TGF-beta1-induced expression of human Mdm2 correlates with late-stage metastatic breast cancer

The E3 ubiquitin ligase human murine double minute (HDM2) is overexpressed in 40%-80% of late-stage metastatic cancers in the absence of gene amplification. Hdm2 regulates p53 stability via ubiquitination and has also been implicated in altering the sensitivity of cells to TGF-beta1. Whether TGF-beta1 signaling induces Hdm2 expression leading to HDM2-mediated destabilization of p53 has not been investigated. In this study, we report that TGF-beta1-activated SMA- and MAD3 (Smad3/4) transcription factors specifically bound to the second promoter region of HDM2, leading to increased HDM2 protein expression and destabilization of p53 in human cancer cell lines. Additionally, TGF-beta1 expression led to Smad3 activation and murine double minute 2 (Mdm2) expression in murine mammary epithelial cells during epithelial-to-mesenchymal transition (EMT). Furthermore, histological analyses of human breast cancer samples demonstrated that approximately 65% of late-stage carcinomas were positive for activated Smad3 and HDM2, indicating a strong correlation between TGF-beta1-mediated induction of HDM2 and late-stage tumor progression. Identification of Hdm2 as a downstream target of TGF-beta1 represents a critical prosurvival mechanism in cancer progression and provides another point for therapeutic intervention in late-stage cancer.

p130Cas is required for mammary tumor growth and transforming growth factor-beta-mediated metastasis through regulation of Smad2/3 activity

During breast cancer progression, transforming growth factor-beta (TGF-beta) switches from a tumor suppressor to a pro-metastatic molecule. Several recent studies suggest that this conversion in TGF-beta function depends upon fundamental changes in the TGF-beta signaling system. We show here that these changes in TGF-beta signaling are concomitant with aberrant expression of the focal adhesion protein, p130Cas. Indeed, elevating expression of either the full-length (FL) or just the carboxyl terminus (CT) of p130Cas in mammary epithelial cells (MECs) diminished the ability of TGF-beta1 to activate Smad2/3, but increased its coupling to p38 MAPK. This shift in TGF-beta signaling evoked (i) resistance to TGF-beta-induced growth arrest, and (ii) acinar filling upon three-dimensional organotypic cultures of p130Cas-FL or -CT expressing MECs. Furthermore, rendering metastatic MECs deficient in p130Cas enhanced TGF-beta-stimulated Smad2/3 activity, which restored TGF-beta-induced growth inhibition both in vitro and in mammary tumors produced in mice. Additionally, whereas elevating TbetaR-II expression in metastatic MECs had no affect on their phosphorylation of Smad2/3, this event markedly enhanced their activation of p38 MAPK, leading to increased MEC invasion and metastasis. Importantly, depleting p130Cas expression in TbetaR-II-expressing metastatic MECs significantly increased their activation of Smad2/3, which (i) reestablished the physiologic balance between canonical and noncanonical TGF-beta signaling, and (ii) reversed cellular invasion and early mammary tumor cell dissemination stimulated by TGF-beta. Collectively, our findings identify p130Cas as a molecular rheostat that regulates the delicate balance between canonical and noncanonical TGF-beta signaling, a balance that is critical to maintaining the tumor suppressor function of TGF-beta during breast cancer progression.

Platelet-derived transforming growth factor-beta down-regulates NKG2D thereby inhibiting natural killer cell antitumor reactivity

Natural killer (NK) cells play an important role in cancer immunosurveillance and may prevent tumor progression and metastasis due to their ability to mediate direct cellular cytotoxicity and by releasing immunoregulatory cytokines, which shape adaptive immune responses. Their reactivity is governed by various activating and inhibitory molecules expressed on target cells and reciprocal interactions with other hematopoietic cells such as dendritic cells. In mice, thrombocytopenia inhibits metastasis, and this is reversed by NK cell depletion, suggesting that platelets are an important additional player in NK cell-tumor interaction. Moreover, it has been shown that metastasizing tumor cells do not travel through the blood alone but are rapidly coated by platelets. However, the knowledge about the molecular mechanisms by which platelets influence NK cells is fragmentary at best. Here we show that platelet-derived soluble factors, secreted on coating of tumor cells or after stimulation with classic platelet agonists, impair NK cell antitumor reactivity resulting in diminished granule mobilization, cytotoxicity, and IFN-gamma production. The impaired NK cell reactivity was not due to induction of apoptosis but mediated by down-regulation of the activating immunoreceptor natural killer group 2, member D (NKG2D) on NK cells by platelet-derived transforming growth factor beta (TGF-beta). Neutralization of TGF-beta in platelet releasate not only prevented NKG2D down-regulation but also restored NK cell antitumor reactivity. Thus, our data elucidate the molecular basis of the previously described influence of platelets on NK cell antitumor reactivity and suggest that therapeutic intervention in tumor cell-platelet interaction and the resulting TGF-beta release by platelets may serve to enhance antitumor immunity.

Hypoxia and TGF-beta drive breast cancer bone metastases through parallel signaling pathways in tumor cells and the bone microenvironment

Background

Most patients with advanced breast cancer develop bone metastases, which cause pain, hypercalcemia, fractures, nerve compression and paralysis. Chemotherapy causes further bone loss, and bone-specific treatments are only palliative. Multiple tumor-secreted factors act on the bone microenvironment to drive a feed-forward cycle of tumor growth. Effective treatment requires inhibiting upstream regulators of groups of prometastatic factors. Two central regulators are hypoxia and transforming growth factor (TGF)- β. We asked whether hypoxia (via HIF-1α) and TGF-β signaling promote bone metastases independently or synergistically, and we tested molecular versus pharmacological inhibition strategies in an animal model.

Methodology/Principal Findings

We analyzed interactions between HIF-1α and TGF-β pathways in MDA-MB-231 breast cancer cells. Only vascular endothelial growth factor (VEGF) and the CXC chemokine receptor 4 (CXCR4), of 16 genes tested, were additively increased by both TGF-β and hypoxia, with effects on the proximal promoters. We inhibited HIF-1α and TGF-β pathways in tumor cells by shRNA and dominant negative receptor approaches. Inhibition of either pathway decreased bone metastasis, with no further effect of double blockade. We tested pharmacologic inhibitors of the pathways, which target both the tumor and the bone microenvironment. Unlike molecular blockade, combined drug treatment decreased bone metastases more than either alone, with effects on bone to decrease osteoclastic bone resorption and increase osteoblast activity, in addition to actions on tumor cells.

Conclusions/Significance

Hypoxia and TGF-β signaling in parallel drive tumor bone metastases and regulate a common set of tumor genes. In contrast, small molecule inhibitors, by acting on both tumor cells and the bone microenvironment, additively decrease tumor burden, while improving skeletal quality. Our studies suggest that inhibitors of HIF-1α and TGF-β may improve treatment of bone metastases and increase survival.

Integrin-mediated transforming growth factor-beta activation, a potential therapeutic target in fibrogenic disorders

A subset of integrins function as cell surface receptors for the profibrotic cytokine transforming growth factor-beta (TGF-beta). TGF-beta is expressed in an inactive or latent form, and activation of TGF-beta is a major mechanism that regulates TGF-beta function. Indeed, important TGF-beta activation mechanisms involve several of the TGF-beta binding integrins. Knockout mice suggest essential roles for integrin-mediated TGF-beta activation in vessel and craniofacial morphogenesis during development and in immune homeostasis and the fibrotic wound healing response in the adult. Amplification of integrin-mediated TGF-beta activation in fibrotic disorders and data from preclinical models suggest that integrins may therefore represent novel targets for antifibrotic therapies.

Lessons in signaling and tumorigenesis from polyomavirus middle T antigen

The small DNA tumor viruses have provided a very long-lived source of insights into many aspects of the life cycle of eukaryotic cells. In recent years, the emphasis has been on cancer-related signaling. Here we review murine polyomavirus middle T antigen, its mechanisms, and its downstream pathways of transformation. We concentrate on the MMTV-PyMT transgenic mouse, one of the most studied models of breast cancer, which permits the examination of in situ tumor progression from hyperplasia to metastasis.

A role for the TGFbeta-Par6 polarity pathway in breast cancer progression

The role of polarity signaling in cancer metastasis is ill defined. Using two three-dimensional culture models of mammary epithelial cells and an orthotopic mouse model of breast cancer, we reveal that Par6 signaling, which is regulated directly by TGFbeta, plays a role in breast cancer metastasis. Interference with Par6 signaling blocked TGFbeta-dependent loss of polarity in acini-like structures formed by non-transformed mammary cells grown in three-dimensional structures and suppressed the protrusive morphology of mesenchymal-like invasive mammary tumor cells without rescuing E-cadherin expression. Moreover, blockade of Par6 signaling in an in vivo orthotopic model of metastatic breast cancer induced the formation of ZO-1-positive epithelium-like structures in the primary tumor and suppressed metastasis to the lungs. Analysis of the pathway in tissue microarrays of human breast tumors further revealed that Par6 activation correlated with markers of the basal carcinoma subtype in BRCA1-associated tumors. These studies thus reveal a key role for polarity signaling and the control of morphologic transformation in breast cancer metastasis.

TGF-beta and fibrosis in different organs - molecular pathway imprints

The action of transforming-growth-factor (TGF)-beta following inflammatory responses is characterized by increased production of extracellular matrix (ECM) components, as well as mesenchymal cell proliferation, migration, and accumulation. Thus, TGF-beta is important for the induction of fibrosis often associated with chronic phases of inflammatory diseases. This common feature of TGF-related pathologies is observed in many different organs. Therefore, in addition to the description of the common TGF-beta-pathway, this review focuses on TGF-beta-related pathogenetic effects in different pathologies/organs, i. e., arthritis, diabetic nephropathy, colitis/Crohn's disease, radiation-induced fibrosis, and myocarditis (including their similarities and dissimilarities). However, TGF-beta exhibits both exacerbating and ameliorating features, depending on the phase of disease and the site of action. Due to its central role in severe fibrotic diseases, TGF-beta nevertheless remains an attractive therapeutic target, if targeted locally and during the fibrotic phase of disease.

Targeting TGF-beta in prostate cancer: therapeutic possibilities during tumor progression

BACKGROUND

TGF-beta regulates prostate growth by inhibiting epithelial cell proliferation and inducing apoptosis through eliciting a dynamic signaling pathway. In metastatic prostate cancer, however, TGF-beta serves as a tumor promoter. TGF-beta engages Smad-dependent and Smad-independent mechanisms to exert its action. During prostate tumorigenesis, prostate cells exhibit loss or mutation of TGF-beta transmembrane receptors. Increased production of TGF-beta causes immunosuppression, extracellular matrix degradation, epithelia to mesenchymal transition and angiogenesis that promotes tumor cell invasion and metastasis.

OBJECTIVE

The molecular basis for effective therapeutic targeting of TGF-beta must be directed towards the double-edge-sword nature of the cytokine: inhibiting the TGF-beta tumor promoter capabilities in advanced metastatic prostate cancer, although retaining the growth-inhibitory abilities exhibited in early stages of prostate tumorigenesis.

RESULTS/CONCLUSION

The current understanding of the therapeutic possibilities of targeting TGF-beta signaling during prostate tumor progression is built on preclinical studies. Studies targeting TGF-beta signaling pathway for the treatment of several human malignancies include the use of neutralizing antibodies, antisense oligonucelotides and small molecule inhibitors of kinase activity of the receptor complex. This review focuses on exploiting the therapeutic potential of targeting TGF-beta signaling in the context of its contribution to prostate cancer initiation and progression to metastasis.

Transforming growth factor-beta induces CD44 cleavage that promotes migration of MDA-MB-435s cells through the up-regulation of membrane type 1-matrix metalloproteinase

CD44, a transmembrane receptor for hyaluronic acid, is implicated in various adhesion-dependent cellular processes, including cell migration, tumor cell metastasis and invasion. Recent studies demonstrated that CD44 expressed in cancer cells can be proteolytically cleaved at the ectodomain by membrane type 1-matrix metalloproteinase (MT1-MMP) to form soluble CD44 and that CD44 cleavage plays a critical role in cancer cell migration. Here, we show that transforming growth factor-beta (TGF-beta), a multifunctional cytokine involved in cell proliferation, differentiation, migration and pathological processes, induces MT1-MMP expression in MDA-MB-435s cells. TGF-beta-induced MT1-MMP expression was blocked by the specific extracellular regulated kinase-1/2 (ERK1/2) inhibitor PD98059 and the specific phosphoinositide 3-OH kinase (PI3K) inhibitor LY294002. In addition, treatment with SP600125, an inhibitor for c-Jun NH(2)-terminal kinase (JNK), resulted in a significant inhibition of MT1-MMP production. These data suggest that ERK1/2, PI3K, and JNK likely play a role in TGF-beta-induced MT1-MMP expression. Interestingly, treatment of MDA-MB-435s cells with TGF-beta resulted in a colocalization of MT1-MMP and CD44 in the cell membrane and in an increased level of soluble CD44. Using an electric cell-substrate impedance sensing cell-electrode system, we demonstrated that TGF-beta treatment promotes MDA-MB-435s cell migration, involving MT1-MMP-mediated CD44 cleavage. MT1-MMP siRNA transfection-inhibited TGF-beta-induced cancer cell transendothelial migration. Thus, this study contributes to our understanding of molecular mechanisms that play a critical role in tumor cell invasion and metastasis.

Diffuse-type gastric carcinoma: progression, angiogenesis, and transforming growth factor beta signaling

Background

Diffuse-type gastric carcinoma is a cancer with poor prognosis that has high levels of transforming growth factor β (TGF-β) expression and thick stromal fibrosis. However, the association of TGF-β signaling with diffuse-type gastric carcinoma has not been investigated in detail.

Methods

We used a lentiviral infection system to express a dominant-negative TGF-β type II receptor (dnTβRII) or green fluorescent protein (GFP) as a control in the diffuse-type gastric carcinoma cell lines, OCUM-2MLN and OCUM-12. These infected cells and the corresponding parental control cells were subcutaneously or orthotopically injected into nude mice. Angiogenesis was inhibited by infecting cells with a lentivirus carrying the gene for angiogenic inhibitor thrombospondin-1 or by injecting mice intraperitoneally with the small-molecule angiogenic inhibitor sorafenib or with anti-vascular endothelial growth factor (VEGF) neutralizing antibody (six or eight mice per group). Expression of phospho-Smad2 and thrombospondin-1 was investigated immunologically in human gastric carcinoma tissues from 102 patients. All statistical tests were two-sided.

Results

Expression of dnTβRII into OCUM-2MLN cells did not affect their proliferation in vitro, but it accelerated the growth of subcutaneously or orthotopically transplanted tumors in vivo (eg, for mean volume of subcutaneous tumors on day 10 relative to that on day 0: dnTβRII tumors = 3.49 and GFP tumors = 2.46, difference = 1.02, 95% confidence interval [CI] = 0.21 to 1.84; P = .003). The tumors expressing dnTβRII had higher levels of angiogenesis than those expressing GFP because of decreased thrombospondin-1 production. Similar results were obtained with OCUM-12 cells. Expression of thrombospondin-1 in the dnTβRII tumor or treatment with sorafenib or anti-VEGF antibody reduced tumor growth, whereas knockdown of thrombospondin-1 expression resulted in more accelerated growth of OCUM-2MLN tumors than of GFP tumors (eg, mean tumor volumes on day 14 relative to those on day 0: thrombospondin-1–knockdown tumors = 4.91 and GFP tumors = 3.79, difference = 1.12, 95% CI = 0.80 to 1.44; P < .001). Positive association between phosphorylated Smad2 and thrombospondin-1 immunostaining was observed in human gastric carcinoma tissues.

Conclusions

Disruption of TGF-β signaling in diffuse-type gastric carcinoma models appeared to accelerate tumor growth, apparently through increased tumor angiogenesis that was induced by decreased expression of thrombospondin-1.

Extracellular matrix-induced gene expression in human breast cancer cells

Extracellular matrix (ECM) molecules modify gene expression through attachment-dependent (focal adhesion-related) integrin receptor signaling. It was previously unknown whether the same molecules acting as soluble peptides could generate signal cascades without the associated mechanical anchoring, a condition that may be encountered during matrix remodeling and degradation and relevant to invasion and metastatic processes. In the current study, the role of ECM ligand-regulated gene expression through this attachment-independent process was examined. It was observed that fibronectin, laminin, and collagen type I and II induce Smad2 activation in MCF-10A and MCF-7 cells. This activation is not caused by transforming growth factor (TGF)-beta ligand contamination or autocrine TGF involvement and is 3- to 5-fold less robust than the TGF-beta1 ligand. The resulting nuclear translocation of Smad4 in response to ECM ligand indicates downstream transcriptional responses occurring. Coimmunoprecipitation experiments determined that collagen type II and laminin act through interaction with integrin alpha(2)beta(1) receptor complex. The ECM ligand-induced Smad activation (termed signaling crosstalk) resulted in cell type and ligand-specific transcriptional changes, which are distinct from the TGF-beta ligand-induced responses. These findings show that cell-matrix communication is more complex than previously thought. Soluble ECM peptides drive transcriptional regulation through corresponding adhesion and non-attachment-related processes. The resultant gene expressional patterns correlate with pathway activity and not by the extent of Smad activation. These results extend the complexity and the existing paradigms of ECM-cell communication to ECM ligand regulation without the necessity of mechanical coupling.

Transforming growth factor beta induces clustering of HER2 and integrins by activating Src-focal adhesion kinase and receptor association to the cytoskeleton

It has been proposed that cross talk between integrin and growth factor receptor signaling such as ErbB2 (HER2) is required for activation of downstream effectors and ErbB2-mediated mammary tumorigenesis. Here we show that transforming growth factor beta (TGF-beta) induced focal adhesion kinase (FAK)-dependent clustering of HER2 and integrins alpha(6), beta(1), and beta(4) in HER2-overexpressing mammary epithelial cells without altering the total and surface levels of HER2 receptors. This effect was mediated by ligand-induced epidermal growth factor receptor (EGFR) activation and the subsequent phosphorylation of Src and FAK. We have previously reported that TGF-beta up-regulates EGFR ligand shedding through a mechanism involving the phosphorylation of tumor necrosis factor-alpha-converting enzyme (TACE/ADAM17). Knockdown of TACE, FAK, or integrin alpha(6) by siRNA or inhibition of EGFR or Src by specific inhibitors abrogated TGF-beta-induced receptor clustering and signaling to phosphatidylinositol 3-kinase-Akt. Finally, inhibition of Src-FAK reversed TGF-beta-induced resistance to the therapeutic HER2 inhibitor trastuzumab in HER2-overexpressing breast cancer cells. Taken together, these data suggest that, by activating Src-FAK, TGF-beta integrates ErbB receptor and integrin signaling to induce cell migration and survival during breast cancer progression.

Disruption of the SRC-1 gene in mice suppresses breast cancer metastasis without affecting primary tumor formation

Steroid receptor coactivator-1 (SRC-1) is a coactivator for nuclear hormone receptors such as estrogen and progesterone receptors and certain other transcription factors such as Ets-2 and PEA3. SRC-1 expression in breast cancer is associated with HER2 and c-Myc expression and with reduced disease-free survival. In this study, SRC-1(-/-) mice were backcrossed with FVB mice and then cross-bred with MMTV-polyoma middle T antigen (PyMT) mice to investigate the role of SRC-1 in breast cancer. Although mammary tumor initiation and growth were similar in SRC-1(-/-)/PyMT and wild-type (WT)/PyMT mice, genetic ablation of SRC-1 antagonized PyMT-induced restriction of mammary ductal differentiation and elongation. SRC-1(-/-)/PyMT mammary tumors were also more differentiated than WT/PyMT mammary tumors. The intravasation of mammary tumor cells and the frequency and extent of lung metastasis were drastically reduced in SRC-1(-/-)/PyMT mice compared with WT/PyMT mice. Metastatic analysis of transplanted WT/PyMT and SRC-1(-/-)/PyMT tumors in SRC-1(-/-) and WT recipient mice revealed that SRC-1 played an intrinsic role in tumor cell metastasis. Furthermore, SRC-1 was up-regulated during mammary tumor progression. Disruption of SRC-1 inhibited Ets-2-mediated HER2 expression and PyMT-stimulated Akt activation in the mammary tumors. Disruption of SRC-1 also suppressed colony-stimulating factor-1 (CSF-1) expression and reduced macrophage recruitment to the tumor site. These results suggest that SRC-1 specifically promotes metastasis without affecting primary tumor growth. SRC-1 may promote metastasis through mediating Ets-2-mediated HER2 expression and activating CSF-1 expression for macrophage recruitment. Therefore, functional interventions for coactivators like SRC-1 may provide unique approaches to control breast cancer progression and metastasis.

Transforming growth factor-beta signaling: emerging stem cell target in metastatic breast cancer?

In most human breast cancers, lowering of TGFbeta receptor- or Smad gene expression combined with increased levels of TGFbetas in the tumor microenvironment is sufficient to abrogate TGFbetas tumor suppressive effects and to induce a mesenchymal, motile and invasive phenotype. In genetic mouse models, TGFbeta signaling suppresses de novo mammary cancer formation but promotes metastasis of tumors that have broken through TGFbeta tumor suppression. In mouse models of "triple-negative" or basal-like breast cancer, treatment with TGFbeta neutralizing antibodies or receptor kinase inhibitors strongly inhibits development of lung- and bone metastases. These TGFbeta antagonists do not significantly affect tumor cell proliferation or apoptosis. Rather, they de-repress anti-tumor immunity, inhibit angiogenesis and reverse the mesenchymal, motile, invasive phenotype characteristic of basal-like and HER2-positive breast cancer cells. Patterns of TGFbeta target genes upregulation in human breast cancers suggest that TGFbeta may drive tumor progression in estrogen-independent cancer, while it mediates a suppressive host cell response in estrogen-dependent luminal cancers. In addition, TGFbeta appears to play a key role in maintaining the mammary epithelial (cancer) stem cell pool, in part by inducing a mesenchymal phenotype, while differentiated, estrogen receptor-positive, luminal cells are unresponsive to TGFbeta because the TGFBR2 receptor gene is transcriptionally silent. These same cells respond to estrogen by downregulating TGFbeta, while antiestrogens act by upregulating TGFbeta. This model predicts that inhibiting TGFbeta signaling should drive the differentiation of mammary stem cells into ductal cells. Consequently, TGFbeta antagonists may convert basal-like or HER2-positive cancers to a more epithelioid, non-proliferating (and, perhaps, non-metastatic) phenotype. Conversely, these agents might antagonize the therapeutic effects of anti-estrogens in estrogen-dependent luminal cancers. These predictions need to be addressed prospectively in clinical trials and should inform the selection of patient populations most likely to benefit from this novel anti-metastatic therapeutic approach.

In vitro generation of cytotoxic and regulatory T cells by fusions of human dendritic cells and hepatocellular carcinoma cells

Background

Human hepatocellular carcinoma (HCC) cells express WT1 and/or carcinoembryonic antigen (CEA) as potential targets for the induction of antitumor immunity. In this study, generation of cytotoxic T lymphocytes (CTL) and regulatory T cells (Treg) by fusions of dendritic cells (DCs) and HCC cells was examined.

Methods

HCC cells were fused to DCs either from healthy donors or the HCC patient and investigated whether supernatants derived from the HCC cell culture (HCCsp) influenced on the function of DCs/HCC fusion cells (FCs) and generation of CTL and Treg.

Results

FCs coexpressed the HCC cells-derived WT1 and CEA antigens and DCs-derived MHC class II and costimulatory molecules. In addition, FCs were effective in activating CD4⁺ and CD8⁺ T cells able to produce IFN-γ and inducing cytolysis of autologous tumor or semiallogeneic targets by a MHC class I-restricted mechanism. However, HCCsp induced functional impairment of DCs as demonstrated by the down-regulation of MHC class I and II, CD80, CD86, and CD83 molecules. Moreover, the HCCsp-exposed DCs failed to undergo full maturation upon stimulation with the Toll-like receptor 4 agonist penicillin-inactivated Streptococcus pyogenes. Interestingly, fusions of immature DCs generated in the presence of HCCsp and allogeneic HCC cells promoted the generation of CD4⁺ CD25^high Foxp3⁺ Treg and inhibited CTL induction in the presence of HCCsp. Importantly, up-regulation of MHC class II, CD80, and CD83 on DCs was observed in the patient with advanced HCC after vaccination with autologous FCs. In addition, the FCs induced WT1- and CEA-specific CTL that were able to produce high levels of IFN-γ.

Conclusion

The current study is one of the first demonstrating the induction of antigen-specific CTL and the generation of Treg by fusions of DCs and HCC cells. The local tumor-related factors may favor the generation of Treg through the inhibition of DCs maturation; however, fusion cell vaccination results in recovery of the DCs function and induction of antigen-specific CTL responses in vitro. The present study may shed new light about the mechanisms responsible for the generation of CTL and Treg by FCs.

Seeding and propagation of untransformed mouse mammary cells in the lung

The acquisition of metastatic ability by tumor cells is considered a late event in the evolution of malignant tumors. We report that untransformed mouse mammary cells that have been engineered to express the inducible oncogenic transgenes MYC and Kras(D12), or polyoma middle T, and introduced into the systemic circulation of a mouse can bypass transformation at the primary site and develop into metastatic pulmonary lesions upon immediate or delayed oncogene induction. Therefore, previously untransformed mammary cells may establish residence in the lung once they have entered the bloodstream and may assume malignant growth upon oncogene activation. Mammary cells lacking oncogenic transgenes displayed a similar capacity for long-term residence in the lungs but did not form ectopic tumors.

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.

An anti-transforming growth factor beta antibody suppresses metastasis via cooperative effects on multiple cell compartments

Overexpression of transforming growth factor beta (TGF-beta) is frequently associated with metastasis and poor prognosis, and TGF-beta antagonism has been shown to prevent metastasis in preclinical models with surprisingly little toxicity. Here, we have used the transplantable 4T1 model of metastatic breast cancer to address underlying mechanisms. We showed that efficacy of the anti-TGF-beta antibody 1D11 in suppressing metastasis was dependent on a synergistic combination of effects on both the tumor parenchyma and microenvironment. The main outcome was a highly significant enhancement of the CD8+ T-cell-mediated antitumor immune response, but effects on the innate immune response and on angiogenesis also contributed to efficacy. Treatment with 1D11 increased infiltration of natural killer cells and T cells at the metastatic site, and enhanced expression of coactivators (NKG2D) and cytotoxic effectors (perforin and granzyme B) on CD8+ T cells. On the tumor cells, increased expression of an NKG2D ligand (Rae1gamma) and of a death receptor (TNFRSF1A) contributed to enhanced immune cell-mediated recognition and lysis. The data suggest that elevated TGF-beta expression in the tumor microenvironment modulates a complex web of intercellular interactions that aggregately promote metastasis and progression. TGF-beta antibodies reverse this effect, and the absence of a major effect of TGF-beta antagonism on any one cell compartment may be critical for a good therapeutic window and the avoidance of autoimmune complications.

The miR-200 family inhibits epithelial-mesenchymal transition and cancer cell migration by direct targeting of E-cadherin transcriptional repressors ZEB1 and ZEB2

MicroRNAs are small non-coding RNA molecules that can regulate gene expression by interacting with multiple mRNAs and inducing either translation suppression or degradation of mRNA. Recently, several miRNAs were identified as either promoters or suppressors of metastasis. However, it is unclear in which step(s) of the multistep metastatic cascade these miRNAs play a defined functional role. To study the functional importance of miRNAs in epithelial-mesenchymal transition (EMT), a process thought to initiate metastasis by enhancing the motility of tumor cells, we used a well established in vitro EMT assay: transforming growth factor-beta-induced EMT in NMuMG murine mammary epithelial cells. We found that members of the miR-200 family, organized as two clusters in the genome, were repressed during EMT. Overexpression of each miRNA individually or as clusters in NMuMG cells hindered EMT by enhancing E-cadherin expression through direct targeting of ZEB1 and ZEB2, which encode transcriptional repressors of E-cadherin. In the 4TO7 mouse carcinoma cell line, which expresses low levels of endogenous E-cadherin and displays a mesenchymal phenotype, ectopic expression of the miR-200 family miRNAs significantly increased E-cadherin expression and altered cell morphology to an epithelial phenotype. Furthermore, ectopic expression of each miR-200 miRNA cluster significantly reduced the in vitro motility of 4TO7 cells in migration assays. These results suggested that loss of expression of the miR-200 family members may play a critical role in the repression of E-cadherin by ZEB1 and ZEB2 during EMT, thereby enhancing migration and invasion during cancer progression.

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.

Evaluation of transforming growth factor and vascular endothelial growth factor polymorphisms in Taiwan Chinese patients with pterygium

PURPOSE

Pterygium is an invasive and highly vascularized growth, thought to arise from activated and proliferating limbal epithelial stem cells. Epidemiologic studies have found the increase of active angiogenic and epithelial growth factors in pterygia, and implicated that these molecules could be involved directly or indirectly in the pathogenesis of pterygia as causative factors. The aim of this study was to investigate the association of polymorphisms of transforming growth factor (TGF) and vascular endothelial growth factor (VEGF) with pterygium.

METHODS

A total of 133 pterygium patients and 105 volunteers without pterygium were enrolled in this study. Polymerase chain reaction based restriction fragment length polymorphism analysis was used to resolve the TGF-Beta1-509 and VEGF-460 genotypes.

RESULTS

There was no significant difference in the allele frequency or genotype of TGF-Beta1-509 or VEGF-460 between total pterygium and the control group. No interaction between TGF-Beta1-509 and VEGF-460 was found either.

CONCLUSIONS

These results indicate that TGF-Beta1-509 and VEGF-460 polymorphisms were not highly associated with the pathology of pterygium. However, it may still be worthwhile to continue to search for angiogenic gene polymorphisms in order to predict the development of pterygium.

New rationales for using TGFbeta inhibitors in radiotherapy

PURPOSE

The first reports that ionizing radiation (IR) induces rapid and persistent activation of transforming growth factor beta1 (TGFbeta) were nearly two decades ago. Subsequent studies have shown that TGFbeta is a major mediator of cellular and tissue responses to IR and have revealed novel facets of its complex biology.

RESULTS

We and others have recently shown that inhibition of production or signaling of TGFbeta in epithelial cells modulates radiosensitivity and impedes activation of the DNA damage response program. The primary transducer of cellular response to DNA damage caused by ionizing radiation is the nuclear protein kinase ataxia telangiectasia mutated, whose activity is severely compromised when TGFbeta is inhibited. Thus, in conjunction, with its well-recognized contribution to normal tissue fibrosis, the role of TGFbeta in the genotoxic stress program provides a previously unsuspected avenue to modulate radiotherapy.

CONCLUSIONS

We hypothesize that identification of the circumstances and tumors in which TGFbeta manipulation enhances tumor cell radiosensitivity, while protecting normal tissues, could significantly increase therapeutic index.

The synthetic triterpenoid 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid-imidazolide alters transforming growth factor beta-dependent signaling and cell migration by affecting the cytoskeleton and the polarity complex

The anti-tumor synthetic triterpenoid 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO)-imidazolide (CDDO-Im) ectopically activates the transforming growth factor beta (TGFbeta)-Smad pathway and extends the duration of signaling by an undefined mechanism. Here we show that CDDO-Imdependent persistence of Smad2 phosphorylation is independent of Smad2 phosphatase activity and correlates with delayed TGFbeta receptor degradation and trafficking. Altered TGFbeta trafficking parallels the dispersal of EEA1-positive endosomes from the perinuclear region of CDDO-Im-treated cells. The effect of CDDO-Im on the EEA1 compartment led to an analysis of the cytoskeleton, and we observed that CDDO-Im alters microtubule dynamics by disrupting the microtubule-capping protein, Clip-170. Interestingly, biotinylated triterpenoid was found to localize to the polarity complex at the leading edge of migrating cells. Furthermore, CDDO-Im disrupted the localization of IQGAP1, PKCzeta, Par6, and TGFbeta receptors from the leading edge of migrating cells and inhibited TGFbeta-dependent cell migration. Thus, the synthetic triterpenoid CDDO-Im interferes with TGFbeta receptor trafficking and turnover and disrupts cell migration by severing the link between members of the polarity complex and the microtubule network.

Antibody-mediated blockade of integrin alpha v beta 6 inhibits tumor progression in vivo by a transforming growth factor-beta-regulated mechanism

The alpha(v)beta(6) integrin is up-regulated on epithelial malignancies and has been implicated in various aspects of cancer progression. Immunohistochemical analysis of alpha(v)beta(6) expression in 10 human tumor types showed increased expression relative to normal tissues. Squamous carcinomas of the cervix, skin, esophagus, and head and neck exhibited the highest frequency of expression, with positive immunostaining in 92% (n = 46), 84% (n = 49), 68% (n = 56), and 64% (n = 100) of cases, respectively. We studied the role of alpha(v)beta(6) in Detroit 562 human pharyngeal carcinoma cells in vitro and in vivo. Prominent alpha(v)beta(6) expression was detected on tumor xenografts at the tumor-stroma interface resembling the expression on human head and neck carcinomas. Nonetheless, coculturing cells in vitro with matrix proteins did not up-regulate alpha(v)beta(6) expression. Detroit 562 cells showed alpha(v)beta(6)-dependent adhesion and activation of transforming growth factor-beta (TGF-beta) that was inhibited >90% with an alpha(v)beta(6) blocking antibody, 6.3G9. Although both recombinant soluble TGF-beta receptor type-II (rsTGF-beta RII-Fc) and 6.3G9 inhibited TGF-beta-mediated Smad2/3 phosphorylation in vitro, there was no effect on proliferation. Conversely, in vivo, 6.3G9 and rsTGF-beta RII-Fc inhibited xenograft tumor growth by 50% (n = 10, P < 0.05) and >90% (n = 10, P < 0.001), respectively, suggesting a role for the microenvironment in this response. However, stromal collagen and smooth muscle actin content in xenograft sections were unchanged with treatments. Although further studies are required to consolidate in vitro and in vivo results and define the mechanisms of tumor inhibition by alpha(v)beta(6) antibodies, our findings support a role for alpha(v)beta(6) in human cancer and underscore the therapeutic potential of function blocking alpha(v)beta(6) antibodies.

Role of transforming growth factor-beta superfamily signaling pathways in human disease

Transforming growth factor beta (TGF-beta) superfamily signaling pathways are ubiquitous and essential regulators of cellular processes including proliferation, differentiation, migration, and survival, as well as physiological processes, including embryonic development, angiogenesis, and wound healing. Alterations in these pathways, including either germ-line or somatic mutations or alterations in the expression of members of these signaling pathways often result in human disease. Appropriate regulation of these pathways is required at all levels, particularly at the ligand level, with either a deficiency or an excess of specific TGF-beta superfamily ligands resulting in human disease. TGF-beta superfamily ligands and members of these TGF-beta superfamily signaling pathways also have emerging roles as diagnostic, prognostic or predictive markers for human disease. Ongoing studies will enable targeting of TGF-beta superfamily signaling pathways for the chemoprevention and treatment of human disease.

Transforming growth factor-beta promotes invasion in tumorigenic but not in nontumorigenic human prostatic epithelial cells

Transforming growth factor-beta (TGF-beta) is a pleiotropic growth factor with actions that are dependent on circumstances, including dose, target cell type, and context. TGF-beta can elicit both growth-promoting and growth-suppressive activities. In normal tissues, TGF-beta generally acts to restrict growth and maintain differentiation. However, during tumorigenesis, changes in TGF-beta expression and cellular responses can promote tumorigenesis. The present study examines the effects of TGF-beta on the nontumorigenic human prostatic epithelial cell line BPH1 and on three derivative tumorigenic sublines BPH1(CAFTD)1, BPH1(CAFTD)3, and BPH1(CAFTD)5. The data show that TGF-beta has different effects on the nontumorigenic and tumorigenic cells. The nontumorigenic cells are growth inhibited by TGF-beta. In contrast, the tumorigenic sublines are not growth inhibited but instead undergo an epithelial to mesenchymal transformation (EMT) in response to TGF-beta. The tumorigenic lines show constitutively elevated levels of phosphorylated Akt, which modulates their response to TGF-beta by blocking Smad3 and p21 nuclear translocation. On TGF-beta stimulation of the tumorigenic sublines, the activated Akt allows the cell to escape cell cycle arrest. The phosphatidylinositol 3-kinase/Akt pathway is also involved in TGF-beta-induced EMT, defined here by induction of vimentin expression and enhanced cellular motility. In vivo, tumorigenic cells with constitutively active TGF-beta signaling show increased invasion with EMT, which express vimentin, located specifically at the invasive front of the tumor. These data indicate that following malignant transformation TGF-beta can play a direct role in promoting prostatic cancer and further that these responses are context specific in vivo.

Modulation of NFkappaB activity and E-cadherin by the type III transforming growth factor beta receptor regulates cell growth and motility

Transforming growth factor beta is growth-inhibitory in non-transformed epithelial cells but becomes growth-promoting during tumorigenesis. The role of the type I and II receptors in tumorigenesis has been extensively studied, but the role of the ubiquitously expressed type III receptor (TbetaRIII) remains elusive. We developed short hairpin RNAs directed against TbetaRIII to investigate the role of this receptor in breast cancer tumorigenesis. Nontumorigenic NMuMG mouse cells stably expressing short hairpin RNA specific to mouse TbetaRIII (NM-kd) demonstrated increased cell growth, motility, and invasion as compared with control cells expressing shRNA to human TbetaRIII (NM-con). Reconstitution of TbetaRIII expression with rat TbetaRIII abrogated the increased growth and motility seen in the NM-kd cells. In addition, the NM-kd cells exhibited marked reduction in the expression of the adherens junction protein, E-cadherin. This loss of E-cadherin was due to increased NFkappaB activity that, in turn, resulted in increased expression of the transcriptional repressors of E-cadherin such as Snail, Slug, Twist, and Sip1. Finally, NMuMG cells in which TbetaRIII had been knocked down formed invasive tumors in athymic nude mice, whereas the control cells did not. These data indicate that TbetaRIII acts as a tumor suppressor in nontumorigenic mammary epithelial cells at least in part by inhibiting NFkappaB-mediated repression of E-cadherin.

Soluble cadherins as cancer biomarkers

Molecular activities, regulating a balanced tissue organisation, are frequently disturbed during cancer progression. These include protein ectodomain shedding, a post-translational process that substantially changes the functional properties of the substrate protein. In comparison with normal epithelia, cancer cells almost invariably show diminished cadherin-mediated intercellular adhesion. This review will address cadherin ectodomain shedding and its functional consequence in normal physiology and in the tumor environment. Soluble cadherin fragments may retain specific biological activities during cancer cell invasion, angiogenesis and perineural invasion. When diffusion barriers disappear, soluble cadherins are detected in sera from cancer patients. Soluble N-(neural) cadherin may represent a novel diagnosis/prognostic biomarker showing a correlation with PSA in sera of prostate cancer patients. Furthermore, therapeutic monitoring in pancreas adenomacarcinoma revealed a correlation between circulating soluble N-cadherin and CA 19-9. A better understanding of cadherin regulation in cancer progression will likely increase our awareness of the importance of the combinatorial signals that regulate tissue integrity and eventually result in the identification of new therapeutics targeting cadherins.

Organotropism of breast cancer metastasis

Breast cancer causes mortality by metastasizing to a variety of vital organs, such as bone, lung, brain and liver. Effective therapeutic intervention of this deadly process relies on a better mechanistic understanding of metastasis organotropism. Recent studies have confirmed earlier speculations that metastasis is a non-random process and is dependent on intricate tumor-stroma interactions at the target organ. Both the intrinsic properties of breast cancer cells and the host organ microenvironment are important in determining the efficiency of organ-specific metastasis. Advances in animal modeling, in vivo imaging and functional genomics have accelerated the discovery of important molecular mediators of organ-specific metastasis. A conceptual framework of breast cancer organotropism is emerging and will be instrumental in guiding future efforts in this exciting research field.

Microglia-derived TGF-β as an important regulator of glioblastoma invasion—an inhibition of TGF-β-dependent effects by shRNA against human TGF-β type II receptor

The invasion of tumor cells into brain tissue is a pathologic hallmark of malignant gliomas and contributes to treatment failures. Diffuse glioblastomas contain numerous microglial cells, which enhance the progression of gliomas; however, factors responsible for invasion-promoting role of microglia are unknown. Transforming growth factor-beta (TGF-beta) can enhance tumor growth, invasion, angiogenesis and immunosuppression. Antagonizing TGF-beta activity has been shown to inhibit tumor invasion in vitro and tumorigenicity, but a systemic inhibition or lack of TGF-beta signaling results in acute inflammation and disruption of immune system homeostasis. We developed plasmid-transcribed small hairpin RNAs (shRNAs) to downregulate the TGF-beta type II receptor (TbetaIIR) expression, which effectively inhibited cytokine-induced signaling pathways and transcriptional responses in transiently transfected human glioblastoma cells. Silencing of TbetaIIR abolished TGF-beta-induced glioblastoma invasiveness and migratory responses in vitro. Moreover, tumorigenicity of glioblastoma cells stably expressing TbetaIIR shRNAs in nude mice was reduced by 50%. Microglia strongly enhanced glioma invasiveness in the co-culture system, but this invasion-promoting activity was lost in glioma cells stably expressing shTbetaRII, indicating a crucial role of microglia-derived TGF-beta in tumor-host interactions. Our results demonstrate a successful targeting of TGF-beta-dependent invasiveness and tumorigenicity of glioblastoma cells by RNAi-mediated gene silencing.

Differential regulation of epithelial and mesenchymal markers by deltaEF1 proteins in epithelial mesenchymal transition induced by TGF-beta

Epithelial-mesenchymal transition (EMT), a crucial event in cancer progression and embryonic development, is induced by transforming growth factor (TGF)-beta in mouse mammary NMuMG epithelial cells. Id proteins have previously been reported to inhibit major features of TGF-beta-induced EMT. In this study, we show that expression of the deltaEF1 family proteins, deltaEF1 (ZEB1) and SIP1, is gradually increased by TGF-beta with expression profiles reciprocal to that of E-cadherin. SIP1 and deltaEF1 each dramatically down-regulated the transcription of E-cadherin in NMuMG cells through direct binding to the E-cadherin promoter. Silencing of the expression of both SIP1 and deltaEF1, but not either alone, completely abolished TGF-beta-induced E-cadherin repression. However, expression of mesenchymal markers, including fibronectin, N-cadherin, and vimentin, was not affected by knockdown of SIP1 and deltaEF1. TGF-beta-induced the expression of Ets1, which in turn activated deltaEF1 promoter activity. Moreover, up-regulation of SIP1 and deltaEF1 expression by TGF-beta was suppressed by knockdown of Ets1 expression. In addition, Id2 suppressed the TGF-beta- and Ets1-induced up-regulation of deltaEF1. Taken together, these findings suggest that the deltaEF1 family proteins, SIP1 and deltaEF1, are necessary, but not sufficient, for TGF-beta-induced EMT and that Ets1 induced by TGF-beta may function as an upstream transcriptional regulator of SIP1 and deltaEF1.

Enhancing Antitumor Immunity: Combining IL-12 With TGFβ1 Antagonism

With respect to CD8 effector T cells, interleukin-12 (IL-12) and transforming growth factor beta (TGFbeta) are 2 cytokines that exert opposing effects. IL-12 promotes antitumor immune responses by augmenting activated CD8 T-cell proliferation and interferon-gamma secretion. Conversely, TGFbeta generates a permissive environment for cancer growth, in part by antagonizing the effects of immunomodulatory cytokines, including IL-12. We demonstrate that TGFbeta-resistant T cells are capable of sustaining IL-12-induced mitogenesis and interferon-gamma secretion in a TGFbeta-rich milieu. Furthermore, in 2 murine tumor models associated with high TGFbeta1 levels in the local microenvironment, treatment with IL-12 and adoptively transferred TGFbeta-resistant T cells provided improved survival times. These results suggest that combining IL-12 with TGFbeta neutralization strategies may be effective in enhancing antitumor immune responses.

Improving T cell therapy for cancer

Adoptive transfer of antigen-specific T lymphocytes is a powerful therapy for the treatment of opportunistic disease and some virus-associated malignancies such as Epstein-Barr virus-positive post-transplant lymphoproliferative disease. However, this strategy has been less successful in patients with nonviral cancers owing to their many and varied immune evasion mechanisms. These mechanisms include downregulation of target antigens and antigen-presenting machinery, secretion of inhibitory cytokines, and recruitment of regulatory immune cells to the tumor site. With increased understanding of the tumor microenvironment and the behavior and persistence of ex vivo-manipulated, adoptively transferred T cells, two novel approaches for increasing the efficacy of T cell therapy have been proposed. The first involves genetic modification of tumor-specific T cells to improve their biological function, for example by augmenting their ability to recognize tumor cells or their resistance to tumor-mediated immunosuppression. The second requires modifications to the host environment to improve the homeostatic expansion of infused T cells or to eliminate inhibitory T cell subsets. In this review, we discuss current, promising strategies to improve adoptive T cell therapy for the treatment of cancer.

Targeting tumor stroma and exploiting mature tumor vasculature to improve anti-cancer drug delivery

The identification of a critical role of tumour stroma in the regulation of tumour interstitial fluid pressure and the simultaneous discovery of the impact of anti-angiogenic drugs on tumour hemodynamics have provided new potential for improving tumour delivery of anti-cancer drugs. Here, we review the most recent studies investigating how tumour-associated fibroblasts and macrophages as well as the extracellular matrix itself may be targeted to facilitate delivery of both low-molecular weight drugs and macromolecules. In addition, we summarize the current understanding of the use of vasoactive compounds, radiotherapy and vascular-disrupting agents as potential adjuvants to maximize tumour delivery of anti-cancer drugs. The impact of these strategies on the diffusive and convective modes of drug transport is discussed in the light of Fick's and Starling's laws. Finally, we discuss how transcytosis through caveolae may also be exploited to optimize the selective delivery of conventional chemotherapy to the subendothelial tumour cell compartment.

Expression profiling of genes regulated by TGF-beta: differential regulation in normal and tumour cells

Background

TGF-beta is one of the key cytokines implicated in various disease processes including cancer. TGF-beta inhibits growth and promotes apoptosis in normal epithelial cells and in contrast, acts as a pro-tumour cytokine by promoting tumour angiogenesis, immune-escape and metastasis. It is not clear if various actions of TGF-beta on normal and tumour cells are due to differential gene regulations. Hence we studied the regulation of gene expression by TGF-beta in normal and cancer cells.

Results

Using human 19 K cDNA microarrays, we show that 1757 genes are exclusively regulated by TGF-beta in A549 cells in contrast to 733 genes exclusively regulated in HPL1D cells. In addition, 267 genes are commonly regulated in both the cell-lines. Semi-quantitative and real-time qRT-PCR analysis of some genes agrees with the microarray data. In order to identify the signalling pathways that influence TGF-beta mediated gene regulation, we used specific inhibitors of p38 MAP kinase, ERK kinase, JNK kinase and integrin signalling pathways. The data suggest that regulation of majority of the selected genes is dependent on at least one of these pathways and this dependence is cell-type specific. Interestingly, an integrin pathway inhibitor, RGD peptide, significantly affected TGF-beta regulation of Thrombospondin 1 in A549 cells.

Conclusion

These data suggest major differences with respect to TGF-beta mediated gene regulation in normal and transformed cells and significant role of non-canonical TGF-beta pathways in the regulation of many genes by TGF-beta.

Molecular interactions between breast cancer cells and the bone microenvironment drive skeletal metastases

Breast cancer cells preferentially spread to bone. Bone metastases are currently incurable and therefore better treatments need to be developed. Metastasis is an inefficient, multi-step process. Specific aspects of both breast cancer cells and the bone microenvironment contribute to the development of bone metastases. Breast cancers express chemokine receptors, integrins, cadherins, and bone-resorbing and bone-forming factors that contribute to the successful and preferential spread of tumor to bone. Bone is rich in growth factors and cell types that make it a hospitable environment for breast cancer growth. Once breast cancer cells enter the bone, a highly complex vicious cycle develops, in which breast cancer cells secrete factors that act on bone cells and other cells within the bone (stem cells, T cells, platelets, adipocytes, fibroblasts, and endothelial cells), causing them to secrete factors that act on adjacent cancer cells. The steps in the metastatic cascade and the vicious cycle within bone offer unique targets for adjuvant treatments to treat and cure bone metastases.

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.

Microenvironment of the involuting mammary gland mediates mammary cancer progression

Breast cancer diagnosed after a completed pregnancy has higher metastatic potential and therefore a much poorer prognosis. We hypothesize that following pregnancy the process of mammary gland involution, which returns the gland to its pre-pregnant state, co-opts some of the programs of wound healing. The pro-inflammatory milieu that results, while physiologically normal, promotes tumor progression. In this review, the similarities between mammary gland involution after cessation of milk-production and pathological tissue remodeling are discussed in light of emerging data demonstrating a role for pathological tissue remodeling in cancer.

A Novel Small-Molecule Inhibitor of Transforming Growth Factor β Type I Receptor Kinase (SM16) Inhibits Murine Mesothelioma Tumor Growth In vivo and Prevents Tumor Recurrence after Surgical Resection

Malignant mesothelioma is an aggressive and lethal pleural cancer that overexpresses transforming growth factor beta (TGFbeta). We investigated the efficacy of a novel small-molecule TGFbeta type I receptor (ALK5) kinase inhibitor, SM16, in the AB12 syngeneic model of malignant mesothelioma. SM16 inhibited TGFbeta signaling seen as decreased phosphorylated Smad2/3 levels in cultured AB12 cells (IC(50), approximately 200 nmol/L). SM16 penetrated tumor cells in vivo, suppressing tumor phosphorylated Smad2/3 levels for at least 3 h following treatment of tumor-bearing mice with a single i.p. bolus of 20 mg/kg SM16. The growth of established AB12 tumors was significantly inhibited by 5 mg/kg/d SM16 (P < 0.001) delivered via s.c. miniosmotic pumps over 28 days. The efficacy of SM16 was a result of a CD8+ antitumor response because (a) the antitumor effects were markedly diminished in severe combined immunodeficient mice and (b) CD8+ T cells isolated from spleens of mice treated with SM16 showed strong antitumor cytolytic effects whereas CD8+ T cells isolated from spleens of tumor-bearing mice treated with control vehicle showed minimal activity. Treatment of mice bearing large tumors with 5 mg/kg/d SM16 after debulking surgery reduced the extent of tumor recurrence from 80% to <20% (P < 0.05). SM16 was also highly effective in blocking and regressing tumors when given p.o. at doses of 0.45 or 0.65 g/kg in mouse chow. Thus, SM16 shows potent activity against established AB12 malignant mesothelioma tumors using an immune-mediated mechanism and can significantly prevent tumor recurrence after resection of bulky AB12 malignant mesothelioma tumors. These data suggest that ALK5 inhibitors, such as SM16, offer significant potential for the treatment of malignant mesothelioma and possibly other cancers.

Stable Overexpression of Smad7 in Human Melanoma Cells Impairs Bone Metastasis

Melanoma has a propensity to metastasize to bone, where it is exposed to high concentrations of transforming growth factor-beta (TGF-beta). Because TGF-beta promotes bone metastases from other solid tumors, such as breast cancer, we tested the role of TGF-beta in melanoma metastases to bone. 1205Lu melanoma cells, stably transfected to overexpress the natural TGF-beta/Smad signaling inhibitor Smad7, were studied in an experimental model of bone metastasis whereby tumor cells are inoculated into the left cardiac ventricle of nude mice. All mice bearing parental and mock-transfected 1205Lu cells developed osteolytic bone metastases 5 weeks post-tumor inoculation. Mice bearing 1205Lu-Smad7 tumors had significantly less osteolysis on radiographs and longer survival compared with parental and mock-transfected 1205Lu mice. To determine if the reduced bone metastases observed in mice bearing 1205Lu-Smad7 clones was due to reduced expression of TGF-beta target genes known to enhance metastases to bone from breast cancer cells, we analyzed gene expression of osteolytic factors, parathyroid hormone-related protein (PTHrP) and interleukin-11 (IL-11), the chemotactic receptor CXCR4, and osteopontin in 1205Lu cells. Quantitative reverse transcription-PCR analysis indicated that PTHrP, IL-11, CXCR4, and osteopontin mRNA steady-state levels were robustly increased in response to TGF-beta and that Smad7 and the TbetaRI small-molecule inhibitor, SB431542, prevented such induction. In addition, 1205Lu-Smad7 bone metastases expressed significantly lower levels of IL-11, connective tissue growth factor, and PTHrP. These data suggest that TGF-beta promotes osteolytic bone metastases due to melanoma by stimulating the expression of prometastatic factors via the Smad pathway. Blockade of TGF-beta signaling may be an effective treatment for melanoma metastasis to bone.

Functional evaluation of novel soluble insulin-like growth factor (IGF)-II–specific ligand traps based on modified domain 11 of the human IGF2 receptor

Ligands transported by the mannose 6-phosphate/insulin-like growth factor (IGF)-II receptor (IGF2R) include IGF-II- and mannose 6-phosphate-modified proteins. Increased extracellular supply of IGF-II, either secondary to loss of the clearance function of IGF2R, loss of IGF binding protein function, or increased IGF2 gene expression, can lead to embryonic overgrowth and cancer promotion. Reduced supply of IGF-II is detrimental to tumor growth, and this suggests that gain of function of IGF-II is a molecular target for human cancer therapy. Domain 11 of IGF2R binds IGF-II with high specificity and affinity. Mutagenesis studies have shown that substitution of glutamic acid for lysine at residue 1554 results in a 6-fold higher affinity for IGF-II (20.5 nmol/L) than native domain 11 (119 nmol/L). Here, we generate a novel high-affinity IGF-II ligand trap by fusion of mutated human 11(E1554K) to a COOH-terminal human IgG1 Fc domain (11(E1554K)-Fc). The resulting homodimer has a significantly increased affinity for IGF-II (1.79 nmol/L) when measured by surface plasmon resonance. IGF-II signaling via the IGF-I receptor and the proliferative effect of IGF-II were specifically inhibited by 11(E1554K)-Fc in both HaCaT and Igf2(-/-) mouse embryonic fibroblast cells. These data confirm that a novel engineered and soluble IGF2R-11(E1554K)-Fc protein functions as an IGF-II-specific and high-affinity ligand trap in vitro and that this protein has potential application as an IGF-II antagonist for cancer therapy following in vivo experimental evaluation.

Inhibition of growth and metastasis of mouse mammary carcinoma by selective inhibitor of transforming growth factor-beta type I receptor kinase in vivo

PURPOSE

Transforming growth factor-beta (TGF-beta) suppresses tumor development by inhibiting cellular proliferation, inducing differentiation and apoptosis, and maintaining genomic integrity. However, once tumor cells escape from the tumor-suppressive effects of TGF-beta, they often constitutively overexpress and activate TGF-beta, which may promote tumor progression by enhancing invasion, metastasis, and angiogenesis and by suppressing antitumor immunity. The purpose of this study was to test this hypothesis using TGF-beta pathway antagonists.

EXPERIMENTAL DESIGN

We examined the effects of selective TGF-beta type I receptor kinase inhibitors, SD-093 and SD-208, on two murine mammary carcinoma cell lines (R3T and 4T1) in vitro and in vivo.

RESULTS

Both agents blocked TGF-beta-induced phosphorylation of the receptor-associated Smads, Smad2 and Smad3, in a dose-dependent manner, with IC50 between 20 and 80 nmol/L. TGF-beta failed to inhibit growth of these cell lines but stimulated epithelial-to-mesenchymal transdifferentiation, migration, and invasiveness into Matrigel in vitro. These effects were inhibited by SD-093, indicating that these processes are partly driven by TGF-beta. Treatment of syngeneic R3T or 4T1 tumor-bearing mice with orally given SD-208 inhibited primary tumor growth as well as the number and size of metastases. In contrast, SD-208 failed to inhibit R3T tumor growth or metastasis in athymic nude mice. Moreover, in vitro anti-4T1 cell cytotoxic T-cell responses of splenocytes from drug-treated animals were enhanced compared with cells from control animals. In addition, SD-208 treatment resulted in a decrease in tumor angiogenesis.

CONCLUSION

TGF-beta type I receptor kinase inhibitors hold promise as novel therapeutic agents for metastatic breast cancer.