Changes in activities and mRNA levels of glycolytic enzymes of ischemia-reperfused rat kidney

Targeting Mitochondria and Metabolism in Acute Kidney Injury

Acute kidney injury (AKI) significantly contributes to morbidity and mortality in critically ill patients. AKI is also an independent risk factor for the development and progression of chronic kidney disease. Effective therapeutic strategies for AKI are limited, but emerging evidence indicates a prominent role of mitochondrial dysfunction and altered tubular metabolism in the pathogenesis of AKI. Therefore, a comprehensive, mechanistic understanding of mitochondrial function and renal metabolism in AKI may lead to the development of novel therapies in AKI. In this review, we provide an overview of current state of research on the role of mitochondria and tubular metabolism in AKI from both pre-clinical and clinical studies. We also highlight current therapeutic strategies which target mitochondrial function and metabolic pathways for the treatment of AKI.

Proximal Tubular Oxidative Metabolism in Acute Kidney Injury and the Transition to CKD

The proximal tubule relies on oxidative mitochondrial metabolism to meet its energy needs and has limited capacity for glycolysis, which makes it uniquely susceptible to damage during AKI, especially after ischemia and anoxia. Under these conditions, mitochondrial ATP production is initially decreased by several mechanisms, including fatty acid-induced uncoupling and inhibition of respiration related to changes in the shape and volume of mitochondria. Glycolysis is initially insufficient as a source of ATP to protect the cells and mitochondrial function, but supplementation of tricarboxylic acid cycle intermediates augments anaerobic ATP production, and improves recovery of mitochondrial oxidative metabolism. Incomplete recovery is characterized by defects of respiratory enzymes and lipid metabolism. During the transition to CKD, tubular cells atrophy but maintain high expression of glycolytic enzymes, and there is decreased fatty acid oxidation. These metabolic changes may be amenable to a number of therapeutic interventions.

Sodium-glucose cotransporter-2 inhibitors for diabetic kidney disease: Targeting Warburg effects in proximal tubular cells

Inhibitors of sodium-glucose cotransporter 2 (SGLT2) have undoubtedly shifted the paradigm for diabetes medicine and research and, especially, diabetic kidney disease (DKD). The pharmacological action of SGLT2 inhibitors is simply the release of glucose into urine; however, precisely how SGLT2 inhibitors contribute to the health of those with diabetes has still not been completely elucidated. Towards this end, the present review provides a novel insight into the action of SGLT2 inhibitors by highlighting a neglected fuel-burning system found in proximal tubular cells-'glycolysis'. In addition, exploring the details of the molecular mechanisms and clinical biomarkers of the organ protection conferred by SGLT2 inhibitors is now required to prepare for the next stage of clinical diabetes medicine-the 'post-SGLT2 inhibitor era'.

Metabolic reprogramming by N-acetyl-seryl-aspartyl-lysyl-proline protects against diabetic kidney disease

BACKGROUND AND PURPOSE

ACE inhibitors (ACEIs) and AT₁ receptor antagonists (ARBs) are first-line drugs that are believed to reduce the progression of end-stage renal disease in diabetic patients. Differences in the effects of ACEIs and ARBs are not well studied and the mechanisms responsible are not well understood.

EXPERIMENTAL APPROACH

Male diabetic CD-1 mice were treated with ACEI, ARB, N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP), ACEI + AcSDKP, ARB + AcSDKP, glycolysis inhibitors or non-treatment. Moreover, prolyl oligopeptidase inhibitor (POPi)-injected male diabetic C57Bl6 mice were treated with ACEI, AcSDKP and ARB or non-treatment. Western blot and immunofluorescent staining were used to examine key enzymes and regulators of central metabolism.

KEY RESULTS

The antifibrotic action of ACEI imidapril is due to an AcSDKP-mediated antifibrotic mechanism, which reprograms the central metabolism including restoring SIRT3 protein and mitochondrial fatty acid oxidation and suppression of abnormal glucose metabolism in the diabetic kidney. Moreover, the POPi S17092 significantly blocked the AcSDKP synthesis, accelerated kidney fibrosis and disrupted the central metabolism. ACEI partly restored the kidney fibrosis and elevated the AcSDKP level, whereas the ARB (TA-606) did not show such effects in the POPi-injected mice. ACE inhibition and AcSDKP suppressed defective metabolism-linked mesenchymal transformations and reduced collagen-I and fibronectin accumulation in the diabetic kidneys.

CONCLUSION AND IMPLICATIONS

The study envisages that AcSDKP is the endogenous antifibrotic mediator that controls the metabolic switch between glucose and fatty acid metabolism and that suppression of AcSDKP leads to disruption of kidney cell metabolism and activates mesenchymal transformations leading to severe fibrosis in the diabetic kidney.

SIRT3 deficiency leads to induction of abnormal glycolysis in diabetic kidney with fibrosis

The regulation of aberrant glucose metabolism in diabetes associated-kidney fibrosis is not well known. In this study we found the suppression of SIRT3 protein level in diabetic kidney, displays responsibility in fibrogenic programming associated with aberrant glycolysis and such abnormal glycolysis is the therapeutic target in diabetes associated-kidney fibrosis. When analyzing different strains of streptozotocin-induced diabetic mice model (fibrotic model: CD-1, less fibrotic model: C57Bl6), we found SIRT3 suppression was associated with kidney fibrosis in fibrotic CD-1; further SIRT3 suppression by systemic administration of SIRT3 siRNA in the diabetic mice, showed profound fibrogenic phenotype in the kidney. Such suppression in SIRT3 was associated with the induction of transforming growth factor-β (TGF-β)/smad signaling, higher level of HIF1α accumulation and PKM2 dimer formation; these alterations subsequently led to abnormal glycolysis and linked abnormal mesenchymal transformations in vivo and in vitro. Inhibition of such aberrant glycolysis suppressed fibrogenic programming and restored SIRT3 level as well. Such aberrant glycolysis was confirmed in the KK/Ta-Ins2Akita mouse, the mouse model of progressive diabetic kidney disease. These data demonstrate that SIRT3 deficiency promotes abnormal glycolysis which is responsible for the fibrogenic pathway in diabetic kidney. Restoration of SIRT3 could be an alternative strategy in combating diabetes associated-kidney fibrosis via inhibition of aberrant glycolysis.

Mitochondrial Pathology and Glycolytic Shift during Proximal Tubule Atrophy after Ischemic AKI

During recovery by regeneration after AKI, proximal tubule cells can fail to redifferentiate, undergo premature growth arrest, and become atrophic. The atrophic tubules display pathologically persistent signaling increases that trigger production of profibrotic peptides, proliferation of interstitial fibroblasts, and fibrosis. We studied proximal tubules after ischemia-reperfusion injury (IRI) to characterize possible mitochondrial pathologies and alterations of critical enzymes that govern energy metabolism. In rat kidneys, tubules undergoing atrophy late after IRI but not normally recovering tubules showed greatly reduced mitochondrial number, with rounded profiles, and large autophagolysosomes. Studies after IRI of kidneys in mice, done in parallel, showed large scale loss of the oxidant-sensitive mitochondrial protein Mpv17L. Renal expression of hypoxia markers also increased after IRI. During early and late reperfusion after IRI, kidneys exhibited increased lactate and pyruvate content and hexokinase activity, which are indicators of glycolysis. Furthermore, normally regenerating tubules as well as tubules undergoing atrophy exhibited increased glycolytic enzyme expression and inhibitory phosphorylation of pyruvate dehydrogenase. TGF-β antagonism prevented these effects. Our data show that the metabolic switch occurred early during regeneration after injury and was reversed during normal tubule recovery but persisted and became progressively more severe in tubule cells that failed to redifferentiate. In conclusion, irreversibility of the metabolic switch, taking place in the context of hypoxia, high TGF-β signaling and depletion of mitochondria characterizes the development of atrophy in proximal tubule cells and may contribute to the renal pathology after AKI.

PARP-1 inhibits glycolysis in ischemic kidneys

After ischemic renal injury (IRI), selective damage occurs in the S(3) segments of the proximal tubules as a result of inhibition of glycolysis, but the mechanism of this inhibition is unknown. We previously reported that inhibition of poly(ADP-ribose) polymerase-1 (PARP-1) activity protects against ischemia-induced necrosis in proximal tubules by preserving ATP levels. Here, we tested whether PARP-1 activation in proximal tubules after IRI leads to poly(ADP-ribosyl)ation of the key glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a modification that inhibits its activity. Using in vitro and in vivo models, under hypoxic conditions, we detected poly(ADP-ribosyl)ation and reduced activity of GAPDH; inhibition of PARP-1 activity restored GAPDH activity and ATP levels. Inhibition of GAPDH with iodoacetate exacerbated ATP depletion, cytotoxicity, and necrotic cell death of LLCPK(1) cells subjected to hypoxic conditions, whereas inhibition of PARP-1 activity was cytoprotective. In conclusion, these data indicate that poly(ADP-ribosyl)ation of GAPDH and the subsequent inhibition of anaerobic respiration exacerbate ATP depletion selectively in the proximal tubule after IRI.