Hormone responsive elements within the upstream sequences of the rabbit whey acidic protein (WAP) gene direct chloramphenicol acetyl transferase (CAT) reporter gene expression in transfected rabbit mammary cells

Implementation and Use of State-of-the-Art, Cell-Based In Vitro Assays

The impressive advances in the generation and interpretation of functional omics data have greatly contributed to a better understanding of the (patho-)physiology of many biological systems and led to a massive increase in the number of specific targets and phenotypes to investigate in both basic and applied research. The obvious complexity revealed by these studies represents a major challenge to the research community and asks for improved target characterisation strategies with the help of reliable, high-quality assays. Thus, the use of living cells has become an integral part of many research activities because the cellular context more closely represents target-specific interrelations and activity patterns. Although still predominant, the use of traditional two-dimensional (2D) monolayer cell culture models has been gradually complemented by studies based on three-dimensional (3D) spheroid (Sutherland 1988) and other 3D tissue culture systems (Santos et al. 2012; Matsusaki et al. 2014) in an attempt to employ model systems more closely representing the microenvironment of cells in the body. Hence, quite a variety of state-of-the-art cell culture models are available for the generation of novel chemical probes or the identification of starting points for drug development in translational research and pharma drug discovery. In order to cope with these information-rich formats and their increasing technical complexity, cell-based assay development has become a scientific research topic in its own right and is used to ensure the provision of significant, reliable and high-quality data outlasting any discussions related to the current "irreproducibility epidemic" (Dolgin 2014; Prinz et al. 2011; Schatz 2014). At the same time the use of cells in microplate assay formats has become state of the art and greatly facilitates rigorous cell-based assay development by providing the researcher with the opportunity to address the multitude of factors affecting the actual assay results in a systematic fashion and a timely manner. This microplate-based assay development strategy should result in the setting up of more robust and reliable test systems that ensure and increase the confidence in the statistical significance of the actual data generated. And, although assay miniaturisation is essential in order to achieve this, most, if not all, cell-based assays can be easily reformatted and adapted to be used in this format in a straightforward manner. This synopsis aims at summarising valuable, general observations made when implementing a diverse set of functional cellular in vitro assays at Bayer Pharma AG without claiming to deeply review all of the literature available in each and every detail. In addition, phenotypic assays (Moffat et al. 2014) or label-free detection methods (Minor 2008) are not discussed. Although this essay tries to cover the most relevant technological developments in the field, it nevertheless may express personal preferences and peculiarities of the author's approach to state-of-the-art cell-based assay development. For additional reviews covering the actual field, see Wunder et al. (2008) and Michelini et al. (2010).

Rabbit milk protein genes: from mRNA identification to chromatin structure

Milk protein genes are among the most intensively expressed and they are active only in epithelial mammary cells of lactating animals. They code for proteins which represent 30% of the proteins consumed by humans in developed countries. Mammary gland development occurs essentially during each pregnancy. This offers experimenters attractive models to study the expression mechanisms of genes controlled by known hormones and factors (prolactin, glucocorticoids, progesterone, insulin-like growth factor-1 and others) as well as extracellular matrix. In the mid-1970s, it became possible to identify and quantify mRNAs from higher living organisms using translation in reticulocyte lysate. A few years later, the use of radioactive cDNAs as probes made it possible for the quantification of mRNA in various physiological situations using hybridisation in the liquid phase. Gene cloning offered additional tools to measure milk protein mRNAs and also to identify transcription factors. Gene transfer in cultured mammary cells and in animals contributed greatly to these studies. It is now well established that most if not all genes of higher eukaryotes are under the control of multiple distal regulatory elements and that local modifications of the chromatin structure play an essential role in the mechanisms of differentiation from embryos to adults. The technique, known as ChIP (chromatin immunoprecipitation), is being implemented to identify the factors that modify chromatin structure at the milk protein gene level during embryo development, mammogenesis and lactogenesis, including the action of hormones and extracellular matrix. Transgenesis is not just a tool to study gene regulation and function, it is also currently used for various biotechnological applications including the preparation of pharmaceutical proteins in milk. This implies the design of efficient vectors capable of directing the secretion of recombinant proteins in milk at a high concentration. Milk protein gene promoters and long genomic-DNA fragments containing essentially all the regulatory elements of milk protein genes are used to optimise recombinant protein production in milk.

Regulation by the extracellular matrix (ECM) of prolactin-induced alpha s1-casein gene expression in rabbit primary mammary cells: role of STAT5, C/EBP, and chromatin structure

The aim of the present study was to understand how the extracellular matrix (ECM) regulates at the gene level the prolactin (Prl)-induced signal transducer and activator of transcription 5 (STAT5)-dependent expression of the alpha s1-casein gene in mammary epithelial cells. CCAAT enhancer binding proteins (C/EBPs) are assumed regulators of beta-casein gene expression. Rabbit primary mammary cells express alpha s1-casein gene when cultured on collagen and not on plastic. Similar C/EBPbeta, C/EBPdelta, STAT5, and Prl-activated STAT5 were found under all culture conditions. Thus the ECM does not act through C/EBPs or STAT5. This was confirmed by transfections of rabbit primary mammary cells by a construct sensitive to ovine prolactin (oPrl) and ECM (6i TK luc) encompassing STAT5 and C/EBP binding sites. The mutation of C/EBPs binding sites showed that these sites were not mandatory for Prl-induced expression of the construct. Interestingly, chromatin immunoprecipitation by the anti-acetylhistone H4 antibody (ChIP) showed that the ECM (and not Prl) maintained a high amount of histone H4 acetylation upstream of the alpha s1-casein gene especially at the level of a distal Prl- and ECM-sensitive enhancer. Alpha6 integrin (a membrane receptor of laminin, the principal active component of the mammary ECM) was found at the surface of cells cultured on collagen but not on plastic. In cells cultured on collagen in the presence of anti-alpha6 integrin antibody, Prl-induced transcription of the endogenous alpha s1-casein gene was significantly reduced, without modifying C/EBPs and STAT5. Besides, histone H4 acetylation was reduced. Thus, we propose that the ECM regulates rabbit alpha s1-casein protein expression by local modification of chromatin structure, independently of STAT5 and C/EBPs.

In vitro and in vivo effects of a multimerized alphas 1-casein enhancer on whey acidic protein gene promoter activity

Experimental data obtained in previous works have led to postulate that enhancers increase the frequency of action of a linked promoter in a given cell and may have some insulating effects. The multimerized rabbit alpha s1-casein gene enhancer, the 6i multimer, was added upstream of the rabbit whey acidic protein gene (WAP) promoter (-6,300; +28 bp) fused to the firefly luciferase (luc) gene (6i WAP-luc construct). The 6i multimer increased reporter gene expression in mouse mammary HC11 cells. In transgenic mice, a very weak but significant increase was also observed. More noticeable, no silent lines were found when the 6i multimer was associated to the WAP-luc construct. This reflects the fact that the 6i multimer tends to prevent the silencing of the WAP-luc construct. After addition of the 5'HS4 insulator region from the chicken beta-globin locus upstream of the 6i multimer, similar luciferase levels were measured in 6i WAP-luc and 5'HS4 WAP-luc transgenic mice. Our present data and previous ones, which show that the 6i multimer has no insulating activity on a TK gene promoter construct indicate that the insulating activity of the 6i multimer is construct-dependent and not amplified by the 5'HS4 insulator.

Hormone-induced modifications of the chromatin structure surrounding upstream regulatory regions conserved between the mouse and rabbit whey acidic protein genes

The upstream regulatory regions of the mouse and rabbit whey acidic protein (WAP) genes have been used extensively to target the efficient expression of foreign genes into the mammary gland of transgenic animals. Therefore both regions have been studied to elucidate fully the mechanisms controlling WAP gene expression. Three DNase I-hypersensitive sites (HSS0, HSS1 and HSS2) have been described upstream of the rabbit WAP gene in the lactating mammary gland and correspond to important regulatory regions. These sites are surrounded by variable chromatin structures during mammary-gland development. In the present study, we describe the upstream sequence of the mouse WAP gene. Analysis of genomic sequences shows that the mouse WAP gene is situated between two widely expressed genes (Cpr2 and Ramp3). We show that the hypersensitive sites found upstream of the rabbit WAP gene are also detected in the mouse WAP gene. Further, they encompass functional signal transducer and activator of transcription 5-binding sites, as has been observed in the rabbit. A new hypersensitive site (HSS3), not specific to the mammary gland, was mapped 8 kb upstream of the rabbit WAP gene. Unlike the three HSSs described above, HSS3 is also detected in the liver, but similar to HSS1, it does not depend on lactogenic hormone treatments during cell culture. The region surrounding HSS3 encompasses a potential matrix attachment region, which is also conserved upstream of the mouse WAP gene and contains a functional transcription factor Ets-1 (E26 transformation-specific-1)-binding site. Finally, we demonstrate for the first time that variations in the chromatin structure are dependent on prolactin alone.

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.

Extracellular matrix regulates alpha s1-casein gene expression in rabbit primary mammary cells and CCAAT enhancer binding protein (C/EBP) binding activity

Previous studies have shown that both the signal transducer and activator of transcription 5 (STAT5) and the CCAAT enhancer binding proteins (C/EBPs) are involved in the regulation of casein gene expression by mammary epithelial cells. Prolactin (Prl) activation of STAT5 is necessary for casein gene expression. The extracellular matrix (ECM) regulates also casein gene expression. Here, we have investigated whether ECM regulates C/EBPs activity in primary rabbit mammary epithelial cells. Isolated primary mammary cells were cultured on plastic or on floating collagen I gel. Prolactin induced alphas 1-casein gene expression when cells were cultured on collagen but not on plastic. It is noteworthy that activated STAT5 was detected in both culture conditions. Several STAT5 isoforms (STAT5a, STAT5b, and other STAT5 related isoforms, some with lower molecular weight than the full-length STAT5a and STAT5b) were detected under the different culture conditions. However, their presence was not related to the expression of alphas 1-casein gene. The binding of nuclear factors to a C/EBP specific binding site and the protein level of C/EBPbeta differed in cells cultured on plastic or on collagen but these parameters were not modified by Prl. This suggests that C/EBP binding activity was regulated by ECM and not by Prl. Interestingly, these modifications were correlated to the expression of the alphas 1-casein gene. Hence, the activation of the alphas 1-casein gene expression depends on two independent signals, one delivered by Prl via the activation of STAT5, the other delivered by ECM via C/EBP.

A transgenic mouse model for investigating the response of the upstream region of whey acidic protein (WAP) gene to various steroid hormones

The limitations of studies of clarification of response elements of whey acidic protein (WAP) gene to hormones using mammary cell lines has been shown. We studied the response of the upstream region (2.6 kb) of WAP to various steroid hormones using gonadectomized mWAP/hGH transgenic mice. Ovariectomy or castration for transgenic mice was performed at 10 days or 30 days post partum. Various steroid hormones were administered daily for 10 days to the gonadectomized transgenic mice after they reached 2 months of age. Prior to the hormonal administration and 24 hr after the final administration, blood was collected and the hGH levels in the plasma was measured by RIA. Daily doses of estradiol-17 beta were significantly more effective at increasing hGH levels in transgenic females ovariectomized at 10 days post partum than progesterone of an equal dose. A combined dose of progesterone and of estradiol-17 beta significantly amplified the increase of hGH levels accompanied by the great development of mammary glands, compared to a dose of progesterone alone. Corticosterone induced only a slight increase of hGH, while testosterone had no effect. The doses of gonadal steroid hormones did not induce an increase in hGH levels and development of mammary glands in the castrated transgenic males. The results showed that the response of 5' region of WAP requires at least some extended development of the mammary gland and that the 2.6 kb upstream region of the exogenous WAP gene contained the element responsive to ovarian hormones.

Glucocorticoid regulation of angiotensin-converting enzyme in primary culture of adult cardiac fibroblasts

The effect of dexamethasone on angiotensin-converting enzyme (ACE) in primary culture of adult cardiac fibroblasts was analyzed in this study. ACE is central to cardiac remodeling in conditions such as myocardial infarction (MI). Some studies indicate that glucocorticoids are often increased post-MI, whereas other studies suggest that glucocorticoids stimulate ACE activity in various cell types. Most cardiac cells are fibroblasts, which have an important function in cardiac remodeling. Therefore, we studied the effects of glucocorticoids on ACE activity and mRNA levels in primary cultures of adult rat cardiac fibroblasts. Steady-state ACE activity was very low, but it increased sixfold with dexamethasone (1 microM for 48 h) treatment. ACE activation occurred within 12 h and peaked at 96 h, after treatment. RNase-protection assays revealed an associated threefold increase (P < 0.05) in ACE mRNA. Dexamethasone's stimulatory effect was abolished by an RNA synthesis inhibitor (actinomycin D, 5 microg/ml) but was potentiated by a protein synthesis inhibitor (cycloheximide, 5 microg/ml). The glucocorticoid-mediated response appears to be specific, because mineralocorticoid treatment did not alter ACE activity. These findings indicate that both transcriptional and posttranscriptional mechanisms are involved in glucocorticoid regulation of ACE expression in rat cardiac fibroblasts.

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.

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.

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.

A MGF/STAT5 binding site is necessary in the distal enhancer for high prolactin induction of transfected rabbit alpha s1-casein-CAT gene transcription

The rabbit alphas1-casein gene contains a distal prolactin-dependent enhancer 3442-3285 bp 5' to the site of initiation of transcription. We have reported previously that four DNA/protein-binding sites (F1-F4) are located within this distal enhancer. We now show that one of this binding site (the F4 site) binds in vitro a MGF/STAT5-like factor. The functional importance of the F4 site was estimated by cotransfection of CHO cells with a chimeric gene containing or not the F4 sequence linked to the (-391/+1774)CAT gene and a plasmid encoding the rabbit mammary prolactin receptor. The F4 site is necessary for maximal response, of the enhancer to prolactin. However, this site has to be associated to the Fl-F3 fragment. It can be replaced by a genuine MGF/STAT5-binding site. A mutational analysis indicates that F4 and F1 sites are simultaneously involved to confer a high prolactin sensitivity.

The effect of matrix attached regions (MAR) and specialized chromatin structure (SCS) on the expression of gene constructs in cultured cells and in transgenic mice

The flanking sequences of several genes have been shown to direct a position independent expression of transgenes. Attempts to completely identify the insulating sequences have failed so far. Some of these sequences contain a matrix attached region (MAR) located in the flanking part of the genes. This article will show that the MARs in cultured cells located in the 3' OH region of the human apolipoprotein B100 (Apo B100) and within the SV40 genome were unable to stimulate and insultate transgene expression directed by the promoters from a rabbit whey acidic protein (WAP) gene or from human cytomegalovirus (hCMV) early genes. In transgenic mice, the MAR from the Apo B100 and SV40 genes did not enhance the expression of a transgene containing the rabbit whey acid protein (WAP) promotor, the late gene SV40 intron (VP1 intron), the bovine growth hormone (bGH) cDNA and the SV40 late gene terminator. This construct was even toxic for embryos. Similarly, the specialized chromatin structure (SCS) from the Drosophila 87A7 HSP70 gene reduced chloramphenicol acetyl transferase (CAT) activity when added between a cytomegalovirus (CMV) enhancer and a Herpes simplex thymidine kinase (TK) gene promoter. This inhibitory action was almost complete when a second SCS sequence was added before the CMV enhancer. Sequences from the firefly luciferase and from the human gene cathepsin D cDNA used as control unexpectedly showed a similar inhibitory effect when added to the CMVTKCAT construct instead of SCS. When added before the CMV enhancer and after the transcription terminator in the CMVTKCAT construct, the SCS sequence was unable to insulate the integrated gene as seen by the fact that the level of CAT in cell extracts were by no means correlated with the number of copies in individual clones. From these data, it is concluded that i) a MAR containing the canonical AT rich sequences does not amplify the expression of all gene constructs ii) At rich MAR sequences do not have per se an insulating effect iii) Drosophila SCS from the 87A7 HSP70 gene has no insulating effect in all gene constructs (at least in mammalian cells) iv) and the addition of a DNA fragment between an enhancer and a promoter in a gene construct cannot be used as a reliable test to evaluate its insulating property.

Rabbit whey acidic protein gene upstream region controls high-level expression of bovine growth hormone in the mammary gland of transgenic mice

Transgenic mice were produced which secreted high levels of bGH into milk. The 6.3-kb upstream region of the rabbit whey acidic protein (rWAP) gene was linked to the structural part of the bovine growth hormone (bGH) gene, and the chimeric gene was radioimmunoassay into mouse oocytes. bGH was detected by radioimmunoassay in the milk of all resulting transgenic mice. bGH concentrations in milk varied from line to line, from 1.0-16 mg/ml. This expression was not correlated to the number of transgene copies. In all lines studied, the mammary gland was the major organ expressing bGH mRNA during lactation. bGH mRNA concentrations were barely detectable in the mammary gland of cyclic females; they increased during pregnancy. These results show that the upstream region of the rWAP gene harbors powerful regulatory elements which target high levels of bGH transgene expression to the mammary gland of lactating transgenic mice.

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.

Expression of whey acidic protein (WAP) genes in tissues other than the mammary gland in normal and transgenic mice expressing mWAP/hGH fusion gene

Whey acidic protein (WAP) is a major whey protein secreted in rodents' milk. Murine WAP (mWAP) genes have been assumed to be expressed solely in the mammary gland. However, several heterologous genes fused with the mWAP promoter and artificially introduced into animal genomes as transgene were expressed not only in the mammary gland but also in other tissues as well. In the present study, we investigated, by means of the reverse transcription polymerase chain reaction (RT-PCR), the patterns of expression of endogenous WAP genes in tissues of normal mice and in transgenic mice carrying hGH gene coupled to the mWAP promoter sequence. The results revealed that the genes driven by the mWAP promoter, regardless of whether they are endogenous genes or transgenes, were transcribed in a variety of tissues other than the mammary gland of lactating normal female mice, although the expression levels are generally low. The expression of WAP genes in the cerebrum and the liver is regulated, as in the mammary gland, according to the reproductive stages. However, the tissue distribution of endogenous WAP gene expression in mature virgin transgenic female mice was the same as that in lactating normal female mice.

The effect of various introns and transcription terminators on the efficiency of expression vectors in various cultured cell lines and in the mammary gland of transgenic mice

Various combinations of promoters, introns and transcription terminators were used to drive the expression of bovine growth hormone (bGH) cDNA in different cell types. In constructs containing the human cytomegalovirus (hCMV) promoter and the SV40 late genes terminator, the intron from SV40 genes (VP1) was much more efficient, than the intron from the early genes (t). The synthetic intron SIS generated by the association of an adenovirus splice donor and an immunoglobulin G splice acceptor showed the highest activity. The respective potency of these introns was similar in several mammalian (CHO, HC11 and COS) and fish (TO2 and EPC) cells. The rabbit whey acidic protein (WAP) gene promoter was highly efficient to drive the expression of bGH gene in the HC11 mammary cell lines. In contrast, the bGH cDNA under the control of the same promoter was much less efficiently expressed when the SV40 VP1 intron and transcription terminator were used. The rabbit WAP gene and the human GH gene terminators did not or only moderately enhanced the expression of the construct WAP bGH cDNA. Introduction of a promoter sequence from the mouse mammary tumor virus (MMTV) LTR in the VP1 intron increased very significantly the expression of the WAP bGH cDNA. Although several of these vectors showed high potency when expressed stably in HC11 cells, all of them were only moderately efficient in transgenic mice. These data indicate that the VP1 and the SIS introns may be used to express foreign cDNAs with good efficiency in different cell types. The addition of an enhancer within an intron may still reinforce its efficiency. However, transfection experiments, even when stable expression is carried out, are poorly predictive of the potential efficiency of a vector in transgenic animals.

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.

Stathmin gene expression in mammary gland and in Nb2 cells

Mammary gland growth occurs essentially during pregnancy and induction of milk synthesis is triggered at parturition. Prolactin is mammogenic in vivo but only marginally in vitro. Prolactin induces milk synthesis in vivo and in cultured mammary cells. Prolactin is also strictly required for the multiplication of the rat lymphoid Nb2 cells. Stathmin is an ubiquitous and highly conserved phosphoprotein which seems to be involved in the intracellular mechanisms which trigger cell multiplication and differentiation. In the present study, the concentration of stathmin mRNA has been evaluated during the pregnancy-lactation-weaning cycle in mouse and rabbit. Stathmin mRNA appeared at its highest level during pregnancy and it was almost undetectable during lactation. Prolactin injected into mid-pregnant rabbits induced milk synthesis and this effect was not accompanied by any modification of stathmin mRNA concentration. In cultured primary rabbit mammary cells, prolactin induced casein gene expression without any alteration of stathmin mRNA concentration. In Nb2 cells, prolactin induced a progressive increase of stathmin mRNA concentration. This effect was not significant until after 4 h of prolactin action. These data suggest that stathmin is involved in mammary and Nb2 cell multiplication but may not be necessary for mammary cell differentiation.

Negative regulatory element in the mammary specific whey acidic protein promoter

Expression of the whey acidic protein (WAP) gene is tightly regulated in a tissue and developmental stage specific manner, in that the WAP gene is exclusively expressed in the mammary gland during pregnancy and lactation. Using both deletion and competition analyses, evidence is provided for the existence of a negative regulatory element (NRE) in the WAP promoter located between -413 and -93 with respect to the WAP transcriptional initiation site. This NRE dramatically decreases transcription from linked heterologous promoter-reporter gene constructs. The activity of NRE requires WAP promoter sequences that are 230 bp apart since subfragments of the NRE fail to inhibit transcription of adjoining reporter genes. Nuclear extracts from different cell types, in which the WAP gene is not active, contain a protein or complex that specifically interacts with the entire NRE but not with subfragments of it. The contact points between this protein (NRE binding factor [NBF]) and the NRE element have been partially determined. Mutation of the implicated nucleotides severely reduces the ability of NBF to bind, and such mutated promoter fragments fail to alleviate transcriptional repression in competition experiments. This suggests that NBF binding to the NRE is at least in part responsible for the negative regulation of the WAP promoter. Since NBF is not detectable in the lactating mammary gland, where the WAP gene is expressed, we speculate that it may be a determinant of the expression spectrum of the WAP gene.

Production of pharmaceutical proteins from transgenic animals

Different systems are being studied and used to prepare recombinant proteins for pharmaceutical use. The blood, and still more the milk, from transgenic animals appear a very attractive source of pharmaceuticals. The cells from these animals are expected to produce well-matured proteins in potentially huge amounts. Several problems remain before this process becomes used in a large scale. Gene transfer remains a difficult and costly task for farm animals. The vectors carrying the genes coding for the proteins of interest are of unpredictable efficiency. Improvement of these vectors includes the choice of efficient promoters, introns and transcription terminators, the addition of matrix attached regions (MAR) and specialized chromatin sequences (SCS) to enhance the expression of the transgenes and to insulate them from the chromatin environment. Mice are routinely used to evaluate the gene constructs to be transferred into larger animals. Mice can also be utilized to prepare amounts as high as a few hundred mg of recombinant proteins from their milk. Rabbit appears adequate for amounts not higher than 1 kg per year. For larger quantities, goat, sheep, pig and cow are required. No recombinant proteins extracted from the blood or milk of transgenic animals are yet on the market. The relatively slow but real progress to improving the efficiency of this process inclines to be reasonably optimistic. Predictive reports suggest that 10% of the recombinant proteins, corresponding to a 100 million dollars annual market, will be prepared from the milk of transgenic animals by the end of the century.

High level production of human growth hormone in the milk of transgenic mice: the upstream region of the rabbit whey acidic protein (WAP) gene targets transgene expression to the mammary gland

The 5' flanking region (6.3 kb) of the rabbit WAP (rWAP) gene possesses important regulatory elements. This region was linked to the human growth hormone (hGH) structural gene in order to target transgene expression to the mammary gland. Thirteen lines of transgenic mice were produced. Milk could be collected from six lines of transgenic mice. In five of them, hGH was present in the milk at high concentrations ranging from 4 to 22 mg ml-1. hGH produced by the mammary gland comigrated with hGH of human origin. It was biologically active, and through its prolactin-like activity induced lactogenesis when introduced into mammary culture media. Two of these mouse lines were studied further. hGH mRNA was only detected in the mammary gland during lactation. In the seven other transgenic lines, hGH was present in the blood of cyclic females. The prolactin-like effect of hGH in these mice probably induced female sterility, and milk could therefore not be obtained. In two lines studied in more detail, the mammary gland was the main organ producing hGH, even in cyclic mice. Low ectopic expression was detected in other organs which varied from one line to the other. This was probably due to the influence on the transgene of the site of integration into the mouse genome. In the 13 lines studied, high mammary-specific hGH expression was not correlated to the transgene copy number. The rWAP-hGH construct thus did not behave as an independent unit of transcription. However, it can be concluded that the 6.3 kb flanking region of the rWAP gene contains regulatory elements responsible for the strong mammary-specific expression of hGH transgene, and that it is a good candidate to control high levels of foreign protein gene expression in the mammary gland of lactating transgenic animals.

Functional analysis of the human somatic angiotensin I-converting enzyme gene promoter

Angiotensin I-converting enzyme (ACE) is a key enzyme in the regulation of systemic blood pressure and plays a major role in the renin-angiotensin and bradykinin-kinin systems, at the luminal surface of the vascular endothelia. To identify the promoter region, the transcription regulatory elements and the cell specificity of the ACE gene, five successive DNA deletions of the 5' upstream region (-1214, -754, -472, -343, -132 bp relative to the start site of transcription) were isolated and fused in sense and antisense orientations to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene in the promoterless plasmid pBLCAT3. Promoter activities were measured in transient transfection assays using three different cell lines from rabbit endothelium (RE), human embryocarcinoma (Tera-1) and hepatocarcinoma cells (HepG2). All five fragments of the ACE promoter region directed expression of the CAT gene when transfected into the endothelial and the embryocarcinoma cells, which contain endogenous ACE mRNA and express ACE activity. In contrast only minimal levels of promoter activity were obtained on transfection into hepatocarcinoma cells in which endogenous ACE mRNA and ACE activity were not detected. Transfection of RE and Tera-1 cells demonstrated that promoter activity was defined by the length of the ACE promoter sequence inserted into the construct. The 132 bases located upstream from the transcription start site were sufficient to confer ACE promoter activity, whereas the sequences upstream from -472 bp and between -343 bp and -132 bp were responsible for a decrease of promoter activity. Furthermore, the minimal 132 bp of the ACE promoter contains elements which direct cell-specific CAT expression. In addition, the DNA transfection study in the presence of dexamethasone suggested that the potential glucocorticoid regulatory elements, located in the sequence of the ACE promoter, are not functional.

[Expression of recombinant proteins in the milk of transgenic animals]

The bulky production of recombinant proteins can be achieved by procaryotes or eucaryotes cells. Cells from higher eucaryotes may be required when proteins have to be modified post-transcriptionally (glycosylation phosphorylation, cleavage, folding...). Cells from higher vertebrates in culture are used to prepare proteins like human factor VIII and erythropoietin. The use of transgenic organism has been suggested to reach the same goal. Indeed a whole living organism allows a very potent amplification, the number of cells involved in the biosynthesis of the recombinant proteins being very numerous and in the best metabolic conditions. Biological fluids (blood, milk, insect hemolymph, egg white...) and possibly organs from transgenic animals are a priori the best sources of recombinant proteins. Blood is abundant and it is a by-product of slaughter house. Its composition is relatively complex and the circulating recombinant proteins may heavily alter health of animals. Milk is very abundant, its composition is relatively simple, it is poor in proteolytic enzymes and it can be collected easily. Hemolymph from insects is relatively scarce. Egg white will be a possible source of recombinant proteins, when transgenesis has become more accessible in birds. Organs from transgenic animals should be solicited only when a particular cell type is required for the biosynthesis of the recombinant proteins. Milk appears therefore, presently, as the best source of recombinant proteins from transgenic animals. About 15 public and private laboratories try to use these techniques. They consist in preparing vectors containing regulatory regions of one of the milk proteins genes and the coding part (cDNA or gene) of the corresponding proteins to be produced. The transfer of these gene constructs to mouse, rabbit, sheep, goat, pig, shows that these techniques are indeed very promising. A single protein, human alpha 1-antitrypsin produced in milk of transgenic sheep, has presently reached the preparation at an industrial scale. This method has two theoretical limitations: 1) some of the proteins secreted in milk may be not matured as their native counterparts. Experiments carried out so far (about 20 proteins has been produced at an experimental scale) indicate that the mammary cell is able to achieve glycosylation in a correct way; 2) a significant proportion of the recombinant proteins migrate from the alveolar compartment of the mammary gland to blood circulation and they can alter health of lactating animals.(ABSTRACT TRUNCATED AT 400 WORDS)

A distal region enhances the prolactin induced promoter activity of the rabbit alpha s1-casein gene

Casein gene expression is induced in the rabbit mammary gland by prolactin (PRL). alpha s1-casein is the major casein secreted into milk. In order to define the position of the DNA sequences involved in the control of rabbit alpha s1-casein gene regulation by PRL, chimeric genes were constructed between upstream regions of the rabbit alpha s1-casein gene and the chloramphenicol acetyl transferase (CAT) reporter gene. A series of 5'-deleted fusion genes was obtained by nuclease digestion of the alpha s1-casein gene upstream region. These gene constructs were transfected into rabbit primary mammary cells, or cotransfected in CHO cells with the plasmid coding for the rabbit mammary receptor (PRL-R). A regulatory region has been located between nt -3768 and -3155. This region enhances the prolactin induced promoter activity of the alpha s1-casein gene. It might possess or cooperate with prolactin responsive elements located further downstream in the alpha s1-casein gene.

The prospects for domesticating milk protein genes

It is possible to convert milk glands of transgenic animals into bioreactors producing heterologous proteins such as scarce human pharmaceuticals. To predictably and successfully engineer the milk gland, we will need a thorough understanding of its physiology. Expression studies in transgenic animals have located mammary specific and hormone inducible transcription elements in the promoter/upstream regions of milk protein genes, and transfection studies in cell lines or primary cells have identified constitutive and hormone inducible elements. Most importantly, it appears that in addition to individual promoter based transcription elements structural features of milk protein chromosomal loci may contribute to the tight developmental and hormonal regulation. I will discuss milk protein gene regulation with emphasis on regulatory differences between genes and species, and the possibility that transcription elements function only properly within genetically defined chromatin domains. Novel strategies to build mammary expression vectors and to test their functionality without pursuing the standard transgenic route will be presented. Finally, I will discuss homologous recombination with the goal to target milk protein genes. Only through the domestication of milk protein genes will we be able to use their full potential in the mammary bioreactor.

A 17.6 kbp region located upstream of the rabbit WAP gene directs high level expression of a functional human protein variant in transgenic mouse milk

We have investigated whether DNA regions present in the rabbit whey acidic protein (WAP) promoter/5' flanking sequence could potentially confer, in vivo, high level expression of reporter genes. Transgenic mice were generated expressing a variant of human alpha 1-antitrypsin, which has inhibitory activity against plasma kallikrein under the control of a 17.6 kbp DNA fragment located upstream of the rabbit WAP gene. Up to 10 mg/ml of active and correctly processed recombinant protein were detected in mouse milk, thus suggesting that the far upstream DNA sequences from the rabbit WAP gene might be useful for engineering efficient protein production in the mammary glands of transgenic animals.

6-Dimethyl amino purine and 2-amino purine inhibit the induction of expression of milk protein genes by prolactin

Two protein kinase-inhibitors, 6-dimethyl amino purine and 2-amino purine inhibited induction of beta-casein synthesis by prolactin when added to the culture medium of rabbit mammary explant and cells. The accumulation of the mRNA for alpha s1- and beta-caseins and for whey acidic protein did not take place in the presence of the inhibitors whereas beta-actin mRNA concentration was not altered. In the same experimental conditions, H7, an inhibitor of protein kinase C and, to a lower extent, of protein kinase A did not prevent prolactin from acting. These data suggest for the first time that specific protein kinases are involved in the transduction of the prolactin signal to milk protein genes.