Merlin-Deficient Human Tumors Show Loss of Contact Inhibition and Activation of Wnt/β-Catenin Signaling Linked to the PDGFR/Src and Rac/PAK Pathways

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.

p21-Activated kinase interacts with Wnt signaling to regulate tissue polarity and gene expression

“…Positive regulation of canonical Wnt signaling by Pak2 has been reported also (31). Although contrary to our findings, this result perhaps is not surprising, given that Rac1 acts positively in canonical Wnt signaling (32) and functions in both canonical and noncanonical Wnt signaling (Fig.…”

Tiam1/Rac1 signals contribute to the proliferation and chemoresistance, but not motility, of chronic lymphocytic leukemia cells

“…Our suggestion of Tiam1 as a critical component of tumorigenesis is consistent with recent reports that attribute the connection of Tiam1/Rac and c-Myc to the Wnt pathway. [52][53][54] For example, in colorectal cancer cells, a complex of Tiam1, Rac1, and the key Wnt-mediator b-catenin binds to the Wnt-responsive promoters of c-Myc and cyclin D1, thereby enhancing their transcription. 53 Furthermore, Rac1 is involved in regulating the phosphorylation of the signal transducer and activator of transcription 3, which results in the transcriptional induction of c-Myc.…”

Merlin inhibits Wnt/β-catenin signaling by blocking LRP6 phosphorylation

“…20 The other study showed that activated Src phosphorylates Tyr654 in β-catenin, which promotes dissociation of the membrane portion of β-catenin, resulting in accumulation of active β-catenin in nuclei of Merlin-deficient human schwannoma cells. 21 In this report, we provide a novel mechanism for the inhibition of Wnt/β-catenin signaling by Merlin. Merlin interacts with and inhibits phosphorylation of LRP6 by blocking the interaction between Axin and LRP6.…”