Puromycin aminonucleoside induces glomerular epithelial cell apoptosis

Low-frequency electromagnetic fields influence the expression of calcium metabolism related proteins in leukocytic cell lines

Our study aimed to verify the hypothesis concerning low-frequency magnetic fields (LF-MFs)-related changes in cell viability through the biomechanism(s) based on calcineurin (CaN)-mediated signaling pathways triggered via ROS-like molecules. For experiments, Mono Mac 6 and U937 leukocytic cell lines were chosen and exposed to various LF-MFs and/or puromycin (PMC). The protein expression level of key regulatory proteins of calcium metabolism was examined by Western Blot analysis. In turn, the reactive oxygen species (ROS) and cell viability parameters were evaluated by cytochrome C reduction assay and flow cytometry, respectively. The simultaneous action of applied MF and PMC influenced cell viability in a MF-dependent manner. The changes in cell viability were correlated with protein expression and ROS levels. It was verified experimentally that applied stress stimuli influence cell susceptibility to undergo cell death. Moreover, the evoked bioeffects might be recognized as specific to both types of leukocyte populations.

In situ evaluation of podocytes in patients with focal segmental glomerulosclerosis and minimal change disease

Podocyte injury in focal segmental glomerulosclerosis (FSGS) and minimal change disease (MCD) results from the imbalance between adaptive responses that maintain homeostasis and cellular dysfunction that can culminate in cell death. Therefore, an in situ analysis was performed to detect morphological changes related to cell death and autophagy in renal biopsies from adult patients with podocytopathies. Forty-nine renal biopsies from patients with FSGS (n = 22) and MCD (n = 27) were selected. In situ expression of Wilms Tumor 1 protein (WT1), light chain microtubule 1-associated protein (LC3) and caspase-3 protein were evaluated by immunohistochemistry. The foot process effacement and morphological alterations related to podocyte cell death and autophagy were analyzed with transmission electronic microscopy. Reduction in the density of WT1-labeled podocytes was observed for FSGS and MCD cases as compared to controls. Foot process width (FPW) in control group was lower than in cases of podocytopathies. In FSGS group, FPW was significantly higher than in MCD group and correlated with proteinuria. A density of LC3-labeled podocytes and the number of autophagosomes in podocytes/ pedicels were higher in the MCD group than in the FSGS group. The number of autophagosomes correlated positively with the estimated glomerular filtration rate in cases of MCD. The density of caspase-3-labeled podocytes in FSGS and MCD was higher than control group, and a higher number of podocytes with an evidence of necrosis was detected in FSGS cases than in MCD and control cases. Podocytes from patients diagnosed with FSGS showed more morphological and functional alterations resulting from a larger number of lesions and reduced cell adaptation.

The impact of steroids on the injured podocytes in nephrotic syndrome

Nephrotic syndrome (NS), a common chronic kidney disease, embraces a variety of kidney disorders. Though Glucocorticoids (GCs) are generally used in the treatment of NS, their mechanism of action is poorly understood. A plethora of evidence indicates that podocytes are considered as the main target cells for the therapeutic strategies to prevent NS. GCs regulate the transactivation and transrepression of genes in podocytes that affect their morphological and cytoskeletal features, motility, apoptosis and survival rate. Moreover, they prevent protein leakage through the glomerular barrier membrane by affecting the synthesis, trafficking and posttranslational modifications of slit diaphragms components, podocytes' intercellular junctions. The response to the treatment is variable among different ethnics and populations and resistance to the steroids is detected in almost 50% of adult patients. Not only do pharmacokinetics and pharmacogenetics of steroids play a role in GC resistance but also the genetic variations in one or more podocyte related genes are connected with the steroid resistance in cases with NS. The focus of this review is to explain the underlying cellular and molecular mechanisms of GCs in podocytes. Understanding the mechanisms by which the GCs and GCs receptors in podocytes regulate the gene expression network and crosstalk with other molecular pathways would guarantee an optimum therapeutic benefit of steroid treatment.

Curvature facilitates podocyte culture in a biomimetic platform

Most kidney diseases begin with abnormalities in glomerular podocytes, motivating the need for podocyte models to study pathophysiological mechanisms and new treatment options. However, podocytes cultured in vitro face a limited ability to maintain appreciable extents of differentiation hallmarks, raising concerns over the relevance of study results. Many key properties such as nephrin expression and morphology reach plateaus that are far from the in vivo levels. Here, we demonstrate that a biomimetic topography, consisting of microhemispheres arrayed over the cell culture substrate, promotes podocyte differentiation in vitro. We define new methods for fabricating microscale curvature on various substrates, including a thin porous membrane. By growing podocytes on our topographic substrates, we found that these biophysical cues augmented nephrin gene expression, supported full-size nephrin protein expression, encouraged structural arrangement of F-actin and nephrin within the cell, and promoted process formation and even interdigitation compared to the flat substrates. Furthermore, the topography facilitated nephrin localization on curved structures while nuclei lay in the valleys between them. The improved differentiation was also evidenced by tracking barrier function to albumin over time using our custom topomembranes. Overall, our work presents accessible methods for incorporating microcurvature on various common substrates, and demonstrates the importance of biophysical stimulation in supporting higher-fidelity podocyte cultivation in vitro.

The Role of Glucocorticoid Receptors in Podocytes and Nephrotic Syndrome

Glucocorticoid receptor (GC), a founding member of the nuclear hormone receptor superfamily, is a glucocorticoid-activated transcription factor that regulates gene expression and controls the development and homeostasis of human podocytes. Synthetic glucocorticoids are the standard treatment regimens for proteinuria (protein in the urine) and nephrotic syndrome (NS) caused by kidney diseases. These include minimal change disease (MCD), focal segmental glomerulosclerosis (FSGS), membranous nephropathy (MN) and immunoglobulin A nephropathy (IgAN) or subsequent complications due to diabetes mellitus or HIV infection. However, unwanted side effects and steroid-resistance remain major issues for their long-term use. Furthermore, the mechanism by which glucocorticoids elicit their renoprotective activity in podocyte and glomeruli is poorly understood. Podocytes are highly differentiated epithelial cells that contribute to the integrity of kidney glomerular filtration barrier. Injury or loss of podocytes leads to proteinuria and nephrotic syndrome. Recent studies in multiple experimental models have begun to explore the mechanism of GC action in podocytes. This review will discuss progress in our understanding of the role of glucocorticoid receptor and glucocorticoids in podocyte physiology and their renoprotective activity in nephrotic syndrome.

Rhinovirus 3C protease suppresses apoptosis and triggers caspase-independent cell death

Apoptosis and programmed necrosis (necroptosis) determine cell fate, and antagonize infection. Execution of these complementary death pathways involves the formation of receptor-interacting protein kinase 1 (RIPK1) containing complexes. RIPK1 binds to adaptor proteins, such as TRIF (Toll-IL-1 receptor-domain-containing-adaptor-inducing interferon-beta factor), FADD (Fas-associated-protein with death domain), NEMO (NF-κB regulatory subunit IKKγ), SQSTM1 (sequestosome 1/p62), or RIPK3 (receptor-interacting protein kinase 3), which are involved in RNA sensing, NF-κB signaling, autophagosome formation, apoptosis, and necroptosis. We report that a range of rhinoviruses impair apoptosis and necroptosis in epithelial cells late in infection. Unlike the double-strand (ds) RNA mimetic poly I:C (polyinosinic:polycytidylic acid), the exposure of dsRNA to toll-like receptor 3 (TLR3) in rhinovirus-infected cells did not lead to apoptosis execution. Accordingly, necroptosis and the production of ROS (reactive oxygen species) were not observed late in infection, when RIPK3 was absent. Instead, a virus-induced alternative necrotic cell death pathway proceeded, which led to membrane rupture, indicated by propidium iodide staining. The impairment of dsRNA-induced apoptosis late in infection was controlled by the viral 3C-protease (3Cpro), which disrupted RIPK1-TRIF/FADD /SQSTM1 immune-complexes. 3Cpro and 3C precursors were found to coimmuno-precipitate with RIPK1, cleaving the RIPK1 death-domain, and generating N-terminal RIPK1 fragments. The depletion of RIPK1 or chemical inhibition of its kinase at the N-terminus did not interfere with virus progeny formation or cell fate. The data show that rhinoviruses suppress apoptosis and necroptosis, and release progeny by an alternative cell death pathway, which is controlled by viral proteases modifying innate immune complexes.

Inhibition of SHIP2 in CD2AP-deficient podocytes ameliorates reactive oxygen species generation but aggravates apoptosis

Lack of CD2-associated protein (CD2AP) in mice increases podocyte apoptosis and leads to glomerulosclerosis and renal failure. We showed previously that SHIP2, a negative regulator of the PI3K/AKT signalling pathway, interacts with CD2AP. Here, we found that the expression level and activity of SHIP2 and production of reactive oxygen species (ROS) are increased in cultured CD2AP knockout (CD2AP−/−) mouse podocytes. Oxidative stress was also increased in CD2AP−/− mouse glomeruli in vivo. We found that puromycin aminonucleoside (PA), known to increase ROS production and apoptosis, increases SHIP2 activity and reduces CD2AP expression in cultured human podocytes. PDK1 and CDK2, central regulators of AKT, were downregulated in CD2AP−/− or PA-treated podocytes. Downregulation of PDK1 and CDK2, ROS generation and apoptosis were prevented by CD2AP overexpression in both models. Notably, inhibition of SHIP2 activity with a small molecule inhibitor AS1949490 ameliorated ROS production in CD2AP−/− podocytes, but, surprisingly, further reduced PDK1 expression and aggravated apoptosis. AKT- and ERK-mediated signalling was diminished and remained reduced after AS1949490 treatment in the absence of CD2AP. The data suggest that inhibition of the catalytic activity of SHIP2 is beneficial in reducing oxidative stress, but leads to deleterious increase in apoptosis in podocytes with reduced expression of CD2AP.

N-Degradomic Analysis Reveals a Proteolytic Network Processing the Podocyte Cytoskeleton

Regulated intracellular proteostasis, controlled in part by proteolysis, is essential in maintaining the integrity of podocytes and the glomerular filtration barrier of the kidney. We applied a novel proteomics technology that enables proteome-wide identification, mapping, and quantification of protein N-termini to comprehensively characterize cleaved podocyte proteins in the glomerulus in vivo We found evidence that defined proteolytic cleavage results in various proteoforms of important podocyte proteins, including those of podocin, nephrin, neph1, α-actinin-4, and vimentin. Quantitative mapping of N-termini demonstrated perturbation of protease action during podocyte injury in vitro, including diminished proteolysis of α-actinin-4. Differentially regulated protease substrates comprised cytoskeletal proteins as well as intermediate filaments. Determination of preferential protease motifs during podocyte damage indicated activation of caspase proteases and inhibition of arginine-specific proteases. Several proteolytic processes were clearly site-specific, were conserved across species, and could be confirmed by differential migration behavior of protein fragments in gel electrophoresis. Some of the proteolytic changes discovered in vitro also occurred in two in vivo models of podocyte damage (WT1 heterozygous knockout mice and puromycin aminonucleoside-treated rats). Thus, we provide direct and systems-level evidence that the slit diaphragm and podocyte cytoskeleton are regulated targets of proteolytic modification, which is altered upon podocyte damage.

Toll-like Receptor 9 Can be Activated by Endogenous Mitochondrial DNA to Induce Podocyte Apoptosis

Toll-like receptor 9 (TLR9) senses bacterial DNA characteristic of unmethylated CpG motifs to induce innate immune response. TLR9 is de novo expressed in podocytes of some patients with glomerular diseases, but its role in podocyte injury remains undetermined. Since TLR9 activates p38 MAPK and NFkB that are known to mediate podocyte apoptosis, we hypothesized that TLR9 induces podocyte apoptosis in glomerular diseases. We treated immortalized podocytes with puromycin aminonucleosides (PAN) and observed podocyte apoptosis, accompanied by TLR9 upregulation. Prevention of TLR9 upregulation by siRNA significantly attenuated NFκB p65 or p38 activity and apoptosis, demonstrating that TLR9 mediates podocyte apoptosis. We next showed that endogenous mitochondrial DNA (mtDNA), whose CpG motifs are also unmethylated, is the ligand for TLR9, because PAN induced mtDNA accumulation in endolysosomes where TLR9 is localized, overexpression of endolysosomal DNase 2 attenuated PAN-induced p38 or p65 activity and podocyte apoptosis, and DNase 2 silencing was sufficient to activate p38 or p65 and induce apoptosis. In PAN-treated rats, TLR9 was upregulated in the podocytes, accompanied by increase of apoptosis markers. Thus, de novo expressed TLR9 may utilize endogenous mtDNA as the ligand to facilitate podocyte apoptosis, a novel mechanism underlying podocyte injury in glomerular diseases.

Role of bad in podocyte apoptosis induced by puromycin aminonucleoside

Apoptosis, which is usually a response to the microenvironment, requires inactivation of prosurvival molecules. Apoptosis contributes to loss of podocytes in the course of renal injury, an event closely associated with the development of proteinuria. Dexamethasone (DEX) is the standard of care for most forms of nephrotic syndrome. However, the precise mechanisms of DEX action on podocytes are unknown. This study examined the hypothesis that cultured podocytes exposed to puromycin aminonucleoside (PAN) showed a reduced rate of apoptosis upon DEX exposure. Apoptotic podocytes seemed to be related to increased Bad mRNA and protein expressions. DEX reduced apoptosis by decreasing Bad mRNA and protein expressions, thereby protecting podocytes. These findings provided insights into the beneficial effects of DEX directly on podocytes. The present study illustrated the signal transduction mechanism of podocyte apoptosis induced by PAN.

c-Abl mediates angiotensin II-induced apoptosis in podocytes

Angiotensin II (Ang II) has been reported to cause podocyte apoptosis in rats both in vivo and in vitro studies. However, the underlying mechanisms are poorly understood. In the present study, we investigated the role of the nonreceptor tyrosine kinase c-Abl in Ang II-induced podocyte apoptosis. Male Sprague-Dawley rats in groups of 12 were administered either Ang II (400 kg/kg/min) or Ang II + STI-571 (50 mg/kg/day) by osmotic minipumps. In addition, 12 rats-receiving normal saline served as the control. Glomeruli c-Abl expression was carried out by real time PCR, Western blotting and immunolabeled, and occurrence of apoptosis was carried out by TUNEL staining and transmission electron microscopic analysis. In vitro studies, conditionally immortalized mouse podocytes were treated with Ang II (10(-9)-10(-6) M) in the presence or absence of either c-Abl inhibitor, Src-I1, specific c-Abl siRNA, or c-Abl plasmid alone. Quantification of podocyte c-Abl expression and c-Abl phosphorylation at Y245 and Y412 was carried out by real time PCR, Western blotting and immunofluorescence imaging. The nuclear c-Abl and p53 were quantified by co-immunoprecipitation and Western blotting studies. Podocyte apoptosis was analysed by flow cytometry and Hoechst-33342 staining. c-Abl expression was demonstrated in rat kidney podocytes in vivo and cultured mouse podocytes in vitro. Ang II-receiving rats displayed enhanced podocyte c-Abl expression. And Ang II significantly stimulated c-Abl expression in cultured podocytes. Furthermore Ang II upregulated podocyte c-Abl phosphorylation at Y245 and Y412. Ang II also induced an increase of nuclear p53 protein and nuclear c-Abl-p53 complexes in podocytes and podocyte apoptosis. Down-regulation of c-Abl expression by c-Abl inhibitor (Src-I1) as well as specific siRNA inhibited Ang II-induced podocyte apoptosis; conversely, podoctyes transfected with c-Abl plasmid displayed enhanced apoptosis. These findings indicate that c-Abl may mediates Ang II-induced podocyte apoptosis, and inhibition of c-Abl expression can protect podocytes from Ang II-induced injury.

Upregulation of nestin protects podocytes from apoptosis induced by puromycin aminonucleoside

BACKGROUND

Nestin is an intermediate filament protein widely used as a marker of stem cells or progenitor cells. Nestin is also highly expressed in the glomerular podocyte, a type of terminally differentiated epithelial cell. Little is known about the significance of nestin in podocytes.

METHODS

Puromycin aminonucleoside (PAN) was injected into the rats to produce a PAN nephrosis model. Transmission electronic microscopy and terminal dUTP nick end-labeling assay were used to examine the podocyte foot process (FP) effacement and apoptosis, respectively. A mouse podocyte cell line was cultured and incubated with PAN. Immunoblot was used to examine the level of nestin expression both in vivo and in vitro. Enhanced green fluorescence protein-tagged plasmids containing nestin shRNA were transfected into the cultured podocytes to silence nestin expression. F-actin arrangement within cultured podocytes was investigated by immunofluorescence, while the apoptosis rate was examined by both Hoechst stain and flow cytometry.

RESULTS

In the PAN-induced rat nephrosis model, podocyte nestin expression was increased in the absence of apparent podocyte apoptosis, even though the FP was significantly effaced. In the cultured mouse podocytes, PAN upregulated nestin expression in a time-dependent manner within 24 h of treatment. Notably, no significant apoptosis occurred, however knocking down nestin expression resulted in a remarkable derangement of actin cytoskeleton and an increase in apoptosis in the cultured podocytes 24 h after being incubated with PAN.

CONCLUSIONS

Upregulation of nestin expression during PAN nephrosis could protect podocytes from apoptosis and that this process is mediated by maintaining the regular arrangement of actin cytoskeleton.

Numb protects renal proximal tubular cells from puromycin aminonucleoside-induced apoptosis through inhibiting Notch signaling pathway

Numb was originally discovered as an intrinsic cell fate determinant in Drosophila by antagonizing Notch signaling. The present study is to characterize the role of Numb in oxidative stress-induced apoptosis of renal proximal tubular cells. Exposure of NRK52E cells to puromycin aminonucleoside (PA) resulted in caspase 3-dependent apoptosis. Numb expression was downregulated by PA in a time- and dose-dependent manner. Knocking down endogenous Numb by siRNA sensitized NRK52E cells to PA-induced apoptosis, whereas overexpressing Numb protected NRK52E cells from PA-induced apoptosis. Moreover, PA activated Notch signaling in a time- and dose-dependent manner as indicated by increased expression of the intracellular domain of Notch and Hes-1. Notch signaling inhibitor DAPT significantly attenuated Numb siRNA-augmented apoptosis. On the other hand, overexpression of intracellular domain of Notch1 could reverse the protective effect of Numb on PA-induced apoptosis. Taken together, our data demonstrated that, in renal proximal tubular cells, Numb functions as a protective molecule on PA-induced apoptosis through antagonizing Notch signaling activity.

Testosterone and 17β-estradiol have opposite effects on podocyte apoptosis that precedes glomerulosclerosis in female estrogen receptor knockout mice

Podocyte damage and apoptosis are thought to be important if not essential in the development of glomerulosclerosis. Female estrogen receptor knockout mice develop glomerulosclerosis at 9 months of age due to excessive ovarian testosterone production and secretion. Here, we studied the pathogenesis of glomerulosclerosis in this mouse model to determine whether testosterone and/or 17β-estradiol directly affect the function and survival of podocytes. Glomerulosclerosis in these mice was associated with the expression of desmin and the loss of nephrin, markers of podocyte damage and apoptosis. Ovariectomy preserved the function and survival of podocytes by eliminating the source of endogenous testosterone production. In contrast, testosterone supplementation induced podocyte apoptosis in ovariectomized wild-type mice. Importantly, podocytes express functional androgen and estrogen receptors, which, upon stimulation by their respective ligands, have opposing effects. Testosterone induced podocyte apoptosis in vitro by androgen receptor activation, but independent of the TGF-β1 signaling pathway. Pretreatment with 17β-estradiol prevented testosterone-induced podocyte apoptosis, an estrogen receptor-dependent effect mediated by activation of the ERK signaling pathway, and protected podocytes from TGF-β1- or TNF-α-induced apoptosis. Thus, podocytes are target cells for testosterone and 17β-estradiol. These hormones modulate podocyte damage and apoptosis.

1, 25-dihydroxyvitamin D3 decreases adriamycin-induced podocyte apoptosis and loss

BACKGROUND

Selective proteinuria is frequently observed in glomerular diseases characterized by podocyte injury. Although, 1,25-dihydroxyvitamin D3 [1,25(OH)(2)D(3)] has potential therapeutic effects on chronic kidney diseases through decreasing podocyte loss, the mechanism underlying the beneficial effects of 1,25(OH)(2)D(3) on podocytes remains still unknown. The present study tested the hypothesis that 1,25(OH)(2)D(3) directly reduced podocyte apoptosis and loss.

METHODS

Sprague-Dawley (SD) rats were randomly assigned into three groups: Adriamycin (ADR) group (n=15), ADR+1,25-(OH)(2)D(3) group (n=16), and control group (n=16). Rats in ADR+1,25-(OH)(2)D(3) group were treated with 1,25(OH)(2)D(3) for 8 weeks. The number of podocytes and foot process width (FPW) were detected by transmission electron microscopy. The number of apoptotic podocytes per glomerulus and that of apoptotic nuclei and caspase-3 activity in cultured podocytes were determined by TUNEL staining. The average number of podocytes per glomerulus was quantified by immunohistochemistry. Expressions of p-Smad2/3, p-Smad1/5/8, Fas, Fas-Associated protein with Death Domain (FADD), Bax, and Bcl-2 proteins were examined by Western blot assay.

RESULTS

Compared with control group, proteinuria, FPW, apoptotic podocytes, caspase-3 activity, the protein expressions of p-Smad2/3, Fas, FADD, and Bax were significantly increased, podocyte density, p-Smad1/5/8 and Bcl-2 expression were decreased in ADR group. 1,25(OH)(2)D(3) significantly reduced proteinuria, FPW, caspase-3 activity, expressions of p-Smad2/3, Fas, FADD, and Bax and apoptosis of podocytes, but increased serum albumin, number of viable podocytes , p-Smad1/5/8 and Bcl-2 expression in ADR treated rats.

CONCLUSION

ADR-induced podocyte apoptosis was associated with the imbalance of p-Smad2/3, p-Smad1/5/8 the activity of caspase-3 and aberrant expressions of, Fas, FADD, Bax and Bcl-2. The beneficial effects of 1,25(OH)(2)D(3 )on podocytes may be attributable to inhibit podocyte apoptosis and the amelioration of podocytopenia.

CDK inhibitor p21 is prosurvival in adriamycin-induced podocyte injury, in vitro and in vivo

In response to injury, the highly specialized and terminally differentiated glomerular visceral epithelial cell, or podocyte, may undergo several cell fates, including dedifferentiation and proliferation, persistent cell cycle arrest, hypertrophy, apoptosis, or necrosis. Common to these potential outcomes of injury is their ultimate regulation at the level of the cell cycle. There is now a large body of literature confirming the importance of cell cycle regulatory proteins in the cellular response to injury. Although CDK inhibitor p21 levels increase in podocytes following injury, the role of p21 is unclear in focal segmental glomerulosclerosis (FSGS), in part because its function depends heavily on the cytotoxic stimulus and the cellular context. Adriamycin (ADR) is a podocyte toxin used to induce experimental FSGS. The purpose of this study was to define the role of p21 in ADR-induced podocyte injury. BALB/c mice, a strain carrying the recessive ADR susceptibility gene, were backcrossed against c57B6 p21-/- mice to yield a 12th generation BALB/c p21-/- strain. Experimental FSGS was induced by injection of ADR 12 mg/kg × 2 doses (n = 8/group), with mice killed at 1, 2, 8, and 11 wk. Diseased p21-/- mice demonstrated worse albuminuria, more widespread glomerulosclerosis, and higher blood urea nitrogen compared with diseased p21+/+ mice. In diseased p21-/- mice vs. p21+/+ mice, apoptosis [measured by TdT-mediated dUTP nick end labeling (TUNEL) assay] was increased, and podocyte number (measured by WT-1 immunostaining) was decreased. To validate these findings in vitro, we utilized differentiated mouse podocytes, p21-/- and p21+/+, exposed to 0.125 μg/ml ADR. Apoptosis, measured by Hoechst 33342 staining and TUNEL assay, was greater in cultured p21-/- podocytes compared with p21+/+ podocytes. Reconstitution of p21 via retroviral transfection rescued the p21-/- podocytes from apoptosis. We conclude that p21 is prosurvival in the podocyte's response to ADR-induced injury. Ongoing studies are defining the mechanisms of this protective effect as it relates to DNA damage and apoptosis.

PPAR-gamma agonist protects podocytes from injury

Podocyte injury and loss contribute to progressive glomerulosclerosis. Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a nuclear hormone receptor, which we have found to be increased in podocytes in a variety of kidney diseases. It is not known if PPAR-gamma contributes to renal injury or if it serves as a countermeasure to limit renal injury during disease progression. We tested these possibilities utilizing the puromycin aminonucleoside (PAN) model of renal injury in immortalized mouse podocytes. The cultured podocytes expressed PPAR-gamma mRNA at baseline but this was decreased by PAN. Pioglitazone, a pharmacologic agonist of PPAR-gamma, increased both PPAR-gamma mRNA and activity in injured podocytes, as assessed by a reporter plasmid assay. Further, pioglitazone significantly decreased PAN-induced podocyte apoptosis and necrosis while restoring podocyte differentiation. The PPAR-gamma agonist significantly restored expression of the cyclin-dependent kinase inhibitor p27 and the antiapoptotic molecule Bcl-xL while significantly decreasing proapoptotic caspase-3 activity. Pioglitazone tended to decrease PAN-induced transforming growth factor-beta (TGF-beta) mRNA expression. Our study shows that PPAR-gamma is normally expressed by podocytes and its activation is protective against PAN-induced apoptosis and necrosis. We postulate that this protective effect may be mediated in part by effects on p27 and TGF-beta expression.

Cumulative excretion of urinary podocytes reflects disease progression in IgA nephropathy and Schönlein-Henoch purpura nephritis

Recent studies have revealed that podocytopenia leads to glomerular scarring and that the loss of podocytes into the urine may be a cause of podocytopenia. The purpose of this study was to examine whether serial examinations of urinary podocytes (u-podo) could be a useful predictor of disease progression in children with glomerulonephritis. Urine samples and renal biopsy specimens from 20 patients (10 males and 10 females; mean age 11.8 yr; range 4 to 24 yr) with IgA nephropathy (n = 17) and Henoch-Schönlein purpura nephritis (n = 3) were analyzed. Forty-four renal biopsies were performed on 20 patients. Proteinuria (g/d per 1.73 m2), hematuria (score), and u-podo (cells/ml) were examined twice a month in 24 intervals between two biopsies (mean 16.7 mo; range 4 to 58 mo) and average and cumulative values were determined for the intervals. Renal histologic changes were scored on the basis of acute intracapillary, acute extracapillary, acute tubulointerstitial, chronic intracapillary, chronic extracapillary, and chronic tubulointerstitial lesions, as well as glomerulosclerosis. It was found that hematuria, proteinuria, u-podo, and acute lesion scores decreased during the intervals examined, whereas chronic lesion scores increased. Changes in acute histology scores correlated well with hematuria, proteinuria, and u-podo excretion, whereas chronic histology scores and glomerulosclerosis both correlated well with cumulative u-podo excretion. Patients with severe histologic progression of disease also had persistent u-podo excretion. These findings provide additional data to support a potential causative role for prolonged urinary loss of podocytes in disease progression in children with IgA nephropathy and Henoch-Schönlein purpura nephritis.

Puromycin aminonucleoside induces oxidant-dependent DNA damage in podocytes in vitro and in vivo

A decline in podocyte number correlates with progression to glomerulosclerosis. A mechanism underlying reduced podocyte number is the podocyte's relative inability to proliferate in response to injury. Injury by the podocyte toxin puromycin aminonucleoside (PA) is mediated via reactive oxygen species (ROS). The precise role of ROS in the pathogenesis of PA-induced glomerulosclerosis remains to be determined. We sought to examine whether PA-induced ROS caused podocyte DNA damage, possibly accounting for the podocyte's inability to proliferate in response to PA. In vitro, podocytes were exposed to PA, with or without the radical scavenger 1,3-dimethyl-2-thiourea (DMTU). In vivo, male Sprague-Dawley rats were divided into experimental groups (n = 6/group/time point): PA, PA with DMTU, and control, killed at days 1.5, 3, or 7. DNA damage was measured by DNA precipitation, apurinic/apyrimidinic site, Comet, and 8-hydroxydeoxyguanosine assays. Cell cycle checkpoint protein upregulation (by immunostaining and Western blotting), histopathology, and biochemical parameters were examined. DNA damage was increased in cultured podocytes that received PA, but not PA with DMTU. PA exposure activated specific cell cycle checkpoint proteins, with attenuation by DMTU. DNA repair enzymes were activated, providing evidence for attempted DNA repair. The PA-treated animals developed worse proteinuria and histopathologic disease and exhibited more DNA damage than the DMTU pretreated group. No significant apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining. A mechanism underlying the lack of podocyte proliferation following PA-induced injury in vitro and in vivo may be ROS-mediated DNA damage, with upregulation of specific cell cycle checkpoints leading to cell cycle arrest.

Cyclin I Protects Podocytes from Apoptosis

The limited regenerative capacity of the glomerular podocyte following injury underlies the development of glomerulosclerosis and progressive renal failure in a diverse range of kidney diseases. We discovered that, in the kidney, cyclin I is uniquely expressed in the glomerular podocyte, and have constructed cyclin I knock-out mice to explore the biological function of cyclin I in these cells. Cyclin I knock-out (-/-) podocytes showed an increased susceptibility to apoptosis both in vitro and in vivo. Following induction of experimental glomerulonephritis, podocyte apoptosis was increased 4-fold in the cyclin I -/- mice, which was associated with dramatically decreased renal function. Our previous data showed that the Cdk inhibitor p21(Cip1/Waf1) protects podocytes from certain apoptotic stimuli. In cultured cyclin I -/- podocytes, the level of p21(Cip1/Waf1) was lower at base line, had a shorter half-life, and declined more rapidly in response to apoptotic stimuli than in wild-type cells. Enforced expression of p21(Cip1/Waf1) reversed the susceptibility of cyclin I -/- podocytes to apoptosis. Cyclin I protects podocytes from apoptosis, and we provide preliminary data to suggest that this is mediated by stabilization of p21(Cip1/Waf1).

The podocyte's response to injury: role in proteinuria and glomerulosclerosis

The terminally differentiated podocyte, also called glomerular visceral epithelial cell, are highly specialized cells. They function as a critical size and charge barrier to prevent proteinuria. Podocytes are injured in diabetic and non-diabetic renal diseases. The clinical signature of podocyte injury is proteinuria, with or without loss of renal function owing to glomerulosclerosis. There is an exciting and expanding literature showing that hereditary, congenital, or acquired abnormalities in the molecular anatomy of podocytes leads to proteinuria, and at times, glomerulosclerosis. The change in podocyte shape, called effacement, is not simply a passive process following injury, but is owing to a complex interplay of proteins that comprise the molecular anatomy of the different protein domains of podocytes. These will be discussed in this review. Recent studies have also highlighted that a reduction in podocyte number directly causes proteinuria and glomerulosclerosis. This is owing to several factors, including the relative inability for these cells to proliferate, detachment, and apoptosis. The mechanisms of these events are being elucidated, and are discussed in this review. It is the hope that by delineating the events following injury to podocytes, therapies might be developed to reduce the burden of proteinuric renal diseases.

The cyclin-dependent kinase inhibitor p21 is required for TGF-beta1-induced podocyte apoptosis

BACKGROUND

Reduced podocyte number is a critical determinant in the development of glomerulosclerosis. Transforming growth factor-beta1 (TGF-beta1) induces podocyte apoptosis, but the cell cycle events are not known. The cyclin-dependent kinase (CDK) inhibitor p21 increases in podocytes in diseases where TGF-beta increases. Accordingly, we studied the role of p21 in podocyte apoptosis.

METHODS

Immortalized and primary p21+/+ and p21-/- mouse podocytes were used. Apoptosis was measured by Hoechst 33342 staining and caspase-3 activity following the exposure to TGF-beta1 or puromycin aminonucleoside. p21 and specific Bcl-2-related family proteins levels were measured by Western blot analysis. To prove a role for p21, we reconstituted p21 expression in p21-/- podocytes utilizing an adenovirus vector.

RESULTS

TGF-beta1 increased the protein levels of p21 in p21+/+ podocytes, and this coincided with apoptosis. In contrast, TGF-beta1 did not induce apoptosis in p21-/- podocytes. Restoring p21 expression increased apoptosis in p21-/- podocytes following exposure to TGF-beta1. TGF-beta1 increased the protein levels of an anti-apoptotic Bcl-2 in p21-/- podocytes, but not in p21+/+ podocytes. Moreover, TGF-beta1 did not increase Bcl-2 expression in p21-/- podocytes in which p21 expression was restored. Finally, puromycin aminonucleoside also induced apoptosis in p21+/+ podocytes, but not in p21-/- podocytes.

CONCLUSION

Podocyte apoptosis induced by TGF-beta1 and puromycin aminonucleoside requires p21, and Bcl-2 plays a crucial role downstream of p21 in mediating this effect. These results suggest that p21 may play a critical role in the decrease in podocyte number in disease status accompanied by increased TGF-beta1.

Cell death effects of resin-based dental material compounds and mercurials in human gingival fibroblasts

In order to test the hypothesis that released dental restorative materials can reach toxic levels in human oral tissues, the cytotoxicities of the resin-based dental (co)monomers hydroxyethylmethacrylate (HEMA), triethyleneglycoldimethacrylate (TEGDMA), urethanedimethacrylate (UDMA), and bisglycidylmethacrylate (BisGMA) compared with methyl mercury chloride (MeHgCl) and the amalgam component mercuric chloride (HgCl2) were investigated on human gingival fibroblasts (HGF) using two different test systems: (1) the modified XTT-test and (2) the modified H 33342 staining assay. The HGF were exposed to various concentrations of the test-substances in all test systems for 24 h. All tested (co)monomers and mercury compounds significantly (P<0.05) decreased the formazan formation in the XTT-test. EC50 values in the XTT assay were obtained as half-maximum-effect concentrations from fitted curves. Following EC50 values were found (mean [mmol/l]; s.e.m. in parentheses; n=12; * significantly different to HEMA): HEMA 11.530 (0.600); TEGDMA* 3.460 (0.200); UDMA* 0.106 (0.005); BisGMA* 0.087 (0.001); HgCl2* 0.013 (0.001); MeHgCl* 0.005 (0.001). Following relative toxicities were found: HEMA 1; TEGDMA 3; UDMA 109; BisGMA 133; HgCl2 887; MeHgCl 2306. A significant (P<0.05) increase of the toxicity of (co)monomers and mercurials was found in the XTT-test in the following order: HEMA < TEGDMA < UDMA < BisGMA < HgCl2 < MeHgCl. TEGDMA and MeHgCl induced mainly apoptotic cell death. HEMA, UDMA, BisGMA, and HgCl2 induced mainly necrotic cell death. The results of this study indicate that resin composite components have a lower toxicity than mercury from amalgam in HGF. HEMA, BisGMA, UDMA, and HgCl2 induced mainly necrosis, but it is rather unlikely that eluted substances (solely) can reach concentrations, which might induce necrotic cell death in the human physiological situation, indicating that other (additional) factors may be involved in the induction of tissue (pulp) inflammation effects after dental restauration.

Dexamethasone Prevents Podocyte Apoptosis Induced by Puromycin Aminonucleoside: Role of p53 and Bcl-2–Related Family Proteins

Nephrotic-range proteinuria is due to glomerular diseases characterized by podocyte injury. Glucocorticoids are the standard of care for most forms of nephrotic syndrome. However, the precise mechanisms underlying the beneficial effects of glucocorticoids on podocytes, beyond its general immunosuppressive and anti-inflammatory effects, are still unknown. This study tested the hypothesis that the synthetic glucocorticoid dexamethasone directly reduces podocyte apoptosis. Growth-restricted immortalized mouse podocytes in culture were exposed to puromycin aminonucleoside (PA) to induce apoptosis. Our results showed that dexamethasone significantly reduced PA-induced apoptosis by 2.81-fold. Dexamethasone also rescued podocyte viability when exposed to PA. PA-induced apoptosis was associated with increased p53 expression, which was completely blocked by dexamethasone. Furthermore, the inhibition of p53 by the p53 inhibitor pifithrin-alpha protected against PA-induced apoptosis. Dexamethasone also lowered the increase in the proapoptotic Bax, which was increased by PA, and increased expression of the antiapoptotic Bcl-xL protein. Moreover, the decrease in p53 by dexamethasone was associated with increased Bcl-xL levels. Podocyte apoptosis induced by PA was caspase-3 independent but was associated with the translocation of apoptosis-inducing factor (AIF) from the cytoplasm to nuclei. AIF translocation was inhibited by dexamethasone. These results show that PA-induced podocyte apoptosis is p53 dependent and associated with changes in Bcl-2-related proteins and AIF translocation. The protective effects of dexamethasone on PA-induced apoptosis were associated with decreasing p53, increasing Bcl-xL, and inhibition of AIF translocation. These novel findings provide new insights into the beneficial effects of corticosteroids on podocytes directly, independent of its immunosuppressive effects.

Cellular Response to Injury in Membranous Nephropathy

The pathogenesis of membranous nephropathy (MN) involves in situ formation of subepithelial immune deposits that produce glomerular injury by damaging and/or activating podocytes through complement-dependent processes. C5b-9 formation and insertion into podocyte cell membranes causes glomerular injury in MN. C5b-9 in sublytic quantities stimulates podocytes to produce proteases, oxidants, prostanoids, extracellular matrix components, and cytokines including TGF-beta. C5b-9 also causes alterations of the cytoskeleton that lead to abnormal distribution of slit diaphragm protein and detachment of viable podocytes that are shed into Bowman's space. These events result in disruption of the functional integrity of the glomerular basement membrane and the protein filtration barrier of podocytes with subsequent development of massive proteinuria. Complement components in proteinuric urine also induce tubular epithelial cell injury and mediate progressive interstitial disease in MN. Measurements of urinary C5b-9 or podocyte excretion in the urine may be useful in the diagnosis of MN and as measures of disease activity and response to therapy. Recent studies of cell-cycle proteins and DNA damage in podocytes have clarified why podocytes fail to proliferate in response to C5b-9-mediated injury and podocyte loss in MN, resulting in the development of glomerular sclerosis and renal failure. Improved understanding of the role of complement in the pathogenesis of MN and of the cellular response to C5b-9 attack creates several new opportunities for therapeutic intervention that may benefit patients with MN in the future.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Angiotensin II induces apoptosis in rat glomerular epithelial cells

ANG II has been shown to modulate kidney cell growth and contribute to the pathobiology of glomerulosclerosis. Glomerular visceral epithelial cell (GEC) injury or loss is considered to play a pivotal role in the initiation and progression of glomerulosclerosis. In the present study, we investigated the effect of ANG II on GEC apoptosis. Rat GECs were incubated with increasing doses of ANG II for variable time periods. Apoptosis was evaluated by cell nucleus staining and DNA fragmentation assay. ANG II induced GEC apoptosis in a dose- and time-dependent manner. The proapoptotic effect was attenuated by the ANG II receptor type 1 antagonist losartan or the ANG II receptor type 2 antagonist PD-123319 and was completely blocked by incubation with the combined antagonists. Moreover, ANG II stimulated transforming growth factor (TGF)-beta1 production as measured by ELISA. GECs exposed to TGF-beta1 demonstrated a dose- and time-dependent increase in apoptosis. ANG II-induced apoptosis was significantly inhibited by addition of anti-TGF-beta1 antibody. ANG II also upregulated the expression of Fas, FasL, and Bax and downregulated the expression of Bcl-2 in GECs. These studies suggest that ANG II induces GEC apoptosis by a mechanism involving TGF-beta1 expression that may, importantly, contribute to the pathogenesis of glomerulosclerosis.