Re-expression of the tumor suppressor NF2/merlin inhibits invasiveness in mesothelioma cells and negatively regulates FAK

Abstract

The neurofibromatosis type 2 NF2 gene product, merlin, is a tumor suppressor frequently inactivated in malignant mesothelioma (MM). To investigate a possible correlation between merlin inactivation and MM invasiveness, we restored merlin expression in NF2-deficient MM cells. Re-expression of merlin markedly inhibited cell motility, spreading and invasiveness, properties connected with the malignant phenotype of MM cells. To test directly whether merlin inactivation promotes invasion in a nonmalignant system, we used small interfering RNA to silence Nf2 in mouse embryonic fibroblasts (MEFs) and found that downregulation of merlin resulted in enhanced cell spreading and invasion. To delineate signaling events connected with this phenotype, we investigated the effect of merlin expression on focal adhesion kinase (FAK), a key component of cellular pathways affecting migration and invasion. Expression of merlin attenuated FAK phosphorylation at the critical phosphorylation site Tyr397 and disrupted the interaction of FAK with its binding partners Src and p85, the regulatory subunit of phosphatidylinositol-3-kinase. In addition, NF2-null MM cells stably overexpressing FAK showed increased invasiveness, which decreased significantly when merlin expression was restored. Collectively, these findings suggest that merlin inactivation is a critical step in MM pathogenesis and is related, at least in part, with upregulation of FAK activity.

HEI10 negatively regulates cell invasion by inhibiting cyclin B/Cdk1 and other promotility proteins

“…In contrast, the phosphorylation of FAK on Y 397 , associated with kinase activation (Ruest et al, 2000) and increased cellular migration, was consistently elevated by 4-to 6-fold following HEI10 depletion ( Figure 4b). FAK is not only regulated by Merlin (Poulikakos et al, 2006), but also by association with focal-adhesion localized signaling proteins such as p130Cas (Brabek et al, 2005) and paxillin (Wade and Vande Pol, 2006). Total levels of paxillin and p130Cas were consistently elevated by 5-to 6-fold in HEI10-versus control-depleted cells (Figure 4c and d).…”

Tumor Suppressor Schwannomin/Merlin Is Critical for the Organization of Schwann Cell Contacts in Peripheral Nerves

“…A partial deficit in defasciculation of axon bundles by Schwann cells was observed in schwannomin mutants. Interestingly, it has been shown recently that focal adhesion kinase is involved in defasciculation (Grove et al, 2007), suggesting a possible link with schwannomin that is associated with focal adhesion kinase (FAK) and ␤1-integrins (Taylor et al, 2003) and can alter FAK activity (Poulikakos et al, 2006). Several other features suggested that Schwann cell membrane interactions were altered in schwannomin mutant mice.…”

Merlin: a tumour suppressor with functions at the cell cortex and in the nucleus

“…On this basis, it has been argued that Merlin mediates both contact inhibition and tumour suppression by directly modulating mitogenic signal transduction at or near the plasma membrane [21,22]. Analysis of various cell types indicated that Merlin can potentially affect a variety of mitogenic pathways, such as Rac-PAK signalling [7,[23][24][25], activation of mTORC1 independently of Akt [26,27], the EGFR-Ras-ERK path- [26,27], the EGFR-Ras-ERK path-,27], the EGFR-Ras-ERK pathway, the PI3K-Akt pathway and FAK-Src signalling [28][29][30][31]. In addition, genetic studies in Drosophila and mice showed that Merlin contributes to the activation of the Hippo tumoursuppressor pathway [18,32,33].…”