br STAR Methods br Acknowledgments
Acknowledgments We thank the City of Hope core facilities, including the Animal Model Core, Bioinformatic Core, NMR Core, Flow Cytometry Core, and Florescence Microscopy Core for excellent technical support, and NIH grants R01GM086171, R01GM102538, and R01CA212119, R01CA216987, and R03DA026556 (to Y.C.) for funding, and R44CA189499 (to S.X.O.). E.S. is a fellowship recipient of the California Institute of Regenerative Medicine. The Conrad Prebys Center for Chemical Genomics wishes to acknowledge the NIH Roadmap Grant U54 HG005033 for providing funds to during its participation as a comprehensive screening center of the Molecular Libraries Probe Production Centers Network (MLPCN). We thank Dr. Sumeet Salaniwal for assistance in preparing Table 1. Dr. Gregory P. Roth, who led the medicinal chemistry studies described here, passed away. Gregory is remembered for his many scientific contributions including those described here.
Introduction The TGFβ signaling pathway has a dual function as both a tumor-suppressor and enhancer, contingent on the stage of tumorigenesis , . In normal LY2603618 and during early tumor initiation, TGFβ signaling acts to inhibit growth by cell-cycle arrest; in contrast, this signaling pathway functions as a critical driver of progression in advanced cancers. In this setting, TGFβ signaling induces migration and invasion of cancer cells and modulates the tumor microenvironment promoting the process of metastasis, where cells spread from the primary tumor to secondary sites. Malignant progression is facilitated through the induction of the epithelial to mesenchymal transition (EMT), a process aberrantly activated in epithelial cancers which leads to a more motile invasive phenotype associated with metastatic spread , . Here, we provide an overview of the regulatory mechanisms that coordinately control the process of EMT, focusing on one important downstream effector of TGFβ-induced EMT, the RNA binding protein hnRNP E1. This factor regulates EMT, cancer cell stemness and metastatic progression through the post-transcriptional regulation of EMT and metastasis-associated genes , , , . The multiple functions of hnRNP E1 in RNA processing and translation, as well as specific examples of hnRNP E1 targets will be discussed in this review (See Fig. 1).
Regulation of EMT and stemness by TGF-β signaling TGFβ ligand acts through the type I and type II TGFβ receptors to activate the canonical Smad2/3 and non-canonical MAPK, RhoA and AKT signaling pathways regulating diverse processes such as proliferation, survival, differentiation and EMT , . EMT normally occurs during embryonic development (Type 1 EMT); during fibrosis, wound healing and tissue regeneration (Type 2 EMT), and is thought to be aberrantly activated during cancer metastasis (Type 3 EMT) , . The mesenchymal phenotype is characterized by a loss of cell-cell contacts and apical-basal polarity and a reorganization of actin, intermediate filament and tubulin cytoskeleton network . In cancer, this transition is associated with a gain in migratory and invasive ability, enhanced chemo-resistance, and immune-suppression , . The induction of EMT is also associated with cancer stemness, where cells exhibit self-renewal properties, which is thought to occur through the reactivation of embryonic signaling , . These cancer stem cells (CSCs) may drive not only metastatic growth but also facilitate cancer recurrence and chemo- and radio-resistance . TGFβ initiates EMT and the associated cancer stem cell phenotype through a variety of mechanisms, including changes at the level of transcription as well as post-transcriptional regulation through non-coding RNAs and RNA binding proteins.
hnRNP E1 as a downstream effector of TGF-β signaling hnRNP E1 (PCBP1 or αCP1) is a member of the hnRNP E family consisting of hnRNP E1, E2, E3 and E4 which contain three nucleic acid-binding K homology (KH) domains . Of these proteins, hnRNP E1 and E2 are the most abundant and share the highest degree of sequence homology; hnRNP E1 is an intronless gene and is thought to be a retro transposition of hnRNP E2 , . Despite their similarity, however, hnRNP E1 and E2′s function appear to be non-redundant with hnRNP E1 knockout being embryonic lethal at the peri-implantation stage in mice, whereas hnRNP E2 remain viable until mid-gestation . In addition to the three KH domains of hnRNP E1 , this RNA binding protein contains a nuclear localization signal between KH II and KH III , which controls localization to either the cytoplasm or nucleus. hnRNP E1 functions in diverse cellular processes, playing a role in embryonic development through the translational regulation of maternal RNAs in oocytes , , and controlling muscle development through its role in miRNA processing . hnRNP E1 also regulates erythropoiesis via its function as an iron chaperone, facilitating the transfer of iron to ferritin in erythroid cells and its role as a translational regulator of 15-lipoxygenase (LOX) during erythroid cell differentiation , . Additionally, hnRNP E1 modulates cellular responses to hypoxia and folate deficient conditions. In response to hypoxia, hnRNP E1 dissociates from the 3′UTR of endothelial nitric oxide synthase (eNOS) mRNA reducing its stability and leading to its degradation ; whereas, hnRNP E1 maintains folate homeostasis by interacting with a C-rich element in the 5′UTR of folate receptor α mRNA. Under folate deficient conditions, hnRNP E1 translationally up-regulates this transcript leading to an increase of cell surface folate receptors which promotes folate up-take , . Numerous studies have linked hnRNP E1 to the process of tumorigenesis and metastasis, demonstrating that hnRNP E1 acts as a tumor suppressor in several cancer types , , , , . Silencing of hnRNP E1 results in the transition of cells to a mesenchymal phenotype, an increase in cell motility and invasiveness, and the promotion of tumor formation and distant metastases , , . In addition, loss of hnRNP E1 has been linked to the acquisition of cancer stem cell-like properties, with hnRNP E1 down-regulation observed in CD44+CD24- populations of cells, which exhibit enhanced tumor initiating properties .