Browsing by Subject "Tumor Suppressor Proteins"
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Item Illuminating the P53 Regulatory Network in Genetic Models(2011-02-01T19:36:06Z) Lu, Wan-Jin; Abrams, JohnThe tumor suppressor gene p53 is mutated in more than 50% of human cancers, and functions as a central component of stress response machinery that mediates a wide variety of downstream responses. Interestingly, the evolutionary appearance of p53 preceded its role in tumor suppression, suggesting that there may be unappreciated functions for this protein. In order to examine physiologic functions of p53 in vivo, a green fluorescent protein (GFP) reporter was designed to follow the activation of this regulatory network in a genetic model, Drosophila melanogaster. By following the reporter during Drosophila development, physiological activation of the p53 regulatory network in the female germ line was discovered. It is provoked by the first enzymatic step for meiotic recombination and conserved in both flies and mice. The functional relevance of the p53 activities in the germ line was shown by the meiotic recombination frequency and genetic interactions with a meiotic effector gene, Rad54. Additionally, genotoxic stress selectively activates p53 in germ line stem cells and promotes regeneration of fertility after IR. Activation of p53 was also found in uncontrolled growth of germ cells by blocked differentiation, and surprisingly by overexpression of oncogenic protein in the germ line. Together, my thesis work indicate that the need for controlling growth by the p53 regulatory network is an evolutionary conserved feature, which may serve as a selective pressure to preserve this network. Future studies on the mechanisms of p53 actvities during meiosis and in response to oncogene activation could provide novel insights on its cancer-related functions.Item The RASSF1A Tumor Suppressor Regulates a Cascade of Oncogenic Signals That Are Restrained by G1(2012-07-17) Ram, Rosalyn Ruanga; White, Michael A.The RASSF1A tumor suppressor is one of the most commonly inactivated genes in cancer. To understand why epigenetic silencing of RASSF1A promotes tumorigenesis, I employed a loss of function approach to elucidate the role of RASSF1A in cancer. RASSF1A is reported to regulate apoptosis, cell cycle progression, and microtubule dynamics. Disruption of these processes by RASSF1A loss may disrupt cellular integrity and promote oncogenesis. I found that RASSF1A depletion elevated oncogenic signaling pathways; however, RASSF1A depletion also induced cell cycle arrest. RASSF1A is a critical regulator in maintaining the balance between pro-growth and anti-growth signals. RASSF1A suppresses proliferative signaling pathways such as the MAPK pathway, promotes apoptosis through MST2, but paradoxically, promotes G1/S progression through modulation of the ubiquitin ligase SCF-BTrCP. Thus, RASSF1A represents a critical line of defense against tumorigenesis as its loss triggers cell arrest; however, loss of RASSF1A also promotes proliferative signaling events, and additional malfunctions in cell cycle regulation will likely drive tumorigenesis. [Keywords: RassflA, mir21, SKp2, REST, SCFBTRCP]Item Studies of the Hippo Signaling Pathway(2012-08-13) Yue, Tao; Jiang, JinHow multicellular organisms control their growth to reach proper organ size during development is a fascinating question. Recent studies, initially from Drosophila, have identified the Hpo tumor suppressor pathway as a crucial mechanism that controls tissue growth by inhibiting cell growth, proliferation and survival. Deregulation of the Hpo pathway has been implicated in various human cancers. Central to the Hpo pathway is a kinase cassette consisting of four tumor suppressor proteins, the Ste20-like kinase Hpo, the WW domain-containing protein Salvador (Sav), the NDR family kinase Warts (Wts) and the Mob family protein Mats. The kinase activities of Hpo and Wts are facilitated by their regulatory proteins Sav and Mats, respectively. Activated Hpo/Sav complex phosphorylates and activates the Wts/Mats complex, which in turn phosphorylates and inactivates the transcriptional coactivator Yorkie (Yki). Phosphorylation of Yki restricts its nuclear localization through recruiting 14-3-3. When the activity of the Hpo/Wts kinase cassette is compromised, Yki forms complexes with transcription factors including Scalloped (Sd) and translocates to the nucleus to activate Hpo pathway target genes, including cyclin E, diap1, and the microRNA bantam that regulate cell growth, proliferation and survival. To identify novel components of the Hpo signaling pathway, I carried out a genetic modifier screen in which flies carrying GMR-Gal4 and UAS-Yki were crossed to a collection of transgenic RNAi lines from Vienna Drosophila RNAi center (VDRC) and Bloomington stock center, and looked for enhancers or suppressors of the overgrown eye phenotype caused by Yki overexpression. Through this screen, I have found that Echinoid (Ed), an immunoglobulin domain-containing cell adhesion molecule, acts as an upstream regulator of the Hpo pathway. Loss of Ed compromises Yki phosphorylation, resulting in elevated Yki activity that drives Hpo target gene expression and tissue overgrowth. Ed physically interacts with and stabilizes the Hpo-binding partner Sav at adherens junctions. Ed/Sav interaction is promoted by cell-cell contact and requires dimerization of Ed cytoplasmic domain. Overexpression of Sav or dimerized Ed cytoplasmic domain suppressed loss-of-Ed phenotypes. I propose that Ed may link cell-cell contact to Hpo signaling through binding and stabilizing Sav, thus modulating the Hpo kinase activity. Furthermore, the Cul4/WDR40A complex has also been identified as a genetic modifier for the Hippo signaling pathway. However, the exact mechanism by which this complex regulates the Hippo signaling pathway need to be further addressed. [Keywords: hippo pathway, echinoid, Salvado, cell adhesion, cell contact]