TGF-β drives epithelial-mesenchymal transition through δEF1-mediated downregulation of ESRP

Hallmarks of alternative splicing in cancer

The immense majority of genes are alternatively spliced and there are many isoforms specifically associated with cancer progression and metastasis. The splicing pattern of specific isoforms of numerous genes is altered as cells move through the oncogenic process of gaining proliferative capacity, acquiring angiogenic, invasive, antiapoptotic and survival properties, becoming free from growth factor dependence and growth suppression, altering their metabolism to cope with hypoxia, enabling them to acquire mechanisms of immune escape, and as they move through the epithelial-mesenchymal and mesenchymal-epithelial transitions and metastasis. Each of the 'hallmarks of cancer' is associated with a switch in splicing, towards a more aggressive invasive cancer phenotype. The choice of isoforms is regulated by several factors (signaling molecules, kinases, splicing factors) currently being identified systematically by a number of high-throughput, independent and unbiased methodologies. Splicing factors are de-regulated in cancer, and in some cases are themselves oncogenes or pseudo-oncogenes and can contribute to positive feedback loops driving cancer progression. Tumour progression may therefore be associated with a coordinated splicing control, meaning that there is the potential for a relatively small number of splice factors or their regulators to drive multiple oncogenic processes. The understanding of how splicing contributes to the various phenotypic traits acquired by tumours as they progress and metastasise, and in particular how alternative splicing is coordinated, can and is leading to the development of a new class of anticancer therapeutics-the alternative-splicing inhibitors.

AKT kinase pathway: a leading target in cancer research

AKT1, a serine/threonine-protein kinase also known as AKT kinase, is involved in the regulation of various signalling downstream pathways including metabolism, cell proliferation, survival, growth, and angiogenesis. The AKT kinases pathway stands among the most important components of cell proliferation mechanism. Several approaches have been implemented to design an efficient drug molecule to target AKT kinases, although the promising results have not been confirmed. In this paper we have documented the detailed molecular insight of AKT kinase protein and proposed a probable doxorubicin based approach in inhibiting miR-21 based cancer cell proliferation. Moreover, the inhibition of miR-21 activation by raising the FOXO3A concentration seems promising in reducing miR-21 mediated cancer activation in cell. Furthermore, the use of next generation sequencing and computational drug design approaches will greatly assist in designing a potent drug molecule against the associated cancer cases.

Four amino acids within a tandem QxVx repeat in a predicted extended α-helix of the Smad-binding domain of Sip1 are necessary for binding to activated Smad proteins

The zinc finger transcription factor Smad-interacting protein-1 (Sip1; Zeb2, Zfhx1b) plays an important role during vertebrate embryogenesis in various tissues and differentiating cell types, and during tumorigenesis. Previous biochemical analysis suggests that interactions with several partner proteins, including TGFβ family receptor-activated Smads, regulate the activities of Sip1 in the nucleus both as a DNA-binding transcriptional repressor and activator. Using a peptide aptamer approach we mapped in Sip1 its Smad-binding domain (SBD), initially defined as a segment of 51 amino acids, to a shorter stretch of 14 amino acids within this SBD. Modelling suggests that this short SBD stretch is part of an extended α-helix that may fit the binding to a hydrophobic corridor within the MH2 domain of activated Smads. Four amino acids (two polar Q residues and two non-polar V residues) that form the tandem repeat (QxVx)₂ in this 14-residue stretch were found to be crucial for binding to both TGFβ/Nodal/Activin-Smads and BMP-Smads. A full-length Sip1 with collective mutation of these Q and V residues (to A) no longer binds to Smads, while it retains its binding activity to its cognate bipartite target DNA sequence. This missense mutant Sip1(AxAx)₂ provides a new molecular tool to identify SBD (in)dependent target genes in Sip1-controlled TGFβ and/or BMP (de)regulated cellular, developmental and pathological processes.

Transcription factors OVOL1 and OVOL2 induce the mesenchymal to epithelial transition in human cancer

Cell plasticity regulated by the balance between the mesenchymal to epithelial transition (MET) and the opposite program, EMT, is critical in the metastatic cascade. Several transcription factors (TFs) are known to regulate EMT, though the mechanisms of MET remain unclear. We demonstrate a novel function of two TFs, OVOL1 and OVOL2, as critical inducers of MET in human cancers. Our findings indicate that the OVOL-TFs control MET through a regulatory feedback loop with EMT-inducing TF ZEB1, and the regulation of mRNA splicing by inducing Epithelial Splicing Regulatory Protein 1 (ESRP1). Using mouse prostate tumor models we show that expression of OVOL-TFs in mesenchymal prostate cancer cells attenuates their metastatic potential. The role of OVOL-TFs as inducers of MET is further supported by expression analyses in 917 cancer cell lines, suggesting their role as crucial regulators of epithelial-mesenchymal cell plasticity in cancer.

Alternative RNA splicing and cancer

Alternative splicing of pre-messenger RNA (mRNA) is a fundamental mechanism by which a gene can give rise to multiple distinct mRNA transcripts, yielding protein isoforms with different, even opposing, functions. With the recognition that alternative splicing occurs in nearly all human genes, its relationship with cancer-associated pathways has emerged as a rapidly growing field. In this review, we summarize recent findings that have implicated the critical role of alternative splicing in cancer and discuss current understandings of the mechanisms underlying dysregulated alternative splicing in cancer cells.

Identification of integrin α3 as a molecular marker of cells undergoing epithelial-mesenchymal transition and of cancer cells with aggressive phenotypes

Epithelial-mesenchymal transition (EMT) is a crucial event in wound healing, tissue repair, and cancer progression in adult tissues. Transforming growth factor (TGF)-β induces EMT in mouse epithelial cells. During prolonged treatment, TGF-β successively induces myofibroblastic differentiation with increased expression of myofibroblast marker proteins, including smooth muscle α actin and calponin. We recently showed that fibroblast growth factor-2 prevented myofibroblastic differentiation induced by TGF-β, and transdifferentiated the cells to those with much more aggressive characteristics (enhanced EMT). To identify the molecular markers specifically expressed in cells undergoing enhanced EMT induced by the combination of TGF-β and fibroblast growth factor-2, we carried out a microarray-based analysis and found that integrin α3 (ITGA3) and Ret were upregulated. Intriguingly, ITGA3 was also overexpressed in breast cancer cells with aggressive phenotypes and its expression was correlated with that of δEF-1, a key regulator of EMT. Moreover, the expression of both genes was downregulated by U0126, a MEK 1/2 inhibitor. Therefore, ITGA3 is a potential marker protein for cells undergoing enhanced EMT and for cancer cells with aggressive phenotypes, which is positively regulated by δEF-1 and the MEK-ERK pathway.

Posttranscriptional regulation by RNA-binding proteins during epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transition (EMT), one of the crucial steps for carcinoma cells to acquire invasive capacity, results from the disruption of cell–cell contacts and the acquisition of a motile mesenchymal phenotype. Although the transcriptional events controlling EMT have been extensively studied, in recent years, several posttranscriptional mechanisms have emerged as critical in the regulation of EMT during tumor progression. In this review, we highlight the regulation of posttranscriptional events in EMT by RNA-binding proteins (RBPs). RBPs are responsible for controlling pre-mRNA splicing, capping, and polyadenylation, as well as mRNA export, turnover, localization, and translation. We discuss the most relevant aspects of RBPs controlling the metabolism of EMT-related mRNAs, and describe the implication of novel posttranscriptional mechanisms regulating EMT in response to different signaling pathways. Novel insight into posttranscriptional regulation of EMT by RBPs is uncovering new therapeutic targets in cancer invasion and metastasis.

TGF-β signaling and epithelial-mesenchymal transition in cancer progression

PURPOSE OF REVIEW

TGF-β acts as a potent driver of cancer progression through the induction of epithelial-mesenchymal transition (EMT), in which epithelial cells acquire mesenchymal phenotype, leading to enhanced motility and invasion. Recent reports highlight the fundamental roles of TGF-β-induced EMT in multiple aspects of cancer progression. In this review, we focus on the novel insights into the roles of TGF-β-induced EMT in cancer progression and the underlying mechanisms that enable TGF-β to activate this epithelial plasticity response at transcription, translation, and posttranslational levels.

RECENT FINDINGS

Smad-mediated transcription regulation is known to activate TGF-β-induced EMT. More recently, novel mechanisms of epigenetic control, alternative splicing, miRNAs, translation control, and posttranslational modifications have been shown to play key roles in the control of EMT. In addition to initiating carcinoma cell invasion, TGF-β-induced EMT can guide cancer cells to de-differentiate and gain cancer stem-cell-like properties. EMT also allows the generation of stromal cells that support and instruct cancer progression.

SUMMARY

The differentiation plasticity of epithelial cells that mediates TGF-β-induced EMT and reversion from mesenchymal to epithelial phenotype are increasingly seen as integral aspects of cancer progression that contribute to survival and dissemination of cancer cells. Further mechanistic insights under physiological conditions may lead to new therapeutic or prognostic strategies in cancer treatment.

The relevance of the TGF-β Paradox to EMT-MET programs

The role of transforming growth factor-β (TGF-β) during tumorigenesis is complex and paradoxical, reflecting its ability to function as a tumor suppressor in normal and early-stage cancers, and as a tumor promoter in their late-stage counterparts. The switch in TGF-β function is known as the "TGF-β Paradox," whose manifestations are intimately linked to the initiation of epithelial-mesenchymal transition (EMT) programs in developing and progressing carcinomas. Indeed, as carcinoma cells emerge from EMT programs stimulated by TGF-β, they readily display a variety of acquired phenotypes that provide a selective advantage to growing carcinomas, including (i) enhanced cell migration and invasion; (ii) heightened resistance to cytotoxic agents, targeted chemotherapeutic, and radiation treatments; and (iv) boosted expansion of cancer-initiating and stem-like cell populations that underlie tumor metastasis and disease recurrence. At present, the molecular, cellular, and microenvironmental mechanisms that enable post-EMT and metastatic carcinoma cells to hijack the oncogenic activities of TGF-β remain incompletely understood. Additionally, the molecular mechanisms that counter EMT programs and limit the aggressiveness of late-stage carcinomas, events that transpire via mesenchymal-epithelial transition (MET) reactions, also need to be further elucidated. Here we review recent advances that provide new insights into how TGF-β promotes EMT programs in late-stage carcinoma cells, as well as how these events are balanced by MET programs during the development and metastatic progression of human carcinomas.

The RNA-binding protein Rbfox2: an essential regulator of EMT-driven alternative splicing and a mediator of cellular invasion

The epithelial-mesenchymal transition (EMT), a prerequisite for cancer progression and metastasis formation, is regulated not only at the transcriptional but also at the post-transcriptional level, including at the level of alternative pre-mRNA splicing. Several recent studies have highlighted the involvement of splicing factors, including epithelial splicing regulatory proteins (Esrps) and RNA-binding Fox protein 2 (Rbfox2), in this process. Esrps regulate epithelial-specific splicing, and their expression is downregulated during EMT. By contrast, the role of Rbfox2 is controversial because Rbfox2 regulates epithelial as well as mesenchymal splicing events. Here, we have used several established cell culture models to investigate the functions of Rbfox2 during EMT. We demonstrate that induction of an EMT upregulates the expression of Rbfox2, which correlates with an increase in Rbfox2-regulated splicing events in the cortactin (Cttn), Pard3 and dynamin 2 (Dnm2) transcripts. At the same time, however, the epithelial-specific ability to splice the Enah, Slk and Tsc2 transcripts is either reduced or lost completely by Rbfox2, which might be due, in part, to downregulation of the expression of the Esrps cooperative factors. Depletion of Rbfox2 during EMT did not prevent the activation of transforming growth factor-β signaling, the upregulation of mesenchymal markers or changes in cell morphology toward a mesenchymal phenotype. In addition, this depletion did not influence cell migration. However, depletion of Rbfox2 in cells that have completed an EMT significantly reduced their invasive potential. Taken together, our results suggest that during an EMT, Rbfox2-regulated splicing shifts from epithelial-to mesenchymal-specific events, leading to a higher degree of tissue invasiveness.

Regulatory networks defining EMT during cancer initiation and progression

Epithelial to mesenchymal transition (EMT) is essential for driving plasticity during development, but is an unintentional behaviour of cells during cancer progression. The EMT-associated reprogramming of cells not only suggests that fundamental changes may occur to several regulatory networks but also that an intimate interplay exists between them. Disturbance of a controlled epithelial balance is triggered by altering several layers of regulation, including the transcriptional and translational machinery, expression of non-coding RNAs, alternative splicing and protein stability.

During embryogenesis, esrp1 expression is restricted to a subset of epithelial cells and is associated with splicing of a number of developmentally important genes

BACKGROUND

Development of a mature organism from a single cell requires a series of important morphological changes, which is in part regulated by alternative splicing. In this article, we report the expression of Esrp1 during early mouse embryogenesis, a splicing factor implicated in epithelial to mesenchymal transitions.

RESULTS

By qRT-PCR, we find higher expression of Esrp1 and Esrp2 in placenta compared to the embryos. We also find a correlation between the expression of Esrp1 and alternative splicing of several known target exons. Using in situ RNA hybridization we show that while Esrp1 expression is ubiquitous in embryonic day (E)6.5 mouse embryos, expression becomes restricted to the chorion and definitive endoderm starting at E7.5. Esrp1 expression was consistently restricted to a subset of epithelial cell types in developing embryos from E9.5 to E13.5.

CONCLUSIONS

Our results suggest that Esrp1 could play an important role in the morphological changes underlying embryogenesis of the placenta and embryo.

Transcriptomic analysis of PNN- and ESRP1-regulated alternative pre-mRNA splicing in human corneal epithelial cells

PURPOSE

We investigated the impact of PININ (PNN) and epithelial splicing regulatory protein 1 (ESRP1) on alternative pre-mRNA splicing in the corneal epithelial context.

METHODS

Isoform-specific RT-PCR assays were performed on wild-type and Pnn knockout mouse cornea. Protein interactions were examined by deconvolution microscopy and co-immunoprecipitation. For genome-wide alternative splicing study, immortalized human corneal epithelial cells (HCET) harboring doxycycline-inducible shRNA against PNN or ESRP1 were created. Total RNA was isolated from four biological replicates of control and knockdown HCET cells, and subjected to hGlue3_0 transcriptome array analysis.

RESULTS

Pnn depletion in developing mouse corneal epithelium led to disrupted alternative splicing of multiple ESRP-regulated epithelial-type exons. In HCET cells, ESRP1 and PNN displayed close localization in and around nuclear speckles, and their physical association in protein complexes was identified. Whole transcriptome array analysis on ESRP1 or PNN knockdown HCET cells revealed clear alterations in transcript profiles and splicing patterns of specific subsets of genes. Separate RT-PCR validation assays confirmed successfully specific changes in exon usage of several representative splice variants, including PAX6(5a), FOXJ3, ARHGEF11, and SLC37A2. Gene ontologic analyses on ESRP1- or PNN-regulated alternative exons suggested their roles in epithelial phenotypes, such as cell morphology and movement.

CONCLUSIONS

Our data suggested that ESRP1 and PNN modulate alternative splicing of a specific subset of target genes, but not general splicing events, in HCET cells to maintain or enhance epithelial characteristics.

Role of TGF-β in breast cancer bone metastases

Breast cancer is the most prevalent cancer among females worldwide leading to approximately 350,000 deaths each year. It has long been known that cancers preferentially metastasize to particular organs, and bone metastases occur in ~70% of patients with advanced breast cancer. Breast cancer bone metastases are predominantly osteolytic and accompanied by increased fracture risk, pain, nerve compression and hypercalcemia, causing severe morbidity. In the bone matrix, transforming growth factor-β (TGF-β) is one of the most abundant growth factors, which is released in active form upon tumor-induced osteoclastic bone resorption. TGF-β, in turn, stimulates bone metastatic tumor cells to secrete factors that further drive osteolytic bone destruction adjacent to the tumor. Thus, TGF-β is a crucial factor responsible for driving the feed-forward vicious cycle of cancer growth in bone. Moreover, TGF-β activates epithelial-to-mesenchymal transition, increases tumor cell invasiveness and angiogenesis and induces immunosuppression. Blocking the TGF-β signaling pathway to interrupt this vicious cycle between breast cancer and bone offers a promising target for therapeutic intervention to decrease skeletal metastasis. This review will describe the role of TGF-β in breast cancer and bone metastasis, and pre-clinical and clinical data will be evaluated for the potential use of TGF-β inhibitors in clinical practice to treat breast cancer bone metastases.

Fluorescence-based alternative splicing reporters for the study of epithelial plasticity in vivo

Alternative splicing generates a vast diversity of protein isoforms from a limited number of protein-coding genes, with many of the isoforms possessing unique, and even contrasting, functions. Fluorescence-based splicing reporters have the potential to facilitate studies of alternative splicing at the single-cell level and can provide valuable information on phenotypic transitions in almost real time. Fibroblast growth factor receptor 2 (FGFR2) pre-mRNA is alternatively spliced to form the epithelial-specific and mesenchymal-specific IIIb and IIIc isoforms, respectively, which are useful markers of epithelial-mesenchymal transitions (EMT). We have used our knowledge of FGFR2 splicing regulation to develop a fluorescence-based reporter system to visualize exon IIIc regulation in vitro and in vivo. Here we show the application of this reporter system to the study of EMT in vitro in cell culture and in vivo in transgenic mice harboring these splicing constructs. In explant studies, the reporters revealed that FGFR2 isoform switching is not required for keratinocyte migration during cutaneous wound closure. Our results demonstrate the value of the splicing reporters as tools to study phenotypic transitions and cell fates at single cell resolution. Moreover, our data suggest that keratinocytes migrate efficiently in the absence of a complete EMT.

EMT-activating transcription factors in cancer: beyond EMT and tumor invasiveness

Cancer is a complex multistep process involving genetic and epigenetic changes that eventually result in the activation of oncogenic pathways and/or inactivation of tumor suppressor signals. During cancer progression, cancer cells acquire a number of hallmarks that promote tumor growth and invasion. A crucial mechanism by which carcinoma cells enhance their invasive capacity is the dissolution of intercellular adhesions and the acquisition of a more motile mesenchymal phenotype as part of an epithelial-to-mesenchymal transition (EMT). Although many transcription factors can trigger it, the full molecular reprogramming occurring during an EMT is mainly orchestrated by three major groups of transcription factors: the ZEB, Snail and Twist families. Upregulated expression of these EMT-activating transcription factors (EMT-ATFs) promotes tumor invasiveness in cell lines and xenograft mice models and has been associated with poor clinical prognosis in human cancers. Evidence accumulated in the last few years indicates that EMT-ATFs also regulate an expanding set of cancer cell capabilities beyond tumor invasion. Thus, EMT-ATFs have been shown to cooperate in oncogenic transformation, regulate cancer cell stemness, override safeguard programs against cancer like apoptosis and senescence, determine resistance to chemotherapy and promote tumor angiogenesis. This article reviews the expanding portfolio of functions played by EMT-ATFs in cancer progression.

Snail represses the splicing regulator epithelial splicing regulatory protein 1 to promote epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT), a tightly regulated process that is critical for development, is frequently re-activated during cancer metastasis and recurrence. We reported previously that CD44 isoform switching is critical for EMT and showed that the splicing factor ESRP1 inhibits CD44 isoform switching during EMT. However, the mechanism by which ESRP1 is regulated during EMT has not been fully understood. Here we show that the transcription repressor Snail binds to E-boxes in the ESRP1 promoter, causing repression of the ESRP1 gene. Biochemically, we define the mechanism by which ESRP1 regulates CD44 alternative splicing: ESRP1 binds to the intronic region flanking a CD44 variable exon and causes increased variable exon inclusion. We further show that ectopically expressing ESRP1 inhibits Snail-induced EMT, suggesting that down-regulation of ESRP1 is required for function by Snail in EMT. Together, these data reveal how the transcription factor Snail mediates EMT through regulation of a splicing factor.

ZEB/miR-200 feedback loop: at the crossroads of signal transduction in cancer

Embryonic differentiation programs of epithelial-mesenchymal and mesenchymal-epithelial transition (EMT and MET) represent a mechanistic basis for epithelial cell plasticity implicated in cancer. Transcription factors of the ZEB protein family (ZEB1 and ZEB2) and several microRNA species (predominantly miR-200 family members) form a double negative feedback loop, which controls EMT and MET programs in both development and tumorigenesis. In this article, we review crosstalk between the ZEB/miR-200 axis and several signal transduction pathways activated at different stages of tumor development. The close association of ZEB proteins with these pathways is indirect evidence for the involvement of a ZEB/miR-200 loop in tumor initiation, progression and spread. Additionally, the configuration of signaling pathways involving ZEB/miR-200 loop suggests that ZEB1 and ZEB2 may have different, possibly even opposing, roles in some forms of human cancer.

Epigenetic reprogramming and post-transcriptional regulation during the epithelial-mesenchymal transition

The epithelial-mesenchymal transition (EMT) is a developmental process that is important for organ development, metastasis, cancer stemness, and organ fibrosis. The EMT process is regulated by different signaling pathways as well as by various epigenetic and post-transcriptional mechanisms. Here, we review recent progress describing the role of different chromatin modifiers in various signaling events leading to EMT, including hypoxia, transforming growth factor (TGF)-β, Notch, and Wnt. We also discuss post-transcriptional mechanisms, such as RNA alternative splicing and the effects of miRNAs in EMT regulation. Furthermore, we highlight on-going and future work aimed at a detailed understanding of the epigenetic and post-transcriptional mechanisms that regulate EMT. This work will shed new light on the cellular and tumorigenic processes affected by EMT misregulation.

Tumor-promoting functions of transforming growth factor-β in progression of cancer

Transforming growth factor-β (TGF-β) elicits both tumor-suppressive and tumor-promoting functions during cancer progression. Here, we describe the tumor-promoting functions of TGF-β and how these functions play a role in cancer progression. Normal epithelial cells undergo epithelial-mesenchymal transition (EMT) through the action of TGF-β, while treatment with TGF-β and fibroblast growth factor (FGF)-2 results in transdifferentiation into activated fibroblastic cells that are highly migratory, thereby facilitating cancer invasion and metastasis. TGF-β also induces EMT in tumor cells, which can be regulated by oncogenic and anti-oncogenic signals. In addition to EMT promotion, invasion and metastasis of cancer are facilitated by TGF-β through other mechanisms, such as regulation of cell survival, angiogenesis, and vascular integrity, and interaction with the tumor microenvironment. TGF-β also plays a critical role in regulating the cancer-initiating properties of certain types of cells, including glioma-initiating cells. These findings thus may be useful for establishing treatment strategies for advanced cancer by inhibiting TGF-β signaling.

Complex changes in alternative pre-mRNA splicing play a central role in the epithelial-to-mesenchymal transition (EMT)

The epithelial-to-mesenchymal transition (EMT) is an important developmental process that is also implicated in disease pathophysiology, such as cancer progression and metastasis. A wealth of literature in recent years has identified important transcriptional regulators and large-scale changes in gene expression programs that drive the phenotypic changes that occur during the EMT. However, in the past couple of years it has become apparent that extensive changes in alternative splicing also play a profound role in shaping the changes in cell behavior that characterize the EMT. While long known splicing switches in FGFR2 and p120-catenin provided hints of a larger program of EMT-associated alternative splicing, the recent identification of the epithelial splicing regulatory proteins 1 and 2 (ESRP1 and ESRP2) began to reveal this genome-wide post-transcriptional network. Several studies have now demonstrated the truly vast extent of this alternative splicing program. The global switches in splicing associated with the EMT add an important additional layer of post-transcriptional control that works in harmony with transcriptional and epigenetic regulation to effect complex changes in cell shape, polarity, and behavior that mediate transitions between epithelial and mesenchymal cell states. Future challenges include the need to investigate the functional consequences of these splicing switches at both the individual gene as well as systems level.