DrugMap: A quantitative pan-cancer analysis of cysteine ligandability.

Cysteine-focused chemical proteomic platforms have accelerated the clinical development of covalent inhibitors for a wide range of targets in cancer. However, how different oncogenic contexts influence cysteine targeting remains unknown. To address this question, we have developed "DrugMap," an atlas of cysteine ligandability compiled across 416 cancer cell lines. We unexpectedly find that cysteine ligandability varies across cancer cell lines, and we attribute this to differences in cellular redox states, protein conformational changes, and genetic mutations. Leveraging these findings, we identify actionable cysteines in NF-κB1 and SOX10 and develop corresponding covalent ligands that block the activity of these transcription factors. We demonstrate that the NF-κB1 probe blocks DNA binding, whereas the SOX10 ligand increases SOX10-SOX10 interactions and disrupts melanoma transcriptional signaling. Our findings reveal heterogeneity in cysteine ligandability across cancers, pinpoint cell-intrinsic features driving cysteine targeting, and illustrate the use of covalent probes to disrupt oncogenic transcription-factor activity.

Detection and alignment of dual-polarization optical quadrature amplitude transmitter IQ and XY skews using reconfigurable interference.

Dual-polarization quadrature amplitude modulation (DP-QAM) is one of the feasible paths towards 100-Gb/s, 400-Gb/s and 1-Tb/s optical fiber communications systems. For DP-QAM transmitter, the time mismatch between the in-phase and quadrature (IQ) or x-polarized and y-polarized (XY) tributary channels is known as the IQ or XY skew. Large uncompensated IQ or XY skew can significantly degrade the optical fiber communications system performance. Sometimes, time-interleaved return-to-zero (RZ) DP signal is preferred with lower nonlinear polarization scattering induced penalty. In this work, detection and alignment of DP-QAM transmitter IQ and XY skews using reconfigurable interference is experimentally demonstrated. For IQ skew detection, a total dynamic range of 26.4 dB is achieved with ~1-dB power change for 0.5-ps skew from well alignment. For XY skew detection, it shows 23.2-dB dynamic range, and ~1.5-dB power change is achieved for 1-ps XY skew. Fast detection algorithm for arbitrary skew is also proposed and experimentally verified. The scheme is compatible with different modulation formats, flexible data sequences, and variable waveforms.

Combination antibody treatment down-regulates epidermal growth factor receptor by inhibiting endosomal recycling.

Due to its common dysregulation in epithelial-based cancers and extensive characterization of its role in tumor growth, epidermal growth factor receptor (EGFR) is a highly validated target for anticancer therapies. There has been particular interest in the development of monoclonal antibodies (mAbs) targeting EGFR, resulting in two approved mAb-based drugs and several others in clinical trials. It has recently been reported that treatment with combinations of noncompetitive mAbs can induce receptor clustering, leading to synergistic receptor down-regulation. We elucidate three key aspects of this phenomenon. First, we show that highly potent combinations consisting of two noncompetitive mAbs that target EGFR domain 3 reduce surface receptor levels by up to 80% with a halftime of 0.5-5 h in both normal and transformed human cell lines to an extent inversely proportional to receptor density. Second, we find the mechanism underlying down-regulation to be consistent with recycling inhibition. Third, in contrast to the agonism associated with ligand-induced down-regulation, we demonstrate that mAb-induced down-regulation does not activate EGFR or its downstream effectors and it leads to synergistic reduction in migration and proliferation of cells that secrete autocrine ligand. These new insights will aid in ongoing rational design of EGFR-targeted antibody therapeutics.

DrugMap: A quantitative pan-cancer analysis of cysteine ligandability.

Cysteine-focused chemical proteomic platforms have accelerated the clinical development of covalent inhibitors of a wide-range of targets in cancer. However, how different oncogenic contexts influence cysteine targeting remains unknown. To address this question, we have developed DrugMap , an atlas of cysteine ligandability compiled across 416 cancer cell lines. We unexpectedly find that cysteine ligandability varies across cancer cell lines, and we attribute this to differences in cellular redox states, protein conformational changes, and genetic mutations. Leveraging these findings, we identify actionable cysteines in NFκB1 and SOX10 and develop corresponding covalent ligands that block the activity of these transcription factors. We demonstrate that the NFκB1 probe blocks DNA binding, whereas the SOX10 ligand increases SOX10-SOX10 interactions and disrupts melanoma transcriptional signaling. Our findings reveal heterogeneity in cysteine ligandability across cancers, pinpoint cell-intrinsic features driving cysteine targeting, and illustrate the use of covalent probes to disrupt oncogenic transcription factor activity.

Direct detection of OXA-48-like carbapenemase variants with and without co-expression of an extended-spectrum ß-lactamase from bacterial cell lysates using mass spectrometry.

INTRODUCTION: Antibiotic-resistant Gram-negative bacteria are of a growing concern globally, especially those producing enzymes conferring resistance. OXA-48-like carbapenemases hydrolyze most ß-lactam antibiotics, with typically low-level hydrolysis of carbapenems, but have limited effect on broad-spectrum cephalosporins. These are frequently co-expressed with extended spectrum ß-lactamases, especially CTX-M-15, which typically shows high level resistance to broad-spectrum cephalosporins, yet is carbapenem susceptible. The combined resistance profile makes the need for successful detection of these specific resistance determinants imperative for effective antibiotic therapy. OBJECTIVES: The objective of this study is to detect and identify OXA-48-like and CTX-M-15 enzymes using mass spectrometry, and to subsequently develop a method for detection of both enzyme types in combination with liquid chromatography. METHODS: Cells grown in either broth or on agar were harvested, lysed, and, in some cases buffer-exchanged. Lysates produced from bacterial cells were separated and analyzed via liquid chromatography with mass spectrometry (LC-MS) and tandem mass spectrometry (LC-MS/MS). RESULTS: The intact proteins of OXA-48, OXA-181, and OXA-232 (collectively OXA-48-like herein) and CTX-M-15 were characterized and detected. Acceptance criteria based on sequence-informative fragments from each protein group were established as confirmatory markers for the presence of the protein(s). A total of 25 isolates were successfully tested for OXA-48 like (2), CTX-M-15 (3), or expression of both (7) enzymes. Thirteen isolates served as negative controls. CONCLUSIONS: Here we present a method for the direct and independent detection of both OXA-48-like carbapenemases and CTX-M-15 ß-lactamases using LC-MS/MS. The added sensitivity of MS/MS allows for simultaneous detection of at least two co-eluting, co-isolated and co-fragmented proteins from a single mass spectrum.

Rapid MRSA detection via tandem mass spectrometry of the intact 80 kDa PBP2a resistance protein.

Treatment of antibiotic-resistant infections is dependent on the detection of specific bacterial genes or proteins in clinical assays. Identification of methicillin-resistant Staphylococcus aureus (MRSA) is often accomplished through the detection of penicillin-binding protein 2a (PBP2a). With greater dependence on mass spectrometry (MS)-based bacterial identification, complementary efforts to detect resistance have been hindered by the complexity of those proteins responsible. Initial characterization of PBP2a indicates the presence of glycan modifications. To simplify detection, we demonstrate a proof-of-concept tandem MS approach involving the generation of N-terminal PBP2a peptide-like fragments and detection of unique product ions during top-down proteomic sample analyses. This approach was implemented for two PBP2a variants, PBP2amecA and PBP2amecC, and was accurate across a representative panel of MRSA strains with different genetic backgrounds. Additionally, PBP2amecA was successfully detected from clinical isolates using a five-minute liquid chromatographic separation and implementation of this MS detection strategy. Our results highlight the capability of direct MS-based resistance marker detection and potential advantages for implementing these approaches in clinical diagnostics.

Direct detection of intact Klebsiella pneumoniae carbapenemase variants from cell lysates: Identification, characterization and clinical implications.

INTRODUCTION: Carbapenemase-producing organisms (CPOs) are a growing threat to human health. Among the enzymes conferring antibiotic resistance produced by these organisms, Klebsiella pneumoniae carbapenemase (KPC) is considered to be a growing global health threat. Reliable and specific detection of this antibiotic resistance-causing enzyme is critical both for effective therapy and to mitigate further spread. OBJECTIVES: The objective of this study is to develop an intact protein mass spectrometry-based method for detection and differentiation of clinically-relevant KPC variants directly from bacterial cell lysates. The method should be specific for any variant expressed in multiple bacterial species, limit false positive results and be rapid in nature to directly influence clinical outcomes. METHODS: Lysates obtained directly from bacterial colonies were used for intact protein detection using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Bottom-up and top-down proteomic methods were used to characterize the KPC protein targets of interest. Comparisons between KPC-producing and KPC-non-producing isolates from a wide variety of species were also performed. RESULTS: Characterization of the mature KPC protein revealed an unexpected signal peptide cleavage site preceding an AXA signal peptide motif, modifying the molecular weight (MW) of the mature protein. Taking the additional AXA residues into account allowed for direct detection of the intact protein using top-down proteomic methods. Further validation was performed by transforming a KPC-harboring plasmid into a negative control strain, followed by MS detection of the KPC variant from the transformed cell line. Application of this approach to clearly identify clinically-relevant variants among several species is presented for KPC-2, KPC-3, KPC-4 and KPC-5. CONCLUSION: Direct detection of these enzymes contributes to the understanding of occurrence and spread of these antibiotic-resistant organisms. The ability to detect intact KPC variants via a simple LC-MS/MS approach could have a direct and positive impact on clinical therapy, by providing both direction for epidemiological tracking and appropriate therapy.

Cox-2 inactivates Smad signaling and enhances EMT stimulated by TGF-beta through a PGE2-dependent mechanisms.

Although it is well established that mammary tumorigenesis converts transforming growth factor-beta (TGF-beta) from a tumor suppressor to a tumor promoter, the molecular, cellular and microenvironmental mechanisms underlying the dichotomous nature of TGF-beta in mammary epithelial cells (MECs) remains to be determined definitively. Aberrant upregulation of the inducible cyclooxygenase, Cox-2, occurs frequently in breast cancers and is associated with increasing disease severity and the acquisition of metastasis; however, the impact of Cox-2 expression on normal and malignant MEC response to TGF-beta remains unknown. We show here that TGF-beta induced Cox-2 expression in normal MECs during their acquisition of an epithelial-mesenchymal transition (EMT) phenotype. Moreover, stable Cox-2 expression in normal MECs stimulated their invasion, EMT and anchorage-independent growth and inhibited their activation of Smad2/3 by TGF-beta. Conversely, antagonizing TGF-beta signaling in malignant, metastatic MECs significantly reduced their expression of Cox-2 as well as enhanced their activation of Smad2/3 by TGF-beta. Along these lines, elevated Cox-2 expression elicited prostaglandin E(2) (PGE(2)) production and the autocrine activation of EP receptors, which antagonized Smad2/3 signaling in normal and malignant MECs. Importantly, rendering normal and malignant MECs Cox-2 deficient inhibited their production of PGE(2) and acquisition of an EMT morphology as well as potentiated their nuclear accumulation of Smad2/3 and transcription of plasminogen activator inhibitor-1 and p15 messenger RNA. Collectively, our findings establish Cox-2 as a novel antagonist of Smad2/3 signaling in normal and malignant MECs; they also suggest that chemotherapeutic targeting of Cox-2 may offer new inroads in restoring the tumor-suppressing activities of TGF-beta in malignant, metastatic breast cancers.

Fibulins 3 and 5 antagonize tumor angiogenesis in vivo.

Lethal tumor growth and progression cannot occur without angiogenesis, which facilitates cancer cell proliferation, survival, and dissemination. Fibulins (FBLN) 5 and 3 are widely expressed extracellular matrix proteins that regulate cell proliferation in a context-specific manner. Reduced FBLN-5 expression has been associated with cancer formation and progression in humans, whereas its constitutive expression antagonizes endothelial cell angiogenic sprouting in vitro. Thus, FBLN-5 may suppress tumorigenesis by preventing tumor angiogenesis. FBLN-3 is homologous to FBLN-5 and expressed in endothelial cells, yet its role in tumorigenesis and angiogenesis is unknown. We find FBLN-3 expression to be altered in some human tumors and that its constitutive expression in endothelial cells inhibited their proliferation, invasion, and angiogenic sprouting, as well as their response to vascular endothelial growth factor as measured by p38 mitogen-activated protein kinase activation. In endothelial cells, both FBLNs (a) reduced angiogenic sprouting stimulated by basic fibroblast growth factor (bFGF); (b) inhibited matrix metalloproteinase expression and activity; and (c) stimulated tissue inhibitor of metalloproteinase expression. More importantly, both FBLNs prevented angiogenesis and vessel infiltration into bFGF-supplemented Matrigel plugs implanted in genetically normal mice, as well as decreased the growth and blood vessel density in tumors produced by MCA102 fibrosarcoma cells implanted s.c. into syngeneic mice. Our findings establish FBLN-3 and FBLN-5 as novel angiostatic agents capable of reducing tumor angiogenesis and, consequently, tumor growth in vivo and suggest that these angiostatic activities may one day be exploited to combat tumor angiogenesis and metastasis in cancer patients.

SPARC inhibits epithelial cell proliferation in part through stimulation of the transforming growth factor-beta-signaling system.

Secreted protein, acidic and rich in cysteine (SPARC) is a multifunctional secreted protein that regulates cell-cell and cell-matrix interactions, leading to alterations in cell adhesion, motility, and proliferation. Although SPARC is expressed in epithelial cells, its ability to regulate epithelial cell growth remains largely unknown. We show herein that SPARC strongly inhibited DNA synthesis in transforming growth factor (TGF)-beta-sensitive Mv1Lu cells, whereas moderately inhibiting that in TGF-beta-insensitive Mv1Lu cells (i.e., R1B cells). Overexpression of dominant-negative Smad3 in Mv1Lu cells, which abrogated growth arrest by TGF-beta, also attenuated growth arrest stimulated by SPARC. Moreover, the extracellular calcium-binding domain of SPARC (i.e., SPARC-EC) was sufficient to inhibit Mv1Lu cell proliferation but not that of R1B cells. Similar to TGF-beta and thrombospondin-1, treatment of Mv1Lu cells with SPARC or SPARC-EC stimulated Smad2 phosphorylation and Smad2/3 nuclear translocation: the latter response to all agonists was abrogated in R1B cells or by pretreatment of Mv1Lu cells with neutralizing TGF-beta antibodies. SPARC also stimulated Smad2 phosphorylation in MB114 endothelial cells but had no effect on bone morphogenetic protein-regulated Smad1 phosphorylation in either Mv1Lu or MB114 cells. Finally, SPARC and SPARC-EC stimulated TGF-beta-responsive reporter gene expression through a TGF-beta receptor- and Smad2/3-dependent pathway in Mv1Lu cells. Collectively, our findings identify a novel mechanism whereby SPARC inhibits epithelial cell proliferation by selectively commandeering the TGF-beta signaling system, doing so through coupling of SPARC-EC to a TGF-beta receptor- and Smad2/3-dependent pathway.

The use of cystatin C to inhibit epithelial-mesenchymal transition and morphological transformation stimulated by transforming growth factor-beta.

INTRODUCTION: Transforming growth factor-beta (TGF-beta) is a potent suppressor of mammary epithelial cell (MEC) proliferation and is thus an inhibitor of mammary tumor formation. Malignant MECs typically evolve resistance to TGF-beta-mediated growth arrest, enhancing their proliferation, invasion, and metastasis when stimulated by TGF-beta. Recent findings suggest that therapeutics designed to antagonize TGF-beta signaling may alleviate breast cancer progression, thereby improving the prognosis and treatment of breast cancer patients. We identified the cysteine protease inhibitor cystatin C (CystC) as a novel TGF-beta type II receptor antagonist that inhibits TGF-beta binding and signaling in normal and cancer cells. We hypothesized that the oncogenic activities of TGF-beta, particularly its stimulation of mammary epithelial-mesenchymal transition (EMT), can be prevented by CystC. METHOD: Retroviral infection was used to constitutively express CystC or a CystC mutant impaired in its ability to inhibit cathepsin protease activity (namely Delta14CystC) in murine NMuMG MECs and in normal rat kidney (NRK) fibroblasts. The effect of recombinant CystC administration or CystC expression on TGF-beta stimulation of NMuMG cell EMT in vitro was determined with immunofluorescence to monitor rearrangements of actin cytoskeletal architecture and E-cadherin expression. Soft-agar growth assays were performed to determine the effectiveness of CystC in preventing TGF-beta stimulation of morphological transformation and anchorage-independent growth in NRK fibroblasts. Matrigel invasion assays were performed to determine the ability of CystC to inhibit NMuMG and NRK motility stimulated by TGF-beta. RESULTS: CystC and Delta14CystC both inhibited NMuMG cell EMT and invasion stimulated by TGF-beta by preventing actin cytoskeletal rearrangements and E-cadherin downregulation. Moreover, both CystC molecules completely antagonized TGF-beta-mediated morphological transformation and anchorage-independent growth of NRK cells, and inhibited their invasion through synthetic basement membranes. Both CystC and Delta14CystC also inhibited TGF-beta signaling in two tumorigenic human breast cancer cell lines. CONCLUSION: Our findings show that TGF-beta stimulation of initiating metastatic events, including decreased cell polarization, reduced cell-cell contact, and elevated cell invasion and migration, are prevented by CystC treatment. Our findings also suggest that the future development of CystC or its peptide mimetics hold the potential to improve the therapeutic response of human breast cancers regulated by TGF-beta.

PTP1B-dependent regulation of receptor tyrosine kinase signaling by the actin-binding protein Mena.

During breast cancer progression, alternative mRNA splicing produces functionally distinct isoforms of Mena, an actin regulator with roles in cell migration and metastasis. Aggressive tumor cell subpopulations express Mena(INV), which promotes tumor cell invasion by potentiating EGF responses. However, the mechanism by which this occurs is unknown. Here we report that Mena associates constitutively with the tyrosine phosphatase PTP1B and mediates a novel negative feedback mechanism that attenuates receptor tyrosine kinase signaling. On EGF stimulation, complexes containing Mena and PTP1B are recruited to the EGFR, causing receptor dephosphorylation and leading to decreased motility responses. Mena also interacts with the 5' inositol phosphatase SHIP2, which is important for the recruitment of the Mena-PTP1B complex to the EGFR. When Mena(INV) is expressed, PTP1B recruitment to the EGFR is impaired, providing a mechanism for growth factor sensitization to EGF, as well as HGF and IGF, and increased resistance to EGFR and Met inhibitors in signaling and motility assays. In sum, we demonstrate that Mena plays an important role in regulating growth factor-induced signaling. Disruption of this attenuation by Mena(INV) sensitizes tumor cells to low-growth factor concentrations, thereby increasing the migration and invasion responses that contribute to aggressive, malignant cell phenotypes.

Epidermal growth factor receptor downregulation by small heterodimeric binding proteins.

No single engineered protein has been shown previously to robustly downregulate epidermal growth factor receptor (EGFR), a validated cancer target. A panel of fibronectin-based domains was engineered to bind with picomolar to nanomolar affinity to multiple epitopes of EGFR. Monovalent and homo- and hetero-bivalent dimers of these domains were tested for EGFR downregulation. Selected orientations of non-competitive heterodimers decrease EGFR levels by up to 80% in multiple cell types, without activating receptor signaling. These heterodimers inhibit autophosphorylation, proliferation and migration, and are synergistic with the monoclonal antibody cetuximab in these activities. These small (25 kDa) heterodimers represent a novel modality for modulating surface receptor levels.

Transforming growth factor-ß and the hallmarks of cancer.

Tumorigenesis is in many respects a process of dysregulated cellular evolution that drives malignant cells to acquire six phenotypic hallmarks of cancer, including their ability to proliferate and replicate autonomously, to resist cytostatic and apoptotic signals, and to induce tissue invasion, metastasis, and angiogenesis. Transforming growth factor-ß (TGF-ß) is a potent pleiotropic cytokine that functions as a formidable barrier to the development of cancer hallmarks in normal cells and tissues. Paradoxically, tumorigenesis counteracts the tumor suppressing activities of TGF-ß, thus enabling TGF-ß to stimulate cancer invasion and metastasis. Fundamental gaps exist in our knowledge of how malignant cells overcome the cytostatic actions of TGF-ß, and of how TGF-ß stimulates the acquisition of cancer hallmarks by developing and progressing human cancers. Here we review the molecular and cellular mechanisms that underlie the ability of TGF-ß to mediate tumor suppression in normal cells, and conversely, to facilitate cancer progression and disease dissemination in malignant cells.

X-linked inhibitor of apoptosis protein and its E3 ligase activity promote transforming growth factor-{beta}-mediated nuclear factor-{kappa}B activation during breast cancer progression.

The precise sequence of events that enable mammary tumorigenesis to convert transforming growth factor-beta (TGF-beta) from a tumor suppressor to a tumor promoter remains incompletely understood. We show here that X-linked inhibitor of apoptosis protein (xIAP) is essential for the ability of TGF-beta to stimulate nuclear factor-kappaB (NF-kappaB) in metastatic 4T1 breast cancer cells. Indeed whereas TGF-beta suppressed NF-kappaB activity in normal mammary epithelial cells, those engineered to overexpress xIAP demonstrated activation of NF-kappaB when stimulated with TGF-beta. Additionally up-regulated xIAP expression also potentiated the basal and TGF-beta-stimulated transcriptional activities of Smad2/3 and NF-kappaB. Mechanistically xIAP (i) interacted physically with the TGF-beta type I receptor, (ii) mediated the ubiquitination of TGF-beta-activated kinase 1 (TAK1), and (iii) facilitated the formation of complexes between TAK1-binding protein 1 (TAB1) and IkappaB kinase beta that enabled TGF-beta to activate p65/RelA and to induce the expression of prometastatic (i.e. cyclooxygenase-2 and plasminogen activator inhibitor-1) and prosurvival (i.e. survivin) genes. We further observed that inhibiting the E3 ubiquitin ligase function of xIAP or expressing a mutant ubiquitin protein (i.e. K63R-ubiquitin) was capable of blocking xIAP- and TGF-beta-mediated activation of NF-kappaB. Functionally xIAP deficiency dramatically reduced the coupling of TGF-beta to Smad2/3 in NMuMG cells as well as inhibited their expression of mesenchymal markers in response to TGF-beta. More importantly, xIAP deficiency also abrogated the formation of TAB1.IkappaB kinase beta complexes in 4T1 breast cancer cells, thereby diminishing their activation of NF-kappaB, their expression of prosurvival/metastatic genes, their invasion through synthetic basement membranes, and their growth in soft agar. Collectively our findings have defined a novel role for xIAP in mediating oncogenic signaling by TGF-beta in breast cancer cells.

Altered TAB1:I kappaB kinase interaction promotes transforming growth factor beta-mediated nuclear factor-kappaB activation during breast cancer progression.

The conversion of transforming growth factor beta (TGF-beta) from a tumor suppressor to a tumor promoter occurs frequently during mammary tumorigenesis, yet the molecular mechanisms underlying this phenomenon remain undefined. We show herein that TGF-beta repressed nuclear factor-kappaB (NF-kappaB) activity in normal NMuMG cells, but activated this transcription factor in their malignant counterparts, 4T1 cells, by inducing assembly of TGF-beta-activated kinase 1 (TAK1)-binding protein 1 (TAB1):I kappaB kinase beta (IKK beta) complexes, which led to the stimulation of a TAK1:IKK beta:p65 pathway. TAB1:IKK beta complexes could only be detected in NMuMG cells following their induction of epithelial-mesenchymal transition (EMT), which, on TGF-beta treatment, activated NF-kappaB. Expression of a truncated TAB1 mutant [i.e., TAB1(411)] reduced basal and TGF-beta-mediated NF-kappaB activation in NMuMG cells driven to undergo EMT by TGF-beta and in 4T1 cells stimulated by TGF-beta. TAB1(411) expression also inhibited TGF-beta-stimulated tumor necrosis factor-alpha and cyclooxygenase-2 expression in 4T1 cells. Additionally, the ability of human MCF10A-CA1a breast cancer cells to undergo invasion in response to TGF-beta absolutely required the activities of TAK1 and NF-kappaB. Moreover, small interfering RNA-mediated TAK1 deficiency restored the cytostatic activity of TGF-beta in MCF10A-CA1a cells. Finally, expression of truncated TAB1(411) dramatically reduced the growth of 4T1 breast cancers in syngeneic BALB/c, as well as in nude mice, suggesting a potentially important role of NF-kappaB in regulating innate immunity by TGF-beta. Collectively, our findings have defined a novel TAB1:TAK1:IKK beta:NF-kappaB signaling axis that forms aberrantly in breast cancer cells and, consequently, enables oncogenic signaling by TGF-beta.

Role of transforming growth factor-beta in cancer progression.

Invasion and metastasis are the most lethal characteristics of cancer and the leading causes of cancer-related death. Transforming growth factor (TGF)-beta is a multifunctional cytokine that normally functions to prevent the uncontrolled proliferation of epithelial, endothelial and hematopoietic cells. Quite dichotomously, however, aberrant genetic or epigenetic events often negate the cytostatic function of TGF-beta in these cells, leading to tumor formation. Once freed from the growth-inhibitory effects of TGF-beta, cancer cells acquire the ability to proliferate, invade and metastasize when stimulated by TGF-beta. A thorough understanding of the molecular mechanisms underlying these paradoxical functions of TGF-beta remains elusive. Here, the authors review the tumor-suppressing and -promoting activities of TGF-beta and discuss the potential use and targeting of the TGF-beta-signaling system to prevent the progression and acquisition of metastatic phenotypes by human malignancies.