The cadherins mediate cell-to-cell adhesion through homophilic binding and their engagement results in growth arrest. Cancer cells are refractory to this contact inhibition of growth. We are examining the signaling mechanisms underlying contact inhibition.
Tomoyo Okada has observed that activation of PAK is sufficient to release normal endothelial cells from contact inhibition. She has discovered that PAK rescues endothelial cells from contact inhibition by phosphorylating and, hence, inactivating the tumor suppressor Merlin.
Merlin is an ERM (Ezrin, Radixin, Moesin) protein, and it is encoded by the NF-2 tumor suppressor gene. This gene is inactivated in Familiar Type II Neurofibromatosis as well as in sporadic Schwannomas, meningiomas, and mesotheliomas. Mouse genetics studies indicate that Merlin may have a broader role in tumor invasion and metastasis than manifested from its specific inactivation in Schwann and mesothelial cells. In spite of the importance of Merlin as a tumor suppressor, the biochemical function of Merlin and, hence, the mechanism through which it suppresses tumorigenesis have remained elusive until recently.
Wei Li and Liru You discovered that the closed, growth inhibitory form of Merlin accumulates in the nucleus, binds to the E3 ubiquitin ligase CRL4DCAF1, and suppresses its activity. Interestingly, depletion of DCAF1 blocked the hyperproliferation caused by inactivation of Merlin. Conversely, enforced expression of a Merlin-insensitive mutant of DCAF1 counteracted the antimitogenic effect of Merlin. Furthermore, we discovered that tumor-derived missense mutations mapping to the FERM domain disrupt the ability of Merlin to enter into the nucleus and/or to bind to DCAF1. In contrast, the most prevalent truncation mutations do not interfere with nuclear entry or binding to DCAF1 but impair the ability of merlin to suppress the E3 ligase activity of CRL4DCAF1. Finally, we found that depletion of DCAF1 inhibits the hyperproliferation of primary Schwannoma cells isolated from NF2 patients and suppresses the oncogenic potential of Merlin-deficient tumor cell lines. These findings indicate that Merlin suppresses tumorigenesis by translocating to the nucleus to inhibit CRL4DCAF1.
Current efforts by Jonathan Cooper and Wei Li are directed toward identifying the physiological substrates of CRL4DCAF1 and examining their role in Merlin-dependent tumorigenesis. In addition, we are testing the effect that deletion of DCAF1 exerts on Merlin-dependent tumorigenesis in genetically engineered mouse models.