Highly metastatic claudin-low mammary cancers can originate from luminal epithelial cells

Claudin-low breast cancer represents an aggressive molecular subtype that is comprised of mostly triple-negative mammary tumor cells that possess stem cell-like and mesenchymal features. Little is known about the cellular origin and oncogenic drivers that promote claudin-low breast cancer. In this study, we show that persistent oncogenic RAS signaling causes highly metastatic triple-negative mammary tumors in mice. More importantly, the activation of endogenous mutant KRAS and expression of exogenous KRAS specifically in luminal epithelial cells in a continuous and differentiation stage-independent manner induces preneoplastic lesions that evolve into basal-like and claudin-low mammary cancers. Further investigations demonstrate that the continuous signaling of oncogenic RAS, as well as regulators of EMT, play a crucial role in the cellular plasticity and maintenance of the mesenchymal and stem cell characteristics of claudin-low mammary cancer cells.

Efficient tissue-type specific expression of target genes in a tetracycline-controlled manner from the ubiquitously active Eef1a1 locus

Using an efficient gene targeting approach, we developed a novel mouse line that expresses the tetracycline-controlled transactivator (tTA) from the constitutively active Eef1a1 locus in a Cre recombinase-inducible manner. The temporally and spatially controlled expression of the EF1-LSL-tTA knockin and activation of tTA-driven responder transgenes was tested using four transgenic lines that express Cre under tissue-specific promoters of the pancreas, mammary gland and other secretory tissues, as well as an interferon-inducible promoter. In all models, the endogenous Eef1a1 promoter facilitated a cell-type-specific activation of target genes at high levels without exogenous enhancer elements. The applicability of the EF1-LSL-tTA strain for biological experiments was tested in two studies related to mammary gland development and tumorigenesis. First, we validated the crucial role of active STAT5 as a survival factor for functionally differentiated epithelial cells by expressing a hyperactive STAT5 mutant in the mammary gland during postlactational remodeling. In a second experiment, we assessed the ability of the EF1-tTA to initiate tumor formation through upregulation of mutant KRAS. The collective results show that the EF1-LSL-tTA knockin line is a versatile genetic tool that can be applied to constitutively express transgenes in specific cell types to examine their biological functions at defined developmental stages.

Janus Kinase 1 Plays a Critical Role in Mammary Cancer Progression

Janus kinases (JAKs) and their downstream STAT proteins play key roles in cytokine signaling, tissue homeostasis, and cancer development. Using a breast cancer model that conditionally lacks Janus kinase 1, we show here that JAK1 is essential for IL-6-class inflammatory cytokine signaling and plays a critical role in metastatic cancer progression. JAK1 is indispensable for the oncogenic activation of STAT1, STAT3, and STAT6 in ERBB2-expressing cancer cells, suggesting that ERBB2 receptor tyrosine kinase complexes do not directly activate these STAT proteins in vivo. A genome-wide gene expression analysis revealed that JAK1 signaling has pleiotropic effects on several pathways associated with cancer progression. We established that FOS and MAP3K8 are targets of JAK1/STAT3 signaling, which promotes tumorsphere formation and cell migration. The results highlight the significance of JAK1 as a rational therapeutic target to block IL-6-class cytokines, which are master regulators of cancer-associated inflammation.

Autocrine IGF1 Signaling Mediates Pancreatic Tumor Cell Dormancy in the Absence of Oncogenic Drivers

Mutant KRAS and c-MYC are oncogenic drivers and rational therapeutic targets for the treatment of pancreatic cancer. Although tumor growth and homeostasis are largely dependent on these oncogenes, a few residual cancer cells are able to survive the ablation of mutant KRAS and c-MYC. By performing a genome-wide gene expression analysis of in vivo-derived bulk tumor cells and residual cancer cells lacking the expression of mutant KRAS or c-MYC, we have identified an increase in autocrine IGF1/AKT signaling as a common survival mechanism in dormant cancer cells. The pharmacological inhibition of IGF-1R reduces residual disease burden and cancer recurrence, suggesting that this molecular pathway is crucial for the survival of cancer cells in the absence of the primary oncogenic drivers.

Generation of Janus kinase 1 (JAK1) conditional knockout mice

The biological functions of the Janus kinase 1 (JAK1) are suggested to be pleiotropic since this signal transducer is ubiquitously expressed and coupled to a variety of cytokine receptors. Consequently, mice that are deficient in this tyrosine kinase were reported to die shortly after birth. To facilitate studies that address the biological and molecular functions of JAK1 during postnatal development, we performed gene targeting in embryonic stem cells and generated a Cre/lox-based conditional knockout mouse model. Expression of Cre recombinase in the germline converted the Jak1 conditional knockout allele (Jak1^fl ) into a null allele (Jak1^- ) that when subsequently crossed into homozygosity led to a complete absence of the JAK1 protein in developing embryos. JAK1 deficient embryos were visibly smaller starting at E15.5. Newborn pups exhibited signs of apnea and died within hours after birth. The examination of fibroblasts from conditional knockout embryos and their littermate wildtype controls expressing JAK1 showed that lack of this Janus kinase resulted in an impaired tyrosine phosphorylation and activation of the downstream Signal Transducers and Activators of Transcription (STATs) 1, 3, and 6. JAK1 conditional knockout mice will be an invaluable tool to study cytokine signaling during normal development and disease progression in adult animals.

Novel transcripts from a distinct promoter that encode the full-length AKT1 in human breast cancer cells

Background

The serine-threonine kinase AKT1 plays essential roles during normal mammary gland development as well as the initiation and progression of breast cancer. AKT1 is generally considered a ubiquitously expressed gene, and its persistent activation is transcriptionally controlled by regulatory elements characteristic of housekeeping gene promoters. We recently identified a novel Akt1 transcript in mice (Akt1m), which is induced by growth factors and their signal transducers of transcription from a previously unknown promoter. The purpose of this study was to examine whether normal and neoplastic human breast epithelial cells express an orthologous AKT1m transcript and whether its expression is deregulated in cancer cells.

Methods

Initial sequence analyses were performed using the UCSC Genome Browser and GenBank to assess the potential occurrence of an AKT1m transcript variant in human cells and to identify conserved promoter sequences that are orthologous to the murine Akt1m. Quantitative RT-PCR was used to determine the transcriptional activation of AKT1m in mouse mammary tumors as well as 41 normal and neoplastic human breast epithelial cell lines and selected primary breast cancers.

Results

We identified four new AKT1 transcript variants in human breast cancer cells that are orthologous to the murine Akt1m and that encode the full-length kinase. These transcripts originate from an alternative promoter that is conserved between humans and mice. Akt1m is upregulated in the majority of luminal-type and basal-type mammary cancers in four different genetically engineered mouse models. Similarly, a subset of human breast cancer cell lines and primary breast cancers exhibited a higher expression of orthologous AKT1m transcripts.

Conclusions

The existence of an alternative promoter that drives the expression of the unique AKT1m transcript may provide a mechanism by which the levels of AKT1 can be temporally and spatially regulated at particular physiological states, such as cancer, where a heightened activity of this kinase is required.

Gain-of-function of Stat5 leads to excessive granulopoiesis and lethal extravasation of granulocytes to the lung

The Signal Transducer and Activator of Transcription 5 (Stat5) plays a significant role in normal hematopoiesis and a variety of hematopoietic malignancies. Deficiency in Stat5 causes impaired cytokine-mediated proliferation and survival of progenitors and their differentiated descendants along major hematopoietic lineages such as erythroid, lymphoid, and myeloid cells. Overexpression and persistent activation of Stat5 are sufficient for neoplastic transformation and development of multi-lineage leukemia in a transplant model. Little is known, however, whether a continuous activation of this signal transducer is essential for the maintenance of hematopoietic malignancies. To address this issue, we developed transgenic mice that express a hyperactive mutant of Stat5 in hematopoietic progenitors and derived lineages in a ligand-controlled manner. In contrast to the transplant model, expression of mutant Stat5 did not adversely affect normal hematopoiesis in the presence of endogenous wildtype Stat5 alleles. However, the gain-of-function of this signal transducer in mice that carry Stat5a/b hypomorphic alleles resulted in abnormally high numbers of circulating granulocytes that caused severe airway obstruction. Downregulation of hyperactive Stat5 in diseased animals restored normal granulopoiesis, which also resulted in a swift clearance of granulocytes from the lung. Moreover, we demonstrate that Stat5 promotes the initiation and maintenance of severe granulophilia in a cell autonomous manner. The results of this study show that the gain-of-function of Stat5 causes excessive granulopoiesis and prolonged survival of granulocytes in circulation. Collectively, our findings underline the critical importance of Stat5 in maintaining a normal balance between myeloid and lymphoid cells during hematopoiesis, and we provide direct evidence for a function of Stat5 in granulophilia–associated pulmonary dysfunction.

Dormant cancer cells contribute to residual disease in a model of reversible pancreatic cancer

The initiation and progression of pancreatic ductal adenocarcinoma (PDAC) is governed by a series of genetic and epigenetic changes, but it is still unknown whether these alterations are required for the maintenance of primary and metastatic PDAC. We show here that the c-Myc oncogene is upregulated throughout the entire process of neoplastic progression in human PDAC and in genetically engineered mice that express mutant Kras. To experimentally address whether c-Myc is essential for the growth and survival of cancer cells, we developed a novel mouse model that allows a temporally and spatially controlled expression of this oncogene in pancreatic progenitors and derived lineages of the exocrine pancreas. Unlike previous reports, upregulation of c-Myc was sufficient to induce the formation of adenocarcinomas after a short latency without additional genetic manipulation of cell survival pathways. Deficiency in Cdkn2a increased the rate of metastasis but had no effect on tumor latency or c-Myc-mediated cancer maintenance. Despite a macroscopically complete regression of primary, metastatic, and transplantable tumors following the ablation of c-Myc, some cancer cells remained dormant. A significant number of these residual neoplastic cells expressed cancer stem cell markers, and re-expression of exogenous c-Myc in these cells led to rapid cancer recurrence. Collectively, the results of this study suggest that c-Myc plays a significant role in the progression and maintenance of PDAC, but besides targeting this oncogene or its downstream effectors, additional therapeutic strategies are necessary to eradicate residual cancer cells to prevent disease recurrence.

Cyclin D3 compensates for the loss of cyclin D1 during ErbB2-induced mammary tumor initiation and progression

Cyclin D1 regulates cell proliferation and is a candidate molecular target for breast cancer therapy. This study addresses whether Cyclin D1 is indispensable for ErbB2-associated mammary tumor initiation and progression using a breast cancer model in which this cell-cycle regulator can be genetically ablated prior to or after neoplastic transformation. Deficiency in Cyclin D1 delayed tumor onset but did not prevent the occurrence of mammary cancer in mice overexpressing wild-type ErbB2. The lack of Cyclin D1 was associated with a compensatory upregulation of Cyclin D3, which explains why the targeted downregulation of Cyclin D1 in established mammary tumors had no effect on cancer cell proliferation. Cyclin D1 and D3 are overexpressed in human breast cancer cell lines and primary invasive breast cancers, and Cyclin D3 frequently exceeded the expression of Cyclin D1 in ErbB2-positive cases. The simultaneous inhibition of both cyclins in mammary tumor cells reduced cancer cell proliferation in vitro and decreased the tumor burden in vivo. Collectively, the results of this study suggest that only the combined inhibition of Cyclin D1 and D3 might be a suitable strategy for breast cancer prevention and therapy.

Abstract 2379: Cyclin D3 is important for ErbB2-induced mammary cancer initiation and progression in the absence of Cyclin D1

The cell cycle regulatory protein Cyclin D1 has been reported to be essential for both normal mammary gland development during pregnancy and the malignant transformation of the mammary epithelium in response to receptor tyrosine kinase (RTK) signaling. We have recently reported that the prolactin receptor-associated kinase Jak2 controls the transcriptional activation and, more importantly, the nuclear accumulation of Cyclin D1 in normal mammary epithelial cells via expression and activation of Akt1. We also demonstrated that the functional ablation of Jak2 and Stat5 in ErbB2-expressing mammary epithelial cells protects against the onset of mammary tumorigenesis. Jak2, however, is not required for the survival and proliferation of neoplastic cells. The constitutive activation of ErbB2 signaling, which is an initial event in the formation of mammary cancer in this model, was able to override the functional role of Jak2 in regulating the expression of Akt1 and Cyclin D1. To further define the role of the downstream effector Cyclin D1 during cancer initiation and progression in response to Jak2 and RTK signaling, we generated an ErbB2-associated mammary cancer model that allows a functional ablation of Cyclin D1 either prior to or following neoplastic transformation. Surprisingly, and in sharp contrast to previous reports, nearly all Cyclin D1-deficient females expressing wildtype ErbB2 developed mammary tumors. While Cyclin D1 deficiency extended the tumor-free survival, this particular cell cycle regulatory protein was clearly not required for tumor maintenance and progression. Growth and proliferation of the vast majority of Cyclin D1-deficient primary mammary cancers was accompanied by an upregulation of Cyclin D3. Since Cyclin D2 expression remained very low, knocking down the last remaining D-type cyclin (i.e. Cyclin D3) in tumor-derived primary cancer cells using three different shRNA constructs was sufficient to attenuate the proliferation rates. Interestingly, the ablation of the pool of all three D-type cyclins did not result in an upregulation of Cyclin E. Collectively, the results of this study show that Cyclin D1 is dispensable for neoplastic transformation of mammary epithelial cells and it might therefore not serve as a sole therapeutic target to treat ErbB2-associated breast cancers as previously suggested. In addition, these observations indicate that Cyclin D1 is most likely not the only downstream effector of Jak2 signaling that mediates the protective effects of a functional ablation of Jak2 in ErbB2-induced mammary carcinogenesis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2379. doi:10.1158/1538-7445.AM2011-2379

Longitudinal analysis of mammogenesis using a novel tetracycline-inducible mouse model and in vivo imaging

We generated a novel mouse model, which expresses the tetracycline-inducible transactivator under the regulation of the endogenous whey acidic protein gene. Using a tet-responsive luciferase reporter transgene, we demonstrated that the Wap-rtTA knockin allele allows a tightly controlled temporal and spatial expression of transgenes in the mammary gland in a ligand-inducible manner. The longitudinal analysis of individual females throughout their reproductive cycles using in vivo bioluminescence imaging confirmed that the expression of the Wap-rtTA knockin allele is highly upregulated during lactation. However, the extent of the transcriptional activation of the targeted Wap locus is dependent on the suckling stimulus and milk retrieval. In addition, we used WAP-rtTA/TetO-H2B-GFP double-transgenic females to monitor the presence of GFP-labeled parity-induced mammary epithelial cells (PI-MECs) during the postlactational involution period. The study shows that, unlike their progeny in mammary epithelial transplants as reported previously, PI-MECs themselves may not belong to the long-term label-retaining epithelial subtype.

Targeting janus kinase 2 in Her2/neu-expressing mammary cancer: Implications for cancer prevention and therapy

The Janus kinase 2 (Jak2) is essential for normal mammary gland development, but this tyrosine kinase and its main effector, signal transducer and activator of transcription 5, are also active in a significant subset of human breast cancers. We have recently reported that Jak2 controls the expression and nuclear accumulation of cyclin D1. Because this particular D-type cyclin has been suggested to be a key mediator for ErbB2-associated mammary tumorigenesis, we deleted Jak2 from ErbB2-expressing mammary epithelial cells prior to tumor onset and in neoplastic cells to address whether this tyrosine kinase plays a role in the initiation as well as progression of mammary cancer. Similar to cyclin D1-deficient mice, the functional ablation of Jak2 protects against the onset of mammary tumorigenesis. In contrast, the deletion of Jak2 from neoplastic cells or the acute, ligand-inducible down-regulation of this tyrosine kinase in an orthotopic transplant model did not affect the growth and survival of cancer cells. The constitutive activation of ErbB2 signaling, which is an initial event in the formation of mammary cancer, was able to override the functional role of Jak2 in regulating the expression of Akt1 and cyclin D1. This might be a compensatory mechanism that explains why Jak2 is a relevant target for preventing the initiation but not the progression of ErbB2-associated mammary cancer.

Expression of the whey acidic protein (Wap) is necessary for adequate nourishment of the offspring but not functional differentiation of mammary epithelial cells

Whey acidic protein (WAP) is the principal whey protein found in rodent milk, which contains a cysteine-rich motif identified in some protease inhibitors and proteins involved in tissue modeling. The expression of the Wap gene, which is principally restricted to the mammary gland, increases more than 1,000-fold around mid-pregnancy. To determine whether the expression of this major milk protein gene is a prerequisite for functional differentiation of mammary epithelial cells, we generated conventional knockout mice lacking two alleles of the Wap gene. Wap-deficient females gave birth to normal litter sizes and, initially, produced enough milk to sustain the offspring. The histological analysis of postpartum mammary glands from knockout dams does not reveal striking phenotypic abnormalities. This suggests that the expression of the Wap gene is not required for alveolar specification and functional differentiation. In addition, we found that Wap is dispensable as a protease inhibitor to maintain the stability of secretory proteins in the milk. Nevertheless, a significant number of litters thrived poorly on Wap-deficient dams, in particular during the second half of lactation. This observation suggests that Wap may be essential for the adequate nourishment of the growing young, which triple in size within the first 10 days of lactation. Important implications of these findings for the use of Wap as a marker for advanced differentiation of mammary epithelial cells and the biology of pluripotent progenitors are discussed in the final section.

The Janus Kinase 2 Is Required for Expression and Nuclear Accumulation of Cyclin D1 in Proliferating Mammary Epithelial Cells

Using a conditional knockout approach, we previously demonstrated that the Janus kinase 2 (Jak2) is crucial for prolactin (PRL) signaling and normal mammary gland development. PRL is suggested to synchronously activate multiple signaling cascades that emerge on the PRL receptor (PRLR). This study demonstrates that Jak2 is essential for the activation of the signal transducer and activator of transcription 5 (Stat5) and expression of Cish (cytokine-inducible SH2-containing protein), a Stat5-responsive negative regulator of Jak/Stat signaling. However, Jak2 is dispensable for the PRL-induced activation of c-Src, focal adhesion kinase, and the MAPK pathway. Despite activation of these kinases that are commonly associated with proliferative responses, the ablation of Jak2 reduces the multiplication of immortalized mammary epithelial cells (MECs). Our studies show that signaling through Jak2 controls not only the transcriptional activation of the Cyclin D1 gene, but, more importantly, it regulates the accumulation of the Cyclin D1 protein in the nucleus by altering the activity of signal transducers that mediate the phosphorylation and subsequent nuclear export of Cyclin D1. In particular, the levels of activated Akt (protein kinase B) and inactive glycogen synthase kinase-3β (i.e. a kinase that regulates the nuclear export and degradation of Cyclin D1) are reduced in MECs lacking Jak2. The proliferation of Jak2-deficient MECs can be rescued by expressing of a mutant form of Cyclin D1 that cannot be phosphorylated by glycogen synthase kinase-3β and therefore constitutively resides in the nucleus. Besides discriminating Jak2-dependent and Jak2-independent signaling events emerging from the PRLR, our observations provide a possible mechanism for phenotypic similarities between Cyclin D1 knockouts and females lacking individual members of the PRLR signaling cascade, in particular the PRLR, Jak2, and Stat5.

Parity-induced mammary epithelial cells are multipotent and express cell surface markers associated with stem cells

Parity-induced mammary epithelial cells (PI-MECs) are defined as a pregnancy hormone-responsive cell population that activates the promoter of late milk protein genes during the second half of pregnancy and lactation. However, unlike their terminally differentiated counterparts, these cells do not undergo programmed cell death during post-lactational remodeling of the gland. We previously demonstrated that upon transplantation into an epithelial-free mammary fat pad, PI-MECs exhibited two important features of multipotent mammary epithelial progenitors: a) self-renewal, and b) contribution to ductal and alveolar morphogenesis. In this new report, we introduce a new method to viably label PI-MECs. Using this methodology, we analyzed the requirement of ovarian hormones for the maintenance of this epithelial subtype in the involuted mammary gland. Furthermore, we examined the expression of putative stem cell markers and found that a portion of GFP-labeled PI-MECs were part of the CD24(+)/CD49f(high) mammary epithelial subtype, which has recently been suggested to contain multipotent stem cells. Subsequently, we demonstrated that isolated PI-MECs were able to form mammospheres in culture, and upon transplantation, these purified epithelial cells were capable of establishing a fully functional mammary gland. These observations suggest that PI-MECs contain multipotent progenitors that are able to self renew and generate diverse epithelial lineages present in the murine mammary gland.

Generation of a conditional knockout allele for the Janus kinase 2 (Jak2) gene in mice

To study biologically relevant functions of the Janus kinase 2 (Jak2) in multiple cytokine and hormone receptor signal transduction pathways, we generated a conditional knockout (floxed) allele of this gene by placing loxP sites around the first coding exon of Jak2. Homozygous floxed animals developed normally and exhibited no phenotypic abnormalities. The conversion of the floxed allele into a null mutation was achieved by transmitting the targeted allele through the female germline of MMTV-Cre (line A) mice. Embryos that carry two Jak2 null alleles died around midgestation and exhibited impaired definitive erythropoiesis, which is a hallmark of Jak2 deficiency reported previously in conventional knockouts. This observation suggested that the Cre-mediated deletion of the first coding exon results in a true null mutation that is incapable of mediating signals through the erythropoietin receptor. Using mouse embryonic fibroblasts derived from Jak2 null embryos and their wildtype littermate controls, we demonstrated that Jak2-deficiency decouples growth hormone-receptor signaling from its downstream mediators, the signal transducer and activator of transcription (Stat) 5a and 5b.

Cell cycle arrest and cell death are controlled by p53-dependent and p53-independent mechanisms in Tsg101-deficient cells

Our previous studies have shown that cells conditionally deficient in Tsg101 arrested at the G(1)/S cell cycle checkpoint and died. We created a series of Tsg101 conditional knock-out cell lines that lack p53, p21(Cip1), or p19(Arf) to determine the involvement of the Mdm2-p53 circuit as a regulator for G(1)/S progression and cell death. In this new report we show that the cell cycle arrest in Tsg101-deficient cells is p53-dependent, but a null mutation of the p53 gene is unable to maintain cell survival. The deletion of the Cdkn1a gene in Tsg101 conditional knock-out cells resulted in G(1)/S progression, suggesting that the p53-dependent G(1) arrest in the Tsg101 knock-out is mediated by p21(Cip1). The Cre-mediated excision of Tsg101 in immortalized fibroblasts that lack p19(Arf) seemed not to alter the ability of Mdm2 to sequester p53, and the p21-mediated G(1) arrest was not restored. Based on these findings, we propose that the p21-dependent cell cycle arrest in Tsg101-deficient cells is an indirect consequence of cellular stress and not caused by a direct effect of Tsg101 on Mdm2 function as previously suggested. Finally, the deletion of Tsg101 from primary tumor cells that express mutant p53 and that lack p21(Cip1) expression results in cell death, suggesting that additional transforming mutations during tumorigenesis do not affect the important role of Tsg101 for cell survival.

Parity-induced mammary epithelial cells facilitate tumorigenesis in MMTV-neu transgenic mice

Using a Cre-lox-based genetic labeling technique, we have recently discovered a parity-induced mammary epithelial subtype that is abundant in nonlactating and nonpregnant, parous females. These mammary epithelial cells serve as alveolar progenitors in subsequent pregnancies, and transplantation studies revealed that they possess features of multipotent progenitors such as self-renewal and the capability to contribute to ductal and alveolar morphogenesis. Here, we report that these cells are the cellular targets for transformation in MMTV-neu transgenic mice that exhibit accelerated mammary tumorigenesis in multiparous animals. The selective ablation of this epithelial subtype reduces the onset of tumorigenesis in multiparous MMTV-neu transgenics. There is, however, experimental evidence to suggest that parity-induced mammary epithelial cells may not be the only cellular targets in other MMTV-promoter-based transgenic strains. In particular, the heterogeneous MMTV-wnt1 lesions predominantly express the ductal differentiation marker Nkcc1 that is absent in MMTV-neu-derived tumors. Our observations support the idea that tumors originate from distinctly different epithelial subtypes in selected MMTV-promoter-driven cancer models and that diverse oncogenes might exert discrete effects on particular mammary epithelial subtypes.

Impaired alveologenesis and maintenance of secretory mammary epithelial cells in Jak2 conditional knockout mice

Jak2 is a hormone-receptor-coupled kinase that mediates the tyrosine phosphorylation and activation of signal transducers and activators of transcription (Stat). The biological relevance of Jak2-Stat signaling in hormone-responsive adult tissues is difficult to investigate since Jak2 deficiency leads to embryonic lethality. We generated Jak2 conditional knockout mice to study essential functions of Jak2 during mammary gland development. The mouse mammary tumor virus-Cre-mediated excision of the first coding exon resulted in a Jak2 null mutation that uncouples signaling from the prolactin receptor (PRL-R) to its downstream mediator Stat5 in the presence of normal and supraphysiological levels of PRL. Jak2-deficient females were unable to lactate as a result of impaired alveologenesis. Unlike Stat5a knockouts, multiple gestation cycles could not reverse the Jak2-deficient phenotype, suggesting that neither other components of the PRL-R signaling cascade nor other growth factors and their signal transducers were able to compensate for the loss of Jak2 function to activate Stat5 in vivo. A comparative analysis of Jak2-deficient mammary glands with transplants from Stat5a/b knockouts revealed that Jak2 deficiency also impairs the pregnancy-induced branching morphogenesis. Jak2 conditional mutants therefore resemble PRL-R knockouts more closely, which suggested that Jak2 deficiency might affect additional PRL-R downstream mediators other than Stat5a and Stat5b. To address whether Jak2 is required for the maintenance of PRL-responsive, differentiating alveolar cells, we utilized a transgenic strain that expresses Cre recombinase under regulatory elements of the whey acidic protein gene (Wap). The Wap-Cre-mediated excision of Jak2 resulted in a negative selection of differentiated alveolar cells, suggesting that Jak2 is required not only for the proliferation and differentiation of alveolar cells but also for their maintenance during lactation.

Developmental outcome of levetiracetam, its major metabolite in humans, 2-pyrrolidinone N-butyric acid, and its enantiomer (R)-alpha-ethyl-oxo-pyrrolidine acetamide in a mouse model of teratogenicity

PURPOSE

The purpose of this study was to test the teratogenic potential of the antiepileptic drug (AED) levetiracetam (LEV), its major metabolite in humans, 2-pyrrolidone-N-butyric acid (PBA), and enantiomer, (R)-alpha-ethyl-oxo-pyrrolidine acetamide (REV), in a well-established mouse model.

METHODS

All compounds were administered by intraperitoneal injections once daily to SWV/Fnn mice on gestational days 8-1/2 to 12-1/2. LEV was administered at doses of 600, 1,200, and 2,000 mg/kg/day, piracetam (PIR) and PBA, at 600 and 1,200 mg/kg/day, and REV, at 600 mg/kg/day. On gestational day 18(1/2), fetuses were examined for gross external malformations and prepared for skeletal analysis by using Alizarin Red S staining.

RESULTS

No significant gross external malformations were observed in any of the study groups. Fetal weights were significantly reduced in most study groups. Resorption rates were significantly increased only in the 2,000-mg/kg/day LEV group. The overall incidence of skeletal abnormalities and specifically of hypoplastic phalanges was significantly increased in both PBA treatments and in the intermediate 1,200-mg/kg/day LEV group. In contrast to that in humans, 24-h urinary excretion analysis in mice showed that 65-100% of the LEV doses were excreted unchanged, whereas only 4% was excreted as the metabolite PBA.

CONCLUSIONS

Results of this study demonstrate that both LEV and its major metabolite in humans, PBA, do not induce major structural malformations in developing SWV/Fnn embryos and suggest that they provide a margin of reproductive safety for the pregnant epileptic population when compared with other AEDs tested in this mouse model.

Tsg101 is essential for cell growth, proliferation, and cell survival of embryonic and adult tissues

Tumor susceptibility gene 101 (Tsg101) was identified in a random mutagenesis screen for potential tumor suppressors in NIH 3T3 cells. Altered transcripts of this gene have been detected in sporadic breast cancers and many other human malignancies. However, the involvement of this gene in neoplastic transformation and tumorigenesis is still elusive. Using gene targeting, we generated genetically engineered mice with a floxed allele of Tsg101. We investigated essential functions of this gene in vivo and examined whether the loss of function of Tsg101 results in tumorigenesis. Conventional knockout mice were generated through Cre-mediated excision of the first coding exon in the germ line of mouse mammary tumor virus (MMTV)-Cre transgenic mice. The complete ablation of Tsg101 in the developing embryo resulted in death around implantation. In contrast, mammary gland-specific knockout mice developed normally but were unable to nurse their young as a result of impaired mammogenesis during late pregnancy. Neither heterozygous null mutants nor somatic knockout mice developed mammary tumors after a latency of 2 years. The Cre-mediated deletion of Tsg101 in primary cells demonstrated that this gene is essential for the growth, proliferation, and survival of mammary epithelial cells. In summary, our results suggest that Tsg101 is required for normal cell function of embryonic and adult tissues but that this gene is not a tumor suppressor for sporadic forms of breast cancer.

Targeted deletion of the Tsg101 gene results in cell cycle arrest at G1/S and p53-independent cell death

The tumor susceptibility gene 101 (Tsg101) was originally discovered in a screen for potential tumor suppressors using insertional mutagenesis in immortalized fibroblasts. To investigate essential functions of this gene in cell growth and neoplastic transformation, we derived primary mouse embryonic fibroblasts from Tsg101 conditional knockout mice. Expression of Cre recombinase from a retroviral vector efficiently down-regulated Tsg101. The deletion of Tsg101 caused growth arrest and cell death but did not result in increased proliferation and cellular transformation. Inactivation of p53 had no influence on the deleterious phenotype, but Tsg101(-/-) cells were rescued through expression of exogenous Tsg101. Fluorescence-activated cell sorting, proliferation assays, and Western blot analysis of crucial regulators of the cell cycle revealed that Tsg101 deficiency resulted in growth arrest at the G(1)/S transition through inactivation of cyclin-dependent kinase 2. As a consequence, DNA replication was not initiated in Tsg101-deficient cells. Our results clearly demonstrate that Tsg101 is not a primary tumor suppressor in mouse embryonic fibroblasts. However, the protein is crucial for cell proliferation and cell survival.

A peritrophin-like protein expressed in the embryonic tracheae of Drosophila melanogaster

We have cloned and sequenced a cDNA from Drosophila melanogaster that encodes a protein homologous to the peritrophins, a family of chitin-binding proteins from the peritrophic matrix of insects. Unexpectedly, the gene, Gasp, is expressed in the embryonic tracheae. We suggest that this family of proteins may be present in other tissues than the peritrophic matrix, particularly where nutrient or gas exchange are important, and/or where invasion by parasites or viruses is possible. We have also mapped two similar genes that had been sequenced by the Berkeley Drosophila Genome Project, and find that these three very similar genes are not clustered, but are located on three different chromosomes.