Acetyl-CoA carboxylase 2 suppression rescues human proximal tubular cells from palmitic acid induced lipotoxicity via autophagy

Autophagy dysfunction links palmitic acid with macrophage inflammatory responses in large yellow croaker (Larimichthys crocea)

Autophagy is a cellular degradation process reliant on lysosome, crucial for preserving intracellular homeostasis. The key saturated fatty acid palmitic acid (PA) has been demonstrated to exert regulatory effects on autophagic activity in mammals. However, the precise impact of PA on autophagy and its role in fish remains incompletely understood. Thus, this study aimed to investigate the regulation of PA on autophagy and explore the role of autophagy in inflammatory responses triggered by PA in the head kidney macrophages of large yellow croaker. This study indicates that PA exposure can inhibit macrophage autophagy by reducing the expression of genes related to autophagy (e.g., beclin1, ulk1, and lc3), activating the negative regulator mTORC1 signaling pathway (p70S6K and S6), and hindering autophagic flux. This effect was observed to be amplified with increasing exposure time and concentration of PA. Similarly to the in vitro results, the palm oil (PO) diet significantly reduced autophagic activity in the head kidney of the croaker in vivo. Subsequent studies demonstrated that restoring autophagy led to a notable reduction in the expression of PA and PO-induced pro-inflammatory genes (il-1β, il-6, tnf-α, and cox-2), the activation of the MAPK signaling pathway (p38 and JNK), and the NLRP3 inflammasome levels, both in vitro and in vivo. In contrast, further inhibition of autophagy produced the opposite effect in vitro. In conclusion, this study demonstrates that PA exerts a dynamic inhibitory effect on autophagy in the head kidney macrophage, which in turn promotes PA-induced inflammatory responses. These findings provide valuable insights into how PA influences autophagy and inflammatory responses in fish immune cells, contributing to the theoretical framework for improving the use of vegetable oils in aquaculture.

Identification of potential key lipid metabolism-related genes involved in tubular injury in diabetic kidney disease by bioinformatics analysis

AIMS

Accumulating evidences indicate that abnormalities in tubular lipid metabolism play a crucial role in the development of diabetic kidney disease (DKD). We aim to identify novel lipid metabolism-related genes associated with tubular injury in DKD by utilizing bioinformatics approaches.

METHODS

Differentially expressed genes (DEGs) between control and DKD tubular tissue samples were screened from the Gene Expression Omnibus (GEO) database, and then were intersected with lipid metabolism-related genes. Hub genes were further determined by combined weighted gene correlation network analysis (WGCNA) and protein-protein interaction (PPI) network. We performed enrichment analysis, immune analysis, clustering analysis, and constructed networks between hub genes and miRNAs, transcription factors and small molecule drugs. Receiver operating characteristic (ROC) curves were employed to evaluate the diagnostic efficacy of hub genes. We validated the relationships between hub genes and DKD with external datasets and our own clinical samples.

RESULTS

There were 5 of 37 lipid metabolism-related DEGs identified as hub genes. Enrichment analysis demonstrated that lipid metabolism-related DEGs were enriched in pathways such as peroxisome proliferator-activated receptors (PPAR) signaling and pyruvate metabolism. Hub genes had potential regulatory relationships with a variety of miRNAs, transcription factors and small molecule drugs, and had high diagnostic efficacy. Immune infiltration analysis revealed that 13 immune cells were altered in DKD, and hub genes exhibited significant correlations with a variety of immune cells. Through clustering analysis, DKD patients could be classified into 3 immune subtypes and 2 lipid metabolism subtypes, respectively. The tubular expression of hub genes in DKD was further verified by other external datasets, and immunohistochemistry (IHC) staining showed that except ACACB, the other 4 hub genes (LPL, AHR, ME1 and ALOX5) exhibited the same results as the bioinformatics analysis.

CONCLUSION

Our study identified several key lipid metabolism-related genes (LPL, AHR, ME1 and ALOX5) that might be involved in tubular injury in DKD, which provide new insights and perspectives for exploring the pathogenesis and potential therapeutic targets of DKD.

The genetic side of diabetic kidney disease: a review

BACKGROUND

Diabetic kidney disease (DKD) is one of the most common complications of diabetes, with approximately 30-40% of patients with type 1 diabetes mellitus and 20% of patients with type 2 diabetes mellitus eventually developing DKD. If DKD is not controlled in the early clinical stage and proteinuria develops, the disease will progress to end-stage renal disease. The pathogenesis of DKD remains largely unknown and is multifactorial, likely due to interactions between genetic and environmental factors. Familial clustering also supports a critical role of hereditary factors in DKD. The development of gene detection technology has promoted the exploration of DKD susceptibility genes in different cohorts of patients with diabetes. Identifying susceptibility genes can provide insights into the pathogenesis of DKD, as well as a basis for its clinical diagnosis and therapy.

RESULTS

Numerous candidate gene loci have been found to be associated with DKD, many of which play critical regulatory roles in the pathogenesis of this disease, including genes involved in glycol-metabolism, lipid metabolism, the renin-angiotensin-aldosterone system, inflammation and oxidative stress. In this review, we summarize the functions of several susceptibility genes involved in the development of DKD.

CONCLUSION

Based on our findings, we recommend that studying susceptibility gene polymorphisms can lead to a better understanding of the pathogenesis of DKD and could help prevent this disease or improve its outcomes.

Acetyl Co-A Carboxylase Inhibition Halts Hyperglycemia Induced Upregulation of De Novo Lipogenesis in Podocytes and Proximal Tubular Cells

The effect of glycemic stress on de novo lipogenesis (DNL) in podocytes and tubular epithelial cells is understudied. This study is aimed (A) to show the effect of glycemic stress on DNL, and (B) to assess the effect of acetyl-Co A (ACC) inhibition on halting upregulation of DNL, on the expression of other lipid regulatory genes in the DNL pathway, and on markers of fibrosis and apoptosis in podocytes and tubular epithelial cells. We used cultured mouse primary tubular epithelial cells, mouse proximal tubular (BUMPT) cells, and immortal mouse podocytes and measured their percentage of labeled 13C2-palmitate as a marker of DNL after incubation with 13C2 acetate in response to high glucose concentration (25 mM). We then tested the effect of ACC inhibition by complimentary strategies utilizing CRISPR/cas9 deletion or incubation with Acaca and Acacb GapmeRs or using a small molecule inhibitor on DNL under hyperglycemic concentration. Exposure to high glucose concentration (25 mM) compared to osmotic controlled low glucose concentration (5.5 mM) significantly increased labeled palmitate after 24 h up to 72 h in podocytes and primary tubular cells. Knocking out of the ACC coding Acaca and Acacb genes by CRISPR/cas9, downregulation of Acaca and Acacb by specific antisense LNA GapmeRs and inhibition of ACC by firsocostat similarly halted/mitigated upregulation of DNL and decreased markers of fibrosis and programmed cell death in podocytes and various tubular cells. ACC inhibition is a potential therapeutic target to mitigate or halt hyperglycemia-induced upregulation of DNL in podocytes and tubular cells.

Thyroid stimulating hormone aggravates diabetic retinopathy through the mitochondrial apoptotic pathway

Diabetic retinopathy (DR) is a common complication of diabetes mellitus. High glucose-induced mitochondrial apoptosis is involved in the loss of retinal pericytes (PCs), which is considered to be a predominant pathologic change of diabetic retinopathy (DR). A high thyroid stimulating hormone (TSH) serum level is associated with an increased prevalence of DR in diabetic patients. Here, we investigated whether TSH regulated glucose-induced PCs loss through TSH-receptor (TSHR)-dependent mitochondrial apoptosis. First, the serum TSH level was found to be an independent risk factor for DR in Type 2 diabetic study participants (odds ratio = 2.294; 95% confidence interval: 1.925-2.733; p ≤ 0.001). Second, human PCs were treated with different concentrations of glucose, with or without bovine TSH (b-TSH). Glucose induced mitochondrial apoptosis through various mechanisms, including through regulating the expression of apoptosis-related proteins and inducing mitochondrial dysfunction, which could be deteriorated by costimulation of glucose and b-TSH. Additionally, we detected functional TSHR in PCs; blocking TSHR significantly restricted TSH-induced apoptosis. Thus, the presence of functional TSHR in human retinal PCs may facilitate the effect of high TSH on high glucose-induced PCs loss through TSHR-dependent mitochondrial apoptosis.

An Overview of Lipid Metabolism and Nonalcoholic Fatty Liver Disease

The occurrence of nonalcoholic fatty liver disease (NAFLD) is associated with major abnormalities of hepatic lipid metabolism. We propose that lipid abnormalities directly or indirectly contribute to NAFLD, especially fatty acid accumulation, arachidonic acid metabolic disturbance, and ceramide overload. The effects of lipid intake and accumulation on NAFLD and NAFLD treatment are explained with theoretical and experimental details. Overall, these findings provide further understanding of lipid metabolism in NAFLD and may lead to novel therapies.

Inhibitor of growth 2 regulates the high glucose-induced cell cycle arrest and epithelial-to-mesenchymal transition in renal proximal tubular cells

The epithelial-to-mesenchymal transition (EMT)-based tubulointerstitial fibrosis is the major pathological feature of diabetic kidney disease (DKD). While several studies have linked cell cycle dysregulation to various kidney injuries in recent years, its involvement in fibrosis of DKD is far from being clarified. ING2 (inhibitor of growth 2) is the second member of the inhibitor of growth family and participates in the regulation of many cellular processes. So far the role of ING2 in DKD remains largely unknown. In the present study, ING2 expression was detected by western blotting and immunofluorescent staining both in vitro high glucose-stimulated human proximal tubular epithelial cells (HK-2) and in vivo streptozotocin-induced diabetic mice. Cell proliferation was analyzed by CCK-8 and EdU assay, and cell cycle arrest was measured by flow cytometry. Quantitative polymerase chain reaction (qPCR) and western blotting were used to detect the EMT markers, and the p53 signaling activation was evaluated by chromatin immunoprecipitation (ChIP), qPCR, and western blotting. We found that the proliferation of the cells was reduced upon high glucose stimulation, which was accompanied by cell cycle arrest. The expression of ING2 was increased in hyperglycemia conditions both in vivo and in vitro. ING2 suppression ameliorated the reduced proliferation and cell cycle arrest induced by high glucose in HK-2 cells. Moreover, ING2 knockdown suppressed p21 expression by reducing p53 acetylation and finally alleviated the EMT progress in the high glucose-stimulated HK-2 cells. Our study demonstrated that cell cycle regulation is bound up with the kidney fibrosis in DKD, suggesting a novel function of ING2 as a potential therapeutic strategy targeting cell cycle arrest for DKD.

GCN5L1 controls renal lipotoxicity through regulating acetylation of fatty acid oxidation enzymes

Dyslipidemia is a common risk factor of chronic kidney disease (CKD). Current notion suggests that insufficient intracellular fatty acid oxidation (FAO) and subsequently enhanced fatty acid esterification within renal resident cells, a process termed as renal lipotoxicity, is the key pathogenic event responsible for dyslipidemia-induced kidney injury. However, the detailed mechanism is not fully elucidated. Recently, accumulating data indicated that acetylation modification is an important regulating manner for both mitochondrial function and energy metabolism, while whether acetylation modification is involved in renal lipotoxicity is of little known. In the present study, the expression level of global lysine acetylation was detected by immunohistochemistry in high-fat diet mice and western blot in palmitic acid (PA) stimulated HK-2 cells. The acetylation levels of long-chain acyl-CoA dehydrogenases (LCAD) and β-hydroxyacyl-CoA dehydrogenase (β-HAD) were measured by immunoprecipitation. And a multifunction microplate reader was applied to detect FAO rate, triglyceride and acyl-CoA contents, and the enzyme activities, with cellular lipid accumulation identified by Oil Red O staining. We evidenced the acetylation levels of LCAD and β-HAD that were enhanced, which led to decreased enzymatic activities and impaired FAO rate. Furthermore, renal protein hyperacetylation induced by lipid overload was associated with increased expression of GCN5L1. And the silence of GCN5L1 in tubular epithelial cells resulted in deacetylation and activation of LCAD and β-HAD. Finally, excess lipids induced lipotoxicity and epithelial-mesenchymal transition (EMT) were ameliorated by GCN5L1 suppression, suggesting GCN5L1-mediated mitochondrial LCAD and β-HAD acetylation might be a key pathogenic event underlying excess lipids induced FAO impairment.

The Susceptibility Genes in Diabetic Nephropathy

Background

Diabetes mellitus (DM) poses a severe threat to global public health. Diabetic nephropathy (DN) is one of the most common complications of diabetes and the leading cause of end-stage renal disease (ESRD). Approximately 30-40% of DM patients in the world progress to ESRD, which emphasizes the effect of genetic factors on DN. Family clustering also supports the important role of hereditary factors in DN and ESRD. Therefore, a large number of genetic studies have been carried out to identify susceptibility genes in different diabetic cohorts. Extensive susceptibility genes of DN and ESRD have not been identified until recently.

Summary and Key Messages

Some of these associated genes function as pivotal regulators in the pathogenesis of DN, such as those related to glycometabolism and lipid metabolism. However, the functions of most of these genes remain unclear. In this article, we review several susceptibility genes according to their genetic functions to make it easier to determine their exact effect on DN and to provide a better understanding of the advancements from genetic studies. However, several challenges associated with investigating the genetic factors of DN still exist. For instance, it is difficult to determine whether these variants affect the expression of the protein they encode or other cytokines. More efforts should be made to determine how these genes influence the progression of DN. In addition, many results could not be replicated among races, suggesting that the association between genetic polymorphisms and DN is race-specific. Therefore, large, well-designed studies involving more relevant variables and ethnic groups and more relevant functional studies are urgently needed. These studies may be beneficial and retard the progression of DN by early intervention, especially for patients who carry certain risk alleles or genotypes.

Notch1 regulates PTEN expression to exacerbate renal tubulointerstitial fibrosis in diabetic nephropathy by inhibiting autophagy via interactions with Hes1

Diabetic nephropathy (DN) is a serious clinical microvascular complication of diabetes mellitus. DN is characterized by the accumulation of extracellular matrix, resulting in progressive fibrosis leading to the loss of renal function. Notch1 and phosphatase and tensin homolog deleted on chromosome ten (PTEN) signaling have been associated with fibrosis. Autophagy serves as an essential regulator of tubular cellular homeostasis. However, how these molecules control the balance between fibrosis and autophagy, the main homeostatic mechanism regulating fibrosis, is not well understood. This association was confirmed using Notch1-siRNA in vitro, which prevented the increase in Hes1 and restored PTEN expression. In contrast, transfection with pHAGE-Hes1 repressed PTEN promoter-driven luciferase activity, implying a direct relationship between Hes1 and PTEN. The expression of Notch1 and Hes1 was increased in diabetic db/db mice by western blotting; in contrast, the expression of PTEN was decreased. Importantly, the dysregulation of these signaling molecules was associated with an increase in extracellular matrix proteins (Collagen-I and III) and the inhibition of autophagy. Similar results were evident in response to high glucose concentrations in vitro in the NRK-52e cells. Therefore, the high glucose concentrations present in diabetes promote fibrosis through the Notch1 pathway via Hes1, while inhibiting the PTEN and autophagy. In conclusion, the inhibition of PTEN by Notch1/Hes1 in response to high glucose concentration inhibits autophagy, which is associated with the progression of fibrosis. Therefore, these signaling molecules may represent novel therapeutic targets in diabetic nephropathy.

Inhibition of insulin resistance by PGE1 via autophagy-dependent FGF21 pathway in diabetic nephropathy

Insulin resistance is a critical process in the initiation and progression of diabetic nephropathy (DN). Alprostadil (Prostaglandin E1, PGE1) had protective effects on renal function. However, it is unknown whether PGE1 inhibited insulin resistance in renal tubule epithelial cells via autophagy, which plays a protective role in DN against insulin resistance. Insulin resistance was induced by palmitic acid (PA) in human HK-2 cells, shown as the decrease of insulin-stimulated AKT phosphorylation, glucose transporter-4 (GLUT4), glucose uptake and enhanced phosphorylation of insulin receptor substrate 1(IRS-1) at site serine 307 (pIRS-1ser307) and downregulated expression of IRS-1. Along with less abundance of p62, autophagy markers LC3B and Beclin-1 significantly increased in HK-2 cells exposed to PA. Such abnormal changes were significantly reversed by PGE1, which mimicked the role of autophagy gene 7 small interfering RNA (ATG7 siRNA). Furthermore, PGE1 promoted the protein expression of autophagy-related fibroblast growth factor-21 (FGF21), which alleviated insulin resistance. Results from western blotting and immunohistochemistry indicated that PGE1 remarkably restored autophagy, insulin resistance and the FGF21 expression in rat kidney of type 2 diabetes mellitus (T2DM). Collectively, we demonstrated the potential protection of PGE1 on insulin resistance in renal tubules via autophagy-dependent FGF21 pathway in preventing the progression of DN.

Combining SGLT2 Inhibition With a Thiazolidinedione Additively Attenuate the Very Early Phase of Diabetic Nephropathy Progression in Type 2 Diabetes Mellitus

Although both sodium glucose co-transporter 2 inhibition by dapagliflozin and thiazolidinedione, pioglitazone have glucose-lowering and anti-inflammatory effects, the therapeutic efficacy of their combination on diabetic nephropathy has not been investigated. 9-week-old male db/db mice were randomly assigned to 4 groups and administrated with (1) vehicle, (2) dapagliflozin, (3) pioglitazone, or (4) dapagliflozin and pioglitazone combination. Human proximal tubule (HK-2) cells were treated with glucose or palmitate acid in the presence of medium, dapagliflozin, pioglitazone, or both. Glomerular tuft area and mesangial expansion of the kidney more reduced in the combination group compared to control and single therapy groups. Podocyte foot process width and glomerular basement membrane thickness decreased regardless of treatment, while the combination group showed the slowest renal hypertrophy progression (P < 0.05). The combination treatment decreased MCP-1, type I and IV collagen expression in the renal cortex. Only the combination treatment decreased the expression of angiotensinogen, IL-6, and TGF-β while it enhanced HK-2 cell survival (all P < 0.05). In conclusion, dapagliflozin and pioglitazone preserved renal function, and combination therapy showed the greatest benefit. These findings suggest that the combination therapy of dapagliflozin with pioglitazone is more effective than the single therapy for preventing the progression of nephropathy in patients with type 2 diabetes.

HRD1 prevents apoptosis in renal tubular epithelial cells by mediating eIF2α ubiquitylation and degradation

Apoptosis of renal tubular epithelial cells is a key feature of the pathogenicity associated with tubulointerstitial fibrosis and other kidney diseases. One factor that regulates important cellular processes like apoptosis and cell proliferation is HRD1, an E3 ubiquitin ligase that acts by promoting ubiquitylation and degradation of its target protein. However, the detailed mechanisms by which HRD1 acts as a regulator of apoptosis in renal tubular epithelial cells have not been established. In our previous liquid chromatography-tandem mass spectrometry (LC-MS/MS) study (Mol Endocrinol. 2016;30:600–613), we demonstrated that one substrate of HRD1 was eIF2α, a critical protein in the PERK-eIF2α-ATF4-CHOP signaling pathway of endoplasmic reticulum (ER) stress. Here, we show that eIF2α expression was increased and HRD1 expression decreased when apoptosis was induced in HKC-8 cells by palmitic acid (PA) or high glucose (HG). HRD1 expression was also lower in kidney tissues from mice with diabetic nephropathy (DN) than in control mice. Forced expression of HRD1 also inhibited apoptosis in HKC-8 cells, while HRD1 overexpression decreased the expression of phosphorylated eIF2α and eIF2α. Further analysis indicated that HRD1 interacted with eIF2α and promoted its ubiquitylation and degradation by the proteasome. Moreover, the HRD1 protection of PA-treated HKC-8 cells was blunted by transfection with Myc-eIF2α. Thus, eIF2α ubiquitylation by HRD1 protects tubular epithelial cells from apoptosis caused by HG and PA, indicating a novel upstream target for therapeutic prevention of renal tubulointerstitial injury.

Glucagon-like peptide-1 analog prevents obesity-related glomerulopathy by inhibiting excessive autophagy in podocytes

To investigate the role of glucagon-like peptide-1 analog (GLP-1) in high-fat diet-induced obesity-related glomerulopathy (ORG). Male C57BL/6 mice fed a high-fat diet for 12 wk were treated with GLP-1 (200 μg/kg) or 0.9% saline for 4 wk. Fasting blood glucose and insulin and the expression of podocin, nephrin, phosphoinositide 3-kinase (PI3K), glucose transporter type (Glut4), and microtubule-associated protein 1A/1B-light chain 3 (LC3) were assayed. Glomerular morphology and podocyte foot structure were evaluated by periodic acid-Schiff staining and electron microscopy. Podocytes were treated with 150 nM GLP-1 and incubated with 400 μM palmitic acid (PA) for 12 h. The effect on autophagy was assessed by podocyte-specific Glut4 siRNA. Insulin resistance and autophagy were assayed by immunofluorescence and Western blotting. The high-fat diet resulted in weight gain, ectopic glomerular lipid accumulation, increased insulin resistance, and fusion of podophyte foot processes. The decreased translocation of Glut4 to the plasma membrane and excess autophagy seen in mice fed a high-fat diet and in PA-treated cultured podocytes were attenuated by GLP-1. Podocyte-specific Glut4 siRNA promoted autophagy, and rapamycin-enhanced autophagy worsened the podocyte injury caused by PA. Excess autophagy in podocytes was induced by inhibition of Glut4 translocation to the plasma membrane and was involved in the pathology of ORG. GLP-1 restored insulin sensitivity and ameliorated renal injury by decreasing the level of autophagy.

Glycyrrhizic Acid Prevents Diabetic Nephropathy by Activating AMPK/SIRT1/PGC-1α Signaling in db/db Mice

Diabetic nephropathy (DN) is a major cause of end-stage renal disease (ESRD). Glycyrrhizic acid (GA) is an effective inhibitor of reactive oxygen species (ROS) production. We investigated the role of GA in the progression of renal injury in DN. Albumin (Alb)/creatinine (crea) levels were significantly lower, and renal histopathology was attenuated in the diabetic db/db mice that were treated with GA (15 mg/kg via intraperitoneal injection) once per day for eight weeks. These changes were associated with significantly lower levels of α-smooth muscle actin (α-SMA) and transforming growth factor β1 (TGF-β1) expression. Additionally, diabetic db/db mice displayed more terminal deoxynucleotidyl transferase-mediated nick-end labeling- (TUNEL-) positive nuclei and diabetes-induced ROS production in the kidneys, and these effects were attenuated by the treatment with GA, which activated adenosine monophosphate-activated protein kinase (AMPK)/silent information regulator 1 (SIRT1)/peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) signaling in the kidneys. In summary, in diabetic db/db mice, the effect of GA on DN involved, in part, the inhibition of ROS and the activation of AMPK/SIRT1/PGC-1α signaling in the kidneys. GA, therefore, shows therapeutic potential for preventing and treating DN.

Fatty Liver and Chronic Kidney Disease: Novel Mechanistic Insights and Therapeutic Opportunities

Chronic kidney disease (CKD) is a risk factor for end-stage renal disease (ESRD) and cardiovascular disease (CVD). ESRD or CVD develop in a substantial proportion of patients with CKD receiving standard-of-care therapy, and mortality in CKD remains unchanged. These data suggest that key pathogenetic mechanisms underlying CKD progression go unaffected by current treatments. Growing evidence suggests that nonalcoholic fatty liver disease (NAFLD) and CKD share common pathogenetic mechanisms and potential therapeutic targets. Common nutritional conditions predisposing to both NAFLD and CKD include excessive fructose intake and vitamin D deficiency. Modulation of nuclear transcription factors regulating key pathways of lipid metabolism, inflammation, and fibrosis, including peroxisome proliferator-activated receptors and farnesoid X receptor, is advancing to stage III clinical development. The relevance of epigenetic regulation in the pathogenesis of NAFLD and CKD is also emerging, and modulation of microRNA21 is a promising therapeutic target. Although single antioxidant supplementation has yielded variable results, modulation of key effectors of redox regulation and molecular sensors of intracellular energy, nutrient, or oxygen status show promising preclinical results. Other emerging therapeutic approaches target key mediators of inflammation, such as chemokines; fibrogenesis, such as galectin-3; or gut dysfunction through gut microbiota manipulation and incretin-based therapies. Furthermore, NAFLD per se affects CKD through lipoprotein metabolism and hepatokine secretion, and conversely, targeting the renal tubule by sodium-glucose cotransporter 2 inhibitors can improve both CKD and NAFLD. Implications for the treatment of NAFLD and CKD are discussed in light of this new therapeutic armamentarium.

Autophagy protects human podocytes from high glucose-induced injury by preventing insulin resistance

OBJECTIVE

Insulin resistance is correlated with the progress of albuminuria in diabetic patients, and podocytes are crucial for maintaining the normal function of the glomerular filtration barrier. In the present study, we aimed to investigate the high glucose-induced insulin resistance and cell injury in human podocytes and the putative role of autophagy in this process.

METHODS

Human podocytes were cultured in high glucose-supplemented medium and low glucose and high osmotic conditions were used for the controls. Autophagy in the podocytes was regulated using rapamycin or 3-methyladenine stimulation. Next, autophagy markers including LC3B, Beclin-1, and p62 were investigated using western blot and qPCR, and the insulin responsiveness was analyzed based on glucose uptake and by using the phosphorylation of the insulin receptor with Nephrin as a podocyte injury marker.

RESULTS

The basal autophagy level decreased under the high glucose conditions, which was accompanied by a decrease in the glucose uptake and phosphorylation of the insulin receptor in the human podocytes. More interestingly, the glucose uptake and the phosphorylation of the insulin receptor were decreased by 3-MA stimulation and increased by rapamycin, illustrating that the responsiveness of insulin was regulated by autophagy. The activation of autophagy by rapamycin also ameliorated cell injury in the human podocytes.

CONCLUSIONS

The presence or activation of autophagy was found to play a protective role in human podocytes against high glucose-induced insulin resistance and cell injury, which indicates a novel cellular mechanism and provides a potential therapeutic target for diabetic nephropathy (DN).

A Molecular Probe for the Detection of Polar Lipids in Live Cells

Lipids have an important role in many aspects of cell biology, including membrane architecture/compartment formation, intracellular traffic, signalling, hormone regulation, inflammation, energy storage and metabolism. Lipid biology is therefore integrally involved in major human diseases, including metabolic disorders, neurodegenerative diseases, obesity, heart disease, immune disorders and cancers, which commonly display altered lipid transport and metabolism. However, the investigation of these important cellular processes has been limited by the availability of specific tools to visualise lipids in live cells. Here we describe the potential for ReZolve-L1^™ to localise to intracellular compartments containing polar lipids, such as for example sphingomyelin and phosphatidylethanolamine. In live Drosophila fat body tissue from third instar larvae, ReZolve-L1^™ interacted mainly with lipid droplets, including the core region of these organelles. The presence of polar lipids in the core of these lipid droplets was confirmed by Raman mapping and while this was consistent with the distribution of ReZolve-L1^™ it did not exclude that the molecular probe might be detecting other lipid species. In response to complete starvation conditions, ReZolve-L1^™ was detected mainly in Atg8-GFP autophagic compartments, and showed reduced staining in the lipid droplets of fat body cells. The induction of autophagy by Tor inhibition also increased ReZolve-L1^™ detection in autophagic compartments, whereas Atg9 knock down impaired autophagosome formation and altered the distribution of ReZolve-L1^™. Finally, during Drosophila metamorphosis fat body tissues showed increased ReZolve-L1^™ staining in autophagic compartments at two hours post puparium formation, when compared to earlier developmental time points. We concluded that ReZolve-L1^™ is a new live cell imaging tool, which can be used as an imaging reagent for the detection of polar lipids in different intracellular compartments.

Autophagy and Tubular Cell Death in the Kidney

Many common renal insults such as ischemia and toxic injury primarily target the tubular epithelial cells, especially the highly metabolically active proximal tubular segment. Tubular epithelial cells are particularly dependent on autophagy to maintain homeostasis and respond to stressors. The pattern of autophagy in the kidney has a unique spatial and chronologic signature. Recent evidence has shown that there is complex cross-talk between autophagy and various cell death pathways. This review specifically discusses the interplay between autophagy and cell death in the renal tubular epithelia. It is imperative to review this topic because recent discoveries have improved our mechanistic understanding of the autophagic process and have highlighted its broad clinical applications, making autophagy a major target for drug development.

The Tm7sf2 Gene Deficiency Protects Mice against Endotoxin-Induced Acute Kidney Injury

Cholesterol is essential for diverse cellular functions and cellular and whole-body cholesterol homeostasis is highly controlled. Cholesterol can also influence cellular susceptibility to injury. The connection between cholesterol metabolism and inflammation is exemplified by the Tm7sf2 gene, the absence of which reveals an essential role in cholesterol biosynthesis under stress conditions but also results in an inflammatory phenotype, i.e. NF-κB activation and TNFα up-regulation. Here, by using Tm7sf2^+/+and Tm7sf2^−/− mice, we investigated whether the Tm7sf2 gene, through its role in cholesterol biosynthesis under stress conditions, is involved in the renal failure induced by the administration of LPS. We found that the loss of Tm7sf2 gene results in significantly reduced blood urea nitrogen levels accompanied by decreased renal inflammatory response and neutral lipid accumulation. The increased expression of fatty acids catabolic enzymes reduces the need of the renal autophagy, a known crucial nutrient-sensing pathway in lipid metabolism. Moreover, we observed that the Tm7sf2 insufficiency is responsible for the inhibition of the NF-κB signalling thus dampening the inflammatory response and leading to a reduced renal damage. These results suggest a pivotal role for Tm7sf2 in renal inflammatory and lipotoxic response under endotoxemic conditions.