Plakoglobin (γ-catenin) is a member of the Armadillo family of proteins and a homolog of β-catenin. As a component of both the adherens junctions and desmosomes, plakoglobin plays a pivotal role in the regulation of cell-cell adhesion. Furthermore, similar to β-catenin, plakoglobin is capable of participating in cell signaling. However, unlike β-catenin that has well-documented oncogenic potential through its involvement in the Wnt signaling pathway, plakoglobin generally acts as a tumor/metastasis suppressor. The exact roles that plakoglobin plays during tumorigenesis and metastasis are not clear; however, recent evidence suggests that it may regulate gene expression, cell proliferation, apoptosis, invasion, and migration. In this paper, we describe plakoglobin, its discovery and characterization, its role in regulating cell-cell adhesion, and its signaling capabilities in regulation of tumorigenesis and metastasis.
Genetically engineered mouse models offer essential opportunities to investigate the mechanisms of initiation and progression in melanoma. Here, we report a new simplified histopathology classification of mouse melanocytic lesions in Tyr::NRASQ61K derived models, using an interactive decision tree that produces homogeneous categories. Reproducibility for this classification system was evaluated on a panel of representative cases of murine melanocytic lesions by pathologists and basic scientists. Reproducibility, measured as inter-rater agreement between evaluators using a modified Fleiss' kappa statistic, revealed a very good agreement between observers. Should this new simplified classification be adopted, it would create a robust system of communication between researchers in the field of mouse melanoma models.
Plakoglobin (γ-catenin) is a member of the Armadillo family of proteins and a homolog of β-catenin with similar dual adhesive and signaling functions. The adhesive function of these proteins is mediated by their interactions with cadherins and their signaling function by association with various intracellular proteins, from signaling molecules to transcription factors. However, while β-catenin has well-documented oncogenic potential, plakoglobin signaling capabilities are typically associated with tumor/metastasis suppression through mechanisms that have remained unclear. The focus of this thesis was to elucidate the molecular mechanisms by which plakoglobin regulates tumorigenesis and metastasis. To this end, we expressed plakoglobin in plakoglobin-null human carcinoma cells and compared the mRNA and protein profiles of plakoglobin expressing cells with those of parental cells. We identified a number of oncogenes and tumor/metastasis suppressors whose mRNA/protein levels were decreased and increased, respectively, upon plakoglobin expression. Extensive characterization of the plakoglobin expressing cells showed that plakoglobin regulates tumorigenesis and metastasis by interacting with and altering the levels, localization and/or function of growth/metastasis regulating proteins and/or by associating with transcription factors that regulate the expression of genes involved in these processes. Plakoglobin interacted with and increased both the protein and mRNA levels of the metastasis suppressor Nm23-H1 while only increasing the protein levels of Nm23-H2. Furthermore, in plakoglobin expressing cells, Nm23-H1/H2 complex was redistributed from the cytoplasm to the adherens junction at the membrane.
We also showed that plakoglobin interacted with p53 and together they regulated the expression of a number of p53-target genes, including tumor suppressors SFN and NME1 and the tumor promoter SATB1. Concurrent with these changes, there was a significant decrease in cell proliferation and in vitro migration and invasion of plakoglobin expressing cells.
These results clearly demonstrate that plakoglobin plays an active role in suppressing tumorigenesis/metastasis through both the regulation of gene expression and by interacting with and altering the levels, localization and function of various intracellular proteins involved in these processes. The larger implication of this work is that plakoglobin may be a useful marker for diagnosis and prognosis as well as a therapeutic target for the treatment of various cancers.
Summary Although tremendous progress has been made in understanding the mechanisms leading to cancer, those governing metastases are still poorly understood. E-cadherin (Ecad) is a cell-cell adhesion molecule essential for tissue homeostasis, and its loss often correlates with the dissemination of human cancers. However, whether and how the loss of Ecad triggers the full metastatic program is largely unknown. Here, we show that the loss of Ecad promotes melanoma lung metastases in females. The loss of Ecad, after the induction of estrogen receptor α (ERα) expression, activates gastrin-releasing peptide receptor (GRPR) expression. GRPR promotes cellular processes essential for metastasis formation through G□ q and YAP1 signaling and its pharmacological inhibition reduces metastasis in vivo . This study reveals an Ecad-ERα-GRPR metastatic sex dimorphism axis in melanoma that is conserved in human breast cancer and provides proof of concept that the G-coupled receptor GRPR is a therapeutic target for metastasis.
Nucleophosmin (NPM) is a nucleolar phosphoprotein that is involved in many cellular processes and has both oncogenic and growth suppressing activities. NPM is localized primarily in nucleoli but shuttles between the nucleus and the cytoplasm, and sustained cytoplasmic distribution contributes to its tumor promoting activities. Plakoglobin (PG, γ-catenin) is a homolog of β-catenin with dual adhesive and signaling functions. These proteins interact with cadherins and mediate adhesion, while their signaling activities are regulated by association with various intracellular partners. Despite these similarities, β-catenin has a well-defined oncogenic activity, whereas PG acts as a tumor/metastasis suppressor through unknown mechanisms. Comparison of the proteomic profiles of carcinoma cell lines with low- or no PG expression with their PG-expressing transfectants has identified NPM as being upregulated upon PG expression. Here, we examined NPM subcellular distribution and in vitro tumorigenesis/metastasis in the highly invasive and very low PG expressing MDA-MB-231 (MDA-231) breast cancer cells and their transfectants expressing increased PG (MDA-231-PG) or NPM shRNA (MDA-231-NPM-KD) or both (MDA-231-NPM-KD+PG). Increased PG expression increased the levels of nucleolar NPM and coimmunoprecipitation studies showed that NPM interacts with PG. PG expression or NPM knockdown decreased the growth rate of MDA-231 cells substantially and this reduction was decreased further in MDA-231-NPM-KD+PG cells. In in vitro tumorigenesis/metastasis assays, MDA-231-PG cells showed substantially lower and MDA-231-NPM-KD cells substantially higher invasiveness relative to the MDA-231 parental cells, and the co-expression of PG and NPM shRNA led to even further reduction of the invasiveness of MDA-231-PG cells. Furthermore, examination of the levels and localization of PG and NPM in primary biopsies of metastatic infiltrating ductal carcinomas revealed coordinated expression of PG and NPM. Together, the data suggest that PG may regulate NPM subcellular distribution, which may potentially change the function of the NPM protein from oncogenic to tumor suppression.
