Increased malignancy of Neu-induced mammary tumors overexpressing active transforming growth factor beta1

TGFbeta in Cancer

The transforming growth factor beta (TGFbeta) signaling pathway is a key player in metazoan biology, and its misregulation can result in tumor development. The regulatory cytokine TGFbeta exerts tumor-suppressive effects that cancer cells must elude for malignant evolution. Yet, paradoxically, TGFbeta also modulates processes such as cell invasion, immune regulation, and microenvironment modification that cancer cells may exploit to their advantage. Consequently, the output of a TGFbeta response is highly contextual throughout development, across different tissues, and also in cancer. The mechanistic basis and clinical relevance of TGFbeta's role in cancer is becoming increasingly clear, paving the way for a better understanding of the complexity and therapeutic potential of this pathway.

TGFbeta in Cancer

The transforming growth factor beta (TGFbeta) signaling pathway is a key player in metazoan biology, and its misregulation can result in tumor development. The regulatory cytokine TGFbeta exerts tumor-suppressive effects that cancer cells must elude for malignant evolution. Yet, paradoxically, TGFbeta also modulates processes such as cell invasion, immune regulation, and microenvironment modification that cancer cells may exploit to their advantage. Consequently, the output of a TGFbeta response is highly contextual throughout development, across different tissues, and also in cancer. The mechanistic basis and clinical relevance of TGFbeta's role in cancer is becoming increasingly clear, paving the way for a better understanding of the complexity and therapeutic potential of this pathway.

Drug development against metastasis-related genes and their pathways: a rationale for cancer therapy

It is well recognized that the majority of cancer related deaths is caused by metastatic diseases. Therefore, there is an urgent need for the development of therapeutic intervention specifically targeted to the metastatic process. In the last decade, significant progress has been made in this research field, and many new concepts have emerged that shed light on the molecular mechanism of metastasis cascade which is often portrayed as a succession of six distinct steps; localized invasion, intravasation, translocation, extravasation, micrometastasis and colonization. Successful metastasis is dependent on the balance and complex interplay of both the metastasis promoters and suppressors in each step. Therefore, the basic strategy of our interventions is aimed at either blocking the promoters or potentiating the suppressors in this disease process. Toward this goal, various kinds of antibodies and small molecules have been designed. These include agents that block the ligand-recepter interaction of metastasis promoters (HGF/c-Met), antagonize the metastasis-promoting enzymes (AMF, uPA and MMP) and inhibit the transcriptional activity of metastasis promoter (beta-Catenin). On the other hand, the intriguing roles of metastasis suppressors and their signal pathways have been extensively studied and various attempts have been made to potentiate these factors. Small molecules have been developed to restore the expression or mimic the function of metastasis-suppressor genes such as NM23, E-cadherin, Kiss-1, MKK4 and NDRG1, and some of them are under clinical trials. This review summarizes our current understanding of the molecular pathway of tumor metastasis and discusses strategies and recent development of anti-metastatic drugs.

Transforming growth factor beta engages TACE and ErbB3 to activate phosphatidylinositol-3 kinase/Akt in ErbB2-overexpressing breast cancer and desensitizes cells to trastuzumab

In HER2-overexpressing mammary epithelial cells, transforming growth factor beta (TGF-beta) activated phosphatidylinositol-3 kinase (PI3K)/Akt and enhanced survival and migration. Treatment with TGF-beta or expression of an activated TGF-beta type I receptor (Alk5 with the mutation T204D [Alk5(T204D)]) induced phosphorylation of TACE/ADAM17 and its translocation to the cell surface, resulting in increased secretion of TGF-alpha, amphiregulin, and heregulin. In turn, these ligands enhanced the association of p85 with ErbB3 and activated PI3K/Akt. RNA interference of TACE or ErbB3 prevented TGF-beta-induced activation of Akt and cell invasiveness. Treatment with TGF-beta or expression of Alk5(T204D) in HER2-overexpressing cells reduced their sensitivity to the HER2 antibody trastuzumab. Inhibition of Alk5, PI3K, TACE, or ErbB3 restored sensitivity to trastuzumab. A gene signature induced by Alk5(T204D) expression correlated with poor clinical outcomes in patients with invasive breast cancer. These results suggest that by acting on ErbB ligand shedding, an excess of TGF-beta may result in (i) conditioning of the tumor microenvironment with growth factors that can engage adjacent stromal and endothelial cells; (ii) potentiation of signaling downstream ErbB receptors, thus contributing to tumor progression and resistance to anti-HER2 therapies; and (iii) poor clinical outcomes in women with breast cancer.

Metastatic cancer cell

Metastasis is the result of cancer cell adaptation to a tissue microenvironment at a distance from the primary tumor. Metastatic cancer cells require properties that allow them not only to adapt to a foreign microenvironment but to subvert it in a way that is conducive to their continued proliferation and survival. Recent conceptual and technological advances have contributed to our understanding of the role of the host tissue stroma in promoting tumor cell growth and dissemination and have provided new insight into the genetic makeup of cancers with high metastatic proclivity.

The receptor tyrosine kinase EphA2 promotes mammary adenocarcinoma tumorigenesis and metastatic progression in mice by amplifying ErbB2 signaling

Overexpression of the receptor tyrosine kinase EPH receptor A2 (EphA2) is commonly observed in aggressive breast cancer and correlates with a poor prognosis. However, while EphA2 has been reported to enhance tumorigenesis, proliferation, and MAPK activation in several model systems, other studies suggest that EphA2 activation diminishes these processes and inhibits the activity of MAPK upon ligand stimulation. In this study, we eliminated EphA2 expression in 2 transgenic mouse models of mammary carcinoma. EphA2 deficiency impaired tumor initiation and metastatic progression in mice overexpressing ErbB2 (also known as Neu) in the mammary epithelium (MMTV-Neu mice), but not in mice overexpressing the polyomavirus middle T antigen in mammary epithelium (MMTV-PyV-mT mice). Histologic and ex vivo analyses of MMTV-Neu mouse mammary epithelium indicated that EphA2 enhanced tumor proliferation and motility. Biochemical analyses revealed that EphA2 formed a complex with ErbB2 in human and murine breast carcinoma cells, resulting in enhanced activation of Ras-MAPK signaling and RhoA GTPase. Additionally, MMTV-Neu, but not MMTV-PyV-mT, tumors were sensitive to therapeutic inhibition of EphA2. These data suggest that EphA2 cooperates with ErbB2 to promote tumor progression in mice and may provide a novel therapeutic target for ErbB2-dependent tumors in humans. Moreover, EphA2 function in tumor progression appeared to depend on oncogene context, an important consideration for the application of therapies targeting EphA2.

Signaling through ShcA is required for transforming growth factor beta- and Neu/ErbB-2-induced breast cancer cell motility and invasion

Cooperation between the Neu/ErbB-2 and transforming growth factor beta (TGF-beta) signaling pathways enhances the invasive and metastatic capabilities of breast cancer cells; however, the underlying mechanisms mediating this synergy have yet to be fully explained. We demonstrate that TGF-beta induces the migration and invasion of mammary tumor explants expressing an activated Neu/ErbB-2 receptor, which requires signaling from autophosphorylation sites located in the C terminus. A systematic analysis of mammary tumor explants expressing Neu/ErbB-2 add-back receptors that couple to distinct signaling molecules has mapped the synergistic effect of TGF-beta-induced motility and invasion to signals emanating from tyrosine residues 1226/1227 and 1253 of Neu/ErbB-2. Given that the ShcA adaptor protein is known to interact with Neu/ErbB-2 through these residues, we investigated the importance of this signaling molecule in TGF-beta-induced cell motility and invasion. The reduction of ShcA expression rendered cells expressing activated Neu/ErbB-2, or add-back receptors signaling specifically through tyrosines 1226/1227 or 1253, unresponsive to TGF-beta-induced motility and invasion. In addition, a dominant-negative form of ShcA, lacking its three known tyrosine phosphorylation sites, completely abrogates the TGF-beta-induced migration and invasion of breast cancer cells expressing activated Neu/ErbB-2. Our results implicate signaling through the ShcA adaptor as a key component in the synergistic interaction between these pathways.

Abrogation of TGF beta signaling in mammary carcinomas recruits Gr-1+CD11b+ myeloid cells that promote metastasis

Aberrant TGFbeta signaling is common in human cancers and contributes to tumor metastasis. Here, we demonstrate that Gr-1+CD11b+ myeloid cells are recruited into mammary carcinomas with type II TGF beta receptor gene (Tgfbr2) deletion and directly promote tumor metastasis. Gr-1+CD11b+ cells infiltrate into the invasive front of tumor tissues and facilitate tumor cell invasion and metastasis through a process involving metalloproteinase activity. This infiltration of Gr-1+CD11b+ cells also results in increased abundance of TGF beta 1 in tumors with Tgfbr2 deletion. The recruitment of Gr-1+CD11b+ cells into tumors with Tgfbr2 deletion involves two chemokine receptor axes, the SDF-1/CXCR4 and CXCL5/CXCR2 axes. Together, these data indicate that Gr-1+CD11b+ cells contribute to TGFbeta-mediated metastasis through enhancing tumor cell invasion and metastasis.

Modeling morphogenesis and oncogenesis in three-dimensional breast epithelial cultures

Three-dimensional (3D) epithelial culture systems recreate the cardinal features of glandular epithelium in vivo and represent a valuable tool for modeling breast cancer initiation and progression in a structurally appropriate context. 3D models have emerged as a powerful method to interrogate the biological activities of cancer genes and oncogenic pathways, and recent studies have poignantly illustrated their utility in dissecting the emerging role of tensional force in regulating epithelial tissue homeostasis. We review how 3D models are being used to investigate fundamental cellular and biophysical mechanisms associated with breast cancer progression that have not been readily amenable to traditional genetic or biochemical analysis.

Pathobiology of transforming growth factor beta in cancer, fibrosis and immunologic disease, and therapeutic considerations

Transforming growth factor beta (TGF-beta) is a highly pleiotropic cytokine that plays an important role in wound healing, angiogenesis, immunoregulation and cancer. The cells of the immune system produce the TGF-beta1 isoform, which exerts powerful anti-inflammatory functions, and is a master regulator of the immune response. However, this is context dependent, because TGF-beta can contribute to the differentiation of both regulatory (suppressive) T cells (Tr cells) and inflammatory Th17 cells. While TGF-beta might be underproduced in some autoimmune diseases, it is overproduced in many pathological conditions. This includes pulmonary fibrosis, glomerulosclerosis, renal interstitial fibrosis, cirrhosis, Crohn's disease, cardiomyopathy, scleroderma and chronic graft-vs-host disease. In neoplastic disease, TGF-beta suppresses the progression of early lesions, but later this effect is lost and cancer cells produce TGF-beta, which then promotes metastasis. This cytokine also contributes to the formation of the tumor stroma, angiogenesis and immunosuppression. In view of this, several approaches are being studied to inhibit TGF-beta activity, including neutralizing antibodies, soluble receptors, receptor kinase antagonist drugs, antisense reagents and a number of less specific drugs such as angiotensin II antagonists and tranilast. It might be assumed that TGF-beta blockade would result in severe inflammatory disease, but this has not been the case, presumably because the neutralization is only partial. In contrast, the systemic administration of TGF-beta for therapeutic purposes is limited by toxicity and safety concerns, but local administration appears feasible, especially to promote wound healing. Immunotherapy or vaccination stimulating TGF-beta production and/or Tr differentiation might be applied to the treatment of autoimmune diseases. The benefits of new therapies targeting TGF-beta are under intense investigation.

Modeling metastatic breast cancer in mice

Metastatic disease is the major cause of death in breast cancer patients. Patients presenting with metastases cannot be cured, and as a consequence, treatment is palliative and focuses on prolonging survival and maintaining quality of life. Numerous mouse models have been generated in which human breast cancer development and metastasis have been studied, ranging from spontaneous and carcinogen-induced models to transplantation models and genetically engineered mouse models. Here, we summarize past progress and highlight present developments in modeling breast cancer invasion and metastasis in genetically modified mice, and the impact it may have on the development of innovative anticancer therapies.

Insights from transgenic mouse models of ERBB2-induced breast cancer

One-third of patients with breast cancer overexpress the ERBB2 receptor tyrosine kinase, which is associated not only with a more aggressive phenotype but also reduced responsiveness to hormonal therapies. Over the past two decades, many ERBB2 mouse models for breast cancer have conclusively shown that this receptor has a causal role in breast cancer development. These mouse models have also enabled the mechanisms controlling tumour growth, angiogenesis, metastasis, dormancy and recurrence in ERBB2-positive breast cancer to be elucidated. In addition, a mouse model has recently been described that accurately recapitulates many of the hallmarks associated with the early stages of the human disease.

Inhibition of TGF-beta with neutralizing antibodies prevents radiation-induced acceleration of metastatic cancer progression

We investigated whether TGF-beta induced by anticancer therapies accelerates tumor progression. Using the MMTV/PyVmT transgenic model of metastatic breast cancer, we show that administration of ionizing radiation or doxorubicin caused increased circulating levels of TGF-beta1 as well as increased circulating tumor cells and lung metastases. These effects were abrogated by administration of a neutralizing pan-TGF-beta antibody. Circulating polyomavirus middle T antigen-expressing tumor cells did not grow ex vivo in the presence of the TGF-beta antibody, suggesting autocrine TGF-beta is a survival signal in these cells. Radiation failed to enhance lung metastases in mice bearing tumors that lack the type II TGF-beta receptor, suggesting that the increase in metastases was due, at least in part, to a direct effect of TGF-beta on the cancer cells. These data implicate TGF-beta induced by anticancer therapy as a pro-metastatic signal in tumor cells and provide a rationale for the simultaneous use of these therapies in combination with TGF-beta inhibitors.

Transforming growth factor-beta in cancer and metastasis

Transforming growth factor-beta (TGF-beta) is a multifunctional regulatory polypeptide that is the prototypical member of a large family of cytokines that controls many aspects of cellular function, including cellular proliferation, differentiation, migration, apoptosis, adhesion, angiogenesis, immune surveillance, and survival. The actions of TGF-beta are dependent on several factors including cell type, growth conditions, and the presence of other polypeptide growth factors. One of the biological effects of TGF-beta is the inhibition of proliferation of most normal epithelial cells using an autocrine mechanism of action, and this suggests a tumor suppressor role for TGF-beta. Loss of autocrine TGF-beta activity and/or responsiveness to exogenous TGF-beta appears to provide some epithelial cells with a growth advantage leading to malignant progression. This suggests a pro-oncogenic role for TGF-beta in addition to its tumor suppressor role. During the early phase of epithelial tumorigenesis, TGF-beta inhibits primary tumor development and growth by inducing cell cycle arrest and apoptosis. In late stages of tumor progression when tumor cells become resistant to growth inhibition by TGF-beta due to inactivation of the TGF-beta signaling pathway or aberrant regulation of the cell cycle, the role of TGF-beta becomes one of tumor promotion. Resistance to TGF-beta-mediated inhibition of proliferation is frequently observed in multiple human cancers, as are various alterations in the complex TGF-beta signaling and cell cycle pathways. TGF-beta can exert effects on tumor and stromal cells as well as alter the responsiveness of tumor cells to TGF-beta to stimulate invasion, angiogenesis, and metastasis, and to inhibit immune surveillance. Because of the dual role of TGF-beta as a tumor suppressor and pro-oncogenic factor, members of the TGF-beta signaling pathway are being considered as predictive biomarkers for progressive tumorigenesis, as well as molecular targets for prevention and treatment of cancer and metastasis.

Functional proteomics approach to investigate the biological activities of cDNAs implicated in breast cancer

Functional proteomics approaches that comprehensively evaluate the biological activities of human cDNAs may provide novel insights into disease pathogenesis. To systematically investigate the functional activity of cDNAs that have been implicated in breast carcinogenesis, we generated a collection of cDNAs relevant to breast cancer, the Breast Cancer 1000 (BC1000), and conducted screens to identify proteins that induce phenotypic changes that resemble events which occur during tumor initiation and progression. Genes were selected for this set using bioinformatics and data mining tools that identify genes associated with breast cancer. Greater than 1000 cDNAs were assembled and sequence verified with high-throughput recombination-based cloning. To our knowledge, the BC1000 represents the first publicly available sequence-validated human disease gene collection. The functional activity of a subset of the BC1000 collection was evaluated in cell-based assays that monitor changes in cell proliferation, migration, and morphogenesis in MCF-10A mammary epithelial cells expressing a variant of ErbB2 that can be inducibly activated through dimerization. Using this approach, we identified many cDNAs, encoding diverse classes of cellular proteins, that displayed activity in one or more of the assays, thus providing insights into a large set of cellular proteins capable of inducing functional alterations associated with breast cancer development.

Extracellular proteolysis in transgenic mouse models of breast cancer

Growth and invasion of breast cancer require extracellular proteolysis in order to physically restructure the tissue microenvironment of the mammary gland. This pathological tissue remodeling process depends on a collaboration of epithelial and stromal cells. In fact, the majority of extracellular proteases are provided by stromal cells rather than cancer cells. This distinct expression pattern is seen in human breast cancers and also in transgenic mouse models of breast cancer. The similar expression patterns suggest that transgenic mouse models are ideally suited to study the role of extracellular proteases in cancer progression. Here we give a status report on protease intervention studies in transgenic models. These studies demonstrate that proteases are involved in all stages of breast cancer progression from carcinogenesis to metastasis. Transgenic models are now beginning to provide vital mechanistic insight that will allow us to combat breast cancer invasion and metastasis with new protease-targeted drugs.

Convergence of p53 and transforming growth factor beta (TGFbeta) signaling on activating expression of the tumor suppressor gene maspin in mammary epithelial cells

Using two-dimensional difference gel electrophoresis, we identified the tumor suppressor gene maspin as a transforming growth factor beta (TGFbeta) target gene in human mammary epithelial cells. TGFbeta up-regulatesMaspin expression both at the RNA and protein levels. This up-regulation required Smad2/3 function and intact p53-binding elements in the Maspin promoter. DNA affinity immunoblot and chromatin immunoprecipitation revealed the presence of both Smads and p53 at the Maspin promoter in TGFbeta-treated cells, suggesting that both transcription factors cooperate to induce Maspin transcription. TGFbeta did not activate Maspin-luciferase reporter in p53-mutant MDA-MB-231 breast cancer cells, which exhibit methylation of the endogenous Maspin promoter. Expression of ectopic p53, however, restored ligand-induced association of Smad2/3 with a transfected Maspin promoter. Stable transfection of Maspin inhibited basal and TGFbeta-stimulated MDA-MB-231 cell motility. Finally, knockdown of endogenous Maspin in p53 wild-type MCF10A/HER2 cells enhanced basal and TGFbeta-stimulated motility. Taken together, these data support cooperation between the p53 and TGFbeta tumor suppressor pathways in the induction of Maspin expression, thus leading to inhibition of cell migration.

Mouse models of breast cancer metastasis

Metastatic spread of cancer cells is the main cause of death of breast cancer patients, and elucidation of the molecular mechanisms underlying this process is a major focus in cancer research. The identification of appropriate therapeutic targets and proof-of-concept experimentation involves an increasing number of experimental mouse models, including spontaneous and chemically induced carcinogenesis, tumor transplantation, and transgenic and/or knockout mice. Here we give a progress report on how mouse models have contributed to our understanding of the molecular processes underlying breast cancer metastasis and on how such experimentation can open new avenues to the development of innovative cancer therapy.

Transcriptional Cooperation between the Transforming Growth Factor-β and Wnt Pathways in Mammary and Intestinal Tumorigenesis

Transforming growth factor-beta (TGF-beta) and Wnt ligands function in numerous developmental processes, and alterations of both signaling pathways are associated with common pathologic conditions, including cancer. To obtain insight into the extent of interdependence of the two signaling cascades in regulating biological responses, we used an oligonucleotide microarray approach to identify Wnt and TGF-beta target genes using normal murine mammary gland epithelial cells as a model. Combination treatment of TGF-beta and Wnt revealed a novel transcriptional program that could not have been predicted from single ligand treatments and included a cohort of genes that were cooperatively induced by both pathways. These included both novel and known components or modulators of TGF-beta and Wnt pathways, suggesting that mutual feedback is a feature of the coordinated activities of the ligands. The majority of the cooperative targets display increased expression in tumors derived from either Min (many intestinal neoplasia) or mouse mammary tumor virus (MMTV)-Wnt1 mice, two models of Wnt-induced tumors, with nine of these genes (Ankrd1, Ccnd1, Ctgf, Gpc1, Hs6st2, IL11, Inhba, Mmp14, and Robo1) showing increases in both. Reduction of TGF-beta signaling by expression of a dominant-negative TGF-beta type II receptor in bigenic MMTV-Wnt1/DNIIR mice increased mammary tumor latency and was correlated with a decrease in expression of Gpc1, Inhba, and Robo1, three of the TGF-beta/Wnt cooperative targets. Our results indicate that the TGF-beta and Wnt/beta-catenin pathways are firmly intertwined and generate a unique gene expression pattern that can contribute to tumor progression.

Multivariable difference gel electrophoresis and mass spectrometry: a case study on transforming growth factor-beta and ERBB2 signaling

Multivariable DIGE/MS was used to investigate proteins altered in expression and/or post-translational modification in response to activation of transforming growth factor (TGF)-beta receptors in MCF10A mammary epithelial cells overexpressing the HER2/Neu (ErbB2) oncogene. Proteome changes were monitored in response to exogenous TGF-beta over time (0, 8, 24, and 40 h), and proteins were resolved using medium range (pH 4-7) and narrow range (pH 5.3-6.5) isoelectric focusing combined with up to 2 mg of protein to allow inspection of lower abundance proteins. Triplicate samples were prepared independently and analyzed together across multiple DIGE gels using a pooled sample internal standard to quantify expression changes with statistical confidence. Unsupervised principle component analysis and hierarchical clustering of the individual DIGE proteome expression maps provided independent confirmation of distinct expression patterns from the individual experiments and demonstrated high reproducibility between replicate samples. Fifty-nine proteins (including some isoforms) that exhibited significant kinetic expression changes were identified using mass spectrometry and database interrogation and were mapped to existing biological networks involved in TGF-beta signaling. Several proteins with a potential role in breast cancer, such as maspin and cathepsin D, were identified as novel molecules associated with TGF-beta signaling.

HER2/Neu (ErbB2) Signaling to Rac1-Pak1 Is Temporally and Spatially Modulated by Transforming Growth Factor β

In HER2 (ErbB2)-overexpressing cells, transforming growth factor beta (TGF-beta), via activation of phosphoinositide-3 kinase (PI3K), recruits actin and actinin to HER2, which then colocalizes with Vav2, activated Rac1, and Pak1 at cell protrusions. This results in prolonged Rac1 activation, enhanced motility and invasiveness, Bad phosphorylation, uncoupling of Bad/Bcl-2, and enhanced cell survival. The recruitment of the HER2/Vav2/Rac1/Pak1/actin/actinin complex to lamellipodia was abrogated by actinin siRNAs, dominant-negative (dn) p85, gefitinib, and dn-Rac1 or dn-Pak1, suggesting that the reciprocal interplay of PI3K, HER2 kinase, and Rac GTPases with the actin cytoskeleton is necessary for TGF-beta action in oncogene-overexpressing cells. Thus, by recruiting the actin skeleton, TGF-beta "cross-links" this signaling complex at cell lamellipodia; this prolongs Rac1 activation and increases metastatic properties and survival of HER2-overexpressing cells.

Novel functional single nucleotide polymorphisms in the latent transforming growth factor-beta binding protein-1L promoter: effect on latent transforming growth factor-beta binding protein-1L expression level and possible prognostic significance in ovarian cancer

Latent transforming growth factor (TGF)-beta binding proteins (LTBPs) play important roles in the secretion and activation of TGF-beta. We previously reported that LTBP-1L is overexpressed in some patients with ovarian cancer. To clarify the molecular mechanism of LTBP-1L regulation, we analyzed DNA sequences in the promoter region of LTBP-1L and identified two novel single nucleotide polymorphisms, -202G/C and +20A/C. While the alleles with -202C and +20C were initially reported, our data demonstrated that -202G and +20A are common in both ovarian cancer patients and healthy patients in the Japanese population. Luciferase reporter assays revealed that the G-A haplotype induced transcriptional activation in a Sp1-dependent manner. Electrophoretic mobility shift assays showed that increased binding affinity of Sp1 to the promoter with -202G and +20A. Interestingly, ovarian cancer patients (n = 42) with G-A/G-A homozygous genotype had increased expression of LTBP-1 and apparently poorer survival than those with other genotypes (P = 0.02). These findings suggest that the single nucleotide polymorphisms -202G/C and +20A/C on the LTBP-1L promoter may affect the clinical outcome of ovarian cancer patients, probably via up-regulating protein expression. Further studies using a larger number of samples will definitively determine the correlation between LTBP-1 haplotype and clinical behavior of ovarian cancer.

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Autocrine PDGFR signaling promotes mammary cancer metastasis

Metastasis is the major cause of cancer morbidity, but strategies for direct interference with invasion processes are lacking. Dedifferentiated, late-stage tumor cells secrete multiple factors that represent attractive targets for therapeutic intervention. Here we show that metastatic potential of oncogenic mammary epithelial cells requires an autocrine PDGF/PDGFR loop, which is established as a consequence of TGF-beta-induced epithelial-mesenchymal transition (EMT), a faithful in vitro correlate of metastasis. The cooperation of autocrine PDGFR signaling with oncogenic Ras hyperactivates PI3K and is required for survival during EMT. Autocrine PDGFR signaling also contributes to maintenance of EMT, possibly through activation of STAT1 and other distinct pathways. Inhibition of PDGFR signaling interfered with EMT and caused apoptosis in murine and human mammary carcinoma cell lines. Consequently, overexpression of a dominant-negative PDGFR or application of the established cancer drug STI571 interfered with experimental metastasis in mice. Similarly, in mouse mammary tumor virus-Neu (MMTV-Neu) transgenic mice, TGF-beta enhanced metastasis of mammary tumors, induced EMT, and elevated PDGFR signaling. Finally, expression of PDGFRalpha and -beta correlated with invasive behavior in human mammary carcinomas. Thus, autocrine PDGFR signaling plays an essential role during cancer progression, suggesting a novel application of STI571 to therapeutically interfere with metastasis.

Actions of TGF-beta as tumor suppressor and pro-metastatic factor in human cancer

Transforming growth factor-beta (TGF-beta) is a secreted polypeptide that signals via receptor serine/threonine kinases and intracellular Smad effectors. TGF-beta inhibits proliferation and induces apoptosis in various cell types, and accumulation of loss-of-function mutations in the TGF-beta receptor or Smad genes classify the pathway as a tumor suppressor in humans. In addition, various oncogenic pathways directly inactivate the TGF-beta receptor-Smad pathway, thus favoring tumor growth. On the other hand, all human tumors overproduce TGF-beta whose autocrine and paracrine actions promote tumor cell invasiveness and metastasis. Accordingly, TGF-beta induces epithelial-mesenchymal transition, a differentiation switch that is required for transitory invasiveness of carcinoma cells. Tumor-derived TGF-beta acting on stromal fibroblasts remodels the tumor matrix and induces expression of mitogenic signals towards the carcinoma cells, and upon acting on endothelial cells and pericytes, TGF-beta regulates angiogenesis. Finally, TGF-beta suppresses proliferation and differentiation of lymphocytes including cytolytic T cells, natural killer cells and macrophages, thus preventing immune surveillance of the developing tumor. Current clinical approaches aim at establishing novel cancer drugs whose mechanisms target the TGF-beta pathway. In conclusion, TGF-beta signaling is intimately implicated in tumor development and contributes to all cardinal features of tumor cell biology.

Tumour microenvironment: TGFbeta: the molecular Jekyll and Hyde of cancer

Transforming growth factor-beta (TGFbeta) signalling regulates cancer through mechanisms that function either within the tumour cell itself or through host-tumour cell interactions. Studies of tumour-cell-autonomous TGFbeta effects show clearly that TGFbeta signalling has a mechanistic role in tumour suppression and tumour promotion. In addition, factors in the tumour microenvironment, such as fibroblasts, immune cells and the extracellular matrix, influence the ability of TGFbeta to promote or suppress carcinoma progression and metastasis. The complex nature of TGFbeta signalling and crosstalk in the tumour microenvironment presents a unique challenge, and an opportunity to develop therapeutic intervention strategies for targeting cancer.

Defining the role of prolactin as an invasion suppressor hormone in breast cancer cells

Prolactin hormone (PRL) is well characterized as a terminal differentiation factor for mammary epithelial cells and as an autocrine growth/survival factor in breast cancer cells. However, this function of PRL may not fully signify its role in breast tumorigenesis. Cancer is a complex multistep progressive disease resulting not only from defects in cell growth but also in cell differentiation. Indeed, dedifferentiation of tumor cells is now recognized as a crucial event in invasion and metastasis. PRL plays a critical role in inducing/maintaining differentiation of mammary epithelial cells, suggesting that PRL signaling could serve to inhibit tumor progression. We show here that in breast cancer cells, PRL and Janus-activated kinase 2, a major kinase involved in PRL signaling, play a critical role in regulating epithelial-mesenchymal transformation (EMT), an essential process associated with tumor metastasis. Activation of the PRL receptor (PRLR), achieved by restoring PRL/JAK2 signaling in mesenchymal-like breast cancer cells, MDA-MB-231, suppressed their mesenchymal properties and reduced their invasive behavior. While blocking PRL autocrine function in epithelial-like breast cancer cells, T47D, using pharmacologic and genetic approaches induced mesenchymal-like phenotypic changes and enhanced their invasive propensity. Moreover, our results indicate that blocking PRL signaling led to activation of mitogen-activated protein kinase (extracellular signal-regulated kinase 1/2) and transforming growth factor-beta/Smad signaling pathways, two major prometastatic pathways. Furthermore, our results indicate that following PRL/JAK2 inhibition, ERK1/2 activation precedes and is required for Smad2 activation and EMT induction in breast cancer cells. Together, these results highlight PRL as a critical regulator of epithelial plasticity and implicate PRL as an invasion suppressor hormone in breast cancer.

The ErbB2 signaling network as a target for breast cancer therapy

Overexpression of the ErbB2/Her2 receptor tyrosine kinase in breast cancers is associated with the most aggressive tumors. Experimental studies have revealed that ErbB2 shows many features of a therapeutic target: ErbB2 is able to confer many of the characteristics of a cancerous cell, including uncontrolled proliferation, resistance to apoptosis and increased motility; ErbB2 overexpression is specific to tumor cells; as a cell surface-associated protein, it is easily accessible to drugs and as a kinase it is amenable to targeted inhibition by small molecules. Recent clinical results demonstrate the efficacy of ErbB2-targeting therapy and promise an expanding use of ErbB2-targeting drugs for breast cancer treatment. However, as only a fraction of patients responds successfully to therapy and risks of recurrence are still high, further investigation is required for an improved understanding of the complex network of signaling pathways underlying ErbB2-driven cancer progression.

Pro-metastasis function of TGFβ mediated by the smad pathway

The transforming growth factor beta (TGFbeta) signaling pathway plays a vital role in the development and homeostasis of normal tissues. Abnormal function of this pathway contributes to the initiation and progression of cancer. Smad proteins are key signal transducers of the TGFbeta pathway and are essential for the growth suppression function of TGFbeta. Smads are bona fide tumor suppressors whose mutation, deletion, and silencing are associated with many types of human cancer. However, the involvement and functional mechanism of Smad proteins in cancer metastasis are poorly defined. Recent studies using genetically modified cancer cells and mouse tumor models have provided concrete evidence for a Smad-dependent mechanism for metastasis promotion by TGFbeta. Understanding the dual roles of Smad proteins in tumor initiation and progression has important implications for cancer therapeutics.

Inhibition of TGFbeta signaling in cancer therapy

Recent evidence continues to support a central role for TGFbeta in tumor maintenance and progression. Although this may involve TGFbeta-mediated paracrine effects that modulate the tumor microenvironment and the host immune system, some studies causally implicate autocrine TGFbeta in cancer cell motility and survival. Other recent evidence indicates synergy between oncogene and TGFbeta signaling in epithelial cell transformation. This suggests opportunities for dissecting molecular mechanisms of cross-talk as well as providing insights into possible combinatorial molecular anticancer therapies that will include TGFbeta inhibitors.

Identification of an epithelial-specific enhancer regulating ESX expression

The Ets transcription factor, ESX, exhibits a unique pattern of epithelial-restricted expression and transactivates genes involved in epithelial differentiation and cancer. The aim of this study was to determine the underlying genetic basis for epithelial-specific expression of ESX. We have identified a 30bp ESX enhancer sequence (EES) approximately 3 kb upstream of the proximal promoter. This region displays enhancer activity in an epithelial-specific manner and deletion of this region abrogates ESX gene transcription. An EES binding protein complex (EBC) was identified through electrophoretic mobility shift assays whose degree of EES binding correlated well with endogenous ESX levels in epithelial cells and was regulated by epithelial differentiation. Understanding the regulation of this element will lend insight into mechanisms of epithelial differentiation and the etiology of breast cancer and may provide novel targets for cancer therapeutic intervention.

HER-2 overexpression differentially alters transforming growth factor-beta responses in luminal versus mesenchymal human breast cancer cells

Introduction

Amplification of the HER-2 receptor tyrosine kinase has been implicated in the pathogenesis and aggressive behavior of approximately 25% of invasive human breast cancers. Clinical and experimental evidence suggest that aberrant HER-2 signaling contributes to tumor initiation and disease progression. Transforming growth factor beta (TGF-β) is the dominant factor opposing growth stimulatory factors and early oncogene activation in many tissues, including the mammary gland. Thus, to better understand the mechanisms by which HER-2 overexpression promotes the early stages of breast cancer, we directly assayed the cellular and molecular effects of TGF-β1 on breast cancer cells in the presence or absence of overexpressed HER-2.

Methods

Cell proliferation assays were used to determine the effect of TGF-β on the growth of breast cancer cells with normal or high level expression of HER-2. Affymetrix microarrays combined with Northern and western blot analysis were used to monitor the transcriptional responses to exogenous TGF-β1 in luminal and mesenchymal-like breast cancer cells. The activity of the core TGF-β signaling pathway was assessed using TGF-β1 binding assays, phospho-specific Smad antibodies, immunofluorescent staining of Smad and Smad DNA binding assays.

Results

We demonstrate that cells engineered to over-express HER-2 are resistant to the anti-proliferative effect of TGF-β1. HER-2 overexpression profoundly diminishes the transcriptional responses induced by TGF-β in the luminal MCF-7 breast cancer cell line and prevents target gene induction by a novel mechanism that does not involve the abrogation of Smad nuclear accumulation, DNA binding or changes in c-myc repression. Conversely, HER-2 overexpression in the context of the mesenchymal MDA-MB-231 breast cell line potentiated the TGF-β induced pro-invasive and pro-metastatic gene signature.

Conclusion

HER-2 overexpression promotes the growth and malignancy of mammary epithelial cells, in part, by conferring resistance to the growth inhibitory effects of TGF-β. In contrast, HER-2 and TGF-β signaling pathways can cooperate to promote especially aggressive disease behavior in the context of a highly invasive breast tumor model.

TGF-beta 1 could be a missing link in the interplay between ER and HER-2 in breast cancer

Breast cancer is a very heterogeneous disease considering a number of biomarkers which are under investigation. However, most important biomarkers in a clinical setting are ER (estrogen receptors) and HER-2 (human epidermal growth factor receptor 2), but still only as predictive markers for tamoxifen and trastuzumab therapy. Their prognostic role is still subject of investigation. On the other hand, although, TGF-beta1 is known as marker of invasiveness and metastatic capacity of breast cancer cells, this marker has never been considered to be introduced in routine clinical setting. TGF-beta1 is ER regulated biomarker which act synergistically with HER-2. Aim of this study is to show that clinical significance of determination ER and HER-2 status could be improved, when they are related with TGF-beta1 as additional biomarker. It seems that TGF-beta1 could make a difference regarding prognosis and prediction in breast cancer patients. This hypothesis could be easily verified in corresponding clinical research.

Activated type I TGFbeta receptor kinase enhances the survival of mammary epithelial cells and accelerates tumor progression

We have examined the effects of transforming growth factor-beta (TGFbeta) signaling on mammary epithelial cell survival. Transgenic mice expressing an active mutant of Alk5 in the mammary gland (MMTV-Alk5(T204D)) exhibited reduced apoptosis in terminal endbuds and during postlactational involution. Transgene-expressing mammary cells contained lower Smad2/3 and higher c-myc levels than controls, high ligand-independent phosphatidylinositol-3 kinase (PI3K) and Akt activities, and were insensitive to TGFbeta-mediated growth arrest. Treatment with a proteasome inhibitor increased Smad2/3 levels and ligand-independent Smad transcriptional reporter activity, as well as reduced both c-myc protein and basal cell proliferation. Treatment with an Alk5 kinase small-molecule inhibitor upregulated Smad2/3 levels, reduced PI3K activity, P-Akt, and c-myc, and inhibited cell survival. Although Alk5(T204D)-expressing mice did not develop mammary tumors, bigenic MMTV-Alk(T204D) x Neu mice developed cancers that were more metastatic than those occurring in MMTV-Neu transgenics. These data suggest that (1) TGFbeta can signal to PI3K/Akt and enhance mammary epithelial cell survival in vivo before cytological or histological evidence of transformation, and (2) TGFbeta signaling can provide epithelial cells with a 'gain-of-function' effect that synergizes with oncogene-induced transformation.

Modelling glandular epithelial cancers in three-dimensional cultures

Little is known about how the genotypic and molecular abnormalities associated with epithelial cancers actually contribute to the histological phenotypes observed in tumours in vivo. 3D epithelial culture systems are a valuable tool for modelling cancer genes and pathways in a structurally appropriate context. Here, we review the important features of epithelial structures grown in 3D basement membrane cultures, and how such models have been used to investigate the mechanisms associated with tumour initiation and progression.

Disruption of the TGF-beta pathway and modeling human cancer in mice

There is considerable complexity underlying the mechanisms through which the TGF-beta signaling pathway regulates the initiation and progression of cancer. Analysis of this pathway and the role that it plays in human malignancy continues to elucidate novel mechanisms through which various genetic and epigenetic events subvert the controls that TGF-beta exerts over cell growth, differentiation, and malignant transformation. Modeling these events in the mouse represents an important goal, as the relevant preclinical models are essential not only for improving our understanding of the role of the TGF-beta pathway in the molecular pathogenesis of cancer, but also as tools for evaluating the impact of novel therapeutics on TGF-beta signaling and the role they may play in the prevention and treatment of malignancies. Here, we consider highlights from a number of in vivo murine model systems and relate a few of the significant observations to what we know about TGF-beta signaling in human cancer.

Gene expression profiling of cancer progression reveals intrinsic regulation of transforming growth factor-beta signaling in ErbB2/Neu-induced tumors from transgenic mice

Upregulation of HER2/ErbB2/Neu occurs in 15-30% of human breast cancers and correlates with poor prognosis. Identification of ErbB2/Neu transcriptional targets should facilitate development of novel therapeutic approaches. Development of breast cancer is a multistep process; thus, to identify the transcriptomes associated with different stages of progression of tumorigenesis, we compared expression profiles of mammary tumors and preneoplastic mammary tissue from MMTV-Neu transgenic mice to expression profiles of wild-type mammary glands using Affymetrix microarrays. We identified 324 candidate genes that were unique to ErbB2/Neu-induced tumors relative to normal mammary gland tissue from wild-type controls. Expression of a subset of these genes (82) was also changed in the preneoplastic mammary glands compared to wild-type controls, indicating that they may play a pivotal role during early events of ErbB2/Neu-initiated mammary tumorigenesis. Further analysis of the microarray data revealed that expression of several known transforming growth factor (TGF)-beta target genes was altered, suggesting that the TGF-beta signaling cascade is downregulated in ErbB2/Neu-induced tumors. Western blot analysis for TGF-beta-Receptor-I/ALK5 and immunohistochemistry for TGF-beta-Receptor-I/ALK5 and phosphorylated/activated Smad2 confirmed that the Smad-dependent TGF-beta signaling cascade was inactive in these tumors. Although absent in most of the tumor, phosphorylated Smad2 was present in the periphery of tumors. Interestingly, presence of phosphorylated/activated Smad2 correlated with expression of Activin-Receptor-IB/ALK4, suggesting that although Smad-dependent TGF-beta signaling is absent in ErbB2/Neu-induced tumors, Activin signaling may be active at the leading edge of these tumors. Cumulatively, these data indicate that the TGF-beta pathway is intrinsically suppressed in ErbB2/Neu tumors via a mechanism involving loss of TGF-beta-Receptor-I/ALK5.

Transforming growth factor {beta} (TGF-{beta})-Smad target gene protein tyrosine phosphatase receptor type kappa is required for TGF-{beta} function

Transforming growth factor beta (TGF-beta) inhibits proliferation and promotes cell migration. In TGF-beta-treated MCF10A mammary epithelial cells overexpressing HER2 and by chromatin immunoprecipitation, we identified novel Smad targets including protein tyrosine phosphatase receptor type kappa (PTPRK). TGF-beta up-regulated PTPRK mRNA and RPTPkappa (receptor type protein tyrosine phosphatase kappa, the protein product encoded by the PTPRK gene) protein in tumor and nontumor mammary cells; HER2 overexpression down-regulated its expression. RNA interference (RNAi) of PTPRK accelerated cell cycle progression, enhanced response to epidermal growth factor (EGF), and abrogated TGF-beta-mediated antimitogenesis. Endogenous RPTPkappa associated with EGF receptor and HER2, resulting in suppression of basal and ErbB ligand-induced proliferation and receptor phosphorylation. In MCF10A/HER2 cells, TGF-beta enhanced cell motility, FAK phosphorylation, F-actin assembly, and focal adhesion formation and inhibited RhoA activity. These responses were abolished when RPTPkappa was eliminated by RNA interference (RNAi). In cells expressing RPTPkappa RNAi, phosphorylation of Src at Tyr527 was increased and (activating) phosphorylation of Src at Tyr416 was reduced. These data suggest that (i) RPTPkappa positively regulates Src; (ii) HER2 signaling and TGF-beta-induced RPTPkappa converge at Src, providing an adequate input for activation of FAK and increased cell motility and adhesion; and (iii) RPTPkappa is required for both the antiproliferative and the promigratory effects of TGF-beta.

Sensitized RNAi screen of human kinases and phosphatases identifies new regulators of apoptosis and chemoresistance

Evasion from apoptosis is a hallmark of cancer, and recent success using targeted therapeutics underscores the importance of identifying anti-apoptotic survival pathways. Here we utilize RNA interference (RNAi) to systematically screen the kinase and phosphatase component of the human genome. In addition to known kinases, we identified several new survival kinases. Interestingly, numerous phosphatases and associated regulatory subunits contribute to cell survival, revealing a previously unrecognized general role for phosphatases as negative regulators of apoptosis. We also identified a subset of phosphatases with tumour-suppressor-like activity. Finally, RNAi targeting of specific protein kinases sensitizes resistant cells to chemotherapeutic agents. The development of inhibitors that target these kinases or phosphatases may lead to new anti-cancer strategies.

Breast cancer cells induce stromal fibroblasts to express MMP-9 via secretion of TNF-alpha and TGF-beta

We used 2D-cocultures employing fibroblasts of different genetic backgrounds and MCF10A-derived human breast epithelial cells of increasingly malignant potential to investigate tumor-stroma interactions in breast cancer and to identify possible signaling pathways involved. Tumor cells induced expression of matrix-metalloproteinase 9 (MMP-9) in fibroblasts in a pattern dependent on the degree of their malignancy. In-situ zymography localized the main gelatinolytic activity around stromal cells in cocultures and xenografted tumors. Use of Smad3 knockout fibroblasts, small molecule inhibitors, and neutralizing antibodies showed that MMP-9 expression was induced by tumor cell-derived TNF-alpha and TGF-beta, dependent on Smad-, Ras-, and PI3-kinase-signaling, and likewise modulated by subsequent HGF- and EGF-signaling. Together, our results indicate that MMP-9 levels in tumor fibroblasts are regulated by a complex tumor-stroma cross-talk, involving multiple ligands and cellular signaling pathways.

Involvement of EGF receptor and c-Src in the survival signals induced by TGF-β1 in hepatocytes

Transforming growth factor beta1 (TGF-beta1) belongs to a family of polypeptide factors, whose cytostatic and apoptotic functions help restrain the growth of mammalian cells. Although solid data established the role of TGF-beta's as suppressor factors in tumorigenic processes, in the context of an advanced stage of disease, TGF-beta's could also play a pro-oncogenic role. We have previously shown that TGF-beta1 induces both pro- and anti-apoptotic signals in foetal rat hepatocytes. In this work, we have focused on its anti-apoptotic mechanism. We show that TGF-beta1 activates the epidermal growth factor receptor (EGFR) and phosphorylates c-Src. EGFR is required for Akt activation. Blocking EGFR signalling amplifies the apoptotic response to TGF-beta1. TGF-beta1 induced a rapid activation of the tumour necrosis factor-alpha-converting enzyme (TACE/ADAM (a disintegrin and metalloprotease) 17). Inhibitors of TACE considerably attenuated Akt activation, which suggests that TGF-beta1 activates EGF signalling in hepatocytes by promoting shedding of EGF-like ligands. The activation of c-Src by TGF-beta1 is EGFR dependent and is required for full Akt phosphorylation and cell survival. Inhibition of EGFR does not block the epithelial-mesenchymal transition (EMT) induced by TGF-beta1 in hepatocytes, which indicates that activation of EGFR plays an essential role in impairing apoptosis, but it is dispensable for the EMT process.

Conditional overexpression of active transforming growth factor beta1 in vivo accelerates metastases of transgenic mammary tumors

To address the role of transforming growth factor (TGF) beta in the progression of established tumors while avoiding the confounding inhibitory effects of TGF-beta on early transformation, we generated doxycycline (DOX)-inducible triple transgenic mice in which active TGF-beta1 expression could be conditionally regulated in mouse mammary tumor cells transformed by the polyomavirus middle T antigen. DOX-mediated induction of TGF-beta1 for as little as 2 weeks increased lung metastases >10-fold without a detectable effect on primary tumor cell proliferation or tumor size. DOX-induced active TGF-beta1 protein and nuclear Smad2 were restricted to cancer cells, suggesting a causal association between autocrine TGF-beta and increased metastases. Antisense-mediated inhibition of TGF-beta1 in polyomavirus middle T antigen-expressing tumor cells also reduced basal cell motility, survival, anchorage-independent growth, tumorigenicity, and metastases. Therefore, induction and/or activation of TGF-beta in hosts with established TGF-beta-responsive cancers can rapidly accelerate metastatic progression.

Development of TGF-beta signalling inhibitors for cancer therapy

The transforming growth factor-beta (TGF-beta) superfamily of ligands has a pivotal role in the regulation of a wide variety of physiological processes from development to pathogenesis. Since the discovery of the prototypic member, TGF-beta, almost 20 years ago, there have been tremendous advances in our understanding of the complex biology of this superfamily. Deregulation of TGF-beta has been implicated in the pathogenesis of a variety of diseases, including cancer and fibrosis. Here we present the rationale for evaluating TGF-beta signalling inhibitors as cancer therapeutics, the structures of small-molecule inhibitors that are in development and the targeted drug discovery model that is being applied to their development.

Early changes in protein expression detected by mass spectrometry predict tumor response to molecular therapeutics

Biomarkers that predict therapeutic response are essential for the development of anticancer therapies. We have used matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to directly analyze protein profiles in mouse mammary tumor virus/HER2 transgenic mouse frozen tumor sections after treatment with the erbB receptor inhibitors OSI-774 and Herceptin. Inhibition of tumor cell proliferation and induction of apoptosis and tumor reduction were predicted by a >80% reduction in thymosin beta4 and ubiquitin levels that were detectable after 16 hours of a single drug dose before any evidence of in situ cellular activity. These effects were time- and dose-dependent, and their spatial distribution in the tumor correlated with that of the small-molecule inhibitor OSI-774. In addition, they predicted for therapeutic synergy of OSI-774 and Herceptin as well as for drug resistance. These results suggest that drug-induced early proteomic changes as measured by MALDI-MS can be used to predict the therapeutic response to established and novel therapies.

Dual Role of Transforming Growth Factor β in Mammary Tumorigenesis and Metastatic Progression

It is generally accepted that transforming growth factor β (TGFβ) is both a tumor suppressor and tumor promoter. Whereas loss or attenuation of TGFβ signal transduction is permissive for transformation, introduction of dominant-negative TGFβ receptors into metastatic breast cancer cells has been shown to inhibit epithelial-to-mesenchymal transition, motility, invasiveness, survival, and metastases. In addition, there is evidence that excess production and/or activation of TGFβ by cancer cells can contribute to tumor progression by paracrine mechanisms involving neoangiogenesis, production of stroma and proteases, and subversion of immune surveillance mechanisms in tumor hosts. These data provide a rationale in favor of blockade of autocrine/paracrine TGFβ signaling in human mammary tumors with therapeutic intent. Several treatment approaches are currently in early clinical development and have been the focus of our laboratory. These include (1) ligand antibodies or receptor-containing fusion proteins aimed at blocking ligand binding to cognate receptors and (2) small-molecule inhibitors of the type I TGFβ receptor serine/threonine kinase. Many questions remain about the viability of anti-TGFβ treatment strategies, the best molecular approach (or combinations) for inhibition of TGFβ function in vivo, the biochemical surrogate markers of tumor response, the molecular profiles in tumors for selection into clinical trials, and potential toxicities, among others.

Oncogenes as Novel Targets for Cancer Therapy (Part I)

In the past 10 years, progress made in cancer biology, genetics, and biotechnology has led to a major transition in cancer drug design and development. There has been a change from an emphasis on non-specific, cytotoxic agents to specific, molecular-based therapeutics. Mechanism-based therapy is designed to act on cellular and molecular targets that are causally involved in the formation, growth, and progression of human cancers. These agents, which may have greater selectivity for cancer versus normal cells, and which may produce better anti-tumor efficacy and lower host toxicity, can be small molecules, natural or engineered peptides, proteins, antibodies, or synthetic nucleic acids (e.g. antisense oligonucleotides, ribozymes, and siRNAs). Novel targets are identified and validated by state-of-the-art approaches, including high-throughput screening, combinatorial chemistry, and gene expression arrays, which increase the speed and efficiency of drug discovery and development. Examples of oncogene-based, molecular therapeutics that show promising clinical activity include trastuzumab (Herceptin), imatinib (Gleevec), and gefitinib (Iressa). However, the full potential of oncogenes as novel targets for cancer therapy has not been realized and many challenges remain, from the validation of novel targets, to the design of specific agents, to the evaluation of these agents in both preclinical and clinical settings. In maximizing the benefits of molecular therapeutics in monotherapy or combination therapy of cancer, it is necessary to have an understanding of the underlying molecular abnormalities and mechanisms involved. This is the first part of a four-part review in which we discuss progress made in the last decade as it relates to the discovery of novel oncogenes and signal transduction pathways, in the context of their potential as targets for cancer therapy. This part delineates the latest discoveries about the potential use of growth factors and protein tyrosine kinases as targets for therapy. Later parts focus on intermediate signaling pathways, transcription factors, and proteins involved in cell cycle, DNA damage, and apoptotic pathways.

Use of three-dimensional basement membrane cultures to model oncogene-induced changes in mammary epithelial morphogenesis

The development of breast carcinomas involves a complex set of phenotypic alterations in breast epithelial cells and the surrounding microenvironment. While traditional transformation assays provide models for investigating certain aspects of the cellular processes associated with tumor initiation and progression, they do not model alterations in tissue architecture that are critically involved in tumor development. In this review, we provide examples of how three-dimensional (3D) cell culture models can be utilized to dissect the pathways involved in the development of mammary epithelial structures and to elucidate the mechanisms responsible for oncogene-induced phenotypic alterations in epithelial behavior and architecture. Many normal mammary epithelial cell lines undergo a stereotypic morphogenetic process when grown in the presence of exogenous matrix proteins. This 3D morphogenesis culminates in the formation of well-organized, polarized spheroids, and/or tubules that are highly reminiscent of normal glandular architecture. In contrast, transformed cell lines isolated from mammary tumors exhibit significant deviations from normal epithelial behavior in 3D culture. We describe the use of 3D models as a method for both reconstructing and deconstructing the cell biological and biochemical events involved in mammary neoplasia.