Dyslipidemia in Kidney Disorders: Perspectives on Mitochondria Homeostasis and Therapeutic Opportunities

Multi-omics and imaging mass cytometry characterization of human kidneys to identify pathways and phenotypes associated with impaired kidney function

Despite the recent advances in our understanding of the role of lipids, metabolites, and related enzymes in mediating kidney injury, there is limited integrated multi-omics data identifying potential metabolic pathways driving impaired kidney function. The limited availability of kidney biopsies from living donors with acute kidney injury has remained a major constraint. Here, we validated the use of deceased transplant donor kidneys as a good model to study acute kidney injury in humans and characterized these kidneys using imaging and multi-omics approaches. We noted consistent changes in kidney injury and inflammatory markers in donors with reduced kidney function. Neighborhood and correlation analyses of imaging mass cytometry data showed that subsets of kidney cells (proximal tubular cells and fibroblasts) are associated with the expression profile of kidney immune cells, potentially linking these cells to kidney inflammation. Integrated transcriptomic and metabolomic analysis of human kidneys showed that kidney arachidonic acid metabolism and seven other metabolic pathways were upregulated following diminished kidney function. To validate the arachidonic acid pathway in impaired kidney function we demonstrated increased levels of cytosolic phospholipase A2 protein and related lipid mediators (prostaglandin E2) in the injured kidneys. Further, inhibition of cytosolic phospholipase A2 reduced injury and inflammation in human kidney proximal tubular epithelial cells in vitro. Thus, our study identified cell types and metabolic pathways that may be critical for controlling inflammation associated with impaired kidney function in humans.

GWAS meta-analysis of kidney function traits in Japanese populations

BACKGROUND

Genetic epidemiological evidence for the kidney function traits in East Asian population including Japanese remain still relatively unclarified. Especially, the number of GWASs for kidney traits reported still remains limited, and the sample size of each independent study is relatively small. Given the genetic variability between ancestries/ethnicities, implementation of GWAS with sufficiently large sample sizes in specific population of Japanese is considered meaningful.

METHODS

We conducted the GWAS meta-analyses of kidney traits by leveraging the GWAS summary data of the representative large genome cohort studies with about 200,000 Japanese participants (n = 202,406 for estimated glomerular filtration rate [eGFR] and n = 200,845 for serum creatinine [SCr]).

RESULTS

In the present GWAS meta-analysis, we identified 110 loci with 169 variants significantly associated with eGFR (on chromosomes 1-13 and 15-22; p < 5×10-8), whereas we also identified 112 loci with 176 variants significantly associated with SCr (on chromosomes 1-22; p < 5×10-8), of which one locus (more than 1Mb distant from known loci) with one variant (CD36 rs146148222 on chromosome 7) for SCr was considered as the truly novel finding.

CONCLUSIONS

The present GWAS meta-analysis of largest genome cohort studies in Japanese provided some original genomic loci associated with kidney function in Japanese, which may contribute to the possible development of personalized prevention of kidney diseases based on genomic information in the near future.

Physiological phenotypes have optimal values relevant to healthy aging: sweet spots deduced from the Canadian Longitudinal Study on Aging

Previous observations on a group of exceptionally healthy "Super-Seniors" showed a lower variance of multiple physiological measures relevant for health than did a less healthy group of the same age. The finding was interpreted as the healthier individuals having physiological measurement values closer to an optimal level, or "sweet spot." Here, we tested the generalizability of the sweet-spot hypothesis in a larger community sample, comparing differences in the variance between healthier and less healthy groups. We apply this method to the Canadian Longitudinal Study on Aging (CLSA) comprehensive cohort of 30,097 participants aged 45 to 85 years with deep phenotype data. Data from both sexes and four age ranges were analyzed. Five instruments were used to represent different aspects of health, physical, and cognitive functioning. We tested 231 phenotypic measures for lower variance in the most healthy vs. least healthy quartile of each sex and age group, as classified by the five instruments. Segmented regression was used to determine sex-specific optimal values. One hundred forty-two physiological measures (61%) showed lower variance in the healthiest than in the least healthy group, in at least one sex and age group. The difference in variance was most significant for hemoglobin A1c and was also significant for many body composition measurements, but not for bone mineral density. Ninety-four phenotypes showed a nonmonotonic relationship with health, consistent with the idea of a sweet spot; for these, we determined optimal values and 95% confidence intervals that were generally narrower than the ranges of current clinical reference intervals. These findings for sweet spot discovery validate the proposed approach for identifying traits important for healthy aging.

2,8-Dihydroxyadenine-induced nephropathy causes hexosylceramide accumulation with increased mTOR signaling, reduced levels of protective SirT3 expression and impaired renal mitochondrial function

AIM

Chronic kidney disease (CKD) is accompanied by increased cardiovascular risk and heart failure (HF). In rodents, 2,8-dihydroxyadenine (DHA)-induced nephropathy is a frequently used CKD model. Cardiac and kidney tubular cells share high energy demand to guarantee constant contractive force of the heart or reabsorption/secretion of primary filtrated molecules and waste products by the kidney. Here we analyze time-dependent mechanisms of kidney damage and cardiac consequences under consideration of energetic pathways with the focus on mitochondrial function and lipid metabolism in mice.

METHODS AND RESULTS

CKD was induced by alternating dietary adenine supplementation (0.2 % or 0.05 % of adenine) in C57BL/6J mice for 9 weeks. Progressive kidney damage led to reduced creatinine clearance, kidney fibrosis and renal inflammation after 3, 6, and 9 weeks. No difference in cardiac function, mitochondrial respiration nor left ventricular fibrosis was observed at any time point. Investigating mechanisms of renal damage, protective SirT3 was decreased in CKD, which contrasted an increase in protein kinase B (AKT) expression, mechanistic target of rapamycin (mTOR) downstream signaling, induction of oxidative and endoplasmic reticulum (ER) stress. This occurred together with impaired renal mitochondrial function and accumulation of hexosylceramides (HexCer) as an established mediator of inflammation and mitochondrial dysfunction in the kidney.

CONCLUSIONS

2,8-DHA-induced CKD results in renal activation of the mTOR downstream signaling, endoplasmic reticulum stress, tubular injury, fibrosis, inflammation, oxidative stress and impaired kidney mitochondrial function in conjunction with renal hexosylceramide accumulation in C57BL/6J mice.

Epigenetic modification in diabetic kidney disease

Diabetic kidney disease (DKD) is a common microangiopathy in diabetic patients and the main cause of death in diabetic patients. The main manifestations of DKD are proteinuria and decreased renal filtration capacity. The glomerular filtration rate and urinary albumin level are two of the most important hallmarks of the progression of DKD. The classical treatment of DKD is controlling blood glucose and blood pressure. However, the commonly used clinical therapeutic strategies and the existing biomarkers only partially slow the progression of DKD and roughly predict disease progression. Therefore, novel therapeutic methods, targets and biomarkers are urgently needed to meet clinical requirements. In recent years, increasing attention has been given to the role of epigenetic modification in the pathogenesis of DKD. Epigenetic variation mainly includes DNA methylation, histone modification and changes in the noncoding RNA expression profile, which are deeply involved in DKD-related inflammation, oxidative stress, hemodynamics, and the activation of abnormal signaling pathways. Since DKD is reversible at certain disease stages, it is valuable to identify abnormal epigenetic modifications as early diagnosis and treatment targets to prevent the progression of end-stage renal disease (ESRD). Because the current understanding of the epigenetic mechanism of DKD is not comprehensive, the purpose of this review is to summarize the role of epigenetic modification in the occurrence and development of DKD and evaluate the value of epigenetic therapies in DKD.

Periodontal disease in chronic kidney disease patients: salivomics by Fourier-transform infrared spectroscopy

It has been reported that 58% of individuals with chronic kidney disease (CKD) have moderate to advanced periodontitis due to alterations of pH and biochemical composition in the saliva. In fact, the composition of this important biofluid may be modulated by systemic disorders. Here we investigate the micro-reflectance Fourier-transform infrared spectroscopy (FTIR) spectra of saliva that CKD patients submitted to periodontal treatment, aiming to identify spectral biomarkers of kidney disease evolution and the effectiveness of periodontal treatment, proposing possible biomarkers of disease evolution. Saliva from 24 CKD patients-stage-5 men, 29 to 64 years old-was evaluated in (i) patients starting periodontal treatment; (ii) patients 30 days after periodontal treatment; and (iii) patients 90 days after periodontal treatment. Our findings indicated that there are statistically relevant changes among the groups after 30 and 90 days of periodontal treatment, when considering the overall spectra in the fingerprint region (800-1800cm^-1). The key bands presenting good prediction power (area under the receiver operating characteristic curve >0.70) were related to poly (ADP-ribose) polymerase (PARP) conjugated to DNA at 883, 1031, and 1060cm^-1 (carbohydrates at 1043 and 1049cm^-1) and triglycerides (1461cm^-1). Interestingly when analyzing the derivative spectra in the secondary structure region (1590-1700cm^-1), we detected over-expression of the β-sheet class of secondary structures in 90 days of periodontal treatment, possibly related to over-expression of human B-defensins. Conformational changes in ribose sugar in this region corroborate the interpretation concerning PARP detection. To our knowledge, PARP was detected for the first time in saliva samples of stage-5 CKD patients by FTIR. All observed changes were correctly interpreted in terms of intensive apoptosis and dyslipidemia due to kidney disease progression. Biomarkers due to CKD predominate in saliva, and the relative improvement in the periodontal state did not cause remarkable changes in the spectra of saliva.

Deciphering the role of lipoproteins and lipid metabolic alterations in ageing and ageing-associated renal fibrosis

Fibrosis is the ultimate pathological feature of many chronic diseases, and ageing a major risk factor for fibrotic diseases. Current therapies are limited to those that reduce the rate of functional decline in patients with mild to moderate disease, but few interventions are available to specifically target the pathogenesis of fibrosis. In this context, new treatments that can significantly improve survival time and quality of life for these patients are urgently needed. In this review, we outline both the synthesis and metabolism of lipids and lipoproteins associated with ageing-associated renal fibrosis and the prominent contribution of lipids and lipidomics in the discovery of biomarkers that can be used for the prevention, diagnosis, and treatment of renal ageing and fibrosis. Next, we describe the effect of dyslipidaemia on ageing-related renal fibrosis and the pathophysiological changes in the kidney caused by dyslipidaemia. We then summarize the enzymes, transporters, transcription factors, and RNAs that contribute to dysregulated lipid metabolism in renal fibrosis and discuss their role in renal fibrosis in detail. We conclude by discussing the progress in research on small molecule therapeutic agents that prevent and treat ageing and ageing-associated renal fibrosis by modulating lipid metabolism. A growing number of studies suggest that restoring aberrant lipid metabolism may be a novel and promising therapeutic strategy to combat ageing and ageing-associated renal fibrosis.

The postprandial actions of GLP-1 receptor agonists: The missing link for cardiovascular and kidney protection in type 2 diabetes

Recent clinical trials in people with type 2 diabetes have demonstrated beneficial actions on heart and kidney outcomes following treatment with GLP-1RAs. In part, these actions are consistent with improved glucose control and significant weight loss. But GLP-1RAs may also have additive benefits by improving postprandial dysmetabolism. In diabetes, dysregulated postprandial nutrient excursions trigger inflammation, oxidative stress, endothelial dysfunction, thrombogenicity, and endotoxemia; alter hormone levels; and modulate cardiac output and regional blood and lymphatic flow. In this perspective, we explore the actions of GLP-1RAs on the postprandial state and their potential role in end-organ benefits observed in recent trials.

Nutrient sensing, signaling transduction, and autophagy in podocyte injury: implications for kidney disease

Podocytes are terminally differentiated epithelial cells of the renal glomerular tuft and these highly specialized cells are essential for the integrity of the slit diaphragm. The biological function of podocytes is primarily based on a complex ramified structure that requires sufficient nutrients and a large supply of energy in support of their unique structure and function in the glomeruli. Of note, the dysregulation of nutrient signaling and energy metabolic pathways in podocytes has been associated with a range of kidney diseases i.e., diabetic nephropathy. Therefore, nutrient-related and energy metabolic signaling pathways are critical to maintaining podocyte homeostasis and the pathogenesis of podocyte injury. Recently, a growing body of evidence has indicated that nutrient starvation induces autophagy, which suggests crosstalk between nutritional signaling with the modulation of autophagy for podocytes to adapt to nutrient deprivation. In this review, the current knowledge and advancement in the understanding of nutrient sensing, signaling, and autophagy in the podocyte biology, injury, and pathogenesis of kidney diseases is summarized. Based on the existing findings, the implications and perspective to target these signaling pathways and autophagy in podocytes during the development of novel preventive and therapeutic strategies in patients with podocyte injury-associated kidney diseases are discussed.

Mechanistic View on the Effects of SGLT2 Inhibitors on Lipid Metabolism in Diabetic Milieu

Chronic hyperglycemia induces pathophysiologic pathways with negative effects on the metabolism of most substrates as well as lipids and lipoproteins, and thereby induces dyslipidemia. Thus, the diabetic milieu is commonly accompanied by different levels of atherogenic dyslipidemia, which is per se a major risk factor for subsequent complications such as atherosclerosis, coronary heart disease, acute myocardial infarction, ischemic stroke, and nephropathy. Therefore, readjusting lipid metabolism in the diabetic milieu is a major goal for preventing dyslipidemia-induced complications. Sodium-glucose cotransporter-2 (SGLT2) inhibitors are a class of relatively newly introduced antidiabetes drugs (including empagliflozin, canagliflozin, dapagliflozin, etc.) with potent hypoglycemic effects and can reduce blood glucose by inducing glycosuria. However, recent evidence suggests that they could also provide extra-glycemic benefits in lipid metabolism. It seems that they can increase fat burning and lipolysis, normalizing the lipid metabolism and preventing or improving dyslipidemia. Nevertheless, the exact mechanisms involved in this process are not well-understood. In this review, we tried to explain how these drugs could regulate lipid homeostasis and we presented the possible involved cellular pathways supported by clinical evidence.

Kidney and lipids: novel potential therapeutic targets for dyslipidemia in kidney disease?

INTRODUCTION

Altered lipid distribution and metabolism may lead to the development and/or progression of chronic kidney disease (CKD). Dyslipidemia is a major risk factor for CKD and increases the risk of cardiovascular events and mortality. Therefore, lipid-lowering treatments may decrease cardiovascular risk and prevent death.

AREAS COVERED

Key players involved in regulating lipid accumulation in the kidney; contribution of lipids to CKD progression, lipotoxicity, and mitochondrial dysfunction in kidney disease; recent therapeutic approaches for dyslipidemia.

EXPERT OPINION

The precise mechanisms for regulating lipid metabolism, particularly in kidney disease, are poorly understood. Guidelines for lipid-lowering therapy for CKD are controversial. Several hypolipemic therapies are available, but compared to others, statin therapy is the most common. No clinical trial has evaluated the efficacy of proprotein convertase subtilisin/kexin type 9 inhibitors (PCSK9i) in preventing cardiovascular events or improving kidney function among patients with CKD or kidney transplant recipients. Attractive alternatives, such as PCSK9-small interfering RNA (siRNA) molecules or evinacumab are available. Additionally, several promising agents, such as cyclodextrins and the FXR/TGR5 dual agonist, INT-767, can improve renal lipid metabolism disorders and delay CKD progression. Drugs targeting mitochondrial dysfunction could be an option for the treatment of dyslipidemia and lipotoxicity, particularly in renal diseases.

Peroxisome proliferator-activated receptor gamma and its natural agonists in the treatment of kidney diseases

Kidney disease is one of the leading non-communicable diseases related to tremendous health and economic burden globally. Diabetes, hypertension, obesity and cardiovascular conditions are the major risk factors for kidney disease, followed by infections, toxicity and autoimmune causes. The peroxisome proliferator-activated receptor gamma (PPAR-γ) is a ligand-activated nuclear receptor that plays an essential role in kidney physiology and disease. The synthetic agonists of PPAR-γ shows a therapeutic effect in various kidney conditions; however, the associated side effect restricts their use. Therefore, there is an increasing interest in exploring natural products with PPARγ-activating potential, which can be a promising solution to developing effective and safe treatment of kidney diseases. In this review, we have discussed the role of PPAR-γ in the pathophysiology of kidney disease and the potential of natural PPAR-γ agonists in treating various kidney diseases, including acute kidney injury, diabetic kidney disease, obesity-induced nephropathy, hypertension nephropathy and IgA nephropathy. PPAR-γ is a potential target for the natural PPAR-γ agonists against kidney disease; however, more studies are required in this direction.

Advances in energy metabolism in renal fibrosis

Renal fibrosis is a common pathway toward chronic kidney disease (CKD) and is the main pathological predecessor for end-stage renal disease; thus, preventing progressive CKD and renal fibrosis is essential to reducing their consequential morbidity and mortality. Emerging evidence has connected renal fibrosis to metabolic reprogramming; abnormalities in energy metabolism pathways, such as glycolysis, the tricarboxylic acid cycle, and lipid metabolism, are known to cause diseases of diverse etiologies. Cytokine interventions in affected metabolic pathways may significantly reduce the degree of fibrosis, highlighting therapeutic targets for drug development for renal fibrosis. Here, we discuss the relationship between glycolysis, lipid metabolism, mitochondrial and peroxisome dysfunction, and renal fibrosis in detail and propose that targeted therapies for specific metabolic pathways are expected to represent the next generation of treatments for renal fibrosis.

Hypercaloric Diet Promotes Metabolic Disorders and Impaired Kidney Function

Poor dietary habits such as overconsumption of hypercaloric diets characterized by a high content of fructose and fat are related to metabolic abnormalities development such as obesity, diabetes, and dyslipidemia. Accumulating evidence supports the hypothesis that if energy intake gradually exceeds the body's ability to store fat in adipose tissue, the prolonged metabolic imbalance of circulating lipids from endogenous and exogenous sources leads to ectopic fat distribution in the peripheral organs, especially in the heart, liver, and kidney. The kidney is easily affected by dyslipidemia, which induces lipid accumulation and reflects an imbalance between fatty acid supply and fatty acid utilization. This derives from tissue lipotoxicity, oxidative stress, fibrosis, and inflammation, resulting in structural and functional changes that lead to glomerular and tubule-interstitial damage. Some authors indicate that a lipid-lowering pharmacological approach combined with a substantial lifestyle change should be considered to treat chronic kidney disease (CKD). Also, the new therapeutic target identification and the development of new drugs targeting metabolic pathways involved with kidney lipotoxicity could constitute an additional alternative to combat the complex mechanisms involved in impaired kidney function. In this review article, we first provide the pathophysiological evidence regarding the impact of hypercaloric diets, such as high-fat diets and high-fructose diets, on the development of metabolic disorders associated with impaired renal function and the molecular mechanisms underlying tissue lipid deposition. In addition, we present the current progress regarding translational strategies to prevent and/or treat kidney injury related to the consumption of hypercaloric diets.

The Roles of Fatty Acids and Apolipoproteins in the Kidneys

The kidneys are organs that require energy from the metabolism of fatty acids and glucose; several studies have shown that the kidneys are metabolically active tissues with an estimated energy requirement similar to that of the heart. The kidneys may regulate the normal and pathological function of circulating lipids in the body, and their glomerular filtration barrier prevents large molecules or large lipoprotein particles from being filtered into pre-urine. Given the permeable nature of the kidneys, renal lipid metabolism plays an important role in affecting the rest of the body and the kidneys. Lipid metabolism in the kidneys is important because of the exchange of free fatty acids and apolipoproteins from the peripheral circulation. Apolipoproteins have important roles in the transport and metabolism of lipids within the glomeruli and renal tubules. Indeed, evidence indicates that apolipoproteins have multiple functions in regulating lipid import, transport, synthesis, storage, oxidation and export, and they are important for normal physiological function. Apolipoproteins are also risk factors for several renal diseases; for example, apolipoprotein L polymorphisms induce kidney diseases. Furthermore, renal apolipoprotein gene expression is substantially regulated under various physiological and disease conditions. This review is aimed at describing recent clinical and basic studies on the major roles and functions of apolipoproteins in the kidneys.

Metabolic Reprogramming and Renal Fibrosis

There are several causes of chronic kidney disease, but all of these patients have renal fibrosis. Although many studies have examined the pathogenesis of renal fibrosis, there are still no effective treatments. A healthy and balanced metabolism is necessary for normal cell growth, proliferation, and function, but metabolic abnormalities can lead to pathological changes. Normal energy metabolism is particularly important for maintaining the structure and function of the kidneys because they consume large amounts of energy. We describe the metabolic reprogramming that occurs during renal fibrosis, which includes changes in fatty acid metabolism and glucose metabolism, and the relationship of these changes with renal fibrosis. We also describe the potential role of novel drugs that disrupt this metabolic reprogramming and the development of fibrosis, and current and future challenges in the treatment of fibrosis.

Lipotoxic Proximal Tubular Injury: A Primary Event in Diabetic Kidney Disease

The pathogenesis of diabetic nephropathy is a complex process that has a great relationship with lipotoxicity. Since the concept of “nephrotoxicity” was proposed, many studies have confirmed that lipotoxicity plays a significant role in the progression of diabetic nephropathy and causes various renal dysfunction. This review will make a brief summary of renal injury caused by lipotoxicity that occurs primarily and predominantly in renal tubules during diabetic progression, further leading to glomerular dysfunction. The latest research suggests that lipotoxicity-mediated tubular injury may be a major event in diabetic nephropathy.

Gene expression changes related to bone mineralization, blood pressure and lipid metabolism in mouse kidneys after space travel

Space travel burdens health by imposing considerable environmental stress associated with radioactivity and microgravity. In particular, gravity change predominantly impacts blood pressure and bone homeostasis, both of which are controlled mainly by the kidneys. Nuclear factor erythroid-2-related transcription factor 2 (Nrf2) plays essential roles in protecting the kidneys from various environmental stresses and injuries. To elucidate the effects of space travel on mammals in preparation for the upcoming space era, our study investigated the contribution of Nrf2 to kidney function in mice two days after their return from a 31-day stay in the International Space Station using Nrf2 knockout mice. Meaningfully, expression levels of genes regulating bone mineralization, blood pressure and lipid metabolism were found to be significantly altered in the kidneys after space travel in an Nrf2-independent manner. In particular, uridine diphosphate-glucuronosyltransferase 1A (Ugt1a) isoform genes were found to be expressed in an Nrf2-dependent manner and induced exclusively in the kidneys after return to Earth. Since spaceflight elevated the concentrations of fatty acids in the mouse plasma, we suggest that Ugt1a isoform expression in the kidneys was induced to promote glucuronidation of excessively accumulated lipids and excrete them into urine after the return from space. Thus, the kidneys were proven to play central roles in adaptation to gravity changes caused by going to and returning from space by controlling blood pressure and bone mineralization. Additionally, kidney Ugt1a isoform induction after space travel implies a significant role of the kidneys for space travelers in the excretion of excessive lipids.

Pleiotropic Effects of Sodium-Glucose Cotransporter-2 Inhibitors: Renoprotective Mechanisms beyond Glycemic Control

Diabetes mellitus is a major cause of chronic kidney disease and end-stage renal disease. However, the management of chronic kidney disease, particularly diabetes, requires vast improvements. Recently, sodium-glucose cotransporter-2 (SGLT2) inhibitors, originally developed for the treatment of diabetes, have been shown to protect against kidney injury via glycemic control, as well as various other mechanisms, including blood pressure and hemodynamic regulation, protection from lipotoxicity, and uric acid control. As such, regulation of these mechanisms is recommended as an effective multidisciplinary approach for the treatment of diabetic patients with kidney disease. Thus, SGLT2 inhibitors are expected to become key drugs for treating diabetic kidney disease. This review summarizes the recent clinical evidence pertaining to SGLT2 inhibitors as well as the mechanisms underlying their renoprotective effects. Hence, the information contained herein will advance the current understanding regarding the pleiotropic effects of SGLT2 inhibitors, while promoting future research in the field.