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From the Department of Pediatric Urology, Children's Hospital of Chongqing Medical University, Chongqing, China.
| Correspondence to: Dr Dawei He, Children's Hospital of Chongqing Medical University, Department of Pediatric Urology, Chongqing, No. 136, Zhongshan 2 RD, Yuzhong District, Chongqing, China 400014 (e-mail: babyuro{at}163.com). |
| Received for publication May 9, 2006; accepted for publication October 9, 2006. |
| Abstract |
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Key words: Testis
Reproductive toxicity is a common side effect of anticancer drug chemotherapy, especially in young men of reproductive age (Byrne et al, 1987; Nicholson and Byrne, 1993; Pauwels et al, 1995; Rueffer et al, 2001). However, the mechanisms underlying the cytotoxic effects of alkylating agents and germ cell loss, as well as treatment effects on the vimentin cytoskeleton, are not completely understood. In the current study nitrogen mustard (bis-[ß-chloroethyl] methylamine; HN2) was used to investigate the effects of alkylating agents on vimentin filament mRNA and protein expression in vitro.
| Materials and Methods |
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Primary Sertoli Cell Culture![]()
Sertoli cells were isolated from 20-day-old mice as previously described
(Takase et al, 1988), without
the use of trypsin digestion. Briefly, 2 decapsulated testes were incubated in
0.5 mg/mL collagenase IV in phosphate-buffered saline (PBS; pH 7.4) at
34°C, shaken for 15 minutes to remove interstitial cells, and then washed
3 times. Sertoli cells and germ cells were separated by incubation with 0.1%
collagenase, 0.2% hyaluronidase, and 0.04% deoxyribonuclease I in PBS (pH 7.4)
at 34°C with shaking for 20 minutes. Sertoli cells were pelleted by
centrifugation (53 x g for 4 minutes), washed in PBS,
repelleted 3 times, resuspended in PBS, and subjected to hypotonic shock in a
dilute PBS solution. Cells were collected by centrifugation (130 x
g for 4 minutes), resuspended in PBS, filtered through a 60-µm
nylon mesh (Small Parts Inc, Miami Lake, Fla), washed, and resuspended in Ham
F12 Nutrient Mixture (F12)/Dulbecco Modified Eagle medium (DMEM) (1:1) tissue
culture media (Invitrogen Corp, Carlsbad, Calif). Sertoli cell number and
purity were estimated by hemocytometer and light microscopic analyses
(Takase et al, 1988),
respectively, with an average of 0.5 to 1 x 105 Sertoli cells
per testis obtained with approximately 90% purity.
Isolated Sertoli cells were cultured (34°C, 95% humidified atmosphere, 5% CO2, vol/vol) at high density (0.5 x 106 cells/cm2) on Matrigel-coated 12-well dishes in serum-free F12/DMEM (1:1, vol/vol) as described above. Increased Sertoli cell purity was obtained by hypotonic treatment (20 mM Tris [pH 7.4]) for 2.5 minutes to lyse contaminating germ cells 36 hours after plating (Galdieri and Zani, 1981), after which cells were washed twice to remove cellular debris. Medium was replaced every 24 hours, and cells were incubated for an additional 5 days. Cultures were terminated at specified time points for RNA extraction or for protein lysate preparation.
Sertoli Cell HN2 Treatment![]()
Sertoli cells isolated from 20-day-old mice testes were cultured for 5 days
and then incubated in DMEM/F12 with 10% fetal bovine serum containing 50, 100,
and 200 µmol/L HN2 (34°C, 5% CO2). Control cells were
brought to equivalent concentrations of isotonic NaCl solution. Morphologic
changes of Sertoli cells were observed per 30-minute intervals at 12-hour
exposure time points to 100 µmol/L HN2. Vimentin expression was
investigated by immunocytochemistry at 6 hours and 24 hours posttreatment and
reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot at
12 hours posttreatment with 50, 100, and 200 µmol/L HN2.
Immunocytochemistry![]()
Slides were air-dried and fixed in 4% paraformaldehyde (wt/vol in PBS; 10
mM sodium phosphate, 0.15 M NaCl [pH 7.4]) at 28°C for 20 minutes. Then
slides were incubated at 4°C overnight with the primary antibody diluted
in PBS containing 10% normal goat serum (NGS). Anti-human vimentin monoclonal
primary antibody and normal IgG for negative control (Santa Cruz
Biotechnology, Santa Cruz, Calif,) were diluted 1:50. After 2 washes in PBS,
sections were incubated with horse anti-mouse biotinylated Ig in 2% NGS
(1:200; Santa Cruz Biotechnology) for 30 minutes. Sections were then washed
twice in PBS, incubated with avidin-biotin complex at room temperature for 30
minutes, and developed with diaminobenzidine. Slides were counterstained with
Mayer hematoxylin, dehydrated, and mounted for microscopic examination.
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Western Blot![]()
Sertoli cells were homogenized in RIPA buffer (1% Triton X-100, 15 mM
HEPES-NaOH [pH 7.5], 0.15 mM NaCl, 1% sodium deoxycholate, 0.1% sodium dodecyl
sulfate [SDS], 1 mM sodium orthovanadate, 10 mM EDTA, and 0.5% protease
inhibitor cocktail) and stored at 80°C until analysis. Protein
concentrations were determined using the bicinchoninic acid method (Pierce,
Rockford, Ill) according to the manufacturer's specifications. Protein samples
were added to an equal volume of 2X loading buffer (100 mM Tris [pH 6.8], 4%
SDS, 0.2% bromophenol blue, and 20% glycerol). Supernatants were obtained by
centrifugation at 10 000 x g for 30 minutes, and samples were
then reduced with 0.1% ß-mercaptoethanol and boiled for 3 minutes.
Protein (30 µg) was separated on SDS-polyacrylamide (12%) slab mini gels.
Separated proteins were transferred (3 hours, 75 V) to Protran nitrocellulose
(Schleicher & Schuell, Keene, NH) with a Trans-Blot SD Semi-Dry
Electrophoretic Transfer Cell (Bio-Rad, Hercules, Calif), and membranes were
blocked at room temperature for 1 hour with 5% nonfat dry milk in PBS and
incubated with anti-vimentin primary antibody (1:1000) at room temperature
overnight in blocking solution. Membranes were washed 3 times with PBS (5
minutes each) and then incubated at room temperature for 30 minutes with
secondary anti-mouse horseradish peroxidase-linked IgG (1:3000, NA 931;
Amersham Pharmacia, Piscataway, NJ) in PBS. Signal was detected using the
SuperSignal WestPico Chemiluminescent kit (Pierce) according to manufacturer's
specifications.
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| Results |
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Sertoli cells treated with HN2 displayed comparatively smaller cell bodies, with disruption of the cytoskeleton and decreased cytoplasm. Our data also suggested that there was a nonsignificant increase in cell length, with narrow columnar or fibroblast-like appearance of cells observed following treatment with low concentrations of HN2 (50 µmol/L) (Figure 2B), while a significant decrease in cellular area was observed (P < .01) (Table 1). Increasing concentrations of HN2 had more dramatic morphologic effects, with reduction in cell size and cytoplasm, atrophy of the cell body, cytoskeletal disruption, gaunter cell shape, and increased detachment (Figure 2C), with a significant increase (P < .01) in cell length and decrease in cell area compared with controls (Table 1). Dramatic effects were observed following 12-hour HN2 treatment (100 µmol/L), with Sertoli cells observed to be much narrower and a noted breakdown of the vimentin cytoskeleton and intercellular conjunction complexes (Figure 2D through F).
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Effects of HN2 Treatment on Sertoli Cell Vimentin Structure and Expression![]()
When a labeled primary antibody to vimentin was used in control Sertoli
cells, vimentin intermediate filaments surrounded the nucleus and radiated out
from the nucleus to the cell periphery, terminating near points of contact
between the Sertoli cell and adjacent cells. The points of contact included
the tight junctions found between neighboring Sertoli cells. Exposure to HN2
caused vimentin intermediate filaments to collapse and Sertoli cell
conjunctions to detach; vimentin intermediate filament network was distributed
between nuclear and cell membranes, gradually notable with increasing
concentrations of HN2. Significant differences in vimentin expression were
induced by 6-hour and 24-hour HN2 treatment (50, 100, and 200 µmol/L) in
vitro. Vimentin expression decreased with increased treatment time at all
doses, compared with the strong staining noted in controls. Positive staining
was observed mostly in the cytoplasm
(Figure 3). Significant
time-dependent and dose-dependent differences were also noted in optical
density values of vimentin antibody (P < .01)
(Table 2).
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Semiquantitative RT-PCR![]()
To further elucidate the mechanisms underlying HN2-mediated Sertoli
cytoskeleton vimentin intermediate filaments disruption, we examined vimentin
mRNA expression using RT-PCR. Our data suggested that vimentin mRNA expression
decreased with HN2 concentration, and there were significant dose-dependent
differences in PCR products following HN2 treatment (P < .01)
(Figure 4).
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Western Blot Characterization of Vimentin![]()
Western blot analysis was used to evaluate vimentin protein expression
levels in Sertoli cells following HN2 treatment. Vimentin and ß-actin
were detected as single bands, as expected, at 57 kd and 42 kd, respectively.
Our data suggested that vimentin protein expression levels gradually decreased
with increasing concentrations of HN2, with significant differences noted
between different HN2 concentrations (P < .01)
(Figure 5).
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| Discussion |
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Previous research has suggested that Sertoli cells, the primary supportive cells of the seminiferous epithelium, play a key role in triggering and regulating spermatogenesis (de Kretser et al, 1998) and have a well-developed cytoskeleton that is responsible for the formation of the seminiferous epithelium and the development of germ cells (Boekelheide et al, 1989; Russell et al, 1989; Muffly et al, 1993; Tung et al, 1993). Morphologic studies have implicated the Sertoli cytoskeleton in the maintenance of cell shape, localization and transformation of cytoplasmic organoids, formation and stabilization of cell-cell or cell-extracellular matrix contacts, localization, anchoring and migration of developed germ cells, and the release of mature sperm from seminiferous epithelium. Vimentin is an important Sertoli cell cytoskeleton component. Vimentin intermediate filaments are mostly located around the nucleus, resulting in a clear zone free of organelles surrounding the nucleus (Aumuller et al, 1988; Aumuller et al, 1992). Although biologic role of intermediate filaments in Sertoli and other cells has not been fully elucidated, it is well established that they provide mechanical resiliency and strength to cells (Klymkowsky et al, 1989; Fuchs, Cleveland, 1998; Show et al, 2003).
We have shown herein that the Sertoli cell cytoskeleton responds to HN2 by exhibiting a marked collapse in vimentin intermediate filaments structure in vitro. Increasing concentrations of HN2 had more dramatic morphologic effects, with reduction in cell size and cytoplasm, atrophy of the cell body, cytoskeletal disruption, gaunter cell shape, and increased detachment. The changes were the result of disruption of vimentin intermediate filament network between nuclear and cell membranes, as seen by immunocytochemistry. Exposure to HN2 causes vimentin intermediate filaments to collapse and Sertoli cell conjunctions to detach, gradually notable with increasing concentrations of HN2. This change was the result of vimentin mRNA and protein expression down-regulation. Similarly, the collapse of the Sertoli intermediate filament cytoskeleton has been observed in cryptorchid testes of immature rats in vivo (Wang et al, 2002); in such testes, immunostaining of vimentin revealed loss of intermediate filament extensions and filament collapse to a perinuclear localization, coinciding with massive germ cell apoptosis. Cytoskeletal intermediate filament breakdown has also been linked to mono-(2-ethylhexyl) phthalate and colchicine treatment (Lloyd and Foster, 1988; Allard et al, 1993; Richburg and Boekelheide, 1996), resulting in loss of structural integrity of the seminiferous epithelium as well as germ cell apoptosis (Richburg and Boekelheide, 1996). Thus, loss of normal Sertoli intermediate filament dynamics has been shown to occur in concert with the failure of spermatogenesis after increased temperature (cryptorchidism) or toxic compounds. If this is generally the case, our data suggests that dysfunction of spermatogenesis induced by HN2 may be due to the disruption of the intermediate filament cytoskeleton in Sertoli cells.
In conclusion, this study demonstrated that exposure of Sertoli cells to HN2 in vitro resulted in intermediate filament cytoskeleton collapse, with a concomitant decrease in vimentin protein expression and down-regulation of vimentin mRNA. The change in the biochemical properties of vimentin may indicate that one of the mechanisms of reproductive toxicity resulting from HN2 is disruption of Sertoli cell vimentin filament structure, accompanied by a down-regulation of vimentin expression. Other testicular insults may result as well. However, future research is needed to confirm these effects in vivo and to further elucidate the role of vimentin intermediate filaments in Sertoli cells.
| Acknowledgments |
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| Footnotes |
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