Despite functional significance of nonmuscle myosin II in cell migration and invasion, its role in epithelial-mesenchymal transition (EMT) or TGF-? signaling is unknown. Analysis of normal mammary gland expression revealed that myosin IIC is expressed in luminal cells, whereas myosin IIB expression is up-regulated in myoepithelial cells that have more mesenchymal characteristics. Furthermore, TGF-? induction of EMT in nontransformed murine mammary gland epithelial cells results in an isoform switch from myosin IIC to myosin IIB and increased phosphorylation of myosin heavy chain (MHC) IIA on target sites known to regulate filament dynamics (S1916, S1943). These expression and phosphorylation changes are downstream of heterogeneous nuclear ribonucleoprotein-E1 (E1), an effector of TGF-? signaling. E1 knockdown drives cells into a migratory, invasive mesenchymal state and concomitantly up-regulates MHC IIB expression and MHC IIA phosphorylation. Abrogation of myosin IIB expression in the E1 knockdown cells has no effect on 2D migration but significantly reduced transmigration and macrophage-stimulated collagen invasion. These studies indicate that transition between myosin IIC/myosin IIB expression is a critical feature of EMT that contributes to increases in invasive behavior.
Epithelial-mesenchymal transition (EMT) and the underlying mechanisms and signaling pathways regulating such transitions have generated a lot of interest among cancer researchers. Much of this can be attributed to the apparent similarities in the molecular processes regulating embryonic EMT that can be recapitulated during tumor progression and metastasis. It appears that both embryonic and oncogenic EMT are regulated by an intricate interplay of transcriptional and post-transcriptional programs, and the recent discovery of a transcript-selective translational regulatory pathway controlling expression of EMT-associated mRNAs demonstrates the high fidelity and tight regulation associated with the process of EMT and metastatic progression. Heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1) is emerging as a critical and integral modulator of TGF?-induced EMT and subsequent tumor metastasis. Through its RNA-binding ability, hnRNP E1 binds distinct 3-UTR structural elements present in mRNA transcripts required for EMT and translationally silences their expression. Translational silencing, mediated by hnRNP E1, occurs specifically at the translation elongation step through effects on the eukaryotic elongation factor-1 A1 (eEF1A1), and is relieved by Akt2-mediated phosphorylation. Interestingly, modulation of either the steady-state expression or the posttranscriptional modification of hnRNP E1 has a temporo-spatial effect on translational repression, tumorigenesis and cancer metastasis.
Transcript-selective translational regulation of epithelial-mesenchymal transition (EMT) by transforming growth factor-? (TGF-?) is directed by the hnRNP E1-containing TGF-?-activated-translational (BAT) mRNP complex. Herein, eukaryotic elongation factor-1 A1 (eEF1A1) is identified as an integral component of the BAT complex. Translational silencing of Dab2 and ILEI, two EMT transcripts, is mediated by the binding of hnRNP E1 and eEF1A1 to their 3UTR BAT element, whereby hnRNP E1 stalls translational elongation by inhibiting the release of eEF1A1 from the ribosomal A site. TGF-?-mediated hnRNP E1 phosphorylation, through Akt2, disrupts the BAT complex, thereby restoring translation of target EMT transcripts. Attenuation of hnRNP E1 expression in two noninvasive breast epithelial cells (NMuMG and MCF-7) not only induced EMT but also enabled cells to form metastatic lesions in vivo. Thus, translational regulation by TGF-? at the elongation stage represents a critical checkpoint coordinating the expression of EMT transcripts required during development and in tumorigenesis and metastatic progression.
Transforming growth factor-beta (TGF-beta) induces epithelial-mesenchymal transdifferentiation (EMT) accompanied by cellular differentiation and migration. Despite extensive transcriptomic profiling, the identification of TGF-beta-inducible, EMT-specific genes has met with limited success. Here we identify a post-transcriptional pathway by which TGF-beta modulates the expression of EMT-specific proteins and of EMT itself. We show that heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1) binds a structural, 33-nucleotide TGF-beta-activated translation (BAT) element in the 3 untranslated region of disabled-2 (Dab2) and interleukin-like EMT inducer (ILEI) transcripts, and represses their translation. TGF-beta activation leads to phosphorylation at Ser 43 of hnRNP E1 by protein kinase Bbeta/Akt2, inducing its release from the BAT element and translational activation of Dab2 and ILEI messenger RNAs. Modulation of hnRNP E1 expression or its post-translational modification alters the TGF-beta-mediated reversal of translational silencing of the target transcripts and EMT. These results suggest the existence of a TGF-beta-inducible post-transcriptional regulon that controls EMT during the development and metastatic progression of tumours.
A major challenge in the clinical management of human cancers is to accurately stratify patients according to risk and likelihood of a favorable response. Stratification is confounded by significant phenotypic heterogeneity in some tumor types, often without obvious criteria for subdivision. Despite intensive transcriptional array analyses, the identity and validation of cancer specific signature genes remains elusive, partially because the transcriptome does not mirror the proteome. The simplification associated with transcriptomic profiling does not take into consideration changes in the relative expression among transcripts that arise due to post-transcriptional regulatory events. We have previously shown that TGF? post-transcriptionally regulates epithelial-mesenchymal transition (EMT) by causing increased expression of two transcripts, Dab2 and ILEI, by modulating hnRNP E1 phosphorylation. Using a genome-wide combinatorial approach involving expression profiling and RIP-Chip analysis, we have identified a cohort of translationally regulated mRNAs that are induced during TGF?-mediated EMT. Coordinated translational regulation by hnRNP E1 constitutes a post-transcriptional regulon inhibiting the expression of related EMT-facilitating genes, thus enabling the cell to rapidly and coordinately regulate multiple EMT-facilitating genes.
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