Pathways of caspase-mediated apoptosis in autosomal-dominant polycystic kidney disease (ADPKD)

Quercetin inhibits caspase-1-dependent macrophage pyroptosis in experimental folic acid nephropathy

BACKGROUND

The role of pyroptosis in kidney disease is limited and incomplete. Quercetin, a flavonoid compound present in a variety of fruits, vegetables, and plants, has shown antioxidant and anti-inflammatory properties. This study was designed to validate the importance of pyroptosis in an experimental model of folic acid nephropathy and to explore the effect of quercetin in protecting against pyroptosis.

METHODS

Gene set enrichment analysis (GSEA) and weighted gene co-expression network analysis (WGCNA) were used to establish the correlation between pyroptosis and folic acid nephropathy. Immune cell infiltration, network pharmacology and single-cell RNA sequencing analysis were utilized to ascertain the specific target of quercetin in relation to pyroptosis. Finally, quercetin's role was verified in vivo and in vitro.

RESULTS

The GSEA analysis revealed a significant correlation between pyroptosis and folic acid nephropathy (NES = 1.764, P = 0.004). The hub genes identified through WGCNA were closely associated with inflammation. Molecular docking demonstrated a strong binding affinity between quercetin and caspase-1, a protein known to be involved in macrophage function, as confirmed by immune cell infiltration and single-cell analysis. Quercetin demonstrated a significant amelioration of kidney injury and reduction in macrophage infiltration in the animal model. Furthermore, quercetin exhibited a significant inhibition of caspase-1 expression, subsequently leading to the inhibition of pro-inflammatory cytokines expression, such as IL-1β, IL-18, TNF-α, and IL-6. The inhibitory effect of quercetin on macrophage pyroptosis was also confirmed in RAW264.7 cells.

CONCLUSION

This study contributes substantial evidence to support the significant role of pyroptosis in the development of folic acid nephropathy, and highlights the ability of quercetin to downregulate caspase-1 in macrophages as a protective mechanism against pyroptosis.

Molecular Mechanisms of Epigenetic Regulation, Inflammation, and Cell Death in ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disorder, which is caused by mutations in the PKD1 and PKD2 genes, characterizing by progressive growth of multiple cysts in the kidneys, eventually leading to end-stage kidney disease (ESKD) and requiring renal replacement therapy. In addition, studies indicate that disease progression is as a result of a combination of factors. Understanding the molecular mechanisms, therefore, should facilitate the development of precise therapeutic strategies for ADPKD treatment. The roles of epigenetic modulation, interstitial inflammation, and regulated cell death have recently become the focuses in ADPKD. Different epigenetic regulators, and the presence of inflammatory markers detectable even before cyst growth, have been linked to cyst progression. Moreover, the infiltration of inflammatory cells, such as macrophages and T cells, have been associated with cyst growth and deteriorating renal function in humans and PKD animal models. There is evidence supporting a direct role of the PKD gene mutations to the regulation of epigenetic mechanisms and inflammatory response in ADPKD. In addition, the role of regulated cell death, including apoptosis, autophagy and ferroptosis, have been investigated in ADPKD. However, there is no consensus whether cell death promotes or delays cyst growth in ADPKD. It is therefore necessary to develop an interactive picture between PKD gene mutations, the epigenome, inflammation, and cell death to understand why inherited PKD gene mutations in patients may result in the dysregulation of these processes that increase the progression of renal cyst formation.

Mining the Biomarker Potential of the Urine Peptidome: From Amino Acids Properties to Proteases

Native biofluid peptides offer important information about diseases, holding promise as biomarkers. Particularly, the non-invasive nature of urine sampling, and its high peptide concentration, make urine peptidomics a useful strategy to study the pathogenesis of renal conditions. Moreover, the high number of detectable peptides as well as their specificity set the ground for the expansion of urine peptidomics to the identification of surrogate biomarkers for extra-renal diseases. Peptidomics further allows the prediction of proteases (degradomics), frequently dysregulated in disease, providing a complimentary source of information on disease pathogenesis and biomarkers. Then, what does urine peptidomics tell us so far? In this paper, we appraise the value of urine peptidomics in biomarker research through a comprehensive analysis of all datasets available to date. We have mined > 50 papers, addressing > 30 different conditions, comprising > 4700 unique peptides. Bioinformatic tools were used to reanalyze peptide profiles aiming at identifying disease fingerprints, to uncover hidden disease-specific peptides physicochemical properties and to predict the most active proteases associated with their generation. The molecular patterns found in this study may be further validated in the future as disease biomarker not only for kidney diseases but also for extra-renal conditions, as a step forward towards the implementation of a paradigm of predictive, preventive and personalized (3P) medicine.

Mechanisms and Efficacy of Chinese Herbal Medicines in Chronic Kidney Disease

As the current treatment of chronic kidney disease (CKD) is limited, it is necessary to seek more effective and safer treatment methods, such as Chinese herbal medicines (CHMs). In order to clarify the modern theoretical basis and molecular mechanisms of CHMs, we reviewed the knowledge based on publications in peer-reviewed English-language journals, focusing on the anti-inflammatory, antioxidative, anti-apoptotic, autophagy-mediated and antifibrotic effects of CHMs commonly used in kidney disease. We also discussed recently published clinical trials and meta-analyses in this field. Based on recent studies regarding the mechanisms of kidney disease in vivo and in vitro, CHMs have anti-inflammatory, antioxidative, anti-apoptotic, autophagy-mediated, and antifibrotic effects. Several well-designed randomized controlled trials (RCTs) and meta-analyses demonstrated that the use of CHMs as an adjuvant to conventional medicines may benefit patients with CKD. Unknown active ingredients, low quality and small sample sizes of some clinical trials, and the safety of CHMs have restricted the development of CHMs. CHMs is a potential method in the treatment of CKD. Further study on the mechanism and well-conducted RCTs are urgently needed to evaluate the efficacy and safety of CHMs.

Autophagy in kidney disease: Advances and therapeutic potential

Autophagy is a highly conserved intracellular catabolic process for the degradation of cytoplasmic components that has recently gained increasing attention for its importance in kidney diseases. It is indispensable for the maintenance of kidney homeostasis both in physiological and pathological conditions. Investigations utilizing various kidney cell-specific conditional autophagy-related gene knockouts have facilitated the advancement in understanding of the role of autophagy in the kidney. Recent findings are raising the possibility that defective autophagy exerts a critical role in different pathological conditions of the kidney. An emerging body of evidence reveals that autophagy exhibits cytoprotective functions in both glomerular and tubular compartments of the kidney, suggesting the upregulation of autophagy as an attractive therapeutic strategy. However, there is also accumulating evidence that autophagy could be deleterious, which presents a formidable challenge in developing therapeutic strategies targeting autophagy. Here, we review the recent advances in research on the role of autophagy during different pathological conditions, including acute kidney injury (AKI), focusing on sepsis, ischemia-reperfusion injury, cisplatin, and heavy metal-induced AKI. We also discuss the role of autophagy in chronic kidney disease (CKD) focusing on the pathogenesis of tubulointerstitial fibrosis, podocytopathies including focal segmental glomerulosclerosis, diabetic nephropathy, IgA nephropathy, membranous nephropathy, HIV-associated nephropathy, and polycystic kidney disease.

Apoptosis and autophagy in polycystic kidney disease (PKD)

Apoptosis in the cystic epithelium is observed in most rodent models of polycystic kidney disease (PKD) and in human autosomal dominant PKD (ADPKD). Apoptosis inhibition decreases cyst growth, whereas induction of apoptosis in the kidney of Bcl-2 deficient mice increases proliferation of the tubular epithelium and subsequent cyst formation. However, alternative evidence indicates that both induction of apoptosis as well as increased overall rates of apoptosis are associated with decreased cyst growth. Autophagic flux is suppressed in cell, zebra fish and mouse models of PKD and suppressed autophagy is known to be associated with increased apoptosis. There may be a link between apoptosis and autophagy in PKD. The mammalian target of rapamycin (mTOR), B-cell lymphoma 2 (Bcl-2) and caspase pathways that are known to be dysregulated in PKD, are also known to regulate both autophagy and apoptosis. Induction of autophagy in cell and zebrafish models of PKD results in suppression of apoptosis and reduced cyst growth supporting the hypothesis autophagy induction may have a therapeutic role in decreasing cyst growth, perhaps by decreasing apoptosis and proliferation in PKD. Future research is needed to evaluate the effects of direct autophagy inducers on apoptosis in rodent PKD models, as well as the cause and effect relationship between autophagy, apoptosis and cyst growth in PKD.

Magnesium lithospermate B attenuates renal injury in 5/6 renal ablation/infarction rats by mitochondrial pathway of apoptosis

Apoptosis is closely interconnected with the severity of renal injury. Magnesium lithospermate B (MLB) extracted from Salviae miltiorrhizae radix showed a wide range of renoprotective effects. However, the underlying mechanisms under beneficial effects have not been elucidated. This study was conducted to investigate whether MLB could mediate renal protection and attenuate apoptosis by mitochondrial pathway. In the present study, the rat model of chronic renal failure (CRF) was established by the 5/6 renal ablation/infarction (A/I) operation. 28 days after the surgery, 30 rats were randomly divided into three groups: sham group, 5/6 (A/I) group and 5/6 (A/I) + MLB group. After 56 days of treatment, we detected the severity of kidney injury, the degree of mitochondrial bax oligomerization and cytochrome c release along with the expression of P53 protein. Our results showed that MLB markedly attenuated kidney injury and apoptosis in 5/6 (A/I) model rats with CRF. MLB effectively inhibited mitochondrial bax accumulation and release of cytochrome c into the cytosol and down-regulated the levels of p53 phosphorylation and acetylation. These data showed that MLB could contribute to renal protection by inhibiting mitochondrial pathway of apoptosis.

Autosomal dominant polycystic kidney disease: Disrupted pathways and potential therapeutic interventions

Autosomal dominant polycystic kidney disease (ADPKD) is a monogenic inherited renal cystic disease that occurs in different races worldwide. It is characterized by the development of a multitude of renal cysts, which leads to massive enlargement of the kidney and often to renal failure in adulthood. ADPKD is caused by a mutation in PKD1 or PKD2 genes encoding the proteins polycystin-1 and polycystin-2, respectively. Recent studies showed that cyst formation and growth result from deregulation of multiple cellular pathways like proliferation, apoptosis, metabolic processes, cell polarity, and immune defense. In ADPKD, intracellular cyclic adenosine monophosphate (cAMP) promotes cyst enlargement by stimulating cell proliferation and transepithelial fluid secretion. Several interventions affecting many of these defective signaling pathways have been effective in animal models and some are currently being tested in clinical trials. Moreover, the stem cell therapy can improve nephropathies and according to studies were done in this field, can be considered as a hopeful therapeutic approach in future for PKD. This study provides an in-depth review of the relevant molecular pathways associated with the pathogenesis of ADPKD and their implications in development of potential therapeutic strategies.

Cell Proliferation and Apoptosis in ADPKD

Increased tubular epithelial cell proliferation with fluid secretion is a key hallmark of autosomal dominant polycystic kidney disease (ADPKD). With disruption of either PKD1 or PKD2, the main causative genes of ADPKD, intracellular calcium homeostasis and cAMP accumulation are disrupted, which in turn leads to altered signaling in the pathways that regulate cell proliferation. These dysregulations finally stimulate the development of fluid-filled cysts originating from abnormally proliferating renal tubular cells. In addition, dysregulated apoptosis is observed in dilated cystic tubules. An imbalance between cell proliferation and apoptosis seems to contribute to cyst growth and renal tissue remodeling in ADPKD. In this section, the mechanisms through which cell proliferation and apoptosis are involved in disease progression, and further, how those signaling pathways impinge on each other in ADPKD will be discussed.

Proteomic characterization of acyclovir-induced nephrotoxicity in a mouse model

Acyclovir (ACV) is an effective and widely used antiviral agent. However, its clinical application is limited by severe nephrotoxicity. We assessed ACV-induced nephrotoxicity and identified the differentially expressed proteins using mass spectrometry-based proteomic analysis. In total, 30 ICR mice were intraperitoneally administrated ACV (150 or 600 mg/kg per day) for 9 days. After administration of ACV, levels of serum creatinine and urea nitrogen increased significantly. In addition, mouse kidneys exhibited histopathological changes and reduced expression levels of vascular endothelial growth factor (VEGF) and its receptor VEGFR2. In the proteomic analysis, more than 1,000 proteins were separated by two-dimensional polyacrylamide gel electrophoresis, and a total of 20 proteins were up- or down-regulated in the ACV group compared with the saline group. Among these, six proteins (MHC class II antigen, glyoxalase 1, peroxiredoxin 1, αB-crystallin, fibroblast growth factor receptor 1-IIIb, and cytochrome c oxidase subunit Vb) were identified in association with ACV-induced nephrotoxicity. These findings were confirmed by Western blotting analysis. The differential expression levels of α-BC, Prx1, Glo I and CcO Vb suggest that oxidative damage and mitochondrial injury may be involved in ACV-induced nephrotoxicity. Furthermore, VEGF and FGF may play a role in tissue repair and the restoration process following ACV nephrotoxicity.

Osteoprotegerin in exosome-like vesicles from human cultured tubular cells and urine

Urinary exosomes have been proposed as potential diagnostic tools. TNF superfamily cytokines and receptors may be present in exosomes and are expressed by proximal tubular cells. We have now studied the expression of selected TNF superfamily proteins in exosome-like vesicles from cultured human proximal tubular cells and human urine and have identified additional proteins in these vesicles by LC-MS/MS proteomics. Human proximal tubular cells constitutively released exosome-like vesicles that did not contain the TNF superfamily cytokines TRAIL or TWEAK. However, exosome-like vesicles contained osteoprotegerin (OPG), a TNF receptor superfamily protein, as assessed by Western blot, ELISA or selected reaction monitoring by nLC-(QQQ)MS/MS. Twenty-one additional proteins were identified in tubular cell exosome-like vesicles, including one (vitamin D binding protein) that had not been previously reported in exosome-like vesicles. Twelve were extracellular matrix proteins, including the basement membrane proteins type IV collagen, nidogen-1, agrin and fibulin-1. Urine from chronic kidney disease patients contained a higher amount of exosomal protein and exosomal OPG than urine from healthy volunteers. Specifically OPG was increased in autosomal dominant polycystic kidney disease urinary exosome-like vesicles and expressed by cystic epithelium in vivo. In conclusion, OPG is present in exosome-like vesicles secreted by proximal tubular epithelial cells and isolated from Chronic Kidney Disease urine.

Comparative proteomic analysis suggests that mitochondria are involved in autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD), characterized by ectatic collecting duct, is an infantile form of PKD occurring in 1 in 20 000 births. Despite having been studied for many years, little is known about the underlying mechanisms. In the current study, we employed, for the first time, a MS-based comparative proteomics approach to investigate the differently expressed proteins between kidney tissue samples of four ARPKD and five control individuals. Thirty two differently expressed proteins were identified and six of the identified protein encoding genes performed on an independent group (three ARPKD subjects, four control subjects) were verified by semi-quantitative RT-PCR, and part of them were further validated by Western blot and immunohistochemistry. Moreover, similar alteration tendency was detected after downregulation of PKHD1 by small interfering RNA in HEK293T cell. Interestingly, most of the identified proteins are associated with mitochondria. This implies that mitochondria may be implicated in ARPKD. Furthermore, the String software was utilized to investigate the biological association network, which is based on known and predicted protein interactions. In conclusion, our findings depicted a global understanding of ARPKD progression and provided a promising resource of targeting protein, and shed some light further investigation of ARPKD.

Polycystins and cellular Ca2+ signaling

The cystic phenotype in autosomal dominant polycystic kidney disease is characterized by a profound dysfunction of many cellular signaling patterns, ultimately leading to an increase in both cell proliferation and apoptotic cell death. Disturbance of normal cellular Ca²⁺ signaling seems to be a primary event and is clearly involved in many pathways that may lead to both types of cellular responses. In this review, we summarize the current knowledge about the molecular and functional interactions between polycystins and multiple components of the cellular Ca²⁺-signaling machinery. In addition, we discuss the relevant downstream responses of the changed Ca²⁺ signaling that ultimately lead to increased proliferation and increased apoptosis as observed in many cystic cell types.

Cytochrome c: potential as a noninvasive biomarker of drug-induced acute kidney injury

INTRODUCTION

Acute kidney injury (AKI) in critically ill patients is closely associated with increased morbidity and mortality, yet there remains continued reliance on increased serum creatinine and blood urea nitrogen to diagnose AKI. These biomarkers increase only after significant renal structural damage has occurred. Recent research efforts have focused on discovery and validation of novel serum and urine biomarkers to detect AKI prior to extensive structural damage. Cytochrome c is best known as an indicator of cell death burden in any organ or tissue. It is released during mitochondrial damage that is associated with processing of apoptosis, cell lysis during necrosis and even reversible mitochondrial and cell injury.

AREAS COVERED

This article reviews the current literature on the potential for cytochrome c as an early biomarker of AKI. The article is based on PubMed searches, using the terms 'acute kidney injury,' 'renal failure,' 'biomarker,' 'toxicity' and 'cytochrome c', with a focus on experimental and clinical data.

EXPERT OPINION

Cytochrome c, as a biomarker, has the potential to improve outcome for AKI patients. Its release indicates mitochondrial damage, one of the earliest changes in cell injury and death. New mitochondrial-targeted therapeutics may be designed around this molecule. Its disadvantages include only transient increase at expression levels that are easily measurable and nonspecificity for kidney injury. The appropriate and optimal utilization of cytochrome c as a biomarker for AKI will be realized only after its complete characterization in experimental and clinical arenas.

Celecoxib inhibits growth of human autosomal dominant polycystic kidney cyst-lining epithelial cells through the VEGF/Raf/MAPK/ERK signaling pathway

Autosomal dominant polycystic kidney disease (ADPKD) is a progressive chronic kidney disease. To date there are no effective medicines to halt development and growth of cysts. In the present study, we explored novel effects of celecoxib (CXB), a COX-2 specific inhibitor, on primary cultures of human ADPKD cyst-lining epithelial cells. Primary cultures of ADPKD cyst-lining epithelial cells were obtained from five patients. Effects of CXB were measured by various assays to detect BrdU incorporation, apoptosis and proliferation in vitro. Additionally, effects of CXB on kidney weight, the cyst index, the fibrosis index, blood urea nitrogen (BUN), serum creatinine (SCr), serum 6-keto-PGF-1α, serum thromboxane-2 (TXB2) and renal PCNA expression were assessed in Han:SPRD rat, a well-characterized rodent model of PKD. CXB inhibited proliferation of ADPKD cyst-lining epithelial cells, blocked the release of VEGF from the cells and induced extensive apoptosis in a time- and dose-dependent manner. Moreover, CXB up-regulated the cell cycle negative regulator p21^CIP/WAF1 and the cell cycle positive regulator Cyclin A, blocked ERK1/2 phosphorylation, induced apoptotic factors (Bax and caspase-3) and reduced Bcl-2. Furthermore, CXB inhibited the expression of VEGFR-2 and Raf-1 in ADPKD cyst-lining epithelial cells. CXB markedly reduced the cyst index, the fibrosis index, leukocyte infiltration, BUN, SCr, serum 6-keto-PGF-1α, TXB2 and renal PCNA expression in Han:SPRD rat. We demonstrated for the first time that CXB could suppress renal cyst-lining growth both in vitro and in vivo in Han:SPRD rat. CXB can inhibit proliferation, suppress cell cycle progression, and induce apoptosis in ADPKD cyst-lining epithelial cells through the inhibition of the VEGF/VEGFR-2/Raf-1/MAPK/ERK signaling pathway.

Hypoxia-inducible factor-1α (HIF-1α) and autophagy in polycystic kidney disease (PKD)

Cyst expansion in polycystic kidney disease (PKD) results in localized hypoxia in the kidney that may activate hypoxia-inducible factor-1α (HIF-1α). HIF-1α and autophagy, a form of programmed cell repair, are induced by hypoxia. The purposes were to determine HIF-1α expression and autophagy in rat and mouse models of PKD. HIF-1α was detected by electrochemiluminescence. Autophagy was visualized by electron microscopy (EM). LC3 and beclin-1, markers of autophagy, were detected by immunoblotting. Eight-week-old male heterozygous (Cy/+) and 4-wk-old homozygous (Cy/Cy) Han:SPRD rats, 4-wk-old cpk mice, and 112-day-old Pkd2WS25/- mice with a mutation in the Pkd2 gene were studied. HIF-1α was significantly increased in massive Cy/Cy and cpk kidneys and not smaller Cy/+ and Pkd2WS25/- kidneys. On EM, features of autophagy were seen in wild-type (+/+), Cy/+, and cpk kidneys: autophagosomes, mitophagy, and autolysosomes. Specifically, autophagosomes were found on EM in the tubular cells lining the cysts in cpk mice. The increase in LC3-II, a marker of autophagosome production and beclin, a regulator of autophagy, in Cy/Cy and cpk kidneys, followed the same pattern of increase as HIF-1α. To determine the role of HIF-1α in cyst formation and/or growth, Cy/+ rats, Cy/Cy rats, and cpk mice were treated with the HIF-1α inhibitor 2-methoxyestradiol (2ME2). 2ME2 had no significant effect on kidney volume or cyst volume density. In summary, HIF-1α is highly expressed in the late stages of PKD and is associated with an increase in LC3-II and beclin-1. The first demonstration of autophagosomes in PKD kidneys is reported. Inhibition of HIF-1α did not have a therapeutic effect.

Apoptosis in polycystic kidney disease

Apoptosis is the process of programmed cell death. It is a ubiquitous, controlled process consuming cellular energy and designed to avoid cytokine release despite activation of local immune cells, which clear the cell fragments. The process occurs during organ development and in maintenance of homeostasis. Abnormalities in any step of the apoptotic process are associated with autoimmune diseases and malignancies. Polycystic kidney disease (PKD) is the most common inherited kidney disease leading to end-stage renal disease (ESRD). Cyst formation requires multiple mechanisms and apoptosis is considered one of them. Abnormalities in apoptotic processes have been described in various murine and rodent models of PKD as well as in human PKD kidneys. The purpose of this review is to outline the role of apoptosis in progression of PKD as well as to describe the mechanisms involved. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Chronic treatment with lisinopril decreases proliferative and apoptotic pathways in autosomal recessive polycystic kidney disease

Angiotensin converting enzyme (ACE) inhibition is a common therapeutic modality in the treatment of autosomal recessive polycystic kidney disease (ARPKD). This study was designed to investigate whether chronic inhibition of ACE would have a therapeutic effect in attenuating the progression of renal cystogenesis in an orthologous rat model of ARPKD, the polycystic kidney (PCK) rat. Lisinopril (3 mg/kg per day) was administered orally for a period of 12 weeks, beginning at post-natal week 4. Lisinopril treatment resulted in an approximately 30% improvement in the collecting duct cystic indices (CT CI) of PCK animals. Activation of extracellular signal-regulated kinase 1 (ERK1) and 2 (ERK2), proliferative signaling markers, and proliferating cell nuclear antigen (PCNA), an end-point marker for proliferation, was reduced following chronic treatment with lisinopril compared to that in vehicle-treated PCK rats. To assess whether apoptotic pathways were altered due to chronic ACE inhibition, we examined p38 mitogen activated protein kinase (MAPK) and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), which are markers of apoptotic signaling cascades. p38 MAPK was significantly reduced (P < 0.0001) following chronic treatment with lisinopril, but no change in the activation of SAPK/JNK could be detected by immunoblot analysis. Lisinopril treatment resulted in a significant reduction (P < 0.01) in cleaved caspase-7 levels, but not caspase-3 activity, in PCK rat kidneys compared to the vehicle-treated PCK rat kidneys. Proteinuria was completely ameliorated in the presence of chronic ACE inhibition in the lisinopril-treated rats compared with the vehicle-treated PCK rats. In all, these findings demonstrated that chronic ACE inhibition can beneficially alter proliferative and apoptotic pathways to promote therapeutic reductions in renal cyst development in ARPKD.

Emerging therapies for chronic kidney disease: what is their role?

The prevalence of chronic kidney disease (CKD) is increasing worldwide. The best therapies currently available focus on the control of blood pressure and optimization of renin-angiotensin-aldosterone system blockade. Currently available agents are only partially effective against hard end points such as the development of end-stage renal disease and are not discussed in this Review. Many other agents have been shown to reduce proteinuria and delay progression in animal models of CKD. Some of these agents, including tranilast, sulodexide, thiazolidinediones, pentoxifylline, and inhibitors of advanced glycation end-products and protein kinase C, have been tested to a limited extent in humans. A small number of randomized controlled human trials of these agents have used surrogate markers such as proteinuria as end points rather than hard end points such as end-stage renal disease or doubling of serum creatinine level. Emerging therapies that specifically target and reverse pathological hallmarks of CKD such as inflammation, fibrosis and atrophy are needed to reduce the burden of this chronic disease and its associated morbidity. This Review examines the evidence for emerging pharmacological strategies for slowing the progression of CKD.

Pathways of apoptosis in human autosomal recessive and autosomal dominant polycystic kidney diseases

Autosomal dominant polycystic kidney disease (ADPKD) is a major cause of end-stage renal disease in adults. Autosomal recessive (AR) PKD affects approximately 1:20,000 live-born children with high perinatal mortality. Both diseases have abnormalities in epithelial proliferation, secretion, and cell-matrix interactions, leading to progressive cystic expansion and associated interstitial fibrosis. Cell number in a kidney reflects the balance between proliferation and apoptosis. Apoptosis results from extrinsic (ligand-induced, expression of caspase-8) and intrinsic (mitochondrial damage, expression of caspase-9) triggers. Previous studies have suggested a role for apoptosis in PKD cyst formation and parenchymal destruction. Mechanisms underlying apoptosis in human ADPKD and ARPKD were examined by quantitative immunohistochemistry and Western immunoblot analyses of age-matched normal and PKD tissues. Caspase-8 expression was significantly greater in small cysts and normal-appearing tubules than in larger cysts in ADPKD kidneys. Caspase-8 also appeared early in the disease process of ADPKD. In ARPKD, expression of caspase-8 was most pronounced in later stages of the disease and was not confined to a specific cyst size. In conclusion, apoptosis in human ADPKD is an early event, occurring predominantly in normal-appearing tubules and small cysts, and is triggered by an extrinsic factor, but it occurs later in ARPKD.

Potential pharmacological interventions in polycystic kidney disease

Polycystic kidney diseases (autosomal dominant and autosomal recessive) are progressive renal tubular cystic diseases, which are characterised by cyst expansion and loss of normal kidney structure and function. Autosomal dominant polycystic kidney disease (ADPKD) is the most common life- threatening, hereditary disease. ADPKD is more prevalent than Huntington's disease, haemophilia, sickle cell disease, cystic fibrosis, myotonic dystrophy and Down's syndrome combined. Early diagnosis and treatment of hypertension with inhibitors of the renin-angiotensin-aldosterone system (RAAS) and its potential protective effect on left ventricular hypertrophy has been one of the major therapeutic goals to decrease cardiac complications and contribute to improved prognosis of the disease. Advances in the understanding of the genetics, molecular biology and pathophysiology of the disease are likely to facilitate the improvement of treatments for these diseases. Developments in describing the role of intracellular calcium ([Ca(2+)](i)) and its correlation with cellular signalling systems, Ras/Raf/mitogen extracellular kinase (MEK)/extracellular signal-regulated protein kinase (ERK), and interaction of these pathways with cyclic adenosine monophosphate (cAMP) levels, provide new insights on treatment strategies. Blocking the vasopressin V(2) receptor, a major adenylyl cyclase agonist, demonstrated significant improvements in inhibiting cytogenesis in animal models. Because of activation of the mammalian target of rapamycin (mTOR) pathway, the use of sirolimus (rapamycin) an mTOR inhibitor, markedly reduced cyst formation and decreased polycystic kidney size in several animal models. Caspase inhibitors have been shown to decrease cytogenesis and renal failure in rats with cystic disease. Cystic fluid secretion results in cyst enlargement and somatostatin analogues have been shown to decrease renal cyst progression in patients with ADPKD. The safety and efficacy of these classes of drugs provide potential interventions for experimental and clinical trials.

Calcium channel inhibition accelerates polycystic kidney disease progression in the Cy/+ rat

In polycystic kidney disease, abnormal epithelial cell proliferation is the main factor leading to cyst formation and kidney enlargement. Cyclic AMP (cAMP) is mitogenic in cystic but antimitogenic in normal human kidney cells, which is due to reduced steady-state intracellular calcium levels in cystic compared to the normal cells. Inhibition of intracellular calcium entry with channel blockers, such as verapamil, induced cAMP-dependent cell proliferation in normal renal cells. To determine if calcium channel blockers have a similar effect on cell proliferation in vivo, Cy/+ rats, a model of dominant polycystic kidney disease, were treated with verapamil. Kidney weight and cyst index were elevated in verapamil-treated Cy/+ rats. This was associated with increased cell proliferation and apoptosis, elevated expression, and phosphorylation of B-Raf with stimulation of the mitogen-activated protein kinase MEK/ERK (mitogen-activated protein kinase kinase/extracellular-regulated kinase) pathway. Verapamil had no effect on kidney morphology or B-Raf stimulation in wild-type rats. We conclude that treatment of Cy/+ rats with calcium channel blockers increases activity of the B-Raf/MEK/ERK pathway accelerating cyst growth in the presence of endogenous cAMP, thus exacerbating renal cystic disease.

Mitochondria are the major targets in albumin-induced apoptosis in proximal tubule cells

Nephrotic-range proteinuria is considered a poor prognostic factor. A correlation between tubulointerstitial injury and the degree of proteinuria is well established. In an attempt to explain the tubular atrophy that is observed in advanced glomerulonephritides, this study investigated apoptotic mechanisms in cultured human proximal tubule cells (HKC-8) that were exposed to endotoxin-free albumin (5, 10, and 20 mg/ml). Apoptosis was detected by Hoechst 33342; annexin staining; and assays for caspases 3, 8, and 9. The apoptotic effect of albumin was maximal at 10 mg/ml albumin, and necrosis prevailed in cells that were incubated with 20 mg/ml. Increase in caspase-9 and -3 activity was observed starting at 6 and maximally at 16 to 24 h. The proapoptotic Bcl-2 protein Bax was upregulated at 6 h, associated with translocation of cytochrome-c from mitochondria to cytosol and alteration in the mitochondrial membrane potential. Production of reactive oxygen species (ROS) was significant at 6 h but declined at 16 and 24 h. Treatment with ROS scavenger dimethylthiourea or antioxidant N-acetylcysteine did not alleviate caspase-3 production. Pan protein kinase C inhibitor bisindolylmaleimide-1 protected the cells from apoptosis. It is concluded that albumin induces apoptosis in human proximal tubule cells by stimulating mitochondrial apoptotic pathway independent of ROS production.

Loss of PKD1 and loss of Bcl-2 elicit polycystic kidney disease through distinct mechanisms

We have recently demonstrated that ablation of one or both alleles of the proapoptotic gene Bim prevents the polycystic kidney disease (PKD) that develops in mice deficient for the prosurvival protein Bcl-2. The aim of the present study was to investigate whether loss of Bim or Bcl-2 could influence the disease in the PKD1del34/del34 mutant mice, a model of autosomal dominant PKD. PKD1del34/del34 mice were intercrossed with Bim-deficient mice and Bcl-2+/- mice to generate double mutants. Loss of Bim does not prevent the development of PKD in PKD1del34/del34 mice. On the C57BL/6 genetic background, most older PKD1del34/+ mice do not develop PKD, but present with liver cysts. Surprisingly, loss of Bim completely prevented liver cysts formation in PKD1del34/+ mice. Loss of one Bcl-2 allele did not influence the PKD1del34 phenotype significantly. We conclude that loss of PKD1 and loss of Bcl-2 elicit PKD through distinct mechanisms.

Caspase inhibition reduces tubular apoptosis and proliferation and slows disease progression in polycystic kidney disease

We have previously demonstrated an increase in proapoptotic caspase-3 in the kidney of Han:SPRD rats with polycystic kidney disease (PKD). The aim of the present study was to determine the effect of caspase inhibition on tubular cell apoptosis and proliferation, cyst formation, and renal failure in the Han:SPRD rat model of PKD. Heterozygous (Cy/+) and littermate control (+/+) male rats were weaned at 3 weeks of age and then treated with the caspase inhibitor IDN-8050 (10 mg/kg per day) by means of an Alzet (Palo Alto, CA) minipump or vehicle [polyethylene glycol (PEG 300)] for 5 weeks. The two-kidney/total body weight ratio more than doubled in Cy/+ rats compared with +/+ rats. IDN-8050 significantly reduced the kidney enlargement by 44% and the cyst volume density by 29% in Cy/+ rats. Cy/+ rats with PKD have kidney failure as indicated by a significant increase in blood urea nitrogen. IDN-8050 significantly reduced the increase in blood urea nitrogen in the Cy/+ rats. The number of proliferating cell nuclear antigen-positive tubular cells and apoptotic tubular cells in non-cystic and cystic tubules was significantly reduced in IDN-8050-treated Cy/+ rats compared with vehicle-treated Cy/+ rats. On immunoblot, the active form of caspase-3 (20 kDa) was significantly decreased in IDN-8050-treated Cy/+ rats compared with vehicle-treated Cy/+ rats. In summary, in a rat model of PKD, caspase inhibition with IDN-8050 (i) decreases apoptosis and proliferation in cystic and noncystic tubules; (ii) inhibits renal enlargement and cystogenesis, and (iii) attenuates the loss of kidney function.

Establishment of a Mouse Macula Densa Cell Line with an nNOS Promoter Driving EGFP Expression

We describe a unique method for establishing a functionally intact macula densa cell line from immortalized renal cells in culture. The macula densa is involved in the tubuloglomerular feedback (TGF) system in the kidney and specifically expresses neuronal nitric oxide synthase (nNOS). A 347 bp portion of the nNOS promoter was used to drive the expression of enhanced green fluorescence protein (EGFP). An immortalized distal tubule (DT) cell line was derived from distal tubules microdissected from the kidneys of SV40 large T antigen transgenic mice. Immunofluorescence labeling using an antibody against nNOS revealed no specific EGFP expression in immunofluorescence-negative DT cells. The established cell line (NE-MD) showed a time-dependent increase in signals of the nNOS protein when they were incubated with 12 microM furosemide (an inhibitor of Na(+)-K(+)-2Cl(-) symporter) for 5 h. In conclusion, this newly developed macula densa cell line will be useful in studies of the TGF stem.