[Show abstract][Hide abstract] ABSTRACT: IntroductionAlthough aberrant tyrosine kinase signaling characterizes particular breast cancer subtypes, a global analysis of tyrosine phosphorylation in mouse models of breast cancer has not been undertaken. This may identify conserved oncogenic pathways and potential therapeutic targets.Methods
We applied an immunoaffinity/mass spectrometry workflow to three mouse models; murine stem cell virus (MSCV)-Neu, expressing truncated Neu, the rat orthologue of human epidermal growth factor receptor 2, Her2 (HER2); mouse mammary tumour virus (MMTV)- polyoma virus middle-T-antigen (PyMT) (PyMT); and the p53 -/- transplant model (p53). Pathways and protein-protein interaction networks were identified by bioinformatics. Molecular mechanisms underpinning differences in tyrosine phosphorylation were characterized by Western blotting and array comparative genomic hybridization. The functional role of Mesenchymal epithelial transition factor (Met) in a subset of p53-null tumours was interrogated using a selective tyrosine kinase inhibitor (TKI), small interfering (si)RNA-mediated knockdown and cell proliferation assays.ResultsThe three models could be distinguished based on tyrosine phosphorylation signatures and signaling networks. HER2 tumours exhibited a protein-protein interaction network centred on avian erythroblastic leukemia viral oncogene homolog 2 (Erbb2), epidermal growth factor receptor (Egfr) and platelet-derived growth factor receptor alpha (PDGFRa) and displayed enhanced tyrosine phosphorylation of ERBB receptor feedback inhibitor 1 (Errfi1). In contrast, the PyMT network displayed significant enrichment for components of the phosphatidylinositol-3-kinase signaling pathway, whilst p53 tumours exhibited increased tyrosine phosphorylation of Met and components or regulators of the cytoskeleton, and shared signaling network characteristics with basal and claudin-low breast cancer cells. A subset of p53 tumours displayed markedly elevated cellular tyrosine phosphorylation and Met expression, and Met gene amplification. Treatment of cultured p53-null cells exhibiting Met amplification with a selective Met TKI abrogated aberrant tyrosine phosphorylation and blocked cell proliferation. The effects on proliferation were re-capitulated when Met was knocked down using siRNA. Additional subtypes of p53 tumours exhibited increased tyrosine phosphorylation of other oncogenes, including Peak1/SgK269 and Prex2.Conclusion
This study provides network-level insights into signaling in these breast cancer models and demonstrates that comparative phosphoproteomics can identify conserved oncogenic signaling pathways. The Met-amplified, p53-null tumours provide a new pre-clinical model for a subset of triple-negative breast cancers.
Breast cancer research: BCR 09/2014; 16(5):437. · 5.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cumulative exposure to estrogen (E) and progesterone (P) over the menstrual cycle significantly influences the risk of developing breast cancer. Despite the dogma that PR in the breast merely serves as a marker of an active estrogen receptor (ER), and as an inhibitor of the proliferative actions of E, it is now clear that in the breast P increases proliferation independently of E action. We show here that the progesterone receptor (PR) and ER are expressed in different epithelial populations, and target non-overlapping pathways in the normal human breast. In breast cancer, PR becomes highly correlated with ER, and this convergence is associated with signaling pathways predictive of disease metastasis. These data challenge the established paradigm that ER and PR function co-operatively in normal breast, and have significant implications not only for our understanding of normal breast biology, but also for diagnosis, prognosis and/or treatment options in breast cancer patients.
[Show abstract][Hide abstract] ABSTRACT: Advances in the study of hematopoietic cell maturation have paved the way to a deeper understanding the stem and progenitor cellular hierarchy in the mammary gland. The mammary epithelium, unlike the hematopoietic cellular hierarchy, sits in a complex niche where communication between epithelial cells and signals from the systemic hormonal milieu, as well as from extra-cellular matrix, influence cell fate decisions and contribute to tissue homeostasis. We review the discovery, definition and regulation of the mammary cellular hierarchy and we describe the development of the concepts that have guided our investigations. We outline recent advances in in vivo lineage tracing that is now challenging many of our assumptions regarding the behavior of mammary stem cells, and we show how understanding these cellular lineages has altered our view of breast cancer.
Cellular and Molecular Life Sciences CMLS 07/2014; 71(22). · 5.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Regulators of differentiated cell fate can offer targets for managing cancer development and progression. Here we identify Runx2 as a new regulator of epithelial cell fate in mammary gland development and breast cancer. Runx2 is expressed in the epithelium of pregnant mice in a strict temporally and hormonally-regulated manner. During pregnancy, Runx2 genetic deletion impaired alveolar differentiation in a manner that disrupted alveolar progenitor cell populations. Conversely, exogenous transgenic expression of Runx2 in mammary epithelial cells blocked milk production, suggesting that the decrease in endogenous Runx2 observed late in pregnancy is necessary for full differentiation. In addition, overexpression of Runx2 drove EMT-like changes in normal mammary epithelial cells, while Runx2 deletion in basal breast cancer cells inhibited cellular phenotypes associated with tumorigenesis. Notably, loss of Runx2 expression increased tumor latency and enhanced overall survival in a mouse model of breast cancer, with Runx2-deficient tumors exhibiting reduced cell proliferation. Together, our results establish a novel function for Runx2 in breast cancer that may offer a novel generalized route for therapeutic interventions.
[Show abstract][Hide abstract] ABSTRACT: The HER2 (ERBB2) and MYC genes are commonly amplified in breast cancer, yet little is known about their molecular and clinical interaction. Using a novel chimeric mammary transgenic approach and in vitro models, we demonstrate markedly increased self-renewal and tumour-propagating capability of cells transformed with Her2 and c-Myc. Coexpression of both oncoproteins in cultured cells led to the activation of a c-Myc transcriptional signature and acquisition of a self-renewing phenotype independent of an epithelial-mesenchymal transition programme or regulation of conventional cancer stem cell markers. Instead, Her2 and c-Myc cooperated to induce the expression of lipoprotein lipase, which was required for proliferation and self-renewal in vitro. HER2 and MYC were frequently coamplified in breast cancer, associated with aggressive clinical behaviour and poor outcome. Lastly, we show that in HER2(+) breast cancer patients receiving adjuvant chemotherapy (but not targeted anti-Her2 therapy), MYC amplification is associated with a poor outcome. These findings demonstrate the importance of molecular and cellular context in oncogenic transformation and acquisition of a malignant stem-like phenotype and have diagnostic and therapeutic consequences for the clinical management of HER2(+) breast cancer.Oncogene advance online publication, 23 September 2013; doi:10.1038/onc.2013.368.
[Show abstract][Hide abstract] ABSTRACT: Overexpression of the anti-apoptotic factor, BCL-2, is a frequent feature of malignant disease and is commonly associated with poor prognosis and resistance to conventional chemotherapy. In breast cancer, however, high BCL-2 expression is associated with favourable prognosis, estrogen receptor (ER) positivity and low tumour grade; whilst low expression is included in several molecular signatures associated with resistance to endocrine therapy. In the present study, we correlate BCL-2 expression and DNA methylation profiles in human breast cancer and in multiple cell models of acquired endocrine-resistance to determine whether BCL-2 hypermethylation could provide a useful biomarker of response to cytotoxic therapy. In human disease, diminished expression of BCL-2 was associated with hypermethylation of the second exon, in a region that overlapped a CpG island and an ER-binding site. Hypermethylation of this region, which occurred in 10% of primary tumours, provided a stronger predictor of patient survival (p=0.019) when compared to gene expression (n=522). In multiple cell-models of acquired endocrine-resistance, BCL-2 expression was significantly reduced in parallel with increased DNA methylation of the exon 2 region. The reduction of BCL-2 expression in endocrine-resistant cells lowered their apoptotic threshold to anti-mitotic agents: nocodazole, paclitaxel and the PLK1 inhibitor, BI2536. This phenomenon could be reversed with ectopic expression of BCL-2, and rescued with the BCL-2 inhibitor, ABT-737. Collectively, these data imply that BCL-2 hypermethylation provides a robust biomarker of response to current and next generation cytotoxic agents in endocrine-resistant breast cancer, which may prove beneficial in directing therapeutic strategy for patients with non-resectable, metastatic disease.
Molecular Cancer Therapeutics 07/2013; · 5.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Alternative splicing of precursor messenger RNA (pre-mRNA) is common in mammalian cells and enables the production of multiple gene products from a single gene, thus increasing transcriptome and proteome diversity. Disturbance of splicing regulation is associated with many human diseases; however, key splicing factors that control tissue-specific alternative splicing remain largely undefined. In an unbiased genetic screen for essential male fertility genes in the mouse, we identified the RNA binding protein RBM5 (RNA binding motif 5) as an essential regulator of haploid male germ cell pre-mRNA splicing and fertility. Mice carrying a missense mutation (R263P) in the second RNA recognition motif (RRM) of RBM5 exhibited spermatid differentiation arrest, germ cell sloughing and apoptosis, which ultimately led to azoospermia (no sperm in the ejaculate) and male sterility. Molecular modelling suggested that the R263P mutation resulted in compromised mRNA binding. Within the adult mouse testis, RBM5 localises to somatic and germ cells including spermatogonia, spermatocytes and round spermatids. Through the use of RNA pull down coupled with microarrays, we identified 11 round spermatid-expressed mRNAs as putative RBM5 targets. Importantly, the R263P mutation affected pre-mRNA splicing and resulted in a shift in the isoform ratios, or the production of novel spliced transcripts, of most targets. Microarray analysis of isolated round spermatids suggests that altered splicing of RBM5 target pre-mRNAs affected expression of genes in several pathways, including those implicated in germ cell adhesion, spermatid head shaping, and acrosome and tail formation. In summary, our findings reveal a critical role for RBM5 as a pre-mRNA splicing regulator in round spermatids and male fertility. Our findings also suggest that the second RRM of RBM5 is pivotal for appropriate pre-mRNA splicing.
[Show abstract][Hide abstract] ABSTRACT: Progesterone-RankL paracrine signaling has been proposed as a driver of stem cell expansion in the mammary gland, and Elf5 is essential for the differentiation of mammary epithelial progenitor cells. We demonstrate that Elf5 expression is induced by progesterone and that Elf5 and progesterone cooperate to promote alveolar development. The progesterone receptor and Elf5 are expressed in a mutually exclusive pattern, and we identify RankL as the paracrine mediator of the effects of progesterone on Elf5 expression in CD61+ progenitor cells and their consequent differentiation. Blockade of RankL action prevented progesterone-induced side branching and the expansion of Elf5+ mature luminal cells. These findings describe a mechanism by which steroid hormones can produce the expansion of steroid hormone receptor-negative mammary epithelial cells.
[Show abstract][Hide abstract] ABSTRACT: ETV5 (Ets variant gene 5) is a transcription factor that is required for fertility. In this study, we demonstrate that ETV5 plays additional roles in embryonic and postnatal developmental processes in the mouse. Through a genome-wide mouse mutagenesis approach, we generated a sterile mouse line that carried a nonsense mutation in exon 12 of the Etv5 gene. The mutation led to the conversion of lysine at position 412 into a premature termination codon (PTC) within the ETS DNA binding domain of the protein. We showed that the PTC-containing allele produced a highly unstable mRNA, which in turn resulted in an undetectable level of ETV5 protein. The Etv5 mutation resulted in male and female sterility as determined by breeding experiments. Mutant males were sterile due to a progressive loss of spermatogonia, which ultimately resulted in a Sertoli cell only phenotype by 8 week-of-age. Further, the ETV5 target genes Cxcr4 and Ccl9 were significantly down-regulated in mutant neonate testes. CXCR4 and CCL9 have been implicated in the maintenance and migration of spermatogonia, respectively. Moreover, the Etv5 mutation resulted in several developmental abnormalities including an increased incidence of embryonic and perinatal lethality, postnatal growth restriction, polydactyly and renal asymmetry. Thus, our data define a physiological role for ETV5 in many aspects of development including embryonic and perinatal survival, postnatal growth, limb patterning, kidney development and fertility.
PLoS ONE 01/2013; 8(10):e77311. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have previously shown that during pregnancy the E-twenty-six (ETS) transcription factor ELF5 directs the differentiation of mammary progenitor cells toward the estrogen receptor (ER)-negative and milk producing cell lineage, raising the possibility that ELF5 may suppress the estrogen sensitivity of breast cancers. To test this we constructed inducible models of ELF5 expression in ER positive luminal breast cancer cells and interrogated them using transcript profiling and chromatin immunoprecipitation of DNA followed by DNA sequencing (ChIP-Seq). ELF5 suppressed ER and FOXA1 expression and broadly suppressed ER-driven patterns of gene expression including sets of genes distinguishing the luminal molecular subtype. Direct transcriptional targets of ELF5, which included FOXA1, EGFR, and MYC, accurately classified a large cohort of breast cancers into their intrinsic molecular subtypes, predicted ER status with high precision, and defined groups with differential prognosis. Knockdown of ELF5 in basal breast cancer cell lines suppressed basal patterns of gene expression and produced a shift in molecular subtype toward the claudin-low and normal-like groups. Luminal breast cancer cells that acquired resistance to the antiestrogen Tamoxifen showed greatly elevated levels of ELF5 and its transcriptional signature, and became dependent on ELF5 for proliferation, compared to the parental cells. Thus ELF5 provides a key transcriptional determinant of breast cancer molecular subtype by suppression of estrogen sensitivity in luminal breast cancer cells and promotion of basal characteristics in basal breast cancer cells, an action that may be utilised to acquire antiestrogen resistance.
[Show abstract][Hide abstract] ABSTRACT: A significant percentage of young men are infertile and, for the majority, the underlying cause remains unknown. Male infertility is, however, frequently associated with defective sperm motility, wherein the sperm tail is a modified flagella/cilia. Conversely, a greater understanding of essential mechanisms involved in tail formation may offer contraceptive opportunities, or more broadly, therapeutic strategies for global cilia defects. Here we have identified Rab-like 2 (RABL2) as an essential requirement for sperm tail assembly and function. RABL2 is a member of a poorly characterized clade of the RAS GTPase superfamily. RABL2 is highly enriched within developing male germ cells, where it localizes to the mid-piece of the sperm tail. Lesser amounts of Rabl2 mRNA were observed in other tissues containing motile cilia. Using a co-immunoprecipitation approach and RABL2 affinity columns followed by immunochemistry, we demonstrated that within developing haploid germ cells RABL2 interacts with intra-flagella transport (IFT) proteins and delivers a specific set of effector (cargo) proteins, including key members of the glycolytic pathway, to the sperm tail. RABL2 binding to effector proteins is regulated by GTP. Perturbed RABL2 function, as exemplified by the Mot mouse line that contains a mutation in a critical protein-protein interaction domain, results in male sterility characterized by reduced sperm output, and sperm with aberrant motility and short tails. Our data demonstrate a novel function for the RABL protein family, an essential role for RABL2 in male fertility and a previously uncharacterised mechanism for protein delivery to the flagellum.
[Show abstract][Hide abstract] ABSTRACT: Grb2-associated binder 1 (Gab1) is a docking protein that transduces signals from a variety of tyrosine kinases, including Met and the epidermal growth factor receptor (EGFR). Although the related protein Gab2 is strongly implicated in human cancer, a role for Gab1 has been less clear. However, a screen for gene mutations in breast cancer identified two somatic mutations in Gab1, Y83C and T387N. In this paper we describe the functional characterization of these Gab1 mutants. MCF-10A immortalized mammary epithelial cells overexpressing Gab1 Y83C and T387N exhibited a more elongated, fibroblastic phenotype compared with wild-type Gab1 controls. Expression of Gab1 or the mutants promoted epidermal growth factor (EGF)-independent proliferation in monolayer culture to a similar degree. However, in Matrigel culture, both mutants enhanced the formation of acini exhibiting an aberrant, branched morphology. In addition, expression of the mutants modestly increased Erk activation. The two mutants also enhanced branching morphogenesis in a different mammary epithelial cell line, HC11. To gain further insights into the mechanism of action of these mutations, we mapped Gab1 phosphorylation sites by mass spectrometry. This detected phosphorylation of T387 but ;not Y83. Cellular stimulation with EGF or hepatocyte growth factor (HGF) led to a transient, or sustained, induction of T387 phosphorylation, respectively. As T387 corresponds in position to Gab2 T391, which suppresses Gab2 signaling in a phosphorylation-dependent manner, these data support a model in which the T387N mutation abrogates negative-feedback regulation of Gab1. Interrogation of publically-available databases revealed additional cancer-associated mutations at, or in close proximity to, identified serine/threonine phosphorylation sites in other docking proteins. These data indicate that aberrant Gab1 signaling can directly contribute to breast cancer progression, and that negative feedback sites in docking proteins can be targeted by oncogenic mutations.Oncogene advance online publication, 2 July 2012; doi:10.1038/onc.2012.271.
[Show abstract][Hide abstract] ABSTRACT: Grb10 is an intracellular adaptor protein that acts as a negative regulator of insulin and insulin-like growth factor 1 (IGF1) receptors. Since global deletion of Grb10 in mice causes hypermuscularity, we have characterized the skeletal muscle physiology underlying this phenotype. Compared to wild-type (WT) controls, adult mice deficient in Grb10 have elevated body mass and muscle mass throughout adulthood, up to 12 mo of age. The muscle enlargement is not due to increased myofiber size, but rather an increase in myofiber number (142% of WT, P<0.01). There is no change in myofiber type proportions between WT and Grb10-deficient muscles, nor are the metabolic properties of the muscles altered on Grb10 deletion. Notably, the weight and cross-sectional area of hindlimbs from neonatal mice are increased in Grb10-deficient animals (198 and 137% of WT, respectively, both P<0.001). Functional gene signatures for myogenic signaling and proliferation are up-regulated in Grb10-deficient neonatal muscle. Our findings indicate that Grb10 plays a previously unrecognized role in regulating the development of fiber number during murine embryonic growth. In addition, Grb10-ablated muscle from adult mice shows coordinate gene changes that oppose those of muscle wasting pathologies, highlighting Grb10 as a potential therapeutic target for these conditions.
The FASEB Journal 05/2012; 26(9):3658-69. · 5.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Salvador/Warts/Hippo (Hippo) signaling pathway defines a novel signaling cascade regulating cell contact inhibition, organ size control, cell growth, proliferation, apoptosis and cancer development in mammals. The upstream regulation of this pathway has been less well defined than the core kinase cassette. KIBRA has been shown to function as an upstream member of the Hippo pathway by influencing the phosphorylation of LATS and YAP, but functional consequences of these biochemical changes have not been previously addressed. We show that in MCF10A cells, loss of KIBRA expression displays epithelial-to-mesenchymal transition (EMT) features, which are concomitant with decreased LATS and YAP phosphorylation, but not MST1/2. In addition, ectopic KIBRA expression antagonizes YAP via the serine 127 phosphorylation site and we show that KIBRA, Willin and Merlin differentially regulate genes controlled by YAP. Finally, reduced KIBRA expression in primary breast cancer specimens correlates with the recently described claudin-low subtype, an aggressive sub-group with EMT features and a poor prognosis.Oncogene advance online publication, 21 May 2012; doi:10.1038/onc.2012.196.
[Show abstract][Hide abstract] ABSTRACT: Recent elucidation of the stem and progenitor cell hierarchies that operate during normal tissue and organ development has provided a foundation for the development of new insights into the disease process. These hierarchies are established by genetic mechanisms, which specify and determine cell fate and act as cell-clade gatekeepers, upon which all multicellular organisms depend for viability. Perturbation of this gatekeeper function characterizes developmentally based diseases, such as cancer. Here, the emerging gatekeeper and master regulatory roles of the ETS transcription factor Elf5 in several diverse developmental scenarios is reviewed, and how this function intersects with hormonal and growth factor mediated regulation of these processes is shown.
Trends in Endocrinology and Metabolism 03/2012; 23(6):292-8. · 8.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Progesterone and prolactin remodel mammary morphology during pregnancy by acting on the mammary epithelial cell hierarchy. The roles of each hormone in mammary development have been well studied, but evidence of signalling cross-talk between progesterone and prolactin is still emerging. Factors such as receptor activator of NFkB ligand (RANKL) may integrate signals from both hormones to orchestrate their joint actions on the epithelial cell hierarchy. Common targets of progesterone and prolactin signalling are also likely to integrate their pro-proliferative actions in breast cancer. Therefore, a thorough understanding of the interplay between progesterone and prolactin in mammary development may reveal therapeutic targets for breast cancer. This review summarises our understanding of Pg and PRL action in mammary gland development before focusing on molecular mechanisms of signalling cross-talk and the implications for breast cancer.
Molecular and Cellular Endocrinology 09/2011; 357(1-2):101-7. · 4.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent characterization of mammary stem and progenitor cells has improved our understanding of the transcriptional network that coordinates mammary development; however, little is known about the mechanisms that enforce lineage commitment and prevent transdifferentiation in the mammary gland. The E-twenty six transcription factor Elf5 forces the differentiation of mammary luminal progenitor cells to establish the milk producing alveolar lineage. Methylation of the Elf5 promoter has been proposed to act as a lineage gatekeeper during embryonic development. We used bisulphite sequencing to investigate in detail whether Elf5 promoter methylation plays a role in lineage commitment during mammary development. An increase in Elf5 expression was associated with decreasing Elf5 promoter methylation in differentiating HC11 mammary cells. Similarly, purified mammary epithelial cells from mice had increased Elf5 expression and decreased promoter methylation during pregnancy. Finally, analysis of epithelial subpopulations revealed that the Elf5 promoter is methylated and silenced in the basal, stem cell-containing population relative to luminal cells. These results demonstrate that Elf5 promoter methylation is lineage-specific and developmentally regulated in the mammary gland in vivo, and suggest that loss of Elf5 methylation specifies the mammary luminal lineage, while continued Elf5 methylation maintains the stem cell and myoepithelial lineages.
[Show abstract][Hide abstract] ABSTRACT: Hedgehog (Hh) signaling plays an important role in several malignancies but its clinical significance in breast cancer is unclear. In a cohort of 279 patients with invasive ductal carcinoma of the breast, expression of Hh ligand was significantly associated with increased risk of metastasis, breast cancer-specific death, and a basal-like phenotype. A paracrine signature, encompassing high epithelial Hh ligand and high stromal Gli1, was an independent predictor for overall survival in multivariate analysis. In 2 independent histological progression series (n = 301), Hh expression increased with atypia. Hh ligand overexpression in a mouse model of basal breast cancer increased growth, induced a poorly differentiated phenotype, accelerated metastasis, and reduced survival. A stromal requirement for these effects was supported by the lack of similar Hh-mediated changes in vitro, and by stromal-specific expression of Hh target genes in vivo. Furthermore, inhibition of Hh ligand with a monoclonal antibody (5E1) inhibited tumor growth and metastasis. These data suggest that epithelial-stromal Hh signaling, driven by ligand expression in carcinoma cells, promotes breast cancer growth and metastasis. Blockade of Hh signaling to peritumoral stromal cells may represent a novel therapeutic approach in some basal-like breast cancers.
Cancer Research 06/2011; 71(11):4002-14. · 9.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Knockout mice have been made which carry null mutations of genes encoding molecules essential for prolactin release, prolactin,
the prolactin receptor and various members of the receptor’s signalling pathway, allowing in vivo genetic analysis of the role of the lactogenic hormones in mammary gland development. In prolactin and prolactin receptor
knockout mice mammary ductal side branching was absent, terminal end bud like structures persisted at the ductal termini well
into maturity and no alveolar buds formed along the ductal tree. Transplants of recombined mammary glands formed from stromal
and epithelial elements with and without the prolactin receptor showed that prolactin plays an indirect role in these processes
and supplementation of progesterone levels in prolactin knockout animals restored ductal side branching but not alveolar bud
formation. Up to mid pregnancy prolactin receptor heterozygous animals showed normal ductal and alveolar development, but
alveolar development stalled during late pregnancy preventing successful lactation. This phenotype was also seen in knockouts
of galanin, a peptide controlling pituitary prolactin release, and Stat5a a member of the receptor’s signalling pathway. Transplants
of recombined glands containing knockout epithelium and wild type stroma showed no lobulo-alveolar development during pregnancy,
while recombinations of wild type epithelium and PRLR knockout stroma showed normal development, demonstrating that a direct
action of lactogenic hormones is confined to the epithelium to promote lobuloalveolar development. The role of prolactin in
mammary tumor progression was analysed by introduction of the polyoma middle T antigen under the control of the MMTV promoter.
Tumors occurred later and grew more slowly in the absence of prolactin, but exhibited identical histopathology to those found
in wild type animals and were ultimately no less injurious, demonstrating that prolactin acts as a tumor promoter without
differentiative activity, in addition to its established role as a factor enhancing the initiation of tumors by chemical carcinogens.