PEA-15 and RSK2 coordinately regulate integrin-mediated cell migration and cancer metastasis

Neuroblastoma is a highly malignant childhood cancer known for its aggressive metastases, which respond very poorly even to intense, combinatorial treatments. Neuroblastoma patients' survival depends on many clinical features which are taken into account in predicting the disease progression and in risk assessment of the treatment. We found that elevated expression of phosphoprotein enriched in astrocytes (PEA-15) correlated with favorable clinical features responsible for a ∼25% increase in patient survival rate. High levels of PEA-15 were found in more differentiated and less metastatic neuroblastic tumors, without MYCN amplification or 1p LOH. Importantly, PEA-15 significantly inhibited neuroblastoma migration. This inhibition required PEA-15 interaction with its binding partners: ERK and RSK2. A similar decrease in neuroblastoma migration was achieved using synthetic inhibitors of RSK2, which suppressed both the integrin activation and integrin-dependent cell migration.;PEA-15 is recognized as a multifunctional protein which serves as a molecular adaptor modulating major cellular functions and has been reported significant in several types of cancer. PEA-15 was originally identified as an inhibitor of the Ras/mitogen-activated protein kinase (MAPK) pathway regulation of integrin activation and integrin-dependent cell migration. The Ras oncogene suppresses integrins by a mechanism that is Raf- but not ERK-dependent. Here, we sought to determine the downstream effector mediating this signaling. RSK2 functions downstream of Ras/Raf and promotes cell motility and tumor metastasis, therefore we investigated whether it affects integrin function. We found that a c-Raf mutant capable of activating RSK2, but not ERK, inactivated beta1 integrins. Importantly, dominant negative RSK2 abrogated Ras suppression of integrins while active RSK2 inactivated integrins. This resulted in impaired cell adhesion and integrin-mediated matrix assembly as well as promoting cell motility. We found that RSK2 might work in part by disrupting focal adhesions and actin stress fibers. RSK2 colocalized with the integrin activator, talin, and associated with integrin cytoplasmic beta tails. It was thereby in a position to modulate integrin activation by altering the interaction of the talin-containing complex at the cytoplasmic tails. We also found that RSK2 was activated in response to integrin ligation to fibronectin. Thus RSK2 could participate in a feedback loop controlling integrin function.;Significance. This thesis provides a significant advancement in cancer biology by determining the role of PEA-15 in modulation of cell migration and revealing its potential as a metastasis suppressor in neuroblastoma. Identification of RSK2 as a potential mediator of this function presents valuable information that can be used in the design of novel therapeutic approaches in the treatment of tumors. PEA-15 also proves to be a useful marker for neuroblastoma prognosis.