Cell lines with extended in vitro growth potential from human renal proximal tubule: characterization, response to inducers, and comparison with established cell lines

The role of efflux transporters in cytotoxicity and intracellular concentration of chlorpyrifos and chlorpyrifos oxon in human cell lines

In this study, we investigated the role of two efflux transporters, p-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), in the cytotoxicity and intracellular accumulation of the organophosphate pesticide chlorpyrifos (CPF) and its active metabolite, CPF-oxon (CPFO), in a human-derived liver cell line (HepG2) and kidney epithelial cell line (HK-2). The cytotoxicity to CPF and CPFO differed between cell lines where HK-2 had lower IC50 values which could be attributed to lower basal expression and inducibility of metabolizing enzymes, transporters, and nuclear receptors in HK-2 cells. In HepG2 cells, co-exposure of CPF with a specific inhibitor of either P-gp or BCRP enhanced the cytotoxicity of CPF while co-exposure of CPFO with VRP enhanced the cytotoxicity of CPFO, suggesting the role of these transporters in the elimination CPF and CPFO. Inhibition of efflux transporters did not affect the cytotoxicity of CPF and CPFO in HK-2 cells. Co-incubation of CPF with P-gp and BCRP inhibitors increased the intracellular concentration of CPF in HepG2 cells suggesting that both transporters play a role in limiting the cellular accumulation of CPF in HepG2 cells. Our results provide evidence that inhibition of efflux transporters can enhance CPF-induced toxicity through enhanced cellular accumulation and raises additional questions regarding how pesticide-transporter interactions may influence toxicity of mixtures containing pesticides and other environmental chemicals.

Targeting alternative splicing of fibronectin in human renal proximal tubule epithelial cells with antisense oligonucleotides to reduce EDA+ fibronectin production and block an autocrine loop that drives renal fibrosis

TGFβ1 is a powerful regulator of fibrosis; secreted in a latent form, it becomes active after release from the latent complex. During tissue fibrosis, the EDA + isoform of cellular fibronectin is overexpressed. In pulmonary fibrosis it has been proposed that the fibronectin splice variant including an EDA domain (FN EDA+) activates latent TGFβ. Our work investigates the potential of blocking the 'splicing in' of EDA with antisense oligonucleotides to inhibit TGFβ1-induced EDA + fibronectin and to prevent the cascade of events initiated by TGFβ1 in human renal proximal tubule cells (PTEC). Human primary PTEC were treated with TGFβ1 for 48 h, medium removed and the cells transfected with RNase H-independent antisense oligonucleotides (ASO) designed to block EDA exon inclusion (ASO5). The efficacy of ASO to block EDA exon inclusion was assessed by EDA + fibronectin RNA and protein expression; the expression of TGFβ, αSMA (α smooth muscle actin), MMP2 (matrix metalloproteinse-2), MMP9 (matrix metalloproteinse-9), Collagen I, K Cadherin and connexin 43 was analysed. Targeting antisense oligonucleotides designed to block EDA exon inclusion in fibronectin pre mRNA were effective in reducing the amount of TGFβ1 -induced cellular EDA + fibronectin RNA and secreted EDA + fibronectin protein (assessed by western immunoblotting and immunocytochemistry) in human proximal tubule cells in an in vitro cell culture model. The effect was selective for EDA + exon with no effect on EDB + fibronectin RNA and total fibronectin mRNA. Exogenous TGFβ1 induced endogenous TGFβ, αSMA, MMP2, MMP9 and Col I mRNA. TGFβ1 treatment for 48h reduced the expression of K-Cadherin and increased the expression of connexin-43. These TGFβ1-induced pro-fibrotic changes were attenuated by ASO5 treatment. 48 h after the removal of exogenous TGFβ, further increases in αSMA, MMP2, MMP9 was observed; ASO5 significantly inhibited this subsequent increase. ASO5 treatment also significantly inhibited ability of the cell culture medium harvested at the end of the experiment (96h) to stimulate SMAD3 reporter cells. The role of endogenous TGFβ1 was confirmed by the use of a TGFβ receptor inhibitor. Our results demonstrate a critical role of FN EDA+ in a cycle of TGFβ driven pro-fibrotic responses in human PTEC and blocking its production with ASO technology offers a potential therapy to interrupt this vicious circle and hence limit the progression of renal fibrosis.

HIF-2α expression and metabolic signaling require ACSS2 in clear cell renal cell carcinoma

Clear cell renal cell carcinoma (ccRCC) is an aggressive cancer driven by VHL loss and aberrant HIF-2α signaling. Identifying means to regulate HIF-2α thus has potential therapeutic benefit. Acetyl-CoA synthetase 2 (ACSS2) converts acetate to acetyl-CoA and is associated with poor patient prognosis in ccRCC. Here we tested the effects of ACSS2 on HIF-2α and cancer cell metabolism and growth in ccRCC models and clinical samples. ACSS2 inhibition reduced HIF-2α levels and suppressed ccRCC cell line growth in vitro, in vivo, and in cultures of primary ccRCC patient tumors. This treatment reduced glycolytic signaling, cholesterol metabolism, and mitochondrial integrity, all of which are consistent with loss of HIF-2α. Mechanistically, ACSS2 inhibition decreased chromatin accessibility and HIF-2α expression and stability. While HIF-2α protein levels are widely regulated through pVHL-dependent proteolytic degradation, we identify a potential pVHL-independent pathway of degradation via the E3 ligase MUL1. We show that MUL1 can directly interact with HIF-2α and that overexpression of MUL1 decreased HIF-2α levels in a manner partially dependent on ACSS2. These findings identify multiple mechanisms to regulate HIF-2α stability and ACSS2 inhibition as a strategy to complement HIF-2α-targeted therapies and deplete pathogenically stabilized HIF-2α.

DCLRE1B/Apollo germline mutations associated with renal cell carcinoma impair telomere protection

Hereditary renal cell carcinoma (RCC) is caused by germline mutations in a subset of genes, including VHL, MET, FLCN, and FH. However, many familial RCC cases do not harbor mutations in the known predisposition genes. Using Whole Exome Sequencing, we identified two germline missense variants in the DCLRE1B/Apollo gene (ApolloN246I and ApolloY273H) in two unrelated families with several RCC cases. Apollo encodes an exonuclease involved in DNA Damage Response and Repair (DDRR) and telomere integrity. We characterized these two functions in the human renal epithelial cell line HKC8. The decrease or inhibition of Apollo expression sensitizes these cells to DNA interstrand crosslink damage (ICLs). HKC8 Apollo-/- cells appear defective in the DDRR and present an accumulation of telomere damage. Wild-type and mutated Apollo forms could interact with TRF2, a shelterin protein involved in telomere protection. However, only ApolloWT can rescue the telomere damage in HKC8 Apollo-/- cells. Our results strongly suggest that ApolloN246I and ApolloY273H are loss-of-function mutants that cause impaired telomere integrity and could lead to genomic instability. Altogether, our results suggest that mutations in Apollo could induce renal oncogenesis.

Generation of proximal tubule-enhanced kidney organoids from human pluripotent stem cells

Kidney organoids derived from human pluripotent stem cells (hPSCs) are now being used as models of renal disease and nephrotoxicity screening. However, the proximal tubules (PTs), which are responsible for most kidney reabsorption functions, remain immature in kidney organoids with limited expression of critical transporters essential for nephron functionality. Here, we describe a protocol for improved specification of nephron progenitors from hPSCs that results in kidney organoids with elongated proximalized nephrons displaying improved PT maturity compared with those generated using standard kidney organoid protocols. We also describe a methodology for assessing the functionality of the PTs within the organoids and visualizing maturation markers via immunofluorescence. Using these assays, PT-enhanced organoids display increased expression of a range of critical transporters, translating to improved functionality measured by substrate uptake and transport. This protocol consists of an extended (13 d) monolayer differentiation phase, during which time hPSCs are exposed to nephron progenitor maintenance media (CDBLY2), better emulating human metanephric progenitor specification in vivo. Following nephron progenitor specification, the cells are aggregated and cultured as a three-dimensional micromass on an air-liquid interface to facilitate further differentiation and segmentation into proximalized nephrons. Experience in culturing hPSCs is required to conduct this protocol and expertise in kidney organoid generation is advantageous.

Prediction of cytotoxicity of heavy metals adsorbed on nano-TiO2 with periodic table descriptors using machine learning approaches

Nanoparticles with their unique features have attracted researchers over the past decades. Heavy metals, upon release and emission, may interact with different environmental components, which may lead to co-exposure to living organisms. Nanoscale titanium dioxide (nano-TiO2) can adsorb heavy metals. The current idea is that nanoparticles (NPs) may act as carriers and facilitate the entry of heavy metals into organisms. Thus, the present study reports nanoscale quantitative structure-activity relationship (nano-QSAR) models, which are based on an ensemble learning approach, for predicting the cytotoxicity of heavy metals adsorbed on nano-TiO2 to human renal cortex proximal tubule epithelial (HK-2) cells. The ensemble learning approach implements gradient boosting and bagging algorithms; that is, random forest, AdaBoost, Gradient Boost, and Extreme Gradient Boost were constructed and utilized to establish statistically significant relationships between the structural properties of NPs and the cause of cytotoxicity. To demonstrate the predictive ability of the developed nano-QSAR models, simple periodic table descriptors requiring low computational resources were utilized. The nano-QSAR models generated good R2 values (0.99-0.89), Q2 values (0.64-0.77), and Q2F1 values (0.99-0.71). Thus, the present work manifests that ML in conjunction with periodic table descriptors can be used to explore the features and predict unknown compounds with similar properties.

Validation of Freshly Isolated Rat Renal Cells as a Tool for Preclinical Assessment of Radiolabeled Receptor-Specific Peptide Uptake in the Kidney

The synthetic analogs of regulatory peptides radiolabeled with adequate radionuclides are perspective tools in nuclear medicine. However, undesirable uptake and retention in the kidney limit their application. Specific in vitro methods are used to evaluate undesirable renal accumulation. Therefore, we investigated the usefulness of freshly isolated rat renal cells for evaluating renal cellular uptake of receptor-specific peptide analogs. Special attention was given to megalin as this transport system is an important contributor to the active renal uptake of the peptides. Freshly isolated renal cells were obtained from native rat kidneys by the collagenase method. Compounds with known accumulation in renal cells were used to verify the viability of cellular transport systems. Megalin expressions in isolated rat renal cells were compared to two other potential renal cell models by Western blotting. Specific tubular cell markers were used to confirm the presence of proximal tubular cells expressing megalin in isolated rat renal cell preparations by immunohistochemistry. Colocalization experiments on isolated rat kidney cells confirmed the presence of proximal tubular cells bearing megalin in preparations. The applicability of the method was tested by an accumulation study with several analogs of somatostatin and gastrin labeled with indium-111 or lutetium-177. Therefore, isolated rat renal cells may be an effective screening tool for in vitro analyses of renal uptake and comparative renal accumulation studies of radiolabeled peptides or other radiolabeled compounds with potential nephrotoxicity.

Multi-level interaction between HIF and AHR transcriptional pathways in kidney carcinoma

Hypoxia-inducible factor (HIF) and aryl hydrocarbon receptor (AHR) are members of the bHLH-PAS family of transcription factors that underpin cellular responses to oxygen and to endogenous and exogenous ligands, respectively, and have central roles in the pathogenesis of renal cancer. Composed of heterodimers, they share a common HIF-1β/ARNT subunit and similar DNA-binding motifs, raising the possibility of crosstalk between the two transcriptional pathways. Here, we identify both general and locus-specific mechanisms of interaction between HIF and AHR that act both antagonistically and cooperatively. Specifically, we observe competition for the common HIF-1β/ARNT subunit, in cis synergy for chromatin binding, and overlap in their transcriptional targets. Recently, both HIF and AHR inhibitors have been developed for the treatment of solid tumours. However, inhibition of one pathway may promote the oncogenic effects of the other. Therefore, our work raises important questions as to whether combination therapy targeting both of these pro-tumourigenic pathways might show greater efficacy than targeting each system independently.

SETD2 regulates the methylation of translation elongation factor eEF1A1 in clear cell renal cell carcinoma

BACKGROUND

SET domain-containing protein 2 (SETD2) is commonly mutated in renal cell carcinoma. SETD2 methylates histone H3 as well as a growing list of non-histone proteins.

OBJECTIVE

Initially, we sought to explore SETD2-dependent changes in lysine methylation of proteins in proximal renal tubule cells. Subsequently, we focused on changes in lysine methylation of the translation elongation factor eEF1A1.

METHODS

To accomplish these objectives, we initially performed a systems-wide analysis of protein lysine-methylation and expression in wild type (WT) and SETD2-knock out (KO) kidney cells and later focused our studies on eEF1A1 as well as the expression of lysine methyltransferases that regulate its lysine methylation.

RESULTS

We observed decreased lysine methylation of the translation elongation factor eEF1A1. EEF1AKMT2 and EEF1AKMT3 are known to methylate eEF1A1, and we show here that their expression is dependent on SET-domain function of SETD2. Globally, we observe differential expression of hundreds of proteins in WT versus SETD2-KO cells, including increased expression of many involved in protein translation. Finally, we observe decreased progression free survival and loss of EEF1AKMT2 gene expression in SETD2-mutated tumors predicted to have loss of function of the SET domain.

CONCLUSION

Overall, these data suggest that SETD2-mutated ccRCC, via loss of enzymatic function of the SET domain, displays dysregulation of protein translation as a potentially important component of the transformed phenotype.

Upregulation of Thioredoxin Reductase 1 Expression by Flavan-3-Ols Protects Human Kidney Proximal Tubular Cells from Hypoxia-Induced Cell Death

Renal hypoxia and its associated oxidative stress is a common pathway for the development of kidney diseases, and using dietary antioxidants such as flavan-3-ols to prevent kidney failure has received much attention. This study investigates the molecular mechanism by which flavan-3-ols prevent hypoxia-induced cell death in renal tubular epithelial cells. Human kidney proximal tubular cells (HKC-8) were exposed to hypoxia (1% O₂) in the presence of flavan-3-ols (catechin, epicatechin, procyanidin B1, and procyanidin B2). Cell death was examined using flow cytometric analysis. Gene expression was determined using a PCR array and Western blotting, and its network and functions were investigated using STRING databases. Here, we show that the cytoprotective activity of catechin was the highest among these flavan-3-ols against hypoxia-induced cell death in cultured HKC-8 cells. Exposure of HKC-8 cells to hypoxia induced oxidative stress leading to up-regulation of DUOX2, NOX4, CYBB and PTGS2 and down-regulation of TXNRD1 and HSP90AA1. Treatment with catechin or other flavan-3-ols prevented the down-regulation of TXNRD1 expression in hypoxic HKC-8 cells. Overexpression of TXNRD1 prevented hypoxia-induced cell death, and inactivation of TXNRD1 with TRi-1, a specific TXNRD1 inhibitor, reduced the catechin cytoprotection against hypoxia-induced HKC-8 cell death. In conclusion, flavan-3-ols prevent hypoxia-induced cell death in human proximal tubular epithelial cells, which might be mediated by their maintenance of TXNRD1 expression, suggesting that enhancing TXNRD1 expression or activity may become a novel therapeutic strategy to prevent hypoxia-induced kidney damage.

Lysophosphatidic Acid Is a Proinflammatory Stimulus of Renal Tubular Epithelial Cells

Chronic kidney disease (CKD) refers to a spectrum of diseases defined by renal fibrosis, permanent alterations in kidney structure, and low glomerular-filtration rate. Prolonged epithelial-tubular damage involves a series of changes that eventually lead to CKD, highlighting the importance of tubular epithelial cells in this process. Lysophosphatidic acid (LPA) is a bioactive lipid that signals mainly through its six cognate LPA receptors and is implicated in several chronic inflammatory pathological conditions. In this report, we have stimulated human proximal tubular epithelial cells (HKC-8) with LPA and 175 other possibly pathological stimuli, and simultaneously detected the levels of 27 intracellular phosphoproteins and 32 extracellular secreted molecules with multiplex ELISA. This quantification revealed a large amount of information concerning the signaling and the physiology of HKC-8 cells that can be extrapolated to other proximal tubular epithelial cells. LPA responses clustered with pro-inflammatory stimuli such as TNF and IL-1, promoting the phosphorylation of important inflammatory signaling hubs, including CREB1, ERK1, JUN, IκΒα, and MEK1, as well as the secretion of inflammatory factors of clinical relevance, including CCL2, CCL3, CXCL10, ICAM1, IL-6, and IL-8, most of them shown for the first time in proximal tubular epithelial cells. The identified LPA-induced signal-transduction pathways, which were pharmacologically validated, and the secretion of the inflammatory factors offer novel insights into the possible role of LPA in CKD pathogenesis.

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.

Transglutaminase 2: Development of therapeutic antibodies reveals four inhibitory epitopes and confirms extracellular function in fibrotic remodelling

BACKGROUND AND PURPOSE

Transglutaminase type 2 (TG2) catalyses formation of ε-(γ-glutamyl)-lysine bonds between proteins, including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlies tissue scarring and fibrosis. Many transglutaminase inhibitors exist and allowed for proof-of-concept studies in disease models, but their lack of specificity for the TG2 isoform, and/or poor pharmacokinetic/pharmacodynamic properties have limited their clinical application. We hypothesised that a high affinity TG2-specific antibody could be developed to specifically inhibit extracellular TG2 activity, with characteristics suitable for therapeutic development.

EXPERIMENTAL APPROACH

Individual human TG2 domains were used to immunise mice and generate hybridomas. Supernatants were screened for inhibition of recombinant human TG2 activity, with TG2 specificity determined by ELISA.

KEY RESULTS

Thirteen TG2-specific supernatants inhibited human transamidation activity. Each hybridoma was cloned and antibody mapped to an epitope in the TG2 core domain, using phage display panning of a TG2 fragment library. Four distinct inhibitory epitopes were determined. The most effective antibodies (AB1, DC1 and BB7) bound to amino acids 313-327 (catalytic core), with an IC₅₀ of approximately 10 nM. The antibodies inhibit TG2 in human cells and block ECM accumulation in a primary human proximal tubular epithelial cell model of fibrosis, only 7 antibodies inhibited rat TG2, and all with higher IC₅₀ values.

CONCLUSIONS AND IMPLICATIONS

We identified a preferred inhibitory epitope in human TG2, developed antibodies with required characteristics for clinical development, and established that targeted inhibition of extracellular TG2 transamidation activity is sufficient to modify fibrotic remodelling.

Diabetic proximal tubulopathy: Can we mimic the disease for in vitro screening of SGLT inhibitors?

Diabetic kidney disease (DKD) is the foremost cause of renal failure. While the glomeruli are severely affected in the course of the disease, the main determinant for disease progression is the tubulointerstitial compartment. DKD does not develop in the absence of hyperglycemia. Since the proximal tubule is the major player in glucose reabsorption, it has been widely studied as a therapeutic target for the development of new therapies. Currently, there are several proximal tubule cell lines available, being the human kidney-2 (HK-2) and human kidney clone-8 (HKC-8) cell lines the ones widely used for studying mechanisms of DKD. Studies in these models have pushed forward the understanding on how DKD unravels, however, these cell culture models possess limitations that hamper research, including lack of transporters and dedifferentiation. The sodium-glucose cotransporters (SGLT) are identified as key players in glucose reabsorption and pharmacological inhibitors have shown to be beneficial for the long-term clinical outcome in DKD. However, their mechanism of action has, as of yet, not been fully elucidated. To comprehend the protective effects of SGLT inhibitors, it is essential to understand the complete functional, structural, and molecular features of the disease, which until now have been difficult to recapitulate. This review addresses the molecular events of diabetic proximal tubulopathy. In addition, we evaluate the protective role of SGLT inhibitors in cardiovascular and renal outcomes, and provide an overview of various in vitro models mimicking diabetic proximal tubulopathy used so far. Finally, new insights on advanced in vitro systems to surpass past limitations are postulated.

Sirtuin 5 depletion impairs mitochondrial function in human proximal tubular epithelial cells

Ischemia is a major cause of kidney damage. Proximal tubular epithelial cells (PTECs) are highly susceptible to ischemic insults that frequently cause acute kidney injury (AKI), a potentially life-threatening condition with high mortality. Accumulating evidence has identified altered mitochondrial function as a central pathologic feature of AKI. The mitochondrial NAD⁺-dependent enzyme sirtuin 5 (SIRT5) is a key regulator of mitochondrial form and function, but its role in ischemic renal injury (IRI) is unknown. SIRT5 expression was increased in murine PTECs after IRI in vivo and in human PTECs (hPTECs) exposed to an oxygen/nutrient deprivation (OND) model of IRI in vitro. SIRT5-depletion impaired ATP production, reduced mitochondrial membrane potential, and provoked mitochondrial fragmentation in hPTECs. Moreover, SIRT5 RNAi exacerbated OND-induced mitochondrial bioenergetic dysfunction and swelling, and increased degradation by mitophagy. These findings suggest SIRT5 is required for normal mitochondrial function in hPTECs and indicate a potentially important role for the enzyme in the regulation of mitochondrial biology in ischemia.

Effects of Charged Oxime Reactivators on the HK-2 Cell Line in Renal Toxicity Screening

Oxime cholinesterase reactivators (oximes) are used to counteract organophosphate intoxication. Charged oximes are administered via intramuscular or intravenous injection when the majority of dose is unmetabolized and is excreted as urine. In this study, the effects of selected double charged oximes were determined in the HK-2 cell line as a model for renal toxicity screening. Some effects on dehydrogenase activity were found for obidoxime, asoxime (syn. HI-6), K027, and K203. The effects of K868 and K869 were found to be unreliable due to rapid degradation of both chlorinated oximes in the assay medium, resulting for K868 in an isoxazole-pyridinium product.

FBXW7 mediates high glucose‑induced SREBP‑1 expression in renal tubular cells of diabetic nephropathy under PI3K/Akt pathway regulation

Diabetic nephropathy (DN) is a severe complication of diabetes mellitus and lipid metabolism abnormality serves a key role in the pathogenesis of DN. Sterol regulatory element-binding protein 1 (SREBP-1) overexpression mediates aberrant lipid accumulation in renal tubular cells of DN. However, the exact mechanism involved in increased SREBP-1 has not been fully elucidated. The aim of the present study was to explore the mechanism involved in SREBP-1 upregulation. Diabetic mice and high glucose-cultured HKC cells were chosen to detect the expression of FBXW7 and SREBP-1 using immunohistochemistry, western blotting and PCR. The present study demonstrated that F-box and WD repeat domain containing 7 (FBXW7) expression was decreased in renal tubular cells of diabetic mice. Moreover, the co-expression of FBXW7 and SREBP-1 was observed in renal tubular cells, but not in the glomeruli. High glucose-induced the downregulation of FBXW7 expression in in vitro cultured HKC cells, which was accompanied by SREBP-1 upregulation. In addition, overexpression of FBXW7 in HKC cells led to SREBP-1 downregulation. By contrast, knockdown of FBXW7 caused SREBP-1 upregulation in HKC cells. It was found that the PI3K/Akt signaling pathway was activated in high glucose-stimulated HKC cells, and inhibition of PI3K/Akt pathway using LY294002 increased FBXW7 expression and decreased SREBP-1 expression. Taken together, the present results suggested that FBXW7 mediated high glucose-induced SREBP-1 expression in renal tubular cells of DN, under the regulation of the PI3K/Akt signaling pathway.

Transcriptomes of Major Proximal Tubule Cell Culture Models

BACKGROUND

Cultured cell lines are widely used for research in the physiology, pathophysiology, toxicology, and pharmacology of the renal proximal tubule. The lines that are most appropriate for a given use depend upon the genes expressed. New tools for transcriptomic profiling using RNA sequencing (RNA-Seq) make it possible to catalog expressed genes in each cell line.

METHODS

Fourteen different proximal tubule cell lines, representing six species, were grown on permeable supports under conditions specific for the respective lines. RNA-Seq followed standard procedures.

RESULTS

Transcripts expressed in cell lines variably matched transcripts selectively expressed in native proximal tubule. Opossum kidney (OK) cells displayed the highest percentage match (45% of proximal marker genes [TPM threshold =15]), with pig kidney cells (LLC-PK1) close behind (39%). Lower-percentage matches were seen for various human lines, including HK-2 (26%), and lines from rodent kidneys, such as NRK-52E (23%). Nominally, identical OK cells from different sources differed substantially in expression of proximal tubule markers. Mapping cell line transcriptomes to gene sets for various proximal tubule functions (sodium and water transport, protein transport, metabolic functions, endocrine functions) showed that different lines may be optimal for experimentally modeling each function. An online resource (https://esbl.nhlbi.nih.gov/JBrowse/KCT/) has been created to interrogate cell line transcriptome data. Proteomic analysis of NRK-52E cells confirmed low expression of many proximal tubule marker proteins.

CONCLUSIONS

No cell line fully matched the transcriptome of native proximal tubule cells. However, some of the lines tested are suitable for the study of particular metabolic and transport processes seen in the proximal tubule.

Renal proximal tubular epithelial cells: review of isolation, characterization, and culturing techniques

The kidney is a complex organ, comprised primarily of glomerular, tubular, mesangial, and endothelial cells, and podocytes. The fact that renal cells are terminally differentiated at 34 weeks of gestation is the main obstacle in regeneration and treatment of acute kidney injury or chronic kidney disease. Furthermore, the number of chronic kidney disease patients is ever increasing and with it the medical community should aim to improve existing and develop new methods of renal replacement therapy. On the other hand, as polypharmacy is on the rise, thought should be given into developing new ways of testing drug safety. A possible way to tackle these issues is with isolation and culture of renal cells. Several protocols are currently described to isolate the desired cells, of which the most isolated are the proximal tubular epithelial cells. They play a major role in water homeostasis, acid-base control, reabsorption of compounds, and secretion of xenobiotics and endogenous metabolites. When exposed to ischemic, toxic, septic, or obstructive conditions their death results in what we clinically perceive as acute kidney injury. Additionally, due to renal cells' limited regenerative potential, the profibrotic environment inevitably leads to chronic kidney disease. In this review we will focus on human proximal tubular epithelial cells. We will cover human kidney culture models, cell sources, isolation, culture, immortalization, and characterization subdivided into morphological, phenotypical, and functional characterization.

New Antimicrobial Bioactivity against Multidrug-Resistant Gram-Positive Bacteria of Kinase Inhibitor IMD0354

Multidrug-resistant pathogens pose a serious threat to human health. For decades, the antibiotic vancomycin has been a potent option when treating Gram-positive multidrug-resistant infections. Nonetheless, in recent decades, we have begun to see an increase in vancomycin-resistant bacteria. Here, we show that the nuclear factor-kappa B (NF-κB) inhibitor N-[3,5-Bis(trifluoromethyl)phenyl]-5-chloro-2-hydroxybenzamide (IMD0354) was identified as a positive hit through a Caenorhabditis elegans–methicillin-resistant Staphylococcus aureus (MRSA) infection screen. IMD0354 was a potent bacteriostatic drug capable of working at a minimal inhibitory concentration (MIC) as low as 0.06 µg/mL against various vancomycin-resistant strains. Interestingly, IMD0354 showed no hemolytic activity at concentrations as high as 16 µg/mL and is minimally toxic to C. elegans in vivo with 90% survival up to 64 µg/mL. In addition, we demonstrated that IMD0354′s mechanism of action at high concentrations is membrane permeabilization. Lastly, we found that IMD0354 is able to inhibit vancomycin-resistant Staphylococcus aureus (VRSA) initial cell attachment and biofilm formation at sub-MIC levels and above. Our work highlights that the NF-κB inhibitor IMD0354 has promising potential as a lead compound and an antimicrobial therapeutic candidate capable of combating multidrug-resistant bacteria.

The effect of repeated passaging on the susceptibility of human proximal tubular HK-2 cells to toxic compounds

The human proximal tubular HK-2 cell line is an immortalized cell line commonly used for studying proximal tubular toxicity. Even as their use is presently increasing, there unfortunately are no studies focused on functional changes in HK-2 cells associated with passaging. The aim of the present study, therefore, was to evaluate the functional stability of HK-2 cells during 13 weeks of continuous passaging after 6 and 24 h of treatment with model nephrotoxic compounds (i.e., acetaminophen, cisplatin, CdCl(2)). Short tandem repeat profile, the doubling time, cell diameter, glutathione concentration, and intracellular dehydrogenase activity were measured in HK-2 cells at each tested passage. The results showed that HK-2 cells exhibit stable morphology, cell size, and cell renewal during passaging. Mean doubling time was determined to be 54 h. On the other hand, we observed a significant effect of passaging on the susceptibility of HK-2 cells to toxic compounds. The largest difference in results was found in both cadmium and cisplatin treated cells across passages. We conclude that the outcomes of scientific studies on HK-2 cells can be affected by the number of passages even after medium-term cultivation and passaging for 13 weeks.

Long Term Culture of Human Kidney Proximal Tubule Epithelial Cells Maintains Lineage Functions and Serves as an Ex vivo Model for Coronavirus Associated Kidney Injury

The mechanism of how SARS-CoV-2 causes severe multi-organ failure is largely unknown. Acute kidney injury (AKI) is one of the frequent organ damage in severe COVID-19 patients. Previous studies have shown that human renal tubule cells could be the potential host cells targeted by SARS-CoV-2. Traditional cancer cell lines or immortalized cell lines are genetically and phenotypically different from host cells. Animal models are widely used, but often fail to reflect a physiological and pathogenic status because of species tropisms. There is an unmet need for normal human epithelial cells for disease modeling. In this study, we successfully established long term cultures of normal human kidney proximal tubule epithelial cells (KPTECs) in 2D and 3D culture systems using conditional reprogramming (CR) and organoids techniques. These cells had the ability to differentiate and repair DNA damage, and showed no transforming property. Importantly, the CR KPTECs maintained lineage function with expression of specific transporters (SLC34A3 and cubilin). They also expressed angiotensin-converting enzyme 2 (ACE2), a receptor for SARS-CoV and SARS-CoV-2. In contrast, cancer cell line did not express endogenous SLC34A3, cubilin and ACE2. Very interestingly, ACE2 expression was around twofold higher in 3D organoids culture compared to that in 2D CR culture condition. Pseudovirion assays demonstrated that SARS-CoV spike (S) protein was able to enter CR cells with luciferase reporter. This integrated 2D CR and 3D organoid cultures provide a physiological ex vivo model to study kidney functions, innate immune response of kidney cells to viruses, and a novel platform for drug discovery and safety evaluation.

Renal ischemia/reperfusion injury: An insight on in vitro and in vivo models

Optimal tissue oxygenation is essential for its normal function. Suboptimal oxygenation or ischemia contributes to increased mortalities during various pathological conditions such as stroke, acute kidney injury (AKI), cardiac failure. Despite the rapid progression of renal tissue injury, the mechanism underlying renal ischemia/reperfusion injury (IRI) remains highly unclear. Experimental in vitro and in vivo models epitomizing the fundamental process is critical to the research of the pathogenesis of IRI and the development of plausible therapeutics. In this review, we describe the in vitro and in vivo models of IRI, ranges from proximal tubular cell lines to surgery-based animal models like clamping of both renal pedicles (bilateral IRI), clamping of one renal pedicle (unilateral IRI), clamping of one/or both renal arteries/or vein, or unilateral IRI with contralateral nephrectomy (uIRIx). Also, advanced technologies like three-dimensional kidney organoids, kidney-on-a-chip are explained. This review provides thoughtful information for establishing reliable and pertinent models for studying IRI-associated acute renal pathologies.

DNA hypomethylation promotes transposable element expression and activation of immune signaling in renal cell cancer

Recently, we reported that expression of endogenous retroviruses (ERVs) is associated with response to immune checkpoint blockade (ICB) in renal cell carcinoma (RCC). We show that decitabine, a DNA hypomethylating agent, activates transposable element (TE) expression (LINE1 and ERVs ERV3-2 and ERV4700) and antiviral signaling to potentially enhance response to ICB in kidney cancer cell lines and primary cells. KO of RIGI and MDA5 dsRNA sensors attenuated activation of antiviral signaling associated with DNA hypomethylation, and RIGI and MDA5 IPs showed increased ERV binding with decitabine treatment. Bioinformatic analyses showed the decitabine-induced signature could be associated with increased immune infiltration and response to ICB. Cytokine secretion induced by decitabine could modestly improve T cell activation and robustly enhanced T cell migration. In a small retrospective cohort of metastatic clear cell RCC (ccRCC) patients treated with anti-PD1/PDL1 blockade, activation of some antiviral genes was significantly higher in responders. Thus, we identified a potential strategy to induce TE expression through inhibition of DNA methylation in modulating T cell action via regulation of the innate antiviral pathway.

Long noncoding RNA NEAT1 sponges miR-129 to modulate renal fibrosis by regulation of collagen type I

The long noncoding RNA nuclear enriched abundant transcript 1 (NEAT1) has been reported to promote liver fibrosis progression. However, its molecular mechanism in renal fibrosis was not elucidated. In the present study, an in vitro model of renal fibrosis was established with HK-2 and HKC-8 cells treated with transforming growth factor-β1. C57BL/6 mice were used for the in vivo model with unilateral ureteral obstruction. Our results indicated that NEAT1 and collagen type I levels were significantly upregulated, whereas miR-129 was obviously downregulated, in the progression of renal fibrosis. Meanwhile, NEAT1 knockdown or miR-129 overexpression inhibited collagen type I deposition, the epithelial-mesenchymal transition process, and the inflammation response to suppress renal fibrosis. NEAT1 directly targeted miR-129, and miR-129 directly bound to collagen type I. Downregulation of miR-129 reversed inhibition of renal fibrosis induced by NEAT1 silencing, and upregulation of collagen type I also reversed inhibition of renal fibrosis caused by miR-129 overexpression. NEAT1 knockdown alleviated renal fibrosis in mice subjected to unilateral ureteral obstruction. In conclusion, NEAT1 sponged miR-129 to modulate the epithelial-mesenchymal transition process and inflammation response of renal fibrosis by regulation of collagen type I. Our study indicates a novel role in the regulation of renal fibrosis and provides a new potential treatment target for renal fibrosis.

Hypoxia drives glucose transporter 3 expression through hypoxia-inducible transcription factor (HIF)-mediated induction of the long noncoding RNA NICI

Hypoxia-inducible transcription factors (HIFs) directly dictate the expression of multiple RNA species including novel and as yet uncharacterized long noncoding transcripts with unknown function. We used pan-genomic HIF-binding and transcriptomic data to identify a novel long noncoding RNA Noncoding Intergenic Co-Induced transcript (NICI) on chromosome 12p13.31 which is regulated by hypoxia via HIF-1 promoter-binding in multiple cell types. CRISPR/Cas9-mediated deletion of the hypoxia-response element revealed co-regulation of NICI and the neighboring protein-coding gene, solute carrier family 2 member 3 (SLC2A3) which encodes the high-affinity glucose transporter 3 (GLUT3). Knockdown or knockout of NICI attenuated hypoxic induction of SLC2A3, indicating a direct regulatory role of NICI in SLC2A3 expression, which was further evidenced by CRISPR/Cas9-VPR-mediated activation of NICI expression. We also demonstrate that regulation of SLC2A3 is mediated through transcriptional activation rather than posttranscriptional mechanisms because knockout of NICI leads to reduced recruitment of RNA polymerase 2 to the SLC2A3 promoter. Consistent with this we observe NICI-dependent regulation of glucose consumption and cell proliferation. Furthermore, NICI expression is regulated by the von Hippel-Lindau (VHL) tumor suppressor and is highly expressed in clear cell renal cell carcinoma (ccRCC), where SLC2A3 expression is associated with patient prognosis, implying an important role for the HIF/NICI/SLC2A3 axis in this malignancy.

The Ser/Thr kinase p90RSK promotes kidney fibrosis by modulating fibroblast-epithelial crosstalk

Healthy kidney structure and environment rely on epithelial integrity and interactions between epithelial cells and other kidney cells. The Ser/Thr kinase 90 kDa ribosomal protein S6 kinase 1 (p90RSK) belongs to a protein family that regulates many cellular processes, including cell motility and survival. p90RSK is predominantly expressed in the kidney, but its possible role in chronic kidney disease (CKD) remains largely unknown. Here, we found that p90RSK expression is dramatically activated in a classic mouse obstructive chronic kidney disease model, largely in the interstitial FSP-1-positive fibroblasts. We generated FSP-1-specific p90RSK transgenic mouse (RSK-Tg) and discovered that these mice, after obstructive injury, display significantly increased fibrosis and enhanced tubular epithelial damage compared with their wt littermates (RSK-wt), indicating a role of p90RSK in fibroblast-epithelial communication. We established an in vitro fibroblast-epithelial coculture system with primary kidney fibroblasts from RSK-Tg and RSK-wt mice and found that RSK-Tg fibroblasts consistently produce excessive H₂O₂ causing epithelial oxidative stress and inducing nuclear translocation of the signaling protein β-catenin. Epithelial accumulation of β-catenin, in turn, promoted epithelial apoptosis by activating the transcription factor forkhead box class O1 (FOXO1). Of note, blockade of reactive oxygen species (ROS) or β-catenin or FOXO1 activity abolished fibroblast p90RSK-mediated epithelial apoptosis. These results make it clear that p90RSK promotes kidney fibrosis by inducing fibroblast-mediated epithelial apoptosis through ROS-mediated activation of β-catenin/FOXO1 signaling pathway.

Heparan sulfate in chronic kidney diseases: Exploring the role of 3-O-sulfation

One of the main feature of chronic kidney disease is the development of renal fibrosis. Heparan Sulfate (HS) is involved in disease development by modifying the function of growth factors and cytokines and creating chemokine gradients. In this context, we aimed to understand the function of HS sulfation in renal fibrosis. Using a mouse model of renal fibrosis, we found that total HS 2-O-sulfation was increased in damaged kidneys, whilst, tubular staining of HS 3-O-sulfation was decreased. The expression of HS modifying enzymes significantly correlated with the development of fibrosis with HS3ST1 demonstrating the strongest correlation. The pro-fibrotic factors TGFβ1 and TGFβ2/IL1β significantly downregulated HS3ST1 expression in both renal epithelial cells and renal fibroblasts. To determine the implication of HS3ST1 in growth factor binding and signalling, we generated an in vitro model of renal epithelial cells overexpressing HS3ST1 (HKC8-HS3ST1). Heparin Binding EGF like growth factor (HB-EGF) induced rapid, transient STAT3 phosphorylation in control HKC8 cells. In contrast, a prolonged response was demonstrated in HKC8-HS3ST1 cells. Finally, we showed that both HS 3-O-sulfation and HB-EGF tubular staining were decreased with the development of fibrosis. Taken together, these data suggest that HS 3-O-sulfation is modified in fibrosis and highlight HS3ST1 as an attractive biomarker of fibrosis progression with a potential role in HB-EGF signalling.

The role of PAK1 in the sensitivity of kidney epithelial cells to ischemia-like conditions

Kidney ischemia, characterized by insufficient supply of oxygen and nutrients to renal epithelial cells, is the main cause of acute kidney injury and an important contributor to mortality world-wide. Earlier research implicated a G-protein coupled receptor (NK1R) in the death of kidney epithelial cells in ischemia-like conditions. P21-associated kinase 1 (PAK1) is involved in signalling by several G-proteins. We explored the consequences of PAK1 inhibition for cell survival under the conditions of reduced glucose and oxygen. Inhibition of PAK1 by RNA interference, expression of a dominant-negative mutant or treatment with small molecule inhibitors greatly reduced the death of cultured kidney epithelial cells. Similar protection was achieved by treating the cells with inhibitors of MEK1, in agreement with the prior reports on PAK1-MEK1 connection. Concomitant inhibition of NK1R and PAK1 offered no better protection than inhibition of NK1R alone, consistent with the two proteins being members of the same pathway. Furthermore, NK1R, PAK and MEK inhibitors reduced the induction of TRAIL in ischemia-like conditions. Considering the emerging role of TRAIL in ischemia-mediated cell death, this phenomenon may contribute to the protective effects of these small molecules. Our findings support further exploration of PAK and MEK inhibitors as possible agents to avert ischemic kidney injury.

DNA-Encoded Library-Derived DDR1 Inhibitor Prevents Fibrosis and Renal Function Loss in a Genetic Mouse Model of Alport Syndrome

The importance of Discoidin Domain Receptor 1 (DDR1) in renal fibrosis has been shown via gene knockout and use of antisense oligonucleotides; however, these techniques act via a reduction of DDR1 protein, while we prove the therapeutic potential of inhibiting DDR1 phosphorylation with a small molecule. To date, efforts to generate a selective small-molecule to specifically modulate the activity of DDR1 in an in vivo model have been unsuccessful. We performed parallel DNA encoded library screens against DDR1 and DDR2, and discovered a chemical series that is highly selective for DDR1 over DDR2. Structure-guided optimization efforts yielded the potent DDR1 inhibitor 2.45, which possesses excellent kinome selectivity (including 64-fold selectivity over DDR2 in a biochemical assay), a clean in vitro safety profile, and favorable pharmacokinetic and physicochemical properties. As desired, compound 2.45 modulates DDR1 phosphorylation in vitro as well as prevents collagen-induced activation of renal epithelial cells expressing DDR1. Compound 2.45 preserves renal function and reduces tissue damage in Col4a3^–/– mice (the preclinical mouse model of Alport syndrome) when employing a therapeutic dosing regime, indicating the real therapeutic value of selectively inhibiting DDR1 phosphorylation in vivo. Our results may have wider significance as Col4a3^–/– mice also represent a model for chronic kidney disease, a disease which affects 10% of the global population.

Inherent DNA-binding specificities of the HIF-1α and HIF-2α transcription factors in chromatin

Hypoxia-inducible factor (HIF) is the major transcriptional regulator of cellular responses to hypoxia. The two principal HIF-α isoforms, HIF-1α and HIF-2α, are progressively stabilized in response to hypoxia and form heterodimers with HIF-1β to activate a broad range of transcriptional responses. Here, we report on the pan-genomic distribution of isoform-specific HIF binding in response to hypoxia of varying severity and duration, and in response to genetic ablation of each HIF-α isoform. Our findings reveal that, despite an identical consensus recognition sequence in DNA, each HIF heterodimer loads progressively at a distinct repertoire of cell-type-specific sites across the genome, with little evidence of redistribution under any of the conditions examined. Marked biases towards promoter-proximal binding of HIF-1 and promoter-distant binding of HIF-2 were observed under all conditions and were consistent in multiple cell type. The findings imply that each HIF isoform has an inherent property that determines its binding distribution across the genome, which might be exploited to therapeutically target the specific transcriptional output of each isoform independently.

Oleanolic Acid and Ursolic Acid Improve Bone Properties and Calcium Balance and Modulate Vitamin D Metabolism in Aged Female Rats

Oleanolic acid (OA) and ursolic acid (UA) are the major chemical constituents in Fructus Ligustri Lucidi (FLL), a kidney-tonifying Chinese herb that is previously shown to improve bone properties and enhance calcium balance in aged female rats. The present study was designed to study if OA and UA act as the active ingredients in FLL to exert the positive effects on bone and mineral metabolism in aged rats. Aged (13-month-old) Sprague-Dawley female rats were randomly assigned to four groups with oral administration of drug or vehicle treatment for 12 weeks: medium calcium diet (MCD, 0.6% calcium), high calcium diet (HCD, 1.2% calcium), MCD + FLL (700 mg/kg/day), MCD + OA (23.6 mg/kg/day) + UA (8.6 mg/kg/day). A group of mature (3-month-old) female rats fed with MCD was included as positive control. The results demonstrated that FLL and OA+UA increased bone mineral density and improved microarchitectural properties of aged female rats. The osteoprotective effects of FLL and OA+UA might be, at least in part, associated with their actions on enhancing calcium balance and suppressing age-induced secondary hyperparathyroidism in aged female rats. FLL and OA+UA also significantly induced renal CYP27B1 protein expression and OA+UA treatment decreased CYP24A1 mRNA and protein expressions in aged female rats. In addition, FLL and OA+UA significantly increased the promoter activity, mRNA and protein expressions of renal CYP27B1 in vitro in human proximal tubule HKC-8 cells. The present findings suggest that OA+UA can be regarded as the active ingredients of FLL and might be a potential drug candidate for prevention and treatment of osteoporosis.

HIF stabilization inhibits renal epithelial cell migration and is associated with cytoskeletal alterations

Acute kidney injury (AKI) is a common and potentially lethal complication in the hospitalized patients, with hypoxic injury being as a major cause. The loss of renal tubular epithelial cells (TEC), one of the AKI hallmarks, is potentially followed by tubular regeneration process orchestrated by the remaining uninjured TECs that undergo proliferation and migration. In this study, we used human primary TEC to investigate the initiation of tubular cell migration and associated cytoskeletal alterations in response to pharmacological HIF stabilization which resembles the pathophysiology of hypoxia. Tubular cells have been shown to migrate as cohorts in a wound healing assay. Importantly, cells of distal tubular origin moved faster than those of proximal origin. HIF stabilization impaired TEC migration, which was confirmed by live single cell tracking. HIF stabilization significantly reduced tubular cell migration velocity and promoted cell spreading. In contrast to the control conditions, HIF stabilization induced actin filaments rearrangement and cell adhesion molecules including paxillin and focal adhesion kinase. Condensed bundling of keratin fibers was also observed, while the expression of different types of keratins, phosphorylation of keratin 18, and the microtubule structure were not altered. In summary, HIF stabilization reduced the ability of renal tubular cells to migrate and led to cytoskeleton reorganization. Our data suggested an important involvement of HIF stabilization during the epithelial migration underlying the mechanism of renal regeneration in response to AKI.

SETD2 Haploinsufficiency for Microtubule Methylation Is an Early Driver of Genomic Instability in Renal Cell Carcinoma

Loss of the short arm of chromosome 3 (3p) occurs early in >95% of clear cell renal cell carcinoma (ccRCC). Nearly ubiquitous 3p loss in ccRCC suggests haploinsufficiency for 3p tumor suppressors as early drivers of tumorigenesis. We previously reported methyltransferase SETD2, which trimethylates H3 histones on lysine 36 (H3K36me3) and is located in the 3p deletion, to also trimethylate microtubules on lysine 40 (αTubK40me3) during mitosis, with αTubK40me3 required for genomic stability. We now show that monoallelic, Setd2-deficient cells retaining H3K36me3, but not αTubK40me3, exhibit a dramatic increase in mitotic defects and micronuclei count, with increased viability compared with biallelic loss. In SETD2-inactivated human kidney cells, rescue with a pathogenic SETD2 mutant deficient for microtubule (αTubK40me3), but not histone (H3K36me3) methylation, replicated this phenotype. Genomic instability (micronuclei) was also a hallmark of patient-derived cells from ccRCC. These data show that the SETD2 tumor suppressor displays a haploinsufficiency phenotype disproportionately impacting microtubule methylation and serves as an early driver of genomic instability.Significance: Loss of a single allele of a chromatin modifier plays a role in promoting oncogenesis, underscoring the growing relevance of tumor suppressor haploinsufficiency in tumorigenesis. Cancer Res; 78(12); 3135-46. ©2018 AACR.

Oleanolic Acid Exerts Osteoprotective Effects and Modulates Vitamin D Metabolism

Oleanolic acid (OA) is a triterpenoid with reported bone anti-resorption activities. The present study aimed to characterize its bone protective effects in vivo and to study its effects on vitamin D metabolism, both in vivo and in vitro. OA significantly increased bone mineral density, improved micro-architectural properties, reduced urinary Ca excretion, increased 1,25(OH)₂D₃ and renal CYP27B1 mRNA expression in mature C57BL/6 ovariectomised (OVX) mice. OA also improved bone properties, Ca balance, and exerted modulatory effects on renal CYP27B1 and CYP24A1 expressions in aged normal female Sprague–Dawley rats. In addition, OA significantly increased renal CYP27B1 mRNA and promoter activity, and suppressed CYP24A1 mRNA and protein expressions in human proximal tubule HKC-8 cells. OA exerted bone protective effects in mature OVX mice and aged female rats. This action on bone might be, at least in part, associated with its effects on Ca and vitamin D metabolism. The present findings suggest that OA is a potential drug candidate for the management of postmenopausal osteoporosis.

Monitoring and manipulating cellular crosstalk during kidney fibrosis inside a 3D in vitro co-culture

In pharmacological research the development of promising lead compounds requires a detailed understanding of the dynamics of disease progression. However, for many diseases, such as kidney fibrosis, gaining such understanding requires complex real-time, multi-dimensional analysis of diseased and healthy tissue. To allow for such studies with increased throughput we established a dextran hydrogel-based in vitro 3D co-culture as a disease model for kidney fibrosis aimed at the discovery of compounds modulating the epithelial/mesenchymal crosstalk. This platform mimics a simplified pathological renal microenvironment at the interface between tubular epithelial cells and surrounding quiescent fibroblasts. We combined this 3D technology with epithelial reporter cell lines expressing fluorescent biomarkers in order to visualize pathophysiological cell state changes resulting from toxin-mediated chemical injury. Epithelial cell damage onset was robustly detected by image-based monitoring, and injured epithelial spheroids induced myofibroblast differentiation of co-cultured quiescent human fibroblasts. The presented 3D co-culture system therefore provides a unique model system for screening of novel therapeutic molecules capable to interfere and modulate the dialogue between epithelial and mesenchymal cells.

Data for automated, high-throughput microscopy analysis of intracellular bacterial colonies using spot detection

Quantification of intracellular bacterial colonies is useful in strategies directed against bacterial attachment, subsequent cellular invasion and intracellular proliferation. An automated, high-throughput microscopy-method was established to quantify the number and size of intracellular bacterial colonies in infected host cells (Detection and quantification of intracellular bacterial colonies by automated, high-throughput microscopy, Ernstsen et al., 2017 [1]). The infected cells were imaged with a 10× objective and number of intracellular bacterial colonies, their size distribution and the number of cell nuclei were automatically quantified using a spot detection-tool. The spot detection-output was exported to Excel, where data analysis was performed. In this article, micrographs and spot detection data are made available to facilitate implementation of the method.

Farnesoid X Receptor Agonism Protects against Diabetic Tubulopathy: Potential Add-On Therapy for Diabetic Nephropathy

Established therapies for diabetic nephropathy (dNP) delay but do not prevent its progression. The shortage of established therapies may reflect the inability to target the tubular compartment. The chemical chaperone tauroursodeoxycholic acid (TUDCA) ameliorates maladaptive endoplasmic reticulum (ER) stress signaling and experimental dNP. Additionally, TUDCA activates the farnesoid X receptor (FXR), which is highly expressed in tubular cells. We hypothesized that TUDCA ameliorates maladaptive ER signaling via FXR agonism specifically in tubular cells. Indeed, TUDCA induced expression of FXR-dependent genes (SOCS3 and DDAH1) in tubular cells but not in other renal cells. In vivo, TUDCA reduced glomerular and tubular injury in db/db and diabetic endothelial nitric oxide synthase-deficient mice. FXR inhibition with Z-guggulsterone or vivo-morpholino targeting of FXR diminished the ER-stabilizing and renoprotective effects of TUDCA. Notably, these in vivo approaches abolished tubular but not glomerular protection by TUDCA. Combined intervention with TUDCA and the angiotensin-converting enzyme inhibitor enalapril in 16-week-old db/db mice reduced albuminuria more efficiently than did either treatment alone. Although both therapies reduced glomerular damage, only TUDCA ameliorated tubular damage. Thus, interventions that specifically protect the tubular compartment in dNP, such as FXR agonism, may provide renoprotective effects on top of those achieved by inhibiting angiotensin-converting enzyme.

Engineering kidney cells: reprogramming and directed differentiation to renal tissues

Growing knowledge of how cell identity is determined at the molecular level has enabled the generation of diverse tissue types, including renal cells from pluripotent or somatic cells. Recently, several in vitro protocols involving either directed differentiation or transcription-factor-based reprogramming to kidney cells have been established. Embryonic stem cells or induced pluripotent stem cells can be guided towards a kidney fate by exposing them to combinations of growth factors or small molecules. Here, renal development is recapitulated in vitro resulting in kidney cells or organoids that show striking similarities to mammalian embryonic nephrons. In addition, culture conditions are also defined that allow the expansion of renal progenitor cells in vitro. Another route towards the generation of kidney cells is direct reprogramming. Key transcription factors are used to directly impose renal cell identity on somatic cells, thus circumventing the pluripotent stage. This complementary approach to stem-cell-based differentiation has been demonstrated to generate renal tubule cells and nephron progenitors. In-vitro-generated renal cells offer new opportunities for modelling inherited and acquired renal diseases on a patient-specific genetic background. These cells represent a potential source for developing novel models for kidney diseases, drug screening and nephrotoxicity testing and might represent the first steps towards kidney cell replacement therapies. In this review, we summarize current approaches for the generation of renal cells in vitro and discuss the advantages of each approach and their potential applications.

Detection and quantification of intracellular bacterial colonies by automated, high-throughput microscopy

To target bacterial pathogens that invade and proliferate inside host cells, it is necessary to design intervention strategies directed against bacterial attachment, cellular invasion and intracellular proliferation. We present an automated microscopy-based, fast, high-throughput method for analyzing size and number of intracellular bacterial colonies in infected tissue culture cells. Cells are seeded in 48-well plates and infected with a GFP-expressing bacterial pathogen. Following gentamicin treatment to remove extracellular pathogens, cells are fixed and cell nuclei stained. This is followed by automated microscopy and subsequent semi-automated spot detection to determine the number of intracellular bacterial colonies, their size distribution, and the average number per host cell. Multiple 48-well plates can be processed sequentially and the procedure can be completed in one working day. As a model we quantified intracellular bacterial colonies formed by uropathogenic Escherichia coli (UPEC) during infection of human kidney cells (HKC-8). Urinary tract infections caused by UPEC are among the most common bacterial infectious diseases in humans. UPEC can colonize tissues of the urinary tract and is responsible for acute, chronic, and recurrent infections. In the bladder, UPEC can form intracellular quiescent reservoirs, thought to be responsible for recurrent infections. In the kidney, UPEC can colonize renal epithelial cells and pass to the blood stream, either via epithelial cell disruption or transcellular passage, to cause sepsis. Intracellular colonies are known to be clonal, originating from single invading UPEC. In our experimental setup, we found UPEC CFT073 intracellular bacterial colonies to be heterogeneous in size and present in nearly one third of the HKC-8 cells. This high-throughput experimental format substantially reduces experimental time and enables fast screening of the intracellular bacterial load and cellular distribution of multiple bacterial isolates. This will be a powerful experimental tool facilitating the study of bacterial invasion, drug resistance, and the development of new therapeutics.

STAT5 drives abnormal proliferation in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) leads to renal failure. The hallmark of ADPKD is increased epithelial proliferation, which has been proposed to be due to atypical signaling including abnormal JAK-STAT activity. However, the relative contribution of JAK-STAT family members in promoting proliferation in ADPKD is unknown. Here, we present siRNA JAK-STAT-focused screens discovering a previously unknown proliferative role for multiple JAK-STAT components (including STAT1, STAT2, STAT4, STAT5a, and STAT5b). Amongst these, we selected to study the growth hormone/growth hormone receptor/STAT5-axis because of its known role as a regulator of growth in nonrenal tissues. Loss of STAT5 function, facilitated by pharmacological inhibition or siRNAs, significantly reduced proliferation with an associated reduction in cyst growth in vitro. To study whether STAT5 is abnormally activated in vivo, we analyzed its expression using two independent mouse models of ADPKD. STAT5 was nuclear, thus activated, in renal epithelial cyst lining cells in both models. To test whether forced activation of STAT5 can modulate proliferation of renal cells in vivo, irrespective of the Pkd1 status, we overexpressed growth hormone. These mice showed increased STAT5 activity in renal epithelial cells, which correlated with de novo expression of cyclin D1, a STAT5 target gene. Chromatin immunoprecipitation experiments revealed that STAT5 transcriptionally activated cyclin D1 in a growth hormone-dependent fashion, thus providing a mechanism into how STAT5 enhances proliferation. Finally, we provide evidence of elevated serum growth hormone in Pkd1 mutant mice. Thus, the growth hormone/STAT5 signaling axis is a novel therapeutic target in ADPKD.

Dynamic NHERF interaction with TRPC4/5 proteins is required for channel gating by diacylglycerol

The activation mechanism of the classical transient receptor potential channels TRPC4 and -5 via the G_q/11 protein-phospholipase C (PLC) signaling pathway has remained elusive so far. In contrast to all other TRPC channels, the PLC product diacylglycerol (DAG) is not sufficient for channel activation, whereas TRPC4/5 channel activity is potentiated by phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion. As a characteristic structural feature, TRPC4/5 channels contain a C-terminal PDZ-binding motif allowing for binding of the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor (NHERF) 1 and 2. PKC inhibition or the exchange of threonine for alanine in the C-terminal PDZ-binding motif conferred DAG sensitivity to the channel. Altogether, we present a DAG-mediated activation mechanism for TRPC4/5 channels tightly regulated by NHERF1/2 interaction. PIP₂ depletion evokes a C-terminal conformational change of TRPC5 proteins leading to dynamic dissociation of NHERF1/2 from the C terminus of TRPC5 as a prerequisite for DAG sensitivity. We show that NHERF proteins are direct regulators of ion channel activity and that DAG sensitivity is a distinctive hallmark of TRPC channels.

Interaction of PPARα With the Canonic Wnt Pathway in the Regulation of Renal Fibrosis

Peroxisome proliferator-activated receptor-α (PPARα) displays renoprotective effects with an unclear mechanism. Aberrant activation of the canonical Wnt pathway plays a key role in renal fibrosis. Renal levels of PPARα were downregulated in both type 1 and type 2 diabetes models. The PPARα agonist fenofibrate and overexpression of PPARα both attenuated the expression of fibrotic factors, and suppressed high glucose-induced or Wnt3a-induced Wnt signaling in renal cells. Fenofibrate inhibited Wnt signaling in the kidney of diabetic rats. A more renal prominent activation of Wnt signaling was detected both in PPARα^-/- mice with diabetes or obstructive nephropathy and in PPARα^-/- tubular cells treated with Wnt3a. PPARα did not block the transcriptional activity of β-catenin induced by a constitutively active mutant of lipoprotein receptor-related protein 6 (LRP6) or β-catenin. LRP6 stability was decreased by overexpression of PPARα and increased in PPARα^-/- tubular cells, suggesting that PPARα interacts with Wnt signaling at the Wnt coreceptor level. 4-Hydroxynonenal-induced reactive oxygen species production, which resulted in LRP6 stability, was suppressed by overexpression of PPARα and dramatically enhanced in PPARα^-/- tubular cells. Diabetic PPARα^-/- mice showed more prominent NADPH oxidase-4 overexpression compared with diabetic wild-type mice, suggesting that the inhibitory effect of PPARα on Wnt signaling may be ascribed to its antioxidant activity. These observations identified a novel interaction between PPARα and the Wnt pathway, which is responsible, at least partially, for the therapeutic effects of fenofibrate on diabetic nephropathy.

Neutrophil gelatinase-associated lipocalin production negatively correlates with HK-2 cell impairment: Evaluation of NGAL as a marker of toxicity in HK-2 cells

Neutrophil gelatinase-associated lipocalin is an extracellular protein produced mostly in kidney. Recently, it has become a promising biomarker of renal damage in vivo. On the other hand, the validation of NGAL as a biomarker for nephrotoxicity estimation in vitro has not been characterized in detail yet. Since the HK-2 cells are frequently used human kidney cell line, we aimed to characterize the production of NGAL in these cells and to evaluate NGAL as a possible marker of cell impairment. We used heavy metals (mercury, cadmium), peroxide, drugs (acetaminophen, gentamicin) and cisplatin to mimic nephrotoxicity. HK-2 cells were incubated with selected compounds for 1-24h and cell viability was measured together with extracellular NGAL production. We proved that HK-2 cells possess a capacity to produce NGAL in amount of 2pg/ml/h. We found a change in cell viability after 24h incubation with all tested toxic compounds. The largest decrease of the viability was detected in mercury, acetaminophen, cisplatin and gentamicin. Unexpectedly, we found also a significant decrease in NGAL production in HK-2 cells treated with these toxins for 24h: to 11±5%, 54±5%, 57±6% and 76±9% respectively, compared with controls (=100%). Our results were followed with qPCR analysis when we found no significant increase in LCN2 gene expression after 24h incubation. We conclude that extracellular NGAL production negatively correlates with HK-2 cell impairment.

Structure/Function Analysis of Recurrent Mutations in SETD2 Protein Reveals a Critical and Conserved Role for a SET Domain Residue in Maintaining Protein Stability and Histone H3 Lys-36 Trimethylation

The yeast Set2 histone methyltransferase is a critical enzyme that plays a number of key roles in gene transcription and DNA repair. Recently, the human homologue, SETD2, was found to be recurrently mutated in a significant percentage of renal cell carcinomas, raising the possibility that the activity of SETD2 is tumor-suppressive. Using budding yeast and human cell line model systems, we examined the functional significance of two evolutionarily conserved residues in SETD2 that are recurrently mutated in human cancers. Whereas one of these mutations (R2510H), located in the Set2 Rpb1 interaction domain, did not result in an observable defect in SETD2 enzymatic function, a second mutation in the catalytic domain of this enzyme (R1625C) resulted in a complete loss of histone H3 Lys-36 trimethylation (H3K36me3). This mutant showed unchanged thermal stability as compared with the wild type protein but diminished binding to the histone H3 tail. Surprisingly, mutation of the conserved residue in Set2 (R195C) similarly resulted in a complete loss of H3K36me3 but did not affect dimethylated histone H3 Lys-36 (H3K36me2) or functions associated with H3K36me2 in yeast. Collectively, these data imply a critical role for Arg-1625 in maintaining the protein interaction with H3 and specific H3K36me3 function of this enzyme, which is conserved from yeast to humans. They also may provide a refined biochemical explanation for how H3K36me3 loss leads to genomic instability and cancer.

Characterization of Acetaminophen Toxicity in Human Kidney HK-2 Cells

Acetaminophen (APAP) overdose causes liver injury, but in some cases it is associated also with renal impairment. While several studies exist in relation to acetaminophen nephrotoxicity, no reports have been published describing intracellular changes related to APAP nephrotoxicity in vitro. Because proximal tubular cells are considered to constitute a secondary site of drug-induced injury after hepatocytes, our study's aim was to estimate the toxicity in the human HK-2 cell line. We used a range of APAP concentrations (1-10 mM) to examine toxicity in the cells (1-48 h). We evaluated cell viability using the WST-1 and LDH tests. Cells impairment was also determined by monitoring ROS production, glutathione levels. We proved that HK-2 cells are able to metabolize acetaminophen. We observed moderate impairment of cells already after 1 h of treatment based on a finding of increased ROS production and decreased cell viability. After 24 h, the results showed significant cellular impairment at all tested concentrations except for 1 mM APAP, but no glutathione depletion was found. We conclude that HK-2 cells are susceptible to acetaminophen toxicity but, unlike hepatocytes, it might be not linked to glutathione depletion.

Kidney-on-a-chip technology for renal proximal tubule tissue reconstruction

The renal proximal tubule epithelium is responsible for active secretion of endogenous and exogenous waste products from the body and simultaneous reabsorption of vital compounds from the glomerular filtrate. The complexity of this transport machinery makes investigation of processes such as tubular drug secretion a continuous challenge for researchers. Currently available renal cell culture models often lack sufficient physiological relevance and reliability. Introducing complex biological culture systems in a 3D microfluidic design improves the physiological relevance of in vitro renal proximal tubule epithelium models. Organ-on-a-chip technology provides a promising alternative, as it allows the reconstruction of a renal tubule structure. These microfluidic systems mimic the in vivo microenvironment including multi-compartmentalization and exposure to fluid shear stress. Increasing data supports that fluid shear stress impacts the phenotype and functionality of proximal tubule cultures, for which we provide an extensive background. In this review, we discuss recent developments of kidney-on-a-chip platforms with current and future applications. The improved proximal tubule functionality using 3D microfluidic systems is placed in perspective of investigating cellular signalling that can elucidate mechanistic aberrations involved in drug-induced kidney toxicity.