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Department of Biology (S.Y., T.U., L.L.E.), Trinity University, San Antonio, Texas 78212; Department of Cell Biology (S.O., D.L.R., J.S.R.), Baylor College of Medicine, Houston, Texas 77030; and Department of Gynecology and Obstetrics (S.F.), Kyoto University School of Medicine, Kyoto 606, Japan
Address all correspondence and requests for reprints to: Lawrence Espey, Ph.D., Department of Biology, Trinity University, San Antonio, Texas 78212. E-mail: lespey{at}trinity.edu
| Abstract |
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| Introduction |
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(TNF-
), which are
primary mediators of the acute phase response (4), have
both been firmly implicated in the ovulatory process
(5, 6, 7). These two cytokines activate a number of
inflammation-related genes, including TNF-stimulated gene-6 (TSG-6),
which is reportedly translated only in inflammatory reactions (4, 8). The present report describes the detection by RT-PCR
differential display of TSG-6 gene expression in rat ovarian follicles
that had been induced to ovulate by treatment with gonadotropin. TSG-6 is a so-called link protein that binds rather specifically to hyaluronan (HA) (4), a glycosaminoglycan that has a central role in the formation and stability of extracellular matrix (9, 10). Interestingly, when HA and TSG-6 exist independently of one another, HA reportedly has a proinflammatory effect, whereas the TSG-6 protein exerts an opposite effect (11). Nevertheless, in spite of its potent antiinflammatory properties, TSG-6 expression is considered to be an integral part of inflammatory processes [presumably affecting the early stages of an acute inflammatory response (4, 11)].
The distribution of HA in ovarian follicles has been studied extensively (10, 12). This glycosaminoglycan is thought to have an especially significant role in the expansion of the cumulus cell-oocyte complex (COC) shortly after the ovulatory process has been initiated by a surge in gonadotropin(s). Such hormone action induces a transient, but detectable, increase in the synthesis of HA by mural granulosa cells, along with a substantial increase in HA production by the cumulus granulosa cells (13, 14). In the latter instance, the accompanying expansion of the COC is thought to be important for ovulation and fertilization (12).
It has been reported recently that TSG-6 is also expressed in the COC of mice at approximately the same time as the ovulatory increase in HA synthesis (9, 15). In the present study, this member of the link module superfamily was discovered by PAGE display of RT-PCR products of messenger RNA (mRNA) extracts from rat ovaries. The results from in situ hybridization show that ovarian expression of TSG-6 mRNA extends beyond the cumulus oophorus and includes the mural granulosa. In addition, the report analyzes the temporal pattern of expression of this link-protein gene, and it assesses the affect of ovulation-inhibiting doses of indomethacin and epostane on ovarian expression of the gene.
| Materials and Methods |
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Differential display protocols that lead to detection of
TSG-6
The steps of the differential display were carried out as
described previously (16). In brief, RNA was extracted by
a standard guanidine isothiocyanate/cesium chloride procedure. RT- PCR
was performed using primers from an RNAimage Kit (G506, GenHunter
Corporation, Nashville, TN). The specific primer set that yielded
differentially expressed complementary DNA (cDNA) for TSG-6 was
5'-HTTTTTTTTTC-3' and 5'-HGGCTGAC-3', where H represents a HindIII
restriction site attached to the primers. After extraction and
reamplification of the differentially expressed cDNA, a standard
Northern analysis was performed to confirm the ovulation-specific
expression of the parent mRNA for TSG-6. The unique cDNA fragment was
cloned using a pCR-TRAP Cloning System (P404, GenHunter Corporation),
and a cloning colony containing the TSG-6 cDNA was identified by
secondary Northern analysis. Manual sequencing of the cDNA was
performed using a Sequenase Version 2.0 DNA Sequencing Kit (US70770,
Amersham Pharmacia Biotech, Piscataway, NJ). In
situ hybridization was performed as described previously
(16).
Statistical analysis
Densitometric analysis of the intensity of the signals from the
Northern blots was analyzed by the NIH-image program, as described
previously (16). Numerical data are presented as
means ± SEM. The significance of the differences
among the six principal time points of 0, 2, 4, 8, 12, and 24 h
after hCG was determined by Duncans multiple-range tests after a
completely randomized one-way ANOVA of the means of the groups. The
probability value used as the cutoff between significant and
not-significant levels was P = 0.05.
| Results |
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Effects of epostane and indomethacin on TSG-6 gene
expression
For these tests, Northern blots were prepared from RNA that
was extracted from control ovaries at 0 and 8 h into the
ovulatory process, or extracted from experimental ovaries that were
taken at 8 h after hCG from rats that had been treated 5 h
earlier with ovulation-inhibiting doses of epostane or indomethacin
(19). As in the Northern blotting tests at the six
different intervals during ovulation, the signal density (normalized
against the ß-actin control) of the 8-h lane was arbitrarily set at
100% (Fig. 3
). There was no detectable
expression of TSG-6 mRNA at 0 h, but substantial expression at
8 h. In animals treated with the antiovulatory agent epostane,
which blocks progesterone synthesis, the signal density of 74.1 ±
6.3% was not significantly different from the 8-h control value.
However, animals treated with the antiovulatory agent indomethacin,
which blocks prostanoid synthesis, had a TSG-6 mRNA level of 49.5
± 11.8%, which was significantly below the 8 h control value,
yet above the 0 h control value.
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| Discussion |
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and IL-1 through a promoter region that includes one AP-1 and
two NF-IL6 binding sites (21, 22). Also, the structure of
the TSG-6 protein has been characterized in detail (4, 9, 23, 24). TSG-6 is now recognized as a 35-kDa glycoprotein that is
one of the shortest members of a superfamily of HA-binding proteins
that includes link protein, aggrecan, versican, brevican, neurocan, and
CD44, along with TSG-6. The TSG-6 protein is composed of two structural
domains: an N-terminal hyaluronin-binding domain consisting of about
100 residues that are characteristic of proteins in the hyaladherin
family (4, 9), and a C-terminal CUB domain consisting of
about 110 residues that are characteristic of a number of diverse
proteins involved in development and differentiation (25, 26). Thus, a basic function of TSG-6 is presumed to be linkage
to hyaluronin and stabilization of the extracellular matrix in various
tissues during morphogenic processes. In the present study, ovarian TSG-6 mRNA expression increased to a peak within 4 h after the ovulatory process had been initiated by hCG. It remained elevated for an additional 4 h but then declined, by 12 h after hCG, to a level that was not significantly different from the 0-h control value. This temporal pattern of expression coincides with the increase in IL-1 in rat ovarian follicles in vivo and in vitro (27, 28, 29, 30). In assessing the spatial distribution of TSG-6 mRNA expression, this transcript followed a pattern most comparable with the increase in ovarian HA synthesis after injection of hCG into PMSG-primed mice (13). Not only was TSG-6 expression observed in COC, it was also intense in the mural granulosa cells that were most adjacent to the antrum but was minimal in the outermost layers of the mural granulosa cells.
Expression of ovarian TSG-6 mRNA also coincides with an increase in
TNF-
concentrations in gonadotropin-treated immature rats
(31). Therefore, IL-1 and TNF-
are both candidates as
promoters of TSG-6 transcription in ovulatory follicles. At the same
time, it is pertinent to take into account that the TNF gene can be
regulated by early growth response protein-1 (Egr-1) (32, 33), and expression of the gene for this zinc-finger
transcription factor precedes expression of ovarian IL-1, TNF-
, or
TSG-6 during the ovulatory process in the rat model (19).
Therefore, it seems likely that Egr-1 has at least an indirect role in
the induction of TSG-6 gene expression during ovulation. In the future,
it would be useful to conduct a well-controlled, comprehensive study of
the temporal and spatial expression of the ovarian genes for Egr-1,
IL-1, TNF-
, and TSG-6 to clarify the relationships among these
agents during ovulation. By analyzing such data concomitantly with
existing knowledge of the promoter regions of the genes for each agent,
their interdependence could be elucidated.
The specific function of TSG-6 in ovulatory follicles has not been
established. It seems relevant that the inflammatory cytokines IL-1 and
TNF-
commonly induce transcription of the gene for this member
of the link-module superfamily (4, 9). It is also relevant
that TSG-6 reportedly is produced only in instances of inflammation
(8). This information suggests that TSG-6 has a role in
acute inflammatory reactions and participates in extracellular matrix
(ECM) degradation associated with inflammatory processes. However, it
is not yet clear whether TSG-6 has a destabilizing, or a stabilizing,
effect on the ECM during local tissue remodeling that is characteristic
of acutely inflamed tissues (9, 11). The preponderance of
the evidence unexpectedly supports a stabilizing role for TSG-6. This
glycoprotein binds to HA and thereby stabilizes the ECM (4, 9). Furthermore, there is substantial evidence to show that
TSG-6 readily forms a stable complex with inter
-inhibitor (I
I), a
Kunitz-type serine protease inhibitor in the plasma (4).
This combination of TSG-6 and I
I inhibits plasmin, a serine protease
that activates matrix metalloproteinases that degrade ECM during
inflammatory reactions (4, 11). Thus, the synergistic
action of TSG-6 and I
I has an antiinflammatory effect, which
suggests a negative feedback role in moderating acute inflammatory
responses.
The hypothesis that TSG-6 binds to HA and interacts with I
I to
protect the COC matrix from degradation is a reasonable deduction
(15), because metalloproteinases (such as the
inflammation-related enzyme ADAMTS-1) are produced in substantial
amounts in the COC and in the stratum granulosum, in response to an
ovulatory dose of gonadotropin (34, 35). In any event,
there are recent reports that binding of I
I to HA in the COC is
important for optimal ovulation (36, 37). Therefore, TSG-6
could have some pivotal association with these factors that effect
stability of the ECM in the vicinity of the oocyte.
In summary, the present results show that ovarian TSG-6 mRNA is expressed chiefly in mural granulosa cells and in cumulus cells around the oocyte. Northern analysis reveals that the mRNA is transcribed relatively early during the ovulatory process, and it declines to nonsignificant levels even before ADAMTS-1 expression has reached its peak at 12 h after hCG (34). Also, the data show that TSG-6 expression is not affected by inhibition of follicular progesterone synthesis, and it is only moderately reduced by inhibition of follicular eicosanoid synthesis. Therefore, TSG-6 probably has a role in ovulation independent of these two well-known mediators of follicular rupture. In any event, the increase in TSG-6 mRNA, in the follicle in response to an ovulatory dose of gonadotropin, provides additional support for the hypothesis that the biochemical events of ovulation are comparable with an acute inflammatory reaction.
| Footnotes |
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Received May 24, 2000.
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-inhibitor
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binding to hyaluronan in the cumulus extracellular matrix is required
for optimal ovulation and development of mouse oocytes. Biol Reprod 61:436443This article has been cited by other articles:
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