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

Dominant negative variants of the SHP-2 tyrosine phosphatase inhibit prolactin activation of Jak2 (janus kinase 2) and induction of Stat5 (signal transducer and activator of transcription 5)-dependent transcription

PRL plays a central role in the regulation of milk protein gene expression in mammary epithelial cells and in the growth and differentiation of lymphocytes. It confers its activity through binding to a specific transmembrane, class I hematopoietic receptor. Ligand binding leads to receptor dimerization and activation of the tyrosine kinase Jak (janus kinase) 2, associated with the membrane-proximal, intracellular domain of the receptor. Jak2 phosphorylates and activates Stat5, a member of the Stat (signal transducers and activators of transcription) family. PRL receptor also activates SHP-2, a cytosolic tyrosine phosphatase. We investigated the connection between these two signaling events and derived a dominant negative mutant of SHP-2 comprising the two SH2 domains [SHP-2(SH2)2]. An analogous variant of the SHP-1 phosphatase [SHP-1(SH2)2] was used as a control. The dominant negative mutant of SHP-2 was found to inhibit the induction of tyrosine phosphorylation and DNA-binding activity of m-Stat5a, m-Stat5b, and the carboxyl-terminal deletion variant m-Stat5adelta749, as well as the transactivation potential of m-Stat5a and m-Stat5b. The dominant negative mutant SHP-1(SH2)2 had no effect. The kinase activity of Jak2 is also dependent on a functional SHP-2 phosphatase. We propose that SHP-2 relieves an inhibitory tyrosine phosphorylation event in Jak2 required for Jak2 activity, Stat5 phosphorylation, and transcriptional induction.

Cytokine receptor-independent, constitutively active variants of STAT5

STAT (signal transducers and activators of transcription) proteins are dual function proteins, which participate in cytokine-mediated signal transduction events at the cell surface and transcriptional regulation in the nucleus. We have exploited insights into the activation mechanism of STAT factors to derive constitutively active variants. Chimeric genes encoding fusion proteins of STAT5 and the kinase domain of JAK2 have been derived. The functional properties of the fusion proteins have been investigated in transiently transfected COS cells or in HeLa cells stably transfected with STAT5-JAK2 gene constructs regulated by a tetracycline-sensitive promoter. The STAT5-JAK2 proteins exhibit tyrosine kinase activity and are phosphorylated on tyrosine. The molecules are activated through an intramolecular or a cross-phosphorylation reaction and exhibit constitutive, STAT5-specific DNA binding activity. The transactivation potentials of three constitutively activated STAT5-JAK2 variants comprising different transactivation domains (TADs) derived from STAT5, STAT6, and VP16 were compared. The chimeric molecule containing the STAT5 TAD had no or only a very low, the molecule with the STAT6 TAD a medium, and the molecule with the VP16 TAD a very high transactivation potential. Transcription from STAT5-responsive gene promoter regions of the beta-casein, oncostatin M, and the cytokine-inducible Src homology 2 domain-containing protein genes was observed. These chimeric STAT molecules allow the study of the function of STAT5 independent of cytokine receptors and the activation of other signal transduction pathways.

Differential activation of the Stat signaling pathway in the liver after burn injury

The liver plays a crucial role in the acute phase response after injury; mechanisms responsible for transducing inflammatory signals to the nucleus to initiate this response are not known. The purpose of this study was to examine the induction of the novel Stat (signal transducer and activator of transcription) pathway in the liver after burn injury. Rats were subjected to either a 60% burn or sham treatment; livers were removed over a time course and extracted for nuclear protein. We found that Stat3, but not Stat5, binding was predominantly increased in the liver after burn injury as assessed by gel mobility and "supershift" analyses. Moreover, Stat3 nuclear protein levels were increased 6- to 14-fold in the livers of burned rats compared with those of sham rats. Stat3 phosphorylation was rapidly induced after burn injury; the subsequent increase of Stat3 binding was completely blocked by preincubation with the antiphosphotyrosine antibody (4G10). We conclude that a differential and early induction of Stat3 binding activity occurs in the liver after burn injury; this induction is mediated by an increase in phosphorylation. These findings suggest an important role for Stat3 in transducting inflammatory signals to the nucleus of liver cells after a systemic burn injury.

Mammary Stat5 abundance and activity are not altered with lactation state in cows

Stat5 is a key intracellular mediator of prolactin signalling and can activate transcription of milk proteins in response to prolactin. Therefore, in animals such as mice where lactation is dependent on prolactin, Stat5 is likely to play an important role in establishing or maintaining lactation in the mammary gland. However, little is known about its role in lactation in the dairy cow. In order to address this, the levels of Stat5a and Stat5b protein, mRNA and Stat5 DNA-binding activity were measured in mammary tissue from mice and cows at different lactational states. In the cow, Stat5a and Stat5b protein and mRNA levels, as well as Stat5 DNA-binding activity were unaltered between pregnancy and established lactation. In contrast, in the mouse Stat5a and Stat5b protein, as well as Stat5 DNA-binding activity were clearly increased during lactation whereas Stat5a and Stat5b mRNA levels were highest during pregnancy as has been previously described. In both species only a minority of the epithelial cell nuclei were Stat5 positive during established lactation. These results suggest that there are significant differences in the biological role of Stat5 in controlling lactation between ruminants and rodents.

Lactation-associated and prolactin-responsive changes in protein synthesis in mouse mammary cells

Lactational function in the mammary epithelial cell is subject to complex regulation, most probably involving multiple extracellular and intracellular proteins that act at any of a number of levels. Although some of these proteins have been identified it is likely that additional controllers of lactation exist, but have yet to be discovered. In an effort to identify such proteins, a search was made for non-milk lactation-associated or prolactin-responsive proteins in primary mouse mammary epithelial cells and the mouse mammary epithelial cell line, COMMA-D using two-dimensional electrophoresis on large-format gels. These analyses revealed 12 proteins whose rate of synthesis was dependent on lactation state or on response to prolactin. Two of these (p77 and p63) were lactation-associated in primary cells and prolactin-responsive in COMMA-D cells. These two proteins were identified by amino acid sequencing as glucose-regulated protein 78 (GRP78) and protein disulphide isomerase (PDI). The localization of these proteins in the endoplasmic reticulum and their presence in other secretory cell types and tissues suggests that they have a function in the processing or secretion of milk proteins.

Requirement of STAT5b for sexual dimorphism of body growth rates and liver gene expression

The signal transducer and activator of transcription, STAT5b, has been implicated in signal transduction pathways for a number of cytokines and growth factors, including growth hormone (GH). Pulsatile but not continuous GH exposure activates liver STAT5b by tyrosine phosphorylation, leading to dimerization, nuclear translocation, and transcriptional activation of the STAT, which is proposed to play a key role in regulating the sexual dimorphism of liver gene expression induced by pulsatile plasma GH. We have evaluated the importance of STAT5b for the physiological effects of GH pulses using a mouse gene knockout model. STAT5b gene disruption led to a major loss of multiple, sexually differentiated responses associated with the sexually dimorphic pattern of pituitary GH secretion. Male-characteristic body growth rates and male-specific liver gene expression were decreased to wild-type female levels in STAT5b-/- males, while female-predominant liver gene products were increased to a level intermediate between wild-type male and female levels. Although these responses are similar to those observed in GH-deficient Little mice, STAT5b-/- mice are not GH-deficient, suggesting that they may be GH pulse-resistant. Indeed, the dwarfism, elevated plasma GH, low plasma insulin-like growth factor I, and development of obesity seen in STAT5b-/- mice are all characteristics of Laron-type dwarfism, a human GH-resistance disease generally associated with a defective GH receptor. The requirement of STAT5b to maintain sexual dimorphism of body growth rates and liver gene expression suggests that STAT5b may be the major, if not the sole, STAT protein that mediates the sexually dimorphic effects of GH pulses in liver and perhaps other target tissues. STAT5b thus has unique physiological functions for which, surprisingly, the highly homologous STAT5a is unable to substitute.

Polymorphisms of bovine beta-lactoglobulin promoter and differences in the binding affinity of activator protein-2 transcription factor

Differential production of the two most common allelic variants of beta-lactoglobulin (LG), beta-LG A and beta-LG B, has been observed using PAGE. This study evaluated 733 bp of the beta-LG promoter region and 92 bp of the first exon for possible polymorphisms using denaturing gradient gel electrophoresis and nucleotide sequence analysis. Within this region, 13 single nucleotide substitution polymorphisms were detected. Twelve polymorphisms were allele specific, and one appeared to be polymorphic only for the B allele. Several potential binding sites for transcription factors were found within the promoter sequence. This study investigated the role of the G to C transversion within a consensus binding site for activator protein-2 at position-430 bp upstream from the transcription initiation site. Using the DNase-I footprint assay, we confirmed the functional importance of this point mutation and showed different binding affinities of activator protein-2 for both alleles. We discuss the possible regulatory role of activator protein-2 in the transcriptional regulation of the beta-LG gene and propose the activator protein-2 transcription factor as a modulator of gene expression of beta-LG.

GAS elements: a few nucleotides with a major impact on cytokine-induced gene expression

Gamma interferon activation site (GAS) elements are short stretches of DNA, originally defined as a requirement for the rapid transcriptional induction of genes in response to interferon-gamma (IFN-gamma). The protein complex binding to GAS sequences in IFN-gamma-treated cells, the gamma interferon activation factor (GAF), is a dimer of Stat1, the prototype of a family of cytokine-responsive transcription factors, the signal transducers and activators of transcription. To date, seven different Stats are known (excluding alternatively spliced or processed forms), six of which recognize the same small palindromic consensus sequence TTCN2-4 GAA that defines a GAS element. Because one or several Stats take part in nuclear signaling in response to most cytokines or growth factors, the GAS sequence has changed from being viewed as a specific site for IFN-activated GAF to becoming the general nuclear end of the Jak-Stat signaling pathways. This review focuses on the identification and definition of GAS elements, their interaction with Stat transcription factors, and their contribution to the specificity of cytokine-induced gene expression.

The Lx1 gene maps to mouse chromosome 17 and codes for a protein that is homologous to glucose and polyspecific transmembrane transporters

A novel mouse gene, provisionally named Lx1, has been cloned and sequenced. Lx1 most likely represents the mouse homolog of the rat gene OCT1, which encodes a polyspecific transmembrane transporter that is possibly involved in drug elimination. The LX1 predicted protein is highly hydrophobic, possesses twelve putative transmembrane domains, and also shares significant homology with members of the sugar transporter family, particularly the novel liver-specific transporter NLT. Lx1 mRNA is expressed at high levels in mouse liver, kidney, and intestine, and at low levels in the adrenals and in lactating mammary glands. The Lx1 gene maps very close to the imprinted Igf2r/Mpr300 gene on mouse Chromosome (Chr) 17, in a region that is syntenic to human Chr 6q. Chr 6q has been previously associated with transient neonatal diabetes mellitus and breast cancer.

Homodimerization of interleukin-4 receptor alpha chain can induce intracellular signaling

The possible role of homodimerization events in intracellular signal transduction triggered by the bipartite human interleukin-4 receptor was addressed. We generated cell lines functionally expressing derivatives of the two receptor subunits alpha and gamma, which allow for a specific and background-free experimental induction of intracellular homo- and heterodimers. A heterodimer of alpha and gamma released an intracellular signal, whereas a gamma-gamma homodimer did not. Unexpectedly, we found the intracellular domain of interleukin-4 receptor alpha chain to evoke cell proliferation and activation of tyrosine kinase Jak1 as well as of transcription factor Stat6 upon homodimerization. Both recruitment of the common gamma chain and activation of kinase Jak3 were shown to be dispensible for these processes.

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.

Regulation of a physiological apoptosis: mouse mammary involution

Continuous milk production during lactation is dependent on a complex interplay of lactogenic hormones and the suckling stimulus exerted by the young. Involution can be initiated in the mouse mammary gland at any stage of lactation by removing the pups; involution then remains reversible for about 30 to 36 h. Involution in the mouse mammary gland is characterized by a massive loss of secretory epithelial cells from programmed cell death. The nuclear activation of protein kinase A and transcription factor activator protein 1 precede the irreversible phase of involution that is characterized by internucleosomal DNA fragmentation. Activation of activator protein 1 and fragmentation of chromosomal DNA can be prevented by lactogenic hormone treatment in explant cultures derived from mammary tissue at lactation. The elevation in activator protein 1 coincides with the epithelial expression of sulfated glycoprotein 2, a potential target gene of activator protein 1. Programmed cell death in the mammary gland is associated with the expression of the growth arrest gene, gas-1, and the integrin-associated protein gene, IAP, which codes for a putative Ca2+ channel that is dependent on integrin. Their potential roles during involution are discussed.

Marelle acts downstream of the Drosophila HOP/JAK kinase and encodes a protein similar to the mammalian STATs

We have identified a putative Drosophila STAT protein named Marelle that exhibits mutant phenotypes identical to mutations in the Hopscotch/JAK kinase. We show that a reduction in the amount of marelle gene activity suppresses the phenotype associated with a gain-of-function mutation in hopscotch and enhances the phenotype associated with a weak hopscotch mutation. We propose that Hopscotch activates Marelle to regulate transcription of target genes such as the pair rule gene even-skipped. Our results demonstrate the existence of an invertebrate JAK/STAT system.

A role for STAT family transcription factors in myeloid differentiation

STAT family transcription factors regulate gene expression in response to a wide variety of cytokines. A transcription factor designated differentiation-induced factor (DIF), activated by treatment of myeloid cells with the differentiating agents interferon-gamma (IFN-gamma), granulocyte-macrophage colony-stimulating factor (GM-CSF), colony-stimulating factor-1 (CSF-1) or during phorbol ester-induced differentiation, was characterized as a 112kDa protein related to, but not identical with known isoforms of STAT 5. Taken together with previously published results, our data suggest an important function for members of the STAT 5 subfamily in regulating gene expression during the process of myeloid differentiation.

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.

Gamma chain-associated cytokine receptors signal through distinct transducing factors

The IL-2, IL-4, and IL-7 signaling pathways have been shown to utilize shared components. The receptors for these cytokines are composed of ligand-specific binding chains that associate with a shared signaling subunit, the common gamma (gamma c) chain. In addition, IL-2, IL-4, and IL-7 induce activation of a common set of nonreceptor tyrosine kinases, Jak-1 and Jak-3. We have further investigated the signaling events induced by these cytokines and find that the gamma c-associated receptors activate distinct signal transducing factors (STFs). In addition, we show that a 94-kDa STAT-related protein (p94) is activated in response to IL-2 and IL-7, but not IL-4. These data indicate that IL-2, IL-4, and IL-7 activate distinct signaling molecules which might be differentially recruited to the receptor complex by the ligand-specific units of the IL-2, IL-4, and IL-7 receptors.

Intermittent plasma growth hormone triggers tyrosine phosphorylation and nuclear translocation of a liver-expressed, Stat 5-related DNA binding protein. Proposed role as an intracellular regulator of male-specific liver gene transcription

Growth hormone (GH) exerts sexually dimorphic effects on liver gene transcription that are regulated by the temporal pattern of pituitary GH release, which is intermittent in male rats and nearly continuous in females. To investigate the influence of these GH secretory patterns on intracellular hepatocyte signaling, we compared the pattern of liver nuclear protein tyrosine phosphorylation in male and female rats. An M(r) approximately 93,000 polypeptide, p93, was found to be tyrosine phosphorylated to a high level in male but not female rats. GH, but not prolactin, rapidly stimulated p93 tyrosine phosphorylation in hypophysectomized rats. Intermittent plasma GH pulses triggered repeated p93 phosphorylation, while continuous GH exposure led to desensitization and a dramatic decline in liver nuclear p93. p93 was cross-reactive with two monoclonal antibodies raised to mammary Stat 5, whose tyrosine phosphorylation is stimulated by prolactin. Intermittent GH pulsation translocated liver Stat 5/p93 protein from the cytosol to the nucleus and also activated its DNA binding activity, as demonstrated using a mammary Stat 5-binding DNA element derived from the beta-casein gene. p93 is thus a liver-expressed, Stat 5-related DNA binding protein that undergoes tyrosine phosphorylation and nuclear translocation in response to intermittent plasma GH stimulation and is proposed to be an intracellular mediator of the stimulatory effects of GH pulses on male-specific liver gene expression.

Colony-stimulating factors and interferon-gamma activate a protein related to MGF-Stat 5 to cause formation of the differentiation-induced factor in myeloid cells

The Jak-Stat pathway of intracellular signals is used by growth factor- and cytokine receptors to induce gene transcription. We have recently reported that differentiation of myeloid cells, induced by phorbol ester, interferon-gamma (IFN-gamma) or colony-stimulating factor-1 (CSF-1) is accompanied by the activation of the differentiation-induced factor (DIF). Activated DIF specifically associates with a subclass of gamma-interferon activation site (GAS)-like DNA elements. We now report that GM-CSF, which like CSF-1 promotes the generation of mature macrophages, activates DIF. No activation was observed after treatment with the granulocyte growth and differentiation factor G-CSF. Antibodies raised against a Stat family protein, designated mammary gland factor-Stat 5 (MGF-Stat 5), reacted with DIF induced by either CSF-1, GM-CSF or IFN-gamma. Antisera to other known Stats were without effect on the DIF complex in electrophoretic mobility shift assays (EMSA). A 112 kDa protein could be isolated from either GM-CSF- or IFN-gamma-treated cells by GAS oligonucleotide precipitation. This protein reacted with antibodies to both MGF-Stat 5 and phosphotyrosine. MGF-Stat 5 and closely related proteins thus define a subfamily of Stat transcription factors that are present in a variety of cell types and are required for the onset of immediate gene expression in response to differentiating stimuli.

Exon organization and sequence of the genes encoding alpha-lactalbumin and beta-lactoglobulin from the tammar wallaby (Macropodidae, Marsupialia)

Clones encompassing the genes encoding alpha-lactalbumin and beta-lactoglobulin were isolated from a tammar wallaby genomic library, the exons localized using end-labeled oligonucleotides and the DNA sequences determined. The tammar beta-lactoglobulin gene has the same 7 exon-6 intron structure as the sheep homologue. Potential binding sites for mammary gland-specific transcription factors were identified, on the basis of similarity to sites in the sheep gene, in the promoter region of the tammar beta-lactoglobulin gene. The tammar gene encoding alpha-lactalbumin appears to contain four introns rather than three as are present in the eutherian homologues, or the evolutionarily related lysozyme gene. The additional intron appears to occur within the 5' noncoding region of the tammar gene.

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

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

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.

Cytokines and growth factors signal through tyrosine phosphorylation of a family of related transcription factors

The ability of cytokines to activate distinct but overlapping sets of genes defines their characteristic biological response. We now show that IFN gamma, IL-3, IL-4, IL-6, erythropoietin, EGF, and CSF-1 activate differing members of a family of latent cytoplasmic transcription factors. Although these factors have distinct physical and functional properties and exhibit different patterns of expression, they share many important features, including recognition of a related set of enhancer elements, rapid activation, tyrosine phosphorylation, and cross-reactivity to antibodies against p91, a cytoplasmic signaling protein activated by IFN alpha, IFN gamma, and IL-6. These shared features point to either parallel or common patterns of signal transduction. A general model of cytokine signal transduction is presented, in which receptor-associated tyrosine kinases activate ligand-specific members of a family of signal-transducing factors. Once activated, these factors carry their signals to the nucleus, where they bind a family of related enhancer elements.

The mammary factor MPBF is a prolactin-induced transcriptional regulator which binds to STAT factor recognition sites

Site-directed mutagenesis of the three binding sites for the mammary factor MPBF in the beta-lactoglobulin (BLG) promoter demonstrates that MPBF is a transcriptional activator of the BLG gene in mammary cells. MPBF requires phosphorylation on tyrosine for maximum binding activity and binds to GAS (interferon gamma-activation site) elements which are similar to the MPBF binding sites. Prolactin induces MPBF binding activity in CHO cells and is not antigenically related to Stat1 (p91) and Stat2 (p113), suggesting that this transcription factor is likely to be another member of the STAT family of cytokine/growth factor-induced transcription factors.

Development of the mammary gland and lactation

Many hormones, growth factors, and protooncogene products involved in mammary development and lactation have been identified; however, the mechanism of their concerted action remains to be explained. In addition to these regulatory factors, normal mammary development and lactation require the cell-cell interaction of stromal and parenchymal elements in the mammary gland. Recent studies indicate that PRL effects on target cells are likely transmitted into cells via activation of tyrosine kinase associated with a protein-designated JAK-2, while other studies have identified stimulatory and inhibitory factors that interact with the 5' promotor regions of milk-product genes.

Laminin and tenascin assembly and expression regulate HC11 mouse mammary cell differentiation

HC11 is a normal mouse mammary epithelial cell line that requires certain growth factors, such as EGF or bFGF, to respond optimally to lactogenic hormones and produce the differentiation marker beta-casein. Growth in insulin (Ins) or PDGF does not produce cells competent to respond to lactogenic hormones. Here we show that competency for differentiation is due at least in part to the modulation of extracellular matrix components. In particular we have studied laminin and tenascin. EGF alters endogenous laminin assembly. In addition, promotion of competency can be partially mimicked by plating HC11 cells on the E8 laminin fragment, which is able to induce lactogenic responsiveness in cells grown in the absence of EGF or bFGF. The production and assembly of tenascin is also dependent upon the growth conditions of the HC11 cells. EGF- or bFGF-grown competent cells produce tenascin but do not assemble it at the extracellular matrix as efficiently as Ins- or PDGF-grown, non-competent cells. This alteration apparently leads to a change in the cellular microenvironment that supports beta-casein production. In addition, when competent cells are plated on dishes coated with tenascin, lactogenic hormone induction of beta-casein is inhibited. The data suggest that tenascin assembly and beta-casein production are opposing features of a coordinated differentiation program of HC11 cells.

Hormonal regulation of transcription factor activity in mammary epithelial cells

The multihormonal control of milk protein gene transcription in mammary epithelial cells has been investigated. Although the hormones regulating milk protein gene expression are known, the interaction of the signal transduction pathways of steroid (glucocorticoids) and peptide (insulin and prolactin) hormones remains undefined in molecular terms. These signals converge on the level of nuclear factors binding to regulatory elements in the beta-casein gene promoter. The promoter has a modular architecture and is composed of positive and negative response elements. Nuclear transcription factors which bind to these elements have been identified. The mammary gland factor, MGF, is an essential mediator of lactogenic hormone action and is itself positively regulated in its DNA binding activity. It binds to the promoter region between positions -80 to -100. MGF counteracts a repressor element, constituted by two components, which is located adjacent to the MGF binding site at positions -100 to -150. The transcription factor YY1 binds to the proximal half of the repressor element which overlaps with the MGF binding site. Specific single-stranded DNA binding proteins contribute to the negative regulation of the promoter by interacting with sequence elements between -160 and -190. DNA binding of these proteins is negatively regulated by the lactogenic hormones.

Phorbol 12-myristate 13-acetate stimulates proliferation and ductal morphogenesis and inhibits functional differentiation of normal rat mammary epithelial cells in primary culture

The effect of the tumor promoter phorbol 12-myristate 13-acetate (PMA) on proliferation and differentiation of normal mammary epithelial cells from 50-day-old virgin rats was investigated using a model system that allows for full morphological and functional development of the cells. In this model, mammary epithelial cells are grown within a reconstituted basement membrane in a defined serum-free medium. PMA at a concentration of 10(-6) M effected translocation of protein kinase C from cytosol to membrane. At the same concentration, it stimulated cell proliferation both in the presence and absence of EGF, and this stimulation was observed even when PMA exposure was limited to 15 min at the time of each media change. In contrast to the effect on proliferation, PMA at concentrations of 10(-7) and 10(-6) M inhibited functional differentiation as assessed by casein accumulation. Phorbol 12,13-dibutyrate at 10(-6) M also stimulated proliferation and inhibited casein accumulation and was more effective than PMA in both cases. In contrast, the nonactive tumor promoter 4-alpha PMA had no effect on either proliferation or differentiation. One of the most striking effects of PMA was its ability to stimulate an atypical ductal morphogenesis, as manifested by the formation of intricate web-like colonies, and to inhibit the development of the well-differentiated alveolar-like multilobular colonies. PMA was also shown to completely suppress the growth of the squamous-like colonies that develop when EGF is absent or deficient. These effects of phorbol esters in mammary epithelial cells to stimulate proliferation, inhibit functional differentiation, and stimulate the development of ductal colonies are consistent with the suggestion that the signal transduction pathways evoked by PMA could act to stimulate the growth of initiated cells or render normal cells more sensitive to carcinogen.

The activated mammary gland specific nuclear factor (MGF) enhances in vitro transcription of the beta-casein gene promoter

The hormonal induction of the beta-casein gene in mammary epithelial cells is dependent on the action of peptide and steroid hormones. Epidermal growth factor, insulin, glucocorticoids and prolactin act in a sequential manner to regulate the transcription of the gene. We have studied the hormonal requirements as well as the nuclear proteins which are involved in the induction process. In vitro transcription in cell free nuclear extracts has been used to demonstrate the central role of the mammary gland specific nuclear factor, MGF, in the mediation of the hormonal signals to the transcription machinery. A gene construct comprising 344 nucleotides of wild type beta-casein promoter sequence and a G-free cassette of 220 nucleotides was used to test the activity of nuclear extracts in the in vitro transcription experiments. A construct in which the proximal MGF binding site in the beta-casein promoter region has been inactivated by mutation and a construct regulated by the adenovirus major late promoter served as controls. Nuclear extracts were prepared from Sf insect cells, HeLa cells and mammary epithelial cells of lactating rats. Strong transcription of the wild type beta-casein promoter construct was observed in the mammary cell extract, weak transcription in the extracts of Sf insect cells and the HeLa cells. The mutation of the MGF binding site drastically reduced the in vitro transcription in the mammary gland cell extract. Beta-casein promoter activity was also compared in nuclear extracts from uninduced and lactogenic hormone induced HC11 mammary epithelial cells. Extracts from induced cells are more efficient in the support of beta-casein gene transcription.