EGF induces the expression of matrilysin in the human prostate adenocarcinoma cell line, LNCaP

Acetylcholine-induced activation of M3 muscarinic receptors stimulates robust matrix metalloproteinase gene expression in human colon cancer cells

Previously, we showed that ACh-induced proliferation of human colon cancer cells is mediated by transactivation of epidermal growth factor (EGF) receptors (EGFRs). In the present study, we elucidate the molecular mechanism underlying this action. ACh-induced proliferation of H508 colon cancer cells, which express exclusively M3 muscarinic receptors (M3Rs), was attenuated by anti-EGFR ligand binding domain antibody, a broad-spectrum matrix metalloproteinase (MMP) inhibitor, anti-MMP7 antibody, a diphtheria toxin analog that blocks release of an EGFR ligand [heparin-binding EGF-like growth factor (HBEGF)], and anti-HBEGF antibody. Conditioned media from ACh-treated H508 cells induced proliferation of SNU-C4 colon cancer cells that express EGFR but not M3R. These actions were attenuated by an EGFR inhibitor and by anti-EGFR and anti-HBEGF antibodies. In H508, but not SNU-C4, colon cancer cells, ACh caused a striking dose- and time-dependent increase in levels of MMP7 mRNA and MMP7 protein. Similarly, ACh induced robust MMP1 and MMP10 gene transcription. ACh-induced MMP1, MMP7, and MMP10 gene transcription was attenuated by atropine, anti-EGFR antibody, and chemical inhibitors of EGFR and ERK activation. In contrast, inhibitors of phosphatidylinositol 3-kinase and NF-kappaB activation did not alter MMP gene transcription. Collectively, these findings indicate that MMP7-catalyzed release of HBEGF mediates ACh-induced transactivation of EGFR and consequent proliferation of colon cancer cells. ACh-induced activation of EGFR and downstream ERK signaling also regulates transcriptional activation of MMP7, thereby identifying a novel feed-forward mechanism for neoplastic cell proliferation.

PEA3 is necessary for optimal epidermal growth factor receptor-stimulated matrix metalloproteinase expression and invasion of ovarian tumor cells

Elevated expression of the epidermal growth factor (EGF) receptor (EGFR) is detected in human ovarian tumors and is associated with decreased recurrence-free and overall survival. EGFR activation affects tumor progression in part by promoting tumor invasion through the induction of prometastatic matrix metalloproteinases (MMP). PEA3, an ETS family transcription factor, is elevated in advanced and metastatic ovarian cancer and regulates MMPs in various cell types, therefore, we investigated whether PEA3 is required for the EGFR-dependent induction of MMP mRNA. MMP-9 and MMP-14 mRNA levels were selectively increased in response to EGFR activity in ovarian tumor cells. EGFR activation resulted in nuclear accumulation of PEA3 and direct binding of PEA3, but not the related protein ETS-1, to the endogenous MMP-9 and MMP-14 promoters. Furthermore, PEA3 overexpression was sufficient to induce MMP-9 and MMP-14 mRNA, tumor cell migration, and invasion, suggesting that PEA3 is an important contributor to the metastatic phenotype. Additionally, inhibition of PEA3 expression via short interfering RNA reduced the EGF induction of MMP-9 and MMP-14 gene expression by 92% and 50%, respectively, and impaired EGF-stimulated tumor cell invasion. These results suggest that PEA3 is regulated by EGFR and that the elevated PEA3 expression detected in human ovarian cancer may divert cells to a more invasive phenotype by regulating MMP-9 and MMP-14.

Coexpression of matrix metalloproteinase-7 (MMP-7) and epidermal growth factor (EGF) receptor in colorectal cancer: an EGF receptor tyrosine kinase inhibitor is effective against MMP-7-expressing cancer cells

PURPOSE

Matrix metalloproteinase-7 (MMP-7) plays an important role in carcinoma invasion and metastasis of cancer. Recent studies focus on diverse roles of MMP-7, other than as a protease, during cancer progression. MMP-7 activates the epidermal growth factor (EGF) receptor by releasing an EGF ligand, tumor growth factor (TGF)-alpha.

EXPERIMENTAL DESIGN

We examined expression of MMP-7 and EGF receptor in an immunohistochemical study of 40 colorectal cancer (CRC) cases. To determine the relationship between the EGF receptor and MMP-7, with a potential curative application, we compared the antitumor activity of the EGF receptor tyrosine kinase inhibitor (gefitinib) between MMP-7 transfectant, KYSE150 and HT29, and control cells.

RESULTS

We found a statistically significant correlation (P = 0.04) between MMP-7 and activated (phosphorylated) EGF receptor expression, both being positive in six (15%) cases. Gefitinib reduced the cell number ratio more for MMP-7 transfectant than mock cells, and the proportion of apoptotic cells was 1.5 times higher in MMP-7 transfectant than mock cells by annexin/propidium iodide staining. This was mediated by activation of a TGF-beta signal as confirmed by the abundant expression of TGF-beta protein, the cytoplasmic to nuclear translocation of Smad4 protein by the administration of gefitinib, and the quantitative assay of the plasminogen activator inhibitor-1 promoter/luciferase construction.

CONCLUSIONS

We propose that there are some cancers with up-regulated MMP-7 expression that leads to the activation of apoptotic activity of TGF-beta, which is susceptible to treatment with EGF receptor tyrosine kinase inhibitor.

Proteolytic-antiproteolytic balance and its regulation in carcinogenesis

Cancer development is essentially a tissue remodeling process in which normal tissue is substituted with cancer tissue. A crucial role in this process is attributed to proteolytic degradation of the extracellular matrix (ECM). Degradation of ECM is initiated by proteases, secreted by different cell types, participating in tumor cell invasion and increased expression or activity of every known class of proteases (metallo-, serine-, aspartyl-, and cysteine) has been linked to malignancy and invasion of tumor cells. Proteolytic enzymes can act directly by degrading ECM or indirectly by activating other proteases, which then degrade the ECM. They act in a determined order, resulting from the order of their activation. When proteases exert their action on other proteases, the end result is a cascade leading to proteolysis. Presumable order of events in this complicated cascade is that aspartyl protease (cathepsin D) activates cysteine proteases (e.g., cathepsin B) that can activate pro-uPA. Then active uPA can convert plasminogen into plasmin. Cathepsin B as well as plasmin are capable of degrading several components of tumor stroma and may activate zymogens of matrix metalloproteinases, the main family of ECM degrading proteases. The activities of these proteases are regulated by a complex array of activators, inhibitors and cellular receptors. In physiological conditions the balance exists between proteases and their inhibitors. Proteolytic-antiproteolytic balance may be of major significance in the cancer development. One of the reasons for such a situation is enhanced generation of free radicals observed in many pathological states. Free radicals react with main cellular components like proteins and lipids and in this way modify proteolytic-antiproteolytic balance and enable penetration damaging cellular membrane. All these lead to enhancement of proteolysis and destruction of ECM proteins and in consequence to invasion and metastasis.

Expression of the disintegrin metalloprotease, ADAM-10, in prostate cancer and its regulation by dihydrotestosterone, insulin-like growth factor I, and epidermal growth factor in the prostate cancer cell model LNCaP

PURPOSE

The disintegrin metalloprotease ADAM-10 is a multidomain metalloprotease that is potentially significant in tumor progression due to its extracellular matrix-degrading properties. Previously, ADAM-10 mRNA was detected in prostate cancer (PCa) cell lines; however, the presence of ADAM-10 protein and its cellular localization, regulation, and role have yet to be described. We hypothesized that ADAM-10 mRNA and protein may be regulated by growth factors such as 5alpha-dihydrotestosterone, insulin-like growth factor I, and epidermal growth factor, known modulators of PCa cell growth and invasion.

EXPERIMENTAL DESIGN

ADAM-10 expression was analyzed by in situ hybridization and immunohistochemistry in prostate tissues obtained from 23 patients with prostate disease. ADAM-10 regulation was assessed using quantitative reverse transcription-PCR and Western blot analysis in the PCa cell line LNCaP.

RESULTS

ADAM-10 expression was localized to the secretory cells of prostate glands, with additional basal cell expression in benign glands. ADAM-10 protein was predominantly membrane bound in benign glands but showed marked nuclear localization in cancer glands. By Western blot, the 100-kDa proform and the 60-kDa active form of ADAM-10 were synergistically up-regulated in LNCaP cells treated with insulin-like growth factor I plus 5alpha-dihydrotestosterone. Epidermal growth factor also up-regulated both ADAM-10 mRNA and protein.

CONCLUSIONS

This study describes for the first time the expression, regulation, and cellular localization of ADAM-10 protein in PCa. The regulation and membrane localization of ADAM-10 support our hypothesis that ADAM-10 has a role in extracellular matrix maintenance and cell invasion, although the potential role of nuclear ADAM-10 is not yet known.

Amphiregulin expression in prostatic intraepithelial neoplasia and adenocarcinoma: a study of 93 cases

BACKGROUND

Amphiregulin (AMP) is a heparin-binding glycoprotein that is structurally and functionally related to epidermal growth factor. Its effects are mediated by the tyrosine kinase activity of the epidermal growth factor receptor (EGF-R), and specific nuclear targeting sequences. AMP induces cell proliferation after androgen stimulation of human prostate cancer cell lines. An autocrine proliferative loop involving AMP, androgen, and EGF-R may, therefore, play a role in prostatic carcinogenesis. The purpose of this study was to compare the expression of AMP in benign prostatic epithelium, high-grade prostatic intraepithelial neoplasia (PIN), and adenocarcinoma.

METHODS

We performed an immunohistochemical study of select sections from 93 radical prostatectomies performed at the Mayo Clinic between 1987 and 1991. All patients were previously untreated and found to have pathologic stage T2N0M0 adenocarcinoma after routine handling of surgical specimens. Affinity-purified polyclonal antibody directed against AMP was applied to tissue sections using the streptavidin-biotin method. For each case, the percentage of immunoreactive cells in benign epithelium, PIN, and adenocarcinoma was estimated in 10% increments. Intensity on a scale from 0 (negative) to 3 (strongly immunoreactive) and pattern of expression (nuclear versus cytoplasmic) also were recorded.

RESULTS

AMP immunoreactivity was present in benign prostatic epithelium, PIN, and prostatic adenocarcinoma in all cases. The mean percentage of AMP-immunoreactive cells was 53.8% in benign epithelium, 65.9% in PIN, and 74.3% in cancer. Intensity was moderate in all cases. The pattern of expression was usually nuclear in benign epithelium (secretory and basal cells), and usually cytoplasmic or nuclear and cytoplasmic in PIN and adenocarcinoma. There were rare scattered immunoreactive cells in the stroma, ejaculatory duct epithelium, and urethral urothelium. Endothelial cells were invariably unstained.

CONCLUSIONS

AMP expression in prostate increases progressively from benign epithelium to PIN and cancer. Increased expression of AMP may contribute to the development of prostatic adenocarcinoma. Predominantly nuclear staining was observed in benign epithelium, whereas cytoplasmic or nuclear and cytoplasmic staining was observed in PIN and adenocarcinoma. The differences in nuclear and cytoplasmic localization of immunoreactivity may reflect the presence of two pathways of activation, and hence varying biological functions of AMP.

Ganglioside GM3 inhibits matrix metalloproteinase-9 activation and disrupts its association with integrin

Gangliosides GM3 and GT1b both inhibit epithelial cell adhesion and migration on fibronectin. GT1b binds to integrin alpha5beta1 and blocks the integrin-fibronectin interaction; GM3 does not interact with integrin, and its effect is poorly understood. We evaluated the effects of endogenous modulation of GM3 expression on epithelial cell motility on several matrices and the mechanism of these effects. Endogenous accumulation of GM3 decreased cell migration on fibronectin, types I, IV, and VII collagen matrices; depletion of GM3 dramatically increased cell migration, regardless of matrix. GM3 overexpression and depletion in vitro correlated inversely with the expression and activity of matrix metalloproteinase-9; consistently, the cell migration stimulated by GM3 depletion is reversed by inhibition of matrix metalloproteinase-9 activity. Accumulation and depletion of GM3 in epithelial cells grown on fibronectin also correlated inversely with epidermal growth factor receptor and mitogen activated protein kinase phosphorylation and with Jun expression. Ganglioside depletion facilitated the co-immunoprecipitation of matrix metal-loproteinase-9 and integrin alpha5beta1, while endogenous accumulation of GM3, but not GT1b, blocked the co-immunoprecipitation. These data suggest modulation of epidermal growth factor receptor signaling and dissociation of integrin/matrix metalloproteinase-9 as mechanisms for the GM3-induced effects on matrix metalloproteinase-9 function.

Regulation of matrix metalloprotease activity in malignant mesothelioma cell lines by growth factors

BACKGROUND

The extracellular matrix metalloproteases (MMPs) produced by tumour and stromal cells are believed to play a key role in tumour cell invasion and metastasis. Malignant mesothelioma is a highly aggressive tumour. Previous studies have shown that malignant mesothelioma cells express MMP-1, -2, -3, -7 and -9. However, the regulation of MMP expression in malignant mesothelioma cells has not been thoroughly investigated.

METHODS

The effects of a number of growth factors on the secretion of MMP-2, -3 and -9 in malignant mesothelioma cells were studied by substrate zymography. The expression of relevant growth factor receptors in malignant mesothelioma cells was also examined using RT-PCR.

RESULTS

The exposure of malignant mesothelioma cells to different growth factors including epidermal growth factor (EGF), transforming growth factor-alpha, amphiregulin, heparin binding EGF, beta-cellulin (BTC), stem cell factor, insulin-like growth factors I and II, acidic and basic fibroblast growth factors, and hepatocyte growth factor increased secretion of MMP-9 and/or MMP-3. EGF receptor ligands BTC and EGF had the most potent effect on MMP-9 and MMP-3 production, respectively. Production of MMP-2 was not affected by any growth factors used in this study. We have also demonstrated mRNA expression of different growth factor receptors in malignant mesothelioma cells, and found that the tyrosine kinase inhibitor genistein inhibited the increased production of MMPs resulting from stimulation with different growth factors.

CONCLUSION

This study shows, for the first time, the broad extent of regulation of MMPs by various growth factors in malignant mesothelioma cells.

Study on the mechanism of epidermal growth factor-induced proliferation of hepatoma cells

AIM: Many growth factors, such as epidermal growth factor (EGF), are associated with the carcinogenesis. EGF plays its role in the proliferation of hepatoma cells through binding with EGF receptor (EGFR) and a series of signal transduction. But the postreceptor pathway is still not clear. In the present experiment, we studied the effect of tyrosine kinase, protein kinase C, Na⁺/H⁺ exchange, calmodulin and voltage-dependent Ca²⁺ channel on EGF-induced hepatoma cell proliferation.

METHODS: Hepatoma cell line SMMC7721 was cultured in RPMI1640 serum-free medium. In order to study the effect of thyrosine kinase, protein kinase C, Na⁺/H⁺ exchange, calmodulin and voltage-dependent Ca²⁺ channel on human heptoma cell proliferation induced by epidermal growth factor (EGF), DNA synthesis rate of hepatoma cells was measured by the method of ³H-TdR incorporation.

RESULTS: EGF (10^-9 M) stimulated the proliferation of heptoma cells significantly (³H-TdR incorporation was 1880 ± 281 cpm/well, P < 0.05), and this effect was significantly inhibited by tyrosine kinase inhibitor genistein (³H-TdR incorporation was 808 ± 209 cpm/well, P < 0.001). Calmodulin inhibitor W-7, protein kinase C inhibitor H-7 and Na⁺/H⁺ exchange inhibitor amiloride individually had significant inhibiting effect on EGF-induced proliferation of hepatoma cells (³H-TdR incorporation was 978 ± 87.3 cpm/well, 1241 ± 147 cpm/well, 1380 ± 189 cpm/well, respectivly, P < 0.001, P < 0.01, P < 0.05), but they all had no effect on the basal level proliferation of cultured hepatoma cells (³H-TdR incorporation was 1284 ± 260 cpm/well, 1179 ± 150 cpm/well, 1392 ± 152 cpm/well, respectivly, ³H-TdR incorporation of the control was 1353 ± 175 cpm/well, P > 0.05). Voltage-dependent Ca²⁺ channel inhibitor verapamil had no inhibition on EGF-induced proliferation of hepatoma cells (³H-TdR incorporation was 1637 ± 133 cpm/well, P > 0.05), it also had no effect on the basal level proliferation of cultured hepatoma cells (³H-TdR incorporation was 1196 ± 12 cpm/well, P > 0.05).

CONCLUSION: Our data suggest that tyrosine kinase, Ca²⁺-calmodulin-dependent pathway, protein kinase C and Na⁺/H⁺ exchange play a critical role in EGF-induced proliferation of hepatoma cells and that the effect of EGF is independent of voltage-dependent Ca²⁺ channel.

Development of matrix metalloproteinase inhibitors in cancer therapy

The matrix metalloproteinases represent an attractive target for cancer treatment, and a number of matrix metalloproteinase inhibitors are undergoing clinical trials. The results of these studies will establish whether any of these compounds are therapeutically useful. Independent of the conclusions from the first generation of studies, the field of matrix metalloproteinase inhibitors remains attractive for creative and innovative research. In the future, the development of novel, less toxic, and more effective matrix metalloproteinase inhibitors, and the combination of conventional agents with these novel anticancer agents will constitute the main focus of research efforts.

Expression profiling of a human cell line model of prostatic cancer reveals a direct involvement of interferon signaling in prostate tumor progression

Cancer-associated fibroblasts induce malignant behavior in genetically initiated but nontumorigenic human prostatic epithelium. The genetic basis for such transformation is still unknown. By using Affymetrix GeneChip technology, we profiled genomewide gene expression of transformed [tumorigenic benign prostatic hyperplasia (BPH1)(CAFTD)] and parental (nontumorigenic BPH1) cells. We identified differentially expressed genes, which are associated with tumorigenesis or tumor progression. One striking finding is that a significant portion of the down-regulated genes belongs to interferon (IFN)-inducible molecules. We show that IFN inhibited the tumorigenic BPH1(CAFTD) cell proliferation and colony formation in vitro and inhibited tumor growth in xenografts in vivo. Expression of the IFN-inducible molecules correlates with the growth-inhibiting effects of IFN. In addition, these genes are reported to be mapped mainly to two chromosomal regions, 10q23-26 and 17q21, which are frequently deleted in human prostate cancers. Furthermore, in silico data-mining with the GeneLogic database revealed that expression of the IFN-inducible genes was down-regulated in approximately 30% of the 49 clinically characterized samples of prostatic adenocarcinomas. Collectively, we show that there seems to be a direct link between IFN-inducible molecules and prostatic tumor progression. These findings suggest IFN-inducible molecules as potential therapeutic targets for the treatment of prostate cancer.

Interleukin-1beta-induced promatrilysin expression is mediated by NFkappaB-regulated synthesis of interleukin-6 in the prostate carcinoma cell line, LNCaP

Previously, our laboratory showed that interleukin-1beta (IL-1beta) secreted by lipopolysaccharide-activated monocytes induces promatrilysin expression in the prostate carcinoma cell line, LNCaP. We now demonstrate that IL-1beta-induced promatrilysin expression is mediated by an indirect mechanism that requires nuclear factor Kappa B (NFkappaB)-dependent synthesis of IL-6. Inhibition of protein synthesis with cycloheximide blocked IL-1beta-mediated induction of matrilysin mRNA suggesting that synthesis of one or more additional factors is required for IL-1beta-induced promatrilysin protein expression. Blockage of NFkappaB transactivation activity abrogated IL-1beta-induced promatrilysin expression to baseline levels suggesting that NFkappaB transactivation activity is necessary. Inhibition of IL-6 activity attenuated IL-1beta-induced promatrilysin, but not NFkappaB transactivation activity indicating that IL-6 acts downstream of NFkappaB in potentiation of IL-1beta-mediated promatrilysin expression. Inhibition of protein synthesis with cycloheximide did not alter IL-6-induced induction of matrilysin mRNA indicating that, contrary to the mechanism by which IL-1beta regulates promatrilysin expression, IL-6-mediated matrilysin mRNA expression does not require new protein synthesis. Transient transfection with dominant negative STAT3 inhibited IL-1beta- and IL-6-induced promatrilysin. These data provide evidence that NFkappaB-mediated IL-6 synthesis is required for IL-1beta-induced promatrilysin expression, and IL-6 signaling through STAT3 plays a role in IL-1beta-induced promatrilysin expression.

Differential regulation of cell proliferation and protease secretion by epidermal growth factor and amphiregulin in tumoral versus normal breast epithelial cells

Amphiregulin (AR) is a heparin-binding epidermal growth factor (EGF)-related peptide that seems to play an important role in mammary epithelial cell growth regulation. We have investigated the regulation of AR-gene expression and -protein secretion by EGF in normal breast epithelial cells (HMECs), as well as in the tumoral breast epithelial cell lines MCF-7 and MDA-MB231. EGF induced a dose-dependent increase of AR mRNA level in both normal and tumoral cells. Thus, 10(-8)M EGF stimulated AR expression in HMECs to 140-300% of control. A similar EGF concentration increased AR mRNA level to 550% and 980% of control in MCF-7 and MDA-MB231 cells, respectively. This was accompanied by an accumulation of AR into conditioned culture media. However, HMECs secreted in response to EGF, 5-10 fold more AR than tumour cells. Furthermore, the potential participation of AR in the regulation of the plasminogen activator (PA)/plasmin system was investigated. Whereas HMEC-proliferation was stimulated by AR, the levels of secreted urokinase-type plasminogen activator (uPA) and type-1 plasminogen activator inhibitor (PAi-1) remained unaffected. Conversely, AR failed to regulate the proliferation of tumoral cell lines but induced an accumulation of uPA and PAi-1 into culture media. This was accompanied by an increase of the number of tumoral cells that invaded matrigel in vitro. Moreover, the presence of a neutralizing anti-uPA receptor antibody reversed the increased invasiveness of MDA-MB231 cells induced by AR. These data reveal differential behaviour of normal versus tumoral breast epithelial cells in regard to the action of AR and demonstrate that, in a number of cases, AR might play a significant role in tumour progression through the regulation of the PA/plasmin system.

Development of matrix metalloproteinase inhibitors in cancer therapy

The matrix metalloproteinases (MMPs) are a family of zinc-dependent proteinases involved in the degradation of the extracellular matrix. The MMPs have been implicated in the processes of tumor growth, invasion, and metastasis; are frequently overexpressed in malignant tumors; and have been associated with an aggressive malignant phenotype and adverse prognosis in patients with cancer. A number of MMP inhibitors are being developed for the treatment of cancer. The most extensively studied class of MMP inhibitors includes collagen peptidomimetics and nonpeptidomimetic inhibitors of the MMP active site, tetracycline derivatives, and bisphosphonates. The hydroxamate peptidomimetic inhibitor batimastat and its orally bioavailable analogue marimastat, which bind covalently to the zinc atom at the MMP-active site, were the first MMP inhibitors to be studied in detail. Marimastat is currently being studied in randomized clinical trials. The nonpeptidic MMP inhibitors were synthesized in an attempt to improve the oral bioavailability and pharmaceutical properties of the peptidic inhibitors. Several members of this class of compounds are undergoing evaluation in phase III clinical trials. The tetracyclines and, particularly, the nonantibiotic chemically modified tetracyclines, interfere with several aspects of MMP expression and activation and inhibit tumor growth and metastases in preclinical models. A representative agent of this class, Col-3, is currently undergoing phase I clinical trials. The development of the MMP inhibitors, like that of other targeted and predominantly antiproliferative compounds, poses a challenge because the paradigms that have governed the design of clinical oncology trials may not be relevant to this new class of agents. The anticipated need for long-term administration of these drugs, together with their cytostatic mechanism of action, will require novel clinical trial design strategies.

Regulation of mouse mammary gland development and tumorigenesis by the ERBB signaling network

The four ERBB receptors and their multiple polypeptide ligands are differentially expressed during development of the mouse mammary gland. Profiles suggest that ERBB1/EGF receptor (EGFR)4 and ERBB2/Neu are required during ductal morphogenesis, whereas the Neuregulin (NRG) receptors, ERBB3 and ERBB4, are preferentially expressed through alveolar morphogenesis and lactation. Consistent with these profiles, recent gene knockouts established that EGFR and its ligand, Amphiregulin (AR), are essential for ductal morphogenesis in the adolescent mouse and likely provide the required epithelial-stromal signal. In contrast, the phenotypes of transgenic mice expressing dominant negative ERBB2 and ERBB4 proteins suggest that these receptors differentially act to promote or maintain alveolar differentiation. This view of ERBB action provides a conceptual framework for future testing using more sophisticated conditional knockout models. New or existing transgenic mice are also being used to better understand the contributions of ERBB receptors and ligands to mammary tumorigenesis, as well as to more closely mimic the human disease. Recent studies have focused on defining molecular events in neoplastic progression, and in the case of ERBB2/Neu, the requirement for ERBB heterodimerization partners as well as the relative importance of gene amplification versus gene mutation. Collectively, these recent studies establish that normal development and homeostasis of the mammary gland is critically dependent on regulated ERBB signaling. They also illustrate the value of animal models in deciphering roles for the complex ERBB network in this dynamic tissue.