The proteasome inhibitor, MG132, attenuates diabetic nephropathy by inhibiting SnoN degradation in vivo and in vitro

DNA Methylation Profiles of PSMA6, PSMB5, KEAP1, and HIF1A Genes in Patients with Type 1 Diabetes and Diabetic Retinopathy

We explored differences in the DNA methylation statuses of PSMA6, PSMB5, HIF1A, and KEAP1 gene promoter regions in patients with type 1 diabetes and different diabetic retinopathy (DR) stages. Study subjects included individuals with no DR (NDR, n = 41), those with non-proliferative DR (NPDR, n = 27), and individuals with proliferative DR or those who underwent laser photocoagulation (PDR/LPC, n = 46). DNA methylation was determined by Zymo OneStep qMethyl technique. The methylation of PSMA6 (NDR 5.9 (3.9-8.7) %, NPDR 4.5 (3.8-5.7) %, PDR/LPC 6.6 (4.7-10.7) %, p = 0.003) and PSMB5 (NDR 2.2 (1.9-3.7) %, NPDR 2.2 (1.9-3.0) %, PDR/LPC 3.2 (2.5-7.1) %, p < 0.01) differed across the groups. Consistent correlations were observed between the methylation levels of HIF1A and PSMA6 in all study groups. DNA methylation levels of PSMA6, PSMB5, and HIF1A genes were positively correlated with the duration of diabetes, HbA1c, and albuminuria in certain study groups. Univariate regression models revealed a significant association between the methylation level z-scores of PSMA6, PSMB5, and HIF1A and severe DR (PSMA6: OR = 1.96 (1.15; 3.33), p = 0.013; PSMB5: OR = 1.90 (1.14; 3.16), p = 0.013; HIF1A: OR = 3.19 (1.26; 8.06), p = 0.014). PSMB5 remained significantly associated with DR in multivariate analysis. Our findings suggest significant associations between the severity of DR and the DNA methylation levels of the genes PSMA6, PSMB5, and HIF1A, but not KEAP1 gene.

Circulating exosomal miR-16-5p and let-7e-5p are associated with bladder fibrosis of diabetic cystopathy

Diabetic cystopathy (DCP) is a prevalent etiology of bladder dysfunction in individuals with longstanding diabetes, frequently leading to bladder interstitial fibrosis. Research investigating the initial pathological alterations of DCP is notably scarce. To comprehend the development of fibrosis and find effective biomarkers for its diagnosis, we prepared streptozotocin-induced long-term diabetic SD rats exhibiting a type 1 diabetes phenotype and bladder fibrosis in histology detection. After observing myofibroblast differentiation from rats' primary bladder fibroblasts with immunofluorescence, we isolated fibroblasts derived exosomes and performed exosomal miRNA sequencing. The co-differentially expressed miRNAs (DEMis) (miR-16-5p and let-7e-5p) were screened through a joint analysis of diabetic rats and long-term patients' plasma data (GES97123) downloaded from the GEO database. Then two co-DEMis were validated by quantitative PCR on exosomes derived from diabetic rats' plasma. Following with a series of analysis, including target mRNAs and transcription factors (TFs) prediction, hubgenes identification, protein-protein interaction (PPI) network construction and gene enrichment analysis, a miRNA-mediated genetic regulatory network consisting of two miRNAs, nine TFs, and thirty target mRNAs were identified in relation to fibrotic processes. Thus, circulating exosomal miR-16-5p and let-7e-5p are associated with bladder fibrosis of DCP, and the crucial genes in regulatory network might hold immense significance in studying the pathogenesis and molecular mechanisms of fibrosis, which deserves further exploration.

MG132, Attenuates the Retinal Vascular Injury Through the Upregulation of Nrf2 Expression

Purpose: This study clarifies the beneficial effects of MG132, a proteasomal inhibitor, on retinal vascular injury mediated by diabetes-induced oxidative stress through nuclear factor erythroid 2-related factor 2 (Nrf2). Methods: Diabetic rats and control animals were randomly assigned to receive MG132 or vehicle for 24 weeks, and human retinal endothelial cells (HRECs) were incubated with normal or high glucose with or without MG132. 26S proteasome activity in the rat retinas or cultured HRECs was measured using Suc-LLVY-7-amido-4-methylcoumarin. NADPH-quinone oxidoreduc-tase (NQO1), heme oxygenase (HO)-1, kelch-like ECH-associated protein 1 (Keap1) and Nrf2 were examined by Western blotting and real-time reverse transcription polymerase chain reaction. Cell apoptosis is measured through flow cytometry assay, mitochondrial reactive oxygen species (ROS) production, and retinal vascular leakage were assayed using CM-H2DCFDA fluorescent probes and Evans blue, respectively. Results: MG132 significantly inhibited the activation of 26S proteasome induced by diabetes or elevated glucose, and subsequently increased the expression of Nrf2, NQO1, and HO-1, and further reduced ROS accumulation. These changes were associated with a decrease of diabetes-induced retinal vascular leakage and retinal capillary cell apoptosis. Conclusions: MG132 decreases diabetes-induced 26S proteasome activation and exerts protective effects against retinal microvascular dysfunction in diabetic rats in association with the alleviation of retinal oxidative stress mediated by Nrf2.

Modeling SILAC Data to Assess Protein Turnover in a Cellular Model of Diabetic Nephropathy

Protein turnover rate is finely regulated through intracellular mechanisms and signals that are still incompletely understood but that are essential for the correct function of cellular processes. Indeed, a dysfunctional proteostasis often impacts the cell's ability to remove unfolded, misfolded, degraded, non-functional, or damaged proteins. Thus, altered cellular mechanisms controlling protein turnover impinge on the pathophysiology of many diseases, making the study of protein synthesis and degradation rates an important step for a more comprehensive understanding of these pathologies. In this manuscript, we describe the application of a dynamic-SILAC approach to study the turnover rate and the abundance of proteins in a cellular model of diabetic nephropathy. We estimated protein half-lives and relative abundance for thousands of proteins, several of which are characterized by either an altered turnover rate or altered abundance between diabetic nephropathic subjects and diabetic controls. Many of these proteins were previously shown to be related to diabetic complications and represent therefore, possible biomarkers or therapeutic targets. Beside the aspects strictly related to the pathological condition, our data also represent a consistent compendium of protein half-lives in human fibroblasts and a rich source of important information related to basic cell biology.

Ubiquitin-proteasome system in diabetic retinopathy

Diabetic retinopathy (DR) is the most common complication of diabetes, being the most prevalent reason for blindness among the working-age population in the developed world. Despite constant improvement of understanding of the pathogenesis of DR, identification of novel biomarkers of DR is needed for improvement of patient risk stratification and development of novel prevention and therapeutic approaches. The ubiquitin-proteasome system (UPS) is the primary protein quality control system responsible for recognizing and degrading of damaged proteins. This review aims to summarize literature data on modifications of UPS in diabetes and DR. First, we briefly review the structure and functions of UPS in physiological conditions. We then describe how UPS is involved in the development and progression of diabetes and touch upon the association of UPS genetic factors with diabetes and its complications. Further, we focused on the effect of diabetes-induced hyperglycemia, oxidative stress and hypoxia on UPS functioning, with examples of studies on DR. In other sections, we discussed the association of several other mechanisms of DR (endoplasmic reticulum stress, neurodegeneration etc) with UPS modifications. Finally, UPS-affecting drugs and remedies are reviewed. This review highlights UPS as a promising target for the development of therapies for DR prevention and treatment and identifies gaps in existing knowledge and possible future study directions.

Mechanisms of gene regulation by histone degradation in adaptation of yeast: an overview of recent advances

Histones are important component of eukaryotic cells chromatin and consist of arginine and lysine residues. Histones play an important role in the protection of DNA. Their contents significantly affect high-level chromatin structure formation, gene expression, DNA replication, and other important life activities. Protein degradation is an important regulatory mechanism of histone content. Recent studies have revealed that modification of amino acid sequence is directly related to histone breakdown. In addition, histone degradation is closely related to covalent modifications, such as ubiquitination and acetylation, which are considered to be driving factors in gene regulation. Gene regulation is an important mechanism in adaptation to the environment and survival of species. With the introduction of highly efficient technology, various mutations in histones have been identified in yeast. In the field of epigenetics and the transmission of chromatin states, two widely used model organisms are the budding yeast Saccharomyces cerevisiae and Schizosaccharomyces pombe. Higher eukaryotes can use their silent loci to maintain their epigenetic states and providing the base to investigate mechanisms underlying development. Therfore, both species have contributed a plethora of information on these mechanisms in both yeast and higher eukaryotes. This study focuses on the role of histone modifications in controlling telomeric silencing in Saccharomyces cerevisiae and centromeric silencing in S. pombe as examples of genetic loci that demonstrate epigenetic inheritance. In view of recent advances, this review focuses on the post-translational modification of histone amino acid residues and reviews the relationship between histone degradation and amino acid residue modification.

ERK and p38 MAPK inhibition controls NF-E2 degradation and profibrotic signaling in renal proximal tubule cells

Aims:

Transforming growth factor-β (TGF-β) mediates fibrotic manifestations of diabetic nephropathy. We demonstrated proteasomal degradation of anti-fibrotic protein, nuclear factor-erythroid derived 2 (NF-E2), in TGF-β treated human renal proximal tubule (HK-11) cells and in diabetic mouse kidneys. The current study examined the role of mitogen-activated protein kinase (MAPK) pathways in mediating NF-E2 proteasomal degradation and stimulating profibrotic signaling in HK-11 cells.

Main methods:

HK-11 cells were pretreated with vehicle or appropriate proteasome and MAPK inhibitors, MG132 (0.5 μM), SB203580 (1 μM), PD98059 (25 μM) and SP600125 (10 μM), respectively, followed by treatment with/without TGF-β (10 ng/ml, 24 h). Cell lysates and kidney homogenates from FVB and OVE26 mice treated with/without MG132 were immunoblotted with appropriate antibodies. pUse vector and pUse-NF-E2 cDNA were transfected in HK-11 cells and effects of TGF-β on JNK MAPK phosphorylation (pJNK) was examined.

Key findings:

We demonstrated activation of p38, ERK, and JNK MAPK pathways in TGF-β treated HK-11 cells. Dual p38 and ERK MAPK blockade prevented TGF-β-induced pSer⁸²Hsp27, fibronectin and connective tissue growth factor (CTGF) expression while preserving NF-E2 expression. Blockade of JNK MAPK inhibited TGF-β-induced CTGF expression without preserving NF-E2 expression. MG132 treatment prevented TGF-β-induced pJNK in HK-11 cells and in type 1 diabetic OVE26 mouse kidneys, demonstrating that TGF-β- and diabetes-induced pJNK occurs downstream of proteasome activation. A direct role for NF-E2 in modulating pJNK activation was demonstrated by NF-E2 over-expression.

Significance:

ERK and p38 MAPK promotes NF-E2 proteasomal degradation while proteasome activation promotes pJNK and profibrotic signaling in renal proximal tubule cells.

Role of Dendritic Cell in Diabetic Nephropathy

Diabetic nephropathy (DN) is one of the most significant microvascular complications in diabetic patients. DN is the leading cause of end-stage renal disease, accounting for approximately 50% of incident cases. The current treatment options, such as optimal control of hyperglycemia and elevated blood pressure, are insufficient to prevent its progression. DN has been considered as a nonimmune, metabolic, or hemodynamic glomerular disease initiated by hyperglycemia. However, recent studies suggest that DN is an inflammatory disease, and immune cells related with innate and adaptive immunity, such as macrophage and T cells, might be involved in its development and progression. Although it has been revealed that kidney dendritic cells (DCs) accumulation in the renal tissue of human and animal models of DN require activated T cells in the kidney disease, little is known about the function of DCs in DN. In this review, we describe kidney DCs and their subsets, and the role in the pathogenesis of DN. We also suggest how to improve the kidney outcomes by modulating kidney DCs optimally in the patients with DN.

Constitutive activity of GPR26 regulated by ubiquitin-dependent degradation and its antitumor role

G protein-coupled receptors (GPCRs) play important roles in many physiological functions and numerous diseases. In addition to the classic ligand-stimulated receptor activity, an increasing number of studies have established that many GPCRs function constitutively in a receptor dose-dependent manner. Previous observations showed that following gene transfection, little or no protein was detectable for certain GPCRs (designated apparent state A), such as GPR26, GPR39, GPR78, GPR133, GPR139, BRS3, and LGR5, which showed strong constitutive activities. When we lysed cells in the immediate presence of western blot loading buffer, a significant increase of protein levels was detected (actual state B), which was much closer to the true expression levels under physiological conditions. GPR26 was chosen for further functional experiments as the actual state B. We identified an important ubiquitination site, K286, as well as the ubiquitin ligase E3 homologous to the E6-associated protein carboxyl terminus domain containing 3 interacting with GPR26. The pronounced differences in the protein expression and constitutive activity of GPR26 were a consequence of the ubiquitin-mediated rapid degradation mechanism. Furthermore, we identified in vitro and in vivo antitumor activity associated with high expression levels and constitutive activity of GPR26 in liver cancer cells. Hence, GPR26 could act as an antitumor gene for hepatocellular carcinoma. This study also represents the actual state B of a batch of GPCRs that actually play potentially important roles in physiological functions by their constitutive activity, which is controlled by rapid ubiquitin-dependent degradation.

miR-130a-3p inhibition protects against renal fibrosis in vitro via the TGF-β1/Smad pathway by targeting SnoN

BACKGROUND

Renal fibrosis, a common pathological outcome of chronic kidney disease (CKD), is characterized by extracellular matrix (ECM) accumulation, damage to the tubular epithelium, and the proliferation and activation of fibroblasts. SnoN, a TGF-β1/Smad transcriptional co-suppressor, is downregulated in obstructive nephropathy. However, the relationship between miR-130a-3p and SnoN expression in the regulation of renal fibrosis is still unknown.

METHODS

We used human renal proximal tubular epithelial cells (HRPTEpiCs, HK-2 and primary HRPTEpiCs) treated with TGF-β1 to establish an in vitro renal fibrosis model. The expression of miR-130a-3p, SnoN and other proteins related to epithelial mesenchymal transition (EMT) and TGF-β1/Smad signalling was investigated by western blotting or qRT-PCR. A luciferase reporter assay was conducted to confirm the interaction of SnoN mRNA and miR-130a-3p. The translocation of p-Smad 2/3 and Smad 7 was determined using immunofluorescence staining.

RESULTS

After TGF-β1 treatment, miR-130a-3p was highly expressed in renal tubular epithelial cells, while SnoN was poorly expressed. The cell morphology changed to fibroblast-like, indicating evidence of EMT. The levels of EMT and fibrosis-related proteins were decreased through miR-130a-3p inhibition. Additionally, miR-130a-3p acted upon the 3'-UTR of SnoN directly to suppress SnoN expression. Furthermore, miR-130a-3p/SnoN promoted the activation of TGF-β1/Smad signalling, as revealed by p-Smad 2/3 and Smad 7 expression levels and distribution patterns.

CONCLUSION

Our study verified that miR-130a-3p facilitates the TGF-β1/Smad pathway in renal tubular epithelial cells and may participate in renal fibrosis by targeting SnoN, which could be a possible strategy for renal fibrosis treatment.

MG132 protects against renal dysfunction by regulating Akt-mediated inflammation in diabetic nephropathy

Diabetic nephropathy (DN), the leading cause of end-stage renal disease (ESRD). To date, mounting evidence has shown that inflammation may contribute to the pathogenesis of DN. Recent reports have shown that proteasome inhibitors display cytoprotection by reducing the phosphorylation of Akt, a serine/threonine kinase, plays a critical role in cellular survival and metabolism and can crosstalk with inflammation. Therefore, we hypothesized that MG132, specific proteasome inhibitor, could provide renoprotection by suppressing Akt-mediated inflammation in DN. In vivo, male Sprague-Dawley rats were divided into normal control group (NC), diabetic nephropathy group (DN), DN model plus MG132 treatment group (MG132), and DN model plus deguelin treatment group (Deguelin)(deguelin, a specific inhibitor of Akt). In vitro, a human glomerular mesangial cell lines (HMCs) was exposed to 5.5 mmol/L glucose (CON), 30 mmol/L glucose (HG), 30 mmol/L glucose with 0.5 umol/L MG132 (MG132) and 30 mmol/L glucose with 5 umol/L deguelin (Deguelin). Compared with NC, DN showed a significant increase in the urinary protein excretion rate and inflammatory cytokines, as well as p-Akt. Compared with CON, HMCs co-cultured with HG was notably proliferated, which is in accord with α-smooth muscle actin (α-SMA) expression. These alterations were inhibited by administration of MG132 or deguelin. In conclusion, MG132 significantly inhibits the development of DN by regulating Akt phosphorylation-mediated inflammatory activation.

Autophagy activation in circulating proangiogenic cells aggravates AKI in type I diabetes mellitus

Acute kidney injury (AKI) occurs frequently in hospitals worldwide, but the therapeutic options are limited. Diabetes mellitus (DM) affects more and more people around the globe. The disease worsens the prognosis of AKI even further. In recent years, cell-based therapies have increasingly been applied in experimental AKI. The aim of the study was to utilize two established autophagy inducers for pharmacological preconditioning of so-called proangiogenic cells (PACs) in PAC treatment of diabetic AKI. Insulin-dependent DM was induced in male C57/Bl6N mice by intraperitoneal injections of streptozotocine. Six weeks later, animals underwent bilateral renal ischemia for 45 min, followed by intravenous injections of either native or zVAD (benzyloxycarbonyl-Val-Ala-Asp-fluoro-methylketone)- or Z-Leu-Leu-Leu-al (MG132)-pretreated syngeneic murine PACs. Mice were analyzed 48 h (short term) and 6 wk (long term) later, respectively. DM worsened postischemic AKI, and PAC preconditioning with zVAD and MG132 resulted in a further decline of excretory kidney function. Injection of native PACs reduced fibrosis in nondiabetic mice, but cell preconditioning promoted interstitial matrix accumulation significantly. Both substances aggravated endothelial-to-mesenchymal transition (EndoMT) under diabetic conditions; these effects occurred either exclusively in the short (zVAD) or in the short and long term (MG132). Preconditioned cells stimulated the autophagocytic flux in intrarenal endothelial cells, and all experimental groups displayed increased endothelial abundances of senescence-associated β-galactosidase, a marker of premature cell senescence. Pharmacological autophagy activation may not serve as an effective strategy for improving PAC competence in diabetic AKI in general. On the contrary, several outcome parameters (excretory function, fibrosis, EndoMT) may even be worsened.

Transcriptional cofactors Ski and SnoN are major regulators of the TGF-β/Smad signaling pathway in health and disease

The transforming growth factor-β (TGF-β) family plays major pleiotropic roles by regulating many physiological processes in development and tissue homeostasis. The TGF-β signaling pathway outcome relies on the control of the spatial and temporal expression of >500 genes, which depend on the functions of the Smad protein along with those of diverse modulators of this signaling pathway, such as transcriptional factors and cofactors. Ski (Sloan-Kettering Institute) and SnoN (Ski novel) are Smad-interacting proteins that negatively regulate the TGF-β signaling pathway by disrupting the formation of R-Smad/Smad4 complexes, as well as by inhibiting Smad association with the p300/CBP coactivators. The Ski and SnoN transcriptional cofactors recruit diverse corepressors and histone deacetylases to repress gene transcription. The TGF-β/Smad pathway and coregulators Ski and SnoN clearly regulate each other through several positive and negative feedback mechanisms. Thus, these cross-regulatory processes finely modify the TGF-β signaling outcome as they control the magnitude and duration of the TGF-β signals. As a result, any alteration in these regulatory mechanisms may lead to disease development. Therefore, the design of targeted therapies to exert tight control of the levels of negative modulators of the TGF-β pathway, such as Ski and SnoN, is critical to restore cell homeostasis under the specific pathological conditions in which these cofactors are deregulated, such as fibrosis and cancer.

Proteins that repress molecular signaling through the transforming growth factor-beta (TGF-β) pathway offer promising targets for treating cancer and fibrosis. Marina Macías-Silva and colleagues from the National Autonomous University of Mexico in Mexico City review the ways in which a pair of proteins, called Ski and SnoN, interact with downstream mediators of TGF-β to inhibit the effects of this master growth factor. Aberrant levels of Ski and SnoN have been linked to diverse range of diseases involving cell proliferation run amok, and therapies that regulate the expression of these proteins could help normalize TGF-β signaling to healthier physiological levels. For decades, drug companies have tried to target the TGF-β pathway, with limited success. Altering the activity of these repressors instead could provide a roundabout way of remedying pathogenic TGF-β activity in fibrosis and oncology.

Involvement of the ubiquitin-proteasome system in the expression of extracellular matrix genes in retinal pigment epithelial cells

Emerging evidence suggests that dysfunction of the ubiquitin-proteasome system is involved in the pathogenesis of numerous senile degenerative diseases including retinal disorders. The aim of this study was to assess whether there is a link between proteasome regulation and retinal pigment epithelium (RPE)-mediated expression of extracellular matrix genes. For this purpose, human retinal pigment epithelial cells (ARPE-19) were treated with different concentrations of transforming growth factor-β (TGFβ), connective tissue growth factor (CTGF), interferon-γ (IFNγ) and the irreversible proteasome inhibitor epoxomicin. First, cytotoxicity and proliferation assays were carried out. The expression of proteasome-related genes and proteins was assessed and proteasome activity was determined. Then, expression of fibrosis-associated factors fibronectin (FN), fibronectin EDA domain (FN EDA), metalloproteinase-2 (MMP-2), tissue inhibitor of metalloproteinases-1 (TIMP-1) and peroxisome proliferator-associated receptor-γ (PPARγ) was assessed. The proteasome inhibitor epoxomicin strongly arrested cell cycle progression and down-regulated TGFβ gene expression, which in turn was shown to induce expression of pro-fibrogenic genes in ARPE-19 cells. Furthermore, epoxomicin induced a directional shift in the balance between MMP-2 and TIMP-1 and was associated with down-regulation of transcription of extracellular matrix genes FN and FN-EDA and up-regulation of the anti-fibrogenic factor PPARγ. In addition, both CTGF and TGFβ were shown to affect expression of proteasome-associated mRNA and protein levels. Our results suggest a link between proteasome activity and pro-fibrogenic mechanisms in the RPE, which could imply a role for proteasome-modulating agents in the treatment of retinal disorders characterized by RPE-mediated fibrogenic responses.

CYLD Deubiquitinase Negatively Regulates High Glucose-Induced NF-κB Inflammatory Signaling in Mesangial Cells

Nuclear factor-kappa B (NF-κB) is the key part of multiple signal transduction of inflammation in the pathogenesis of diabetic nephropathy (DN). The ubiquitin-proteasome system is extensively involved in the regulation of the NF-κB pathway. Cylindromatosis (CYLD) has deubiquitinase activity and acts as a negative regulator of the NF-κB signaling pathway. However, the association between CYLD and NF-κB inflammatory signaling in DN is unclear. In the present study, mouse glomerular mesangial cells (GMCs) and rat GMCs were stimulated by elevated concentrations of glucose (10, 20, and 30 mmol/L high glucose) or mannitol as the osmotic pressure control. CYLD was overexpressed or suppressed by transfection with a CYLD expressing vector or CYLD-specific siRNA, respectively. Our data showed that high glucose significantly inhibited the protein and mRNA expression of CYLD in a dose- and time-dependent manner (both p < 0.05). siRNA-mediated knockdown CYLD facilitated the high glucose-induced activation of NF-κB signaling and triggered the release of MCP-1, IL-6, and IL-8 (all p < 0.05). However, these high glucose-mediated effects were blunted by overexpression of CYLD (p < 0.05). The present results support the involvement of CYLD in the regulation of NF-κB inflammatory signaling induced by elevated glucose, implicating CYLD as a potential therapeutic target of DN.

Hidden targets of ubiquitin proteasome system: To prevent diabetic nephropathy

Diabetic nephropathy (DN) is the major cause of end stage renal failure. Although, several therapeutic targets have emerged to prevent the progression of DN, the number of people with DN still continues to rise worldwide, suggesting an urgent need of novel targets to prevent DN completely. Currently, the role of ubiquitin proteasome system (UPS) has been highlighted in the pathogenesis and progression of various diseases like obesity, insulin resistance, atherosclerosis, cancers, neurodegerative disorders and including secondary complications of diabetes. UPS mainly involves in protein homeostatis through ubiquitination (post translational modification) and proteasomal degradation of various proteins. Ubiquitination, not only involves in proteasomal degradation, but also directs the substrate proteins to participate in multitude of cell signalling pathways. However, very little is known about ubiquitination and UPS in the progression of DN. This review mainly focuses on UPS and its components including E2 conjugating enzymes, E3 ligases and deubiquitinases (DUBs) in the development of DN and thus may help us to find novel therapeutic targets with in UPS to prevent DN completely in future.

Prevention of Streptozotocin-Induced Diabetic Nephropathy by MG132: Possible Roles of Nrf2 and IκB

Our previous study showed that proteasomal inhibitor MG132 can prevent diabetic nephropathy (DN) along with upregulation of nuclear factor (erythroid-derived 2)-like 2 (Nrf2). The present study was to investigate whether MG132 can prevent DN in wild-type and Nrf2-KO mice. Type 1 diabetes was induced in wild-type and Nrf2-KO mice by multiple low doses of streptozotocin. Two weeks after streptozotocin injection, both wild-type and Nrf2-KO mice were randomly divided into four groups: control, MG132, DM, and DM/MG132. MG132 (10 μg/kg/day) or vehicle was administered intraperitoneally for 4 months. Renal function, morphology, and biochemical changes were measured after 4-month treatment with MG132. MG132 treatment suppressed proteasomal activity in the two genotypes. In wild-type mice, MG132 attenuated diabetes-induced renal dysfunction, fibrosis, inflammation, and oxidative damage along with increased Nrf2 and IκB expression. Deletion of Nrf2 gene resulted in a partial, but significant attenuation of MG132 renal protection in Nrf2-KO mice compared with wild-type mice. MG132-increased IκB expression was not different between wild-type and Nrf2-KO mice. This work indicates that MG132 inhibits diabetes-increased proteasomal activity, resulting in Nrf2 and IκB upregulation and renal protection, which could be used as a strategy to prevent diabetic nephropathy.

Proteasome Activators, PA28α and PA28β, Govern Development of Microvascular Injury in Diabetic Nephropathy and Retinopathy

Diabetic nephropathy (DN) and diabetic retinopathy (DR) are major complications of type 1 and type 2 diabetes. DN and DR are mainly caused by injury to the perivascular supporting cells, the mesangial cells within the glomerulus, and the pericytes in the retina. The genes and molecular mechanisms predisposing retinal and glomerular pericytes to diabetic injury are poorly characterized. In this study, the genetic deletion of proteasome activator genes, PA28α and PA28β genes, protected the diabetic mice in the experimental STZ-induced diabetes model against renal injury and retinal microvascular injury and prolonged their survival compared with wild type STZ diabetic mice. The improved wellbeing and reduced renal damage was associated with diminished expression of Osteopontin (OPN) and Monocyte Chemoattractant Protein-1 (MCP-1) in the glomeruli of STZ-injected PA28α/PA28β double knockout (Pa28αβDKO) mice and also in cultured mesangial cells and retinal pericytes isolated from Pa28αβDKO mice that were grown in high glucose. The mesangial PA28-mediated expression of OPN under high glucose conditions was suppressed by peptides capable of inhibiting the binding of PA28 to the 20S proteasome. Collectively, our findings demonstrate that diabetic hyperglycemia promotes PA28-mediated alteration of proteasome activity in vulnerable perivascular cells resulting in microvascular injury and development of DN and DR.

Rapid Identification of Potential Drugs for Diabetic Nephropathy Using Whole-Genome Expression Profiles of Glomeruli

Objective. To investigate potential drugs for diabetic nephropathy (DN) using whole-genome expression profiles and the Connectivity Map (CMAP). Methodology. Eighteen Chinese Han DN patients and six normal controls were included in this study. Whole-genome expression profiles of microdissected glomeruli were measured using the Affymetrix human U133 plus 2.0 chip. Differentially expressed genes (DEGs) between late stage and early stage DN samples and the CMAP database were used to identify potential drugs for DN using bioinformatics methods. Results. (1) A total of 1065 DEGs (FDR < 0.05 and fold change > 1.5) were found in late stage DN patients compared with early stage DN patients. (2) Piperlongumine, 15d-PGJ2 (15-delta prostaglandin J2), vorinostat, and trichostatin A were predicted to be the most promising potential drugs for DN, acting as NF-κB inhibitors, histone deacetylase inhibitors (HDACIs), PI3K pathway inhibitors, or PPARγ agonists, respectively. Conclusion. Using whole-genome expression profiles and the CMAP database, we rapidly predicted potential DN drugs, and therapeutic potential was confirmed by previously published studies. Animal experiments and clinical trials are needed to confirm both the safety and efficacy of these drugs in the treatment of DN.

High Glucose and Lipopolysaccharide Prime NLRP3 Inflammasome via ROS/TXNIP Pathway in Mesangial Cells

While inflammation is considered a central component in the development in diabetic nephropathy, the mechanism remains unclear. The NLRP3 inflammasome acts as both a sensor and a regulator of the inflammatory response. The NLRP3 inflammasome responds to exogenous and endogenous danger signals, resulting in cleavage of procaspase-1 and activation of cytokines IL-1β, IL-18, and IL-33, ultimately triggering an inflammatory cascade reaction. This study observed the expression of NLRP3 inflammasome signaling stimulated by high glucose, lipopolysaccharide, and reactive oxygen species (ROS) inhibitor N-acetyl-L-cysteine in glomerular mesangial cells, aiming to elucidate the mechanism by which the NLRP3 inflammasome signaling pathway may contribute to diabetic nephropathy. We found that the expression of thioredoxin-interacting protein (TXNIP), NLRP3, and IL-1β was observed by immunohistochemistry in vivo. Simultaneously, the mRNA and protein levels of TXNIP, NLRP3, procaspase-1, and IL-1β were significantly induced by high glucose concentration and lipopolysaccharide in a dose-dependent and time-dependent manner in vitro. This induction by both high glucose and lipopolysaccharide was significantly inhibited by N-acetyl-L-cysteine. Our results firstly reveal that high glucose and lipopolysaccharide activate ROS/TXNIP/ NLRP3/IL-1β inflammasome signaling in glomerular mesangial cells, suggesting a mechanism by which inflammation may contribute to the development of diabetic nephropathy.

Participation of proteasome-ubiquitin protein degradation in autophagy and the activation of AMP-activated protein kinase

Although activation of the AMP-activated protein kinase (AMPK) as well as of ubiquitin/proteasome degradative pathways play an essential role in the preservation of metabolic homeostasis, little is known concerning interactions between protein turnover and AMPK activity. In the present studies, we found that inhibition of the 26S proteasome resulted in rapid activation of AMPK in macrophages, epithelial and endothelial cells. This was associated with increased levels of non-degraded Ub-protein conjugates, in both cytosolic and mitochondrial fractions. Selective inhibitors of ubiquitination or siRNA-dependent knockdown of Ub-ligase E1 diminished AMPK activation in cells treated with MG132, a 26S proteasome inhibitor. In addition to inhibition of AMPK activation by Ub-ligase E1 inhibitors, deficiency in Park2 mitochondria-associated Ub-ligase E3 also reduced AMPK activation upon dissipation of mitochondrial membrane potential (Δψm). Accumulation of Ub-proteins was correlated with decreases in cellular bioenergetics, including mitochondria oxidative phosphorylation, and an increase in ROS formation. Antioxidants, such as N-acetyl-L-cysteine or mitochondria-targeted MitoTEMPO, effectively diminished MG132-induced AMPK activation. Glucose-dependent regulation of AMPK or AMPK-mediated autophagy was modulated by alterations in intracellular levels of Ub-protein conjugates. Our results indicate that accumulation of ubiquitinated proteins alter cellular bioenergetics and redox status, leading to AMPK activation.