Glucocorticoid receptor binding sites in the promoter region of milk protein genes

Buffalo alpha S1-casein gene 5'-flanking region and its interspecies comparison

The expression of milk protein genes is tightly regulated in a spatio-temporal manner through the combinatorial interaction of lactogenic hormones and a set of transcription factors mediating developmental and tissue-specific gene expression. The recruitment of a unique set of transcription factors is determined by the cis-regulatory motifs present in the gene promoter region. Here, we report the isolation, sequencing, structural analysis and interspecies comparison of the 5'cis-regulatory region of the buffalo alpha S1 (αS1)-casein gene. The proximal promoter region of the buffalo αS1-casein gene harbored the insertion of a 72-bp fragment of long interspersed nuclear element of the L1_BT retrotransposon family. Among the core and vertebrate-specific promoter elements, the motifs for the binding of Brn POU domain factors (BRNF), Lim homeodomain factors (LHXF), NK6 homeobox transcription factors (NKX6), nuclear factor kappa B/c-rel (NFKB), AT-rich interactive domain factor (ARID), Brn POU domain factor 5 (BRN5), pancreatic and intestinal homeodomain transcription factor (PDX1), Distal-less homeodomain transcription factors (DLXF), T-cell factor/lymphoid enhancer-binding factor-1 (LEFF) and GHF-1 pituitary-specific POU domain transcription factor (PIT1) were over-represented in the αS1-casein gene regulatory region (Z score >4.0). The Multiple EM for Motif elicitation predicted three motifs which consisted of the sequences known to bind mammary gland factor/signal transducer and activator of transcription 5 (MGF/STAT5), estrogen receptor-related alpha (ERα), steroidogenic factor 1 (SF1) and glucocorticoid receptor (GR), indicating their potential role in the mammary gland-specific gene expression. The interspecies comparison of the proximal promoter region revealed conserved sequences for TATA boxes and MGF/STAT5 in all species, whereas activator protein 1 (AP1), pregnancy-specific mammary nuclear factor (PMF), CCAAT/enhancer binding protein (C/EBP), double-stranded and single-stranded DNA-binding protein 1 (DS1 and SS), ying and yang factor 1 (YY1), and GR half-sites were among ruminants. The functional significance of the L1_BT retrotransposon insertion on the buffalo αS1-casein gene expression needs to be experimentally validated.

Glucocorticoids regulate the human occludin gene through a single imperfect palindromic glucocorticoid response element

The 65kDa protein occludin is an essential element of the blood-brain barrier. This integral membrane protein represents an important part of the tight junctions, which seal and protect the blood brain barrier against paracellular diffusion of solutes to the brain parenchyme and are therefore responsible for the high resistance and low permeability between cerebral capillary endothelial cells. However, the molecular basis for the regulation of occludin gene expression is only incompletely understood. In former projects we showed that treatment of a brain microvascular cell line, cEND, with glucocorticoids resulted in increased occludin expression in cell-cell-contacts [Förster, C., Silwedel, C., Golenhofen, N., Burek, M., Kietz, S., Mankertz, J., Drenckhahn, D., 2005. Occludin as direct target for glucocorticoid-induced improvement of blood-brain barrier properties in a murine in vitro system. J. Physiol. 565, Pt 2, 475-486]. Induction of occludin expression by glucocorticoids was shown to be dependent on the glucocorticoid receptor. This study aims to identify the underlying molecular mechanism of gene expression and to identify potential glucocorticoid receptor binding sites within the occludin promoter, the glucocorticoid response elements. We identified one candidate glucocorticoid response element within the distal part of the occludin promoter that differs from the consensus glucocorticoid response element by the presence of a 4-basepair instead of a 3-basepair spacer between two highly degenerate halfsites (5'-ACATGTGTTTACAAAT-3'). Chromatin immunoprecipitation assay and site-directed mutagenesis confirmed binding of the glucocorticoid receptor to this site. The need for glucocorticoid receptor dimerization to induce gene expression was further confirmed by transfection studies using wild type and glucocorticoid receptor dimerization-deficient expression vectors, indicating that transactivation of occludin occurs through the glucocorticoid response element (GRE).

The role of glucocorticoids in secretory activation and milk secretion, a historical perspective

In this review we present our current understanding of the role of glucocorticoids in secretory activation and milk secretion by looking at the literature from a historical perspective. We begin with the early endocrine ablation experiments and continue from there to show that glucocorticoids are not just necessary for secretory activation and milk secretion--but mandatory. Specifically, we discuss the importance of glucocorticoids to: (1) induce the formation of ultrastructural components necessary to support milk synthesis and secretion, including rough endoplasmic reticulum and tight junction sealing; (2) regulate milk protein gene expression; and (3) prevent the second phase of involution, possibly by preventing the breakdown of the extracellular matrix.

Involvement of the ubiquitous Oct-1 transcription factor in hormonal induction of beta-casein gene expression

Transcription of the milk protein beta-casein gene is induced by the lactogenic hormones Prl (prolactin) and glucocorticoids. Multiple transcription factors involved in this induction have been identified, including the STAT5 (signal transducer and activator of transcription 5) and the GR (glucocorticoid receptor). Our previous studies have identified a binding site for the ubiquitous Oct-1 (octamer-binding transcription factor 1) protein in the lactogenic hormonal regulatory region of the mouse beta-casein promoter. In the present study, we report that Oct-1 is indeed expressed and binds to the beta-casein promoter in mammary epithelial cells. Oct-1 activates hormonally induced beta-casein promoter activity in a dose-dependent manner. Hormonal induction of promoter activity was decreased not only by mutating the Oct-1-binding site from ATTAGCAT to GCTAGCAT, which abolishes Oct-1 binding (50% decrease, P<0.01), but also by changing the site to the consensus Oct-1-binding motif ATTTGCAT (40% decrease, P<0.01). Reversing the Oct-1-binding site reduced hormonal induction by 70% (P<0.01), showing that orientation of Oct-1 binding is also critical in hormonal action. In transient transfection experiments, Oct-1 collaboratively transactivated the beta-casein gene promoter with STAT5 and/or GR in the presence of Prl receptor in cells treated with the lactogenic hormones. The C-terminus of Oct-1 was not essential to its function. The results of the present study provide biochemical evidence that the ubiquitous Oct-1 transcription factor may be involved in hormonally regulated, tissue-specific beta-casein gene expression.

In silico modelling of hormone response elements

BACKGROUND

An important step in understanding the conditions that specify gene expression is the recognition of gene regulatory elements. Due to high diversity of different types of transcription factors and their DNA binding preferences, it is a challenging problem to establish an accurate model for recognition of functional regulatory elements in promoters of eukaryotic genes.

RESULTS

We present a method for precise prediction of a large group of transcription factor binding sites - steroid hormone response elements. We use a large training set of experimentally confirmed steroid hormone response elements, and adapt a sequence-based statistic method of position weight matrix, for identification of the binding sites in the query sequences. To estimate the accuracy level, a table of correspondence of sensitivity vs. specificity values is constructed from a number of independent tests. Furthermore, feed-forward neural network is used for cross-verification of the predicted response elements on genomic sequences.

CONCLUSION

The proposed method demonstrates high accuracy level, and therefore can be used for prediction of hormone response elements de novo. Experimental results support our analysis by showing significant improvement of the proposed method over previous HRE recognition methods.

Integration of prolactin and glucocorticoid signaling at the beta-casein promoter and enhancer by ordered recruitment of specific transcription factors and chromatin modifiers

Lactogenic hormone regulation of beta-casein gene expression in mammary epithelial cells provides an excellent system in which to perform kinetic studies of chromatin remodeling and transcriptional activation. Using HC11 cells as a model, we have investigated the effects of prolactin (Prl) and glucocorticoids both singly and in combination at different time points after hormone treatment. Using chromatin immunoprecipitation analysis, we have determined the dynamics of assembly and disassembly of signal transducer and activator of transcription 5, glucocorticoid receptor, CCAAT enhancer binding protein beta, and Ying Yang-1 at the hormonally activated beta-casein proximal promoter as well as the distal mouse beta-casein enhancer located approximately -6 kb upstream of the transcription start site. Prl alone resulted in a rapid recruitment of both signal transducer and activator of transcription 5 and histone deacetylase 1 to the beta-casein promoter and enhancer, and reciprocally the dissociation of Ying Yang-1 from the proximal promoter. In addition, we have examined the recruitment of coactivator p300 and determined chromatin acetylation status as a function of hormonal treatment. Finally, we have established the time course of RNA polymerase II and phospho-RNA polymerase II accumulation at the beta-casein promoter and enhancer after stimulation with hydrocortisone and Prl. Although glucocorticoids alone led to a rapid increase in histone H3 acetylation, treatment with both hormones was required for stable association of p300 and phospho-RNA polymerase II at both the promoter and enhancer. Collectively, these data suggest a model for the assembly of a multiprotein complex that helps to define how the signaling pathways controlled by these lactogenic hormones are integrated to regulate beta-casein gene expression.

Glucocorticoids enhance activation of the human type II 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase gene

Glucocorticoids indirectly alter adrenocortical steroid output through the inhibition of ACTH secretion by the anterior pituitary. However, previous studies suggest that glucocorticoids can directly affect adrenocortical steroid production. Therefore, we have investigated the ability of glucocorticoids to affect transcription of adrenocortical steroid biosynthetic enzymes. One potential target of glucocorticoid action in the adrenal is an enzyme critical for adrenocortical steroid production: 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase (3beta-HSD). Treatment of the adrenocortical cell line (H295R) with the glucocorticoid agonist dexamethasone (DEX) increased cortisol production and 3beta-HSD mRNA levels alone or in conjunction with phorbol ester. This increase in 3beta-HSD mRNA was paralleled by increases in Steroidogenic Acute Regulatory Protein (StAR) mRNA levels. The human type II 3beta-HSD promoter lacks a consensus palindromic glucocorticoid response element (GRE) but does contain a Stat5 response element (Stat5RE) suggesting that glucocorticoids could affect type II 3beta-HSD transcription via interaction with Stat5. Transfection experiments show enhancement of human type II 3beta-HSD promoter activity by coexpression of the glucocorticoid receptor (GR) and Stat5A and treatment with 100nM dexamethasone. Furthermore, removal of the Stat5RE either by truncation of the 5' flanking sequence in the promoter or introduction of point mutations to the Stat5RE abolished the ability of DEX to enhance 3beta-HSD promoter activity. These studies demonstrate the ability of glucocorticoids to directly enhance the expression of an adrenal steroidogenic enzyme gene albeit independent of a consensus palindromic glucocorticoid response element.

The mineralocorticoid receptor may compensate for the loss of the glucocorticoid receptor at specific stages of mammary gland development

To study the role of glucocorticoid receptor (GR) at different stages of mammary gland development, mammary anlage were rescued from GR-/- mice by transplantation into the cleared fat pad of wild-type mice. In virgin mice, GR-/- outgrowths displayed abnormal ductal morphogenesis characterized by distended lumena, multiple layers of luminal epithelial cells in some regions along the ducts, and increased periductal stroma. In contrast, the loss of GR did not result in overt phenotypic changes in mammary gland development during pregnancy, lactation, and involution. Surprisingly, despite the known synergism between glucocorticoids and prolactin in the regulation of milk protein gene expression, whey acidic protein and beta-casein mRNA levels were unaffected in GR-/- transplants as compared with wild-type transplants. That mineralocorticoid receptor (MR) might compensate for the loss of GR was suggested by the detection of MR in the mammary gland at d 1 of lactation. This hypothesis was tested using explant cultures derived from the GR-/- transplants in which the mineralocorticoid fludrocortisone was able to synergistically induce beta-casein gene expression in the presence of prolactin and insulin. These studies suggest that MR may compensate for the absence of GR at some, but not at all stages of mammary gland development.

Nuclear factor 1-C2 contributes to the tissue-specific activation of a milk protein gene in the differentiating mammary gland

Members of the nuclear factor 1 (NF1) transcription factor family have been postulated to be involved in the regulation of milk genes. In this work we have been able to identify the splice variant NF1-C2 as an important member of a tissue-specific activating complex that regulates the milk gene encoding carboxyl ester lipase (CEL). Mutation of the NF1-binding site in the CEL gene promoter results in a drastic reduction of the gene expression to about 15% in mammary epithelial cells. Furthermore, we demonstrate that the NF1-C2 protein interacts with a higher affinity to the NF1-binding site in the CEL gene promoter than other NF1 family members do and that NF1-C2 in the mouse mammary gland is a phosphorylated protein. During development of the mouse mammary gland, binding of NF1-C2 to the CEL gene promoter is induced at midpregnancy, in correlation with the induction of CEL gene expression. The fact that the NF1-C2 involving complex remains throughout the lactation period and decreases during the weaning period, when the CEL gene is down-regulated, supports its importance in the regulation of CEL gene expression. To our knowledge, this is the first report identifying a specific, endogenously expressed NF1 isoform to be involved in the tissue-specific activation of a gene.

A distal region, hypersensitive to DNase I, plays a key role in regulating rabbit whey acidic protein gene expression

The aim of the present study was to identify the functional domains of the upstream region of the rabbit whey acidic protein (WAP) gene, which has been used with considerable efficacy to target the expression of several foreign genes to the mammary gland. We have shown that this region exhibits three sites hypersensitive to DNase I digestion in the lactating mammary gland, and that all three sites harbour elements which can bind to Stat5 in vitro in bandshift assays. However, not all hypersensitive regions are detected at all stages from pregnancy to weaning, and the level of activated Stat5 detected in the rabbit mammary gland is low except during lactation. We have studied the role of the distal site, which is only detected during lactation, in further detail. It is located within a 849 bp region that is required to induce a strong expression of the chloramphenicol acetyltransferase reporter gene in transfected mammary cells. Taken together, these results suggest that this region, centred around a Stat5-binding site and surrounded by a variable chromatin structure during the pregnancy-lactation cycle, may play a key role in regulating the expression of this gene in vivo. Furthermore, this distal region exhibits sequence similarity with a region located around 3 kb upstream of the mouse WAP gene. The existence of such a distal region in the mouse WAP gene may explain the differences in expression between 4.1 and 2.1 kb mouse WAP constructs.

Expression level-dependent contribution of glucocorticoid receptor domains for functional interaction with STAT5

The action of the glucocorticoid receptor (GR) on beta-casein gene transcription serves as a well-studied example of a case where the action of the GR is dependent on the activity of another transcription factor, STAT5. We have investigated the domain-requirement of the GR for this synergistic response in transfection experiments employing GR mutants and CV-1 or COS-7 cells. The results were influenced by the expression levels of the GR constructs. At low expression, STAT5-dependent transactivation by mutants of the GR DNA binding domain or N-terminal transactivation domain was impaired and the antiglucocorticoid RU486 exhibited a weak agonistic activity. When the N-terminal region of the GR was exchanged with the respective domain of the progesterone receptor, STAT5-dependent transactivation was reduced at low and high expression levels. Only at high expression levels did the GR exhibit the properties of a coactivator and enhanced STAT5 activity in the absence of a functional DNA binding domain and of GR binding sites in the proximal region of the beta-casein gene promoter. Furthermore, at high GR expression levels RU486 was nearly as efficient as dexamethasone in activating transcription via the STAT5 dependent beta-casein gene promoter. The results reconcile the controversial issue regarding the DNA binding-independent action of the GR together with STAT5 and provide evidence that the mode of action of the GR depends not only on the type of the particular promoter at which it acts but also on the concentration of the GR. GR DNA binding function appears to be mandatory for beta-casein gene expression in mammary epithelial cells, since the promoter function is completely dependent on the integrity of GR binding sites in the promoter.

Synergistic and antagonistic interactions of transcription factors in the regulation of milk protein gene expression. Mechanisms of cross-talk between signalling pathways

The stage and tissue specific expression of milk protein genes in the mammary gland is controlled by modular response regions with multiple binding sites for distinct classes of transcription factors, which either co-operate or are antagonistic. In addition, the activity of some of these factors is individually control-led by diverse extracellular signals. A well studied paradigm for a synergistic co-operation is the activation of beta-casein gene transcription by prolactin and glucocorticoids mediated by the signal transducer and activator of transcription STAT5 and the glucocorticoid receptor (GR). As an example for an antagonistic interaction we can demonstrate inhibition of prolactin signalling by TNF-alpha, which is mediated by NF-kappa B. In both cases, the interactions occur at several levels: For GR and STAT5, the synergy is discussed to be promoted by protein-protein interactions. Furthermore, we can demonstrate a co-operation between GR and STAT5 in DNA binding by a mechanism, which is dependent on the integrity of the DNA binding domain of the GR and on the existence of half-palindromic GR binding sites in the hormone response region. Indirect effects of glucocorticoids by modulation of the expression of secondary genes are also important. They might account for the observed enhancement of prolactin induced tyrosine phosphorylation of STAT5 by glucocorticoids. For NF-kappa B and STAT5, one component of the antagonism is the inhibition of STAT5 tyrosine phosphorylation by activation of NF-kappa B. Another potential mechanism is the inhibition of DNA binding of STAT5 due to overlapping binding sites for STAT5 and NF-kappa B in the beta-casein gene promoter. Thus, synergistic and antagonistic interactions between GR, NF-kappa B, and STAT5 involve (a) cross-talk mechanisms influencing the activation of STAT5 and (b) promoter-dependent interactions modulating the DNA binding activity of the transcription factors.

Cooperative effects of STAT5 (signal transducer and activator of transcription 5) and C/EBPbeta (CCAAT/enhancer-binding protein-beta) on beta-casein gene transcription are mediated by the glucocorticoid receptor

Beta-casein gene transcription is controlled primarily by a composite response element (CoRE) that integrates signaling from the lactogenic hormones, PRL, insulin, and hydrocortisone, in mammary epithelial cells. This CoRE contains binding sites for STAT5 (signal transducer and activator of transcription 5) and C/EBPbeta (CCAAT/enhancer-binding protein-beta) and several half-sites for glucocorticoid receptor (GR). To examine how interactions among these three transcription factors might regulate beta-casein gene transcription, a COS cell reconstitution system was employed. Cooperative transactivation was observed when all three factors were expressed, but unexpectedly was not seen between STAT5 and C/EBPbeta in the absence of full-length, transcriptionally active GR. Cooperativity required the amino-terminal transactivation domain of C/EBPbeta, and neither C/EBPalpha nor C/EBPdelta was able to substitute for C/EBPbeta when cotransfected with STAT5 and GR. Different GR determinants were needed for transcriptional cooperation between STAT5 and GR as compared with those required for all three transcription factors. These studies provide some new insights into the mechanisms responsible for high level, tissue-specific expression conferred by the beta-casein CoRE.

Cross-talk between signal transducer and activator of transcription (Stat5) and thyroid hormone receptor-beta 1 (TRbeta1) signaling pathways

PRL and T3 are involved in antagonistic regulations during various developmental processes in vertebrate species. We have studied cross-talk between transcription factors activated by these signaling pathways, i.e. signal transducer and activator of transcription 5 (Stat5) and thyroid hormone receptor beta1 (TRbeta1). Liganded TRbeta1 in the presence of its heterodimeric partner, retinoid X receptor gamma (RXRgamma), inhibited the PRL-induced Stat5a- and Stat5b-dependent reporter gene expression by up to 60%. This T3-inhibitory effect studied on Stat5 activity was partly reversed by overexpression of a TRbeta1 dominant negative variant mutated within its nuclear localization signal (TR2A). We next showed that TRbeta1 and TR2A in the presence of RXRgamma increased and decreased, respectively, Stat5 localization into the nucleus regardless of hormonal stimulation. Thus, our data suggest that TRbeta1 can be associated with Stat5 in the cytoplasm and may be involved in Stat5 nuclear translocation. In PRL-treated cells overexpressing TRbeta1/RXRgamma, both Stat5 and TRbeta1 were coimmunoprecipitated, indicating physical association of the two transcription factors. In these cells, addition of T3 with ovine (o)PRL decreased the amounts of total and tyrosine-phosphorylated Stat5 in the cytoplasm compared with oPRL-treated cells. In the nucleus, no clear difference was observed on Stat5 DNA-binding after treatment with PRL and T3 vs. PRL alone in TRbeta1/RXRgamma transfected cells. However, antibodies directed against TRbeta1 lowered Stat5-DNA binding and addition of the deacetylase inhibitor trichostatin A (TSA) relieved T3 inhibition on Stat5 transcriptional activity. Thus, we postulated that the negative cross-talk between TR and Stat5 on target genes could involve histone deacetylase recruitment by liganded TRbeta1.

Regulation of milk protein gene expression

Studies using both transgenic mice and transfected mammary epithelial cells have established that composite response elements containing multiple binding sites for several transcription factors mediate the hormonal and developmental regulation of milk protein gene expression. Activation of signal transduction pathways by lactogenic hormones and cell-substratum interactions activate transcription factors and change chromatin structure and milk protein gene expression. The casein promoters have binding sites for signal transducers and activators of transcription 5, Yin Yang 1, CCAAT/enhancer binding protein, and the glucocorticoid receptor. The whey protein gene promoters have binding sites for nuclear factor I, as well as the glucocorticoid receptor and the signal transducers and activators of transcription 5. The functional importance of some of these factors in mammary gland development and milk protein gene expression has been elucidated by studying mice in which some of these factors have been deleted.

Transgenic animal bioreactors in biotechnology and production of blood proteins

The regulatory elements of genes used to target the tissue-specific expression of heterologous human proteins have been studied in vitro and in transgenic mice. Hybrid genes exhibiting the desired performance have been introduced into large animals. Complex proteins like protein C, factor IX, factor VIII, fibrinogen and hemoglobin, in addition to simpler proteins like alpha 1-antitrypsin, antithrombin III, albumin and tissue plasminogen activator have been produced in transgenic livestock. The amount of functional protein secreted when the transgene is expressed at high levels may be limited by the required posttranslational modifications in host tissues. This can be overcome by engineering the transgenic bioreactor to express the appropriate modifying enzymes. Genetically engineered livestock are thus rapidly becoming a choice for the production of recombinant human blood proteins.

A distal enhancer region in the human beta-casein gene mediates the response to prolactin and glucocorticoid hormones

The 5' flanking region of the human beta-casein gene was investigated for the presence of regulatory sequences mediating the action of the lactogenic hormones prolactin and dexamethasone. DNA encompassing 9389 base pairs of the flanking region was isolated and a sequence comparison performed with regulatory regions previously identified in the beta-casein gene of rodents and ruminants. The analysis revealed the presence of a distal region between -4700 and -4550 with a high percentage of identity to the bovine beta-casein enhancer region, and a proximal region between -1 and -200 similar to the proximal promoter regions found in rodents and ruminants. Reporter gene constructs under the control of the distal or the proximal region of the human beta-casein gene were tested for their responsiveness to prolactin and dexamethasone. In transfection experiments, the distal region functioned as a lactogenic hormone inducible enhancer, whereas the proximal region exhibited low activity. In electromobility shift assays, multiple binding sites for Stat5, CCAAT/enhancer-binding proteins, and Ets domain proteins were identified in the distal human enhancer. These transcription factors have already been demonstrated as important regulators of the transcription of milk protein genes in rodents. Thus, a common set of transcription factors appears to be required for the expression of the human beta-casein gene and of milk protein genes in other species.

Characterization of Stat5a and Stat5b homodimers and heterodimers and their association with the glucocortiocoid receptor in mammary cells

The lactogenic hormones, i.e., prolactin and glucocorticoids, act in concert to stimulate transcription factors responsible for hormone-dependent milk protein gene expression. In the mammary gland, prolactin activates Stat5a and Stat5b and glucocorticoids activate the glucocorticoid receptor (GR). Immunoprecipitation experiments revealed that in mammary cells, Stat5a, Stat5b, and the GR are physically associated in vivo. The association is not dependent on lactogenic hormone treatment and is evident at all stages of mammary gland development. Immunodepletion experiments indicated that a fraction of GR and Stat5 proteins are not associated, suggesting that there are different intracellular pools of these proteins. Lactogenic hormone treatment of HC11 mammary cells resulted in tyrosine phosphorylation of Stat5a and Stat5b, dimerization, and rapid nuclear translocation of both Stat5 proteins. Following hormone treatment, Stat5a-Stat5b heterodimers were detected by their coimmunoprecipitation. In addition, immunodepletion experiments followed by gel shift analyses revealed the presence of active Stat5a and Stat5b homodimers. In mammary cells, Stat5b homodimers are less abundant than Stat5a homodimers. Although the GR does not bind the Stat5 DNA binding site directly, it could be detected with the Stat5-DNA complex. These results suggest that glucocorticoids affect milk protein gene expression via association of the GR with Stat5. Thus, there is a functional coupling between Stat-dependent and nuclear hormone receptor-dependent gene transcription.

Computer analysis of distribution of putative cis- and trans-regulatory elements in milk protein gene promoters

Multiple alignment of 28 milk protein gene promoters belonging to seven protein superfamilies is described. In these gene promoters three groups of common motifs were found: group I--specific for all milk protein gene promoters; group II--specific only for one gene superfamily; and group III--motifs shared by several gene superfamilies. Motifs of group I and III do not have any preferential location in the promoters, while group II motifs are located in the proximal part, from -36 to -224. Milk protein gene promoters were analysed for presence of putative binding sites for nine transcription factors important for the expression of this group of genes. The transcription factor binding sites for C/EBP, CTF/NF1, MAF and MGF were found in all promoters investigated. The set of putative transcription factor binding sites or response elements for GRE, IRE, PMF, STR and YY1 is unique for every gene superfamily.

Mechanism of interaction between the glucocorticoid receptor and Stat5: role of DNA-binding

The functional interaction between the glucocorticoid receptor (GR) and the signal transducer and activator of transcription-5 (Stat5) was investigated by studying the synergistic activation of beta-cascin gene transcription by prolactin and glucocorticoids. The synergism was shown to be mediated by a complex hormone response region with multiple binding sites for Stat5, the glucocorticoid receptor, and CCAAT/enhancer binding proteins (C/EBP). HC11 mammary epithelial cells, which contain physiological levels of GR and Stat5, and COS-7 cells overexpressing GR and Stat5 were employed. In both cell types intact binding sites for Stat5 and the GR were a prerequisite for the synergism, whereas C/EBP sites were only required in HC11 cells. Interestingly, the GR sites employed for the synergism were nonclassical, half palindromic sites, which did not function in the absence of activated Stat5 to mediate the action of the GR on transcription. The interaction of GR and Stat5 triggered by the unusual configuration of binding sites appears to represent a novel mechanism by which these two distinct types of transcription factors cooperate. The mode of interaction provides an efficient means to restrict gene expression to conditions where both Stat5 and the GR are activated.

Specific DNA binding of Stat5, but not of glucocorticoid receptor, is required for their functional cooperation in the regulation of gene transcription

Prolactin and glucocorticoid hormone are signals which regulate the transcription of milk protein genes in mammary epithelial cells. We have investigated the molecular mechanisms by which these hormones cooperate in the induction of transcription. Both hormones activate latent transcription factors in the cytoplasm of mammary epithelial cells. Prolactin exerts its effect through binding to the extracellular domain of the prolactin receptor and through receptor dimerization. This leads to the activation of a protein tyrosine kinase (Jak2), which is noncovalently associated with the cytoplasmic domain of the prolactin receptor. Jak2 phosphorylates the signal transducer and transcription activator (Stat5) which causes its dimerization and nuclear translocation where Stat5 specifically binds to sequence elements in the promoter regions of milk protein genes. In comparison, the glucocorticoid receptor is activated by a lipophilic steroid ligand in the cytoplasm which causes allosteric changes in the molecule, dimerization, and nuclear localization. It has been demonstrated that Stat5 and the glucocorticoid receptor form a molecular complex which cooperates in the induction of transcription of the beta-casein gene. We have defined the DNA sequence requirements for this cooperative mechanism and have delimited the functional domains in Stat5 and the glucocorticoid receptor that are necessary for the functional interaction. We find that the Stat5 response element (Stat5RE) within the beta-casein gene promoter is sufficient to elicit the cooperative action of Stat5 and the glucocorticoid receptor on transcription. Activation of Stat5 through phosphorylation of tyrosine 694 is an absolute prerequisite for transcription. Deletion of the transactivation domain of Stat5 results in a molecule which cannot mediate transactivation by itself but can still cooperate with the glucocorticoid receptor. Mutated variants of the glucocorticoid receptor with a nonfunctional DNA binding domain or a DNA binding domain contributed by the estrogen receptor are still able to cooperate with Stat5 in transcriptional induction. Deletion of the ligand binding domain of the glucocorticoid receptor does not impede cooperation with Stat5, whereas deletion of the AF-1 transactivation domain does prevent cooperation. Our results indicate that the glucocorticoid receptor acts as a ligand-dependent coactivator of Stat5 independently of its DNA binding function.

Promoter-dependent synergy between glucocorticoid receptor and Stat5 in the activation of beta-casein gene transcription

Steroid hormone receptors and Stat factors comprise two distinct families of inducible transcription factors. Activation of a member of each family, namely the glucocorticoid receptor by glucocorticoids and Stat5 by prolactin, is required for the efficient induction of the expression of milk protein genes in the mammary epithelium. We have studied the mode of interaction between Stat5 and the glucocorticoid receptor in the activation of beta-casein gene transcription. The functional role of potential half-palindromic glucocorticoid receptor-binding sites mapped previously in the promoter region was investigated. beta-Casein gene promoter chloramphenicol acetyltransferase constructs containing mutations and deletions in these sites were tested for their responsiveness to the synergistic effect of prolactin and dexamethasone employing COS-7 cells or HC11 mammary epithelial cells. Synergism depended on promoter regions containing intact binding sites for the glucocorticoid receptor and Stat5. The carboxyl-terminal transactivation domains of Stat5a and Stat5b were not required for this synergism. Our results suggest that in lactogenic hormone response elements glucocorticoid receptor molecules bound to nonclassical half-palindromic sites gain competence as transcriptional activators by the interaction with Stat5 molecules binding to vicinal sites.

Functional interactions between Stat5 and the glucocorticoid receptor

Signal transduction pathways enable extracellular signals to activate latent transcription factors in the cytoplasm of cells. Dimerization, nuclear localization and binding to specific DNA sequences result in the induction of gene transcription by these proteins. These events are necessary for the functioning of the JAK/STAT pathway and of the glucocorticoid-receptor pathway. In the former, the protein Stat5, which is a member of a family of signal transducers and activators of transcription, is activated by cytokines, hormones and growth factors. These polypeptide ligands bind at the outside of the cell to specific transmembrane receptors and activate intracellular Janus protein tyrosine kinases (JAKs) to tyrosine-phosphorylate STAT proteins; interaction with the SH2 domain of the dimerization partner then confers the ability to bind to DNA at the STAT-response element and induce transcription. In the glucocorticoid-receptor pathway, the receptor interacts with its steroid hormone ligand in the cytoplasm, undergoes an allosteric change that enables the hormone receptor complex to bind to specific DNA-response elements (glucocorticoid response elements, or GRE) and modulate transcription. Although these pathways appear to be unrelated, we show here that the glucocorticoid receptor can act as a transcriptional co-activator for Stat5 and enhance Stat5-dependent transcription. Stat5 forms a complex with the glucocorticoid receptor which binds to DNA independently of the GRE. This complex formation between Stat5 and the glucocorticoid receptor diminishes the glucocorticoid response of a GRE-containing promoter.

Prolactin-mediated gene activation in mammary epithelial cells

Mammary epithelial cells grow and develop with the onset of sexual maturity. In addition, lobular alveolar structures are formed during pregnancy, and quiescent differentiated cells secrete high levels of milk proteins after parturition. These events are governed by multiple hormones and growth factors and involve the sequential and synergistic action of functionally distinct signal transduction pathways. Milk protein genes have been analyzed and composite response elements have been identified in the promoter sequences. Transcription factors, which relay the hormonal signals, bind to these sequences. The factor that confers prolactin simulation to milk protein gene transcription has recently been identified. MGF/Stat5 is a latent transcription factor that becomes activated by a tyrosine-specific protein kinase, Jak2, associated with the prolactin receptor. Tyrosine phosphorylation converts the latent factor into one with DNA-binding and transcriptional activation potential. The regulation of MGF/Stat5 in vitro and in vivo indicates that it is a central component of the lactogenic hormone signaling pathway. Involvement of MGF/Stat5 in the signaling by other cytokines indicates that the same factor might be involved in regulation of growth-promoting genes, primarily in hematopoietic cells.

CCAAT/enhancer-binding protein isoforms beta and delta are expressed in mammary epithelial cells and bind to multiple sites in the beta-casein gene promoter

Lactogenic hormone-dependent expression of the rat beta-casein gene in mammary epithelial cells is controlled via a complex regulatory region in the promoter. The sequence between -176 and -82 is the minimal region to confer the response to glucocorticoid hormone and prolactin on a heterologous promoter. The response is further enhanced by the region between -282 and -176. DNase I footprinting experiments and electromobility shift assays revealed the presence of four binding sites for CCAAT/enhancer-binding protein (C/EBP) isoforms in the hormone response region between -220 and -132. In nuclear extracts from mammary epithelial cells, the prevalent C/EBP isoform binding to these sites is beta (C/EBP-beta). C/EBP-delta is also present in mammary epithelial cells, whereas C/EBP-alpha is not detectable. The C/EBP sites are located in close proximity to the previously characterized binding sites for the prolactin-inducible mammary gland factor/signal transducer and activator of transcription-5, the nuclear factor YY1, and the glucocorticoid receptor. The importance of the two proximal C/EBP binding sites at the 5' border of the minimal region was tested by mutational analysis. Mutations of each site were found to inhibit strongly both the basal and the lactogenic hormone-induced transcription of a beta-casein gene promoter chloramphenicol acetyltransferase construct. The results implicate C/EBPs as important regulators of beta-casein gene expression in the mammary epithelium.

Nuclear factor I and mammary gland factor (STAT5) play a critical role in regulating rat whey acidic protein gene expression in transgenic mice

The rat whey acidic protein (WAP) gene contains a mammary gland-specific and hormonally regulated DNase I-hypersensitive site 830 to 720 bp 5' to the site of transcription initiation. We have reported previously that nuclear factor I (NFI) binding at a palindromic site and binding at a half-site are the major DNA-protein interactions detected within this tissue-specific nuclease-hypersensitive region. We now show that point mutations introduced into these NFI-binding sites dramatically affect WAP gene expression in transgenic mice. Transgene expression was totally abrogated when the palindromic NFI site or both binding sites were mutated, suggesting that NFI is a key regulator of WAP gene expression. In addition, a recognition site for mammary gland factor (STAT5), which mediates prolactin induction of milk protein gene expression, was also identified immediately proximal to the NFI-binding sites. Mutation of this site reduced transgene expression by approximately 90% per gene copy, but did not alter tissue specificity. These results suggest that regulation of WAP gene expression is determined by the cooperative interactions among several enhancers that constitute a composite response element.

Complete sequence of the ovine beta-casein-encoding gene and interspecies comparison

The 9149-bp transcription unit encoding ovine beta-casein (Cas) and 4636 bp of 5' flanking region were completely sequenced. The gene is composed of nine exons and its overall organization is similar to that of its counterparts from other species. Intron 4, the largest, shares three similar stretches (sizes ranging from 0.1 to 0.3 kb) with the region upstream from the transcription unit. These common sequences are part of short interspersed nuclear elements (SINE) specific to Bovidae (Bov). Intron 4 contains two 274-bp Bov-A2 SINE in opposite orientation, as well as a full-length 569-bp Bov-B SINE. This latter SINE, also present in caprine intron 4, is missing in cattle. This suggests that the amplification of Bov-SINE has continued after the divergence of cattle from sheep and goats, assuming that the presently known sequences are representative of these species.

Computer-aided analysis of potential transcription-factor binding sites in the rabbit beta-casein gene promoter

Computer analysis of putative cis- and trans-regulatory sequences in the promoter region of the rabbit beta-casein gene is described. Nucleotide sequences up to 2096 bp upstream of the initiation site were compared to known consensus sequences of both ubiquitous and specific transcription factor motifs as well as to those described as characteristic for milk protein genes. Our analysis demonstrated that 5 independent motifs described for milk protein gene promoters exist. Four of them--'Groenen structure', 'Yu-Lee' 1 and 6 sequence and 'Oka box A'--were found in the beta-casein gene promoter; surprisingly, no typical milk box was found. More than one hundred and fifty putative binding motifs were found, representing 56 various consensus sequences. These sequences are located both inside and outside structures typical of milk protein gene promoters and include sequences homologous to mammary gland specific, hormone specific and ubiquitous transcription factors. Our analysis suggests that milk protein gene promoters can bind many transcription factors and assure complex regulation by hormonal and tissue specific factors.

Involvement of Ets-related proteins in hormone-independent mammary cell-specific gene expression

Regulatory regions have been located in the 5' flanking sequence of the mouse whey acidic protein gene which contribute to its tissue- and stage-specific expression in the mammary gland. They can be functionally separated into elements which mediate the action of lactogenic hormones prolactin and glucocorticoids and elements which control mammary cell-specific transcription in the absence of hormones. By mutational analysis, we have located a site in the whey acidic protein promoter between -120 and -100 which is important for hormone-independent promoter function. In stably transfected HC11 mammary epithelial cells, the hormone-independent activity of the mutated promoter was reduced 40-fold, whereas the capability to respond to lactogenic hormones was retained. The site was specifically recognised by two nuclear factors contained in extracts of cultivated mammary epithelial cells or mammary glands. Electrophoretic mobility shift assay, DNase I footprinting and methylation interference experiments indicated a relation of both factors to the Ets family of DNA-binding proteins. One of these factors also recognised a functionally important site in the mammary cell-specific enhancer of the mouse mammary tumor virus long terminal repeat. The results suggest that factors related to the Ets family are important determinants in mammary cell-specific gene expression.