Modulation of the oncogenic potential of β‐catenin by the subcellular distribution of plakoglobin

Abstract

Plakoglobin (Pg) and beta-catenin are homologous proteins that function in cell-cell adhesion and signaling. The cadherin-associated form of these proteins mediates adhesion, whereas the cytosolic/nuclear form has a signaling role. Despite their interactions with common cellular partners, beta-catenin has a well-documented oncogenic potential while Pg has a less characterized tumor suppressor activity. We showed previously that Pg overexpression in Pg-deficient SCC9 cells (SCC9-Pg-WT) induced Bcl-2 expression and inhibited apoptosis. To assess the exact role of Pg in Bcl-2 expression, we generated and characterized SCC9 transfectants expressing Pg with a restricted cytoplasmic (Pg-NES) or nuclear (Pg-NLS) distribution. We show that Bcl-2 was expressed regardless of Pg localization, although its level was substantially lower in SCC9-Pg-NLS cells. Bcl-2 expression coincided with increased nuclear beta-catenin levels (Pg-NES) or a decrease in the level of total and nuclear beta-catenin associated with N-cadherin and alpha-catenin (Pg-WT and -NLS) cells. Bcl-2 expression also was induced in SCC9 cells overexpressing beta-catenin. In contrast, SCC9 cells expressing mutant Pg proteins, unable to interact with N-cadherin and alpha-catenin, had noticeably lower Bcl-2 levels. Our data suggest that Bcl-2 expression is induced by beta-catenin and modulated by Pg. We show that the inhibition of beta-catenin-dependent TCF transactivation had no effect on Bcl-2 levels, suggesting that induction of Bcl-2 expression by beta-catenin and its modulation by Pg may involve factors other than, or in addition, to, TCF. These results provide a possible mechanism for the tumor suppressor activity of Pg via its role as a regulator of the oncogenic potential beta-catenin.

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

“…The colon carcinoma cell line SW620 was used because it expresses p53 and transcriptionally active b-catenin (Lamy et al, 2010;El-Bahrawy et al, 2004;Li et al, 2007). In agreement with the coimmunoprecipitation data, we did not observe an association between the 14-3-3s gene promoter and b-catenin in any cell line (Fig.…”

“…We chose to focus on 14-3-3s because (a) its mRNA levels were increased over 30-fold in SCC9-PG cells, one of the most notable increases in any of the identified p53-target genes, (b) it is a welldocumented tumor and metastasis suppressor (Lodygin and Hermeking, 2006;Lee and Lozano, 2006;Yi et al, 2009), (c) members of the 14-3-3 family are known to interact with a wide range of cellular partners and regulate several biological processes (Obsilová et al, 2008;Morrison, 2009; van Heusden, 2009), (d) 14-3-3s itself has been shown to interact with plakophilin, a component of the desmosomal plaque, which also contains plakoglobin (Benzinger et al, 2005) and (e) more recently it has been shown that various 14-3-3 proteins can regulate the Wnt pathway and b-catenin signaling , functionally linking these proteins to catenin proteins. Furthermore, we and others have shown that plakoglobin also regulates b-catenin subcellular localization and in turn its transcriptional activity (Salomon et al, 1997;Klymkowsky et al, 1999;Zhurinsky et al, 2000a;Li et al, 2007), thereby suggesting that both plakoglobin and 14-3-3s act to regulate the Wnt signaling pathway in similar, albeit not identical ways.…”

Plakoglobin interacts with and increases the protein levels of metastasis suppressor Nm23-H2 and regulates the expression of Nm23-H1

“…plakoglobin can modulate the amount of b-catenin and its signaling function in a cell context-dependent manner (Li et al, 2007).…”

“…We then asked whether a-catenin could be a bridge between Nm23 and plakoglobin-Ncadherin complexes. To clarify this point, we assessed the interactions between Nm23, plakoglobin and Ncadherin in SCC9 cells expressing Flag-tagged wild-type plakoglobin or a mutant plakoglobin with a deletion in the N-terminal a-catenin binding domain (Pg-DN123; Li et al, 2007;Kolligs et al, 2000). Soluble and insoluble cell extracts from Pg-Flag and Pg-DN123 transfectants were processed for sequential immunoprecipitation and immunoblotting with Flag and Nm23 antibodies, respectively.…”

“…The human tongue squamous cell carcinoma cell line SCC9, plakoglobin-expressing SCC9, MDCK, MCF-7, MCF-10-2A and PC3 cells have been described previously (Pasdar and Nelson, 1988a;Torlakovic et al, 2002;Li et al, 2007;Lam et al, 2009). The colon carcinoma cell line SW620 was obtained from the American Type Culture Collection (ATCC, Manassas, VA, USA), and was maintained in Leibovitz's L-15 medium supplemented with 2 mM L-glutamine, 10% FBS and 1% antibiotics.…”