Ribosomal biogenesis induction by high glucose requires activation of upstream binding factor in kidney glomerular epithelial cells

Extracellular signal-regulated kinase is activated in podocytes from patients with diabetic nephropathy

In the past few decades, the global prevalence of diabetes has provided us with a warning about future chronic complications. Diabetic nephropathy (DN) is the main cause of end-stage kidney disease. Podocytes in the glomerulus play a critical role in regulating glomerular permeability, and podocyte injury is one of the main causes of DN. Extracellular signal-regulated kinase (ERK) is a member of the mitogen-activated protein kinase family that plays critical roles in intracellular signal transduction. In human patients with DN, phosphorylated ERK (pERK), the active form of ERK, is increased in the glomeruli. However, information on the expression of pERK, specifically in podocytes in DN, is limited. Meanwhile, high glucose induces ERK activation in immortalized podocyte cell lines, suggesting the involvement of podocytic ERK in DN. We performed an immunohistochemical study using Wilms' tumor-1 (WT-1) as a podocyte-specific marker to investigate whether podocytic pERK levels are increased in patients with DN. In the glomeruli of the DN group, we observed remarkable co-staining for WT-1 and pERK. In contrast, the glomeruli of the control group contained only a few pERK-positive podocytes. Statistical analyses revealed that, relative to healthy controls, patients with DN showed significantly increased pERK expression levels in cells that were positive for WT-1 (DN: 51.3 ± 13.1% vs. control: 7.3 ± 1.6%, p = 0.0158, t-test, n = 4 for each group). This suggests that ERK activation in podocytes is involved in the pathogenesis of DN.

Glucose ingestion before and after resistance training sessions does not augment ribosome biogenesis in healthy moderately trained young adults

PURPOSE

Resistance training-induced skeletal muscle hypertrophy seems to depend on ribosome biogenesis and content. High glucose treatment may augment ribosome biogenesis through potentiating resistance training-induced adaptations. This was investigated with total RNA and ribosomal RNA abundances as main outcomes, with relevant transcriptional/translational regulators (c-Myc/UBF/rpS6) as a secondary outcome.

METHODS

Sixteen healthy, moderately trained individuals [male/female, n = 9/7; age, 24.1 (3.3)] participated in a within-participant crossover trial with unilateral resistance training (leg press and knee extension, 3 sets of 10 repetitions maximum) and pre- and post-exercise ingestion of either glucose (3 × 30 g, 90 g total) or placebo supplements (Stevia rebaudiana, 3 × 0.3 g, 0.9 g total), together with protein (2 × 25 g, 50 g total), on alternating days for 12 days. Six morning resistance exercise sessions were conducted per condition, and the sessions were performed in an otherwise fasted state. Micro-biopsies were sampled from m. vastus lateralis before and after the intervention.

RESULTS

Glucose ingestion did not have beneficial effects on resistance training-induced increases of ribosomal content (mean difference 7.6% [- 7.2, 24.9], p = 0.34; ribosomal RNA, 47S/18S/28S/5.8S/5S, range 7.6-37.9%, p = 0.40-0.98) or levels of relevant transcriptional or translational regulators (c-MYK/UBF/rpS6, p = 0.094-0.292). Of note, both baseline and trained state data of total RNA showed a linear relationship with UBF; a ∼14% increase in total RNA corresponded to 1 SD unit increase in UBF (p = 0.003).

CONCLUSION

Glucose ingestion before and after resistance training sessions did not augment ribosomal RNA accumulation during twelve days of heavy-load resistance training in moderately trained young adults.

An RNA Damage Response Network Mediates the Lethality of 5-FU in Clinically Relevant Tumor Types

5-fluorouracil (5-FU) is a successful and broadly used anti-cancer therapeutic. A major mechanism of action of 5-FU is thought to be through thymidylate synthase (TYMS) inhibition resulting in dTTP depletion and activation of the DNA damage response. This suggests that 5-FU should synergize with other DNA damaging agents. However, we found that combinations of 5-FU and oxaliplatin or irinotecan failed to display any evidence of synergy in clinical trials, and resulted in sub-additive killing in a panel of colorectal cancer (CRC) cell lines. In seeking to understand this antagonism, we unexpectedly found that an RNA damage response during ribosome biogenesis dominates the drug's efficacy in tumor types for which 5-FU shows clinical benefit. 5-FU has an inherent bias for RNA incorporation, and blocking this greatly reduced drug-induced lethality, indicating that accumulation of damaged RNA is more deleterious than the lack of new RNA synthesis. Using 5-FU metabolites that specifically incorporate into either RNA or DNA revealed that CRC cell lines and patient-derived colorectal cancer organoids are inherently more sensitive to RNA damage. This difference held true in cell lines from other tissues in which 5-FU has shown clinical utility, whereas cell lines from tumor tissues that lack clinical 5-FU responsiveness typically showed greater sensitivity to the drug's DNA damage effects. Analysis of changes in the phosphoproteome and ubiquitinome shows RNA damage triggers the selective ubiquitination of multiple ribosomal proteins leading to autophagy-dependent rRNA catabolism and proteasome-dependent degradation of ubiquitinated ribosome proteins. Further, RNA damage response to 5-FU is selectively enhanced by compounds that promote ribosome biogenesis, such as KDM2A inhibitors. These results demonstrate the presence of a strong RNA damage response linked to apoptotic cell death, with clear utility of combinatorially targeting this response in cancer therapy.

Identification of molecular pathways and protein-protein interactions in adipose tissue-derived mesenchymal stromal cells (ASCs) under physiological oxygen concentration in a diabetic rat model

Objectives

Adipose tissue-derived mesenchymal stromal cells (ASCs) are useful in cell-based therapy. However, it is well known that diabetes mellitus (DM) alters ASCs' functionality. The majority of in vitro studies related to ASCs are developed under non-physiological oxygen conditions. Therefore, they may not reflect the full effects of DM on ASCs, in vivo. The main aim of the current study is to identify molecular pathways and underlying biological mechanisms affected by diabetes on ASCs in physiological oxygen conditions.

Materials and Methods

ASCs derived from healthy (ASCs-C) and diabetic (ASCs-D) rats were expanded under standard culture conditions (21% O2) or cultured in physiological oxygen conditions (3% O2) and characterized. Differential gene expressions (DEGs) of ASCs-D with respect to ASCs-C were identified and analyzed with bioinformatic tools. Protein-protein interaction (PPI) networks, from up- and down-regulated DEGs, were also constructed.

Results

The bioinformatic analysis revealed 1354 up-regulated and 859 down-regulated DEGs in ASCs-D, with 21 and 78 terms over and under-represented, respectively. Terms linked with glycosylation and ribosomes were over-represented and terms related to the activity of RNA-polymerase II and transcription regulation were under-represented. PPI network disclosed RPL11-RPS5 and KDR-VEGFA as the main interactions from up- and down-regulated DEGs, respectively.

Conclusion

These results provide valuable information about gene pathways and underlying molecular mechanisms by which diabetes disturbs ASCs biology in physiological oxygen conditions. Furthermore, they reveal, molecular targets to improve the use of ASCs in autologous transplantation.

Human type 2 diabetes mellitus-associated transcriptional disturbances in a high-sugar diet long-term exposed Drosophila melanogaster

Type 2 Diabetes mellitus (T2DM) is a multifactorial and polygenic disorder with the molecular bases still idiopathic. Experimental analyses and tests are quite limited upon human samples due to the access, variability of patient's conditions, and the size and complexity of the genome. Therefore, high-sugar diet exposure is commonly used for modeling T2DM in non-human animals, which includes invertebrate organisms like the fruit fly Drosophila melanogaster. Interestingly, high-sugar diet (HSD) induces delayed time for pupation and reduced viability in fruit fly larvae hatched from a 30% sucrose-containing medium (HSD-30%). Here we carried out an mRNA-deep sequencing study to identify differentially transcribed genes in adult fruit fly hatched and reared from an HSD-30%. Seven days after hatching, flies reared on control and HSD-30% were used to glucose and triglyceride level measurements and RNA extraction for sequencing. Remarkably, glucose levels were about 2-fold higher than the control group in fruit flies exposed to HSD-30%, whereas triglycerides levels increased 1.7-fold. After RNA-sequencing, we found that 13.5% of the genes were differentially transcribed in the dyslipidemic and hyperglycaemic insects. HSD-30% up-regulated genes involved in ribosomal biogenesis (e.g. dTOR, ERK and dS6K) and down-regulated genes involved in energetic process (e.g. Pfk, Gapdh1, and Pyk from pyruvate metabolism; kdn, Idh and Mdh2 from the citric acid cycle; ATPsynC and ATPsynẞ from ATP synthesis) and insect development. We found a remarkable down-regulation for Actin (Act88F) that likely impairs muscle development. Moreover, HSD-30% up-regulated both the insulin-like peptides 7 and 8 and down-regulated the insulin receptor substrate p53, isoform A and insulin-like peptide 6 genes, whose functional products are insulin signaling markers. All these features pointed together to a tightly correlation of the T2DM-like phenotype modeled by the D. melanogaster and an intricate array of phenomena, which includes energetic processes, muscle development, and ribosomal synthesis as that observed for the human pathology.

Zika Virus Growth in Human Kidney Cells Is Restricted by an Elevated Glucose Level

Mosquito-borne Zika virus (ZIKV) became a real threat to human health due to the lack of vaccine and effective antiviral treatment. The virus has recently been responsible for a global outbreak leading to millions of infected cases. ZIKV complications were highlighted in adults with Guillain–Barré syndrome and in newborns with increasing numbers of congenital disorders ranging from mild developmental delays to fatal conditions. The ability of ZIKV to establish a long-term infection in diverse organs including the kidneys has been recently documented but the consequences of such a viral infection are still debated. Our study aimed to determine whether the efficiency of ZIKV growth in kidney cells relates to glucose concentration. Human kidney HK-2 cells were infected with different ZIKV strains in presence of normal and high glucose concentrations. Virological assays showed a decrease in viral replication without modifying entry steps (viral binding, internalization, fusion) under high glucose conditions. This decrease replication was associated with a lower virus progeny and increased cell viability when compared to ZIKV-infected HK-2 cells in normal glucose concentration. In conclusion, we showed for the first time that an elevated glucose level influences ZIKV replication level with an effect on kidney cell survival.

Transcriptome of CD8+ tumor-infiltrating T cells: a link between diabetes and colorectal cancer

There is an increased risk of colorectal cancer (CRC) development in patients with non-insulin-dependent type 2 diabetes. CD8⁺ T cells have been implicated in diabetes and are crucial for anti-tumor immunity. However, transcriptomic profiling for CD8⁺ T cells from CRC diabetic patients has not been explored. We performed RNA sequencing and compared transcriptomic profiles of CD8⁺ tumor-infiltrating T lymphocytes (CD8⁺ TILs) in CRC diabetic patients with CRC nondiabetic patients. We found that genes associated with ribogenesis, epigenetic regulations, oxidative phosphorylation and cell cycle arrest were upregulated in CD8⁺ TILs from diabetic patients, while genes associated with PI3K signaling pathway, cytokine response and response to lipids were downregulated. Among the significantly deregulated 1009 genes, 342 (186 upregulated and 156 downregulated) genes were selected based on their link to diabetes, and their associations with the presence of specific CRC pathological parameters were assessed using GDC TCGA colon database. The 186 upregulated genes were associated with the presence of colon polyps history (P = 0.0007) and lymphatic invasion (P = 0.0025). Moreover, CRC patients with high expression of the 186 genes were more likely to have poorer disease-specific survival (DSS) (Mantel-Cox log-rank P = 0.024) than those with low score. Our data provide novel insights into molecular pathways and biological functions, which could be altered in CD8⁺ TILs from CRC diabetic versus nondiabetic patients, and reveal candidate genes linked to diabetes, which could predict DSS and pathological parameters associated with CRC progression. However, further investigations using larger patient cohorts and functional studies are required to validate these findings.

A Drug Repurposing and Protein-Protein Interaction Network Study of Ribosomopathies Using Yeast as a Model System

Ribosomopathies result in various cancers, neurodegenerative and viral diseases, and other pathologies such as Diamond-Blackfan anemia and Shwachman-Diamond syndrome. Their pathophysiology at a proteome and functional level remains to be determined. Protein networks and highly connected hub proteins for ribosome biogenesis in Saccharomyces cerevisiae offer a potential as a model system to inform future therapeutic innovation in ribosomopathies. In this context, we report a ribosome biogenesis protein-protein interaction network in S. cerevisiae, created with 1772 proteins and 22,185 physical interactions connecting them. Moreover, by network decomposition analysis, we determined the linear pathways between the transcription factors and target proteins with a view to drug repurposing. While considering only the paths containing the three C/D box proteins (Nop56, Nop58, and Nop1), the most frequently encountered proteins were Aft1, Htz1, Ssa1, Ssb1, Ssb2, Gcn5, Cka1, Tef1, Nop1, Cdc28, Act1, Krr1, Rpl8B, and Tor1, which were then identified as potential drug targets. For drug repurposing, these candidate proteins were further searched in the DrugBank to find other diseases associated with them, as well as the drugs used to treat these diseases. To support the computational results, an experimental study was conducted using in-house manufactured microfluidic bioreactor platform, while the effect of the drug temsirolimus, Tor1 inhibitor, on yeast cells was investigated by following Nop56 protein expression. In conclusion, these results inform the ways in which ribosomopathies and associated common complex human diseases materialize and how drug repurposing might accelerate therapeutic innovation through bioinformatic studies of yeast.

Microparticles as Potential Mediators of High Glucose-Induced Renal Cell Injury

Diabetic nephropathy (DN) is the most common cause of chronic kidney disease worldwide. Activation of signaling pathways such as the mammalian target of rapamycin (mTOR), extracellular signal-regulated kinases (ERK), endoplasmic reticulum (ER) stress, transforming growth factor-beta (TGF-β), and epithelial-mesenchymal transition (EMT), are thought to play a significant role in the etiology of DN. Microparticles (MPs), the small membrane vesicles containing bioactive signals shed by cells upon activation or during apoptosis, are elevated in diabetes and were identified as biomarkers in DN. However, their exact role in the pathophysiology of DN remains unclear. Here, we examined the effect of MPs shed from renal proximal tubular cells (RPTCs) exposed to high glucose conditions on naïve RPTCs in vitro. Our results showed significant increases in the levels of phosphorylated forms of 4E-binding protein 1 and ERK1/2 (the downstream targets of mTOR and ERK pathways), phosphorylated-eIF2α (an ER stress marker), alpha smooth muscle actin (an EMT marker), and phosphorylated-SMAD2 and nuclear translocation of SMAD4 (markers of TGF-β signaling). Together, our findings indicate that MPs activate key signaling pathways in RPTCs under high glucose conditions. Pharmacological interventions to inhibit shedding of MPs from RPTCs might serve as an effective strategy to prevent the progression of DN.

Association of TATA box-binding protein-associated factor RNA polymerase I subunit C (TAF1C) with T2DM

Proteins differential expression in type 2 diabetes mellitus (T2DM) can be due to etiological factors or pathological changes, such proteins can be utilized as biomarkers. Identification of a marker protein out of thousands became a feasible task during the proteomics era by using liquid chromatography-tandem mass spectrometry (LC-MS/MS). In this study, blood samples were obtained from 80 Bahraini subjects with and without T2DM, a subset was used for proteomic analysis by LC-MS/MS, while all samples were used for ELISA analysis of 3 proteins, TATA-box binding protein-associated factor RNA polymerase-1-C (TAF1C), ceruloplasmin (CERP) and fibronectin (FN). The former 2 proteins were selected from the T2DM-protein-panel identified by LC-MS/MS, and the latter was analyzed for validation of the setting. The main findings of the proteomic analysis are i. Identifications of 62 differentially expressed proteins in T2DM, ii. Upregulation of 71% of the identified proteins. While the ELISA analysis showed that; both TAF1C and FN were significantly increased in T2DM (P0.015 and P0.001, respectively), while CERP was not (P0.088). Logistic regression analysis: i. confirmed the above associations after correction for covariates, ii. Revealed an interaction between age and gender that affect the association of the proteins with T2DM. In conclusion, knowing that TAF1C is a prerequisite in ribosomal biogenesis, our ELISA results are suggestive of increased protein synthesis in T2DM, explaining the observed upregulation of the proteins identified by LC-MSMS. The association between T2DM and TAF1C is a novel finding that might open a new avenue in DM research.

Albumin induces CD44 expression in glomerular parietal epithelial cells by activating extracellular signal-regulated kinase 1/2 pathway

De novo expression of CD44 in glomerular parietal epithelial cells (PECs) leads to a prosclerotic and migratory PEC phenotype in glomerulosclerosis. However, the regulatory mechanisms underlying CD44 expression by activated PECs remain largely unknown. This study was performed to examine the mediators responsible for CD44 induction in glomerular PECs in association with diabetes. CD44 expression and localization were evaluated in the glomeruli of Zucker diabetic rat kidneys and primary cultured PECs upon albumin stimulation. Real-time polymerase chain reaction confirmed an albuminuria-associated upregulation of the CD44 gene in the glomeruli of diabetic rats. Immunostaining analysis of diabetic kidneys further revealed an increase in CD44 in hypertrophic PECs, which often contain albumin-positive vesicles. Losartan treatment significantly attenuated albuminuria and lowered CD44 protein levels in the diabetic kidneys. In primary cultured rat PECs, rat serum albumin (0.25-1 mg/ml) caused a dose-dependent upregulation of CD44, claudin-1, and megalin protein expression, which was accompanied by an activation of extracellular signal-regulated kinase1/2 (ERK1/2) signaling. Albumin-induced CD44 and claudin-1 expression were greatly suppressed in the presence of the ERK1/2 inhibitor, U0126. In addition, knockdown of megalin by small interfering RNA interference in PECs resulted in a significant reduction of albumin-induced CD44 and claudin-1 proteins. Taken together, our results demonstrate that albumin induces CD44 expression by PECs via the activation of the ERK signaling pathway, which is partially mediated by endocytic receptor megalin.

Deacetylation of S6 kinase promotes high glucose-induced glomerular mesangial cell hypertrophy and matrix protein accumulation

S6 kinase acts as a driver for renal hypertrophy and matrix accumulation, two key pathologic signatures of diabetic nephropathy. As a post-translational modification, S6 kinase undergoes acetylation at the C terminus. The role of this acetylation to regulate kidney glomerular cell hypertrophy and matrix expansion is not known. In mesangial cells, high glucose decreased the acetylation and enhanced phosphorylation of S6 kinase and its substrates rps6 and eEF2 kinase that lead to dephosphorylation of eEF2. To determine the mechanism of S6 kinase deacetylation, we found that trichostatin A, a pan-histone deacetylase (HDAC) inhibitor, blocked all high glucose-induced effects. Furthermore, high glucose increased the expression and association of HDAC1 with S6 kinase. HDAC1 decreased the acetylation of S6 kinase and mimicked the effects of high glucose, resulting in mesangial cell hypertrophy and expression of fibronectin and collagen I (α2). In contrast, siRNA against HDAC1 inhibited these effects by high glucose. A C-terminal acetylation-mimetic mutant of S6 kinase suppressed high glucose-stimulated phosphorylation of S6 kinase, rps6 and eEF2 kinase, and inhibited the dephosphorylation of eEF2. Also, the acetylation mimetic attenuated the mesangial cell hypertrophy and fibronectin and collagen I (α2) expression. Conversely, an S6 kinase acetylation-deficient mutant induced all the above effects of high glucose. Finally, in the renal glomeruli of diabetic rats, the acetylation of S6 kinase was significantly reduced concomitant with increased HDAC1 and S6 kinase activity. In aggregate, our data uncovered a previously unrecognized role of S6 kinase deacetylation in high glucose-induced mesangial cell hypertrophy and matrix protein expression.

The increase in fiber size in male rat gastrocnemius after chronic central leptin infusion is related to activation of insulin signaling

Insulin potentiates leptin effects on muscle accrual and glucose homeostasis. However, the relationship between leptin's central effects on peripheral insulin sensitivity and the associated structural changes remain unclear. We hypothesized that central leptin infusion modifies muscle size through activation of insulin signaling. Muscle insulin signaling, enzymes of fatty acid metabolism, mitochondrial respiratory chain complexes, proliferating cell nuclear antigen (PCNA) and fiber area were analyzed in the gastrocnemius of chronic central infused (L), pair-fed (PF) and control rats. PCNA-positive nuclei, fiber area, GLUT4 and glycogen levels and activation of Akt and mechanistic target of rapamycin were increased in L, with no changes in PF. Acetyl-CoA carboxylase-β mRNA levels and non-esterified fatty acid and triglyceride content were reduced and carnitine palmitoyltransferase-1b expression and mitochondrial complexes augmented in L. These results suggest that leptin promotes an increase in muscle size associated with improved insulin signaling favored by lipid profile.

microRNA-181a downregulates deptor for TGFβ-induced glomerular mesangial cell hypertrophy and matrix protein expression

TGFβ contributes to mesangial cell hypertrophy and matrix protein increase in various kidney diseases including diabetic nephropathy. Deptor is an mTOR-interacting protein and suppresses mTORC1 and mTORC2 activities. We have recently shown that TGFβ-induced inhibition of deptor increases the mTOR activity. The mechanism by which TGFβ regulates deptor expression is not known. Here we identify deptor as a target of the microRNA-181a. We show that in mesangial cells, TGFβ increases the expression of miR-181a to downregulate deptor. Decrease in deptor augments mTORC2 activity, resulting in phosphorylation/activation of Akt kinase. Akt promotes inactivating phosphorylation of PRAS40 and tuberin, leading to stimulation of mTORC1. miR-181a-mimic increased mTORC1 and C2 activities, while anti-miR-181a inhibited them. mTORC1 controls protein synthesis via phosphorylation of translation initiation and elongation suppressors 4EBP-1 and eEF2 kinase. TGFβ-stimulated miR-181a increased the phosphorylation of 4EBP-1 and eEF2 kinase, resulting in their inactivation. miR-181a-dependent inactivation of eEF2 kinase caused dephosphorylation of eEF2. Consequently, miR-181a-mimic increased protein synthesis and hypertrophy of mesangial cells similar to TGFβ. Anti-miR-181a blocked these events in a deptor-dependent manner. Finally, TGFβ-miR-181a-driven deptor downregulation increased the expression of fibronectin. Our results identify a novel mechanism involving miR-181a-driven deptor downregulation, which contributes to mesangial cell pathologies in renal complications.

Identification of key gene pathways and coexpression networks of islets in human type 2 diabetes

PURPOSE

The number of people with type 2 diabetes (T2D) is growing rapidly worldwide. Islet β-cell dysfunction and failure are the main causes of T2D pathological processes. The aim of this study was to elucidate the underlying pathways and coexpression networks in T2D islets.

MATERIALS AND METHODS

We analyzed the differentially expressed genes (DEGs) in the data set GSE41762, which contained 57 nondiabetic and 20 diabetic samples, and developed Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses. Protein-protein interaction (PPI) network, the modules from the PPI network, and the gene annotation enrichment of modules were analyzed as well. Moreover, a weighted correlation network analysis (WGCNA) was applied to screen critical gene modules and coexpression networks and explore the biological significance.

RESULTS

We filtered 957 DEGs in T2D islets. Then GO and KEGG analyses identified that key pathways like inflammatory response, type B pancreatic cell differentiation, and calcium ion-dependent exocytosis were involved in human T2D. Three significant modules were filtered from the PPI network. Ribosome biogenesis, extrinsic apoptotic signaling pathway, and membrane depolarization during action potential were associated with the modules, respectively. Furthermore, coexpression network analysis by WGCNA identified 13 distinct gene modules of T2D islets and revealed four modules, which were strongly correlated with T2D and T2D biomarker hemoglobin A1c (HbA1c). Functional annotation showed that these modules mainly enriched KEGG pathways such as NF-kappa B signaling pathway, tumor necrosis factor signaling pathway, cyclic adenosine monophosphate signaling pathway, and peroxisome proliferators-activated receptor signaling pathway.

CONCLUSION

The results provide potential gene pathways and underlying molecular mechanisms for the prevention, diagnosis, and treatment of T2D.

Hydrogen Sulfide in Renal Physiology and Disease

SIGNIFICANCE

Hydrogen sulfide (H2S) has only recently gained recognition for its physiological effects. It is synthesized widely in the mammalian tissues and regulates several biologic processes ranging from development, angiogenesis, neurotransmission to protein synthesis. Recent Advances: The aim of this review is to critically evaluate the evidence for a role for H2S in kidney function and disease.

CRITICAL ISSUES

H2S regulates fundamental kidney physiologic processes such as glomerular filtration and sodium reabsorption. In kidney disease states H2S appears to play a complex role in a context-dependent manner. In some disease states such as ischemia-reperfusion and diabetic kidney disease it can serve as an agent that ameliorates kidney injury. In other diseases such as cis-platinum-induced kidney disease it may mediate kidney injury although more investigation is needed. Recent studies have revealed that the actions of nitric oxide and H2S may be integrated in kidney cells.

FUTURE DIRECTIONS

Further studies are needed to understand the full impact of H2S on kidney physiology. As it is endowed with the properties of regulating blood flow, oxidative stress, and inflammation, H2S should be investigated for its role in inflammatory and toxic diseases of the kidney. Such in-depth exploration may identify specific kidney diseases in which H2S may constitute a unique target for therapeutic intervention. Antioxid. Redox Signal. 25, 720-731.

Translational control by oncogenic signaling pathways

Messenger RNA (mRNA) translation is highly regulated in cells and plays an integral role in the overall process of gene expression. The initiation phase of translation is considered to be the most rate-limiting and is often targeted by oncogenic signaling pathways to promote global protein synthesis and the selective translation of tumor-promoting mRNAs. Translational control is a crucial component of cancer development as it allows cancer cells to adapt to the altered metabolism that is generally associated with the tumor state. The phosphoinositide 3-kinase (PI3K)/Akt and Ras/mitogen-activated protein kinase (MAPK) pathways are strongly implicated in cancer etiology, and they exert their biological effects by modulating both global and specific mRNA translation. In addition to having respective translational targets, these pathways also impinge on the mechanistic/mammalian target of rapamycin (mTOR), which acts as a critical signaling node linking nutrient sensing to the coordinated regulation of cellular metabolism. mTOR is best known as a central regulator of protein synthesis and has been implicated in an increasing number of pathological conditions, including cancer. In this article, we describe the current knowledge on the roles and regulation of mRNA translation by various oncogenic signaling pathways, as well as the relevance of these molecular mechanisms to human malignancies. This article is part of a Special Issue entitled: Translation and cancer.

Pharmacological AMP kinase activators target the nucleolar organization and control cell proliferation

AIMS

Phenformin, resveratrol and AICAR stimulate the energy sensor 5'-AMP activated kinase (AMPK) and inhibit the first step of ribosome biogenesis, de novo RNA synthesis in nucleoli. Nucleolar activities are relevant to human health, because ribosome production is crucial to the development of diabetic complications. Although the function of nucleoli relies on their organization, the impact of AMPK activators on nucleolar structures is not known. Here, we addressed this question by examining four nucleolar proteins that are essential for ribosome biogenesis.

METHODS

Kidney cells were selected as model system, because diabetic nephropathy is one of the complications associated with diabetes mellitus. To determine the impact of pharmacological agents on nucleoli, we focused on the subcellular and subnuclear distribution of B23/nucleophosmin, fibrillarin, nucleolin and RPA194. This was achieved by quantitative confocal microscopy at the single-cell level in combination with cell fractionation and quantitative Western blotting.

RESULTS

AMPK activators induced the re-organization of nucleoli, which was accompanied by changes in cell proliferation. Among the compounds tested, phenformin and resveratrol had the most pronounced impact on nucleolar organization. For B23, fibrillarin, nucleolin and RPA194, both agents (i) altered the nucleocytoplasmic distribution and nucleolar association and (ii) reduced significantly the retention in the nucleus. (iii) Phenformin and resveratrol also increased significantly the total concentration of B23 and nucleolin.

CONCLUSIONS

AMPK activators have unique effects on the subcellular localization, nuclear retention and abundance of nucleolar proteins. We propose that the combination of these events inhibits de novo ribosomal RNA synthesis and modulates cell proliferation. Our studies identified nucleolin as a target that is especially sensitive to pharmacological AMPK activators. Because of its response to pharmacological agents, nucleolin represents a potential biomarker for the development of drugs that diminish diabetic renal hypertrophy.

Ribosomal protein S6 kinase activity controls the ribosome biogenesis transcriptional program

S6 kinases (S6Ks) are mechanistic target of rapamycin substrates that participate in cell growth control. S6Ks phosphorylate ribosomal protein S6 (rpS6) and additional proteins involved in the translational machinery, although the functional roles of these modifications remain elusive. Here we analyze the S6K-dependent transcriptional and translational regulation of gene expression by comparing whole-genome microarray of total and polysomal mouse liver RNA after feeding. We show that tissue lacking S6Ks 1 and 2 (S6K1 and S6K2), displays a defect in the ribosome biogenesis (RiBi) transcriptional program after feeding. Over 75% of RiBi factors are controlled by S6K, including Nop56, Nop14, Gar1, Rrp9, Rrp15, Rrp12 and Pwp2 nucleolar proteins. Importantly, the reduced activity of RiBi transcriptional promoters in S6K1;S6K2(-/-) cells is also observed in rpS6 knock-in mutants that cannot be phosphorylated. As ribosomal protein synthesis is not affected by these mutations, our data reveal a distinct and specific aspect of RiBi under the control of rpS6 kinase activity, that is, the RiBi transcriptional program.

Molecular events in matrix protein metabolism in the aging kidney

We explored molecular events associated with aging-induced matrix changes in the kidney. C57BL6 mice were studied in youth, middle age, and old age. Albuminuria and serum cystatin C level (an index of glomerular filtration) increased with aging. Renal hypertrophy was evident in middle-aged and old mice and was associated with glomerulomegaly and increase in mesangial fraction occupied by extracellular matrix. Content of collagen types I and III and fibronectin was increased with aging; increment in their mRNA varied with the phase of aging. The content of ZEB1 and ZEB2, collagen type I transcription inhibitors, and their binding to the collagen type Iα2 promoter by ChIP assay also showed age-phase-specific changes. Lack of increase in mRNA and data from polysome assay suggested decreased degradation as a potential mechanism for kidney collagen type I accumulation in the middle-aged mice. These changes occurred with increment in TGFβ mRNA and protein and activation of its SMAD3 pathway; SMAD3 binding to the collagen type Iα2 promoter was also increased. TGFβ-regulated microRNAs (miRs) exhibited selective regulation. The renal cortical content of miR-21 and miR-200c, but not miR-192, miR-200a, or miR-200b, was increased with aging. Increased miR-21 and miR-200c contents were associated with reduced expression of their targets, Sprouty-1 and ZEB2, respectively. These data show that aging is associated with complex molecular events in the kidney that are already evident in the middle age and progress to old age. Age-phase-specific regulation of matrix protein synthesis occurs and involves matrix protein-specific transcriptional and post-transcriptional mechanisms.

Dysregulation of RNA polymerase I transcription during disease

Transcription of the ribosomal RNA genes by the dedicated RNA polymerase I enzyme and subsequent processing of the ribosomal RNA are fundamental control steps in the synthesis of functional ribosomes. Dysregulation of Pol I transcription and ribosome biogenesis is linked to the etiology of a broad range of human diseases. Diseases caused by loss of function mutations in the molecular constituents of the ribosome, or factors intimately associated with RNA polymerase I transcription and processing are collectively termed ribosomopathies. Ribosomopathies are generally rare and treatment options are extremely limited tending to be more palliative than curative. Other more common diseases are associated with profound changes in cellular growth such as cardiac hypertrophy, atrophy or cancer. In contrast to ribosomopathies, altered RNA polymerase I transcriptional activity in these diseases largely results from dysregulated upstream oncogenic pathways or by direct modulation by oncogenes or tumor suppressors at the level of the RNA polymerase I transcription apparatus itself. Ribosomopathies associated with mutations in ribosomal proteins and ribosomal RNA processing or assembly factors have been covered by recent excellent reviews. In contrast, here we review our current knowledge of human diseases specifically associated with dysregulation of RNA polymerase I transcription and its associated regulatory apparatus, including some cases where this dysregulation is directly causative in disease. We will also provide insight into and discussion of possible therapeutic approaches to treat patients with dysregulated RNA polymerase I transcription. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Mammalian target of rapamycin and the kidney. II. Pathophysiology and therapeutic implications

The mTOR pathway plays an important role in a number of common renal diseases, including acute kidney injury (AKI), diabetic nephropathy (DN), and polycystic kidney diseases (PKD). The activity of mTOR complex 1 (mTORC1) is necessary for renal regeneration and repair after AKI, and inhibition of mTORC1 by rapamycin has been shown to delay recovery from ischemic AKI in animal studies, and to prolong delayed graft function in humans who have received a kidney transplant. For this reason, administration of rapamycin should be delayed or discontinued in patients with AKI until full recovery of renal function has occurred. On the other hand, inappropriately high mTORC1 activity contributes to the progression of the metabolic syndrome, the development of type 2 diabetes, and the pathogenesis of DN. In addition, chronic hyperactivity of mTORC1, and possibly also mTORC2, contributes to cyst formation and enlargement in a number of forms of PKD. Inhibition of mTOR, using either rapamycin (which inhibits predominantly mTORC1) or "catalytic" inhibitors (which effectively inhibit both mTORC1 and mTORC2), provide exciting possibilities for novel forms of treatment of DN and PKD. In this second part of the review, we will examine the role of mTOR in the pathophysiology of DN and PKD, as well as the potential utility of currently available and newly developed inhibitors of mTOR to slow the progression of DN and/or PKD.

Elevated levels of renal and circulating Nop-7-associated 2 (NSA2) in rat and mouse models of diabetes, in mesangial cells in vitro and in patients with diabetic nephropathy

AIMS/HYPOTHESIS

We previously found that Nop-7-associated 2 (NSA2), which is involved in ribosomal biogenesis in yeast and is a putative cell cycle regulator in mammalian cells, is elevated in the kidney of Goto-Kakizaki (GK) rat, a spontaneous model of type 2 diabetes. Here we tested the hypothesis that elevated NSA2 is involved in diabetic nephropathy (DN).

METHODS

We examined Nsa2/NSA2 expression and NSA2 production in two rodent models of diabetes, in cultured renal glomerular cells, and in diabetic patients with and without nephropathy. Patients with nephropathy who had a history of albuminuria were further divided as responders (DN-NA; DN patients normoalbuminuric at the time of this study with a history of albuminuria) and non-responders (DN-A; diabetic nephropathy patients with albuminuria) to current treatment for albuminuria.

RESULTS

Renal Nsa2/NSA2 mRNA increased in tandem with hyperglycaemia in GK rats, in a streptozotocin-induced mouse model of diabetes, and in human mesangial cells (HMCs) grown in high glucose (p < 0.05). In the mouse model of diabetes, hyperglycaemia resulted in increased Nsa2 expression and NSA2 levels in tubular and glomerular cells and in circulating cells; this increase was normalised by diabetes treatment. Circulating NSA2 mRNA levels were elevated in patients with DN independently of body weight (BMI), glycaemic (HbA(1c)) and haemodynamic (blood pressure) control, and showed an inverse correlation with renal function (GFR, p < 0.05). NSA2 levels were the only variable that showed a significant difference between patients with albuminuria (DN-A) compared with non-albuminuric patients (DN-NA) and diabetic controls (p < 0.05), this increase being independent of all other variables, including GFR.

CONCLUSION

We show for the first time that renal and circulating NSA2/NSA2 levels are increased in hyperglycaemia in experimental models of diabetes, and that circulating NSA2 is elevated in DN patients with albuminuria. Further studies will be required to assess whether NSA2 plays a role in the pathogenesis of DN.

Signaling mechanisms in the regulation of renal matrix metabolism in diabetes

Renal hypertrophy and accumulation of extracellular matrix proteins are among cardinal manifestations of diabetic nephropathy. TGF beta system has been implicated in the pathogenesis of these manifestations. Among signaling pathways activated in the kidney in diabetes, mTOR- (mammalian target of rapamycin-)regulated pathways are pivotal in orchestrating high glucose-induced production of ECM proteins leading to functional and structural changes in the kidney culminating in adverse outcomes. Understanding signaling pathways that influence individual matrix protein expression could lead to the development of new interventional strategies. This paper will highlight some of the diverse components of the signaling network stimulated by hyperglycemia with an emphasis on extracellular matrix protein metabolism in the kidney in diabetes.

HIV-1 promotes renal tubular epithelial cell protein synthesis: role of mTOR pathway

Tubular cell HIV-infection has been reported to manifest in the form of cellular hypertrophy and apoptosis. In the present study, we evaluated the role of mammalian target of rapamycin (mTOR) pathway in the HIV induction of tubular cell protein synthesis. Mouse proximal tubular epithelial cells (MPTECs) were transduced with either gag/pol-deleted NL4-3 (HIV/MPTEC) or empty vector (Vector/MPTEC). HIV/MPTEC showed enhanced DNA synthesis when compared with Vector/MPTECs by BRDU labeling studies. HIV/MPTECs also showed enhanced production of β-laminin and fibronection in addition to increased protein content per cell. In in vivo studies, renal cortical sections from HIV transgenic mice and HIVAN patients showed enhanced tubular cell phosphorylation of mTOR. Analysis of mTOR revealed increased expression of phospho (p)-mTOR in HIV/MPTECs when compared to vector/MPTECs. Further downstream analysis of mTOR pathway revealed enhanced phosphorylation of p70S6 kinase and associated diminished phosphorylation of eEF2 (eukaryotic translation elongation factor 2) in HIV/MPTECs; moreover, HIV/MPTECs displayed enhanced phosphorylation of eIF4B (eukaryotic translation initiation factor 4B) and 4EBP-1 (eukaryotic 4E binding protein). To confirm our hypothesis, we evaluated the effect of rapamycin on HIV-induced tubular cell downstream signaling. Rapamycin not only attenuated phosphorylation of p70S6 kinase and associated down stream signaling in HIV/MPTECs but also inhibited HIV-1 induced tubular cell protein synthesis. These findings suggest that mTOR pathway is activated in HIV-induced enhanced tubular cell protein synthesis and contributes to tubular cell hypertrophy.

Hydrogen sulfide inhibits high glucose-induced matrix protein synthesis by activating AMP-activated protein kinase in renal epithelial cells

Hydrogen sulfide, a signaling gas, affects several cell functions. We hypothesized that hydrogen sulfide modulates high glucose (30 mm) stimulation of matrix protein synthesis in glomerular epithelial cells. High glucose stimulation of global protein synthesis, cellular hypertrophy, and matrix laminin and type IV collagen content was inhibited by sodium hydrosulfide (NaHS), an H(2)S donor. High glucose activation of mammalian target of rapamycin (mTOR) complex 1 (mTORC1), shown by phosphorylation of p70S6 kinase and 4E-BP1, was inhibited by NaHS. High glucose stimulated mTORC1 to promote key events in the initiation and elongation phases of mRNA translation: binding of eIF4A to eIF4G, reduction in PDCD4 expression and inhibition of its binding to eIF4A, eEF2 kinase phosphorylation, and dephosphorylation of eEF2; these events were inhibited by NaHS. The role of AMP-activated protein kinase (AMPK), an inhibitor of protein synthesis, was examined. NaHS dose-dependently stimulated AMPK phosphorylation and restored AMPK phosphorylation reduced by high glucose. Compound C, an AMPK inhibitor, abolished NaHS modulation of high glucose effect on events in mRNA translation as well as global and matrix protein synthesis. NaHS induction of AMPK phosphorylation was inhibited by siRNA for calmodulin kinase kinase β, but not LKB1, upstream kinases for AMPK; STO-609, a calmodulin kinase kinase β inhibitor, had the same effect. Renal cortical content of cystathionine β-synthase and cystathionine γ-lyase, hydrogen sulfide-generating enzymes, was significantly reduced in mice with type 1 diabetes or type 2 diabetes, coinciding with renal hypertrophy and matrix accumulation. Hydrogen sulfide is a newly identified modulator of protein synthesis in the kidney, and reduction in its generation may contribute to kidney injury in diabetes.

Erk in kidney diseases

Acute or chronic kidney injury results from various insults and pathological conditions, and is accompanied by activation of compensatory repair mechanisms. Both insults and repair mechanisms are initiated by circulating factors, whose cellular effects are mediated by activation selective signal transduction pathways. Two main signal transduction pathways are activated during these processes, the phosphatidylinositol 3' kinase (PI-3K)/mammalian target of rapamycin (mTOR) and the mitogen-activated protein kinase (MAPK) cascades. This review will focus on the latter, and more specifically on the role of extracellular signal-regulated kinase (ERK) cascade in kidney injury and repair.