Post-translational modification of proteins with ubiquitin regulates many aspects of cell physiology, including protein degradation. A uniform polyubiquitin chain that is linked through Lys48 has been widely accepted as central for recognition and destruction by the 26S proteasome. Work in more recent years has demonstrated that the repertoire of proteolytic signals may encompass chains of other linkage types, including Lys11-linked ubiquitin chains and short assemblies of mixed linkage. In this dissertation I examine whether catalysis mediated by the Anaphase-Promoting Complex/Cyclosome (APC/C) is dependent on polyubiquitination and whether the proteolytic machinery exerts a requirement for specific ubiquitin linkages to efficiently degrade cyclin B1. In chapter II, I describe a novel method in which Xenopus cell-cycle extracts are made largely dependent on exogenous ubiquitin by inhibiting ubiquitin recycling, allowing us to evaluate the relative contribution of distinct ubiquitin linkages in APC/C-mediated ubiquitination and degradation. Utilizing this approach, in chapter III, I found that the conjugation of single ubiquitin moieties to multiple lysine residues in cyclin promotes efficient degradation of cyclin B1 in mitotic Xenopus extracts. Lysine11-ubiquitin chain-formation becomes essential to proteasomal targeting only when the number of available lysine residues in cyclin B1 is restricted. Analysis in a reconstituted system revealed that APC/C catalyzes multiple monoubiquitination with rapid kinetics and species bearing four or more monoubiquitins on distinct lysines are recognized by ubiquitin receptors. These multiply monoubiquitinated species are rapidly degraded by purified proteasomes. In chapter IV, I examine the role of distinct E2 enzymes in APC/C-dependent proteolysis. I demonstrate that the chain-extending E2 UBE2S and long Lys11-linked ubiquitin assemblies are dispensable for cyclin B1 degradation, but become increasingly important with restriction of the number of ubiquitination sites. Our findings support a model where through attachment of monoubiquitin to multiple lysine residues, and possibly elaboration of some short chains, UBCH10, or possibly members of the UBC4/5 family, cooperate with the APC/C to generate a robust proteolytic signal on cyclin B1.

Thesis (Ph. D.)--Case Western Reserve University, 2005. / [School of Medicine] Department of Environmental Health Sciences. Includes bibliographical references. Available online via OhioLINK's ETD Center.

The aim of this thesis was to investigate the prognostic role of the proliferation markers cyclin B1 and Phosphorylated Histone 3 (PPH3) in breast cancer (BC). In paper I we used an experimental study design, we compared women dying early from their BC with women free from relapse more than eight years after initial diagnosis. All women had stage I, node-negative and hormone receptor positive disease. None had received adjuvant chemotherapy. We found that low-risk node negative patients with high expression of cyclin B1 had a significantly worse outcome than patients with low expression of cyclin B1. In paper II a population-based case control study was performed to further investigate the prognostic value of cyclin B1. One hundred and ninety women who died from BC were defined as cases and 190 women alive at the time for the corresponding case’s death were defined as controls. Inclusion criteria were tumor size 50 mm, no lymph node metastases, and no adjuvant chemotherapy. Two investigators evaluated the stainings independently. Cyclin B1 was found to be a prognostic factor for BC death that could identify high-risk patients with a good to very good reproducibility. Paper III aimed to investigate the role of proliferation in male breast cancer (MBC). One hundred and ninety-seven MBC tumors were stained for cyclin A, B1, D1 and Ki67. Overexpression of cyclin A and B1 and elevated mitotic count were predictive of breast cancer death. Ki67 was re-evaluated and different cut-offs were used, but no prognostic value could be demonstrated. On the other hand high levels of cyclin D1 were associated with better outcome in MBC. In paper IV we applied the immunohistochemichal panel suggested from international guidelines to the same patient material as in paper II, to discriminate luminal A from luminal B BC. We wanted to evaluate if different cut-off values of Ki67, cyclin A or B1 could more clearly separate luminal A from B. Cyclin A, B1 and Ki67 (cut-off 20%) could detect difference in outcome between these subtypes with cyclin A showing greater prognostic value. The aim of paper V was to examine the prognostic role of PPH3 compared to the proliferation markers Ki67, cyclin A and cyclin B1 with focus on ER positive disease. PPH3 was found to be a prognostic factor for breast cancer death but in the multivariate analysis including all proliferation markers, only cyclin A remained a prognostic factor. Finally, we conclude that both cyclin B1 and PPH3 are prognostic factors for breast cancer death, but are outperformed by cyclin A in ER positive patients. In male breast cancer prognostic factors need to be further studied.

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La toxoplasmose, déterminée par le parasite Toxoplasma gondii, est l'une des infections les plus répandues au monde, en raison de la persistance à vie sous forme latente de ce parasite au sein de ces hôtes. Ce parasite fait partie des Apicomplexa et détourne les voies de signalisation de l'hôte par des mécanismes épigénétiques qui convergent vers des protéines nucléaires clés. Nous rapportons ici une nouvelle stratégie de persistance parasitaire impliquant la protéine de rhoptries ROP16 de T. gondii, sécrétée précocement lors de l'invasion, qui cible le facteur de transcription UHRF1 (Ubiquitin-like containing PHD and RING fingers domain 1) et induit un arrêt du cycle de la cellule-hôte. Ceci est induit par l'activité de la DNMT et le remodelage de la chromatine au niveau du promoteur du gène de la cycline B1 par le recrutement d’UHRF1 phosphorylé associé à un complexe protéique multienzymatique répressif. Cela conduit à la désacétylation et à la méthylation de l'histone H3 entourant le promoteur de la cycline B1 pour réduire de manière épigénétique son activité transcriptionnelle. De plus, l'infection par T. gondii provoque une hyper-méthylation de l'ADN dans la cellule hôte par la régulation positive des DNMTs. ROP16 est déjà connue pour activer et phosphoryler des facteurs de transcription de l'immunité protectrice tels que STAT 3/6/5 et le suppresseur tumoral p53 impliqué dans la progression du cycle cellulaire. De plus, ROP16 module ces voies de signalisation de l'hôte de manière souche-dépendante. Comme dans le cas de STAT3, les effets de ROP16 sur UHRF1 dépendent du polymorphisme d'un seul acide aminé du domaine kinase de ROP16. Ce travail montre que Toxoplasma module un nouvel initiateur épigénétique, UHRF1, via un événement précoce initié par la kinase parasitaire ROP16. / Toxoplasmosis, caused by the apicomplexan parasite Toxoplasma gondii, is one of the most common infections in the world due to the lifelong persistence of this parasite in a latent stage in its hosts. T. gondii hijacks host signaling pathways through epigenetic mechanisms which converge on key nuclear proteins. Here we report a new parasite persistence strategy involving Toxoplasma rhoptry protein ROP16 secreted early during invasion, which targets the transcription factor UHRF1 (Ubiquitin-like containing PHD and RING fingers domain 1), and leads to host cell cycle arrest. This is mediated by DNMT activity and chromatin remodeling at the cyclin B1 gene promoter through recruitment of phosphorylated UHRF1 associated with a repressive multienzymatic protein complex. This leads to deacetylation and methylation of histone H3 surrounding the cyclin B1 gene promoter to epigenetically silence its transcriptional activity. Moreover, T. gondii infection causes DNA hypermethylation in its host cell, by upregulation of DNMTs. ROP16 is already known to activate and phosphorylate protective immunity transcription factors such as STAT 3/6/5 and the tumor suppressor p53 involved in cell cycle progression. Moreover, ROP16 modulates host signaling pathways in a strain-dependent manner. Like in the case of STAT3, the strain-dependent effects of ROP16 on UHRF1 can be attributed to a single amino-acid polymorphism in ROP16. This study demonstrates that Toxoplasma hijacks a new epigenetic initiator, UHRF1, through an early event initiated by the ROP16 parasite kinase.

In vertebrate oocytes, meiotic progression is driven by the sequential translational activation of maternal messenger RNAs stored in the cytoplasm. This activation is mainly induced by the cytoplasmic elongation of their poly(A) tails, which is mediated by the cytoplasmic polyadenylation element (CPE) present in their 3’ untranslated regions (3´ UTRs). Sequential, phase-specific translation of these maternal mRNAs is required to complete the two meiotic divisions. Although the earlier polyadenylation events in prophase I and metaphase I are driven by the CPE-binding protein 1 (CPEB1), 90% of this protein is degraded by the anaphase promoting complex in the first meiotic division. The low levels of CPEB1 during interkinesis and in metaphase II raise the question of how the cytoplasmic polyadenylation required for the second meiotic division is achieved. In this work, we demonstrate that CPEB1 activates the translation of the maternal mRNA encoding CPEB4, which, in turn, recruits the cytoplasmic poly(A) polymerase GLD2 to “late” CPE-regulated mRNAs driving the transition from metaphase I to metaphase II, and, therefore, replacing CPEB1 for “late” meiosis polyadenylation.

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