Molecular cloning and characterization of a zinc finger protein involved in Id-1-stimulated mammary epithelial cell growth

Genomic Analysis of the BMP Family in Glioblastomas

Glioblastoma multiforme (GBM) is a grade IV glioma with a median survival of 15 months. Recently, bone morphogenetic protein (BMP) signaling has been shown to promote survival in xenograft murine models. To gain a better understanding of the role of BMP signaling in human GBMs, we examined the genomic alterations of 90 genes associated with BMP signaling in GBM patient samples. We completed this analysis using publically available datasets compiled through The Cancer Genome Atlas and the Glioma Molecular Diagnostic Initiative. Here we show how mRNA expression is altered in GBM samples and how that is associated with patient survival, highlighting both known and novel associations between BMP signaling and GBM biology.

Hypoxic conditions induce a cancer-like phenotype in human breast epithelial cells

INTRODUCTION

Solid tumors are less oxygenated than their tissue of origin. Low intra-tumor oxygen levels are associated with worse outcome, increased metastatic potential and immature phenotype in breast cancer. We have reported that tumor hypoxia correlates to low differentiation status in breast cancer. Less is known about effects of hypoxia on non-malignant cells. Here we address whether hypoxia influences the differentiation stage of non-malignant breast epithelial cells and potentially have bearing on early stages of tumorigenesis.

METHODS

Normal human primary breast epithelial cells and immortalized non-malignant mammary epithelial MCF-10A cells were grown in a three-dimensional overlay culture on laminin-rich extracellular matrix for up to 21 days at normoxic or hypoxic conditions. Acinar morphogenesis and expression of markers of epithelial differentiation and cell polarization were analyzed by immunofluorescence, immunohistochemistry, qPCR and immunoblot.

RESULTS

In large ductal carcinoma in situ patient-specimens, we find that epithelial cells with high HIF-1α levels and multiple cell layers away from the vasculature are immature compared to well-oxygenated cells. We show that hypoxic conditions impaired acinar morphogenesis of primary and immortalized breast epithelial cells grown ex vivo on laminin-rich matrix. Normoxic cultures formed polarized acini-like spheres with the anticipated distribution of marker proteins associated with mammary epithelial polarization e.g. α6-integrin, laminin 5 and Human Milk Fat Globule/MUC1. At hypoxia, cells were not polarized and the sub-cellular distribution pattern of the marker proteins rather resembled that reported in vivo in breast cancer. The hypoxic cells remained in a mitotic state, whereas proliferation ceased with acinar morphogenesis at normoxia. We found induced expression of the differentiation repressor ID1 in the undifferentiated hypoxic MCF-10A cell structures. Acinar morphogenesis was associated with global histone deacetylation whereas the hypoxic breast epithelial cells showed sustained global histone acetylation, which is generally associated with active transcription and an undifferentiated proliferative state.

Zinc in specialized secretory tissues: roles in the pancreas, prostate, and mammary gland

Zinc (Zn) is an essential micronutrient required for over 300 different cellular processes, including DNA and protein synthesis, enzyme activity, and intracellular signaling. Cellular Zn homeostasis necessitates the compartmentalization of Zn into intracellular organelles, which is tightly regulated through the integration of Zn transporting mechanisms. The pancreas, prostate, and mammary gland are secretory tissues that have unusual Zn requirements and thus must tightly regulate Zn metabolism through integrating Zn import, sequestration, and export mechanisms. Recent findings indicate that these tissues utilize Zn for basic cellular processes but also require Zn for unique cellular needs. In addition, abundant Zn is transported into the secretory pathway and a large amount is subsequently secreted in a tightly regulated manner for unique biological processes. Expression of numerous members of the SLC30A (ZnT) and SLC39A (Zip) gene families has been documented in these tissues, yet there is limited understanding of their precise functional role in Zn metabolism or their regulation. Impairments in Zn secretion from the pancreas, prostate, and mammary gland are associated with disorders such as diabetes, infertility, and cancer, respectively. In this review, we will provide a brief summary of the specific role of Zn in each tissue and describe our current knowledge regarding how Zn metabolism is regulated. Finally, in each instance, we will reflect upon how this information shapes our current understanding of the role of Zn in these secretory tissues with respect to human health and disease.

ARFGAP2 and ARFGAP3 are essential for COPI coat assembly on the Golgi membrane of living cells

Coat protein complex I (COPI) vesicles play a central role in the recycling of proteins in the early secretory pathway and transport of proteins within the Golgi stack. Vesicle formation is initiated by the exchange of GDP for GTP on ARF1 (ADP-ribosylation factor 1), which, in turn, recruits the coat protein coatomer to the membrane for selection of cargo and membrane deformation. ARFGAP1 (ARF1 GTPase-activating protein 1) regulates the dynamic cycling of ARF1 on the membrane that results in both cargo concentration and uncoating for the generation of a fusion-competent vesicle. Two human orthologues of the yeast ARFGAP Glo3p, termed ARFGAP2 and ARFGAP3, have been demonstrated to be present on COPI vesicles generated in vitro in the presence of guanosine 5'-3-O-(thio)triphosphate. Here, we investigate the function of these two proteins in living cells and compare it with that of ARFGAP1. We find that ARFGAP2 and ARFGAP3 follow the dynamic behavior of coatomer upon stimulation of vesicle budding in vivo more closely than does ARFGAP1. Electron microscopy of ARFGAP2 and ARFGAP3 knockdowns indicated Golgi unstacking and cisternal shortening similarly to conditions where vesicle uncoating was blocked. Furthermore, the knockdown of both ARFGAP2 and ARFGAP3 prevents proper assembly of the COPI coat lattice for which ARFGAP1 does not seem to play a major role. This suggests that ARFGAP2 and ARFGAP3 are key components of the COPI coat lattice and are necessary for proper vesicle formation.

Smad3 mediates immediate early induction of Id1 by TGF-beta

Id1 is a member of the inhibitor of differentiation (Id) protein family that regulates a wide range of cell functions. Previous studies have shown that expression of the Id1 gene is down-regulated by TGF-beta in epithelial cells, whereas it is up-regulated by BMP in a variety of cell types. During our study of the biological function of TGF-beta1, we found that Id1 can be strongly up-regulated by TGF-beta1 in the human mammary gland epithelial cell line MCF10A. Quantitative real-time RT-PCR has revealed as high as 7.5-fold induction of Id1 mRNA by TGF-beta1 in MCF10A cells after 1 h of TGF-beta1 stimulation, and this induction does not require de novo protein synthesis. Using Smad knockdown and knockout approaches, we have identified Smad3 as the responsible R-Smad for mediating transcriptional activation of the Id1 gene. Chromatin immunoprecipitation assay confirms that Smad3 and Smad4 bind to the upstream region of the Id1 gene. Our results demonstrate that Smad3, but not Smad2, mediates TGF-beta1-dependent early transcriptional induction of Id1.

Differential roles of ArfGAP1, ArfGAP2, and ArfGAP3 in COPI trafficking

The formation of coat protein complex I (COPI)–coated vesicles is regulated by the small guanosine triphosphatase (GTPase) adenosine diphosphate ribosylation factor 1 (Arf1), which in its GTP-bound form recruits coatomer to the Golgi membrane. Arf GTPase-activating protein (GAP) catalyzed GTP hydrolysis in Arf1 triggers uncoating and is required for uptake of cargo molecules into vesicles. Three mammalian ArfGAPs are involved in COPI vesicle trafficking; however, their individual functions remain obscure. ArfGAP1 binds to membranes depending on their curvature. In this study, we show that ArfGAP2 and ArfGAP3 do not bind directly to membranes but are recruited via interactions with coatomer. In the presence of coatomer, ArfGAP2 and ArfGAP3 activities are comparable with or even higher than ArfGAP1 activity. Although previously speculated, our results now demonstrate a function for coatomer in ArfGAP-catalyzed GTP hydrolysis by Arf1. We suggest that ArfGAP2 and ArfGAP3 are coat protein–dependent ArfGAPs, whereas ArfGAP1 has a more general function.

Two human ARFGAPs associated with COP-I-coated vesicles

ADP-ribosylation factors (ARFs) are critical regulators of vesicular trafficking pathways and act at multiple intracellular sites. ADP-ribosylation factor-GTPase-activating proteins (ARFGAPs) are proposed to contribute to site-specific regulation. In yeast, two distinct proteins, Glo3p and Gcs1p, together provide overlapping, essential ARFGAP function required for coat protein (COP)-I-dependent trafficking. In mammalian cells, only the Gcs1p orthologue, named ARFGAP1, has been characterized in detail. However, Glo3p is known to make the stronger contribution to COP I traffic in yeast. Here, based on a conserved signature motif close to the carboxy terminus, we identify ARFGAP2 and ARFGAP3 as the human orthologues of yeast Glo3p. By immunofluorescence (IF), ARFGAP2 and ARFGAP3 are closely colocalized with coatomer subunits in NRK cells in the Golgi complex and peripheral punctate structures. In contrast to ARFGAP1, both ARFGAP2 and ARFGAP3 are associated with COP-I-coated vesicles generated from Golgi membranes in the presence of GTP-γ-S in vitro. ARFGAP2 lacking its zinc finger domain directly binds to coatomer. Expression of this truncated mutant (ΔN-ARFGAP2) inhibits COP-I-dependent Golgi-to-endoplasmic reticulum transport of cholera toxin (CTX-K63) in vivo. Silencing of ARFGAP1 or a combination of ARFGAP2 and ARFGAP3 in HeLa cells does not decrease cell viability. However, silencing all three ARFGAPs causes cell death. Our data provide strong evidence that ARFGAP2 and ARFGAP3 function in COP I traffic.

Regulation of clusterin expression in mammary epithelial cells

Mammary epithelial cells undergo changes in growth, invasion, differentiation, and dedifferentiation throughout much of adult hood, and most strikingly during pregnancy, lactation, and involution. Clusterin is a multifunctional glycoprotein that is involved in the differentiation and morphogenesis of epithelia, and that is important in the regulation of postnatal mammary gland development. However, the mechanisms that regulate clusterin expression are still poorly understood. Here, we show that clusterin is up-regulated twice during mouse mammary gland development, a first time at the end of pregnancy and a second time at the beginning of the involution. These points of clusterin up-regulation coincide with the dramatic phenotypic and functional changes occurring in the mammary gland. Using cell culture conditions that resemble the regulatory microenvironment in vivo, we determined that the factors responsible for the first up-regulation of clusterin levels can include the extracellular matrix component, laminin, and the lactogenic hormones, prolactin and hydrocortisone. On the other hand, the second and most dramatic up-regulation of clusterin can be due to the potent induction by TGF-beta1, and this up-regulation by TGF-beta1 is dependent on beta1 integrin ligand-binding activity. Moreover, the level of expression of beta-casein, a marker of mammary epithelial cell differentiation, was decreased upon treatment of cells with clusterin siRNA. Overall, these findings reveal several novel pathways for the regulation of clusterin expression during mammary gland development, and suggest that clusterin is a morphogenic factor that plays a key role during differentiation.

Angiogenesis and capillary maturation phenotypes associated with the Edpm3 locus on rat chromosome 3

The quantitative trait locus (QTL) Edpm3 is one of a group of additively acting QTL \responsible for the difference in estrogen-induced pituitary tumor growth between the tumor-susceptible F344 and tumor-resistant BN rat strains. The F344.BN-Edpm3(BN) rat strain was produced by moving the segment of rat Chr 3 between D3Mgh7 and D3Mgh13, which contains the Edpm3 QTL, from the BN strain into the F344 genetic background. In a previous study, we used this congenic line to find that the BN allele of the Edpm3 QTL reduces tissue mass and S-phase fraction in the estrogen-induced rat pituitary tumor. We now report on the use of this congenic line to investigate the linkage of Edpm3 to tumor angiogenesis. Contrary to expectation, the F344.BN-Edpm3(BN) strain has significantly greater angiogenic activity than does F344 in both treated and untreated rats. Microvessel count (MVC), perivascular space, and number of nonattached pericytes/pericapillary fibroblasts are all elevated in the pituitary by chronic estrogen treatment and their values are significantly greater in F344.BN-Edpm3(BN) than F344. Thus, although there is greater angiogenic activity in the pituitary of estrogen-treated F344.BN-Edpm3(BN) rats, there is a deficiency in capillary maturation compared with F344.

A role for Id proteins in mammary gland physiology and tumorigenesis

Id helix-loop-helix (HLH) proteins are regulators of cell growth and differentiation in embryonic and adult tissues. They are members of the basic HLH family of transcription factors but lack a DNA binding domain. By binding to basic HLH transcription factors, Id proteins regulate gene expression. Id1 and Id3 have extensive sequence homology and similar patterns of expression during embryogenesis and in adult tissues. They are also expressed at high levels in the endothelial cells of tumor-infiltrating blood vessels, and breast tumors spontaneously arising in MMTV-neu mice demonstrate impaired angiogenesis when growing in an Id1- and/or Id3-deficient background. These lesions are typically cystic with a small rim of viable tumor cells surrounding an acellular necrotic core. Id2 plays a critical role in breast differentiation and lactation. Id4 regulates BRCA1 expression and may be involved in hormone-dependent regulation of BRCA1 homeostasis. Thus, all four members of the Id protein family play pivotal roles in distinct aspects of normal and malignant breast biology, the subject of this review.

Reduction of Human Metastatic Breast Cancer Cell Aggressiveness on Introduction of Either Form A or B of the Progesterone Receptor and Then Treatment with Progestins

The sex steroid hormone progesterone (Pg) is critically involved in the development of the mammary gland, and it also is thought to play a role in breast cancer progression. However, the effect of Pg on malignant phenotypes is not fully understood in breast cancer. We previously reported that in Pg receptor (PR)-positive T47D breast cancer cells, Pg was able to counterbalance the stimulatory effect of estrogen or serum on proliferation and on expression level of Id-1, which generally stimulates cell proliferation and inhibits differentiation. Conversely, metastatic MDA-MB231 breast cancer cells lack PR and express high levels of Id-1 constitutively, and Pg showed no effect on Id expression, proliferation, and invasion in these cells. However, after introducing PR (either PR-A or PR-B) into MDA-MB231 cells, Pg inhibited the expression of Id-1 mRNA drastically. PR-transfected MDA-MB231 cells exhibited less proliferative activity after Pg treatment than parental or control MDA-MB231 cells, an effect which correlated well with reduction of Id-1 mRNA. This inhibitory effect on proliferation was accompanied by p21 up-regulation and c-myc down-regulation. Moreover, Pg-treated PR transfectants showed significant morphologic change, appearing more flattened and spread out than control ethanol-treated cells. Boyden chamber invasion assay revealed that PR-transfected MDA-MB231 cells also lost most of their invasive properties after Pg treatment. Zymographic analysis revealed that Pg drastically inhibited matrix metalloproteinase-9 (MMP-9) activity in cells transfected with either PR-A or PR-B. To determine whether Id-1 could act as a key mediator of the effects of Pg, we prepared cells transfected with Id-1 and PR. The morphologic change and p21 up-regulation still were observed after Pg treatment. However, c-myc down-regulation was not observed; the proliferative and invasive activities were mostly recovered; and MMP-9 down-regulation could not be detected anymore. From these observations, we conclude that either form of the PR is sufficient to reduce the malignant phenotypes on treatment with Pg and that Id-1 plays an important role as a mediator of the effects of Pg on breast cancer cell proliferation and invasion.

Id4 regulates mammary epithelial cell growth and differentiation and is overexpressed in rat mammary gland carcinomas

Id4 belongs to a family of helix-loop-helix (HLH) proteins that impact cellular growth and differentiation via regulation of basic HLH transcription factors. Herein the rat Id4 gene was cloned (GenBank Accession No. AF468681). The expression of rat Id4 was examined in rat mammary gland tumors induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a carcinogen found in the human diet. By real-time polymerase chain reaction analysis, relative expression of Id4 mRNA in carcinomas, adenomas, and normal tissue was 27, 6, and 1, respectively. Immunohistochemical analysis indicated statistically elevated nuclear expression for Id4 protein in carcinomas in comparison to adenomas and normal mammary gland. In carcinomas, Id4 nuclear expression was positively correlated with proliferation, invasiveness, and tumor weight (Fisher Exact Test or Spearman Correlation, P < 0.05). The consequence of enforced expression of Id4 on mammary epithelial cell proliferation, differentiation, and growth in soft agar was examined in HC11 cells, a well-characterized model for studying various aspects of mammary epithelial cell biology. After transient and stable transfection of HC11 cells, Id4 overexpression increased cell proliferation and inhibited lactogenic hormone-mediated differentiation as revealed by inhibition of beta-casein promoter activity and beta-casein expression. In addition, enforced expression of Id4 in HC11 cells induced a statistically significant increase in colony growth in soft agar. The results implicate Id4 in rat mammary gland carcinogenesis and suggest that Id4 may contribute to carcinogenesis by inhibiting mammary epithelial cell differentiation and stimulating mammary epithelial cell growth.

Id-1 as a molecular target in therapy for breast cancer cell invasion and metastasis

Mammary epithelial cells constitutively expressing Id-1 protein are unable to differentiate, acquire the ability to proliferate, and invade the extracellular matrix. In addition, Id-1 is aberrantly over-expressed in aggressive and metastatic breast cancer cells, as well as in human breast tumor biopsies from infiltrating carcinomas, suggesting Id-1 might be an important regulator of breast cancer progression. We show that human metastatic breast cancer cells become significantly less invasive in vitro and less metastatic in vivo when Id-1 is down-regulated by stable transduction with antisense Id-1. Expression of the matrix metalloproteinase MT1-MMP is decreased in proportion to the decrease in Id-1 protein levels, representing a potential mechanism for the reduction of invasiveness. Further, to more accurately recapitulate the biology of and potential therapeutic approaches to tumor metastasis, we targeted Id-1 expression systemically in tumor-bearing mice by using a nonviral approach. We demonstrate significant reduction of both Id-1 and MT1-MMP expressions as well as the metastatic spread of 4T1 breast cancer cells in syngeneic BALB/c mice. In conclusion, our studies have identified Id-1 as a critical regulator of breast cancer progression and suggest the feasibility of developing novel therapeutic approaches to target Id-1 expression to reduce breast cancer metastasis in humans.

Overexpression of Id-1 is associated with poor clinical outcome in node negative breast cancer

Id-1 is an important regulator of cellular growth and differentiation and controls malignant progression of breast cancer cells. The aim of our study was to assess the clinical impact of Id-1 expression in breast cancer, i.e., its potential impact on prognosis and prediction of treatment response. Id-1 protein expression was determined immunohistochemically in 191 patients with lymph-node negative breast cancer, and univariate and multivariate survival analysis was carried out. Fifteen (7.9%) specimens showed strong expression, 75 (39.3%) moderate, 55 (28.8%) weak expression and 46 (24.1%) cases no expression of Id-1. Patients with strong or moderate Id-1 expression had a significant shorter overall (p = 0.003, Cox regression) and disease-free survival (p = 0.01, Cox regression) compared to those with absent or low expression. Progesterone receptor density was significantly higher in breast cancers with absent/low Id-1 expression compared to those with moderate/strong expression (p < 0.001, t-test). Id-1 expression was significantly stronger in cases positive for p16(INK4a) expression compared to those negative for p16 (p = 0.049, Mann-Whitney test). The influence of Id-1 on clinical outcome seems much stronger in patients with negative estrogen receptor status compared to those with positive status, who received receptor antagonists as adjuvant therapy in most cases. Overexpression of Id-1 protein represents a strong independent prognostic marker in node negative breast cancer, and future therapies inhibiting Id-1 expression might be beneficial for these patients. Our results also suggest that due to the apparent interaction of Id-1 with the steroid-receptor system in breast cancer, hormonal therapies might influence Id-1 expression and its impact on clinical outcome.

Id proteins in epithelial cells

Id helix-loop-helix (Id HLH) proteins are negative regulators of basic HLH transcription factors. They are expressed during embryonic development and are important for the regulation of cell phenotypes in adults. They participate in the molecular networks controlling cell growth, differentiation, and carcinogenesis, through specific basic HLH and non-basic HLH protein interactions. Recent in vitro and in vivo data implicate Id HLH as important orchestrating proteins of homeostasis in glandular and protective epithelia. In particular, Id proteins have been reported to be involved in cell behavior in epidermis, respiratory system, digestive tract, pancreas, liver, thyroid, urinary system, prostate, testis, endometrium, cervix, ovary, and mammary gland. The purpose of this review is to summarize the evidence implicating Id proteins in the regulation of mammalian epithelial cell phenotypes.

Helix-loop-helix proteins in mammary gland development and breast cancer

The basic helix-loop-helix (bHLH) family of transcription factors functions in the coordinated regulation of gene expression, cell lineage commitment, and cell differentiation in most mammalian tissues. Helix-loop-helix Id (Inhibitor of DNA binding) proteins are distinct from bHLH transcription factors in that they lack the basic domain necessary for DNA binding. Id proteins thus function as dominant negative regulators of bHLH transcription factors. The inhibition of bHLH factor activity by forced constitutive expression of Id proteins is closely associated with the inhibition of differentiation in a number of different cell types, including mammary epithelial cells. Moreover, recent literature suggests important roles of HLH proteins in many normal and transformed tissues, including mammary gland. Therefore, future directions for prognosis or therapeutic treatments of breast cancer may be able to exploit bHLH and Id genes as useful molecular targets. The purpose of this review is to summarize the evidence implicating HLH proteins in the regulation of normal and transformed mammary epithelial cell phenotypes.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.

cDNA microarray analysis of changes in gene expression induced by neuronal hypoxia in vitro

We used cDNA microarray gene expression profiling to characterize the transcriptional response to exposure of cultured mouse cerebral cortical neurons to hypoxia for 24 hr. Of 11,200 genes examined, 1,405 (12.5%) were induced or repressed at least 1.5-fold, whereas 26 known genes were induced and 20 known genes were repressed at least 2.5-fold. The most strongly induced genes included genes coding for endoplasmic reticulum proteins (Ero1L/Giig11, Sac1p, Ddit3/Gadd153), proteins involved in ubiquitination (Arih2, P4hb), proteins induced by hypoxia in non-neuronal systems (Gpi1, Aldo1, Anxa2, Hig1), and proteins that might promote cell death (Gas5, Egr1, Ndr1, Vdac2). These findings reinforce the importance of endoplasmic reticulum-based mechanisms and of protein-ubiquitination pathways in the neuronal response to hypoxia.

Constitutive expression of the Id-1 promoter in human metastatic breast cancer cells is linked with the loss of NF-1/Rb/HDAC-1 transcription repressor complex

The helix-loop-helix protein Id-1 is a dominant negative regulator of basic helix-loop-helix transcription factors, and plays a key role in the control of breast epithelial cell growth, invasion and differentiation. Previous investigations in our laboratory have shown that Id-1 mRNA was constitutively expressed in highly aggressive and invasive human breast cancer cells in comparison to non-transformed or non-aggressive cancerous cells, and that this loss of regulation is mediated by a 2.2-kb region of the human Id-1 promoter. Here we show that a 31 bp sequence within this 2.2-kb promoter, located 200 bp upstream of the initiation of transcription, is responsible for the constitutive expression of Id-1 in metastatic human breast cancer cells. Using gel shift experiments, we identified a high molecular weight complex present only in non-aggressive breast cancer cells cultured in serum-free medium and which appear to be necessary for proper Id-1 repression. In contrast, nuclear extracts from highly aggressive and metastatic cell lines do not contain this large molecular weight complex. Using DNA affinity precipitation assays (DAPA), we show that this complex contains SP-1, NF-1, Rb and HDAC-1 proteins. On the basis of these findings, we propose a mechanism for the loss of regulation of Id-1 promoter in invasive and metastatic human breast cancer cells.

Id-1, ITF-2, and Id-2 comprise a network of helix-loop-helix proteins that regulate mammary epithelial cell proliferation, differentiation, and apoptosis

Mammary epithelial cells proliferate, invade the stroma, differentiate, and die in adult mammals by mechanisms that are poorly understood. We found that Id-1, an inhibitor of basic helix-loop-helix transcription factors, regulates mammary epithelial cell growth, differentiation, and invasion in culture. Here, we show that Id-1 is expressed highly during mammary development in virgin mice and during early pregnancy, when proliferation and invasion are high. During mid-pregnancy, Id-1 expression declined to undetectable levels as the epithelium differentiated fully. Surprisingly, Id-1 increased during involution, when the epithelium undergoes extensive apoptosis. To determine whether Id-1 regulates both proliferation and apoptosis, we constitutively expressed Id-1 in mammary epithelial cell cultures. Id-1 stimulated proliferation in sparse cultures but induced apoptosis in dense cultures, which reflect epithelial cell density during early pregnancy and involution, respectively. To understand how Id-1 acts, we screened a yeast two-hybrid library from differentiating mammary epithelial cells and identified ITF-2, a basic helix-loop-helix transcription factor, as an Id-1-interacting protein. Overexpression of ITF-2 significantly reduced Id-1-stimulated proliferation and apoptosis. We show further that, in contrast to Id-1, Id-2 was expressed highly in differentiated mammary epithelial cells in vivo and in culture. In culture, Id-2 antisense transcripts blocked differentiation. Our results suggest that Id-1, ITF-2, and Id-2 comprise a network of interacting molecular switches that govern mammary epithelial cell phenotypes.