Thesis (Ph. D.)--Case Western Reserve University, 2006. / [School of Medicine] Department of Pharmacology. Includes bibliographical references. Available online via OhioLINK's ETD Center.

Recent developments in neurobiology make it clear that glia play fundamental and active roles, in the adult and in development. Many hereditary cognitive disorders have been linked to developmental defects, and in at least two cases, Rett Syndrome and Fragile X Mental Retardation, glia are important in pathogenesis. However, most studies of developmental disorders, in particular intellectual disability, focus on neuronal defects. An example is intellectual disability caused by mutations in ACSL4, a metabolic enzyme that conjugates long-chain fatty acids to Coenzyme A (CoA). Depleting ACSL4 in neurons is associated with defects in dendritic spines, a finding replicated in patient tissue, but the etiology of this disorder remains unclear. In a genetic screen to discover genes necessary for visual function, I identified the Drosophila homolog of ACSL4, Acsl, as a gene important for the magnitude of neuronal transmission, and found that it is required in glia. I determined that Acsl is required in a specific subtype of glia in the Drosophila optic lobe, and that depletion of Acsl from this population causes morphological defects. I demonstrated that Acsl is required in development, and that the phenotype can be rescued by human ACSL4. Finally, I discovered that ACSL4 is expressed in astrocytes in the mouse hippocampus. This study is highly significant for understanding glial biology and neurodevelopment. It provides information on the role of glia in development, substantiates a novel role for Acsl in glia, and advances our understanding of the potential role that glia play in the pathogenesis of intellectual disability.

In response to telomere shortening, oxidative stress, DNA damage or aberrant activation of oncogenes, normal somatic cells exit the cell cycle and enter an irreversible growth arrest termed senescence. The limited proliferative capacity imposed by senescence on cells impedes the accumulation of mutations necessary for tumorigenesis and prevents proliferation of cells at risk of neoplastic transformation. Opposite to the tumor suppressor function, accumulation of senescent cells in adult organisms is thought to contribute to aging by depleting the renewal capacity of tissues and stem/progenitor cells, and by interfering with tissue homeostasis and functions. The Antagonistic Pleiotropy Theory of senescence proposes that senescence is beneficial early in life by acting as a tumor suppressor, but harmful late in life by contributing to aging. Recent studies have provided evidence strongly supporting the tumor suppressor function of senescence, however, direct evidence supporting the role of senescence in aging remains largely elusive. In this thesis, I describe studies to test the Antagonistic Pleiotropy Theory of senescence in tumorigenesis and aging. The approach that I have taken is to alter the senescence response in vivo by changing the expression of a senescence regulator in mice. The consequence of altered senescence response on tumorigenesis and stem cell self-renewal was investigated. The senescence regulator I studied is Smurf2, which has been shown previously to activate senescence in culture. I hypothesized that the senescence response will be impaired by Smurf2 deficiency in vivo. Consequently, Smurf2-deficient mice will develop tumors at an increased frequency, but also gain enhanced self-renewal capacity of stem/progenitor cells with age. I generated a Smurf2-deficient mouse model, and found that Smurf2 deficiency attenuated p16 expression and impaired the senescence response in primary cells and tissues. Smurf2-deficient mice exhibited an increased susceptibility to spontaneous tumorigenesis, indicating that Smurf2 is a tumor suppressor. At the premalignant stage of tumorigenesis, a defective senescence response was documented in the Smurf2-deficient mice, providing a mechanistic link between impaired senescence response and increased tumorigenesis. The majority of tumors developed in Smurf2-deficent mice were B-cell lymphomas with an origin in germinal centers of the spleen and a phenotype resembling human diffuse large B-cell lymphoma (DLBCL). I discovered that Smurf2 mediated ubiquitination of YY1, a master regulator of germinal centers. Stabilization of YY1 in the absence of Smurf2 was responsible for increased cell proliferation and drove lymphomagenesis in Smurf2-deficient mice. Consistently, a significant decrease of Smurf2 expression was observed in human primary DLBCL samples, and more importantly, a low level of Smurf2 expression in DLBCL correlated with poor survival prognosis. Moreover, I found that hematopoietic stem cells (HSCs) in Smurf2-deficient mice had enhanced function compared to wild-type controls. This enhanced stem cell function was associated with increased cell proliferation and decreased p16 expression, suggesting that defective senescence response in Smurf2-deficient mice leads to increased self-renewal capacity of HSCs. My study, for the first time, offers direct genetic evidence of an important tumor suppressor function for Smurf2 as well as its function in contributing to stem cell aging. Collectively, these findings provide strong evidence supporting the Antagonistic Pleiotropy Theory of senescence in tumorigenesis and aging.

Nierenzellkarzinome (RCC) gehören zu den häufigsten malignen Tumoren weltweit. Aufgrund der alarmierend hohen Inzidenz- und Sterberate besteht ein dringender Bedarf an neuen therapeutischen Targets zur Behandlung von RCCs. Punktmutationen in der Codesequenz von Proteinen führen zu einer Anhäufung von fehlgefalteten Proteinen in Tumorzellen und erfordern eine stärkere Kontrolle der Proteinqualität. Das Ubiquitin-Proteasome-System (UPS) bietet daher ein ideales therapeutisches Target für die RCC Therapie. Aktuelle Veröffentlichungen deuten auf eine Deregulation des COP9 Signalosome (CSN)-Cullin-RING-Ubiquitin-Ligase-(CRL)-Signalweges hin, einem Bestandteil des UPS. In der vorliegenden Arbeit wurden ausgewählte Komponenten des CSN-CRL Signalweges im RCC Gewebe und in vier RCC Zelllinien untersucht. In immunohistochemischen Studien am klarzelligen RCC-Gewebe konnte keine Hochregulierung einer einzelnen CSN-Untereinheit gezeigt werden. Höchstwahrscheinlich ist der gesamte CSN-Komplex im klarzelligen Nierenkarzinom im Vergleich zu nicht-malignem Nierengewebe stärker exprimiert. Die Untersuchung von vier RCC-Zelllinien zeigte eine interessante Deregulierung der CAND1-Skp2-p27 Achse in einer der Zelllinien. 786-O Zellen wiesen zwei p27Kip1 (p27) Varianten (p27V109G und p27I119T), eine Erhöhung des Skp2 und eine Reduktion des CAND1 Levels auf. Die Expression und Lokalisation von CAND1 wurde weiter in einer größeren RCC-Kohorte untersucht. Dabei zeigte sich eine negative Korrelation zwischen einer hohen zytosolischen CAND1 Expression und dem Gesamtüberleben von Patienten mit klarzelligen renalen Tumoren. Beide p27 Varianten werden durch das UPS abgebaut und binden an das CSN, Skp2, Cdks sowie an Cyclin E. Interessanterweise zeigte die p27 Mutanten beinhaltende Zelllinie 786-O eine höhere Proliferationsrate als die p27-Wildtyp-Zelllinie A498. In einem im Rahmen dieser Arbeit entwickelten Genotypisierungs-Assay konnte eine große RCC-Kohorte nach den beiden p27-Mutanten untersucht werden. In 42,5% der RCC Patienten konnte die Mutante p27V109G heterozygot nachgewiesen werden. Die Präsenz der beiden Mutanten p27V109 und p27I119T im RCC-Gewebe sowie die veränderte Expression von Skp2 und CAND1 machen den CSN-CRL Signalweg zu einem attraktiven therapeutischen Target für die Behandlung von Patienten mit Nierenzellkarzinom. / Renal cell carcinomas (RCC) belong to the most common malignant tumors worldwide. Alarming high incidence and mortality rates elucidate the urgent need for new therapeutic targets in RCCs. Point mutations in protein coding sequences lead to numerous unfolded proteins in cancer cells, requiring effective protein quality control. Therefore, components of the ubiquitin proteasome system (UPS) might be a promising new approach for RCC therapy. Recent publications in renal cancers point to a deregulated COP9 signalosome (CSN)-Cullin-RING Ubiquitin Ligase (CRL) pathway, a segment of the UPS. In the present thesis, selected components of the CSN-CRL pathway were studied in RCC tissues and four RCC cell lines. Immunohistochemistry results did not show an overexpression of a single CSN subunit in clear cell RCC tissues (ccRCC). However, it seems that the CSN holo complex is upregulated in analyzed ccRCCs. Examination of four RCC cell lines revealed a deregulation of the CAND1-Skp2-p27 axis in 786-O cells. These cells harbor two p27Kip1 (p27) mutants (p27V109G and p27I119T), high Skp2 and decreased CAND1 levels. Expression and localization of CAND1 was studied in a larger cohort of RCC tissues and revealed high cytosolic levels of CAND1 to be negatively correlated with overall survival in ccRCC patients. Both p27 variants were found to be degraded by the UPS and bound to the CSN, Skp2, Cdks and cyclin E. Interestingly, 786-O cells appear to grow 3-fold faster than A496 cells expressing p27wt. Further, a large cohort of RCC was screened for both p27 variants using a genotyping assay, specifically designed within the present thesis. 42.5% of the RCC patients harbor p27V109G heterozygously. The occurrence of p27V109G and p27I119T in RCC tissues as well as changed expression of Skp2 and CAND1 make the CSN-CRL pathway an attractive therapeutic target for the treatment of patients with RCC.

No description available.

Diffuse large B cell lymphoma (DLBCL) is the most common non-Hodgkin’s lymphoma. Patients who fail conventional therapy (~50%) have a poor prognosis and few treatment options. It is essential to understand the underlying biological processes, the progression of the disease, and utilize this information to develop new therapeutics. DLBCL patients with high C-MYC expression have a poor prognosis and new therapeutics for these patients are needed. This thesis describes work testing the hypothesis that JQ1, which can indirectly inhibit C-MYC in some tumors, can be used as an effective treatment for DLBCL. Some tumors have an unknown mechanism causing high C-MYC expression, leading me to investigate the underlying mechanisms. YY1 is a transcriptional regulator of c- Myc and has been implicated in DLBCL and as a potential regulator of the germinal center (GC) reaction. DLBCL arises from GC cells or post-GC cells. I tested the hypothesis that YY1 regulates the GC reaction. SMURF2 is an E3-ubiquitin ligase for YY1 and a tumor suppressor for DLBCL. I was interested in examining the mechanism underlying the suppression of DLBCL by SMURF2 leading to the hypothesis that SMURF2 regulates the GC. This thesis shows JQ1 leads to cell death and cellular senescence in human DLBCL cells. I conclude that BRD4 inhibition by JQ1 or derivatives could provide a new therapeutic avenue for DLBCL patients. I also show loss of YY1 perturbs the GC by decreasing the dark zone and increasing apoptosis. Finally I show modulation of SMURF2 does not affect the GC, suggesting SMURF2 utilizes a different mechanism to act as a tumor suppressor and may not modulate YY1 in the context of the GC.

Replication of HIV-1 requires the assembly and release of mature and infectious viral particles. In order to accomplish this goal, HIV-1 has evolved multiple methods to interact with the host cell. HIV-1 recruits the host cell ESCRT machinery to facilitate the release of nascent viral particles from the host cell membrane. Recruitment of these cellular factors is dependent on the presence of short motifs in Gag referred to as Late-domains. Deletion or mutation of these domains results in substantial decrease in the release of infectious virions. However, previously published work has indicated that over-expression of the E3 ubiquitin ligase, NEDD4.2s is able to robustly rescue release of otherwise budding-defective HIV-1 particles. This rescue is specific to the NEDD4.2s isoform as related E3 ubiquitin ligases display no ability to rescue particle release. In addition, rescue of particle release is dependent on the presence of the partial C2 domain and a catalytically active HECT domain of NEDD4.2s. Here I provide evidence supporting the hypothesis that a partial C2 domain of NEDD4.2s constitutes a Gag interacting module capable of targeting the HECT domains of other E3 ubiquitin ligases to HIV-1 Gag. Also, by generating chimeras between HECT domains shown to form poly-ubiquitin chains linked through either K48 or K63 of ubiquitin, I demonstrate that the ability of NEDD4.2s to catalyze the formation of K63-polyubiquitin chains is required for its stimulation of HIV-1 L-domain mutant particle release. In addition, I present findings from on-going research into the role of the HIV-1 accessory protein Nef during viral replication using the culture T-cell line, MOLT3. My current findings indicate that downregulation of CD4 from the host cell membrane does not solely account for the dramatic dependence of HIV-1 replication on Nef expression in this system. In addition, I present evidence indicating that Nef proteins from diverse HIV-1 Groups and strains are capable of enhancing HIV-1 replication in this system. Analysis of a range of mutations in Nef known to impact interaction with cellular proteins suggest that the observed replication enhancement requires Nef targeting to the host cell membrane and may also require the ability to interact with select Src-kinases. Lastly, we find that the ability of Nef to enhance replication in this system is separate from any increase in viral particle infectivity, in agreement with current literature.

Indiana University-Purdue University Indianapolis (IUPUI) / Murine double minute (Mdm2) is a highly modified and multi-faceted protein that is overexpressed in numerous human malignancies. It engages in many cellular activities and is essential for development since deletion of mdm2 is lethal in early stages of embryonic development. The most studied function of Mdm2 is as a negative regulator of the tumor suppressor protein p53. Mdm2 achieves this regulation by binding to p53 and inhibiting p53 transcriptional activity. Mdm2 also functions as an E3 ubiquitin ligase that signals p53 for destruction by the proteasome. Interestingly recent evidence has shown that Mdm2 can also function as an E3 neddylating enzyme that can conjugate the ubiquitin-like molecule, nedd8, to p53. This modification results in inhibition of p53 activity, while maintaining p53 protein levels. While the signaling events that regulate Mdm2 E3 ubiquitin ligase activity have been extensively studied, what activates the neddylating activity of Mdm2 has remained elusive. My investigations have centered on understanding whether tyrosine kinase signaling could activate the neddylating activity of Mdm2. I have shown that c-Src, a non-receptor protein tyrosine kinase that is involved in a variety of cellular processes, phosphorylates Mdm2 on tyrosines 281 and 302. This phosphorylation event increases the half-life and neddylating activity of Mdm2 resulting in a neddylation dependent reduction of p53 transcriptional activity. Mdm2 also has many p53-independent cellular functions that are beginning to be linked to its role as an oncogene. There is an emerging role for Mdm2 in tumor metastasis. Metastasis is a process involving tumor cells migrating from a primary site to a distal site and is a major cause of morbidity and mortality in cancer patients. To date, the involvement of Mdm2 in breast cancer metastasis has only been correlative, with no in vivo model to definitively define a role for Mdm2. Here I have shown in vivo that Mdm2 enhances breast to lung metastasis through the up regulation of multiple angiogenic factors, including HIF-1 alpha and VEGF. Taken together my data provide novel insights into important p53-dependent and independent functions of Mdm2 that represent potential new avenues for therapeutic intervention.

[ES] El crecimiento de las plantas se ve afectado por el estrés abiótico, sequía, salinidad o altas temperaturas. La transducción de señales de estrés abiótico es fundamental para generar una respuesta fisiológica adecuada, que implica la participación de diferentes hormonas vegetales, siendo el ácido abscísico (ABA) el regulador hormonal crítico en la regulación de la respuesta de la planta a situaciones de estrés por déficit hídrico. La vía de señalización de ABA y los componentes principales están bien caracterizados molecular y bioquímicamente. Los receptores de ABA "Pyrabactin Resistance 1"(PYR)/"PYR1-LIKE" (PYL)/ "Regulatory Component of ABA Receptor" (RCAR) juegan un papel importante en la regulación cuantitativa de la señalización ABA tanto en semillas como en tejidos vegetativos. Aunque la función bioquímica de los receptores PYR/PYL/RCARs de ABA, está bien caracterizada, se conoce poco sobre otros aspectos con relevancia biológica, como sus modificaciones postraduccionales o la regulación de su vida media. Uno de los avances recientes en este campo ha sido el descubrimiento de una nueva familia de E3 ligasas llamadas RSL1/RFAs ("RING-finger-ABA-related") que consta de al menos 10 miembros, reguladores clave de la estabilidad de los receptores PYR/PYL/RCAR de ABA en tejidos de raíces y hojas, regulando su degradación en diferentes ubicaciones celulares. Un estudio detallado de esta familia génica reveló que RSL1/RFA se caracterizan estructuralmente por la presencia de tres dominios RING putativos en tándem, denominados "RING1-IN BETWEEN RING-RING2" (RBR), y en consecuencia pertenecen a la familia de E3 ligasas de tipo RBR. Cinco miembros de la familia RSL1/RFA, RSL1 y RFA6-RFA9, contienen un dominio TM en el extremo C-terminal, lo que sugiere que RFA6-RFA9 también se localizan en la membrana plasmática. Sin embargo, otros miembros de las E3 ligasas como RFA1-RFA5 carecen del dominio TM C-terminal y su caracterización funcional, así como su ubicación celular, aún no se conocen. Nosotros mostramos que la E3 ligasa RFA1 se localiza en núcleo y citosol, mientras que RFA4 muestra una localización específica en el núcleo promoviendo la degradación nuclear de los receptores ABA. Por lo tanto, los miembros de la familia RSL1/RFA interactúan con los receptores ABA en la membrana plasmática, el citosol y el núcleo, dirigiéndolos a su degradación a través de la vía endosomal/vacuolar (en el caso de RSL1) o el proteosoma 26S (para RFA1 y RFA4). Proporcionamos información sobre la función fisiológica de estas E3 ligasas de tipo RBR. Realizando tanto mutagénesis como ensayos bioquímicos para identificar la cisteína 361 (Cys361) en RFA4 como la Cys del sitio activo, que es una característica distintiva de las E3 ligasas de tipo RBR. Demostramos mediante análisis de inmunotransferencia del mutante con pérdida de función de rfa1rfa4 que los niveles endógenos de los receptores de ABA PYR1 y PYL4 aumentan en comparación con las plantas de tipo silvestre. Hemos identificado una enzima E2, "Ubiquitin Conjugating Enzyme 26" (UBC26), como la enzima nuclear canónica E2 que interactúa con la E3 ligasa RFA4 y forma complejos UBC26-RFA4-Receptor, formando agregados nucleares. Generamos alelos ubc26 con pérdida de función que mostraban una mayor sensibilidad a ABA y acumulación de receptores ABA en comparación con el tipo silvestre. En definitiva, hemos revelado un sofisticado sistema de ubiquitinación de receptores ABA en diferentes ubicaciones subcelulares llevado a cabo a través de la familia de E3 ligasas RSL1/RFA de tipo RBR. Por otro lado, hemos iniciado pruebas bioquímicas para identificar la S-acilación en el dominio TM de RSL1. Generando RSL1C334S, RSL1 C5S y RSL1C6S mediante mutagénesis y RSL1ΔTM que presenta una delección del dominio TM. Los estudios iniciales han demostrado que los residuos de Cys cercanos al dominio TM están S-acilados. Finalmente, generamos nu / [CA] El creixement de les plantes es pot veure afectat per l'estrès abiòtic, sequera, salinitat o altes temperatures. La transducció de senyals d'estrès abiòtic és fonamental per a generar una resposta fisiològica adequada, que implica la participació de diferents hormones vegetals, sent l'àcid abscísic (ABA) el regulador hormonal crític en la regulació de la resposta de la planta a situacions d'estrès per dèficit hídric. La ruta de senyalització d'ABA i els components principals de la ruta estan ben caracteritzats molecularment i bioquímica. Els receptors "Pyrabactin Resistance 1"(PYR)/"PYR1-LIKE"(PYL)/"Regulatory Component of ABA Receptor" (RCAR) exerceixen un paper important en la regulació quantitativa en resposta a l'estrès tant en llavors com en planta. Encara que la funció bioquímica dels receptors PYR/PYL/RCARs d'ABA, està ben caracteritzada en els últims anys, es coneix poc sobre altres aspectes amb rellevància biològica, com les seues modificacions postraduccionals o la regulació de la seua vida mitjana. Un dels avanços recents en aquest camp ha sigut el descobriment d'una nova família d'E3 ligases anomenades RSL1/RFAs ("RING-finger-ABA-related") que consta d'almenys 10 membres, que són reguladors clau de l'estabilitat dels receptors PYR/PYL/RCAR d'ABA en teixits d'arrels i fulles, regulant la seua degradació en diferents ubicacions cel·lulars. Un estudi més detallat d'aquesta família gènica va revelar que RSL1/RFAs es caracteritzen estructuralment per la presència de tres dominis RING putatius en tàndem, denominats "RING1-IN BETWEEN RING-RING2" (RBR), i en conseqüència pertanyen a la família d'E3 ligases de tipus RBR. Cinc membres de la família RSL1/RFA, RSL1 i RFA6-RFA9, contenen un domini TM en l'extrem C-terminal, la qual cosa suggereix que RFA6-RFA9 també es localitzen en la membrana plasmàtica. No obstant això, altres membres d'aquesta família d'E3 ligases com RFA1-RFA5 manquen del domini TM C-terminal i la seua caracterització funcional, així com la seua ubicació cel·lular, encara no ha sigut investigada. Vam mostrar que l'E3 ligasa RFA1 es localitza tant en el nucli com en el citosol, mentre que RFA4 mostra una localització específica en el nucli promovent la degradació nuclear dels receptors ABA. Per tant, els membres de la família RSL1/RFA interactuen amb els receptors ABA en la membrana plasmàtica, el citosol i el nucli, dirigint-los a la seua degradació a través de la vía endosomal/vacuolar (en el cas de RSL1) o el proteosoma 26S (per a RFA1 i RFA4). Proporcionem informació sobre la funció fisiològica d'aquestes E3 ligases de tipus RBR. Realitzant tant mutagènesis com a assajos bioquímics per a identificar la cisteïna 361 (Cys361) en RFA4 com la Cys del lloc actiu, que és una característica distintiva de les E3 ligases de tipus RBR. Hem demostrat mitjançant una anàlisi d'immuno-transferència del mutant amb pèrdua de funció de rfa1rfa4 que els nivells endògens dels receptors d'ABA PYR1 i PYL4 augmenten en comparació amb les plantes de tipus silvestre. D'altra banda, hem identificat un enzim E2, "Ubiquitin Conjugating Enzyme 26" (UBC26), com l'enzim nuclear canònic E2 que interactua amb l'E3 ligasa RFA4 i forma complexos UBC26-RFA4-Receptor, formant agregats nuclears. També generem al·lels ubc26 amb pèrdua de funció que mostraven una major sensibilitat a ABA i acumulació de receptors ABA en comparació amb el tipus silvestre. En definitiva, hem revelat un sofisticat sistema d'ubiquitinació de receptors ABA en diferents ubicacions subcel·lulars dut a terme a través de la família d'E3 ligases RSL1/RFA de tipus RBR. Hem iniciat proves bioquímiques per a identificar la S-acilació en el domini TM de RSL1. Hem generat RSL1C334S, RSL1 C5S i RSL1C6S mitjançant mutagènesis, així com RSL1ΔTM que presenta una delecció del domini TM. Els estudis inicials han demostrat que els residus de Cys pròxims al domini TM estan S-acilados. Final / [EN] Plant growth is affected by abiotic stress, drought, salinity or high temperature. Signal transduction of abiotic stress is crucial to generate an appropriated physiological response, which involves the participation of different plant hormones, being abscisic acid (ABA) the critical hormonal regulator in regulating the plant's response to situations of stress due to water deficit. The ABA signaling pathway and the major components of the pathway are well characterized molecularly and biochemically. Pyrabactin Resistance 1 (PYR)/PYR1-LIKE (PYL)/Regulatory Component of ABA Receptor (RCAR) ABA receptors play an important role in quantitative regulation of ABA signaling both in seeds and vegetative tissues. Although the biochemical function of the PYR/PYL/RCAR ABA receptors has been well established in recent years, little is known about other aspects with biological relevance, such as their post-translational modifications or the regulation of their half-life. One of the recent advances in this field has been the discovery of a new family of E3 ligases called RSL1/RFAs (RING-finger-ABA-related) that consists of at least 10 members, which are key regulators of the stability of PYR/PYL/RCARs in root and leaf tissues, and regulate the degradation of ABA receptors at different cellular locations. Further inspection of the gene family revealed that RSL1/RFAs are structurally characterized by the presence of three putative RING domains in tandem, named as RING1-IN BETWEEN RING (IBR)-RING2, and accordingly they belong to the RBR-type E3 ligase family. Five members of the RSL1/RFA family, that is, RSL1 and RFA6-RFA9, contain a TM domain at the C-terminal end of the proteins, which suggests that RFA6-RFA9 are also localized in plasma membrane. However, other members of this family of E3 ligases such as RFA1-RFA5 lack the C-terminal TM domain and their functional characterization, as well as their cellular location, has not been investigated yet. In this study we show that the E3 ligase RFA1 is localized both in the nucleus and in the cytosol, while RFA4 shows a specific localization in the nucleus promoting the nuclear degradation of ABA receptors. Therefore, we members of the RSL1/RFA family interact with ABA receptors at the plasma membrane, cytosol and nucleus, targeting them for degradation via the endosomal/vacuolar pathway (in the case of RSL1) or the 26S- proteasome (for RFA1 and RFA4). We provide information on the physiological function of these RBR-type E3 ligases, which are hardly explored in plants. Additionally, we performed mutagenesis and biochemical assays to identify Cys361 in RFA4 as the active site cysteine, which is a distinctive feature of RBR-type E3 ligases. We have shown by immunoblot analysis of the rfa1rfa4 loss-of-function mutant that endogenous levels of ABA receptors PYR1 and PYL4 are increased compared to wild-type plants. On the other hand, we have identified an E2 enzyme, Ubiquitin Conjugating Enzyme 26 (UBC26), as the canonical nuclear enzyme E2 that interacts with the E3 ligase RFA4 and forms UBC26-RFA4-Receptor complexes, forming nuclear aggregates. We also generated loss-of function ubc26 alleles that exhibited higher sensitivity to ABA and accumulation of ABA receptors compared to wild type. We have revealed a sophisticated ubiquitination system of ABA receptors in different subcellular locations carried out through the RBR-type RSL1/RFA family of E3 ligases. We have proceeded with the biochemical and genetic study of the different members of the family. We have started biochemical tests to identify the S-acylation in the TM domain of RSL1. To this end, we have generated RSL1C334S, RSL1 C5S and RSL1C6S by mutagenesis as well as RSL1ΔTM, a deletion of the TM domain. Initial studies have shown that Cys residues close to the TM domain are S-acylated. Finally, we have also generated new combined mutants: rsl1rfa1, rsl1rfa5, rfa1rfa5 and rsl1rfa1rfa5. / Fernández López, MA. (2021). Regulación de la señalización del ABA mediante mecanismos que controlan vida media y actividad de los receptores PYR/PYL [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/172364

Plus de 70% des tumeurs mammaires expriment le récepteur des œstrogènes alpha (ERα), un facteur de transcription dépendant de ses ligands, les œstrogènes. Deux types d’anti-œstrogènes (AE) peuvent être utilisés en clinique pour traiter ces tumeurs : les SERM (Selective Estrogen Receptor Modulators) qui sont des agonistes partiels, tels que le tamoxifène, le SERM le plus communément utilisé en première ligne de traitement. Les SERD (Selective Estrogen Receptor Degraders) sont des AE purs et induisent la dégradation de ERα. Le fulvestrant a été le premier SERD utilisé en clinique, en seconde ligne de traitement après rechute ou dans un contexte métastatique. Cependant, il est très peu biodisponible oralement. ERα est ubiquitiné en présence de SERD, ce qui conduit à sa dégradation par la voie du protéasome. De plus, nous avons montré qu’ERα est également SUMOylé en présence de fulvestrant et d’autres AE purs. Cette SUMOylation contribue à supprimer ses propriétés transcriptionnelles. La SUMOylation et l’ubiquitination sont deux modifications post-traductionnelles similaires et consistent en la conjugaison d’une petite protéine (SUMO1/2/3 ou ubiquitine) sur des résidus d’une protéine cible lors d’une cascade de réactions impliquant trois enzymes : une enzyme activatrice E1, une enzyme de conjugaison E2 et une ligase E3. Ces processus peuvent être réversibles à l’aide de déSUMOylases (SENP) ou déubiquitinases (DUB). Dans un premier temps, nous avons étudié les déterminants structuraux de ERα nécessaires pour sa SUMOylation. Nos analyses Western et nos tests BRET (Bioluminescence Resonance Energy Transfer), dans lesquels nous avons mesuré le transfert d’énergie entre un donneur luminescent (ERα ou ERβ fusionnés à la luciférase) et un accepteur fluorescent (SUMO1 ou SUMO3 fusionnée à la YFP), ont montré que ERα est SUMOylé en présence de fulvestrant, mais pas son paralogue ERβ. Nous avons ensuite créé des chimères dans lesquelles nous avons échangé les domaines de ERα et de ERβ et avons étudié le profil de SUMOylation de ces chimères à l’aide des mêmes essais. Nous avons ainsi mis en évidence que les séquences spécifiques à ERα dans les hélices H3-H4 du domaine de liaison au ligand sont nécessaires et suffisantes pour sa SUMOylation en présence de fulvestrant. Dans cette région, les acides aminés spécifiques à ERα ne sont pas de potentiels substrats de modification. De plus, ces hélices font partie du sillon de recrutement de cofacteurs de ERα, ce qui suggère qu’il y a un recrutement différentiel de la machinerie de SUMOylation par ERα et ERβ en présence de fulvestrant. Nous avons montré que la surexpression de PIAS1 et PIAS2 peut augmenter le signal de SUMOylation. Nous avons ensuite étudié le lien entre la SUMOylation et l’ubiquitination de ERα induites par les AE. Nous avons adapté nos tests BRET pour mesurer l’ubiquitination de ERα, en réalisant des essais Ubi-BRET (Ubiquitine fusionnée à la YFP). Nous avons ainsi pu observer que la SUMOylation et l’ubiquitination de ERα sont affectées par les mêmes mutations en présence d’AE, suggérant un lien entre ces deux voies de modification de ERα induites par les AEs purs. Des essais en cinétique BRET ont montré que ces modifications se font en parallèle et que l’utilisation d’inhibiteurs de SUMOylation chimiques (ML-792 et TAK-981) ou protéiques (déSUMOylase SENP1/2) abrogent la SUMOylation de ERα. En revanche, l’ubiquitination de ERα par différents AE est partiellement diminuée lorsque ces inhibiteurs sont utilisés. La surexpression des protéines RNF4 et RNF111, qui sont des enzymes E3 ubiquitine ligases qui vont ubiquitiner spécifiquement les protéines SUMOylées (STUbL), augmente l’ubiquitination de ERα en présence de fulvestrant et d’autres SERD, de manière dépendante de la SUMOylation. Cependant, l’inhibition de la SUMOylation par les inhibiteurs ML-792 et TAK-781 n’a pas eu d’effet sur la dégradation de ERα par la voie du protéasome dans les cellules ER-positives MCF-7 et T47D. Ces résultats suggèrent que la SUMOylation et l’ubiquitination de ERα ont lieu en parallèle et nécessitent la même conformation de ERα induite par les AE. Enfin, ces deux modifications peuvent être reliées, grâce aux protéines STUbL de manière spécifique des cellules. En résumé, ces projets nous ont permis de mieux comprendre les mécanismes d’action des anti-œstrogènes purs utilisés dans le cancer du sein ER-positif. Nos découvertes pourront contribuer à anticiper des mécanismes possibles de résistance à ces molécules et permettre d’optimiser le développement de nouveaux anti-œstrogènes plus efficaces dans l’induction de la SUMOylation. / More than 70% of mammary tumors are positives for the expression of the estrogen receptor alpha (ERα), a ligand-dependent transcription factor, activated by estrogens. Two types of antiestrogens (AE) are used in the clinic to treat these tumors: the SERMs (Selective Estrogen Receptor Modulators), which are partial agonists, with the tamoxifen, the most common SERM used in first line of treatment. The SERDs (Selective Estrogen Receptor Degraders) are pure antiestrogens that induced ERα degradation. Fulvestrant has been the first SERD used in the clinic, in second line of treatment after relapse or in a metastatic setting. However, fulvestrant is poorly orally bioavailable. ERα is ubiquitinated in the presence of SERDs and this induce its degradation via the proteasome pathway. In our lab, we have showed that ERα is also SUMOylated in the presence of fulvestrant and other pure antiestrogens. This SUMOylation suppresses its transcriptional properties. SUMOylation and ubiquitination are two similar post-translational modifications that consist in the conjugation of a small protein (SUMO1/2/3 or ubiquitin) on residues of a target protein during an enzymatic cascade implicating three enzymes: an E1 activating, an E2 conjugating and an E3 ligase. These processes are reversible thanks to deSUMOylases (SENPs) and deubiquitinases (DUBs). First, we have studied the structural determinants of ERα required for its SUMOylation. Our Western and BRET (Bioluminescence Resonance Energy Transfer) analyses, where we measured the energy transfer between a luminescent donor (ERα or ERβ fused to the luciferase) to a fluorescent acceptor (SUMO1 or SUMO3 fused to the YFP), have showed that ERα is SUMOylated in the presence of fulvestrant but not its paralog ERβ. We have created chimeras by exchanging domains between ERα and ERβ et we have studied the SUMOylation profile of these chimeras with these assays. We showed that the specific sequence to ERα in the H3-H4 helices of the ligand binding domain are necessary and sufficient to induce its SUMOylation in the presence of fulvestrant. In this region, the amino acids that are specifics to ERα are not potential substrates of modification. Moreover, these helices are part of the cofactor binding groove of ERα, suggesting that there is a differential recruitment of the SUMOylation machinery by ERα compared to ERβ in the presence of fulvestrant. We have also showed that the overexpression of the E3 SUMO ligases PIAS1 and PIAS2 can increase the SUMOylation signal. We then studied the parallel between SUMOylation and ubiquitination of ERα induced by AE. We adapted our BRET assays to measure the ubiquitination of ERα, by performing Ubi-BRET assays (ubiquitin fused to the YFP). We observed that SUMOylation and ubiquitination of ERα are affected by the same mutations in the presence of AE, suggesting that there is a crosstalk between these two modification pathways of ERα induced by AE. BRET kinetic assays showed that these modifications happened in parallel and that the use of SUMOylation inhibitors (chemicals ML-792 and TAK-981; or deSUMOylases SENP1/2) abrogated the SUMOylation of ERα. However, the ubiquitination of ERα by different AE is partially decreased when these inhibitors are used. The overexpression of RNF4 and RNF111, which are SUMO-targeted ubiquitin ligases (STUbLs), that will specifically ubiquitinate SUMOylated target proteins, increased the ubiquitination of ERα in the presence of fulvestrant and other SERDs, in a SUMO-dependent manner. However, the inhibition of SUMOylation by ML-792 and TAK-981 inhibitors did not impact the degradation of ERα induced by the proteasome in ER-positive breast cancer cells MCF7 and T47D. These results suggest that SUMOylation and ubiquitination of ERα happened in parallel and required the same conformation of ERα induced by AE. Finally, these two modifications can be linked thanks to STUbLs in a cell-specific manner. These projects led us to better understand the mechanisms of action of pure AE used in the treatment of ER-positive breast cancer. Our findings will contribute to anticipate possible mechanisms of resistance to AE et will help for the optimization and the development of new AE, more efficient in inducing ERα SUMOylation.