Renal metabolism in 2017: Glycolytic adaptation and progression of kidney disease

Fatty acid binding protein 4 regulates doxorubicin-induced renal injury via mediating lipid metabolism and apoptosis

Doxorubicin(DOX) is an anthracycline chemotherapeutic drug used in the treatment of a wide range of tumors. However, the nephrotoxicity has limited the clinical application of DOX. Fatty acid-binding protein 4 (FABP4), a key adipokine mainly expressed in adipocytes, is strongly associated with kidney dysfunction and kidney disease. This study aims to investigate the role of FABP4 in DOX-induced nephrotoxicity. In vivo experiments, the mice were divided into 4 groups randomly: CON group, DOX group, 4T1 group, and 4T1 + DOX group respectively. DOX was given by intraperitoneal injection at a dose of 15 mg/kg. The kidney function indicators, the morphology and lipid deposition in renal tissues and the preliminary mechanism were assessed. In vitro experiment, HK-2 cell was used to detect DOX-induced kidney cell injury with or without BMS309403. DOX caused renal dysfunction in both DOX group and 4T1 + DOX group, with a more severe kidney injury in DOX group. DOX also induced kidney lipid deposition, glycerophospholipids metabolism dysfunction, apoptosis accompanied by increased FABP4 and decreased Peroxisome Proliferator Activated Receptor- γ (PPAR-γ) levels in the kidney tissues. The decreased cell viability, increased apoptotic ratio, elevated protein levels of apoptosis and the lipid deposition caused by DOX were all alleviated by BMS309403. FABP4 mediated DOX induced kidney damage in normal mice and tumor-bearing mice by lipid metabolism disorders and cell apoptosis. This study may enhance the clinical management of DOX-induced kidney injury and provide new therapeutic targets and preventive strategies for the clinical application of DOX.

ATGL regulates renal fibrosis by reprogramming lipid metabolism during the transition from AKI to CKD

Acute kidney injury (AKI) can progress to chronic kidney disease (CKD) and subsequently to renal fibrosis. Poor repair of renal tubular epithelial cells (TECs) after injury is the main cause of renal fibrosis. Studies have shown that restoring damaged fatty acid β-oxidation (FAO) can reduce renal fibrosis. Adipose triglyceride lipase (ATGL) is a key enzyme that regulates lipid hydrolysis. This study, for the first time, demonstrated that ATGL was downregulated in the renal TEC in the AKI-CKD transition mouse model. Moreover, treatment with the ATGL inhibitor atglistatin exacerbated lipid accumulation and downregulated the FAO level and mitochondrial function, while it increased the level of oxidative stress injury and apoptosis, resulting in aggravated renal fibrosis. In contrast, ATGL overexpression suppressed lipid accumulation, improved the FAO level and mitochondrial function, and attenuated oxidative stress and apoptosis, thereby ameliorating fibrosis in vitro and in vivo. In summary, ATGL regulates renal fibrosis by reprogramming lipid metabolism in renal TECs. This study provided new avenues and targets for treating CKD.

Role of mitochondria in reno-cardiac diseases: A study of bioenergetics, biogenesis, and GSH signaling in disease transition

Acute kidney injury (AKI) and chronic kidney disease (CKD) are global health burdens with rising prevalence. Their bidirectional relationship with cardiovascular dysfunction, manifesting as cardio-renal syndromes (CRS) types 3 and 4, underscores the interconnectedness and interdependence of these vital organ systems. Both the kidney and the heart are critically reliant on mitochondrial function. This organelle is currently recognized as a hub in signaling pathways, with emphasis on the redox regulation mediated by glutathione (GSH). Mitochondrial dysfunction, including impaired bioenergetics, redox, and biogenesis pathways, are central to the progression of AKI to CKD and the development of CRS type 3 and 4. This review delves into the metabolic reprogramming and mitochondrial redox signaling and biogenesis alterations in AKI, CKD, and CRS. We examine the pathophysiological mechanisms involving GSH redox signaling and the AMP-activated protein kinase (AMPK)-sirtuin (SIRT)1/3-peroxisome proliferator-activated receptor-gamma coactivator (PGC-1α) axis in these conditions. Additionally, we explore the therapeutic potential of GSH synthesis inducers in mitigating these mitochondrial dysfunctions, as well as their effects on inflammation and the progression of CKD and CRS types 3 and 4.

Targeting pyruvate kinase M2 for the treatment of kidney disease

Pyruvate kinase M2 (PKM2), a rate limiting enzyme in glycolysis, is a cellular regulator that has received extensive attention and regards as a metabolic regulator of cellular metabolism and energy. Kidney is a highly metabolically active organ, and glycolysis is the important energy resource for kidney. The accumulated evidences indicates that the enzymatic activity of PKM2 is disturbed in kidney disease progression and treatment, especially diabetic kidney disease and acute kidney injury. Modulating PKM2 post-translational modification determines its enzymatic activity and nuclear translocation that serves as an important interventional approach to regulate PKM2. Emerging evidences show that PKM2 and its post-translational modification participate in kidney disease progression and treatment through modulating metabolism regulation, podocyte injury, fibroblast activation and proliferation, macrophage polarization, and T cell regulation. Interestingly, PKM2 activators (TEPP-46, DASA-58, mitapivat, and TP-1454) and PKM2 inhibitors (shikonin, alkannin, compound 3k and compound 3h) have exhibited potential therapeutic property in kidney disease, which indicates the pleiotropic effects of PKM2 in kidney. In the future, the deep investigation of PKM2 pleiotropic effects in kidney is urgently needed to determine the therapeutic effect of PKM2 activator/inhibitor to benefit patients. The information in this review highlights that PKM2 functions as a potential biomarker and therapeutic target for kidney diseases.

Untargeted and spatial-resolved metabolomics characterize serum and tissue-specific metabolic reprogramming in acute kidney injury

BACKGROUND

Acute kidney injury (AKI) is one of the most common clinical emergencies characterized by rapid progression, difficulty in early diagnosis, and high mortality. Currently, there are no effective AKI early diagnostic methods and treatments. Therefore, identifying new mechanisms of AKI have become urgent for development new targets for early diagnosis and treatment of AKI in the current clinical setting.

METHODS

In this study, systematic analysis and comparison of serum metabolic profiles of clinical AKI patients, chronic kidney disease (CKD) patients, and healthy subjects were performed using untargeted metabolomics. Moreover, the first spatial metabolomic analysis of kidney tissues in an AKI mouse model using MALDI-TOF MS technology was conducted. Differentially expressed metabolites were identified using a comprehensive, publicly available database. The metabolic data obtained were evaluated using principal component analysis, (orthogonal) partial least squares discriminant analysis, and metabolic pathway analysis to explore the unique serum metabolic profile of the patients, as well as to characterize the spatial distribution of differential metabolites in the kidneys of AKI mice.

RESULTS

Significant changes in the metabolite levels of amino acids, carnitine, and lipids were observed in the AKI and CKD groups versus the healthy population, suggesting that kidney injury may lead to abnormalities in various metabolic pathways, such as amino acids, fatty acids, and lipids. The significant difference between the AKI and CKD groups were found for the first time in these indexes including amino acid, carnitine, fatty acid, and lipid levels. Additionally, spatial metabolomics results revealed that amino acid, carnitine, organic acid, and fatty acid metabolites were more likely significantly altered in the renal cortex, while lipid metabolites were both differentially distributed in the cortex and medulla of the AKI group.

CONCLUSION

Abnormalities in the serum metabolism of amino acids, carnitine, and lipids in patients with kidney diseases, such as AKI and CKD, are closely associated with the physiological dysfunction of kidney injury. Metabolic differences between patients with AKI and CKD were compared for the first time, showing that fatty acid oxidative inhibition was more severe in patients with AKI. Furthermore, spatial metabolomics has revealed metabolic reprogramming with tissue heterogeneity in AKI mice model. Our study provides valuable information in the molecular pathological features of AKI in the kidney tissues.

Dynamic cellular changes in acute kidney injury caused by different ischemia time

Ischemia reperfusion injury (IRI), often related to surgical procedures, is one of the important causes of acute kidney injury (AKI). To decipher the dynamic process of AKI caused by IRI (with prolonged ischemia phase), we performed single-cell RNA sequencing (scRNA-seq) of clinically relevant IRI murine model with different ischemic intervals. We discovered that Slc5a2^hi proximal tubular cells were susceptible to AKI and highly expressed neutral amino acid transporter gene Slc6a19, which was dramatically decreased over the time course. With the usage of mass spectrometry-based metabolomic analysis, we detected that the level of neutral amino acid isoleucine dropped off in AKI mouse plasma metabolites. And the reduction of plasma isoleucine was also verified in patients with cardiac surgery-associated acute kidney injury (CSA-AKI). The findings advanced the understanding of dynamic process of AKI and introduced reduction of isoleucine as a potential biomarker for CSA-AKI.

Orphan nuclear receptor COUP-TFII enhances myofibroblast glycolysis leading to kidney fibrosis

Recent studies demonstrate that metabolic disturbance, such as augmented glycolysis, contributes to fibrosis. The molecular regulation of this metabolic perturbation in fibrosis, however, has been elusive. COUP-TFII (also known as NR2F2) is an important regulator of glucose and lipid metabolism. Its contribution to organ fibrosis is undefined. Here, we found increased COUP-TFII expression in myofibroblasts in human fibrotic kidneys, lungs, kidney organoids, and mouse kidneys after injury. Genetic ablation of COUP-TFII in mice resulted in attenuation of injury-induced kidney fibrosis. A non-biased proteomic study revealed the suppression of fatty acid oxidation and the enhancement of glycolysis pathways in COUP-TFII overexpressing fibroblasts. Overexpression of COUP-TFII in fibroblasts also induced production of alpha-smooth muscle actin (αSMA) and collagen 1. Knockout of COUP-TFII decreased glycolysis and collagen 1 levels in fibroblasts. Chip-qPCR revealed the binding of COUP-TFII on the promoter of PGC1α. Overexpression of COUP-TFII reduced the cellular level of PGC1α. Targeting COUP-TFII serves as a novel treatment approach for mitigating fibrosis in chronic kidney disease and potentially fibrosis in other organs.

Kidney physiology and susceptibility to acute kidney injury: implications for renoprotection

Kidney damage varies according to the primary insult. Different aetiologies of acute kidney injury (AKI), including kidney ischaemia, exposure to nephrotoxins, dehydration or sepsis, are associated with characteristic patterns of damage and changes in gene expression, which can provide insight into the mechanisms that lead to persistent structural and functional damage. Early morphological alterations are driven by a delicate balance between energy demand and oxygen supply, which varies considerably in different regions of the kidney. The functional heterogeneity of the various nephron segments is reflected in their use of different metabolic pathways. AKI is often linked to defects in kidney oxygen supply, and some nephron segments might not be able to shift to anaerobic metabolism under low oxygen conditions or might have remarkably low basal oxygen levels, which enhances their vulnerability to damage. Here, we discuss why specific kidney regions are at particular risk of injury and how this information might help to delineate novel routes for mitigating injury and avoiding permanent damage. We suggest that the physiological heterogeneity of the kidney should be taken into account when exploring novel renoprotective strategies, such as improvement of kidney tissue oxygenation, stimulation of hypoxia signalling pathways and modulation of cellular energy metabolism.

Relieving lipid accumulation through UCP1 suppresses the progression of acute kidney injury by promoting the AMPK/ULK1/autophagy pathway

Rationale: Acute kidney injury (AKI) is a serious clinical emergency with an acute onset, rapid progression, and poor prognosis. Recent evidence suggests that AKI is accompanied by significant metabolic abnormalities, including alterations in lipid metabolism. However, the specific changes in lipids in AKI, and their role and regulation mechanisms are currently unclear.

Methods: Quantitative metabolomics was performed in AKI models to reveal the differences of lipid metabolism-related products. Regulated pathway was detected by western blot, qRT-PCR, immunoblot analysis and immunohistochemistry.

Results: The present study systematically analyzes the changes in lipid composition in AKI for the first time and find that the degree of lipid accumulation was highly correlated with uncoupling protein 1 (UCP1). Importantly, relieving lipid accumulation in AKI by upregulating UCP1 can significantly inhibit the progression of AKI through promoting AMPK/ULK1/autophagy pathway.

Conclusions: The present findings suggest that lipid accumulation in AKI is directly regulated by UCP1, which can activate cell autophagy and thus significantly inhibit disease progression. It will provide new ideas and targets for the treatment of AKI.

The pHLIP system as a vehicle for microRNAs in the kidney

MicroRNAs (miRNAs) are small endogenous RNAs that regulate gene expression through post-transcriptional repression of their target messenger RNAs. A study of changes in expression of certain miRNAs in the kidney has supplied evidence on their pathogenic role and therapeutic potential in nephrology. This review proposes a nanotechnology approach based on the binding of analogs or inhibitors of miRNAs formed by peptide nucleic acids (PNAs) to peptides with a transmembrane structure sensitive to a low pH, called pHLIPs (pH [low] insertion peptides). The review draws on the concept that an acidic pH in the microenvironment of the renal tubule may facilitate concentration and distribution of the pHLIP-PNA complex in this organ. In this context, we have demonstrated for the first time that targeted administration of miR-33 inhibitors with the pHLIP system effectively prevents the development of renal fibrosis, thus opening up this technology to new strategies for diagnosis and treatment of kidney diseases.

Endothelial-to-mesenchymal transition compromises vascular integrity to induce Myc-mediated metabolic reprogramming in kidney fibrosis

Endothelial-to-mesenchymal transition (EndMT) is a cellular transdifferentiation program in which endothelial cells partially lose their endothelial identity and acquire mesenchymal-like features. Renal capillary endothelial cells can undergo EndMT in association with persistent damage of the renal parenchyma. The functional consequence(s) of EndMT in kidney fibrosis remains unexplored. Here, we studied the effect of Twist or Snail deficiency in endothelial cells on EndMT in kidney fibrosis. Conditional deletion of Twist1 (which encodes Twist) or Snai1 (which encodes Snail) in VE-cadherin⁺ or Tie1⁺ endothelial cells inhibited the emergence of EndMT and improved kidney fibrosis in two different kidney injury/fibrosis mouse models. Suppression of EndMT limited peritubular vascular leakage, reduced tissue hypoxia, and preserved tubular epithelial health and function. Hypoxia, which was exacerbated by EndMT, resulted in increased Myc abundance in tubular epithelial cells, enhanced glycolysis, and suppression of fatty acid oxidation. Pharmacological suppression or epithelial-specific genetic ablation of Myc in tubular epithelial cells ameliorated fibrosis and restored renal parenchymal function and metabolic homeostasis. Together, these findings demonstrate a functional role for EndMT in the response to kidney capillary endothelial injury and highlight the contribution of endothelial-epithelial cross-talk in the development of kidney fibrosis with a potential for therapeutic intervention.

MicroRNA and mRNA analysis of angiotensin II-induced renal artery endothelial cell dysfunction

Continuous activation of angiotensin II (Ang II) induces renal vascular endothelial dysfunction, inflammation and oxidative stress, all of which may contribute to renal damage. MicroRNAs (miRs/miRNAs) play a crucial regulatory role in the pathogenesis of hypertensive nephropathy (HN). The present study aimed to assess the differential expression profiles of potential candidate genes involved in Ang II-induced rat renal artery endothelial cell (RRAEC) dysfunction and explore their possible functions. In the present study, the changes in energy metabolism and autophagy function in RRAECs were evaluated using the Seahorse XF Glycolysis Stress Test and dansylcadaverine/transmission electron microscopy following exposure to Ang II. Subsequently, mRNA-miRNA sequencing experiments were performed to determine the differential expression profiles of mRNAs and miRNAs. Integrated bioinformatics analysis was applied to further explore the molecular mechanisms of Ang II on endothelial injury induced by Ang II. The present data supported the notion that Ang II upregulated glycolysis levels and promoted autophagy activation in RRAECs. The sequencing data demonstrated that 443 mRNAs and 58 miRNAs were differentially expressed (DE) in response to Ang II exposure, where 66 mRNAs and 55 miRNAs were upregulated, while 377 mRNAs and 3 miRNAs were downregulated (fold change >1.5 or <0.67; P<0.05). Functional analysis indicated that DE mRNA and DE miRNA target genes were mainly associated with cell metabolism (metabolic pathways), differentiation (Th1 and Th2 cell differentiation), autophagy (autophagy-animal and autophagy-other) and repair (RNA-repair). To the best of the authors' knowledge, this is the first report on mRNA-miRNA integrated profiles of Ang II-induced RRAECs. The present results provided evidence suggesting that the miRNA-mediated effect on the ‘mTOR signaling pathway’ might play a role in Ang II-induced RRAEC injury by driving glycolysis and autophagy activation. Targeting miRNAs and their associated pathways may provide valuable insight into the clinical management of HN and may improve patient outcome.

Tubule-derived lactate is required for fibroblast activation in acute kidney injury

Acute kidney injury (AKI) is a highly prevalent medical syndrome associated with high mortality and morbidity. Several types of cells, including epithelial cells, vascular endothelial cells, pericytes, and macrophages, participate in the development of AKI. Recently, renal fibroblasts were found to play an important role in the regulation of tubular injury, repair, and recovery after AKI. However, the mechanisms underlying fibroblast activation and proliferation during the progression of AKI remain unclear. In the present study, we found many activated myofibroblasts located in the renal interstitium with an abundance of extracellular matrix deposition following folic acid-induced AKI. The proliferative pattern of tubular epithelial cells and interstitial cells following acute injury was different, indicating that the proliferation of fibroblasts followed the proliferation of tubular epithelial cells. Furthermore, we observed that proliferative tubular epithelial cells preferred aerobic glycolysis as the dominating metabolic pathway in the progression of AKI. Lactate generated from injured tubules was taken up by interstitial fibroblasts in the later stages of AKI, which induced fibroblast activation and proliferation in vitro. Early inhibition of lactate production in tubules by glycolytic inhibitors suppressed fibroblast activation after folic acid-induced injury. Collectively, these results support the important role of fibroblasts in the development of AKI and suggest that lactate produced by glycolysis in tubular epithelial cells is a novel regulator of fibroblast activation and proliferation.

Comparative effect of indomethacin (IndoM) on the enzymes of carbohydrate metabolism, brush border membrane and oxidative stress in the kidney, small intestine and liver of rats

Indomethacin (IndoM) has prominent anti-inflammatory and analgesic-antipyretic properties. However, high incidence and severity of side-effects on the structure and functions of the kidney, liver and intestine limits its clinical use. The present study tested the hypothesis that IndoM causes multi-organ toxicity by inducing oxidative stress that alters the structure of various cellular membranes, metabolism and hence functions. The effect of IndoM was determined on the enzymes of carbohydrate metabolism, brush border membrane (BBM) and oxidative stress in the rat kideny, liver and intestine to understand the mechanism of IndoM induced toxicity. Adult male Wister rats were given IndoM (20 mg/kg) intra-peritoneally in sodium bicarbonate twice a day for 3 d. The body weights of the rats were recorded before and after experimental procedure. IndoM administration significantly increased blood urea nitrogen, serum creatinine, cholesterol and alkaline phosphatase but inorganic phosphate indicating IndoM induced renal, hepatic and intestinal toxicity. Activity of lactate dehydrogenase along with glucose-6- and fructose-1, 6-bis phosphatase, glucose-6-phosphate dehydrogenase and NADP-malic enzyme increased but malate dehydrogenase decreased in all tissues. Lipid peroxidation (LPO) significantly increased whereas the antioxidant enzymes decreased in all rat tissues studied. The results indicate that IndoM administration caused severe damage to kidney, liver and intestine by icreasing LPO, suppressing antioxidant enzymes and inhibiting oxidative metablolism. The energy dependence was shifted to anaerobic glycolysis due to mitochondrial damage supported by increased gluconeogenesis to provide more glucose to meet energy requirements.