Mammary gland-specific nuclear factor is present in lactating rodent and bovine mammary tissue and composed of a single polypeptide of 89 kDa

Impact of STAT Proteins in Tumor Progress and Therapy Resistance in Advanced and Metastasized Prostate Cancer

Signal transducers and activators of transcription (STATs) are a family of transcription factors involved in several biological processes such as immune response, cell survival, and cell growth. However, they have also been implicated in the development and progression of several cancers, including prostate cancer (PCa). Although the members of the STAT protein family are structurally similar, they convey different functions in PCa. STAT1, STAT3, and STAT5 are associated with therapy resistance. STAT1 and STAT3 are involved in docetaxel resistance, while STAT3 and STAT5 are involved in antiandrogen resistance. Expression of STAT3 and STAT5 is increased in PCa metastases, and together with STAT6, they play a crucial role in PCa metastasis. Further, expression of STAT3, STAT5, and STAT6 was elevated in advanced and high-grade PCa. STAT2 and STAT4 are currently less researched in PCa. Since STATs are widely involved in PCa, they serve as potential therapeutic targets. Several inhibitors interfering with STATs signaling have been tested unsuccessfully in PCa clinical trials. This review focuses on the respective roles of the STAT family members in PCa, especially in metastatic disease and provides an overview of STAT-inhibitors evaluated in clinical trials.

Genome Wide Identification of Novel Long Non-coding RNAs and Their Potential Associations With Milk Proteins in Chinese Holstein Cows

Long non-coding RNAs (lncRNAs) have emerged as a novel class of regulatory molecules involved in various biological processes. However, their role in milk performance is unknown. Here, whole transcriptome RNA sequencing was used to generate the lncRNA transcriptome profiles in mammary tissue samples from 6 Chinese Holstein cows with 3 extremely high and 3 low milk protein percentage phenotypes. In this study, 6,450 lncRNA transcripts were identified through 5 stringent steps and filtration by coding potential. In total, 31 lncRNAs and 18 novel genes were identified to be differentially expressed in high milk protein samples (HP) relative to low milk protein samples (LP), respectively. Differentially expressed lncRNAs were selected to predict target genes through bioinformatics analysis, followed by the integration of differentially expressed mRNA data, gene function, gene ontology (GO) and pathway, genome wide association study (GWAS) and quantitative trait locus (QTL) information, as well as network analysis to further characterize potential interactions. Several lncRNAs were found (such as XLOC_059976) that could be used as candidate markers for milk protein content prediction. This is the first study to perform global expression profiling of lncRNAs and mRNAs related to milk protein traits in dairy cows. These results provide important information and insights into the synthesis of milk proteins, and potential targets for the future improvement of milk quality.

STAT3 activation in infection and infection-associated cancer

The Janus kinase/signal transducers and activators for transcription (JAK/STAT) pathway plays crucial roles in regulating apoptosis, proliferation, differentiation, and the inflammatory response. The JAK/STAT families are composed of four JAK family members and seven STAT family members. STAT3 plays a key role in inducing and maintaining a pro-carcinogenic inflammatory microenvironment. Recent evidence suggests that STAT3 regulates diverse biological functions in pathogenesis of diseases, such as infection and cancer. In the current review, we will summarize the research progress of STAT3 activation in infection and cancers. We highlight our recent study on the novel role of STAT3 in Salmonella infection-associated colon cancer. Infection with bacterial AvrA-expressing Salmonella activates the STAT3 pathway, which induces the β-catenin signals and enhances colonic tumorigenesis. STAT3 may be a promising target in developing prevention and treatment for infectious diseases and infection-associated cancers.

Crosstalk between STAT5 activation and PI3K/AKT functions in normal and transformed mammary epithelial cells

Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) have been shown to function downstream of several peptide hormones and cytokines that are required for postnatal development and secretory function of the mammary gland. As part of an extended network, these signal transducers can engage in crosstalk with other pathways to facilitate synergistic, and sometimes antagonistic, actions of different growth factors. Specifically, signaling through the JAK2/STAT5 cascade has been demonstrated to be indispensable for the specification, proliferation, differentiation, and survival of secretory mammary epithelial cells. Following a concise description of major cellular programs in mammary gland development and the role of growth factors that rely on JAK/STAT signaling to orchestrate these programs, this review highlights the significance of active STAT5 and its crosstalk with the PI3 kinase and AKT1 for mediating the proliferation of alveolar progenitors and survival of their functionally differentiated descendants in the mammary gland. Based on its ability to provide self-sufficiency in growth signals that are also capable of overriding intrinsic cell death programs, persistently active STAT5 can serve as a potent oncoprotein that contributes to the genesis of breast cancer. Recent experimental evidence demonstrated that, similar to normal developmental programs, oncogenic functions of STAT5 rely on molecular crosstalk with PI3K/AKT signaling for the initiation, and in some instances the progression, of breast cancer. The multitude by which STATs can interact with individual mediators of the PI3K/AKT signaling cascade may provide novel avenues for targeting signaling nodes within molecular networks that are crucial for the survival of cancer cells.

STAT5 and CD4 + T Cell Immunity

STAT5 plays a critical role in the development and function of many cell types. Here, we review the role of STAT5 in the development of T lymphocytes in the thymus and its subsequent role in the differentiation of distinct CD4 + helper and regulatory T-cell subsets.

Stat5 Exerts Distinct, Vital Functions in the Cytoplasm and Nucleus of Bcr-Abl+ K562 and Jak2(V617F)+ HEL Leukemia Cells

Signal transducers and activators of transcription (Stats) play central roles in the conversion of extracellular signals, e.g., cytokines, hormones and growth factors, into tissue and cell type specific gene expression patterns. In normal cells, their signaling potential is strictly limited in extent and duration. The persistent activation of Stat3 or Stat5 is found in many human tumor cells and contributes to their growth and survival. Stat5 activation plays a pivotal role in nearly all hematological malignancies and occurs downstream of oncogenic kinases, e.g., Bcr-Abl in chronic myeloid leukemias (CML) and Jak2(V617F) in other myeloproliferative diseases (MPD). We defined the mechanisms through which Stat5 affects growth and survival of K562 cells, representative of Bcr-Abl positive CML, and HEL cells, representative for Jak2(V617F) positive acute erythroid leukemia. In our experiments we suppressed the protein expression levels of Stat5a and Stat5b through shRNA mediated downregulation and demonstrated the dependence of cell survival on the presence of Stat5. Alternatively, we interfered with the functional capacities of the Stat5 protein through the interaction with a Stat5 specific peptide ligand. This ligand is a Stat5 specific peptide aptamer construct which comprises a 12mer peptide integrated into a modified thioredoxin scaffold, S5-DBD-PA. The peptide sequence specifically recognizes the DNA binding domain (DBD) of Stat5. Complex formation of S5-DBD-PA with Stat5 causes a strong reduction of P-Stat5 in the nuclear fraction of Bcr-Abl-transformed K562 cells and a suppression of Stat5 target genes. Distinct Stat5 mediated survival mechanisms were detected in K562 and Jak2(V617F)-transformed HEL cells. Stat5 is activated in the nuclear and cytosolic compartments of K562 cells and the S5-DBD-PA inhibitor most likely affects the viability of Bcr-Abl⁺ K562 cells through the inhibition of canonical Stat5 induced target gene transcription. In HEL cells, Stat5 is predominantly present in the cytoplasm and the survival of the Jak2(V617F)⁺ HEL cells is impeded through the inhibition of the cytoplasmic functions of Stat5.

Stat5 regulates the phosphatidylinositol 3-kinase/Akt1 pathway during mammary gland development and tumorigenesis

Stat5 (signal transducer and activator of transcription 5) is an essential mediator of cytokine receptor signaling and plays important roles in the proliferation of alveolar progenitors and the survival of functionally differentiated epithelial cells in the mammary gland. A deregulated expression and activation of Stat5 leads to precocious alveolar development in the absence of pregnancy hormones, impaired mammary gland remodeling following the cessation of lactation, and mammary tumor formation. We reported previously that Stat5 induces the transcription of the Akt1 gene from a novel promoter. In this report, we provide experimental evidence that Akt1 is an essential mediator for the biological function of Stat5 as a survival factor. Additionally, Stat5 controls the expression of the regulatory and catalytic subunits of the phosphatidylinositol 3-kinase (PI3K) (p85α and p110α), thereby greatly augmenting signaling through the prosurvival PI3K/Akt pathway. In agreement with this model, we observed that the constitutive activation of Stat5 cooperates with the loss of function of the tumor suppressor PTEN by accelerating the formation of preneoplastic lesions and mammary tumors. The mammary gland-specific ablation of Stat5 is sufficient to prevent mammary carcinogenesis in a genuine mouse model for Cowden syndrome. Therefore, targeting the Jak2/Stat5 pathway might be a suitable strategy to prevent breast cancer in patients that carry a mutant PTEN allele.

Epigenetic regulation of milk production in dairy cows

It is well established that milk production of the dairy cow is a function of mammary epithelial cell (MEC) number and activity and that these factors can be influenced by diverse environmental influences and management practises (nutrition, milk frequency, photoperiod, udder health, hormonal and local effectors). Thus, understanding how the mammary gland is able to respond to these environmental cues provides a huge potential to enhance milk production of the dairy cow. In recent years our understanding of molecular events within the MEC underlying bovine lactation has been advanced through mammary microarray studies and will be further advanced through the recent availability of the bovine genome sequence. In addition, the potential of epigenetic regulation (non-sequence inheritable chemical changes in chromatin, such as DNA methylation and histone modifications, which affect gene expression) to manipulate mammary function is emerging. We propose that a substantial proportion of unexplained phenotypic variation in the dairy cow is due to epigenetic regulation. Heritability of epigenetic marks also highlights the potential to modify lactation performance of offspring. Understanding the response of the MEC (cell signaling pathways and epigenetic mechanisms) to external stimuli will be an important prerequisite to devising new technologies for maximising their activity and, hence, milk production in the dairy cow.

Visualization of Stat3 and Stat5 transactivation activity with specific response element dependent reporter constructs integrated into lentiviral gene transfer vectors

BACKGROUND

Signal transducer and activator of transcription 3 and 5 (Stat3 and Stat5) play important roles in cell differentiation, proliferation, apoptosis and inflammation. They are transiently activated by ligand-receptor interactions in normal cells but are often found to be constitutively active in cancer cells. Analysis of their activation pattern is therefore important for the description of developmental processes and the understanding of cellular transformation.

MATERIALS AND METHODS

To visualize Stat3 and Stat5 transactivation activity in different cell types, we designed novel reporter constructs. These constructs comprise Stat3 or Stat5 specific promoter elements and reporter genes encoding β-galactosidase or fluorescent proteins. These constructs were integrated into lentiviral gene transfer vectors facilitating efficient transduction of most cell types.

RESULTS

The lentiviral reporter constructs were used to infect different cell types and their inducibility by activated Stat3 or Stat5 was measured. The Stat3-mCherry reporter was active in transduced tumor cells, which exhibit high levels of phosphorylated Stat3 and it was inducible in HepG2 liver cells by interleukin-6 treatment. The Stat5-LacZ reporter was active in cultured cells upon hormone induction of Stat5 and in primary mammary epithelial cells transplanted into cleared fat pads of mice during late pregnancy.

CONCLUSION

These novel reporter constructs are valuable tools to investigate and to distinguish between Stat3 and Stat5 activity in primary cells and cancer cells. They will also be useful in the discovery of drugs targeting Stat3 or Stat5. They can also be employed to generate transgenic mice and track Stat activity during development.

Transcription factor Stat5a/b as a therapeutic target protein for prostate cancer

Prostate cancer is the most common non-cutaneous cancer in Western males. The majority of prostate cancer fatalities are caused by development of castration-resistant growth and metastatic spread of the primary tumor. The average duration of the response of primary prostate cancer to hormonal ablation is less than 3 years, and 75% of prostate cancers in the United States progress to castration-resistant disease. The existing pharmacological therapies for metastatic and/or castration-resistant prostate cancer do not provide significant survival benefit. This review summarizes the importance of transcription factor Stat5 signaling in the pathogenesis of prostate cancer and discusses the molecular basis of Stat5a/b inhibition as a therapeutic strategy for prostate cancer.

Heat shock protein-90-alpha, a prolactin-STAT5 target gene identified in breast cancer cells, is involved in apoptosis regulation

INTRODUCTION

The prolactin-Janus-kinase-2-signal transducer and activator of transcription-5 (JAK2-STAT5) pathway is essential for the development and functional differentiation of the mammary gland. The pathway also has important roles in mammary tumourigenesis. Prolactin regulated target genes are not yet well defined in tumour cells, and we undertook, to the best of our knowledge, the first large genetic screen of breast cancer cells treated with or without exogenous prolactin. We hypothesise that the identification of these genes should yield insights into the mechanisms by which prolactin participates in cancer formation or progression, and possibly how it regulates normal mammary gland development.

METHODS

We used subtractive hybridisation to identify a number of prolactin-regulated genes in the human mammary carcinoma cell line SKBR3. Northern blotting analysis and luciferase assays identified the gene encoding heat shock protein 90-alpha (HSP90A) as a prolactin-JAK2-STAT5 target gene, whose function was characterised using apoptosis assays.

RESULTS

We identified a number of new prolactin-regulated genes in breast cancer cells. Focusing on HSP90A, we determined that prolactin increased HSP90A mRNA in cancerous human breast SKBR3 cells and that STAT5B preferentially activated the HSP90A promoter in reporter gene assays. Both prolactin and its downstream protein effector, HSP90alpha, promote survival, as shown by apoptosis assays and by the addition of the HSP90 inhibitor, 17-allylamino-17-demethoxygeldanamycin (17-AAG), in both untransformed HC11 mammary epithelial cells and SKBR3 breast cancer cells. The constitutive expression of HSP90A, however, sensitised differentiated HC11 cells to starvation-induced wild-type p53-independent apoptosis. Interestingly, in SKBR3 breast cancer cells, HSP90alpha promoted survival in the presence of serum but appeared to have little effect during starvation.

CONCLUSIONS

In addition to identifying new prolactin-regulated genes in breast cancer cells, we found that prolactin-JAK2-STAT5 induces expression of the HSP90A gene, which encodes the master chaperone of cancer. This identifies one mechanism by which prolactin contributes to breast cancer. Increased expression of HSP90A in breast cancer is correlated with increased cell survival and poor prognosis and HSP90alpha inhibitors are being tested in clinical trials as a breast cancer treatment. Our results also indicate that HSP90alpha promotes survival depending on the cellular conditions and state of cellular transformation.

The Stat family of transcription factors have diverse roles in mammary gland development

The Stat family of transcription factors have diverse roles in mammary gland development. Genetic studies in mice have revealed an essential requirement for Stat5a in development of secretory alveolar cells during pregnancy while Stat6, which is normally associated with differentiation of T helper cells, is important in the commitment of luminal cells to this alveolar lineage. In contrast, Stat3 is specifically activated at the initiation of post-lactational regression when it has an essential function in the regulation of cell death and tissue remodelling. Stat1 and Stat4 have been shown to be regulated during a mammary developmental cycle although whether they have specific, non-redundant roles is not clear. Thus, the adult mammary gland is somewhat unusual in that it is a tissue where different Stats are sequentially activated to orchestrate the processes of functional differentiation, cell death and tissue remodelling.

STAT5 activity in pancreatic β-cells

Signal transducer and activator of transcription (STAT)5A and -5B are latent transcription factors activated by cytokines and hormones of the cytokine family. In pancreatic insulin-secreting β-cells, STAT5A and -5B are activated primarily by prolactin and growth hormone stimulation and are important mediators of the potent stimulation of proliferation and insulin production caused by these hormones. STAT5A and -5B are both expressed in β-cells and control the expression of a number of mRNAs implicated in cell replication control, insulin biosynthesis and secretion. In addition to STAT5A and -5B being transcriptional activators, they may also repress gene transcription. By these means, STAT5 proteins increase the levels of anti-apoptotic transcripts in β-cells and repress expression of pro-apoptotic genes. This review focuses on the anti-apoptotic role of STAT5 signaling, providing a mechanism for β-cell resistance to pro-apoptotic cytokines, Type 1 diabetes mellitus and obesity-associated β-cell stress. It is clear from studies of STAT5 signaling in pancreatic β-cells that STAT5 is important for postnatal β-cell compensatory growth (as in pregnancy or obesity) and in the defense against β-cell stress factors.

Prostate epithelial cell fate

Androgen receptor (AR) within prostatic mesenchymal cells, with the absence of AR in the epithelium, is still sufficient to induce prostate development. AR in the luminal epithelium is required to express the secretory markers associated with differentiation. Nkx3.1 is expressed in the epithelium in early prostatic embryonic development and expression is maintained in the adult. Induction of the mouse prostate gland by the embryonic mesenchymal cells results in the organization of a sparse basal layer below the luminal epithelium with rare neuroendocrine cells that are interdispersed within this basal layer. The human prostate shows similar glandular organization; however, the basal layer is continuous. The strong inductive nature of embryonic prostatic and bladder mesenchymal cells is demonstrated in grafts where embryonic stem (ES) cells are induced to differentiate and organize as a prostate and bladder, respectively. Further, the ES cells can be driven by the correct embryonic mesenchymal cells to form epithelium that differentiates into secretory prostate glands and differentiated bladders that produce uroplakin. This requires the ES cells to mature into endoderm that gives rise to differentiated epithelium. This process is control by transcription factors in both the inductive mesenchymal cells (AR) and the responding epithelium (FoxA1 and Nkx3.1) that allows for organ development and differentiation. In this review, we explore a molecular mechanism where the pattern of transcription factor expression controls cell determination, where the cell is assigned a developmental fate and subsequently cell differentiation, and where the assigned cell now emerges with it's own unique character.

Historical perspectives of prolactin and growth hormone as mammogens, lactogens and galactagogues--agog for the future!

Around 80 years ago researchers first established that the pituitary gland regulates mammary gland function as demonstrated by the ability of its extracts to promote both mammogenesis and lactogenesis in animal models. Little did they realize that in fact two hormones, prolactin (PRL) and growth hormone (GH), were contributing to these effects. By the mid 1930s PRL had been purified as a distinct lactogen, while the galactopoietic effect of GH was confirmed after its purification in the 1940s. Interest in these hormones initially centered about their potential for increasing milk production, while in the latter half of the twentieth century it became obvious that these hormones also had the potential to influence mammary cancer development. During the past 50 years large strides have been made into understanding how these hormones signal to, and within, cells of the mammary gland, paralleling rapid developments in the fields of cellular and molecular biology. In compiling this review we have summarized the progress that has been made to date regarding roles for these hormones in the mammary gland, with a goal of ensuring that some of the seminal literature is not diluted or forgotten. In doing so it is clear that there are lessons to be learned from past experiences, where new methods and technologies will continue to present exciting new opportunities to revisit lingering questions regarding these fascinating hormones and this fascinating organ.

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.

The role of forkhead box A2 to restrict androgen-regulated gene expression of lipocalin 5 in the mouse epididymis

Murine epididymal retinoic acid-binding protein [or lipocalin 5 (Lcn5)] is synthesized and secreted by the principal cells of the mouse middle/distal caput epididymidis. A 5-kb promoter fragment of the Lcn5 gene can dictate androgen-dependent and epididymis region-specific gene expression in transgenic mice. Here, we reported that the 1.8-kb Lcn5 promoter confers epididymis region-specific gene expression in transgenic mice. To decipher the mechanism that directs transcription, 14 chimeric constructs that sequentially removed 100 bp of 1.8-kb Lcn5 promoter were generated and transfected into epididymal cells and nonepididymal cells. Transient transfection analysis revealed that 1.3 kb promoter fragment gave the strongest response to androgens. Between the 1.2-kb to 1.3-kb region, two androgen receptor (AR) binding sites were identified. Adjacent to AR binding sites, a Foxa2 [Fox (Forkhead box) subclass A] binding site was confirmed by gel shift assay. Similar Foxa binding sites were also found on the promoters of human and rat Lcn5, indicating the Foxa binding site is conserved among species. We previously reported that among the three members of Foxa family, Foxa1 and Foxa3 were absent in the epididymis whereas Foxa2 was detected in epididymal principal cells. Here, we report that Foxa2 displays a region-specific expression pattern along the epididymis: no staining observed in initial segment, light staining in proximal caput, gradiently heavier staining in middle and distal caput, and strongest staining in corpus and cauda, regions with little or no expression of Lcn5. In transient transfection experiments, Foxa2 expression inhibits AR induction of the Lcn5 promoter, which is consistent with the lack of expression of Lcn5 in the corpus and cauda. We conclude that Foxa2 functions as a repressor that restricts AR regulation of Lcn5 to a segment-specific pattern in the epididymis.

Coactivators in Gene Regulation by STAT5

Signal transducer and activator of transcription 5 (STAT5) is a member of the STAT family of transcription factors that relay the effect of diverse cytokines, hormones, and growth factors by regulating the transcription of distinct target genes. This function is emphasized by its crucial role in the development of the mammary gland and the hematopoietic system. Cytokine receptor-associated Janus kinases (JAKs) induce dimerization, nuclear translocation, and DNA binding through tyrosine phosphorylation of STAT5. STAT5 regulates the expression of cytokine target genes by binding to gamma interferon-activated sequence (GAS) motifs. Transcriptional activation requires the contact of STAT5 to coactivators and components of the transcription machinery. Another important point in transcriptional activation is the cooperation with other transcription factors that bind in close vicinity to the target gene promoters and enhancers. Their concerted action can result in an enhanced binding to the promoters or in cooperative recruitment of coactivators. In addition, cross-talk with other signaling pathways as well as secondary modifications of STAT5 have been described to affect transactivation function.

Cloning and characterization of 5' upstream promoter region of rat WAP gene

Regulatory regions of genes encoding milk proteins are frequently used to produce in the mammary gland of transgenic animals a variety of pharmaceutically and medically important human proteins. One such example is the whey acidic protein (WAP) promoter region, identified so far in the genome of mouse, rat, rabbit, camel, pig, brushtail possum and Tammar wallaby. The aim of the present study was cloning and characterization of the 5' upstream promoter region of rat WAP gene. Using Genome Walking procedure, we cloned the region extending from -849 to -3671 bp. We have shown that there are two conserved regions highly similar to hypersensitive sites present in mouse and rabbit upstream region of WAP gene with binding sites for STAT5 transcription factor, essential for expression of WAP gene in mammary glands during lactation. We characterized dispersed and tandem repeats in the upstream region of rat WAP gen localized not far away from the translation initiation site.

NCoA-1/SRC-1 is an essential coactivator of STAT5 that binds to the FDL motif in the alpha-helical region of the STAT5 transactivation domain

Signal transducer and activator of transcription 5 (STAT5) is a transcription factor that activates prolactin (PRL)-dependent gene expression in the mammary gland. For the activation of its target genes, STAT5 recruits coactivators like p300 and the CREB-binding protein (CBP). In this study we analyzed the function of p300/CBP-associated members of the p160/SRC/NCoA-family in STAT5-mediated transactivation of beta-casein expression. We found that only one of them, NCoA-1, acts as a coactivator for both STAT5a and STAT5b. The two coactivators p300/CBP and NCoA-1 cooperatively enhance STAT5a-mediated transactivation. For NCoA-1-dependent coactivation of STAT5, both the activation domain 1 and the amino-terminal bHLH/PAS domain are required. The amino-terminal region mediates the interaction with STAT5a in cells. A motif of three amino acids in an alpha-helical region of the STAT5a-transactivation domain is essential for the binding of NCoA-1 and for the transcriptional activity of STAT5a. Moreover we observed that NCoA-1 is involved in the synergistic action of the glucocorticoid receptor and STAT5a on the beta-casein promoter. These findings support a model in which STAT5, in concert with the glucocorticoid receptor, recruits a multifunctional coactivator complex to initiate the PRL-dependent transcription.

Essential functions of p21-activated kinase 1 in morphogenesis and differentiation of mammary glands

Although growth factors have been shown to influence mammary gland development, the nature of downstream effectors remains elusive. In this study, we show that the expression of p21-activated kinase (Pak)1, a serine/threonine protein kinase, is activated in mammary glands during pregnancy and lactation. By targeting an ectopic expression of a kinase-dead Pak1 mutant under the control of ovine beta-lactoglobulin promoter, we found that the mammary glands of female mice expressing kinase-dead Pak1 transgene revealed incomplete lobuloalveolar development and impaired functional differentiation. The expression of whey acidic protein and beta-casein and the amount of activated Stat5 in the nuclei of epithelial cells in transgenic mice were drastically reduced. Further analysis of the underlying mechanisms revealed that Pak1 stimulated beta-casein promoter activity in normal mouse mammary epithelial cells and also cooperated with Stat5a. Pak1 directly interacted with and phosphorylated Stat5a at Ser 779, and both COOH-terminal deletion containing Ser 779 of Stat5a and the Ser 779 to Ala mutation completely prevented the ability of Pak1 to stimulate beta-casein promoter. Mammary glands expressing inactive Pak1 exhibited a reduction of Stat5a Ser 779 phosphorylation. These findings suggest that Pak1 is required for alveolar morphogenesis and lactation function, and thus, identify novel functions of Pak1 in the mammary gland development.

Stimulation of hepatic signal transducer and activator of transcription 5b by GH is not altered by 3-methylcholanthrene

We are investigating the mechanisms by which aromatic hydrocarbons, such as 3-methylcholanthrene (MC), suppress hepatic cytochrome P450 2C11 (CYP2C11) gene expression. CYP2C11 is an enzyme expressed in the liver of male rats and is regulated by a pulsatile pattern of GH secretion. We have previously shown that MC attenuates the stimulatory effect of GH on CYP2C11 expression in hypophysectomized male rats. In follow-up studies we evaluated the effect of MC on GH-stimulated signal transducer and activator of transcription 5b (STAT5b) phosphorylation, nuclear translocation, and DNA-binding activity. GH-stimulated increases in hepatic nuclear STAT5b and phospho-STAT5b levels were not different between groups of hypophysectomized rats receiving MC or vehicle. This observation was corroborated at the DNA-binding level by EMSA. We also measured GH-induced STAT5b activation in the H4IIE rat hepatoma cell line. STAT5b DNA-binding activity detected in GH-treated cells was not affected by MC. Immunocytochemistry experiments revealed no effect of MC on GH-stimulated STAT5b nuclear translocation in H4IIE cells. These in vivo and in vitro data suggest that interference with GH-stimulated STAT5b activation does not constitute a mechanism by which MC attenuates the stimulatory effect of GH on CYP2C11 gene expression.

Effects of hyperactive Janus kinase 2 signaling in mammary epithelial cells

Prolactin, the Janus kinase 2 (JAK2) and the signal transducer and activator of transcription 5 (STAT5) are important for mammary gland development and have also been implicated in development and growth of breast tumors. In the present study we have investigated the role for JAK2 in proliferation, differentiation, and apoptosis of the mammary epithelial cell line HC11 by stably overexpressing two hyperactive JAK2 mutants. Cells expressing a JAK2 mutant consisting of only the kinase domain had high amount of nuclear STAT5 protein with low DNA-binding activity, which was rapidly induced to a DNA-binding state by prolactin treatment. Cells expressing JAK2 deleted of the kinase-like domain showed increased sensitivity to prolactin treatment compared to wild type cells. Proliferation was not affected by any of the mutants whereas the ability to undergo apoptosis was decreased implicating a transforming potential of the JAK2 mutants.

Synergistic action of prolactin (PRL) and androgen on PRL-inducible protein gene expression in human breast cancer cells: a unique model for functional cooperation between signal transducer and activator of transcription-5 and androgen receptor

The signal transducer and activator of transcription 5 (Stat5) has been shown to cooperate with some nuclear receptors. However, an interaction has never been demonstrated with the androgen receptor (AR). Given that the PRL-inducible protein/gross cystic disease fluid-15 (PIP/GCDFP-15) is both a PRL-controlled and an androgen-controlled protein, we used its promoter region to investigate the potential interaction between Stat5 and androgen receptor. Dihydrotestosterone or PRL alone slightly modulated or did not modulate the luciferase activity of all reporter gene constructs. In contrast, a maximal increase was observed using the -1477+42 reporter gene construct after exposure to both dihydrotestosterone and PRL. The requirement of half-site androgen-responsive elements and two consensus Stat5-binding elements, Stat5#1 and Stat5#2, was determined by site-directed mutagenesis. Activated Stat5B binds with a higher affinity to Stat5#2 than to Stat5#1. Stat5ADelta749 and Stat5BDelta754 mutants demonstrated that the Stat5 trans-activation domain is involved in the hormonal cooperation. The cooperation depends on the PRL-induced phosphorylation on Tyr(694) in Stat5A and Tyr(699) in Stat5B, as demonstrated using the Stat5AY694F and Stat5BY699F proteins. The use of AR Q798E, C619Y, and C784Y mutants showed that trans-activation, DNA-binding, and ligand-binding domains of AR are essential. Our study thus suggests a functional cooperation between AR and Stat5.

Differential induction of transcription factors and expression of milk protein genes by prolactin and growth hormone in the mammary gland of rabbits

Previously we demonstrated that administration of lactogenic hormones - prolactin (PRL) and growth hormone (GH) - to pregnant rabbits differentially induces expression of casein and whey proteins in the mammary gland. Now we extend these observations to transcription factors (TFs) that are responsive for differential induction of milk protein genes. Analysis of correlation between the number of putative TF binding sites in 5'-upstream sequences and the levels of induction of milk protein genes allowed preselection of the TFs involved. An electrophoretic mobility shift assay with nuclear proteins derived from rabbit mammary glands showed changes in the patterns of Stat5, MAF, NF1 and Oct1 DNA-protein binding during progression of pregnancy and transition to lactation. Administration of lactogenic hormones - PRL or GH - to early-pregnant rabbits induced DNA-protein complexes similar to those formed by nuclear proteins from the mammary glands of lactating (Stat5, MAF, NF1) or late-pregnant (Oct1) animals. Induction of milk protein genes by PRL was several-fold greater than that by GH. However, PRL and GH similarly induced MAF DNA-protein complexes, thus suggesting that the amount of MAF factor in the mammary gland can be limiting for expression of these genes. Our study for the first time provided the evidence that in the mammary gland both PRL and GH can induce DNA-binding activity of transcription factors other than Stats.

Specific DNA binding and transactivation potential of recombinant, purified Stat5

The signal transducers and activators of transcriptions (Stats) are central mediators of cytokine responses especially in hematopoietic cells. The detailed molecular mechanisms of Stat activation, particularly the role of post-translational modifications and co-operation with cellular transcription factors are subject to intense investigation. The phosphorylation of a tyrosine residue in the carboxyl terminal domain is a common characteristic for the biologically active state of all known Stats. We studied the biological potential of purified recombinant murine Stat5a and Stat5b. These proteins were expressed in Sf9 insect cells upon infection with Stat5 encoding baculoviruses. We also obtained the tyrosine phosphorylated, activated forms of the Stat5 proteins by expressing the tyrosine kinase Janus kinase2 (Jak) in the same cells through co-infection with a kinase encoding virus. After purification, only the tyrosine phosphorylated form was able to bind specifically in vitro to the Stat5 DNA response element. This activated form of Stat5 is also able to support specific cell free in vitro transcription of a gene with a Stat5 response element in its promoter region. The recombinant purified Stat5 proteins were treated with the tyrosine specific protein phosphatase or with potato acidic phosphatase, which removes phosphate groups from serine and tyrosine residues. Phosphatase treatment resulted in the loss of specific DNA binding ability. This property could be restored by an in vitro reaction with recombinant, purified EGF or PDGF receptor kinases. Tyrosine rephosphorylation in vitro also restored the transactivation potential of Stat5. This modification is, therefore, a sufficient prerequisite for transcriptional induction by Stat5.

Stat5 phosphorylation status and DNA-binding activity in the bovine and murine mammary glands

The transcription factors Stat5a and Stat5b are mediators of prolactin signalling in mammary epithelial cells, and are thought to play a role in lactogenesis. In cultured cells, activation of Stat5 activity through phosphorylation results in Stat5 binding to the promoters of at least some of the milk protein genes, thereby stimulating their transcription. However, the mammary biology of Stat5 differs between species, and the role of Stat5 in the bovine mammary gland is not fully understood. We have generated an antibody that specifically recognises the phosphorylated forms of Stat5a and Stat5b and used it to compare the levels of phosphorylated Stat5 with Stat5 DNA-binding activity in bovine and murine mammary tissue. Both Stat5 DNA-binding activity and phosphorylation status in the bovine mammary gland were at near-maximal levels at late pregnancy (27-35 days prior to calving), when at least three of the major milk proteins are not highly expressed. In addition, these studies revealed significant animal-to-animal variation in the level of Stat5 activity in both species. The results are consistent with a role in terminal differentiation of mammary epithelial cells. They also suggest that the stimulation of high-level expression of milk protein genes in the bovine mammary gland is not through activation of the prolactin receptor-Jak2-Stat5 pathway.

DNA binding specificity of different STAT proteins. Comparison of in vitro specificity with natural target sites

STAT transcription factors are expressed in many cell types and bind to similar sequences. However, different STAT gene knock-outs show very distinct phenotypes. To determine whether differences between the binding specificities of STAT proteins account for these effects, we compared the sequences bound by STAT1, STAT5A, STAT5B, and STAT6. One sequence set was selected from random oligonucleotides by recombinant STAT1, STAT5A, or STAT6. For another set including many weak binding sites, we quantified the relative affinities to STAT1, STAT5A, STAT5B, and STAT6. We compared the results to the binding sites in natural STAT target genes identified by others. The experiments confirmed the similar specificity of different STAT proteins. Detailed analysis indicated that STAT5A specificity is more similar to that of STAT6 than that of STAT1, as expected from the evolutionary relationships. The preference of STAT6 for sites in which the half-palindromes (TTC) are separated by four nucleotides (N(4)) was confirmed, but analysis of weak binding sites showed that STAT6 binds fairly well to N(3) sites. As previously reported, STAT1 and STAT5 prefer N(3) sites; however, STAT5A, but not STAT1, weakly binds N(4) sites. None of the STATs bound to half-palindromes. There were no specificity differences between STAT5A and STAT5B.

Regulation of the trans-activation potential of STAT5 through its DNA-binding activity and interactions with heterologous transcription factors

Extracellular hormones, growth factors and cytokines relay their effects on the transcription of genes through the recognition of specific receptors and intracellular signalling molecules. Signal transducers and activators of transcription (STATs) have been recognized as crucial intracellular signalling molecules. The cytokine receptor-associated Janus kinases (JAKs) convert the latent monomeric form of the STAT molecules to the activated dimeric form through tyrosine phosphorylation. The dimers bind to specific DNA response elements and are able to induce transcription. This induction requires the full-length form of the STAT molecules. Negative regulatory potential is exerted by the short form of the molecule, which lacks the trans-activation domain. This short form is activated and dimerized, but dephosphorylation is impaired. The short form of STAT occupies the DNA-binding sites in a stable fashion and acts as a strong suppressor of wild-type action. Positive enhancement of STAT5 trans-activation potential is provided by the glucocorticoid receptor. Ligand activation of this receptor causes the formation of a complex with STAT5 and deviation to the STAT5 DNA-binding site. An additional regulatory loop is provided by the reactivation of the short form of STAT5 through glucocorticoid receptor association. Conversely, classical glucocorticoid-responsive genes are negatively affected by STAT5 activation.

STAT5b-deficient mice are growth hormone pulse-resistant. Role of STAT5b in sex-specific liver p450 expression

The signal transducer and transcriptional activator STAT5b is required to maintain the adult male pattern of liver gene expression and whole body pubertal growth rates, as demonstrated by the loss of these growth hormone (GH) pulse-dependent responses in mice with a targeted disruption of the STAT5b gene. The present study investigates whether these phenotypes of STAT5b-deficient mice result from impaired intracellular GH signaling associated with a loss of GH pulse responsiveness, as contrasted with a feminization of the pituitary GH secretory profile leading to the observed feminization of body growth and liver gene expression. Pulsatile GH replacement in hypophysectomized mice stimulated body weight gain in wild-type but not in STAT5b-deficient mice. Expression of the male-specific liver P450 enzyme CYP2D9, which is reduced to female levels in hypophysectomized male mice, was restored to male levels by GH pulse replacement in wild-type but not in STAT5b-deficient mice. Similarly, a female-specific liver CYP2B P450 enzyme that was up-regulated to female levels following hypophysectomy of males was suppressed to normal basal male levels by GH pulses only in wild-type hypophysectomized mice. Finally, urinary excretion of the male-specific, GH pulse-induced major urinary protein was restored to normal male levels following pulsatile GH treatment only in the case of wild-type hypophysectomized mice. STAT5b-deficient mice are thus GH pulse-resistant, supporting the proposed role of STAT5b as a key intracellular mediator of the stimulatory effects of plasma GH pulses on the male pattern of liver gene expression.

ErbB4 signaling in the mammary gland is required for lobuloalveolar development and Stat5 activation during lactation

Signaling by members of the epidermal growth factor receptor family plays an important role in breast development and breast cancer. Earlier work suggested that one of these receptors, ErbB4, is coupled to unique responses in this tissue. To determine the function of ErbB4 signaling in the normal mouse mammary gland, we inactivated ErbB4 signaling by expressing a COOH terminally deleted dominant-negative allele of ErbB4 (ErbB4DeltaIC) as a transgene in the mammary gland. Despite the expression of ErbB4DeltaIC from puberty through later stages of mammary development, an ErbB4DeltaIC-specific phenotype was not observed until mid-lactation. At 12-d postpartum, lobuloalveoli expressing ErbB4DeltaIC protein were condensed and lacked normal lumenal lactation products. In these lobuloalveoli, beta-casein mRNA, detected by in situ hybridization, was normal. However, whey acidic protein mRNA was reduced, and alpha-lactalbumin mRNA was undetectable. Stat5 expression was detected by immunohistochemistry in ErbB4DeltaIC-expressing tissue. However, Stat5 was not phosphorylated at Y694 and was, therefore, probably inactive. When expressed transiently in 293T cells, ErbB4 induced phosphorylation of Stat5. This phosphorylation required an intact Stat5 SH2 domain. In summary, our results demonstrate that ErbB4 signaling is necessary for mammary terminal differentiation and Stat5 activation at mid-lactation.

Transcription originating in the long terminal repeats of the endogenous mouse mammary tumor virus MTV-3 is activated in Stat5a-null mice and picks Up hitchhiking exons

The enhancer within the long terminal repeats (LTRs) of acquired somatic mouse mammary tumor viruses (MMTV) can activate juxtaposed genes and induce mammary tumors. In contrast, germ line proviral MMTV genomes are integrated in the host genome and considered to be genetically confined transcription units. Here we demonstrate that transcription initiated in an MMTV provirus proceeds into flanking host sequences. We discovered multiple polyadenylated transcripts which are induced in Stat5a null mice. These range from 1.5 kb to more than 8 kb and are specifically expressed in mammary tissue from pregnant and lactating mice from the 129 but not C57BL/6 strain. The RNAs emanate from both LTRs of the endogenous MTV-3 provirus on chromosome 11 and proceed at least 10 kb into the juxtaposed genomic territory. Transcripts originating in the 5' LTR splice from the native splice site within the MMTV envelope gene into at least six exons, three of which contain functional internal splice sites. The combination of alternative splicing and the use of several polyadenylation sites ensure the generation of multiple transcripts. To date no significant open reading frame has been discovered. Furthermore, we demonstrate that transcription from the MMTV 5' LTR is highly active in the absence of Stat5a, a transcription factor that had been shown previously to be required for transcription from the MMTV LTR.

Prolactin and insulin synergize to regulate the translation modulator PHAS-I via mitogen-activated protein kinase-independent but wortmannin- and rapamycin-sensitive pathway

The synergism between insulin and prolactin (PRL) in their effect on protein synthesis in the mammary gland was studied in differentiating mammary epithelial CID-9 cells. Both hormones were needed to induce phosphorylation of PHAS-I which resulted in its dissociation from the eIF-4E translation initiation factor. This step is crucial for the initiation of translation. The induction of PHAS-I phosphorylation was rapid and its rate matched that demonstrated for the JAK2/STAT5a and the binding of STAT5a to its DNA binding motif. However, 120 min was needed for complete phosphorylation of the PHAS-I protein. In the presence of insulin, PRL induced MAP kinase activity, initiated at a comparable rate to that of PHAS-I phosphorylation. However, a line of evidence suggested that although this kinase phosphorylates PHAS-I in vitro, it does not actively participate in its phosphorylation in vivo: (a) the level of insulin needed to enable PRL-induced ERK-1/ERK-2 activation was one order of magnitude higher than that needed for PHAS-I phosphorylation; and (b) PD 098059, a MEK-1 inhibitor, completely inhibited insulin-dependent, PRL-induced ERK-1/ERK-2 activation but had no effect on the PRL-induced PHAS-I phosphorylation. In contrast, wortmannin, a phosphatidylinositol 3-kinase (PI 3'-kinase) inhibitor and the immunosuppressant rapamycin abrogated PHAS-I phosphorylation and caused a reciprocal shift between the fully phosphorylated PHAS-I gamma form and its non-phosphorylated alpha form. Since the partly phosphorylated PHAS-I beta form was not significantly affected by these inhibitors, it is possible that more than a single kinase mediates the synergistic effect of prolactin and insulin on PHAS-I phosphorylation.

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.

Induction of germline transcription in the TCRgamma locus by Stat5: implications for accessibility control by the IL-7 receptor

IL-7 receptor (IL-7R) plays critical roles in lymphocyte development by promoting survival and proliferation and by inducing V(D)J recombination in TCR and Ig loci. Here, we demonstrate that IL-7R-activated Stat5 binds to consensus motifs in the 5' regions of Jgamma segments and induces germline transcripts. We also show that a constitutively active form of Stat5 restores V-J recombination of TCRgamma genes and partially rescues T cell development from IL-7R(-/-) T cell precursors, especially in favor of gammadelta T cells. Therefore, this study reveals a potential role of Stat5 in T cell development and also implies that IL-7R may control the accessibility of the TCRgamma locus through Stat5-induced germline transcription.

Specific activation of a STAT family member in Xiphophorus melanoma cells

Melanoma formation in the teleost fish Xiphophorus is caused by the uncontrolled activity of the genetically defined tumor locus Tu. The critical component of this locus is the Xmrk oncogene encoding a subclass I receptor tyrosine kinase. Overexpression and constitutive activation of the Xmrk receptor triggers a set of specific signal transduction events eventually resulting in the malignant phenotype. We have identified a melanoma-specific DNA-protein complex which seems to depend on Xmrk activation as shown in a heterologous cell system. The critical component of this complex, which directs transcriptional activation in the melanoma cells, proved to be a fish homologue of STAT5. Two other STAT factors, STAT1 and STAT3, implied in signaling by the Xmrk-related EGF receptor, were not activated in this particular cell type. Thus, Xmrk initiates very specific signaling pathways and transcriptional responses in Xiphophorus melanoma.

A functional DNA binding domain is required for growth hormone-induced nuclear accumulation of Stat5B

The mechanisms regulating the cellular distribution of STAT family transcription factors remain poorly understood. To identify regions of Stat5B required for ligand-induced nuclear accumulation, we constructed a cDNA encoding green fluorescent protein (GFP) fused to the N terminus of Stat5B and performed site-directed mutagenesis. When co-expressed with growth hormone (GH) receptor in COS-7 cells, GFP-Stat5B is tyrosyl-phosphorylated, forms dimers, and binds DNA in response to GH in a manner indistinguishable from untagged Stat5B. In multiple cell types, laser scanning confocal imaging of GFP-Stat5B co-expressed with GH receptor shows that GFP-Stat5B undergoes a rapid, dramatic accumulation in the nucleus upon GH stimulation. We introduced alanine substitutions in several regions of Stat5B and assayed for GH-dependent nuclear localization. Only the mutation that prevented binding to DNA (466VVVI469) abrogated GH-stimulated nuclear localization. This mutant fusion protein is tyrosyl-phosphorylated and dimerizes in response to GH. These results suggest that either high affinity binding to DNA contributes to nuclear accumulation of Stat5B or that this region is crucial for two functions, namely accumulation of Stat5B in the nucleus and DNA binding. Thus, we have identified a mutant Stat5 defective in nuclear localization despite its ability to be tyrosyl-phosphorylated and to dimerize.

Mammary gland expression of mouse mammary tumor virus is regulated by a novel element in the long terminal repeat

Mouse mammary tumor virus (MMTV) infects both lymphoid tissue and lactating mammary gland during its infectious cycle, but some endogenous MMTVs are transcribed only in lymphoid cells. We found a lymphoid cell-specific endogenous MMTV that was converted to a milk-borne, infectious virus through recombination with an exogenously transmitted MMTV. The changed expression pattern correlated with the alteration of a single base pair in the long terminal repeat of the lymphoid cell-specific virus. Transgenic mice with the element from either the milk-borne or lymphoid cell-specific virus upstream of the chloramphenicol acetyltransferase reporter gene showed the same pattern of expression as the virus from which the regulatory sequences were derived. Electrophoretic mobility shift assays with mammary cell extracts showed that the site from the milk-borne virus was preferentially bound by a prolactin-inducible factor that poorly bound the altered site from the lymphoid cell-specific virus. The complex that formed on the milk-borne virus-specific oligonucleotide supershifted with anti-Stat5b antibody. Mice lacking either Stat5a or Stat5b had dramatically reduced levels of MMTV transcripts in mammary gland but not in lymphoid tissue. Thus, a member of the STAT family of transcription factors is involved in the tissue-specific expression of mouse mammary tumor virus in vivo. This is the first example of the involvement of a member of the STAT family of transcription factors in the control of tissue-specific expression.

Molecular mechanisms of ovulation and luteinization

Ovulation is a complex process initiated by the mid-cycle surge of luteinizing hormone (LH). Once initiated, a cascade of events occurs that culminates in the release of a fertilizable oocyte. The complex series of events involves specific ovarian cell types, diverse signaling pathways and temporally controlled expression of specific genes. This review will focus on several genes shown to control the ovulation process.

p53 suppresses cytokine induced, Stat5 mediated activation of transcription

p53 is a tumor suppressor which exerts its function through the regulation of genes mediating cell cycle arrest and the induction of apoptosis. Cellular survival and proliferation can be positively regulated through the action of cytokines. These signals act through the activation of cell surface receptors, and the phosphorylation of intracellular signaling components, e.g. members of the Stat family (signal transducers and activators of transcription). The signaling effects of p53 and the cytokine receptors on the cellular phenotype are counteracting. We investigated the influence of p53 on the transactivation potential of Stat5. p53 repressed the prolactin induction of the Stat5 mediated transcription of the beta-casein promoter-luciferase reporter gene, but did not affect IFN-gamma induced, Stat1 dependent transcription of the IRF-1 promoter. The inhibition was not due to a decrease in the cellular concentration of Stat5 or to interference with its specific DNA binding activity. No repression of the basal transcriptional activity of the beta-casein promoter was observed. p53 mutants defective in their DNA binding or oligomerization functions had only weak inhibitory effects, but a mutant of p53 in the transactivation domain, efficiently repressed Stat5 dependent induction. The repressive function of p53 on Stat5 activity is independent of the amino-terminal transactivation domain, but requires a functional DNA binding domain and the carboxyl-terminal domain. Our experiments show that p53 counteracts Stat5 mediated cytokine induction of gene transcription. The effect is specific for Stat5 and independent of p53 induced apoptosis.

Cloning and sequencing of the bovine STAT5A cDNA reveals significant sequence divergence with ovine

The transcription factors STAT5 mediates prolactin signals in mammary epithelial cells. The cDNA of bovine STAT5A was cloned, sequenced and compared to other species. The encoded protein proves to be > 95% homologous to other mammals. We show that the STAT5A mRNA of the closely related ovine species contains an extended (by 130 nt) 5'-untranslated region, being encoded by an extra-exon, and accounts, possibly, for improved translation efficiency.