ABSTRACT
Epithelial-to-mesenchymal transition (EMT) is an important physiologic process that drives tissue formation during development, but also contributes to disease pathogenesis, including fibrosis and cancer metastasis. Elevated expression of the FOXC1 transcription factor has been detected in several metastatic cancers that have undergone EMT. Therefore, mechanistic insight into the role of FOXC1 in the initiation of the EMT process was sought. It was determined that although Foxc1 transcript expression was elevated following TGFß1-induced EMT of NMuMG cells, FOXC1 was not required for this induction. RNA sequencing revealed that the mRNA levels of FGF receptor 1-isoform IIIc (Fgfr1-IIIc), normally activated upon TGFß1 treatment, were reduced in Foxc1 knockdown cells, and overexpression of Foxc1 was sufficient to induce Fgfr1-IIIc expression, but not EMT. Chromatin immunoprecipitation experiments demonstrated that FOXC1 binds to an Fgfr1 upstream regulatory region and that FOXC1 activates an Fgfr1 promoter element. Furthermore, elevated expression of Foxc1 led to increased Fgfr1-IIIc transcript. Foxc1 knockdown impaired the FGF2-mediated three-dimensional migratory ability of NMuMG cells, which was rescued by expression of FGFR1. In addition, elevated expression of FOXC1 and FGFR1 was also observed in migratory mesenchymal MDA-MB-231 breast cancer cells. Together, these results define a role for FOXC1 in specifying an invasive mesenchymal cell type by promoting FGFR1 isoform switching following induction of TGFß1-mediated EMT. Mol Cancer Res; 15(10); 1341-53. ©2017 AACR.
Subject(s)
Breast Neoplasms/genetics , Forkhead Transcription Factors/genetics , Receptor, Fibroblast Growth Factor, Type 1/genetics , Sequence Analysis, RNA/methods , Transforming Growth Factor beta1/pharmacology , Animals , Breast Neoplasms/metabolism , Cell Line, Tumor , Cell Movement , Epithelial-Mesenchymal Transition/drug effects , Female , Forkhead Transcription Factors/metabolism , Gene Expression Regulation, Neoplastic/drug effects , Humans , Mice , Neoplasm Invasiveness , RNA Isoforms/geneticsABSTRACT
Using a combination of biochemical, structural probing and rapid kinetics techniques we reveal for the first time that the universally conserved translational GTPase (trGTPase) HflX binds to the E-site of the 70S ribosome and that its GTPase activity is modulated by peptidyl transferase centre (PTC) and peptide exit tunnel (PET) binding antibiotics, suggesting a previously undescribed mode of action for these antibiotics. Our rapid kinetics studies reveal that HflX functions as a ribosome splitting factor that disassembles the 70S ribosomes into its subunits in a nucleotide dependent manner. Furthermore, our probing and hydrolysis studies show that the ribosome is able to activate trGTPases bound to its E-site. This is, to our knowledge, the first case in which the hydrolytic activity of a translational GTPase is not activated by the GTPase activating centre (GAC) in the ribosomal A-site. Furthermore, we provide evidence that the bound state of the PTC is able to regulate the GTPase activity of E-site bound HflX.
Subject(s)
Escherichia coli Proteins/metabolism , GTP Phosphohydrolases/metabolism , GTP-Binding Proteins/metabolism , GTPase-Activating Proteins/genetics , Ribosomes/genetics , Binding Sites/genetics , Escherichia coli/genetics , Escherichia coli Proteins/genetics , GTP Phosphohydrolases/genetics , GTP-Binding Proteins/genetics , GTPase-Activating Proteins/metabolism , Hydrolysis , Kinetics , Peptidyl Transferases/genetics , Peptidyl Transferases/metabolism , Protein Binding , Ribosomes/enzymologyABSTRACT
The universally conserved GTPase HflX is a putative translation factor whose GTPase activity is stimulated by the 70S ribosome as well as the 50S but not the 30S ribosomal subunit. However, the details and mechanisms governing this interaction are only poorly understood. In an effort to further elucidate the functional mechanism of HflX, we examined its interaction with the 70S ribosome, the two ribosomal subunits (50S and 30S), as well as its ability to interact with guanine nucleotides in the respective ribosomal complexes using a highly purified in vitro system. Binding studies reported here demonstrate that HflX not only interacts with 50S and 70S particles, but also with the 30S subunit, independent of the nucleotide-bound state. A detailed pre-steady-state kinetic analysis of HflX interacting with a non-hydrolyzable analog of mant-GTP, coupled with an enzymatic probing assay utilizing limited trypsinolysis, reveal that HflX·GTP exists in a structurally distinct 50S- and 70S-bound form that stabilizes GTP binding up to 70 000-fold and that may represent the "GTPase-activated" state. This activation is likely required for efficient GTP-hydrolysis, and may be similar to that observed in elongation factor G. Results reported here address the surprising low affinity of free HflX for GTP and suggest that cellular HflX will mainly exist in the HflX·GTP·ribosome-bound form. A minimal model for the functional cycle of HflX is proposed.
