Internal ribosome entry site (IRES)-mediated translation is critical for the cell’s ability to respond to stress. Understanding how RNA binding proteins (IRES trans-acting factors; ITAFs) regulate IRESes is crucial to elucidating the mechanism of alternative translation initiation. Furthermore, determining how these ITAFs are regulated is central to understanding their functions in diseased states. I have identified the tumour suppressor programmed cell death 4 (PDCD4) as a novel ITAF of the XIAP and Bcl-xL IRES elements. I demonstrate that under normal conditions, PDCD4 acts to inhibit translation from these IRES elements by preventing formation of the 48S translation initiation complex. Furthermore, I show that in response to treatment with the pro-survival fibroblast growthfactor-2 (FGF-2), S6 kinase 2 (S6K2) phosphorylates PDCD4 leading to its degradation and the subsequent de-repression of XIAP and Bcl-xL translation. Importantly, I demonstrate the clinical significance of this regulation in glioblastoma multiforme (GBM) tumours where the loss of PDCD4 expression correlates with an increase in Bcl-xL protein and poor patient outcome. Additionally, re-expression of PDCD4 down-regulates Bcl-xL and decreases cell viability, and direct inhibition of Bcl-xL by a small molecule antagonist ABT-737 sensitizes GBM cells to the chemotherapeutic doxorubicin. Finally, I demonstrate that PDCD4 can be regulated at multiple levels. Importantly, I identify the RNA binding protein HuR as a regulator of microRNA (miR) -21 induced silencing of PDCD4. I show that HuR can bind the PDCD4 3'UTR and prevent miR-21 binding, and that a loss of PDCD4 expression following H2O2 treatment is mediated via miR-21. These results provide novel insight into the role of PDCD4 as a tumour suppressor and highlight the importance of ITAFs in cancer progression.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第22334号 / 医博第4575号 / 新制||医||1041(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 河本 宏, 教授 濵﨑 洋子, 教授 羽賀 博典 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

In several human cancers, ErbB2 over-expression facilitates the formation of constitutively active homodimers resistant to internalization which results in progressive signal amplification from the receptor, conducive to cell survival, proliferation, or metastasis. Here we report on studies of the influence of ErbB2 over-expression on localization and signaling in polarized Caco-2 and MDCK cells, two established models to study molecular trafficking. In these cells, ErbB2 is not over-expressed and shares basolateral localization with ErbB3. Over-expression of ErbB2 by transient transfection resulted in partial separation of the receptors by relocalization of ErbB2, but not ErbB3, to the apical surface, as shown by biotinylation of the apical or basolateral surfaces. These results were confirmed by immunofluorescence and confocal microscopy. Polarity controls indicated that the relocalization of ErbB2 is not the result of depolarization of the cells. Biotinylation and confocal microscopy also showed that apical, but not basolateral ErbB2 is activated at tyrosine 1139. This phosphotyrosine binds adaptor protein Grb2, as confirmed by immunoprecipitation. However, we found that it does not initiate the canonical Grb2-Ras-Raf-Erk pathway. Instead, our data supports the activation of a survival pathway via Bcl-2. The effects of ErbB2 over-expression were abrogated by the humanized anti-ErbB2 monoclonal antibody Herceptin added only from the apical side. The ability of apical ErbB2 to initiate an altered downstream cascade suggests that subcellular localization of the receptor plays an important role in regulating ErbB2 signaling in polarized epithelia.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第20566号 / 医博第4251号 / 新制||医||1022(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 竹内 理, 教授 小柳 義夫, 教授 秋山 芳展 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

The treatment and/or prevention of cancer by selective down regulation of cancer causing gene (oncogene) transcription would represent a significant advance in the area of anticancer drug design. Non-canonical higher order DNA structures formed in oncogene promoter regions are novel targets for the modulation of oncogene expression. An obvious advantage of selectively targeting oncogene expression would be that general cytotoxicity would be minimized and the negative side effects of current chemotherapy approaches could be minimized or eliminated. To provide a foundation for the design of drugs that target oncogene promoter G-quadruplexes and i-Motifs, the basic understanding is required of the folding of guanine and cytosine rich sequences and how small molecules bind to these structures. The research reported here focuses on higher order DNA structures of two oncogenes, K-ras that is overexpressed in pancreatic cancer, and Bcl-2 that is overexpressed in a number of cancers, and one non-oncogene, HAR1. We have probed the overall structure, stability, and binding of a model drug compounds to G-quadruplex and i-Motif DNA structures in these genes. The overall objectives of this work were: 1) to understand the relationship between oligonucleotide sequence and intramolecular folding topology and stability, and 2) to understand the mechanisms for the selective binding of small molecules to these structures. Biophysical techniques including: microcalorimetry, spectroscopy, analytical ultracentrifugation, gel electrophoresis, and computational methods were used to characterize both the folding and the binding interactions. We have shown that the native K-ras purine and pyrimidine rich sequences form stable G-quadruplexes and i-Motifs. We have also characterized four G-rich sequences found within the reading frame of the human HAR1 gene. This is the first report on the formation of stable G-quadruplex motifs within the RF of any gene. The model drug, TMPyP4, binds to the Bcl-2, K-ras, and HAR1 G-quadruplexes by two different binding modes, end binding and intercalation. The significance of this research is that the results of the K-ras and Bcl-2 studies could lead to the design of drugs that selectively target oncogenes while the HAR1 results could provide new approaches to the treatment of Schizophrenia and Alzheimer’s disease.

Autophagy is a lysosomal degradation pathway wherein cytoplasmic components not needed by or harmful to the cell are degraded and recycled. BECN homologs are key autophagy proteins consisting of an intrinsically disordered region (IDR), flexible helical domain (FHD), coiled-coil domain (CCD) and β-α repeated, autophagy-specific domain (BARAD). Diverse proteins modulate autophagy by binding BECN1. Understanding the mechanisms by which these proteins regulate BECN1-mediated autophagy is important for developing therapeutics targeting these proteins. Toward this goal, we have developed purification protocols for multi-domain BECN1 fragments to explore the conformational flexibility and interactions. We show that a BECN1 helix transitions between mutually exclusive packing states, wherein it either forms part of the CCD homodimer or packs against the BARAD, but predominantly packs against the BARAD. The same set of residues on this helix contribute to the CCD homodimer or packing with the BARAD, and mutation of these residues abrogates starvation-induced up-regulation of autophagy. Next, we show the equatorial groove of GAPR-1 may be responsible for binding BECN1. The five conserved residues lining the GAPR-1 equatorial groove are essential for the interaction, as mutation of these residues disrupts GAPR-1:BECN1 interaction. We also solved the structure of this pentad mutant, which indicates the changes in the equatorial groove and the improved dimerization of pentad mutant likely abrogates BECN1-binding. We then show that BH3D is not required for BECN1 to up-regulate autophagy, though it is required for binding BCL2 homologs. Therefore, we investigated the interactions between BH3D-containing BECN1 fragments and the BCL2 homolog, M11. BECN1 regions outside the BH3D increase binding to M11 by 5-10 fold. In addition, M11-binding increases flexibility of the nuclear export sequence (NES). Further, homodimerization and thermostability of BECN1 BH3D-FHD-CCD increases upon M11-binding. Lastly, the M11:BH3D-FHD-CCD complex appears to fluctuate between two major types of conformations, which may be mediated by the increased flexibility of BECN1 NES upon binding M11. Lastly, we investigated the interactions between BH3D-containing BECN1 fragments and Bcl-XL. Our results indicate that BECN1 regions outside the BH3D do not affect BECN1 interaction with Bcl-XL. Together, these studies are important for better understanding how proteins down-regulate BECN1-mediate autophagy. / NIH: RO3 NS090939, R15 GM122035, P20 RR015566, and R21 AI078198 (S.S). R15 GM113227, P30 GM103332-01, P41 GM103622, and P41 GM103403.; NSF: MCB-1413525 (S.S.); ND Dept. of Commerce: Award #14-11-J1-73 (S.S.)

Apoptosis is essential for cellular homeostasis and is also a pathologic feature of various diseases. The balance between Bcl-2 family proteins determines whether a cell will live or die. Bax, a member of the BCL-2 family proteins, is a pro-apoptotic protein that exists in both a soluble, cytoplasmic form and a membrane-bound form. Upon apoptotic stimuli, Bax undergoes a conformational change and translocates to the mitochondria, initiating apoptotic events. However, little is known about whether Bax is involved in the regulation of mitochondrial function under non-apoptotic conditions, and how Bax binds to mitochondria to exert its activity. Here, we investigate the role of Bax in the regulation of mitochondrial function under non-apoptotic conditions and explore the molecular mechanisms for Bax binding mitochondria under apoptotic stimuli. Using Bax-containing and Bax-deficient (Bax⁻/⁻) HCT-116 cells, we examined Bax cellular localization and its effects on mitochondria bioenergetics, and also tested whether over-expression of full-length Bax in Bax⁻/⁻ cells would recover mitochondrial metabolic activity. To determine the effects of Bax localization upon mitochondrial function, we measured citrate synthase activity and ATP generation. We showed that Bax localized to the outer and inner mitochondrial membranes in non-apoptotic cells, enabling the activity of citrate synthase and the generation of ATP. Loss of Bax led to impairment of respiring mitochondria morphology and reduced oxidative capacity, all of which was restored by expression of full-length or C-terminal-deleted Bax. These findings indicate that under non-apoptotic conditions, the constitutive expression of Bax is necessary for mitochondrial bioenergetics. To determine the molecular mechanisms for Bax binding mitochondria under apoptotic stimuli, we previously performed in silico-mutagenesis and predicted that Lysines 189/190, in the C-terminal [alpha]9 helix, could regulate Bax binding to mitochondria. We demonstrated here that these lysines are the structural elements responsible for controlling how Bax interacts with the mitochondrial membrane. Expression of full-length Bax led to mitochondrial translocation and apoptosis, whereas deletion of the [alpha]9 helix resulted in cytosolic retention and dramatically reduced cell death. Mutation of the two lysine residues changed how Bax bound to mitochondrial membranes. We replicated the results achieved with full-length Bax by attaching the [alpha]9 helix of Bax to GFP or to a regulatory element, the degradation domain (DD), and induced apoptosis upon expression in cells. We demonstrated that the [alpha]9 helix alone promoted the mitochondrial translocation of Bax and increased apoptosis. These results indicate that the C-terminal [alpha]9 helix could be further studied for use in cancer therapies. Overall, we have demonstrated that the constitutive expression of the inactive form of Bax in non-apoptotic cells is necessary for mitochondrial bioenergetics, and have identified the C-terminal [alpha]9 helix of Bax as the effector domain of apoptotic function.

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