Reproduction de : Thèse doctorat : Pharmacie. Biologie moléculaire et physiologie : Reims : 2005. / Bibliogr. p.180-217. Index.

Murine plasmacytoma is induced by plastic implants, injection of paraffin oil or pristane, or through viral infection, and Myc is invariably overexpressed in the tumour cells. Although translocation and juxtaposition of the Myc locus to an immunoglobulin locus is the prominent nonrandom cytogenetic aberration observed, the significance of other karyotypic instabilities in murine plasmacytoma is not clear, including the previously observed occurrence of trisomy 11. As well as identifying new cytogenetic mutations in murine plasmacytomagenesis, this study provides evidence for their combined and sequential accumulation that may offer new parallels to human B-cell malignancies. Plasmacytomas were induced in Balb/c Rb6.15 mice by intraperitoneal (i.p.) pristane injection prior to infection with the ABL-MYC retrovirus, and confirmed by histological examination. Spectral karyotype analysis of tumour samples identified frequent aneuploidy, tetraploidy, and amplification of chromosomes 11, 12 and 16. In contrast, control mice treated by i.p. pristane injection did not develop plasmacytoma, and lipopolysaccharide-stimulated splenocytes from control mice had mainly normal diploid karyotypes. However, karyotypic instability in a minority of splenocytes indicated that control mice showing no signs of plasmacytoma development nevertheless are prone to numerical and structural cytogenetic mutations that may possibly result in plasmacytoma initiation and progression under favourable conditions, such as infection with ABL-MYC virus with the resulting high expression of v-abl and Myc in target cells. These results indicate the possible existence of proto-oncogenes present on murine chromosomes 11, 12, and 16 that are important for plasmacytoma initiation and/or progression. There are also indications that T(1;6) and monosomy of the X chromosome may also play roles in plasmacytomagenesis, and that trisomy 12 may only occur in cells with pre-existing nonrandom mutations, thereby acting as a late mutation event. As other experimental models of murine plasmacytoma have not shown a similar karyotypic etiology, there appears to be several possible redundant cytogenetic mutation events that lead to plasmacytoma. Also, as tumours in this study present various combinations of the aforementioned amplified chromosomes, their combined amplification may serve redundant purposes as well.

Murine plasmacytoma is induced by plastic implants, injection of paraffin oil or pristane, or through viral infection, and Myc is invariably overexpressed in the tumour cells. Although translocation and juxtaposition of the Myc locus to an immunoglobulin locus is the prominent nonrandom cytogenetic aberration observed, the significance of other karyotypic instabilities in murine plasmacytoma is not clear, including the previously observed occurrence of trisomy 11. As well as identifying new cytogenetic mutations in murine plasmacytomagenesis, this study provides evidence for their combined and sequential accumulation that may offer new parallels to human B-cell malignancies. Plasmacytomas were induced in Balb/c Rb6.15 mice by intraperitoneal (i.p.) pristane injection prior to infection with the ABL-MYC retrovirus, and confirmed by histological examination. Spectral karyotype analysis of tumour samples identified frequent aneuploidy, tetraploidy, and amplification of chromosomes 11, 12 and 16. In contrast, control mice treated by i.p. pristane injection did not develop plasmacytoma, and lipopolysaccharide-stimulated splenocytes from control mice had mainly normal diploid karyotypes. However, karyotypic instability in a minority of splenocytes indicated that control mice showing no signs of plasmacytoma development nevertheless are prone to numerical and structural cytogenetic mutations that may possibly result in plasmacytoma initiation and progression under favourable conditions, such as infection with ABL-MYC virus with the resulting high expression of v-abl and Myc in target cells. These results indicate the possible existence of proto-oncogenes present on murine chromosomes 11, 12, and 16 that are important for plasmacytoma initiation and/or progression. There are also indications that T(1;6) and monosomy of the X chromosome may also play roles in plasmacytomagenesis, and that trisomy 12 may only occur in cells with pre-existing nonrandom mutations, thereby acting as a late mutation event. As other experimental models of murine plasmacytoma have not shown a similar karyotypic etiology, there appears to be several possible redundant cytogenetic mutation events that lead to plasmacytoma. Also, as tumours in this study present various combinations of the aforementioned amplified chromosomes, their combined amplification may serve redundant purposes as well.

Thesis (Ph. D.)--University of Hawaii at Manoa, 2003. / Includes bibliographical references (leaves 122-177).

Thesis (Ph. D.)--University of Hawaii at Manoa, 2003. / Includes bibliographical references (leaves 122-177). Also available by subscription via World Wide Web.

Im Neuroblastom ist die Amplifikation des MYCN-Gens, eines Mitglieds der MYC-Onkogenfamilie, mit einer ungünstigen Prognose assoziiert. Der von dem Gen kodierte Transkriptionsfaktor N-Myc ist für die Proliferation der MYCN-amplifizierten Neuroblastomzelllinien notwendig und seine Depletion oder Destabilisierung führen zum Proliferationsarrest (Otto et al., 2009). Da N-Myc auf Proteinebene durch die Interaktion mit der mitotischen Kinase Aurora-A stabilisiert wird, bewirkt deren Depletion oder die Hemmung der Interaktion der beiden Proteine mittels spezieller Aurora- A-Inhibitoren (z.B. MLN8054 und MLN8237) ebenso eine Hemmung der Proliferation – in vitro und in vivo (Brockmann et al., 2013). Bisher ist jedoch unklar, über welchen Mechanismus Aurora-A die Stabilisierung von N-Myc erreicht, die Kinaseaktivität spielt hierbei jedoch keine Rolle (Otto et al., 2009). Eine Möglichkeit stellt die Rekrutierung von Usps dar, die das angehängte Ubiquitinsignal so modifizieren, dass die Erkennung und der Abbau des Proteins durch das Proteasom verringert werden. In der vorliegenden Arbeit wurde die Wirkung von Usp7 und Usp11 auf die Stabilität von N-Myc untersucht. Für beide konnte in Immunpräzipitationen die Interaktion mit N-Myc gezeigt werden. Ebenso erhöhten beide Proteasen in Überexpressionsexperimenten die vorhandene Menge an NMyc. Die Depletion von Usp7 mittels shRNAs führte in IMR-32 zu einem Arrest in der G1-Phase und zur Differenzierung der Zellen. Gleichzeitig wurden stark erniedrigte mRNA- und Proteinmengen von N-Myc und Aurora-A nachgewiesen. Es konnte jedoch nicht eindeutig gezeigt werden, ob die beobachteten zellulären Effekte durch eine vermehrte proteasomale Degradation von N-Myc begründet sind oder ob dabei die veränderte Regulation weiterer Zielproteine von Usp7 eine Rolle spielt. Die Depletion von Usp11 mit shRNAs bewirkte eine Abnahme der N-Myc-Mengen auf posttranslationaler Ebene. Somit stellen beide Usps vielversprechende Angriffspunkte einer gezielten Therapie in MYCN-amplifizierten Neuroblastomen dar und sollten deshalb Gegenstand weiterführender Untersuchungen sein. Über welche Proteindomäne in N-Myc die Interaktion mit Aurora-A stattfindet ist nicht bekannt. Eine mögliche Pseudosubstratbindungssequenz in Myc-Box I (Idee Richard Bayliss, University of Leicester) wurde in der vorliegenden Arbeit untersucht. Durch Mutation dieser Sequenz sollte die Bindung von Aurora-A unmöglich gemacht werden. Allerdings wurde die erwartete Abnahme der Stärke der Interaktion von Aurora-A und N-Myc durch die Mutation ebensowenig beobachtet wie eine verringerte Stabilität. Die Regulation der Phosphorylierung von N-Myc im Verlauf des Zellzyklus wurde durch die Mutation beeinträchtigt. Wie diese Veränderung exakt zu begründen ist bedarf weiterer Experimente / Neuroblastomas with an amplification of the MYCN-gene, a member of the MYC-oncogene family, are associated with a poor prognosis. The transcription factor encoded by this gene, N-Myc, is essential for the proliferation of MYCN-amplified neuroblastoma cell lines and its depletion or destabilization leads to an arrest of proliferation (Otto et al., 2009). Since N-Myc is stabilized by the interaction with the mitotic kinase Aurora-A, the depletion of the kinase or the inhibition of the interaction with N-Myc using a special class of Aurora-A inhibitors (e.g. MLN8054 and MLN8237) inhibits proliferation – in vitro and in vivo (Brockmann et al., 2013). Up to date it is not known by which mechanism Aurora-A is able to stabilize N-Myc preventing it from Fbxw7-mediated proteasomal degradation, interestingly the Aurora-A kinase activity is not necessary (Otto et al., 2009). One possible explanation is the recruitment of Usps, which modify the attached ubiquitin signal and therefore reduce the recognition and degradation of the protein by the proteasome. In this thesis the influence of Usp7 and Usp11 on N-Myc stability was studied. For both the interaction with N-Myc was shown in immune precipitations. Furthermore the overexpression of both proteases increased the amount of N-Myc protein in transfection experiments. The depletion of Usp7 via shRNAs caused the arrest of IMR-32 cells in G1-phase and the differentiation of the cells. Simultaneously strongly reduced amounts of N-Myc and Aurora-A mRNA and proteins were observed. However it could not be shown that the observed effects were mediated by an increased proteasomal degradation of N-Myc and not via the changed regulation of other targets of Usp7. The depletion of Usp11 led to a decrease of the N-Myc amounts, whereas mRNA-levels were unaffected. Thus both Usps are promising targets for a targeted therapy of MYCN-amplified neuroblastomas and the underlying mechanism should be the object of further research. Furthermore the N-Myc domain binding to Aurora-A still remains to be idientified. A possible pseudosubstrate binding site in Myc-Box I (idea of Richard Bayliss, University of Leicester) was investigated in this thesis. To inhibit the possible binding of Aurora-A to this site, two lysines in Myc-Box I were mutated to glutamate (KK51EE). Nonetheless neither the expected decrease of the intensity of interaction of N-Myc and Aurora-A was observed nor was a decrease of the stability of N-Myc. The regulation of the phosphorylation of N-Myc during the cell cycle was changed through the mutagenesis however. It must be clarified in further experiments, what the reasons for this change are.

Interactions between DNA and ligands are important in the rational design of drugs and in research into DNA function. In particular, the interaction of DMZ with DNA structures named “G-quadruplexes” was considered. G-quadruplexes are structures present in telomeres and several oncogenes. The main purpose of this project was to provide a computational tool to study DNA ligand interactions using a variety of molecular modeling techniques that include molecular docking, molecular dynamics simulations (MD) and MM/PBSA (Molecular Mechanics/Poisson Boltzmann Surface Area). We investigated the binding modes and binding affinities of DMZ with c-MYC G-quadruplexes (G4s). We found that the conformation and structural design of the quadruplex can dramatically influence the binding profiles of the ligand. The binding free energies for each site were estimated by the MM/PBSA method. The binding of small molecules to DNA can result in the disruption of oncogene transcription, making it an effective anticancer strategy.

Programmed cell death, known as apoptosis, is widely accepted as a key tumour suppression mechanism. The oncogene MYC promotes cell growth and proliferation but also sensitises cells to apoptosis, which limits its oncogenic potential. MYC-induced apoptosis requires the pro-apoptotic BCL2 family proteins BAX/BAK and can be blocked by anti-apoptotic family members such as BCL2 and BCL-xL. Previous studies have identified glutamine withdrawal as a trigger for MYC-induced apoptosis. Through untargeted metabolomic analyses of cells with perturbed BCL2 family member composition and of cells undergoing glutamine-dependent MYC-induced apoptosis, we found that nucleosides and nucleotides were altered in correlation with apoptotic status. Glutamine is an important biosynthetic substrate and energy source and we show global transcription and translation of the cells decreased upon glutamine withdrawal. However, MYC-activated cells promote transcription and translation even in the absence of glutamine and thus still drive huge demand for energy. Deregulated MYC promotes nucleotide catabolism and depletes cellular energy charge upon glutamine withdrawal, indicating energy shortage driven by MYC. Nucleotide conversion and remodeling by adenylate kinase 2 (AK2) protects cellular energy charge and inhibits MYC-induced apoptosis. These results indicate a homeostatic model for MYC-induced apoptosis based upon mitochondrial energy supply and demand. We propose that the transcriptional activity of MYC drives huge demand for energy to support global transcription and translation and thereby sensitises cells to apoptosis under conditions of limiting energy supply.

The transcription factor c-Myc plays a central role in cell growth and proliferation owing to the large number of genes it transactivates or transrepresses and to the fact that these genes are in turn implicated in these cellular processes. Also, c-Myc's deregulation and/or overexpression contribute to most aspects of tumoral cellular biology. As a heterodimer with Max, c-Myc activates the transcription of genes leading to cell proliferation and represses the transcription of cytostatic genes such as p15[superscript ink4b] and p21[superscript CiP1]. In contrast to the transactivation mechanism, our current understanding of the transrépression by c-Myc is still incomplete, aside from the fact that an interaction with Miz-1 is essential. Coupling preliminary results from a collaboration with Martin Eilers' group to data obtained following a bioinformatics' approach to predict Miz-1 DNA binding, we were able to elaborate a now transrepression mechanism for c-Myc/Miz-1. In this mechanism, the c-Myc/Max heterodimer directly binds the noncanonical E-box sequences present in the promoters and provoke the supercoiling of DNA assisted by the interaction between c-Myc and Miz-1. This supercoiling impairs accessibility to the initiation site to the transcriptional machinery. This thesis aims at the study on a structural and biophysics viewpoint of the determinants for the specific heterodimerization and DNA binding by c-Myc and Max and the interaction between c-Myc and Miz-1 in order to validate our mechanistic model for the transrépression by c-Myc. In chapter 1, we present an overview of the actual knowledge on Myc and the repression model along with some of the results that led to its elaboration. Chapters 2 and 3 report the study of the structural determinants for the heterodimerization and E-box binding by the b-HLH-LZ domains of c-Myc and Max. Our model allows to predict that b-HLH-LZ peptides able to bind the E-box present in the repressed promoters without interaction with Miz-1 could reverse the inaccessibility and reactivate p15[superscript ink4b] and p21[superscript Cip1] expression in cancer cells where c-Myc is overexpressed.The results presented in this thesis will find application in the development of new inhibitors of c-Myc eventually leading to novel therapies to fight cancer.

Currently there is no curative therapy for Chronic Myelogenous Leukemia (CML), and patients must remain on the current prescribed treatment, tyrosine kinase inhibitors (TKI), indefinitely. Although many patients can survive in the chronic phase of the disease under TKI treatment, some patients do progress to the terminal blast crisis phase of the disease. Patients in this terminal phase do not respond to TKI treatment. We evaluated the therapeutic benefit of targeting the oncogene c-MYC in CML, using the CML cell line K562. This was achieved by inhibiting the enzyme O-linked β-N-acetylglucosamine Transferase (OGT), using two indirect inhibitors 2-deoxyglucose and Azaserine, and the direct inhibitor ST078925. Treatment with these inhibitors resulted in decreased half-life of c-MYC protein in K562, reduced c-MYC protein in K562 cells, and reduced K562 cell growth. Together these results suggest that targeting c-MYC through OGT may be a potential therapeutic option for patients with CML.