Molecular Characterization of the Tumor and Metastasis Suppressor Activity of Plakoglobin in Mutant p53 Expressing Carcinoma Cells

  • Author / Creator
    Alaee, Mahsa
  • Plakoglobin (γ-catenin) is an Armadillo protein family member and a paralog of β-catenin with similar dual cell-cell adhesion and signaling activities. These proteins interact with cadherins at the membrane and mediate cell-cell adhesion. In the cytoplasm, they interact with an array of cellular protein partners to regulate signaling pathways involved in tumorigenesis and metastasis. Recently, our laboratory identified p53 as one of the interacting partners of plakoglobin. p53 is a tumor suppressor and transcription factor that in response to various stress signals activates physiological pathways that regulate cell cycle arrest, DNA repair and apoptosis. More than half of all cancers harbor a mutant form of p53. In addition to the loss of tumor suppressor activity, a number of most frequent mutant p53 proteins acquire oncogenic properties and are known as gain of function mutants. Here, we first assessed the in vitro tumor and metastasis suppressive functions of plakoglobin in high-grade ovarian serous carcinoma cell lines expressing wild type or mutant p53 proteins with different adhesion profiles. We showed that plakoglobin-deficient ovarian cancer cells that express N-cadherin and mutant p53 were highly migratory and invasive, whereas those that express mutant p53 and plakoglobin were not. Exogenously expressed plakoglobin colocalized with cadherins in adhesion complexes, interacted with wild type and mutant p53 proteins and significantly reduced growth, migration and invasion of ovarian cancer cells expressing N-cadherin and mutant p53 in vitro. Next, we mapped the interacting domain of p53 and plakoglobin and showed that p53/plakoglobin interaction was mediated by the DNA binding domain of p53 and the C-terminal transactivation domain of plakoglobin. We showed that wild type plakoglobin and wild type p53 acted synergistically to significantly reduce in vitro growth, migration and invasion of transfectants relative to parental cells. Additionally, the C-terminal of plakoglobin was necessary for its invasion suppressor activity. We examined the effects of one of the most frequently expressed p53 mutations p53R175H (Arginine 175 to Histidine) on β-catenin accumulation and transcriptional activation and their modifications by plakoglobin co-expression. p53R175H expression in plakoglobin null cells increased total and nuclear levels of β-catenin and its transcriptional activity. Co-expression of plakoglobin in these cells promoted β-catenin’s proteasomal degradation, and decreased its nuclear levels and transactivation. Wnt/β-catenin targets, c-MYC and S100A4 were upregulated in p53R175H cells and were downregulated when plakoglobin was co-expressed. The plakoglobin-p53R175H cells also showed significant reduction in their migration and invasion in vitro. Taken together, the experimental evidence from this PhD project strongly suggest that underlying mechanisms for tumor and metastasis suppressor effects of plakoglobin may be its interaction with mutant p53 proteins and down-modulation of β-catenin-TCF axis.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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