[Show abstract][Hide abstract] ABSTRACT: Neurofibromatosis type 2 (NF2) is an inherited predisposition cancer syndrome characterized by the development of multiple benign tumors in the nervous system including schwannomas, meningiomas, and ependymomas. Using a disease model comprising primary human schwannoma cells, we previously demonstrated that adherens junctions (AJs) are impaired in schwannoma cells because of a ubiquitous, upregulated Rac activity. However, the mechanism by which loss of contact inhibition leads to proliferation remains obscure in merlin-deficient tumors. In this study, we show that proliferative Wnt/β-catenin signaling is elevated as active β-catenin (dephosphorylated at serine 37 and threoine 41) localizes to the nucleus and the Wnt targets genes c-myc and cyclin D1 are upregulated in confluent human schwannoma cells. We demonstrate that Rac effector p21-activated kinase 2 (PAK2) is essential for the activation of Wnt/β-catenin signaling because depletion of PAK2 suppressed active β-catenin, c-myc, and cyclin D1. Most importantly, the link between the loss of the AJ complex and the increased proliferation in human schwannoma cells is connected by Src and platelet-derived growth factor receptor-induced tyrosine 654 phosphorylation on β-catenin and associated with degradation of N-cadherin. We also demonstrate that active merlin maintains β-catenin and N-cadherin complex at the plasma membrane through direct regulation. Finally, we demonstrate that phosphorylation of tyrosine 654 is critical for the increased proliferation in human schwannoma cells because overexpression of a Y654F mutant β-catenin reduces hyperproliferation of schwannoma cells. We suggest a model that these pathways are coordinated and relevant for proliferation in merlin-deficient tumors.
Neoplasia (New York, N.Y.) 12/2011; 13(12):1101-12. · 5.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Merlin is a tumour suppressor involved in the development of a variety of tumours including mesotheliomas. Neurofibromatosis type 2 (NF2), a dominantly inherited tumour disease, is also caused by loss of merlin. NF2 patients suffer from multiple genetically well-defined tumours, schwannomas are most frequent among those. Using our in vitro model for human schwannoma, we found that schwannoma cells display enhanced proliferation because of the overexpression/activation of platelet-derived growth factor receptor and ErbB2/3, increased cell-matrix adhesion because of the overexpression of integrins, and decreased apoptosis. Mechanisms underlying schwannomas basal proliferation and cell-matrix adhesion are not understood. Here, we investigated insulin-like growth factor-binding protein-1 (IGFBP-1), which is expressed and released from central nervous system tumours and strongly overexpressed in schwannoma at the mRNA level. IGFBP-1 acts via β1-integrin and focal-adhesion-kinase (FAK), which are strongly overexpressed and basally activated in schwannoma. Using short hairpin RNA knockdown, small inhibitors and recombinant IGFBP-1, we demonstrate that schwannoma cells, in contrast to Schwann cells, release IGFBP-1 that activates the Src/FAK pathway, via integrin β1, potentiating schwannoma's proliferation and cell-matrix adhesion. We show that FAK localizes to the nucleus and Src triggers IGFBP-1 production. Further, we observed downregulation of the tumour-suppressor phosphatase and tensin homolog in schwannoma cells leading to increased activity of anti-apoptotic AKT. Thus, IGFBP-1/integrin β1/Src/FAK pathway has a crucial role in merlin-related tumourigenesis and therefore represents an important therapeutic target in the treatment of merlin-deficient tumours.