Regulation of subcellular distribution and oncogenic potential of nucleophosmin by plakoglobin

Abstract

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.

Plakoglobin interacts with the transcription factor p53 and regulates the expression of 14-3-3σ

“…4A). Also, plakoglobin may play a role in regulating the subcellular distribution of p53 as was recently demonstrated for NPM (Lam et al, 2012), since there was considerably more p53 in the nuclear fractions of SCC9-PG cells compared with SCC9 cells (compare Fig. 3,IB: p53).…”

“…In contrast, plakoglobin has often been associated with tumor/metastasis suppressor activity, although the mechanisms underlying this activity remain unclear. A number of studies have suggested that plakoglobin decreases cell proliferation, migration and invasion and induces apoptosis (Simcha et al, 1996;Parker et al, 1998;Pantel et al, 1998;Charpentier et al, 2000;Winn et al, 2002;Rieger-Christ et al, 2005;Yin et al, 2005;Dusek et al, 2007;Kanazawa et al, 2008;Närkiö-Mäkelä et al, 2009;Todorović et al, 2010;Aktary et al, 2010;Aktary and Pasdar, 2012;Bailey et al, 2012;Holen et al, 2012;Franzen et al, 2012;Lam et al, 2012).…”

Desmosome regulation and signaling in disease

“…PG has also been reported to be downregulated in esophageal cancer in which it is associated with a decrease in the Triton-X 100 insoluble pool of E-cadherin and desmocollin-2 and decreased E-cadherin at the cell membrane, suggesting decreased cell-cell adhesion (Fang et al 2014). The nucleolar phosphoprotein nucleophosmin interacts directly with PG in MDA-MB-231 carcinoma cells and PG expression inhibits nucleophosmin-mediated cell growth and invasion, potentially through the regulation of its subcellular localization (Lam et al 2012). …”

Nuclear Signaling from Cadherin Adhesion Complexes

“…In addition to TCF/LEF, plakoglobin is reported to associate with p53 and coregulate some p53 target genes (Aktary, Kulak, Mackey, Jahroudi, & Pasdar, 2013), as well as to associate with other proteins having nuclear activity (Aktary & Pasdar, 2013; Lam et al, 2012). Certain mutations in human desmosomal proteins, including desmosomal cadherins (e.g., desmoglein2 and desmocollin2), reduce plakoglobin’s junctional association, enhance its nuclear localization, and thereby contribute to the heart pathology arrhythmogenic right ventricular cardiomyopathy (ARVC) (Garcia-Gras et al, 2006).…”