Identification of MAGEA antigens as causal players in the development of tamoxifen-resistant breast cancer

The antiestrogen tamoxifen is a well-tolerated, effective treatment for estrogen receptor-α-positive (ER+) breast cancer, but development of resistance eventually limits its use. Here we show that expression of MAGEA2, and related members of this cancer-testis antigen family, is upregulated in tamoxifen-resistant tumor cells. Expression of MAGEA2 in tumor lines grown in vitro or as xenografts led to continued proliferation in the presence of tamoxifen. At the molecular level, we demonstrate that MAGEA2 protein localizes to the nucleus and forms complexes with p53 and ERα, resulting in repression of the p53 pathway but increased ER-dependent signaling. In a series of ER+, tamoxifen-treated breast cancer patients, we show a highly significant (P=0.006) association between MAGEA (melanoma-associated antigen) expression and reduced overall survival, confirming the clinical significance of our observations.

Functional analysis of a breast cancer-associated FGFR2 single nucleotide polymorphism using zinc finger mediated genome editing

Genome wide association studies have identified single nucleotide polymorphisms (SNP) within fibroblast growth factor receptor 2 (FGFR2) as one of the highest ranking risk alleles in terms of development of breast cancer. The potential effect of these SNPs, in intron two, was postulated to be due to the differential binding of cis-regulatory elements, such as transcription factors, since all the SNPs in linkage disequilibrium were located in a regulatory DNA region. A Runx2 binding site was reported to be functional only in the minor, disease associated allele of rs2981578, resulting in increased expression of FGFR2 in cancers from patients homozygous for that allele. Moreover, the increased risk conferred by the minor FGFR2 allele associates most strongly in oestrogen receptor alpha positive (ERα) breast tumours, suggesting a potential interaction between ERα and FGFR signalling. Here, we have developed a human cell line model system to study the effect of the putative functional SNP, rs2981578, on cell behaviour. MCF7 cells, an ERα positive breast cancer cell line homozygous for the wild-type allele were edited using a Zinc Finger Nuclease approach. Unexpectedly, the acquisition of a single risk allele in MCF7 clones failed to affect proliferation or cell cycle progression. Binding of Runx2 to the risk allele was not observed. However FOXA1 binding, an important ERα partner, appeared decreased at the rs2981578 locus in the risk allele cells. Differences in allele specific expression (ASE) of FGFR2 were not observed in a panel of 72 ERα positive breast cancer samples. Thus, the apparent increased risk of developing ERα positive breast cancer seems not to be caused by rs2981578 alone. Rather, the observed increased risk of developing breast cancer might be the result of a coordinated effect of multiple SNPs forming a risk haplotype in the second intron of FGFR2.

Role of the HSP90-associated cochaperone p23 in enhancing activity of the androgen receptor and significance for prostate cancer

Prostate tumor growth initially depends on androgens, which act via the androgen receptor (AR). Despite androgen ablation therapy, tumors eventually progress to a castrate-resistant stage in which the AR remains active. The mechanisms are poorly understood but it may be that changes in levels or activity of AR coregulators affect trafficking and activation of the receptor. A key stage in AR signaling occurs in the cytoplasm, where unliganded receptor is associated with the heat shock protein (HSP)90 foldosome complex. p23, a key component of this complex, is best characterized as a cochaperone for HSP90 but also has HSP90-independent activity and has been reported as having differential effects on the activity of different steroid receptors. Here we report that p23 increases activity of the AR, and this appears to involve steps both in the cytoplasm (increasing ligand-binding capacity, possibly via direct interaction with AR) and the nucleus (enhancing AR occupancy at target promoters). We show, for the first time, that AR and p23 can interact, perhaps directly, when HSP90 is not present in the same complex. The effects of p23 on AR activity are at least partly HSP90 independent because a mutant form of p23, unable to bind HSP90, nevertheless increases AR activity. In human prostate tumors, nuclear p23 was higher in malignant prostate cells compared with benign/normal cells, supporting the utility of p23 as a therapeutic target in prostate cancer.

Dual association by TFAP2A during activation of the p21cip/CDKN1A promoter

The cyclin-dependent kinase inhibitor p21cip/CDKN1A is induced to promote growth arrest in response to a variety of stimuli in normal cells and loss of correct regulation of this gene is frequently observed in cancer. In particular, the upregulation of CDKN1A by p53 is considered to be a central mechanism of tumour suppression. Other transcription factors with tumour suppressor activity can also regulate CDKN1A, including the developmentally regulated factor, TFAP2A. Here we identify a novel AP-2 binding site within the proximal promoter of the CDKN1A gene and show this is required for optimal, p53-independent expression of p21cip/CDKN1A. We further describe a non-tumourgenic breast epithelial cell line model to study the role of endogenous TFAP2A and p53 in the control of drug-induced p21cip expression using ChIP. Maximal expression of CDKN1A requires TFAP2A which binds to two regions of the promoter: the proximal region where the AP-2 site lies and upstream near the major p53 binding site. The pattern of binding alters with time post-induction, with the proximal, p53-independent site becoming more important at later stages of p21cip induction. This pattern of promoter interaction by TFAP2A is distinct from that seen for the TFAP2C family member which represses CDKN1A expression.

Alternative TFAP2A isoforms have distinct activities in breast cancer

Introduction

AP-2α is a transcription factor implicated in the regulation of differentiation and proliferation in certain tissues, including the mammary gland. In breast tumours, continued expression of AP-2α has been correlated with a better prognosis, but this is hard to reconcile with a reported role in the upregulation of the ERBB2 oncogene. The existence of TFAP2A isoforms, deriving from alternative first exons and differing in their N-terminal sequence, has been described in some mammals, but their relative abundance and activity has not been investigated in the human breast.

Methods

Expression levels of four TFAP2A isoforms were assayed at the level of RNA and protein (via the generation of isoform-specific antibodies) in a panel of breast tumour cell lines and in tissue from normal breast and primary tumour samples. Expression constructs for each isoform were used in reporter assays with synthetic and natural promoters (cyclin D3 and ERBB2) to compare the activation and repression activity of the isoforms.

Results

We demonstrate that the two isoforms AP-2α 1b and AP-2α 1c, in addition to the originally cloned, AP-2α 1a, are conserved throughout evolution in vertebrates. Moreover, we show that isoform 1c in particular is expressed at levels at least on a par with the 1a isoform in breast epithelial lines and tissues and may be more highly expressed in tamoxifen resistant tumours. The isoforms share a similar transactivation mechanism involving the recruitment of the adaptors CITED2 or 4 and the transactivators p300 or CBP. However, isoform 1b and 1c are stronger transactivators of the ERBB2 promoter than isoform 1a. In contrast, AP-2α 1a is the only isoform able to act as a repressor, an activity that requires an intact sumoylation motif present within the N-terminus of the protein, and which the other two isoforms lack.

Conclusions

Our findings suggest that TFAP2A isoforms may be differentially regulated during breast tumourigenesis and this, coupled with differences in their transcriptional activity, may impact on tumour responses to tamoxifen therapy. These data also have implications for the interpretation of tumour studies that seek to correlate outcomes with TFAP2A expression level.

AP-2gamma promotes proliferation in breast tumour cells by direct repression of the CDKN1A gene

Overexpression of the activator protein (AP)-2gamma transcription factor in breast tumours has been identified as an independent predictor of poor outcome and failure of hormone therapy. To understand further the function of AP-2gamma in breast carcinoma, we have used an RNA interference and gene expression profiling strategy with the MCF-7 cell line as a model. Gene expression changes between control and silenced cells implicate AP-2gamma in the control of cell cycle progression and developmental signalling. A function for AP-2gamma in cell cycle control was verified using flow cytometry: AP-2gamma silencing led to a partial G1/S arrest and induction of the cyclin-dependent kinase inhibitor, p21cip/CDKN1A. Reporter and chromatin immunoprecipitation assays demonstrated a direct, functional interaction by AP-2gamma at the CDKN1A proximal promoter. AP-2gamma silencing coincided with acquisition of an active chromatin conformation at the CDKN1A locus and increased gene expression. These data provide a mechanism whereby AP-2gamma overexpression can promote breast epithelial proliferation and, coupled with previously published data, suggest how loss of oestrogen regulation of AP-2gamma may contribute to the failure of hormone therapy in patients.

Androgen receptor is targeted to distinct subcellular compartments in response to different therapeutic antiandrogens

PURPOSE

Antiandrogens are routinely used in the treatment of prostate cancer. Although they are known to prevent activation of the androgen receptor (AR), little is known about the mechanisms involved. This report represents the first study of the localization of wild-type AR following expression at physiologic relevant levels in prostate cells and treatment with androgen and antiandrogens.

EXPERIMENTAL DESIGN

We have characterized a cellular model for prostate cancer using in situ cellular fractionation, proteomics, and confocal microscopy and investigated the effect of antiandrogens in clinical use on the subcellular localization of the AR.

RESULTS

Different antiandrogens have diverse effects on the subcellular localization of the AR. Treatment with androgen results in translocation from the cytoplasm to the nucleoplasm, whereas the antiandrogens hydroxyflutamide and bicalutamide lead to reversible association with the nuclear matrix. In contrast, treatment with the antiandrogen cyproterone acetate results in AR association with cytoplasmic membranes and irreversible retention within the cytoplasm. In addition, we demonstrate that AR translocation requires ATP and the cytoskeleton, regardless of ligand.

CONCLUSIONS

These results reveal that not all antiandrogens work via the same mechanism and suggest that an informed sequential treatment regime may benefit prostate cancer patients. The observed subnuclear and subcytoplasmic associations of the AR suggest new areas of study to investigate the role of the AR in the response and resistance of prostate cancer to antiandrogen therapy.

Competition on Nitrocellulose-immobilized Antibody Arrays

Large scale comparative evaluation of protein expression requires miniaturized techniques to provide sensitive and accurate measurements of the abundance of molecules present as individual and/or assembled protein complexes in cells. The principle of competition between target molecules for binding to arrayed antibodies has recently been proposed to assess differential expression of numerous proteins with one-color or two-color fluorescence detection methods. To establish the limiting factors and to validate the use of alternative detection for protein profiling, we performed competitive binding assays under different conditions. A model experimental protocol was developed whereby the competitive displacement of multi-subunit bacterial RNA polymerase and/or its subunits was evaluated through binding to subunit-specific immobilized monoclonal antibodies. We show that the difference in physico-chemical properties of unlabeled and labeled molecules significantly affects the performance of one-color detection, whereas epitope inaccessibility in the protein complex can prohibit the assessment of competition by both detection methods. Our data also demonstrate that antibody cross-reactivity, target protein truncation and abundance, as well as the cellular compartment of origin are major factors that affect protein profiling on antibody arrays. The experimental conditions established for prokaryotic proteins were adopted to compare protein profiles in the breast tumor-derived cell lines MDA MB-231 and SKBR3. Competitive displacement was detected and confirmed for a number of proteins using both detection methods; however, we show that overall the two-color method is better suited for accurate expression profile evaluation of a large, complex set of proteins. Antibody array data confirm the functional linkage between the ErbB2 receptor and AP-2 transcription factors in these cell lines and highlight unexpected differences in G1 cyclin expression.

Neuregulin 1-alpha expression in locally advanced breast cancer

The human Neuregulin 1 (NRG1) gene encodes several alternatively spliced ligands that bind to both c-erbB-3 and c-erbB-4, members of the family of type 1 tyrosine kinase growth factor receptors. Antibodies raised to a synthetic peptide recognize selectively the alpha variant of NRG1. The NRG1-alpha isoforms' expression was studied in 115 locally advanced adenocarcinomas of the breast using immunohistochemistry. Absent or low levels of NRG1-alpha were found to be associated with poorer prognosis compared to tumours that had moderate to high levels of the protein.

Activator protein-2gamma (AP-2gamma) expression is specifically induced by oestrogens through binding of the oestrogen receptor to a canonical element within the 5'-untranslated region

The activator protein 2 (AP-2) transcription factors are essential proteins for oestrogenic repression of the ERBB2 proto-oncogene in breast cancer cells. In the present study, we have examined the possible oestrogenic regulation of AP-2 genes themselves in breast-tumour-derived lines. As early as 1 h after oestrogen treatment, AP-2gamma mRNA was markedly increased, whereas AP-2alpha was down-regulated, but with slower kinetics, and AP-2beta was not affected at all. Addition of anti-oestrogens ablated these effects. Modulation of the protein levels corresponded to changes in the transcript levels, thus suggesting that in oestrogen-treated cells, an inversion of the balance between AP-2alpha and AP-2gamma isoforms occurs. The 5'-untranslated region (5'-UTR) of the human AP-2gamma gene contains one consensus and one degenerate oestrogen-responsive element (ERE). Reporter constructs carrying the AP-2gamma promoter and the 5'-UTR were up-regulated by oestrogens in transient transfection assays. Deletion of the most conserved (but not of the degenerate) ERE from reporter constructs abrogated the oestrogenic response, although both ERE-containing segments were footprinted in DNaseI protection assays. In vitro binding assays demonstrated the ability of oestrogen receptor alpha (ERalpha) to bind to this site, and chromatin immunoprecipitation analysis of the endogenous gene showed that ERalpha occupies this region in response to oestrogens. We conclude that AP-2gamma is a primary oestrogen-responsive gene and suggest that AP-2 proteins may mediate some oestrogenic responses.

Characterization of the human activator protein-2gamma (AP-2gamma) gene: control of expression by Sp1/Sp3 in breast tumour cells

The activator protein-2 (AP-2) family of DNA-binding transcription factors are developmentally regulated and also play a role in human neoplasia. In particular, the AP-2gamma protein has been shown to be overexpressed in a high percentage of breast tumours. In the present study, we report the complete sequence determination of the human TFAP2C gene encoding the AP-2gamma transcription factor plus the mapping of the transcription start site used in breast tumour-derived cells. The 5'-end of the gene lies within a CpG island and transcription is initiated at a single site within a classical initiator motif. We have gone on to investigate why some breast tumour-derived cell lines readily express AP-2gamma, whereas others do not, and show that the proximal promoter (+191 to -312) is differentially active in the two cell phenotypes. DNase footprinting led to the identification of three Sp1/Sp3-binding sites within this region, two of which are absolutely required both for promoter function and cell-type-specific activity. By Western blotting a panel of expressing and non-expressing breast tumour lines we show that the latter have higher levels of Sp3. Furthermore, increasing Sp3 levels in AP-2gamma-expressing cells led to the repression of AP-2gamma promoter activity, particularly when Sp3 inhibitory function was maximized through sumoylation. We propose that differences in the level and activity of Sp3 between breast tumour lines can determine the expression level of their AP-2gamma gene.

Physical and functional interactions among AP-2 transcription factors, p300/CREB-binding protein, and CITED2

The transcriptional co-activators and histone acetyltransferases p300/CREB-binding protein (CBP) interact with CITED2, a transcription factor AP-2 (TFAP2) co-activator. p300/CBP, CITED2, and TFAP2A are essential for normal neural tube and cardiac development. Here we show that p300 and CBP co-activate TFAP2A in the presence of CITED2. TFAP2A transcriptional activity was modestly impaired in p300(+/-) and CBP(+/-) mouse embryonic fibroblasts; this was rescued by ectopic expression of p300/CBP. p300, TFAP2A, and endogenous CITED2 could be co-immunoprecipitated from transfected U2-OS cells indicating that they can interact physically in vivo. CITED2 interacted with the dimerization domain of TFAP2C, which is highly conserved in TFAP2A/B. In mammalian two-hybrid experiments, full-length p300 and TFAP2A interacted only when CITED2 was co-transfected. N-terminal residues of TFAP2A, containing the transactivation domain, are both necessary and sufficient for interaction with p300, and this interaction was independent of CITED2. Consistent with this, N-terminal residues of TFAP2A were required for p300- and CITED2-dependent co-activation. A histone acetyltransferase-deficient p300 mutant (D1399Y) did not co-activate TFAP2A and did not affect the expression or cellular localization of TFAP2A or CITED2. In mammalian two-hybrid experiments p300D1399Y failed to interact with TFAP2A, explaining, at least in part, its failure to function as a co-activator. Our results suggest a model wherein interactions among TFAP2A, CITED2, and p300/CBP are necessary for TFAP2A-mediated transcriptional activation and for normal neural tube and cardiac development.

FGFR4 overexpression in pancreatic cancer is mediated by an intronic enhancer activated by HNF1alpha

Fibroblast growth factor receptor 4 (FGFR4) is expressed in 50-70% of pancreatic carcinomas (PC) and a similar proportion of derived cell lines. Here we determine the sites of FGFR4 transcriptional initiation which show a pattern characteristic of genes with GC-rich, TATA-less promoters. We have examined the chromatin structure around the FGFR4 gene in a panel of expressing and non-expressing PC lines using the DNase I hypersensitive site assay. One region of hypersensitivity, located largely within intron 1, was found to be greatly extended in expressing cells. Subsequent functional analyses using reporter assays demonstrated that this region was able to act as a cell-specific enhancer, only showing significant activity in PC lines expressing endogenous FGFR4. Transcription factors able to bind to the enhancer were investigated using footprinting and mobility shift assays and two binding sites for Sp1 proteins and two sites able to bind hepatic nuclear factor 1 (HNF1) proteins were identified. Further reporter assays using constructs mutated in each binding site demonstrated that HNF1 binding was essential for enhancer activity in expressing cells, an observation that correlated with the increased abundance of HNF1alpha in these same cells as measured by Western blotting. Finally we show that exogenous expression of HNF1 factors in an FGFR4 non-expressing line led to an induction of enhancer activity in reporter assays and also activated expression of the endogenous gene. We conclude that HNF1alpha is a major determinant of FGFR4 expression in PC.

Transcription factor AP-2 interacts with the SUMO-conjugating enzyme UBC9 and is sumolated in vivo

The members of the AP-2 family of transcription factors are developmentally regulated and have distinct yet overlapping functions in the regulation of many genes governing growth and differentiation. All AP-2 factors appear to be capable of binding very similar DNA recognition sites, and the determinants of functional specificity remain to be elucidated. AP-2 transcription factors have been shown to act both as transcriptional activators and repressors in a promoter-specific manner. Although several mediators of their activation function have been suggested, few mechanisms for the repression or down-regulation of transactivation have been described. In a two-hybrid screen for proteins interacting with AP-2 factors, we have identified the UBC9 gene that encodes the E2 (ubiquitin carrier protein)-conjugating enzyme for the small ubiquitin-like modifier, SUMO. The interaction domain resides in the C-terminal half of AP-2, which contains the conserved DNA binding and dimerization domains. We have detected sumolated forms of endogenous AP-2 in mammalian cells and have further mapped the in vivo sumolation site to conserved lysine 10. Transient transfection studies indicate that sumolation of AP-2 decreases its transcription activation potential, and we discuss the possible mechanisms for the observed suppression of AP-2 transactivation.

Targeting of SWI/SNF chromatin remodelling complexes to estrogen-responsive genes

SWI/SNF complexes are ATP-dependent chromatin remodelling enzymes that have been implicated in the regulation of gene expression in yeast and higher eukaryotes. BRG1, a catalytic subunit in the mammalian SWI/SNF complex, is required for transcriptional activation by the estrogen receptor, but the mechanisms by which the complex is recruited to estrogen target genes are unknown. Here, we have identified an interaction between the estrogen receptor and BAF57, a subunit present only in mammalian SWI/SNF complexes, which is stimulated by estrogen and requires both a functional hormone-binding domain and the DNA-binding region of the receptor. We also found an additional interaction between the p160 family of coactivators and BAF57 and demonstrate that the ability of p160 coactivators to potentiate transcription by the estrogen receptor is dependent on BAF57 in transfected cells. Moreover, chromatin immunoprecipitation assays demonstrated that BAF57 is recruited to the estrogen-responsive promoter, pS2, in a ligand-dependent manner. These results suggest that one of the mechanisms for recruiting SWI/SNF complexes to estrogen target genes is by means of BAF57.

Human CREB-binding protein/p300-interacting transactivator with ED-rich tail (CITED) 4, a new member of the CITED family, functions as a co-activator for transcription factor AP-2

Members of the CREB-binding protein/p300-interacting transactivator with ED-rich tail (CITED) family bind CREB-binding protein and p300 with high affinity and regulate gene transcription. Gene knockout studies indicate that CITED2 is required for neural crest and neural tube development and that it functions as a co-activator for transcription factor AP-2 (TFAP2). Here we describe human CITED4, a new member of this family, which is encoded by a single exon mapping to chromosome 1p34--1p35. CITED4 and p300/CREB-binding protein are present in endogenous naturally occurring complexes, indicating that they interact physiologically. The interaction occurs between the cysteine-histidine-rich domain 1 of p300 and the carboxyl terminus of CITED4. In keeping with this, CITED4 functions as a transactivator when artificially targeted to a promoter element. CITED4 physically interacts with all TFAP2 isoforms in vitro and strongly co-activates all TFAP2 isoforms in Hep3B cells. Co-activation of TFAP2 requires amino-terminal and carboxyl-terminal residues of CITED4. In HepG2 cells, CITED4 is significantly weaker than CITED2 for TFAP2C co-activation. These results suggest that CITED4 may function as a co-activator for TFAP2. They also suggest the existence of cell type- and TFAP2 isoform-specific co-activation by CITED2 and CITED4, which may result in differential modulation of TFAP2 function.

Cardiac malformations, adrenal agenesis, neural crest defects and exencephaly in mice lacking Cited2, a new Tfap2 co-activator

The protein EP300 and its paralog CREBBP (CREB-binding protein) are ubiquitously expressed transcriptional co-activators and histone acetyl transferases. The gene EP300 is essential for normal cardiac and neural development, whereas CREBBP is essential for neurulation, hematopoietic differentiation, angiogenesis and skeletal and cardiac development. Mutations in CREBBP cause Rubinstein-Taybi syndrome, which is characterized by mental retardation, skeletal abnormalities and congenital cardiac defects. The CBP/p300-interacting transactivator with ED-rich tail 2 (CITED2) binds EP300 and CREBBP with high affinity and regulates gene transcription. Here we show that Cited2-/- embryos die with cardiac malformations, adrenal agenesis, abnormal cranial ganglia and exencephaly. The cardiac defects include atrial and ventricular septal defects, overriding aorta, double-outlet right ventricle, persistent truncus arteriosus and right-sided aortic arches. We find increased apoptosis in the midbrain region and a marked reduction in ErbB3-expressing neural crest cells in mid-embryogenesis. We show that CITED2 interacts with and co-activates all isoforms of transcription factor AP-2 (TFAP2). Transactivation by TFAP2 isoforms is defective in Cited2-/- embryonic fibroblasts and is rescued by ectopically expressed CITED2. As certain Tfap2 isoforms are essential in neural crest, neural tube and cardiac development, we propose that abnormal embryogenesis in mice lacking Cited2 results, at least in part, from its role as a Tfap2 co-activator.

Intracellular expression of the truncated extracellular domain of c-erbB-3/HER3

The ERBB3 gene is expressed as a 6.2- and a 1.4-kb transcript. The former encodes the full-length transmembrane protein and the latter a truncated extracellular fragment consisting of 140 amino acids of the c-erbB-3 protein followed by 43 unique residues. We have examined the expression of the two ERBB3 transcripts by Northern blotting in cancer cell lines and normal human fetal and adult tissues. We expressed the truncated receptor fragment and showed that it was glycosylated, probably with a single N-linked complex sugar chain, and that the protein was a 58-kDa disulphide-linked dimer. We were able to crosslink iodinated neuregulin (NRG)-1beta to the full-length solubilised receptor but not to the truncated dimeric protein. Using Western blot analysis, the truncated protein was shown to be present in cell lysates and, using immunoelectron microscopy, in vesicular structures within cells and associated with the plasma cell membrane.

Expression pattern of AP-2 transcription factors in cervical cancer cells and analysis of their influence on human papillomavirus oncogene transcription

The AP-2 family of transcription factors consists of three known members, namely AP-2alpha, AP-2beta, and AP-2gamma. In experimental systems AP-2 factors possess tumor suppressor-like activities, and alterations in the AP-2 expression pattern have been described for some tumor entities. In addition, AP-2 has been implicated in the transcriptional control of human papillomaviruses (HPVs). We investigated here the expression pattern of AP-2alpha, AP-2beta, and AP-2gamma, as well as that of the cellular AP-2 target gene c-erbB-2, in a series of cervical cancer cell lines. In addition, we analyzed the influence of AP-2 factors on the activity of the HPV16 and HPV18 E6/E7 oncogene promoter. We found that, with the exception of HPV-negative C33A cells, all investigated cervical cancer cell lines expressed all three AP-2 family members, although at varying levels. No linear correlation between AP-2 and c-erbB-2 levels was observed. Although AP-2alpha, AP-2beta, and AP-2gamma can activate the c-erbB-2 promoter in reporter gene assays, they do not stimulate the HPV16 or HPV18 E6/E7 promoter. These results indicate that, although a rare event, loss of AP-2 expression occurs in cervical cancer cells. Moreover, AP-2alpha, AP-2beta, and AP-2gamma are neither sufficient nor required to activate the viral E6/E7 promoter.

Update on HER-2 as a target for cancer therapy: the ERBB2 promoter and its exploitation for cancer treatment

Overexpression of the ERBB2 proto-oncogene is associated with amplification of the gene in breast cancer but increased activity of the promoter also plays a significant role. Members of two transcription factor families (AP-2 and Ets) show increased binding to the promoter in over-expressing cells. Consequently, strategies have been devised to target promoter activity, either through the DNA binding sites for these factors, or through another promoter sequence, a polypurine-polypyrimidine repeat structure. The promoter has also been exploited for its tumour-specific activity to direct the accumulation of cytotoxic compounds selectively within cancer cells. Our current understanding of the ERBB2 promoter is reviewed and the status of these therapeutic avenues is discussed.

Cofactor competition between the ligand-bound oestrogen receptor and an intron 1 enhancer leads to oestrogen repression of ERBB2 expression in breast cancer

Overexpression of the ERBB2 proto-oncogene in breast tumours, which occurs in 25-30% of patients, correlates with poor prognosis. In oestrogen receptor (ER) positive breast epithelial cells oestrogens reduce ERBB2 mRNA and protein levels, an effect that is reversed in the presence of anti-oestrogens such as tamoxifen and ICI 182780. Our previous studies have shown that the major effect of oestrogen on ERBB2 expression is at the level of transcription and that this is mediated through a region within the ERBB2 first intron which can act as an oestrogen-suppressible enhancer in ER positive breast cells. In vitro footprinting of the smallest DNA fragment that retained full activity revealed four transcription factor binding sites. We report here that two of these sites are recognized by AP-2 proteins and the other two are bound by a variety of bZIP factors, including CREB and ATFI, with a major complex containing ATFa/ JunD. However, by using ER mutants it is clear that repression occurs essentially off the DNA. Indeed, the essential domain of the ER responsible for repression of the ERBB2 enhancer is a region termed AF2 which is required for the ligand-dependent association of non-DNA binding cofactors. We further demonstrate that one of these ER cofactors, SRC-1, can relieve oestrogen repression of the ERBB2 enhancer and conclude that these data fit with a model whereby the ER and the ERBB2 enhancer compete for this limiting, non-DNA binding cofactor. Thus, in oestrogenic conditions SRC-1 preferentially binds to the ER which effectively sequesters it thereby reducing enhancer activity, but in antioestrogenic media the cofactor is released from the ER and is therefore available to activate the ERBB2 enhancer.

Immunohistochemical analysis reveals a tumour suppressor-like role for the transcription factor AP-2 in invasive breast cancer

This paper describes the generation and characterization of a monoclonal antibody specific for two members of the AP-2 family of transcription factors, AP-2alpha and AP-2beta, and its subsequent application to archival primary breast tumour material. Nuclear localization of AP-2 was found in all expressing cases, but in general levels of immunostaining were low, with only 17 per cent of the 86 tumours examined showing very high expression levels. Nevertheless, data analysis of the whole patient series allowed the identification of significant relationships between levels of AP-2 and other important breast markers. Thus, expression of AP-2alpha/beta was found to correlate significantly with expression of both ER ( p=0.036*) and the universal cell-cycle inhibitor p21(cip) ( p=0.03*), but was inversely related to levels of the proto-oncogene ErbB2 ( p=0.008*). AP-2-positive tumours also showed a low rate of proliferation, with significantly reduced mitotic count and a lower tumour grade. There was no significant relationship with clinical parameters, but samples with adjacent normal tissue indicated that loss of the AP-2 marker was associated with disease progression from normal breast through to invasive disease. This was confirmed by examining separate series of pure normal and pure DCIS samples, both of which expressed significantly higher levels of AP-2 ( p=0.0001* in each case) than the invasive tumours. Overall, these findings implicate AP-2alpha/beta as having a role akin to that of a tumour suppressor in breast cancer.

Genetic prodrug activation therapy for breast cancer: A phase I clinical trial of erbB-2-directed suicide gene expression

PURPOSE

This trial was designed to test the safety and efficacy of a tumor-specific genetic prodrug activation therapy targeted by use of the human erbB-2 gene promoter. The erbB-2 oncogene is overexpressed in approximately 20% of cases of breast cancer and is associated with poor prognosis.

PATIENTS AND METHODS

Twelve breast cancer patients received transcriptionally targeted gene therapy in a phase I clinical trial using direct intratumoral injection of plasmid construct combined with systemic administration of prodrug. The genetic prodrug activation therapy is specifically targeted to erbB-2-overexpressing breast cancer cells by use of a therapeutic cassette that contains the Escherichia coli cytosine deaminase gene driven by the tumor-specific erbB-2 promoter, thus allowing activation of fluorocytosine to the active cytotoxic fluorouracil only within tumor cells that express the oncogene.

RESULTS

The approach was shown to be safe and to result in targeted gene expression in up to 90% of cases. Using a number of different assays, we demonstrated that significant levels of expression of the suicide gene were specifically restricted to erbB-2-positive tumor cells, confirming the selectivity of the approach.

CONCLUSION

The results of this study, the first targeted gene therapy for breast cancer and the first to use the cytosine deaminase system in human subjects, are encouraging for the development of genetic prodrug activation therapies that exploit the transcriptional profile of cancer cells.

Insulation of a conditionally expressed transgene in an adenoviral vector

Replication-defective recombinant adenoviruses provide an efficient system for in vivo gene transfer and numerous studies have demonstrated that this vector can accommodate tissue-specific promoters to restrict the expression of a transgene to a particular subset of cells. However, in some cases the selectivity of expression is lost when the tissue-specific promoter is placed in an adenoviral environment. In an attempt to restore the conditionality of expression of the transgene driven by the human ERBB2 promoter, we have flanked the expression cassette in 5' and 3' orientations with a 250 bp sequence containing the bovine growth hormone transcriptional stop signal for cloning into a recombinant adenovirus. The data presented here clearly demonstrate that these 'insulator' elements are able to restrict the expression of the transgene (herpes simplex thymidine kinase) to ERBB2-expressing cells and therefore to restore the selectivity mediated by the ERBB2 promoter. This approach could be generally useful to insulate expression cassettes in adenoviral vectors.

Expression of AP-2 transcription factors in human breast cancer correlates with the regulation of multiple growth factor signalling pathways

The AP-2 transcription factors are required for normal growth and morphogenesis during mammalian development. Previous in vitro studies have also indicated that the AP-2 family of proteins may be involved in the etiology of human breast cancer. The AP-2 genes are expressed in many human breast cancer cell lines, and critical AP-2-binding sites are present in both the ERBB-2 (HER2/neu) and estrogen receptor promoters. We have now characterized immunological reagents that enable specific AP-2 family members, including AP-2alpha and AP-2gamma, to be detected in human breast cancer epithelium. Data obtained with these reagents demonstrate that whereas AP-2alpha and AP-2gamma are both present in benign breast epithelia, there is a significant up-regulation of AP-2gamma expression in breast cancer specimens (P = 0.01). There was also a significant correlation between the presence of the AP-2alpha protein and estrogen receptor expression (P = 0.018) and between specimens containing both AP-2alpha/AP-2gamma proteins and ERBB-2 expression (P = 0.003). Furthermore, we detected an association (P = 0.04) between the expression of AP-2gamma and the presence of an additional signal transduction molecule implicated in breast cancer, the insulin-like growth factor I receptor. Analysis of the proximal promoter of the insulin-like growth factor I receptor revealed a novel AP-2-binding site. Thus, AP-2 proteins may directly regulate the transcription of this growth factor receptor. Taken together, these data strongly support a role for the AP-2 gene family in the control of cell growth and differentiation in breast cancer.

Expression of the c-erbB-4/HER4 protein and mRNA in normal human fetal and adult tissues and in a survey of nine solid tumour types

The c-erbB-4/HER4 receptor belongs to the family of the type I growth factor receptors. Mouse monoclonal antibodies have been raised to the cytoplasmic domain of the c-erbB-4 receptor and characterized; the antibody HFR-1 has been used to determine the pattern of expression of the c-erbB-4 protein immunohistochemically in formalin-fixed, paraffin-embedded adult and fetal tissues. The expression of c-erbB-4 mRNA was determined by using 35S-labelled riboprobes and tissue in situ hybridization. c-erbB-4 is widely expressed in many adult and fetal tissues, including the lining epithelia of the gastrointestinal, urinary, reproductive, and respiratory tracts, as well as the skin, skeletal muscle, circulatory, endocrine, and nervous systems. The developing brain and heart notably express high levels of this receptor. The pattern of c-erbB-4 protein expression is also reported in a survey of common solid human cancers. Loss of expression was noted in 40-80 per cent of adenocarcinomas and up to 100 per cent of squamous cell carcinomas, whereas overexpression was observed in about 10-20 per cent of adenocarcinomas and astrocytomas. In general, the pattern of c-erbB-4 expression in normal tissues and cancers suggests that it tends to be associated with the differentiated compartment.

RB and c-Myc activate expression of the E-cadherin gene in epithelial cells through interaction with transcription factor AP-2

E-cadherin plays a pivotal role in the biogenesis of the first epithelium during development, and its down-regulation is associated with metastasis of carcinomas. We recently reported that inactivation of RB family proteins by simian virus 40 large T antigen (LT) in MDCK epithelial cells results in a mesenchymal conversion associated with invasiveness and a down-regulation of c-Myc. Reexpression of RB or c-Myc in such cells allows the reexpression of epithelial markers including E-cadherin. Here we show that both RB and c-Myc specifically activate transcription of the E-cadherin promoter in epithelial cells but not in NIH 3T3 mesenchymal cells. This transcriptional activity is mediated in both cases by the transcription factor AP-2. In vitro AP-2 and RB interaction involves the N-terminal domain of AP-2 and the oncoprotein binding domain and C-terminal domain of RB. In vivo physical interaction between RB and AP-2 was demonstrated in MDCK and HaCat cells. In LT-transformed MDCK cells, LT, RB, and AP-2 were all coimmunoprecipitated by each of the corresponding antibodies, and a mutation of the RB binding domain of the oncoprotein inhibited its binding to both RB and AP-2. Taken together, our results suggest that there is a tripartite complex between LT, RB, and AP-2 and that the physical and functional interactions between LT and AP-2 are mediated by RB. Moreover, they define RB and c-Myc as coactivators of AP-2 in epithelial cells and shed new light on the significance of the LT-RB complex, linking it to the dedifferentiation processes occurring during tumor progression. These data confirm the important role for RB and c-Myc in the maintenance of the epithelial phenotype and reveal a novel mechanism of gene activation by c-Myc.

Expression of the c-erbB-4/HER4 protein and mRNA in normal human fetal and adult tissues and in a survey of nine solid tumour types

The c-erbB-4/HER4 receptor belongs to the family of the type I growth factor receptors. Mouse monoclonal antibodies have been raised to the cytoplasmic domain of the c-erbB-4 receptor and characterized; the antibody HFR-1 has been used to determine the pattern of expression of the c-erbB-4 protein immunohistochemically in formalin-fixed, paraffin-embedded adult and fetal tissues. The expression of c-erbB-4 mRNA was determined by using 35S-labelled riboprobes and tissue in situ hybridization. c-erbB-4 is widely expressed in many adult and fetal tissues, including the lining epithelia of the gastrointestinal, urinary, reproductive, and respiratory tracts, as well as the skin, skeletal muscle, circulatory, endocrine, and nervous systems. The developing brain and heart notably express high levels of this receptor. The pattern of c-erbB-4 protein expression is also reported in a survey of common solid human cancers. Loss of expression was noted in 40-80 per cent of adenocarcinomas and up to 100 per cent of squamous cell carcinomas, whereas overexpression was observed in about 10-20 per cent of adenocarcinomas and astrocytomas. In general, the pattern of c-erbB-4 expression in normal tissues and cancers suggests that it tends to be associated with the differentiated compartment.

Use of transcriptional regulatory elements of the MUC1 and ERBB2 genes to drive tumour-selective expression of a prodrug activating enzyme

In order to exploit differences in gene expression between normal and malignant cells for genetic prodrug-activation therapy, we have generated recombinant retroviruses containing the herpes simplex virus thymidine kinase coding region cloned downstream of sequences derived from the 5'-flanking regions of the MUC1 and ERBB2 genes. Transduction with retroviruses containing MUC1 promoters resulted in an increase in GCV sensitivity in MUC1 positive cells. A further increase in GCV sensitivity was achieved when MUC1-positive cells were transduced with retroviruses containing chimeric-MUC1/ERBB2 promoters. No significant sensitization to GCV was observed when MUC1-negative cells were transduced with these recombinant retroviruses. These results suggest that one may be able to develop a tumour-selective therapy by utilizing the transcriptional regulatory regions of the MUC1 and ERBB2 genes to drive the expression of suicide genes.

An intron 1 enhancer element mediates oestrogen-induced suppression of ERBB2 expression

Overexpression of the ERBB2 gene in human breast cancer is associated with a poor prognosis and resistance to hormonal treatment and chemotherapy. Oestrogen receptor (ER) positive tumour-derived cell lines are known to express relatively low levels of ERBB2 protein under oestrogenic conditions, but markedly higher levels following withdrawal of oestrogens or administration of tamoxifen. Expression of the closely related ERBB3 gene, which co-operates with ERBB2 in cellular transformation, is now shown to respond to oestrogenic manipulation in a similar way, both responses being mediated largely by transcriptional changes. Six previously undescribed DNase I hypersensitive sites occur within the first intron of ERBB2 in cells that overexpress the gene. A 409 base pair DNA fragment containing one of these sites conferred ER dependent oestrogen inhibition on the ERBB2 promoter in two types of transient transfection assay. DNase I footprinting revealed four separate transcription factor binding sites within this fragment consistent with a role as a transcriptional enhancer. These findings implicate intron 1 sequences in the control of ERBB2 expression for the first time and demonstrate that one site within this region is involved in mediating the transcriptional response to oestrogens. Additionally, there is likely to be synergism between ERBB2 and ERBB3 signalling when both are overexpressed in response to oestrogen inhibition, thereby driving transformed cell behaviour.

Transcriptional regulation of type I receptor tyrosine kinases in the mammary gland

The transcriptional regulation of the human EGFR3 and ERBB2 genes has been extensively studied, particularly in the context of their overexpression in breast cancer. Here we summarize published work detailing the transcription factors which interact with the promoters of these and the rat ERBB2 homologue, neu, genes and discuss their possible relevance to gene activation in cancer. In addition we review the biologically significant molecules which modulate expression of these genes and discuss the nuclear factors involved in mediating these responses. We also describe novel therapies which may result from these studies and highlight directions for future research into the control of expression of the EGFR and ERBB2 genes in the normal mammary gland and in breast cancer.

Suicide gene expression induced in tumour cells transduced with recombinant adenoviral, retroviral and plasmid vectors containing the ERBB2 promoter

In order to exploit the tumour-specific nature of ERBB2 expression for genetic prodrug-activation therapy, we have generated recombinant adenoviral, retroviral and plasmid vectors containing an expression cassette consisting of the ERBB2 promoter and herpes simplex virus thymidine kinase coding sequence. In the case of the adenoviral vectors, the expression cassette was introduced into the E1 or E3 region of the genome. All of the vectors were capable of sensitizing ERBB2-positive cells to the action of ganciclovir. In contrast to the retroviral and plasmid vectors, however, transduction with the adenoviral vectors also resulted in sensitization of ERBB2-negative cells to ganciclovir, infection of cell lines with a beta-galactosidase expressing adenovirus showed that the sensitizing effect was not due to adenoviral infection per as in all but one of the cell lines tested. This study demonstrates that the ERBB2 promoter can be used to induce ERBB2-dependent sensitization to ganciclovir when in the context of retroviral and plasmid vectors. Observations made in this study do, however, suggest that adenoviral vectors may not be the ideal system to engineer conditional expression, and possible explanation for this phenomenon are discussed.

A family of AP-2 proteins regulates c-erbB-2 expression in mammary carcinoma

The proto-oncogene c-erbB-2 is overexpressed in 25-30% of breast cancers through increased transcription and amplification of the gene. We have previously described a factor, OB2-1 which upregulates c-erbB-2 transcription and which is closely related to the developmentally regulated transcription factor, AP-2. Further analysis of affinity purified OB2-1 has now shown that it is in fact a combination of proteins from three AP-2-related genes, the previously described AP-2alpha gene and two new human family members, AP-2beta and AP-2gamma whose cloning and characterisation are described here. All three AP-2 proteins show a high degree of homology and are capable of binding to the c-erbB-2 promoter as homo- or heterodimers. The three proteins can also activate a c-erbB-2 reporter construct, but AP-2alpha and AP-2gamma are 3-4 times more active in this regard than AP-2beta. In addition both AP-2alpha and AP-2gamma are expressed at elevated levels in the majority of c-erbB-2 overexpressing mammary tumour lines examined. Mechanisms which may have led to the increased AP-2 levels in these cells are discussed.

Transcription factors as drug targets in cancer

Within multicellular organisms, cells are continually signalling to each other to keep in tune with their environment. The ultimate targets for the majority of these signal pathways are upstream transcription factors, whose activity is thereby modulated, resulting in a new pattern of gene expression suitably coupled to the needs of the cell. It has been estimated that up to 10% of human genes may encode transcription factors, thus emphasising how fundamental the control of gene expression is to the processes of cellular division and differentiation during normal development. As a corollary of this, transcriptional regulation can also profoundly affect the course of growth-related diseases such as cancer. Of course it has been realised for some time that the normal counterparts of many oncogenes are transcription factors whose proper role is in the control of normal cell growth. More recent work has begun to identify several other transcription factors which may play a role in cancer, and strategies are now being developed which are designed to use our growing knowledge of transcriptional control mechanisms in the development of novel cancer therapies.

Protein-protein interaction between the transcriptional repressor E4BP4 and the TBP-binding protein Dr1

We have previously mapped a repression domain from the active transcriptional repressor E4BP4 to a 65 amino acid segment near the C-terminus of the polypeptide. Here we show that the E4BP4 repression domain interacts specifically with the TBP binding repressor protein Dr1. Mutants that affect the ability of E4BP4 to bring about transcriptional repression are also deficient in their binding of Dr1. The results are discussed in the light of evidence for squelching of a 'global' repressor by a DNA binding defective E4BP4 mutant.

Chromosomal mapping of the human and mouse homologues of two new members of the AP-2 family of transcription factors

The AP-2 transcription factor has been shown to play an important role in the development of tissues of ectodermal origin and has also been implicated in mammary oncogenesis. It has recently been found that AP-2 is encoded by a family of related genes, AP-2alpha, AP-2beta, and AP-2gamma. As a further step in understanding the role each of these genes has in development, we have used fluorescence in situ hybridization to map the chromosomal locations of the mouse and human homologues of the newly isolated AP-2beta and AP-2gamma genes. Tcfap2b and Tcfap2c map to mouse chromosomes 1A2-4 and 2H3-4, respectively, while TFAP2B and TFAP2C map to human chromosomes 6p12 and 20q13.2, the latter being a region that is frequently amplified in breast carcinoma.

Copurification of casein kinase II with transcription factor ATF/E4TF3

We have developed a simple method to purify sequence-specific DNA-binding proteins directly from crude cell extracts by using DNA affinity latex beads. The method enabled us to purify not only DNA-binding proteins, but also their associated proteins. Using beads bearing the ATF/E4TF3 site from the adenovirus E4 gene promoter, a protein kinase activity was copurified with the ATF/E4TF3 family. We found that the kinase interacted with ATF1 in vitro efficiently. The kinase did not bind directly to DNA. The kinase mainly phosphorylated ATF1 on serine 36, which was one of target amino acids for casein kinase (CK) II. Biological features of the kinase were the same as those of CKII and an anti-CKII serum reacted with the kinase, indicating that the kinase was CKII. Moreover, it was clearly shown that one of CKII subunits, the CKII alpha protein bound to glutathione-S-transferase (GST) fusion ATF1 but not GST in vitro. It has been reported that a specific CKII inhibitor, 5,6-dichloro-1-beta-D-ribo-furanosylbenzimidazole (DRB) inhibits transcription by RNA polymerase II [Zandomeni et al., (1986) J. Biol. Chem. 261, 3414-3419]. Taken together, these results suggest that ATF/E4TF3 may recruit the CKII activity to a transcription initiation machinery and stimulate transcription.

Targeting gene transcription: a new strategy to down-regulate c-erbB-2 expression in mammary carcinoma

Overexpression of the c-erbB-2 proto-oncogene in mammary carcinoma is frequently associated with amplification of the c-erbB-2 gene, but it also occurs from single-copy gene. Studies in mammary-derived cell lines have shown that, whether or not the gene is amplified, there is a 6- to 8-fold increase in the accumulation of c-erbB-2 mRNA per gene copy in overexpressing cells. We have recently shown that this phenomenon is due to increased activity of the c-erbB-2 promoter mediated by the binding of a novel transcription factor, OB2-1, which is present at higher levels in overexpressing cells than in low expressors. OB2-1 activity therefore represents a novel therapeutic target for the down-regulation of c-erbB-2 levels in human cells. As a prototype for this strategy, we show here that the drug sodium aurothiomalate is able to inhibit the DNA-binding activity of OB2-1 in vitro and also to interfere with c-erbB-2 promoter activity in cell-based transfection assays. In addition, endogenous c-erbB-2 immunoreactivity was reduced in cells treated with aurothiomalate as compared with the levels observed in control cells.

A novel cyclic AMP response element-binding protein-1 (CREB-1) splice product may down-regulate CREB-1 activity

In this report we identify novel spliced forms of cyclic AMP (cAMP) response element-binding protein-1 (CREB-1) mRNA. These forms contained an additional 17 nucleotide insert, which we refer to as the beta exon, located between exons 4 and 7 of the delta, and 5 and 7 of the alpha forms of CREB-1 transcript (nomenclature of Ruppert et al. 1992; EMBO Journal 11, 1503-1512). The inclusion of the beta exon led to the generation of mRNAs in which the frame of CREB-1 sequences 3' to the exon was shifted such that the encoded proteins terminate after the transactivation domain, but before the target serine for cAMP-dependent protein kinase. The beta exon-containing CREB-1 mRNAs were more abundant in tissues that respond poorly to cAMP, suggesting that the generation of beta CREB-1 mRNAs may contribute to the down-regulation of CREB-1 activity and cAMP responsiveness.

The developmentally regulated transcription factor AP-2 is involved in c-erbB-2 overexpression in human mammary carcinoma

Overexpression of the c-erbB-2/HER2 protooncogene in breast carcinoma is controlled not only by the degree of amplification of the gene but also at the level of gene transcription. Thus, whether or not the gene is amplified, the activity of the c-erbB-2 promoter is enhanced in overexpressing cells through the binding of an additional transcription factor, OB2-1, whose activity is increased in these lines. Here we describe further characterization of OB2-1 and show that it is identical to the developmentally regulated transcription factor AP-2. Functional assays confirm that AP-2 is able to regulate c-erbB-2 expression in mammary-derived cell lines. Furthermore, although AP-2 is barely detectable in cells with the low c-erbB-2 expression phenotype, protein levels are clearly elevated in a panel of c-erbB-2-overexpressing lines. These findings demonstrate an important role for this transcription factor in human cancer.

Identification of a distinctive P-glycoprotein-mediated resistance phenotype in human ovarian carcinoma cells after their in vitro exposure to fractionated X-irradiation

BACKGROUND

Clinical drug resistance is recognized in patients previously treated with radiotherapy and after chemotherapy. In vitro exposure of mammalian tumor cells to fractionated X-irradiation also resulted in the expression of drug resistance. Analysis of the resistance phenotype of irradiated Chinese hamster ovary sublines revealed P-glycoprotein overexpression, without any concomitant increase in P-glycoprotein messenger RNA, under posttranslational regulation. This study aimed to determine whether this distinctive resistance phenotype could also be identified in irradiated human tumor cells.

METHODS

Irradiated sublines established from two human ovarian tumor cell lines, SK-OV-3 and JA-T, which showed resistance to vincristine and to etoposide, were studied. Protein and RNA expression were quantitated by Western and Northern blotting or RNase protection assays. P-glycoprotein turnover was measured after immunoprecipitation of metabolically labelled cells.

RESULTS

Significant P-glycoprotein overexpression was detected using the C219 and C494 monoclonal antibodies in the two irradiated human ovarian tumor sublines. No concomitant increase in P-glycoprotein messenger RNA was detectable in the SK-OV-3/DXR10 subline, contrasting with the increased message characteristic of vincristine-selected SKVCR sublines. In addition, turnover of P-glycoprotein was significantly reduced in these DXR10 cells when compared with that measured in a vincristine-selected subline. These irradiated sublines showed reduced levels of epidermal growth factor receptors and unchanged levels of topoisomerase II, but they overexpressed c-erbB2 marginally and heat shock protein 27 significantly. These latter elevations in protein levels, however, were associated with concomitant increases in their respective messenger RNAs, implicating regulation at the transcriptional level.

CONCLUSIONS

Exposure of human ovarian tumor cells to fractionated X-irradiation in vitro resulted in the expression of a distinctive multiple drug resistance phenotype unusually involving posttranslational regulation of P-glycoprotein. Monitoring tumor biopsies for P-glycoprotein-associated drug resistance in patients treated with radiotherapy should evaluate protein levels rather than, or as well as, MDR1 mRNA expression.

Gene therapy for cancer using tumour-specific prodrug activation

Current treatments for metastatic malignant disease are often ineffective. One of the most promising of the selective genetic strategies against cancer is VDEPT (virally directed enzyme prodrug therapy). This uses a viral vector to carry a prodrug-activating enzyme gene into both tumour and normal cells. By linking the foreign gene downstream of tumour-specific transcription units, tumour-specific expression of the foreign enzyme gene can be achieved. We have developed a genetic therapy strategy using VDEPT against cancers that overexpress the oncogene ERBB2. This occurs in approximately one-third of breast and pancreatic tumours (and in a smaller proportion of other tumours) and involves transcriptional up-regulation of the ERBB2 gene with or without gene amplification. We have constructed a chimeric minigene consisting of the proximal ERBB2 promoter linked to the gene encoding cytosine deaminase, an enzyme that can deaminate the prodrug 5-fluorocytosine (5-FC) to form cytotoxic 5-fluorouracil (5-FU). We have constructed a double-copy recombinant retrovirus to deliver the enzyme gene under the control of the ERBB2 promoter into a panel of ERBB2 expression-positive (ERBB2+) and -negative (ERBB2-) pancreatic and breast cell lines. Cytosine deaminase activity was high in ERBB2+ transduced cells but was not detected in ERBB2- transduced cells. Significant cell death was observed in ERBB2+ transduced cells treated with 5-FC whereas ERBB2- cells were not affected. Hence we present a novel gene therapy strategy that is potentially tumour-specific and could be used against a range of tumour types that overexpress the ERBB2 oncogene.

Transcriptional repression by the human bZIP factor E4BP4: definition of a minimal repression domain

The bZIP factor E4BP4 overlaps in DNA binding site specificity with the transcriptional activator CREB and members of the ATF family of transcription factors, but is an active transcriptional repressor. In this study we have mapped the repressing activity of E4BP4 to a small 'domain' of 65 amino acids that retains its ability to repress transcription when transferred to the heterologous DNA binding domain of the yeast transcriptional activator GAL4. This segment of the E4BP4 polypeptide contains a high proportion of charged amino acids and does not resemble the repression domains that have been characterized so far from other active transcriptional repressors such as the Drosophila Krüppel, Engrailed or Even-skipped proteins. A mutation which changes the charge configuration of this repression module resulted in a complete loss of repressor activity. The E4BP4-GAL4 fusion protein is able to repress the residual transcription from minimal promoters containing the adenovirus E4 or E1b TATA box. This is consistent with a mechanism of action whereby E4BP4 interacts with some component of the general transcription machinery to cause repression of basal and activated transcription. Although a number of nuclear proteins are able to interact with the E4BP4 repression domain in vitro, these proteins do not appear to include the general transcription factors TFIIB or TBP.

Transcriptional regulation of the c-erbB-3 gene in human breast carcinoma cell lines

The c-erbB-3 gene is a member of the EGF receptor family of membrane tyrosine kinases. Previous work has shown that the expression of this gene is frequently unregulated in human breast carcinoma-derived cell lines and in tumour biopsy material. As there is no evidence of gene amplification, we decided to examine the control of c-erbB-3 transcription in overexpressing cells. In this paper we describe the cloning of the c-erbB-3 promoter and its functional analysis within mammary-derived cell lines. We show that the promoter is more active in overexpressing cells and this is largely due to the action of the OB2-1 transcription factor which we have recently identified as mediating overexpression of the c-erbB-2 promoter in similar cell lines.

Specific short transmembrane sequences can inhibit transformation by the mutant neu growth factor receptor in vitro and in vivo

The neu oncogene is activated by a point mutation within its transmembrane domain that results in the substitution of glutamic acid for valine at position 664, and is associated with constitutive activation of the tyrosine kinase. It has been proposed that the mutation allows for stabilization of homodimers of the receptor that are necessary for transduction of the mitogenic signal. To investigate the role of the alpha-helical transmembrane sequence in the function of neu, we constructed an expression vector to produce a variety of short transmembrane neu proteins, lacking ligand binding or intracellular kinase domains. Such sequences should interact with full-length receptors and prevent receptor dimerization and thus act as specific inhibitors of function. These small proteins all included a pentapeptide from position 661-665, which has been proposed to be necessary for packing. We show that the short transmembrane molecules are expressed at the cell surface and can retard the growth of neu-transformed cells in monolayers, as colonies in soft agar and as tumours in animals. As predicted by molecular modelling, the magnitude of inhibition depended on the nature of the packing surface, suggesting that the neu transmembrane domain is directly involved in neu protein dimerization.