Transient expression of some proto-oncogenes, cytokines, and transcription factors occurs as a cellular response to growth factors, 12-O-tetradecanoylphorbol-13-acetate, antigen stimulation, or inflammation. Expression of these genes is mediated in part by the rapid turnover of their mRNAs. A + U-rich elements in the 3' untranslated regions of these mRNAs serve as one recognition signal targeting the mRNAs for rapid degradation. I report the identification of a cytosolic factor that both binds to the proto-oncogene c-myc A + U-rich element and specifically destabilizes c-myc mRNA in a cell-free mRNA decay system which reconstitutes mRNA decay processes found in cells. Proteinase K treatment of the factor abolishes its c-myc mRNA degradation activity without affecting its RNA-binding capacity. Thus, RNA substrate binding and degradation appear to be separable functions. These findings should aid in understanding how the cell selectively targets mRNAs for rapid turnover.

Which genetic alterations drive tumorigenesis and how they evolve over the course of disease and therapy are central questions in cancer biology. Here we identify 44 recurrently mutated genes and 11 recurrent somatic copy number variations through whole-exome sequencing of 538 chronic lymphocytic leukaemia (CLL) and matched germline DNA samples, 278 of which were collected in a prospective clinical trial. These include previously unrecognized putative cancer drivers (RPS15, IKZF3), and collectively identify RNA processing and export, MYC activity, and MAPK signalling as central pathways involved in CLL. Clonality analysis of this large data set further enabled reconstruction of temporal relationships between driver events. Direct comparison between matched pre-treatment and relapse samples from 59 patients demonstrated highly frequent clonal evolution. Thus, large sequencing data sets of clinically informative samples enable the discovery of novel genes associated with cancer, the network of relationships between the driver events, and their impact on disease relapse and clinical outcome.

There appear to be two pathways involved in the pathogenesis of premalignant non-immunoglobulin M (IgM) monoclonal gammopathy of undetermined significance (MGUS) and multiple myeloma (MM). Nearly half of tumors are nonhyperdiploid, and mostly have one of five recurrent IgH translocations: 16% 11q13 (CCN D1), 3% 6p21 (CCN D3), 5% 16q23 (MAF), 2% 20q12 (MAFB), and 15% 4p16 (FGFR3 and MMSET). The remaining hyperdiploid tumors have multiple trisomies involving chromosomes 3, 5, 7, 9, 11, 15, 19, and 21, and infrequently one of these five translocations. Although cyclin D1 is not expressed by healthy lymphoid cells, it is bi-allelically dysregulated in a majority of hyperdiploid tumors. Virtually all MM and MGUS tumors have dysregulated and/or increased expression of cyclin D1, D2, or D3, providing an apparent early, unifying event in pathogenesis. The patterns of translocations and cyclin D expression (TC) define a novel classification that includes eight groups: 11q; 6p; MAF; 4p; D1 (34%); D1+D2 (6%); D2 (17%); and none (2%). The hyperdiploid D1 group is virtually absent in extramedullary MM and MM cell lines, suggesting a particularly strong dependence on interaction with the bone marrow microenvironment. Despite shared progression events (RAS mutations, MYC dysregulation, p53 mutations, and additional disruption of the retinoblastoma pathway), the phenotypes of MGUS and MM tumors in the eight TC groups is determined mainly by early oncogenic events. Similar to acute lymphocytic leukemia, MM seems to include several diseases (groups) that have differences in early or initiating events, global gene expression patterns, bone marrow dependence, clinical features, prognosis, and response to therapy.

Oncogenes influence nutrient metabolism and nutrient dependence. The oncogene c-Myc stimulates glutamine metabolism and renders cells dependent on glutamine to sustain viability ("glutamine addiction"), suggesting that treatments targeting glutamine metabolism might selectively kill c-Myc-transformed tumor cells. However, many current or proposed cancer therapies interfere with the metabolism of glucose, not glutamine. Here, we studied how c-Myc-transformed cells maintained viability when glucose metabolism was impaired. In SF188 glioblastoma cells, glucose deprivation did not affect net glutamine utilization but elicited a switch in the pathways used to deliver glutamine carbon to the tricarboxylic acid cycle, with a large increase in the activity of glutamate dehydrogenase (GDH). The effect on GDH resulted from the loss of glycolysis because it could be mimicked with the glycolytic inhibitor 2-deoxyglucose and reversed with a pyruvate analogue. Furthermore, inhibition of Akt signaling, which facilitates glycolysis, increased GDH activity whereas overexpression of Akt suppressed it, suggesting that Akt indirectly regulates GDH through its effects on glucose metabolism. Suppression of GDH activity with RNA interference or an inhibitor showed that the enzyme is dispensable in cells able to metabolize glucose but is required for cells to survive impairments of glycolysis brought about by glucose deprivation, 2-deoxyglucose, or Akt inhibition. Thus, inhibition of GDH converted these glutamine-addicted cells to glucose-addicted cells. The findings emphasize the integration of glucose metabolism, glutamine metabolism, and oncogenic signaling in glioblastoma cells and suggest that exploiting compensatory pathways of glutamine metabolism can improve the efficacy of cancer treatments that impair glucose utilization.

The MYC proto-oncogene is frequently activated in human cancers through a variety of mechanisms. Its deregulated expression, unconstrained by inactivation of key checkpoints, such as p53, contributes to tumorigenesis. Unlike its normal counterpart, which is restrained by negative regulators, the unleashed MYC oncogene produces a transcription factor that alters global gene expression through transcriptional regulation, resulting in tumorigenesis. Key genes involved in ribosomal and mitochondrial biogenesis, glucose and glutamine metabolism, lipid synthesis, and cell-cycle progression are robustly activated by MYC, contributing to the acquisition of bioenergetics substrates for the cancer cell to grow and proliferate.

Histone deacetylase inhibitors (HDACis) inhibit tumor cell growth and survival, possibly through their ability to regulate the expression of specific proliferative and/or apoptotic genes. However, the HDACi-regulated genes necessary and/or sufficient for their biological effects remain undefined. We demonstrate that the HDACis suberoylanilide hydroxamic acid (SAHA) and depsipeptide regulate a highly overlapping gene set with at least 22% of genes showing altered expression over a 16-h culture period. SAHA and depsipeptide coordinately regulated the expression of several genes within distinct apoptosis and cell cycle pathways. Multiple genes within the Myc, type beta TGF, cyclin/cyclin-dependent kinase, TNF, Bcl-2, and caspase pathways were regulated in a manner that favored induction of apoptosis and decreased cellular proliferation. APAF-1, a gene central to the intrinsic apoptotic pathway, was induced by SAHA and depsipeptide and shown to be important, but not essential, for HDACi-induced cell death. Overexpression of p16(INK4A) and arrest of cells in G(1) can suppress HDACi-mediated apoptosis. Although p16(INK4A) did not affect the genome-wide transcription changes mediated by SAHA, a small number of apoptotic genes, including BCLXL and B-MYB, were differentially regulated in a manner consistent with attenuated HDACi-mediated apoptosis in arrested cells. We demonstrate that different HDACi alter transcription of a large and common set of genes that control diverse molecular pathways important for cell survival and proliferation. The ability of HDACi to target multiple apoptotic and cell proliferation pathways may provide a competitive advantage over other chemotherapeutic agents because suppression/loss of a single pathway may not confer resistance to these agents.

The multifunctional, stress-inducible molecular chaperone HSP70 has important roles in aiding protein folding and maintaining protein homeostasis. HSP70 expression is elevated in many cancers, contributing to tumor cell survival and resistance to therapy. We have determined that a small molecule called 2-phenylethynesulfonamide (PES) interacts selectively with HSP70 and leads to a disruption of the association between HSP70 and several of its cochaperones and substrate proteins. Treatment of cultured tumor cells with PES promotes cell death that is associated with protein aggregation, impaired autophagy, and inhibition of lysosomal function. Moreover, this small molecule is able to suppress tumor development and enhance survival in a mouse model of Myc-induced lymphomagenesis. The data demonstrate that PES disrupts actions of HSP70 in multiple cell signaling pathways, offering an opportunity to better understand the diverse functions of this molecular chaperone and also to aid in the development of new cancer therapies.

Inflammatory cytokine interleukin (IL)-6 is elevated in the serum and lungs of patients with pulmonary artery hypertension (PAH). Several animal models of PAH cite the potential role of inflammatory mediators. We investigated role of IL-6 in the pathogenesis of pulmonary vascular disease. Indices of pulmonary vascular remodeling were measured in lung-specific IL-6-overexpressing transgenic mice (Tg(+)) and compared to wild-type (Tg(-)) controls in both normoxic and chronic hypoxic conditions. The Tg(+) mice exhibited elevated right ventricular systolic pressures and right ventricular hypertrophy with corresponding pulmonary vasculopathic changes, all of which were exacerbated by chronic hypoxia. IL-6 overexpression increased muscularization of the proximal arterial tree, and hypoxia enhanced this effect. It also reproduced the muscularization and proliferative arteriopathy seen in the distal arteriolar vessels of PAH patients. The latter was characterized by the formation of occlusive neointimal angioproliferative lesions that worsened with hypoxia and were composed of endothelial cells and T-lymphocytes. IL-6-induced arteriopathic changes were accompanied by activation of proangiogenic factor, vascular endothelial growth factor, the proproliferative kinase extracellular signal-regulated kinase, proproliferative transcription factors c-MYC and MAX, and the antiapoptotic proteins survivin and Bcl-2 and downregulation of the growth inhibitor transforming growth factor-beta and proapoptotic kinases JNK and p38. These findings suggest that IL-6 promotes the development and progression of pulmonary vascular remodeling and PAH through proproliferative antiapoptotic mechanisms.

Cancer susceptibility genes have been classified into two groups: gatekeepers and caretakers. Gatekeepers are genes that control cell proliferation and death, whereas caretakers are DNA repair genes whose inactivation leads to genetic instability. Abrogation of both caretaker and gatekeeper function markedly increases cancer susceptibility. Although the importance of Ku80 in DNA double-strand break repair is well established, neither Ku80 nor other components of the non-homologous end-joining pathway are known to have a caretaker role in maintaining genomic stability. Here we show that mouse cells deficient for Ku80 display a marked increase in chromosomal aberrations, including breakage, translocations and aneuploidy. Despite the observed chromosome instabilities, Ku80-/- mice have only a slightly earlier onset of cancer. Loss of p53 synergizes with Ku80 to promote tumorigenesis such that all Ku80-/- p53-/- mice succumb to disseminated pro-B-cell lymphoma before three months of age. Tumours result from a specific set of chromosomal translocations and gene amplifications involving IgH and c-Myc, reminiscent of Burkitt's lymphoma. We conclude that Ku80 is a caretaker gene that maintains the integrity of the genome by a mechanism involving the suppression of chromosomal rearrangements.

The MYC oncogene codes for a transcription factor that is overexpressed in many human cancers. Here we show that MYC regulates the expression of two immune checkpoint proteins on the tumor cell surface: the innate immune regulator CD47 (cluster of differentiation 47) and the adaptive immune checkpoint PD-L1 (programmed death-ligand 1). Suppression of MYC in mouse tumors and human tumor cells caused a reduction in the levels of CD47 and PD-L1 messenger RNA and protein. MYC was found to bind directly to the promoters of the Cd47 and Pd-l1 genes. MYC inactivation in mouse tumors down-regulated CD47 and PD-L1 expression and enhanced the antitumor immune response. In contrast, when MYC was inactivated in tumors with enforced expression of CD47 or PD-L1, the immune response was suppressed, and tumors continued to grow. Thus, MYC appears to initiate and maintain tumorigenesis, in part, through the modulation of immune regulatory molecules.

Transforming growth factor-beta (TGFbeta) is a cytokine that arrests epithelial cell division by switching off the proto-oncogene c-myc and rapidly switching on cyclin-dependent kinase (CDK) inhibitors such as p15INK4b. Gene responses to TGFbeta involve Smad transcription factors that are directly activated by the TGFbeta receptor. Why downregulation of c-myc expression by TGFbeta is required for rapid activation of p15INK4b has remained unknown. Here we provide evidence that TGFbeta signalling prevents recruitment of Myc to the p15INK4b transcriptional initiator by Myc-interacting zinc-finger protein 1 (Miz-1). This relieves repression and enables transcriptional activation by a TGFbeta-induced Smad protein complex that recognizes an upstream p15INK4b promoter region and contacts Miz-1. Thus, two separate TGFbeta-dependent inputs - Smad-mediated transactivation and relief of repression by Myc - keep tight control over p15INK4b activation.

Genetically modified mice have been extensively used for analyzing the molecular events that occur during tumor development. In many, if not all, cases, however, it is uncertain to what extent the mouse models reproduce features observed in the corresponding human conditions. This is due largely to lack of precise methods for direct and comprehensive comparison at the molecular level of the mouse and human tumors. Here we use global gene expression patterns of 68 hepatocellular carcinomas (HCCs) from seven different mouse models and 91 human HCCs from predefined subclasses to obtain direct comparison of the molecular features of mouse and human HCCs. Gene expression patterns in HCCs from Myc, E2f1 and Myc E2f1 transgenic mice were most similar to those of the better survival group of human HCCs, whereas the expression patterns in HCCs from Myc Tgfa transgenic mice and in diethylnitrosamine-induced mouse HCCs were most similar to those of the poorer survival group of human HCCs. Gene expression patterns in HCCs from Acox1(-/-) mice and in ciprofibrate-induced HCCs were least similar to those observed in human HCCs. We conclude that our approach can effectively identify appropriate mouse models to study human cancers.

Regulating ribosome number is thought to control cellular growth. Synthesis of ribosomal RNA (rRNA) is a limiting step in ribosome biogenesis and rates of rRNA synthesis are generally altered depending on the growth status of a cell. Although studies in unicellular systems have addressed the mechanisms by which this occurs, few studies have applied a genetic approach to examine growth-dependent control of rRNA synthesis in metazoans. Here, we show that in Drosophila melanogaster Myc (dMyc) is a regulator of rRNA synthesis. Expression of dMyc is both necessary and sufficient to control rRNA synthesis and ribosome biogenesis during larval development. Stimulation of rRNA synthesis by dMyc is mediated through a rapid, coordinated increase in the levels of the Pol I transcriptional machinery. In addition, the growth effects of dMyc in larval wing imaginal discs require de novo rRNA synthesis. We suggest that during animal development, the control of rRNA synthesis and ribosome biogenesis is an essential Myc function.

The c-Myc oncoprotein regulates transcription of genes that are associated with cell growth, proliferation and apoptosis. c-Myc levels are modulated by ubiquitin/proteasome-mediated degradation. Proteasome inhibition leads to c-Myc accumulation within nucleoli, indicating that c-Myc might have a nucleolar function. Here we show that the proteins c-Myc and Max interact in nucleoli and are associated with ribosomal DNA. This association is increased upon activation of quiescent cells and is followed by recruitment of the Myc cofactor TRRAP, enhanced histone acetylation, recruitment of RNA polymerase I (Pol I), and activation of rDNA transcription. Using small interfering RNAs (siRNAs) against c-Myc and an inhibitor of Myc-Max interactions, we demonstrate that c-Myc is required for activating rDNA transcription in response to mitogenic signals. Furthermore, using the ligand-activated MycER (ER, oestrogen receptor) system, we show that c-Myc can activate Pol I transcription in the absence of Pol II transcription. These results suggest that c-Myc coordinates the activity of all three nuclear RNA polymerases, and thereby plays a key role in regulating ribosome biogenesis and cell growth.

Abiotic stress is severe environmental stress, which impairs crop production on irrigated land worldwide. Overall, the susceptibility or tolerance to the stress in plants is a coordinated action of multiple stress responsive genes, which also cross-talk with other components of stress signal transduction pathways. Plant responses to abiotic stress can be determined by the severity of the stress and by the metabolic status of the plant. Abscisic acid (ABA) is a phytohormone critical for plant growth and development and plays an important role in integrating various stress signals and controlling downstream stress responses. Plants have to adjust ABA levels constantly in responce to changing physiological and environmental conditions. To date, the mechanisms for fine-tuning of ABA levels remain elusive. The mechanisms by which plants respond to stress include both ABA-dependent and ABA-independent processes. Various transcription factors such as DREB2A/2B, AREB1, RD22BP1 and MYC/MYB are known to regulate the ABA-responsive gene expression through interacting with their corrosponding cis-acting elements such as DRE/CRT, ABRE and MYCRS/MYBRS, respectively. Understanding these mechanisms is important to improve stress tolerance in crops plants. This article first describes the general pathway for plant stress response followed by roles of ABA and transcription factors in stress tolerance including the regulation of ABA biosynthesis.

X-ray structures of the basic/helix-loop-helix/leucine zipper (bHLHZ) domains of Myc-Max and Mad-Max heterodimers bound to their common DNA target (Enhancer or E box hexanucleotide, 5'-CACGTG-3') have been determined at 1.9 A and 2.0 A resolution, respectively. E box recognition by these two structurally similar transcription factor pairs determines whether a cell will divide and proliferate (Myc-Max) or differentiate and become quiescent (Mad-Max). Deregulation of Myc has been implicated in the development of many human cancers, including Burkitt's lymphoma, neuroblastomas, and small cell lung cancers. Both quasisymmetric heterodimers resemble the symmetric Max homodimer, albeit with marked structural differences in the coiled-coil leucine zipper regions that explain preferential homo- and heteromeric dimerization of these three evolutionarily related DNA-binding proteins. The Myc-Max heterodimer, but not its Mad-Max counterpart, dimerizes to form a bivalent heterotetramer, which explains how Myc can upregulate expression of genes with promoters bearing widely separated E boxes.

Phosphorylation on a serine or threonine residue preceding proline (Ser/Thr-Pro) is a key regulatory mechanism, and the conformation of certain phosphorylated Ser/Thr-Pro bonds is regulated specifically by the prolyl isomerase Pin1. Whereas the inhibition of Pin1 induces apoptosis, Pin1 is strikingly overexpressed in a subset of human tumours. Here we show that Pin1 regulates beta-catenin turnover and subcellular localization by interfering with its interaction with adenomatous polyposis coli protein (APC). A differential-display screen reveals that Pin1 increases the transcription of several beta-catenin target genes, including those encoding cyclin D1 and c-Myc. Manipulation of Pin1 levels affects the stability of beta-catenin in vitro. Furthermore, beta-catenin levels are decreased in Pin1-deficient mice but are increased and correlated with Pin1 overexpression in human breast cancer. Pin1 directly binds a phosphorylated Ser-Pro motif next to the APC-binding site in beta-catenin, inhibits its interaction with APC and increases its translocation into the nucleus. Thus, Pin1 is a novel regulator of beta-catenin signalling and its overexpression might contribute to the upregulation of beta-catenin in tumours such as breast cancer, in which APC or beta-catenin mutations are not common.

The c-myc gene comprises three exons with a single large AUG-initiated open reading frame extending from exon 2 through exon 3. Exon 1 lacks any AUG codons. Cells from a wide range of species produce two c-myc proteins that, while highly related, do not appear to arise from posttranslational interconversion. To understand the origin of the two proteins, we mapped them and analyzed the in vitro protein-coding capacity of c-myc cDNAs. Our findings show that the two proteins are derived from alternative translational initiations at the exon 2 AUG and at a non-AUG codon near the 3' end of exon 1, resulting in the production of proteins with distinct N termini. In Burkitt's lymphomas, the removal or specific mutation of exon 1 in c-myc translocations correlates with suppression of synthesis of the larger protein, and thus may contribute to the oncogenic activation of c-myc.

Ovarian cancer accounts for more deaths than all other gynecological cancers combined. To identify common low-penetrance ovarian cancer susceptibility genes, we conducted a genome-wide association study of 507,094 SNPs in 1,768 individuals with ovarian cancer (cases) and 2,354 controls, with follow up of 21,955 SNPs in 4,162 cases and 4,810 controls, leading to the identification of a confirmed susceptibility locus at 9p22 (in BNC2). Here, we report on nine additional candidate loci (defined as having P ≤ 10⁻⁴) identified after stratifying cases by histology, which we genotyped in an additional 4,353 cases and 6,021 controls. We confirmed two new susceptibility loci with P ≤ 5 × 10⁻⁸ (8q24, P = 8.0 × 10⁻¹⁵ and 2q31, P = 3.8 × 10⁻¹⁴) and identified two additional loci that approached genome-wide significance (3q25, P = 7.1 × 10⁻⁸ and 17q21, P = 1.4 × 10⁻⁷). The associations of these loci with serous ovarian cancer were generally stronger than with other cancer subtypes. Analysis of HOXD1, MYC, TIPARP and SKAP1 at these loci and of BNC2 at 9p22 supports a functional role for these genes in ovarian cancer development.

A paradox for the cancer biology field has been the revelation that oncogenes, once thought to simply provide advantages to a cancer cell, actually put it at dire risk of cell suicide. Myc is the quintessential oncogene in this respect, as in normal cells it is required for cell cycle traverse, whereas in cancers it is overexpressed and functions as the angiogenic switch. Nonetheless, Myc overexpression kills normal cells dead in their tracks. Here we review Myc-induced pathways that contribute to the apoptotic response. Molecular analysis of Myc-induced tumors has established that some of these apoptotic pathways are essential checkpoints that guard the cell from cancer, as they are selectively bypassed during tumorigenesis. The precise mechanism(s) by which Myc targets these pathways are largely unresolved, but we propose that they involve crosstalk and feedback regulatory loops between arbiters of cell death.

The centromere and its binding proteins constitute the kinetochore structure of metaphase chromosomes, which is crucial for the high accuracy of the chromosome segregation process. Isolation and analysis of the gene encoding a centromere binding protein from the yeast S. cerevisiae, CBF1, are described in this paper. DNA sequence analysis of the CBF1 gene reveals homology with the transforming protein myc and a family of regulatory proteins known as the helix-loop-helix (HLH) proteins. Disruption of the CBF1 gene caused a decrease in the growth rate, an increase in the rate of chromosome loss/nondisjunction, and hypersensitivity to the antimitotic drug thiabendazole. Unexpectedly, the cbf1 null mutation concomitantly resulted in a methionine auxotrophic phenotype, which suggests that CBF1, like other HLH proteins in higher eukaryotic cells, participates in the regulation of gene expression.

The Myc/Max/Mad transcription factor network is critically involved in cell behavior; however, there is relatively little information on its genomic binding sites. We have employed the DamID method to carry out global genomic mapping of the Drosophila Myc, Max, and Mad/Mnt proteins. Each protein was tethered to Escherichia coli DNA adenine-methyltransferase (Dam) permitting methylation proximal to in vivo binding sites in Kc cells. Microarray analyses of methylated DNA fragments reveals binding to multiple loci on all major Drosophila chromosomes. This approach also reveals dynamic interactions among network members as we find that increased levels of dMax influence the extent of dMyc, but not dMnt, binding. Computer analysis using the REDUCE algorithm demonstrates that binding regions correlate with the presence of E-boxes, CG repeats, and other sequence motifs. The surprisingly large number of directly bound loci ( approximately 15% of coding regions) suggests that the network interacts widely with the genome. Furthermore, we employ microarray expression analysis to demonstrate that hundreds of DamID-binding loci correspond to genes whose expression is directly regulated by dMyc in larvae. These results suggest that a fundamental aspect of Max network function involves widespread binding and regulation of gene expression.

Since the poor prognosis associated with HER2 amplified breast cancers might be explained by a mechanistic association between p185HER2 overexpression and therapeutic resistance, we assessed the chemo-endocrine sensitivity of estrogen receptor (ER) containing MCF-7 breast cancer cells transfected with full-length HER2 cDNA. Of the 36 isolated MCF/HER2 subclones, 7 were found to overexpress p185HER2 surface receptor at levels 3 to 45-fold greater than parental or control transfected cells (MCF/neo). The overexpressing transfectants possessed increased inositol-1,4,5-triphosphate-3'-kinase activity comparable to enzyme activity in the endogenously HER2 amplified breast cancer cell lines SK-Br-3 and BT-474. The anti-p185HER2 monoclonal antibody and receptor-specific partial agonist, muMAb4D5 (4D5), known to inhibit growth of SK-Br-3 and BT-474 cells, produced no significant growth inhibitory effect on any of the transfectants including the 45-fold overexpressing MCF/HER2-18 cells which were studied in greater detail. MCF/HER2-18 cells contained at least partially functioning exogenous receptor since 4D5 (3 micrograms/ml) specifically stimulated phosphorylation of p185HER2 and its co-precipitating ptyr56 substrate within 5 min, and this was followed at 1 h by a transient induction of c-myc but not c-fos mRNA. ER content and the in vitro sensitivity of MCF/HER2-18 cells to 5-fluorouracil and adriamycin were identical to those of control transfectants and parental cells. However, these highly overexpressing transfectants had acquired low level (2 to 4-fold) resistance to cisplatin and were no longer sensitive to the antiestrogen tamoxifen (TAM). To compare the hormone-dependent tumorigenicity of the HER2 transfectants, MCF/HER2-18 and control cells (MCF, MCF/neo-3) were implanted into ovariectomized athymic nude mice. No tumors were produced in the absence of estradiol (E2) administration. In E2 supplemented mice, MCF/HER2-18 tumors grew most rapidly. When E2 treatment was stopped and daily TAM injections were initiated, MCF-7 and MCF/neo-3 tumor growth ceased immediately, while MCF/HER2-18 tumors continued to show an accelerated growth rate lasting weeks. This pattern of hormone-dependent, TAM-resistant growth exhibited by the MCF/HER2-18 tumors in nude mice supports the possibility that p185HER2 overexpression in human breast cancers may be linked to therapeutic resistance.