Abstract P1-03-01: Withdrawn

This abstract was withdrawn by the authors.

Abstract S1-8: Prolactin-humanized mice: an improved animal recipient for therapy response-testing of patient-derived breast cancer xenotransplants

Eighty percent of newly diagnosed breast cancer represents estrogen receptor(ER)-positive luminal subtypes. Many patients with luminal breast cancer develop antiestrogen resistant disease. It has historically been particularly difficult to establish ER-positive breast cancer lines from primary breast cancer in the laboratory or in mice. Murine and bovine prolactins, the major lactogens in current laboratory experimental in vivo and in vitro conditions, fail to activate human prolactin receptors because of species incompatibility. In fact, murine prolactin is a potent antagonist for human prolactin receptors. Because ER-positive, luminal breast cancers also express prolactin receptors, we hypothesized that lack of human lactogenic activity under experimental conditions selected against establishment of ER-positive breast cancer in the laboratory. We therefore genetically engineered mice to express physiological levels of human prolactin in place of mouse prolactin and backcrossed the mice for ten generations into the immunodeficient NSG strain. The resulting hPRL.NSG mice have a greatly improved take rate for ER positive, luminal type of breast cancer, suggesting key tumor-promoting roles for prolactin in luminal breast cancer. A panel of novel transplantable human breast cancer lines has been established in hPrl.NSG mice, the majority of which are ER-positive. The transplantable lines maintain key histopathological characteristics and expression of major marker proteins of the primary patient tumors. Intriguingly, initial tumor establishment and growth rates of breast cancer xenografts were consistently greater in the hPrl.NSG mice than in wildtype NSG mice. Furthermore, tumors grown in hPrl.NSG were more responsive to tamoxifen than size-matched tumors grown in wildtype NSG mice. At least two new tumor lines examined so far develop spontaneous distant metastases in hPrl.NSG mice, with evidence of prolactin-dependent progression of ER-positive disease. Collectively, these observations validate the hPrl.NSG mice as an improved recipient for preclinical modeling of human breast cancer in vivo, both for therapeutic targeting of prolactin-pathways and other growth and survival pathways, as well as overcoming anti-estrogen resistance.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr S1-8.

P4-03-01: Stat5 Regulates the PI3-Kinase/Akt1 Pathway during Mammary Gland Development and Promotes Neoplastic Transformation in a Mouse Model for Cowden Syndrome

Background: The Jak/Stat pathway responds to extracellular signals (i.e. cytokines) to promote physiological responses at the transcriptional level. In the developing mammary gland, Jak2/Stat5 signaling is critical for transducing prolactin signals by promoting the proliferation, differentiation, and survival of alveolar epithelial cells during pregnancy and lactation. Additionally, hyper-activation of Jak2/Stat5 activity has also been shown to cause mammary tumors in a number of transgenic models and we have previously shown it plays a crucial role in the early neoplastic events of prolactin and ErbB2-induced tumorigenesis. However, the mechanisms and target genes by which Stat5 promotes cell survival and neoplastic transformation remain to be elucidated and are the focus of this current study. Materials and Methods: To assess the functions of Stat5 during mammary development, gain-of-function Stat5 was expressed in a spatial and temporal manner within the mammary epithelial cells of double transgenic females (Wap-rtTA TetO-Stat5S710F model) during lactation and involution in a doxycycline (dox)-inducible manner. Mammary tissues were collected for histological examination along with mRNA, protein, and chromatin immunprecipitation (ChIP) analyses. HC11 mouse mammary epithelial cells were infected to overexpress wildtype or gain-of-function Stat5, a constitutively active prolactin receptor, or combinations of both. Finally, the functional cooperation between Stat5 and PI3-kinase/Akt signaling in promoting mammary cancer was assessed by expressing hyperactive Stat5 specifically within the mammary epithelium of females harboring a knock-in PtenG129E mutation found in patients with Cowden Syndrome (PtenG129E MMTV-tTA TetO-Stat5S710F model).

Results: Dox-induced expression of active Stat5 during involution overrides pro-apoptotic signaling and sustains p-Akt1 levels along with expression of the PI3-kinase and Akt1, which are normally downregulated at the transcriptional and protein levels during involution. ChIP analysis revealed that Stat5 binds two consensus sites in the Akt1 promoter during lactation whereas this is lost by the second day of involution. Furthermore, Stat5 promotes the transcriptional activation of Akt1 through a novel mammary-specific promoter and this suggests a mechanism for Stat5 in cell survival both in mammary development and breast cancer. Preliminary results also suggest that forced expression of active Stat5 in the mammary glands of PtenG129E females accelerates the development of mammary intraepithelial neoplasia (MIN) lesions predisposed by the loss of PI3K/Akt inhibition.

Discussion: In this study we demonstrate that Stat5 regulates PI3-kinase/Akt1 signaling during mammary gland development and this provides a mechanism for enhancing cell survival during lactation and for allowing apoptosis through loss of this regulation during involution. These findings further suggest that Jak2/Stat5 signaling may play an important role in the development of breast cancers induced through aberrant PI3-kinase/Akt signaling (i.e. Cowden Syndrome) and that targeting Jak2/Stat5 activity might be a suitable strategy for prophylactic intervention.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-03-01.

Deletion of Tip30 leads to rapid immortalization of murine mammary epithelial cells and ductal hyperplasia in the mammary gland

Transformation of mammary epithelial cells (MECs) from the normal to the neoplastic stage requires the dysregulation of tumor suppressor genes and proto-oncogenes. Tip30 is a tumor suppressor that can inhibit estrogen receptor-mediated transcription in MECs, but its role in MEC proliferation remains unknown. Here, we show that deleting the Tip30 gene leads to ductal hyperplasia in mouse mammary glands early in life and extensive mammary hyperplasia with age. Tip30(-/-) mammary glands transplanted into wild-type mammary fat pads also display mammary trees with extensive ductal hyperplasia. Strikingly, Tip30 deletion promotes proliferation of primary MECs and results in rapid immortalization of MECs in vitro relative to wild-type cells. Gene array analysis identified significant increases in the expression of mammary epithelial growth factors Wisp2 and Igf-1 in Tip30(-/-) cells. Knockdown of either Wisp2 or Igf-1 using short interfering RNA dramatically inhibited proliferation of Tip30(-/-) cells. Together, these results suggest that Tip30 is an intrinsic and negative regulator of MEC proliferation partly through the inhibition of Wisp2 and Igf-1 expression, and its absence in the mammary gland may predispose MECs to neoplastic transformation.

Tsg101 is upregulated in a subset of invasive human breast cancers and its targeted overexpression in transgenic mice reveals weak oncogenic properties for mammary cancer initiation

Previous studies reported that the Tumor Susceptibility Gene 101 (TSG101) is upregulated in selected human malignancies, and the expression of exogenous Tsg101 was suggested to transform immortalized fibroblasts in culture. To date, the potential oncogenic properties of Tsg101 have not been examined in vivo owing to the lack of appropriate model systems. In this study, we show that Tsg101 is highly expressed in a subset of invasive human breast cancers. Based on this observation, we generated the first transgenic mouse model with a targeted overexpression of Tsg101 in the developing mammary gland to test whether exogenous Tsg101 is capable of initiating tumorigenesis. Normal functionality of exogenous Tsg101 was tested by rescuing the survival of Tsg101-deficient mammary epithelial cells in conditional knockout mice. The overexpression of Tsg101 resulted in increased phosphorylation of the epidermal growth factor receptor and downstream activation of MAP kinases. Despite an increase in the activation of these signal transducers, the mammary gland of females expressing exogenous Tsg101 developed normally throughout the reproductive cycle. In aging females, the overexpression of Tsg101 seemed to increase the susceptibility of mammary epithelia toward malignant transformation. However, owing to the long latency of tumor formation and the sporadic occurrence of bona fide mammary cancers, we conclude that the Tsg101 protein has only weak oncogenic properties. Instead of cancer initiation, it is therefore likely that Tsg101 plays a more predominant role in the progression of a subset of spontaneously arising breast cancers.

Human prolactin receptors are insensitive to mouse prolactin: implications for xenotransplant modeling of human breast cancer in mice

Experimental testing of growth, metastatic progression and drug responsiveness of human breast cancer in vivo is performed in immunodeficient mice. Drug candidates need to show promise against human breast cancer in mice before being allowed into clinical trials. Breast cancer growth is under endocrine control by ovarian steroids and the pituitary peptide hormone prolactin. While it is recognized that the most relevant biologic effects of prolactin are achieved with prolactin from the matching species, the biologic efficacy of mouse prolactin for human prolactin receptors has not been recorded. Thus, it is unclear whether the mouse endocrine environment adequately reflects the hormonal environment in breast cancer patients with regard to prolactin. We now show both recombinant and natural pituitary-derived mouse prolactin to be a poor agonist for human prolactin receptors. Mouse prolactin failed to induce human prolactin receptor-mediated biologic responses of cell clustering, proliferation, gene induction and signal transduction, including activation of Stat5, Stat3, Erk1/2 and Akt pathways. Consistent data were derived from human breast cancer lines T-47D, MCF-7 and ZR-75.1, as well as human prolactin receptor-transfected COS-7 and 32D cells. Failure of mouse prolactin to activate human prolactin receptors uncovers a key deficiency of the mouse endocrine environment for human xenotransplant studies. Since most human breast cancers express prolactin receptors, human breast cancer transferred into mice is unnaturally selected for growth in the absence of circulating prolactin. The new insight raises concerns about the validity of analyzing biology and drug responsiveness of human breast cancer in existing mouse xenotransplant models.

Conditional deletion of the bcl-x gene from mouse mammary epithelium results in accelerated apoptosis during involution but does not compromise cell function during lactation

In the mammary gland Bcl-x is the most abundant cell survival factor from the Bcl-2 family. Since Bcl-x null mice die around day 12 of embryogenesis, the relevance of this protein in organ development and function is poorly understood. In erythroid cells bcl-x gene expression is controlled by cytokines and the transcription factor Stat5 (signal transducer and activator of transcription). However, we identified that bcl-x RNA levels in mammary tissue from prolactin receptor- and Stat5-null mice were indistinguishable from wild type mice. We have proposed that Bcl-x might control the survival of mammary epithelial cells throughout pregnancy, lactation, and the early stages of involution, and we have now tested this hypothesis through the conditional deletion of the bcl-x gene from mouse mammary epithelium. Conditional (floxed) bcl-x alleles were excised from alveolar cells during pregnancy using a Cre transgene under the control of the whey acidic protein gene promoter. Deletion of the bcl-x gene from the entire epithelial compartment (ducts and alveoli) was achieved by expressing Cre-recombinase under control of the mouse mammary tumor virus long terminal repeat. The absence of Bcl-x did not compromise proliferation and differentiation of mammary ductal and alveolar epithelial cells in virgin mice and during pregnancy and lactation. However, epithelial cell death and tissue remodeling were accelerated in the bcl-x conditional knockout mice during the first stage of involution. Concomitant deletion of the bax gene did not significantly modify the Bcl-x phenotype. Our results suggest that Bcl-x is not essential during mammopoiesis, but is critical for controlled apoptosis during the first phase of involution.

Spatial and temporal expression of the Cre gene under the control of the MMTV-LTR in different lines of transgenic mice

Cre-loxP based gene deletion approaches hold great promise to enhance our understanding of molecular pathways controlling mammary development and breast cancer. We reported earlier the generation of transgenic mice that express the Cre recombinase under the control of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). These mice have become a valuable research tool to delete genes specifically in the mammary gland, other secretory organs, and the female germline. We have now characterized in depth the expression of the MMTV-Cre transgene using the ROSA26-lox-Stop-lox-LacZ reporter strain to determine the temporal and spatial activation of Cre on the level of single cells. Our results show that MMTV-mediated Cre-activation is restricted to specific cell types of various secretory tissues and the hematopoietic system. Secondly, the timing of Cre expression varies between tissues and cell types. Some tissues express Cre during embryonic development, while other selected cell types highly activate Cre around puberty, suggesting a strong influence of steroid hormones on the transcriptional activation of the MMTV-LTR. Thirdly, Cre expression in the female germline is restricted to individual mouse lines and is therefore dependent on the site of integration of the transgene. Information provided by this study will guide the researcher to those cell types and developmental stages at which a phenotype can be expected upon deletion of relevant genes.

Signal transducer and activator of transcription (Stat) 5 controls the proliferation and differentiation of mammary alveolar epithelium

Functional development of mammary epithelium during pregnancy depends on prolactin signaling. However, the underlying molecular and cellular events are not fully understood. We examined the specific contributions of the prolactin receptor (PrlR) and the signal transducers and activators of transcription 5a and 5b (referred to as Stat5) in the formation and differentiation of mammary alveolar epithelium. PrlR- and Stat5-null mammary epithelia were transplanted into wild-type hosts, and pregnancy-mediated development was investigated at a histological and molecular level. Stat5-null mammary epithelium developed ducts but failed to form alveoli, and no milk protein gene expression was observed. In contrast, PrlR-null epithelium formed alveoli-like structures with small open lumina. Electron microscopy revealed undifferentiated features of organelles and a perturbation of cell-cell contacts in PrlR- and Stat5-null epithelia. Expression of NKCC1, an Na-K-Cl cotransporter characteristic for ductal epithelia, and ZO-1, a protein associated with tight junction, were maintained in the alveoli-like structures of PrlR- and Stat5-null epithelia. In contrast, the Na-Pi cotransporter Npt2b, and the gap junction component connexin 32, usually expressed in secretory epithelia, were undetectable in PrlR- and Stat5-null mice. These data demonstrate that signaling via the PrlR and Stat5 is critical for the proliferation and differentiation of mammary alveoli during pregnancy.

Role of serine phosphorylation of Stat5a in prolactin-stimulated beta-casein gene expression

Milk production remains suppressed in mammals during late pregnancy despite high levels of lactogenic polypeptide hormones. At parturition, associated with a precipitous fall in circulating progesterone, rising glucocorticoid levels synergize with prolactin to initiate copious milk production. This synergy is mediated at least in part through the coordinated activation of glucocorticoid receptors and transcription factor Stat5, particularly Stat5a. Here we show that two proline-juxtaposed serine residues within the transactivation domain of Stat5a are phosphorylated in the mammary gland during late gestation and lactation, and that these phosphorylation sites inhibit the transcriptional activity of Stat5a in the absence of glucocorticoid receptor costimulation. Specifically, transfection assays revealed that phosphorylation of residues S725 and S779 of Stat5a cooperatively suppressed prolactin-stimulated transcription from the beta-casein promoter in both COS-7 kidney and MCF-7 mammary cells. This suppression was associated with shortened duration and reduced amplitude of nuclear DNA binding activity of wild type Stat5a relative to that of the serine phosphorylation-defective Stat5 mutant. However, costimulation of glucocorticoid receptors completely reversed the suppressive effect of Stat5a serine phosphorylation on beta-casein gene transcription. We propose that serine phosphorylation within the transactivation domain may limit the activity of Stat5a in the absence of proper coactivation by glucocorticoid receptors.

Functional mammary gland development and oncogene-induced tumor formation are not affected by the absence of the retinoblastoma gene

Loss of cell cycle regulation in mammary epithelium results in impaired mammary gland development and neoplasia. We investigated the consequences of the absence of pRb in mammary epithelial cells during normal development and in mice that express an oncogene in the mammary epithelium. Since pRb-deficiency results in embryonic lethality, we transplanted pRb-null mammary anlagen into wild hosts. pRb-deficient mammary epithelia were capable of functional differentiation in term animals and they regenerated a differentiated gland even after multiple pregnancies. In serial transplantations no significant differences were found in outgrowth of pRb-deficient and wild type epithelia indicating that the absence of pRb does not lead to transformation. Likewise the effect of a TGFbeta1 transgene was not altered in the absence of pRb. The susceptibility of mammary epithelium to form tumors was assessed in three different models. No differences in tumor incidence were found between wild type and Rb +/- WAP-int3, MMTV-PyMT transgenic and Brcal-/- epithelia. These results demonstrate that the absence of pRb does not affect normal mammary gland development and tumorigenesis in three different mouse models investigated and suggest that loss of more than one member of the pRb pathway is required to induce mammary tumors.

Targeted reduction of oxytocin expression provides insights into its physiological roles

Oxytocin is a nonapeptide hormone that participates in the regulation of parturition and lactation. It has also been implicated in various behaviors, such as mating and maternal, and memory. To investigate whether or not oxytocin (OT) is essential for any of these functions, we eliminated, by homologous recombination, most of the first intron and the last two exons of the OT gene in mice. Those exons encode the neurophysin portion of the oxytocin preprohormone which is hypothesized to help in the packaging and transport of OT. The homozygous mutant mice have no detectable neurophysin or processed oxytocin in the paraventricular nucleus, supraoptic nucleus or posterior pituitary. Interestingly, homozygous mutant males and females are fertile and the homozygous mutant females are able to deliver their litters. However, the pups do not successfully suckle and die within 24 hours without milk in their stomachs. OT injection into the dams or rescue with the rat OT gene restores the milk ejection in response to suckling. OT is also needed for post-partum alveolar proliferation. These results indicate an absolute requirement for oxytocin for successful milk ejection, but not for mating, parturition and milk production, in mice. Furthermore, homozygous mutant mice show reduced aggression in some tests.

Conditional deletion of the Bcl-x gene from erythroid cells results in hemolytic anemia and profound splenomegaly

Bcl-x is a member of the Bcl2 family and has been suggested to be important for the survival and maturation of various cell types including the erythroid lineage. To define the consequences of Bcl-x loss in erythroid cells and other adult tissues, we have generated mice conditionally deficient in the Bcl-x gene using the Cre-loxP recombination system. The temporal and spatial excision of the floxed Bcl-x locus was achieved by expressing the Cre recombinase gene under control of the MMTV-LTR. By the age of five weeks, Bcl-x conditional mutant mice exhibited hyperproliferation of megakaryocytes and a decline in the number of circulating platelets. Three-month-old animals suffered from severe hemolytic anemia, hyperplasia of immature erythroid cells and profound enlargement of the spleen. We demonstrate that Bcl-x is only required for the survival of erythroid cells at the end of maturation, which includes enucleated reticulocytes in circulation. The extensive proliferation of immature erythroid cells in the spleen and bone marrow might be the result of a fast turnover of late red blood cell precursors and accelerated erythropoiesis in response to tissue hypoxia. The increase in cell death of late erythroid cells is independent from the proapoptotic factor Bax, as demonstrated in conditional double mutant mice for Bcl-x and Bax. Mice conditionally deficient in Bcl-x permitted us for the first time to study the effects of Bcl-x deficiency on cell proliferation, maturation and survival under physiological conditions in an adult animal.

Bcl-x and Bax regulate mouse primordial germ cell survival and apoptosis during embryogenesis

Restricted germ cell loss through apoptosis is initiated in the fetal gonad around embryonic day 13.5 (E13.5) as part of normal germ cell development. The mechanism of this germ cell attrition is unknown. We show that Bcl-x plays a crucial role in maintaining the survival of mouse germ cells during gonadogenesis. A bcl-x hypomorphic mouse was generated through the introduction of a neomycin (neo) gene into the promoter of the bcl-x gene by homologous recombination. Mice that contained two copies of the hypomorphic allele had severe reproductive defects attributed to compromised germ cell development. Males with two mutant alleles lacked spermatogonia and were sterile; females showed a severely reduced population of primordial and primary follicles and exhibited greatly impaired fertility. Primordial germ cells (PGCs) in bcl-x hypomorph mice migrated to the genital ridge by E12.5 but were depleted by E15.5, a time when Bcl-x and Bax were present. Two additional bcl-x transcripts were identified in fetal germ cells more than 300 bp upstream of previously reported start sites. Insertion of a neo cassette led to a down-regulation of the bcl-x gene at E12.5 in the hypomorph. Bax was detected by immunohistochemistry in germ cells from bcl-x hypomorph and control testes at E12.5 and E13.5. Bcl-x function was restored, and animals of both genders were fertile after removal of the neo selection cassette using Cre-mediated recombination. Alternatively, the loss of Bcl-x function in the hypomorph was corrected by the deletion of both copies of the bax gene, resulting in a restoration of germ cell survival. These findings demonstrate that the balance of Bcl-x and Bax control PGC survival and apoptosis.

Conditional mutation of Brca1 in mammary epithelial cells results in blunted ductal morphogenesis and tumour formation

Cre-mediated excision of exon 11 of the breast-tumour suppressor gene Brca1 in mouse mammary epithelial cells causes increased apoptosis and abnormal ductal development. Mammary tumour formation occurs after long latency and is associated with genetic instability characterized by aneuploidy, chromosomal rearrangements or alteration of Trp53 (encoding p53) transcription. To directly test the role of p53 in Brca1-associated tumorigenesis, we introduced a Trp53-null allele into mice with mammary epithelium-specific inactivation of Brca1. The loss of p53 accelerated the formation of mammary tumours in these females. Our results demonstrate that disruption of Brca1 causes genetic instability and triggers further alterations, including the inactivation of p53, that lead to tumour formation.

Genomic architecture and transcriptional activation of the mouse and human tumor susceptibility gene TSG101: common types of shorter transcripts are true alternative splice variants

The functional inactivation of the tumor susceptibility gene tsg101 in mouse NIH3T3 cells leads to cell transformation and the formation of metastatic tumors in nude mice. We cloned, mapped and sequenced the mouse tsg101 gene and further identified a processed pseudogene that is 98% identical to the tsg101 cDNA. Based on Northern blot analysis, tsg101 is expressed ubiquitously in mouse tissues. A comparison of the coding region of the mouse tsg101 gene with the human TSG101 cDNA revealed that both the mouse and human gene encode ten additional highly conserved amino acids at the N-terminus. Based on the mouse tsg101 genomic structure, we predicted four additional introns within the human TSG101 gene. Their location was confirmed using PCR and sequencing analysis. The presence of these so far unidentified introns now explains published data on aberrantly spliced mRNA products that were frequently observed in primary breast tumors. We show that a majority of shorter TSG101 transcripts are not the result of aberrant splicing events, but represent a fraction of true alternative splice variants. Finally, we examined tsg101 expression patterns during different stages of mammary gland development and in different transgenic mouse models for breast tumorigenesis.

Oxytocin and milk removal are required for post-partum mammary-gland development

The oxytocin (OT)-neurophysin preprohormone is synthesized in the paraventricular and supraoptic nuclei of the hypothalamus. OT is cleaved from its precursor, transported from the magnocellular neurons to the posterior pituitary and secreted during labour and upon the suckling stimulus of pups. OT induces the contraction of myoepithelial cells surrounding the mammary alveoli, which leads to the ejection of milk. Mice deficient in OT are unable to nurse their young. Administration of OT enabled OT-deficient dams to nurse. We now show that OT and milk removal are also required for post-partum alveolar proliferation and mammary-gland function. Alveolar density and mammary epithelial-cell differentiation at parturition was similar in wild-type and OT-deficient dams. However, within 12 h after parturition approx. 2% of the alveolar cells in wild-type dams incorporated DNA and proliferated, but virtually no proliferation was detected in OT-deficient dams. Continuous suckling of pups led to the expansion of lobulo-alveolar units in wild-type but not in OT-deficient dams. Despite suckling and the presence of systemic lactogenic hormones, mammary tissue in OT-deficient dams partially involuted. Our studies demonstrate that post-partum alveolar proliferation requires not only systemic lactogenic hormones, such as prolactin, but also the presence of OT in conjunction with continued milk removal.

Cre-mediated gene deletion in the mammary gland

To delete genes specifically from mammary tissue using the Cre-lox system, we have established transgenic mice expressing Cre recombinase under control of the WAP gene promoter and the MMTV LTR. Cre activity in these mice was evaluated by three criteria. First, the tissue distribution of Cre mRNA was analyzed. Second, an adenovirus carrying a reporter gene was used to determine expression at the level of single cells. Third, tissue specificity of Cre activity was determined in a mouse strain carrying a reporter gene. In adult MMTV-Cre mice expression of the transgene was confined to striated ductal cells of the salivary gland and mammary epithelial cells in virgin and lactating mice. Expression of WAP-Cre was only detected in alveolar epithelial cells of mammary tissue during lactation. Analysis of transgenic mice carrying both the MMTV-Cre and the reporter transgenes revealed recombination in every tissue. In contrast, recombination mediated by Cre under control of the WAP gene promoter was largely restricted to the mammary gland but occasionally observed in the brain. These results show that transgenic mice with WAP-Cre but not MMTV-Cre can be used as a powerful tool to study gene function in development and tumorigenesis in the mammary gland.

Developing a mammary gland is a stat affair

The mammary gland is a recent acquisition on the phylogenetic scale of organ evolution and is characterized by an unparalleled regenerative capacity. With each pregnancy an expanded lobulo-alveolar compartment rises on the ductal compartment and differentiates to secrete large amounts of milk during lactation. After weaning of the young the entire alveolar compartment undergoes apoptosis and is remodeled to return to a virgin-like state. Evolution recruited old hands from existing signaling pathways to guide and accomplish the extraordinary task of repeatedly building and destroying this highly specialized tissue. Seventy years ago it was known that the presence of estrogen, progesterone, and prolactin (PRL) was essential for ductal and alveolar development. The recent ability to generate mice from which genes have been deleted by homologous recombination has made it possible to gain molecular insight into the signaling pathways used by these hormones to effect mammary differentiation. In the cast of characters progesterone and PRL are on center stage. After binding to its receptor, PRL activates the JAK-STAT pathway leading to transcription of genes which induce alveolar proliferation and differentiation. In vivo experiments have shown that JAK-Stat signaling is mandatory for adult mammary gland development and lactation. Two Stat molecules, Stat3 and Stat5, appear to have opposite functions and their relative activity may serve to control developmental cycles of mammary tissue. While Stat5 activity has been linked to alveolar proliferation and function, Stat3 activity correlates with the loss of alveolar function, cell death and the initiation of mammary tissue remodeling.

Mammary-derived signals activate programmed cell death during the first stage of mammary gland involution

Programmed cell death (PCD) of mammary alveolar cells during involution commences within hours of the end of suckling. Locally, milk accumulates within alveolar lumens; systemically, levels of lactogenic hormones fall. Four experimental models were used to define the role of local factors as compared with systemic hormones during the first and second stages of involution. In three models, milk release was disrupted in the presence of systemic lactogenic hormones: (i) sealing of the teats, (ii) mammary gland transplants that cannot release milk due to the absence of a teat connection, and (iii) inactivation of the oxytocin gene. The ability of systemic hormones to preserve lobular-alveolar structure without blocking PCD was illustrated using a fourth transgenic model of lactation failure. During the first stage of involution, local signals were sufficient to induce alveolar PCD even in the presence of systemic lactogenic hormones. PCD coincided with bax induction, decreased expression of milk proteins, block of prolactin signal transduction through Stat5a and 5b, and activation of Stat3. The two stages of mammary gland involution are regulated by progressive gain of death signals and loss of survival factors. This study demonstrates that genetic events that occur during the first reversible stage are controlled by local factors. These mammary-derived death signals are dominant over protective effects related to systemic hormone stimulation.

Stat5a is mandatory for adult mammary gland development and lactogenesis

Prolactin (PRL) induces mammary gland development (defined as mammopoiesis) and lactogenesis. Binding of PRL to its receptor leads to the phosphorylation and activation of STAT (signal transducers and activators of transcription) proteins, which in turn promote the expression of specific genes. The activity pattern of two STAT proteins, Stat5a and Stat5b, in mammary tissue during pregnancy suggests an active role for these transcription factors in epithelial cell differentiation and milk protein gene expression. To investigate the function of Stat5a in mammopoiesis and lactogenesis we disrupted this gene in mice by gene targeting. Stat5a-deficient mice developed normally and were indistinguishable from hemizygous and wild-type littermates in size, weight, and fertility. However, mammary lobuloalveolar outgrowth during pregnancy was curtailed, and females failed to lactate after parturition because of a failure of terminal differentiation. Although Stat5b has a 96% similarity with Stat5a and a superimposable expression pattern during mammary gland development it failed to counterbalance for the absence of Stat5a. These results document that Stat5a is the principal and an obligate mediator of mammopoietic and lactogenic signaling.

Deficiency in mouse oxytocin prevents milk ejection, but not fertility or parturition

Oxytocin is a nonapeptide hormone that participates in the regulation of parturition and lactation. It has also been implicated in various behaviors, such as mating and maternal, and memory. To investigate whether or not oxytocin (OT) is essential for any of these functions, we eliminated, by homologous recombination, most of the first intron and the last two exons of the OT gene in mice. Those exons encode the neurophysin portion of the oxytocin preprohormone which is hypothesized to help in the packaging and transport of OT. The homozygous mutant mice have no detectable neurophysin or processed oxytocin in the paraventricular nucleus, supraoptic nucleus or posterior pituitary. Interestingly, homozygous mutant males and females are fertile and the homozygous mutant females are able to deliver their litters. However, the pups do not successfully suckle and die within 24 h without milk in their stomachs. OT injection into the dams restores the milk injection in response to suckling. These results indicate an absolute requirement for oxytocin for successful milk injection, but not for mating, parturition and milk production, in mice.

The cyanobacterium Synechococcus sp. strain PCC 7942 contains a second alkaline phosphatase encoded by phoV

A gene (phoV) encoding an alkaline phosphatase from Synechococcus sp. strain PCC 7942 was isolated by screening a plasmid gene bank for expression of alkaline phosphatase activity in Escherichia coli JM103. Two independent clones carrying the same alkaline-phosphatase-encoding gene were isolated. One of these clones (pKW1) was further analysed and the nucleotide sequence of a contiguous 3234 bp DNA fragment was determined. Two complete open reading frames (ORF1 and phoV) and an incomplete ORF3 were identified reading in the same direction. The deduced phoV gene product showed 34% identity to the alkaline phosphatase PhoA from Zymomonas mobilis, and the N-terminal part of the putative ORF3 protein exhibited 57% identity to a protein of unknown function from Frankia sp. Insertional inactivation of the Synechococcus PCC 7942 phoV gene failed, indicating an essential role for either the phoV or the ORF3 gene product. PhoV consists of 550 amino acid residues, resulting in a molecular mass of 61.3 kDa. To overexpress the Synechococcus PCC 7942 phoV gene in E. coli, plasmid pKW1 was transformed into a phoA mutant of E. coli (CC118). In E. coli strain CC118(pKW1) PhoV was expressed constitutively with high rates of activity, and was shown to be membrane associated in the periplasmic space. After partial purification of the recombinant PhoV, it was shown that, like other alkaline phosphatases, the Synechococcus PhoV had a broad pH optimum in the alkaline region and a broad substrate specificity for phosphomonoesters, required Zn2+ for activity, and was inhibited by phosphate. In contrast to several other alkaline phosphatases, PhoV was inhibited by Mn2+. Due to the lack of a Synechococcus PCC 7942 phoV mutant strain, the function of PhoV remains uncertain. However, the present results show that Synechococcus PCC 7942 has a second, probably phosphate-irrepressible, alkaline phosphatase (PhoV, 61.3 kDa) in addition to the phosphate-repressible enzyme (PhoA, 145 kDa) already described.