Analysis of urinary sphingolipids using liquid chromatography-tandem mass spectrometry in diabetic nephropathy

Nanopore-based full-length transcriptome sequencing for understanding the underlying molecular mechanisms of rapid and slow progression of diabetes nephropathy

BACKGROUND

Diabetic nephropathy (DN) has been a major factor in the outbreak of end-stage renal disease for decades. As the underlying mechanisms of DN development remains unclear, there is no ideal methods for the diagnosis and therapy.

OBJECTIVE

We aimed to explore the key genes and pathways that affect the rate progression of DN.

METHODS

Nanopore-based full-length transcriptome sequencing was performed with serum samples from DN patients with slow progression (DNSP, n = 5) and rapid progression (DNRP, n = 6).

RESULTS

Here, transcriptome proclaimed 22,682 novel transcripts and obtained 45,808 simple sequence repeats, 1,815 transcription factors, 5,993 complete open reading frames, and 1,050 novel lncRNA from the novel transcripts. Moreover, a total of 341 differentially expressed transcripts (DETs) and 456 differentially expressed genes (DEGs) between the DNSP and DNRP groups were identified. Functional analyses showed that DETs mainly involved in ferroptosis-related pathways such as oxidative phosphorylation, iron ion binding, and mitophagy. Moreover, Functional analyses revealed that DEGs mainly involved in oxidative phosphorylation, lipid metabolism, ferroptosis, autophagy/mitophagy, apoptosis/necroptosis pathway.

CONCLUSION

Collectively, our study provided a full-length transcriptome data source for the future DN research, and facilitate a deeper understanding of the molecular mechanisms underlying the differences in fast and slow progression of DN.

Sphingolipids and Chronic Kidney Disease

Sphingolipids (SLs) are bioactive signaling molecules essential for various cellular processes, including cell survival, proliferation, migration, and apoptosis. Key SLs such as ceramides, sphingosine, and their phosphorylated forms play critical roles in cellular integrity. Dysregulation of SL levels is implicated in numerous diseases, notably chronic kidney disease (CKD). This review focuses on the role of SLs in CKD, highlighting their potential as biomarkers for early detection and prognosis. SLs maintain renal function by modulating the glomerular filtration barrier, primarily through the activity of podocytes. An imbalance in SLs can lead to podocyte damage, contributing to CKD progression. SL metabolism involves complex enzyme-catalyzed pathways, with ceramide serving as a central molecule in de novo and salvage pathways. Ceramides induce apoptosis and are implicated in oxidative stress and inflammation, while sphingosine-1-phosphate (S1P) promotes cell survival and vascular health. Studies have shown that SL metabolism disorders are linked to CKD progression, diabetic kidney disease, and glomerular diseases. Targeting SL pathways could offer novel therapeutic approaches for CKD. This review synthesizes recent research on SL signaling regulation in kidney diseases, emphasizing the importance of maintaining SL balance for renal health and the potential therapeutic benefits of modulating SL pathways.

Association of plasma ceramide with decline in kidney function in patients with type 2 diabetes

Circulating ceramide levels are dysregulated in kidney disease. However, their associations with rapid decline in kidney function (RDKF) and end-stage kidney disease (ESKD) in patients with type 2 diabetes (T2D) are unknown. In this prospective study of 1746 T2D participants, we examined the association of plasma ceramide Cer16:0, Cer18:0, Cer24:0, and Cer24:1 with RDKF, defined as an estimated glomerular filtration rate (eGFR) decline of 5 ml/min/1.73 m2 per year or greater, and ESKD defined as eGFR <15/min/1.73 m2 for at least 3 months, on dialysis or renal death at follow-up. During a median follow-up period of 7.7 years, 197 patients experienced RDKF. Ceramide Cer24:0 (odds ratio [OR] = 0.71, 95% CI 0.56-0.90) and ratios Cer16:0/Cer24:0 (OR = 3.54 [1.70-7.35]), Cer18:0/Cer24:0 (OR = 1.89 [1.10-3.25]), and Cer24:1/Cer24:0 (OR = 4.01 [1.93-8.31]) significantly associated with RDKF in multivariable analysis; 124 patients developed ESKD. The ratios Cer16:0/Cer24:0 (hazard ratio [HR] = 3.10 [1.44-6.64]) and Cer24:1/Cer24:0 (HR = 4.66 [1.93-11.24]) significantly associated with a higher risk of ESKD. The Cer24:1/Cer24:0 ratio improved risk discrimination for ESKD beyond traditional risk factors by small but statistically significant margin (Harrell C-index difference: 0.01; P = 0.022). A high ceramide risk score also associated with RDKF (OR = 2.28 [1.26-4.13]) compared to lower risk score. In conclusion, specific ceramide levels and their ratios are associated with RDKF and conferred an increased risk of ESKD, independently of traditional risk factors, including baseline renal functions in patients with T2D.

Urinary sphingolipids in adolescents and young adults with youth-onset diabetes

BACKGROUND

This study evaluated urinary sphingolipids as a marker of diabetic kidney disease (DKD) in adolescents and young adults with youth-onset type 1 and type 2 diabetes.

METHODS

A comprehensive panel of urinary sphingolipids, including sphingomyelin (SM), glucosylceramide (GC), ceramide (Cer), and lactosylceramide (LC) species, was performed in patients with youth-onset diabetes from the SEARCH for Diabetes in Youth cohort. Sphingolipid levels, normalized to urine creatinine, were compared in 57 adolescents and young adults with type 1 diabetes, 59 with type 2 diabetes, and 44 healthy controls. The association of sphingolipids with albumin-to-creatinine (ACR) ratio and estimated glomerular filtration rate (eGFR) was evaluated.

RESULTS

The median age (interquartile range [IQR]) of participants was 23.1 years (20.9, 24.9) and the median duration of diabetes was 9.3 (8.5, 10.2) years. Urinary sphingolipid concentrations in patients with and without DKD (ACR ≥ 30 mg/g) were significantly elevated compared to healthy controls. There were no significant differences in sphingolipid levels between participants with type 1 and type 2 diabetes. In multivariable analysis, many sphingolipid species were positively correlated with ACR. Most significant associations were evident for the following species: C18 SM, C24:1 SM, C24:1 GC, and C24:1 Cer (all p < 0.001). Sphingolipid levels were not associated with eGFR. However, several interaction terms (diabetes type*sphingolipid) were significant, indicating diabetes type may modify the association of sphingolipids with eGFR.

CONCLUSION

Urinary sphingolipids are elevated in adolescents and young adults with youth-onset diabetes and correlate with ACR. Urinary sphingolipids may therefore represent an early biomarker of DKD.

Lipid metabolism disorder in diabetic kidney disease

Diabetic kidney disease (DKD), a significant complication associated with diabetes mellitus, presents limited treatment options. The progression of DKD is marked by substantial lipid disturbances, including alterations in triglycerides, cholesterol, sphingolipids, phospholipids, lipid droplets, and bile acids (BAs). Altered lipid metabolism serves as a crucial pathogenic mechanism in DKD, potentially intertwined with cellular ferroptosis, lipophagy, lipid metabolism reprogramming, and immune modulation of gut microbiota (thus impacting the liver-kidney axis). The elucidation of these mechanisms opens new potential therapeutic pathways for DKD management. This research explores the link between lipid metabolism disruptions and DKD onset.

Development of an advanced liquid chromatography-tandem mass spectrometry measurement system for simultaneous sphingolipid analysis

Mass spectrometry-based lipidomics approaches offer valuable tools for the detection and quantification of various lipid species, including sphingolipids. The present study aimed to develop a new method to simultaneously detect various sphingolipid species that applies to diverse biological samples. We developed and validated a measurement system by employing a single-column liquid chromatography-mass spectrometry system utilizing a normal-phase separation mode with positive ionization. The measurement system provided precision with a coefficient of variant below 20% for sphingolipids in all types of samples, and we observed good linearity in diluted serum samples. This system can measure the following sphingolipids: sphingosine 1-phosphate (S1P), sphingosine (Sph), dihydroS1P (dhS1P), dihydroSph (dhSph), ceramide 1-phosphate (Cer1P), hexosylceramide (HexCer), lactosylceramide (LacCer), dh-ceramide, deoxy-ceramide, deoxy-dh-ceramide, and sphingomyelin (SM). By measuring these sphingolipids in cell lysates where S1P lyase expression level was modulated, we could observe significant and dynamic modulations of sphingolipids in a comprehensive manner. Our newly established and validated measurement system can simultaneously measure many kinds of sphingolipids in biological samples. It holds great promise as a valuable tool for laboratory testing applications to detect overall modulations of sphingolipids, which have been proposed to be involved in pathogenesis processes in a series of elegant basic research studies.

Alterations in urinary ceramides, sphingoid bases, and their phosphates among patients with kidney disease

Background

To avoid an invasive renal biopsy, noninvasive laboratory testing for the differential diagnosis of kidney diseases is a desirable goal. As sphingolipids are demonstrated to be involved in the pathogenesis of various kidney diseases, we investigated the possible usefulness of the simultaneous measurement of urinary sphingolipids for differentiating kidney diseases.

Materials and methods

Residual urine specimens were collected from patients who had been clinically diagnosed with chronic glomerulonephritis (CGN), diabetic mellitus (DM), systemic lupus erythematosus (SLE), and arterial hypertension (AH). The urinary sphingolipids-CERs C16:0, C18:0, C18:1, C20:0, C22:0, and C24:0; sphingosine [Sph]; dihydrosphingosine; sphingosine 1-phosphate [S1P]; and dihydroS1P [dhS1P]-were measured by liquid chromatography-tandem mass spectrometry. Based on the results, machine learning models were constructed to differentiate the various kidney diseases.

Results

The urinary S1P was higher in patients with DM than in other participants (P < 0.05), whereas dhS1P was lower in the CGN and AH groups compared with control participants (P < 0.05). Sph and dhSph were higher in patients with CGN, AH, and SLE than in those with control participants (P < 0.05). The urinary CERs were significantly higher in patients with CGN, AH, and SLE than in those with control participants (P < 0.05). As a results of constructing a machine learning model discriminating kidney diseases, the resulting diagnostic accuracy and precision were improved from 94.03% and 66.96% to 96.10% and 78.26% respectively, when the urinary CERs, Sph, dhSph, S1P, dhS1P, and their ratios were added to the models.

Conclusion

The urinary CERs, sphingoid bases, and their phosphates show alterations among kidney diseases, suggesting their potential involvement in the development of kidney injury.

Immunolocalization of Sphingolipid Catabolism Enzymes along the Nephron: Novel Early Urinary Biomarkers of Renal Damage

The objective of this study was to investigate whether the activity of enzymes involved in sphingolipid catabolism could be biomarkers to predict early renal damage in streptozotocin (STZ)-induced diabetic rats and Angiotensin II (Ang II)-induced hypertension rats. Diabetic and hypertensive rats had no changes in plasma creatinine concentration. However, transmission electron microscopy (TEM) analysis showed slight ultrastructural changes in the glomeruli and tubular epithelial cells from diabetic and hypertensive rats. Our results show that the acid sphingomyelinase (aSMase) and neutral sphingomyelinase (nSMase) activity increased in the urine of diabetic rats and decreased in hypertensive rats. Only neutral ceramidase (nCDase) activity increased in the urine of diabetic rats. Furthermore, the immunofluorescence demonstrated positive staining for the nSMase, nCDase, and sphingosine kinase (SphK1) in glomerular mesangial cells, proximal tubule, ascending thin limb of the loop of Henle, thick ascending limb of Henle's loop, and principal cells of the collecting duct in the kidney. In conclusion, our results suggest that aSMase and nCDase activity in urine could be a novel predictor of early slight ultrastructural changes in the nephron, aSMase and nCDase as glomerular injury biomarkers, and nSMase as a tubular injury biomarker in diabetic and hypertensive rats.

Modulations of urinary lipid mediators in acute bladder cystitis

Bioactive lipids, such as lysophospholipids, ceramides, and eicosanoids and related mediators, have been demonstrated to be involved in inflammation. We aimed to investigate the possible orchestral modulations of these bioactive lipids in human inflammation. We simultaneously measured the urinary levels of lysophospholipids, ceramides, and eicosanoids and related mediators by a liquid chromatography-mass spectrometry method in patients with cystitis and control subjects. The urinary levels of lysophosphatidylcholine, lysophosphatidylethanolamine, sphingosine 1-phosphate, ceramides, prostaglandin (PG)E₂ and its metabolites represented by tetranor-PGEM, several oxylipins, DHA, and lysoPAF were higher in patients with cystitis. Urinary levels of some species of glycerolysophospholipids were highly positively correlated with those of other species of the same glycerolysophospholipids. Cluster analyses revealed that lysophosphatidylcholine was close to a PGE₂ metabolite, lysophosphatidylethanolamine was close to DHA, and sphingosine 1-phosphate and ceramides were close to lysoPAF. The orchestral dynamism of the lipid mediators was observed in the urine of cystitis, suggesting the necessity for simultaneous investigation of lipid mediators for translational research.

Deficiency of CFB attenuates renal tubulointerstitial damage by inhibiting ceramide synthesis in diabetic kidney disease

Accumulating evidence suggests the pathogenic role of immunity and metabolism in diabetic kidney disease (DKD). Herein, we aimed to investigate the effect of complement factor B (CFB) on lipid metabolism in the development of DKD. We found that in patients with diabetic nephropathy, the staining of Bb, CFB, C3a, C5a, and C5b-9 was markedly elevated in renal tubulointerstitium. Cfb-knockout diabetic mice had substantially milder tubulointerstitial injury and less ceramide biosynthesis. The in vitro study demonstrated that cytokine secretion, endoplasmic reticulum stress, oxidative stress, and cell apoptosis were ameliorated in HK-2 cells transfected with siRNA of CFB under high-glucose conditions. Exogenous ceramide supplementation attenuated the protective effect of CFB knockdown in HK-2 cells, while inhibiting ceramide synthases (CERS) with fumonisin B1 in CFB-overexpressing cells rescued the cell injury. CFB knockdown could downregulate the expression of NF-κB p65, which initiates the transcription of CERS3. Furthermore, C3 knockdown abolished CFB-mediated cytokine secretion, NF-κB signaling activation, and subsequently ceramide biosynthesis. Thus, CFB deficiency inhibited activation of the complement alternative pathway and attenuated kidney damage in DKD, especially tubulointerstitial injury, by inhibiting the NF-κB signaling pathway, further blocking the transcription of CERS, which regulates the biosynthesis of ceramide. CFB may be a promising therapeutic target of DKD.

Modulations of bioactive lipids and their receptors in postmortem Alzheimer's disease brains

Background

Analyses of brain samples from Alzheimer's disease (AD) patients may be expected to help us improve our understanding of the pathogenesis of AD. Bioactive lipids, including sphingolipids, glycerophospholipids, and eicosanoids/related mediators have been demonstrated to exert potent physiological actions and to be involved in the pathogenesis of various human diseases. In this cross-sectional study, we attempted to elucidate the associations of these bioactive lipids with the pathogenesis/pathology of AD through postmortem studies of human brains.

Methods

We measured the levels of glycerophospholipids, sphingolipids, and eicosanoids/related mediators in the brains of patients with AD (AD brains), patients with Cerad score B (Cerad-b brains), and control subjects (control brains), using a liquid chromatography-mass spectrometry method; we also measured the mRNA levels of specific receptors for these bioactive lipids in the same brain specimens.

Results

The levels of several species of sphingomyelins and ceramides were higher in the Cerad-b and AD brains. Levels of several species of lysophosphatidic acids (LPAs), lysophosphatidylcholine, lysophosphatidylserine, lysophosphatidylethanolamine (LPE), lysophosphatidylinositol, phosphatidylcholine, phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylinositol, and phosphatidylglycerol were especially high in the Cerad-b brains, while those of lysophosphatidylglycerol (LPG) were especially high in the AD brains. Several eicosanoids, including metabolites of prostaglandin E2, oxylipins, metabolites of epoxide, and metabolites of DHA and EPA, such as resolvins, were also modulated in the AD brains. Among the lipid mediators, the levels of S1P2, S1P5, LPA1, LPA2, LPA6, P2Y10, GPR174, EP1, DP1, DP2, IP, FP, and TXA2r were lower in the AD and/or Cerad-b brains. The brain levels of ceramides, LPC, LPI, PE, and PS showed strong positive correlations with the Aβ contents, while those of LPG showed rather strong positive correlations with the presence of senile plaques and neurofibrillary tangles. A discriminant analysis revealed that LPG is especially important for AD and the LPE/PE axis is important for Cerad-b.

Conclusions

Comprehensive lipidomics, together with the measurement of lipid receptor expression levels provided novel evidence for the associations of bioactive lipids with AD, which is expected to facilitate future translational research and reverse translational research.

Dynamic modulations of urinary sphingolipid and glycerophospholipid levels in COVID-19 and correlations with COVID-19-associated kidney injuries

Background

Among various complications of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), renal complications, namely COVID-19-associated kidney injuries, are related to the mortality of COVID-19.

Methods

In this retrospective cross-sectional study, we measured the sphingolipids and glycerophospholipids, which have been shown to possess potent biological properties, using liquid chromatography-mass spectrometry in 272 urine samples collected longitudinally from 91 COVID-19 subjects and 95 control subjects without infectious diseases, to elucidate the pathogenesis of COVID-19-associated kidney injuries.

Results

The urinary levels of C18:0, C18:1, C22:0, and C24:0 ceramides, sphingosine, dihydrosphingosine, phosphatidylcholine, lysophosphatidylcholine, lysophosphatidic acid, and phosphatidylglycerol decreased, while those of phosphatidylserine, lysophosphatidylserine, phosphatidylethanolamine, and lysophosphatidylethanolamine increased in patients with mild COVID-19, especially during the early phase (day 1–3), suggesting that these modulations might reflect the direct effects of infection with SARS-CoV-2. Generally, the urinary levels of sphingomyelin, ceramides, sphingosine, dihydrosphingosine, dihydrosphingosine l-phosphate, phosphatidylcholine, lysophosphatidic acid, phosphatidylserine, lysophosphatidylserine, phosphatidylethanolamine, lysophosphatidylethanolamine, phosphatidylglycerol, lysophosphatidylglycerol, phosphatidylinositol, and lysophosphatidylinositol increased, especially in patients with severe COVID-19 during the later phase, suggesting that their modulations might result from kidney injuries accompanying severe COVID-19.

Conclusions

Considering the biological properties of sphingolipids and glycerophospholipids, an understanding of their urinary modulations in COVID-19 will help us to understand the mechanisms causing COVID-19-associated kidney injuries as well as general acute kidney injuries and may prompt researchers to develop laboratory tests for predicting maximum severity and/or novel reagents to suppress the renal complications of COVID-19.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00880-5.

Dynamic modulations of sphingolipids and glycerophospholipids in COVID-19

BACKGROUND

A heterogeneous clinical phenotype is a characteristic of coronavirus disease 2019 (COVID-19). Therefore, investigating biomarkers associated with disease severity is important for understanding the mechanisms responsible for this heterogeneity and for developing novel agents to prevent critical conditions. This study aimed to elucidate the modulations of sphingolipids and glycerophospholipids, which have been shown to possess potent biological properties.

METHODS

We measured the serum sphingolipid and glycerophospholipid levels in a total of 887 samples from 215 COVID-19 subjects, plus 115 control subjects without infectious diseases and 109 subjects with infectious diseases other than COVID-19.

RESULTS

We observed the dynamic modulations of sphingolipids and glycerophospholipids in the serum of COVID-19 subjects, depending on the time course and severity. The elevation of C16:0 ceramide and lysophosphatidylinositol and decreases in C18:1 ceramide, dihydrosphingosine, lysophosphatidylglycerol, phosphatidylglycerol and phosphatidylinositol were specific to COVID-19. Regarding the association with maximum severity, phosphatidylinositol and phosphatidylcholine species with long unsaturated acyl chains were negatively associated, while lysophosphatidylethanolamine and phosphatidylethanolamine were positively associated with maximum severity during the early phase. Lysophosphatidylcholine and phosphatidylcholine had strong negative correlations with CRP, while phosphatidylethanolamine had strong positive ones. C16:0 ceramide, lysophosphatidylcholine, phosphatidylcholine and phosphatidylethanolamine species with long unsaturated acyl chains had negative correlations with D-dimer, while phosphatidylethanolamine species with short acyl chains and phosphatidylinositol had positive ones. Several species of phosphatidylcholine, phosphatidylethanolamine and sphingomyelin might serve as better biomarkers for predicting severe COVID-19 during the early phase than CRP and D-dimer. Compared with the lipid modulations seen in mice treated with lipopolysaccharide, tissue factor, or histone, the lipid modulations observed in severe COVID-19 were most akin to those in mice administered lipopolysaccharide.

CONCLUSION

A better understanding of the disturbances in sphingolipids and glycerophospholipids observed in this study will prompt further investigation to develop laboratory testing for predicting maximum severity and/or novel agents to suppress the aggravation of COVID-19.

Combination of mulberry leaf active components possessed synergetic effect on SD rats with diabetic nephropathy by mediating metabolism, Wnt/β-catenin and TGF-β/Smads signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Mulberry leaf has attracted much attention due to its excellent curative effect on diabetes and its complications, whether the combination of its effective components have protective and synergistic effect on diabetic nephropathy (DN) in vivo remain unclear.

AIM OF THE STUDY

The aim of this study was to investigate the protective and synergistic effect of the combination (MAF_1:1 and MAF_1:5) of mulberry leaf alkaloids (MA) and flavonoids extract (MF) on DN.

MATERIALS AND METHODS

A step by step method consisted of network pharmacological prediction, animal in vivo validation and metabolic mechanism research was used to construct the multi-component-target-pathway network of mulberry leaf against DN. Firstly, the potential components and mechanism of mulberry leaf against DN was explored by network pharmacology analysis. Secondly, DN animal model was established to validate the anti-DN activity of these potential compounds. Thirdly, the metabolomics of serum and urine samples from animal experiments was analyzed to explore the anti-DN mechanism of these potential compounds.

RESULTS

The results of network pharmacology demonstrated that a total of 7 compounds detected in MA and MF exhibited anti-DN activity, their mechanism were strongly in connection with metabolic pathways, arachidonic acid metabolism, sphingolipid signaling pathway, etc. The results of animal experiment indicated that MAF_1:1 and MAF_1:5 significantly relieved metabolic disorders through regulating Wnt/β-catenin and TGF-β/Smads signaling pathway, just like MF or MA alone. Metabolomics suggested they could regulate 16 serum and 7 urine endogenous metabolites through arachidonic acid metabolism, phenylalanine metabolism and sphingolipid metabolism, thus alleviated DN. Significantly, MAF_1:1 and MAF_1:5 might possess synergistic effect considering their therapeutic effects on DN rats were superior to the single use of MA or MF.

CONCLUSIONS

MAF_1:1 and MAF_1:5 possessed protective and synergistic effect on DN rats through multi-target and multi-pathways. These findings were of great scientific significance and application value to reveal the advantage of mulberry leaf in preventing and treating DN.

Implications of Sphingolipid Metabolites in Kidney Diseases

Sphingolipids, which act as a bioactive signaling molecules, are involved in several cellular processes such as cell survival, proliferation, migration and apoptosis. An imbalance in the levels of sphingolipids can be lethal to cells. Abnormalities in the levels of sphingolipids are associated with several human diseases including kidney diseases. Several studies demonstrate that sphingolipids play an important role in maintaining proper renal function. Sphingolipids can alter the glomerular filtration barrier by affecting the functioning of podocytes, which are key cellular components of the glomerular filtration barrier. This review summarizes the studies in our understanding of the regulation of sphingolipid signaling in kidney diseases, especially in glomerular and tubulointerstitial diseases, and the potential to target sphingolipid pathways in developing therapeutics for the treatment of renal diseases.

A Rheostat of Ceramide and Sphingosine-1-Phosphate as a Determinant of Oxidative Stress-Mediated Kidney Injury

Reactive oxygen species (ROS) modulate sphingolipid metabolism, including enzymes that generate ceramide and sphingosine-1-phosphate (S1P), and a ROS-antioxidant rheostat determines the metabolism of ceramide-S1P. ROS induce ceramide production by activating ceramide-producing enzymes, leading to apoptosis, while they inhibit S1P production, which promotes survival by suppressing sphingosine kinases (SphKs). A ceramide-S1P rheostat regulates ROS-induced mitochondrial dysfunction, apoptotic/anti-apoptotic Bcl-2 family proteins and signaling pathways, leading to apoptosis, survival, cell proliferation, inflammation and fibrosis in the kidney. Ceramide inhibits the mitochondrial respiration chain and induces ceramide channel formation and the closure of voltage-dependent anion channels, leading to mitochondrial dysfunction, altered Bcl-2 family protein expression, ROS generation and disturbed calcium homeostasis. This activates ceramide-induced signaling pathways, leading to apoptosis. These events are mitigated by S1P/S1P receptors (S1PRs) that restore mitochondrial function and activate signaling pathways. SphK1 promotes survival and cell proliferation and inhibits inflammation, while SphK2 has the opposite effect. However, both SphK1 and SphK2 promote fibrosis. Thus, a ceramide-SphKs/S1P rheostat modulates oxidant-induced kidney injury by affecting mitochondrial function, ROS production, Bcl-2 family proteins, calcium homeostasis and their downstream signaling pathways. This review will summarize the current evidence for a role of interaction between ROS-antioxidants and ceramide-SphKs/S1P and of a ceramide-SphKs/S1P rheostat in the regulation of oxidative stress-mediated kidney diseases.

Understanding modulations of lipid mediators in cancer using a murine model of carcinomatous peritonitis

Background

Numerous studies have investigated the possible involvement of eicosanoids, lysophospholipids, and sphingolipids in cancer. We considered that comprehensive measurement of these lipid mediators might provide a better understanding of their involvement in the pathogenesis of cancer. In the present study, we attempted to elucidate the modulations of sphingolipids, lysophospholipids, diacyl‐phospholipids, eicosanoids, and related mediators in cancer by measuring their levels simultaneously by a liquid chromatography‐mass spectrometry method in a mouse model of carcinomatous peritonitis.

Methods

We investigated the modulations of these lipids in both ascitic fluid and plasma specimens obtained from Balb/c mice injected intraperitoneally with Colon‐26 cells, as well as the modulations of the lipid contents in the cancer cells obtained from the tumor xenografts.

Results

The results were as follows: the levels of sphingosine 1‐phosphate were increased, while those of lysophosphatidic acid (LysoPA), especially unsaturated long‐chain LysoPA, tended to be increased, in the ascitic fluid. Our findings suggested that ceramides, sphingomyelin, and phosphatidylcholine, their precursors, were supplied by both de novo synthesis and from elsewhere in the body. The levels of lysophosphatidylserine (LysoPS), lysophosphatidylinositol, lysophosphatidylglycerol, and lysophosphatidylethanolamine were also increased in the ascitic fluid, while those of phosphatidylserine (PS), a precursor of LysoPS, were markedly decreased. The levels of arachidonic acid derivatives, especially PGE2‐related metabolites, were increased, while the plasma levels of eicosanoids and related mediators were decreased. Comprehensive statistical analyses mainly identified PS in the ascitic fluid and eicosanoids in the plasma as having highly negative predictive values for cancer.

Conclusions

The results proposed many unknown associations of lipid mediators with cancer, underscoring the need for further studies. In particular, the PS/LysoPS pathway could be a novel therapeutic target, and plasma eicosanoids could be useful biomarkers for cancer.

Isoform-Dependent Effects of Apolipoprotein E on Sphingolipid Metabolism in Neural Cells

BACKGROUND

Sphingosine 1-phosphate (S1P) and ceramides have been implicated in the development of Alzheimer's disease. Apolipoprotein E (ApoE) isoforms are also involved in the development of Alzheimer's disease.

OBJECTIVE

We aimed at elucidating the potential association of the ApoE isoforms with sphingolipid metabolism in the central nervous system.

METHODS

We investigated the modulations of apolipoprotein M (apoM), a carrier of S1P, S1P, and ceramides in Apoeshl mice, which spontaneously lack apoE, and U251 cells and SH-SY5Y cells infected with adenovirus vectors encoding for apoE2, apoE3, and apoE4.

RESULTS

In the brains of Apoeshl mice, the levels of apoM were lower, while those of ceramides were higher. In U251 cells, cellular apoM and S1P levels were the highest in the cells overexpressing apoE2 among the apoE isoforms. The cellular and medium contents of ceramides decreased in the order of the cells overexpressing apoE3 > apoE2 and increased in the cells overexpressing apoE4. In SH-SY5Y cells, apoM mRNA and medium S1P levels were also the highest in the cells overexpressing apoE2. The cellular contents of ceramides decreased in the order of the cells overexpressing apoE3 > apoE2 = apoE4 and those in medium decreased in the order of the cells overexpressing apoE3 > apoE2, while increased in the cells overexpressing apoE4.

CONCLUSION

The modulation of apoM and S1P might partly explain the protective effects of apoE2 against Alzheimer's disease, and the modulation of ceramides might be one of the mechanisms explaining the association of apoE4 with the development of Alzheimer's disease.

Podocyte Sphingolipid Signaling in Nephrotic Syndrome

Podocytes play a vital role in the pathogenesis of nephrotic syndrome (NS), which is clinically characterized by heavy proteinuria, hypoalbuminemia, hyperlipidemia, and peripheral edema. The pathogenesis of NS has evolved through several hypotheses ranging from immune dysregulation theory and increased glomerular permeability theory to the current concept of podocytopathy. Podocytopathy is characterized by dysfunction or depletion of podocytes, which may be caused by unknown permeability factor, genetic disorders, drugs, infections, systemic disorders, and hyperfiltration. Over the last two decades, numerous studies have been done to explore the molecular mechanisms of podocyte injuries or NS and to develop the novel therapeutic strategies targeting podocytopathy for treatment of NS. Recent studies have shown that normal sphingolipid metabolism is essential for structural and functional integrity of podocytes. As a basic component of the plasma membrane, sphingolipids not only support the assembly of signaling molecules and interaction of receptors and effectors, but also mediate various cellular activities, such as apoptosis, proliferation, stress responses, necrosis, inflammation, autophagy, senescence, and differentiation. This review briefly summarizes current evidence demonstrating the regulation of sphingolipid metabolism in podocytes and the canonical or noncanonical roles of podocyte sphingolipid signaling in the pathogenesis of NS and associated therapeutic strategies.

Neuroprotective role of sphingolipid rheostat in excitotoxic retinal ganglion cell death

The glutamate excitotoxicity has been suggested as a factor involved in the loss of retinal neuronal cells, including retinal ganglion cell (RGC), in various retinal degenerative diseases including ischemia-reperfusion injury, diabetic retinopathy, and glaucoma. Excitotoxic RGC death is caused not only by direct damage to RGCs but also by indirect damage due to the inflammation of retinal glial cells. Sphingosine 1-phosphate (S1P) and ceramides are bioactive sphingolipids which have been shown to possess important physiological roles in cellular survival and apoptosis, and the balance between S1P and ceramide, sphingolipid rheostat, has been suggested to be important for determining cellular fate. Therefore, we conducted the present study to clarify the neuroprotective role of sphingolipid rheostat in excitotoxic RGC death in vivo and in vitro. Acute RGC death was induced by intravitreal N-methyl-d-aspartate (NMDA) injection in the mouse. The mRNA expression of sphingosine kinase (SphK1/SphK2) was examined by quantitative real-time polymerase chain reaction (qRT-PCR). The expressions of SphK1/2, S1P, S1P-receptor (S1PR), glial fibrillary acidic protein (GFAP), Iba1, and CD31 were examined by immunostaining. Retinal sphingolipids and ceramides were quantified by liquid chromatography with tandem mass spectrometry. The neuroprotective effect of the sphingosine kinase inhibitor (SKI) on RGC death was assessed by RGC count and Terminal deoxynucleotidyl transferase dUTP nick-end labeling assay. Further, the in vitro effect of SKI was investigated using rat primary cultured RGCs and glial cells. In addition, MG5 cells and A1 cells, which were mouse microglia and astrocyte cell-line, were also used. The expression of cleaved-caspase-3, GFAP, and Iba1 in RGCs, primary glial cells, MG5 cells, and A1 cells was assessed by immunostaining. NMDA injection resulted in mRNA upregulation of SphK1; however, SphK2 was reduced in the mouse retina. SphKs, S1P, S1PR1, S1PR2, and GFAP expression increased in the early-stage NMDA group, whereas S1P and GFAP were higher in the late-stage NMDA + SKI group. In the NMDA group, S1P expression was lower whereas sphingosine, C20, C22, and C24 ceramides showed higher levels. The proportion of very-long-chain ceramide was elevated in the NMDA group but reduced in the NMDA + SKI group. SKI treatment significantly increased RGC survival in retinal wholemount analysis and decreased apoptosis in the ganglion cell layer and inner nuclear layer. In vitro, SKI suppressed excitotoxic RGC death, cleaved-caspase-3 expression, and activated glial cells. The findings in the present study provide the first evidence demonstrating the involvement of sphingolipid rheostat in the neuroprotection against excitotoxic RGC death. Therefore, regulation of sphingolipid rheostat might serve as a potential therapy for retinal degenerative disease.

New insights into renal lipid dysmetabolism in diabetic kidney disease

Lipid dysmetabolism is one of the main features of diabetes mellitus and manifests by dyslipidemia as well as the ectopic accumulation of lipids in various tissues and organs, including the kidney. Research suggests that impaired cholesterol metabolism, increased lipid uptake or synthesis, increased fatty acid oxidation, lipid droplet accumulation and an imbalance in biologically active sphingolipids (such as ceramide, ceramide-1-phosphate and sphingosine-1-phosphate) contribute to the development of diabetic kidney disease (DKD). Currently, the literature suggests that both quality and quantity of lipids are associated with DKD and contribute to increased reactive oxygen species production, oxidative stress, inflammation, or cell death. Therefore, control of renal lipid dysmetabolism is a very important therapeutic goal, which needs to be archived. This article will review some of the recent advances leading to a better understanding of the mechanisms of dyslipidemia and the role of particular lipids and sphingolipids in DKD.

Development and validation of a RPLC-MS/MS method for the quantification of ceramides in human serum

Ceramides are key-role lipids involved in numerous central cellular processes. A plethora of studies have demonstrated that the levels of ceramides in blood circulation are related to different disease states, such as type 2 diabetes, cardiovascular diseases, ovarian cancer, multiple sclerosis and others. Herein, a RPLC-MS/MS method for the rapid quantification of ceramides Cer(d18:1/16:0), Cer(d18:1/18:0), Cer(d18:1/24:0) and Cer(d18:1/24:1) in human blood serum was developed and validated. Different sample preparation strategies including SLE, LLE and QuECheRS were tested with the aim to attain effective, accurate and reproducible determination of ceramides in serum samples. Intra and inter-day accuracy were found to be between 80.0-111% and 87.8-106%, respectively, for all ceramides, while intra and inter-day precision were found to vary from 0.05% to 10.2% %RSD and 2.2% to 14.0% %RSD, respectively. The lower limits of quantification were 2.3 ng/mL for Cer(d18:1/16:0) and Cer(d18:1/18:0) and 1.4 ng/mL for Cer(d18:1/24:0) and Cer(d18:1/24:1). The method was evaluated in accordance to bioanalytical method guidelines and was used for the determination of serum ceramides of patients with coronary artery disease to evaluate its utility in clinical analyses.

Molecular and functional profiling of apical versus basolateral small extracellular vesicles derived from primary human proximal tubular epithelial cells under inflammatory conditions

Proximal tubular epithelial cells (PTEC) are central players in inflammatory kidney diseases. However, the complex signalling mechanism/s via which polarized PTEC mediate disease progression are poorly understood. Small extracellular vesicles (sEV), including exosomes, are recognized as fundamental components of cellular communication and signalling courtesy of their molecular cargo (lipids, microRNA, proteins). In this study, we examined the molecular content and function of sEV secreted from the apical versus basolateral surfaces of polarized human primary PTEC under inflammatory diseased conditions. PTEC were cultured under normal and inflammatory conditions on Transwell inserts to enable separate collection and isolation of apical/basolateral sEV. Significantly increased numbers of apical and basolateral sEV were secreted under inflammatory conditions compared with equivalent normal conditions. Multi‐omics analysis revealed distinct molecular profiles (lipids, microRNA, proteins) between inflammatory and normal conditions for both apical and basolateral sEV. Biological pathway analyses of significantly differentially expressed molecules associated apical inflammatory sEV with processes of cell survival and immunological disease, while basolateral inflammatory sEV were linked to pathways of immune cell trafficking and cell‐to‐cell signalling. In line with this mechanistic concept, functional assays demonstrated significantly increased production of chemokines (monocyte chemoattractant protein‐1, interleukin‐8) and immuno‐regulatory cytokine interleukin‐10 by peripheral blood mononuclear cells activated with basolateral sEV derived from inflammatory PTEC. We propose that the distinct molecular composition of sEV released from the apical versus basolateral membranes of human inflammatory PTEC may reflect specialized functional roles, with basolateral‐derived sEV pivotal in modulating tubulointerstitial inflammatory responses observed in many immune‐mediated kidney diseases. These findings provide a rationale to further evaluate these sEV‐mediated inflammatory pathways as targets for biomarker and therapeutic development.

Lysosome function in glomerular health and disease

The lysosome represents an important regulatory platform within numerous vesicle trafficking pathways including the endocytic, phagocytic, and autophagic pathways. Its ability to fuse with endosomes, phagosomes, and autophagosomes enables the lysosome to break down a wide range of both endogenous and exogenous cargo, including macromolecules, certain pathogens, and old or damaged organelles. Due to its center position in an intricate network of trafficking events, the lysosome has emerged as a central signaling node for sensing and orchestrating the cells metabolism and immune response, for inter-organelle and inter-cellular signaling and in membrane repair. This review highlights the current knowledge of general lysosome function and discusses these findings in their implication for renal glomerular cell types in health and disease including the involvement of glomerular cells in lysosomal storage diseases and the role of lysosomes in nongenetic glomerular injuries.

Role of Sphingolipid Signaling in Glomerular Diseases: Focus on DKD and FSGS

Sphingolipids are well-recognized as major players in the pathogenesis of many human diseases, including chronic kidney disease. The kidney is a very sensitive organ to alterations in sphingolipid metabolism. The critical issues to be addressed in this review relate to the role of sphingolipids and enzymes involved in sphingolipid metabolism in the pathogenesis of glomerular diseases with a special focus on podocytes, a key cellular component of the glomerular filtration barrier. Among several sphingolipids, we will highlight the role of ceramide, sphingosine, sphingosine-1-phosphate and ceramide-1-phosphate. Additionally, we will summarize the current knowledge with regard to the use of sphingolipids as therapeutic agents for the treatment of podocyte injury in kidney disease.

Podocyte Lysosome Dysfunction in Chronic Glomerular Diseases

Podocytes are visceral epithelial cells covering the outer surface of glomerular capillaries in the kidney. Blood is filtered through the slit diaphragm of podocytes to form urine. The functional and structural integrity of podocytes is essential for the normal function of the kidney. As a membrane-bound organelle, lysosomes are responsible for the degradation of molecules via hydrolytic enzymes. In addition to its degradative properties, recent studies have revealed that lysosomes may serve as a platform mediating cellular signaling in different types of cells. In the last decade, increasing evidence has revealed that the normal function of the lysosome is important for the maintenance of podocyte homeostasis. Podocytes have no ability to proliferate under most pathological conditions; therefore, lysosome-dependent autophagic flux is critical for podocyte survival. In addition, new insights into the pathogenic role of lysosome and associated signaling in podocyte injury and chronic kidney disease have recently emerged. Targeting lysosomal functions or signaling pathways are considered potential therapeutic strategies for some chronic glomerular diseases. This review briefly summarizes current evidence demonstrating the regulation of lysosomal function and signaling mechanisms as well as the canonical and noncanonical roles of podocyte lysosome dysfunction in the development of chronic glomerular diseases and associated therapeutic strategies.

Rotten to the Cortex: Ceramide-Mediated Lipotoxicity in Diabetic Kidney Disease

Diabetic kidney disease (DKD) is a prevalent and progressive comorbidity of diabetes mellitus that increases one's risk of developing renal failure. Progress toward development of better DKD therapeutics is limited by an incomplete understanding of forces driving and connecting the various features of DKD, which include renal steatosis, fibrosis, and microvascular dysfunction. Herein we review the literature supporting roles for bioactive ceramides as inducers of local and systemic DKD pathology. In rodent models of DKD, renal ceramides are elevated, and genetic and pharmacological ceramide-lowering interventions improve kidney function and ameliorate DKD histopathology. In humans, circulating sphingolipid profiles distinguish human DKD patients from diabetic controls. These studies highlight the potential for ceramide to serve as a central and therapeutically tractable lipid mediator of DKD.