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Germ-Inserm U435 (C.C., H.K., A.-M.T., B.J.), Université de Rennes I, Campus de Beaulieu, Avenue du Général Leclerc, 35042 Rennes, Bretagne, France; Rhône-Poulenc Rorer (C.C., F.B.), Centre de Recherche de Vitry-Alfortville, Département de la Sécurité du Médicament, 94403 Vitry sur Seine Cedex, France
Address all correspondence and requests for reprints to: Dr. Bernard Jégou, Germ-Inserm U 435, Université de Rennes I, Campus de Beaulieu, 35042 Rennes, Bretagne, France. E-mail: bernard.jegou{at}univ-rennes1.fr
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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, induced IL-6 secretion in the compartment of
stimulation. In conclusion, the present study demonstrates that
vectorial secretory patterns of IL-1 and IL-6 production greatly differ
and that these cytokines are also differently regulated. These results
suggest that Sertoli IL-1 and IL-6 have different targets within the
testis and that, in normal and pathophysiological conditions, both the
tubular and the interstitial compartments may be influenced by the
action of these paracrine factors. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Recently, our laboratory and others have demonstrated that rat Sertoli
cells constitute an important source of interleukin-1 (IL-1) and
interleukin-6 (IL-6) within the testis (6, 7, 8). Furthermore, residual
bodies shed by late spermatids at the time of spermiation were found to
trigger Sertoli cell IL-1 production which, in turn, was found to
stimulate, by an autocrine mechanism, IL-6 secretion through activation
of leukotriene production (9, 10). IL-1 and IL-6 are generally known
to be key mediators of inflammation (11). In fact, in cultures of
monocytes/macrophages or lymphocytes, these cytokines are strongly
induced by lipopolysaccharide (LPS), a component of the cell wall of
Gram-negative bacteria (12, 13). Within the testis, in vivo
administration of phthalate esters, which are plasticizers present in
polyvinyl chloride plastics, was found to enhance the release of
Sertoli cell IL-1, concomitantly to an interstitial leukocyte
infiltration (14). Furthermore, in vitro treatments by
inflammatory mediators, including IL-1 and LPS, are known to increase
surface expression of Sertoli cell integrin ligands and to stimulate
IL-1 and IL-6 productions, as well as lymphocyte adhesion to Sertoli
cells (7, 8, 9, 10, 15). In addition to their involvement in inflammatory
events, IL-1 and IL-6 are also most probably involved in the local
control of spermatogenesis. In fact, IL-1 was found to stimulate DNA
replication in mitotic spermatogonia and in the meiotic spermatocytes
in vitro (16), and other experiments, in vitro,
have shown that IL-6 has the opposite effects (17).
Considering this dual involvement of cytokines in the pathophysiology (inflammation) and physiology (spermatogenesis) of the testis, it seems possible that chemical or infectious agents prone to increase the production of Sertoli IL-1 and IL-6 may also interfere with the spermatogenetic process. With this hypothesis in mind, we believe that the elucidation of the activating and secretory pathways of Sertoli cell cytokines would be very useful for the understanding of the IL-1 and IL-6 testicular sites of action and regulatory mechanisms.
In the present study, we have investigated the vectorial production of
IL-1 and IL-6 by rat Sertoli cells using a dual compartment culture
system and their possible regulation by factors originating from
infectious agents such as lipopolysaccharide (bacteria) and zymosan
(yeast), by FSH, or by the phagocytosis of latex beads or residual
bodies. In addition, we have examined the ability of IL-1 and -ß
to regulate vectorial IL-6 production by polarized Sertoli cells.
Transferrin being a major Sertoli cell product (1–4% of total
glycoproteins secreted by Sertoli cells; 1), vectorial transferrin
production was also measured in all culture conditions, as a
conventional marker of Sertoli cell polarized secretion.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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and -1ß (rhIL-1 and
-1ß), insulin, soybean trypsin inhibitor, DNAse, collagenase, trypsin
and hyaluronidase were obtained from Sigma Chemical Co. (St. Louis,
MO). Ovine FSH (NIDDK oFSH-17 was kindly provided by the NIH (Bethesda,
MD).
Preparation of Matrigel
Matrigel was extracted from EHS tumors according to the method
described previously by Kleinman et al. (18, 19). All the
extraction procedures were performed at 4 C. Briefly, tumors were
washed, homogenized in 3.4 M NaCl, 0.05 M
Tris-HCl, 0.004 M EDTA buffer, pH 7.4, containing 0.002
M N-ethylmaleimide and then centrifuged (15 min,
12000 x g) three times. The pellets were washed
overnight in 2 M urea, 0.05 M Tris-HCl, 0.15
M NaCl buffer, pH 7.4, and centrifuged twice (20 min,
24000 x g). The supernatants were pooled and dialyzed
against 0.15 M NaCl, 0.05 M Tris-HCl buffer, pH
7.4, containing chloroform (0.5%) for sterilization, for 2 h.
Supernatants were dialyzed against Tris-NaCl buffer twice more and a
last dialysis step was performed against culture medium. The Matrigel
was then aliquoted and stored at -20 C.
Isolation of residual bodies
Residual bodies were isolated from crude germ cell suspensions
prepared from adult rat testes according to methods previously
described by Meistrich et al. (20) and Pineau et
al. (21), combining a mechanical and enzymatical dissociation of
testicular tissues and centrifugal elutriation. Centrifugal elutriation
yields high enrichment of the residual body fraction (around 80–85%),
the major contaminants observed being early spermatids (5–10%), late
spermatids and sperm heads (10–15%).
Isolation and culture of Sertoli cells
Sertoli cells were prepared from testes of 20-day-old
Sprague-Dawley rats according to the method described by Toebosch
et al. (22) that reduced germ and peritubular cell
contaminations to, respectively, about 2% and 0.5%. Sertoli cells
were seeded at the density of 2.5 x 106 cells onto
Millipore inserts culture CM (polytetrafluoroethylene membrane, 
: 12
mm, pore size: 0.40 µm), previously coated with Matrigel diluted 1:8
with sterile water, and air-dried for 6 h, in 600 µl of DMEM
containing sodium bicarbonate (3.7 g/liter), HEPES (25 mM),
gentamycin (50 mg/liter) and insulin (10 mg/liter). The inserts (upper
compartment of the bicameral culture system) were placed in 24-well
Nunclon culture plates (lower compartment) containing 600 µl
DMEM/well. Cultures were maintained at 32 C in a humidified atmosphere
of 95% air-5% CO2 and media in each compartment were
changed every day. By days 2 and 3 of culture, the ability of Sertoli
cells to form a confluent monolayer allowing the creation of a
permeability barrier was assessed by the prevention of a hydrodynamic
equilibrium of media between the upper and lower chambers, as
previously described (23). On day 4 of culture, LPS (5–20 µg/ml),
zymosan (800 µg/ml), IL-1 or IL-1ß (30 U/ml) were added
separately to either the upper or the lower compartment, for 12 h.
In other experiments, Sertoli cells were also exposed, either from the
upper or the lower compartment, to LPS (20 µg/ml), for 3, 6, and
12 h. As phagocytosis of residual bodies or latex beads had been
shown to induce IL-1 and IL-6 production in previous experiments done
in a conventional culture system (7, 10) and because phagocytosis of
residual bodies occurs at the Sertoli cell apical pole in
situ (24), we performed another series of experiments where latex
beads (3.5 x 108/ml) or residual bodies (4.2 x
106/ml) were added to the upper compartment for 24 and
36 h. Sertoli cells were also exposed to increasing concentrations
of FSH (10–1000 ng/ml) added to the lower chamber for 12 h, FSH
receptors being, in situ, localized at the baso-lateral pole
of Sertoli cells (25). At the end of each experiment, media of the
upper and the lower compartments were collected separately, centrifuged
and stored at -20 C until IL-1, IL-6 and transferrin assays.
Bioassays of IL-1 and IL-6
IL-1 was quantified in the culture media using the murine
thymocyte assay (26) as described by Syed et al. (10) and
validated for Sertoli cell culture media by Gérard et
al. (6, 9). One unit (U) of IL-1 was defined as the quantity of
tested material required to double 3H-thymidine
incorporation into thymocytes, when compared with phytohemagglutinin
(PHA) stimulated cultures, PHA being a co-mitogen factor added to every
assay dish. In our assay, one unit of IL-1 corresponds to 0.32 ±
0.10 ng.
IL-6 was measured using the specific IL-6-dependent 7TD1 mouse hybridoma cell line proliferation assay (27) described and validated previously for Sertoli cell culture media by Syed et al. (7). In this assay, one unit was defined as the IL-6 concentration that gave half maximal proliferation and corresponds to 1 ± 0.27 pg of IL-6.
The intra- and interassay coefficients of variation were < 20 and 28% for the IL-1 bioassay, respectively, and < 10% for the IL-6 bioassay, for both coefficients of variation. All samples from each independent study were run in the same IL-1 or IL-6 bioassay. The results are expressed as U/600 µl of cell culture media of each compartment (U/chamber).
Neither IL- 1 nor IL-6 were detected in Matrigel coating the insert culture. None of the agents tested on Sertoli cells in the present study had any effect on proliferation of murine cells used for IL-1 and IL-6 bioassays.
Transferrin RIA
Rat transferrin (rTF) was measured using a specific
double-antibodies RIA method described previously (28). The intra- and
interassay coefficients of variation were < 6 and < 8%,
respectively. The results are expressed as ng/600 µl of cell culture
media of each chamber (ng/chamber).
Statistical analyses
The data are the mean ± SEM of groups
consisting of three dishes, each assayed for IL-1, IL-6, and
transferrin in triplicate. Each experiment was performed independently
at least twice, with representative results reported. Statistical
analyses were performed using a Student’s t test after
ANOVA. Significance was defined as P < 0.05.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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, A and B). Addition of LPS
markedly stimulated IL-1 production when added to the upper or to the
lower compartment (Fig. 1, A and B). Interestingly, IL-1 levels only
increased in the compartment where LPS had been added, whereas in the
opposite chamber, IL-1 production was only marginally stimulated.
Between the concentrations of 10 and 20 µg/ml of LPS which, in
control experiments, had no direct effect in the IL-1 bioassay, IL-1
levels plateaued at a value corresponding to 60 U/chamber, when LPS was
added to the upper chamber (Fig. 1A). However, when added to the lower
chamber, the same concentrations of LPS induced much higher IL-1
release: 100 and 140 U/chamber, at 10 and 20 µg/ml of LPS,
respectively (Fig. 1B).
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, C and D; P < 0.01). The
ratio of IL-6 between the upper and the lower chambers was unchanged
whatever compartment LPS was added to. However, apical exposure of
Sertoli cells to LPS induced only a slight but significant increase in
basal IL-6 production (Fig. 1C), whereas a marked stimulation in IL-6
secretion was measured in the upper compartment, when 20 µg/ml of LPS
was added to the lower chamber (Fig. 1D). At this concentration, LPS
had no direct effect in the IL-6 bioassay.
Under all culture conditions tested, Sertoli cell transferrin
production exhibited a marked apical preference, with an apical
versus basal ratio (RA/B) of about 2 (Fig. 1, E
and F) and no change in transferrin production was observed after
either apical or basal LPS stimulation.
To study the kinetics of vectorial IL-1, IL-6, and transferrin
production, Sertoli cells were cultured for different periods of time
in the presence or absence of 20 µg/ml of LPS. Similar to the results
obtained at 12 h (Fig. 1, A and B), after 3 h of culture,
Sertoli cells produced constitutive low levels of IL-1, with an equal
distribution between the apical and basal poles (Fig. 2A). Apical and basal IL-1 production increased
significantly with the time of culture, with no significant
modification of the apical vs. basal ratio. In confirmation
of our previous experiments (Fig. 1, A and B), exposure of Sertoli
cells to LPS markedly increased IL-1 production in the compartment
where LPS had been added, independent of the duration of the treatment.
LPS-stimulated IL-1 production (apical + basal) increased weakly
between 3 and 6 h, when it reached maximal levels, which were
maintained for 12 h for both apical and basal stimulation.
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), whatever the time of culture. Total IL-6
production (apical + basal) increased with the time of culture (4-fold
between 3 and 12 h). As previously seen (Fig. 1, C and D),
addition of 20 µg/ml of LPS to the apical compartment had no effect
on IL-6 secretion, whereas its addition to the lower chamber induced a
3-fold increase in the release of IL-6 in the upper chamber after
12 h.
Under control conditions, as well as after stimulation in the upper or
lower compartments, transferrin production occurred preferentially at
the apical pole (RA/B of about 2; Fig. 2C). Total
transferrin production and apical/basal ratio increased in a
time-dependent manner, and independent of the protocol used, no effect
of LPS was ever observed on transferrin release.
Vectorial production of IL-1, IL-6 and transferrin after zymosan
treatment
These experiments were designed to assess whether zymosan, a yeast
component known to induce IL-1 and IL-6 production by macrophages, like
LPS (29), would be able to stimulate production of these cytokines in
Sertoli cells. In fact, zymosan also markedly stimulated the release of
IL-1 in the compartment where it was added, without significant
modification of IL-1 production in the opposite chamber (Fig. 3A). Notably, as with LPS, basal stimulations
consistently induced higher IL-1 levels than apical stimulations, the
factors of stimulation being 6.5 and 18 for the apical and basal
stimulation, respectively.
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), whereas the addition of zymosan to the apical side
had no effect on IL-6 secretion.
No change in transferrin production was ever observed after either
apical or basal exposure of Sertoli cells to zymosan (Fig. 3C).
Effects of IL-1 on vectorial IL-6 and transferrin production
IL-6 production is known to be controlled by IL-1 in different
cell systems including Sertoli cells (7, 10). Therefore, we assessed
the effect of IL-1 on IL-6 production in the bicameral culture system.
As IL-1 exists in two forms (30) - IL-1 and IL-1ß - both these
forms were tested.
Whatever the compartment of stimulation, no effect of IL-1 was
observed on Sertoli cell vectorial IL-6 production (Fig. 4A). In contrast, exposure of Sertoli cells to IL-1ß
at their apical pole induced a 2.4-fold increase of IL-6 production in
the upper chamber. The addition of IL-1ß to the lower chamber also
induced a significant increase of IL-6 production in the same
compartment (2.4-fold) (Fig. 4A). Interestingly, under certain culture
conditions, IL-1 was found to stimulate Sertoli cell transferrin
production (Fig. 4B). However, the response to an apical stimulation by
IL-1 differed according to the IL-1 form used: IL-1 added to the
upper compartment had no effect on transferrin production, while
IL-1ß added to this compartment significantly enhanced both basal and
apical transferrin secretion (Fig. 4B). In contrast, a basal
stimulation by both IL-1 and -ß induced an increase in transferrin
release in the lower compartment, with no effect observed in the
opposite compartment.
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, A and B). In contrast, a significant increase in
transferrin production was observed in both upper and lower chambers
after a stimulation by 1 µg/ml of FSH at the basal Sertoli cell pole
(Fig. 5C), that is where FSH receptors are located in situ
(25).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The present study for the first time provides evidence that Sertoli cell IL-1 and IL-6 productions are also bidirectional. Interestingly, under control conditions, the apical/basal ratio of IL-1 and IL-6 differs considerably: whereas IL-1 production is not significantly different between the apical and the basal poles, IL-6 secretion always occurs with a marked apical preference. The observation that IL-1 production is evenly distributed between both poles, added to that establishing that type I IL-1 receptors are widely distributed among the various testicular cell types (36A ) supports the hypothesis of multiple targets of IL-1 within the testis (for review see Ref. 37). The fact that IL-1 is secreted at the basal pole of the Sertoli cells is consistent with the observation that type I IL-1 receptors are present on spermatogonia (36A ), which are situated in the basal compartment of the seminiferous tubules, and with the other observation showing that IL-1 can stimulate DNA replication in this germ cell category (16). In the interstitial compartment, the Leydig cells are another important target for IL-1 (36A, 37). Leydig cell testosterone production and proliferation are regulated by this cytokine (38, 39). It is therefore possible that basal Sertoli cell IL-1 also plays a role in the paracrine control of Leydig cell activity. In accordance with the ability of Sertoli cells to constitutively produce IL-1 in the apical compartment, IL-1 was detected in the efferent duct fluid, which supports a previous hypothesis by Syed et al. (40) that, in vivo, IL-1 is also secreted through the Sertoli cell apical pole into the tubule lumen.
As stated above, IL-6 constitutive production is clearly preferentially directed at the apical side. From this, it is tempting to hypothesize that IL-6 is particularly required within the adluminal compartment of the seminiferous tubules and may be in the seminiferous fluid. The presence of high concentrations of IL-6 in the human seminiferous plasma is consistent with this observation (41, 42). The fact that IL-6 is also produced basally is in accordance with the observation that this cytokine is a potent negative regulator of the replication of DNA in preleptotene spermatocytes and, to a lesser extent, in the A3-B spermatogonia (17), which are situated within the basal compartment of the tubules. Because IL-6 receptor messenger RNA have been detected in both Leydig cells and Sertoli cells (43), the bidirectional production of IL-6 also indicates that IL-6, like IL-1, probably has the potential for both autocrine and paracrine action within the testis.
In accordance with our previous studies in which Sertoli cells were
cultured in monolayers (9), our present results indicate that the
endotoxin LPS markedly stimulates the release of Sertoli IL-1 and IL-6.
However, the present results also demonstrate that LPS concentration
curves are different for the two cytokines: whereas LPS was stimulatory
for IL-1 at the lowest concentration used (5 µg/ml), higher
concentrations were required for IL-6 to be stimulated (10 µg/ml and
more). Furthermore, whereas near maximum IL-1 production was observed
after only 3 h of incubation of Sertoli cells with LPS, 12 h
were required for IL-6 to be stimulated. In addition to this, it also
clearly appears that vectorial production of both cytokines are
regulated differentially. Indeed, we demonstrate that IL-1 production
increases almost exclusively in the compartment of the bicameral
culture system to which LPS has been added. In contrast, IL-6 release
essentially occurs at the apical pole of Sertoli cells, and only when
these cells have been exposed to LPS in the basal compartment. That
IL-1 and IL-6 fluxes are differentially directed and regulated strongly
suggests that the targets of these cytokines are, at least to some
extent, different within the testis. We have also observed that unlike
LPS, IL-1ß increases Sertoli cell IL-6 production at the same pole it
had been added to. That Sertoli cell IL-6 can be stimulated by IL-1 has
been previously shown (7). However, when Sertoli cells were cultured in
a monolayer, we also shown that LPS effects on IL-6 were mediated
through Sertoli cell IL-1 release (10). This finding is in accordance
with our observations showing that, after addition of LPS, IL-1
stimulation occurs before IL-6 increases (10). The reason why, in the
present study, a dissociation was observed between LPS action on IL-6,
which occurred through a stimulation at the basal pole, and IL-1
action, which was mediated through the apical side, is obscure. It is
possible that, under the present experimental conditions, IL-6
secretion observed after either direct (IL-1) or indirect (LPS)
stimulations is, at least for a part, dependent on distinct regulatory
pathways. Despite the fact that both IL-1 and IL-1ß bind to and
act through the same receptor, the reason why in these studies IL-1
lacks effect on vectorial Sertoli IL-6 secretion, whereas it was very
potent in a previous work on Sertoli cell monolayers (7), is unknown.
It is noticeable though, that such divergences between actions of
IL-1ß and IL-1 have already previously been observed in other
studies on the testis (44, 45), for reasons that have not been
elucidated yet. Leydig cells and macrophages (46, 47) and possibly also
germ cells (48) have now been suggested to be sources of IL-1. The fact
that, in vitro, Sertoli cells have the potential to be
stimulated both through their basal and their apical poles, in turn,
suggests that in vivo Sertoli cells have the ability to
respond to IL-1 originating from the interstitial compartment, as well
as from Sertoli cell or germ cell IL-1 within the seminiferous
tubules.
The effects of LPS on vectorial production of IL-1 and IL-6 are identical to those of the yeast extract zymosan. We propose that the ability of Sertoli cells to produce these cytokines after exposure to these two activators originating from bacteria and yeasts reflects their capability to respond to an infectious aggression in situ, as monocytes/macrophages do (11). Infectious agents can penetrate the testis through blood and lymphatic vessels located in the interstitial compartment, which corresponds to the lower compartment of our culture system. By analogy to what is observed in the bicameral system in this study, we hypothesize that if these infectious agents reach the basal portion of the seminiferous tubules, they would trigger a basal flux of IL-1 from the Sertoli cells. It is, therefore, tempting to speculate that, in this pathological situation, the directioning of IL-1 by the Sertoli cell toward the interstitial compartment would result in a marked reduction in the intratubular concentrations of IL-1 and, thus, would prevent the occurrence of unwanted actions of this cytokine within the tubules. We have also observed that basal stimulations by both LPS and zymosan consistently induced higher release of IL-1 and IL-6 than apical stimulation by the same activators. Therefore, it appears that Sertoli cells have a relatively greater ability to respond to stimuli originating from the interstitial compartment, or from their base, than from apical stimuli. Because it has been previously demonstrated that IL-1 is a potent inhibitor of Leydig cell steroidogenesis (38, 49, 50), it is likely that the reduced steroidogenesis observed after an in vivo endotoxin treatment (38, 51, 52) could be due, to some extent, to the basal release of Sertoli cell IL-1. This, in turn, suggests that Sertoli cell cytokines are involved in the altered testicular functions frequently observed in male patients with serious illness (53, 54).
In contrast to previous results obtained in conventional
Sertoli cell cultures, when Sertoli cells were cultured in the
presence of residual bodies or latex beads (7, 9, 10) in the bicameral
culture system described here, no stimulation in bioactive IL-1 and
IL-6 levels was observed. This was unexpected, as there are strong
indications that, in addition to our own previous results (7, 9, 10),
residual bodies are indeed important regulators of Sertoli cell IL-1
production (55). These effects of residual bodies on both IL-1
bioactivity and messenger RNA levels most likely reflects the
physiological situation since, within the seminiferous epithelium, IL-1
levels rise dramatically between stages VII and VIII, when spermiation
and consequently phagocytosis of residual bodies occurs in
situ (56). In the bicameral system, the absence of cytokine
activation by latex beads and residual bodies could result from the
high density of Sertoli cells used. In fact, if this density is
required for optimal vectorial secretion of Sertoli cell products (23
and our own data), it leads to the superposition of two to several
layers of Sertoli cells. Under these conditions, our histological
studies show that phagocytosis of residual bodies and of the latex
beads almost exclusively occurred in Sertoli cells situated at the
surface of the cultures, (data not shown). Therefore, it may be that
not enough Sertoli cells were activated for changes in IL-1 and IL-6 to
be detectable. Furthermore, it is also possible that this high density
of Sertoli cells disrupts the transduction machinery induced by
residual bodies and latex beads phagocytosis. This shows that if the
bicameral culture system represents a very important tool to study the
vectorial secretion of Sertoli cell products, it does not necessarily
represent a simple reconstitution of the in situ situation,
as often thought. This is not the first time that marked differences in
the functionality of cultured Sertoli cells have been observed between
the conventional and the bicameral culture system. In addition to the
discrepancy described in the above discussion for IL-1, in contrast
to the situations observed in the monolayers, in the bicameral system,
no estrogen production was observed, and effects of FSH on transferrin
production were only seen with high concentrations of this pituitary
hormone (35, 36). The latter results on transferrin are in agreement
with the results herein presented. Also of note is the absence of
effect of FSH on IL-6 levels in our bicameral system, when a
stimulatory action of this hormone was previously shown in the
conventional culture system (7).
In conclusion, our results show that the site of activation and the vectorial production of IL-1 and IL-6 greatly differ, which suggests that these cytokines have different targets and functions. Furthermore, the patterns of IL-1 and IL-6 vectorial productions also suggest that both the tubular and the interstitial compartments are influenced by these local factors in the normal and pathological testis.
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Acknowledgments |
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Footnotes |
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2 During the course of this work, C. Cudicini was the recipient of a
Rhône-Poulenc Rorer fellowship.
Received October 17, 1996.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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production. Biochem Biophys
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release which triggers IL-6 production by an
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and ß on Leydig
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is
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stimulate expression of IL-1 mRNA in rat
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to LPS (20 µg/ml) added to either the apical
or the basal 
compartment for 3, 6, and 12 h. IL-1 (A),
IL-6 (B), and transferrin (C) were assayed in culture media from both
upper and lower chambers using the respective assays. Values are the
mean ± SEM of three dishes, each assayed in
triplicate. *, P < 0.05; **, P
< 0.01; ***, P < 0.001, as compared with the
respective unstimulated IL-1, IL-6 and transferrin levels.
