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Department of Veterinary Biosciences, University of Illinois, Urbana, Illinois 61802
Address all correspondence and requests for reprints to: Dr. Paul Cooke, Department of Veterinary Biosciences, University of Illinois, 2001 South Lincoln Avenue, Urbana, Illinois 61802. E-mail: p-cooke{at}uiuc.edu
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mRNA in these same cultures.
FSH (2.5–100 ng/ml) also produced a dose-responsive decrease in MIS
mRNA levels, and FSH and T3 together had an additive
inhibitory effect on MIS mRNA levels, indicating that these hormones
may act through distinct mechanisms. In summary, this is the first
primary culture system in which sustained MIS mRNA production can be
demonstrated, and it should prove useful for understanding the
regulation of MIS in developing Sertoli cells. In addition,
T3 and FSH are major regulators of the postnatal decrease
in MIS production by the rat Sertoli cell, and these hormones may act
through separate pathways. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In the male, MIS is produced only by Sertoli cells (5). In the testis, MIS receptor messenger RNA (mRNA) expression was originally reported to occur only in Sertoli cells (5, 9), but a recent report has indicated that Leydig cells also express MIS receptor mRNA (7). Thus, the testicular actions of MIS may result from both autocrine actions on the Sertoli cells as well as paracrine actions on adjacent Leydig cells.
The expression of MIS protein and mRNA is high before birth in the rat, then drops sharply during the postnatal period (10, 11). The apparent importance of MIS for various aspects of testicular development makes it critical to understand the factors that regulate the decline in the production of this hormone during neonatal life. One potential endocrine regulator of MIS expression during Sertoli cell development is thyroid hormone.
Extensive work in the past few years has indicated that thyroid
hormones are major regulators of Sertoli cell development. Transient
neonatal hypothyroidism in rats produces unprecedented increases of
80% and 140% in adult testis weight and sperm production,
respectively (12, 13). This increase in testis size results primarily
from increased Sertoli cell proliferation during neonatal and juvenile
life (14) and the consequently larger populations of adult Sertoli and
germ cells (14, 15). Sertoli cells express thyroid hormone receptor and
its mRNA during the neonatal period (16, 17, 18), and T3, the
biologically active thyroid hormone, inhibits Sertoli cell
proliferation in vitro (19). T3 stimulates
maturation of Sertoli cells (20) and has been shown to increase overall
protein synthesis and production of certain Sertoli cell proteins (21)
and decrease aromatase activity (22, 23). T3 treatment of
cultured neonatal Sertoli cells also increases their levels of mRNA for
inhibin-, a marker of Sertoli cell maturation
(19).2
Knowledge of the factors that regulate the postnatal decline in MIS is limited, due in large part to the inability of primary cell cultures to maintain MIS production in vitro (reviewed in Ref. 1). The aim of the present study was to develop a Sertoli cell culture system for examining hormonal regulation of MIS mRNA, then to test the effects of T3 and FSH, alone and in combination, on MIS mRNA production.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Culture of Sertoli cells from 2- and 5-day-old rats
Sertoli cells from 2- and 5-day-old Sprague-Dawley rats (day of
birth = day 0) were isolated using a sequential enzymatic
procedure that has been previously described (24, 25) and used
extensively in our laboratory to examine the effects of T3
on neonatal Sertoli cells (19, 26). Briefly, for each culture, pools of
Sertoli cells were obtained from 8–10 males from 2 litters. Sertoli
cells were grown in 24-well plates coated with Matrigel
(Collaborative Research, Waltham, MA) diluted 1:5 with
HBSS. Cells were plated at a density of 4 x 105
cells/well. The nutritive medium was DMEM supplemented with sodium
pyruvate (1 mM), nonessential amino acids (0.1
mM), and an antimicrobial solution (24, 25). Cells were
normally grown for 4 days in a humidified atmosphere of 95% air-5%
CO2 at 34 C. Medium was changed every 24 h.
Some cultures received T3 (Sigma Chemical Co., Inc., St. Louis, MO) at 0.01–1 nM or FSH (ovine FSH-18, USDA Animal Hormone Program) at 2.5–100 ng/ml. To determine possible additive effects of these hormones, some cultures received both T3 (0.1 nM) and FSH (100 ng/ml). T3 was dissolved in 0.025 N NaOH, then diluted with physiological saline to make a stock solution; FSH was dissolved in physiological saline. Other cultures were grown without hormonal supplementation (controls) and received only vehicle. In all cases, T3 was added at the initiation of the culture, whereas FSH was added at the beginning of the third day, and fresh hormone was added when medium was changed. After 4 days in vitro, some wells derived from the testes of 2-day-old rats were used to determine the purity of the cultured cells, as described below; the remainder were used to obtain RNA for Northern analysis.
Viability and purity of cultured cells
For all cell cultures described, the percentage of viable cells
was determined by trypan blue exclusion before plating. At the end of
the culture, wells derived from 2-day testes were stained for alkaline
phosphatase and 3ß-hydroxysteroid dehydrogenase (3ßHSD) to
determine peritubular and Leydig cell contamination, respectively, as
previously described (26). Leydig cells differentiate during early
postnatal life and may not stain as intensely for 3ßHSD during this
period as fully mature adult Leydig cells. In addition, recent results
have indicated that MIS may suppress 3ßHSD in Leydig cells (7).
Therefore, both intensely and weakly stained cells were considered
positive.
Northern analysis
Northern analyses were performed as described previously (26).
Briefly, total RNA was prepared from all cell cultures using the RNeasy
Mini Kit (Qiagen, Chatsworth, CA). Total RNA was also isolated from
2-day-old testis and from freshly isolated preparations of Sertoli
cells from 2-day-old testis, which served as positive controls for the
MIS and inhibin- probes, and from adult spleen, which served as a
negative control for these probes. Purified total RNA was dissolved in
diethylpyrocarbonate-treated water. The purity and concentration of the
RNA were determined by UV (260/280 nm) absorbance in a
spectrophotometer. Equal amounts of total RNA (8 µg) from the various
treatment groups were electrophoresed on a 1.5% agarose formaldehyde
gel. Gels were blotted to nylon membrane, and the RNA was fixed onto
the membrane by UV cross-linking.
The following probes were used in this study: 1) rat MIS (11); 2) rat
inhibin- (27), and 3) human 28S ribosomal RNA (rRNA) (28). The
complementary DNA (cDNA) inserts were isolated from the plasmid vector
by restriction digestion and gel purification. The insert was labeled
with [32P]deoxy-CTP using the Multiprime DNA labeling
system (Amersham, Arlington Heights, IL) and used to probe
the membrane.
All hybridizations were carried out in QuikHyb
(Stratagene, La Jolla, CA) according to the
manufacturer’s recommendations at 68 C in a Robbins Scientific
hybridization oven (Sunnyvale, CA). The hybridized membrane was washed,
covered with plastic wrap, and exposed to Kodak X-Omat x-ray film
(Eastman Kodak Co., Rochester, NY) with intensifying
screens. After hybridization with the MIS cDNA probe, some membranes
were stripped of probe by incubation in 50% formamide at 65 C for
1 h and then rehybridized with inhibin- cDNA probe. For
normalization of RNA load levels between lanes, the membrane was
reprobed a final time with labeled 28S rRNA cDNA probe.
The mRNA bands on the autoradiograms were scanned and quantitated using a computer-linked scanning laser densitometer and RFLPrint software (Pdi, Huntington Station, NY). Relative levels of mRNA transcripts were adjusted to compensate for differences in total RNA loaded per gel lane as determined by densitometry of 28S rRNA hybridization signals (18). All statistical analyses were performed using the SYSTAT statistical package (29). The optical densities of the bands in the various treatment groups were compared by two-way ANOVA, and differences between various treatment groups were compared using Tukey’s honest significant difference test. Differences were considered significant when P < 0.05.
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Results |
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5%) similar to those we have previously observed in Sertoli cells
from 5-day-old rats grown under the same culture conditions (26). A
small number of Leydig cells (<1% of the overall cell population)
could be identified; although these cells stained relatively lightly
for 3ßHSD, possibly as a result of the suppressive effect of MIS on
the expression of this steroidogenic enzyme (7), they could be clearly
identified among the surrounding unstained Sertoli cells.
Expression of MIS mRNA in Sertoli cell cultures from 2- and
5-day-old rats
Preliminary experiments detected only a faint 1.8-kb band,
previously shown to correspond to MIS mRNA (11), in 5-day Sertoli cells
cultured for 4 days. T3 treatment of these cultures
resulted in an undetectable MIS mRNA signal (data not shown). As
expression of MIS and its mRNA is high in the rat until birth, then
declines sharply during the neonatal period (10, 11), we reasoned that
utilization of Sertoli cells from younger rats might result in greater
MIS mRNA expression in vitro. MIS mRNA expression in freshly
isolated Sertoli cells from 2-day rats and in Sertoli cells from
2-day-old rats that had been cultured for 2 and 4 days is shown in Fig. 1
. High levels of a 1.8-kb band
corresponding to MIS mRNA were detected in freshly isolated Sertoli
cells from 2-day-old rats. The level of MIS mRNA in the cultured
Sertoli cells after 2 days in vitro was only 36% of that
measured in freshly isolated 2-day Sertoli cells, but rebounded to 67%
of the level seen in freshly isolated 2-day Sertoli cells after 4 days
in vitro. Additional work indicated that MIS mRNA declined
between days 4 and 6 in culture (data not shown), so the 4-day culture
period was used for subsequent experiments.
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).
However, the highest FSH dose (100 ng/ml), previously shown to produce
maximal effects on Sertoli cell proliferation and inhibin- mRNA
levels (19), produced a decrease in MIS mRNA expression of only 43%,
compared with the 86% decrease observed with T3 doses of 1
nM or higher.
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). T3 was given at a
0.1-nM dose in this experiment because earlier studies
indicated that the maximally effective T3 dose (1
nM) produced 86% suppression of MIS mRNA levels, and the
intensity of the Northern bands in this treatment group were near the
limit of detectability. Therefore, any additive effects of FSH with
this high dose of T3 would have been obscured by
difficulties in attempting to measure the MIS mRNA signal in this low
range, so a lower 0.1 nM T3 dose was chosen for
the studies involving treatment with both T3 and FSH. In
combination, T3 and FSH treatment resulted in an inhibition
of MIS mRNA (76%) that was significantly greater than that with either
hormone alone (Fig. 4).
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mRNA (Fig. 5). Therefore, the decreased MIS mRNA
levels in response to increasing T3 or FSH treatment do not
reflect decreased viability or activity of the cultured Sertoli cells,
but, rather, show an effect of the hormonal treatments.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The present results are the first to show that T3 can regulate MIS mRNA production in Sertoli cells. MIS mRNA levels in cultured Sertoli cells are very sensitive to T3, in that decreases of more than 50% are produced by as little as 0.01 nM T3, and maximal decreases are produced by 1 nM. The effect is also extremely robust, in that 1 nM T3 decreases MIS mRNA by over 85% compared with the control value.
The genes for MIS have been cloned in the rat and mouse (31, 32) as well as in other species, such as cattle and humans (33). Although full promoter sequences are not available for all of these genes, analysis of the promoter sequences available in GenBank did not reveal the presence of any of the various thyroid hormone response elements that have been previously described (34). In addition, T3 induces changes associated with increasing maturation in the neonatal Sertoli cell, such as decreased proliferation, altered production of various basement membrane components, increased production of secretory proteins typical of the mature Sertoli cell, and changes in the production of steroid hormones and their receptors (14, 19, 20, 22, 23, 26, 35, 36, 37). The lack of known thyroid hormone response elements in the available MIS promoter sequences in conjunction with the known stimulation of Sertoli cell development by T3 indicate that it is likely that the decreases in steady state MIS mRNA levels in response to T3 reflect general stimulatory effects of thyroid hormone on Sertoli cell maturation rather than direct effects on MIS mRNA production.
Thyroid hormone levels are low in the rat before and at birth, then rise sharply (4- to 5-fold) during the immediate postnatal period (30). Conversely, MIS production is high until birth, then falls dramatically shortly after birth (10). The temporal correlation between the increase in thyroid hormones and the decrease in MIS production by Sertoli cells during early neonatal life coupled with the ability of physiological levels of thyroid hormones to produce marked decreases in MIS mRNA production in cell culture suggest that T3 is a major regulator of the postnatal decrease in MIS production in vivo. T3 appears to act in concert with FSH, which also produces significant declines in MIS mRNA at physiological doses, to induce the decrease in MIS mRNA and protein production during the neonatal period. However, as low T3 doses produce significant declines in MIS mRNA levels, and physiological doses induce substantially greater suppression of MIS mRNA than even supraphysiological FSH doses, T3, rather than FSH, may be the main regulator of the progressive postnatal MIS decline.
Understanding of the factors that regulate MIS production in the Sertoli cell has been limited due to the lack of cell culture systems in which MIS production is maintained (reviewed in Ref. 1). Dutertre et al. (38) recently used targeted oncogenesis to develop a transformed Sertoli cell line that stably expresses MIS and its receptor, but previous attempts to examine hormonal regulation of MIS by primary Sertoli cell cultures were hampered by rapid declines in MIS mRNA and protein when the cells were placed in vitro. Vigier et al. (39), using Sertoli cells isolated from 15- to 25-day calves, reported that MIS in both the culture medium and the cells themselves fell approximately 95% between days 1 and 4 in vitro. Similarly, MIS production by cultured Sertoli cells taken from calves between birth and 1 week of age showed an approximately 95% decline in MIS production between days 1 and 3 of culture, and MIS production by these cells became undetectable by day 4 of culture (40). The one report looking at MIS mRNA production from human tissue in vitro reported similar large decreases. MIS mRNA levels in testis cells from 20-week-old human fetuses were decreased almost to the limit of detectability after 3 days of culture compared with the relatively high expression in intact human fetal testis (41).
In contrast, although MIS mRNA levels in 2-day-old Sertoli cells following 2 days of culture are only one third of those seen in freshly isolated Sertoli cells from 2-day-old rats, between days 2 and 4 MIS mRNA levels recover, and by 4 days of culture they are approximately two thirds of the level seen in the freshly isolated Sertoli cells from 2-day-old rats. This 33% fall over the 4-day culture period is very similar in magnitude to the normal fall in MIS expression that occurs between days 2 and 6 in neonatal rats (10). MIS mRNA expression in our cultured cells is therefore present at levels similar to what would be seen in Sertoli cells at a similar age, and this MIS mRNA expression is also relatively stable throughout the culture period, as opposed to the precipitous and progressive declines in either MIS protein or mRNA reported in other culture systems (39, 40, 41). Thus, this is the first primary culture system in which sustained MIS mRNA production can be demonstrated, and it should prove useful for studying the regulation of MIS in developing Sertoli cells.
Our cultures were grown on Matrigel, a reconstituted basement membrane derived from the EHS tumor cell line, whereas Sertoli cells in the earlier reports were grown on plastic (39, 40, 41). Sertoli cells grown on Matrigel are morphologically and functionally more similar to these cells in vivo than those grown on plastic (42), which become flattened and show pronounced changes in growth factor responsiveness and vectorial protein production. Thus, the Matrigel used in these cultures may be a critical factor in the maintenance of MIS mRNA production in our culture system.
Previous work in vivo has indicated that FSH treatment of newborn rat pups decreased MIS bioactivity compared with that in testes that did not receive the hormonal treatment, indicating that FSH may normally inhibit MIS expression (43). This conclusion has been confirmed by Kuroda et al. (44), who reported that FSH injection into neonatal rats reduced both MIS protein and mRNA. Similar results were later reported for rat fetuses injected with FSH during the last 2 days of gestation (45). In contrast, the previous studies with cultured neonatal calf Sertoli cells (39, 40) or human fetal testis cells (41) did not show an effect of FSH on MIS protein or mRNA production.
The present results are therefore the first demonstration that the inhibitory effects of FSH on MIS seen in vivo can also be obtained in cell culture. Furthermore, the magnitude of the decrease in MIS immunostaining obtained by Kuroda et al. (44) in neonatal rats following 4 days of FSH injection (40%) is in good agreement with the 35–55% decreases in MIS mRNA levels reported here for neonatal rat Sertoli cells exposed to various doses of FSH during a 4-day culture. More significantly, our present results indicate that FSH treatment of Sertoli cells results in decreased MIS mRNA levels, and therefore, the FSH effects seen in previous in vivo studies at least predominately reflect FSH actions on Sertoli cells, rather than a secondary effect(s) of FSH at a site other than the Sertoli cell, which then results in decreased MIS expression.
The reasons for the discrepancy between our results and those of the previous in vitro studies may involve the difficulties in assessing the effects of a hormonal treatment such as FSH when the overall production of MIS protein or mRNA is declining sharply, which would have made it difficult to resolve an effect of FSH in the earlier studies. The maintenance of a more normal morphology in the Sertoli cells cultured on Matrigel may also be a factor in allowing them to respond to FSH in vitro as they do in vivo, as discussed above.
The ability of T3 plus FSH to cause greater suppression of
MIS mRNA than a maximal dose of FSH alone suggests that these hormones
may be exerting their effects through different pathways. The ability
of the T3 and FSH combination to produce greater
suppression than only 0.1 nM T3 is clearly
consistent with this conclusion. However, as the administered dose of
T3 used was less than the maximally effective
T3 dose, it could not be concluded from comparing the
T3 and the T3 plus FSH groups alone that FSH
was acting through a separate pathway than T3, as a
T3 dose higher than the 0.1-nM dose used in
this experiment would itself produce greater suppression. Similar
results showing the additive effects of T3 and FSH on other
Sertoli cell parameters, such as production of inhibin- mRNA (19)
and androgen receptor mRNA (26), have been observed, although
T3 and FSH have opposite effects on Sertoli cell
proliferation (19).
The developmental changes seen in the expression of MIS closely parallel those reported for MIS mRNA (10, 11), and FSH treatment of rats decreases both MIS and its mRNA in vivo (44). These results suggest that the normal developmental changes in MIS production as well as hormonal effects on this process primarily reflect transcriptional changes. Thus, the effects of T3 and FSH on MIS mRNA levels reported here are presumably accompanied by similar changes in MIS production, although that cannot be established from the present data.
In conclusion, our understanding of the role of T3 in Sertoli cell development is still incomplete, but T3 appears to exert pleiotropic effects on Sertoli cell maturation. T3 induces production of secretory proteins, but also has effects on estrogen receptor expression and the production of aromatase, the enzyme that converts androgens to estrogens (22, 23, 35). T3 also stimulates the production of androgen receptor and its mRNA in vitro (26, 35), another process believed to be important for the normal maturation of Sertoli cells. The present results indicate that T3 also regulates MIS, most likely by an indirect mechanism related to its overall stimulatory effects on Sertoli cell maturation. Therefore, in addition to the effects of T3 on critical maturational processes, T3 appears to induce complex changes in the production and/or receptor expression for other hormones that themselves may be important modulators of Sertoli cell activity.
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Acknowledgments |
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cDNA
probes, respectively, and David Buchanan for proofreading the
manuscript. |
Footnotes |
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2 We inadvertently reversed the identification of
inhibin- and inhibin-ßB mRNA in our original paper on
this topic (18 ). Therefore, the transcript identified as
inhibin-ßB mRNA in that and a subsequent paper (18 19 )
is actually inhibin- mRNA, whereas the transcript identified as
inhibin- mRNA in the former paper is actually inhibin-ßB mRNA (see
erratum, Biol Reprod 59:216, 1998).
Received January 30, 1998.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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1 mRNA
expression in rat testis. Mol Endocrinol 8:89–96[Abstract]
and ß subunit
complementary deoxyribonucleic acids and expression in the ovary. Mol
Endocrinol 1:561–568[Abstract]
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