ErbB Family Members Research Articles

High Expression of ErbB Family Members and Their Ligands in Lung Adenocarcinomas That Are Sensitive to Inhibition of Epidermal Growth Factor Receptor

Recent findings in tumor biopsies from lung adenocarcinoma patients suggest that somatic mutations in the genes encoding epidermal growth factor receptor (EGFR) and Kirsten ras (KRAS) confer sensitivity and resistance, respectively, to EGFR inhibition. Here, we provide evidence that these genetic mutations are not sufficient to modulate the biological response of lung adenocarcinoma cells to EGFR inhibition. We found high expression of ErbB family members, ErbB ligands, or both in three models that were sensitive to EGFR inhibition, including alveolar epithelial neoplastic lesions in mice that develop lung adenocarcinoma by oncogenic KRAS, human lung adenocarcinoma cell lines, and tumor biopsies from lung adenocarcinoma patients. Thus, lung adenocarcinoma cells that depend on EGFR for survival constitutively activate the receptor through a combination of genetic mutations and overexpression of EGFR dimeric partners and their ligands.

The ErbB-4 s80 intracellular domain is a constitutively active tyrosine kinase

The ErbB-4 receptor tyrosine kinase homo- and heterodimerizes following heregulin binding, which provokes increased levels of tyrosine autophosphorylation. Unique to the ErbB family, ErbB-4 is then proteolytically cleaved by alpha- and gamma-secretase to produce an 80 kDa intracellular domain (s80 ICD) fragment. This fragment is found in both the cytoplasm and nucleus of many normal and cancer cells and can interact with transcription factors in the cytoplasm and nucleus. Since the s80 ICD lacks ectodomain sequences known to play a major role in dimerization of ErbB family members, we asked whether the s80 ICD is an active tyrosine kinase. Here, we demonstrate that the s80 ICD is a constitutively active tyrosine kinase and can form homodimers. The s80 ICD is autophosphorylated in cells and can phosphorylate an exogenous substrate in vitro. Also, the s80 ICD can coassociate and dimers are detected by chemical crosslinking. This is the first example of constitutive kinase activation and dimerization totally within the cytoplasmic domain of an ErbB receptor and suggests that the s80 ICD may function to phosphorylate substrates in the cytoplasm or nucleus.

Secretase-dependent Tyrosine Phosphorylation of Mdm2 by the ErbB-4 Intracellular Domain Fragment

Heregulin activation of the endogenous receptor tyrosine kinase ErbB-4 in ZR-75-1 breast cancer cells provokes tyrosine phosphorylation of Hdm2 in a manner that is sensitive to inhibition of alpha- or gamma-secretase activity, indicating that liberation of the tyrosine kinase intracellular domain (ICD) fragment is required. Similar results are obtained when Erbb-4 is exogenously expressed in 32D cells, which do not otherwise express any ErbB family members. Expression of the ErbB-4 ICD fragment leads to its constitutive association with Mdm2 and tyrosine phosphorylation of Mdm2, a protein that is predominantly localized in the nucleus and that regulates p53 levels. When the ErbB-4 ICD fragment was expressed in H1299 cells, it promoted Hdm2 ubiquitination and increased the levels of p53 and p21, a transcriptional target of p53. In addition, expression of the ICD fragment increased p53 activity toward the p21 promoter in a luciferase reporter assay.

ErbB3 Expression Predicts Tumor Cell Radiosensitization Induced by Hsp90 Inhibition

The ability to identify tumors that are susceptible to a given molecularly targeted radiosensitizer would be of clinical benefit. Towards this end, we have investigated the effects of a representative Hsp90 inhibitor, 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17DMAG), on the radiosensitivity of a panel of human tumor cell lines. 17DMAG was previously shown to enhance the radiosensitivity of a number of human cell lines, which correlated with the loss of ErbB2. We now report on cell lines in which 17DMAG induced the degradation of ErbB2, yet had no effect on radiosensitivity. In a comparison of ErbB family members, ErbB3 protein was only detectable in cells resistant to 17DMAG-induced radiosensitization. To determine whether ErbB3 plays a casual role in this resistance, short interfering RNA (siRNA) was used to knockdown ErbB3 in the resistant cell line AsPC1. Whereas individual treatments with siRNA to ErbB3 or 17DMAG had no effect on radiosensitivity, the combination, which reduced both ErbB2 and ErbB3, resulted in a significant enhancement in AsPC1 radiosensitivity. In contrast to siRNA to ErbB3 or 17DMAG treatments only, AsPC1 cell exposure to the combination also resulted in a decrease in ErbB1 kinase activity. These results indicate that ErbB3 expression predicts for tumor cell susceptibility to and suggests that the loss of ErbB1 signaling activity is necessary for 17DMAG-induced radiosensitization. However, for cell lines sensitized by 17DMAG, treatment with siRNA to ErbB2, which reduced ErbB1 activity, had no effect on radiosensitivity. These results suggest that, whereas the loss of ErbB1 signaling may be necessary for 17DMAG-induced radiosensitization, it is not sufficient.

Synergy of Epidermal Growth Factor Receptor Kinase Inhibitor AG1478 and ErbB2 Kinase Inhibitor AG879 in Human Colon Carcinoma Cells Is Associated with Induction of Apoptosis

Previous studies have shown that constitutive activation of epidermal growth factor receptor (EGFR) and ErbB2 by elevated autocrine transforming growth factor-alpha (TGF-alpha) expression plays an important role in colon cancer progression. Coexpression of EGFR and ErbB2 is found in a subset of colon cancers and may cooperatively promote cancer cell growth and survival, as heterodimerization is known to provide for diversification of signal transduction. In this study, the EGFR-selective tyrosine kinase inhibitor (TKI) AG1478 inhibited cell growth of an aggressive human colon carcinoma cell line, FET6alphaS26X, which harbors constitutively activated EGFR after stable transfection with TGF-alpha cDNA. However, AG1478 failed to induce apoptosis in FET6alphaS26X cells at concentrations sufficient for cell growth inhibition and complete suppression of EGFR phosphorylation. Similarly, AG879, a selective ErbB2 TKI, was incapable of inducing apoptosis in FET6alphaS26X cells at concentrations sufficient to inhibit cell growth and ErbB2 phosphorylation. To test the hypothesis that targeting both ErbB family members would show better efficacy than targeting the single receptors, combinations of inhibitors at fixed ratios of 1:1, 5:1, and 10:1 of AG1478 and AG879, respectively, were compared with single drugs for inhibition of cell growth. All combinations resulted in synergistic effects as indicated by combination index analysis. Synergistic inhibition was associated with induction of apoptosis as reflected by poly(ADP-ribose) polymerase cleavage, caspase-3 activation, and Annexin V staining. Finally, Western blot analysis showed significant inhibition of phosphorylation of both EGFR and ErbB2 by the combination treatment. These data suggest that the strategy to target both EGFR and ErbB2 simultaneously might result in more efficient inhibition of tumor growth than to target single receptor alone.

Synergistic growth inhibition by Iressa and Rapamycin is modulated by VHL mutations in renal cell carcinoma

Epidermal growth factor receptor (EGFR) and tumour growth factor alpha (TGFα) are frequently overexpressed in renal cell carcinoma (RCC) yet responses to single-agent EGFR inhibitors are uncommon. Although von Hippel–Lindau (VHL) mutations are predominant, RCC also develops in individuals with tuberous sclerosis (TSC). Tuberous sclerosis mutations activate mammalian target of rapamycin (mTOR) and biochemically resemble VHL alterations. We found that RCC cell lines expressed EGFR mRNA in the near-absence of other ErbB family members. Combined EGFR and mTOR inhibition synergistically impaired growth in a VHL-dependent manner. Iressa blocked ERK1/2 phosphorylation specifically in wt-VHL cells, whereas rapamycin inhibited phospho-RPS6 and 4E-BP1 irrespective of VHL. In contrast, phospho-AKT was resistant to these agents and MYC translation initiation (polysome binding) was similarly unaffected unless AKT was inhibited. Primary RCCs vs cell lines contained similar amounts of phospho-ERK1/2, much higher levels of ErbB-3, less phospho-AKT, and no evidence of phospho-RPS6, suggesting that mTOR activity was reduced. A subset of tumours and cell lines expressed elevated eIF4E in the absence of upstream activation. Despite similar amounts of EGFR mRNA, cell lines (vs tumours) overexpressed EGFR protein. In the paired cell lines, PRC3 and WT8, EGFR protein was elevated post-transcriptionally in the VHL mutant and EGF-stimulated phosphorylation was prolonged. We propose that combined EGFR and mTOR inhibitors may be useful in the subset of RCCs with wt-VHL. However, apparent differences between primary tumours and cell lines require further investigation.

Biological properties of a human compact anti-ErbB2 antibody

ErbB2 is a prognostic factor and target of therapy for many carcinomas. In contrast with the other ErbB receptors, ErbB2 lacks a soluble direct ligand, but it is the preferred co-receptor for the ErbB family members, forming heterodimers with more potent and prolonged signalling activity than that of homodimers. We recently produced a new anti-ErbB2 antibody, Erb-hcAb, by fusion of Erbicin, a human, anti-ErbB2 scFv, selectively cytotoxic to ErbB2-positive cells, and a human Fc domain. This fully human antitumour antibody represents a compact version of an IgG1, with the cytotoxicity of the scFv moiety on target cells, combined with the ability of the Fc moiety to induce both antibody- and complement-dependent cytotoxicity. Here, we describe the main properties of Erb-hcAb, using as a reference Herceptin, an anti-ErbB2 humanized monoclonal currently employed in clinical immunotherapy. We found that both bivalent Erb-hcAb and Herceptin increase receptor phosphorylation and downregulation, whereas monovalent Erbicin does not. These results correlate with the finding that Erb-hcAb is capable of inducing apoptosis and inhibiting cell cycle progression in ErbB2-positive cells. Its powerful in vitro antitumour action matched that observed in vivo in experiments with human ErbB2-positive tumour xenografts established in athymic mice. Finally, Erb-hcAb displays a glycosylation profile virtually superimposable to that of a human IgG. These findings suggest that Erb-hcAb is a very promising new agent for the immunotherapy of carcinomas that overexpress the ErbB2 receptor.

EGF-induced proliferation of adult human pancreatic duct cells is mediated by the MEK/ERK cascade

Human postnatal pancreatic duct cells are a potential source of new beta cells. Factors regulating proliferation of human pancreatic duct cells in vitro are unknown. In several other cell types, this process is influenced by ligands of the ErbB receptor family. The expression and functionality of the ErbB family members and their possible role in duct cell proliferation were determined. In cultured adult human pancreatic duct cells the different members of the ErbB family (ErbB1-4) were present at transcript and protein level. Stimulation of the duct cells with epidermal growth factor (EGF) and betacellulin results in Tyr-phosphorylation of ErbB1 and ErbB2, followed by activation of Shc, MEK1/2 and ERK1/2. Duct cells with activated ErbB signaling changed morphology and motility. EGF induced proliferation of a fraction of the duct cells and treatment with PD98059 prevented Ki67 expression in EGF-supplemented cells. When transduced with recombinant adenovirus expressing constitutively activated MEK1, duct cells proliferate and spread even in the absence of EGF. Importantly, the adult human duct cells retain their capacity to recapitulate ngn3-induced embryonic (neuro)endocrine differentiation after proliferation. Therefore, the present data support a possible role for human adult pancreatic duct cells, following expansion and transdifferentiation, as a source of insulin by transplantation to type I diabetes patients.

Aberrant Epidermal Growth Factor Receptor Signaling and Enhanced Sensitivity to EGFR Inhibitors in Lung Cancer

Epidermal growth factor receptor (EGFR) is occasionally amplified and/or mutated in non-small cell lung cancer (NSCLC) and can be coexpressed with other members of the HER receptor family to form functional heterodimers. We therefore investigated lung cancer cell lines for alterations in EGFR gene copy number, enhanced expression of EGFR and other HER family members, and EGFR coding sequence mutations and correlated these findings with response to treatment with the EGFR inhibitors and the kinetics of ligand-induced signaling. We show here that somatic deletions in the tyrosine kinase domain of EGFR were associated with increased EGFR gene copy number in NSCLC. Treatment with the specific EGFR tyrosine kinase inhibitors (TKI) gefitinib or erlotinib or the EGFR inhibitory antibody cetuximab induced apoptosis of HCC827, a NSCLC cell line with EGFR gene amplification and an exon 19 deletion. H1819, a NSCLC cell line that expresses high levels of EGFR, ErbB2, and ErbB3 but has wild-type EGFR, showed intermediate sensitivity to TKIs. In both cell lines, ligand-induced receptor tyrosine phosphorylation was delayed and prolonged and AKT was constitutively phosphorylated (but remained inhibitable by EGFR TKI). Thus, in addition to EGFR mutations, other factors in NSCLC cells, such as high expression of ErbB family members, may constitutively activate AKT and sensitize cells to EGFR inhibitors.

G1 cell cycle arrest due to the inhibition of erbB family receptor tyrosine kinases does not require the retinoblastoma protein

The erbB receptor family (EGFr, erbB-2, erbB-3, and erbB-4) consists of transmembrane glycoproteins that transduce extracellular signals to the nucleus when activated. erbB family members are widely expressed in epithelial, mesenchymal, and neuronal cells and contribute to the proliferation, differentiation, migration, and survival of these cell types. The present study evaluates the effects of erbB family signaling on cell cycle progression and the role that pRB plays in regulating this process. ErbB family RTK activity was inhibited by PD 158780 in the breast epithelial cell line MCF10A. PD 158780 (0.5 μM) inhibited EGF-stimulated and heregulin-stimulated autophosphorylation and caused a G1 cell cycle arrest within 24 h, which correlated with hypophosporylation of pRB. MCF10A cells lacking functional pRB retained the ability to arrest in G1 when treated with PD 158780. Both cell lines showed induction of p27 KIP1 protein when treated with PD 158780 and increased association of p27 KIP1 with cyclin E–CDK2. Furthermore, CDK2 kinase activity was dramatically inhibited with drug treatment. Changes in other pRB family members were noted with drug treatment, namely a decrease in p107 and an increase in p130. These findings show that the G1 arrest induced through inhibition of erbB family RTK activity does not require functional pRB.

Co-expression of ErbB family members in human breast cancer: Her-2/neu is the preferred dimerization candidate in nodal-positive tumors

9522 Background: Overexpression of members of the ErbB receptor family has been asssociated with malignant transformation and the amplification of Her-2/neu in tumor tissue is now an established prognostic factor in breast cancer. In order to initiate signal transduction, ErbB receptor monomers need to form homo- or hetero- dimers. The composition of these dimers is thought to greatly influence both quality and quantity of downstream signalling pathways, and to ultimately determine the elicited biological response. Methods: The protein expression pattern of all four ErbB receptors EGF-R, Her-2/neu, Her-3 and Her-4, and expression of their putative ligands EGF, TGF-α and HRG was investigated in 74 women with invasive breast cancer by membrane isolation and Western blot analysis. Results: The co-expression of all four ErbB family members was detected in 79,7% of cases, and of all of the three investigated ligands in 82,4%. While we did not observe a correlation between EGFR and Her-2/neu or Her-4 protein expression, EGFR and Her-3 (p=0.005), and Her-3 and Her-4 (p=0.05) were clearly co-expressed. The strongest overall correlation, however, was found between Her-2/neu and Her-3 (p=0.001) and between Her-2/neu and Her-4 (p=0.001). This was particularly true in nodal-positive tumors (p<0.001 and p=0.002, respectively) whereas in nodal-negative tumors the co-expression was either less significant (Her-2/neu and Her-3: p=0.01) or not significant at all (Her-2/neu and Her-4). The co-expression of EGFR/Her-3 was associated with the expression of all three ligands, whereas the Her-2/neu/Her-3 was correlated with HRG (p=0.002), thereby indicating a functional relation between specific receptor-dimer combinations and putative ligands. Conclusion: Taken together, we have performed a comprehensive survey of ErbB system expression in breast cancer, and have demonstrated the presence of a co-regulated receptor/ligand system in vivo. Her-2/neu appears to function as the preferred co-expression partner in nodal-positive tumors, thus rendering him the most likely dimerization candidate in malignant breast tumors. No significant financial relationships to disclose.

Co-expression of ErbB family members in human breast cancer: Her-2/neu is the preferred dimerization candidate in nodal-positive tumors

Co-expression of ErbB family members in human breast cancer: Her-2/neu is the preferred dimerization candidate in nodal-positive tumors

The ErbB/HER receptor protein-tyrosine kinases and cancer*1

The ErbB/HER protein-tyrosine kinases, which include the epidermal growth factor receptor, consist of a growth-factor-binding ectodomain, a single transmembrane segment, an intracellular protein-tyrosine kinase catalytic domain, and a tyrosine-containing cytoplasmic tail. The genes for the four members of this family, ErbB1–ErbB4, are found on different human chromosomes. Null mutations of any of the ErbB family members result in embryonic lethality. ErbB1 and ErbB2 are overexpressed in a wide variety of tumors including breast, colorectal, ovarian, and non-small cell lung cancers. The structures of the ectodomains of the ErbB receptors in their active and inactive conformation have shed light on the mechanism of receptor activation. The extracellular component of the ErbB proteins consists of domains I–IV. The activating growth factor, which binds to domains I and III, selects and stabilizes a conformation that allows a dimerization arm to extend from domain II to interact with an ErbB dimer partner. As a result of dimerization, protein kinase activation, trans-autophosphorylation, and initiation of signaling occur. The conversion of the inactive to active receptor involves a major rotation of the ectodomain. The ErbB receptors are targets for anticancer drugs. Two strategies for blocking the action of these proteins include antibodies directed against the ectodomain and drugs that inhibit protein-tyrosine kinase activity. A reversible ATP competitive inhibitor of ErbB1 (ZD1839, or Iressa) and an ErbB1 ectodomain directed antibody (IMC-C225, or Erbitux) have been approved for the treatment of non-small cell lung cancer and colorectal cancer, respectively. An ErbB2/HER2 ectodomain directed antibody (trastuzumab, or Herceptin) has also been approved for the treatment of breast cancer. Current research promises to produce additional agents based upon these approaches.

Expression of erbB receptors mRNA in thyroid tissues.

ErbB family members, such as epidermal growth factor receptor 1 (erbB1), erbB2, erbB3 and erbB4, are widely distributed in organ tissues, and these receptors are suspected tumorigenesis factors. We measured erbB mRNA in thyroid tissues of benign and malignant thyroid tumors or Graves' disease using Genescan. ErbB2 is associated with aggressive cancers and is used as a biological marker for the disease; Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses have shown it to be increased in Graves' disease. Additional studies indicated a similar result in papillary carcinoma cells; mRNAs of erbB2 and erbB3 were increased but erbB4 mRNA was decreased, suggesting distorted erbB expression may be associated with tumorigenesis. However, only erbB2 overexpression is associated with Graves' disease. These data further implicate transmembrane-type receptors in tumorigenesis in the thyroid.

Expression of erbB receptors mRNA in thyroid tissues

ErbB family members, such as epidermal growth factor receptor 1 (erbB1), erbB2, erbB3 and erbB4, are widely distributed in organ tissues, and these receptors are suspected tumorigenesis factors. We measured erbB mRNA in thyroid tissues of benign and malignant thyroid tumors or Graves' disease using Genescan. ErbB2 is associated with aggressive cancers and is used as a biological marker for the disease; Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses have shown it to be increased in Graves' disease. Additional studies indicated a similar result in papillary carcinoma cells; mRNAs of erbB2 and erbB3 were increased but erbB4 mRNA was decreased, suggesting distorted erbB expression may be associated with tumorigenesis. However, only erbB2 overexpression is associated with Graves' disease. These data further implicate transmembrane-type receptors in tumorigenesis in the thyroid.

Expression of ErBb family members and levels of phosphorylated ErBb2 in breast tumours

Expression of ErBb family members and levels of phosphorylated ErBb2 in breast tumours

MUC1: A multifunctional cell surface component of reproductive tissue epithelia

MUC1 is a large, transmembrane mucin glycoprotein expressed at the apical surface of a variety of reproductive tract epithelia. Functions attributed to MUC1 include those generally associated with mucins such as lubrication and hydration of cell surfaces as well as protection from microorganisms and degradative enzymes. In addition, MUC1 is an effective inhibitor of both cell-cell and cell-extracellular matrix interactions in both normal and malignant contexts. Moreover, a series of recent studies has shown that the highly conserved cytoplasmic tail of MUC1 interacts specifically with a series of important signal transducing molecules including β-catenin, Grb2 and erbB family members. MUC1 expression in normal epithelia can be quite dynamic, varying in response to steroid hormone or cytokine influences. Following malignant transformation, MUC1 often becomes highly overexpressed, loses its apical restriction, and displays aberrant glycosylation and altered mRNA splice variants. Regulation of MUC1 expression can occur at the transcriptional level. In addition, post-translational regulation of cell surface expression occurs via the activity of cell surface proteases or "sheddases" that release soluble forms of the large ectodomains. This review will briefly summarize studies of MUC1 expression and function in reproductive tissues with particular emphasis on the uterus. In addition, current knowledge of the mechanisms of MUC1 gene regulation, metabolic processing and potential signal transducing functions will be presented.

CI-1033 An pan Erb-b tyrosine kinase inhibitor (TKI) induces apoptosis and radiosenstization in human glioblastoma cells

Purpose/Objective: Glioblastoma Multiforme (GBM) is the most common malignant intracranial neoplasm and presents a challenge to oncologists. Investigators have shown expression and amplification of EGFR and its mutant form EGFRvIII in GBM both in-vitro and in-vivo. CI-1033 is an irreversible TKI of all 4 ErbB family members. We hypothesized that CI-1033 would be effective against GBM via blockade of EGFR signaling and in turn produce radiosensitivity.

Extracellular domain determinants of LET-23 (EGF) receptor tyrosine kinase activity in Caenorhabditis elegans.

Negative regulation of ErbB/EGFR signalling pathways is important for normal development and the prevention of cancer. In a genetic screen to uncover mechanisms that negatively regulate ErbB signalling in Caenorhabditis elegans, we isolated a second-site mutation (sy621) that promotes the activity of a gain-of-function allele (sa62gf) of the let-23 (EGF) receptor tyrosine kinase. We show that activation by the sa62 mutation (C359Y) likely results from a break in the conserved disulphide-bonded eighth module at the junction of CR1 and L2. The sy621 mutation causes a G270E change in the third disulphide-bonded module of CR1, and causes no phenotype on its own, but cooperates with the sa62 mutation to promote receptor activity. Although both sa62 single- and double-mutant receptors can function in the absence of ligand, they can be further activated by ligand. Our results support the current model for ligand-induced dimerization based on the recent crystal structures of HER3 and the EGFR, and provide more evidence for the generation of distinctly activated ErbB family members through mutation.

Co-expression of ErbB-family members in human breast cancer: Her-2/neu is the preferred dimerization candidate in nodal-positive tumors.

Over-expression of members of the ErbB-receptor family has been associated with malignant transformation. The amplification of Her-2/neu in tumor tissue is now an established prognostic factor in breast cancer. In order to initiate signal transduction, ErbB-receptor monomers need to form homo- or heterodimers. The composition of these dimers is thought to influence both quality and quantity of downstream signaling pathways, and to determine the biological response. We have investigated the protein expression pattern of the four ErbB-receptors EGFR, Her-2/neu, Her-3 and Her-4, and correlated it with their putative ligands EGF, TGF-alpha and HRG in 74 women with invasive breast cancer. Using western blot-analysis on cell membrane isolates, we detected the co-expression of all four ErbB-family members in 79.7% of cases, and of all of the three investigated ligands in 82.4%. We did not observe a correlation between EGFR and Her-2/neu or Her-4 protein expression, EGFR and Her-3 (p = 0.005), and Her-3 and Her-4 (p = 0.05) were clearly co-expressed. The strongest overall correlation, was found between Her-2/neu and Her-3 (p < 0.001) and between Her-2/neu and Her-4 (p = 0.001). This was particularly true in nodal-positive tumors (p < 0.001 and p = 0.002) whereas in nodal-negative tumors the co-expression was either less significant (Her-2/neu and Her-3; p = 0.01) or not significant (Her-2/neu and Her-4). The co-expression of EGFR/Her-3 was associated with the expression of all ligands, whereas the Her-2/neu/Her-3 was correlated with HRG (p = 0.002), thereby indicating a functional relation between specific receptor-dimer combinations and putative ligands. Taken together, we have performed the first comprehensive survey of ErbB-system expression in breast cancer, and have demonstrated the presence of a co-regulated receptor/ligand system in vivo. We have further shown that Her-2/neu is the preferred co-expression partner in nodal-positive tumors and thus the most likely dimerization candidate in malignant breast tumors.