be under purinergic regulation via P1 receptors in different cell types, which further supports its relevance in purinergic signaling. Thus, several transporter proteins regulate extracellular adenosine levels. Moreover, CNT and ENT proteins are differentially expressed in tissues but also in particular cell types. Accordingly, transporter-mediated fine tuning of adenosine levels is cell and tissue specific. Future developments focusing on CNT pharmacology are needed to unveil transporter subtype-specific events.

Orally active, species-independent novel A3 adenosine receptor antagonist protects against kidney injury in db/db mice

Diabetic kidney disease (DKD) is the leading cause of end-stage kidney disease, and the current pharmacological treatment for DKD is limited to renin-angiotensin system (RAS) inhibitors. Adenosine is detectable in the kidney and is significantly elevated in response to cellular damage. While all 4 known subtypes of adenosine receptors, namely, A₁AR, A_2aAR, A_2bAR, and A₃AR, are expressed in the kidney, our previous study has demonstrated that a novel, orally active, species-independent, and selective A₃AR antagonist, LJ-1888, ameliorates unilateral ureteral obstruction-induced tubulointerstitial fibrosis. The present study examined the protective effects of LJ-2698, which has higher affinity and selectivity for A₃AR than LJ-1888, on DKD. In experiment I, dose-dependent effects of LJ-2698 were examined by orally administering 1.5, 5, or 10 mg/kg for 12 weeks to 8-week-old db/db mice. In experiment II, the effects of LJ-2698 (10 mg/kg) were compared to those of losartan (1.5 mg/kg), which is a standard treatment for patients with DKD. LJ-2698 effectively prevented kidney injuries such as albuminuria, glomerular hypertrophy, tubular injury, podocyte injury, fibrosis, inflammation, and oxidative stress in diabetic mice as much as losartan. In addition, inhibition of lipid accumulation along with increases in PGC1α, a master regulator of mitochondrial biogenesis, were demonstrated in diabetic mice treated with either LJ-2698 or losartan. These results suggest that LJ-2698, a selective A₃AR antagonist, may become a novel therapeutic agent against DKD.

A therapeutic treatment targeting a protein involved in the progression of diabetic kidney disease (DKD) shows promise in mouse trials. Between 30 and 40 per cent of diabetic patients suffer from DKD, a common cause to fatal end-stage kidney disease. Protein receptors, commonly expressed on cell surfaces throughout the body, play both positive and negative roles in diseases. The A₃ adenosine receptor (A₃AR) is highly expressed in diabetic kidney tissue, and is linked to disease progression. Hunjoo Ha at Ewha Womans University in Seoul, Republic of Korea, and co-workers demonstrated the positive effect of a novel drug in targeting A₃AR in mice with DKD. A 12-week treatment of the drug prevented kidney injury, lowered oxidative stress and inflammation, and improved kidney function. It may prove an invaluable drug, particularly in combination with an existing DKD drug.

P2X7 receptor and klotho expressions in diabetic nephropathy progression

Diabetes mellitus is characterized by increased levels of reactive oxygen species (ROS), leading to high levels of adenosine triphosphate (ATP) and the activation of purinergic receptors (P2X₇), which results in cell death. Klotho was recently described as a modulator of oxidative stress and as having anti-apoptotic properties, among others. However, the roles of P2X₇ and klotho in the progression of diabetic nephropathy are still unclear. In this context, the aim of the present study was to characterize P2X₇ and klotho in several stages of diabetes in rats. Diabetes was induced in Wistar rats by streptozotocin, while the control group rats received the drug vehicle. From the 1st to 8th weeks after the diabetes induction, the animals were placed in metabolic cages on the 1st day of each week for 24 h to analyze metabolic parameters and for the urine collection. Then, blood samples and the kidneys were collected for biochemical analysis, including Western blotting and qPCR for P2X₇ and klotho. Diabetic rats presented a progressive loss of renal function, with reduced nitric oxide and increased lipid peroxidation. The P2X₇ and klotho expressions were similar up to the 4th week; then, P2X₇ expression increased in diabetes mellitus (DM), but klotho expression presented an opposite behavior, until the 8th week. Our data show an inverse correlation between P2X₇ and klotho expressions through the development of DM, which suggests that the management of these molecules could be useful for controlling the progression of this disease and diabetic nephropathy.

Mitochondrial dysfunction in diabetic kidney disease

Globally, diabetes is the leading cause of chronic kidney disease and end-stage renal disease, which are major risk factors for cardiovascular disease and death. Despite this burden, the factors that precipitate the development and progression of diabetic kidney disease (DKD) remain to be fully elucidated. Mitochondrial dysfunction is associated with kidney disease in nondiabetic contexts, and increasing evidence suggests that dysfunctional renal mitochondria are pathological mediators of DKD. These complex organelles have a broad range of functions, including the generation of ATP. The kidneys are mitochondrially rich, highly metabolic organs that require vast amounts of ATP for their normal function. The delivery of metabolic substrates for ATP production, such as fatty acids and oxygen, is altered by diabetes. Changes in metabolic fuel sources in diabetes to meet ATP demands result in increased oxygen consumption, which contributes to renal hypoxia. Inherited factors including mutations in genes that impact mitochondrial function and/or substrate delivery may also be important risk factors for DKD. Hence, we postulate that the diabetic milieu and inherited factors that underlie abnormalities in mitochondrial function synergistically drive the development and progression of DKD.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

Deficient Insulin-mediated Upregulation of the Equilibrative Nucleoside Transporter 2 Contributes to Chronically Increased Adenosine in Diabetic Glomerulopathy

Deficient insulin signaling is a key event mediating diabetic glomerulopathy. Additionally, diabetic kidney disease has been related to increased levels of adenosine. Therefore, we tested a link between insulin deficiency and dysregulated activity of the equilibrative nucleoside transporters (ENTs) responsible for controlling extracellular levels of adenosine. In ex vivo glomeruli, high D-glucose decreased nucleoside uptake mediated by ENT1 and ENT2 transporters, resulting in augmented extracellular levels of adenosine. This condition was reversed by exposure to insulin. Particularly, insulin through insulin receptor/PI3K pathway markedly upregulated ENT2 uptake activity to restores the extracellular basal level of adenosine. Using primary cultured rat podocytes as a cellular model, we found insulin was able to increase ENT2 maximal velocity of transport. Also, PI3K activity was necessary to maintain ENT2 protein levels in the long term. In glomeruli of streptozotocin-induced diabetic rats, insulin deficiency leads to decreased activity of ENT2 and chronically increased extracellular levels of adenosine. Treatment of diabetic rats with adenosine deaminase attenuated both the glomerular loss of nephrin and proteinuria. In conclusion, we evidenced ENT2 as a target of insulin signaling and sensitive to dysregulation in diabetes, leading to chronically increased extracellular adenosine levels and thereby setting conditions conducive to kidney injury.

Pathological overproduction: the bad side of adenosine

Adenosine is an endogenous ubiquitous purine nucleoside, which is increased by hypoxia, ischaemia and tissue damage and mediates a number of physiopathological effects by interacting with four GPCRs, identified as A1 , A2A , A2B and A3 . Physiological and acutely increased adenosine is mostly associated with beneficial effects that include vasodilatation and a decrease in inflammation. In contrast, chronic overproduction of adenosine occurs in important pathological states, where long-lasting increases in the nucleoside levels are responsible for the bad side of adenosine associated with chronic inflammation, fibrosis and organ damage. In this review, we describe and critically discuss the pathological overproduction of adenosine and analyse when, where and how adenosine exerts its detrimental effects throughout the body.

Signalling pathways involved in hypoxia-induced renal fibrosis

Renal fibrosis is the common pathological hallmark of progressive chronic kidney disease (CKD) with diverse aetiologies. Recent researches have highlighted the critical role of hypoxia during the development of renal fibrosis as a final common pathway in end‐stage kidney disease (ESKD), which joints the scientist's attention recently to exploit the molecular mechanism underlying hypoxia‐induced renal fibrogenesis. The scaring formation is a multilayered cellular response and involves the regulation of multiple hypoxia‐inducible signalling pathways and complex interactive networks. Therefore, this review will focus on the signalling pathways involved in hypoxia‐induced pathogenesis of interstitial fibrosis, including pathways mediated by HIF, TGF‐β, Notch, PKC/ERK, PI3K/Akt, NF‐κB, Ang II/ROS and microRNAs. Roles of molecules such as IL‐6, IL‐18, KIM‐1 and ADO are also reviewed. A comprehensive understanding of the roles that these hypoxia‐responsive signalling pathways and molecules play in the context of renal fibrosis will provide a foundation towards revealing the underlying mechanisms of progression of CKD and identifying novel therapeutic targets. In the future, promising new effective therapy against hypoxic effects may be successfully translated into the clinic to alleviate renal fibrosis and inhibit the progression of CKD.

Adenosine contribution to normal renal physiology and chronic kidney disease

Adenosine is a nucleoside that is particularly interesting to many scientific and clinical communities as it has important physiological and pathophysiological roles in the kidney. The distribution of adenosine receptors has only recently been elucidated; therefore it is likely that more biological roles of this nucleoside will be unveiled in the near future. Since the discovery of the involvement of adenosine in renal vasoconstriction and regulation of local renin production, further evidence has shown that adenosine signaling is also involved in the tubuloglomerular feedback mechanism, sodium reabsorption and the adaptive response to acute insults, such as ischemia. However, the most interesting finding was the increased adenosine levels in chronic kidney diseases such as diabetic nephropathy and also in non-diabetic animal models of renal fibrosis. When adenosine is chronically increased its signaling via the adenosine receptors may change, switching to a state that induces renal damage and produces phenotypic changes in resident cells. This review discusses the physiological and pathophysiological roles of adenosine and pays special attention to the mechanisms associated with switching homeostatic nucleoside levels to increased adenosine production in kidneys affected by CKD.