Proximal tubule hypertrophy and hyperfunction: a novel pathophysiological feature in disease states.

The role of proximal tubules (PTs), a major component of the renal tubular structure in the renal cortex, has been examined extensively. Along with its physiological role in the reabsorption of various molecules, including electrolytes, amino acids and monosaccharides, transcellular transport of different hormones and regulation of homeostasis, pathological events affecting PTs may underlie multiple disease states. PT hypertrophy or a hyperfunctioning state, despite being a compensatory mechanism at first in response to various stimuli or alterations at tubular transport proteins, have been shown to be critical pathophysiological events leading to multiple disorders, including diabetes mellitus, obesity, metabolic syndrome and congestive heart failure. Moreover, pharmacotherapeutic agents have primarily targeted PTs, including sodium-glucose cotransporter 2, urate transporters and carbonic anhydrase enzymes. In this narrative review, we focus on the physiological role of PTs in healthy states and the current understanding of the PT pathologies leading to disease states and potential therapeutic targets.

Renal Proximal Tubule Cell-specific Megalin Deletion Does Not Affect Atherosclerosis But Induces Tubulointerstitial Nephritis in Mice Fed Western Diet.

Background: Pharmacological inhibition of megalin (also known as low-density lipoprotein receptor-related protein 2: LRP2) attenuates atherosclerosis in hypercholesterolemic mice. Since megalin is abundant in renal proximal tubule cells (PTCs), the purpose of this study was to determine whether PTC-specific deletion of megalin reduces hypercholesterolemia-induced atherosclerosis in mice. Methods: Female Lrp2 f/f mice were bred with male Ndrg1-Cre ERT2 +/0 mice to develop PTC-LRP2 +/+ and -/- littermates. To study atherosclerosis, all mice were to bred to an LDL receptor -/- background and fed a Western diet to induce atherosclerosis. Results: PTC-specific megalin deletion did not attenuate atherosclerosis in LDL receptor -/- mice in either sex. Serendipitously, we discovered that PTC-specific megalin deletion led to interstitial infiltration of CD68+ cells and tubular atrophy. The pathology was only evident in male PTC-LRP2 -/- mice fed the Western diet, but not in mice fed a normal laboratory diet. Renal pathologies were also observed in male PTC-LRP2 -/- mice in an LDL receptor +/+ background fed the same Western diet, demonstrating that the renal pathologies were dependent on diet and not hypercholesterolemia. By contrast, female PTC-LRP2 -/- mice had no apparent renal pathologies. In vivo multiphoton microscopy demonstrated that PTC-specific megalin deletion dramatically diminished albumin accumulation in PTCs within 10 days of Western diet feeding. RNA sequencing analyses demonstrated the upregulation of inflammation-related pathways in kidney. Conclusions: PTC-specific megalin deletion does not affect atherosclerosis, but leads to tubulointerstitial nephritis in mice fed Western diet, with severe pathologies in male mice.

Therapeutic α-1-microglobulin ameliorates kidney ischemia-reperfusion injury.

α-1-Microglobulin (A1M) is a circulating glycoprotein with antioxidant, heme-binding, and mitochondrial protection properties. The investigational drug RMC-035, a modified therapeutic A1M protein, was assessed for biodistribution and pharmacological activity in a broad set of in vitro and in vivo experiments, supporting its clinical development. Efficacy and treatment posology were assessed in various models of kidney ischemia and reperfusion injury (IRI). Real-time glomerular filtration rate (GFR), functional renal biomarkers, tubular injury biomarkers (NGAL and KIM-1), and histopathology were evaluated. Fluorescently labeled RMC-035 was used to assess biodistribution. RMC-035 demonstrated consistent and reproducible kidney protection in rat IRI models as well as in a model of IRI imposed on renal impairment and in a mouse IRI model, where it reduced mortality. Its pharmacological activity was most pronounced with combined dosing pre- and post-ischemia and weaker with either pre- or post-ischemia dosing alone. RMC-035 rapidly distributed to the kidneys via glomerular filtration and selective luminal uptake by proximal tubular cells. IRI-induced expression of kidney heme oxygenase-1 was inhibited by RMC-035, consistent with its antioxidative properties. RMC-035 also dampened IRI-associated inflammation and improved mitochondrial function, as shown by tubular autofluorescence. Taken together, the efficacy of RMC-035 is congruent with its targeted mechanism(s) and biodistribution profile, supporting its further clinical evaluation as a novel kidney-protective therapy.NEW & NOTEWORTHY A therapeutic A1M protein variant (RMC-035) is currently in phase 2 clinical development for renal protection in patients undergoing open-chest cardiac surgery. It targets several key pathways underlying kidney injury in this patient group, including oxidative stress, heme toxicity, and mitochondrial dysfunction. RMC-035 is rapidly eliminated from plasma, distributing to kidney proximal tubules, and demonstrates dose-dependent efficacy in numerous models of ischemia-reperfusion injury, particularly when administered before ischemia. These results support its continued clinical evaluation.

Role of Organic Anion Transporter NPT4 in Renal Handling of Uremic Toxin 3-indoxyl Sulfate.

Sodium-phosphate transporter NPT4 (SLC17A3) is a membrane transporter for organic anionic compounds localized on the apical membranes of kidney proximal tubular epithelial cells and plays a role in the urinary excretion of organic anionic compounds. However, its physiological role has not been sufficiently elucidated because its substrate specificity is yet to be determined. The present study aimed to comprehensively explore the physiological substrates of NPT4 in newly developed Slc17a3-/- mice using a metabolomic approach. Metabolomic analysis showed that the plasma concentrations of 11 biological substances, including 3-indoxyl sulfate, were more than two-fold higher in Slc17a3-/- mice than in wild-type mice. Moreover, urinary excretion of 3-indoxyl sulfate was reduced in Slc17a3-/- mice compared to that in wild-type mice. The uptake of 3-indoxyl sulfate by NPT4-expressing Xenopus oocytes was significantly higher than that by water-injected oocytes. The calculated Km and Vmax values for NPT4-mediated 3-indoxyl sulfate uptake were 4.52 ± 1.18 mM and 1.45 ± 0.14 nmol/oocyte/90 min, respectively. In conclusion, the present study revealed that 3-indoxyl sulfate is a novel substrate of NPT4 based on the metabolomic analysis of Slc17a3-/- mice, suggesting that NPT4 regulates systemic exposure to 3-indoxyl sulfate by regulating its urinary excretion.

Tubular Endogenous Erythropoietin Protects Renal Function against Ischemic Reperfusion Injury.

Many large-scale studies show that exogenous erythropoietin, erythropoiesis-stimulating agents, lack any renoprotective effects. We investigated the effects of endogenous erythropoietin on renal function in kidney ischemic reperfusion injury (IRI) using the prolyl hydroxylase domain (PHD) inhibitor, Roxadustat (ROX). Four h of hypoxia (7% O2) and 4 h treatment by ROX prior to IRI did not improve renal function. In contrast, 24-72 h pretreatment by ROX significantly improved the decline of renal function caused by IRI. Hypoxia and 4 h ROX increased interstitial cells-derived Epo production by 75- and 6-fold, respectively, before IRI, and worked similarly to exogenous Epo. ROX treatment for 24-72 h increased Epo production during IRI by 9-fold. Immunohistochemistry revealed that 24 h ROX treatment induced Epo production in proximal and distal tubules and worked similarly to endogenous Epo. Our data show that tubular endogenous Epo production induced by 24-72 h ROX treatment results in renoprotection but peritubular exogenous Epo production by interstitial cells induced by hypoxia and 4 h ROX treatment did not. Stimulation of tubular, but not peritubular, Epo production may link to renoprotection.

Manipulation of components of the renin angiotensin system in renal proximal tubules fails to alter atherosclerosis in hypercholesterolemic mice.

Background and objective: Whole body manipulation of the renin-angiotensin system (RAS) consistently exerts profound effects on experimental atherosclerosis development. A deficit in the literature has been a lack of attention to the effects of sex. Also, based on data with gene-deleted mice, the site of RAS activity that influences lesion formation is at an unknown distant location. Since angiotensin (AngII) concentrations are high in kidney and the major components of the RAS are present in renal proximal tubule cells (PTCs), this study evaluated the role of the RAS in PTCs in atherosclerosis development. Methods and results: Mice with an LDL receptor -/- background were fed Western diet to induce hypercholesterolemia and atherosclerosis. We first demonstrated the role of AT1 receptor antagonism on atherosclerosis in both sexes. Losartan, an AngII type 1 (AT1) receptor blocker, had greater blood pressure-lowering effects in females than males, but equivalent effects between sexes in reducing atherosclerotic lesion size. To determine the roles of renal AT1a receptor and angiotensin-converting enzyme (ACE), either component was deleted in PTCs after weaning using a tamoxifen-inducible Cre expressed under the control of an Ndrg1 promoter. Despite profound deletion of AT1a receptor or ACE in PTCs, the absence of either protein did not influence development of atherosclerosis in either sex. Conversely, mice expressing human angiotensinogen and renin in PTCs or expressing human angiotensinogen in liver but human renin in PTCs did not change atherosclerotic lesion size in male mice. Conclusion: Whole-body AT1R inhibition reduced atherosclerosis equivalently in both male and female mice; however, PTC-specific manipulation of the RAS components had no effects on hypercholesterolemia-induced atherosclerosis.

Tubular Secretion and Estimated GFR Decline in the Jackson Heart Study.

Introduction: Secretion of solutes by the proximal tubules represents an intrinsic kidney function not directly reflected by the glomerular filtration rate (GFR). The early loss of secretory clearance may reflect unrecognized kidney dysfunction, portending future disease progression. Methods: We designed a nested case-control study within the Jackson Heart Study (JHS), a prospective study of African American adults in Mississippi, to associate baseline differences in proximal tubular secretion of 5 endogenously produced solutes with future estimated glomerular rate (eGFR) decline. We matched 127 pairs by creatinine-eGFR, age, diabetes, and sex among the patients who provided a 24-hour urine collection; cases had a ≥25% decline in eGFR compared to <10% in controls over 10 years of follow-up. We measured baseline plasma and urine concentrations of secretory solutes using liquid chromatography-mass spectrometry to determine the odds ratio of kidney disease progression. Results: Mean age was 60 years; 76% were women; 30% had diabetes; mean baseline eGFR was 94±20 ml/min per 1.73 m2. The eGFR decline over 10 years was 38±13% in cases and 0±10% in controls. After adjustment for the matching variables plus albuminuria, systolic blood pressure, body mass index, and smoking, each 50% lower kidney clearance of isovalerylglycine, kynurenic acid, and xanthosine were associated with 1.4 to 2.2 greater odds of eGFR decline. Kynurenic acid exhibited the strongest association; each 50% lower clearance of this secretory solute was associated with 2.20-fold higher odds of eGFR decline (95% confidence interval [CI] 1.32-3.67). Conclusion: We found that in this community-based study of adults without significant kidney disease, lower proximal tubular secretory solute clearance is associated with future eGFR decline.

Sodium arsenite but not aluminum chloride stimulates ABC transporter activity in renal proximal tubules of killifish (Fundulus heteroclitus).

ABC export proteins including Multidrug resistance-related protein 2 (Mrp2) serve as detoxification mechanism in renal proximal tubules due to active transport of xenobiotics and metabolic waste products into primary urine. The environmental pollutants aluminum and arsenic interfere with a multitude of regulatory mechanisms in the body and here their impact on ABC transporter function was studied. NaAsO2 but not AlCl3 rapidly stimulated Mrp2-mediated Texas Red (TR) transport in isolated renal proximal tubules from killifish, a well-established laboratory model for the determination of efflux transporter activity by utilizing fluorescent substrates for the ABC transporters of interest and confocal microscopy followed by image analysis. This observed stimulation remained unaffected by the translation inhibitor cycloheximide (CHX), but it was abrogated by antagonists and inhibitors of the endothelin receptor type B (ETB)/nitric oxide synthase (NOS)/protein kinase C (PKC) signaling pathway. NaAsO2-triggered effects were abolished as a consequence of PKCα inhibition through Gö6976 and PKCα inhibitor peptide C2-4. Phosphatidylinositol 3-kinase (PI3K) inhibitor LY 294,002 as well as the mammalian target of rapamycin (mTOR) inhibitor rapamycin suppressed NaAsO2-triggered stimulation of luminal TR transport. In addition, the stimulatory effect of NaAsO2 was abolished by GSK650394, an inhibitor of serum- and glucocorticoid-inducible kinase 1 (SGK1), which is an important downstream target. Environmentally relevant concentrations of NaAsO2 further stimulated transport function of P-glycoprotein (P-gp), Multidrug resistance-related protein 4 (Mrp4) and Breast cancer resistance protein (Bcrp) while AlCl3 was ineffective. To our knowledge, this is the first report engaging in the impact of NaAsO2 on efflux transporter signaling and it may contribute to the understanding of defense mechanisms versus this worrying pollutant.

Insulin-induced mTOR signaling and gluconeogenesis in renal proximal tubules: A mini-review of current evidence and therapeutic potential.

Energy is continuously expended in the body, and gluconeogenesis maintains glucose homeostasis during starvation. Gluconeogenesis occurs in the liver and kidneys. The proximal tubule is the primary location for renal gluconeogenesis, accounting for up to 25% and 60% of endogenous glucose production during fasting and after a meal, respectively. The mechanistic target of rapamycin (mTOR), which exists downstream of the insulin pathway, plays an important role in regulating proximal tubular gluconeogenesis. mTOR is an atypical serine/threonine kinase present in two complexes. mTORC1 phosphorylates substrates that enhance anabolic processes such as mRNA translation and lipid synthesis and catabolic processes such as autophagy. mTORC2 regulates cytoskeletal dynamics and controls ion transport and proliferation via phosphorylation of SGK1. Therefore, mTOR signaling defects have been implicated in various pathological conditions, including cancer, cardiovascular disease, and diabetes. However, concrete elucidations of the associated mechanisms are still unclear. This review provides an overview of mTOR and describes the relationship between mTOR and renal.

Ceramide-1-Phosphate as a Potential Regulator of the Second Sodium Pump from Kidney Proximal Tubules by Triggering Distinct Protein Kinase Pathways in a Hierarchic Way.

Kidney proximal tubules are a key segment in the reabsorption of solutes and water from the glomerular ultrafiltrate, an essential process for maintaining homeostasis in body fluid compartments. The abundant content of Na+ in the extracellular fluid determines its importance in the regulation of extracellular fluid volume, which is particularly important for different physiological processes including blood pressure control. Basolateral membranes of proximal tubule cells have the classic Na+ + K+-ATPase and the ouabain-insensitive, K+-insensitive, and furosemide-sensitive Na+-ATPase, which participate in the active Na+ reabsorption. Here, we show that nanomolar concentrations of ceramide-1 phosphate (C1P), a bioactive sphingolipid derived in biological membranes from different metabolic pathways, promotes a strong inhibitory effect on the Na+-ATPase activity (C1P50 ≈ 10 nM), leading to a 72% inhibition of the second sodium pump in the basolateral membranes. Ceramide-1-phosphate directly modulates protein kinase A and protein kinase C, which are known to be involved in the modulation of ion transporters including the renal Na+-ATPase. Conversely, we did not observe any effect on the Na+ + K+-ATPase even at a broad C1P concentration range. The significant effect of ceramide-1-phosphate revealed a new potent physiological and pathophysiological modulator for the Na+-ATPase, participating in the regulatory network involving glycero- and sphingolipids present in the basolateral membranes of kidney tubule cells.

Proximal-tubule molecular relay from early Protein diaphanous homolog 1 to late Rho-associated protein kinase 1 regulates kidney function in obesity-induced kidney damage.

The small GTPase protein RhoA has two effectors, ROCK (Rho-associated protein kinase 1) and mDIA1 (protein diaphanous homolog 1), which cooperate reciprocally. However, temporal regulation of RhoA and its effectors in obesity-induced kidney damage remains unclear. Here, we investigated the role of RhoA activation in the proximal tubules at the early and late stages of obesity-induced kidney damage. In mice, a three-week high-fat-diet induced proximal tubule hypertrophy and damage without increased albuminuria, and RhoA/mDIA1 activation without ROCK activation. Conversely, a 12-week high-fat diet induced proximal tubule hypertrophy, proximal tubule damage, increased albuminuria, and RhoA/ROCK activation without mDIA1 elevation. Proximal tubule hypertrophy resulting from cell cycle arrest accompanied by downregulation of the multifunctional cyclin-dependent kinase inhibitor p27Kip1 was elicited by RhoA activation. Mice overexpressing proximal tubule-specific and dominant-negative RHOA display amelioration of high-fat diet-induced kidney hypertrophy, cell cycle abnormalities, inflammation, and renal impairment. In human proximal tubule cells, mechanical stretch mimicking hypertrophy activated ROCK, which triggered inflammation. In human kidney samples from normal individuals with a body mass index of about 25, proximal tubule cell size correlated with body mass index, proximal tubule cell damages, and mDIA1 expression. Thus, RhoA activation in proximal tubules is critical for the initiation and progression of obesity-induced kidney damage. Hence, the switch in the downstream RhoA effector in proximal tubule represents a transition from normal to pathogenic kidney adaptation and to body weight gain, leading to obesity-induced kidney damage.

Reducing lipid bilayer stress by monounsaturated fatty acids protects renal proximal tubules in diabetes.

In diabetic patients, dyslipidemia frequently contributes to organ damage such as diabetic kidney disease (DKD). Dyslipidemia is associated with both excessive deposition of triacylglycerol (TAG) in lipid droplets (LDs) and lipotoxicity. Yet, it is unclear how these two effects correlate with each other in the kidney and how they are influenced by dietary patterns. By using a diabetes mouse model, we find here that high-fat diet enriched in the monounsaturated oleic acid (OA) caused more lipid storage in LDs in renal proximal tubular cells (PTCs) but less tubular damage than a corresponding butter diet with the saturated palmitic acid (PA). This effect was particularly evident in S2/S3 but not S1 segments of the proximal tubule. Combining transcriptomics, lipidomics, and functional studies, we identify endoplasmic reticulum (ER) stress as the main cause of PA-induced PTC injury. Mechanistically, ER stress is caused by elevated levels of saturated TAG precursors, reduced LD formation, and, consequently, higher membrane order in the ER. Simultaneous addition of OA rescues the cytotoxic effects by normalizing membrane order and increasing both TAG and LD formation. Our study thus emphasizes the importance of monounsaturated fatty acids for the dietary management of DKD by preventing lipid bilayer stress in the ER and promoting TAG and LD formation in PTCs.

New Mutants of Epsilon Toxin from Clostridium perfringens with an Altered Receptor-Binding Site and Cell-Type Specificity.

Epsilon toxin (Etx) from Clostridium perfringens is the third most potent toxin after the botulinum and tetanus toxins. Etx is the main agent of enterotoxemia in ruminants and is produced by Clostridium perfringens toxinotypes B and D, causing great economic losses. Etx selectively binds to target cells, oligomerizes and inserts into the plasma membrane, and forms pores. A series of mutants have been previously generated to understand the cellular and molecular mechanisms of the toxin and to obtain valid molecular tools for effective vaccination protocols. Here, two new non-toxic Etx mutants were generated by selective deletions in the binding (Etx-ΔS188-F196) or insertion (Etx-ΔV108-F135) domains of the toxin. As expected, our results showed that Etx-ΔS188-F196 did not exhibit the usual Etx binding pattern but surprisingly recognized specifically an O-glycoprotein present in the proximal tubules of the kidneys in a wide range of animals, including ruminants. Although diminished, Etx-ΔV108-F135 maintained the capacity for binding and even oligomerization, indicating that the mutation particularly affected the pore-forming ability of the toxin.

Effects of Roxadustat on Erythropoietin Production in the Rat Body.

Anemia is a major complication of chronic renal failure. To treat this anemia, prolylhydroxylase domain enzyme (PHD) inhibitors as well as erythropoiesis-stimulating agents (ESAs) have been used. Although PHD inhibitors rapidly stimulate erythropoietin (Epo) production, the precise sites of Epo production following the administration of these drugs have not been identified. We developed a novel method for the detection of the Epo protein that employs deglycosylation-coupled Western blotting. With protein deglycosylation, tissue Epo contents can be quantified over an extremely wide range. Using this method, we examined the effects of the PHD inhibitor, Roxadustat (ROX), and severe hypoxia on Epo production in various tissues in rats. We observed that ROX increased Epo mRNA expression in both the kidneys and liver. However, Epo protein was detected in the kidneys but not in the liver. Epo protein was also detected in the salivary glands, spleen, epididymis and ovaries. However, both PHD inhibitors (ROX) and severe hypoxia increased the Epo protein abundance only in the kidneys. These data show that, while Epo is produced in many tissues, PHD inhibitors as well as severe hypoxia regulate Epo production only in the kidneys.

Targeted proteomic analysis dataset of archival core human kidney biopsies to investigate the biology of hypertensive nephropathy.

Hypertensive nephropathy is the second most frequent cause of end-stage renal disease in western societies. In previous experiments in our laboratory with proteomic analysis of renal parenchyma of SHR hypertensive animals, we identified two molecules, namely SGLT2 and CLIC4, associated with the development of hypertension. Here, we apply the methodology of targeted proteomic analysis in kidney biopsies from patients with hypertensive nephropathy to study the role of SGLT2 and CLIC4 in the pathogenesis of the disease. Relative quantification data of SGLT2 and CLIC4 via means of targeted proteomic analysis in human kidney biopsies from hypertensive patients and normotensive controls are reported. In addition, validation data of the proteomic results via immunofluorescence are presented. Renal tissue biopsies (N = 17) from archival material of patients with hypertensive nephropathy and normotensive controls were used. Targeted proteomic analysis was performed using the method: ``Parallel Reaction Monitoring'' (PRM) in renal parenchyma of hypertensive and normotensive patients for the selective identification of SGLT2 and CLIC4 and the relative quantification of their expression using proteotypic peptides for each protein. The expression of SGLT2 molecule was also confirmed by immunofluorescence followed by quantification of fluorescence intensity. According to PRM, the SGLT2 protein was found with reduced and the CLIC4 protein with increased expression levels in hypertensive patients compared to normotensive controls. Comparison of representative immunofluorescence images confirmed a decrease in the expression of SGLT2 in the brush border of proximal tubular epithelial cells in hypertensive patients. Our data show changes in the tubular compartment of the kidney and especially in the proximal tubules associated with the hypertensive nephropathy. The clinical significance of these findings should be further explored for the discovery and/or confirmation of novel therapeutic approaches and biomarkers in the development of hypertensive kidney disease.

Mitochondrial Ndufa4l2 Enhances Deposition of Lipids and Expression of Ca9 in the TRACK Model of Early Clear Cell Renal Cell Carcinoma.

Mitochondrial dysfunction and aberrant glycolysis are hallmarks of human clear cell renal cell carcinoma (ccRCC). Whereas glycolysis is thoroughly studied, little is known about the mitochondrial contribution to the pathology of ccRCC. Mitochondrial Ndufa4l2 is predictive of poor survival of ccRCC patients, and in kidney cancer cell lines the protein supports proliferation and colony formation. Its role in ccRCC, however, remains enigmatic. We utilized our established ccRCC model, termed Transgenic Cancer of the Kidney (TRACK), to generate a novel genetically engineered mouse model in which dox-regulated expression of an shRNA decreases Ndufa4l2 levels specifically in the renal proximal tubules (PT). This targeted knockdown of Ndufa4l2 reduced the accumulation of neutral renal lipid and was associated with decreased levels of the ccRCC markers carbonic anhydrase 9 (CA9) and Enolase 1 (ENO1). These findings suggest a link between mitochondrial dysregulation (i.e. high levels of Ndufa4l2), lipid accumulation, and the expression of ccRCC markers ENO1 and CA9, and demonstrate that lipid accumulation and ccRCC development can potentially be attenuated by inhibiting Ndufa4l2.

Transcription Factor ChREBP Mediates High Glucose-Evoked Increase in HIF-1α Content in Epithelial Cells of Renal Proximal Tubules.

Hyperglycemia/diabetes appears to be accompanied by the state of hypoxia, which especially affects kidneys. The aim of the study was to elucidate the mechanism of high glucose action on HIF-1α expression in renal proximal tubule epithelial cells. The research hypotheses included: (1) the participation of transcription factor ChREBP; and (2) the involvement of the effects resulting from pseudohypoxia, i.e., lowered intracellular NAD+/NADH ratio. The experiments were performed on HK-2 cells and primary cells: D-RPTEC (Diseased Human Renal Proximal Tubule Epithelial Cells-Diabetes Type II) and RPTEC (Renal Proximal Tubule Epithelial Cells). Protein and mRNA contents were determined by Western blot and RT-qPCR, respectively. ChREBP binding to DNA was detected applying chromatin immunoprecipitation, followed by RT-qPCR. Gene knockdown was performed using siRNA. Sirtuin activity and NAD+/NADH ratio were measured with commercially available kits. It was found that high glucose in HK-2 cells incubated under normoxic conditions: (1) activated transcription of HIF-1 target genes, elevated HIF-1α and ChREBP content, and increased the efficacy of ChREBP binding to promoter region of HIF1A gene; and (2), although it lowered NAD+/NADH ratio, it affected neither sirtuin activity nor HIF-1α acetylation level. The stimulatory effect of high glucose on HIF-1α expression was not observed upon the knockdown of ChREBP encoding gene. Experiments on RPTEC and D-RPTEC cells demonstrated that HIF-1α content in diabetic proximal tubular cells was lower than that in normal ones but remained high glucose-sensitive, and the latter phenomenon was mediated by ChREBP. Thus, it is concluded that the mechanism of high glucose-evoked increase in HIF-1α content in renal proximal tubule endothelial cells involves activation of ChREBP, indirectly capable of HIF1A gene up-regulation.

High-Fat Diet-Induced Renal Proximal Tubular Inflammatory Injury: Emerging Risk Factor of Chronic Kidney Disease.

Nowadays, with the improvements in living standards and changes in living habits, high-fat diet (HFD) has become much more common in the populations worldwide. Recent studies have shown that HFD could induce lipid accumulation, and structural and functional abnormalities, accompanied by the release of large amounts of pro-inflammatory cytokines, in proximal tubular epithelial cells (PTECs). These findings indicate that, as an emerging risk factor, PTEC injury-induced by HFD may be closely related to inflammation; however, the potential mechanisms underlying this phenomenon is still not well-known, but may involve the several inflammatory pathways, including oxidative stress-related signaling pathways, mitochondrial dysfunction, the myeloid differentiation factor 2/Toll like receptor 4 (MD2/TLR4) signaling pathway, the ERK1/2-kidney injury molecule 1 (KIM-1)-related pathway, and nuclear factor-κB (NF-κB) activation, etc., and the detailed molecular mechanisms underlying these pathways still need further investigated in the future. Based on lipid abnormalities-induced inflammation is closely related to the development and progression of chronic kidney disease (CKD), to summarize the potential mechanisms underlying HFD-induced renal proximal tubular inflammatory injury, may provide novel approaches for CKD treatment.

Fructose in the kidney: from physiology to pathology.

The Warburg effect is a unique property of cancer cells, in which glycolysis is activated instead of mitochondrial respiration despite oxygen availability. However, recent studies found that the Warburg effect also mediates non-cancer disorders, including kidney disease. Currently, diabetes or glucose has been postulated to mediate the Warburg effect in the kidney, but it is of importance that the Warburg effect can be induced under nondiabetic conditions. Fructose is endogenously produced in several organs, including the kidney, under both physiological and pathological conditions. In the kidney, fructose is predominantly metabolized in the proximal tubules; under normal physiologic conditions, fructose is utilized as a substrate for gluconeogenesis and contributes to maintain systemic glucose concentration under starvation conditions. However, when present in excess, fructose likely becomes deleterious, possibly due in part to excessive uric acid, which is a by-product of fructose metabolism. A potential mechanism is that uric acid suppresses aconitase in the Krebs cycle and therefore reduces mitochondrial oxidation. Consequently, fructose favors glycolysis over mitochondrial respiration, a process that is similar to the Warburg effect in cancer cells. Activation of glycolysis also links to several side pathways, including the pentose phosphate pathway, hexosamine pathway, and lipid synthesis, to provide biosynthetic precursors as fuel for renal inflammation and fibrosis. We now hypothesize that fructose could be the mediator for the Warburg effect in the kidney and a potential mechanism for chronic kidney disease.