HDAC inhibition induces expression of scaffolding proteins critical for tumor progression in pediatric glioma: focus on EBP50 and IRSp53

BACKGROUND

Diffuse midline glioma (DMG) is a pediatric malignancy with poor prognosis. Most children die less than one year after diagnosis. Recently, mutations in histone H3 have been identified and are believed to be oncogenic drivers. Targeting this epigenetic abnormality using histone deacetylase (HDAC) inhibitors such as panobinostat (PS) is therefore a novel therapeutic option currently evaluated in clinical trials.

METHODS

BH3 profiling revealed engagement in an irreversible apoptotic process of glioma cells exposed to PS confirmed by annexin-V/propidium iodide staining. Using proteomic analysis of 3 DMG cell lines, we identified 2 proteins deregulated after PS treatment. We investigated biological effects of their downregulation by silencing RNA but also combinatory effects with PS treatment in vitro and in vivo using a chick embryo DMG model. Electron microscopy was used to validate protein localization.

RESULTS

Scaffolding proteins EBP50 and IRSp53 were upregulated by PS treatment. Reduction of these proteins in DMG cell lines leads to blockade of proliferation and migration, invasion, and an increase of apoptosis. EBP50 was found to be expressed in cytoplasm and nucleus in DMG cells, confirming known oncogenic locations of the protein. Treatment of glioma cells with PS together with genetic or chemical inhibition of EBP50 leads to more effective reduction of cell growth in vitro and in vivo.

CONCLUSION

Our data reveal a specific relation between HDAC inhibitors and scaffolding protein deregulation which might have a potential for therapeutic intervention for cancer treatment.

Vestigial-like 3 is a novel Ets1 interacting partner and regulates trigeminal nerve formation and cranial neural crest migration

Drosophila Vestigial is the founding member of a protein family containing a highly conserved domain, called Tondu, which mediates their interaction with members of the TEAD family of transcription factors (Scalloped in Drosophila). In Drosophila, the Vestigial/Scalloped complex controls wing development by regulating the expression of target genes through binding to MCAT sequences. In vertebrates, there are four Vestigial-like genes, the functions of which are still not well understood. Here, we describe the regulation and function of vestigial-like 3 (vgll3) during Xenopus early development. A combination of signals, including FGF8, Wnt8a, Hoxa2, Hoxb2 and retinoic acid, limits vgll3 expression to hindbrain rhombomere 2. We show that vgll3 regulates trigeminal placode and nerve formation and is required for normal neural crest development by affecting their migration and adhesion properties. At the molecular level, vgll3 is a potent activator of pax3, zic1, Wnt and FGF, which are important for brain patterning and neural crest cell formation. Vgll3 interacts in the embryo with Tead proteins but unexpectedly with Ets1, with which it is able to stimulate a MCAT driven luciferase reporter gene. Our findings highlight a critical function for vgll3 in vertebrate early development.

Overexpression of Leap2 impairs Xenopus embryonic development and modulates FGF and activin signals

Besides its widely described function in the innate immune response, no other clear physiological function has been attributed so far to the Liver-Expressed-Antimicrobial-Peptide 2 (LEAP2). We used the Xenopus embryo model to investigate potentially new functions for this peptide. We identified the amphibian leap2 gene which is highly related to its mammalian orthologues at both structural and sequence levels. The gene is expressed in the embryo mostly in the endoderm-derived tissues. Accordingly it is induced in pluripotent animal cap cells by FGF, activin or a combination of vegT/β-catenin. Modulating leap2 expression level by gain-of-function strategy impaired normal embryonic development. When overexpressed in pluripotent embryonic cells derived from blastula animal cap explant, leap2 stimulated FGF while it reduced the activin response. Finally, we demonstrate that LEAP2 blocks FGF-induced migration of HUman Vascular Endothelial Cells (HUVEC). Altogether these findings suggest a model in which LEAP2 could act at the extracellular level as a modulator of FGF and activin signals, thus opening new avenues to explore it in relation with cellular processes such as cell differentiation and migration.

MRAS GTPase is a novel stemness marker that impacts mouse embryonic stem cell plasticity and Xenopus embryonic cell fate

Pluripotent mouse embryonic stem cells (mESCs), maintained in the presence of the leukemia inhibitory factor (LIF) cytokine, provide a powerful model with which to study pluripotency and differentiation programs. Extensive microarray studies on cultured cells have led to the identification of three LIF signatures. Here we focus on muscle ras oncogene homolog (MRAS), which is a small GTPase of the Ras family encoded within the Pluri gene cluster. To characterise the effects of Mras on cell pluripotency and differentiation, we used gain- and loss-of-function strategies in mESCs and in the Xenopus laevis embryo, in which Mras gene structure and protein sequence are conserved. We show that persistent knockdown of Mras in mESCs reduces expression of specific master genes and that MRAS plays a crucial role in the downregulation of OCT4 and NANOG protein levels upon differentiation. In Xenopus, we demonstrate the potential of Mras to modulate cell fate at early steps of development and during neurogenesis. Overexpression of Mras allows gastrula cells to retain responsiveness to fibroblast growth factor (FGF) and activin. Collectively, these results highlight novel conserved and pleiotropic effects of MRAS in stem cells and early steps of development.

Comparative functional analysis of ZFP36 genes during Xenopus development

ZFP36 constitutes a small family of RNA binding proteins (formerly known as the TIS11 family) that target mRNA and promote their degradation. In mammals, ZFP36 proteins are encoded by four genes and, although they show similar activities in a cellular RNA destabilization assay, there is still a limited knowledge of their mRNA targets and it is not known whether or not they have redundant functions. In the present work, we have used the Xenopus embryo, a model system allowing gain- and loss-of-function studies, to investigate, whether individual ZFP36 proteins had distinct or redundant functions. We show that overexpression of individual amphibian zfp36 proteins leads to embryos having the same defects, with alteration in somites segmentation and pronephros formation. In these embryos, members of the Notch signalling pathway such as hairy2a or esr5 mRNA are down-regulated, suggesting common targets for the different proteins. We also show that mouse Zfp36 protein overexpression gives the same phenotype, indicating an evolutionary conserved property among ZFP36 vertebrate proteins. Morpholino oligonucleotide-induced loss-of-function leads to defects in pronephros formation, reduction in tubule size and duct coiling alterations for both zfp36 and zfp36l1, indicating no functional redundancy between these two genes. Given the conservation in gene structure and function between the amphibian and mammalian proteins and the conserved mechanisms for pronephros development, our study highlights a potential and hitherto unreported role of ZFP36 gene in kidney morphogenesis.

Induction and modulation of smooth muscle differentiation in Xenopus embryonic cells

By comparison with skeletal or cardiac developmental programs, little is known regarding the specific factors that promote specification and differentiation of smooth muscle cells from pluripotent cells. We have analyzed the developmental expression of a subset of smooth muscle genes during Xenopus early development and showed that similar to mammals and avians, Xenopus smooth muscle myosin heavy chain (SM-MHC) is a highly specific marker of smooth muscle differentiation. Embryonic cells from animal pole explants of Xenopus blastula can be induced by basic fibroblast growth factor, Wnt, and bone morphogenetic protein signals to adopt the smooth muscle pathway. Explants from early embryos that contain neural crest cells can also differentiate into cells expressing smooth muscle genes. We examined the interplay of several transcription factors, that is SRF, myocardin, and GATA6, that induce the expression of SM-MHC in animal cap cells and found that myocardin-dependent expression of smooth muscle genes in animal cap cells is synergized by SRF but is strongly antagonized by GATA6.

Differential expression of two TEF-1 (TEAD) genes during Xenopus laevis development and in response to inducing factors

Transcription enhancer factors 1 (TEF-1 or TEAD) make a highly conserved family of eukaryotic DNA binding proteins that activate not only viral regulatory elements but muscle specific genes and are involved in several developmental processes. In this study, we report the identification and the expression pattern of NTEF-1 (TEAD1) and DTEF-1 (TEAD3), two members of this family in Xenopus laevis. Both X. laevis NTEF-1 (XNTEF-1 or XTEAD1) and DTEF-1 (XDTEF-1 or XTEAD3) possess a 72 amino acid TEA domain characteristic of TEF-1 proteins. XNTEF-1 is a 426 amino acid protein that has 96% identity with the avian or the mammalian NTEF-1 proteins while XDTEF-1 is a 433 amino acid protein with 77 to 80% identity with the avian and mammalian DTEF-1 sequences respectively. Temporal expression analysis by RT-PCR indicated that the two genes are expressed maternally and throughout embryonic development. In the adult, the two genes are broadly expressed although they showed differences of expression between tissues. Spatial expression analysis by whole mount in situ hybridization showed that the XNTEF-1 and XDTEF-1 mRNAS were predominantly detected in eye, embryonic brain, somites and heart. In animal cap assay, the two genes are activated by bFGF but are differently regulated by BMP4, and the muscle regulatory factor Mef2d.