A positive role for Myc in TGFβ-induced Snail transcription and epithelial-to-mesenchymal transition

MYC: connecting selective transcriptional control to global RNA production

Two opposing models have been proposed to describe the function of the MYC oncoprotein in shaping cellular transcriptomes: one posits that MYC amplifies transcription at all active loci; the other that MYC differentially controls discrete sets of genes, the products of which affect global transcript levels. Here, we argue that differential gene regulation by MYC is the sole unifying model that is consistent with all available data. Among other effects, MYC endows cells with physiological and metabolic changes that have the potential to feed back on global RNA production, processing and turnover. The field is progressing steadily towards a full characterization of the MYC-regulated genes and pathways that mediate these biological effects and - by the same token - endow MYC with its pervasive oncogenic potential.

UTX inhibits EMT-induced breast CSC properties by epigenetic repression of EMT genes in cooperation with LSD1 and HDAC1

The histone H3K27 demethylase, UTX, is a known component of the H3K4 methyltransferase MLL complex, but its functional association with H3K4 methylation in human cancers remains largely unknown. Here we demonstrate that UTX loss induces epithelial-mesenchymal transition (EMT)-mediated breast cancer stem cell (CSC) properties by increasing the expression of the SNAIL, ZEB1 and ZEB2 EMT transcription factors (EMT-TFs) and of the transcriptional repressor CDH1. UTX facilitates the epigenetic silencing of EMT-TFs by inducing competition between MLL4 and the H3K4 demethylase LSD1. EMT-TF promoters are occupied by c-Myc and MLL4, and UTX recognizes these proteins, interrupting their transcriptional activation function. UTX decreases H3K4me2 and H3 acetylation at these promoters by forming a transcriptional repressive complex with LSD1, HDAC1 and DNMT1. Taken together, our findings indicate that UTX is a prominent tumour suppressor that functions as a negative regulator of EMT-induced CSC-like properties by epigenetically repressing EMT-TFs.

The Placental Gene PEG10 Promotes Progression of Neuroendocrine Prostate Cancer

More potent targeting of the androgen receptor (AR) in advanced prostate cancer is driving an increased incidence of neuroendocrine prostate cancer (NEPC), an aggressive and treatment-resistant AR-negative variant. Its molecular pathogenesis remains poorly understood but appears to require TP53 and RB1 aberration. We modeled the development of NEPC from conventional prostatic adenocarcinoma using a patient-derived xenograft and found that the placental gene PEG10 is de-repressed during the adaptive response to AR interference and subsequently highly upregulated in clinical NEPC. We found that the AR and the E2F/RB pathway dynamically regulate distinct post-transcriptional and post-translational isoforms of PEG10 at distinct stages of NEPC development. In vitro, PEG10 promoted cell-cycle progression from G0/G1 in the context of TP53 loss and regulated Snail expression via TGF-β signaling to promote invasion. Taken together, these findings show the mechanistic relevance of RB1 and TP53 loss in NEPC and suggest PEG10 as a NEPC-specific target.

TGF-beta1 pathway activation and adherens junction molecular pattern in nonsyndromic mitral valve prolapse

AIMS

Dysregulation of the transforming growth factor beta (TGF-β) 1 pathway has been associated with either syndromic or isolated mitral valve (MV) prolapse due to myxoid degeneration (floppy MV). The activation of Smad receptor-mediated intracellular TGF-β pathway and its effect on adherens junction (AJ) molecular pattern of activated valvular interstitial cells (VICs) in MV prolapse are herein investigated.

METHODS

Floppy MV leaflets were obtained from 30 patients (24 males, mean age 55.5±12.7 years) who underwent surgical repair, and 10 age- and sex-matched Homograft Tissue Bank samples served as controls. MV leaflet cellular and extracellular matrix composition, including collagen I and III, was evaluated by histology and transmission electron microscopy. Smad2 active phosphorylated form (p-Smad2), α-smooth muscle actin (α-SMA), and junctional proteins (N-cadherin, cadherin-11, β-catenin, plakoglobin, plakophilin-2) in VICs were assessed by immunohistochemistry and immunofluorescence and confirmed by immunoblotting. Quantitative real-time polymerase chain reaction was carried out for components of TGF-β pathway cascade and filamin A (FLN-A).

RESULTS

Floppy MV leaflets were thicker (P<.001) and had higher α-SMA+ cell density (P=.002) and collagen III expression (P<.001) than controls. Enhanced p-Smad2 nuclear immunoreactivity (P<.001) and TGF-β1 gene (P=.045), TIMP1 (P=.020), and CTGF (P=.047) expression but no differences in FLN-A and total Smad2 gene expression levels were found between floppy MV and controls. Higher expression of cadherin-11, either exclusively or in colocalization with N-cadherin, and aberrant presence of plakophilin-2 at the AJ were found in floppy MV vs.

CONCLUSIONS

TGF-β1 pathway activation in nonsyndromic MV prolapse induces VICs differentiation into contractile myofibroblasts and is associated with changes in the molecular pattern of the AJ, with increased cadherin-11 and aberrant plakophilin-2 expression. AJ reinforcement might promote latent TGF-β1 activation leading to extracellular matrix remodeling in floppy MV.

DOT1L cooperates with the c-Myc-p300 complex to epigenetically derepress CDH1 transcription factors in breast cancer progression

DOT1L has emerged as an anticancer target for MLL-associated leukaemias; however, its functional role in solid tumours is largely unknown. Here we identify that DOT1L cooperates with c-Myc and p300 acetyltransferase to epigenetically activate epithelial–mesenchymal transition (EMT) regulators in breast cancer progression. DOT1L recognizes SNAIL, ZEB1 and ZEB2 promoters via interacting with the c-Myc-p300 complex and facilitates lysine-79 methylation and acetylation towards histone H3, leading to the dissociation of HDAC1 and DNMT1 in the regions. The upregulation of these EMT regulators by the DOT1L-c-Myc-p300 complex enhances EMT-induced breast cancer stem cell (CSC)-like properties. Furthermore, in vivo orthotopic xenograft models show that DOT1L is required for malignant transformation of breast epithelial cells and breast tumour initiation and metastasis. Clinically, DOT1L expression is associated with poorer survival and aggressiveness of breast cancers. Collectively, we suggest that cooperative effect of DOT1L and c-Myc-p300 is critical for acquisition of aggressive phenotype of breast cancer by promoting EMT/CSC.

DOT1L is an anti-cancer therapeutic target in leukaemia but has a poorly understood role in solid tumours. Here the authors show that DOT1L expression is associated with poor survival and aggressive cancers by helping to epigenetically activate the epithelial-mesenchymal transition during breast cancer progression.

HPIP is upregulated in colorectal cancer and regulates colorectal cancer cell proliferation, apoptosis and invasion

Hematopoietic pre-B cell leukemia transcription factor (PBX)-interacting protein (HPIP) was shown to play a role in cancer development and progression. However, the role of HPIP in colorectal cancer (CRC) is unknown. Here, we report that HPIP is overexpressed in most of CRC patients and predicts poor clinical outcome in CRC. HPIP promotes CRC cell proliferation via activation of G1/S and G2/M checkpoint transitions, concomitant with a marked increase of the positive cell cycle regulators, including cyclin D1, cyclin A, and cyclin B1. HPIP inhibits CRC cell apoptosis accompanied by the decreased levels of BAX and PIG3, the inducers of apoptosis, and the increased level of the apoptosis inhibitor BCL2. HPIP blocks caspase-3-mediated cleavage of PARP, an important apoptosis marker. HPIP promotes CRC cell migration and invasion, and regulates epithelial-mesenchymal transition (EMT), which plays a critical role in cancer cell migration and invasion. Activation of MAPK/ERK1/2 and PI3k/AKT pathways is required for HPIP modulation of CRC cell proliferation, migration and EMT. Moreover, HPIP knockdown suppresses colorectal tumor growth in nude mice. These data highlight the important role of HPIP in CRC cell proliferation and progression and suggest that HPIP may be a useful target for CRC therapy.

IGFBP-rP1 suppresses epithelial-mesenchymal transition and metastasis in colorectal cancer

Epithelial-mesenchymal transition (EMT) was initially recognized during organogenesis and has recently been reported to be involved in promoting cancer invasion and metastasis. Cooperation of transforming growth factor-β (TGF-β) and other signaling pathways, such as Ras and Wnt, is essential to inducing EMT, but the molecular mechanisms remain to be fully determined. Here, we reported that insulin-like growth factor binding protein-related protein 1 (IGFBP-rP1), a potential tumor suppressor, controls EMT in colorectal cancer progression. We revealed the inhibitory role of IGFBP-rP1 through analyses of clinical colorectal cancer samples and various EMT and metastasis models in vitro and in vivo. Moreover, we demonstrated that IGFBP-rP1 suppresses EMT and tumor metastasis by repressing TGF-β-mediated EMT through the Smad signaling cascade. These data establish that IGFBP-rP1 functions as a suppressor of EMT and metastasis in colorectal cancer.

TGF-β1 induces epigenetic silence of TIP30 to promote tumor metastasis in esophageal carcinoma

TGF-β1, a potent EMT (epithelial-mesenchymal transition) inducer present in the tumor microenvironment, is involved in the metastasis and progression of various carcinomas, including esophageal squamous cell carcinoma (ESCC). TIP30 (30kDa HIV-1 Tat interacting protein) is a putative tumor metastasis suppressor. Here, we found TIP30 was decreased in cells undergoing EMT induced by TGF-β1, an occurrence that was related to promoter hypermethylation. TGF-β1 induced TIP30 hypermethylation via increasing DNMT1 and DNMT3A expression, which could be restored by TGF-β antibodies. In our in vitro and in vivo studies, we showed that silence of TIP30 led to EMT, enhanced migrative and invasive abilities of ESCC cells, promoted tumor metastasis in xenografted mice; alternatively, overexpression of TIP30 inhibited TGF-β1-induced EMT, and metastatic abilities of ESCC cells. Mechanically, TIP30 silencing induced the nuclear translocation and transcriptional activation of β-catenin in an AKT-dependent manner, which further resulted in the initiation of EMT. Consistently, TIP30 was frequently methylated and downregulated in ESCC patients. Loss of TIP30 correlated with nuclear β-catenin and aberrant E-cadherin expression. TIP30 was a powerful marker in predicting the prognosis of ESCC. Taken together, our results suggest a novel and critical role of TIP30 involved in TGF-β1-induced activation of AKT/β-catenin signaling and ESCC metastasis.

Reprogramming during epithelial to mesenchymal transition under the control of TGFβ

Epithelial-mesenchymal transition (EMT) refers to plastic changes in epithelial tissue architecture. Breast cancer stromal cells provide secreted molecules, such as transforming growth factor β (TGFβ), that promote EMT on tumor cells to facilitate breast cancer cell invasion, stemness and metastasis. TGFβ signaling is considered to be abnormal in the context of cancer development; however, TGFβ acting on breast cancer EMT resembles physiological signaling during embryonic development, when EMT generates or patterns new tissues. Interestingly, while EMT promotes metastatic fate, successful metastatic colonization seems to require the inverse process of mesenchymal-epithelial transition (MET). EMT and MET are interconnected in a time-dependent and tissue context-dependent manner and are coordinated by TGFβ, other extracellular proteins, intracellular signaling cascades, non-coding RNAs and chromatin-based molecular alterations. Research on breast cancer EMT/MET aims at delivering biomolecules that can be used diagnostically in cancer pathology and possibly provide ideas for how to improve breast cancer therapy.

Intratumoral heterogeneity: Clonal cooperation in epithelial-to-mesenchymal transition and metastasis

Although phenotypic intratumoral heterogeneity was first described many decades ago, the advent of next-generation sequencing has provided conclusive evidence that in addition to phenotypic diversity, significant genotypic diversity exists within tumors. Tumor heterogeneity likely arises both from clonal expansions, as well as from differentiation hierarchies existent in the tumor, such as that established by cancer stem cells (CSCs) and non-CSCs. These differentiation hierarchies may arise due to genetic mutations, epigenetic alterations, or microenvironmental influences. An additional differentiation hierarchy within epithelial tumors may arise when only a few tumor cells trans-differentiate into mesenchymal-like cells, a process known as epithelial-to-mesenchymal transition (EMT). Again, this process can be influenced by both genetic and non-genetic factors. In this review we discuss the evidence for clonal interaction and cooperation for tumor maintenance and progression, particularly with respect to EMT, and further address the far-reaching effects that tumor heterogeneity may have on cancer therapy.

An atypical canonical bone morphogenetic protein (BMP) signaling pathway regulates Msh homeobox 1 (Msx1) expression during odontogenesis

Bone morphogenetic protein (BMP) signaling plays an essential role in early tooth development, evidenced by disruption of BMP signaling leading to an early arrested tooth development. Despite being a central mediator of BMP canonical signaling pathway, inactivation of Smad4 in dental mesenchyme does not result in early developmental defects. In the current study, we investigated the mechanism of receptor-activated Smads (R-Smads) and Smad4 in the regulation of the odontogenic gene Msx1 expression in the dental mesenchyme. We showed that the canonical BMP signaling is not operating in the early developing tooth, as assessed by failed activation of the BRE-Gal transgenic allele and the absence of phospho-(p)Smad1/5/8-Smad4 complexes. The absence of pSmad1/5/8-Smad4 complex appeared to be the consequence of saturation of Smad4 by pSmad2/3 in the dental mesenchyme as knockdown of Smad2/3 or overexpression of Smad4 led to the formation of pSmad1/5/8-Smad4 complexes and activation of canonical BMP signaling in dental mesenchymal cells. We showed that Smad1/5 but not Smad4 are required for BMP-induced expression of Msx1 in dental mesenchymal cells. We further presented evidence that in the absence of Smad4, BMPs are still able to induce pSmad1/5/8 nuclear translocation and their binding to the Msx1 promoter directly in dental mesenchymal cells. Our results demonstrate the functional operation of an atypical canonical BMP signaling (Smad4-independent and Smad1/5/8-dependent) pathway in the dental mesenchyme during early odontogenesis, which may have general implication in the development of other organs.

Snail promotes epithelial mesenchymal transition in breast cancer cells in part via activation of nuclear ERK2

Snail transcription factor is up-regulated in several cancers and associated with increased tumor migration and invasion via induction of epithelial-to-mesenchymal transition (EMT). MAPK (ERK1/2) signaling regulates cellular processes including cell motility, adhesion, and invasion. We investigated the regulation of ERK1/2 by Snail in breast cancer cells. ERK1/2 activity (p-ERK) was higher in breast cancer patient tissue as compared to normal tissue. Snail and p-ERK were increased in several breast cancer cell lines as compared to normal mammary epithelial cells. Snail knockdown in MDA-MB-231 and T47-D breast cancer cells decreased or re-localized p-ERK from the nuclear compartment to the cytoplasm. Snail overexpression in MCF-7 breast cancer cells induced EMT, increased cell migration, decreased cell adhesion and also increased tumorigenicity. Snail induced nuclear translocation of p-ERK, and the activation of its subcellular downstream effector, Elk-1. Inhibiting MAPK activity with UO126 or knockdown of ERK2 isoform with siRNA in MCF-7 Snail cells reverted EMT induced by Snail as shown by decreased Snail and vimentin expression, decreased cell migration and increased cell adhesion. Overall, our data suggest that ERK2 isoform activation by Snail in aggressive breast cancer cells leads to EMT associated with increased cell migration and decreased cell adhesion. This regulation is enhanced by positive feedback regulation of Snail by ERK2. Therefore, therapeutic targeting of ERK2 isoform may be beneficial for breast cancer.

SHON, a novel secreted protein, regulates epithelial-mesenchymal transition through transforming growth factor-β signaling in human breast cancer cells

The epithelial-mesenchymal transition (EMT) is one of the main mechanisms contributing to the onset of cancer metastasis, and has proven to be associated with breast cancer progression. SHON is a novel secreted hominoid-specific protein we have previously identified; it is specifically expressed in all human cancer cell lines tested and is oncogenic for human mammary carcinoma cells. Here, we show that ectopic overexpression of SHON in immortalized human mammary epithelial cells is sufficient for cells to acquire the mesenchymal traits, as well as the enhanced cell migration and invasion, along with the epithelial stem cell properties characterized by increased CD44(high) /CD24(low) subpopulation and mammosphere-forming ability. Moreover, we demonstrate that SHON positively activates the autocrine transforming growth factor-β (TGF-β) pathway to contribute to EMT, while SHON itself is induced by TGF-β in mammary epithelial cells. These data are in favor of a SHON-TGFβ-SHON-positive feedback loop that regulates EMT program in breast cancer progression. Finally, examination of the human clinic breast cancer specimens reveals that tumor cells may extracellularly release SHON protein to promote the cancerization of surrounding cells. Together, our findings define an important function of SHON in regulation of EMT via TGF-β signaling, which is closely associated with the invasive subtypes of human breast cancer.

A high-resolution whole-genome map of the distinctive epigenomic landscape induced by butyrate in bovine cells

This report presents a study utilizing next-generation sequencing technology, combined with chromatin immunoprecipitation (ChIP-seq) technology to analyze histone modification induced by butyrate and to construct a high-definition map of the epigenomic landscape with normal histone H3 and H4 and their variants in bovine cells at the whole-genome scale. A total of 10 variants of histone H3 and H4 modifications were mapped at the whole-genome scale (acetyl-H3K18-ChIP-seq, trimethy-H3K9, histone H4 ChIP-seq, acetyl-H4K5 ChIP-seq, acetyl-H4K12 ChIP-seq, acetyl-H4K16 ChIP-seq, histone H3 ChIP-seq, acetyl H3H9 ChIP-seq, acetyl H3K27 ChIP-seq and tetra-acetyl H4 ChIP-seq). Integrated experiential data and an analysis of histone and histone modification at a single base resolution across the entire genome are presented. We analyzed the enriched binding regions in the proximal promoter (within 5 kb upstream or at the 5'-untranslated region from the transcriptional start site (TSS)), and the exon, intron and intergenic regions (defined by regions 25 kb upstream and 10 kb downstream from the TSS). A de novo search for the binding motif of the 10 ChIP-seq datasets discovered numerous motifs from each of the ChIP-seq datasets. These consensus sequences indicated that histone modification at different locations changes the histone H3 and H4 binding preferences. Nevertheless, a high degree of conservation in histone binding also was presented in these motifs. This first extensive epigenomic landscape mapping in bovine cells offers a new framework and a great resource for testing the role of epigenomes in cell function and transcriptomic regulation.

Role of p38 MAP Kinase Signal Transduction in Solid Tumors

Mitogen-activated protein kinases (MAPKs) mediate a wide variety of cellular behaviors in response to extracellular stimuli. One of the main subgroups, the p38 MAP kinases, has been implicated in a wide range of complex biologic processes, such as cell proliferation, cell differentiation, cell death, cell migration, and invasion. Dysregulation of p38 MAPK levels in patients are associated with advanced stages and short survival in cancer patients (e.g., prostate, breast, bladder, liver, and lung cancer). p38 MAPK plays a dual role as a regulator of cell death, and it can either mediate cell survival or cell death depending not only on the type of stimulus but also in a cell type specific manner. In addition to modulating cell survival, an essential role of p38 MAPK in modulation of cell migration and invasion offers a distinct opportunity to target this pathway with respect to tumor metastasis. The specific function of p38 MAPK appears to depend not only on the cell type but also on the stimuli and/or the isoform that is activated. p38 MAPK signaling pathway is activated in response to diverse stimuli and mediates its function by components downstream of p38. Extrapolation of the knowledge gained from laboratory findings is essential to address the clinical significance of p38 MAPK signaling pathways. The goal of this review is to provide an overview on recent progress made in defining the functions of p38 MAPK pathways with respect to solid tumor biology and generate testable hypothesis with respect to the role of p38 MAPK as an attractive target for intervention of solid tumors.

A novel PTEN/mutant p53/c-Myc/Bcl-XL axis mediates context-dependent oncogenic effects of PTEN with implications for cancer prognosis and therapy

Phosphatase and tensin homolog located on chromosome 10 (PTEN) is one of the most frequently mutated tumor suppressors in human cancer including in glioblastoma. Here, we show that PTEN exerts unconventional oncogenic effects in glioblastoma through a novel PTEN/mutant p53/c-Myc/Bcl-XL molecular and functional axis. Using a wide array of molecular, genetic, and functional approaches, we demonstrate that PTEN enhances a transcriptional complex containing gain-of-function mutant p53, CBP, and NFY in human glioblastoma cells and tumor tissues. The mutant p53/CBP/NFY complex transcriptionally activates the oncogenes c-Myc and Bcl-XL, leading to increased cell proliferation, survival, invasion, and clonogenicity. Disruption of the mutant p53/c-Myc/Bcl-XL axis or mutant p53/CBP/NFY complex reverses the transcriptional and oncogenic effects of PTEN and unmasks its tumor-suppressive function. Consistent with these data, we find that PTEN expression is associated with worse patient survival than PTEN loss in tumors harboring mutant p53 and that a small molecule modulator of p53 exerts greater antitumor effects in PTEN-expressing cancer cells. Altogether, our study describes a new signaling pathway that mediates context-dependent oncogenic/tumor-suppressive role of PTEN. The data also indicate that the combined mutational status of PTEN and p53 influences cancer prognosis and anticancer therapies that target PTEN and p53.

JAK/STAT3 signaling is required for TGF-β-induced epithelial-mesenchymal transition in lung cancer cells

Epithelial-mesenchymal transition (EMT), a key step in the early stages of cancer metastasis, is orchestrated by several signaling pathways, including IL-6/JAK/STAT3 and TGF-β/Smad signaling. However, an association between the two signaling pathways during the EMT process is largely unknown. Here, we focused on lung cancer and demonstrated that TGF-β1 induced the phosphorylation of Smad3 (p-Smad3), upregulation of Snail, a fibroblast-like morphology, and downregulation of E-cadherin as well as upregulation of vimentin in lung cancer cell lines. SIS3 (an inhibitor of Smad3) suppressed TGF-β1-induced activation of Smad3, upregulation of Snail and the EMT process. Importantly, the JAK2/STAT3-specific inhibitor AG490 blocked Stat3 phosphorylation, resulting in attenuated levels of TGF-β1-induced p-Smad3, Snail, MMP2, and Smad-mediated PAI-1 promoter reporter gene activity in A549 and H1650 cells. Subsequently, AG490 inhibited TGF-β-induced cell migration and invasion. Moreover, exogenous IL-6 treatment stimulated Stat3 activation, enhanced TGF-β-induced expression of p-Smad3 and Snail, aggravated the EMT process, and increased lung cancer cell migration and invasion induced by TGF-β1. Our findings show that the JAK/STAT3 pathway is required for TGF-β-induced EMT and cancer cell migration and invasion via upregulation of the expression of p-Smad3 and Snail, and the IL-6/JAK/STAT3 and TGF-β/Smad signaling synergistically enhance EMT in lung carcinomas. The present study suggests a novel rationale for inhibiting cancer metastasis using anti-IL-6/JAK/STAT3 and anti-TGF-β/Smad therapeutic strategies.

TGFβ and matrix-regulated epithelial to mesenchymal transition

BACKGROUND

The progression of cancer through stages that guide a benign hyperplastic epithelial tissue towards a fully malignant and metastatic carcinoma, is driven by genetic and microenvironmental factors that remodel the tissue architecture. The concept of epithelial-mesenchymal transition (EMT) has evolved to emphasize the importance of plastic changes in tissue architecture, and the cross-communication of tumor cells with various cells in the stroma and with specific molecules in the extracellular matrix (ECM).

SCOPE OF THE REVIEW

Among the multitude of ECM-embedded cytokines and the regulatory potential of ECM molecules, this article focuses on the cytokine transforming growth factor β (TGFβ) and the glycosaminoglycan hyaluronan, and their roles in cancer biology and EMT. For brevity, we concentrate our effort on breast cancer.

MAJOR CONCLUSIONS

Both normal and abnormal TGFβ signaling can be detected in carcinoma and stromal cells, and TGFβ-induced EMT requires the expression of hyaluronan synthase 2 (HAS2). Correspondingly, hyaluronan is a major constituent of tumor ECM and aberrant levels of both hyaluronan and TGFβ are thought to promote a wounding reaction to the local tissue homeostasis. The link between EMT and metastasis also involves the mesenchymal-epithelial transition (MET). ECM components, signaling networks, regulatory non-coding RNAs and epigenetic mechanisms form the network of regulation during EMT-MET.

GENERAL SIGNIFICANCE

Understanding the mechanism that controls epithelial plasticity in the mammary gland promises the development of valuable biomarkers for the prognosis of breast cancer progression and even provides new ideas for a more integrative therapeutic approach against disease. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Mammalian MYC Proteins and Cancer

The MYC family of proteins is a group of basic-helix-loop-helix-leucine zipper transcription factors that feature prominently in cancer. Overexpression of MYC is observed in the vast majority of human malignancies and promotes an extraordinary set of changes that impact cell proliferation, growth, metabolism, DNA replication, cell cycle progression, cell adhesion, differentiation, and metastasis. The purpose of this review is to introduce the reader to the mammalian family of MYC proteins, highlight important functional properties that endow them with their potent oncogenic potential, describe their mechanisms of action and of deregulation in cancer cells, and discuss efforts to target the unique properties of MYC, and of MYC-driven tumors, to treat cancer.

AP4 is a mediator of epithelial-mesenchymal transition and metastasis in colorectal cancer

The basic helix-loop-helix transcription factor AP4/TFAP4/AP-4 is encoded by a c-MYC target gene and displays up-regulation concomitantly with c-MYC in colorectal cancer (CRC) and numerous other tumor types. Here a genome-wide characterization of AP4 DNA binding and mRNA expression was performed using a combination of microarray, genome-wide chromatin immunoprecipitation, next-generation sequencing, and bioinformatic analyses. Thereby, hundreds of induced and repressed AP4 target genes were identified. Besides many genes involved in the control of proliferation, the AP4 target genes included markers of stemness (LGR5 and CD44) and epithelial-mesenchymal transition (EMT) such as SNAIL, E-cadherin/CDH1, OCLN, VIM, FN1, and the Claudins 1, 4, and 7. Accordingly, activation of AP4 induced EMT and enhanced migration and invasion of CRC cells. Conversely, down-regulation of AP4 resulted in mesenchymal-epithelial transition and inhibited migration and invasion. In addition, AP4 induction was required for EMT, migration, and invasion caused by ectopic expression of c-MYC. Inhibition of AP4 in CRC cells resulted in decreased lung metastasis in mice. Elevated AP4 expression in primary CRC significantly correlated with liver metastasis and poor patient survival. These findings imply AP4 as a new regulator of EMT that contributes to metastatic processes in CRC and presumably other carcinomas.

A signal transduction pathway from TGF-β1 to SKP2 via Akt1 and c-Myc and its correlation with progression in human melanoma

Both SKP2 (S-phase kinase-associated protein 2) and transforming growth factor-β1 (TGF-β1) play important roles in cancer metastasis through different mechanisms: TGF-β1 via induction of epithelial-mesenchymal transition (EMT) and SKP2 via downregulating p27(kip1). Recent studies indicated that c-Myc and Akt1 were active players in metastasis. In this study we demonstrated a crosstalk between these pathways. Specifically, we found that TGF-β1 treatment increased SKP2 expression accompanied with increased phosphorylation of Akt1 and c-Myc protein accumulation during EMT. We demonstrated that Akt1 was required for TGF-β1-mediated SKP2 upregulation and that c-Myc transcription factor specifically bound to the promoter of SKP2 for its enhanced transcription. Analysis of 25 samples of normal human skin, nevi, and melanomas revealed a positive correlation between c-Myc and SKP2 accumulation. Furthermore, accumulation of SKP2 and c-Myc proteins was significantly higher in metastatic melanoma samples as compared with that in primary melanomas, which again was higher than that in normal skin or nevi. In summary, our results integrated TGF-β1 signals to SKP2 via Akt1 and c-Myc during EMT, and provided, to our knowledge, a previously unreported mechanistic molecular event for TGF-β1-induced metastasis in human melanoma.

Mammary epithelial cell interactions with fibronectin stimulate epithelial-mesenchymal transition

In the mammary gland, the stromal extracellular matrix (ECM) undergoes dramatic changes during development and in tumorigenesis. For example, normal adult breast tissue is largely devoid of the ECM protein fibronectin (FN) whereas high FN levels have been detected in the stroma of breast tumors. FN is an established marker for epithelial-mesenchymal transition (EMT), which occurs during development and has been linked to cancer. During EMT, epithelial cell adhesion switches from cell-cell contacts to mainly cell-ECM interactions raising the possibility that FN may have a role in promoting this transition. Using MCF-10A mammary epithelial cells, we show that exposure to exogenous FN induces an EMT response including up-regulation of the EMT markers FN, Snail, N-cadherin, vimentin, the matrix metalloprotease MMP2, α-smooth muscle actin, and phospho-Smad2 as well as acquisition of cell migratory behavior. FN-induced EMT depends on Src kinase and ERK/MAP kinase signaling but not on the immediate early gene EGR-1. FN initiates EMT under serum-free conditions; this response is partially reversed by a TGFβ neutralizing antibody suggesting that FN enhances the effect of endogenous TGFβ. EMT marker expression is up-regulated in cells on a fragment of FN containing the integrin-binding domain but not other domains. Differences in gene expression between FN and MG are maintained with addition of a sub-threshold level of TGFβ1. Together, these results show that cells interacting with FN are primed to respond to TGFβ. The ability of FN to induce EMT shows an active role for the stromal ECM in this process and supports the notion that the increased levels of FN observed in breast tumors facilitate tumorigenesis.

Hepatitis B virus X protein represses miRNA-148a to enhance tumorigenesis

MicroRNAs (miRNAs) have been shown to be dysregulated in virus-related cancers; however, miRNA regulation of virus-related cancer development and progression remains poorly understood. Here, we report that miR-148a is repressed by hepatitis B virus (HBV) X protein (HBx) to promote cancer growth and metastasis in a mouse model of hepatocellular carcinoma (HCC). Hematopoietic pre-B cell leukemia transcription factor-interacting protein (HPIP) is an important regulator of cancer cell growth. We used miRNA target prediction programs to identify miR-148a as a regulator of HPIP. Expression of miR-148a in hepatoma cells reduced HPIP expression, leading to repression of AKT and ERK and subsequent inhibition of mTOR through the AKT/ERK/FOXO4/ATF5 pathway. HBx has been shown to play a critical role in the molecular pathogenesis of HBV-related HCC. We found that HBx suppressed p53-mediated activation of miR-148a. Moreover, expression of miR-148a was downregulated in patients with HBV-related liver cancer and negatively correlated with HPIP, which was upregulated in patients with liver cancer. In cultured cells and a mouse xenograft model, miR-148a reduced the growth, epithelial-to-mesenchymal transition, invasion, and metastasis of HBx-expressing hepatocarcinoma cells through inhibition of HPIP-mediated mTOR signaling. Thus, miR-148a activation or HPIP inhibition may be a useful strategy for cancer treatment.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

CIP2A is a predictor of survival and a novel therapeutic target in bladder urothelial cell carcinoma

Cancerous inhibitor of protein phosphatase 2A (CIP2A) is a recently identified human oncoprotein that stabilizes the c-MYC protein. Herein, we aimed to investigate its expression pattern, clinical significance, and biological function in urothelial cell carcinoma (UCC) of the bladder. CIP2A expression was examined in 20 fresh bladder UCC tissues and paired adjacent normal bladder tissues by RT-PCR and Western blot. Immunohistochemistry for CIP2A was performed on additional 117 bladder UCC tissues. The clinical significance of CIP2A expression was analyzed. CIP2A downregulation was performed in bladder UCC cell line T24 with high abundance of CIP2A, and the effects of CIP2A silencing on cell proliferation, migration, invasion in vitro, and tumor growth in vivo were evaluated. We found that CIP2A expression was upregulated in bladder UCC tissues relative to adjacent normal bladder tissues. Clinicopathological analysis showed that CIP2A expression was significantly associated with tumor stage (P = 0.004), histological grade (P = 0.007), and lymph node status (P = 0.001). The Kaplan-Meier survival curves revealed that CIP2A expression was associated with poor prognosis in bladder UCC patients (log-rank value = 14.704, P < 0.001). CIP2A expression was an independent prognostic marker of overall patient survival in a multivariate analysis (P = 0.015). Knockdown of the CIP2A expression reduced cell proliferation, anchorage-independent growth, migration, invasion, and tumor growth in xenograft model mice. Our findings suggest that CIP2A is an independent predictor of poor prognosis of bladder UCC patients, and inhibition of its expression might be of therapeutic significance.

Histone demethylase KDM6B promotes epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is a critical event that occurs in embryonic development, tissue repair control, organ fibrosis, and carcinoma invasion and metastasis. Although significant progress has been made in understanding the molecular regulation of EMT, little is known about how chromatin is modified in EMT. Chromatin modifications through histone acetylation and methylation determine the precise control of gene expression. Recently, histone demethylases were found to play important roles in gene expression through demethylating mono-, di-, or trimethylated lysines. KDM6B (also known as JMJD3) is a histone demethylase that might activate gene expression by removing repressive histone H3 lysine 27 trimethylation marks from chromatin. Here we report that KDM6B played a permissive role in TGF-β-induced EMT in mammary epithelial cells by stimulating SNAI1 expression. KDM6B was induced by TGF-β, and the knockdown of KDM6B inhibited EMT induced by TGF-β. Conversely, overexpression of KDM6B induced the expression of mesenchymal genes and promoted EMT. Chromatin immunoprecipitation (ChIP) assays revealed that KDM6B promoted SNAI1 expression by removing histone H3 lysine trimethylation marks. Consistently, our analysis of the Oncomine database found that KDM6B expression was significantly increased in invasive breast carcinoma compared with normal breast tissues. The knockdown of KDM6B significantly inhibited breast cancer cell invasion. Collectively, our study uncovers a novel epigenetic mechanism regulating EMT and tumor cell invasion, and has important implication in targeting cancer metastasis.

Far upstream element binding protein 1: a commander of transcription, translation and beyond

The far upstream binding protein 1 (FBP1) was first identified as a DNA-binding protein that regulates c-Myc gene transcription through binding to the far upstream element (FUSE) in the promoter region 1.5 kb upstream of the transcription start site. FBP1 collaborates with TFIIH and additional transcription factors for optimal transcription of the c-Myc gene. In recent years, mounting evidence suggests that FBP1 acts as an RNA-binding protein and regulates mRNA translation or stability of genes, such as GAP43, p27(Kip) and nucleophosmin. During retroviral infection, FBP1 binds to and mediates replication of RNA from Hepatitis C and Enterovirus 71. As a nuclear protein, FBP1 may translocate to the cytoplasm in apoptotic cells. The interaction of FBP1 with p38/JTV-1 results in FBP1 ubiquitination and degradation by the proteasomes. Transcriptional and post-transcriptional regulations by FBP1 contribute to cell proliferation, migration or cell death. FBP1 association with carcinogenesis has been reported in c-Myc dependent or independent manner. This review summarizes biochemical features of FBP1, its mechanism of action, FBP family members and the involvement of FBP1 in carcinogenesis.

c-Myc expression and MEK1-induced Erk2 nuclear localization are required for TGF-beta induced epithelial-mesenchymal transition and invasion in prostate cancer

Understanding the initial mechanisms by which epithelial cells transform to an invasive phenotype is critical to the development of diagnostics that can identify the metastatic potential of cancers as well as therapeutic agents that can prevent metastases. Changes in cellular response to the transforming growth factor-beta (TGF-β) cytokine are known to promote epithelial cell invasion and metastasis in part through induction of epithelial-mesenchymal transitions (EMTs). In this report, we demonstrate that non-metastatic human prostate cancer cell lines of increasing Gleason score can be induced to undergo EMT when treated with TGF-β in combination with epidermal growth factor. Mechanistic studies revealed that in cells stably transfected with activated Ras, TGF-β alone induced EMT and that a Ras-Raf-MEK1, but not MEK2, signaling cascade is necessary and sufficient for Erk2 nuclear localization that works in concert with TGF-β to promote EMT. Furthermore, we show for the first time that expression of the transcription factor c-myc, which is phosphorlyated by Erk2, is required for EMT. Characteristically, EMT involved adoption of a spindle-shaped morphology, loss of E-cadherin and increased expression of Vimentin, Fibronectin and Fibroblast Specific Protein-1 (S100A4). Prostate cells undergoing EMT became invasive and expressed several genes associated with metastasis, including MT-MMP1, MMP-2/9, the MMP-9 homodimer, Slug and Twist2. In sum, we demonstrate a novel mechanism by which non-invasive primary prostate tumor cells transition to an invasive phenotype characteristic of malignant tumor cells in response to TGF-β signaling.

USP4 is regulated by AKT phosphorylation and directly deubiquitylates TGF-β type I receptor

The stability and membrane localization of the transforming growth factor-β (TGF-β) type I receptor (TβRI) determines the levels of TGF-β signalling. TβRI is targeted for ubiquitylation-mediated degradation by the SMAD7-SMURF2 complex. Here we performed a genome-wide gain-of-function screen and identified ubiquitin-specific protease (USP) 4 as a strong inducer of TGF-β signalling. USP4 was found to directly interact with TβRI and act as a deubiquitylating enzyme, thereby controlling TβRI levels at the plasma membrane. Depletion of USP4 mitigates TGF-β-induced epithelial to mesenchymal transition and metastasis. Importantly, AKT (also known as protein kinase B), which has been associated with poor prognosis in breast cancer, directly associates with and phosphorylates USP4. AKT-mediated phosphorylation relocates nuclear USP4 to the cytoplasm and membrane and is required for maintaining its protein stability. Moreover, AKT-induced breast cancer cell migration was inhibited by USP4 depletion and TβRI kinase inhibition. Our results uncover USP4 as an important determinant for crosstalk between TGF-β and AKT signalling pathways.

From oncogene to tumor suppressor: the dual role of Myc in leukemia

The transcription factor c-Myc strongly stimulates cell proliferation but also regulates apoptosis, senescence, cell competition and cell differentiation, and its elevated activity is a hallmark for human tumorigenesis. c-Myc induces transcription by forming heterodimers with Max and then directly binding DNA at E-box sequences. Conversely, transcription repression depends primarily on the inhibitory interaction of c-Myc/Max with Miz-1 at DNA initiator elements. We recently described a distinct mechanism of c-Myc gene regulation, in which c-Myc interacts with the retinoic acid receptor α (RARα) and is recruited to RAR DNA binding sequences (RAREs). In leukemia cells, this c-Myc/RARα complex functions either as an activator or a repressor of RARα-dependent targets through a phosphorylation switch. Unphosphorylated c-Myc interacts with RARα to repress the expression of RAR targets required for differentiation, thereby aggravating leukemia malignancy. However, if c-Myc is phosphorylated by the kinase Pak2, the c-Myc/RARα complex activates transcription of those same genes to stimulate differentiation, thus reducing tumor burden. Here, we discuss the role of c-Myc in balancing proliferation and differentiation and how modulating this previously unidentified c-Myc activity might provide alternative therapies against leukemia and possibly other types of tumors.

Ubiquitination and the Ubiquitin-Proteasome System as regulators of transcription and transcription factors in epithelial mesenchymal transition of cancer

Epithelial to Mesenchymal Transition (EMT) in cancer is a process that allows cancer cells to detach from neighboring cells, become mobile and metastasize and shares many signaling pathways with development. Several molecular mechanisms which regulate oncogenic properties in neoplastic cells such as proliferation, resistance to apoptosis and angiogenesis through transcription factors or other mediators are also regulators of EMT. These pathways and downstream transcription factors are, in their turn, regulated by ubiquitination and the Ubiquitin-Proteasome System (UPS). Ubiquitination, the covalent link of the small 76-amino acid protein ubiquitin to target proteins, serves as a signal for protein degradation by the proteasome or for other outcomes such as endocytosis, degradation by the lysosome or directing these proteins to specific cellular compartments. This review discusses aspects of the regulation of EMT by ubiquitination and the UPS and underlines its complexity focusing on transcription and transcription factors regulating EMT and are being regulated by ubiquitination.

Key signaling nodes in mammary gland development and cancer: Smad signal integration in epithelial cell plasticity

Smad proteins are the key intermediates of transforming growth factor-beta (TGF-β) signaling during development and in tissue homeostasis. Pertubations in TGF-β/Smad signaling have been implicated in cancer and other diseases. In the cell nucleus, Smad complexes trigger cell type- and context-specific transcriptional programs, thereby transmitting and integrating signals from a variety of ligands of the TGF-β superfamily and other stimuli in the cell microenvironment. The actual transcriptional and biological outcome of Smad activation critically depends on the genomic integrity and the modification state of genome and chromatin of the cell. The cytoplasmic and nuclear Smads can also modulate the activity of other signal transducers and enzymes such as microRNA-processing factors. In the case of breast cancer, the role of Smads in epithelial plasticity, tumor-stroma interactions, invasion, and metastasis seems of particular importance.

Proteomic analysis identifies differentially expressed proteins after HDAC vorinostat and EGFR inhibitor gefitinib treatments in Hep-2 cancer cells

Several solid tumors are characterized by poor prognosis and few effective treatment options, other than palliative chemotherapy in the recurrent/metastatic setting. Epidermal growth factor receptor (EGFR) has been considered an important anticancer target because it is involved in the development and progression of several solid tumors; however, only a subset of patients show a clinically meaningful response to EGFR inhibition, particularly to EGFR tyrosine kinase inhibitors such as gefitinib. We have recently demonstrated synergistic antitumor effect of the histone deacetylase inhibitor vorinostat combined with gefitinib. To further characterize the interaction between these two agents, cellular extracts from Hep-2 cancer cells that were untreated or treated for 24 h with either vorinostat or gefitinib alone or with a vorinostat/gefitinib combination were analyzed using 2-D DIGE. Software analysis using DeCyder was performed, and numerous differentially expressed protein spots were visualized between the four examined settings. Using MALDI-TOF MS and ESI-Ion trap MS/MS, several differentially expressed proteins were identified; some of these were validated by Western blotting. Finally, a pathway analysis of experimental data performed using MetaCore highlighted a relevant relationship between the identified proteins and additional potential effectors. In conclusion, we performed a comprehensive analysis of proteins regulated by vorinostat and gefitinib, alone and in combination, providing a useful insight into their mechanisms of action as well as their synergistic interaction.

Regulation of gene transcription by the oncoprotein MYC

The proteins of the MYC/MAX/MAD network are central regulators of many key processes associated with basic cell physiology. These include the regulation of protein biosynthesis, energy metabolism, proliferation, and apoptosis. Molecularly the MYC/MAX/MAD network achieves these broad activities by controlling the expression of many target genes, which are primarily responsible for the diverse physiological consequences elicited by the network. The MYC proteins of the network possess oncogenic activity and their functional deregulation is associated with the majority of human tumors. Over the last years we have witnessed the accumulation of a considerable number of molecular observations that suggest many different biochemical means and tools by which MYC controls gene expression. We will summarize the more recent findings and discuss how these different building blocks might come together to explain how MYC regulates gene transcription. We note that despite the many molecular details known, we do not have an integrated view of how MYC uses the different tools, neither in a spatial nor in a temporal order.

Transcriptional repression: the dark side of myc

The (c-)Myc oncoprotein and its cousins, the N-Myc and L-Myc proteins, show all hallmarks of transcriptional activator proteins: Myc carries a carboxy-terminal DNA binding domain, which mediates sequence-specific binding to DNA. At its amino-terminus, Myc carries a transcriptional regulatory domain that strongly activates transcription when fused to an ectopic DNA binding domain; moreover, the strength of activation of different members of the Myc family correlates with their ability to transform rodent cells. Furthermore, activation of conditional alleles of Myc, either tetracycline or estrogen inducible, upregulates expression of a large number of genes, both in tissue culture and in transgenic animals. Indeed, many of these genes have essential roles in cell proliferation, cell growth, and metabolism; two of them, odc, encoding ornithine decarboxylase, a rate-limiting enzyme of polyamine biosynthesis, and rpl24, encoding a constituent of the large ribosomal subunit, are haploinsufficient for Myc-induced lymphomagenesis but not for normal development, arguing very strongly that upregulation of both genes is critical for Myc-dependent tumor formation. Undoubtedly, therefore, Myc exerts part of its biological activities via transcriptional upregulation of a large number of target genes. One of the key issues in the field is whether there are additional biochemical activities of the Myc protein and, if so, whether and how they contribute to Myc biology. This review summarizes evidence demonstrating that Myc has the ability to repress transcription and that this may be an important function during oncogenic transformation.

Expression of CIP2A in renal cell carcinomas correlates with tumour invasion, metastasis and patients' survival

Background:

Cancerous inhibitor of protein phosphatase 2A (CIP2A) drives cellular transformation. The objective of this study was to detect the potential effects of CIP2A in renal cell carcinomas (RCCs).

Methods:

A total of 107 RCC patients were involved in the study. Cancerous inhibitor of protein phosphatase 2A expression was investigated by real-time PCR and immunohistochemistry. In vitro, we examined the expression of CIP2A and c-Myc and tested the migration and invasion capability of A498 and KRC/Y cells with scratch migration assay and Matrigel invasion assay after down-regulating CIP2A expression using siRNA.

Results:

Cancerous inhibitor of protein phosphatase 2A was over-expressed in RCC tissues. Clear cell RCC showed an even higher-CIP2A expression level than papillary or chromophobe RCC did. The CIP2A immunostaining level was positively correlated with primary tumour stage, lymph node metastasis, distant metastasis, TNM stage and histological grade (all P<0.05). High-CIP2A expression implied poor survival for patients (P<0.05). Cancerous inhibitor of protein phosphatase 2A depletion by siRNA down-regulated c-Myc expression and attenuated the migration and invasion of RCC cells.

Conclusion:

Higher-CIP2A expression positively correlates with the aggressive phenotype of RCCs, and predicts poor prognosis for patients. Cancerous inhibitor of protein phosphatase 2A may be a novel target for prevention and treatment of RCC metastasis and recurrence.

Transforming growth factor-β2 promotes Snail-mediated endothelial-mesenchymal transition through convergence of Smad-dependent and Smad-independent signalling

EndMT (endothelial-mesenchymal transition) is a critical process of cardiac development and disease progression. However, little is know about the signalling mechanisms that cause endothelial cells to transform into mesenchymal cells. In the present paper we show that TGF-β2 (transforming growth factor-β2) stimulates EndMT through the Smad, MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase], PI3K (phosphinositide 3-kinase) and p38 MAPK signalling pathways. Inhibitors of these pathways prevent TGF-β2-induced EndMT. Furthermore, we show that all of these pathways are essential for increasing expression of the cell-adhesion-suppressing transcription factor Snail. Inhibition of Snail with siRNA (small interfering RNA) prevents TGF-β2-induced EndMT. However, overexpression of Snail is not sufficient to cause EndMT. Chemical inhibition of GSK-3β (glycogen synthase kinase-3β) allows EndMT to be induced by Snail overexpression. Expression of a mutant Snail protein that is resistant to GSK-3β-dependent inactivation also promotes EndMT. These results provide the foundation for understanding the roles of specific signalling pathways in mediating EndMT.

Genome-wide mapping of Myc binding and gene regulation in serum-stimulated fibroblasts

The transition from quiescence to proliferation is a key regulatory step that can be induced by serum stimulation in cultured fibroblasts. The transcription factor Myc is directly induced by serum mitogens and drives a secondary gene expression program that remains largely unknown. Using mRNA profiling, we identify close to 300 Myc-dependent serum response (MDSR) genes, which are induced by serum in a Myc-dependent manner in mouse fibroblasts. Mapping of genomic Myc-binding sites by ChIP-seq technology revealed that most MDSR genes were directly targeted by Myc, but represented a minor fraction (5.5%) of all Myc-bound promoters (which were 22.4% of all promoters). Other target loci were either induced by serum in a Myc-independent manner, were not significantly regulated or were negatively regulated. MDSR gene products were involved in a variety of processes, including nucleotide biosynthesis, ribosome biogenesis, DNA replication and RNA control. Of the 29 MDSR genes targeted by RNA interference, three showed a requirement for cell-cycle entry upon serum stimulation and 11 for long-term proliferation and/or survival. Hence, proper coordination of key regulatory and biosynthetic pathways following mitogenic stimulation relies upon the concerted regulation of multiple Myc-dependent genes.

MYC and metastasis

Aggressive primary tumors express transcriptional signatures that correlate with their metastatic propensity. A number of these signatures have been deployed in the clinic as risk stratification tools. However, the molecular basis of these clinically useful prognostic signatures has remained a largely unresolved area of controversy. We recently found that many prognostic signatures reflect the activity of the MYC oncogene, which in turn regulates tumor metastasis through specific effects on cancer cell invasion and migration. These findings offer a general framework for understanding the molecular basis of clinically prognostic transcriptional signatures and suggest potentially new avenues for studying metastasis.

Addicted to Myc--but why?

Many tumors continuously depend on the initiating oncogenes, but whether this extends to their downstream targets is unclear. In this issue of Genes & Development, Sodir and colleagues (pp. 907-916) demonstrate an essential role for endogenous Myc proteins in maintaining the tumor microenvironment, providing an unexpected molecular explanation for addiction to Myc.

Role of Smads in TGFβ signaling

Transforming growth factor-β (TGFβ) is the prototype for a large family of pleiotropic factors that signal via heterotetrameric complexes of type I and type II serine/threonine kinase receptors. Important intracellular mediators of TGFβ signaling are members of the Smad family. Smad2 and 3 are activated by C-terminal receptor-mediated phosphorylation, whereafter they form complexes with Smad4 and are translocated to the nucleus where they, in cooperation with other transcription factors, co-activators and co-repressors, regulate the transcription of specific genes. Smads have key roles in exerting TGFβ-induced programs leading to cell growth arrest and epithelial-mesenchymal transition. The activity and stability of Smad molecules are carefully regulated by a plethora of post-translational modifications, including phosphorylation, ubiquitination, sumoylation, acetylation and poly(ADP)-ribosylation. The Smad function has been shown to be perturbed in certain diseases such as cancer.

IKK(α) controls canonical TGF(ß)-SMAD signaling to regulate genes expressing SNAIL and SLUG during EMT in panc1 cells

The epithelial to mesenchymal transition (EMT) is a crucial step in tumor progression, and the TGFβ-SMAD signaling pathway is an inductor of EMT in many tumor types. One hallmark of EMT is downregulation of the adherens junction protein E-cadherin, a process mediated by transcription factors such as the zinc fingers SNAIL and SLUG. Here, we report that the catalytic IκB kinase (IKK) subunit IKKα is necessary for the silencing of E-cadherin in a Panc1 cell model of TGFβ-SMAD-mediated EMT, independently of NFκB. IKKα regulates canonical TGFβ-SMAD signaling by interacting with SMAD3 and controlling SMAD complex formation on DNA. Furthermore, we demonstrate that the TGFβ-IKKα-SMAD signaling pathway induces transcription of the genes encoding SNAIL and SLUG. In addition, we demonstrate that IKKα also modulates canonical TGFβ-SMAD signaling in human MDA-MB231 breast cancer cells, arguing for a more general impact of IKKα on the control of TGFβ-SMAD signaling. Taken together, these findings indicate that IKKα contributes to the tumor-promoting function of the TGFβ-SMAD signaling pathway in particular cancers.

The pathophysiology of epithelial-mesenchymal transition induced by transforming growth factor-beta in normal and malignant mammary epithelial cells

Epithelial-mesenchymal transition (EMT) is an essential process that drives polarized, immotile mammary epithelial cells (MECs) to acquire apolar, highly migratory fibroblastoid-like features. EMT is an indispensable process that is associated with normal tissue development and organogenesis, as well as with tissue remodeling and wound healing. In stark contrast, inappropriate reactivation of EMT readily contributes to the development of a variety of human pathologies, particularly those associated with tissue fibrosis and cancer cell invasion and metastasis, including that by breast cancer cells. Although metastasis is unequivocally the most lethal aspect of breast cancer and the most prominent feature associated with disease recurrence, the molecular mechanisms whereby EMT mediates the initiation and resolution of breast cancer metastasis remains poorly understood. Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that is intimately involved in regulating numerous physiological processes, including cellular differentiation, homeostasis, and EMT. In addition, TGF-beta also functions as a powerful tumor suppressor in MECs, whose neoplastic development ultimately converts TGF-beta into an oncogenic cytokine in aggressive late-stage mammary tumors. Recent findings have implicated the process of EMT in mediating the functional conversion of TGF-beta during breast cancer progression, suggesting that the chemotherapeutic targeting of EMT induced by TGF-beta may offer new inroads in ameliorating metastatic disease in breast cancer patients. Here we review the molecular, cellular, and microenvironmental factors that contribute to the pathophysiological activities of TGF-beta during its regulation of EMT in normal and malignant MECs.

Snail family regulation and epithelial mesenchymal transitions in breast cancer progression

Since its initial description, the interconversion between epithelial and mesenchymal cells (designed as epithelial-mesenchymal or mesenchymal-epithelial transition, EMT or MET, respectively) has received special attention since it provides epithelial cells with migratory features. Different studies using cell lines have identified cytokines, intercellular signaling elements and transcriptional factors capable of regulating this process. Particularly, the identification of Snail family members as key effectors of EMT has opened new ways for the study of this cellular process. In this article we discuss the molecular pathways that control EMT, showing a very tight and interdependent regulation. We also analyze the contribution of EMT and Snail genes in the process of tumorigenesis using the mammary gland as cellular model.

Key signalling nodes in mammary gland development and cancer. Mitogen-activated protein kinase signalling in experimental models of breast cancer progression and in mammary gland development

Seven classes of mitogen-activated protein kinase (MAPK) intracellular signalling cascades exist, four of which are implicated in breast disease and function in mammary epithelial cells. These are the extracellular regulated kinase (ERK)1/2 pathway, the ERK5 pathway, the p38 pathway and the c-Jun N-terminal kinase (JNK) pathway. In some forms of human breast cancer and in many experimental models of breast cancer progression, signalling through the ERK1/2 pathway, in particular, has been implicated as being important. We review the influence of ERK1/2 activity on the organised three-dimensional association of mammary epithelial cells, and in models of breast cancer cell invasion. We assess the importance of epidermal growth factor receptor family signalling through ERK1/2 in models of breast cancer progression and the influence of ERK1/2 on its substrate, the oestrogen receptor, in this context. In parallel, we consider the importance of these MAPK-centred signalling cascades during the cycle of mammary gland development. Although less extensively studied, we highlight the instances of signalling through the p38, JNK and ERK5 pathways involved in breast cancer progression and mammary gland development.