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From the Renal Cell Biology Laboratory*
and the
Division of Pulmonary and Critical Care
Medicine,
Department of Medicine, University
of Miami School of Medicine, Miami, Florida; and the Division of
Nephrology and Dialysis,
IRCCS Policlinico
San Matteo, University of Pavia, Pavia, Italy
| Abstract |
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| Introduction |
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A common finding in kidney biopsies of CsA-treated animals is the presence of apoptosis, and several models of CsA-induced apoptosis have been described. Apoptosis has been shown in tubular and interstitial cells of CsA-treated animals3 and in tubular cells in vitro.4,5 In glomeruli, CsA treatment is also associated with altered expression of apoptosis regulatory genes such as Bcl-2, Bax, and Fas-L.6 In addition, renal tubular cells treated with CsA undergo apoptosis and show increased Fas expression.4 Although a decrease in apoptosis was observed in the kidneys of CsA-treated animals after nitric oxide blockade or angiotensin II type I receptor antagonism,3 a rescue factor from CsA-induced apoptosis has not been identified.
Several growth factors have been shown to provide protection against programmed cell death.7,8 HGF prevented the acute side effects of CsA in mice.9 Both HGF and IGF-I are modulated by CsA in vivo,10,11 and their ability to decrease apoptosis has been well established.12,13 Recently, it has been proposed that HGF protects tubular cells from apoptosis by a dual mechanism, one involving PI3'-kinase/Akt/Bad, and a second one mediated by a later induction of Bcl-xL expression.14 Bcl-xL, but not other members of the Bcl-2 family, were altered by HGF. It is not known whether the expression of Bcl-xL induced by HGF could be the effect of long-term PI3'-K activation by HGF. Here we examine whether CsA induces apoptosis in cultured podocytes and whether a regulation of Bcl-xL, Fas, and Fas-ligand occurs. Because epithelial cells express c-met, the receptor for HGF,15 and podocytes bind IGF-I,16 we study whether HGF and IGF-I prevent apoptosis, and whether Bcl-xL is regulated in a PI3'-K or a MEK-1-dependent manner.
| Materials and Methods |
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Glomerular epithelial cells derived from normal 4- to 6-week-old female B6SJLF1/J mice were used at passages 23 to 25. These cells have been described in detail previously.17 Cells were maintained in DMEM/F12 (3:1) medium (Gibco BRL, Rockville, MD) supplemented with 5% fetal bovine serum (Gibco BRL), 1 mmol/L glutamine (Biosource International, Camarillo, CA), 0.075% Na2HCO3 (Gibco BRL), 100 µg/ml penicillin-streptomycin (100 U/ml) (Biosource International), and trace elements (Biosource International). Cells were propagated in fibronectin-coated Nunc flasks.
Apoptosis Detection by ELISA
Cells were plated in 24-well plates. Cyclosporin A (IVAX Corporation, Miami, FL) was diluted in dimethylsulfoxide (DMSO). For dose-dependent experiments, CsA was added at concentrations of 0.1, 0.5, and 1 µg/ml when cells reached confluence. DMSO alone (final concentration 0.1%) was used to treat control cells. Cell lysates were collected after 24 hours of CsA exposure to investigate the presence of apoptosis. Apoptosis was quantitated by ELISA for the determination of histone-associated DNA fragments released into the cytoplasm during the apoptotic process, according to the manufacturers suggestions (Cell Death Detection ELISA PLUS, Boehringer Mannheim, Indianapolis, IN). Results are expressed as percent of control. For the time course study of apoptosis, a dose of 0.5 µg/ml was used, and apoptosis was examined after 1, 2, 3, 4, 6, 12, 24, and 48 hours of CsA exposure.
For protection experiments, recombinant human IGF-I (Biosource International) or recombinant heterodimeric human HGF-1 (kindly provided by Dr. Ralph Schwall, Genentech, San Bruno, CA, through the Miami VA Geriatric Research, Education, and Clinical Center, Miami, FL) were used. The protective effect of HGF or IGF-I on CsA-induced apoptosis was tested by pretreatment with either 50 or 100 ng/ml of both growth factors. A pretreatment for 1, 12, and 48 hours was performed with HGF or IGF-I (100 ng/ml). Cell layers were collected after 24 hours of CsA exposure.
Apoptosis Detection by Flow Cytometry
Cell death was determined by cell cycle analysis using fluorescence-activated cell sorter flow cytometry (FACS). Cell were plated onto fibronectin-coated 6-well plates and treated for 24 hours with either CsA (0.5 µg/ml) or DMSO (0.1%) as control. The cells were trypsinized and stained with propidium iodide plus RNase. The stained cells were analyzed by FACS (FACScan cytofluorometer, Becton Dickinson and Co., Franklin Lakes, NJ) and a CELL Quest software (Becton Dickinson) was used for analysis of the data. Cell cycle analysis indicates that the cells in G1, S, and G2/M phase were alive whereas the cells in pre-G1 phase (apoptotic phase) were not. The percentage of live or dead cells was calculated as cell number in pre-G1 phase/total cell number x 100.
Bcl-xL, Fas, and Fas-Ligand Analysis by Western Immunoblot
Cells plated onto fibronectin-coated 6-well plates were treated for 1, 12, or 48 hours with HGF or IGF-I (100 ng/ml). For blocking experiments, inhibitors of MEK-1 (PD098059, Calbiochem-Novabiochem Corp., San Diego, CA) and of PI3'-K (LY29400, Calbiochem-Novabiochem) were added 30 minutes before adding the growth factors. After 24 hours of CsA treatment, each well of a 6-well plate was washed twice with cold PBS and 100 µl of cold lysis buffer (20 mmol/L Tris, 140 mmol/L NaCl, 3 mmol/L EDTA, 10 mmol/L NaF, 10 mmol/L Na4P2O7, 2 mmol/L NaVO4, 10% glycerol, pH 7.4, 1% Triton X-100, aprotinin, leupeptin, PMSF) was added for 2 minutes before scraping the cells. Cell lysates were incubated 45 minutes at 4°C and the insoluble material was removed by centrifugation at 20,000 x g for 30 minutes at 4°C. Ten or 50 µg of protein for each sample were loaded on a 15% SDS-PAGE under reducing conditions after being heated for 3 minutes at 100°C. The proteins were electrotransferred to a nitrocellulose membrane (Amersham, Arlington Heights, IL). Blots were blocked for 1 hour at room temperature in Tris-buffered saline (TBS), 5% milk, 0.05% Tween-20. The membrane was incubated overnight at 4°C with a mouse monoclonal antibody against Bcl-xL (1:500 dilution, Santa Cruz Biotechnology, Santa Cruz, CA), followed by three washes (total time 35 minutes) with TBS 0.05% Tween 20. For Fas and Fas-ligand detection, rabbit polyclonal antibodies (Santa Cruz Biotechnology) against Fas and Fas-ligand were used at a concentration of 1:500 and processed as for Bcl-xL. The membranes were incubated with horseradish peroxidase-conjugated secondary antibodies (1:1000 dilution, Santa Cruz Biotechnology) for 1 hour at room temperature. Peroxidase activity was detected using a chemiluminescence kit (Santa Cruz Biotechnology). Data were analyzed on a Macintosh computer using the public domain NIH Image program (developed at the National Institutes of Health and available on the Internet at http://rsb.info.nih.gov/nih-image/).
Data Analysis
Each experiment was carried out in duplicate or triplicate and three or four independent experiments were performed. Results are expressed as means ± SD. Results were compared using analysis of variance (ANOVA). When ANOVA showed a statistically significant difference, a group-by-group comparison was performed using a t-test with Tukeys correction for multiple comparison. Statistical significance was set at P < 0.05.
| Results |
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CsA exposure induced apoptosis in podocytes in a dose- and
time-dependent manner. Apoptosis was detected after 24 hours of
treatment with CsA at concentrations
0.5 µg/ml (Figure 1A)
. Time course experiments using 0.5
µg/ml CsA for 1, 2, 4, 6, and 12 hours showed apoptosis only after 24
hours of exposure (Figure 1B)
. DNA fragmentation was detected at 24
hours, but there was no change in cell number. After 48 hours, both
apoptosis and a decrease in cell number were observed. CsA-induced
apoptosis was confirmed by FACS analysis. The percentage of cells
undergoing apoptosis increased to 10.5 ± 0.1% as compared to
6.1 ± 1.4% in control cells (P < 0.001)
(Figure 2)
. In addition, a significant
decrease in the percentage of cells in the S phase of the cell cycle
was observed (16.41 ± 4.1% versus 26.39 ±
1.5%, CsA-treated cells versus control cells respectively,
P < 0.05).
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Pretreatment with HGF for 1, 12, and 48 hours before CsA exposure
reduced apoptosis to control levels (Figure 3A)
. The percentage of cells undergoing
CsA-mediated apoptosis increased to 173.9 ± 13.1%
(P < 0.05 as compared to 100 ± 34.4%
control) an effect which was reversed by HGF pretreatment for 1 hour
(95.7 ± 20.6% versus 173.9 ± 13.1%,
P < 0.01), 12 hours (73.3 ± 10.2%
versus 173.9 ± 13.1%, P < 0.01) and
48 hours (69 ± 11.4% versus 173.9 ± 13.1%,
P < 0.05). The percentage of cells undergoing
apoptosis was decreased by pretreatment with 50 ng/ml HGF (140.7
± 14.76% versus 186.6 ± 8.00%, HGF + CsA
versus CsA alone, respectively), and was reduced to control
levels by 100 ng/ml HGF (85.98 ± 6.47% versus
186.6 ± 8.00%, HGF + CsA versus CsA alone,
respectively) (Figure 3B)
. When HGF and CsA were administered
simultaneously, there was no decrease in apoptosis.
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Effect of CsA and HGF or IGF-I on Bcl-xL, Fas and Fas-Ligand Production
CsA exposure (0.5 µg/ml for 48 hours) did not change Fas or
Fas-ligand production (Figure 5)
, but
reduced Bcl-xL production to 40.4 ± 15.7% of control (100
± 3.7%) levels (P < 0.05) (Figure 6A)
. However, in the presence of HGF
pretreatment, the reductions in Bcl-xL levels were inhibited
(72.88 ± 27.2% at 1 hour, 69.9 ± 12.4% at 12 hours, and
92.8 ± 8.2% at 48 hours, respectively) (Figure 6A)
. In contrast,
IGF-I pretreatment for 1, 12, or 48 hours did not affect the
CsA-induced Bcl-xL down-regulation (Figure 6B)
.
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PI3'-kinase inhibition by LY29400 completely abolished Bcl-xL
up-regulation by HGF in CsA-treated podocytes, although the inhibitors
of MEK-1 (PD098059) did not have this effect. In particular, a 50
± 9.6% reduction in Bcl-xL production in CsA-treated samples
receiving HGF pretreatment was observed when PI3'-kinase pathway was
inhibited by LY29400 pretreatment (Figure 7)
, which was not significantly different
compared to CsA-treated samples without HGF pretreatment.
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| Discussion |
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The mechanisms for CsA-induced apoptosis and for HGF and IGF-I protection are still a matter of investigation. A recent study suggested that CsA-induced apoptosis in the kidney could be mediated by a down-regulation of Bax/Bcl-2 as well as by an up-regulation of Fas-L.6 Immunofluorescence experiments conducted in this latter study showed that Bcl-2 and Fas-L were altered in the glomeruli as well as in the interstitial and vascular compartment of CsA-treated rats, suggesting that several pathways were probably involved. Although the ability of podocytes to express Fas and Fas-ligand is not known,23 we also tested if CsA regulates Fas/Fas-ligand. In contrast to what has been previously shown in renal tubular cells,4 we did not find changes in either Fas or Fas-ligand production in CsA-treated podocytes.
Although evidence exists for a PI3'-kinase and MEK-1 dependent regulation of apoptosis by IGF-I,24 less is known about the means by which HGF exerts a protective effect. A recent study suggests that both a rapid posttranslational PI3-K/AKT activation and a later Bcl-xL de novo gene expression could be involved in the HGF protection from apoptosis of human proximal tubular epithelial HKC cells.12 This specific regulation of Bcl-xL among various apoptosis regulatory proteins by HGF led us to hypothesize that Bcl-xL was involved in podocyte apoptosis, and that its levels were maintained by HGF when given before CsA treatment. Thus, we investigated whether CsA-induced apoptosis in podocytes was accompanied by an alteration in the levels of Bcl-xL. We found that CsA-induced apoptosis was associated with a down-regulation of Bcl-xL, and that HGF protects podocytes from CsA-induced apoptosis, probably by regulation of Bcl-xL in a PI3'-K dependent but MEK-1 independent manner. In addition, this effect was specific for HGF, because IGF-I did not protect from apoptosis in podocytes and did not modulate Bcl-xL production. However, the intracellular signaling pathways of both growth factors were functional in podocytes, as shown by the preservation of their effect on cell proliferation.
In conclusion, we found that CsA can induce apoptosis in podocytes in vitro and that this phenomenon could be completely prevented by HGF, but not IGF-I, pretreatment. In addition, we propose that the preservation of Bcl-xL levels by HGF could be responsible for its protection from CsA-induced apoptosis in podocytes. Our data suggest that a direct effect of CsA on podocytes may contribute to the glomerular damage observed in some patients, and that HGF could have a protective effect. Further studies will be needed to assess the clinical relevance of HGF in CsA-induced nephropathy, as well as in other glomerular diseases, such as rapidly progressive glomerulonephritis, HIV nephropathy, or lupus nephritis, where podocyte apoptosis is observed and could play a role in disease initiation and progression. Finally, the ability of CsA to induce apoptosis in podocytes raises some concerns about the widespread use of this drug in glomerulopathy with nephrotic syndrome resistant to conventional therapy,25 and should be further investigated.
| Footnotes |
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Accepted for publication September 21, 2000.
| References |
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