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Endocrinology Vol. 139, No. 5 2493-2500
Copyright © 1998 by The Endocrine Society


ARTICLES

Expression and Hormonal Regulation of Coactivator and Corepressor Genes

Silvia Misiti, Lutz Schomburg, Paul M. Yen and William W. Chin

Division of Genetics, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts 02115

Address all correspondence and requests for reprints to: Dr. Silvia Misiti, G. W. Thorn Research Building, Room 1005, Brigham and Women’s Hospital, 20 Shattuck Street, Boston, Massachusetts 02115. E-mail: misiti{at}rascal.med.harvard.edu


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Steroid/thyroid/retinoid receptors are members of the nuclear receptor superfamily and ligand-inducible transcription factors. These receptors modulate transcription of various cellular genes, either positively or negatively, by interacting with specific hormone-response elements located in the target gene promoters. Recent data show that nuclear receptors enhance or inhibit transcription by recruiting an array of coactivator and corepressor proteins to the transcription complex. We examined and compared the expression of four coactivator (steroid receptor coactivator-1 and E1A-associated 300-kDa protein) and corepressor (SMRT and N-CoR) genes in a number of tissues including several endocrine glands and cell lines. We also addressed whether their messenger RNA levels are hormonally regulated by studying the effects of thyroid hormone (T3) and estrogen (E2) treatment in rat pituitary cells (GH3) in vitro and in anterior pituitary in vivo. Our studies show that there are distinct tissue-specific expression patterns of these genes. We show that T3 and E2 regulate the expression of steroid receptor coactivator-1 messenger RNA in the anterior pituitary in addition to a gender-related difference. These tissue variations may have physiological implications for heterogeneity of hormone responses that are observed in normal and malignant tissues.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
STEROID hormones [estrogen receptor, thyroid hormone receptor (TR), and vitamin D3] receptors belong to a large superfamily of nuclear receptors (NR) that display substantial specificity in the control of gene expression (1). They also regulate many biological processes, including reproduction, differentiation, development, and homeostasis, by stimulating or repressing target gene expression. Transcriptional activation by steroid/thyroid hormone receptors may be divided into two different steps: derepression and activation. Repression is effected in part by association of unliganded receptors with NR corepressors such as N-CoR and SMRT (2, 3). The addition of a ligand results in conformational changes in specific receptor domains, with the displacement of corepressors, and recruitment of coactivators for trans-activation. The hormone drives the equilibrium in the direction of activation by destabilizing the repressive state and/or stabilizing the activation state (4). Recently, several coactivators that associate with these receptors and enhance their ability to trans-activate target genes have been cloned and characterized (for review, see Ref.5).

The recent discoveries that NR-associated coactivators and corepressors appear to be directly involved in chromatin remodeling have complicated this picture. The corepressors, SMRT and N-CoR, form complexes with histone deacetylases, suggesting that chromatin remodeling by histone deacetylation is a possible mechanism for receptor-mediated repression. Further, most of the coactivators have been found to possess intrinsic histone acetyltransferase activities (6, 7, 8, 9), in addition to several putative activation domains.

At the tissue/organ messenger RNA (mRNA) level, the coactivators and corepressors identified to date are ubiquitously expressed (2, 3, 10, 11), and most, if not all, cells contain both types of coregulators. However, the relative expression of coactivator and corepressor mRNAs has not been determined. In this study we examined the expression of known coactivator [steroid receptor coactivator-1 (SRC-1) (10) and E1A-associated 300-kDa protein (p300) (11)] and corepressor [SMRT) (3) and (N-CoR) (2)] genes in a large number of tissues, including several endocrine glands and cell lines, to provide a comparative analysis of their mRNA expression patterns.

The mRNAs encoding several members of the steroid/thyroid hormone receptor superfamily [TRs, retinoic acid receptors (RARs), and retinoid X receptors (RXRs)] have been shown either to increase or decrease in response to treatment with their cognate hormones (12, 13, 14) or with heterologous hormones [i.e. effect of estradiol (E2) on different TR and RXR isoforms (Schomburg, L., manuscript in preparation)]. In addition, the regulation of estrogen receptor mRNA by E2 differs in direction depending on the tissue (15), and there is a tissue-specific regulation of glucocorticoid receptor mRNA levels in states of glucocorticoid excess and depletion (16).

Thus, we next addressed whether coactivator and corepressor mRNA levels are hormonally modulated. We examined the effects of thyroid hormone (T3) treatment on the levels of mRNAs encoding SRC-1, SMRT, and N-CoR, which are factors that directly interact with TR, and of p300, which binds with SRC-1 (17, 18), in GH3 cells and in the anterior pituitary (AP), where all TR isoforms are present (19).

Inasmuch as there are examples of multihormonal regulation of genes in the pituitary (20, 21, 22), and TR and estrogen receptor (ER) both belong to the NR superfamily, we also studied the effects of E2 on the expression of SRC-1, p300, SMRT, and N-CoR mRNAs. Additionally, because the AP is influenced by sex steroids (23), we compared the levels of mRNAs for these cofactors in the pituitary glands of male and female rats.

In this report we show that there is a marked variation in the tissue-specific expression and differential hormonal regulation of the mRNAs encoding SRC-1, p300, SMRT, and N-CoR.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Chemicals
All chemicals were analytical grade. T3 and 17ß-E2 were obtained from Sigma Chemical Co. (St. Louis, MO); [{alpha}-32P]deoxy-CTP was purchased from New England Nuclear Corp. (Boston, MA), and the random primed DNA labeling kit was obtained from Stratagene (La Jolla, CA).

In vitro studies
GH3 cells were grown in DMEM supplemented with 10% FCS at 37 C in a 5% CO2 atmosphere. When the cells reached 50–75% confluence, the medium was exchanged for DMEM containing 10% charcoal- and AG1-X8 (Bio-Rad, Richmond, CA)-stripped FCS. After 48 h, the stripped medium was exchanged; vehicle, E2 (10 nM), or T3 (100 nM) was then added, and the dishes were incubated for the times indicated in Results.

In vivo studies
Adult male and female Sprague-Dawley rats (250 g; Charles River, Wilmington, MA) were used. The animals, maintained according to the guidelines of the Harvard Medical Area Standing Committee on Animals, had access to water and standard laboratory chow ad libitum. The animals were housed at an ambient temperature of 22 C with alternating 12-h light, 12-h dark cycles. Euthyroid animals received sc injections of E2 (0.5 µg/100 g BW), T3 (30 µg/100 g BW), or vehicle (sesame oil and 15% dimethylsulfoxide in 0.9% saline) and were killed after the time periods indicated. Immediately after decapitation, the tissues of interest were removed and frozen in liquid nitrogen.

Northern blot analysis
The frozen tissues (a pool of six APs or pineal glands, or 100 mg pooled hypothalamic fragments/preparation) or pellets containing approximately 5 x 107 GH3 (rat pituitary tumor), AtT20 (mouse pituitary tumor), Rat-1 (rat embryo), NIH-3T3 (mouse fibroblast), 293 (human embryonic kidney cells transformed by the early region of adenovirus), COS-7 (simian virus 40-transformed African green monkey kidney), CHOK1 (Chinese hamster ovary), CV1 (African green monkey kidney), and HeLa (human cervix epitheloid carcinoma) cells were homogenized in 5 ml SDS-Tris-based buffer [0.1 M Tris-HCl (pH 8.0), 0.5 M LiCl, 10 mM EDTA, 1% SDS, and 5 mM dithiothreitol] with the aid of a Teflon-glass homogenizer. Polyadenylated [poly(A)+]-enriched RNA was isolated directly from the homogenates using magnetic oligo-(deoxythymidine)25 polystyrene beads (Deutsche Dynal, Hamburg, Germany) according to the manufacturer’s instructions. Total RNA was prepared from cultured cells by the acid guanidinium thiocynate-phenol-chloroform procedure using RNAzol B (Biotex Laboratories, Houston, TX). Total and poly(A)+ RNA (10 µg/lane) was separated by electrophoresis in denaturing agarose gels (2.2 M formaldehyde and 1.5% agarose), transferred to nylon membranes by diffusion (Nytran NY 12 N, Schleicher and Schuell, Dassel, Germany), and cross-linked by UV irradiation. Hybridizations were performed under high stringency conditions (42 C, 16 h; in 50% formamide, 0.5% SDS, 100 µg salmon DNA, 0.9 M NaCl, 12 mM EDTA, and 0.09 M sodium phosphate, pH 7.4) with 50 ng complementary DNA (cDNA) fragments randomly labeled with [32P]deoxy-CTP to high specific activities (>109 cpm/µg). The following cDNA fragments were used as probes: a 0.8-kb HindIII fragment of human cDNA encoding SRC-1 (24), a 2.5-kb SacI fragment of human cDNA encoding TRAM-1 (25), a 0.8-kb XbaI-SmaI fragment of human cDNA encoding p300 (11), a 0.8-kb BamHI fragment of mouse cDNA encoding N-CoR (2), a 1.4-kb BamHI fragment of human cDNA encoding SMRT (3), and as a standard, a 0.7-kb fragment of rat cDNA encoding cyclophilin (26). The membranes were washed at a final stringency of 0.2 x SSPE-0.3% SDS (0.2 x SSPE = 30 mM NaCl, 2 mM sodium phosphate, and 0.2 mM EDTA, pH 7.4) at 60 C (mouse and human cDNA probes) and 65 C (rat cDNA probes), respectively. The membranes were analyzed by a PhosphorImager (Molecular Dynamics, Sunnyvale, CA; >24-h exposure) and then subjected to autoradiography (X-Omat, Eastman Kodak, Rochester, NY). All data were corrected for variability in loading by normalization to the amount of cyclophilin mRNA. Thereafter, blots were stripped by washing in 0.02 x SSPE-0.3% SDS at 90 C for 15 min. The complete removal of hybridizing cDNA probes was confirmed by autoradiography. The same membrane was used for the subsequent hybridizations with the other probes.

Statistical analysis
ANOVA was used to assess the statistical significance, and post-hoc comparison was made using Duncan’s new multiple range test.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Tissue distribution
We first studied the expression of coactivator and corepressor genes in different tissues and cell lines. The coactivators, SRC-1 and p300, and the corepressors, SMRT and N-CoR, were chosen for these studies. Northern blot analysis, using poly(A)+ RNA from various rat tissues, was performed with SRC-1, p300, SMRT, and N-CoR cDNA probes.

SRC-1 is expressed as a major RNA transcript of 7.5 kb in many tissues, including thyroid, pineal gland, adrenal, and hypothalamus. SRC-1 mRNA was present in all of the rat tissues examined, although the amounts varied among the organs tested, with relatively high levels detected in brain and pituitary. Next, we analyzed the expression of p300, SMRT, and N-CoR mRNAs, each migrating as an approximate 9-kb species. These mRNAs were detected in anterior pituitary, on which more extensive studies of hormonal regulation were performed, and in every rat tissue analyzed. The expression pattern for each cofactor mRNA was different (Fig. 1Go). Thus, SRC-1, p300, SMRT, and N-CoR genes are ubiquitously expressed, but also display tissue-specific expression.



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Figure 1. Northern blot analysis of SRC-1, p300, SMRT, and N-CoR mRNA levels in different rat tissues. Poly(A)+ RNA (10 µg/lane) from different rat tissues was fractionated on a denaturing gel, transferred to membrane, and hybridized against specific cDNA probes, as described in Materials and Methods. The cDNAs are SRC-1, p300, SMRT, N-CoR, and cyclophilin (Cyclo; a control for equal loading). The same blot was stripped and rehybridized with each probe to show the relative differences in expression; the blots shown were subjected to a final wash with a stringency of 0.2 x SSPE-0.3% SDS for 30 min at 60–65 C, and the hybridization signals were analyzed by a PhosphorImager (>24-h exposure).

 
Cell line distribution
Northern blot analysis of poly(A)+ RNA from several cell lines was performed using randomly labeled human SRC-1, p300, SMRT, and mouse N-CoR cDNAs. Figure 2Go shows a marked variation in the levels of coactivator and corepressor mRNA among the cell lines examined. For SRC-1, the major transcript (~7.5 kb) was highly expressed in GH3 cells (Fig. 2Go); this finding prompted us to choose this system for further hormonal regulation studies (Figs. 3Go and 5Go). In keeping with the ubiquitous expression of the p300 and N-CoR proteins in cell lines (2, 27), we detected their 9-kb mRNAs in all cell lines examined, with p300 mRNA being highly expressed in 293 cells and CV-1 (Fig. 2Go). The major N-CoR mRNA is about 9 kb; a minor species (~5.0 kb) was present in all cell lines. As the same band was not present in the tissue distribution blot (Fig. 1Go), it is unclear if represents a nonspecific signal or a minor transcript (Fig. 2Go). The highest level of SMRT mRNA (~9 kb) was detected in CV1 and COS-7 cells (Fig. 2Go), and no signal was present in GH3 cells. For all probes, additional, shorter transcripts could also be detected at lower abundance in different cell lines. SRC-1 belongs to a novel 160-kDa NR coactivator gene family that includes TIF2 (28) and TRAM-1 (25). To compare the expression of different members of this family, TRAM-1 recently cloned in our laboratory was included in these experiments (Fig. 2Go). The expression pattern of TRAM-1 mRNA (~9 kb) was different from that of SRC-1 in the same cell lines. The highest level of TRAM-1 mRNA was detected in COS-7 cells, and no signal was present in GH3 cells.



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Figure 2. Northern blot analysis comparing the relative expressions of SRC-1, TRAM-1, p300, SMRT, and N-CoR mRNA in different cell lines. Poly(A)+ RNA (10 µg/lane) from pellets containing approximately 5 x 107 cells was fractionated on a denaturing gel, transferred to membrane, and hybridized against specific cDNA probes as described in Materials and Methods. The cDNAs are SRC-1, TRAM-1, p300, SMRT, N-CoR, and Cyclo (a control for equal loading). The same blot was stripped and rehybridized with each probe to show the relative differences in expression. The blots shown were then subjected to a final wash with a stringency of 0.2 x SSPE-0.3% SDS for 30 min at 60–65 C, and the hybridization signals were analyzed by a PhosphorImager (>24-h exposure). Ethidium bromide staining of 28S and 18S ribosomal RNA appeared equivalent in each lane, except for GH3 RNA (~50%). TRAM-1 and SMRT mRNA signals were not detected in GH3 cells.

 


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Figure 3. T3 up-regulates SRC-1 mRNA levels in rat pituitary GH3 cells. Time course of the effect of T3 on the expression of SRC-1 in GH3 cells is shown. GH3 were cultured in medium containing stripped serum and 100 nM T3 for the indicated periods. The total RNA was extracted and fractionated on an agarose gel, transferred to a membrane, and hybridized with cDNA probes of SRC-1, as described in Materials and Methods. SRC-1 mRNA levels were internally standardized relative to levels of cyclophilin mRNA. Results are expressed as a percentage of the control value. Each bar represents the mean ± SD of six samples from three independent experiments. *, P < 0.01 vs. control.

 


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Figure 5. E2 down-regulates SRC-1 mRNA levels in rat pituitary GH3 cells. A time course of the effect of E2 on the expression of SRC-1 mRNA in GH3 cells is shown. GH3 were cultured in medium containing stripped serum and 10 nM E2 for the indicated periods. The total RNA was extracted and fractionated on an agarose gel, transferred to a membrane, and hybridized with cDNA probes of SRC-1, as described in Fig. 3Go. SRC-1 mRNA levels were internally standardized relative to levels of cyclophilin mRNA. Results are expressed as a percentage of the control value. Each bar represents the mean ± SD of six samples from three independent experiments. *, P < 0.01 vs. control.

 
Regulation by T3: in vitro
The rat pituitary GH3 cells were chosen as in vitro system to study the effects of hormones on the expression of SRC-1, p300, and N-CoR mRNAs. GH3 cells were maintained in medium containing stripped serum for 48 h and then were incubated for 6 and 24 h in the presence and absence of T3 (100 nM). Total RNA was isolated, and Northern blot analysis was performed using specific probes for SRC-1, p300, and N-CoR mRNAs as described in Materials and Methods. T3 treatment produced a significant increase in the SRC-1 mRNA level. The effect was 2.8-fold after 6 h of treatment and reached a maximum 4-fold above control values after 24 h (Fig. 3Go), with a return to basal values after 48 h.

In contrast, T3 treatment did not alter the p300 and N-CoR mRNA levels present in GH3 cells (data not shown). We also attempted to study SMRT mRNA levels in this system. However, the hybridization signals were not sufficient to be analyzed quantitatively. As a positive control, T3 treatment of GH3 cells resulted in an approximately 4-fold increase in GH mRNA in the same RNA samples, similar to previous reports (29).

Regulation by T3: in vivo
To study the effects of thyroid hormone on coactivator and corepressor mRNAs in vivo, euthyroid male rats received single (ip) injections of T3 and were killed 0, 2, 4, 6, 8, 24, 48, and 96 h after treatment. Pharmacological doses of T3 (30 µg/100 g BW) were injected to examine the rapid, hence probably direct, effects of thyroid hormone (30). Pituitary SRC-1 mRNA levels showed a slow and slight, but statistically significant, induction, reaching 1-fold above control values 24 h (Fig. 4Go). The same blots were stripped and rehybridized with the other probes. No significant T3-dependent changes were observed for p300, SMRT, and N-CoR mRNA levels in the anterior pituitary (data not shown).



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Figure 4. Time course of the effect of T3 on pituitary SRC-1 mRNA levels. Male Sprague-Dawley rats (250 g BW) received a single sc injection of T3 (30 µg/100 g BW). After the indicated time periods, the pituitaries were removed, and mRNA was prepared and analyzed by Northern blot hybridization, as described in Materials and Methods. For each point, 10 µg poly(A)+-enriched RNA were fractionated, blotted onto a nylon membrane, and probed successively with 32P-labeled cDNA fragments of the genes indicated. C, Control; Cyclo, cyclophilin. The same pattern was observed when this experiment was repeated.

 
Thus, SRC-1 was the only coactivator/corepressor mRNA shown to be regulated by T3 in these in vivo experiments. Although the induction is small, this effect correlates well with the T3 up-regulation of SRC-1 mRNA levels observed in GH3 cells in vitro (Fig. 3Go).

Regulation by E2: in vitro
GH3 cells were propagated in growth medium containing FCS, as described above, and then maintained in a defined medium containing stripped serum for 48 h before treatment. The cells were incubated in the presence or absence of E2 (10 nM) for 6 and 24 h. Total RNA was isolated and then subjected to RNA blot hybridization using specific 32P-labeled cDNA fragments. Treatment of GH3 cells with E2 decreased SRC-1 mRNA levels; within 6 h after E2, a decrease to 34% of the control levels was observed, which reached a minimum value (28% of the control) after 24 h (Fig. 5Go).

In contrast, E2 (10 nM) did not significantly alter the levels of the single, approximately 9-kb p300 and N-CoR mRNA species present in GH3 cells (Fig. 2Go). SMRT mRNA levels were not analyzed in this system because of the low expression in this cell line.

Regulation by E2: in vivo
We next evaluated the effect of estrogen (E2) on the expression of coactivator and corepressor mRNAs in vivo. Estrogen (single ip injection; 0.5 µg/100 g BW) regulated the expression of pituitary SRC-1 and SMRT mRNA levels in male rats. Two hours after injection, a significant decrease in SRC-1 mRNA levels was evident (Fig. 6AGo). The maximum suppressive effect was obtained 4–6 h after injection, reaching 56% of control values. SRC-1 mRNA returned to basal levels 48 h after hormone treatment. The effects of E2 on SMRT mRNA levels were more rapid and greater than those on SRC-1. Two hours after the injection of E2, SMRT mRNA levels increased to 1.8-fold the control levels, returned to basal values only 4 h after injection, and showed a significant decrease to 65% of control values 6 h after E2 injection (Fig. 6BGo). However, these SMRT mRNA levels did not return to basal levels even 48 h after injection.



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Figure 6. Time course of the effect of E2 on pituitary SRC-1, p300, SMRT, and N-CoR mRNA levels. Male Sprague-Dawley rats (250 g BW) received a single sc injection of E2 (0.5 µg/100 g BW) or vehicle (sesame oil). After the indicated time periods, the pituitaries were removed, and mRNA was prepared and analyzed by Northern blot hybridization, as described in Materials and Methods. For each point, 10 µg poly(A)+-enriched RNA was fractionated, blotted onto a nylon membrane, and probed successively with 32P-labeled cDNA fragments corresponding to the indicated genes. A, SRC-1; B, SMRT; C, p300; D, N-CoR. The same blot was stripped and rehybridized with each subsequent probe to show the relative differences in expression of each mRNA. The cyclophilin mRNA signals were used as internal standards. The two curves in each panel represent replicate data for the same treatments. The time point of each curve represents the mean of two samples internally standardized relative to levels of cyclophilin mRNA. The 0, 6, and 24 h points represent the mean of 10 samples, and the bars indicate the SD *, P < 0.01 vs. time zero.

 
When the poly(A)+ RNA of the hypothalamus and the pineal gland of these rats was analyzed by Northern blot hybridization 6 and 24 h after the injection of E2, both SRC-1 and SMRT mRNA levels were unaltered.

The same blots used to show down-regulation of SRC-1 and up-regulation of SMRT by E2 were stripped and rehybridized with p300- and N-CoR-specific probes. Again, E2 did not change p300 and N-CoR mRNA levels (Fig. 6Go, C and D), as expected from the in vitro studies.

Gender-related difference
A tissue-specific gender difference was observed in the expression of SRC-1 steady state mRNA levels in anterior pituitary. Preparations from female APs contained 40% less SRC-1 mRNA than those from male rats (Table 1Go). This result may be correlated with the down-regulation of SRC-1 mRNA levels by E2 treatment that we observed in GH3 cells and in AP by Northern blot analysis. These gender differences were specific for SRC-1 mRNA levels and were not observed in hypothalamic and pineal gland preparations. In addition, the levels of p300, N-CoR, and SMRT mRNAs did not display gender differences in AP (Table 1Go), hypothalamus, or pineal gland (data not shown).


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Table 1. Anterior pituitary (AP) SRC-1 mRNA levels display gender differences

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
A number of coactivators and corepressors that interact with NR has recently been identified and characterized (5). From in vitro binding studies, it appears that both can bind simultaneously to NR under appropriate conditions (4). Thus, one can hypothesize that the complex process of transcriptional activation of the steroid/thyroid hormone receptors is influenced by the relative amounts of these components. These ratios may be modulated by the type of ligand (agonist, antagonist, etc.), the structure of the target gene promoter, and tissue-specific expression of endogenous coactivators and corepressors. Other studies suggest that NR with their coregulators may form multiprotein complexes to regulate the expression of their target genes (9). Different cell types may have different relative compositions of these coregulators associated with NR. Accordingly, we have investigated the expression and hormonal regulation of four coactivator (SRC-1 and p300) and corepressor (SMRT and N-CoR) genes in a number of tissues and cell lines.

Our studies show that the tissue-expression patterns of these genes vary among the tissues and cell lines examined, suggesting that although the coactivator and corepressor genes are ubiquitously expressed, their relative expression is tissue dependent. Additionally, the presence of smaller mRNA transcripts that seem to be tissue specific raises the possibility of tissue-specific alternatively spliced mRNA isoforms encoding these coregulators. Hayashi et al. suggested that a splicing variant of SRC-1, named SRC-1E, is expressed more abundantly than SRC-1, and this variant enhances T3/TR-mediated trans-activation more efficiently than other isoforms (31). Although generally the abundance of coactivator and corepressor mRNAs reflects the abundance at the protein level, further studies are required to determine the protein content for each coregulator and its variants.

The marked variation observed in coactivator and corepressor mRNA expression prompted us to study the possible hormonal modulation of their genes. Regulation of the NR expression is a common mechanism for modulating cell responses to hormones, neurotransmitters, and growth factors, and it is well known that the NR gene expression is hormonally regulated (32). Individual factors in a given multiprotein complex may serve as communicators that integrate distinct physiological signals. This concept has been suggested by the recent findings that the activity of the TR coactivator, Trip 230, is negatively regulated by Rb, a protein for cell cycle progression and cellular differentiation (33), and that the retinoids up-regulate corepressor SMRT, but not coactivator Trip1, mRNA levels in neuroblastoma cell lines (34).

We have investigated the effects of T3 and E2 on SRC-1, p300, SMRT, and N-CoR gene expression in rat pituitary cells (GH3) in vitro and in AP in vivo. The AP is a good system for our studies because it expresses a wide variety of peptide and steroid hormone receptors, and it is the only gland in which all TR isoforms are present (35, 36).

Our results show that T3 up-regulates SRC-1 mRNA levels in GH3 cells and in the AP. However, the magnitude of the effect in vivo was lower and slower compared with the in vitro effect. The difference may be explained by the presence of a mixed population of five different endocrine cell types in the AP gland and by the fact that the somatotrope, from which GH3 is derived, accounts for only 50% of the adenohypophyseal cells (37). Further immunocytochemistry studies are in progress to assess the cellular distribution of SRC-1 expression in the AP. In addition, Hodin et al. have shown that T3-mediated changes in ligand binding in the pituitary may provide only a partial reflection of the changes in TR expression occurring within the pituitary because of the differential effect of T3 on TR isoform mRNA expression in the pituitary (32). We did not observe any major T3 effects on p300, SMRT, and N-CoR mRNA levels when the same experimental conditions were used. Nonetheless, SRC-1 mRNA levels were stimulated by T3, although the magnitude of the change in vivo was slight.

We have shown that estrogen has opposing effects on SRC-1 and SMRT gene expression. In fact, we found a consistent decrease in SRC-1 mRNA levels within 4–6 h after treating rats with E2 and a rapid increase in SMRT mRNA levels within 2 h of treatment, followed by an equally rapid decrease. E2 also did not affect the p300 and N-CoR gene expression in our in vitro and in vivo experiments. Although the physiological relevance of these coactivators and corepressors remains poorly understood, we suggest that tissue sensitivity to hormones may be modified by the relative abundance of SRC-1, p300, SMRT, and N-CoR. In tissue in which the coactivators and corepressors may be limiting, even small variations in their mRNA levels could influence the balance of these proteins, and thus the hormonal response. For example, the responses to ligands of the estrogen receptor is tissue dependent (38).

In agreement with our finding that the SRC-1 mRNA levels are down-regulated by estrogen, we observed a significant gender difference in SRC-1 gene expression in the pituitary, with higher mRNA levels found in preparations from male rats than in those from female rats. The biological significance of this gender difference in SRC-1 mRNA levels is also unclear. We believe that it might be related to a sex-specific difference in the distribution of subpopulations of pituitary cells. Additional in situ hybridization and immunohistochemistry studies must be performed to determine the cellular distribution and the relative levels of coactivator and corepressor mRNAs and proteins in the different populations of AP cells.

In conclusion, we show tissue to tissue variation in the expression of SRC-1, p300, SMRT, and N-CoR genes. These findings are consistent with our hypothesis that the relative expression of coactivator and corepressor genes may contribute to the heterogeneity of hormonal responses observed in normal and malignant tissues. We also show that the hormonal status differentially affects the expression of the coactivator and corepressor genes. These hormonal effects may not only modulate target gene regulation, but also may have physiological consequences for normal programs of growth and development and for cellular function.


    Acknowledgments
 
We thank Dr. N. Koibuchi for helpful discussions. We also thank Drs. A. Takeshita, D. Livingston, R. Evans, and C. Glass for providing the h-SRC-1 and h-TRAM-1, h-p300, h-SMRT, and m-N-CoR cDNAs, respectively.

Received October 13, 1997.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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A Corepressor and Chicken Ovalbumin Upstream Promoter Transcriptional Factor Proteins Modulate Peroxisome Proliferator-Activated Receptor-{gamma}2/Retinoid X Receptor {alpha}-Activated Transcription from the Murine Lipoprotein Lipase Promoter
Endocrinology, April 1, 1999; 140(4): 1586 - 1593.
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J. Biol. Chem.Home page
A. Z. Chaudhry, A. D. Vitullo, and R. M. Gronostajski
Nuclear Factor I-mediated Repression of the Mouse Mammary Tumor Virus Promoter Is Abrogated by the Coactivators p300/CBP and SRC-1
J. Biol. Chem., March 12, 1999; 274(11): 7072 - 7081.
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J. Biol. Chem.Home page
E. Treuter, L. Johansson, J. S. Thomsen, A. Warnmark, J. Leers, M. Pelto-Huikko, M. Sjoberg, A. P. H. Wright, G. Spyrou, and J.-A. Gustafsson
Competition between Thyroid Hormone Receptor-associated Protein (TRAP) 220 and Transcriptional Intermediary Factor (TIF) 2 for Binding to Nuclear Receptors. IMPLICATIONS FOR THE RECRUITMENT OF TRAP AND p160 COACTIVATOR COMPLEXES
J. Biol. Chem., March 5, 1999; 274(10): 6667 - 6677.
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Proc. Natl. Acad. Sci. USAHome page
W.-S. Shim, J. DiRenzo, J. A. DeCaprio, R. J. Santen, M. Brown, and M.-H. Jeng
Segregation of steroid receptor coactivator-1 from steroid receptors in mammary epithelium
PNAS, January 5, 1999; 96(1): 208 - 213.
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Mol. Endocrinol.Home page
T. Tagami, W.-X. Gu, P. T. Peairs, B. L. West, and J. L. Jameson
A Novel Natural Mutation in the Thyroid Hormone Receptor Defines a Dual Functional Domain That Exchanges Nuclear Receptor Corepressors and Coactivators
Mol. Endocrinol., December 1, 1998; 12(12): 1888 - 1902.
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J. Biol. Chem.Home page
S. Chen, N. J. Sarlis, and S. S. Simons Jr.
Evidence for a Common Step in Three Different Processes for Modulating the Kinetic Properties of Glucocorticoid Receptor-induced Gene Transcription
J. Biol. Chem., September 22, 2000; 275(39): 30106 - 30117.
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J. Biol. Chem.Home page
L.-N. Song, B. Huse, S. Rusconi, and S. S. Simons Jr.
Transactivation Specificity of Glucocorticoid Versus Progesterone Receptors. ROLE OF FUNCTIONALLY DIFFERENT INTERACTIONS OF TRANSCRIPTION FACTORS WITH AMINO- AND CARBOXYL-TERMINAL RECEPTOR DOMAINS
J. Biol. Chem., June 29, 2001; 276(27): 24806 - 24816.
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Proc. Natl. Acad. Sci. USAHome page
J. Xu, L. Liao, G. Ning, H. Yoshida-Komiya, C. Deng, and B. W. O'Malley
The steroid receptor coactivator SRC-3 (p/CIP/RAC3/AIB1/ACTR/TRAM-1) is required for normal growth, puberty, female reproductive function, and mammary gland development
PNAS, June 6, 2000; 97(12): 6379 - 6384.
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