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

Gene Expression Profiling Reveals Cross-talk between Melanoma and Fibroblasts: Implications for Host-Tumor Interactions in Metastasis

Host-tumor interaction is considered critical in carcinogenesis, tumor invasion, and metastasis. To explore the reciprocal effects of host-tumor interaction, we developed a system to assess the gene expression patterns of A2058 human melanoma cells cocultured in fibrillar collagen with HS-68 primary human fibroblasts. The gene expression pattern of the cocultured A2058 cells was only modestly affected, whereas the HS-68 fibroblast gene expression pattern was significantly altered. Interleukin-11 and inhibitor of DNA-binding domain-1 gene expression in the cocultured A2058 cells was down-regulated, indicative of a proinflammatory response and resistance to apoptosis, respectively. The overall pattern of up-regulated genes indicated triggering of the proinflammatory process. In addition, the melanoma growth and migration stimulatory chemokines CXCL1 and CXCL2 were significantly up-regulated in the cocultured fibroblasts. These results were corroborated by additional coculture experiments with the melanoma cell lines WM-164, BLM, and SK-Mel-28 and immunohistochemistry on invasive human melanoma sections. Taken together, these results indicate that tumor cells cause a proinflammatory and melanoma growth-promoting response in stromal fibroblasts. The role of inflammation in carcinogenesis, tumor promotion, invasion, and metastasis is viewed as being increasingly important and the results of these studies underscore this as well as identify certain key proteins that are expressed as a result of the complex interactive processes in the host-tumor microenvironment.

Differences in gene expression in the proliferative human endometrium

OBJECTIVE

To use microdissection and DNA microarray technology to demonstrate differences in gene expression between epithelial and stromal areas in the proliferative human endometrium.

DESIGN

Pilot study.

SETTING

University hospital.

PATIENT(S)

Patients with normal menstrual cycles and at least one previous intrauterine pregnancy.

INTERVENTION(S)

Uterine endometrial biopsy.

MAIN OUTCOME MEASURE(S)

Gene expression.

RESULT(S)

From a total of 1,200 genes, 14 were strongly expressed in epithelial areas and 12 were strongly expressed in stromal areas. Among the genes strongly expressed in the stroma, expressions of decorin and discoidin domain receptor were confirmed by real-time polymerase chain reaction. Decorin was localized in the stromal areas by immunohistochemical staining. To confirm the effects of estrogen on gene expression, stromal cells were cultured. When E(2) was added to the culture media, expression of decorin mRNA was increased.

CONCLUSION(S)

The data demonstrated in this study help to understand the physiology of human endometrium. Decorin was strongly expressed in the stromal areas and was regulated by estrogen, and therefore it may be involved in restoration of the endometrium.

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.

Characterization of two APP gene promoter polymorphisms that appear to influence risk of late-onset Alzheimer's disease

Alzheimer's disease (AD) is characterized by formation of plaques of amyloid beta peptide (Abeta). Autosomally-inherited or "familial" AD had been demonstrated only in connection with coding sequence mutations. We characterized DNA-protein interaction and expression influence of two polymorphisms that occur in the promoter (C<-->T at -3829 and T<-->C at -1023, +1 transcription start site) of the Abeta precursor protein (APP) gene. We report distinct functional differences in reporter expression and in DNA-protein interaction for variant sequences in both -3829 and -1023 polymorphic regions. The -3829T variant has reduced DNA-protein interaction and reporter expression compared to -3829C, while -1023C has greater DNA-protein interaction and reporter expression than -1023T. Our predictions for likely transcription factors for loss of function (-3829T) are ADR1, MIG1, and PuF, and for gain of function (-1023C) are E12/E47, ITF-2, and RFX2. Characterization of the activity of a regulatory polymorphism of the APP gene points towards understanding mechanisms that likely underlie the majority of AD cases and may contribute to promoter-based drug design.

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.

Regulation of cyclin expression and cell cycle progression in breast epithelial cells by the helix–loop–helix protein Id1

The helix-loop-helix protein Id1 has been implicated in regulating mammary epithelial cell proliferation and differentiation but the underlying molecular mechanisms are not well characterized. Under low serum conditions, ectopic expression of Id1, but not Id2, allowed continued proliferation of immortalized mammary epithelial cells and breast cancer cells. Conversely, downregulation of Id1 impaired proliferation. The effects of short interfering RNA (siRNA)-mediated downregulation of Id1 were the same as those following downregulation of c-Myc: decreased expression of cyclins D1 and E, reduced phosphorylation of pRb at Ser780 (a site targeted by cyclin D1-Cdk4) and reduced cyclin E-Cdk2 activity. Decreased cyclin D1 expression was an early response to Id1 antisense oligonucleotide treatment. Inhibition of c-Myc function by siRNA, antisense oligonucleotides or a dominant repressor resulted in downregulation of Id1, while ectopic expression of c-Myc resulted in rapid induction of Id1, suggesting that Id1 may be downstream of c-Myc. These data indicate that in mammary epithelial cells, Id1 has cell cycle regulatory functions that are similar to those of c-Myc, and suggest that cyclin D1 may be involved in Id1 regulation of cell cycle progression.

Id2 is a primary partner for the E2-2 basic helix-loop-helix transcription factor in the human placenta

We screened a term placental cDNA library by the yeast two-hybrid approach with Id2, a negative regulator of basic helix-loop-helix (bHLH) factors. Of the clones obtained, approximately one-third were the E2-2 bHLH transcription factor. Id2 and E2-2 were shown to interact in direct two-hybrid assays in yeast cells, as well as immunoprecipitation assays in mammalian cells. Immunohistochemical analysis demonstrated co-localization of both Id2 and E2-2 in placental trophoblasts. Co-transfection of JEG-3 cells with E2-2 and Id2, and a luciferase reporter construct under the control of the human chorionic gonadotropin alpha-subunit promoter revealed that E2-2 had a negative effect on CGalpha-subunit transcription, which could be relieved by overexpression of Id2. The library was in turn rescreened with E2-2, and Id2 and Id1 were essentially the only clones obtained. We conclude that Id2 is a primary binding partner for the bHLH transcription factor E2-2 in the human placenta.

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.

Transcriptional control of the cell cycle in mammary gland development and tumorigenesis

Over the past several years it has become increasingly evident that normal development and cancer share many properties. Both processes involve alterations in cell proliferation and differentiation, cell death, neovascularization, and cell motility and invasion. Thus, genes involved in normal development are frequently utilized in neoplasia. During development, numerous transcriptional regulatory mechanisms are used to ensure tight control over cellular proliferation. In this review we focus on a number of transcription factor families (homeobox, STAT, and Ets), and on inhibitors of transcription factors (Id), which have been implicated in controlling the cell cycle not only in normal mammary gland development but also in breast tumorigenesis.

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.

Identification of genetic pathways activated by the androgen receptor during the induction of proliferation in the ventral prostate gland

The androgen receptor (AR), when complexed with 5alpha-dihydrotestosterone (DHT), supports the survival and proliferation of prostate cells, a process critical for normal development, benign prostatic hypertrophy, and tumorigenesis. However, the androgen-responsive genetic pathways that control prostate cell division and differentiation are largely unknown. To identify such pathways, we examined gene expression in the ventral prostate 6 and 24 h after DHT administration to androgen-depleted rats. 234 transcripts were expressed significantly differently from controls (p < 0.05) at both time points and were subjected to extensive data mining. Functional clustering of the data reveals that the majority of these genes can be classified as participating in induction of secretory activity, metabolic activation, and intracellular signaling/signal transduction, indicating that AR rapidly modulates the expression of genes involved in proliferation and differentiation in the prostate. Notably AR represses the expression of several key cell cycle inhibitors, while modulating members of the wnt and notch signaling pathways, multiple growth factors, and peptide hormone signaling systems, and genes involved in MAP kinase and calcium signaling. Analysis of these data also suggested that p53 activity is negatively regulated by AR activation even though p53 RNA was unchanged. Experiments in LNCaP prostate cancer cells reveal that AR inhibits p53 protein accumulation in the nucleus, providing a post-transcriptional mechanism by which androgens control prostate cell growth and survival. In summary these data provide a comprehensive view of the earliest events in AR-mediated prostate cell proliferation in vivo, and suggest that nuclear exclusion of p53 is a critical step in prostate growth.

TGF-β cytokines increase senescence-associated beta-galactosidase activity in human prostate basal cells by supporting differentiation processes, but not cellular senescence

The family of transforming growth factors betas (TGF-betas) comprises molecules involved in growth inhibition, stress-induced premature senescence, epithelial mesenchymal transition and differentiation processes. The aim of this study was to clarify the effect of long term exposure of human prostate basal cells to TGF-betas, which are found in high concentrations in prostatic fluid and areas of benign prostatic hyperplasia (BPH). Basal cell cultures established from prostate explants (n=3) were either grown into cellular senescence, or stimulated with TGF-beta1, beta2 and beta3. Similar to cellular senescence, TGF-beta stimulation resulted in an increase of SA-beta galactosidase (SA-beta-gal) activity, flattened and enlarged cell morphology, and down-regulation of the inhibitor of differentiation Id-1. TGF-beta-treated prostate epithelial cells neither showed terminal growth arrest nor induction of important senescence-relevant genes, such as p16(INK4A), IFI-6-16, IGFBP-3 or Dkk-3. Cells stained positive for cytokeratins 8/18, but did not express other lumenal markers, such as prostate-specific antigen and androgen-receptors. TGF-betas increased also the expression of the mesenchymal marker vimentin, indicating that basal epithelial cells underwent differentiation with lumenal and mesenchymal features. In contrast, in vitro-differentiated neuroendocrine-like cells from prostate organoide cultures, expressing chromogranin A and cytokeratin 18, strongly stained positive for SA-beta-gal. Thus, SA-beta-gal activity is not only a marker for senescence, but also for differentiation of human prostate epithelial cells. With regard to the in vivo situation, in addition to cellular senescence, TGF-beta could contribute to the increased number of SA-beta-gal positive epithelial cells in BPH.

Mad upregulation and Id2 repression accompany transforming growth factor (TGF)-beta-mediated epithelial cell growth suppression

The growth inhibitory cytokine TGF-beta enforces homeostasis of epithelia by activating processes such as cell cycle arrest and apoptosis. Id2 expression is often highest in proliferating epithelial cells and declines during differentiation. Recently, Id2 expression has been found to depend on Myc-Max transcriptional complexes. We observed that TGF-beta signaling inhibits Id2 expression in human and mouse epithelial cell lines from different tissue origins. Furthermore, the observed Id2 down-regulation by TGF-beta in mouse mammary epithelial cells occurs without a concurrent drop in c-Myc levels. However, sustained Id2 repression in these cells and in human keratinocytes coincides with induction of the Myc antagonistic repressors Mad2 and Mad4, decreased formation of Myc-Max heterodimers and the replacement of Myc-Max complexes with Mad-Max complexes on the Id2 promoter. These results argue that induction of Mad expression and Id2 down-regulation are important events during the TGF-beta cytostatic program in epithelial cells.

Forced expression of cyclin D1 does not compensate for Id2 deficiency in the mammary gland

Id2 and cyclin D1 share several biological activities, including inhibition of differentiation, stimulation of the G1-S transition in the cell cycle and stimulation of tumorigenesis. Mammary glands of Id2(-/-) mice display severely impaired lobulo-alveolar development during pregnancy, similarly to those of cyclin D1 null females. We investigated the functional relationship between Id2 and cyclin D1 in the mammary gland. Id2(-/-) mammary glands expressed a normal level of cyclin D1. No direct interaction of Id2 with cyclin D1 or its binding partner cdk4 was detected in mammalian two-hybrid assays. Ectopic expression of a cyclin D1 transgene did not rescue the mammary phenotype of Id2(-/-) mice. These results suggest that Id2 acts downstream or independently of cyclin D1 in the control of mammary cell proliferation during pregnancy.

Microarray analysis uncovers retinoid targets in human bronchial epithelial cells

Retinoids, the natural and synthetic derivatives of vitamin A, have a role in cancer treatment and prevention. There is a need to reveal mechanisms that account for retinoid response or resistance. This study identified candidate all-trans-retinoic acid (RA) target genes linked to growth suppression in BEAS-2B human bronchial epithelial cells. Microarray analyses were performed using Affymetrix arrays. A total of 11 RA-induced species were validated by reverse transcription polymerase chain reaction (RT-PCR), Western or Northern analyses. Three of these species were novel candidate RA-target genes in human bronchial epithelial cells. These included: placental bone morphogenetic protein (PLAB), polyamine oxidase isoform 1 (PAOh1) and E74-like factor 3 (ELF3). Expression patterns were studied in RA-resistant BEAS-2B-R1 cells. In BEAS-2B-R1 cells, RA dysregulated the expression of the putative lymphocyte G0/G1 switch gene (G0S2), heme oxygenase 1 (HMOX1), tumor necrosis factor-alpha-induced protein 2 (TNFAIP2), inhibitor of DNA binding 1(Id1), fos-like antigen 1 (FOSL1), transglutaminase 2 (TGM2), asparagine synthetase (ASNS), PLAB, PAOh1 and ELF3, while prominent induction of insulin-like growth-factor-binding protein 6 (IGFBP6) still occurred. In summary, this study identified 11 candidate RA-target genes in human bronchial epithelial cells including three novel species. Expression studies in BEAS-2B-R1 cells indicated that several were directly implicated in RA signaling, since their aberrant expression was linked to RA resistance of human bronchial epithelial cells.

Id-1 expression promotes cell survival through activation of NF-kappaB signalling pathway in prostate cancer cells

The growth-promoting effect of Id-1 (inhibitor of differentiation/DNA binding) has been demonstrated in a number of human cancers. However, the mechanisms responsible for its action are not clear. In this study, we report that in prostate cancer cells, Id-1 promotes cell survival through activation of nuclear factor-kappaB (NF-kappaB) signalling pathway. After stable expression of Id-1 protein in LNCaP cells, we found that the Id-1 transfectants showed increased resistance to apoptosis induced by TNFalpha through inactivation of Bax and caspase 3. In addition, in the LNCaP cells expressing ectopic Id-1 protein, we also observed increased NF-kappaB transactivation activity and nuclear translocation of the p65 and p50 proteins, which was accompanied by upregulation of their downstream effectors Bcl-xL and ICAM-1. These results indicate that the Id-1-induced antiapoptotic effect may be via NF-kappaB signalling transduction pathway in these cells. In addition, inactivation of Id-1 by its antisense oligonucleotide and retroviral construct in DU145 cells resulted in the decrease of nuclear level of p65 and p50 proteins, which was associated with increased sensitivity to TNFalpha-induced apoptosis. Our results strongly suggest that Id-1 may be one of the upstream regulators of NF-kappaB and activation of NF-kappaB signalling pathway may be essential for Id-1 induced cell proliferation through protection against apoptosis. Our findings also suggest a potential therapeutic strategy in which inactivation of Id-1 may lead to sensitization of prostate cancer cells to chemotherapeutic drug-induced apoptosis.

Id proteins in cell growth and tumorigenesis

Since the gene encoding Id1 was cloned in 1990, Id proteins have been implicated in regulating a variety of cellular processes, including cellular growth, senescence, differentiation, apoptosis, angiogenesis, and neoplastic transformation. The development of knockout and transgenic animal models for many members of the Id gene family has been particularly useful in sorting out the biologic relevance of these genes and their expression during normal development, malignant transformation, and tumor progression. Here we review the current understanding of Id gene function, the biologic consequences of Id gene expression, and the implications for Id gene regulation of cell growth and tumorigenesis.

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.

C-terminal-binding protein corepresses epithelial and proapoptotic gene expression programs

The genesis of carcinoma cells often involves epithelial-to-mesenchymal transitions and the acquisition of apoptosis resistance, but it is unclear whether these alterations are controlled coordinately or independently. Our previously reported effects of adenovirus E1a in human tumor cells raised the possibility that the E1a-interacting corepressor protein C-terminal-binding protein (CtBP) might selectively repress epithelial cell adhesion and proapoptotic genes. Here, we report that CtBP-knockout cells were hypersensitive to apoptosis. Correspondingly, microarray analysis of CtBP-knockout vs. CtBP-rescued mouse embryo fibroblasts revealed that many epithelial-specific and proapoptotic genes were indeed regulated by CtBP. Neither the apoptosis nor the repression activities of CtBP required histidine-315, suggesting that the proposed dehydrogenase activity is not essential for CtBP function. The results presented herein establish two functional roles of CtBP: to corepress epithelial genes, thus permitting epithelial-to-mesenchymal transitions, and to modulate the cellular threshold for apoptotic responses.

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.

A beta-catenin survival signal is required for normal lobular development in the mammary gland

The Wnt (wingless) family of secreted glycoproteins initiates a signalling pathway implicated in the regulation of both normal mouse mammary gland development and tumorigenesis. Multiple Wnt signals ultimately converge on the multifunctional protein beta-catenin to activate the transcription of target genes. Although beta-catenin plays a crucial role in canonical Wnt signalling, it also functions in epithelial cell-cell adhesion at the adherens junctions. This study was designed to isolate beta-catenin's signalling function from its role in adherence during mouse mammary gland development. A transgenic dominant-negative beta-catenin chimera (beta-eng), which retains normal protein-binding properties of wild-type beta-catenin but lacks its C-terminal signalling domain, was expressed preferentially in the mammary gland. Thus, beta-eng inhibits the signalling capacity of endogenous beta-catenin, while preserving normal cell-cell adhesion properties. Analysis of the mammary gland in transgenic mice revealed a severe inhibition of lobuloalveolar development and a failure of the mice to nurse their young. Expression of beta-eng resulted in an induction of apoptosis both in transgenic mice and in retrovirally transduced HC11 cells. Thus, endogenous beta-catenin expression appears to be required to provide a survival signal in mammary epithelial cells, which can be suppressed by transgenic expression of beta-eng. Comparison of the timing of transgene expression with the transgenic phenotype suggested a model in which beta-catenin's survival signal is required in lobular progenitors that later differentiate into lobuloalveolar clusters. This study illustrates the importance of beta-catenin signalling in mammary lobuloalveolar development.

Id2 expression during apoptosis and satellite cell activation in unloaded and loaded quail skeletal muscles

Inhibitor of differentiation-2 (Id2) is a basic helix-loop-helix protein that acts as a negative regulator of the myogenic regulatory transcription factor family, but Id2 has also been implicated in apoptosis in several cell lines. In this study, we tested the hypothesis that Id2 has a role in both apoptosis-associated muscle atrophy and muscle hypertrophy. A weight corresponding to 12% of the body weight was attached to one wing of Japanese quail to induce hypertrophy in the patagialis (PAT) muscle. Birds in group 1 were killed after 5 (n = 8), 7 (n = 10), or 14 days (n = 10) of loading. The left wing was loaded for 14 days in group 2 birds, and then the weight was removed and the PAT was examined after 7 (n = 10), 14 (n = 10), or 21 (n = 5) days of unloading. A time-released bromodeoxyuridine (BrdU) pellet was implanted subcutaneously with wing weighting to identify activated satellite cells during loading. The left wing was loaded for 14 days, unloaded for 14 days, and then the weight was reattached for a subsequent 7 (n = 10) or 14 days (n = 10) in group 3 birds. BrdU was implanted on the second loading phase in this group. Id2 mRNA as measured by kinetic PCR increased by 3.9-, 2.7-, and 1.6-fold, relative to control levels after 7, 14, and 21 days of unloading (group 2). Id2 protein as estimated by Western blots increased by 1.5-, 1.4-, and 0.75-fold after 7, 14, and 21 days of unloading (group 2). Muscle unloading induced apoptosis, because poly(ADP-ribose) polymerase-(PARP)-positive nuclei increased and caspase 8 levels increased by 2.6- and 1.7-fold after 7 or 14 days of unloading, respectively (group 2). Although BrdU-positive nuclei increased during loading (groups 1 and 3), 50% failed to survive during unloading (group 2). Id2 mRNA increased by 2.2- and 1.8-fold after 5 and 7 days of loading, respectively, but decreased to control levels by 14 days of loading in group 1. Id2 protein levels increased 2.1-fold after 5 days of loading (group 1). In contrast, Id2 did not increase in reloaded muscles of group 3 birds. These data suggest that Id2 may have a role in apoptosis-associated atrophy of skeletal muscles, but its role in muscle hypertrophy is less clear.

Id-1 is not expressed in the luminal epithelial cells of mammary glands

BACKGROUND

The family of inhibitor of differentiation/DNA binding (Id) proteins is known to regulate development in several tissues. One member of this gene family, Id-1, has been implicated in mammary development and carcinogenesis. Mammary glands contain various cell types, among which the luminal epithelial cells are primarily targeted for proliferation, differentiation and carcinogenesis. Therefore, to assess the precise significance of Id-1 in mammary biology and carcinogenesis, we examined its cellular localization in vivo using immunohistochemistry.

METHODS

Extracts of whole mammary glands from wild type and Id-1 null mutant mice, and tissue sections from paraffin-embedded mouse mammary glands from various developmental stages and normal human breast were subjected to immunoblot and immunohistochemical analyses, respectively. In both these procedures, an anti-Id-1 rabbit polyclonal antibody was used for detection of Id-1.

RESULTS

In immunoblot analyses, using whole mammary gland extracts, Id-1 was detected. In immunohistochemical analyses, however, Id-1 was not detected in the luminal epithelial cells of mammary glands during any stage of development, but it was detected in vascular endothelial cells.

CONCLUSION

Id-1 is not expressed in the luminal epithelial cells of mammary glands.

Regulation of BRCA1 expression and its relationship to sporadic breast cancer

Germ-line mutations in the BRCA1 tumour suppressor gene contribute to familial breast tumour formation, but there is no evidence for direct mutation of the BRCA1 gene in the sporadic form of the disease. In contrast, decreased expression of the BRCA1 gene has been shown to be common in sporadic tumours, and the magnitude of the decrease correlates with disease progression. BRCA1 expression is also tightly regulated during normal breast development. Determining how these developmental regulators of BRCA1 expression are co-opted during breast tumourigenesis could lead to a better understanding of sporadic breast cancer aetiology and the generation of novel therapeutic strategies aimed at preventing sporadic breast tumour progression.

Mammary epithelial cells are not able to undergo pregnancy-dependent differentiation in the absence of the helix-loop-helix inhibitor Id2

Mammary alveolar development during pregnancy is triggered by hormone signals. The prolactin receptor/Jak2/signal transducer and activator of transcription (Stat) 5 signal transduction pathway is the principal mediator of these cues and alveolar development is abrogated in its absence. The loss of the basic helix-loop-helix protein inhibitor of differentiation (Id)2 results in a similar defect. To investigate the role of Id2 in mammary epithelium, we performed structural and molecular analyses. Id2-null mammary epithelial cells were unable to form alveoli; the epithelial architecture was disorganized and dissimilar from early stages of alveologenesis in wild-type glands. The epithelial cells retained the ductal marker Na-K-Cl cotransporter (NKCC)1. Nuclear localization of Stat5a and down-regulation of NKCC1 was observed in some areas, indicating a limited response to pregnancy signals. The differentiation status of Id2-null tissue at term was further characterized with cDNA microarrays enriched in mammary specific sequences (mammochip). Some of the early differentiation markers for mammary epithelium were expressed in the Id2-null tissue, whereas genes that are expressed at later stages of pregnancy were not induced. From these results, we conclude that, in the absence of Id2, mammary epithelial development is arrested at an early stage of pregnancy.

Identification of Myc-mediated death response pathways by microarray analysis

To understand the mechanisms of Myc-mediated apoptosis induced by DNA damage, we have characterized the death kinetics of three Rat-1 fibroblast cell lines that either overexpress Myc or lack Myc and their parental wild-type cells following exposure to the DNA-damaging agent VP-16, and we monitored the changes in gene expression using microarray. We have identified three groups of genes whose expressions are distinctly regulated during this process. One cluster (Cluster A) revealed a VP-16-dependent but Myc-independent induction of a set of genes that is not linked to the apoptotic response. Two other gene clusters, however, were associated with VP-16-induced apoptosis. Cluster B, which includes p53-responsive genes, was associated with the temporal onset of apoptosis but accounted for only the basal apoptosis. However, Cluster C, which includes c-jun, was highly regulated by Myc and appeared to be critical to mounting the maximal apoptotic response in Myc-expressing cells. Furthermore, the Myc level dropped sharply following VP-16 exposure, which varied inversely with the induction of Cluster C genes, suggesting Myc normally represses their transcription. Thus, we have proposed that removal of Myc-mediated repression of apoptotic signals, combined with Myc-associated acceleration of the p53 responsive pathway, results in complete and rapid cell death following DNA damage.

ITF-2, a downstream target of the Wnt/TCF pathway, is activated in human cancers with beta-catenin defects and promotes neoplastic transformation

In many cancers, inactivation of the adenomatous polyposis coli (APC) or Axin tumor suppressor proteins or activating mutations in beta-catenin lead to elevated beta-catenin levels, enhanced binding of beta-catenin to T cell factor (TCF) proteins, and increased expression of TCF-regulated genes. We found that the gene for the basic helix-loop-helix transcription factor ITF-2 (immunoglobulin transcription factor-2) was activated in rat E1A-immortalized RK3E cells following neoplastic transformation by beta-catenin or ligand-induced activation of a beta-catenin-estrogen receptor fusion protein. Human cancers with beta-catenin regulatory defects had elevated ITF-2 expression, and ITF-2 was repressed by restoring wild-type APC function or inhibiting TCF activity. Of note, ITF-2 promoted neoplastic transformation of RK3E cells. We propose that ITF-2 is a TCF-regulated gene, which functions in concert with other TCF target genes to promote growth and/or survival of cancer cells with defects in beta-catenin regulation.