Pulsatile gonadotropin-releasing hormone stimulation of gonadotropin subunit transcription in rat pituitaries: evidence for the involvement of Jun N-terminal kinase but not p38.

We investigated whether Jun N-terminal kinase (JNK) and p38 mediate gonadotropin subunit transcriptional responses to pulsatile GnRH in normal rat pituitaries. A single pulse of GnRH or vehicle was given to female rats in vivo, pituitaries collected, and phosphorylated JNK and p38 measured. GnRH stimulated an increase in JNK phosphorylation within 5 min, which peaked 15 min after GnRH (3-fold). GnRH also increased p38 phosphorylation 2.3-fold 15 min after stimulus. Rat pituitary cells were given 60-min pulses of GnRH or media plus the JNK inhibitor SP600125 (SP, 20 microM), p38 inhibitor SB203580 (20 microM), or vehicle. In vehicle-treated groups, GnRH pulses increased LHbeta and FSHbeta primary transcript (PT) levels 3-fold. SP suppressed both basal and GnRH-induced increases in FSHbeta PT by half, but the magnitude of responses to GnRH was unchanged. In contrast, SP had no effect on basal LHbeta PT but suppressed the stimulatory response to GnRH. SB203580 had no effect on the actions of GnRH on either LH or FSHbeta PTs. Lbeta-T2 cells were transfected with dominant/negative expression vectors for MAPK kinase (MKK)-4 and/or MKK-7 plus a rat LHbeta promoter-luciferase construct. GnRH stimulated a 50-fold increase in LHbeta promoter activity, and the combination of MKK-4 and -7 dominant/negatives suppressed the response by 80%. Thus, JNK (but not p38) regulates both LHbeta and FSHbeta transcription in a differential manner. For LHbeta, JNK is essential in mediating responses to pulsatile GnRH. JNK also regulates FSHbeta transcription (i.e. maintaining basal expression) but does not play a role in responses to GnRH.

Relationship between mezerein-mediated biological responses and phorbol ester receptor occupancy.

The phorbol ester analog, mezerein, is a weak complete and Stage 1 tumor promoter; however, it is as potent as the most active phorbol esters as a second stage promoter and inflammatory agent. Therefore, mezerein is a useful compound for studying responses associated with Stage 1 or Stage 2 promotion. In this paper, we show that in G-292 osteosarcoma cells in culture, mezerein is 25-fold more potent in causing a decrease in binding of epidermal growth factor to its specific cellular receptor than in inducing prostaglandin E2 production. This differential potency for these two actions was not noted for other phorbol esters. Our findings indicate that mezerein interacts with the major phorbol dibutyrate receptor to increase prostaglandin E2 production and also either with a distinct cellular target with a higher affinity or the same target with increased efficacy to cause a decrease in the binding of epidermal growth factor. These human osteosarcoma cells thus provide a model system to facilitate analysis of phorbol ester receptor heterogeneity.

Selective loss of estrogen receptor beta in malignant human colon.

Epidemiological data suggest a protective effect for estrogen replacement therapy on colon cancer. The estrogen receptor (ER) is required for the action of estrogen. The ER-beta isoform is functionally similar to ER-alpha but has a distinct pattern of expression and transcriptional response to selective estrogen response modulators. Our goal was to investigate the presence of ER-alpha and ER-beta in normal and malignant colon tissue. Human colon cancer tissue and adjacent normal colon tissue were harvested from five male and six female patients undergoing segmental colon resection for colon cancer. Western blot analysis revealed very low levels of ER-alpha protein in tumor and normal colon tissue. In both male and female patients, malignant colon tissue showed a selective loss of ER-beta protein expression when compared to normal colon tissue in the same patient. Semiquantitative reverse transcription-PCR revealed no difference in ER-beta mRNA levels between normal and malignant colon tissue. Malignant transformation of the colon is associated with a marked diminution of ER-beta protein expression, possibly through a posttranscriptional mechanism.

Well-differentiated endometrial adenocarcinomas and poorly differentiated mixed mullerian tumors have altered ER and PR isoform expression.

Both the estrogen receptor (ER) and the progesterone receptor (PR) have two subtypes: ER-alpha and beta, and PR-A and -B, respectively. These subtypes differ in function and expression, and recent reports have correlated changes in the normal proportions of these isoforms with neoplastic states. We investigated ER and PR isoform expression in normal pre- and post-menopausal endometrium, well-differentiated endometrial adenocarcinoma, and poorly differentiated malignant mixed mullerian tumors (MMMTs). Semi-quantitative RT-PCR and immunoblotting were used to measure receptor mRNA and protein expression. Estrogen receptor-alpha/beta mRNA ratios were significantly higher in postmenopausal (27.3) compared to premenopausal endometrium (4.9) mainly as a result of lower ER-beta expression in the former. Compared to age-matched postmenopausal controls, the ER-alpha/beta ratio was reduced in both grade I adenocarcinoma and MMMT specimens (3.3 and 6.8, respectively), due to a selective loss of ER-alpha. The relative abundance of PR-A and PR-B mRNA remained unchanged between all tissue subtypes. Total PR protein, however, was significantly reduced in MMMTs compared to all other groups. Thus, sex steroid receptor expression is significantly and differentially altered in well-differentiated and poorly-differentiated endometrial cancers. Both cancers exhibit decreased ER-alpha expression and the MMMTs also demonstrate a significant loss of PR protein.

Gonadal hormone feedback on pituitary gonadotropin genes.

The pituitary gonadotropins LH and FSH, which act on the ovaries and testes to promote gametogenesis and sex steroid production, are regulated by changes in the levels of steroids and gonadal peptides. Steroid feedback can be positive, as demonstrated by the estrogen and LH surge at ovulation, or negative, as demonstrated by the rise in LH and FSH after gonadectomy or reductions in steroid synthesis. Modulatory effects of the steroids estrogen and testosterone may be mediated directly at the level of the pituitary cells, or by alterations in the release of the hypothalamic releasing factor GnRH. Gonadal peptides, including activin and inhibin, have been shown to have direct effects on pituitary cells to alter FSH synthesis specifically, with no effects on LH. Changes in gonadotropin subunit gene transcription and mRNA levels occur very rapidly and have profound effects on physiological levels of the hormones. In this article, the effects of the gonadal steroids and peptides as modifiers of the rat gonadotropin genes in a subunit specific manner are reviewed, and the physiological implications discussed.

Effects of gonadotropin-releasing hormone on rat gonadotropin gene transcription in vitro: requirement for pulsatile administration for luteinizing hormone-beta gene stimulation.

The transcriptional regulation of the rat gonadotropin subunit genes was investigated under different regimens of GnRH administration in vitro. Anterior pituitary fragments (8-10/gland) from either intact or ovariectomized CD female rats were treated in static culture with 0.1 or 1 nM GnRH or on perifusion columns with pulsatile GnRH (25 ng pulse every 30 or 60 min) for 1-6 h. Gene transcription rates were measured in nuclear run-off assays, and hemipituitaries from the same animals were matched in control and treatment groups. In static culture, only rates of alpha-subunit mRNA synthesis were stimulated at 1, 3, and 6 h from 64 +/- 10 (control) to 170 +/- 29 (3 or 6 h of GnRH) parts/million (ppm). There was no change in FSH beta mRNA synthesis (28 +/- 6 ppm), and significant stimulation of LH beta was seen only at 1 h (98 +/- 10 vs. 34 +/- 1 ppm for control) with continuous GnRH. Similar results were obtained with both GnRH doses and with pituitaries from either intact or ovariectomized rats. In addition, continuous 1 nM GnRH administration to perifusion columns for 4 or 6 h resulted in no changes in the transcription rate for LH beta (44 +/- 10 vs. 40 +/- 12 ppm for control) or FSH beta (29 +/- 6 vs. 36 +/- 9 ppm for control), but consistent stimulation for alpha-subunit (240 +/- 29 vs. 71 +/- 16 ppm for control). Markedly different results were observed with pulsatile GnRH administration. In perifusion studies, LH beta mRNA synthesis was stimulated 2- to 2.5-fold after 1 h of pulses and 3- to 4-fold after 3 or 6 h. A slight (2-fold) stimulation was noted for FSH beta mRNA synthesis only after 1 h of pulsatile GnRH, while alpha-subunit gene transcription was elevated 2-fold after 1 h and 4- to 5-fold after 3 or 6 h of pulsatile GnRH. GnRH pulses in vivo may also be crucial to maintain gonadotropin mRNA synthesis, since administration of a GnRH antagonist ([Nal-Lys] GnRH; 20 micrograms/100 g BW) suppressed the transcription rate of all three genes to 10-25% of control values after 4 or 24 h. TSH beta mRNA synthesis was not changed by any GnRH treatment, and LH secretion was consistently stimulated by GnRH. No significant differences in transcription rate were noted between GnRH pulse intervals of 30 or 60 min in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyrotropin (TSH)-releasing hormone stimulates TSH beta promoter activity by two distinct mechanisms involving calcium influx through L type Ca2+ channels and protein kinase C.

TRH stimulates rat (r) TSH beta gene promoter activity at two distinct response elements, which also respond to protein kinase C-signaling pathways. The dependence of TRH-stimulated transcription of the TSH beta gene on a rise in intracellular calcium [Ca2+]i, and on the necessity for Ca2+ influx through L-type voltage-gated calcium channels was investigated in two transfected cell lines and in normal thyrotropes. The transcription rate of the homologous gene in normal thyrotropes was measured by nuclear run-off assays. Bay K8644, an L channel agonist, stimulated TSH beta gene transcription 6-fold, and TRH stimulation of TSH beta gene transcription was partially blocked by nimodipine, an L channel antagonist, while phorbol 12-myristate-13-acetate (PMA)-stimulated transcription was not. Bay K8644 plus TRH had a greater effect than either treatment alone. Constructs of the 5'-flanking region of the TSH beta gene fused to the luciferase reporter (TSH beta LUC) were then transfected into excitable GH3 pituitary cells. TSH beta LUC was stimulated 2- to 5-fold by 1 nM TRH or 100 nM Bay K8644, and the TRH effect was nearly abolished by nimodipine or chelation of external Ca2+. Constructs containing isolated TRH-responsive elements fused to a heterologous promoter responded similarly. The protein kinase C activator, PMA (100 nM) also stimulated TSH beta LUC transcription, but its effect was not inhibited by nimodipine. A stable heterologous cell line containing the mouse TRH receptor was constructed by transfection of nonexcitable 293 cells, which lack L channel activity. In the resultant 301 cells, TSH beta LUC activity was increased 2- to 3-fold by TRH or PMA; nimodipine, Bay K8644, and removal of extracellular Ca2+ had no effect. We conclude that TRH stimulation of TSH beta gene transcription requires Ca2+ release from inositol triphosphate-sensitive stores and Ca2+ influx via L-type calcium channels in GH3 cells, but in transfected 293 cells TRH activation of protein kinase C plays a predominant role in activating TSH beta. Both mechanisms appear to be operative in normal thyrotropes.

Divergent effects of estradiol on gonadotropin gene transcription in pituitary fragments.

Our previous work demonstrated that in vivo estradiol (E2) administration to ovariectomized rats suppressed the transcription of the LH subunit genes within 4 h. To determine whether these effects were mediated directly at the level of the gonadotrope, the transcription rates of the LH beta, FSH beta, and alpha-subunits were measured in short term cultures of pituitary fragments from female rats in various physiological states. In each in vitro experiment, fragments from matched sets of hemipituitaries were used for control and treatment (10(-8)ME2) groups. In pituitaries from ovariectomized animals, treated in vitro with or without E2, there was no significant effect of 2 h or 6 h of E2 on FSH beta or alpha-subunit gene transcription, but a consistent 2- to 3-fold stimulation of LH beta mRNA synthesis. In pituitaries from intact randomly cycling rats, E2 in culture had no effect on the transcription rate of alpha-subunit, FSH beta, or TSH beta genes, but stimulated LH beta and PRL transcription 2-fold. Thyroid hormone treatment specifically suppressed alpha-subunit (38% of control) and TSH beta (17% of control) mRNA synthesis, indicating that the culture system is responding in an appropriate physiological manner and that decreases in transcription can be easily and accurately measured with this system. Basal and E2-treated transcription rates for each gonadotropin subunit gene were measured as a function of stage of the estrous cycle. Basal transcription of the alpha-subunit gene did not vary significantly throughout the cycle and did not respond to E2 treatment in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone stimulates the activity of the rat thyrotropin beta-subunit gene promoter transfected into pituitary cells.

In order to investigate the molecular mechanism(s) by which TRH regulates the biosynthesis of TSH, we are studying the effects of TRH on the expression of the TSH subunit genes (alpha and TSH beta). To study the structure-function relation of TRH stimulation of the activity of the single rat TSH beta gene, chimaeric plasmids were constructed. The 5'-flanking region of the rat TSH beta gene including exon 1 (5'-untranslated region) was inserted into a promoterless, modified pBR, chloramphenicol acetyltransferase (CAT) expression vector. After transfection, specific TSH beta promoter activity was evident in both TRH-responsive pituitary-derived GH3 and primary pituitary cell cultures. To determine potential regulation of TSH beta promoter-directed activity in these cells by TRH, cells were incubated with media containing TRH (10(-7) to 10(-11) M) for 1 to 48 h. TRH stimulated a 1.5- to 3-fold increase in TSH beta promoter activity. Concomitant with an increase in CAT activity was an anticipated increase in PRL synthesis in the GH3 cells in response to TRH. The TRH effect on the TSH beta gene was specific; no increase in CAT activity was detected for TKCAT (thymidine kinase of herpes simplex virus promoter), pBRCAT (no promoter), or TSH beta CAT (3'-5'-orientation). Similar results were obtained using primary pituitary cell cultures. Deletion mutation analysis indicated that TRH sensitivity was detected in a 1.1 kilobase, but not in a 0.38 kilobase TSH beta gene fragment suggesting that the TRH responsive element(s) resides at least in part within the 700 base pairs of the 5'-flanking sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential gonadotropin-releasing hormone stimulation of rat luteinizing hormone subunit gene transcription by calcium influx and mitogen-activated protein kinase-signaling pathways.

Gonadotropin secretion and gene expression are differentially regulated by hypothalamic GnRH pulses by unknown mechanisms. GnRH stimulates calcium influx through L-type voltage-gated channels and activates phospholipase C, leading to increased protein kinase C (PKC) and mitogen-activated protein kinase activity. We found differential contributions of these pathways to GnRH-stimulated rat LH subunit transcription in pituitary gonadotropes and cell lines. Endogenous transcription of the alpha- and LHbeta-subunits in rat pituitary cells was stimulated by GnRH. Independent PKC activation by phorbol myristate acid stimulated only the alpha-subunit gene. In contrast, an L-channel antagonist (nimodipine) inhibited only LHbeta stimulation by GnRH, and an L-channel agonist (BayK 8644) stimulated only basal LHbeta transcription. GnRH induction of a rat alpha-subunit promoter construct in alphaT3 cells was unaffected by nimodipine or elimination of external calcium, while both treatments eliminated the LHbeta response. Application of a mitogen-activated kinase kinase (MEK) inhibitor (PD098059) decreased basal and GnRH-stimulated alpha-subunit promoter activity and had no effect on LHbeta promoter activity. In pituitary cells from mice bearing an LHbeta promoter-luciferase reporter transgene, GnRH stimulation was inhibited by nimodipine but not by PD098059. Thus, GnRH induction and basal control of the alpha-subunit gene seem to occur through the PKC/mitogen-activated protein kinase pathway, while induction of the LHbeta gene is dependent on calcium influx. Differential signaling from the same receptor may be a mechanism for preferential regulation of transcription.

Interactions of thyrotropin-releasing hormone, phorbol ester, and forskolin-sensitive regions of the rat thyrotropin-beta gene.

Our previous studies demonstrated TRH stimulation of TSH beta gene transcription in rat pituitary cell cultures and in transient expression assays, with the TRH-sensitive region located between -1.3 kilobases and -204 basepairs (bp) relative to the major transcriptional start site. Using nuclear runoff and transient expression assays, we have analyzed the interactions among TRH, the phorbol ester 12-myristate 13-acetate (PMA), and the adenylate cyclase activator forskolin on TSH beta gene transcription. In cultured pituitary cells, TSH beta gene transcription was stimulated by 2 h of 10(-9) M TRH (2- to 4-fold), 100 nM PMA (2- to 6-fold), or 2 microM forskolin (1.5- to 2.5-fold) treatment, with additive interactions among all three effectors. Chimeric plasmids containing various 5'-flanking portions of the TSH beta gene and both transcriptional start sites, fused to the chloramphenicol acetyltransferase (CAT) gene, were transfected into the clonal pituitary GH3 cell line to delineate DNA sequences conferring this regulation. Transfected TSH beta CAT constructs containing TSH beta gene sequences from -2100/+27I150, -1295/+27I150, and -520/+27I150 expressed CAT enzyme activity which was stimulated by 24 h of TRH (2- to 3-fold), PMA (3- to 6-fold), or forskolin (1.5- to 3-fold) treatment, similar to observations in normal pituitary cells. In addition, a CAT expression vector construct containing only upstream TSH beta gene sequences from -703 to -85 bp, fused to the heterologous thymidine kinase promoter (tkCAT), exhibited similarly stimulated transcription in a transfection assay in response to TRH, PMA, and forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue-specific regulation of rat estrogen receptor mRNAs.

The estrogen receptor (ER) is present in a wide variety of mammalian tissues and is required for physiological estrogen responses, including estrogen-induced tissue-specific changes in gene expression. We studied the estrogen regulation of the mRNAs encoding the ER in rat uterus, liver, and pituitary. Ovariectomized (21-28 day post surgery) female CD-1 rats were injected daily with 17 beta-estradiol (E2, 10 micrograms/100 g BW) for 0, 1, or 4 h, 1, 3, or 7 days and compared with intact controls. Steady-state levels of ER mRNA were quantified using a human ER cDNA probe. Only one hybridizing species of approximately 6.2 kilobase (kb) was detected in uterine and liver RNA, similar to that observed in MCF7 human breast cancer cells. However, the ER mRNA regulation by E2 differed in direction depending on the tissue examined. In uterus, ER mRNA increased 3- to 6-fold after ovariectomy, and returned to intact levels within 24 h of E2 replacement. In contrast, liver ER mRNA declined 1.5- to 3-fold after ovariectomy and returned to intact levels after 1-3 days of E2. In pituitary tissue two hybridizing forms of ER mRNA were observed, with one species migrating at 6.2 kb, equivalent to the form in other tissues, and a second smaller species at approximately 5.5 kb. The lower molecular weight species varied somewhat in abundance from animal to animal, averaging about 20% of the intensity of the 6.2 kb band. The ER mRNA forms were regulated positively with E2.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin (TSH)-releasing hormone-responsive elements in the rat TSH beta gene have distinct biological and nuclear protein-binding properties.

Two TRH-responsive elements have been identified in the rat TSH beta gene by deletion/mutation analysis of the 5'-flanking region of the gene and transfection of TSH beta-luciferase constructs into the GH3 pituitary cell line. Biological responsiveness was confirmed by inserting synthetic oligonucleotides next to the heterologous viral thymidine kinase (tk) promoter in tk luciferase (tkLUC) constructs. Both DNA regions, termed TSH A (at -274 to -258 bp) and TSH C (-402 to -385 bp), have a high level of sequence similarity to binding sites for the POU domain pituitary transcription factor Pit-1/GHF-1. In transfection assays, the TSH A region had no basal enhancer activity, but did confer 3- to 6-fold TRH- and PMA-stimulated transcriptional responses to the tk promoter. The TSH C region conferred basal enhancer activity (3- to 10-fold above control tkLUC) as well as a 2- to 3-fold TRH or PMA response. Combinations of TSH A and TSH C elements conferred both enhancer activity and a TRH- or PMA-stimulated response, but more than two copies of the regions resulted in no further stimulatory effect. Both TSH beta gene regions bound to nuclear proteins from GH3 cells, as determined by gel retardation analysis. The TSH A region DNA formed three prominent DNA-protein complexes, ranging from slowly to rapidly migrating bands and with calculated affinities of 32, 0.5, and 208 nM, respectively. The TSH C region formed two major complexes, which corresponded on the basis of mobility to the most slowly and rapidly migrating complexes formed by TSH A, but with calculated affinities of 3.1 and 33 nM. TSH C also formed a rapidly migrating minor complex unique for this gene region. The more rapidly migrating complexes appeared to be specific to nuclear proteins from GH3 cells. Treatment of cells with TRH did not significantly alter the affinity of protein binding. Mutation of TSH A and TSH C DNA by T to G substitutions abolished the ability of the DNA to confer a TRH response and severely inhibited the ability of the DNA to bind to GH3 nuclear proteins. Thus, transcriptional regulation of the rat TSH beta gene by TRH is correlated with the ability of the two TRH-sensitive elements to bind nuclear proteins. The differences noted in basal enhancer activity or the degree of TRH responsiveness may be related to some unique proteins bound to each DNA or to the differences in affinity of binding of the proteins common to both elements.

Divergent and composite gonadotropin-releasing hormone-responsive elements in the rat luteinizing hormone subunit genes.

GnRH pulses regulate gonadotropin subunit gene transcription in a frequency-dependent, subunit-specific manner. The alpha-subunit gene is stimulated by constant GnRH and by rapid to intermediate pulse frequencies, while stimulation of LHbeta subunit gene transcription requires intermediate frequency pulses. We have defined the GnRH-responsive elements of the rat LH subunit gene promoters by deletion/mutation analysis and transfection studies in rat pituitary cells and two clonal gonadotrope cell lines. The alpha-subunit gene GnRH-responsive region lies between -411 and -375 bp. The region contains two Ets-domain protein binding sites, and mutating either site obliterates the response. DNA protein binding studies demonstrate the two sites are not equivalent, and that Ets-1 does not mediate this response. Studies of the LHbeta promoter reveal a major GnRH-responsive region between -456 and -342 bp. Within this region, two Sp1 binding sites contribute to the GnRH response, and the 3'Sp1 site is also critical for basal expression. The 5'Sp1 site partially overlaps a CArG box, and mutating the CArG element specifically eliminates the response to pulsatile GnRH. DNA containing this mutation cannot form intermediate mobility complexes with nuclear proteins, but retains Sp1 binding. Mutation of the 3'Sp1 site and either the 5'Sp1 or CArG element partially restores GnRH stimulation, suggesting a downstream element contributes to the full GnRH response. These studies demonstrate that unique composite elements and transcription factors are responsible for GnRH stimulation of the LH subunit genes and may contribute to their differential responses to GnRH pulses.

Transcriptional regulation by a naturally occurring truncated rat estrogen receptor (ER), truncated ER product-1 (TERP-1).

Truncated estrogen receptor product-1 (TERP-1) is a naturally occurring rat estrogen receptor (ER) variant transcribed from a unique start site and containing a unique 5'-untranslated region fused to exons 5-8 of ERalpha. TERP-1 is detected only in the pituitary, and TERP-1 mRNA levels are highly regulated during the estrous cycle, exceeding those of the full-length ERalpha on proestrus. These data suggest that TERP-1 may play a role in estrogen- regulated feedback in the pituitary. We examined the ability of TERP-1 to modulate gene transcription in transiently transfected ER-negative (Cos-1) and ER-positive pituitary (alphaT3 and GH3) cell lines. In Cos-1 cells transiently cotransfected with TERP-1 and either ERalpha or ERbeta, low levels of TERP-1 (ratios of < 1:1 with ER) enhanced transcription of model promoters containing estrogen response elements by an average of 3- to 4-fold above that seen with ER alone. At higher concentrations of TERP-1 (> 1:1 with ER) transcription was inhibited. TERP-1 also had a biphasic action on transcription in the alphaT3 and GH3 pituitary cell lines, although the stimulatory action was less pronounced. TERP-1 actions were dependent on ligand-activated ER as TERP-1 did not bind estradiol in transfected Cos-1 cells or in vitro, and estrogen antagonists prevented the stimulatory effects of TERP-1. Coimmunoprecipitation studies suggest that TERP-1 does not bind with high affinity to the full-length ERalpha. However, TERP-1 may compete with ER for binding sites of receptor cofactors because steroid receptor coactivator-1 (SRC-1) rescued the inhibitory actions of TERP-1. The ability of TERP-1 to both enhance and inhibit ER-dependent promoter activity suggests that TERP-1 may play a physiological role in estrogen feedback in the rat pituitary.

Gonadotropin subunit messenger RNA concentrations after blockade of gonadotropin-releasing hormone action: testosterone selectively increases follicle-stimulating hormone beta-subunit messenger RNA by posttranscriptional mechanisms.

Regulation of gonadotropin gene expression by sex steroids may occur via direct effects on the pituitary and/or indirect effects of steroids mediated through hypothalamic GnRH. We aimed to define the effects of testosterone (T) on alpha, LH beta, and FSH beta mRNA expression in the male rat after blockade of GnRH action on the gonadotrope. A water-soluble GnRH antagonist was administered iv to castrate male rats (increased endogenous GnRH secretion) and to castrate T-replaced rats in which gonadotropin subunit mRNAs had been increased by prior treatment with exogenous GnRH pulses. In castrate male rats, GnRH antagonist resulted in a fall in all three subunit mRNAs. Alpha and LH beta declined at slower rates (half-disappearance after 50 and 65 h, respectively), and neither fell to values present in intact rats over 84 h. In contrast, FSH beta mRNA declined more rapidly, with a half-disappearance after 20 h. In castrate T-replaced rats, alpha mRNA declined at a rate similar to that in castrates (half-disappearance after 50 h). LH beta declined more slowly, and the rate of FSH beta decline was markedly prolonged in the presence of T (half-disappearance time increased from 20 to 50 h). These results suggest that T exerts direct effects on FSH beta transcription or mRNA stability which are independent of GnRH action. To assess these possibilities, a long-acting GnRH antagonist (Detirelix) was administered to castrate male rats, which also received T or sham implants 4 days after castration. FSH beta mRNA levels fell during the 4 days of Detirelix alone, but the addition of T on day 4 resulted in a 2-fold rise in FSH beta mRNA, restoring FSH beta mRNA to levels present in intact rats. Serum FSH closely paralleled FSH beta mRNA concentrations. Alpha mRNA was reduced by 25%, and LH beta mRNA concentrations were unchanged in the presence of T. The rate of alpha mRNA transcription was markedly reduced and that of LH beta tended to fall in T-treated rats, but T had no significant effect on the FSH beta transcription rate. Thus, the action of T to increase concentrations of cytosolic FSH beta mRNA appears to be exerted at a posttranscriptional level, possibly via effects of T on FSH beta mRNA stability. This may represent a mechanism by which T can effect differential regulation of gonadotropin subunit mRNA concentrations.

Androgen suppression of GnRH-stimulated rat LHbeta gene transcription occurs through Sp1 sites in the distal GnRH-responsive promoter region.

Steroids may regulate LH subunit gene transcription by modulating hypothalamic GnRH pulse patterns or by acting at the pituitary gonadotrope to alter promoter activity. We tested direct pituitary effects of the androgen dihydrotestosterone (DHT) to modulate the rat LHbeta promoter in transfected LbetaT2 clonal gonadotrope cells and in pituitaries of transgenic mice expressing LHbeta-luciferase. The LHbeta promoter (-617 to +44 bp)-luciferase construct was stimulated in LbetaT2 cells 7- to 10-fold by GnRH. Androgen treatment had little effect on basal promoter activity but suppressed GnRH stimulation by approximately 75%. GnRH stimulation of LHbeta was also suppressed by DHT in isolated pituitary cells from male or female mice with functional nuclear ARs, but not in male littermates with mutant AR. GnRH stimulation of the LHbeta promoter requires interactions between a complex distal response element containing two specificity protein-1 (Sp1) binding sites and a CArG box, and a proximal element with two bipartite binding sites for steroidogenic factor-1 and early growth response protein-1 (Egr-1). DHT effectively suppressed promoter constructs with an intact distal response element. The distal response element does not bind AR, but AR reduces Sp1 binding to this region. Glutathione-S-transferase pull-down studies demonstrated direct interactions of AR with Sp1, which requires the DNA-binding domain of AR, and weaker interactions with Egr-1. We conclude that androgen suppression of the rat LHbeta promoter occurs primarily through direct interaction of AR with Sp1, with some possible role through binding to Egr-1. These interactions result in interference with GnRH-stimulated gene transcription by reducing cooperation between the distal and proximal GnRH response elements.

Pulsatile GnRH regulation of gonadotropin subunit gene transcription.

We investigated the requirement for gonadotropin-releasing hormone (GnRH) release in vivo and the pattern of GnRH administration in vitro on the expression of the gonadotropin subunit genes in female rats. Injection of the GnRH antagonist ([Nal-Lys] GnRH; 30 micrograms/100g bw) to ovariectomized rats rapidly suppressed transcription of the alpha-subunit and LH beta genes to 10-25% of control after 24 h, as measured by nuclear run-off assays. The rate of FSH beta gene transcription was also suppressed, but to a lesser extent (to 60-75% of control). Administration of 17 beta-estradiol (20 micrograms/100 g bw) in addition to antagonist did not suppress transcription of the genes beyond that seen with the antagonist alone. Administration of constant levels of GnRH (0.1, 1, or 10 nM) to pituitary fragments in static culture stimulated alpha-subunit mRNA synthesis 2- to 3-fold, but had no significant sustained effects on LH beta or FSH beta transcription. In contrast, pulsatile GnRH administered once per hour (25 ng over 10 min) to pituitary fragments mounted on perifusion columns stimulated both alpha-subunit and LH beta gene transcription 3-fold after 3 h, with inconsistent stimulation of FSH beta. Pulsatile GnRH appears to be crucial for LH beta gene stimulation, as continuous GnRH on the columns stimulated only alpha-subunit mRNA synthesis. Thus, pulsatile GnRH in vivo is required to maintain transcription of the alpha-subunit and LH beta genes, with lesser effects on FSH beta. While continuous GnRH can stimulate alpha-subunit mRNA synthesis, a pulsatile GnRH is required to stimulate the LH beta gene.

Thyroid hormone control of thyrotropin gene expression in rat anterior pituitary cells.

The relationship between thyroid hormone and TSH subunit mRNA levels and gene transcription in rat pituitaries was investigated. Synthesis of alpha-subunit and TSH-beta mRNAs were measured by incorporation of [32P]UTP into isolated nuclei, followed by hybridization of labeled transcripts to specific cDNAs. Steady state levels of mRNA were measured by Northern blot analysis of mRNA followed by hybridization with labeled TSH beta and alpha-subunit cDNAs. Hybridizing bands were quantitated by densitometry. TSH subunit mRNA synthesis was suppressed by T3 treatment in a manner directly related to T3 receptor occupancy; this effect occurred both in vivo and in cell culture. In the euthyroid rat, alpha-subunit mRNA had a higher level of synthesis [126 parts/million (ppm)] than did TSH beta mRNA (24 ppm). In the hypothyroid animal this situation was reversed, and the synthesis of both alpha-subunit and TSH beta mRNAs was increased to 230 and 331 ppm, respectively. T3 administration to hypothyroid animals suppressed mRNA synthesis maximally to 69 ppm for alpha-subunit and 10 ppm for TSH beta. The alpha-subunit to TSH beta mRNA synthesis ratio can thus vary from 0.7 in the hypothyroid animal to 6.9 in the hyperthyroid animal. The transcription rate for both genes decreased in linear fashion with increased T3 nuclear receptor occupancy. A 50% decrease in transcription occurred at a T3 dose (10(-9) M) similar to the Kd of the thyroid receptor. This agrees with our previous findings in thyrotropic tumors indicating a time- and dose-dependent suppression of the TSH subunit genes with increasing levels of nuclear receptor-bound T3. In primary rat pituitary cultures, T3 directly regulated steady state levels and synthesis of TSH subunit mRNA. With euthyroid pituitary cultures, alpha-subunit mRNA synthesis (50 ppm) was greater than that of TSH beta (12 ppm), but with hypothyroid pituitary cultures both mRNAs were synthesized at nearly equal rates (alpha-subunit, 120 ppm; TSH beta, 105 ppm). T3 (10(-8) M) treatment of hypothyroid cultures for 3 days decreased alpha-subunit (60% of control) and TSH beta (35% of control) mRNAs. Thus, mRNA synthesis is changed coordinately for both TSH subunits, but TSH beta is altered to a greater extent by thyroid status.

Triiodothyronine rapidly decreases transcription of the thyrotropin subunit genes in thyrotropic tumor explants.

We have investigated the direct effect of the thyroid hormone T3 on the TSH subunit genes in tissue explants. Minces of TtT 97 thyrotropic tumor were treated with 5 nM T3 for varying periods of time. Nuclei were then isolated from the tumor cells and allowed to continue RNA synthesis in the presence of [alpha-32P]UTP. Newly synthesized RNA sequences were quantified by hybridization to immobilized cloned cDNAs containing sequences specific for either TSH beta or alpha-subunit mRNA. Basal TSH beta and alpha-subunit mRNA synthesis rates were both approximately 300-400 parts/million, the same as in vivo values. After 15 min of T3 treatment, TSH beta mRNA synthesis was significantly decreased by 42% and was maximally decreased by 95% after 1 or more hours of T3. Synthesis of alpha-subunit mRNA was decreased by 38% after 30 min of T3 treatment and by 78% after 1 h or more of T3. The suppressive effects of T3 on transcription correlated with the time course of T3 binding to its nuclear receptor. These changes are quantitatively similar to those observed after in vivo T3 treatment. Decreases in mRNA synthesis preceded significant decreases in tissue steady state mRNA levels or subunit protein levels. The presence of the protein synthesis inhibitor cycloheximide (25 micrograms/ml) during a 4-h incubation with T3 did not change the T3-mediated decreases in TSH beta or alpha-subunit mRNA synthesis or the decreases in cellular mRNA levels. Therefore, T3 can act directly on the thyrotrope to suppress TSH beta and alpha-subunit mRNA synthesis, and protein synthesis is not necessary for the T3-mediated decreases in gene transcription. The data suggest that T3 may act directly at the level of the TSH subunit genes to modulate their expression.