The dysregulation of metabolic pathways and induction of the pentose phosphate pathway in renal ischaemia-reperfusion injury

Metabolism and bioenergetics in the pathophysiology of organ fibrosis

Fibrosis is the tissue scarring characterized by excess deposition of extracellular matrix (ECM) proteins, mainly collagens. A fibrotic response can take place in any tissue of the body and is the result of an imbalanced reaction to inflammation and wound healing. Metabolism has emerged as a major driver of fibrotic diseases. While glycolytic shifts appear to be a key metabolic switch in activated stromal ECM-producing cells, several other cell types such as immune cells, whose functions are intricately connected to their metabolic characteristics, form a complex network of pro-fibrotic cellular crosstalk. This review purports to clarify shared and particular cellular responses and mechanisms across organs and etiologies. We discuss the impact of the cell-type specific metabolic reprogramming in fibrotic diseases in both experimental and human pathology settings, providing a rationale for new therapeutic interventions based on metabolism-targeted antifibrotic agents.

Comprehensive overview of the role of mitochondrial dysfunction in the pathogenesis of acute kidney ischemia-reperfusion injury: a narrative review

Acute kidney ischemia-reperfusion (IR) injury is a life-threatening condition that predisposes individuals to chronic kidney disease. Since the kidney is one of the most energy-demanding organs in the human body and mitochondria are the powerhouse of cells, mitochondrial dysfunction plays a central role in the pathogenesis of IR-induced acute kidney injury. Mitochondrial dysfunction causes a reduction in adenosine triphosphate production, loss of mitochondrial dynamics (represented by persistent fragmentation), and impaired mitophagy. Furthermore, the pathological accumulation of succinate resulting from fumarate reduction under oxygen deprivation (ischemia) in the reverse flux of the Krebs cycle can eventually lead to a burst of reactive oxygen species driven by reverse electron transfer during the reperfusion phase. Accumulating evidence indicates that improving mitochondrial function, biogenesis, and dynamics, and normalizing metabolic reprogramming within the mitochondria have the potential to preserve kidney function during IR injury and prevent progression to chronic kidney disease. In this review, we summarize recent advances in understanding the detrimental role of metabolic reprogramming and mitochondrial dysfunction in IR injury and explore potential therapeutic strategies for treating kidney IR injury.

The metabolic pathway regulation in kidney injury and repair

Kidney injury and repair are accompanied by significant disruptions in metabolic pathways, leading to renal cell dysfunction and further contributing to the progression of renal pathology. This review outlines the complex involvement of various energy production pathways in glucose, lipid, amino acid, and ketone body metabolism within the kidney. We provide a comprehensive summary of the aberrant regulation of these metabolic pathways in kidney injury and repair. After acute kidney injury (AKI), there is notable mitochondrial damage and oxygen/nutrient deprivation, leading to reduced activity in glycolysis and mitochondrial bioenergetics. Additionally, disruptions occur in the pentose phosphate pathway (PPP), amino acid metabolism, and the supply of ketone bodies. The subsequent kidney repair phase is characterized by a metabolic shift toward glycolysis, along with decreased fatty acid β-oxidation and continued disturbances in amino acid metabolism. Furthermore, the impact of metabolism dysfunction on renal cell injury, regeneration, and the development of renal fibrosis is analyzed. Finally, we discuss the potential therapeutic strategies by targeting renal metabolic regulation to ameliorate kidney injury and fibrosis and promote kidney repair.

Energy metabolic reprogramming regulates programmed cell death of renal tubular epithelial cells and might serve as a new therapeutic target for acute kidney injury

Acute kidney injury (AKI) induces significant energy metabolic reprogramming in renal tubular epithelial cells (TECs), thereby altering lipid, glucose, and amino acid metabolism. The changes in lipid metabolism encompass not only the downregulation of fatty acid oxidation (FAO) but also changes in cell membrane lipids and triglycerides metabolism. Regarding glucose metabolism, AKI leads to increased glycolysis, activation of the pentose phosphate pathway (PPP), inhibition of gluconeogenesis, and upregulation of the polyol pathway. Research indicates that inhibiting glycolysis, promoting the PPP, and blocking the polyol pathway exhibit a protective effect on AKI-affected kidneys. Additionally, changes in amino acid metabolism, including branched-chain amino acids, glutamine, arginine, and tryptophan, play an important role in AKI progression. These metabolic changes are closely related to the programmed cell death of renal TECs, involving autophagy, apoptosis, necroptosis, pyroptosis, and ferroptosis. Notably, abnormal intracellular lipid accumulation can impede autophagic clearance, further exacerbating lipid accumulation and compromising autophagic function, forming a vicious cycle. Recent studies have demonstrated the potential of ameliorating AKI-induced kidney damage through calorie and dietary restriction. Consequently, modifying the energy metabolism of renal TECs and dietary patterns may be an effective strategy for AKI treatment.

Insulin-induced gene 2 protects against hepatic ischemia-reperfusion injury via metabolic remodeling

BACKGROUND

Hepatic ischemia-reperfusion (IR) injury is the primary reason for complications following hepatectomy and liver transplantation (LT). Insulin-induced gene 2 (Insig2) is one of several proteins that anchor the reticulum in the cytoplasm and is essential for metabolism and inflammatory responses. However, its function in IR injury remains ambiguous.

METHODS

Insig2 global knock-out (KO) mice and mice with adeno-associated-virus8 (AAV8)-delivered Insig2 hepatocyte-specific overexpression were subjected to a 70% hepatic IR model. Liver injury was assessed by monitoring hepatic histology, inflammatory responses, and apoptosis. Hypoxia/reoxygenation stimulation (H/R) of primary hepatocytes and hypoxia model induced by cobalt chloride (CoCl2) were used for in vitro experiments. Multi-omics analysis of transcriptomics, proteomics, and metabolomics was used to investigate the molecular mechanisms underlying Insig2.

RESULTS

Hepatic Insig2 expression was significantly reduced in clinical samples undergoing LT and the mouse IR model. Our findings showed that Insig2 depletion significantly aggravated IR-induced hepatic inflammation, cell death and injury, whereas Insig2 overexpression caused the opposite phenotypes. The results of in vitro H/R experiments were consistent with those in vivo. Mechanistically, multi-omics analysis revealed that Insig2 is associated with increased antioxidant pentose phosphate pathway (PPP) activity. The inhibition of glucose-6-phosphate-dehydrogenase (G6PD), a rate-limiting enzyme of PPP, rescued the protective effect of Insig2 overexpression, exacerbating liver injury. Finally, our findings indicated that mouse IR injury could be attenuated by developing a nanoparticle delivery system that enables liver-targeted delivery of substrate of PPP (glucose 6-phosphate).

CONCLUSIONS

Insig2 has a protective function in liver IR by upregulating the PPP activity and remodeling glucose metabolism. The supplementary glucose 6-phosphate (G6P) salt may serve as a viable therapeutic target for alleviating hepatic IR.

Metabolic reprogramming heterogeneity in chronic kidney disease

Fibrosis driven by excessive accumulation of extracellular matrix (ECM) is the hallmark of chronic kidney disease (CKD). Myofibroblasts, which are the cells responsible for ECM production, are activated by cross talk with injured proximal tubule and immune cells. Emerging evidence suggests that alterations in metabolism are not only a feature of but also play an influential role in the pathogenesis of renal fibrosis. The application of omics technologies to cell-tracing animal models and follow-up functional data suggest that cell-type-specific metabolic shifts have particular roles in the fibrogenic response. In this review, we cover the main metabolic reprogramming outcomes in renal fibrosis and provide a future perspective on the field of renal fibrometabolism.

Transcriptional regulation of proximal tubular metabolism in acute kidney injury

The kidney, and in particular the proximal tubule (PT), has a high demand for ATP, due to its function in bulk reabsorption of solutes. In normal PT, ATP levels are predominantly maintained by fatty acid β-oxidation (FAO), the tricarboxylic acid (TCA) cycle, and oxidative phosphorylation. The normal PT also undertakes gluconeogenesis and metabolism of amino acids. Acute kidney injury (AKI) results in profound PT metabolic alterations, including suppression of FAO, gluconeogenesis, and metabolism of some amino acids, and upregulation of glycolytic enzymes. Recent studies have elucidated new transcriptional mechanisms regulating metabolic pathways in normal PT, as well as the metabolic switch in AKI. A number of transcription factors have been shown to play important roles in FAO, which are themselves downregulated in AKI, while hypoxia-inducible factor 1α, which is upregulated in ischemia-reperfusion injury, is a likely driver of the upregulation of glycolytic enzymes. Transcriptional regulation of amino acid metabolic pathways is less well understood, except for catabolism of branched-chain amino acids, which is likely suppressed in AKI by upregulation of Krüppel-like factor 6. This review will focus on the transcriptional regulation of specific metabolic pathways in normal PT and in AKI, as well as highlighting some of the gaps in knowledge and challenges that remain to be addressed.

Disturbing effect of cepharanthine on valve interstitial cells calcification via regulating glycolytic metabolism pathways

Osteogenic differentiation of valve interstitial cells (VICs) directly leads to aortic valve calcification, which is a common cardiovascular disease caused by inflammation and metabolic disorder. There is still no ideal drug for its treatment and prevention. The purpose of this study was to explore the effect and molecular mechanism of cepharanthine (CEP), a natural product, on inhibiting the osteogenic differentiation of VICs. First, CCK8 assay was used to evaluate cell viability of CEP on VICs. CEP concentration of 10 μM was the effective dose with slight cytotoxicity, which was used for further study. The alizarin red staining analysis showed that CEP significantly inhibited calcium deposition caused by osteogenic medium related calcification induction. In order to explore the anti-calcification molecular mechanism of CEP, transcriptome and metabolome were synchronously used to discover the possible molecular mechanism and target of CEP. The results showed that CEP inhibited valve calcification by regulating the glycolytic pathway. The molecular docking of CEP and selected key factors in glycolysis showed significant binding energies for GLUT1 (-11.3 kcal/mol), ENO1 (-10.6 kcal/mol), PKM (-9.8 kcal/mol), HK2 (-9.2 kcal/mol), PFKM (-9.0 kcal/mol), and PFKP (-8.9 kcal/mol). The correlation analysis of RUNX2 expression and cellular lactate content showed R2 of 0.7 (p < 0.001). In conclusion, this study demonstrated that CEP inhibited osteoblastic differentiation of VICs by interfering with glycolytic metabolisms via downregulation of the production of lactate and glycolysis-associated metabolites.

An integrated network pharmacology and cell metabolomics approach to reveal the role of rhein, a novel PPARα agonist, against renal fibrosis by activating the PPARα-CPT1A axis

BACKGROUND

Rhein, an anthraquinone compound, displays extensive antifibrotic effects; however, its potential mechanisms are not fully understood. In this study, we explored the underlying molecular mechanism of action of rhein.

METHOD

An integrated network pharmacology and cell metabolomics approach was developed based on network pharmacology and bioinformatics method, and then successfully applied to speculate the potential targets of rhein and construct a rhein-target-metabolic enzyme-metabolite network. Thereafter, the antifibrotic mechanism of rhein was validated in TGF-β- and oleic acid- induced HK-2 and NRK-52E cells in vitro as well as a unilateral ischemia-reperfusion injury Sprague-Dawley rat model.

RESULTS

Based on the construction of the rhein-target-metabolic enzyme-metabolite network, we found that rhein played an antifibrotic role through the PPAR-α-CPT1A-l-palmitoyl-carnitine axis. In vitro experiments demonstrated that rhein effectively activated the expression of PPARα and its downstream proteins (CPT1A and ACOX1) to alleviate lipid accumulation and fibrosis development. In vivo experiments indicated that rhein attenuated renal fibrosis mainly by activating the fatty acid oxidation pathway and improving lipid metabolism.

CONCLUSION

Taken together, our findings reveal that rhein is a novel agonist of PPARα, which contributes to its renoprotection through the regulation of the PPARα-CPT1A axis. Moreover, our study provides a novel insight into an integrated network pharmacology-metabolomics strategy for uncovering the pharmacological mechanisms of drugs from the system perspective.

Immunometabolic rewiring of tubular epithelial cells in kidney disease

Kidney tubular epithelial cells (TECs) have a crucial role in the damage and repair response to acute and chronic injury. To adequately respond to constant changes in the environment, TECs have considerable bioenergetic needs, which are supported by metabolic pathways. Although little is known about TEC metabolism, a number of ground-breaking studies have shown that defective glucose metabolism or fatty acid oxidation in the kidney has a key role in the response to kidney injury. Imbalanced use of these metabolic pathways can predispose TECs to apoptosis and dedifferentiation, and contribute to lipotoxicity and kidney injury. The accumulation of lipids and aberrant metabolic adaptations of TECs during kidney disease can also be driven by receptors of the innate immune system. Similar to their actions in innate immune cells, pattern recognition receptors regulate the metabolic rewiring of TECs, causing cellular dysfunction and lipid accumulation. TECs should therefore be considered a specialized cell type - like cells of the innate immune system - that is subject to regulation by immunometabolism. Targeting energy metabolism in TECs could represent a strategy for metabolically reprogramming the kidney and promoting kidney repair.

Comprehensively characterizing cellular changes and the expression of THSD7A and PLA2R1 under multiple in vitro models of podocyte injury

The unique morphology and gene expression of podocytes are critical for kidney function, and their abnormalities lead to nephropathies such as diabetic nephropathy and membranous nephropathy. Podocytes cultured in vitro are valuable tools to dissect the molecular mechanism of podocyte injury relative to nephropathy, however, these models have never been comprehensively compared. Here, we comprehensively compared the morphology, cytoskeleton, cell adhesion, cell spreading, cell migration, and lipid metabolism under five commonly used in vitro models including lipopolysaccharide (LPS), puromycin aminonucleoside (PAN), doxorubicin (Dox), high glucose, and glucose deprivation. Our results indicate that all stimulations significantly downregulate the expression of synaptopodin both in human and mouse podocytes. All stimulations affect podocyte morphology but show different intensity and phenotypes. In general, the five stimulations reduce cell adhesion, cell spreading, and cell migration, but the effect in human and mouse podocytes is slightly different. Human podocytes show high expression of genes enriched in the pentose phosphate pathway. Dox and PAN treatment show a strong effect on gene expression in lipid metabolism, while the other three stimulations show minimal effect. The expression of phospholipase A2 receptor (PLA2R1) and type-1 domain-containing protein 7 A (THSD7A) show opposite trends in given cells. Stimulations can dramatically affect the expression of PLA2R1 and THSD7A. Inhibition of super-enhancers reduces PLA2R1 and THSD7A expression, but ERK inhibition enhances their expression. Our results demonstrate distinctive responses in five commonly used in vitro podocyte injury models and the dynamic expression of PLA2R1 and THSD7A, which supply novel information to select suitable podocyte injury models.

Gene expression profile and injury sites in mice treated with Shiga toxin 2 and lipopolysaccharide as a Shiga toxin-associated hemolytic uremic syndrome model

Shiga toxin 2 (Stx2) and lipopolysaccharide (LPS) contribute to the development of hemolytic uremic syndrome (HUS). Mouse models of HUS induced by LPS/Stx2 have been used for elucidating HUS pathophysiology and for therapeutic development. However, the underlying molecular mechanisms and detailed injury sites in this model remain unknown. We analyzed mouse kidneys after LPS/Stx2 administration using microarrays. Decreased urinary osmolality and urinary potassium were observed after LPS/Stx2 administration, suggestive of distal nephron disorders. A total of 1212 and 1016 differentially expressed genes were identified in microarrays at 6 and 72 h after LPS/Stx2 administration, respectively, compared with those in controls. Ingenuity pathway analysis revealed activation of TNFR1/2, iNOS, and IL-6 signaling at both time points, and inhibition of pathways associated with lipid metabolism at 72 h only. The strongly downregulated genes in the 72-h group were expressed in the distal nephrons. In particular, genes associated with distal convoluted tubule (DCT) 2 /connecting tubule (CNT) and principal cells of the cortical collection duct (CCD) were downregulated to a greater extent than those associated with DCT1 and intercalated cells. Stx receptor globotriaosylceramide 3 (Gb3) revealed no colocalization with DCT1-specific Pvalb and intercalated cell-specific Slc26a4 but did present colocalization with Slc12a3 (present in both DCT1 and DCT2), and Aqp2 in principal cells. Gb3 localization tended to coincide with the segment in which the downregulated genes were present. Thus, the LPS/Stx2-induced kidney injury model represents damage to DCT2/CNT and principal cells in the CCD, based on molecular, biological, and physiological findings.

Metabolic Regulation of Fibroblast Activation and Proliferation during Organ Fibrosis

Background

Activated fibroblasts are present in the injury response, tumorigenesis, fibrosis, and inflammation in a variety of tissues and myriad disease types.

Summary

During normal tissue repair, quiescent fibroblasts transform into a proliferative and contractile phenotype termed myofibroblasts and are then lost as repair resolves to form a scar. When excessive levels are reached, activated fibroblasts proliferate and produce large amounts of extracellular matrix, which accumulates in the interstitial space of different organs. This accumulation leads to fibrotic dysfunction and multiple-organ dysfunction syndrome. To date, there are limited effective treatments for these conditions. Cellular metabolism is the cornerstone of all biological activities. Emerging evidence shows that metabolic alterations in fibroblasts are important for the activation process and illness progression. These discoveries, along with current clinical advances showing decreased lung fibrosis after targeting specific metabolic pathways, thus offer new possibilities for therapeutic interventions. The purpose of this review was to summarize the most recent knowledge of the major metabolic changes that occur during fibroblast transition from quiescent to activated states and the evidence linking alterations in fibroblast metabolism to the pathobiology of several common fibrotic diseases and tumor-related diseases.

Key Messages

Metabolic disorders are associated with the progression of chronic kidney diseases. Interfering with fibroblast metabolism may be a promising therapeutic strategy for renal fibrosis and other fibrosis-related diseases.

Carnitine Palmitoyltransferase System: A New Target for Anti-Inflammatory and Anticancer Therapy?

Lipid metabolism involves multiple biological processes. As one of the most important lipid metabolic pathways, fatty acid oxidation (FAO) and its key rate-limiting enzyme, the carnitine palmitoyltransferase (CPT) system, regulate host immune responses and thus are of great clinical significance. The effect of the CPT system on different tissues or organs is complex: the deficiency or over-activation of CPT disrupts the immune homeostasis by causing energy metabolism disorder and inflammatory oxidative damage and therefore contributes to the development of various acute and chronic inflammatory disorders and cancer. Accordingly, agonists or antagonists targeting the CPT system may become novel approaches for the treatment of diseases. In this review, we first briefly describe the structure, distribution, and physiological action of the CPT system. We then summarize the pathophysiological role of the CPT system in chronic obstructive pulmonary disease, bronchial asthma, acute lung injury, chronic granulomatous disease, nonalcoholic fatty liver disease, hepatic ischemia–reperfusion injury, kidney fibrosis, acute kidney injury, cardiovascular disorders, and cancer. We are also concerned with the current knowledge in either preclinical or clinical studies of various CPT activators/inhibitors for the management of diseases. These compounds range from traditional Chinese medicines to novel nanodevices. Although great efforts have been made in studying the different kinds of CPT agonists/antagonists, only a few pharmaceuticals have been applied for clinical uses. Nevertheless, research on CPT activation or inhibition highlights the pharmacological modulation of CPT-dependent FAO, especially on different CPT isoforms, as a promising anti-inflammatory/antitumor therapeutic strategy for numerous disorders.

Hepatocyte nuclear factor-1β shapes the energetic homeostasis of kidney tubule cells

Energetic metabolism controls key steps of kidney development, homeostasis, and epithelial repair following acute kidney injury (AKI). Hepatocyte nuclear factor-1β (HNF-1β) is a master transcription factor that controls mitochondrial function in proximal tubule (PT) cells. Patients with HNF1B pathogenic variant display a wide range of kidney developmental abnormalities and progressive kidney fibrosis. Characterizing the metabolic changes in PT cells with HNF-1β deficiency may help to identify new targetable molecular hubs involved in HNF1B-related kidney phenotypes and AKI. Here, we combined ¹ H-NMR-based metabolomic analysis in a murine PT cell line with CrispR/Cas9-induced Hnf1b invalidation (Hnf1b^-/- ), clustering analysis, targeted metabolic assays, and datamining of published RNA-seq and ChIP-seq dataset to identify the role of HNF-1β in metabolism. Hnf1b^-/- cells grown in normoxic conditions display intracellular ATP depletion, increased cytosolic lactate concentration, increased lipid droplet content, failure to use pyruvate for energetic purposes, increased levels of tricarboxylic acid (TCA) cycle intermediates and oxidized glutathione, and a reduction of TCA cycle byproducts, all features consistent with mitochondrial dysfunction and an irreversible switch toward glycolysis. Unsupervised clustering analysis showed that Hnf1b^-/- cells mimic a hypoxic signature and that they cannot furthermore increase glycolysis-dependent energetic supply during hypoxic challenge. Metabolome analysis also showed alteration of phospholipid biosynthesis in Hnf1b^-/- cells leading to the identification of Chka, the gene coding for choline kinase α, as a new putative target of HNF-1β. HNF-1β shapes the energetic metabolism of PT cells and HNF1B deficiency in patients could lead to a hypoxia-like metabolic state precluding further adaptation to ATP depletion following AKI.