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Messenger Ribonucleic Acid and Protein in Human and Rat Testis
Tampere University Hospital (R.S.), Division of Pediatrics, Tampere, Finland; Institut f. Anatomie und Zellbiologie II Heidelberg (A.V.), Germany; Departments of Pediatrics and Physiology (W.Y., J.T.), University of Turku, Finland; Department of Medical Nutrition, Huddinge (J.-Å.G.), Sweden; Department of Developmental Biology, Medical School (M.P-H.), University of Tampere and Tampere University Hospital (M.-P.H.), Department of Pathology, Finland
Address all correspondence and requests for reprints to: Dr. M. Pelto-Huikko, Medical School, University of Tampere, P.O. Box 607, 33101 Tampere, Finland. E-mail: blmapel{at}poph.uta.fi
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Abstract |
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(PPAR), a member of
the steroid hormone receptor superfamily, has been linked to lipid
homeostasis and tumorigenesis in tissues with high expression of
receptor protein. On the other hand, the role of PPAR in tissues
with a lower expression is not well known. Here we demonstrate the
localization of PPAR messenger RNA (mRNA) and protein in developing
and adult rat testis. Additionally, we demonstrate the expression of
PPAR protein in adult human testis. Our experiments with Northern
analysis, in situ hybridization and immunocytochemistry
reveal a complex distribution of PPAR in tubular and interstitial
cells of both adult and developing rat testis. The overall expression
is rather low but may be modified by exogenous or endogenous stimuli.
An up-regulation of PPAR mRNA could be observed after stimulation
with FSH. In the developing rat testis, a clear expression of PPAR
mRNA was present from the first days after birth. Additionally, PPAR
mRNA and protein increased toward adulthood. In adult human testis
PPAR immunoreactivity (IR) was present in interstitial Leydig cells
and tubular cells. In the seminiferous epithelium of adult human testis
the expression of PPAR-IR could be seen in meiotic spermatocytes,
spermatids and myoid peritubular cells. The findings of our study
suggest that PPAR may be involved in the regulation of growth and
differentiation of tubular and interstitial cells in rat and human
testis. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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,ß- or NUC1/
and 
) have been discovered of
which PPAR is the most investigated. Several peroxisome
proliferators including hypolipidemic drugs, plasticizers, synthetic
and naturally occurring fatty acids, herbicides, prostaglandins, and
leukotriene antagonists are able to activate liver PPAR (3, 4, 5).
Activation of PPAR in the liver enhances the transcription of genes
encoding several peroxisomal enzymes which are active in fatty acid
ß-oxidation. In case of chronic stimulation, this process leads to
peroxisome proliferation and hepatomegaly (6, 7, 8). The fact that chronic
activation of the receptor could induce hepatocellular carcinoma in
rodents fed with peroxisome proliferators linked PPARs also to
carcinogenesis (2). Concerning the effects on peroxisomes, there are
certain species differences. Peroxisome proliferation is mainly seen in
mouse and rat but not in guinea pig and monkey. In man, hypolipidemic
drugs reduce triglyceride levels but are unable to stimulate peroxisome
proliferation (2). According to the wide tissue distribution of PPARs
other functions like glucose and lipid homeostasis have been suggested
(9). Furthermore, PPAR prevents programmed cell death (apoptosis)
and induces cell growth (10). PPAR binds to a special response
element on target DNA (PPRE) by forming heterodimers with the retinoid
X receptor (RXR) (11, 12, 13). In addition to high expression in liver,
kidney, and adipocytes, lower expression of PPAR is found in heart,
skeletal muscle, small intestine, thymus, and testis (1, 9, 14, 15).
Little is known about the role of PPAR in tissues with a low
expression of the receptor like the testis. To investigate the cellular
localization and possible regulation of PPAR in developing and adult
rat and human testis, we employed Northern analysis, in situ
hybridization and immuno-cytochemistry. |
Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Human testis
Adult human testis material was obtained from biopsies of 27-
and 35- year-old healthy men who had previously been examined on
fertility at the laboratory of andrology of the University of
Tampere.
In situ hybridization and immunocytochemistry
Immunocytochemistry. For immunocytochemistry the animals
were perfused through the ascending aorta under fentanyl-fluanisone
anesthesia, first with physiological saline and then with a fixative
containing 4% paraformaldehyde in 0.1 M PBS for 3 min.
Subsequently, the testes were excised and further fixed by immersion at
4 C in the same fixative for 60 min. The samples were cryoprotected
with 20% sucrose in PBS, frozen with carbon dioxide, and 10 µm
sections were cut with a Microm HM 500 cryostat.
The human testicular biopsies were immersion fixed in Bouin’s fixative and embedded in paraffin. Thereafter, the sections were deparaffinized with xylene and rehydrated through graded series of ethanol. Subsequently the sections were subjected to microwave antigen retrieval treatment as described earlier by Shi et al. (16). Endogenous peroxidase activity was blocked by treating the sections with 0.1% hydrogen peroxide in PBS for 20 min.
The sections were incubated for 12–24 h at 4 C with rabbit antiserum
to rat PPAR (1:250–500) followed by biotinylated goat antirabbit
IgG and the ABC-complex (Vector Laboratories, Inc.,
Burlingame, CA). Diaminobenzidine was used as chromogen to visualize
the PPAR-IR. The characterization of the PPAR antibody used has been
published earlier (13). Controls included the omission of the primary
antibody and staining with nonimmunized rabbit serum (1:100). In
addition, the antiserum was preabsorbed with a lysate of Sf-cells
transfected with PPAR complementary DNA (cDNA) or with a lysate from
untransfected cells. Only the lysate from transfected cells abolished
all staining and only immunoreactivities not seen in the controls were
considered specific.
In situ hybridization. After decapitation of the animals the
testes were excised and frozen on dry ice. Serial 14-µm thick
sections were cut with a Microm HM-500 cryostat (Microm, Heidelberg,
Germany) and the sections were thawed on Probe On glasses
(Fischer Scientific, Pittsburgh, PA). Two oligonucleotide
probes directed against nucleotides 562–609 and 843–889
(corresponding to amino acids 62–78 and 116–131, respectively) of the
rat PPAR-cDNA (17) were used in this study. The sequences exhibited
less than 60% homology with any other known gene, when compared with
the known sequences in the GenBank database. Several control probes
with the same length, similar GC-content and specific activity were
used to ascertain the specificity of the hybridizations. The in
situ hybridization was carried out as described in detail
previously (18). The probes were labeled with
[-33P]dATP (NEN Life Science Products,
Boston, MA) using terminal deoxynucleotidyltransferase (Amersham Pharmacia Biotech, Buckinghamshire, UK) to a specific activity
of 6 x 109 cpm µg-1. The sections were
briefly air dried and hybridized at 42 C for 18 h with 5 ng
ml-1 of the probes in the hybridization cocktail. After
hybridization, the sections were rinsed four times at 55 C in 1 x
SSC for 15 min each and subsequently left to cool down for 1 h at
room temperature. The sections were dipped in distilled water,
dehydrated with 60% and 90% ethanol and air dried. Thereafter, the
sections were covered with Amersham Pharmacia Biotech
ß-max autoradiography film (Amersham Pharmacia Biotech,
Buckinghamshire, UK). Films were developed using LX24 developer and AL4
fixative (Eastman Kodak, Rochester, NY). Alternatively,
sections were dipped in NTB2 emulsion (Kodak) diluted 1:1
with distilled water and exposed at -20 C. The sections were developed
with D19 developer (Kodak), fixed with G333 fixative (Agfa
Gevaert, Germany), counterstained with cresyl violet and
coverslipped.
Sections from immunohistochemistry and in situ hybridization were examined under a Nikon FX microscope equipped with a PCO Sensicam digital camera (PCO, Kelheim, Germany). Images were processed using Corel Draw software (Corel Corporation Ltd., Ontario, Canada) and printed with a ALPS MD-2300 printer (ALPS Electric Ltd., Ireland).
Northern analysis
Transillumination-assisted microdissection of seminiferous
tubules. The rats were killed by CO2 asphyxiation and
the testes were excised and decapsulated. The seminiferous tubules were
teased free by fine forceps under a transilluminating stereomicroscope
in DMEM/F12 medium (1:1) (DMEM/F12; Gibco BRL, Paisley,
Scotland, UK) supplemented with 15 mM HEPES, 1.25 g/l
sodium bicarbonate, 10 mg/liter gentamycin sulfate, 60 mg/liter
G-penicillin, 1 g/liter BSA and 0.1 mM
3-isobutyl-1-methylxanthin (MIX; Aldrich Chemie, Steinheim,
Germany). The stages were recognized according to light absorption
criteria (19). For Northern analysis, pools of stages II-VI, VII-VIII,
IX-XII, and XIII-I, each containing a total of 10 cm of seminiferous
tubule segments were collected.
Tissue culture and stimulation. Twenty pieces of 5-mm-long seminiferous tubule segments were incubated in 1 ml above-mentioned culture medium in the presence or absence of FSH (rh FSH, Org 32489, 10.000 IU/mg; Organon, Oss, The Netherlands) in a concentration of 10 ng/ml for 30 h. The tubules were then collected for isolation of total RNA.
Northern blot hybridization. Total RNAs were isolated from
testes of rats at different ages and from the cultured seminiferous
tubules by a single step method (20). Ten to 16 µg of total RNA was
size fractioned in 1% denaturing agarose gel and transferred onto a
Hybond-N+ nylon membrane (Amersham Pharmacia Biotech). A 861-bp-long cDNA fragment of PPAR cut with
EcoRI and SacI was subcloned in a pGEM 4Z vector
for preparing a cRNA probe. The riboprobe was sunthesized using a
Riboprobe system II kit (Promega Corp., Madison, WI) and
32P-UTP (Amersham Pharmacia Biotech,
Aylesbury, UK). Hybridizations were performed according to the
instructions of the membrane manufacturer. After baking for 2 h at
80 C the filters were prehybridized in 50% formamide, 3x SSC, 5x
Denhardt‘s solution (1 mg/ml Ficoll, 1 mg/ml polyvinylpyrrolide and 1
mg/ml BSA), 1% SDS, and 10% dextran sulfate containing 100 µg/ml
yeast transfer RNA at 65 C for 6–16 h. Hybridization was performed at
the same temperature for 16–24 h by adding 32P-labeled
probe. After hybridization with PPAR-riboprobe, the blots were
stripped by pouring the boiling 0.1 SDS onto the membrane and cooled
down onto room temperature. The stripped membrane was subsequently used
for hybridization with a 28S cDNA probe labeled with
32P-dCTP (Amersham Pharmacia Biotech) by
random priming method (Prime-a-Gene Kit, Promega Corp.) at
45 C overnight. The filters were exposed to a Kodak XAR-5
film at -80 C between intensifying screens.
Densitometric analysis. The x-ray films of Northern hybridization were first scanned by a UMAX scanner (Super Vista S-20, Binuscan, Inc., NY) and a Binuscan Photoperfect software package (Binuscan). The images were analyzed using a TINA 2.0 densitometric analytical system (Raytest Isotopenmeßgeräte GmbH, Straubenhardt, Germany) according to the manufacturer’s instruction.
Replication of experiments and statistical analysis. All
experiments were repeated independently three times. In all Northern
hybridization analyses, the densitometric values of the signals of
PPAR messenger RNA (mRNA) were first normalized to 28S signals and
then the highest densitometric value was designated as 100%. Other
values were expressed as the percentages of the highest one. The values
from all the experiments were pooled for the calculation of the means
and their standard errors and for one way ANOVA and Duncan’s new
multiple range test to determine the significant differences between
different experimental groups using StatView 4.51 statistic program
(Abacus Concepts Inc., Berkeley, CA). The P values less than
0.05 were considered as statistically significant.
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Results |
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mRNA mRNA in all
stages of the seminiferous epithelium. Both of the transcripts showed
highest expression during stages II-VI, a slightly lower expression
during stages XIII-I and the lowest expression of PPAR mRNA during
stages VII-XII (Fig. 1
, A and B). An
up-regulation of PPAR mRNA expression could be seen in seminiferous
tubules cultured with FSH during all stages of the cycle but the
changes seen after FSH stimulation were clearly stronger with the
shorter transcript of 7.6 kb (Fig. 2, A
and B). Northern blot analysis of the developing rat showed the
strongest expression of PPAR mRNA at day 1 and 60 after birth. A
slight reduction occurred from days 5–30, whereas an increase could be
observed from day 30 toward day 60 (Fig. 3, A and B).
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mRNA could be seen in seminiferous tubules and insterstitial
cells of adult and developing rat testis (Fig. 4, a–e). In the adult rat, the highest
expression of PPAR mRNA was detected in cells of seminiferous
tubules located close to the basal membrane (Fig. 4c). An accumulation
of grains was seen over Sertoli cells and primary pachytene
spermatocytes. In the interstitium labeling was present in most of the
Leydig cells (Fig. 4d).
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mRNA could be seen in most of the
tubules and some interstitial cells (Fig. 4e
). Compared with the
1-week-old animal, expression of PPAR mRNA in the 2-week-old animal
was only slightly stronger (not shown). No labeling was present in the
control hybridization (Fig. 4b).
Immunocytochemistry. In the adult rat testis, PPAR
immunoreactivity (IR) was present in most of the seminiferous tubules.
The expression of PPAR-IR appeared in a stage-specific manner (Fig. 5a). In the tubules, the nuclei of
Sertoli cells and primary pachytene spermatocytes were intensely
stained during stages XIII-VI, whereas spermatogonia, other
spermatocytes, spermatids, or spermatozoa were devoid of staining (Fig. 5c). In the interstitium most of the Leydig cells showed strong nuclear
IR (Fig. 5d). Additionally, PPAR-IR could be seen in the nuclei of
some peritubular myoid cells (Fig. 5c) and in the interstitium in the
majority of endothelial and smooth muscle cells of blood vessels (Fig. 5d). The controls were devoid of PPAR-IR (Fig. 5b).
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-IR was seen in the seminiferous
tubules of 2-week-old animals in a stage-specific manner (Fig. 5e).
Also, some peritubular cells showed nuclear staining for PPAR. In
the tubules, PPAR-IR was localized in the nuclei of pachytene
spermatocytes and Sertoli cells (Fig. 5d). In the interstitium some of
the Leydig cell precursors showed PPAR-IR.
In adult human testis, PPAR-IR could be detected in the seminiferous
tubules and interstitial cells (Fig. 6, a–e). Strong PPAR-IR was present in leptotene, pachytene
spermatocytes and spermatids (Fig. 6, b–d). Sertoli cells,
spermatogonia, and spermatozoa were devoid of staining. In the
interstitium, Leydig cells demonstrated strong IR for PPAR (Fig. 6e). Also, nuclei of endothelial cells and smooth muscle cells of blood
vessels showed PPAR-IR (Fig. 6e). Additionally, PPAR-IR could be
observed in some peritubular myoid cells (Fig. 6d). No specific
staining could be observed in the controls (Fig. 6f).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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mRNA and protein in developing rat
testis during the differentiation of those cells in rat testis.
The finding that PPAR-IR could be observed in the nuclei of
pachytene spermatocytes and supporting cells in a stage-specific manner
suggests a specific receptor function during certain stages of the
cycle. The role of PPAR in the postnatal development of rat testis
is not known. However, since null mutant mice lacking PPAR protein
remain viable and fertile, PPAR does not seem to be essential for
rodent testicular development and fertility (8). Another possibility is
that compensatory mechanisms are activated which ensure a proper
testicular development in knockout animals.
Expression of PPAR mRNA and protein in the adult rat
testis
We observed two different transcripts of 8.5 and 7.6 kb,
respectively. This is not surprising because multiple transcripts of
mRNAs that are usually expressed as single ones have been reported in
testis for several proteins (21, 22). The expression of PPAR mRNA
and protein occurred in a stage-specific manner. Strong PPAR-IR
could be seen during stages XIII-VI in the nuclei of Sertoli cells
which are most sensitive for FSH during these stages (23). Northern
analysis also revealed the highest amount of PPAR mRNA during stages
II-VI and XIII-I of the cycle. This suggests that PPAR is controlled
by FSH and has a special functional role at the stages of the
seminiferous epithelial cycle. Indeed, Northern analysis showed an
up-regulation of PPAR mRNA after treatment with FSH. Surprisingly,
in comparison with the larger fragment of 8.5 kb the smaller fragment
of 7.6 kb showed stronger changes after FSH stimulation. It seems that
in rodent testis the two gene products of PPAR are differently
regulated suggesting also different functions for the transcripts in
the seminiferous epithelium.
In addition to expression of PPAR mRNA and protein in Sertoli cells
and some peritubular cells a strong expression could be observed in
primary pachytene spermatocytes. In contrast to our observations,
Braissant et al. (9) reported a weak expression of PPAR
mRNA and protein in Leydig and Sertoli cells, whereas a signal in germ
cells could not be observed. The physiological role of the receptor in
germ cells is not known. However, an important factor for optimal sperm
motility and fertility seems to be the fatty acid composition of
phospholipids in mammalian spematozoa (24). Especially the proportion
of long chain fatty acids of the n-3 series seems to improve avian and
mammalian sperm number and motility (25). In mammals the conversion to
n-3 long chain fatty acids occurs via peroxisomal ß-oxidation (26, 27). Because PPAR plays a key role in lipid ß-oxidation, it may be
an important factor in this process. It has to be kept in mind that
interactions between PPAR and other nuclear hormone receptors like
RXR, the thyroid hormone receptor and the estrogen receptor have been
established (28, 29). Because some of those steroid receptors are also
present in interstitial and tubular cells of the rat testis (9), it is
possible that PPAR in testicular cells has merely a modulatory
effect on other hormonal pathways. In addition to the possible
physiological role of the receptor several toxic effects of phthalate
esters on tubular and interstitial cells of rat testis may be explained
by the presence of PPAR in the affected cells (30, 31, 32).
The finding that PPAR protein could be seen in the nuclei of Leydig
cells is in concordance with the findings of Braissant et
al. (9). The possible physiological role of the receptor in Leydig
cells has been studied by Hegardt et al. (33). A diet with
the potent peroxisome proliferator DEHP (di-2-ethylhexyl phtalate)
could not induce HMG-CoA-reductase, a key enzyme in cholesterol
biosynthesis in Leydig cells. Neither could Etomoxir, an inhibitor of
fatty acid oxidation cause any decline in cholesterol biosynthesis.
This suggests that the control of steroidogenesis in Leydig cells is
not a main target for PPAR. PPAR-IR could also be observed in
endothelial and smooth muscle cells of blood vessels but a putative
role of the receptor in lipid homeostasis and possible
pathophysiological processes in these cells needs further
investigation.
Expression of PPAR protein in adult human testis
The pattern of PPAR-IR in the adult human testis differed to
some extent from the distribution of PPAR-IR in adult rat testis. In
human testis, a wider distribution could be observed in germ cells. In
addition to primary pachytene spermatocytes PPAR-IR was present in
the nuclei of leptotene spermatocytes and round spermatids. On the
other hand, in human testis PPAR-IR could not be seen in the nuclei
of Sertoli cells. Distribution of PPAR-IR in the interstitium and
peritubular cells was similar to the pattern seen in adult rat testis.
Previous investigations on human PPAR have shown that human and
rodent PPAR's differ in structure, function, and distribution (34).
The strongest tissue expression was observed in muscle, kidney, and
pancreas whereas the liver showed only a weak expression of PPAR
mRNA (34). To the best of our knowledge, the expression of PPAR in
human testis has not been previously reported. The function of PPAR
in human testis remains to be clarified. However, the more widespread
expression of PPAR-IR in human germ cells suggests a stronger
implication of PPAR on fertility in man than in rodents.
In conclusion, the present findings demonstrate the localization of
PPAR mRNA and protein in the nuclei of interstitial and tubular
cells of the developing and adult rat testis. Despite the fact that
PPAR is not essential for male rodent fertility, the changes in the
amount of receptor mRNA and protein observed in developing rat testis
may be important for the differentiation of testicular cells. In the
adult animal a stage-specific expression of PPAR mRNA and protein in
seminiferous tubules could be observed. Our observation that the
strongest PPAR-IR is present in Sertoli cells in a stage-specific
manner suggests a specific role for PPAR in the function of these
cells during certain stages of the cycle. This is also supported by the
finding that expression of receptor mRNA was up-regulated in animals
treated with FSH. Additionally, the stronger changes after FSH
stimulation seen with the shorter fragment of 7.6 kb suggests a
differential hormonal regulation and function of the two gene products
of PPAR in the seminiferous epithelium of rodents. The role of
PPAR in germ cells is not clear but may suggest a task for the
receptor in maintaining an optimal lipid content during maturation of
these cells. Because PPAR interacts with other steroid receptors it
is also possible that it may modify the signaling pathways of other
steroid receptors in tubular and interstitial cells. In addition to the
findings in rat testis, we demonstrate for the first time PPAR-IR in
adult human testis. Because PPAR-IR could be observed in most of the
meiotic germ cells, a more important role of PPAR in spermatogenesis
in adult human testis than in rat may be suggested.
Received September 28, 1998.
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References |
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, -ß and - in the adult rat. Endocrinology 137:354–366[Abstract]
-linoleic acid
on the phospholipid fatty acid composition and quality of spermatozoa
in cockerel from 24 to 72 weeks of age. J Reprod Fertil 110:53–59This article has been cited by other articles:
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