Interaction of a tissue-specific factor with an essential rat growth hormone gene promoter element

The gene regulating activity of thyroid hormone nuclear receptors is modulated by cell-type specific factors

To understand whether the transcriptional activity of thyroid hormone nuclear receptors (TRs) is modulated by cell-type specific factors, full length TR subtype alpha1 (TRalpha1) and beta1 (TRbeta1) cDNAs were cloned from human hepatoma cell lines: HA22T, SK-Hep-1 and HepG2. The cloned receptor bound to the thyroid hormone 3,3',5-triiodo-L-thyronine (T3) and the thyroid hormone response elements (TREs) similarly to those cloned from other tissues. They exhibited T3- and TRE-dependent transactivation activities, indicating these TRs were transcriptionally active. The lipogenic malic enzyme (ME), a T3-target gene in liver, was stimulated approximately 3- and 1.5-fold by T3 in HA22T and SK-Hep-1, respectively. The T3-stimulated ME gene expression was inhibited in HA22T, but stimulated in SK-Hep-1 cells by insulin. These results suggest that the gene regulating activity of TRs was modulated by cell-type specific factors. Furthermore, these cell-type specific factors could modulate the cross talk between TR- and insulin receptor-mediated pathways.

CCAAT/enhancer-binding protein alpha activation of the rat growth hormone promoter in pituitary progenitor GHFT1-5 cells

High level, anterior pituitary-specific expression of the rat growth hormone (rGH) promoter requires cooperative actions of several different transcription factors. Previously, we described a series of multisubunit, tissue-general, transcription factor complexes that bound to the GHF3 activation site and strongly regulated rGH promoter activity. A 43-kDa DNA-binding subunit common to each of the different GHF3 complexes is identified here as the transcription factor, CCAAT/Enhancer-binding Protein alpha (C/EBPalpha). In human monocyte U937 cells, which do not express the endogenous or transfected GH genes, co-expression of C/EBPalpha and Pit-1 synergistically activated the transfected rGH promoter. Full-length C/EBPalpha was present in the GH-secreting GC, and prolactin-secreting 235-1, pituitary cell lines, but not in GHFT1-5 cells, which are transformed at a stage in development immediately prior to GH expression. Transient expression of C/EBPalpha in GHFT1-5 cells strongly activated the co-transfected rGH promoter through the GHF3 binding site; a second activation site mapped to evolutionary conserved GH promoter sequences between -106 and -33. C/EBPalpha activation was synergistic with phorbol 12-myristate 13-acetate and forskolin, activators of protein kinases C and A, respectively. Thus, C/EBPalpha is an important regulator of rGH promoter activity that appears to function in synergy with Pit-1, activators of A and C protein kinases and possibly other factors.

An E-box motif conveys inhibitory activity on the atrial natriuretic peptide gene

Atrial natriuretic peptide (ANP) is a potent diuretic, natriuretic, and vasorelaxant hormone that is expressed early in ventricular hypertrophy. Expression of human ANP is controlled by a series of regulatory elements located in the 5' flanking sequence of its gene. We generated a series of 5' deletion mutations extending from -2600 to -1150 relative to the transcription start site and linked them to a chloramphenicol acetyltransferase reporter gene. Using transient transfection analysis, we have identified a negative regulatory element between -1206 and -1152 relative to the start site. Each of a series of 5' deletion mutants, when introduced into fibroblast cultures, expressed the reporter function at a level that was significantly less (< 20%) than that seen with the -1152 reporter construct, whereas comparably transfected atrial cardiocytes demonstrated no change in reporter activity, implying that the repressor function is specific to cell type. The critical region (from -1206 to -1152) associates with a soluble protein present in cardiac fibroblast extracts in a sequence-specific fashion. Deoxyribonuclease I footprint analysis demonstrated the presence of several protected regions, including one that overlies an E-box motif (CAACTG), an element that in other systems has been implicated in promoting differentiation in the myocyte lineage. Site-directed mutagenesis of the E-box motif suppressed both the protein-binding and inhibitory activities of the 54-bp fragment. In summary, we have found a region in the 5' flanking sequence of the human ANP gene that represses transcriptional activity in nonmyocardial cells. This element may play an important role in the restriction of ANP gene expression to cardiac myocytes.

An activation function in Pit-1 required selectively for synergistic transcription

Synergistic transcription activation is a key component in the generation of the spectrum of eukaryotic promoter activities by a limited number of transcription factors. Various mechanisms could account for synergy, but a central question remains of whether synergism requires transcription factor functions that differ from those that direct independent activation. The rat growth hormone promoter is synergistically activated by the pituitary-specific transcription factor, Pit-1, and the thyroid hormone receptor (TR). Mutations that disrupted the previously described DNA binding and transcriptional activation domains of both Pit-1 and TR reduced Pit-1/TR synergy in parallel with their effects on the much weaker, independent Pit-1 and TR activations of the rat growth hormone promoter. Thus, Pit-1 and TR amplify each other's intrinsic activities. Mutations of Pit-1 that selectively inhibited synergism with the TR without affecting independent Pit-1 activity were also identified. Pit-1/TR synergy is therefore a consequence of a novel synergism-selective activity and synergism-independent Pit-1 and TR functions.

Control of growth hormone synthesis

A large body of research, primarily in the rodent and human species, has elucidated many of the details regarding the control of GH synthesis and release. Cell type-specific transcriptional control has been identified as the main mechanism of the somatotroph-specific expression of GH. The recent detailed analysis in rodents and humans of a highly specific transcriptional activator protein, PIT-1, has opened several new areas of study. This is especially true for research in the farm animal species, where PIT-1 has been cloned and its binding elements on the GH gene are being investigated in a number of economically important species. Genetic and biochemical analyses of PIT-1 and other GH regulators have shown the central role of PIT-1 not only in the cell-autonomous stimulation of GH gene transcription, but also in the participation of PIT-1 in the response at the GH gene to exogenous hormones such as RA and TH. PIT-1 has been implicated in the proliferative development of the pituitary itself, in the maintenance of anterior pituitary cell types once cell types are defined, and in the mechanism by which the hypothalamic signal for GH release is transduced. However, PIT-1 by itself does not activate the GH gene, so that additional unknown factors exist that need to be identified to fully understand the cell type-specific activation of the GH gene. In addition, GH gene regulatory elements acting through well-characterized systems such as TH have seemingly different effects; the specific context of the regulatory elements relative to the promoter elements appear to be crucial. These contextual details of GH gene regulation are not well understood for any species and need to be further studied to be able to make predictions for particular elements and regulatory mechanisms across species. The regulation of the pulsatile secretion of GH by GHRH and SRIH is reasonably well understood after the cloning and analysis of the two releasing factors and their receptors. Modification or manipulation of the pathways involved in the regulation of GH secretion is a potential means of enhancing the lean tissue growth of meat animals. However, further understanding of the systems controlling the in vivo release of GH is needed before such manipulations are likely to be productive. Several other research questions regarding the control of GH expression and release remain to be answered. What is the biochemical connection between exogenous signal transduction (i.e., GRH/GHRH-R, TR, ER, RAR) and PIT-1 at the GH gene? Are there additional coactivators or repressors of GH that respond to cAMP levels? Do ubiquitous regulatory factors such as GHF-3 and Zn-15, identified thus far only in the rat, exist in humans or livestock? Zn-15 is expected to be found in many mammalian species, because its recognition sequence between the PIT-1 binding sites is highly conserved across mammals (Figure 2). What is the mechanism causing GH levels to drop during aging? Does PIT-1 expression decrease during the lifespan of animals? Is it possible to increase GH gene expression within target tissues by directing the expression of PIT-1 to these tissues via transgenesis, or are other factors limiting in peripheral tissues so that the lack of PIT-1 expression is not the deciding factor? Finally, is there genetic variation in the expression of GHRH and/or SRIH or in their respective receptors? These questions are relevant to and could be investigated in several of the livestock species.

Pit-1 exhibits a unique promoter spacing requirement for activation and synergism

The developmentally regulated Pit-1 transcription factor is involved in the activation of prolactin, growth hormone, and TSH beta expression. Using templates with spacing mutations to program an in vitro transcription system, the activity of a single Pit-1 proximal binding site within the rat prolactin promoter was shown to have a unique bimodal distance requirement. Transcription activity rapidly decreased with each 5-base pair (bp) addition to the spacing between the binding site and the TATA box. When positioned 20 bp upstream from its normal -36 position in the prolactin promoter, the activity of the Pit-1 binding site is reduced to basal levels. Placement of the site at a position 30 bp upstream resulted in a return of Pit-1-mediated activation. Using transient transfection assays in GH3 cells, the prime bimodal sites are also a requirement for optimum expression of chimeric prolactin-luciferase reporter constructs. Interestingly, optimal synergism of transcription in vivo by the prolactin distal enhancer, containing four Pit-1 binding sites and an estrogen-responsive element, is also sensitive to the placement of the proximal Pit-1 binding site. These data have important implications for Pit-1 activator function in pituitary cells and for general models of transcription synergism.

Brain and testis selective expression of the glutathione S-transferase Yb3 subunit is governed by tandem direct repeat octamer motifs in the 5'-flanking region of its gene

To gain insight into mechanisms of cell type-specific transcription of class mu-glutathione S-transferase genes, the gene encoding the Yb3 subunit was cloned. Yb3 subunits are selectively expressed at high levels in rat brain and testis but not in liver or kidney. The Yb3 subunit gene spans over 6 kb and consists of 8 exons and 7 introns and a sequence consisting of tandem direct repeat consensus octamer DNA binding motifs separated by a 6 base pair (bp) spacer was identified in its 5'-flanking region. Gel shift assays with a 40 bp segment of DNA containing the two consensus octamer sequences, revealed the presence of specific binding proteins in nuclear extracts of rat brain, testis and C6 glioma cells. DNA binding activity was greatly reduced in liver, kidney and HTC cells. Reporter vectors carrying segments of the 5'-flanking region of the Yb3 subunit gene fused to a luciferase gene were introduced into C6 glioma cells which express high levels of Yb3 subunits, and into HTC cells which do not. The plasmids consisting of the Yb3 gene promoter up to, but not including, the octamer motifs did not support luciferase transcription in the C6 glioma cells, but larger fragments that included the octamer repeat sequences, effectively directed transcription in the C6 glioma cells. With mutated octameric sequences transcriptional activity was greatly reduced, and none of the same Yb3 constructs directed substantial luciferase transcription in the HTC cells. The results show that octamer motifs in the 5'-flanking region of the Yb3 subunit gene are functional and are the principal cis-acting elements that account for its discrete cell type-selective expression. This gene is one of the few known targets for octamer DNA binding transcription factors in brain.

The rat growth hormone proximal silencer contains a novel DNA-binding site for multiple nuclear proteins that represses basal promoter activity

Cell-type-specific expression of the rat growth hormone (rGH) gene is determined by the interaction of both positive as well as negative regulatory proteins with cis-acting elements located upstream of the rGH mRNA start site. We have recently shown that the rat liver transcription factor NF1-L binds to the proximal rGH silencer (called silencer-1) to repress its transcriptional activity. However, this single factor proved to be insufficient by itself to confer cell-specific gene repression. We therefore attempted to identify other regulatory proteins interacting with silencer 1, which might be needed to achieve full cell-specific repression of that gene. A common recognition site for three yet uncharacterized nuclear proteins (designated as SBP1, SBP2 and SBP3) which bind a DNA sequence adjacent to the NF1-L-binding site in the rGH silencer-1 element were identified. UV crosslinking of DNA/protein complexes and nuclear protein fractionation/renaturation from SDS/polyacrylamide gels further indicated that the molecular masses for SBP1-3 are 41, 26 and 17 kDa respectively, the major species being the 26-kDa protein (SBP2) which account for 83% of the shifted SBP double-stranded oligonucleotide in gel mobility-shift assays. For this reason, most of this study focussed on the characterization of SBP2. We demonstrated that binding of NF1-L and SBP2 to their respective recognition sequence is a mutually exclusive event. Although an SBP-binding activity has been found in every non-pituitary tissue or cell line tested, no such activity could be detected in either rat pituitaries or rat pituitary GH4C1 cells. Insertion of the SBP element upstream of the basal promoter of the mouse p12 heterologous gene resulted in a consistent decrease in chloramphenicol acetyl transferase reporter gene expression following transient transfections in non-pituitary cells only, suggesting that the related SBP1-3 proteins might be involved in generally repressing gene transcription in a cell-specific manner.

c-Fos transcriptional activity stimulated by H-Ras-activated protein kinase distinct from JNK and ERK

Ras proteins exert their mitogenic and oncogenic effects through activation of downstream protein kinases. An important question is how Ras-generated signals reach the nucleus to activate downstream target genes. AP-1, a heterodimeric complex of Jun and Fos proteins, which activates mitogen-inducible genes, is a major nuclear target of Ras. Ras can stimulate AP-1 activity by inducing c-fos transcription, a process which is probably mediated by the ERK1 and -2 mitogen-activated protein (MAP) kinases, which phosphorylate the transcription factor Elk-1/TCF. Besides inducing transcription from fos and jun genes, mitogens and Ras proteins enhance AP-1 activity through phosphorylation of c-Jun. Phosphorylation of the c-Jun activation domain leads to c-jun induction through an autoregulatory loop. Ras- and ultra-violet-responsive protein kinases that phosphorylate c-Jun on serine residues at positions 63 and 73 and stimulate its transcriptional activity have been identified. These proline-directed kinases, termed JNKs, are novel MAP kinases. It is not clear, however, whether c-Jun is the only recipient and JNK the only transducer of the Ras signal to AP-1 proteins. A short sequence surrounding the major JNK phosphorylation site of c-Jun is conserved in c-Fos and is part of its activation domain, suggesting that c-Fos may be similarly regulated. Here we show that Ras does indeed augment the transcriptional activity of c-Fos through phosphorylation at Thr 232, the homologue of Ser 73 of c-Jun. However, this is mediated by a novel Ras- and mitogen-responsive proline-directed protein kinase that is different from JNKs and ERKs. Therefore, at least three types of proline-directed kinases transmit Ras- and mitogen-generated signals to the transcriptional machinery.

The 30-kDa rat liver transcription factor nuclear factor 1 binds the rat growth-hormone proximal silencer

Transcription of the gene encoding rat growth hormone is under the influence of cis-acting negative regulatory elements termed silencers. We showed previously that one such element, designated the rat growth hormone proximal silencer-1 site, binds a nuclear protein, the nuclear-factor-1-like protein that is probably a member of the CAAT transcription factor/nuclear-factor-1 (CTF/NF-I) family of transcription factors. This nuclear protein possesses DNA-binding activity as well as biochemical properties similar to those reported for the 30-kDa rat liver form of nuclear factor 1 (NF1-L). Results from both gel mobility supershift assays and Western-blot analyses, performed in combination with a polyclonal antibody directed against the DNA-binding domain of NF1-L, indicated that rat liver nuclear factor 1 might indeed correspond to one of the transcription factors interacting with the rat growth-hormone proximal silencer element. Further experiments using gel mobility shift assays also indicated that, as for NF1-L, multiple proteins among the 52-66-kDa CTF/NF-I isoforms from human HeLa cells also possess the ability to bind the rat growth-hormone silencer.

Differential binding of rat pituitary-specific nuclear factors to the 5'-flanking region of pituitary and placental members of the human growth hormone gene family

Placental chorionic somatomammotropin (hCS-A or B) and growth hormone variant (hGH-V) are members of the human growth hormone family, and are related by structure and function to pituitary growth hormone (hGH-N). However, while the hGH-N gene is expressed specifically in the anterior pituitary, hGH-V and hCS are produced in the placenta. Hybrid hGH-N, hGH-V and hCS-A genes containing 5'-flanking sequences, including the endogenous promoter, are preferentially expressed in rat pituitary tumor (GC) cells, after gene transfer. Since interaction with a pituitary-specific protein (Pit 1) is required for efficient hGH-N as well as rat growth hormone (rGH) gene expression in GC cells, binding of pituitary proteins to the hGH-V and hCS-A promoter sequences was investigated. Rat Pit 1 binds at two locations on the hGH-N gene, a distal (-140/-107) and proximal site (-97/-66), in a similar manner to that observed with the rGH gene. By contrast, efficient Pit 1 binding was seen only to the distal site of the hGH-V gene and the proximal site of the hCS-A gene. Although binding of a protein to the distal hCS-A sequences was observed, the site of interaction was truncated (-140/-116), not pituitary-specific, and was more consistent with the binding of Sp1. These data indicate that rat Pit 1 binds to the placental hGH-V and hCS-A genes and correlates with their promoter activity in GC cells after gene transfer.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin-deficient GH3B3 cells are defective in the utilization of the endogenous prolactin promoter yet are fully competent to initiate transcription from a transfected prolactin promoter

Transcription of the prolactin (PRL) gene has been analyzed in wild-type D6, PRL-deficient B3, and revertant r16 GH3 cells. Levels of processed nuclear transcripts from the PRL gene were substantially reduced in the deficient line compared to wild-type cells and returned to greater than wild-type levels in the revertant line. Rare PRL transcripts in the deficient line contained the same 5' end found on transcripts in wild-type and revertant cells as judged by primer extension and S1 nuclease protection assays, implying that the cells are deficient in utilization of the normal wild-type promoter. Deficient cells also contained wild-type levels of the PRL- and growth hormone-specific transcription factor pit-1/GHF-1, and no difference was found in the ability of extracts from wild-type and deficient cells to retard various restriction fragments from both the proximal and the distal PRL promoter regions. The deficient and wild-type cells were equally competent in initiating transcription from a transfected rat PRL promoter containing both the distal and proximal promoter elements. These observations imply that PRL-deficient cells are not defective in a trans-activating factor functioning on these PRL promoter fragments (trans model). Rather, inefficient use of the PRL promoter in the variant cells may reflect an increased methylation state of the PRL gene itself (cis model).

Functional glucocorticoid inducible enhancer activity in the 5'-flanking sequences of the rat growth hormone gene

Glucocorticoid regulation of rat growth hormone (rGH) gene expression has been investigated in a series of gene transfer studies into cells in culture. It has been established that sequences (-12 to -523) immediately flanking the start site for rGH gene transcription behave as a functional glucocorticoid inducible enhancer when associated with a heterologous promoter (RSV), displaying independence of orientation and position in mediating the glucocorticoid effect. The induction of chloramphenicol acetyl transferase (CAT) gene expression in these constructs by dexamethasone was established at the enzyme and mRNA levels and was inhibited in the presence of the antiglucocorticoid, RU 38486. The glucocorticoid inducible enhancer activity was not restricted to pituitary cells. The constructs containing the rGH-5'-flanking sequences, associated with the RSV promoter, also mediated glucocorticoid induction of CAT gene expression when transiently transfected into MH1C1 cells, a hepatoma cell line. The effect was similarly demonstrable on co-transfection of these constructs with a glucocorticoid receptor expression vector into receptor deficient COS cells. Two elements within these rGH sequences (-97 to -111 and -250 to -264) display partial homology with a consensus sequence computed for a group of glucocorticoid regulatory elements. Mutation of both of these elements or of the more proximal element alone (-97/-111) led to a complete loss of ability to mediate glucocorticoid induction of gene expression. However, the rGH sequences still mediated glucocorticoid induction of gene expression when the distal GRE-like element was mutated or deleted. Thus, the proximal rGH GRE-like element is absolutely required to mediate this glucocorticoid inducible enhancer activity.

Dwarf locus mutants lacking three pituitary cell types result from mutations in the POU-domain gene pit-1

Mutations at the mouse dwarf locus (dw) interrupt the normal development of the anterior pituitary gland, resulting in the loss of expression of growth hormone, prolactin and thyroid-stimulating hormone, and hypoplasia of their respective cell types. Disruptions in the gene encoding the POU-domain transcription factor, Pit-1, occur in both characterized alleles of the dwarf locus. The data indicate that Pit-1 is necessary for the specification of the phenotype of three cell types in the anterior pituitary, and directly link a transcription factor to commitment and progression events in mammalian organogenesis.

Tissue-specific expression of the rat growth hormone gene is due to the interaction of multiple promoter, not enhancer, elements

Expression of the rat growth hormone (rGH) gene is highly tissue-specific, being limited to a subset of cells in the anterior pituitary. DNA sequences within 237 bp of the transcription start site of the rGH gene play a major role in directing the expression of this gene in the pituitary. Transfection studies in cultured rat pituitary (GC) cells demonstrate that optimal expression of rGH requires the binding of at least two non-tissue-specific factors whose contribution to rGH expression is dependent on the binding of the pituitary-specific factor, Pit-1. Although the segment of DNA containing the elements to which these factors bind can direct pituitary-specific expression of a gene lacking upstream promoter elements, it cannot confer stimulation to either a heterologous or homologous promoter when placed downstream from the coding sequences. These results suggest that expression of the rGH gene exclusively in the pituitary is due to the activity of a tissue-specific promoter element, not an enhancer.

Characterisation of tissue-specific trans-acting factor binding to a proximal element in the rat growth hormone gene promoter

Using an exonuclease III protection assay, tissue-specific binding of rat pituitary tumour cell (GH3 cell) nuclear factors to a proximal region (-68 to -138) of the rat growth hormone gene promoter has been detected. The binding is particularly strong between the borders -68 to -102. The binding is eliminated in the presence of excess unlabelled rat growth hormone gene promoter sequences but also by proximal (-423 to +38) or distal (-1960 to -1260) rat prolactin gene promoter sequences and simian virus 40 enhancer/promoter sequences. Extracts of rat pituitaries showed identical binding characteristics. Methylation interference analysis indicated that the contact points between the pituitary-specific factor and the proximal rat growth hormone gene promoter-binding element (-65 to -95) are over a conserved sequence which occurs twice in the rat growth hormone gene promoter and at least eight times in the rat prolactin gene 5'-flanking sequences. This sequence has previously been proposed to constitute the binding site for the somatotroph/lactotroph tissue-specific transcription factor. Gel-retardation and exonuclease III competition analysis showed that three of the rat prolactin gene promoter elements (-46 to -71, -156 to -180 and -174 to -204) share the ability to bind the pituitary-specific factor. The binding to the most proximal rat prolactin gene promoter element (-46 to -71) was clearly more avid than to the rat growth hormone gene promoter (-65 to -95) proximal element. However, both these elements displayed the formation of two gel-retarded complexes while the more distal rat prolactin gene binding elements (-156 to -180 and -174 to -204) formed only the smaller of the two complexes. Finally, we demonstrated by co-transfection competition analysis that plasmids containing the most proximal rat prolactin gene promoter binding element completely inhibited transcription from the rat growth hormone gene promoter while rat growth hormone gene promoter sequences only partially inhibited transcription from the rat prolactin gene promoter. This suggests that the higher affinity for factor binding displayed by the proximal rat prolactin gene promoter binding site in vitro is reflected in factor binding activity in vivo.

Triiodothyronine inhibits transcription from the human growth hormone promoter

Three DNA constructs, the natural human growth hormone gene (hGH-hGH) its 500 bp promoter linked to the chloramphenicol acetyl transferase reporter gene (hGH-CAT), and its structural part linked to the herpes virus thymidine kinase promoter (TK-hGH) were introduced into rat pituitary GC cells by DEAE-dextran transfection. Transient expression was followed as a function of triiodothyronine (T3) concentration. The hGH-CAT expression was specifically inhibited by T3 following a typical dose-response curve while hGH-GH gene expression was not significantly modified. The transient expression of TK-hGH increased as a function of T3 concentration. These results indicate that T3 exerts two opposite effects on hGH gene expression. First, it down-regulates expression by acting on the promoter; second, it up-regulates expression by acting on the structural part of the gene. These action could be due to regulation of transcription and mRNA stabilization, respectively.

Human growth hormone gene expression in rat but not human non-pituitary cells after stable gene transfer

Tissue-specific expression of the rat growth hormone (rGH) gene requires binding of a pituitary-specific factor. Binding of this factor has been used to explain tissue-specific expression of the human growth hormone (hGH-N) gene in transfected rat pituitary (GC) tumour cells. Neither rat fibroblast (R2) nor human cervical carcinoma (HeLa) cells contain the rat pituitary-specific factor. Thus, no expression of hGH-N or rGH would be expected in these cells. R2 cell lines containing stably integrated hGH-N or rGH genes were generated. Expression of hGH-N but not rGH was detected. By contrast, stably transfected HeLa cells did not express the endogenous or transfected hGH-N genes. However, an hGH-N transcript was detected when hGH-N gene expression was directed by a viral promoter. This suggests that the block in expression occurs at the level of transcription and not mRNA stability. Hybrid genes containing 496 base pairs (bp) of hGH-N or 234 bp of rGH 5'-flanking DNA, including promoter sequences, fused to the bacterial gene coding for chloramphenicol acetyltransferase were used to stably transfect R2 cells. The hybrid hGH-N gene was more active than a promoterless construction in these cells. By contrast, the hybrid rGH gene was not. These data suggest that the hGH-N gene can be activated by rat transcription factors other than those found in pituitary cells.

Regulation of cell-type-specific transcription and differentiation of the pituitary

The transcription of rat prolactin and growth hormone genes in vitro requires a pituitary transcription factor, specific to certain cell types in the pituitary, which currently appears to be the PUF-I/Pit-1/GHF-1 protein. This factor binds to cis-regulatory elements in the 5' region of both genes and exerts a positive influence on transcription initiation presumably by interacting with general transcription factors. The PUF-I/Pit-1/GHF-1 transcriptional regulatory protein probably has an important role in not only the differentiation of the pituitary lactotroph/somatotroph cell lineage; it is also expressed in the early development of the nervous system but its function there is less well documented. It appears to be one member of a family of trans-activator proteins involved in differential gene expression in several cell types.

Dissection of functional domains of the pituitary-specific transcription factor GHF-1

The specific expression of growth hormone (GH) in the somatotrophic cells of the anterior pituitary is largely attributable to a short promoter in the 5' flanking region of the GH gene. This promoter contains two binding sites for the transcription factor GHF-1, the expression of which is also specific to cells of the somatotrophic lineage and correlates with activation of the GH gene in the developing mouse pituitary. Various studies indicate that GHF-1 is the main determinant of cell type-specific expression of the GH gene. GHF-1 is a member of the POU-domain class of proteins that each contain two highly conserved sequence motifs, the homoeodomain and the POU-specific domain. Here we report that the GHF-1 homoeodomain is sufficient for sequence-specific DNA binding, although its activity is stimulated by the POU-specific domain, which does not interact directly with the DNA. Transcriptional activation is mediated by a separate domain rich in hydroxylated amino-acid residues. Similar sequences are present in other cell type-specific transcription factors.

Dexamethasone control of growth hormone mRNA levels in GH3 pituitary cells is cycloheximide-sensitive and primarily posttranscriptional

To clarify the mechanism of growth hormone (GH) gene activation by glucocorticoids in GH3 pituitary cells, GH mRNA accumulation in nuclear and cytoplasmic compartments was measured in the presence and absence of cycloheximide. In dexamethasone-treated cells, levels of GH mRNA were increased in the nucleus by 6 h and in the cytoplasm by 12 h. Dexamethasone treatment caused a 5- to 24-fold rise in total GH mRNA levels by 48-72 h. The differential elevation of nuclear levels of GH mRNA relative to the amount of cytoplasmic GH mRNA persisted for 48 h. A transient accumulation of GH mRNA in the nucleus was followed by a brief rise in cytoplasmic GH mRNA levels in GH3 cells treated simultaneously with dexamethasone and cycloheximide. In GH3 cells pretreated for 2 h with cycloheximide, the rise in nuclear and cytoplasmic GH mRNA levels mediated by dexamethasone was blocked completely. Levels of glucocorticoid receptor were unaffected by cycloheximide. These data suggest that the stimulation of GH mRNA levels by glucocorticoids is initiated within the nucleus and that cycloheximide-sensitive events are essential for this stimulation to occur. To assess the importance of GH gene transcriptional activation by glucocorticoids, nuclear transcription run-on reactions and assays of GH promoter activity in an aminoglycoside 3'-phosphotransferase (Neo) fusion gene within stably transformed GH3 cells were performed. Evidence for a weak, transient transcriptional activation of the GH gene by dexamethasone in nuclear run-on assays was obtained. Consistent with this idea, a 30-72 h exposure to dexamethasone raised levels of Neo mRNA in GH-Neo GH3 cell transformants by less than or equal to 2-fold. We conclude that glucocorticoid stimulation of GH mRNA in GH3 cells requires ongoing protein synthesis and can occur largely independently of GH gene transcriptional activation.

Role of the octamer motif in hybrid cell extinction of immunoglobulin gene expression: extinction is dominant in a two enhancer system

We have shown previously that genes activated by the immunoglobulin heavy chain (IgH) enhancer or promoter in mouse myeloma cells are extinguished upon fusion of the myeloma with a mouse T cell lymphoma. Here we show that the conserved octamer sequence shared by the IgH enhancer and promoter, when multimerized to form a tissue-specific enhancer, can also render a gene extinguishable under the same experimental conditions. Extinction, however, is not correlated with either absence of the tissue-specific transcription factor OTF-2 or loss of its ability to bind the octamer sequence. It was also found that extinction mediated by the IgH enhancer is dominant to transcriptional activation by the SV40 enhancer. We propose, therefore, that the T cell-negative regulator responsible for IgH gene extinction does not simply prevent IgH enhancer activation but interferes with gene expression more directly, perhaps by disrupting the transcription complex established as a result of tissue-specific enhancer activation.

Inhibition of rat growth hormone promoter activity by site-specific DNA methylation

The effect of methylation on rat growth hormone (rGH) promoter activity was determined in GH3 cells by measuring rGH-Neo and rGH-CAT fusion gene expression with or without prior in vitro treatment with the site-specific DNA methyltransferases, M-BsuE and M-HhaI. To assay for rGH-promoter-specific effects of DNA methylation, RSV-Neo and RSV-CAT activities with or without M-BsuE, M-HhaI and M-HpaII treatment were measured in parallel cultures of GH3 cells. GH1-Neo and RSV-Neo fusion gene expression was inhibited by in vitro methylation from 44 to 83% as measured by the number of Geneticin-resistant GH3 cell colonies. Methylation of the GH1 promoter by M-BsuE exhibited some selective inhibition of Neo expression as determined by colony numbers, although extensive methylation of non-promoter DNA in GH1-Neo and RSV-Neo by M-HhaI and M-HpaII also inhibited Neo expression. Southern blot analysis of genomic DNA isolated from the Geneticin-resistant GH3 cells indicated that Geneticin-resistance was accompanied by demethylation of the BsuE (ThaI) sites in stably incorporated GH1-Neo DNA but not RSV-Neo DNA. Transient expression of the CAT gene in GH3 cells was selectively inhibited by 60% upon methylation of two BsuE (ThaI) sites in the GH1 promoter of GH1-CAT by M-BsuE. The data demonstrate, for the first time, to our knowledge, a direct effect of DNA methylation on the activity of the rat growth hormone promoter.

The complete nucleotide sequence of the growth-hormone gene from Atlantic salmon (Salmo salar)

We report here the complete genomic nucleotide sequence for the Atlantic salmon growth-hormone gene (asGH), including 600 bp of 5' flanking sequences. The primary transcription (3651 nt) is significantly longer than that of the mammalian genes, mainly because of larger intron sizes, but also because the asGH gene contains an additional intron (intron 5). The coding regions of the asGH gene have been compared to the corresponding regions from rainbow trout (cDNA and genomic), coho salmon (cDNA) and chum salmon (cDNA). With the exception of the rainbow trout cDNA sequence, all results were in agreement with current classification of the four species. The results of a similar comparison with the mRNA leader and trailer regions were also consistent with current classification. Sequences upstream from the transcription start point have been compared to the corresponding regions from rainbow trout and mammalian GH gene (maGH) upstream sequences. The results showed that the upstream sequences in the two fish species were very similar, while short stretches similar to conserved upstream sequences in the maGH genes were also found. Some of these conserved sequences are known to be involved in the specificity of expression of the mammalian genes.

Pituitary-specific factor binding to the human prolactin, growth hormone, and placental lactogen genes

The human genes coding for growth hormone (GH), chorionic somatomammotropin (placental lactogen, CS), and prolactin (Prl) are related evolutionarily but are expressed in phenotypically distinct cell types despite their nucleotide sequence homology. We show here that the promoters of the human Prl and CS genes contain cis-acting sequences that confer pituitary-specific expression in a cell-free transcription assay. Similar data are obtained with the human GH gene, consistent with earlier work by others. Footprinting analysis shows that neighboring sequences in each of these three promoters are protected from deoxyribonuclease I digestion by rat pituitary cell extracts. Footprinting competition experiments and gel retardation assays with synthetic oligonucleotides suggest that a single factor is responsible for the pituitary-specific footprints seen on the human Prl, CS, and GH genes. They also suggest that this factor is identical or closely related to the trans-acting factor GHF-1/Pit-1. Similarities with and differences from the rat GH and Prl genes are discussed.

Thyroid hormone action and the erbA oncogene family

In this review, we discuss the biological action and biochemical function of the v-erbA oncogene product, and the role of c-erbA proto-oncogene products as thyroid hormone receptors, as related to the molecular structure and function of the nuclear hormone receptors at large.

A tissue-specific transcription factor containing a homeodomain specifies a pituitary phenotype

Multiple related cis-active elements required for cell-specific activation of the rat prolactin gene appear to bind a pituitary-specific positive transcription factor(s), referred to as Pit-1. DNA complementary to Pit-1 mRNA, cloned on the basis of specific binding to AT-rich cell-specific elements in the rat prolactin and growth hormone genes, encodes a 33 kd protein with significant similarity at its carboxyl terminus to the homeodomains encoded by Drosophila developmental genes. Pit-1 mRNA is expressed exclusively in the anterior pituitary gland in both somatotroph and lactotroph cell types, which produce growth hormone and prolactin, respectively. Pit-1 expression in heterologous cells (HeLa) selectively activates prolactin and growth hormone fusion gene expression, suggesting that Pit-1 is sufficient to confer a characteristic pituitary phenotype. The structure of Pit-1 and its recognition elements suggests that metazoan tissue phenotype is controlled by a family of transcription factors that bind to related cis-active elements and contain several highly conserved domains.

Extinction of growth hormone expression in somatic cell hybrids involves repression of the specific trans-activator GHF-1

Growth hormone (GH) expression in pituitary-derived cells has been attributed to the presence of a positive trans-activator, GHF-1, which binds to two sites on the GH promoter. Somatic cell hybridization of non-GH-expressing L cells with pituitary-derived GH3 cells usually results in extinction of GH production. While previous studies showed that extinction occurs at the level of GH transcription, the exact mechanism remained elusive. We therefore characterized two parental cell lines and three hybrids, two of which extinguish GH expression and one in which GH is reexpressed after loss of mouse chromosomal material. Using in vivo transfections, in vitro transcription, DNAase I footprints, and immunoblotting experiments, no evidence for a direct repressor of GH transcription was found. Rather, extinction of GH expression in fibroblast x pituitary hybrids was accompanied by loss of GHF-1 protein and mRNA expression, suggesting that extinction occurs by repression of this trans-activator.

Effect of butyrate on thyroid hormone-mediated gene expression in rat pituitary tumour cells

These studies correlate the effects of (sodium) butyrate on intranuclear thyroid hormone receptor levels, with influences on both endogenous and transfected rat growth hormone (rGH) gene expression and regulation by L-triiodothyronine (T3). In rat anterior pituitary tumour (GH3) cells, 5.0 mM butyrate elicits a biphasic reduction in the number of nuclear T3 receptors. About 75% are depleted rapidly (t1/2 = 7 h), and the remaining receptors are depleted more slowly (t1/2 = 59 h). GH3 cells were treated with increasing concentrations of butyrate (0-5.0 mM), plus or minus 10 nM T3 for 48 h. Total cytoplasmic RNA, cellular protein and medium were analysed for rGH levels with radiolabelled rGH cDNA or antibodies. A greater than 50-fold increase in rGH mRNA level was seen after T3 treatment in the absence or presence of 0.1 mM butyrate. However, 1.0 and 5.0 mM butyrate decreased the stimulation of rGH mRNA levels by T3 to 10- and less than 2-fold, respectively. Control mRNA levels were decreased slightly by increasing butyrate concentrations; rGH mRNA level was 2- to 3-fold higher in the absence of 5 mM butyrate. The pattern of butyrate/T3 response displayed by both cellular and secreted rGH was similar to that seen with mRNA levels. Thus, the predominant effect of butyrate on T3-mediated regulation of growth hormone gene expression is at the level of transcription or mRNA accumulation. A hybrid gene containing 5'-flanking DNA from the rGH gene fused to the bacterial gene coding for chloramphenicol acetyl transferase (CAT), was used to transfect rat pituitary tumour cells with or without butyrate and T3 treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

A c-erb-A binding site in rat growth hormone gene mediates trans-activation by thyroid hormone

The substance 3,5,3-triiodothyronine (T3) stimulates growth hormone gene transcription in rat pituitary tumour cells. This stimulation is thought to be mediated by the binding of nuclear T3 receptors to regulatory elements 5' to the transcriptional start site. Understanding of the mechanism by which thyroid hormone activates gene transcription has been limited by failure to purify nuclear T3 receptors because of their low abundance, and by the absence of defined T3 receptor-DNA binding sites affecting T3 regulation. Recently, human and avian c-erb-A gene products have been shown to bind thyroid hormone with high affinity and to have a molecular weight and nuclear association characteristic of the thyroid hormone receptor. In the present report, we describe the development of an avidin-biotin complex DNA-binding assay which can detect specific, high-affinity binding of rat pituitary cell T3 receptors to the sequence 5'CAGGGACGTGACCGCA3', located 164 base pairs 5' to the transcriptional start site of the rat growth hormone gene. An oligonucleotide containing this sequence transferred T3 regulation to the herpes simplex virus thymidine kinase promoter in transfected rat pituitary GC2 cells, and specifically bound an in vitro translation product of the human placental c-erb-A gene. The data provide supporting evidence that the human c-erb-A gene product mediates the transcriptional effects of T3 and also that GC2 cell nuclear extracts contain additional factors that modify the binding of pituitary T3 receptors to the rat growth hormone gene T3 response element.